Umami Sensation and Taste-mGluR4

Study Notes

Umami receptors are a modified form of mGluR4 called 'taste-mGluR4', missing part of its molecule.
Activation of G protein–coupled receptors by specific molecules is crucial for umami, sweet, and bitter tastes.
L-glutamate, an amino acid, is the primary molecule activating umami receptors.
Nucleotides IMP (disodium 5’-inosine monophosphate) and GMP (disodium 5’-guanosine monophosphate) also contribute to umami taste.
These nucleotides are found in many protein-rich foods, enhancing the umami flavor.
The combination of MSG, IMP, and GMP results in a potent umami taste due to their synergistic effect.
Specific taste buds respond to glutamate similarly to how sweet taste buds respond to sugar.
Glutamate binds to a variant of G protein coupled glutamate receptors, potentially interacting with Ca2+ channel proteins.

Bitter taste receptors activate via G-proteins, particularly gustducin, affecting ~24 other G-proteins.
Saltiness is perceived through ion channels, allowing Na+ and Cl– ions to generate action potentials.
Sour tastes, caused by acids, increase H+ concentration in taste neurons, leading to depolarization.
Sweet, bitter, and umami tasting involve G-protein-coupled receptors to transmit taste signals, while sour and salty involve direct ion channel activity.

Bitter taste is associated with a variety of nitrogen-containing molecules called alkaloids found in coffee, hops, tea, and more.
The gag reflex can be stimulated by bitter taste to avoid poison ingestion, with the highest concentration of bitter receptors located on the posterior tongue.
Three mechanisms contribute to bitter taste transduction:
- K+ efflux blockage creating depolarization.
- G protein gustducin activation leading to phospholipase C response.
- Hydrolysis of substrates generating intracellular calcium release.

Saltiness originates from Na+ ions dissolving in saliva, creating strong concentration gradients that facilitate ion entry into gustatory cells.
Sodium's entry depolarizes cell membranes, generating receptor potentials.

Seven kinds of tastes recognized: Astringent, Bitter, Pungent, Salty, Sour, Sweet, and Umami.
Taste buds detect taste and are situated around papillae, with food particles interacting with taste receptors through taste pores.
Information is transmitted to the brain via the seventh, ninth, and tenth cranial nerves.
Taste cells in taste buds are replaced every 10 days, ensuring continued sensitivity to food chemicals.
Four types of papillae exist based on appearance; most individuals have 10 to 14 Circumvallate papillae.