Astrocytic α2A-adrenergic signalling in the hypothalamic paraventricular nucleus contributes to dexmedetomidine-induced sedation in mice.

Abstract

Dexmedetomidine (Dex) is a selective α2-adrenergic receptor (α2-AR) agonist widely used for sedation, but its underlying mechanisms remain incompletely understood. We aimed to identify novel brain regions and cellular pathways mediating Dex-induced sedation in mice. Brain-wide α2A-ARs expression was mapped in adult male C57BL/6 mice using RNAscope in situ hybridisation with immunofluorescence. Functional expression was confirmed by intracellular Ca²⁺ imaging in astrocytes. Sedation was assessed by rotarod, open-field tests, and electroencephalography (EEG). Astrocyte-specific gene knockdown, chemogenetics, and electrophysiology were used to explore underlying mechanisms. Expression of α2A-ARs was enriched in hypothalamic paraventricular nucleus (PVH) astrocytes, where Dex triggered α2A-AR-dependent Ca²⁺ elevations. Knockdown of astrocyte α2A-ARs reduced Dex-induced sedation, evidenced by increased rotarod fall latency (mean [standard deviation] 228 [52] vs 166 [62] s, P=0.026), increased open-field distance (3640 [950] vs 2680 [850] cm, P=0.029) and decreased EEG delta power (54.2 [11.0] vs 62.0 [5.9]%, P=0.024). Inhibition of astrocyte Ca²⁺ signalling produced similar effects. Mechanistically, Dex enhanced tonic gamma-aminobutyric acid (GABA) currents in local Vglut2⁺ neurones (15.5 [9.6] vs 7.8 [6.6] pA, P=0.035) and suppressed neuronal firing (0.5 [0.5] vs 1.8 [1.3] Hz, P<0.001); both effects were attenuated by inhibition of astrocyte Ca²⁺ levels. Furthermore, knockdown of the Ca²⁺-activated anion channel bestrophin-1 (BEST1) in PVH astrocytes reduced tonic GABA currents (3.5 [3.4] vs 7.5 [7.2] pA, P=0.024) and diminished sedation. An astrocytic α2A-AR-Ca²⁺-BEST1 pathway in hypothalamic paraventricular nucleus neurones mediates Dex-induced sedation, which identifies a critical role for astrocytes in pharmacologically induced sedation.