'One receptor' rules in sensory neurons

With the recent explosion in the characterization of different sensory systems, a general rule is emerging: only one type of sensory receptor molecule is expressed per receptor neuron. The visual system is no exception and, in most cases, photoreceptors express only one visual pigment per cell. However, the mechanisms underlying the exclusion of sensory receptors are poorly understood. As expression of a given receptor in a given cell is often stochastic, a decision must first be made to express one of the many receptors of the same family (i.e. one particular rhodopsin) and this expression must correlate with the silencing of the other receptors. Furthermore, the projection center for the receptors in the brain must be informed of the decision in order to process this information. Although cells can choose from up to hundreds of sensory receptors (e.g. in the olfactory system), they make almost no mistakes. Evidence has recently emerged that the exclusion mechanism involves the sensory receptor molecules themselves. Here, we describe the findings from various systems in mammals and Drosophila, and review evidence that in the simple visual system of the fly, rhodopsin molecules play an important role in sensory receptor exclusion.