Lecture 10 - Taste and Smell - Chemosensation Flashcards
What is the main difference in chemosensation between animals, single cell prokaryotes and plants?
Single cell prokaryotes and plants have chemosensation but it is not neuronal based - organised by other more acient processes
SC prokaryotes - can orient towards and move up a gradient towards nutrients through chemical sensing
Plants: can orient towards air-borne chemicals allowing growth towards food sources
What actions do smell and taste guide?
- food (nutritive value)
- mate selection
- danger
- poison
What is they typical response to bitter/sour and sugary food?
Bitter/sour - rejection, bitter compounds often toxic
Sugars: appetitive, contain imp nutrients
What is the main difference between Olfaction and gustation?
Olfaction is the detection of chemicals at a distance and gustation requires direct contact with the relelvent chemical
What is the organisation of detection in olfaction and gustation?
- Olfaction is optimised for combinatorial detection of vast numbers of odourants
- Gustation is required to categorise tastants into defined non-overlapping modalities (bitter, sweet, sour, salty umami)
In insects and mammals, why are the major gustatory and olfactory organs closely associated?
may be due to evolutionary mechanisms
also functional - odour is used to attract but still assess before material is placed near the gustatory system
What is flavour a fusion of?
taste and odour also texture (somatosensory)
Why does eating glucose result in more insulin production that direct injection into the brain?
Taste receptors are present in the gut in endocrine cells
- cells produce hormones = incretins
- incretins stimulate the secretion of insulin
- this helps to prime the metabolic system and signals satiety in the brain when sufficient incretin is produced
What do taste receptors in the gut for ‘sour’ mediate?
a slowing down of stomach emptying
Where are taste receptors present?
Mouth
Gut
Brain
How do taste receptors in the brain react to a 24hr period of food deprivation?
Increase in T1R1 and T1R2 expression (involved in metabolic regulation)
What is throught to be a ‘taste’ receptor for fat?
Not entirely sure
-‘filliform’ papillae detect texture and fat may be partially a texture within food
What are our innnate respnses to sugar and tastants such as coffee?
Innate response to sugar is to eat a LOT of it, without any prior exposure
-On first exposure to coffee we will often reject it, although learn to like it on subsequent exposures
Taste is a driver of both innate and learnt behaviours
What are the functions of taste and what are the taste qualitites of bitter, sweet, sour, salty, and unami - what are these mediated by?
Function: prepare the gut for digestion (TIR and TIIR), and other organ function/activation
Bitter - poison? Inially reject e.g. coffee, learn to like
Sweet - sugar, carbohydrate (energy)
Sour - organic acids
Salty - Sodium (ion channel transduction)
Umami - L-amino acids, nucleotides
What is the average lifespan of taste receptors and how might this contribute to loss of taste with age?
- taste receptors are constantly renewed throughout life
- lifespan of two weeks
What are the structures of taste buds, where are these found on the tounge and what do they all contain?
3 structures in the tongue Circumvallate Foliate Fungiform All contain taste buds which contain taste receptors to allow solubalised (by saliva) taste molecules through Circumvillate -taste buds on side of the pore -located at the back of the tongue Foliate -taste buds on side of the pore -located at the middle edges of the tongue Fungiform -taste bud on top of the pore Located at the front of the tongue
What are the different receptors for sweet, umami, sour and bitter - how was this discovered experimantlly?
T1R receptors: GPCRs -sweet and umami
T1R1+T1R3 = umami
T1R2+T1R3 = sweet
T1R3 is the common receptor
T2R receptors:GPCRs bitter
PKD1L3+PKD2L1: TRP receptors - sour
Experimentally
Recordings from single afferent fibres in receptor KO mice
T1Rs are a major difference in taste preference between mammals - what does this suggest about these G protein coupled receptors?
- appear to evolve rapidly
- v large sequence differences between mouse and human in the extracellular sequence
- reflects differences in diet and niche
What are the features of T1Rs and T2Rs?
- T1Rs - low affinity, so saturation not easily reached for an appetitive stimulus
- T2Rs - higher affinity, necessary to detech noxious foodstuffs more rapidly and at lower concentrations to generate a safety margin
What does the taste of saccharine elicit at different concentrations?
Elicits sweet taste at lowconcentrations, sour at high conc
-may be due to the broad tuning of both sweet and sour receptors and the differing affintiy of the ligand
How is the action of PKD2L1 a clear indicator of the labelled line theory?
Knocking out neurons expressing this channel abolishes the ability to taste ‘sour’ without affecting other tastes
What experiment was done to show strong evidence for the labelled line model for tastants?
- took sweet promotor and conjugated to the bitter receptor so that the bitter receptor was expressed in the sweet cell
- generated at attractive response to a bitter tastant that would normally be shown in response to a sweet tastant
What are the features of taste buds~?
- up to 100 polarised neuroepithelial cells which form columnar islands in the oral cavity
- have a neuronal and glial characterisitics
- tips directly contact apical surface - cell exposed to damage therefore continuously renewing
- although there is some regional sensitivity there is no taste map
- Types I, II, III, IV (Type I-III imp)
What techniques can be used to show the localisation of the different types of tastebuds?
- electromicrograph
- fluoresence labelling with GFP
- immunostaining
