Acute tolerance to ethanol inhibition of NMDA-mediated EPSPs in the CA1 region of the rat hippocampus

Synaptic Mechanisms of Ethanol Tolerance and Neuroplasticity: Insights from Invertebrate Models

Alcohol tolerance is a neuroadaptive response that leads to a reduction in the effects of alcohol caused by previous exposure. Tolerance plays a critical role in the development of alcohol use disorder (AUD) because it leads to the escalation of drinking and dependence. Understanding the molecular mechanisms underlying alcohol tolerance is therefore important for the development of effective therapeutics and for understanding addiction in general. This review explores the molecular basis of alcohol tolerance in invertebrate models, Drosophila and C. elegans, focusing on synaptic transmission. Both organisms exhibit biphasic responses to ethanol and develop tolerance similar to that of mammals. Furthermore, the availability of several genetic tools makes them a great candidate to study the molecular basis of ethanol response. Studies in invertebrate models show that tolerance involves conserved changes in the neurotransmitter systems, ion channels, and synaptic proteins. These neuroadaptive changes lead to a change in neuronal excitability, most likely to compensate for the enhanced inhibition by ethanol.

Chronic alcohol disrupts hypothalamic responses to stress by modifying CRF and NMDA receptor function

The chronic inability of alcoholics to effectively cope with relapse-inducing stressors has been linked to dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis and corticotropin-releasing factor (CRF) signaling. However, the cellular mechanisms responsible for this dysregulation are yet to be identified. After exposure of male Sprague Dawley rats to chronic intermittent ethanol (CIE; 5-6 g/kg orally for 35 doses over 50 days) or water, followed by 40-60 days of protracted withdrawal, we investigated CIE effects on glutamatergic synaptic transmission, stress-induced plasticity, CRF- and ethanol-induced NMDAR inhibition using electrophysiological recordings in parvocellular neurosecretory cells (PNCs) of the paraventricular nucleus. We also assessed CIE effects on hypothalamic mRNA expression of CRF-related genes using real-time polymerase chain reaction, and on HPA axis function by measuring stress-induced increases in plasma adrenocorticotropic hormone, corticosterone, and self-grooming. In control rats, ethanol-mediated inhibition of NMDARs was prevented by CRF1 receptor (CRFR1) blockade with antalarmin, while CRF/CRFR1-mediated NMDAR blockade was prevented by intracellularly-applied inhibitor of phosphatases PP1/PP2A, okadaic acid, but not the selective striatal-enriched tyrosine protein phosphatase inhibitor, TC-2153. CIE exposure increased GluN2B subunit-dependent NMDAR function of PNCs. This was associated with the loss of both ethanol- and CRF-mediated NMDAR inhibition, and loss of stress-induced short-term potentiation of glutamatergic synaptic inputs, which could be reversed by intracellular blockade of NMDARs with MK801. CIE exposure also blunted the hormonal and self-grooming behavioral responses to repeated restraint stress. These findings suggest a cellular mechanism whereby chronic alcohol dysregulates the hormonal and behavioral responses to repetitive stressors by increasing NMDAR function and decreasing CRFR1 function.

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".

Ethanol neurotoxicity in the developing cerebellum: underlying mechanisms and implications

Ethanol is the main constituent of alcoholic beverages that exerts toxicity to neuronal development. Ethanol affects synaptogenesis and prevents proper brain development. In humans, synaptogenesis takes place during the third trimester of pregnancy, and in rodents this period corresponds to the initial few weeks of postnatal development. In this period neuronal maturation and differentiation begin and neuronal cells start migrating to their ultimate destinations. Although the neuronal development of all areas of the brain is affected, the cerebellum and cerebellar neurons are more susceptible to the damaging effects of ethanol. Ethanol’s harmful effects include neuronal cell death, impaired differentiation, reduction of neuronal numbers, and weakening of neuronal plasticity. Neuronal development requires many hormones and growth factors such as retinoic acid, nerve growth factors, and cytokines. These factors regulate development and differentiation of neurons by acting through various receptors and their signaling pathways. Ethanol exposure during development impairs neuronal signaling mechanisms mediated by the N-methyl-d-aspartate (NMDA) receptors, the retinoic acid receptors, and by growth factors such as brain-derived neurotrophic factor (BDNF), insulin-like growth factor 1 (IGF-I), and basic fibroblast growth factor (bFGF). In combination, these ethanol effects disrupt cellular homeostasis, reduce the survival and migration of neurons, and lead to various developmental defects in the brain. Here we review the signaling mechanisms that are required for proper neuronal development, and how these processes are impaired by ethanol resulting in harmful consequences to brain development.

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.

Prion protein is a key determinant of alcohol sensitivity through the modulation of N-methyl-D-aspartate receptor (NMDAR) activity

The prion protein (PrP) is absolutely required for the development of prion diseases; nevertheless, its physiological functions in the central nervous system remain elusive. Using a combination of behavioral, electrophysiological and biochemical approaches in transgenic mouse models, we provide strong evidence for a crucial role of PrP in alcohol sensitivity. Indeed, PrP knock out (PrP^−/−) mice presented a greater sensitivity to the sedative effects of EtOH compared to wild-type (wt) control mice. Conversely, compared to wt mice, those over-expressing mouse, human or hamster PrP genes presented a relative insensitivity to ethanol-induced sedation. An acute tolerance (i.e. reversion) to ethanol inhibition of N-methyl-D-aspartate (NMDA) receptor-mediated excitatory post-synaptic potentials in hippocampal slices developed slower in PrP^−/− mice than in wt mice. We show that PrP is required to induce acute tolerance to ethanol by activating a Src-protein tyrosine kinase-dependent intracellular signaling pathway. In an attempt to decipher the molecular mechanisms underlying PrP-dependent ethanol effect, we looked for changes in lipid raft features in hippocampus of ethanol-treated wt mice compared to PrP^−/− mice. Ethanol induced rapid and transient changes of buoyancy of lipid raft-associated proteins in hippocampus of wt but not PrP^−/− mice suggesting a possible mechanistic link for PrP-dependent signal transduction. Together, our results reveal a hitherto unknown physiological role of PrP on the regulation of NMDAR activity and highlight its crucial role in synaptic functions.

