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A synaptosomal preparation was employed to pharmacologically characterize the role of presynaptic nociceptin/orphanin FQ (N/OFQ) receptors (NOP receptors) in the regulation of 5-hydroxytryptamine release in the Swiss mouse neocortex. In the present study, the NOP receptor ligands N/OFQ, Ac-RYYRWK-NH(2) and [Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)-NH(2) inhibited the K(+)-induced [(3)H]-5-HT overflow with similar maximal effects ( approximately -35%) but different potencies (pEC(50) of 8.56, 8.35 and 7.23, respectively). The novel agonist [Arg(14),Lys(15)]N/OFQ also inhibited [(3)H]-5-HT overflow, but the concentration-response curve was biphasic and the efficacy higher ( approximately -45%). Receptor selectivity of NOP receptor agonists was demonstrated by showing that synaptosomes from NOP receptor knockout mice were unresponsive to N/OFQ, [Arg(14),Lys(15)]N/OFQ and [Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)-NH(2) but maintained full responsiveness to endomorphin-1. Moreover, the inhibitory effect of N/OFQ was prevented by peptide ([Nphe(1)]N/OFQ(1-13)-NH(2) and UFP-101) and nonpeptide (J-113397 and JTC-801) NOP receptor selective antagonists. Desensitization occurred under perfusion with high (3 and 10 microm) N/OFQ concentrations. This phenomenon was prevented by the protein kinase C inhibitor, bisindolylmaleimide. Moreover, N/OFQ-induced desensitization did not affect mu opioid receptor responsiveness. Finally, it was observed in a similar preparation of rat cerebrocortical synaptosomes, although it was induced by higher N/OFQ concentrations than that used in the mouse. Together, these findings indicate that presynaptic NOP receptors inhibit 5-hydroxytryptamine release in the mouse neocortex. Based on present and previous studies, we conclude that NOP receptors in the mouse are subtly different from the homologous receptor population in the rat, strengthening the view that there exist species differences in the pharmacology of central NOP receptors.

Rat striatal synaptosomes and slices were used to investigate the responsiveness of different populations of nerve terminals to 3-nitropropionic acid (3-NP), a suicide inhibitor of the mitochondrial enzyme succinate dehydrogenase, and to elucidate the ionic mechanisms involved. 3-NP (0.3-3 mm) stimulated spontaneous gamma-aminobutyric acid (GABA), glutamate and [3H]-dopamine efflux but left unchanged acetylcholine efflux from synaptosomes. This effect was associated with a >70% inhibition of succinate dehydrogenase, as measured in the whole synaptosomal population. The facilitation was not dependent on extracellular Ca2+ but relied on voltage-dependent Na+ channel opening, because it was prevented by tetrodotoxin and riluzole. 3-NP also elevated spontaneous glutamate efflux from slices but in a tetrodotoxin-insensitive way. To investigate whether energy depletion could change the responsiveness of nerve endings to a depolarizing stimulus, synaptosomes were pretreated with 3-NP and challenged with pulses of KCl evoking 'quasi-physiological' neurotransmitter release. 3-NP potentiated the K+-evoked GABA, glutamate and [3H]-dopamine release but inhibited the K+-evoked acetylcholine release. The 3-NP induced potentiation of GABA release was Ca2+-dependent and prevented by tetrodotoxin and riluzole whereas the 3-NP-induced inhibition of acetylcholine release was tetrodotoxin- and riluzole-insensitive but reversed by glipizide, an ATP-dependent K+ channel inhibitor. We conclude that the responsiveness of striatal nerve endings to 3-NP relies on activation of different ionic conductances, and suggest that the selective survival of striatal cholinergic interneurons following chronic 3-NP treatment (as in models of Huntington's disease) may rely on the opening of ATP-dependent K+ channels, which counteracts the fall in membrane potential as a result of mitochondrial impairment.