Elsevier

Neuroscience

Volume 118, Issue 2, 8 May 2003, Pages 547-562
Neuroscience

Original contribution
N-methyl-d-aspartate-induced excitation and sensitization of normal and inflamed nociceptors

https://doi.org/10.1016/S0306-4522(03)00009-5Get rights and content

Abstract

The present study investigates the contribution of peripheral N-methyl-d-aspartate (NMDA) receptors to acute nociception and persistent inflammatory pain in the rat. Immunohistochemical localization of the NMDA receptor one (NMDAR1) subunit demonstrates that 47% of unmyelinated axons in the normal digital nerve are positively labeled. In concert with the overall progression of inflammation following injection of complete Freund’s adjuvant (CFA) in the hind paw, a significant increase in the proportion of NMDAR1-labeled unmyelinated digital axons occurs at 2 and 7, but not 14 days following hind-paw inflammation. In behavioral studies, we confirm an increased mechanical sensitivity in CFA-injected hind paws. Furthermore, activation of NMDA receptors following intraplantar NMDA (1.0 mM) in normal animals results in a mechanical sensitivity similar to that observed in inflamed animals. Conversely, a low concentration of NMDA (0.5 mM) that has little affect on mechanical thresholds in normal animals produces a significant increase in mechanical sensitivity in the inflamed state. CFA-induced mechanical sensitivity involves NMDA-receptor activation demonstrated by the observation that injection of MK-801 alone into the inflamed hind paw returns mechanical sensitivity to normal (pre-inflammation) levels. In single-unit studies, there is a dose-dependent increase in NMDA-induced nociceptor activity in both normal and inflamed skin, but the amount of NMDA required to induce activation is reduced in inflamed skin. In addition, NMDA-induced discharge rates and percentage of NMDA-activated nociceptors are significantly increased in inflamed compared with normal skin, and this activation can be blocked by co-administration of MK-801. Exposure of nociceptors in normal skin to 1 mM NMDA sensitizes the units to reapplication of NMDA and to heat. Nociceptors that demonstrate sensitization to heat in persistent inflammation show an enhanced sensitization when exposed to exogenous NMDA. Thus, peripheral NMDA receptors not only play an important role in modulating the responses of nociceptors in normal skin, but their upregulation and activation on peripheral nociceptors contributes significantly to the mechanical sensitivity and heat sensitization that accompanies persistent inflammation.

Section snippets

Experimental procedures

All experiments were approved by the University Animal Care and Use Committee and followed the guidelines for the ethical care and use of laboratory animals (Zimmermann, 1983). Steps were taken to minimize both the number of animals used and their discomfort.

Results

Intraplantar injection of 100 μl CFA resulted in a considerable increase in hind-paw swelling (thickness), redness (rubor) and temperature (calor) by 2 days post-injection compared with baseline (P<0.05, Friedman’s test, Fig. 1A–C). The increased hind-paw thickness and redness persisted at 7 days and the inflamed paw was still significantly different from baseline and from the contralateral non-inflamed paw (P<0.05, t-test). Although the temperature of the inflamed hind paw at 7 days was no

Discussion

The presence of NMDA receptors on peripheral nociceptors in normal skin is confirmed (Carlton et al., 1995) as is the increase in the proportion of NMDAR1-labeled axons that occurs 2 days post-CFA injection (Carlton and Coggeshall, 1999). New findings are that the proportion of NMDA-expressing axons is still significantly increased at 7 days post-CFA injection but returns to normal after 14 days, in concert with the overall progression of the persistent inflammation. In behavioral studies, we

Conclusion

In summary, peripheral NMDA receptor activation contributes to acute nociception and to the pain of persistent inflammation. The increase in number of NMDAR1-expressing peripheral axons and the enhanced responses of nociceptors following NMDA receptor activation in inflamed animals indicates that these receptors are important in the maintenance of inflammatory pain and thus may be peripheral targets for analgesics. Targeting peripheral NMDA receptors will avoid the serious side effects that

Acknowledgements

The authors thank Zhixia Ding for assistance in the immunohistochemistry and electron microscopy and Vicki Wilson for her excellent secretarial assistance. This work was supported by NS27910 and NS40700 to S.M.C., NS10161 to R.E.C. and NS11255 to S.M.C. and R.E.C.

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