Age-related changes in serotonin-immunoreactivity in the telencephalon and diencephalon of rats.

Comparison of the appearance and density of the serotoninergic fibers and terminals in some tel- and diencephalon areas of young adult (3 months) and aged (28 months) rats was made immunohistochemically using an antibody to serotonin. In young adult rats a characteristic arrangement of the serotonin-immunoreactive (SER-IR) fibers and terminals in the tel- and diencephalon was observed. In aged rats the distribution pattern was the same but changes in the appearance and density of the SER-IR fibers were demonstrated. Many swollen and folded fibers forming cluster-like structures as well as reduced fiber networks were seen in the neocortex, hippocampal formation and striatum of aged rats. Aberrant fibers and decreased fiber density were also observed in the bed nucleus of the stria terminalis, lateral septum, nucleus accumbens and thalamic nuclei. Reduced fine granular immunostaining and scattered swollen varicosities were found in some amygdaloid and hypothalamic nuclei of aged rats. These results provide further morphological evidence for changes in the serotoninergic innervation during aging.