Concentration-dependent isoflurane effects on depolarization-evoked glutamate and GABA outflows from mouse brain slices

Effects of isoflurane and urethane anesthetics on glutamate neurotransmission in rat brain using in vivo amperometry

BACKGROUND

Aspects of glutamate neurotransmission implicated in normal and pathological conditions are predominantly evaluated using in vivo recording paradigms in rats anesthetized with isoflurane or urethane. Urethane and isoflurane anesthesia influence glutamate neurotransmission through different mechanisms; however, real-time outcome measures of potassium chloride (KCl)-evoked glutamate overflow and glutamate clearance kinetics have not been compared within and between regions of the brain. In order to maintain rigor and reproducibility within the literature between the two most common methods of anesthetized in vivo recording of glutamate, we compared glutamate signaling as a function of anesthesia and brain region in the rat strain most used in neuroscience.

METHODS

In the following experiments, in vivo amperometric recordings of KCl-evoked glutamate overflow and glutamate clearance kinetics (uptake rate and T80) in the cortex, hippocampus, and thalamus were performed using glutamate-selective microelectrode arrays (MEAs) in young adult male, Sprague-Dawley rats anesthetized with either isoflurane or urethane.

RESULTS

Potassium chloride (KCl)-evoked glutamate overflow was similar under urethane and isoflurane anesthesia in all brain regions studied. Analysis of glutamate clearance determined that the uptake rate was significantly faster (53.2%, p < 0.05) within the thalamus under urethane compared to isoflurane, but no differences were measured in the cortex or hippocampus. Under urethane, glutamate clearance parameters were region-dependent, with significantly faster glutamate clearance in the thalamus compared to the cortex but not the hippocampus (p < 0.05). No region-dependent differences were measured for glutamate overflow using isoflurane.

CONCLUSIONS

These data support that amperometric recordings of KCl-evoked glutamate under isoflurane and urethane anesthesia result in similar and comparable data. However, certain parameters of glutamate clearance can vary based on choice of anesthesia and brain region. In these circumstances, special considerations are needed when comparing previous literature and planning future experiments.

Effects of isoflurane and urethane anesthetics on glutamate neurotransmission in rat brain using in vivo amperometry

Aspects of glutamate neurotransmission implicated in normal and pathological conditions are often evaluated using in vivo recording paradigms in rats anesthetized with isoflurane or urethane. Urethane and isoflurane anesthesia influence glutamate neurotransmission through different mechanisms; however real-time outcome measures of potassium chloride (KCl)-evoked glutamate overflow and glutamate clearance kinetics have not been compared within and between regions of the brain. In the following experiments, in vivo amperometric recordings of KCl-evoked glutamate overflow and glutamate clearance kinetics (uptake rate and T80) in the cortex, hippocampus and thalamus were performed using glutamate-selective microelectrode arrays (MEAs) in young adult male, Sprague-Dawley rats anesthetized with isoflurane or urethane. Potassium chloride (KCl)-evoked glutamate overflow was similar under urethane and isoflurane anesthesia in all brain regions studied. Analysis of glutamate clearance determined that the uptake rate was significantly faster (53.2%, p<0.05) within the thalamus under urethane compared to isoflurane, but no differences were measured in the cortex or hippocampus. Under urethane, glutamate clearance parameters were region dependent, with significantly faster glutamate clearance in the thalamus compared to the cortex but not the hippocampus (p<0.05). No region dependent differences were measured for glutamate overflow using isoflurane. These data support that amperometric recordings of glutamate under isoflurane and urethane anesthesia result in mostly similar and comparable data. However, certain parameters of glutamate uptake vary based on choice of anesthesia and brain region. Special considerations must be given to these areas when considering comparison to previous literature and when planning future experiments.

