Comparative effects of continuous infusion of mCPP, Ro 60-0175 and d-fenfluramine on food intake, water intake, body weight and locomotor activity in rats.

1. The aim of the study was to compare the effects of 14 day subcutaneous infusion of the 5-HT(2C) receptor agonists, m-chlorophenylpiperazine (mCPP, 12 mg kg(-1) day(-1)) and Ro 60-0175 (36 mg kg(-1) day(-1)) and the 5-HT releasing agent and re-uptake inhibitor, d-fenfluramine (6 mg kg(-1) day(-1)), on food and water intake, body weight gain and locomotion in lean male Lister hooded rats. 2. Chronic infusion of all three drugs significantly reduced food intake and attenuated body weight gain. In contrast, drug infusion did not lead to significant reductions in locomotor activity in animals assessed 2 and 13 days after pump implantation. 3. In a subsequent 14 day study that was designed to identify possible tolerance during days 7 - 14, animals were given a subcutaneous infusion of mCPP (12 mg kg(-1) day(-1)) or d-fenfluramine (6 mg kg(-1) day(-1)) for either 7 or 14 days. During the first 7 days both drugs significantly reduced body weight gain compared to saline-infused controls; however, from day 7 onwards animals withdrawn from drug treatment exhibited an increase in body weight such that by day 14 they were significantly heavier than their 14-day drug-treated counterparts. 4. Both mCPP and d-fenfluramine reduced daily food intake throughout the infusion periods. For 14-day treated animals this hypophagia was marked during the initial week of the study but only minor during the second week. In light of the sustained drug effect on body weight, the data suggest that weight loss by 5-HT(2C) receptor stimulation may be only partly dependent on changes in food consumption and that 5-HT(2C) receptor agonists may have effects on thermogenesis. 5. These data suggest tolerance does not develop to the effects of d-fenfluramine, mCPP and Ro 60-0175 on rat body weight gain.