Inhibition of N-methyl-D-aspartate (NMDA)- and L-glutamate-induced noradrenaline and acetylcholine release in the rat brain by ethanol

Serotonin and beyond-a tribute to Manfred Göthert (1939-2019)

Manfred Göthert, who had served Naunyn-Schmiedeberg's Arch Pharmacol as Managing Editor from 1998 to 2005, deceased in June 2019. His scientific oeuvre encompasses more than 20 types of presynaptic receptors, mostly on serotoninergic and noradrenergic neurones. He was the first to identify presynaptic receptors for somatostatin and ACTH and described many presynaptic receptors, known from animal preparations, also in human tissue. In particular, he elucidated the pharmacology of presynaptic 5-HT receptors. A second field of interest included ligand-gated and voltage-dependent channels. The negative allosteric effect of anesthetics at peripheral nACh receptors is relevant for the peripheral clinical effects of these drugs and modified the Meyer-Overton hypothesis. The negative allosteric effect of ethanol at NMDA receptors in human brain tissue occurred at concentrations found in the range of clinical ethanol intoxication. Moreover, the inhibitory effect of gabapentinoids on P/Q Ca2+ channels and the subsequent decrease in AMPA-induced noradrenaline release may contribute to their clinical effect. Another ligand-gated ion channel, the 5-HT3 receptor, attracted the interest of Manfred Göthert from the whole animal via isolated preparations down to the cellular level. He contributed to that molecular study in which 5-HT3 receptor subtypes were disclosed. Finally, he found altered pharmacological properties of 5-HT receptor variants like the Arg219Leu 5-HT1A receptor (which was also shown to be associated with major depression) and the Phe124Cys 5-HT1B receptor (which may be related to sumatriptan-induced vasospasm). Manfred Göthert was a brilliant scientist and his papers have a major impact on today's pharmacology.

The Neurotransmitters Involved in Drosophila Alcohol-Induced Behaviors

Alcohol is a widely used and abused substance with numerous negative consequences for human health and safety. Historically, alcohol's widespread, non-specific neurobiological effects have made it a challenge to study in humans. Therefore, model organisms are a critical tool for unraveling the mechanisms of alcohol action and subsequent effects on behavior. Drosophila melanogaster is genetically tractable and displays a vast behavioral repertoire, making it a particularly good candidate for examining the neurobiology of alcohol responses. In addition to being experimentally amenable, Drosophila have high face and mechanistic validity: their alcohol-related behaviors are remarkably consistent with humans and other mammalian species, and they share numerous conserved neurotransmitters and signaling pathways. Flies have a long history in alcohol research, which has been enhanced in recent years by the development of tools that allow for manipulating individual Drosophila neurotransmitters. Through advancements such as the GAL4/UAS system and CRISPR/Cas9 mutagenesis, investigation of specific neurotransmitters in small subsets of neurons has become ever more achievable. In this review, we describe recent progress in understanding the contribution of seven neurotransmitters to fly behavior, focusing on their roles in alcohol response: dopamine, octopamine, tyramine, serotonin, glutamate, GABA, and acetylcholine. We chose these small-molecule neurotransmitters due to their conservation in mammals and their importance for behavior. While neurotransmitters like dopamine and octopamine have received significant research emphasis regarding their contributions to behavior, others, like glutamate, GABA, and acetylcholine, remain relatively unexplored. Here, we summarize recent genetic and behavioral findings concerning these seven neurotransmitters and their roles in the behavioral response to alcohol, highlighting the fitness of the fly as a model for human alcohol use.

Serotonin discovery and stepwise disclosure of 5-HT receptor complexity over four decades. Part II. Some contributions of Manfred Göthert

About 40% of the papers within the scientific oeuvre of Manfred Göthert (1939-2019) were dedicated to serotonin (5-hydroxytryptamine, 5-HT). He was not only the witness of the gradual definition of the fourteen 5-HT receptor subtypes but also was involved directly by identifying 5-HT_1B, 5-HT_1D and 5-HT₃ receptors. Moreover, he identified presynaptic 5-HT receptors on central and/or peripheral serotoninergic, noradrenergic and/or cholinergic neurones. Two inhibitory (5-HT_1B, 5-HT_1D) and two facilitatory (5-HT₃, 5-HT₄) receptors were found, the 5-HT_1B receptor representing a possible target for antidepressant drugs. Ten years earlier than electrophysiologists, he identified ligand-gated receptors like the 5-HT₃ and the nicotinic acetylcholine (nACh) receptor as targets of halothane. Simultaneously with, but independent of, other authors he found that ethanol allosterically inhibits N-methyl-D-aspartate (NMDA) receptors, which are affected at an even lower concentration than 5-HT₃ and nACh receptors. The latter two receptors were shown to be subject to allosteric inhibition also by cannabinoids via a mechanism unrelated to cannabinoid CB₁ or CB₂ receptors; cannabinoid inhibition of 5-HT₃ receptors may represent a new target for the treatment of neuropathic pain.

Cholinesterase inhibitors, donepezil and rivastigmine, attenuate spatial memory and cognitive flexibility impairment induced by acute ethanol in the Barnes maze task in rats

Central cholinergic dysfunction contributes to acute spatial memory deficits produced by ethanol administration. Donepezil and rivastigmine elevate acetylcholine levels in the synaptic cleft through the inhibition of cholinesterases—enzymes involved in acetylcholine degradation. The aim of our study was to reveal whether donepezil (acetylcholinesterase inhibitor) and rivastigmine (also butyrylcholinesterase inhibitor) attenuate spatial memory impairment as induced by acute ethanol administration in the Barnes maze task (primary latency and number of errors in finding the escape box) in rats. Additionally, we compared the influence of these drugs on ethanol-disturbed memory. In the first experiment, the dose of ethanol (1.75 g/kg, i.p.) was selected that impaired spatial memory, but did not induce motor impairment. Next, we studied the influence of donepezil (1 and 3 mg/kg, i.p.), as well as rivastigmine (0.5 and 1 mg/kg, i.p.), given either before the probe trial or the reversal learning on ethanol-induced memory impairment. Our study demonstrated that these drugs, when given before the probe trial, were equally effective in attenuating ethanol-induced impairment in both test situations, whereas rivastigmine, at both doses (0.5 and 1 mg/kg, i.p.), and donepezil only at a higher dose (3 mg/kg, i.p.) given prior the reversal learning, attenuated the ethanol-induced impairment in cognitive flexibility. Thus, rivastigmine appears to exert more beneficial effect than donepezil in reversing ethanol-induced cognitive impairments—probably due to its wider spectrum of activity. In conclusion, the ethanol-induced spatial memory impairment may be attenuated by pharmacological manipulation of central cholinergic neurotransmission.

Different sites of alcohol action in the NMDA receptor GluN2A and GluN2B subunits

The NMDA receptor is a major target of alcohol action in the CNS, and recent behavioral and cellular studies have pointed to the importance of the GluN2B subunit in alcohol action. We and others have previously characterized four amino acid positions in the third and fourth membrane-associated (M) domains of the NMDA receptor GluN2A subunit that influence both ion channel gating and alcohol sensitivity. In this study, we found that substitution mutations at two of the four corresponding positions in the GluN2B subunit, F637 and G826, influence ethanol sensitivity and ion channel gating. Because position 826 contains a glycine residue in the native protein, we focused our attention on GluN2B(F637). Substitution mutations at GluN2B(F637) significantly altered ethanol IC50 values, glutamate EC50 values for peak (Ip) and steady-state (Iss) current, and steady-state to peak current ratios (Iss:Ip). Changes in apparent glutamate affinity were not due to agonist trapping in desensitized states, as glutamate Iss EC50 values were not correlated with Iss:Ip values. Ethanol sensitivity was correlated with values of both Ip and Iss glutamate EC50, but not with Iss:Ip. Values of ethanol IC50, glutamate EC50, and Iss:Ip for mutants at GluN2B(F637) were highly correlated with the corresponding values for mutants at GluN2A(F636), consistent with similar functional roles of this position in both subunits. These results demonstrate that GluN2B(Phe637) regulates ethanol action and ion channel function of NMDA receptors. However, despite highly conserved M domain sequences, ethanol's actions on GluN2A and GluN2B subunits differ.

