Ethanol-GABA interactions at the rat Purkinje cell

Ethanol decreases Purkinje neuron excitability by increasing GABA release in rat cerebellar slices

Cerebellar Purkinje neurons (PNs) receive inhibitory GABAergic input from stellate and basket cells, which are located in the outer and inner portions of the molecular layer, respectively. Ethanol (EtOH) was recently shown to increase GABAergic transmission at PNs via a mechanism that involves enhanced calcium release from presynaptic internal stores (J Pharmacol Exp Ther 323:356-364, 2007). Here, we further characterized the effect of EtOH on GABA release and assessed its impact on PN excitability. Using whole-cell patch-clamp electrophysiological techniques in cerebellar vermis parasagittal slices, we found that EtOH acutely increases the frequency but not the amplitude or half-width of miniature and spontaneous inhibitory postsynaptic currents (IPSCs). EtOH significantly increased the amplitude and decreased the paired pulse ratio of IPSCs evoked by stimulation in the outer but not inner molecular layer. In current clamp, EtOH decreased both the amplitude of excitatory postsynaptic potentials evoked in PNs by granule cell axon stimulation and the number of action potentials triggered by these events; these effects depended on GABA(A) receptor activation because they were not observed in presence of bicuculline. Loose-patch cell-attached PN recordings revealed that neither the spontaneous action potential firing frequency nor the coefficient of variation of the interspike interval was altered by acute EtOH exposure. These findings suggest that EtOH differentially affects GABAergic transmission at stellate cell- and basket cell-to-PN synapses and that it modulates PN firing triggered by granule cell axonal input. These effects could be in part responsible for the cerebellar impairments associated with acute EtOH intoxication.

Chronic exposure to ethanol alters GABA(A) receptor-mediated responses of layer II pyramidal cells in adult rat piriform cortex

This study examined the effect of chronic exposure to ethanol on gamma-aminobutyric acid type-A (GABA(A)) receptor-mediated responses of layer II pyramidal neurons of the piriform cortex. Slices containing the piriform cortex were derived from pair-fed adult rats maintained on ethanol-supplemented or control liquid diet for 30 days. Responses of identified layer II pyramidal neurons to exogenously applied GABA were monitored by whole-cell patch-clamp recording. Chronic exposure to ethanol resulted in a rightward shift in the EC(50) of GABA and a decrease in the amplitude of maximal GABA response. GABA-induced responses were modulated by acutely applied ethanol (10-100 mM) in both chronic ethanol-treated and control groups. No significant difference was found in the average change in GABA response, suggesting that tolerance to acute ethanol exposure did not develop. When the modulatory responses of individual cells were classified and grouped as either being attenuating, potentiating, or having no effect, the incidence of potentiation in the ethanol-treated group was significantly higher. Consistent with the absence of tolerance to acute ethanol, cross-tolerance to diazepam was not observed following 30 days of treatment with ethanol. These results are discussed in light of regionally specific effects of chronic ethanol treatment on GABA(A) receptor-mediated responses of layer II piriform cortical neurons.

The contribution of electrophysiology to knowledge of the acute and chronic effects of ethanol

This review describes the effects of ethanol on the components of neuronal transmission and the relationship of such effects to the behavioural actions of ethanol. The concentrations of ethanol with acute actions on voltage-sensitive ion channels are first described, then the actions of ethanol on ligand-gated ion channels, including those controlled by cholinergic receptors, 5-hydroxytryptamine receptors, the various excitatory amino acid receptors, and gamma-aminobutyric acid receptors. Acute effects of ethanol are then described on brain areas thought to be involved in arousal and attention, the reinforcing effects of ethanol, the production of euphoria, the actions of ethanol on motor control, and the amnesic effects of ethanol; the acute effects of ethanol demonstrated by EEG studies are also discussed. Chronic effects of alcohol on neuronal transmission are described in the context of the various components of the ethanol withdrawal syndrome, withdrawal hyperexcitability, dysphoria and anhedonia, withdrawal anxiety, craving, and relapse drinking. Electrophysiological studies on the genetic influences on the effects of ethanol are discussed, particularly the acute actions of ethanol and electrophysiological differences reported in individuals predisposed to alcoholism. The conclusion notes the concentration of studies on the classical transmitters, with relative neglect of the effects of ethanol on peptides and on neuronal interactions between brain areas and integrated patterns of neuronal activity.

