Neurobiology Select

Studying the molecular mechanisms that make the senses respond precisely to external stimuli continues to generate surprising results. This issue's Neurobiology Select discusses studies of the five senses that reveal new insights into protein function, physiology, and animal behavior. As a testament to their biological importance, odorant receptors are the largest gene family in vertebrates, with roughly 1000 being encoded per mamma-lian genome. Given this diversity, the existence of other types of chemore-ceptors should not need to be invoked to envisage how the panoply of molecules recognized by the olfactory epithelium is discriminated. Thus, the discovery by Liberles and Buck (2006) of a new class of chemosensory receptors in the olfactory epithelium of mice, called TAARs (trace amine-associated receptors), provides fresh insights into our sense of smell. Like ca-nonical olfactory receptors, TAARs are G protein-coupled receptors, but unlike the canonical receptors, TAARs bear close homology to the receptors for the biogenic amines, such as those that bind the neurotransmitters serotonin and dopamine. In mice, Liberles and Buck show that TAARs can bind to specific amine compounds, such as those found in urine, suggesting that TAARs may be important in the pheromonal response to social cues. It is compelling that a subpopulation of olfactory neurons has recently been shown to receive pheromonal input, which had been previously thought to be the sole provenance of the vomeronasal organ. However, whether this subpopulation of olfactory neurons express TAARs remains to be determined. As TAARs are also found in the human genome, this work hints that TAARs could mediate behavioral responses of humans to pheromones. The G protein-coupled photoreceptor rhodopsin is remarkably sensitive, capable of initiating the hyperpolarization of a rod cell in response to a single photon of light. Yet it is also unusually reliable, according to new work by Doan and colleagues (2006). By studying the responses of rod photoreceptor cells to single photons, the authors examined the variability in the length of time that rhodopsin is active when stimulated by light. Normally, upon absorbing a photon of light, rhodopsin initiates a highly stereotypical electrical response in the rod cell, with current returning to baseline after approximately one second. Rhodopsin signaling is terminated by phosphorylation at multiple serine and thre-onine residues in its cytoplasmic tail and by the recruitment of arrestin. Doan et al. examined the variability of rho-dopsin signaling following the removal of the inhibitory phosphorylation sites. They found that it …