Early specification of GAD67 subventricular derived olfactory interneurons

Olfactory bulb interneurons are continuously generated in the subventricular zone (SVZ) and migrate along the rostral migratory stream (RMS) into the olfactory bulb (OB) where the majority becomes local GABAergic interneurons. We previously showed that SVZ-derived progenitor cells expressed glutamic acid decarboxylase 65 kDa (GAD65) very early in the migratory pathway. However, only approximately half of OB GABAergic interneurons use GAD65, an equal number express the 67 kDa GAD enzyme. To investigate the differentiation of these GABAergic interneurons we examined their migration in a transgenic mouse expressing green fluorescent protein (GFP) under the control of the GAD67 promoter. In adult, GFP was expressed by a subpopulation of migratory cells in the SVZ and along the RMS. Using Doublecortin (DCX) as a marker of migrating neuroblasts and bromodeoxyuridine (BrdU) incorporation, we show that these GAD67-GFP neurons co-express DCX and incorporate BrdU indicating they are newly born migratory neuroblasts. This is similar to GAD65 transgene expression, and in contrast to dopaminergic interneuron transgene expression which occurs only after cells reach the olfactory bulb. Although the GAD65/67 transgenes are expressed early in migration, there is minimal protein production in the cells prior to reaching the OB. These results suggest that migrating SVZ-derived neuroblasts acquire GABAergic identity prior to reaching their final location in the olfactory bulb.     

The localization of neuronal nitric oxide synthase may influence its role in neuronal precursor proliferation and synaptic maintenance

Neuronal nitric oxide synthase (nNOS) is implicated in some developmental processes, including neuronal survival, differentiation, and precursor proliferation. To define the roles of nNOS in neuronal development, we utilized the olfactory system as a model. We hypothesized that the role of nNOS may be influenced by its localization. nNOS expression was developmentally regulated in the olfactory system. During early postnatal development, nNOS was expressed in developing neurons in the olfactory epithelium (OE), while in the adult its expression was restricted to periglomerular (PG) cells in the olfactory bulb (OB). At postnatal week 1 (P1W), loss of nNOS due to targeted gene deletion resulted in a decrease in immature neurons in the OE due to decreased proliferation of neuronal precursors. While the pool of neuronal precursors and neurogenesis normalized in the nNOS null mouse by P6W, there was an overgrowth of mitral or tufted cells dendrites and a decreased number of active synapses in the OB. Cyclic GMP (cGMP) immunostaining was reduced in the OE and in the glomeruli of the OB at early postnatal and adult ages, respectively. Our results suggest that nNOS appears necessary for neurogenesis in the OE during early postnatal development and for glomerular organization in the OB in the adult. Thus, the location of nNOS, either within cell bodies or perisynaptically, may influence its developmental role.