On the role of anesthesia on the body/brain temperature differential in rats

Transient Changes in Cerebral Tissue Oxygen, Glucose, and Temperature by Microstrokes: A Computational Study

OBJECTIVE

This study focuses on evaluating the disruptions in key physiological parameters during microstroke events to assess their severity.

METHODS

A mathematical model was developed to simulate the changes in cerebral tissue pO2, glucose concentration, and temperature due to blood flow interruptions. The model considers variations in baseline cerebral blood flow (CBF), capillary density, and blood oxygen/glucose levels, as well as ambient temperature changes.

RESULTS

Simulations indicate that complete blood flow obstruction still allows for limited glucose availability, supporting nonoxidative metabolism and potentially exacerbating lactate buildup and acidosis. Partial obstructions decrease tissue pO2, with minimal impact on glucose level, which can remain almost unchanged or even slightly increase. Reduced CBF, capillary density, or blood oxygen due to aging or disease enhances hypoxia risk at lower obstruction levels, with capillary density having a significant effect on stroke severity by influencing both pO2 and glucose levels. Conditions could lead to co-occurrence of hypoxia/hypoglycemia or hypoxia/hyperglycemia, each worsening outcomes. Temperature effects were minimal in deep brain regions but varied near the skull by 0.2-0.8°C depending on ambient temperature.

CONCLUSIONS

The model provides insights into the conditions driving severe stroke outcomes based on estimated levels of hypoxia, hypoglycemia, hyperglycemia, and temperature changes.

Autonomous animal heating and cooling system for temperature-regulated magnetic resonance experiments

Temperature is a hallmark parameter influencing almost all magnetic resonance properties (e.g., T1 , T2 , proton density, and diffusion). In the preclinical setting, temperature has a large influence on animal physiology (e.g., respiration rate, heart rate, metabolism, and oxidative stress) and needs to be carefully regulated, especially when the animal is under anesthesia and thermoregulation is disrupted. We present an open-source heating and cooling system capable of regulating the temperature of the animal. The system was designed using Peltier modules capable of heating or cooling a circulating water bath with active temperature feedback. Feedback was obtained using a commercial thermistor, placed in the animal rectum, and a proportional-integral-derivative controller was used to modulate the temperature. Its operation was demonstrated in a phantom as well as in mouse and rat animal models, where the standard deviation of the temperature of the animal upon convergence was less than a 10th of a degree. An application where brain temperature of a mouse was modulated was demonstrated using an invasive optical probe and noninvasive magnetic resonance spectroscopic thermometry measurements.

In vivo brain temperature mapping using polymer optical fiber Bragg grating sensors

Variation of the brain temperature is strongly affected by blood flow, oxygen supply, and neural cell metabolism. Localized monitoring of the brain temperature is one of the most effective ways to correlate brain functions and diseases such as stroke, epilepsy, and mood disorders. While polymer optical fibers (POFs) are considered ideal candidates for temperature sensing in the brain, they have never been used so far in vivo. Here, we developed for the first, to the best of our knowledge, time an implantable probe based on a microstructured polymer optical fiber Bragg grating (FBG) sensor for intracranial brain temperature mapping. The temperature at different depths of the brain (starting from the cerebral cortex) and the correlation between the brain and body core temperature of a rat were recorded with a sensitivity of 33 pm/°C and accuracy <0.2°C. Our in vivo experimental results suggest that the proposed device can achieve real-time and high-resolution local temperature measurement in the brain, as well as being integrated with existing neural interfaces.

In Vivo Magnetic Resonance Thermometry for Brain and Body Temperature Variations in Canines under General Anesthesia

The core body temperature tends to decrease under general anesthesia. Consequently, monitoring the core body temperature during procedures involving general anesthesia is essential to ensure patient safety. In veterinary medicine, rectal temperature is used as an indicator of the core body temperature, owing to the accuracy and convenience of this approach. Some previous studies involving craniotomy reported differences between the brain and core temperatures under general anesthesia. However, noninvasive imaging techniques are required to ascertain this because invasive brain temperature measurements can cause unintended temperature changes by inserting the temperature sensors into the brain or by performing the surgical operations. In this study, we employed in vivo magnetic resonance thermometry to observe the brain temperatures of patients under general anesthesia using the proton resonance frequency shift method. The rectal temperature was also recorded using a fiber optic thermometer during the MR thermometry to compare with the brain temperature changes. When the rectal temperature decreased by 1.4 ± 0.5 °C (mean ± standard deviation), the brain temperature (white matter) decreased by 4.8 ± 0.5 °C. Furthermore, a difference in the temperature reduction of the different types of brain tissue was observed; the reduction in the temperature of white matter exceeded that of gray matter mainly due to the distribution of blood vessels in the gray matter. We also analyzed and interpreted the core temperature changes with the body conditioning scores of subjects to see how the body weight affected the temperature changes.

