The mouse 5-HT2B receptor: homologous subtype or species variant?

The serotonin binding site of human and murine 5-HT2B receptors: molecular modeling and site-directed mutagenesis

Bacteriorhodopsin and rhodopsin crystal structures were used as templates to build structural models of the mouse and human serotonin (5-HT)-2B receptors (5-HT(2B)Rs). Serotonin was docked to the receptors, and the amino acids predicted to participate to its binding were subjected to mutagenesis. 5-HT binding affinity and 5-HT-induced inositol triphosphate production were measured in LMTK(-) cells transfected with either wild-type or mutated receptor genes. According to these measurements, the bacteriorhodopsin-based models of the 5-HT(2B)Rs appear more confident than the rhodopsin-based ones. Residues belonging to the transmembrane domains 3 and 6, i.e. Asp(3.32), Ser(3.36), Phe(6.52), and Asn(6.55), make direct contacts with 5-HT. In addition, Trp(3.28), Phe(3.35), Phe(6.52), and Phe(7.38) form an aromatic box surrounding 5-HT. The specificity of human and mouse 5-HT(2B)Rs may be reflected by different rearrangements of the aromatic network upon 5-HT binding. Two amino acids close to Pro(5.50) in the human transmembrane domain 5 sequence were permuted to introduce a "mouse-like" sequence. This change was enough to confer the human 5-HT(2B)R properties similar to those of the mouse. Taken together, the computed models and the site-directed mutagenesis experiments give a structural explanation to (i) the different 5-HT pK(D) values measured with the human and mouse 5-HT(2B)Rs (7.9 and 5.8, respectively) and (ii) the specificity of 5-HT binding to 5-HT(2B)Rs as compared with other serotonergic G-protein coupled receptors.

Expression of serotonin receptors in bone

The 5-hydroxytryptamine (5-HT) receptors 5-HT(2A), 5-HT(2B), and 5-HT(2C) belong to a subfamily of serotonin receptors. Amino acid and mRNA sequences of these receptors have been published for several species including man. The 5-HT(2) receptors have been reported to act on nervous, muscle, and endothelial tissues. Here we report the presence of 5-HT(2B) receptor in fetal chicken bone cells. 5-HT(2B) receptor mRNA expression was demonstrated in osteocytes, osteoblasts, and periosteal fibroblasts, a population containing osteoblast precursor cells. Pharmacological studies using several agonists and antagonists showed that occupancy of the 5-HT(2B) receptor stimulates the proliferation of periosteal fibroblasts. Activity of the 5-HT(2A) receptor could however not be excluded. mRNA for both receptors was shown to be equally present in adult mouse osteoblasts. Osteocytes, which showed the highest expression of 5-HT(2B) receptor mRNA in chicken, and to a lesser extent osteoblasts, are considered to be mechanosensor cells involved in the adaptation of bone to its mechanical usage. Nitric oxide is one of the signaling molecules that is released upon mechanical stimulation of osteocytes and osteoblasts. The serotonin analog alpha-methyl-5-HT, which preferentially binds to 5-HT(2) receptors, decreased nitric oxide release by mechanically stimulated mouse osteoblasts. These results demonstrate that serotonin is involved in bone metabolism and its mechanoregulation.

The molecular biology of serotonin receptors: therapeutic implications for the interface of mood and psychosis

This review summarizes the molecular biology of serotonin (5-HT; 5-hydroxytryptamine) receptors and indicates the potential relevance of this information for the treatment of mood and psychotic disorders. At least 15 separate subtypes of 5-HT receptors have been identified by molecular cloning techniques to be distinct genetic entities. Subtle differences in the primary amino acid sequences of these receptors can yield large differences in ligand selectivity. Additionally, it has recently been discovered that drugs such as atypical antipsychotic drugs and serotonin-selective reuptake inhibitors may interact with a large number of heretofore unknown 5-HT receptors. Thus clozapine, for instance, has high affinity for at least four separate 5-HT receptors, and it is unknown which of these receptors is essential for its unique therapeutic efficacy. One way to approach these questions is to test subtype-selective agents, although there are few of these currently available. Approaches to the design of subtype-selective ligands are described, including structure-based drug design and combinatorial approaches. Modes of regulation of 5-HT receptors are also summarized, and it is emphasized that antipsychotic drugs and antidepressants likely exert their effects via nontranscriptional and posttranslational means. Understanding the cellular mechanisms by which 5-HT receptors are regulated by psychopharmacologic agents is likely to yield novel insights into drug action.

Activation of 5-HT2B receptors in the medial amygdala causes anxiolysis in the social interaction test in the rat

In a recent study, we reported the presence of neurones expressing 5-HT2B receptor protein in the medial amygdaloid nucleus of the adult rat brain. In the present study, bilateral micro-injection of the 5-HT2B receptor agonist 1-[5-(2-thienylmethoxy)-1H-3-indolyl]propan-2-amine hydrochloride (BW 723C86, 0.09 and 0.93 nmol, 5 min pretest) into the medial amygdaloid nuclei increased the total interaction time of a pair of male rats in the social interaction test, to a comparable extent to chlordiazepoxide (5 mg/kg p.o., 1 hr pretest) without altering locomotor activity; indicative of anxiolytic activity. The increase in social interaction was prevented by pretreatment with the 5-HT2C/2B receptor antagonist N-(1-methyl-5-indoyl)-N'-(3-pyridyl) urea hydrochloride (SB 200646A, at 2 but not 1 mg/kg p.o., 1 hr pretest), which did not alter behaviour when given alone. Intra-amygdala BW 723C86 (0.09, 0.31 and 0.93 nmol, 5 min pretest) did not significantly alter the number of punished responses made when the same rats were examined seven days later in a Vogel punished drinking test, although chlordiazepoxide (5 mg/kg p.o., 1 hr pretest) produced the expected anxiolytic profile. The results are consistent with the proposal that activation of 5-HT2B receptors in the medial amygdala induces anxiolysis in the social interaction model but has little effect on behaviour in a punished conflict model of anxiety. These data suggest that serotonergic neurotransmission in this nucleus may selectively affect specific kinds of anxiety generated by different animal models.

Evidence for expression of the 5-hydroxytryptamine-2B receptor protein in the rat central nervous system

The 5-hydroxytryptamine-2B receptor is the most recent addition to the 5-hydroxytryptamine-2 family of G-protein-coupled receptors. In the rat stomach fundus, 5-hydroxytryptamine-2B receptor activation causes contraction; however, its distribution and function in the rat CNS are unclear. By performing immunohistochemistry with an antiserum raised against the N-terminus of the 5-hydroxytryptamine-2B receptor protein, this study identifies receptor expression in longitudinal and circular smooth muscle in the rat stomach fundus and in neurons in discrete nuclei in the cerebellum, lateral septum, dorsal hypothalamus and medial amygdala. The potential function of this receptor in the CNS is discussed.

Immunohistochemical localisation of the serotonin 5-HT2B receptor in mouse gut, cardiovascular system, and brain

We recently reported the cloning of a new member of the serotonin 5-HT2 family, the 5-HT2B receptor. We now report the production and characterisation of a specific antiserum directed against the C-terminal portion of the mouse 5-HT2B receptor. After affinity purification, this polyclonal antibody recognises specifically the mouse 5-HT2B receptor. Immunohistochemical analysis of cryosections from various adult mouse tissues reveals a major 5-HT2B receptor expression in stomach, intestine and pulmonary smooth muscles as well as in myocardium. Furthermore, the antiserum recognises specific areas of the mouse brain, including cerebellar Purkinje cells and their projection areas.