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Several identified photoresponsive neurons (or extraocular photoreceptors) exist in the ganglion (CNS) of the sea slug Onchidium. The named A-P-1/Es-1 of these neurons responded to light with a depolarization, caused by closing of the cGMP-gated K+ channels, as in vertebrate phototransduction. The hyperpolarizing photoresponse of the others Ip-2/Ip-1 was produced by opening of the same cGMP-gated K+ channels as above following activation of a G-protein, Go coupled with guanylate cyclase. The amount of light required to stimulate these neurons covered in situ could be easily provided by the transmission of living daylight through the animal's body wall. The first order photosensory cells, A-P-1/Es-1 and Ip-2/Ip-1 were not only the second order interneurons relaying several kinds of sensory inputs, but also motoneurons innervating the mantle and the pneumostome. Thus, it is suggested that the depolarizing photoresponse of A-P-1/Es-1 plays a role in facilitating the synaptic transmission of sensory inputs and the following outputs, i.e. the mantle movements and that the hyperpolarizing one of Ip-2/Ip-1 in depressing a transmission similar to above and the following pneumostome ones. Similarly, it is possible that the photoresponse of photoresponsive neurons, ipRGCs in mammalian retina operates also in the general regulation of synaptic transmission and behavioral activities. [ABSTRACT FROM AUTHOR]