Alterations of rat corticostriatal synaptic plasticity after chronic ethanol exposure and withdrawal

BACKGROUND: The purpose of this study was to investigate the effects of chronic ethanol exposure (CEE) and withdrawal on corticostriatal plasticity in rats. METHODS: We established an animal model of alcoholism using the method of Turchan et al. (1999). A synaptic model of long-term memory (long-term depression, LTD) was used as an index and the striatum, which is related to habit learning, was selected as a target region in the present study. The effects of CEE and withdrawal on the LTD were studied in striatal slices of ethanol-dependent rats using the extracellular recording method. RESULTS: A stable LTD can be induced after high-frequency stimulation (HFS) in the slices of control rats. Chronic ethanol exposure and withdrawal suppressed the induction of corticostriatal LTD to different extents, with the strongest suppressive effects on LTD occurring in the slices of rats exposed to ethanol for 10 days and in those withdrawn from ethanol for 1 day. Notably, 3 days of withdrawal resulted in the shift of corticostriatal synaptic plasticity from LTD to long-term potentiation, and the peak latencies of the population spikes were obviously shortened compared with those of control rats. After 7 days of withdrawal, ethanol's effects tended to disappear. CONCLUSIONS: These results suggest that the alterations of corticostriatal synaptic plasticity produced by CEE and withdrawal may play a prominent role in alcohol abuse and alcoholism.     

Age-Dependent Inhibition of Long-Term Potentiation by Ethanol in Immature Versus Mature Hippocampus

The goal of this study was to assess the effects of ethanol on the induction of long-term potentiation (LTP) in hippocampal slices from immature versus mature rats. Population excitatory postsynaptic potentials (pEPSPs) were recorded from stratum radiatum of area CA1 of hippocampal slices using electrical stimulation of the Schaffer collateral/commissural fiber pathway. The slices were prepared from rats aged 15 to 25 or from 70 to 100 days. Long-term potentiation (LTP) of the pEPSP slope was induced using a single, θ-burst stimulus train in the presence or absence of 60 mM ethanol. Under control conditions, the stimulus train induced LTP in slices from both immature and mature animals. However, the magnitude of LTP was greater in slices from immature rats. When ethanol was present during the stimulus train, the magnitude of LTP in slices from mature animals did not differ significantly from the magnitude of LTP in control slices. However, ethanol virtually blocked the induction of LTP in slices from immature animals. These results indicate that memory-related synaptic plasticity in the hippocampus is attenuated by ethanol to a greater degree in immature versus mature animals.     

ERK activation in axonal varicosities modulates presynaptic plasticity in the CA3 region of the hippocampus through synapsin I

Activity-dependent changes in the strength of synaptic connections in the hippocampus are central for cognitive processes such as learning and memory storage. In this study, we reveal an activity-dependent presynaptic mechanism that is related to the modulation of synaptic plasticity. In acute mouse hippocampal slices, high-frequency stimulation (HFS) of the mossy fiber (MF)-CA3 pathway induced a strong and transient activation of extracellular-regulated kinase (ERK) in MF giant presynaptic terminals. Remarkably, pharmacological blockade of ERK disclosed a negative role of this kinase in the regulation of a presynaptic form of plasticity at MF-CA3 contacts. This ERK-mediated inhibition of post-tetanic enhancement (PTE) of MF-CA3 synapses was both frequency- and pathway-specific and was observed only with HFS at 50 Hz. Importantly, blockade of ERK was virtually ineffective on PTE of MF-CA3 synapses in mice lacking synapsin I, 1 of the major presynaptic ERK substrates, and triple knockout mice lacking all synapsin isoforms displayed PTE kinetics resembling that of wild-type mice under ERK inhibition. These findings reveal a form of short-term synaptic plasticity that depends on ERK and is finely tuned by the firing frequency of presynaptic neurons. Our results also demonstrate that presynaptic activation of the ERK signaling pathway plays part in the activity-dependent modulation of synaptic vesicle mobilization and transmitter release.     

