| Peripheral opiates attenuate hyperalgesia in experimental models of inflammation but have little discernable effect on nociceptive thresholds in normal animals, suggesting that opioid receptors are localized to a subset of sensory neurons that have distinct functions under inflammatory conditions. The objective of this thesis was to investigate whether activation of endogenous opioid receptors alters the response properties of peripheral sensory neurons in normal and inflamed tissues. Inflammation was induced by injection of complete Freund's adjuvant (CFA) into the hindpaw of rats. An in vitro skin-nerve preparation was used to examine opiate effects on the response properties of single units in healthy and CFA-inflamed skin. The excitability of a subpopulation of sensory neurons innervating inflamed skin was attenuated by direct application of morphine to the cutaneous receptive fields. No morphine effect was observed on units innervating control skin. Morphine effects were concentration-dependent and prevented by pre-treatment of the receptive field with naloxone, indicating that the effects were receptor-mediated. Peripheral application of deltorphin II, a delta opioid receptor (DOR)-selective agonist, inhibited the mechanical responsiveness of nociceptors innervating inflamed skin. These effects were concentration-dependent and blocked by co-application with naltrindole, a DOR-selective antagonist, suggesting that inhibitory effects of peripherally delivered morphine on single fiber activity were mediated, in part, by activation of DORs. Next, microfluorimetry was used to compare the responses of dorsal root ganglia neurons from control and CFA-inflamed rats in the presence and absence of morphine. Direct application of morphine attenuated responses to KCl-evoked increases in intracellular calcium in acutely dissociated neurons from control rats. No morphine effect was observed in neurons from the inflamed groups. Finally, immunohistochemistry was used to localize a variant of the mu opioid receptor, MOR-1C, to small diameter, varicose axons innervating rat glabrous skin and lip. Localization of MOR-1C to peripheral tissues provided an additional anatomical substrate for the receptor-mediated functions that are described. The experiments described here identified and characterized a population of opiate-sensitive primary afferent neurons in the peripheral nervous system. Together, the anatomical, functional, and electrophysiological data presented in this thesis establish endogenous opioid receptors as critical components of a peripheral analgesic system. |
