Mechanisms of etomidate potentiation of GABAA receptor-gated currents in cultured postnatal hippocampal neurons

Etomidate enhances cerebellar CF-PC synaptic plasticity through CB1 receptor/PKA cascade in vitro in mice

Etomidate (ET) is a widely used intravenous imidazole general anesthetic, which depresses the cerebellar neuronal activity by modulating various receptors activity and synaptic transmission. In this study, we investigated the effects of ET on the cerebellar climbing fiber-Purkinje cells (CF-PC) plasticity in vitro in mice using whole-cell recording technique and pharmacological methods. Our results demonstrated that CF tetanic stimulation produced a mGluR1-dependent long-term depression (LTD) of CF-PC excitatory postsynaptic currents (EPSCs), which was enhanced by bath application of ET (10 µM). Blockade of mGluR1 receptor with JNJ16259685, ET triggered the tetanic stimulation to induce a CF-PC LTD accompanied with an increase in paired-pulse ratio (PPR). The ET-triggered CF-PC LTD was abolished by extracellular administration of an N-methyl-(D)-aspartate (NMDA) receptor antagonist, D-APV, as well as by intracellular blockade of NMDA receptors activity with MK801. Furthermore, blocking cannabinoids 1 (CB1) receptor with AM251 or chelating intracellular Ca2+ with BAPTA, ET failed to trigger the CF-PC LTD. Moreover, the ET-triggered CF-PC LTD was abolished by inhibition of protein kinase A (PKA), but not by inhibition of protein kinase C inhibiter. The present results suggest that ET acts on postsynaptic NMDA receptor resulting in an enhancement of the cerebellar CF-PC LTD through CB1 receptor/PKA cascade in vitro in mice. These results provide new evidence and possible mechanism for ET anesthesia to affect motor learning and motor coordination by regulating cerebellar CF-PC LTD.

The Modulation by Anesthetics and Analgesics of Respiratory Rhythm in the Nervous System

Rhythmic eupneic breathing in mammals depends on the coordinated activities of the neural system that sends cranial and spinal motor outputs to respiratory muscles. These outputs modulate lung ventilation and adjust respiratory airflow, which depends on the upper airway patency and ventilatory musculature. Anesthetics are widely used in clinical practice worldwide. In addition to clinically necessary pharmacological effects, respiratory depression is a critical side effect induced by most general anesthetics. Therefore, understanding how general anesthetics modulate the respiratory system is important for the development of safer general anesthetics. Currently used volatile anesthetics and most intravenous anesthetics induce inhibitory effects on respiratory outputs. Various general anesthetics produce differential effects on respiratory characteristics, including the respiratory rate, tidal volume, airway resistance, and ventilatory response. At the cellular and molecular levels, the mechanisms underlying anesthetic-induced breathing depression mainly include modulation of synaptic transmission of ligand-gated ionotropic receptors (e.g., γ-aminobutyric acid, N-methyl-D-aspartate, and nicotinic acetylcholine receptors) and ion channels (e.g., voltage-gated sodium, calcium, and potassium channels, two-pore domain potassium channels, and sodium leak channels), which affect neuronal firing in brainstem respiratory and peripheral chemoreceptor areas. The present review comprehensively summarizes the modulation of the respiratory system by clinically used general anesthetics, including the effects at the molecular, cellular, anatomic, and behavioral levels. Specifically, analgesics, such as opioids, which cause respiratory depression and the "opioid crisis", are discussed. Finally, underlying strategies of respiratory stimulation that target general anesthetics and/or analgesics are summarized.

Etomidate Depresses Spontaneous Complex Spikes Activity of Cerebellar Purkinje Cells via Cannabinoid 1 Receptor in vivo in Mice

INTRODUCTION

Complex spikes (CSs) activity of cerebellar Purkinje cells plays critical roles in motor coordination and motor learning by transferring information to cerebellar cortex, which is an accessible and useful model for neurophysiological investigation. Etomidate is an ultrashort-acting nonbarbiturate intravenous anesthetic, which inhibits the spontaneous activity of cerebellar Purkinje cells through activation of GABAA and glycine receptors in vivo in mice. However, the effect of etomidate on the spontaneous CSs activity of cerebellar Purkinje cells in living mouse is not clear.

METHODS

We here investigated the effects of etomidate on spontaneous CSs activity of cerebellar Purkinje cell in urethane-anesthetized mice by electrophysiology recording technique and pharmacological methods.

RESULTS

Our results showed that cerebellar surface perfusion of etomidate significantly depressed the activity of spontaneous CSs, which exhibited decreases in the number of spikelets and the area under curve (AUC) of the CSs. The etomidate-produced inhibition of CSs activity was persisted in the presence of GABAA and glycine receptors antagonists. However, application of cannabinoid 1 (CB1) receptor antagonist, AM-251, completely blocked the etomidate-induced inhibition of CSs. Furthermore, application of the CB1 receptor agonist, WIN55212-2, induced a decrease of CSs. Moreover, in the presence of a specific protein kinase A (PKA) inhibitor, KT5720, etomidate failed to produce decreases in the spikelets number and the AUC of the spontaneous CSs.

CONCLUSION

These results indicate that cerebellar surface application of etomidate facilitates CB1 receptor activity resulting in a depression of spontaneous CSs activity of Purkinje cells via PKA signaling pathway in mouse cerebellar cortex. Our present results suggest that the etomidate administration may impair the function of cerebellar cortical neuronal circuitry by inhibition of the climbing fiber - Purkinje cells synaptic transmission through activation of CB1 receptors in vivo in mice.

Elucidating Pathway and Anesthetic Mechanism of Action of Clove Oil Nanoformulations in Fish

Clove oil (CO), an essential oil of Syzygium aromaticum, has been reported as an anesthetic for many fish species. However, its insoluble properties require a suitable delivery system for its application. In the present study, nanoformulations of CO as a nanoemulsion (CO-NE), a self-microemulsifying drug-delivery system (CO-SMEDDS), and a self-nanoemulsifying drug-delivery system (CO-SNEDDS) were prepared for delivering CO. Zebrafish were used as a fish model to investigate oil pathways. The result shows fluorescence spots of fluorescence-labeled CO accumulate on the gills, skin, and brain. All CO nanoformulations significantly increased penetration flux compared to CO ethanolic solution. Investigation of the anesthetic mechanism of action using a rat brain γ-aminobutyric acid subtype A (GABA_A) receptor-binding test demonstrates that CO and its major compound, eugenol, modulate [³H]muscimol binding. CO-NE exhibited a concentration-dependent binding activity with an EC₅₀ value of 175 µg/mL, significantly higher than CO solution in dimethyl sulfoxide. In conclusion, CO enters the fish through the skin and gills. The anesthetic mechanism of action of CO is based on modulation of [³H] muscimol binding to GABA_A receptors. Among three nanoformulations tested, CO-NE is the most effective at increasing permeability and enhancing the receptor-binding activity of the oil.

GABAA Receptors in Astrocytes Are Targets for Commonly Used Intravenous and Inhalational General Anesthetic Drugs

Background: Perioperative neurocognitive disorders (PNDs) occur commonly in older patients after anesthesia and surgery. Treating astrocytes with general anesthetic drugs stimulates the release of soluble factors that increase the cell-surface expression and function of GABA_A receptors in neurons. Such crosstalk may contribute to PNDs; however, the receptor targets in astrocytes for anesthetic drugs have not been identified. GABA_A receptors, which are the major targets of general anesthetic drugs in neurons, are also expressed in astrocytes, raising the possibility that these drugs act on GABA_A receptors in astrocytes to trigger the release of soluble factors. To date, no study has directly examined the sensitivity of GABA_A receptors in astrocytes to general anesthetic drugs that are frequently used in clinical practice. Thus, the goal of this study was to determine whether the function of GABA_A receptors in astrocytes was modulated by the intravenous anesthetic etomidate and the inhaled anesthetic sevoflurane.

