Chemoreception

Distinguish between taste and smell

Gustation (taste)

• chemicals in contact with the animal

• dissolved chemicals at relatively high concentrations

Olfaction (smell)

• chemicals concentrated at a distance

• airborne or dissolved chemicals at low concentrations

Physiological distinction

• Different sense organs, different receptor cells

• Different signal transduction mechanism

• Separate integrating centers

Know chemoreceptive organs in terrestrial vertebrates and mammals

Vertebrates

Gustation (taste)

• Taste buds

  • Oral cavity (tongue, larynx, soft palate)

  • Esophagus

Olfaction (smell)

• Odorants: olfactory epithelium

• Pheromones: Vomeronasal organ (vestigial in humans)

Mammals

Gustation (taste)

• Taste receptors: epithelial sensory cells

  • Receptors potential

• Taste bud:

  • Cluster of taste receptors (50-100)

  • Apical surface folded into microvilli (contain receptor proteins)

  • Pore

  • Supporting cells

  • Rapid turnover (10-14 days)

Describe the 5 categories of taste

• Sweet

  • monosaccharides, polysaccharides, artificial sweeteners

• Salty

  • Na+

• Sour

  • HCl

• Bitter

  • Caffeine, quinine

• Umami

  • L-glutamate, amino acids, MSG

Describe the salty transduction mechanism

Na + channels (ENaC) also permeable to H+ (possible to detect sour taste?)

• Na + compete with H +

• Detection of sourness in species with low Na + in saliva (e.g. hamster)

Mechanism

1. Na + from salty food enters through a Na + channel

2. The resulting depolarization opens voltage-gated Ca 2+ channels

3. The influx of Ca 2+ causes neurotransmitter release

Taste preferences based on nutritional deficiencies

• Salt deficiency triggers aldosterone secretion

• Aldosterone enhances Na+ retention by kidney and increases ENaC expression in taste receptor cells:

  • Induces craving for salty foods

Describe sour transduction mechanism

• Sour taste caused by H+ ions in food

Different sour transduction mechanisms

• Mammal: PKD2L1 channel, acid-sensing ion channels (ASlCs)

  • involve pH-sensitive Na+ channels

• Salamander: taste receptor cells express K+ channels

  • K+ channels blocked by H+

  • Decreased K+ permeability = depolarization

  • VG-Ca2+ channels opens

  • Increase [Ca2+], Nt released

• Frogs: H+ - gated Ca2+ channels and H+ transporters

Describe the sweet transduction mechanism

• Broad spectrum receptors: detect many kind of sweet substances

• Chemical bind to receptor, causing conformational change, activating gustducin (G-protein)

  • Activates AC

  • AC catalyze conversion of ATP to cAMP

  • Increase in cAMP: activates protein kinase: phosphorylates K+ channels (close)

  • Depolarization opens VG Ca2+ channels

  • Neurotransmitter release

Describe the bitter transduction mechanism

• Sensation: unpleasant but bearable when we; repulsive when strong; prevents ingestion of harmful compounds

• Many genes (~30) coding for bitter taste receptors

• Binding causing conformational change activates transducin (G-protein)

  • Activates phospholipase C (PLC)

  • Catalyzes conversion of PIP2 into IP3 (inositol triphosphate)

  • IP3 releases Ca2+ from intracellular stores (ER_

  • Increase in [Ca2+], causes neurotransmitter release

Describe the umami transduction mechanism

Umami taste comes from foods containing L-glutamate, other amino acids, and MSG

• Glutamate binds to glutamate receptor, conformational change, activating G protein

• G protein activates phosphodiesterase (PDE) that degrades cAMP into AMP

• decreases in cAMP thought to trigger neurotransmitter release

- precise but not fully described yet

- it appears there are several pathways involved

Understand olfactory reception and combinatorial encoding

• Bipolar sensory neuron

• Generator potential

Mechanism

1. Binding of odorant to an odorant receptor causes conformational change

2. The activated G protein (Golf) moves through the membrane and activates adenylate cyclase

3. Adenylate cyclase converts ATP into cAMP

4. cAMP opens cAMP-gated ion channels

5. Ca2+ and Na+ enter the cell, causing a generator potential

6. The Ca2+ also opens Ca2+ activated Cl- channels, causing Cl- to leave the cell, increasing the depolarization

7. The generator potential opens voltage-gated Na+ channels, triggering action potentials

Combinatorial encoding: pattern of activation of olfactory receptor cells codes for particular odorants

• Most odors composed of multiple odorant molecules and each activates several odorant receptors

• 1 type of receptor per receptor cell, but receptor proteins can recognize > 1 odorant

Describe the vomeronasal organ

• Accessory olfactory structure in terrestrial vertebrates

  • highly sensitive to pheromones: intraspecific chemical communication

• Opens to oral or nasal cavity but isolated from main airstream

  • air entry via pumping (Flehmen response)

• Different from olfactory epithelium

  • Different receptors

  • Different receptor gene families

  • Different signal transduction pathway