Function and distribution of three rat 5-hydroxytryptamine(7) (5-HT7) receptor isoforms produced by alternative splicing

Serotonin (5-HT7) receptor pre-mRNA is alternatively spliced in rat tissue to produce three isoforms, 5-HT(7a), 5-HT(7b) and 5-HT(7c), which differ in the amino acid sequences of their carboxyl terminal tails. Substantial spe cies differences in structure and expression patterns exist for 5-HT7 isofo rms. We have now compared some of the functional characteristics and level of expression for the three rat 5-HT7 receptor isoforms, Recombinant recept or isoforms were expressed in COS-7 cells for examination of [H-3]5-HT bind ing characteristics and in JEG-3 cells to ascertain their ability to stimul ate cAMP production. These studies showed that all three isoforms are funct ionally active and have similar agonist binding characteristics. Distributi on of expression of the three rat receptor isoforms were examined in severa l blain regions and peripheral tissues using RT-PCR and in situ hybridizati on. The relative proportions of total 5-HT7 receptor message lent by each i soform varied little between these areas. In contrast to what has been obse rved in human tissue. the 5-HT(7a) isoform predominated in all regions exam ined? while the 5-HT(7c) isoform revealed a low level of expression (3% of total transcript). In situ hybridization was used to determine if the overa ll low level of expression of the 5-HT(7c) isoform by RT-PCR could be attri buted to a small localized subpopulation of cells expressing high levels 5- HT(7c) message. In situ hybridization results indicate a generalized low le vel of expression of the 5-HT(7c) isoform throughout the CNS. These data su ggest that while all three known 5-HT7 receptor isoforms in the rat are fun ctionally competent, any functionally important differences between the thr ee isoforms are not likely to involve differences in ligand binding or gros s differences in adenylate cyclase coupling. However, differences in recept or phosphorylation, regulation or coupling to other effectors or cell traff icking could still exist. (C) 1998 Elsevier Science Ltd. All rights reserve d.