Volatile anesthetic inhibition of neuronal Ca channel currents expressed in Xenopus oocytes

Nicardipine constant rate infusion alleviates the cardiovascular effects of dexmedetomidine infusions without affecting the minimal alveolar concentration in sevoflurane-anesthetized dogs

Two randomized crossover trials evaluated the effects of nicardipine constant rate infusion (CRI) on 1) the anesthetic potency of sevoflurane and 2) the ability to attenuate dexmedetomidine-induced cardiovascular depression in anesthetized dogs. First, six healthy Beagle dogs weighing 11.7 ± 0.9 kg were allocated to one of three treatments that administered a CRI of carrier (saline) or dexmedetomidine 0.5 or 3.0 μg/kg/h following a loading dose. The minimum alveolar concentration (MAC) of sevoflurane was determined utilizing electric stimuli before and after the loading dose of nicardipine (20 μg/kg intravenously for 10 min), followed by CRI at 40 μg/kg/h with 60 min of equilibration. Subsequently, cardiovascular and blood gas variables were evaluated in another trial under sevoflurane anesthesia at the individual 1.5 MAC. After baseline measurements, the dogs were assigned to two treatments (dexmedetomidine CRI at 0.5 or 3.0 μg/kg/h following a loading dose) with sevoflurane doses adjusted to 1.5 times of MAC equivalent, and the measurements were repeated every 15 min for 120 min. After 60 min, nicardipine CRI at 40 μg/kg/h with a loading dose was added to the dexmedetomidine CRI. Dexmedetomidine infusions significantly decreased the sevoflurane MAC but nicardipine did not significantly alter the MAC either with or without dexmedetomidine CRI in dogs. Dexmedetomidine dose-dependently decreased the cardiac index and increased the systemic vascular resistance index; these effects were fully counteracted by concomitant nicardipine CRI. Nicardipine CRI can be useful for controlling the cardiovascular depression elicited by dexmedetomidine in anesthetized dogs without affecting the anesthetic potency of sevoflurane.

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.

Frequency-Dependent Block of Excitatory Neurotransmission by Isoflurane via Dual Presynaptic Mechanisms

Volatile anesthetics are widely used for surgery, but neuronal mechanisms of anesthesia remain unidentified. At the calyx of Held in brainstem slices from rats of either sex, isoflurane at clinical doses attenuated EPSCs by decreasing the release probability and the number of readily releasable vesicles. In presynaptic recordings of Ca²⁺ currents and exocytic capacitance changes, isoflurane attenuated exocytosis by inhibiting Ca²⁺ currents evoked by a short presynaptic depolarization, whereas it inhibited exocytosis evoked by a prolonged depolarization via directly blocking exocytic machinery downstream of Ca²⁺ influx. Since the length of presynaptic depolarization can simulate the frequency of synaptic inputs, isoflurane anesthesia is likely mediated by distinct dual mechanisms, depending on input frequencies. In simultaneous presynaptic and postsynaptic action potential recordings, isoflurane impaired the fidelity of repetitive spike transmission, more strongly at higher frequencies. Furthermore, in the cerebrum of adult mice, isoflurane inhibited monosynaptic corticocortical spike transmission, preferentially at a higher frequency. We conclude that dual presynaptic mechanisms operate for the anesthetic action of isoflurane, of which direct inhibition of exocytic machinery plays a low-pass filtering role in spike transmission at central excitatory synapses.

SIGNIFICANCE STATEMENT Synaptic mechanisms of general anesthesia remain unidentified. In rat brainstem slices, isoflurane inhibits excitatory transmitter release by blocking presynaptic Ca²⁺ channels and exocytic machinery, with the latter mechanism predominating in its inhibitory effect on high-frequency transmission. Both in slice and in vivo, isoflurane preferentially inhibits spike transmission induced by high-frequency presynaptic inputs. This low-pass filtering action of isoflurane likely plays a significant role in general anesthesia.

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.

Role of specific presynaptic calcium channel subtypes in isoflurane inhibition of synaptic vesicle exocytosis in rat hippocampal neurones

BACKGROUND

P/Q- and N-type voltage-gated calcium channels (VGCC) are the principal subtypes mediating synaptic vesicle (SV) exocytosis. Both the degree of isoflurane inhibition of SV exocytosis and VGCC subtype expression vary between brain regions and neurotransmitter phenotype. We hypothesised that differences in VGCC subtype expression contribute to synapse-selective presynaptic effects of isoflurane.

METHODS

We used quantitative live-cell imaging to measure exocytosis in cultured rat hippocampal neurones after transfection of the fluorescent biosensor vGlut1-pHluorin. Selective inhibitors of P/Q- and N-type VGCCs were used to isolate subtype-specific effects of isoflurane.

