CENTRAL ADMINISTRATION OF THE ENDOPEPTIDASE-24.15 INHIBITOR CFP-AAF-PAB SUGGESTS DYNORPHIN AS THE ENDOGENOUS LIGAND UNDERLYING BEHAVIORAL-EFFECTS OF MILK IN THE FETAL-RAT

Intraoral infusion of milk to the rat fetus promotes opioid activity t hat results in reduced responsiveness in a behavioral bioassay involvi ng perioral cutaneous stimulation. Intracisternal administration of cF P-AAF-pAB, an inhibitor of endopeptidase 24.15, prolonged the opioid a ctivity induced by milk infusion. Treatment with the selective kappa o pioid antagonist nor-binaltorphimine blocked the effect of cFP-AAF-pAB on milk-induced opioid activity, but treatment with the mu antagonist CTOP or the delta antagonist naltrindole did not. These findings impl y that milk may exert its effect on fetal behavior by increasing level s of dynorphin in the fetal central nervous system.