Figure 7. Cocaine results in the formation of large swellings filled with mitochondria in Apex2+ dopaminergic (DA) axons.

(A) Left: 2D electron microscopy (EM) image of Apex2 labeled DA axon bulb in cocaine-treated mouse (square) as compared to a neighboring neuronal soma (circle). Right: zoomed in 2D EM image of Apex2 DA axon bulb from left panel. (B) Scatter plot of the number of large swellings versus axon length (μm) + saline: 0.00 ± 0.0 swellings/µm length of axon, n = 29 axons; two mice + cocaine: 0.04 ± 0.02 swellings/µm length of axon, n = 30 axons, two mice. p = 1.7e-5. (C) Reconstructions of two representative Apex+ dopamine axons with large swellings (asterisk) and medium sized swellings (green spheres). Top reconstruction depicts an axon with a terminal axon bulb and bottom reconstruction shows one along the axon. (D) Left: 2D EM image of Apex+ large DA axon swellings (red) filled with mitochondria (two examples highlighted in purple) found in the nucleus accumbens (NAc) of cocaine-treated animals. Both swellings are filled with mitochondria (examples highlighted in blue). Right: 3D segmentation of swelling and extremely long and coiled mitochondria found inside. Only the top half of the DA axon swelling is depicted to illustrate the mitochondria contained within. In this example, the swelling is at the end of the DA axon where it is attached to a thinner portion of the axon (arrowheads). Scale bar: (A) left: 10 µm, right: 5 µm, (C) 20 μm, (D) 500 nm. Data: mean ± SEM. p-Values: two-tailed Mann-Whitney U test.