GABAA subtype-specific pharmacological modulation of reward-related behavior

Mclean Hospital, Belmont MA

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Abstract

DESCRIPTION (provided by applicant): Human genetic studies have linked genes encoding GABAA receptor ? subunits to mental illnesses like major depression, bipolar disorder, schizophrenia, alcohol dependence, and illicit drug dependence. However, the functions of the products of these genes with respect to depressive-like behavior and abuse potential are unknown. In this application, we propose to study the role of different GABAA receptor subtypes defined by the presence of the ?1, ?2 or ?3 subunits in reward-related behavior using the intracranial self-stimulation paradigm. Diazepam (ValiumR), a non-subtype-selective benzodiazepine decreases the reward threshold, however, the GABAA receptor subtype mediating this action is unknown. The effects of zolpidem (AmbienR), a partially ?1-selective hypnotic agent that - like diazepam - is also self-administered on reward have not been examined. We hypothesize that ?1- containing GABAA receptors on GABAergic interneurons in the ventral tegmental area and ?2-containing GABAA receptors on GABAergic neurons in the nucleus accumbens mediate a decrease of the reward threshold ("antidepressant-like" action), while ?3-containing GABAA receptors in dopaminergic neurons of the ventral tegmental area mediate an increase in the reward threshold ("pro-depressant-like action"). Using knock-in mice carrying point mutations in the ?1, ?2, or ?3 subunit rendering the respective GABAA receptors insensitive to modulation by diazepam and zolpidem, we will dissect the contribution of individual receptor subtypes to the reward-modulating action of these agents. The identification of the GABAA receptor subtypes regulating reward-related behaviors is of relevance for the development of novel antidepressant drugs and also for the prediction of the dependence and abuse liability of novel subtype-selective compounds, e.g. for the treatment of anxiety disorders, insomnia or chronic pain, based on specific efficacy at defined GABAA receptor subtypes. PUBLIC HEALTH RELEVANCE: The proposed project will investigate the roles of three GABAA receptor subtypes which are expressed in the mesolimbic dopamine system in reward-related behavior with the hypothesis that some GABAA receptor subtypes will be reward-reducing while others will be reward-enhancing. Specific agonists at reward- enhancing GABAA receptor subtypes might be suitable for the treatment of depression, whereas specific agonists at reward-reducing GABAA receptor subtypes might be suitable for the treatment of manic episodes. Moreover, as GABAA receptor subtype-selective agents are currently being developed for the treatment of anxiety, insomnia, and chronic pain, knowledge on the reward-modulating functions of individual GABAA receptor subtypes will be important for the design of compounds with a low abuse liability.