Abstract

The epilepsies are one of the most common neurologic disorders with a diversity of etiologies. Despite tremendous advances in both medical and surgical therapies, availability of preventive and disease-modifying therapies is an important unmet medical need. Insight into the molecular and cellular mechanisms underlying epileptogenesis will hopefully provide novel targets for such therapies. To this end, multiple investigations using a variety of acquired epilepsy models deploying diverse etiologies (traumatic, ischemic, or seizure-induced) have identified the neurotrophin BDNF and its cognate receptor TrkB as key molecules in the pathogenesis. However, as detailed in this chapter, BDNF/TrkB signaling plays unique roles in each model. In addition to its contributions to epileptogenesis, BDNF/TrkB signaling contributes to neuronal survival following some epilepsy-inducing insults. Selective inhibition of distinct signaling pathways downstream of TrkB enables limiting TrkB-mediated epileptogenesis while preserving the protective functions. This chapter provides a critical review of the data supporting this diversity of roles for both BDNF and TrkB in the epilepsies and proposes strategies to disentangle these varying roles to develop therapies appropriate for clinical application.

Introduction

Affecting 1% of the world’s population and accounting for at least 0.5% of the global disease burden, the epilepsies represent a serious neurologic disorder with far-reaching epidemiologic impact (Begley and Durgin, 2015; Bialer et al., 2009; Chang and Lowenstein, 2003, Forsgren et al., 2005, Löscher et al., 2013). Causes are numerous and include traumatic (traumatic brain injury, subdural hematoma, intracerebral hemorrhage), vascular (stroke), immune (autoimmune encephalitis), infectious (encephalitis of varying sources, neurocysticercosis), and genetic etiologies, to name a few (Devinsky et al., 2018; Scheffer et al., 2017). For the large majority of patients with epilepsy, first-line therapies involve antiseizure medications of which over 20 have been developed for use worldwide (Scheffer et al., 2017). Despite the variety of drugs available, these therapies remain limited in that at least 30% of patients prove unable to obtain seizure control and, for the patients who do achieve benefit, these medicines provide only symptomatic relief (preventing seizures) without addressing the underlying pathology (Dichter, 2009; Loscher and Schmidt, 2011; Löscher et al., 2013). Surgical intervention is an increasingly available option thanks to development of several new techniques and approaches (Englot et al., 2011; Halpern et al., 2008; Jobst and Cascino, 2015; Le et al., 2018, Noachtar and Borggraefe, 2009; Skarpaas et al., 2019); however, surgery remains an invasive procedure carrying significant risk and morbidity, often requiring removal or destruction of brain tissue, and without any guarantee of success (Brodie et al., 2012; Englot et al., 2011; Heck et al., 2014; Löscher et al., 2013; Noachtar and Borggraefe, 2009; Téllez-Zenteno et al., 2005; Wiebe et al., 2001). As such, nondestructive, preventive, and/or disease-modifying therapies are needed. Understanding the cellular and molecular mechanisms underlying why and how a normal brain becomes epileptic, namely epileptogenesis, in a variety of clinical contexts will hopefully reveal novel targets for therapy development.

To this end, multiple investigations have identified an important role of the neurotrophin brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase, tropomyosin-related kinase B (TrkB), in epileptogenesis following a diversity of insults. Interestingly, many of these same investigations have also revealed a role for BDNF/TrkB signaling in promoting neuronal survival following insult. In this chapter, we review these studies linking BDNF/TrkB signaling with epileptogenesis and neuronal survival and then propose a model for how to disentangle these distinct functions in designing BDNF/TrkB-targeted therapies.

BDNF and TrkB Biology

As its name implies, BDNF is a member of the neurotrophin family of growth factors. The first member of this family, nerve growth factor (NGF), was discovered by Rita Levi-Montalcini, Stanley Cohen, and Viktor Hamburger as a soluble protein within mouse sarcoma tissue that could promote the growth and survival of neurons from cultured chick sensory and sympathetic ganglia (Cohen et al., 1954; Levi-Montalcini and Hamburger, 1951; Levi-Montalcini, 1952; Levi-Montalcini and Hamburger, 1953; Levi-Montalcini, 1987). Motivated by this discovery, Yves Barde and colleagues purified a similar protein from pig brain extracts that could promote the survival of cultured dorsal root ganglion neurons, a molecule they named BDNF (Barde et al., 1982). The receptors for these neurotrophins were subsequently identified, thanks to contemporaneous studies in cancer research that discovered a novel receptor containing the extracellular domain of a nonmuscle tropomyosin with the transmembrane and cytoplasmic domains of a tyrosine kinase, a receptor aptly named tropomyosin-related kinase (Trk) (Martin-Zanca et al., 1986). Despite their initial identification in cancer tissue, anatomic mapping of the Trk receptors revealed striking enrichment in neural tissues, thus providing the clue linking them to their neurotrophin ligands—TrkA for NGF and TrkB for BDNF (Kaplan et al., 1991; Klein et al., 1991).

BDNF is robustly expressed in the central nervous system (CNS) throughout development and most prominently in principal (excitatory) neurons (Conner et al., 1997; Leibrock et al., 1989, Nawa et al., 1995). It is synthesized as a precursor protein—proBDNF—and then cleaved into the mature BDNF form by proteases including furin and proprotein convertase 7 intracellularly and matrix metalloproteinases and the serine protease plasmin extracellularly (Lee et al., 2001; Wetsel et al., 2013). This mature form of BDNF complexes noncovalently with other mature BDNF molecules to form a homodimer, which is released into the extracellular space, allowing it to bind to and activate TrkB (Balkowiec and Katz, 2002; Cunningham and Greene, 1998; Griesbeck et al., 1999; Hartmann et al., 2001; Harward et al., 2016).

TrkB is expressed in virtually all neurons—both excitatory and inhibitory—within the CNS (Klein et al., 1991). Binding of the BDNF homodimer to the ectodomain of TrkB induces dimerization of two TrkB monomers. Dimerization in turn triggers activation of the intrinsic TrkB kinase and subsequent autophosphorylation of various tyrosine residues within the intracellular domain of TrkB (e.g., tyrosine 515 and tyrosine 816) (Cunningham and Greene, 1998; Middlemas et al., 1994; Minichiello, 2009). Once phosphorylated, these residues serve as binding sites for adaptor proteins such as Shc and the enzyme PLCγ1 (Begni et al., 2017; Kavanaugh and Williams, 1994; Minichiello, 2009, Reichardt, 2006). All of these adaptor proteins and enzymes, once bound to TrkB, are phosphorylated by TrkB, leading to their activation. The result is initiation of multiple downstream signaling pathways with distinct biological consequences (Fig. 32–1).

Figure 32–1.

Signal transduction pathways driven by BDNF-mediated TrkB activation.

BDNF/TrkB signaling has been implicated in numerous physiologic functions throughout development. A detailed description of each function is beyond the scope of this chapter, and interested readers are directed to the following reviews (Cappoli et al., 2020; Colucci-D’Amato et al., 2020; Minichiello, 2009; Reichardt, 2006). However, two functions of particular significance in epilepsy are its role in neuronal survival as well as synaptic plasticity. Regarding survival, BDNF was first discovered because of its pro-survival effect on dorsal root ganglion neurons (Barde et al., 1982). Since its discovery, additional investigations have validated the pro-survival effects of BDNF and TrkB in multiple contexts, including excitotoxicity and hypoxic-ischemic injury (Almeida et al., 2005; Cheng and Mattson, 1994; Han and Holtzman, 2000) and have linked such effects to TrkB-mediated Shc signaling with subsequent activation of both ERK and Akt (Atwal et al., 2000; Minichiello et al., 1998). Regarding synaptic plasticity, in vitro studies have shown that BDNF treatment of cultured neurons promotes dendritic outgrowth and spine formation (Kellner et al., 2014).

Further, experiments utilizing acute hippocampal slice preparations have confirmed that BDNF/TrkB signaling is key for the activity-dependent strengthening of multiple hippocampal synapses, a process termed long-term potentiation (LTP) (Bramham and Messaoudi, 2005; Harward et al., 2016, Huang et al., 2008; Minichiello et al., 1998, 2002). These effects have been specifically linked to TrkB-mediated PLCγ1 signaling (He et al., 2010).

BDNF/TrkB Signaling: Epileptogenesis Caused by Trauma

Head trauma is another common cause of epilepsy. Of the traumas encountered in the clinical setting, one of the most frequent is trauma secondary to nonpenetrating, concussive forces, which in turn leads to traumatic brain injury (TBI). Following TBI, 5%–7% of patients will develop seizures after a latent period of months to years (Fordington and Manford, 2020; Pitkänen and Immonen, 2014; Teasell et al., 2007; Temkin et al., 1990). One proposed mechanism underlying these seizures has been the shearing of axons of cortical neurons that can occur during the initial injury (Ding et al., 2016; Jasper, 1970; Lamar et al., 2014). The potential candidacy of axonal transection as a causal event for posttraumatic epilepsy together with the ability to study it with in vitro, in vivo, and ex vivo preparations has fueled substantial work in this area (Lucke-Wold et al., 2015). Multiple investigations utilizing these various preparations have perturbed BDNF/TrkB signaling and identified a variety of effects on the development of hyperexcitability following injury (Lin et al., 2020).

