Analgesic tolerance produced by morphine pellets is facilitated by analgesic testing.

Analgesic tolerance induced by morphine pellets was examined in rats using the nociceptive tail flick reflex. Analgesic responses of animals who received preliminary tail flick tests after morphine implantation were significantly lower than responses of naive, nontested animals. Previously tested animals were also significantly more tolerant to a morphine challenge than nontested animals. A dose response curve to morphine was not obtained, at the doses used here, from previously tested animals, where as naive animals responded to in a dose dependent manner. Environmental modulation of the tail flick represents an elementary form of behavioral plasticity which may prove useful for neural analyses of simple reflex systems.