Nicotine administration increases serotonin synthesis and tryptophan hydroxylase expression in dorsal raphe of food-deprived rats

The effects of nicotine on the synthesis of serotonin (5-hydroxytryptamine; 5-HT), known to inhibit food intake, and the expression of tryptophan hydroxylase (TPH), the rate-limiting enzyme of 5-HT biosynthesis, in the dorsal raphe were investigated via immunohistochemistry. Male Sprague-Dawley rats were divided into 4 groups: The fed group, the fed and nicotine-treated group, the food-deprived group, and the food-deprived and nicotine-treated group. Food was withheld from the food-deprived groups for 72 h. Animals of the nicotine-treated groups were injected intraperitoneally with 1 mg/kg of nicotine twice per day for 3 days. The numbers of 5-HT-positive and TPH-positive cells in the dorsal raphe were decreased in food-deprived rats. Nicotine administration exerted no significant effect on the numbers of 5-HT-positive and TPH-positive cells in fed rats, while it increased those numbers in food-deprived rats. These observations suggest that smoking has a negative bearing on the augmented desire for food under food-restricted conditions.