Effects of heating and cooling the spinal cord and medulla oblongata on thermoregulation in monkeys

Safety Review of Therapeutic Ultrasound for Spinal Cord Neuromodulation and Blood-Spinal Cord Barrier Opening

New focused ultrasound spinal cord applications have emerged, particularly those improving therapeutic agent delivery to the spinal cord via blood-spinal cord barrier opening and the neuromodulation of spinal cord tracts. One hurdle in the development of these applications is safety. It may be possible to use safety trends from seminal and subsequent works in focused ultrasound to guide the development of safety guidelines for spinal cord applications. We collated data from decades of pre-clinical studies and illustrate a clear relationship between damage, time-averaged spatial peak intensity and exposure duration. This relationship suggests a thermal mechanism underlies ultrasound-induced spinal cord damage. We developed minimum and mean thresholds for damage from these pre-clinical studies. When these thresholds were plotted against the parameters used in recent pre-clinical ultrasonic spinal cord neuromodulation studies, the majority of the neuromodulation studies were near or above the minimum threshold. This suggests that a thermal neuromodulatory effect may exist for ultrasonic spinal cord neuromodulation, and that the thermal dose must be carefully controlled to avoid damage to the spinal cord. By contrast, the intensity-exposure duration threshold had no predictive value when applied to blood-spinal cord barrier opening studies that employed injected contrast agents. Most blood-spinal cord barrier opening studies observed slight to severe damage, except for small animal studies that employed an active feedback control method to limit pressures based on measured bubble oscillation behavior. The development of new focused ultrasound spinal cord applications perhaps reflects the recent success in the development of focused ultrasound brain applications, and recent work has begun on the translation of these technologies from brain to spinal cord. However, a great deal of work remains to be done, particularly with respect to developing and accepting safety standards for these applications.

Pulmonary ventilation and gas exchange during prolonged exercise in humans: Influence of dehydration, hyperthermia and sympathoadrenal activity

NEW FINDINGS

What is the central question of the study? Ventilation increases during prolonged intense exercise, but the impact of dehydration and hyperthermia, with associated blunting of pulmonary circulation, and independent influences of dehydration, hyperthermia and sympathoadrenal discharge on ventilatory and pulmonary gas exchange responses remain unclear. What is the main finding and its importance? Dehydration and hyperthermia led to hyperventilation and compensatory adjustments in pulmonary CO2 and O2 exchange, such that CO2 output increased and O2 uptake remained unchanged despite the blunted circulation. Isolated hyperthermia and adrenaline infusion, but not isolated dehydration, increased ventilation to levels similar to combined dehydration and hyperthermia. Hyperthermia is the main stimulus increasing ventilation during prolonged intense exercise, partly via sympathoadrenal activation.

ABSTRACT

The mechanisms driving hyperthermic hyperventilation during exercise are unclear. In a series of retrospective analyses, we evaluated the impact of combined versus isolated dehydration and hyperthermia and the effects of sympathoadrenal discharge on ventilation and pulmonary gas exchange during prolonged intense exercise. In the first study, endurance-trained males performed two submaximal cycling exercise trials in the heat. On day 1, participants cycled until volitional exhaustion (135 ± 11 min) while experiencing progressive dehydration and hyperthermia. On day 2, participants maintained euhydration and core temperature (Tc ) during a time-matched exercise (control). At rest and during the first 20 min of exercise, pulmonary ventilation (V ̇ E ${\skew2\dot V_{\rm{E}}}$ ), arterial blood gases, CO2 output and O2 uptake were similar in both trials. At 135 ± 11 min, however,V ̇ E ${\skew2\dot V_{\rm{E}}}$ was elevated with dehydration and hyperthermia, and this was accompanied by lower arterial partial pressure of CO2 , higher breathing frequency, arterial partial pressure of O2 , arteriovenous CO2 and O2 differences, and elevated CO2 output and unchanged O2 uptake despite a reduced pulmonary circulation. The increasedV ̇ E ${\skew2\dot V_{\rm{E}}}$ was closely related to the rise in Tc and circulating catecholamines (R2  ≥ 0.818, P ≤ 0.034). In three additional studies in different participants, hyperthermia independently increasedV ̇ E ${\skew2\dot V_{\rm{E}}}$ to an extent similar to combined dehydration and hyperthermia, whereas prevention of hyperthermia in dehydrated individuals restoredV ̇ E ${\skew2\dot V_{\rm{E}}}$ to control levels. Furthermore, adrenaline infusion during exercise elevated both Tc andV ̇ E ${\skew2\dot V_{\rm{E}}}$ . These findings indicate that: (1) adjustments in pulmonary gas exchange limit homeostatic disturbances in the face of a blunted pulmonary circulation; (2) hyperthermia is the main stimulus increasing ventilation during prolonged intense exercise; and (3) sympathoadrenal activation might partly mediate the hyperthermic hyperventilation.

3-Week passive acclimation to extreme environmental heat (100± 3 °C) in dry sauna increases physical and physiological performance among young semi-professional football players

This manuscript aims to evaluate the influence of a novel passive heat acclimation program among human participants in the physical performance, as well as in several physiological parameters. 36 male football players were acclimated using a dry sauna bath to extreme hot (100 ± 3 °C), performing a total of nine sauna sessions with a weekly frequency of three sessions. The players were randomly into the sauna group (SG; n = 18; age: 20.69 ± 2.09 years) and the control group (CG; n = 18; age: 20.23 ± 1.98 years). All participants performed maximal effort test until exhaustion as well as hamstring flexibility test before and after the acclimation program. Anthropometric, respiratory, circulatory, hematological and physiological variables were evaluated at the beginning and at the end of the survey. Statistical analysis consisted of a Mann-Whitney U test to determine differences between groups at the beginning and at the end of the survey and a Wilcoxon test for paired samples to compare the differences for each group separately. Additionally, size effects of the pre-post acclimation changes were calculated. After the acclimation program SG participants experienced a diminution in body weight (p < 0.01), body mass index (p < 0.01), body fat (p < 0.05) and fat percentage (p < 0.05) decreased. Hamstring flexibility (p < 0.05) and work capacity (p < 0.05) increased. External basal temperature decreased (p < 0.05) as well as post-exercise systolic and diastolic blood pressures (p < 0.05). Finally, maximal oxygen uptake (ml Kg^-1 min^-1) (p < 0.05), maximal minute ventilation (p < 0.05) and maximal breath frequency (p < 0.05) increased at the end of the intervention. There were no significant changes in the CG in any variable. Favorable adaptations have been observed in this survey, suggesting a beneficial effect of extreme heat acclimation on physical performance. Several of the observed responses seem interesting for sport performance and health promotion as well. However, this is a novel, extreme protocol which requires further research.

