Responses of suprachiasmatic nucleus neurons to light and dark adaptation: relative contributions of melanopsin and rod-cone inputs.

The circadian oscillator in the suprachiasmatic nucleus (SCN) is entrained to the environmental light/dark cycle through photic information conveyed from the retina. The vast majority of projections to the SCN arise from melanopsin-expressing ganglion cells that are intrinsically light sensitive and that receive inputs from both rods and cones. To investigate the relative contributions of the different photoreceptive systems in shaping the photic signal influencing the circadian clock, we analyzed neuronal responses of single SCN neurons using extracellular electrophysiological recordings under different conditions of light adaptation. In the majority of neurons (78%), the spike rate is increased by light stimulation whereas the remainder are light-inhibited. The neuronal response to light is composed of several components distinguished by their temporal dynamics and degree of alteration after previous light exposure. SCN neurons display a sustained response to light followed by persistence of the response after light offset. These responses are sluggish and relatively unaffected by previous light exposures. Neurons also respond with a brisk, excitatory ON response and often an OFF response that is either excitatory or inhibitory. ON-OFF responses are transient and strongly reduced by previous bright white light exposure. Furthermore, two types of neuronal response patterns can be distinguished by the presence or absence of a slow-transient component that follows the transient ON response. The transient ON-OFF components express light adaptation properties characteristic of retinal channels involving cones, whereas the sustained and persistent components are consistent with in vitro response properties reported for melanopsin-expressing ganglion cells.