Local Cerebral Metabolism Research Articles

Mild Hyperthermia Aggravates Glucose Metabolic Consequences in Repetitive Concussion

Traumatic brain injury (TBI) is one of the leading causes of mortality and disability around the world. Mild TBI (mTBI) makes up approximately 80% of reported cases and often results in transient psychological abnormalities and cognitive disruption. At-risk populations for mTBI include athletes and other active individuals who may sustain repetitive concussive injury during periods of exercise and exertion when core temperatures are elevated. Previous studies have emphasized the impact that increased brain temperature has on adverse neurological outcomes. A lack of diagnostic tools to assess concussive mTBI limits the ability to effectively identify the post-concussive period during which the brain is uniquely susceptible to damage upon sustaining additional injury. Studies have suggested that a temporal window of increased vulnerability that exists corresponds to a period of injury-induced depression of cerebral glucose metabolism. In the current study, we sought to evaluate the relationship between repetitive concussion, local cerebral glucose metabolism, and brain temperature using the Marmarou weight drop model to generate mTBI. Animals were injured three consecutive times over a period of 7 days while exposed to either normothermic or hyperthermic temperatures for 15 min prior to and 1 h post each injury. A 14C-2-deoxy-d-glucose (2DG) autoradiography was used to measure local cerebral metabolic rate of glucose (lCMRGlc) in 10 diverse brain regions across nine bregma levels 8 days after the initial insult. We found that repetitive mTBI significantly decreased glucose utilization bilaterally in several cortical areas, such as the cingulate, visual, motor, and retrosplenial cortices, as well as in subcortical areas, including the caudate putamen and striatum, compared to sham control animals. lCMRGlc was significant in both normothermic and hyperthermic repetitive mTBI animals relative to the sham group, but to a greater degree when exposed to hyperthermic conditions. Taken together, we report significant injury-induced glucose hypometabolism after repetitive concussion in the brain, and additionally highlight the importance of temperature management in the acute period after brain injury.

Consequences of mTOR inhibition on epileptogenesis and neuroprotection in lithium-pilocarpine model of epilepsy in rats

Natural history of Mesial Lobe Temporal Epilepsy (mLTE) starts with the SE followed by a histological restructuration within the hippocampal and para-hippocampal regions, which creates a hyper-excitable neo-circuit implementing recurring spontaneous seizures. The mTor pathway is a proteical intracellular complex that regulates synaptogenesis, neuronal plasticity, inflammation and proliferation of cerebral gliosis. This pathway is activated at different stages of the mLTE. The goal of the present pre-clinical study was to evaluate the impact of inhibiting the mTor pathway by Rapamycin over epileptogenesis and neuroprotection in the lithium- pilocarpine model. For this study, 44 adult rats were divided into 3 groups: treated SE (i), untreated SE (ii) and healthy witnesses (iii). Each of them received Rapamycin per os at 3 stages: H+2 at 6 mg per kg per day, then D1 and D2 at 3 mg per kg per day. The treatment was evaluated by studying metabolic modifications, using positon emission tomography and 2-[18F]-fluoro-2-deoxy-d-glucose, at H+4, D2 and D8 (1), the research for spontaneous crisis from D8 (2), and eventually by counting neurons in the interest regions after sacrifice at D60 (3). In the acute phase, we show an increase of consequences of the SE on glucose local cerebral metabolism, in the hippocampus and parahippocampal cortex, by the use of Rapamycin. However, an increased hippocampal hypometabolism at D2 suggests that Rapamycin may have a neuroprotecting potential. We don't observe any treatment effect at D8 but we show at D60 a significant smaller neuronal loss in thalamic region (p = 0,003) and a trend in decreasing in all histologically studied regions. We don't show either any influence of Rapamycin treatment over epileptogenesis. Our results along with previous studies outcomes recommend evaluating Rapamycin administration from the beginning of SE and by the intra-peritoneal pathway, as well as treatment prolongation during the whole epileptogenesis silent phase.

Early vs late cranioplasty: what is better?

