Adenosine 2A Receptors Link Astrocytic Alpha-1 Adrenergic Signaling to Wake-Promoting Dopamine Neurons.

Background: Sleep and arousal disorders are common, but the underlying physiology of wakefulness is not fully understood. The locus coeruleus promotes arousal via alpha-1 adrenergic receptor (α₁AR) driven recruitment of wake-promoting dopamine (DA) neurons in the ventral periaqueductal gray (vPAG^DA neurons). α₁AR expression is enriched on vPAG astrocytes, and chemogenetic activation of astrocytic G_q signaling promotes wakefulness. Astrocytes can release extracellular "gliotransmitters," such as ATP and adenosine, but the mechanism underlying how vPAG astrocytic α₁ARs influence sleep/wake behavior and vPAG^DA neuron physiology is unknown.

Methods: In this study, we utilized genetic manipulations with ex vivo calcium imaging in vPAG^DA neurons and astrocytes, patch-clamp electrophysiology, and behavioral experiments in mice to probe our hypothesis that astrocytic α₁ARs mediate noradrenergic modulation of wake-promoting vPAG^DA neurons via adenosine signaling.

Results: Activation of α₁ARs with phenylephrine increased calcium transients in vPAG^DA neurons and vPAG astrocytes, and increased vPAG^DA neuron excitability ex vivo. Chemogenetic Gq-DREADD activation of vPAG astrocytes similarly increased vPAG^DA neuron calcium activity and intrinsic excitability. Conversely, shRNA knockdown of vPAG astrocytic α₁ARs reduced the excitatory effect of phenylephrine on vPAG^DA neurons and blunted arousal during the wake phase. Pharmacological blockade of adenosine 2A (A_2A) receptors precludes the α₁AR-induced increase in vPAG^DA calcium activity and excitability in brain slices, as well as the wake-promoting effects of vPAG α₁AR activation in vivo.

Conclusions: We have identified a crucial role for vPAG astrocytic α₁AR receptors in sustaining arousal through heightened excitability and activity of vPAG^DA neurons mediated by local A_2A receptors.