Changes in the masseteric reflex during hypothalamic aphagia in cats

Abstract

Experiments were conducted on 13 adult cats, each bearing a bipolar stimulating electrode in the mesencephalic nucleus of the trigeminal nerve, a bipolar recording electrode in the ipsilateral masseter muscle, bipolar cortical recording electrodes, and two monopolar electrodes implanted bilaterally in hypothalamic sites. In each animal, the masseteric reflex (MR), evoked by stimulation of the mesencephalic nucleus, was recorded during five daily sessions, which included periods of eating and sleeping. Subsequently, unilateral or bilateral electrolytic lesions were made in the hypothalamus through the implanted electrodes. Recording was continued for 24 h immediately after the lesions and then daily for 3 h. It was found that bilateral lesions in the lateral or far-lateral hypothalamus resulted both in aphagia and a decrease in the MR in nine of 13 cats. In two cats a strong aphagia was accompanied by a large decrease in the MR, and in three cats a mild aphagia or hypophagia was observed with a small decrease in the MR. The changes in feeding behavior in the remaining cats were not proportional to the changes in the MR. In three of these cats, the lesions resulted in a strong decrease in the MR but only in a mild decrease in feeding. In another animal a strong aphagia was accompanied only by a mild decrease in the MR. In two other cats an increase in the MR was observed while the animals were hypophagic. Unilateral lesions in the far-lateral hypothalamus in two cats resulted in hypophagia, but produced no change in the MR. From these data we conclude that alterations in the masseteric reflex, which is known to be directly related to food intake, may be dependent on brain mechanisms different from those responsible for the control of food intake, and that changes in oral motor activity related to mastication are not an indispensable component of hypothalamic aphagia.