Translational Intracerebral Hemorrhage Research: Has Current Neuroprotection Research ARRIVEd at a Standard for Experimental Design and Reporting?

One major aim of preclinical intracerebral hemorrhage (ICH) research is to develop and test potential neuroprotectants. Published guidelines for experimental design and reporting stress the importance of clearly and completely reporting results and methodological details to ensure reproducibility and maximize information availability. The current review has two objectives: first, to characterize current ICH neuroprotection research and, second, to analyze aspects of translational design in preclinical ICH studies. Translational design is the adoption and reporting of experimental design characteristics that are thought to be clinically relevant and critical to reproducibility in animal studies (e.g., conducting and reporting experiments according to the STAIR and ARRIVE guidelines, respectively). Given that ICH has no current neuroprotective treatments and an ongoing reproducibility crisis in preclinical research, translational design should be considered by investigators. We conducted a systematic review of ICH research from 2015 to 2019 using the PubMed database. Our search returned 281 published manuscripts studying putative neuroprotectants in animal models. Contemporary ICH research predominantly uses young, healthy male rodents. The collagenase model is the most commonly used. Reporting of group sizes, blinding, and randomization are almost unanimous, but group size calculations, mortality and exclusion criteria, and animal model characteristics are infrequently reported. Overall, current ICH neuroprotection research somewhat aligns with experimental design and reporting guidelines. However, there are areas for improvement. Because failure to consider translational design is associated with inflation of effect sizes (and possibly hindered reproducibility), we suggest that researchers, editors, and publishers collaboratively consider enhanced adherence to published guidelines.

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.

Physiological and Pathological Brain Activation in the Anesthetized Rat Produces Hemodynamic-Dependent Cortical Temperature Increases That Can Confound the BOLD fMRI Signal

Anesthetized rodent models are ubiquitous in pre-clinical neuroimaging studies. However, because the associated cerebral morphology and experimental methodology results in a profound negative brain-core temperature differential, cerebral temperature changes during functional activation are likely to be principally driven by local inflow of fresh, core-temperature, blood. This presents a confound to the interpretation of blood-oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) data acquired from such models, since this signal is also critically temperature-dependent. Nevertheless, previous investigation on the subject is surprisingly sparse. Here, we address this issue through use of a novel multi-modal methodology in the urethane anesthetized rat. We reveal that sensory stimulation, hypercapnia and recurrent acute seizures induce significant increases in cortical temperature that are preferentially correlated to changes in total hemoglobin concentration (Hbt), relative to cerebral blood flow and oxidative metabolism. Furthermore, using a phantom-based evaluation of the effect of such temperature changes on the BOLD fMRI signal, we demonstrate a robust inverse relationship between both variables. These findings suggest that temperature increases, due to functional hyperemia, should be accounted for to ensure accurate interpretation of BOLD fMRI signals in pre-clinical neuroimaging studies.

The IMPROVE Guidelines (Ischaemia Models: Procedural Refinements Of in Vivo Experiments)

Most in vivo models of ischaemic stroke target the middle cerebral artery and a spectrum of stroke severities, from mild to substantial, can be achieved. This review describes opportunities to improve the in vivo modelling of ischaemic stroke and animal welfare. It provides a number of recommendations to minimise the level of severity in the most common rodent models of middle cerebral artery occlusion, while sustaining or improving the scientific outcomes. The recommendations cover basic requirements pre-surgery, selecting the most appropriate anaesthetic and analgesic regimen, as well as intraoperative and post-operative care. The aim is to provide support for researchers and animal care staff to refine their procedures and practices, and implement small incremental changes to improve the welfare of the animals used and to answer the scientific question under investigation. All recommendations are recapitulated in a summary poster (see supplementary information).

Body Temperature Modulates Infarction Growth following Endovascular Reperfusion

BACKGROUND AND PURPOSE

The neuronal substrate is highly sensitive to temperature elevation; however, its impact on the fate of the ischemic penumbra has not been established. We analyzed interactions between temperature and penumbral expansion among successfully reperfused patients with acute ischemic stroke, hypothesizing infarction growth and worse outcomes among patients with fever who achieve full reperfusion.

