Dose dependent reduction of glucose utilization by pentobarbital in rat brain

Stroke and Translational Research - Review of Experimental Models with a Focus on Awake Ischaemic Induction and Anaesthesia

Animal models are an indispensable tool in the study of ischaemic stroke with hundreds of drugs emerging from the preclinical pipeline. However, all of these drugs have failed to translate into successful treatments in the clinic. This has brought into focus the need to enhance preclinical studies to improve translation. The confounding effects of anaesthesia on preclinical stroke modelling has been raised as an important consideration. Various volatile and injectable anaesthetics are used in preclinical models during stroke induction and for outcome measurements such as imaging or electrophysiology. However, anaesthetics modulate several pathways essential in the pathophysiology of stroke in a dose and drug dependent manner. Most notably, anaesthesia has significant modulatory effects on cerebral blood flow, metabolism, spreading depolarizations, and neurovascular coupling. To minimise anaesthetic complications and improve translational relevance, awake stroke induction has been attempted in limited models. This review outlines anaesthetic strategies employed in preclinical ischaemic rodent models and their reported cerebral effects. Stroke related complications are also addressed with a focus on infarct volume, neurological deficits, and thrombolysis efficacy. We also summarise routinely used focal ischaemic stroke rodent models and discuss the attempts to induce some of these models in awake rodents.

Basic physiological effects of ketamine-xylazine mixture as a general anesthetic preparation for rodent surgeries

Among the numerous general anesthetics utilized in rodent surgical procedures, the co-administration of ketamine and xylazine is the current standard for induction and maintenance of surgical planes of anesthesia and pain control. In contrast to classical GABAergic anesthetics, which act to inhibit CNS activity, inducing muscle relaxation, sedation, hypothermia, and brain hypoxia, ketamine and xylazine act through different mechanisms to induce similar effects while also providing potent analgesia. By using three-point thermorecording in freely moving rats, we show that the ketamine-xylazine mixture induces modest brain hyperthermia, resulting from increased intra-cerebral heat production due to metabolic brain activation and increased heat loss due to skin vasodilation. The first effect derives from ketamine, which alone increases brain and body temperatures due to brain metabolic activation and skin vasoconstriction. The second effect derives from xylazine, which increases heat loss due to potent skin vasodilation. By using oxygen sensors coupled with amperometry, we show that the ketamine-xylazine mixture modestly decreases brain oxygen levels that results from relatively weak respiratory depression. This tonic pharmacological effect was preceded by a strong but transient oxygen increase that may result from a stressful injection or unknown, possibly peripheral action of this drug combination. This pattern of physiological effects elicited by the ketamine-xylazine mixture differs from the effects of other general anesthetic drugs, particularly barbiturates.

Changes in breathing pattern during severe hypothermia and autoresuscitation from hypothermic respiratory arrest in anesthetized mice

Some evidence suggests that both hypothermia and anesthesia can exert similar effects on metabolism and ventilation. This study examined the synergistic effects of anesthesia and hypothermia on ventilation in spontaneously breathing adult mice under three different conditions, that is, (1) pentobarbital group (n = 7) in which mice were anesthetized with intraperitoneal pentobarbital of 80 mg/kg, (2) sevoflurane-continued group (n = 7) in which mice were anesthetized with 1 MAC sevoflurane, and (3) sevoflurane-discontinued group (n = 7) in which sevoflurane was discontinued at a body temperature below 22˚C. We cooled mice in each group until breathing ceased and followed this with artificial rewarming while measuring changes in respiratory variables and heart rate. We found that the body temperature at which respiration arrested is much lower in the sevoflurane-discontinued group (13.8 ± 2.0˚C) than that in the sevoflurane-continued group (16.7 ± 1.2˚C) and the pentobarbital group (17.0 ± 1.4˚C). Upon rewarming, all animals in all three groups spontaneously recovered from respiratory arrest. There was a considerable difference in breathing patterns between sevoflurane-anesthetized mice and pentobarbital-anesthetized mice during progressive hypothermia in terms of changes in tidal volume and respiratory frequency. The changes in the respiratory pattern during rewarming are nearly mirrored images of the changes observed during cooling in all three groups. These observations indicate that adult mice are capable of autoresuscitation from hypothermic respiratory arrest and that anesthesia and hypothermia exert synergistic effects on the occurrence of respiratory arrest while the type of anesthetic affects the breathing pattern that occurs during progressive hypothermia leading to respiratory arrest.

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.

GnRH Pulse Generator Activity Across the Estrous Cycle of Female Mice

A subpopulation of kisspeptin neurons located in the arcuate nucleus (ARN) operate as the GnRH pulse generator. The activity of this population of neurons can be monitored in real-time for long periods using kisspeptin neuron-selective GCaMP6 fiber photometry. Using this approach, we find that ARN kisspeptin neurons exhibit brief (∼50 seconds) periods of synchronized activity that precede pulses of LH in intact female mice. The dynamics and frequency of these synchronization episodes (SEs) are stable at approximately one event every 40 minutes throughout metestrus, diestrus, and proestrus, but slow considerably on estrus to occur approximately once every 10 hours. Evaluation of ARN kisspeptin neuron activity across the light-dark transition, including the time of onset of the proestrus LH surge, revealed no changes in SE frequency. Longer 24-hour recordings across proestrus into estrus demonstrated that an abrupt decrease in SEs occurred ∼4 to 5 hours after the onset of the LH surge to reach the low frequency of SEs observed on estrus. The frequency of SEs was stable across the 24-hour period from metestrus to diestrus. Administration of progesterone to diestrus mice resulted in the abrupt slowing of SEs. These observations show that the GnRH pulse generator exhibits an unvarying pattern of activity from metestrus through to the late evening of proestrus, at which time it slows dramatically, likely in response to postovulation progesterone secretion. The GnRH pulse generator maintains a constant frequency of activity across the time of the LH surge, demonstrating that it is not involved directly in surge generation.

Energy metabolism in the rat cortex under thiopental anaesthesia measured In Vivo by 13 C MRS

Barbiturates, commonly used as general anaesthetics, depress neuronal activity and thus cerebral metabolism. Moreover, they are likely to disrupt the metabolic support of astrocytes to neurons, as well as the uptake of nutrients from circulation. By employing ¹³ C magnetic resonance spectroscopy (MRS) in vivo at high magnetic field, we characterized neuronal and astrocytic pathways of energy metabolism in the rat cortex under thiopental anaesthesia. The neuronal tricarboxylic acid (TCA) cycle rate was 0.46 ± 0.02 µmol/g/min, and the rate of the glutamate-glutamine cycle was 0.09 ± 0.02 µmol/g/min. In astrocytes, the TCA cycle rate was 0.16 ± 0.02 µmol/g/min, accounting for a quarter of whole brain glucose oxidation, pyruvate carboxylase rate was 0.02 ± 0.01 µmol/g/min, and glutamine synthetase was 0.12 ± 0.01 µmol/g/min. Relative to previous experiments under light α-chloralose anaesthesia, thiopental reduced oxidative metabolism in neurons and even more so in astrocytes. Interestingly, total oxidative metabolism in the cortex under thiopental anaesthesia surpassed the rate of pyruvate production by glycolysis, indicating substantial utilisation of substrates other than glucose, likely plasma lactate. © 2017 Wiley Periodicals, Inc.

