A review of terms for regulated vs. forced, neurochemical-induced changes in body temperature

A randomized controlled trial of sweet basil leaf powder-enriched cookies for anemia management in adolescent girls

The study aimed to evaluate the effectiveness of sweet basil leaf powder as a natural source of iron for the treatment of anemia in adolescent girls. Purposive sampling technique of two-stage sampling; part of the nonprobability sampling approach. Out of 2400 approached adolescent girls, 1645 agreed to participate and their nutritional status was assessed. Of these, 89.95% had clinical signs and symptoms of anemia, and 59.79% were found to be anemic based on Hb levels. From the anemic group, 65.18% were randomly selected to receive either B0 (Control), B1 (12.699 g FeSO4.7H2O/100 g), and B3 (16 g SBLP/100 g) cookies for 4 months. At the end of the intervention, the assessment of nutritional status, complete blood count, serum iron, serum ferritin, serum total iron-binding capacity (TIBC), and transferrin saturation was explored. Hematological parameters such as Hb, Hct, TIBC, MCV, MCH, MCHC, serum iron, and serum ferritin were significant (p ≤ .05). The result showed that the serum Fe was highest in group B3 while a significant decline was noted for group B0. Serum ferritin for B1 was better than B3. The entire treatment for transferrin saturation showed a highly significant increasing trend in B3 and B1, regardless of the control. TIBC levels raised in the control group while in all other treatments, it declined. The study demonstrated that SBLP-fortified cookies can be an effective treatment option for anemia, as evidenced by significant improvements in key hematological parameters.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 1: Foundational principles and theories of regulation

This contribution is the first of a four-part, historical series encompassing foundational principles, mechanistic hypotheses and supported facts concerning human thermoregulation during athletic and occupational pursuits, as understood 100 years ago and now. Herein, the emphasis is upon the physical and physiological principles underlying thermoregulation, the goal of which is thermal homeostasis (homeothermy). As one of many homeostatic processes affected by exercise, thermoregulation shares, and competes for, physiological resources. The impact of that sharing is revealed through the physiological measurements that we take (Part 2), in the physiological responses to the thermal stresses to which we are exposed (Part 3) and in the adaptations that increase our tolerance to those stresses (Part 4). Exercising muscles impose our most-powerful heat stress, and the physiological avenues for redistributing heat, and for balancing heat exchange with the environment, must adhere to the laws of physics. The first principles of internal and external heat exchange were established before 1900, yet their full significance is not always recognised. Those physiological processes are governed by a thermoregulatory centre, which employs feedback and feedforward control, and which functions as far more than a thermostat with a set-point, as once was thought. The hypothalamus, today established firmly as the neural seat of thermoregulation, does not regulate deep-body temperature alone, but an integrated temperature to which thermoreceptors from all over the body contribute, including the skin and probably the muscles. No work factor needs to be invoked to explain how body temperature is stabilised during exercise.

The role of somatosensory innervation of adipose tissues

Adipose tissues communicate with the central nervous system to maintain whole-body energy homeostasis. The mainstream view is that circulating hormones secreted by the fat convey the metabolic state to the brain, which integrates peripheral information and regulates adipocyte function through noradrenergic sympathetic output¹. Moreover, somatosensory neurons of the dorsal root ganglia innervate adipose tissue². However, the lack of genetic tools to selectively target these neurons has limited understanding of their physiological importance. Here we developed viral, genetic and imaging strategies to manipulate sensory nerves in an organ-specific manner in mice. This enabled us to visualize the entire axonal projection of dorsal root ganglia from the soma to subcutaneous adipocytes, establishing the anatomical underpinnings of adipose sensory innervation. Functionally, selective sensory ablation in adipose tissue enhanced the lipogenic and thermogenetic transcriptional programs, resulting in an enlarged fat pad, enrichment of beige adipocytes and elevated body temperature under thermoneutral conditions. The sensory-ablation-induced phenotypes required intact sympathetic function. We postulate that beige-fat-innervating sensory neurons modulate adipocyte function by acting as a brake on the sympathetic system. These results reveal an important role of the innervation by dorsal root ganglia of adipose tissues, and could enable future studies to examine the role of sensory innervation of disparate interoceptive systems.

Individual differences in biological regulation: Predicting vulnerability to drug addiction, obesity, and other dysregulatory disorders

Physiological regulation is so fundamental to survival that natural selection has greatly favored the evolution of robust regulatory systems that use both reactive and preemptive responses to mitigate the disruptive impact of biological and environmental challenges on physiological function. In good health, robust regulatory systems provide little insight into the typically hidden complex array of sensor-effector interactions that accomplish successful regulation. Numerous health disorders have been traced to defective regulatory mechanisms, and generations of scientists have worked to discover ways to correct these defects and restore normal physiological function. Despite progress, numerous chronic health disorders remain resistant to treatment, and indeed for some disorders the incidence is increasing. We propose that an individual's susceptibility to acquire certain persistent dysregulatory disorders can be traced to interindividual variation in how that individual's regulatory system responds to challenges. Preexisting reliable individual differences among regulatory systems are typically unrecognized until appropriate regulatory challenges (e.g., exposure to a drug of abuse) lead to dysregulation (e.g., drug addiction). Specific characteristics of an individual's regulatory responsiveness may include etiological factors that participate in the acquisition, escalation and maintenance of health disorders characterized by dysregulation. By appropriately challenging a healthy individual's regulatory systems to identify its underlying characteristics, it is possible to ascertain whether an individual has an elevated risk for acquiring a dysregulated health condition and thereby enable strategies designed to prevent, rather than treat, the condition. This model is applied to drug addiction, and in addition we relate this approach to other dysregulated conditions such as obesity. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

The Effect of Behaviour and Diet on the Rumen Temperature of Holstein Bulls

Rumen temperature boluses are becoming increasingly used as a means of monitoring core body temperature for the detection of ill health. However, the effect of behavior on rumen temperature is largely unknown. This research investigates the impact of behaviour and diet on the rumen temperature of Holstein bulls, both at grass, and in a housed environment. Rumen temperature was recorded at five-minute intervals using a bolus. Direct observations were conducted on young bulls in two studies (i) at grass (n = 30) and (ii) while housed (n = 32). In addition, activity monitors were attached to bulls at grass (n = 24). Within each study, diet differed by the level of concentrate supplementation. There was no effect of diet on rumen temperature. Significant differences in rumen temperature were observed between behaviour groups for bulls at grass (p < 0.001) and housed (p < 0.001). Furthermore, drinking resulted in the lowest rumen temperature (grass 35.97 °C; housed 36.70 °C). Therefore, rumen temperature is affected by behavior; however, the temperatures recorded were not outside the normal temperature range for healthy cattle.

