The central neurology of mammalian thermoregulation

Thyroid hormone regulation of thermal acclimation in ectotherms: Physiological mechanisms and ecoevolutionary implications

The pathways that regulate adaptive thermal plasticity in ectothermic vertebrates have received little attention relative to those in birds and mammals. However, there is increasing evidence that thyroid hormone represents a critical regulator of thermal plasticity in both ectothermic and endothermic vertebrates. In this review, I summarize the evidence for thyroid hormone-mediated thermal compensation responses in ectothermic vertebrates, with specific focus on effects on the whole animal, skeletal muscle, and cardiac muscle. Interestingly, these effects can differ wildly between focal tissues and species. I move on to discuss what the role of thyroid hormone in ectotherm thermal plasticity can reveal about stressor interactions and central vs. peripheral levels of thyroid hormone regulation. Lastly, I focus on the conserved nature of thyroid hormone signaling in animal thermal responses, with specific reference to the ectotherm → endotherm spectrum. I use this framework to highlight research avenues that will further resolve the evolutionary trajectory of thyroid hormone actions across animals. I hope to emphasize what thyroid hormone-mediated cold acclimation in a 3 cm fish can contribute to ongoing debates surrounding the impacts of stressor interactions, the potential costs of plasticity, the evolution of endothermy, and the impacts of global change.

Understanding the unexplained: The magnitude and correlates of individual differences in residual variance

Behavioral and physiological ecologists have long been interested in explaining the causes and consequences of trait variation, with a focus on individual differences in mean values. However, the majority of phenotypic variation typically occurs within individuals, rather than among individuals (as indicated by average repeatability being less than 0.5). Recent studies have further shown that individuals can also differ in the magnitude of variation that is unexplained by individual variation or environmental factors (i.e., residual variation). The significance of residual variation, or why individuals differ, is largely unexplained, but is important from evolutionary, methodological, and statistical perspectives. Here, we broadly reviewed literature on individual variation in behavior and physiology, and located 39 datasets with sufficient repeated measures to evaluate individual differences in residual variance. We then analyzed these datasets using methods that permit direct comparisons of parameters across studies. This revealed substantial and widespread individual differences in residual variance. The magnitude of individual variation appeared larger in behavioral traits than in physiological traits, and heterogeneity was greater in more controlled situations. We discuss potential ecological and evolutionary implications of individual differences in residual variance and suggest productive future research directions.

Circadian rhythmicity of body temperature and metabolism

This article reviews the literature on the circadian rhythms of body temperature and whole-organism metabolism. The two rhythms are first described separately, each description preceded by a review of research methods. Both rhythms are generated endogenously but can be affected by exogenous factors. The relationship between the two rhythms is discussed next. In endothermic animals, modulation of metabolic activity can affect body temperature, but the rhythm of body temperature is not a mere side effect of the rhythm of metabolic thermogenesis associated with general activity. The circadian system modulates metabolic heat production to generate the body temperature rhythm, which challenges homeothermy but does not abolish it. Individual cells do not regulate their own temperature, but the relationship between circadian rhythms and metabolism at the cellular level is also discussed. Metabolism is both an output of and an input to the circadian clock, meaning that circadian rhythmicity and metabolism are intertwined in the cell.

Effects of capsaicin application on the skin during resting exposure to temperate and warm conditions

We investigated thermoregulatory and cardiovascular responses at rest in a temperate (20°C) and in a warm (30°C) environment (40% RH) without and with the application of capsaicin on the skin. We hypothesized that regardless of environmental temperature, capsaicin application would stimulate heat loss and concomitantly deactivate heat conservation mechanisms, thus resulting in rectal temperature (Tre) and mean blood pressure decline due to excitation of heat-sensitive TRPV1. Ten male subjects were exposed, while seated, for 30 minutes to 20.8 ± 1.0°C or to 30.6 ± 1.1°C: without (NCA) and with (CA) application of capsaicin patches on the skin. Thermoregulatory (Tre, proximal-distal skin temperature gradient) and cardiovascular variables (modelflow technique) as well as oxygen uptake were continuously measured. The area under the curve for Tre decline at 20°C was smaller in CA (-2.1 ± 1.3 a.u.) than in NCA (-0.6 ± 1.1 a.u., P < 0.01, r = 0.8). Likewise, at 30°C it was smaller in CA (-2.2 ± 2.1 a.u.) compared to NCA (-0.8 ± 2.0 a.u., P = 0.02, r = 0.7). Local vasomotor tone and oxygen uptake, were significantly lower by 36.7% ± 94.2% and 12.3% ± 12.3%, respectively, with capsaicin compared to NCA (P = 0.05 and P < 0.01, respectively). Additionally, in 30°C CA mean arterial pressure was lower by 10.7% ± 5.9%, 8.9% ± 5.9%, and 10.6% ± 7.0% compared to 30°C NCA, 20°C NCA, and 20°C CA, respectively (P < 0.01, P = 0.02, and P < 0.01, respectively, d = 1.4-1.8). In conclusion, capsaicin application on the skin induced vasodilation and Tre decline. At 30°C CA, thermal responses were accompanied by arterial hypotension most likely due to the interactive effects of both stressors (warm environment and capsaicin) on cutaneous vascular regulation.

Mechanism underlying berberine's effects on HSP70/TNFα under heat stress: Correlation with the TATA boxes

Heat stress can stimulate an increase in body temperature, which is correlated with increased expression of heat shock protein 70 (HSP70) and tumor necrosis factor α (TNFα). The exact mechanism underlying the HSP70 and TNFα induction is unclear. Berberine (BBR) can significantly inhibit the temperature rise caused by heat stress, but the mechanism responsible for the BBR effect on HSP70 and TNFα signaling has not been investigated. The aim of the present study was to explore the relationship between the expression of HSP70 and TNFα and the effects of BBR under heat conditions, using in vivo and in vitro models. The expression levels of HSP70 and TNFα were determined using RT-PCR and Western blotting analyses. The results showed that the levels of HSP70 and TNFα were up-regulated under heat conditions (40 °C). HSP70 acted as a chaperone to maintain TNFα homeostasis with rising the temperature, but knockdown of HSP70 could not down-regulate the level of TNFα. Furthermore, TNFα could not influence the expression of HSP70 under normal and heat conditions. BBR targeted both HSP70 and TNFα by suppressing their gene transcription, thereby decreasing body temperature under heat conditions. In conclusion, BBR has a potential to be developed as a therapeutic strategy for suppressing the thermal effects in hot environments.