Effects of ethanol and acetaldehyde in zebrafish brain structures: an in vitro approach on glutamate uptake and on toxicity-related parameters

Ethanol (EtOH) and its metabolite, acetaldehyde (ALD), induce deleterious effects on central nervous system (CNS). Here we investigate the in vitro toxicity of EtOH and ALD (concentrations of 0.25%, 0.5%, and 1%) in zebrafish brain structures [telencephalon (TE), opticum tectum (OT), and cerebellum (CE)] by measuring the functionality of glutamate transporters, MTT reduction, and extracellular LDH activity. Both molecules decreased the activity of the Na(+)-dependent glutamate transporters in all brain structures. The strongest glutamate uptake inhibition after EtOH exposure was 58% (TE-1%), and after ALD, 91% (CE-1%). The results of MTT assay and LDH released demonstrated that the actions of EtOH and its metabolite are concentration and structure-dependent, in which ALD was more toxic than EtOH. In summary, our findings demonstrate a differential toxicity in vitro of EtOH and ALD in zebrafish brain structures, which can involve changes on glutamatergic parameters. We suggest that this species may be an interesting model for assessing the toxicological actions of alcohol and its metabolite in CNS.

Alterations in ethanol-induced behaviors and consumption in knock-in mice expressing ethanol-resistant NMDA receptors

Ethanol's action on the brain likely reflects altered function of key ion channels such as glutamatergic N-methyl-D-aspartate receptors (NMDARs). In this study, we determined how expression of a mutant GluN1 subunit (F639A) that reduces ethanol inhibition of NMDARs affects ethanol-induced behaviors in mice. Mice homozygous for the F639A allele died prematurely while heterozygous knock-in mice grew and bred normally. Ethanol (44 mM; ∼0.2 g/dl) significantly inhibited NMDA-mediated EPSCs in wild-type mice but had little effect on responses in knock-in mice. Knock-in mice had normal expression of GluN1 and GluN2B protein across different brain regions and a small reduction in levels of GluN2A in medial prefrontal cortex. Ethanol (0.75-2.0 g/kg; i.p.) increased locomotor activity in wild-type mice but had no effect on knock-in mice while MK-801 enhanced activity to the same extent in both groups. Ethanol (2.0 g/kg) reduced rotarod performance equally in both groups but knock-in mice recovered faster following a higher dose (2.5 g/kg). In the elevated zero maze, knock-in mice had a blunted anxiolytic response to ethanol (1.25 g/kg) as compared to wild-type animals. No differences were noted between wild-type and knock-in mice for ethanol-induced loss of righting reflex, sleep time, hypothermia or ethanol metabolism. Knock-in mice consumed less ethanol than wild-type mice during daily limited-access sessions but drank more in an intermittent 24 h access paradigm with no change in taste reactivity or conditioned taste aversion. Overall, these data support the hypothesis that NMDA receptors are important in regulating a specific constellation of effects following exposure to ethanol.

A novel alcohol-sensitive position in the N-methyl-D-aspartate receptor GluN2A subunit M3 domain regulates agonist affinity and ion channel gating

Abundant evidence supports a role for N-methyl-d-aspartate (NMDA) receptor inhibition in the behavioral actions of ethanol, but the underlying molecular mechanisms have not been fully elucidated. We recently found that clusters of five positions in the third and fourth membrane-associated domains (M3 and M4) at the intersubunit interfaces form putative sites of alcohol action. In the present study, we found that one of these positions, NMDA receptor subunit, GluN2A(F636), can strongly regulate ethanol sensitivity, glutamate potency, and apparent desensitization: ethanol IC50 values, peak (Ip) and steady-state (Iss) glutamate EC50 values, and steady-state to peak current ratio (Iss:Ip) values differed significantly among the mutants tested. Changes in glutamate affinity among the various mutants were not attributable to agonist trapping due to desensitization, as glutamate peak EC50 values were correlated with values of both steady-state EC50 and Iss:Ip. The mean open times determined in selected mutants could be altered up to 4-fold but did not account for the changes in ethanol sensitivity. Ethanol sensitivity was significantly correlated with glutamate EC50 and Iss:Ip values, but the changes in ethanol IC50 among mutants at this position do not appear to be secondary to changes in ion channel kinetics. Substitution of the isomeric amino acids leucine and isoleucine had markedly different effects on ethanol sensitivity, agonist potency, and desensitization, which is consistent with a stringent structural requirement for ion channel modulation by the side chain at this position. Our results indicate that GluN2A(F636) plays an important role in both channel function and ethanol inhibition in NMDA receptors.

Interactions among positions in the third and fourth membrane-associated domains at the intersubunit interface of the N-methyl-D-aspartate receptor forming sites of alcohol action

The N-methyl-D-aspartate (NMDA) glutamate receptor is a major target of ethanol in the brain. Previous studies have identified positions in the third and fourth membrane-associated (M) domains of the NMDA receptor GluN1 and GluN2A subunits that influence alcohol sensitivity. The predicted structure of the NMDA receptor, based on that of the related GluA2 subunit, indicates a close apposition of the alcohol-sensitive positions in M3 and M4 between the two subunit types. We tested the hypothesis that these positions interact to regulate receptor kinetics and ethanol sensitivity by using dual substitution mutants. In single-substitution mutants, we found that a position in both subunits adjacent to one previously identified, GluN1(Gly-638) and GluN2A(Phe-636), can strongly regulate ethanol sensitivity. Significant interactions affecting ethanol inhibition and receptor deactivation were observed at four pairs of positions in GluN1/GluN2A: Gly-638/Met-823, Phe-639/Leu-824, Met-818/Phe-636, and Leu-819/Phe-637; the latter pair also interacted with respect to desensitization. Two interactions involved a position in M4 of both subunits, GluN1(Met-818) and GluN2A(Leu-824), that does not by itself alter ethanol sensitivity, whereas a previously identified ethanol-sensitive position, GluN2A(Ala-825), did not unequivocally interact with any other position tested. These results also indicate a shift by one position of the predicted alignment of the GluN1 M4 domain. These findings have allowed for the refinement of the NMDA receptor M domain structure, demonstrate that this region can influence apparent agonist affinity, and support the existence of four sites of alcohol action on the NMDA receptor, each consisting of five amino acids at the M3-M4 domain intersubunit interfaces.

Suicidal behavior and alcohol abuse

Suicide is an escalating public health problem, and alcohol use has consistently been implicated in the precipitation of suicidal behavior. Alcohol abuse may lead to suicidality through disinhibition, impulsiveness and impaired judgment, but it may also be used as a means to ease the distress associated with committing an act of suicide. We reviewed evidence of the relationship between alcohol use and suicide through a search of MedLine and PsychInfo electronic databases. Multiple genetically-related intermediate phenotypes might influence the relationship between alcohol and suicide. Psychiatric disorders, including psychosis, mood disorders and anxiety disorders, as well as susceptibility to stress, might increase the risk of suicidal behavior, but may also have reciprocal influences with alcohol drinking patterns. Increased suicide risk may be heralded by social withdrawal, breakdown of social bonds, and social marginalization, which are common outcomes of untreated alcohol abuse and dependence. People with alcohol dependence or depression should be screened for other psychiatric symptoms and for suicidality. Programs for suicide prevention must take into account drinking habits and should reinforce healthy behavioral patterns.