RESULTS

Inhibition of N-type channels by 1 μM ω-conotoxin GVIA reduced SV exocytosis to 81±5% of control (n=10). Residual exocytosis mediated by P/Q-type channels was further inhibited by isoflurane to 42±4% of control (n=10). The P/Q-type channel inhibitor ω-agatoxin IVA at 0.4 μM inhibited SV exocytosis to 29±3% of control (n=10). Residual exocytosis mediated by N-type channels was further inhibited by isoflurane to 17±3% of control (n=10). Analysis of isoflurane effects at the level of individual boutons revealed no difference in sensitivity to isoflurane between P/Q- or N-type channel-mediated SV exocytosis (P=0.35). There was no correlation between the effect of agatoxin (P=0.91) or conotoxin (P=0.15) and the effect of isoflurane on exocytosis.

CONCLUSIONS

Sensitivity of SV exocytosis to isoflurane in rat hippocampal neurones is independent of the specific VGCC subtype coupled to exocytosis. The differential sensitivity of VGCC subtypes to isoflurane does not explain the observed neurotransmitter-selective effects of isoflurane in hippocampus.

Editor's Highlight: Genetic Targets of Acute Toluene Inhalation in Drosophila melanogaster

Interpretation and use of data from high-throughput assays for chemical toxicity require links between effects at molecular targets and adverse outcomes in whole animals. The well-characterized genome of Drosophila melanogaster provides a potential model system by which phenotypic responses to chemicals can be mapped to genes associated with those responses, which may in turn suggest adverse outcome pathways associated with those genes. To determine the utility of this approach, we used the Drosophila Genetics Reference Panel (DGRP), a collection of ∼200 homozygous lines of fruit flies whose genomes have been sequenced. We quantified toluene-induced suppression of motor activity in 123 lines of these flies during exposure to toluene, a volatile organic compound known to induce narcosis in mammals via its effects on neuronal ion channels. We then applied genome-wide association analyses on this effect of toluene using the DGRP web portal (http://dgrp2.gnets.ncsu.edu), which identified polymorphisms in candidate genes associated with the variation in response to toluene exposure. We tested ∼2 million variants and found 82 polymorphisms located in or near 66 candidate genes that were associated with phenotypic variation for sensitivity to toluene at P < 5 × 10-5, and human orthologs for 52 of these candidate Drosophila genes. None of these orthologs are known to be involved in canonical pathways for mammalian neuronal ion channels, including GABA, glutamate, dopamine, glycine, serotonin, and voltage sensitive calcium channels. Thus this analysis did not reveal a genetic signature consistent with processes previously shown to be involved in toluene-induced narcosis in mammals. The list of the human orthologs included Gene Ontology terms associated with signaling, nervous system development and embryonic morphogenesis; these orthologs may provide insight into potential new pathways that could mediate the narcotic effects of toluene.

Isoflurane causes concentration-dependent inhibition of medullary raphé 5-HT neurons in situ

BACKGROUND

Anesthetics have a profound influence on a myriad of autonomic processes. Mechanisms of general anesthesia, and how these mechanisms give rise to the multifaceted state of anesthesia, are largely unknown. The ascending and descending serotonin (5-HT) networks are key modulators of autonomic pathways, and are critically involved in homeostatic reflexes across the motor, somatosensory, limbic and autonomic systems. These 5-HT networks are thought to contribute to anesthetic effects, but how anesthetics affect 5-HT neuron function remains a pertinent question. We hypothesized that the volatile anesthetic isoflurane inhibits action potential discharge of medullary raphé 5-HT neurons.

METHODS

We conducted extracellular recordings on individual neurons in the medullary raphé region of the unanesthetized in situ perfused brainstem preparation to determine how exposure to isoflurane affects 5-HT neurons. We examined changes in 5-HT neuron baseline firing in response to treatment with either 1, 1.5, or 2% isoflurane. We measured isoflurane concentrations by gas chromatography-mass spectrometry (GC-MS) analysis.

RESULTS

Exposure to isoflurane inhibited action potential discharge in raphé 5-HT neurons. We document a concentration-dependent inhibition over a range of concentrations approximating isoflurane MAC (minimum alveolar concentration required for surgical anesthesia). Delivered concentrations of isoflurane were confirmed using GC-MS analysis.

CONCLUSIONS

These findings illustrate that halogenated anesthetics greatly affect 5-HT neuron firing and suggest 5-HT neuron contributions to mechanisms of general anesthesia.

PKC independent inhibition of voltage gated calcium channels by volatile anesthetics in freshly isolated vascular myocytes from the aorta

In this study we used barium currents through voltage gated L-type calcium channels (recorded in freshly isolated cells with a conventional patch-clamp technique) to elucidate the cellular action mechanism for volatile anesthetics. It was found that halothane and isoflurane inhibited (dose-dependently and voltage independently) Ba2+ currents through voltage gated Ca2+ channels. Half maximal inhibitions occurred at 0.64 ± 0.07 mM and 0.86 ± 0.1 mM. The Hill slope value was 2 for both volatile anesthetics, suggesting the presence of more than one interaction site. Current inhibition by volatile anesthetics was prominent over the whole voltage range without changes in the peak of the current voltage relationship. Intracellular infusion of the GDPβS (100 μM) together with staurosporine (200 nM) did not prevent the inhibitory effect of volatile anesthetics. Unlike pharmacological Ca2+ channel blockers, volatile anesthetics blocked Ca2+ channel currents at resting membrane potentials. In other words, halothane and isoflurane induced an 'initial block'. After the first 4-7 control pulses, the cells were left unstimulated and anesthetics were applied. The first depolarization after the pause evoked a Ca2+ channel current whose amplitude was reduced to 41 ± 3.4% and to 57 ± 4.2% of control values. In an analysis of the steady-state inactivation curve for voltage dependence, volatile anesthetics induced a negative shift of the 50% inactivation of the calcium channels. By contrast, the steepness factor characterizing the voltage sensitivity of the channels was unaffected. Unitary L-type Ca2+ channels blockade occurred under cell-attached configuration, suggesting a possible action of volatile anesthetics from within the intracellular space or from the part of the channel inside the lipid bilayer.

