Chronic blockade of N-methyl-D-aspartate receptors alters gamma-aminobutyric acid type A receptor peptide expression and function in the rat

Differential expression of miR-148b, miR-129-2 and miR-296 in animal models of schizophrenia-Relevance to NMDA receptor hypofunction

Hypofunction of N-methyl-d-aspartate receptors (NMDAR) is a key component in the pathophysiology of schizophrenia. Alterations in the regulation of NMDARs by microRNAs (miRNAs) are possible since numerous miRNAs are differentially expressed in post mortem schizophrenia brain samples. We screened the miRNAs that are altered in schizophrenia against the targets, Grin2A and Grin2B subunits of NMDAR using bioinformatic tools. Among the predicted miRNAs some interacted with the 3'-UTR sequences of Grin2A (miR-296, miR-148b, miR-129-2, miR-137) and Grin2B (miR-296, miR-148b, miR-129-2, miR-223) in dual luciferase assays. This was supported by downregulation of the GluN2B protein in primary hippocampal neurons upon overexpressing Grin2B targeting miRNAs. In two models of schizophrenia-pharmacological MK-801 model and neurodevelopmental methylazoxymethanol acetate (MAM) model which showed cognitive deficits - protein levels of GluN2A and GluN2B were downregulated but their transcript levels were upregulated. miR-296-3p, miR-148b-5p and miR-137 levels showed upregulation in both models which could have interacted with Grin2A/Grin2B transcripts resulting in translational arrest. In MAM model, reciprocal changes in the expression of the 3p and 5p forms of miR-148b and miR-137 were observed. Expression of some genes implicated in schizophrenia such as Neuregulin 1 (NRG1), BDNF and CaMKIIα, were also altered in these models. This is the first report showing downregulation of GluN2A and GluN2B by miR-296, miR-148b and miR-129-2 in vitro and association between them in animal models. Mining miRNAs regulating NMDA receptors might give insights into the pathophysiology of this disorder, providing avenues in therapeutics.

The positive allosteric modulation of GABAA receptors mRNA in immature hippocampal rat neurons by midazolam affects receptor expression and induces apoptosis

Background: Numerous experimental studies show that anesthetics are potentially toxic to the immature brain. Even though benzodiazepines are widely used in pediatric anesthesia and intensive care medicine, only a few studies examine the effects of these drugs on immature neurons. Methods: Hippocampal neuronal cell cultures of embryonic Wistar rats (15 days in culture) were incubated with midazolam 100 or 300 nM for either 30 min or 4 h. The time course of the mRNA expression of the glutamate receptors subunits NR1, NR2A and NR2B of the NMDA receptor, the GluA-1 and A-2 subunits of the AMPA receptor as well as the alpha 1 subunit of the GABA_A receptor were examined by PCR. Apoptosis was detected using Western blot analysis for BAX, Bcl-2 and Caspase-3. Results: Midazolam at 100 and 300 nM applied for 30 min and 100 nM for 4 h affected glutamate receptor and GABA_A receptor subunit expression. However, these effects were reversible within 72 h following washout. When 300 nM midazolam was applied for 4 h a significant increase in the NR 1 and NR 2A mRNA subunit expression could be detected. The increase in NR 2B receptor subunit expression as well as the GluA1 subunit expression was not reversible within 72 h following washout. This increase in mRNA glutamate receptor subunit expression was associated with a significant increase in neuronal apoptosis. Conclusion: In immature neurons midazolam altered GABA and glutamate mRNA receptor subunit expression. Prolonged increase in midazolam-induced glutamate receptor expression was associated with apoptosis.

A rational approach to the treatment of alcohol withdrawal in the ED

Approximately 7% of the US population abuses or is dependent on alcohol. Patients with alcohol disorders often seek medical attention in Emergency Departments (EDs) for complications directly related to alcohol use or due to other medical issues associated with alcohol use. Because of increasing lengths of stay in EDs, alcohol-dependent patients are at high risk of developing alcohol withdrawal syndrome (AWS) during their ED visit. This article reviews the physiology of alcohol withdrawal as well as the symptoms of this potentially deadly illness for the practicing emergency physician (EP). We provide evidence-based guidelines for the appropriate ED treatment of moderate to severe AWS, including pharmacologic interventions, adjunctive therapies, and disposition of these patients.

