The Circuit And Mechanism Of VGLUT3 Neurons In Median Raphe Nucleus Involved In Spatial Memory Retrieval

Background:Human brain contains about 100 billion neurons.Understanding how these neurons are organized to perform the behavioral function is one of the most important research topics in the field of brain sciences.Median raphe nucleus（MnR）is a nucleus located in the midbrain.To date,the most studies of the MnR have focused on 5-hydroxytryptamine（5-HT）neurons and reported their involvements in various functions such as feeding.Recently,brain single cell RNA sequencing has shown that a fraction of neurons in the MnR express Vesicular glutamate transporter 3（VGLUT3）,thereafter named as VGLUT3 neurons.The vesicular glutamate transporters play a key role in maintaining normal neuronal function.However,the circuit mechanisms underlying the function of VGLUT3 neurons in the MnR are yet to be studied.Alzheimer’s Disease（AD）is one of the major neurodegenerative disorders.The early symptoms of AD are progressive and irreversible memory loss.But,whether VGLUT3neurons are involved in the disease progression of AD has remained unknown.Aim:To determine the circuit mechanisms underlying the function of VGLUT3 neurons.To explore the role of VGLUT3 neurons in the disease progression of AD.Methods:Development of VGLUT3-CRE mice,in which CRE was expressed under control of VGLUT3 promoter.Generation of a series of the CRE-recombination dependent recombinant adeno-associated virus vectors to selectively label and manipulate VGLUT3neurons in adult mice in vivo.Using genetically-modified monosynaptic tracing strategies to determine synaptic targets of VGLUT3 and identify the circuit mechanisms underlying the behavioral function of VGLUT3 neurons.Combination of AD model mouse with optogenetic and electrophysiological studies to determine roles of VGLUT3 neurons in the disease progression of AD.Results:We identified that VGLUT3 neurons in the MnR are excitatory glutamatergic neurons.Our data revealed that VGLUT3 neurons synapsed with parvalbumin-expressingγ-amino-butyric acid inhibitory neurons in the dentate gyrus（PGIS）.The synaptic transmission between VGLUT3 and PGIs underwent activity-dependent long term potentiation（LTP）that acts as a cellular substrate for spatial memory retrieval via activation of Ca²⁺-permesble Gria2-lacking AMPA receptor(Ca²⁺AR).In AD mice,the synaptic transmission between VGLUT3 and PGIs was impaired and this impairment contribute to the spatial memory loss.Activation of the synaptic transmission between VGLUT3 and PGIs effectively rescued the spatial memory deficits in the early stage of AD.Conclusions:VGLUT3 neurons in the MnR synapse with PGIs.LTP of VGLUT3→PGIS encodes spatial memory via activation of Ca²⁺AR.Degeneration of VGLUT3→PGIS synaptic transmission contributes to the spatial memory deficits in the early stage of AD.