Methods: Whole-cell voltage-clamp recordings were performed in astrocytes in the stratum radiatum of the CA1 region of hippocampal slices isolated from C57BL/6 male mice. Astrocytes were identified by their morphologic and electrophysiologic properties. Focal puff application of GABA (300 μM) was applied with a Picospritzer system to evoke GABA responses. Currents were studied before and during the application of the non-competitive GABA_A receptor antagonist picrotoxin (0.5 mM), or etomidate (100 μM) or sevoflurane (532 μM).

Results: GABA consistently evoked inward currents that were inhibited by picrotoxin. Etomidate increased the amplitude of the peak current by 35.0 ± 24.4% and prolonged the decay time by 27.2 ± 24.3% (n = 7, P < 0.05). Sevoflurane prolonged current decay by 28.3 ± 23.1% (n = 7, P < 0.05) but did not alter the peak amplitude. Etomidate and sevoflurane increased charge transfer (area) by 71.2 ± 45.9% and 51.8 ± 48.9% (n = 7, P < 0.05), respectively.

Conclusion: The function of astrocytic GABA_A receptors in the hippocampus was increased by etomidate and sevoflurane. Future studies will determine whether these general anesthetic drugs act on astrocytic GABA_A receptors to stimulate the release of soluble factors that may contribute to PNDs.

Effects of general anesthetics on synaptic transmission and plasticity

General anesthetics depress excitatory and/or enhance inhibitory synaptic transmission principally by modulating the function of glutamatergic or GABAergic synapses, respectively, with relative anesthetic agent-specific mechanisms. Synaptic signaling proteins, including ligand- and voltage-gated ion channels, are targeted by general anesthetics to modulate various synaptic mechanisms, including presynaptic neurotransmitter release, postsynaptic receptor signaling, and dendritic spine dynamics to produce their characteristic acute neurophysiological effects. As synaptic structure and plasticity mediate higher-order functions such as learning and memory, long-term synaptic dysfunction following anesthesia may lead to undesirable neurocognitive consequences depending on the specific anesthetic agent and the vulnerability of the population. Here we review the cellular and molecular mechanisms of transient and persistent general anesthetic alterations of synaptic transmission and plasticity.

Physiological role for GABAA receptor desensitization in the induction of long-term potentiation at inhibitory synapses

GABAA receptors (GABAARs) are pentameric ligand-gated ion channels distributed throughout the brain where they mediate synaptic and tonic inhibition. Following activation, these receptors undergo desensitization which involves entry into long-lived agonist-bound closed states. Although the kinetic effects of this state are recognised and its structural basis has been uncovered, the physiological impact of desensitization on inhibitory neurotransmission remains unknown. Here we describe an enduring form of long-term potentiation at inhibitory synapses that elevates synaptic current amplitude for 24 h following desensitization of GABAARs in response to agonist exposure or allosteric modulation. Using receptor mutants and allosteric modulators we demonstrate that desensitization of GABAARs facilitates their phosphorylation by PKC, which increases the number of receptors at inhibitory synapses. These observations provide a physiological relevance to the desensitized state of GABAARs, acting as a signal to regulate the efficacy of inhibitory synapses during prolonged periods of inhibitory neurotransmission.

Mechanism and Development of Modern General Anesthetics

BACKGROUND

Before October 1846, surgery and pain were synonymous but not thereafter. Conquering pain must be one of the very few strategies that has potentially affected every human being in the world of all milestones in medicine.

METHODS

This review article describes how various general anesthetics were discovered historically and how they work in the brain to induce sedative, hypnosis and immobility. Their advantages and disadvantages will also be discussed.

RESULTS

Anesthesia is a relatively young field but is rapidly evolving. Currently used general anesthetics are almost invariably effective, but nagging side effects, both short (e.g., cardiac depression) and long (e.g., neurotoxicity) term, have reawakened the call for new drugs.

CONCLUSION

Based on the deepening understanding of historical development and molecular targets and actions of modern anesthetics, novel general anesthetics are being investigated as potentially improved sedative-hypnotics or a key to understand the mechanism of anesthesia.

Neonatal Propofol and Etomidate Exposure Enhance Inhibitory Synaptic Transmission in Hippocampal Cornus Ammonis 1 Pyramidal Neurons

Background:

Propofol and etomidate are the most important intravenous general anesthetics in the current clinical use and that mediate gamma-aminobutyric acid's (GABAergic) synaptic transmission. However, their long-term effects on GABAergic synaptic transmission induced by neonatal propofol or etomidate exposure remain unclear. We investigated the long-term GABAergic neurotransmission alterations, following neonatal propofol and etomidate administration.

Methods:

Sprague-Dawley rat pups at postnatal days 4–6 were underwent 6-h-long propofol-induced or 5-h-long etomidate-induced anesthesia. We performed whole-cell patch-clamp recording from pyramidal cells in the cornus ammonis 1 area of acute hippocampal slices of postnatal 80–90 days. Spontaneous and miniature inhibitory GABAergic currents (spontaneous inhibitory postsynaptic currents [sIPSCs] and miniature inhibitory postsynaptic currents [mIPSCs]) and their kinetic characters were measured. The glutamatergic tonic effect on inhibitory transmission and the effect of bumetanide on neonatal propofol exposure were also examined.

Results:

Neonatal propofol exposure significantly increased the frequency of mIPSCs (from 1.87 ± 0.35 Hz to 3.43 ± 0.51 Hz, P < 0.05) and did not affect the amplitude of mIPSCs and sIPSCs. Both propofol and etomidate slowed the decay time of mIPSCs kinetics (168.39 ± 27.91 ms and 267.02 ± 100.08 ms vs. 68.18 ± 12.43 ms; P < 0.05). Bumetanide significantly blocked the frequency increase and reversed the kinetic alteration of mIPSCs induced by neonatal propofol exposure (3.01 ± 0.45 Hz and 94.30 ± 32.56 ms).

Conclusions:

Neonatal propofol and etomidate exposure has long-term effects on inhibitory GABAergic transmission. Propofol might act at pre- and post-synaptic GABA receptor A (GABA_A) receptors within GABAergic synapses and impairs the glutamatergic tonic input to GABAergic synapses; etomidate might act at the postsynaptic site.

Astrocytes contribute to the effects of etomidate on synaptic transmission in rat primary somatosensory cortex

Little is known about the mechanisms of unconsciousness induced by general anesthetics. Previous studies have shown that the primary somatosensory cortex (S1) is a sensitive region to a variety of intravenous general anesthetics. Etomidate is a widely used intravenous anesthetic that can influence synaptic transmission. Recently, there are some evidences suggesting that astrocytes, a type of glia cell, also contribute to information transmission in the brain, and modulate synaptic function by releasing neuroactive substances. However, it is unknown whether astrocytes influence the effects of etomidate on information transmission in S1 pyramidal neurons. In the present study, the role of astrocytes in etomidate-induced unconsciousness was investigated by using the whole-cell patch clamp technique. We observed etomidate at clinically relevant concentrations inhibited the spontaneous postsynaptic currents (sPSCs) of rat S1 pyramidal neurons in a concentration-dependent manner, and the EC50 value of etomidate for inhibiting sPSCs from the concentration-effect curve was 6.9μM. Furthermore, in the presence of fluorocitrate, a glia-selective metabolism inhibitor that blocks the aconitase enzyme, both the amplitude and frequency of sPSCs in rat S1 pyramidal neurons were reduced, and the inhibitory effects of etomidate on sPSCs amplitude was strengthened without affecting the effects of etomidate on frequency. From these data, we deduce that etomidate suppresses synaptic activity via presynaptic and postsynaptic components. Furthermore, astrocytes participate in synaptic transmission and influence the effects of etomidate on postsynaptic receptors. This study provides new insight into the role of astrocytes in etomidate-induced unconsciousness.

Effect of gabapentin pretreatment on myoclonus after etomidate: a randomized, double-blind, placebo-controlled study

AIM

To evaluate the effects of three different doses of gabapentin pretreatment on the incidence and severity of myoclonic movements linked to etomidate injection.

