Thermosensitivity of neurons in the sensorimotor cortex of the cat

Regulation of Body Temperature by the Nervous System

The regulation of body temperature is one of the most critical functions of the nervous system. Here we review our current understanding of thermoregulation in mammals. We outline the molecules and cells that measure body temperature in the periphery, the neural pathways that communicate this information to the brain, and the central circuits that coordinate the homeostatic response. We also discuss some of the key unresolved issues in this field, including the following: the role of temperature sensing in the brain, the molecular identity of the warm sensor, the central representation of the labeled line for cold, and the neural substrates of thermoregulatory behavior. We suggest that approaches for molecularly defined circuit analysis will provide new insight into these topics in the near future.

Central thermoreceptors

Homeotherms maintain their core body temperature within a narrow range by employing multiple redundant mechanisms to control heat production and dissipation. Preoptic area/anterior hypothalamic (PO/AH) neurons receive thermal signals from peripheral and deep-body thermoreceptors as well as hormonal and metabolic signals. A population of PO/AH neurons termed warm-sensitive increase their firing temperature with warming and are considered central thermoreceptors. Electrophysiologic and pharmacologic experiments have provided descriptions of their characteristics and signaling mechanisms. These studies have also allowed insights into the mechanisms by which neurochemicals important in thermoregulation exert their influence. Finally, the cellular mechanism involved in the interactions between thermoregulation and other aspects of homeostasis, such as energy metabolism and osmoregulation, have started to be unraveled.

Histamine receptor signaling in energy homeostasis

Histamine modulates several aspects of energy homeostasis. By activating histamine receptors in the hypothalamus the bioamine influences thermoregulation, its circadian rhythm, energy expenditure and feeding. These actions are brought about by activation of different histamine receptors and/or the recruitment of distinct neural pathways. In this review we describe the signaling mechanisms activated by histamine in the hypothalamus, the evidence for its role in modulating energy homeostasis as well as recent advances in the understanding of the cellular and neural network mechanisms involved. This article is part of the Special Issue entitled 'Histamine Receptors'.

Modulation of body temperature and LH secretion by hypothalamic KNDy (kisspeptin, neurokinin B and dynorphin) neurons: a novel hypothesis on the mechanism of hot flushes

Despite affecting millions of individuals, the etiology of hot flushes remains unknown. Here we review the physiology of hot flushes, CNS pathways regulating heat-dissipation effectors, and effects of estrogen on thermoregulation in animal models. Based on the marked changes in hypothalamic kisspeptin, neurokinin B and dynorphin (KNDy) neurons in postmenopausal women, we hypothesize that KNDy neurons play a role in the mechanism of flushes. In the rat, KNDy neurons project to preoptic thermoregulatory areas that express the neurokinin 3 receptor (NK3R), the primary receptor for NKB. Furthermore, activation of NK₃R in the median preoptic nucleus, part of the heat-defense pathway, reduces body temperature. Finally, ablation of KNDy neurons reduces cutaneous vasodilatation and partially blocks the effects of estrogen on thermoregulation. These data suggest that arcuate KNDy neurons relay estrogen signals to preoptic structures regulating heat-dissipation effectors, supporting the hypothesis that KNDy neurons participate in the generation of flushes.

Single cell transcriptomics of hypothalamic warm sensitive neurons that control core body temperature and fever response Signaling asymmetry and an extension of chemical neuroanatomy

We report on an 'unbiased' molecular characterization of individual, adult neurons, active in a central, anterior hypothalamic neuronal circuit, by establishing cDNA libraries from each individual, electrophysiologically identified warm sensitive neuron (WSN). The cDNA libraries were analyzed by Affymetrix microarray. The presence and frequency of cDNAs were confirmed and enhanced with Illumina sequencing of each single cell cDNA library. cDNAs encoding the GABA biosynthetic enzyme Gad1 and of adrenomedullin, galanin, prodynorphin, somatostatin, and tachykinin were found in the WSNs. The functional cellular and in vivo studies on dozens of the more than 500 neurotransmitters, hormone receptors and ion channels, whose cDNA was identified and sequence confirmed, suggest little or no discrepancy between the transcriptional and functional data in WSNs; whenever agonists were available for a receptor whose cDNA was identified, a functional response was found. Sequencing single neuron libraries permitted identification of rarely expressed receptors like the insulin receptor, adiponectin receptor 2 and of receptor heterodimers; information that is lost when pooling cells leads to dilution of signals and mixing signals. Despite the common electrophysiological phenotype and uniform Gad1 expression, WSN transcriptomes show heterogeneity, suggesting strong epigenetic influence on the transcriptome. Our study suggests that it is well-worth interrogating the cDNA libraries of single neurons by sequencing and chipping.