Time to wake up: Studying neurovascular coupling and brain-wide circuit function in the un-anesthetized animal

Functional magnetic resonance imaging (fMRI) has allowed the noninvasive study of task-based and resting-state brain dynamics in humans by inferring neural activity from blood-oxygenation-level dependent (BOLD) signal changes. An accurate interpretation of the hemodynamic changes that underlie fMRI signals depends on the understanding of the quantitative relationship between changes in neural activity and changes in cerebral blood flow, oxygenation and volume. While there has been extensive study of neurovascular coupling in anesthetized animal models, anesthesia causes large disruptions of brain metabolism, neural responsiveness and cardiovascular function. Here, we review work showing that neurovascular coupling and brain circuit function in the awake animal are profoundly different from those in the anesthetized state. We argue that the time is right to study neurovascular coupling and brain circuit function in the awake animal to bridge the physiological mechanisms that underlie animal and human neuroimaging signals, and to interpret them in light of underlying neural mechanisms. Lastly, we discuss recent experimental innovations that have enabled the study of neurovascular coupling and brain-wide circuit function in un-anesthetized and behaving animal models.

A Fatal Adverse Effect of Barbiturate Coma Therapy: Dyskalemia

The management guideline for traumatic brain injury (TBI) recommends high-dose barbiturate therapy to control increased intracranial pressure refractory to other therapeutic options. High-dose barbiturate therapy, however, may cause many severe side effects; the commonly recognized ones include hypotension, immunosuppression, hepatic dysfunction, renal dysfunction, and prolonged decrease of cortical activity. Meanwhile, dyskalemia remains relatively uncommon. In this study, we report the case of a hypokalemic patient with severe rebound hyperkalemia, which occurred as a result of barbiturate coma therapy administered for TBI treatment.

MRS glucose mapping and PET joining forces: re-evaluation of the lumped constant in the rat brain under isoflurane anaesthesia

Although numerous positron emission tomography (PET) studies with (18) F-fluoro-deoxyglucose (FDG) have reported quantitative results on cerebral glucose kinetics and consumption, there is a large variation between the absolute values found in the literature. One of the underlying causes is the inconsistent use of the lumped constants (LCs), the derivation of which is often based on multiple assumptions that render absolute numbers imprecise and errors hard to quantify. We combined a kinetic FDG-PET study with magnetic resonance spectroscopic imaging (MRSI) of glucose dynamics in Sprague-Dawley rats to obtain a more comprehensive view of brain glucose kinetics and determine a reliable value for the LC under isoflurane anaesthesia. Maps of Tmax /CMRglc derived from MRSI data and Tmax determined from PET kinetic modelling allowed to obtain an LC-independent CMRglc . The LC was estimated to range from 0.33 ± 0.07 in retrosplenial cortex to 0.44 ± 0.05 in hippocampus, yielding CMRglc between 62 ± 14 and 54 ± 11 μmol/min/100 g, respectively. These newly determined LCs for four distinct areas in the rat brain under isoflurane anaesthesia provide means of comparing the growing amount of FDG-PET data available from translational studies.

Mapping phosphorylation rate of fluoro-deoxy-glucose in rat brain by (19)F chemical shift imaging

(19)F magnetic resonance spectroscopy (MRS) studies of 2-fluoro-2-deoxy-d-glucose (FDG) and 2-fluoro-2-deoxy-d-glucose-6-phosphate (FDG-6P) can be used for directly assessing total glucose metabolism in vivo. To date, (19)F MRS measurements of FDG phosphorylation in the brain have either been achieved ex vivo from extracted tissue or in vivo by unusually long acquisition times. Electrophysiological and functional magnetic resonance imaging (fMRI) measurements indicate that FDG doses up to 500 mg/kg can be tolerated with minimal side effects on cerebral physiology and evoked fMRI-BOLD responses to forepaw stimulation. In halothane-anesthetized rats, we report localized in vivo detection and separation of FDG and FDG-6P MRS signals with (19)F 2D chemical shift imaging (CSI) at 11.7 T. A metabolic model based on reversible transport between plasma and brain tissue, which included a non-saturable plasma to tissue component, was used to calculate spatial distribution of FDG and FDG-6P concentrations in rat brain. In addition, spatial distribution of rate constants and metabolic fluxes of FDG to FDG-6P conversion were estimated. Mapping the rate of FDG to FDG-6P conversion by (19)F CSI provides an MR methodology that could impact other in vivo applications such as characterization of tumor pathophysiology.

In vivo recordings of GnRH neuron firing reveal heterogeneity and dependence upon GABAA receptor signaling

The gonadotropin-releasing hormone (GnRH) neurons are the key cells regulating fertility in all mammalian species. The scattered distribution of these neurons has made investigation of their properties extremely difficult and the key goal of recording their electrical activity in vivo near impossible. The caudal-most extension of the GnRH neuron continuum brings some cells very close to the base of the brain at the level of the anterior hypothalamic area. Taking insight from this, we developed an experimental procedure in anesthetized GnRH-GFP mice that allows the electrical activity of these GnRH neurons to be recorded in vivo. On-cell recordings revealed that the majority of GnRH neurons (86%) were spontaneously active, exhibiting a range of firing patterns, although only a minority (15%) exhibited burst firing. Mean firing frequencies ranged from 0.06 to 3.65 Hz, with the most common interspike interval being ~500 ms. All GnRH neurons tested were activated by AMPA and kisspeptin. Whereas the GABAA receptor agonist muscimol evoked excitatory, inhibitory, or mixed effects on GnRH neuron firing, the GABAA receptor antagonist picrotoxin resulted in a consistent suppression of firing. These observations represent the first electrical recordings of GnRH neurons in vivo. They reveal that GnRH neurons in vivo exhibit considerable heterogeneity in their firing patterns with both similarities and differences to firing in vitro. These variable patterns of firing in vivo are found to be critically dependent upon ongoing GABAA receptor signaling.

The hidden side of drug action: brain temperature changes induced by neuroactive drugs

RATIONALE

Most neuroactive drugs affect brain metabolism as well as systemic and cerebral blood flow, thus altering brain temperature. Although this aspect of drug action usually remains in the shadows, drug-induced alterations in brain temperature reflect their metabolic neural effects and affect neural activity and neural functions.

OBJECTIVES

Here, I review brain temperature changes induced by neuroactive drugs, which are used therapeutically (general anesthetics), as a research tool (dopamine agonists and antagonists), and self-administered to induce desired psychic effects (cocaine, methamphetamine, ecstasy). I consider the mechanisms underlying these temperature fluctuations and their influence on neural, physiological, and behavioral effects of these drugs.

RESULTS

By interacting with neural mechanisms regulating metabolic activity and heat exchange between the brain and the rest of the body, neuroactive drugs either increase or decrease brain temperatures both within (35-39 °C) and exceeding the range of physiological fluctuations. These temperature effects differ drastically depending upon the environmental conditions and activity state during drug administration. This state-dependence is especially important for drugs of abuse that are usually taken by humans during psycho-physiological activation and in environments that prevent proper heat dissipation from the brain. Under these conditions, amphetamine-like stimulants induce pathological brain hyperthermia (>40 °C) associated with leakage of the blood-brain barrier and structural abnormalities of brain cells.

