Influence of chloralose on brain regional glucose utilization

Retinal Vessel Responses to Flicker Stimulation Are Impaired in Ca v 2.3-Deficient Mice-An in-vivo Evaluation Using Retinal Vessel Analysis (RVA)

Objective: Metabolic demand increases with neuronal activity and adequate energy supply is ensured by neurovascular coupling (NVC). Impairments of NVC have been reported in the context of several diseases and may correlate with disease severity and outcome. Voltage-gated Ca2+-channels (VGCCs) are involved in the regulation of vasomotor tone. In the present study, we compared arterial and venous responses to flicker stimulation in Cav2.3-competent (Cav2.3[+/+]) and -deficient (Cav2.3[-/-]) mice using retinal vessel analysis. Methods: The mice were anesthetized and the pupil of one eye was dilated by application of a mydriaticum. An adapted prototype of retinal vessel analyzer was used to perform dynamic retinal vessel analysis. Arterial and venous responses were quantified in terms of the area under the curve (AUCart/AUCven) during flicker application, mean maximum dilation (mMDart/mMDven) and time to maximum dilation (tMDart/tMDven) during the flicker, dilation at flicker cessation (DFCart/DFCven), mean maximum constriction (mMCart/mMCven), time to maximum constriction (tMCart/tMCven) after the flicker and reactive magnitude (RMart/RMven). Results: A total of 33 retinal scans were conducted in 22 Cav2.3[+/+] and 11 Cav2.3[-/-] mice. Cav2.3[-/-] mice were characterized by attenuated and partially reversed arterial and venous responses, as reflected in significantly lower AUCart (p = 0.031) and AUCven (p = 0.047), a trend toward reduced DFCart (p = 0.100), DFCven (p = 0.100), mMDven (p = 0.075), and RMart (p = 0.090) and a trend toward increased tMDart (p = 0.096). Conclusion: To our knowledge, this is the first study using a novel, non-invasive analysis technique to document impairment of retinal vessel responses in VGCC-deficient mice. We propose that Cav2.3 channels could be involved in NVC and may contribute to the impairment of vasomotor responses under pathophysiological conditions.

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.

Quantitative separation of arterial and venous cerebral blood volume increases during voluntary locomotion

Voluntary locomotion is accompanied by large increases in cortical activity and localized increases in cerebral blood volume (CBV). We sought to quantitatively determine the spatial and temporal dynamics of voluntary locomotion-evoked cerebral hemodynamic changes. We measured single vessel dilations using two-photon microscopy and cortex-wide changes in CBV-related signal using intrinsic optical signal (IOS) imaging in head-fixed mice freely locomoting on a spherical treadmill. During bouts of locomotion, arteries dilated rapidly, while veins distended slightly and recovered slowly. The dynamics of diameter changes of both vessel types could be captured using a simple linear convolution model. Using these single vessel measurements, we developed a novel analysis approach to separate out spatially and temporally distinct arterial and venous components of the location-specific hemodynamic response functions (HRF) for IOS. The HRF of each pixel of was well fit by a sum of a fast arterial and a slow venous component. The HRFs of pixels in the limb representations of somatosensory cortex had a large arterial contribution, while in the frontal cortex the arterial contribution to the HRF was negligible. The venous contribution was much less localized, and was substantial in the frontal cortex. The spatial pattern and amplitude of these HRFs in response to locomotion in the cortex were robust across imaging sessions. Separating the more localized arterial component from the diffuse venous signals will be useful for dealing with the dynamic signals generated by naturalistic stimuli.

Selective potentiation of the metabotropic glutamate receptor subtype 2 blocks phencyclidine-induced hyperlocomotion and brain activation

Previous preclinical and clinical studies have demonstrated the efficacy of group II metabotropic glutamate receptor (mGluR) agonists as potential antipsychotics. Recent studies utilizing mGluR2-, mGluR3-, and double knockout mice support that the antipsychotic effects of those compounds are mediated by mGluR2. Indeed, biphenyl indanone-A (BINA), an allosteric potentiator of mGluR2, is effective in experimental models of psychosis, blocking phencyclidine (PCP)-induced hyperlocomotion and prepulse inhibition deficits in mice. In this study, we administered the NMDA receptor antagonist PCP (5.6 mg/kg i.p.) to rats, an established animal model predictive of schizophrenia. Here, we show that BINA (32 mg/kg i.p.) attenuated PCP-induced locomotor activity in rats. Using behaviorally relevant doses of BINA and PCP, we performed pharmacological magnetic resonance imaging (phMRI) to assess the specific brain regions that underlie the psychotomimetic effects of PCP, and examined how BINA modulated the PCP-induced functional changes in vivo. In anesthetized rats, acute administration of PCP produced robust, sustained blood oxygenation level-dependent (BOLD) activation in specific cortical, limbic, thalamic, and striatal regions. Pretreatment with BINA suppressed the amplitude of the BOLD response to PCP in the prefrontal cortex, caudaute-putamen, nucleus accumbens, and mediodorsal thalamus. Our results show key brain structures underlying PCP-induced behaviors in a preclinical model of schizophrenia, and, importantly, its reversal by potentiation of mGluR2 by BINA, revealing specific brain regions functionally involved in its pharmacological action. Finally, our findings bolster the growing body of evidence that mGluR2 is a viable target for the treatment of schizophrenia.

Energetics of neuronal signaling and fMRI activity

Energetics of resting and evoked fMRI signals were related to localized ensemble firing rates (nu) measured by electrophysiology in rats. Two different unstimulated, or baseline, states were established by anesthesia. Halothane and alpha-chloralose established baseline states of high and low energy, respectively, in which forepaw stimulation excited the contralateral primary somatosensory cortex (S1). With alpha-chloralose, forepaw stimulation induced strong and reproducible fMRI activations in the contralateral S1, where the ensemble firing was dominated by slow signaling neurons (SSN; nu range of 1-13 Hz). Under halothane, weaker and less reproducible fMRI activations were observed in the contralateral S1 and elsewhere in the cortex, but ensemble activity in S1 was dominated by rapid signaling neurons (RSN; nu range of 13-40 Hz). For both baseline states, the RSN activity (i.e., higher frequencies, including the gamma band) did not vary upon stimulation, whereas the SSN activity (i.e., alpha band and lower frequencies) did change. In the high energy baseline state, a large majority of total oxidative energy [cerebral metabolic rate of oxygen consumption (CMR(O2))] was devoted to RSN activity, whereas in the low energy baseline state, it was roughly divided between SSN and RSN activities. We hypothesize that in the high energy baseline state, the evoked changes in fMRI activation in areas beyond S1 are supported by rich intracortical interactions represented by RSN. We discuss implications for interpreting fMRI data where stimulus-specific DeltaCMR(O2) is generally small compared with baseline CMR(O2).

Alpha-chloralose is a suitable anesthetic for chronic focal cerebral ischemia studies in the rat: a comparative study

alpha-Chloralose is widely used as an anesthetic in studies of the cerebrovasculature because it provides robust metabolic and hemodynamic responses to functional stimulation. However, there have been no controlled studies of focal ischemia in the rat under alpha-chloralose anesthesia. Artificially ventilated rats were prepared using 1.2-1.5% isoflurane anesthesia for filament occlusion of the right middle cerebral artery (MCA), and anesthesia was either switched to alpha-chloralose (60 mg/kg bolus, 30 mg/kg/h; n=10) or was maintained on 1% isoflurane (n=10). Following temporary MCA occlusion EEG was monitored from a screw electrode and changes in cerebral blood flow (rCBF) measured with a laser Doppler probe placed over the ischemic cortex. This study shows that alpha-chloralose is a safe anesthetic for ischemia studies and provides excellent survival. Compared with isoflurane, the cortical and total infarct volumes are larger in the alpha-chloralose-anesthetized animals, while the functional outcome at 72 h is similar. The total duration of peri-infarct flow transients (PIFTs) is also significantly longer in alpha-chloralose-anesthetized animals. The average amplitude of the flow transients showed a good correlation with the extent of edema in all animals as did the total duration of non-convulsive seizures (NCS) in the alpha-chloralose-anesthetized animals.

