General anesthetic action on gamma-aminobutyric acid-activated channels

Insights into the structure and pharmacology of GABA(A) receptors

GABA is the major inhibitory neurotransmitter in the adult mammalian CNS. The ionotropic GABA type A receptors (GABA(A)Rs) belong to the Cys-loop family of receptors. Each member of the family is a large pentameric protein in which each subunit traverses the cell membrane four times. Within this family, the GABA type A receptors are particularly important for their rich pharmacology as they are targets for a range of therapeutically important drugs, including the benzodiazepines, barbiturates, neuroactive steroids and anesthetics. This review discusses new insights into receptor properties that allow us to begin to relate the structure of an individual receptor to its functional and pharmacological properties.

Halothane and propofol modulation of gamma-aminobutyric acidA receptor single-channel currents

Halothane and propofol enhance the activity of the gamma-aminobutyric acid (GABA) system, which is one of the most important systems in the mechanism of anesthesia. To determine whether halothane and propofol enhance GABAergic responses by the same mechanism, we performed single-channel patch-clamp experiments with rat cortical neurons in primary culture. Each of the open-time and closed-time distributions of GABA(A) receptor single channels was expressed by a sum of fast and slow time constants. Neither halothane nor propofol changed the single-channel conductance. Halothane increased the probability of the channel being open via a prolongation of the slow phase of open time, whereas propofol increased the channel open probability via a shortening of the slow phase of closed time. Thus, although both halothane and propofol augmented the channel open probability, thereby causing an increase in charge transfer during inhibitory transmitter action, they acted by different mechanisms.

Anesthetic and ethanol effects on spontaneously opening glycine receptor channels

Strychnine-sensitive glycine receptors mediate inhibitory neurotransmission occurring in the brain stem and spinal cord. Alcohols, volatile anesthetics and inhaled drugs of abuse are positive allosteric modulators of glycine receptor function, normally enhancing function only in the presence of glycine. A complication in studying allosteric actions on ligand-gated ion channels is in the dissection of their effects on neurotransmitter binding from their effects on channel opening. Mutation of an aspartate residue at position 97 to arginine in the glycine receptor alpha1 subunit simulated the effects of glycine binding, producing receptors that exhibited tonic channel opening in the absence of neurotransmitter; i.e. these receptors demonstrated a dissociation of channel opening from neurotransmitter binding. In these receptors, ethanol, enflurane, chloroform, halothane, 1,1,1-trichloroethane and toluene elicited inward currents in the absence of glycine. We previously identified mutations on ligand-gated ion channels that eliminate ethanol, anesthetic and inhalant actions (such as S267I on alpha1 glycine receptors). The double mutant (D97R and S267I) receptors were both constitutively active and resistant to the enhancing effects of ethanol and enflurane. These data demonstrate that ethanol and volatile anesthetics can affect glycine receptor channel opening independently of their effects on enhancing neurotransmitter binding.

Modulation of neuronal nicotinic acetylcholine receptors by halothane in rat cortical neurons

Inhalational general anesthetics have recently been shown to inhibit neuronal nicotinic acetylcholine (ACh) receptors (nnAChRs) expressed in Xenopus laevis oocytes and in molluscan neurons. However, drug actions on these systems are not necessarily the same as those seen on native mammalian neurons. Thus, we analyzed the detailed mechanisms of action of halothane on nnAChRs using rat cortical neurons in long-term primary culture. Currents induced by applications of ACh via a U-tube system were recorded by the whole-cell, patch-clamp technique. ACh evoked two types of currents, alpha-bungarotoxin-sensitive, fast desensitizing (alpha 7-type) currents and alpha-bungarotoxin-insensitive, slowly desensitizing (alpha 4 beta 2-type) currents. Halothane suppressed alpha 4 beta 2-type currents more than alpha 7-type currents with IC(50) values of 105 and 552 microM, respectively. Halothane shifted the ACh dose-response curve for the alpha 4 beta 2-type currents in the direction of lower ACh concentrations and slowed its apparent rate of desensitization. The rate of recovery after washout from halothane block was much faster than the rate of recovery from ACh desensitization. Thus, the halothane block was not caused by receptor desensitization. Chlorisondamine, an irreversible open channel blocker for nnAChRs, caused a time-dependent block that was attenuated by halothane. These results could be accounted for by kinetic simulation based on a model in which halothane causes flickering block of open channels, as seen in muscle nAChRs. Halothane block of nnAChRs is deemed to play an important role in anesthesia via a direct action on the receptor and an indirect action to suppress transmitter release.

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.

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.

