κ₁- and κ₂-opioid receptors mediating presynaptic inhibition of dopamine and acetylcholine release in rat neostriatum

Abstract

1. The effects of selective opioid receptor agonists and antagonists on N-methyl-D-aspartate (NMDA, 10 μM)-induced release of [³H]-dopamine and [¹⁴C]-acetylcholine (ACh) from superfused neostriatal slices were studied to investigate the possible occurrence of functional κ-opioid receptor subtypes in rat brain.

2. The κ receptor agonists (−)-ethylketocyclazocine ((−)-EKC), U69593 and the endogenous opioid peptide dynorphin A_1–13 caused a naloxone-reversible inhibition of NMDA-induced [³H]-dopamine release, with pD₂ 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-Pen²-D-Pen⁵-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 [³H]-dopamine release were not changed by the δ₁/δ₂-opioid receptor antagonist naltrindole (up to a concentration of 1 μM), but reversed by the κ receptor antagonist nor-binaltorphimine (nor-BNI), with an IC₅₀ as low as 0.02 nM, indicating the involvement of U69593-sensitive κ₁-opioid receptors.

3. NMDA-induced [¹⁴C]-ACh release was reduced in a naloxone-reversible manner by DPDPE (pD₂ about 7.2), dynorphin A_1–13 (pD₂ 6.7) and EKC (pD₂ 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 [¹⁴C]-ACh release was antagonized by naltrindole with an IC₅₀ of 1 nM, 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 [¹⁴C]-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 κ₂-opioid receptors in the inhibitory effects of dynorphin A_1–13 and (−)-EKC.

4. Although naloxone benzoylhydrazone (NalBzoH), displaying high affinity towards the putative κ₃-opioid receptor, antagonized the inhibitory effects of dynorphin A_1–13 and (−)-EKC on [³H]-dopamine and [¹⁴C]-ACh release as well as that of U69593 on [³H]-dopamine release, it displayed a low apparent affinity (IC₅₀ about 100 nM) in each case.

5. In conclusion, whereas activation of κ₁-opioid receptors causes presynaptic inhibition of NMDA-induced dopamine release, κ₂ 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 κ-opioid receptor subtypes in the brain.

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