Fig. 4.

Submaximal FM1–43 loading of vesicle pools during 10 min of miniature synaptic activity. The amount of synaptic FM1–43 loading was compared between two groups of synapses subjected to different loading conditions. In the first group (gray bars; n = 335 boutons), FM1–43 uptake was achieved by 10 min of dye exposure in a miniature activity-promoting medium (5 mm Ca²⁺/0.6 μmTTX). In the second (control) group (white bars;n = 822 boutons) FM1–43 uptake was stimulated by 1200 AP-inducing field stimuli. Responsive boutons were selected by an automated criterion (Fig. 2). Distributions of the FM1–43 uptake amounts in the two groups indicate that AP stimulation results on average in 40% more synaptic FM1–43 uptake than miniature activity during 10 min. For comparison, each histogram is scaled to its maximal bin value (bin size = 2.5 pixel value).