Anisatin modulation of the gamma-aminobutyric acid receptor-channel in rat dorsal root ganglion neurons

1. Anisatin, a toxic, insecticidally active component of Sikimi plant, is known to act on the GABA system. In order to elucidate the mechanism of anisatin interaction with the GABA system, whole-cell and single-channel patch clamp experiments were performed with rat dorsal root ganglion neurons in primary culture. 2. Repeated co-applications of GABA and anisatin suppressed GABA-induced whole-cell currents with an EC50 of 1.10 microM. No recovery of currents was observed after washout with anisatin-free solution. 3. However, pre-application of anisatin through the bath had no effect on GABA-induced currents. The decay phase of currents was accelerated by anisatin. These results indicate that anisatin suppression of GABA-induced currents requires opening of the channels and is use-dependent. 4. Anisatin suppression of GABA-induced currents was not voltage dependent. 5. Picrotoxinin attenuated anisatin suppression of GABA-induced currents. [3H]-EBOB binding to rat brain membranes was competitively inhibited by anisatin. These data indicated that anisatin bound to the picrotoxinin site. 6. At the single-channel level, anisatin did not alter the open time but prolonged the closed time. The burst duration was reduced and channel openings per burst were decreased indicating that anisatin decreased the probability of openings.

Brain mechanisms of propofol-induced loss of consciousness in humans: a positron emission tomographic study

In the present study, we used positron emission tomography to investigate changes in regional cerebral blood flow (rCBF) during a general anesthetic infusion set to produce a gradual transition from the awake state to unconsciousness. Five right-handed human volunteers participated in the study. They were given propofol with a computer-controlled infusion pump to achieve three stable levels of plasma concentrations corresponding to mild sedation, deep sedation, and unconsciousness, the latter defined as unresponsiveness to verbal commands. During awake baseline and each of the three levels of sedation, two scans were acquired after injection of an H215O bolus. Global as well as regional CBF were determined and correlated with propofol concentrations. In addition, blood flow changes in the thalamus were correlated with those of the entire scanned volume to determine areas of coordinated changes. In addition to a generalized decrease in global CBF, large regional decreases in CBF occurred bilaterally in the medial thalamus, the cuneus and precuneus, and the posterior cingulate, orbitofrontal, and right angular gyri. Furthermore, a significant covariation between the thalamic and midbrain blood flow changes was observed, suggesting a close functional relationship between the two structures. We suggest that, at the concentrations attained, propofol preferentially decreases rCBF in brain regions previously implicated in the regulation of arousal, performance of associative functions, and autonomic control. Our data support the hypothesis that anesthetics induce behavioral changes via a preferential, concentration-dependent effect on specific neuronal networks rather than through a nonspecific, generalized effect on the brain.

Post-synaptic inhibitory mechanisms of anaesthesia; glycine receptors

(1) The effects on human homomeric alpha1 glycine receptors of 11 general anaesthetics; four barbiturates, two other intravenous anaesthetics, three volatile anaesthetics and two simple gaseous anaesthetics are described. (2) Pentobarbital and thiopental potentiate the current response to bath applied glycine (50 microM) by 200 and 300%, respectively, at clinically relevant concentrations. (3) Neither methohexital nor phenobarbital had any effect on the current response to bath applied glycine (50 microM). (4) Using maximal doses of applied glycine (1 mM) all the barbiturates acted as non-competitive antagonists. (5) Propofol and etomidate potentiate the current response to bath applied glycine (50 microM) by 200 and 10%, respectively, at clinically relevant concentrations. (6) Etomidate acts as a non-competitive antagonist for doses of glycine above the EC50 (197 microM). Propofol was without effect using maximal doses of applied glycine (1 mM). (7) Halothane, chloroform and ether potentiated the response to bath applied glycine (50 microM) by 200, 100 and 200%, respectively, at clinically relevant doses. (8) None of the volatile anaesthetics had any effect using maximal doses of applied glycine (1 mM). (9) Nitrous oxide and xenon potentiated the response to bath applied glycine (50 microM) by 75 and 50%, respectively, at clinically relevant doses. Nitrous oxide also potentiated the response using maximal doses of applied glycine (1 mM). (10) These results suggest a role for glycinergic neurotransmission in the production of anaesthesia.

Interactions of general anesthetics within the pore of an ion channel

(1) We review the electrophysiological evidence that the ion channel pore is the site at which general anesthetics bind to inhibit muscle-type acetylcholine receptor channels. (2) The amphipathic character of a pore certainly offers a suitable environment for the binding of amphipathic anesthetics. (3) The absence of direct information about the binding sites of these rather non-specific drugs, forces us to rely on indirect information provided by kinetic experiments. (4) We also discuss the implications of these findings for the interaction of general anesthetics with other ion channels.

