We describe the cloning and characterization of a human 5‐HT6 serotonin receptor. The open reading frame is interrupted by two introns in positions corresponding to the third cytoplasmic loop and the third extracellular loop. The human 5‐HT6 cDNA encodes a 440‐amino‐acid polypeptide whose sequence diverges significantly from that published for the rat 5‐HT6 receptor. Resequencing of the rat cDNA revealed a sequencing error producing a frame shift within the open reading frame. The human 5‐HT6 amino acid sequence is 89% similar to the corrected rat sequence. The recombinant human 5‐HT6 receptor is positively coupled to adenylyl cyclase and has pharmacological properties similar to the rat receptor with high affinity for several typical and atypical antipsychotics, including clozapine. The receptor is expressed in several human brain regions, most prominently in the caudate nucleus. The gene for the receptor maps to the human chromosome region 1p35–p36. This localization overlaps that established for the serotonin 5‐HT1Dα receptor, suggesting that these may be closely linked. Comparison of genomic and cDNA clones for the human 5‐HT6 receptor also reveals an RsaI restriction fragment length polymorphism within the coding region.

The concentrations of HHV-6 genome in lung tissue and their relation to changes in serologic titers support an association between HHV-6 infection and idiopathic pneumonitis in immunocompromised hosts.

The serotonin (5-HT) 5-HT7 receptor subtype is thought to mediate a number of physiological effects in mammalian brain and periphery. Previous studies suggested that alternative splicing might contribute to 5-HT7 receptor diversity as well. We now report that alternative splicing in human and rat tissues produces four 5-HT7 receptor isoforms that differ in their predicted C-terminal intracellular tails. Human and rat partial 5-HT7 cDNAs and intronic sequences were identified and compared. In rat tissues, three 5-HT7 isoforms, here called 5~HT7iai,5 HT7(b), and 5-HT7101, are found. Rat 5HT71a1 [448-amino acid (aa)] and 5-HT7(b) (435-aa) forms arise from alternative splice donor sites. A third new isoform found in rat, 5-HTJ(C) (470-aa), results from a retained exon cassette. Three 5-HT7 mRNA isoforms were also identified in human tissues, where only one isoform was previously described. Two human isoforms represent 5-HT71~1and 5-HT7(b) forms (445-and 432-aa), but the third form does not correspond to 5-HT7101. Instead, it constitutes a distinct isoform, 5-HT7(d) (479-aa), resulting from retention of a separate exon cassette. 5-HT7(d) transcripts are not present in rat because the 5-HT71~1-specifyingexon is absent from the rat 5-HT7 gene. A frame-shifting homologue of the rat 5-HT71~1-specifyingexon is present in the human gene but is not used in the human tissues examined. Tissue-specific splicing differences are present in human between brain and spleen. These studies suggest that alternative splicing may contribute to diversity of 5-HT7 receptor action and that the human and rat repertoires of 5-HT7 splice variants are substantially different.

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The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) plays an important regulatory role in developing and adult nervous systems. With the exception of the 5-HT 3 receptor, all of the cloned serotonin receptors belong to the G protein-coupled receptor superfamily. Subtypes 5-HT 6 and 5-HT 7 couple to stimulation of adenylyl cyclases through G s and display high affinities for antipsychotic and antidepressant drugs. In the brain, mRNA for 5-HT 6 is found at high levels in the hippocampus, striatum, and nucleus accumbens. 5-HT 7 mRNA is most abundant in the hippocampus, neocortex, and hypothalamus. To better understand how serotonin might control cAMP levels in the brain, we coexpressed 5-HT 6 or 5-HT 7A receptors with specific isoforms of adenylyl cyclase in HEK 293 cells. The 5-HT 6 receptor functioned as a typical G s -coupled receptor in that it stimulated AC5, a G s -sensitive adenylyl cyclase, but not AC1 or AC8, calmodulin (CaM)-stimulated adenylyl cyclases that are not activated by G s -coupled receptors in vivo. Surprisingly, serotonin activation of 5-HT 7A stimulated AC1 and AC8 by increasing intracellular Ca 2؉ . 5-HT also increased intracellular Ca 2؉ in primary neuron cultures. These data define a novel mechanism for the regulation of intracellular cAMP by serotonin. Serotonin (5-hydroxytryptamine, 5-HT)1 is a ubiquitous neurotransmitter that elicits a variety of physiological effects peripherally and centrally (1-3). A growing family of plasma membrane receptors bind 5-HT and mediate its cellular effects (4). All of the 5-HT receptors except 5-HT 3 belong to the superfamily of G protein-coupled receptors. The recently cloned 5-HT 6 (5, 6) and 5-HT 7 (7-10) receptors activate adenylyl cyclase(s) through the heterotrimeric G protein G s , and expression of these receptors in cultured cells couples serotonin to increases in cAMP (6,8). 5-HT 6 and 5-HT 7 display high affinities for antipsychotic and antidepressant drugs including clozapine, amoxapine, and amitryptiline (5-8), suggesting a role for these receptors in affective function. In the brain, the 5-HT 6 receptor is most highly expressed in the hippocampus, nucleus accumbens, striatum, and limbic regions, and the 5-HT 7 receptor is found in the hypothalamus, hippocampus, and cortex. The distribution of these receptors in brain is consistent with the hypothesis that they play a role in mood and affect (8,(11)(12)(13). In addition, the expression of the 5-HT 7 receptor in the suprachiasmatic nucleus (SCN) and the ability of serotonin and cAMP to advance the mammalian circadian rhythm indicates that 5-HT 7 plays an important role in circadian physiology (10, 14, 15). 5-HT 7 receptors are also expressed in glial cells (16,17) and at lower levels in peripheral tissues including the spleen, intestine, and vascular smooth muscle (7-9, 18).Many psychotherapeutic agents modulate the cAMP signaling pathway (19) and perturbation of this signal transduction system may contribute to some affective disorders. To date, nine distinct cDNA clones for mammalian...

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