THE ROLE OF EPSILON SUBUNIT IN GABAA RECEPTOR FUNCTION

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DESCRIPTION: (Applicant's Abstract) The gamma-aminobutyric type A (GABAA) receptor is the major inhibitory neurotransmitter receptor in the brain. General anesthetics modulate GABAA receptors through an unknown site and it is widely believed that this is their mechanism of action. This theory has been impossible to test by virtue of a lack of antagonists to anesthetic binding sites. GABAA receptors each consist of five subunits and there are 15 known varieties of these polypeptides in mammals, providing the potential for numerous receptor subtypes. Different subunit combinations can form receptors with distinct properties, and heterogeneous subunit distribution throughout the brain provides regional differences in the sensitivity of GABAA receptors to various therapeutic agents. The sensitivity of GABAA receptors to anesthetics was thought to lack subunit specificity until the identification of a new polypeptide (e). The e subunit has a distinctive distribution in the brain and may confer novel properties to the neurons in which it is expressed. Indeed, when the subunit is combined with others to form recombinant receptors in cell lines, these GABAA receptors exhibit a resistance to modulation by anesthetics. The e subunit shares most sequence identity with the gamma subunit, although the properties conferred to GABAA receptors by these two polypeptides are quite distinct. Most notably the gamma subunit supports anesthetic and benzodiazepine modulation of GABAA receptors while the e subunit does not. This project seeks to determine the role of the e subunit in controlling GABAA receptor function. The subunit will be used to define the amino acid motifs involved in the regulation of GABAA receptor function by anesthetics. A major goal of the project is to produce a subunit that can in future studies be introduced into transgenic mice which will functionally antagonize anesthetic regulation of GABAA receptors causing the minimum possible change in other GABAA receptor properties. Specific aim 1 will determine whether the e subunit displaces the gamma subunit from GABAA receptors. In Aim 2 chimeric e/gamma2 constructs will be used to probe the relationship between structure and the properties conferred by these subunits, with particular emphasis on identifying the anesthetic site. Specific aim 3 will examine whether general anesthetics use similar mechanisms to regulate the function of other ionotropic receptors related to the GABAA receptor. Particular emphasis will be placed on the 5-HT3 receptor. This receptor is of considerable interest as it is thought to be the site of action of several anti-emetic agents. Human homomeric and heteromeric 5-HT3 receptors are modulated by the intravenous anesthetic propofol which is reported to have anti-emetic properties.