Effects of diazepam and ketamine administered individually or in combination on regional rates of glucose utilization in rat brain

Several reasons why ketamine as a neuroplastic agent may have failed to prevent postoperative delirium: Implications for future protocols

Ketamine exerts anti-inflammatory, neuroprotective and neuroplastic activity, therefore it may counteract the neurotoxic processes underlying postoperative delirium. However, the majority of studies in this field failed. We identified several pharmacological reasons why these studies may have failed, together with suggestions of how to remediate them. Among them, the interaction with intravenous general anesthetics exerting the opposite effect on GABA interneurons than ketamine may be of principal importance. We suggest biomarkers which may elucidate the influence of this interaction on the different steps of neuroplastic pathways. We hypothesize that administering ketamine before or after general anesthesia could both prevent the interactions and strengthen the effect of ketamine by timing surgery within the climax of ketamine-induced neuroplastic changes or by stabilizing AMPA receptors. It is vital to deal with these questions because the protocols of ongoing studies are based again on the administration of ketamine during general anesthesia (the major identified pitfall).

Pharmacogenetic and drug interaction aspects on ketamine safety in its use as antidepressant - implications for precision dosing in a global perspective

Ketamine and its enantiomer S-ketamine (esketamine) are known to produce rapid-onset antidepressant effects in major depression. Intranasal esketamine has recently come into the market as an antidepressant. Besides experience from short-term use in anesthesia and analgesia, the experience with ketamine as long-term medication is rather low. The use of ketamine and esketamine is limited due to potential neurotoxicity, psychocomimetic side effects, potential abuse and interindividual variability in treatment response including cessation of therapy. Therefore, taking a look at individual patient risks and potential underlying variability in pharmacokinetics may improve safety and dosing of these new antidepressant drugs in clinical practice. Differential drug metabolism due to polymorphic cytochrome P450 (CYP) enzymes and gene-drug interactions are known to influence the efficacy and safety of many drugs. Ketamine and esketamine are metabolized by polymorphic CYP enzymes including CYP2B6, CYP3A4, CYP2C9 and CYP2A6. In antidepressant drug therapy, usually multiple drugs are administered which are substrates of CYP enzymes, increasing the risk for drug-drug interactions (DDIs). We reviewed the potential impact of polymorphic CYP variants and common DDIs in antidepressant drug therapy affecting ketamine pharmacokinetics, and the role for dose optimization. The use of ketamine or intranasal esketamine as antidepressants demands a better understanding of the factors that may impact its metabolism and efficacy in long-term use. In addition to other clinical and environmental confounders, prior information on the pharmacodynamic and pharmacokinetic determinants of response variability to ketamine and esketamine may inform on dose optimization and identification of individuals at risk of adverse drug reactions.

Pharmacodynamic Interactions Between Ketamine and Psychiatric Medications Used in the Treatment of Depression: A Systematic Review

BACKGROUND

The use of ketamine for depression has increased rapidly in the past decades. Ketamine is often prescribed as an add-on to other drugs used in psychiatric patients, but clear information on drug-drug interactions is lacking. With this review, we aim to provide an overview of the pharmacodynamic interactions between ketamine and mood stabilizers, benzodiazepines, monoamine oxidase-inhibitors, antipsychotics, and psychostimulants.

METHODS

MEDLINE and Web of Science were searched.

RESULTS

Twenty-four studies were included. For lithium, no significant interactions with ketamine were reported. Two out of 5 studies on lamotrigine indicated that the effects of ketamine were attenuated. Benzodiazepines were repeatedly shown to reduce the duration of ketamine's antidepressant effect. For the monoamine oxidase-inhibitor tranylcypromine, case reports showed no relevant changes in vital signs during concurrent S-ketamine use. One paper indicated an interaction between ketamine and haloperidol, 2 other studies did not. Four papers investigated risperidone, including 3 neuroimaging studies showing an attenuating effect of risperidone on ketamine-induced brain perfusion changes. Clozapine significantly blunted ketamine-induced positive symptoms in patients with schizophrenia but not in healthy participants. One paper reported no effect of olanzapine on ketamine's acute psychotomimetic effects.

CONCLUSION

Current literature shows that benzodiazepines and probably lamotrigine reduce ketamine's treatment outcome, which should be taken into account when considering ketamine treatment. There is evidence for an interaction between ketamine and clozapine, haloperidol, and risperidone. Due to small sample sizes, different subject groups and various outcome parameters, the evidence is of low quality. More studies are needed to provide insight into pharmacodynamic interactions with ketamine.

Digital autoradiography for efficient functional imaging without anesthesia in experimental animals: Reversing phencyclidine-induced functional alterations using clozapine

Autoradiography (ARG) is a high-resolution imaging method for localization of radiolabeled biomarkers in ex vivo specimen. ARG using 2-deoxy-d-glucose (2-DG) method is used in to study drug actions on brain functional activity, as it provides results comparable to clinically used functional positron-emission tomography (PET). The requirement of slow analog detection methods and emerging advances in small animal PET imaging have, however, reduced the interest in ARG. In contrast to ARG, experimental animals need to be restrained or sedated/anesthetized for PET imaging, which strongly influence functional activity and thus complicate the interpretation of the results. Digital direct particle-counting ARG systems have gained attraction during the last decade to overcome the caveats of conventional ARG methods. Here we demonstrate that the well-established 2-DG imaging method can be adapted into use with contemporary digital detectors. This method readily and rapidly captures the characteristic effects of phencyclidine (5 mg/kg, i.p.), a dissociative agent targeting the NMDAR (N-methyl-d-aspartate receptor), on regional glucose utilization in the adult mouse brain. Pretreatment with antipsychotic drug clozapine (6 mg/kg, i.p.) essentially abolishes these effects of phencyclidine on brain functional activity. Digital ARG produces viable data for the regional analysis of functional activity in a fraction of time required for film development. These results support the use of digital ARG in preclinical drug research, where high throughput and response linearity are preferred and use of sedation/anesthesia has to be avoided.

