Olfactory transduction

Olfactory transduction is the sensory transduction of the olfactory system. It is a series of events in which odor molecules are detected by the olfactory receptors (in olfactory mucosa or insect antennae) and chemical signals are transformed into electrical signals to the brain where they are perceived as smells.^[1]

Once ligands (odorant particles) bind to specific receptors on the external surface of cilia, olfactory transduction is initiated. In mammals, olfactory receptors have been shown to signal via G protein. This is a similar type of signaling of other known G protein-coupled receptors (GPCR). The binding of an odorant particle on an olfactory receptor activates a particular G protein (Gαolf), which then activates adenylate cyclase, leading to cAMP production. cAMP then binds and opens Cyclic nucleotide-gated ion channel. This opening allows for an influx of both Na⁺ and Ca²⁺ ions into the cell, thus depolarizing it. The Ca²⁺ in turn activates chloride channels, causing efflux of Cl⁻, which results in a further depolarization of the cell.^[2]

The odorant-activated cAMP cascade in the olfactory sensory neuron is subject to negative feedback regulation, like all other G-protein mediated pathways. This contributes to response deactivation and adaptation to stimulus.^[1]

Individual odorants activate subsets of receptors. Receptors also have varying affinities for odorant molecules. In addition, higher concentrations of odorants elicit activity from greater a number of receptors than do lower concentrations. Thus, odor intensity as well as odor identity is represented by combination of a number of activated receptors.^[3]

References

1 2 M. Ma (2007) Encoding olfactory signals via multiple chemosensory systems. Crit. Rev. Biochem. Mol. Bio. 42 (6): 463-480. PMID 18066954
↑ A.Kato & K. Touhara (2009) Mammalian olfactory receptors: Pharmacology, G protein coupling and desensitization. Cel. Mol. Life Sciences. 66(23): 3743-3753. PMID 19652915
↑ S. Chig-Ying, K. Menus, & J.R. Carlson (2009) Olfactory Perception: Receptors. Cells and Circuits. Cell 139(1): 45-59. PMID 19804753

External links

Olfactory transduction entry in the public domain NCI Dictionary of Cancer Terms

This article incorporates public domain material from the U.S. National Cancer Institute document "Dictionary of Cancer Terms".

Anatomy of smell

Microanatomy

Olfactory nerve: 1° neuron

Olfactory receptor neurons (Olfactory receptor) → Olfactory bulb (Glomeruli)

Olfactory nerve: 2° neuron

Mitral cells → Olfactory tract → Olfactory trigone

Lateral olfactory stria/
Primary olfactory cortex

Piriform cortex EC-hippocampus system Entorhinal cortex Hippocampal formation Prepyriform area Periamygdaloid cortex

Stria medullaris → Habenular nuclei

Amygdala → Stria terminalis → Hypothalamus

Medial forebrain bundle → Hypothalamus

Medial olfactory stria

Anterior olfactory nucleus

G protein-coupled receptors: olfactory receptors

Class I
(fish-like specific receptors)

Family 51	A2 A4 A7 B2 B4 B5 B6 D1 E1 E2 F1 F2 G1 G2 H1 I1 I2 L1 M1 Q1 S1 T1 V1

Family 52	A1 A4 A5 B2 B4 B6 D1 E1 E2 E4 E5 E6 E8 H1 I1 I2 J3 K1 K2 L1 L2 M1 N1 N2 N4 N5 P1 R1 W1

Family 56	A1 A3 A4 B1 B4

Class II
(tetrapod specific receptors)

Family 1	A1 A2 B1 C1 D2 D4 D5 E1 E2 F1 F2 G1 I1 J1 J2 J4 K1 L1 L3 L4 L6 L8 M1 N1 N2 Q1 S1 S2

Family 2	A1 A2 A4 A5 A7 A12 A14 A25 A42 AE1 AG1 AG2 AJ1 AK2 AP1 AT4 B2 B3 B6 B8 B11 C1 C3 D2 D3 F1 F2 G2 G3 G6 H1 H2 J1 J2 J3 K2 L2 L3 L5 L8 L13 M2 M3 M4 M5 M7 S2 T1 T2 T3 T4 T5 T6 T8 T10 T11 T12 T27 T29 T33 T34 T35 V1 V2 W1 W3 W5 Y1 Z1

Family 3	A1 A2 A3 A4

Family 4	A5 A15 A16 A47 B1 C3 C5 C6 C11 C12 C13 C15 C16 C45 C46 D1 D2 D5 D6 D9 D10 D11 E2 F3 F4 F5 F6 F15 F16 F17 F21 F29 K1 K2 K3 K5 K13 K14 K15 K17 L1 M1 M2 N2 N4 N5 P4 Q3 S1 S2 X1 X2

Family 5	A1 A2 AC2 AK2 AK3 AN1 AP2 AR1 AS1 AT1 AU1 AY1 B2 B3 B12 B17 B21 BF1 BU1 C1 D13 D14 D16 D18 F1 H1 H2 H6 H14 H15 I1 J2 K1 K2 K3 K4 L1 L2 M1 M3 M8 M9 M10 M11 P2 P3 R1 T1 T2 T3 U1 V1 W2

