Is there a genetic control of morphine preference in rat?

Morphine preference was tested in two-bottle, voluntary-choice situations on physically dependent Sprague-Dawley rats. The animals ingested morphine which was dissolved in a fluid diet. Choice tests were performed under similar experimental conditions as the ingestions. Approximately 10% of the physically dependent rats voluntarily preferred large amounts of morphine already after a short treatment. The preference level was found to correlate with the animals' requirement for the drug. There was a gradual increase in morphine preference in F1 and F2 offspring of extremely high morphine preference rats. In F3 (of such extremely high morphine preference rats) up to 65% died shortly after birth. The surviving rats showed a low morphine preference after ingestion when adult. Our result of increasing preference over two generations and death of a large number of rats in the third generation with a low morphine preference of the surviving rats was seen also in F4-F8. The data suggest that high morphine preference is under genetic control.