Ethanol in low doses augments calcium-mediated mechanisms measured intracellularly in hippocampal neurons

Ethanol modulation of mammalian BK channels in excitable tissues: molecular targets and their possible contribution to alcohol-induced altered behavior

In most tissues, the function of Ca²⁺- and voltage-gated K⁺ (BK) channels is modified in response to ethanol concentrations reached in human blood during alcohol intoxication. In general, modification of BK current from ethanol-naïve preparations in response to brief ethanol exposure results from changes in channel open probability without modification of unitary conductance or change in BK protein levels in the membrane. Protracted and/or repeated ethanol exposure, however, may evoke changes in BK expression. The final ethanol effect on BK open probability leading to either BK current potentiation or BK current reduction is determined by an orchestration of molecular factors, including levels of activating ligand (Ca²⁺_i), BK subunit composition and post-translational modifications, and the channel's lipid microenvironment. These factors seem to allosterically regulate a direct interaction between ethanol and a recognition pocket of discrete dimensions recently mapped to the channel-forming (slo1) subunit. Type of ethanol exposure also plays a role in the final BK response to the drug: in several central nervous system regions (e.g., striatum, primary sensory neurons, and supraoptic nucleus), acute exposure to ethanol reduces neuronal excitability by enhancing BK activity. In contrast, protracted or repetitive ethanol administration may alter BK subunit composition and membrane expression, rendering the BK complex insensitive to further ethanol exposure. In neurohypophyseal axon terminals, ethanol potentiation of BK channel activity leads to a reduction in neuropeptide release. In vascular smooth muscle, however, ethanol inhibition of BK current leads to cell contraction and vascular constriction.

Alcohol withdrawal is associated with a downregulation of large-conductance Ca²⁺-activated K⁺ channels in rat inferior colliculus neurons

RATIONALE

Large conductance calcium-activated potassium (BK(Ca) or K(Ca)1.1) channels are well-known molecular targets for the action of alcohol and therefore may play an important role in the pathogenesis of alcohol withdrawal syndrome.

OBJECTIVES

We evaluate the modifications of total outward K⁺ currents and protein expression of BK(Ca) channels α-subunit in inferior colliculus (IC) neurons obtained from controls and rats subjected to alcohol withdrawal associated with enhanced susceptibility to seizures.

METHODS

Outward K⁺ currents and BK(Ca) channel proteins were measured using the whole cell configuration of patch clamp techniques and Western blot analysis, respectively.

RESULTS

Total outward K⁺ current density was significantly reduced in IC neurons at 24 and 48 h during the alcohol withdrawal period when the susceptibility to seizures was maximal and absent, respectively. The iberiotoxin-sensitive (BK(Ca)) current density and conductance also were significantly reduced at 24 h following alcohol withdrawal. Consistent with functional data, the levels of protein expression of α-subunit associated with BK(Ca) channels also was significantly reduced in IC neurons at 24 and 48 h following alcohol withdrawal.

CONCLUSIONS

The downregulation of BK(Ca) channels outlasts the finite period of elevated susceptibility to alcohol withdrawal seizures. These findings indicate that BK(Ca) channels, per se, may not be fundamentally important for the generation of alcohol withdrawal seizures.

Ethanol-enhanced GABA release: a focus on G protein-coupled receptors

While research on the actions of ethanol at the GABAergic synapse has focused on postsynaptic mechanisms, recent data have demonstrated that ethanol also facilitates GABA release from presynaptic terminals in many, but not all, brain regions. The ability of ethanol to increase GABA release can be regulated by different G protein-coupled receptors (GPCRs), such as the cannabinoid-1 receptor, corticotropin-releasing factor 1 receptor, GABA(B) receptor, and the 5-hydroxytryptamine 2C receptor. The intracellular messengers linked to these GPCRs, including the calcium that is released from internal stores, also play a role in ethanol-enhanced GABA release. Hypotheses are proposed to explain how ethanol interacts with the GPCR pathways to increase GABA release and how this interaction contributes to the brain region specificity of ethanol-enhanced GABA release. Defining the mechanism of ethanol-facilitated GABA release will further our understanding of the GABAergic profile of ethanol and increase our knowledge of how GABAergic neurotransmission may contribute to the intoxicating effects of alcohol and to alcohol dependence.

Alcohol Pathophysiology

There is no specialized alcohol addiction area in the brain; rather, alcohol acts on a wide range of excitatory and inhibitory nervous networks to modulate neurotransmitters actions by binding with and altering the function of specific proteins. With no hemato-encephalic barrier for alcohol, its actions are strongly related to the amount of intake. Heavy alcohol intake is associated with both structural and functional changes in the central nervous system with long-term neuronal adaptive changes contributing to the phenomena of tolerance and withdrawal. The effects of alcohol on the function of neuronal networks are heterogeneous. Because ethanol affects neural activity in some brain sites but is without effect in others, its actions are analyzed in terms of integrated connectivities in the functional circuitry of neuronal networks, which are of particular interest because of the cognitive interactions discussed in the manuscripts contributing to this review. Recent molecular data are reviewed as a support for the other contributions dealing with cognitive disturbances related to alcohol acute and addicted consumption.

Detoxification: the basis of current practice

With chronic use of a psychoactive drug the central nervous system undergoes a series of changes. These changes vary both in nature and in extent with each drug, but in general, they alter the responsiveness of the nervous system to the drug so as to reduce the perceived clinical effect, and also alter the innate responsiveness of the affected neurons to various stimuli. With cessation of drug use the nervous system undergoes a natural healing which consists of a restoration of normal responsiveness, but this process takes some time during which the patient's responses are abnormal and they are vulnerable to a variety of stressors. Detoxification is the safe negotiation of this period.

