Characterization of α 3 Glycine Receptors with Ginkgolide B and Picrotoxin
Ginkgolide B (GB) and picrotoxin (PTX) are antagonists of the major inhibitory receptors of the central nervous system: GABA and glycine receptors (GlyRs). GlyRs contain one or more of the four alpha subunit isoforms of which 1 and 2 have been extensively studied. This report compares GB and PTX block of 3 GlyRs expressed in HEK 293 cells, using whole-cell patch clamp techniques. In CNS, 3 exists as a heteropentamer in conjunction with beta subunits in a 2:3{beta} ratio. Thus, the nature of block was also tested in 3{beta} heteromeric glycine receptors. GB and PTX blocked 3 GlyRs both in the presence (liganded state) and absence of glycine (unliganded state). This property is unique to 3 subunits; 1 and 2 subunits are only blocked in the liganded state. The GB block of 3 GlyRs is voltage-dependent (more effective when the cell is depolarized) and non-competitive, while the PTX block is competitive and not voltage-dependent. The heteromeric and homomeric 3 GlyRs recovered significantly faster from unliganded GB block compared to liganded GB block, but no such distinction was found for PTX block suggesting more than one binding site for GB. This study sheds light on features of the 3 GlyR that distinguish it from the more widely studied 1 and 2 subunits. Understanding these properties can help decipher the physiological functioning of GlyRs in the CNS and may permit development of subunit specific drugs.