Effect of ethanol on gamma-aminobutyric acid and glycine receptor-coupled Cl- fluxes in rat brain synaptoneurosomes

Combining ERAP1 silencing and entinostat therapy to overcome resistance to cancer immunotherapy in neuroblastoma

BACKGROUND

Checkpoint immunotherapy unleashes tumor control by T cells, but it is undermined in non-immunogenic tumors, e.g. with low MHC class I expression and low neoantigen burden, such as neuroblastoma (NB). Endoplasmic reticulum aminopeptidase 1 (ERAP1) is an enzyme that trims peptides before loading on MHC class I molecules. Inhibition of ERAP1 results in the generation of new antigens able of inducing potent anti-tumor immune responses. Here, we identify a novel non-toxic combinatorial strategy based on genetic inhibition of ERAP1 and administration of the HDAC inhibitor (HDACi) entinostat that increase the immunogenicity of NB, making it responsive to PD-1 therapy.

METHODS

CRISPR/Cas9-mediated gene editing was used to knockout (KO) the ERAP1 gene in 9464D NB cells derived from spontaneous tumors of TH-MYCN transgenic mice. The expression of MHC class I and PD-L1 was evaluated by flow cytometry (FC). The immunopeptidome of these cells was studied by mass spectrometry. Cocultures of splenocytes derived from 9464D bearing mice and tumor cells allowed the assessment of the effect of ERAP1 inhibition on the secretion of inflammatory cytokines and activation and migration of immune cells towards ERAP1 KO cells by FC. Tumor cell killing was evaluated by Caspase 3/7 assay and flow cytometry analysis. The effect of ERAP1 inhibition on the immune content of tumors was analyzed by FC, immunohistochemistry and multiple immunofluorescence.

RESULTS

We found that inhibition of ERAP1 makes 9464D cells more susceptible to immune cell-mediated killing by increasing both the recall and activation of CD4+ and CD8+ T cells and NK cells. Treatment with entinostat induces the expression of MHC class I and PD-L1 molecules in 9464D both in vitro and in vivo. This results in pronounced changes in the immunopeptidome induced by ERAP1 inhibition, but also restrains the growth of ERAP1 KO tumors in vivo by remodelling the tumor-infiltrating T-cell compartment. Interestingly, the absence of ERAP1 in combination with entinostat and PD-1 blockade overcomes resistance to PD-1 immunotherapy and increases host survival.

CONCLUSIONS

These findings demonstrate that ERAP1 inhibition combined with HDACi entinostat treatment and PD-1 blockade remodels the immune landscape of a non-immunogenic tumor such as NB, making it responsive to checkpoint immunotherapy.

Alcohol and the dopamine system

The mesolimbic dopamine pathway plays a major role in drug reinforcement and is likely involved also in the development of drug addiction. Ethanol, like most addictive drugs, acutely activates the mesolimbic dopamine system and releases dopamine, and ethanol-associated stimuli also appear to trigger dopamine release. In addition, chronic exposure to ethanol reduces the baseline function of the mesolimbic dopamine system. The molecular mechanisms underlying ethanol´s interaction with this system remain, however, to be unveiled. Here research on the actions of ethanol in the mesolimbic dopamine system, focusing on the involvement of cystein-loop ligand-gated ion channels, opiate receptors, gastric peptides and acetaldehyde is briefly reviewed. In summary, a great complexity as regards ethanol´s mechanism(s) of action along the mesolimbic dopamine system has been revealed. Consequently, several new targets and possibilities for pharmacotherapies for alcohol use disorder have emerged.

Ectopic GABAA receptor β3 subunit determines Cl- / HCO 3 - -ATPase and chloride transport in HEK 293FT cells

Neuronal intracellular chloride concentration ([Cl^- ]_i ) is a crucial determinant of transmission mediated by the γ-aminobutyric acid type A receptor (GABA_A R), which subserves synaptic and extrasynaptic inhibition as well as excitation. The Cl^- ion is the main carrier of charge through the GABA_A R; however, bicarbonate ions ( HCO 3 - ) flowing in the opposite direction can also contribute to the net current. The direction of Cl^- and HCO 3 - fluxes is determined by the underlying electrochemical gradient, which is controlled by Cl^- transporters and channels. Accumulating evidence suggests that active mechanisms of chloride transport across the GABA_A R pore can underlie the regulation of [Cl^- ]_i . Measurement of Cl^- / HCO 3 - -ATPase activity and Cl^- transport in HEK 293FT cells expressing homomeric or heteromeric GABA_A R ensembles (α2, β3, or γ2) with fluorescent dye for chloride demonstrated that receptor subtypes containing the β3 subunit show enzymatic activity and participate in GABA-mediated or ATP-dependent Cl^- transport. GABA-mediated flow of Cl^- ions into and out of the cells occurred for a short time period but then rapidly declined. However, Cl^- ion flux was stabilized for a long time period in the presence of HCO 3 - ions. The reconstituted β3 subunit isoform, purified as a fusion protein, confirmed that β3 is critical for ATPase; however, only the triplet variant showed the full receptor function. The high sensitivity of the enzyme to γ-phosphate inhibitors led us to postulate that the β3 subunit is catalytic. Our discovery of a GABA_A R type that requires ATP consumption for chloride movement provides new insight into the molecular mechanisms of inhibitory signaling.

Phytochemical analysis of Ficus carica L. active compounds possessing anticonvulsant activity

The anticonvulsant potential of Ficus carica methanol-extract (Fc) has been studied. It was found that Fc most active fraction is rich in oligosaccharides (OFG). ¹H, ¹³C NMR and Nano-ESI, MALDI MS, and LC-MS techniques proved that OFG contains alpha-glucopyranoside oligomer in high amounts. Both Fc and OFG reduced strychnine (STR) convulsion-action. Fc and OFG fully protected the experimental-animals from STR-lethality. The intracerebroventricular-administration (ICV) of Fc or OFG in combination with glycine in ethanol-treated mice caused a dose-dependent returning to a 2nd-loss of righting-reflex (LORR), and was antagonized by STR. FC and OFG ICV injection counteracted STR-inhibition, confirming that Fc/OFG anticonvulsant mechanism of action was mediated by potentiation of glycine receptor. These results support Fc and OFG potential anticonvulsant-activity with good safety-profile.

The Glycine Receptor-A Functionally Important Primary Brain Target of Ethanol

Identification of ethanol's (EtOH) primary molecular brain targets and determination of their functional role is an ongoing, important quest. Pentameric ligand-gated ion channels, that is, the nicotinic acetylcholine receptor, the γ-aminobutyric acid type A receptor, the 5-hydroxytryptamine₃ , and the glycine receptor (GlyR), are such targets. Here, aspects of the structure and function of these receptors and EtOH's interaction with them are briefly reviewed, with special emphasis on the GlyR and the importance of this receptor and its ligands for EtOH pharmacology. It is suggested that GlyRs are involved in (i) the dopamine-activating effect of EtOH, (ii) regulating EtOH intake, and (iii) the relapse preventing effect of acamprosate. Exploration of the GlyR subtypes involved and efforts to develop subtype specific agonists or antagonists may offer new pharmacotherapies for alcohol use disorders.

Differential regulation of NMDA receptors by d-serine and glycine in mammalian spinal locomotor networks

We provide evidence that NMDARs within murine spinal locomotor networks determine the frequency and amplitude of ongoing locomotor-related activity in vitro and that NMDARs are regulated by d-serine and glycine in a synapse-specific and activity-dependent manner. In addition, glycine transporter-1 is shown to be an important regulator of NMDARs during locomotor-related activity. These results show how excitatory transmission can be tuned to diversify the output repertoire of spinal locomotor networks in mammals.