Reorganization of sensory processing below the level of spinal cord injury as revealed by fMRI

The adult mammalian CNS undergoes plastic changes in response to injury. To investigate such changes in spinal cord, functional magnetic resonance imaging (fMRI) was applied in rats subjected to complete transection of the mid-thoracic spinal cord. Blood oxygenation level-dependent (BOLD) contrasts were recorded in the distal spinal cord different times after injury (3, 7, and 14 days, and 1, 3, and 6 months) in response to electrical hind limb stimulation. Functional MRI demonstrated a substantial increase of neuronal activation in the ipsilateral dorsal horn after injury. Notably, 0.5 mA, which did not evoke activation in the normal spinal cord and was considered a non-painful stimulus, induced significant BOLD responses in the dorsal horn after injury. Increased sensitivity was also seen in response to 1.0 mA stimulation. Our results suggest exaggerated responsiveness of spinal neurons after spinal cord injury. Reorganization in the injured spinal cord has been shown to involve the amplification of peripheral inputs and implicated as one underlying mechanism causing neuropathic pain and autonomic dysreflexia. Since BOLD signals can demonstrate such plastic changes in spinal cord parenchyma, we propose fMRI as a method to monitor functional reorganization in the spinal cord after injury. Combining brain and spinal cord fMRI allows the visualization of neuronal activities along the entire neuroaxis and thereby an evaluation of the different plastic responses to CNS injuries that occur in the brain and the spinal cord.

Comparison of functional activity in the rat cervical spinal cord during alpha-chloralose and halothane anesthesia

Alpha-chloralose is commonly used during animal fMRI studies for anesthesia, however, recovery of animals is difficult, limiting experimental design. The use of a less invasive anesthetic would enable chronic experiments. The present study compares functional activity in the spinal cord of the alpha-chloralose and halothane-anesthetized rat. Functional MRI of the rat cervical spinal cord was performed on 6 alpha-chloralose and 6 halothane-anesthetized rats in a Bruker 7 T MR system during electrical stimulation of the right forepaw. Following imaging, four animals from each group were perfused and spinal cords removed for immunohistochemical analysis. Areas of c-fos expression were identified with immunofluorescent labeling to confirm the presence of neuronal activity. Functional activity and c-fos expression were observed predominantly between the fifth and seventh cervical spinal cord segments. Areas of fMRI activation in the spinal cord correspond well with spinal cord physiology. Areas of c-fos expression confirmed that neuronal activity was present in the regions of fMRI activity. The regions and amount of fMRI activity observed were similar for both anesthetics. Functional magnetic resonance imaging of the spinal cord can be achieved using both alpha-chloralose and halothane anesthesia in rats. We therefore suggest that halothane may be used as an anesthetic agent for chronic fMRI studies of the spinal cord.

Small animal, whole brain fMRI: innocuous and nociceptive forepaw stimulation

Supra-spinal pain processing involves a number of extensive networks. An examination of these networks using small animal functional magnetic resonance imaging (fMRI) is difficult. While prior studies have successfully delineated regions consistent with known pain processing pathways, they have been restricted to acquisitions of limited spatial extent with coarse in-plane resolution to achieve a high temporal resolution. An isotropic, whole brain fMRI protocol has been developed for the examination of the supra-spinal consequences of innocuous and nociceptive electrical stimulation of the rat forepaw. Innocuous electrical stimulation of the rat forepaw delineated BOLD contrast responses consistent with known somatosensory processing pathways (contralateral primary somatosensory cortex (S1), a region consistent with secondary somatosensory cortex, the ventral posterolateral thalamic nucleus and ipsilateral cuneate nucleus), providing face validity for the technique. The putative noxious stimulus delineated additional regions consistent with the classical lateral and medial pain systems as well as secondarily associated areas: the aversion and descending inhibition systems. These included the ipsilateral inferior colliculus, anterior pretectal nucleus, mediodorsal thalamic nucleus, with regions in the pre-frontal, cingulated, ventral orbital and infra-limbic cortices, nucleus accumbens all exhibiting negative BOLD changes. Such regions are in agreement with, and extend, those previously reported. Acquisition, post-processing and analysis methodologies undertaken in this study constitute a marked extension of previous fMRI in the rat, enabling whole brain coverage at a spatial resolution sufficient to delineate regional changes in BOLD contrast consistent with somatosensory and nociceptive networks.

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

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

5-Hydroxytryptamine2C Receptor Contribution to m-Chlorophenylpiperazine and N-Methyl-β-carboline-3-carboxamide-Induced Anxiety-Like Behavior and Limbic Brain Activation

Activation of 5-hydroxytryptamine2C (5-HT(2C)) receptors by the 5-HT(2) receptor agonist m-chlorophenylpiperazine (m-CPP) elicits anxiety in humans and anxiety-like behavior in animals. We compared the effects of m-CPP with the anxiogenic GABA(A) receptor inverse agonist N-methyl-beta-carboline-3-carboxamide (FG-7142) on both anxiety-like behavior and regional brain activation using functional magnetic resonance imaging (fMRI) in the rat. We also determined whether the selective 5-HT(2C) receptor antagonist SB 242084 [6-chloro-2,3-dihydro-5-methyl-N-[6-[(2-methyl-3-pyridinyl)oxy]-3-pyridinyl]-1H-indole-1-carboxyamide dihydrochloride] would blunt m-CPP or FG-7142-induced neuronal activation. Both m-CPP (3 mg/kg i.p.) and FG-7142 (10 mg/kg i.p.) elicited anxiety-like behavior when measured in the social interaction test, and pretreatment with SB 242084 (1 mg/kg i.p.) completely blocked the behavioral effects of both anxiogenic drugs. Regional brain activation in vivo in response to anxiogenic drug challenge was determined by blood oxygen level-dependent (BOLD) fMRI using a powerful 9.4T magnet. Region of interest analyses revealed that m-CPP and FG-7142 significantly increased BOLD signals in brain regions that have been linked to anxiety, including the amygdala, dorsal hippocampus, and medial hypothalamus. These BOLD signal increases were blocked by pretreatment with SB 242084. In contrast, injection of m-CPP and FG-7142 resulted in BOLD signal decreases in the medial prefrontal cortex that were not blocked by SB 242084. In conclusion, the brain activation signals produced by anxiogenic doses of both m-CPP and FG-7142 are mediated at least partially by the 5-HT(2C) receptor, indicating that this receptor is a key component in anxiogenic neural circuitry.

Alpha-chloralose poisoning in dogs and cats: a retrospective study of 33 canine and 13 feline confirmed cases

Alpha-chloralose (AC) is an anaesthetic compound also used as a rodenticide, and has dose-dependent central nervous system mixed effects of excitation and depression. The objectives of this study were to detail the clinical and clinicopathological characteristics, as well as the treatment and prognosis, of AC toxicosis in dogs and cats. Medical records were retrospectively reviewed for AC poisoning between the years 1989 and 2004, and 33 dogs and 13 cats were included in the study. The most common clinical signs were seizures, muscle tremor, hyperaesthesia, hypothermia, salivation, myosis, stupor, coma and ataxia. Coma was more common, while salivation and ataxia were less common in cats compared to dogs. Although hypothermia was very common, especially in cats (90.9%), hyperthermia was frequently observed in dogs (21%). Treatment in all patients was supportive and symptomatic, and the most commonly used anticonvulsants were diazepam and barbiturates; however, severe unresponsive seizures in three dogs had to be controlled with inhalant gas anaesthesia. The hospitalisation period was 1-3 days, and the overall mortality rate was 6.5%. Alpha-chloralose poisoning seems to have a favourable prognosis in dogs and cats.

