Thermoregulatory responses of decerebrate and spinal cats

A computationally informed distinction of interoception and exteroception

While interoception is of major neuroscientific interest, its precise definition and delineation from exteroception continue to be debated. Here, we propose a functional distinction between interoception and exteroception based on computational concepts of sensor-effector loops. Under this view, the classification of sensory inputs as serving interoception or exteroception depends on the sensor-effector loop they feed into, for the control of either bodily (physiological and biochemical) or environmental states. We explain the utility of this perspective by examining the perception of skin temperature, one of the most challenging cases for distinguishing between interoception and exteroception. Specifically, we propose conceptualising thermoception as inference about the thermal state of the body (including the skin), which is directly coupled to thermoregulatory processes. This functional view emphasises the coupling to regulation (control) as a defining property of perception (inference) and connects the definition of interoception to contemporary computational theories of brain-body interactions.

Efferent neural pathways for the control of brown adipose tissue thermogenesis and shivering

The fundamental central neural circuits for thermoregulation orchestrate behavioral and autonomic repertoires that maintain body core temperature during thermal challenges that arise from either the ambient or the internal environment. This review summarizes our understanding of the neural pathways within the fundamental thermoregulatory reflex circuitry that comprise the efferent (i.e., beyond thermosensory) control of brown adipose tissue (BAT) and shivering thermogenesis: the motor neuron systems consisting of the BAT sympathetic preganglionic neurons and BAT sympathetic ganglion cells, and the alpha- and gamma-motoneurons; the premotor neurons in the region of the rostral raphe pallidus, and the thermogenesis-promoting neurons in the dorsomedial hypothalamus/dorsal hypothalamic area. Also included are inputs to, and neurochemical modulators of, these efferent neuronal populations that could influence their activity during thermoregulatory responses. Signals of metabolic status can be particularly significant for the energy-hungry thermoeffectors for heat production.

Cortical control of thermoregulatory sympathetic activation

Thermoregulation enables adaptation to different ambient temperatures. A complex network of central autonomic centres may be involved. In contrast to the brainstem, the role of the cortex has not been clearly evaluated. This study was therefore designed to address cerebral function during a whole thermoregulatory cycle (cold, neutral and warm stimulation) using 18-fluordeoxyglucose-PET (FDG-PET). Sympathetic activation parameters were co-registered. Ten healthy male volunteers were examined three times on three different days in a water-perfused whole-body suit. After a baseline period (32 degrees C), temperature was either decreased to 7 degrees C (cold), increased to 50 degrees C (warm) or kept constant (32 degrees C, neutral), thereafter the PET examination was performed. Cerebral glucose metabolism was increased in infrapontine brainstem and cerebellar hemispheres during cooling and warming, each compared with neutral temperature. Simultaneously, FDG uptake decreased in the bilateral anterior/mid-cingulate cortex during warming, and in the right insula during cooling and warming. Conjunction analyses revealed that right insular deactivation and brainstem activation appeared both during cold and warm stimulation. Metabolic connectivity analyses revealed positive correlations between the cortical activations, and negative correlations between these cortical areas and brainstem/cerebellar regions. Heart rate changes negatively correlated with glucose metabolism in the anterior cingulate cortex and in the middle frontal gyrus/dorsolateral prefrontal cortex, and changes of sweating with glucose metabolism in the posterior cingulate cortex. In summary, these results suggest that the cerebral cortex exerts an inhibitory control on autonomic centres located in the brainstem or cerebellum. These findings may represent reasonable explanations for sympathetic hyperactivity, which occurs, for example, after hemispheric stroke.

