Differential effects of cuts through the posterior hypothalamus on food intake and body weight in male and female rats

Large knife cuts in the coronal plane through the posterior hypothalamus which transected the caudal connections of the medial hypothalamus without producing significant direct damage to cellular components of the region resulted in a very marked increase in food intake and rate of weight gain in female rats, but had little or no effect in male rats of the same strain. Water intake was equally elevated in both sexes. A detailed analysis of the effects of smaller cuts in the same region indicated that the pathways that are responsible for the effectiveness of the larger cuts do not course through the region immediately behind the ventromedial nuclei (VMN) but, instead, through the perifornical region lateral and posterior to the VMN. In the female, hyperphagia is not seen after cuts behind the VMH or lateral to the fornix but all cuts that invade the perifornical region to a significant extent result in overeating and obesity. In the male, a very mild hyperphagia (but no obesity) is seen after cuts that are restricted to this perifornical region but larger cuts that involve even minor aspects of the lateral hypothaalmus (LH) are entirely ineffective. Analysis of the effects of cuts that involve the medial, perifornical, and lateral hypothalamus to varying extent suggested that the disinhibitory effects of perifornical transections may be counteracted by relatively strong inhibitory effects due to damage to fibers in the lateral hypothalamus in the male, where the influence of the perifornical system appears to be weak, but not in the female, where the influence of the perifornical system appears to be very strong. The marked sex differences which characterize the effects of all cuts on body weight indicate that the perifornical system may be concerned specifically with metabolic functions that contribute to the development and/or maintenance of the organism's setpoint for body weight.