Effect of hypothalamic lesions on temperature regulation in hibernating ground squirrels

Molecular Prerequisites for Diminished Cold Sensitivity in Ground Squirrels and Hamsters

Thirteen-lined ground squirrels and Syrian hamsters are known for their ability to withstand cold during hibernation. We found that hibernators exhibit cold tolerance even in the active state. Imaging and electrophysiology of squirrel somatosensory neurons reveal a decrease in cold sensitivity of TRPM8-expressing cells. Characterization of squirrel and hamster TRPM8 showed that the channels are chemically activated but exhibit poor activation by cold. Cold sensitivity can be re-introduced into squirrel and hamster TRPM8 by transferring the transmembrane domain from the cold sensitive rat ortholog. The same can be achieved in squirrel TRPM8 by mutating only six amino acids. Reciprocal mutations suppress cold sensitivity of the rat ortholog, supporting functional significance of these residues. Our results suggest that ground squirrels and hamsters exhibit reduced cold sensitivity, partially due to modifications in the transmembrane domain of TRPM8. Our study reveals molecular adaptations that accompany cold tolerance in two species of mammalian hibernators.

The amplitude and period of circannual cycles of body weight in golden-mantled ground squirrels with medial hypothalamic lesions

Golden-mantled ground squirrels, Citellus lateralis, were kept for several years in conditions of constant temperature and photoperiod. Control animals showed the expected circannual cycles of body weight, becoming fat in the autumn and thin in the spring. Animals with lesions in the ventromedial region of the hypothalamus also had circannual cycles of weight but in most animals the amplitude was increased. At one time of year the lesioned ground squirrels were grossly obese while often at other times their weight was close to that of control animals or their own preoperative weights. There were unusually early peaks in weight after the lesion in some animals and it is probable that a phase shift occurred. Observations on hibernation, food intake, transient weight gains and delayed weight gains following the lesions are also reported. Evidently during the winter phase of the annual cycle in these ground squirrels there exist mechanisms that lead not only to a loss of weight gained naturally before the time for hibernation but also to a temporary attenuation of additional obesity produced by ventromedial hypothalamic lesions.