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Purpose: Serotonin (5-hydroxytryptamine), a monoamine neurotransmitter released by prostate neuroendocrine cells, has a fundamental role in tumor growth, differentiation and gene expression. We investigated the effect of 5-hydroxytryptamine and 5-hydroxytryptamine antagonists on the growth of prostate cancer cells and we identified 5-hydroxytryptamine receptor expression in PC3 cells and in human hormone refractory prostate cancer tissue. Materials and Methods: A total of 12 preparations of hormone refractory PC3 human prostate cancer cells were incubated with 5-hydroxytryptamine, or the 5-hydroxytryptamine receptor antagonists 5-hydroxytryptamine1A, 1B, 1D, 2, 3 or 4. After 72 hours cell viability was assessed using the crystal violet assay. PC3 cells treated with 5-hydroxytryptamine1A and 1B antagonists were investigated for apoptosis using flow cytometry. PC3 cells and sections of hormone refractory human prostate cancer tissue were studied by immunohistochemistry and Western blot analysis. Results: In PC3 cells 5-hydroxytryptamine caused dose dependent proliferation with a maximum increase of 15% in 12 preparations at a concentration of 10−8 M at 72 hours compared to controls (p <0.0001). At a concentration of 10−4 M at 72 hours the 5HT1A antagonist NAN-190 hydrobromide and the 5-hydroxytryptamine1B antagonist SB224289 HCl (Tocris Laboratories, Bristol, United Kingdom) induced a 20% and 78% inhibitory effect, respectively, on PC3 cell growth compared to that in controls (p <0.0001). In PC3 cells 5-hydroxytryptamine1A and 1B antagonists demonstrated apoptosis after 24 and 48 hours of incubation. Immunostaining for 5-hydroxytryptamine1A and 1B receptors was seen in PC3 cells and prostate cancer tissue. Western blot analysis demonstrated 5-hydroxytryptamine1A and 1B receptor proteins with 46 and 43 kDa bands, respectively. Conclusions: In PC3 prostate cancer cells 5-hydroxytryptamine1A and to a greater extent 5-hydroxytryptamine1B antagonists significantly inhibit growth and induce apoptosis. To our knowledge growth inhibition caused by the 5-hydroxytryptamine1B antagonist SB224289 HCl is a novel finding, as is apoptosis caused by the 2 antagonists 5-hydroxytryptamine1A and 1B. This effect is most likely mediated via 5-hydroxytryptamine1A and 1B receptors. Therefore, our results imply that 5-hydroxytryptamine1A and in particular 5-hydroxytryptamine1B receptor antagonists warrant further investigations as potential anti-neoplastic agents. [Copyright &y& Elsevier]