At thermoneutrality, acute thyroxine-induced thermogenesis and pyrexia are independent of UCP1

OBJECTIVE

Hyperthyroidism is associated with increased metabolism ("thyroid thermogenesis") and elevated body temperature, often referred to as hyperthermia. Uncoupling protein-1 (UCP1) is the protein responsible for nonshivering thermogenesis in brown adipose tissue. We here examine whether UCP1 is essential for thyroid thermogenesis.

METHODS

We investigated the significance of UCP1 for thyroid thermogenesis by using UCP1-ablated (UCP1 KO) mice. To avoid confounding factors from cold-induced thermogenesis and to approach human conditions, the experiments were conducted at thermoneutrality, and to resemble conditions of endogenous release, thyroid hormone (thyroxine, T4) was injected peripherally.

RESULTS

Both short-term and chronic thyroxine treatment led to a marked increase in metabolism that was largely UCP1-independent. Chronic thyroxine treatment led to a 1-2 °C increase in body temperature. This increase was also UCP1-independent and was maintained even at lower ambient temperatures. Thus, it was pyrexia, i.e. a defended increase in body temperature, not hyperthermia. In wildtype mice, chronic thyroxine treatment induced a large relative increase in the total amounts of UCP1 in the brown adipose tissue (practically no UCP1 in brite/beige adipose tissue), corresponding to an enhanced thermogenic response to norepinephrine injection. The increased UCP1 amount had minimal effects on thyroxine-induced thermogenesis and pyrexia.

CONCLUSIONS

These results establish that thyroid thermogenesis is a UCP1-independent process. The fact that the increased metabolism coincides with elevated body temperature and thus with accelerated kinetics accentuates the unsolved issue of the molecular background for thyroid thermogenesis.

Concentration-related metabolic rate and behavioral thermoregulatory adaptations to serial administrations of nitrous oxide in rats

Background

Initial administration of ≥60% nitrous oxide (N₂O) to rats evokes hypothermia, but after repeated administrations the gas instead evokes hyperthermia. This sign reversal is driven mainly by increased heat production. To determine whether rats will behaviorally oppose or assist the development of hyperthermia, we previously performed thermal gradient testing. Inhalation of N₂O at ≥60% causes rats to select cooler ambient temperatures both during initial administrations and during subsequent administrations in which the hyperthermic state exists. Thus, an available behavioral response opposes (but does not completely prevent) the acquired hyperthermia that develops over repeated high-concentration N₂O administrations. However, recreational and clinical uses of N₂O span a wide range of concentrations. Therefore, we sought to determine the thermoregulatory adaptations to chronic N₂O administration over a wide range of concentrations.

Methods

This study had two phases. In the first phase we adapted rats to twelve 3-h N₂O administrations at either 0%, 15%, 30%, 45%, 60% or 75% N₂O (n = 12 per group); outcomes were core temperature (via telemetry) and heat production (via respirometry). In the second phase, we used a thermal gradient (range 8°C—38°C) to assess each adapted group’s thermal preference, core temperature and locomotion on a single occasion during N₂O inhalation at the assigned concentration.

Results

In phase 1, repeated N₂O administrations led to dose related hyperthermic and hypermetabolic states during inhalation of ≥45% N₂O compared to controls (≥ 30% N₂O compared to baseline). In phase 2, rats in these groups selected cooler ambient temperatures during N₂O inhalation but still developed some hyperthermia. However, a concentration-related increase of locomotion was evident in the gradient, and theoretical calculations and regression analyses both suggest that locomotion contributed to the residual hyperthermia.

Conclusions

Acquired N₂O hyperthermia in rats is remarkably robust, and occurs even despite the availability of ambient temperatures that might fully counter the hyperthermia. Increased locomotion in the gradient may contribute to hyperthermia. Our data are consistent with an allostatic dis-coordination of autonomic and behavioral thermoregulatory mechanisms during drug administration. Our results have implications for research on N₂O abuse as well as research on the role of allostasis in drug addiction.

2-Deoxy-D-glucose, not mercaptoacetate, induces a reversible reduction of body temperature in male desert hamsters (Phodopus roborovskii)

The initiation of torpor is supposed to be related to the availability of metabolic fuels. Studies on metabolic fuel inhibition of glucose by using 2-deoxy-D-glucose (2DG) or fatty acid by mercaptoacetate (MA) in heterothermic mammals produced mixed outcomes. To examine the roles of availability of glucose and fatty acid in the initiation of torpor in desert hamsters (Phodopus roborovskii), we intraperitoneally administrated 2DG and MA to summer-acclimated male hamsters while body temperature (T_b), metabolic rate (MR) and respiratory quotient (RQ) were simultaneously recorded to monitor their thermoregulatory response. 2DG induced a reversible reduction of T_b in desert hamsters both at ambient temperature (T_a) of 23°C and 5°C. At T_a of 23°C, T_b, MR and RQ decreased in a dose-dependent manner with a large T_b-T_a differential (> 6.5°C) and a lowest T_b of 28.0°C which were comparable to those in fasted hamsters. At T_a of 5°C, 2DG-treated hamsters also decreased T_b to the same level as at T_a 23°C, but MR was significantly higher than that at T_a of 23°C at each dose, suggesting doses of 2DG directly affected the hypothalamic T_b set-point. Different from fasted hamsters which maintain normothermic at T_a of 5°C, 2DG-treated hamsters showed a substantial reduction of T_b at T_a 5°C, indic_ating an overwhelming effect on the thermoregulatory system regardless of T_a. Furthermore, the rapid decrease of T_b and outstretched body posture in 2DG-treated hamsters suggest that the effects of 2DG were not simply mimicking the torpor pathways but that other mechanisms are involved. Interestingly, MA failed to induce a torpor-like state in male desert hamsters. Our results suggest that availability of glucose rather than fatty acid plays an important role for initiation of torpor in desert hamsters.

