Effects of the volatile anesthetic enflurane on spontaneous discharge rate and GABA(A)-mediated inhibition of Purkinje cells in rat cerebellar slices

Inhibition of astrocyte metabolism is not the primary mechanism for anaesthetic hypnosis

Astrocytes have been promoted as a possible mechanistic target for anaesthetic hypnosis. The aim of this study was to explore this using the neocortical brain slice preparation. The methods were in two parts. Firstly, multiple general anaesthetic compounds demonstrating varying in vivo hypnotic potency were analysed for their effect on "zero-magnesium" seizure-like event (SLE) activity in mouse neocortical slices. Subsequently, the effect of astrocyte metabolic inhibition was investigated in neocortical slices, and compared with that of the anaesthetic drugs. The rationale was that, if suppression of astrocytes was both necessary and sufficient to cause hypnosis in vivo, then inhibition of astrocytic metabolism in slices should mimic the anaesthetic effect. In vivo anaesthetic potency correlated strongly with the magnitude of reduction in SLE frequency in neocortical slices (R(2) 37.7 %, p = 0.002). Conversely, SLE frequency and length were significantly enhanced during exposure to both fluoroacetate (23 and 20 % increase, respectively, p < 0.01) and aminoadipate (12 and 38 % increase, respectively, p < 0.01 and p < 0.05). The capacity of an anaesthetic agent to reduce SLE frequency in the neocortical slice is a good indicator of its in vivo hypnotic potency. The results do not support the hypothesis that astrocytic metabolic inhibition is a mechanism of anaesthetic hypnosis.

4-bromopropofol decreases action potential generation in spinal neurons by inducing a glycine receptor-mediated tonic conductance

BACKGROUND AND PURPOSE

Impaired function of spinal strychnine-sensitive glycine receptors gives rise to chronic pain states and movement disorders. Therefore, increased activity of glycine receptors should help to treat such disorders. Although compounds targeting glycine receptors with a high selectivity are lacking, halogenated analogues of propofol have recently been considered as potential candidates. Therefore we asked whether 4-bromopropofol attenuated the excitability of spinal neurons by promoting glycine receptor-dependent inhibition.

EXPERIMENTAL APPROACH

The actions of sub-anaesthetic concentrations of propofol and 4-bromopropofol were investigated in spinal tissue cultures prepared from mice. Drug-induced alterations in action potential firing were monitored by extracellular multi-unit recordings. The effects on GABAA and glycine receptor-mediated inhibition were quantified by whole-cell voltage-clamp recordings.

KEY RESULTS

Low concentrations of 4-bromopropofol (50 nM) reduced action potential activity of ventral horn neurons by about 30%, compared with sham-treated slices. This effect was completely abolished by strychnine (1 μM). In voltage-clamped neurons, 4-bromopropofol activated glycine receptors, generating a tonic current of 65 ± 10 pA, while GABAA - and glycine receptor-mediated synaptic transmission remained unaffected.

CONCLUSIONS AND IMPLICATIONS

The highest glycine levels in the CNS are found in the ventral horn of the spinal cord, a region mediating pain-induced motor reflexes and participating in the control of muscle tone. 4-Bromopropofol may serve as a starting point for the development of non-sedative, non-addictive, muscle relaxants and analgesics to be used to treat low back pain.

Opposing actions of sevoflurane on GABAergic and glycinergic synaptic inhibition in the spinal ventral horn

BACKGROUND

The ventral horn is a major substrate in mediating the immobilizing properties of the volatile anesthetic sevoflurane in the spinal cord. In this neuronal network, action potential firing is controlled by GABA(A) and glycine receptors. Both types of ion channels are sensitive to volatile anesthetics, but their role in mediating anesthetic-induced inhibition of spinal locomotor networks is not fully understood.