Hypothalamic and dietary control of temperature-mediated longevity

Temperature is an important modulator of longevity and aging in both poikilotherms and homeotherm animals. In homeotherms, temperature homeostasis is regulated primarily in the preoptic area (POA) of the hypothalamus. This region receives and integrates peripheral, central and environmental signals and maintains a nearly constant core body temperature (T(core)) by regulating the autonomic and hormonal control of heat production and heat dissipation. Temperature sensitive neurons found in the POA are considered key elements of the neuronal circuitry modulating these effects. Nutrient homeostasis is also a hypothalamically regulated modulator of aging as well as one of the signals that can influence T(core) in homeotherms. Investigating the mechanisms of the regulation of nutrient and temperature homeostasis in the hypothalamus is important to understanding how these two elements of energy homeostasis influence longevity and aging as well as how aging can affect hypothalamic homeostatic mechanisms.

Central mechanisms for thermoregulation in a hot environment

Homeothermic animals regulate body temperature by autonomic and behavioral thermoeffector responses. The regulation is conducted mainly in the brain. Especially, the preoptic area (PO) in the hypothalamus plays a key role. The PO has abundant warm-sensitive neurons, sending excitatory signals to the brain regions involved in heat loss mechanisms, and inhibitory signals to those involved in heat production mechanisms. The sympathetic fibers determine tail blood flow in rats, which is an effective heat loss process. Some areas in the midbrain and medulla are involved in the control of tail blood flow. Recent study also showed that the hypothalamus is involved in heat escape behavior in rats. However, our knowledge about behavioral regulation is limited. The central mechanism for thermal comfort and discomfort, which induce various behavioral responses, should be clarified. In the heat, dehydration affects both autonomic and behavioral thermoregulation by non-thermoregulatory factors such as high Na+ concentration. The PO seems to be closely involved in these responses. The knowledge about the central mechanisms involved in thermoregulation is important to improve industrial health, e.g. preventing accidents associated with the heat or organizing more comfortable working environment.

Altered sympathetic thermoregulation in patients with hypothalamic dysfunction following resection of suprasellar tumors

In patients with suprasellar tumors, both the tumor and its surgical resection may produce hypothalamic dysfunction including thermoregulatory disturbances. We quantitated skin sympathetic nerve activity using microneurography in eight patients with hypothalamic dysfunction following resection. Skin sympathetic nerve activity, skin blood flow (by laser Doppler flowmetry), sweating, blood pressure, and pulse rate were monitored during changes of core (tympanic) temperature in the patients and seven healthy controls. Core temperature was decreased by 0.3 degrees C and increased by 0.5 degrees C relative to baseline using a cooling/heating blanket. The increase in sympathetic nerve activity in response to cooling did not differ between controls and patients (41.0+/-13.1 vs. 38.1+/-7.7 bursts [min degrees C](-1)), but an increase upon heating in controls (45.1+/-5.4 bursts [min degrees C](-1)) was absent in the patients (-26.0+/-17.5 bursts [(min degrees C](-1)). Skin blood flow during heating increased significantly in controls but not in patients (35.6+/-14.6 vs. 15.8+/-5.8 ml [min 100 g tissue degrees C](-1)). The patients thus showed impaired heat loss during body temperature elevation. Microneurography proved to be a sensitive measure of hypothalamic autonomic function.

Some historical perspectives on thermoregulation

In this paper, selected historical aspects of thermoregulation and fever are presented as background to the application of molecular biology to thermoregulation. Temperature-sensing mechanisms, coordination of thermal information, thermoregulatory circuitry, efferent responses to thermal stimuli, set point mechanisms, and some of the mechanisms and consequences of fever and hyperthermia are highlighted. Neurotransmitters used in thermoregulatory circuits are also discussed. An attempt is made to include information from comparative physiological sources. Possible future avenues of research in the light of recent new technologies are also presented.

Neuronal circuitries involved in thermoregulation

The body temperature of homeothermic animals is regulated by systems that utilize multiple behavioral and autonomic effector responses. In the last few years, new approaches have brought us new information and new ideas about neuronal interconnections in the thermoregulatory network. Studies utilizing chemical stimulation of the preoptic area revealed both heat loss and production responses are controlled by warm-sensitive neurons. These neurons send excitatory efferent signals for the heat loss and inhibitory efferent signals for the heat production. The warm-sensitive neurons are separated and work independently to control these two opposing responses. Recent electrophysiological analysis have identified some neurons sending axons directly to the spinal cord for thermoregulatory effector control. Included are midbrain reticulospinal neurons for shivering and premotor neurons in the medulla oblongata for skin vasomotor control. As for the afferent side of the thermoregulatory network, the vagus nerve is recently paid much attention, which would convey signals for peripheral infection to the brain and be responsible for the induction of fever. The vagus nerve may also participate in thermoregulation in afebrile conditions, because some substances such as cholecyctokinin and leptin activate the vagus nerve. Although the functional role for this response is still obscure, the vagus may transfer nutritional and/or metabolic signals to the brain, affecting metabolism and body temperature.

