Central and peripheral relationships between morphine and glucose on antinociception in rats

Previous research from our laboratory has determined that in the absence of a gustatory response or taste hedonics, intraperitoneal (i.p.) glucose administration enhanced morphine-mediated analgesia in rats and had antinociceptive actions on its own. Two experiments examined the potential of a central mechanism for glucose's actions on morphine-mediated antinociception. Morphine (2.5 µg) was infused into the periaqueductal gray (PAG) while glucose (300 mg/kg) was injected into the peritoneal cavity, or glucose (32 nmol) was infused into the PAG while morphine (3.2 mg/kg) was injected i.p. Doses of morphine and glucose were selected based on our own prior research for being below the threshold for analgesic efficacy. Antinociception was assessed using the hot-water tail-withdrawal procedure. Tail-withdrawal latency was tested at baseline (before), and 12, 24 and 36 minutes after the i.p. injection. The results indicated that 300 mg/kg glucose, administered i.p. effectively increased the antinociceptive potency of a low dose of centrally administered morphine, while central infusion of glucose enhanced peripheral morphine-mediated antinociception. These outcomes support previous evidence of glucose's influence on the antinociception actions of opioid drugs. Furthermore, they suggest that glucose produces its enhancing actions on morphine-mediated antinociception in the brain. These results support the hypothesis that glucose does not need to go through a gustatory mechanism or taste hedonics to alter morphine's antinociceptive actions.