METHODOLOGY/PRINCIPAL FINDINGS

To compare the effects of sevoflurane on GABAergic and glycinergic inhibitory postsynaptic currents (IPSCs) whole-cell voltage-clamp recordings from ventral horn interneurons were carried out in organotypic spinal cultures. At concentrations close to MAC (minimum alveolar concentration), decay times of both types of IPSCs were significantly prolonged. However, at 1.5 MAC equivalents, GABAergic IPSCs were decreased in amplitude and reduced in frequency. These effects counteracted the prolongation of the decay time, thereby decreasing the time-averaged GABAergic inhibition. In contrast, amplitudes and frequency of glycinergic IPSCs were not significantly altered by sevoflurane. Furthermore, selective GABA(A) and glycine receptor antagonists were tested for their potency to reverse sevoflurane-induced inhibition of spontaneous action potential firing in the ventral horn. These experiments confirmed a weak impact of GABA(A) receptors and a prominent role of glycine receptors at a high sevoflurane concentration.

CONCLUSIONS

At high concentrations, sevoflurane mediates neuronal inhibition in the spinal ventral horn primarily via glycine receptors, and less via GABA(A) receptors. Our results support the hypothesis that the impact of GABA(A) receptors in mediating the immobilizing properties of volatile anesthetics is less essential in comparison to glycine receptors.

Behavior-related pauses in simple-spike activity of mouse Purkinje cells are linked to spike rate modulation

Purkinje cells (PCs) in the mammalian cerebellum express high-frequency spontaneous activity with average spike rates between 30 and 200 Hz. Cerebellar nuclear (CN) neurons receive converging input from many PCs, resulting in a continuous barrage of inhibitory inputs. It has been hypothesized that pauses in PC activity trigger increases in CN spiking activity. A prediction derived from this hypothesis is that pauses in PC simple-spike activity represent relevant behavioral or sensory events. Here, we asked whether pauses in the simple-spike activity of PCs related to either fluid licking or respiration, play a special role in representing information about behavior. Both behaviors are widely represented in cerebellar PC simple-spike activity. We recorded PC activity in the vermis and lobus simplex of head-fixed mice while monitoring licking and respiratory behavior. Using cross-correlation and Granger causality analysis, we examined whether short interspike intervals (ISIs) had a different temporal relationship to behavior than long ISIs or pauses. Behavior-related simple-spike pauses occurred during low-rate simple-spike activity in both licking- and breathing-related PCs. Granger causality analysis revealed causal relationships between simple-spike pauses and behavior. However, the same results were obtained from an analysis of surrogate spike trains with gamma ISI distributions constructed to match rate modulations of behavior-related Purkinje cells. Our results therefore suggest that the occurrence of pauses in simple-spike activity does not represent additional information about behavioral or sensory events that goes beyond the simple-spike rate modulations.

Isoflurane enhances both fast and slow synaptic inhibition in the hippocampus at amnestic concentrations

BACKGROUND

Inhibition mediated by γ-aminobutyric acid type A (GABA A) receptors has long been considered an important target for a variety of general anesthetics. In the hippocampus, two types of phasic GABA A receptor-mediated inhibition coexist: GABA A,fast, which is expressed primarily at peri-somatic sites, and GABAA,slow, which is expressed primarily in the dendrites. Their spatial segregation suggests distinct functions: GABA A,slow may control plasticity of dendritic synapses, whereas GABA A,fast controls action potential initiation at the soma. We examined modulation of GABA A,fast and GABA A,slow inhibition by isoflurane at amnesic concentrations, and compared it with modulation by behaviorally equivalent doses of the GABA A receptor-selective drug etomidate.

METHODS

Whole cell recordings were obtained from pyramidal cells in organotypic hippocampal cultures prepared from C57BL/6 × 129/SvJ F1 hybrid mice. GABA A receptor-mediated currents were isolated using glutamate receptor antagonists. GABAA,slow currents were evoked by electrical stimulation in the stratum lacunosum-moleculare. Miniature GABA A,fast currents were recorded in the presence of tetrodotoxin.

RESULTS

100 μM isoflurane (approximately EC50,amnesia) slowed fast- and slow-inhibitory postsynaptic current decay by approximately 25%. Higher concentrations, up to 400 μM, produced proportionally greater effects without altering current amplitudes. The effects on GABA A,slow were approximately one-half those produced by equi-amnesic concentrations of etomidate.

CONCLUSIONS

Isoflurane enhances both types of phasic GABA A receptor-mediated inhibition to similar degrees at amnesic concentrations. This pattern differs from etomidate, which at low concentrations selectively enhances slow inhibition. These effects of isoflurane are sufficiently large that they may contribute substantially to its suppression of hippocampal learning and memory.