Functional magnetic resonance imaging and c-Fos mapping in rats following an anorectic dose of m-chlorophenylpiperazine

We have used blood-oxygenation-level-dependent (BOLD) contrast functional magnetic resonance imaging (fMRI) to characterise brain regions responsive to a regulator of appetite. An anorectic dose of the 5-HT(1B/2C) receptor agonist m-chlorophenylpiperazine (mCPP; 3 mg/kg s.c.) was used to compare BOLD contrast fMRI with expression of the c-Fos protein. mCPP was administered to rats, which were then anaesthetised and perfused with fixative 90 min later to allow immunohistochemistry. In a separate experiment, rats were imaged using a T(2)*-weighted gradient echo in a 7 T magnet for 70 min under alpha-chloralose anaesthesia. Both methods detected positive activation in areas of the limbic system: cingulate and orbitofrontal cortices, nucleus accumbens, paraventricular and dorsomedial regions of the hypothalamus. fMRI detected increased signal in the pontine nuclei, the hippocampal formation and olfactory cortex, areas that did not display c-Fos. In addition, BOLD signal was diminished in the ventral tegmental area, preoptic area and the cerebellum-presumably due to decreased neuronal signalling and, therefore, unlikely to display c-Fos. Activity in the limbic system may reflect the appetitive agonist activity of mCPP at the 5-HT(2C) receptor. We conclude that c-Fos provides excellent spatial information but is less useful for detecting inhibited regions, whereas fMRI provides greater temporal resolution. Thus, the two methodologies provide complementary details of brain activity following pharmacological challenge.

Methodological considerations in rat brain BOLD contrast pharmacological MRI

RATIONALE AND OBJECTIVES

Blood oxygen level dependent (BOLD) contrast pharmacological magnetic resonance imaging (phMRI) is an increasingly popular technique that allows the non-invasive investigation of spatial and temporal changes in rat brain function in response to pharmacological stimulation in vivo. Rat brain BOLD contrast phMRI is, at present, established in few neuropharmacological laboratories, and various issues associated with the technique require attention. The present review is primarily aimed at psychopharmacologists with no previous experience of phMRI, who are interested in the practical aspects that phMRI studies entail.

RESULTS AND DISCUSSION

Experimental and analytical considerations, including anaesthesia, physiological monitoring, drug dose and delivery, scanning protocols, statistical approaches and the interpretation of phMRI data, are discussed.

Confounding effects of anesthesia on functional activation in rodent brain: a study of halothane and α-chloralose anesthesia

Functional magnetic resonance imaging (fMRI) in animal models provides a platform for more extensive investigation of drug effects and underlying physiological mechanisms than is possible in humans. However, it is usually necessary for the animal to be anesthetized. In this study, we have used a rat model of direct cortical stimulation to investigate the effects of anesthesia in rodent fMRI. Specifically, we have sought to answer two questions (i) what is the relationship between baseline neuronal activity and the BOLD response to stimulation under halothane anesthesia? And (ii) how does the BOLD response change after transferring from halothane to the commonly used anesthetic alpha-chloralose? In the first set of experiments, we found no significant differences in the amplitude of the BOLD response at the different halothane doses studied, despite electroencephalography (EEG) recordings indicating a dose-dependent reduction in baseline neuronal activity with increasing halothane levels. In the second set of experiments, a reduction in the spatial extent of the BOLD response was apparent immediately after transfer from halothane to alpha-chloralose anesthesia, although no change in the peak signal change was evident. However, several hours after transfer to alpha-chloralose, a significant increase in both the spatial extent and peak height of the BOLD response was observed, as well as an increased sensitivity to secondary cortical and subcortical activation. These findings suggest that, although alpha-chloralose anesthesia is associated with a greater BOLD response for a fixed stimulus relative to halothane, there is substantial variation in the extent and magnitude of the response over time that could introduce considerable variability in studies using this anesthetic.

Functional magnetic resonance imaging and somatosensory evoked potentials in rats with a neonatally induced freeze lesion of the somatosensory cortex

Brain plasticity is an important mechanism for functional recovery from a cerebral lesion. The authors aimed to visualize plasticity in adult rats with a neonatal freeze lesion in the somatosensory cortex using functional magnetic resonance imaging (fMRI), and hypothesized activation outside the primary projection area. A freeze lesion was induced in the right somatosensory cortex of newborn Wistar rats (n = 12). Sham-operated animals (n = 7) served as controls. After 6 or 7 months, a neurologic examination was followed by recording of somatosensory evoked potentials (SSEPs) and magnetic resonance experiments (anatomical images, fMRI with blood oxygen level-dependent contrast and perfusion-weighted imaging) with electrical forepaw stimulation under alpha-chloralose anesthesia. Lesioned animals had no obvious neurologic deficits. Anatomical magnetic resonance images showed a malformed cortex or hyperintense areas (cysts) in the lesioned hemisphere. SSEPs were distorted and smaller in amplitude, and fMRI activation was significantly weaker in the lesioned hemisphere. Only in a few animals were cortical areas outside the primary sensory cortex activated. The results are discussed in respect to an apparent absence of plasticity, loss of excitable tissue, the excitability of the lesioned hemisphere, altered connectivity, and a disturbed coupling of increased neuronal activity to the hemodynamic response.

Neuroimaging With Calibrated fMRI

The conventional functional MRI (fMRI) map offers information indirectly about localized changes in neuronal activity because it reflects changes in blood oxygenation, not actual neuronal activity. To provide a neurophysiological basis of fMRI, researchers have used electrophysiology to show correlations of fMRI and electric signals. However, quantitative interpretation of the degree to which neuronal activity has changed still cannot be made from conventional fMRI data. The fMRI signal has 2 parts: one describes the correlation between oxidative metabolism (cerebral metabolic rate of oxygen [CMRO 2 ]) and cerebral blood flow (CBF), which supports the bioelectric work to sustain neuronal excitability; the other is the requisite dilation of blood vessels (cerebral blood volume [CBV]), which is the mechanical response involved in removal of waste while providing nutrients. Since changes in energy metabolism are related to bioelectric work, we tested whether spiking frequency of a neuronal ensemble (ν) is reflected by local energy metabolism (CMRO 2 ) in rat brain. We used extracellular recordings to measure Δν/ν and calibrated fMRI (ie, using fMRI signal, CBF, and CBV maps) to measure ΔCMRO 2 /CMRO 2 during sensory stimulation. We found that ΔCMRO 2 /CMRO 2 is ≈Δν/ν, which suggests efficient energy use during brain work. Thus, calibrated fMRI provides data on where and by how much the neuronal activity has changed. Possibilities of utilizing calibrated fMRI as a neuroimaging method are discussed.

Total neuroenergetics support localized brain activity: implications for the interpretation of fMRI

In alpha-chloralose-anesthetized rats, changes in the blood oxygenation level-dependent (BOLD) functional MRI (fMRI) signal (DeltaS/S), and the relative spiking frequency of a neuronal ensemble (Deltanu/nu) were measured in the somatosensory cortex during forepaw stimulation from two different baselines. Changes in cerebral oxygen consumption (DeltaCMR(O2)/CMR(O2)) were derived from the BOLD signal (at 7T) by independent determinations in cerebral blood flow (DeltaCBF/CBF) and volume (DeltaCBV/CBV). The spiking frequency was measured by extracellular recordings in layer 4. Changes in all three parameters (CMR(O2), nu, and S) were greater from the lower baseline (i.e., deeper anesthesia). For both baselines, DeltaCMR(O2)/CMR(O2) and Deltanu/nu were approximately one order of magnitude larger than DeltaS/S. The final values of CMR(O2) and nu reached during stimulation were approximately the same from both baselines. If only increments were required to support functions then their magnitudes should be independent of the baseline. In contrast, if particular magnitudes of activity were required, then sizes of increments should inversely correlate with the baseline (being larger from a lower baseline). The results show that particular magnitudes of activity support neural function. The disregard of baseline activity in fMRI experiments by differencing removes a large and necessary component of the total activity. Implications of these results for understanding brain function and fMRI experiments are discussed.

