Opioid modulation of scratching and spinal c-fos expression evoked by intradermal serotonin

Spinal Glycine Receptor Alpha 3 Cells Communicate Sensations of Chemical Itch in Hairy Skin

Glycinergic neurons regulate nociceptive and pruriceptive signaling in the spinal cord, but the identity and role of the glycine-regulated neurons are not fully known. Herein, we have characterized spinal glycine receptor alpha 3 (Glra3) subunit-expressing neurons in Glra3-Cre female and male mice. Glra3-Cre(+) neurons express Glra3, are located mainly in laminae III-VI, and respond to glycine. Chemogenetic activation of spinal Glra3-Cre(+) neurons induced biting/licking, stomping, and guarding behaviors, indicative of both a nociceptive and pruriceptive role for this population. Chemogenetic inhibition did not affect mechanical or thermal responses but reduced behaviors evoked by compound 48/80 and chloroquine, revealing a pruriceptive role for these neurons. Spinal cells activated by compound 48/80 or chloroquine express Glra3, further supporting the phenotype. Retrograde tracing revealed that spinal Glra3-Cre(+) neurons receive input from afferents associated with pain and itch, and dorsal root stimulation validated the monosynaptic input. In conclusion, these results show that spinal Glra3(+) neurons contribute to acute communication of compound 48/80- and chloroquine-induced itch in hairy skin.

Formalin Itch Test: Low-Dose Formalin Induces Histamine-Independent, TRPA1-Mediated Itch in Mice

Chronic itch is a common distressing symptom of many diseases, which reduced patient's quality of life. The mechanistic study on itch and screening for new anti-itch drugs require the development of new pre-clinical itch animal models. Herein, we established an acute itch model by intradermal (i.d.) injection of low-dose formalin into the neck or cheek in mice. In mice, i.d. injection of formalin (0.1–5%) in the nape of the neck evoked robust scratching behavior in a dose-dependent manner and the dose–response curves showed an inverted “U” shape. I.d. injection of formalin (0.3–0.6%) into the cheek evoked scratching in mice but wiping in rats, while formalin (1.25–5%) induced mixed wiping and scratching behavior in both mice and rats. Further, we found that 0.3% formalin-induced scratching was histamine-independent and significantly attenuated by transient receptor potential ion channel A1 (TRPA1) inhibitor (HC030031) or in TRPA1 knockout (KO) mice, but not affected by transient receptor potential ion channel V1 (TRPV1) inhibitor (capsazepine) or in TRPV1 KO mice. Additionally, 0.3% formalin-induced up-regulation of phosphorylation of extracellular regulated protein kinases (p-ERK) in the dorsal root ganglion (DRG) and scratching were suppressed by intrathecal injection of MEK inhibitor U0126 in mice. Incubation of 0.03% formalin induced the accumulation of intracellular reactive oxygen species (ROS) in the cultured DRG-derived cell line ND7-23, and formalin-induced itch was suppressed by antioxidants in mice. Finally, perfusion of 0.03% formalin induced elevation of intracellular calcium in a subset of primary cultured DRG neurons of mice. Thus, these results indicate that low-dose formalin induced non-histaminergic itch by activation of TRPA1 in mice, which may be employed as a useful acute itch model for screening potential anti-itch drugs.

Spinal neurons that contain gastrin-releasing peptide seldom express Fos or phosphorylate extracellular signal-regulated kinases in response to intradermal chloroquine

Background

Gastrin-releasing peptide (GRP) is thought to play a role in the itch evoked by intradermal injection of chloroquine. Although some early studies suggested that GRP was expressed in pruriceptive primary afferents, it is now thought that GRP in the spinal cord is derived mainly from a population of excitatory interneurons in lamina II, and it has been suggested that these are involved in the itch pathway. To test this hypothesis, we used the transcription factor Fos and phosphorylation of extracellular signal-regulated kinases (ERK) to look for evidence that interneurons expressing GRP were activated following intradermal injection of chloroquine into the calf, in mice that express enhanced green fluorescent protein (EGFP) in these cells.