Increased Kv1 channel expression may contribute to decreased sIPSC frequency following chronic inhibition of NR2B-containing NMDAR

Numerous studies have documented the effects of chronic N-methyl-D-aspartate receptor (NMDAR) blockade on excitatory circuits, but the effects on inhibitory circuitry are not well studied. NR2A- and NR2B-containing NMDARs play differential roles in physiological processes, but the consequences of chronic NR2A- or NR2B-containing NMDAR inhibition on glutamatergic and GABAergic neurotransmission are unknown. We investigated altered GABAergic neurotransmission in dentate granule cells and interneurons following chronic treatment with the NR2B-selective antagonist, Ro25,6981, the NR2A-prefering antagonist, NVP-AAM077, or the non-subunit-selective NMDAR antagonist, D-APV, in organotypic hippocampal slice cultures. Electrophysiological recordings revealed large reductions in spontaneous inhibitory postsynaptic current (sIPSC) frequency in both granule cells and interneurons following chronic Ro25,6981 treatment, which was associated with minimally altered sIPSC amplitude, miniature inhibitory postsynaptic current (mIPSC) frequency, and mIPSC amplitude, suggesting diminished action potential-dependent GABA release. Chronic NVP-AAM077 or D-APV treatment had little effect on these measures. Reduced sIPSC frequency did not arise from downregulated GABA(A)R, altered excitatory or inhibitory drive to interneurons, altered interneuron membrane properties, increased failure rate, decreased action potential-dependent release probability, or mGluR/GABA(B) receptor modulation of GABA release. However, chronic Ro25,6981-mediated reductions in sIPSC frequency were occluded by the K+ channel blockers, dendrotoxin, margatoxin, and agitoxin, but not dendrotoxin-K or XE991. Immunohistochemistry also showed increased Kv1.2, Kv1.3, and Kv1.6 in the dentate molecular layer following chronic Ro25,6981 treatment. Our findings suggest that increased Kv1 channel expression/function contributed to diminished action potential-dependent GABA release following chronic NR2B-containing NMDAR inhibition and that these Kv1 channels may be heteromeric complexes containing Kv1.2, Kv1.3, and Kv1.6.

Prolonged exposure to NMDAR antagonist induces cell-type specific changes of glutamatergic receptors in rat prefrontal cortex

N-methyl-d-aspartic acid (NMDA) receptors are critical for both normal brain functions and the pathogenesis of schizophrenia. We investigated the functional changes of glutamatergic receptors in the pyramidal cells and fast-spiking (FS) interneurons in the adolescent rat prefrontal cortex in MK-801 model of schizophrenia. We found that although both pyramidal cells and FS interneurons were affected by in vivo subchronic blockade of NMDA receptors, MK-801 induced distinct changes in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and NMDA receptors in the FS interneurons compared with pyramidal cells. Specifically, the amplitude, but not the frequency, of AMPA-mediated miniature excitatory postsynaptic currents (mEPSCs) in FS interneurons was significantly decreased whereas both the frequency and amplitude in pyramidal neurons were increased. In addition, MK-801-induced new presynaptic NMDA receptors were detected in the glutamatergic terminals targeting pyramidal neurons but not FS interneurons. MK-801 also induced distinct alterations in FS interneurons but not in pyramidal neurons, including significantly decreased rectification index and increased calcium permeability. These data suggest a distinct cell-type specific and homeostatic synaptic scaling and redistribution of AMPA and NMDA receptors in response to the subchronic blockade of NMDA receptors and thus provide a direct mechanistic explanation for the NMDA hypofunction hypothesis that have long been proposed for the schizophrenia pathophysiology.

Group II metabotropic glutamate receptor agonist ameliorates MK801-induced dysfunction of NMDA receptors via the Akt/GSK-3β pathway in adult rat prefrontal cortex

