Different roles of alpha-noradrenergic receptor subtypes in regulating lordosis.

Ovariectomized guinea pigs were given free estradiol (E) at hr 0 and 28, and progesterone (P) at hr 39. Experiment 1: The alpha-1 noradrenergic antagonist prazosin, but not the alpha-2 noradrenergic antagonist idazoxan, prevented display of lordosis behavior when administered systemically 30 min prior to E injections at either hr 0 or 28, or 30 min prior to both injections. Experiment 2: Multiple injections of idazoxan (i.e., 30 min prior to and 60 and 120 min after each E injection) failed to prevent display of lordosis. Experiment 3: E-primed animals were given a single injection of idazoxan 1 hr before P, or 5 hr after P. In those animals given idazoxan 1 hr before P, display of lordosis was not prevented. However, in animals given idazoxan once lordosis display had begun (i.e., 5 hr after P) ongoing lordosis behavior was blocked. These data suggest differential roles of alpha-noradrenergic receptor subtypes in regulation of lordosis: noradrenergic transmission through the alpha-1 subtype may mediate hormone-priming processes leading to lordosis, whereas transmission through the alpha-2 subtype may modulate ongoing lordosis responses.