The mouse thermoregulatory system: Its impact on translating biomedical data to humans

The laboratory mouse has become the predominant test species in biomedical research. The number of papers that translate or extrapolate data from mouse to human has grown exponentially since the year 2000. There are many physiological and anatomical factors to consider in the process of extrapolating data from one species to another. Body temperature is, of course, a critical determinant in extrapolation because it has a direct impact on metabolism, cardiovascular function, drug efficacy, pharmacokinetics of toxins and drugs, and many other effects. While most would consider the thermoregulatory system of mice to be sufficiently stable and predictable as to not be a cause for concern, the thermal physiology of mice does in fact present unique challenges to the biomedical researcher. A variable and unstable core temperature, high metabolic rate, preference for warm temperatures, large surface area: body mass ratio, and high rate of thermal conductance, are some of the key factors of mice that can affect the interpretation and translation of data to humans. It is the intent of this brief review to enlighten researchers studying interspecies translation of biomedical data on the salient facets of the mouse thermal physiology and show how extrapolation in fields such as physiology, psychology, nutrition, pharmacology, toxicology, and pathology.

Spontaneous hypothermia in human sepsis is a transient, self-limiting, and nonterminal response

Hypothermia in sepsis is generally perceived as something dysregulated and progressive although there has been no assessment on the natural course of this phenomenon in humans. This was the first study on the dynamics of hypothermia in septic patients not subjected to active rewarming, and the results were surprising. A sample of 50 subjects presenting with spontaneous hypothermia during sepsis was drawn from the 2005-2012 database of an academic hospital. Hypothermia was defined as body temperature below 36.0°C for longer than 2 h, with at least one reading of 35.5°C or less. The patients presented with 138 episodes of hypothermia, 21 at the time of the sepsis diagnosis and 117 with a later onset. However, hypothermia was uncommon in the final 12 h of life of the patients that succumbed. The majority (97.1%) of the hypothermic episodes were transient and self-limited; the median recovery time was 6 h; body temperature rarely fell below 34.0°C. Bidirectional oscillations in body temperature were evident in the course of hypothermia. Nearly half of the hypothermic episodes had onset in the absence of shock or respiratory distress, and the incidence of hypothermia was not increased during either of these conditions. Usage of antipyretic drugs, sedatives, neuroleptics, or other medications did not predict the onset of hypothermia. In conclusion, hypothermia appears to be a predominantly transient, self-limiting, and nonterminal phenomenon that is inherent to human sepsis. These characteristics resemble those of the regulated hypothermia shown to replace fever in animal models of severe systemic inflammation.

Drug-induced therapeutic hypothermia after asphyxial cardiac arrest in swine

A feasibility study was performed to compare an investigational drug, HBN-1, to forced cooling to induce hypothermia after resuscitation in a translation model of asphyxial cardiac arrest in swine. Serum and cerebral spinal fluid neuron-specific enolase activity (sNSE and csfNSE) were measured after cardiac arrest as surrogate markers of brain injury. In a block design, swine resuscitated from 10 minutes of asphyxial cardiac arrest were infused intravenously with HBN-1 or iced saline vehicle (forced hypothermia [FH]) 5 to 45 minutes after return of spontaneous circulation (ROSC). External cooling in both groups was added 45 minutes after ROSC until hypothermia (T=4°C below baseline) was attained. Esophageal (core) temperature, shivering, cardiopulmonary parameters, and time to hypothermia after ROSC were monitored. sNSE and csfNSE were measured 180 minutes after ROSC. HBN-1 induced hypothermia significantly lowered temperature compared to FH 5-45 minutes after ROSC (p<0.0001). Time to hypothermia was reduced by HBN-1 (93±6 minutes) compared to FH (177±10 minutes) (p<0.0001). HBN-1 sNSE (0.7±1.9 ng/mL) and csfNSE (17.3±1.9 ng/mL) were lower compared to FH (6±1.6 ng/mL) and (49.7±32.0 ng/mL) (p<0.0001, p=0.022, respectively). There was no shivering with HBN-1 cooling while all FH cooled swine shivered (p<0.0001). The time to reach target hypothermia after cardiac arrest was reduced by nearly 50% with HBN-1 compared to the FH method of inducing hypothermia. Moreover, surrogate biomarkers of brain injury were significantly reduced with HBN-1 as compared to FH. While HBN-1-induced hypothermia shows promise for being neuroprotective, survival studies are needed to confirm these preliminary findings.

Methamphetamine-induced toxicity: an updated review on issues related to hyperthermia

Reports of methamphetamine-related emergency room visits suggest that elevated body temperature is a universal presenting symptom, with lethal overdoses generally associated with extreme hyperthermia. This review summarizes the available information on methamphetamine toxicity as it pertains to elevations in body temperature. First, a brief overview of thermoregulatory mechanisms is presented. Next, central and peripheral targets that have been considered for potential involvement in methamphetamine hyperthermia are discussed. Finally, future areas of investigation are proposed, as further studies are needed to provide greater insight into the mechanisms that mediate the alterations in body temperature elicited by methamphetamine.

Clarifying the roles of homeostasis and allostasis in physiological regulation

Homeostasis, the dominant explanatory framework for physiological regulation, has undergone significant revision in recent years, with contemporary models differing significantly from the original formulation. Allostasis, an alternative view of physiological regulation, goes beyond its homeostatic roots, offering novel insights relevant to our understanding and treatment of several chronic health conditions. Despite growing enthusiasm for allostasis, the concept remains diffuse, due in part to ambiguity as to how the term is understood and used, impeding meaningful translational and clinical research on allostasis. Here, we provide a more focused understanding of homeostasis and allostasis by explaining how both play a role in physiological regulation, and a critical analysis of regulation suggests how homeostasis and allostasis can be distinguished. Rather than focusing on changes in the value of a regulated variable (e.g., body temperature, body adiposity, or reward), research investigating the activity and relationship among the multiple regulatory loops that influence the value of these regulated variables may be the key to distinguishing homeostasis and allostasis. The mechanisms underlying physiological regulation and dysregulation are likely to have important implications for health and disease.