Functional interactions of alcohol-sensitive sites in the N-methyl-D-aspartate receptor M3 and M4 domains

The N-methyl-D-aspartate receptor is an important mediator of the behavioral effects of ethanol in the central nervous system. Previous studies have demonstrated sites in the third and fourth membrane-associated (M) domains of the N-methyl-D-aspartate receptor NR2A subunit that influence alcohol sensitivity and ion channel gating. We investigated whether two of these sites, Phe-637 in M3 and Met-823 in M4, interactively regulate the ethanol sensitivity of the receptor by testing dual substitution mutants at these positions. A majority of the mutations decreased steady-state glutamate EC(50) values and maximal steady-state to peak current ratios (I(ss)/I(p)), whereas only two mutations altered peak glutamate EC(50) values. Steady-state glutamate EC(50) values were correlated with maximal glutamate I(ss)/I(p) values, suggesting that changes in glutamate potency were attributable to changes in desensitization. In addition, there was a significant interaction between the substituents at positions 637 and 823 with respect to glutamate potency and desensitization. IC(50) values for ethanol among the mutants varied over the approximate range 100-325 mm. The sites in M3 and M4 significantly interacted in regulating ethanol sensitivity, although this was apparently dependent upon the presence of methionine in position 823. Molecular dynamics simulations of the NR2A subunit revealed possible binding sites for ethanol near both positions in the M domains. Consistent with this finding, the sum of the molecular volumes of the substituents at the two positions was not correlated with ethanol IC(50) values. Thus, there is a functional interaction between Phe-637 and Met-823 with respect to glutamate potency, desensitization, and ethanol sensitivity, but the two positions do not appear to form a unitary site of alcohol action.

Cholinergic cells in the nucleus basalis of mice express the N-methyl-d-aspartate-receptor subunit NR2C and its replacement by the NR2B subunit enhances frontal and amygdaloid acetylcholine levels

It is known that glutamatergic and cholinergic systems interact functionally at the level of the cholinergic basal forebrain. The N-methyl-d-aspartate receptor (NMDA-R) is a multiprotein complex composed of NR1, NR2 and/or NR3 subunits. The subunit composition of NMDA-R of cholinergic cells in the nucleus basalis has not yet been investigated. Here, by means of choline acetyl transferase and NR2B or NR2C double staining, we demonstrate that mice express both the NR2C and NR2B subunits in nucleus basalis cholinergic cells. We generated NR2C-2B mutant mice in which an insertion of NR2B cDNA into the gene locus of the NR2C gene replaced NR2C by NR2B expression throughout the brain. This NR2C-2B mutant was used to examine whether a subunit exchange in cholinergic neurons would affect acetylcholine (ACh) content in several brain structures. We found increased ACh levels in the frontal cortex and amygdala in the brains of NR2C-2B mutant mice. Brain ACh has been implicated in neuroplasticity, novelty-induced arousal and encoding of novel stimuli. We therefore assessed behavioral habituation to novel environments and objects as well as object recognition in NR2C-2B subunit exchange mice. The behavioral analysis did not indicate any gross behavioral alteration in the mutant mice compared with the wildtype mice. Our results show that the NR2C by NR2B subunit exchange in mice affects ACh content in two target areas of the nucleus basalis.

Effects of amino acid substitutions in transmembrane domains of the NR1 subunit on the ethanol inhibition of recombinant N-methyl-D-aspartate receptors

BACKGROUND

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor is involved in a variety of processes that regulate neuronal plasticity and is an important target for the acute and chronic effects of ethanol. However, the specific sites where ethanol interacts with the receptor protein have yet to be fully elucidated. We previously demonstrated that a phenylalanine to alanine mutation in the third transmembrane domain (TM3) of the NR1 subunit decreased the ethanol inhibition of NMDA receptors expressed in HEK293 cells. In this study, we characterized the ethanol inhibition of NMDA receptors containing additional mutations within the TM3 and TM4 domains of the NR1 subunit.

METHODS

Site-directed mutagenesis was used to alter specific amino acid residues in the TM3 and TM4 domains of the NR1 subunit. Mutant NR1 subunits were coexpressed with the NR2A subunit in HEK293 cells and examined for alterations in ethanol sensitivity using whole-cell voltage-clamp electrophysiology.

RESULTS

Replacing phenylalanine at TM3 position 639 in the NR1 subunit (F639) with 9 different amino acids produced functional receptors when coexpressed with the NR2A subunit. All mutants showed a concentration-dependent inhibition by ethanol (10-100 mM), with the alanine and serine mutants being significantly less sensitive to ethanol. Amino acid substitutions at the F639 site also produced variable changes in the concentration-response relationship to glycine. However, no significant correlation between glycine EC(50) values and the magnitude of ethanol inhibition was observed. Alanine mutations at TM4 positions 813 (M813A) and 819 (L819A), but not at 817 (F817A), of the NR1 subunit enhanced ethanol inhibition. Substitution of tryptophan for TM4 residues in the NR1 subunit (positions 820-822) that are homologous to a site in the NR2A subunit shown to reduce ethanol inhibition (A825W) had no effect on ethanol sensitivity. However, these NR1 TM4 tryptophan mutants restored the ethanol inhibition of the NR1 TM3 F639A mutant to wild-type levels in a stepwise fashion.

CONCLUSIONS

These results indicate that the ethanol sensitivity of NMDA receptors may be regulated by discrete sites within the TM3 and TM4 domains of the NR1 subunit.

Sites in the fourth membrane-associated domain regulate alcohol sensitivity of the NMDA receptor

N-methyl-D-aspartate (NMDA) receptors are important target sites of alcohol action in the central nervous system. Alcohol inhibits NMDA receptor current by an action on ion channel gating, apparently through a direct action on a region of the NMDA receptor accessible from the extracellular environment. Our previous studies have revealed an important role for a methionine residue (Met823) in membrane-associated domain 4 (M4) of the NR2A subunit in channel gating as well as alcohol sensitivity of the NMDA receptor. The role of sites in M4 of the NMDA receptor NR2A subunit adjacent to Met823 was investigated using tryptophan-scanning mutagenesis and electrophysiological recording. Receptors containing NR1 and NR2A(V820W) or NR2A(M817W) mutant subunits expressed in HEK 293 cells were not functional. The mutation Ala826Trp modified apparent desensitization, and the mutations Ala825Trp and Ala826Trp changed the mean open time of the channel as determined by fluctuation analysis. In addition, the mutations Tyr822Trp and Ala825Trp significantly altered the concentration-response curves for ethanol inhibition. The changes in mean open time did not appear to be able to account for the observed differences in ethanol sensitivity. These results indicate that this region in M4 of the NR2A subunit may be involved in the action of alcohol.

Effect of the NR3 subunit on ethanol inhibition of recombinant NMDA receptors

N-Methyl-D-aspartate (NMDA) receptors are a subtype of glutamate receptor that serve important functions at glutamatergic synapses in the brain. NMDA receptors are inhibited by concentrations of ethanol that are associated with intoxication and chronic exposure of neurons to ethanol enhances NMDA receptor function. The factors that underlie the acute inhibition of NMDA receptors by ethanol are not completely known, but ethanol sensitivity is influenced by receptor subunit composition. In this study, the effect of the regulatory subunit, NR3, on ethanol inhibition of NMDA receptors was examined. Recombinant NMDA receptors comprised of NR1 and NR2 (A-D) subunits were transiently transfected into HEK293 cells in the absence or presence of the NR3 subunit. In the absence of NR3, all NMDA receptor subunit combinations were inhibited by 100 mM ethanol. Co-expression of NR3 or an NR3-GFP fusion protein with NR1/NR2 (A-D) subunits did not alter the inhibitory effects of ethanol. In addition, the inhibition of NR1/NR2B receptors by the NR2B subunit-selective antagonist, ifenprodil, was not altered by co-expression of the NR3 subunit. Overall, these results suggest that the NR3A subunit is not a determinant of ethanol sensitivity in recombinant NMDA receptors.