The brain-derived neurotrophic factor enhances synthesis of Arc in synaptoneurosomes

Protein synthesis in neurons is essential for the consolidation of memory and for the stabilization of activity-dependent forms of synaptic plasticity such as long-term potentiation (LTP). Activity-dependent translation of dendritically localized mRNAs has been proposed to be a critical source of new proteins necessary for synaptic change. mRNA for the activity-regulated cytoskeletal protein, Arc, is transcribed during LTP and learning, and disruption of its translation gives rise to deficits in both. We have found that selective translation of Arc in a synaptoneurosomal preparation is induced by the brain-derived neurotrophic factor, a neurotrophin that is released during high-frequency stimulation patterns used to elicit LTP. This effect involves signaling through the TrkB receptor and is blocked by the N-methyl-d-aspartate-type glutamate receptor antagonist, MK801. The results suggest there is a synergy between neurotrophic and ionotropic mechanisms that may influence the specificity and duration of changes in synaptic efficacy at glutamatergic synapses.     

Ethanol exposure in early adolescence inhibits intrinsic neuronal plasticity via sigma-1 receptor activation in hippocampal CA1 neurons

Background: We demonstrated previously that rats exposed to chronic intermittent ethanol (CIE) vapors in early adolescence show increased magnitudes of long‐term potentiation (LTP) of excitatory transmission when recorded at dendritic synapses in hippocampus. Large amplitude LTP following CIE exposure is mediated by sigma‐1 receptors; however, not yet addressed is the role of sigma‐1 receptors in modulating the intrinsic properties of neurons to alter their action potential firing during LTP. Methods: Activity‐induced plasticity of spike firing was investigated using rat hippocampal slice recordings to measure changes in both field excitatory postsynaptic potentials (fEPSPs) and population spikes (pop. spikes) concomitantly at dendritic inputs and soma of CA1 pyramidal neurons, respectively. Results: We observed unique modifications in plasticity of action potential firing in hippocampal slices from CIE exposed adolescent rats, where the induction of large amplitude LTP by 100 Hz stimulations was accompanied by reduced CA1 neuronal excitability––reflected as decreased pop. spike efficacy and impaired activity‐induced fEPSP‐to‐spike (E‐S) potentiation. In contrast, LTP induction in ethanol‐naïve control slices resulted in increased spike efficacy and robust E‐S potentiation. E‐S potentiation impairments emerged at 24 hours after CIE treatment cessation, but not before the alcohol withdrawal period, and were restored with bath‐application of the sigma‐1 receptor selective antagonist BD1047, but not the NMDA receptor antagonist d ‐AP5. Further evidence revealed a significantly shortened somatic fEPSP time course in adolescent CIE‐withdrawn hippocampal slices during LTP; however, paired‐pulse data show no apparent correspondence between E‐S dissociation and altered recurrent feedback inhibition. Conclusions: Results here suggest that acute withdrawal from adolescent CIE exposure triggers sigma‐1 receptors that act to depress the efficacy of excitatory inputs in triggering action potentials during LTP. Such withdrawal‐induced depression of E‐S plasticity in hippocampus probably entails sigma‐1 receptor modulation of 1 or several voltage‐gated ion channels controlling the neuronal input–output dynamics.     

Regulation of a protein phosphatase cascade allows convergent dopamine and glutamate signals to activate ERK in the striatum

Many drugs of abuse exert their addictive effects by increasing extracellular dopamine in the nucleus accumbens, where they likely alter the plasticity of corticostriatal glutamatergic transmission. This mechanism implies key molecular alterations in neurons in which both dopamine and glutamate inputs are activated. Extracellular signal-regulated kinase (ERK), an enzyme important for long-term synaptic plasticity, is a good candidate for playing such a role. Here, we show in mouse that d-amphetamine activates ERK in a subset of medium-size spiny neurons of the dorsal striatum and nucleus accumbens, through the combined action of glutamate NMDA and D1-dopamine receptors. Activation of ERK by d-amphetamine or by widely abused drugs, including cocaine, nicotine, morphine, and Delta(9)-tetrahydrocannabinol was absent in mice lacking dopamine- and cAMP-regulated phosphoprotein of M(r) 32,000 (DARPP-32). The effects of d-amphetamine or cocaine on ERK activation in the striatum, but not in the prefrontal cortex, were prevented by point mutation of Thr-34, a DARPP-32 residue specifically involved in protein phosphatase-1 inhibition. Regulation by DARPP-32 occurred both upstream of ERK and at the level of striatal-enriched tyrosine phosphatase (STEP). Blockade of the ERK pathway or mutation of DARPP-32 altered locomotor sensitization induced by a single injection of psychostimulants, demonstrating the functional relevance of this regulation. Thus, activation of ERK, by a multilevel protein phosphatase-controlled mechanism, functions as a detector of coincidence of dopamine and glutamate signals converging on medium-size striatal neurons and is critical for long-lasting effects of drugs of abuse.     

