Co-expression of μ and δ opioid receptors by mouse colonic nociceptors

Background and Purpose: To better understand opioid signalling in visceral nociceptors, we examined the expression and selective activation of μ and δ opioid receptors by dorsal root ganglia (DRG) neurons innervating the mouse colon. Experimental Approach: DRG neurons projecting to the colon were identified by retrograde tracing. δ receptor-GFP reporter mice, in situ hybridization, single-cell RT-PCR and μ receptor-specific antibodies were used to characterize expression of μ and δ receptors. Voltage-gated Ca ²⁺ currents and neuronal excitability were recorded in small diameter nociceptive neurons (capacitance <30 pF) by patch clamp and ex vivo single-unit afferent recordings were obtained from the colon. Key Results: In situ hybridization of oprm1 expression in Fast Blue-labelled DRG neurons was observed in 61% of neurons. μ and δ receptors were expressed by 36–46% of colon DRG neurons, and co-expressed by ~25% of neurons. μ and δ receptor agonists inhibited Ca ²⁺ currents in DRG, effects blocked by opioid antagonists. One or both agonists inhibited action potential firing by colonic afferent endings. Incubation of neurons with supernatants from inflamed colon segments inhibited Ca ²⁺ currents and neuronal excitability. Antagonists of μ, but not δ receptors, inhibited the effects of these supernatant on Ca ²⁺ currents, whereas both antagonists inhibited their actions on neuronal excitability. Conclusions and Implications: A significant number of small diameter colonic nociceptors co-express μ and δ receptors and are inhibited by agonists and endogenous opioids in inflamed tissues. Thus, opioids that act at μ or δ receptors, or their heterodimers may be effective in treating visceral pain.