SYNAPTIC VESICLE LIFE-CYCLE AND SYNAPTIC TURNOVER

Cholinergic synaptic vesicles contain a mixture of soluble low molecul ar mass constituents. Besides acetylcholine these include Ca2+, ATP, G TP, small amounts of ADP and AMP, and also the diadenosine polyphospha tes Ap4A and Ap5A. In synaptic vesicles isolated from the electric ray these diadenosine polyphosphates occur in mmol concentrations and mig ht represent a novel cotransmitter. The membrane proteins of cholinerg ic synaptic vesicles presumably are identical to those in other types of electron-lucent synaptic vesicles. A presumptive exception are the transmitter-specific carriers. The life cycle of the synaptic vesicle in intact neurons and in situ was investigated by analysis of all cyto plasmic membrane compartments that share membrane integral proteins wi th synaptic vesicles. The results suggest that the synaptic vesicle me mbrane compartment might originate from the trans-Golgi network and, a fter cycles of exo- and endocytosis in the nerve terminal, might fuse into an endosomal membrane compartment early on retrograde transport. Tracer experiments using membrane proteins and soluble contents sugges t that the synaptic vesicle membrane compartment does not intermix wit h the presynaptic plasma membrane on repeated cycles of exo- and endoc ytosis if low frequency stimulation is applied. A cDNA has been isolat ed from the electric ray electric lobe that codes for o-rab3, a small GTP-binding protein highly homologous to mammalian rab3. While abundan t in the nerve terminals of the electric organ and at the neuromuscula r junction this protein occurs only in limited subpopulations of nerve terminals in electric ray brain. Immunocytochemical analysis using th e colloidal gold technique and a monospecific antibody against o-rab3 suggests that the GTP-binding protein remains attached to recycling sy naptic vesicles. No evidence was found for a major contribution of an intraterminal endosomal sorting compartment involved in synaptic vesic le recycling.