Opioids in the milk.

In various studies, the milk has been screened for the presence of free or precursor-bound opioids. In fact, various opioid receptor ligands with agonistic or even antagonistic activity were found. Besides the alkaloid morphine, peptides derived from alpha-casein (alpha-casein exorphins), beta-casein (beta-casomorphins; beta-casorphin), alpha-lactalbumin (alpha-lactorphins) and beta-lactoglobulin (beta-lactorphin) were among the agonists. In addition, certain peptides derived from k-casein (casoxins) or from lactoferrin (lactoferroxins) were found to behave like opioid antagonists. Although a functional role in the mammalian organism for all of these compounds appears to be well possible, evidence has only been presented for the functional significance of beta-casomorphins, so far. These peptides might play a role in reproduction or nutrition in the female, in the newborn's or in a milk consumer's organism, respectively. Thus, opioids related to milk might represent essential exogenous extensions of the endogenous opiodergic systems.