Activation of N-methyl-d-aspartate receptors (NMDARs) requires the binding of a coagonist, either d-serine or glycine, in addition to glutamate. Changes in occupancy of the coagonist binding site are proposed to modulate neural networks including those controlling swimming in frog tadpoles. Here, we characterize regulation of the NMDAR coagonist binding site in mammalian spinal locomotor networks. Blockade of NMDARs by d(−)-2-amino-5-phosphonopentanoic acid (d-APV) or 5,7-dichlorokynurenic acid reduced the frequency and amplitude of pharmacologically induced locomotor-related activity recorded from the ventral roots of spinal-cord preparations from neonatal mice. Furthermore, d-APV abolished synchronous activity induced by blockade of inhibitory transmission. These results demonstrate an important role for NMDARs in murine locomotor networks. Bath-applied d-serine enhanced the frequency of locomotor-related but not disinhibited bursting, indicating that coagonist binding sites are saturated during the latter but not the former mode of activity. Depletion of endogenous d-serine by d-amino acid oxidase or the serine-racemase inhibitor erythro-β-hydroxy-l-aspartic acid (HOAsp) increased the frequency of locomotor-related activity, whereas application of l-serine to enhance endogenous d-serine synthesis reduced burst frequency, suggesting a requirement for d-serine at a subset of synapses onto inhibitory interneurons. Consistent with this, HOAsp was ineffective during disinhibited activity. Bath-applied glycine (1–100 µM) failed to alter locomotor-related activity, whereas ALX 5407, a selective inhibitor of glycine transporter-1 (GlyT1), enhanced burst frequency, supporting a role for GlyT1 in NMDAR regulation. Together these findings indicate activity-dependent and synapse-specific regulation of the coagonist binding site within spinal locomotor networks, illustrating the importance of NMDAR regulation in shaping motor output.

NEW & NOTEWORTHY We provide evidence that NMDARs within murine spinal locomotor networks determine the frequency and amplitude of ongoing locomotor-related activity in vitro and that NMDARs are regulated by d-serine and glycine in a synapse-specific and activity-dependent manner. In addition, glycine transporter-1 is shown to be an important regulator of NMDARs during locomotor-related activity. These results show how excitatory transmission can be tuned to diversify the output repertoire of spinal locomotor networks in mammals.

Potentiation of Gamma Aminobutyric Acid Receptors (GABAAR) by Ethanol: How Are Inhibitory Receptors Affected?

In recent years there has been an increase in the understanding of ethanol actions on the type A γ-aminobutyric acid chloride channel (GABA_AR), a member of the pentameric ligand gated ion channels (pLGICs). However, the mechanism by which ethanol potentiates the complex is still not fully understood and a number of publications have shown contradictory results. Thus many questions still remain unresolved requiring further studies for a better comprehension of this effect. The present review concentrates on the involvement of GABA_AR in the acute actions of ethanol and specifically focuses on the immediate, direct or indirect, synaptic and extra-synaptic modulatory effects. To elaborate on the immediate, direct modulation of GABA_AR by acute ethanol exposure, electrophysiological studies investigating the importance of different subunits, and data from receptor mutants will be examined. We will also discuss the nature of the putative binding sites for ethanol based on structural data obtained from other members of the pLGICs family. Finally, we will briefly highlight the glycine gated chloride channel (GlyR), another member of the pLGIC family, as a suitable target for the development of new pharmacological tools.

Brucine suppresses ethanol intake and preference in alcohol-preferring Fawn-Hooded rats

AIM

Brucine (BRU) extracted from the seeds of Strychnos nux-vomica L is glycine receptor antagonist. We hypothesize that BRU may modify alcohol consumption by acting at glycine receptors, and evaluated the pharmacodynamic profiles and adverse effects of BRU in rat models of alcohol abuse.

METHODS

Alcohol-preferring Fawn-Hooded (FH/Wjd) rats were administered BRU (10, 20 or 30 mg/kg, sc). The effects of BRU on alcohol consumption were examined in ethanol 2-bottle-choice drinking paradigm, ethanol/sucrose operant self-administration paradigm and 5-d ethanol deprivation test. In addition, open field test was used to assess the general locomotor activity of FH/Wjd rats, and conditioned place preference (CPP) was conducted to assess conditioned reinforcing effect.

RESULTS

In ethanol 2-bottle-choice drinking paradigm, treatment with BRU for 10 consecutive days dose-dependently decreased the ethanol intake associated with a compensatory increase of water intake, but unchanged the daily total fluid intake and body weight. In ethanol/sucrose operant self-administration paradigms, BRU (30 mg/kg) administered before each testing session significantly decreased the number of lever presses for ethanol and the ethanol intake, without affecting the number of sucrose (10%) responses, total sucrose intake, and the number of lever presses for water. Acute treatment with BRU (30 mg/kg) completely suppressed the deprivation-induced elevation of ethanol consumption. Treatment with BRU (10, 20, and 30 mg/kg) did not alter locomotion of FH/Wjd rats, nor did it produce place preference or aversion.

CONCLUSION

BRU selectively decreases ethanol consumption with minimal adverse effects. Therefore, BRU may represent a new pharmacotherapy for alcoholism.

Ethanol, not detectably metabolized in brain, significantly reduces brain metabolism, probably via action at specific GABA(A) receptors and has measureable metabolic effects at very low concentrations

Ethanol is a known neuromodulatory agent with reported actions at a range of neurotransmitter receptors. Here, we measured the effect of alcohol on metabolism of [3-¹³C]pyruvate in the adult Guinea pig brain cortical tissue slice and compared the outcomes to those from a library of ligands active in the GABAergic system as well as studying the metabolic fate of [1,2-¹³C]ethanol. Analyses of metabolic profile clusters suggest that the significant reductions in metabolism induced by ethanol (10, 30 and 60 mM) are via action at neurotransmitter receptors, particularly α4β3δ receptors, whereas very low concentrations of ethanol may produce metabolic responses owing to release of GABA via GABA transporter 1 (GAT1) and the subsequent interaction of this GABA with local α5- or α1-containing GABA(A)R. There was no measureable metabolism of [1,2-¹³C]ethanol with no significant incorporation of ¹³C from [1,2-¹³C]ethanol into any measured metabolite above natural abundance, although there were measurable effects on total metabolite sizes similar to those seen with unlabelled ethanol.

Acute effects of ethanol on GABAA and glycine currents in the lateral habenula neurons of young rats

Compelling evidence has shown a pivotal role of dopaminergic function in drug addiction. Recently, the lateral habenula (LHb) has attracted a great deal of attention as another target for abused drugs in the brain because its role in regulating dopaminergic system, among others. GABA and glycine are major inhibitory neurotransmitters. Their corresponding receptors are key targets of ethanol. The properties of these receptors in LHb neurons and their responses to ethanol in particular however, remain unknown. Using the patch clamp techniques, we examined the effects of ethanol on the chloride currents elicited by GABA and glycine in LHb neurons acutely dissociated from 10-20 day-old Sprague-Dawley rats. We show that GABA concentration-dependently elicited a bicuculline sensitive inward current in 96% (130/140) of the neurons tested. Ethanol (43.2 mM) suppressed current elicited by a wide range of concentrations (1-300 μM) of GABA in 74% (35/47) cells tested. Ethanol suppression is dependent on its concentrations but not on membrane potentials of the neurons. Moreover, glycine concentration-dependently elicited an inward current in 94% (112/120) of the neurons tested. Both strychnine and picrotoxin concentration dependently suppressed glycine current with IC₅₀ of 220 nM and 813 μM, respectively. Ethanol (43.2 mM) potentiated current elicited by unsaturated but not saturated concentrations of glycine. Thus, the LHb neurons of young rats contain both functional GABA_A and glycine receptors which are sensitive to ethanol at pharmacologically relevant concentrations. These effects of ethanol might be important in the control of the activity and output of LHb neurons.

