PHGY 210 Lecture Notes - Lecture 7: Tas1R2, Amiloride, Inositol Trisphosphate

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31 Jan 2013

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Mechanisms of Taste Transduction
The process by which an environmental stimulus causes an electrical response in a sensory
receptor cell is called transduction. Taste transduction involves several different processes and
each basic taste uses one or more of these mechanisms:
Directly pass through ion channels (salt and sour)
Bind and block ion channels (sour)
Bind to G-protein-coupled receptors in the membrane that activate second messenger
The taste of salt is mostly the taste of the cation Na+ and its concentration must be quite high in
order to taste it (at least 10 nM). Salt-sensitive taste cells have a special Na+-selective channel
that is blocked by the drug amiloride (see Fig. 8.5). This channel is insensitive to voltage and is
always open. When you sip chick soup, for example, the Na+ concentration rises outside the
receptor cell. Na+ then diffuses down its concentration gradient which results in an inward
current and depolarization (receptor potential) of the membrane. The receptor potential causes
the voltage-gated sodium and calcium channels to open and trigger the release of
neurotransmitter onto the gustatory afferent axon.
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
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