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. 2021 Apr 1;15:662453. doi: 10.3389/fncel.2021.662453

Table 2.

Overview and comparison of the adaptation and feedback mechanisms in invertebrate and vertebrate phototransduction and olfactory transduction cascades.

Invertebrate Vertebrate
Vision SM homeostasis DAG kinase (DKG) and DAG lipase (DAGL) regulate breakdown of DAG. Guanylyl Cyclase GC1 and GC2: synthesis of cGMP
Calcium feedback In microvilli: light ↑Ca2+ from 0.1 to 1,000 μM.
In high Ca2+: first, positive feedback on TRP channel and IP3 receptor activation; Then, acceleration of inactivation of TRP channels potentially via PKC (shortens bump duration); CaM-mediated acceleration of GPCR* inactivation (via arrestin) and Ca2+-dependent PKC-mediated inactivation of PLC*.
Light ↓Ca2+ in the outer segment from ~0.5 to ~0.05 μM (rods) or ~0.005 μM (cones).
In low Ca2+: GCAP1/GCAP2 accelerate cGMP synthesis, Ca2+-dependent acceleration of GPCR* and potentially PDE6*via recoverin.
Calcium regulation Diffusion, ER sequestration, Na+/Ca2+ exchange Na+/Ca2+, K+ exchangers: NCKX1 (rod), NCKX2/4 (cone)
Channel co-operativity Ca2+-dependent sensitization of channels -> Thresholding: opening of 1 channel leads to abrupt opening of ~15 channels. 3
Pigment regeneration/dark adaptation/vision in bright light Bistable pigment that can be regenerated by long wavelength light but also in total darkness, regulation of refractory period by accelerating inactivation of GPCR*, PLC* and clearance of DAG and/or replenishment of PIP2 enable vision in bright light. Visual cycles via RPE and/or Müller cells to regenerate visual pigment rate-limiting for dark adaptation and potentially required to prevent pigment depletion in bright light for cones.
Light adaptation Dynamic range: from single photon up to 105-106 photons/s.
Ca2+ feedback by inactivation of GPCR* by arrestin, acceleration of GTPase activity of Gq* by PLC.
Dynamic range: Rods: 1–~1,000 photons/s; Cones: ~100–107 photons/s.
Ca2+ feedbacks; GRK-mediated phosphorylation and arrestin binding to GPCR*; acceleration of GTPase activity of G* by PDE6 and GAP complex (RGS9, Gb5).
Smell Channel co-operativity 1.5
Calcium feedback Ca2+/CaM mediate positive feedback on OR-Orco and PPK25 Na+ channels. Odor stimulation increases Ca2+ through CNG channels by several-fold from basal level of ~0.05 μM (in the absence of odorants). In high Ca2+: Positive feedback to activate Ca2+-activated Cl channels; Negative feedback: CaMKII-mediated inhibition of ACIII; Ca2+/CaM-mediated activation of PDE1C.
Calcium regulation CALX Na+/Ca2+, K+ exchanger: NCKX4
Adaptation Dephosphorylation of Orco. Ca2+ feedback; Phosphorylation by cAMP-dependent kinase PKA.

*activated; ↓decreased.