Alcohol-binding sites in distinct brain proteins: the quest for atomic level resolution

Defining the sites of action of ethanol on brain proteins is a major prerequisite to understanding the molecular pharmacology of this drug. The main barrier to reaching an atomic-level understanding of alcohol action is the low potency of alcohols, ethanol in particular, which is a reflection of transient, low-affinity interactions with their targets. These mechanisms are difficult or impossible to study with traditional techniques such as radioligand binding or spectroscopy. However, there has been considerable recent progress in combining X-ray crystallography, structural modeling, and site-directed mutagenesis to define the sites and mechanisms of action of ethanol and related alcohols on key brain proteins. We review such insights for several diverse classes of proteins including inwardly rectifying potassium, transient receptor potential, and neurotransmitter-gated ion channels, as well as protein kinase C epsilon. Some common themes are beginning to emerge from these proteins, including hydrogen bonding of the hydroxyl group and van der Waals interactions of the methylene groups of ethanol with specific amino acid residues. The resulting binding energy is proposed to facilitate or stabilize low-energy state transitions in the bound proteins, allowing ethanol to act as a "molecular lubricant" for protein function. We discuss evidence for characteristic, discrete alcohol-binding sites on protein targets, as well as evidence that binding to some proteins is better characterized by an interaction region that can accommodate multiple molecules of ethanol.

Neuronal nicotinic acetylcholine receptors as pharmacotherapeutic targets for the treatment of alcohol use disorders

Alcohol use disorders (AUDs) are complex, and developing effective treatments will require the combination of novel medications and cognitive behavioral therapy approaches. Epidemiological studies have shown there is a high correlation between alcohol consumption and tobacco use, and the prevalence of smoking in alcoholics is as high as 80% compared to about 30% for the general population. Both preclinical and clinical data provide evidence that nicotine administration increases alcohol intake and non-specific nicotinic receptor antagonists reduce alcohol-mediated behaviors. As nicotine interacts specifically with the neuronal nicotinic acetylcholine receptor (nAChR) system, this suggests that nAChRs play an important role in the behavioral effects of alcohol. In this review, we discuss the importance of nAChRs for the treatment of AUDs and argue that the use of FDA approved nAChR ligands, such as varenicline and mecamylamine, approved as smoking cessation aids may prove to be valuable treatments for AUDs. We also address the importance of combining effective medications with behavioral therapy for the treatment of alcohol dependent individuals.

Molecular targets and mechanisms for ethanol action in glycine receptors

Glycine receptors (GlyRs) are recognized as the primary mediators of neuronal inhibition in the spinal cord, brain stem and higher brain regions known to be sensitive to ethanol. Building evidence supports the notion that ethanol acting on GlyRs causes at least a subset of its behavioral effects and may be involved in modulating ethanol intake. For over two decades, GlyRs have been studied at the molecular level as targets for ethanol action. Despite the advances in understanding the effects of ethanol in vivo and in vitro, the precise molecular sites and mechanisms of action for ethanol in ligand-gated ion channels in general, and in GlyRs specifically, are just now starting to become understood. The present review focuses on advances in our knowledge produced by using molecular biology, pressure antagonism, electrophysiology and molecular modeling strategies over the last two decades to probe, identify and model the initial molecular sites and mechanisms of ethanol action in GlyRs. The molecular targets on the GlyR are covered on a global perspective, which includes the intracellular, transmembrane and extracellular domains. The latter has received increasing attention in recent years. Recent molecular models of the sites of ethanol action in GlyRs and their implications to our understanding of possible mechanism of ethanol action and novel targets for drug development in GlyRs are discussed.

Single-channel analysis of ethanol enhancement of glycine receptor function

The glycine receptor (GlyR) is a ligand-gated ion channel and member of the nicotinic acetylcholine receptor superfamily. Acting as allosteric modulators of receptor function, drugs such as alcohol and volatile anesthetics enhance the function of GlyRs. The actions of these drugs at inhibitory receptors in the brain and spinal cord are thought to produce many of the physiological effects associated with their use. The actions of ethanol on the GlyR have been well studied on the macroscopic, whole cell level. We examined the effects of 3 microM glycine +/- 50 or 200 mM ethanol on outside-out patches pulled from Xenopus laevis oocytes expressing wild-type alpha1 GlyR, to determine the effects of alcohol at the single-channel level. Alcohol enhanced GlyR function in a very specific manner. It had minimal effects on open and closed dwell times and likelihood. Instead, ethanol potentiated GlyR function almost exclusively by increasing burst durations and increasing the number of channel openings per burst, without affecting the percentage of open time within bursts. Kinetic modeling suggests that ethanol increases burst durations by decreasing the rate of glycine unbinding.

Targets for ethanol action and antagonism in loop 2 of the extracellular domain of glycine receptors

The present studies used increased atmospheric pressure in place of a traditional pharmacological antagonist to probe the molecular sites and mechanisms of ethanol action in glycine receptors (GlyRs). Based on previous studies, we tested the hypothesis that physical-chemical properties at position 52 in extracellular domain Loop 2 of alpha1GlyRs, or the homologous alpha2GlyR position 59, determine sensitivity to ethanol and pressure antagonism of ethanol. Pressure antagonized ethanol in alpha1GlyRs that contain a non-polar residue at position 52, but did not antagonize ethanol in receptors with a polar residue at this position. Ethanol sensitivity in receptors with polar substitutions at position 52 was significantly lower than GlyRs with non-polar residues at this position. The alpha2T59A mutation switched sensitivity to ethanol and pressure antagonism in the WTalpha2GlyR, thereby making it alpha1-like. Collectively, these findings indicate that (i) polarity at position 52 plays a key role in determining sensitivity to ethanol and pressure antagonism of ethanol; (ii) the extracellular domain in alpha1- and alpha2GlyRs is a target for ethanol action and antagonism and (iii) there is structural-functional homology across subunits in Loop 2 of GlyRs with respect to their roles in determining sensitivity to ethanol and pressure antagonism of ethanol. These findings should help in the development of pharmacological agents that antagonize ethanol.

Measurement of chloride movement in neuronal preparations

In this unit, protocols are described for biochemical and optical techniques that have been used by investigators to measure ligand-gated chloride movement in vesicular structures called synaptoneurosomes (also referred to as microsacs), in cultured neurons, and in the acute brain slice. These techniques can be applied to other ions as well. The measurement of uptake and efflux of radioisotopic chloride in synaptoneurosomes is used to study the responses of gamma-aminobutyric acid (GABA) receptors, which are coupled to chloride channels. Similar chloride flux assays for primary neuronal cultures are also presented. Alternatively, the efflux of chloride from synaptoneurosomes and primary neuronal cultures can be studied using fluorescent dyes and photometry. Finally, the measurement of chloride uptake can be studied in individual neurons in brain slices using fluorescent dyes and optical imaging by nonconfocal and confocal microscopy. Several support protocols are provided as well, outlining the preparation of synaptoneurosomes from specific brain regions, and the preparation, loading, and calibration of chloride-sensitive fluorescent dyes.

