The present studies were undertaken to determine whether neuronal subsets in normal brains constitutively express functionally competent C5a receptors. In situ hybridization studies coupled with immunohistochemical approaches revealed that most neurons in the hippocampal formation, many pyramidal cortical neurons, and cerebellar Purkinje neurons in normal human and murine brains constitutively express C5a receptors. Neuronal C5a receptors bound C5a-coated fluorescent microspheres, and primary rodent hippocampal neurons responded to C5a with increased calcium fluxes via a pertussis-sensitive, presumably Gi-coupled protein. Additional studies with human neuroblastoma cells conducted to address the functional role of C5a receptors revealed that C5a triggered rapid activation of protein kinase C and activation and nuclear translocation of the NF-kappa B transcription factor. In addition, C5a was found to be mitogenic for undifferentiated human neuroblastoma cells, a novel action for the C5aR. In contrast, C5a protected terminally differentiated human neuroblastoma cells from toxicity mediated by the amyloid A beta peptide. Thus, normal rodent hippocampal neurons as well as undifferentiated and differentiated human neuroblastoma cells express functional C5a receptors. These results have implications for understanding the role of neuronal C5aR receptors in normal neuronal development, neuronal homeostasis, and neuroinflammatory conditions such as Alzheimer's disease.