Role of Serotonin in Feedback Control of Hypothalamic-Pituitary-Testicular Complex in Male Rats

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Abstract
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.