Effects of ethanol on GABA(A) receptors in GABAergic and glutamatergic presynaptic nerve terminals

Ethanol (EtOH) has a number of behavioral effects, including intoxication, amnesia, and/or sedation, that are thought to relate to the activation of GABA(A) receptors. However, GABA(A) receptors at different cellular locations have different sensitivities to EtOH. The present study used the "synaptic bouton" preparation where we could stimulate nerve endings on mechanically dissociated single rat hippocampal CA1 and CA3 pyramidal neurons and investigate the effects of EtOH on presynaptic and postsynaptic GABA(A) receptors. Low concentrations of EtOH (10 mM) had no effect on postsynaptic GABA(A) and glutamate receptors or voltage-dependent Na(+) and Ca(2+) channels. Higher concentrations (≥100 mM) could significantly inhibit these current responses. EtOH at 10 mM had no direct effect on inhibitory postsynaptic currents (IPSCs) and excitatory postsynaptic currents (EPSCs) evoked by focal stimulation of single boutons [evoked IPSCs (eIPSCs) and evoked EPSCs (eEPSCs)]. However, coapplication of 10 mM EtOH with muscimol decreased the amplitude of eIPSCs and eEPSCs and increased their paired-pulse ratio. The effects on eEPSCs were reversed by bicuculline. Coapplication of muscimol and EtOH significantly increased the frequency of spontaneous IPSCs and EPSCs. The EtOH effects on the postsynaptic responses and eEPSCs were similar in neurons from neonatal and mature rats. These results revealed that low concentrations of EtOH can potentiate the activation of presynaptic GABA(A) receptors to inhibit evoked GABA and glutamate release. These results indicate a high sensitivity of presynaptic GABA(A) receptor to EtOH, which needs to be accounted for when considering the cellular mechanisms of EtOH's physiological responses.

Functional adaptation of the N-methyl-D-aspartate receptor to inhibition by ethanol is modulated by striatal-enriched protein tyrosine phosphatase and p38 mitogen-activated protein kinase

The hippocampal N-methyl-D-aspartate receptor (NMDAR) activity plays important roles in cognition and is a major substrate for ethanol-induced memory dysfunction. This receptor is a glutamate-gated ion channel, which is composed of NR1 and NR2 subunits in various brain areas. Although homomeric NR1 subunits form an active ion channel that conducts Na⁺ and Ca²⁺ currents, the incorporation of NR2 subunits allows this channel to be modulated by the Src family of kinases, phosphatases, and by simple molecules such as ethanol. We have found that short-term ethanol application inhibits the NMDAR activity via striatal enriched protein tyrosine phosphatase (STEP)-regulated mechanisms. The genetic deletion of the active form of STEP, STEP61, leads to marked attenuation of ethanol inhibition of NMDAR currents. In addition, STEP61 negatively regulates Fyn and p38 mitogen-activated protein kinase (MAPK), and these proteins are members of the NMDAR super molecular complex. Here we demonstrate, using whole-cell electrophysiological recording, Western blot analysis, and pharmacological manipulations, that neurons exposed to a 3-h, 45 mM ethanol treatment develop an adaptive attenuation of short-term ethanol inhibition of NMDAR currents in brain slices. Our results suggest that this adaptation of NMDAR responses is associated with a partial inactivation of STEP61, an activation of p38 MAPK, and a requirement for NR2B activity. Together, these data indicate that altered STEP61 and p38 MAPK signaling contribute to the modulation of ethanol inhibition of NMDARs in brain neurons.

Correlated changes in NMDA receptor phosphorylation, functional activity, and sedation by chronic ethanol consumption

Alcohol abuse leads to tolerance, dependence, and memory impairments that involve excitatory glutamatergic NMDA synaptic transmission. The NMDA receptor (NMDAR) is known to undergo activity-dependent adaptive functional changes. Since we observed that acute ethanol inhibition of the NMDAR was regulated by protein tyrosine phosphorylation, we investigated the role of protein tyrosine kinases and phosphatases on the NMDAR functions by chronic ethanol treatment. We carried out whole-cell recording, western blotting, and behavioral righting reflex measurements to assess the impact of chronic ethanol treatment on NMDAR function. Our results indicated that these receptors became resistant to the acute ethanol inhibition following chronic ethanol consumption. This resistance occurred without an increase in the NMDAR subunit expression but was associated with decreases in the levels of phospho-Y-1472 NR2B, increases in the levels of STEP33, increases in phospho-p38 mitogen-activated protein kinase (pp38 MAPK), and acquisition of tolerance to the sedative effects of ethanol. These data suggested that altered protein tyrosine phosphorylation of the NMDAR subunits significantly contributes to functional changes of this receptor by chronic ethanol ingestion. Therefore, preservation of the integrity of tyrosine phosphorylation mechanisms of the NMDAR may be important in controlling the progression of alcohol tolerance and dependence.