Molecular pathogenesis of alcohol withdrawal seizures: the modified lipid-protein interaction mechanism

The phrase alcohol withdrawal seizures (AWS) refers to seizures that result from the withdrawal of alcohol after a period of chronic alcohol administration. A mechanism of AWS is postulated, namely the modified lipid-protein interaction (MLPI) mechanism. This hypothesis is based upon an evaluation of the mechanisms of membrane fluidity, calcium channels, gamma-aminobutyric acid (GABA) and glutamate in the molecular pathogenesis of AWS. The mechanism hypothesizes that acute ethanol treatment alters the neuronal membrane lipids which then perturbs protein events, such as affecting the GABAA receptors, NMDA receptors and voltage-dependent Ca2+ channels synergistically or in combination. Subsequent adaptations in these systems occur after prolonged administration of ethanol. A sudden withdrawal of ethanol then leads to hyperexcitability which results in AWS.

Potentiation by ethanol of GABA-induced current and facilitation of its desensitization in cultured rat cortical neurons

1. Patch-clamp whole cell recording was performed to elucidate whether or not ethanol, at low concentration, has an effect on the GABAA receptor in cultured rat cortical neurons as compared with flunitrazepam. 2. Bath application of ethanol (0.01%) or flunitrazepam (1 mM) potentiated the peak amplitude of GABA-induced (10 microM) current without affecting the equilibrium potential. 3. The decay time constant and time to peak of GABA-induced current were shortened in the presence of ethanol or flunitrazepam. 4. These findings indicate that a low concentration of ethanol and flunitrazepam potentiates the GABA-induced current concomitantly with acceleration of desensitization to the drug.

8-Bromo-cAMP mimics beta-adrenergic sensitization of GABA responses to ethanol in cerebellar Purkinje neurons in vivo

Previous studies in our laboratory indicated that electrophysiological responses of cerebellar Purkinje neurons to GABA were not routinely potentiated by ethanol (EtOH), and the potentiation was not large when it occurred. In the presence of beta-adrenergic agonists, such as isoproterenol, however, GABA inhibitions became sensitive to potentiation by EtOH in nearly every Purkinje neuron tested. beta-adrenergic receptor activation alone also modulates (potentiates) GABA responses on Purkinje neurons, and this has been reported to be mediated by a cAMP second messenger system. Herein, we report that the membrane-permeable cAMP analog, 8-bromoadenosine-3',5'-cyclic monophosphate (8-Br-cAMP), but not the membrane-impermeable cAMP, can also modulate GABA responses and that EtOH potentiates this facilitatory action of 8-Br-cAMP. These effects are not likely caused by adenosine receptor mechanisms, because this 8-bromoadenosine mediated modulation and sensitization was observed in the presence of systemic theophylline. These data suggest that the beta-adrenergic modulation and sensitization to EtOH of cerebellar Purkinje neuron GABA responses occur via a cAMP second messenger mechanism.

How has molecular pharmacology contributed to our understanding of the mechanism(s) of general anesthesia?

This review discusses the mechanism(s) of general anesthesia from a pharmacological viewpoint; in particular, the ability of drugs to produce many different effects is emphasised. The problems of experimental measurement of general anesthesia are discussed, and the possibilities for antagonism and potentiation of anesthesia considered. Physicochemical studies on anesthesia are described, as are the advancement of ideas beyond consideration of lipids and proteins as separate sites of action. The importance of studies on different areas of the brain is highlighted, and the review finishes with a survey of the effects of general anesthetics on synaptic transmission which emphasises the problems of extrapolation from in vitro to in vivo.