From Nanowarming to Thermoregulation: New Multiscale Applications of Bioheat Transfer

This review explores bioheat transfer applications at multiple scales from nanoparticle (NP) heating to whole-body thermoregulation. For instance, iron oxide nanoparticles are being used for nanowarming, which uniformly and quickly rewarms 50-80-mL (≤5-cm-diameter) vitrified systems by coupling with radio-frequency (RF) fields where standard convective warming fails. A modification of this approach can also be used to successfully rewarm cryopreserved fish embryos (∼0.8 mm diameter) by heating previously injected gold nanoparticles with millisecond pulsed laser irradiation where standard convective warming fails. Finally, laser-induced heating of gold nanoparticles can improve the sensitivity of lateral flow assays (LFAs) so that they are competitive with laboratory tests such as the enzyme-linked immunosorbent assay. This approach addresses the main weakness of LFAs, which are otherwise the cheapest, easiest, and fastest to use point-of-care diagnostic tests in the world. Body core temperature manipulation has now become possible through selective thermal stimulation (STS) approaches. For instance, simple and safe heating of selected areas of the skin surface can open arteriovenous anastomosis flow in glabrous skin when it is not already established, thereby creating a convenient and effective pathway to induce heat flow between the body core and environment. This has led to new applications of STS to increase or decrease core temperatures in humans and animals to assist in surgery (perioperative warming), to aid ischemic stress recovery (cooling), and even to enhance the quality of sleep. Together, these multiscale applications of nanoparticle heating and thermoregulation point to dramatic opportunities for translation and impact in these prophylactic, preservative, diagnostic, and therapeutic applications of bioheat transfer.

Respiratory mechanics and cerebral blood flow during heat-induced hyperventilation and its voluntary suppression in passively heated humans

We investigated whether heat-induced hyperventilation can be voluntarily prevented, and, if so, how this modulates respiratory mechanics and cerebral blood flow in resting heated humans. In two separate trials, 10 healthy men were passively heated using lower body hot-water immersion and a water-perfused garment covering their upper body (both 41°C) until esophageal temperature (Tes ) reached 39°C or volitional termination. In each trial, participants breathed normally (normal-breathing) or voluntarily controlled minute ventilation (VE ) at a level equivalent to that observed after 5 min of heating (controlled-breathing). Respiratory gases, middle cerebral artery blood velocity (MCAV), work of breathing, and end-expiratory and inspiratory lung volumes were measured. During normal-breathing, VE increased as Tes rose above 38.0 ± 0.3°C, whereas controlled-breathing diminished the increase in VE (VE at Tes  = 38.6°C: 25.6 ± 5.9 and 11.9 ± 1.3 L min-1 during normal- and controlled-breathing, respectively, P < 0.001). During normal-breathing, end-tidal CO2 pressure and MCAV decreased with rising Tes , but controlled-breathing diminished these reductions (at Tes  = 38.6°C, 24.7 ± 5.0 vs. 39.5 ± 2.8 mmHg; 44.9 ± 5.9 vs. 60.2 ± 6.3 cm sec-1 , both P < 0.001). The work of breathing correlated positively with changes in VE (P < 0.001) and was lower during controlled- than normal-breathing (16.1 ± 12.6 and 59.4 ± 49.5 J min-1 , respectively, at heating termination, P = 0.013). End-expiratory and inspiratory lung volumes did not differ between trials (P = 0.25 and 0.71, respectively). These results suggest that during passive heating at rest, heat-induced hyperventilation increases the work of breathing without affecting end-expiratory lung volume, and that voluntary control of breathing can nearly abolish this hyperventilation, thereby diminishing hypocapnia, cerebral hypoperfusion, and increased work of breathing.

From Nanowarming to Thermoregulation: New Multiscale Applications of Bioheat Transfer

This review explores bioheat transfer applications at multiple scales from nanoparticle (NP) heating to whole-body thermoregulation. For instance, iron oxide nanoparticles are being used for nanowarming, which uniformly and quickly rewarms 50-80-mL (≤5-cm-diameter) vitrified systems by coupling with radio-frequency (RF) fields where standard convective warming fails. A modification of this approach can also be used to successfully rewarm cryopreserved fish embryos (∼0.8 mm diameter) by heating previously injected gold nanoparticles with millisecond pulsed laser irradiation where standard convective warming fails. Finally, laser-induced heating of gold nanoparticles can improve the sensitivity of lateral flow assays (LFAs) so that they are competitive with laboratory tests such as the enzyme-linked immunosorbent assay. This approach addresses the main weakness of LFAs, which are otherwise the cheapest, easiest, and fastest to use point-of-care diagnostic tests in the world. Body core temperature manipulation has now become possible through selective thermal stimulation (STS) approaches. For instance, simple and safe heating of selected areas of the skin surface can open arteriovenous anastomosis flow in glabrous skin when it is not already established, thereby creating a convenient and effective pathway to induce heat flow between the body core and environment. This has led to new applications of STS to increase or decrease core temperatures in humans and animals to assist in surgery (perioperative warming), to aid ischemic stress recovery (cooling), and even to enhance the quality of sleep. Together, these multiscale applications of nanoparticle heating and thermoregulation point to dramatic opportunities for translation and impact in these prophylactic, preservative, diagnostic, and therapeutic applications of bioheat transfer.

Panting as a human heat loss thermoeffector

The human autonomic nervous system participates in the control of thermoregulatory responses that are employed to regulate core temperature following deviations of skin temperature and/or core temperature from their respective resting values. This permits a regulation of the core temperature (T_C) at 37.0 ± 1°C with superimposed circadian variations in both sexes and menstrual cycle-associated variations in premenopausal women. When rendered hyperthermic, passively by heat exposure while at rest or actively during exercise, humans engage heat loss or thermolytic responses, including eccrine sweating and cutaneous vasodilatation. A third, less studied, human thermolytic response is thermal panting, and this response is the focus of this review. Human thermal panting was first described over a century ago. It has since been shown to be a reproducible response showing some similar patterns of breathing in species that employ panting as their sole thermolytic heat loss response. The contribution of human panting as a thermolytic response, however, remains controversial. This review highlights both past and recent evidence supporting that hyperthermic humans have a panting pattern of breathing that plays an important role in human thermoregulation.

