Kristal M B, Thompson A C, Grishkat H L
Physiol Behav. 1985 Oct;35(4):481-6. doi: 10.1016/0031-9384(85)90127-1.
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.
在未怀孕的雌性大鼠中监测吗啡注射或足部电击产生的镇痛作用（采用甩尾试验）。给大鼠喂食几种物质中的一种后，几分钟内就会诱导出镇痛作用。当所吃的物质是大鼠胎盘时，吗啡诱导型和电击诱导型镇痛作用都明显强于摄入其他物质（或未摄入任何物质）的对照组。当将足部电击（后爪）与阿片拮抗剂纳曲酮联合使用时，产生的镇痛作用减弱但仍可检测到；在这种情况下，摄入胎盘并没有增强镇痛作用，这表明胎盘的作用特定于阿片介导的镇痛。在没有产生镇痛作用的操作时，摄入胎盘不会提高痛阈。镇痛作用的这种增强可能是哺乳动物母亲在分娩时摄入胎盘和产液（食胎盘行为）的主要益处之一。