Control of the Cutaneous Circulation by the Central Nervous System

The central nervous system (CNS), via its control of sympathetic outflow, regulates blood flow to the acral cutaneous beds (containing arteriovenous anastomoses) as part of the homeostatic thermoregulatory process, as part of the febrile response, and as part of cognitive-emotional processes associated with purposeful interactions with the external environment, including those initiated by salient or threatening events (we go pale with fright). Inputs to the CNS for the thermoregulatory process include cutaneous sensory neurons, and neurons in the preoptic area sensitive to the temperature of the blood in the internal carotid artery. Inputs for cognitive-emotional control from the exteroceptive sense organs (touch, vision, sound, smell, etc.) are integrated in forebrain centers including the amygdala. Psychoactive drugs have major effects on the acral cutaneous circulation. Interoceptors, chemoreceptors more than baroreceptors, also influence cutaneous sympathetic outflow. A major advance has been the discovery of a lower brainstem control center in the rostral medullary raphé, regulating outflow to both brown adipose tissue (BAT) and to the acral cutaneous beds. Neurons in the medullary raphé, via their descending axonal projections, increase the discharge of spinal sympathetic preganglionic neurons controlling the cutaneous vasculature, utilizing glutamate, and serotonin as neurotransmitters. Present evidence suggests that both thermoregulatory and cognitive-emotional control of the cutaneous beds from preoptic, hypothalamic, and forebrain centers is channeled via the medullary raphé. Future studies will no doubt further unravel the details of neurotransmitter pathways connecting these rostral control centers with the medullary raphé, and those operative within the raphé itself. © 2016 American Physiological Society. Compr Physiol 6:1161-1197, 2016.

Direct animal calorimetry, the underused gold standard for quantifying the fire of life

Direct animal calorimetry, the gold standard method for quantifying animal heat production (HP), has been largely supplanted by respirometric indirect calorimetry owing to the relative ease and ready commercial availability of the latter technique. Direct calorimetry, however, can accurately quantify HP and thus metabolic rate (MR) in both metabolically normal and abnormal states, whereas respirometric indirect calorimetry relies on important assumptions that apparently have never been tested in animals with genetic or pharmacologically-induced alterations that dysregulate metabolic fuel partitioning and storage so as to promote obesity and/or diabetes. Contemporary obesity and diabetes research relies heavily on metabolically abnormal animals. Recent data implicating individual and group variation in the gut microbiome in obesity and diabetes raise important questions about transforming aerobic gas exchange into HP because 99% of gut bacteria are anaerobic and they outnumber eukaryotic cells in the body by ∼10-fold. Recent credible work in non-standard laboratory animals documents substantial errors in respirometry-based estimates of HP. Accordingly, it seems obvious that new research employing simultaneous direct and indirect calorimetry (total calorimetry) will be essential to validate respirometric MR phenotyping in existing and future pharmacological and genetic models of obesity and diabetes. We also detail the use of total calorimetry with simultaneous core temperature assessment as a model for studying homeostatic control in a variety of experimental situations, including acute and chronic drug administration. Finally, we offer some tips on performing direct calorimetry, both singly and in combination with indirect calorimetry and core temperature assessment.

The role of monoamines in the changes in body temperature induced by 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) and its derivatives

Hyperthermia is probably the most widely known acute adverse event that can follow ingestion of 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) by recreational users. The effect of MDMA on body temperature is complex because the drug has actions on all three major monoamine neurotransmitters [5-hydroxytryptamine (5-HT), dopamine and noradrenaline], both by amine release and by direct receptor activation. Hyperthermia and hypothermia can be induced in laboratory animals by MDMA, depending on the ambient temperature, and involve both central thermoregulation and peripheral changes in blood flow and thermogenesis. Acute 5-HT release is not directly responsible for hyperthermia, but 5-HT receptors are involved in modulating the hyperthermic response. Impairing 5-HT function with a neurotoxic dose of MDMA or p-chlorophenylalanine alters the subsequent MDMA-induced hyperthermic response. MDMA also releases dopamine, and evidence suggests that this transmitter is involved in both the hyperthermic and hypothermic effects of MDMA in rats. The noradrenergic system is also involved in the hyperthermic response to MDMA. MDMA activates central alpha(2A)-adrenoceptors and peripheral alpha(1)-adrenoceptors to produce cutaneous vasoconstriction to restrict heat loss, and beta(3)-adrenoceptors in brown adipose tissue to increase heat generation. The hyperthermia occurring in recreational users of MDMA can be fatal, but data reviewed here indicate that it is unlikely that any single pharmaceutical agent will be effective in reversing the hyperthermia, so careful body cooling remains the principal clinical approach. Crucially, educating recreational users about the potential dangers of hyperthermia and the control of ambient temperature should remain key approaches to prevent this potentially fatal problem.

Altered thermoregulation in ambulatory schizophrenia patients: a naturalistic study

BACKGROUND

Schizophrenia patients may exhibit alterations in core/body temperature. Hence, we intended to examine the potential existence of thermoregulatory abnormalities in ambulatory schizophrenia patients.

METHODS

Anonymous electronic patient record data of the Leumit Health Fund (Israel) were screened for all schizophrenia patients who have no other apparent chronic co-morbidity (mental or non-mental) and had their oral temperature assessed during routine follow-ups (Schiz-rFUs) or for various transitory infectious/inflammatory processes (Schiz-Infect) during the years 1999-2005 (n = 535). The comparison group consisted of a comparable sample (n = 560) of healthy subjects (Control-rFUs and Control-Infect).

RESULTS

The sub-group of Schiz-rFUs (n = 216) exhibited significantly lower mean oral temperature compared to the matched group of Control-rFUs (n = 140) (36.72 +/- 0.54 vs. 36.94 +/- 0.64C, respectively; P<0.05). There was no significant difference in mean oral temperatures between the Schiz-Infect (n = 319) and the Control-Infect (n = 420) (37.32 +/- 0.92 vs. 37.28 +/- 0.98C, respectively; NS).

CONCLUSIONS

Ambulatory schizophrenia patients without a concomitant infectious/inflammatory process exhibit altered thermoregulation manifested by a substantial (about 0.2 C) and significantly lower oral temperature compared to healthy comparison subjects as well as a potential exaggerated increase in oral temperature during transitory infectious/inflammatory processes. The relevance of these phenomena to the pathophysiology of schizophrenia as well as the potential immune-mediated pathologies in schizophrenia merit further investigation.

In vivo occipital-frontal temperature-gradient in schizophrenia patients and its possible association with psychopathology: a magnetic resonance spectroscopy study

BACKGROUND

Accumulating data suggest that schizophrenia patients' mental status might be modulated by their core/brain temperature. Hence, we intended to assess in vivo brain temperature (Tb) of schizophrenia patients vs. healthy subjects and to evaluate its potential association with patients' mental status.

METHODS

Absolute values of Tb were measured in 9 neuroleptic-treated schizophrenia patients and 10 healthy comparison subjects using 1H magnetic resonance spectroscopy (MRS). Values were extracted by measuring the chemical shift between the peaks of water and N-acetyl-aspartate in the 1H MRS spectra.