CONCLUSIONS

The knowledge on brain temperature fluctuations induced by neuroactive drugs provides new information to understand how they influence metabolic neural activity, why their effects depend upon the behavioral context of administration, and the mechanisms underlying adverse drug effects including neurotoxicity.

Rat brain slices oxidize glucose at high rates: a (13)C NMR study

Since glucose is the main cerebral substrate, we have characterized the metabolism of various (13)C glucose isotopomers in rat brain slices. For this, we have used our cellular metabolomic approach that combines enzymatic and carbon 13 NMR techniques with mathematical models of metabolic pathways. We identified the fate and the pathways of the conversion of glucose carbons into various products (pyruvate, lactate, alanine, aspartate, glutamate, GABA, glutamine and CO(2)) and determined absolute fluxes through pathways of glucose metabolism. After 60 min of incubation, lactate and CO(2) were the main end-products of the metabolism of glucose which was avidly metabolized by the slices. Lactate was also used at high rates by the slices and mainly converted into CO(2). High values of flux through pyruvate carboxylase, which were similar with glucose and lactate as substrate, were observed. The addition of glutamine, but not of acetate, stimulated pyruvate carboxylation, the conversion of glutamate into succinate and fluxes through succinate dehydrogenase, malic enzyme, glutamine synthetase and aspartate aminotransferase. It is concluded that, unlike brain cells in culture, and consistent with high fluxes through PDH and enzymes of the tricarboxylic acid cycle, rat brain slices oxidized both glucose and lactate at high rates.

Phenobarbital augments hypothermic neuroprotection

Seizures are associated with adverse outcome in infants with hypoxic-ischemic encephalopathy. We hypothesized that early administration of the anticonvulsant phenobarbital after cerebral hypoxia ischemia could enhance the neuroprotective efficacy of delayed-onset hypothermia. We tested this hypothesis in a neonatal rodent model. Seven-d-old rats (n = 104) underwent right carotid ligation, followed by 90 min 8% O2 exposure; 15 min later, they received injections of phenobarbital (40 mg/kg) or saline. One or 3 h later, all were treated with hypothermia (30 degrees C, 3 h). Function and neuropathology were evaluated after 7 d (early outcomes) or 1 mo (late outcomes). Early outcome assessment demonstrated better sensorimotor performance and less cortical damage in phenobarbital-treated groups; there were no differences between groups in which the hypothermia delay was shortened from 3 to 1 h. Late outcome assessment confirmed sustained benefits of phenobarbital + hypothermia treatment; sensorimotor performance was better (persistent attenuation of contralateral forepaw placing deficits and absence of contralateral forepaw neglect); neuropathology scores were lower (median, phenobarbital 2 and saline 8.5, p < 0.05); and less ipsilateral cerebral hemisphere %Damage (mean +/- SD, 11 +/- 17 versus 28 +/- 22, p < 0.05). These results suggest that early posthypoxia-ischemia administration of phenobarbital may augment the neuroprotective efficacy of therapeutic hypothermia.

Brain temperature fluctuations during physiological and pathological conditions

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

Physiological and pathological brain hyperthermia

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

Effects of anesthesia on the responses to cortical spreading depression in the rat brain in vivo

OBJECTIVES

The aim of this study was to evaluate the effect of cortical spreading depression (CSD) on the metabolic, hemodynamic, electrical and ionic properties during anesthesia as compared with the awake state.

METHODS

The mitochondrial NADH redox state, reflected light, direct current (DC) potential, electrocorticography (ECoG), cerebral blood flow (CBF) and volume (CBV), and extracellular K(+) concentrations ([K(+)](e)), were measured continuously and simultaneously in real time using two unique monitoring systems that evaluate brain function. Three consecutive CSD waves were initiated using a KCl solution in both awake and anesthetized rats.

RESULTS AND DISCUSSION

CSD caused typical amplitude changes: biphasic waves in reflectance, oxidation cycles in NADH, an increase in CBF, CBV and in [K(+)](e), a negative shift in DC potential and depression in ECoG. Anesthesia by equithesin decreased significantly the baseline levels of CBF and [K(+)](e), showing a reduction in oxygen supply and demand. After anesthesia, CSD significantly decreased [K(+)](e) and NADH oxidation cycles, indicating a reduction in oxygen demand and in oxygen balance, respectively. Furthermore, anesthesia reduced CSD wave frequencies by slowing the recovery period, showing a decline in energy production during brain activation, or by changing electrophysiological properties of the tissue. No changes were found in the propagation rate and in the initiation period of CSD, which may indicate that equithesin does not block CSD initiation. In addition, we found that the whole cerebral cortex reacts homogenously to CSD and that equithesin may reduce oxygen demand and energy production, which may have a protective effect on the brain exposed to pathophysiological conditions.

Reversible inactivation of the dorsal hippocampus by tetrodotoxin or lidocaine: A comparative study on cerebral functional activity and motor coordination in the rat

Reversible inactivation of the hippocampus by lidocaine or tetrodotoxin is used to investigate implications of this structure in memory processes. Crucial points related to such inactivation are the temporal and spatial extents of the blockade. We compared effects of intrahippocampal infusions of commonly-used doses of lidocaine (5 or 10 mug) or tetrodotoxin (5 or 10 ng) in rats at two post-infusion delays (5 or 30 min), using 2-deoxyglucose autoradiography to visualize local cerebral glucose metabolism, and beam-walking performance to assess motor coordination. In addition, memory retrieval was evaluated in a water maze after bilateral infusions of 10 mug lidocaine. A unilateral tetrodotoxin infusion induced dose- and time-dependent reductions of 2-deoxyglucose uptake in the vicinity of the infusion site (dorsal hippocampus: -29% to -67%) and in other ipsi- and contralateral brain regions (ventral hippocampus, lateral thalamus, cortical regions). The maximal effect was at 10 ng, at the delay of 30 min between the tetrodotoxin infusion and the 2-deoxyglucose injection. Uni- and bilateral infusions of tetrodotoxin induced dramatic motor coordination deficits. Conversely, lidocaine reduced 2-deoxyglucose uptake (-19%) in the dorsal hippocampus only at 10 mug, with weak extrahippocampal effects. Whether infused uni- or bilaterally and regardless of the dose, lidocaine did not alter motor coordination. When infused bilaterally, however, 10 microg of lidocaine impaired short-term retrieval of spatial information in a water maze. Because lidocaine i) induced a weak though significant functional blockade mainly restricted to the infusion site, ii) had no consequences on motor coordination and, nevertheless iii) altered short-term spatial memory retrieval, we conclude that acute intrahippocampal infusions of lidocaine may offer some advantages over tetrodotoxin at the doses used herein.

