Section on Light and Circadian Rhythms

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Research projects, Fiscal Year 2018, can be divided into four major areas listed below: 1- The Development and the role of the SCN in regulating several output functions The suprachiasmatic nucleus (SCN) is believed to influence circadian rhythms primarily by direct efferent connections with downstream brain regions. The SCN produces several neuropeptides including vasoactive intestinal peptide (VIP), gastrin releasing peptide (Grp), prokineticin-2 (Prok2), and arginine-vasopressin (Avp). These neuropeptides are thought to synchronize cellular oscillators within the SCN. However, whether they are capable of entraining downstream oscillators is unclear. A major obstacle to the study of SCN neuropeptide functions in controlling downstream oscillators has been the availability of animal models that still have SCN synchrony but lack neuropeptide expression. In collaboration with Seth Blackshaw's lab, we have generated a mouse model that lacks the expression of major SCN neuropeptides but maintains synchronous cellular oscillations in the SCN. This allows us, for the first time, to specifically address the necessity and sufficiency of SCN neuropeptides in regulating circadian behaviors. In collaboration with Seth Blackshaw (Johns Hopkins School of Medicine), we conditionally deleted a LIM homeodomain transcription factor, Lhx1, specifically in the SCN (Six3-Cre;Lhx1lox/lox henceforth called Lhx1 mutant mice). We found that Lhx1-mutants exhibit a dramatic downregulation in the levels of the major SCN neuropeptides, Vip, Grp, Avp and Prok2. Notably, the maturation of SCN, the core SCN clock-work and clock-controlled gene expression are still preserved (Bedont et al., Cell Reports 2014). Lhx1-mutant mice exhibit modest deficits in the circadian output behavior of wheel running activity, but importantly, we observed profound deficits in sleep regulation by light (Bedont et al., Current Biology 2017). We will determine the necessity of the SCN neuropeptides in controlling sleep in future publications. 2- Diversity and function of ipRGCs We have generated genetically modified mouse lines to uncover the contribution of ipRGC subtypes and the corresponding brain circuits to the synchronization of the internal biological clock to the solar day. We have animals that either harbor only the SCN-projecting ipRGCs (Chen et al., Nature 2011) or lack the ipRGCs that project to the SCN (unpublished). This will allow us to determine the contribution of individual subtypes of ipRGCs to circadian photoentrainment and phase shifts. The phase of the circadian oscillator can be advanced or delayed by acute pulses of light, known as phase shifts. Our exciting preliminary data reveal that different populations of ipRGCs control phase delays versus phase advances in the circadian oscillator. This finding challenges the current view in the field that similar mechanisms underlie phase advances and phase delays, and that light has a simple ON/OFF effect on the clock. Future studies will determine which ipRGC populations are necessary for advances versus delays, and map the brain regions that are influenced by light to cause changes in the phase of the circadian oscillator. We recently made a startling discovery that a subpopulation of ipRGCs (200 M1-Brn3b-negative, which we called circadian photoreceptors) is critical for the development of the circadian clock as well as vision, although they do not project to visual centers (Chew et al., eLife 2017). An exciting hypothesis is that these 200 ipRGCs (Chen et al., Nature 2011) represent an evolutionary ancient photoreceptor class given their broad influence on several distinct behaviors (photoentrainment, development of the clock and vision as well as local pupillary light reflex). Therefore, it is critical to understand the molecular and functional specification of this population in relation to other ipRGCs and conventional ganglion cells. Thus, we have started to examine the transcriptome and epigenetic marks of this population using our resources at NIMH. This project will provide the molecular handles to understand the ontogeny and the functional specialization of the 200 M1 ipRGCs in relation to other ipRGCs and conventional ganglion cells. 3- Uncovering the retinal and brain circuits that allow light to influence mood and learning and memory It is well established that light therapy can be used to treat several types of major depression in humans. However, it has been hard to ascertain whether these effects of light are purely placebo effects. We recently discovered a new brain region that allows light to directly regulate mood in rodents. This new region is termed the perihabenular complex and it connects to several area in the brain essential for mood regulation such as the medial prefrontal cortex and the nucleus accumbens. 4- Uncovering new brain areas that regulate sleep In an exciting collaboration with Seth Blackshaw's lab, we found an undefined population, Lhx6-neuronal population of GABA neurons in the zona incerta regulate sleep (Liu, Nature 2017). Together, we will continue to break new ground about how light signaling from the environment regulates several functions that are essential for the well-being of humans.