Reversibility of alcohol-related brain damage: clinical and experimental observations

Chronic alcoholics who maintain abstinence often demonstrate remarkable improvement of neurological and mental dysfunction. This paper presents an overview of the clinical and laboratory work of our group. Reversible clinical manifestations include psychometric scores, ataxia, tremor, Parkinsonism, dyskinesia, cerebral atrophy, EEG parameters, and a CSF acidosis. Electrophysiological investigations showed that in the in vitro hippocampus of rats fed ethanol for several months there was evidence for diminished long-term potentiation, impaired neuronal inhibitory mechanisms (diminished inhibitory post-synaptic potentials and post-spike after hyperpolarisations), decreased neuronal specific membrane capacitance and increased specific membrane resistance. Golgi stains showed attenuation of hippocampal CA1 neuronal dendrites in rats fed ethanol for five months, which reverted to control size in rats permitted two months of alcohol withdrawal.

Sizing up ethanol-induced plasticity: the role of small and large conductance calcium-activated potassium channels

Small (SK) and large conductance (BK) Ca(2+)-activated K(+) channels contribute to action potential repolarization, shape dendritic Ca(2+)spikes and postsynaptic responses, modulate the release of hormones and neurotransmitters, and contribute to hippocampal-dependent synaptic plasticity. Over the last decade, SK and BK channels have emerged as important targets for the development of acute ethanol tolerance and for altering neuronal excitability following chronic ethanol consumption. In this mini-review, we discuss new evidence implicating SK and BK channels in ethanol tolerance and ethanol-associated homeostatic plasticity. Findings from recent reports demonstrate that chronic ethanol produces a reduction in the function of SK channels in VTA dopaminergic and CA1 pyramidal neurons. It is hypothesized that the reduction in SK channel function increases the propensity for burst firing in VTA neurons and increases the likelihood for aberrant hyperexcitability during ethanol withdrawal in hippocampus. There is also increasing evidence supporting the idea that ethanol sensitivity of native BK channel results from differences in BK subunit composition, the proteolipid microenvironment, and molecular determinants of the channel-forming subunit itself. Moreover, these molecular entities play a substantial role in controlling the temporal component of ethanol-associated neuroadaptations in BK channels. Taken together, these studies suggest that SK and BK channels contribute to ethanol tolerance and adaptive plasticity.

Ethanol modulation of GABAergic transmission: the view from the slice

For almost three decades now, the GABAergic synapse has been the focus of intense study for its putative role in mediating many of the behavioral consequences associated with acute and chronic ethanol exposure. Although it was initially thought that ethanol interacted solely with the postsynaptic GABAA receptors that mediate the majority of fast synaptic inhibition in the mammalian central nervous system (CNS), a number of recent studies have identified novel pre- and postsynaptic mechanisms that may contribute to the acute and long-term effects of ethanol on GABAergic synaptic inhibition. These mechanisms appear to differ in a brain region specific manner and may also be influenced by a variety of endogenous neuromodulatory factors. This article provides a focused review of recent evidence, primarily from in vitro brain slice electrophysiological studies, that offers new insight into the mechanisms through which acute and chronic ethanol exposures modulate the activity of GABAergic synapses. The implications of these new mechanistic insights to our understanding of the behavioral and cognitive effects of ethanol are also discussed.

Ca2+ -activated K+ channels of the BK-type in the mouse brain

An antibody against the 442 carboxy-terminal amino acids of the BK channel alpha-subunit detects high immunoreactivity within the telencephalon in cerebral cortices, olfactory bulb, basal ganglia and hippocampus, while lower levels are found in basal forebrain regions and amygdala. Within the diencephalon, high density was found in nuclei of the ventral and dorsal thalamus and the medial habenular nucleus, and low density in the hypothalamus. The fasciculus retroflexus and its termination in the mesencephalic interpeduncular nucleus are prominently stained. Other mesencephalic expression sites are periaquaeductal gray and raphe nuclei. In the rhombencephalon, BK channels are enriched in the cerebellar cortex and in the locus coeruleus. Strong immunoreactivity is also contained in the vestibular nuclei, but not in cranial nerves and their intramedullary course of their roots. On the cellular level, BK channels show pre- and postsynaptic localizations, i.e., in somata, dendrites, axons and synaptic terminals.

A conceptualization of integrated actions of ethanol contributing to its GABAmimetic profile: a commentary

Early behavioral investigations supported the contention that systemic ethanol displays a GABAmimetic profile. Microinjection of GABA agonists into brain and in vivo electrophysiological studies implicated a regionally specific action of ethanol on GABA function. While selectivity of ethanol to enhance the effect of GABA was initially attributed an effect on type-I-benzodiazepine (BZD)-GABA(A) receptors, a lack of ethanol's effect on GABA responsiveness from isolated neurons with this receptor subtype discounted this contention. Nonetheless, subsequent work identified GABA(A) receptor subtypes, with limited distribution in brain, sensitive to enhancement of GABA at relevant ethanol concentrations. In view of these data, it is hypothesized that the GABAmimetic profile for ethanol is due to activation of mechanisms associated with GABA function, distinct from a direct action on the majority of postsynaptic GABA(A) receptors. The primary action proposed to account for ethanol's regional specificity on GABA transmission is its ability to release GABA from some, but not all, presynaptic GABAergic terminals. As systemic administration of ethanol increases neuroactive steroids, which can enhance GABA responsiveness, this elevated level of neurosteroids is proposed to magnify the effect of GABA released by ethanol. Additional factors contributing to the degree to which ethanol interacts with GABA function include an involvement of GABA(B) and other receptors that influence ethanol-induced GABA release, an effect of phosphorylation on GABA responsiveness, and a regional reduction of glutamatergic tone. Thus, an integration of these consequences induced by ethanol is proposed to provide a logical basis for its in vivo GABAmimetic profile.