How "Pharmacoresistant" is Cav2.3, the Major Component of Voltage-Gated R-type Ca2+ Channels?

Membrane-bound voltage-gated Ca²⁺ channels (VGCCs) are targets for specific signaling complexes, which regulate important processes like gene expression, neurotransmitter release and neuronal excitability. It is becoming increasingly evident that the so called “resistant” (R-type) VGCC Ca_v2.3 is critical in several physiologic and pathophysiologic processes in the central nervous system, vascular system and in endocrine systems. However its eponymous attribute of pharmacologic inertness initially made in depth investigation of the channel difficult. Although the identification of SNX-482 as a fairly specific inhibitor of Ca_v2.3 in the nanomolar range has enabled insights into the channels properties, availability of other pharmacologic modulators of Ca_v2.3 with different chemical, physical and biological properties are of great importance for future investigations. Therefore the literature was screened systematically for molecules that modulate Ca_v2.3 VGCCs.

P/Q-type calcium channel modulators

P/Q-type calcium channels are high-voltage-gated calcium channels contributing to vesicle release at synaptic terminals. A number of neurological diseases have been attributed to malfunctioning of P/Q channels, including ataxia, migraine and Alzheimer's disease. To date, only two specific P/Q-type blockers are known: both are peptides deriving from the spider venom of Agelenopsis aperta, ω-agatoxins. Other peptidic calcium channel blockers with activity at P/Q channels are available, albeit with less selectivity. A number of low molecular weight compounds modulate P/Q-type currents with different characteristics, and some exhibit a peculiar bidirectional pattern of modulation. Interestingly, there are a number of therapeutics in clinical use, which also show P/Q channel activity. Because selectivity as well as the exact mode of action is different between all P/Q-type channel modulators, the interpretation of clinical and experimental data is complicated and needs a comprehensive understanding of their target profile. The situation is further complicated by the fact that information on potency varies vastly in the literature, which may be the result of different experimental systems, conditions or the splice variants of the P/Q channel. This review attempts to provide a comprehensive overview of the compounds available that affect the P/Q-type channel and should help with the interpretation of results of in vitro experiments and animal models. It also aims to explain some clinical observations by implementing current knowledge about P/Q channel modulation of therapeutically used non-selective drugs. Chances and challenges of the development of P/Q channel-selective molecules are discussed.

Are neuronal voltage-gated calcium channels valid cellular targets for general anesthetics?

The effects of anesthetics and analgesics on ion channels have been the subject of intense research since recent reports of direct actions of anesthetic molecules on ion channel proteins.  It is now known that ligand-gated channels, particularly γ-amino-butyric acid (GABAA) and N-methyl-D-aspartate (NMDA) receptors, play a key role in mediating anesthetic actions, but these channels are unable to account for all aspects of clinical anesthesia such as loss of consciousness, immobility, analgesia, amnesia, and muscle relaxation.  Furthermore, an assortment of voltage-gated and background channels also display anesthetic sensitivity and a key question arises: What role do these other channels play in clinical anesthesia? These channels have overlapping physiological roles and pharmacological profiles, making it difficult to assign aspects of the anesthetic state to individual channel types.  Here, we will focus on the function of neuronal voltage-gated calcium channels in mediating the effects of general anesthetics.

Molecular mechanisms of general anesthesia

General anesthetics produce a widespread neurodepression in the central nervous system by enhancing inhibitory neurotransmission and reducing excitatory neurotransmission. However, the action mechanisms of general anesthetics are not completely understood. Moreover, the general anesthetic state comprises multiple components (amnesia, unconsciousness, analgesia, and immobility), each of which is mediated by different receptors and neuronal pathways. Recently, neurotransmitter- and voltage-gated ion channels have emerged as the most likely molecular targets for general anesthetics. The γ-aminobutyric acid type A (GABA_A) receptors are leading candidates as a primary target of general anesthetics. This review summarizes current knowledge on how anesthetics modify GABA_A receptor function.