Anesthesia and Preconditioning Induced Changes in Mouse Brain [18F] FDG Uptake and Kinetics

PURPOSE

2-Deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) has been widely used for imaging brain metabolism. Tracer injection in anesthetized animals is a prerequisite for performing dynamic positron emission tomography (PET) scanning. Since preconditioning, as well as anesthesia, has been described to potentially influence brain [¹⁸F] FDG levels, this study evaluated how these variables globally and regionally affect both [¹⁸F] FDG uptake and kinetics in murine brain.

PROCEDURES

Sixty-minute dynamic [¹⁸F] FDG PET scans were performed in adult male C57BL/6 mice anesthetized with isoflurane [control (in 100 % O₂), in medical air, in 100 % O₂ + insulin pre-treatment, and in 100 % O₂ after 18 h fasting], ketamine/xylazine, sevoflurane, and chloral hydrate. An additional group was scanned after awake uptake. Blood glucose levels were determined, and data was analyzed by comparing percent injected dose per cc tissue (%ID/cc) and glucose influx rate and metabolic rate (MRGlu) calculated by Patlak plot.

RESULTS

Ketamine/xylazine and chloral hydrate anesthesia induced a lower whole-brain uptake of [¹⁸F] FDG (2.86 ± 0.67 %ID/cc, p < 0.001; 4.25 ± 0.28 %ID/cc, p = 0.0179, respectively) compared to isoflurane anesthesia (5.04 ± 0.19 %ID/cc). In addition, protocols affected differently distribution of [¹⁸F] FDG uptake in brain regions. Ketamine/xylazine reduced [¹⁸F] FDG influx rate in murine brain (0.0135 ± 0.0009 vs 0.0247 ± 0.0014 ml/g/min; p < 0.005) and chloral hydrate increased MRGlu (66.72 ± 3.75 vs 41.55 ± 3.06 μmol/min/100 ml; p < 0.01) compared to isoflurane. Insulin-pretreated animals showed a higher influx rate (0.0477 ± 0.0101 ml/min/g; p < 0.05) but a reduced MRGlu (21.92 ± 3.12 μmol/min/100 ml; p < 0.01). Blood glucose levels were negatively correlated to [¹⁸F] FDG uptake and influx rate, but positively correlated to MRGlu.

CONCLUSIONS

Choice of anesthesia and pre-conditioning affect not only [¹⁸F] FDG uptake but also kinetics and regional distribution in the mouse brain. Both anesthesia and pre-conditioning should be carefully considered in the interpretation of [¹⁸F] FDG studies due to its great influence on the uptake and distribution of the tracer along the brain regions.

The Antidepressant Effect of Ketamine Is Dampened by Concomitant Benzodiazepine Medication

The rapid antidepressant effect of ketamine has become a breakthrough in the research and treatment of depression. Although predictive and modulating factors of the response to ketamine are broadly studied, little is known about optimal concurrent medication protocols. Concerning gamma-aminobutyric acid neurotransmission being a shared target for both ketamine and benzodiazepines (BZD), we evaluated the influence of BZD on the antidepressant effect of a single ketamine infusion in depressed patients. Data from 47 patients (27 females) with major depression (MADRS ≥ 20, ≥ 1 prior nonresponse to antidepressant treatment in current episode) who participated in two previous studies (EudraCT Number: 2009-010625-39 and 2013-000952-17) entered the analysis. All of the subjects were given an infusion of a subanesthetic dose of racemic ketamine (0.54 mg per kg) as an add-on medication to ongoing antidepressant treatment. Thirteen patients (28%) reached ≥ 50% reduction in MADRS within one week after ketamine administration. Nineteen (40%) patients took concomitant benzodiazepines on a daily basis. The doses of BZDs were significantly higher in nonresponders (p=0.007). ROC analysis distinguished responders from nonresponders by a criterion of >8mg of diazepam equivalent dose (DZ equivalent) with a sensitivity of 80% and a specificity of 85% (p<0.001). RM-ANOVA revealed a different time pattern of response to ketamine between the BZD+ (>8mg of DZ equivalent) and BZD- (≤8mg of DZ equivalent) groups, with a significantly worse outcome in BZD+ on day 3 (p=0.04) and day 7 (p=0.02). The results of the study indicate that concomitant benzodiazepine treatment in higher doses may attenuate ketamine's antidepressant effect. The pathophysiological, clinical and methodological implications of this finding should be considered in future research and ketamine treatment.

Standardization of Small Animal Imaging-Current Status and Future Prospects

The benefit of small animal imaging is directly linked to the validity and reliability of the collected data. If the data (regardless of the modality used) are not reproducible and/or reliable, then the outcome of the data is rather questionable. Therefore, standardization of the use of small animal imaging equipment, as well as of animal handling in general, is of paramount importance. In a recent paper, guidance for efficient small animal imaging quality control was offered and discussed, among others, the use of phantoms in setting up a quality control program (Osborne et al. 2016). The same phantoms can be used to standardize image quality parameters for multi-center studies or multi-scanners within center studies. In animal experiments, the additional complexity due to animal handling needs to be addressed to ensure standardized imaging procedures. In this review, we will address the current status of standardization in preclinical imaging, as well as potential benefits from increased levels of standardization.

Ketamine Reverses Lateral Habenula Neuronal Dysfunction and Behavioral Immobility in the Forced Swim Test Following Maternal Deprivation in Late Adolescent Rats

Mounting evidence suggests that the long-term effects of adverse early life stressors on vulnerability to drug addiction and mood disorders are related to dysfunction of brain monoaminergic signaling in reward circuits. Recently, there has been a growing interest in the lateral habenula (LHb) as LHb dysfunction is linked to the development of mental health disorders through monoaminergic dysregulation within brain reward/motivational circuits and may represent a critical target for novel anti-depressants, such as ketamine. Here, we show that maternal deprivation (MD), a severe early life stressor, increases LHb intrinsic excitability and LHb bursting activity, and is associated with the development of increased immobility in the forced swim test (FST) in late-adolescent male rats. A single in vivo injection of ketamine is sufficient to exert prolonged antidepressant effects through reversal of this early life stress-induced LHb neuronal dysfunction and the response in the FST. Our assessment of ketamine’s long-lasting beneficial effects on reversal of MD-associated changes in LHb neuronal function and behavior highlights the critical role of the LHb in pathophysiology of depression associated with severe early life stress and in response to novel fast-acting antidepressants.

