Fast and reversible trapping of surface glycine receptors by gephyrin

Variations in receptor number at a given synapse are known to contribute to synaptic plasticity, but methods used to establish this idea usually do no t allow for the determination of the dynamics of these phenomena. We used s ingle-particle tracking to follow in real time, on the cell surface, moveme nts of the glycine receptor (GlyR) with or without the GlyR stabilizing pro tein gephyrin. GlyR alternated within seconds between diffusive and confine d states. In the absence of gephyrin, GlyR were mostly freely diffusing. Ge phyrin induced long confinement periods spatially associated with submembra nous clusters of gephyrin. However, even when most receptors were stabilize d, they still frequently made transitions through the diffusive state. Thes e data show that receptor number in a cluster results from a dynamic equili brium between the pools of stabilized and freely mobile receptors. Modifica tion of this equilibrium could be involved in regulation of the number of r eceptors at synapses.