Sensation and Sensory Processing: Perceiving the world Module 404 Sean Sweeney

1. Sensation and Sensory Processing: Perceiving the world Module 404 Sean Sweeney

2. Learning Outcomes: Understand the purpose of the sensory system to an organism Differentiate between different sensory ‘modalities’ Understand that the sensory system is organised in a logical manner, tuned to life strategy Understand that sensory stimuli are transduced, encoded and finally perceived Understand the basis of light perception in the insect eye Appreciate the different organisation between the insect and mammalian olfactory and gustatory systems while also appreciating that the systems follow a similar logic for coding and perception

3. Sensation involves the ability to transduce, encode and perceive informationgenerated by stimuli arising from both the external and internal environment. Questions: 1) How is the stimulus detected (at a molecular level)? 2) How is the stimulus transduced and encoded? 3) How does the CNS perceive the incoming information? for 2) and 3), how is the wiring diagram organised? 4) How can qualitative and quantitative information about the stimuli be represented? 5) for some modalities, how can location be represented

4. The stimuli touch movement mechanosensation imbalance sound temperature thermosensation (Pain!) light photoreception pain nociception taste smell chemoception moisture Monitoring the internal environment for homeostatic regulation internal or external? ?

5. The anatomy of detection touch body surface movement body surface, muscle spindles, chordotonal organs imbalance auditory organ sound auditory organ temperature body surface, hypothalamus, gustatory system light eye/photoreceptive organ pain body surface, internal pain receptors taste integrated w/ the gustatory system (gut?) smell olfactory organ, body surface? moisture body surface? gustatory system? Dedicated Organs v Dispersed Receptors

6. The detection and transduction of the stimuli The TRP receptor paradigm Superfamily of channels (found in yeast to humans) Six transmembrane domains (with varying degrees of homology) Permeability to cations (varying cation selectivity) A single channel can be activated by disparate mechanisms TRP channels play critical roles in responses to all major classes of external stimuli. TRP channels work as heteromultimers in supramolecular complexes

7. ChannelPca:Pnamodulation TRPC1 nonselective store depletion, stretch conformational coupling TRPC2 2.7 DAG TRPC3 1.6 store depletion, DAG conformational coupling exocytosis TRPV1 3.8(heat) Heat (43oC), vanilloids 9.6(vanilloids) anandamide, camphor piperine, allacin, EtOH proinflammatory cytokines nicotine, protons, PIP2 TRPV2 3 Heat (53oC), osmotic cell swelling, exocytosis TRPV3 2.6 PUFAs, menthol, compounds from oregano, cloves, thyme

10. The capsaicin receptor capsaicin, the active ingredient of capiscum or chili peppers Strength measured in ‘Scoville Units’ (Wilbur Scoville, 1912) Jalapeño, 5000 Scoville units Habañero, 300,000 Scoville units Expression cloning of the receptor TRPV1: Caterina et al., (1997) Nature 389: 816-24 In vivo function of the receptor (KO mice): Caterina et al., (2000) Science 288: 306-313 TRPV1 activated by capsaicin, anandamide, heat (>43oC), camphor, piperine, garlic Mice lacking TRPV1 are deficient for vanilloid ellicited pain, thermal sensation, and tissue injury-induced thermal hyperalgesia

11. Known sensory modalities mediated by TRP channels: KO organisms, experimental evidence Chemosensation osm-9, ocr-2 (C. elegans) TRPV response to odorants (and other modalities) TRPM5 (mammals) TRPM sweet, bitter and a.a. taste TRPC2 (mouse!!) TRPC pheromone (in VNO!!) Thermosensation/nociception TRPV1 (mouse) TRPV >43oC TRPV2 (mouse) TRPV >52oC TRPV3 (mouse) TRPV >30-39oC TRPV4 (mouse) TRPV ~25-34oC TRPM8 (mouse) TRPM <28oC TRPA1 (mouse) TRPA ?? dTRPA1 (Drosophila) TRPA >35-41oC painless (Drosophila) TRPA >39-41oC pyrexia (Drosophila) TRPA >39oC ThermoTRPs are also required for response to chemical stimuli

