Sensitivity of postsynaptic GABAB receptors on hippocampal CA1 and CA3 pyramidal neurons to ethanol

GABAergic mechanisms in alcohol dependence

The target of alcohol's effect on the central nervous system has been sought for more than 50 years in the brain's GABA system. The behavioral and emotional effects of alcohol in humans and rodents are very similar to those of barbiturates and benzodiazepines, and GABAA receptors have been shown to be one of the sites of alcohol action. The mechanisms of GABAergic inhibition have been a hotspot of research but have turned out to be complex and controversial. Genetics support the involvement of some GABAA receptor subunits in the development of alcohol dependence and in alcohol use disorders (AUD). Since the effect of alcohol on the GABAA system resembles that of a GABAergic positive modulator, it may be possible to develop GABAergic drug treatments that could substitute for alcohol. The adaptation mechanisms of the GABA system and the plasticity of the brain are a big challenge for drug development: the drugs that act on GABAA receptors developed so far also may cause adaptation and development of additional addiction. Human polymorphisms should be studied further to get insight about how they affect receptor function, expression or other factors to make reasonable predictions/hypotheses about what non-addictive interventions would help in alcohol dependence and AUD.

Concomitant taurine exposure counteracts ethanol-induced changes in locomotor and anxiety-like responses in zebrafish

Taurine (TAU) is a β-amino sulfonic acid with pleiotropic roles in the brain, including the neuromodulatory activity via GABAergic and glycinergic agonism. This molecule is found at high concentrations in energy drinks and is often mixed with alcohol in beverages. Although TAU has a neuroprotective role in the brain, the putative risks of mixing TAU and EtOH are not fully understood. Here, we investigated whether TAU modulates locomotor and anxiety-like behavior in adult zebrafish by using the novel tank and light-dark tests following acute EtOH exposure at anxiogenic and anxiolytic concentrations. Zebrafish were individually exposed to water (control), TAU (42, 150, and 400 mg/L), and EtOH (0.25% (v/v) and 1% (v/v)) both independently and cotreated for 1 h. EtOH 0.25% and TAU produced U-shape anxiolytic-like behavior in the light-dark test, TAU 42 and 400 positively modulated EtOH effects, and TAU 150 exerted a protective effect. All TAU concentrations counteracted EtOH 1%-induced locomotion impairment, as well as the anxiogenic-like behavior. Finally, all TAU concentrations when given independently or cotreated with EtOH 0.25% and 1% decreased the risk assessment of the lit compartment. Principal component analyses revealed that exploration and anxiety-like responses were the main behaviors that contribute to the effects of TAU and EtOH. Overall, we demonstrate that TAU differently modulates EtOH-induced anxiolytic- and anxiogenic-like behaviors depending on the concentration, suggesting a complex mechanism underlying TAU and EtOH interactions.

Synaptic targets: Chronic alcohol actions

Alcohol acts on numerous cellular and molecular targets to regulate neuronal communication within the brain. Chronic alcohol exposure and acute withdrawal generate prominent neuroadaptations at synapses, including compensatory effects on the expression, localization and function of synaptic proteins, channels and receptors. The present article reviews the literature describing the synaptic effects of chronic alcohol exposure and their relevance for synaptic transmission in the central nervous system. This review is not meant to be comprehensive, but rather to highlight the effects that have been observed most consistently and that are thought to contribute to the development of alcohol dependence and the negative aspects of withdrawal. Specifically, we will focus on the major excitatory and inhibitory neurotransmitters in the brain, glutamate and GABA, respectively, and how their neuroadaptations after chronic alcohol exposure contributes to alcohol reinforcement, dependence and withdrawal. This article is part of the Special Issue entitled "Alcoholism".

