Role of serotonin in feedback control of hypothalamic-pituitary-testicular complex in male rats

Hemicastration of male rats produced rapid changes in plasma testosterone (T) levels both in summer and in winter. However, in winter, after unilateral castration, the duration of T maximum decrease was found to be longer, but the compensatory increase of blood T following its drop was more dramatic as compared to the summer season. The intraperitoneal (i.p.) 5-hydroxytryptophan (5-HTP) administration blocked the compensatory increase in plasma T that followed its drop induced by hemicastration. At the same time, under similar experimental conditions, a preliminary injection of p-chlorophenyl-alanine (PCPA) was accompanied by a more marked rise of plasma T than in control unilaterally castrated rats. After the injection of serotonin (5-HT) into the lateral ventricle of the brain there was no effect on the blood T content in rats with intact gonads. However, 5-HT injection to hemicastrated animals under similar conditions completely prevented the compensatory increase of T level in blood after its fall caused by the operation. On the other hand, double intraventricular injection of methysergide was followed by a more marked rise of blood T content as compared with the control hemicastrated rats. The conclusion is drawn that brain serotoninergic neurons are involved in feedback mechanisms related to the control of hypothalamic-pituitary-testicular complex.