Ethanol effects on synaptic glutamate receptor function and on membrane lipid organization

The striatal adenosinergic modulation of ethanol-induced motor incoordination in rats: possible role of chloride flux

Previous studies from our laboratory have provided strong evidence that brain adenosine modulates acute ethanol (i.p.)-induced motor incoordination (MI) through receptor mediated mechanism(s). Recently, we have reported the involvement of the striatum in ethanol-induced MI as well as the striatal adenosinergic modulation of the ethanol-induced motor deficit. The present study was thus designed to further characterize the modulatory effect of striatal adenosine on ethanol-induced MI and to look for its functional correlation with chloride flux within the rat striatum. Intrastriatal microinfusion of adenosine A1 receptor agonist N6-cyclohexyladenosine (CHA) and antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), significantly accentuated and attenuated, respectively, the motor incoordinating effect of ethanol while having no effect on the normal motor coordination in saline-treated control animals. These data confirmed the role of striatal adenosine in ethanol-induced MI. The selectivity of interactions between adenosine A1 agonist and antagonist and ethanol was further confirmed by the study in which neither intrastriatal CHA nor DPCPX significantly altered the MI induced by sodium pentobarbital. Previously, we have shown that intrastriatal Ro15-4513 not only significantly attenuated ethanol-induced MI but also blocked its accentuation by intrastriatal CHA. It is well known that Ro15-4513 antagonizes many, but not all, CNS effects of ethanol by blocking the ethanol potentiation of GABA-stimulated uptake of chloride. Therefore, experiments using striatal microsac preparations were carried out to investigate the possible modulation of chloride conductance by CHA and its relationship to ethanol. High concentrations of CHA (10 and 100 nM) increased the total chloride uptake by the striatal microsacs. Corresponding to the ethanol-adenosine interaction observed behaviorally, a much lower concentration (1 nM) of CHA, being ineffective itself, significantly enhanced the stimulatory action of ethanol on chloride uptake. This effect was blocked by either Ro15-4513 (100 nM) or DPCPX (10 nM). The modulatory effect of GABA and/or ethanol on chloride influx was also evaluated, and the results supported the appropriateness to use striatal microsac preparations in the present study. Overall, the data suggested a functional interaction between ethanol and striatal adenosine and further supported the hypothesis that striatal adenosine might, in part, modulate ethanol-induced MI through its effect on chloride conductance through chloride channels coupled to GABA-benzodiazepine receptor complex.

The membrane disordering effect of ethanol on neural crest cells in vitro and the protective role of GM1 ganglioside

The teratogenic effect of ethanol appears to be related to excessive cell death in selected cell populations including craniofacial neural crest. Because there is a large body of evidence suggesting that a primary site of action of ethanol is at the membrane level, the current study was designed to examine and attempt to ameliorate ethanol-induced neural crest cell membrane changes that proceed cell death. To this end, neural crest cells were grown as primary cultures from mouse cranial neural tube be explants. In these cultured cells, the relationships between changes in membrane lipid lateral mobility (a measure of membrane fluidity) as determined using the technique of fluorescence recovery after photobleaching (FRAP), ethanol-induced cell death, and the protective role of GM1 ganglioside were examined. A dose-response study showed that treatment with 50, 100, 150, or 200 mM ethanol respectively, for 24 h was positively correlated with membrane lipid lateral mobility and negatively correlated with cell viability. Pre- or co-treatment of the cells with GM1 ganglioside diminished the ethanol-induced increases in membrane fluidity and decreases in cell viability. The results of this study suggest that change in membrane fluidity can account, in part, for ethanol-induced neural crest cell death and that the protection conferred by GM1 ganglioside may result from membrane stabilization and subsequent preservation of the biophysical properties and biological function of the ethanol-exposed cell membranes.

Correlation between [125I]calmodulin binding and lipid fluidity in synaptic plasma membranes: effects of ethanol and other short-chain alcohols

Ethanol inhibits [125I]calmodulin binding to synaptic plasma membranes from rat brain, and this inhibition is correlated in a concentration-dependent manner with the increase of membrane fluidity, as determined by diphenylhexatriene fluorescence polarization. Moreover, several short-chain alcohols that increase membrane fluidity are also effective inhibitors of [125I]calmodulin binding. These data support the notion that ethanol inhibits calmodulin binding by increasing lipid fluidity of the synaptic membranes.

Role of gangliosides in behavioral and biochemical actions of alcohol: cell membrane structure and function

Alcohol exerts its pharmacological effects in adult brain by altering the physicochemical properties of cellular plasma membranes. Although alcohol does induce changes in membrane lipid composition, studies to relate these alterations to the development of behavioral tolerance to alcohol and the withdrawal effects have been unsuccessful. Actions of alcohol on developing brain are even more complex. Some of the reported effects include inhibition of embryogenesis, cell migration, and differentiation, including synaptogenesis. Gangliosides have neuroprotective action against a variety of neural insults (e.g., mechanical injury, drug toxicity, or hypoxic insult). This review addresses the role and significance of gangliosides in the CNS pathophysiology of alcohol exposure, as well as the effect of changes in endogenous gangliosides on membrane structure and function. We also describe the role of exogenous gangliosides in prevention of alcohol (acute and/or chronic)-induced CNS (prenatal and postnatal) neurotoxicity through their action on cellular plasma membranes. We propose that ganglioside's neuroprotective effects against alcohol neurotoxicity involve protection and restoration of plasma membrane structure (proteins and lipids) and thereby its function (ionic homeostasis, neurotransmitter receptor-mediated signal transduction). Thus gangliosides may have potential therapeutic use in treatment of alcohol-related problems.

