Temperature regulation in laboratory mammals following acute toxic insult

Impact of the hypothermic response in inhalation toxicology studies

Previous studies from this laboratory showed that the decreases in Tco and associated functional parameters often observed in rodents following exposure to xenobiotic agents are capable of modulating the subsequent toxic response and that the magnitude of this induced hypothermic response may itself be modified by a number of experimental conditions. A moderate hypothermic response, characterized by a temperature drop of approximately 2 degrees C, appears to afford the optimal protection. Studies in which exposures occur through inhalation of harmful gases or particles present a special set of problems. In such studies, the dose of the toxic agent to which the animal is exposed is a function of the concentration of the agent in the atmosphere and the minute ventilation of the animal. Although ambient concentrations is generally held constant in laboratory studies, minute ventilation varies directly with metabolism, and both of these parameters may change significantly across experimental conditions. Thus, at low Tas, metabolism and minute ventilation are relatively high and uptake of inhalable toxic agents is increased. However, the development of the hypothermic response during the exposure entails a directly correlated reduction in these parameters and, presumably, in dose. For the most part, inhalation toxicological studies are conducted using resting animals or exercising humans. Animals are sometimes concurrently exposed to CO2 to simulate the increased ventilation of exercise and more closely mimic human studies. The experimental protocols employed in the above inhalation studies permitted examination of (1) the impact of species, size, handling stress, and changes in Ta on both the induced hypothermic response and the concomitant pulmonary toxicity; (2) the additive impact of exercise stress on O3 toxicity; and (3) the toxicity of ambient-derived particulate matter in normal rats and in rats with preexisting pulmonary inflammation. The results of these studies demonstrate that the magnitude of the induced hypothermic response is directly proportional to the uptake of the toxic agent by the lung and inversely proportional to the mass of the animal and the ambient temperature at which the exposure is conducted. The hypothermic response is sensitive to a number of experimental stresses including handling and changes in cage conditions. Exercise attenuates the hypothermic response, whereas CO2-stimulated increases in ventilation employed as an exercise surrogate may potentiate the response. Toxic exposures conducted in animals with lung disease or compromised pulmonary function may induce a severe hypothermic response while comparable exposures in normal animals produce only mild or moderate responses. In general, the development of the hypothermic response in the presence of ambient pollutants serves to decrease the minute ventilation of the animal and therefore limits the uptake and dose of the airborne toxicant. The results of these inhalation studies support our previous conclusions concerning the impact of the hypothermic response on toxicity and emphasize the need to monitor and incorporate these changes in functional parameters into analyses of toxicological data. Furthermore, because humans do not demonstrate a robust hypothermic response following exposure to toxic agents, extrapolation of the results obtained from animal studies and comparisons with data from human studies are considerably more complicated.

Thermoregulatory aspects of environmental exposure to anticholinesterase agents

Anticholinesterase (antiChE) agents can be highly toxic to birds and mammals and constitute a major proportion of the pesticides used throughout the world. AntiChEs consist of the organophosphates (OP), which irreversibly inhibit the enzyme acetylcholinesterase (AChE), and the carbamates (CB), which reversibly inhibit AChE. AChE inhibition elicits cholinergic stimulation in the central nervous system and in peripheral tissues and organs, which can lead to marked dysfunction of homeostatic systems, including temperature regulation. The control of body temperature uses cholinergic pathways in the integration and central processing of thermal information, as well as in the control of thermoeffector responses. Hence, the cholinergic stimulation elicited from exposure to antiChEs has profound effects on body temperature at rest as well as during exercise. Ambient heat and cold stress can also modulate the animal's sensitivity to antiChE exposure. After exposure to most OPs, rodents and other small species undergo a marked hypothermic response lasting up to 24 hours. On the other hand, humans exposed to OP pesticides rarely become hypothermic but rather experience a fever that may last many days. Recent studies monitoring body temperature in OP-exposed, telemetered rats demonstrated that the initial hypothermic response is followed by a period of hyperthermia lasting several days. That the hyperthermia can be blocked with administration of sodium salicylate suggests that the hyperthermia is a fever. Thus, the antiChE-induced effects on body temperature and other physiological systems cannot be explained solely by the immediate consequences of AChE inhibition and stimulation of cholinergic systems. Research into the mechanisms of action of antiChE toxicity will be improved with a better understanding of their effects on temperature regulation.

