Dorsal raphe nucleus–hippocampus serotonergic circuit underlies the depressive and cognitive impairments in 5×FAD male mice

Abstract Background Depressive symptoms often occur in patients with Alzheimer's disease (AD) and exacerbate the pathogenesis of AD. However, the neural circuit mechanisms underlying the AD-associated depression remain unclear. The serotonergic system plays crucial roles in both AD and depression. Methods We used a combination of in vivo trans-synaptic circuit-dissecting anatomical approaches, chemogenetic manipulations, optogenetic manipulations, pharmacological methods, behavioral testing, and electrophysiological recording to investigate dorsal raphe nucleus serotonergic circuit in AD-associated depression in AD mouse model. Results We found that the activity of dorsal raphe nucleus serotonin neurons (DRN 5-HT ) and their projections to the dorsal hippocampal CA1 (dCA1) terminals (DRN 5-HT -dCA1 CaMKII ) both decreased in brains of early 5×FAD mice. Chemogenetic or optogenetic activation of the DRN 5-HT -dCA1 CaMKII neural circuit attenuated the depressive symptoms and cognitive impairments in 5×FAD mice through serotonin receptor 1B (5-HT 1B R) and 4 (5-HT 4 R). Pharmacological activation of 5-HT 1B R or 5-HT 4 R attenuated the depressive symptoms and cognitive impairments in 5×FAD mice by regulating the DRN 5-HT -dCA1 CaMKII neural circuit to improve synaptic plasticity. Conclusions These findings provide a new mechanistic connection between depression and AD and provide potential pharmaceutical prevention targets for AD.