Ethanol Alters Trafficking and Functional N-Methyl-D-aspartate Receptor NR2 Subunit Ratio via H-Ras

The N-methyl-D-aspartate receptor (NMDAR) plays a critical role in synaptic plasticity and is one of the main targets for alcohol (ethanol) in the brain. Trafficking of the NMDAR is emerging as a key regulatory mechanism that underlies channel activity and synaptic plasticity. Here we show that exposure of hippocampal neurons to ethanol increases the internalization of the NR2A but not NR2B subunit of the NMDAR via the endocytic pathway. We further observed that ethanol exposure results in NR2A endocytosis through the activation of H-Ras and the inhibition of the tyrosine kinase Src. Importantly, ethanol treatment alters functional subunit composition from NR2A/NR2B- to mainly NR2B-containing NMDARs. Our results suggest that addictive drugs such as ethanol alter NMDAR trafficking and subunit composition. This may be an important mechanism by which ethanol exerts its effects on NMDARs to produce alcohol-induced aberrant plasticity.

Acute tolerance to ethanol inhibition of NMDA-induced responses in rat rostral ventrolateral medulla neurons

The present study was performed to examine the effects of acute ethanol exposure on N-methyl-D-aspartate (NMDA)-induced responses and the development of acute tolerance in rat rostral ventrolateral medulla (RVLM) in vivo and in vitro. Repeated microinjections of NMDA (0.14 nmol) into the RVLM every 30 min caused reproducible increases in mean arterial pressure in urethane-anesthetized rats weighing 325-350 g. Intravenous injections of ethanol (0.16 or 0.32 g, 1 ml) inhibited NMDA-induced pressor effects in a blood-concentration-dependent and reversible manner. The inhibitory effect of ethanol was reduced over time during continuous infusion of ethanol or on the second injection 3.5 h after prior injection of a higher dose of ethanol (0.32 g). A high dose of ethanol (0.32 g) had no significant effects on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, gamma-aminobutyric acid and glycine-induced changes in blood pressure. In vitro studies showed that ethanol (10- 100 mM) dose-dependently inhibited inward currents elicited by pressure ejection of NMDA (10 mM) in RVLM neurons of neonatal brainstem slice preparations. When the superfusion time of ethanol (100 mM) was increased to 50 min, its inhibitory effect decreased gradually after 30-40 min in 60% of RVLM neurons examined. These data suggested that ethanol inhibition and subsequent tolerance development is associated with changed sensitivity to NMDA in the RVLM, which may play important roles in the ethanol regulation of cardiovascular function.

Inhibition by ethanol of NMDA-induced responses and acute tolerance to the inhibition in rat sympathetic preganglionic neurons in vitro and in vivo

N-methyl-d-aspartate (NMDA) receptors have been demonstrated to be a pivotal target for ethanol action. The present study examined the actions of acute ethanol exposure on NMDA-induced responses and the acute tolerance to ethanol actions in rat sympathetic preganglionic neurons (SPNs) in vitro and in vivo. NMDA (50 microM) applied every 5 min induced reproducible membrane depolarizations of SPNs in neonatal spinal cord slice preparations. Ethanol (50 - 100 mM) applied by superfusion for 15 min caused a sustained decrease in NMDA-induced depolarizations in a dose-dependent and reversible manner. When the superfusion time of ethanol (100 mm) was increased to 50 min, NMDA-induced depolarizations were attenuated initially but a gradual recovery was seen in approximately 40% of SPNs tested. Repeated injections of NMDA (2 nM) intrathecally at 30 min interval caused reproducible increases in mean arterial pressure (MAP) in urethane-anesthetized rats. Intravenous injections of ethanol (0.16 or 0.32 g, 1 ml) inhibited NMDA-induced pressor effects in a blood concentration-dependent manner. The inhibition by ethanol of NMDA-induced pressor effects was reduced over time during continuous infusion of ethanol or on the second injection 3.5 h after prior injection of a higher dose of ethanol. Ethanol, at concentrations significantly inhibited NMDA-induced responses, had no significant effects on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced responses. The study demonstrated the selective inhibition by ethanol of NMDA-induced responses and the development of acute tolerance to the inhibitory effects in SPNs both in vitro and in vivo. These effects may play important roles in the ethanol regulation of cardiovascular function.

Deletion of the fyn-kinase gene alters behavioral sensitivity to ethanol

BACKGROUND

An earlier study showed that deletion of the fyn-kinase gene enhanced sensitivity to ethanol's sedative hypnotic effects and suggested that this was associated with diminished fyn-kinase phosphorylation of NMDA receptors. The authors of that study speculated that this resulted in an inability of the null mutants to develop acute tolerance to ethanol, leading to the longer ethanol-induced sleep times. However, in vivo acute tolerance to ethanol was not examined directly.

METHODS

To address the role of fyn-kinase in mediating acute tolerance, as well as sensitivity to several other behavioral effects of ethanol, we studied an independently generated population of fyn null mutant and wild-type mice.

RESULTS

Homozygous mutants exhibited longer ethanol sleep times that could not be attributed to differences in initial sensitivity, and impaired acute tolerance to the motor incoordinating effects of ethanol as measured by using the stationary dowel, but not the rotarod. Fyn-kinase null mutants were more sensitive to the anxiolytic effects of ethanol when tested using the elevated plus maze, and males displayed a lower preference for ethanol in a two-bottle choice paradigm. Finally, mutant and wild-type mice did not differ in sensitivity to the hypothermic effects of ethanol. The genotypes also did not differ in blood-ethanol clearance, eliminating a metabolic explanation for these behavioral differences.

CONCLUSIONS

These results show that fyn-kinase modulates acute tolerance to ethanol and suggest a role for fyn in mediating ethanol's anxiolytic and reinforcing properties.