Diurnal variation in the control of ventilation in response to rising body temperature during exercise in the heat

We investigated whether heat-induced hyperventilation during exercise is affected by time of day, as diurnal variation leads to higher core temperatures in the evening. Nineteen male subjects were divided into two experiments (protocol 1, n = 10 and protocol 2, n = 9). In protocol 1, subjects performed cycle exercise at 50% peak oxygen uptake in the heat (37°C and 50% RH) in the morning (0600) and evening (1800). Results showed that baseline resting and exercising esophageal temperature (Tes) were significantly (0.5°C) higher in the evening than morning. Minute ventilation (V̇e) increased from 54.3 ± 7.9 and 54.9 ± 6.8 l/min at 10 min to 71.4 ± 8.1 and 76.5 ± 11.8 l/min at 48.5 min in the morning and evening, respectively (both P < 0.01). Time of day had no effect on V̇e (P = 0.44). When V̇e as the output response was plotted against Tes as thermal input, the Tes threshold for increases in V̇e was higher in the evening than morning (37.2 ± 0.7 vs. 36.6 ± 0.6°C, P = 0.009), indicating the ventilatory response to the same core temperature is smaller in the evening. In protocol 2, the circadian rhythm-related higher resting Tes seen in the evening was adjusted down to the same temperature seen in the morning by immersing the subject in cold water. Importantly, the time course of changes in V̇e during exercise were smaller in the evening, but the threshold for V̇e remained higher in the evening than morning (P < 0.001). Collectively, those results suggest that time of day has no effect on time course hyperventilation during exercise in the heat, despite the higher core temperatures in the evening. This is likely due to diurnal variation in the control of ventilation in response to rising core temperature.

Characteristics of hyperthermia-induced hyperventilation in humans

In humans, hyperthermia leads to activation of a set of thermoregulatory responses that includes cutaneous vasodilation and sweating. Hyperthermia also increases ventilation in humans, as is observed in panting dogs, but the physiological significance and characteristics of the hyperventilatory response in humans remain unclear. The relative contribution of respiratory heat loss to total heat loss in a hot environment in humans is small, and this hyperventilation causes a concomitant reduction in arterial CO₂ pressure (hypocapnia), which can cause cerebral hypoperfusion. Consequently, hyperventilation in humans may not contribute to the maintenance of physiological homeostasis (i.e., thermoregulation). To gain some insight into the physiological significance of hyperthermia-induced hyperventilation in humans, in this review, we discuss 1) the mechanisms underlying hyperthermia-induced hyperventilation, 2) the factors modulating this response, and 3) the physiological consequences of the response.

Fever, immunity, and molecular adaptations

The heat shock response (HSR) is an ancient and highly conserved process that is essential for coping with environmental stresses, including extremes of temperature. Fever is a more recently evolved response, during which organisms temporarily subject themselves to thermal stress in the face of infections. We review the phylogenetically conserved mechanisms that regulate fever and discuss the effects that febrile-range temperatures have on multiple biological processes involved in host defense and cell death and survival, including the HSR and its implications for patients with severe sepsis, trauma, and other acute systemic inflammatory states. Heat shock factor-1, a heat-induced transcriptional enhancer is not only the central regulator of the HSR but also regulates expression of pivotal cytokines and early response genes. Febrile-range temperatures exert additional immunomodulatory effects by activating mitogen-activated protein kinase cascades and accelerating apoptosis in some cell types. This results in accelerated pathogen clearance, but increased collateral tissue injury, thus the net effect of exposure to febrile range temperature depends in part on the site and nature of the pathologic process and the specific treatment provided.

Metabolism, temperature, and ventilation

In mammals and birds, all oxygen used (VO2) must pass through the lungs; hence, some degree of coupling between VO2 and pulmonary ventilation (VE) is highly predictable. Nevertheless, VE is also involved with CO2 elimination, a task that is often in conflict with the convection of O2. In hot or cold conditions, the relationship between VE and VO2 includes the participation of the respiratory apparatus to the control of body temperature and water balance. Some compromise among these tasks is achieved through changes in breathing pattern, uncoupling changes in alveolar ventilation from VE. This article examines primarily the relationship between VE and VO2 under thermal stimuli. In the process, it considers how the relationship is influenced by hypoxia, hypercapnia or changes in metabolic level. The shuffling of tasks in emergency situations illustrates that the constraints on VE-VO2 for the protection of blood gases have ample room for flexibility. However, when other priorities do not interfere with the primary goal of gas exchange, VE follows metabolic rate quite closely. The fact that arterial CO2 remains stable when metabolism is changed by the most diverse circumstances (moderate exercise, cold, cold and exercise combined, variations in body size, caloric intake, age, time of the day, hormones, drugs, etc.) makes it unlikely that VE and metabolism are controlled in parallel by the condition responsible for the metabolic change. Rather, some observations support the view that the gaseous component of metabolic rate, probably CO2, may provide the link between the metabolic level and VE.

Heat stress does not augment ventilatory responses to presyncopal limited lower body negative pressure

Simulated haemorrhage, e.g. lower body negative pressure (LBNP), reduces central blood volume and mean arterial pressure, while ventilation increases. Passive whole-body heat stress likewise increases ventilation. The objective of this project was to test the hypothesis that ventilatory responses to reductions in central blood volume and arterial pressure during simulated haemorrhage are enhanced when individuals are heat stressed rather than normothermic. Eight healthy men (34 ± 9 years old, 176 ± 6 cm tall and 80.2 ± 4.2 kg body weight) underwent a simulated haemorrhagic challenge via LBNP until presyncope on two separate occasions, namely normothermic control and whole-body heat-stress trials. Baseline ventilation and core and mean skin temperatures were not different between trials (all P > 0.05). Prior to LBNP, heat stress increased core (from 36.8 ± 0.2 to 38.2 ± 0.2°C, P < 0.05) and mean skin temperatures (from 33.9 ± 0.5 to 38.1 ± 0.6°C, P < 0.05), as well as minute ventilation (from 8.01 ± 2.63 to 13.68 ± 6.68 l min(-1), P < 0.01). At presyncope, mean arterial pressure and middle cerebral artery blood velocity decreased in both trials (P < 0.05). At presyncope, ventilation increased to 23.22 ± 6.78 (P < 0.01) and 25.88 ± 10.16 l min(-1) (P < 0.01) in the normothermic and hyperthermic trials, respectively; however, neither the increase in ventilation from the pre-LBNP period nor the absolute ventilation was different between normothermic and hyperthermic trials (P > 0.05). These data suggest that the increase in ventilation during simulated haemorrhage induced via LBNP is not altered in heat-stressed humans.

