Pharmacology, Institute of Psychiatry, De Crespigny Park, London SE5 8AF.

Size: px

Start display at page:

Download "Pharmacology, Institute of Psychiatry, De Crespigny Park, London SE5 8AF."

Daisy Sharp
6 years ago
Views:

Quarterly Journal of Experimental Physiology (1977) 62, 143-149 BODY TEMPERATURE REGULATION AND THERMONEUTRALITY IN RATS. By S. POOLE and J. D. STEPHENSON.

1 Quarterly Journal of Experimental Physiology (1977) 62, BODY TEMPERATURE REGULATION AND THERMONEUTRALITY IN RATS. By S. POOLE and J. D. STEPHENSON. From the Department of Pharmacology, Institute of Psychiatry, De Crespigny Park, London SE5 8AF. (Received for publication 9th August 1976 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 C for rats housed at 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, 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

2 144 Poole and Stephenson METHODS Materials Thermistor probes were constructed as previously described [Poole and Stephenson, 1977]. Electrodes for recording electromyographic (EMG) activity were constructed in a similar manner, except that the last 1-2 mm of each wire was stripped of its insulation instead of a thermistor being attached. Methods Male Wistar rats ( g), anaesthetized with chloral hydrate (30-40 mg.100 g-1 i.p.), were positioned on a stereotactic instrument and, under aseptic conditions, the skull exposed with a midline incision. Three 12BA stainless steel screws were screwed into the skull away from the midline and set in dental cement. Penicillin (Triplopen 40,000 units) was sprinkled on to the wound, which was then sutured around the implant and the rat allowed to recover. After further growth to g (4-5 weeks after initial implantation) the rats were reanaesthetized with chloral hydrate (30-40 mg.100 g-1 i.p.) and thermistor probes implanted into their thoracic cavities so that the thermistor tip lay adjacent to the pulmonary artery, where blood from all regions of the body mixes [Poole & Stephenson, 1977]. Seven rats were also implanted with EMG electrodes: a midline incision, 10 mm in length, was made in the back of the neck and the bared ends of electrodes inserted 2-3 mm apart into the dorsal neck muscles. The incision was closed and the connector pins and an earth pin (soldered to one of the screws previously implanted in the skull) were attached with dental cement to the existing implant. Before and after implantation the rats were housed singly in plastic cages (33 5 x 21 x 17 cm) at 23 ± 1-50C with water and food (Diet 41B, Dixons Foods Ltd.) ad libitum, and subject to a 12 h light ( )/dark ( ) cycle. Cooling experiments. At least 2 weeks after implantation of the thermistor probes, naive rats were placed singly in a 2 litre glass experimental chamber (12 cm diameter, 21 cm high). The chest probe and a second thermistor probe, taped 2 cm from the base of the tail, were connected to either a bridge circuit [Allen and Lanworn, 1968] and Devices Pen Recorder or to a Grants Temperature recorder. In 7 experiments EMG was also recorded and integrated, via screened microcable, by a Devices AC8 preamplifier and pen recorder. The chamber was immersed in a circulating fluid (water and ethylene glycol) and dry, CO2-free air passed through it via a long copper coil that was also immersed in the fluid. Ambient temperature, i.e. chamber-temperature was initially maintained at 23 ± 0 5 C by controlling the temperature of the circulating fluid. The air leaving the chamber was passed first through a tube containing dry and wet thermistors, then over CaCl2 and finally to a CO2 analyzer (Hartmann & Braun, Ltd.) The analyzer output was displayed on a potentiometric recorder from which mean values for CO2 elimination were obtained by weighing the area under the curve. A quantity of light mineral oil beneath the grid floor of the chamber reduced evaporation of water from urine and faeces. When the rat had been in the chamber for 1 h at 23 ± 0-5 C, core and tail temperatures, electromyographic activity, water output (calculated from the difference between dry and wet thermistor temperatures, [Kaye and Laby, 1936]) and CO2 content of expired air were monitored for a further hour at this temperature and during the subsequent reduction of ambient temperature from 23 to 0 C. This reduction of 1 C every min was effected by decreasing the temperature of the circulating fluid by 1 C every 10 minutes. Warming experiments. These experiments were performed in a similar way to the cooling experiments except that ambient temperature was progressively elevated from 23 to 400C. This elevation of 1 C every min was effected by increasing the temperature of the circulating fluid by 1 C every 10 minutes. Activity experiments. In 3 experiments activity was also monitored by removing the experimental chamber from the water bath and placing it on an activity meter (Animex Ltd.) in a room maintained at 23 ± 1C.