Physiological significance of hypotonicity-induced regulatory volume decrease: reduction in intracellular Cl- concentration acting as an intracellular signaling

Regulatory volume decrease (RVD) occurs after hypotonicity-caused cell swelling. RVD is caused by activation of ion channels and transporters, which cause effluxes of K(+), Cl(-), and H(2)O, leading to cell shrinkage. Recently, we showed that hypotonicity stimulated transepithelial Na(+) reabsorption via elevation of epithelial Na(+) channel (alpha-ENaC) expression in renal epithelia A6 cells in an RVD-dependent manner and that reduction of intracellular Cl(-) concentration ([Cl(-)](i)) stimulated the Na(+) reabsorption. These suggest that RVD would reveal its stimulatory action on the Na(+) reabsorption by reducing [Cl(-)](i). However, the reduction of [Cl(-)](i) during RVD has not been definitely analyzed due to technical difficulties involved in halide-sensitive fluorescent dyes. In the present study, we developed a new method for the measurement of [Cl(-)](i) change during RVD by using a high-resolution flow cytometer with a halide-specific fluorescent dye, N-(6-methoxyquinolyl) acetoethyl ester. The [Cl(-)](i) in A6 cells in an isotonic medium was 43.6 +/- 3.1 mM. After hypotonic shock (268 to 134 mosmol/kgH(2)O), a rapid increase of cell volume followed by RVD occurred. The RVD caused drastic diminution of [Cl(-)](i) from 43.6 to 10.8 mM. Under an RVD-blocked condition with NPPB (Cl(-) channel blocker) or quinine (K(+) channel blocker), we did not detect the reduction of [Cl(-)](i). Based on these observations, we conclude that one of the physiological significances of RVD is the reduction of [Cl(-)](i) and that RVD shows its action via reduction of [Cl(-)](i) acting as an intracellular signal regulating cellular physiological functions.

Glycine receptors regulate dopamine release in the rat nucleus accumbens

BACKGROUND

The mesolimbic dopamine (DA) system seems to be centrally involved in regulating reward-related behavior and consequently has been implicated in addictive processes, such as alcoholism and drug addiction. This DA system has also been implicated in psychosis and in regulating hedonia/anhedonia, important components of mania and depression. Given the potentially great importance of the mesolimbic DA system for several psychiatric disorders, it is of major interest to delineate the mechanisms and dynamics underlying DA regulation and release. Recently strychnine-sensitive glycine receptors (GlyR) have attracted some interest in this matter.

METHODS

Western blot and in vivo microdialysis (couplied to high-pressure liquid chromatography with electrochemical detection), as well as reversed microdialysis, in awake, freely moving, adult male Wistar rats.

RESULTS

Here we demonstrate by means of Western blot that alpha GlyR subunit proteins are expressed in the rat nucleus accumbens (nAc), a major target of the mesolimbic DA system. We further show that reversed microdialysis of the competitive GlyR antagonist strychnine into the nAc concentration-dependently (2-200 microM) and in a reversible manner decreases accumbal extracellular DA levels. Conversely, reversed microdialysis of the agonist glycine increases accumbal DA levels in some rats but not others. The strychnine-induced depression of the accumbal DA levels is antagonized by simultaneous local perfusion of glycine.

CONCLUSIONS

The present results indicate that GlyRs in the nAc are tonically activated and of importance for regulating extracellular DA levels. The possibility of pharmacologically interfering with GlyRs to combat psychiatric disorders, in which the mesolimbic DA system is implicated, such as alcoholism, drug addiction, and psychosis, should be explored.

The role of G proteins in the activity and ethanol modulation of glycine-induced currents in rat neurons freshly isolated from the ventral tegmental area

In freshly isolated neurons of the ventral tegmental area of young rats, we first examined the role of G proteins in the functional modulation of the glycine receptor (GlyR). GTP-gamma-S [guanosine-5'-0-(2-thiotriphosphate)] (2 mM) or GDP-beta-S [guanosine 5'-0-(2-thiodiphosphate)] (2 mM) was added to the pipette solution of whole-cell recordings to regulate G protein activities. GTP-gamma-S enhanced the amplitude of glycine-induced current (I(Gly)), suggesting modulation of GlyRs via a G protein-coupled pathway. GDP-beta-S suppressed I(Gly), suggesting that basal G protein activity positively modulates the GlyRs. We next examined effects of G proteins in ethanol potentiation of GlyR function. Activation of G proteins with 2 mM GTP-gamma-S attenuated, but did not eliminate, ethanol-induced potentiation of I(Gly). These results suggest that GTP-gamma-S and ethanol share the same pathway of activating GlyRs. When G proteins are maximally activated by GTP-gamma-S, the action of ethanol was partially occluded. When 2 mM GDP-beta-S was added in pipette solution, ethanol-induced potentiation of I(Gly) was significantly attenuated, suggesting that GDP-beta-S partially blocked the action of ethanol. However, the inability of GTP-gamma-S (or GDP-beta-S) to eliminate completely the potentiating effect of ethanol indicates that some other factors, in addition to G proteins, may also contribute to the action of ethanol on GlyRs.

Accumbal Strychnine-Sensitive Glycine Receptors: An Access Point for Ethanol to the Brain Reward System

BACKGROUND

Ethanol (EtOH), like other drugs of abuse, increases extracellular dopamine (DA) levels in the nucleus accumbens (nAc) of the brain reward system, an effect that may be of importance for alcohol addiction. How this DA increase is produced is not fully understood, although previous studies from the present laboratories indicate that nicotinic acetylcholine receptors in the ventral tegmental area play an important role in mediating this effect. Furthermore, activation of these receptors may be secondary to some priming effect produced by EtOH in the nAc. We recently demonstrated that strychnine-sensitive glycine receptors (GlyRs) are present in the nAc and that they are involved in regulating extracellular DA levels. Here we examine the tentative role of these accumbal GlyRs in the above-mentioned priming mechanism of EtOH.

METHOD

In vivo microdialysis (coupled to high pressure liquid chromatography with electrochemical detection) and reversed microdialysis, in awake, freely moving adult male Wistar rats.

RESULTS

Local perfusion of strychnine decreased accumbal DA levels per se and completely prevented the increase of accumbal DA levels after both local and systemic EtOH administration. Accumbal perfusion of the GlyR agonist glycine instead increased DA levels in a subpopulation of rats and prevented the EtOH-induced increase after local but not systemic EtOH in all animals.

CONCLUSION

The present results suggest that GlyRs in the nAc might constitute targets for EtOH in its mesolimbic DA-activating effect. Gene polymorphism and drug developmental studies that focus on this receptor population and its relation to alcohol dependence are warranted.

Channel Gating of the Glycine Receptor Changes Accessibility to Residues Implicated in Receptor Potentiation by Alcohols and Anesthetics

The glycine receptor is a target for both alcohols and anesthetics, and certain amino acids in the alpha1 subunit transmembrane segments (TM) are critical for drug effects. Introducing larger amino acids at these positions increases the potency of glycine, suggesting that introducing larger residues, or drug molecules, into the drug-binding cavity facilitates channel opening. A possible mechanism for these actions is that the volume of the cavity expands and contracts during channel opening and closing. To investigate this hypothesis, mutations for amino acids in TM1 (I229C) and TM2 (G256C, T259C, V260C, M263C, T264C, S267C, S270C) and TM3 (A288C) were individually expressed in Xenopus laevis oocytes. The ability of sulfhydryl-specific alkyl methanethiosulfonate (MTS) compounds of different lengths to covalently react with introduced cysteines in both the closed and open states of the receptor was determined. S267C was accessible to short chain (C3-C8) MTS in both open and closed states, but was only accessible to longer chain (C10-C16) MTS compounds in the open state. Reaction with S267C was faster in the open state. I229C and A288C showed state-dependent reaction with MTS only in the presence of agonist. M263C and S270C were also accessible to MTS labeling. Mutated residues more intracellular than M263C did not react, indicating a floor of the cavity. These data demonstrate that the conformational changes accompanying channel gating increase accessibility to amino acids critical for drug action in TM1, TM2, and TM3, which may provide a mechanism by which alcohols and anesthetics can act on glycine (and likely other) receptors.