Cerebral energetics and spiking frequency: the neurophysiological basis of fMRI

Functional MRI (fMRI) is widely assumed to measure neuronal activity, but no satisfactory mechanism for this linkage has been identified. Here we derived the changes in the energetic component from the blood oxygenation level-dependent (BOLD) fMRI signal and related it to changes in the neuronal spiking frequency in the activated voxels. Extracellular recordings were used to measure changes in cerebral spiking frequency (Deltanu/nu) of a neuronal ensemble during forepaw stimulation in the alpha-chloralose anesthetized rat. Under the same conditions localized changes in brain energy metabolism (DeltaCMR(O2)/CMR(O2)) were obtained from BOLD fMRI data in conjunction with measured changes in cerebral blood flow (DeltaCBF/CBF), cerebral blood volume (DeltaCBV/CBV), and transverse relaxation rates of tissue water (T(2)(*) and T(2)) by MRI methods at 7T. On stimulation from two different depths of anesthesia DeltaCMR(O2)/CMR(O2) approximately Deltanu/nu. Previous (13)C magnetic resonance spectroscopy studies, under similar conditions, had shown that DeltaCMR(O2)/CMR(O2) was proportional to changes in glutamatergic neurotransmitter flux (DeltaV(cyc)/V(cyc)). These combined results show that DeltaCMR(O2)/CMR(O2) approximately DeltaV(cyc)/V(cyc) approximately Deltanu/nu, thereby relating the energetic basis of brain activity to neuronal spiking frequency and neurotransmitter flux. Because DeltaCMR(O2)/CMR(O2) had the same high spatial and temporal resolutions of the fMRI signal, these results show how BOLD imaging, when converted to DeltaCMR(O2)/CMR(O2), responds to localized changes in neuronal spike frequency.

Mapping of brain activation in response to pharmacological agents using fMRI in the rat

Functional MRI (fMRI) was used to investigate the effects of psychotropic compound activity in the rat brain in vivo. The effects of dizocilpine (MK-801) an N-methyl-D-aspartate receptor antagonist and m-chlorophenylpiperazine (mCPP), a 5-HT(2b/2c)-receptor agonist on rat brain activity were investigated over a time interval of about 1 h and the results were compared to published glucose utilisation and cerebral blood flow data. Signal magnitude increases were observed predominantly in limbic regions following MK-801 administration (0.5 mg/kg i.v) whereas signal decreases were restricted to neocortical areas; a characteristic, time dependent pattern of regional changes evolved from the thalamic nuclei to cortical regions. In contrast, mCPP (25 mg/kg i.p) produced gradual signal intensity increases in limbic and motor regions with signal decreases restricted to the visual, parietal and motor cortices. The results from both compounds show remarkable similarity with autoradiographic measurements of cerebral blood flow and glucose uptake. These experiments suggest that the spatio-temporal capabilities of fMRI may be applied to the in vivo investigation of psychoactive compound activity with potential for clinical applications.

Effects of anesthesia on functional activation of cerebral blood flow and metabolism

Functional brain mapping based on changes in local cerebral blood flow (lCBF) or glucose utilization (lCMR(glc)) induced by functional activation is generally carried out in animals under anesthesia, usually alpha-chloralose because of its lesser effects on cardiovascular, respiratory, and reflex functions. Results of studies on the role of nitric oxide (NO) in the mechanism of functional activation of lCBF have differed in unanesthetized and anesthetized animals. NO synthase inhibition markedly attenuates or eliminates the lCBF responses in anesthetized animals but not in unanesthetized animals. The present study examines in conscious rats and rats anesthetized with alpha-chloralose the effects of vibrissal stimulation on lCMR(glc) and lCBF in the whisker-to-barrel cortex pathway and on the effects of NO synthase inhibition with N(G)-nitro-L-arginine methyl ester (L-NAME) on the magnitude of the responses. Anesthesia markedly reduced the lCBF and lCMR(glc) responses in the ventral posteromedial thalamic nucleus and barrel cortex but not in the spinal and principal trigeminal nuclei. L-NAME did not alter the lCBF responses in any of the structures of the pathway in the unanesthetized rats and also not in the trigeminal nuclei of the anesthetized rats. In the thalamus and sensory cortex of the anesthetized rats, where the lCBF responses to stimulation had already been drastically diminished by the anesthesia, L-NAME treatment resulted in loss of statistically significant activation of lCBF by vibrissal stimulation. These results indicate that NO does not mediate functional activation of lCBF under physiological conditions.

Toward a unified theory of narcosis: brain imaging evidence for a thalamocortical switch as the neurophysiologic basis of anesthetic-induced unconsciousness

A unifying theory of general anesthetic-induced unconsciousness must explain the common mechanism through which various anesthetic agents produce unconsciousness. Functional-brain-imaging data obtained from 11 volunteers during general anesthesia showed specific suppression of regional thalamic and midbrain reticular formation activity across two different commonly used volatile agents. These findings are discussed in relation to findings from sleep neurophysiology and the implications of this work for consciousness research. It is hypothesized that the essential common neurophysiologic mechanism underlying anesthetic-induced unconsciousness is, as with sleep-induced unconsciousness, a hyperpolarization block of thalamocortical neurons. A model of anesthetic-induced unconsciousness is introduced to explain how the plethora of effects anesthetics have on cellular functioning ultimately all converge on a single neuroanatomic/neurophysiologic system, thus providing for a unitary physiologic theory of narcosis related to consciousness.

Evolution of microcirculatory disturbances after permanent middle cerebral artery occlusion in rats

Nonischemic brain capillaries show a continuous and heterogeneous plasma perfusion. In the current study, plasma perfusion was investigated in rats during 2 to 168 hours of permanent middle cerebral artery occlusion. Perfused capillaries were detected in brain cryosections by fluorescein isothiocyanate (FITC) dextran after 10 minutes of circulation time. Heterogeneity of capillary perfusion was identified by Evans blue (EB), which circulated for 3 seconds. In this setting, the heterogeneity of intracapillary EB concentrations reflects heterogeneities in capillary flow velocities. The CBF was quantified by simultaneous iodo[14C]antipyrine autoradiography. When moving from normal flow to low-flow areas in the ischemic hemisphere, three states of capillary filling could be distinguished: state 1--fast perfusion, filling by FITC dextran and EB (CBF 0.33 mL x g(-1) x min(-1)); state 2--delayed perfusion, only FITC dextran filling (CBF 0.104 mL x g(-1) x min(-1)); state 3--minimal perfusion, no dye filling (CBF 0.056 mL x g(-1) x min(-1)). In tissue of state 1 at the borderline to ischemic tissue, a higher heterogeneity of intracapillary EB concentration (85.7%) was found than in the contralateral nonischemic hemisphere (76.4%) (P < 0.05), indicating a compromised microcirculation. The adjacent ischemic areas were filled by FITC dextran (state 2) 2 to 4 hours after middle cerebral artery occlusion, indicating a maintained, although slow, perfusion at this time. Later, minimal perfused areas (state 3) progressively replaced the delayed perfused areas (state 2). This study shows, for the first time, the evolution of microvascular disturbances in relation to CBF. In the low-flow areas, an early residual plasma perfusion is later followed by a lack of perfusion or minimal perfusion. In areas of higher, although reduced flow at the border between normal and ischemic tissue, an extreme capillary perfusion heterogeneity indicates permanent microcirculatory abnormalities.

FOS induction in brain associated with seizure and sustained cortical vasodilation in anesthetized rat

PURPOSE

By estimating the anatomical distribution of neurons expressing c-fos protein, we sought to establish whether the intrinsic neural systems known to be implicated in the cerebrovascular regulation were activated during the increase in cortical blood flow associated with epileptic seizures.

METHODS

A single unilateral microinjection of the cholinergic agonist, carbachol, in the thalamic generalized convulsive seizure area was used in anesthetized rats to elicit recurrent episodes of electrocortical epileptiform activity and an increase in cortical blood flow. Neuronal expression of Fos protein was analyzed to identify activated brain regions.

RESULTS

We identified two cortical vasodilatory responses: a sustained cortical vasodilatory response associated with the continuous low-frequency, high-amplitude spiking and a transient cortical vasodilatory response invariably related to the recurrent spike-burst activity. The sustained cortical blood flow began to increase at 55-65 min, remaining significantly (p < 0.05) increased and reaching at the end of the experiment < or =182+/-17% of the prestimulated control. The electrocortical epileptic activity and the cerebral cortical vasodilation were associated with a marked increase in Fos immunoreactivity in the entorhinal and piriform cortices, the dentate gyrus, the hippocampus, and the amygdala. Fos-positive neurons also were found in specific thalamic nuclei, the cerebral cortex, the caudate-putamen, the hypothalamus, the pontine parabrachial nuclei, the dorsal raphe, and the rostral ventrolateral medulla.

