Effect of chronic administration of ethanol on GABAA receptor assemblies derived from α2-, α3-, β2- and γ2-subunits in the rat cerebral cortex

GABAa receptor density alterations revealed in a mouse model of early moderate prenatal ethanol exposure using [18F]AH114726

INTRODUCTION

Prenatal ethanol exposure (PEE) has been shown to alter the level and function of receptors in the brain, one of which is GABA_a receptors (GABA_aR), the major inhibitory ligand gated ion channels that mediate neuronal inhibition. High dose PEE in animals resulted in the upregulation of GABA_aR, but the effects of low and moderate dose PEE at early gestation have not been investigated. This study aimed at examining GABA_aR density in the adult mouse brain following PEE during a period equivalent to the first 3 to 4 weeks in human gestation. It was hypothesized that early moderate PEE would cause alterations in brain GABA_aR levels in the adult offspring.

METHODS

C57BL/6J mice were given 10% v/v ethanol during the first 8 gestational days. Male offspring were studied using in-vivo Positron Emission Tomography (PET)/Magnetic Resonance Imaging (MRI), biodistribution, in-vitro autoradiography using [¹⁸F]AH114726, a novel flumazenil analogue with a high affinity for the benzodiazepine-binding site, and validated using immunohistochemistry.

RESULTS

In vivo PET and biodistribution did not detect alteration in brain tracer uptake. In vitro radiotracer studies detected significantly reduced GABA_aR in the olfactory bulbs. Immunohistochemistry detected reduced GABA_aR in the cerebral cortex, cerebellum and hippocampus, while Nissl staining showed that cell density was significantly higher in the striatum following PEE.

CONCLUSION

Early moderate PEE may induce long-term alterations in the GABA_aR system that persisted into adulthood.

Activation of GABA-A receptors during postnatal brain development increases anxiety- and depression-related behaviors in a time- and dose-dependent manner in adult mice

Disturbances of the gamma-amino butyric acid-ergic (GABAergic) system during postnatal development can have long-lasting consequences for later life behavior, like the individual's response to stress. However, it is unclear which postnatal windows of sensitivity to GABA-ergic modulations are associated with what later-life behavioral outcomes. Therefore, we sought to determine whether neonatal activation of the GABA-A receptor during two postnatal periods, an early window (postnatal day 3-5) and a late window (postnatal day 14-16), can affect anxiety- and depression-related behaviors in male mice in later life. To this end, mice were treated with either saline or muscimol (50, 100, 200, 300 and 500μg/kg) during the early and late postnatal periods. An additional group of mice was treated with the GABA-A receptor antagonist bicuculline+muscimol. When grown to adulthood male mice were exposed to behavioral tests to measure anxiety- and depression-related behaviors. Baseline and stress-induced corticosterone (CORT) levels were also measured. The results indicate that early postnatal and to a lesser extent later postnatal exposure to the GABA-A receptor agonist muscimol increased anxiety-like behavior and stress-induced CORT levels in adults. Moreover, the early postnatal treatment with muscimol increased depression-like behavior with increasing baseline CORT levels. The anxiogenic and depression-like later-life consequences could be antagonized by bicuculline. Our findings suggest that GABA-A receptor signaling during early-life can influence anxiety- and depression-related behaviors in a time- and dose-dependent manner in later life. Our findings help to increase insight in the developmental mechanisms contributing to stress-related disorders.

The TM2 6' position of GABA(A) receptors mediates alcohol inhibition

Ionotropic GABA(A) receptors (GABA(A)Rs), which mediate inhibitory neurotransmission in the central nervous system, are implicated in the behavioral effects of alcohol and alcoholism. Site-directed mutagenesis studies support the presence of discrete molecular sites involved in alcohol enhancement and, more recently, inhibition of GABA(A)Rs. We used Xenopus laevis oocytes to investigate the 6' position in the second transmembrane region of GABA(A)Rs as a site influencing alcohol inhibition. We asked whether modification of the 6' position by substitution with larger residues or methanethiol labeling [using methyl methanethiosulfonate (MMTS)] of a substituted cysteine, reduced GABA action and/or blocked further inhibition by alcohols. Labeling of the 6' position in either α2 or β2 subunits reduced responses to GABA. In addition, methanol and ethanol potentiation increased after MMTS labeling or substitution with tryptophan or methionine, consistent with elimination of an inhibitory site for these alcohols. Specific alcohols, but not the anesthetic etomidate, competed with MMTS labeling at the 6' position. We verified a role for the 6' position in previously tested α2β2 as well as more physiologically relevant α2β2γ2s GABA(A)Rs. Finally, we built a novel molecular model based on the invertebrate glutamate-gated chloride channel receptor, a GABA(A)R homolog, revealing that the 6' position residue faces the channel pore, and modification of this residue alters volume and polarity of the pore-facing cavity in this region. These results indicate that the 6' positions in both α2 and β2 GABA(A)R subunits mediate inhibition by short-chain alcohols, which is consistent with the presence of multiple counteracting sites of action for alcohols on ligand-gated ion channels.

