The reinforcing action of morphine and its paradoxical side effect.

Rats were trained to run down a runway for food in the goal box, and were then tested with one trial per day for 5 days. After running in the runway and eating in the goal box each rat was injected with a drug and returned to the empty goal box for 50 min. Over the 5 trials, rats that received morphine sulphate increased their running speed approximately 400% while the amount of food they ate in the goal box decreased to about 70% of baseline values. The running speed of rats that received lithium chloride decreased to about 30%, while the amount of food they ate decreased to less than 10% of baseline. These two variables did not change for rats that received saline injections. The large increases in running speed observed in the rats that received morphine injections were attributed to an interaction (but not simple summation) between the positive reinforcing effects of morphine and food. The accompanying paradoxical decrease in amount eaten was discussed in terms of the complex pharmacological properties of morphine and it was suggested that morphine may have a reinforcing effect on behavior that is independent of its affective properties.