Orphanin FQ/nociceptin in the ventromedial nucleus facilitates lordosis in female rats.
The localization of opioid receptor-like (ORL-1) orphan receptor in the ventromedial nucleus of the hypothalamus (VMH) suggested a role for this opioid system in the regulation of lordosis behavior. Recently, the ligand for ORL-1, orphanin FQ/nociceptin (OFQ/N), has been characterized and also demonstrated to be present in the VMH. The present experiments examined whether OFQ/N in the VMH facilitates lordosis behavior in estrogen-primed, sexually unreceptive female rats, and whether estrogen regulates ORL-1 levels in the VMH. Estrogen was shown to increase ORL-1 immunoreactivity in the VMH, and microinfusions of OFQ/N into the VMH facilitated lordosis behavior in a dose-dependent manner.