Movement of mitochondria in the axons and dendrites of cultured hippocampal neurons

Mitochondria generate ATP and are involved in the regulation of cytoplasmic calcium levels. It is thought that local demand for mitochondria differs between axons and dendrites. Moreover, it has been suggested that the distribution of both energy need and calcium flux in dendrites changes with patterns of synaptic activation, whereas the distribution of these demands in axons is stable. The present study sought to determine whether there are differences in mitochondrial movements between axons and dendrites that may relate to differences in local mitochondrial demand. We labeled the mitochondria in cultured hippocampal neurons with a fluorescent dye and used time-lapse microscopy to examine their movements. In both axons and dendrites, approximately one-third of the mitochondria were in motion at any one time. In both domains, approximately 70% of the mitochondria moved in the anterograde direction, whereas the remainder moved in the retrograde direction. The velocity of the movements in each direction in each domain ranged from 0.1 microm/sec to approximately 2 microm/sec, and the means and distributions of the velocities were similar. Only one difference in the behavior of mitochondria between axons and dendrites emerged from this analysis. Mitochondria in axons were more likely to move with a consistently rapid velocity than were those in dendrites. As a result, mitochondria in axons tended to travel farther than mitochondria in dendrites. These results suggest that the transport of mitochondria in axons and dendrites is similar despite any differences in mitochondrial demand between the two domains.