Ethanol Modulation of Excitatory and Inhibitory Synaptic Transmission in Rat and Monkey Dentate Granule Neurons

BACKGROUND

The physiological mechanisms underlying the behavioral and cognitive effects of ethanol are not fully understood. However, there is now compelling evidence that ethanol acts, at least in part, by modulating the function of a small group of proteins that mediate excitatory and inhibitory synaptic transmission. For example, intoxicating concentrations of ethanol have been shown to enhance GABAergic synaptic inhibition and depress glutamatergic excitatory neurotransmission in a number of brain regions. Because all of these electrophysiological studies have been performed in rodent brain slice or neuronal culture preparations, direct evidence that ethanol exerts similar effects on synaptic transmission in the primate central nervous system is lacking.

METHODS

We have therefore developed methods to perform patch-clamp electrophysiological recordings from neurons in acutely prepared monkey (Macaca fascicularis) hippocampal slices. We have used these methods to compare the acute effects of ethanol on excitatory and inhibitory synaptic transmission in rat and monkey dentate granule neurons.

RESULTS

Under our recording conditions, ethanol significantly potentiated gamma-aminobutyric acid type A inhibitory postsynaptic currents in both rat and monkey neurons. In addition, ethanol significantly inhibited NMDA, but not AMPA, excitatory postsynaptic currents in dentate granule neurons from both species. Notably, no significant differences were observed in any of the pharmacological properties of inhibitory or excitatory synaptic responses recorded from rat and monkey neurons.

CONCLUSIONS

These data suggest that the differences in the behavioral effects of ethanol that have been observed between rats and higher-order mammals, such as monkeys and humans, may not reflect differences in the sensitivity of some of the major synaptic sites of ethanol action. Moreover, our results provide empirical evidence for the use of rodent brain slice preparations in elucidating synaptic mechanisms of ethanol action in the primate central nervous system.

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.

Patch-clamp analysis of anesthetic interactions with recombinant SK2 subtype neuronal calcium-activated potassium channels

Small conductance calcium-activated potassium channels (SK) mediate spike frequency adaptation and underlie the slow afterhyperpolarization in central neurons. We tested the actions of several anesthetics on the SK2 subtype of recombinant SK channels, cloned from rat brain and functionally expressed in a mammalian cell line. Butanol, ethanol, ketamine, lidocaine, and methohexital blocked recombinant SK2 channel currents, measured in the whole-cell patch clamp recording mode. The block was reversible, dose-dependent, and of variable efficacy. The inhaled anesthetics chloroform, desflurane, enflurane, halothane, isoflurane, and sevoflurane produced little or no block when applied at 1 minimum alveolar anesthetic concentration; varying degrees of modulation were observed at very large concentrations (10 minimum alveolar concentration). The extent of block by inhaled anesthetics did not appear to depend on concentration or membrane voltage.

IMPLICATIONS

We describe differential effects of anesthetics on cloned small conductance calcium-activated potassium channels from brain that may play a role in generating the effects or side effects of anesthetics.

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.

Ethanol opens G-protein-activated inwardly rectifying K+ channels

Ethanol affects many functions of the brain and peripheral organs. Here we show that ethanol opens G-protein-activated, inwardly rectifying K + (GIRK) channels, which has important implications for inhibitory regulation of neuronal excitability and heart rate. At pharmacologically relevant concentrations, ethanol activated both brain-type GIRK1/2 and cardiac-type GIRK1/4 channels without interaction with G proteins or second messengers. Moreover, weaver mutant mice, which have a missense mutation in the GIRK2 channel, showed a loss of ethanol-induced analgesia. These results suggest that the GIRK channels in the brain and heart are important target sites for ethanol.

Reduction of voltage-dependent currents by ethanol contributes to inhibition of NMDA receptor-mediated excitatory synaptic transmission

Previous studies have shown inhibitory effects of EtOH on NMDA receptor-mediated synaptic transmission in several brain regions. We examined this effect of EtOH under both current clamp and voltage clamp conditions in the basolateral amygdala because of the putative role of the amygdala in mediating anxiolytic effects of EtOH. We found that EtOH reduced NMDA receptor-mediated synaptic responses. In addition, we found that NMDA receptor-mediated depolarizations could also activate a voltage-dependent regenerative potential which was also sensitive to EtOH. Pharmacological characterization of this current was consistent with a high-threshold Ca2+ current. This current also exhibited a pronounced tendency towards transient enhancement upon withdrawal of EtOH.

Effects of ethanol on rat somatosensory cortical neurons

In this study, we characterized the local effects of ethanol (EtOH) on postsynaptic potentials (PSPs) and membrane properties of layer II-III (L2-3) and layer V (L5) somatosensory cortical neurons. Intracellular recordings were done using the in vitro slice preparation of rat somatosensory cortex. Our results show that EtOH exerts local effects on cortical cell membrane at physiologically relevant concentrations. A predominant effect of EtOH was to reduce excitability of L2-3 and L5 neurons by increasing the rheobase, decreasing input resistance and repetitive firing, reducing PSPs amplitude and the probability of evoking action potentials. Early (6 ms) and late (18 ms) PSP components were affected differentially by EtOH, the late components being more suppressed. Overall, EtOH-mediated suppression of PSPs was stronger in L5 neurons. Cortical neurons were divided into three subtypes: regular spiking adapting (RS-A), regular spiking non-adapting (RS-NA) and bursting (D-IB) neurons. PSPs evoked in RS-A neurons were more sensitive to EtOH suppressant effects. EtOH effects on input resistance were distributed differentially among the three groups of neurons. These results support the notion that EtOH disrupts higher processing of somatosensory information via a differential alteration of cortical neuron's membrane properties and synaptic transmission.