Ultrastructural investigation of lead-induced intranuclear inclusion bodies in mice

By means of optical and electron microscopy and by atomic absorption spectrophotometry in a graphite chamber, this study evaluated the effect of temperature (22-35 degrees C) on lesions in the kidney, liver, and brain, and on concentrations of lead caused by the administration of 2 and 5 mg/kg/IP of lead acetate to Swiss mice. The most pronounced effects were observed in the kidney and in groups of animals receiving the highest doses (5 mg/kg at 22 and 35 degrees C). These effects consisted of significantly higher (p < .05) lead concentrations in the tissues, a significant decrease (p < .05) in kidney weights, and progressive kidney atrophy and fibrosis with, at the ultrastructural level, constant intranuclear inclusions, which were also observed in the cytoplasm of renal and endothelial cells.

Hypothermia in hypoxic animals: mechanisms, mediators, and functional significance

A basic tenet of biology is that body temperature (Tb) has a marked effect on oxygen uptake of resting animals. For most animals, the temperature coefficient (Q10) is >> 2.5; e.g., resting oxygen uptake changes about 11% per degree C change in Tb. An important consequence of this dependence is that hyperthermia could be deleterious for hypoxic animals, particularly for oxygen sensitive organs, e.g., heart and brain. Conversely, a moderate degree of hypothermia could be beneficial during hypoxia. This concept is not new. Forced hypothermia is sometimes used in surgical procedures, particularly for heart and brain surgery. However, in many situations where hypothermia might have benefits, e.g., pediatric intensive care, it is not permitted. This is due in part to dogma and in part to the real and potential disadvantages of hypothermia, even in severely hypoxic animals. Among these in ventricular fibrillation. This is apparently preventable if blood pH is allowed to rise following the "Buffalo Curve." Another important disadvantage, were it to occur, is elevation of oxygen demand due to a thermogenic responses. However, at least in some species, the thermogenic response is blunted during hypoxia; e.g., in young rats. Furthermore, even if a thermogenic response occurs, this takes place primarily in muscles (shivering) and brown fat (non-shivering) and not in the O2-sensitive organs, heart and brain. A third disadvantage, for prolonged hypothermia, might be impairment of the immune response, a serious problem if hypoxia is combined with infection. This paper will review four aspects of behavioral fever and hypothermia: the occurrence among animals, the mechanisms and mediators that might trigger behavioral responses, and the functional significance.

Strain as a determinant factor in the differential responsiveness of rats to chemicals

The beneficial effects derived from the use of chemicals in agriculture, energy production, transportation, pharmaceuticals, and other products that improve the quality of life are clearly established. However, continued exposure to these chemicals is only advantageous in conditions where the benefit far outweighs toxic manifestations. By law, determination of risk of toxicity necessitates the use of laboratory animals to establish whether chemical exposure is safe for humans. To simulate the human condition, it is incumbent upon investigators to choose a species in which pharmacokinetic and toxicokinetic principles are established and resemble those of humans. Some of the advantages to the use of rat in chemical toxicity testing include (a) similarities in metabolism, anatomy, and physiological parameters to humans; (b) the short life span, especially for carcinogenesis study; (c) the availability, ease of breeding, and maintenance at a relatively low cost; and (d) the existence of a large database to enable comparison of present to reported literature findings. However, the choice of rat can be complicated by several factors such as sex, age, and nutrition, but especially strain, where currently there are over 200 different strains of rat known to exist. The aim of this review is to demonstrate that there are differences in the responsiveness of rat strains to chemicals and that the susceptibility observed is dependent on the tissue examined. It is evident that the genotype differs among strains, and this may be responsible for differences in sensitivities to chemicals. Awareness of strain as a factor in susceptibility to toxicant action needs to be taken into account in interpretation of relevance of risk of toxicity for humans.

Effect of environmental temperature on tissue lead accumulation in mice repeatedly treated with lead acetate

The effect of environmental temperature on lead accumulation in tissues of mice repeatedly treated with lead acetate (2 mg/kg per day and 5 mg/kg per day) for 3 or 6 weeks was studied. In blood, kidney and liver, the amount of lead accumulated after 3 weeks of treatment was markedly higher in animals exposed to 22 degrees C than those maintained at 35 degrees C. Conversely, when the treatment was extended to 6 weeks, lead concentrations in the liver and kidney were equal or higher respectively, in the mice exposed to 35 degrees C. In the brain, lead concentration was lower than that found in kidney and liver and it was independent of dose and ambient temperature of lead being higher at 35 degrees C than at 22 degrees C. These results demonstrate that environmental temperature influences the amount of lead accumulated in some rodent tissues, and that the duration of the treatment modifies the effect produced by temperature, suggesting that the changes elicited during the period of acclimation to the hot environment could be responsible for these findings.

