The proposed research is a logical expansion of Aims 2 and 3 of the parent grant, R01AA027768- 02, which determines whether "excessive ethanol intake compromises thalamic regulation of striatal cholinergic interneurons (CINs) and CIN-mediated regulation of direct-pathway medium spiny neurons (dMSNs), leading to inflexible behaviors." This expansion includes how the alcohol-induced reduction of cholinergic activity contributes to the onset and progression of cognitive deficit in Alzheimer's disease and related dementias (ADRD), which further improve our understanding of the pathogenesis of alcohol use disorder superimposed on ADRD. Therefore, this research is relevant to ADRD. Alcohol use disorder is characterized by inflexible compulsive drinking, despite negative consequences. Similar cognitive inflexibility is a hallmark of Alzheimer's disease. Deficits in cognitive flexibility in both diseases are likely mediated by reduced cholinergic activity. The striatum contains dMSNs and CINs. dMSN activity is enhanced by excessive alcohol intake. Preliminary studies found that dMSNs sent GABAergic projections to inhibit striatal CINs and cholinergic projection neurons (CPNs) in the basal forebrain. Striatal CINs also receive glutamatergic inputs from the thalamus. Studies from the parent grant found that excessive alcohol intake reduces thalamic inputs to CINs and CIN activity, impairing flexibility in reversal learning. It is not known how alcohol regulates cholinergic activity in the context of Alzheimer's disease. This research explores how excessive alcohol intake reduces CIN and CPN activity in the 5xFAD mouse model of Alzheimer's disease. The hypothesis is that excessive alcohol intake exacerbates the onset and extent of enhanced inhibitory dMSN→CIN (or CPN) transmission and reduces glutamatergic thalamic inputs to CINs, leading to hypocholinergic activity in the 5xFAD mouse model of Alzheimer's disease. We will test this hypothesis via the following two specific aims: (1) Test whether excessive ethanol intake enhances dMSN→CIN or dMSN→CPN transmission to a greater extent in 5xFAD mice than in wild-type controls; (2) Determine whether excessive ethanol intake reduces thalamostriatal transmission in DMS CINs to a greater extent in 5xFAD mice than in wild-type controls. This research is conceptually innovative because it focuses on the relatively neglected area of the striatum-mediated hypocholinergic mechanisms in both disorders. It is technically innovative in its combined use of triple transgenic mice and optogenetic stimulation to selectively measure three neural circuits and determine their contributions to hypocholinergic activity in both diseases. Knowledge generated from this proposal will provide novel strategies for improving cholinergic activity and cognitive flexibility, thereby informing the treatment of alcohol use disorder and Alzheimer's disease.