We investigated roles for spinal neurons expressing the neurokinin-1 receptor (NK1R) and/or gastrin releasing peptide receptor (GRPR) in a mouse model of ovalbumin (OVA)-induced chronic atopic dermatitis (AD). Mice receiving repeated topical application of OVA exhibited atopic-like skin lesions and behavioral signs of chronic itch including spontaneous scratching, touch-evoked scratching (alloknesis), and enhancement of chloroquine-evoked scratching (hyperknesis). Substance P-saporin (SP-SAP) and bombesin-saporin (BB-SAP) were intrathecally injected into OVA-sensitized mice to neurotoxically ablate NK1R- or GRPR-expressing spinal neurons, respectively. SP-SAP diminished the expression of NK1R in the superficial spinal dorsal horn, and significantly attenuated all behavioral signs of chronic itch. BB-SAP reduced the spinal dorsal horn expression of GRPR and significantly attenuated hyperknesis, with no effect on spontaneous scratching or alloknesis. To investigate whether NK1R-expressing spinal neurons project in ascending somatosensory pathways, we performed a double-label study. The retrograde tracer, Fluorogold (FG), was injected into either the somatosensory thalamus or lateral parabrachial nucleus. In the upper cervical (C1-2) spinal cord, most neurons retrogradely labeled with FG were located in the dorsomedial aspect of the superficial dorsal horn. Of FG-labeled spinal neurons, 89-94% were double-labeled for NK1R. These results indicate that NK1R-expressing spinal neurons play a major role in the expression of symptoms of chronic itch, and give rise to ascending somatosensory projections. GRPR-expressing spinal neurons contribute to hyperknesis but not alloknesis or ongoing itch. NK1R-expressing spinal neurons represent a potential target to treat chronic itch.

Itch is relayed to higher centers by projection neurons in the spinal and medullary dorsal horn. We employed a double-label method to map the ascending projections of pruriceptive and nociceptive trigeminal and spinal neurons. The retrograde tracer fluorogold (FG) was stereotaxically injected into the right thalamus or lateral parabrachial area (LPb) in mice. Seven days later, mice received intradermal (id) microinjection of histamine, chloroquine, capsaicin, or vehicle into the left cheek. Id histamine, chloroquine and capsaicin elicited similar distributions of Fos-positive neurons in the medial aspect of the superficial medullary and spinal dorsal horn from the trigeminal subnucleus caudalis to C2. Of neurons retrogradely labeled from the thalamus, 43, 8 and 22% were Fos-positive following id histamine, chloroquine or capsaicin. Of the Fos-positive neurons following pruritic or capsaicin stimuli, ∼1–2% were retrogradely labeled with FG. Trigeminoparabrachial projection neurons exhibited a higher incidence of double-labeling in the superficial dorsal horn. Of the neurons retrogradely labeled from LPb, 36, 29 and 33% were Fos-positive following id injection of histamine, chloroquine or capsaicin, respectively. Of Fos-positive neurons elicited by id histamine, chloroquine and capsaicin, respectively, 3.7, 4.3 and 4.1% were retrogradely labeled from LPb. The present results indicate that, overall, relatively small subpopulations of pruriceptive and/or nociceptive neurons innervating the cheek project to thalamus or LPb. These results imply that the vast majority of pruritogen- and algogen-responsive spinal neurons are likely to function as interneurons relaying information to projection neurons and/or participating in segmental nocifensive circuits.

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