Control of seizures by ketogenic diet-induced modulation of metabolic pathways

Epilepsy is too complex to be considered as a disease; it is more of a syndrome, characterized by seizures, which can be caused by a diverse array of afflictions. As such, drug interventions that target a single biological pathway will only help the specific individuals where that drug's mechanism of action is relevant to their disorder. Most likely, this will not alleviate all forms of epilepsy nor the potential biological pathways causing the seizures, such as glucose/amino acid transport, mitochondrial dysfunction, or neuronal myelination. Considering our current inability to test every individual effectively for the true causes of their epilepsy and the alarming number of misdiagnoses observed, we propose the use of the ketogenic diet (KD) as an effective and efficient preliminary/long-term treatment. The KD mimics fasting by altering substrate metabolism from carbohydrates to fatty acids and ketone bodies (KBs). Here, we underscore the need to understand the underlying cellular mechanisms governing the KD's modulation of various forms of epilepsy and how a diverse array of metabolites including soluble fibers, specific fatty acids, and functional amino acids (e.g., leucine, D-serine, glycine, arginine metabolites, and N-acetyl-cysteine) may potentially enhance the KD's ability to treat and reverse, not mask, these neurological disorders that lead to epilepsy.

Ketamine modulates TRH and TRH-like peptide turnover in brain and peripheral tissues of male rats

Major depression is the largest single healthcare burden with treatments of slow onset and often limited efficacy. Ketamine, a NMDA antagonist used extensively as a pediatric and veterinary anesthetic, has recently been shown to be a rapid acting antidepressant, making it a potential lifesaver for suicidal patients. Side effects and risk of abuse limit the chronic use of ketamine. More complete understanding of the neurobiochemical mechanisms of ketamine should lead to safer alternatives. Some of the physiological and pharmacological actions of ketamine are consistent with increased synthesis and release of TRH (pGlu-His-Pro-NH2), and TRH-like peptides (pGlu-X-Pro-NH2) where "X" can be any amino acid residue. Moreover, TRH-like peptides are themselves potential therapeutic agents for the treatment of major depression, anxiety, bipolar disorder, epilepsy, Alzheimer's and Parkinson's diseases. For these reasons, male Sprague-Dawley rats were anesthetized with 162 mg/kg ip ketamine and then infused intranasally with 20 μl of sterile saline containing either 0 or 5 mg/ml Glu-TRH. One, 2 or 4h later, the brain levels of TRH and TRH-like peptides were measured in various brain regions and peripheral tissues. At 1h in brain following ketamine only, the levels of TRH and TRH-like peptides were significantly increased in 52 instances (due to increased biosynthesis and/or decreased release) or decreased in five instances. These changes, listed by brain region in order of decreasing number of significant increases (↑) and/or decreases (↓), were: hypothalamus (9↑); piriform cortex (8↑); entorhinal cortex (7↑); nucleus accumbens (7↑); posterior cingulate (5↑); striatum (4↑); frontal cortex (2↑,3↓); amygdala (3↑); medulla oblongata (1↑,2↓); cerebellum (2↑); hippocampus (2↑); anterior cingulate (2↑). The corresponding changes in peripheral tissues were: adrenals (8↑); epididymis (4↑); testis (1↑,3↓); pancreas (1↑); prostate (1↑). We conclude that TRH and TRH-like peptides may be downstream mediators of the rapid antidepressant actions of ketamine.

Transient postnatal fluoxetine leads to decreased brain arachidonic acid metabolism and cytochrome P450 4A in adult mice

Fetal and perinatal exposure to selective serotonin (5-HT) reuptake inhibitors (SSRIs) has been reported to alter childhood behavior, while transient early exposure in rodents is reported to alter their behavior and decrease brain extracellular 5-HT in adulthood. Since 5-HT2A/2C receptor-mediated neurotransmission can involve G-protein coupled activation of cytosolic phospholipase A2 (cPLA2), releasing arachidonic acid (ARA) from synaptic membrane phospholipid, we hypothesized that transient postnatal exposure to fluoxetine would alter brain ARA metabolism in adult mice. Brain ARA incorporation coefficients k* and rates Jin were quantitatively imaged following intravenous [1-(14)C]ARA infusion of unanesthetized adult mice that had been injected daily with fluoxetine (10mg/kg i.p.) or saline during postnatal days P4-P21. Expression of brain ARA metabolic enzymes and other relevant markers also was measured. On neuroimaging, k* and Jin was decreased widely in early fluoxetine- compared to saline-treated adult mice. Of the enzymes measured, cPLA2 activity was unchanged, while Ca(2+)-independent iPLA2 activity was increased. There was a significant 74% reduced protein level of cytochrome P450 (CYP) 4A, which can convert ARA to 20-HETE. Reduced brain ARA metabolism in adult mice transiently exposed to postnatal fluoxetine, and a 74% reduction in CYP4A protein, suggest long-term effects independent of drug presence in brain ARA metabolism, and in CYP4A metabolites. These changes might contribute to reported altered behavior following early SSRI in rodents.

Brain docosahexaenoic acid [DHA] incorporation and blood flow are increased in chronic alcoholics: a positron emission tomography study corrected for cerebral atrophy

OBJECTIVE

Chronic alcohol dependence has been associated with disturbed behavior, cerebral atrophy and a low plasma concentration of docosahexaenoic acid (DHA, 22∶6n-3), particularly if liver disease is present. In animal models, excessive alcohol consumption is reported to reduce brain DHA concentration, suggesting disturbed brain DHA metabolism. We hypothesized that brain DHA metabolism also is abnormal in chronic alcoholics.