RESULTS

A substantial (about 1.1 degrees C) and significantly higher occipital-frontal temperature-gradient was found in the schizophrenia patients compared to the healthy controls (1.27 degrees C vs. 0.18 degrees C; p=0.032). Furthermore, a trend was found between the above mentioned occipital-frontal temperature-gradient in the schizophrenia patients and the severity of their psychopathology, as assessed by the total Positive and Negative Syndrome Scale (PANSS) scores (r=0.61; p=0.08).

CONCLUSIONS

Our findings corroborate previous results indicating putative correlation between core/brain temperature and the mental status of schizophrenia patients, emphasizing the possible role of within patients decreased frontal temperature and a significant occipital-frontal temperature-gradient as modulators of psychopathology. In addition, the MRS technique used for brain temperature assessment seems to be a potential non-invasive method to assess in vivo absolute Tb in schizophrenia.

Effect of chronic cold exposure on noradrenergic modulation in the preoptic area of thermoregulation in freely moving rats

For this study, we compared the thermoregulatory involvement of noradrenaline (NA) in the medial preoptic area (mPOA) of non-cold acclimated rats to that of cold-acclimated rats. We quantified the release of NA in the mPOA during 3 h cold (5 degrees C) exposure in room-temperature-acclimated rats (RA group, kept at 23 degrees C for 2 weeks) and cold-acclimated rats (CA group, kept at 5 degrees C for 2 weeks). We concurrently monitored the core body temperature (Tc), heart rate (HR), and tail skin temperature (Tt). Cold exposure significantly increased Tc and HR, and decreased Tt in both groups. However, the cold-induced increase of the extracellular NA levels in mPOA was observed only in the RA group: not in the CA group. To elucidate these different results in NA levels further, and to evaluate participation of the mPOA in thermoregulation in the cold, we measured Tc, HR, and Tt during perfusion of alpha-adrenoceptor antagonist phenoxybenzamine during cold exposure (5 degrees C). This pharmacological procedure induced marked hypothermia, with decreases in HR only in the RA group; no changes were observed in Tc or any thermoregulatory parameter in the CA group. These results suggest that NA in the mPOA modulates heat production in response to acute cold stress in the RA group. However, this thermoregulatory action of NA in the mPOA was attenuated in the CA group.

Baseline temperature in an animal model of schizophrenia: long-term effects of perinatal phencyclidine administration

Phencyclidine (PCP), a dissociative anaesthetic, acts as a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist. PCP is a psychostimulant capable of producing both positive and negative symptoms of schizophrenia, including cognitive dysfunction in normal humans. Perinatal phencyclidine administration to rats has been widely accepted as an animal model of schizophrenia. It has been known for a long time that schizophrenia patients may develop various thermoregulatory disturbances. The aim of this study was to assess the acute effects of phencyclidine administration on the temperature of newborn rats, the long-term effects on the baseline temperature of perinatal phencyclidine administration and the effects of a PCP challenge on the temperature of adult perinatally treated rats. The animals were treated on the 2nd, 6th, 9th and 12th postnatal (PN) days with either phencyclidine (10 mg/kg) or saline. The interscapular skin temperature was measured during the first 40 postnatal days and subsequently the colonic temperature until PN day 62. The immediate effect of phencyclidine administration to pups was a significant decrease of the body temperature, while the application of PCP to adult rats perinatally treated with either saline or PCP caused a significant increase of the baseline temperature. Perinatal phencyclidine administration to rat pups produced a long lasting effect on the baseline temperature. It can be concluded that the nature of the response to acute phencyclidine administration differs between newborn and adult rats. Further experiments are necessary in order to clarify the role of specific neurotransmitter systems in the changes of temperature regulation provoked by phencyclidine administration.

Studies on the effect of MDMA ('ecstasy') on the body temperature of rats housed at different ambient room temperatures

3,4-Methylenedioxymethamphetamine (MDMA, 'ecstasy') administration to rats produces hyperthermia if they are housed in normal or warm ambient room temperature (Ta) conditions (>or=20 degrees C), but hypothermia when in cool conditions (Ta<or=17 degrees C). We have now investigated some of the mechanisms involved. MDMA (5 mg kg(-1) i.p.) produced a rapid decrease in rectal temperature in rats at Ta 15 degrees C. This response was blocked by pretreatment with the dopamine D2 receptor antagonist remoxipride (10 mg kg(-1) i.p.), but unaltered by pretreatment with the D1 antagonist SCH23390 (1.1 mg kg(-1) i.p). MDMA (5 mg kg(-1)) did not alter the tail temperature of rats at Ta 15 degrees C, but decreased the tail temperature of rats at Ta 30 degrees C. A neurotoxic dose of MDMA (three doses of 5 mg kg(-1) given 3 h apart) decreased cortical and hippocampal 5-HT content by approximately 30% 7 days later. This lesion did not influence the rise in tail temperature when rats were moved from Ta 20 degrees C to 30 degrees C compared to nonlesioned controls, but did result in a lower tail temperature than that of controls when they were returned to Ta 24 degrees C. Acute administration of MDMA (5 mg kg(-1)) to MDMA-lesioned rats produced a sustained decrease in tail temperature in rats housed at Ta 30 degrees C compared to nonlesioned controls. These data suggest that the thermoregulatory problems previously observed in MDMA-lesioned rats housed at Ta 30 degrees C result, partially, from their inability to lose heat by vasodilation of the tail, a major heat-loss organ in this species.

A Pharmacodynamic Turnover Model Capturing Asymmetric Circadian Baselines of Body Temperature, Heart Rate and Blood Pressure in Rats: Challenges in Terms of Tolerance and Animal-handling Effects

This study presents development and behaviour of a feedback turnover model that mimics asymmetric circadian oscillations of body temperature, blood pressure and heart rate in rats. The study also includes an application to drug-induced hypothermia, tolerance and handling effects. Data were collected inn normotensive Sprague-Dawley rats, housed at 25 degrees C with a 12:12 hr light dark cycle (light on at 06:00 am) and with free access of food and water. The model consisted of two intertwined parallel compartments which captured a free-running rhythm with a period close to but not exactly 24 hrs. The free-running rhythm was synchronised to exactly 24 hrs by the environmental timekeeper (12:12 hr light on/off cycle) in experimental settings. The baseline model was fitted to a standardised 24-hr period derived from mean data of six animals over a period of nine consecutive days. The first-order rate constants related to the turnover of the baseline temperature, alpha and beta, were 0.026 min(-1) (+/-5%) and 0.0037 min(-1) (+/-3%). The alpha and beta parameters are approximately 2/transition time between day and night and 2/night time, respectively. The day:night timekeeper g(t), reference point T(ref) and amplitude were 0.053(+/-2%), 37.3(+/-0.02%) and 3.3% (+/-2%), respectively. Simulations with the baseline model revealed stable oscillations (free-running rhythm) in the absence of the timekeeper. This temperature-time profile was then symmetric and had a smaller amplitude, with a slightly shorter period and less pronounced temperature shift as compared to the profile in the presence of an external Timekeeper. Fitting the model to 96 hr mean profiles of blood pressure and heart rate from 10 control animals demonstrated the usefulness of the model. Simulations of the integrated temperature model succeeded in mimicking other modes of administration such as oral dosing.