In one injury-based model, the white matter underlying the cerebral cortex is transected, thereby isolating the overlying cortex but leaving the vascular supply intact (Hoffman et al., 1994; Prince and Tseng, 1993). Such an injury has been shown to induce hyperexcitability of the overlying cortex in multiple species, including cats, monkeys, and rodents (Hoffman et al., 1994; Prince and Futamachi, 1970; Prince and Tseng, 1993; Purpura and Housepian, 1961; Salin et al., 1995; Sharpless and Halpern, 1962). Specifically, in vitro studies utilizing field potential or whole-cell recordings of isolated slices weeks after the injury have revealed epileptiform events in >80% of animals (Hoffman et al., 1994). Lasting only a few hundred milliseconds, the duration of such events parallels that seen for interictal spikes on scalp electroencephalogram (EEG) in patients with epilepsy. Frank electrographic seizures can be induced in this model by application of low concentrations of the GABAA receptor antagonist, bicuculline (Gu et al., 2018). Whether these animals develop spontaneous seizures following injury (and thus become epileptic) is not entirely clear since long-term video-EEG studies of this model are not commonly reported. However, these animals do exhibit enhanced sensitivity to chemoconvulsant-induced seizures, confirming that the injury does lower their seizure threshold, thereby promoting a state of hyperexcitability (Gu et al., 2018).

The cellular and circuit mechanisms underlying these epileptiform events have been extensively investigated by David Prince and colleagues (Prince et al., 2016). To begin, using slices isolated weeks after the initial undercutting of cortex in conjunction with current source density analyses of field potential recordings, Prince and colleagues pinpointed the origin of the epileptiform activity to layer 5 of the cortex (L5) (Hoffman et al., 1994). Subsequent whole-cell recordings of L5 principal neurons demonstrated increased frequency of both spontaneous and miniature excitatory postsynaptic currents (sEPSCs), enhanced amplitude of the α-amoino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainic acid receptor (AMPA/KA) component of the evoked EPSC, and reduced paired pulse depression of the evoked EPSC. These electrophysiologic findings collectively point toward an increase in release probability from glutamatergic terminals as one mechanism underlying the enhanced excitability seen after injury (Li and Prince, 2002; Li et al., 2005). Additionally, analyses of biocytin-filled L5 principal neurons revealed enhanced length and branching of axons (Salin et al., 1995) while immunohistochemical analysis demonstrated increased immunoreactivity of GAP-43, a marker of axonal sprouting (Prince et al., 2009). These morphologic changes in parallel with activation of L5 principal neurons via glutamate uncaging provided direct evidence of enhanced synaptic connectivity between L5 principal neurons, thereby supporting the hypothesis that emergence of an enhanced recurrent excitatory synaptic network involving pyramidal neurons could be an additional mechanism underlying the increased excitability seen after undercutting the cortex (Jin et al., 2006).

Interestingly, in addition to modifications in excitatory synaptic connectivity, Prince and colleagues found that undercutting cortex led to multiple changes in synaptic inhibition of L5 principal neurons. First, whole-cell recordings of L5 principal cells revealed reduced frequency of both spontaneous and miniature IPSCs (sIPSCs and mIPScs) but no changes in amplitude, a finding consistent with a reduced release probability from inhibitory synaptic terminals (Li and Prince, 2002). Second, morphologic analysis of interneurons, specifically parvalbumin (PV) interneurons, found no change in absolute number of interneurons but did show that these interneurons have thinner dendrites and fewer large boutons (Gu et al., 2017). And third, immunohistochemical analysis centered on the soma of L5 principal neurons identified reduced immunoreactivity of the vesicular GABA transporter (VGAT) and glutamic acid decarboxylase 65 kD and 67 kD isoforms (GAD65 and GAD67) (Gu et al., 2017). Collectively, these findings indicate a reduction in function of inhibitory interneurons following injury and suggest that undercutting cortex may have somehow led to reduced trophic support for these cells.

As noted above, BDNF and TrkB contribute to a variety of physiologic functions, including neuronal development and survival. Normally, TrkB is robustly expressed throughout the adult CNS in both principal cells and interneurons (Masana et al., 1993) while BDNF expression is largely restricted to principal cells with minimal if any expression in interneurons (Conner et al., 1997; Will et al., 2013). In the undercut cortex injury model, BDNF mRNA levels in L5 principal cells decrease at 3 weeks post injury. Such a reduction raises the possibility that following injury, BDNF-mediated trophic support and subsequent activation of TrkB receptors declines and thus could be a causal mechanism underlying the changes in PV interneuron function and morphology detailed above.

To explore this possibility further, Prince and colleagues sought to modulate BDNF/TrkB signaling following injury with a small-molecule BDNF mimetic with partial agonist properties for TrkB but not TrkA or TrkC in cell-based assays, a molecule named LM22A-4 (Massa et al., 2010). Subsequent systemic administration in vivo confirmed activation of TrkB plus the downstream effectors AKT and ERK reported by Western blotting of cortical lysates. Utilizing this small molecule, Prince and colleagues started treatment immediately after undercutting and continued for 2 weeks (Gu et al., 2018). They found that, at the termination of treatment, LM22A-4 had increased the frequency of mIPSCs in whole-cell recordings of L5 pyramidal cells in acutely isolated slices, did not change mEPSCs, reduced the frequency of interictal bursts in field potential recordings from these slices, and reduced the incidence of bicuculine-induced electrographic seizures. In parallel with these selective enhancements of synaptic inhibition, Prince and colleagues also found enhanced immunoreactivity of VGAT, GAD65, and PV around the soma of L5 principal cells. All these findings associated with LM22A-4 treatment were reversed when LM22A-4 was given in conjunction with selective inhibition of the TrkB kinase using a chemical genetic approach. This approach involves study of mice in which alanine is substituted for phenylalanine in the ATP-binding domain of TrkB kinase (TrkB^F616A), thereby rendering the mutant receptor sensitive to inhibition by the blood–brain barrier-permeable, small-molecule 1NMPP1 (Chen et al., 2005). Although the effects of inhibiting TrkB kinase alone were not reported, these results suggest that the effects of LM22A-4 on synaptic inhibition in acute slices occur through a TrkB-dependent mechanism. Unfortunately, whether these ex vivo effects of LM22A-4 impact the development of spontaneous recurrent seizures in vivo in this model was not reported, thus precluding determination of whether this treatment is truly antiepileptogenic. However, in vivo treatment with an analog of LM22A-4 did reduce the enhanced sensitivity to the chemoconvulsant pentylenetetrazol typically seen after undercutting the cortex (Gu et al., 2018). In sum, studies of this model support the idea that reduced activation of TrkB within PV interneurons contributes to increased excitability in a small cortical circuit. Selectively inhibiting TrkB signaling within PV interneurons alone will enable testing the hypothesis that loss of BDNF/TrkB-mediated trophic support of PV interneurons causes a reduction of synaptic inhibition of principal neurons and the observed hyperexcitability.

In contrast to the neocortical axonal transection model described above, similar studies have examined the effects of axonal transection within the hippocampus, in particular transection of the Shaffer collateral axons by which CA3 pyramidal cells innervate CA1 pyramidal cells (McKinney et al., 1997). Transecting these axons in cultured hippocampal slices induced sprouting of CA3 pyramidal cell axons as well as marked increases in synaptophysin and GAP-43 immunoreactivity within axons (markers of synaptic vesicles and axon growth, respectively). These increases in GAP-43 immunoreactivity correlated with enhanced sEPSPs and sIPSPs seen with whole-cell recordings of CA3 pyramidal cells in the lesioned hippocampi. Further, dual whole-cell recordings of distinct CA3 pyramidal cells revealed increased synaptic coupling between cells in the lesioned versus nonlesioned hippocampi. Unlike these observations for excitatory transmission, synaptic inhibition in lesioned hippocampi was unaltered as evident by normal numbers of spontaneous and evoked IPSPs and levels of GABA immunoreactivity. These findings collectively suggest that transection of CA3 pyramidal cell axons leads to axonal sprouting, increased synaptic connectivity between pyramidal neurons, and enhanced excitability throughout the hippocampal circuitry.

In search of a molecular mechanism, prior evidence linking neurotrophins to axonal growth led to the study of BDNF/TrkB signaling in this model of trauma-induced hyperexcitability (Huang and Reichardt, 2001). Consistent with a possible role following injury, both BDNF and TrkB protein levels increase after transection of CA3 pyramidal axons. Such increases peaked in the hours immediately after the lesion before returning to normal after approximately 72 hours (Dinocourt et al., 2006). Additionally, the previously documented increases in GAP-43 immunoreactivity seen after transecting the CA3 to CA1 connection were virtually eliminated in slices cultured from genetically modified mice exhibiting low levels of TrkB. These in vitro data supported a causal role of BDNF/TrkB signaling in the lesion-induced axonal sprouting and provided the rationale for investigating the contribution of BDNF/TrkB signaling to axonal sprouting and hyperexcitability induced by a knife cut in vivo.