Molecular interactions of ethanol with GABAergic system and potential of RO15-4513 as an ethanol antagonist

The behavioral and biochemical effects of ethanol in man and animals have been investigated for a long time. A role of catecholamines in the central stimulatory action and during withdrawal has been envisaged, but more recent observations have revealed the involvement of inhibitory synaptic transmitter, GABA, in the actions of ethanol. Ethanol-induced motor incoordination, hypnosedation, antianxiety, and anticonvulsant actions are reported to be GABA-mediated. Involvement of the GABA system has been implicated in ethanol withdrawal-induced seizures in animals. More direct evidences using Cl- influx studies in synaptoneurosomes and spinal neuronal culture studies confirm such a mode of action of ethanol, probably influencing the chloride channel modulation at the GABA-benzodiazepine receptor ionophore complex. RO15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-Oxo-4H-imidazo [1,5-alpha], [1,4] benzodiazepine-3-carboxylate), a novel imidazobenzodiazepine, an analogue of the classical benzodiazepine antagonist is reported to possess alcohol antagonistic properties. RO15-4513 reverses both the behavioral and biochemical effects of ethanol, including the action of GABA-induced Cl- fluxes. But its potential clinical application may be restricted due to its inverse agonistic property. The present review focuses on the GABA-linked behavioral and biochemical actions of ethanol and discusses the potential of RO15-4513 as an alcohol antagonist.

Ethanol-induced changes in neuronal membrane order. An NMR study

The effects of ethanol-d6 on the lipid matrix of rat brain neuronal membranes were investigated by delayed Fourier transform 1H-NMR techniques. At 24 degrees C, neither 0.1 nor 0.2% (v/v) ethanol-d6 measurably affected the methylene resonance intensity. However, 0.4 and 1.0% ethanol-d6 increased resonance intensity, 35 and 51%, respectively. With increasing temperature, a decrease in resonance intensity for 0.1% ethanol-d6 was observed reaching a maximum of 20% at 42 degrees C. Furthermore, increasing temperature attenuated the increases in resonance intensity seen with 0.4 and 1.0% ethanol-d6. At 24 degrees C, no concentration of ethanol-d6 had a significant effect on the choline methyl resonance. However, with increasing temperature both 0.1 and 0.2% ethanol-d6 decreased this resonance's intensity. The intensity of the terminal methyl resonance was increased in a dose related fashion by ethanol-d6, reaching a maximum of +41% at 1.0% (24 degrees C). Increasing temperature attenuated this effect, but no concentration of ethanol-d6 significantly decreased resonance intensity. The increases and decreases in resonance intensity induced by ethanol-d6 are interpreted in terms of a decrease and an increase in membrane order, respectively. It is proposed that ethanol-d6 exerts two effects on neuronal membranes, an ordering effect on the membrane surface and a disordering effect in the membrane interior. A higher enthalpy of ethanol binding to the surface as compared to the interior of the membrane leads to an attenuation of the ethanol disordering effect with increasing temperature.

Ethanol stimulates gamma-aminobutyric acid receptor-mediated chloride transport in rat brain synaptoneurosomes

The effects of ethanol on Cl- uptake were studied using a cell-free subcellular preparation from brain that contains a gamma-aminobutyric acid (GABA)/barbiturate receptor-sensitive Cl- transport system. In isolated vesicles prepared from rat cerebral cortex, ethanol, at concentrations that are present during acute intoxication (20-50 mM), stimulated 36Cl- uptake in a concentration-dependent and biphasic manner. The ethanol-stimulated uptake of 36Cl- was markedly inhibited by the GABA antagonists picrotoxin and bicuculline but not by a variety of other neurotransmitter receptor antagonists. The effects of ethanol in stimulating 36Cl- uptake in isolated brain vesicles were qualitatively and quantitatively similar to that of pentobarbital. Ethanol also markedly potentiated both muscimol- and pentobarbital-stimulated 36Cl- uptake at concentrations below those that directly stimulate 36Cl- uptake. Under our incubation conditions, ethanol did not release GABA, suggesting that it interacts with the postsynaptic GABA/barbiturate receptor complex. The ability of ethanol to stimulate GABA/barbiturate receptor-mediated Cl- transport may explain many of its pharmacological properties and provides a mechanism for the common psychopharmacological actions of ethanol, barbiturates, and benzodiazepines.