Running wheel activity protects against increased seizure susceptibility in ethanol withdrawn male rats

Ethanol withdrawal is a dysphoric condition that arises from termination of ethanol intake by dependent individuals. Common withdrawal symptoms include anxiety, increased reactivity to stimuli and increased seizure susceptibility as well as the risk of increased seizure severity. We use an animal model of dependence and withdrawal to study withdrawal behaviors and potential underlying neurobiological mechanisms. For a number of years, we have quantified pentylenetetrazol seizure thresholds as an assessment of ethanol withdrawal at both one day and three days of withdrawal. Typically, we see a significant decrease in seizure threshold (increased sensitivity to seizure induction) that persists through three days of withdrawal for male rats. Increasing evidence indicates that voluntary exercise affords protection against various challenges to physical and psychological health, including ethanol-related challenges. Therefore, the current study investigated the effect of voluntary wheel running on seizure susceptibility following chronic ethanol administration and withdrawal. We found that voluntary wheel running attenuated the increased sensitivity to pentylenetetrazol-induced seizures observed with ethanol withdrawal, at both the one-day and three-day time points. This result was especially interesting as animals with access to the running wheels consumed more of the ethanol-containing diet. These findings showed that chronic voluntary wheel running reduces the severity of ethanol withdrawal in our animal model and suggest that exercise-based interventions may have some utility in the clinical management of heavy drinking and alcohol withdrawal.

Ethanol-induced loss-of-righting response during ethanol withdrawal in male and female rats: associations with alterations in Arc labeling

BACKGROUND

There is increasing evidence for relevant sex differences in responses to ethanol. Several investigations have found differences in expression and recovery from ethanol withdrawal (EW) in people and across various animal models. We have found that female rats recover more quickly than male rats and show differential responses to various behavioral assessments and pharmacological challenges during withdrawal. The purpose of this study was to determine whether sex differences in EW behaviors extend to the hypnotic effects of acute ethanol administration.

METHODS

We used a repeated measures design to assess duration and latency for loss-of-righting reflex following an acute injection of ethanol (4.2 g/kg; 20% w/v) to pair-fed control or ethanol-withdrawn animals at 1 and 3 days EW in male, female, and ovariectomized female (OVX) rats. We determined protein levels of the activity-regulated cytoskeletal protein (Arc), used as a marker for synaptic activity in glutamatergic synapses, in the motor cortex and prefrontal cortex across these same treatment conditions.

RESULTS

Ethanol-withdrawn animals had a reduced ethanol-induced sleep time compared to controls at 1 day EW. Sleep time remained shortened at 3 days EW for males and OVX, but not females. Arc protein levels in motor cortex and preoptic nuclei significantly increased at 1 day EW across all sex conditions, suggestive of an association with the reduced ethanol-induced sleep times during EW. Arc levels increased further for males and OVX, but not females, at the 3 days EW time point.

CONCLUSIONS

These findings add further support to sex differences in effects of and responses to ethanol. They suggest that the more rapid recovery from EW for females than males also includes expression of tolerance to the hypnotic effects of ethanol. These sex differences may involve some differential neuroadaptations in glutamatergic signaling.

Effect of chronic administration of ethanol on the regulation of the delta-subunit of GABA(A) receptors in the rat brain

In the present study, we investigated the effect of chronic ethanol (CE) administration on the polypeptide levels of the delta-subunit of GABA(A) receptors and [(3)H]muscimol binding to the immunoprecipitated delta-subunit-containing GABA(A) receptor assemblies in the rat brain. CE administration resulted a down-regulation of polypeptide levels of the delta-subunit of GABA(A) receptors in the rat cerebellum and hippocampus, whereas there were no changes in the delta-subunit polypeptide levels in the rat cerebral cortex. Further, CE administration caused a down-regulation of native delta-subunit-containing GABA(A) receptor assemblies in the rat cerebellum as determined by [(3)H]muscimol binding to the immunoprecipitated receptor assemblies. These results indicate that the delta-subunit-containing GABA(A) receptors may play a role in chronic ethanol-induced tolerance and dependence.

