Modulation of morphine-induced antinociception by palatable solutions in male and female rats.

The analgesic potency of opioid drugs varies as a function of gender, and can be modified by the intake of palatable sweet-tasting solutions. To determine if gender interacts with diet-induced changes in antinociceptive responses, male and female Long-Evans rats were fed laboratory chow and water alone, or chow, water and either a 32% w/v sucrose solution or a 0.15% w/v saccharin solution, and tested in two analgesic paradigms, the tail-flick test and the hot-plate test. For both tests, antinociceptive responses of male and female rats were tested following administration of cumulative doses (1.25, 2.5, 5.0, and 10.0 mg/kg, SC) of morphine sulfate. On the tail-flick test, morphine produced dose-related increases in antinociceptive responses. In addition, relative to both the chow only and saccharin conditions, chronic intake of the sucrose solution access significantly augmented morphine's antinociceptive properties. On the hot-plate test, when the plate was heated to 51 degrees C, morphine led to significant dose-related increases in antinociceptive responses, but diet did not affected antinociceptive responses. When the temperature of the hot plate was increased to 53 degrees C, there was a trend for animals given sucrose to have greater antinociceptive responses than those given either chow alone or saccharin. No differences in baseline pain sensitivity or morphine-induced analgesia were observed as a function of gender.