Differential effects of adrenergic blockade on seasonal changes in core temperatures and thermal conductances of deer mice maintained in thermal neutral environments.

1. Resting, daytime, thermal conductances and metabolic rates of mice conditioned to winter (10:14LD) and summer (14:10LD) photoperiods were reduced by social huddling; huddling resulted in group size related elevations in core temperature during summer, but not with winter light-dark cycle exposures. 2. Core temperatures of resting, solitary winter animals were lower than those of summer; both summer and winter animals' core temperatures were further reduced by increased thermal conductance resulting from (phentolamine) alpha receptor blockade. 3. Social huddling reduction of the heat loss from phentolamine treatment was more effective for winter (10:14LD) animals. 4. While phentolamine treatment resulted in increased thermal conductance and lower core temperatures of the mice, propranalol treatment resulted in lower core temperatures and resting metabolic rates, with a resulting decrease in thermal conductance. 5. Since adrenergic blockade was less dose-effective on winter animals, we reasoned that winter animals display higher levels of endogenous adrenergic capacity than summer animals and that lower winter thermoregulatory set points provide for energy conservation with enhanced capacity for meeting cold challenge.