Chin tremor in full-term neonate after hypoxia

CONTEXT

Newborns may present a range of motor phenomena that are not epileptic in nature. Chin tremor is an unusual movement disorder that typically starts in early childhood and may be precipitated by stress and emotion. Its pathophysiology has not been fully elucidated.

CASE REPORT

We describe a full-term newborn that, immediately after neonatal anoxia, presented body and chin tremors that were unresponsive to anti-epileptic drugs. Subsequent neurological evaluation revealed signs of pyramidal tract damage and chin tremor triggered by percussion and crying. We discuss the hypothesis that the anatomopathological abnormality may lie at the level of the higher cortical centers or midbrain.

CONCLUSIONS

Further studies are needed in order to gain greater comprehension of neonatal tremors. Recognition of the various etiological possibilities and consequent management of treatable causes is essential for care optimization.

Effect of initial core temperature on hyperthermic hyperventilation during prolonged submaximal exercise in the heat

We investigated whether a core temperature threshold for hyperthermic hyperventilation is seen during prolonged submaximal exercise in the heat when core temperature before the exercise is reduced and whether the evoked hyperventilatory response is affected by altering the initial core temperature. Ten male subjects performed three exercise trials at 50% of peak oxygen uptake in the heat (37°C and 50% relative humidity) after altering their initial esophageal temperature (T(es)). Initial T(es) was manipulated by immersion for 25 min in water at 18°C (Precooling), 35°C (Control), or 40°C (Preheating). T(es) after the water immersion was significantly higher in the Preheating trial (37.5 ± 0.3°C) and lower in the Precooling trial (36.1 ± 0.3°C) than in the Control trial (36.9 ± 0.3°C). In the Precooling trial, minute ventilation (Ve) showed little change until T(es) reached 37.1 ± 0.4°C. Above this core temperature threshold, Ve increased linearly in proportion to increasing T(es). In the Control trial, Ve increased as T(es) increased from 37.0°C to 38.6°C after the onset of exercise. In the Preheating trial, Ve increased from the initially elevated levels of T(es) (from 37.6 to 38.6°C) and Ve. The sensitivity of Ve to increasing T(es) above the threshold for hyperventilation (the slope of the T(es)-Ve relation) did not significantly vary across trials (Precooling trial = 10.6 ± 5.9, Control trial = 8.7 ± 5.1, and Preheating trial = 9.2 ± 6.9 L·min(-1)·°C(-1)). These results suggest that during prolonged submaximal exercise at a constant workload in humans, there is a clear core temperature threshold for hyperthermic hyperventilation and that the evoked hyperventilatory response is unaffected by altering initial core temperature.

Tympanic temperature is not suited to indicate selective brain cooling in humans: a re-evaluation of the thermophysiological basics

Selective brain cooling in humans, with venous blood returning from the head surface as the relevant heat sink, was proposed more than two decades ago as a mechanism protecting the brain against damage in hyperthermic conditions. Brain cooling was inferred from decreases of tympanic temperature under the premise that it reflected brain temperature closely, even in conditions of external head cooling. In mammals with a well-developed carotid rete selective brain cooling and its quantitative relevance are experimentally well established by directly monitoring brain temperature. For humans, however, the dispute about the existence and physiological relevance of selective brain cooling has remained unsettled, especially, as far as arguments have been exchanged on the basis of thermophysiological data and model calculations considering brain metabolism, brain hemodynamics and the anatomical preconditions for arterio-venous heat exchange. In this essay two seminal studies in support of the existence of human selective brain cooling in the condition of exercise hyperthermia, with and without dehydration, are re-examined from two points of view: first the stringency of the working hypotheses underlying data evaluation and their subsequent fate. Second the minimum theoretical requirements for data interpretation. The working hypotheses supporting data interpretation in favor of selective brain cooling in humans were heuristic and/or had become questionable at the dates of their application; today, they may be considered as outdated. Data interpretation becomes most conclusive, if tympanic temperature simply is not taken into account.

Components and mechanisms of thermal hyperpnea

The pattern of breathing during a hyperthermia-induced hyperventilation varies across different species. Thermal tachypnea is a first phase panting response adopted during hyperthermia when tidal volume is minimized and the frequency of breathing is maximized. Blood-gas tensions and pH are maintained during this hyperventilation, and the associated heat loss helps the animal regulate its body temperature. A second pattern of breathing adopted in hyperthermia is thermal hyperpnea; this response is the focus of this review. This form of hyperventilation is evident after an increase in core temperature and it is apparent in humans. Increases of tidal volume as well as frequency of breathing are evident during this response that results in a respiratory alkalosis. The cause of thermal hyperpnea is not resolved; evidence of the potential mechanisms underlying this response support that modulators of the response act in either a multiplicative or additive manner with body temperatures. The details of the designs and methodologies of the studies supporting or refuting these two views are discussed. A physiological rationale for thermal hyperpnea is presented in which it is suggested this response serves a heat-loss role and contributes to selective brain cooling in hyperthermic humans. Ongoing research in this area is focused on resolving the mechanisms underlying thermal hyperpnea and its contribution to cranial thermoregulation. The direct application of this research is for the care of febrile and hyperthermic patients.

c-Fos generation in the dorsal vagal complex after systemic endotoxin is not dependent on the vagus nerve

The present study used activation of the c-Fos oncogene protein within neurons in the dorsal vagal complex (DVC) as a marker of neuronal excitation in response to systemic endotoxin challenge [i.e. , lipopolysaccharide (LPS)]. Specifically, we investigated whether vagal connections with the brain stem are necessary for LPS cytokine- induced activation of DVC neurons. Systemic exposure to LPS elicited a significant activation of c-Fos in neurons in the nucleus of the solitary tract (NST) and area postrema of all thiobutabarbital-anesthetized rats examined, regardless of the integrity of their vagal nerves. That is, rats with both vagi cervically transected were still able to respond with c-Fos activation of neurons in the DVC. Unilateral cervical vagotomy produced a consistent but small reduction in c-Fos activation in the ipsilateral NST of all animals within this experimental group. Given that afferent input to the NST is exclusively excitatory, it is not surprising that unilateral elimination of all vagal afferents would diminish NST responsiveness (on the vagotomized side). These data lead us to conclude that the NST itself is a primary central nervous system detector of cytokines.

