Opioid receptor-mediated control of acetylcholine release in human neocortex tissue

The effects of various opioid receptor agonists and antagonists on evoked acetylcholine release were studied in slices of human neocortex prelabelled with [3H]-choline, superfused and depolarized electrically (2 min, 3 Hz, 2 ms, 24 mA) or by K+ (20 mM). The delta-opioid receptor agonist DPDPE and the kappa-opioid receptor agonist U50488 reduced the evoked [3H]-overflow (acetylcholine release) in a concentration-dependent fashion; the delta-opioid receptor antagonist naltrindole and the kappa-opioid receptor antagonist norbinaltorphimine, respectively, antagonized these effects. Application of the mu-opioid receptor agonist DAGO also resulted in an inhibition of acetylcholine release; however, both delta- and kappa-opioid receptor antagonists were able to block this effect. The mu-opioid receptor agonists morphine and (+)-nortilidine had no effect. These results indicate that acetylcholine release in human neocortex is inhibited through delta- and kappa-opioid receptors, but not through mu-opioid receptors. Acetylcholine release was significantly increased by the delta-opioid receptor antagonist naltrindole in the presence of a mixture of peptidase inhibitors providing evidence for a delta-opioid receptor-mediated inhibition of acetylcholine release by endogenous enkephalin. K(+)-evoked acetylcholine release in the presence of TTX was inhibited by U50488, but not by DPDPE, suggesting the presence of kappa-opioid receptors on cholinergic terminals and the localization of delta-receptors on cortical interneurons. Therefore, the potent effect of DPDPE on acetylcholine release is likely to be indirect, by modulation of intrinsic cortical neurons. These interneurons probably do not use GABA as neurotransmitter since both GABAA and GABAB receptor agonists (muscimol and baclofen, respectively) were without effect on acetylcholine release.