Acute desensitization of nociceptin/orphanin FQ inhibition of voltage-gated calcium channels in freshly dissociated hippocampal neurons

Nociceptin/orphanin FQ (N/OFQ), an endogenous ligand for opioid receptor-li ke receptor, has been shown to inhibit high-voltage-gated calcium channels (VGCCs) in acutely dissociated rat hippocampal pyramidal cells [Knoflach, F ., Reinscheid, R.K. Civelli, O. & Kemp, J.A. (1996), J. Neurosci., 16, 6657 ]. In this study, it was further demonstrated that N/OFQ inhibition of calc ium channel current was blocked by its specific antagonist PGN, [Phe(1)-psi (CH2-NH)-Gly(2)]nociceptin (1-13)-NH2, and the EC50 of the N/OFQ inhibition was similar to 10 nM, indicating that this effect was really mediated via the opioid receptor-like receptor. The N/OFQ inhibition of the calcium chan nel current was significantly reduced, as the maximal inhibition decreased from 36 to 23%, by 1-min pretreatment of freshly dissociated hippocampal ne urons with the same peptide. The inhibition completely recovered from this acute desensitization in les than 20 min. The N/OFQ inhibition was also gre atly attenuated by pretreatment of the neurons with the GABA(B) (gamma-amin obutyric acid) agonist baclofen while the baclofen inhibition of the calciu m channel current was significantly reduced by N/OFQ pretreatment, revealin g the agonist-induced desensitization was heterologous in nature. This dens ensitization was blocked by pretreating the neurons with the sodium channel blocker tetrodotoxin (TTX), or by removing the extracellular calcium, whic h indicates the necessity of membrane depolarization and extracellular calc ium influx in the process. Furthermore, pretreatment of the neurons wit the protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (PMA), a ttenuate the N/OFQ inhibition of the calcium channel current whereas the cA MP-dependent kinase A activator, forskolin, showed no effect, suggesting th e probable involvement of PKC in the N/OFQ-induced desensitization.