Lecture Notes

Vertebrate olfactory receptor cells: molecular mechanisms of
transduction

Learning outcomes
1. Knowledge of the cellular organization of the olfactory system in mammals and insects
2. Knowledge of the mechanisms of transduction in olfactory receptor cells in mammals and insects
3. Knowledge of how of olfactory receptor genes were discovered and their patterns of expression in olfactory system.
4. Knowledge of how olfactory receptor expression maps onto glomerular organization of the olfactory bulbs / antennal
lobes
5. Knowledge of specific examples of identified odorants that activate specific receptors.
6. Understanding of mechanisms of olfactory coding – how odorants can activate different combinations of ORNs and
glomeruli
7. An ability to compare and contrast mechanisms of olfaction in mammals and insects


Vertebrate olfactory receptors

 Looking at the olfactory system (another sensory modality) to compare to the visual system
 diagram of nasal epithelium
 At the roof of the nasal cavity Is the nasal epithelium
 Within the nasal epithelium shown in red is the olfactory receptor neurons, which are enabling smell and have
dendritic process, which is modified cilia on the surface
 The axons of olfactory receptor cells travel a short distance
 The axons make synapses with the second neurons of the olfactory system in the olfactory bulb





Olfactory receptor cells (neurons) or ORNs in vertebrates

 Humans have ~ 10 million ORNs; dogs have ~4000 million ORNs
 ORNs have a short axon that projects to glomeruli of the ofactory bulb
 ORNs undergo continuous turnover in adult life
- Constantly replacing olfactory receptor neurons, which is a good thing or
otherwise end up losing our sense of smell
 ORNs have a dendritic knob with 20-30 cilia that project into the mucous layer
over the
 olfactory epithelium (see picture)

,Odorant excitation of a vertebrate olfactory receptor cell opens individual ion

 if get an individual olfactory receptor cell and puff some odorant onto the cilia will observe inward currents
 Image showing patch clamp recordings of individual ion channels opening and closing




Olfactory transduction mechanisms in cilia membranes of olfactory receptor cells

 The basic signal transduction cascade in an olfactory receptor cell involves an olfactory receptor protein
 Olfactory receptors are GCPR coupled to G-proteins
 Adenylyl cyclase generates cyclic AMP, which goes on to activate nucleotide-gated ion channel, which is a cation
channel.
- This is all similar to what occurs in a photoreceptor cell.


,  4 of these sub-units combine to form a tetrameric cation channel
 Reminder of how the cyclic nucleotide-gated channels look like




Olfactory transduction mechanism in cilia membrane of olfactory receptor cells

 Olfactory receptors or odorant receptors
 Focus on the proteins that are essential for smell




Potentially confusing terminology

 Olfactory receptor cells or olfactory receptor neurons (ORNs)
 These cells express proteins that are known as olfactory receptors or odorant receptors (ORs)
 The word olfactory receptor can be used both for the cell type and the protein

The nobel prize in physiology or medicine 2004

 ‘’For their discoveries of odorant receptors and the organisation of the olfactory system’’