Nitrous oxide causes a regulated hypothermia: rats select a cooler ambient temperature while becoming hypothermic

An initial administration of 60% nitrous oxide (N(2)O) evokes hypothermia in rats and if the administration continues for more than 1-2h, acute tolerance typically develops such that the initial reduction in core temperature (Tc) reverses and Tc recovers toward control values. Calorimeter studies at normal ambient temperature indicate that hypothermia results from a transient reduction in heat production (HP) combined with an elevation in heat loss. Acute tolerance develops primarily due to progressive increases in HP. Our aim was to determine whether rats provided a choice of ambient temperatures would behaviorally facilitate or oppose N(2)O-induced hypothermia. A gas-tight thermally-graded alleyway (range, 6.7-37.0°C) enabled male Long-Evans rats (n=12) to select a preferred ambient temperature during a 5-hour steady-state administration of 60% N(2)O and a separate paired control gas exposure (order counterbalanced). Tc was measured telemetrically from a sensor surgically implanted into the peritoneal cavity >7days before testing. Internal LED lighting maintained the accustomed day:night cycle (light cycle 0700-1900h) during sessions lasting 45.5h. Rats entered the temperature gradient at 1100h, and the 5-h N(2)O or control gas period did not start until 23h later to provide a long habituation/training period. Food and water were provided ad libitum at the center of the alleyway. The maximum decrease of mean Tc during N(2)O administration occurred at 0.9h and was -2.05±0.25°C; this differed significantly (p<0.0001) from the corresponding Tc change at 0.9h during control gas administration (0.01±0.14°C). The maximum decrease of the mean selected ambient temperature during N(2)O administration occurred at 0.7h and was -13.58±1.61°C; this differed significantly (p<0.0001) from the corresponding mean change in the selected ambient temperature at 0.7h during control gas administration (0.30±1.49°C). N(2)O appears to induce a regulated hypothermia because the selection of a cool ambient temperature facilitates the reduction in Tc. The recovery of Tc during N(2)O administration (i.e., acute tolerance development) could have been facilitated by selection of ambient temperatures that were warmer than those chosen during control administrations, but interestingly, this did not occur.

Effects of GABA-transaminase inhibitor Vigabatrin on thermoregulation in rats

Vigabatrin is a GABA derivative (gamma-vinyl GABA) which inhibits irreversibly the enzyme activity of GABA transaminase and thus increased indirectly brain GABA concentrations. We have used body temperature assay to examine the effects of Vigabatrin on thermoregulation in intact rats. In order to understand the mechanism of thermoregulatory action of Vigabatrin at cellular level, we have investigated its effect on individual warm-sensitive preoptic area/anterior hypothalamus (PO/AH) neurons in rat brain slice preparations. The results of the present study suggest that Vigabatrin produced dose-dependent hypothermia in rats and also increased temperature sensitivity of warm-sensitive PO/AH neurons.

Hypoxia reduces the hypothalamic thermogenic threshold and thermosensitivity

Hypoxia is well known to reduce the body temperature (T(b)) of mammals, although the neural origins of this response remain uncertain. Short-term hypoxic exposure causes a reduction in the lower critical temperature of the thermal neutral zone and a reduction in whole body thermal conductance of rodents, providing indirect support that hypoxia lowers T(b) in a regulated manner. In this study, we examined directly the potential for changes in central thermosensitivity to evoke the hypoxic metabolic response by heating and cooling the preoptic area of the hypothalamus (the area which integrates thermoreceptor input and regulates thermoeffector outputs) using chronic, indwelling thermodes in ground squirrels during normoxia and hypoxia (7, 10 and 12% O(2)). We found that the threshold hypothalamic temperature for the metabolic response to cooling (T(th)) of approximately 38 degrees C in normoxia was proportionately reduced in hypoxia (down to 28-31 degrees C at 7% O(2)) and that the metabolic thermosensitivity (alpha; the change in metabolic rate for any given change in hypothalamic temperature below the lower critical temperature) was comparatively reduced by 5 to 9 times. This provides strong support for the hypothesis that the fall in temperature that occurs during hypoxia is the result of a reduction in the activation of thermogenic mechanisms. The decrease in the central thermosensitivity in hypoxia, however, appears to be a critical factor in the alteration of mammalian T(b). We suggest, therefore, that an altered central thermosensitivity may provide a proximate explanation of how low oxygen and similar stressors reduce normal fluctuations in T(b) (i.e. circadian), in addition to the depression in regulated T(b).

Homeostasis: beyond Curt Richter

Curt Richter introduced behavioral control into the concept of homeostasis, thereby opening entire fields of research. The prevailing dogma, and the techniques he used, conspired to lead Richter and others to interpret regulation in strict negative feedback terms. Although this point of view continues to be embraced by many contemporary biologists, we believe that prevailing sentiment favors a broader view in which organisms integrate anticipatory pre-emptive control over regulated variables whenever possible.

Regulated hypothermia reduces brain oxidative stress after hypoxic-ischemia

Regulated hypothermia produces a decrease in core temperature by lowering the brain's temperature set-point while maintaining thermoregulation at that lower set point. In contrast, forced hypothermia lowers core temperature by overwhelming the body's capacity to thermoregulate, but does not change the set-point. Regulated hypothermia has been shown to be cerebral protective in hibernating mammals. The effect of regulated hypothermia on the brain during reperfusion from hypoxic-ischemia has not been well studied. We induced regulated hypothermia with a neurotensin analogue (NT77) to determine whether it could reduce oxidative stress in the brain during reperfusion from asphyxial cardiac arrest (ACA) in rats. Mild hypothermia (32-34 degrees C) was induced by brief (4 h) external cooling (BC), NT77 or prolonged external cooling (24 h) (PC) 30 min after resuscitation from 8 min of ACA in rats. Malondialdehyde (MDA) levels in the brain were measured during reperfusion to quantitate oxidative stress.

RESULTS

MDA levels in the hippocampus were elevated at 16 h of normothermic reperfusion versus 48 h with BC reperfusion. There was no increase in hippocampal MDA levels in the NT77 and PC groups at 24-72 h of reperfusion. Regulated hypothermia induced by NT77 reduced oxidative stress in the hippocampus during reperfusion from hypoxic-ischemia in comparison to forced brief external cooling of the same duration. In addition, the duration of external cooling after resuscitation also alters oxidative stress in the brain during reperfusion.

