Mast cell degranulation mediates compound 48/80-induced hyperalgesia in mice.

Immune cells contribute to acute and chronic pain in diverse and nuanced ways [ 1 ]. Mast cells are versatile effectors [ 2 ] whose roles have been studied in rodent models of migraine [ 3 ], interstitial cystitis [ 4 ], and postoperative pain [ 5 ], as well as in clinical studies of irritable bowel syndrome [ 6 ], endometriosis [ 7 ], and thermal capsaicin pain [ 8 ]. Peripherally located at tissue-environment interfaces, mast cells respond to a wide array of stimuli by promptly releasing a spectrum of both preformed and newly synthesized mediators [ 1 ]. Treatments that modulate the activities of mast cell mediators can alter inflammation-associated thermal and mechanical pain responses induced by zymosan and acetic acid [ 9 ], formalin [ 10 ], and venom [ 11 ]. Systemic mast cell degranulation in rats produced c-fos activation in medullary and dorsal horn neurons in a rat model of mechanical sensitivity [ 12 ]. Mast cell-deficient C57BL/6- Kit W-sh/W-sh (Wsh/Wsh) mice exhibited decreased pseudorabies virus-induced pelvic pain compared to wild-type (WT) C57BL/6 mice, and injections of WT bone marrow restored the histamine-mediated pain responses [ 4 ]. However, there has been little evidence in mice that localized mast cell degranulation can directly cause measurable pain. We found that localized plantar mast cell degranulation produced thermal hyperalgesia, edema, and neutrophil influx in ND4 Swiss mice. Mast cell granule stabilization, inhibition of neutrophil influx, and histamine receptor antagonism inhibited nociceptive behaviors. Compound 48/80-mediated thermal and mechanical hyperalgesia observed in WT C57BL/6 mice were abrogated in mast cell-deficient Wsh/Wsh mice and restored in Wsh/Wsh mice with hindpaws locally reconstituted with cultured mast cells.

3-6 month old male ND4 Swiss and C57BL/6 mice (Harlan Laboratories, Indianapolis, IN), mast cell-deficient C57BL/6- Kit W-sh/W-sh (Wsh/Wsh) mice and mast cell-reconstituted C57BL/6- Kit W-sh/W-sh (Wsh/Wsh:WT) (gift of Dr. Stephen Galli, Stanford University) were housed in Macalester College’s animal facility with a 12-hour light/dark cycle with food and water ad libitum . Bone marrow-derived cultured mast cells (2×10 6 /hindpaw) were transplanted into Wsh/Wsh mice [ 13 ]; age-matched controls received saline [ 14 ]. Reconstituted mice were used >12 weeks post-transplant. Macalester College’s Institutional Animal Care and Use Committee approved all experimental procedures. All drugs (Sigma-Aldrich, St. Louis, MO) were administered using 0.9% saline vehicle. Mice received bilateral intraplantar (i.pl.) treatments with compound 48/80 (c48/80; 0.3μg or 1.5μg/paw; 10μl) or saline alone [ 15 ]. Sodium cromoglycate (SCG; 80mg/kg), diphenhydramine (H1R antagonist; 20mg/kg), thioperamide maleate (H3R/H4R antagonist, 10mg/kg), or 100μl vehicle was injected intraperitoneally (i.p.) one hour prior [ 16 , 17 ], and fucoidan (20mg/kg; 200μl) or vehicle was administered retro-orbitally (r.o.) 30 minutes prior to c48/80 injection [ 18 ]. To assess thermal sensitivity single mice treated with i.pl. c48/80 or vehicle were placed in a Plexiglas cylinder on a hotplate analgesia meter (Harvard Laboratories, Edenbridge, KY) maintained at 51.0 ± 0.5°C for ND4 Swiss mice and 53.0 ± 0.5°C for C57/BL6 mice and removed when prolonged retraction, flipping/licking of the hindpaw, or jumping with both hindpaws off the hotplate were observed, but no later than 40 seconds (adapted from [ 19 ]). Two baseline hotplate