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.

Effects of dihydropyridines on the components of the ethanol withdrawal syndrome: possible evidence for involvement of potassium, as well as calcium?

Comparison was made of the ability of two dihydropyridine calcium channel antagonists, nitrendipine and felodipine, to prevent a range of signs of ethanol withdrawal. The increases in handling-induced behavior seen in mice during withdrawal from chronic ethanol treatment were prevented by administration of nitrendipine, 50 mg/kg, but not by, felodipine, 10 mg/kg, a dose that caused a similar displacement of dihydropyridine binding in central nervous system tissue, in vivo and in vitro. A higher dose of felodipine, 20 mg/kg, also had no effects. Nitrendipine, but not felodipine, prevented audiogenic seizures during the withdrawal phase. Similarly, nitrendipine prevented both the decrease in thresholds for N-methyl-DL-aspartate seizures and the increase in thresholds for convulsions due to 4-aminopyridine, which were seen during the withdrawal period, while felodipine did not alter either of these changes. Withdrawal from the ethanol chronic treatment increased the thresholds to seizures produced by intravenous aminophylline; this change was also prevented by nitrendipine. The significance of this increase in thresholds was lost after felodipine administration. In naive mice (not treated with ethanol) the doses of nitrendipine and felodipine used in the withdrawal studies were tested against the effects of convulsant drugs. Both dihydropyridines increased, to similar extents, the thresholds for seizures produced by bicuculline, pentylenetetrazol, and by N-methyl-DL-aspartate. The thresholds for aminophylline were unaltered by either dihydropyridine. In contrast, the thresholds for seizures due to 4-aminopyridine in the naive animals were not changed by felodipine, but were increased by nitrendipine. The results suggest that changes in potassium, as well as calcium, may possibly be involved in some of the stages of the ethanol withdrawal syndrome.

Fatty acid ethyl esters, nonoxidative metabolites of ethanol, accelerate the kinetics of activation of the human brain delayed rectifier K+ channel, Kv1.1

Herein we demonstrate that the major metabolites of ethanol in neural tissues, fatty acid ethyl esters, dramatically accelerate the kinetics of the voltage-induced activation of the human brain delayed rectifier potassium channel, Kv1.1. Specifically, the external application of ethyl oleate (20 microM) to Sf9 cells expressing the recombinant Kv1.1 channel resulted in a decrease in the rise times of the macroscopic current (e.g. from 51.7 +/- 13.1 to 12.8 +/- 3.0 ms at 0 mV for 10-90% rise times) and a 10-mV hyperpolarizing shift (at 0 mV) in the voltage dependence of channel activation. These effects were dose-dependent (half-maximal effect at 7 microM), saturable and specific (i.e. fatty acid methyl esters were without effect). Although application of either ethanol or oleic acid alone did not result in alterations of the activation kinetics, the concomitant application of ethanol and oleic acid reproduced the effects of fatty acid ethyl esters with a temporal course which paralleled the intracellular accumulation of fatty acid ethyl esters in Sf9 cells. Moreover, application of fatty acid ethyl esters (but not ethanol) to rat hippocampal cells in culture produced similar effects on hippocampal delayed rectifier currents. Collectively, these results demonstrate that pathophysiologically relevant concentrations of metabolites of ethanol, fatty acid ethyl esters, modulate the function of a prototypic neuronal ion channel and thus likely contribute to the pathophysiologic sequelae of ethanol abuse in excitable tissues.

Hippocampal-dependent learning and experience-dependent activation of the hippocampus are preferentially disrupted by ethanol

A classical fear conditioning paradigm was used to examine the effect of acute ethanol on the acquisition of context conditioning, a hippocampal-dependent associative task, and tone conditioning, a hippocampal-independent task. Administration of ethanol before the presentation of seven tone-shock pairings severely disrupted the acquisition of context conditioning, but had only a slight effect on tone conditioning, when conditioned fear was measured 48 h later. This effect was dose dependent: a dose of 0.5 g/kg had no effect on either context or tone conditioning, while doses of 1.0 and 1.5 g/kg disrupted context conditioning by 78-86%, and tone conditioning by 9-17%. Subsequent experiments indicated that ethanol's preferential effect on context conditioning could not be attributed to the fact that context conditioning is weaker than tone conditioning, ethanol-induced changes in motivational state or state-dependent learning. The effect of ethanol on stimulus-induced increases in hippocampal and neocortical expression of c-fos mRNA, a marker for changes in metabolic neuronal activity, was also examined. Ethanol completely blocked the induction of hippocampal c-fos mRNA by exposure to the conditioning context alone or seven tone-shock pairings, but only attenuated neocortical responses to these stimuli. Together, these results suggest that ethanol disrupts hippocampal-dependent learning by preferentially impairing stimulus processing at the level of the hippocampus.

