Inducing Daytime Circadian Phase Shifts Using Chemogenetic and Spectral Approaches.

In mammals, circadian phase shifting during the daytime is limited by reduced photic responsiveness of the suprachiasmatic nucleus (SCN), restricting the ability to experimentally manipulate the circadian clock during this phase. Reliable methods to induce daytime phase shifts are therefore essential for investigating mechanisms of circadian plasticity and photic entrainment. Complementary genetic and spectral strategies are described to enable robust, temporally precise manipulation of the circadian clock during the day. The first approach, currently limited to mice, employs a chemogenetic strategy involving intravitreal delivery of Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to selectively activate intrinsically photosensitive retinal ganglion cells (ipRGCs). This approach permits controlled activation of the retinohypothalamic pathway and induces reproducible daytime phase shifts independent of ambient lighting conditions. The second approach utilizes wavelength-specific optical stimulation as a non-invasive alternative. Exposure to violet light exploits the spectral sensitivity of ipRGCs to reduce depolarization block and promote sustained activation, enabling reliable phase resetting during the subjective day. This method is broadly applicable across mammalian systems and does not require genetic manipulation or pharmacological intervention. Detailed protocols are provided for experimental preparation, stimulation timing, validation of phase shifts using locomotor activity, and assessment of neuronal activation via c-Fos immunohistochemistry. Key considerations, including circadian timing, stimulus parameters, and experimental controls, are outlined to facilitate reproducibility. Together, these approaches provide versatile and experimentally tractable tools for inducing daytime circadian phase shifts and enable direct investigation of mechanisms underlying daytime circadian responsiveness.