DYNAMICS OF SYNAPTIC VESICLES IN CULTURED SPINAL-CORD NEURONS IN RELATIONSHIP TO SYNAPTOGENESIS

The dynamics of synaptic vesicles (SVs) during the development of pres ynaptic specializations in cultured Xenopus spinal cord neurons was st udied with the fluorescent vesicular probe FM1-43. In naive neurons th at have not contacted synaptic targets, packets of SVs are distributed along the entire neurite and are quite mobile. The interaction with t he synaptic target, such as a muscle cell or a latex bead coated with basic fibroblast growth factor, results in the localization and immobi lization of SV packets at the contact site. Depolarization resulted in exocytosis of SVs in both naive and target-contacted neurites. Okadai c acid, a phosphatase inhibitor, caused a dispersal of SV packets in b oth naive and target-contacted neurites. Thus, prior to target contact , SVs are already organized into packets capable of release and recycl ing by a phosphorylation-dependent mechanism. Target interaction then recruits and anchors these functional SV packets into forming the pres ynaptic nerve terminal. With fluorescent phalloidin as a probe, F-acti n was found to colocalize with SV clusters at bead-neurite contacts. A lthough okadaic acid caused a dispersal of SVs at the beads, F-actin l ocalization there was relatively resistant to this drug treatment. Thi s suggests that SVs become localized at the target by interacting with an actin-based cytoskeletal specialization in a phosphorylation-sensi tive manner. The induction of this cytoskeletal specialization by the target may be an early event in presynaptic differentiation.