The immediate behavioral and physiological effects of hypothalamic knife cuts in rats

Abstract

An experiment to test the immediate food intake behavior and subsequent physiological effects on rats of DC lesions of the ventromedial hypothalamus (VMH), circular knife cuts isolating the VMH, parasagittal knife cuts lateral to the VMH, and knife cuts anterior to the VMH was conducted. Surgery was performed while the animals were anesthetized with ether. Immediately upon arousal, the animals were placed in automatic pellet dispenser cages and food intake (number of pellets called for minus spillage) was measured every two hours for eight hours postsurgery and again at twenty-four hours postsurgery. During this period (the immediate or acute stage of hyperphagia) an order effect on food intake was observed, with the lesion group consuming the greatest amount of food, followed by the circular cut group, the anterior cut group, and the lateral cut group. A peak in food intake was noted for all but the lateral cut animals within the first four hours after surgery. The hyperkinesis common to animals with DC lesions was noted to some extent in the circular cut group. The behavior of the lateral cut animals was generally depressed, most probably because of cannula-induced damage to the motor cortex and at the base of the brain. It was concluded from these results that the acute stage of hyperphagia is not merely an artifact of the DC lesion procedure, but is observed in some measure following hypothalamic knife cuts as well. The degree of immediate hyperphagia is related to the extent and location of damage to the VMH area. The underlying mechanism posited to account for these results is an irritation or excitation of the lateral hypothalamic area induced by the surgical methods. Weight gains were measured weekly for eight weeks following surgery. Again, an order effect was found. The heaviest animals were those with lesions of the VMH, followed by circular cut, lateral cut, and anterior cut groups. At eight weeks the animals were sacrificed and fat content, water content, and residue were measured. Increases in fat content were noted for all experimental groups, and the increased weight gain in all cases was attributed to increases in fat deposition. In addition, free fatty acid mobilization rates attributable to epinephrine stimulation, adrenal glands, ovaries, and naso-occipital growth changes were analyzed. Order effects similar to the effect discussed for weight gains were observed. In most instances, differences between groups were not statistically significant, but were suggestive nevertheless. For each measurement a physiological mechanism responsible for the changes was suggested. In every instance except naso-occipital growth change, the lesion and circular cut groups were at the same end of the continuum. In line with the findings of other investigators, the circular cuts are followed by accelerated growth, while lesions are followed by decreases in naso-occipital growth. An ovariectomized group was included in this study based on the premise that the anterior cuts may disrupt the gonadal-hypothalamic-pituitary axis, and the animals may therefore resemble those with the ovaries removed. The mean growth curves, total weight gains, and fat content of the lateral cut group, the anterior cut group, and the ovariectomized group were remarkably similar. These findings, as well as the results of the free fatty acid mobilization analysis suggest the severance of fibers lateral-anterior to the VMH may be responsible for the weight gain in the anterior and lateral cut groups, for a portion of the weight gain in the circular cut group, as well as responsible perhaps for some alteration in hormone cyclicity.