MATERIALS AND METHODS

Data from 129 successfully reperfused (modified TICI 2b/3) patients (mean age, 65 ± 15 years) presenting within 12 hours of onset were examined from a prospectively collected acute ischemic stroke registry. CT perfusion was analyzed to produce infarct core, hypoperfusion, and penumbral mismatch volumes. Final DWI infarction volumes were measured, and relative infarction growth was computed. Systemic temperatures were recorded throughout hospitalization. Correlational and logistic regression analyses assessed the associations between fever (>37.5°C) and both relative infarction growth and favorable clinical outcome (90-day mRS of ≤2), corrected for NIHSS score, reperfusion times, and age. An optimized model for outcome prediction was computed by using the Akaike Information Criterion.

RESULTS

The median presentation NIHSS score was 18 (interquartile range, 14-22). Median (interquartile range) CTP-derived volumes were: core = 9.6 mL (1.5-25.3 mL); hypoperfusion = 133 mL (84.2-204 mL); and final infarct volume = 9.6 mL (8.3-45.2 mL). Highly significant correlations were observed between temperature of >37.5°C and relative infarction growth (Kendall τ correlation coefficient = 0.24, P = .002). Odds ratios for favorable clinical outcome suggested a trend toward significance for fever in predicting a 90-day mRS of ≤2 (OR = 0.31, P = .05). The optimized predictive model for favorable outcomes included age, NIHSS score, procedure time to reperfusion, and fever. Likelihood ratios confirmed the superiority of fever inclusion (P < .05). Baseline temperature, range, and maximum temperature did not meet statistical significance.

CONCLUSIONS

These findings suggest that imaging and clinical outcomes may be affected by systemic temperature elevations, promoting infarction growth despite reperfusion.

Cellular and molecular cues of glucose sensing in the rat olfactory bulb

In the brain, glucose homeostasis of extracellular fluid is crucial to the point that systems specifically dedicated to glucose sensing are found in areas involved in energy regulation and feeding behavior. Olfaction is a major sensory modality regulating food consumption. Nutritional status in turn modulates olfactory detection. Recently it has been proposed that some olfactory bulb (OB) neurons respond to glucose similarly to hypothalamic neurons. However, the precise molecular cues governing glucose sensing in the OB are largely unknown. To decrypt these molecular mechanisms, we first used immunostaining to demonstrate a strong expression of two neuronal markers of glucose-sensitivity, insulin-dependent glucose transporter type 4 (GLUT4), and sodium glucose co-transporter type 1 (SGLT1) in specific OB layers. We showed that expression and mapping of GLUT4 but not SGLT1 were feeding state-dependent. In order to investigate the impact of metabolic status on the delivery of blood-borne glucose to the OB, we measured extracellular fluid glucose concentration using glucose biosensors simultaneously in the OB and cortex of anesthetized rats. We showed that glucose concentration in the OB is higher than in the cortex, that metabolic steady-state glucose concentration is independent of feeding state in the two brain areas, and that acute changes in glycemic conditions affect bulbar glucose concentration alone. These data provide new evidence of a direct relationship between the OB and peripheral metabolism, and emphasize the importance of glucose for the OB network, providing strong arguments toward establishing the OB as a glucose-sensing organ.

Altered functional connectivity in an aged rat model of postoperative cognitive dysfunction: a study using resting-state functional MRI

Background

Postoperative cognitive impairment is a common complication after cardiac and major non-cardiac surgery in the elderly, but its causes and mechanisms remain unclear. The purpose of the current study was to use resting-state functional magnetic resonance imaging (fMRI) to explore changes in the functional connectivity, i.e. the synchronization of low frequency fluctuation (LFF), in an animal model of cognitive impairment in aged rats.

Methods

Aged (22 months) rats were anaesthetized with 40 µg/kg fentanyl and 500 µg/kg droperidol (intraperitoneal) for splenectomy. Cognitive function was assessed using Y maze prior to operation and on postoperative days 1, 3 and 9. To evaluate functional connectivity, resting-state fMRI data were acquired using a 3T MR imaging system with a 4 channel phase array rat head coil.

Results

Cognitive function was impaired at postoperative days 1 and 3 compared with preoperative. Significant synchronized LFF was detected bilaterally in the primary somatosensory cortex and hippocampus preoperatively. By contrast, no significant LFF synchronization was detected in the right primary somatosensory cortex and right hippocampus on postoperative days 1 and 3, although the pattern of functional connectivity had become almost normal by day 9.

