Milk-induced, opioid-mediated antinociception in rats at the time of cesarean delivery.

Four experiments were conducted in rats within 2 hr of cesarean delivery to assess antinociception at birth and its possible opioid bases. Morphine antinociception was established in a dose-dependent fashion (0.0625-5.0 mg/kg bw). Analgesia was naloxone (1.0 mg/kg) reversible. In succeeding experiments, antinociception equivalent to that produced by 0.0625-0.125 mg/kg morphine injections was induced by a single 20-microliters bolus of milk (commercial half-and-half) that was delivered over 1-2 s to the middle of the tongue. This, too, was naloxone reversible. Milk-induced antinociception was maintained for at least 4 min. Finally, baseline latencies were progressively reduced during the first 2 hr after delivery to levels (8-10 s) that are typically obtained in older (10-day-old) rats. This decline was not opioid mediated because it was not affected by naloxone. Thus, at birth, delivering milk to the mouth in physiological volumes can exert opioid-mediated antinociceptive effects in rats born by cesarean delivery that had never suckled or experienced any other form of maternal contact.