Skeletal effects of serotonin (5-hydroxytryptamine) transporter inhibition: evidence from in vitro and animal-based studies.

The regulation of bone metabolism continues to be an area of intense investigation, with recent evidence indicating a potential contribution from the neural system. In particular, the neurotransmitter serotonin (5-hydroxytryptamine [5-HT]) has been hypothesized to play a role in skeletal metabolism via its transporter (5-HTT). The 5-HTT is a plasma membrane transporter that is highly specific for the uptake of extracellular 5-HT, thereby facilitating the intracellular storage and/or degradation of 5-HT. The 5-HTT is clinically important as it is the key target of pharmaceutical agents aimed at treating affective disorders, such as major depressive disorder. By antagonizing the 5-HTT, selective serotonin reuptake inhibitors (SSRIs) potentiate 5-HT activity and effectively relieve the symptoms of depression. However, questions have been raised regarding the potential skeletal effects of SSRIs given the recent identification of a functional 5-HTT and functional 5-HT receptors in bone cells. This paper discusses the preclinical evidence for the skeletal effects of 5-HT and the inhibition of the 5-HTT. In particular, it discusses the: (1) role of 5-HT and the function of the 5-HTT; (2) presence of functional 5-HTTs in bone; (3) potential sources and response mechanisms for 5-HT in bone, and; (4) in vitro and in vivo skeletal effects of 5-HT and 5-HTT inhibition.