Similarities between aberrant serotonergic fibers in the aged and 5,7-DHT denervated young adult rat brain.

Recent morphological observations have suggested neurotransmitter specific degeneration of amongst others, the serotonergic system in the aged rat brain. However, morphological studies can only give a static picture of the events that take place over a period of several months. In the present study we used an experimental model in which degeneration of the serotonergic system in the young adult rat brain was produced on a short time scale. Morphological changes were studied 2 h and 1 or 14 days after intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT). Non-specific damage and severe depletion of serotonergic fibers was observed in the immediate surroundings of the injection site, representing the effects of high local concentrations of 5,7-DHT. Sometime after injection swollen varicosities and dilated non-varicose fibers were observed. Fourteen days after the 5,7-DHT treatment cluster-like fibers appeared. It is argued that these swollen and crumpled fiber knots are slowly degenerating fibers. A comparison is made with the abnormal serotonergic fibers in the aged rat brain and it is concluded that these aged abnormal fibers represent axonal degeneration of the serotonergic system in the senescent rat brain.