Family 6	A2 B1 B2 B3 C1 C2 C3 C4 C6 C65 C68 C70 C74 C75 C76 F1 J1 K2 K3 K6 M1 N1 N2 P1 Q1 S1 T1 V1 X1 Y1

Family 7	A2 A5 A10 A17 C1 C2 D2 D4 E24 G1 G2 G3

Family 8	A1 B2 B3 B4 B8 B12 D1 D2 D4 G1 G2 G5 H1 H2 H3 I2 J1 J3 K1 K3 K5 S1 U1

Family 9	A2 A4 G1 G4 I1 K2 Q1 Q2

Family 10	A2 A3 A4 A5 A6 A7 AD1 AG1 C1 D4 G2 G3 G4 G6 G7 G8 G9 H1 H2 H3 H4 H5 J1 J3 J5 J6 K1 K2 P1 Q1 R2 S1 T2 V1 W1 X1 Z1

Family 11	A1 G2 H1 H4 H6 L1

Family 12	D2 D3

Family 13	A1 C2 C3 C4 C5 C8 C9 D1 F1 G1 H1 J1

Cell surface receptor: G protein-coupled receptors

Class A: Rhodopsin like

Neurotransmitter

Adrenergic	α1 (A B D) α2 (A B C) β1 β2 β3

Purinergic	Adenosine (A1 A2A A2B A3) P2Y (1 2 4 5 6 8 9 10 11 12 13 14)

Serotonin	(all but 5-HT3) 5-HT1 (A B D E F) 5-HT2 (A B C) 5-HT (4 5A 6 7)

Other	Acetylcholine (M1 M2 M3 M4 M5) Dopamine (D1 D2 D3 D4 D5) Histamine (H1 H2 H3 H4) Melatonin (1A 1B 1C) TAAR (1 2 3 5 6 8 9)

Metabolites and
signaling molecules

Eicosanoid	CysLT (1 2) LTB4 (1 2) FPRL1 OXE Prostaglandin (DP (1 2), EP (1 2 3 4), FP) Prostacyclin Thromboxane

Other	Bile acid Cannabinoid (CB1 CB2, GPR (18 55 119)) EBI2 Estrogen Free fatty acid (1 2 3 4) Lactate Lysophosphatidic acid (1 2 3 4 5 6) Lysophospholipid (1 2 3 4 5 6 7 8) Niacin (1 2) Oxoglutarate PAF Sphingosine-1-phosphate (1 2 3 4 5) Succinate

Peptide

Neuropeptide	B/W (1 2) FF (1 2) S Y (1 2 4 5) Neuromedin (B U (1 2)) Neurotensin (1 2)

Other	Anaphylatoxin (C3a C5a) Angiotensin (1 2) Apelin Bombesin (BRS3 GRPR NMBR) Bradykinin (B1 B2) Chemokine Cholecystokinin (A B) Endothelin (A B) Formyl peptide (1 2 3) FSH Galanin (1 2 3) GHB receptor Gonadotropin-releasing hormone (1 2) Ghrelin Kisspeptin Luteinizing hormone/choriogonadotropin MAS (1 1L D E F G X1 X2 X3 X4) Melanocortin (1 2 3 4 5) MCHR (1 2) Motilin Opioid (Delta Kappa Mu Nociceptin & Zeta, but not Sigma) Orexin (1 2) Oxytocin Prokineticin (1 2) Prolactin-releasing peptide Relaxin (1 2 3 4) Somatostatin (1 2 3 4 5) Tachykinin (1 2 3) Thyrotropin Thyrotropin-releasing hormone Urotensin-II Vasopressin (1A 1B 2)

Miscellaneous

Orphan	GPR (1 3 4 6 12 15 17 18 19 20 21 22 23 25 26 27 31 32 33 34 35 37 39 42 44 45 50 52 55 61 62 63 65 68 75 77 78 81 82 83 84 85 87 88 92 101 103 109A 109B 119 120 132 135 137B 139 141 142 146 148 149 150 151 152 153 160 161 162 171 173 174 176 177 182 183)

Other	Adrenomedullin Olfactory Opsin (3 4 5 1LW 1MW 1SW RGR RRH) Protease-activated (1 2 3 4) SREB (1 2 3)

Class B: Secretin like

Adhesion	ADGRG (1 2 3 4 5 6 7)

Orphan	GPR (56 64 97 98 110 111 112 113 114 115 116 123 124 125 126 128 133 143 144 155 157)

Other	Brain-specific angiogenesis inhibitor (1 2 3) Cadherin (1 2 3) Calcitonin CALCRL CD97 Corticotropin-releasing hormone (1 2) EMR (1 2 3) Glucagon (GR GIPR GLP1R GLP2R) Growth hormone releasing hormone PACAPR1 GPR Latrophilin (1 2 3 ELTD1) Methuselah-like proteins Parathyroid hormone (1 2) Secretin Vasoactive intestinal peptide (1 2)

Class C: Metabotropic glutamate / pheromone

Taste	TAS1R (1 2 3) TAS2R (1 3 4 5 7 8 9 10 13 14 16 19 20 30 31 38 39 40 41 42 43 45 46 50 60)

Other	Calcium-sensing receptor GABA B (1 2) Glutamate receptor (Metabotropic glutamate (1 2 3 4 5 6 7 8)) GPRC6A GPR (156 158 179) RAIG (1 2 3 4)

Class F: Frizzled / Smoothened

Frizzled	Frizzled (1 2 3 4 5 6 7 8 9 10)

Smoothened	Smoothened

Membrane proteins, receptors: cell surface receptors

G protein-coupled receptor

Class A	Eicosanoid receptor (Prostaglandin receptor) Protease-activated receptor Neurotransmitter receptor Purinergic receptor Biogenic amine receptor Olfactory receptor

Class B	Secretin receptor

Class C	Metabotropic glutamate receptor

Class D	Pheromone receptor

Class E	cAMP receptor

Class F	Frizzled/smoothened

Ligand-gated ion channel

Purinergic receptor

Enzyme-linked receptor

Other/ungrouped

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