Electrophysiological interactions of ethanol with GABAergic mechanisms in the rat cerebellum in vivo

Biochemical studies indicate that ethanol (EtOH) will facilitate the activation of the GABAA/Cl- channel, and behavioral studies demonstrate that EtOH-induced sedative and incoordinating effects can be potentiated by GABA mimetics and blocked by GABA antagonists. It has been difficult, however, to demonstrate an EtOH-induced potentiation of the depressant electrophysiological effects of locally applied GABA in mammalian brain in vivo. Similarly, in this study, local EtOH applications only infrequently caused potentiations of the depressant effects of microiontophoretically applied GABA on cerebellar Purkinje neurons, and this interaction was modest when present. The predominant interaction of locally applied EtOH was an antagonism of GABA-induced depressions of neuronal activity. However, the GABAA receptor antagonist bicuculline reversibly and apparently competitively blocked the depressant effects of locally applied EtOH on single cerebellar Purkinje neurons. Our data suggest that EtOH potentiation of GABA responses alone is insufficient to account for EtOH-induced depressions of cerebellar Purkinje neurons. However, these data clearly imply that activation of a GABAA receptor is required for the expression of EtOH-induced depressions of neuronal activity in this brain area. It is less clear how lower, nondepressant doses of EtOH interact with GABA mechanisms. We hypothesize that either the GABAA receptor mechanism must be sensitized to the potentiative effects of EtOH through the influences of neuromodulatory and/or hormonal regulation, or that EtOH interacts directly with these regulatory processes.

Comparison of the effects of ethanol and chlordiazepoxide on electrophysiological activity in the fascia dentata and hippocampus regio superior

Acute intoxicating doses of ethanol-producing blood alcohol levels of 120-200 mg% increase paired-pulse (PP) inhibition in the dentate gyrus of anesthetized rats suggesting that ethanol increases recurrent inhibitory processes (Wiesner, J.B., and S.J. Henriksen (1987) Ethanol enhances recurrent inhibition in the dentate gyrus of the hippocampus. Neurosci. Lett. 79:169-173). To further understanding of the neuronal mechanisms underlying this phenomenon, the authors studied the effects of the benzodiazepine (BZ), chlordiazepoxide, and acute intoxicating levels of ethanol on extracellular field potential recordings and single-unit activity in the dentate gyrus and area CA1 of the hippocampus. In the dentate, ethanol had no effect on population excitatory postsynaptic potential (pEPSP) amplitudes or slopes; decreased population spike (PS) amplitudes (25%); increased PP inhibition; decreased dentate granule cell (DGC) spontaneous activity (58%); had no effect on putative interneuron spontaneous activity; and markedly increased post field potential-evoked interneuron discharges (IDs, 218%). Chlordiazepoxide had no effect on pEPSP amplitudes or slopes or PS amplitudes; increased PP inhibition; decreased DGC (62%) and interneuron (72%) spontaneous activity; and markedly decreased IDs (89%). In CA1, ethanol had no effect on pEPSP amplitudes or slopes; decreased PS amplitudes (26%); had no effect on PP responses; decreased pyramidal cell (PC) spontaneous activity (39%); had no effect on interneuron spontaneous activity; and markedly increased IDs (97%). Chlordiazepoxide had no effect on pEPSP amplitudes or slopes or PS amplitudes; had no effect on PP responses; decreased PC spontaneous activity (41%); and had no effect on interneuron spontaneous activity or IDs. The results suggest that the BZs decrease principal cell excitability by postsynaptic facilitation of inhibitory processes, whereas ethanol decreases principal cell excitability indirectly by increasing the excitability of inhibitory interneurons.