Sodium channels and the synaptic mechanisms of inhaled anaesthetics

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

Isoflurane-sensitive presynaptic R-type calcium channels contribute to inhibitory synaptic transmission in the rat thalamus

Because inhibitory synaptic transmission is a major mechanism of general anesthesia, we examined the effects of isoflurane on properties of GABAergic inhibitory currents in the reticular thalamic nucleus (nRT) in brain slices. The evoked IPSCs (eIPSCs) and spontaneous miniature synaptic currents (mIPSCs) of visualized nRT cells in young and adult rats were recorded. Consistent with postsynaptic effects on GABA(A) receptors, isoflurane prolonged the decay-time constants of both eIPSCs and mIPCSs. Surprisingly, isoflurane completely inhibited the amplitude of eIPSCs at clinically relevant concentrations (IC(50) of 240+/-20 microm), increased the paired-pulse ratio, and decreased the frequency of mIPSCs, indicating that presynaptic mechanisms may also contribute to the effects of isoflurane on IPSCs. The overall effect of isoflurane on eIPSCs in nRT cells was a decrease of net charge-transfer across the postsynaptic membrane. The application of 100 microm nickel (Ni(2+)) and the more specific R-type Ca(2+) channel blocker SNX-482 (0.5 microm) decreased eIPSC amplitudes, increased the paired-pulse ratio, and attenuated isoflurane-induced inhibition of eIPSCs. In addition, isoflurane potently blocked currents in recombinant human Ca(V)2.3 (alpha1E) channels with an IC(50) of 206 +/- 22 mum. Importantly, in vivo electroencephalographic (EEG) recordings in adult Ca(V)2.3 knock-out mice demonstrated alterations in isoflurane-induced burst-suppression activity. Because the thalamus has a key function in processing sensory information, sleep, and cognition, modulation of its GABAergic tone by presynaptic R-type Ca(2+) channels may contribute to the clinical effects of general anesthesia.

Perturbation of Voltage-Sensitive Ca2+ Channel Function by Volatile Organic Solvents

The mechanisms underlying the acute neurophysiological and behavioral effects of volatile organic compounds (VOCs) remain to be elucidated. However, the function of neuronal ion channels is perturbed by VOCs. The present study examined effects of toluene (TOL), trichloroethylene (TCE), and perchloroethylene (PERC) on whole-cell calcium current (ICa) in nerve growth factor-differentiated pheochromocytoma (PC12) cells. All three VOCs affected ICa in a reversible, concentration-dependent manner. At +10-mV test potentials, VOCs inhibited ICa, whereas at test potentials of -20 and -10 mV, they potentiated it. The order of potency for inhibition (IC50) was PERC (270 microM) > TOL (720 microM) > TCE (1525 microM). VOCs also changed ICa inactivation kinetics from a single- to double-exponential function. Voltage-ramp experiments suggested that VOCs shifted ICa activation in a hyperpolarizing direction; this was confirmed by calculating the half-maximal voltage of activation (V1/2, act) in the absence and presence of VOCs using the Boltzman equation. V(1/2, act) was shifted from approximately -2 mV in control to -11, -12, and -16 mV by TOL, TCE, and PERC, respectively. Similarly, VOCs shifted the half-maximal voltage of steady-state inactivation (V1/2, inact) from approximately -16 mV in control to -32, -35, and -20 mV in the presence of TOL, TCE, and PERC, respectively. Inhibition of ICa by TOL was confirmed in primary cultures of cortical neurons, where 827 microM TOL inhibited current by 61%. These data demonstrate that VOCs perturb voltage-sensitive Ca2+ channel function in neurons, an effect that could contribute to the acute neurotoxicity of these compounds.

Contrasting anesthetic sensitivities of T-type Ca2+ channels of reticular thalamic neurons and recombinant Ca(v)3.3 channels

Reticular thalamocortical neurons express a slowly inactivating T-type Ca(2+) current that is quite similar to that recorded from recombinant Ca(v)3.3b (alpha1Ib) channels. These neurons also express abundant Ca(v)3.3 mRNA, suggesting that it underlies the native current. Here, we test this hypothesis by comparing the anesthetic sensitivities of recombinant Ca(v)3.3b channels stably expressed in HEK 293 cells to native T channels in reticular thalamic neurons (nRT) from brain slices of young rats. Barbiturates completely blocked both Ca(v)3.3 and nRT currents, with pentobarbital being about twice more potent in blocking Ca(v)3.3 currents. Isoflurane had about the same potency in blocking Ca(v)3.3 and nRT currents, but enflurane, etomidate, propofol, and ethanol exhibited 2-4 fold higher potency in blocking nRT vs Ca(v)3.3 currents. Nitrous oxide (N(2)O; laughing gas) blocked completely nRT currents with IC(50) of 20%, but did not significantly affect Ca(v)3.3 currents at four-fold higher concentrations. In addition, we observed that in lower concentration, N(2)O reversibly increased nRT but not Ca(v)3.3 currents. In conclusion, contrasting anesthetic sensitivities of Ca(v)3.3 and nRT T-type Ca(2+) channels strongly suggest that different molecular structures of Ca(2+) channels give rise to slowly inactivating T-type Ca(2+) currents. Furthermore, effects of volatile anesthetics and ethanol on slowly inactivating T-type Ca(2+) channel variants may contribute to the clinical effects of these agents.

