Endocytotic mechanisms in synapses.

Nerve terminals are highly enriched in proteins needed for endocytosis. Although constitutive and ligand-stimulated endocytosis take place in nerve terminals, the primary type is compensatory endocytosis--the process by which a cell retrieves the additional membrane added to cell surface by a regulated secretory event. This process has been extensively characterized using electrophysiological techniques. Except for an unusual form of coupled exo- and endocytosis called kiss-and-run release, compensatory endocytosis appears to use basically the same clathrin-mediated mechanisms as the constitutive and ligand stimulated type. The remarkable speed and selectivity of compensatory endocytosis may be achieved by concentrating the machinery at specialized sites in the nerve terminal adjacent to exocytosis sites and by the use of neuronal isoforms of the proteins that mediate endocytosis.