What the actions of anaesthetics on fast synaptic transmission reveal about the molecular mechanism of anaesthesia

1. Synapses with the brain are important components of the networks responsible for higher nervous function and current evidence suggests that general anaesthetics modulate synaptic transmission in the brain. 2. Analysis of anaesthetic action on these synapses not only defines the cellular mechanisms involved in anaesthesia but also reveals much about the molecular targets of anaesthetic action. 3. It appears that while anaesthetics affect a wide variety of processes, the most sensitive are those which are directly linked to the activity of ligand-gated ion channels. Moreover, both single channel patch clamp studies and the molecular biological investigations of the sub-unit specificity of the sensitivity to anaesthetics indicate that anaesthetics interact directly with these functional proteins.

Cutaneous responsiveness of lumbar spinal dorsal horn neurons is reduced by general anesthesia, an effect dependent in part on GABAA mechanisms

Extracellular activity was recorded from single spinal dorsal horn neurons in both chronic cat and acute rat models. This was done to define the effects of anesthesia on the processing of sensory information elicited by nonnoxious tactile stimulation of peripheral receptive fields (RFs). In the chronic cat model, baseline data were obtained in physiologically intact, awake, drug-free animals before anesthetic administration (halothane 1.0-2.0%). This made it possible to compare and contrast activity of each cell in the drug-free and anesthetized state. Halothane effects were confirmed in the acute rat model (anesthetized, spinally transected, and in some cases decerebrate). In addition, the gamma-aminobutyic acid-A (GABAA)-receptor antagonist picrotoxin (2 mg/kg) was administered intravenously to verify that the observed halothane effect on spinal dorsal horn neurons was mediated by an interaction with GABAA-receptor systems. Halothane effects on three separate measures of response to nonnoxious tactile stimuli were observed in the chronic cat model. Halothane produced a significant, dose-dependent reduction in the low-threshold RF area of the neurons studied. Halothane also caused a significant reduction in neuronal response to RF brushing (dynamic stimulus) and to maintained contact with the RF (static stimulus). A dose dependency was not observed with these latter two effects. Neurons with a predominant rapidly adapting response seemed to be less susceptible to halothane suppression than slowly adapting cells. In the acute rat model an increase in halothane caused a reduction in neuronal response similar to that seen in the cat. The intravenous administration of 2 mg/kg of picrotoxin by itself caused no significant change in RF size or response to brushing. However, the same amount of picrotoxin did cause a 50% reversal of the halothane-induced reduction in RF size without causing a significant change in the halothane effect on response to RF brushing. In contrast to work recently reported in a chronic sheep model, halothane causes a significant reduction in spinal dorsal horn neuronal response to tactile stimulation of peripheral RFs. This effect is caused by, in part, but not exclusively, to GABAA-neurotransmitter systems. However, the relative influence of GABAA systems may vary with the nature of the stimulus.

Potentiation by sevoflurane of the gamma-aminobutyric acid-induced chloride current in acutely dissociated CA1 pyramidal neurones from rat hippocampus

1. The effects of a new kind of volatile anaesthetic, sevoflurane (Sev), on gamma-aminobutyric acid (GABA)-gated chloride current (Icl) in single neurones dissociated from the rat hippocampal CA1 area were examined using the nystatin perforated patch recording configuration under the voltage-clamp condition. All drugs were applied with a rapid perfusion system, termed the "Y-tube' method. 2. When the concentrations were higher than 3 x 10(-4) M, Sev, itself, induced an inward current (ISev) at a holding potential (VH) of -40 mV. The concentration-response curve of ISev was bell-shaped, with a suppressed peak and plateau currents at high concentrations (above 2 x 10(-3) M). The reversal potential of ISev (ESev) was close to the theoretical Cl- equilibrium potential, indicating that ISev was carried mainly by Cl-. 3. ISev was reversibly blocked by bicuculline (Bic), an antagonist of the GABAA receptor, in a concentration-dependent manner with a half-inhibitory concentration (IC50) of 7.2 x 10(-7) M. But ISev was insensitive to strychnine (Str), an antagonist of the glycine receptor. 4. At low concentrations (between 3 x 10(-4) and 10(-3) M), Sev markedly enhanced the 10(-6) M GABA induced current (IGABA) but reduced the IGABA with accelerating desensitization accompanied by a "hump' current after washout at high concentrations (higher than 2 x 10(-3) M). 5. Sev, 10(-3) M potentiated the current induced by low concentrations of GABA (between 10(-7) and 3 x 10(-6) M) but reduced the current induced by high concentrations (higher than 10(-5) M) of GABA with a clear acceleration of IGABA desensitization. 6. Sev, like pentobarbitone (PB), pregnanolone (PGN) or diazepam (DZP), potentiated the 10(-6) M GABA-induced response without shifting the reversal potential of IGABA. 7. ISev was augmented by PB, PGN, or DZP at concentrations that maximally potentiated IGABA, suggesting that Sev enhanced IGABA at a binding site distinct from that for PB, PGN, or DZP. 8. It is concluded that Sev acts on the GABAA receptor complex mimicking the GABA-induced chloride current at high concentrations. At low concentrations, Sev enhances GABA-gated chloride current at a binding site independent of the allosteric modulator sites of barbiturates, benzodiazepines or neurosteroids. The reversible potentiation of the inhibitory GABAA receptor-mediated Cl- current may result in the depressing of postsynaptic excitability and may, at least in part, underlie the anaesthetic action of Sev.