Aim of the study: Cranioplasty is the surgical repair of skull defects, which commonly is performed after traumatic skull injuries due to tumor removal or decompressive craniectomy. Several studies reported improvement in cognitive functions following cranioplasty in patients with severe brain damage. The reasons why exist such clinical improvement is not completely understood, although the increase in cerebrospinal fluid hydrodynamics with the potential improvement of local and global cerebral hemodynamics, blood flow, and metabolism may play a pivotal role. We investigated whether the cranioplasty improved neurological recovery and the whole array of cognitive functions or just some specific domains. Materials and methods: A total of 30 consecutive brain-injured subjects with craniectomy were enrolled and underwent a structured neuropsychological assessment immediately before the cranioplasty, 1 month after the cranioplasty and 1 year after the surgical procedure. Results: Our results showed that cranioplasty may facilitate the cognitive recovery, independently from the surgical timing. Particularly, we observed an important cognitive recovery in the period immediately after cranioplasty, while the improvement trend settles after a lapse of time, and the recovery starts to slow down. Conclusions: Cranioplasty seems to significantly improve neuropsychological and motor status in the patients with skull defects, independently from cranioplasty timing and patient's clinical status.

Investigation on the Effects of Brain Retraction on Local Cerebral Metabolism Utilizing Microdialysis

In order to study the effect of brain retraction on local biochemical metabolism in brain tissue and to investigate the potential application of microdialysis in monitoring local cerebral metabolic and biochemical variations owing to brain retraction injury. Brain tissues were retracted based on different retraction pressures (30 mmHg, 40 mmHg, 50 mmHg) and retraction modes (intermittent and continuous tractions). Microdialysis was utilized for the dynamic collection of the dialysis fluid of extracellular fluid (ECF), and [Glu]d, [Lac]d, [Gly]d, [Gluta]d and L/P changes were observed. Different brain retraction pressures led to the changes of [Glu]d, [Lac]d, [Gly]d, [Gluta]d and L/P to various extents in ECF. Differences between the retraction group and control group before and after injury were significantly different (P<0.05). A higher retraction pressure resulted in a more significant change. Continuous retraction led to more serious brain damage than intermittent retraction. [Glu]d, [Lac]d, [Gly]d, [Gluta]d and L/P of the continuous retraction group changed more apparently, and the differences between the groups were significantly different (P<005). Microdialysis technique is ideal for the dynamic monitoring of local biochemical changed in brain tissues resulted from brain injury, which can be utilized as a valuable tool in monitoring brain retraction injury during neurosurgical operations.

Glucose consumption of inflammatory cells masks metabolic deficits in the brain

Inflammatory cells such as microglia need energy to exert their functions and to maintain their cellular integrity and membrane potential. Subsequent to cerebral ischemia, inflammatory cells infiltrate tissue with limited blood flow where neurons and astrocytes died due to insufficient supply with oxygen and glucose. Using dual tracer positron emission tomography (PET), we found that concomitant with the presence of inflammatory cells, transport and consumption of glucose increased up to normal levels but returned to pathological levels as soon as inflammatory cells disappeared. Thus, inflammatory cells established sufficient glucose supply to satisfy their energy demands even in regions with insufficient supply for neurons and astrocytes to survive. Our data suggest that neurons and astrocytes died from oxygen deficiency and inflammatory cells metabolized glucose non-oxidatively in regions with residual availability. As a consequence, glucose metabolism of inflammatory cells can mask metabolic deficits in neurodegenerative diseases. We further found that the PET tracer did not bind to inflammatory cells in severely hypoperfused regions and thus only a part of the inflammation was detected. We conclude that glucose consumption of inflammatory cells should be taken into account when analyzing disease-related alterations of local cerebral metabolism.

