FIGURE 5.

Three models for the ipRGC photocycle. In all models of the ipRGC photocycle above, apo-melanopsin can only bind the cis conformation of retinal, but recycling of all-trans-retinal differs. The models are not mutually exclusive. A, after binding, melanopsin may have a bistable photocycle with a reversal wavelength so close to the forward wavelength that the two are nearly indistinguishable under normal circumstances. B, melanopsin may also possess dark isomerase activity that fully resets melanopsin activity to dark-adapted levels. C, there may also exist a photoisomerase that reisomerizes all-trans-retinal into 11-cis-retinal similar to the retinochrome in squid.