Increasing isoflurane dose reduces homotopic correlation and functional segregation of brain networks in mice as revealed by resting-state fMRI

Effects of anesthetics on brain functional networks are not fully understood. In this work, we investigated functional brain networks derived from resting-state fMRI data obtained under different doses of isoflurane in mice using stationary and dynamic functional connectivity (dFC) analysis. Stationary network analysis using FSL Nets revealed a modular structure of functional networks, which could be segregated into a lateral cortical, an associative cortical network, elements of the prefrontal network, a subcortical network, and a thalamic network. Increasing isoflurane dose led to a loss of functional connectivity between the bilateral cortical regions. In addition, dFC analysis revealed a dominance of dynamic functional states (dFS) exhibiting modular structure in mice anesthetized with a low dose of isoflurane, while at high isoflurane levels dFS showing widespread unstructured correlation displayed highest weights. This indicates that spatial segregation across brain functional networks is lost with increasing dose of the anesthetic drug used. To what extent this indicates a state of deep anesthesia remains to be shown. Combining the results of stationary and dynamic FC analysis indicates that increasing isoflurane levels leads to loss of modular network organization, which includes loss of the strong bilateral interactions between homotopic brain areas.

Endocannabinoid signaling in hypothalamic circuits regulates arousal from general anesthesia in mice

Consciousness can be defined by two major attributes: awareness of environment and self, and arousal, which reflects the level of awareness. The return of arousal after general anesthesia presents an experimental tool for probing the neural mechanisms that control consciousness. Here we have identified that systemic or intracerebral injection of the cannabinoid CB1 receptor (CB1R) antagonist AM281 into the dorsomedial nucleus of the hypothalamus (DMH) - but not the adjacent perifornical area (Pef) or the ventrolateral preoptic nucleus of the hypothalamus (VLPO) - accelerates arousal in mice recovering from general anesthesia. Anesthetics selectively activated endocannabinoid (eCB) signaling at DMH glutamatergic but not GABAergic synapses, leading to suppression of both glutamatergic DMH-Pef and GABAergic DMH-VLPO projections. Deletion of CB1R from widespread cerebral cortical or prefrontal cortical (PFC) glutamatergic neurons, including those innervating the DMH, mimicked the arousal-accelerating effects of AM281. In contrast, CB1R deletion from brain GABAergic neurons or hypothalamic glutamatergic neurons did not affect recovery time from anesthesia. Inactivation of PFC-DMH, DMH-VLPO, or DMH-Pef projections blocked AM281-accelerated arousal, whereas activation of these projections mimicked the effects of AM281. We propose that decreased eCB signaling at glutamatergic terminals of the PFC-DMH projection accelerates arousal from general anesthesia through enhancement of the excitatory DMH-Pef projection, the inhibitory DMH-VLPO projection, or both.

Sensitivity to isoflurane anesthesia increases in autism spectrum disorder Shank3+/∆c mutant mouse model

Autism is a heterogeneous developmental disorder characterized by impaired social interaction, impaired communication skills, and restricted and repetitive behavior. The abnormal behaviors of these patients can make their anesthetic and perioperative management difficult. Evidence in the literature suggests that some patients with autism or specific autism spectrum disorders (ASD) exhibit altered responses to pain and to anesthesia or sedation. A genetic mouse model of one particular ASD, Phelan McDermid Syndrome, has been developed that has a Shank3 haplotype truncation (Shank3^+/Δc). These mice exhibit important characteristics of autism that mimic human autistic behavior. Our study demonstrates that a Shank3^+/ΔC mutation in mice is associated with a reduction in both the MAC and RREC50 of isoflurane and down regulation of NR1 in vestibular nuclei and PSD95 in spinal cord. Decreased expression of NR1 and PSD95 in the central nervous system of Shank3^+/ΔC mice could help reduce the MAC and RREC50 of isoflurane, which would warrant confirmation in a clinical study. If Shank3 mutations are found to affect anesthetic sensitivity in patients with ASD, better communication and stricter monitoring of anesthetic depth may be necessary.

