Anesthetic technique influences brain temperature, independently of core temperature, during craniotomy in cats

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.

Rethinking the Conditions and Mechanism for Glymphatic Clearance

Critical studies that form the foundation of the glymphatic system and the clearance of metabolic by-products of unwanted proteins from the brain are reviewed. Concerns are raised about studying glymphatic flow in anesthetized animals and making assumptions about the whole brain based upon data collected from a cranial window on the cortex. A new model is proposed arguing that the flow of cerebral spinal fluid and parenchymal clearance in the perivascular system of unwanted proteins is regulated by circadian changes in brain temperature and blood flow at the level of the microvasculature.

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.

Dynamic Thermal Mapping of Localized Therapeutic Hypothermia in the Brain

Although whole body cooling is used widely to provide therapeutic hypothermia for the brain, there are undesirable clinical side effects. Selective brain cooling may allow for rapid and controllable neuroprotection while mitigating these undesirable side effects. We evaluated an innovative cerebrospinal fluid (CSF) cooling platform that utilizes chilled saline pumped through surgically implanted intraventricular catheters to induce hypothermia. Magnetic resonance thermal imaging of the healthy sheep brain (n = 4) at 7.0T provided dynamic temperature measurements from the whole brain. Global brain temperature was 38.5 ± 0.8°C at baseline (body temperature of 39.2 ± 0.4°C), and decreased by 3.1 ± 0.3°C over ∼30 min of cooling (p < 0.0001). Significant cooling was achieved in all defined regions across both the ipsilateral and contralateral hemispheres relative to catheter placement. On cooling cessation, global brain temperature increased by 3.1 ± 0.2°C over ∼20 min (p < 0.0001). Rapid and synchronized temperature fall/rise on cooling onset/offset was observed reproducibly with rates ranging from 0.06-0.21°C/min, where rewarming was faster than cooling (p < 0.0001) signifying the importance of thermoregulation in the brain. Although core regions (including the subcortex, midbrain, olfactory tract, temporal lobe, occipital lobe, and parahippocampal cortex) had slightly warmer (∼0.2°C) baseline temperatures, after cooling, temperatures reached the same level as the non-core regions (35.6 ± 0.2°C), indicating the cooling effectiveness of the CSF-based cooling device. In summary, CSF-based intraventricular cooling reliably reduces temperature in all identified brain regions to levels known to be neuroprotective, while maintaining overall systemic normothermia. Dynamic thermal mapping provides high spatiotemporal temperature measurements that can aid in optimizing selective neuroprotective protocols.

In Vivo Detection of Perinatal Brain Metabolite Changes in a Rabbit Model of Intrauterine Growth Restriction (IUGR)

Background

Intrauterine growth restriction (IUGR) is a risk factor for abnormal neurodevelopment. We studied a rabbit model of IUGR by magnetic resonance imaging (MRI) and spectroscopy (MRS), to assess in vivo brain structural and metabolic consequences, and identify potential metabolic biomarkers for clinical translation.

Methods

IUGR was induced in 3 pregnant rabbits at gestational day 25, by 40–50% uteroplacental vessel ligation in one horn; the contralateral horn was used as control. Fetuses were delivered at day 30 and weighted. A total of 6 controls and 5 IUGR pups underwent T2-w MRI and localized proton MRS within the first 8 hours of life, at 7T. Changes in brain tissue volumes and respective contributions to each MRS voxel were estimated by semi-automated registration of MRI images with a digital atlas of the rabbit brain. MRS data were used for: (i) absolute metabolite quantifications, using linear fitting; (ii) local temperature estimations, based on the water chemical shift; and (iii) classification, using spectral pattern analysis.

Results

Lower birth weight was associated with (i) smaller brain sizes, (ii) slightly lower brain temperatures, and (iii) differential metabolite profile changes in specific regions of the brain parenchyma. Specifically, we found estimated lower levels of aspartate and N-acetylaspartate (NAA) in the cerebral cortex and hippocampus (suggesting neuronal impairment), and higher glycine levels in the striatum (possible marker of brain injury). Our results also suggest that the metabolic changes in cortical regions are more prevalent than those detected in hippocampus and striatum.

Conclusions

IUGR was associated with brain metabolic changes in vivo, which correlate well with the neurostructural changes and neurodevelopment problems described in IUGR. Metabolic parameters could constitute non invasive biomarkers for the diagnosis and abnormal neurodevelopment of perinatal origin.

Local O2 Balance in Cerebral Ischemia-Reperfusion Improved during Pentobarbital Compared with Isoflurane Anesthesia

BACKGROUND

Most anesthetics affect cerebral blood flow and metabolism. We compared microregional O2 balance in cerebral ischemia-reperfusion during pentobarbital and isoflurane anesthesia.

