kappa(1)- and kappa(2)-opioid receptors mediating presynaptic inhibition of dopamine and acetylcholine release in rat neostriatum

1 The effects of selective opioid receptor agonists and antagonists on N-methyl-D-aspartate (NMDA, 10 mu M)-induced release of [H-3]-dopamine and [C-14]-acetylcholine (ACh) from superfused neostriatal slices were studied to investigate the possible occurrence of functional kappa-opioid receptor subtypes in rat brain. 2 The kappa receptor agonists (-)-ethylketocyclazocine ((-)-EKC), U69593 and the endogenous opioid peptide dynorphin A(1-13) caused a naloxone-reversible inhibition of NMDA-induced [H-3]-dopamine release, with pD(2) values of about 9, 8.5 and 8.2, respectively, whereas both the mu agonist Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO) and the delta agonist D-Pen(2)-D-Pen(5)-enkephalin (DPDPE) were ineffective in this respect. The inhibitory effect of submaximally effective concentrations of dynorphin A(1-13), U69593 and (-)-EKC on NMDA-induced [H-3]-dopamine release were not changed by the delta(1)/delta(2)-opioid receptor antagonist naltrindole (up to a concentration of 1 mu M), but reversed by the kappa receptor antagonist norbinaltorphimine (nor-BNI), with an IC50 as low as 0.02 nM, indicating the involvement of U69593-sensitive kappa(1)-opioid receptors. 3 NMDA-induced [C-14]-ACh release was reduced in a naloxone-reversible manner by DPDPE (pD(2) about 7.2), dynorphin A(1-13) (pD(2) 6.7) and EKC (pD(2) 6.2), but not by U69593 and DAMGO. The inhibitory effect of a submaximally effective concentration of DPDPE, unlike those of dynorphin A(1-13) and (-)-EKC, on NMDA-induced [C-14]-ACh release was antagonized by naltrindole with an IC50 of 1 nM, indicating the involvement of delta-opioid receptors in the inhibitory effect of DPDPE. On the other hand, the inhibitory effects of dynorphin A(1-13) and (-)-EKC on [C-14]-ACh release were readily antagonized by nor-BNI with an IC50 of about 3 nM. A 100 fold higher concentration of nor-BNI also antagonized the inhibitory effect of DPDPE, indicating the involvement of U69593-insensitive kappa(2)-opioid receptors in the inhibitory effects of dynorphin A(1-13) and (-)-EKC. 4 Although naloxone benzoylhydrazone (NalBzoH), displaying high affinity towards the putative kappa(3)-opioid receptor, antagonized the inhibitory effects of dynorphin A(1-13) and (-)-EKC on [H-3]-dopamine and [C-14]-ACh release as well as that of U69593 on [H-3]-dopamine release, it displayed a low apparent affinity (IC50 about 100 nM) in each case. 5 In conclusion, whereas activation of kappa(1)-opioid receptors causes presynaptic inhibition of NMDA-induced dopamine release, kappa(2) receptor activation results in inhibition of ACh release in rat neostriatum. As such, this study is the first to provide unequivocal in vitro evidence for the existence of functionally distinct kappa-opioid receptor subtypes in the brain.