A method for measuring cerebral glucose metabolism in vivo by 13C-NMR spectroscopy

Current methods for estimating the rate of cerebral glucose utilization (CMR(glc)) typically measure metabolic activity for 40 min or longer subsequent to administration of [(13)C]glucose, 2-[(14)C]deoxyglucose, or 2-[(18)F]deoxyglucose. We report preliminary findings on estimating CMR(glc) for a period of 15 min by use of 2-[6-(13)C]deoxyglucose. After a 24-hr fast, rats were anesthetized, infused with [1-(13)C]glucose for 50 min, and injected with 2-[6-(13)C]deoxyglucose (500 mg/kg). During the subsequent 12.95 min the estimated value of CMR(glc) was 0.6 +/- 0.4 micromol/min/g (mean +/- SD, N = 7), in agreement with values reported for anesthetized rats studied with the 2-[(14)C]deoxyglucose method and other (13)C-NMR methods that measure CMR(glc). In rats injected with bicuculline methiodide (a known stimulant of CMR(glc)), CMR(glc) increased by more than 75% during 12.95 min following injection of bicuculline (Wilcoxon signed rank test, P = 0.042, N = 8).

Perinatal brain injury: from pathogenesis to neuroprotection

Brain injury secondary to hypoxic-ischemic disease is the predominant form of all brain injury encountered in the perinatal period. The focus of this article is the most recent research developments in this field and especially those developments that should lead to the most profound effects on interventions in the first years of the new millennium. Neuronal injury is the predominant form of cellular injury in the term infant. The principal mechanisms leading to neuronal death after hypoxia-ischemia/reperfusion are initiated by energy depletion, accumulation of extracellular glutamate, and activation of glutamate receptors. The cascade of events that follows involves accumulation of cytosolic calcium and activation of a variety of calcium-mediated deleterious events. Notably this deleterious cascade, which evolves over many hours, may be interrupted even if interventions are instituted after termination of the insult, an important clinical point. Of the potential interventions, the leading candidates for application to the human infant in the relative short-term are mild hypothermia, inhibitors of free radical production, and free radical scavengers. Promising clinical data are available for the use of mild hypothermia.

Performance of appetitive or consummatory components of male sexual behavior is mediated by different brain areas: a 2-deoxyglucose autoradiographic study

The in vivo autoradiographic deoxyglucose method was used to identify the functional brain circuits that are involved in the performance of appetitive and consummatory components of male sexual behavior in Japanese quail (Coturnix japonica). Two groups of castrated, testosterone-treated male quail were trained during 12 sessions to associate the view of a female behind a window with the opportunity to interact freely and to copulate with her. They developed, as a consequence, a social proximity response (staying close and looking through the window providing a view of the female) that has been used in previous experiments to measure appetitive sexual behavior. A third control group (also castrated and treated with testosterone) was allowed to view the female but not to copulate with her and therefore did not develop this proximity response. 2-14C-deoxyglucose was then injected i.p. to these birds and they were allowed to either copulate freely with a female (consummatory sexual behavior group) or express the social proximity response (appetitive sexual behavior group). The control group was provided a view of the female but these birds, although they were exposed to the same stimuli as birds in the appetitive group, did not express the social proximity response because they had never learned the association with the opportunity to copulate. Birds were killed 45 min after the deoxyglucose injection and their brains were processed for autoradiography. Densitometric analyses of the autoradiograms revealed that the expression of appetitive or consummatory aspects of male sexual behavior was associated with significant increases by comparison with the control group in the deoxyglucose incorporation in the nucleus mesencephalicus lateralis, pars dorsalis and in the nucleus leminsci lateralis. In addition, an increase in the deoxyglucose incorporation was specifically observed in the paleostriatum primitivum, rostral preoptic area, nucleus intercollicularis, nucleus interpeduncularis and third nerve but a decrease was observed in the dorsomedial part of the hippocampus and in the nucleus nervi oculomotori in birds of the consummatory sexual behavior group by comparison with controls. By contrast, in the appetitive sexual behavior group, significant increases in deoxyglucose incorporation were observed in two telencephalic areas, the intermediate hyperstriatum ventrale and neostriatum caudolaterale by comparison with the controls, but decreases were detected in the stratum griseum et fibrosum superficiale of optic tectum by comparison with the consummatory behavior group. These studies demonstrate that the performance of appetitive or consummatory components of male sexual behavior affects in a specific manner the deoxyglucose uptake and accumulation in specific regions of the quail brain. Changes in metabolic activity were observed in steroid-sensitive areas, in auditory, visual and vocal brain regions, and in brain nuclei related to motor behavior but also in association telencephalic and limbic structures. These changes in oxidative metabolism overlap to some extent with metabolic changes as revealed by immunocytochemistry for the immediate early gene products Fos and Zenk, but many specific reactions are also detected indicating that these techniques are not necessarily redundant and, together, they can provide a more complete picture of the brain circuits that are implicated in the control and performance of complex behaviors.

Reversible neural inactivation reveals hippocampal participation in several memory processes

Studies of patients and animals with brain lesions have implicated the hippocampal formation in spatial, declarative/relational and episodic types of memory. These and other types of memory consist of a series of interdependent but potentially dissociable memory processes-encoding, storage, consolidation and retrieval. To identify whether hippocampal activity contributes to these processes independently, we used a novel method of inactivating synaptic transmission using a water-soluble antagonist of AMPA/kainate glutamate receptors. Once calibrated using electrophysiological and two-deoxyglucose techniques in vivo, drug or vehicle was infused chronically or acutely into the dorsal hippocampus of rats at appropriate times during or after training in a water maze. Our findings indicate that hippocampal neural activity is necessary for both encoding and retrieval of spatial memory and for either trace consolidation or long-term storage.

Reduced palmitate turnover in brain phospholipids of pentobarbital-anesthetized rats

Our laboratory has reported that pentobarbital-induced anesthesia reduced the incorporation of intravenously injected radiolabeled palmitic acid into brain phospholipids. To determine if this decrease reflected a pentobarbital-induced decrease in palmitate turnover in phospholipids, we applied our method and model to study net flux and turnover of palmitate in brain phospholipids (1). Awake, light and deep pentobarbital (25-70 mg/kg, iv) anesthetized rats were infused with [9,10-3H]palmitate over a 5 min period. Brain electrical activity was monitored by electroencephalography. An isoelectric electroencephalogram characterized deep pentobarbital anesthesia. Net incorporation rates (J(FA,i)) and turnover rates (Fi) of palmitate were calculated. J(FA,i) for palmitate incorporated into phospholipids was dramatically reduced by pentobarbital treatment in a dose-dependent manner, by 70% and 90% respectively for lightly and deeply anesthetized animals, compared with awake controls. Turnover rates for palmitate in total phospholipid and individual phospholipid classes were decreased by nearly 70% and 90% for lightly and deeply anesthetized animals, respectively. Thus, pentobarbital decreases, in a dose-dependent manner, the turnover of palmitate in brain phospholipids. This suggests that palmitate turnover is closely coupled to brain functional activity.