METHOD

One hundered patients, between 18 and 60 years of age and risk category American Society of Anesthesiologists I-II, with planned elective surgery under general anesthetic were included in the study. The patients were randomly divided into four groups and 2h before the operation were given oral capsules of placebo (Group P, n=25), 400mg gabapentin (Group G400, n=25), 800mg gabapentin (Group G800, n=25) or 1200mg gabapentin (Group G1200, n=25). Side effects before the operation were recorded. After preoxygenation for anesthesia induction 0.3mgkg(-1) etomidate was administered for 10s. A single anesthetist with no knowledge of the study medication evaluated sedation and myoclonic movements on a scale between 0 and 3. Two minutes after induction, 2μgkg(-1) fentanyl and 0.8mgkg(-1) rocuronium were administered for tracheal intubation.

RESULTS

Demographic data were similar. Incidence and severity of myoclonus in Group G1200 and Group G800 were significantly lower than in Group P; sedation incidence and level were appreciably higher compared to Group P and Group G400. While there was no difference in the incidence of myoclonus between Group P and Group G400, the severity of myoclonus in Group G400 was lower than in the placebo group. In the two-hour period before induction other than sedation none of the side effects related to gabapentin were observed in any patient.

CONCLUSION

Pretreatment with 800mg and 1200mg gabapentin 2h before the operation increased the level of sedation and reduced the incidence and severity of myoclonic movements due to etomidate.

Etomidate uniquely modulates the desensitization of recombinant α1β3δ GABA(A) receptors

Central GABA(A) receptors mediate GABAergic phasic and tonic inhibition. While synaptic αβγ GABA(A) receptors primarily mediate phasic inhibition, extrasynaptic αβδ receptors play an important role in mediating tonic inhibition. Etomidate is a general anesthetic that produces its effects by enhancing GABA(A) receptor activity. We previously showed that etomidate modulates the gating of oocyte-expressed αβγ and αβδ receptors with similar overall allosteric impact, but different pharmacological patterns. In αβγ receptors, etomidate enhances apparent GABA sensitivity (reduces GABA EC50), modestly increases maximal GABA efficacy, and slows current deactivation without affecting desensitization (Zhong et al., 2008). In αβδ receptors characterized by low GABA efficacy, etomidate dramatically increases responses to both low and maximal GABA. The effects of etomidate on desensitization and deactivation of αβδ receptors are unknown. To investigate the kinetic effects of etomidate on α1β3δ receptors of defined subunit arrangement, we expressed concatenated trimer (β3-α1-δ) and dimer (β3-α1) GABA(A) receptor subunit assemblies in human embryonic kidney (HEK)293T cells and recorded whole-cell voltage-clamp currents during rapid external solution exchanges. As expected, etomidate substantially increased maximal GABA-induced currents and prolonged deactivation. Moreover, desensitization was significantly decreased by etomidate. During prolonged GABA applications, etomidate enhanced steady-state currents more than peak currents. Thus, etomidate enhances tonic GABAergic inhibition through extrasynaptic αβδ receptors by both augmenting gating and reducing desensitization.

The general anaesthetic etomidate inhibits the excitability of mouse thalamocortical relay neurons by modulating multiple modes of GABAA receptor-mediated inhibition

Modulation of thalamocortical (TC) relay neuron function has been implicated in the sedative and hypnotic effects of general anaesthetics. Inhibition of TC neurons is mediated predominantly by a combination of phasic and tonic inhibition, together with a recently described ‘spillover’ mode of inhibition, generated by the dynamic recruitment of extrasynaptic γ-aminobutyric acid (GABA)_A receptors (GABA_ARs). Previous studies demonstrated that the intravenous anaesthetic etomidate enhances tonic and phasic inhibition in TC relay neurons, but it is not known how etomidate may influence spillover inhibition. Moreover, it is unclear how etomidate influences the excitability of TC neurons. Thus, to investigate the relative contribution of synaptic (α1β2γ2) and extrasynaptic (α4β2δ) GABA_ARs to the thalamic effects of etomidate, we performed whole-cell recordings from mouse TC neurons lacking synaptic (α1^0/0) or extrasynaptic (δ^0/0) GABA_ARs. Etomidate (3 μm) significantly inhibited action-potential discharge in a manner that was dependent on facilitation of both synaptic and extrasynaptic GABA_ARs, although enhanced tonic inhibition was dominant in this respect. Additionally, phasic inhibition evoked by stimulation of the nucleus reticularis exhibited a spillover component mediated by δ-GABA_ARs, which was significantly prolonged in the presence of etomidate. Thus, etomidate greatly enhanced the transient suppression of TC spike trains by evoked inhibitory postsynaptic potentials. Collectively, these results suggest that the deactivation of thalamus observed during etomidate-induced anaesthesia involves potentiation of tonic and phasic inhibition, and implicate amplification of spillover inhibition as a novel mechanism to regulate the gating of sensory information through the thalamus during anaesthetic states.

Evaluation of metomidate hydrochloride as an anesthetic in leopard frogs (Rana pipiens)

Metomidate hydrochloride is an imidazole-based, nonbarbiturate hypnotic drug primarily used as an immersion sedation and anesthetic agent in freshwater and marine finfish. To the authors' knowledge, there is no documentation in the literature of its use in amphibians. In this study, 7 male and 4 female leopard frogs (Rana pipiens) were induced with metomidate hydrochloride via immersion bath at a concentration of 30 mg/L for 60 min. The pH of the induction solution ranged from 7.63 to 7.75. Each frog was then removed from the induction solution, rinsed, and recovered in 26.6 degrees C amphibian Ringer's solution. After 210 min in the Ringer's solution, the frogs were transferred to moist paper towels for recovery. Heart rate, gular and abdominal respiration rates, righting reflex, superficial and deep pain withdrawal reflexes, corneal and palpebral reflexes, and escape response were monitored and recorded at defined intervals during both induction and recovery. The average time to loss of righting reflex and escape response was 17.36 min and 17.82 min, respectively. Metomidate produced clinical sedation in all frogs (n = 11). Surgical anesthesia was achieved in only 27% (3/11), with an anesthetic duration that ranged from 9 to 20 min. Recovery times were extremely prolonged and varied, with a range from 313 min to longer than 600 min. The findings of this study indicate that metomidate hydrochloride is unsuitable as a sole anesthetic agent in leopard frogs, and further research is needed to evaluate its suitability in other amphibians.

Etomidate--a review of robust evidence for its use in various clinical scenarios

Etomidate is an intravenous hypnotic with a favourable clinical profile in haemodynamic high-risk scenarios. Currently, there is an active debate about the clinical significance of the drug's side effects and its overall risk-benefit ratio. Etomidate-induced transient adrenocortical suppression is well documented and has been associated with increased mortality in sepsis. In surgical patients at risk of hypotensive complications, however, a review of current literature provides no robust evidence to contraindicate a single-bolus etomidate induction. Large randomised controlled trials as well as additional observational data are required to compare safety of etomidate and its alternatives.

Etomidate evokes synaptic vesicle exocytosis without increasing miniature endplate potentials frequency at the mice neuromuscular junction

Etomidate is an intravenous anesthetic used during anesthesia induction. This agent induces spontaneous movements, especially myoclonus after its administration suggesting a putative primary effect at the central nervous system or the periphery. Therefore, the aim of this study was to investigate the presynaptic and postsynaptic effects of etomidate at the mouse neuromuscular junction (NMJ). Diaphragm nerve muscle preparations were isolated and stained with the styryl dye FM1-43, a fluorescent tool that tracks synaptic vesicles exo-endocytosis that are key steps for neurotransmission. We observed that etomidate induced synaptic vesicle exocytosis in a dose-dependent fashion, an effect that was independent of voltage-gated Na(+) channels. By contrast, etomidate-evoked exocytosis was dependent on extracellular Ca(2+) because its effect was abolished in Ca(2+)-free medium and also inhibited by omega-Agatoxin IVA (30 and 200nM) suggesting the participation of P/Q-subtype Ca(2+) channels. Interestingly, even though etomidate induced synaptic vesicle exocytosis, we did not observe any significant difference in the frequency and amplitude of miniature end-plate potentials (MEPPs) in the presence of the anesthetic. We therefore investigated whether etomidate could act on nicotinic acetylcholine receptors labeled with α-bungarotoxin-Alexa 594 and we observed less fluorescence in preparations exposed to the anesthetic. In conclusion, our results suggest that etomidate exerts a presynaptic effect at the NMJ inducing synaptic vesicle exocytosis, likely through the activation of P-subtype voltage gated Ca(2+) channels without interfering with MEPPs frequency. The present data contribute to a better understanding about the effect of etomidate at the neuromuscular synapse and may help to explain some clinical effects of this agent.

