Interactions between contact and chemosensory mechanisms in pain modulation in 10-day-old rats.

Antinociception was evaluated in 10-day-old rats while suckling or in contact with the mother. Testing occurred during, immediately after, or at 30 and 60 s following milk-induced hyperextension. Hyperextension-induced hypoalgesia terminated immediately with stretch cessation. For suckling rats, baseline escape levels were reacheived within 1 min. For contact rats, baseline levels were also manifest at 30 s but were elevated by 1 min. Sublingual infusions into the anterior portion of the mouth in rats that were either suckling or in contact caused a 20-25-s increase in escape latencies. For suckling rats, escape latencies returned to baseline levels immediately at infusion termination. For contact rats, latencies continued to be elevated for at least 5 min postinfusion. Thus, 3 classes of mother-infant interactions, contact, suckling, and hyperextension during milk letdown, cause varying degrees of hypoalgesia in 10-day-old rats.