Cortical inhibition during burst suppression induced with isoflurane anesthesia

Isoflurane is a widely used anesthetic which safely and reversibly induces deep coma and associated burst suppression (BS) electroencephalographic patterns. Here we investigate possible underlying causes for the state of cortical hyperexcitability which was recently shown to be one of the characteristics of BS. Our hypothesis was that cortical inhibition is diminished during isoflurane-induced BS. Experiments were performed in vivo using intracellular recordings of cortical neurons to assess their responsiveness to stimulations of connected thalamic nuclei. We demonstrate that during BS EPSPs were diminished by 44%, whereas inhibitory potentials were completely suppressed. This finding was supported by additional results indicating that a decrease in neuronal input resistance normally found during inhibitory responses under low isoflurane conditions was abolished in the BS condition. Moreover, removal of inhibition occasionally revealed excitatory components which were absent during recordings before the induction of BS. We also show that the absence of inhibition during BS is not caused by a blockage of GABA receptors, since iontophoretically applied GABA shows receptor availability. Moreover, the concentration of extracellular chloride was increased during BS, as would be expected after reduced flow of chloride through GABA(A) receptors. Also inhibitory responses were reinstated by selective blockage of glial glutamate transporters with dihydrokainate. These results suggest that the lack of inhibition during BS is caused by reduced excitation, probably resulting from increased glial uptake of glutamate stimulated by isoflurane, which creates a diminished activation of cortical interneurons. Thus cortical hyperexcitability during BS is favored by suppressed inhibition.

Brain activity modeling in general anesthesia: enhancing local mean-field models using a slow adaptive firing rate

In this paper, an enhanced local mean-field model that is suitable for simulating the electroencephalogram (EEG) in different depths of anesthesia is presented. The main building elements of the model (e.g., excitatory and inhibitory populations) are taken from Steyn-Ross [M. L. Steyn-Ross, Phys. Rev. E 64, 011917 (2001), D. A. Steyn-Ross, Phys. Rev. E 64, 011918 (2001)] and Bojak and Liley [I. Bojak and D. T. Liley, Phys. Rev. E 71, 041902 (2005)] mean-field models and a new slow ionic mechanism is included in the main model. Generally, in mean-field models, some sigmoid-shape functions determine firing rates of neural populations according to their mean membrane potentials. In the enhanced model, the sigmoid function corresponding to excitatory population is redefined to be also a function of the slow ionic mechanism. This modification adapts the firing rate of neural populations to slow ionic activities of the brain. When an anesthetic drug is administered, the slow mechanism may induce neural cells to alternate between two levels of activity referred to as up and down states. Basically, the frequency of up-down switching is in the delta band (0-4 Hz) and this is the main reason behind high amplitude, low frequency fluctuations of EEG signals in anesthesia. Our analyses show that the enhanced model may have different working states driven by anesthetic drug concentration. The model is settled in the up state in the waking period, it may switch to up and down states in moderate anesthesia while in deep anesthesia it remains in the down state.

Etomidate reduces initiation of backpropagating dendritic action potentials: implications for sensory processing and synaptic plasticity during anesthesia

Anesthetics may induce specific changes that alter the balance of activity within neural networks. Here we describe the effects of the GABA(A) receptor potentiating anesthetic etomidate on sensory processing, studied in a cerebellum-like structure, the electrosensory lateral line lobe (ELL) of mormyrid fish, in vitro. Previous studies have shown that the ELL integrates sensory input and removes predictable features by comparing reafferent sensory signals with a descending electromotor command-driven corollary signal that arrives in part through parallel fiber synapses with the apical dendrites of GABAergic interneurons. These synapses show spike timing-dependent depression when presynaptic activation is associated with postsynaptic backpropagating dendritic action potentials. Under etomidate, almost all neurons become tonically hyperpolarized. The threshold for action potential initiation increased for both synaptic activation and direct intracellular depolarization. Synaptically evoked inhibitory postsynaptic potentials (IPSPs) were also strongly potentiated and prolonged. Current source density analysis showed that backpropagation of action potentials through the apical dendritic arborization in the molecular layer was reduced but could be restored by increasing stimulus strength. These effects of etomidate were blocked by bicuculline or picrotoxin. It is concluded that etomidate affects both tonic and phasic inhibitory conductances at GABA(A) receptors and that increased shunting inhibition at the level of the proximal dendrites also contributes to increasing the threshold for action potential backpropagation. When stimulus strength is sufficient to evoke backpropagation, repetitive association of synaptic excitation with postsynaptic action potential initiation still results in synaptic depression, showing that etomidate does not interfere with the molecular mechanism underlying plastic modulation.