Pharmacological intervention targeting mGluRs has emerged as a potential treatment for schizophrenia, whereas the mechanisms involved remain elusive. We explored the antipsychotic effects of an mGluR2/3 agonist in the MK-801 model of schizophrenia in the rat prefrontal cortex. We found that the mGluR2/3 agonist LY379268 effectively recovered the disrupted expression of NMDA receptors induced by MK-801 administration. This effect was attributable to the direct regulatory action of LY379268 on NMDA receptors via activation of the Akt/GSK-3β signaling pathway. As occurs with the antipsychotic drug clozapine, acute treatment with LY379268 significantly increased the expression and phosphorylation of NMDA receptors, as well as Akt and GSK-3β. Physiologically, LY379268 significantly enhanced NMDA-induced current in prefrontal neurons and a GSK-3β inhibitor occluded this effect. In contrast to the widely proposed mechanism of modulating presynaptic glutamate release, our results strongly argue that mGluR2/3 agonists modulate the function of NMDA receptors through postsynaptic actions and reverse the MK-801-induced NMDA dysfunction via the Akt/GSK-3β pathway. This study provides novel evidence for postsynaptic mechanisms of mGluR2/3 in regulation of NMDA receptors and presents useful insights into the mechanistic actions of mGluR2/3 agonists as potential antipsychotic agents for treating schizophrenia.

Traumatic brain injury and the effects of diazepam, diltiazem, and MK-801 on GABA-A receptor subunit expression in rat hippocampus

Background

Excitatory amino acid release and subsequent biochemical cascades following traumatic brain injury (TBI) have been well documented, especially glutamate-related excitotoxicity. The effects of TBI on the essential functions of inhibitory GABA-A receptors, however, are poorly understood.

Methods

We used Western blot procedures to test whether in vivo TBI in rat altered the protein expression of hippocampal GABA-A receptor subunits α1, α2, α3, α5, β3, and γ2 at 3 h, 6 h, 24 h, and 7 days post-injuy. We then used pre-injury injections of MK-801 to block calcium influx through the NMDA receptor, diltiazem to block L-type voltage-gated calcium influx, or diazepam to enhance chloride conductance, and re-examined the protein expressions of α1, α2, α3, and γ2, all of which were altered by TBI in the first study and all of which are important constituents in benzodiazepine-sensitive GABA-A receptors.

Results

Western blot analysis revealed no injury-induced alterations in protein expression for GABA-A receptor α2 or α5 subunits at any time point post-injury. Significant time-dependent changes in α1, α3, β3, and γ2 protein expression. The pattern of alterations to GABA-A subunits was nearly identical after diltiazem and diazepam treatment, and MK-801 normalized expression of all subunits 24 hours post-TBI.

Conclusions

These studies are the first to demonstrate that GABA-A receptor subunit expression is altered by TBI in vivo, and these alterations may be driven by calcium-mediated cascades in hippocampal neurons. Changes in GABA-A receptors in the hippocampus after TBI may have far-reaching consequences considering their essential importance in maintaining inhibitory balance and their extensive impact on neuronal function.

Subchronic MK-801 treatment and post-weaning social isolation in rats: differential effects on locomotor activity and hippocampal long-term potentiation

Subchronic NMDA receptor antagonist treatment and post-weaning social isolation are two animal models of schizophrenia symptoms. However, behavioral and physiological changes following a combination of these two procedures have not been investigated. Thus, we examined effects of a novel, "double hit" model combining these two treatments, comparing them to standard models involving only NMDA antagonist treatment or social isolation. Male, Sprague-Dawley rats were either group-housed or maintained in social isolation (starting at postnatal day [PD] 21 and continuing throughout the study). Each housing condition was further subdivided into two groups, receiving either subchronic treatment with either saline or MK-801 (0.5mg/kg, i.p., 2xday for seven days starting at PD 56). Post-weaning social isolation increased locomotor activity (assessed at PD 70) in response to a novel environment and an acute amphetamine injection, while subchronic MK-801 increased only amphetamine induced locomotor activity. Subsequent electrophysiological experiments (under urethane anesthesia) assessing changes in plasticity of hippocampal synapses showed that subchronic MK-801 treatment resulted in an increase in long-term potentiation in area CA1 in response to high frequency stimulation of the contralateral CA3 area, while housing condition had no effect. No other changes in hippocampal electrophysiology (input-output curves, paired-pulse facilitation) were observed. These data are the first to demonstrate an enhancement in hippocampal long-term plasticity in vivo following subchronic MK-801 administration, an effect that may be related to the well-characterized changes in glutamatergic and GABAergic systems seen after subchronic NMDA receptor blockade. That lack of additive or synergistic effects in the "double hit model" suggests that combining isolation and subchronic MK-801 treatment does not necessarily produce greater behavioral or physiological dysfunction than that seen with either treatment alone.

