Locus Coeruleus-CA1 Projections Are Involved In Chronic Major Depressive Disorder-induced Hippocampal Vulnerability To Transient Global Ischaemia

Background: Major depressive disorder(MDD)and transient global ischaemia(TGI)represent the most common psychological and neurological diseases,respectively,and are tightly associated.Emerging diseases are increasing,causing a burden for both families and societies.However,studies of ischaemia-aggravated depression have been well studied,evidences of depression-affected ischaemic attacks have been limited to recent epidemiological studies.TGI results in delayed cell death,including hippocampal CA1 pyramidal neurons,and is tightly associated with impaired cognition and memory defects in both humans and animals.The degree of damage is positively correlated with the duration of ischemia.In addition,dopaminergic axons originate from locus coeruleus neurons,as opposed to those of the ventral tegmental area(VTA),to modulate spatial learning and memory.Imaging studies have shown that patients with depression have abnormal activity of the locus coeruleus compared with controls.Therefore,we used the mice model to explore the the potential effects of depression on TGI.Methods: 1.First,mice were subjected to consecutive time-points(sham,10,20,and 30 min)of transient ischaemia(bilateral common carotid artery occlusion)followed by full reperfusion.Behavioural tests such as open field test,grip strength test,morris water maze were then performed.Finally,the mice were sacrificed,and the numbers of pyramidal neurons in the CA1 region were counted.We choose the most suitable time point of TGI based on the experimental results.2.Mice were subjected to consecutive 10-day repeated social defeat stress to establish the mouse model of MDD.Social interaction test,forced swimming test were performed to assess the depression behaviors.Then mice were subjected to TGI-10 min or sham operation.Morris water maze and the immunofluorescence staining of the CA1 were used to evaluate whether depression aggravates TGI-induced hippocampal damage.3.We unilaterally injected a mixture of two viruses into the LC region,which resulted in the expression of EYFP fluorescence in the whole brain.After four weeks of viral injection,mice were subjected to consecutive 10-day repeated social defeat stress.Next,we quantified the EYFP-positive axonal in the group of control,susceptible and resilient throughout the brain.4.We utilized the technique of DREADDs,and bilateral injected the virus to label the TH neurons.After a few weeks of viral injection,mice were again injected CNO to select inhibit or activate the neuronal circuit.The methods were to investigate the role of the Th:LC-CA1 circuit in mediating TGI-induced spatial memory impairment and CA1 neuron loss.Results: 1.Mice subjected to consecutive time-points(sham,10,20,and 30 min)of transient ischaemia showed no difference in the open field test and grip strength test.Compared to the group of Sham and TGI-10 min,TGI-20 min and TGI-30 min displayed spatial memory impairment and CA1 neuronal death.2.After ten days of CSDS,mice could be separated into resilient and susceptible groups,based on social avoidance testing.Susceptible mice showed robust decreases in social avoidance test in the presence of CD1 mice.The forced swimming test results indicated the susceptible mice displayed marked desperate behaviour.In the morris water maze test and the immunofluorescence staining of the CA1,the susceptible/TGI-10 min mice showed spatial memory dysfunction and CA1 neuronal loss.These data demonstrated that CSDS confers TGI susceptibility.3.We utilized a cell type-specific tracing technique and immunofluorescence staining to illuminate the brain-wide mapping of direct output projections of the locus coeruleus.These results indicated that the Th:LC–CA1 circuit is selectively reduced in susceptible mice.4.The group of hM4Di/TGI-10 min showed a markedly worsening learning ability during the six days of the learning test.In the probe trial,hM4Di/ TGI-10 min mice spent less time in the target quadrant than others.Representative images of Neu Nstaining showed numbers of CA1 pyramidal neurons in hM4Di/TGI-10 min mice were markedly decreased compared with other groups.5.The hM3Dq/TGI-10 min exhibited the better performances in all measured indexes,including the escape latency in the learning phase and the latency and swimming length to reach a hidden platform than the group of mCherry /TGI-10 min.In the probe trial,hM3Dq/TGI-10 min mice spent more time in the target quadrant compared with the mCherry /TGI-10 min mice.Neu N staining indicated that a lot of surviving CA1 neurons were prominently increased in hM3Dq/TGI-10 min mice compared with mCherry /TGI-10 min mice.6.DREADDs-activation of the Th:LC-CA1 circuit rescued the TGI-induced spatial memory impairment and CA1 neuronal death in CSDS susceptible mice rather than resilient mice.Conclusions: We utilized animal modeling,virus tracing,the technique of DREADDs,immunofluorescence,behavioral testing technologies find: 1.CSDS aggravated TGI-induced spatial memory loss and CA1 neuronal death.2.The projection strength of the Th:LC-CA1 passway was prominently reduced in mice susceptible to CSDS,as evidenced compared with the control and resilient group the susceptible mice showed fewer axons in the CA1 region.3.The Th:LC-CA1 circuit involved in mediating TGI-induced spatial memory impairment and CA1 neuron loss.