Ethanol Acutely Inhibits Ionotropic Glutamate Receptor‐Mediated Responses and Long‐Term Potentiation in the Developing CA1 Hippocampus

Background: Developmental ethanol (EtOH) exposure damages the hippocampus, causing long‐lasting alterations in learning and memory. Alterations in glutamatergic synaptic transmission and plasticity may play a role in the mechanism of action of EtOH. This signaling is fundamental for synaptogenesis, which occurs during the third trimester of human pregnancy (first 12 days of life in rats). Methods: Acute coronal brain slices were prepared from 7‐ to 9‐day‐old rats. Extracellular and patch‐clamp electrophysiological recording techniques were used to characterize the acute effects of EtOH on α‐amino‐3‐hydroxyl‐5‐methyl‐4‐isoxazole‐propionate receptor (AMPAR)‐ and N‐methyl‐d ‐aspartate receptor (NMDAR)‐mediated responses and long‐term potentiation (LTP) in the CA1 hippocampal region. Results: Ethanol (40 and 80 mM) inhibited AMPAR‐ and NMDAR‐mediated field excitatory postsynaptic potentials (fEPSPs). EtOH (80 mM) also reduced AMPAR‐mediated fEPSPs in the presence of an inhibitor of Ca²⁺ permeable AMPARs. The effect of 80 mM EtOH on NMDAR‐mediated fEPSPs was significantly greater in the presence of Mg²⁺. EtOH (80 mM) neither affected the paired‐pulse ratio of AMPAR‐mediated fEPSPs nor the presynaptic volley. The paired‐pulse ratio of AMPAR‐mediated excitatory postsynaptic currents was not affected either, and the amplitude of these currents was inhibited to a lesser extent than that of fEPSPs. EtOH (80 mM) inhibited LTP of AMPAR‐mediated fEPSPs. Conclusions: Acute EtOH exposure during the third‐trimester equivalent of human pregnancy inhibits hippocampal glutamatergic transmission and LTP induction, which could alter synapse refinement and ultimately contribute to the pathophysiology of fetal alcohol spectrum disorder.     

Locomotor activity induced by noncompetitive NMDA receptor antagonists versus dopamine transporter inhibitors: opposite strain differences in inbred long-sleep and short-sleep mice

Background: The actions of ethanol in the brain involve multiple neuroreceptor systems, including glutamatergic N‐methyl‐D‐aspartate receptor (NMDAR) channels. In a novel environment, both ethanol and the noncompetitive NMDAR antagonist MK‐801 stimulate locomotor activity to a lesser extent in inbred long‐sleep (ILS) mice compared with inbred short‐sleep (ISS) mice. The behaviorally activating effects of noncompetitive NMDAR antagonists are thought to involve increased monoamine neurotransmission. Thus, in this study, we sought to determine whether: (1) habituation to the behavioral environment alters the differential locomotor‐stimulant effects of noncompetitive NMDAR antagonists in ILS and ISS mice and (2) the differential behavioral sensitivity of ILS and ISS mice to noncompetitive NMDAR antagonists is mediated through direct inhibition of the dopamine transporter (DAT). Methods: Open field locomotor activity was measured following acute systemic injection of saline or drug. [³H]DA uptake parameters were determined in striatal synaptosomes prepared from drug‐naïve mice. Results: Habituation to the testing environment abolished the strain differences in saline‐induced locomotor activity. However, ethanol‐ as well as MK‐801‐treated ILS mice still exhibited reduced locomotor activity compared with ISS mice, suggesting that a drug‐environment interaction is not the primary explanation for the strain differences. The noncompetitive NMDAR antagonists phencyclidine and ketamine also induced significantly lower locomotor activity in ILS than in ISS mice. In contrast, the DAT inhibitors cocaine and GBR 12909 and the DA releaser amphetamine induced greater locomotor activity in ILS than in ISS mice, a strain difference opposite that of the noncompetitive NMDAR antagonists. Furthermore, the differential behavioral effect found with DAT inhibitors was not mediated by differences in the affinity nor number of striatal DATs between ILS and ISS mice. Conclusions: Our results support the conclusion that the differential locomotor‐stimulant effects of ethanol and noncompetitive NMDAR antagonists in ILS and ISS mice are not mediated through direct inhibition of DAT.     