Acute and chronic actions of ethanol on CA1 hippocampal responses to serotonin

The effects of acute or chronic ethanol on serotonin (5-HT)-induced membrane hyperpolarization and inhibition of the slow Ca2(+)-dependent after hyperpolarization (sAHP) were recorded in rat CA1 pyramidal neurons in hippocampal slices using sharp intracellular electrodes. 5-HT (1-100 microM) caused concentration-dependent hyperpolarization of the membrane that was not altered by simultaneous 30 mM ethanol treatment, but blunted by 10 microM buspirone, a weak 5-HT1A agonist. 5-HT (1-30 microM) also partially inhibited (approximately 40%) the sAHP following a burst of five or more action potentials. Initially ethanol (30 mM) alone did not alter the sAHP, but over a period of 38 min, a slow increase in amplitude (approximately 40%) was observed. 5-HT-mediated inhibition of the sAHP was significantly greater with ethanol present, regardless of the length of exposure. Pyramidal neurons in hippocampal slices prepared from ethanol-dependent animals showed no obvious signs of withdrawal related hyperexcitability and neither concentration-dependent membrane hyperpolarization nor sAHP inhibition caused by 5-HT were significantly changed from responses in controls. These results suggest that hyperpolarizing responses to 5-HT in hippocampal CA1 pyramidal neurons are functionally resistant to acute or chronic ethanol treatment. 5-HT-mediated inhibition of the sAHP is enhanced by ethanol acutely, but does not show an adaptive change as a result of ethanol dependence.

Acamprosate (calcium acetylhomotaurinate) enhances the N-methyl-D-aspartate component of excitatory neurotransmission in rat hippocampal CA1 neurons in vitro

The taurinate analog acamprosate (calcium acetylhomotaurinate) has received considerable attention in Europe for its ability to prevent relapse in abstained alcoholics. To determine the mechanism of acamprosate actions in the CNS, we superfused acamprosate onto rat hippocampal CA1 pyramidal neurons using an in vitro slice preparation. In current-and voltage-clamp recordings, acamprosate (100 to 100 microM) superfusion had little effect on resting membrane potential or input slope resistance. Acamprosate had no effect on Ca(2+)-dependent action potentials when tetrodotoxin was used to block Na+ spikes. In whole-cell voltage-clamp recordings, and in the presence of tetraethylammonium and Cs+ to block K+ channels, acamprosate had little effect on a Cd(2+)-sensitive inward current likely to be a high voltage-activated Ca2+ current. However, in both current- and voltage-clamp recordings, acamprosate significantly increased the N-methyl-D-aspartate (NMDA) component of excitatory postsynaptic potentials evoked by stimulation of Schaffer collaterals in the stratum radiatum, in the presence of the selective non-NMDA (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid kainate) glutamate receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione and the GABAA receptor antagonist bicuculline. Acamprosate had inconsistent or no effects on the stratum radiatum-evoked non-NMDA component of the excitatory postsynaptic potentials, in the presence of bicuculline and the NMDA antagonist DL-2-amino-5-phosphonovalerate. Acamprosate, on average, had little effect on the late inhibitory postsynaptic potentials thought to be mediated by GABAB receptors. In the presence of tetrodotoxin to block synaptic transmission, acamprosate dramatically increased inward current responses in most CA1 neurons to exogenous NMDA applied by pressure or superfusion, with reversal on washout of acamprosate. These data suggest that acamprosate may act postsynaptically to increase the NMDA component of excitatory transmission to hippocampal CA1 pyramidal neurons. Considering the known interaction of ethanol with NMDA receptors, this acamprosate modulation of NMDA receptor-mediated neurotransmission could provide a mechanism of action underlying the clinical efficacy of acamprosate.

Metabolic mapping of the effects of chronic voluntary ethanol consumption in rats

The 2-[14C]deoxyglucose method was used to examine the effects of chronic, voluntary ethanol consumption on rates of local cerebral glucose utilization (LCGU). LCGU was measured in male Long-Evans rats immediately following the completion of a 60-min schedule-induced polydipsia drinking session. Three groups of animals were examined: animals with a history of ethanol consumption that received ethanol on the test day (ethanol-ethanol), animals with a similar ethanol history that were presented with water on the test day (ethanol-water), and a control group that received water throughout the experiment (water-water). Ethanol consumption on the test day resulted in a highly discrete pattern of metabolic changes, with significant decreases in glucose utilization in the hippocampal complex, habenula, anterior ventral thalamus, and mammillary bodies, whereas increases were observed in the nucleus accumbens and locus coeruleus. Rates of LCGU in the ethanol-water group were increased throughout all regions of the central nervous system examined, indicating that the long-term consumption of moderate ethanol doses that do not produce physical dependence can cause significant changes in functional brain activity.

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.

Acute alcohol blocks neurosteroid modulation of synaptic transmission and long-term potentiation in the rat hippocampal slice

Effects of ethanol (22 mM) on the modulation of synaptic transmission and long-term potentiation (LTP) by the neurosteroid dehydroepiandrosterone sulfate (DHEAS; 10 microM) was examined in the in vitro rat hippocampal slice preparation. The synaptic responses were elicited by Schaffer collateral stimulation and recorded extracellularly in the somatic and dendritic regions of CA1 pyramidal neurons. LTP induction produced an increase (approximately 55% to 75%) in the amplitude of synaptic responses in ethanol and ethanol plus DHEAS (ethanol/DHEAS) treated slices. These increases were significantly smaller than the approximately 130% increase observed previously in slices treated with DHEAS, but were not significantly different from the approximately 82% increase observed in control slices. These results indicate that an ethanol/DHEAS interaction prevents the enhancement of LTP normally observed with DHEAS treatment of hippocampal slices. An ethanol/DHEAS interaction also altered DHEAS's effects on individual synaptic components of the synaptic response to Schaffer collateral stimulation. Ethanol applied before but not after DHEAS prevented DHEAS's enhancement of the NMDA receptor-mediated synaptic component. DHEAS's depression of the GABAA receptor-mediated synaptic component was also blocked by ethanol. Ethanol or DHEAS individually had no effect on the AMPA receptor-mediated synaptic component, but application of ethanol after DHEAS resulted in a small enhancement of this synaptic component, an effect that was not observed if ethanol was applied before DHEAS. These results show that ethanol and DHEAS interact, altering DHEAS's effects on synaptic transmission and LTP in the hippocampus. Such an interaction may be involved in ethanol's actions on the CNS and raises the possibility that ethanol and DHEAS may act via a common site or pathway.

