Where are neurotransmitters stored?

Neurotransmitters are stored within small, membrane-bound sacs known as synaptic vesicles. The synaptic vesicles are concentrated in high density in the ends of presynaptic neurons or the axon terminals. 

The neurotransmitters are released into the synaptic cleft of the synapse when the thin-walled synaptic vesicle fuses with the cell membrane in a process known as exocytosis.