Are exocytosis mechanisms neurotransmitter specific?

Neurotransmission is a multistage regulated process in which a variety of active molecules contained in vesicles are liberated in response to specific stimuli from different types of neurone or related cells. This includes the release of fast neurotransmitters such as amino acids and acetylcholine from central and peripheral synapses, but also that of relatively slow-acting polypeptides from central and peripheral neurones or neuroendocrine cells. Considerable progress has been made over recent years in the understanding at a molecular level of the mechanism of regulated exocytosis, a crucial phase in this phenomenon. The currently proposed overall mechanism, which incorporates the "SNARE" hypothesis for vesicle-membrane docking and fusion, is based on data from experimental models ranging from brain synaptosomes to mast cells. Since the kinetics of the models studied and the physiological effects of the neurotransmitters implicated vary so much, it is pertinent to question whether a general mechanism can be proposed from such experimental data. This review examines known differences in putative exocytotic mechanisms for the various systems studied and attempts to relate these to the nature of the active substances released. Differences exist in each step of the exocytosis process and include the channel through which Ca2+ enters to trigger it or the internal Ca2- source, the type of vesicle in which the transmitter is packaged, the way vesicles are translocated to the surface membrane or how they dock and fuse with it. Major differences have been reported in release mechanisms of different types of vesicle, but minor differences also exist within the same vesicle class. Thus small synaptic vesicles and large dense core vesicles are translocated by distinct processes and the Ca2+ channels, Ca2+ sensors and docking proteins involved in other steps are not identical in all neuronal phenotypes. It may be concluded that each of these differences has evolved to accommodate the different physiological requirements of the neuromodulator released.