Conclusion

Splenectomy performed under neuroleptic anaesthesia triggers a cognitive decline that is associated with altered spontaneous neuronal activity in the cortex and hippocampus.

Physiological fluctuations in brain temperature as a factor affecting electrochemical evaluations of extracellular glutamate and glucose in behavioral experiments

The rate of any chemical reaction or process occurring in the brain depends on temperature. While it is commonly believed that brain temperature is a stable, tightly regulated homeostatic parameter, it fluctuates within 1-4 °C following exposure to salient arousing stimuli and neuroactive drugs, and during different behaviors. These temperature fluctuations should affect neural activity and neural functions, but the extent of this influence on neurochemical measurements in brain tissue of freely moving animals remains unclear. In this Review, we present the results of amperometric evaluations of extracellular glutamate and glucose in awake, behaving rats and discuss how naturally occurring fluctuations in brain temperature affect these measurements. While this temperature contribution appears to be insignificant for glucose because its extracellular concentrations are large, it is a serious factor for electrochemical evaluations of glutamate, which is present in brain tissue at much lower levels, showing smaller phasic fluctuations. We further discuss experimental strategies for controlling the nonspecific chemical and physical contributions to electrochemical currents detected by enzyme-based biosensors to provide greater selectivity and reliability of neurochemical measurements in behaving animals.

Brain temperature: physiology and pathophysiology after brain injury

The regulation of brain temperature is largely dependent on the metabolic activity of brain tissue and remains complex. In intensive care clinical practice, the continuous monitoring of core temperature in patients with brain injury is currently highly recommended. After major brain injury, brain temperature is often higher than and can vary independently of systemic temperature. It has been shown that in cases of brain injury, the brain is extremely sensitive and vulnerable to small variations in temperature. The prevention of fever has been proposed as a therapeutic tool to limit neuronal injury. However, temperature control after traumatic brain injury, subarachnoid hemorrhage, or stroke can be challenging. Furthermore, fever may also have beneficial effects, especially in cases involving infections. While therapeutic hypothermia has shown beneficial effects in animal models, its use is still debated in clinical practice. This paper aims to describe the physiology and pathophysiology of changes in brain temperature after brain injury and to study the effects of controlling brain temperature after such injury.

Electromagnetic field effect or simply stress? Effects of UMTS exposure on hippocampal longterm plasticity in the context of procedure related hormone release

Harmful effects of electromagnetic fields (EMF) on cognitive and behavioural features of humans and rodents have been controversially discussed and raised persistent concern about adverse effects of EMF on general brain functions. In the present study we applied radio-frequency (RF) signals of the Universal Mobile Telecommunications System (UMTS) to full brain exposed male Wistar rats in order to elaborate putative influences on stress hormone release (corticosteron; CORT and adrenocorticotropic hormone; ACTH) and on hippocampal derived synaptic long-term plasticity (LTP) and depression (LTD) as electrophysiological hallmarks for memory storage and memory consolidation. Exposure was computer controlled providing blind conditions. Nominal brain-averaged specific absorption rates (SAR) as a measure of applied mass-related dissipated RF power were 0, 2, and 10 W/kg over a period of 120 min. Comparison of cage exposed animals revealed, regardless of EMF exposure, significantly increased CORT and ACTH levels which corresponded with generally decreased field potential slopes and amplitudes in hippocampal LTP and LTD. Animals following SAR exposure of 2 W/kg (averaged over the whole brain of 2.3 g tissue mass) did not differ from the sham-exposed group in LTP and LTD experiments. In contrast, a significant reduction in LTP and LTD was observed at the high power rate of SAR (10 W/kg). The results demonstrate that a rate of 2 W/kg displays no adverse impact on LTP and LTD, while 10 W/kg leads to significant effects on the electrophysiological parameters, which can be clearly distinguished from the stress derived background. Our findings suggest that UMTS exposure with SAR in the range of 2 W/kg is not harmful to critical markers for memory storage and memory consolidation, however, an influence of UMTS at high energy absorption rates (10 W/kg) cannot be excluded.

