Figure 10. Schematic of proposed geniculohypothalamic circuitry and its impact on SCN output.

A, based on the data reported in the present study, we propose that GABAergic neurons contributing to the GHT are primarily excited by contralateral retina inputs and convey the resulting inhibitory signals to SCN neurons that receive input from either (or both) eyes. An SCN‐intrinsic mechanism then adjusts SCN responses to retinal and GHT inputs according to time of day. B and C, a schematic illustrating inferred excitatory and inhibitory synaptic influences on SCN neurons (B) and the resulting spike output under varying illumination (C) for SCN cells receiving input from just the contra‐ (upper) and or ipsilateral (lower) eye, respectively. Following experimental illumination of just the contralateral retina (left), GHT input is activated and inhibitory inputs reach the SCN 50–100 ms after arrival of excitatory retinal inputs. For SCN cells that receive contralateral retinal inputs, the resulting long‐lasting inhibitory influence attenuates the excitatory retinal drive, leading to a suppression of steady state light‐driven firing rates (especially during the projected day when SCN neurons are more sensitive to GHT signals). By contrast, in the absence of any driving excitatory input (SCN cells receiving only ipsilateral input), spontaneous firing is unaffected. When only the ipsilateral retina is illuminated, the activity of the GHT is very low and therefore does not influence SCN responses (middle). Under the more commonly encountered situation in the natural world where both eyes are illuminated (right), inhibitory GHT signals provided by the GHT are able to reduce SCN responses regardless of which eye a cell receives input from.