The synapsins are a family of proteins that have long been implicated in the regulation of neurotransmitter release at synapses. Specifically, they are thought to be involved in regulating the number of synaptic vesicles available for release via exocytosis at any one time.Synapsins are present in invertebrates and vertebrates and are somewhat homologous across evaluated vertebrates.
Gene knockout studies in mice (where the mouse is unable to produce synapsin) have had some surprising results. Mice lacking all three synapsins are prone to seizures, and experience learning defects. These results suggest that while synapsins are not essential for synaptic function, they do serve an important modulatory role. Conversely, studies using transgenic mice in which neuronal signaling is abolished in specific circuitries showed that synaptic activity regulates, but is not essential to maintain, the expression of these proteins.
Different neuron terminals will express varying amounts of each of these synapsin proteins and collectively these synapsins will comprise 1% of the total expressed protein at any one time. Synapsin Ia has been implicated in bipolar disorder and schizophrenia.
^Vawter, MP et al. (April 2002). "Reduction of synapsin in the hippocampus of patients with bipolar disorder and schizophrenia". Mol. Psychiatry7 (6): 571–8. doi:10.1038/sj.mp.4001158. PMID12140780.CS1 maint: Explicit use of et al. (link)