3 Temperature regulation 145 The reported experiments were all performed using a 2 litre experimental chamber. This volume was a necessary compromise between a larger volume, to allow more freedom of movement, and a smaller volume, for more rapid wash-out and hence faster response times on the carbon dioxide analyzer. However to ensure that the results were not -influenced by the volume of thechamber, theexperiments were repeated in a large chamber (33.5cm x 29.0cm x 28-0 cm, approx. 27 litres) when similar results were obtained. RESULTS Effects of decreasing ambient temperature from 23 C At 23'C, rats displayed periods of motor activity or grooming alternating with inactivity and sleep. These different behavioural states were mirrored by fluctuations in carbon dioxide elimination and core temperature, both of which CO2 ELI MINATION (ml/h.w073) 8-10 min 38 CORE TEMPERATURE (OC) 36 ACTIVITY (counts/ min) FIG F o L Jl J111 T Effects of locomotor activity of a rat on the recordings of core temperature and carbon dioxide elimination. Ambient temperature, 230C. increased with these activities (Fig. 1). Mean core temperature and mean carbon dioxide elimination were C (mean ± S.D., n = 14) and 5-8 ±0O8 ml/ h.wt (n = 24), respectively. On lowering ambient temperature, carbon dioxide elimination remained within this range until 180O ±1 90C, after which it gradually increased (Fig. 2); in 7 rats, in which electromyographic activity was recorded, the first burst of shivering occurred at C. As ambient temperature was decreased further, the rats became more active, as shown by increases in carbon dioxide elimination and related increments in muscle tone and core temperature although bursts of shivering were not sufficiently intense or frequent to restrict other activities until ambient temperature had decreased to C. Between 40C and -50C, the lowest temperature to which rats were exposed, core temperature was stable at 38-2±+060C. A typical experiment illustrating these points is shown in Fig. 3.

4 146 Poole and Stephenson Effects of increasing ambient temperature from 23 C From 23 C to about 30 C tail temperature fluctuated between 0 5 C and 2 C above ambient temperature. In contrast, both core temperature and carbon dioxide elimination remained stable at 37T9 +±04 C and 5 30 ±0 4 ml/h.wt. 73 (n = 12) respectively, until ambient temperature had increased to approximately FIG. 2. Effects of decreasing and increasing ambient temperature on mean core temperature and mean carbon dioxide elimination of rats. The stippled columns represent mean ambient temperature ± S.D. Within the zone of minimum and constant metabolic rate (28-1 ± 1 O0C to 32-0 ± 1 0 C) rats were inactive and the tail blood vessels dilated. Since these are both responses to heat stress an alternative zone was defined (18&0 ± 1V90C to 28-0 ± 10'C) in which these responses were absent. The lower limit was arbitrarily defined as the ambient temperature at which mean carbon dioxide elimination exceeded the mean carbon dioxide elimination + 1 S.D. at 23 C (the mid-point of the thermoneutral zone), the limit coinciding with onset of shivering (18-3±1-2'C). The upper limit was defined as the lowest ambient temperature at which minimum carbon dioxide elimination occurred. Vertical bars indicate S.D., N = no. of rats. 28 C (Fig. 2). Between ambient temperatures of I 0 C and 32-0 ± I 0 C carbon dioxide elimination was minimum and constant at 4-1 ± 0 5 ml/h.wt.07 3, the rats were inactive and core temperature gradually declined to 37-3 ±0 4 C. At an ambient temperature of 30 1 ± 2*l C, a marked elevation of tail temperature was observed in 10 out of 12 rats; core temperature at this point was 380±+0'5 C. As ambient temperature was elevated from 32 C to 40 C (the highest temperature to which rats were exposed), core temperature increased from 380±+0*5 C to C; carbon dioxide elimination also increased.