Cysteine substitutions define etomidate binding and gating linkages in the α-M1 domain of γ-aminobutyric acid type A (GABAA) receptors

Etomidate is a potent general anesthetic that acts as an allosteric co-agonist at GABAA receptors. Photoreactive etomidate derivatives labeled αMet-236 in transmembrane domain M1, which structural models locate in the β+/α- subunit interface. Other nearby residues may also contribute to etomidate binding and/or transduction through rearrangement of the site. In human α1β2γ2L GABAA receptors, we applied the substituted cysteine accessibility method to α1-M1 domain residues extending from α1Gln-229 to α1Gln-242. We used electrophysiology to characterize each mutant's sensitivity to GABA and etomidate. We also measured rates of sulfhydryl modification by p-chloromercuribenzenesulfonate (pCMBS) with and without GABA and tested if etomidate blocks modification of pCMBS-accessible cysteines. Cys substitutions in the outer α1-M1 domain impaired GABA activation and variably affected etomidate sensitivity. In seven of eight residues where pCMBS modification was evident, rates of modification were accelerated by GABA co-application, indicating that channel activation increases water and/or pCMBS access. Etomidate reduced the rate of modification for cysteine substitutions at α1Met-236, α1Leu-232 and α1Thr-237. We infer that these residues, predicted to face β2-M3 or M2 domains, contribute to etomidate binding. Thus, etomidate interacts with a short segment of the outer α1-M1 helix within a subdomain that undergoes significant structural rearrangement during channel gating. Our results are consistent with in silico docking calculations in a homology model that orient the long axis of etomidate approximately orthogonal to the transmembrane axis.

Two etomidate sites in α1β2γ2 γ-aminobutyric acid type A receptors contribute equally and noncooperatively to modulation of channel gating

BACKGROUND

Etomidate is a potent hypnotic agent that acts via γ-aminobutyric acid receptor type A (GABA(A)) receptors. Evidence supports the presence of two etomidate sites per GABA(A) receptor, and current models assume that each site contributes equally and noncooperatively to drug effects. These assumptions remain untested.

METHODS

We used concatenated dimer (β2-α1) and trimer (γ2-β2-α1) GABA(A) subunit assemblies that form functional α1β2γ2 channels, and inserted α1M236W etomidate site mutations into both dimers (β2-α1M236W) and trimers (γ2-β2-α1M236W). Wild-type or mutant dimers (D(wt) or D(αM236W)) and trimers (T(wt) or T(αM236W)) were coexpressed in Xenopus oocytes to produce four types of channels: D(wt)T(wt), D(αM236W)T(wt), D(wt)T(αM236W), and D(αM236W)T(αM236W). For each channel type, two-electrode voltage clamp was performed to quantitatively assess GABA EC(50), etomidate modulation (left shift), etomidate direct activation, and other functional parameters affected by αM236W mutations.

RESULTS

Concatenated wild-type D(wt)T(wt) channels displayed etomidate modulation and direct activation similar to α1β2γ2 receptors formed with free subunits. D(αM236W)T(αM236W) receptors also displayed altered GABA sensitivity and etomidate modulation similar to mutated channels formed with free subunits. Both single-site mutant receptors (D(αM236W)T(wt) and D(wt)T(αM236W)) displayed indistinguishable functional properties and equal gating energy changes for GABA activation (-4.9 ± 0.48 vs. -4.7 ± 0.48 kJ/mol, respectively) and etomidate modulation (-3.4 ± 0.49 vs. -3.7 ± 0.38 kJ/mol, respectively), which together accounted for the differences between D(wt)T(wt) and D(αM236W)T(αM236W) channels.

CONCLUSIONS

These results support the hypothesis that the two etomidate sites on α1β2γ2 GABA(A) receptors contribute equally and noncooperatively to drug interactions and gating effects.

An allosteric coagonist model for propofol effects on α1β2γ2L γ-aminobutyric acid type A receptors

BACKGROUND

Propofol produces its major actions via γ-aminobutyric acid type A (GABA(A)) receptors. At low concentrations, propofol enhances agonist-stimulated GABA(A) receptor activity, and high propofol concentrations directly activate receptors. Etomidate produces similar effects, and there is convincing evidence that a single class of etomidate sites mediate both agonist modulation and direct GABA(A) receptor activation. It is unknown if the propofol binding site(s) on GABA(A) receptors that modulate agonist-induced activity also mediate direct activation.

METHODS

GABA(A) α1β2γ2L receptors were heterologously expressed in Xenopus oocytes and activity was quantified using voltage clamp electrophysiology. We tested whether propofol and etomidate display the same linkage between agonist modulation and direct activation of GABA(A) receptors by identifying equiefficacious drug solutions for direct activation. We then determined whether these drug solutions produce equal modulation of GABA-induced receptor activity. We also measured propofol-dependent direct activation and modulation of low GABA responses. Allosteric coagonist models similar to that established for etomidate, but with variable numbers of propofol sites, were fitted to combined data.

RESULTS

Solutions of 19 μM propofol and 10 μM etomidate were found to equally activate GABA(A) receptors. These two drug solutions also produced indistinguishable modulation of GABA-induced receptor activity. Combined electrophysiological data behaved in a manner consistent with allosteric coagonist models with more than one propofol site. The best fit was observed when the model assumed three equivalent propofol sites.

CONCLUSIONS

Our results support the hypothesis that propofol, like etomidate, acts at GABA(A) receptor sites mediating both GABA modulation and direct activation.

Anaesthesia of farmed fish: implications for welfare

During their life cycle as farmed animals, there are several situations in which fish are subjected to handling and confinement. Netting, weighing, sorting, vaccination, transport and, at the end, slaughter are frequent events under farming conditions. As research subjects, fish may also undergo surgical procedures that range from tagging, sampling and small incisions to invasive procedures. In these situations, treatment with anaesthetic agents may be necessary in order to ensure the welfare of the fish. The main objective of this paper is to review our knowledge of the effects of anaesthetic agents in farmed fish and their possible implications for welfare. As wide variations in response to anaesthesia have been observed both between and within species, special attention has been paid to the importance of secondary factors such as body weight, water temperature and acute stress. In this review, we have limited ourselves to the anaesthetic agents such as benzocaine, metacaine (MS-222), metomidate hydrochloride, isoeugenol, 2-phenoxyethanol and quinaldine. Anaesthetic protocols of fish usually refer to one single agent, whereas protocols of human and veterinary medicine cover combinations of several drugs, each contributing to the effects needed in the anaesthesia. As stress prior to anaesthesia may result in abnormal reactions, pre-anaesthetic sedation is regularly used in order to reduce or avoid stress and is an integral part of the veterinary protocols of higher vertebrates. Furthermore, the anaesthetic agents that are used in order to obtain general anaesthesia are combined with analgesic agents that target nociception. The increased use of such combinations in fish is therefore included as a special section. Anaesthetic agents are widely used to avoid stress during various farming procedures. While several studies report that anaesthetics are effective in reducing the stress associated with confinement and handling, there are indications that anaesthesia may in itself induce a stress response, measured by elevated levels of cortisol. MS-222 has been reported to elicit high cortisol release rates immediately following exposure, while benzocaine causes a bimodal response. Metomidate has an inhibitory effect on cortisol in fish and seems to induce the lowest release of cortisol of the agents reported in the literature. Compared to what is observed following severe stressors such as handling and confinement, the amount of cortisol released in response to anaesthesia appears to be low but may represent an extra load under otherwise stressful circumstances. Furthermore, anaesthetics may cause secondary adverse reactions such as acidosis and osmotic stress due to respiratory arrest and insufficient exchange of gas and ions between the blood and the water. All in all, anaesthetics may reduce stress and thereby improve welfare but can also have unwanted side effects that reduce the welfare of the fish and should therefore always be used with caution. Finally, on the basis of the data reported in the literature and our own experience, we recommend that anaesthetic protocols should always be tested on a few fish under prevailing conditions in order to ensure an adequate depth of anaesthesia. This recommendation applies whether a single agent or a combination of agents is used, although it appears that protocols comprising combinations of agents provide wider safety margins. The analgesic effects of currently used agents, in spite of their proven local effects, are currently being debated as the agents are administrated to fish via inhalation rather than locally at the target site. We therefore recommend that all protocols of procedures requiring general anaesthesia should be complemented by administration of agents with analgesic effect at the site of tissue trauma.

