Beyond sweet taste: saccharin, sucrose, and polycose differ in their effects upon morphine-induced analgesia

The effects of saccharin, sucrose, or Polycose intake on morphine-induced analgesia (MIA) were examined in 40 adult male Long-Evans rats. Rats were tested for MIA on a tail-flick apparatus following acute (5-h) and chronic (3-wk) intake of a 0.15% saccharin solution, a 32% sucrose solution, a 33.68% Polycose solution, or water. During the chronic phase, all rats were given a choice between the test solution and water. Morphine sulfate was administered according to a cumulative dosing procedure beginning with 2.5 mg/kg morphine. The same dose was administered every 30 min. Tail-flick latencies were measured immediately prior to injections and 30 min following each injection. After acute intake of flavored solutions or water, there were no differences in MIA as a function of diet. However, after drinking the flavored solutions or water for three weeks rats drinking Polycose or sucrose showed significantly enhanced MIA relative to rats drinking saccharin. Rats drinking Polycose also showed enhanced MIA relative to rats drinking water. Comparison between the acute and chronic phases of the study demonstrated that tolerance to morphine's analgesic effects did not develop in rats drinking Polycose or sucrose, but did develop in rats drinking saccharin or water. The results support the hypothesis that, in addition to palatability, the nutritive value of flavored solutions influences MIA.