Neurotensin-induced hypothermia improves neurologic outcome after hypoxic-ischemia

OBJECTIVE

External cooling is commonly used to force induction of mild hypothermia but requires equipment, has a slow onset of action, and must be prolonged to provide permanent neurologic benefits after hypoxic-ischemia. It is unknown whether the method for inducing mild hypothermia affects neurologic outcome after near-drowning. The objective of the study was to induce mild hypothermia with neurotensin analog NT77 or external cooling in a rat model of near-drowning. We hypothesize that NT77 would be more effective for improving neurologic outcome than external cooling of the same duration.

DESIGN

Rats were randomized to a normothermic control, neurotensin-induced hypothermia, brief external cooling, or prolonged external cooling group after asphyxial cardiac arrest.

SETTING

Laboratory investigation.

SUBJECTS

Forty-eight rats.

INTERVENTIONS

Mild hypothermia was induced by external cooling for 4 hrs (brief external cooling) or 24 hrs (prolonged external cooling) or by neurotensin-induced hypothermia administration 30 mins after asphyxial cardiac arrest in rats.

MEASUREMENTS

Outcome was assessed by a neurologic deficit score, the Morris water maze, and CA1 hippocampus histology 15 days after resuscitation.

MAIN RESULTS

Neurologic deficit score at 72 hrs after asphyxial cardiac arrest was lower with neurotensin-induced hypothermia (score, 0) and prolonged external cooling (score, 0) vs. normothermic control (score, 20) and brief external cooling (score, 18; p <.05). Latency time in the Morris water maze 15 days after asphyxial cardiac arrest was decreased with neurotensin-induced hypothermia (14+/-11 secs) and prolonged external cooling (18+/-9 secs) vs. normothermic control (74+/-17 secs) and brief external cooling (78+/-18 secs, p <.05). There was less ischemic neuronal damage with neurotensin-induced hypothermia (28+/-24%) and prolonged external cooling (21+/-14%) vs. normothermic control (61+/-32%) and brief external cooling (51+/-32%).

CONCLUSIONS

Neurotensin-induced hypothermia improved neurologic outcome after asphyxial cardiac arrest in rats vs. brief external cooling but was comparable to prolonged external cooling.

Neurotensin analog NT77 induces regulated hypothermia in the rat

The potential use of hypothermia as a therapeutic treatment for stroke and other pathological insults has prompted the search for drugs that can lower core temperature. Ideally, a drug is needed that reduces the set-point for control of core temperature (T(c)) and thereby induces a regulated reduction in T(c). To this end, a neurotensin analog (NT77) that crosses the blood brain barrier and induces hypothermia was assessed for its effects on the set-point for temperature regulation in the Sprague-Dawley rat by measuring behavioral and autonomic thermoregulatory responses. Following surgical implanation of radiotransmitters to monitor T(c), rats were placed in a temperature gradient and allowed to select from a range of ambient temperatures (T(a)) while T(c) was monitored by radiotelemetry. There was an abrupt decrease in selected T(a) from 29 to 16 degrees C and a concomitant reduction in T(c) from 37.4 to 34.0 degrees C 1 hr after IP injection of 5.0 mg/kg NT77. Selected T(a) and T(c) then recovered to control levels by 1.5 hr and 4 hr, respectively. Oxygen consumption (M) and heat loss (H) were measured in telemetered rats housed in a direct calorimeter maintained at a T(a) of 23.5 degrees C. Injection of NT77 initially led to a reduction in M, little change in H, and marked decrease in T(c). H initially rose but decreased around the time of the maximal decrease in T(c). Overall, NT77 appears to induce a regulated hypothermic response because the decrease in T(c) was preceded by a reduction in heat production, no change in heat loss, and preference for cold T(a)'s. Inducing a regulated hypothermic response with drugs such as NT77 may be an important therapy for ischemic disease and other insults.

Merriam's kangaroo rats (Dipodomys merriami) voluntarily select temperatures that conserve energy rather than water

Desert endotherms such as Merriam's kangaroo rat (Dipodomys merriami) use both behavioral and physiological means to conserve energy and water. The energy and water needs of kangaroo rats are affected by their thermal environment. Animals that choose temperatures within their thermoneutral zone (TNZ) minimize energy expenditure but may impair water balance because the ratio of water loss to water gain is high. At temperatures below the TNZ, water balance may be improved because animals generate more oxidative water and reduce evaporative water loss; however, they must also increase energy expenditure to maintain a normal body temperature. Hence, it is not possible for kangaroo rats to choose thermal environments that simultaneously minimize energy expenditure and increase water conservation. I used a thermal gradient to test whether water stress, energy stress, simultaneous water and energy stress, or no water/energy stress affected the thermal environment selected by D. merriami. During the night (i.e., active phase), animals in all four treatments chose temperatures near the bottom of their TNZ. During the day (i.e., inactive phase), animals in all four treatments settled at temperatures near the top of their TNZ. Thus, kangaroo rats chose thermal environments that minimized energy requirements, not water requirements. Because kangaroo rats have evolved high water use efficiency, energy conservation may be more important than water conservation to the fitness of extant kangaroo rats.

Prenatal exposure to nicotine attenuates stress-induced hyperthermia in 7- to 8-week-old rats upon exposure to a novel environment

Given that approximately 25% of women in the United States continue to smoke cigarettes during pregnancy, it is important to know if exposure to nicotine during development alters the physiological response of the adult to the "stressors" of everyday life. Our current experiments were carried out to determine if prenatal exposure to nicotine alters "stress-induced hyperthermia" in adult rats upon exposure to a novel environment such as a simulated open field. Forty-eight rats (23 males and 25 females) were exposed to a simulated open field or left in their home cage at 7 to 8 weeks of postnatal life (i.e., adulthood as defined by the ability to reproduce) following prenatal exposure to vehicle or nicotine (6 mg of nicotine tartrate per kilogram of maternal body weight per day) via a maternally implanted osmotic minipump from Day 6 or 7 of gestation. The simulated open field consisted of a 30(W)x60(L)x24(H)-in. white acrylic finish box illuminated by two hanging fluorescent lights and core temperature was measured by telemetry. Exposure to a simulated open field following prenatal exposure to vehicle elicited an increase in core temperature in male and female rats with a magnitude of approximately 1.2 degrees C and a duration of greater than 170 min. Prenatal exposure to nicotine significantly attenuated the thermogenic response of both genders; this was not only evident in the latency, magnitude and duration of the core temperature response but also in the core temperature index determined from the 3-h period following exposure to a simulated open field. Thus, our data provide evidence that prenatal exposure to nicotine attenuates stress-induced hyperthermia in male and female 7- to 8-week-old rats upon exposure to a "stressor" of everyday life (i.e., a novel environment).

