EXPERIENCE WITH MILK AND AN ARTIFICIAL NIPPLE PROMOTES CONDITIONED OPIOID ACTIVITY IN THE RAT FETUS

Milk promotes activity in the kappa opioid system of the rat fetus and reduced responsiveness in a behavioral bioassay of cutaneous sensitiv ity. In this study, E20 rat fetuses were presented with an artificial nipple (CS) and intraoral infusions of milk (US) to condition opioid a ctivity. Paired presentations of the CS and US resulted in conditioned changes in perioral cutaneous sensitivity, which was not evident in g roups exposed to the CS alone, US alone, or explicitly unpaired presen tations of US and CS. Selective opioid antagonists administered betwee n conditioning and testing revealed that reexposure to the nipple resu lted in activation of the endogenous opioid system. The nipple promote d activity at mu, not kappa, opioid receptors. Conditioned opioid acti vity to an artificial nipple in the fetus, which lacks experience with milk or other suckling stimuli, implies that conditioned changes in t he endogenous opioid system of the newborn rat may develop quickly dur ing the first suckling episodes. (C) 1993 John Wiley & Sons, Inc.