Table 1.
Overview and comparison table of the transduction cascades in vertebrates and invertebrates in vision and olfaction.
| Invertebrate | Vertebrate | ||
|---|---|---|---|
| Vision | Transduction compartment/volume (femtoliter, fL) | Microvillus/~10−2–10−3 | Modified cilium (outer segment)/~10–1,000 |
| Receptor | r-opsin, Gq-coupled GPCR | c-opsin, GT (Gi/o family)-coupled GPCR | |
| Effector enzyme (E) | PLCβ4: synthesizes DAG, IP3; catabolizes PIP2 | PDE6: hydrolysis of cGMP | |
| Second messenger (SM) | DAG/PUFA/PIP2/IP3 | cGMP | |
| SM homeostasis | DAG kinase (DKG) and DAG lipase (DAGL) regulate breakdown of DAG. | Guanylyl Cyclase GC1 and GC2: synthesis of cGMP | |
| Transduction channel | TRP/TRPL | CNGA1/B1 (rod), CNGA3/B3 (cone) | |
| Sensitivity (unitary response) | ~10 pA/~10 mV | ~1 pA/1 mV | |
| Kinetics (time-to-peak, ms) | 20 (Drosophila)-500 (nocturnal spider) | 100 (cone)−500 (amphibian rod) | |
| Lifetime of GPCR* (tGPCR*), ms | – | 40 (mouse)−400 (amphibian) | |
| Amount of E*s per GPCR* | ~10 (Drosophila) | ~50 | |
| Lifetime of E* (tE*), ms | – | 200 (mouse)−2,000 (amphibian) | |
| SMs hydrolyzed/synthesized | 500 (Drosophila) | 100–1,000 | |
| Channels opened by single photon | ~15 | ~250 | |
| Smell | Transduction compartment/volume (femtoliter, fL) | 1–4 dendrites/cilia of OSNs are inside each hair-like sensillum (diameter ~1 μm; length ~10–20 μm) | Cilium, 0.4 |
| Receptor | Odorant receptor (OR), Ionotropic Receptor (IR), Gustatory Receptor (GR), Drosophila genome encodes ~60 OR receptors | Golf-coupled GPCR, hundreds or even >thousand different receptors | |
| Effector enzyme (E) | OR/IR can act as ligand-gated channel, potential regulation by G proteins. | Adenylate cyclase III (AC3) | |
| Second messenger (SM) | Directly ligand gated. | cAMP | |
| Transduction channel | OR-Orco, IR | CNGA2/A4/B1b | |
| Sensitivity (unitary response) | – | ~0.05 pA/0.25 mV (assuming 5 GΩ input resistance), threshold for spike: ~40 odor molecules | |
| Kinetics (time-to-peak, ms) | ~200 for ORs | ~250 (mouse) | |
| Lifetime of GPCR* (tGPCR*), ms | N/A | 1 or less potentially due to fast dissociation of the odor molecule from the receptor | |
| Amount of E*s per GPCR* | N/A | <1 |
*activated.