[5][6] Synapsins associate as endogenous substrates to the surface of synaptic vesicles and act as key modulators in neurotransmitter release across the presynaptic membrane of axonal neurons in the nervous system.

Family members are characterized by common protein domains, and they are implicated in synaptogenesis and the modulation of neurotransmitter release, suggesting a potential role in several neuropsychiatric diseases.

At its B domain, between amino acids 43 and 121, synapsin II binds to a protein component in the cytosolic surface membrane of synaptic vesicles, organelles in neurons which carry neurotransmitters.

Because of its restorative effect, synapsin IIa is believed to play a fundamental role in synaptic vesicle mobilization and reserve pool regulation in presynaptic nerve terminals.

Initial signal transduction appears to be unaffected by the lack of synapsins, but repeated stimulation of cultured synapsinless hippocampal neurons subsequently showed depressed responses at the excitatory synapse.

Through fluorescence and staining, it has been demonstrated that synapsin IIa increases the number and density of glutamatergic synaptic vesicles in the nerve terminal of neural axons.