A direct involvement of the central nervous system in hypophagia and inhibition of respiratory rate in rats after treatment with O,O,S-trimethyl phosphorothioate

O,O,S-Trimethyl phosphorothioate (OOS-TMP) is known to induce unique symptoms, which are characterized by hypophagia, progressive weight loss, and hypothermia. To determine whether there is the possibility of a causal relationship between these toxic symptoms and a direct action of OOS-TMP on the central nervous system, we investigated the development of these symptoms in Fischer 344 female rats after oral or intracerebral treatment with OOS-TMP. Oral administration of OOS-TMP at 20 mg/kg induced marked hypophagia, progressive weight loss and hypothermia. Moreover, inhibition of respiratory rate was observed immediately after treatment. It lasts during the entire experimental period. Profound hypothermia below 34 degrees C was observed more frequently in the rats, which became hypercapnic (PaCO2 > or = 50 mmHg). In contrast, administration of OOS-TMP at 20 mg/kg (as much as the oral dose) into the cerebral lateral ventricle succeeded in inducing hypophagia, progressive weight loss and lowered respiratory rates. On the other hand, by this route of administration, OOS-TMP at 20 mg/kg failed to induce hypothermia, hypercapnia and lung injury. The present results suggest that hypophagia and inhibitions of respiratory rate are attributable to the direct action of OOS-TMP on the central nervous system, while other symptoms are associated with lung injury.

Reductions in body temperature and spontaneous activity in rats exposed to horizontal rotation: abolition following chemical labyrinthectomy

The effect of horizontal rotation of male rats (70 rpm) on core temperature and spontaneous motor activity levels was examined. In Experiment 1, subjects were chemically labyrinthectomized (VNX) by intratympanic (IT) injections of sodium arsanilate and control rats (VNS) received IT injections of saline. Half of the rats in each group were subsequently rotated and the other half sham rotated. Measurement of body temperature prior to, immediately after, and 20 min following rotation revealed significant (all p < 0.01) reductions in temperature immediately after treatment, and 20 min later, in VNS rats. Sham-rotated VNS and all VNX rats failed to exhibit any significant changes in temperature following treatment. In Experiment 2, motor activity level was monitored in chemically labyrinthectomized (VNX) and control (VNS) rats prior to, and following, horizontal rotation. The VNS rats exhibited large (all p < 0.01) depressions in measures of horizontal and vertical spontaneous motor activity following rotation treatment, whereas VNX rats exhibited similar levels of activity in the pre- and postrotation period. These experiments show that, as in humans, exposing rats to horizontal rotation results in reduction of body temperature and motor activity, and that these physiological and behavioral changes require a functional vestibular system.

The relationship between traumatic brain injury-induced changes in brain temperature and behavioral and anatomical outcome

Alteration of brain temperature, experimentally induced or spontaneous, has been shown to affect the symptoms resulting from a variety of cerebral insults. This study examined the effect of traumatic brain injury (TBI) on brain and body temperature in rats and the relationship between TBI-induced temperature changes, neuropathology, and behavioral recovery. Anesthesia, surgery and TBI all caused changes in brain and body temperatures. The level of brain (but not body) temperature at the time of TBI was positively correlated with the severity of hippocampal and thalamic pathology. In contrast, the measured levels of both brain and body temperatures after TBI were not related to behavioral or neuroanatomical outcome. Interestingly, the increase in brain (but not body) temperature from the time of TBI to 5 to 10 minutes after termination of anesthesia was negatively correlated with behavioral and anatomical outcome. Simply stated, the more rapidly brain temperature returned toward normal, the better the rats' behavioral and anatomical outcome. This rate of return toward normal brain temperature is not interpreted as causally related to outcome but rather as an index of the severity of brain injury.

Effects of different factors in lead- and cadmium-induced hypothermia in mice

The aim of this study was to investigate the relationship between the hypothermic effect induced by lead or cadmium chloride treatments and the cerebral metal levels attained. Mice were injected intraperitoneally with different doses of lead acetate or cadmium chloride at ambient temperatures of 22 degrees C and 35 degrees C, and rectal temperatures and brain metal levels were determined. At 22 degrees C, doses of lead acetate exceeding 25 mg/kg caused significant hypothermia and a rise of lead levels in the brain; this hypothermic effect was significantly inhibited at 35 degrees C. In mice repeatedly treated with 2 or 5 mg/kg lead acetate and exposed to 22 degrees C or 35 degrees C, no significant changes were observed in body temperature after the repeated exposure, although brain lead levels increased significantly. Cadmium chloride at 22 degrees C produced a significant fall in body temperature, and a rise in brain cadmium levels at doses of 2-4 mg/kg; the decrease in body temperature was partially inhibited at 35 degrees C. The results of the present study suggest that the hypothermic effect elicited by these heavy metals is related to both the level of metal absorbed by the brain and its rate of uptake.

