Abstract
Three weeks after complete deafferentation of the medial basal hypothalamus (MBH) of adult female rats, the hypothalamic arcuate nucleus (ARCN) was examined ultrastructurally. Axodendritic and axosomatic synapses were counted in a field of 18,000 μm 2 in the middle part of the ARCN in each brain. Intraventricular infusion of 5-hydroxydopamine before autopsy resulted in the differentiation of two types of axon terminals. One axon terminal contained small spherical vesicles (SSVs, about 50 nm in diameter) and the other contained 5-hydroxydopamine-labeled small granular vesicles (SGVs) which were regarded as monoaminergic terminals. In the completely deafferented MBH, mean numbers of SSV and SGV synapses of the ARCN decreased to about one-half and one-fourth, respectively, compared to those of the unoperated rats. However, considerable numbers of intact SSV and SGV synapses were still encountered in the ARCN following deafferentation. There was no significant difference in the number of synapses between the large MBH island (including ARCN and ventromedial nucleus) and the small MBH island (mostly localized in ARCN). These results suggested that numerous converging nonmonoaminergic and monoaminergic fibers terminated in the ARCN and also suggested that nonmonoaminergic and monoaminergic arcuate neurons connected directly with other arcuate neurons.
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