latencies were taken 24 and 48 hours before the experiment. Mice with >10 second differences between baselines or <15 second averages were excluded. Nociceptive behavior was quantified by subtracting the mean baseline thermal latency from the experimental thermal latency at each time point for each mouse. Mechanical sensitivity was measured with an Electronic von Frey Anesthesiometer (IITC Corporation, Woodland Hills, CA) as the pressure required to evoke either sharp retraction of the hindpaw, jumping with all four paws, or licking of the stimulated hindpaw [ 20 ]. Baseline latencies were calculated as the mean of the three readings closest to the median out of five readings taken 24 and 48 hours before the experiment. Experimental measurements were calculated as the average of 3-4 readings per mouse without exclusions. Average baseline was subtracted from average experimental withdrawal threshold to calculate the delta withdrawal threshold for each mouse. All behavioral experiments used a minimum of 10 mice per treatment group. Change in hindpaw width measured using digital calipers (±0.1mm; VWR) was calculated as an average of the left and right paw widths. Baseline paw widths for each mouse were taken pre-treatment and subtracted from post-treatment paw widths to calculate tissue edema. Excised footpads were frozen at -80°C in 50mM K 2 HPO 4 buffer (pH 6.0) containing 0.05% hexadecyl trimethylammonium bromide (HTAB), thawed, homogenized in 5x volumes of HTAB buffer, sonicated 3x for 10 seconds, freeze-thawed 3x, re-sonicated, and centrifuged for 4 minutes at 4750 rpm (adapted from [ 21 , 22 ]). Absorbance was recorded at 450nm after a 20-minute incubation in 50mM phosphate buffer (pH 6.0) with 0.025% hydrogen peroxide and 0.167mg/ml o -dianisidine dihydrochloride at room temperature in the dark. Myeloperoxidase (MPO) levels were normalized to tissue weight and presented as OD/g of wet tissue. Excised hindpaws were fixed in 10% buffered formalin for 24 hours, transferred to 70% ethanol, decalcified for 1-2 weeks in 15% EDTA, hydrated, and embedded in paraffin. 4μm sagittal sections were stained with Toluidine blue (Tb) and Hematoxylin/Eosin (H&E) for quantification of mast cells and neutrophils, respectively, at 400x magnification with ≥3 biological replicates per treatment. Tb-stained mast cells were counted in 10 fields/section and degranulation was scored based on the number of granules observed outside the boundary of the cell: intact (0), mild (1-10), moderate (10-20), and extensive (20+). Neutrophils were counted in each of 10 fields/section. Researchers performing cell counts remained blind to treatment. Excised hindpaws were flash-frozen in liquid nitrogen and homogenized in Cell Lysis Buffer (Cell Signaling Technology, Beverly, MA) supplemented with protease inhibitor (Cocktail Set IV; EMD Biosciences, Billerica, MA) using a Tissue Tearor (BioSpec; Model 985370). Homogenates were incubated on ice for 20 minutes, centrifuged at 2000 rpm for 10 minutes at 4°C, and lysates stored at −80°C. Histamine levels were quantified by ELISA according to manufacturers’ (Neogen Corporation, Lansing, MI) instructions. Data were processed using Microsoft Excel and graphed with Graphpad PRISM 5.0 (San Diego, CA); statistical analyses were performed using JMP 9.0 (SAS; Cary, NC). All data are presented as the mean ± SEM, analyzed by one-way ANOVA and the Tukey-Kramer HSD post hoc test at each timepoint. Statistical significance was defined as p <0.05, and denoted in the figures as: p <0.05 [*, #], p <0.01 [**, ##], p <0.001 [***, ###].

