Differential Effects of CB1 and Opioid Agonists on Two Populations of Adult Rat Dorsal Root Ganglion Neurons

Abstract

Inhibition of primary afferent neurons contributes to the antihyperalgesic effects of opioid and CB1 receptor agonists. Two bioassays were used to compare the effects of the CB1 receptor agonist CP 55,940 and morphine on dissociated adult rat DRG neurons. Both agonists inhibited the increase in free intraceUular Ca2+ concentration evoked by depolarization; however, effects of CP 55,940 occurred primarily in large neurons (cell area, >80 μm2), whereas morphine inhibited the response in smaller neurons. Cotreatment with selective blockers of L-, N-, and P/Q-type voltage-dependent Ca2+ channels indicated that CB1 receptors on DRG neurons couple solelywith N-type channels but opioid receptors couple with multiple subtypes. Experiments with selective agonists and antagonists of opioid receptors indicated that μ and δ, but not κ, receptors contributed to the inhibitory effect of morphine on voltage-dependent Ca2+ influx. Because Ca2+ channels underlie release of transmitters from neurons, the effects of opioid agonists and CP 55,940 on depolarization-evoked release of calcitonin gene-related peptide (CGRP) were compared. Morphine inhibited release through δ receptors but CP 55,940 had no effect. Colocalization of CGRP with δ-opioid but not μ-opioid or CB1 receptor immunoreactivity in superficial laminae of the dorsal horn of the spinal cord was consistent with the data for agonist inhibition of peptide release. Therefore, CB1 and opioid agonists couple with different voltage-dependent Ca2+ channels in different populations of DRG neurons. Furthermore, differences occur in the distribution of receptors between the cell body and terminals of DRG neurons. The complementary action of CB1 and opioid receptor agonists on populations of DRG neurons provides a rationale for their combined use in modulation of somatosensory input to the spinal cord.