Core temperature is regulated, although at a lower temperature, in rats exposed to hypergravic fields

1. In rats acclimated to 23 degrees C (RT rats) or 5 degrees C (CA rats), core temperature (Tc), tail temperature (Tt) and oxygen consumption (VO2) were measured during exposure to a hypergravic field. 2. Rats were exposed for 5.5 h to a 3 g field while ambient temperature (Ta) was varied. For the first 2 h, Ta was 25 degrees C; then Ta was raised to 34 degrees C for 1.5 h. During this period of warm exposure, Tc increased 4 degrees C in both RT and CA rats. Finally, Ta was returned to 25 degrees C for 2 h, and Tc decreased toward the levels measured prior to warm exposure. 3. In a second experiment at 3 g, RT and CA rats were exposed to cold (12 degrees C) after two hours at 25 degrees C. During the one hour cold exposure, Tc fell 1.5 degrees C in RT and 0.5 degree C in CA rats. After cold exposure, when ambient temperature was again 25 degrees C, Tc of RT and CA rats returned toward the levels measured prior to the thermal disturbance. 4. Rats appear to regulate their temperature, albeit at a lower level, in a 3 g field.

Mechanisms and mediators of psychological stress-induced rise in core temperature

OBJECTIVE

Despite numerous case reports on "psychogenic fever," it remains uncertain how psychological stress raises core temperature and whether the rise in core temperature is a real fever or a hyperthermia. This article reviews studies on the psychological stress-induced rise in core temperature (PSRCT) in animals with the aim to facilitate studies on the mechanisms of so-called psychogenic fever in humans.

METHODS

To address this question, we reviewed the mechanisms and mediators of the PSRCT and classic conditioning of the fever response in animals.

RESULTS

The PSRCT is not due to the increased locomotor activity during stress, and the magnitude of the PSRCT is the same in warm and cold environments, indicating that it is a centrally regulated rise in temperature due to an elevated thermoregulatory "set point." The PSRCT caused by conventional psychological stress models, such as open-field stress, is attenuated by cyclooxygenase inhibitors, which block prostaglandin synthesis. On the other hand, the PSRCT elicited by an "anticipatory anxiety stress" is not inhibited by cyclooxygenase inhibitors but by benzodiazepines and serotonin Type 1A receptor agonists. The febrile response can be conditioned to neutral stimuli after paired presentation with unconditioned stimuli such as injection of lipopolysaccharide, a typical pyrogen.

CONCLUSIONS

Most findings indicate that the PSRCT is a fever, a rise in the thermoregulatory set point. The PSRCT may occur through prostaglandin E2-dependent mechanisms and prostaglandin E2-independent, 5-HT-mediated mechanisms. The febrile response can be conditioned. Thus, these mechanisms might be involved in psychogenic fever in humans.

A peripheral mechanism of fever: differential sensitivity to the antipyretic action of methyl scopolamine

The organophosphate pesticide (OP) chlorpyrifos leads to an acute period of hypothermia followed by a delayed fever in the rat. Methyl scopolamine, a peripheral muscarinic antagonist, is thought to have little effect on body temperature of the rat because it does not cross the blood brain barrier. However, administration of methyl scopolamine (1 mg/kg, i.p.) during the period of chlorpyrifos-induced fever results in a rapid recovery of core temperature. This indicates a peripheral cholinergic pathway is operative in the febrile response to chlorpyrifos and possibly other modes of fever. In this study, we evaluated the possible antipyretic role of methyl scopolamine (i.p.) to a variety of stimuli that lead to fever-like responses in the rat: stress-induced (handling and cage switch), chlorpyrifos-induced (15 mg/kg, p.o.), nocturnal-induced, and lipopolysaccharide (LPS)-induced fever (50 microg/kg, i.p.). Methyl scopolamine led to marked reversal in the elevated core temperature caused by handling, cage switch, and during the nocturnal phase. It is of interest to note that all these elevations of core body temperature are prostaglandin mediated and are blocked with the antipyretic drug, sodium salicylate. However, LPS-induced fever, also a prostaglandin dependent fever, was unaffected by methyl scopolamine. Methyl scopolamine also lowered baseline core temperature when administered during the afternoon, but not during the morning in unstressed animals. It is proposed that a peripheral cholinergic pathway, possibly mediated through afferent vagal pathways, is operative in controlling core temperature during fevers associated with stress, nocturnal phase, and a pesticide. During recovery from exposure to a LPS, the fever appears to be mediated independently of peripheral cholinergic activation.

The vagus nerve in thermoregulation and energy metabolism

The vagus nerve may indirectly influence thermoregulation by modulation of energy balance: its afferent fibers convey signals that represent information on feeding state, resulting in either depression or stimulation of metabolic processes. A regulated metabolic depression can be detected in the background of fasting-induced hypometabolism and hypothermia. In its development (besides humoral signals) vagally transmitted neural signals of gastrointestinal and hepatoportal origin are important. These signals are related to hunger, to decrease of mechanical/chemical stimuli from the gut, to decline of blood glucose; they alter discharge rates of vagal afferents and activity of the nucleus of the solitary tract, eliciting inhibition of metabolic rate and enhancement of food intake. In this hunger-related metabolic inhibition the nucleus of the solitary tract is in interaction with hypothalamic nuclei, that contribute to neuropeptide changes characterized by high neuropeptide Y activity (with energy-conserving type of regulation) and depressed cholecystokinin and corticotropin releasing hormone activities (with depressed energy-expenditure). In postalimentary states the hypermetabolism and hyperthermia are due to opposite changes in metabolic regulation. Satiety-related stimulatory signals of abdominal origin, transmitted via hepatic vagal afferents to the nucleus of the solitary tract, contribute to enhancement of sympathetic activity and stress-responsiveness, leading to hypermetabolism and hyperthermia. Depressed neuropeptide Y release and enhanced cholecystokinin and corticotropin releasing hormone activities participate in the central regulatory changes, and in the high energy-expenditure. The biological role of these vagal functions is not directly the regulation of body temperature, rather the regulation of energy balance and energy content in the body.

