Serotonin in golden hamster testes: Testicular levels, immunolocalization and role during sexual development and photoperiodic regression-recrudescence transition

Serotonin (5-HT) is found in the gonads and accessory reproductive organs o f several species. The golden (Syrian) hamster is a seasonal breeder. Expos ure of male adult hamsters to short days for 14 weeks results in a severe g onadal regression, while after a photoinhibition period of 22 weeks a spont aneous testicular recrudescence occurs. The aim of this study was to invest igate the presence of 5-HT and its major metabolite 5-hydroxyindoleacetic a cid (5-HIAA) in the gonads of golden hamsters, its immunolocation and its p hysiological role in the testis. The influence of age and photoperiod was a lso analyzed. Hamsters of 23, 36, 46, 60 and 90 days of age were kept in lo ng photoperiod (LP: 14:10 h light/dark), and adult animals were exposed eit her to LP or to short photoperiod (SP: 6:18 h light/dark) for 14 and 22 wee ks. Testicular parenchyma and capsule levels of 5-HT and 5-HIAA increased s ignificantly at ages of 36 and 60-90 days, but decreased markedly during th e exposure of adult hamsters to SP for 14 and 22 weeks. Mast cells were fou nd exclusively in the testicular capsule. The testicular number of mast cel ls increased concomitantly with age, but decreased in adult hamsters expose d to SP. Mast and Leydig cells presented 5-HT-positive immunoreactivity. Du ring sexual maturation as well as during the transfer of adult hamsters fro m LP to SP, the 5-HIAA/5-HT ratio showed the highest values in active adult animals, indicating that the increase in testicular 5-HT levels in adultho od is accompanied by an augment in 5-HT turnover. In vitro basal and hCG-st imulated testosterone production was significantly inhibited in presence of physiological concentrations of 5-HT. In conclusion, the present studies d emonstrate the existence of 5-HT in mast cells and Leydig cells of hamster testes, as well as describe an inhibitory action of this neurotransmitter o n gonadal testosterone production. Furthermore, the age-dependent and photo periodic-related changes detected in testicular 5-HT levels suggest that th is neurotransmitter might act as an important local modulator of the action of gonadotropins on steroidogenesis during sexual development and du ring the photoperiodic regression-recrudescence transition in the golden hamster .