Mechanisms for the modulation of native glycine receptor channels by ethanol

Previously, we showed that ethanol increases synaptic glycine currents, an effect that depends on ethanol concentration and developmental age of the preparation. Glycine receptor (GlyR) subunits undergo a shift from alpha2/beta to alpha1/beta from neonate to juvenile ages, with synaptic glycine currents from neonate hypoglossal motoneurons (HMs) being less sensitive to ethanol than those from juvenile HMs. Here we investigate whether these dose and developmental effects are also present in excised membrane patches containing GlyRs and if ethanol changes response kinetics. We excised outside-out patches from rat HM somata and applied glycine using either a picospritzer or piezo stack translator. Ethanol (100 mM) increased the response to glycine (200 microM) of patches from neonate and juvenile HMs. However, 30 mM ethanol increased the response from only juvenile HM patches. Using a lower concentration of glycine (30 microM) to observe single channel openings, we found that 100 mM ethanol increased the number of GlyRs that open in response to glycine and decreased first latency to channel opening. To investigate GlyR kinetic properties, we rapidly applied 1 mM glycine for 1 ms and found that glycine currents were increased by ethanol (100 mM) at both ages. For patches from juvenile HMs, ethanol consistently decreased response rise-time and increased response decay time. Using kinetic modeling, we determined that ethanol's potentiation of the glycine response arises from an increase in the glycine association (k(on)) and a decrease in the dissociation (k(off)) rate constants, resulting in increased glycine affinity of the GlyR.

Ethanol potentiation of glycine receptors expressed in Xenopus oocytes antagonized by increased atmospheric pressure

BACKGROUND

Behavioral and biochemical studies indicate that exposure to 12 times normal atmospheric pressure (12 ATA) of helium-oxygen gas (heliox) is a direct, selective ethanol antagonist. The current study begins to test the hypothesis that ethanol acts by a common mechanism on ligand-gated ion channels by expanding previous hyperbaric investigations on gamma-aminobutyric acid type A (GABA(A)) receptors (GABA(A)Rs) at the biochemical level to alpha(1)glycine (GlyRs) expressed in Xenopus oocytes.

METHODS

Oocytes expressing wild-type alpha(1) homomeric GlyRs were voltage-clamped (-70 mV) and tested in the presence of glycine (EC(2)) +/- ethanol (50-200 mM) under 1 ATA control and 3 to 12 ATA heliox conditions. Glycine concentration response curves, strychnine/glycine interactions, and zinc (Zn2+) modulation of GlyR function was also tested.

RESULTS

Pressure reversibly antagonized the action of ethanol. The degree of antagonism increased as pressure increased. Pressure did not significantly alter the effects of glycine, strychnine, or Zn2+, indicating that ethanol antagonism by pressure cannot be attributed to alterations by pressure of normal GlyR function. The antagonism did not reflect tolerance to ethanol, receptor desensitization, or receptor rundown.

CONCLUSION

This is the first use of hyperbarics to investigate the mechanism of action of ethanol in recombinant receptors. The findings indicate that pressure directly and selectively antagonizes ethanol potentiation of alpha(1)GlyR function in a reversible and concentration- and pressure-dependent manner. The sensitivity of ethanol potentiation of GlyR function to pressure antagonism indicates that ethanol acts by a common, pressure-antagonism-sensitive mechanism in GlyRs and GABA(A)Rs. The findings also support the hypothesis that ethanol potentiation of GlyR function plays a role in mediating the sedative-hypnotic effects of ethanol.

Ethanol dual modulatory actions on spontaneous postsynaptic currents in spinal motoneurons

Recently we have shown that acute ethanol (EtOH) exposure suppresses dorsal root-evoked synaptic potentials in spinal motoneurons. To examine the synaptic mechanisms underlying the reduced excitatory activity, EtOH actions on properties of action potential-independent miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) were studied in spinal motoneurons of newborn rats. Properties of mEPSCs generated by activation of N-methyl-D-aspartate receptors (NMDARs) and non-NMDA receptors and of mIPSCs mediated by glycine and gamma-aminobutyric acid-A receptors (GlyR and GABA(A)R) were examined during acute exposure to 70 and 200 mM EtOH. In the presence of 70 mM EtOH, the frequency of NMDAR- and non-NMDAR-mediated mEPSCs decreased to 53 +/- 5 and 45 +/- 7% (means +/- SE) of control values, respectively. In contrast, the frequency of GlyR- and GABA(A)R-mediated mIPSCs increased to 138 +/- 15 and 167 +/- 23% of control, respectively. Based on the quantal theory of transmitter release, changes in the frequency of miniature currents are correlated with changes in transmitter release, suggesting that EtOH decreased presynaptic glutamate release and increased the release of both glycine and GABA. EtOH did not change the amplitude or rise and decay times of either mEPSCs or mIPSCs, indicating that the presynaptic changes were not associated with changes in the properties of postsynaptic receptors/channels. Acute exposure to 200 mM EtOH increased mIPSC frequency two- to threefold, significantly higher than the increase induced by 70 mM EtOH. However, the decrease in mEPSC frequency was similar to that observed in 70 mM EtOH. Those findings implied that the regulatory effect of EtOH on glycine and GABA release was dose-dependent. Exposure to the higher EtOH concentration had opposite actions on mEPSC and mIPSC amplitudes: it attenuated the amplitude of NMDAR- and non-NMDAR-mediated mEPSCs to ~80% of control and increased GlyR- and GABA(A)R-mediated mIPSC amplitude by ~20%. EtOH-induced changes in the amplitude of postsynaptic currents were not associated with changes in their basic kinetic properties. Our data suggested that in spinal networks of newborn rats, EtOH was more effective in modulating the release of excitatory and inhibitory neurotransmitters than changing the properties of their receptors/channels.

Decay of ethanol-induced suppression of glycine-activated current of ventral tegmental area neurons

We demonstrated previously that ethanol depresses glycine-induced currents in 45% of neurons freshly isolated from the ventral tegmental area (VTA) of rats (), and that protein kinase C (PKC) modulates this action of ethanol (). In the present study, we investigated the time course of this effect of ethanol on VTA neurons from young rats. For 70% of the neurons in which ethanol reduced glycine-evoked currents, this depressant effect gradually diminished during continuous superfusion with ethanol. Its action decayed faster when ethanol was applied in several brief pulses than by continuous superfusion. On the other hand, the decay was especially slower when ethanol was applied in pulses at longer intervals or by preincubation. Phorbol ester 12,13-dibutyrate (PDBu, 1 microM), an activator of PKC, also depressed glycine-induced currents. In approximately 40% (6/15) of the neurons, the effect of PDBu diminished with time and was antagonized by the specific PKC inhibitor, chelerythrine (7 microM). Chelerythrine also attenuated the ethanol-induced depression of glycine-induced currents and its time-dependent decay, thus confirming our previous evidence that PKC mediates, at least in part, the decay of the depressant effect of ethanol on glycine-induced currents of VTA neurons.

Inhibition of glycine receptor function of native neurons by aliphatic n-alcohols

The inhibitory effects of n-alcohols (methanol to dodecanol) on glycine-activated currents were studied in neurons freshly dissociated from the ventral tegmental area of neonatal rats using whole-cell patch-clamp recording technique. Ethanol enhanced and depressed glycine-activated currents in 35% and 45%, respectively, of neurons of ventral tegmental area of neonatal rats. In this report, we extended our focus of ethanol-induced inhibition of glycine currents to other straight-chain alcohols. Aliphatic n-alcohols, which have carbon numbers less than nine, suppressed glycine currents in 45% (71/158) of the neurons. All results from this study are obtained from the 45% of cells displaying inhibition; the other 55% of the neurons were not studied. Alcohol potency increased as the number of carbon atoms increased from one to five, and was at a maximal plateau from five to nine; alcohols with 10 or more carbons did not inhibit glycine-activated currents. Thus, a 'cutoff' point in their potency for inhibition of glycine receptor function occurred at about decanol. A coapplication of dodecanol with ethanol eliminated the inhibition resulting from ethanol. Thus, dodecanol may bind to the receptor silently and compete with ethanol. These observations indicate that straight-chain n-alcohols exhibit a 'cutoff' point in their potency for inhibition of the glycine receptor function between nine and 10 carbon atoms. The inability of longer alcohols to change the activation properties of the receptors may contribute to the cutoff effect.