Body and brain temperature coupling: the critical role of cerebral blood flow

Direct measurements of deep-brain and body-core temperature were performed on rats to determine the influence of cerebral blood flow (CBF) on brain temperature regulation under static and dynamic conditions. Static changes of CBF were achieved using different anesthetics (chloral hydrate, CH; alpha-chloralose, alphaCS; and isoflurane, IF) with alphaCS causing larger decreases in CBF than CH and IF; dynamic changes were achieved by inducing transient hypercapnia (5% CO(2) in 40% O(2) and 55% N(2)). Initial deep-brain/body-core temperature differentials were anesthetic-type dependent with the largest differential observed with rats under alphaCS anesthesia (ca. 2 degrees C). Hypercapnia induction raised rat brain temperature under all three anesthesia regimes, but by different anesthetic-dependent amounts correlated with the initial differentials--alphaCS anesthesia resulted in the largest brain temperature increase (0.32 +/- 0.08 degrees C), while CH and IF anesthesia lead to smaller increases (0.12 +/- 0.03 and 0.16 +/- 0.05 degrees C, respectively). The characteristic temperature transition time for the hypercapnia-induced temperature increase was 2-3 min under CH and IF anesthesia and approximately 4 min under alphaCS anesthesia. We conclude that both, the deep-brain/body-core temperature differential and the characteristic temperature transition time correlate with CBF: a lower CBF promotes higher deep-brain/body-core temperature differentials and, upon hypercapnia challenge, longer characteristic transition times to increased temperatures.

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.

Methods to produce hyperthermia-induced brain dysfunction

The recent increase in the frequency and intensity of killer heat waves across the globe has aroused worldwide medical attention to exploring therapeutic strategies to attenuate heat-related morbidity and/or mortality. Death due to heat-related illnesses often exceeds >50% of heat victims. Those who survive are crippled with lifetime disabilities and exhibit profound cognitive, sensory, and motor dysfunction akin to premature neurodegeneration. Although more than 50% of the world populations are exposed to summer heat waves; our understanding of detailed underlying mechanisms and the suitable therapeutic strategies have still not been worked out. One of the basic reasons behind this is the lack of a reliable experimental model to simulate clinical hyperthermia. This chapter describes a suitable animal model to induce hyperthermia in rats (or mice) comparable to the clinical situation. The model appears to be useful for studying the effects of heat-related illnesses on changes in various organs and systems, including the central nervous system (CNS). Since hyperthermia is often associated with profound brain dysfunction, additional methods to examine some crucial parameters of brain injury, e.g., blood-brain barrier (BBB) breakdown and brain edema formation, are also described.

Exercise and heat stress: cerebral challenges and consequences

This review deals with new aspects of exercise in the heat as a challenge that not only influences the locomotive and cardiovascular systems, but also affects the brain. Activation of the brain during such exercise is manifested in the lowering of the cerebral glucose to oxygen uptake ratio, the elevated ratings of perceived exertion and increased release of hypothalamic hormones. While the slowing of the electroencephalographic (EEG), the decreased endurance and hampered ability to activate the skeletal muscles maximally during sustained isometric and repeated isokinetic contractions appear to relate to central fatigue arising as the core/brain increases, the central fatigue during exercise with hyperthermia thus can be considered as the ultimate safety break against catastrophic hyperthermia. This would force the subject to stop exercising or decrease the internal heat production. It appears that the dopaminergic system is important, but several other factors may interact and feedback from the skeletal muscles and internal temperature sensors are probably also involved. The complexity of brain fatigue response is discussed based on our own investigations and in the light of recent literature.

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.

Theoretical model of temperature regulation in the brain during changes in functional activity

The balance between metabolic heat production, heat removal by blood flow, and heat conductance defines local temperature distribution in a living tissue. Disproportional local increases in blood flow as compared with oxygen consumption during functional brain activity disturb this balance, leading to temperature changes. In this article we have developed a theoretical framework that allows analysis of temperature changes during arbitrary functional brain activity. We established theoretical boundaries on temperature changes and explained how these boundaries depend on physiology (blood flow and metabolism) and external (heat exchange with the environment) experimental conditions. We show that, in regions located deep in the brain, task performance should be accompanied by temperature decreases in regions where blood flow increases (activated regions) and by temperature increases in regions where blood flow decreases (deactivated regions). The sign of temperature effect may be reversed for superficial cortex regions, where the baseline brain temperature is lower than the temperature of incoming arterial blood due to the heat exchange with the environment. Importantly, due to heat conductance, the temperature effect is not localized to the activated region but extends to a surrounding tissue at rest over the distances regulated by the temperature-shielding effect of blood flow. This temperature-shielding effect quantifies the means by which cerebral blood flow prevents "temperature perturbations" from propagating away from the perturbed regions. For small activated regions, this effect also substantially suppresses the magnitude of the temperature response, making it especially important for small animal brains.

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.