Attenuation of metabolic heat production and cold-escape/warm-seeking behaviour during a cold exposure following systemic salt loading in rats

The reduction of body core temperature (T_core) after salt loading has been reported. In this study, we tested the hypothesis that, during a cold exposure in rats, (1) salt loading would decrease metabolic rate (MR), reducing T_core but (2) T_core would be maintained when cold-escape/warm-seeking behaviour is available. In the first experiment (n = 7), MR and T_core were measured by indirect calorimetry and telemetry, respectively, during 26, 20 and 10 °C exposure for 1 h each, in that order. In the second experiment (n = 7), each rat was placed in an operant system during the same exposure protocol as in the first experiment, where it could trigger a 40 °C air reward for 30 s at 20 and 10 °C by moving into specific areas (operant behaviour). In each experiment, rats repeated the same protocol twice with a subcutaneous injection (10 ml kg^-1) of either isotonic saline (154 mM) or hypertonic saline (2500 mM). In the first experiment, MR in the isotonic-saline trial increased (P < 0.05) at 20 and 10 °C compared with that at 26 °C by 21 ± 5 and 48 ± 6%, respectively (means ± S.E.M.), with T_core unchanged. However, values for MR and T_core in the hypertonic-saline trial were lower (P < 0.05) than those in the isotonic-saline trial in any ambient temperature. In the second experiment, T_core was also lower (P < 0.05) in the hypertonic-saline trial than in the isotonic-saline trial. The counts of the operant behaviour in the hypertonic-saline trial remained unchanged in each exposure period, but those in the isotonic-saline trial increased (P < 0.05) at 10 °C. These results may suggest that salt loading attenuates both metabolic and behavioural thermoregulatory responses to the cold.