The Kety-Schmidt technique for repeated measurements of global cerebral blood flow and metabolism in the conscious rat

Cerebral activation will increase cerebral blood flow (CBF) and cerebral glucose uptake (CMRglc) more than it increases cerebral uptake of oxygen (CMR(O2)). To study this phenomenon, we present an application of the Kety-Schmidt technique that enables repetitive simultaneous determination of CBF, CMR(O2), CMRglc and CMRlac on awake, non-stressed animals. After constant intravenous infusion with 133Xenon, tracer infusion is terminated, and systemic arterial blood and cerebral venous blood are continuously withdrawn for 9 min. In this paper, we evaluate if the assumptions applied with the Kety-Schmidt technique are fulfilled with our application of the method. When measured twice in the same animal, the intra-individual variation for CBF, CMR(O2), and CMRglc were 10% (SD: 25%), 8% (SD: 25%), and 9% (SD: 28%), respectively. In the awake rat the values obtained for CBF, CMR(O2) and CMRglc were 106 mL [100 g](-1) min(-1), 374 micromole [100 g](-1) min(-1) and 66 micromole [100 g](-1) min(-1), respectively. The glucose taken up by the brain during wakefulness was fully accounted for by oxidation and cerebral lactate efflux. Anaesthesia with pentobarbital induced a uniform reduction of cerebral blood flow and metabolism by approximately 40%. During halothane anaesthesia CBF and CMRglc increased by approximately 50%, while CMR(O2) was unchanged.

Stoichiometric coupling of brain glucose metabolism and glutamatergic neuronal activity

To determine the relationship between cerebral Glc metabolism and glutamatergic neuronal function, we used 13C NMR spectroscopy to measure, simultaneously, the rates of the tricarboxylic acid cycle and Gln synthesis in the rat cortex in vivo. From these measurements, we calculated the rates of oxidative Glc metabolism and glutamate-neurotransmitter cycling between neurons and astrocytes (a quantitative measure of glutamatergic neuronal activity). By measuring the rates of the tricarboxylic acid cycle and Gln synthesis over a range of synaptic activity, we have determined the stoichiometry between oxidative Glc metabolism and glutamate-neurotransmitter cycling in the cortex to be close to 1:1. This finding indicates that the majority of cortical energy production supports functional (synaptic) glutamatergic neuronal activity. Another implication of this result is that brain activation studies, which map cortical oxidative Glc metabolism, provide a quantitative measure of synaptic glutamate release.

Determination of cerebral glucose transport and metabolic kinetics by dynamic MR spectroscopy

A new in vivo nuclear magnetic resonance (NMR) spectroscopy method is introduced that dynamically measures cerebral utilization of magnetically labeled [1-13C]glucose from the change in total brain glucose signals on infusion. Kinetic equations are derived using a four-compartment model incorporating glucose transport and phosphorylation. Brain extract data show that the glucose 6-phosphate concentration is negligible relative to glucose, simplifying the kinetics to three compartments and allowing direct determination of the glucose-utilization half-life time [t1/2 = ln2/(k2 + k3)] from the time dependence of the NMR signal. Results on isofluorane (n = 5)- and halothane (n = 7)-anesthetized cats give a hyperglycemic t1/2 = 5.10 +/- 0.11 min-1 (SE). Using Michaelis-Menten kinetics and an assumed half-saturation constant Kt = 5 +/- 1 mM, we determined a maximal transport rate Tmax = 0.83 +/- 0.19 mumol.g-1.min-1, a cerebral metabolic rate of glucose CMRGlc = 0.22 +/- 0.03 mumol.g-1.min-1, and a normoglycemic cerebral influx rate CIRGlc = 0.37 +/- 0.05 mumol.g-1.min-1. Possible extension of this approach to positron emission tomography and proton NMR is discussed.

Determination of local brain glucose level with [14C]methylglucose: effects of glucose supply and demand

Methylglucose can be used to assay brain glucose levels because the equilibrium brain-to-plasma distribution ratio for methylglucose (Ce*/Cp*) is quantitatively related to brain (Ce) and plasma (Cp) glucose contents. The relationship between Ce and Ce*/Cp* predicted by Michaelis-Menten kinetics has been experimentally confirmed when glucose utilization rate (CMRGlc) is maintained at normal, resting levels and Cp is varied in conscious rats. Theoretically, however, Ce and Ce*/Cp* should change when CMRGlc is altered and Cp is held constant; their relationship in such conditions was, therefore, examined experimentally. Drugs were applied topically to brains of conscious rats with fixed levels of Cp to produce focal alterations in CMRGlc, and Ce and Ce*/Cp* were measured. Plots of Ce as a function of Ce*/Cp* for each Cp produced straight lines; their slopes decreased as Cp increased. The results confirm that a single theoretical framework describes the relationship between Ce and Ce*/Cp* as either glucose supply or demand is altered over a wide range; they also validate the use of methylglucose to estimate local Ce under abnormal conditions.

Interictal cerebral metabolic levels in Wistar rats sensitive to audiogenic seizures

In the present study, we compared interictal local cerebral metabolic rates for glucose (LCMRglcs) in a strain of audiogenic rats (Wistar AS) selected in our laboratory to interictal LCMRglcs in a strain of control non-epileptic (NE) rats. Two groups of Wistar AS were studied, one group exposed to a single audiogenic seizure and one group of kindled rats exposed to 40 daily repetitive seizures. Control NE animals were exposed to a single sound exposure which did not induce any behavioral disturbance. Interictal LCMRglcs were measured by the quantitative autoradiographic [14C]2-deoxyglucose technique 5 days after the last sound exposure. LCMRglcs were similar in the three groups of rats in 80% of the structures. Compared to the control NE strain, interictal metabolic levels were mainly decreased in auditory structures of Wistar AS, either naive or kindled, thus confirming auditory impairment in audiogenic animals. LCMRglcs were increased over control levels in both groups of Wistar AS in cerebellar regions. This increase of cerebellar functional activity in Wistar AS compared to control NE rats might reflect an increased cerebellar input which, together with auditory impairment, may facilitate the induction of seizure activity in Wistar AS. Finally, there was no difference between the interictal cerebral metabolic level of naive and kindled Wistar AS, except in the cerebellar dentate nucleus where LCMRglc was significantly higher in kindled than in naive animals.

Positron emission tomographic imaging of serotonin activation effects on prefrontal cortex in healthy volunteers

Serotonergic system abnormalities have been implicated in major depression, suicide, violence, alcoholism, and other psychopathologies. The prolactin response to fenfluramine has been widely used as a neuroendocrine probe to study brain serotonin responsivity. We have extended this methodology by using the positron emission tomography (PET) 18F-fluorodeoxyglucose (18FDG) method to examine the fenfluramine-induced changes in regional cerebral glucose metabolism (rCMRglu), an indicator of changes in regional neuronal activity. We report results on 16 healthy controls, each of whom underwent two PET studies. One group of six subjects had a placebo on day 1 and a single 60 mg oral dose of fenfluramine on day 2. The second group, of 10 subjects, was tested on two consecutive occasions without drug or placebo. Data were analyzed for significant rCMRglu changes on day 2 vs day 1 using the statistical parametric mapping method (p < 0.01). Subjects who did not receive drugs showed no statistically significant areas of rCMRglu increase or decrease on day 2 versus day 1. In contrast, the group that received fenfluramine showed significant fenfluramine-induced responses. Areas of rCMRglu increases involved mainly the left prefrontal and left temperoparietal cortex. Within the prefrontal cortex, two major areas of rCMRglu increase included, first, an area centered on the anterior cingulate and, second, an area in the lateral prefrontal cortex involving principally the inferior, middle, and superior frontal gyri. Some decreases in rCMRglu were observed, principally in the right hemisphere. This PET-fenfluramine paradigm is a potentially useful method for studying abnormalities of serotonin function in the prefrontal cortex.

