ELECTRON MICROSCOPY OF DEGENERATION WITHIN SEROTONIN PATHWAY OF RAT BRAIN

Two large aggregates of serotonin-containing neurons are situated in the dorsal and median raphe nuclei of the caudal midbrain. Small electrolytic lesions placed in these nuclei and the suprachiasmaticnucleus (which receives axon terminals from the raphe nuclei) and the paraventricular nucleus of the hypothalamus were examined with the electron microscope 2-7 days post lesion. Many degenerating terminals were found in the suprachiasmatic nucleus but few were noted in the paraventricular nucleus. The degeneration was manifested by increased osmiophilia of nerve-ending ground substance, a dissolution of vesicular and mitochondrial membranes, and later, by a marked proliferation of glial lamellae. Both axosomatic and axodendritic terminals showed such changes; large granular vesicles were frequently observed in the involved endings. These results are in accord with the delineation of the serotonin-containing neuronal pathway based on the fluorescence histochemical method. © 1969.