The neuropeptide vasoactive intestinal peptide (VIP) is widely distributed in the adult central nervous system where this peptide functions to regulate synaptic transmission and neural excitability. The expression of VIP and its receptors in brain regions implicated in learning and memory functions, including the hippocampus, cortex, and amygdala, raise the possibility that this peptide may function to modulate learned behaviors. Among other actions, the loss of VIP has a profound effect on circadian timing and may specifically influence the temporal regulation of learning and memory functions.
Chaudhury, D., Loh, D. H., Dragich, J. M., Hagopian, A., & Colwell, C. S. (2008). Select cognitive deficits in Vasoactive Intestinal Peptide deficient mice. BMC Neuroscience, 9. https://doi.org/10.1186/1471-2202-9-63