Aphasic or amnesic status epilepticus detected on PET but not EEG

To describe five patients with ictal aphasia and one patient with ictal amnesia, who had focal positron emission tomography (PET) hypermetabolism but no clear ictal activity on electroencephalography (EEG). (18)F-Fluorodeoxyglucose (FDG)-PET scans with concomitant EEG were obtained in five patients with suspected ictal aphasia or ictal amnesia without ictal activity on EEG. We reviewed medical history, EEG, imaging data, and treatment outcome. Brain magnetic resonance imaging (MRI) showed no structural abnormalities in any of the patients. EEG showed left temporal irregular delta activity in three patients, with aphasia and nonspecific abnormalities in two other patients, all without clear ictal pattern. All patients demonstrated focal hypermetabolism on PET scan. The hypermetabolism was in the left frontotemporal region in patients with ictal aphasia and in the bilateral hippocampal region in the patient with amnesia. Three patients who received intravenous benzodiazepines during their episodes had transient clinical improvement. With antiepileptic drug (AED) treatment, symptoms gradually resolved in all patients. Concomitant resolution of PET hypermetabolism was documented in three patients who had follow up scans. One patient with ictal aphasia later developed recurrent episodes, each with recurrent PET hypermetabolism. This patient and one other patient required immune-modulating therapy in addition to AEDs. FDG-PET imaging should be considered as a diagnostic tool in patients with suspected ictal aphasia or amnesia, who fail to show clear evidence of ictal activity on EEG.

Dissociation of Metabolic and Neurovascular Responses to Levodopa in the Treatment of Parkinson's Disease

We compared the metabolic and neurovascular effects of levodopa (LD) therapy for Parkinson's disease (PD). Eleven PD patients were scanned with both [15O]-H2O and [18F]-fluorodeoxyglucose positron emission tomography in the unmedicated state and during intravenous LD infusion. Images were used to quantify LD-mediated changes in the expression of motor- and cognition-related PD covariance patterns in scans of cerebral blood flow (CBF) and cerebral metabolic rate for glucose (CMR). These changes in network activity were compared with those occurring during subthalamic nucleus (STN) deep brain stimulation (DBS), and those observed in a test-retest PD control group. Separate voxel-based searches were conducted to identify individual regions with dissociated treatment-mediated changes in local cerebral blood flow and metabolism. We found a significant dissociation between CBF and CMR in the modulation of the PD motor-related network by LD treatment (p < 0.001). This dissociation was characterized by reductions in network activity in the CMR scans (p < 0.003) occurring concurrently with increases in the CBF scans (p < 0.01). Flow-metabolism dissociation was also evident at the regional level, with LD-mediated reductions in CMR and increases in CBF in the putamen/globus pallidus, dorsal midbrain/pons, STN, and ventral thalamus. CBF responses to LD in the putamen and pons were relatively greater in patients exhibiting drug-induced dyskinesia. In contrast, flow-metabolism dissociation was not present in the STN DBS treatment group or in the PD control group. These findings suggest that flow-metabolism dissociation is a distinctive feature of LD treatment. This phenomenon may be especially pronounced in patients with LD-induced dyskinesia.

Einteilung und Therapie des Schädel-Hirn-Traumas (SHT)

In spite of great success in research severe traumatic brain injury (TBI) remains the most frequent cause for morbidity and mortality in the age < 45 years. The primary lesion emerges at the moment of trauma. Due to several pathophysiological mechanisms secondary lesions occur that enlarge size of contusions significantly. As a consequence of intracranial bleedings and brain edema intracranial pressure (ICP) increases and threaten the patient. Extent of severity (declared in Glasgow Coma Scale Score [GCS]), expansion and type of bleedings (acute and chronic subdural hemorrhage, epidural bleeding, contusion bleedings and intracerebral hemorrhage) determinate operative and conservative therapy as well as intensive care medicine. A specific feature represents frontobasal lesions that, apart of penetrating injuries, are treated interdisciplinary not before ICP is stable, brain edema declining and coagulation sufficient several days after trauma. A persisting rhinoliquorrhoe cause meningitis up to 85 % within 10 years. Patient with GCS < 8 have to be intubated and controlled ventilated. Basic monitoring does not differ from those of other patients treated at the intensive care ward (sufficient breathing [pO (2), pCO (2)], arterial blood pressure, CBC and coagulation parameters, fluid monitoring and nutrition). Additionally, ICP have to be measured and be treated corresponding to the algorithm of ICP treatment. Complementary, oxygen saturation of brain tissue (ptiO (2)), local cerebral blood flow (r-CBF) and cerebral metabolism (micro dialysis) can be measured. Just the combination of the single monitoring parameters gives evidence of the functional condition of the injured brain and relieved planning and performing of the appropriate therapy.