CONCLUSIONS

These results provide evidence that convulsive seizures elicited by cholinergic stimulation of the thalamus, in addition to limbic and somatic motor systems, activate central autonomic nuclei and their pathways, including those implicated in cerebrovascular regulation.

Local uncoupling of the cerebrovascular and metabolic responses to somatosensory stimulation after neuronal nitric oxide synthase inhibition

It has recently been shown, using either genetically engineered mutant mice (nitric oxide synthase [NOS] knockout) or specific pharmacological tools, that type I NOS (neuronal isoform of NOS, [nNOS]) participates in coupling cerebral blood flow to functional activation. However, it has not been clearly established whether the associated metabolic response was preserved under nNOS inhibition and whether this action was exerted homogeneously within the brain. To address these issues, we analyzed the combined circulatory and metabolic consequences of inhibiting the nNOS both at rest and during functional activation in the rat anesthetized with alpha-chloralose. Cerebral blood flow and cerebral glucose use (CGU) were measured autoradiographically using [14C]iodoantipyrine and [14C]2-deoxyglucose during trigeminal activation induced by unilateral whiskers stimulation in vehicle- and 7-nitroindazole-treated rats. Our data show that inhibition of nNOS globally decreased CBF without altering CGU, indicating that NO-releasing neurons play a significant role in maintaining a resting cerebrovascular tone in the whole brain. During whisker stimulation, nNOS inhibition totally abolished the cerebrovascular response only in the second order relay stations (thalamus and somatosensory cortex) of the trigeminal relay without altering the metabolic response. These findings provide evidence that the involvement of neurally-derived NO in coupling flow to somatosensory activation is region-dependent, and that under nNOS inhibition, CBF and CGU may vary independently during neuronal activation.

Oxidative glucose metabolism in rat brain during single forepaw stimulation: a spatially localized 1H[13C] nuclear magnetic resonance study

In the alpha-chloralose-anesthetized rat during single forepaw stimulation, a spatially localized 1H[13C] nuclear magnetic resonance spectroscopic method was used to measure the rate of cerebral [C4]-glutamate isotopic turnover from infused [1,6-(13)C]glucose. The glutamate turnover data were analyzed using a mathematical model of cerebral glucose metabolism to evaluate the tricarboxylic acid (TCA) cycle flux (V(TCA)). During stimulation the value of V(TCA) in the sensorimotor region increased from 0.47 +/- 0.06 (at rest) to 1.44 +/- 0.41 micromol x g(-1) x min(-1) (P < 0.01) in the contralateral hemispheric compartment (24 mm3) and to 0.65 +/- 0.10 micromol x g(-1) x min(-1) (P < 0.03) in the ipsilateral side. Each V(TCA) value was converted to the cerebral metabolic rates of glucose oxidation (oxidative-CMR(glc)) and oxygen consumption (CMR(O2)). These rates were corrected for partial-volume based on activation maps obtained by blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI). The percent increase and the absolute value of oxidative-CMR(glc) in the activated regions are similar to values reported previously for total-CMR(glc) using the same activation paradigm. This indicates that the large majority of energy required for brain activation, in going from the resting to an activated state, is supplied by glucose oxidation. The level of activity during stimulation is relevant to awake animals because the oxidative-CMR(glc) (1.05 +/- 0.28 micromol x g(-1) x min(-1); current study) is in the range of total-CMR(glc) previously reported for awake rats undergoing physiologic activation (0.7-1.4 micromol x g(-1) x min(-1)). It is concluded that oxidative glycolysis is the main source of energy for increased brain activity and a positive BOLD fMRI signal-change occurs in conjunction with a large increase in CMR(O2).

Increased tricarboxylic acid cycle flux in rat brain during forepaw stimulation detected with 1H[13C]NMR

NMR spectroscopy was used to test recent proposals that the additional energy required for brain activation is provided through nonoxidative glycolysis. Using localized NMR spectroscopic methods, the rate of C4-glutamate isotopic turnover from infused [1-(13)C]glucose was measured in the somatosensory cortex of rat brain both at rest and during forepaw stimulation. Analysis of the glutamate turnover data using a mathematical model of cerebral glucose metabolism showed that the tricarboxylic acid cycle flux [(V(TCA)] increased from 0.49 +/- 0.03 at rest to 1.48 +/- 0.82 micromol/g/min during stimulation (P < 0.01). The minimum fraction of C4-glutamate derived from C1-glucose was approximately 75%, and this fraction was found in both the resting and stimulated rats. Hence, the percentage increase in oxidative cerebral metabolic rate of glucose use (CMRglc) equals the percentage increases in V(TCA) and cerebral metabolic rate of oxygen consumption (CMRO2). Comparison with previous work for the same rat model, which measured total CMRglc [Ueki, M., Linn, F. & Hossman, K. A. (1988) J. Cereb. Blood Flow Metab. 8, 486-4941, indicates that oxidative CMRglc supplies the majority of energy during sustained brain activation.

Is alpha-chloralose plus halothane induction a suitable anesthetic regimen for cerebrovascular research?

The aim of this study was to determine whether alpha-chloralose, when associated with an initial period of halothane, is a suitable anesthetic regimen for cerebrovascular studies. For this purpose, rats anesthetized with alpha-chloralose plus halothane induction were first subjected to noxious stimuli, and the behavior, EEG and systemic variables were recorded. During a second step, cortical blood flow was measured with laser-Doppler flowmetry and the time-course of the cerebrovascular reactivity to hypercapnia were measured in artificially ventilated rats anesthetized with either alpha-chloralose (40 mg.kg-1, s.c.) plus halothane induction (1.5% given during the first 45-60 min) or halothane alone (1.5%). Finally, an experimental paradigm was developed that allowed the comparison of the hypercapnic reactivity, both in awake and anesthetized conditions in the same animal. Our results show that the association of alpha-chloralose with halothane leads to stable cardiovascular parameters and immobility of ventilated rats, placed in ear bars without curare, for 3 h without any sign of discomfort. Based on EEG criteria, we found that halothane induction lengthens the duration of alpha-chloralose anesthesia (253 +/- 19 vs. 200 +/- 15 min, P < 0.01). Under alpha-chloralose alone or in association with halothane induction, the vascular reactivity to hypercapnia was considerably impaired (-85% compared to the awake state, P < 0.01), but this impairment was transient, since a control reactivity was restored 150-190 min after induction of anesthesia. Under halothane alone, the vascular reactivity remained reduced throughout the experiment. These results provide evidence that alpha-chloralose plus halothane induction is a suitable anesthetic regimen which displays a temporal window of normal cerebrovascular reactivity.

Phrenic afferent stimulation by bradykinin and the distribution of the inspiratory motor drive

Activation of thin-fiber (groups III and IV) afferents from the diaphragm using capsaicin or ischemia increases the respiratory muscle activity. To assess whether bradykinin causes similar effects, we injected boluses of bradykinin into the phrenic artery of in situ, isolated and innervated left hemi-diaphragm preparations in 8 alpha-chloralose anesthetized, vagotomized, mechanically ventilated dogs. Inspiratory motor drive during spontaneous breathing attempts was assessed from the integrated EMG activity of several inspiratory muscles. Fifty micrograms of bradykinin increased peak integrated EMG activities of alae nasi to 110%, genioglossus to 189%, left diaphragm to 115% (P < 0.05) and parasternal to 109% (P < 0.01) of baseline activity 60 sec after the injection. Inspiratory time decreased by 10% (P < 0.01). The mean arterial blood pressure increased by about 10 mmHg. Responses were similar with 10, 25 and 100 micrograms of bradykinin. After left phrenicotomy, bradykinin did not affect inspiratory muscle EMG or respiratory timing. In conclusion, thin-fiber phrenic afferent activation by bradykinin exerts an excitatory but disproportionate influence on the inspiratory motor drive.

