Effects of Tryptophan and of 5-Hydroxytryptamine Receptor Subtype Agonists on Feeding

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Abstract
Most research on the relationships between tryptophan (TRP), 5-hydroxytryptamine (5-HT) and feeding comes from laboratories centered on either neurochemistry or pharmacology. Neurochemists have usually studied how feeding affects brain TRP concentration and the synthesis and availability of 5-HT. Pharmacologists have focussed on the use of 5-HTergic drugs to control appetite. Consideration of possible relationships between the neurochemical and pharmacological findings leads to other important questions, e.g. do effects of feeding on TRP and on 5-HT function have roles in the normal control of appetite and in its disorders such as anorexia nervosa. Research in this area has been stimulated in recent years by evidence for numerous 5-HT receptor subtypes and by the availability of drugs with selectivity towards them. The main subject of this chapter will be how some of these drugs affect feeding in the rat. While it is easy to show that many 5-HTergic drugs affect food intake in laboratory animals it is less easy to be sure that appetite changes are involved. However, the fact that the drug most commonly used clinically to decrease appetite (fenfluramine, Ponderax) acts by releasing 5-HT (Rowland and Carlton, 1986), encourages the belief that other 5-HTergic drugs can also directly influence appetite in the rat and do not decrease food intake merely, for example as a result of general malaise. Before describing research on this topic however, the effects of feeding on brain TRP and of TRP administration on feeding must be considered.