Effects of hypothalamic thermal stimuli on sympathetic neurones innervating skin and skeletal muscle of the cat hindlimb

1. Postganglionic neurones supplying hairless and hairy skin of the cat hindlimb were analysed for their responses to thermal stimuli applied to the anterior hypothalamus and spinal cord in anaesthetized and artificially ventilated cats. Activity was recorded from multi- and single-unit bundles which were isolated from peripheral nerves. The neurones were functionally identified as cutaneous vasoconstrictor (CVC) and muscle vasoconstrictor (MVC) neurones. Activity in sudomotor (SM) neurones was either monitored indirectly by recording the phasic negative deflections of the skin potential from the surface of the hairless skin, or in some experiments additionally by recording activity directly from the SM axons. 2. The activity in forty-one out of forty-four multi-unit and six out of six single-unit CVC bundles was inhibited, in a graded manner, by hypothalamic warming. An increase in the temperature of the surface of hairless skin followed the decrease in activity of the CVC neurones supplying it. Large changes in skin temperature only followed decreases in CVC activity of more than 40%. Cooling of the hypothalamus had only weak transient effects on CVC neurones. 3. Simultaneous warming of hypothalamus and spinal cord had multiplicative effects on the activity in CVC neurones. Subthreshold warming of one structure increased the response to warming of the other one and reduced the threshold temperature. 4. SM neurones were not affected by hypothalamic warming, but activated during hypothalamic cooling. 5. MVC neurones were weakly activated during hypothalamic warming only if arterial blood pressure decreased, otherwise they were unaffected. It is likely that this activation was due to secondary unloading of arterial baroreceptors. 6. Two silent postganglionic neurones projecting to skin were activated during hypothalamic warming. These neurones may have had a vasodilatory function. 7. Rhythmicity of the activity in CVC neurones, related to the cycle of artificial ventilation, increased during hypothalamic warming whereas that of MVC neurones was unchanged. 8. The functionally highly specific responses to hypothalamic warming in CVC neurones indicate a pathway from the hypothalamus that is specific for CVC neurones, in contrast to MVC and SM neurones. This central pathway is integrated with other spinal and supraspinal reflex pathways that determine the characteristic reflex pattern of CVC neurones to somatic and visceral stimuli and possibly with pathways that generate other physiological changes during hypothalamic warming (e.g. increase in respiratory drive).

Release of heat-loss responses in paradoxical sleep by thermal loads and by pontine tegmental lesions in cats

Thermoregulatory heat-loss responses at high ambient temperatures were studied in intact cats and those with bilateral electrolytic lesions in the pontine tegmentum during wakefulness (W), slow-wave sleep (SWS), paradoxical sleep (PS) and PS without atonia induced by the lesions. Panting (respiratory rate greater than or equal to 90/min) was present during W, SWS, and in some cases, during PS. The percentage of the PS episodes with panting was directly related to ambient temperature. In intact cats at 30 degrees C, panting occurred in 8% of the PS episodes; at 35 degrees C, in 52%, and at 40 degrees C, in 77%. The percentage of PS episodes with panting was higher in the pontine-lesioned cats (90% at 35 degrees C), probably another indication of the altered thermoregulation of such animals. Thermoregulatory responses to heat load, and thermoregulation in general, have previously been shown to be suppressed in PS. Because hypothalamic thermosensitive neurons lack thermal responses during PS, the partial activation of heat-loss responses observed here may depend upon the function of extrahypothalamic brainstem areas.

Convergence of thermal signals on the reticulospinal neurons in the midbrain, pons and medulla oblongata

A total of 670 reticulospinal (RfS) and non-RfS neurons in the mesencephalic, pontile and medullary reticular formation (mRf, pRf and mdRf) were studied for the responsiveness to changes in temperatures of local brain sites, preoptic and anterior hypothalamus (PO/AH) and skin in the urethane anesthetized rat. Local thermoresponsiveness was found in 49.6% of 139 mRf neurons, 61.9% of 160 pRf neurons and 75.4% of 126 mdRf neurons. While the ventromedial region of pRf and mdRf contained predominantly warm-responsive neurons (54.8% and 62.5%), cold-responsive neurons were much more frequently found in the mRf (33.8%) and the dorsolateral region of pRf (41.9%) and mdRf (50.0%). Responsiveness to hypothalamic temperature and/or skin temperature was observed in about 40-74% of Rf neurons. Higher incidence of responsiveness to remote temperatures was found among locally thermoresponsive neurons than among locally thermounresponsive neurons in all three areas. Particularly, there was a high degree of convergence of 'cold' signals from local and remote sites on the RfS neurons in the mRf and the dorsolateral pRf and mdRf. Microinjections of procaine and glutamate into these regions decreased and increased the cold-induced increase in EMG activity and shivering without any correlated changes in cardiovascular and respiratory parameters and pilomotor activity. The results suggest that RfS and non-RfS neurons in the mRf and the dorsolateral pRf and mdRf are involved in the control of thermoregulatory muscle tone and shivering.

