A new method for determining co-operativity in neurotransmitter release

It has been accepted for some time that neurotransmitter release exhibits a co-operative dependence on calcium. Here we suggest a new procedure for estimating the co-operativity, based on the early rise of synaptic delay histograms of induced release at low quantal content. Measurements of such histograms at the lobster neuromuscular junction are reported. On the basis of this data, and also of data from the literature for other species, a re-examination is made of the conventional hypothesis that the kinetics of release is primarily determined by the time course of entry and removal of calcium ions. Two major new hypotheses for the nature of co-operativity are discussed, both containing the additional feature that membrane depolarization activates a molecule or complex that only then can bind calcium and induce release. The measurements confirm the hypothesis that the co-operativity arises from the action of several complexes between calcium and adepolarization-activated molecule to initiate the release of a vesicle. The co-operativity exponent is estimated to be between three and five in lobster neuromuscular junction and also in crayfish, macrobrachium, and frog.