As predicted, extracellular recordings of area CA3 from slices isolated after disruption of the CA3-CA1 pathway in vivo revealed epileptiform bursting, a finding absent in slices isolated from sham-treated animals (Aungst et al., 2013). Such abnormal bursting was first detected 7–21 days after lesioning but not earlier, suggesting a latent period between injury and emergence of hyperexcitability. The authors also found that GAP-43 immunoreactivity was increased at 7 but not 1, 14, or 21 days following injury. With respect to TrkB, Western blot analyses revealed increased TrkB phosphorylation in the first 24 hours following injury but not afterward. To probe whether the observed TrkB activation is critical for the changes seen in extracellular recordings and GAP-43 immunoreactivity, the authors selectively inhibited TrkB kinase through a chemical genetic approach detailed above. As predicted, the previously seen increases in GAP-43 immunoreactivity and the associated signs of hyperexcitability in this model were prevented by TrkB inhibition, supporting a critical role for TrkB signaling in promoting both axonal sprouting and hyperexcitability following hippocampal axonal transection. These results were reinforced by experiments using the BDNF scavenger, TrkB-Fc. In these experiments, TrkB-Fc treatment prevented the increase in actional potential firing, evoked fEPSPs, and axonal sprouting previously seen after injury in this model (Gill et al., 2013).

In sum—and in stark contrast to the neocortical axonal transection—studies of this hippocampal axon transection model reveal that enhanced TrkB signaling causes increased excitability in a small cortical circuit. Moreover, the data are consistent with the idea that enhanced BDNF/TrkB signaling promotes axonal sprouting and development of the enhanced recurrent excitatory synaptic connections underlying the observed increases in excitability.

Collectively, these two models provide compelling evidence implicating BDNF/TrkB signaling in these models of trauma-induced hyperexcitability. That said, the results differ drastically in that BDNF/TrkB signaling promotes increased excitability following axonal transection in the hippocampus but decreased excitability after axonal transection in the neocortex. Additionally, the cellular consequences of BDNF/TrkB signaling in the neocortex appear to impact trophic support of interneurons but in hippocampus impact axonal sprouting among excitatory neurons. Differences in the organization of neocortex and hippocampus notwithstanding, the opposite consequences of BDNF/TrkB signaling on the lesion-induced hyperexcitability in these locales are unexpected.

BDNF/TrkB Signaling: Development of Epilepsy Caused by Hypoxic/Ischemic Insults

Like trauma, hypoxic-ischemic injury can lead to development of epilepsy later in life. In neonates, for example, hypoxic-ischemic injury accounts for more than 50% of neonatal seizures, with half of these patients eventually developing epilepsy (Boylan and Pressler, 2013; Kang and Kadam, 2015). Neonatal seizures often prove refractory to common antiseizure medications, a clinical observation thought to reflect the depolarizing (and thus activating) nature of GABA during early development (Ben-Ari et al., 2011; Dzhala et al., 2005).

In the adult brain, activation of GABAA receptors induces neuronal hyperpolarization. However, in the developing brain, GABA binding to its receptor has the opposite effect, resulting in neuronal depolarization. The mechanism underlying these age-dependent differences lies in the chloride equilibrium potential. In early development, the chloride equilibrium potential favors chloride efflux, and thus GABAA receptor activation is depolarizing. In the mature brain, the chloride equilibrium potential favors chloride influx, and GABAA receptor activation is hyperpolarizing. One factor influencing the chloride equilibrium potential is the potassium/chloride co-transporter KCC2, which extrudes chloride from neurons. BDNF-mediated TrkB activation reduces KCC2 expression (Rivera et al., 2004), which can promote accumulation of intracellular chloride and render GABAA receptor activation depolarizing. This provided the rationale for investigating BDNF/TrkB signaling and KCC2 expression in the context of hypoxic-ischemic injury-induced seizures in the developing brain.

In one hypoxia-based model, postnatal day 10 (P10) rat pups were exposed to 15 minutes of graded global hypoxia, which in turn led to increased hippocampal excitability and enhanced sensitivity to the chemoconvulsant kainic acid (KA) at postnatal day 14 (P14) (Rakhade et al., 2008; Talos et al., 2012). Western blot analyses of hippocampal membranes revealed increased TrkB phosphorylation, a surrogate for activation of TrkB, that peaked at 12 hours post insult and then normalized by 24 hours, suggesting a role in epileptogenesis after hypoxia (Obeid et al., 2014). Consistent with such a role, administration of a single dose of lestaurtinibid, an FDA-approved inhibitor of tyrosine kinases,15 minutes after the hypoxic insult prevented the insult-induced increases in TrkB activation as well as the enhanced sensitivity to KA (Obeid et al., 2014). TrkB is one of the kinases inhibited by lestaurtinibid (Shabbir and Stuart, 2010), raising the possibility that enhanced TrkB signaling contributed to hypoxia-induced enhanced sensitivity to KA.

An alternative model deployed unilateral carotid ligation of postnatal day 7 (P7) mice causing an ischemic insult (Carter et al., 2018). Consistent with clinical observations, these mice developed seizures that proved refractory to phenobarbital, an antiseizure agent that enhances GABAA receptor–mediated responses (Löscher and Rogawski, 2012). This was paralleled by enhanced TrkB phosphorylation and reduced expression of KCC2. Initiating treatment with an antagonist of TrkB (ANA12) immediately following carotid ligation prevented the injur-induced biochemical changes and enhanced the sensitivity to the antiseizure effects of phenobarbital (Cazorla et al., 2011). Taken together, the authors conclude that hypoxic-ischemic insult leads to TrkB activation, which in turn downregulates KCC2, leading to a chloride ion gradient that renders GABA depolarizing. Such a chloride ion gradient also explains the resistance to phenobarbital, an antiseizure agent that acts as a positive allosteric modulator of GABAA receptors.

In a follow-up study, these same investigators examined the effects of partial and full TrkB agonists—LM22A-4, HIOC, and deoxygedunin (DG)—on hyperexcitability following hypoxic insult (Kipnis et al., 2020). Surprisingly, they found that these three agents that activate TrkB enhanced sensitivity to the antiseizure effects of phenobarbital, effects paralleled by rescuing insult-mediated reductions of KCC2 expression. Effects on TrkB activation assessed by ratio of pTrkB/TrkB were not reported. These results are surprising because these authors previously reported similar effects with an antagonist of TrkB, ANA12. The in vitro work demonstrating ability of LM22A-4, HIOC, and DG (Jang et al., 2010; Massa et al., 2010; Shen et al., 2012) to activate TrkB notwithstanding, the effects of these molecules on TrkB activation in vivo and in this mouse model were not reported. While these studies suggest a role for BDNF/TrkB signaling in modulating KCC2 expression and sensitivity to antiseizure effects of phenobarbital following this ischemic insult, whether the role involves activation or inhibition of TrkB signaling remains to be elucidated.

BDNF/TrkB Signaling: Development of Epilepsy Caused by Seizures

A third cause of epilepsy appears to be seizures themselves. In the late nineteenth century, the astute British neurologist Sir William Gowers noted that “seizures beget seizures” in that “the minor fits may occur alone at the commencement of the disease, and then, after months or years of slight seizures, the severe fits may occur [ . . . ] The tendency of the disease is to self-perpetuation; each attack facilitates the occurrence of another” (Gowers, 1881).

Almost a century later, Graham Goddard discovered the kindling model providing experimental evidence in support of Gowers’s hypothesis (Goddard, 1969). In this model, a small and brief electrical stimulus is introduced through an implanted electrode. Response to the stimulus varies with the anatomic area targeted. Initial stimulation of the amygdala, for example, results in a brief (a few seconds) electrographic seizure localized to the area of stimulation and without any overt behavioral change. However, with repeated stimulations, electrographic seizures increase in duration and anatomic spread. After about 10–15 stimulations, electrographic seizures are accompanied by behavioral seizures that secondarily generalize, resulting in tonic-clonic muscle contractions. After about 70 stimulations, animals develop spontaneous seizures that are often quite severe and at times fatal.

Whereas studies of the kindling model demonstrated that >70 or so isolated seizures can induce epilepsy, a single episode of prolonged seizures (aka “status epilepticus” [SE]) can be sufficient to induce lifelong epilepsy. Jackie French and colleagues found that more than half of patients with medically refractory temporal lobe epilepsy (TLE) had experienced an episode of prolonged seizures as an infant or child, often in the setting of a febrile illness (French et al., 1993). Subsequent clinical observations and longitudinal studies reveal that SE is often followed by TLE after a variable latent period of months to years depending on age (Lewis et al., 2014; Shinnar et al., 2008; VanLandingham et al., 1998). In children, 13%–82% who experience afebrile convulsive SE eventually develop epilepsy after a long latent period of up to 10 years (Raspall-Chaure et al., 2006), whereas in adults and teens, only 30% who experience SE develop epilepsy within 2 years (Hesdorffer et al., 1998).

These distinct lines of evidence supporting seizures as both a cause of epilepsy and a contributor to its progression have provided a strong rationale for investigating the molecular mechanisms underlying its pathogenesis. As such, the question arose: how does a transient episode of intense neuronal activity such as a seizure result in long-lasting changes in both the structure and function of the brain? Because of its known regulation by neuronal activity plus its critical roles in synaptic plasticity, learning, and memory, BDNF was identified as one possible answer.

