The classification of 5-hydroxytryptamine receptors.

1. The current classification of receptors for 5-hydroxytryptamine (5-HT) is based on functional studies, and encompasses three main receptor types. 2. 5-HT1-like receptors mediate inhibition of release of various neurotransmitters from central and peripheral sites, smooth muscle contraction and relaxation (and release of endothelium-derived relaxing factor), tachycardia, a variety of behavioural actions (for example, forepaw treading, hypothermia, hyperphagia, drug discriminative stimulus properties, nociceptive pathway modulation, and anxiolytic, anti-aggressive and prosexual effects), and central neuronal excitatory and inhibitory activity. Selective antagonists for this receptor are not yet available, but the 5-HT2 receptor antagonists methysergide and methiothepin have appreciable affinity for 5-HT1-like receptors, and 5-carboxamidotryptamine is a selective agonist. 3. 5-HT2 receptors mediate smooth muscle contraction, platelet aggregation, increased capillary permeability, some behavioural syndromes (for example, head twitch and wet-dog shakes) and drug discriminative stimulus properties, central neuroexcitatory effects, and some neuroendocrine functions. Ketanserin and cyproheptadine are selective antagonists. 4. 5-HT3 receptors mediate peripheral afferent and efferent neuroexcitatory actions, anxiogenic effects, and modulation of cytotoxic drug-induced emesis, gastric emptying, and dopamine-related mesolimbic hyperactivity. Selective antagonists include cocaine, MDL 72222 and ICS 205-930; 2-methyl-5-HT is a selective agonist.