Protein kinase C modulation of ethanol inhibition of glycine-activated current in dissociated neurons of rat ventral tegmental area

The brain is particularly sensitive to alcohol during its growth spurt period. To better understand the mechanism(s) involved, we studied the effects of ethanol on neurons freshly dissociated from the ventral tegmental area (VTA) in neonatal rats. Ethanol enhanced (35%) or depressed (45%) glycine-induced responses in VTA neurons (Ye et al., 2001a, 2001b). In this report, we investigated the role of protein kinase C (PKC) and protein kinase A (PKA) in ethanol-induced inhibition of glycine-activated current, using whole-cell patch-clamp technique. Ethanol inhibited glycine-activated current when it was coapplied with the agonist. This inhibition was enhanced when neurons were pretreated with ethanol before the subsequent coapplication of ethanol and glycine. Ethanol's inhibition of glycine-activated currents increased with the length of ethanol pretreatment time (ranging from 1 to 30 s), and reached the maximum at 30 s. However, this enhanced inhibition was not seen in the absence of internal ATP. In addition, phorbol-12-myristate-13-acetate (PMA, 100 nM), a PKC activator, markedly inhibited glycine-activated current. Blockade of PKC by chelerythrine or by PKC inhibitor peptide significantly attenuated ethanol-induced inhibition. Although partial increase of PKC activity by 1 nM PMA enhanced ethanol inhibition, pretreatment of ethanol did not increase ethanol inhibition after the neurons were treated with 100 nM PMA. These data suggest that ethanol and PKC share the same pathway to suppress glycine receptors. H-89 (1 microM), a selective PKA inhibitor, did not alter glycine-activated current or ethanol inhibition. Our observations suggest that activation of PKC (but not PKA) contributes to ethanol-induced inhibition of glycine receptors.

Transgenic expression of a mutant glycine receptor decreases alcohol sensitivity of mice

Glycine receptors (GlyRs) are pentameric ligand-gated ion channels that inhibit neurotransmission in the adult brainstem and spinal cord. GlyR function is potentiated by ethanol in vitro, and a mutant GlyR subunit alpha(1)(S267Q) is insensitive to the potentiating effects of ethanol. To test the importance of GlyR for the actions of ethanol in vivo, we constructed transgenic mice with this mutation. Under the control of synapsin I regulatory sequences, transgenic expression of S267Q mutant GlyR alpha(1) subunits in the nervous system was demonstrated using [(3)H]strychnine binding and immunoblotting. These mice showed decreased sensitivity to ethanol in three behavioral tests: ethanol inhibition of strychnine seizures, motor incoordination (rotarod), and loss of righting reflex. There was no change in ethanol sensitivity in tests of acute functional tolerance or body temperature, and there was no change in ethanol metabolism. Transgene effects were pharmacologically specific for ethanol, compared with pentobarbital, flurazepam, and ketamine. These results support the idea that glycine receptors contribute to some behavioral actions of ethanol and that ethanol sensitivity can be changed in vivo by transgenic expression of a single receptor subunit.

Ethanol inhibition of glycine-activated responses in neurons of ventral tegmental area of neonatal rats

The brain is particularly sensitive to alcohol during the period of its rapid growth. To better understand the mechanism(s) involved, we studied ethanol effects on glycine-activated responses of ventral tegmental area (VTA) neurons isolated from the newborn rat, using whole cell and gramicidin perforated patch-clamp techniques. Previously we reported that 0.1-40 mM ethanol enhances glycine-induced responses of 35% of VTA neurons. We now direct our attention to the inhibitory effects of ethanol observed in 45% (312 of 694) of neonatal VTA neurons. Under current-clamp conditions, 1 mM ethanol had no effect on the membrane potential of these cells, but it decreased glycine-induced membrane depolarization and the frequency of spontaneous action potentials. Under voltage-clamp conditions, 0.1-10 mM ethanol did not elicit a current but depressed the glycine-induced currents. The ethanol-induced inhibition of glycine current was independent of membrane potential (between -60 and +60 mV). Likewise, ethanol did not alter the reversal potential of the glycine-activated currents. Ethanol-mediated inhibition of glycine current depended on the glycine concentration. While ethanol strongly depressed currents activated by 30 microM glycine, it had no appreciable effect on maximal currents activated by 1 mM glycine. In the presence of ethanol (1 mM), the EC(50) for glycine increased from 32 +/- 5 to 60 +/- 3 microM. Thus ethanol may decrease the agonist affinity of glycine receptors. A kinetic analysis indicated that ethanol shortens the time constant of glycine current deactivation but has no effect on activation. In conclusion, by altering VTA neuronal function, ethanol-induced changes in glycine receptors may contribute to neurobehavioral manifestations of the fetal alcohol syndrome.

Developmental changes in the modulation of synaptic glycine receptors by ethanol

During postnatal motoneuron development, the glycine receptor (GlyR) alpha subunit changes from alpha2 (fetal) to alpha1 (adult). To study the effect this change has on ethanol potentiation of GlyR currents in hypoglossal motoneurons (HMs), we placed neurons into two groups: neonate [postnatal day 1 to 3 (P1-3)], primarily expressing alpha2, and juvenile (P9-13), primarily expressing alpha1. We found that glycinergic spontaneous miniature inhibitory postsynaptic currents (mIPSCs) in neonate HMs are less sensitive to ethanol than in juveniles. Thirty millimolar ethanol increased the amplitude of juvenile mIPSCs but did not significantly change neonatal mIPSCs. However, 100 mM ethanol increased the amplitudes of both neonate and juvenile mIPSCs. There was a significant difference between age groups in the average ethanol-induced increase in mIPSC amplitude for 10, 30, 50, and 100 mM ethanol. In both age groups ethanol increased the frequency of glycinergic mIPSCs, but there was no difference in the amount of frequency increase between age groups. Ethanol (100 mM) also potentiated evoked IPSCs (eIPSCs) in both neonate and juvenile HMs. As we observed for mIPSCs, 30 mM ethanol increased the amplitude of juvenile eIPSCs, but had no significant effect on eIPSCs in neonate HMs. Ethanol also potentiated currents induced by exogenously applied glycine in both neonate and juvenile HMs. These results suggest that ethanol directly modulates the GlyR. To investigate possible mechanisms for this, we analyzed the time course of mIPSCs and single-channel conductance of the GlyR in the presence and absence of ethanol. We found that ethanol did not significantly change the time course of mIPSCs. We also determined that ethanol did not significantly change the single-channel conductance of synaptic GlyRs, as estimated by nonstationary noise analysis of mIPSCs. We conclude that the adult form of the native GlyR is more sensitive to ethanol than the fetal form. Further, enhancement of GlyR currents involves mechanisms other than an increase in the single-channel conductance or factors that alter the decay kinetics.

