Changes in the connections of the main olfactory bulb after mitral cell selective neurodegeneration

Abstract

[EN]The connections of the main olfactory bulb (OB) of themouse were studied with iontophoretic injections of bi-otinylated dextran amine. To sort efferences from mitralcells and tufted cells, the Purkinje cell degeneration(PCD) mouse was used. This mutant animal undergoesa specific neurodegeneration of mitral cells, whereastufted cells do not degenerate. The unilateral tracerinjections used were small and confined largely to theOB of both PCD and control mice at P120. Seven daysafter tracer injection, the efferences from the OB andthe centrifugal afferences from secondary olfactorystructures to it were studied. Although there is a largeoverlap of their target fields, mitral cell axons inner-vated more caudal regions of the olfactory cortex thantufted cell axons, thus providing definitive evidence ofthe differential projections of olfactory output neurons.Additionally, an important increase in retrogradely-labeled neurons was detected in the ipsilateral anteriorolfactory nucleus of the mutant animals. This was notobserved in any other secondary olfactory structure,suggesting a strengthening of the centrifugal input tothe OB from that central area after mitral cell loss.Moreover, we recorded a complete loss of bilaterality inthe olfactory connections of the PCD mice due todegeneration of the anterior commissure. These resultspoint to an important reorganization of this essentialolfactory circuit between the anterior olfactory nucleusand the OB, and hint at a transsynaptic level of plastic-ity not considered previously in literature