Scaffolding of Fyn kinase to the NMDA receptor determines brain region sensitivity to ethanol

Alcohol (ethanol) abuse is a major societal problem. Although ethanol is a structurally simple, diffusible molecule, its sites of action are surprisingly selective, and the molecular mechanisms underlying specificity in ethanol actions are not understood. The NMDA receptor channel is one of the main targets for ethanol in the brain. We report here that the brain region-specific compartmentalization of Fyn kinase determines NMDA receptor sensitivity to ethanol. We demonstrate that, in the hippocampus but not in the cerebral cortex, Fyn is targeted to the NR2B subunit of the NMDA receptor by the scaffolding protein RACK1. During acute exposure to ethanol, RACK1 is dissociated from the complex, thereby facilitating Fyn-mediated phosphorylation of NR2B, which enhances channel activity, counteracting the inhibitory actions of ethanol. In this way, the selective scaffolding can account for the ethanol-induced acute tolerance of NMDA receptor activity that is detected in the hippocampus but not in the cerebral cortex. The phosphorylation-dependent, region-specific activities of ethanol on the NMDA receptor provide a compelling molecular explanation that accounts for the selective activities of ethanol and may have important implications for elucidating pathways leading to alcohol addiction.

AMPA receptors on developing medial septum/diagonal band neurons are sensitive to early postnatal binge-like ethanol exposure

The impact of binge-like, early postnatal ethanol treatment on AMPA or kainate whole cell currents was examined in acutely isolated medial septum/diagonal band (MS/DB) neurons. AMPA (10 or 100 microM) current was inhibited by GYKI 52466, a selective AMPA receptor (AMPAR) antagonist, in all neurons isolated on postnatal day (PD) 5-8, PD 12-15 or PD 32-35. Cyclothiazide, a selective inhibitor of AMPAR desensitization, also effectively potentiated AMPA currents. This suggests that non-NMDA, ionotropic glutamate receptors on immature MS/DB neuron are predominantly AMPARs. Concentration-dependent kainate (10-1000 microM) application evoked nondesensitizing currents that exhibited an increase in the maximum response by the end of first postnatal month, consistent with developmental regulation of AMPAR function. Acute 3 s ethanol application (100 mM) consistently blunted AMPA- and kainate currents approximately 20-30% across age groups. Inhibition was sustained during continuous ethanol superfusion lasting 10-12 min without evidence of acute tolerance. Repeated oral intubation of rat pups with ethanol (5.25 g/kg/day on PD 4-9), which models third trimester human binge drinking, resulted in peak blood ethanol levels of approximately 350 mg/dl (measured 90 min after PD 6 dosing). AMPA or kainate currents were upregulated in neurons isolated on PD 32-35 by earlier ethanol intubation suggesting that binge-like intoxication augments developing AMPAR function. Despite this augmentation of AMPAR function, no significant changes were found in the sensitivity of AMPA currents to GYKI 52466, cyclothiazide or acute ethanol (100 mM) sensitivity or in the levels of GluR1/GluR2 subunit proteins from MS/DB tissue. These results indicate that non-NMDA ionotrophic glutamate receptors on immature MS/DB neurons, which are largely of the AMPAR subtype, are moderately sensitive to immediate inhibition by ethanol. Repeating this inhibition during early postnatal binge-like intoxication can augment normal development of AMPAR function.

Ethanol tachyphylaxis in spinal cord motorneurons: role of metabotropic glutamate receptors

1. Ethanol (EtOH) tachyphylaxis (acute tolerance), a time-dependent decrease in apparent potency, is known in vivo and in some neuronal preparations. The present studies characterize EtOH tachyphylaxis in spinal motorneurons and test the hypothesis that metabotropic glutamate receptors (mGluRs) play a role. 2. Patch clamp studies were carried out in motorneurons in rat spinal cord slices. Currents were evoked by pulses of glutamate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) or N-methyl-D-aspartic acid (NMDA). 3. In nine of 15 cells, ethanol depression of glutamate-evoked currents was time-dependent. EtOH depressed current area 36.9+/-3% at 8-10 min, but only 16.8+/-3% at 20 min. Mean reduction in depression was 20.1+/-1%, N=9. Tachyphylaxis was less prominent in currents evoked by AMPA or NMDA, appearing in two of 10 AMPA and three of 11 NMDA currents. 4. The mGluR agonist trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD) increased, the antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG) decreased the area of glutamate-evoked currents. ACPD also increased the area of NMDA- and AMPA-evoked currents. 5. ACPD increased the incidence of tachyphylaxis in glutamate-evoked currents to 100% (N=9); MCPG markedly reduced tachyphylaxis. ACPD also increased the incidence of tachyphylaxis in currents evoked by NMDA and AMPA to five of eight and four of seven neurons, respectively. 6. Block of G-protein pathways by intracellular GDP-beta-s abolished tachyphylaxis in glutamate-evoked currents (N=8); however, currents recovered only partially following EtOH washout. 7. Activation of mGluRs contributes to neuronal tachyphylaxis to EtOH in spinal cord motorneurons, probably via G-protein pathways.

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.

