Central modulators of thermoregulation

Disturbance of thermal homeostasis following dynamic exercise

Recovery from dynamic exercise results in significant perturbations of thermoregulatory control. These perturbations evoke a prolonged elevation in core body temperature and a concomitant decrease in sweating, skin blood flow, and skin temperature to pre-exercise baseline values within the early stages of recovery. Cutaneous vasodilation and sweating are critical responses necessary for effective thermoregulation during heat stress in humans. The ability to modulate the rate of heat loss through adjustments in vasomotor and sudomotor activity is a fundamental mechanism of thermoregulatory homeostasis. There is a growing body of evidence in support of a possible relationship between hemodynamic changes postexercise and heat loss responses. Specifically, nonthermoregulatory factors, such as baroreceptors, associated with hemodynamic changes, influence the regulation of core body temperature during exercise recovery. The following review will examine the etiology of the post-exercise disturbance in thermal homeostasis and evaluate possible thermal and nonthermal factors associated with a prolonged hyperthermic state following exercise.

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.

Vasopressin and sensory circumventricular organs

The subfornical organ, the area postrema and the organum vasculosum of the lamina terminalis are considered to be sensory circumventricular organs as they contain neuronal somata which are located outside the blood-brain barrier and are thus capable of serving as 'sensors' for blood-borne humoral messengers. The endocrine hormone, vasopressin (VP), not only causes strong antidiuresis by acting on the kidney, but also exerts centrally mediated effects as a neuromodulator. Several lines of evidence suggest that VP can influence regulatory functions mediated by the sensory circumventricular organs, since vasopressinergic somata and terminals as well as VP receptors have been reposted to be present in these structures. These biochemical prerequisites offer the possibility that blood-borne VP might on the one hand act as a feedback signal from the periphery and, on the other hand, synaptically released or locally produced VP could modulate the known functions of sensory circumventricular organs, such as thirst, fever or cardiovascular regulation. This review focuses on the possible physiological relevance of VP acting on sensory circumventricular organs in view of recent evidence obtained from biochemical and electrophysiological studies at the cellular level.

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).

Iron and thermoregulation: a review

Poor temperature regulation in both human and animal models has been associated with iron deficiency anemia. The review examines the evidence for causality in both animals and humans, as well as provides an overview of temperature regulation in homeotherms. A number of investigations conclude that anemia is a central component of the inability of iron-deficient individuals to temperature regulate when they are cold stressed. This is apparently due to effects both on heat production and heat loss rates. The former is related to poor thyroid function in iron deficiency and the latter to the competing demands for tissue oxygenation vs. decreased flood flow to minimize heat losses to the environment. Future research needs are outlined that require integration of endocrinology, physiology, and nutrition disciplines.

A comparison of human thermoregulatory response following dynamic exercise and warm-water immersion

We tested the hypothesis that the prolonged elevated plateau of esophageal temperature (Tes) following moderate exercise is a function of some exercise-related factors and not the increase in heat content and Tes during exercise, by comparing the response to increase Tes during exercise (endogenous heating) and warm-water immersion (exogenous heating). Nine healthy, young [24.0 (1.9) years] subjects performed two separate experiments: (1) 15 min of treadmill exercise at 70% (VO2max) and 15 min rest in a climatic chamber at 29 degrees C, followed by 15 min of immersion in a 42 degrees C water bath and a further 60 min of recovery in the climatic chamber [exercise-water (EW)]; and (2) 15 min of immersion in a 42 degrees C water bath followed by 60 min of recovery in the climatic chamber [water-only (WO)]. Esophageal (Tes) and skin (Tsk) temperatures were recorded at 5-s intervals throughout. The Tes at which the forearm to finger temperature gradient (Tfa-Tfi) abruptly decreases was used to identify the threshold for forearm cutaneous vessel dilation (Thdil) during exercise. Pre-exercise Tes values were 36.64 degrees C and 36.74 degrees C for EW and WO respectively. The EW post-exercise Tes value fell to a stable level of 37.12 degrees C and this value differed by 0.48 degree C (P < 0.05) from baseline, but was similar to Thdil (37.09 degrees C). Despite a 1.2 degrees C increase in Tes during the subsequent warm-water immersion, Tes returned to the post-exercise value (37.11 degrees C). The WO post-immersion Tes fell to a stable plateau of 36.9 degrees C, which was not statistically different from the pre-immersion Tes. The data for both warm-water treatments support the hypothesis that increases in Tes and heat content alone are not the primary mechanisms for the post-exercise elevation in Tes and Thdil. These data also support our previous observation that the exercise-induced elevation in Thdil persists into recovery.

Post-exercise thermal homeostasis as a function of changes in pre-exercise core temperature

We have previously reported that, following continuous exercise, a prolonged elevated plateau of esophageal temperature (Tes) was directly related to the Tes at the time of cutaneous vasodilation (Thdil) during exercise. In order to investigate the hypothesis that the factors which result in an increase of the post-exercise Thdil and define the post-exercise Tes elevation are related to pre-exercise Tes, nine healthy, young [24.0 (1.9) years], non-training males rested at 29 degrees C, 50% humidity for > 1 h (control). They then completed three successive cycles of 15 min treadmill running at 70% maximal oxygen consumption (VO2max) followed by 30 min rest. Esophageal, rectal (Tre) and skin (Tsk) temperatures and forearm cutaneous blood flow were recorded at 5-s intervals throughout. Laser-Doppler flowmetry of forearm skin blood flow was used to identify the Thdil during exercise. Pre-exercise Tes was 36.74 (0.25) degrees C and post-exercise Tes fell to stable and significant (P < 0.05) elevations above pre-exercise values at 37.22 (0.27) degrees C, 37.37 (0.27) degrees C and 37.48 (0.26) degrees C following each successive work bout respectively. Correspondingly, Thdil during each work bout rose in proportion to, and was not different than, the post-exercise Tes in the following recovery [37.20(0.23) degrees C, 37.41 (0.24) degrees C and 37.58 (0.24) degrees C]. Although the increases were less with each successive exercise bout, the differences between each exercise bout, in terms of post-exercise Tes and Thdil values, were significant (P < 0.05). These results reinforce our previous observations of elevations in Thdil and post-exercise Tes after a single exercise bout and lead to the tentative conclusions that (1) pre-exercise Tes has a direct influence on Thdil and post-exercise Tes, and (2) the exercise-induced increase of Thdil persists into recovery, influencing post-exercise thermal recovery.