Use of cerebral microdialysis during wake-up craniotomy to monitor local cerebral metabolism

Use of cerebral microdialysis during wake-up craniotomy to monitor local cerebral metabolism

Neuroprotection by a Central Nervous System–Type Prostacyclin Receptor Ligand Demonstrated in Monkeys Subjected to Middle Cerebral Artery Occlusion and Reperfusion

Recently, we found that a novel subtype of prostacyclin (PGI(2)) receptor clearly distinct from the peripheral subtype in terms of ligand specificity is expressed in the central nervous system (CNS). (15R)-16-m-tolyl-17,18,19,20-tetranorisocarbacyclin (15R-TIC) was synthesized and demonstrated to be a specific ligand for this CNS-type PGI(2) receptor. Previously, we demonstrated 15R-TIC to be neuroprotective in vivo during transient forebrain ischemia in gerbils and permanent middle cerebral artery occlusion (MCAO) in rats. Furthermore, this compound was shown to exert an anti-apoptotic effect on primary cultured hippocampal neurons, indicating its neuroprotective effect against ischemic insults occurs via direct action on CNS-type PGI(2) receptor. Local cerebral hemodynamics and oxygen metabolism were measured simultaneously by using positron emission tomography with the (15)O steady-state method, before and up to 18 hours after 3-hour transient MCAO reperfusion in cynomolgus monkeys. Methyl ester of 15R-TIC (50 microg/kg, n=4) or its vehicle (10% Intralipos, n=4) was injected intravenously within 5 minutes after onset of MCAO and continuously infused for 5 hours (50 microg/kg per hour). Neuropathology showed that 15R-TIC significantly reduced cortical damage after 3-hour MCAO. Positron emission tomography results showed 15R-TIC significantly reduced the volume of "infarct" region of interest and attenuated the decrease in cerebral metabolic rate of oxygen and oxygen extraction fraction, and these protective effects were not attributable to improvement of cerebral circulation. These results suggest that 15R-TIC has a potent neuroprotective effect against focal cerebral ischemia in a monkey MCAO via its direct action on CNS-type PGI(2) receptors.

Neuropeptide Y suppresses glucose utilization in the dorsal optic tectum towards visual stimulation in the toad Bombina orientalis: A [ 14C]2DG study

Neuropeptide Y (NPY) experimentally administered to the surface of the optic tectum in visually stimulated fire bellied toads diminishes local glucose utilization in the retinorecipient tectal laminae. Strong NPY-induced suppression of tectal glucose utilization was found even when visual retinal input to the tectum was boosted pharmacologically under systemic apomorphine treatment. These novel results on the local cerebral energy metabolism contribute to the concept that NPY controls retinotectal visual processing via an inhibitory mechanism.

NMDA and age dependent cerebral hemodynamics after traumatic brain injury

Activation of N-methyl- D-aspartate (NMDA) glutamatergic receptors elicits cerebrovascular dilation, may couple local cerebral metabolism to blood flow but contribute to excitotoxic neuronal cell death. While cerebral hemodynamics following traumatic brain injury may correlate with neurologic status, the role of NMDA vascular activity is uncertain in the sequelae of brain injury. NMDA dilation was impaired following fluid percussion brain injury (FPI) in an age dependent manner in the pig and the newly described opioid nociceptin/orphanin FQ (NOC/oFQ) contributes to such impairment via the cyclooxygenase dependent generation of superoxide. Further, hypotensive pial artery dilation (PAD) was blunted after FPI but partially protected by pretreatment with the NMDA antagonist MK801. Cerebral blood flow (CBF) was reduced during normotension by FPI, further reduced by hypotension, but both were partially protected by MK801 in the newborn. In contrast, blunted hypotensive PAD was protected significantly less by MK801 in the juvenile pig. Similarly, MK801 had less protective effect on normotensive and hypotensive CBF values post FPI in the juvenile. These data indicate that NMDA receptor activation contributes to impaired hypotensive cerebral hemodynamics following FPI in an age dependent manner. Further, these data suggest that NMDA receptor activation, NOC/oFQ, and prostaglandins dynamically interact to impair cerebral hemodynamics following FPI.