METHODS

We compared 15 non-smoking chronic alcoholics, studied within 7 days of their last drink, with 22 non-smoking healthy controls. Using published neuroimaging methods with positron emission tomography (PET), we measured regional coefficients (K*) and rates (J(in)) of DHA incorporation from plasma into the brain of each group using [1-(11)C]DHA, and regional cerebral blood flow (rCBF) using [(15)O]water. Data were partial volume error corrected for brain atrophy. Plasma unesterified DHA concentration also was quantified.

RESULTS

Mean K* for DHA was significantly and widely elevated by 10-20%, and rCBF was elevated by 7%-34%, in alcoholics compared with controls. Unesterified plasma DHA did not differ significantly between groups nor did whole brain J(in), the product of K* and unesterified plasma DHA concentration.

DISCUSSION

Significantly higher values of K* for DHA in alcoholics indicate increased brain avidity for DHA, thus a brain DHA metabolic deficit vis-à-vis plasma DHA availability. Higher rCBF in alcoholics suggests increased energy consumption. These changes may reflect a hypermetabolic state related to early alcohol withdrawal, or a general brain metabolic change in chronic alcoholics.

Glutamatergic and GABAergic TCA cycle and neurotransmitter cycling fluxes in different regions of mouse brain

The (13)C nuclear magnetic resonance (NMR) studies together with the infusion of (13)C-labeled substrates in rats and humans have provided important insight into brain energy metabolism. In the present study, we have extended a three-compartment metabolic model in mouse to investigate glutamatergic and GABAergic tricarboxylic acid (TCA) cycle and neurotransmitter cycle fluxes across different regions of the brain. The (13)C turnover of amino acids from [1,6-(13)C2]glucose was monitored ex vivo using (1)H-[(13)C]-NMR spectroscopy. The astroglial glutamate pool size, one of the important parameters of the model, was estimated by a short infusion of [2-(13)C]acetate. The ratio Vcyc/VTCA was calculated from the steady-state acetate experiment. The (13)C turnover curves of [4-(13)C]/[3-(13)C]glutamate, [4-(13)C]glutamine, [2-(13)C]/[3-(13)C]GABA, and [3-(13)C]aspartate from [1,6-(13)C2]glucose were analyzed using a three-compartment metabolic model to estimate the rates of the TCA cycle and neurotransmitter cycle associated with glutamatergic and GABAergic neurons. The glutamatergic TCA cycle rate was found to be highest in the cerebral cortex (0.91 ± 0.05 μmol/g per minute) and least in the hippocampal region (0.64 ± 0.07 μmol/g per minute) of the mouse brain. In contrast, the GABAergic TCA cycle flux was found to be highest in the thalamus-hypothalamus (0.28 ± 0.01 μmol/g per minute) and least in the cerebral cortex (0.24 ± 0.02 μmol/g per minute). These findings indicate that the energetics of excitatory and inhibitory function is distinct across the mouse brain.

Fiber pathway pathology, synapse loss and decline of cortical function in schizophrenia

A quantitative cortical model is developed, based on both computational and simulation approaches, which relates measured changes in cortical activity of gray matter with changes in the integrity of longitudinal fiber pathways. The model consists of modules of up to 5,000 neurons each, 80% excitatory and 20% inhibitory, with these having different degrees of synaptic connectiveness both within a module as well as between modules. It is shown that if the inter-modular synaptic connections are reduced to zero while maintaining the intra-modular synaptic connections constant, then activity in the modules is reduced by about 50%. This agrees with experimental observations in which cortical electrical activity in a region of interest, measured using the rate of oxidative glucose metabolism (CMR_glc(ox)), is reduced by about 50% when the cortical region is isolated, either by surgical means or by transient cold block. There is also a 50% decrease in measured cortical activity following inactivation of the nucleus of Meynert and the intra-laminar nuclei of the thalamus, which arise either following appropriate lesions or in sleep. This occurs in the model if the inter-modular synaptic connections require input from these nuclei in order to function. In schizophrenia there is a 24% decrease in functional anisotropy of longitudinal fasciculi accompanied by a 7% decrease in cortical activity (CMR_glc(ox)).The cortical model predicts this, namely for a 24% decrease in the functioning of the inter-modular connections, either through the complete loss of 24% of axons subserving the connections or due to such a decrease in the efficacy of all the inter-modular connections, there will be about a 7% decrease in the activity of the modules. This work suggests that deterioration of longitudinal fasciculi in schizophrenia explains the loss of activity in the gray matter.

[18F]FDG-PET as an imaging biomarker to NMDA receptor antagonist-induced neurotoxicity

Positron emission tomography (PET) is an effective tool for noninvasive examination of the body and provides a range of functional information. PET imaging with [(18)F]fluoro-2-deoxy-d-glucose ([(18)F]FDG) has been used to image alterations in glucose metabolism in brain or cancer tissue in the field of clinical diagnosis but not in the field of toxicology. A single dose of N-methyl-d-aspartate (NMDA) receptor antagonist induces neuronal cell degeneration/death in the rat retrosplenial/posterior cingulate (RS/PC) cortex region. These antagonists also increase local cerebral glucose utilization. Here, we examined the potential of [(18)F]FDG-PET as an imaging biomarker of neurotoxicity induced by an NMDA receptor antagonist, MK-801. Using [(18)F]FDG-PET, we determined that increased glucose utilization involved the neurotoxicity induced by MK-801. The accumulation of [(18)F]FDG was increased in the rat RS/PC cortex region showing neuronal cell degeneration/death and detected before the onset of neuronal cell death. This effect increased at a dose level at which neuronal cell degeneration recovered 24h after MK-801 administration. Scopolamine prevented the neurotoxicity and [(18)F]FDG accumulation induced by MK-801. Furthermore, in cynomolgus monkeys that showed no neuronal cell degeneration/death when treated with MK-801, we noted no differences in [(18)F]FDG accumulation between test and control subjects in any region of the brain. These findings suggest that [(18)F]FDG-PET, which is available for clinical trials, may be useful in generating a predictive imaging biomarker for detecting neurotoxicity against NMDA receptor antagonists with the same pharmacological activity as MK-801.

