The cloned human 5-HT 7 receptor splice variants: a comparative characterization of their pharmacology, function and distribution

Abstract

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.

Ras-dependent ERK Activation by the Human Gs-coupled Serotonin Receptors 5-HT4(b) and 5-HT7(a)

“…Plasmids-The pcDNA3.1(Ϫ) vectors encoding the human 5-HT 4(b) and 5-HT 7(a) receptors were described previously (39,40). The pcDNA3.1 vector encoding the dominant negative Rap1, RapN17, was provided by Dr. Philip J. S. Stork (Oregon).…”

“…Thus, there is a need to determine whether 5-HT 4 and 5-HT 7 receptors can activate ERK, as well as the mechanism for such activation. We have previously characterized activation of adenylyl cyclase by different splice variants of the human G s -coupled 5-HT 4 (39) and 5-HT 7 (40) receptors. We now demonstrate that both the human G s -coupled serotonin receptors 5-HT 4(b) and 5-HT 7(a) , when transiently expressed in HEK293 and COS-7 cells, activate ERK1/2 through a mechanism dependent on Ras and independent of Rap1.…”

Serotonin decreases generation of dopaminergic neurons from mesencephalic precursors via serotonin type 7 and type 4 receptors

“…In addition to its effect on 5-HT 2 receptors, Ritanserin is also a 5-HT 7 antagonist. However, the affinity for 5-HT 2 receptors is much higher than for 5-HT 7 receptors (Hoyer et al, 1993;Krobert et al, 2001). Any possible effect on 5-HT 7 receptors (i.e., increase in generation of TH-ir cells) may therefore be counteracted by the effect on 5-HT 2 receptors (i.e., decrease in generation of TH-ir cells).…”

Distribution of 5-HT3, 5-HT4, and 5-HT7Receptors Along the Human Colon

“…[22][23][24][25][26] Immunohistochemistry studies have pinpointed co-expression of A, B, C, D, and E subunits in enterocytes and also the nerves of the myenteric plexus 24,27 indicating that heteromeric 5-HT 3 receptors are likely to form in the human colon. Although 5-HT 4 and 5-HT 7 receptor splice variants have been identified in separate studies in the small intestine and colon, 22,[28][29][30] no study has looked at their tissue distribution along the colon. However, one earlier study showed that the 5-HT 3 receptor subunit transcript distribution differed in the sigmoid colon 22 providing evidence that specific subunits may be potential targets.…”