Mutations in the GABAA receptor that mimic the allosteric ligand etomidate

Etomidate is a hydrophobic molecule, a potent general anesthetic, and the best understood drug in this group. Etomidate's target molecules are GABA(A) receptors, its site of action has been identified with photolabeling, and a quantitative allosteric coagonist model has emerged for etomidate effects on GABA(A) receptors. We have shown that when methionine residues that are thought to be adjacent to the etomidate site are mutated to tryptophan, that the bulky hydrophobic side-chains alter mutant GABA(A) receptor function in ways that mimic the effects of etomidate binding to wild-type receptors. Furthermore, these mutations reduce receptor modulation by etomidate. Both of these observations support the hypothesis that these methionine residues form part of the etomidate binding pocket.

Clinical and molecular pharmacology of etomidate

This review focuses on the unique clinical and molecular pharmacologic features of etomidate. Among general anesthesia induction drugs, etomidate is the only imidazole, and it has the most favorable therapeutic index for single-bolus administration. It also produces a unique toxicity among anesthetic drugs: inhibition of adrenal steroid synthesis that far outlasts its hypnotic action and that may reduce survival of critically ill patients. The major molecular targets mediating anesthetic effects of etomidate in the central nervous system are specific γ-aminobutyric acid type A receptor subtypes. Amino acids forming etomidate binding sites have been identified in transmembrane domains of these proteins. Etomidate binding site structure models for the main enzyme mediating etomidate adrenotoxicity have also been developed. Based on this deepening understanding of molecular targets and actions, new etomidate derivatives are being investigated as potentially improved sedative-hypnotics or for use as highly selective inhibitors of adrenal steroid synthesis.

Anesthetic sites and allosteric mechanisms of action on Cys-loop ligand-gated ion channels

PURPOSE

The Cys-loop ligand-gated ion channel superfamily is a major group of neurotransmitter-activated receptors in the central and peripheral nervous system. The superfamily includes inhibitory receptors stimulated by γ-aminobutyric acid (GABA) and glycine and excitatory receptors stimulated by acetylcholine and serotonin. The first part of this review presents current evidence on the location of the anesthetic binding sites on these channels and the mechanism by which binding to these sites alters their function. The second part of the review addresses the basis for this selectivity, and the third part describes the predictive power of a quantitative allosteric model showing the actions of etomidate on γ-aminobutyric acid type A receptors (GABA(A)Rs).

PRINCIPAL FINDINGS

General anesthetics at clinical concentrations inhibit the excitatory receptors and enhance the inhibitory receptors. The location of general anesthetic binding sites on these receptors is being defined by photoactivable analogues of general anesthetics. The receptor studied most extensively is the muscle-type nicotinic acetylcholine receptor (nAChR), and progress is now being made with GABA(A)Rs. There are three categories of sites that are all in the transmembrane domain: 1) within a single subunit's four-helix bundle (intrasubunit site; halothane and etomidate on the δ subunit of AChRs); 2) between five subunits in the transmembrane conduction pore (channel lumen sites; etomidate and alcohols on nAChR); and 3) between two subunits (subunit interface sites; etomidate between the α1 and β2/3 subunits of the GABA(A)R).

CONCLUSIONS

These binding sites function allosterically. Certain conformations of a receptor bind the anesthetic with greater affinity than others. Time-resolved photolabelling of some sites occurs within milliseconds of channel opening on the nAChR but not before. In GABA(A)Rs, electrophysiological data fit an allosteric model in which etomidate binds to and stabilizes the open state, increasing both the fraction of open channels and their lifetime. As predicted by the model, the channel-stabilizing action of etomidate is so strong that higher concentrations open the channel in the absence of agonist. The formal functional paradigm presented for etomidate may apply to other potent general anesthetic drugs. Combining photolabelling with structure-function mutational studies in the context of allosteric mechanisms should lead us to a more detailed understanding of how and where these important drugs act.

Anesthesia induces stress in Atlantic salmon (Salmo salar), Atlantic cod (Gadus morhua) and Atlantic halibut (Hippoglossus hippoglossus)

Stress in response to anesthesia with benzocaine, MS-222, metomidate and isoeugenol was studied in Atlantic salmon (Salmo salar), Atlantic halibut (Hippoglossus hippoglossus), and Atlantic cod (Gadus morhua) with no concomitant stress from handling or confinement in association with anesthesia or sampling. All of the anesthetics tested induced a stress response in all species, displayed by a release of cortisol to the water. MS-222 anesthesia elicited the highest cortisol release rates, reaching maximum levels 0.5 h post-exposure and returning to basal levels after 3-4 h. Benzocaine anesthesia caused a bimodal response where the initial peak in cortisol release rate was followed by a second increase lasting towards the end of the trial (6 h). This bimodality was more profound in Atlantic salmon than in Atlantic halibut and Atlantic cod. Metomidate anesthesia induced the lowest release of cortisol of the agents tested in both Atlantic halibut and Atlantic cod, but resulted in a bimodal response in Atlantic salmon where the initial increase in cortisol release was followed by a larger increase peaking at 2-2.5 h post exposure before returning to basal levels after 5 h. The stress induced in Atlantic salmon by isoeugenol anesthesia resembled that of MS-222, but did not reach the same elevated level. Overall, the cortisol release was most profound in Atlantic salmon followed by Atlantic halibut and Atlantic cod.

Gamma-amino butyric acid type A receptor mutations at beta2N265 alter etomidate efficacy while preserving basal and agonist-dependent activity

BACKGROUND

Etomidate acts at gamma-Aminobutyric acid type A (GABAA) receptors containing beta2 or beta3, but not beta1 subunits. Mutations at beta residue 265 (Ser in beta1; Asn in beta2 or beta3) profoundly affect etomidate sensitivity. Whether these mutations alter etomidate binding remains uncertain.

METHODS

Heterologously expressed alpha1beta2gamma2L GABAA receptors and receptors with beta2(N265S) or beta2(N265M) mutations were studied electrophysiologically in both Xenopus oocytes and HEK293 cells. Experiments quantified the impact of beta2N265 mutations or substituting beta1 for beta2 on basal channel activation, GABA EC50, maximal GABA efficacy, etomidate-induced leftward shift in GABA responses, etomidate direct activation, and rapid macrocurrent kinetics. Results were analyzed in the context of an established allosteric co-agonist mechanism.

RESULTS

Mutations produced only small changes in basal channel activity, GABA EC50, maximal GABA efficacy, and macrocurrent kinetics. Relative to wild-type, beta2(N265S) reduced etomidate enhancement of apparent GABA affinity six-fold, and it reduced etomidate direct activation efficacy 14-fold. beta2(N265M) totally eliminated both etomidate modulation of GABA responses and direct channel activation. Mechanism-based analysis showed that the function of both mutants remains consistent with the allosteric co-agonist model and that beta2(N265S) reduced etomidate allosteric efficacy five-fold, whereas etomidate-binding affinity dropped threefold. Experiments swapping beta2 subunits for beta1 indicated that etomidate efficacy is reduced 34-fold, whereas binding affinity drops less than two-fold.

CONCLUSIONS

Mutations at beta2N265 profoundly alter etomidate sensitivity with only small changes in basal and GABA-dependent channel activity. Mutations at the beta2N265 residue or replacement of beta2 with beta1 influence etomidate efficacy much more than binding to inactive receptors.