Low concentrations of ethanol do not affect radioligand binding to the delta-subunit-containing GABAA receptors in the rat brain

In the present study, we investigated the co-localization pattern of the delta subunit with other subunits of GABA(A) receptors in the rat brain using immunoprecipitation and Western blotting techniques. Furthermore, we investigated whether low concentrations of ethanol affect the delta-subunit-containing GABA(A) receptor assemblies in the rat brain using radioligand binding to the rat brain membrane homogenates as well as to the immunoprecipitated receptor assemblies. Our results revealed that delta subunit is not co-localized with gamma(2) subunit but it is associated with the alpha(1), alpha(4) or alpha(6), beta(2) and/or beta(3) subunit(s) of GABA(A) receptors in the rat brain. Ethanol (1-50 mM) neither affected [(3)H]muscimol (3 nM) binding nor diazepam-insensitive [(3)H]Ro 15-4513 (2 nM) binding in the rat cerebellum and cerebral cortex membranes. However, a higher concentration of ethanol (500 mM) inhibited the binding of these radioligands to the GABA(A) receptors partially in the rat cerebellum and cerebral cortex. Similarly, ethanol (up to 50 mM) did not affect [(3)H]muscimol (15 nM) binding to the immunoprecipitated delta-subunit-containing GABA(A) receptor assemblies in the rat cerebellum and hippocampus but it inhibited the binding partially at a higher concentration (500 mM). These results suggest that the native delta-subunit-containing GABA(A) receptors do not play a major role in the pharmacology of clinically relevant low concentrations of ethanol.

The lack of CB1 receptors prevents neuroadapatations of both NMDA and GABA(A) receptors after chronic ethanol exposure

As the contribution of cannabinoid (CB1) receptors in the neuroadaptations following chronic alcohol exposure is unknown, we investigated the neuroadaptations induced by chronic alcohol exposure on both NMDA and GABA(A) receptors in CB1-/- mice. Our results show that basal levels of hippocampal [(3)H]MK-801 ((1)-5-methyl-10,11-dihydro-5Hdibenzo[a,d]cyclohepten-5,10-imine) binding sites were decreased in CB1-/- mice and that these mice were also less sensitive to the locomotor effects of MK-801. Basal level of both hippocampal and cerebellar [(3)H]muscimol binding was lower and sensitivity to the hypothermic effects of diazepam and pentobarbital was increased in CB1-/- mice. GABA(A)alpha1, beta2, and gamma2 and NMDA receptor (NR) 1 and 2B subunit mRNA levels were altered in striatum of CB1-/- mice. Our results also showed that [(3)H]MK-801 binding sites were increased in cerebral cortex and hippocampus after chronic ethanol ingestion only in wild-type mice. Chronic ethanol ingestion did not modify the sensitivity to the locomotor effects of MK-801 in both genotypes. Similarly, chronic ethanol ingestion reduced the number of [(3)H]muscimol binding sites in cerebral cortex, but not in cerebellum, only in CB1+/+ mice. We conclude that lifelong deletion of CB1 receptors impairs neuroadaptations of both NMDA and GABA(A) receptors after chronic ethanol exposure and that the endocannabinoid/CB1 receptor system is involved in alcohol dependence.

Effect of chronic administration of ethanol on the regulation of tyrosine kinase phosphorylation of the GABAA receptor subunits in the rat brain

One of the many pharmacological targets of ethanol is the GABA inhibitory system, and chronic ethanol (CE) is known to alter the polypeptide levels of the GABA(A )receptor subunits in rat brain regions. In the present study, we investigated the regulation of the tyrosine kinase phosphorylation of the GABA(A) receptor alpha(1)-, beta(2)- and gamma(2)-subunits in the rat cerebellum, cerebral cortex and hippocampus following chronic administration of ethanol to the rats. We observed either down-regulation or no change in the tyrosine kinase phosphorylation of the alpha(1) subunit, whereas there was an up-regulation or no change in the case of beta(2)- and gamma(2)-subunits of the GABA(A) receptors depending on the brain region following chronic administration of ethanol to the rats. These changes reverted back to the control level following 48 h of ethanol-withdrawal. These results suggest that tyrosine kinase phosphorylation of GABA(A )receptors may play a significant role in ethanol dependence.