Synaptic effects induced by alcohol

Ethanol (EtOH) has effects on numerous cellular molecular targets, and alterations in synaptic function are prominent among these effects. Acute exposure to EtOH activates or inhibits the function of proteins involved in synaptic transmission, while chronic exposure often produces opposing and/or compensatory/homeostatic effects on the expression, localization, and function of these proteins. Interactions between different neurotransmitters (e.g., neuropeptide effects on release of small molecule transmitters) can also influence both acute and chronic EtOH actions. Studies in intact animals indicate that the proteins affected by EtOH also play roles in the neural actions of the drug, including acute intoxication, tolerance, dependence, and the seeking and drinking of EtOH. This chapter reviews the literature describing these acute and chronic synaptic effects of EtOH and their relevance for synaptic transmission, plasticity, and behavior.

A transcriptional study in mice with different ethanol-drinking profiles: possible involvement of the GABA(B) receptor

Previous studies have suggested that γ-aminobutyric acid-B (GABA(B)) receptor agonists effectively reduce ethanol intake. The quantification using real-time polymerase chain reaction of Gabbr1 and Gabbr2 mRNA from the prefrontal cortex, hypothalamus, hippocampus, and striatum in mice exposed to an animal model of the addiction developed in our laboratory was performed to evaluate the involvement of the GABA(B) receptor in ethanol consumption. We used outbred, Swiss mice exposed to a three-bottle free-choice model (water, 5% v/v ethanol, and 10% v/v ethanol) that consisted of four phases: acquisition (AC), withdrawal (W), reexposure (RE), and quinine-adulteration (AD). Based on individual ethanol intake, the mice were classified into three groups: "addicted" (A group; preference for ethanol and persistent consumption during all phases), "heavy" (H group; preference for ethanol and a reduction in ethanol intake in the AD phase compared to AC phase), and "light" (L group; preference for water during all phases). In the prefrontal cortex in the A group, we found high Gabbr1 and Gabbr2 transcription levels, with significantly higher Gabbr1 transcription levels compared with the C (ethanol-naive control mice), L, and H groups. In the hippocampus in the A group, Gabbr2 mRNA levels were significantly lower compared with the C, L, and H groups. In the striatum, we found a significant increase in Gabbr1 transcription levels compared with the C, L, and H groups. No differences in Gabbr1 or Gabbr2 transcription levels were observed in the hypothalamus among groups. In summary, Gabbr1 and Gabbr2 transcription levels were altered in cerebral areas related to drug taking only in mice behaviorally classified as "addicted" drinkers, suggesting that these genes may contribute to high and persistent ethanol consumption.

Baclofen in the management of inhalant withdrawal: a case series

INTRODUCTION

Abuse of inhalants and solvents is a significant public health problem. There is no specific treatment for inhalant withdrawal.

OBJECTIVE

To study the effect of baclofen in treating craving and withdrawal symptoms in patients with inhalant dependence.

CASE REPORTS

Case studies of 3 young male patients with DSM-IV diagnoses of inhalant dependence treated in an inpatient setting with baclofen are presented. All patients had nonspecific withdrawal symptoms in the form of irritability, insomnia, and craving. Baclofen was given in doses up to 50 mg/day and was continued throughout the period of hospitalization.

DISCUSSION

All patients reported significant reduction in withdrawal symptoms within 48 hours of treatment and were free of symptoms for the duration of their hospital stay. One patient continued the medication as an outpatient and has remained abstinent to date. Baclofen was well tolerated by all patients. Our results suggest that baclofen may be an effective treatment modality in this patient population. These effects are possibly due to the agonistic action of baclofen at gamma-aminobutyric acid B receptors in the ventral tegmental area.

Mapping a barbiturate withdrawal locus to a 0.44 Mb interval and analysis of a novel null mutant identify a role for Kcnj9 (GIRK3) in withdrawal from pentobarbital, zolpidem, and ethanol