12. Mechanosensation TRPV4 (mouse) TRPV hypotonicity osm-9 TRPV osmotic change ocr-2 TRPV osmotic change TRPY (yeast) hyperosmotic conditions TRPA1 (mouse) TRPA hearing???? TRPML3 (mouse) TRPML hearing? TRPN1 (mouse zebrafish) TRPN hearing? NOMPC (Drosophila) TRPN hearing, mechanosensation Nanchung (Drosophila) TRPV hearing, hygrosensation Inactive (Drosophila) TRPV hearing, proprioception? TRP-4 (C. elegans) TRPN mechanosensation water witch (Drosophila) TRPML hygrosensation (moist air) Phototransduction TRP (Drosophila) TRPC phototransduction TRPL (Drosophila) TRPC phototransduction TRP (Drosophila) TRPC phototransduction TRPC3 (mouse) TRPC phototransduction????

13. The tuned sensitivity of TRP channels to ranges of temperatures ensures efficient detection across a range of temperatures for thermosensation/nociception TRP channels transduce many environmental signals into a physiological response. Responses may be specific or may be multi-modal depending on the activator or (possibly) the heteromultimerisation of the channel subunits or the sensory neurons in which the receptors are expressed (?).

14. The original Transient Receptor Potential: Drosophila phototransduction seven rhabdomeres per ommatidium, some are sensitive to different wavelengths SMC: submicrovillar cisternae ROS: Rod outer segments

15. Rhodopsin, the light sensing molecule (ancient!)

16. 1 photon generates 1 ‘quantum bump’ ~20ms duration, ~10pA amplitude (in Ca2+) = opening of ~15 TRP channels within one villus Short latency (20-100ms) - time for DAG to accumulate and activate TRP channels Whole cell patch clamp

17. The Drosophila Signalplex

18. 1) Photoisomeration of rhodopsin to meta- rhodopsin activates heterotimeric Gq - releases Gq 2) Gq activates phospholipase C generating InsP3 and DAG from PIP2. DAG also releases PUFAs by activation of DAG lipase 3) TRP and TRPL activated by PUFAs (?) and/or DAG. TRPs, PKC, PLC organised in a complex by inaD (5x PDZ domains) 4) SMC (submicrovillar cisternae) Ca2+ stores? Insp3 gated? 5) DAG converted to PA via DAG kinase and CDP-DAG by CD synthase, PI regenerated And transported back to microvillar membrane By PI transfer protein and converted to PIP2

19. Things that go ‘bump’: a) 20ms after absorbtion of photon metarhodopsin activates G-protein, activating PLC generating membrane 2nd messenger (red) - threshold for activating one channel is reached b) Ca2+ influx sensitises other channels - rising phase of bump c) Ca2+ floods microvillus (>200µM) leading to rapid inactivation and refractory period, Ca2+ returns to resting levels (~150nM) within ~100ms. M, Gq and PLC are deactivated and PIP2 resynthesised. The Ca2+/Na+ exchanger Calx extrudes Ca2+

20. TRPless vision: the mammalian phototransduction cascade The human eye light sensitive protein outer segment of rod Retina Rhodopsin catalyses the only light sensitive step in vision. 11-cis-retinal chromophore lies in a pocket of the protein and is isomerised to all-trans retinal when light is absorbed. The isomerisation of retinal leads to a change of the shape of rhodopsin which triggers a cascade of reactions which lead to a nerve impulse which is transmitted to the brain by the optical nerve.

21. Rods (100 million) detect degree of lightness bleached by light sensitivity determined by amount of rhodopsin Low sensitivity Cones (3 million) sensitive to light but retain function in high illumination, use pigment iodopsin red green blue

22. Activated rhodopsin binds to transducin (a trimeric G-protein), activated -transducin removes the inhibitory subunit of phosphodiesterase E PDE hydrolyses cGMP to GMP Dark - cGMP high cGMP binds to cyclic nucleotide gated channels (CNG) ‘dark current’ flows releasing glutamate to the horizontal and bipolar cells In light, cGMP is hydrolysed by PDE, the CNG channel closes, inhibiting glutamate release, bipolar cells relay this to ganglion cells

