Microglia are believed to play an important role in the regulation of phagocytosis, neuronal survival, neuronal cell death, and inflammation. Recent studies have demonstrated that microglia are multipotential stem cells that give rise to neurons, astrocytes, and oligodendrocytes. However, the functional properties of neurons derived from microglia are poorly understood. In this study, we investigated the possibility that microglia differentiate into functional neurons. Immunocytochemical study demonstrated that microtubule-associated protein 2 (MAP2)-positive cells were derived from microglia under differentiation conditions. Intracellular Ca2+ imaging study demonstrated that KCl caused no significant changes in [Ca2+](i) in microglia, whereas it caused a remarkable increase in [Ca2+](i) in microglia-derived cells. Furthermore, electrophysiological study demonstrated that the spike waveform, firing rate, and tetrodotoxin sensitivity of extracellular action potentials evoked by 4-aminopyridine from microglia-derived NIAP2-positive cells were nearly identical to those from cultured cortical neurons. These results suggest that microglia-derived NIAP2-positive cells possess properties of functional neurons. (C) 2008 Elsevier Inc. All rights reserved.