Dizocilpine (MK-801) induces distinct changes of N-methyl-D-aspartic acid receptor subunits in parvalbumin-containing interneurons in young adult rat prefrontal cortex

N-methyl-D-aspartic acid receptor (NMDAR) hypofunction has long been implicated in schizophrenia and NMDARs on gamma-aminobutyric acid (GABA)ergic interneurons are proposed to play an essential role in the pathogenesis. However, controversial results have been reported regarding the regulation of NMDAR expression, and direct evidence of how NMDAR antagonists act on specific subpopulations of prefrontal interneurons is missing. We investigated the effects of the NMDAR antagonist dizocilpine (MK-801) on the expression of NMDAR subtypes in the identified interneurons in young adult rat prefrontal cortex (PFC) by using laser microdissection and real-time polymerase chain reaction, combined with Western blotting and immunofluorescent staining. We found that MK-801 induced distinct changes of NMDAR subunits in the parvalbumin-immunoreactive (PV-ir) interneurons vs. pyramidal neurons in the PFC circuitry. The messenger RNA (mRNA) expression of all NMDAR subtypes, including NR1 and NR2A to 2D, exhibited inverted-U dose-dependent changes in response to MK-801 treatment in the PFC. In contrast, subunit mRNAs of NMDARs in PV-ir interneurons were significantly down-regulated at low doses, unaltered at medium doses, and significantly decreased again at high doses, suggesting a biphasic dose response to MK-801. The differential effects of MK-801 in mRNA expression of NMDAR subunits were consistent with the protein expression of NR2A and NR2B subunits revealed with Western blotting and double immunofluorescent staining. These results suggest that PV-containing interneurons in the PFC exhibit a distinct responsiveness to NMDAR antagonism and that NMDA antagonist can differentially and dose-dependently regulate the functions of pyramidal neurons and GABAergic interneurons in the prefrontal cortical circuitry.

The role of GABA(A) receptors in the acute and chronic effects of ethanol: a decade of progress

The past decade has brought many advances in our understanding of GABA(A) receptor-mediated ethanol action in the central nervous system. We now know that specific GABA(A) receptor subtypes are sensitive to ethanol at doses attained during social drinking while other subtypes respond to ethanol at doses attained by severe intoxication. Furthermore, ethanol increases GABAergic neurotransmission through indirect effects, including the elevation of endogenous GABAergic neuroactive steroids, presynaptic release of GABA, and dephosphorylation of GABA(A) receptors promoting increases in GABA sensitivity. Ethanol's effects on intracellular signaling also influence GABAergic transmission in multiple ways that vary across brain regions and cell types. The effects of chronic ethanol administration are influenced by adaptations in GABA(A) receptor function, expression, trafficking, and subcellular localization that contribute to ethanol tolerance, dependence, and withdrawal hyperexcitability. Adolescents exhibit altered sensitivity to ethanol actions, the tendency for higher drinking and longer lasting GABAergic adaptations to chronic ethanol administration. The elucidation of the mechanisms that underlie adaptations to ethanol exposure are leading to a better understanding of the regulation of inhibitory transmission and new targets for therapies to support recovery from ethanol withdrawal and alcoholism.

Prenatal alcohol exposure alters GABA(A)alpha5 expression: a mechanism of alcohol-induced learning dysfunction

OBJECTIVE

In a model for fetal alcohol syndrome (FAS), we have previously found an alteration in NMDA receptors suggesting mediation, at least in part, of alcohol-related learning deficit. NMDA and GABA receptors interact in a multisynaptic circuit for the regulation of the inhibitory tone through the CNS. The GABA receptor subunit GABA(A)alpha5 is involved in learning and is developmentally regulated, as it is excitatory in the perinatal brain and inhibitory in the adult. We were interested to evaluate alcohol's effect on GABA(A)alpha5 expression to further understand alcohol-induced learning dysfunction.

STUDY DESIGN

Timed, pregnant C57B16/J mice were treated on gestational day 8 with alcohol (25% alcohol, 0.03 mL/kg i.p.) or control (saline). Embryos and brains were harvested 10 days after treatment, and brains from adult offspring were collected after evaluation in the Morris Water Maze, a well-established test for spatial learning. Gene expression included samples from at least 3 litters per timepoint, and calibrator-normalized relative real-time polymerase chain reaction (PCR) was performed to quantify GABA(A)alpha5 with GAPDH standardization. Statistical analysis included analysis of variance (ANOVA).

