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 spliced 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 the three recombinant h5-HT7 splice variants we compared their pharmacological profiles and ability to activate adenylyl cyclase. Using PCR on cDNA derived 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 detected in 13 of 16 tissues examined, with predominant expression in the heart, small intestine, colon, ovary and testis. All three h5-HT7 splice variants displayed high affinity binding for [3H]5-HT (pKd=8.8-8.9) in the presence and absence of 100 microM GTP and had similar binding affinities for all 17 ligands evaluated. In HEK293 cells expressing similar, high levels of receptor (approximately 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 with relative efficacy of approximately 0.8. Even at this high receptor level, EC50 values for stimulation of adenylyl cyclase were 10- to 50-fold higher than the Kd values, indicating a lack of spare receptors. No significant differences in coupling to adenylyl cyclase were observed between the three splice variants over a wide range of receptor expression levels. For antagonists, binding affinities determined by displacement of [3H]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 indistinguishable and that modifications of the carboxyl tail do not influence coupling to adenylyl cyclase.