Effects of dopamine and kappa opioid receptors on fetal responsiveness to perioral stimuli

Opioid ligands or drugs that act on dopaminergic pathways effect changes in the motor behavior of near‐term rat fetuses. In this study, selective manipulation of kappa opioid and D1 DA receptors was used to investigate opioid and dopamine involvement in mediating the behavioral effects of milk infusion. Milk reduced expression of the fetal facial wiping response to a tactile probe, and this effect was blocked by the kappa antagonist nor‐binaltorphimine as well as the D1 antagonist SCH‐23390. Administration of either the kappa agonist U50,488 or the D1 agonist SKF‐38393 mimicked the effects of milk and reduced fetal responsiveness to the probe in the absence of milk infusion. Administration of nor‐binaltorphimine reversed the effects of SKF‐38393 and resulted in a high incidence of the wiping response. However, administration of SCH‐23390 did not reverse the low incidence of facial wiping induced by U50,488. These findings suggest that milk modulates activity at dopamine receptors, which in turn promotes activity at kappa receptors of the opioid system to bring about changes in cutaneous responsiveness in the fetal rat. © 1993 Johan Wiley & Sons, Inc.