Further Evidence That Serotonin Mediates the Steroid-Independent Inhibition of Luteinizing Hormone Secretion in Anestrous Ewes1

Two photoperiod-conzrolled neuroendocrine systems appear to suppress secretion of tonic luteinizing hor-mone (LII) in anestrous ewes: a steroid-independent system that decreases LII pulse frequency in ovariectomized ewes and a steroid-dependent system whereby estradiol gains the capacity to suppress LH pulse frequency in anestrus. This study was designed to test the hypothesis that serotonergic neurons inhibit LII pulse frequency in ovariectomized ewes and to examine the possible interaction of this system with the steroid-dependent inhibition of LII pulse frequency in the anestrous season. in Experiment 1, i.v. injection of serotonin receptor antagonist, methysergide, significantly increased LI-I pulse frequency in ovariectomized ewes during the anestrous season. in Experiment 2, we examined the effects of oral administration of parachiorophenylalanine for 5 days on the synthesis of serotonin. This treatment significantly increased LII pulse frequency in ovariectomized ewes, but had no effect on the negative feed back action of estradiol. These data support the hypothesis that a serotonergic neural system mediates the steroid-independent inhibition of LII pulse frequency in anestrous ewes and suggest that this system is not absolutely essentiai for the functioning of the steroid-dependent system responsible for the negative feedback action during the anestrous season