Absence of metabotropic glutamate receptor-mediated plasticity in the neocortex of fragile X mice

Fragile X syndrome is a common heritable form of mental retardation in humans. Recent neuroanatomical studies indicate an apparent immature appearance of neurons in fragile X syndrome patients and fragile X mental retardation protein (FMRP)-knockout mice, an animal model of this condition. In this work, we investigated possible alterations in synaptic plasticity in the neocortex of FMRP-knockout mice. Extracellular field potentials were recorded from the deep-layer visual neocortex. Long-term potentiation (LTP) was severely attenuated in brain slices from knockout mice relative to that observed in slices from wild-type mice. Considering that neocortical LTP can involve both NMDA receptor-dependent and -independent mechanisms, we attempted to distinguish the nature of LTP attenuated in the knockout condition. In slices from wild-type mice, LTP was partially attenuated by the NMDA receptor antagonist 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphate (CPP); however, the general metabotropic glutamate receptor (mGluR) antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) strongly attenuated LTP, resulting in a response indistinguishable from that observed in slices from knockout mice. The selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) attenuated LTP to a similar degree as did MCPG in wild-type slices, but MPEP did not alter the reduced potentiation in knockout slices. Our results suggest that LTP in layer V visual neocortex depends primarily on mGluR5 activation. Our data also indicate that mGluR5-mediated synaptic plasticity is absent in the neocortex of FMRP-knockout mice. Such an alteration may contribute to the cognitive and learning deficits exhibited in these mice as well as in fragile X syndrome.     

Region-specific restoration of striatal synaptic plasticity by dopamine grafts in experimental parkinsonism

Intrastriatal transplantation of dopaminergic neurons can restore striatal dopamine levels and improve parkinsonian deficits, but the mechanisms underlying these effects are poorly understood. Here, we show that transplants of dopamine neurons partially restore activity-dependent synaptic plasticity in the host striatal neurons. We evaluated synaptic plasticity in regions distal or proximal to the transplant (i.e., dorsolateral and ventrolateral striatum) and compared the effects of dopamine- and serotonin-enriched grafts using a rat model of Parkinson disease. Naïve rats showed comparable intrinsic membrane properties in the two subregions but distinct patterns of long-term synaptic plasticity. The ventrolateral striatum showed long-term potentiation using the same protocol that elicited long-term depression in the dorsolateral striatum. The long-term potentiation was linked to higher expression of postsynaptic AMPA and N2B NMDA subunits (GluN2B) and was dependent on the activation of GluN2A and GluN2B subunits and the D1 dopamine receptor. In both regions, the synaptic plasticity was abolished after a severe dopamine depletion and could not be restored by grafted serotonergic neurons. Solely, dopamine-enriched grafts could restore the long-term potentiation and partially restore motor deficits in the rats. The restoration could only be seen close to the graft, in the ventrolateral striatum where the graft-derived reinnervation was denser, compared with the distal dorsolateral region. These data provide proof of concept that dopamine-enriched transplants are able to functionally integrate into the host brain and restore deficits in striatal synaptic plasticity after experimental parkinsonism. The region-specific restoration might impose limitations in symptomatic improvement following neural transplantation.Nonpharmacological dopamine (DA) replacement approaches to the therapy of Parkinson disease (PD) focus on the transplantation of DA-producing neurons into the striatum. Parkinson disease is indeed viewed as the disease of choice to develop intracerebral transplantation therapies, and promising results have been obtained both in experimental models and in some patients using embryonic DA neurons (1, 2). Embryonic DA neurons are able to innervate the host striatum, release DA, and reverse alterations in neuropeptide expression after a parkinsonian lesion (3). There is a continuous debate about whether these effects are sufficient for transplanted neurons to partially restore clinical symptoms or whether other underlying mechanisms also are required. In particular, a functional integration of the graft into the host microcircuits, with bidirectional synaptic contacts between the host and grafted neurons, may give superior therapeutic benefit than a mere neurochemical restoration. Transplanted DA neurons are able to form synapses with the surrounding striatal medium-sized spiny neurons (MSNs) (4) and receive innervation from the host neurons with bidirectional synaptic interactions (5–7). It is, however, unknown whether these plastic changes are sufficient to restore the basic functional properties of the host neurons essential for corticostriatal control of movements (8). This study attempts to understand whether neural transplants have the ability to restore activity-dependent synaptic plasticity in the host corticostriatal pathway.We have herein investigated corticostriatal plasticity after transplantation of DA and 5-HT neurons in host MSNs in an experimental model of PD. Dopamine is critical for inducing long-term striatal plasticity, i.e., long-term potentiation (LTP) and long-term depression (LTD) in MSNs (9). The changes are mediated by the activation of ionotropic glutamate receptors, i.e., alpha-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-d-aspartate (NMDA) receptors, as well as by the activation of DA receptors. Consequently, animal models of severe DA denervation have demonstrated a loss of both forms of corticostriatal plasticity in the dorsolateral (DL) striatum (10, 11). A partial DA denervation, on the other hand, spares LTD in the DL striatum (12). Also clinical studies have revealed an impairment of synaptic plasticity in the corticostriatal pathway (13).Using an experimental model of PD, we here demonstrate that transplanted DA neurons are efficient in restoring corticostriatal plasticity in the densely innervated area close to the graft whereas more sparsely innervated areas remain unaffected. This restoration is in contrast to the effect of transplanted 5-HT neurons that were unable to restore any type of plasticity.     