Age-dependent core temperature responses of conscious rabbits to acute hypoxemia

Experiments were carried out on chronically instrumented newborn and older rabbits to characterize their core temperature (T(c)) responses to acute hypoxemia and to differentiate "forced" vs. "regulated" thermoregulatory responses. Three age ranges of kits were studied: 4-6, 9-11, and 28-30 days of age. During an experiment, T(c), selected ambient temperature (T(a)), and oxygen consumption were measured from kits studied in a thermocline during a control period of normoxemia, an experimental period of normoxemia or hypoxemia (fraction of inspired oxygen 0.10), and a recovery period of normoxemia. We reasoned that no change or a decrease in T(a) while T(c) decreased during hypoxemia would indicate a regulated thermoregulatory response, whereas an increase in T(a) while T(c) decreased during hypoxemia would indicate a forced thermoregulatory response. T(c) decreased during acute hypoxemia in the older kits but not in the 4- to 6-day-old kits; the decrease in T(c) was accentuated on postnatal days 28-30 compared with postnatal days 9-11. T(a) decreased or stayed the same during exposure to acute hypoxemia. Our data provide evidence that postnatal maturation influences the T(c) response of rabbits to acute hypoxemia and that the decrease in T(c) during hypoxemia in the older kits results from a regulated thermoregulatory response.

Normal body temperature of rats: the setpoint controversy

Emotional hyperthermia, circadian variations and the rise of body temperature related to exercise, have all been attributed to setpoint temperature shifts. The accepted theory holds that core temperature is regulated by corrective thermoregulatory responses opposing the core temperature deviations from the setpoint level. However, in fever and anapyrexia the thermoregulatory responses appear to be not corrective but helping, that is in the same direction as the core temperature deviation. A supplementary ad hoc hypothesis that setpoint level shifts explains why the thermoregulatory responses still could be considered "corrective" in spite of being in the same direction as the core temperature deviation. But supplementary ad hoc hypotheses immunize a theory to experimental challenges and therefore can no longer be considered a scientific theory. The present work shows that most of the arguments adduced to explain almost every biothermal phenomenon as being due to setpoint shifts cannot withstand a critical analysis.

Cerebral cortex does not modulate "regulated" decrease in core temperature during hypoxemia in rats

In newborns and adults of a number of species including humans, exposure to acute hypoxemia produces a "regulated" decrease in core temperature, the mechanism of which is unknown. Considering that various cortical areas participate in autonomic regulation including thermoregulation, the present experiments were carried out to test the hypothesis that the cerebral cortex plays a role in modulating the regulated decrease in core temperature during acute hypoxemia. This hypothesis was tested by determining the core temperature response to acute hypoxemia in chronically instrumented adult Sprague-Dawley rats before and after cortical spreading depression (i.e., functional decortication) was produced by the local application of potassium chloride to the dura overlying the cerebral hemispheres. There was no effect of cortical spreading depression on baseline core temperature. Core temperature decreased during acute hypoxemia in a similar fashion when the cerebral cortex was intact as well as during functional decortication. Thus our data do not support the hypothesis that the cerebral cortex modulates the regulated decrease in core temperature that occurs in adult rats during acute hypoxemia.

Thermoregulatory control during pregnancy and lactation in rats

Although the mechanisms remain unknown, maternal core temperature (Tc) decreases near term of pregnancy and is increased throughout lactation in rats. The purpose of our present experiments was to determine whether pregnancy and lactation shift the thermoneutral zone of rats and to investigate whether the changes in maternal Tc during pregnancy and lactation result from "forced" or "regulated" thermoregulatory responses. Conscious, chronically instrumented nonpregnant and pregnant and lactating rats were studied both in a thermocline (a chamber with a linear temperature gradient from 12 to 36 degrees C) and in a metabolic chamber to determine the influence of pregnancy and lactation on selected ambient temperature as well as the thermoregulatory response to changes in ambient temperature. We found that selected ambient temperature, oxygen consumption, and thermal conductance did not change in rats studied in a thermocline as Tc decreased near term of pregnancy. There was, however, a downward shift in the thermoneutral zone of rats studied in a metabolic chamber near term of pregnancy. During lactation, selected ambient temperature decreased in rats studied in a thermocline as oxygen consumption and Tc increased. The thermoneutral zone of lactating rats was not different from that of nonpregnant animals. Thus our data provide evidence that the decrease in Tc near term of pregnancy in rats results from a regulated thermoregulatory response, whereas the increase in Tc during lactation results from a forced thermoregulatory response.

Brain selenium status and behavioral development in selenium-deficient preweanling mice

The influence of Se deficiency on behavioral development in preweanling mice was evaluated. Female ICR mice were fed either Se-deficient or control diet (containing < 20 or 400 ng Se/g diet, respectively) from 4 weeks before conception to the end of the suckling period. In the offspring of Se-deficient dams, liver and brain Se levels were reduced to < 5% and 60% of those in the control offspring, respectively, from as early as the third postnatal day. At weaning, brain Se content exceeded the hepatic one in Se-deficient offspring, whereas in the control offspring the liver contained 10 times more Se than the brain did. Thus, tissue-specific metabolism of Se was already functioning during the neonatal period. When placed in a thermogradient and allowed to move along the gradient, Se-deficient offspring exhibited a preference for a significantly warmer environment than did the controls. They also showed slightly retarded development of walking ability. These results showed that Se-deficient offspring differed from the controls in behavioral development. Possible mechanisms of these alterations are discussed.

Autonomic and behavioral thermoregulation in the golden hamster during subchronic administration of clorgyline

Chronic administration of clorgyline, a type-A monoamine oxidase inhibitor, leads to a decrease in peritoneal (i.e., core) temperature of golden hamsters. To better understand the mechanisms of clorgyline's thermoregulatory effects, autonomic and behavioral thermoregulatory effectors were measured in Syrian hamsters following chronic infusion of clorgyline via a minipump (2 mg/kg/day). Metabolic rate, evaporative water loss, motor activity, and core temperature were measured after 60 min of exposure to ambient temperatures (Ta) of 5, 20, 30, and 35 degrees C. Behavioral thermoregulatory responses were assessed by measuring selected Ta and motor activity of the same animals in a temperature gradient over the course of 23 h. Metabolic rate and motor activity were significantly elevated in clorgyline-treated hamsters exposed to a Ta of 5 degrees C. There were no effects of clorgyline on evaporative water loss. In the temperature gradient the mean selected Ta of clorgyline-treated hamsters was nearly equal to that of the saline-treated hamsters, 30.7 and 31.2 degrees C, respectively. On the other hand, the mode of selected Ta in the clorgyline group was 2.8 degrees C higher than that of the saline group. Motor activity in the gradient was significantly elevated and food consumption was depressed by clorgyline treatment. Overall, these findings indicate that chronic clorgyline treatment in the golden hamster results in novel autonomic and behavioral modification; it stimulates metabolic thermogenesis during cold exposure, but appears to increase the behavioral zone of thermoneutrality. This latter effect may mean an improvement in heat tolerance, suggesting that this drug might assist in the adaptation to warm temperatures.