Unmasking the mysteries of the habenula in pain and analgesia

The habenula is a small bilateral structure in the posterior-medial aspect of the dorsal thalamus that has been implicated in a remarkably wide range of behaviors including olfaction, ingestion, mating, endocrine and reward function, pain and analgesia. Afferent connections from forebrain structures send inputs to the lateral and medial habenula where efferents are mainly projected to brainstem regions that include well-known pain modulatory regions such as the periaqueductal gray and raphe nuclei. A convergence of preclinical data implicates the region in multiple behaviors that may be considered part of the pain experience including a putative role in pain modulation, affective, and motivational processes. The habenula seems to play a role as an evaluator, acting as a major point of convergence where external stimuli is received, evaluated, and redirected for motivation of appropriate behavioral response. Here, we review the role of the habenula in pain and analgesia, consider its potential role in chronic pain, and review more recent clinical and functional imaging data of the habenula from animals and humans. Even through the habenula is a small brain structure, advances in structural and functional imaging in humans should allow for further advancement of our understanding of its role in pain and analgesia.

Memantine-induced brain activation as a model for the rapid screening of potential novel antipsychotic compounds: exemplified by activity of an mGlu2/3 receptor agonist

RATIONALE

Schizophrenia is a severe, disabling chronic disorder affecting approximately 1% of the population. Improvements and development of more robust and hopefully predictive screening assays for this disease should enhance the identification and development of novel treatments. The present study describes a rapid and robust method for the testing of potential novel antipsychotics by utilising a simplified [(14)C]2-deoxyglucose (2-DG) autoradiography method following memantine-induced brain activation.

METHODS

Male C57BL/6JCRL mice were given vehicle, ketamine or memantine (10, 20 and 30 mg/kg, subcutaneously (s.c.)) and sacrificed 45 min post-[(14 C)]2-DG administration. In subsequent reversal studies, the memantine challenge was further validated with haloperidol (0.32 mg/kg, s.c.) and clozapine (2.5 and 10 mg/kg, s.c.) in parallel with the ketamine model (Duncan et al. 1998a). Lastly, the effects of an mGlu2/3 receptor agonist, LY404039 (10 mg/kg, s.c.), on both ketamine and memantine-induced brain activation was determined.

RESULTS

Both N-methyl-d-aspartate (NMDA) antagonists dose-dependently induced significant region-specific increases in 2-DG uptake. Interestingly, memantine elicited a considerably greater brain activation signature with a larger dynamic window than ketamine. The "atypical" antipsychotic clozapine significantly reversed memantine-induced 2-DG uptake whilst the "typical" antipsychotic haloperidol was inactive. Pre-treatment with LY404039 fully reversed both the ketamine- and memantine-induced increase in 2-DG uptake without effects on basal 2-DG uptake.

CONCLUSION

This novel pre-clinical imaging methodology displays potential for the screening of compounds targeting the NMDA receptor hypofunction hypothesis of schizophrenia and should assist in developing compounds from the bench to clinic.

MicroPET imaging of noxious thermal stimuli in the conscious rat brain

Small animal positron emission tomography (microPET) has been utilized in the investigation of nociception. However, a possible drawback from previous studies is the reduced activation pattern due to the application of anesthesia. The purpose of the present study was to demonstrate a potential means of avoiding anesthesia during stimulation, as well as minimizing the confounding anesthetic effect. Sodium pentobarbital and ketamine were first evaluated to determine their effect on microPET images in the current study. [(18)F]-Fluorodeoxyglucose ((18)F-FDG) was an appropriate radiotracer to reveal activated regions in rat brains. Pentobarbital anesthesia significantly reduced (18)F-FDG uptake in neural tissues, blurrier to lower contrast; therefore, ketamine was used to anesthetize animals during microPET. After the rats were anesthetized and secured in a laboratory-made stereotaxic frame, a simple, noninvasive stereotaxic technique was used to position their heads in the microPET scanner and to roughly conform the images in the stereotaxic atlas. For functional imaging, conscious rats were restrained in cages with minimal ambient noise; short repetitive thermal stimuli were applied to each rat's tail subsequently. The rats were adequately anesthetized with ketamine following 30 min of scanning without stimulation. An activation index (AI) was calculated from microPET data to quantify the local metabolic activity changes according to the normalized (18)F-FDG dosage. The average AI indicated a side-to-side difference for all innocuous stimulations in the thalamus. However, such side-to-side difference was only observed for noxious heat and cold stimulations in primary somatosensory cortex (SI), secondary somatosensory cortex (SII), and agranular insular cortex (AIC). The present study demonstrated the feasibility of the microPET technique to image metabolic functions of the conscious rat brain, offering better rationale and protocol designs for future pain studies.

Effects of anesthetic protocol on normal canine brain uptake of 18F-FDG assessed by PET/CT

The purpose of this study was to assess the effects of four anesthetic protocols on normal canine brain uptake of 2-deoxy-2-[18F]fluoro-D-glucose (FDG) using positron emission tomography/computed tomography (PET/CT). Five clinically normal beagle dogs were anesthetized with (1) propofol/isoflurane, (2) medetomidine/pentobarbital, (3) xylazine/ketamine, and (4) medetomidine/tiletamine-zolazepam in a randomized cross-over design. The standard uptake value (SUV) of FDG was obtained in the frontal, parietal, temporal and occipital lobes, cerebellum, brainstem and whole brain, and compared within and between anesthetic protocols using the Friedman test with significance set at P < 0.05. Significant differences in SUVs were observed in various part of the brain associated with each anesthetic protocol. The SUV for the frontal and occipital lobes was significantly higher than in the brainstem in all dogs. Dogs receiving medetomidine/tiletamine-zolazepam also had significantly higher whole brain SUVs than the propofol/isoflurane group. We concluded that each anesthetic protocol exerted a different regional brain glucose uptake pattern. As a result, when comparing brain glucose uptake using PET/CT, one should consider the effects of anesthetic protocols on different regions of the glucose uptake in the dog's brain.