METHODS

After 1 hour of middle cerebral artery occlusion and a 2-hour reperfusion under isoflurane (1.4%, n = 14) or pentobarbital (50 mg/kg, n = 14) anesthesia in rats, regional cerebral blood flow using (14)C-iodoantipyrine autoradiography, microregional arterial and venous O2 saturation (20-60 μm in diameter) using cryomicrospectrophotometry, and the size of cortical infarct were determined.

RESULTS

Ischemia-reperfusion decreased the average cortical venous O2 saturation in both pentobarbital and isoflurane groups (P < .0001), which was higher (P < .05) with pentobarbital despite a similar average regional cerebral blood flow and O2 consumption. The heterogeneity of venous O2 saturation reported as a coefficient of variation (100 × standard deviation/mean) was smaller (P < .005) with pentobarbital than that with isoflurane (7.5 versus 16.1). The number of veins with low venous O2 saturation (<50%) was smaller (P < .005) with pentobarbital (5 of 80 versus 24 of 80). The percentage of cortical infarct in total cortex was smaller with pentobarbital (5.2 ± 2.5% versus 12.3 ± 2.6%, P < .001).

CONCLUSIONS

In the cerebral ischemic-reperfused cortex, the average venous O2 saturation was higher, and its heterogeneity and the number of veins with low O2 saturation were smaller under pentobarbital than isoflurane anesthesia. This improvement in microregional O2 balance with pentobarbital was accompanied by the reduced cortical infarct. Our data suggest that the neurologic outcome could vary during cerebral ischemia-reperfusion depending on the anesthetics used.

Pathological alterations and stress responses near DBS electrodes after MRI scans at 7.0T, 3.0T and 1.5T: an in vivo comparative study

OBJECTIVE

The purpose of this study was to investigate the pathological alterations and the stress responses around deep brain stimulation (DBS) electrodes after magnetic resonance imaging (MRI) scans at 7.0T, 3.0T and 1.5T.

MATERIALS AND METHODS

DBS devices were stereotactically implanted into the brains of New Zealand rabbits, targeting the left nucleus ventralis posterior thalami, while on the right side, a puncture passage pointing to the same target was made. MRI scans at 7.0T, 3.0T and 1.5T were performed using transmit/receive head coils. The pathological alterations of the surrounding tissue were evaluated by hematoxylin and eosin staining (H&E staining) and transmission electron microscopy (TEM). The levels of the 70 kDa heat shock protein (HSP-70), Neuronal Nuclei (NeuN) and Caspase-3 were determined by western-blotting and quantitative polymerase chain reaction (QPCR) to assess the stress responses near the DBS electrodes.

RESULTS

H&E staining and TEM showed that the injury around the DBS electrodes was featured by a central puncture passage with gradually weakened injurious alterations. Comparisons of the injury across the groups manifested similar pathological alterations near the DBS electrodes in each group. Moreover, western-blotting and QPCR assay showed that the level of HSP-70 was not elevated by MRI scans (p>0.05), and the levels of NeuN and Caspase-3 were equal in each group, regardless of the field strengths applied (p>0.05).

CONCLUSIONS

Based on these findings, it is reasonable to conclude that in this study the MRI scans at multiple levels failed to induce additional tissue injury around the DBS electrodes. These preliminary data furthered our understanding of MRI-related DBS heating and encouraged revisions of the current MRI guidelines for patients with DBS devices.

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.

Measurements of RF heating during 3.0-T MRI of a pig implanted with deep brain stimulator

PURPOSE

To present preliminary, in vivo temperature measurements during MRI of a pig implanted with a deep brain stimulation (DBS) system.

MATERIALS AND METHODS

DBS system (Medtronic Inc., Minneapolis, MN) was implanted in the brain of an anesthetized pig. 3.0-T MRI was performed with a T/R head coil using the low-SAR GRE EPI and IR-prepped GRE sequences (SAR: 0.42 and 0.39 W/kg, respectively), and the high-SAR 4-echo RF spin echo (SAR: 2.9 W/kg). Fluoroptic thermometry was used to directly measure RF-related heating at the DBS electrodes, and at the implantable pulse generator (IPG). For reference the measurements were repeated in the same pig at 1.5 T and, at both field strengths, in a phantom.

RESULTS

At 3.0T, the maximal temperature elevations at DBS electrodes were 0.46 °C and 2.3 °C, for the low- and high-SAR sequences, respectively. No heating was observed on the implanted IPG during any of the measurements. Measurements of in vivo heating differed from those obtained in the phantom.

CONCLUSION

The 3.0-T MRI using GRE EPI and IR-prepped GRE sequences resulted in local temperature elevations at DBS electrodes of no more than 0.46 °C. Although no extrapolation should be made to human exams and much further study will be needed, these preliminary data are encouraging for the future use 3.0-T MRI in patients with DBS.

