Alterations of GABAA receptor subunit mRNA levels associated with increases in punished responding induced by acute alprazolam administration: an in situ hybridization study

Training-induced changes in the expression of GABAA-associated genes in the amygdala after the acquisition and extinction of Pavlovian fear

Previous work suggests the gamma-aminobutyric acid (GABA)ergic system may be dynamically regulated during emotional learning. In the current study we examined training-induced changes in the expression of GABA(A)-related genes and the binding of GABA receptor radioligands in the amygdala after the acquisition and extinction of Pavlovian fear. Using in situ hybridization, we examined the expression pattern changes of mRNAs for GABAergic markers in the lateral, basolateral and central subdivisions of the amygdala in C57Bl/6J mice. These markers included GABA-synthesizing enzymes (GAD67 and GAD65), major GABA(A) receptor subunits (alpha1, alpha2, alpha3, alpha5, beta2 and gamma2) and the expression of mRNAs that are involved in a variety of GABA-related intracellular processes, including GABA transporter-1 (GAT1), GABA(A) receptor-associated protein and the GABA(A) clustering protein, gephyrin. With fear conditioning, we found decreased mRNA levels of alpha1, alpha5 and GAD67, as well as deceased benzodiazepine binding in the amygdala. Fear extinction induced an increase in mRNA levels of alpha2, beta2, GAD67 and gephyrin, as well as a decrease in GAT1. Together, these findings indicate that the acquisition of fear induced a downregulation of mRNA markers related to a decrease in amygdala GABAergic function, whereas the acquisition of fear extinction produced an upregulation of GABAergic markers related to enhanced GABAergic transmission.

Differential expression of GABAA receptor subunits in the distinct nuclei of the rat amygdala

Detailed knowledge of the anatomical distribution of different GABA(A) receptor subunits is crucial for understanding the physiological actions of GABA in individual brain areas and for developing drugs acting through the individual GABA receptor subtypes. Since the amygdala is a key brain structure in the processing of emotional information with distinct functions in each nucleus, GABA(A) receptors in the amygdala are an important target of treatment for emotional disorders. In this study, we analyzed by quantitative RT-PCR the expression levels of all GABA(A) receptor subunits in distinct nuclei of the amygdala, the central (Ce) and the lateral/basolateral (LA/BLA) amygdala. We found the strongest expression of the gamma(2) subunit mRNA in both the Ce and LA/BLA, modest expressions of alpha(1), alpha(2) and alpha(3) mRNAs in the LA/BLA and alpha(2) and gamma(1) mRNAs in the Ce, and weak expressions of alpha(6), rho(2) and rho(3) mRNAs in both regions. We further revealed the significantly different expressions of alpha(1), alpha(3), alpha(5), gamma(1), gamma(2), delta, epsilon and theta subunit mRNAs in the Ce and LA/BLA. Differences in the expression levels of GABA(A) receptor subunits suggest different sensitivity to a variety of drugs including benzodiazepines and anesthetics in amygdala nuclei with distinct functions.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Effects of pre- and postnatal corticosterone exposure on the rat hippocampal GABA system

Several lines of evidence have implicated prenatal stress and the hippocampal GABA system in the pathophysiology of schizophrenia, and prenatal stress is believed to increase the risk for schizophrenia through alterations of this neurotransmitter. To explore this hypothesis, we treated male rats pre- and/or postnatally (P48 and P60) with either corticosterone (CORT) or vehicle to establish three study groups: VVV, receiving vehicle at all three time points; VCC, receiving vehicle prenatally and CORT at both postnatal timepoints; and CCC, receiving CORT at all three timepoints. Animals were sacrificed at either 24 h or 5 days after final injection and examined for mRNA levels of GAD65, GAD67, and the GABA(A) receptor subunits alpha2 and gamma2. At 24 h, GAD65 mRNA was decreased in CA1, CA2, CA4, and dentate gyrus (DG) of VCC rats; this effect was either decreased or reversed in CCC-treated animals. No effect was detected in GAD67 mRNA at 24 h. At 5 days, CORT treatment increased GAD67 mRNA levels in CA1, CA3, and DG. Prenatal treatment with CORT was associated with increased responsiveness only in CA3 and DG. For the GABAA receptor, alpha2 subunit mRNA did not show any change in response to CORT treatment, while that for the gamma2 subunit was decreased in CA2 of both VCC- and CCC-treated animals. Consistent with gamma2 subunit mRNA decreases, benzodiazepine (BZ) receptor binding activity was decreased in CA2 with CORT treatment. Prenatal CORT exposure neither increased nor decreased this effect. These results demonstrate that CORT administration is associated with a complex regulation of mRNA expression for pre- and postnatal aspects of the hippocampal GABA system. Under these conditions, prenatal exposure to CORT may sensitize some of these effects, but does not fundamentally alter the nature of this response.

Regulation of benzodiazepine receptor binding and GABA(A) subunit mRNA expression by punishment and acute alprazolam administration

Quantitative autoradiography of benzodiazepine (BZ) receptors and competitive reverse transcription-polymerase chain reaction were used to characterize changes in BZ binding and GABA(A) receptor subunit transcription levels associated with the anxiolytic effects of alprazolam. Effects were assessed on punished and non-suppressed water consumption using a lick suppression (Vogel) paradigm. Alprazolam had no effect on non-suppressed licking, [(3)H]Ro 15-1788 binding or receptor subunit transcript levels, compared to non-drug controls. When each fifth lick produced a shock (0-0.5 mA), responding was suppressed in an intensity-related manner. The highest intensity significantly decreased licking (85%), [(3)H]Ro 15-1788 binding (12%) and alpha1 transcript levels (63%) in the basolateral nucleus of the amygdala, and [(3)H]Ro 15-1788 binding in the mediodorsal thalamic nucleus (15%), compared to non-punished controls. Punishment increased the ratio of gamma2L/S transcripts in the basolateral nucleus of the amygdala. Alprazolam blocked or reversed each of these effects. These results show that punishment has similar effects on BZ binding and GABA(A) receptor subunit expression and that alprazolam can block or reverse those effects. Such changes may be related to the anxiolytic effects of alprazolam.