A site of alcohol action in the fourth membrane-associated domain of the N-methyl-D-aspartate receptor

The N-methyl-d-aspartate (NMDA) subtype of ionotropic glutamate receptor is an important mediator of the behavioral effects of ethanol in the central nervous system. Although ethanol is known to inhibit NMDA receptors by influencing ion-channel gating, its molecular site of action and the mechanism underlying this effect have not been established. We have previously identified a conserved methionine residue in the fourth membrane-associated domain of the NMDA receptor NR2A subunit (Met823) that influences desensitization and gating of the ion channel. Here we report that this residue plays an important role in mediating the effect of ethanol on the NMDA receptor. Ethanol IC50 values among functional substitution mutants at this position varied over the range approximately 130-225 mm. There was a weak correlation between ethanol IC50 and mean open time of NR2A(Met823) mutants that was dependent on inclusion of the value for the tryptophan mutant. In the absence of this value, there was no trend toward a correlation among the remaining mutants. Desensitization appeared to influence the action of ethanol, because ethanol IC50 of the mutants was correlated with the steadystate to peak current ratio. With the exception of tryptophan, ethanol sensitivity was significantly related to the molecular volume and hydrophobicity of the substituent. The relation between ethanol sensitivity and the molecular volume and hydrophobicity at this position suggests that this residue interacts with or forms part of a site of ethanol action and that the presence of a tryptophan residue in this site disrupts its ability to interact with ethanol.

Ethanol differentially inhibits homoquinolinic acid- and NMDA-induced neurotoxicity in primary cultures of cerebellar granule cells

The potency of ethanol to inhibit N-methyl-D-aspartate (NMDA) receptor functions may depend on the subunit composition of the NMDA receptors. We used a NR2A-B subunit-selective NMDA receptor agonist, homoquinolinic acid (HQ), and a subunit-unselective agonist, NMDA, to induce neurotoxicity in cerebellar granule cells and examined the neuroprotective actions of ethanol, as well as NR2A- and NR2B-subunit selective antagonists, respectively. HQ was a more potent neurotoxic agent than NMDA, as measured by the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay. NR2A- and NR2B-selective NMDA receptor antagonists displayed quite similar neuroprotective potencies against the NMDA- and HQ-produced cell death, indicating that the higher potency of HQ to induce neurotoxicity cannot be simply explained by NR2A- or NR2B-subunit selectivity. As expected, ethanol (25 and 50 mM) attenuated the NMDA-induced neurotoxicity in a non-competitive manner by significantly reducing the maximum neurotoxicity produced by NMDA. By contrast, ethanol inhibited the HQ-induced neurotoxicity in a manner resembling a competitive-like interaction significantly increasing the EC50 value for HQ, without reducing the maximum neurotoxicity produced by HQ. These results suggest that HQ reveals either a novel site or a not previously observed mechanism of interaction between ethanol and NMDA receptors in rat cerebellar granule cell cultures.

Nerve growth factor and chronic ethanol treatment alter calcium homeostasis in developing rat septal neurons

Chronic ethanol treatment (CET) during development produces cellular adaptations resulting in tolerance to the acute effects of ethanol (EtOH). The objectives of this study were to determine whether CET during the prenatal period (PCET) followed by a period of in vitro CET (PCET-CET) altered intracellular calcium [Ca(2+)](i) and produced tolerance to acute EtOH treatment (AET), and whether nerve growth factor (NGF) modulated the effects of PCET-CET in cultured developing rat septal neurons. Fetuses were obtained from EtOH-fed and sucrose-fed (diet-control) female rats. Neurons from PCET fetuses were cultured in the presence of NGF (+NGF) and 200 mg/dl (mg %) EtOH and diet-control cultures received NGF and no EtOH. PCET and diet-control cultures were then divided into two groups, +NGF and -NGF (withdrawn from NGF), and exposed acutely to one of five doses of EtOH during stimulation with potassium (K(+)) chloride. [Ca(2+)](i) was measured using fura-2. PCET-CET did not affect resting [Ca(2+)](i). PCET-CET decreased and acute EtOH withdrawal increased overall K(+)-stimulated changes in [Ca(2+)](i), but only in +NGF PCET neurons. Reducing the level of EtOH from 200 to 100 mg % decreased overall K(+)-stimulated [Ca(2+)](i) in -NGF PCET neurons. The effects of PCET-CET or PCET-CET combined with NGF on overall K(+)-stimulated changes in [Ca(2+)](i) occurred mostly in the early and middle phases of the K(+)-response. NGF reduced overall K(+)-stimulated changes in [Ca(2+)](i) in PCET neurons during EtOH withdrawal and during AET with 200 mg % EtOH and increased overall K(+)-stimulated changes in [Ca(2+)](i) during AET with 400 and 800 mg % EtOH. There was no effect of NGF on overall K(+)-stimulated changes in [Ca(2+)](i) in diet-control neurons with the exception that NGF-treatment decreased overall K(+)-stimulated changes in [Ca(2+)](i) during AET with 400 mg % EtOH. The effects of AET on overall K(+)-stimulated changes in [Ca(2+)](i) mostly occurred in +NGF PCET neurons. In conclusion, CET during development of the brain could adversely affect Ca(2+)-dependent functions such as neuronal migration, neurite outgrowth, and synaptogenesis in neurons even in the presence of neurotrophin support.

Functional cooperation between neurosteroids and D2 dopamine antagonists on KCl-evoked [3H]noradrenaline release: modulation by calcium channel blockers

It has recently been proposed that neurosteroids, such as dehydroepiandrosterone sulphate and pregnenolone sulphate, interfere with the dopamine system in the central nervous system. According to our previous report showing that the butyrophenone, spiperone, slightly enhances the evoked release of [3H]-noradrenaline ([3H]NA) in the presence of these sulphated steroids, the present study was carried out to document the putative interplay between steroids and spiperone, which is known to be a prototypic D2 dopamine antagonist and also a 5-HT2 serotonin antagonist. For this purpose, the paradigm of KCl-evoked [3H]NA release from preloaded rat hippocampal slices was used to investigate the interactions between neurosteroids, spiperone and the voltage-sensitive calcium channels (VSCCs). The selective 5-HT2 serotonin antagonist ritanserine was ineffective, whereas sulpiride, a selective D2 dopamine antagonist mimicked the action of spiperone, thus suggesting that the blockade of D2 dopamine receptors accounted for the modulatory effect of spiperone on neurosteroid-induced modulation of evoked [3H]NA release. In addition, this facilitation of KCl-evoked [3H]NA release by the combination of a steroid and a D2 dopamine antagonist was partially inhibited by the L- and N-type VSCC blockers nifedipine and omega-conotoxin GVIA, respectively. The present results provide in-vitro functional evidence for the putative role of VSCCs in the interplay between steroids and D2 dopamine receptors.

Ethanol sensitivity of NMDA receptors

NMDA receptors are ionotropic glutamate receptors assembled of subunits of the NR1 and of the NR2 family (NR2A-NR2D). The subunit diversity largely affects the pharmacological properties of NMDA receptors and, hence, gives rise to receptor heterogeneity. As an overall result of studies on recombinant and native NMDA receptors, ethanol inhibits the function of receptors containing the subunits NR2A and/or NR2B to a greater extent than those containing NR2C or NR2D. For example, in rat cultured mesencephalic neurons, NR2C expression was developmentally increased, whereas expression of NR2A and NR2B was decreased. These changes coincided with a developmental loss of sensitivity of NMDA responses to ethanol and ifenprodil, a non-competitive NMDA receptor antagonist that shows selectivity for NR2B-containing receptors. Also in rat locus coeruleus neurons, the low ethanol sensitivity of somatic NMDA receptors could be explained by a prominent expression of NR2C. The inhibitory site of action for ethanol on the NMDA receptor is not yet known. Patch-clamp studies suggest a target site exposed to or only accessible from the extracellular environment. Apparently, amino acid residue Phe(639), located in the TM3 domain of NR1, plays a crucial role in the inhibition of NMDA receptor function by ethanol. Since this phenylalanine site is common to all NMDA and non-NMDA receptor (AMPA/kainate receptor) subunits, this observation is consistent with accumulating evidence for a similar ethanol sensitivity of a variety of NMDA and non-NMDA receptors, but it cannot explain the differences in ethanol sensitivity observed with different NR2 subunits.

