Influence of aging on the neurochemical organization of the rat paraventricular nucleus.

The paraventricular nucleus (PVH) of the hypothalamus is a key region for the integration of the autonomic and neuroendocrine mechanisms. This integration becomes less reliable with age. Some critical functions, such as eating and drinking, body-temperature regulation, autonomic and endocrine responses which regulate the cardiovascular system seem to be particularly affected by the aging-related processes. In this paper, we analysed by means of immunocytochemistry the neurochemical organization of the magnocellular and parvocellular component of the PVH in old male rats. The main results concerning the neurohormones and the carrier proteins are the following: a significant decrease in the number of the oxytocin- (OXY) like immunoreactive neurons of the medial and lateral parvocellular nuclei; a decrease in the vasopressin- (VAS) like immunoreactive neurons of the medial and lateral parvocellular nuclei and also of the medial magnocellular nucleus; a decrease in the neurophysin- (NRP) like immunoreactive neurons of the lateral parvocellular nucleus. We also found a decrease in the mean area of magnocellular OXY- and VAS-like immunoreactive neurons, a decrease in the extension of the dendritic tree sampled in the medial part of the nucleus; a decrease in the number of varicosities along the neurons coming from the PVH, and in the density of axons in the median eminence and in the vagal complex. The NRP-like immunoreactive structures in the substantia gelatinosa of the spinal cord of old rats were also decreased in respect to younger adult animals. Among the neuropeptides investigated (corticotropin-releasing factor, leu-enkephalin, somatostatin, cholecystokinin and neurotensin) we found a decrease in the leu-enkephalin-like immunoreactive neurons of the dorsal and medial parvocellular nuclei. Our data support--from a morphological point of view--the existence of involution processes in the neurochemical organization of the PVH during aging.