Lesion‐Induced synaptogenesis in brain: A study of dynamic changes in neuronal membrane specializations

Abstract

When incoming fibers to a given brain region are damaged and degenerate, the remaining undamaged fibers can, in some cases, form new synapses, and restore physiologically functional circuitry. Synaptic membrane events underlie this reconstruction: the connection between membranes is broken and reformed. In order to understand these membrane events, it is necessary to know the molecular composition of the synapse and the nature of the interaction between pre- and postsynaptic membranes. The synaptic membranes are probably joined by proteins extending from their surfaces. The postsynaptic membrane has on its outer surface an array of lectin receptors, probably glycoproteins. On its inner surface, juxtaposed to the bilayer, the membrane has an electron-dense structure called the postsynaptic density which, from studies on the isolated structure, is composed of a few polypeptides. On the basis of the molecular composition and structure of CNS synapses and ultrastructural studies of the lesion-induced synaptogenesis, some of the underlying dynamic events at synaptic membranes are inferred. New synapses are formed either by reutilization of the old contact sites or by generation of new ones. The protein and carbohydrates in the cleft are enzymatically degraded and a new synapse is generated in response to ingrowing fibers by the addition or reutilization of the specialized proteins of postsynaptic membrane, which differentiate a small segment of the postsynaptic membrane.