Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2006 Jul 5;26(27):7172-80.
doi: 10.1523/JNEUROSCI.1101-06.2006.

Impaired firing and cell-specific compensation in neurons lacking nav1.6 sodium channels

Audra Van Wart  1 , Gary Matthews
Affiliations

Impaired firing and cell-specific compensation in neurons lacking nav1.6 sodium channels

Audra Van Wart et al. J Neurosci. .

Abstract

The ability of neurons to fire precise patterns of action potentials is critical for encoding inputs and efficiently driving target neurons. At the axon initial segment and nodes of Ranvier, where nerve impulses are generated and propagated, a high density of Na(v)1.2 sodium channels is developmentally replaced by Na(v)1.6 channels. In retinal ganglion cells (GCs), this isoform switch coincides with the developmental transition from single spikes to repetitive firing. Also, Na(v)1.6 channels are required for repetitive spiking in cerebellar Purkinje neurons. These previous observations suggest that the developmental appearance of Na(v)1.6 underlies the transition to repetitive spiking in GCs. To test this possibility, we recorded from GCs of med (Na(v)1.6-null) and wild-type mice during postnatal development. By postnatal day 18, when the switch to Na(v)1.6 at GC initial segments is normally complete, the maximal sustained and instantaneous firing rates were lower in med than in wild-type GCs, demonstrating that Na(v)1.6 channels are necessary to attain physiologically relevant firing frequencies in GCs. However, the firing impairment was milder than that reported previously in med Purkinje neurons, which prompted us to look for differences in compensatory sodium channel expression. Both Na(v)1.2 and Na(v)1.1 channels accumulated at initial segments and nodes of med GCs, sites normally occupied by Na(v)1.6. In med Purkinje cells, only Na(v)1.1 channels were found at initial segments, whereas in other brain regions, only Na(v)1.2 was detected at med initial segments and nodes. Thus, compensatory mechanisms in channel isoform distribution are cell specific, which likely results in different firing properties.