Early experiments found striking increases in BDNF mRNA and protein following seizures evoked in animal models, providing additional evidence of activity-dependent regulation of expression of this gene (Gall et al., 1991; Isackson et al., 1991). Such increases were widespread in neurons throughout neocortex and particularly robust within the hippocampus, specifically the dentate granule cells and CA3 and CA1 pyramidal cells. As noted above, activation of TrkB signaling requires the phosphorylation of tyrosine residues in its cytoplasmic domain; thus, antibodies selectively detecting phosphorylated TrkB can provide a surrogate measure of its activation. Using such antibodies in both biochemical and immunohistochemical analyses revealed that seizures also enhanced TrkB activation and localized it to two distinct populations of neurons within hippocampus: the dentate granule cells and CA1 pyramidal cells. Notably, the enhanced phosphorylation seen for TrkB was localized to synapses, specifically the presynaptic mossy fiber boutons of the granule cells and dendritic spines of apical but not basal dendrites of CA1 pyramidal cells (Binder and Routbort, 1999; He et al., 2010; Helgager and Liu, 2013).

The seizure-induced expression of BDNF and parallel enhancement of TrkB activation advanced BDNF/TrkB signaling as one candidate molecular mechanism by which seizures could promote epileptogenesis. To investigate this possibility, initial work utilized the kindling model.

An absence of pharmacologic inhibitors specific for BDNF/TrkB signaling led investigators to probe genetic perturbations. Using mice heterozygous for BDNF (BDNF^+/–), Kokaia and colleagues found a marked impairment in epileptogenesis in the kindling model compared to wild-type animals (Kokaia et al., 1995). This result was confirmed by additional experiments utilizing direct injection of BDNF scavenging proteins into the ventricles of the brain (Binder and Routbort, 1999). Intraventricular infusion of the BDNF scavenger markedly impaired seizure development, while infusion of scavengers of other neurotrophins (NGF or NT-3) had no effect. Collectively, these results provided compelling evidence of a requirement for endogenous BDNF in epileptogenesis. Moreover, transgenic overexpression of BDNF proved sufficient to induce spontaneous recurrent seizures as well as enhanced sensitivity to chemoconvulsant-induced seizures (Croll et al., 1999). Genetic and pharmacological evidence implicating BDNF was paralleled by studies of TrkB itself. Conditional deletion of TrkB from forebrain neurons using a synapsin promoter-driven expression of Cre recombinase revealed striking impairments of epileptogenesis in the kindling model (He et al., 2004). Collectively, these results provided strong evidence linking both BDNF and TrkB to epileptogenesis in this model. As such, the question was raised as to whether BDNF/TrkB signaling could also promote epileptogenesis induced by an episode of SE.

In the studies of the kindling model considered above, all perturbations of BDNF or TrkB were instituted prior to induction of epileptogenesis. Because perturbation of BDNF/TrkB signaling attenuates seizures evoked in the kindling model, inhibiting BDNF/TrkB prior to SE may attenuate the SE itself, thereby confounding interpretation of results. To determine whether a perturbation can prevent epilepsy induced by SE, it is essential to introduce the perturbation after the SE and continue it for short duration, the latter enabling clear distinction between antiseizure and antiepileptogenic effects. To this end, Liu et al. (2013) utilized a chemical-genetic approach for inhibiting TrkB activation (Chen et al., 2005). Initiating treatment following SE and continuing it for 2 weeks, 1NMPP1 prevented development of seizures in TrkB^F616A but not wild-type animals in the weeks following termination of treatment. These results implicate a causal role of TrkB signaling in SE-induced epileptogenesis and advance TrkB signaling as a target for preventive interventions following an insult like SE. The seizure-evoked increase of expression of BDNF (but neither NT-3 nor NT-4) renders BDNF the neurotrophin most likely responsible for activating TrkB in this context (Ernfors et al., 1991; Gall et al., 1991; Isackson et al., 1991). Further strengthening the candidacy of BDNF are the facts that ligands scavenging TrkB (but not TrkA or C) inhibit seizure-induced epileptogenesis (Binder and Routbort, 1999) and that null mutations of NT-4 have no effect on seizure-induced epileptogenesis (He et al., 2006).

BDNF/TrkB Signaling: A Role in Neuronal Survival

One consequence of SE is excitotoxic injury culminating in death of neurons as revealed in preclinical studies (Sankar et al., 1998). Massive swelling of a hippocampus has been identified in magnetic resonance imaging shortly after SE in children (VanLandingham et al., 1998), sequential studies revealing the progressive atrophy consistent with mesial temporal sclerosis (Lewis et al., 2014). Because BDNF and TrkB have known roles in neuronal survival, the question arose as to whether activation of TrkB signaling by SE might seek to limit the damage and promote neuronal survival in this context.

To explore this possibility, Gu et al. (2015) assessed SE-induced damage with Fluoro-Jade C staining of hippocampal neurons 24 hours after SE. Indeed, chemical genetic inhibition of TrkB kinase following SE led to significantly higher numbers of Fluoro-Jade C-positive neurons. This finding revealed a potential hazard of inhibiting TrkB signaling immediately following a CNS insult, namely exacerbating insult-induced cell death.

Might it be possible to disentangle the beneficial (antiepileptogenic) from the deleterious (enhanced cell death) effects of TrkB kinase inhibition following SE? Insight into the pleiotropic consequences of TrkB activation and the responsible downstream signaling pathways suggested that this may be possible. BDNF-mediated TrkB activation triggers multiple signaling pathways through phosphorylation of its various residues, most notably tyrosine 515 and 816 (Y515 and Y816, respectively). For Y515, phosphorylation leads to binding of the Shc adaptor protein and activation of the Ras/MAP kinase and subsequently of both Erk and Akt, signaling known to promote neuronal survival in both in vitro and in vivo experiments (Atwal et al., 2000; Huang et al., 2019; Minichiello et al., 1998). By contrast, phosphorylation of Y816 of TrkB leads to binding and phosphorylation of the enzyme PLCγ1, its activation mediating hydrolysis of phosphatidyl inositol bisphosphate and formation of diacylglycerol and inositol phosphate 3. This signaling cascade has been implicated in synaptic plasticity, specifically potentiation of the synaptic connections between excitatory neurons (LTP) (Bramham and Messaoudi, 2005; Harward et al., 2016; Huang et al., 2008; Minichiello et al., 1998; Minichiello et al., 2002). Collectively these findings led to the hypothesis that pro-survival consequences of TrkB activation induced by SE are mediated through TrkB/Shc signaling, whereas the epileptogenic consequences of TrkB activation are mediated by TrkB/PLCγ1 signaling.

To explore this hypothesis, researchers turned to genetically modified mice in which TrkB/Shc signaling was selectively eliminated by substituting phenylalanine for tyrosine 515 in the TrkB intracellular domain (TrkB^Y515F). Behavioral and electrographic features of SE in these mice did not differ from wild-type controls (He et al., 2006), consistent with prior work demonstrating similar rates of kindling between wild-type and TrkB^Y515F mice (He et al., 2010). However, following SE, the TrkB^Y515F mutant mice exhibited significantly higher amounts of cell death in conjunction with a lower degree of Akt activation (He et al., 2006). In parallel to these findings, mice in which TrkB/PLCγ1 signaling is selectively eliminated by substituting phenylalanine for tyrosine 816 (TrkB^Y816F) exhibited a striking decrease in epileptogenesis in the kindling model (He et al., 2010). Collectively, these results supported the notion of a dichotomy of function arising from TrkB, namely one signaling pathway contributing to neuronal survival after an insult (TrkB-Shc signaling) and another pathway leading to epileptogenesis (TrkB-PLC signaling).

As noted above, interpreting the results of experiments relying on genetic perturbations of BDNF/TrkB are confounded by the fact that inhibition of TrkB, specifically TrkB-PLCγ1 signaling itself, limits the severity of SE. Consequently, Gu et al. (2015) developed a pharmacologic approach to specifically inhibit TrkB-PLCγ1 signaling, enabling initiation of inhibition after the SE. Knowledge of the motif of TrkB that bound the SH2 domain of PLCγ1 led the authors to design a small peptide (pY816) that contained these amino acids. Fusing this peptide to the HIV-1 Tat sequence enabled it to traverse cell membranes as well as the blood–brain barrier. The idea was that pY816 itself would bind PLCγ1 and prevent its binding to and subsequent activation by TrkB without impacting TrkB-mediated Shc signaling. Initiating treatment with pY816 following SE and continuing it for just two additional days led to striking reduction of epilepsy assessed several weeks later in comparison to control animals treated with a scrambled peptide sequence. Importantly, no differences in SE-induced cell death were identified between pY816 and control peptide-treated animals, demonstrating preservation of TrkB/Shc signaling in this context. Together these findings established the feasibility of inhibiting distinct signaling pathways downstream of TrkB in vivo while preserving others, thereby enabling dissociation of the beneficial and deleterious consequences of TrkB activation following SE.

Summary and Perspective

Analyses of models of epileptogenesis caused by trauma (axonal transection), hypoxic ischemic injury in neonates, and seizures have revealed multiple contributions of BDNF/TrkB signaling. In the neocortical axonal transection model, reduced BDNF/TrkB signaling promotes hyperexcitabilty, potentially through impaired trophic support of inhibitory interneurons. By contrast, in the hippocampal axonal transection model, enhanced BDNF/TrkB signaling promotes hyperexcitability, potentially through promoting formation of recurrent excitatory circuits. In the hypoxic injury model of neonates, enhanced BDNF/TrkB signaling may promote hyperexcitability. In the carotid ligation ischemic injury model of neonates, BDNF/TrkB signaling enhances antiseizure effects of phenobarbital; whether enhanced or reduced TrkB signaling is responsible is uncertain. Finally, in epileptogenesis induced by isolated seizures or SE, activation of BDNF/TrkB signaling promotes epileptogenesis.