Sensory and motor effects of etomidate anesthesia

The effects of anesthesia with etomidate on the cellular mechanisms of sensory processing and sensorimotor coordination have been studied in the active electric sense of the mormyrid fish Gnathonemus petersii. Like many anesthetics, etomidate is known to potentiate GABA(A) receptors, but little is known about the effects on sensory processing at the systems level. A better understanding is necessary for experimental studies of sensory processing, in particular regarding possible effects on the dynamic structure of excitatory and inhibitory receptive fields and to improve the knowledge of the mechanisms of anesthesia in general. Etomidate slowed the electromotor discharge rhythm, probably because of feedback inhibition at the premotor level, but did not alter the structure of the electromotor command. Sensory translation through primary afferents projecting to the cerebellum-like electrosensory lobe (ELL) was not changed. However, central interneurons and projection neurons were hyperpolarized under etomidate, and their spiking activity was reduced. Although the spatial extent and the center/surround organization of sensory receptive fields were not changed, initial excitatory responses were followed by prolonged inhibition. Corollary discharge input to ELL was maintained, and the temporal sequence of excitatory and inhibitory components of this descending signal remained intact. Later inhibitory corollary discharge responses were prolonged by several hundred milliseconds. The result was that excitatory reafferent sensory input was conserved with enhanced precision of timing, whereas background activity was greatly reduced. Anti-Hebbian synaptic plasticity evoked by association of sensory and corollary discharge input was still present under anesthesia, and differences compared with the nonanesthetized condition are discussed.

Subunit-dependent block by isoflurane of wild-type and mutant α1S270H GABAA receptor currents in Xenopus oocytes

The volatile anesthetic isoflurane both prolongs and reduces the amplitude of GABA-mediated inhibitory postsynaptic currents (IPSCs) recorded in neurons. To explore the latter effect, we investigated isoflurane-induced inhibition of steady-state desensitized GABA currents in Xenopus oocytes expressing wild-type alpha(1)beta(2), alpha(1)beta(2)gamma(2s), mutant alpha(1)(S270H)beta(2) (serine to histidine at residue 270) or alpha(1)(S270H)beta(2)gamma(2s) receptors. The alpha(1) serine 270 site in TM2 (second transmembrane domain of the subunit) is postulated as a binding site for some volatile agents and is critical for positive modulation of sub-maximal GABA responses by isoflurane. For all receptor combinations, at < or =0.6 mM isoflurane (< or =2 minimum alveolar concentration (MAC)) current inhibitions were not pronounced ( approximately 10%) with block reaching half-maximal levels at supraclinical concentrations ( approximately 2 mM isoflurane, 6 MAC). Comparisons with other GABA(A) receptor blockers indicated that isoflurane blocks in a similar manner to picrotoxin, possibly via the pore of the receptor. The extent of isoflurane-induced inhibition was significantly attenuated by inclusion of the gamma(2s)-subunit but was unaffected by introduction of the S270H mutation in the alpha(1)-subunit. In conclusion, isoflurane binds with low affinity and with subunit-specificity to an inhibitory site on the GABA(A) receptor that is distinct from the site that facilitates positive modulation at the extracellular end of the pore.

Modulation of gamma-aminobutyric acid type A receptor-mediated spontaneous inhibitory postsynaptic currents in auditory cortex by midazolam and isoflurane

BACKGROUND

Anesthetic agents that target gamma-aminobutyric acid type A (GABA(A)) receptors modulate cortical auditory evoked responses in vivo, but the cellular targets involved are unidentified. Also, for agents with multiple protein targets, the relative contribution of modulation of GABA(A) receptors to effects on cortical physiology is unclear. The authors compared effects of the GABA(A) receptor-specific drug midazolam with the volatile anesthetic isoflurane on spontaneous inhibitory postsynaptic currents (sIPSCs) in pyramidal cells of auditory cortex.

