Arterial baroreflex modulation of heat-induced vasodilation in the rabbit ear

Brain temperature and its role in physiology and pathophysiology: Lessons from 20 years of thermorecording

It is well known that temperature affects the dynamics of all physicochemical processes governing neural activity. It is also known that the brain has high levels of metabolic activity, and all energy used for brain metabolism is finally transformed into heat. However, the issue of brain temperature as a factor reflecting neural activity and affecting various neural functions remains in the shadow and is usually ignored by most physiologists and neuroscientists. Data presented in this review demonstrate that brain temperature is not stable, showing relatively large fluctuations (2-4°C) within the normal physiological and behavioral continuum. I consider the mechanisms underlying these fluctuations and discuss brain thermorecording as an important tool to assess basic changes in neural activity associated with different natural (sexual, drinking, eating) and drug-induced motivated behaviors. I also consider how naturally occurring changes in brain temperature affect neural activity, various homeostatic parameters, and the structural integrity of brain cells as well as the results of neurochemical evaluations conducted in awake animals. While physiological hyperthermia appears to be adaptive, enhancing the efficiency of neural functions, under specific environmental conditions and following exposure to certain psychoactive drugs, brain temperature could exceed its upper limits, resulting in multiple brain abnormalities and life-threatening health complications.

Fluctuations in central and peripheral temperatures associated with feeding behavior in rats

We examined the pattern of temperature fluctuations in the nucleus accumbens (NAcc), temporal muscle, and skin, along with locomotion in food-deprived and nondeprived rats following the presentation of an open or closed food container and during subsequent eating or food-seeking behavior without eating. Although rats in food-deprived, quiet resting conditions had more than twofold lower spontaneous locomotion and lower temperature values than in nondeprived conditions, after presentation of a container, they consistently displayed food-seeking behavior, showing much larger and longer temperature changes. When the container was open, rats rapidly retrieved food and consumed it. Food consumption was preceded and accompanied by gradual increases in brain and muscle temperatures ( approximately 1.5 degrees C) and a weaker, delayed increase in skin temperature ( approximately 0.8 degrees C). All temperatures began to rapidly fall immediately after eating was completed, but NAcc and muscle temperatures returned to baseline after approximately 35 min. When the container was closed and rats were unable to obtain food, they continued food-seeking activity during the entire period of presentation. Similar to eating, this activity was preceded and accompanied by gradual temperature increases in the brain and muscle, which were somewhat smaller than those during eating ( approximately 1.2 degrees C), with no changes in skin temperature. In contrast to trials with eating, NAcc and muscle temperatures continued to increase for approximately 10 min after the container was removed from the cage and the rat continued food-seeking behavior, with a return to baselines after approximately 50 min. These temperature fluctuations are discussed with respect to alterations in metabolic brain activity associated with feeding behavior, depending upon deprivation state and food availability.

Brain temperature fluctuations during physiological and pathological conditions

This review discusses brain temperature as a physiological parameter, which is determined primarily by neural metabolism, regulated by cerebral blood flow, and affected by various environmental factors and drugs. First, we consider normal fluctuations in brain temperature that are induced by salient environmental stimuli and occur during motivated behavior at stable normothermic conditions. Second, we analyze changes in brain temperature induced by various drugs that affect brain and body metabolism and heat dissipation. Third, we consider how these physiological and drug-induced changes in brain temperature are modulated by environmental conditions that diminish heat dissipation. Our focus is psychomotor stimulant drugs and brain hyperthermia as a factor inducing or potentiating neurotoxicity. Finally, we discuss how brain temperature is regulated, what changes in brain temperature reflect, and how these changes may affect neural functions under normal and pathological conditions. Although most discussed data were obtained in animals and several important aspects of brain temperature regulation in humans remain unknown, our focus is on the relevance of these data for human physiology and pathology.

Physiological and pathological brain hyperthermia

While brain temperature is usually considered a stable, tightly regulated parameter, recent animal research revealed relatively large and rapid brain temperature fluctuations (approximately 3 degrees C) during various forms of naturally occurring physiological and behavioral activities. This work demonstrates that physiological brain hyperthermia has an intra-brain origin, resulting from enhanced neural metabolism and increased intra-brain heat production, and discusses its possible mechanisms and functional consequences. This work also shows that brain hyperthermia may also be induced by various drugs of abuse. While each individual drug (i.e., heroin, cocaine, meth-amphetamine, MDMA) has its own, dose-dependent effects on brain and body temperatures, these effects are strongly modulated by the individual's activity state and environmental conditions, showing dramatic alterations during the development of drug-taking behavior. While brain temperatures may also increase due to environmental overheating and diminished heat dissipation from the brain, adverse environmental conditions and physiological activation strongly potentiate thermal effects of psychomotor stimulant drugs, resulting in dangerous brain overheating. Since hyperthermia exacerbates drug-induced toxicity and is destructive to neural cells and brain functions, use of these drugs under conditions that restrict heat loss may pose a significant health risk, resulting in both acute life-threatening complications and chronic destructive CNS changes. We argue that brain temperature is an important physiological parameter, affecting various neural functions, and show the potential of brain temperature monitoring for studying alterations in metabolic neural activity under physiological and pathological conditions. Finally, we discuss brain temperature as a factor affecting various neuronal and neurochemical evaluations made in different animal preparations (in vitro slices, general anesthesia, awake, freely moving conditions) and consider a possible contribution of temperature fluctuations to behavior-related and drug-induced alterations in neuronal and neurochemical parameters.