Fever: causes and consequences

The present review distinguishes pathogenic, neurogenic, and psychogenic fever, but focuses largely on pathogenic fever, the hallmark of infectious disease. The data presented show that a complex cascade of events underlies pathogenic fever, which in broad outline - and with frank disregard of contradictory data - can be described as follows. An invading microorganism releases endotoxin that stimulates macrophages to synthesize a variety of pyrogenic compounds called cytokines. Carried in blood, these cytokines reach the perivascular spaces of the organum vasculosum laminae terminalis (OVLT) and other regions near the brain where they promote the synthesis and release of prostaglandin (PGE2). This prostaglandin then penetrates the blood-brain barrier to evoke the autonomic and behavioral responses characteristic of fever. But then once expressed, fever does not continue unchecked; endogenous antipyretics likely act on the septum to limit the rise in body temperature. The present review also examines fever-resistance in neonates, the blunting of fever in the aged, and the behaviorally induced rise in body temperature following infection in ectotherms. And finally it takes up the question of whether fever enhances immune responsiveness, and through such enhancement contributes to host survival.

Twenty-four hour rhythms of selected ambient temperature in rat and hamster

The purpose of this study was to assess the effect of time of day on the behavioral thermoregulatory patterns of nocturnal rodents, the Long-Evans (LE) rat, Fischer 344 (F344) rat, and the golden hamster. Individual animals were placed in a temperature gradient for 4 days while selected ambient temperature (STa) and motor activity (MA) were monitored. Food was provided at the cold and warm ends of the gradient and water was provided ad lib. All animals eventually showed a 24-h rhythm of STa and MA characterized by a preference for cooler TaS during the dark period which coincided with an increase in MA. Both rat strains had STaS of approximately 28 degrees C during the light period that decreased to 22-24 degrees C during the dark period. The F344 rat developed a STa rhythm by the second day in the gradient, whereas the LE strain required 4 days. The hamster exhibited relatively warm STaS of 32-33 degrees C during the light period that decreased to 26-28 degrees C during the dark period. The nocturnal preference for cooler STaS contradicts a current concept of an elevation in set point of the thermoregulatory system. However, the data also suggest that behavioral and autonomic thermoregulatory effectors may operate independently in the control of night time elevations in body temperature.

Effect of mu-, kappa-, and delta-selective opioid agonists on thermoregulation in the rat

The effect of selective mu-, kappa-, and delta-agonists on brain surface temperature (Tb), oxygen consumption (Vo2), and heat exchange (Q) was studied in unrestrained, male Sprague-Dawley rats using whole-body calorimetry. Hyperthermia, produced by PL-017 (1.86 nM) given ICV, resulted from increased Vo2 and reduced Q during the first 15-45 min postinjection. Tb returned to control levels due to a combination of increased Q and reduced Vo2. PL-017-induced hyperthermia was abolished by the mu-selective antagonist CTAP (0.75 nM). Dynorphin A1-17 (4.65 nM), a kappa-selective agonist, reduced both Vo2 and Q, resulting in hypothermia that was blocked by the kappa-selective antagonist nor-binaltorphimine (25 nM). The delta-selective agonist DPDPE (4.64 nM) caused no significant changes in Tb, Vo2, or Q. The data indicate that central stimulation of the mu- and kappa-opioid receptors affects both oxidative metabolism and heat exchange, which result in a change in Tb. These alterations can be prevented with selective opioid antagonist pretreatment.

Stress hyperthermia: physiological arguments that it is a fever

The theory that stress (or emotional) rise in central temperature (Tc) in rats is a fever with an upward shift of the set-point temperature was tested with three experiments: 1) Measurement of tail skin temperature and Tc during the emotional Tc rise; 2) Investigation of the effect of ambient temperature on the emotional Tc rise; and 3) The assessment of emotional Tc rise during daytime and nighttime. Skin vasomotor responses helped the increase of Tc toward a higher level and contributed to the regulation of central temperature at this new higher level. The cold environment did not diminish the emotional rise of central temperature as it would be expected in the case of a hyperthermia. However, at night emotional fever reached a higher level than during the daytime, suggesting that prostaglandin rise in Tc is distinct from emotional or stress-induced hyperthermia. In conclusion, the experiments reported here confirm the hypothesis that the rise of Tc induced by handling or disturbance of the rats is regulated, and is due to a shift of the set-point as occurs in fever.

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.

Components of the opioid withdrawal syndrome in mice are thermoregulatory responses

C57BL/6J mice were rendered physically dependent on morphine by giving them ad lib access to a drinking fluid containing 0.2% saccharin and morphine for 14 days at 20-22 degrees C. Core body temperatures were monitored by radio telemetry, which obviated the need for restraint, handling, or otherwise disturbing the animals. Consistent hyperthermia was present throughout the morphine intoxication phase, followed by hypothermia after the withdrawal syndrome had been precipitated by naloxone challenge (2.0 mg/kg, IP) at 22.5 degrees C. The hypothermia could be blocked by exposing the animals to a 34.5 degrees C ambient temperature, which also prevented the occurrence of tremor and "wet dog shakes." In contrast, the other withdrawal signs monitored were not significantly affected. In a second experiment, mice were given the same morphine-saccharin drinking fluid as before, except that a choice was provided between two interconnected home cages (23 degrees C vs. 35 degrees C) throughout the experiment. A marked preference for the 35 degrees C cage was seen during intoxication, which served to enhance the hyperthermia due to morphine. Following withdrawal, when hypothermia is evident, the preference for the 35 degrees C cage declined to control levels. These results suggest that hypothermia is both a consequence and a contributor to the opioid withdrawal syndrome.