Naloxone inhibition of stress-induced daily torpor

Peromyscus maniculatus, deermice , were induced into daily torpor by restricting food to one-half daily ration. Intraperitoneal injection of naloxone (20 mg/kg) into mice habituated to daily IP injections of saline inhibited or modified the expression of daily torpor. In those individuals demonstrating long duration/deep bouts (greater than 300 min/body temperature 20 degrees C or below) naloxone administration resulted in 1) a significant decrease in the duration of torpor, 2) a significant elevation in minimum body temperatures attained during torpor and 3) a significant delay in the initiation time of torpor. In those individuals demonstrating short duration/shallow bouts (less than 300 min/body temperatures above 20 degrees C), naloxone administration resulted only in a significant delay of initiation time. Upon subsequent return to saline administration, however, these mice displayed a significant increase in the duration and depth of torpor. The results suggest that the endogenous opiates modulate the state of daily torpor.