Odour-induced c-fos expression in the rat olfactory bulb: involvement of centrifugal afferents

Expression of the proto-oncogene c-fos is known to increase in granule cells of the olfactory bulb following a sustained olfactory stimulation. Most granule cells displaying high levels of Fos accumulation are located in the bulbar columns defined by the odour-induced foci of high 2-deoxyglucose glomerular uptake. The present studies were undertaken in order to assess the possible involvement of centrifugal afferents in the modulation of odour-induced patterns of either 2-deoxyglucose accumulation or c-fos expression in the olfactory bulb. A unilateral olfactory peduncle section had no effect on the odour-induced 2-deoxyglucose foci but induced a significant decrease in the number of Fos-containing neurons in odour-selective areas of both olfactory bulbs, ipsilateral and contralateral to the lesion. This suppressive effect was much more pronounced in the side ipsilateral to the peduncle section. It is concluded that c-fos expression induced by a sustained stimulation with propionic acid vapours is not only determined by the olfactory peripheral input but also by afferents of central origin. In order to estimate the contingent involvements of the cholinergic and noradrenergic afferents in this control of c-fos expression, we attempted to mimic the effects of the surgical deafferentation on odour-induced c-fos expression by using a pharmacological approach with selective cholinergic and noradrenergic antagonists. The beta-adrenergic antagonist propanolol induced a suppression of the odour-related patterns of Fos accumulation similar to the one caused by the surgical deafferentation of the olfactory bulb. The muscarinic antagonist scopolamine did not alter c-fos expression in the odour-selective area but increased significantly Fos labelling in the other bulbar aspects. Pharmacological investigations indicate that the noradrenergic and cholinergic centrifugal systems are likely involved in the central modulation of c-fos expression in the OB. The Fos protein could be expressed as an early nuclear signal triggering further long-term modifications of the neuronal phenotype, in certain conditions of sensory stimulation involving the activation of centrifugal systems.