23. Invertebrate photoreception uses the phosphoinositide pathway Best characterised genetic model of this pathway! vertebrate rods use the phosphodiesterase pathway. Both G-protein signal transduction methods employ arrestin to terminate the signal, also rhodopsin kinase arrests rhodopsin function. Invertebrates activate TRP channels to activate an electrical response Vertebrates inactivate CNG channels, inhibiting glutamate release to activate an appropriate response Invertebrates employ a highly structured signalling complex Vertebrates employ a diffusion process

24. Olfactory and gustatory processing Single cell prokaryotes can orient towards and move up a gradient towards nutrients: chemical sensing Plants can orient towards air-borne chemicals allowing growth to food sources Smell and taste guide food and mate selection, danger, nutritive value, poison ‘Flavour’ is a fusion of taste and odour Olfaction: detection of chemicals at a distance Gustation: requires direct contact with relevant chemical Sugars are appetitive: important nutrients bitter or sour elicit rejection: bitter compounds often toxic Olfaction: optimised for combinatorial detection of vast numbers of odorants Gustation: organised to categorise tastants into defined non-overlapping modalities (sweet, bitter, sour, salty, umami)

25. Flies can taste the world with more than their ‘tongue’ and ‘nose’.

26. Drosophila olfactory receptors (OR) and gustatory receptors (GR) cloned by expression G-protein coupled???? 7 transmembrane inverse comformation to mammalian ORs ORs, GRs: one large family Obligate heterodimer 62 ORs in Drosophila (60 genes) 79 ORs in Mosquito 157 ORs in Honeybee 68 GRs (60 genes) Or83b mutant flies are anosmic two receptors per chemosensory neuron, OR83b + 2nd CO2!!!! some ORs v. specific for one chemical some broadly tuned for class

27. Black = posterior, grey intermediate, white posterior sensilla from similar anatomical regions send projections to closely associated glomeruli glomeruli are dendrites for the projection neurons (PNs) PNs then project to the Mushroom body calyx and lateral horn OR67d responds to 11,cis-vaccenyl-acetate the glomeruli are fruitless positive and sexually dimorphic Each OR target a unique and sterotyped glomerulus Transgenic reporters uncover the odour code GRs project to the suboesophageal ganglion (somatotopy?)

28. The mammalian olfactory system: Closely linked with the respiratory and gustatory apparatus Aided by turbulent air eddies

29. The mammalian olfactory epithelia lines the nasal cavity - allows direct access to odorant molecules mucus protects, neurons are turned over Each olfactory sensory neuron expresses only one type of olfactory receptor ORs in human: 950 ORs in mouse 1500

30. Mammalian Odorant receptors Identified 1991: Buck and Axel, Cell, 65 175-187 G-protein coupled receptors, 7 transmembrane. 1000s of ORs expressed in millions of neurons projecting to 2000 glomeruli ORs are involved in regulating axon guidance and glomerular targeting cAMP gates the Ca2+/Na+ channel, depolarisation aided by the Ca2+-gated Cl- channel. rectified by Ca2+/Na+ exchanger

32. Gustatory Transduction in Mammals: No Taste map!!

33. T1R receptors: GPCRs - sweet and umami T1R1 + T1R3 - umami T1R2 + T1R3 - sweet T1R3 - common receptor T2R receptors: GPCRs - bitter PKD1L3 + PKD2L1: TRP receptors - sour

34. Two models of taste perception: the ‘labeled line’ and ‘across fibre’ models

35. Expressing the bitter receptor in the ‘sweet’ cells generates an attractive response to a bitter tastant : favours the labelled line model

36. Conclusions: Insects and vertebrates employ remarkably similar strategies for sensory transduction and coding suggesting ancient origins for sensory systems Sensory transduction is mediated by ‘molecular sensors’ which detect specific sensory stimuli and transduce this signal to a generate a neuronal code The neuronal code is processed in secondary and tertiary order neurons Questions How are strength, quality and direction of sensory cues transduced and detected? How are these properties of the sensory cue coded?