Caveats regarding the use of the laboratory rat as a model for acute toxicological studies: modulation of the toxic response via physiological and behavioral mechanisms

The rodent, specifically the laboratory rat, is the primary experimental animal used in toxicology testing. Despite its popularity, recent studies from our laboratory and others raise a number of questions concerning the rat's appropriateness as an animal model for toxicological studies. While there may be additional areas in which the rat and other small rodents fail to adequately mimic the human response to xenobiotic agents, this article will focus on the area of temperature regulation. Thus, this article will review the thermoregulatory response of the laboratory rat following acute exposure to toxic agents and examine the impact of this response on the extrapolation of toxicological data from experimental animals to humans. In general, the rat responds to acute intoxication by lowering its core temperature via both physiological and behavioral mechanisms, thereby attenuating the induced toxicity. Similar responses have not been reported in humans.

O,O,S-Trimethyl phosphorothioate induces hypothermia in Fischer 344 rats in a manner dependent on both doses and housing temperatures

We explored the effects of O,O,S-trimethyl phosphorothioate (OOS-TMP) on body temperatures in Fischer 344 female rats. The 7-day LD50 p.o. for Fischer 344 female rats was found to be 11.8 mg/kg. OOS-TMP induced long-lasting (more than 48 h) and extensive hypothermia at doses > or = 14 mg/kg at a typical laboratory temperature (22 degrees C) while it produced typical symptoms at 10 mg/kg without hypothermia. In contrast, pair-fed (to 20 mg/kg rats) rats (n = 4) did not become hypothermic, negating any role of hypophagia in OOS-TMP associated hypothermia. We next investigated the effects of housing temperatures on toxicities at a LD50 dose (12 mg/kg). At 30 degrees C (n = 11) and 22 degrees C (n = 13), rats did not have hypothermic bouts but at 15 degrees C, eight out of ten rats had. Evidence that changes of housing temperatures neither modified clinical symptoms nor changed mortality rates discards a possibility of hypothermia being involved in delayed toxicity. A novel result of the present study suggests that thermoregulation may be heavily impaired by a special class of organophosphorus compounds.

Behavioral thermoregulation in mice inoculated with influenza virus

Mice housed at 30 degrees C and inoculated with a mouse-adapted influenza virus show a fall in body temperature (Tb) and a decrease in food intake to almost 0 grams per day. This study tested whether the fall in Tb could be accounted for by the decreased food intake and whether the fall in Tb was due to a decrease of thermoregulatory set point or to an inability to maintain Tb at set point level. The fall in Tb of influenza-infected mice was greater than that of food-deprived mice. When food deprived, mice given access to a thermal gradient increased their preference for warmer areas in the gradient and, as a result, Tb did not fall as much as Tb of starved mice not given access to a thermal gradient. When infected with influenza virus, mice given a thermal gradient decreased Tb less and at a slower rate than mice not given a gradient. However, this fall in Tb of influenza-infected mice was greater than that of food-deprived mice given a thermal gradient. Mice given a thermal gradient increased their preference for the warmer temperatures after inoculation; this returned to preinoculation preference for cooler temperatures during the later days of infection despite a continuous fall in Tb. Influenza-infected mice given a thermal gradient survived significantly fewer days than infected mice not given a thermal gradient. We conclude that the influenza-induced fall of Tb in mice cannot be explained solely by the decrease in food intake, and is partially due to a decrease in thermoregulatory set point.

Thermal influences on nervous system function

The various effects of temperature change are only partially predictable. Temporal measures relevant to membrane activity, action potentials, synaptic transmission, and evoked potentials are all consistently increased with cooling and decreased by warming. However, the various measures of amplitude at different levels, and even within similar preparations, are contradictory: Some laboratories report increased amplitudes with cooling and others report decreased amplitudes under similar conditions. Emphasis is given to identifying factors that may resolve the differences. These include: (a) the rate of temperature change, (b) sites of cooling, stimulation and recording, (c) stimulus characteristics, and (d) fundamental differences in temperature sensitivities of different neural tissue. Other factors that may affect the ability to predict thermal influences on neural function from existing formulations are: relative ion permeabilities, metabolic ion pumps, the resting potential at the onset of cooling, and an animal's acclimated temperature at sacrifice.

