Spinal N-methyl-d-aspartate receptors and nociception-evoked release of primary afferent substance P

Abstract

Dorsal horn NMDA receptors contribute significantly to spinal nociceptive processing through an effect postsynaptic to non-primary glutamatergic axons, and perhaps presynaptic to the primary afferent terminals. The present study sought to examine the regulatory effects of NMDA receptors on primary afferent release of SP, as measured by neurokinin 1 receptor (NK1r) internalization in the spinal dorsal horn of rats. The effects of intrathecal NMDA alone or in combination with D-serine (a glycine site agonist) was initially examined on basal levels of NK1r internalization. NMDA alone or when co-administered with D-serine failed to induce NK1r internalization, whereas activation of spinal TRPV1 receptors by capsaicin resulted in a notable NK1r internalization. To determine whether NMDA receptor activation could potentiate NK1r internalization or pain behavior induced by a peripheral noxious stimulus, intrathecal NMDA was given prior to an intraplantar injection of formalin. NMDA did not alter the formalin-induced NK1r internalization nor did it enhance the formalin paw flinching behavior. To further characterize the effects of presynaptic NMDA receptors, the NMDA antagonists AP-5 and MK-801 were intrathecally administered to assess their regulatory effects on formalin-induced NK1r internalization and pain behavior. AP-5 had no effect on formalininduced NK1r internalization, whereas MK-801 produced only a modest reduction. Both antagonists, however, reduced the formalin paw flinching behavior. In subsequent in vitro experiments, perfusion of NMDA in spinal cord slice preparations did not evoke basal release of SP or calcitonin generelated peptide (CGRP). Likewise, perfusion of NMDA did not enhance capsaicin-evoked release of the two peptides. These results suggest that presynaptic NMDA receptors in the spinal cord play little if any role on the primary afferent release of SP. KeywordsNeurokinin 1 receptor; internalization; dorsal horn; glutamate; C-fiber *Correspondence should be addressed to: Dr. Arbi Nazarian, Department of Pharmaceutical Sciences, Western University of Health Sciences, 309 E. Second Street, Pomona, CA 91766-1854, Tel: (909) Fax: (909) 469-5600, Email: anazarian@westernu.edu. a These authors have contributed equally to this manuscript. Section Editor: Dr. Richard J. Weinberg, Department of Cell and Developmental Biology, CB7090, University of North Carolina, Chapel Hill, NC, 27599 USA Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Small diameter primary afferents terminate superficially in lamina I and II and exert frequencydependent depolarizin...

Activation of Spinal μ- and δ-Opioid Receptors Potently Inhibits Substance P Release Induced by Peripheral Noxious Stimuli

“…Using NK 1 R internalization as a tool to evaluate SP release, we demonstrated that selective activation of either MOPRs or DOPRs was sufficient to abolish formalin-and capsaicin-induced SP release. Interestingly, it was observed by others that MOPR agonists can decrease NK 1 R internalization induced by paw compression (Kondo et al, 2005) or intraplantar formalin (Nazarian et al, 2008). Similarly, Kondo et al (2005) have also observed DOPR-mediated inhibition of mechanical stimulus-induced NK 1 R internalization.…”

“…Following activation of primary afferent fibers by formalin and capsaicin, SP is released in the superficial laminae of the spinal cord, where it binds to and activates NK 1 R. Upon activation, NK 1 Rs are rapidly internalized (Mantyh et al, 1995;Abbadie et al, 1997;Marvizó n et al, 2003;Nazarian et al, 2008). NK 1 R internalization can therefore be used as an indication of SP release.…”

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“…Studies in dorsal horn neurons have established that activation of presynaptic NMDA receptors on the central terminals of peptidergic primary afferents causes an increased release of SP after dorsal root stimulation or administration of capsaicin (Afrah et al 2001;Liu et al 1997;Malcangio et al 1998;Marvizon et al 1997; but see Nazarian et al 2008). A similar mechanism may be invoked in the RVM in which synaptically released glutamate acts at presynaptic NMDA receptors on SP-containing afferents to RVM neurons.…”

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“…24 Previous reports on NMDA and SP interaction suggest a regulatory role of NMDA in SP release which was also observed peripherally. [25][26][27] We can therefore speculate that the increased NMDAR1 levels have a role in SP elevation observed in tendinosis, however, interventional studies are essential in order to draw certain conclusions.…”