Chronic sucrose intake augments antinociception induced by injections of mu but not kappa opioid receptor agonists into the periaqueductal gray matter in male and female rats

Chronic intake of a palatable sucrose solution enhances the antinociceptive potency of systemically administered mu, and kappa opioid receptor agonists. To investigate whether the effects of sucrose on the actions of opioid drugs are mediated within the central nervous system (CNS), antinociception was examined following the administration of mu and kappa opioid receptor agonists into the periaqueductal gray area (PAG). Male and female Long-Evans rats consumed either water and ground chow, or water, chow and a 32% (w/v) sucrose solution. After adaptation to the dietary conditions, a guide cannula was stereotaxically implanted into the PAG. Injections of the mu agonist, morphine (0.0, 2.5, 5.0, 10.0 and 20.0 mg), into the PAG led to dose-related increases in antinociceptive responses on a tail flick test in both male and female rats. Rats which had consumed sucrose displayed significantly greater levels of antinociception than rats not given the sugar. Antinociceptive responses to morphine did not differ as a function of sex. Injections of the kappa agonist, spiradoline (0, 100, 300, 600 mg), into the PAG increased tail flick latencies in male and female rats. However, antinociceptive responses did not vary as a function of diet in rats injected with spiradoline. In both diet conditions, spiradoline led to greater levels of antinociception in female rats than in male rats. These results support the hypothesis that intake of palatable foods and fluids act within the CNS to moderate the behavioral actions of opioid drugs.