RESULTS

Prenatal alcohol exposure significantly decreased GABA(A)alpha5 expression in the embryo (P < .02) and fetal brains (P < .01) 10 days after therapy. However, in adult brains GABA(A)alpha5 expression was increased versus controls (P < .01). As previously demonstrated, prenatal alcohol exposure resulted in deficits in adults learning the Morris Water Maze with controls learning faster (P < .05).

CONCLUSION

Prenatal alcohol exposure alters developmental GABA(A)alpha5 expression. This may further explain the long-lasting damage of alcohol on learning skills. Both the alcohol-induced reduction in the GABA(A)alpha5 subunit during development and up-regulation in adult brain may be related to learning deficits resulting in decreased learning potential caused by the developmental defect and an increased inhibition of learning resulting from increased expression as an adult. In combination with our previous findings, these suggest that alcohol-induced learning impairment is likely the result of alterations of both NMDA and GABA expression and function.

NMDA receptor antagonism disrupts the development of morphine analgesic tolerance in male, but not female C57BL/6J mice

Multiple studies demonstrate that coadministration of N-methyl-D-aspartate (NMDA) receptor antagonists with the opioid agonist morphine attenuates the development of analgesic tolerance. Sex differences in the effects of noncompetitive, but not competitive NMDA receptor antagonists on acute morphine analgesia, have been reported in mice, yet the role of sex in modulation of morphine tolerance by NMDA receptor antagonists has yet to be addressed. Therefore, we tested whether there is a sex difference in the effect of NMDA receptor antagonists on the development of morphine analgesic tolerance in C57BL/6J mice. Acutely, at a dose required to affect morphine tolerance in male mice, the noncompetitive NMDA receptor antagonist dizocilpine (MK-801) prolonged morphine analgesia similarly in both sexes in the hot plate and tail withdrawal assays. In the hot plate assay, coadministration of MK-801 or the competitive antagonist 3-(2-carboxpiperazin-4-yl)propyl-1-phosphanoic acid (CPP) with morphine attenuated the development of tolerance in male mice, while having no effect in females. Like normal and sham females, ovariectomized mice were similarly insensitive to the attenuation of morphine tolerance by MK-801 in the hot plate assay. Surprisingly, in the tail withdrawal assay, MK-801 facilitated the development of morphine-induced hyperalgesia and tolerance in males but not females. The results demonstrate that male mice are more sensitive to modulation of nociception and morphine analgesia after repeated coadministration of NMDA receptor antagonists. Furthermore, the underlying mechanisms are likely to be different from those mediating the sex difference in the modulation of acute morphine analgesia that has previously been reported.

Selective reduction of γ-aminobutyric acid type A receptor δ subunit mRNA levels by MK-801 in rat dentate gyrus

The influence of excitatory blockade elicited by uncompetitive N-methyl-D-aspartate (NMDA)/glutamate receptor antagonists on inhibitory GABAergic systems is not well understood. Adult male rats were injected i.p. with a single dose of the prototypical uncompetitive antagonist MK-801 (0.2-10 mg/kg) and in situ hybridization was performed to measure mRNA levels of gamma-aminobutyric acid type A (GABAA) receptor subunits (alpha1-6, beta1-3, gamma1-3, delta, and theta). A significant decrease in delta subunit mRNA levels, that reached approximately 70% of saline-treated values, was observed in the hippocampal dentate gyrus following MK-801 administration. Other subunits did not display statistically significant alterations. These data demonstrate selective actions on GABAA receptor subunit levels that result from blockade of excitation by MK-801.

The N-methyl-D-aspartate receptor subunit NR2B: localization, functional properties, regulation, and clinical implications

The N-methyl-D-aspartate (NMDA) receptor is an example of a heteromeric ligand-gated ion channel that interacts with multiple intracellular proteins by way of different subunits. NMDA receptors are composed of seven known subunits (NR1, NR2A-D, NR3A-B). The present review focuses on the NR2B subunit of the receptor. Over the last several years, an increasing number of reports have demonstrated the importance of the NR2B subunit in a variety of synaptic signaling events and protein-protein interactions. The NR2B subunit has been implicated in modulating functions such as learning, memory processing, pain perception, and feeding behaviors, as well as being involved in a number of human disorders. The following review provides a summary of recent findings regarding the structural features, localization, functional properties, and regulation of the NR2B subunit. The review concludes with a section discussing the role of NR2B in human diseases.