Adolescent cortical development: a critical period of vulnerability for addiction

Cortical growth and remodeling continues from birth through youth and adolescence to stable adult levels changing slowly into senescence. There are critical periods of cortical development when specific experiences drive major synaptic rearrangements and learning that only occur during the critical period. For example, visual cortex is characterized by a critical period of plasticity involved in establishing visual acuity. Adolescence is defined by characteristic behaviors that include high levels of risk taking, exploration, novelty and sensation seeking, social interaction and play behaviors. In addition, adolescence is the final period of development of the adult during which talents, reasoning and complex adult behaviors mature. This maturation of behaviors corresponds with periods of marked changes in neurogenesis, cortical synaptic remodeling, neurotransmitter receptors and transporters, as well as major changes in hormones. Frontal cortical development is later in adolescence and likely contributes to refinement of reasoning, goal and priority setting, impulse control and evaluating long and short term rewards. Adolescent humans have high levels of binge drinking and experimentation with other drugs. This review presents findings supporting adolescence as a critical period of cortical development important for establishing life long adult characteristics that are disrupted by alcohol and drug use.

Tyrosine kinase phosphorylation of GABA(A) receptor alpha1, beta2 and gamma2 subunits following chronic intermittent ethanol (CIE) exposure of cultured cortical neurons of mice

There is evidence that many of the GABA(A) receptor subunits contain consensus sequence for tyrosine kinase, and phosphorylation may play a key role in ethanol's regulation of GABA(A) receptors. Recently, we investigated the effect of chronic exposure of ethanol (CE) on tyrosine kinase phosphorylation and reported that there was an up-regulation in tyrosine kinase phosphorylation of the beta(2)- and gamma(2)- subunits and no effect on alpha(1)-subunit of the GABA(A) receptor in the cultured cortical neurons of mice. In the present study, we have further investigated the effect of chronic intermittent administration of ethanol (CIE) on tyrosine kinase phosphorylation of the GABA(A) receptor subunits (alpha(1), beta(2), and gamma(2)) in the mouse cultured cortical neurons by immunoprecipitation and Western blot techniques. We observed that there was an up-regulation in the tyrosine kinase phosphorylation of the GABA(A )receptor beta(2)- and gamma(2)-subunits following CIE exposure, and no effect on alpha(1)-subunit in the cultured cortical neurons of mice. These CIE changes, unlike CE, were not reverted back to the control level following ethanol withdrawal even after 7 days. Acute exposure of ethanol did not cause any change in the tyrosine kinase regulation of the GABA(A) receptor subunits. In conclusion, the CIE exposure, unlike chronic/acute ethanol exposure, regulates the tyrosine kinase phosphorylation of the selective population of GABA(A )receptors in a long lasting manner.

Repeated ethanol treatment in adolescent rats alters cortical NMDA receptor

Earlier we have reported that repeated ethanol treatment during adolescence causes long-lasting impairments in spatial learning and memory. The present study was undertaken to determine the cellular mechanisms underlying the persistent ethanol-induced cognitive dysfunction in adolescent male rats. Since in adult animals ethanol is known to affect the N-methyl-d-aspartate (NMDA) receptor-gated ion channel, the hypothesis tested here was that adolescent ethanol exposure modulates NMDA receptor (NR) regulation in the brain. Adolescent male rats were injected daily with ethanol (2g/kg intraperitoneally) for 5 consecutive days. Control rats received isovolumetric saline for the same number of days. Groups of control and experimental rats were sacrificed 7 days after the last ethanol/saline administration, and NR activity was measured in specific brain regions (frontal cortex, hippocampus) using the [(3)H]MK-801 binding assay. In addition, some rats were sacrificed and their brains were used to investigate changes in NR pharmacology by measuring specific NR2 subunits immunohistochemically. Compared to saline-treated controls, ethanol-treated rats showed significant increases in [(3)H]MK-801 maximal binding in the frontal cortex. This was associated with increased cortical NR2B subunit protein. [(3)H]MK-801 binding in the hippocampus was minimally affected. These results indicate that ethanol exposure during the adolescent period produces brain region-specific alterations in NR activity. These changes are different from those reported in literature for ethanol administration during the perinatal period or adulthood. Together, these data suggest that adolescence represents a unique stage in brain development in its long-term sensitivity to ethanol.

