Milk is a mammalian characteristic and is of particular importance for humans: Mother's milk or its substitutes from cows' milk are absolutely essential nutriments for the neonate and cows' milk also represents a basic foodstuff for adults.
However, in addition to their well-known nutritive role, milk constituents apparently are also able to carry specific information from the milk producer's to the milk receiver's organism: Thus, a number of milk protein fragments has been shown to behave like opioid receptor ligands able to address opioidergic systems in the adult's or in the neonate's organism.
With respect to the proteins, which they are derived off, these peptides have been named α-casein exorphins or casoxin D (α-casein), β-casomorphins or β-casorphin (β-casein), casoxin or casoxin A, B, or C (κ-casein), α-lactorphins (α-lactalbumin), β-lactorphin (β-lactoglobulin) or lactoferroxins (lactoferrin). Only casoxins and lactoferroxins display antagonistic properties; the other peptides behave like opioid receptor agonists.
Most of the information available so far has been collected about β-casomorphins. These peptides obviously can be released from β-casein in the adult's or in the neonate's organism, where they might elicit opioid effects in the frame of a regulatory role as “food hormones”. Several synthetic β-casomorphin derivatives have been shown to be highly specific and potent μ-type opioid receptor ligands which frequently have been used as standard tools in opioid research. © 1997 John Wiley & Sons, Inc. Biopoly 43: 99–117, 1997