Energetic responses to cold temperatures in rats lacking forebrain-caudal brain stem connections

Hypothalamic neurons are regarded as essential for integrating thermal afferent information from skin and core and issuing commands to autonomic and behavioral effectors that maintain core temperature (T(c)) during cold exposure and for the control of energy expenditure more generally. Caudal brain stem neurons are necessary elements of the hypothalamic effector pathway and also are directly driven by skin and brain cooling. To assess whether caudal brain stem processing of thermal afferent signals is sufficient to drive endemic effectors for thermogenesis, heart rate (HR), T(c), and activity responses of chronic decerebrate (CD) and control rats adapted to 23 degrees C were compared during cold exposure (4, 8, or 12 degrees C) for 6 h. Other CDs and controls were exposed to 4 or 23 degrees C for 2 h, and tissues were processed for norepinephrine turnover (NETO), a neurochemical measure of sympathetic drive. Controls maintained T(c) for all temperatures. CDs maintained T(c) for the 8 and 12 degrees C exposures, but T(c) declined 2 degrees C during the 4 degrees C exposure. Cold exposure elevated HR in CDs and controls alike. Tachycardia magnitude correlated with decreases in environmental temperature for controls, but not CDs. Cold increased NETO in brown adipose tissue, heart, and some white adipose tissue pads in CDs and controls compared with their respective room temperature controls. These data demonstrate that, in neural isolation from the hypothalamus, cold exposure drives caudal brain stem neuronal activity and engages local effectors that trigger sympathetic energetic and cardiac responses that are comparable in many, but not in all, respects to those seen in neurologically intact rats.

Physiology of thermoregulatory dysfunction and current approaches to the treatment of vasomotor symptoms

Vasomotor symptoms (VMS), including hot flushes and night sweats, are the most common symptoms associated with menopause. Although the physiology of hot flushes is not fully defined, understanding the complex thermoregulatory circuitry that underlies VMS is important for the development of new therapies. This circuitry is composed of three distinct, yet interconnected, components: core body temperature, neurochemical messaging and peripheral vasculature. Evidence suggests that multiple physiological systems, including the neuroendocrine system, are important in the maintenance of thermoregulatory control. Causative roles of declining ovarian steroid levels in initiating thermoregulatory dysfunction are well documented. This paper reviews the physiology involved in the underlying thermoregulatory dysfunction that presumably causes VMS and discusses how this physiology relates to current and future VMS treatment options.

Amino acid concentrations in hypothalamic and caudate nuclei during microwave-induced thermal stress: analysis by microdialysis

Exposure to radiofrequency radiation (RFR) may produce thermal responses. Extracellular amino acid concentrations in the hypothalamus (Hyp) and caudate nucleus (CN) were measured by using in vivo microdialysis before and during exposure to RFR. Under urethane anesthetic, each rat was implanted stereotaxically with a nonmetallic microdialysis probe and temperature probe guides and then placed in the exposure chamber. The rat laid on its right side with its head and neck placed directly under the wave guide. Temperature probes were placed in the left brain, right brain, face (subcutaneously), left tympanum, and rectum. Each microdialysis sample was collected over a 20 min period. The microdialysis probe was perfused for 2 h before the rat was exposed to 5.02 GHz radiation (10 microseconds pulse width, 1000 pulses/s). The right and left sides of the brain were maintained at approximately 41.2 and 41.7 degrees C, respectively, throughout a 40 min exposure period. Initially when the brain was being heated to these temperatures, the time-averaged specific absorption rates (SARs) for the right and left sides of the brain were 29 and 40 W/kg, respectively. Concentrations of aspartic acid, glutamic acid, serine, glutamine, and glycine in dialysate were determined by using high-pressure liquid chromatography with electrochemical detection. In the Hyp and CN, the concentrations of aspartic acid, serine, and glycine increased significantly during RFR exposure (P < .05). These results indicate that RFR-induced thermal stress produces a general change in the amino acid concentrations that is not restricted to thermoregulatory centers. Changes in the concentrations of glutamic acid (Hyp, P = .16; CN, P = .34) and glutamine (Hyp, P = .13; CN, P = .10) were not statistically significant. Altered amino acid concentrations may reveal which brain regions are susceptible to damage in response to RFR-induced thermal stress.

Does the preoptic anterior hypothalamus receive thermoafferent information?

