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
. 2024 Jan-Dec:20:17448069241227922.
doi: 10.1177/17448069241227922.

Sensitization of human and rat nociceptors by low dose morphine is toll-like receptor 4-dependent

Eugen V Khomula  1 , Dionéia Araldi  1 , Paul G Green  1   2 , Jon D Levine  1   3
Affiliations

Sensitization of human and rat nociceptors by low dose morphine is toll-like receptor 4-dependent

Eugen V Khomula et al. Mol Pain. 2024 Jan-Dec.

Abstract

While opioids remain amongst the most effective treatments for moderate-to-severe pain, their substantial side effect profile remains a major limitation to broader clinical use. One such side effect is opioid-induced hyperalgesia (OIH), which includes a transition from opioid-induced analgesia to pain enhancement. Evidence in rodents supports the suggestion that OIH may be produced by the action of opioids at Toll-like Receptor 4 (TLR4) either on immune cells that, in turn, produce pronociceptive mediators to act on nociceptors, or by a direct action at nociceptor TLR4. And, sub-analgesic doses of several opioids have been shown to induce hyperalgesia in rodents by their action as TLR4 agonists. In the present in vitro patch-clamp electrophysiology experiments, we demonstrate that low dose morphine directly sensitizes human as well as rodent dorsal root ganglion (DRG) neurons, an effect of this opioid analgesic that is antagonized by LPS-RS Ultrapure, a selective TLR4 antagonist. We found that low concentration (100 nM) of morphine reduced rheobase in human (by 36%) and rat (by 26%) putative C-type nociceptors, an effect of morphine that was markedly attenuated by preincubation with LPS-RS Ultrapure. Our findings support the suggestion that in humans, as in rodents, OIH is mediated by the direct action of opioids at TLR4 on nociceptors.

Keywords: Morphine; excitability; human and rat nociceptors; opioid-induced hyperalgesia; patch-clamp electrophysiology; sensitization; toll-like receptor 4.

PubMed Disclaimer

Conflict of interest statement

Declaration of conflicting interestsThe author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Figures

