Descending locus coeruleus noradrenergic signaling to spinal astrocyte subset is required for stress-induced mechanical pain hypersensitivity.

Abstract

It is known that stress powerfully alters pain, but its underlying mechanisms remain elusive. Here, we identified a circuit, locus coeruleus descending noradrenergic neurons projecting to the spinal dorsal horn (LC→SDH-NA neurons), that is activated by acute exposure to restraint stress and is required for stress-induced mechanical pain hypersensitivity in mice. Interestingly, the primary target of spinal NA released from descending LC→SDH-NAergic terminals causing the stress-induced pain hypersensitivity was α1A-adrenaline receptors (α1ARs) in Hes5-positive (Hes5+) astrocytes located in the SDH, an astrocyte subset that has an ability to induce pain sensitization. Furthermore, activation of Hes5+ astrocytes reduced activity of SDH-inhibitory neurons (SDH-INs) that have an inhibitory role in pain processing. This astrocytic reduction of IN activity was canceled by an A1-adenosine receptor (A1R)-knockdown in SDH-INs, and the A1R-knockdown suppressed pain hypersensitivity caused by acute restraint stress. Therefore, our findings suggest that LC→SDH-NA neuronal signaling to Hes5+ SDH astrocytes and subsequent astrocytic reduction of SDH-IN activity are essential for mechanical pain facilitation caused by stress.