Olfactory neurons are interdependent in maintaining axonal projections

In mice, individual olfactory neurons express one of the thousand distinct olfactory receptor genes [1-3]. Neurons that express a given receptor conve rge on distinct loci in the olfactory bulb to form structures called glomer uli [4-6]. The olfactory receptor is involved in an Instructive manner in t his axonal convergence [6,7] but little is known about the mechanisms invol ved in maintaining convergence. We have previously created a transgenic olf actory receptor locus that functions independently of the endogenous loci [ 8]. Here, we show that, although the projections of neurons expressing this ectopic transgenic olfactory receptor always converse in newborn mice, sur prisingly, in adult mice, convergence is not always maintained. Moreover, i n adult mice there is a positive correlation between the number of neurons expressing the transgenic receptor and the probability of maintaining conve rgence. These observations, taken together with the variability observed in wild-type [4,6] and genetically manipulated mice ([6] and our unpublished observations), suggest that olfactory neurons require the presence of other similar axons to maintain a glomerulus. We call this phenomenon interdepen dence.