Acute alcohol alters the excitability of cerebellar Purkinje neurons and hippocampal neurons in culture

Acute exposure to ethanol at 22 and 44 mM concentrations altered several features of the current-evoked voltage responses of cerebellar Purkinje neurons and hippocampal neurons studied in culture model systems. Whole cell current clamp techniques were used. At 22 mM, ethanol depressed current-evoked spiking in the hippocampal neurons but enhanced the current-evoked spiking in the Purkinje neurons. In both neuronal types, 44 mM ethanol depressed spiking, the amplitude of the afterhyperpolarization generated at the termination of a current pulse and the amplitude of the off-response generated at the termination of a hyperpolarizing pulse. Ethanol had little or no effect on resting membrane potential or the passive membrane properties measured near resting level in either neuronal type. Some changes in the current-voltage curves were observed at more depolarized or hyperpolarized potentials in both neuronal types. In the Purkinje neurons, where spontaneous activity was a prominent feature of some recordings, exposure to ethanol reduced the frequency of the spontaneous events. These results indicate that acute exposure to ethanol at intoxicating doses alters the membrane excitability of these two CNS neuronal types. The ethanol induced changes in neuronal excitability presumably contribute to the changes in firing properties observed in extracellular recordings from these neuronal types in vivo and the behavioral effects observed during alcohol intoxication in animal models.

Correlation between morphological and biochemical effects of ethanol on neuroblast-enriched cultures derived from three-day-old chick embryos

We have shown that ethanol exposure during embryogenesis affects a variety of parameters of neuronal growth. In this study we examined the direct effects of ethanol exposure on developing neuroblasts in culture. Neuroblast-enriched cultures derived from 3-day-old whole chick embryos were grown in the presence of ethanol at doses ranging from 12.5 to 50 mM from culture day 3-14. Cholinergic and GABAergic phenotypic expression were both significantly reduced following ethanol exposure as assessed by the activities of choline acetyltransferase and glutamate decarboxylase, respectively. Morphometric analysis of the growth patterns showed significant differences between control and ethanol-treated cultures. Control cultures exhibited the characteristic pattern of growth consisting of neuronal aggregation with neuritic arborization, i.e., neuritic bundles and fasciculation. Cultures grown in ethanol from culture day 3 consisted of aggregates that measured significantly greater in size than those observed in control cultures. In addition, in ethanol-treated cultures, the primary pattern of neuritic bundles was replaced by a complex network of individual neurites radiating from the central aggregate, forming a defined "neuritic field." Morphometric analysis revealed that both neurite number and neurite length were significantly reduced in ethanol-treated cultures. The biochemical data confirm earlier reports from this laboratory suggesting that ethanol exposure during early embryogenesis alters the normal neuronal pattern of phenotypic expression. In addition, we have presented evidence in this study that ethanol alters the morphological growth patterns of developing neurons. Although ethanol does not alter the ability of these cells to aggregate, there is a significant alteration in neuritic outgrowth.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol potentiation of GABA-induced electrophysiological responses in cerebellum: requirement for catecholamine modulation

In this study, we confirmed that microiontophoretically applied norepinephrine (NE) and isoproterenol potentiate the depressant effects of locally-applied gamma-aminobutyric acid (GABA) on cerebellar Purkinje neurons of anesthetized rats. Although ethanol (EtOH) does not reliably or efficaciously potentiate GABA-induced depressions of neuronal activity, we found that systemic or locally-applied EtOH does markedly potentiate GABA-induced inhibitions of Purkinje neuron firing rate if that response is concomitantly modulated by NE or isoproterenol. This study suggests that the EtOH sensitivity of the GABA mechanism of electrophysiological responses in the cerebellar cortex is regulated by the neuromodulatory effect of beta-adrenergic receptor activation.

