Effects of amniotic fluid on opioid activity and fetal responses to chemosensory stimuli.

The present study provides evidence that milk or amniotic fluid (AF) can promote activity in the endogenous opioid system of the E20 rat fetus. Fetal responses to a chemosensory test stimulus (lemon) were reduced after intraoral infusion of milk (Experiment 1). The effect of milk was mimicked by the kappa opioid agonist U50,488 (Experiment 2), and blocked by pretreatment with naloxone (Experiment 3), confirming opioid involvement. E20 fetuses also showed reduced responses after exposure to AF collected on E20 or E21, but not to AF collected on E19 (Experiment 4). The effects of AF on fetal responses were blocked by pretreatment with naloxone (Experiment 5), and by a selective kappa opioid antagonist, but not by a mu antagonist (Experiment 6). These findings suggest that the fetus may experience activation of the kappa opioid system for several days before birth as a consequence of its exposure to AF in utero.