GABAergic miniature postsynaptic currents in septal neurons show differential allosteric sensitivity after binge-like ethanol exposure

Binge-like ethanol treatment of septal neurons blunts GABAAR-mediated miniature postsynaptic currents (mPSCs), suggesting it arrests synaptic development. Ethanol may disrupt postsynaptic maturation by blunting feedback signaling through immature GABAARs. Here, the impact of ethanol on the sensitivity of mPSCs to zolpidem, zinc and 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha-OH-DHP) was tested. The decay phase of mPSCs showed concentration-dependent potentiation by zolpidem (0.03-100 microM), which was substantially blunted after ethanol exposure. Since zolpidem potentiation exhibited a substantial age-dependent increase in untreated neurons, this finding supported the idea that ethanol arrests synaptic development. GABAAR alpha1 subunit protein also increased with age in untreated neurons, paralleling enhanced sensitivity to zolpidem. Surprisingly, alpha1 levels were not reduced by binge ethanol even though mPSCs were relatively zolpidem-insensitive. Zinc (3-30 microM) decreased mPSC parameters in a concentration- and age-related manner with older untreated cells showing less inhibition. However, there was no increase in mPSC zinc sensitivity after binge ethanol as would be expected if a general arrest of synaptic maturation had occurred. 3alpha-OH-DHP (3-1000 nM) induced concentration-dependent potentiation of mPSC decay. Although potentiation was age-independent, binge ethanol treatment exaggerated sensitivity to this neurosteroid. Finally, chronic picrotoxin pretreatment (100 microM) intended to mimic GABAAR inhibition from ethanol pretreatment did not significantly change mPSC modulation by zolpidem, zinc or 3alpha-OH-DHP. These results suggest that binge ethanol treatment selectively arrests a subset of processes important for maturation of postsynaptic GABAA Rs. However, it is unlikely that ethanol causes a broad arrest of postsynaptic development through a direct inhibition of GABAAR signaling.

Tyrosine kinase phosphorylation of GABAA receptor subunits following chronic ethanol exposure of cultured cortical neurons of mice

Previous studies from our laboratory revealed that acute ethanol exposure inhibits phosphorylation of mitogen-activated protein (MAP) kinase and extracellular signal-regulated kinases (ERK) in mice. In the present study, we have further investigated effect of chronic administration of ethanol on tyrosine kinase phosphorylation of GABA(A) receptor subunits in the mouse cultured cortical neurons. We observed that there was an up-regulation in tyrosine kinase phosphorylation of the GABA(A) receptor beta(2) and gamma(2) subunits following chronic ethanol exposure, whereas there was no effect on alpha(1) subunit of the GABA(A) receptor in the cultured cortical neurons of mice as determined by Western blotting. These results suggest a potential role for tyrosine kinase phosphorylation of some of the GABA(A) receptor subunits in chronic ethanol-induced tolerance and dependence.

Role of AP-1 in ethanol-induced N-methyl-d-aspartate receptor 2B subunit gene up-regulation in mouse cortical neurons

Activator protein 1 (AP-1) has been reported to regulate the gene expression in a wide variety of cellular processes in response to stimuli. In this study, we investigated the DNA-protein binding activities and promoter activity in the N-methyl-D-aspartate R2B (NR2B) gene AP-1 site in normal and ethanol-treated cultured neurons. The identity of the AP-1 site as the functional binding factor is suggested by the specific binding of nuclear extract derived from cultured cortical neurons to the labeled probes and the specific antibody-induced supershift. Mutations in the core sequence resulted in a significantly reduced promoter activity and the ability to compete for the binding. Moreover, treatment of the cultured neuron with 75 mm ethanol for 5 days caused a significant increase in the AP-1 binding activity and promoter activity. The AP-1 DNA-binding complex in control and ethanol-treated nuclear extract was composed of c-Fos, FosB, c-Jun, JunD, and phosphorylated CREB (p-CREB). Western blot analysis showed that p-CREB and FosB significantly increased, whereas c-Jun decreased. The DNA affinity precipitation assay indicated that FosB, p-CREB, and c-Jun increased in the AP-1 complex following ethanol treatment. These results suggest that AP-1 is an active regulator of the NR2B transcription and ethanol-induced changes may result at multiple levels in the regulation including AP-1 proteins expression, CREB phosphorylation and perhaps reorganization of dimmers.