Prevention of carbon tetrachloride-induced liver necrosis by the chelator alizarin sodium sulfonate

The administration of the calcium chelator alizarin sodium sulfonate (ASR) (100 mg/kg ip in saline) 30 min before or 6 or 10 hr after CCl4 (1 ml/kg ip as a 20% v/v solution in olive oil) partially prevents the necrogenic effect of the hepatotoxin at 24 hr, but prevention of CCl4 fat accumulation was not observed. Protective action cannot be attributed to potential decreasing effects of ASR on CCl4 levels reaching the liver, on the covalent binding of CCl4-reactive metabolites to cellular components, or on CCl4-induced lipid peroxidation because ASR does not modify these parameters significantly. ASR administration increases GSH levels in livers of both control and CCl4-poisoned animals and decreases the calcium content of intoxicated animals at 24 hr of poisoning. ASR significantly lowers the body temperature of CCl4-treated animals at different times of the intoxication process. Present and previous results from our laboratory on the preventive effects of another very specific calcium chelator, calcion, and several anticalmodulins suggest that the beneficial effects of ASR might be associated with its calcium chelating ability. Other protective effects of ASR, such as lowering body temperature or increasing GSH content in liver, cannot be excluded.

The circadian rhythm of body temperature

This paper reviews the literature on the circadian rhythm of body temperature (CRT). The review starts with a brief discussion of methodological procedures followed by the description of known patterns of oscillation in body temperature, including ultradian and infradian rhythms. Special sections are devoted to issues of species differences, development and aging, and the relationships between the CRT and the circadian rhythm of locomotor activity, between the CRT and the thermoregulatory system, and between the CRT and states of disease. A section on the nervous control of the CRT is followed by summary and conclusions.

Differential impact of hypothermia and pentobarbital on brain-stem auditory evoked responses

Two experiments were conducted to determine the effects of hypothermia and pentobarbital anesthesia, alone and in combination, on the brain-stem auditory evoked responses (BAERs) of rats. In experiment I, unanesthetized rats were cooled to colonic temperatures 0.5 and 1.0 degrees C below normal. In experiment II, 2 groups of rats were cooled and tested at 37.5, 36.0, 34.5 and 31.5 degrees C. One group was anesthetized during testing and the other group was awake. The rat BAER was sensitive to cooling of 1 degree C or less. Peak latencies were prolonged and peak-to-peak amplitudes were increased by hypothermia alone. The effect on amplitude may be related to the time course of temperature change or to stimulus level. Pentobarbital significantly affected both latencies and amplitudes over and above the effects of cooling. The specific effects of pentobarbital differed by BAER peak and by temperature. The findings point up the importance of the potential confound of anesthetic drugs in most of the evoked potential literature on hypothermia and, for the first time, quantify the complex interactions between pentobarbital and temperature which affect the BAER wave form.

Acute effects of diisopropyl fluorophosphate (DFP) on autonomic and behavioral thermoregulatory responses in the Long-Evans rat

Experiments were designed to assess the mechanisms of diisopropyl fluorophosphate (DFP)-induced changes in thermoregulation of the rat. In one study, male rats of the Long-Evans strain were injected with DFP (s.c.) at doses ranging from 0 to 2.0 mg/kg while maintained at an ambient temperature (Ta) of 20--24 degrees C. Body (Tb) and tail skin (Tt) temperatures were recorded for 5 h post-injection. DFP doses of greater than or equal to 1.0 mg/kg resulted in significant decreases in Tb lasting up to 5 h and increases in Tt lasting up to 1 h post-injection. In a second study, metabolic rate (MR), evaporative water loss (EWL), motor activity (MA), Tb, and Tt were measured at 2 h post-injection of 0, 0.5, 1.0, and 1.5 mg/kg DFP (s.c.) at Ta values of 10, 20, and 30 degrees C. DFP treatment resulted in hypothermia at all three Ta values, but the effect was attenuated at 30 degrees C. MR was significantly reduced at a Ta of 20 degrees C following 1.5 mg/kg, unaffected by DFP at a Ta of 30 degrees C, and stimulated at 10 degrees C following 0.5 mg/kg DFP. EWL was significantly elevated at 30 degrees C following 1.5 mg/kg DFP. MA was significantly reduced following greater than or equal to 1.0 mg/kg DFP at 20 and 30 degrees C and 1.5 mg/kg at 10 degrees C. Tt was elevated and reduced by DFP at Ta values of 30 and 10 degrees C, respectively. In a third study, rats were injected with DFP and placed in a temperature gradient for 1 to 2 h post-injection while selected Ta and Tb were monitored. While both control and DFP-treated rats remained in the cool end of the gradient, rats administered DFP at doses of 1.0 and 1.5 mg/kg were significantly hypothermic. It was also found that Ta values of 10, 20, and 30 degrees C had no effect on DFP-induced inhibition of cholinesterase activity of plasma and erythrocyte fractions of whole blood. Overall, these data support the hypothesis that acute DFP may lower the set-point for the control of body temperature in the rat and demonstrates that the toxicity of DFP is modified by changes in Ta.

