Dopamine neurones form a discrete plexus with melanopsin cells in normal and degenerating retina

In addition to rods and cones of the outer retina, a third class of photoreceptive cell has recently been described in the inner retina of mammals. These intrinsically photosensitive retinal ganglion cells (ipRGCs) have been shown to relay luminance information to the mammalian brain. In addition to their intrinsic photosensitivity, the function of ipRGCs may also be modulated by signals from within the retina itself. Such signals may emanate from classical photoreceptors in the outer retina or from the circadian activity of adjacent inner retinal neurones. Prime candidates for the latter are the retinal dopamine neurones which ramify at the border of the inner plexiform and inner nuclear layers. In order to investigate the nature of any interaction between dopamine and ipRGC populations in normal retina and to assess the impact of outer retinal degeneration on this interrelationship, we examined the retinae of normal and RCS dystrophic rats. We report a direct interaction between the dendrites of ipRGCs and dopaminergic neurones which is conserved across species. Triple immunolabelling using synaptic markers provides evidence for the unidirectionality of information transfer between the two cell types, with processes of ipRGCs being directly adjacent to sites of dopamine release. This fundamental architectural feature of the mammalian retina appears resistant to degeneration of classical photoreceptors and may provide the anatomical substrate by which dopamine cells influence the physiology of central circadian targets in the brain.