Electrophysiological signature of homomeric and heteromeric glycine receptor channels

Glycine receptors are chloride-permeable, ligand-gated ion channels and contribute to the inhibition of neuronal firing in the central nervous system or to facilitation of neurotransmitter release if expressed at presynaptic sites. Recent structure-function studies provided detailed insights into the mechanisms of channel gating, desensitization and ion permeation. However, most of the work focused only on comparing few isoforms; and amongst studies, different cellular expression systems were used. Here, we performed a series of experiments using recombinantly expressed homomeric and heteromeric glycine receptor channels including their splice variants in the same cellular expression system to investigate and compare their electrophysiological properties. Our data show that the current-voltage relationships of channels formed by the {alpha}2 or {alpha}3 subunits change upon receptor desensitization from a linear to an inwardly-rectifying shape, in contrast to their heteromeric counterparts. We demonstrate that inward rectification depends on a single amino acid (A254) at the inner pore mouth of the channels and is closely linked to chloride permeation. We also show that the current-voltage relationships of glycine-evoked currents in primary hippocampal neurons are inwardly-rectifying upon desensitization. Thus, the alanine residue A254 determines voltage-dependent rectification upon receptor desensitization and provides a physio-molecular signature of homomeric glycine receptor channels which provides unprecedented opportunities for the identification of these channels at the single cell level.