Ionic channels in synaptic vesicles: are they involved in transmitter release?

Synaptic vesicles were isolated from the nerve terminals of Torpedo electric organ. After fusion, 'giant' vesicles were formed which could be examined by the patch clamp technique. One of the cationic channels, the P channel, shows a small preference for K+ compared to Na+ and has multiple conductance levels. Its rate of opening is voltage and calcium dependent. Fractal analysis of the P channels reveals that its behaviour does not seem to be fractal in nature. At voltages where only one conductance level is observed, fractal analysis shows at least one discrete open state and at least two discrete closed states. There are considerable similarities between the P channel and channels found in granules from the hypophysis. These channels resemble, in turn, the channels found in gap junctions. Therefore, it is not unwarranted to speculate that a gap-junction-like communication between the secretory vesicle and the extracellular space may occur during exocytosis.