INGESTION OF AMNIOTIC-FLUID BY POSTPARTUM RATS ENHANCES MORPHINE ANTINOCICEPTION WITHOUT LIABILITY TO MATERNAL-BEHAVIOR

Ingestion of amniotic fluid or placenta by rats has been shown to enha nce opioid-mediated analgesia induced by morphine injection, foot shoc k, vaginal/cervical stimulation, or late pregnancy. The present study was designed to determine whether this mechanism might be a means of p roviding greater analgesia during the periparturitional period without contributing to the disruption of maternal behavior (measured primari ly as retrieval) that can result from excessive opioid levels. Postpar tum primiparous rats, injected with either 2 or 3 mg/kg morphine sulfa te or vehicle and given orogastric infusions of either amniotic fluid or saline, were tested for maternal behavior. Pain threshold (determin ed by tail-flick latency test) in rats injected with 2 mg/kg morphine and infused with amniotic fluid was elevated to a level that did not d iffer significantly from that of a separate group of rats injected wit h 3 mg/kg morphine and infused with saline. This enhanced analgesia wa s not, however, accompanied by the significant disruption of maternal behavior found among the rats receiving the higher morphine dose.