Modulation of pain-associated hyperexcitability at central synapses of capsaicin-sensitive nociceptors

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

TRPV1-expressing (capsaicin-sensitive) afferents correspond largely to peptidergic nociceptors, which
play an important role in both acute pain and chronic hyper-sensitive pain states. Investigation of
processes that can modulate function of their early central synapses in spinal dorsal horn could point the
way to novel analgesics for chronic pain. We have developed a new method to quantify receptor-evoked
Ca2+ fluorescence responses of ex vivo synaptic preparations and use it here to measure
capsaicin-evoked responses in dorsal horn from control and pain state animals.
Synaptoneurosomes (re-sealed presynaptic and closely apposed postsynaptic compartments) were
prepared from dorsal lumbar spinal cord of male Sprague-Dawley rats, under conditions designed to
maintain functional integrity, and loaded with a no-wash Ca2+ fluorophore (Calcium 5). Capsaicin or
other agents (including ionomycin as a positive control) were added in vitro and responses measured by
fluorometric plate reader.
Responses to capsaicin showed concentration-dependent increases from 0.2-10 microM, were 5-6 fold
greater in dorsal than in ventral horn and were largely reversed by the TRPV1 antagonist AMG9810 or
presynaptically acting tetanus toxin. In addition the responses were inhibited by antagonists of AMPAor
NMDA-type glutamate receptors, consistent with glutamatergic transmission from capsaicin-activated
presynaptic terminals. Agents selective for several distinct subtypes of GluN2 subunit showed
differential ability to inhibit capsaicin responses.
We further explored the effects of endogenous analgesic mechanisms. In vitro addition of mu (and to a
lesser extent delta) opioids strongly attenuated capsaicin responses. In a model of chronic inflammatory
pain (intraplantar Complete Freund’s Adjuvant), ex vivo responsiveness to capsaicin was increased in a
manner completely reversed by NMDA receptor antagonists. This inflammation-induced
hypersensitivity at TRPV1 afferent central synapses was strongly attenuated by prior in vivo
administration of the TRPM8 agonist, icilin (200 microM topical to hindpaws, 15 min).
These observations reveal quantifiable actions of established or novel analgesic targets impacting on
central synapses of TRPV1-expressing nociceptors.
Original languageEnglish
Title of host publicationProceedings of the British Neuroscience Association 2015 Festival of Neuroscience
Number of pages1
Publication statusPublished - 12 Apr 2015

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