Selective enhancement of AMPA receptor-mediated function in hippocampal CA1 neurons from chronic benzodiazepine-treated rats

Two days following one-week administration of the benzodiazepine, flurazepam (FZP), rats exhibit anticonvulsant tolerance in vivo, while reduced GABA(A) receptor-mediated inhibition and enhanced EPSP amplitude are present in CA1 pyramidal neurons in vitro. AMPA receptor (AMPAR)-mediated synaptic transmission in FZP-treated rats was examined using electrophysiological techniques in in vitro hippocampal slices. In CA1 pyramidal neurons from FZP-treated rats, the miniature excitatory postsynaptic current (mEPSC) amplitude was significantly increased (33%) without change in frequency, rise time or decay time. Moreover, mEPSC amplitude was not elevated in dentate granule neurons following 1-week FZP treatment or in CA1 pyramidal neurons following acute desalkyl-FZP treatment. Regulation of AMPAR number was assessed by quantitative autoradiography with the AMPAR antagonist, [(3)H]Ro48-8587. Specific binding was significantly increased in stratum pyramidale of hippocampal areas CA1 and CA2 and in proximal dendritic fields of CA1 pyramidal neurons. Regulation of AMPAR subunit proteins was examined using immunological techniques. Neither abundance nor distribution of GluR1-3 subunit proteins was different in the CA1 region following FZP treatment. These findings suggest that enhanced AMPAR currents, mediated at least in part by increased AMPAR number, may contribute to BZ anticonvulsant tolerance. Furthermore, these studies suggest an interaction between GABAergic and glutamatergic systems in the CA1 region which may provide novel therapeutic strategies for restoring BZ effectiveness.

GABA(A) receptor changes in delta subunit-deficient mice: altered expression of alpha4 and gamma2 subunits in the forebrain

The delta subunit is a novel subunit of the pentameric gamma-aminobutyric acid (GABA)(A) receptor that conveys special pharmacological and functional properties to recombinant receptors and may be particularly important in mediating tonic inhibition. Mice that lack the delta subunit have been produced by gene-targeting technology, and these mice were studied with immunohistochemical and immunoblot methods to determine whether changes in GABA(A) receptors were limited to deletion of the delta subunit or whether alterations in other GABA(A) receptor subunits were also present in the delta subunit knockout (delta-/-) mice. Immunohistochemical studies of wild-type mice confirmed the restricted distribution of the delta subunit in the forebrain. Regions with moderate to high levels of delta subunit expression included thalamic relay nuclei, caudate-putamen, molecular layer of the dentate gyrus, and outer layers of the cerebral cortex. Virtually no delta subunit labeling was evident in adjacent regions, such as the thalamic reticular nucleus, hypothalamus, and globus pallidus. Comparisons of the expression of other subunits in delta-/- and wild-type mice demonstrated substantial changes in the alpha4 and gamma2 subunits of the GABA(A) receptor in the delta-/- mice. gamma2 Subunit expression was increased, whereas alpha4 subunit expression was decreased in delta-/- mice. Importantly, alterations of both the alpha4 and the gamma2 subunits were confined primarily to brain regions that normally expressed the delta subunit. This suggests that the additional subunit changes are directly linked to loss of the delta subunit and could reflect local changes in subunit composition and function of GABA(A) receptors in delta-/- mice.

Effects of bilateral olfactory bulbectomy on N-methyl-D-aspartate receptor function: autoradiographic and behavioral studies in the rat

Rat bilateral olfactory bulbectomy (OBX) serves as a useful model in the study of depression and the mechanisms of action of antidepressant treatments. Considering the evidence of NMDA receptors involvement in depression, the present study was undertaken in order to investigate the time-course effects of OBX on the NMDA receptor function. Following bilateral olfactory bulbectomy, rats display an increase in locomotor activity and changes in other types of behavior in a novel environment. Autoradiographic experiments using the noncompetitive NMDA antagonist [(125)I]-iodo-MK-801 as the labeling agent showed that this increase in behavioral activities corresponds to a decrease in [(125)I]-iodo-MK-801 binding in a number of brain regions. In most regions, this reduction reached significance by the third week following OBX. However, in some cortical areas-a nucleus of the thalamus (AV) and one of the amygdala (LA)-this reduction was already significant in the first or second week following OBX and lasted throughout the 4 weeks of the study. We also compared the behavioral modifications induced by a challenge injection of MK-801 (0.2 mg/kg i.p.) in OBX and sham-operated rats. This challenge is known to induce hyperlocomotion and a number of stereotypies in naive rats. These effects were drastically reduced in OBX as compared to sham-operated rats. These data are consistent with the above-mentioned decrease in cerebral binding of MK-801 to NMDA receptors.

