Participation of Placental Opioid-Enhancing Factor in Opioid-Modulated Events at Parturition

Endogenous opioids play a significant role in mammalian parturition as well as in the behaviors associated with delivery itself and toward the emerging young. Distention of the uterus, and distention and mechanical stimulation of the vaginal/cervical area are critical stimuli in the positive feedback loop associated with expulsion of the fetus, and in the onset of maternal behavior upon emergence of the fetus [9, 11, 25]. These stimuli are sufficiently "stressful", "painful", or "aversive" so that they occur during a period of elevated endogenous opioid levels and elevated pain threshold [8, and see 12 for review]. Furthermore, the elevation of endogenous opioids in certain brain areas at delivery, namely the ventral tegmental area, is associated with the rapid onset of appropriate maternal caretaking behavior at that time [24]. One might hypothesize that if endogenous opioids facilitate maternal behavior and delivery, that the level of opioid present would be the same for the optimization of both dimensions of parturition. In fact, increasing pain threshold beyond that observed immediately prior to delivery, in rats, by administration of exogenous opioid (morphine), interferes with the onset of proper maternal care [1, 10, 19, 21]. How then is antinociception increased, at parturition, in a way that does not interfere with the expression of proper maternal behavior?