Toxic-induced hypothermia and hypometabolism: do they increase uncertainty in the extrapolation of toxicological data from experimental animals to humans?

Commonly used experimental mammals, such as the rat and mouse, exhibit hypothermia and hypometabolism when exposed acutely to many drugs and other chemical substances. This toxic-induced hypothermic/hypometabolic state may be an inherently protective response that can reduce the lethality of a toxic insult. However, as body mass increases, the ability to lower body temperature in response to toxic insult is diminished. Hence, the presence of a protective hypothermic/hypometabolic response in small laboratory mammals and apparent lack thereof in larger species, such as humans, may represent an additional physiological dissimilarity which may underestimate the risk assessment of acute toxicological data. It is proposed that acute toxicological studies in rodents be performed at relatively warm ambient temperatures (ca. 28 to 32 degrees C) to prevent toxic-induced hypothermia. This would assure a more uniform internal thermal environment between species, thus reducing a major physiological variable in species-to-species extrapolation.

Hypothermic effects of a homologous series of short-chain alcohols in rats

The purpose of this study was to assess the utility of the thermoregulatory system as an end point in predicting the toxicity of various short-chain alcohols. Male Fischer rats developed significant hypothermia following acute administration (ip) of methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, or 2-butanol. The hypothermic responses to the six alcohols all showed similar segmented responses characterized by a threshold dose below which no change in body temperature occurred, and a suprathreshold regression with increasing dose causing greater hypothermia. Relative potency of the alcohols was assessed using both the threshold dose to cause hypothermia and the dose that would cause body temperature to decrease by 1 degree C. Both measures gave the progression of toxicity from least to most potent of methanol less than ethanol less than 2-propanol less than 1-propanol less than 2-butanol less than 1-butanol. The effective dose of each alcohol was compared to its membrane/buffer partition coefficient (Pm/b), and there was a high inverse correlation between the hypothermic dose of an alcohol and its lipid solubility. That the potency of an alcohol was strongly correlated with its Pm/b suggests that the membrane disordering theory of narcosis may also be used to explain the hypothermic action of alcohols.

Comparison of rats of the Fischer 344 and Long-Evans strains in their autonomic thermoregulatory response to trimethyltin administration

The purpose of this study was to assess the effect of genetic strain on the acute and long-term thermoregulatory response to the neurotoxicant trimethyltin (TMT) in rats of the Long-Evans (LE) and Fischer 344 (FCH) strains. In one study basic thermoregulatory responses including colonic temperature (Tc), metabolic rate (MR), evaporative water loss (EWL), motor activity (MA), and thermal conductance (Cd) were measured in both rat strains at ambient temperatures (Ta) of 10, 28, and 37 degrees C. It was found that the LE rat has a significantly higher Tc when it is measured in their home cage. Because of its smaller body mass the FCH rat has a higher MR at all Ta values. The FCH rat also has a greater rate of EWL during exposure to a Ta of 37 degrees C. Following iv administration of 8.0 mg/kg TMT both rat strains become significantly hypothermic; however, the effect differed significantly between the strains. At 26-34 d after TMT exposure thermoregulatory responses at Ta values of 10-37 degrees C were generally similar to that of the saline controls. Overall, the LE and FCH strains of rat exhibit some distinct thermoregulatory differences in response to changes in Ta.

Toxicity modification of sodium selenite by a brief exposure to heat or cold in mice

The effect of a brief exposure to heat/cold on the subsequent development of toxicity of sodium selenite (SS) was evaluated in male ICR mice. Mice were exposed to one of three ambient temperatures (Ta; approx. 8 degrees, 22 degrees and 33 degrees C). One and a half hours after the beginning of the exposure, they were injected with 45 mumol/kg of SS subcutaneously. The exposure was terminated 3 h after injection and the mice were returned to Ta of 22 degrees C. Heat-induced enhancement of toxicity was recognized in some plasma enzyme activities 3 days after injection and in the suppression of body weight for up to 3 weeks. On the other hand, cold exposure alleviated SS toxicity in terms of these indices. Thus, the Ta during this short period was recognized to be important in determining subsequent development of SS toxicity. At the end of the thermal exposure, heat increased renal concentration of the injected Se. On the other hand, in the liver and the other organs examined, the highest Se concentration was found in the cold-exposed group, followed by the control (room temperature) and the heat-exposed. The relation between the modification of toxicity and the altered distribution was not clear. Neither glutathione level in the liver nor that in the kidney at the time of SS injection could explain the observed modification of the toxicity.