Audiogenic seizure susceptibility is induced by termination of continuous infusion of gamma-aminobutyric acid or an N-methyl-D-aspartic acid Antagonist into the inferior colliculus

The inferior colliculus (IC) is strongly implicated in seizure initiation in a genetic form of audiogenic seizures (AGS) and in AGS observed during ethanol withdrawal (ETX). Ethanol is known to block the actions of excitatory amino acids (EAA) and enhance the actions of gamma-aminobutyric acid (GABA) in several brain areas, including the IC. The present study investigated the effects on susceptibility to AGS following withdrawal from continuous blockade of N-methyl-D-aspartic acid (NMDA) receptors or continuous activation of GABA receptors in the IC. This involved infusion of GABA (1 M) or a competitive NMDA antagonist, DL-2-amino-7-phosphonoheptanoic acid (AP7, 1 mM), at 0.25 microl/h for 7 days using an Alzet osmotic minipump. Following abrupt termination of the infusion, AGS susceptibility began at 30 min. The incidence of AGS was 38.9 and 56.3% following GABA and AP7 withdrawal, respectively. The AGS behaviors observed during withdrawal, which included wild running and bouncing clonus, were very similar to those evoked by acoustic stimuli during ETX. AGS susceptibility lasted for several hours and in 13% of animals persisted for up to 6 months. The current results support diminished GABAergic and elevated glutamatergic function in the IC as the critical mechanisms and sites for AGS initiation. The present study, coupled with previous evidence that chronic ethanol exposure reduced GABA-mediated inhibition and enhanced EAA-mediated excitation, suggests that these amino acid receptor-mediated alterations in the IC are key elements in initiating AGS during ethanol withdrawal.

Distinguishing between GABA(A) receptors responsible for tonic and phasic conductances

Cell-to-cell communication in the mammalian nervous system does not solely involve direct synaptic transmission. There is considerable evidence for a type of communication between neurons through chemical means that lies somewhere between the rapid synaptic information transfer and the relatively non-specific neuroendocrine secretion. Here I review some of the experimental evidence accumulated for the GABA system indicating that GABA(A) receptor-gated Cl-channels localized at synapses differ significantly from those found extrasynaptically. These two types of GABA(A) receptor are involved in generating distinctly different conductances. Thus, the development and search for pharmacological agents specifically aimed at selectively altering synaptic and extrasynaptic GABA(A) conductances is within reach, and is expected to provide novel insights into the regulation of neuronal excitability.

Modulation of the levels of NMDA receptor subunit mRNA and the bindings of [3H]MK-801 in rat brain by chronic infusion of subtoxic dose of MK-801

The effects of continuous infusion of NMDA receptor antagonist MK-801 on the modulation of NMDA receptor subunits NR1, NR2A, NR2B, and NR2C were investigated by using in situ hybridization study. Differential assembly of NMDA receptor subunits determines their functional characteristics. Continuous intracerebroventricular (i.c.v.) infusion with MK-801 (1 pmol/10 microl/h) for 7 days resulted in significant modulations in the NR1, NR2A, and NR2B mRNA levels without producing stereotypic motor syndromes. The levels of NR1 mRNA were significantly increased (9-20%) in the cerebral cortex, striatum, septum, and CA1 of hippocampus in MK-801-infused rats. The levels of NR2A mRNA were significantly decreased (11-16%) in the CA3 and dentate gyrus of hippocampus in MK-801-infused rats. In contrast to NR2A, NR2B subunit mRNA levels were increased (10-14%) in the cerebral cortex, caudate putamen, and thalamus. However, no changes of NR2C subunits in cerebellar granule layer were observed. Using quantitative ligand autoradiography, the binding of NMDA receptor ligand [3H]MK-801 was increased (12-25%) significantly in almost all brain regions except in the thalamus and cerebellum after 7 days infusion with MK-801. These results suggest that region-specific changes of NMDA receptor subunit mRNA and [3H]MK-801 binding are involved in the MK-801-infused adult rats.

