On the implications of bistability of visual pigment systems.

The characteristics of different responses of invertebrate photoreceptors are reviewed. Invertebrate photopigment bistability has made possible the functional operational dissection of the pigment transition scheme. Outlasting the usual stimulus-coincident late receptor potential (LRP), additional antagonistic responses have been found: the prolonged depolarizing after-potential (PDA) arising from a net rhodopsin to metarhodopsin pigment shift, and a PDA-depression and an anti-PDA effect which arise from a reverse shift and cancel the PDA when induced during or closely before it. The characteristics of these aftereffects and of the LRP are reviewed, analyzed and compared. Both potentials require rhodopsin activation and they share the characteristics of a common ionic conductance-change mechanism. However, for the LRP response to weak stimuli, no antagonistic metarhodopsin-dependent effect has been found analogous to PDA-depression and the anti-PDA. However, this is just the response level where interactive effects would be weakest. For more intense stimuli, pigment-state effects on the shape of the LRP have been found, and net pigment shifts affect the strength of a facilitatory effect.