Gotow T, Nishi T. Simple photoreceptors in some invertebrates: physiological properties of a new photosensory modality.
Brain Res 2008;
1225:3-16. [PMID:
18538313 DOI:
10.1016/j.brainres.2008.04.059]
[Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2008] [Revised: 03/23/2008] [Accepted: 04/15/2008] [Indexed: 10/22/2022]
Abstract
Simple photoreceptors, namely photoresponsive neurons without microvilli and/or cilia have long been known in the central ganglion of crayfish, Aplysia, Onchidium and Helix. Recently, similar simple photoreceptors, ipRGCs were discovered in the mammalian retinas. A characteristic common to all of their photoreceptor potentials shows a slow kinetics and little adaptation, contrasting with the fast and adaptive photoresponses in eye photoreceptors. Furthermore, these simple photoreceptors are not only first-order photosensory cells, but also second-order interneurons. Such characteristics suggested that simple photoreceptors function as a new sensory modality, non-image-forming vision, which is different from the image-forming vision of eye photoreceptors. The Onchidium simple photoreceptors A-P-1 and Es-1 respond to light with a depolarizing receptor potential, caused by closing of light-dependent, cGMP-gated K+ channels, as in vertebrate cGMP cascade mediated by Gt-type G-protein. The same simple photoreceptors Ip-2 and Ip-1 are hyperpolarized by light, owing to opening of the same K+ channels. This shows the first demonstration of a new type of cGMP cascade, in which Ip-2/Ip-1 are hyperpolarized when light activates guanylate cyclase (GC) through a Go-type G-protein. The ipRGCs, as involved in non-imaging function of ipRGCs, contribute to pupillary light reflex and circadian clocks. However, their function as interneurons has not been ascertained. In Onchidium simple photoreceptors, A-P-1/Es-1 and Ip-2/Ip-1 cells the photoreceptor potentials play a role in LTP-like long-lasting potentiation (LLP) of the non-imaging functions, e.g., excitatory tactile or inhibitory pressure synaptic transmission and the subsequent behavioral responses. It was also shown that this LLP is effective, even if their photoresponse is subthreshold.
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