Fig. 7.

Facilitation of dopamine release was demonstrated by lowering release probability. Extracellular calcium and magnesium concentrations were altered to manipulate release probability (P_R): (A) low P_R (1.2 mm Ca, 5 mm Mg), (B) control P_R (2.4 mm Ca, 1.2 mm Mg) and (C) high P_R (5 mm Ca, 1.2 mm Mg). Dopamine IPSCs were evoked with single stimuli or trains of five rapid (10 ms interval) stimuli. (A, top) When P_R was low, one stimulus produced very little current while five stimuli still produced a substantial IPSC. If the current produced by a single stimulus (‘1’) was constant over the next four stimuli, the IPSC would be the amplitude and time course predicted by the closed circles labelled ‘5 × 1’. (A, bottom, ‘5’) This predicted IPSC is smaller than the measured IPSC evoked by five stimuli. (B and C, bottom) When P_R was increased the predicted IPSC ‘5 × 1’ was larger than the measured IPSC evoked by five stimuli, suggesting that facilitation of dopamine release occurred only when P_R was low (n = 7–9). Total charged passed by one vs. five stimuli: low P_R, 8.8 ± 1.6%; normal P_R, 27.6 ± 2.3%; high P_R, 28.8 ± 2.9%; n = 7–9.