THE localization of opioid receptor-like (ORL-1) orphan receptor in th
e ventromedial nucleus of the hypothalamus (VMH) suggested a role for
this opioid system in the regulation of lordosis behavior. Recently, t
he ligand for ORL-1, orphanin FQ/nociceptin (OFQ/N), has been characte
rized and also demonstrated to be present in the VMH. The present expe
riments examined whether OFQ/N in the VMH facilitates lordosis behavio
r in estrogen-primed, sexually unreceptive female rats, and whether es
trogen regulates ORL-1 levels in the VMH. Estrogen was shown to increa
se ORL-1 immunoreactivity in the VMH, and microinfusions of OFQ/N into
the VMH facilitated lordosis behavior in a dose-dependent manner.