Deafening decreases neuronal incorporation in the zebra finch caudomedial nidopallium (NCM)

New neurons formed in the adult brain are incorporated into existing circuits. However, the number of new neurons recruited into a given brain region varies widely depending on the experience of the animal. An emerging general principle is that recruitment and early neuronal survival may be correlated with activity or use of the brain region. Here we show that use-dependent neuronal survival also occurs in the higher order auditory processing region of the songbird caudomedial nidopallium (NCM). We suggest that retention of young neurons may in part be influenced by use of the system without an increased demand for learning or behavioral plasticity.

Stimulus-dependent changes of extracellular glucose in the rat hippocampus determined by in vivo microdialysis

Neuronal activity is tightly coupled with brain energy metabolism; and glucose is an important energy substrate for neurons. The present in vivo microdialysis study was aimed at investigating changes in extracellular glucose concentrations in the rat ventral hippocampus due to exposure to the elevated plus maze. Determination of basal hippocampal glucose and lactate/pyruvate ratio in male Wistar rats was conducted in the home cage using in vivo microdialysis. Rats were exposed to the elevated plus maze, a rodent model of anxiety-related behaviour, or to unspecific stress induced by white noise (95dB) as a control condition. Basal hippocampal levels of glucose, as determined by zero-net-flux, and the basal lactate/pyruvate ratio were 1.49+/-0.05mmol/l and 13.8+/-1.1, respectively. In rats without manipulation, glucose levels remained constant throughout the experiment (120min). By contrast, exposure to the elevated plus maze led to a temporary decline in hippocampal glucose (-33.2+/-4.4%) which returned to baseline level in the home cage. White noise caused only a non-significant decrease in extracellular glucose level (-9.3+/-3.5%). In all groups, the lactate/pyruvate ratio remained unchanged by the experimental procedures. Our microdialysis study demonstrates that exposure to the elevated plus maze induces a transient decrease in extracellular hippocampal glucose concentration. In contrast, an unspecific stimulus did not change hippocampal glucose. The latter suggests that only specific behavioural stimuli increase hippocampal glucose utilization in the ventral hippocampus.

Neuroprotection afforded by diazepam against oxygen/glucose deprivation-induced injury in rat cortical brain slices

The aim of the present investigation was to assess neuroprotection exerted by diazepam (0.1-25 microM) in rat cortical brain slices subjected to oxygen-glucose deprivation and reoxygenation. Neuronal injury and neuroprotection were assessed by measuring the release of glutamate and lactate dehydrogenase and tissue water content. Results demonstrate that diazepam exerted neuroprotective effects according to a "U-shaped", hormetic-like, concentration-response curve, with an efficacy window of 0.5-5 microM concentration. Flumazenil (20 microM) fully antagonised neuroprotection afforded by 5 microM diazepam. In conclusion, the hormetic response of diazepam should be taken into consideration when designing experiments aimed at assessing diazepam neuroprotection against ischemia/reoxygenation injury.

Cerebral metabolism of glucose and pyruvate in soman poisoning

Organophosphates, such as the nerve gas soman, cause inhibition of acetylcholine esterase, accumulation of acetylcholine in synaptic clefts, and excessive activation of cholinergic receptors, causing central nervous symptoms such as tremor and seizures. Soman-poisoned animals have low brain levels of ATP, indicating that energy demand is greater than energy supply. We investigated whether soman poisoning is accompanied by an increased brain metabolism of glucose, as can be inferred from the accumulation of radiolabeled 2-deoxyglucose found in previous studies, or whether soman poisoning entails impairment of cerebral energy metabolism. We performed 13C nuclear magnetic resonance spectroscopy on brain extracts from soman-poisoned mice (160 microg/kg; 1 LD50) that had been dosed with 13C-labeled glucose or pyruvate intravenously. Formation of 13C-labeled glutamate, GABA and glutamine from [1-(13)C]glucose was reduced by approximately 30% in awake, soman-intoxicated animals, but formation of these amino acids from [3-(13)C]pyruvate was not different in soman-intoxicated animals and controls. These results suggest that soman intoxication entails inhibition of glycolysis, but not of tricarboxylic acid cycle activity in the brain. However, when brain metabolism was depressed by a sedative dose of diazepam (5 mg/kg) soman intoxication caused increased metabolism of 13C-labeled glucose. The latter finding shows that the soman-poisoned brain has a high energy requirement even during anticonvulsant therapy. We conclude that metabolic inhibition, as seen in awake, soman-intoxicated animals, may lower seizure threshold and contribute to soman-related neurodegeneration and lethality.

Efflux of drugs and solutes from brain: the interactive roles of diffusional transcapillary transport, bulk flow and capillary transporters

We examined the roles of diffusion, convection and capillary transporters in solute removal from extracellular space (ECS) of the brain. Radiolabeled solutes (eight with passive distribution and four with capillary or cell transporters) were injected into the brains of rats (n=497) and multiple-time point experiments measured the amount remaining in brain as a function of time. For passively distributed compounds, there was a relationship between lipid:water solubility and total brain efflux:diffusional efflux, which dominated when k(p), the transcapillary efflux rate constant, was >10(0) h(-1); when 10(-1)<k(p)<10(-2) h(-1) both diffusion and convection contributed, and when k(p)<10(-3) h(-1), convective efflux dominated. Para-aminohippuric acid (PAH) experiments (n=112) showed that PAH entered the brain passively, but had efflux transporters. The total efflux rate constant, k(eff), was the sum of a passive component (k(p)=0.0018 h(-1)), a convective component (k(csf)=0.2 h(-1)), and a variable, concentration-dependent component (k(x)=0 to 0.45 h(-1)). Compounds with cell membrane transporters had longer clearance half times as did an oligonucleotide, which interacted with cell surface receptors. Manipulation of physiologic state (n=35) did not affect efflux, but sucrose efflux half time was longer with pentobarbital anesthesia (24 h) than with no anesthesia or ketamine-xylazine anesthesia (2 to 3 h). These results show that solute clearance from normal brain ECS may involve multiple physiologic pathways, may be affected by anesthesia, and suggests that convection-mediated efflux may be manipulated to increase or decrease drug clearance from brain.

