It has been recently described that a small percentage of ganglion cells of the mammalian retina are able to express an opsin (melanopsin) that make them to be intrinsically sensitive to light: intrinsically photosensitive retinal ganglion cells (ipRGC). These cells send their axons to different areas of the diencephalon and midbrain and contribute to a series of non visual physiological processes (pupillary reflex, circadian rhythms, ...).

However, the functional role of the ipRGC on the dark/light adaptation processes has not yet been studied. By the use of electroretinogram (ERG) recordings on anaesthetized mice, we have tested the rod and cone responses from living animals in four experimental groups under different day/night and dark/light conditions. Our experiments demonstrate that ERG b-wave maximal amplitude of the cone response was significant higher when the animals were maintained under light adapted conditions, both in day or night phase of their circadian rhythm. Moreover, this effect of light adaptation was observed only when the intensity of the adaptation light source was enough to stimulate the ipRGC (>= 200 cd·m-2). When mice were light adapted with a light source of such intensity not enough to stimulate the ipRGC (~10 cd·m-2), the amplitudes of the cone mediated b-wave were similar to those obtained under dark adapted conditions. From the present experiments we conclude that ipRGC are able to increase the light sensitivity of cone photoreceptors, thus showing an unknown mechanism of neural mediated light adaptation.