We have recently shown that beta-casomorphin-7, a milk opioid peptide, strongly stimulates mucin secretion in the rat jejunum through a nervous pathway and opioid receptor activation. In this study, the hypothesis that beta-casomorphin-7 may also act directly on intestinal goblet cells was investigated in vitro in rat and human intestinal mucin-producing cells (DHE and HT29-MTX) using quantitative and semiquantitative RT-PCR and ELISA. The presence of mu-opioid receptors was demonstrated in rat goblet cells in the upper half of the colonic crypt and in the two cell lines by immunohistochemistry and RT-PCR. In rat DHE cells, beta-casomorphin-7 increased the expression of rat mucin (rMuc)2 and rMuc3 but not rMuc1, rMuc4, and rMuc5AC. This effect was time and dose dependent, with the maximum of increase in transcripts being noticed for a concentration of 10(-4) M after 2 h of stimulation for rMuc2 (225% of controls) and 4 h of stimulation for rMuc3 (208% of controls). Mucin secretion was maximally increased after 8 h of stimulation. Interestingly, these effects were prevented by pretreatment of the cells with the mu-opioid antagonist cyprodime. In human HT29-MTX cells, beta-casomorphin-7 (10(-4) M) also increased MUC5AC mRNA levels (219% after 24 h of stimulation) and the secretion of this mucin (169% of controls). In conclusion, beta-casomorphin-7 may contribute significantly to mucin production via a direct effect on intestinal goblet cells and the activation of mu-opioid receptors. Because intestinal mucins have a crucial mucosal protective function, dairy products containing beta-casomorphin-7 may improve intestinal protection and could have dietary and health applications.