MODULATION OF DOPAMINE BINDING IN THE FETAL-RAT - EFFECTS OF MILK AND EXOGENOUS OPIOID MANIPULATION

Presentation of milk to the newborn rat promotes changes in sensory and motor behavior and concomitant changes in both endogenous opioid and dopamine systems. The present study employed an in vivo binding technique with a tritiated ligand for the D-2 receptor ([H-3]raclopride) to examine the effects of intraoral milk infusion and opioid manipulations on dopamine activity in the term rat fetus (E21). In Experiment 1, fetuses received a series of milk infusions, which resulted in decreased occupancy at D-2 receptors by the endogenous ligand, dopamine, in striatal, septal, and hypothalamic brain regions. In Experiment 2, fetuses were pretreated with morphine, the mu-agonist [D-Ala(2),NMe-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), the K-agonist U50,488, or saline. Opioid manipulations had no effect on binding at D-2 receptors. These results confirm that intraoral milk infusion can modulate activity in the dopamine system of the near-term fetal rat.