Nicotinic mechanisms involved in the dopamine activating and reinforcing properties of ethanol

Ethanol shares with all major dependence producing drugs the ability to activate brain mesocorticolimbic dopamine neurons, an important part of the brain reward systems. This dopamine activation may be involved in mediating the positive reinforcing effects of ethanol. The mechanisms of action of ethanol in its activation of this dopamine system remain, however, to be elucidated. A selective pharmacological interference with these mechanisms may offer a possibility to reduce the reinforcing properties of ethanol without simultaneously interfering with the reinforcing properties of natural rewards. Ethanol has been shown to directly influence the function of various ligand-gated ion-channels. Several of these are located on or nearby mesocorticolimbic dopamine neurons. One such receptor is the nicotinic acetylcholine receptor (nAChR). The present article reviews a series of investigations aimed at investigating whether nAChRs are involved in the dopamine activating and reinforcing properties of ethanol. To this end acute and chronic behavioral and neurochemical experiments were performed in mice and rats. The results obtained indicate that central nAChRs in the ventral tegmental area are involved in mediating the mesolimbic dopamine activating and reinforcing effects of ethanol. Furthermore, the ethanol-induced activation of these receptors is probably indirect, subsequent to a primary interference of ethanol in the nucleus accumbens. Moreover, subchronic nicotine treatment enhances the reinforcing and dopamine activating properties of ethanol. This long-term effect may, however, derive from autonomic adaptations in response to intermittent blockade of peripheral nAChRs (rather than from intermittent stimulation of central receptors), and appears to be associated with development of a disinhibitory behavior that could involve also other neurotransmitters, e.g. serotonin. Taken together, these findings could provide a neurobiological explanation to the often observed co-abuse of nicotine and ethanol in man. Furthermore, since the behavioral models applied previously have predicted therapeutic drug effects in the clinic, the results suggest that selective blockade of the ventral tegmental nAChRs that are involved in the above effects may provide a new pharmacological alternative in the treatment of alcoholism.

Ethanol and amino acids in the central nervous system: assessment of the pharmacological actions of acamprosate

Ethanol induces alterations in the central nervous system by differentially interfering with a number of neurotransmitter systems, although the mechanisms by which such effects are executed are not well understood. The present review therefore, is designed to ascertain the effect of ethanol on both excitatory and inhibitory amino acid neurotransmitters, as well as the sulphonated amino acid taurine, assayed by the microdialysis technique within specific brain regions of rat during different types of alcohol intoxication, acute and chronic, as well as during the withdrawal period. Such an understanding of these pharmacological actions of ethanol on neurotransmitters is essential in order to provide the impetus for the development of appropriate therapeutic intervention to ameliorate the multitude of neurochemical disorders induced by ethanol. In addition the possible mode of action of a new therapeutic drug for the treatment of alcoholism, acamprosate will be discussed. The first part of this review will be limited to studies of the effect of ethanol on both amino acid neurotransmitters and the sulphonated amino acid taurine, a possible neuromodulator. While, the second part will seek to establish the possible mechanism of action of a new therapeutic drug, acamprosate, which is used to combat the effects of ethanol, particularly during the craving period, as well as maintaining abstinence in weaned alcoholics.

Differential effects of ethanol on glycine uptake mediated by the recombinant GLYT1 and GLYT2 glycine transporters

The effects of ethanol on the function of recombinant glycine transporter 1 (GLYT1) and glycine transporter 2 (GLYT2) have been investigated. GLYT1b and GLYT2a isoforms stably expressed in human embryonic kidney 293 (HEK 293) cells showed a differential behaviour in the presence of ethanol; only the GLYT2a isoform was acutely inhibited. The 'cut-off' (alcohols with four carbons) displayed by the n-alkanols on GLYT2a indicates that a specific binding site for ethanol exists on GLYT2a or on a GLYT2a-interacting protein. The non-competitive inhibition of GLYT2a indicates an allosteric modulation by ethanol of GLYT2a activity. Chronic treatment with ethanol caused differential adaptive responses on the activity and the membrane expression levels of these transporters. The neuronal GLYT2a isoform decreased in activity and surface expression and the mainly glial GLYT1b isoform slightly increased in function and surface density. These changes may be involved in some of the modifications of glycinergic or glutamatergic neurotransmitter systems produced by ethanol intoxication.

Effect of ethanol upon respiratory-related hypoglossal nerve output of neonatal rat brain stem slices

The actions of ethanol (EtOH) on the respiratory output of the neonatal rat brain stem slice preparation in vitro are described. Ethanol inhibited respiratory-related hypoglossal nerve activity in a dose-dependent manner. The effect of EtOH was evident within 5 min and was reversible on EtOH washout. The actions of EtOH were qualitatively similar to those of two other alcohols, methanol and octanol. We investigated the dose-response relationship for each alcohol and determined that the order of potency was methanol < EtOH << octanol, with EC(50) values of 291 mM, 39.7 mM, and 49.2 microM respectively. Application of either strychnine (5 microM) or bicuculline (5 microM) alone, partially but not significantly, reversed the inhibition of respiratory-related hypoglossal nerve activity produced by 50 mM EtOH. Preincubation of rhythmic slices with a combination of both strychnine and bicuculline (both 5 microM) partially, but significantly, blocked the inhibitory actions of EtOH, suggesting that other mechanisms also play a role in the action of EtOH. Preincubation of the slices with 25 microM APV reduced the relative degree of inhibition caused by EtOH suggesting that N-methyl-D-aspartate (NMDA)-receptor-mediated events can be affected by EtOH. Furthermore inhibition of protein kinase C by incubation with 100 nM staurosporine also reduced the efficacy of EtOH. These results suggest that the actions of EtOH may be mediated via glycine, GABA(A), and NMDA receptors and that activation of protein kinase C is involved in the EtOH-induced inhibition of respiratory-related hypoglossal nerve activity.

Intracellular chloride and calcium transients evoked by gamma-aminobutyric acid and glycine in neurons of the rat inferior colliculus

Microfluorometric recordings showed that the inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine activated transient increases in the intracellular Cl- concentration in neurons of the inferior colliculus (IC) from acutely isolated slices of the rat auditory midbrain. Current recordings in gramicidin-perforated patch mode disclosed that GABA and glycine mainly evoked inward or biphasic currents. These currents were dependent on HCO3- and characterized by a continuous shift of their reversal potential (E(GABA/gly)) in the positive direction. In HCO3- -buffered saline, GABA and glycine could also evoke an increase in the intracellular Ca2+ concentration. Ca2+ transients occurred only with large depolarizations and were blocked by Cd2+, suggesting an activation of voltage-gated Ca2+ channels. However, in the absence of HCO3-, only a small rise, if any, in the intracellular Ca2+ concentration could be evoked by GABA or glycine. We suggest that the activation of GABAA or glycine receptors results in an acute accumulation of Cl- that is enhanced by the depolarization owing to HCO3- efflux, thus shifting E(GABA/gly) to more positive values. A subsequent activation of these receptors would result in a strenghtened depolarization and an enlarged Ca2+ influx that might play a role in the stabilization of inhibitory synapses in the auditory pathway.

Molecular simulation of the effects of alcohols on peptide structure

The effects of alcohols on local protein structure have been simulated using computational approaches and model peptides. Molecular simulations were carried out on a 7-residue peptide created in both an extended conformation and an alpha-helix to explore alcohol-induced changes in peptide structure. It was assumed that alcohols hydrogen bond at peptide carbonyl groups with an optimum geometry and compete with water molecules at these site. Energy minimization of the peptide/alcohol assemblies revealed that alcohols induced a twist in the peptide backbone as a function of (1) the methylene chain length, (2) the hydrogen-bond geometry, (3) halogenation of the molecule, (4) concentration, and (5) the dielectric constant. The rank ordering of the potencies of the alcohols was hexafluoroisopropanol > trifluoroethanol approximately pentanol > butanol > ethanol > methanol. Helix destabilization by cosolvent was measured by examining the hydrogen-bond lengths in peptide structures that resulted from a combination of energy minimization and molecular dynamics simulations. Destabilization was also found to be dependent upon the chemical nature of the alcohol and the hydrogen-bond geometry. The data suggest that alcohols at low concentrations affect protein structure mainly through a combination of hydrogen-bonding and hydrophobic interactions that are influenced by the properties of the solvent.

