Development of wiring specificity in the olfactory system.

The olfactory system discriminates a large number of odorants using precisely wired neural circuits. It offers an excellent opportunity to study mechanisms of neuronal wiring specificity at the single synapse level. Each olfactory receptor neuron typically expresses only one olfactory receptor from many receptor genes (1000 in mice). In mice, this striking singularity appears to be ensured by a negative feedback mechanism. Olfactory receptor neurons expressing the same receptor converge their axons to stereotypical positions with high precision, a feature that is conserved from insects to mammals. Several molecules have recently been identified that control this process, including olfactory receptors themselves in mice. The second order neurons, mitral cells in mammals and projection neurons in insects, have a similar degree of wiring specificity: studies in Drosophila suggest that projection neuron-intrinsic mechanisms regulate their precise dendritic targeting. Finally, recent studies have revealed interactions of different cell types during circuit assembly, including axon-axon interactions among olfactory receptor neurons and dendro-dendritic interactions of projection neurons, that are essential in establishing wiring specificity of the olfactory circuit.