GABAA Receptors and Tonic Inhibition: Towards an Improved Understanding of Agonist Binding and in vivo Expression of the Extrasynaptic alpha 4beta3delta Subtype

The GABAA receptor is the major inhibitory neurotransmitter receptor in the central nervous system. Receptors containing the delta subunit generate tonic inhibition due to their extrasynaptic expression, high affinity for gamma-aminobutyric acid (GABA), and slow densensitization kinetics. The present work had two goals: first, compare structural elements involved in agonist binding in the alpha1beta2gamma2 and alpha4beta3delta receptors, which are model synaptic and extrasynaptic receptor subtypes, respectively; second, develop an immunoassay using two-step fluorescence resonance energy transfer to detect the incorporation of three subunits into one receptor complex. The structural studies showed that the loop D region participates in agonist activity at both receptor subtypes, and that the agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) may function through a distinct subsite from that of GABA. Inadequate expression of the subunit constructs limited progress on the immunoassay, requiring more work to optimize the expression system before proceeding to proof-of-principle studies using two-step FRET.