Here, we map a quantitative trait locus (QTL) with a large effect on predisposition to barbiturate (pentobarbital) withdrawal to a 0.44 Mb interval of mouse chromosome 1 syntenic with human 1q23.2. We report a detailed analysis of the genes within this interval and show that it contains 15 known and predicted genes, 12 of which demonstrate validated genotype-dependent transcript expression and/or nonsynonymous coding sequence variation that may underlie the influence of the QTL on withdrawal. These candidates are involved in diverse cellular functions including intracellular trafficking, potassium conductance and spatial buffering, and multimolecular complex dynamics, and indicate both established and novel aspects of neurobiological response to sedative-hypnotics. This work represents a substantial advancement toward identification of the gene(s) that underlie the phenotypic effects of the QTL. We identify Kcnj9 as a particularly promising candidate and report the development of a Kcnj9-null mutant model that exhibits significantly less severe withdrawal from pentobarbital as well as other sedative-hypnotics (zolpidem and ethanol) versus wild-type littermates. Reduced expression of Kcnj9, which encodes GIRK3 (Kir3.3), is associated with less severe sedative-hypnotic withdrawal. A multitude of QTLs for a variety of complex traits, including diverse responses to sedative-hypnotics, have been detected on distal chromosome 1 in mice, and as many as four QTLs on human chromosome 1q have been implicated in human studies of alcohol dependence. Thus, our results will be primary to additional efforts to identify genes involved in a wide variety of behavioral responses to sedative-hypnotics and may directly facilitate progress in human genetics.

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.

Ethanol potentiation of GABAergic synaptic transmission may be self-limiting: role of presynaptic GABA(B) receptors

Ethanol enhances GABAergic synaptic inhibition, and this interaction contributes to many of the behavioral and cognitive effects of this drug. Most studies suggest that ethanol enhances GABAergic neurotransmission via an allosteric potentiation of the postsynaptic GABA(A) receptors that mediate fast synaptic inhibition in the mammalian CNS. Despite widespread acceptance of this hypothesis, direct support for such a mechanism has been difficult to obtain. Ethanol does not enhance GABA(A) receptor function in all brain regions or under all experimental conditions, and factors responsible for this variability remain mostly unknown. Notably, blockade of GABA(B) receptors dramatically enhances ethanol potentiation of hippocampal GABA(A) IPSPs and IPSCs, suggesting that some unknown GABA(B) receptor mechanism limits the overall potentiating effect of ethanol on GABAergic synapses. In this study, we demonstrate that, at perisomatic synapses in the rat hippocampus, ethanol enhances presynaptic GABA(B) autoreceptor function and that this interaction reduces the overall potentiating effect of ethanol at these synapses. We further show that ethanol significantly elevates basal presynaptic GABA(B) receptor tone, possibly via an increase in spontaneous GABA release, and that pretreatment with a subthreshold concentration of the GABA(B) receptor agonist baclofen blocks ethanol but not flunitrazepam or pentobarbital potentiation of GABA(A) IPSCs. These data suggest that an interaction between ethanol and presynaptic GABA(B) autoreceptor activity regulates the ethanol sensitivity of GABAergic synapses. Given that the in vitro ethanol sensitivity of these synapses correlates with in vivo ethanol responsiveness in a number of rodent lines, our data further suggest that presynaptic GABA(B) receptor activity may play a role in regulating behavioral sensitivity to ethanol.

Differential GABAB Receptor Modulation of Ethanol Effects on GABAA Synaptic Activity in Hippocampal CA1 Neurons

We tested the hypothesis that differential sensitivity to ethanol of synaptic GABA(A) somatic and dendritic inhibitory postsynaptic currents (IPSCs) in hippocampal CA1 pyramidal neurons could be due to differences in the extent of GABA(B) receptor activity at GABAergic synapses in these two hippocampal subfields. Our present results show that dendritic (distally evoked) GABA IPSCs contain a larger GABA(B) IPSC component of the total GABA IPSC than the somatic (proximally evoked) subfield. The inhibition of GABA(B) receptors by pretreatment of hippocampal slices with CGP-52432 [3[[(3,4-dichlorophenyl)methyl]amino]propyl](diethoxymethyl) phosphinic acid], a selective GABA(B) receptor antagonist, changes the basal ethanol-insensitive, distally evoked GABA(A) IPSCs to become more sensitive to ethanol. In addition, paired-pulse stimulation of the proximal and distal subfields of hippocampal pyramidal neurons shows that ethanol alone increases the probability of GABA release at proximal but not distal regions. Changes by ethanol on the probability of GABA release are only seen at distal locations during GABA(B) blockade. Finally, when the modulation of presynaptic GABA(B) receptors is minimized by the local application of 10 mM GABA directly onto somatic or dendritic GABAergic synaptic regions, postsynaptic GABA(B) receptors seem to exert significant negative (inhibiting) influence on the effects of ethanol on GABA(A) IPSCs in the distal subfields of CA1 pyramidal neurons. Together, our data suggest that differences in both presynaptic and postsynaptic GABA(B) receptor activity at these GABAergic synapses may modulate the differential ethanol sensitivity of proximal and distal GABA IPSCs(A) in hippocampal CA1 pyramidal neurons.