Noxious stimulation and local cerebral metabolism during Propofol anesthesia in the rat

Noxious stimulation and local cerebral metabolism during Propofol anesthesia in the rat

Intracerebral temperature alterations associated with focal seizures

Because focal seizures produce an increase in local cerebral metabolism and blood flow, we wanted to determine whether they might lead to changes in brain temperature. We induced focal neocortical seizures by microinjection of 4-aminopyridine (4-AP) into the rat motor cortex. The temperature on the dura immediately over the injection site, or 8 mm away, was measured with a thermocouple and in some experiments relative blood flow was monitored with a laser Doppler probe. In animals that did not receive 4-AP, brain and rectal temperature remained fairly constant at 33.5 and 37.2 °C, respectively, over a 2 h monitoring period. In animals treated with 4-AP, brain temperature over the seizure focus rose an average of 0.3 °C, within a few seconds of seizure onset, while rectal temperature remained constant. The seizure-induced temperature rise was preceded by an increase in cortical blood flow. The temperature, but not blood flow, was also elevated 8 mm away from the seizure focus. When blood flow was increased independently of neuronal activity, by elevating pCO 2, brain temperature also rose by about 0.3 °C. Focal seizures in anesthetized rats produce a small, but statistically significant increase in local brain temperature, as a result of increased blood flow that brings brain temperature closer to body temperature. In humans, seizures could actually cause a reduction in brain temperature, because brain temperature is normally higher than body temperature.

In vitro biotransformation of the selective serotonin reuptake inhibitor citalopram, its enantiomers and demethylated metabolites by monoamine oxidase in rat and human brain preparations.

This study was conducted to identify enzyme systems eventually catalysing a local cerebral metabolism of citalopram, a widely used antidepressant of the selective serotonin reuptake inhibitor type. The metabolism of citalopram, of its enantiomers and demethylated metabolites was investigated in rat brain microsomes and in rat and human brain mitochondria. No cytochrome P-450 mediated transformation was observed in rat brain. By analysing H2O2 formation, monoamine oxidase A activity in rat brain mitochondria could be measured. In rat whole brain and in human frontal cortex, putamen, cerebellum and white matter of five brains monoamine oxidase activity was determined by the stereoselective measurement of the production of citalopram propionate. All substrates were metabolised by both forms of MAO, except in rat brain, where monoamine oxidase B activity could not be detected. Apparent Km and Vmax of S-citalopram biotransformation in human frontal cortex by monoamine oxidase B were found to be 266 microM and 6.0 pmol min(-1) mg(-1) protein and by monoamine oxidase A 856 microM and 6.4 pmol min(-1) mg(-1) protein, respectively. These Km values are in the same range as those for serotonin and dopamine metabolism by monoamine oxidases. Thus, the biotransformation of citalopram in the rat and human brain occurs mainly through monoamine oxidases and not, as in the liver, through cytochrome P-450.

Influence of early posttraumatic hypothermia therapy on local cerebral blood flow and glucose metabolism after fluid-percussion brain injury

