Physiological separation of vesicle pools in low- and high-output nerve terminals.

Physiological differences in low- (tonic like) and high-output (phasic like) synapses match many of the expected anatomical features of these terminals. However, investigation in the recruitment of synaptic vesicles from a reserve pool (RP) to a readily releasable pool (RRP) of synaptic vesicles within these types of nerve terminals has not been fully addressed. This study highlights physiological differences and differential modulation of the vesicles in a RP for maintaining synaptic output during evoked depression of the RRP. With the use of bafilomycin A1, a vacuolar ATPase blocker, recycling vesicles are blocked in refilling with transmitter. The tonic terminal is fatigue resistant due to a large RRP, whereas the phasic depresses rapidly upon continuous stimulation. These differences in rates of depression appear to be in the size and degree of utilization of the RRP of vesicles. The working model is that upon depression of the tonic terminal, serotonin (5-HT) has a large RP to act on in order to recruit vesicles to the RRP; whereas, the phasic terminal, 5-HT can recruit RP vesicles to the RRP prior to synaptic depression but not after depression. The vesicle pools are physiologically differentiated between phasic and tonic output terminals.