Background & Aims

Glutamate and its receptors, particularly the N-methyl-D-aspartate receptor (NMDAr), contribute to sensitization and hyperalgesia in spinal cord neurons, especially in inflammatory and neuropathic pain conditions [1]. Moreover, in a previous study [1], our research group investigated the involvement of peripheral glutamatergic NMDAr, present in satellite glial cells (SGC) of the dorsal root ganglion (DRG), in the modulation of nociceptive impulses originating from peripheral tissue inflammation [2]. Despite the widespread clinical use of NMDAr antagonists like ketamine as anesthetics and analgesics, their application is limited by side effects resulting from central nervous system NMDAr blockade [3,4]. The aim of this study is to test the hypothesis that the analgesic effect of NMDA antagonists is, at least partially, dependent on peripheral receptor blockade.

Methods

Male C57BL/6 mice and Wistar rats were utilized to investigate the role of peripheral NMDAr in pain transmission. We assessed the analgesic effects of the NMDAr antagonists ketamine (KET), 2-amino-5-phosphopentanoic acid (AP-5) and 5,7-Dichlorokynurenic acid (5,7-DCK). Unlike KET, these substances are unable to penetrate the blood-brain barrier. AP-5 (9 μg/5μl) was administered via intraganglionar (i.gl.), using the direct injection technique described by Ferrari et al. in 2007 [5], at the right L5 dorsal root ganglia in rats to test the analgesic potential of i.gl. administered antagonists in carrageenan-induced paw hyperalgesia, capsaicin-induced nociception, and sciatic nerve constriction-induced neuropathic pain. Additionally, we compared the effects of systemically administered (i.p.) KET (10 mg/kg and 30 mg/kg) with the peripheral antagonist 5,7-DCK (15 mg/kg and 30 mg/kg) in capsaicin-induced nociception in mouse paws and carrageenan-induced paw hyperalgesia. The open filed test was used to evaluate possible central effects of KET and 5,7-DCK by registering the motor activity of mice.

Results

AP-5 i.gl. significantly increased mechanical nociception thresholds (MNT) after 3 hours (p<0.001, Control Group (CG) vs. AP-5 i.gl.) and 4,5 hours (p<0.001, CG vs. AP-5 i.gl.) of carrageenan injection. AP-5 inhibited nociceptive behaviors induced by capsaicin (p<0.05, CG vs AP-5 i.gl.) and neuropathic pain induced by chronic sciatic nerve
constriction. The AP-5 i.gl. group showed a progressive increase in the threshold from day 7 and on the 21st day there was a significantly increase in MNT (p<0.01, CG vs AP-5 i.gl.). Systemic administration of both KET and 5,7-DCK similarly reduced capsaicin-induced nociception in mice paws (p<0.05, CG vs KET 10 and 30 mg/kg; p<0.05, CG vs 5,7-DCK 15 and 30 mg/kg). 5,7-DCK i.p. significantly increased MNT after 3 hours of carrageenan-induced paw hyperalgesia (p<0.001, CG vs. 5,7-DCK 15 and 30 mg/kg), and after 4,5 hours (p<0.01, CG vs. 5,7-DCK 30 mg/kg), offering an analgesic effect similar to KET after three hours (p<0.001, CG vs KET 10 and 30 mg/kg). The Open Field Test indicated influence of the antagonists on the motor activity of mice (p<0.05, CG vs KET 30 mg/kg; CG vs. 5,7-DCK 30 mg/kg).

Conclusions

In conclusion, peripheral NMDA receptors are implicated in pain transmission, suggesting that peripheral-acting NMDA antagonists may be an alternative for pain treatment while avoiding adverse central nervous system effects.

References

[1] D’Mello R, Dickenson AH. Spinal cord mechanisms of pain. Br J Anaesth. 2008 Jul;101(1):8-16. doi: 10.1093/bja/aen088. Epub 2008 Apr 15.

[2] Ferrari LF, Lotufo CM, Araldi D, Rodrigues MA, Macedo LP, Ferreira SH, Parada CA. Inflammatory sensitization of nociceptors depends on activation of NMDA receptors in DRG satellite cells. Proc Natl Acad Sci U S A. 2014 Dec 23;111(51):18363-8. doi: 10.1073/pnas.1420601111.

[3] Traynelis SF, Wollmuth LP, McBain CJ, Menniti FS, Vance KM, Ogden KK, Hansen KB, Yuan H, Myers SJ, Dingledine R. Glutamate receptor ion channels: structure, regulation, and function. Pharmacol Rev. 2010 Sep;62(3):405-96. doi: 10.1124/pr.109.002451. Erratum in: Pharmacol Rev. 2014 Oct;66(4):1141.

[4] Andreeva D, Murashova L, Burzak N, Dyachuk V. Satellite Glial Cells: Morphology, functional heterogeneity, and role in pain. Front Cell Neurosci. 2022 Oct 6;16:1019449. doi: 10.3389/fncel.2022.1019449.

[5] Ferrari LF, Cunha FQ, Parada CA, Ferreira SH. A novel technique to perform direct intraganglionar injections in rats. Journal of Neuroscience Methods 2007, 159(2):236–243. https://doi.org/10.1016/j.jneumeth.2006.07.025

Presenting Author

Juliana Fernandes

Poster Authors

Juliana Juliate Damacena Fernandes

OTHR

Departament of Physiological Science, Institute of Biomedical Science, Federal University of Uberlândia, 38400-902, Minas Gerais, Brazil

Lead Author

Diego Fernandes de Araújo

Msc Health Sciences

Departament of Physiological Science, Institute of Biomedical Science, Federal University of Uberlândia, 38400-902, Minas Gerais, Brazil

Lead Author

Celina Monteiro da Cruz Lotufo

Ph.D. in Sciences

Departament of Physiological Science, Institute of Biomedical Science, Federal University of Uberlândia, 38400-902, Minas Gerais, Brazil

Lead Author

Topics

  • Mechanisms: Biological-Molecular and Cell Biology