Inhibition of N-methyl-D-aspartate receptors by straight-chain diols: implications for the mechanism of the alcohol cutoff effect

n-Alkanol inhibition of N-methyl-D-aspartate (NMDA) receptors exhibits a "cutoff" effect: alcohols with up to eight to nine carbon atoms inhibit the receptor, whereas larger alcohols do not. This phenomenon was originally proposed to result from size exclusion; i.e., alcohols above the cutoff are too large to bind to an amphiphilic site on the receptor. In the present study, 1,Omega-diols with 3 to 14 carbon atoms inhibited NMDA-activated current in Chinese hamster ovary and human embryonic kidney 293 cells transiently expressing NR1 and NR2B NMDA receptor subunits. Results of fluctuation analysis experiments were consistent with a similar mechanism of inhibition of NMDA-activated current by alcohols and diols. The average change in apparent energy of binding of the diols caused by addition of a methylene group was 2.1 kJ/mol, which is consistent with an important role of hydrophobic interactions. Because 1,Omega-diols with 9 to 14 carbons inhibited NMDA-activated current, despite having molecular volumes exceeding that at the cutoff point for 1-alkanols, a size exclusion mechanism seems inadequate to explain the cutoff effect. A disparity in hydrophobicity values at the cutoff for alcohols and diols, however, revealed that hydrophobicity could also not entirely explain the cutoff phenomenon. From these results, it seems that the cutoff effect on NMDA receptors results primarily from the inability of long-chain alcohols to achieve adequate concentrations at their site of action due to low aqueous solubility, although other factors may also contribute to the effect.

Ethanol inhibition of N-methyl-D-aspartate receptors is reduced by site-directed mutagenesis of a transmembrane domain phenylalanine residue

N-Methyl-D-aspartate (NMDA) receptors (NRs) are ionotropic receptors activated by glutamate and the co-agonist glycine. Ethanol inhibits NMDA receptor function, although its site of action is undefined. We hypothesized that ethanol acts at specific amino acids contained within the transmembrane (TM) domains of the receptor. In this study, NR1 and NR2A subunits were altered by mutagenesis and tested for sensitivity to ethanol. Three NR1 mutants (W636A, F817A, and L819A) and one NR2A mutant (F637A) failed to generate functional receptors. Pre-TM1 (I546A, L551A, F554A, and F558A), TM1 (W563A), and TM2 (W611A) NR1 mutations did not affect ethanol sensitivity of heteromeric receptors. In contrast, altering a TM3 phenylalanine to alanine (F639A) reduced the ethanol inhibition of NMDA receptors expressed in oocytes and human embryonic kidney 293 cells. Mutation of the nearby methionine (M641) to alanine did not affect ethanol sensitivity, whereas changing Phe(639) to tryptophan slightly enhanced ethanol inhibition. NR1(F639A) did not alter the agonist potency of glutamate but did produce a leftward shift in the glycine concentration response for receptors containing NR2A and NR2B subunits. NR1(F639A) also reduced the potency of the competitive glycine antagonist 5,7-dichlorokynurenic acid and increased the efficacy of the glycine partial agonist 3-amino-1-hydroxy-2-pyrrolidinone ((+)-HA-966). These results suggest that ethanol may interact with amino acids contained in the TM3 domain of NMDA subunits that are involved in transducing agonist binding to channel opening.

Ethanol sensitivity of recombinant human N-methyl-D-aspartate receptors

In this study, the ethanol sensitivity of human N-methyl-D-aspartate (NMDA) receptors stably expressed in L(tk-) cells, or transiently expressed in HEK 293 cells and Xenopus oocytes was determined. NMDA receptor function was measured using fura-2 calcium imaging for L(tk-) cells, whole cell voltage-clamp for HEK 293 cells, and two-electrode voltage clamp for oocytes. Ethanol inhibited NMDA receptor function in all three expression system, but was less potent for receptors expressed in L(tk-) cells. NMDA receptors composed of NR1a/2B subunits were inhibited to a greater extent by ethanol than NR1a/2A receptors when expressed in L(tk-) cells and HEK 293 cells, but not in oocytes. These results suggest that the method of receptor expression and assay system used may influence the degree of ethanol inhibition of recombinant NMDA receptors.

Water maze and radial maze learning and the density of binding sites of glutamate, GABA, and serotonin receptors in the hippocampus of inbred mouse strains

Correlations between the densities of ionotropic glutamate, GABA(A), and serotonin binding sites in the hippocampus of seven inbred mouse strains and strain-specific learning capacities in two types of maze were studied. Binding site densities were measured with quantitative receptor autoradiography. Learning capacities were determined in a water maze task as well as in spatial and nonspatial versions of an eight-arm radial maze. The densities of most binding sites differed significantly between the strains in the subfields of Ammon's horn (CA1 and CA3) and the dentate gyrus, except for serotonin binding sites in CA1. By comparing the different strains, significant receptor-behavioral correlations between the densities of the GABA(A) receptors and the activity-dependent behavior in the water maze as well as the spatial learning in the radial maze were found. The densities of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionate (AMPA) and kainate receptors correlated positively with learning capacity in the spatial eight-arm radial maze. We conclude that hereditary variations mainly in AMPA, kainate, and GABA(A) receptor densities are involved in behavioral variations in spatial and nonspatial learning tasks.

Glutamate-mediated transmission, alcohol, and alcoholism

Glutamate-mediated neurotransmission may be involved in the range of adaptive changes in brain which occur after ethanol administration in laboratory animals, and in chronic alcoholism in human cases. Excitatory amino acid transmission is modulated by a complex system of receptors and other effectors, the efficacy of which can be profoundly affected by altered gene or protein expression. Local variations in receptor composition may underlie intrinsic regional variations in susceptibility to pathological change. Equally, ethanol use and abuse may bring about alterations in receptor subunit expression as the essence of the adaptive response. Such considerations may underlie the regional localization characteristic of the pathogenesis of alcoholic brain damage, or they may form part of the homeostatic change that constitutes the neural substrate for alcohol dependence.

Alcohols inhibit N-methyl-D-aspartate receptors via a site exposed to the extracellular environment

N-Methyl-D-aspartate (NMDA) receptors are important CNS target sites of alcohols, but the site and mechanism of action of alcohols on NMDA receptors remains unclear. In CHO-K1 cells transfected with NR1/NR2B NMDA receptor subunits, ethanol inhibited NMDA-activated current with an IC(50) of 138 mM. Truncation of the intracellular C-terminal domain of the NR1 subunit (NR1T) did not alter ethanol sensitivity when combined with the NR2B subunit, but a similar truncation of the NR2B subunit (NR2BT) slightly enhanced ethanol sensitivity of receptors formed from coexpression with either NR1 or NR1T subunits. 1-Pentanol applied externally inhibited NMDA receptors with an IC(50) of 9.9 mM, but intracellular application of 1-pentanol (25 mM) did not alter NMDA receptor inhibition by externally applied ethanol or 1-pentanol. In addition, the amplitude of NMDA-activated current did not decrease during the time required for 1-pentanol (25 mM) to diffuse throughout the cytoplasm. Ethanol did not inhibit NMDA receptors when bath-applied in cell-attached patches or when applied to the cytoplasmic face of inside-out membrane patches. These results appear to be best explained by an action of alcohols on the NMDA receptor-channel protein, at a site located in a domain exposed to, or only accessible from, the extracellular environment.