METHODS

Whole cell recordings were obtained in murine brain slices at 34 degrees C. GABA(A) sIPSCs were isolated by blocking ionotropic glutamate receptors. Effects of midazolam and isoflurane on time course, amplitude, and frequency of sIPSCs were measured.

RESULTS

The authors detected no effect of midazolam at 0.01 microM on sIPSCs, whereas midazolam at 0.1 and 1 microM prolonged the decay of sIPSCs by approximately 25 and 70%, respectively. Isoflurane at 0.1, 0.25, and 0.5 mm prolonged sIPSCs by approximately 45, 150, and 240%, respectively. No drug-specific effects were observed on rise time or frequency of sIPSCs. Isoflurane at 0.5 mm caused a significant decrease in sIPSC amplitude.

CONCLUSIONS

The dose dependence of isoflurane effects on GABA(A) sIPSCs in pyramidal cells is consistent with effects on auditory evoked response in vivo. By contrast, comparable effects of midazolam on GABA(A) sIPSCs arise at concentrations exceeding those currently thought to be achieved in vivo, suggesting that the cellular targets of midazolam reside elsewhere in the thalamocortical circuit or that the concentration of midazolam reached in the brain is higher than currently believed.

GABAA receptor modulation by the novel intravenous general anaesthetic E-6375

E-6375 (4-butoxy-2-[4-(2-cyanobenzoyl)-1-piperazinyl] pyrimidine hydrochloride) is a new intravenous general anaesthetic with an anaesthetic potency, in mice, comparable to propofol, or etomidate. Here, we examined the effect of E-6375 upon the GABAA receptor, a putative target of intravenous anaesthetic action. E-6375 reversibly enhanced GABA-evoked currents mediated by recombinant GABAA (alpha1beta2gamma2L) receptors expressed in Xenopus laevis oocytes, with little effect on NMDA- and kainate-evoked currents mediated by NR1a/NR2A and GluR1o/GluR2o glutamate receptors, respectively. E-6375 prolonged the decay of GABA-evoked miniature inhibitory postsynaptic currents recorded from rat Purkinje neurones demonstrating the anaesthetic also enhanced the activity of synaptic GABAA receptors. The GABA enhancing action of E-6375 on recombinant GABAA receptors was unaffected by the subtype of the alpha isoform (i.e. alphaxbeta2gamma2L; x=1-3) within the receptor, but was increased by the omission of the gamma2L subunit. Receptors incorporating beta2, or beta3, subunits were more sensitive to modulation by E-6375 than those containing the beta1 subunit. The selectivity of E-6375 was largely governed by the identity (serine or asparagine) of a single amino acid residue within the second transmembrane domain of the beta-subunit. The various in vivo actions of general anaesthetics may be mediated by GABAA receptor isoforms that have a differential distribution within the CNS. The identification of agents, such as E-6375, that discriminate between GABAA receptor subtypes may augur the development of general anaesthetics with an improved therapeutic profile.

The effects of general anaesthetics on carbachol-evoked gamma oscillations in the rat hippocampus in vitro

The effects of general anaesthetics and temperature on carbachol-evoked gamma oscillations in the rat hippocampal brain slice preparation were investigated. The frequency of the oscillations was found to be dependent on temperature in the range 32-25 degrees C, with a linear reduction in frequency from 40-17 Hz over this temperature range. The volatile anaesthetics isoflurane and halothane, and the intravenous anaesthetics thiopental, propofol and R(+)-etomidate caused a reduction in the frequency of the oscillations, in a concentration-dependent manner, over a range of clinically relevant concentrations. On the other hand, the intravenous agent ketamine and the "inactive" S(-)-isomer of etomidate had no significant effect on the oscillation frequency. The oscillations were markedly asymmetric over one cycle with a relatively rapid "rising" phase followed by a slower "decaying" phase. The decrease in oscillation frequency was due to an increase in the time-course of the "decaying phase" of the oscillation with little effect on the "rising" phase, consistent with the idea that carbachol-evoked gamma oscillations are trains of GABAergic inhibitory postsynaptic potentials and that the anaesthetics are acting postsynaptically at the GABA(A) receptor.