Altered effects of ethanol in NR2A(DeltaC/DeltaC) mice expressing C-terminally truncated NR2A subunit of NMDA receptor

Phosphorylation of C-termini of receptor subunits is thought to play a significant role in modulation of N-methyl-D-aspartic acid (NMDA) receptor function. To investigate whether the C-terminus of the NR2A subunit is involved in determining the sensitivity of NMDA receptors to ethanol we compared the effects of ethanol in vitro on NMDA-mediated field excitatory postsynaptic potentials (fEPSPs) in the CA1 and dentate gyrus (DG) of adult male NR2A(DeltaC/DeltaC) mice lacking the C-terminus of NR2A subunit and in their parental strain C57Bl/6. We also tested the in vivo effects of a hypnotic dose of ethanol in C57Bl/6 and NR2A(DeltaC/DeltaC) mice and their F2 offspring. Ifenprodil (10 microM) was used to distinguish between the NR2A and NR2B components of NMDA fEPSPs. Ethanol (100 mM) in the presence of ifenprodil inhibited the CA1 NR2A-mediated component of NMDA fEPSPs two times more in NR2A(DeltaC/DeltaC) than in C57Bl/6. Ethanol inhibition of the CA1 NR2B-mediated component was five to seven times lower in NR2A(DeltaC/DeltaC) than in C57Bl/6. In the DG ethanol had similar effects in the two strains. In vivo administration of ethanol (4 g/kg) induced sedation of similar duration in both strains of mice. A second administration of ethanol 7 days after the initial injection revealed an increased ethanol sensitivity of NR2A(DeltaC/DeltaC) and F2(DeltaC/DeltaC) mice including a shortened time to loss of righting reflex and an increased sleep time. The sensitization of NR2A(DeltaC/DeltaC) mice to alcohol was not accompanied by an altered ethanol sensitivity of NMDA fEPSPs recorded in vitro. Our data are consistent with the inhibitory action of ethanol on NMDA receptors being mediated by a site other than the intracellular C-terminus of the NR2A subunit. The altered sensitivities to ethanol of both NR2A- and NR2B-mediated responses in the CA1 of NR2A(DeltaC/DeltaC) imply that NR2A- and NR2B subunit-containing NMDA receptors may be linked by a common target of ethanol.

Genetic-environment analysis of sensitivity and acute tolerance to ethanol in mice

The purpose of this study was to characterize initial sensitivity (IS), acute functional tolerance (AFT), and rate of tolerance development to ethanol in lines of mice selected for aggression mice as well as to investigate the impact of isolate housing on these phenotypes. The results showed that for IS, there were no differences among treatment groups. For acute tolerance and rate of tolerance development, a Line x Sex x Housing interaction was present, with the response to housing being more pronounced in the low aggressive line than the high aggressive line, and the females being more affected than the males. Correlational analysis showed low to moderate associations between rate of tolerance development and IS, as well as between rate of tolerance and AFT. Housing condition significantly influenced female expression of ethanol phenotypes as compared to males. The line of the subject also influenced the magnitude of expression of these phenotypes. These findings suggest that environmental and genetic influences interact to influence acute tolerance and rate of tolerance development.

In vitro tolerance to inhibition by ethanol of N-methyl-D-aspartate-induced depolarization in locus coeruleus neurons of behaviorally ethanol-tolerant rats

Intracellular recordings were made in pontine slice preparations of the rat brain containing the locus coeruleus (LC). Ethanol at 100 mM, but not at 10 or 30 mM inhibited depolarizing responses to pressure-applied N-methyl-D-aspartate (NMDA) in LC neurons of ethanol-naive rats. Ethanol (100 mM) had a similar effect in LC neurons of ethanol-naive rats, of rats treated with ethanol for 14 days (3 g/kg daily, i.p.) and of rats treated with equicaloric amounts of saccharose (5 g/kg daily, i.p.). The blood concentration of ethanol was markedly decreased at 4 h, and was below the detection limit at 24 h after the last injection. Behavioral measurements in the open-field system demonstrated the development of tolerance in rats receiving ethanol for 14 days. Moreover, an anxiety-related reaction was shown to develop when the acute effect of the last ethanol injection vanished. Therefore, in subsequent in vitro experiments, ethanol (10 mM) was continuously present in the superfusion medium in order to mimic a steady blood concentration and to prevent a withdrawal-like situation. Under these conditions, ethanol (100 mM) still continued to inhibit the NMDA-induced depolarization in slices of untreated rats, but became ineffective in slices of ethanol-treated rats at 4 h after the last injection. By contrast, a supersensitivity to ethanol developed in brain slices at 24 h after the last ethanol injection. In conclusion, in vitro tolerance between systemically and locally applied ethanol at LC neurons could only be demonstrated when a low concentration of ethanol was added to the superfusion medium to simulate the blood concentration of this compound.

Evidence for the ability of hippocampal neurons to develop acute tolerance to ethanol in behaving rats

BACKGROUND

The cellular mechanisms underlying acute tolerance to alcohol are unclear. This study aimed to determine whether hippocampal neurons have the ability to develop acute tolerance to alcohol in behaving rats.

METHODS

Intrahippocampal microdialysis was performed in freely behaving rats, and the firing of single neurons in the dialysis area was recorded. The control microdialysis fluid, artificial cerebrospinal fluid (ACSF), was replaced with 1 M ethanol in ACSF for a 30 min period. One hour later, the ethanol perfusion was repeated. To test the functional integrity of the microdialysis probe in situ, each microdialysis session was completed with recording the effect of a 10-20 min perfusion of 500 microM N-methyl-D-aspartate (NMDA). The extracellular concentration profile of ethanol during intrahippocampal microdialysis with 1 M ethanol was estimated in a separate study in anesthetized rats. The ethanol content was measured in tissue slices surrounding the probe with gas chromatography (GC), and the generated data were analyzed with a mathematical model for microdialysis to estimate the concentration of ethanol at the recording site.

RESULTS

The predominant effect of the first intrahippocampal microdialysis with ethanol was a decrease in firing rate in both pyramidal cells and interneurons. In contrast, such firing rate decrease did not develop during the second ethanol perfusion. Subsequent NMDA perfusion still induced robust changes in the electrical activity of the neurons. The estimated extracellular ethanol concentration at the recording site was 45-70 mM.

