Systemic administration of beta-casein heptapeptide beta-casomorphin-7 (YPFPGPI, 1 mg/kg daily) to 10-23-day-old albino rat pups produced delayed anxiolytic effects, which were more pronounced in female than in male rats. Experimental findings confirm our assumption on the important role of nutritional opioids in brain development in newborn mammals.

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We studied the effect of nonselective antagonist of opioid receptor naloxone on the behavior of albino female rats on days 4-6 after delivery. Intraperitoneal injection of naloxone (5 mg/kg) significantly stimulated maternal reactions (increased the number of approaches to pups, decreased the latency of their transfer into new location). Intranasal naloxone (1 mg/kg) produced similar changes. Naloxone in intraperitoneal dose of 1 mg/kg and intranasal dose of 0.2 mg/kg virtually did not modify maternal behavior.

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