In the present study we compared the affinity of various drugs for the high affinity "agonist-preferring" binding site of human recombinant 5-HT(2A), 5-HT(2B) and 5-HT(2C) receptors stably expressed in monoclonal mammalian cell lines. To ensure that the "agonist-preferring" conformation of the receptor was preferentially labelled in competition binding experiments, saturation analysis was conducted using antagonist and agonist radiolabels at each receptor. Antagonist radiolabels ([3H]-ketanserin for 5-HT(2A) receptor and [3H]-mesulergine for 5-HT(2B) and 5-HT(2C) receptor) bound to a larger population of receptors in each preparation than the corresponding agonist radiolabel ([125I]-DOI for 5-HT(2A) receptor binding and [3H]-5-HT for 5-HT(2B) and 5-HT(2C) receptor binding). Competition experiments were subsequently conducted against appropriate concentrations of the agonist radiolabels bound to the "agonist-preferring" subset of receptors in each preparation. These studies confirmed that there are a number of highly selective antagonists available to investigate 5-HT2 receptor subtype function (for example, MDL 100907, RS-127445 and RS-102221 for 5-HT(2A), 5-HT(2B) and 5-HT(2C) receptors respectively). There remains, however, a lack of highly selective agonists. (-)DOI is potent and moderately selective for 5-HT(2A) receptors, BW723C86 has poor selectivity for human 5-HT(2B) receptors, while Org 37684 and VER-3323 display some selectivity for the 5-HT(2C) receptor. We report for the first time in a single study, the selectivity of numerous serotonergic drugs for 5-HT2 receptors from the same species, in mammalian cell lines and using, exclusively, agonist radiolabels. The results indicate the importance of defining the selectivity of pharmacological tools, which may have been over-estimated in the past, and highlights the need to find more selective agonists to investigate 5-HT2 receptor pharmacology.

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An association between the brain serotonin (5-HT) system and feeding has been postulated since the 1970's but it has only been in recent years that the nature of 5-HT-mediated hypophagia has become well understood, and the receptor subtypes responsible for the effect better defined. The invention and utilisation of subtype-selective 5-HT receptor antagonists has demonstrated that the 5-HT(2C) receptor is of paramount importance in this regard. Importantly, ethological studies of animal behaviour have shown that the hypophagia resulting from 5-HT(2C) receptor activation is likely to be a consequence of increased satiety and this is in contrast to hypophagia following 5-HT(2A) receptor activation. Furthermore, recent studies have also shown that 5-HT(2C) receptor agonists not only reduce feeding when acutely administered to rats or mice, they can also reduce body weight without inducing tolerance when administered chronically to obese animals. These observations have led researchers to conclude that selective 5-HT(2C) receptor agonists have the potential to be effective anti-obesity agents. Encouragingly, this suggestion is supported by both direct and indirect evidence from clinical studies. Indirect evidence stems from recent observations that the clinically effective anorectic agent d-fenfluramine exerts its hypophagic and weight-loss effects via 5-HT(2C) receptor activation. More direct clinical evidence derives from the use of the prototypical 5-HT(2C) receptor agonist m-chlorophenylpiperazine (mCPP), with which both acute hypophagia and body-weight loss have been observed. The current paper therefore reviews both the pre-clinical and clinical evidence supporting the use of 5-HT(2C) receptor agonists for the treatment of obesity and assesses the developments that have been made in this regard to date.

1 The aim of the study was to compare the e ects of 14 day subcutaneous infusion of the 5-HT 2C receptor agonists, m-chlorophenylpiperazine (mCPP, 12 mg kg 71 day 71 ) and Ro 60-0175 (36 mg kg 71 day 71 ) and the 5-HT releasing agent and re-uptake inhibitor, d-fen¯uramine (6 mg kg 71 day 71 ), on food and water intake, body weight gain and locomotion in lean male Lister hooded rats. 2 Chronic infusion of all three drugs signi®cantly reduced food intake and attenuated body weight gain. In contrast, drug infusion did not lead to signi®cant 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 71 day 71 ) or d-fen¯uramine (6 mg kg 71 day 71 ) for either 7 or 14 days. During the ®rst 7 days both drugs signi®cantly 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 signi®cantly heavier than their 14-day drug-treated counterparts. 4 Both mCPP and d-fen¯uramine 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 e ect 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 e ects on thermogenesis. 5 These data suggest tolerance does not develop to the e ects of d-fen¯uramine, mCPP and Ro 60-0175 on rat body weight gain.

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