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In order to explore the morphological basis of the altered feeding behaviour of old rats, an ultrastructural investigation of the magnocellular neurons of the hypothalamic paraventricular nucleus (PVN) was performed. Young and old male Wistar rats, 5 and 24 months old, respectively, and with each age group comprising 12 animals, were divided into 3 groups. The rats in Group I were used as controls (normally fed), the rats of Group II were fasted for 48 hours and in Group III the rats were fasted for 48 hours and then refed for 24 hours. The brains were fixed by perfusion and histological and ultrathin sections were obtained by routine methods. Common features of the magnocellular PVN neurons of young and old rats were abundant Golgi complexes and short fragments of RER localised at the cell periphery. In contrast to young rats, the PVN neurons of old animals showed deep indentations of the nuclear envelope and age-related residual bodies. In both age groups fasting for 48 hours led to the expansion of the Golgi complexes and dilatation of RER cisternae. In contrast to those in fed rats, RER cisternae in the neurons of old fasted animals were situated between the nuclear envelope and the Golgi zone. Prolonged RER cisternae were distributed in the peripheral cytoplasm of refed old rats. Our observations suggest that at the ultrastructural level the process of ageing does not change the responsiveness of magnocellular PVN neurons to fasting-refeeding.

Many hypothalamic nuclei are involved in the regulation of food intake and energy homeostasis. An ultrastructural investigation of the hypothalamic ventromedial nucleus (VMN), a hypothetical "satiety centre" was performed to explore the morphological basis of altered feeding behaviour of old rats in an experimental model of fasting/refeeding. Young (5 months old, n=12) and old (24 months old, n=12) male Wistar rats were fasted for 48 hours, then refed for 24 hours and sampled thereafter. Brain tissue was fixed by perfusion, histological and ultrathin sections were obtained by routine methods. Although food intake was similar in control young and old rats, during refeeding old animals consumed less chow than young ones. The EM analysis of VMN neurones of old control rats revealed, besides typical age-related residual bodies, deep indentations of the nuclear envelope and the presence of long, undulating rough endoplasmic reticulum cisternae in the cell periphery. In both young and old rats fasting for 48 hours led to the expansion of Golgi complexes and increased folds of the nuclear envelope, which is suggestive of enhanced cellular activity of the VMN neurones. These fasting-induced alterations were sustained in the VMN neurones of refed rats in both age groups. The results showed that the VMN neurones of old control rats differ at the ultrastructural level from young ones. However, starvation and subsequent refeeding cause similar alterations in the hypothalamic neurones of "satiety centre" of both young and old rats.