MORPHINE DEPRESSION AND TOLERANCE OF NERVE-INDUCED PAROTID SECRETION

The nerve‐induced secretion produced by the rat parotid gland is proportional to the frequency of stimulation. Morphine decreased the flow rate during stimulation at 2.5 and 5 Hz, but not at 20 Hz. This frequency‐dependent action of morphine was partially reversed by naloxone. The secretion produced by the rat parotid gland during an intravenous infusion of acetylcholine was not diminished by morphine. Therefore, the action of morphine on nerve‐induced secretion is most probably on the motor nerve terminals, which release acetylcholine. Animals that had been implanted with morphine base pellets tolerated 4 times as much morphine as controls; after 6 days the minute ventilation was less depressed by graded doses of morphine than non‐implanted controls. Nerve‐induced secretion in morphine‐implanted animals was less depressed by morphine than control animals 6 and 24 h after the pellets were removed. The flow rates in the 6 h group treated with morphine were greater after naloxone than control (precipitated withdrawal) but at 24 h when withdrawal symptoms were no longer evident, naloxone produced only a slight reversal. 1979 British Pharmacological Society