ND4 mice treated with bilateral intraplantar injections of mast cell secretagogue c48/80 (0.3μg/paw) were significantly more sensitive to thermal stimuli compared to saline-treated mice at 0.5 hours and 1.5 hours ( Figure 1A ). The difference between latencies of saline- and c48/80-treated mice was resolved by 2.5 hours post-treatment ( Figure 1A ). Intraperitoneal pre-treatment with 80mg/kg of mast cell granule stabilizer SCG blocked the early induction of nociceptive behavior at 0.5 hours post intraplantar c48/80 treatment, suggesting that inhibiting mast cell degranulation abrogates thermal hyperalgesia. SCG pre-treatment alone did not result in significantly longer thermal latencies compared to saline-treated controls at any time point, thus SCG did not have a thermal analgesic effect independent of blocking degranulation ( Supplementary Figure 1 ). Mice treated with c48/80 had significant paw edema compared to saline-treated mice at 0.5 hours after treatment; edema was maintained through 2.5 hours post-treatment ( Figure 1B ) and was blocked at all time points by SCG. We observed increased mast cell degranulation in histological sections of hindpaws injected with c48/80 ( Figure 1C ) but not in hindpaws of mice given SCG prior to intraplantar c48/80 ( Figure 1D ) or in saline-treated paws ( Figure 1E ). ~50% of the mast cells were intact and 50% mildly degranulated in saline-treated paws ( Figure 1F ). With c48/80 treatment, intact mast cells were reduced to 20% with 8-10% of mast cells showing moderate or extensive levels of degranulation. SCG pre-treatment blocked moderate and extensive degranulation and restored intact cell counts to control levels ( Figure 1F ). The front paws of mice injected with either saline or c48/80 in the hindpaw showed no difference in levels of degranulation ( Supplementary Figure 2 ), demonstrating that intraplantar c48/80 administration had a strictly local effect. Tissue enzymes quickly degrade histamine released from mast cell granules leading to a reduction in histamine levels after mast cell degranulation [ 23 ]. Compound 48/80 treatment caused a 30% decrease in tissue histamine levels that was largely restored by SCG pre-treatment to stabilize mast cell granules ( Figure 1G ). H&E-stained histological sections showed clear evidence of neutrophil infiltration in c48/80-treated but not SCG pre-treated paws compared to controls ( Figure 2A-C ). Levels of MPO, an enzyme frequently used to quantify neutrophil infiltration [ 21 , 22 ], were significantly elevated in the plantar tissue following c48/80 administration; this increase was reduced partially but significantly by pre-treatment with SCG ( Figure 2D ). We blocked neutrophil influx into the plantar tissue by systemic pre-treatment with 20mg/kg fucoidan, an agent known to inhibit selectin-mediated leukocyte infiltration [ 24 ]. There was a marked reduction in plantar MPO activity in fucoidan pre-treated mice ( Figure 2D ), which was confirmed by neutrophil counts in histological sections ( Supplementary Figure 3 ). Inhibition of neutrophil influx into the plantar tissue significantly reduced c48/80-induced thermal hyperalgesia at 0.5 and 1.5 hours post-treatment ( Figure 2E ) and partially abrogated tissue edema at 0.5 hours ( Figure 2F ). Mice treated with fucoidan alone did not show significantly longer thermal latencies compared to saline controls, indicating the absence of an analgesic effect independent of neutrophil blockade ( Supplementary Figure 1 ). Blockade of histamine signaling can abrogate thermal and mechanical hypernociception in rodents [ 4 , 25 ]. We found that intraperitoneal pre-treatment with H1R and H3R/H4R antagonists, diphenhydramine and thioperamide, respectively, significantly reduced c48/80-induced thermal hyperalgesia 0.5 hours after plantar c48/80 challenge ( Figure 3A ), partially blocked tissue edema ( Figure 3B ), and reduced tissue MPO activity ( Figure 3C ). We confirmed that neither diphenhydramine nor thioperamide administration caused drowsiness as neither drug significantly changed cage-crossing activity 0-4 hours post-treatment (data not shown). We observed that diphenhydramine, but not thioperamide, on its own, caused mice to remain on the hotplate longer than counterparts injected with saline alone ( Supplementary Figure 1 ), suggesting that the analgesic effect of diphenhydramine is only partially dependent on its effects on mast