Inhibition of the preoptic area and anterior hypothalamus by tetrodotoxin alters thermoregulatory functions in exercising rats

We have previously demonstrated a functional role of the preoptic area and anterior hypothalamus (PO/AH) in thermoregulation in freely moving rats at various temperature conditions by using microdialysis and biotelemetry methods. In the present study, we perfused tetrodotoxin (TTX) solution into the PO/AH to investigate whether this manipulation can modify thermoregulation in exercising rats. Male Wistar rats were trained for 3 wk by treadmill running. Body core temperature (Tb), heart rate (HR), and tail skin temperature (Ttail) were measured. Rats ran for 120 min at speed of 10 m/min, with TTX (5 μM) perfused into the left PO/AH during the last 60 min of exercise through a microdialysis probe (control, n = 12; TTX, n = 12). Tb, HR, and Ttail increased during the first 20 min of exercise. Thereafter, Tb, HR, and Ttail were stable in both groups. Perfusion of TTX into the PO/AH evoked an additional rise in Tb (control: 38.2 ± 0.1°C, TTX: 39.3 ± 0.2°C; P < 0.001) with a significant decrease in Ttail (control: 31.2 ± 0.5°C, TTX: 28.3 ± 0.7°C; P < 0.01) and a significant increase in HR (control: 425.2 ± 12 beats/min, TTX: 502.1 ± 13 beats/min; P < 0.01). These results suggest that the TTX-induced hyperthermia was the result of both an impairment of heat loss and an elevation of heat production during exercise. We therefore propose the PO/AH as an important thermoregulatory site in the brain during exercise.

Morphine and d-amphetamine nullify each others' hypothermic effects in mice

We have examined the effects of the psychostimulant d-amphetamine and the neuroleptic haloperidol on hypothermia induced by intraperitoneal injection of the centrally penetrating opioids morphine, fentanyl and sufentanil and the peripherally acting opioid loperamide. Measuring rectal body temperatures, dose-response relationships were established for all compounds. Morphine and sufentanil produced hyperthermia at low doses and dose-related hypothermia at higher doses. Fentanyl and loperamide produced dose-related hypothermia. Fixed doses of each opioid producing significant hypothermia were selected for interaction studies. The psychostimulant d-amphetamine was found t o produce biphasic effects with low doses inducing hypothermia and higher doses inducing hyperthemia. Haloperidol produced dose-related hypothermia. The selected doses of the opioids were then injected followed after 15 min. by injection of hypothermia producing doses of d-amphetamine or haloperidol. Hypothermia induced by morphine, fentanyl and sufentanil was reversed by d-amphetamine whereas loperamide-induced hypothermia was unaffected. Rebound hyperthermia was also measured with fentanyl and sufentanil. Haloperidol increased the hypothermic effects of morphine, fentanyl and sufentanil but not of loperamide. In conclusion, the central stimulating effects of opioids and amphetamine may combine resulting in thermogenesis and reversal of hypothermia. Central mechanisms of opioid-induced hypothermia in mice are influenced by drugs which alter the dopamine system, whereas peripheral mechanisms are unaffected. A possible clinical implication for this dopaminergic interaction may be toxicity associated with hyperpyrexia caused by psychostimulant misuse, which is increasingly occurring concomitantly with abuse of opioids.

A competitive interaction model predicts the effect of WAY-100,635 on the time course of R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin-induced hypothermia

The objective of this investigation was to characterize quantitatively the pharmacodynamic interaction between N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexanecarboxamide (WAY-100,635) and R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin (R-8-OH-DPAT) in vivo. The 8-OH-DPAT-induced change in body temperature was used as a pharmacodynamic endpoint. Four groups of rats each received 1 mg/kg 8-OH-DPAT in 5 min during computer-controlled infusions of physiological saline or WAY-100,635, targeted at steady-state concentrations of 20, 85, and 170 ng/ml. Body temperature was monitored continuously with a telemetric system, and frequent blood samples were obtained to determine the pharmacokinetics of both drugs. Large differences in pharmacokinetics were observed between WAY-100,635 and R-8-OH-DPAT, reflected in values of the terminal elimination half-life of 33 and 143 min, respectively. Infusion of WAY-100,635 had no influence on the pharmacokinetics of R-8-OH-DPAT. With regard to the pharmacodynamics, clear antagonism of the R-8-OH-DPAT-induced hypothermia was observed. The complex pharmacological effect versus time profiles of R-8-OH-DPAT were analyzed on the basis of an indirect physiological response model with set point control coupled to a competitive interaction model for an agonist and antagonist acting at a common receptor. This model converged, yielding precise estimates of the pharmacodynamic parameters of both WAY-100,635 and R-8-OH-DPAT, which were independent of the infusion rate of WAY-100,635. The estimated in vivo binding constant of WAY-100,635 was 0.98 ng/ml (2.3 nM), which is very similar to the reported value from in vitro receptor binding assays. The findings of this investigation show that, in contrast to earlier reports in the literature, WAY 100,635 behaves as a pure competitive antagonist at the 5-hydroxytryptamine(1A) receptor in vivo.

A set-point model with oscillatory behavior predicts the time course of 8-OH-DPAT-induced hypothermia

Agonists for the 5-hydroxytryptamine (HT)(1A) receptor induce a hypothermic response that is believed to occur by lowering of the body's set-point temperature. We have developed a physiological model that can be used to predict the complex time course of the hypothermic response after administration of 5-HT(1A) agonists to rats. In the model, 5-HT(1A) agonists exert their effect by changing heat loss through a control mechanism with a thermostat signal that is proportional to the difference between measured and set-point temperature. Agonists exert their effect in a direct concentration-dependent manner, with saturation occurring at higher concentrations. On the basis of simulations, it is shown that, depending on the concentration and the intrinsic efficacy of a 5-HT(1A) agonist, the model shows oscillatory behavior. The model was successfully applied to characterize the complex hypothermic response profiles after administration of the reference 5-HT(1A) agonists R-8-hydroxy-2-(di-n-propylamino)tetralin (R-8-OH-DPAT) and S-8-OH-DPAT. This analysis revealed that the observed difference in effect vs. time profile for these two reference agonists could be explained by a difference in in vivo intrinsic efficacy.