Passive spatial and temporal integration of excitatory synaptic inputs in cerebellar Purkinje cells of young rats

We have investigated the integration of excitatory (parallel fiber) synaptic inputs in cerebellar Purkinje cells of young rats in vitro and in a compartmental model of such a cell, based on 3D morphological reconstruction. Excitatory synaptic inputs at two independent dendritic sites were activated by electrical stimulation with various delays between the two stimuli. Population postsynaptic potentials summed linearly under current clamp condition when the two dendritic input sites were spatially separated (>200 microm) but sublinearly, in a delay dependent manner, when the input sites were close (<50 microm) to each other. Population postsynaptic currents measured under voltage clamp conditions summed linearly independent of the spatial and temporal separation of inputs. Summation of inputs in a passive compartmental model of a Purkinje cell was similar to that of Purkinje cells in vitro. We show that sublinear summation of neighboring inputs is independent of inhibitory mechanisms and suggest that sublinearity is mainly due to a locally reduced driving force.

Pre- and postsynaptic volatile anaesthetic actions on glycinergic transmission to spinal cord motor neurons

1. A common anaesthetic endpoint, prevention of withdrawal from a noxious stimulus, is determined primarily in spinal cord, where glycine is an important inhibitory transmitter. To define pre- and postsynaptic anaesthetic actions at glycinergic synapses, the effects of volatile anaesthetic agents on spontaneous and evoked glycinergic currents in spinal cord motor neurons from 6 - 14-day old rats was investigated. 2. The volatile anaesthetic agents enflurane, isoflurane and halothane significantly increased the frequency of glycinergic mIPSCs, enflurane to 190.4% of control+/-22.0 (mean+/-s.e.m., n=7, P<0.01), isoflurane to 199.0%+/-28.8 (n=7, P<0.05) and halothane to 198.2%+/-19.5 (n=7, P<0.01). However without TTX, isoflurane and halothane had no significant effect and enflurane decreased sIPSC frequency to 42.5% of control+/-12.4 (n=6, P<0.01). All the anaesthetics prolonged the decay time constant (tau) of both spontaneous and glycine-evoked currents without increasing amplitude. With TTX total charge transfer was increased; without TTX charge transfer was unchanged (isoflurane and halothane) or decreased (enflurane). 3. Enflurane-induced mIPSC frequency increases were not significantly affected by Cd(2+) (50 microM), thapsigargin (1 - 5 microM), or KB-R7943 (5 microM). KB-R7943 and thapsigargin together abolished the enflurane-induced increase in mIPSC frequency. 4. There are opposing facilitatory and inhibitory actions of volatile anaesthetics on glycine release dependent on calcium homeostatic mechanisms and sodium channels respectively. Under normal conditions (no TTX) the absolute amount of glycinergic inhibition does not increase. The contribution of glycinergic inhibition to anaesthesia may depend on its duration rather than its absolute magnitude.

Sevoflurane potentiates and blocks GABA-induced currents through recombinant alpha1beta2gamma2 GABAA receptors: implications for an enhanced GABAergic transmission

BACKGROUND AND OBJECTIVE

The gamma-aminobutyric acidA receptor (GABAAR) is a target for anaesthetic agents. We investigated the interactions of sevoflurane with a recombinant GABAAR. Emphasis was on the mechanism of block, as relevant open-channel block by a volatile anaesthetic would possibly explain prolonged GABAergic postsynaptic currents.

METHODS

The effect of sevoflurane on GABA-induced currents through recombinant alpha1beta2gamma2 GABAAR channels was studied (patch clamp; HEK293 cells). GABA 0.01 mM or 1 mM was applied alone or together with sevoflurane (0.05 mM to 5 mM).

