Effect of changes of ambient temperature on spontaneous activity of hypothyroid rats.

Abstract

Administration of the antithyroid drug, propylthiouracil (PTU), to male rats during 4 weeks did not reduce spontaneous running activity in air at 25 °C. When room temperature was lowered to 8 °C for 4 days, PTU-treated rats increased their activity level during the 1st day of exposure but decreased it thereafter. Control rats responded similarly on the first day but continued to increase activity during each subsequent day in cold. In other experiments PTU-treated and control rats were lightly restrained and exposed acutely to air at 5 °C in individual cages. A sensitive transducer converted movement of the cage to electrical impulses which were recorded on a polygraph. Colonic temperature of each rat was recorded simultaneously by a potentiometer. Mean colonic temperature of control rats decreased 2 to 3 °C during the first 2 hours of cold exposure but was maintained at this subnormal level for an additional 2.5 hours by adjustment of activity. Activity increased immediately with cold exposure and became maximal when colonic temperature fell 0.5 to 1 °C. Mean colonic temperature of PTU-treated rats continued to fall throughout the cold exposure and decreased approximately 6 °C in 3 hours. These rats also responded to cold initially with an increase in activity which became maximal when colonic temperature fell 3 to 4 °C. However, this maximal effort occurred too late to prevent further fall of colonic temperature. The results suggest that regulation of body temperature in cold air by adjustment of activity level is an important aspect of the total regulatory mechanism and that hypothyroidism alters the relationship between colonic temperature and activity level.