Synaptic changes in the hypothalamic arcuate nucleus of old male rats

The hypothalamic arcuate nucleus (ARCN) was examined ultrastructurally in young (90 to 100 days of age) and old (720 to 930 days of age) male rats. Axodendritic (ADSs) and axosomatic synapses (ASSs) were counted in a field of 18,000 μm² of the ARCN in each brain. In old male rats, the mean numbers of ADSs and ASSs were reduced to approximately 51 and 57% of those of young rats, respectively. Neuronal cell bodies in the ARCN of old male rats were significantly decreased in number compared with those in young male rats. In the ARCN of old animals, increasing numbers of large spherical vesicles or large granular vesicles coexistent with small spherical vesicles were detected in the axon terminals. In some cases, dense inclusion bodies or membranous multilamellar bodies were encountered in the nerve endings. Synaptic thickenings of old male rats were thin, short, and/or discontinuous. As for the dendrites in old male rats, irregular, electron-dense dendritic shafts with a paucity of neurotubules were often observed. Because the ARCN is considered to be one of the principal neural components which regulate secretory functions of the anterior pituitary, loss in the number of synapses and some pathologic changes in its neuropil during the process of aging may apparently impair its integrative control mechanisms. These age-related changes in the ARCN seem to correlate with the aging of the neuroendocrine system.