Ability of baclofen in reducing alcohol intake and withdrawal severity: I--Preclinical evidence

BACKGROUND

The similarities between the pharmacological effects of the gamma-aminobutyric acid receptor agonist, baclofen, and the alcohol-substituting agent, gamma-hydroxybutyric acid, led us to investigate whether baclofen was capable of reducing (a) ethanol withdrawal syndrome in ethanol-dependent rats and (b) voluntary ethanol intake in ethanol-preferring rats.

METHODS

In experiment 1, Wistar rats were rendered physically dependent on ethanol by the repeated administration of intoxicating doses of ethanol for 6 consecutive days. Baclofen was acutely administered intraperitoneally at doses of 10, 20, and 40 mg/kg. In experiment 2, baclofen (0, 2.5, 5, and 10 mg/kg, intraperitoneally) was administered once a day for 14 consecutive days to ethanol-preferring sP rats that had continuous access to ethanol (10%, v/v) and water under the two-bottle free choice regimen.

RESULTS

In experiment 1, baclofen dose-dependently decreased the intensity of ethanol withdrawal signs; furthermore, 20 mg/kg of baclofen protected from audiogenic seizures in ethanol-withdrawn rats. In experiment 2, baclofen selectively and dose-dependently reduced voluntary ethanol intake; a compensatory increase in water intake left total fluid intake virtually unchanged.

CONCLUSIONS

These results are in close agreement with those of a preliminary clinical study and suggest that baclofen may constitute a novel therapeutic agent for alcoholism.

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 and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Ethanol inhibition of NMDA currents in acutely dissociated medial septum/diagonal band neurons from ethanol dependent rats

The effect of acutely applied ethanol and the impact of chronic ethanol treatment, sufficient to induce tolerance and physical dependence, on N-methyl-D-aspartate (NMDA) receptor function were studied in acutely isolated neurons from the medial septum/diagonal band (MS/DB) of adult rats using whole cell, patch-clamp electrophysiology. There was a small positive correlation for capacitance and current amplitude activated by 100 microM NMDA for all groups. Also, cell membrane capacitance was significantly smaller for Ethanol Dependent (approximately 80-84%) than either Naive or Control cells. Therefore NMDA-activated responses were normalized for capacitance (current density, pA/pF) across all three groups. NMDA-activated (30-1000 microM) responses were significantly larger in cells from Control and Ethanol Dependent rats relative to those from Naives. In addition, estimated maximal responses were significantly larger for Ethanol Dependent cells, compared to either Control or Naive, respectively, while EC50s and slopes were not significantly different. Acute 60 mM ethanol significantly inhibited responses to 100 microM NMDA in all three groups, however, mean ethanol inhibition was 12-25% smaller after ethanol dependence. There was no evidence of acute tolerance to ethanol inhibition for any group, but examination of patterns of inhibition for individual neurons showed a few cells were resistant to ethanol or exhibited progressive loss of ethanol inhibition. These results suggest that NMDA receptor function in acutely isolated MS/DB neurons is increased following in vivo chronic ethanol treatment, and shows resistance to acute ethanol inhibition suggesting NMDA receptor-mediated cellular tolerance.