Attenuation of the acute amnestic effect of ethanol by ifenprodil: comparison with ondansetron and dizocilpine

The aim of the present study was to examine the influence of ifenprodil, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist which also blocks 5-HT3 receptors, on the amnestic effect of ethanol in a passive avoidance task in mice. The anti-amnestic action of ifenprodil was compared with the effects of the 5-HT3 receptor antagonist ondansetron and the non-competitive NMDA-receptor antagonist dizocilpine (MK-801). Ethanol, 2 g/kg and dizocilpine 0.1 mg/kg significantly impaired the passive avoidance response. In contrast, ifenprodil (0.1-10 mg/kg), ondansetron (0.03-0.3 mg/kg) and dizocilpine (0.01 and 0.03 mg/kg) did not alter passive avoidance by themselves. Dizocilpine did not diminish the amnestic action of ethanol when administered at doses of 0.03-0.1 mg/kg. However, the amnestic effect of ethanol was partially restored towards normal by ifenprodil 0.3 mg/kg and by ondansetron 0.03 mg/kg (alone or together with dizocilpine 0.01 mg/kg) but it was not affected by other doses of ifenprodil (0.1, 1 and 10 mg/kg) and ondansetron (0.1 and 0.3 mg/kg). In conclusion, ifenprodil at an appropriate dose reduced ethanol-induced amnesia in a step-through passive avoidance task. The results are compatible with the assumption that the anti-amnestic action of ifenprodil may be (at least partially) due to an antagonism at 5-HT3 receptors.

Ifenprodil influences changes in mouse behaviour related to acute and chronic ethanol administration

The aim of the present study was to examine the influence of ifenprodil (a non-competitive NMDA receptor antagonist which also blocks 5-HT3 receptors and alpha1-adrenoceptors) on the effects of ethanol in the mouse in vivo and to elucidate the role of various receptors in these actions. The ethanol (4 g/kg i.p.)-induced sleeping time was shortened by ifenprodil 1 mg/kg but was not affected by ifenprodil 0.3 mg/kg, the 5-HT3 receptor antagonist ondansetron 0.03 mg/kg and the non-competitive NMDA receptor antagonist MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cycloheptan-5,10-imine maleate) 0.01 mg/kg. Ifenprodil 10 mg/kg mimicked the alpha1-adrenoceptor antagonist prazosin 1 mg/kg in that it prolonged the hypnotic response to ethanol (no additive effect when both drugs were given in combination); this is compatible with an involvement of alpha1-adrenoceptors in this effect of ifenprodil. Chronic exposure to ethanol (7%) induced physical dependence. The severity of ethanol withdrawal was suppressed by ifenprodil 1 and 10 mg/kg. In conclusion, ifenprodil influences ethanol-related changes in mouse behaviour and may prove to be useful in the treatment of alcoholism.

Calcium influx through NMDA receptors, chronic receptor inhibition by ethanol and 2-amino-5-phosponopentanoic acid, and receptor protein expression

Chronic treatment of neurons with either ethanol or competitive and noncompetitive antagonists of NMDA receptors leads to enhanced expression of NMDA receptor density and function in neurons. The signal transduction pathways for such receptor up-regulation are not known. The focus of the present study was on the role of Ca2+ entry into neurons, either through receptor or voltage-gated channels, in the expression of the NMDA receptor subunit NR1 and the 71-kDa glutamate-binding protein (GBP) of a glutamate/NMDA receptor-like complex. Chronic inhibition of NMDA receptors in cortical neurons in primary cultures by either 100 mM ethanol or 100 microM 2-amino-5-phosphonopentanoic acid (2-AP5) increased the expression of NR1 and GBP. The effect of 2-AP5 on the expression of the two proteins was not additive with that of ethanol when neuronal cultures were treated with both agents at the same time. However, the effects of ethanol on NR1 and GBP expression were blocked by the simultaneous treatment with NMDA (50 microM). Activation or inhibition of other glutamate ionotropic receptors had no effect on the expression of NR1 and GBP. The inhibition of L- or N-type voltage-sensitive Ca2+ channels and voltage-gated Na+ channels also had little effect on the expression of either protein; neither did exposure of neurons to elevated extracellular Ca2+ concentrations (3 or 5 mM). On the other hand, treatment of neurons for 48 h with the intracellular Ca2+ chelator BAPTA-AM as well as partial chelation of extracellular Ca2+ with EGTA caused an up-regulation in NR1 and GBP expression. The enhanced expression of NR1 in neurons treated for 48 h with either ethanol or EGTA was correlated with increases in the activity of NMDA receptors demonstrated as a doubling of the NMDA-stimulated rise in intracellular free Ca2+ concentration. The effects of chronic administration of EGTA on both NR1 expression as well as NMDA receptor function were probably related to an acute inhibition by EGTA of NMDA-induced Ca2+ influx into neurons. It appears that the expression of both the NR1 subunit of NMDA receptors and the GBP of a receptor-like complex is regulated by intracellular Ca2+, especially that entering through NMDA receptor ion channels.

NMDA receptor characterization and subunit expression in rat cultured mesencephalic neurones

1. NMDA-induced changes in free intracellular Ca2+ concentration ([Ca2+]i) were determined in individual cultured rat mesencephalic neurones by the fura-2 method. mRNA expression encoding NMDA receptor subunits (NR1, NR2A-D) was examined by RT-PCR. 2. NMDA (1-100 microM, plus 10 microM glycine) induced a concentration-dependent increase in [Ca2+]i (EC50 = 5.7 microM). The effect of NMDA was virtually insensitive to tetrodotoxin (0.3 microM) and nitrendipine (1 microM), but dependent on extracellular Ca2+. 5,7-Dichlorokynurenic acid (10 microM), a specific antagonist at the glycine binding site on the NMDA receptor, abolished the NMDA response. 3. Memantine, an open-channel blocker, and ifenprodil, a preferential non-competitive NR1/NR2B receptor antagonist diminished the NMDA effect with an IC50 value of 0.17 and 1 microM, respectively. Ethanol at 50 and 100 mM caused about 25 and 45%-inhibition, respectively. 4. Agarose gel analysis of the PCR products followed by ethidium bromide fluorescence or CSPD chemiluminescence detection revealed an almost exclusive expression of the NR1 splice variants lacking exon (E) 5 and E22. The 3' splice form without both E21 and E22 exceeded that containing E21 by approximately 4 fold. The relative amounts of NR2A, NR2B, NR2C corresponded to approximately 1:2:1. NR2D mRNA was also detectable. 5. In conclusion, mesencephalic neurones bear ethanol-sensitive NMDA receptors which might be involved in the development of ethanol dependence and withdrawal. The high affinity of NMDA to this receptor, its sensitivity to ifenprodil and memantine may suggest that the mesencephalic NMDA receptor comprises the NR1 splice variant lacking E5, NR2B, and NR2C, respectively.

