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 sensitivity. In this study, E20 rat fetuses were presented with an artificial nipple (CS) and intraoral infusions of milk (US) to condition opioid activity. Paired presentations of the CS and US resulted in conditioned changes in perioral cutaneous sensitivity, which was not evident in groups exposed to the CS alone, US alone, or explicitly unpaired presentations of US and CS. Selective opioid antagonists administered between conditioning and testing revealed that reexposure to the nipple resulted in activation of the endogenous opioid system. The nipple promoted activity at mu, not kappa, opioid receptors. Conditioned opioid activity to an artificial nipple in the fetus, which lacks experience with milk or other suckling stimuli, implies that conditioned changes in the endogenous opioid system of the newborn rat may develop quickly during the first suckling episodes.
Robinson
S R
SR
Department of Psychology, Binghamton University, New York 13902-6000.
Arnold
H M
HM
Spear
N E
NE
Smotherman
W P
WP
D000818:Animals; D001070:Appetitive Behavior; D001522:Behavior, Animal; D003214:Conditioning, Classical; D005260:Female; D005333:Fetus; D007858:Learning; D008892:Milk; D009294:Narcotics / Q000378:metabolism*; D009558:Nipples; D011247:Pregnancy; D051381:Rats; D017207:Rats, Sprague-Dawley; D012026:Reflex, Stretch
D009294:Narcotics