Lesions of the rostral ventrolateral medulla reduce the cerebrovascular response to hypoxia

Sympathoexcitatory neurons of the rostral ventrolateral medulla are tonically active and required for maintenance of resting levels of arterial pressure. They are also selectively excited by hypoxia and responsible for the associated sympathoexcitation. Since electrical or chemical stimulation of RVL will increase regional cerebral blood flow (rCBF) independently of changes in regional cerebral glucose utilization (rCGU) we investigated whether the RVL was also required to maintain resting levels of rCBF and also participated in the cerebrovascular vasodilation elicited by hypoxia. Rats were anesthetized (chloralose; 40 mg/kg, s.c.), paralyzed (tubocurarine) and ventilated (100% O2). rCBF was measured in 10 dissected brain regions using [14C]iodoantipyrine; rCGU was measured by 2-deoxy-D-[14C]glucose. In controls (n = 6) rCBF ranged from 56 +/- 5 in corpus callosum to 101 +/- 6 ml/min x 100 g in inferior colliculus. Hypoxic-hypoxia (PaO2 = 36 +/- 1 mmHg, n = 6) increased rCBF in all structures maximally, at 204% of control, in occipital cortex. Hypercapnia (PaCO2 = 63.5 +/- 0.9, n = 5) also increased rCBF (P < 0.01) maximally to 299% of control in superior colliculus. Spinal cord transection with maintenance of arterial pressure did not affect resting rCBF and increased the vasodilation to hypoxia (PaO2 = 39 +/- 1 mmHg, n = 5) from 2- to 3-fold in all structures (P < 0.01). Bilateral lesions within the RVL had no effect on resting rCBF or rCGU. However, they significantly reduced, in all areas by 50-69% (P < 0.01, n = 5), the cerebrovascular dilation elicited by hypoxia but not hypercapnia.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential cerebrovascular and metabolic responses in specific neural systems elicited from the centromedian-parafascicular complex

The effect of electrical stimulation of the centromedian-parafascicular complex on local cerebral blood flow and local cerebral glucose utilization was investigated in anesthetized, paralysed and ventilated rats. Local cerebral blood flow and local cerebral glucose utilization were measured in separate groups of animals using the autoradiographic (14C)iodoantipyrine and (14C)2-deoxyglucose methods, respectively. Because of the well-established centromedian-parafascicular complex neuroanatomical connections, three functional neuronal systems were analysed and compared: the extrapyramidal motor system the limbic system and the reticular formation, also known as the ascending activating system. Cortical regions not included in the limbic system were considered separately. The validity of comparisons between changes in local cerebral blood flow and local cerebral glucose utilization across the brain was verified by assessing the reactivity and stability of the cortical blood flow during long-term centromedian-parafascicular complex stimulation. Centromedian-parafascicular complex stimulation elicited a marked but heterogeneous increase in local cerebral blood flow in 50 of the 52 cerebral structures measured. The most pronounced increases were seen in the lateral habenular nucleus (331 +/- 30% of control), the zona incerta (400 +/- 55%), the mesencephalic reticular formation (415 +/- 122%) and the parietal cortex (211 +/- 35%). In contrast, local cerebral glucose utilization remained statistically unchanged (P greater than 0.05) in 28 of these 50 individual brain regions during centromedian-parafascicular complex stimulation. The most pronounced increases in local cerebral glucose utilization were seen in the zona incerta (123 +/- 28%) and the mesencephalic reticular formation (193 +/- 26%). Local cerebral blood flow and local cerebral glucose utilization were linearly related in unstimulated controls, considering either all brain regions taken as a whole or the three systems separately. The significant increase in the slopes of the regression line between local cerebral blood flow and local cerebral glucose utilization for the reticular formation and the limbic system during centromedian-parafascicular complex stimulation indicates, however, that the coupling mechanisms for these systems, but not for the extrapyramidal motor system, were reset. The local cerebral blood flow to local cerebral glucose utilization ratio was heterogeneous in controls and differentially increased during centromedian-parafascicular complex stimulation, being markedly pronounced in the parietal cortex and in the reticular formation. We conclude that these results, for the first time, provide evidence that, the functionally well-defined neural networks may have different mechanisms whereby changes in vascular and metabolic demands are regulated.

Effect of alpha-chloralose, halothane, pentobarbital and nitrous oxide anesthesia on metabolic coupling in somatosensory cortex of rat

The effect of various anesthetics on the functional-metabolic coupling of cerebral cortex was studied in rats submitted to unilateral somatosensory stimulation. The regional cerebral metabolic rate of glucose (CMRglc) was measured autoradiographically using the 2-deoxyglucose method, and somatosensory activation was carried out by electrical stimulation of the left forepaw. In animals treated with 70% nitrous oxide, 0.5% halothane/70% nitrous oxide or 40 mg/kg pentobarbital, CMRglc of somatosensory cortex did not change despite generation of primary evoked cortical potentials. Anesthesia with 80 mg/kg alpha-chloralose, in contrast, led to a focal increase of CMRglc in the primary somatosensory cortex from 52.1 +/- 18.3 to 73.1 +/- 18.9 mumol/100 g/min (means +/- s.d.). Metabolic activation was strictly confined to the forelimb (FL) area of somatosensory cortex, and it exhibited a laminar pattern with maximal activation in layers I, II and IV. The preservation of functional-metabolic coupling under a surgical dose of chloralose renders this anesthetic particularly suited for the investigation of coupling processes under conditions where the experimental requirements preclude the use of unanaesthetized animals.

Dissociation by chloralose of the cardiovascular and cerebrovascular responses evoked from the cerebellar fastigial nucleus

We studied the effects of chloralose anesthesia on the elevation in arterial pressure (AP), heart rate (HR), and regional CBF (rCBF) elicited by stimulation of the cerebellar fastigial nucleus (FN). Rats were anesthetized with an initial dose of chloralose (40 mg/kg s.c.), paralyzed, and artificially ventilated. The FN was stimulated (50-100 microA, 50 Hz, 1 s on/1 s off) with microelectrodes stereotaxically implanted. During the stimulation AP was carefully maintained within cerebrovascular autoregulation. CBF was measured by the [14C]iodoantipyrine technique with regional dissection. In rats that received only the initial dose of chloralose, FN stimulation elevated rCBF in brain and spinal cord, up to 209 +/- 13% of control in frontal cortex (n = 5; p less than 0.01, analysis of variance). Administration of additional chloralose (10 mg/kg i.v., 30 min prior to measurement of CBF) did not affect resting rCBF (n = 5), the EEG, or the elevation in AP and HR elicited by FN stimulation (n = 4). However, the additional chloralose abolished the elevations in rCBF (n = 5; p greater than 0.05). Thus, the cerebrovasodilation elicited from the FN is more susceptible to the effects of additional anesthesia than the elevation in AP and HR. These results indicate that the cerebrovascular and cardiovascular responses elicited from the FN are functionally distinct and provide additional evidence for the notion that these responses are mediated by different neural pathways and transmitters.

Effects of MK-801 upon local cerebral glucose utilisation in conscious rats and in rats anaesthetised with halothane

The effects of MK-801 (0.5 mg/kg i.v.), a non-competitive N-methyl-D-aspartate (NMDA) antagonist, upon local cerebral glucose utilisation were examined in conscious, lightly restrained rats and in rats anaesthetised with halothane in nitrous oxide by means of the quantitative autoradiographic [14C]-2-deoxyglucose technique. In the conscious rats, MK-801 produced a heterogenous pattern of altered cerebral glucose utilisation with significant increases being observed in 12 of the 28 regions of gray matter examined and significant decreases in 6 of the 28 regions. Pronounced increases in glucose use were observed after MK-801 in the olfactory areas and in a number of brain areas in the limbic system (e.g., hippocampus molecular layer, dentate gyrus, subicular complex, posterior cingulate cortex, and mammillary body). In the cerebral cortices, large reductions in glucose use were observed after administration of MK-801, whereas in the extrapyramidal and sensory-motor areas, glucose use remained unchanged after MK-801 administration in conscious rats. In the halothane-anaesthetised rats, the pattern of altered glucose use after MK-801 differed qualitatively and quantitatively from that observed in conscious rats. In anaesthetised rats, significant reductions in glucose use were noted after MK-801 in 10 of the 28 regions examined, with no area displaying significantly increased glucose use after administration of the drug. In halothane-anaesthetised rats, MK-801 failed to change the rates of glucose use in the olfactory areas, the hippocampus molecular layer, and the dentate gyrus.(ABSTRACT TRUNCATED AT 250 WORDS)