Influence of glucose supply and demand on determination of brain glucose content with labeled methylglucose

The equilibrium brain/plasma distribution ratio for 3-0-methyl-D-glucose (methylglucose) varies with plasma and tissue glucose contents and can be used to determine local glucose levels in brain. This ratio was previously found to rise as brain glucose concentration fell in response to lowered plasma glucose content. The ratios, however, differed with the same tissue glucose levels in conscious and pentobarbital-sedated rats, suggesting that changes in metabolic demand might alter the quantitative relationship between the methylglucose distribution ratio and brain glucose concentration. To examine this possibility, metabolic rate was varied by focal drug application, and hexose concentrations measured in treated and surrounding tissue. When tissue glucose levels were reduced by raised metabolic demand, methylglucose distribution ratios also fell. When brain glucose levels rose due to reduced consumption, the methylglucose distribution ratio also rose. Thus, in contrast to the inverse relationship between brain/plasma methylglucose ratio and brain glucose concentration when brain glucose content is altered secondarily to changes in plasma glucose level, changes in brain glucose content induced by altered glucose utilization cause the brain glucose level and methylglucose distribution ratio to rise and fall in a direct relationship. Determination of brain glucose content from methylglucose distribution ratios must take into account rates of glucose delivery and consumption.

Carotid endarterectomy without a shunt for symptomatic lesions associated with contralateral severe stenosis or occlusion

PURPOSE

The purpose of this study was to assess the adequacy of thiopental protection against ischemic cerebral damage in patients undergoing carotid endarterectomy for symptomatic stenosis greater than 70% in association with contralateral stenosis greater than 70% or contralateral occlusion.

METHODS

All patients (n=259) with severe bilateral carotid disease who underwent carotid endarterectomy for symptomatic stenosis greater than 70% were extracted from the database of an ongoing prospective carotid surgery study. Large-dose thiopental sodium without shunting was used for cerebral protection during endarterectomy. Asymmetric electroencephalogram changes during the operation, carotid occlusion time, stroke onset, and neuropathologic outcomes were analyzed.

RESULTS

Three contralateral strokes occurred in the series, producing a cerebral morbidity/mortality rate of 1.2% (major 0.4%, minor 0.8%). Transient morbidity was 1.9% made of two reversible ischemic neurologic deficits and three transient ischemic attacks. New asymmetric electroencephalography changes were seen in 49 (19% patients, one of whom had transient deficit. Average occlusion time was 35 minutes. All strokes occurred within 24 hours of the procedure. Patients with previous stroke and and systemic hypertension seemed at greatest risk, and the contralateral hemisphere was the area at greatest risk. All transient deficits were ipsilateral and related to technical complications rather failed protection.

CONCLUSIONS

Thiopental cerebral protection eliminates strokes caused by complications of shunting, prevents ischemic stroke during carotid occlusion for periods up to 67 minutes (average 35 minutes), allows meticulous management of the operative site, may modify or minimize clinical neurologic deficit, and in our experience has rendered intraluminal shunting obsolete.

Effects of repeated treatments with an extract of Ginkgo biloba (EGb 761) on cerebral glucose utilization in the rat: an autoradiographic study

1. The autoradiographic method based on 2-deoxy-D[1-14C]glucose ([14C]DG) was used to determine glucose utilization in 49 discrete structures of rat brain under control conditions and after the animals had received repeated treatment with an extract of Ginkgo biloba (EGb 761). 2. Oral administration of EGb 761 (50 or 150 mg/kg/day) to adult male rats for 15 days did not modify body weight, mean arterial blood pressure, the concentrations of glucose or hemoglobin in blood, blood gases or arterial pH. 3. EGb 761 treatments produced only slight-to-moderate changes in glucose utilization in the various brain structures; i.e. decreases to an extent not exceeding 18.4% at the 50 mg/kg dose or 11.7% at the 150 mg/kg dose. 4. Glucose utilization was significantly decreased only in the frontoparietal somatosensory cortex, nucleus accumbens, cerebellar cortex and pons and only with the 50 mg/kg dose of EGb 761. 5. Although the four brain structures affected by EGb 761 treatment do not, in themselves, constitute a specific functional system of the CNS, these effects appear useful in explaining mechanisms underlying the clinical use of EGb 761 in treating problems associated with deficient somatosensory processing (e.g. impairment of "vigilance") and vestibular mechanisms (e.g. vertiginous syndromes).

Decreased cerebral glucose metabolism in patients with brain tumors: an effect of corticosteroids

The authors measured cerebral glucose metabolism (CMRglu) using [18F]fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) in patients with brain tumors to evaluate the effect of exogenous corticosteroids (in this instance, dexamethasone) on glucose metabolism. Fifty-six FDG-PET studies obtained in 45 patients with unilateral supratentorial brain tumors were analyzed. Patients with brain tumors were divided into three groups: 1) patients with cushingoid symptoms, who had been treated with combinations of radiotherapy and chemotherapy taking oral dexamethasone; 2) patients not taking dexamethasone but treated with radiotherapy; and 3) patients not taking dexamethasone who had not been treated with radiotherapy. Serial FDG-PET scans were obtained in eight of the cushingoid patients. Glucose metabolism was measured in the contralateral cerebral and ipsilateral cerebellar hemispheres in patients and compared to measurements taken from 19 normal volunteers. The authors found that in the cushingoid brain tumor patients there was a marked reduction in CMRglu compared to normal volunteers and other brain tumor patients (Kruskal-Wallis test; p 0.001). In the majority of patients who had serial FDG-PET scans, there was a decline in glucose metabolism over time and in one patient, in whom dexamethasone was reduced in dosage, there was a subsequent increase in CMRglu. The authors conclude that there is a generalized reduction in CMRglu in brain tumor patients taking dexamethasone compared to other brain tumor patients and normal volunteers, and that this effect is independent of radiotherapy, concurrent anticonvulsant medication, and transhemispheric functional disconnection (transhemispheric diaschisis).

AMPA receptor antagonism attenuates MK-801-induced hypermetabolism in the posterior cingulate cortex

The effect of pretreatment with an AMPA receptor antagonist, NBQX, on MK-801-induced alterations in glucose use was examined using [14C]-2-deoxyglucose autoradiography. NBQX (7 mg/kg) had minimal effect on glucose utilisation in all anatomical regions examined. The intravenous administration of MK-801 (0.2 mg/kg) induced increases in glucose use in the limbic system and cingulate cortex. MK-801 reduced glucose utilisation in the sensory motor and auditory cortices. Pretreatment with NBQX attenuated the MK-801-induced hypermetabolism in the posterior cingulate cortex. The decreases in glucose utilisation induced by MK-801 were not exacerbated by the pretreatment with NBQX. The interaction between NBQX and MK-801 suggests a possible method of attenuating some of the adverse effects of the non-competitive NMDA receptor antagonists in the posterior cingulate cortex.

