Abstract

Ro 64-6198 [(1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triaza-spiro[4.5]decan-4-one] was developed as a nonpeptide agonist of nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptors, using bioassays at cloned receptors expressed in cell cultures. We have investigated the actions of Ro 64-6198 at native NOP receptors of the ventrolateral periaqueductal gray (PAG), a crucial site for N/OFQ-induced reversal of opioid analgesia, using the patch-clamp recording technique in rat brain slices. Ro 64-6198, like N/OFQ, activated G protein-coupled inwardly rectifying K⁺ channels (GIRK) in ventrolateral PAG neurons but displayed only 60% efficacy and 22% potency of N/OFQ. Unlike N/OFQ that activated GIRK through NOP receptors in almost all tested neurons, Ro 64-6198 affected only 62% (114/185) of the neurons recorded, among which 57% were sensitive to CompB (J-113397), a selective NOP receptor antagonist. The effect of Ro 64-6198 was not affected by naloxone (1 μM), sulpiride (10 μM), and [1-(2-methoxyphenyl)-4-[4-2-phthalimido)butyl]piperazine (NAN-190) (1 μM), respectively, the antagonist of opioid, dopamine D₂, and 5-HT_1A receptors. In Ro 64-6198-unresponsive neurons, N/OFQ activated GIRK through NOP receptors. It is concluded that Ro 64-6198 is a weak agonist of NOP receptors both in terms of potency and efficacy in ventrolateral PAG neurons. Heterogeneity of NOP receptors has been proposed from binding studies and in vivo functional studies. The possibility was discussed that two subsets of NOP receptors exist in ventrolateral PAG neurons, and Ro 64-6198 activates only one subset but N/OFQ activates both of them.