Genetic differences in the ethanol sensitivity of GABAA receptors expressed in Xenopus oocytes

Animal lines selected for differences in drug sensitivity can be used to help determine the molecular basis of drug action. Long-sleep (LS) and short-sleep (SS) mice differ markedly in their genetic sensitivity to ethanol. To investigate the molecular basis for this difference, mRNA from brains of LS and SS mice was expressed in Xenopus oocytes and the ethanol sensitivity of gamma-aminobutyric acid A (GABAA)- and N-methyl D-aspartate (NMDA)-activated ion channels was tested. Ethanol facilitated GABA responses in oocytes injected with mRNA from LS mice but antagonized responses in oocytes injected with mRNA from SS animals. Ethanol inhibited NMDA responses equally in the two lines. Thus, genes coding for the GABAA receptor or associated proteins may be critical determinants of individual differences in ethanol sensitivity.

Enhancement of current induced by superfusion of GABA in locus coeruleus neurons by pentobarbital, but not ethanol

gamma-Aminobutyric acid (GABA), pentobarbital and ethanol were applied by bath superfusion to rat locus coeruleus (LC) neurons in a brain slice preparation. The GABA-induced current and conductance increase was measured with single-electrode voltage clamp. Pentobarbital potentiated the GABA-induced current and conductance increase in all LC neurons tested. In contrast, ethanol did not alter the current and conductance increase induced by bath application of GABA to LC neurons.

GABAergic mechanisms in the electrophysiological actions of ethanol on cerebellar neurons

We have found that the partial inverse benzodiazepine agonists Ro 15-4513 and FG 7142 antagonize the depressant electrophysiological effects of locally applied ethanol in the cerebellum. Although absolute tissue concentrations are not known, dose-response curves constructed using pressure-ejection doses as previously described we found that FG 7142 was more efficacious, but less potent than Ro 15-4513. Our observation that ethanol and inverse benzodiazepine agonists have interactions which are not competitive might suggest that these two drugs act through separate, but interactive mechanisms in order to produce the observed ethanol antagonism. If such independent interactions were mediated at different sites on a given macromolecular complex, such as the GABAa/Cl- channel, then one might expect to find allosteric interactions between those sites as well as with the functional response of the complex to GABA activation. Indeed, this hypothesis is consistent with the recent finding of Harris and collaborators that ethanol potentiates the inverse agonist actions of Ro 15-4513 and FG 7142. On the other hand, we were unable to find large ethanol-induced potentiations of GABA effects on all neurons which showed depressant responses to ethanol administration in rat cerebellum. However we did find that the GABAa antagonist, bicuculline, blocks the depressant effects of ethanol on the same neurons. We conclude that the interaction between ethanol and GABA probably does not occur directly at the GABAa receptor site, but that the GABAa mechanism does play a permissive role in the ethanol-induced depressions of cerebellar Purkinje neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol inhibits NMDA-activated current but does not alter GABA-activated current in an isolated adult mammalian neuron

The effects of ethanol (EtOH) on membrane ion currents activated by N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid (GABA) were studied under voltage-clamp conditions in isolated sensory neurons within hours of being dissociated from adult rats. The amplitude of the ion current activated by NMDA was decreased in the presence of 2.5-100 mM EtOH (IC50, 10 mM or 0.05% EtOH), a concentration range that produces intoxication. The amplitude of the GABA-activated Cl- current, on the other hand, was not significantly affected by this concentration range of EtOH. The observations suggest that some of the neural and cognitive impairments associated with EtOH intoxication may result from inhibition of the NMDA-activated ion current.

Effect of ethanol on hippocampal GABA receptors in the rat brain

The effect of ethanol on GABA receptors was studied in hippocampal slices and membrane preparations of the rat brain. In slice preparation, ethanol enhanced the GABA inhibition of the population spikes evoked in CA 1 pyramidal cells by Schaffer collateral stimulation. The effect of ethanol was dose-dependent being observed with minimal concentration of 70 mM. In 3H-GABA binding experiments, ethanol enhanced the binding to fresh membranes with no change in the affinity. Maximal stimulation was observed at ethanol concentration of 70 mM. In hippocampal slices or membrane preparations obtained from ethanol- or barbital-administered rats chronically, ethanol failed to enhance GABA function. Thus, the present study demonstrated the involvement of GABA in central effect of ethanol. Also suggested in GABA was the involvement in the development of cross-tolerance between ethanol and barbiturates.

