Oral intake of morphine in selectively bred rats.

Oral intake of morphine was investigated in selectively bred strains of rats. It was possible to induce all the five genetic lines to consume morphine in distilled water. Withdrawal of morphine resulted in significant decrease in the body weights of all the genetic lines in both the experiments. Significant drug-preference behavior was found only in three genetic lines with a common characteristic of relatively high emotional reactivity as compared to the two lines not showing such a behavior.