Abnormal thermoregulation in drug-free male schizophrenia patients

Schizophrenia patients may develop various thermoregulatory disturbances. We hypothesized that a standardized exercise-heat tolerance test [two 50-min bouts of walking a motor-driven treadmill at 40 degrees C (relative humidity=40%)] would reveal abnormal thermoregulation in drug-free schizophrenia patients. Six drug-free schizophrenia outpatients and seven healthy comparison subjects participated in this study. The schizophrenia patients exhibited significantly higher baseline and exertion-related rectal temperature. The relevance of these findings to the pathophysiology of schizophrenia-related thermoregulatory disorders is as yet unclear.

Changes in monoaminergic activity in the anterior, medium and posterior hypothalamus, gonadotropins levels and ovarian hormones during puberty of the female rat

The aim of present study is the analysis of monoamines concentrations changes in the anterior, medium and posterior hypothalamus, as well as changes in serum gonadotropins levels, ovarian steroids and follicular growth during the prepubertal development of the female rat. Noradrenergic activity in the anterior, medium and posterior hypothalamus reached highest level at day 13 after birth, followed by a subsequent decrease from day 15 to 19 and an increase on days 22 and 27 postnatal. At day 1, neural activity in the medium hypothalamus was higher than the activity in the anterior and posterior hypothalamus. Serotoninergic activity in three portions of the hypothalamus was higher throughout the prepubertal development. Follicle-stimulating hormone and luteinizing hormone serum levels increased between days 11 and 17 and decreased from day 19 to 36. The concentration of 17beta-estradiol was consistently low throughout the prepubertal development and increased at day 39 after birth. These results indicate that during the prepubertal development of the rat, the three regions of the hypothalamus show significant changes in the monoaminergic neural activity. There is an inverse relationship between the noradrenergic activity on the anterior and medium hypothalamus and serotoninergic activity in the posterior hypothalamus with ovarian steroids during sexual maturation. These changes may be linked to the development of the neuroendocrine processes that modulate gonadotropin secretion and ovarian function.

Influence of bombesin on neuronal hypothalamic thermosensitivity during the early postnatal period in the Muscovy duck (Cairina moschata)

The influence of bombesin (1 microg/0.1 ml artificial cerebrospinal fluid) on neuronal thermosensitivity of the preoptic area of the anterior hypothalamus in brain slices of 5- (n = 7 neurons) and 10-day-old (n = 36 neurons) Muscovy ducks (Cairina moschata) was investigated. Similar to adult mammals, most of the neurons investigated increased the firing rate (FR) after bombesin application. Changes in FR were not related to changes in thermal coefficient (TC). The neurons react to bombesin also under synaptic blockade. The bombesin-induced effect on TC (increase or decrease in nearly the same number of neurons, e.g. nine neurons increased and ten decreased TC in 10-day-old ducklings) in the postnatal bird neurons investigated was different from the results described in adult mammals, where the main reaction to bombesin was an increase of TC in warm-sensitive and temperature-insensitive-neurons and a transformation of temperature-insensitive-neurons into warm-sensitive ones. This may be related to the assumption that during early ontogeny, body functions react to exogenous and endogenous factors nonspecifically. It is to speculate, that later, probably at the end of embryonic development or during the early postnatal period, the reactivity of these functions changes qualitatively, so that the reaction of an individual function to different factors becomes specific (ultimately adaptive).

Sympathoadrenal hyperactivity and the etiology of neuroleptic malignant syndrome

OBJECTIVE

The author's goal was to develop a pathophysiological model for neuroleptic malignant syndrome with greater explanatory power than the alternative hypotheses of hypothalamic dopamine antagonism (elevated set point) and direct myotoxicity (malignant hyperthermia variant).

METHOD

Published clinical findings on neuroleptic malignant syndrome were integrated with data from human and animal studies of muscle physiology, thermoregulation, and autonomic nervous system function.

RESULTS

The data show that the sympathetic nervous system's latent capacity for autonomous activity is expressed when tonic inhibitory inputs from higher central nervous system centers are disrupted. These tonic inhibitory inputs are relayed to preganglionic sympathetic neurons by way of dopaminergic hypothalamospinal tracts. The sympathetic nervous system mediates hypothalamic coordination of thermoregulatory activity and is a primary regulator of muscle tone and thermogenesis, augmenting both of these when stimulated. In addition, the sympathetic nervous system modulates all of the other end-organs that function abnormally in neuroleptic malignant syndrome.

CONCLUSIONS

There is substantial evidence to support the hypothesis that dysregulated sympathetic nervous system hyperactivity is responsible for most, if not all, features of neuroleptic malignant syndrome. A predisposition to more extreme sympathetic nervous system activation and/or dysfunction in response to emotional or psychological stress may constitute a trait vulnerability for neuroleptic malignant syndrome, which, when coupled with state variables such as acute psychic distress or dopamine receptor antagonism, produces the clinical syndrome of neuroleptic malignant syndrome. This hypothesis provides a more comprehensive explanation for existing clinical data than do the current alternatives.

Biogenic amines and thermoregulatory changes

This review recapitulates the general principles of the organization of the thermoregulatory system, describes the thermoregulatory reactions of small and large mammals to hot environment and analyzes the probable roles of biogenic amines in these responses. Catecholamines found in peripheral blood plasma or excreted in urine represent a spillover of mediators released partly from sympathetic nerve endings and partly from the adrenal medulla. Since the thermoregulatory efforts differ between small and large mammals in cold and hot environments, the peripheral release of catecholamines is also different in these animals. The levels of these signal substances in the blood, as well as their peripheral metabolic and functional effects, serve as feedback signals for the hypothalamic integrative circuitry. The roles of antagonistic modulatory monoaminergic systems, ascending from the lower brain stem to the hypothalamus, in these integrations were discussed only partly, because this was a topic of another recent review (Zeisberger and Roth, 1996).

Hypothermic effects of neuropeptide FF analogues in mice

The effects of neuropeptide FF (NPFF) and its analogues on mouse body temperature were examined. In a thermoneutral environment, administration of NPFF (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2), 1DMe ([D.Tyr1, (N.Me)Phe3] NPFF), and 3D ([D.Tyr1, D.Leu2, D.Phe3] NPFF) in the third ventricle produced marked hypothermia. The effect of 1DMe was dose-dependent, and 45 nmol decreased body temperature by 5.6 degrees C. This effect was more pronounced when mice were placed at 4 degrees C. Hypothermia was not reversed by naloxone, an opioid antagonist, and was not modified by morphine. After 5 days of chronic treatment with 1DMe, mice did not became tolerant to the hypothermic effect. These results indicate that central NPFF receptors may control body temperature independently from opioid functions.

