Slow oscillatory activity (SOA) has been described in several structures of the subcortical visual system, including the olivary pretectal nucleus (OPN) and lateral geniculate nucleus (LGN). Since rhythmic spiking in the OPN is dependent on contralateral retinal output, we set out to investigate whether rod-cone and/or melanopsin photoreceptors are important for SOA generation in the OPN and LGN. Two different approaches were used: extracellular single-unit recordings in rats combined with intraviteral injections of photoreceptor blockers, and extracellular multi-electrode recordings from wild-type or genetically modified mice, lacking either melanopsin (Opn4-/-) or rods and cones (rd/rd cl). The first of these methods indicated a major contribution of melanopsin to SOA generation, since this was abolished in the OPN after intravitreal 2-APB injection. This melanopsin contribution was confirmed by the second approach, with Opn4-/- mice having the lowest percentage of oscillatory cells and also the shortest average period in both structures. This is the first study to show SOA in the mouse visual system and to characterize its retinal origins, revealing melanopsin expressing retinal ganglion cells as the main driving force underlying this activity.
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