5-Hydroxytryptamine-receptor subtypes.

5-Hydroxytryptamine (5-HT) receptors were originally subclassified into the subtypes M and D based on the findings that 5-HT contracted the guinea-pig ileum by two different mechanisms: (a) directly by an effect on receptors located on smooth muscles (via D receptors), and (b) indirectly by an effect on neuronal receptors (M receptors), the activation of which caused acetylcholine release. With the introduction of radioligand-binding studies and the development of more selective 5-HT agonists and antagonists, it rapidly became apparent that this subclassification is an oversimplification, and it is now accepted that at least three, possibly four main families of 5-HT receptors exist: 5-HT1, 5-HT2, 5-HT3 and possibly 5-HT4 receptors. Furthermore, 5-HT1 receptors are not a homogeneous class, but are subdivided further into four subtypes: 5-HT1A, 5-HT1B, 5-HT1C and 5-HT1D. Whether 5-HT2 and 5-HT3 receptors are also a heterogeneous class of receptors is still a matter of controversy. Besides the differences in specific agonists and antagonists, 5-HT-receptor subtypes seem to differ also in their signal-transduction mechanisms. 5-HT1 receptors (with the exception of 5-HT1C) are coupled to adenylate cyclase, predominantly in an inhibitory fashion, but 5-HT1-mediated activation of adenylate cyclase has been also described. 5-HT2 receptors (and 5-HT1C) are coupled to PI turnover, while 5-HT3 receptors appear to be coupled directly to fast ion channels. On the other hand, 5-HT4 receptors couple obviously in an excitatory fashion to adenylate cyclase.