Neurite outgrowth-stimulating activities of β-casomorphins in neuro-2a mouse neuroblastoma cells

Abstract

Endogenous opioid peptides and opiate drugs are known to affect the development of the nervous system. β-Casomorphins (β-CMs) belong to a family of exogenous opioid peptides derived from the milk protein β-casein by proteolytic fragmentation. We investigated the effects of various fragments and analogues of β-CM on neurite outgrowth in Neuro-2a mouse neuroblastoma cells. The fragments β-CM-5 to -9 and β-CM-5 amide stimulated neurite outgrowth. Fragments shorter than β-CM-5 (β-CM-3, -4, and β-CM-4 amide) and longer than β-CM-9 (β-CM-13 and -21) had no effects. The activity of β-CMs to promote neurite outgrowth does not correlate with their opioid activity in guinea-pig ileum. The effect of the most potent fragment, β-CM-5, was prevented by the μ-opioid receptor-selective antagonist D-Phe-Cys2-Tyr3-D-Trp-Orn5-Thr 6-Pen7-Thr8-NH2 (CTOP), or by pretreatment with pertussis toxin. These results suggest that the stimulatory effects of β-CMs on neurite outgrowth were mediated through G protein-coupled μ-opioid receptors.