Anthropometric status and resting metabolic rate in users of the areca nut and smokers of tobacco in rural Sarawak

The areca nut is chewed by many of the world's population, mainly in South and Southeast Asia. Anthropometric data for 458 Sarawaki adults aged over 24 years, measured both in 1990 and in 1996, were examined in relation to use of tobacco and areca nut. Compared to non-smokers, smoking men were significantly taller and slightly (not significantly) thinner in both years, while smoking women were thinner in 1990 and slightly (not significantly) thinner in 1996. In both sexes there was an increase in the mean and range of body mass index (BMI, W/H2) over the 6-year interval. Smoking women showed a significantly smaller increment in BMI after allowing for areca nut use, which was associated with a similar trend, and this finding depended on including areca use in the model. The trend for men was similar. Possible effects of areca use could reflect variation in 'affluence' or conservatism, or appetite suppression. However, resting metabolic rate in 54 men and 70 women aged 24-60 years was associated with areca use. This association appeared to be mediated by the maximum room temperature of the 24 h preceding measurement. In women, a significant curvilinear association of RMR with maximum temperature was found in users of areca nut but not in non-users. In men, RMR was 7% higher (p < 0.05) in users of areca nut than in non-users, after allowing for age, height, weight, the sum of four skinfold thicknesses, and haemoglobin, but the association with maximum temperature was similar in both groups. It is speculated that constituents of areca nut modulate thermoregulatory pathways, resulting in prolonged temperature-dependent and hyperthermic heat production in this population; that males are more responsive to this effect than females; and that by this mechanism, and possibly also through centrally mediated effects on appetite for food, areca use could contribute to long-term variation in energy balance represented by change in BMI.

Heat intolerance induced by antidepressants

A case in which prescription medications induced heat intolerance which led to heat stroke is presented. A subject who suffered from depression and was treated with fluoxetine HCL (prozac) and lithium carbonate was engaged in mild intermittent work for 4 hours under hot/dry climatic conditions (Ta = 37 degrees C, rh = 15%). The subject lost consciousness, was hyperthermic and suffered from disseminated intravascular coagulation. A year later residual cerebellar symptoms were still evident and severe atrophy of the cerebellar tissue was demonstrated in a CT scan. It is suggested that drug-induced heat intolerance was the predisposing factor that reduced the patient ability to sustain exercise-heat stress, and under the favorable environmental circumstances led to excessive heat accumulation which ultimately caused heat stroke. This is the first description, to our knowledge, of heat intolerance of a patient treated by a combination of fluoxetine and lithium carbonate.

Influences of melatonin on human circadian rhythms

Administration of melatonin is useful in the treatment of desynchronized conditions. The mechanisms through which melatonin exerts its effect are not completely clear. Melatonin exerts direct effects on several biological functions, such as the regulation of body temperature, but there is no proof that these actions are important in the indirect regulation of main pacemaker activity. By contrast, it is very likely that melatonin exerts direct effects on circadian clocks, and that depending on the time of its administration/presence, it antagonizes or promotes the phase-shifting effects exerted by light. It is possible that melatonin regulates its own secretion and that its prolonged or shortened secretion in the period of the night-day transition is responsible for the lengthening or shortening, respectively, of the nocturnal melatonin rise. This possibility that needs to be confirmed by extensive studies may represent a physiological mechanism through which photoperiodic information is more rapidly and efficiently transformed by melatonin in a circadian signal to all the body.

Thermoregulatory dysfunction in neuroleptic malignant syndrome

Hyperthermia is the central feature of neuroleptic malignant syndrome (NMS), but its etiology remains elusive. Two competing hypotheses implicate either hypothalamic dysfunction (inappropriate "set point") or direct myotoxicity (excessive peripheral heat production). These two models have distinct implications for thermoregulatory activity in NMS. The first predicts that the individual should respond as to a hypothermic threat or infection (the hypothalamus signals the body to raise its temperature). The second implies that an excessive heat load is perceived by the hypothalamus, which responds to this hyperthermic threat (it signals the body to lower its temperature). To assess the validity of these two hypotheses the thermoregulatory responses of a series of NMS patients (36 patients, 46 episodes) were examined using standard statistical methods. In contrast to normal mammalian thermoregulatory behavior, thermoeffector responses were not organized into either mode, but appeared to function somewhat independently and paradoxically. We conclude that neither hypothesis is sufficient to explain altered thermoregulation in NMS, and that the loss of integrated thermoeffector activity may be unique to this disorder.

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.

alpha 2-, alpha 2A-, alpha 2B/2C-Adrenoceptor subtype antagonists prevent lipopolysaccharide-induced fever response in rabbits

Exogenous pyrogens, e.g., bacterial lipopolysaccharides (LPS), are thought to stimulate macrophages to release endogenous pyrogens, e.g., TNF alpha, IL-1 beta, and IL-6, which act in the hypothalamus to produce fever. We studied the effect of different alpha 1- and alpha 2-adrenoceptor subtype antagonists, applied intraperitoneally, on the febrile response induced by LPS in rabbits. Evidence was obtained that prazosin, an alpha 1- and alpha 2B/2C-adrenoceptor antagonist; WB-4101, an alpha 1- and alpha 2A-adrenoceptor antagonist; CH-38083, a highly selective alpha 2-adrenoceptor antagonist (alpha 2:alpha 1 > 2000); BRL-44408, an alpha 2A-adrenoceptor antagonist; and ARC-239, an alpha 2B/2C- and also alpha 1-adrenoceptor antagonist, blocked the increase of colonic temperature of the rabbit produced by 2 micrograms/kg LPS administered intravenously without being able in themselves to affect colonic temperature. In addition, prazosin, WB-4101 and CH-38083 antagonized the fall in skin temperature that occurred at the time when the colonic temperature was rising in control animals injected with LPS. All these results suggest that norepinephrine, through stimulation of both alpha 1- and alpha 2- (alpha 2A- and alpha 2B/2C-) adrenoceptor subtypes, is involved in producing fever in response to bacterial LPS.

Higher environmental temperature-induced increase of body temperature: involvement of central opioidergic-GABAergic interaction

Exposure (2 h) of male albino rats to higher environmental temperature (HET, 40 degrees C) significantly increased the body temperature (BT). Administration of bicuculline (1 mg/kg, i.p.), physostigmine (0.2 mg/kg, i.p.), or their combination significantly raised the BT of normal rats (kept at 28 degrees C) or of HET-exposed rats. Atropine (5 mg/kg, i.p.) abolished the hyperthermic effect of bicuculline in normal and HET-exposed rats. The BT of normal and HET-exposed rat was increased with morphine (1 mg/kg, i.p.) and was reduced with naloxone (1 mg/kg, i.p.). Bicuculline or physostigmine-induced rise in BT of HET-exposed rats was potentiated following cotreatment of physostigmine with morphine. Atropine-induced hypothermia was abolished due to the cotreatment of atropine with morphine with physostigmine but was attenuated with atropine. In normal rats (kept at 28 degrees C), only atropine attenuated (morphine + bicuculline)-induced hyperthermia. L-Dopa + carbidopa or haloperidol did not significantly affect the BT of rats under similar conditions. These results suggest that short-term (2 h) exposure to HET activates the opioidergic neuron, which activates cholinergic activity through the inhibition of GABAergic system and, thus, enhances the BT.

