Sleep deprivation in the rat: XIII. The effect of hypothyroidism on sleep deprivation symptoms

Abstract

Previous studies of rats subjected to total sleep deprivation by the disk-over-water method had shown a large increase in energy expenditure (EE) and an initial increase followed by a later decrease in body temperature (Tb). It had been proposed that the increase in Tb resulted from regulation toward a higher temperature or setpoint, that the later decline in Tb resulted from excessive heat loss, and that the increase in EE supported both of these thermoregulatory changes. To evaluate this proposed role of the increase in EE, we examined whether blunting the EE rise in sleep-deprived rats by making them hypothyroid attenuated and/or shortened the initial increase in Tb and accelerated the later decline in Tb. Rats made hypothyroid by propylthiouracil administration (TxD rats) were totally sleep deprived and compared to hypothyroid yoked control (TxC) rats and to previously studied, untreated, totally sleep-deprived (TSD) rats. Neither TxD nor TxC rats showed large increases in EE like those of TSD rats. TxD rats did not initially increase Tb, as TSD rats had. Presumably, TSD rats had been able to support an initially elevated Tb, in spite of excessive heat loss, by large increases in EE, although even these increases were eventually insufficient. TxD rats showed much earlier and greater declines in Tb than TxC and TSD rats, eventually becoming severely hypothermic. These results support the interpretation that the large increase in EE previously seen in TSD rats had been compensatory for deprivation-induced thermoregulatory deficits. TxD rats survived an average of 17.1 days, which was not significantly different from survival time in TSD rats. However, there were differences in mortal processes between the two groups. TxD rats died or were sacrificed after chronic, severe hypothermia without observable signs of other morbid pathology. TSD rats had not shown similarly low Tb until just prior to death, but had shown signs of severe pathology, including severely debilitated appearance, disheveled fur, and severe lesions on their tails and on the plantar surfaces of their paws. These signs were diminished or absent in TxD rats, possibly due to blunted EE, lower Tb, or other effects of hypothyroidism. Because the skin changes seen in TSD rats were minimal in TxD rats, they could not have been responsible for the excessive heat loss.