Bitter sensation, the detection of a diverse array of organic compounds, is one of the five basic tastes. Bitter is an aversive sensation in that it is regarded as unpleasant. Of the three tastes that sense organic compounds (bitter, sweet, and umami), it is the only one that is aversive, and the only taste that senses a broad range of unrelated molecules. It has likely been advantageous in human evolution by facilitating the detection and avoidance of toxic organic compounds. Most medications are considered to have a bitter taste, as well as coffee, unsweetened cacao, various leaf vegetables, and chicory. The standard substance used to compare the bitterness of compounds is quinine.
Bitterness is determined by affinity to bitter receptors, which are the taste receptor 2 family of receptors (Tas2R). There are approximately 25 functional Tas2R genes in humans. The receptor is expressed on the surface of taste receptor cells (TRCs). Only TRCs that express the Tas2R receptors sense bitterness, and TRCs that detect bitterness do not detect other tastes. Different TRCs are responsible for all five of the primary tastes. TRCs are organized into taste buds, which contain 50—150 TRCs each. Taste buds throughout the tongue contain TRCs for all five primary tastes; therefore, there is no topographic taste map. The specificity of the TRC to one taste alone is essential for the way that the different taste sensations are encoded.
Depolarization of the bitter TRCs following a signaling cascade triggered by the taste receptor transmits a signal to afferent neurons. The signal is relayed by the facial nerve (cranial nerve VII) in the anterior two-thirds of the tongue and glossopharyngeal nerve (cranial nerve IX) in the posterior third of the tongue. Sensory afferents synapse in the rostral portion of the nucleus of the solitary tract in the brainstem and are relayed to the thalamus with projections to the primary gustatory cortex.
The Tas2R receptors include receptors that detect specific compounds with high affinity, and others that detect a broad range of compounds with lower affinity. Though many different molecules may be detected, the primary bitter taste of one compound is indistinguishable from another. The affinity required for detection of bitter is much lower than those of the sweet and umami receptors. These attributes are consistent with the likely role of bitter taste to indicate the presence of potentially harmful molecules, which could be toxic at miniscule amounts and thus would need to be detected within the same range.
Bitterness is not directly correlated with toxicity. While many toxic substances are bitter, bitterness is a poor predictor of toxicity. This likely reflects a system designed to err on the side of caution—substances that are unfamiliar and do not have sufficient caloric or protein content are detected as bitter to facilitate rejection, regardless of actual toxicity. There is evidence that the full spectrum of bitter taste has lost some of its evolutionary importance in humans, as there has been some loss of function within the Tas2R receptor family. An example of loss of function is the population variation in phenylthiocarbamide (PTC) detection. PTC is sensed as bitter by the receptor T2R38, but not all humans have a functional version of this receptor, leading some people to identify the normally bitter PTC as tasteless.
Michael S. Harper
See also: Facial Nerve; Glossopharyngeal Nerve; Supertaster; Taste Aversion; Taste Bud; Taste System; Type II Taste Cells
Meyerhof, Wolfgang, Maik Behrens, Anne Brockhoff, Bernd Bufe, & Christina Kuhn. (2005). Human bitter taste perception. Chemical Senses, 30 (Suppl. 1), i14—i15.
Wang, Xiaoxia, Stephanie D. Thomas, & Jianzhi Zhang. (2004). Relaxation of selective constraint and loss of function in the evolution of human bitter taste receptor genes. Human Molecular Genetics, 13(21), 2671—2678.