Thermogenesis, myoclonic twitching, and ultrasonic vocalization in neonatal rats during moderate and extreme cold exposure

Physiological and behavioral responses of 2- and 7-8-day-old rats were monitored during moderate and extreme cold exposure. During moderate cold exposure (30 degrees C less than or equal to air temperature less than or equal to 32.5 degrees C), pups at both ages increased heat production, maintained an elevated interscapular temperature, and maintained baseline levels of myoclonic twitching, a behavior commonly associated with active sleep. During extreme cold exposure (21 degrees C less than or equal to air temperature less than or equal to 25 degrees C), pups at both ages continued producing metabolic heat, but now exhibited pronounced decreases in interscapular temperature and decreased rates of myoclonic twitching. Furthermore, the 7-8-day-old pups exhibited significant increases in ultrasound production, and males vocalized more than females. These results suggest the presence of a narrow subthermoneutral zone in neonates in which nonshivering thermogenesis is regulated and sleep-related behaviors are protected.