cell-derived histamine signaling. As neither c48/80 nor SCG acts exclusively on mast cells [ 26 , 27 ], we asked whether c48/80-mediated hyperalgesic responses were abrogated in genetically mast cell-deficient Wsh/Wsh mice [ 13 ] and whether hyperalgesia could be restored with tissue-specific mast cell reconstitution. We adapted our protocol to the C57BL/6 background by adjusting the c48/80 dose to 1500ng/paw and the observation period to 4 hours. Thermal hyperalgesia was markedly decreased in mast cell-deficient Wsh/Wsh mice compared to WT mice at 1.5-4 hours post-treatment ( Figure 4A ). Compound 48/80-challenged Wsh/Wsh:WT mice engrafted with 2×10 6 WT cultured mast cells in the hindpaws had significantly lower delta latencies compared to non-transplanted counterparts ( Figure 4A ). As hypoalgesic responses to vertically applied punctate thermal stimuli have been reported in kit-mutant mice [ 28 ], we compared baselines to confirm that Wsh/Wsh mice were at least as sensitive to the thermal stimulus applied across all four paws as WT mice ( Supplementary Fig 4 ). We independently confirmed mast cell-dependent restoration of hyperalgesia with mechanical sensitivity measurements. Mechanical pain responses following c48/80 challenge were also significantly abrogated in Wsh/Wsh mice compared to their WT counterparts and restored to WT levels upon plantar mast cell reconstitution ( Figure 4B ). Both thermal and mechanical latencies of c48/80-treated WT C57BL/6 were significantly lower than saline controls (data not shown). Mast cells were present in representative sections of plantar tissue from WT and Wsh/Wsh:WT mice but not in Wsh/Wsh mice ( Figure 4C-E ); absolute counts confirmed that plantar mast cells were reconstituted to approximately 70% of WT levels in mast cell-reconstituted paws ( Table 1 ). Neutrophil influx was restored to about 70% of WT levels in histological sections of mast cell-reconstituted mice ( Table 1 ). We found comparable levels ( p >0.05) of mild, moderate, and extensive degranulation of mast cells in WT and mast cell-reconstituted Wsh/Wsh:WT mice ( Figure 4F ). Compound 48/80-induced tissue edema was absent in Wsh/Wsh mice but restored to approximately half of WT levels in plantar mast cell-reconstituted mice ( Figure 4G ). Taken together, these data showed that c48/80-induced pain and edema responses could be depleted and restored in a mast cell-dependent manner.

Here we show that stabilization of plantar mast cell granules limits c48/80-mediated hindpaw hyperalgesia and edema in ND4 mice, demonstrating that mast cell degranulation can initiate inflammatory pain. Systemic mast cell degranulation can cause cranial, tailbase, and hindpaw sensitivity in rats through central sensitization mechanisms [ 12 ]. Our findings indicate that localized mast cell activation in the hindpaw leads to pain responses via local neutrophil influx and histamine release. Taken together, these two studies provide strong evidence that mast cells contribute to both central and peripheral pain pathways in rodents. Recruitment of neutrophils into inflamed tissue has been shown to be dependent upon mast cell activation in host defense against bacterial infection [ 29 ] and collagen-induced arthritis, [ 30 ]. Neutrophils have been previously shown to mediate carrageenan-induced mechanical hyperalgesia in mice [ 18 ] and allergen-evoked thermal hyperalgesia in rats [ 25 ]. In our study, c48/80-mediated neutrophil infiltration into the hindpaw tissue of ND4 mice was partially, but significantly, inhibited by mast cell granule stabilization with SCG pre-treatment. Fucoidan pre-treatment blocked neutrophil influx and significantly reduced thermal hyperalgesia in c48/80-treated ND4 mice. Compound 48/80-mediated mast cell degranulation also decreased the total histamine content of the hindpaw, consistent with the rapid degradation of free histamine by tissue enzymes [ 23 ]. Histamine signaling has been shown to mediate formalin- and antigen-induced pain and sensitize meningeal nociceptors [ 25 , 31 ]. Here, pre-treatment with H3R/H4R dual antagonist thioperamide attenuated c48/80-induced thermal pain 30 minutes after challenge without causing analgesia when administered alone, implicating H3R and H4R in pro-nociceptive mast cell signaling. H3R is primarily located in the central nervous system and is not accessible by histamine released locally in the hindpaw [ 32 ]. H4R has