Cerebral blood flow (CBF) is tightly coupled to neuronal activity. Excitatory glutamatergic activity induces local vessel dilation, while monoamines such as noradrenaline and serotonin (5-HT) stemming from brainstem neurons induce vessel constriction to maintain vascular tone. Despite its well-described role as a vasoconstrictor, the exact mechanism through which 5-HT affects blood flow remains largely understudied. Here, we show that serotonergic nerve terminals are in direct contact with capillary pericytes and astrocytic endfeet. Applying 5-HT to brain slices and in vivo induces potent pericyte constriction by activating the 5HT_2C receptor, which is expressed in neurons and astrocytes. Exogenous 5-HT application induces a significant increase in the frequency, duration and amplitude of calcium transients in astrocytes and pericytes. Importantly, blocking production of 20-HETE, a potent arachidonic acid derivative that is released by astrocytes to constrict vessels, abolished the 5-HT-mediated pericyte constriction in brain slices, suggesting that 5-HT modulates pericyte contraction through astrocytes. Altogether, these data suggest that in homeostasis, 5-HT exerts its vasoactive effects on CBF by regulating astrocyte-to-pericyte communication.

In the context of Alzheimer’s disease (AD), the association between serotonergic axons, pericytes and astrocytes is altered, owing to the development of markedly enlarged neuronal processes, termed axonal spheroids (AxS), that are found near Aβ plaques. AxS have been primarily studied in excitatory neurons and their development in monoaminergic and cholinergic axons remains largely unexplored. Here, we show that monoaminergic and cholinergic AxS have striking anatomical differences to glutamatergic AxS, with far greater axon swelling and distinct molecular constituents. We classify amyloid plaques based on the type of affected axons in their vicinity and explore its association with plaque compaction. Relevantly, using in vivo two-photon microscopy in anaesthetized animals expressing GRAB_5-HT sensors, we show that some cortical plaques are linked to dynamic fluctuations in local 5-HT levels, possibly reflecting altered neurotransmitter release or diffusion through AxS. These findings suggest that the peri-plaque environment could be shaped by the affected axon type and raise the possibility that acetylcholinesterase inhibitors may exert their therapeutic effect by restoring peri-plaque neurotransmitter balance. We are currently investigating how the presence of cholinergic and monoaminergic AxS might influence local inflammatory responses and blood flow.