Excitation of mouse superficial dorsal horn neurons by histamine and/or PAR-2 agonist: potential role in itch

Abstract

Recent studies have suggested the existence of separate transduction mechanisms and sensory pathways for histamine and nonhistaminergic types of itch. We studied whether histamine and an agonist of the protease-activated receptor (PAR)-2, associated with nonhistaminergic itch, excite murine dorsal horn neurons. Single units were recorded in superficial lumbar dorsal horn of adult ICR mice anesthetized with pentobarbital. Unit activity was searched using a small intradermal hindpaw injection of histamine or the PAR-2 agonist SLIGRL-NH2. Isolated units were subsequently challenged with intradermal histamine followed by SLIGRL-NH2 (each 50 microg/1 microl) or reverse order, followed by mechanical, thermal, and algogenic stimuli. Forty-three units were classified as wide dynamic range (62%), nociceptive specific (22%), or mechano insensitive (16%). Twenty units gave prolonged (mean, 10 min) discharges to intradermal injection of histamine; 76% responded to subsequent SLIGRL-NH2, often more briefly. Units additionally responded to noxious heat (63%), cooling (43%), topical mustard oil (53%), and intradermal capsaicin (67%). Twenty-two other units gave prolonged (mean, 5 min) responses to initial intradermal injection of SLIGRL-NH2; 85% responded to subsequent intradermal histamine. They also responded to noxious heat (75%), mustard oil (93%), capsaicin (63%), and one to cooling. Most superficial dorsal horn neurons were excited by both histamine and the PAR-2 agonist, suggesting overlapping pathways for histamine- and non-histamine-mediated itch. Because the large majority of pruritogen-responsive neurons also responded to noxious stimuli, itch may be signaled at least partly by a population code.

Fig. 1.

Examples of units isolated using histamine search strategy. A: peristimulus-time histogram (PSTH; bins 1 s) of a unit response to histamine (left PSTH). This mechanically insensitive unit responded weakly, if at all, to subsequent intradermal injection of SLIGRL-NH2 or to topical application of mustard oil (MO) or intradermal capsaicin. Inset: left: drawing of left hindpaw with arrow indicating intradermal injection site. Right: drawing of histological section showing recording site (open circle) in lamina I of superficial dorsal horn. B: a different wide dynamic range (WDR) lamina I unit that responded weakly to histamine and more strongly to SLIGRL-NH2 (format as in A). This unit also responded robustly to capsaicin and weakly to mustard oil. Top left inset: drawing of left hindpaw receptive field (gray) and injection site (arrow). C: a different WDR lamina I unit that responded equally to intradermal histamine and SLIGRL-NH2. This unit also responded phasically to capsaicin and mustard oil (format as in A and B).

Fig. 2.

Averaged responses of units isolated using histamine (A) or SLIGRL-NH2 search strategy (B). A: histamine search. Averaged PSTHs (bins: 1 s) show, from left to right, unit responses to histamine, SLIGRL-NH2, MO, capsaicin, brush, pinch, noxious heat, and cooling. Error bars (gray): SE. Numbers in parentheses give the number of responders in relation to the total number tested. B: SLIGRL-NH2 search. Format as in A for 20 different units tested with SLIGRL-NH2 1st.

Fig. 3.

Examples of units isolated using SLIGRL-NH2 search strategy. A: PSTH shows robust response to SLIGRL-NH2, followed by a somewhat weaker response to histamine. This WDR unit responded to graded mechanical stimuli, heating and cooling, weakly to intradermal capsaicin, and robustly to topical MO. Inset: drawing of left hindpaw receptive field (gray) and injection site (arrow). B: different WDR lamina I unit responded robustly to SLIGRL-NH2 and noxious heat but not to histamine or cooling and only weakly to capsaicin. C: different WDR lamina I units that responded weakly, if at all, to SLIGRL-NH2 but more to histamine injected in right hindpaw (top left inset). It responded weakly to capsaicin and MO.

Fig. 4.

Evidence for cross-tachyphylaxis between histamine and SLIGRL-NH2. Bar graphs plot mean responses over a 13-min postinjection period for histamine tested 1st (black bar) or 2nd after SLIGRL-NH2 (open bar) and for SLIGRL-NH2 tested 1st (hatched bar) or 2nd (gray bar). Error bars: SE. *Significantly different (P < 0.01; unpaired t-test).

Fig. 5.

Schematic Venn diagram showing overlapping populations of central neurons responsive to histamine (His+), PAR-2 agonist (PAR−²⁺), or noxious stimuli (Noci). Numbers: percentage of neurons within each overlap region. White: mechanically insensitive units. Gray: WDR and nociceptive-specific (NS) units that responded to histamine and/or PAR-2 agonist. Black: WDR and NS units unresponsive to histamine or PAR-2 agonist.