been shown to modulate mast cells, eosinophils, T cells, and dendritic cells [ 32 ], as well as carrageenan- and Complete Freund’s Adjuvant-induced mechanical hyperalgesia in rats [ 33 ]. H4R, therefore, may be the primary receptor involved in the early pro-nociceptive signaling in the periphery. We found that H4R (and H1R) antagonism significantly reduced the infiltration of neutrophils into c48/80-treated paws, supporting earlier observations that H4R blockade prevents neutrophil influx in zymosan peritonitis and carrageenan-treated plantar tissue [ 33 ]. Mast cell degranulation-initiated pain, therefore, is crucially mediated through histamine signaling and neutrophil influx. Interestingly, our findings suggest that pain and edema resulting from c48/80-induced mast cell degranulation may, in part, be differentially regulated. While SCG pre-treatment reduced pain and edema responses, H3/4R antagonist thioperamide blocked thermal pain to control levels but only partially reduced edema. Blocking neutrophil influx, similarly, markedly reduced pain responses but only partially blocked tissue edema. In a rat model of allergen-evoked pain, bradykinin and histamine receptor antagonists can alleviate pain but not edema [ 25 ]. Mast cell deficiency can inhibit hyperalgesia, but not vasodilation, in murine interstitial cystitis [ 4 ]. Our findings support independent regulation of pain and edema despite their co-occurrence in inflammatory cascades. We found that genetically mast cell-deficient Wsh/Wsh mice have reduced thermal and mechanical pain and edema, and local plantar reconstitution with bone marrow-derived mast cells can restore these responses. While Rudick et al. found that whole bone marrow transplantation restored cystitis pain in Wsh/Wsh mice [ 4 ], their study did not specifically isolate mast cells as the relevant cellular players. Repair of other c-Kit-related defects may have played unspecified roles. We provide the first evidence that tissue-specific reconstitution of mast cells restores the c48/80-induced thermal and mechanical pain responses abrogated in mast cell-deficient mice. Future experiments in recently described kit-independent mouse strains [ 34 ] with conditional mast cell deficiencies will further elucidate contributions of these effectors to pain responses in the absence of systemic c-Kit defects. As mast cells are the primary source of histamine in the tissue, we were not surprised to find virtually no detectable histamine in the hindpaws of Wsh/Wsh mice (data not shown). Furthermore, mast cell-deficient Wsh/Wsh mice showed significantly decreased neutrophil numbers in the hindpaw after c48/80-injection, and neutrophil infiltration was partially, but significantly, restored with plantar mast cell reconstitution. These findings demonstrate that the contributions of histamine signaling and neutrophil influx to c48/80-mediated inflammatory pain are significant and mast cell-dependent. In recent years, mast cell contributions to human pain pathologies have been explored. Cutaneous administration of c48/80 to ten healthy men caused increased thermal sensitivity after two consecutive doses at 0 and 3 hours, while continued administration of c48/80 at 3-hour intervals over 24 hours decreased subjects’ thermal sensitivity and hyperalgesic response to capsaicin [ 8 ]. The early response seen in this study is consistent with the rapid thermal hyperalgesia we observed in mice, and the later response could reflect the gradual depletion of mast cell mediators after chronic exposure to c48/80. Mast cells have been implicated in vulvodynia and vestibulitis with increased numbers of degranulated mast cells in vulvar biopsies of patients [ 35 ]. Increased levels of mast cell tryptase are found in prostatic secretions of men with pelvic pain [ 36 ]. Additionally, increased dermal mast cell degranulation co-occurs with increased sensitivity to heat and pressure in children with self-injurious behaviors [ 37 ]. Here we have demonstrated that mast cells are necessary for c48/80-induced inflammatory pain responses in mice. Tissue-specific mast cell reconstitution restores neutrophil and histamine-mediated, c48/80-induced, localized hyperalgesia in mast cell-deficient mice. Our findings contribute to a growing body of literature that establishes mast cells as important and necessary players and therapeutic targets in the initiation of inflammatory pain responses.