The preoptic anterior hypothalamus (POAH) is considered the thermointegrative center of the mammalian brain. Studies on anesthetized and unanesthetized animals have demonstrated neurons in the POAH that respond to changes in both POAH temperature (TPOAH) and skin temperature (Ts). In these studies, however, electroencephalographic (EEG) activity was not monitored. Recent work has revealed the potential for arousal state selectivity of neurons combined with thermal influences on arousal state to create the appearance that cells are thermosensitive or thermoresponsive when in fact they may not be responding directly to temperature or to thermoafferent input. It is therefore necessary to reexamine the influence of central and peripheral temperature on POAH cells. In the present study, 66 POAH cells were recorded from urethan-anesthetized rats while EEG, TPOAH, and Ts were monitored. Seventy-five percent (41 of 55) of the cells were EEG state responsive; 22% (6 of 27) were TPOAH sensitive; and 33% (19 of 58) appeared to be Ts responsive. However, when EEG state changes were taken into account, none of the cells that appeared to be Ts responsive were responding to Ts within any uniform EEG state. All changes in their firing rates were associated with EEG state changes. This study raises a question as to whether or not peripheral temperature information is integrated in the POAH. Consideration should be given to the possibility that Ts information is integrated lower in the neuroaxis. Monitoring EEG is essential in studies attempting to characterize the integrative properties of POAH neurons of anesthetized or unanesthetized animals. This caveat applies not just to thermoregulatory studies but to investigations of other integrative functions of the hypothalamus and many other brain regions as well.

Procaine microinjection into the lower midbrain increases brown fat and body temperatures in anesthetized rats

A tonic inhibitory mechanism on heat production was studied by microinjecting procaine into various regions of the brain while recording temperature changes of the interscapular brown adipose tissue (IBAT) and rectum in urethane-anesthetized rats at room temperature of 23-25 degrees C. Procaine microinjected bilaterally (10%, 1.0 mu l/site, 1.5 mm to midline) into the midbrain and the upper- to mid-pontine area of the reticular formation increased temperatures of the IBAT and rectum. The highest temperature rise (1.02 +/- 0.11 degrees C for IBAT, 0.64 +/- 0.06 degrees C for rectum) with the shortest onset latency (1.5 +/- 0.3 min for IBAT, 4.6 +/- 1.1 min for rectum) was observed when procaine was injected into the lower midbrain (the area between 6 and 7 mm posterior to the bregma, and 6.5 to 8.5 mm deep from the cortical surface). These regions include the retrorubral field, pedunculopontine tegmental nucleus, and rubrospinal tract. Procaine-induced IBAT and rectal temperature increases were dose-dependent, and reproduced reliably from the same injection site of the same animal. Intravenous indomethacin, a prostaglandin H synthase inhibitor, did not affect procaine-induced temperature rise, and propranolol, a beta-blocker, completely blocked it. These results suggest that microinjected procaine exerts its local anesthetic effect and release a tonic inhibition resulting in a disinhibition-induced temperature increase through the enhanced central sympathetic outflow. They support the hypothesis that a bilateral tonic inhibitory mechanism on heat production exists in the lower midbrain.

Fatal hyperthermia in a quadriplegic man. Possible evidence for a peripheral action of haloperidol in neuroleptic malignant syndrome

A patient with a cervical cord transection isolating his hypothalamic thermoregulatory centers from peripheral effectors suffered a fatal hyperthermic episode after receiving haloperidol. This suggests that neuroleptic malignant syndrome is caused by a peripheral, not central, effect of haloperidol.

Dissociation of thermoregulation in cats with cytotoxic pontine lesions

Neurons of the pontine tegmentum of the cat were lesioned by microinjection of ibotenic acid into the brainstem. The threshold ambient temperatures for heat-gain (shivering) and for heat-loss (panting) responses, together with brain and skin temperatures, were measured in intact animals and after the neurotoxic lesioning. After the lesioning the shivering threshold was altered but the panting threshold did not change. The results indicate that certain neurons involved in the shivering response reside in the pontine tegmentum. Neurons involved in the panting response, however, may lie outside the lesioned areas.

