Effect of ketamine on early synaptic mistuning and emotional behavior deficits in App^NL-Fmouse model of Alzheimer’s disease

Alzheimers disease is the most common neurodegenerative disease and constitute 75% of dementia cases worldwide. Unfortunately, efficient and affordable treatments are still lacking for this mental illness, it is therefore urgent to identify new pharmacological targets. Whereas the late phases of the disease are well described, recent evidence suggest synaptic impairments at a pre-amyloid β; (Aβ) plaque stage. Astrocytes are playing a crucial role in the tuning of synaptic transmission and several studies have pointed out severe astrocyte reactivity in Alzheimers disease, especially around Aβ; plaques. Reactive astrocytes show altered physiology and function, suggesting they could have a role in the early pathophysiology of Alzheimers disease. In this study we used the AppNL-F knock-in mouse model of Alzheimers disease which carries two disease-causing mutations inserted in the amyloid precursor protein (App) gene. This strain does not start to develop Aβ; plaques until nine months of age. To better understand early changes in Alzheimer's disease, we investigated synaptic function, at both neuronal and astrocytic levels, in six months old AppNL-F mice and correlate the synaptic dysfunction with emotional behavior. Electrophysiological recordings in the hippocampus revealed an overall synaptic mistuning at a pre-plaque stage of the pathology, associated to an intact social memory but a stronger depressive-like behavior. Astrocytes displayed a reactive-like morphology and a higher tonic GABA current compared to control mice. Interestingly, we here show that the synaptic impairments in hippocampal slices are partially corrected by a pre-treatment with the monoamine oxidase B (MAO-B) blocker deprenyl or the fast-acting antidepressant ketamine (5mg/kg). Thus, we propose that reactive astrocytes can induce synaptic mistuning early in Alzheimers disease, before plaques deposition, and that these changes are associated with emotional symptoms.