ABSTRACT
PURPOSE: Recent research has suggested that serotonin may play an important role in the expression of radiation-induced bystander effects. Serotonin levels in serum were reported to range from 6-22 µM and to correlate inversely with the magnitude of cellular colony-forming ability in medium transfer bystander assays. That is, high serotonin concentration correlated with a low cloning efficiency in cultures receiving medium derived from irradiated cells. METHODS: Because of the potential importance of this observation, the European Union's Non-targeted Effects Integrated Project (NOTE) performed an inter-comparison exercise where serum samples with high and low serotonin levels were distributed to seven laboratories which then performed their own assay to determine the magnitude of the bystander effect. RESULTS: The results provided some support for a role for serotonin in four of the laboratories. Two saw no difference between the samples and one gave inconclusive results. In this summary paper, full data sets are presented from laboratories whose data was inconclusive or insufficient for a full paper. Other data are published in full in the special issue. CONCLUSION: The data suggest that there may be multiple bystander effects and that the underlying mechanisms may be modulated by both the culture conditions and the intrinsic properties of the cells used in the assay.
