Postnatal alterations in noradrenaline nerve terminals in the periventricular hypothalamus of the rat.

Abstract

Postnatal changes in aminergic terminals were studied electron microscopically in the periventricular region of the rat hypothalamus by means of a potassium permanganate fixation technique. By this method, a NA terminal could be recognized by the presence of small cored vesicles within the terminals. In neonatal rats, NA terminals were already present, although no special contact with other neuronal elements was found. In three day old rats, the NA terminals seemed to gradually make special contact with dendritic processes. Such contacts were similar to the synaptic specialization of Gray's type I, and their membrane specializations existed in all part of apposing membranes of synapses. Hence, they were termed an “immature synapse”. Subsequently, such immature synapses appeared to increase markedly in number with the development until two weeks after the birth. In 21 day old rats, the NA terminals possessing typical synapses of Gray's type I decreased in population of total NA terminals. A large number of synapses in NA terminals displayed partial membrane specializations in this stage, but differed from those of “mature type” in electon-opacities of both intracellular materials in synaptic clefts and cytoplasmic materials associated with postsynaptic membrane, so that they were termed a “transient type”. In 35 day old rats, NA terminals possessed synapses of a “mature type” which were characterized by both few electron-dense intercellular materials in synaptic clefts and poor development of postsynaptic thickening. As several features were found in synapses revealing intermingled characters of all three types, it is suggested that the synapse of the immature type gradually converted to the mature type synapse. These results were confirmed by another approach employing the conventional aldehyde-osmium fixation method in rats pretreated with 5-OH dopamine.