Two other cloned receptors, 5-HT6 and 5-HT-, are linked to activation of adenylyl cyclase. Multiple splice variants of the 5-HT7 receptor have been found, although functional distinctions are not clear. Circumstantial evidence suggests that 5-HT7 receptors play a role in smooth-muscle relaxation in the GI tract and the vasculature. The atypical antipsychotic drug clozapine has a high affinity for 5-HT6 and 5-HT7 receptors, but whether this relates to the broader effectiveness of clozapine as an antipsychotic is not known (see Chapter 18). Two subtypes of the 5-HT5 receptor have been cloned; the 5-HT5A receptor couples to inhibit adenylyl cyclase; functional coupling of the cloned 5-HT5B receptor has not been described.
Sites of 5-HT Action
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