Sleep Deprivation in the Rat: XV. Ambient Temperature Choice in Paradoxical Sleep-Deprived Rats

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Abstract
Previous studies of total sleep deprivation (TSD) and paradoxical sleep deprivation (PSD) in the rat by the disk-over-water method have indicated that both produce changes in thermoregulation. In both kinds of deprivation, there was a progressive, large increase in heat production as indicated by measures of energy expenditure (EE). In TSD there was an initial increase in waking body temperature (Tb) followed by a later decrease; in PSD there was only a progressive decrease. The increases in heat production far in excess of heat storage indicated increased heat loss in both groups. Because the increase in Tb in TSD rats was supported by ambient temperature choices (Tch) in a thermal gradient that became progressively higher during deprivation, an increase in waking temperature setpoint (TSET) was indicated. Because the rats resorted to behavioral warming in spite of greatly increased thermogenesis, they must have had some failure to retain body heat. Prior to the present study, changes in TSET, had not been evaluated in PSD rats. Because they had not shown increases in Tb, PSD rats might not have an elevated TSET, which would indicate a functional difference between PS and nonrapid eye movement (NREM) sleep. Also, an evaluation of behavioral thermoregulation in PSD rats would clarify whether their Tb decline resulted from excessive heat loss or from a lowered TSET. To evaluate changes in heat flow and TSET, EE, Tb and Tch were measured in five PSD rats and their yoked control (PSC) rats. PSD rats showed progressive increases in EE and decreases in Tb as in the earlier PSD study; Tch rose progressively. PSC rats showed minimal changes in all three parameters. Because PSD rats showed increased EE and Tch, which oppose Tb decline, their decline in Tb must be ascribed to excessive heat loss rather than to decreased thermogenesis or a lowered TSET. Heat loss deficits and TSET change in PSD rats were compared to those of previously studied TSD rats. A positive correlation between EE and Tch in both groups was consistent with an elevation of thermogenesis and behavioral warming in response to a heat retention deficit. At Tch near baseline, where Tb was assumed to be near TSET, Tb was increased in TSD rats but not in PSD rats, indicating that TSET was increased by TSD and not by PSD. Therefore, in TSD rats the increased Tch and EE may be viewed as attempts to reach an elevated TSET in the face of excessive heat loss. In PSD rats, elevated Tch and EE are also responses to excessive heat loss, but the apparent target is to maintain baseline Tb rather than an elevated TSET. Because both TSD and PSD rats lost PS, but NREM sleep was well preserved in PSD, the elevation of TSET in TSD rats was more closely associated with the loss of NREM sleep, whereas the difficulty in retaining body heat can be ascribed to the loss of PS.

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