Epilepsy and SPECT

SPECT studies for rCBF imaging in clinical epilepsy are reviewed divided into interictal, ictal and postictal SPECT studies in partial and generalized epilepsies, as compared with the corresponding PET studies, and at times referring to SPECT studies in experimental models of epilepsy. All of the interictal, ictal and postictal SPECT scans in partial epilepsy are useful for the regional determination of epileptic foci, in view of the high appearance rates of focal abnormalities on these SPECT scans and good correlation with epileptic EEG foci. Mechanisms underlying the focal abnormality on interictal SPECT scans (i.e., focal hypoperfusion) image are complicative, and therefore, studies concerning the underlying mechanisms are reviewed with regard to correlations of the SPECT abnormality with the following various clinical factors: focal organic lesions on X-ray CT or MRI scans, frequencies of clinical seizures or interictal seizure discharges on EEGs, background EEG activity, propagation of epileptic activity, antiepileptic drugs, and cognitive functions. Further, new neurochemical SPECT studies using 123I-Iomazenil for benzodiazepine receptor imaging in clinical epilepsy are reviewed and compared with earlier SPECT studies for rCBF imaging with regard to their utility in regional determination of epileptic foci. In addition, the few SPECT or PET studies available on epileptic psychosis are reviewed.

AMPA receptor antagonists and local cerebral glucose utilization in the rat

Local cerebral glucose utilization was examined using [14C]2-deoxyglucose autoradiography following systemic administration of the AMPA antagonists 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) and 6-(2-(1H-tetrazol-5-yl)ethyl)decahydroisoquinoline-3-carboxy lic acid (LY-293558) in conscious rats. Both NBQX (10, 30 and 100 mg/kg) and LY-293558 (10, 30 and 100 mg/kg) produced marked, anatomically widespread, dose-dependent reductions in glucose utilization throughout the brain. In none of the 50 regions investigated were elevations in glucose use observed at any dose of either agent. The reductions in glucose use were accompanied by sedation, suppression of spontaneous behaviour, and respiratory depression after NBQX (30 and 100 mg/kg) and LY-293558 (30 and 100 mg/kg) administration. The greatest reductions in glucose use after NBQX or LY-293558 occurred in primary auditory regions, limbic structures (particularly hippocampal regions and cingulate cortex), neocortex and some thalamic nuclei. However, a small number of regions were found to be insensitive to either NBQX or LY-293558, most notably the superior colliculus superficial layer which failed to display significant alterations in glucose use following any concentration of either AMPA antagonist. The anatomical pattern of altered glucose use was essentially similar following either agent although the cerebral cortex, thalamus and auditory regions were more sensitive to LY-293558 and subcortical regions more sensitive to NBQX. The anatomical pattern of glucose use alterations after AMPA receptor antagonists differs from that described previously for competitive and non-competitive NMDA receptor antagonists.

Behavioural and biochemical effects of acute central metabolic inhibition: effects of acetyl-l-carnitine

In the present study we evaluated a new method to assess the behavioural and biochemical effects of a brief period of acute hypoxia in the brain. In this method, cyanide is injected into the lateral ventricles. Spatial navigation performance in a Morris task was found to be impaired 1 and 5 min after an i.c.v. injection of 5.0 micrograms cyanide but not after 2.5 micrograms cyanide. Increased rate of phosphatidic acid formation, reflecting increased phospholipase C activity, were observed after injection of 5.0 micrograms cyanide, indicating that energy-dependent phosphoinositide metabolism was affected. Chronic treatment with acetyl-l-carnitine attenuated the cyanide-induced behavioural deficit, but had no effect on energy-dependent phosphoinositide metabolism. The results suggest that, in this model, acetyl-l-carnitine may act via free fatty acid metabolism, by increasing the reservoir of activated acyl groups which are involved in the reacylation of membrane phospholipids.

An experimental model of generalized seizures for the measurement of local cerebral glucose utilization in the immature rat. I. Behavioral characterization and determination of lumped constant

An experimental model of status epilepticus has been developed in the immature rat by administration of pentylenetetrazol (PTZ) using repetitive, timed intraperitoneal injections of subconvulsive doses. The pattern of behavioral signs has been well characterized in each age group, i.e. 10 (P10), 14 (P14), 17 (P17) and 21 postnatal days (P21). In this model, the dose of convulsant could be adjusted as a function of interindividual sensitivity and status epilepticus lated for quite a long duration to allow the measurement of local cerebral metabolic rates for glucose (LCMRglc) by means of the [14C]2-deoxyglucose method [J. Neurochem., 28 (1977) 897-916]. To estimate LCMRglc during status epilepticus, the lumped constant (LC) was re-calculated in controls and PTZ-treated rats. The control LC was 0.54 at P10 and 0.50-0.51 at the three older ages studied (P14, P17 and P21). During status epilepticus, it increased to 0.64 in P10 rats and decreased to 0.42 and 0.40, respectively, in P17 and P21 animals. At P14, LC was not affected by seizures. The measurements of brain lactate levels showed a large 4.5-10-fold increase in PTZ-treated rats as compared to controls at all ages. The results of the present study show that the immature brain responds to sustained seizure activity in a specific way according to its postnatal age. Moreover, our results underscore the necessity of re-calculation of LC to the quantification of LCMRglc in such pathological states, particularly in immature animals.

Microdetermination of 2-deoxyglucose and 2-deoxyglucose 6-phosphate to determine glucose utilization rates in single neurons and small CNS regions after injecting nontracer amounts of 2-deoxyglucose

A nontracer amount (0.25 mmol/kg of body weight) of 2-deoxyglucose (DG) was intravenously injected into rats, which were frozen 2 and 4 min later in liquid nitrogen. Freeze-dried samples of CNS regions and cell bodies of spinal motor neurons were prepared, and the concentrations of glucose, glucose 6-phosphate, DG, and DG 6-phosphate (DG6P) in them were microassayed after 3,000-1,500,000-fold amplification using an enzymatic amplification reaction, NADP cycling. Based on the time course of glucose, DG, and DG6P concentrations in arterial plasma and the anterior horn of the spinal cord, the Sokoloff-type rate equations for DG and DG6P concentrations were mathematically solved, and the resultant DG and DG6P concentration functions were fitted to the data points using the nonlinear least-squares fitting SALS package program. This fitting provided four rate constants for the functions and supported the theoretical basis for our calculations of glucose utilization rate (GUR) when DG was administered in nontracer amounts. The GUR was highest in the spinal motor neurons and lowest in the white matter of the cerebellum. Neuron-rich structures, such as the cerebellar molecular and granular layers and the anterior horn of the spinal cord, had higher GUR values than the white matter of the cerebellum and spinal cord.

