REPRODUCTIVE BEHAVIORAL RESPONSIVENESS TO NORADRENERGIC STIMULATION IN DEVELOPING GUINEA-PIGS

The stimulatory effects of ovarian hormones on sexual receptivity in g uinea pigs may be mediated by norepinephrine. Juvenile females rarely exhibit steroid-induced receptivity and also respond poorly to the lor dosis-enhancing action of alpha-noradrenergic receptor stimulation. Th is experiment was designed to chart the development of behavioral resp onsiveness to the alpha-noradrenegic agonist, clonidine, and to test t he hypothesis that higher doses of estradiol and/or clonidine are requ ired to stimulate lordosis in juvenile compared to adult guinea pigs. Ovariectomized females received estradiol benzoate (10 mu g or 50 mu g SC) 40-48 h before administration of clonidine (1 mg/kg or 5 mg/kg IF ) or saline at 14-17-day intervals. Regardless of treatment, few anima ls (0-36%) displayed lordosis at 20, 34, or 48 days of age. At 65 days of age, in both estradiol dose groups significantly more clonidine- ( 1 mg/kg) than saline-injected animals displayed lordosis (80-91 vs. 0- 33%, respectively). Clonidine (5 mg/kg) was ineffective at all ages. T hese data do not support the hypothesis that behavioral responsiveness to alpha-noradrenergic receptor stimulation in immature females can b e elicited by increasing the doses of estradiol and/or clonidine. Thes e results suggest the occurrence of a maturational change in the neura l systems governing noradrenergic involvement in steroid-induced sexua l behavior in guinea pigs. (C) 1998 Elsevier Science Inc.