Fluorescence imaging of changes in intracellular chloride in living brain slices

In brain slice preparations, chloride movements across the cell membrane of living cells are measured traditionally with 36Cl- tracer methods, Cl--selective microelectrodes, or whole-cell recording using patch clamp analysis. We have developed an alternative, noninvasive technique that uses the fluorescent Cl- ion indicator, 6-methoxy-N-ethylquinolinium iodide (MEQ), to study changes in intracellular Cl- by epifluorescence or UV laser scanning confocal microscopy. In brain slices taken from rodents younger than 22 days of age, excellent cellular loading is achieved with the membrane-permeable form of the dye, dihydro-MEQ. Subsequent intracellular oxidation of dihydro-MEQ to the Cl--sensitive MEQ traps the polar form of the dye inside the neurons. Because MEQ is a single-excitation and single-emission dye, changes in intracellular Cl- concentrations can be calibrated from the Stern-Volmer relationship, determined in separate experiments. Using MEQ as the fluorescent indicator for Cl-, Cl- flux through the gamma-aminobutyric acid (GABA)-gated Cl- channel (GABAA receptor) can be studied by dynamic video imaging and either nonconfocal (epifluorescence) or confocal microscopy in the acute brain slice preparation. Increases in intracellular Cl- quench MEQ fluorescence, thereby reflecting GABAA receptor activation. GABAA receptor functional activity can be measured in discrete cells located in neuroanatomically defined populations within areas such as the neocortex and hippocampus. Changes in intracellular Cl- can also be studied under various conditions such as oxygen/glucose deprivation ("in vitro ischemia") and excitotoxicity. In such cases, changes in cell volume may also occur due to the dependence of cell volume regulation on Na+, K+, and Cl- flux. Because changes in cell volume can affect optical fluorescence measurements, we assess cell volume changes in the brain slice using the fluorescent indicator calcein-AM. Determination of changes in MEQ fluorescence versus calcein fluorescence allows one to distinguish between an increase in intracellular Cl- and an increase in cell volume.

Enhancement of glycine receptor function by ethanol: role of phosphorylation

1. The effects of several kinase inhibitors (staurosporine, GF 109203X, H89, KN62, genistein) and of the phosphatase inhibitor calyculin A were studied on the ethanol potentiation and on the function of homomeric alpha1 glycine receptor expressed in Xenopus oocytes using a two electrode voltage clamp recording technique. 2. The function of the homomeric alpha1 glycine receptor was not modified in Xenopus oocytes pretreated with kinase inhibitors or with the phosphatase inhibitor calyculin A. 3. The potentiation of the glycine receptor function induced by ethanol (10-200 mM) was significantly reduced in Xenopus oocytes pretreated with the PKC inhibitors staurosporine or GF 109203X. 4. No differences in propofol (2.5 microM) or halothane (250 microM) actions were found after exposure of Xenopus oocytes to staurosporine. 5. No differences in ethanol sensitivity were found after exposure of Xenopus oocytes expressing glycine alpha1 receptors to H89, KN62, genistein or to the phosphatase inhibitor calyculin A. 6. The mutant alpha1 (S391A), in which the PKC phosphorylation site at serine 391 was mutated to alanine, was less sensitive to the effects of ethanol than was the alpha1 wild type receptor. Moreover, the ethanol potentiation of the glycine receptor function was not affected by treatment with staurosporine in oocytes expressing alpha1 (S391A). 7. The splice variant of the alpha1 glycine receptor subunit, alpha1ins, containing eight additional amino acids and a potential phosphorylation site for PKA, did not differ from wild type for sensitivity to ethanol. 8. These results indicate that phosphorylation by PKC of the homomeric alpha1 glycine receptor subunit modulates ethanol potentiation, but not the function of the glycine receptor.

Enhancement of glycine receptor function by ethanol is inversely correlated with molecular volume at position alpha267

Glycine and gamma-aminobutyric acid (GABA)A receptors are members of the "superfamily" of ion channels, and are sensitive to allosteric modulation by n-alcohols such as ethanol and butanol. We recently demonstrated that the mutation of Ser-267 to Ile in the alpha1 subunit abolished ethanol regulation of glycine receptors (Gly-R). In the present study, a pair of chimeric receptors was studied, in which a 45-amino acid domain comprising transmembrane domains 2 and 3 was exchanged between the Gly-Ralpha1 and gamma-aminobutyric acid rho1 subunits. Detailed pharmacologic analysis of these chimeras confirmed that this domain of the Gly-R confers enhancement of receptor function by ethanol and butanol. An extensive series of mutations at Ser-267 in the Gly-Ralpha1 subunit was also prepared, and the resulting homomeric receptors were expressed and tested for sensitivity to glycine, and allosteric modulation by alcohols. All of the mutant receptors expressed successfully in Xenopus oocytes. Mutation of Ser-267 to small amino acid residues such as Gly or Ala produced receptors in which glycine responses were potentiated by ethanol. As we have reported previously, the mutant Gly-Ralpha1 (Ser-267 --> Ile) was completely insensitive to ethanol; mutation of Ser-267 to Val had a similar effect. Mutation of Ser-267 to large residues such as His, Cys, or Tyr resulted in inhibition of Gly-R function by ethanol. These results demonstrate that the size of the amino acid residue at position alpha267 plays a crucial role in determining the functional consequences of allosteric modulation of the Gly-R by alcohols.

Confocal imaging of intracellular chloride in living brain slices: measurement of GABAA receptor activity

We have developed a method using UV laser-scanning confocal microscopy and the fluorescent chloride ion indicator, 6-methoxy-N-ethylquinolinium chloride (MEQ), to image GABA-mediated changes in intracellular chloride (Cli-) in individual neurons of the rat acute brain slice. After bath-loading slices with the cell-permeant form (reduced) of MEQ, there was intense fluorescence within neurons of diverse morphologies in the hippocampus, neocortex and cerebellum. MEQ fluorescence localized to the cytosolic compartment of both the somata and proximal dendrites. MEQ fluorescence was calibrated using the ionophores nigericin and tributyltin in the presence of varying extracellular Cl- concentrations. Neuronal MEQ fluorescence was inversely related to intracellular Cl-, with a Stern-Volmer constant of 16 M-1 (50% quench by 61 mM Cl-). Application of GABA in the perfusate produced a concentration-dependent decrease in MEQ fluorescence (EC50 = 40 microM) that was blocked in the presence of the Cl- channel antagonist, picrotoxin. Bath perfusion of hippocampal slices with modulators of the GABAA receptor, pentobarbital and diazepam, potentiated the GABA-mediated response by 85 and 44%, respectively. A regional comparison identified larger GABA responses for both cerebellar Purkinje and granule cells relative to pyramidal neurons of the hippocampus and neocortex and to hippocampal interneurons. Pressure ejection of the GABAA agonist, muscimol (40 microM), from a micropipet onto individual hippocampal neurons allowed the measurement of rapid responses (1-5 s), compared to those obtained with bath application. Thus, optical imaging of [Cl-]i using MEQ and UV-laser-scanning confocal microscopy provides investigators with a new method to study GABAA pharmacology in neighboring neurons and perhaps even in the soma versus dendrites simultaneously, within living brain slices.

Glycine receptors from long-sleep and short-sleep mice: genetic differences in drug sensitivity

The long-sleep (LS) and short-sleep (SS) mice were selected for differences in sensitivity to ethanol but also differ in response to propofol and some neurosteroids. To determine the role of strychnine-sensitive glycine receptors in genetic differences between these mice, effects of propofol, ethanol and pregnenolone sulfate on glycine responses were compared in Xenopus oocytes expressing mRNA extracted from spinal cord of LS and SS mice. The two lines of mice did not differ in sensitivity to glycine, ethanol or pregnenolone sulfate. However, receptors expressed from LS mRNA were more sensitive to the potentiation induced by propofol than those from SS. Binding of [3H]strychnine to spinal cord membranes demonstrated a similar affinity and density of receptors in LS and SS. These results suggest that glycine receptor function could account for differences in propofol sensitivity between LS and SS mice, but may not be responsible for the differences in behavioral sensitivity to ethanol or steroids previously reported.

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.