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 < or = air temperature < or = 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 < or = air temperature < or = 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.