Differential drug responses on native GABA(A) receptors revealing heterogeneity in extrasynaptic populations in cultured hippocampal neurons

Hippocampal pyramidal neurons potentially express multiple subtypes of GABA(A) receptors at extrasynaptic locations that could therefore respond to different drugs. We activated extrasynaptic GABA(A) receptors in cultured rat hippocampal pyramidal neurons and measured single-channel currents in order to compare the actions of two drugs that potentially target different GABA(A) receptor subtypes. Despite the possible difference in receptor targets of etomidate and diazepam, the two drugs were similar in their actions on native extrasynaptic GABA(A) receptors. Each drug produced three distinct responses that differed significantly in current magnitude, implying heterogeneous GABA(A) receptor populations. In the majority of patches, drug application increased both the single-channel conductance (>40 pS) and the open probability of the channels. By contrast, in the minority of patches, drug application caused an increase in open probability only. In the third group high-conductance channels were observed upon GABA activation and drug application increased their open probability only. The currents potentiated by etomidate or diazepam were substantially larger in patches displaying high-conductance GABA channels compared to those displaying only low-conductance channels. Factors contributing to the large magnitude of these currents were the long mean open time of high-conductance channels and the presence of multiple channels in these patches. In conclusion, we suggest that the local density of extrasynaptic GABA(A) receptors may influence their single-channel properties and may be an additional regulating factor for tonic inhibition and, importantly, differential drug modulation.

Tryptophan mutations at azi-etomidate photo-incorporation sites on alpha1 or beta2 subunits enhance GABAA receptor gating and reduce etomidate modulation

The potent general anesthetic etomidate produces its effects by enhancing GABA(A) receptor activation. Its photolabel analog [(3)H]azi-etomidate labels residues within transmembrane domains on alpha and beta subunits: alphaMet236 and betaMet286. We hypothesized that these methionines contribute to etomidate sites formed at alpha-beta subunit interfaces and that increasing side-chain bulk and hydrophobicity at either locus would mimic etomidate binding and block etomidate effects. Channel activity was electrophysiologically quantified in alpha(1)beta(2)gamma(2L) receptors with alpha(1)M236W or beta(2)M286W mutations, in both the absence and the presence of etomidate. Measurements included spontaneous activation, GABA EC(50), etomidate agonist potentiation, etomidate direct activation, and rapid macrocurrent kinetics. Both alpha(1)M236W and beta(2)M286W mutations induced spontaneous channel opening, lowered GABA EC(50), increased maximal GABA efficacy, and slowed current deactivation, mimicking effects of etomidate on alpha(1)beta(2)gamma(2L) channels. These changes were larger with alpha(1)M236W than with beta(2)M286W. Etomidate (3.2 muM) reduced GABA EC(50) much less in alpha(1)M236Wbeta(2)gamma(2L) receptors (2-fold) than in wild type (23-fold). However, etomidate was more potent and efficacious in directly activating alpha(1)M236Wbeta(2)gamma(2L) compared with wild type. In alpha(1)beta(2)M286Wgamma(2L) receptors, etomidate induced neither agonist-potentiation nor direct channel activation. These results support the hypothesis that alpha(1)Met236 and beta(2)Met286 are within etomidate sites that allosterically link to channel gating. Although alpha(1)M236W produced the larger impact on channel gating, beta(2)M286W produced more profound changes in etomidate sensitivity, suggesting a dominant role in drug binding. Furthermore, quantitative mechanistic analysis demonstrated that wild-type and mutant results are consistent with the presence of only one class of etomidate sites mediating both agonist potentiation and direct activation.

Ischemic insult to cerebellar Purkinje cells causes diminished GABAA receptor function and allopregnanolone neuroprotection is associated with GABAA receptor stabilization

Cerebellar Purkinje cells (PC) are particularly vulnerable to ischemic injury and excitotoxicity, although the molecular basis of this sensitivity remains unclear. We tested the hypothesis that ischemia causes rapid down-regulation of GABA(A) receptors in cerebellar PC, thereby increasing susceptibility to excitotoxicity. Oxygen-glucose deprivation (OGD) caused a decline in functional GABA(A) receptors, within the first hour of re-oxygenation. Decreased amplitude of miniature inhibitory post-synaptic potentials confirmed that OGD caused a significant decrease in functional synaptic GABA(A) receptors and quantitative Western blot analysis demonstrated the loss of GABA(A) receptor current was associated with a decline in total receptor protein. Interestingly, the potent neuroprotectant allopregnanolone (ALLO) prevented the decline in GABA(A) receptor current and protein. Consistent with our in vitro data, global ischemia in mice caused a significant decline in total cerebellar GABA(A) receptor protein and PC specific immunoreactivity. Moreover, ALLO provided strong protection of PC and prevented ischemia-induced decline in GABA(A) receptor protein. Our findings indicate that ischemia causes a rapid and sustained loss of GABA(A) receptors in PC, whereas ALLO prevents the decline in GABA(A) receptors and protects against ischemia-induced damage. Thus, interventions which prevent ischemia-induced decline in GABA(A) receptors may represent a novel neuroprotective strategy.

Photo-activated azi-etomidate, a general anesthetic photolabel, irreversibly enhances gating and desensitization of gamma-aminobutyric acid type A receptors

BACKGROUND

The general anesthetic etomidate acts via gamma-aminobutyric acid type A (GABA(A)) receptors, enhancing activation at low GABA and prolonging deactivation. Azi-etomidate is a photo-reactive etomidate derivative with similar pharmacological actions, which has been used to identify putative binding sites. The authors examine the irreversible effects of azi-etomidate photo-modification on functional GABA(A) receptors in cell membranes.

METHODS

GABA(A) receptors (alpha1beta2gamma2L) were expressed in both Xenopus oocytes and human embryonic kidney cells exposed to 365 nm light-activated azi-etomidate with or without GABA, then extensively washed. Receptor-mediated chloride currents were measured using voltage clamp electrophysiology to assess the ratio of peak responses at 10 microm and 1 mm GABA (I10/I1000) and deactivation time course.

RESULTS

After azi-etomidate photo-modification, I10/I1000 ratios were persistently enhanced and deactivation was prolonged, mimicking reversible azi-etomidate actions. Azi-etomidate and ultraviolet light were required to produce irreversible receptor modulation. Adding GABA during photo-modification greatly enhanced irreversible modulation. Azi-etomidate modification also dose-dependently reduced maximal GABA-activated currents, consistent with accumulation of permanently desensitized receptors. Excess etomidate during azi-etomidate photo-modification competitively reduced permanent desensitization. Persistent channel modulation was blocked by 320-fold excess etomidate but enhanced when 32-fold excess etomidate was present.

CONCLUSIONS

Azi-etomidate efficiently photo-modifies etomidate sites on GABA(A) receptors in intact cells, producing persistent functional changes that mimic its reversible effects. The results demonstrate sequential modification at more than one etomidate site per receptor. The sites display reciprocal positive cooperativity. In combination with focal photo-activation, azi-etomidate may prove useful for studies of anesthetic actions in neural circuits.

Functional asymmetry of the conserved cystine loops in alphabetagamma GABA A receptors revealed by the response to GABA activation and drug potentiation

Ligand-gated ion channels respond to specific neurotransmitters by transiently opening an integral membrane ion-selective pore, allowing ions to move down their electrochemical gradient. A distinguishing feature of all members of the ligand-gated ion channel superfamily is the presence of a 13-amino acid disulfide loop (Cys-loop) in the extracellular ligand-binding domain. Structural data derived from the acetylcholine receptor place this loop at the interface between the ligand-binding domain and the transmembrane pore-forming domain where it is ideally located to participate in coupling ligand binding to channel opening. We have introduced specific mutations into a conserved motif at the mid-point of the Cys-loop of the GABA A receptor subunits alpha1, beta2 and gamma2S where the sequence reads aromatic, proline, aliphatic (ArProAl motif). Receptors carrying a mutation in the Cys-loop of one of their subunits were expressed in L929 cells and responses to both GABA and drugs were assessed using the whole-cell patch clamp technique. Drug potentiation and direct activation were significantly enhanced by mutations in this Cys-loop but these effects were subunit-dependent. Currents in response to agonists were larger when mutations were carried in the alpha and beta subunits but not in the gamma subunit. In contrast, potentiation of current responses by diazepam, etomidate and pentobarbital were all enhanced when mutations were carried in the alpha and gamma subunits, but not the beta subunit. Since the disruption of interactions mediated through the ArProAl motif enhances the mutant receptor's response to both agonist and drugs we suggest that this motif in the Cys-loop of the wild-type receptor participates in interactions that create activation barriers to conformational changes during channel gating.