Effects of moderate alcohol consumption on the central nervous system

The concept of moderate consumption of ethanol (beverage alcohol) has evolved over time from considering this level of intake to be nonintoxicating and noninjurious, to encompassing levels defined as "statistically" normal in particular populations, and the public health-driven concepts that define moderate drinking as the level corresponding to the lowest overall rate of morbidity or mortality in a population. The various approaches to defining moderate consumption of ethanol provide for a range of intakes that can result in blood ethanol concentrations ranging from 5 to 6 mg/dl, to levels of over 90 mg/dl (i.e., approximately 20 mM). This review summarizes available information regarding the effects of moderate consumption of ethanol on the adult and the developing nervous systems. The metabolism of ethanol in the human is reviewed to allow for proper appreciation of the important variables that interact to influence the level of exposure of the brain to ethanol once ethanol is orally consumed. At the neurochemical level, the moderate consumption of ethanol selectively affects the function of GABA, glutamatergic, serotonergic, dopaminergic, cholinergic, and opioid neuronal systems. Ethanol can affect these systems directly, and/or the interactions between and among these systems become important in the expression of ethanol's actions. The behavioral consequences of ethanol's actions on brain neurochemistry, and the neurochemical effects themselves, are very much dose- and time-related, and the collage of ethanol's actions can change significantly even on the rising and falling phases of the blood ethanol curve. The behavioral effects of moderate ethanol intake can encompass events that the human or other animal can perceive as reinforcing through either positive (e.g., pleasurable, activating) or negative (e.g., anxiolysis, stress reduction) reinforcement mechanisms. Genetic factors and gender play an important role in the metabolism and behavioral actions of ethanol, and doses of ethanol producing pleasurable feelings, activation, and reduction of anxiety in some humans/animals can have aversive, sedative, or no effect in others. Research on the cognitive effects of acute and chronic moderate intake of ethanol is reviewed, and although a number of studies have noted a measurable diminution in neuropsychologic parameters in habitual consumers of moderate amounts of ethanol, others have not found such changes. Recent studies have also noted some positive effects of moderate ethanol consumption on cognitive performance in the aging human. The moderate consumption of ethanol by pregnant women can have significant consequences on the developing nervous system of the fetus. Consumption of ethanol during pregnancy at levels considered to be in the moderate range can generate fetal alcohol effects (behavioral, cognitive anomalies) in the offspring. A number of factors--including gestational period, the periodicity of the mother's drinking, genetic factors, etc.--play important roles in determining the effect of ethanol on the developing central nervous system. A series of recommendations for future research endeavors, at all levels, is included with this review as part of the assessment of the effects of moderate ethanol consumption on the central nervous system.

Inhibition of excitatory amino acid-activated currents by trichloroethanol and trifluoroethanol in mouse hippocampal neurones

1. The effects of the active metabolite of chloral derivative sedative-hypnotic agents, 2,2,2-trichloroethanol (trichloroethanol), and its analog 2,2,2-trifluoroethanol (trifluoroethanol), were studied on ion current activated by the excitatory amino acids N-methyl-D-aspartate (NMDA) and kainate in mouse hippocampal neurones in culture using whole-cell patch-clamp recording. 2. Both trichloroethanol and trifluoroethanol inhibited excitatory amino acid-activated currents in a concentration-dependent manner. Trichloroethanol inhibited NMDA- and kainate-activated currents with IC50 values of 6.4 and 12 mM, respectively, while trifluoroethanol inhibited NMDA- and kainate-activated currents with IC50 values of 28 and 35 mM, respectively. 3. Both trichloroethanol and trifluoroethanol appeared to be able to inhibit excitatory amino acid-activated currents by 100 per cent. 4.Concentration-response analysis of NMDA- and kainate-activated current revealed that trichloroethanol decreased the maximal response to both agonists without significantly affecting their EC50 values. 5. Both trichloroethanol and trifluoroethanol inhibited excitatory amino acid-activated currents more potently than did ethanol. The inhibitory potency of trichloroethanol and trifluoroethanol appears to be associated with their increased hydrophobicity. 6. The observation that trichloroethanol inhibits excitatory amino acid-activated currents at anaesthetic concentrations suggests that inhibition of excitatory amino acid receptors may contribute to the CNS depressant effects of chloral derivative sedative-hypnotic agents.

The role of N-methyl-D-aspartate (NMDA) receptor-mediated neurotransmission in the pathophysiology and therapeutics of psychiatric syndromes

The study of excitatory amino acids (EAAs) has recently resulted in new and fundamental concepts in neuroscience. This progress has led to a growing awareness of the crucial role that brain EAAs systems play in a variety of physiological and pathological processes. The N-methyl-D-aspartate (NMDA) receptor, presently the most well understood subtype of EAAs receptors, has been implicated in crucial physiological processes such as synaptogenesis, learning and memory. Dysfunctions of NMDA receptors seem to play a crucial role in the neurobiology of disorders such as Parkinson's disease, Alzheimer's disease, epilepsy and ischemic stroke. This paper is a review of emerging data indicating that alterations of NMDA receptor function may be pivotal to the pathophysiology of four common psychiatric syndromes: schizophrenia, major depression, posttraumatic stress disorder, and alcoholism. Special emphasis is placed on the current state of development of pharmacological strategies aiming at the modulation of NMDA receptor-mediated neurotransmission in these disorders.

Effect of acute, but not chronic ethanol treatment on somatostatin secretion in rat hypothalamic neurons

To examine the possible involvement of somatostatin in growth hormone modifications induced by ethanol, we examined: (1) the effects of chronic ethanol exposure of cultured hypothalamic neurons on somatostatin content and mRNA levels; (2) the acute effect of ethanol on somatostatin release stimulated by N-methyl-D-aspartate (NMDA). The results showed that 8 days of ethanol exposure (10-100 mM) did not decrease somatostatin content or somatostatin mRNA levels. Ethanol treatment alone had no significant effect on cell morphology or on protein content. In contrast, acute application of ethanol in 8 day-old cultures significantly reduced (50 mM) or completely blocked (100 mM) somatostatin release elicited by 50 microM NMDA without modifying basal release. We conclude that chronic ethanol treatment to concentrations up to 100 mM has no effect on somatostatin biosynthesis in fetal rat hypothalamic neurons, while weaker concentrations decrease NMDA-induced somatostatin release.

Ethanol inhibition of N-methyl-D-aspartate-activated current in mouse hippocampal neurones: whole-cell patch-clamp analysis

1. The action of ethanol on N-methyl-D-aspartate (NMDA)-activated ion current was studied in mouse hippocampal neurones in culture using whole-cell patch-clamp recording. 2. Ethanol inhibited NMDA-activated current in a voltage-independent manner, and did not alter the reversal potential of NMDA-activated current. 3. Concentration-response analysis of NMDA- and glycine-activated current revealed that ethanol decreased the maximal response to both agonists without affecting their EC50 values. 4. The polyamine spermine (1 microM) increased amplitude of NMDA-activated current but did not alter the percentage inhibition of ethanol. 5. Compared to an extracellular pH of 7.0, pH 6.0 decreased and pH 8.0 increased the amplitude of NMDA-activated current, but these changes in pH did not significantly alter the percentage inhibition by ethanol. 6. The sulphydryl reducing agent dithiothreitol (2 mM) increased the amplitude of NMDA-activated current, but did not affect the percentage inhibition by ethanol. 7. Mg2+ (10, 100, 500 microM), (5, 20 microM) or ketamine (2, 10 microM) decreased the amplitude of NMDA-activated current, but did not affect the percentage inhibition by ethanol. 8. The observations are consistent with ethanol inhibiting the function of NMDA receptors by a non-competitive mechanism that does not involve several modulatory sites on the NMDA receptor-ionophore complex.

Molecular pathogenesis of alcohol withdrawal seizures: the modified lipid-protein interaction mechanism

The phrase alcohol withdrawal seizures (AWS) refers to seizures that result from the withdrawal of alcohol after a period of chronic alcohol administration. A mechanism of AWS is postulated, namely the modified lipid-protein interaction (MLPI) mechanism. This hypothesis is based upon an evaluation of the mechanisms of membrane fluidity, calcium channels, gamma-aminobutyric acid (GABA) and glutamate in the molecular pathogenesis of AWS. The mechanism hypothesizes that acute ethanol treatment alters the neuronal membrane lipids which then perturbs protein events, such as affecting the GABAA receptors, NMDA receptors and voltage-dependent Ca2+ channels synergistically or in combination. Subsequent adaptations in these systems occur after prolonged administration of ethanol. A sudden withdrawal of ethanol then leads to hyperexcitability which results in AWS.

