AGING OF THE SEROTONERGIC SYSTEM IN THE RAT FOREBRAIN - AN IMMUNOCYTOCHEMICAL AND NEUROCHEMICAL STUDY

Age-related changes in both morphological and neurochemical parameters of indol- and catecholaminergic system in the rat brain were examined . A qualitative histochemical survey of the occurrence of aberrant ser otonergic fibers in the aged rat brain suggests region-specificity in the process of degeneration. Forebrain areas, such as the caudate-puta men complex, globus pallidus, prefrontal and frontoparietal cortices w ere consistently affected, whereas serotonergic fibers were only infre quently affected in other areas like septal and amygdaloid nuclei. Neu rochemical data similarly revealed regional differences. 5-Hydroxytryp tamine levels were increased in the frontoparietal cortex, hippocampus , hypothalamus and the mesencephalic raphe region but remained unchang ed in the caudate-putamen complex. 5-Hydroxyindolacetic acid levels we re also enhanced in all these areas. Examination of brains of 12-, 18- and 24-month-old rats revealed that aberrant serotonergic fibers were already present at the age of 12 months and their incidence increase with age. There was no difference in the number of serotonergic cells in the dorsal raphe nucleus of young and aged rats. Aberrant tyrosine hydroxylase-immunoreactive fibers were observed only infrequently. The ir occurrence showed no overlap with the areas containing aberrant ser otonergic fibers. Neurochemical estimates of the levels of catecholami nes in young versus aged rat brain areas similarly revealed regional a nd neurotransmitter specific differences to occur during the process o f aging.