Oxidative metabolism of limbic structures after acute administration of diazepam, alprazolam and zolpidem

The effects of acute administration of two benzodiazepines and a non-benzodiazepine hypnotic on behavior and brain metabolism were evaluated in rats. After testing the behavioral action of the benzodiazepines on the open field and the elevated plus-maze, the effects of the three drugs on neuronal metabolism of particular limbic regions were measured using cytochrome c oxidase (CO) histochemistry. Diazepam (5 mg/kg i.p.) and alprazolam (0.5 mg/kg i.p.) induced clear anxiolytic effects and a decrease in locomotion, whereas zolpidem (2 mg/kg i.p.) caused an intense hypnotic effect. The anxiolytic effects of alprazolam were distinguishable from diazepam due to the pharmacological and clinical profile of this triazolobenzodiazepine. CO activity decreased significantly in almost all the limbic regions evaluated after zolpidem administration. However, significant prominent decreases in CO activity were found after diazepam treatment in the medial mammillary nucleus, anteroventral thalamus, cingulate cortex, dentate gyrus and basolateral amygdala. Alprazolam caused similar decreases in CO activity, with the exception of the prelimbic and cingulate cortices, where significant increases were detected. In agreement with previous studies using other functional mapping techniques, our results indicate that particular benzodiazepines and non-benzodiazepine hypnotics induce selective changes in brain oxidative metabolism.

Special cases: Ketamine, nitrous oxide and xenon

Most general anaesthetic agents produce anaesthesia by increasing the activity of inhibitory gamma-aminobutyric acid type A receptors. The effects of ketamine, xenon and nitrous oxide on these receptors are, however, negligible. These anaesthetic agents potently inhibit excitatory N-methyl-D-aspartate receptors. Although these anaesthetic agents display some similar clinical features, such as potent analgesic effects, there are some important differences. Ketamine and nitrous oxide produce sympathomimetic effects, whereas xenon produces a sympatholytic effect. In addition, these anaesthetic agents return differential signals on clinical available anaesthetic depth monitors such as the bispectral index and mid-latency auditory evoked potential. Ketamine and nitrous oxide do not per se decrease the bispectral index. However, xenon decreases the bispectral index in a concentration-dependent manner. Similarly, ketamine and nitrous oxide do not suppress the mid-latency auditory evoked potential whereas xenon does. Thus, anaesthetic depth monitors fail to describe consciousness accurately when ketamine and nitrous oxide are used.

A null mutation for Fmr1 in female mice: effects on regional cerebral metabolic rate for glucose and relationship to behavior

As a measure of functional activity we determined regional cerebral metabolic rate for glucose (rCMR(glc)) in adult, female wild type and fragile X (Fmr1 null) mice homozygous and heterozygous for the null mutation. To ascertain if the sexes differ with respect to the severity of the effects of the mutation we compared our results with results of our previous study on male Fmr1 null mice [Qin M, Kang J, Smith CB (2002) Increased rates of cerebral glucose metabolism in a mouse model of fragile X mental retardation. Proc Natl Acad Sci U S A 99:15758-15763.]. In contrast to the male Fmr1 null mouse, rCMR(glc) was unchanged in the homozygous female except in the dorsal raphe where rCMR(glc) was increased by 36%. There were no differences in rCMR(glc) between heterozygous and wild type female mice. We compared male and female mice for effects of the null mutation on behavior. We found that the female Fmr1 null mouse is similar to the male with deficits in performance on a passive avoidance task, general hyperactivity, and increased susceptibility to audiogenic seizures. Both homozygous and heterozygous female mice exhibited hyperactivity and increased susceptibility to seizures, whereas only the homozygous mice had a deficit on the passive avoidance test. Male Fmr1 null mice had a tendency for lower anxiety-like behavior in an open field, whereas this was not evident in females. Compared with male wild type, male Fmr1 null mice also had a diminished acoustic startle response at higher stimulus intensities, whereas all three female genotypes had responses similar to those of male Fmr1 null mice. Whether estrogen affords female Fmr1 null mice some protection from the effects of the mutation remains to be determined.

Critical period for sensory experience-dependent survival of newly generated granule cells in the adult mouse olfactory bulb

Granule cells in the olfactory bulb (OB) are continually produced and added into the neuronal circuit in the adult brain. Sensory input to the OB plays a key role in the survival of newly generated granule cells. Here, we examined in the adult mice whether there is a time window after the generation of new granule cells when their survival is strongly influenced by sensory input. New granule cells were labeled by BrdUrd injection, and the mice were deprived of sensory input unilaterally by naris cauterization. During the initial 14 days after BrdUrd labeling, the number of BrdUrd-positive granule cells was similar for deprived and undeprived OBs. At 28 days or later, the BrdUrd-positive cell number remarkably decreased in the deprived OB. Cauterization at days 14-28 effectively reduced the number of BrdUrd-positive granule cells, whereas 2-week cauterization before or after this period produced little effect. Administration of diazepam, a GABAA receptor modulator, decreased the number of BrdUrd-positive granule cells. The diazepam administration was most effective at days 14-28. Histochemical examination showed that activation of caspase-3 was accompanied by apoptotic cell death of granule cells that was induced by sensory deprivation or diazepam administration. Double labeling with activated caspase-3 and BrdUrd indicated that granule cells at days 14-20 were most susceptible to cell death. These results indicate that there is a critical period when the survival of new granule cells is determined in a sensory experience-dependent manner and that the pharmacological manipulation can mimic the effect of sensory deprivation.

Evaluation of anesthesia effects on [18F]FDG uptake in mouse brain and heart using small animal PET

This study evaluates effects of anesthesia on (18)F-FDG (FDG) uptake in mouse brain and heart to establish the basic conditions of small animal PET imaging. Prior to FDG injection, 12 mice were anesthetized with isoflurane gas; 11 mice were anesthetized with an intraperitoneal injection of a ketamine/xylazine mixture; and 11 mice were awake. In isoflurane and ketamine/xylazine conditions, FDG brain uptake (%ID/g) was significantly lower than in controls. Conversely, in the isoflurane condition, %ID/g in heart was significantly higher than in controls, whereas heart uptake in ketamine/xylazine mice was significantly lower. Results suggest that anesthesia impedes FDG uptake in mouse brain and affects FDG uptake in heart; however, the effects in the brain and heart differ depending on the type of anesthesia used.

