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The delayed effect of food-derived opioid peptides (exorphins) after chronic administration on postnatal days 1-14 on the learning of albino rat pups has been studied. Heptapeptide YPFPGPI (beta-casomorphin-7), pentapeptide YPLDL (rubiscolin-5) and pentapeptide YPISL (exorphin C) improved the development of the conditioned foraging reflex in a complex maze. Hexapeptide PFPGPI lacking the N-terminal tyrosine proved inefficient. Only beta-casomorphin-7 had an effect (negative) on passive avoidance conditioning. The obtained data confirm that exorphins (particularly, milk-derived beta-casomorphins) can have significant and long-term effects on the environmental adaptation of young mammals.

The influence of food-derived opioid peptides beta-casomorphines on the manifestation of nursing albino rats maternal behavior was investigated. It was shown that both acute and chronic (during the postnatal period) administration of beta-casomorphin-7 (Tyr-Pro-Phe-Pro-Gly-Pro), the typical representative of this group of peptides, decreases the level of the parental motivation. The effects of beta-casomorphin-7 were naloxone-dependent; N-terminal-reduced analogues had a significantly lesser activity. The obtained results testify in favor of the probable role of casein opioid fragments which were formed in mammary glands of a nursing female, in the development of maternal behavior abnormalities. At the same time, beta-casomorphins could be considered as the limiting factors to the excessive manifestation of the parental motivation.