Spinal neuronal thermosensitivity in vivo and in vitro in relation to hypothalamic neuronal thermosensitivity

In the spinal cord, temperature signals are generated which serve as specific inputs in the central nervous control of body temperature. Because of the spatially distinct organization of afferent and efferent neuronal systems at the spinal level, the afferent pathway for temperature signal transmission could be identified in vivo in the ascending, anterior and lateral tracts with a relationship of about 75:25% between warm and cold sensitive neuraxons. Analysis of spinal neuronal thermosensitivity in vitro on spinal cord tissue slices has been concerned, so far, with the superficial laminae of the dorsal horn as the site of origin of ascending nerve fibers conveying mostly temperature and pain signals, and with lamina X as a site of origin of afferent as well as efferent neurons. A relationship of about 95:5% between warm and cold sensitive neurons was found at the segmental level, indicating that warm sensitivity is the prevailing, primary property of spinal neurons, whereas cold sensitivity seems to be mainly generated by synaptic interaction as a secondary modality. Dynamic responses to temperature changes were frequently displayed in vitro at the spinal segmental level in lamina I + II but not in lamina X, even by neurons whose static activity was little influenced by local temperature. Dynamic thermosensitivity was found less frequently in ascending tract neuraxons and was not observed in hypothalamic neurons receiving temperature signal inputs from the spinal cord, and thus, does not seem to be relevant for the thermosensory function of spinal cord neurons, unlike peripheral warm and cold receptors. A majority of spinal warm sensitive neurons displayed both static and dynamic warm sensitivity as an inherent property after synaptic blockade. In the further analysis of spinal cord thermosensitivity, the in vitro approach permits application of the same electrophysiological and neuropharmacological methods as were established for the analysis of hypothalamic thermosensitivity. In addition, the topography of the spinal cord will provide additional structural and possibly histochemical information to characterize the functions of neurons independently of their thermal properties.

Whole-cell properties of temperature-sensitive neurons in rat hypothalamic slices

Hypothalamic temperature-sensitive neurons have a pivotal role in body temperature regulation. To study their thermal transduction mechanism, we made nystatin-perforated patch-clamp recordings from warm-responsive (WR) and cold-responsive (CR) neurons in rat hypothalamic slices after blocking synaptic transmission in low Ca2+ high Mg2+ solution. Warming depolarized the WR neurons and increased their firing frequency, whereas the same procedure suppressed firing in CR neurons. Warming increased the voltage-gated sodium and potassium currents and the input conductance in both types of neuron. The warm-activated current in WR neurons had a reversal potential that was significantly more positive than that of CR neurons. We suggest that the different thermosensitivity of resting ionic conductances underlie the differential behaviours of WR and CR neurons.

Pitfalls in the use of brain slices

In vitro brain slices are the preparation of choice for the detailed examination of local circuit properties in mammalian brain. However it is the investigator's responsibility to verify that the circuits under investigation are indeed confined within the boundaries of the functional region of the slice used. The medium in which the slice is maintained is under the full control of the investigator. This places the burden on the investigator to ensure that: (1) the properties of the medium are fully under control; (2) the effects of the medium on the slice are known; (3) the conditions under which the slice is being maintained bear some reasonable relation to those it enjoys (or endures) in vivo. Generalizations to in vivo conditions must be made with caution. If at all possible, similar studies (perhaps less extensive, due to the greater technical difficulties) should be done in vivo to provide a basis for comparison. Investigators using drugs should be aware of, and respect, the basic pharmacological principles cited in the text. In particular, the substantial freedom the investigator has in defining the extracellular medium should not be abused.

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.

The effects of temperature on synaptic transmission in hippocampal tissue slices

Fully submerged rat hippocampal tissue slices were exposed to temperature changes, and the effects on CA1 pyramidal cell electrophysiology studied. Raising the temperature from 29 to 33 or 37 degrees C simultaneously increased the focal-excitatory postsynaptic potentials and decreased the population spikes. These changes were largely reversible for slices warmed to 33 degrees C, but not for slices warmed to 37 degrees C. During warming transiently increased excitatory transmission was observed; the degree of increased transmission was related to the rate of temperature rise. It is postulated that neuronal membrane hyperpolarization with warming is responsible for several of the effects seen.