Analyses of the signaling pathways by which TrkB activation promotes epileptogenesis has been elucidated in the models in which seizures themselves are the causal factor. While seizure-induced TrkB signaling activates multiple downstream pathways, use of genetic and pharmacological tools revealed that TrkB/PLCγ1 signaling promotes epileptogenesis. By contrast, TrkB/Shc signaling pathways promote neuronal survival but have no detectable effect on epileptogenesis. We suggest that SE activates TrkB signaling in an effort to limit death of CNS neurons induced by this insult; unfortunately, the parallel activation of PLCγ1 signaling promotes an unwanted consequence, namely epileptogenesis. In this view, development of TLE represents a maladaptive consequence of a homeostatic response to the insult of SE; restated, development of epilepsy is a side effect of the brain’s protective response to an insult. We suspect that a similar scenario underlies pathogenesis of many diseases.

Elucidating the causal role of TrkB-mediated activation of PLCγ1 signaling to epileptogenesis caused by seizures provides an opportunity for therapeutic intervention. Inhibiting TrkB/PLCγ1 signaling for just 3 days following SE prevented development of epilepsy while sparing neuronal survival benefits, thereby providing the rationale for prevention of epilepsy caused by SE. Inhibiting TrkB/PLCγ1 for just 3 days following an evoked seizure-induced regression of epileptogenesis in the kindling model, raising the possibility of attenuating the severity of TLE after its emergence (Krishnamurthy et al., 2019). These findings underscore the importance of determining whether briefly inhibiting TrkB/PLCγ1 following a spontaneous (not evoked) seizure reduces severity of epilepsy and potentially transforms a patient from medical refractoriness to medically responsiveness.