CONCLUSION

This study revealed that hippocampal neurons have the ability to develop acute tolerance to a single exposure of clinically relevant concentrations of ethanol in behaving rats, without influences from the rest of the body.

Transgenic and gene "knockout" models in alcohol research

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Paula L. Hoffman and Takeshi Yagi. The presentations were (1) cAMP signaling in ethanol sensitivity and tolerance, by Boris Tabakoff; (2) Synaptic signaling pathways of Fyn-tyrosine kinase, by Takeshi Yagi; (3) Ethanol drinking and sensitization in dopaminergic and serotonergic receptor knockouts, by Tamara J. Phillips; (4) ICAM-1 is involved in early alcohol-induced liver injury in the mouse given enteral alcohol, by Hiroshi Kono; and (5) Strategies for targeted and regulated knockouts, by Robert O. Messing and Doo-Sup Choi.

Peptide signaling paths related to intoxication, memory and addiction

Abstract Many peptides bind to G protein-coupled receptors and activate intracellular signaling paths for adaptive cellular responses. The components of these paths can be affected by signals from other neurotransmitters to produce overall integrated results not easily predicted from customary a priori considerations. This intracellular cross-talk among signaling paths provides a "filter" through which long-term tonic signals affect short-term phasic signals as they progress toward the nucleus and induce long-term adaptation of gene expression which provide enduring attributes of acquired memories and addictions. Peptides of the PACAP family provide intracellular signaling that involves kinases, scaffolding interactions, Ca2 + mobilization, and gene expression to facilitate development of tolerance to alcohol and development of associative memories. The peptide-induced enhancement of NMDA receptor responses to extracellular glutamate also may increase behavioral sensitization to the low doses of alcohol that occur at the onset of each bout of drinking. Because many gene products participate in each signaling path, each behavioral response to alcohol is a polygenic process of many steps with no single gene product sufficient to interpret fully the adaptive response to alcohol. Different susceptibility of individuals to alcohol addiction may be a cumulative result of small differences among the many signaling components. Understanding this network of signals may help interpret future "magic bullets" proposed to treat addiction.

Sustained ethanol inhibition of native AMPA receptors on medial septum/diagonal band (MS/DB) neurons

The direct impact of ethanol on native, non-NMDA glutamate receptors was examined in acutely isolated MS/DB neurons from rat. The impact of ethanol functional tolerance and physical dependence on non-NMDA receptor function was also determined. Non-NMDA receptors were defined pharmacologically as predominantly the AMPA subtype, because both AMPA- or kainate-activated currents were blocked by GYKI 52466, a selective AMPA receptor antagonist. The relative magnitude of potentiation of AMPA-activated currents by 10 or 100 microM cyclothiazide was consistent with recombinant AMPA flop-subtype receptors. Finally, the selective kainate receptor agonist, SYM 8021, induced little current in MS/DB neurons. AMPA receptor currents when activated by kainate were sensitive to ethanol, showing inhibition of approximately 5 - 50% when 10 - 300 mM ethanol and kainate were briefly co-applied (3 s). Ethanol (100 mM) also inhibited both the initial transient peak and sustained currents activated by AMPA. Inhibition was sustained during continuous ethanol superfusions of 5 min, suggesting a lack of acute tolerance to ethanol-induced AMPA receptor blockade. Rapid application of 3 - 3000 microM kainate activated concentration-dependent currents in MS/DB neurons from Control and Ethanol Dependent animals that were not significantly different. Also, direct ethanol inhibition (300 mM) of kainate-activated currents was not reduced by ethanol dependence, suggesting a lack of functional tolerance. These results suggest that native AMPA receptors on MS/DB neurons are inhibited by pharmacologically-relevant concentrations of ethanol. However, these receptors, unlike NMDA receptors, do not undergo adaptation with sustained ethanol exposure sufficient to induce physical dependence. British Journal of Pharmacology (2000) 129, 87 - 94

Fyn tyrosine kinase reduces the ethanol inhibition of recombinant NR1/NR2A but not NR1/NR2B NMDA receptors expressed in HEK 293 cells

NMDA receptors are potentiated by phosphorylation in a subunit- and kinase-specific manner. Both native and recombinant NMDA receptors are inhibited by behaviorally relevant concentrations of ethanol. Whether the phosphorylation state of individual subunits modulates the ethanol sensitivity of these receptors is not known. In this study, the effects of Fyn tyrosine kinase on the ethanol sensitivity of specific recombinant NMDA receptors expressed in HEK 293 cells were investigated. Whole-cell mode patch clamp and ratiometric calcium imaging demonstrated that the degree of ethanol inhibition of NR1/NR2B receptors was unaffected by Fyn tyrosine kinase. In contrast, the inhibition of NR1/NR2A receptors by ethanol (100 mM) was significantly reduced under conditions of enhanced Fyn-mediated tyrosine phosphorylation of the NR2A subunit. This effect was not observed at lower concentrations of ethanol (< or = 50 mM). These results suggest that tyrosine phosphorylation of specific NMDA receptors by Fyn tyrosine kinase may regulate the sensitivity of these receptors to the sedative/hypnotic concentrations of ethanol.

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

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

Ethanol tolerance and synaptic plasticity

Current concepts of the mechanisms underlying many of the pharmacological effects of ethanol on the CNS involve disruption of ion channel function via the interaction of ethanol with specific hydrophobic sites on channel subunit proteins. Of particular clinical importance is the development of tolerance and dependence to ethanol, and it is likely that adaptive changes in synaptic function in response to ethanol's actions on ion channels play a role in this process. In this article, Judson Chandler, Adron Harris and Fulton Crews discuss potential mechanisms of ethanol-induced changes in synaptic function that might provide a cellular basis for ethanol tolerance and dependence. It is proposed that multiple mechanisms are involved that include both transcriptional and post-translational modifications in NMDA and GABAA receptors.

