High rates of excitatory miniature currents in crayfish claw opener muscle evoked by high concentrations of gamma-aminobutyric acid (GABA) in normal and Ca2+-deficient superfusions

High concentrations (0.5 mol/l) of the neutral amino acid GABA were used to evoke release of transmitter quanta from excitatory terminals at voltage clamped crayfish muscle fibres in normal and Ca²⁺-deficient superfusions. An experiment in which the release of transmitter quanta proceeded at high rates in both normal and Ca²⁺-deficient superfusion was analyzed in detail indicating a Ca²⁺-independent mechanism of release. In the normal superfusion, on application of GABA, the release rates ñ increased within a few seconds up to about 6000 quanta/s and thereafter declined exponentially with a time constant $\text{τ}{}_{\mathrm{<mtext>q</mtext>}}\text{) = 18.5}\text{s}$, most likely due to depletion of a readily releasable store of transmitter in the excitatory nerve terminals comprising at least 110,000 quanta per muscle fibre. Assuming that about 1900 excitatory synapses exist per muscle fibre 9], it results that about 58 quanta can be associated with each synapse in agreement with morphological data 15] which show that between 47–117 vesicles exist in a single glutamatergic synapse of crayfish.