SUPRACHIASMATIC NUCLEUS - ROLE IN CIRCANNUAL BODY-MASS AND HIBERNATION RHYTHMS OF GROUND-SQUIRRELS

Female golden-mantled ground squirrels that sustained complete ablatio n of the suprachiasmatic nucleus (SCNx) were housed pre- and post-oper atively at 23 degrees C and then at 6.5 degrees C for 5-7 yr. SCNx and control animals held at the higher temperature manifested circannual rhythms (CARs) in body mass. In contrast, body mass CARs were not expr essed in 50% of SCNx squirrels during cold exposure; rhythm amplitude was reduced to 25-40% of pre-operative values and the interval between successive peaks in body mass fell outside the circannual range. Unli ke normal squirrels that hibernate for about 6 months during each circ annual cycle, these SCNx squirrels expressed bouts of torpor nearly co ntinuously throughout 2.5 yr of cold exposure. Body mass increases wer e often observed during hibernation-a phenomenon never observed in con trol animals. The remaining SCNx squirrels that did not hibernate cont inuously displayed CARs in body mass within the normal range. The effe cts of SCN ablation on body mass rhythms presumably are related to dis rupted patterns of hibernation, food intake, and metabolism. The SCN, which sustains neural and metabolic activity at low tissue temperature s, may exert greater influence on thermoregulation and metabolism duri ng the hibernation season than at other times of year, thereby account ing for the greater effect of SCN ablation in squirrels maintained at low ambient temperatures. (C) 1998 Elsevier Science B.V.