Thermoregulation at a high ambient temperature following the oral administration of ethanol in the rat

This study was designed to assess the thermoregulatory mechanisms responsible for the elevation in body temperature following ethanol administration when exposed to a high ambient temperature (Ta). Male rats of the Fischer 344 strain were gavaged with 20% ethanol at doses of 0, 2.0, 4.0, 6.0, or 8.0 g/kg and were then placed in an environmental chamber set at a Ta of 37 degrees C. Metabolic rate normalized to body mass0.75 (MR), evaporative water loss (EWL), and motor activity were recorded for 60 min. Ethanol elicited a significant increase in colonic temperature and decrease in MR, EWL, and motor activity. Ethanol also significantly reduced the quantity of evaporated water per milliliter of oxygen consumed (E/M). Multiple linear regression analysis indicated that the two major factors which were associated with the ethanol-induced elevation in body temperature were an increase in MR and a decrease in E/M. Visual observation of behavior indicated that the normal grooming of saliva onto the fur during heat stress was impaired in ethanol-treated animals. Thus, during exposure to a high Ta, the acute ethanol-induced elevation in body temperature appears to be attributed to a suppression in both autonomic and behavioral mechanisms of heat dissipation.

Effects of methanol on autonomic thermoregulation of rats at different ambient temperatures

To measure the effect of methanol on autonomic thermoregulation, male Fischer rats were injected intraperitoneally with saline or 1 or 3 g/kg methanol (20% w/v in saline). The rats were then placed in a chamber, set at an ambient temperature (Ta) of 5, 15, 25, or 35 degrees C, for 60 min while total activity, metabolic rate (MR), evaporative water loss (EWL), and dry thermal conductance were measured. After 60 min, the rat was removed from the chamber and colonic temperature (Tc) was measured. The rats developed a significant hypothermia following the 3 g/kg dosage of methanol at Ta's from 5 to 25 degrees C, while the change in Tc at 35 degrees C was not significant. Total activity decreased following 3 g/kg methanol at Ta's from 15 to 35 degrees C. At a Ta of 25 degrees C, MR was at basal levels and methanol had no effect, but at the other Ta's, where MR was elevated, methanol caused a significant reduction in MR. EWL and thermal conductance were elevated at 35 degrees C in control rats, and methanol reduced EWL at this Ta while it had no effect on conductance at this or any other Ta. These data suggest that the methanol-induced hypothermia and its depressive effect on activity and MR were related. The effect of methanol on the heat loss effectors (EWL, conductance) was not significant or was in the direction of heat conservation. Thus, methanol exposure leads to a significant hypothermia in rats by an inhibitory action on heat production pathways.

Thermal biology of the laboratory rat

The purpose of this paper is to thoroughly review the literature and present a data base of the basic thermoregulatory parameters of the laboratory rat. This review surveys the pertinent papers dealing with various aspects of the thermal biology of the laboratory rat, including: metabolism, thermoneutrality, core and brain temperature, thermal tolerance, thermal conductance and insulation, thermoregulatory effectors (i.e., thermogenesis, peripheral vasomotor tone, evaporation, and behavior), thermal acclimation, growth and reproduction, ontogeny, aging, motor activity and exercise, circadian rhythm and sleep, gender differences, and other parameters. It is shown that many facets of the thermoregulatory system of the laboratory rat are typical to that of most homeothermic species. However, is several instances the rat exhibits unique thermoregulatory responses which are not comparable to other species.

Hypothermia and hypometabolism: sensitive indices of whole-body toxicity following exposure to metallic salts in the mouse

To investigate the practicality of hypothermia and hypometabolism as sensitive indices of toxicity in the mouse, oxygen consumption was monitored continuously and body temperature was measured at 30 min postinjection following the intraperitoneal administration of various metal salts. Eleven metal ions were tested: Al3+, Cd2+, Co2+, Cr2+, Cu2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, and Zn2+. All metals induced dose-dependent reductions in both oxygen consumption (hypometabolism) and deep body (colonic) temperature. Comparative toxicity of the metal ions was evaluated by calculating the dose of metal ion in dimensions of mmol/kg body mass needed to reduce colonic temperature to 35 degrees C. The order of toxicity from lowest to highest was as follows: Cr less than Al less than Pb less than Mn less than Mg less than Zn less than Cu less than Co less than Ni less than Hg less than Cd. The threshold doses for reducing body temperature were less than 5% of the LD50 in 6 of the metals studied. Metal salts with relatively low LD50 doses such as Hg, Cd, and Ni were most efficacious in inducing hypothermia and hypometabolism. Moreover, there was a direct linear relationship between dose for inducing hypothermia or hypometabolism and the reported LD50. Hence, the hypothermia and hypometabolism test may prove to be a sensitive and rapid test for the evaluation of toxicity of environmental contaminants.