PubMed Disclaimer

Figures

Figure 1.
Figure 1.
Nav channel localization is normal in P18 medTg (Nav1.6-null) mice. A, B, Wild-type (A) and Nav1.6-null (B) P17 flat-mount retina stained with an antibody against AnkG (red) and a pan-specific sodium channel antibody (PAN, green). Projections through nerve fiber layer span 3 μm. In both genotypes, sodium channels cluster at high density at axon initial segments of GCs, which are marked by ankyrin-G immunoreactivity. C, D, Wild-type P20 (C) and Nav1.6-null P21 (D) optic nerve stained for sodium channels (red) and the paranodal marker Caspr (green). Sodium channel localization appears normal at nodes of Ranvier in both WT and Nav1.6-null optic nerves. Images span 4 μm. Scale bars, 10 μm.
Figure 2.
Figure 2.
Both wild-type (+/+) and Nav1.6-null (−/−) ganglion cells can support repetitive firing at P18 and encode a similar range of input intensities. Because of the high mortality rate beyond P18 (lethal by P21), this was the oldest age used for analysis. The traces show voltage recordings from one of the fastest cells from each genotype in response to a 60 pA current pulse lasting 1 s.
Figure 3.
Figure 3.
Nav1.6-null GCs do not undergo the same developmental increase in firing rates as their wild-type counterparts. Developmental comparison of average sustained firing rates (maximum number of spikes fired during a 1 s current step). Only cells that reproducibly fired throughout the step were included (>90% of cells did so at all three ages). At P18, the average WT GC fired significantly more spikes during the pulse than the average medTg GC [73 vs 52 spikes/s, p < 0.005; an increase from the firing rate at P12 of 93% (WT) vs 35% (medTg)]. At each age, the level of statistical significance of differences between WT (+/+) and medTg (−/−) cells is indicated above each pair of bars (n.s., not significant). The asterisks indicate the statistical significance of differences in firing rates for cells of each genotype at P14 and P18 compared with the same genotype at P12 (∗p < 0.03; ∗∗p < 0.001).
Figure 4.
Figure 4.
Wild-type GCs tend to have higher instantaneous spike frequencies and fewer failures when driven to these frequencies. A, Each data point represents results from a single P18 WT GC (+/+; filled circles) or medTg GC (−/−; open circles). The position along the abscissa indicates the maximum instantaneous spike frequency (i.e., the reciprocal of the minimum interval between the first 2 spikes) observed for each cell. The ordinate indicates the number of spikes fired before failure during a 1 s depolarization at the level necessary to achieve the maximum instantaneous frequency. Instances in which the cell continued to fire throughout the 1 s depolarization are plotted along the top (No failure). B, Examples of typical firing patterns of WT (left) and medTg (right) GCs in response to increasing depolarization, from levels just above threshold (bottom) to those required to reach maximum instantaneous frequency.
Figure 5.
Figure 5.
The remaining Nav1.1 and Nav1.2 channels in medTg GCs accumulate at sites normally occupied by Nav1.6. P20 Nav1.6-null retina and optic nerve. A, Flat-mount retinas stained for Nav1.2 and ankyrin-G. B, Section of retina stained for Nav1.1 and ankyrin-G. C, D, Optic nerve stained for Caspr and either Nav1.2 or Nav1.1. Both channel types are found throughout the ankyrin-G-defined initial segments, as well as at nodes of Ranvier. Images span 4 μm (A), 1 μm (B), 5 μm (C), 2 μm (D). Scale bars, 10 μm.
Figure 6.
Figure 6.
Cerebellar Purkinje neurons of Nav1.6-null animals express Nav1.1 but not Nav1.2 at their axon initial segments. Sections of P20 medTg cerebellum stained with anti-Nav1.1 (A, B; green) or anti-Nav1.2 antibody (C, D; green), together with immunostaining for either ankyrin-G to mark initial segments (B, C; red), or the Purkinje cell marker calbindin (A, D; red). A, Arrowhead indicates the initial segment of a calbindin-labeled Purkinje cell, with an associated cluster of Nav1.1 immunoreactivity. Calbindin-negative cells lack Nav1.1 immunostaining. B, Arrowheads indicate two examples of large initial segments that are positive for both ankyrin-G and Nav1.1 channels. Numerous smaller initial segments likely originate from cerebellar granule cells and lack detectable Nav1.1 immunofluorescence. C, Nav1.2 antibody labels most initial segments in the granule cell layer but does not label large initial segments (arrowheads) near Purkinje cell bodies. D, Nav1.2 immunostaining is not detectable in axon initial segments (e.g., arrowhead) of Purkinje cells labeled with anti-calbindin. Images are projections of confocal Z-series spanning 4 μm (A), 3 μm (B), 8 μm (C), or 5 μm (D). Scale bars: A, D, 10 μm; B, C, 20 μm.
Figure 7.
Figure 7.
Nav1.1 channels are located at nodes of Ranvier in the cerebellum of Nav1.6-null mice. Sections of P20 medTg cerebellum stained with Nav1.1 antibody (green) and either anti-calbindin (A, B; red) or ankyrin-G antibody (C; red). A, B, Nav1.1 immunostaining is associated with calbindin-labeled axons of Purkinje cells at the initial segment (A, arrowhead) and at focal spots deeper in the granule-cell layer (box, A; arrowheads, B). Insets a and b show a higher-magnification view of the region indicated by the box in A, to illustrate a focal spot of Nav1.1 immunofluorescence associated with the axon membrane (arrows). C, Nodes of Ranvier in the cerebellar white matter marked with anti-ankyrin-G (red). Focal spots of Nav1.1 immunofluorescence colocalize with ankyrin-G immunostaining (middle, yellow). Images are projections of confocal Z-series spanning 3 μm (A, C) or 4 μm (B). Scale bars: A, B, 10 μm; C, 20 μm.
Figure 8.
Figure 8.
Nav1.2 is found at initial segments and nodes of Ranvier in the cortex, hippocampus, and corpus callosum of Nav1.6-null mice. Sections of P20 medTg brain stained with antibodies against ankyrin-G (red) and Nav1.2 (green). Extensive colocalization of immunofluorescence is seen at initial segments of neurons in the cortex (A) and area CA2/3 of the hippocampus (B), as well as virtually all nodes of Ranvier in the corpus callosum (C). Nav1.1 immunoreactivity is not detectable at any of these sites (data not shown). Images span 7 μm (A), 8 μm (B), and 4 μm (C). Scale bars, 20 μm.
See this image and copyright information in PMC

Similar articles

See all similar articles

Cited by

See all "Cited by" articles

References

    1. Boiko T, Rasband MN, Levinson SR, Caldwell JH, Mandel G, Trimmer JS, Matthews G (2001). Compact myelin dictates the differential targeting of two sodium channel isoforms in the same axon. Neuron 30:91–104. - PubMed
    1. Boiko T, Van Wart A, Caldwell JH, Levinson SR, Trimmer JS, Matthews G (2003). Functional specialization of the axon initial segment by isoform-specific sodium channel targeting. J Neurosci 23:2306–2313. - PMC - PubMed
    1. Buchner DA, Seburn KL, Frankel WN, Meisler MH (2004). Three ENU-induced neurological mutations in the pore loop of sodium channel Scn8a (Nav 1.6) and a genetically linked retinal mutation, rd13. Mamm Genome 15:344–351. - PubMed
    1. Burgess DL, Kohrman DC, Galt J, Plummer NW, Jones JM, Spear B, Meisler MH (1995). Mutation of a new sodium channel gene, Scn8a, in the mouse mutant “motor endplate disease.”. Nat Genet 10:461–465. - PubMed
    1. Caldwell JH, Schaller KL, Lasher RS, Peles E, Levinson SR (2000). Sodium channel Nav 1.6 is localized at nodes of Ranvier, dendrites, and synapses. Proc Natl Acad Sci USA 97:5616–5620. - PMC - PubMed

Publication types

MeSH terms

LinkOut - more resources