Pathophysiological Study of Dopamine in Alzheimer's Disease and Related Demantia

National Institute On Aging

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Abstract

To determine the role of midbrain DAN afferents in episodic memory formation, we began by expressing inhibitory (Gi) or facilitatory (Gq) chemogenetic modulator DREADD in DATIRESCre-positive afferents to LEC to modulate afferent activity during behavior. Although we suspect low retrograde transfection by the CAV2 vectors used for this study, we were nonetheless able to detect changes in tests designed to reveal LEC function. We administered DREADD ligands to disrupt LEC DA afferents during the learning phase of an episodic-like memory for novel object (ELMNO) paradigm designed to demonstrate recency and novelty discrimination. Although no mice (including controls) successfully discriminated objects based on recency or novelty, both Gi and Gq mice displayed increased exploration overall compared to controls, suggesting a general memory impairment. To test temporal interval recall, we used a Peak Responding operant protocol and found that impairing dopamine afferents to LEC slowed acquisition of a time-based task but was not critical for task recall. Lastly, we tested associative memory using Trace Fear Conditioning and found that Gq mice showed increased freezing and Gi mice showed decreased freezing relative to controls, supporting a positive correlation between midbrain afferents to LEC and the strength of associative learning. To identify modulatory factors and timing of DAN afferent activity in LEC, we will use fiber photometry to record neuronal activity in these afferents within LEC during behavior. Guided by our preliminary data, we will run modified ELMNO and Trace-fear tasks. We will add a foraging-based episodic memory task on a Radial Arm Maze, and a Serial Odor Discrimination task shown to engage the LEC. We will replace Peak Responding with a simplified cue-delay-reward Pavlovian task to capture temporal encoding. Next, we will parse contributions of the midbrain afferents transmitters dopamine and glutamate. To single out the effect of dopamine itself within LEC, we generated a cohort of dopamine D1-type receptor (Drd1) cKO mice, in which Drd1 is excised in the presence of Cre. We will inject Cre-carrying virus locally to LEC in these mice before testing their episodic memory performance using behaviors described above. To examine the effect of glutamate from VTA afferents to LEC, we will perform analogous experiments in a cohort of glutamate transporter VGlut2 cKO mice. The above experiments will discern PDD-relevant behaviors which rely on midbrain afferents to LEC, when and how these afferents are modulated, and whether dopamine and/or glutamate are critically involved. Moving forward, increasingly precise, genetically defined lesions and imaging of neuronal subpopulations within SNc and VTA can be used to further pinpoint the exact midbrain DAN populations responsible.