Entorhinal Cholecystokinin in Alzheimer's Disease: Its Earliest Vulnerability and Rescue Effects Across Different Disease Stages in a Mouse Model.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by cognitive decline. The entorhinal cortex (Ent) is among the earliest affected regions, and its neuropeptide cholecystokinin (CCK) supports neocortical-associated memory. Although our previous work demonstrated that CCK treatment rescues cognition and neuroplasticity in aged AD mice, the correlations among CCK expression, synaptic function, and cognitive decline with aging remain poorly understood. Using 3xTg-AD mice (2-18 months), we performed stereological, histological, and molecular analyses to evaluate brain atrophy, neuronal loss, glial responses, and gene expression. Cognitive function was assessed using the novel object recognition test (NOR), motor learning was evaluated using the rotarod test, and synaptic integrity was measured via electrophysiological recordings. We also investigated the therapeutic potential of CCK-B receptor (CCK-BR) agonists on learning and memory and neuroplasticity across disease stages of AD. In 3xTg-AD mice, the Ent exhibits significant atrophy and excitatory neuronal loss as early as 7 months of age, confirming its role as one of the earliest and most severely affected regions in AD. CCK was downregulated earlier than other synaptic genes in the Ent and other brain regions. Cholecystokinin tetrapeptide (CCK-4) treatment rescued deficits in synaptic plasticity, cognition, and motor learning in 3xTg-AD mice across multiple disease stages. Long-term administration of HT-267, a CCK-BR agonist with a prolonged half-life, in young 3xTg-AD mice delayed cognitive decline, enhanced synaptic scaffolding, and restored long-term potentiation in both the cortex and hippocampus (HPC). Our findings identify CCK downregulation as an early biomarker in AD and demonstrate the therapeutic effects of CCK-BR agonists in mitigating cognitive and synaptic deficits from mild to severe disease stages. The ability of long-term CCK-4 analogue treatment to decelerate AD progression indicates its promise as an early intervention strategy.