REGULATORY ROLES OF COMPLEXINS IN NEUROTRANSMITTER RELEASE FROM MATURE PRESYNAPTIC NERVE-TERMINALS

Complexins are presynaptic proteins whose functional roles in synaptic transmission are still unclear. In cultured rat hippocampal neurons, complexins are distributed throughout the cell bodies, dendrites and a xons, whereas synaptotagmin I and synaptobrevin/VAMP-2, essential prot eins for neurotransmitter release, accumulated in the synaptic-releasi ng sites as early as 1 week in culture. With a maturation of synapses in vitro, complexins also accumulated in the synaptic release sites an d co-localized with synaptotagmin I and synaptobrevin/VAMP-2 after 3-4 weeks in culture, Complexins I and II were expressed in more than 90 and 70% of the cultured neurons, respectively; however, they were larg ely distributed in different populations of synaptic terminals. In the developing rat brain, complexins were distributed in neuronal cell bo dies in the early stage of postnatal development, but gradually accumu lated in the synapse-enriched regions with development. In mature pres ynaptic neurons of Aplysia buccal ganglia, injection of anticomplexin II antibody caused a stimulation of neurotransmitter release. Injectio n of recombinant complexin II and alpha SNAP caused depression and fac ilitation of neurotransmitter release from nerve terminals, respective ly. The effect of complexin was reversed by a subsequent injection of recombinant alpha SNAP, and vice versa. These results suggest that com plexins are not essential but have some regulatory roles in neurotrans mitter release from presynaptic terminals of mature neurons.