Ethanol enhances muscarinic cholinergic neurotransmission in rat hippocampus in vitro

Previous studies from our laboratory showed that ethanol enhances muscarinic excitatory responses in rat hippocampal neurons in vivo and, like muscarinic agonists, reduces the M-current (IM) in these neurons in vitro. Therefore, we used extracellular and intracellular recording techniques in the hippocampal slice preparation to examine the mechanisms underlying this ethanol-muscarinic interaction. Surprisingly, superfusion or local application of low concentrations of acetylcholine (ACh), carbachol (CCh) or muscarine reduced the amplitudes of CA1 field potentials evoked by stratum radiatum (SR) stimulation. This effect was blocked by 1 microM atropine but was independent of the method of agonist application, the site of application or the SR stimulus paradigm. In intracellular and extracellular single unit recordings, cholinergic depressions of field potentials were correlated with: (1) depolarization of pyramidal neurons; (2) spike discharge increases; (3) reduction of amplitudes of postsynaptic potentials and (4) reduction of late afterhyperpolarizations (AHPs). Superfusion of low ethanol concentrations (11-22 mM) alone had little effect on SR-evoked field potentials but enhanced (by 10-90%) both the depressions of evoked field potentials and depolarizations elicited by the muscarinic agonists. Ethanol (22-44 mM) also enhanced both the amplitude and duration of the muscarinic slow excitatory postsynaptic potentials (sEPSPs) recorded intracellularly in CA1 and CA3 neurons. This effect was enhanced by eserine and blocked by atropine, verifying involvement of muscarinic receptors. These results suggest that: (1) caution be used in interpreting results of field potential studies regarding drug-induced excitability changes; and (2) ethanol in just-intoxicating concentrations enhances endogenous muscarinic synaptic transmission as well as responses to exogenous muscarinic agonists.

Calcium channel blockers improve hypoxia/hypoglycemia-induced impairment of rat hippocampal 2-deoxyglucose uptake in vitro after ethanol withdrawal

The aim of the present study was to determine whether calcium channel antagonists attenuated hypoxia/hypoglycemia- or glutamate-induced reduction in 2-deoxyglucose (2-DG) uptake of hippocampal slices obtained from ethanol withdrawal rats. Ethanol withdrawal significantly potentiated the hypoxia/hypoglycemia- and glutamate-induced reductions in 2-DG uptake of hippocampal slices. Both nifedipine and flunarizine exhibited attenuating effects on ethanol withdrawal-induced potentiation of impairment of 2-DG uptake caused by hypoxia/hypoglycemia or glutamate. Hypoxia/hypoglycemia-induced deficit of 2-DG uptake was prevented by ethanol, but chronic consumption of ethanol resulted in the development of tolerance to neuroprotective effect. These findings suggest that the increased sensitivity of neurons to ischemic damage by ischemia may involve in the increased activity of calcium channels in the hippocampus.

Guanosine phosphate analogs modulate ethanol potentiation of GABAA-mediated synaptic currents in hippocampal CA1 neurons

Recent studies have demonstrated that ethanol potentiation of GABAA receptor function can be regulated by second-messenger-dependent processes. As a preliminary step to further characterize this interaction, we used the whole-cell patch-clamp technique to study the effects of guanosine phosphate analogs on ethanol potentiation of GABAA-mediated synaptic transmission in rat hippocampal CA1 neurons. Intracellular dialysis with 400 microM GDP beta S, an analog that inhibits G-protein coupled events, significantly reduced ethanol, but not pentobarbital, potentiation of IPSCs. In contrast, dialysing neurons with 100 microM GTP gamma S, an irreversible G-protein activator, selectively facilitated ethanol potentiation of GABAA IPSCs. These results suggest that one or more G-protein coupled events regulate the ethanol sensitivity of synaptic GABAA receptors.

Actions of cromakalim on outward currents of CA1 neurones in hippocampal slices

1. Membrane effects of cromakalim (Crom; 50-300 microM) were examined in CA1 neurones recorded mainly by intracellular, single-electrode voltage-clamping in slices (from Sprague-Dawley rats) kept in an interface chamber at 33 degrees C. 2. In 14 cells held at -63 +/- 3.5 mV, in the presence of tetrodotoxin, kynurenic acid and (in most cases) bicuculline, bath applied Crom produced no consistent change in holding current (-59 +/- 66 pA) or input conductance (GN) (-3.9 +/- 5.2%). 3. Overall there were no significant changes in instantaneous inward rectification or in Q-current inward relaxations. 4. In 18 out of 22 cells, outward currents, evoked by 0.5 s pulses to voltages > -50 and < -20 mV, were depressed by Crom (by 42 +/- 11%, for n = 22). Because this effect was consistently seen in Ca current-blocking media, containing either Mn and low Ca, or Cd (and also carbachol), the K channels depressed by Crom were probably of the delayed rectifier (IDR) type. 5. The Crom-control difference current (ICrom), obtained with slow depolarizing ramps, had a biphasic character, inward in the voltage (V) range > -50 < -20 mV (where outward currents are depressed by Crom) and tending outward for V > or = -20 mV. 6. In 10 out of 11 cells, Crom potentiated a D-like, slowly-inactivating outward current (by 88 +/- 31%, for n = 11). 7 The effects of Crom and of 2 min periods of anoxia were compared in 12 cells: unlike anoxia, Cromproduced no consistent increases in GN; the currents evoked in the same cells by anoxia differed significantly from those evoked by Crom (by 150 +/- 60 pA); the directions of current changes induced byCrom and anoxia respectively were not significantly correlated. Crom strongly depressed anoxic outward currents (by 80 +/- 12%, n = 4).8 Some Crom-induced effects (increases in D-like current and the outward current elicited at V>- 20 mV) were always reversed by tolbutamide (1 mM), but much less consistently by glibenclamide(10-30 microM).9 In conclusion, the effects of Crom, recorded with intracellular electrodes in CA1 neurones in slices,show little resemblance to the effects of activation of ATP-sensitive K channels.