Effects of heating on electrical activities of guinea pig olfactory cortical slices

We examined the effect of heating on electrical activity of neurons in the guinea pig olfactory cortex slice. At the control temperature (37 degree C) the potential evoked by stimulation of the lateral olfactory tract consisted of an initial spike (IS) potential and a negative (N) potential. The IS potential is considered to be presynaptic and the other transsynaptic. The IS potential decreased in amplitude on heating and completely disappeared at 49 degree C. However, it recovered when the temperature was lowered to 37 degree C after five minutes of incubation at 49 degree C. In contrast, the N potential increased in amplitude at 39 degree C, was completely suppressed at 47 degree C and did not recover when the temperature was dropped to the control temperature. The maximum temperature from which the N potential recovered was 43 degree C. Unit activity was extracellularly recorded from neurons in the slice. On heating the brain slice some neurons showed an increase in activity others a decrease, and the rest were unaffected. We conclude that neurons in the olfactory cortex have different thermal sensitivities.

Hypothermia-induced changes in rat short latency somatosensory evoked potentials

Under general anesthesia, rats were gradually cooled from 37 degrees C to 24 degrees C, Slowly cooling avoided large temperature gradients between central and peripheral nervous systems. Short latency somatosensory potentials were evoked by forepaw stimulation and recorded from skull and depth structures. Cooling progressively increased onset and peak latencies and duration of all potentials. Amplitude of surface and depth recorded potentials decreased with decreasing temperatures except amplitude of surface component I increased. The response of surface and depth components to different rates of stimulation and cooling clearly indicates that cooling slows synaptic transmission much more than fiber conduction. The response of surface and depth recorded potentials to hypothermia suggests that evoked activity in cervical dorsal column and cuneate nucleus contributes to surface component I, that activity in cuneate nucleus, medial lemniscus, and inferior cerebellar peduncle contributes to surface component II, and that activity in thalamocortical fibers and probably cerebellum contributes to surface component III. These conclusions agree with our previous thoughts about the origin of short latency, surface recorded somatosensory evoked potentials.

Thermosensitivity of the extrahypothalamic brain stem in conscious goats

In 5 conscious goats, 84 experiments with 881 perfusion periods were performed to explore the brain stem between the rostral medulla and preoptic region for thermo-sensitive structures involved in temperature regulation. The chronically implanted thermodes consisted of 24 or 27 single probes, which were arranged in 8 or 9 rows. The rows of probes were individually perfused with water of 25-46 degrees C to produce discrete temperature stimuli along the brain stem. When the animals were exposed to an air temperature of +4 degrees C, local cooling at various levels of the lower brain stem augmented shivering and increased heat production, which was not regularly followed by a rise in rectal temperature. Ongoing shivering was reduced by local warming of the same sites. In comparison to the effects of hypothalamic thermal stimuli, the magnitude of the lower brain stem responses was reduced. At an air temperature of +30 degrees C local warming of discrete areas of the lower brain stem increased panting and caused a significant rise in respiratory evaporative heat loss. However, panting and shivering were not affected by the same site, and the effective sites of the various animals were not found at corresponding anatomical positions. Thus, thermosensitive sites which are not associated with defined anatomical structures, appear to be dispersed in the lower brain stem of the goat and to interfere with the temperature regulating system.

The influence of deep body temperatures and skin temperatures on respiratory frequency in the pig

1. The influences on respiratory frequency of ambient temperature, the temperature of the skin, the temperature and humidity of the inspired air, hypothalamic temperature, the temperature of the spinal cord, rectal temperature and some temperatures in the abdomen have been studied in the pig.2. At a constant ambient temperature the effect on respiratory frequency of heating a thermode in the hypothalamus was modified by the temperature of the skin of the trunk which was varied independently by means of a temperature-controlled coat. A cold skin inhibited panting; a warm skin enhanced panting. The effect of heating a thermode over the spinal cord was similarly modified by skin temperatures.3. Simultaneous heating of thermodes in the hypothalamus and spinal cord increased respiratory frequency more than heating either alone, and in a warm environment the rectal temperature influenced the extent to which respiratory frequency increased on heating the thermodes.4. Cooling the thermodes decreased respiratory frequency in a warm environment and the cooling of one thermode enhanced the effect of cooling the other.5. At a constant trunk skin temperature the effect on respiratory frequency of heating the thermode in the hypothalamus depended on ambient temperature.6. Changing the temperature of thermodes in the abdomen did not affect respiration nor was there any evidence that the temperature and humidity of the inspired air had a direct effect on respiration.