Capsaicin attenuates feeding suppression but not reinforcement by intestinal nutrients

During and after a meal, the presence of nutrients in the intestine can be rapidly detected and signaled to the brain via visceral afferents. Intestinal infusions of carbohydrate and fat suppress food intake, and this satiety action is inhibited by capsaicin, a neurotoxin that produces a partial visceral deafferentation. This study determined if the postingestive reinforcing action of nutrients is also suppressed by capsaicin treatment. Food-restricted control and capsaicin-treated rats were trained to drink a flavored solution paired with intraduodenal infusions of carbohydrate (Polycose solution) on some days and, on other days, a differently flavored solution paired with intraduodenal water infusions. In a choice test, both groups displayed a strong preference for the Polycose-paired flavor. In a subsequent satiety test, however, intraduodenal Polycose suppressed sucrose sham feeding in the controls but not in capsaicin-treated rats. The same rats were next trained to associate new flavors with intraduodenal fat (corn oil emulsion) and intraduodenal water infusions. During training, oil infusions reduced oral intakes in the controls much more than in the capsaicin-treated rats. Both groups reliably preferred the oil-paired flavor in the subsequent choice test. These results indicate that, unlike the satiating effect of intestinal carbohydrate and fat, the reinforcing actions of these nutrients are not mediated by capsaicin-sensitive visceral afferents. The data also imply that the postingestive reinforcement produced by nutrients is not dependent on the nutrients' satiating quality.