Interhemispheric resting-state functional connectivity of the claustrum in the awake and anesthetized states

The claustrum is a brain region whose function remains unknown, though many investigators suggest it plays a role in conscious attention. Resting-state functional magnetic resonance imaging (RS-fMRI) has revealed how anesthesia alters many functional connections in the brain, but the functional role of the claustrum with respect to the awake versus anesthetized states remains unknown. Therefore, we employed a combination of seed-based RS-fMRI and neuroanatomical tracing to reveal how the anatomical connections of the claustrum are related to its functional connectivity during quiet wakefulness and the isoflurane-induced anesthetic state. In awake rats, RS-fMRI indicates that the claustrum has interhemispheric functional connections with the mediodorsal thalamus (MD) and medial prefrontal cortex (mPFC), as well as other known connections with cortical areas that correspond to the connections revealed by neuroanatomical tracing. During deep isoflurane anesthesia, the functional connections of the claustrum with mPFC and MD were significantly attenuated, while those with the rest of cortex were not significantly altered. These changes in claustral functional connectivity were also observed when seeds were placed in mPFC or MD during RS-fMRI comparisons of the awake and deeply anesthetized states. Collectively, these data indicate that the claustrum has functional connections with mPFC and MD-thalamus that are significantly lessened by anesthesia.

Magnetic resonance spectroscopic analysis of neurometabolite changes in the developing rat brain at 7T

We utilized proton magnetic resonance spectroscopy to evaluate the metabolic profile of the hippocampus and anterior cingulate cortex of the developing rat brain from postnatal days 14-70. Measured metabolite concentrations were modeled using linear, exponential, or logarithmic functions and the time point at which the data reached plateau (i.e. when the portion of the data could be fit to horizontal line) was estimated and was interpreted as the time when the brain has reached maturity with respect to that metabolite. N-acetyl-aspartate and myo-inositol increased within the observed period. Gluthathione did not vary significantly, while taurine decreased initially and then stabilized. Phosphocreatine and total creatine had a tendency to increase towards the end of the experiment. Some differences between our data and the published literature were observed in the concentrations and dynamics of phosphocreatine, myo-inositol, and GABA in the hippocampus and creatine, GABA, glutamine, choline and N-acetyl-aspartate in the cortex. Such differences may be attributed to experimental conditions, analysis approaches and animal species. The latter is supported by differences between in-house rat colony and rats from Charles River Labs. Spectroscopy provides a valuable tool for non-invasive brain neurochemical profiling for use in developmental neurobiology research. Special attention needs to be paid to important sources of variation like animal strain and commercial source.

Effects of isoflurane anesthesia on ensemble patterns of Ca2+ activity in mouse v1: reduced direction selectivity independent of increased correlations in cellular activity

Anesthesia affects brain activity at the molecular, neuronal and network level, but it is not well-understood how tuning properties of sensory neurons and network connectivity change under its influence. Using in vivo two-photon calcium imaging we matched neuron identity across episodes of wakefulness and anesthesia in the same mouse and recorded spontaneous and visually evoked activity patterns of neuronal ensembles in these two states. Correlations in spontaneous patterns of calcium activity between pairs of neurons were increased under anesthesia. While orientation selectivity remained unaffected by anesthesia, this treatment reduced direction selectivity, which was attributable to an increased response to the null-direction. As compared to anesthesia, populations of V1 neurons coded more mutual information on opposite stimulus directions during wakefulness, whereas information on stimulus orientation differences was lower. Increases in correlations of calcium activity during visual stimulation were correlated with poorer population coding, which raised the hypothesis that the anesthesia-induced increase in correlations may be causal to degrading directional coding. Visual stimulation under anesthesia, however, decorrelated ongoing activity patterns to a level comparable to wakefulness. Because visual stimulation thus appears to 'break' the strength of pairwise correlations normally found in spontaneous activity under anesthesia, the changes in correlational structure cannot explain the awake-anesthesia difference in direction coding. The population-wide decrease in coding for stimulus direction thus occurs independently of anesthesia-induced increments in correlations of spontaneous activity.

