Dual effect of morphiceptin on lordosis behavior: possible mediation by different opioid receptor subtypes.

Intracerebroventricular (ICV) infusions of the selective mu receptor agonist morphiceptin produce a dual effect on lordosis behavior in ovariectomized, steroid-primed rats. Low doses of morphiceptin (20 ng) inhibit lordosis whereas higher doses (2000 ng) facilitate this behavior. The present experiment tested whether naloxone, an antagonist of both high- and low-affinity mu receptors, or the long-acting high-affinity mu receptor antagonist naloxazone could block the dual effect of morphiceptin on lordosis. Ovariectomized rats primed with estrogen and progesterone received naloxone, naloxazone, or a control solution prior to ICV infusions of either 0, 20, or 2000 ng of morphiceptin. Naloxone reversed both the inhibition and facilitation of lordosis produced by morphiceptin, but had no effect on lordosis when administered before control infusions. In contrast, naloxazone reversed the inhibition but not the facilitation of lordosis. These results indicate that the inhibitory effect of morphiceptin on lordosis reflects the activation of high-affinity mu receptors whereas the facilitatory effect reflects either the activation of low-affinity mu receptors or other opioid receptor subtypes. The failure of naloxone or naloxazone to affect lordosis in rats receiving control infusions of saline further suggests that endogenous opioid systems do not exert a tonic inhibitory or facilitatory action on lordosis behavior.