Smell and Taste Program
NIDCD investigators study the chemical senses of olfaction (smell) and gustation (taste) to enhance our understanding of how individuals communicate with their environment. Smell and taste play important roles in preferences and aversions for aromas, specific foods and flavors. Sweet-tasting substances are generally consumed and contribute to caloric intake and proper nutrition; bitter-tasting substances are typically avoided because bitterness is often associated with toxic compounds that cause illness. The NIDCD is supporting research on the development of bitter-taste blockers in an effort to identify compounds that can mask the bitter taste of essential medications, especially for children.
Both the olfactory and gustatory systems offer special approaches for the understanding of the fundamental mechanisms of neural plasticity. NIDCD scientists have found that smell and taste receptor cells are continually replaced and have the further capacity to replace themselves rapidly in response to injury. With every hard sneeze and with every burnt tongue from a hot cup of coffee, olfactory and taste receptor cells are destroyed and are then replaced. These are the only known mammalian sensory cells with this native regenerative capability, and the olfactory system is now used as a model system in the study of the biology of multi-potent stem cells. Unfortunately, the plasticity of the olfactory system declines with age, with important consequences to the increasingly aged population. The perceived quality of foods moves towards blandness in the elderly and this affects food intake, diet and overall nutrition, and health status. Prevention of this age-related decline in olfactory sensitivity is being studied by NIDCD investigators.
Advances in molecular and cellular biology, biophysics and biochemistry of the olfactory and gustatory systems are paving the way for improved diagnosis, prevention, and treatment of chemosensory disorders. The vertebrate olfactory receptor neuron has become an important model system in molecular and cellular biology. The olfactory receptor gene family has been described in several mammalian species, including humans, and may contain as many as 1,000 members. NIDCD scientists are presently characterizing genetic mechanisms of olfaction, which will provide the opportunity to study the molecular pharmacology of the process of smell. More recently, a family of about 80 taste receptor genes has been identified by NIDCD investigators. Interestingly, both olfactory and sweet and bitter taste receptors are structurally similar and activate similar second messenger signal transduction cascades, which ultimately generates neural activity in the central nervous system. The characterization of these receptor genes was greatly facilitated by the genetic database provided by the NIH's human and mouse genome projects.
The molecular biological studies of olfactory and taste receptor cells have provided essential information about the sensitivities of the chemical senses at the first level of neural integration. The coding of odorants and tastants by the central nervous system begins at the level of the receptor cell. In addition, in both the olfactory and gustatory systems, odor and taste quality coding is further refined by a synthetic process of the central nervous system. NIDCD-funded projects are examining the nature of the central coding. In the olfactory system, odor coding appears very complex because of the numerous types of odors that must be detected and because of the complicated neuroanatomical organization of the olfactory system. We are just beginning to understand the nature of the olfactory code. On the other hand, in the taste system, significant progress has been made in our understanding of how the four taste qualities of sweet, salty, sour, and bitter are coded centrally. Recent work suggests a fifth taste quality, umami, which is familiar to many as the taste of monosodium glutamate (MSG). The nature of the gustatory code and the high degree of central processing makes the gustatory system very resistant to damage. Consequently, the taste system is less often affected by aging as is the olfactory system.
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