Protons are the causative agents of acidity and sourness. They are known to affect sensitive
taste receptors in at least two ways (see Fig. 8.5):
First, H+ can permeate the amiloride-sensitive sodium channel and cause an inward H+
current and depolarize the cell.
Second, hydrogen ions can bind to and block K+-selective channels. When the K+
permeability of a membrane is decreased, it depolarizes.
There are two families of taste receptor genes (T1R and T2R) which encode for a variety of G-
protein-coupled taste receptors. Bitter substances are detected by the 30 or so different types of
T2R receptors, however, animals are not very good at telling different bitter tastants apart
because each bitter taste cell expresses many, and perhaps all, of the 30 bitter receptor
proteins. Because each taste cell can send only one type of signal to its afferent nerve, a
chemical binding to one of the 30 receptors will trigger the same response as a different
chemical that binds to another bitter receptor. This is important because it conveys to the brain
that a bitter substance is poisonous and should be avoided regardless of how bitter it is.
Bitter receptors use a second messenger pathway to carry their signal to the gustatory afferent
axon. Bitter, sweet, and Umami receptors all seem to use exactly the same second messenger
pathway to carry their signals to the afferent axons (see Fig. 8.6).
When a tastants binds to a bitter (or sweet or umami) receptor, it activates its G-proteins
The G-proteins stimulate the enzyme phospholipase C and thereby increasing the
production of inositol triphosphate (IP3).
IP3 activates a special type of ion channel that is unique to taste cells
o This allows Na+ to enter and depolarizing the taste cell
The voltage-gate calcium channels open and allow Ca2+ to enter the cell
o IP3 also triggers the release of Ca2+ from intracellular storage sites
o These two sources of Ca2+ help trigger neurotransmitter release
Sweet receptors resemble bitter receptors, in that they are both G-protein-coupled receptors,
but they are different in that sweet receptors are formed from two such proteins bound tightly
together (the bitter receptor is only a single protein) (see Fig. 8.7). A functioning sweet receptor
requires two very particular members of the T1R receptor family: T1R2 and T1R3.
Chemicals binding to the T1R2 + T1R3 receptor activate exactly the same second messenger
system as the bitter receptors. Why don’t we confuse bitter chemicals with sweet ones then?
Bitter receptor proteins and sweet receptor proteins are expressed in different taste cells
and also connect to different gustatory axons.