. 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 $G_{q}^{*}$ 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.