Changes in calcium currents during ethanol withdrawal in a genetic mouse model

Changes in voltage dependent calcium currents in the dentate gyrus of the hippocampal slice during ethanol withdrawal were studied using an alcohol withdrawal seizure prone mouse strain (WSP) and compared to a withdrawal-resistant strain (WSR). There was a significant increase in the high voltage activated calcium currents during the withdrawal period in the WSP strain, while those of the resistant strain showed no significant enhancement. These results suggest that an increase in calcium currents is one factor involved in the alcohol withdrawal hyperexcitability of the prone strain observed in vivo.

Role of the GTP-binding protein G(o) in the suppressant effect of ethanol on voltage-activated calcium channels of murine sensory neurons

Whole-cell and single-channel recording techniques were used to investigate the acute, in vitro effects of ethanol on the function of voltage-activated Ca2+ channels in cultured neurons derived from dorsal root ganglia (DRG) of embryonic mice. Although 5.4 mM ethanol produced a sustained increase of the amplitude of the whole-cell Ca2+ current (ICa), 43.2 mM ethanol had a time-dependent biphasic effect. That is, within 0.5 min of exposure to 43.2 mM ethanol, the maximal amplitude of ICa initially increased before declining to a new steady-state value. As anticipated, the facilitatory and inhibitory effects of ethanol on ICa were associated with an increase and decrease, respectively, in the probability of single-channel open events. Pretreatment of DRG with 200 ng/ml of pertussis toxin abolished the inhibitory, but not the facilitatory, effect of 43.2 mM ethanol on ICa. Pretreatment with pertussis toxin also prevented the reduction of the probability of single-channel opening caused by 43.2 mM ethanol. Similarly, dialysis of neurons with polyclonal antibodies against the alpha-subunit of G(o) but not Gs, abolished the inhibitory effect of 43.2 mM ethanol on ICa. These data demonstrate concentration- and time-dependent biphasic effects of ethanol on the activity of Ca2+ channels. The inhibitory effect of ethanol requires activation of the alpha-subunit of G(o), which then decreases the probability of Ca2+ channel opening.

Interaction of ethanol and allosteric modulators with GABAA-activated currents in adult medial septum/diagonal band neurons

Behavioral and electrophysiological studies suggest that neurons in the medial septum may express ethanol sensitive GABAA receptors. In the present study, patch-clamp recordings of whole-cell currents were used to directly characterize the ethanol sensitivity of GABAA receptors on acutely dissociated neurons, isolated from the medial septum/nucleus of the diagonal band (MS/nDB) of the adult rat brains. MS/nDB neurons displayed inward currents in response to GABA applied rapidly with a large-bore dual pipette system. The currents were mediated by the activation of GABAA receptors, since they reversed near the calculated reversal potential for chloride and were completely blocked by bicuculline. GABA responses were concentration dependent with an EC50 of 8.7 microM GABA and a slope of 1.35 suggesting cooperativity. Pharmacologically relevant concentrations of ethanol (3-300 mM) neither significantly increased nor decreased mean responses to GABA in neurons from Sprague Dawley or High Alcohol Sensitivity (HAS) rats. Mean GABA currents were significantly increased by 300 mM ethanol in neurons from 'ethanol sensitive' Fischer 344, ACI and Wistar Kyoto inbred rats. In subsets of neurons, 12.5 to 57.1% of those tested from these 5 rats strains, ethanol (30-300 mM) significantly increased GABA currents by > or = 20%. An additional, 10 percent of cells from Sprague Dawley rats showed ethanol-induced inhibition of GABA-activated current by < or = 20%. Allosteric modulators pentobarbital (10 microM), midazolam (1 microM) and lanthanum (300 microM), enhanced, while zinc (30 microM) decreased GABA-activated currents in all neurons, consistent with the well-known actions of these agents. These results suggest that GABAA receptors on MS/dDB neurons are pharmacologically similar to those on other neurons with respect to regulation by allosteric modulators. On the other hand, ethanol sensitivity of GABAA receptors varies considerably from cell to cell ranging from significant enhancement to inhibition of GABA-activated current.

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.