Effect of ambient temperature on sleep-waking cycle in cats with electrolytic dorsolateral pontine tegmental lesions

The effects of ambient temperature on the sleep-waking cycle were studied in intact cats and those with bilateral electrolytic lesions in the pontine tegmentum. At a room temperature of 23 degrees C, the percentage of time spent in paradoxical sleep was significantly lower in the lesioned cats than in intact animals. The mean duration of paradoxical sleep episodes was also decreased in the lesioned animals. The reduction in slow-wave sleep was not significant. At a slightly warmer ambient temperature of 30 degrees C, both the mean duration of paradoxical sleep episodes and the total duration of paradoxical sleep in the lesioned animals were increased toward normal values. Slow-wave sleep increased slightly but not significantly. At a higher ambient temperature of 35 degrees C, as well as at colder ambient temperatures of 15 and 7 degrees C, the durations of both paradoxical sleep and slow-wave sleep were significantly reduced. Under these thermal loads, the reduction in the duration of sleep was significantly greater in the lesioned cats than in the intact animals. The results suggest that: (i) pontine lesions alter the sleep cycle of cats and ambient temperature influences this alteration; (ii) the effects of thermal loads on the sleep cycle are more severe in the lesioned cats; and (iii) a moderately warm ambient temperature (30 degrees C) improves the sleep of pontine-lesioned cats.

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.

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.

Respiratory modulation of shivering intensity in the pigeon

Respiration and shivering were measured in unanaesthetized, cold-exposed pigeons using pneumotachography and electromyography, respectively. The instantaneous intensity of shivering in the pectoral muscle varied in phase with respiration. Power spectral analysis showed that the main frequency components of respiration and demodulated EMG coincided exactly. The intensity of shivering was highest during end-expiration and lowest at end-inspiration. This was confirmed by cross-correlation analysis of respiration and demodulated EMG. The absolute level of modulation remained constant (c. 10 microV peak-to-peak) despite changes in the general intensity of shivering. On the other hand, the relative depth of modulation was highest during incipient shivering. These facts indicate that only a part of the motor units recruited for shivering is susceptible to respiratory modulation and that this part is first recruited during incipient shivering. Inhalation of 5% CO2 did not affect the interaction between respiration and shivering although respiration frequency varied from 25 to 60 min-1. Thus, pulmonary chemoreceptors do not mediate this effect. It is suggested that the interaction between respiration and shivering occurs directly in the CNS. The question whether the interaction is adaptive for the animal or merely reflects a common evolutionary history of the underlying neural circuits is discussed.

Anaesthetic temperature and shivering in epidural anaesthesia

The mechanism of shivering during epidural analgesia remains unclear. This study investigates the role of the temperature of the local anaesthetic injected extradurally. Forty patients admitted for elective caesarean section under epidural anaesthesia were studied; 20 were given bupivacaine warmed to 37 degrees C (warm group) and 20 were given bupivacaine stored at 4 degrees C (cold group); the occurrence of shivering in both groups was recorded. The overall incidence of shivering was 27.5%; two patients of the warm group and nine patients of the cold group shivered. This difference was statistically significant (P less than 0.03). The results suggest that there are thermosensory mechanisms in the human spinal canal. In our view, epidural anaesthetic solutions should be warmed to body temperature prior to injection to reduce the incidence of shivering.

Effects of pontine tegmental lesions that induce paradoxical sleep without atonia on thermoregulation in cats during wakefulness

The characteristics of thermoregulation during wakefulness of cats with pontine tegmental lesions that induced paradoxical sleep (PS) without atonia have been studied. Thresholds for shivering and for panting, brain temperature and body posture at different ambient temperatures were noted and compared with those in intact animals. Intact cats began to shiver, piloerect and curl their bodies at ambient temperatures (Ta) of 8-10 degrees C, whereas after the lesions these responses began at Ta of 15-17 degrees C. Body temperature (Tb) did not change. Cats with lesions panted and extended their bodies much earlier at high Ta than did normal cats, and Tb began rising at a lower Ta and reached a higher value. Therefore, the cats with lesions were more susceptible to thermal loads; the thresholds for shivering (heat-gain) and panting (heat-loss) were both lowered. These results obtained in wakefulness suggest that the absence of shivering previously shown in cats during PS without atonia cannot simply be the result of an overall increased threshold for heat-gain responses but, rather, are in keeping with the observation that thermoregulation is suppressed in PS.

