Figure 2. Probability of transmitter release at a giant terminal using variance-mean analysis.

A, glutamatergic AMPA current evoked by focal stimulation (arrow) and isolated by addition of strychnine (Sty; 1 μM), bicuculline (Bic; 10 μM) and D-AP5 (30 μM). Mean amplitude of 2.0 nA (from 185 traces). B, AMPA current responded in an all-or-none fashion to a graded stimulus. Stimulation intensity was set at 1.5 times threshold for all experiments (arrow). C, P_r was altered by bath application of cadmium (Cd; 4 or 8 μM), normal calcium (Ca; 2 mM), high calcium (Hi Ca; 4 mM), and phorbol ester (PDBu; 0.5 μM). Mean amplitude and variance of evoked EPSC was measured during a stable epoch of 30–150 responses for each condition (a, b and c). D, variance-mean relationship fitted with a simple parabola estimates Q_av (50.6 pA) and N_min (160). Estimates of P_r were 0.04 in cadmium (a), 0.31 under physiological conditions (b; 2 mM Ca²⁺, 1.3 mM Mg²⁺) and 0.88 in the presence of elevated calcium (4 mM) and phorbol ester PDBu (c). Same cell as in C.