What is the source of the proximal excitatory input to granule cells?
Previous work suggested two potential sources of excitatory, glutamatergic inputs to the proximal dendrites of granule cells: local collaterals of mitral cell axons and centrifugal feedback projections from cortical regions. We used two-photon imaging and a combined olfactory bulb/piriform cortex slice preparation to determine whether centrifugal projections from piriform cortex could generate proximal 2PGMS-like postsynaptic responses in granule cells. We first asked whether these slices maintained any of the feedback projections from piriform cortex that normally innervate the granule cell layer in the olfactory bulb (de Olmos et al., 1978; Haberly and Price, 1978; Shipley and Adamek, 1984). We tested this by making focal DiI injections in APC in fixed slices (3 mM; 500 ms pressure pulse duration; 2 psi; n = 8 slices). After waiting 14 d for DiI to diffuse throughout the axonal arborizations, we were able to visualize abundant labeled axons, many with en passant terminals, in the gra
