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 appli ed focal heat (34-51 degrees C) to the ear or shoulder of 10-day-old r ats that were isolated, in contact among themselves or with their moth er, suckling nonnutritively, or in the hyperextension position normall y caused by milk letdown. Relative to isolated rats: contact doubled w ithdrawal latencies from heat (43 or 45 degrees C) applied to the ear. Suckling quadrupled heat-escape latencies. During hyperextension. rat s essentially did not escape from thermal stimulation of up to 48 degr ees C. Protection provided by maternal contact, especially suckling, w as not mediated by either mu or kappa opioid receptors: Neither system ic injections of naltrexone nor norbinaltorphimine reduced heat-escape latencies. Morphine (0.125 and 0.250 mg/kg) added to the effects of c ontact but multiplied the effects of suckling to produce heat-escape l atencies that were upward of 2 min.