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 ␣-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 main highly differentiated cell types in bone tissue are the osteoclast, the osteoblast (OB), 1 and the osteocyte (OCY). Osteoclasts are multinucleated cells of hemopoietic origin and are related to the monocyte/macrophage (1, 2, 3). Osteoblasts on the other hand are of mesenchymal stock. Their stem and progenitor cells are present in the periosteum, endosteum, and bone marrow stroma (4). Osteocytes differentiate from osteoblasts. During the process of bone formation, a number of cells from the osteoblast layer is encapsulated in the newly formed bone matrix, whereas the other cells retract with the bone formation front. The cells that are incorporated in the bone matrix change from a cuboidal into a more stellate-shaped morphology and become osteocytes. The result of this process is a three-dimensional cellular network of osteocytes within the mineralized matrix connected to each other and to the bone surface cells by thin cell processes (5).Together these three cell types produce, maintain, and adapt or repair, if necessary, the mineralized bone matrix. Osteoclasts are capable of bone resorption, osteoblasts of bone formation. The function of osteocytes is less well understood. Because of their location within the bone matrix, osteocytes are in a prime position for the detection of mechanical inadequacies of the tissue. They are believed to act as sensor cells that translate mechanical stimuli resulting from gravitational and muscular forces on the skeleton into biochemical signals. These biochemical signals in turn activate the effectors of bone turnover, osteoclasts...

Recent studies have proposed a role for serotonin and its transporter in regulation of bone cell function. In the present study, we examined the in vitro effects of serotonin and the serotonin transporter inhibitor fluoxetine "Prozac" on osteoblasts and osteoclasts. Human mononuclear cells were differentiated into osteoclasts in the presence of serotonin or fluoxetine. Both compounds affected the total number of differentiated osteoclasts as well as bone resorption in a bell-shaped manner. RT-PCR on the human osteoclasts demonstrated several serotonin receptors, the serotonin transporter, and the rate-limiting enzyme in serotonin synthesis, tryptophan hydroxylase 1 (Tph1). Tph1 expression was also found in murine osteoblasts and osteoclasts, indicating an ability to produce serotonin. In murine pre-osteoclasts (RAW264.7), serotonin as well as fluoxetine affected proliferation and NFkappaB activity in a biphasic manner. Proliferation of human mesenchymal stem cells (MSC) and primary osteoblasts (NHO), and 5-HT2A receptor expression was enhanced by serotonin. Fluoxetine stimulated proliferation of MSC and murine preosteoblasts (MC3T3-E1) in nM concentrations, microM concentrations were inhibitory. The effect of fluoxetine seemed direct, probably through 5-HT2 receptors. Serotonin-induced proliferation of MC3T3-E1 cells was inhibited by the PKC inhibitor (GF109203) and was also markedly reduced when antagonists of the serotonin receptors 5-HT2B/C or 5-HT2A/C were added. Serotonin increased osteoprotegerin (OPG) and decreased receptor activator of NF-kappaB ligand (RANKL) secretion from osteoblasts, suggesting a role in osteoblast-induced inhibition of osteoclast differentiation, whereas fluoxetine had the opposite effect. This study further describes possible mechanisms by which serotonin and the serotonin transporter can affect bone cell function.

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