Hyperthermia induced by pre-pontine knife-cut: evidence for a tonic inhibition of non-shivering thermogenesis in anaesthetized rat

Temperature of colon, interscapular brown adipose tissue (IBAT) and paw skin (index of vasomotor activity) were monitored before and after microwire knife lesions at the pre-pontine or/and the post-mammillary levels in the urethane-anaesthetized rats at room temperature of 23-24 degrees C. Following the pre-pontine, but not the post-mammillary cut, colonic and IBAT temperatures increased by 3-4 degrees C within 90-240 min. IBAT temperature rose faster with a shorter latency and attained a higher steady-state value than colonic temperature; skin temperature, however rose by only 0.8 degrees C. A procaine microinjection into the pre-pontine area transiently increased by more than 1 degree C both colonic and IBAT temperatures, with similar kinetics as for the knife cut. Cardiac output distribution was measured using radiolabelled microspheres. Brown adipose tissue (BAT) was found to be the only organ to which the fractional blood flow increased dramatically (12 times over baseline value) during the development of hyperthermia. Propanolol, injected after the hyperthermia had fully developed, decreased IBAT and then colonic temperatures. Hexamethonium decreased both colonic and IBAT temperatures with a concomitant rise in skin temperature while tubocurarine was without effect. It is concluded that the hyperthermia observed after the pre-pontine lesion results from an increased sympathetic stimulation of BAT thermogenesis triggered by the release of a tonic inhibitory control on its heat production. Such an inhibitory system would be located somewhere between the lower midbrain and the upper pons.

Warm- and cold-sensitive neurons inactive at normal core temperature in rat hypothalamic slices

Electrical activities of thermosensitive neurons were recorded extracellularly in slices of rat preoptic area and anterior hypothalamus. Of 63 spontaneously firing neurons found at high searching temperature (37-40 degrees C), 33% were warm-sensitive, 8% were cold-sensitive and the remaining 59% were thermally insensitive. In particular, 6 warm-sensitive neurons were active only above 38 degrees C of rat normal core temperature. In contrast, of 38 spontaneously firing neurons found at low searching temperature (32-36 degrees C), 8% were warm-sensitive, 29% were cold-sensitive and the remaining 63% were thermally insensitive. Furthermore, all these cold-sensitive neurons were active only below 38 degrees C. Therefore, the warm- and cold-sensitive neurons active at 38 degrees C would be functioning for narrow band control and the remaining warm- and cold-sensitive neurons inactive at 38 degrees C would be recruited for wide band control when core temperature was changed critically from 38 degrees C. Their firing rate activities often showed obvious threshold responses, large hysteresis of the threshold responses and remarkable transient responses to slice temperature changes. From aspects of automatic control theory, these warm- and cold-sensitive neurons themselves may be thermostats to regulate the brain temperature rather than thermosensors to monitor it.

Operant control of convective cooling and microwave irradiation by the squirrel monkey

Adult male squirrel monkeys (Saimiri sciureus) were individually chair-restrained in an air-conditioned Styrofoam box in the far field of a horn antenna. Each monkey first received extensive training to regulate the temperature of the air circulating through the box by selecting between 10 and 50 degrees C air source temperatures. Then, to investigate the ability of the animals to utilize microwaves as a source of thermalizing energy, 2450-MHz continuous wave microwaves accompanied by thermoneutral (30 degrees C) air were substituted for the 50 degrees C air. Irradiation at each of three power densities was made available, ie, at 20, 25, and 30 mW/cm2 [SAR = 0.15 (W/kg)/(mW/cm2)]. The percentage of time that the monkeys selected microwave irradiation paired with thermoneutral air averaged 90% at 20 and at 25 mW/cm2. The mean percentage declined reliably (p less than 0.001) to 81% at 30 mW/cm2, confirming the monkey's ability to utilize microwave irradiation as a source of thermal energy during the course of behavioral thermoregulation. All animals readily made the warm-air to microwave-field transition, regulating rectal temperature with precision by sequentially selecting 10 degrees C air, then microwave irradiation accompanied by 30 degrees C air. Although the selection of cooler air resulted in a slight reduction of skin temperatures, normal rectal temperature was maintained. The results indicate that the squirrel monkey can utilize a microwave source in conjunction with convective cooling to regulate body temperature behaviorally.

Thermosensitive neurons in slice preparations of rat medulla oblongata

With extracellular recording, we examined thermosensitive neurons in slices of a possible thermosensitive region in the dorsal medulla oblongata of rats. In addition to temperature insensitive neurons, warm- and cold-sensitive neurons were recorded in vitro. The firing rates of many warm- and cold-sensitive neurons responded transiently to temporal changes in slice temperature with "overshoots" and/or "reverse overshoots." In a steady state, warm-sensitive neurons were similar in thermoresponsiveness to those studied previously in the CNS in vivo. Cold-sensitive neurons were unique in that. Most of them were silent at the normal core temperature of 38 degrees C and fired only below their own threshold temperatures (2-6 degrees C below normal).

Mutual connections of the raphe system and hypothalamus in relation to fever

Previous studies in our laboratory have shown that lesions in certain brain stem regions prevent elevations of body temperature after administration of bacterial pyrogen. In the present experiments wer examined the morphology of the connections of these "brain stem thermoregulatory centers" which are represented in all raphe and paraphe nuclei of the brain stem reticular formation. The methods of anterograde degeneration and tracing of retrograde transport of horseradish peroxidase were used to identify afferent and efferent connections within this "thermoregulatory field." Abundant mutual connections between the raphe nuclei and hypothalamus were found. All nuclei of the raphe system receive afferents from the medial and lateral hypothalamus. All raphe nuclei have efferent projections to the medial reticular formation, and the raphe nuclei of the pons and mesencephalon provide ascending fibers to the hypothalamus. A lesion of any part (origin, course, termination field) of this mutual raphe-hypothalamic pathway system will prevent development of fever in response to bacterial pyrogen.

Total body thermosensitivity and its spinal and supraspinal fractions in the conscious goose

1. Effects of general body cooling on heat production: an intravascular heat exchanger was used to alter total body temperature. Heat production increased with decreasing body temperature at an average rate of -12W/kg x degree C. The rate of rise was independent of air temperature. The threshold body temperature below which heat production rose was lower at higher air temperature. 2. Effects of spinal cord cooling: heat production increased with decreasing spinal temperature at an average rate of -0.3 W/kg x degree C. The rate of rise was not clearly affected by air temperature. The spinal threshold temperature was lower at warm ambient conditions. The results suggest that spinal thermosensitivity in the goose represents only a minor fraction of total body thermosensitivity. 3. Effects of head cooling: heat exchangers closing the carotid arteries were used to alter the temperature of the blood supplied to the head. Cooling increased heat production. When the thermosensitivity of the area, which was affected by heat exchanger, was calculated from the relationship between changes of heat production and brain temperature, values between -0.74 and -1.65 W/kg x degree C were obtained. Measurements of brain, spinal cord and head skin temperatures suggest that the thermosensitive structures which mediated the responses, were predominantly situated in the brain.

