Plasticity of dendrodendritic microcircuits following mitral cell loss in the olfactory bulb of the murine mutant purkinje cell degeneration

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Abstract
Mitral cells of the olfactory bulb typically form reciprocal dendrodendritic synapses with anaxonic interneurons, granule cells, within a sublamina of the external plexiform layer. As a result of mitral cell loss in the murine mutant Purkinjie cell degeneration (PCD), subpopulations of these granule cells are denervated. The present report examines the capacity of these denervated interneurons to form new dendrodendritic microcircuits with a second population of olfactory bulb neurons, tufted cells. Quantitative ultrastructural assessments were made of the morphology and distribution of dendrodendritic circuits in the olfactory bulbs of normal heterozygous litter mates and affected homozygous recessive PCD mice following mitral cell loss. There were no apparent morphological characteristics that distinguished the reciprocal synaptic connections formed by mitral cells from those formed by tufted cells. However, the segregation of mitral cell dendrodendritic circuits in the deep sublamina of the external plexiform layer (EPL) and tufted cell circuits in the superficial sublamina provided the basis for a comparative analysis of synaptic organization following mitral cell loss. Following mitral cell loss there was a significant reduction in the area occupied by characteristic mitral cell dendrites within the deep sublamina of the EPL. A slight but nonsignificant increase in the area occupied by granule cell spines was also observed. The number of synaptic appositions involving granule cells decreased slightly, the number involving tufted cells increased significantly in the mutant mice. This indicates that many granule cell spines survive denervation and establish new reciprocal dendrodendritic synapses at available sites on tufted cells. In both the control and mutant mice the ratios of symmetrical: asymmetrical dendrodendritic synapses closely approached 1. This demonstrates that not only do the denervated spines receive new afferent input from tufted cell dendrites, but they also establish the reciprocal efferent projection. These data are discussed in terms of the sublaminar organization of dendrodendritic microcircuits in the olfactory bulb and their capacity of plasticity and reorganization following pertubation.