Surface protein phosphorylation by ecto-protein kinase is required for the maintenance of hippocampal long-term potentiation.

During the induction of long-term potentiation (LTP) in hippocampal slices adenosine triphosphate (ATP) is secreted into the synaptic cleft, and a 48 kDa/50 kDa protein duplex becomes phosphorylated by extracellular ATP. All the criteria required as evidence that these two proteins serve as principal substrates of ecto-protein kinase activity on the surface of hippocampal pyramidal neurons have been fulfilled. This phosphorylation activity was detected on the surface of pyramidal neurons assayed after synaptogenesis, but not in immature neurons nor in glial cells. Addition to the extracellular medium of a monoclonal antibody termed mAb 1.9, directed to the catalytic domain of protein kinase C (PKC), inhibited selectively this surface protein phosphorylation activity and blocked the stabilization of LTP induced by high frequency stimulation (HFS) in hippocampal slices. This antibody did not interfere with routine synaptic transmission nor prevent the initial enhancement of synaptic responses observed during the 1-5 min period immediately after the application of HFS (the induction phase of LTP). However, the initial increase in the slope of excitatory postsynaptic potentials, as well as the elevated amplitude of the population spike induced by HFS, both declined gradually and returned to prestimulus values within 30-40 min after HFS was applied in the presence of mAb 1.9. A control antibody that binds to PKC but does not inhibit its activity had no effect on LTP. The selective inhibitory effects observed with mAb 1.9 provide the first direct evidence of a causal role for ecto-PK in the maintenance of stable LTP, an event implicated in the process of learning and the formation of memory in the brain.     

Role of major NMDA or AMPA receptor subunits in MK-801 potentiation of ethanol intoxication