References

1. Almeida, R. D., Manadas, B. J., Melo, C. V., Gomes, J. R., Mendes, C. S., Grãos, M. M., Carvalho, R. F., Carvalho, A. P. & Duarte, C. B.  2005. Neuroprotection by BDNF against glutamate-induced apoptotic cell death is mediated by ERK and PI3 kinase pathways.  Cell Death and Differentiation, 12, 1329–1343. [PubMed: 15905876]
2. Atwal, J. K., Massie, B., Miller, F. D. & Kaplan, D. R.  2000. The TrkB-Shc site signals neuronal survival and local axon growth via MEK and P13-kinase.  Neuron, 27, 265–277. [PubMed: 10985347]
3. Aungst, S., England, P. M. & Thompson, S. M.  2013. Critical role of trkB receptors in reactive axonal sprouting and hyperexcitability after axonal injury.  Journal of Neurophysiology, 109, 813–824. [PMC free article: PMC3567381] [PubMed: 23155176]
4. Balkowiec, A. & Katz, D. M.  2002. Cellular mechanisms regulating activity-dependent release of native brain-derived neurotrophic factor from hippocampal neurons.  The Journal of Neuroscience, 22, 10399–10407. [PMC free article: PMC6758764] [PubMed: 12451139]
5. Barde, Y. A., Edgar, D. & Thoenen, H.  1982. Purification of a new neurotrophic factor from mammalian brain.  The EMBO Journal, 1, 549–553. [PMC free article: PMC553086] [PubMed: 7188352]
6. Begley, C. E. & Durgin, T. L.  2015. The direct cost of epilepsy in the United States: A systematic review of estimates.  Epilepsia, 56, 1376–1387. [PubMed: 26216617]
7. Begni, V., Riva, M. A. & Cattaneo, A.  2017. Cellular and molecular mechanisms of the brain-derived neurotrophic factor in physiological and pathological conditions.  Clinical Science, 131, 123–138. [PubMed: 28011898]
8. Ben-Ari, Y., Tyzio, R. & Nehlig, A.  2011. Excitatory action of GABA on immature neurons is not due to absence of ketone bodies metabolites or other energy substrates.  Epilepsia, 52, 1544–1558. [PubMed: 21692780]
9. Bialer, M., Johannessen, S. I., Levy, R. H., Perucca, E., Tomson, T. & White, H. S.  2009. Progress report on new antiepileptic drugs: a summary of the Ninth Eilat Conference (EILAT IX).  Epilepsy Research, 83, 1–43. [PubMed: 19008076]
10. Binder, D. K. & Routbort, M. J. A.  1999. Immunohistochemical evidence of seizure-induced activation of trk receptors in the mossy fiber pathway of adult rat hippocampus.  The Journal of Neuroscience, 19, 4616–4626. [PMC free article: PMC6782602] [PubMed: 10341259]
11. Boylan, G. B. & Pressler, R. M.  2013. Neonatal seizures: the journey so far.  Seminars in Fetal \& Neonatal Medicine, 18, 173–174. [PubMed: 23790527]
12. Bramham, C. R. & Messaoudi, E.  2005. BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis.  Progress in Neurobiology, 76, 99–125. [PubMed: 16099088]
13. Brodie, M. J., Barry, S. J. E., Bamagous, G. A., Norrie, J. D. & Kwan, P.  2012. Patterns of treatment response in newly diagnosed epilepsy.  Neurology, 78, 1548–1554. [PMC free article: PMC3348850] [PubMed: 22573629]
14. Cappoli, N., Tabolacci, E., Aceto, P. & Dello Russo, C.  2020. The emerging role of the BDNF-TrkB signaling pathway in the modulation of pain perception.  Journal of Neuroimmunology, 349, 577406. [PubMed: 33002723]
15. Carter, B. M., Sullivan, B. J., Landers, J. R. & Kadam, S. D.  2018. Dose-dependent reversal of KCC2 hypofunction and phenobarbital-resistant neonatal seizures by ANA12.  Scientific Reports, 8, 11987. [PMC free article: PMC6086916] [PubMed: 30097625]
16. Cazorla, M., Prémont, J., Mann, A., Girard, N., Kellendonk, C. & Rognan, D.  2011. Identification of a low-molecular weight TrkB antagonist with anxiolytic and antidepressant activity in mice.  The Journal of Clinical Investigation, 121, 1846–1857. [PMC free article: PMC3083767] [PubMed: 21505263]
17. Chang, B. S. & Lowenstein, D. H.  2003. Epilepsy.  The New England Journal of Medicine, 349, 1257–1266. [PubMed: 14507951]
18. Chen, X., Ye, H., Kuruvilla, R., Ramanan, N., Scangos, K. W., Zhang, C., Johnson, N. M., England, P. M., Shokat, K. M. & Ginty, D. D.  2005. A chemical-genetic approach to studying neurotrophin signaling.  Neuron, 46, 13–21. [PubMed: 15820690]
19. Cheng, B. & Mattson, M. P.  1994. NT -3 and BDNF protect CNS neurons against metabolic/excitotoxic insults.  Brain Research, 640, 56–67. [PubMed: 7911729]
20. Cohen, S., Levi-Montalcini, R. & Hamburger, V.  1954. A nerve growth-stimulating factor isolated from sarcom as 37 and 180.  Proceedings of the National Academy of Sciences of the United States of America, 40, 1014–1018. [PMC free article: PMC534215] [PubMed: 16589582]
21. Colucci-D’amato, L., Speranza, L. & Volpicelli, F.  2020. Neurotrophic factor BDNF, physiological functions and therapeutic potential in depression, neurodegeneration and brain cancer.  International Journal of Molecular Sciences, 21(20), 7777. [PMC free article: PMC7589016] [PubMed: 33096634]
22. Conner, J. M., Lauterborn, J. C., Yan, Q., Gall, C. M. & Varon, S.  1997. Distribution of brain-derived neurotrophic factor (BDNF) protein and mRNA in the normal adult rat CNS : evidence for anterograde axonal transport.  The Journal of Neuroscience, 17, 2295–2313. [PMC free article: PMC6573520] [PubMed: 9065491]
23. Croll, S. D., Suri, C., Compton, D. L., Simmons, M. V., Yancopoulos, G. D., Lindsay, R. M., Wiegand, S. J., Rudge, J. S. & Scharfman, H. E.  1999. Brain-derived neurotrophic factor transgenic mice exhibit passive avoidance deficits, increased seizure severity and in vitro hyperexcitability in the hippocampus and entorhinal cortex.  Neuroscience, 93, 1491–1506. [PMC free article: PMC2504500] [PubMed: 10501474]
24. Cunningham, M. E. & Greene, L. A.  1998. A function-structure model for NGF-activated TRK.  The EMBO Journal, 17, 7282–7293. [PMC free article: PMC1171074] [PubMed: 9857185]
25. Devinsky, O., Vezzani, A., O’brien, T. J., Jette, N., Scheffer, I. E., De Curtis, M. & Perucca, P.  2018. Epilepsy.  Nature Reviews. Disease Primers, 4, 18024. [PubMed: 29722352]
26. Dichter, M. A.  2009. Posttraumatic epilepsy: the challenge of translating discoveries in the laboratory to pathways to a cure.  Epilepsia, 50 Suppl 2, 41–45. [PubMed: 19187293]
27. Ding, K., Gupta, P. K. & Diaz-Arrastia, R.  2016. Epilepsy after traumatic brain injury. In D. Laskowitz (Eds.) et. al., Translational Research in Traumatic Brain Injury. CRC Press/Taylor and Francis Group. [PubMed: 26583175]
28. Dinocourt, C., Gallagher, S. E. & Thompson, S. M.  2006. Injury-induced axonal sprouting in the hippocampus is initiated by activation of trkB receptors.  The European Journal of Neuroscience, 24, 1857–1866. [PubMed: 17040478]
29. Dzhala, V. I., Talos, D. M., Sdrulla, D. A., Brumback, A. C., Mathews, G. C., Benke, T. A., Delpire, E., Jensen, F. E. & Staley, K. J.  2005. NKCC1 transporter facilitates seizures in the developing brain.  Nature Medicine, 11, 1205–1213. [PubMed: 16227993]
30. Englot, D. J., Chang, E. F. & Auguste, K. I.  2011. Vagus nerve stimulation for epilepsy: a meta-analysis of efficacy and predictors of response.  Journal of Neurosurgery, 115, 1248–1255. [PubMed: 21838505]
31. Ernfors, P., Bengzon, J., Kokaia, Z., Persson, H. & Lindvall, O.  1991. Increased levels of messenger RNAs for neurotrophic factors in the brain during kindling epileptogenesis.  Neuron, 7, 165–76. [PubMed: 1829904]
32. Fordington, S. & Manford, M.  2020. A review of seizures and epilepsy following traumatic brain injury.  Journal of Neurology, 267, 3105–3111. [PMC free article: PMC7501105] [PubMed: 32444981]
33. Forsgren, L., Hauser, W. A., Olafsson, E., Sander, J. W. A. S., Sillanpää, M. & Tomson, T.  2005. Mortality of epilepsy in developed countries: a review.  Epilepsia, 46 Suppl 11, 18–27. [PubMed: 16393174]
34. French, J. A., Williamson, P. D., Thadani, V. M., Darcey, T. M., Mattson, R. H., Spencer, S. S. & Spencer, D. D.  1993. Characteristics of medial temporal lobe epilepsy: I. Results of history and physical examination.  Annals of Neurology, 34, 774–780. [PubMed: 8250525]
35. Gall, C., Lauterborn, J., Bundman, M., Murray, K. & Isackson, P.  1991. Seizures and the regulation of neurotrophic factor and neuropeptide gene expression in brain.  Epilepsy Research. Supplement, 4, 225–245. [PubMed: 1815605]
36. Gill, R., Chang, P. K. Y., Prenosil, G. A. & Deane, E. C. A.  2013. Blocking brain-derived neurotrophic factor inhibits injury-induced hyperexcitability of hippocampal CA3 neurons.  The European Journal of Neuroscience, 38, 3554–3566. [PubMed: 24118418]
37. Goddard, G. V. A.  1969. A permanent change in brain function resulting from daily electrical stimulation.  Experimental Neurology, 25, 295–330. [PubMed: 4981856]
38. Gowers, W. R. 1881. Epilepsy and other chronic convulsive diseases their causes, symptoms.
39. Griesbeck, O., Canossa, M., Campana, G., Gärtner, A., Hoener, M. C., Nawa, H., Kolbeck, R. & Thoenen, H.  1999. Are there differences between the secretion characteristics of NGF and BDNF? Implications for the modulatory role of neurotrophins in activity-dependent neuronal plasticity.  Microscopy Research and Technique, 45, 262–275. [PubMed: 10383119]
40. Gu, B., Huang, Y. Z., He, X.-P., Joshi, R. B. & Jang, W. A.  2015. A peptide uncoupling BDNF receptor TrkB from phospholipase C\gamma1 prevents epilepsy induced by status epilepticus.  Neuron, 88, 484–491. [PMC free article: PMC4636438] [PubMed: 26481038]
41. Gu, F., Parada, I., Shen, F., Li, J., Bacci, A., Graber, K., Taghavi, R. M., Scalise, K., Schwartzkroin, P., Wenzel, J. & Prince, D. A.  2017. Structural alterations in fast-spiking GABAergic interneurons in a model of posttraumatic neocortical epileptogenesis.  Neurobiology of Disease, 108, 100–114. [PMC free article: PMC5927780] [PubMed: 28823934]
