Transitory and activity-dependent expression of Neurogranin in olfactory bulb tufted cells during mouse postnatal development.

Neurogranin (Ng) is a brain specific post-synaptic calmodulin-binding protein involved in synaptic activity-dependent plasticity. In the adult olfactory bulb (OB), Ng is expressed by a large population of GABAergic interneurons in the granule cell layer. In this study we show that during postnatal development Ng is also expressed by OB neurons in the superficial external plexiform layer (sEPL) and glomerular layer (GL). These Ng-positive neurons display morphological and neurochemical features of superficial and external tufted cells. Ng expression in these cells is transient during OB development: few elements express Ng at postnatal day (P) 5, to increase in number reaching a peak at P10 and then progressively decrease. At P30 Ng is rarely detectable in these neurons. Ng expression in developing tufted cells is also modulated at the cellular level: while at earlier stages Ng labelling is distributed throughout the cell body and dendritic arborization in the GL, at P20, when the glomerular circuits are fully matured, Ng becomes restricted to the soma and proximal portion of tufted cell apical dendrites. We show that olfactory deprivation at early postnatal stages induces a strong increase in Ng-positive tufted cells from P10 to P20, while no changes have been observed following olfactory deprivation in adult mice. These findings demonstrate that Ng expression in sEPL-GL is restricted to developmental stages and indicate its activity-dependent regulation in a time window critical for glomerular circuit development, suggesting a role for Ng in maturation and dendritic remodeling of tufted cells.