Background: The glutamate system plays a major role in mediating EtOH’s effects on brain and behavior, and is implicated in the pathophysiology of alcohol‐related disorders. N‐methyl‐D‐aspartate receptor (NMDAR) antagonists such as MK‐801 (dizocilpine) interact with EtOH at the behavioral level, but the molecular basis of this interaction is unclear. Methods: We first characterized the effects of MK‐801 treatment on responses to the ataxic (accelerating rotarod), hypothermic and sedative/hypnotic effects of acute EtOH administration in C57BL/6J and 129/SvImJ inbred mice. Effects of another NMDAR antagonist, phencyclidine, on EtOH‐induced sedation/hypnosis were also assessed. Gene knockout of the NMDAR subunit NR2A or l ‐alpha‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate GluR1 or pharmacological antagonism of the NMDAR subunit NR2B (via Ro 25‐6981) was employed to examine whether inactivating any one of these glutamate signaling molecules modified MK‐801’s effect on EtOH‐related behaviors. Results: MK‐801 markedly potentiated the ataxic effects of 1.75 g/kg EtOH and the sedative/hypnotic effects of 3.0 g/kg EtOH, but not the hypothermic effects of 3.0 g/kg EtOH, in C57BL/6J and 129/SvImJ mice. Phencyclidine potentiated EtOH‐induced sedation/hypnosis in both inbred strains. Neither NR2A nor GluR1 KO significantly altered basal EtOH‐induced ataxia, hypothermia, or sedation/hypnosis. Ro 25‐6981 modestly increased EtOH‐induced sedation/hypnosis. The ability of MK‐801 to potentiate EtOH‐induced ataxia and sedation/hypnosis was unaffected by GluR1 KO or NR2B antagonism. NR2A KO partially reduced MK‐801 + EtOH‐induced sedation/hypnosis, but not ataxia or hypothermia. Conclusions: Data confirm a robust and response‐specific potentiating effect of MK‐801 on sensitivity to EtOH’s intoxicating effects. Inactivation of three major components of glutamate signaling had no or only partial impact on the ability of MK‐801 to potentiate behavioral sensitivity to EtOH. Further work to elucidate the mechanisms underlying NMDAR × EtOH interactions could ultimately provide novel insight into the role of NMDARs in alcoholism and its treatment.     

N-methyl-D-aspartate receptor responses are differentially modulated by noncompetitive receptor antagonists and ethanol in inbred long-sleep and short-sleep mice: behavior and electrophysiology

BACKGROUND: Short-sleep (SS) mice exhibit higher locomotor activity than do long-sleep (LS) mice when injected with low doses of ethanol or the noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist MK-801 (dizocilpine). SS mice also have higher densities of brain NMDARs. However, two strains of LS X SS recombinant inbred (RI) mice also show differential activation to ethanol and MK-801, but have similar numbers of NMDARs. Here we used inbred LS (ILS) and SS (ISS) mice to investigate further the relationship between NMDARs and sensitivity to the stimulant effects of low doses of ethanol. METHODS: Open field activity and spontaneous alternations were measured after saline or drug injection. [3H]MK-801 binding parameters were determined in hippocampus, cortex, striatum, and nucleus accumbens. Extracellular field excitatory postsynaptic potentials (fEPSPs) were recorded in the CA1 region of hippocampal slices. RESULTS: Systemic injection of either ethanol or MK-801 increased locomotor activity to a greater extent in ISS mice than in ILS mice. The competitive NMDAR antagonist 2-carboxypiperazin-4-yl-propyl-1-lphosphonic acid (+/- CPP) depressed activity of ILS, but not ISS, mice. No strain differences were observed in spontaneous alternations or in the number or affinity of NMDARs in the brain regions examined. Likewise, the magnitudes of hippocampal NMDAR-mediated fEPSPs were similar in ILS and ISS mice and were inhibited to the same extent by a competitive NMDAR antagonist. However, both ethanol and the NMDAR NR2B receptor antagonist ifenprodil inhibited the late component of hippocampal NMDAR fEPSPs to a greater extent in ISS, than in ILS, mice. CONCLUSIONS: Differential ethanol- and MK-801-induced behavioral activation in ILS and ISS mice was not associated with differences in NMDAR number. Nonetheless, pharmacological differences in hippocampal NMDAR responsiveness suggest that ISS mice express NMDARs that have a greater sensitivity to noncompetitive, but not competitive, NMDAR antagonists. These differences, which may reflect differences in NMDAR subunit composition, could underlie the differential responsiveness to low doses of ethanol in ILS and ISS mice.     

