Lecture 2 - REVISION Flashcards
What are taste and smell useful for?
They allow organisms to distinguish between dangerous substances and food resources
Central mechanisms in brain used to identify attractive and aversive smells not well understood
What model organisms are used in sensory coding
Drosophila, C. elegans, rodents
Higher animals anatomical and functional homology too complex
Explain olfactory systems
- Detect airborne organic/inorganic chemicals
- Odors mediate aversion and attraction, mating, foraging, feeding, escape from predators
- They have complex mechanisms to distinguish between odorants and measure intensity
4 principles of odor discrimination
- Organisms have a large number of olfactory receptor genes e.g. 1000 in mammals, 100 in insects
- Olfactory sensory neurons express usually only one olfactory receptor and each receptor has a specific ligand profile
- Olfactory sensory neurons which express the same receptor are located on the same glomerulus in the brain
- ORs and odors are combinatorially - multiple ORs can detect the same odor, and odors can bind to multiple different ORs.
The olfactory systems of mice and flies
- Olfactory circuits wired according to axon convergence from OSNs expressing same OR
- Synapses are organised into spherical neuropils - olfactory glomeruli - connect sensory input with output neurons and local modulatory interneurons
- These glomeruli solve discontinuous chemical space mapping in brain
Mammalian and C. elegans odorant receptors
- 7 transmembrane G-protein coupled receptors
- Odor binding causes GPCR signalling and neuronal depolarization
Insect odorant receptors
- Similar to mammalian OSNs
- However evolutionary distinct from mammalian
- Thought to be odor gated cation channels modulated by G-protein signalling
Drosophila odorant receptors
- Flies attracted to vinegar, rotting fruit, each other
- acj6 gene was first mutant studied, and a key transcription factor in regulation of a subset of odorant receptor genes
- Fly genome has 62 ORs that we know of encoded by 60 genes
Analysed by:
- Expression pattern of OSNs
- Functional response to odors
- Anatomical mapping to antennal lobe
Where are olfactory and gustatory neurons expressed in Drosophila
Basiconic sensilla - Antenna and Palp
Trichoid sensilla - Antenna
Coeloconic sensilla - Antenna
Dorsal organ dome - Larva
Or83b receptor
Obligate coreceptor for all olfactory neurons except CO2-sensitive neurons expressing Gr21a and Gr63a CO2 receptor
Unknown Receptors in Antennal Coeloconic Neurons
Receptors for sensing water vapor, ammonia, and putrescine remain unidentified
Ionotropic Receptors (IRs)
- Encoded by 66 genes - detect amines, acids and salts
- Structure related to ionotropic glutamate receptors, but low sequence similarity (<34%)
- Share structures such as extracellular N-terminus, two-lobed ligand-binding domain, intracellular C-terminus
Function of IRs
- Tetrameric ligand-gated channels
- 16 IRs expressed in fly antenna
- Some act as odorant receptors for amines/acids
- Thermosensation and hygrosensation
Sensillar lymph and peri-receptor events
- Sensory neuron dendrites bathed in sensillar lymph
- Chemical compounds solubilized and transported across aqueous phase to reach sensory receptors
- Peri-receptor events involve Odorant binding proteins
Odorant binding proteins
- Small soluble protein found in chemo-sensilla lymph and nasal mucus in vertebrates
- Bind, solubilize and transport hydrophobic stimuli to chemoreceptors
- buffer odorant fluctuations, involved in hygroreception
- Initally found in olfactory appendages, now chemo and non-chemosensory organs
OBP expression in D. melanogaster
Gustatory:
- Proboscis
- Pharynx
- Leg tarsi
- Wing margins
- Female genitalia
Olfactory:
- Antenna
- Maxillary palps
Unexplored roles of OBPs
Mouthparts
Pheromone glands
Reproductive organs
Digestive tract
Venom glands
Physiological function remains unknown in these tissues
Drosophila odor coding
Transition to second stage of olfactory processing
- Axons of neurins expessing same OR converge on glomeruli in antennal lobe
- Excitatory projection neuron send its dendrites to single glomerulus
- Inhibitory neurons project multiple glomeruli
- Similar organisation of OR axons and mitral cells found on vertebrate olfactory bulb
Drosophila olfactory circuit
1,300 ORNS
30:1 convergence
43 AL glomeruli
1:3 divergence
150 PNs
PN:MB neurons is 1:15
Hundreds of calyx glomeruli
2500 MB neurons
Olfactory processing stages
Encoder stage:
- Begins at antenna and maxillaru palp, ORNs detect odors
- ORNs signal to antennal love
Decoder stage:
- processed odor information replayed to mushroom body (learning/memory) and lateral horn (innate behavioural responses
Neural circuitry of olfactory processing
- Olfactory receptor neurons (~1400)
First-order sensory neurons that detect odors
Send signals to projection neurons - Projection neurons (~150)
Transmit signals from antennal love to MB and LH - Kenyon cells (~2000)
Located in MB and involved in associative learning - Mushroom body output neurons (~34)
Receive signals from kenyon cells and influence behavioural responses - Lateral horn neurons (~1400)
- Directly process innate odor-driven behaviours
- Local interneurons (~200)
Modulate and refine odor processing
Dopaminergic neurons (~160)
Modulate learning and memory by influencing kenyon cells
Anterior paired lateral neuron (1)
Single inhibitory neuron that regulayes kenyon cell activity
Gustation
- Insects and mammals show same attractive response to sugar, and aversion to bitters, but gustatory systems different
- Mammals have single gustatory organ in head
- Insects have taste receptors all over body
- GRNs and gustatory system not well understood
taste pathways in CNS
- Taste neurons send axons directly to CNS
- Most 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 SEZ - sensorimotor centre for feeding
What gene in Drosophila detects water
pickpocket 28
member of PPK channel family
