Doublecortin expression in unipolar brush cells of the dorsal cochlear nucleus and the vestibulocerebellum: neurogenesis?
Paolone, Nicholas A.
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In our earlier investigations, we saw immunoreactivity to doublecortin (DCX), a microtubule-associated protein critical for neuronal migration and protection of the cytoskeleton, in unipolar brush cells (UBCs) of the dorsal cochlear nucleus (DCN), and in selected regions of the cerebellum including the ventral paraflocculus (vPFL), the transition zone (tz), the flocculus (FL), the uvula (lobule IX), and the nodulus (lobule X). To explore the possibility of continued neurogenesis in the brainstem and vestibulocerebellum, closely spaced sections from the caudal emergence of the DCN and the rostral disappearance of the PFL were immunolabeled for DCX and bromodeoxyuridine (BrdU), a thymidine analog. BrdU is injected systemically and taken up by dividing cells. Three groups of adult rats (ages 3 months–5 months) with different numbers of injections, and time delays after last injection until animals were sacrificed, were administered i.p. injections of BrdU. BrdU-labeled cells were seen in all groups. As time between last injection and sacrifice increased, the number of BrdU immunoreactive (-ir) cells decreased, suggesting death of some of the newly-born cells. To examine the colocalization of DCX and BrdU, we examined closely spaced sections in the brainstem and vestibulocerebellum with immunofluorescence label. Both DCX and BrdU expression were seen in the vPFL, the tz, the flocculus FL, and lobules IX and X but we did not see colocalization. We also looked for DCX expression in the other species. We used immunolabel for DCX in two cats and one monkey ( Macaca maura ) we saw DCX expression in the DCN, vPFL, FL, and tz in one section. We looked for expression of epidermal growth factor receptor kinase substrate 8 (Eps8), a UBC marker and found UBCs. These data suggest that UBCs have the capacity for neuronal plasticity well into adulthood. Since DCX and BrdU were not colocalized in the brainstem and vestibulocerebellum, further investigations need to be done with double-label immunofluorescence for BrdU and glial fibrillary acidic protein (GFAP), to determine if the BrdU-labeled elements are glial cells.