Global reduction in cerebral blood flow and metabolism elicited from intrinsic neurons of fastigial nucleus

We sought to determine whether the global increase in regional cerebral blood flow (rCBF) produced by electrical stimulation of the rostral cerebellar fastigial nucleus (FN) is a consequence of excitation of intrinsic neurons of the FN or of axons of fibers passing through or projecting into it. Studies were conducted in rats anesthetized with chloralose, paralyzed and ventilated. rCBF was measured with [14C]iodoantipyrine as tracer and regional cerebral glucose utilization (rCGU) by [14C]2-deoxyglucose in homogenates of 11 brain regions. Neuronal perikarya in FN were excited chemically by local microinjection of the glutamate analogue kainic acid (KA) (5 nmol in 100 nl). KA elicited a transient and significant fall of arterial pressure and heart rate, the fastigial depressor response (FDR). Associated was a significant and symmetrical reduction in rCBF, to 44% of control in all regions except medulla. The response was site- and agent-specific and unrelated to the hypotension. KA also significantly and proportionally reduced, to 52% of control, rCGU in the same 10 areas of brain. In all regions the magnitudes of the reductions in rCBF and rCGU elicited by KA were linearly related. Intrinsic neurons of FN were chronically destroyed by local microinjection of the excitotoxin ibotenic acid (IBO) (10 micrograms/microliters in 0.4 microliter). Destruction of intrinsic FN neurons had no effect on resting rCBF nor on the global cerebrovascular vasodilation elicited by electrical stimulation of the FN. We conclude that: (a) excitation of intrinsic neurons of FN elicits a widespread reduction of cerebral metabolism and, secondarily, blood flow; (b) FN neurons do not exert a long-term tonic influence on brain blood flow nor metabolism; (c) the global increase in rCBF elicited by electrical stimulation of the FN is a consequence of excitation of axons projecting into or through the nucleus.

Computer-assisted three-dimensional reconstructions of [14C]-2-deoxy-D-glucose metabolism in cat lumbosacral spinal cord following cutaneous stimulation of the hindfoot

We report on computer-assisted three-dimensional reconstruction of spinal cord activity associated with stimulation of the plantar cushion (PC) as revealed by [14C]-2-deoxy-D-glucose (2-DG) serial autoradiographs. Moderate PC stimulation in cats elicits a reflex phasic plantar flexion of the toes. Four cats were chronically spinalized at about T6 under barbiturate anesthesia. Four to 11 days later, the cats were injected (i.v.) with 2-DG (100 microCi/kg) and the PC was electrically stimulated with needle electrodes at 2-5 times threshold for eliciting a reflex. Following stimulation, the spinal cord was processed for autoradiography. Subsequently, autoradiographs, representing approximately 8-18 mm from spinal segments L6-S1, were digitized for computer analysis and 3-D reconstruction. Several strategies of analysis were employed: 1) Three-dimensional volume images were color-coded to represent different levels of functional activity. 2) On the reconstructed volumes, "virtual" sections were made in the horizontal, sagittal, and transverse planes to view regions of 2-DG activity. 3) In addition, we were able to sample different regions within the grey and white matter semi-quantitatively (i.e., pixel intensity) from section to section to reveal differences between ipsi- and contralateral activity, as well as possible variation between sections. These analyses revealed 2-DG activity associated with moderate PC stimulation, not only in the ipsilateral dorsal horn as we had previously demonstrated, but also in both the ipsilateral and contralateral ventral horns, as well as in the intermediate grey matter. The use of novel computer analysis techniques--combined with an unanesthetized preparation--enabled us to demonstrate that the increased metabolic activity in the lumbosacral spinal cord associated with PC stimulation was much more extensive than had heretofore been observed.

Functional activation of cerebral blood flow and metabolism before and after global ischemia of rat brain

The effect of somatosensory stimulation on the local CBF (LCBF), CMRglu (LCMRglu), tissue pH, and tissue content of ATP, glucose, and lactate was studied in chloralose-anesthetized rats before and after 30 min of near-complete forebrain ischemia. In nonischemic rats LCBF in primary somatosensory cortex increased by 33%, LCMRglu increased by 55%, tissue glucose content decreased by 21%, and lactate increased by 30%. Local ATP and tissue pH did not change. Functional activation of the intact chloralose-anesthetized rat, in consequence, is associated with the stimulation of "aerobic" glycolysis but does not result in disturbances of energy or acid-base homeostasis. After 30-min ischemia and 3-h recirculation, somatosensory stimulation did not evoke any metabolic or hemodynamic alterations, although EEG and primary somatosensory evoked potentials recovered. The maintenance of normal energy state despite constant metabolic rate suggests that the postischemic generation of evoked potentials does not require measurable amounts of energy. Stimulation of glycolysis in the intact animal, therefore, may serve other purposes than fueling the energy requirements of evoked cortical activity.

Ketamine effects on local cerebral blood flow and metabolism in the rat

The effects of an anesthetic dose (100 mg/kg) of ketamine, a phencyclidine derivative, on local rates of cerebral glucose utilization (LCGU) and CBF (LCBF) have been investigated by the quantitative [14C]2-deoxy-glucose and [14C]iodoantipyrine techniques in the unparalyzed, spontaneously breathing rat. In ketamine-injected animals, LCGU was significantly increased in some limbic structures and decreased in inferior colliculus, vestibular, and cerebellar nuclei. The degree and spatial distribution of drug-induced changes was similar for local blood flow rates, LCBF being increased in limbic regions and decreased in the inferior colliculus. Although Paco2 values were higher in anesthetized animals, the pattern of LCBF/LCGU ratios was not significantly affected by ketamine in the 36 brain regions examined in this study. So, at least in the rat and at the anesthetic level studied here, a net vasodilatory in vivo effect was not observed. These results support the hypothesis that CBF changes induced by the drug in animals and man are primarily related to the metabolic effects exerted by ketamine on cerebral structures.

Differential metabolic activity in the brain during deep halothane anesthesia. A qualitative study using [3H]deoxyglucose

Alteration of metabolic activity during deep halothane anesthesia (3%) was analyzed qualitatively in the rat using a modified 2-deoxyglucose technique using the tritiated compound. The grey matter exhibited a homogeneous low level of metabolic activity in most places but some nuclei or subnuclear structures showed a much higher activity. These included in particular the locus coeruleus, the dorsal raphé, the substantia nigra pars compacta, the CA3 region of the hippocampus and the nucleus reticularis thalami. In the nucleus reticularis thalami, the use of 2-deoxy-[3H]glucose allowed us to demonstrate a high cellular metabolic activity, which is in agreement with several electrophysiological studies which showed a high rate of spontaneous activity under the same anesthetics.

Widespread reductions in cerebral blood flow and metabolism elicited by electrical stimulation of the parabrachial nucleus in rat

We have studied the effect of electrical stimulation of the parabrachial nucleus (PBN) and adjacent areas of dorsal pons on regional cerebral blood flow (rCBF) and glucose utilization (rCGU) in anesthetized (chloralose), paralyzed (tubocurarine) rats. rCBF and rCGU were measured in dissected tissue samples of 9 brain regions by the [14C]iodoantipyrine and [14C]2-deoxyglucose method, respectively. Electrical stimulation restricted to the medial parabrachial nucleus (PBNm, n = 5) elicited significant (P less than 0.05) reductions in rCBF in 7 out of 9 brain regions. Reductions were greatest in cerebral cortex (up to 35% in occipital cortex) and least in the white matter of the corpus callosum (23%). The effect on rCBF persisted after transection of the cervical sympathetic trunk (n = 5). In contrast, stimulation of the lateral portion of PBN (n = 5), periventricular gray (n = 5) and interestingly, the nucleus locus coeruleus (n = 5) failed to elicit similar changes in rCBF. PBNm stimulation also elicited decreases in rCGU (n = 4) in 5 out of 9 brain areas, most notably regions of cerebral cortex. The decreases in rCGU (delta rCGU) were linearly related to the decreases in rCBF (delta rCBF) according to the equation delta rCBF = 2.37 delta rCGU + 2.1 (r = 0.72; P less than 0.001). We conclude that excitation of neural pathways originating in, or passing through, PBNm elicits a widespread reduction in cerebral metabolism and secondarily in blood flow (secondary vasoconstriction). Since projections of the PBNm do not involve the entire cortex, it seems likely that the effect is mediated via inhibition of diffuse cortical projections through a subcortical site.

