Placenta ingestion enhances opiate analgesia in rats.

Analgesia, produced by either a morphine injection or footshock, was monitored (using a tail-flick test) in nonpregnant female rats. Analgesia was induced within minutes of having the rats eat one of several substances. When the substance eaten was rat placenta, both the morphine- and shock-induced types of analgesia were significantly greater than in controls that ingested other substances (or nothing). When footshock (hind-paw) was administered in conjunction with the opiate antagonist naltrexone, the analgesia produced was attenuated but detectable; in this case, placenta ingestion did not enhance the analgesia, suggesting that the effect of placenta is specific to opiate-mediated analgesia. Placenta ingestion, in the absence of an analgesia-producing manipulation, did not elevate pain threshold. It is possible that this enhancement of analgesia is one of the principal benefits to mammalian mothers of ingesting the placenta and birth fluids (placentophagia) at delivery.