REGULATION OF SYNAPTIC VESICLE RECYCLING BY CALCIUM AND SEROTONIN

Serotonin, a neuromodulator at the crayfish neuromuscular junction, re gulates neurotransmission without changing intracellular calcium level s. However, the mechanism of this regulation remains unclear. By analy sis of synaptic depression using a depletion model and measurement of vesicle recycling using the styryl dye FM1-43, we show that serotonin increases the number of vesicles available for transmitter release (to tal synaptic vesicle pool size). This regulation is due either to an i ncrease in the number of vesicles at each release site or to an activa tion of previously nonsecreting or silent synapses. We also observed t hat low calcium medium rendered part of the vesicle pool unavailable f or release. These results suggest a new mechanism for regulating synap tic transmission.