Etomidate revisited

The major advantage of etomidate is its lack of cardiovascular side effects. In addition, etomidate is supposed to be neuroprotective. The side effects of etomidate include adrenal suppression and myocloni. A review of the recent literature on etomidate, its clinical use, its side effects and its mechanism of action was performed. Among others, major recent advances include a new drug preparation devoid of propylene glycol and its side effects, a new pretreatment technique that may reduce the incidence of myocloni, and the identification of its site of action in the central nervous system.

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.

Potentiation, activation and blockade of GABAA receptors by etomidate in the rat sacral dorsal commissural neurons

Etomidate (ET), an imidazole general anesthetic, has been medically widely used. Recent evidence suggests that the inhibitory neurotransmitter GABA receptor may be one of the important molecular target(s) of general anesthetics. Up to date, little attention has been directed toward the sacral dorsal commissural nucleus (SDCN), which serves as a relay of sensory information from the pelvic viscera in the spinal cord. Therefore, the effect of ET on GABA(A) receptor function in neurons acutely dissociated from the SDCN was investigated using the nystatin-perforated patch-recording configuration under voltage-clamp conditions. At a holding potential of -40 mV, ET (above 10 microM) induced an inward ET-activated current (I(ET)) with the EC(50) value of 33 +/- 3 microM, which was reversibly blocked by bicuculline and picrotoxin. The reversal potential of I(ET) was close to the Cl(-) equilibrium potential. ET also displayed a biphasic modulatory effect on GABA responses. At lower concentrations (0.1-100 microM), ET reversibly potentiated GABA (1 microM)-activated Cl(-) currents in a bell-shaped manner, with the maximal facilitative effect at 10 microM, whereas at concentrations >100 microM, the peak of the ET-induced current was suppressed in the absence or presence of GABA (1 microM). These results suggest that in SDCN, in addition to the potentiation of GABA(A) receptor-mediated responses at low concentrations and the direct activation of GABA(A) receptors at moderate concentrations as expected, ET produced a fast blocking action at high concentrations. The general anesthetic-induced effects in SDCN, at least the potentiation of GABA responses, may significantly contribute to anesthesia of pelvic viscera during the general anesthesia.

Etomidate elevates intracellular calcium levels and promotes catecholamine secretion in bovine chromaffin cells

Etomidate, an intravenous imidazole general anaesthetic, is thought to produce anaesthesia by modulating or activating ionotropic Cl(-)-permeable GABA(A) receptors. Chromaffin cells are known to express functional GABA(A) receptors with properties similar to their neuronal counterparts. We have shown that activation of the GABA(A) receptors, with specific GABA(A) agonists, leads to cellular excitation. Our goal was to determine whether etomidate mimicked this response and to explore the functional consequences of this activation. Imaging experiments with the Ca(2+)-indicator dye fura-2 were used to assay [Ca(2+)](i). Bovine adrenal chromaffin cells were superfused with a variety of GABA(A)-selective drugs to determine their effects on [Ca(2+)](i). Amperometric measurements were used to assay catecholamine release in real-time. We show that bovine adrenal chromaffin cells were excited by etomidate at clinically relevant concentrations. Etomidate directly activated GABA(A) receptors found in chromaffin cells thereby elevating [Ca(2+)](i). The effects of etomidate were mimicked by the specific GABA(A) agonist muscimol and blocked by the specific antagonist bicuculline. Our data show that low concentrations of etomidate modulated GABA(A) receptor activation by muscimol. Blockade of voltage-dependent Ca(2+) channels prevented the elevation of [Ca(2+)](i) by GABA. Application of etomidate directly to the chromaffin cells elicited robust catecholamine secretion from these cells. The data indicate that clinically relevant concentrations of etomidate can directly activate GABA(A) receptors, which, due to the positive anion equilibrium potential, depolarizes chromaffin cells. This depolarization activates voltage-dependent Ca(2+) channels thereby stimulating catecholamine release. Our data suggest that circulating catecholamine levels may be elevated after etomidate application.

Gating Allosterism at a Single Class of Etomidate Sites on α1β2γ2L GABAA Receptors Accounts for Both Direct Activation and Agonist Modulation

At clinical concentrations, the potent intravenous general anesthetic etomidate enhances gamma-aminobutyric acid, type A (GABA(A)) receptor activity elicited with low gamma-aminobutyric acid (GABA) concentrations, whereas much higher etomidate concentrations activate receptors in the absence of GABA. Therefore, GABA(A) receptors may possess two types of etomidate sites: high affinity GABA-modulating sites and low affinity channel-activating sites. However, GABA modulation and direct activation share stereoselectivity for the (R)(+)-etomidate isomer and display parallel dependence on GABA(A) beta subunit isoforms, suggesting that these two actions may be mediated by a single class of etomidate site(s) that exert one or more molecular effects. In this study, we assessed GABA modulation by etomidate using leftward shifts of electrophysiological GABA concentration responses in cells expressing human alpha1beta2gamma2L receptors. Etomidate at up to 100 microm reduced GABA EC(50) values by over 100-fold but without apparent saturation, indicating the absence of high affinity etomidate sites. In experiments using a partial agonist, P4S, etomidate both reduced EC(50) and increased maximal efficacy, demonstrating that etomidate shifts the GABA(A) receptor gating equilibrium toward open states. Results were quantitatively analyzed using equilibrium receptor gating models, wherein a postulated class of equivalent etomidate sites both directly activates receptors and enhances agonist gating. A Monod-Wyman-Changeux co-agonist mechanism with two equivalent etomidate sites that allosterically enhance GABA(A) receptor gating independently of agonist binding most simply accounts for direct activation and agonist modulation. This model also correctly predicts the actions of etomidate on GABA(A) receptors containing a point mutation that increases constitutive gating activity.

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.

Kinetics of etomidate actions on GABA(A) receptors in the rat spinal dorsal horn neurons

Electrophysiological properties of etomidate (ET)-induced current (I(ET)) at different concentrations and effects of ET at clinically relevant concentrations (1-10 microM) on postsynaptic GABA(A) receptor function were investigated using whole-cell patch-clamp technique in mechanically dissociated rat spinal dorsal horn neurons. The results showed that ET actions were concentration-dependent: low concentrations (10 microM) of ET potentiated GABA-activated current (I(GABA)), slowed activation, desensitization and deactivation of GABA(A) receptors; moderate concentrations (10-1,000 microM) of ET directly activated and desensitized GABA(A) receptors; high concentrations (>1,000 microM) of ET produced an inhibitory effect on I(ET). In addition, ET prolonged the duration of GABAergic miniature inhibitory postsynaptic currents (mIPSCs) in the mechanically dissociated rat dorsal horn neurons. These results suggest that general anesthetics-induced changes at spinal level could significantly contribute to analgesia and general anesthesia.

Inhibitory effect of theophylline on recombinant GABA(A) receptor

Convulsions due to systemic toxicity are a major and frequently fatal side effect of theophylline. The cause of theophylline-induced convulsions is not clear, but antagonism of the inhibitory nervous system may be implicated, so we examined the effects of theophylline on GABA-induced currents using recombinant GABA(A) receptor (GABA(A)-R). Theophylline dose-dependently inhibited GABA-induced currents: the IC50 value was 1841+/-63 microM and Hill coefficient 1.09+/-0.03. The inhibitory action of theophylline on GABA-induced currents was competitive and voltage dependent. The inhibition of GABA-induced currents by theophylline may be a primary mechanism underlying theophylline-induced convulsions.