Thermoregulatory effects of methanol in Fischer and Long Evans rats

While methanol neurotoxicity has been studied for decades, there are very few data available on the thermoregulatory effects of methanol exposure. This paper will present the results of three studies designed to assess the effects of methanol on body temperature and behavioral thermoregulation in Fischer and Long Evans rats. The first study measured the onset of body temperature changes following methanol exposure. Following gavage of 3 g/kg methanol (20% w/v in saline), brain temperature (Tbr) of Fischer rats decreased 1.5 degrees C within 35 min. A similar volume of saline led to transient increases in Tbr. A second study assessed the time course of changes in body temperature by measuring colonic temperature (Tc) hourly following IP injection of saline or 1 or 3 g/kg methanol. The highest dosage of methanol caused a significant hypothermia in both Fischer and Long Evans rats. The hypothermia reached its nadir in both strains at 1-2 hours and partially recovered within the 6 hour experiment. The third study measured the effects of methanol on behavioral thermoregulation. Fischer and Long Evans rats were gavaged with saline or 1-3 g/kg methanol and placed in a temperature gradient. After 90 min in the gradient, rats of both strains which received 2 or 3 g/kg methanol had a significantly lower Tc than control rats. However, the methanol-treated rats remained in the cool end of the gradient and did not prevent the hypothermic effect of the alcohol. The absence of an observed effect on behavioral temperature selection suggests that methanol may interfere with thermal sensation.

Modulating effect of body temperature on the toxic response produced by the pesticide chlordimeform in rats

Previous studies from this laboratory have demonstrated significant deficits in cardiovascular function in rats exposed to the pesticide chlordimeform (CDM) when body core temperature (TCO) was maintained at 37 degrees C. To investigate the role of TCO on CDM toxicity, similar experiments were conducted over a range of TCO values. Adult rats (n = 30) were anesthetized with sodium pentobarbital (35 mg/kg) and randomly assigned to one of six equal groups. Groups were paired and TCO was maintained in the rats in each of the respective group pairs at one of three levels (37, 35, or 33 degrees C). Rats in one group at each temperature level (groups T37, T35, and T33) were injected intraperitoneally with 60 mg/kg of CDM. Animals in the corresponding temperature-matched groups (groups C37, C35, and C33, respectively) received volume-matched injections of normal saline vehicle and served as time-paired controls. The electrocardiogram and heart rate (HR) were monitored throughout the experimental procedure. There was a significant decrease in HR in all CDM-treated groups when compared to the control group animals. The magnitude of the observed cardiac effect was attenuated in the T35 group when compared to that of the other treated groups. Similarly, lethality rates (number of deaths/total) for the T37, T35, and T33 groups were 2/5, 0/5, and 3/5, respectively; there were no deaths among the control-group animals. From these and previous data from this laboratory, we conclude there may be a beneficial effect of moderate hypothermia in rats exposed to toxic agents while more severe hypothermia appears to offer no advantage and may actually exacerbate the toxic effect.

Effect of nickel chloride on body temperature and behavioral thermoregulation in the rat

This study was designed to assess the effects of acute nickel chloride administration on behavioral and autonomic thermoregulation in the rat. In one experiment, male rats of the Fischer 344 strain were injected with nickel chloride (IP) at dosages of 0 to 24.0 mg/kg and placed in an environmental chamber maintained at an ambient temperature (Ta) of 10 or 20 degrees C. Colonic temperature was measured 60 min postinjection. Nickel chloride caused a dose-related decrease in colonic temperature, and the hypothermia was accentuated at the cooler Ta. In a second study, rats injected with 0, 6.0, 12.0, or 24.0 mg/kg nickel chloride were placed in a temperature gradient which allowed the rats to select their preferred thermal environment. Nickel chloride at dosages of 12.0 and 24.0 mg/kg caused a significant reduction in the selected Ta. At these dosages the rats were also significantly hypothermic at 60 min postinjection. In a third experiment, whole-body oxygen consumption (i.e., metabolic rate) was measured at Ta's of 10, 20, and 30 degrees C following a 12.0 mg/kg injection of nickel chloride. Nickel chloride caused an initial depression in metabolic rate and hypothermia at Ta's of 10 and 20 degrees C but not at 30 degrees C. In conclusion, (a) nickel chloride affects both behavioral and autonomic control of thermoregulation in the rat and appears to induce a regulated decrease in body temperature and (b) the behavioral thermoregulatory response of the rat is less sensitive to nickel chloride when compared to the mouse.