Developing an exposure-dose-response model for the acute neurotoxicity of organic solvents: overview and progress on in vitro models and dosimetry

We are developing an exposure-dose-response (EDR) model for volatile organic compounds (VOCs) to predict acute effects of VOCs on nervous system function from exposure data (concentration and duration of inhalation). This model contains both toxicokinetic and toxicodynamic components. One advantage of the EDR model will be its ability to relate in vitro effects of solvents on cellular ion channels (putative targets) to in vivo effects, using a combination of physiologically-based toxicokinetic (PBTK) modeling (to estimate VOC concentrations in the blood and brain) and in vitro studies to clarify the mode of action of the VOCs. Recent work in vitro has focused on quantifying the inhibitory effects of toluene, trichloroethylene (TCE) and perchloroethylene (PERC) on ion channel currents. All three VOCs inhibit current through voltage-sensitive calcium channels (VSCCs) in pheochromocytoma cells; PERC blocked calcium currents and altered the current-voltage relationship at lower concentrations than did toluene or TCE. Recombinant nicotinic acetylcholine receptors (nAChRs), expressed in Xenopus oocytes, were also inhibited by PERC and toluene in a concentration-dependent manner. PERC inhibited α7 receptors more than α4β2 receptors in recombinant human and rat nAChRs. However, human and rat α7 receptors were equally sensitive to PERC and TOL. These in vitro studies will be used to identify an appropriate neuronal receptor system to serve as an index of acute effects of VOCs in vivo. The PBTK model incorporates physiological input parameters derived from radiotelemetered heart rate data from rats performing operant tests of cognitive and motor functions. These studies should improve predictions of target organ concentrations of inhaled VOCs in subjects actively performing behavioral tests over a range of physical activity levels.

Effects of isoflurane and xenon on Ba2+-currents mediated by N-type calcium channels

BACKGROUND

Isoflurane and xenon are inhalation general anaesthetics with differing clinical profiles and contrasting synaptic actions. Both agents have been shown to depress excitatory synaptic responses. Whether this is via pre-synaptic or post-synaptic mechanisms has not been determined clearly. N-type calcium channels are a putative pre-synaptic target for these agents. We tested whether N-type calcium channels were sensitive to isoflurane and xenon and whether there was any stereoselectivity in the effect of isoflurane.

METHODS

We used patch-clamp electrophysiology on isolated HEK293 cells stably expressing N-type calcium channels to investigate the effects of isoflurane and xenon on barium currents mediated by N-type calcium channels.

RESULTS

Racemic isoflurane caused a concentration-dependent reduction (11-35%) in the peak current through the N-type channels in the concentration range 0.15-1.22 mM. In the clinically relevant concentration range the inhibition was small. At an isoflurane concentration of 0.31 mM (equivalent to 1 MAC), the peak N-type current was inhibited by 14 (1)%. The optical isomers of isoflurane were found to be equally potent at inhibiting currents through N-type channels. The inert gas anaesthetic xenon was found to have no measureable effect on N-type channels at a concentration of 3.4 mM (approximately 1 MAC).

CONCLUSIONS

These results suggest that N-type calcium channels are not the targets mediating general anaesthesia with these two inhalation agents.

The General Anesthetic Isoflurane Depresses Synaptic Vesicle Exocytosis

General anesthetics have marked effects on synaptic transmission, but the mechanisms of their presynaptic actions are unclear. We used quantitative laser-scanning fluorescence microscopy to analyze the effects of the volatile anesthetic isoflurane on synaptic vesicle cycling in cultured neonatal rat hippocampal neurons monitored using either transfection of a pH-sensitive form of green fluorescent protein fused to the luminal domain of VAMP (vesicle-associated membrane protein), (synapto-pHluorin) or vesicle loading with the fluorescent dye FM 1-43. Isoflurane reversibly inhibited action potential-evoked exocytosis over a range of concentrations, with little effect on vesicle pool size. In contrast, exocytosis evoked by depolarization in response to an elevated extracellular concentration of KCl, which is insensitive to the selective Na+ channel blocker tetrodotoxin, was relatively insensitive to isoflurane. Inhibition of exocytosis by isoflurane was resistant to bicuculline, indicating that this presynaptic effect is not caused by the well known GABA(A) receptor modulation by volatile anesthetics. Depression of exocytosis was mimicked by a reduction in stimulus frequency, suggesting a reduction in action potential initiation, conduction, or coupling to Ca2+ channel activation. There was no evidence for a direct effect on endocytosis. The effects of isoflurane on synaptic transmission are thus caused primarily by inhibition of action potential-evoked synaptic vesicle exocytosis at a site upstream of Ca2+ entry and exocytosis, possibly as a result of Na+ channel blockade and/or K+ channel activation, with the possibility of lesser contributions from Ca2+ channel blockade and/or soluble N-ethylmaleimide-sensitive factor attachment protein receptor-mediated vesicle fusion.

Multiple synaptic and membrane sites of anesthetic action in the CA1 region of rat hippocampal slices

BACKGROUND

Anesthesia is produced by a depression of central nervous system function, however, the sites and mechanisms of action underlying this depression remain poorly defined. The present study compared and contrasted effects produced by five general anesthetics on synaptic circuitry in the CA1 region of hippocampal slices.