Prostaglandins and hypothalamic neurotransmitter receptors involved in hyperthermia: a critical evaluation

The role of a prostaglandin of the E series (PGE) in the hypothalamic mechanisms underlying a fever continues to be controversial. This paper reviews the historical literature and current findings on the central action of the PGEs on body temperature (Tb). New experiments were undertaken to examine the local effect of muscarinic, nicotinic, serotonergic, alpha-adrenergic, or beta-adrenergic receptor antagonists at hypothalamic sites where PGE1 caused a rise in Tb of the primate. Guide tubes for microinjection were implanted stereotaxically above sites in and around the anterior hypothalamic, preoptic area (AH/POA) of male Macaque monkeys. Following postoperative recovery, 30-100 ng of PGE1 was micro-injected unilaterally in a volume of 1.0-1.5 microliter at sites in the AH/POA to evoke a rise in Tb, and once identified, pretreated with a receptor antagonist. PGE1 hyperthermia was significantly reduced by microinjections of the muscarinic and nicotinic antagonists, atropine, or mecamylamine, at PGE1 reactive sites in the AH/POA. The serotonergic antagonist, methysergide, injected at PGE1 sensitive sites in the ventromedial hypothalamus also attenuated the rise in Tb. However, the 5-HT reuptake blocker, fluoxetine, and the beta-adrenergic receptor antagonist, propranolol, injected in the AH/POA failed to alter the PGE1 hyperthermia. In contrast, the alpha-adrenergic antagonist, phentolamine, potentiated the increase in Tb at all PGE1 reactive sites in the hypothalamus. An updated model is presented to explain how the concurrent actions of aminergic neurotransmitters acting on their respective receptors in the hypothalamus can interact with a PGE to elicit hyperthermia. Finally, an evaluation of the current literature including recent findings on macrophage inflammatory protein (MIP-1) supports the conclusion that a PGE in the brain is neither an obligatory nor essential factor for the expression of a pyrogen fever.

Zucker obese rats: defect in brain histamine control of feeding

Manipulation of hypothalamic histamine produced different effects on feeding between the Zucker obese (fa/fa) and their lean littermate rats (Fa/-). Infusion of a histamine H1-receptor antagonist into the third cerebroventricle elicited feeding in the lean and Wistar King A rats, but it did not affect feeding in the obese rats. To enhance hypothalamic neuronal histamine, thioperamide, and H3-receptor antagonist, was similarly infused. The lean and Wistar rats decreased their food intake after the infusion, but thioperamide produced no significant effect on feeding in the obese rats. Infusion of histamine into the third cerebroventricle mimicked the effects of thioperamide on feeding: reduction of food intake in the lean and Wistar rats, but no significant change in the obese rats. Hypothalamic histamine of the obese rats (0.430 nmol/g) was significantly lower than the lean (1.209 nmol/g) and Wistar rats (4.838 nmol/g). The histamine concentration of the cerebral cortex in the obese rats was also lower than the non-obese animals. The results indicate that the feeding abnormality of Zucker obese rats may be at least due to disturbance of histamine suppressive signals both at presynaptic and postsynaptic levels.

Thermogenic, thermolytic and body temperature effects of fenfluramine, a 5-hydroxytryptamine agonist, in the domestic fowl (Gallus domesticus)

1. Intramuscular injection of the 5-HT agonist DL-fenfluramine increased the metabolic rate of mature cockerels by about 25% over the following 22 hr. The acute effect, over the 3 hr following injection, attained 40% at 10 degrees C, 35% at 20 degrees C and 25% at 32 degrees C. 2. Heat loss mechanisms (peripheral vasodilatation, postural change and panting) were also stimulated, to an extent which varied with ambient temperature. 3. Food intake, respiratory quotient and locomotor activity were significantly reduced by fenfluramine injection. 4. The opposing effects on heat production and heat loss had an influence on deep-body temperature which varied from a reduction of 0.5 degrees C at 10 degrees C ambient to an increase of about 0.5 degrees C at 32 degrees C. 5. The 5-HT blocker, methysergide, prevented the effects of fenfluramine on both heat production and heat loss for about 5 hr after injection. 6. The autonomic ganglion blocker, hexamethonium chloride, reduced the thermogenic but not the heat-loss effects of fenfluramine. 7. The results suggest that DL-fenfluramine has centrally-occurring but independent effects on heat production and a number of heat loss effectors, and that the heat production effect is mediated by the autonomic nervous system. The effect of fenfluramine on deep-body temperature appears to result from an altered equilibrium between heat production and heat loss.

Time course of induction of a heat shock gene (hsp70) in the rabbit cerebellum after LSD in vivo: involvement of drug-induced hyperthermia

In situ hybridization studies were carried out to determine whether induction of hsp70 mRNA in various cellular layers of the rabbit cerebellum was due to hyperthermic effects of the psychotropic drug LSD. Results indicated that induction was not present when LSD-induced hyperthermia was blocked. The pattern of induction of hsp70 mRNA in various cell types of the cerebellum was similar when hyperthermia was induced by either drug (LSD) or nondrug means (placement of animals in a warm incubator). A time course analysis of the induction of hsp70 mRNA following LSD-induced hyperthermia revealed maximal levels of mRNA at 1 hr in all cerebellar cell layers except the Purkinje layer where highest levels were attained at 5 hr. By 10 hr hsp70 mRNA had returned to constitutive levels in all cellular layers of the cerebellum.

Microdialysis measurement of monoamine and amino acid release from the medial preoptic region of the sheep in response to heat exposure

Concentrations of monoamines and metabolites and amino acids were measured in microdialysis samples taken from the medial preoptic area of 5 conscious sheep before, during and after exposure to an ambient temperature of 45 degrees C. Concentrations of dopamine, noradrenaline and aspartate significantly increased, and those of the serotonin metabolite, 5-hydroxyindole-3-acetic acid (5-HIAA), significantly decreased during heat exposure and although panting was induced, body temperature did not change. Concentrations of noradrenaline and aspartate declined and 5-HIAA increased to preheat exposure levels during the 60 min after the ambient temperature was reduced but levels of dopamine and its metabolite, homovanillic acid, remained elevated. Dopamine, noradrenaline, 5-HIAA and aspartate concentrations were not significantly altered by isolation stress and did not show significant changes in the cortex following heat exposure. These experiments provide further support for the proposed roles of dopamine, noradrenaline, serotonin and aspartate in the neural control of autonomic thermoregulatory responses.