42. Gu, F., Parada, I., Yang, T., Longo, F. M. & Prince, D. A.  2018. Partial TrkB receptor activation suppresses cortical epileptogenesis through actions on parvalbumin interneurons.  Neurobiology of Disease, 113, 45–58. [PubMed: 29408225]
43. Halpern, C. H., Samadani, U., Litt, B., Jaggi, J. L. & Baltuch, G. H.  2008. Deep brain stimulation for epilepsy.  Neurotherapeutics, 5, 59–67. [PMC free article: PMC2941772] [PubMed: 18164484]
44. Han, B. H. & Holtzman, D. M.  2000. BDNF protects the neonatal brain from hypoxic-ischemic injury in vivo via the ERK pathway.  The Journal of Neuroscience, 20, 5775–5781. [PMC free article: PMC6772561] [PubMed: 10908618]
45. Hartmann, M., Heumann, R. & Lessmann, V.  2001. Synaptic secretion of BDNF after high-frequency stimulation of glutamatergic synapses.  The EMBO Journal, 20, 5887–5897. [PMC free article: PMC125691] [PubMed: 11689429]
46. Harward, S. C., Hedrick, N. G., Hall, C. E., Parra-Bueno, P., Milner, T. A., Pan, E., Laviv, T., Hempstead, B. L. & Yasuda, R. A.  2016. Autocrine BDNF-TrkB signalling within a single dendritic spine.  Nature, 538, 99–103. [PMC free article: PMC5398094] [PubMed: 27680698]
47. He, X.-P., Kotloski, R., Nef, S., Luikart, B. W. & Parada, L. F. A.  2004. Conditional deletion of TrkB but not BDNF prevents epileptogenesis in the kindling model.  Neuron, 43, 31–42. [PubMed: 15233915]
48. He, X. P., Butler, L., Liu, X. & Mcnamara, J. O.  2006. The tyrosine receptor kinase B ligand, neurotrophin-4, is not required for either epileptogenesis or tyrosine receptor kinase B activation in the kindling model.  Neuroscience, 141, 515–20. [PubMed: 16650613]
49. He, X. P., Pan, E., Sciarretta, C. & Minichiello, L. A.  2010. Disruption of TrkB -mediated phospholipase Cgamma signaling inhibits limbic epileptogenesis.  The Journal of Neuroscience, 30, 6188–6196. [PMC free article: PMC2872928] [PubMed: 20445044]
50. Heck, C. N., King-Stephens, D., Massey, A. D., Nair, D. R., Jobst, B. C., Barkley, G. L., Salanova, V., Cole, A. J., Smith, M. C., Gwinn, R. P., Skidmore, C., Van Ness, P. C., Bergey, G. K., Park, Y. D., Miller, I., Geller, E., Rutecki, P. A., Zimmerman, R., Spencer, D. C., Goldman, A., Edwards, J. C., Leiphart, J. W., Wharen, R. E., Fessler, J., Fountain, N. B., Worrell, G. A., Gross, R. E., Eisenschenk, S., Duckrow, R. B., Hirsch, L. J., Bazil, C., O’donovan, C. A., Sun, F. T., Courtney, T. A., Seale, C. G. & Morrell, M. J.  2014. Two-year seizure reduction in adults with medically intractable partial onset epilepsy treated with responsive neurostimulation: final results of the RNS System Pivotal trial.  Epilepsia, 55, 432–441. [PMC free article: PMC4233950] [PubMed: 24621228]
51. Helgager, J. & Liu, G. A.  2013. The cellular and synaptic location of activated TrkB in mouse hippocampus during limbic epileptogenesis.  The Journal of Comparative Neurology, 521, 499–521, Spc1. [PMC free article: PMC3527653] [PubMed: 22987780]
52. Hesdorffer, D. C., Logroscino, G., Cascino, G., Annegers, J. F. & Hauser, W. A.  1998. Incidence of status epilepticus in Rochester, Minnesota, 1965–1984.  Neurology, 50, 735–741. [PubMed: 9521266]
53. Hoffman, S. N., Salin, P. A. & Prince, D. A.  1994. Chronic neocortical epileptogenesis in vitro.  Journal of Neurophysiology, 71, 1762–1773. [PubMed: 8064347]
54. Huang, E. J. & Reichardt, L. F.  2001. Neurotrophins: roles in neuronal development and function.  Annual Review of Neuroscience, 24, 677–736. [PMC free article: PMC2758233] [PubMed: 11520916]
55. Huang, Y. Z., He, X.-P. & Krishnamurthy, K. A.  2019. TrkB-Shc signaling protects against hippocampal injury following status epilepticus.  The Journal of Neuroscience, 39, 4624–4630. [PMC free article: PMC6554629] [PubMed: 30926745]
56. Huang, Y. Z., Pan, E. & Xiong, Z.-Q. A.  2008. Zinc-mediated transactivation of TrkB potentiates the hippocampal mossy fiber- CA3 pyramid synapse.  Neuron, 57, 546–558. [PubMed: 18304484]
57. Isackson, P. J., Huntsman, M. M., Murray, K. D. & Gall, C. M.  1991. BDNF mRNA expression is increased in adult rat forebrain after limbic seizures: temporal patterns of induction distinct from NGF.  Neuron, 6, 937–948. [PubMed: 2054188]
58. Jang, S.-W., Liu, X., Chan, C. B., France, S. A., Sayeed, I., Tang, W., Lin, X., Xiao, G., Andero, R., Chang, Q., Ressler, K. J. & Ye, K.  2010. Deoxygedunin, a natural product with potent neurotrophic activity in mice.  Plos One, 5, e11528. [PMC free article: PMC2903477] [PubMed: 20644624]
59. Jasper, H. H.  1970. Physiopathological mechanisms of post-traumatic epilepsy.  Epilepsia, 11, 73–80. [PubMed: 4987162]
60. Jin, X., Prince, D. A. & Huguenard, J. R.  2006. Enhanced excitatory synaptic connectivity in layer v pyramidal neurons of chronically injured epileptogenic neocortex in rats.  The Journal of Neuroscience, 26, 4891–4900. [PMC free article: PMC6674164] [PubMed: 16672663]
61. Jobst, B. C. & Cascino, G. D.  2015. Resective epilepsy surgery for drug-resistant focal epilepsy: a review.  The Journal of the American Medical Association, 313, 285–293. [PubMed: 25602999]
62. Kang, S. K. & Kadam, S. D.  2015. Neonatal seizures: impact on neurodevelopmental outcomes.  Frontiers in Pediatrics, 3, 101. [PMC free article: PMC4655485] [PubMed: 26636052]
63. Kaplan, D. R., Martin-Zanca, D. & Parada, L. F.  1991. Tyrosine phosphorylation and tyrosine kinase activity of the trk proto-oncogene product induced by NGF.  Nature, 350, 158–160. [PubMed: 1706478]
64. Kavanaugh, W. M. & Williams, L. T.  1994. An alternative to SH2 domains for binding tyrosine-phosphorylated proteins.  Science, 266, 1862–1865. [PubMed: 7527937]
65. Kellner, Y., Gödecke, N., Dierkes, T., Thieme, N., Zagrebelsky, M. & Korte, M.  2014. The BDNF effects on dendritic spines of mature hippocampal neurons depend on neuronal activity.  Frontiers in Synaptic Neuroscience, 6, 5. [PMC free article: PMC3960490] [PubMed: 24688467]
66. Kipnis, P. A., Sullivan, B. J., Carter, B. M. & Kadam, S. D.  2020. TrkB agonists prevent postischemic emergence of refractory neonatal seizures in mice.  JCI Insight, 5, e136007. [PMC free article: PMC7406252] [PubMed: 32427585]
67. Klein, R., Nanduri, V., Jing, S. A., Lamballe, F., Tapley, P., Bryant, S., Cordon-Cardo, C., Jones, K. R., Reichardt, L. F. & Barbacid, M.  1991. The trkB tyrosine protein kinase is a receptor for brain-derived neurotrophic factor and neurotrophin-3.  Cell, 66, 395–403. [PMC free article: PMC2710095] [PubMed: 1649702]
68. Kokaia, M., Ernfors, P., Kokaia, Z., Elmér, E., Jaenisch, R. & Lindvall, O.  1995. Suppressed epileptogenesis in BDNF mutant mice.  Experimental Neurology, 133, 215–224. [PubMed: 7649227]
69. Krishnamurthy, K., Huang, Y. Z., Harward, S. C., Sharma, K. K., Tamayo, D. L. & Mcnamara, J. O.  2019. Regression of epileptogenesis by inhibiting tropomyosin kinase B signaling following a seizure.  Ann Neurol, 86, 939–950. [PMC free article: PMC7531259] [PubMed: 31525273]
70. Lamar, C. D., Hurley, R. A., Rowland, J. A. & Taber, K. H.  2014. Post-traumatic epilepsy: review of risks, pathophysiology, and potential biomarkers.  The Journal of Neuropsychiatry and Clinical Neurosciences, 26, iv–113. [PubMed: 24763802]
71. Le, S., Ho, A. L., Fisher, R. S., Miller, K. J., Henderson, J. M., Grant, G. A., Meador, K. J. & Halpern, C. H.  2018. Laser interstitial thermal therapy (LITT): Seizure outcomes for refractory mesial temporal lobe epilepsy.  Epilepsy \& Behavior, 89, 37–41. [PubMed: 30384097]
72. Lee, R., Kermani, P., Teng, K. K. & Hempstead, B. L.  2001. Regulation of cell survival by secreted proneurotrophins.  Science, 294, 1945–1948. [PubMed: 11729324]
73. Leibrock, J., Lottspeich, F., Hohn, A., Hofer, M., Hengerer, B., Masiakowski, P., Thoenen, H. & Barde, Y. A.  1989. Molecular cloning and expression of brain-derived neurotrophic factor.  Nature, 341, 149–152. [PubMed: 2779653]
74. Levi-Montalcini, R.  1952. Effects of mouse tumor transplantation on the nervous system.  Annals of the New York Academy of Sciences, 55, 330–344. [PubMed: 12977049]
75. Levi-Montalcini, R.  1987. The nerve growth factor 35 years later.  Science, 237, 1154–1162. [PubMed: 3306916]
76. Levi-Montalcini, R. & Hamburger, V.  1951. Selective growth stimulating effects of mouse sarcoma on the sensory and sympathetic nervous system of the chick embryo.  The Journal of Experimental Zoology, 116, 321–361. [PubMed: 14824426]
77. Levi-Montalcini, R. & Hamburger, V.  1953. A diffusible agent of mouse sarcoma, producing hyperplasia of sympathetic ganglia and hyperneurotization of viscera in the chick embryo.  Journal of Experimental Zoology, 123, 233–287.
78. Lewis, D. V., Shinnar, S., Hesdorffer, D. C., Bagiella, E., Bello, J. A., Chan, S. & Xu, Y. A.  2014. Hippocampal sclerosis after febrile status epilepticus: the FEBSTAT study.  Annals of Neurology, 75, 178–185. [PMC free article: PMC3980500] [PubMed: 24318290]
79. Li, H., Bandrowski, A. E. & Prince, D. A.  2005. Cortical injury affects short-term plasticity of evoked excitatory synaptic currents.  Journal of Neurophysiology, 93, 146–156. [PubMed: 15342719]
80. Li, H. & Prince, D. A.  2002. Synaptic activity in chronically injured, epileptogenic sensory-motor neocortex.  Journal of Neurophysiology, 88, 2–12. [PubMed: 12091528]
81. Lin, T. W., Harward, S. C. & Huang, Y. Z. A.  2020. Targeting BDNF/TrkB pathways for preventing or suppressing epilepsy.  Neuropharmacology, 167, 107734. [PMC free article: PMC7714524] [PubMed: 31377199]
82. Löscher, W., Klitgaard, H., Twyman, R. E. & Schmidt, D.  2013. New avenues for anti-epileptic drug discovery and development.  Nature Reviews. Drug Discovery, 12, 757–776. [PubMed: 24052047]
83. Löscher, W. & Rogawski, M. A.  2012. How theories evolved concerning the mechanism of action of barbiturates.  Epilepsia, 53 Suppl 8, 12–25. [PubMed: 23205959]