The effect of epidural analgesia on maternal thermoregulation in labor

Core and peripheral body temperatures were recorded in 24 parturients in labor with epidural analgesia and in 9 control parturients without analgesia. Shivering occurred in 11 (46%) of the women with epidural analgesia and in one parturient without analgesia. Twelve patients (50%) showed increase in leg and chest temperatures after injection of bupivacaine 0.35% into the epidural space. Lumbar epidural analgesia may produce shivering and temporary temperature changes in the periphery. Core temperature is not significantly altered.

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.

Behavioral functions of the reticular formation

Studies of the behavioral correlates of activity in reticular formation cells, usually performed in restrained animals, have found units whose discharge relates to sensory stimuli, pain and escape behavior, conditioning and habituation, arousal, complex motivational states, REM sleep, eye movements, respiration and locomotion. Units with these different behavioral correlates were found in the same anatomical areas. Most studies report that a large proportion of encountered cells related to the behavior being studied. If one adds up the reported percentages, the total far exceeds 100%. Therefore it appears that many investigators are looking at the same cells and reaching very different conclusions about their behavioral roles. On the basis of observations in unrestrained cats, it is hypothesized that discharge in most RF cells is primarily related to the excitation of small groups of muscles. This hypothesis can parsimoniously explain many previous observations on the behavioral correlates of these cells, and is consistent with anatomical, physiological and phylogenetic studies of the reticular formation. The hypothesized simplicity of reticular formation unit function is contrasted with the complexity of the behavioral functions mediated by the RF, and the implications of this contrast discussed.

Neural organization and evolution of thermal regulation in mammals

This article proposes a modification of the currently accepted view of the central neural integration of body temperature. In place of a single integrator with multiple inputs and outputs, the new model includes as many integrators as there are thermoregulatory responses. Futhermore, these integrators are postulated to be represented at many levels of the nervous system, with each level facilitated or inhibited by levels above and below. The purpose of such a complicated arrangement is to achieve finer and finer control over body temperature. A consideration of how endothermy might have evolved, with originally nonthermally related responses gradually coming under thermal control, makes such a brain organization highly reasonable.

Thermosensitive midbrain neurons in the cat

Responses of single neurons in the midbrain of cats anesthetized with chloral hydrate were studied during manipulations of midbrain temperature produced with a bilateral water-perfused thermode. Temperatures of the thermodes and the anterior hypothalamus were monitored while single neuron activity was recorded between the thermodes and correlated with the midbrain temperature. Q10's and thermal coefficients were calculated from the estimated temperature at the neuron itself. A surprisingly high percentage (72%) of the 72 neurons recorded in the caudal paramedian midbrain of 11 cats were thermoresponsive. Most of these were heat sensitive and exhibited a variety of frequency/temperature curves. Explorations of more rostral regions of the midbrain in 9 cats yielded only 18% thermoresponsive units out of the 99 neurons sampled. We suggest that the concentrated pool of warm-sensitive neurons in the caudal midbrain is part of an extensive system of brain stem thermosensors which are involved in establishing and controlling normal brain temperature.

Organization of brainstem behavioral systems

A review of recent literature suggests that the brainstem may play a more fundamental role in the elaboration of adaptive behaviors than has often been assumed. This view is indicated by current reports documenting the substantial behavioral repertoire of decerebrate animals and by the recent findings that electrical stimulation of localized areas in all major levels of the brainstem can induce complex and coordinated behaviors, including eating, grooming and attack. Indeed, behaviors elicited from sites in the caudal brainstem evidence unexpected goal specificity and stimulus control over response topography. Additional neuroanatomical and behavioral data are reviewed which further implicate caudal brainstem networks in process of reward and aversion. From these and other findings it is argued that integrating mechanisms for the expression of many aspects of species-characteristic behaviors are intrinsic to the brainstem. In line with this view, rostral hypothalamic-limbic mechanisms, while perhaps contributing refinement to the integration of behaviors, may best be viewed as phylogenetically newer control mechanisms making the expression of species-characteristic behaviors subordinate to additional class of exteroceptive and interoceptive stimuli.

