ABSTRACT

Serotonin (5-hydroxytryptamine; 5-HT) is a monoamine neurotransmitter primarily present in the enterochromaffin cells (ECs) of gastrointestinal tract (GT), Central Nervous System (CNS), blood platelets and of gastrointestinal 202mucosa (Saxena PR, 1995). It was first isolated and characterized by Rapport et al. (1948a–c). Serotonin exerts its effects through a variety of membrane-bound receptors, present in the central and peripheral nervous system, and in nonneuronal tissues in the gut, cardiovascular system and blood (Hoyer et al., 2002). It belongs to a complex family of neurotransmitters consisting of seven families of 5-HT (5-HT1 −5-HT7) receptors (Hoyer et al., 2002), and at least 14 different serotonin receptor subtypes (Nichols DE and Nichols CD, 2008). Except for 5-HT3 receptor, which is a ligand-gated ion channel mediating fast depolarization (Sugita et al., 1992), all other 5-HT receptors belong to the G-protein-coupled receptor (GPCR) superfamily (Hoyer et al., 2002). The 5-HT2-receptor family is positively linked to phospholi-pase C, whereas 5-HT1 and 5-HT5 receptors inhibit adenylate cyclase and 5-HT4, 5-HT6 and 5-HT7 receptors are known to stimulate adenylate cyclase (Hannon and Hoyer, 2008; Millan et al., 2008; Pytliak et al., 2011). These receptors are classified according to their structure, their pharmacology and the transduction signal initiated by them. Serotonin present in CNS is responsible for a number of behavioral functions, including the regulation of mood, sleep or wakefulness, appetite, nociception, and sexual behavior (Jacobs and Azmitia, 1992). Other than CNS, it is involved in the regulation of GI, endocrine, respiratory, cardiovascular system (Berger et al., 2009). Because of its multiple regulatory roles in the human body, serotonin is an important target for a variety of therapeutic diseases including depression, migraines, schizophrenia, anxiety, and learning and memory disorders (Durham and Russo, 2002; Lesch, 2001; Mann et al., 2001).