Molecular basis for the high THIP/gaboxadol sensitivity of extrasynaptic GABA(A) receptors

作者：Meera Pratap; Wallner Martin; Otis Thomas S^*

来源：Journal of Neurophysiology, 2011, 106(4): 2057-2064.

DOI：10.1152/jn.00450.2011

• delta subunit
• cerebellar granule cell
• tonic inhibition

Meera P, Wallner M, Otis TS. Molecular basis for the high THIP/gaboxadol sensitivity of extrasynaptic GABA(A) receptors. J Neurophysiol 106: 2057-2064, 2011. First published July 27, 2011; doi: 10.1152/jn.00450.2011.-Extrasynaptic GABA(A) receptors (eGABARs) allow ambient GABA to tonically regulate neuronal excitability and are implicated as targets for ethanol and anesthetics. These receptors are thought to be heteropentameric proteins made up of two alpha subunits-either alpha 4 or alpha 6-two beta 2 or beta 3 subunits, and one delta subunit. The GABA analog 4,5,6,7-tetrahydroisoxazolo (5,4-c) pyridin-3(-ol) (THIP) has been proposed as a selective ligand for eGABARs. Behavioral and in vitro studies suggest that eGABARs have nanomolar affinity for THIP; however, all published studies on recombinant versions of eGABARs report micromolar affinities. Here, we examine THIP sensitivity of native eGABARs on cerebellar neurons and on reconstituted GABARs in heterologous systems. Concentrationresponse data for THIP, obtained from cerebellar granule cells and molecular layer interneurons in wild-type and delta subunit knockout slices, confirm that submicromolar THIP sensitivity requires delta subunits. In recombinant experiments, we find that delta subunit coexpression leads to receptors activated by nanomolar THIP concentrations (EC50 of 30-50 nM for alpha 4 beta 3 delta and alpha 6 beta 3 delta), a sensitivity almost 1,000-fold higher than receptors formed by alpha 4/6 and beta 3 subunits. In contrast, gamma 2 subunit expression significantly reduces THIP sensitivity. Even when delta subunit cDNA or cRNA was supplied in excess, high-and low-sensitivity THIP responses were often apparent, indicative of variable mixtures of low-affinity alpha beta and high-affinity alpha beta delta receptors. We conclude that delta subunit incorporation into GABARs leads to a dramatic increase in THIP sensitivity, a defining feature that accounts for the unique behavioral and neurophysiological properties of THIP.

• 出版日期2011-10