The distribution of vesicles on the SB changes sharply in response to stimulation, as seen in electron tomography maps of inhibited versus strongly stimulated synapses. Vesicles near the plasma membrane side of the SB were released most rapidly. This generated a gradient of vesicles, most of which remained on the cytoplasmic face of the SB. “The redistribution is what you expect if vesicles bind to the synaptic body and then move along to reach the release site,” says Roberts.
After stimulation, fewer vesicles were found at the plasma membrane just below the SB than in a surrounding ring. Thus, stimulation establishes vesicle gradients on both the SB and the plasma membrane that may be sources for ongoing release. Roberts hopes to determine whether the movement of vesicles down the SB is passive or is powered by a transporter. So far, only three SB components are known, which limits the number of proteins that could be tested for transporter activity. ▪