κ 1 ‐ and κ 2 ‐opioid receptors mediating presynaptic inhibition of dopamine and acetylcholine release in rat neostriatum

The effects of selective opioid receptor agonists and antagonists on N‐methyl‐ D ‐aspartate (NMDA, 10 μ M )‐induced release of [ 3 H]‐dopamine and [ 14 C]‐acetylcholine (ACh) from superfused neostriatal slices were studied to investigate the possible occurrence of functional κ‐opioid receptor subtypes in rat brain. The κ receptor agonists (−)‐ethylketocyclazocine ((−)‐EKC), U69593 and the endogenous opioid peptide dynorphin A 1–13 caused a naloxone‐reversible inhibition of NMDA‐induced [ 3 H]‐dopamine release, with pD 2 values of about 9, 8.5 and 8.2, respectively, whereas both the μ agonist Tyr‐ D ‐Ala‐Gly‐(NMe)Phe‐Gly‐ol (DAMGO) and theδ 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 [ 3 H]‐dopamine release were not changed by the δ 1 /δ 2 ‐opioid receptor antagonist naltrindole (up to a concentration of 1 μ M ), but reversed by the κ receptor antagonist nor‐binaltorphimine (nor‐BNI), with an IC 50 as low as 0.02 n M , indicating the involvement of U69593‐sensitive κ 1 ‐opioid receptors. NMDA‐induced [ 14 C]‐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 [ 14 C]‐ACh release was antagonized by naltrindole with an IC 50 of 1 n M , indicating the involvement of δ‐opioid receptors in the inhibitory effect of DPDPE. On the other hand, the inhibitory effects of dynorphin A 1–13 and (−)‐EKC on [ 14 C]‐ACh release were readily antagonized by nor‐BNI with an IC 50 of about 3 n M . A 100 fold higher concentration of nor‐BNI also antagonized the inhibitory effect of DPDPE, indicating the involvement of U69593‐insensitive κ 2 ‐opioid receptors in the inhibitory effects of dynorphin A 1–13 and (−)‐EKC. Although naloxone benzoylhydrazone (NalBzoH), displaying high affinity towards the putative κ 3 ‐opioid receptor, antagonized the inhibitory effects of dynorphin A 1–13 and (−)‐EKC on [ 3 H]‐dopamine and [ 14 C]‐ACh release as well as that of U69593 on [ 3 H]‐dopamine release, it displayed a low apparent affinity (IC 50 about 100 n M ) in each case. In conclusion, whereas activation of κ 1 ‐opioid receptors causes presynaptic inhibition of NMDA‐induced dopamine release, κ