Abstract
Four projects relating to the vesicle hypothesis of neurotransmitter release were pursued. In the first, the subunit hypothesis of transmitter release, which proposes that transmitter quanta are composed of 1-30 smaller subunits, was examined by recording miniature end-plate potentials (m.e.p.p.s) from small muscle cells in frogs and mice. No evidence was found to support the subunit hypothesis, suggesting that the ealier data which were used to support it can not be used to contradict the vesicle hypothesis.In the second project the effect of increased Mg('2+) in fixative solutions on motor nerve terminal ultrastructure was examined. 15 mM Mg('2+) produced an aggregation of synaptic vesicles. The mechanism of this effect was shown to be unrelated to the blockade of transmitter release produced by elevated concentrations of Mg('2+); this finding suggests that previous proposals (Birks, 1971,1974) that elevated Mg('2+) levels revealed the 'true' ultrastructure of nerve terminals is incorrect. Therefore the Mg('2+) effect does not reveal an unusually large number of synaptic vesicles, and as a result calculations comparing numbers of synaptic vesicles with known stores of transmitter quanta ought to be compatible, supporting the vesicle hypothesis.In the third project the distributions of synaptic vesicle diameters and volumes were compared to distributions of m.e.p.p. amplitudes to test whether a one-to-one correspondence between volumes and amplitudes existed. A lack of correspondence between vesicle volumes and m.e.p.p. amplitudes was found; due to the large number of factors which could affect the size of m.e.p.p.s these data do not support but can not exclude the vesicle hypothesis as the mechanism for transmitter release.In the fourth project membrane fusion events were studied in vitro by rapid mixing techniques. It was found that substantial fusion could occur in 10 msec or less, which supports the proposal that exocytosis of synaptic vesicle contents can account for the rapidity of transmitter release. True fusion was morphologically demonstrated by freeze-fracure electron microscopic techniques to occur between liposomes composed of pure phosphatidylserine, and also between liposomes composed of a mixture of phospholipids including all the phospholipid components of chromaffin granule membranes except for lysolecithin. These latter results do not support the proposal that lysolecithin is required for membrane fusion.