PC12 (undifferentiated and differentiated) and C6 cells have been used to investigate kinetics, morphological and functional endpoints following exposure to MnCl(2) and manganic transferrin (Mn-Tf). [Mn](i) in undifferentiated (non-differentiated cells) exposed to both free (MnCl(2)) and bound Mn (Mn-Tf), was three- to fivefold lower as compared to differentiated (differentiated) PC12 cells and higher by one order of magnitude as compared to glial C6 cells. Exposure to both MnCl(2) and Mn-Tf was followed by time- and dose-dependent morphological changes characteristic of apoptosis, which was never observed in Mn-exposed C6 glial cells. Results from cell viability assays were consistent with apoptotic response rates quantified by cell count. Threshold concentrations for undifferentiated and differentiated PC12 cells were 10(-6) and 10(-5)m, respectively. Thus, despite their greater ability to accumulate Mn, differentiated PC12 cells are less sensitive to Mn-induced apoptosis. This model might be relevant to neuronal degeneration induced by Mn occurring in the developing brain and possibly in clinical manganism. Such critical doses at the cellular level seem to be consistent with Mn levels (5x10(-6)m) recorded in the basal ganglia of monkeys chronically exposed to Mn and developing clinical signs of manganism.