Exogenous lactate sustains synaptic activity and neuronal viability, but fails to induce long-term potentiation (LTP)

In Alzheimer's disease, brain glucose metabolic ratio decreases, whereas the brain lactate metabolic ratio increases. To investigate possible synaptic dysfunction in Alzheimer's disease, we examined the effects of exogenous glucose deprivation and replacement of glucose with lactate on the synaptic transmission, synaptic plasticity, and the morphological integrity of hippocampal neurons. Synaptic activity was estimated by the amplitude of the population spike (PS) recorded in the granular cell layer in the hippocampal slices from guinea pig and rat. Exogenous glucose deprivation caused the immediate depression of PS. Replacement of glucose with lactate induced transient decrease of PS, followed by spontaneous recovery of synaptic transmission. Neural activity recovered from transient glucose deprivation became resistant to the replacement of glucose with exogenous lactate. Glucose-supported synaptic transmission exhibited approximately 140% enhancement of PS (LTP). However, lactate-supported synaptic activity yielded approximately 110% potentiation of PS. Effects of exogenous glucose and lactate on the cell viability were examined by the propidum iodide uptake and LDH release in the organotypic hippocampal slice cultures. Hippocampal slice cultures incubated in medium containing 10 mM lactate suppressed the cell death during 48 h observation as well as those in the 10-30 mM glucose containing medium. These results indicate that lactate can sustain the neural transmission and support the morphological integrity of hippocampal neurons, but failed to induce LTP, which could at least in part, cause the memory impairment in Alzheimer's disease.     

Striatal involvement in human alcoholism and alcohol consumption, and withdrawal in animal models

Background: Different regions of the striatum may have distinct roles in acute intoxication, alcohol seeking, dependence, and withdrawal. Methods: The recent advances are reviewed and discussed in our understanding of the role of the dorsolateral striatum (DLS), dorsomedial striatum (DMS), and ventral striatum in behavioral responses to alcohol, including alcohol craving in abstinent alcoholics, and alcohol consumption and withdrawal in rat, mouse, and nonhuman primate models. Results: Reduced neuronal activity as well as dysfunctional connectivity between the ventral striatum and the dorsolateral prefrontal cortex is associated with alcohol craving and impairment of new learning processes in abstinent alcoholics. Within the DLS of mice and nonhuman primates withdrawn from alcohol after chronic exposure, glutamatergic transmission in striatal projection neurons is increased, while GABAergic transmission is decreased. Glutamatergic transmission in DMS projection neurons is also increased in ethanol withdrawn rats. Ex vivo or in vivo ethanol exposure and withdrawal causes a long‐lasting increase in NR2B subunit‐containing NMDA receptor activity in the DMS, contributing to ethanol drinking. Analyses of neuronal activation associated with alcohol withdrawal and site‐directed lesions in mice implicate the rostroventral caudate putamen, a ventrolateral segment of the DMS, in genetically determined differences in risk for alcohol withdrawal involved in physical association of the multi‐PDZ domain protein, MPDZ, with 5‐HT_2C receptors and/or NR2B. Conclusions: Alterations of dopaminergic, glutamatergic, and GABAergic signaling within different regions of the striatum by alcohol is critical for alcohol craving, consumption, dependence, and withdrawal in humans and animal models.     

Alcohol withdrawal-induced hippocampal neurotoxicity in vitro and seizures in vivo are both reduced by memantine

Background: The ethanol withdrawal (EWD) syndrome is typically treated using benzodiazepines such as diazepam. However there is concern that benzodiazepines may not prevent neurotoxicity associated with EWD. Antagonists of glutamate/N‐Methyl‐D‐Aspartate receptors (NMDARs) such as MK801 have been shown to be effective against both EWD‐induced neurotoxicity in vitro and seizures in vivo. However, most of these agents have adverse side effects. An exception is the moderate affinity NMDAR channel blocker memantine, used in Alzheimer’s dementia. The present studies examined the ability of memantine to protect against EWD‐related toxicity in vitro and seizures in vivo. Methods: Organotypic hippocampal slice cultures from neonatal rat pups were treated starting at 15 days in vitro with 100 mM ethanol for 10 days followed by a 24‐hour EWD period. During the 24‐hour EWD period cultures were treated with memantine (15 or 30 μM). MK801 (10 μM) was utilized as a positive control. For the in vivo studies, the ability of memantine (2, 5, 10, and 15 mg/kg) to reduce convulsions was analyzed in Swiss‐Webster mice using the handling induced convulsion test paradigm. Results: In vitro studies demonstrated that memantine is effective at blocking EWD‐induced neurotoxicity. In vivo experiments showed that memantine also significantly reduced convulsions induced by EWD in mice. Conclusions: Memantine may be of therapeutic value during alcohol detoxification by virtue of its having neuroprotective effects in addition to anti‐seizure activity. The potential role of memantine in treatment of alcoholism is deserving of further study.     