Interaction of anesthetics with neurotransmitter release machinery proteins

General anesthetics produce anesthesia by depressing central nervous system activity. Activation of inhibitory GABA(A) receptors plays a central role in the action of many clinically relevant general anesthetics. Even so, there is growing evidence that anesthetics can act at a presynaptic locus to inhibit neurotransmitter release. Our own data identified the neurotransmitter release machinery as a target for anesthetic action. In the present study, we sought to examine the site of anesthetic action more closely. Exocytosis was stimulated by directly elevating the intracellular Ca(2+) concentration at neurotransmitter release sites, thereby bypassing anesthetic effects on channels and receptors, allowing anesthetic effects on the neurotransmitter release machinery to be examined in isolation. Three different PC12 cell lines, which had the expression of different release machinery proteins stably suppressed by RNA interference, were used in these studies. Interestingly, there was still significant neurotransmitter release when these knockdown PC12 cells were stimulated. We have previously shown that etomidate, isoflurane, and propofol all inhibited the neurotransmitter release machinery in wild-type PC12 cells. In the present study, we show that knocking down synaptotagmin I completely prevented etomidate from inhibiting neurotransmitter release. Synaptotagmin I knockdown also diminished the inhibition produced by propofol and isoflurane, but the magnitude of the effect was not as large. Knockdown of SNAP-25 and SNAP-23 expression also changed the ability of these three anesthetics to inhibit neurotransmitter release. Our results suggest that general anesthetics inhibit the neurotransmitter release machinery by interacting with multiple SNARE and SNARE-associated proteins.

Repeated whisker stimulation evokes invariant neuronal responses in the dorsolateral striatum of anesthetized rats: a potential correlate of sensorimotor habits

The dorsolateral striatum (DLS) receives extensive projections from primary somatosensory cortex (SI), but very few studies have used somesthetic stimulation to characterize the sensory coding properties of DLS neurons. In this study, we used computer-controlled whisker deflections to characterize the extracellular responses of DLS neurons in rats lightly anesthetized with isoflurane. When multiple whiskers were synchronously deflected by rapid back-and-forth movements, whisker-sensitive neurons in the DLS responded to both directions of movement. The latency and magnitude of these neuronal responses displayed very little variation with changes in the rate (2, 5, or 8 Hz) of whisker stimulation. Simultaneous recordings in SI barrel cortex and the DLS revealed important distinctions in the neuronal responses of these serially connected brain regions. In contrast to DLS neurons, SI neurons were activated by the initial deflection of the whiskers but did not respond when the whiskers moved back to their original position. As the rate of whisker stimulation increased, SI responsiveness declined, and the latencies of the responses increased. In fact, when whiskers were deflected at 5 or 8 Hz, many neurons in the DLS responded before the SI neurons. These results and earlier anatomic findings suggest that a component of the sensory-induced response in the DLS is mediated by inputs from the thalamus. Furthermore, the lack of sensory adaptation in the DLS may represent a critical part of the neural mechanism by which the DLS encodes stimulus-response associations that trigger motor habits and other stimulus-evoked behaviors that are not contingent on rewarded outcomes.

Rapid microelectrode measurements and the origin and regulation of extracellular glutamate in rat prefrontal cortex

Glutamate in the prefrontal cortex (PFC) plays a significant role in several mental illnesses, including schizophrenia, addiction and anxiety. Previous studies on PFC glutamate-mediated function have used techniques that raise questions on the neuronal versus astrocytic origin of glutamate. The present studies used enzyme-based microelectrode arrays to monitor second-by-second resting glutamate levels in the PFC of awake rats. Locally applied drugs were employed in an attempt to discriminate between the neuronal or glial components of the resting glutamate signal. Local application of tetrodotoxin (sodium channel blocker), produced a significant (∼ 40%) decline in resting glutamate levels. In addition significant reductions in extracellular glutamate were seen with locally applied ω-conotoxin (MVIIC; ∼ 50%; calcium channel blocker), and the mGluR(2/3) agonist, LY379268 (∼ 20%), and a significant increase with the mGluR(2/3) antagonist LY341495 (∼ 40%), effects all consistent with a large neuronal contribution to the resting glutamate levels. Local administration of D,L-threo-β-benzyloxyaspartate (glutamate transporter inhibitor) produced an ∼ 120% increase in extracellular glutamate levels, supporting that excitatory amino acid transporters, which are largely located on glia, modulate clearance of extracellular glutamate. Interestingly, local application of (S)-4-carboxyphenylglycine (cystine/glutamate antiporter inhibitor), produced small, non-significant bi-phasic changes in extracellular glutamate versus vehicle control. Finally, pre-administration of tetrodotoxin completely blocked the glutamate response to tail pinch stress. Taken together, these results support that PFC resting glutamate levels in rats as measured by the microelectrode array technology are at least 40-50% derived from neurons. Furthermore, these data support that the impulse flow-dependent glutamate release from a physiologically -evoked event is entirely neuronally derived.