Object Using autoradiographic image averaging, the authors recently described prominent foci of marked glucose metabolism-greater-than-blood-flow uncoupling in the acutely traumatized rat brain. Because hypothermia is known to ameliorate injury in this and other injury models, the authors designed the present study to assess the effects of post-traumatic therapeutic hypothermia on the local cerebral metabolic rate of glucose (LCMRglu) and local cerebral blood flow (LCBF) following moderate parasagittal fluid-percussion head injury (FPI) in rats. Methods Either cranial hypothermia (30°C) or normothermia (37°C) was induced for 3 hours in matched groups of rats immediately after FPI; LCMRglu and LCBF were assessed 3 hours after concluding these temperature manipulations. In rats subjected to FPI, regardless of whether normothermia or hypothermia ensued, LCBF was reduced relative to the sham-injury groups. In addition, when FPI was followed by hypothermia (FPI–30°C group), the subsequent LCBF was significantly lower (35–38% on average) than in FPI–37°C rats. Statistical mapping of LCBF difference imaging data revealed confluent cortical and subcortical zones of significantly reduced LCBF (largely ipsilateral to the prior injury) in FPI–30°C rats relative to the FPI–37°C group. Local glucose utilization was reduced in both hemispheres of FPI–37°C rats relative to the sham-injury group and was lower in the right (traumatized) hemisphere than in the left. However, LCMRglu values were largely unaffected by temperature manipulation in either the FPI or sham-injury groups. The LCMRglu/LCBF ratio was nearly doubled in FPI–30°C rats relative to the FPI–37°C group, in a diffuse and bihemispheric fashion. Linear regression analysis comparing LCMRglu and LCBF revealed that the FPI–37°C and FPI–30°C data sets were completely nonoverlapping, whereas the two sham-injury data sets were intermixed. Conclusions Despite its proven neuroprotective efficacy, early posttraumatic hypothermia (30°C for 3 hours) nonetheless induces a moderate decline in cerebral perfusion without the (anticipated) improvement in cerebral glucose utilization, so that a state of mild metabolism-greater-than-blood-flow dissociation is perpetuated.

Mechanisms of peritumoural brain dysfunction: metabolic and neuroreceptor findings in striatal C6 glioma

The aetiology of the peritumoural brain dysfunction that is rectified by steroids is unknown. To determine potential aspects of its pathophysiological basis we performed metabolic, histochemical and neuroreceptor studies in rodents with striatal C6 gli oma. This model is known to cause focal neurobehavioural and electrophysiological dysfunction. The fully quantitative [14C]-2-deoxyglucose autoradiographic technique of measuring local cerebral metabolism of glucose (LCMRglu) showed raised LCMRglu (22–29%) in the pallidum, substantia nigra and endopeduncular nucleus. Acetylcholinesterase (AChE) histochemistry and a range of ligand binding studies for dopamine type 1 and 2, and serotonergic 5-HT2receptors were negative in the tumour and normal in peritumoural brain. 5-HT uptake sites and strong peripheral benzodiazepine receptor expression were present in the tumour. There was extensive up-regulation of peripheral benzodiazepine receptor expression in the peritumoural brain. These studies show there is metabolic dysregulation in brain regions functionally connected to, but anatomically distant from the striatum. There is also a peritumoural region of up-regulated receptors that have many, predominantly inhibitory, functions. The relationship of these findings to peritumoural brain dysfunction is discussed.

Induced mitochondrial failure in the feline brain: implications for understanding acute post-traumatic metabolic events

Objective: Recently, evidence has become available implicating mitochondrial failure as a crucial factor in the pathogenesis of acute brain damage following severe traumatic brain injury (TBI). However, it remains unclear how mitochondrial dysfunction affects cerebral metabolism. Therefore the aim of the study was to evaluate the impact of ‘isolated’ mitochondrial failure on local cerebral metabolism. Methods: Cerebral mitochondrial metabolism was blocked by local microdialysis perfusion with cyanide in seven cats. Local brain tissue oxygen tension ( p tiO 2 ), carbon dioxide tension ( p tiCO 2 ) and pH, as well as extracellular cerebral fluid, glucose, lactate, pyruvate and glutamate were monitored, using a Neurotrend sensor and microdialysis, respectively. Tissue oxygen consumption was measured in a microrespirometric system, and ultrastructural changes evaluated via electron microscopy. Results: Brain tissue oxygen tension increased from a baseline of 31±9 mmHg to 84±30 mmHg after 60 min of cyanide perfusion ( P<0.05), concomitant a decrease in oxygen consumption from 14.45±3.91 μl/h/mg to 10.83±1.74 μl/h/mg ( P<0.05). Brain tissue pH was decreased after 60 min of cyanide perfusion (6.83±0.16) compared to baseline (7.07±0.39) ( P<0.05), whereas p tiCO 2 did not show significant changes. Lactate massively increased from a baseline of 599±270 μmol/l to 2609±1188 μmol/l immediately after cyanide perfusion ( P<0.05). The lactate:glucose ratio increased from 0.79±0.15 before cyanide perfusion to 6.40±1.44 at 40 min after cyanide perfusion ( P<0.05), while no significant changes in the lactate:pyruvate ratio could be observed. Glutamate increased from a baseline of 11.6±7.2 μmol/l to 61.4±44.7 μmol/l after cyanide perfusion ( P<0.05). Conclusion: The results of this study show that ‘isolated’ cerebral mitochondrial failure initiates changes in cerebral substrates and biochemistry, which are very similar to most of the changes seen after severe human head injury, except for the early fall in p tiO 2 , further indicating a crucial involvement of mitochondrial impairment in the development of brain damage after TBI.

