Immunocytochemical changes in serotonin in the forebrain and pituitary of aging fish

The immunocytochemical distribution of immunoreactive serotonin (ir-5HT) was studied in the forebrain of male platyfish ranging in age from 5 to 30 months (mean life span, 30 months). In fish at all ages, ir-5HT is found in the forebrain in the wall of the third ventricle and its lateral and posterior recesses and in the pituitary in PAS-positive cells of the pars intermedia (PI). With increasing age, ir-5HT first appears in perikarya of the nucleus preopticus (18 months) and in gonadotropes of the caudal pars distalis. Between 8 and 12 months of age, some fish display pale 5HT immunoreactivity in pituitary gonadotropes while in all fish of 18 months and older, the gonadotropes as well as the PI cells show intense 5HT immunostaining. It is suggested that these modifications in serotonin localization may be related to reproductive senescence in platyfish and that they may be associated with other age-related changes in neurohormone and neurotransmitter immunoreactivity. These results are compared with similar age-related changes in the metabolism of neurotransmitters which have been found in mammals.