Lecture 2 Flashcards
What are olfaction and gustation important for and how well understood are the central mechanisms?
Chemical senses, taste and smell allow organisms to recognise, evaluate and distinguish between dangerous substances and food resources
The central mechanisms for how attraction and aversion are distinguished are not well understood
Due to anatomical and functional homology of humans, model organisms are used instead like Drosophila, C. elegans and rodents
Explain olfactory systems
- Detect inorganic/organic airborne organic chemicals
- Odors mediate innate and learned behaviours
- Olfactory systems have complex mechanisms to distinguish between odorants and measure intensity
Four principles of olfactory systems
- Large number of odorant receptor genes per organism (1000 in mammals, 100 in insects)
- Each OSN expresses usually 1 OR which has distinct ligand profile
- OSNs expressing same receptor have axons that converge on the same glomerulus
- Each odor is encoded combinatorially - one odor activates multiple ORs, and each OR can respond to multiple odors
Organisation of olfactory systems
OSNs expressing same receptor have axons that converge on the same glomerulus
Synapses are organised into spherical neuropils - connect sensory input with output neurons/local modulatory interneurons
Olfactory glomeruli solve problem of discontinous chemical space on brain
Odorant receptor structure
Mammals:
- 7 transmembrane G protein coupled receptors
- Binding of odor causes GPCR signalling and neuronal depolarization
Insects:
- 7 transmembrane G protein coupled receptors
- Evolutionarily distinct from mammals
- Odor-gated cation channels modulated by G protein signalling
Drosophila odorant receptors
- Flies attracted to rotting fruit, each other and vinegar
- acj6 mutant was found to be a key transcription factor in regulation of a subset of ORGs.
- Fly genome has 62 ORs encoded for by 60 genes
- This was discovered by OSN expression patterns, odor response and anatomical mapping of antennal lobe
Where are Drosophila olfacotry organs located
All over the flies body:
- taste with their legs and wings
- Functional homologue of a human tongue is on their proboscis - houses multiple GSNs on labial palps
- Basiconic (food, Co2), trichoid (pheromones) and coeloconic sensilla (food, water vapour, ammonia) all house a variety of olfactory and gustatory (only basiconic) receptors
Palp receptors
Basiconic sensilla
Detect food odors using Or receptors
Larva receptors
Dorsal organ dome
Food odors using Or receptors
Drosophila odor coding
- Axons of neurons expressing same OR converge on same glomeruli
- Excitable projection neurons send dendrites to a single glomerulus
- Inhibitory neurons project to multiple glomeruli
- Similar organisation of OR axons and mitral cells found in vertebrate olfactory bulb
What is co-convergence and co-expression
More receptors than glomeruli
Co-convergence - More than 1 class of OSN is located on the same glomerulus
Co-expression - Multiple OSNs of the same class are expressed on the same glomerulus
Functional overview of the Drosophila Olfactory circuit
OSNs (1300) detect odors and transmit them to the antennal lobe (43) via antennal nerve
Projection neurons (150) and local interneurons process the signal
Projection neurons send the signal to the mushroom body neurons (learning and memory, 2500 MBNs, and hundreds of calyx glomeruli) and the lateral horn (innate behaviour)
Output of higher brain regions drive behavioural responses
Mammals vs insects in olfactory circuit wiring
Mammals:
Olfactory receptor neurons -> Olfactory bulb (primary center) -> Secondary centers like piriform/entorhinal cortex or olfactory tubercle
Insects:
antenna and maxillary palp -> Antennal lobe via ORNs -> MB calyx and lateral horn via PNs
Gustation
- Insects and mammals show attractive responses to sugars and aversive taste to bitters
- Gustatory system organisation differs
- Mammals have gustatory organ in head, insects have taste organs all over their body
- GRNs and organisation of gustatory system not well understood
Where do different neurons project to pathway wise?
- Taste neurons send axons directly to CNS
- Sensory neurons in legs and wings project to ventral nerve chord
- Sensory neurons from proboscis project to suboesophageal zone
- Gustatory neurons and motor neurons aborize in the SEZ - sensorimotor centre for feeding
Name the three types of gustatory receptors in flies
Gustatory receptors - 68 - detect sweet and bitter tastes
Ionotropic receptors - 66 - detect amines, acids and salt
pickpocket 28 - Member of PPK channel family, detects water
- Elements of feeding program elicited by activation of GNs
- Detection of food usually begins with leg GNs
- Fly extends proboscis and ingests upon detecting sucrose
- Proboscis retracts when detecting bitter compounds
Gustation in mammal
- Taste receptor cells located in papillae of tongue and oral cavity
- Taste bud has 3 TRCs and basal cells involved in genesis of new TRCs
- Three cranial nerves (7, 9 and 10) innervate oral cavity and convey taste to rNST
rNST recieves input from pontine parabrachial nucleus, lateral hypothalamus, gustatory cortex, amygdala and drom caudal NST
PBN -> GC -> OFC
OFC neurons project and receive input from dorsolateral prefrontal cortex
Taste bud TLCs
Type I TRC -> low salt conc
Type II TRC -> sweet, umami, bitter and kokumi
Type III TRC -> Sours
Tastant binding pathway
- GPCRs activate intracellular signalling
- G-protein activates phospholipase C
- PLC produced IP3
- Calcium ions released and enter through voltage gated calcium ion channels
- This triggers neurotransmitter release
- Activate afferent sensory fibres, sending signals to brain
- Sodium ion channels detect salty taste via depolarization
