Fig. 7.

BoNT/A or BoNT/C treatment decreases the maximum number of quanta released and the Ca²⁺sensitivity, but not cooperativity, of the exocytotic machinery.A, Action potentials elicited in a presynaptic CA3 pyramidal cell and unitary EPSCs recorded in a postsynaptic CA3 pyramidal cell in control (left traces) and BoNT/A-treated cultures (right traces). Each trace is a representative example from 60 trials for each [Ca²⁺]_o, which was decreased from 2.8 to 0.5 mm. [Mg²⁺]_o was kept constant throughout the experiment at 2.5 mm. B, Log–log plot of mean unitary EPSC amplitudes, evoked by single action potentials, as a function of [Ca²⁺]_o in individual cell pairs in control, BoNT/A-treated, and BoNT/C-treated cultures. The control and BoNT/A data are from the cell pairs illustrated inA. Note that EPSC amplitudes approach a maximum, presumably corresponding to successful release at all synapses. In BoNT/A- or BoNT/C-treated cultures, the relation was shifted to the right, indicating a decrease in Ca²⁺ sensitivity, and the maximum EPSC amplitude was decreased, indicating a reduction in the maximum number of quanta released. The slope of this relation (for [Ca²⁺]_o = 0.25–1 mm in the control culture; [Ca²⁺]_o = 0.5–2.8 mm in the BoNT/A- or BoNT/C-treated culture) was not affected by toxin treatment, indicating that Ca²⁺ cooperativity (Dodge and Rahamimoff, 1967) was not changed. Failures of transmission were included in the calculation of mean EPSC amplitude.