Ingestion of amniotic fluid enhances the facilitative effect of VTA morphine on the onset of maternal behavior in virgin rats

Previous research has shown that injection of morphine into the ventral tegmental area (VTA) facilitates the onset of maternal behavior in virgin female rats, and injection of the opioid antagonist naltrexone into the VTA disrupts the onset of maternal behavior in parturient rats. Placentophagia - ingestion of placenta and amniotic fluid, usually at parturition - modifies central opioid processes. Ingestion of the active substance in placenta and amniotic fluid, Placental Opioid-Enhancing Factor (POEF), enhances the hypoalgesic effect of centrally administered morphine, and more specifically, enhances δ- and κ-opioid-receptor-mediated hypoalgesia and attenuates μ-opioid-receptor-mediated hypoalgesia. POEF (in placenta or amniotic fluid) ingestion does not, by itself, produce hypoalgesia. In the present study, we tested the hypothesis that ingestion of amniotic fluid enhances the facilitative effect of opioid activity (unilateral morphine injection) in the VTA on the rate of onset of maternal behavior. Virgin female Long-Evans rats were given one intra-VTA injection of morphine sulfate (0.0, 0.01, or 0.03 μg, in saline) and an orogastric infusion of 0.25 ml amniotic fluid or saline once each day of the first three days of the 10-day testing period. Subjects were continuously exposed to foster pups that were replaced every 12 h; replacement of pups was followed by a 15-min observation period. Maternal behavior latency was determined by the first of two consecutive tests wherein the subject displayed pup retrieval, pup licking in the nest, and crouching over all foster pups, during the 15-min observation. We confirmed the previous finding that the VTA injection, alone, of 0.03 μg morphine shortened the latency to show maternal behavior and that 0.0 μg and 0.01 μg morphine did not. Ingestion of amniotic fluid (and therefore POEF) facilitated the onset of maternal behavior in rats receiving an intra-VTA microinjection of an otherwise subthreshold dose of morphine (0.01 μg).