Circadian photoentrainment mechanism in mammals

Abstract

Proper alignment of an organism's behavior, physiology, and metabolism to the daily cyclical changes in light and dark is essential for health and wellbeing. This process is dependent upon a small subset of intrinsically photosensitive retinal ganglion cells (ipRGCs) that express the photopigment, melanopsin. The signaling pathways involved in melanopsin phototransduction are distinct from those used in rod and cone opsins and are more reminiscent of invertebrate opsins. Distributed throughout the retina, ipRGCs send monosynaptic projections from the retina to many regions of the brain to facilitate non-image-forming visual processes, but their glutamatergic inputs to the hypothalamic suprachiasmatic nucleus (SCN) are essential for entraining circadian rhythms in behavior and physiology to environmental lighting conditions. While nearly all cells in the mammalian body have cell-autonomous molecular clocks, the remarkable intercellular coupling among SCN neurons allows for the SCN to act as the master circadian oscillator and transduces light information into rhythmic electrical and chemical signals that facilitate photoentrainment. This chapter delves into the role of ipRGCs in circadian photoentrainment, the melanopsin signaling pathway, the interaction between ipRGCs and the heterogeneous SCN neurons, and the network properties and signaling pathways that underlie SCN function.