Recurrent coma

Recurrent episodes of coma are usually associated with a metabolic disorder. A healthy 9-year-old boy of normal intellect and intact corpus callosum on neuroimaging had recurrent episodes of coma associated with profound spontaneous hypothermia. An evaluation, differential diagnosis and insights into the pathogenesis of this disorder are discussed.

Effect of various pretreatments on the hypothermic activity of repin in naive rats

Repin is a sesquiterpene lactone found in Centaurea solstitialis, a plant responsible for Parkinson-like disease in horses. Repin, on intraperitoneal (i.p.) injection, produces a dose-dependent and highly significant hypothermia in naive rats. The hypothermia is long-lasting with a peak 3 h after the injection and a return to normal temperature after more than 8 h. The effects of atropine sulfate, atropine methylbromide, propranolol, metergoline, ketanserin, diphenhydramine and apomorphine pretreatment on repin-induced hypothermia were investigated. None of the pretreatments directly antagonized repin's hypothermic effect. However, partial but significant reversals of hypothermia by atropine sulfate, metergoline, ketanserin, diphenhydramine and apomorphine were observed 2-4 h after the repin injection. These late-onset effects are probably due to secondary physiological mechanisms. On the other hand, propranolol, at 20 mg/kg i.p., clearly accentuated both the early-onset (30-90 min) and late-onset (2-4 h) hypothermic effects of repin.

Episodic spontaneous hypothermia with hyperhidrosis: implications for pathogenesis

Unprovoked hypothermia is an unusual presenting sign. When occurring with diaphoresis it has been referred to as episodic spontaneous hypothermia with hyperhidrosis. Earlier reports described episodic, spontaneous hypothermia with hyperhidrosis in patients with agenesis of the corpus callosum and postulated a midline congenital malformation of the central nervous system. Since then, various endocrine, electrolyte, autonomic, and sleep disturbances have been described but the etiology remains undetermined. Three unrelated children are reported each of whom had an intact corpus callosum and normal endocrine function. Shivering was consistently absent despite marked symptomatic hypothermia. One child had spontaneous resolution of episodic spontaneous hypothermia with hyperhidrosis and two children responded to the antiserotonergic, cyproheptadine. It is hypothesized that specific serotonergic dysfunction in the anterior hypothalamic extrapyramidal shivering mechanism is central in the pathogenesis of this condition.

Disturbance of thermal homeostasis during post-exercise hyperthermia

The response of core temperature to exercise was investigated during recovery in order to avoid the antagonistic competition between exercise and thermal reflexes for the same effector systems which control skin blood flow. Five healthy, non-training males [mean (SD) age, 23.8 (2.04) years] were habituated to 29 degrees C at relative 50% humidity for more than 2 h and then exercised by treadmill running at about 75% maximum oxygen uptake for 18 min. They then remained at 29 degrees C for up to 65 min of recovery. Oesophageal (T(es)), rectal (Tre) and skin temperatures (Tsk) were recorded at 5-s intervals throughout. The abrupt fall of temperature gradient from the forearm to finger was used to identify the T(es) for skin vessel dilatation (Tdil) during exercise. Mean (SE) Tes rose from a resting value of 36.67 (0.15) degrees C to 38.22 (0.24) degrees C, mean Tre rose from 37.09 (0.25) degrees C to 38.23 (0.15) degrees C, and Tdil occurred at 37.39 (0.32) degrees C. Within 10 min of recovery mean T(es) fell to 37.31 (0.24) degrees C, where it remained a significant 0.64 degrees C above its pre-exercise (PrEx) level (P < or = 0.018) but insignificantly different from Tdil for the remaining 55 min of recovery. Meanwhile, Tre fell gradually throughout recovery to 37.64 (0.18) degrees C. The Tsk at all non-acral sites except the thigh had recovered to PrEx levels by 20-30 min post-exercise (PoEx).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of central vasopressin administration to infant rats

The neurohypophyseal peptide hormone arginine-vasopressin functions as a neuropeptide in several brain areas in addition to its role as a posterior pituitary hormone. Several studies indicate that arginine-vasopressin and arginine-vasopressin receptors appear early in the infant rat brain. To determine if arginine-vasopressin receptors in the infant were responsive to exogenous peptides, we compared the behavioral effects of central or peripheral administration of arginine-vasopressin, arginine vasotocin, the oxytocin precursor oxytocin-Gly-Lys-Arg, and arginine-vasopressin receptor antagonists in socially isolated rat pups. Central administration of arginine-vasopressin decreased the number of rat pup ultrasonic vocalizations, reduced locomotor activity and decreased the latency to express a response to negative geotaxis. Temperature was also reduced at all doses tested. Co-administration of arginine-vasopressin and receptor antagonists suggested that changes in vocal behavior were mediated by the V1 receptor subtype. Changes in core temperature appeared to be mediated by a V2 receptor subtype. Peripheral arginine-vasopressin administration increased calling and decreased core body temperature. Neither effect was blocked by central receptor antagonist administration. The results are consistent with the hypothesis that arginine-vasopressin receptors in the infant rat brain are functional.