84. Löscher, W. & Schmidt, D.  2011. Modern antiepileptic drug development has failed to deliver: ways out of the current dilemma.  Epilepsia, 52, 657–678. [PubMed: 21426333]
85. Lucke-Wold, B. P., Nguyen, L., Turner, R. C., Logsdon, A. F., Chen, Y.-W., Smith, K. E., Huber, J. D., Matsumoto, R., Rosen, C. L., Tucker, E. S. & Richter, E.  2015. Traumatic brain injury and epilepsy: Underlying mechanisms leading to seizure. Seizure, 33, 13–23. [PubMed: 26519659]
86. Martin-Zanca, D., Hughes, S. H. & Barbacid, M.  1986. A human oncogene formed by the fusion of truncated tropomyosin and protein tyrosine kinase sequences.  Nature, 319, 743–748. [PubMed: 2869410]
87. Masana, Y., Wanaka, A., Kato, H., Asai, T. & Tohyama, M.  1993. Localization of trkB mRNA in postnatal brain development.  Journal of Neuroscience Research, 35, 468–479. [PubMed: 8377221]
88. Massa, S. M., Yang, T., Xie, Y., Shi, J., Bilgen, M., Joyce, J. N., Nehama, D., Rajadas, J. & Longo, F. M.  2010. Small molecule BDNF mimetics activate TrkB signaling and prevent neuronal degeneration in rodents.  The Journal of Clinical Investigation, 120, 1774–1785. [PMC free article: PMC2860903] [PubMed: 20407211]
89. Mckinney, R. A., Debanne, D., Gahwiler, B. H. & Thompson, S. M.  1997. Lesion-induced axonal sprouting and hyperexcitability in the hippocampus in vitro: implications for the genesis of posttraumatic epilepsy.  Nat Med, 3, 990–6. [PubMed: 9288725]
90. Middlemas, D. S., Meisenhelder, J. & Hunter, T.  1994. Identification of TrkB autophosphorylation sites and evidence that phospholipase C-gamma 1 is a substrate of the TrkB receptor.  The Journal of Biological Chemistry, 269, 5458–5466. [PubMed: 8106527]
91. Minichiello, L.  2009. TrkB signalling pathways in LTP and learning.  Nature Reviews. Neuroscience, 10, 850–860. [PubMed: 19927149]
92. Minichiello, L., Calella, A. M., Medina, D. L., Bonhoeffer, T., Klein, R. & Korte, M.  2002. Mechanism of TrkB-mediated hippocampal long-term potentiation.  Neuron, 36, 121–137. [PubMed: 12367511]
93. Minichiello, L., Casagranda, F., Tatche, R. S., Stucky, C. L., Postigo, A., Lewin, G. R., Davies, A. M. & Klein, R.  1998. Point mutation in trkB causes loss of NT4 -dependent neurons without major effects on diverse BDNF responses.  Neuron, 21, 335–345. [PubMed: 9728915]
94. Nawa, H., Carnahan, J. & Gall, C.  1995. BDNF protein measured by a novel enzyme immunoassay in normal brain and after seizure: partial disagreement with mRNA levels.  The European Journal of Neuroscience, 7, 1527–1535. [PubMed: 7551179]
95. Noachtar, S. & Borggraefe, I.  2009. Epilepsy surgery: a critical review.  Epilepsy \& Behavior, 15, 66–72. [PubMed: 19236942]
96. Obeid, M., Rosenberg, E. C., Klein, P. M. & Jensen, F. E.  2014. Lestaurtinib (CEP-701) attenuates second hit kainic acid-induced seizures following early life hypoxic seizures.  Epilepsy Research, 108, 806–810. [PubMed: 24582454]
97. Pitkänen, A. & Immonen, R.  2014. Epilepsy related to traumatic brain injury.  Neurotherapeutics, 11, 286–296. [PMC free article: PMC3996118] [PubMed: 24554454]
98. Prince, D. A. & Futamachi, K. J.  1970. Intracellular recordings from chronic epileptogenic foci in the monkey.  Electroencephalography and Clinical Neurophysiology, 29, 496–510. [PubMed: 4097440]
99. Prince, D. A., Gu, F. & Parada, I.  2016. Antiepileptogenic repair of excitatory and inhibitory synaptic connectivity after neocortical trauma.  Progress in Brain Research, 226, 209–227. [PubMed: 27323945]
100. Prince, D. A., Parada, I., Scalise, K., Graber, K., Jin, X. & Shen, F.  2009. Epilepsy following cortical injury: cellular and molecular mechanisms as targets for potential prophylaxis.  Epilepsia, 50 Suppl 2, 30–40. [PMC free article: PMC2710960] [PubMed: 19187292]
101. Prince, D. A. & Tseng, G. F.  1993. Epileptogenesis in chronically injured cortex: in vitro studies.  Journal of Neurophysiology, 69, 1276–1291. [PubMed: 8492163]
102. Purpura, D. P. & Housepian, E. M.  1961. Alterations in corticospinal neuron activity associated with thalamocortical recruiting responses.  Electroencephalography and Clinical Neurophysiology, 13, 365–381. [PubMed: 14489254]
103. Rakhade, S. N., Zhou, C., Aujla, P. K., Fishman, R., Sucher, N. J. & Jensen, F. E.  2008. Early alterations of AMPA receptors mediate synaptic potentiation induced by neonatal seizures.  The Journal of Neuroscience, 28, 7979–7990. [PMC free article: PMC2679369] [PubMed: 18685023]
104. Raspall-Chaure, M., Chin, R. F. M., Neville, B. G. & Scott, R. C.  2006. Outcome of paediatric convulsive status epilepticus: a systematic review.  Lancet Neurology, 5, 769–779. [PubMed: 16914405]
105. Reichardt, L. F.  2006. Neurotrophin-regulated signalling pathways.  Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 361, 1545–1564. [PMC free article: PMC1664664] [PubMed: 16939974]
106. Rivera, C., Voipio, J., Thomas-Crusells, J., Li, H., Emri, Z., Sipilä, S., Payne, J. A., Minichiello, L., Saarma, M. & Kaila, K.  2004. Mechanism of activity-dependent downregulation of the neuron-specific K-Cl cotransporter KCC2.  The Journal of Neuroscience, 24, 4683–4691. [PMC free article: PMC6729393] [PubMed: 15140939]
107. Salin, P., Tseng, G. F., Hoffman, S., Parada, I. & Prince, D. A.  1995. Axonal sprouting in layer V pyramidal neurons of chronically injured cerebral cortex.  The Journal of Neuroscience, 15, 8234–8245. [PMC free article: PMC6577943] [PubMed: 8613757]
108. Sankar, R., Shin, D. H., Liu, H., Mazarati, A., Pereira De Vasconcelos, A. & Wasterlain, C. G.  1998. Patterns of status epilepticus-induced neuronal injury during development and long-term consequences.  The Journal of Neuroscience, 18, 8382–8393. [PMC free article: PMC6792849] [PubMed: 9763481]
109. Scheffer, I. E., Berkovic, S., Capovilla, G., Connolly, M. B., French, J., Guilhoto, L., Hirsch, E., Jain, S., Mathern, G. W., Moshé, S. L., Nordli, D. R., Perucca, E., Tomson, T., Wiebe, S., Zhang, Y.-H. & Zuberi, S. M.  2017. ILAE classification of the epilepsies: Position paper of the ILAE Commission for Classification and Terminology.  Epilepsia, 58, 512–521. [PMC free article: PMC5386840] [PubMed: 28276062]
110. Shabbir, M. & Stuart, R.  2010. Lestaurtinib, a multitargeted tyrosine kinase inhibitor: from bench to bedside.  Expert Opinion on Investigational Drugs, 19, 427–436. [PubMed: 20141349]
111. Sharpless, S. K. & Halpern, L. M.  1962. The electrical excitability of chronically isolated cortex studied by means of permanently implanted electrodes.  Electroencephalography and Clinical Neurophysiology, 14, 244–255. [PubMed: 13911417]
112. Shen, J., Ghai, K., Sompol, P., Liu, X., Cao, X., Iuvone, P. M. & Ye, K.  2012. N-acetyl serotonin derivatives as potent neuroprotectants for retinas.  Proceedings of the National Academy of Sciences of the United States of America, 109, 3540–3545. [PMC free article: PMC3295250] [PubMed: 22331903]
113. Shinnar, S., Hesdorffer, D. C., Nordli, D. R., Pellock, J. M., O’dell, C., Lewis, D. V., Frank, L. M., Moshé, S. L., Epstein, L. G., Marmarou, A., Bagiella, E. & Study, T.  2008. Phenomenology of prolonged febrile seizures: results of the FEBSTAT study.  Neurology, 71, 170–176. [PubMed: 18525033]
114. Skarpaas, T. L., Jarosiewicz, B. & Morrell, M. J.  2019. Brain-responsive neurostimulation for epilepsy (RNS\text-registered system).  Epilepsy Research, 153, 68–70. [PubMed: 30850259]
115. Talos, D. M., Sun, H., Zhou, X., Fitzgerald, E. C., Jackson, M. C., Klein, P. M., Lan, V. J., Joseph, A. & Jensen, F. E.  2012. The interaction between early life epilepsy and autistic-like behavioral consequences: a role for the mammalian target of rapamycin (mTOR) pathway.  Plos One, 7, e35885. [PMC free article: PMC3342334] [PubMed: 22567115]
116. Teasell, R., Bayona, N., Lippert, C., Villamere, J. & Hellings, C.  2007. Post-traumatic seizure disorder following acquired brain injury.  Brain Inj, 21, 201–14. [PubMed: 17364531]
117. Téllez-Zenteno, J. F., Dhar, R. & Wiebe, S.  2005. Long-term seizure outcomes following epilepsy surgery: a systematic review and meta-analysis.  Brain: A Journal of Neurology, 128, 1188–1198. [PubMed: 15758038]
118. Temkin, N. R., Dikmen, S. S., Wilensky, A. J., Keihm, J., Chabal, S. & Winn, H. R.  1990. A randomized, double-blind study of phenytoin for the prevention of post-traumatic seizures.  N Engl J Med, 323, 497–502. [PubMed: 2115976]
119. Vanlandingham, K. E., Heinz, E. R., Cavazos, J. E. & Lewis, D. V.  1998. Magnetic resonance imaging evidence of hippocampal injury after prolonged focal febrile convulsions.  Ann Neurol, 43, 413–26. [PubMed: 9546321]
120. Wetsel, W. C., Rodriguiz, R. M., Guillemot, J., Rousselet, E., Essalmani, R., Kim, I. H., Bryant, J. C., Marcinkiewicz, J., Desjardins, R., Day, R., Constam, D. B., Prat, A. & Seidah, N. G.  2013. Disruption of the expression of the proprotein convertase PC7 reduces BDNF production and affects learning and memory in mice.  Proceedings of the National Academy of Sciences of the United States of America, 110, 17362–17367. [PMC free article: PMC3808632] [PubMed: 24101515]
121. Wiebe, S., Blume, W. T., Girvin, J. P., Eliasziw, M., Effectiveness & Efficiency Of Surgery For Temporal Lobe Epilepsy Study, G. 2001. A randomized, controlled trial of surgery for temporal-lobe epilepsy.  The New England Journal of Medicine, 345, 311–318. [PubMed: 11484687]
122. Will, T. J., Tushev, G., Kochen, L., Nassim-Assir, B., Cajigas, I. J., Tom Dieck, S. & Schuman, E. M.  2013. Deep sequencing and high-resolution imaging reveal compartment-specific localization of Bdnf mRNA in hippocampal neurons.  Science Signaling, 6, rs16. [PMC free article: PMC5321484] [PubMed: 24345682]