5 Temperature regulation 147 Grooming was observed at intervals throughout the experiment but it was not accompanied by increased water vapour output, indicative of thermoregulatory grooming (saliva spreading), until ambient temperature was 33' C when core temperature was 39* C. Sometimes this marked increase was preceded by smaller episodes of saliva spreading. A representative experiment illustrating these effects is shown in Fig Co2 13' ELIMINATION (ml./h.w 073) EMG (!v2) 11A CORE 3 TEMPERATURE 38E (OC ) 37L AM BIENT TEMPERATURE FiG min Typical effects of gradually decreasing ambient temperature from 23 to 0 C on carbon dioxide elimination, EMG activity and core temperature of a rat. Note the first bout of shivering at approximately 17-50C and becoming sufficiently intense to restrict other motor activities at an ambient temperature of approximately 11C. Core temperature increased gradually in response to increased muscle tone despite the declining ambient temperature. The isolated bouts of shivering were associated with increments in carbon dioxide elimination. DIscussIoN Thermoneutrality may be defined as the range of ambient temperatures in which the animal neither combats cold by raising its heat production nor heat by increasing evaporative heat loss, and in which behavioural thermoregulation is normally absent (Bianca, 1974). Rats housed at 23 ±1 50C were thermoneutral between ±1 9 C and C. Occurrence of bursts of shivering and elevation of metabolic rate indicated that the lower limit had been reached whereas entry to the upper limit was marked by a pronounced decrease in activity, a concomitant fall in metabolic rate and a decline in core temperature. Delineation of the upper limit of thermoneutrality by a decrease in metabolic rate contradicts the classic definition of thermoneutrality, that is, the ambient temperature range over which metabolic rate is minimum and constant (in these experiments, OOC to 32 ± 1 0C). In rats a resting state in which postural and locomotor activity was markedly attenuated-essential if a minimal metabolic rate is to be recorded-was consistently induced by elevating ambient

6 148 Poole and Stephenson temperature above 28 C. However, this resting state was not of a rat in a 'neutral' condition since the inactivity appeared to be the first physiological response to a high ambient temperature, that is, an attempt to reduce heat production and so prevent body temperature rising; activity elevates body temperature, the increment in body temperature increasing with increased ambient temperature [Hart and Jansky, 1963]. Therefore the classic definition of thermoneutrality is inappropriate for rats and probably other animals, particularly so if they are not trained to remain immobile throughout the determination. Instead, the ELIM/INATIO3N 7tv 3 WATER 8 r OUTPUT 4 (mg.g-l.h-1) 0 42 CORE 40 TEMPERATURE 38 (OC) 36 TAIL 42 B l TEMPERATURE AM BIE NT 421 _ TEMPERATURE L- (0C) FIG ni Typical effects of gradually increasing ambient temperature from 23 to 400C on carbon dioxide elimination, water vapour output, core temperature and tail temperature of a rat. Note that the zone of minimal carbon dioxide elimination is associated with a reduced core temperature. Within the range there was a marked increase in tail temperature occurring at an ambient temperature of approximately 30 C. Increased water vapour output, caused by salivary grooming, became continuous at an ambient temperature of 34 C; prior to this there were smaller increments in water vapour output commencing at 320C. defined range should permit normal postural and locomotor activities but not thermoregulatory behaviour. Within this range, the increases in core temperature resulting from bouts of motor activity probably contribute to the maintenance of body temperature. Tail vasodilatation occurred at a core temperature not significantly different from its value at 23 C (P < 005) and therefore appears to be a response to increased ambient temperature via skin thermal receptors. This finding is in agreement with the results of Rand et al. [1965] who demonstrated that tail vasodilatation, with a 15 to 20 fold increase in tail blood flow, occurred at a critical ambient temperature, this depending on the temperature to which the rats had been acclimated. Despite obvious tail vasodilatation at 30 C, the