Effect of locus coeruleus stimulation on regional cerebral oxygen consumption in the cat

Regional cerebral oxygen consumption was determined during stimulation of the intra-axial noradrenergic pathway to quantitate the metabolic effects of this manipulation on cerebral oxygen extraction, cerebral blood flow (CBF) and its regional distribution. Regional arterial and venous oxygen saturation were examined microspectrophotometrically. Regional CBF was examined using radioactively tagged microspheres (15 +/- 3 microns in diameter). Oxygen consumption was calculated as the regional product of CBF and oxygen extraction. Bipolar concentric electrodes were stereotaxically implanted bilaterally in the locus coeruleus of alpha-chloralose anesthetized, artificially respired adult mongrel cats. The control group was killed after hemodynamic and CBF measurements were taken. The experimental group was sacrificed after these same measurements were taken before and during 10 min of bilateral locus coeruleus stimulation. The cats' heads were simultaneously sawed in 3 places and quickly frozen in liquid nitrogen-cooled propane. Systolic blood pressure was significantly increased during treatment. The heterogeneity of venous oxygen saturation was significantly reduced by stimulation. Average CBF and oxygen consumption were significantly decreased to 57% and 59% of control, respectively. Oxygen consumption was significantly reduced in the hypothalamus from 1.5 +/- 0.3 to 0.9 +/- 0.3 ml O2/min/100 g and from 3.5 +/- 0.9 to 1.2 +/- 0.4 ml O2/min/100 g in the cerebellum by treatment. Changes in the neuronal and/or synthetic cerebral activity produced regional decreases in cerebral oxygen consumption and secondarily altered CBF. These changes are probably due to interaction of the intraparenchymal noradrenergic pathways with other systems or processes in the brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular pH, potassium, and calcium activities in progressive ischaemia of rat cortex

We measured the relationships between changes in extracellular pH (pHe), potassium (Ke), and calcium (Cae) activities and DC potential (DCe) in progressive ischaemia of rat cerebral cortex. pHe and Ke, or Cae and Ke, were measured at the same point simultaneously, using triple-barrelled, double-ion-sensitive microelectrodes. Ischaemia was produced using bilateral carotid artery occlusion and hypotension in rats under 50% N2O-0.4% halothane anaesthesia. Unilateral carotid artery occlusion did not affect blood flow, but bilateral occlusion reduced flow to approximately 40% of normal. Autoregulation of blood pressure (BP) changes was lost after bilateral occlusion, and so progressive hypotension produced a linear decrease in flow. pHe began to decrease at high levels of flow (30-35 ml 100 g-1 min-1) and showed stepwise acidotic shifts with reductions in BP. Ke was affected at flows of approximately 15 ml 100 g-1 min-1, during which time it was critically dependent on BP. When Ke reached 6 mM, it increased rapidly to 40 mM and was associated with a negative shift in DCe. When Ke reached approximately 10 mM, Cae decreased rapidly to approximately 0.1 mM. pHe had reached 6.87 when Ke increased rapidly and showed a transient alkalotic shift of approximately 0.14 units at that time. Possible mechanisms for the sequence of ion changes described are discussed.

Global increase in cerebral metabolism and blood flow produced by focal electrical stimulation of dorsal medullary reticular formation in rat

We sought to determine whether the increases in local cerebral blood flow (LCBF) elicited by focal electrical stimulation within the dorsal medullary reticular formation (DMRF), are secondary to or independent of, increased local cerebral glucose utilization (LCGU). Rats were anesthetized (chloralose), paralyzed, artificially ventilated and arterial pressure and blood gases controlled. LCBF and LCGU were determined in two separate groups of animals, using the autoradiographic [14C]iodoantipyrine and [14C]2-deoxyglucose methods, respectively. In unstimulated controls, LCBF (n = 5) and LCGU (n = 5) were linearly related (r = 0.780; P less than 0.001) in the 27 brain regions studied. During DMRF stimulation LCGU increased significantly in 21 of the 27 regions, including cerebral cortex (up to 168% of control), thalamic nuclei (up to 161%) and selected ponto-medullary regions (e.g. parabrachial complex: 212%; vestibular complex: 147%). Along with LCGU, LCBF rose significantly in 25 regions (sensory motor cortex: 163%; anterior thalamus: 161%; parabrachial complex: 186%). Correlation analysis demonstrated that, during DMRF stimulation, the close relationship between LCBF and LCGU is preserved (r = 0.845; P less than 0.001) and that, in addition, the increase in LCBF (delta LCBF) is proportional to the increase in LCGU (delta LCGU) (delta LCBF = 2.18 delta LCGU + 6.92; r = 0.7729; P less than 0.001). Excitation of neurons or fibers within DMRF increases brain metabolism globally and blood flow secondarily. The DMRF appears to modulate cerebral metabolism globally, by as yet undefined pathways.

The relationship of local cerebral glucose utilization to optical density ratios

The validity of optical density ratios used in [14C]2-deoxyglucose neuroanatomical mapping experiments is evaluated by comparing local cerebral glucose utilization (LCGU) and optical density (OD) ratios in the same animals. OD ratios are calculated by dividing the optical density of different gray matter structures by the optical density of a single white matter structure in each animal. OD ratios are linearly related to LCGU within a given animal including stimulated, highly activated regions. Anesthesia profoundly affects the relationship between LCGU and OD ratios, however, showing that OD ratios do not provide an accurate index of LCGU between animals in different physiological states. Anesthesia had only a slight effect on OD ratios, however, indicating that OD ratios may be helpful in assessing whether structures are functionally activated between animals in different physiological states.

The effects of apomorphine upon local cerebral glucose utilization in conscious rats and in rats anesthetized with chloral hydrate

The effects of the dopaminergic agonist apomorphine (1 mg . kg-1 i.v.) upon local cerebral glucose utilization in 43 anatomically discrete regions of the CNS were examined in conscious, lightly restrained rats and in rats anesthetized with chloral hydrate by means of the quantitative autoradiographic [14C]2-deoxyglucose technique. In animals anesthetized with chloral hydrate, glucose utilization was reduced throughout all regions of the CNS from the levels observed in conscious animals, although the magnitude of the reductions in glucose use displayed considerable regional heterogeneity. With chloral hydrate anesthesia, the proportionately most marked reductions in glucose use (by 40-60% from conscious levels) were noted in primary auditory nuclei, thalmaic relay nuclei, and neocortex, and the least pronounced reductions in glucose use (by 15-25% from conscious levels) were observed in limbic areas, some motor relay nuclei, and white matter. In conscious, lightly restrained rats, the administration of apomorphine (1 mg . kg-1) effected significant increased in glucose utilization in 15 regions of the CNS (e.g., subthalamic nucleus, ventral thalamic nucleus, rostral neocortex, substantia nigra, pars reticulata), and significant reductions in glucose utilization in two regions of the CNS (lateral habenular nucleus and anterior cingulate cortex). In rats anesthetized with chloral hydrate, the effects of apomorphine upon local glucose utilization were less widespread and less marked than in conscious animals. In only two of the regions (the globus pallidus and septal nucleus), which displayed increased glucose use following apomorphine in conscious rats, were significant increases in local glucose utilization observed with this agent in chloral hydrate-anesthetized rats. In the pars compacta of the substantia nigra, in which apomorphine increased glucose utilization in conscious animals, significant reductions in glucose utilization were observed following apomorphine in rats anesthetized with chloral hydrate. The profound effects of chloral hydrate anesthesia upon local cerebral glucose use, and the modification by this anesthetic regime of the local metabolic responses to apomorphine, emphasize the difficulties which exists in the extrapolation of data from anesthetized animals to the conditions which prevail in the conscious animal.