LECTURE 18: OLFACTION AND TASTE

1. LECTURE 18: OLFACTION AND TASTE REQUIRED READING: Kandel text, Chapter 32 Smell and Taste are the chemical senses Smell (olfaction) is the discriminating sensation of volatile chemical odorants by the olfactory system Taste is discriminating sensation of soluble chemicals by the gustatory system Olfaction is far more discriminating than taste, and much of our subtle perceptual distinctions in flavors require integrating gustatory, olfactory & somatosensory information

2. ODORS ARE DETECTED BY NASAL OLFACTORY SENSORY NEURONS Apical dendrite of sensory neuron projects through support cells to nasal cavity, and is capped by dendritic cilia projecting into specialized mucus in the cavity Olfactory sensory neurons are fairly short-lived (1-2 months), and regenerate from basal stem cells Each sensory neuron responds to a single odorant or a specific repertoire of chemically related odorants An odor is ENCODED by the specific combination of neurons which respond to it Sensory neurons respond to odorant by inward current flow, which depolarizes neuron. There is a relationship between odorant concentration and size/duration of inward current; sufficient depolarization triggers action potential.

3. OLFACTORY SENSORY NEURONS EXPRESS ODORANT RECEPTORS Clue to discovery of odorant receptors: Odors trigger cAMP synthesis in olfactory sensory neurons Linda Buck and Richard Axel reasoned odorant receptors were G-protein-coupled receptors They searched for novel GPCRs expressed in subsets of olfactory sensory neurons using the techniques of reverse transcription + polymerase chain reaction (RT-PCR) combined with in situ hybridization (ISH) Method led to discovery of ~1000 odorant receptor genes in mammals, each encoding a 7-TM GPCR Odorant receptors can be classified into subfamilies, each having somewhat greater amino acid sequence similarity

4. ODORANT RECEPTOR STIMULATION OPENS cAMP-GATED CATION CHANNEL

5. RULES OF ODORANT RECEPTOR GENE EXPRESSION SOME RULES OF VERTEBRATE ODORANT RECEPTOR (OR) GENE EXPRESSION ARE SIMILAR TO ANTIBODY GENE EXPRESSION One olfactory neuron ----------------> One OR gene expressed Allelic exclusion: Only one of two alleles of an OR gene expressed in a neuron; Allelic choice is random MORE RULES Each OR gene expressed in neurons interspersed within one of four domains of nasal olfactory epithelium All axons from sensory neurons expressing the same receptor converge on one or a few glomeruli within the olfactory bulb THEREFORE, CELLS FOR DETECTING AN ODOR ARE DISPERSED IN EPITHELIUM, AND ALL DETECTION OF THE ODOR IS GATHERED AND SUMMATED INTO A SPECIFIC CLUSTER OF OLFACTORY BULB NEURONS

6. OR GENES EXPRESSED IN EACH OF FOUR SECTORS OF OLFACTORY EPITHELIUM

7. CONVERGENCE OF AXONS EXPRESSING A SPECIFIC ODORANT RECEPTOR TO ONE OR A FEW GLOMERULI IN OLFACTORY BULB OE OB The P2 OR gene was genetically tagged with to coexpress a Tau-bGAL protein, which binds to axon microtubules and is detected With X-GAL. All axons converge to a single glomerulus (from Wang et al, Cell 93:47;1998) Peppermint odor activates a repertoire of odorant receptors to stimulate a distinct set of olfactory bulb glomeruli (from Guthrie et al, PNAS 90:3329;1993) All M50 OR-expressing axons project to one glomerulus, as detected by ISH (from Ressler et al, Cell 79:1245;1994)

8. OLFACTORY INFORMATION IN GLOMERULI IS INTEGRATED AND DISTRIBUTED TO DIFFERENT BRAIN CENTERS

9. PHEROMONES ARE SPECIES-SPECIFIC ODORANTS SENSED THROUGH A PARALLEL OLFACTORY SYSTEM Specific pheromone receptors expressed in dispersed sensory neurons within the veromonasal organ

10. OLFACTORY RECEPTORS ARE USED TO GUIDE AXONS TO PROPER GLOMERULI: OLFACTORY SENSORY AXONS LACKING ODORANT RECEPTOR WANDER AND DIE (from Wang et al, Cell 93:47;1998)

11. OLFACTORY RECEPTORS ARE USED TO GUIDE AXONS TO PROPER GLOMERULI: CHANGING ODORANT RECEPTOR EXPRESSED IN A NEURON CHANGES ITS PROJECTION When different odorant receptors are engineered to be expressed in cells that would normally express the P2 receptor, the axons of these neurons project to a new glomerular “address”. In P3 ---> P2 neurons, axons project to where P3 neuron axons go. But for other misexpressions, the glomerular address is different from both that of P2 or of the replacement OR. Therefore, while the specific OR is a determinant of axonal pathfinding, it is NOT the only determinant.

12. ORGANIZATION OF THE TASTE BUD Each taste bud contains ~100 taste cells Mature taste cells are very short-lived, and are continuously regenerated from basal cells Apical microvilli of taste cells are exposed to saliva through the taste pore Tasty substance is sensed at microvilli by several mechanisms, but always induces depolarization and action potential generation Taste cell action potential releases neurotransmitter which activates gustatory afferent fiber Taste cells can detect one of five known tastes: SOUR SALTY SWEET BITTER UMAMI

13. DIFFERENT TASTE STIMULI USE DIFFERENT SIGNAL TRANSDUCTION METHODS SALTY-sensing taste cells express amiloride-sensitive sodium channels. Sodium in salts enters through channel to depolarize cell. Potassium-type salts also stimulate these cells because of leak potassium channels and change in EK SOUR-sensing taste cells express proton-sensitive potassium leak channels. Acid H+ ions (protons) block these potassium channels, reducing gK and depolarizing cell.

14. DIFFERENT TASTE STIMULI USE DIFFERENT SIGNAL TRANSDUCTION METHODS BITTER-sensing taste cells use 7-TM receptors coupled to various G proteins. Bitter sensation is method for recognizing TOXIC compounds. There is a family of related bitter receptors. Some receptors couple to Gq which activates PLC to increase Ca+2 through IP3 Other receptors couple to gustducin, which activates cyclic nucleotide phosphodiesterases A few bitter compounds act by directly blocking leak potassium channels SWEET-sensing taste cells use 7-TM receptor coupled to Gs. Sugars act through Gs to produce cAMP, and PKA phosphorylates and closes potassium leak channels, causing depolarization Alternatively, some substances (artificial sweeteners) bind receptors coupled to Gq which activates PLC to increase Ca+2 through IP3

15. TRANSMISSION OF TASTE INFORMATION TO THE BRAIN AND PERCEPTION OF TASTE Integration of taste stimuli first occurs in afferant gustatory fibers, since each fiber receives input from multiple taste cells of different types. Each gustatory afferent fiber has a response profile to 5 tastes Taste stimuli project to the gustatory cortex, where it is consciously perceived. Taste stimuli are integrated with somatosensory inputs to generate perception of where tasty substance is. Taste perception is also shaped by parallel olfactory input; the somatosensory stimulus “fools” us to perceive the olfaction as part of the taste.