Brain hyperthermia as physiological and pathological phenomena

Although brain metabolism consumes high amounts of energy and is accompanied by intense heat production, brain temperature is usually considered a stable, tightly "regulated" homeostatic parameter. Current research, however, revealed relatively large and rapid brain temperature fluctuations (3-4 degrees C) in animals during various normal, physiological, and behavioral activities at stable ambient temperatures. This review discusses these data and demonstrates that physiological brain hyperthermia has an intra-brain origin, resulting from enhanced neural metabolism and increased intra-brain heat production. Therefore, brain temperature is an important physiological parameter that both reflects alterations in metabolic neural activity and affects various neural functions. This work also shows that brain hyperthermia may be induced by various drugs of abuse that cause metabolic brain activation and impair heat dissipation. While individual drugs (i.e., heroin, cocaine, methamphetamine, MDMA) have specific, dose-dependent effects on brain and body temperatures, these effects are strongly modulated by an individual's activity state and environmental conditions, and change dramatically during the development of drug self-administration. Thus, brain thermorecording may provide new information on the central effects of various addictive drugs, drug-activity-environment interactions in mediating drugs' adverse effects, and alterations in metabolic neural activity associated with the development of drug-seeking and drug-taking behavior. While ambient temperatures and impairment of heat dissipation may also affect brain temperature, these environmental conditions strongly potentiate thermal effects of psychomotor stimulant drugs, resulting in pathological brain overheating. Since hyperthermia exacerbates drug-induced toxicity and is destructive to neural cells and brain functions, use of these drugs under activated conditions that restrict heat loss may pose a significant health risk, resulting in both acute life-threatening complications and chronic destructive CNS changes.

Carotid-Sinus Baroreflex Modulation of Core and Skin Temperatures in Rats: An Open-Loop Approach

The neural mechanisms of the thermoregulatory control of core and skin temperatures in response to heat and cold stresses have been well clarified. However, it has been unclear whether baroreceptor reflexes are involved in the control of core and skin temperatures. To investigate how the arterial baroreceptor reflex modulates the body temperatures, we examined the effect of pressure changes of carotid sinus baroreceptors on core and skin temperatures in halothane-anesthetized rats. To open the baroreflex loop and control arterial baroreceptor pressure (BRP), we cut vagal and aortic depressor nerves and isolated carotid sinuses. We sequentially altered BRP in 20-mmHg increments from 60 to 180 mmHg and then in 20-mmHg decrements from 180 to 60 mmHg while measuring systemic arterial pressure (SAP), heart rate (HR), and core blood temperature (T(core)) at the aortic arch and skin temperature (T(skin)) at the tail. In response to the incremental change in BRP by 120 mmHg, SAP, HR, and T(core) fell by 90.3 +/- 5.1 mmHg, 60.3 +/- 10.5 beats min(-1), and 0.18 +/- 0.01 degrees C, respectively. T(skin) rose by 0.84 +/- 0.10 degrees C. The maximum rate of change per unit BRP change was -2.1 +/- 0.2 for SAP, -1.5 +/- 0.4 beats min(-1) mmHg(-1) for HR, -0.003 +/- 0.001 degrees C mmHg(-1) for T(core), and 0.011 +/- 0.002 degrees C mmHg(-1) for T(skin). After the administration of hexamethonium or bretylium, these baroreflexogenic responses were completely abolished. We concluded that T(core) and T(skin) are modulated by the arterial baroreceptor reflex.

Reflex control of the cutaneous circulation during passive body core heating in humans

The impact of body core heating on the interaction between the cutaneous and central circulation during blood pressure challenges was examined in eight adults. Subjects were exposed to -10 to -90 mmHg lower body negative pressure (LBNP) in thermoneutral conditions and -10 to -60 mmHg LBNP during heat stress. We measured forearm vascular conductance (FVC; ml. min(-1). 100 ml(-1). mmHg(-1)) by plethysmography; cutaneous vascular conductance (CVC) by laser-Doppler techniques; and central venous pressure, arterial blood pressure, and cardiac output by impedance cardiography. Heat stress increased FVC from 5.7 +/- 0.9 to 18.8 +/- 1.3 conductance units (CU) and CVC from 0.21 +/- 0.07 to 1.02 +/- 0.20 CU. The FVC-CVP relationship was linear over the entire range of LBNP and was shifted upward during heat stress with a slope increase from 0. 46 +/- 0.10 to 1.57 +/- 0.3 CU/mmHg CVP (P < 0.05). Resting CVP was lower during heat stress (6.3 +/- 0.6 vs. 7.7 +/- 0.6 mmHg; P < 0. 05) but fell to similar levels during LBNP as in normothermic conditions. Data analysis indicates an increased capacity, but not sensitivity, of peripheral baroreflex responses during heat stress. Laser-Doppler techniques detected thermoregulatory responses in the skin, but no significant change in CVC occurred during mild-to-moderate LBNP. Interestingly, very high levels of LBNP produced cutaneous vasodilation in some subjects.