The preventive effect of NR2B and NR2D-containing NMDAR antagonists on Aβ-induced LTP disruption in the dentate gyrus of rats

Amyloid β-protein (Aβ) in the brain of Alzheimer’s disease (AD) potently inhibits the synaptic plasticity subsequently causing the cognitive deficits. Long-term potentiation (LTP) of synaptic transmission is thought to be an important cellular mechanism underlying memory formation. Different NR2 subunits are involved in NMDA receptor-dependent LTP. In the present study, we investigated the roles of NR2B and NR2D-containing NMDAR on Aβ(1–42)-induced LTP deficits in the hippocampal slices of rats by using selective NMDAR antagonists. First, we found that Aβ(1–42) significantly inhibited the LTP in the dentate gyrus of slices as reported before. Following that the Aβ(1–42)-induced LTP inhibition was prevented by the pre-perfusion of the specific NR2B-containing NMDAR antagonists ifenprodil (approximately >200-fold selectivity for NR2B) and Ro25-6981 (>3,000-fold selectivity for NR2B), as well as PPDA, a specific NR2D receptor antagonist. Meanwhile, the antagonists on their own had no or only partial effects on the normal LTP in the same dose condition. These findings not only support the effects of NR2B and NR2D subunits on Aβ(1–42)-induced LTP deficits, but also imply that preferentially targeting NR2B- and NR2D-containing NMDARs may provide an effective means to prevent cognitive deficits in the early AD.     

GluN2B subunit deletion reveals key role in acute and chronic ethanol sensitivity of glutamate synapses in bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) is a critical region for alcohol/drug-induced negative affect and stress-induced reinstatement. NMDA receptor (NMDAR)-dependent plasticity, such as long-term potentiation (LTP), has been postulated to play key roles in alcohol and drug addiction; yet, to date, little is understood regarding the mechanisms underlying LTP of the BNST, or its regulation by ethanol. Acute and chronic exposure to ethanol modulates glutamate transmission via actions on NMDARs. Despite intense investigation, tests of subunit specificity of ethanol actions on NMDARs using pharmacological approaches have produced mixed results. Thus, we use a conditional GluN2B KO mouse line to assess both basal and ethanol-dependent function of this subunit at glutamate synapses in the BNST. Deletion of GluN2B eliminated LTP, as well as actions of ethanol on NMDAR function. Further, we show that chronic ethanol exposure enhances LTP formation in the BNST. Using KO-validated pharmacological approaches with Ro25-6981 and memantine, we provide evidence suggesting that chronic ethanol exposure enhances LTP in the BNST via paradoxical extrasynaptic NMDAR involvement. These findings demonstrate that GluN2B is a key point of regulation for ethanol's actions and suggest a unique role of extrasynaptic GluN2B-containing receptors in facilitating LTP.     

Morphine induces c-fos and junB in striatum and nucleus accumbens via D1 and N-methyl-D-aspartate receptors.

Morphine induced the c-fos and junB immediate early genes in neurons of the medial and ventral striatum and nucleus accumbens. Induction of c-fos and junB mRNA and Fos protein was blocked by naloxone, the D1 dopamine (DA) receptor antagonists SCH23390 and SCH39166, and the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK801. SCH23390 attenuated morphine induction of AP-1 binding in striatum, suggesting that c-fos and junB contribute to AP-1 binding. SCH23390 and MK801 did not block morphine induction of c-fos and junB in septum. Since the morphine induction of c-fos and junB in striatum and nucleus accumbens (NA) was similar to that observed with cocaine and amphetamine, these data support current concepts that limbic striatum and NA are among the brain regions that mediate drug abuse. Furthermore, since DA and NMDA receptors may mediate opiate reward and opiate induction of c-fos and junB, the DA/NMDA regulation of c-fos and junB and their target genes may produce long-term changes in the striatal and NA circuits that contribute to opiate drug abuse.