Differences in the Central Nervous System Distribution and Pharmacology of the Mouse 5-Hydroxytryptamine-6 Receptor Compared with Rat and Human Receptors Investigated by Radioligand Binding, Site-Directed Mutagenesis, and Molecular Modeling

Abstract

There is increasing evidence for a role of 5-hydroxytryptamine-6 (5-HT 6 ) receptors in cognitive function. In the rat and human brain, 5-HT 6 receptors are widely expressed and highly enriched in the basal ganglia. However, in the mouse brain, only very low levels of 5-HT 6 receptor mRNA and receptor protein, measured by TaqMan reverse transcriptase-polymerase chain reaction and selective radioligand binding, could be detected, with no evidence of enrichment in the basal ganglia. The mouse receptor was cloned and transiently expressed in human embryonic kidney 293 cells to characterize its pharmacological profile. Despite significant sequence homology between human, rat, and mouse 5-HT 6 receptors, the pharmacological profile of the mouse receptor was significantly different from the rat and human receptors. Four amino acid residues, conserved in rat and human and divergent in mouse receptors, were identified, and various mutant receptors were generated and their pharmacologies studied. Residues 188 (tyrosine in mouse, phenylalanine in rat and human) in transmembrane region 5 and 290 (serine in mouse, asparagine in rat and human) in transmembrane region 6 were identified as key amino acids responsible for the different pharmacological profiles. Molecular modeling of the receptor and docking of selective and nonselective compounds was undertaken to elucidate the ligand receptor interactions. The binding pocket was predicted to be different in the mouse compared with rat and human 5-HT 6 receptors, and the models were in excellent agreement with the observed mutation results and have been used extensively in the design of further selective 5-HT 6 antagonists.

A Null Mutation of the Serotonin 6 Receptor Alters Acute Responses to Ethanol

“…We attempted to confirm loss of protein product via autoradiography using the ligand 125 I-SB258585 (Roberts et al, 2002); however, we were unable to detect specific binding in wild-type brains. This finding is consistent with recent observations indicating that the affinities of ligands used to characterize 5-HT 6 Rs in the rat (such as 125 I-SB258585) demonstrate reduced affinity for the mouse homolog, and that 5-HT 6 R expression levels in the mouse are reduced relative to those in the rat (Hirst et al, 2003). Heterozygote crosses produced wild-type, heterozygous, and homozygous mutant mice in the expected Mendelian ratios, indicating that the mutation does not impair embryonic or postnatal viability.…”

“…Additionally, species differences in 5-HT 6 R mRNA expression may contribute to species differences in behaviors sensitive to 5-HT 6 R function. Recent evidence indicates that differences between mouse and rat/human 5-HT 6 R receptor sequences are associated with differences in their affinities for 5-HT 6 R antagonist compounds, while not altering binding affinities for serotonin (Hirst et al, 2003).…”

Serotonin controlling feeding and satiety

“…In the latter study, however, there was no change of habitual feeding on a normal diet in 5-HT6 receptor knockout mice. The interpretation of these findings should be seen with some caution, as the central distribution of this receptor is different in mice compared to rats and humans [115]. Whereas 5-HT2C…”

Increased Expression of 5-HT6 Receptors in the Rat Dorsomedial Striatum Impairs Instrumental Learning

“…receptors are one of the most recently identified serotonin receptor subtypes (Monsma et al, 1993;Plassat et al, 1993;Ruat et al, 1993), and are coupled to a G stimulatory protein, inducing cAMP through adenylate cyclase stimulation (Monsma et al, 1993). Several in situ hybridization studies have shown robust mRNA expression in rat and human striatum, nucleus accumbens, cortex and olfactory tubercle, with moderate expression in the hippocampus and thalamus (Gerard et al, 1996;Ward and Dorsa, 1996), whereas expression in mouse brain is very low compared to rat or human brain (Hirst et al, 2003). Immunohistochemical localization of 5-HT 6 receptors has been detected in the above regions as well as the cerebellum and substantia nigra (Gerard et al, 1997;Hamon et al, 1999).…”