WINSLOW, JL; COOPER, RL; ATWOOD, HL; Univ. of Toronto; Univ. of Kentucky; Univ. of Toronto: Variations of Presynaptic Active Zones and Ca2+ Induced Transmitter Release

Presynaptic terminals have synapses which contain multiple active zones (AZs) at which neurotransmitter is released. Freeze fracture micrographs of lobster neuromuscular junction show AZs within growth plateaus and containing intramembraneous particles (IMPs) which are putative voltage activated calcium (CaV) and calcium activated potassium channels. These putative CaV channels are in close to impressions of vesicles. We analyzed the sizes of the AZs and the geometric distributions of the putative channels. The computational geometry technique of Voronoi neighbors gave, per channel, the number of neighbors and their distances. This suggests the number of links and distances between channels within an AZ compared to membrane not included in AZs. The diffent configuration of CaV channels within the AZs were used in a compuational preparation for calcium and calcium binding proteins (immobile buffers). The reaction-diffusion equations were solved numerically. We found that the different configurations of CaV channels within AZs give different spatio-temporal profiles of calcium concentration that evoke vesicle fusion, thence neurotransmitter release. Thus, the probability of vesicle release does not obey a uniform distribution across different AZs and synapses on a presynaptic terminal. The pattern of plateaus containing the AZs can be viewed as net growth morphology of synaptic membrane.