Olfactory system, neuropeptides and behaviour Flashcards
How are odors detected?
- Olfactory sensory neurons project to olfactory bulb
- Vomeronasal organ (VNO) sensory neurons project to accessory OB, and then onto amygdala and hypothalamus
These neurons express olfacotry receptors (ie G.olf) which are stimulated by odourant, which triggers cAMP production and opens Na and Ca channels (influx into cell) and Ca triggers Cl channel (efflux out of cell) -> depolarization
VNO involved in pheramone detection
How are specific odors identified?
Each odourant binds a specific receptor (one receptor per neuron aka allelic exlcusion)
And each receptor gene is expressed within a spatial zone on the olfactory epithelium
- There are ~1000 different olfactory receptorr genes
- never two different receptors on one neuron
- Although they are expressed in regions, but within the region they are randomly scattered (ie there is no specific area that is expressing specific OR for specific odors)
What cell types compose the olfactory epithelium?
- Olfactory receptor cells (site of olfactory transduction and cross CNS)
- supporting cells (like glia)
- basal cells (make new receptor cells)
- supporting cells provide mucus and antibodies and odourant binding proteins
- olfactory cells are replaced every 4-6 weeks and only cell in nervous system that is continously replaced throughout life
What is the anotomical pathway of odor detection?
odor-> odor receptor on sensory neuron in olfactory epithelium -> olfactory nerve -> synapse onto 1-3 glomeruli in olfactory bulb-> higher brain processing
How were odorant receptors first identified?
Buck and Axel 1991 identified large gene family of potential odourants
?
How is odor signal transduced?
Odourant binds OR, which activates G.olf, which is coupled to adenylyl cyclase, which produces cAMP, which opens Na and Ca2+ channels (influx), and Ca2+ opens Cl- channels (efflux), which cause depolarization and thus AP firing
What are spatial zones of the olfactory epithelium?
A zone of the OE containing sensory neurons that express non-overlapping sets of olfactory receptors (aka one OR per neuron)
Each zone in OE projects to a coressponding zone in the OB (synapses onto 1-3 glomeruli in mitral cells)
Describe the colormetric assay used in investigating G(olf) expression
Replaced G.olf gene with Tau-lacZ-> tau bind microtubules and carries lacZ into neurons, where its gene product B-galactosidase converts xgal into blue and can assess where G.olf is being expressed
Saw G.olf promoter was driving lacZ expression in every nasal epithelium cell-> shows that neurons were projecting from olfactory epithelium into olfactory bulb and thats why OB is blue
This was done in transgenic animals
neurons from OE innervate OB and thats why its blue, not bc OB expresses G.olf
each glomerular is cluster of synapses from single mitral cell
What were the results from the two colormetric assays done to visualize olfactory system?
- use G.olf promoter with Tau-lacZ gene: found OE neurons innervate OB, and G.olf is expressed in every nasal epithelium cell
- use specific OR promoter with Tau-lacZ gene: found that all OE neurons come together at glomeruli in mitral cells in OB
OR= odorant receptor
- shows that activation of receptor neurons is relfected in OB
How are odorant receptors organised in the olfactory bulb?
specific OR are scattered within zones in the OE, then project to OB where each receptor type neuron synapses onto 1-3 glomeruli in the OB
receptor cells regenerate 3-4 wks but must maintain spatial map
How did researchers investigate how spatial map patterns persist?
1) Used diptheria toxin to destroy cells expressing one receptor type and see (via lacZ) which neurons regenrate: found that regenerating neurons innervate same glomerulus -> means that signals for patterning do not come from the cells themselves
2) Added lacZ in a way that destroyed receptor (P2) coding sequence: found that cells projected to OB but lost patterning (did not localise to specific glomerulus), this was reversed when P2 was functional -> means that patterning depends on receptor activity
3) swapped receptor genes (M1 for P2): found that convergence on glomeruli happened, but at a new location to P2 or M1-> means that yes receptor is needed for innervation, BUT additional factors (eg interplay btw environement and genes) dictates which glomerulus is innervated
whatever cues that are required to form pattern are still there even in absense of cells that had established this pattern (meaning signals not coming from cells themselves)
How can we differentiate between more than 16,000 odors?
- Each glomerulus only responds to the chemicals that activate a specific receptor
- Population coding: each odourant stimulates a population of receptors- odourants have varying affinities for each receptor
- Spatial coding: an odourant activates specific patterns of glomeruli within the OB: different combo of glomeruli respond to different odors
What were the fruit fly experiements and what did they tell us?
1) used a Ca2+ reporter to measure synaptic transmisson of glomeruli in antenal lobes in response to different odors: found that the pattern of glomeruli being activated by a smell was the same in two diff species of flies-> means that patterning is same across diff animals
2) used Ca2+ reporter to measure glomeruli activation in response to increasing odor conc: found that at low conc glomeruli activation was discrete, but as conc increased more glomeruli were recruited-> means that grades of activity for different glomeruli for a single odor (ie population coding)
used confocal microscopy to image glomeruli in the antenal lobe from Ca2+
How can three different receptor types encode four different smells? example
Range of receptors responding to odors based on their level of activation means that you can have a lot of responses to lots of odors from a limited number of receptors
Example:
- Citrus has highest affinity for OR1, then OR2 but zero for OR3: this ‘codes’ a citrus smell to the brain
- Floral has highest affinity for OR2, then OR1 and OR3: this ‘codes’ floral smell
- Peppermint has strong affinity for OR3 and OR2 but not for OR1
- Almond has strong affinity for OR3 and very little for OR2 and none for OR1
What are glomelular receptive feilds?
glomeruli that are receptive to chemically similar odors tend to cluster together
How are complex odors interpreted in mamillian brains?
Complex odors are made up of lots of chemicals, and each chemical is sensed by particular patterns of glomerular activity, which is combined together makes a map (ie spatial coding)
What are trace amine associated receptors (TAARs)?
Small family of GCPRs, expressed in v discrete patterns in nasal epithelium, very strongly linked to particular behavioural responses in animals
trace amines are really volatile nasty smells
Give two examples of TAARs and their link to behavior
Eg TAAR4: senses 2-phenylethylamine (found in carnivore urine) and enables mice to avoid (taar4-/- mice dont avoid)
Eg TAAR5: senses trimethylamine (in male mice urine) and is involved in mating (taar5-/- arent attracted to males)
What is chemosensation?
- Sense of smell detects volatile compounds for identification of substances that may be beneficial or harmful
- Needed for mate selection in sexual reproduction, essential impetus for evolution
What is a pheramone?
What are the two kinds?
any chemical signal conveying information between members of same species
releaser pheramone= immediate effect
primer pheramone= long term effect (eg puberty)
What is the evolutionary purpose of olfaction?
in bacteria and animals
Olfaction is linked to limbic system and memory centers, its important for behavioural responses (eg mating, fleeing predators, sexual maturation, fitness) and also memory
olfaction is one of the best senses to recall memory, and olfaction memory is the strongest