RESULTS

Currents elicited by GABA 0.01 mM were increased by low sevoflurane concentrations to 183% and decreased by high sevoflurane concentrations (> 1 mM) to 34% (P < 0.05). Ten- to 90%-rise times of the currents were reduced by sevoflurane concentration dependently. At GABA (1 mM), peak currents and 10-90%-rise times decreased with increasing sevoflurane concentrations. A transient current increase was induced by discontinuation of GABA and sevoflurane. Such rebound currents indicate a reversal of an open-channel block by sevoflurane.

CONCLUSIONS

Sevoflurane (a) increases the apparent affinity of GABA to the GABAAR, as suggested by the decreased current rise times. This explains the enhancement of the currents induced by low GABA concentrations (0.01 mM). Additionally, sevoflurane (b) induces a picrotoxin-like open-channel block at the GABAAR. The reversal of the open-channel block elicits a delayed GABA response. These findings indicate at least two different sites of action of sevoflurane at this receptor that are both important for an enhanced GABAergic synaptic transmission.

Is anesthesia caused by potentiation of synaptic or intrinsic inhibition? Recent insights into the mechanisms of volatile anesthetics

Volatile anesthetics modulate synaptic (GABAA receptor-mediated) and intrinsic (K+ channel-controlled) neuronal inhibition. GABAA receptor activity is enhanced, leading to increased charge transfer and prolonged synaptic inhibition, and members of the two pore domain family of potassium channels are activated, leading to neuronal hyperpolarization and reduced excitability. These effects may underlie different components of the complex anesthetic state.

Membrane and synaptic actions of halothane on rat hippocampal pyramidal neurons and inhibitory interneurons

A relatively small number of inhibitory interneurons can control the excitability and synchronization of large numbers of pyramidal neurons in hippocampus and other cortical regions. Thus, anesthetic modulation of interneurons could play an important role during anesthesia. The aim of this study was to investigate effects of a general anesthetic, halothane, on membrane and synaptic properties of rat hippocampal interneurons. GABA receptor-mediated IPSCs were recorded with whole-cell patch-clamp techniques in visually identified CA1 pyramidal cells and interneurons located at the border of stratum lacunosum-moleculare and stratum radiatum. Halothane (0.35 mm congruent with 1.2 vol%) depressed evoked IPSC amplitudes recorded from both pyramidal cells and inhibitory interneurons. Also, halothane considerably prolonged the decay time constant of evoked IPSCs in pyramidal cells and interneurons. The frequencies of miniature IPSCs were increased by halothane (two- to threefold) in both types of neuron. On the other hand, halothane effects on resting membrane potentials were variable but minimal in both types of neurons. In current-clamp recordings, halothane depressed EPSP amplitudes and increased IPSP amplitudes recorded from both types of neurons. In addition, halothane increased the failure rate of synaptically evoked action potentials. Taken together, these data provide evidence that halothane increases GABA(A) receptor-mediated synaptic inhibition between synaptically connected interneurons and depresses excitatory transmission, similar to effects observed in pyramidal neurons.

Localization of the tandem pore domain K+ channel TASK-1 in the rat central nervous system

Recently, a new family of potassium channels with two pore domains in tandem and four transmembrane segments has been identified. Seven functional mammalian channels have been reported at this time. These channels give rise to baseline potassium currents because they are not gated by voltage and exhibit spontaneous activity at all membrane potentials. Although the physiological role of these ion channels has yet to be determined, three mammalian members of this family (TREK-1, TASK-1, TASK-2) are activated by volatile anesthetics and may therefore contribute to the central nervous system (CNS) depression produced by volatile anesthetics. In this study we used northern blot analysis and immunohistochemical localization to determine the expression of TASK-1 subunits in the CNS. TASK-1 immunoreactivity was prominently found in astrocytes of the hippocampus, in the median eminence, in the choroid plexus, and the granular layer, Purkinje cell layer, and molecular layer of the cerebellum. In the spinal cord, strong TASK-I immunoreactivity was seen in ependymal cells lining the central canal and in white matter. These findings suggest a role for the TASK-1 channel in the production of cerebrospinal fluid and function of hypothalamic neurosecretory cells.