GABA/benzodiazepine receptor/chloride ionophore complex in brains of rat lines selectively bred for differences in ethanol-induced motor impairment

In vitro ligand binding studies were used to compare GABA/benzodiazepine receptor/chloride ionophore complexes in various brain regions of ethanol-sensitive ANT and ethanol-insensitive AT rats. In naive rats, there were several, but fairly small line differences in the binding parameters of [3H]muscimol and [3H]flunitrazepam to cerebral cortical, cerebellar or hippocampal membranes washed with or without a detergent. GABA-stimulation of flunitrazepam binding in the cerebral cortex membranes was slightly greater in the AT than ANT rats. In detergent solubilized receptors, the GABA-stimulation of flunitrazepam binding emerged only in the presence of ethanol in most AT samples, whereas the GABA-stimulation was always observed in ANT samples and ethanol had no further effect. Pharmacological characteristics of [3H]t-butylbicycloorthobenzoate binding displaceable by picrotoxin were similar in both lines. Chronic ethanol administration tended to increase the number of these binding sites in the cerebral cortex of AT rats and to decrease them in the ANT rats. Although many differences between the lines were observed, our results indicate that the ethanol-sensitivity difference between the AT and ANT rat lines cannot be explained by enhanced function of the GABA/benzodiazepine receptor/chloride ionophore complex as far as this is revealed by in vitro binding studies. It remains to be studied whether these lines differ in presynaptic GABAergic mechanisms or in the actual function of the postsynaptic chloride channels before the role of GABA can be more accurately assessed in this genetic model for ethanol-induced motor impairment.

Effect of ethanol on GABA uptake and release from hypothalamic fragments

A study was performed on the effect of ethanol on the basal and K+-evoked efflux of endogenous GABA from rat hypothalamic fragments. The amount of GABA present in the medium and in the tissue was measured by radioreceptor assay. In vitro addition of ethanol (50 and 100 mM) enhanced the K+-evoked efflux of GABA in a Ca++-dependent manner, and increased tissue GABA content. Since K+-evoked outflow induced by ethanol was not affected by the presence of nipecotic acid, ethanol appears to alter the uptake of endogenous GABA. An inhibitory effect of ethanol on 3H-GABA uptake was observed under K+ depolarization. On the other hand, acute ethanol administration produced a decrease in basal and K+-evoked efflux from hypothalamic fragments and in tissue GABA concentration. Changes in GABA efflux may lie behind some of the neuropharmacological effects of ethanol.

Involvement of neuronal chloride channels in ethanol intoxication, tolerance, and dependence

Studies of the role of neuronal chloride channels in ethanol action have focused on chloride channels coupled to gamma-aminobutyric acid (GABA) receptors. Ethanol intoxication is hypothesized to result from enhancement of GABA action, leading to increased chloride conductance and decreased neuronal excitability. Chronic ethanol treatment is suggested to produce a subsensitivity to GABA, leading to decreased action of ethanol and hyperexcitability on withdrawal of ethanol. Behavioral, electrophysiological, and biochemical studies of acute and chronic ethanol treatments on the GABA-regulated chloride channels of brain are reviewed.

Ethanol depresses inferior olive neurones and reduces Purkinje cell complex spike activity evoked by cerebral cortical stimulation

Complex spike activity was evoked in cerebellar Purkinje cells by submaximal stimulation of the sensory cerebral cortex of urethane anaesthetised rats. Ethanol (1.5 g/kg, i.v. over 10 min) produced a parallel decrease in spontaneous and cerebral cortex-evoked CS activity. The spontaneous firing rate of inferior olive neurones was decreased by ethanol administered intravenously (1.5 g/kg, over 10 min) or locally by micropressure ejection. Thus, the decrease in CS activity produced by ethanol is due to a depression of inferior olive neurones, possibly by a direct action of ethanol on these neurones.