Temperature regulation 149 immediate increase in temperature gradient between ambient and tail temperature was not sustained, probably because after vasodilatation rats tended to extend their tails

7 Temperature regulation 149 immediate increase in temperature gradient between ambient and tail temperature was not sustained, probably because after vasodilatation rats tended to extend their tails away from their bodies, markedly increasing heat loss. This increased heat loss from the tail, resulting in a lower tail temperature, was enhanced by the air flowing through the chamber. In contrast to tail vasodilatation, saliva spreading commenced when rats were hyperthermic and may therefore be a response to elevated core temperature and/or increased ambient temperature. The above data pertain to conscious unrestrained rats since anaesthetics are known to cause hypothermia and restraint impairs not only growth [Frankel, 1959] but also temperature regulation at both high [Frankel, 1959] and low [Bartlett, Bohr, Helmendach, Foster and Miller, 1954] ambient temperatures. ACKNOWLEGMENT This work, which was supported by a grant from the Medical Research Council, forms part of S. P.'s Ph.D. Thesis, University of London. The authors thank Professor E. Marley for his interest and advice in this study. REFERENCES ALLEN, D. J. and LANWORN, B. K. (1968). A design for a linear output thermistor bridge circuit. Journal of Science Technology, 14, 5-6. BARTLETr, J. R., BOHR, V. C., HELMENDACH, R. H., FOsTER, G. L. and MILLER, M. A. (1954). Evidence of an emotional factor in hypothermia produced by restraint. American Journal ofphysiology, 179, BIANCA, W. (1974). 'Heat loss from animals and man'. Ed. MONTEITH, J. L. and MouNT, L. E. p Butterworth, London. FRANKEL, H. M. (1959). Effects of restraint on rats exposed to high temperature. Journal of Applied Physiology, 14, HAINSWORTH, F. R. (1967). Saliva spreading, activity and body temperature regulation in the rat. American Journal ofphysiology, 212, HAINSWORTH, F. R. (1968). Evaporative water loss from rats in the heat. American Journal of Physiology, 214, HART, J. S. and JANSKY, L. (1963). Thermogenesis due to exercise and cold in warm- and cold-acclimated rats. Canadian Journal of Biochemistry and Physiology, 41, HELLSTR6M, B. (1975). Heat vasodilatation of the rat tail. Canadian Journal of Physiolo and Pharmacology, 53, HERRINGTON, L. P. (1940). The heat regulation of small laboratory animals at various environmental temperatures. American Journal ofphiysiology, 129, KAYE, G. W. C. and LABY, T. H. (1936). 'Physical and chemical constants and some mathematical functions.' 8th Ed. Longmans, Green & Co., London. MOUNT, L. E. (1974). The concept of thermoneutrality. In 'Heat loss from animals and man'. Ed. MONTEITH, J. L. & MouNT, L. E. pp Butterworth, London. POOLE, S. and STEPHENSON, J. D. (1977). Core temperature: some shortcomings of rectal temperature measurements. Physiology and Behaviour. In press. RAND, R. P., BURTON, A. C. and ING, T. (1965). The tail of the rat, in temperature regulation and acclimatization. Canadian Journal of Physiology and Pharmacology, 43, SwIFT, R. W. and FORBES, R. M. (1939). The heat production of the fasting rat in relation to the environmental temperature. Journal of Nutrition, 18,

Tioga ISD Athletic Department Heat Policy

Tioga ISD Athletic Department Heat Policy Practice of competition in hot and humid environmental conditions poses special problems for student-athletes. Heat stress and resulting heat illness is a primary