Integration of ocular and non-ocular photosensory information in the brain of the terrestrial slug Limax

In the terrestrial slugs Limax , most of the photosensory information is thought to be acquired by an eye located on the superior tentacles, by which the slugs avoid light. Recent studies, however, suggested that the brain also plays a role as a photosensor in their negative phototaxis behavior. In the present study, we investigated how the photosensory information acquired by the eye and brain is integrated. The visual pathway in the brain was traced by incorporating tracer molecules from the cut end of an optic nerve, and commissural interactions were found in optic neuropiles located in the lateral regions of the cerebral ganglia. A cluster of neuronal cell bodies located near the dorsal surface of the cerebral ganglion had connections with the contralateral optic neuropile via gap junctions. Some of these neuronal cell bodies were Opn5A-immunoreactive, and contained numerous photic vesicle-like structures. Light-induced spikes were recorded extracellularly from the dorsal surface of these neuronal clusters, and they were synchronous with the spikes recorded from the cut end of the cerebral commissure. This study suggests that both the light information from the eye and the contralateral cerebral ganglion are integrated in the optic neuropile.