Modulation of tolerance to the lethal effect of morphine by extinction

In contrast with previous demonstrations that tolerance to the analgesic and thermic effects of morphine can be extinguished by repeated placebo administrations, Sklar and Amit (1978, Behavioral Biology, 22, 509–514) purportedly demonstrated that tolerance to morphine-induced mortality is not affected by such an extinction procedure. The results of the present experiment demonstrated that rats administered a fatal dose of morphine following a tolerance-extinction procedure died more quickly than rats with the same pharmacological history but without the extinction experience. The results suggest that learning is involved in tolerance to lethal doses of morphine.