Phenobarbital and temperature profile during hypothermia for hypoxic-ischemic encephalopathy

Data from the whole-body hypothermia trial was analyzed to examine the effects of phenobarbital administration prior to cooling (+PB) on the esophageal temperature (T (e)) profile, during the induction phase of hypothermia. A total of 98 infants were analyzed. At enrollment, +PB infants had a higher rate of severe hypoxic-ischemic encephalopathy and clinical seizures and lower T (e) and cord pH than infants that have not received phenobarbital (-PB). There was a significant effect of phenobarbital itself and an interaction between phenobarbital and time in the T (e) profile. Mean T (e) in the +PB group was lower than in the -PB group, and the differences decreased over time. In +PB infants, the time to surpass target T (e) of 33.5°C and to reach the minimum T (e) during overshoot were shorter. In conclusion, the administration of phenobarbital before cooling was associated with changes that may reflect a reduced thermogenic response associated with barbiturates.

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.

Temperature management in studies of barbiturate protection from focal cerebral ischemia: systematic review and speculative synthesis

Our goal was to test the hypothesis that-given the barbiturates' novel ability to reduce brain temperature-the high prevalence of reports describing cerebral protection by barbiturates in animal models are, in part, the result of inadvertent cerebral hypothermia. We reviewed all published reports evaluating barbiturate protection in animal models of focal cerebral ischemia where functional or anatomic endpoints were assessed. Presence or absence of protection, and additionally the year of publication, were tabulated. Temperature monitoring was categorized as: (a) not monitored, (b) inadequately monitored (ie, temperature monitored, but not at appropriate sites or times), or (c) adequately monitored (brain or cranial temperature monitored at appropriate times, with or without core temperature). Twenty eight references published between 1974 and 2008 described 57 separate protocols. Cerebral protection by barbiturates was reported in 35 of 57 (61%) protocols. Temperature was not monitored in 10 protocols (18%), inadequately monitored in 32 (56%), and adequately monitored in 15 (26%). Although the majority (32 of 57; 56%) of the protocols were published before December 1987, none of these properly monitored temperature. In the protocols published in 1988 or later, 15 of 25 (60%) had proper temperature monitoring and 9 of the 15 (60%) reported protection by the barbiturates. Very few (ie, 15 of 57; 26%) protocols were capable of distinguishing between direct cerebral protection by the barbiturates and an artifactual, hypothermia-related, effect. However, among those protocols having proper temperature monitoring, there remained considerable evidence of barbiturate protection.

1H-MRSI pattern perturbation in a mouse glioma: the effects of acute hyperglycemia and moderate hypothermia

MR spectroscopic Imaging (MRSI), with PRESS localization, is used here to monitor the effects of acute hyperglycemia in the spectral pattern of 11 mice bearing GL261 gliomas at normothermia (36.5-37.5 degrees C) and at hypothermia (28.5-29.5 degrees C). These in vivo studies were complemented by ex vivo high resolution magic angle spinning (HR-MAS) analysis of GL261 tumor samples from 6 animals sacrificed by focused microwave irradiation, and blood glucose measurements in 12 control mice. Apparent glucose levels, monitored by in vivo MRSI in brain tumors during acute hyperglycemia, rose to an average of 1.6-fold during hypothermia (p < 0.05), while no significant changes were detected at normothermia, or in control experiments performed at euglycemia, or in normal/peritumoral brain regions. Ex vivo analysis of glioma-bearing mouse brains at hypothermia revealed higher glucose increases in distinct regions during the acute hyperglycemic challenge (up to 6.6-fold at the tumor center), in agreement with maximal in vivo blood glucose changes (5-fold). Phantom studies on taurine plus glucose containing solutions explained the differences between in vivo and ex vivo measurements. Our results also indicate brain tumor heterogeneity in the four animal tumors investigated in response to a defined metabolic challenge.

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.

Independence of brain and trunk temperature during hypothermic preconditioning in rats

Hypothermic preconditioning is rapid cooling and warming to induce tolerance to ischemia. The purpose of the study was to examine differences in brain and trunk temperature during hypothermic preconditioning.

METHODS

Rats (n=18) were implanted with telemetric probes for simultaneous measure of brain and trunk temperature. Hypothermic preconditioning was produced by exposing rats to cool and warm environments that produced rapid cooling to 30 degrees C and warming to 35 degrees C.

RESULTS

Brain temperature was warmer (37.56+/-0.45 degrees C) than trunk (37.17+/-0.29 degrees C) temperature in unanesthetized, free roaming rats at room temperature (t-test p=0.04). The brain cooled (0.59+/-0.1 degrees C/min) quicker than the trunk (0.44+/-0.19 degrees C/min) during cooling cycles of hypothermic preconditioning and the brain (0.28+/-0.04 degrees C/min) warmed quicker than the trunk (0.18+/-0.07 degrees C/min) during the warming cycle of hypothermic preconditioning (t-test p<0.0001). When the trunk temperature probe was designated to reach the target temperature of 35 degrees C during warming, the brain temperature (38.1+/-0.44 degrees C) was warmer than trunk temperature (34.95+/-0.16 degrees C) during the peak of warming (t-test p<0.0001).

