Sensitization of spinal itch transmission neurons in a mouse model of chronic itch requires an astrocytic factor

Chronic itch is a highly debilitating symptom among patients with inflammatory skin diseases. Recent studies have revealed that gastrin-releasing peptide (GRP) and its receptor (gastrin-releasing peptide receptor [GRPR]) in the spinal dorsal horn (SDH) play a central role in itch transmission.

We aimed to investigate whether GRP-GRPR signaling is altered in SDH neurons in a mouse model of chronic itch and to determine the potential mechanisms underlying these alterations.

Patch-clamp recordings from enhanced green fluorescent protein (EGFP)-expressing (GRPR⁺) SDH neurons were used to examine GRP-GRPR signaling in spinal cord slices obtained from Grpr-EGFP mice. Immunohistochemical, genetic (gene expression and editing through adeno-associated virus vectors), and behavioral approaches were also used for in vivo experiments.

We observed potentiation of GRP-evoked excitation in the GRPR⁺ SDH neurons of mice with contact dermatitis, without concomitant changes in GRPR expression. Interestingly, increases in excitation were attenuated by suppressing the reactive state of SDH astrocytes, which are known to be reactive in patients with chronic itch conditions. Furthermore, CRISPR-Cas9-mediated astrocyte-selective in vivo editing of a gene encoding lipocalin-2 (LCN2), an astrocytic factor implicated in chronic itch, suppressed increases in GRP-induced excitation of GRPR⁺ neurons, repetitive scratching, and skin damage in mice with contact dermatitis. Moreover, LCN2 potentiated GRP-induced excitation of GRPR⁺ neurons in normal mice.

Our findings indicate that, under chronic itch conditions, the GRP-induced excitability of GRPR⁺ SDH neurons is enhanced through a non-cell-autonomous mechanism involving LCN2 derived from reactive astrocytes.