Duration of sucrose availability differentially alters morphine-induced analgesia in rats.

The effects of duration of sucrose consumption on morphine-induced analgesia (MIA) were examined in 20 adult male Long-Evans rats. Ten rats were tested for MIA on a tail-flick apparatus following acute (5 h), chronic (3 weeks) intake, and subsequent removal of a 32% sucrose solution. Ten rats that never received the sucrose solution served as controls. 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 until a total dose of 15 mg/kg was achieved. Tail-flick latencies were measured immediately prior to injections, and 30 min following each injection. After acute intake of sucrose, there was a trend for animals drinking the sugar solution to show suppressed MIA relative to animals drinking water. In contrast, after drinking the sucrose for 3 weeks, rats showed an enhanced MIA relative to rats drinking water. Three weeks after sucrose removal, there were no differences in MIA as a function of prior dietary conditions. The results support the hypothesis that length of exposure to sucrose influences morphine-induced analgesia and suggest that any change in physiology resulting from sucrose exposure may be reversible.