RESULTS

At clinically relevant and equi-effective concentrations, presynaptic and postsynaptic anesthetic actions were evident at glutamate-mediated excitatory synapses and at GABA-mediated inhibitory synapses. In addition, depressant effects on membrane excitability were observed for CA1 neuron discharge in response to direct current depolarization. Combined actions at several of these sites contributed to CA1 circuit depression, but the relative degree of effect at each site was different for each anesthetic studied. For example, most of propofol's depressant effect (> 70 %) was reversed with a GABA antagonist, but only a minor portion of isoflurane's depression was reversed (< 20 %). Differences were also apparent on glutamate synapses-pentobarbital depressed transmission by > 50 %, but thiopental by only < 25 %.

CONCLUSIONS

These results, in as much as they may be relevant to anesthesia, indicate that general anesthetics act at several discrete sites, supporting a multi-site, agent specific theory for anesthetic actions. No single effect site (e.g. GABA synapses) or mechanism of action (e.g. depressed membrane excitability) could account for all of the effects produced for any anesthetic studied.

Acid-sensitive two-pore domain potassium (K2P) channels in mouse taste buds

Sour (acid) taste is postulated to result from intracellular acidification that modulates one or more acid-sensitive ion channels in taste receptor cells. The identity of such channel(s) remains uncertain. Potassium channels, by regulating the excitability of taste cells, are candidates for acid transducers. Several 2-pore domain potassium leak conductance channels (K(2)P family) are sensitive to intracellular acidification. We examined their expression in mouse vallate and foliate taste buds using RT-PCR, and detected TWIK-1 and -2, TREK-1 and -2, and TASK-1. Of these, TWIK-1 and TASK-1 were preferentially expressed in taste cells relative to surrounding nonsensory epithelium. The related TRESK channel was not detected, whereas the acid-insensitive TASK-2 was. Using confocal imaging with pH-, Ca(2+)-, and voltage-sensitive dyes, we tested pharmacological agents that are diagnostic for these channels. Riluzole (500 microM), selective for TREK-1 and -2 channels, enhanced acid taste responses. In contrast, halothane (< or = approximately 17 mM), which acts on TREK-1 and TASK-1 channels, blocked acid taste responses. Agents diagnostic for other 2-pore domain and voltage-gated potassium channels (anandamide, 10 microM; Gd(3+), 1 mM; arachidonic acid, 100 microM; quinidine, 200 microM; quinine, 100 mM; 4-AP, 10 mM; and TEA, 1 mM) did not affect acid responses. The expression of 2-pore domain channels and our pharmacological characterization suggest that a matrix of ion channels, including one or more acid-sensitive 2-pore domain K channels, could play a role in sour taste transduction. However, our results do not unambiguously identify any one channel as the acid taste transducer.

Increased sensitivity to halothane but decreased sensitivity to propofol in mice lacking the N-type Ca2+ channel

Volatile anesthetics are known to depress excitatory synaptic transmission. Inhibition of voltage-dependent Ca2+ channels is speculated to underlie this mechanism, which remains to be clarified in vivo. We examined the sensitivity to halothane in mice lacking the N-type Ca2+ channel, a major contributor of presynaptic neurotransmitter release. Sensitivity to halothane was significantly increased in the knockout mice compared with the wild-type littermates. Halothane also depressed field excitatory postsynaptic potentials recorded from the Schaffer collateral-CA1 hippocampal synapses more greatly in the knockout mice. We further examined sleep time induced by injection of propofol, an intravenous anesthetic that mainly affects inhibitory synaptic transmission. In contrast, sensitivity to propofol was significantly decreased in the knockout mice. We suggest that inhibition of the N-type Ca2+ channel underlies mechanisms of halothane anesthesia but counteracts propofol anesthesia.

Anesthetic sensitivities to propofol and halothane in mice lacking the R-type (Cav2.3) Ca2+ channel

Because inhibition of voltage-dependent Ca(2+) channels can be a mechanism underlying general anesthesia, we examined sensitivities to propofol and halothane in mice lacking the R-type (Ca(v)2.3) channel widely expressed in neurons. Sleep time after propofol injection (26 mg/kg IV) and halothane MAC(RR) and MAC (50% effective concentrations for the loss of the righting reflex and for the tail pinch/withdrawal response, respectively) were determined. Significantly shorter propofol-induced sleep time (291.6 +/- 16.8 s versus 344.4 +/- 12.1 s) and larger halothane MAC(RR) (1.11% +/- 0.04% versus 0.98% +/- 0.03%) were observed in Ca(v)2.3 channel knockouts (Ca(v)2.3(-/-)) than in wild-type (Ca(v)2.3(+/+)) litter mates. To investigate the basis of the decreased anesthetic sensitivities in vivo, field excitatory postsynaptic potentials and population spikes (PSs) were recorded from Schaffer collateral CA1 synapses in hippocampal slices. Propofol (10-30 micro M) inhibited PSs by potentiating gamma-aminobutyric acid-ergic inhibition, and this potentiation was markedly smaller at 30 micro M in Ca(v)2.3(-/-) mice, possibly accounting for the decreased propofol sensitivity in vivo. Halothane (1.4%-2.2%) inhibited field excitatory postsynaptic potentials similarly in both genotypes, whereas 1%-2% halothane depressed PSs more in Ca(v)2.3(-/-) mice, suggesting the postsynaptic role of the R-type channel in the propagation of excitation and other mechanisms underlying the increased halothane MAC(RR) in Ca(v)2.3(-/-) mice.

