PROBABILITY OF QUANTAL SECRETION AND THE MOBILIZATION OF VESICLES AT THE ACTIVE ZONES OF END-PLATES

A vesicle mobilization model is developed for active zones of motor-ne rve terminals. In the model, vesicles from a storage pool are recruite d to an available pool where they are docked at the active zone, the l atter pool being replenished by endocytosis. The predictions of this m odel are then compared with experimental results on these active zones . If a styryl dye (FM1-43) is used to stain all of the vesicles at mot or-nerve terminals then the stores of vesicles associated with differe nt active zones are shown to vary considerably in size. Under the assu mption that the available pool size is proportional to the storage poo l size, the mobilization model shows that the extent of destaining of these stores during a particular period of stimulation is proportional to the size of the storage pool, this size being indicated by the ini tial staining. This is not so under the alternative assumption that th e available pool size does not vary proportionally with store size. If the two pools are proportional then the number of quanta released fro m the available pool is proportional to the storage pool size. Experim ental results indicate that there is a linear relationship between des taining and staining, as well as between quantal release and staining at different sites along motor-nerve terminals, thus favouring the hyp othesis that available pool size (or the size of the active zone) and store size are proportional. Given that there are very different store sizes along the length of motor-nerve terminals, this analysis provid es further evidence for the non-uniform probability of quantal secreti on at different active zones within a single motor-nerve terminal. (C) 1998 Academic Press Limited.