Learning takes place in rats with deafferentation of the medial basal hypothalamus

Abstract

The present experiments were designed primarily to determine whether or not the neural connections of the hypophysiotrophic region of the hypothalamus are essential for learning (habituation, acquisition, retention, and extinction) in the male rat. The paradigms employed measured learning of a food-related habit, inhibitory and passive shock avoidance, and swim escape. In addition, measurements were made of body weights throughout the entire experimental period, of open field activity, of deposition of fecal boli, and of male sexual behavior. The results showed that learning took place approximately to the same extent in rats whether or not the hypophysiotrophic region was isolated from the rest of the brain. Thus, if the operation of the hypophysiotrophic region is essential at all for learning, humoral communication to and from the isolated region may be sufficient to maintain the required processes at a normal level. When placed on an ad libitum food intake, the hypothalamic island animals became obese. Impairment in male sexual function also was noted. In the feeding paradigm and five of seven measurements made in the other behavioral studies, the variabilities (standard deviations) were less among the animals with hypothalamic islands than in the operated controls. In measurements of the feeding habit, the variability in response rates among individual rats in the hypothalamic island group was significantly lower than among sham-operated animals during the times when high response rates were observed. One of the roles of the hypothalamus is to help maintain homeostatic constancy. It is suggested that another role, largely mediated by the activities of the hypophysiotrophic region, might be to emit neural and humoral signals that increase the number of behavioral options available to an individual organism, concomitantly increasing the variability among individuals, viz., the hypophysiotrophic region may be viewed as a variability generator. Increases in behavioral variability among members of a particular species in response to increasing environmental exigencies may enhance the ability of that species to survive.