nd a directionally moving population. A mitochondrion was classified as stationary if it moved less than 0.2 mm over the period of observation. A mitochondrion was considered to be oscillatory if it moved bidirectionally at least 0.2 mm–but not more than 2.5 mm–around a fixed point during the period of observation. A mitochondrion was considered to move directionally, either anterogradely or retrogradely, if it was observed to travel a distance greater than 2.5 mm within the axon. The same classification scheme was also applied in the analysis of dendritic mitochondria. Following a method described previously, the mean speed of each individual mitochondrion in the axonal segment under observation was calculated from the cumulative distance traveled over the entire period of observation. The initial relative position of each mitochondrion along the axonal segment is shown on the y-axis in Fig. 3A and B. The average numbers of stationary, oscillatory, and directionally moving mitochondria were obtained from time-lapse 
images taken in the first and last 15 minutes of each experiment. Prior to treatment with dopamine, most axonal mitochondria were either stationary or moved in an oscillatory manner. D1 receptors are shown in red; D2 receptors are shown in green; MAP2 expression is shown in blue; the merged image indicates colocalization. doi:10.1371/journal.pone.0002804.g001 2 Dopamine and Mitochondria chart insets a and b; Movie S1). Following the administration of dopamine, the directionally moving mitochondria slowed down dramatically, and at the end of two hours, all directional movement was inhibited. The oscillatory population was also significantly diminished. To determine the time of onset of the effect of dopamine on mitochondrial motility, the mean speeds of all directionally moving mitochondria were binned every 15 minutes two hours before and two hours after treatment. As shown in Fig. 3C, there was already a dramatic decrease in the rate of mitochondrial movement 15 minutes after the administration of dopamine. Within 45 minutes following treatment, virtually all mitochondria had essentially stopped moving. Each data set was obtained from five paired time-lapse imaging experiments. Given the observed inhibitory effect of dopamine on mitochondrial motility, we were concerned that the mitochondria in our cultured neurons had been directly impaired by exposure to exogenous monoamine. In other studies of mitochondrial motility, inhibitory effects on mitochondrial movement have generally been consistent with mitochondrial impairment, indicated by a loss of membrane potential and drastic changes in mitochondrial morphology. However, we did not observe any obvious signs of toxicity in neurons treated with the amount of dopamine that we administered; nor did we see any change in mitochondrial membrane potential as visualized with the vital dye JC-1. As expected, when carbonylcyanide p-trifluoromethoxyphenylhydrazone, an uncoupler, was added to a parallel culture as a positive control, JC-1 fluorescence was extinguished over time, indicating a loss of membrane potential. A D2R agonist inhibits mitochondrial transport, whereas a 10884520 D2R antagonist promotes mitochondrial transport Administration of the dopamine 2 receptor agonist, AS-703026 bromocriptine, in the absence of added dopamine markedly inhibited both directional and oscillatory movement of mitochondria. Based on an analysis of 9874164 time-lapse image sequences from five separate paired experiment