The cloned human 5-HT7 receptor splice variants: a comparative characterization of their pharmacology, function and distribution

Serotonin (5-hydroxytryptamine, 5-HT) receptor pre-mRNA is alternatively sp liced in human tissue to produce three splice variants, h5-HT7(a), h5-HT7(b ) and h5-HT7(d), which differ only in their carboxyl terminal tails. Using membranes from transiently and stably transfected HEK293 cells expressing t he three recombinant h5-HT7 splice variants we compared their pharmacologic al profiles and ability to activate adenylyl cyclase. Using PCR on cDNA der ived from various human tissues, the 5-HT7(a) and 5-HT7(b) splice variants were detected in every tissue examined. The h5-HT7(d) splice variant was de tected in 13 of 16 tissues examined, with predominant expression in the hea rt, small intestine, colon, ovary and testis. All three h5-HT7 splice varia nts displayed high affinity binding for [H-3]5-HT (pK(d)=8.8-8.9) in the pr esence and absence of 100 muM GTP and had similar binding affinities for al l 17 ligands evaluated. In HEK293 cells expressing similar, high levels of receptor (similar to 10,000 fmol/mg protein), 5-CT (5-carboxamidotryptamine ), 5-MeOT (5-methoxytryptamine) and 5-HT were full agonists while 8-OH-DPAT ((2R)-(+)8-hydroxy-2-(di-n-propylamino)tetralin) was a partial agonist wit h relative efficacy of similar to0.8. Even at this high receptor level, EC5 0 values for stimulation of adenylyl cyclase were 10- to 50-fold higher tha n the K-d values, indicating a lack of spare receptors. No significant diff erences in coupling to adenylyl cyclase were observed between the three spl ice variants over a wide range of receptor expression levels. For antagonis ts, binding affinities determined by displacement of [H-3]5-HT binding and by competitive inhibition of 5-HT-stimulated adenylyl cyclase activity were essentially identical amongst the splice variants. These studies indicate that the three human splice variants are pharmacologically indistinguishabl e and that modifications of the carboxyl tail do not influence coupling to adenylyl cyclase.