Effects of undernutrition during suckling and of training on the hypothalamic beta-endorphin of young and adult rats.

The involvement of the hypothalamic beta-endorphinergic system in behavioral processes has previously been studied in adult rats. In the present report, we studied the effects of undernutrition and of inhibitory avoidance training on the hypothalamic beta-endorphin-like immunoreactivity of 21-day-old and adult rats. Rats were undernourished by feeding their dams an 8% protein diet from the day of delivery until weaning (21 days of age). The beta-endorphin-like immunoreactivity was measured by radioimmunoassay. In adult rats, undernutrition decreased the basal level hypothalamic beta-endorphin. Avoidance training decreased the content of beta-endorphin in the hypothalamus of well-nourished adults, but had no effect on the levels of previously undernourished rats. In 21-day-old rats, neither undernutrition nor avoidance training altered the levels of beta-endorphin. These results suggest that the hypothalamic beta-endorphinergic system of weaning rats is not yet functional in relation to the parameters analyzed. Probably, other developmental factors are necessary for the emergence of the effects of undernutrition found in adult rats and for the emergence of the response of this system to training (novelty).