Time-dependent effects of acute ethanol administration on regional cerebral blood flow in the rat

The present study investigated the role of the postinjection interval in determining the functional consequences of acute ethanol administration in the CNS. Regional cerebral blood flow (RCBF) was determined by the [14C]iodoantipyrine method in 33 brain structures of ethanol-naive Sprague-Dawley rats. In the first experiment, changes in RCBF were assessed 5 and 15 min after a 0.8 g/kg (i.p.) dose of ethanol or water. Five minutes after treatment, rates of RCBF were increased in the motor cortex, agranular insular cortex, and the olfactory tubercle compared to water controls. No significant differences compared to control were found at the 15-min time point, despite the continued presence of ethanol in the blood. Experiment 2 tested whether blood ethanol level was the sole determinant of this response to ethanol by comparing animals with the same blood ethanol level at the 5- and 15-min time points. Greater rates of RCBF were found at 5 min postinjection compared to 15 min, in the motor cortex, agranular insular cortex, caudate/putamen, cerebellum, and the lateral septum. These data demonstrate that the rates of cerebral blood flow are increased in regionally discrete portions of the rat brain shortly after ethanol administration. Furthermore, blood ethanol level is not the exclusive factor governing this functional response.

Multiphasic consequences of the acute administration of ethanol on cerebral glucose metabolism in the rat

The present study investigated the role of the postinjection interval in determining the functional consequences of acute ethanol administration in the CNS. Local cerebral metabolic rates for glucose (LCMRglc) were determined by the 2[14C]deoxyglucose method in 48 brain structures of ethanol-naive Sprague-Dawley rats. Tracer was injected 1 or 45 min after a 0.8 g/kg intragastric dose of ethanol or water. At the early time point, LCMRglc was increased in a highly restricted portion of the basal ganglia that included the dorsal striatum, globus pallidus, and core of the nucleus accumbens, compared to water controls. No significant decreases were found at this early time point. At the later time point, by contrast, LCMRglc was decreased in a different set of brain structures. These sites were limbic in nature and included the infralimbic and anterior cingulate cortices, dentate gyrus, lateral septum, and the bed nucleus of the stria terminalis. These data indicate that there are multiple phases that can be detected during the time course of an acute dose of ethanol. They further demonstrate the involvement of different neural systems at the two time points. Increased activity in basal ganglia is consistent with stimulated motor activity, whereas diminished activity in limbic sites may correspond to changes in mood and motivation.

Brain imaging. Functional consequences of ethanol in the central nervous system

In recent years, sophisticated methods have been developed to view structure and function within the living brain. Functional imaging methods are used to visualize dynamic chemical processes that are linked to brain activity. Increased neural activity, for example, leads to greater glucose and oxygen consumption and greater regional rates of blood flow to meet elevated energy demands. Mapping these changes provides quantitative visual descriptions of localized changes in brain activity that result from behavioral or pharmacological manipulations. This chapter first describes several current methods and how they are used to study the effects of alcohol on brain function. In the second part, the effects of acute intoxication are discussed with emphasis on the complex nature of alcohol's effects in the central nervous system, which depend on dose, time since administration, and environmental context. In the final part, the functional consequences of long-term exposure to alcohol as well as diseases associated with chronic alcoholism are reviewed.

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.

A study of the duration of acute tolerance induced with hexobarbital in male rats

Male rats were infused i.v. with hexobarbital to obtain a burst suppression of 1 s or more in the EEG (SS). At SS the rats were killed and the concentration of hexobarbital was determined by HPLC in three parts of the brain. Acute tolerance (induced by a 1-h exposure at the SS level) was recorded as an approximately 20% increase in brain concentrations of hexobarbital at the last SS during the exposure when compared with concentrations recorded at the first SS in the controls. Increased brain concentrations (approximately 8%) at SS were recorded 24 h after induction of acute tolerance. After 48 h the increase was uncertain. Thus, acute tolerance to hexobarbital could have cumulative properties if new exposures are imposed after 24 h.

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.

Halogenated aromatic hydrocarbons suppress CA1 field excitatory postsynaptic potentials in rat hippocampal slices

Halogenated aromatic hydrocarbons (HAHs), such as polychlorinated biphenyls (PCBs) and dibenzo-p-dioxins (PCDDs), alter cognitive function and learning. The cellular basis of HAH-induced alteration of brain function is not well-understood. The hippocampus is a likely site of toxic action because of its well-known roles in learning and memory, as well as its propensity to accumulate environmental neurotoxicants. A hippocampal function that can be measured readily is evoked excitatory postsynaptic potentials (EPSPs), which are an index of excitatory synaptic function. In this study, effects of HAHs on EPSPs were characterized in hippocampal slices from adolescent to adult male Sprague-Dawley rats. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 1,2,3,4-TCDD were used because these HAHs are prototypical potent and weak aryl hydrocarbon (Ah) receptor agonists, respectively. 2,2',5,5'-Tetrachlorobiphenyl (TCB) was used as a prototypical ortho-substituted PCB, which acts through Ah receptor-independent pathways. For each hippocampal slice, peak amplitudes of EPSPs during a 15-min recording period (1 recording/min) were averaged and used as baseline (100%). Subsequent EPSPs were expressed as percentage of baseline. TCDD and 1,2,3,4-TCDD did not alter EPSPs in slices from the middle third of the hippocampus. However, in ventral slices, TCDD significantly decreased EPSPs, whereas 1,2,3,4-TCDD was inactive. TCB decreased EPSPs in both middle and ventral slices at half-maximal stimulation. An unexpected reversal of inhibition was observed within 30 min of continuous application of TCDD or TCB. In ventral slices, L-type calcium channel blocker nifedipine blocked inhibition of EPSPs induced by TCDD but not EPSPs inhibited by TCB. These results suggest that, while TCB-induced inhibition of EPSPs occurs through an unknown mechanism, TCDD-induced inhibition of EPSPs was mediated by L-type calcium channel activity in a congener-specific manner.