Mu opioid receptor modulation of somatodendritic dopamine overflow: GABAergic and glutamatergic mechanisms

Mu opioid receptor (MOR) regulation of somatodendritic dopamine neurotransmission in the ventral tegmental area (VTA) was investigated using conventional microdialysis in freely moving rats and mice. Reverse dialysis of the MOR agonist DAMGO (50 and 100 microm) into the VTA of rats produced a concentration-dependent increase in dialysate dopamine concentrations. Basal dopamine overflow in the VTA was unaltered in mice lacking the MOR gene. However, basal gamma-aminobutyric acid (GABA) overflow in these animals was significantly increased, whereas glutamate overflow was decreased. Intra-VTA perfusion of DAMGO into wild-type (WT) mice increased dopamine overflow. GABA concentrations were decreased, whereas glutamate concentrations in the VTA were unaltered. Consistent with the loss of MOR, no effect of DAMGO was observed in MOR knockout (KO) mice. These data provide the first direct demonstration of tonically active MOR systems in the VTA that regulate basal glutamatergic and GABAergic neurotransmission in this region. We hypothesize that increased GABAergic neurotransmission following constitutive deletion of MOR is due to the elimination of a tonic inhibitory influence of MOR on GABAergic neurons in the VTA, whereas decreased glutamatergic neurotransmission in MOR KO mice is a consequence of intensified GABA tone on glutamatergic neurons and/or terminals. As a consequence, somatodendritic dopamine release is unaltered. Furthermore, MOR KO mice do not exhibit the positive correlation between basal dopamine levels and the glutamate/GABA ratio observed in WT mice. Together, our findings indicate a critical role of VTA MOR in maintaining an intricate balance between excitatory and inhibitory inputs to dopaminergic neurons.

Isoflurane inhibits the neurotransmitter release machinery

Despite their importance, the mechanism of action of general anesthetics is still poorly understood. Facilitation of inhibitory GABA(A) receptors plays an important role in anesthesia, but other targets have also been linked to anesthetic actions. Anesthetics are known to suppress excitatory synaptic transmission, but it has been difficult to determine whether they act on the neurotransmitter release machinery itself. By directly elevating [Ca(2+)](i) at neurotransmitter release sites without altering plasma membrane channels or receptors, we show that the commonly used inhalational general anesthetic, isoflurane, inhibits neurotransmitter release at clinically relevant concentrations, in a dose-dependent fashion in PC12 cells and hippocampal neurons. We hypothesized that a SNARE and/or SNARE-associated protein represents an important target(s) for isoflurane. Overexpression of a syntaxin 1A mutant, previously shown in Caenorhabditis elegans to block the behavioral effects of isoflurane, completely eliminated the reduction in neurotransmitter release produced by isoflurane, without affecting release itself, thereby establishing the possibility that syntaxin 1A is an intermediary in isoflurane's ability to inhibit neurotransmitter release.

Sodium channels and the synaptic mechanisms of inhaled anaesthetics

General anaesthetics act in an agent-specific manner on synaptic transmission in the central nervous system by enhancing inhibitory transmission and reducing excitatory transmission. The synaptic mechanisms of general anaesthetics involve both presynaptic effects on transmitter release and postsynaptic effects on receptor function. The halogenated volatile anaesthetics inhibit neuronal voltage-gated Na(+) channels at clinical concentrations. Reductions in neurotransmitter release by volatile anaesthetics involve inhibition of presynaptic action potentials as a result of Na(+) channel blockade. Although voltage-gated ion channels have been assumed to be insensitive to general anaesthetics, it is now evident that clinical concentrations of volatile anaesthetics inhibit Na(+) channels in isolated rat nerve terminals and neurons, as well as heterologously expressed mammalian Na(+) channel alpha subunits. Voltage-gated Na(+) channels have emerged as promising targets for some of the effects of the inhaled anaesthetics. Knowledge of the synaptic mechanisms of general anaesthetics is essential for optimization of anaesthetic techniques for advanced surgical procedures and for the development of improved anaesthetics.

