Milk-induced analgesia and comforting in 10-day-old rats: Opioid mediation

The effects of slow intraoral milk infusions on the opioid-mediated behaviors of ultrasonic vocalizations and paw removal from a hot plate (48-49 degrees C) were evaluated in 10-day-old rats. Milk reduced distress vocalization by circa 30% while increasing paw lift latencies by about 60%. Alterations in both behaviors were fully reversed by naltrexone (0.5 mg/kg) pretreatments. These data demonstrate the calming and analgesic effects of milk. Implications for a possible role of opioid peptides in mother-young relationships are discussed.