Design Principles of Sensory Receptors

Organisms continuously detect and process physical and chemical signals from their external and internal world, and they monitor their interaction with the environment. Aristotle was the first who defined the five external senses in humans: sight, hearing, smell, taste, and touch. In addition, we consider balance as the sixth external sense. Similarly important for the control of movement is the sense of body position, the proprioception. The physiological state of the organism is reported by a variety of internal receptors including those for gases, temperature, or pH. According to the activating stimulus sensory receptors can be classified into electromagnetic receptors (photoreceptor, thermoreceptor), mechanoreceptors (hearing, touch, balance, osmoreceptor), and chemoreceptors (odorant receptor, gustatory receptor). Sensory signals are perceived by specialized neurons equipped with one type of receptor molecules as photoreceptor cells or with various types of receptors as nociceptive neurons to detect different noxious stimuli including heat, pressure, pH, or chemical signals. Most receptor molecules are tuned to a single sensory modality but some are polymodal as the vanilloid receptor VR1 which is activated by heat, pungent chemicals, acids, or lipids. Activation of receptor molecules by an adequate stimulus initiates a signal transduction process in the sensory neuron in which the physical or chemical signal is amplified and converted into an electrical signal that depolarizes or hyper-polarizes the cell. The properties of the stimulus such as strength and duration are then translated into a specific temporal pattern of action potentials which is further processed in the brain. In recent years the research on design and function of sensory receptors and their signal transduction machinery has largely extended our knowledge in this field. The present special issue is aimed of presenting an actual view on photoreceptors and chemoreceptors. Photoreceptors and many chemoreceptors belong to the G protein-coupled receptor (GPCR) family. GPCRs are seven trans-membrane (7-TM)-spanning proteins that interact with heter-otrimeric G proteins. According to structural similarities GPCRs are grouped into families. Photoreceptors and most vertebrate olfactory receptors belong to the rhodopsin receptor/class A family. Ligand binding to a GPCR causes a conformational change thereby activating a trimeric G protein which disensembles into the G α-and the G βγ-subunit. Independently, both subunits can affect various targets such as enzymes or ion channels. G protein signal-ing is usually associated with signal amplification. An exception are mammalian olfactory receptors in which the dwell time of odorant molecules is too short to efficiently activate G proteins …