The action of anaesthetics and high pressure on neuronal discharge patterns

1. The membrane actions of both anaesthetics and high pressure have been studied in rat hippocampal slices in experiments where either field potential (orthodromic or antidromic) responses from the CA1 region are recorded or intracellular measurements in CA1 neurones are made. 2. It is clear that anaesthetics have multiple post-synaptic actions in CA1 pyramidal neurones. For example, the amplitude of antidromic field potential responses are depressed (e.g. 2.5% enflurane) or increased (20 mM ketamine) or recruitment of a second population spike occurs (2.5% halothane). 3. In separate intracellular experiments anaesthetics have been shown to hyperpolarize [e.g. inhalations (2.5%), or methohexitone, 50-100 microM), or depolarize (ketamine, > or = 20 microM), CA1 pyramidal neurones decreasing or increasing respectively the responses to weak depolarizing synaptic input, or direct stimulation. 4. In the case of some anaesthetics (inhalation agents or ketamine), the accommodation of action potential discharge is also decreased, this being accompanied by a reduction in the amplitude of the associated slow after hyperpolarization (AHP). Consequently many neurones fire repetitively in response to long lasting, strong depolarizing inputs. Sub-classes of K+ channel such as the M (channel closed by muscarinic agonists) and the SK (Ca(2+)-dependent K+ channel of small conductance) type are currently believed to control the accommodation of spike discharge and the AHP. Consistent with this idea is the finding that in voltage-clamp experiments enflurane decreased reversibly the M current. 5. Methohexitone (50 microM) does not block the accommodation of action potential discharge but interestingly induces a long lasting depolarization and action potential bursting. 6. In comparison, high pressure (51-101 ATA) can induce a second population spike in antidromic field potential recordings, indicative also of a post-synaptic action. 7. There is no apparent change in the resting membrane potential or input resistance of CA1 neurones at high pressure, however 51 or 101 ATA produced a reduction in the accommodation of action potential discharge and the associated AHP leading to repetitive discharge in response to strong (0.7 nA) depolarizing currents. 8. In a few neurones spontaneous depolarizations and action potential discharge also occurred during compression between 51-101 ATA. 9. Our working hypothesis is that certain subclasses of neuronal K+ channel represent interesting targets for both anaesthetic and high pressure action.

Electrophysiological actions of acetate, a metabolite of ethanol, on hippocampal dentate granule neurons: interactions with adenosine

Acetate is the primary breakdown product of ethanol metabolism in the liver and has been found in the brain following ethanol ingestion in rats. Systemically administered acetate has been shown to cause motor impairment, an effect which is blocked by the adenosine receptor blocker, 8-phenyltheophylline (8-PT). The effects of sodium acetate were investigated in this study using intracellular recording techniques in rat hippocampal dentate granule cells, and were compared to the actions of ethanol and adenosine individually and in conjunction with 8-PT. Acetate hyperpolarized the membrane at 0.4-0.8 mM. The amplitude and duration of the postspike train afterhyperpolarization (AHP) were increased by acetate when the cell was repolarized to the control resting membrane potential. Comparable results were seen in voltage clamp. Acetate also decreased spike frequency adaptation. The effects of acetate were mimicked by adenosine (50 microM) and ethanol (20 mM). The ethanol effects occluded those produced by acetate. All of the effects of acetate, adenosine and ethanol could be inhibited with prior perfusion of 8-PT (1-10 microM). These data suggest that the actions of the major metabolite of ethanol, acetate, may be mediated by adenosine receptor activation.

Brain region-dependent sensitivity of GABAA receptor-mediated responses to modulation by ethanol

Simultaneous extracellular and intracellular electrophysiological recordings were made from the CA1 region of rat hippocampal brain slices during superfusion with ethanol. Ethanol (80 mM) had a biphasic effect on the extracellularly recorded population spike, with an initial increase followed by a significant reduction (38%) in this response, which was maximal 10 to 15 min after the start of ethanol application. Concurrent intracellular recordings in the CA1 showed a small (0.7 mV) hyperpolarization of the resting membrane potential, with no significant change in the input impedance, EPSP, GABAA and GABAB IPSPs, or after hyperpolarization (AHP) following depolarizing current injection. Ethanol reduced the amplitude and duration of depolarizing responses to brief, localized pressure-ejection of N-methyl-D-aspartate (NMDA) onto pyramidal neuron dendrites, but did not affect the GABAA receptor-mediated depolarizing responses to the dendritic application of GABA. In parallel studies, the effect of ethanol on GABA-stimulated 36Cl- flux was measured in microsac preparations from rat hippocampus, cerebellum, and cerebral cortex. Ethanol application caused substantial enhancement of the chloride uptake from cerebellar and cerebral cortical microsacs, but had no effect on 36Cl- influx in hippocampal microsacs. These results suggest that there are important brain region-dependent differences in the sensitivity of the GABAA receptor/chloride channel to modulation by ethanol.

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.

Differential effects of picrotoxin and RO 15-1788 on high and low ethanol concentrations on rat locus coeruleus in vitro

In an in vitro electrophysiological single-cell recording model, ethanol had an inhibitory effect on locus coeruleus (LC) neurons at both low (0.1 mmol/l) and high (500 mmol/l) concentrations. In order to test if the benzodiazepine-GABA (gamma-aminobutyric acid) receptor complex could be implicated in this effect, we tested the interaction of these ethanol concentrations with picrotoxin (100 mmol/l) and RO 15-1788 (10 nmol/l). RO 15-1788 reversed the inhibitory effect induced by ethanol 500 mmol/l, but not by ethanol 0.1 mmol/l; picrotoxin reversed the effects of both concentrations. This indicates that the mechanisms of action of ethanol on LC neurons are not the same for high and low concentrations. Furthermore, the effect of concentrations related to a behavioral effect (greater than 10 mmol/l) was reversed by a low-calcium medium that abolishes transmitter release. Therefore, the inhibition induced by ethanol 500 mmol/l seems to be due to the release of an endogenous benzodiazepine-like compound.

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.