Body Temperature Regulation and Thermoneutrality in Rats

Abstract

Various concepts of thermoneutrality were considered for a proposed study of the role of hypothalamic amines in temperature regulation of rats. The classic definition, the ambient temperature over which metabolic rate is minimum and constant, gave a range of approximately 28 to 32'C. However, within this temperature range rats were inactive, the inactivity apparently representing a behavioural response to heat stress and itself responsible for the reduced metabolic rate; certain thermoregulatory effectors were also activated to increase heat loss. Therefore an alternative range, 180O± 1-9 (mean ± S.D.) to 28-1 ± 1 0°C, was defined in which rats displayed normal activity, behavioural thermoregulation being absent.For a study of the thermoregulatory effects of intrahypothalamic infusions of amines in conscious unrestrained rats, knowledge of their thermoneutral range is essential. Various concepts of thermoneutrality have been advanced. The classic definition, which considers only metabolic rate, is the ambient temperature range over which metabolic rate is minimum and constant. This gives a range of 28-33°C for rats housed at 23-25°C [Herrington, 1940;Swift and Forbes, 1939]. Outside this range, metabolic rate is elevated at the lower limit by enhanced heat production and at the upper limit by increased motor activity due to escape behaviour and thermoregulatory grooming (saliva spreading). However, this definition was considered unsuitable for the purposes of the proposed study, since within the temperature range defined (28-330C), body temperature is elevated above normal [Hainsworth, 1967; Hellstrom, 1975], the tail vessels are markedly dilated [Rand, Burton and Ing, 1965] and saliva spreading has been reported [Hainsworth, 1967[Hainsworth, , 1968; these effects would obviously distort the thermoregulatory responses to the amines. Clearly, a rat attempting to maintain a normal body temperature by increasing evaporative (saliva spreading) and non-evaporative (a sustained tail vasodilatation) heat loss is not in a 'neutral' condition [Mount, 1974]. A definition which also considers behavioural thermoregulation is likely to be more appropriate, for example that of Bianca [1974]: the range of ambient temperature in which the animal neither combats cold by raising its heat production nor heat by evaporative heat loss, and in which behavioural thermoregulation is normally absent. In an attempt to delineate this range in rats, metabolic rate and activities of thermoregulatory effectors (shivering, tail vasomotor tone and behavioural thermoregulation including saliva spreading) were monitored at different ambient temperatures.143

Effects of Noradrenaline and Carbachol on Temperature Regulation of Rats

“…At least 2 weeks after implantation of the thermistor probes, rats were placed singly in a perspex environmental chamber (335 mm x 290 mm x 280 mm) at 23 + 0.5°C (the mid-point of their thermoneutral range; Poole & Stephenson, 1977c) through which air flowed at 2 1/min. The chest probe and a second thermistor probe taped to the dorsal surface of the tail, 20 mm from its base, were connected to a Devices pen-recorder via a microcable (Radiospares, Ltd.), a mercury concentric swivel (Campden Instruments Ltd.) and bridge circuits (Allen & Lanworn, 1968).…”

Facial thermal input in the caudal trigeminal nucleus of rats reared at 30 degrees C.

“…Similarly, our results show that rats reared at 30 sC have a considerably reduced capacity to thermoregulate when exposed to TRLIEMINAL THERMAL INPUT IN HEAT-REARED RATS 10 0C compared to controls reared at 20 0C. The temperature of 30 0C represents a heat stress for the rat, since the zone of thermoneutrality in which behavioural thermoregulation is absent lies approximately between 18 and 28 0C (Poole & Stephenson, 1977). A temperature of 33 0C was not used for heat adaptation in the present work as it is known that a temperature of 34 0C results in the eventual death ofall pre-weanling offspring, apart from depressive and deleterious effects ofsuch high temperatures on a wide range of physiological functions in the adult (Purves, 1964;Pennycuick, 1964a-d).…”

Mouse strain differences in the effects of corticotropin releasing hormone (CRH) on sleep and wakefulness

“…The difference between studies in rats and mice could be related to species differences and possibly to differences in ambient temperature. The rats in the studies conducted in Opp's lab were maintained at 22-23° C which is within the thermoneutral range for rats (Poole and Stephenson, 1977) whereas our mouse colony is maintained at 24.5±0.5° C which is below the 29-30° thermoneutral temperatures typically reported for mice (Williams et al, 2003;DeRuisseau et al, 2004;Overton and Williams, 2004). Ambient temperature can significantly affect sleep in mice (Roussel et al, 1984), thus, there is the possibility that relative temperature played a role in the species differences we saw.…”