TRPC (‘Canonical’) * TRPV (‘Vanilloid’) * TRPM (‘Melastatin’) * TRPP (‘Polycystin’) * TRPML (‘Mucolipin’) * TRPA (‘Ankyrin’) * TRPN (‘NOMPC’) transient receptor potential

Animals rely on chemical senses to identify nourishment, poison, and potential mates.
Chemical sensation is the oldest and most common sensory system.
It is essential for monitoring the changing environment, both internally and externally.
Chemical senses include:
- Gustation (taste)
- Olfaction (smell)
- Chemoreceptors: internal probes in the gut, muscle, and blood.

The basic tastes are saltiness, sourness, sweetness, bitterness, and umami.
Possible additional tastes include:
- Fat (FFA receptor in gut).
- Starch (starch receptor; detection of glucose oligomers is independent of the hT1R2/hT1R3 sweet taste receptor).
- Carbonation (taste cell with carbonic anhydrase, which catalyzes the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid, i.e., bicarbonate and hydrogen ions; sour taste cells can detect carbonation).
- Calcium (sensed as a combination of bitter and sour tastes, possibly via T1R3).
- Water (water receptor in insects; acid-sensing taste receptor cells (TRCs) that were previously suggested as the sour taste sensors also mediate taste responses to water in mice, Yuki Oka, Nature Neurosci. 2017).
Examples of correspondence between chemistry and taste:
- Sour: acids.
- Salty: salts.
- Sweet: sugars like fructose, sucrose, and artificial sweeteners (saccharin and aspartame).
- Bitter: ions like $K^+$ and $Mg^{2+}$ , quinine, and caffeine.
There is an advantage for survival as poisonous substances are often bitter.

Epiglottis: a thin flap that covers the entrance to the larynx during swallowing, preventing food from entering the trachea.
Taste receptor cells reside primarily in the tongue, but are also in the soft palate, esophagus, epiglottis, and larynx in significant numbers.

Papillae (미각 돌기) are the organs of taste.
Types of papillae:
- Foliate papillae: edge of tongue, bilaterally along the posterolateral margins of the tongue surface.
- Vallate papillae: back of tongue.
- Fungiform papillae: most anterior part of tongue.
Threshold concentration: just enough exposure of a single papilla to detect taste.

One to several hundred taste buds (미뢰) per papilla.
50-150 taste receptor cells per taste bud.
2000-5000 taste buds per person.
Taste tuning (specificity) at the papilla level is present but not absolute exclusiveness (at higher concentrations of taste stimuli).
Real sensitivity seems to be at the taste receptor cell level.

Apical ends have microvilli that project to the taste pore.
Receptor potential: Voltage shift that may fire action potentials.
Most receptor cells respond primarily to just one of the basic tastes.
They are not neurons by standard histological criteria but do form synapses to gustatory ganglion neurons and basal cells.
Receptor cells have apical microvilli (increase absorptive area) where most receptors/channels are located.

Different sensitivity of TR cells.
Action potential of sensory axons.
More than 90% of receptor cells respond to two or more of the basic tastes (old view).
Recent developments of molecular genetic tools dispute the old beliefs of broad tuning of TRCs.
Depolarization: voltage-gated $Ca^{2+}$ channels open, $Ca^{2+}$ entry, neurotransmitter release.

Transduction process:
- Taste stimuli (tastants) pass directly through ion channels (salt and sour).
- Bind to and block ion channels (sour).
- Bind to G-protein-coupled receptors (bitter, sweet, umami).
Different neurotransmitters:
- Sour and salty taste cells: serotonin.
- Sweet, bitter, umami: ATP.
- Acetylcholine, glutamate, GABA?

Taste stimuli interact with ion channels or receptors on the apical membrane (small ionic stimuli may permeate tight junctions).
This interaction typically depolarizes the taste cell (directly or via 2nd messengers).
Some stimuli release $Ca^{2+}$ from stores.
Receptor potential opens voltage-activated $Na^+$ & $K^+$ channels (depolarization).
Voltage-activated $Ca^{2+}$ channels open, allowing $Ca^{2+}$ inside and a net rise in $[Ca^{2+}]_{in}$ .
As in typical chemical synaptic transmission, $Ca^{2+}$ rise triggers exocytosis of transmitter-containing vesicles.

Salt-sensitive taste cells have a special $Na^+$ selective channel.
Blocked by the drug amiloride.
Open all the time.
Requires the concentration to be quite high (at least 10mM).
Epithelial sodium channel (ENaC): low salt sensor.
High salt sensing is more elusive. High conc. of salt appear to activate bitter cells (TRPM5 or PLCβ2 expressing) and PKD2L1-expressing sour cells.
Anions of salts affect saltiness – anion inhibition (the larger anions inhibit the salt taste of cations).
Some anions could produce their own Taste (e.g. NaCl vs. CH3COONa).

Sourness is related to acidity and low pH.
Protons are causative agents of acidity and sourness.
Amiloride-sensitive sodium channels allow the influx of protons - inward current produces membrane depolarization.
Hydrogen ions bind and block $K^+$ -selective channels - depolarization.
PKD2L1 (Polycystic kidney disease 2-like 1 protein), a member of the TRP (transient receptor potential) ion channel family, specifies sour-tasting cells.

A group of ion channels located mostly on the plasma membrane of numerous animal cell types.
Mediate a variety of sensations such as pain, temperature, different kinds of tastes, pressure, and vision.
These ion channels have a relatively non-selective permeability to cations, including sodium, calcium, and magnesium.
TRP channels were initially discovered in the