IMPLICATIONS

Because inhibition of neuronal Ca(2+) currents can be a mechanism underlying general anesthesia, we examined anesthetic sensitivities in mice lacking the R-type (Ca(v)2.3) Ca(2+) channels both in vivo and in hippocampal slices. Decreased sensitivities in mutant mice imply a possibility that agents blocking this channel may increase the requirements of anesthetics/hypnotics.

Differential interaction of anaesthetics and antiepileptic drugs with neuronal Na+ channels, Ca2+ channels, and GABA(A) receptors

BACKGROUND

Current theories favour multiple agent-specific neuronal actions for both general anaesthetics and antiepileptic drugs, but the pharmacological properties that distinguish them are poorly understood. We compared the interactions of representative agents from each class on their putative targets using well-characterized radioligand binding assays.

METHODS

Synaptosomes or membranes prepared from rat cerebral cortex were used to analyse drug effects on [(35)S]t-butyl bicyclophosphorothionate ([(35)S]TBPS) binding to the picrotoxinin site of GABA(A) receptors, [(3)H]batrachotoxinin A 20-alpha benzoate ([(3)H]BTX-B) binding to site 2 of voltage-gated Na(+) channels, (+)-[methyl-(3)H]isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-5-methoxycarboxyl-2,6-dimethyl-3-pyridinecarboxylate ([(3)H]PN200-110; isradipine) binding to L-type Ca(2+) channels, and [cyclohexyl-2,3-(3)H](N)glibenclamide ([(3)H]GB) binding to K(ATP) channels.

RESULTS

I.V. anaesthetics other than ketamine preferentially inhibited [(35)S]TBPS binding (etomidate approximately equal alphaxalone > propofol > thiopental > pentobarbital). Volatile anaesthetics inhibited both [(35)S]TBPS and [(3)H]BTX-B binding with comparable potencies (halothane approximately equal isoflurane approximately equal enflurane). Antiepileptic drugs preferentially antagonized either [(35)S]TBPS (diazepam > phenobarbital) or [(3)H]BTX-B (phenytoin > carbamazepine) binding. Local anaesthetics (lidocaine, tertracaine) selectively antagonized [(3)H]BTX-B binding. None of the drugs tested were potent antagonists of [(3)H]PN200-110 or [(3)H]GB binding.

CONCLUSIONS

Comparative radioligand binding assays identified distinct classes of general anaesthetic and antiepileptic drugs based on their relative specificities for a defined target set. I.V. anaesthetics interacted preferentially with GABA(A) receptors, while volatile anaesthetics were essentially equipotent at Na(+) channels and GABA(A) receptors. Antiepileptic drugs could be classified by preferential actions at either Na(+) channels or GABA(A) receptors. Anaesthetics and antiepileptic drugs have agent-specific effects on radioligand binding. Both general anaesthetics and antiepileptic drugs interact with Na(+) channels and GABA(A) receptors at therapeutic concentrations, in most cases with little selectivity.

Effects of halothane and propofol on excitatory and inhibitory synaptic transmission in rat cortical neurons

General anesthetics are thought to act on both excitatory and inhibitory neuronal pathways at both post- and presynaptic sites. However, the literature in these regards is somewhat controversial. The aim of the present study was to reassess the relative importance of the various anesthetic actions using a common preparation. Rat cortical neurons in primary culture were used to record spontaneous miniature postsynaptic currents by the whole-cell patch-clamp technique. Halothane at clinically relevant concentrations prolonged the decay phase of spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded in the presence of tetrodotoxin and at higher concentrations decreased the frequency of mIPSCs. The mIPSC amplitudes underwent little change. Spontaneous action potential-dependent IPSCs recorded in the absence of tetrodotoxin were similarly affected by halothane. Halothane also decreased the frequency of spontaneous miniature non-N-methyl-D-aspartate (NMDA) excitatory postsynaptic currents (mEPSCs) as well as spontaneous action potential-dependent NMDA EPSCs and non-NMDA EPSCs without affecting their decay phase. The halothane effect on mIPSC and mEPSC frequency was dependent on the external calcium concentration. In contrast to halothane, the only effect of propofol was the prolongation of the decay phase of mIPSCs and IPSCs. The prolongation of mIPSCs and IPSCs by halothane and propofol coupled with the ineffectiveness on mEPSCs and EPSCs suggests a selective postsynaptic modulation of GABA(A) receptors. The additional calcium-dependent inhibition of mIPSC and mEPSC frequency by halothane (but not propofol) suggests a more general mechanism by this anesthetic on presynaptic transmitter release.

