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The effect of intratesticular administration of serotonin (5-HT), ketanserin (5-HT2 receptor antagonist), and 5,7-dihydroxytryptamine (5,7-DHT) (the neurotoxin that destroys serotoninergic neural elements) on steroidogenesis was studied in immature and adult rats. In adults, bilateral intratesticular injection of 5-HT resulted in a significant decrease in basal but not in hCG-stimulated testosterone secretion and in serum testosterone concentration, whereas ketanserin induced a significant rise in steroidogenesis 1 h post-treatment. There was no effect 1 day after administration of 5-HT or ketanserin, and 7 days after the injection of 5,7-DHT. In immature rats 1 day after bilateral testicular administration of ketanserin, basal testosterone secretion in vitro was significantly suppressed. In immature hemicastrates, local injection of 5-HT resulted (1 day post-treatment) in a significant rise in steroidogenesis while administration of 5,7-DHT decreased testosterone secretion 7 days after the injection of the neurotoxin. The results indicate that in adult rats 5-HT exerts a suppressive, whereas in immature rats, a stimulatory action on steroidogenesis occurs. Data also suggest that, in both age groups, the effect of 5-HT is mediated through 5-HT2 receptors. The observation that in immatures administration of the neurotoxin resulted in an effect similar to that found following the treatment with the receptor antagonist suggests that, in this age group, 5-HT derived from local neural elements might also be involved in the control of 5-HT on Leydig cell steroidogenesis.

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