Differential regulation of opioid receptors during inflammation

Properties of the opium poppy have been exploited for centuries for the alleviation of pain and to induce euphoria. Classically thought to produce its effects solely in the central nervous system, peripheral opioid analgesic systems are now widely accepted. The activation of these systems leads to a reduction in primary afferent fiber excitability leading to the inhibition of sensory transduction. Opioid receptors function is modulated by a variety of mechanisms. An example of this is enhanced peripheral opioid receptor function following inflammation. The present study examined peripheral opioid receptor regulation in early and late stages of CFA inflammation. Additionally, a new model of UVB of inflammation was characterized. Peripheral MOR receptors are differentially regulated in late and early CFA inflammation. Peripheral MOR is not responsible for attenuated responses of nociceptors to mechanical stimuli 18 hours after CFA inflammation. DAMGO reduced mechanical responsiveness of nociceptors at 72 hours after CFA inflammation in a concentration and antagonist reversible manner indicating that MOR efficacy is enhanced during later stages of CFA inflammation. UVB produced severe but localized inflammation that differed from inflammation produced by CFA. This inflammation sensitizes nociceptor units innervating irradiated skin and results in enhanced peripheral opioid receptor efficacy.