Excitatory responses evoked in prefrontal cortex by mediodorsal thalamic nucleus stimulation: influence of anaesthesia

The prefrontal cortex and the mediodorsal thalamic nucleus are reciprocally connected through excitatory amino acid pathways. Cortical excitatory responses resulting from activation of either the mediodorsal thalamic nucleus-prefrontal cortex pathway (short latency) or the recurrent collaterals of prefrontal cortex-mediodorsal thalamic nucleus neurons (long latency) can be discriminated mainly by their latency. The present study was undertaken to compare the effects of halothane and ketamine anaesthesia on these cortical excitatory responses and to establish their pharmacological characteristics using microiontophoretic application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D,L-2-amino-5-phosphonovaleric acid (APV), the specific antagonists of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptors respectively. The number of prefrontal cortex cells which presented short or long latency excitatory responses was smaller in halothane- than in ketamine-anaesthetized rats. Whatever the anaesthetic used, short latency responses were blocked by CNQX and not affected by APV. Long latency responses were mainly blocked by APV and occasionally by CNQX in halothane-anaesthetized rats, while they were only blocked by CNQX in ketamine-anaesthetized animals. Therefore, halothane seems to preferentially reduce evoked responses mediated by AMPA receptors while ketamine completely abolishes evoked responses involving NMDA receptors. Moreover, the present data confirm that excitatory responses resulting from the activation of the mediodorsal thalamic nucleus-prefrontal cortex pathway are mainly mediated by AMPA receptors. In addition, they demonstrate that cortical responses linked to the activation of recurrent collaterals from prefrontal cortex-mediodorsal thalamic nucleus neurons involve both AMPA and NMDA receptors.

Effects of propofol, pentobarbital and alphaxalone on t-[35S]butylbicyclophosphorothionate binding in rat cerebral cortex

The effects of propofol, pentobarbital, alphaxalone, etomidate and diazepam on t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to membrane preparations from rat cerebral cortex were studied in the absence of gamma-aminobutyric acid (GABA). Addition of low concentrations (3-10 microM) of propofol to washed membrane preparations (devoid of GABA) markedly enhanced [35S]TBPS binding (maximal enhancement, 85%), whereas higher concentrations (50-100 microM) inhibited this parameter. Diazepam also enhanced [35S]TBPS binding in this preparation (maximal enhancement, 38%). In contrast, pentobarbital, alphaxalone and etomidate decreased [35S]TBPS binding in a concentration-dependent manner. The propofol-induced increase in [35S]TBPS binding in washed membranes was completely reversed by the addition of GABA at a concentration (0.3 microM) that alone did not modify [35S]TBPS binding (78% increase with 10 microM propofol alone, 33% decrease in the additional presence of GABA). The ability of GABA to reverse the effect of propofol on [35S]TBPS binding in washed membranes was shared by pentobarbital (200 microM) and alphaxalone (3 microM); etomidate (20 microM) only partially antagonized the effect of propofol. Diazepam at a concentration (30 microM) that alone had no effect on [35S]TBPS binding failed to modify the propofol-induced increase in [35S]TBPS binding, whereas at a concentration (3 microM) that alone increased [35S]TBPS binding the effect of diazepam was additive with that of propofol. The addition of bicuculline to washed membranes failed to abolish the increase in [35S]TBPS binding induced by propofol or diazepam, but completely antagonized the effects of pentobarbital, alphaxalone and etomidate.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular and cellular mechanisms of general anaesthesia

General anaesthetics are much more selective than is usually appreciated and may act by binding to only a small number of targets in the central nervous system. At surgical concentrations their principal effects are on ligand-gated (rather than voltage-gated) ion channels, with potentiation of postsynaptic inhibitory channel activity best fitting the pharmacological profile observed in general anaesthesia. Although the role of second messengers remains uncertain, it is now clear that anaesthetics act directly on proteins rather than on lipids.