Acute hypoxia induces specific changes in local cerebral glucose utilization at different postnatal ages in the rat

The quantitative autoradiographic 2-[14C]-deoxyglucose technique (2-DG) was applied to measure the effects of an acute hypoxic exposure on local cerebral metabolic rates for glucose (LCMRglcs) in the 10 (P10)-, 14 (P14)-, and 21 (P21)-day-old rat. The animals were exposed to hypoxic (7% O2/93% N2) or control gas mixture (21% O2/79% N2) for 20 min before the initiation and for the duration of the 2-DG procedure. Lumped constants were not affected by hypoxia at any age. At P10, the exposure to the hypoxic gas mixture induced a generalized increase in LCMRglc which affected 41 structures of the 45 studied. At P14, average cerebral glucose utilization was similar in hypoxic and control rats. LCMRglc increased in 5 areas and decreased in 11 regions, mainly brainstem and respiratory areas in hypoxic rats. Finally, at P21, LCMRglc decreased in 11 structures of hypoxic rats. The increase in LCMRglc in the hypoxic 10-day-old rat likely reflects stimulation of anaerobic glycolysis. Conversely, at P14 and P21, when the brain has become more dependent upon oxygen supply for its energy metabolism, levels of LCMRglc are similar in both groups of animals or decreased in a few structures of hypoxic compared to normoxic rats. The results of the present study show that the immature brain responds to an acute hypoxic insult in a specific way according to its maturational state. They are also in good accordance with the higher resistance of the immature animal to oxygen deprivation.

Atrial natriuretic peptide augments the blood-brain transfer of water but not leucine and glucose

Recent evidence predicts an effect of atrial natriuretic peptide (ANP) on the blood-brain transfer of water. To test this prediction, we measured the blood-brain transfer of water, L-leucine, and D-glucose in 9 brain regions of male rats after intravenous injection of 10 pmol ANP. The peptide elicited an increase of the permeability-surface area (PaS) product of labeled water by 28-108% while the PaS products of leucine and glucose remained unchanged. Cerebral blood flow increased 15-48% while cardiac output and plasma volume in brain did not alter, indicating no change of capillary surface area (CSA). Regionally, the CSA varied from 63 cm2/g (striatum) to 97 cm2/g (colliculi) and the fraction of capillaries contributing to the total vascular volume varied from 29% (olfactory bulb/lobe) to 62% (striatum). The blood-brain barrier (BBB) permeability to water (5.7 micron/s) was an order of magnitude higher than to glucose (0.4 micron/s) or to leucine (0.3 micron/s).

In vivo location of the rate-limiting step of hexose uptake in muscle and brain tissue of rats

The uptake of glucose proceeds via facilitated transport from the plasma followed by phosphorylation of intracellular glucose. We have quantified the relative contribution of transport and phosphorylation to the overall rate of hexose utilization into the quadriceps muscle (red and white) and cerebellum of rats anesthetized with pentobarbital sodium. The method employed simultaneous infusions of radiolabeled 3-O-methyl-D-glucose and 2-deoxy-D-glucose. Results were expressed in terms of a parameter ft*, which has theoretical limits of 0 and 1 corresponding to phosphorylation and transport limitation, respectively. In cerebellum, basal rates of transport and phosphorylation were comparable (ft* = 0.32 +/- 0.02). Under conditions of hyperglycemia plus maximum insulin stimulation, phosphorylation limited glucose utilization to a greater extent (ft* = 0.12 +/- 0.02). No effect of hyperinsulinemia alone was observed. In red muscle, transport determined overall glucose utilization in the basal (ft* = 0.96 +/- 0.05) and euglycemic insulin-stimulated states (ft* = 0.90 +/- 0.02). A shift of the rate-limiting step from transport toward phosphorylation was observed in insulin-stimulated red muscle when blood glucose (ft* = 0.64 +/- 0.05) or epinephrine levels (ft* = 0.66 +/- 0.07) were elevated. Neither effect was seen in white muscle. We conclude that the transport step dominates but is not the only determinant of muscle hexose utilization under all conditions.

Modeling the dependence of hexose distribution volumes in brain on plasma glucose concentration: implications for estimation of the local 2-deoxyglucose lumped constant

The steady-state distribution volumes of glucose, 3-O-methylglucose, and 2-deoxyglucose (2DG) are known to change as the concentration of glucose in plasma ranges from hypo- to hyperglycemic values. Model estimates of the three distribution volumes were compared with distribution volume values experimentally measured in the brains of conscious rats as the concentration of glucose in plasma was varied from 2 to 28 mM. The dependence on plasma glucose concentration of the 2DG lumped constant, the factor that relates the phosphorylation rate of 2DG to the net rate of glucose utilization at unit specific radioactivity in the plasma, had been determined previously in separate series of experiments. The model was extended to incorporate this dependence of the lumped constant. In the model both the transport and the phosphorylation barriers were assumed to be single and saturable. The values of their respective half-saturation concentrations and the ratio of the two maximum velocities for glucose were assumed to be invariant over the entire range of plasma glucose concentration. Good agreement between measured and estimated values for the distribution volumes and the lumped constant was attained over the full range of plasma glucose concentration. The model estimates reflected the progressive transport limitation of the brain glucose content as plasma glucose levels were reduced to hypoglycemic values. The results also indicated that these changes should be evident in the time course of 2DG in brain following administration by bolus or continuous infusion, and thus that indexes of local lumped constant change could be derived from the time course data.

Influence of early chronic phenobarbital treatment on cerebral arteriovenous differences of glucose and ketone bodies in the developing rat

The influence of an early chronic phenobarbital treatment on cerebral arteriovenous differences of glucose, lactate, pyruvate, beta-hydroxybutyrate and acetoacetate was studied in suckling rats. The animals were treated from day 2 to 21 after birth by a daily injection of 50 mg/kg phenobarbital or by saline and were studied at 10, 14 and 21 days. Phenobarbital treatment induced a decrease in cerebral arteriovenous difference of glucose at P14 and no change at P10 and P21. The barbiturate did not have any influence on cerebral arteriovenous difference of lactate and pyruvate at the three stages studied. Cerebral uptake of beta-hydroxybutyrate was unchanged at P10 and increased by two-fold at P14 and by threefold at P21 by phenobarbital. Cerebral arteriovenous difference of acetoacetate was low and did not change with the pharmacological treatment. At P14 and P21, the calculated amount of oxygen used by the brain for the oxidation of ketone bodies was twice as high in barbiturate- as in saline-treated rats and reached values of 47 and 16% respectively in phenobarbital-exposed animals. In addition, the barbiturate seemed to affect the carrier process of beta-hydroxybutyrate from blood to brain. The results of the present study are in good agreement with previous data from our laboratory showing that an early chronic phenobarbital treatment is able to induce a shift in the cerebral energy metabolism balance in favor of ketone bodies.

The effect of pentobarbital and isoflurane on glucose metabolism in thermally injured rat brain

The effect of pentobarbital and isoflurane on cerebral glucose metabolism (CMRglc) was studied in thermally injured rat brain using quantitative autoradiography. In awake lesioned animals, CMRglc in cortical regions ipsilateral to the injury was reduced to 50% of normal while little if any decrease was observed in contralateral cortical regions and subcortical regions bilaterally. Treatment of lesioned animals with pentobarbital or isoflurane further reduced CMRglc, but more in the hemisphere contralateral to the injury than on the injured side. Thus, the side-to-side difference in cortical CMRglc present in the awake lesioned animals was abolished by the anesthetics. The results support the hypothesis that CMRglc depression associated with a focal cold injury is functional in nature. Reduction of metabolism by anesthetics in functionally depressed brain is limited by the decrease in CMRglc associated with the injury.