Fyn-kinase as a determinant of ethanol sensitivity: relation to NMDA-receptor function

Animals vary in their sensitivity to ethanol, a trait at least partly determined by genetic factors. In order to identify possible responsible genes, mice lacking Fyn, a non-receptor type tyrosine kinase, were investigated. These mice were hypersensitive to the hypnotic effect of ethanol. The administration of ethanol enhanced tyrosine phosphorylation of the N-methyl-D-aspartate receptor (NMDAR) in the hippocampus of control mice but not in Fyn-deficient mice. An acute tolerance to ethanol inhibition of NMDAR-mediated excitatory postsynaptic potentials in hippocampal slices developed in control mice but not in Fyn-deficient mice. These results indicate that Fyn affects behavioral, biochemical, and physiological responses to ethanol.

Single-channel and whole-cell analysis of ethanol inhibition of NMDA-activated currents in cultured mouse cortical and hippocampal neurons

The effects of 0.1 to 500 mM ethanol on NMDA-activated currents were studied in primary cultures of mouse cortical and hippocampal neurons. In whole-cell recordings the IC50S for inhibition of NMDA-activated currents by ethanol were 129 mM +/- 20 mM in hippocampal neurons and 126 +/- 18 mM in cortical neurons. In single-channel recordings from excised outside-out patches of cortical neurons, ethanol inhibited total charge per minute with an IC50 of 174 +/- 23 mM, which was not significantly different from the IC50S for inhibition of whole-cell current. The reduction in mean open channel lifetime by ethanol was fit by the logistic equation with an apparent IC50 of 340 +/- 28 mM. Analysis of single-channel data indicated that ethanol inhibition of NMDA currents did not involve substantial changes in fast closed state kinetics, changes in open channel conductance, or block of the open channel. At the whole-cell IC50 of ethanol, mean open channel lifetime would decrease by 28% and frequency of opening would decline by 31% to account for the reduction in current. Single-channel data were consistent with ethanol being an allosteric modulator of gating which reduces agonist efficacy.

Chronic alcohol reduces calcium signaling elicited by glutamate receptor stimulation in developing cerebellar neurons

The effect of chronic alcohol (33 mM ethanol) on Ca2+ signals elicited by glutamate receptor agonists (quisqualate and NMDA) was examined in developing cerebellar Purkinje and granule neurons in culture. The neurons were exposed to alcohol during the second week in culture, the main period of morphological and physiological development. The Ca2+ signals were measured with fura-2 based microscopic video imaging. Chronic exposure to alcohol during development significantly reduced the peak amplitude of the Ca2+ signals to quisqualate (1 microM; Quis) in both the somatic and dendritic regions of the Purkinje neurons. The dendritic region was affected to a greater extent than the somatic region. Granule neurons also showed a reduced somatic Ca2+ signal to Quis (dendrites not measured) in the alcohol-treated cultures, indicating that the effect was not limited to Purkinje neurons. In addition to the effects on in the response to Quis, the peak amplitude of the Ca2+ signals to NMDA (100 microM) was reduced by chronic alcohol exposure during development in both the cultured Purkinje and granule neurons. Resting Ca2+ levels were not consistently affected by alcohol treatment in either neuronal type. These results indicate that Ca2+ signaling linked to glutamate receptor activation is an important target of alcohol in the developing nervous system and could be a contributing factor in the altered CNS function and development observed in animal models of fetal alcohol syndrome.

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

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

Alteration of [3H]MK-801 binding associated with the N-methyl-D-Aspartate receptor complex by acute ethanol in rat cortex and hippocampus in vitro

We investigated the effect of ethanol on specific binding of [3H]MK-801 to the intrachannel phencyclidine (PCP) receptor site, as an index of change in the functional response of the N-methyl-D-Aspartate (NMDA)-associated ion channel. Saturation binding experiments were performed on synaptic membrane homogenates from adult rat cortex and hippocampus. [3H]MK-801 binding assays were conducted under conditions of basal, 10 microM glutamate, or 10 microM glutamate + 30 microM D-serine, with and without 50 or 100 mM ethanol. Association experiments of [3H]MK-801 binding (5 nM) were conducted under conditions of 0 or 10 microM glutamate, with varying concentrations of glycine (0.01, 0.10, and 10 microM) with and without 100 mM ethanol. Ethanol (50 and 100 mM) significantly decreased the percentage of high-affinity (open-channel state) MK-801 receptors with a concomitant increase in percentage of low-affinity receptors, but did not change high- and low-affinity constants of the two binding states. An ethanol-induced increase in the closed-channel receptor density in basal and activated conditions was suggested by the saturation experiments. Association experiments further explained this finding, in that ethanol (100 mM) significantly decreased fast component (open-channel) [3H]MK-801 binding in conditions of glycine (0.01-10 microM) only and activated conditions of glutamate + glycine (0.01-0.10 microM). However, the observed fast and slow kinetic rate constants of [3h]MK-801 binding, as well as total specific binding (fast + slow components), were not altered.(ABSTRACT TRUNCATED AT 250 WORDS)