Abstract Serotonin (5-hydroxytryptamine, 5-HT) recep- tor pre-mRNA is alternatively spliced in human tissue to produce three splice variants, h5-HT 7(a) , h5-HT 7(b) and h5-HT 7(d) , which differ only in their carboxyl terminal tails. Using membranes from transiently and stably transfected HEK293 cells expressing the three recombinant h5-HT 7 splice variants we compared their pharmacological pro- files and ability to activate adenylyl cyclase. Using PCR on cDNA derived from various human tissues, the 5-HT 7(a) and 5-HT 7(b) splice variants were detected in every tissue examined. The h5-HT 7(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-HT 7 splice variants displayed high affinity binding for [ 3 H]5-HT (pK d =8.8–8.9) in the presence and absence of 100 μM GTP and had similar binding affinities for all 17 ligands evaluated. In HEK293 cells expressing similar, high levels of receptor (~10,000 fmol/mg protein), 5-CT (5-car- boxamidotryptamine), 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 ag- onist with relative efficacy of ~0.8. Even at this high re- ceptor level, EC 50 values for stimulation of adenylyl cy- clase were 10- to 50-fold higher than the K d values, indi- cating 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 ex- pression levels. For antagonists, binding affinities deter- mined by displacement of [ 3 H]5-HT binding and by com- petitive inhibition of 5-HT-stimulated adenylyl cyclase activity were essentially identical amongst the splice vari- ants. These studies indicate that the three human splice variants are pharmacologically indistinguishable and that modifications of the carboxyl tail do not influence cou- pling to adenylyl cyclase. Keywords 5-Hydroxytryptamine · Serotonin · 5-HT 7 receptor · Splice variants · Adenylyl cyclase · Pharmacology · Receptor-effector coupling Introduction Serotonin mediates its diverse physiological effects through at least 14 different receptor subtypes, of which 13 belong to the G-protein-coupled or seven transmembrane-span- ning receptor family (Hoyer et al. 1994). 5-HT receptors are defined with operational, structural and transductional criteria (Hoyer et al. 1994), and form seven discrete fami- lies, including three subtypes positively coupled to ade- nylyl cyclase (5-HT 4 , 5-HT 6 and 5-HT 7 ) through G s . Where- as many of the 5-HT receptors are encoded by a single exon, it is now recognized that RNA editing (e.g. 5-HT 2C ; Burns et al. 1997) and alternative mRNA splicing of 5-HT receptor subtypes (Gerald et al. 1995; Olsen et al. 1999) produce splice variants adding another level to receptor complexity. RT-PCR studies have assisted in the identifi- cation of four mammalian 5-HT 7 receptor splice variants that are structurally divergent in their predicted intracellu- lar carboxyl termini (Heidmann et al. 1997). The 5-HT 7(a) receptor was the first splice variant cloned from human with a predicted length of 445 amino acids (h5-HT 7(a) ; Bard et al. 1993). Alternative splicing of a second intron in the coding region located near the carboxy terminus of the human gene generates a 432-amino-acid truncated re- ceptor splice variant (h5-HT 7(b) ) due to a five-nucleotide base insertion which introduces an in-frame stop codon (Heidmann et al. 1997; Jasper et al. 1997; Stam et al. 1997). Heidmann et al. (1997) have identified two additional splice variants, designated 5-HT 7(c) (rat) and 5-HT 7(d) (hu- Kurt A. Krobert · Trond Bach · Trygve Syversveen · Ane Marit Kvingedal · Finn Olav Levy The cloned human 5-HT 7 receptor splice variants: a comparative characterization of their pharmacology, function and distribution Naunyn-Schmiedeberg’s Arch Pharmacol (2001) 363 : 620–632 DOI 10.1007/s002100000369 Received: 30 June 2000 / Accepted: 20 October 2000 / Published online: 22 March 2001 ORIGINAL ARTICLE K.A. Krobert · F.O. Levy (✉) MSD Cardiovascular Research Center, Rikshospitalet University Hospital and Department of Pharmacology, University of Oslo, P.O. Box 1057 Blindern, 0316 Oslo, Norway e-mail: f.o.levy@klinmed.uio.no, Tel.: +47-22840237, Fax: +47-22840202 T. Bach · T. Syversveen · A.M. Kvingedal MSD Cardiovascular Research Center and Institute for Surgical Research, University of Oslo, Rikshospitalet University Hospital, 0027 Oslo, Norway © Springer-Verlag 2001