We show for the first time that the neuropeptide orexin modulates pupillary light response (PLR), a non-image forming visual function, in mice of either sex. Intravitreal injection of the orexin receptor (OXR) antagonist TCS1102 and orexin-A reduced and enhanced pupillary constriction in response to light, respectively. Orexin-A activated OX1Rs on M2-type intrinsically photosensitive retinal ganglion cells (ipRGCs) (M2 cells), and caused membrane depolarization of these cells by modulating inward rectifier potassium channels and non-selective cation channels, thus resulting in an increase in intrinsic excitability. The increased intrinsic excitability could account for the orexin-A-evoked increase in spontaneous discharges and light-induced spiking rates of M2 cells, leading to an intensification of pupillary constriction. Orexin-A did not alter the light response of M1 cells, which could be due to no or weak expression of OX1Rs on them, as revealed by RNAscope in situ hybridization. In sum, orexin-A is li...