Alterations in nociception and morphine antinociception in mice fed a high-fat diet.

Currently, more than 78.6 million adults in the United States are obese. A majority of the patient population receiving treatment for pain symptoms is derived from this subpopulation. Environmental factors, including the increased availability of food high in fat and sugar, contribute to the continued rise in the rates of obesity. The focus of this study was to investigate whether long-term exposure to a high-fat, energy-dense diet enhances baseline thermal and inflammatory nociception while reducing sensitivity to morphine-induced antinociception. Antinociceptive and hypothermic responses to morphine were determined in male and female C57BL/6N mice fed either a "western-style" diet high in fat and sucrose (HED) or a standard low-fat chow diet for 15 weeks. Antinociception was assessed using both the hot plate and tail flick tests of acute thermal pain and the formalin test of inflammatory pain. Acute administration of morphine dose-dependently increased antinociception in the hot plate and tail flick assays for mice of both sexes fed chow and HED. However, female mice displayed lower antinociceptive response to morphine compared to males in the tail-flick test. Hypothermic responses to acute morphine were also assessed in mice fed chow or HED. Male and female mice fed chow, and female mice fed HED displayed similar hypothermic responses to morphine. However, males fed HED did not exhibit morphine-induced hypothermia. Tolerance to the antinociceptive and hypothermic effects of morphine was assessed after ten days of repeated daily administration (10mg/kg morphine). Male mice fed chow or HED developed tolerance to morphine in the hot plate test. However, females fed HED did not. In the tail flick assay, only mice fed HED developed tolerance to morphine. All groups showed tolerance to morphine-induced hypothermia. In the formalin test, we found that both male and female mice fed HED had reduced sensitivity to the antinociceptive effects of morphine (6mg/kg). Collectively, these data suggest that sensitivity and tolerance to the antinociceptive effects of morphine may be dependent on diet and sex in the hot plate and tail flick thermal pain models, and that the acute antinociceptive effects of morphine in the formalin inflammatory pain model may also be dependent on these two factors. In addition, diet and sex can influence morphine-induced hypothermia. Exposure to an HED may lead to changes in neuronal signaling pathways that alter nociceptive responses to noxious stimuli in a sex-specific manner. Thus, dietary modifications might be a useful way to impact pain therapy.