Lecture 2

Question 1

Olfaction (smell) and gustation (taste)

Answer 1

• Chemical senses, taste and smell allow organisms to distinguish food from dangerous substances
• Recognising attractive and aversive tastes and smells not very well understood in animals
• Drosophila, C elegans and rodents used to understand mechanism of sensory coding due to anatomical and functional homology

Question 2

Olfaction

Answer 2

• Detect airborne organic and inorganic chemicals
• Odors mediate innate and learned behaviours e.g. aversion and attraction, mating, feeding, and escape from predators
• Olfactory systems evolved complex mechanisms to discriminate between them and measure intensity due to large numbers of odors

Question 3

4 principles of olfaction

Answer 3

1. Large numbers of odorant receptor genes dedicated to olfaction (1000 in mammals, 100 in insects)
2. Olfactory sensory neurons usually expresses only a single OR and each OR has a distinct ligand profile
3. OSNs expressing same receptors extend axons that converge on same glomerulus in brain
4. Each odor encoded combinatorically - one odor activates multiple ORs and each OR can respond to multiple odors

Question 4

Olfactory systems in mice/flies

Answer 4

• Olfactory circuits are wired in accordance to axon convergence from sensory neurons expressing the same OR
• Synapses organised into spherical neuropils - olfactory glomeruli - that connect sensory input with output neurons and local modulatory interneurons
• Olfactory glomeruli solve the problem of mapping discontinuous chemical space onto the brain

Question 5

Mammalian and C elegans ORs

Answer 5

7 transmembrane G protein coupled receptors (GCPRs)

Binding of odor to OR results in GPCR signalling and neuronal depolarisation

Question 6

Insect OSNs

Answer 6

7 transmembrane protein ORs evolutionary distinct from mammalian

Thought to be odor-gated cation channels that are modulated by G-protein signalling

Question 7

Drosophila Odorant receptors (ORs)

Answer 7

• Attracted to smell of vinegar, yeast, rotting fruit, and each other
• Obaid Siddiqi found acj6 was a key transcription factor in a subset of odorant receptor gene regulation
• Fly genome has 62 ORs encoded by 60 genes, almost all analysed at level of:
  > Expression pattern on OSNs
  > Functional response to odors
  > Anatomical mapping to the antennal lobe (higher brain centre)

Question 8

Distribution of Drosophila odorant receptors

Answer 8

Antenna formed from basiconic sensilla (food odors, CO2), trichoid sensilla (Pheromones), and coeioconic sensilla (food odors, water vapour ammonia)

Palp has basiconic sensilla (Food odors)

Larva has dorsal organ dome (Food odors)

Question 9

Drosophila Odor Coding - Transition to secondary stage of olfactory processing

Answer 9

• Axons of neurons expressing the same OR converge on glomeruli in antennal lobe
• Each excitatory projection neuron sends its
• Inhibitory neurons project to multiple glomeruli
• Organisation of axons or mitral cells found in vertebrate olfactory bulb

Question 10

Drosophila odor coding

Answer 10

Co-convergence or Co-expression can occur