Morphometric modifications associated with early sensory experience in the rat olfactory bulb: I. Volumetric study of the bulbar layers.

Current models of sensory coding in the olfactory bulb are based on the notion of topographical specificity in the processing of stimuli. Part of this hypothesis comes from studies of patterns of radiolabelled 2-deoxyglucose uptake, and local morphometric variations of the mitral cell size observed following prolonged exposure to an odor. The present study explored the possibility that exposing young rats to a long-term stimulation with an odor would induce spatially distributed volumetric variations of the bulbar layers. Three groups of 5 rats have been studied: (1) stimulated with ethyl acetoacetate from birth to 1 month of age, (2) unilaterally deprived following early cauterization of one nostril, and (3) normal animals of same age. Fourteen frontal histological sections uniformly distributed along the rostrocaudal axis of the olfactory bulb were used for this study. Areas of the bulbar layers were measured with the aid of an image analyser and the volume of the corresponding layers deduced by computation. Following complete sensory deprivation, the volume of all bulbar layers, except the periventricular core, was homogeneously reduced along the rostrocaudal axis of the olfactory bulb. Following long-term stimulation with ethyl acetoacetate, volume reduction was significantly higher in anterior and middle regions than in the posterior part of the olfactory bulb. It is assumed that neuronal pathways activated by ethyl acetoacetate stimulation are mainly located in the posterior part of the olfactory bulb. Functional interpretations of these results are discussed with respect to the spatial dimension in olfactory coding.