  The density of synaptic receptors in front of presynaptic release sites is
stabilized in the presence of scaffold proteins, but the receptors and scaffold
molecules have local exchanges with characteristic times shorter than that of
the receptor-scaffold assembly. We propose a mesoscopic model to account for
the regulation of the local density of receptors as quasiequilibrium. It is
based on two zones (synaptic and extrasynaptic) and multi-layer (membrane,
sub-membrane and cytoplasmic) topological organization. The model includes the
balance of chemical potentials associated with the receptor and scaffold
protein concentrations in the various compartments. The model shows highly
cooperative behavior including a "phase change" resulting in the formation of
well-defined post-synaptic domains. This study provides theoretical tools to
approach the complex issue of synaptic stability at the synapse, where
receptors are transiently trapped yet rapidly diffuse laterally on the plasma
membrane.
