Influence of maternal milk on functional activation of delta-opioid receptors in postnatal rats.

Weaning rat pups at day 21 activates a delta-opioid receptor that mediates swim-stress-induced analgesia (swim SIA). We have addressed the possibility that removal of maternal milk is the stimulus for the weaning-induced delta-receptor activation by studying the effect of lactating and nonlactating surrogate mothers and two milk substitutes (casein-rich and casein-free) on opioid receptor control of swim SIA. The delta-receptor antagonist naltrindole (1 mg/kg) significantly antagonized swim SIA in 25-day-old weaned rats, in rats provided with a nonlactating surrogate, and those provided with casein-free milk substitute. Naltrindole had no effect in nonweaned pups, pups given a casein-rich substitute, or in pups from litters provided with a lactating surrogate from day 21 to day 25. Weaning-induced activation of delta-receptors involved in mediating swim SIA appears to be dependent on the loss of dietary casein, which is known to produce peptide fragments that can exert opioid activity. The data suggest that exposure to exogenous opioid peptides can influence the ontogenesis of mu- and delta-opioid receptors.