Implantation of a glass capillary-based enzyme electrode in mouse hippocampal slices for monitoring of L-glutamate release

A glass capillary-based enzyme electrode (tip size approximately 10 microm) was implanted in the target neuronal region, i.e., dentate gyrus (DG) or cornu ammonis 1 (CA1), of acute brain slices at a depth of approximately 10 microm from the slice surface in order to allow the monitoring of chemical stimulant-induced changes in L-glutamate levels. First, the sampling behavior of a glass capillary in a slice was investigated by visualizing the transport of a fluorescence dye. Then, the electrode was applied to real-time monitoring of L-glutamate release in acute hippocampal slices stimulated by surface application of a stimulant solution. The extracellular application of KCl (0.10 M) increased the glutamate levels in the DG and CA1 regions, respectively. The enhancement of L-glutamate concentration at DG was much larger than at CA1. The application of tetraethylammonium chloride (TEA) (25 mM) enhanced the L-glutamate level in the DG region and the enhanced level did not return to the initial value before TEA application even when washed with an artificial cerebrospinal fluid (ACSF). The usefulness of a surface-implanted capillary electrode for monitoring L-glutamate release in acute brain slices is discussed.

CB(1) cannabinoid receptors inhibit the glutamatergic component of KCl-evoked excitation of locus coeruleus neurons in rat brain slices

CB(1) cannabinoid receptors located at presynaptic sites suppress synaptic transmission in the rat brain. The aim of this work was to examine by single-unit extracellular techniques the effect of the synthetic cannabinoid receptor agonist WIN 55212-2 on KCl-evoked excitation of locus coeruleus neurons in rat brain slices. Short applications of KCl (30 mM) increased by 9-fold the firing rate of locus coeruleus cells. Perfusion with the GABA(A) receptor antagonist picrotoxin (100 microM) increased KCl-evoked effect, whereas NMDA and non-NMDA glutamate receptor antagonists (D-AP5 100 microM and CNQX 30 microM, respectively) were able to decrease KCl-evoked effect only in the presence of picrotoxin (100 microM). Bath application of WIN 55212-2 (10 microM) inhibited KCl-evoked effect; this inhibition was blocked by the CB(1) receptor antagonist AM 251 (1 microM). However, a lower concentration of WIN 55212-2 (1 microM) did not significantly change KCl effect. In the presence of picrotoxin (100 microM), perfusion with D-AP5 (100 microM) or CNQX (30 microM) blocked WIN 55212-2-induced inhibition, although picrotoxin (100 microM) itself failed to affect cannabinoid effect. In conclusion, GABAergic and glutamatergic components are both involved in KCl-evoked excitation of LC neurons, although CB(1) receptors only seem to inhibit the glutamatergic component of KCl effect in the locus coeruleus.

Neuronal activity and cortical perfusion determined by quantitative EEG analysis and laser doppler flowmetry are uncoupled in brain injured rats

Following traumatic brain injury uncoupling of metabolism from perfusion, reflected by an increase in cellular activity in face of reduced perfusion contributes to secondary tissue damage. Standard autoradiographic techniques employed to study metabolism and perfusion are limited by the inability to perform longitudinal investigations. Therefore, the aims of this study were to investigate if metabolic uncoupling can be studied by applying noninvasive and non-radioactive methods. For this, quantitative EEG analysis and laser Doppler flowmetry were employed to determine changes in EEG activity as a global measure of neuronal activity and pericontusional cortical perfusion during the early phase following controlled cortical impact injury (CCII). In eight rats changes in neuronal activity and pericontusional cortical perfusion were determined before, at 4 and 24 hours after CCII. Neuronal activity was significantly increased by 40% at 4 hours after CCII followed by a significant decrease by 60% at 24 hours compared to pre-trauma levels. Pericontusional cortical perfusion was significantly reduced by 45% and 18% at 4 and 24 hours after CCII. respectively. Non-radioactive methods like quantitative EEG analysis and laser Doppler flowmetry can be used to reveal underlying uncoupling of EEG activity from cortical perfusion which is mostly sustained in the early phase following CCII.

