Effect of a high-fat diet on firing rate of sympathetic nerves innervating brown adipose tissue in anesthetized rats

Three experiments have examined the effects of ad lib and forced intake of a high-fat diet on sympathetic firing rate to brown adipose tissue. Seven days after beginning of ad lib intake of either a low-fat or high-fat diet, sympathetic activity was not significantly different in either group nor was it significantly different from the values obtained in animals measured at the switch from the chow to a semisynthetic high- or low-fat diet. After 22 days on the semisynthetic diet, however, the sympathetic firing rate of animals eating the high-fat diet had decreased nearly 25% and was significantly lower than the animals maintained on the semisynthetic low-fat diet or animals studied at the transition from the chow to the low-fat diet. In a second experiment animals were tube-fed for 3, 6 or 9 weeks on a high- or low-fat diet. Sympathetic firing rate of the rats eating the low-fat diet was higher at all three times, but the difference decreased with longer feeding. To eliminate differences in food intake, animals were tube-fed a moderate- or high-fat liquid diet three times a day for six days. The 80 kcal/day intake produced a steady weight gain in both groups. Liver weight, retroperitoneal white adipose tissue weight, and interscapular brown adipose tissue weight were all significantly greater in the animals fed the high-fat diet. Sympathetic firing rate, however, was significantly lower in the animals fed the high-fat semisynthetic diet as compared to animals fed the moderate-fat diet. These data show the high-fat diets are associated with a reduction in sympathetic activity to brown adipose tissue.