Age-dependent differences in the thermoregulatory response of the immature rat to ethanol.

Major improvement in the homeothermic ability of the rat occurs during the first 2 weeks of postnatal development. Changes in thermoregulatory responsiveness to a single injection of ethanol (EtOH) may occur during this period. Immature rats (2-3, 8-9, and 14-15 days of age) were administered either saline or EtOH (2 or 4 g/kg BW; ip) at thermoneutral ambient temperatures (Ta). In one experiment, metabolic rate (MR) and body temperatures (colonic and skin) were recorded for 1-3 hr postinjection. A second experiment determined blood EtOH concentration in rats from the 3 age groups over an 8-hr period following injection of EtOH. 4 g EtOH/kg produced few significant reductions in thermoregulatory function of 2-3 day-old rats, but decreased MR by 16% and colonic temperature by 0.5-0.7 degrees C in 8-15 day-old animals. 2 g EtOH/kg had no effect on 8-9 day-old rats, but reduced MR and colonic temperature in rats aged 14-15 days. In every case, the hypothermic response to EtOH was correlated with a reduction in MR. Back and abdominal skin temperatures decreased with colonic temperature, and tail skin temperature indicated EtOH-induced vasoconstriction in older rats. Blood EtOH concentrations were similar in the three age groups during the first 2 hr postinjection and did not explain differences in metabolic response. The magnitude and duration of thermoregulatory responsiveness to EtOH increases with age in the immature rat.