Glycine does not reverse the inhibitory actions of ethanol on NMDA receptor functions in cerebellar granule cells

The effects of ethanol and/or glycine on NMDA-induced enhancement of cytoplasmic free Ca2+ concentrations ([Ca2+]i), 45Ca2+ influx, 4-b-[3H]phorbol-12,13-dibutyrate ([3H]PDBu) binding, and neuronal necrosis in cultured rat cortical and cerebellar granule neurons were examined. Using microfluorimetric techniques in combination with rapid perfusion of single brain neurons, we found that glycine (10 microM) was a necessary co-agonist for NMDA-induced depolarization in cerebellar granule cells. In contrast, depolarization with NMDA in cortical cells was observed even without the addition of exogenous glycine as well as in the absence or presence of 1 mM MgCl2. Ethanol (50 mM) inhibited the effects of NMDA in some, but not all, neurons indicative of the existence of ethanol-sensitive and ethanol-insensitive cortical and cerebellar granule neurons. In studies performed in monolayers of cortical and cerebellar granule cells, we observed that the presence of glycine (10 microM) was a necessary prerequisite to unmask inhibitory actions of ethanol on 45Ca2+ influx induced by NMDA. In another set of experiments, we noted that NMDA-induced stimulation of [3H]PDBu binding to monolayers of intact cerebellar granule cells was inhibited by ethanol (50 mM). Finally, we report that ethanol caused a concentration-dependent inhibition of NMDA-induced necrotic cell death, assessed by measuring the ability of cerebellar granule cells to transform 3-[4, 5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) into formazan. In none of the four assays used to demonstrate the inhibitory effects of ethanol on NMDA receptor activity, the ethanol-induced inhibition was reversed by glycine (up to 100 microM). Thus, in contrast to earlier reports, our data suggest that ethanol and glycine produce their effects by acting at different regulatory sites within the NMDA receptor system in brain neurons.

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.

Differentiation of sigma ligand-activated receptor subtypes that modulate NMDA-evoked [3H]-noradrenaline release in rat hippocampal slices

1. It is now widely accepted that there are two classes of sigma (sigma) binding sites, denoted sigma(1) and sigma(2), and recently sigma(3) subtype has been proposed. Selective sigma(1) and sigma(2) receptor agonists are known to modulate the neuronal response to N-methyl-D-aspartate (NMDA) in vivo and in vitro. To identify the site of action of a series of recently synthesised high affinity sigma ligands, the present in vitro series of experiments was carried out on NMDA-evoked [3H]-noradrenaline ([3H]-NA) overflow from preloaded hippocampal slices of the rat. 2. The ligands (+)-cis-N-methyl-N-[2,(3,4-dichlorophenyl) ethyl]-2-(1-pyrrolidinyl) cyclohexylamine (BD-737) and (+)-pentazocine, considered as the prototypic sigma(1) agonists, potentiated the NMDA response from 10 nM to 100 nM. This potentiation faded between 100 nM and 1 microM ligand concentrations. On the other hand, 1,3-di(2-tolyl)guanidine (DTG), a mixed sigma(1)/sigma(2) agonist, at concentrations greater than 100 nM inhibited the NMDA-evoked [3H]-NA release. Spiperone, considered as active on putative sigma(3) receptors, was without effect on the NMDA response, or on the potentiating effect of BD-737. 3. The high affinity sigma antagonists haloperidol and 1[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine (BD-1063), inactive by themselves on the NMDA-induced response, at concentrations above 30 nM totally prevented the potentiating effect of (+)-pentazocine (100 nM) as well as the inhibitory effect of DTG (300 nM) on NMDA-evoked [3H]-NA release. Whereas haloperidol and BD-1063, at concentrations < 1 microM, were inactive on the potentiating effect of BD-737 (100 nM). 4. 4-(4-Chlorophenyl)-alpha-4-fluorophenyl-4-hydroxy-1-piperidinebutanol (reduced haloperidol), N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine (BD-1008), inactive by themselves on the NMDA-evoked [3H]-NA release, failed to reverse the effects of (+)-pentazocine and DTG, but at concentrations of 30 nM to 1 microM antagonised the BD-737-induced potentiation of the NMDA response. Conversely, N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100) blocked the effects of (+)-pentazocine as well as those of BD-737, but not those of DTG. 5. The present results provide in vitro functional evidence for a sigma receptor type preferentially sensitive to BD-737, reduced haloperidol, BD-1008 and also to NE-100, that differs from the already identified sigma(1), sigma(2) and sigma(3) sites.

Modulation of NMDA and dopaminergic neurotransmissions by sigma ligands: possible implications for the treatment of psychiatric disorders

Sigma (sigma) receptors, improperly classified as belonging to the opiate receptor family when discovered in 1976, were subsequently confused with phencyclidine binding sites for several years. It's only recently, with the emergence of new selective ligands that their functional significance could be meaningfully addressed. Several subtypes of sigma receptors are present in high densities in the limbic structures as well as in motor-related areas of the CNS. Different lines of evidence suggest that a major role for sigma receptors might be to regulate the activity of the glutamatergic system via the modulation one of its subtype of receptor, the NMDA receptor. This modulation of the glutamatergic system could in turn interfere with the dopaminergic neurotransmission with which, however, sigma ligands could also interact directly. The potential involvement of sigma receptors in schizophrenia has been considered ever since their discovery. The initial suggestion to this respect emerged from the observation that several of the earliest sigma ligands induced psychotomimetic symptoms such as delusions, hallucinations and depersonalization. This link was later reinforced with the demonstration that several neuroleptics, such as haloperidol, have a high affinity for sigma receptors, whereas, some new molecules with a high affinity for sigma receptors, but a low affinity for dopaminergic receptors demonstrated a "neuroleptic-like" pharmacological profile. However, the therapeutic efficacy of selective sigma ligands in schizophrenia has not yet been established and it has even been suggested that sigma receptors might be responsible for some side effects of the classical neuroleptics. The possible implication of sigma receptors in affective disorders has also been suggested by reports showing that some antidepressant drugs have a high affinity for sigma receptors and that long-term treatments with anti- depressant drugs, even with those devoid of affinity for sigma receptors, modify their binding characteristics. In conclusion, indirect evidence suggests possible etiological and/or therapeutic roles for sigma receptors in some psychiatric disorders. However, despite several attempts, no clear indications of a therapeutic efficacy of sigma ligands has yet emerged. More selective ligands and fundamental studies on the respective role of the different subtypes of sigma receptors are needed before clear concepts can be formulated. p3

FOS and JUN as markers for ethanol-sensitive pathways in the rat brain

The expression of proteins coded by the immediate early genes of the fos family and c-jun was used to study the effect of acute ethanol administration on convulsant-induced neuronal activity in rat brain. Immunoreactivity for both types of protein was induced by either SC injection of pentylenetetrazole or by IP injection of N-methyl-D-aspartic acid. Both agents elicited distinct patterns of behaviour and a high level of FOS-immunoreactivity in the cerebral cortex and hippocampus. Acute IP doses of ethanol (1.0-3.0 g/kg) significantly reduced the behaviours and FOS-immunoreactivity induced in the cerebral cortex by both pentylenetetrazole and N-methyl-D-aspartic acid. Pentylenetetrazole-induced FOS-immunoreactivity in the hippocampus was also inhibited by ethanol. In contrast, N-methyl-D-aspartic acid-induced FOS-immunoreactivity in the hippocampus was not inhibited by any dose of ethanol. c-JUN immunoreactivity showed a distinct pattern of induction in the hippocampus after injection of N-methyl-D-aspartic acid. Ethanol (3.0 g/kg) inhibited N-methyl-D-aspartic acid-induced c-JUN-immunoreactivity in the hippocampus and cerebral cortex. The differences in inhibition of immunoreactivity suggest that the sensitivity of the NMDA- and GABAA-related neuronal pathways to ethanol varies among different anatomical structures.