Acetaminophen (paracetamol) in the management of burned children with fever

Acetaminophen (paracetamol 12 mg/kg p.o.) was administered on a total of 33 occasions to 12 children (13-36 months of age) during the first 60 h after burn injury covering 10-44 per cent of the body surface area (BSA). The drug was effective in lowering rectal temperature on most (80 per cent) occasions; failure to elicit a response was not restricted to particular patients or patterns of injury. There was no evidence of tachyphylaxis to the antipyretic effects of acetaminophen. The pattern of change in the rectal and toe temperature after acetaminophen suggested that it may be acting by either increasing heat loss or by lowering heat production, both of which are consistent with a reduction in thermoregulatory setpoint mediated by inhibition of prostaglandin synthetase.

Thermal characterization and transmitter analysis of single units in the preoptic and anterior hypothalamus of conscious ducks

With a multibarrel assembly combining one carbon fiber micropipette as recording electrode and 6 pipettes for microiontophoretic application of drugs, the activity of neurons in the preoptic and anterior hypothalamic (POAH) region was extracellularly recorded in situ in conscious ducks implanted chronically with a device permitting hypothalamic thermal stimulation. Among 355 neurons 17% were identified as warm-responsive (warm units) and 20% as cold-responsive (cold units). In 58 warm and 56 cold units control discharge rates at 40 degrees C local temperature (F40) and temperature coefficients (delta F/delta T) were determined and presented as means +/- SEM. The F40 values of warm units (35.2 +/- 2.3 Imp . s-1) were significantly higher than of cold units (16.3 +/- 1.8 Imp . s-1). The delta F/delta T values (+1.77 +/- 0.15 and -1.77 +/- 0.19 Imp . s-1 . degree C-1) of warm and cold units were not different in absolute terms. In pilot experiments either activation or inhibition by lowering whole-body temperature was observed in both warm and cold units. Microiontophoretic application of one or more of the amines acetylcholine (ACh), 5-hydroxytryptamine (5-HT), and noradrenaline (NA) to warm and cold units revealed differences in their responsiveness to ACh, which more consistently stimulated cold units. NA inhibited the majority of warm units; 5-HT stimulated the majority of cold units. In both warm and cold units NA and ACh differed in their actions, with the latter amine more consistently producing activation.

Neural mechanisms mediating the hyperthermia elicited by prostaglandin E2 injected into the preoptic-anterior hypothalamus

The neuromechanism mediating the hyperthermia induced by injection of PGE2 into the preoptic/anterior hypothalamus (PO/AH) was investigated in the rat. Pretreatment of rats with intraperitoneal injection of atropine sulfate blocked, whereas pretreatment with atropine methyl bromide had no significant effect on the hyperthermia. In a second series of experiments, atropine sulfate was microinjected into different regions of the hypothalamus and the thalamus in an attempt to locate the central cholinergic synapses involved in the PGE2-induced hyperthermia. The hyperthermia was blocked by atropine injection into the dorsal/dorsomedial hypothalamic area (DH), but was not significantly affected by injection into the PO/AH, ventromedial hypothalamus, or the thalamic area above the DH. Moreover, microinjection of the cholinergic agonist carbachol (0.5 microgram) into the DH could also elicit hyperthermia. Thus, our data suggest that the hyperthermia induced by PGE2 administration into the PO/AH is mediated by a cholinergic mechanism in the DH.

Thermosensitivity of dorsal raphe neurons in vitro

Thermosensitivity of raphe neurons was studied in tissue slices of rat brainstems (400-500 micron). Measurement of activity of single cells in the dorsal raphe region of the slices revealed that the majority of neurons (89%) were sensitive to changes in temperature. Over the range 34 to 42 degrees C, 3 classes of thermosensitive cells were found: warm (61%), cold (15%) and biphasic type cells (13%). Many dorsal raphe neurons may be intrinsically temperature sensitive and may serve as extrahypothalamic thermodetector components in the integrative process of central thermoregulation.

Adaptive changes in thermoregulation and their neuropharmacological basis

Adaptive changes of the thermoregulatory system include morphological and functional modifications. The morphological modifications such as changes in body shape and insulation need time periods of months to years to develop, unless they are genetically fixed and appear seasonally. In general, they are preceded by functional modifications, including changes in capacity of the effector systems and changes in regulatory characteristics, which need much less time to develop. These early changes in regulatory characteristics, which can be defined as deviations in threshold and gain of the thermoregulatory responses, have been described and subdivided into short-term (minutes) and long-term (weeks) modifications. Evidence for the participation of monoaminergic brain stem systems in these modifications has been reviewed. On the basis of recent insights into the organization of the thermoregulatory system, and of evaluation of experimental evidence from electrophysiological, neuropharmacological, and neuroanatomical studies it can be concluded that these systems are involved in adaptive modifications. Receiving information from several sensory systems they seem to deliver additional modulatory signals, which may interfere with the processing of specific thermal information at several sites. Theoretically, the central monoamines may participate in the control of thermal input, in the central integration of thermal signals, and in modification of output signals to thermoregulatory effectors. Best documented is their modulatory action on thermosensitive and thermointegrative hypothalamic neurons. There, the monoamines 5-hydroxytryptamine and noradrenaline act as antagonists, which enhance or diminish the effects of thermal afferents mediated by other transmitters. Moreover, the antagonistic monoaminergic systems are interconnected and can influence each other at the level of lower brain stem. The activity in central monoaminergic systems can also be modified by neurohumoral feedback mechanisms from the periphery. By means of these interrelations the vegetative responses of the organism can be corrected and optimized. These interrelations can explain also some cross-adaptive changes in the thermoregulatory threshold for shivering evoked by nonthermal factors such as food intake or long-distance running.

Modification of the circadian body temperature rhythm of the spontaneously hypertensive rat during and following cessation of continuous clonidine infusion

The effects of continuous clonidine infusion (10 micrograms/kg/h for 10 days) and the cessation of this infusion on the circadian body temperature rhythm of the spontaneously hypertensive (SH) rat were examined. This circadian rhythm was blunted significantly during the infusion of clonidine. The fall in body temperature which normally occurs at the onset of each light phase was attenuated during the clonidine infusion, and as such, these rats displayed a relative hyperthermia over the light but not the dark phases. On cessation of infusion (24:00 h), a distinct hyperthermia occurred within the immediate dark phase and the subsequent light phase. The results demonstrate that the circadian control of body temperature is disturbed both during and after continuous clonidine infusion.