Effects of local and intravenous anesthetics on the activity of glutamate transporter type 2

Glutamate transporters may be important targets for anesthetic action in the central nervous system. The authors investigated the effects of the intravenous anesthetics, thiopental and ketamine, and the local anesthetics, lidocaine and bupivacaine, on the activity of glutamate transporter type 2, EAAT2. EAAT2 was expressed in Xenopus oocytes by injection of its mRNA. By using two-electrode voltage clamping, membrane currents were recorded after the application of L-glutamate (30 microM) in the presence or absence of various concentrations of anesthetics. Lidocaine and bupivacaine did not change glutamate-induced inward currents at the tested concentrations (1-1000 microM). Thiopental and ketamine also did not affect the activity of EAAT2 at the tested concentrations (0.3-300 microM). Our results suggest that the two commonly used local anesthetics (lidocaine and bupivacaine) and intravenous anesthetics (thiopental and ketamine) do not affect the activity of EAAT2 expressed in oocytes. EAAT2 may not be a target for these anesthetics.

Selective depression by general anesthetics of glutamate versus GABA release from isolated cortical nerve terminals

The role of presynaptic mechanisms in general anesthetic depression of excitatory glutamatergic neurotransmission and facilitation of GABA-mediated inhibitory neurotransmission is unclear. A dual isotope method allowed simultaneous comparisons of the effects of a representative volatile (isoflurane) and intravenous (propofol) anesthetic on the release of glutamate and GABA from isolated rat cerebrocortical nerve terminals (synaptosomes). Synaptosomes were prelabeled with L-[(3)H]glutamate and [(14)C]GABA, and release was determined by superfusion with pulses of 30 mM K(+) or 1 mM 4-aminopyridine (4AP) in the absence or presence of 1.9 mM free Ca(2+). Isoflurane maximally inhibited Ca(2+)-dependent 4AP-evoked L-[(3)H]glutamate release (99 +/- 8% inhibition) to a greater extent than [(14)C]GABA release (74 +/- 6% inhibition; P = 0.023). Greater inhibition of L-[(3)H]glutamate versus [(14)C]GABA release was also observed for the Na(+) channel antagonists tetrodotoxin (99 +/- 4 versus 63 +/- 5% inhibition; P < 0.001) and riluzole (84 +/- 5 versus 52 +/- 12% inhibition; P = 0.041). Propofol did not differ in its maximum inhibition of Ca(2+)-dependent 4AP-evoked L-[(3)H]glutamate release (76 +/- 12% inhibition) compared with [(14)C]GABA (84 +/- 31% inhibition; P = 0.99) release. Neither isoflurane (1 mM) nor propofol (15 microM) affected K(+)-evoked release, consistent with a molecular target upstream of the synaptic vesicle exocytotic machinery or voltage-gated Ca(2+) channels coupled to transmitter release. These findings support selective presynaptic depression of excitatory versus inhibitory neurotransmission by clinical concentrations of isoflurane, probably as a result of Na(+) channel blockade.

Neuronal activity determined by quantitative EEG and cortical microdialysis is increased following controlled cortical impact injury in rats

Following brain injury increased glutamate release is linked to sustained neuronal activation resulting in excitotoxic tissue damage. Isoflurane anesthesia has been shown to decrease electroencephalographic (EEG) activity and extracellular (e.c.) glutamate, possibly attenuating excitotoxic tissue damage. However, based on clinical experience EEG activity can fluctuate despite stable isoflurane concentrations. Therefore, the aims of this study were to investigate the impact of neuronal activity assessed by quantitative EEG on e.c. glutamate during isoflurane anesthesia following controlled cortical impact injury (CCII). In 10 rats balanced anesthesia using isoflurane was induced at 4 hours after CCII. Under steady-state conditions maintaining isoflurane at 1.8 vol%, EEG was recorded for 3 hours. During this period, e.c. glutamate was sampled in the pericontusional cortex by microdialysis. Despite maintaining isoflurane at 1.8 vol%, neuronal activity expressed as low frequency EEG power showed marked fluctuations. Spontaneous increases in neuronal activity coincided with elevated e.c. glutamate levels and vice versa. Overall, EEG power correlated significantly with pathologically elevated e.c. glutamate levels (n = 58; R2 = 0.54; p < 0.05). Despite unchanged isoflurance concentrations fluctuations in neuronal activity were reflected by altered EEG power and e.c. glutamate concentrations. Therefore, neuronal activity needs to be considered for the interpretation of e.c. glutamate levels.