Piracetam and TRH analogues antagonise inhibition by barbiturates, diazepam, melatonin and galanin of human erythrocyte D-glucose transport

1 Nootropic drugs increase glucose uptake into anaesthetised brain and into Alzheimer's diseased brain. Thyrotropin-releasing hormone, TRH, which has a chemical structure similar to nootropics increases cerebellar uptake of glucose in murine rolling ataxia. This paper shows that nootropic drugs like piracetam (2-oxo 1 pyrrolidine acetamide) and levetiracetam and neuropeptides like TRH antagonise the inhibition of glucose transport by barbiturates, diazepam, melatonin and endogenous neuropeptide galanin in human erythrocytes in vitro. 2 The potencies of nootropic drugs in opposing scopolamine-induced memory loss correlate with their potencies in antagonising pentobarbital inhibition of erythrocyte glucose transport in vitro (P<0.01). Less potent nootropics, D-levetiracetam and D-pyroglutamate, have higher antagonist Ki's against pentobarbital inhibition of glucose transport than more potent L-stereoisomers (P<0.001). 3 Piracetam and TRH have no direct effects on net glucose transport, but competitively antagonise hypnotic drug inhibition of glucose transport. Other nootropics, like aniracetam and levetiracetam, while antagonising pentobarbital action, also inhibit glucose transport. Analeptics like bemigride and methamphetamine are more potent inhibitors of glucose transport than antagonists of hypnotic action on glucose transport. 4 There are similarities between amino-acid sequences in human glucose transport protein isoform 1 (GLUT1) and the benzodiazepine-binding domains of GABAA (gamma amino butyric acid) receptor subunits. Mapped on a 3D template of GLUT1, these homologies suggest that the site of diazepam and piracetam interaction is a pocket outside the central hydrophilic pore region. 5 Nootropic pyrrolidone antagonism of hypnotic drug inhibition of glucose transport in vitro may be an analogue of TRH antagonism of galanin-induced narcosis.

Assessment of microPET performance in analyzing the rat brain under different types of anesthesia: comparison between quantitative data obtained with microPET and ex vivo autoradiography

MicroPET (positron emission tomography) has been implemented for use in experiments with small animals. However, the quantification and optimal conditions for scanning are not established yet. The aim of this study was to compare the results obtained by microPET with those by ex vivo autoradiography of rat brain slices, based on the 2-[18F]fluoro-2-deoxy-D-glucose (FDG) method, and to establish the optimal conditions for scanning. As an example, we examined glucose metabolism in the rat brain under 6 types of anesthesia and in the conscious state. The scanning conditions for the rat brain were (1) use of a 4-mm-thick leaden jacket, (2) an energy window of 350-650 keV, and (3) a coincidence time window of 6 ns. Under these conditions, the quantitative ROI data from microPET showed a good correlation with the corresponding ROI data from FDG autoradiography in the animal study (r2=0.81). With our protocol, when anesthesia was started 40 min after the FDG injection, the glucose metabolism was almost the same as that in the conscious rat brain.

Opposite effects of ethanol and ketamine in the elevated plus-maze test in Wistar rats undergoing a chronic oral voluntary consumption procedure

The anxiolytic effects of ethanol (EtOH) have been involved in the vulnerability to EtOH drinking in humans. However, the role of the anxiolytic effects of EtOH during a chronic ingestion of the drug has not been extensively addressed, either in humans or in animal models. Since it was first shown that EtOH interacts with the N-methyl-d-aspartate (NMDA) receptor, a growing body of evidence demonstrating the involvement of this receptor in a wide range of EtOH effects has been reported. The present study aimed to investigate the ability of a voluntary consumption of EtOH to exert its putative anxiolytic-like activity in non-selected male Wistar rats held under a voluntary chronic oral consumption procedure using the elevated plus-maze (EPM) test. The effects of EtOH were compared with those of the noncompetitive NMDA receptor antagonist ketamine (KET), and with a mixture of both drugs. Rats were provided with 1-h limited access to one of the following sweetened (10% w/v glucose) solutions: (i) control; (ii) EtOH (ethanol, 10% v/v); (iii) KET (ketamine HCl, 0.28 mg/ml); or (iv) mixed (EtOH 10% v/v plus ketamine HCl 0.28 mg/ml) for 35 consecutive days. At the end of this period, and immediately after the last 1-h access to the respective solution, animals were independently tested in either EPM or open field tests. Previously, rats were tested on the inclined screen test during 15 consecutive days. The opposite effects were observed with EtOH and KET consumption in the EPM test, with EtOH decreasing and KET increasing the percentage of time spent in the open arms of the EPM, which was shown to be independent of any locomotor impairment, whereas consumption of the mixed solution did not significantly affect any test. Since the EtOH did not exhibit anxiolytic-like effects after its chronic oral consumption, it might be hypothesized that the anxiolytic activity of the EtOH is not critically involved in the maintenance of a voluntary EtOH consumption in non-selected rats. On the other hand, the lack of effects from mixed solution consumption suggests that EtOH and KET may interact in such a way that their effects are neutralized.

Estimation of the number of "true" null hypotheses in multivariate analysis of neuroimaging data

The repeated testing of a null univariate hypothesis in each of many sites (either regions of interest or voxels) is a common approach to the statistical analysis of brain functional images. Procedures, such as the Bonferroni, are available to maintain the Type I error of the set of tests at a specified level. An initial assumption of these methods is a "global null hypothesis," i.e., the statistics computed on each site are assumed to be generated by null distributions. This framework may be too conservative when a significant proportion of the sites is affected by the experimental manipulation. This report presents the development of a rigorous statistical procedure for use with a previously reported graphical method, the P plot, for estimation of the number of "true" null hypotheses in the set. This estimate can then be used to sharpen existing multiple comparison procedures. Performance of the P plot method in the multiple comparison problem is investigated in simulation studies and in the analysis of autoradiographic data.