Role of glutamatergic and GABAergic systems in alcoholism

The pharmacological effects of ethanol are complex and widespread without a well-defined target. Since glutamatergic and GABAergic innervation are both dense and diffuse and account for more than 80% of the neuronal circuitry in the human brain, alterations in glutamatergic and GABAergic function could affect the function of all neurotransmitter systems. Here, we review recent progress in glutamatergic and GABAergic systems with a special focus on their roles in alcohol dependence and alcohol withdrawal-induced seizures. In particular, NMDA-receptors appear to play a central role in alcohol dependence and alcohol-induced neurological disorders. Hence, NMDA receptor antagonists may have multiple functions in treating alcoholism and other addictions and they may become important therapeutics for numerous disorders including epilepsy, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's chorea, anxiety, neurotoxicity, ischemic stroke, and chronic pain. One of the new family of NMDA receptor antagonists, such as DETC-MESO, which regulate the redox site of NMDA receptors, may prove to be the drug of choice for treating alcoholism as well as many neurological diseases.

Regional variations in the effects of chronic ethanol administration on GABA(A) receptor expression: potential mechanisms

Gamma-aminobutyric acid type A (GABA(A)) receptors in brain adapt to chronic ethanol exposure via changes in receptor function and subunit expression. The present review summarizes currently available data regarding changes in GABA(A) receptor subunit mRNA and peptide expression. Data are presented from various different brain regions and the variations between specific brain regions used to draw conclusions about mechanisms that may underlie GABA(A) receptor adaptations during chronic ethanol exposure. In the whole cerebral cortex, chronic ethanol exposure leads to a reduction of GABA(A) receptor alpha1 subunit mRNA and peptide levels and a near equivalent increase in alpha4 subunit mRNA and peptide levels. This observation is the primary support for the hypothesis that altered receptor composition is a mechanism for GABA(A) receptor adaptation produced by chronic ethanol exposure. However, other brain regions do not display similar patterns of subunit changes. Moreover, subregions within cortex (prefrontal, cingulate, parietal, motor, and piriform) exhibit patterns of changes in subunit expression that differ from whole cortex. Therefore, regional differences in GABA(A) receptor subunit expression are evident following chronic ethanol administration, thus suggesting that multiple mechanisms contribute to the regulation of GABA(A) receptor expression. These mechanisms may include the involvement of other neurotransmitter systems, endogenous steroids and second or third messenger cross-talk.

Changes of GABA(A) receptor binding and subunit mRNA level in rat brain by infusion of subtoxic dose of MK-801

In the present study, we have investigated the effects of prolonged inhibition of NMDA receptor by infusion of subtoxic dose of MK-801 to examine the modulation of GABA(A) receptor binding and GABA(A) receptor subunit mRNA level in rat brain. It has been reported that NMDA-selective glutamate receptor stimulation alters GABA(A) receptor pharmacology in cerebellar granule neurons in vitro by altering the levels of selective subunit. However, we have investigated the effect of NMDA antagonist, MK-801, on GABA(A) receptor binding characteristics in discrete brain regions by using autoradiographic and in situ hybridization techniques. The GABA(A) receptor bindings were analyzed by quantitative autoradiography using [3H]muscimol, [3H]flunitrazepam, and [35S]TBPS in rat brain slices. Rats were infused with MK-801 (1 pmol/10 microl per h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps (Alzet, model 2 ML). The levels of [3H]muscimol binding were highly elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, and cerebellum. However, the [3H]flunitrazepam binding and [35S]TBPS binding were increased only in specific regions; the former level was increased in parts of the cortex, thalamus, and hippocampus, while the latter binding sites were only slightly elevated in parts of thalamus. The levels of beta2-subunit were elevated in the frontal cortex, thalamus, hippocampus, brainstem, and cerebellar granule layers while the levels of beta3-subunit were significantly decreased in the cortex, hippocampus, and cerebellar granule layers in MK-801-infused rats. The levels of alpha6- and delta-subunits, which are highly localized in the cerebellum, were increased in the cerebellar granule layer after MK-801 treatment. These results show that the prolonged suppression of NMDA receptor function by MK-801-infusion strongly elevates [3H]muscimol binding throughout the brain, increases regional [3H]flunitrazepam and [35S]TBPS binding, and alters GABA(A) receptor subunit mRNA levels in different directions. The chronic MK-801 treatment has differential effect on various GABA(A) receptor subunits, which suggests involvement of differential regulatory mechanisms in interaction of NMDA receptor with the GABA receptors.