Use of sedative and analgesic agents in neurotrauma patients: effects on cerebral physiology

Sedation and analgesia is used primarily in the intensive care unit (ICU) to limit the stress response to critical illness, provide anxiolysis, improve ventilatory support, and facilitate adequate ICU care. However, in the neurotrauma ICU there are many other reasons for the use of these agents. The primary aim is to prevent secondary cerebral damage by maintaining adequate cerebral perfusion pressures. This is accomplished in several different ways. Controlling intracranial pressure (ICP) and maintaining an adequate mean arterial pressure (MAP) is at the cornerstone of this management. Lowering the metabolic demands of the brain is also an important consideration as a treatment strategy. Analgesic and sedative agents are utilized to prevent undesirable increases in ICP and to lower cerebral metabolic demands. Concerns surrounding the use of these agents include time to awakening after discontinuation, effect on the cerebrovasculature, and the effect on patient outcome. There are many different pharmacological agents available, each with their distinct advantages and disadvantages. The purpose of this review is to evaluate the pharmacokinetic and pharmacological effects of each of these agents when used in neurotrauma patients.

Protracted postnatal development of inhibitory synaptic transmission in rat hippocampal area CA1 neurons

In the CNS, inhibitory synaptic function undergoes profound transformation during early postnatal development. This is due to variations in the subunit composition of subsynaptic GABA(A) receptors (GABA(A)Rs) at differing developmental stages as well as other factors. These include changes in the driving force for chloride-mediated conductances as well as the quantity and/or cleft lifetime of released neurotransmitter. The present study was undertaken to investigate the nature and time course of developmental maturation of GABAergic synaptic function in hippocampal CA1 pyramidal neurons. In neonatal [postnatal day (P) 1-7] and immature (P8-14) CA1 neurons, miniature inhibitory postsynaptic currents (mIPSCs) were significantly larger, were less frequent, and had slower kinetics compared with mIPSCs recorded in more mature neurons. Adult mIPSC kinetics were achieved by the third postnatal week in CA1 neurons. However, despite this apparent maturation of mIPSC kinetics, significant differences in modulation of mIPSCs by allosteric agonists in adolescent (P15-21) neurons were still evident. Diazepam (1-300 nM) and zolpidem (200 nM) increased the amplitude of mIPSCs in adolescent but not adult neurons. Both drugs increased mIPSC decay times equally at both ages. These differential agonist effects on mIPSC amplitude suggest that in adolescent CA1 neurons, inhibitory synapses operate differently than adult synapses and function as if subsynaptic receptors are not fully occupied by quantal release of GABA. Rapid agonist application experiments on perisomatic patches pulled from adolescent neurons provided additional support for this hypothesis. In GABA(A)R currents recorded in these patches, benzodiazepine amplitude augmentation effects were evident only when nonsaturating GABA concentrations were applied. Furthermore nonstationary noise analysis of mIPSCs in P15-21 neurons revealed that zolpidem-induced mIPSC augmentation was not due to an increase in single-channel conductance of subsynaptic GABA(A)Rs but rather to an increase in the number of open channels responding to a single GABA quantum, further supporting the hypothesis that synaptic receptors may not be saturated during synaptic function in adolescent neurons. These data demonstrate that inhibitory synaptic transmission undergoes a markedly protracted postnatal maturation in rat CA1 pyramidal neurons. In the first two postnatal weeks, mIPSCs are large in amplitude, are slow, and occur infrequently. By the third postnatal week, mIPSCs have matured kinetically but retain distinct responses to modulatory drugs, possibly reflecting continued immaturity in synaptic structure and function persisting through adolescence.

The alpha and gamma subunit-dependent effects of local anesthetics on recombinant GABA(A) receptors

Although convulsions due to local anesthetic systemic toxicity are thought to be due to inhibition of GABA(A) receptor-linked currents in the central nervous system, the mechanism of action remains unclear. We therefore examined the effects of local anesthetics on gamma-aminobutyric acid (GABA)-induced currents using recombinant GABA(A) receptors with specific combinations of subunits. Murine GABA(A) receptors were expressed by injection of cRNAs encoding each subunit into Xenopus oocytes. The effects of local anesthetics (lidocaine, bupivacaine, procaine and tetracaine) on GABA-induced currents of receptors expressing different subunit combinations (alpha1beta2, alpha1beta2gamma2s, alpha4beta2gamma2s and beta2) were examined via the two electrode voltage clamp method. At alpha1beta2, alpha1beta2gamma2s and alpha4beta2gamma2s GABA(A) receptors, all local anesthetics inhibited GABA-induced currents in a dose-dependent manner. The presence of the gamma2s subunit resulted in a greater inhibition by all local anesthetics, but the presence of the alpha4 subunit resulted in less inhibition. At beta2 homomeric receptors, local anesthetics directly induced an outward current similar to that of picrotoxin. These data indicated that (1) the alpha and gamma subunits of GABA(A) receptors modulated the inhibitory effects of local anesthetics on GABA(A) function, and (2) local anesthetics can activate the beta2 subunit and may block the GABA(A) receptor channel pore.

Identification of an amino acid defining the distinct properties of murine beta1 and beta3 subunit-containing GABA(A) receptors

Murine gamma-aminobutyric acid (GABA) type A homomeric receptors made of beta1 subunits are profoundly different, when expressed in Xenopus oocytes, from beta3 homomeric receptors. Application of the intravenous general anesthetic pentobarbital, etomidate, or propofol to beta3 homomeric receptors allows current flow. In contrast, beta1 homomers do not respond to any of these agents. Through construction of chimeric beta1/beta3 receptors, we identified a single amino acid that determines the pharmacological difference between the two beta subunits. When the serine residue present in the wild-type nonresponsive beta1 subunit is replaced by an asparagine found in the same position in the beta3 subunit, the resulting point-mutated beta1S265N forms receptors responsive to intravenous general anesthetics, like the wild-type beta3 subunits. Conversely, after mutation of the wild-type beta3 to beta3N265S, the homomeric receptor loses its ability to respond to these same general anesthetics. Wild-type-to-mutant titration experiments showed that the nonresponsive phenotype is dominant: A single nonresponsive residue within a pentameric receptor is sufficient to render the receptor nonresponsive. In alpha1betax or alpha1betaxgamma2 heteromeric receptors, the same residue manifests as a partial determinant of the degree of potentiation of the GABA-induced current by some general anesthetics. The location of this amino acid at the extracellular end of the second transmembrane segment, its influence in both homomeric and heteromeric receptor function, and its dominant behavior suggest that this residue of the beta subunit is involved in an allosteric modulation of the receptor.

General anaesthetic actions on ligand-gated ion channels

The molecular mechanisms of general anaesthetics have remained largely obscure since their introduction into clinical practice just over 150 years ago. This review describes the actions of general anaesthetics on mammalian neurotransmitter-gated ion channels. As a result of research during the last several decades, ligand-gated ion channels have emerged as promising molecular targets for the central nervous system effects of general anaesthetics. The last 10 years have witnessed an explosion of studies of anaesthetic modulation of recombinant ligand-gated ion channels, including recent studies which utilize chimeric and mutated receptors to identify regions of ligand-gated ion channels important for the actions of general anaesthetics. Exciting future directions include structural biology and gene-targeting approaches to further the understanding of general anaesthetic molecular mechanisms.

Blocking effects of the anaesthetic etomidate on human brain sodium channels

Sodium channels from human brain tissue were incorporated into voltage-clamped planar lipid bilayers in presence of batrachotoxin and exposed to increasing concentrations of the intravenous anaesthetic drug etomidate (0.03-1.02 mM). Etomidate interacted with the sodium-conducting pathway of the channel causing a concentration-dependent block of the time-averaged sodium conductance (computer fit of the concentration-response curve: half-maximal blocking concentration, EC50, 0.19 mM; maximal block, block(max), 38%). This block of sodium-conductance resulted from two distinct effects (I) major effect: reduction of the sodium-channel amplitude and (II) minor effect: reduction of the fractional channel open-time. These results were observed at concentrations above clinically-relevant serum concentrations (up to 0.01 mM), suggesting only a limited role for human brain sodium channels in the mechanism of action of etomidate during clinical anaesthesia.