L12: Signalling pathways in practice (perception of odorants and light)

Question 1

Odorant molecules

Answer 1

• Odorant molecules must be volatile (diffuse into nostrils for perception)
• Whilst they are simple, they have great structural diversity

Question 2

Structures involved in odorant perception

Answer 2

• Found in main olfactory epithelium
• Sensory cilia in mucous lining of nose, 12 for each olfactory neuron (1x10^6), converging to an olfactory bulb

Question 3

Conserved structure of an odorant receptor, variability

Answer 3

• Members of the 7TM-receptor (superfamily of GPCRs)
• > 1000 diff. odorant receptor genes

Question 4

Odorant perception signalling pathway

Answer 4

• Odorant binds to BS inside barrel-shaped receptor
  -> conformational change in rec.
• Recruits and activates heterotrimeric G-olf protein, exchange of GDP for GTP on Guanyl BS of a-subunit, activates protein
  -> a-subunit dissociates
  -> activates adenylate cyclase which catalyses ATP->cAMP
• cAMP conc. peak prompts action potential (binds to cAMP-gated ion channel, causing influx of lots of Na+ w/ small amounts of Ca2+/depolarisation, Ca2+ in turn activates voltage gated ion channel causing Cl- efflux/further depolarisation)

Question 5

Dark-adapted rod cells, impact on polarisation

Answer 5

• Rod cells have an abnormal resting state; unstimulated rod cell membrane potential at -30 mV (dark-adapted cell) -> always depolarised, open cGMP-gated ion channel allowing Na+ and Ca2+ influx
• Hyperpolarisation occurs in light perception

Question 6

Rhodopsin activation

Answer 6

• Instead of ligand binding as in the typical GPCR, rhodopsin (7TM rec.) undergoes light-induced isomerisation (at 11-cis pstn)
• Results in an all-trans configuration; Meta-rhodopsin II/activated opsin

Question 7

Light perception signalling pathway

Answer 7

• Light activation of rhodopsin to meta-rhodopsin II recruits G protein (‘transducin’)
• Activation -> GDP exchange for GTP, a-subunit dissociates
• Downstream effector is phosphodiesterase, transducin activates and rapidly degrades cGMP
• Lowered cGMP conc. closes cGMP gated ion channels, stops Ca2+ and a+ influx -> hyperpolarisation
• Hyperpolarisation of PM reduces rate of neurotransmitter (glutamate) release -> signal