P.I. Hanson, T. Meyer, L. Stryer, and  H. Schulman, Neuron 12:943–956, 1994. With permission from Elsevier.) they activate an olfactory-specific G protein (known as Golf), which in turn activates adenylyl cyclase. The resulting increase in cAMP opens cyclic-AMP-gated cation channels, thereby allowing an influx of Na+, which depolarizes the olfactory receptor neuron and initiates a nerve impulse that travels along its axon to the brain.
There are about 1000 different olfactory receptors in a mouse and about 350 in a human, each encoded by a different gene and each recognizing a different set of odorants. Each olfactory receptor neuron produces only one of these receptors; the neuron responds to a specific set of odorants by means of the specific receptor it displays, and each odorant activates its own characteristic set of olfactory receptor neurons. The same receptor also helps direct the elongating axon of each developing olfactory neuron to the specific target neurons that it will connect to in the brain. A different set of GPCRs acts in a similar way in some vertebrates to mediate responses to pheromones, chemical signals detected in a different part of the nose that are used in communication between members of the same species. Humans, however, are thought to lack functional pheromone receptors.
Vertebrate vision employs a similarly elaborate, highly sensitive, signal-detection process. Cyclic-nucleotide-gated ion channels are also involved, but the crucial cyclic nucleotide is cyclic GMP (Figure 15–37) rather than cAMP. As with cAMP, a continuous rapid synthesis (by guanylyl cyclase) and rapid degradation (by cyclic GMP phosphodiesterase) controls the concentration of cyclic GMP in the cytosol.
In visual transduction responses, which are the fastest G-protein-mediated responses known in vertebrates, the receptor activation stimulated by light causes a fall rather than a rise in the level of the cyclic nucleotide. The pathway has been especially well studied in rod photoreceptors (rods) in the vertebrate retina. Rods are responsible for noncolor vision in dim light, whereas cone photoreceptors (cones) are responsible for color vision in bright light. A rod photoreceptor is a highly specialized cell with outer and inner segments, a cell body, and a synaptic region where the rod passes a chemical signal to a retinal nerve cell (Figure 15–38). This nerve cell relays the signal to another nerve cell in the retina, which in turn relays it to the brain.
The phototransduction apparatus is in the outer segment of the rod, which contains a stack of discs, each formed by a closed sac of membrane that is densely packed with photosensitive rhodopsin molecules. The plasma membrane surrounding the outer segment contains cyclic-GMP-gated cation channels. Cyclic GMP bound to these channels keeps them open in the dark. Paradoxically, light causes a hyperpolarization (which inhibits synaptic signaling) rather than a depolarization of the plasma membrane (which would stimulate synaptic signaling). Hyperpolarization (that is, the membrane potential moves to a more negative value—discussed in Chapter 11) results because the light-induced activation of rhodopsin molecules in the disc membrane decreases the cyclic GMP concentration and closes the cation channels in the surrounding plasma membrane (Figure 15–39).