The different responses of rat glutamate transporter type 2 and its mutant (tyrosine 403 to histidine) activity to volatile anesthetics and activation of protein kinase C

Glutamate transporters play an important role in homeostasis of extracellular glutamate, a major excitatory neurotransmitter and a potential neurotoxin. In mammalian brain, glutamate transporter type 2 (EAAT2) is the most abundant form. Studies of molecular structures demonstrated that tyrosine 403 is critical in regulating the ion selectivity and transport mode of EAAT2. We hypothesized that wild type EAAT2 and its mutant at tyrosine 403 have different responses to volatile anesthetics, commonly used anesthetics that have been shown to affect glutamate transporter activity and decrease extracellular glutamate concentrations. We used site-directed mutagenesis and oocyte expression systems to test the hypothesis. Volatile anesthetics did not affect the activity of wild type EAAT2, isolated from rat hippocampus. When tyrosine 403 was replaced by histidine (Y403H), volatile anesthetics (isoflurane or halothane) at clinically relevant concentrations significantly decreased the transporter activity. Okadaic acid, a phosphatase inhibitor, significantly prolonged the isoflurane-induced inhibition. This inhibition was reversed by staurosporine and calphostin C, two protein kinase C (PKC) inhibitors, but not by the third PKC inhibitor, chelerythrine. Phorbol 12-myristate 13-acetate, a PKC activator, inhibited the activity of both wild type and Y403H EAAT2. This inhibition was also reversed by the same two PKC inhibitors but not by the third one. These results suggest that the switch of tyrosine 403 to histidine rendered EAAT2 sensitive to volatile anesthetics, a phenomenon that may require protein phosphorylation. PKC may be involved in the regulation of the activity of both wild type and Y403H EAAT2.

General anesthetic actions on norepinephrine, dopamine, and gamma-aminobutyric acid transporters in stably transfected cells

The effects of general anesthetics on neurotransmitter uptake by plasma membrane transporters are controversial. We analyzed the effects of representative volatile and IV general anesthetics on recombinant transporters for norepinephrine (human NET), dopamine (rat DAT), or gamma-aminobutyric acid (rat GAT-1) stably expressed in a porcine kidney cell line (LLC-PK(1)). This approach avoids complicating factors associated with neuronal preparations, such as the involvement of multiple transporters and the indirect effects of membrane potential. At clinical concentrations, human NET was inhibited only by halothane (50% inhibitory concentration [IC(50)] = 0.54 mM), rat DAT was sensitive to both halothane and isoflurane (IC(50) = 0.60 and 0.64 mM, respectively), and rat GAT-1 was insensitive to both volatile anesthetics. Human NET was inhibited in a dose-dependent fashion by propofol (IC(50) = 41 micro M), ketamine (IC(50) = 150 micro M), and etomidate (IC(50) > 200 micro M), but not by pentobarbital. Only propofol inhibited NET at a clinically relevant concentration (5 micro M). Rat DAT was inhibited in a dose-dependent fashion by propofol (IC(50) = 120 micro M), etomidate (IC(50) = 100 micro M), and ketamine (IC(50) = 210 micro M), but not by pentobarbital. None of these anesthetics was predicted to inhibit DAT at concentrations that produce anesthesia. Propofol inhibited rat GAT-1, but only at the largest concentration tested. General anesthetics have drug- and subtype-selective actions on neurotransmitter transporters. We conclude that effects on catecholamine, but not gamma-aminobutyric acid, transporters may contribute to secondary synaptic actions of certain anesthetics but are unlikely to be essential to their anesthetic properties.

IMPLICATIONS

Previous studies have implicated neurotransmitter transporters as targets for general anesthetic effects on synaptic transmission. Recombinant transporters for norepinephrine and dopamine were sensitive to certain volatile and IV anesthetics, whereas gamma-aminobutyric acid transporters were insensitive. These anesthetic- and neurotransmitter-specific effects may underlie some of the secondary effects of general anesthetics.