Effects of halothane on GABA(A) receptor kinetics: evidence for slowed agonist unbinding

Many anesthetics, including the volatile agent halothane, prolong the decay of GABA(A) receptor-mediated IPSCs at central synapses. This effect is thought to be a major factor in the production of anesthesia. A variety of different kinetic mechanisms have been proposed for several intravenous agents, but for volatile agents the kinetic mechanisms underlying this change remain unknown. To address this question, we used rapid solution exchange techniques to apply GABA to recombinant GABA(A) receptors (alpha(1)beta(2)gamma(2s)) expressed in HEK 293 cells, in the absence and presence of halothane. To differentiate between different microscopic kinetic steps that may be altered by the anesthetic, we studied a variety of measures, including peak concentration-response characteristics, macroscopic desensitization, recovery from desensitization, maximal current activation rates, and responses to the low-affinity agonist taurine. Experimentally observed alterations were compared with predictions based on a kinetic scheme that incorporated two agonist binding steps, and open and desensitized states. We found that, in addition to slowing deactivation after a brief pulse of GABA, halothane increased agonist sensitivity and slowed recovery from desensitization but did not alter macroscopic desensitization or maximal activation rate and only slightly slowed rapid deactivation after taurine application. This pattern of responses was found to be consistent with a reduction in the microscopic agonist unbinding rate (k(off)) but not with changes in channel gating steps, such as the channel opening rate (beta), closing rate (alpha), or microscopic desensitization. We conclude that halothane slows IPSC decay by slowing dissociation of agonist from the receptor.

Effects of volatile anaesthetics on the membrane potential and ion channels of cultured neocortical astrocytes

Volatile anaesthetics cause changes in the membrane resting potential of central neurons. This effect probably arises from actions on neuronal ion channels, but may also involve alterations in the ion composition of the extracellular space. Since glial cells play a key role in regulating the extracellular ion composition in the brains of mammals, we analyzed the effects of halothane, isoflurane and enflurane on the membrane conductances and ion channels of cultured cortical astrocytes. Astrocytes were dissociated from the neocortex of 0-2-day old rats and grown in culture for 3-4 weeks. Anaesthetic-induced changes in the membrane potential were recorded in the whole cell current-clamp configuration of the patch-clamp technique. We further studied the effects of halothane and enflurane on single ion channels in excised membrane patches. At concentrations corresponding to 1-2 MAC (1 MAC induces general anaesthesia in 50% of the patients and rats), membrane potentials recorded in the presence of enflurane, isoflurane and halothane did not differ significantly from the control values. At higher concentrations, effects of enflurane and halothane, but not of isoflurane, were statistically significant. Single-channel recordings revealed that halothane and enflurane activated a high conductance anion channel, which possibly mediated the effects observed during whole cell recordings. In less than 10% of the membrane patches, volatile anaesthetics either increased or decreased the mean open time of K+-selective ion channels without altering single-channel conductances. In summary, it seems unlikely that the actions of volatile anaesthetics described here are involved in the state of general anaesthesia. Statistically significant effects occurred at concentrations ten times higher than those required to cause half-maximal depression of action potential firing of neocortical neurons in cultured brain slices. However, it cannot be excluded that the changes observed in the membrane conductance of cortical astrocytes disturb the physiological function of these cells, thereby influencing the membrane resting potential of neurons.

Cellular mechanisms of gamma rhythms in rat neocortical brain slices probed by the volatile anaesthetic isoflurane

In hippocampus slice preparations, the frequency of gamma oscillations (30-80 Hz) is inversely related to the decay time of GABA(A)-mediated inhibitory postsynaptic currents (IPSCs), suggesting a causal relationship [Jefferys, J.R.G., Traub, R.D. and Whittington, M.A., Neuronal networks for induced '40 Hz' rhythms, Trends Neurosci., 19 (1996) 202-207]. We determined the effects of the commonly used volatile anaesthetic isoflurane on IPSCs monitored from neocortical and cerebellar neurons, as well as on gamma oscillations detected in the field potential of neocortical brain slices. At 0.16 mM, the anaesthetic lengthened IPSC-decays by a factor of 1.7 and decreased the frequency of gamma oscillations by 41%. Quantitatively equivalent effects of isoflurane on gamma rhythms were discovered during encephalographic recordings from human patients [Munglani, R., Andrade, J., Sapsford, D.J., Baddeley, A. and Jones, J.G., Br. J. Anaesth., 71 (1993) 633-641].