Relationship between local cerebral blood flow and metabolism during mild and moderate hypothermia in rats.

Hypothermia may interfere with the relationship between cerebral blood flow (CBF) and metabolism. Because this conclusion was based on the analysis of global values, the question remains whether hypothermic CBF/metabolism uncoupling exists on a local cerebral level. This study investigated the effects of hypothermic anesthesia on local cerebral blood flow (LCBF) and local cerebral glucose utilization (LCGU). Thirty-six rats were anesthetized with isoflurane (1 minimum alveolar concentration) and artificially ventilated to maintain normal arterial carbon dioxide partial pressure (pH-stat). Pericranial temperature was maintained as normothermic (37.5 degrees C, n = 12) or was reduced to 35 degrees C (n = 12) or 32 degrees C (n = 12). Pericranial temperature was maintained constant for 60 min until LCBF or LCGU were measured by autoradiography. Twelve conscious rats served as normothermic controls. Compared with conscious animals, mean CBF remained unchanged during normothermic anesthesia. Mean CBF significantly increased during mild hypothermia but was unchanged during moderate hypothermia. During normothermic anesthesia, mean CGU was 45% lower than in conscious controls (P < 0.05). No further CGU reduction was found during mild hypothermia, whereas CGU further decreased during moderate hypothermia (48%; P < 0.05). Local analysis showed a linear LCBF/LCGU relationship in conscious (r = 0.94) and anesthetized (r = 0.94) normothermic animals, as well as in both hypothermic groups (35 degrees C: r = 0.92; 32 degrees C: r = 0.95; P < 0.05). The LCBF-to-LCGU ratio increased from 1.4 (conscious controls) to 2.4 (normothermic isoflurane) and 3.6 ml/micromol (mild and moderate hypothermia, P < 0.05). Decrease of mean CGU at unchanged or increased mean CBF during hypothermic anesthesia may not indicate uncoupling. Local analysis shows a maintained linear relationship that is reset to a higher CBF/CGU ratio.

Cytochrome P-450 activities in human and rat brain microsomes

The role of cytochrome P450 in the metabolism of dextromethorphan, amitriptyline, midazolam, S-mephenytoin, citalopram, fluoxetine and sertraline was investigated in rat and human brain microsomes. Depending on the parameters, the limit of quantification using gas chromatography–mass spectrometry methods was between 1.6 and 20 pmol per incubation, which generally contained 1500 μg protein. Amitriptyline was shown to be demethylated to nortriptyline by both rat and human microsomes. Inhibition studies using ketoconazole, furafylline, sulfaphenazole, omeprazole and quinidine suggested that CYP3A4 is the isoform responsible for this reaction whereas CYP1A2, CYP2C9, CYP2C19 and CYP2D6 do not seem to be involved. This result was confirmed by using a monoclonal antibody against CYP3A4. Dextromethorphan was metabolized to dextrorphan in rat brain microsomes and was inhibited by quinidine and by a polyclonal antibody against CYP2D6. Only the addition of exogenous reductase allowed the measurement of this activity in human brain microsomes. Metabolites of the other substrates could not be detected, possibly due to an insufficiently sensitive method. It is concluded that cytochrome P450 activity in the brain is very low, but that psychotropic drugs could undergo a local cerebral metabolism which could have pharmacological and/or toxicological consequences.