Brain stem sites mediating specific and non-specific temperature effects on thermoregulation in the pekin duck

1. Thermodes were chronically implanted into various levels of the brain stem of sixteen Pekin ducks. The effects of local thermal stimulation on metabolic heat production, core temperature, peripheral skin temperature and respiratory frequency were investigated. 2. Four areas of thermode positions were determined according to the responses observed and were histologically identified at the end of the investigation. 3. Thermal stimulation of the lower mid-brain/upper pontine brain stem (Pos. III) elicited an increase in metabolic heat production, cutaneous vasoconstriction and rises in core temperature in response to cooling at thermoneutral and cold ambient conditions and, further, inhibition of panting by cooling and activation of panting by heating at warm ambient conditions. The metabolic response to cooling this brain stem section amounted to -0.1 W/kg. degrees C as compared with -7 W/kg. degrees C in response to total body cooling. 4. Cooling of the anterior and middle hypothalamus (Pos. II) caused vasodilatation in the skin and did not elicit shivering. The resulting drop in core temperature at a given degree of cooling was greater than the rise in core temperature in response to equivalent cooling of the lower mid-brain/upper pontine brain stem. 5. Cooling of the preoptic forebrain (Pos. I) and of the myelencephalon (Pos. IV) did not elicit thermoregulatory reactions. 6. It is concluded that the duck's brain stem contains thermoreceptive structures in the lower mid-brain/upper pontine section. However, the brain stem as a whole appears to contribute little to cold defence during general hypothermia because of the inhibitory effects originating in the anterior and middle hypothalamus. Cold defence in the duck, which is comparable in strength to that in mammals, has to rely on extracerebral thermosensory structures.

Evidence for a caudal brainstem site of action for cannabinoid induced hypothermia

delta 9-Tetrahydrocannabinol (THC), 11-hydroxy delta 9-tetrahydrocannabinol (11-OH-THC) and the synthetic dimethylheptyl analogue of THC (DMHP) were injected intracerebrally into proven chemosensitive sites in the hypothalamus of unanesthetized cats with implanted microinjection guide tubes. 100 micrograms of each compound was administered in a volume of 8 microliters. Chemosensitivity of all injection sites was established by microinjection of carbamylcholine to induce hyperthermia and tetrodotoxin to induce hypothermia. THC or its analogues produced no significant change in body temperature when injected intracerebrally. However, in the same animals, parenteral administration of THC, 11-OH-THC or DMHP (0.5 to 2.0 mg/kg) induced hypothermic responses ranging from -2.0 to -7.0 degrees C. Intravenous administration of THC was effective in blocking shivering induced by cooling the preoptic region in unanesthetized cats with implanted thermodes. In cats with mid-pontine transections, cooling of the spinal cord by perfusion with an epidural double wall cannula at temperatures of 30, 20, 10 and 0 degrees C produced graded shivering which was recorded electromyographically. Intravenous THC, (0.25-2.0 mg/kg) produced a dose-dependent attenuation of spinal cord induced shivering. These data plus results of prior studies suggest that the tetrahydrocannabinols produce their hypothermic effect at sites in the caudal brainstem. Suppression of shivering at the ponto medullary or spinal cord level may represent an important mechanism which contributes to the lowering of body temperature.

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.

Microwaves modify thermoregulatory behavior in squirrel monkey

Squirrel monkeys (Saimiri sciureus) trained to regulate environmental temperature (Ta) behaviorally were exposed in the far field of a horn antenna to ten-minute periods of 2,450 MHz CW microwaves. Incident power density ranged from 1 to 22 mW/cm2. The corresponding specific absorption rate (SAR), derived from temperature increments in saline-filled styrofoam models, ranged from 0.15 to 3.25 W/kg. Controls included exposure to infrared radiation equivalent incident energy and no radiation exposure. Normal thermo-regulatory behavior produces tight control over environmental and body temperatures; most monkeys select a Ta of 34-36 degrees C. Ten-minute exposures to 2,450 MHz CW microwaves at an incident power density of 6-8 mW/cm2 stimulated all animals to select a lower Ta. This threshold energy represents a whole-body SAR of 1.1 W/kg, about 20% of the resting metabolic rate of the monkey. Thermoregulatory behavior was highly efficient, and skin and rectal temperatures remained stable, even at 22 mW/cm2 where the preferred Ta was lowered by as much as 4 degrees C. No comparable reduction in selected Ta below control levels occurred during exposure to infrared radiation of equal incident power density.

Graded changes in central chemoceptor input by local temperature changes on the ventral surface of medulla

1. In cats under pentobarbitone anaesthesia the effects of focal temperature changes of the ;chemoceptive' areas on the ventral surface of medulla, described by Loeschcke and his associates, were studied with respect to tidal volume, V(T), tidal variation in efferent phrenic activity, Phr(T), and respiratory rate. The cats were either paralysed and ventilated at various constant P(A,CO2) and P(a,O2) levels, or breathing spontaneously.2. It was confirmed that focal bilateral cooling of the intermediate, ;I((S))', areas caused rapid depression of respiration even at constant artificial ventilation. In normocapnic and normoxic conditions apnoea usually ensued at brain surface temperatures of 20-22 degrees C.3. The effects were graded along continuous temperature-response curves with enhancements of ventilation above and depression below normal body temperature.4. The strongest effects on V(T) and Phr(T) were obtained from the I((S)) areas with no or only small effects on inspiratory or expiratory timing in the vagotomized animal. The Hering-Breuer inflation reflex and its effects on timing and amplitudes were not affected by cooling this area.5. Focal cooling of the caudal or the rostral ;chemoceptive' areas, ;C((L))' and ;R((M))' areas, caused smaller effects on V(T) and Phr(T) but produced significant effects on respiratory rate even after vagotomy.6. The effects of focal cooling of these areas could be mimicked by topical application of procaine solution which has been shown not to penetrate deeper than 100 mum from the surface.7. Moderate focal cooling of area I((S)) to temperatures above 28-30 degrees C caused a parallel shift in the CO(2)-response (V(T), Phr(T)) curves to the right with little change in slope. The P(CO2) thresholds for apnoea were correspondingly raised. These focal temperature effects could be compensated by changes in P(CO2) with, on the average, 2.7 torr/ degrees C. Focal temperatures below 28 degrees C usually caused some decrease in slope of the CO(2)-response curves in addition to further shifts.8. Added hypoxic stimulus or electrical stimulation of the carotid sinus nerves caused an almost parallel increase of Phr(T) at all P(CO2) levels and all focal temperatures suggesting an additive type of interaction between the input from the peripheral chemoreceptors and that from the central (CO(2), H(+)) sensing structures whether the latter was altered by changing P(CO2) or by focal temperature changes on the I((S)) areas.9. In contrast to these effects of hypoxia and stimulation of the carotid sinus nerves the reflex increase of inspiratory activity caused by lung deflation or by electrical stimulation of the glossopharyngeal nerve distal to the carotid sinus nerves was CO(2) dependent. These reflex effects decreased with focal cooling of the I((S)) areas as with hypocapnia, suggesting a mainly multiplicative or ;gain-changing' type of interaction with the central chemoceptive drive.10. The close similarities in effect of focal cooling and of hypocapnia on the different respiratory parameters even during constant artificial ventilation indicate that focal temperature changes of the I((S)) areas intervene effectively with the normal ventilatory response to CO(2) without changing the chemical or physical environment of those neural structures in the brain stem which set respiratory pattern.

