Morphine tolerance in genetically selected rats induced by chronically elevated saccharin intake.

Rats of line LC2-Hi that drank about 50 milliliters of a highly palatable saccharin solution daily for 28 consecutive days did not show morphine analgesia or an opioid form of stress-induced analgesia and were not responsive to naloxone. These findings support the idea that chronically elevated saccharin intake may cause increased release and utilization of endogenous opiates.