Single‐dose tolerance to morphine‐induced analgesic and hypoactive effects in infant rats

A single injection of morphine or isotonic saline was administered to rats at either 1, 5, 9, or 13 days of age. At 26–29 days of age, animals were tested for pain responsivity and activity following either morphine (5 mg/kg) or saline. Animals which received morphine at 1 day of age displayed tolerance at 26 days of age, whereas animals which received morphine at either 5, 9, or 13 days of age failed to display tolerance. This age‐dependent change in susceptibility to single‐dose morphine tolerance may involve alterations in opiate receptor ontogeny.