Differential antagonism by naloxone benzoylhydrazone of the activation of inward rectifying K+ channels by nociceptin and a μ-opioid in rat periaqueductal grey slices

A novel receptor, the opioid receptor-like orphan receptor (ORL1), is homologous to, but distinct from, classical opioid receptors. Although initially developed as an opioid receptor ligand, naloxone benzoylhydrazone (NalBzOH) is one of the few antagonists at ORL1. The present electrophysiological study of the effects of NalBzOH on the activation of ORL1 and mu -opioid receptors was performed in brain slices of the ventrolateral periaqueductal grey (PAG), a crucial site for opioid-induced supraspinal analgesia. Both orphanin FQ/nociceptin (OFQ/N), an ORL1 agonist, and [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), a mu -opioid receptor agonist, activated inwardly rectifying K+ (Kir) channels in the ventrolateral PAG. Of the neurons tested, 96% responded to OFQ/N, but only 65% to DAMGO. NalBzOH (3-30 muM) antagonized the effect of OFQ/N competitively with a pA(2) of 5.67. NalBzOH also antagonized, but more potently and non-competitively, the effect of DAMGO. In contrast, NalBzOH did not affect baclofen-induced activation of Kir channels. NalBzOH alone, at concentrations up to 30 muM, had little effect on this inwardly rectifying channel. It is concluded that NalBzOH antagonizes the activation of Kir channels mediated by both ORL1 and mu -opioid receptors in the ventrolateral PAG. It acts not only as a competitive antagonist at ORL1, but also as a more potent and non-competitive antagonist at mu -opioid receptors.