Membrane fluidity and alcohol dependence

Rats were chronically intoxicated with alcohol by exposing them to increasing concentrations of ethanol vapor over a 4-week period. They were tested for alcohol consumption in a free choice situation of water and 10% (v/v) alcohol. On the basis of their intakes they were divided into alcohol-dependent and nondependent groups. Synaptosome membrane fluidity evaluated by fluorescence polarization was compared between the two groups and against nonintoxicated controls. The intoxicated animals had a lower membrane fluidity than controls, mainly because of a highly significant increase of rigidity in the alcohol-dependent group. Furthermore, membrane fluidity was found to be correlated with the degree of behavioral dependence (i.e., alcohol intake during the free choice period).

Maternal ethanol consumption: binding of L-glutamate to synaptic membranes from whole brain, cortices, and cerebella of offspring

We examined the influence of chronic maternal ethanol consumption on the Na+- and Ca2+-independent binding of L-glutamate to synaptic plasma membranes from whole brain as well as from cortices and cerebella of developing offspring. The maximum specific binding (Bmax) of L-glutamate to the Na+- and Ca2+-independent binding sites in synaptic plasma membranes of brain peaked at 17 days of age in the offspring of both control and ethanol-fed rats, although at that age there were significantly fewer binding sites in the brains of the offspring of ethanol-fed rats. The regional localization of this deficit is not now known. However, it appears that one major glutamatergic region (the cortex) does not reflect the transient deficiency of L-glutamate sites in brain. In fact, the concentration of L-glutamate binding sites in cortical synaptic plasma membranes was significantly increased in the 20-day-old offspring of ethanol-fed rats. In contrast to the cortex, binding to cerebellar synaptic plasma membranes was comparable in 20-day-old offspring of control and ethanol-fed rats. Despite transient alterations in the concentrations of L-glutamate binding sites in brain and synaptic plasma membranes, the affinity of the sites for L-glutamate (Kd) was consistently normal in the 14- to 26-day-old offspring of ethanol-fed rats.

Ethanol and membrane lipids

Although ethanol is known to exert its primary mode of action on the central nervous system, the exact molecular interaction underlying the behavioral and physiological manifestations of alcohol intoxication has not been elucidated. Chronic ethanol administration results in changes in organ functions. These changes are reflective of the adaptive mechanisms in response to the acute effects of ethanol. Biophysical studies have shown that ethanol in vitro disorders the membrane and perturbs the fine structural arrangement of the membrane lipids. In the chronic state, these membranes develop resistance to the disordering effects. Tolerance development is also accompanied by biochemical changes. Although ethanol-induced changes in membrane lipids have been implicated in both biophysical and biochemical studies, measurements of membrane lipids, such as cholesterol content, fatty acid unsaturation, phospholipid distribution, and ganglioside profiles, have not produced conclusive evidence that any of these parameters are directly involved in the action of ethanol. On the other hand, there is increasing evidence indicating that although ethanol in vitro produces a membrane-fluidizing effect, the chronic response to this effect is not to change the membrane bulk lipid composition. Instead, changes in membrane lipids may pertain to small metabolically active pools located in certain subcellular fractions. Most likely, these lipids are involved in important membrane functions. For example, the increase in PS in brain plasma membranes may provide an explanation for the adaptive increase in synaptic membrane ion transport activity, especially (Na,K)-ATPase. There is also evidence that the lipid pool involved in the deacylation-reacylation mechanism (i.e., PI and PC with 20:4 groups) is altered after ethanol administration. An increase in metabolic turnover of these phospholipid pools may have important implications for the membrane functional changes. Obviously, there are other lipid-metabolizing enzyme systems that may exert similar effects but have not yet been investigated in detail. From the results of these studies, it is concluded that the multiple actions of ethanol are associated with changes in enzymic systems important in the functional expression of the membranes.

Alcohol effects on synaptic membrane calcium ion fluxes

The effects of ethanol on Na+-dependent CA2+ fluxes have been examined in resealed synaptic membrane vesicles assayed at 3 different temperatures. Sodium chloride-loaded vesicles were preincubated with various concentrations of ethanol for 120 sec prior to being diluted into a 45CaCl2-containing medium in the presence or absence of an outward-directed Na+ gradient. The effect of ethanol on Na+-dependent Ca2+ transport measured at 23 degrees C was biphasic. However, when the assay was conducted either at 16 degrees C or at 35 degrees C, all ethanol concentrations tested (10-300 mM) produced only inhibition of Ca2+ influx. The role of membrane fluidization in the ethanol-induced inhibition was explored by determining the effects of incorporating various fatty acids into the membranes. Membrane fluidizing agents such as cis-vaccenic acid stimulated Ca2+ influx whereas trans-vaccenic and saturated fatty acids had little effect. The fluidizing effect of incorporating cis-vaccenic acid into the membranes was confirmed with electron paramagnetic resonance (EPR) spectroscopy. The data obtained from these studies suggest that the inhibition of Ca2+ fluxes produced by alcohol and local anesthetics is not the result of a general increase in bulk phase synaptic membrane fluidity.