Acquistion and extinction of a conditioned hyperthermic response to a tone paired with morphine administration

Following a 7-day baseline period, five male hooded rats were injected daily with a constant dose (20 mg/kg) of morphine sulfate. Immediately following the injections they were taken to a sound-attenuated room and exposed to a 5,000 cps tone for 90 min. On certain predetermined days of the morphine treatment, rectal temperatures were measured before each injection and 30 min following the onset of the tone. Also, after the 11th, 19th, and 57th morphine-tone pairing, the rectal temperatures were measured before and after a saline injection which was paired with tone. On these days tone presentations by themselves were sufficient to cause a reliable increase in rectal temperature resembling that seen following an injection of morphine. When tone presentations were continued, without further pairing with morphine, the hyperthermic response to the tone was gradually extinguished. However, a significant increase in rectal temperature in response to the tone was again observed following a second set of five morphine-tone pairings.