Pii: S0166-2236(03)00037-7

The olfactory system permits animals to recognize and discriminate a vast number of different odorous molecules. This sensitivity is achieved by the selective expression of a single odorant receptor, specific to a small subset of the universe of odors, in each olfactory sensory neuron [1,2]. The topography of synaptic connections at the first relay in the olfactory system is organized according to the type of odorant receptor a particular olfactory sensory neuron expresses. All neurons expressing a given member of the odorant receptor gene superfamily extend axons that synapse in one medial and one lateral glomerulus in the olfactory bulb [3,4]. The position of the glomerulus is sensitive to the particular odorant receptor a neuron expresses, as shown by experiments that exchanged the coding region of odorant receptors and produced predictable distortions in the olfactory bulb map [5,6]. These experiments have led to the model that the odorant receptor itself plays a role in organizing the connectivity of olfactory map. A recent article by Belluscio et al. [7] demonstrated for the first time that olfactory neurons expressing a novel odorant receptor not only target a novel glomerulus, but also functionally engage the circuitry of the olfactory bulb. This is the first direct evidence that a glomerulus identified by the molecular properties of the olfactory sensory neurons that innervate it responds to a given odorant. Therefore, these results serve as a starting point to unify the molecular and functional maps of the olfactory bulb.