Statistical analysis of the discharge patterns of medullary temperature-responsive neurones in rabbits

To characterize the medullary temperature-responsive neurones in rabbits, statistical analysis of interspike intervals was performed on 24 neurones recorded at various temperatures of the medulla. It was found that the medullary neurone fired at a relatively regular interval and 11 of 24 neurones displayed an invariable precision in thermoreception over a broad range of medullary temperatures; also, that the temporal distribution of interspike intervals of medullary neurones was markedly different from that of hypothalamic neurones.

Central thermosensitivity in conscious goats: hypothalamus and spinal cord versus residual inner body

Experiments were performed on conscious goats to confirm the suggestion that in this species the inner body contains more thermosensitive structures than those residing in the hypothalamus and spinal cord. For this purpose goats were chronically implanted with local thermodes and intravascular heat exchangers to allow independent temperature control of the hypothalamus, spinal cord and residual inner body. With the hypothalamus and spinal cord clamped simultaneously at different levels between 32 degrees C and 40 degrees C, residual internal temperature was lowered by subtracting heat via the intravascular heat exchanger. The residual internal temperature at which shivering and increased heat production occured due to heat extraction, was directly related to the value of the combined hypothalamic and spinal cord clamp temperature. The higher hypothalamic and spinal cord clamp temperatures were, the lower residual internal temperature fell before shivering occurred and heat production rose. Plots relating residual internal temperature to hypothalamic and spinal cord temperature at different levels of heat production showed the signal input generated within the residual inner body to be of nearly the same order of magnitude as that from the hypothalamus and spinal cord.

Effects of total body core cooling on heat production of conscious goats

Experiments were performed to investigate the effect of general body core cooling on heat production at various air temperatures between +1 degree C and +56 degrees C in conscious goats. An intravascular heat exchanger (IVHE) was used to alter body core temperature independently of air temperature. Heat loss via the IVHE caused a fall in body core temperature, the extent of which depended on the rate of extraction and air temperature. Irrespective of air temperature the decrease in body core temperature resulted in shivering and an increase in heat production, which eventually balanced the heat loss. During steady state conditions the extra heat production was approximately equal to that lost via the IVHE. The threshold body core temperature at which heat production increased in response to central cooling did not significantly alter with air temperature. However, the slopes of the curves describing this response were smaller at higher than at lower air temperatures, which indicated that central thermosensitivity decreased with increasing air temperature. Irrespective of air temperature the threshold temperatures for shivering were higher and the slopes of the curves were steeper than those previously found with combined cooling of the hypothalamus and spinal cord in the same species which indicated the existence of central thermosensors outside the above two mentioned areas.

Response of temperature-sensitive medullary neurons to acetylcholine, norepinephrine, and 5-hydroxytryptamine

Single unit activity was recorded by means of five-barrel micropipettes from temperature-sensitive neurons in the reticular formation of the medulla oblongata in urethanized cats. Acetylcholine (ACh), norepinephrine (NE) and 5-hydroxytryptamine (5-HT) were applied microiontophoretically to the immediate vicinity of the neurons. Both thermally sensitive and insensitive units responded to ACh (82.0%) and 5-HT (93.0%) by increasing discharge rate. Iontophoretically applied atropine, but not hexamethonium, antagonized the excitatory responses to ACh. NE was shown to have different effects on the medullary neurons. Most temperature-insensitive units were either nonresponsive (52.4%) or excited (28.6%) by NE, while a majority of warm-sensitive (61.8%) and cold-sensitive (55.6%) neurons were inhibited by NE. Iontophoretically application of the alpha-adrenergic blocking agent, phentolamine, or the beta-antagonists, propranolol, or sotalol, produced no effect on the inhibitory responses to NE. These results tend to support the current concept of the transmitter role of monoamines in thermoregulation.

Medullary unit responses to changes in local and hypothalamic temperatures in the cat

Single unit activity was recorded with glass microelectrodes in the reticular formation of the medulla oblongata of cats lightly anesthetized with urethan, while medullary temperature was changed by surface irrigation with warm or cold artificial cerebrospinal fluid. In addition, water-perfusion thermodes were implanted over the preoptic anterior hypothalamus (POAH) region, and the effects of heating and cooling of the POAH on firing rate of medullary units were studied. One hundred and twenty-five temperature-sensitive neurons were studied in the medullary reticular formation. Out of these 125 neurons, 80 were warm-sensitive and 45 were cold-sensitive. Sixteen warm-sensitive medullary neurons were examined in responses to changes of POAH temperature. Ten units (62.5%) responded, and the remaining 6 units were not responsive to changes of the POAH temperature. Of 13 cold-sensitive neurons examined, 10 units (77.0%) responded. On the other hand, only 1 out of 7 (14.3%) temperature-insensitive neurons tested did respond to changes in the POAH temperature. These results suggest that the temperature signals sent out from thermosensitive structures in the hypothalamus might be transmitted to a major portion of temperature-sensitive neurons in the medullary reticular formation.