CONCLUSION

The brain cools and warms quicker than the trunk during hypothermic preconditioning. Failure to anticipate these differences could lead to unrecognized brain hyperthermia during warming. Appreciation of differences in rates of change between brain and trunk temperature may be important when designing hypothermic preconditioning experiments.

Cerebral microvascular dilation during hypotension and decreased oxygen tension: a role for nNOS

Endothelial (eNOS) and neuronal nitric oxide synthase (nNOS) are implicated as important contributors to cerebral vascular regulation through nitric oxide (NO). However, direct in vivo measurements of NO in the brain have not been used to dissect their relative roles, particularly as related to oxygenation of brain tissue. We found that, in vivo, rat cerebral arterioles had increased NO concentration ([NO]) and diameter at reduced periarteriolar oxygen tension (Po(2)) when either bath oxygen tension or arterial pressure was decreased. Using these protocols with highly selective blockade of nNOS, we tested the hypothesis that brain tissue nNOS could donate NO to the arterioles at rest and during periods of reduced perivascular oxygen tension, such as during hypotension or reduced local availability of oxygen. The decline in periarteriolar Po(2) by bath manipulation increased [NO] and vessel diameter comparable with responses at similarly decreased Po(2) during hypotension. To determine whether the nNOS provided much of the vascular wall NO, nNOS was locally suppressed with the highly selective inhibitor N-(4S)-(4-amino-5-[aminoethyl]aminopentyl)-N'-nitroguanidine. After blockade, resting [NO], Po(2), and diameters decreased, and the increase in [NO] during reduced Po(2) or hypotension was completely absent. However, flow-mediated dilation during occlusion of a collateral arteriole did remain intact after nNOS blockade and the vessel wall [NO] increased to approximately 80% of normal. Therefore, nNOS predominantly increased NO during decreased periarteriolar oxygen tension, such as that during hypotension, but eNOS was the dominant source of NO for flow shear mechanisms.

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.

Limits on activation-induced temperature and metabolic changes in the human primary visual cortex

Changes in cerebral blood flow (CBF) and metabolism are now widely used to map and quantify neural activity, although the underlying mechanism for these changes is still incompletely understood. Magnetic resonance spectroscopy (MRS) at 3T, synchronized with a 32-s block design visual stimulation paradigm, was employed to investigate activation-induced changes in temperature and metabolism in the human primary visual cortex. A marginally significant increase in the local temperature of the visual cortex was found (0.1 degrees C, P = 0.09), excluding the possibility of a temperature decrease (95% confidence interval (CI) = 0.0-0.2 degrees C), which was previously suggested. A comparison with models of thermal equilibrium in the presence of blood flow suggests that an increase in heat production during activation, greater than or at least equal to that produced by the complete oxidative metabolism of the elevated glucose (Glc) utilization accompanying activation, would be required to offset the cooling effects of the increased blood flow. The total pools of glutamate (Glu), glutamine (Gln), myo-Inositol (mI), N-acetylaspartate (NAA), choline (Cho), and lactate (Lac) were not significantly affected by activation. Limits on Lac concentration changes were too weak to constrain theories of the metabolic use of elevated Glc consumption during stimulation, and emphasize the challenges of measuring even large Lac changes accompanying stimulation.

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.

Rat-PET study without anesthesia: Anesthetics modify the dopamine D1 receptor binding in rat brain

Positron emission tomography (PET) measurements in 6-month-old F344/N rats were performed in the conscious state and the influence of chloral hydrate, ketamine, and pentobarbital anesthesia on dopamine D(1) (DA-D(1)) receptor binding was evaluated using [(11)C]SCH23390, a selective DA-D(1) receptor ligand. To perform the PET study in conscious rats, an original fixation apparatus was developed and the animals were trained to acclimate to the scanning atmosphere for 3 h. This training was carried out twice a day for 2 weeks. PET measurements in conscious rats were successful, since the trained rats scarcely moved during the scanning (as monitored by video camera) and since highly reproducible measurements of binding potential (BP) were derived from their scanning. Chloral hydrate and ketamine anesthesia significantly increased the striatal BP of DA-D(1) receptors by 36% and 46%, respectively, compared to that observed in the conscious state. In contrast, pentobarbital markedly decreased the BP by 41%. These BP values of DA-D(1) receptors were calculated using a curve-fitting method. Our results indicate that PET studies in rats should be performed in the conscious state since the anesthetics dramatically modified ligand-receptor bindings, such as DA-D(1) receptor binding, in rat brain.