Circadian rhythms of neuronal and astrocytic glutamate transporters in the suprachiasmatic nucleus of mice.

Retinal ganglion cells release glutamate at their synapses, transmitting visual signals via the optic nerve to brain regions involved in vision and light-dependent functions. Glutamatergic input to the circadian pacemaker, the suprachiasmatic nucleus (SCN), is essential for photoentrainment of circadian rhythms. Glutamate transporters are crucial for efficient transmission in the tripartite synapse formed by presynaptic terminals, postsynaptic neurons and astrocytes. However, it remains unclear whether this glutamatergic synapse is modulated by rhythmic environmental lighting conditions. In this study, neuronal (VGLUT1 and VGLUT2) and astrocytic (GLAST) glutamate transporters of 8-12 weeks old male C57Bl/6 J mice were analyzed in the SCN of mice exposed to different lighting paradigms: a standard 12:12-h light-dark cycle (LD), constant darkness (DD) and constant light (LL), using immunofluorescence and confocal laser microscopy. Under LD, all three transporters exhibited time-of-day-dependent rhythms: VGLUT1 and GLAST peaked during the light phase, while VGLUT2 peaked during the dark phase. This suggests that VGLUT1 and GLAST are more strongly coupled and that VGLUT2 is a complementary system, so that the strength or efficiency of the light information can be modulated depending on the time of day. Furthermore, the rhythms of neuronal glutamate transporters persisted under DD and LL, indicating robust circadian control. Our study thus supports the hypothesis that the circadian system may regulate its own input.