Hypothermia induced by delta9-tetrahydrocannabinol in rats with electrolytic lesions of preoptic region

The preoptic region (POR) is a primary central site for thermoregulation. Bilateral lesions of POR disrupt thermoregulation, and in rats, produce a characteristic syndrome including hyperthermia. delta9-Tetrahydrocannabinol (delta9-THC), a potent hypothermic agent, appears to mediate this effect via some central mechanism. The studies reported here suggest that delta9-THC induces hypothermia at a site other than POR. Male Sprague-Dawley rats were divided into 2 groups, one with subsequently confirmed bilateral POR lesions and a sham operated group. The lesioned animals developed hyperthemia (+2.1degrees +/- 0.1degreesC, p less than 0.01) within 2 hr after surgery when compared to the sham operated controls. delta9-THC was administered intraperitoneally (5 and 10 mg/kg). Rectal temperature was recorded at 30 min intervals for 2.5 hr. Both lesioned and nonlesioned rats exhibited hypothermia within 30 min of delta9-THC administration. The hypothermic response to 5 and 10 mg/kg delta9-THC in the lesioned animals was significantly greater (p less than 0.05) and showed a trend toward longer duration than the hypothermia induced in the sham operated controls. These data demonstrate that delta9-THC is able to induce a hypothermic response in rats whose body temperatures were elevated by POR ablation. Although delta9-THC does not appear to act primarily at POR to induce hypothermia, it is evident than an intact POR plays a role in modifying the duration and magnitude of delta9-THC induced hypothermia.

Patterns of differentiation in various sympathetic efferents induced by hypoxic and by central thermal stimulation in decerebrated rabbits

The patterns of regional changes of sympathetic efferent activity evoked by thermal stimulation of the spinal cord and by arterial and primary tissue hypoxia were investigated in decerebrated, anesthetized and immobilized rabbits. Decerebration was performed either at the mid- or infracollicular level. The responses of the decerebrated rabbits evoked by spinal thermal stimulation were the same as those of intact rabbits, i.e., splanchnic and cardiac sympathetic activity increased and cutaneous sympathetic activity decreased during warming, while the reverse response was elicited by cooling. It is concluded that the typical thermoregulatory response pattern of the sympathetic nervous system can be produced also after the loss of hypothalamic integration, i.e., by integrative mechanisms in the lower brain stem and the spinal cord. In contrast, the responses of decerebrated rabbits to arterial and primary tissue hypoxia differed from those of intact rabbits in that they consisted in an overall activation in all investigated sympathetic branches. It is confirmed by this result that suprabulbar integration is essential for the generation of the inhibitory components in the differential sympathetic responses to hypoxia, which typically consist in cutaneous and cardiac sympathetic inhibition with splanchnic activation during arterial hypoxia and in cutaneous sympathetic inhibition with cardiac and splanchnic sympathetic activation during primary tissue hypoxia.

Interaction between the effects of spinal heating and cooling and of injections into a lateral cerebral ventricle of noradrenaline, 5-hydroxytryptamine and carbachol on thermoregulation in sheep

1. A study has been made of the interactions of the thermoregulatory effects of spinal cord heating and cooling and of the injections into the cerebral ventricle of noradrenaline, 5-hydroxytryptamine (5-HT) and carbamylcholine in sheep. 2. The interactions of spinal cord heating and the injections into the cerebral ventricle of noradrenaline, 5-HT and carbamylcholine were very similar to those of hypothalamic heating or of high ambient temperature and the injections into the cerebral ventricle of these substances. These results are interpreted as evidence of the synaptic convergence of the pathways from peripheral, spinal cord and hypothalamic warm-sensors at or before the points of action of these synaptically active substances. 3. The only definite thermoregulatory effect of spinal cooling was the onset of shivering which could be due to a purely spinal effect of cold. No substantial evidence was obtained of an interaction between spinal cooling and an injection of noradrenaline, 5-HT or carbamylcholine into the cerebral ventricle. Thus there was no clear indication of centripetal pathways from spinal cold sensors converging with those from the skin and the hypothalamus for which evidence of convergence was obtained in an earlier study. 4. The results of this study are expressed in terms of the neuronal model of Bligh, Cottle & Maskrey (1971) and Maskrey & Bligh (1971), appropriately modified.