Figure 1.
Figure 1.
Low dose morphine induces TLR4-dependent sensitization of rat and human nociceptors. (a) Examples of low dose morphine-induced reduction in rheobase of putative C-type rat nociceptors, and its prevention by a selective TLR4 antagonist (LPS-RS Ultrapure). Electrophysiological traces (upper, grey) show APs generated in response to stimulation of a small-diameter DRG neuron (depicted in the inset image) with a square wave current pulse (shown below AP recordings, black). The height of the pulse represents rheobase. The scale is indicated by corresponding scale bars and, if not indicated by a different scale bar, is the same for left and right traces. Dotted line shows level of 0 mV. Left panel shows traces from a low dose morphine-treated neuron, of the control group, with no TLR4 antagonist added. Note the reduction in the height of the current pulse (right traces) after application of morphine (100 nM), compared to the baseline value (left trace). Right panel shows traces from a neuron of the “prevention” protocol group, preincubated for 30 min with the selective TLR4 antagonist, LPS-RS Ultrapure (10 μg/mL). Note, only a small reduction in rheobase in response to morphine (100 nM), being markedly attenuated compared to the control effect of morphine in the absence of LPS-RS Ultrapure. (b) Reduction in rheobase, relative to its baseline value, produced by morphine (100 nM) in rat small-diameter DRG neurons, without (control group; left white bar, “-“) and after preincubation with the selective TLR4 antagonist, LPS-RS Ultrapure (10 μg/mL; “prevention” protocol group; right grey bar, “+”). Bars show mean ± S.E.M. Symbols show effect in individual neurons. Morphine produced a significant reduction in rheobase in the control group (one-sample two-tailed Student's t-test for zero effect: *p = .013, t(5) = 3.8) that was significantly attenuated in the “prevention” group (two-sample unpaired two-tailed Student's t-test: #p = .018, t(9) = 2.9), became not significantly different from baseline (one-sample two-tailed Student's t-test for zero effect: p = .96, t(4) = 0.05), supporting the suggestion that sensitization produced by low dose of morphine, in rat nociceptors, is dependent on TLR4 that is expressed in the nociceptor. Number of cells: six in control, five in the “prevention” protocol group. (c) Examples of low dose morphine-induced reduction in rheobase in putative C-type human nociceptors, and its prevention by a selective TLR4 antagonist (LPS-RS Ultrapure). Electrophysiological traces (upper, grey) show APs generated in response to stimulation of a small-diameter DRG neuron (depicted in the inset image) with a square wave current pulse (shown below AP recordings, black). The height of the pulse represents rheobase. The scale is indicated by corresponding scale bars and, if not indicated by a different scale bar, is the same for left and right traces. Dotted line shows level of 0 mV. Left panel shows traces from a low dose morphine-treated neuron of the control group, with no TLR4 antagonist added. Note the reduction in the height of the current pulse (right traces) after application of morphine (100 nM) compared to baseline value (left trace). Right panel shows traces from a neuron of the “prevention” protocol group, preincubated for 30 min with the selective TLR4 antagonist, LPS-RS Ultrapure (10 μg/mL). In this cell no reduction in rheobase in response to morphine (100 nM) is observed, being markedly attenuated compared to the control effect of morphine in the absence of LPS-RS Ultrapure. (d) Reduction in rheobase, relative to its baseline value, produced by morphine (100 nM) in human small-diameter DRG neurons, without (control group; left white bar, “-“) and after preincubation with the selective TLR4 antagonist, LPS-RS Ultrapure (10 μg/mL; “prevention” protocol group; right grey bar, “+”). Bars show mean ± S.E.M. Symbols show effect in individual neurons. Morphine produced a significant reduction in rheobase in the control group (one-sample two-tailed Student's t-test for zero effect: **p = .0014, t(8) = 4.8) that was significantly attenuated in the “prevention” group (two-sample unpaired two-tailed Student's t-test: & p = .031, t(13) = 2.4), became not significantly different from baseline (one-sample two-tailed Student's t-test for zero effect: p = .29, t(5) = 1.2), supporting the suggestion that sensitization produced by low dose of morphine, in human nociceptors, is dependent on TLR4 that is expressed in the nociceptor. Number of cells: 9 in control, six in the “prevention” protocol group.
See this image and copyright information in PMC

Update of

Similar articles

See all similar articles

References

    1. Goldberg DS, McGee SJ. Pain as a global public health priority. BMC Publ Health 2011; 11: 770. - PMC - PubMed
    1. Pathan H, Williams J. Basic opioid pharmacology: an update. Br J Pain 2012; 6: 11–16. - PMC - PubMed
    1. Hutchinson MR, Northcutt AL, Hiranita T, Wang X, Lewis SS, Thomas J, van Steeg K, Kopajtic TA, Loram LC, Sfregola C, Galer E, Miles NE, Bland ST, Amat J, Rozeske RR, Maslanik T, Chapman TR, Strand KA, Fleshner M, Bachtell RK, Somogyi AA, Yin H, Katz JL, Rice KC, Maier SF, Watkins LR. Opioid activation of toll-like receptor 4 contributes to drug reinforcement. J Neurosci 2012; 32: 11187–11200. - PMC - PubMed
    1. Due MR, Piekarz AD, Wilson N, Feldman P, Ripsch MS, Chavez S, Yin H, Khanna R, White FA. Neuroexcitatory effects of morphine-3-glucuronide are dependent on Toll-like receptor 4 signaling. J Neuroinflammation 2012; 9: 200. - PMC - PubMed
    1. Hutchinson MR, Zhang Y, Shridhar M, Evans JH, Buchanan MM, Zhao TX, Slivka PF, Coats BD, Rezvani N, Wieseler J, Hughes TS, Landgraf KE, Chan S, Fong S, Phipps S, Falke JJ, Leinwand LA, Maier SF, Yin H, Rice KC, Watkins LR. Evidence that opioids may have toll-like receptor 4 and MD-2 effects. Brain Behav Immun 2010; 24: 83–95. - PMC - PubMed

LinkOut - more resources