Mother as shield: differential effects of contact and nursing on pain responsivity in infant rats--evidence for nonopioid mediation.

To determine how rat mothers protect their pups against pain, we applied focal heat (34-51 degrees C) to the ear or shoulder of 10-day-old rats that were isolated, in contact among themselves or with their mother, suckling nonnutritively, or in the hyperextension position normally caused by milk letdown. Relative to isolated rats, contact doubled withdrawal latencies from heat (43 or 45 degrees C) applied to the ear. Suckling quadrupled heat-escape latencies. During hyperextension, rats essentially did not escape from thermal stimulation of up to 48 degrees C. Protection provided by maternal contact, especially suckling, was not mediated by either mu or kappa opioid receptors: neither systemic injections of naltrexone nor norbinaltorphimine reduced heat-escape latencies. Morphine (0.125 and 0.250 mg/kg) added to the effects of contact but multiplied the effects of suckling to produce heat-escape latencies that were upward of 2 min.