A Circadian Rhythm-gated Subcortical Pathway For Nighttime-light-induced Depressive-like Behaviors In Mice

Vision is the most indispensable sense of human perception,and it accounts for at least 80%of the acquisition of external information.For mammals,the eyes are the only organ that receives external light signals.Among the eyes,the rods and cones on the retina mediate image-forming vision and enable individuals to recognize the color and movement of objects;A third type of photoreceptor-intrinsically photosensitive Retinal Ganglion Cells(ipRGC)mediate non-image forming vision,such as circadian photoentrainment,pupil light reflection,sleep-wake cycle,body temperature,mood,etc.With the development of industry and the progress of society,the heavy use of electronic equipment makes people inevitably exposed to artificial light sources for longer periods of time at night.Epidemiological data shows that light input at night increases the risk of depression.In fact,people have long been aware of the role of light in regulating emotions and cognition,but previous studies have focused on light conditions changing the rhythm of the biological clock and sleep patterns,which further affects emotions and cognitive functions.However,evidence from human and animal studies suggest that in addition to the indirect regulation of "light-emotion" involved in the rhythms,there are also direct "light-emotion" neural circuits,and ipRGC is likely to be involved.Unfortunately,there is still a lack of in-depth and comprehensive understanding of the organization and neurophysiological characteristics of such direct pathway,and its relationship with the regulation of the biological clock.In order to address this question,we establish a light at night(LAN)model to simulate the abnormal light pattern in the current society.Two hours of blue light(480nm)exposure from 9 to 11 p.m.every night for three weeks caused the mice to exhibit depressive-like behavior without disrupting circadian rhythm or sleep pattern.It is determined that ipRGC plays a sufficient and necessary role in the influence of LAN on depressive mood through the behavior testing of transgenic animals which lack rods,cones or ipRGCs.In order to identify the downstream area projections of ipRGC achieved the above performance,we used the excitotoxicity of NMDA to destroy the peripheral habenular nucleus(pHb),and the LAN-induced depressive-like phenotype disappear.At the same time,we used optogenetics to activate the ipRGC-pHb projections,which can also induce depressive-like performance in mice.It has been verified from both positive and negative that pHb plays an essential role in the circuit that mediates the LAN.The tracking results of the combination of the anterograde trans-synaptic virus AAV1-Cre and the retrograde labeling tool CTB suggest that dorsal pHb(dpHb)projects to the nucleus accumbens core(NAc),and the ventral pHb(vpH b)projects to medial prefrontal cortex(mPFC).In order to explore which of these two downstream nucleus plays a key role,we used the AAV virus to inhibit the input of pHb to mPFC and NAc,respectively.Under LAN conditions,only operations that occur in NAc could affect the emotional impact of LAN.The above evidence identifies the neural circuits underlying the influences of LAN on the brain.On the other hand,we conducted a long-term experiment with chronic chemogenetic activation of the dpHb-NAc projection during the period 20:00 to 23:00 for three consecutive weeks to simulate the light stimulation of LAN to mice,depressive-like performance can also be induced under normal light conditions.It proves that the circuit is necessary and sufficient in the LAN.Finally,we want to further explore why blue light at night affects the mood of mice.By single cell electrophysiological recording,we found that NAc-projecting dpHb neuron were capable of sustaining spiking activities at night than during the day when injected with the depolarizing current,the cells are more likely to be activated,but not mPFC-projecting vpHb neurons.In order to verify whether endogenous excitability also exists in vivo,we hence performed in vivo fiber photometry experiments monitored calcium signal and found that under the environment light stimulation,the calcium signal of dpHb neurons responded more strongly at night,indicating that these neurons may be regulated by circadian rhythms.In summary,through animal models,electrophysiology,optogenetic and chemogenetic methods,we have elucidated a new circuit,the pHb inhibitory neurons projected from the ipRGC of the retina then innervate the nucleus accumbens.This circuit mediates negative emotions induced by light at night and which is regulated by circadian rhythms.These findings can indicate to a certain extent the mechanism of negative emotions such as depression caused by light at night(urban lighting or the use of electronic devices such as mobile phones and computers).It is of great significance for people to correctly understand the potential hazards of excessive lighting at night and explore prevention and treatment methods.