Secondary reinforcement property of a stimulus paired with morphine administration in the rat

Rats learned to run to the correct arm of a Y-maze. Correct responses were reinforced with morphine injection paired with a conditional tone stimulus. After the maze response was well established, extinction trials were run. During extinction half of the animals received neither morphine nor tone as a consequence of a correct response, while the other half received the tone but no morphine. Rats receiving the tone during extinction required significantly more trials to reach the extinction criteria than rats not receiving tone presentations. Extinction with the tone also facilitated relearning of the maze response. The results support the view that morphone is a potent reinforcer, and that stimuli paired with morphine administration acquire the properties of a secondary reinforcer.