ABSTRACT
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 D2 receptor ([3H]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 infusion, which resulted in decreased occupancy at D2 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-Ala2,NMe-Phe4,Gly5-ol]-enkephalin (DAMGO), the kappa-agonist U50,488, or saline. Opioid manipulations had no effect on binding at D2 receptors. These results confirm that intraoral milk infusion can modulate activity in the dopamine system of the near-term fetal rat.
Subject(s)
Dopamine/metabolism , Fetus/metabolism , Milk/physiology , Narcotics/pharmacology , Animals , Brain Chemistry/drug effects , Brain Chemistry/physiology , Chemoreceptor Cells/drug effects , Chemoreceptor Cells/metabolism , Female , Pregnancy , Rats , Rats, Sprague-Dawley , Receptors, Dopamine D2/drug effects , Receptors, Dopamine D2/metabolism , Receptors, Opioid, kappa/antagonists & inhibitors , Receptors, Opioid, kappa/physiology , Receptors, Opioid, mu/antagonists & inhibitors , Receptors, Opioid, mu/physiologyABSTRACT
Although born blind and deaf, newborn rats exhibit a remarkable capacity to recognize and gain access to the nipples of the lactating mother. However, it is well-known that full-term rat neonates will not attach to an artificial nipple. In the present study, an artificial nipple fashioned from soft vinyl was presented to rat fetuses from E17-E21 of gestation. Fetuses showed side-to-side head movements that resulted in oral capture of the nipple on E18 and exhibited a direct nipple-grasping response from E19 through term. Frame-by-frame analysis of videotape records of E21 rat fetuses revealed that tactile contact with the artificial nipple elicited mouthing, licking directed at the nipple, forelimb treadling, and grasping of the nipple. Fetuses also exhibited components of aversive behavior, including facial wiping and head turning, that appeared to terminate oral contact with the nipple. Morphine pretreatment reduced the expression of aversive responses and promoted licking and grasping of the artificial nipple. In addition to documenting the prenatal ontogeny of this important neonatal behavior, these findings imply a role for endogenous opioids in the newborn rat's first suckling episode.
