Effect of acute and repeated immobilization stress on the ultrastructure of neurons of the rat hypothalamic ventromedial nucleus.

The fine structure of neuronal somata of the central subdivision of the hypothalamic ventromedial nucleus (VMN) was studied in acutely and repeatedly immobilized rats. Four categories of neurons were distinguished in control animals based mainly on ultrastructural appearance of rough endoplasmic reticulum (RER): 1. the neurons with longer segments of RER distributed throughout the cytoplasm which, at the same time, clearly dominated in this area, 2. the neurons with RER arranged into several groups located at different sites of the soma, 3. the neurons with only a few of short fragments of RER scattered in the cytoplasm, and 4. the neurons with an extensive parallel system of RER in the cell periphery. After acute stress only three categories were found (the 4th category being absent), whereby the number of the 2nd category of neurons evidently increased. Following repeated immobilization a further category of neurons occurred, which represented 9...11% of the neuronal population and showed a wide variety of RER modifications including whorls. The presented morphological evidence suggest that the central subdivision of rat VMN might be involved in neuroendocrine processes activated during increased pituitary-adrenocortical response under immobilization stress.