Mitochondria mass is low in mouse substantia nigra dopamine neurons: Implications for Parkinson's disease

In Parkinson's disease (PD) there is a selective loss of certain midbrain dopaminergic (DA) neurons. The most vulnerable neurons reside in the substantia nigra zona compacta (SNC), whereas the DA neurons in the ventral tegmental area (VTA) and interfascicular (IF) nucleus are less vulnerable to degeneration. Many sporadic PD patients have a defect in mitochondria respiration, and some of the genes that cause PD are mitochondrial-related (e.g., PINK1, Parkin, DJ1). The present study sought to determine whether mitochondria mass is different in SNC neurons compared to other midbrain DA neurons and to non-DA neurons in the mouse. At the electron microscopic level, mitochondria in the SN DA neurons occupy 40% less of the soma and dendritic area than in the SN non-DA neurons. The area occupied by mitochondria in the SN DA neurons is also lower than in the VTA neurons, although not different from the IF neurons. The red nucleus somata have the largest percentage of the somata occupied by mitochondria (12%). Mitochondria size is related to somata size; the largest mitochondria are found in the red nucleus neurons and the smallest mitochondria are found in the IF neurons. At the light microscopic level, SNC, VTA and IF DA neurons have <50% of the cytoplasm immunostained with the mitochondrial antibody 1D6, whereas non-DA neurons in the same midbrain regions contain mitochondria areas up to >65% of the cytoplasm area. These data indicate that mitochondria size and mass are not the same for all neurons, and the SNC DA neurons have relatively low mitochondria mass. The low mitochondria mass in SNC DA neurons may contribute to the selective vulnerability of these neurons in certain rodent models of PD.