The Light‐Induced FOS Response in Melanopsin Expressing HEK‐293 Cells is Correlated with Melanopsin Quantity and Dependent on Light Duration and Irradiance

We established a cell line (HEK‐hMel) expressing melanopsin in a tetracycline dependent manner to elucidate new aspects of melanopsin's light response. Different light stimuli were evaluated using FOS expression as response parameter. Immunoblotting was used to evaluate expression of melanopsin and FOS and qPCR to quantify FOS mRNA responses. The magnitude of the FOS response was found to correlate with the amount of melanopsin expressed by the cells, and a transient FOS mRNA induction followed by FOS protein still elevated after 24 h of illumination was revealed. Exposing the cells to darkness after light resulted in reduction of the response compared to exposure to light solely showing dependency on continuous light. Increasing irradiances of blue light (480 nm) up to 10¹¹ quanta cm^?2 s^?1 elicited steep increases in FOS mRNA, while increases between 10¹² and 5 × 10¹³ quanta  cm^?2 s^?1 resulted in equally high FOS expression. The HEK‐hMel cells were used to characterize facets of melanopsin's light‐induced FOS response not approachable in vivo. Novel information such as dependency of the FOS response on both melanopsin amount and light intensity in addition to a detailed time‐course of both FOS mRNA and protein were revealed.