Halothane attenuates the cerebroprotective action of several Na+ and Ca2+ channel blockers via reversal of their ion channel blockade

We have previously shown the involvement of Na(+) channel as well as N-type and P/Q-type Ca(2+) channels in the oxygen and glucose deprivation-induced injury in rat cerebrocortical slices. In the present study, we investigated the influence of halothane on the cerebroprotective effects of a variety of Na(+) and Ca(2+) channel blockers in rat cerebrocortical slices. The hypoxic injury was attenuated by Na(+) channel blockers including tetrodotoxin, lidocaine and dibucaine, and Ca(2+) channel blockers, such as verapamil, omega-agatoxin IVA and omega-conotoxin GVIA. Halothane abolished the protective effects of lidocaine, dibucaine and verapamil, all of which block the respective cation channels in a voltage-dependent manner, without affecting the actions of tetrodotoxin, omega-agatoxin IVA and omega-conotoxin GVIA, which reveal voltage-independent blockade. On the other hand, the nitric oxide synthesis estimated from the extracellular cyclic GMP formation was elevated during exposure to hypoxia. All channel blockers tested here attenuated hypoxia-evoked nitric oxide synthesis. Halothane blocked almost completely these actions of lidocaine and verapamil. Moreover, the Na(+) and Ca(2+) channel blockade by these compounds, as determined by veratridine- and KCl-stimulated nitric oxide synthesis, respectively, was also reversed by halothane. These findings suggest that an anesthetic agent halothane reversed the Na(+) and Ca(2+) channel blockade of several voltage-dependent ion channel blockers, leading to the attenuation of their cerebroprotective actions. Therefore, the influence of halothane anesthesia should be taken into consideration for the evaluation of neuroprotective action of Na(+) and Ca(2+) channel blockers.

The effects of isoflurane on native and chimeric muscarinic acetylcholine receptors: the role of protein kinase C

By using two electrode voltage clamps, we investigated the effects of isoflurane on m3 and chimeric m1/m3 muscarinic receptors and the role of protein kinase C (PKC) in the effects. Muscarinic receptors were expressed by injection of mRNA into Xenopus oocytes, and Ca(2+)-activated Cl(-) currents were measured after the application of acetyl-beta-methylcholine. We constructed chimeric m1/m3 receptor DNA encoding the third intracellular loop of m1 and the remainder from the m3 receptor. Chimeric and m3 receptors were inhibited by isoflurane, but the m1 receptor was not. PKC activation with phorbol-12-myrisate-13-acetate (50 nM) decreased signaling of both chimeric and m3 receptors significantly. Chelerythrine (20 microM, PKC inhibitor) abolished the effect of isoflurane on chimeric and m3 signaling. Whereas isoflurane inhibition of chimeric and m3 receptors was completely reversible after washout with Tyrode's solution for 3 min, treatment with okadaic acid (500 nM, protein phosphatase inhibitor) rendered the inhibition irreversible. Taken together, our results suggest that isoflurane inhibits m3 and chimeric m1/m3 muscarinic signaling by enhancing PKC activity and that the site of action is located outside of the third intracellular loop.

IMPLICATIONS

By use of the Xenopus oocyte expression system, we investigated the effects of isoflurane on muscarinic signaling and the role of protein kinase C in these effects. Our findings suggest that isoflurane inhibits muscarinic receptors through activation of protein kinase C and that the relevant phosphorylation sites are located outside the third intracellular loop.

Isoflurane improves long-term neurologic outcome versus fentanyl after traumatic brain injury in rats

Despite routine use of fentanyl in patients after traumatic brain injury (TBI), it is unclear if it is the optimal sedative/analgesic agent. Isoflurane is commonly used in experimental TBI. We hypothesized that isoflurane would be neuroprotective versus fentanyl after TBI. Rats underwent controlled cortical impact (CCI) and received 4 h of N2O/O2 (2:1) and either fentanyl (10 microg/kg i.v. bolus, 50 microg/kg/h infusion) or isoflurane (1% by inhalation) with controlled ventilation. Shams underwent identical preparation, without CCI. Functional outcome (beam balance, beam walking, Morris water maze [MWM] tasks) was assessed over 20 days. Lesion volume and hippocampal neuron survival were quantified on day 21. Additional rats underwent identical CCI and anesthesia with intracranial pressure (ICP) monitoring, and brain water content was assessed. Motor and MWM performances were better in injured rats treated with isoflurane versus fentanyl (p < 0.05). CA1 hippocampal damage was attenuated in isoflurane-treated rats (p < 0.05). Fentanyl-treated rats had higher mean arterial blood pressure after injury (p < 0.05); however, ICP and brain water were similar between groups. Isoflurane improved functional outcome and attenuated damage to CA1 versus fentanyl in rats subjected to CCI. Isoflurane may be neuroprotective by augmenting cerebral blood flow and/or reducing excitotoxicity, not by reducing ICP or brain water content. Alternatively, fentanyl may be detrimental. Isoflurane may mask beneficial effects of novel agents tested in TBI models. Additionally, fentanyl may not be optimal early after TBI in humans.