Results

Injection of chloroquine resulted in numerous Fos- or phospho-ERK (pERK) positive cells in the somatotopically appropriate part of the superficial dorsal horn. The proportion of all neurons in this region that showed Fos or pERK was 18% and 21%, respectively. However, among the GRP–EGFP, only 7% were Fos-positive and 3% were pERK-positive. As such, GRP–EGFP cells were significantly less likely than other neurons to express Fos or to phosphorylate ERK.

Conclusions

Both expression of Fos and phosphorylation of ERK can be used to identify dorsal horn neurons activated by chloroquine injection. However, these results do not support the hypothesis that interneurons expressing GRP are critical components in the itch pathway.

Systemic and spinal administration of FAAH, MAGL inhibitors and dual FAAH/MAGL inhibitors produce antipruritic effect in mice

The increase of endocannabinoid tonus by inhibiting fatty acid amide hydrolase (FAAH) or monoacylglycerol lipase (MAGL) represents a promising therapeutic approach in a variety of disease to overcome serious central side effects of exocannabinoids. Recent studies reported that systemic administration of FAAH and MAGL inhibitors produce antipruritic action. Dual FAAH/MAGL inhibitors have also been described to get enhanced endocannabinoid therapeutic effect. In this study, we examined and compared dose-related antipruritic effects of systemic (intraperitoneal; ip) or intrathecal (it) administration of selective FAAH inhibitor PF-3845 (5, 10, and 20 mg/kg, i.p.; 1, 5, and 10 µg, i.t.), MAGL inhibitor JZL184 (4, 20, and 40 mg/kg, i.p.; 1, 5, and 10 µg, i.t.) and dual FAAH/MAGL inhibitor JZL195 (2, 5, and 20 mg/kg, i.p.; 1, 5, and 10 µg, i.t.) on serotonin (5-HT)-induced scratching model. Serotonin (25 μg) was injected intradermally in a volume of 50 μl into the rostral part of skin on the back of male Balb-C mice. Both systemic or intrathecal administration of PF-3845, JZL184 or JZL195 produced similar dose-dependent antipruritic effects. Our results suggest that endocannabinoid-degrading enzymes FAAH and MAGL are involved in pruritic process at spinal level. FAAH, MAGL or dual FAAH/MAGL inhibitors have promising antipruritic effects, at least, in part through spinal site of action.

Spinal Functions of B-Type Natriuretic Peptide, Gastrin-Releasing Peptide, and Their Cognate Receptors for Regulating Itch in Mice

B-type natriuretic peptide (BNP)-natriuretic peptide receptor A (NPRA) and gastrin-releasing peptide (GRP)-GRP receptor (GRPR) systems contribute to spinal processing of itch. However, pharmacological and anatomic evidence of these two spinal ligand-receptor systems are still not clear. The aim of this study was to determine the spinal functions of BNP-NPRA and GRP-GRPR systems for regulating scratching activities in mice by using pharmacological and immunohistochemical approaches. Our results showed that intrathecal administration of BNP (0.3-3 nmol) dose dependently elicited scratching responses, which could be blocked by the NPRA antagonist (Arg6,β-cyclohexyl-Ala8,D-Tic16,Arg17,Cys18)-atrial natriuretic factor(6-18) amide (A71915). However, A71915 had no effect on intrathecal GRP-induced scratching. In contrast, pretreatment with a GRPR antagonist (D-Tpi6,Leu13ψ(CH2-NH)-Leu14)bombesin(6-14) (RC-3095) inhibited BNP-induced scratching. Immunostaining revealed that NPRA proteins colocalize with GRP, but not GRPR, in the superficial area of dorsal horn, whereas BNP proteins do not colocalize with either GRP or GRPR in the dorsal horn. Intradermal administration of ligands including endothelin-1, U-46619, bovine adrenal medulla 8-22, and Ser-Leu-Ile-Gly-Arg-Leu-NH2 (SLIGRL) increased scratching bouts at different levels of magnitude. Pretreatment with intrathecal A71915 did not affect scratching responses elicited by all four pruritogens, whereas pretreatment with RC-3095 only inhibited SLIGRL-induced scratching. Interestingly, immunostaining showed that RC-3095, but not A71915, inhibited SLIGRL-elicited c-Fos activation in the spinal dorsal horn, which was in line with behavioral outcomes. These findings demonstrate that: 1) BNP-NPRA system may function upstream of the GRP-GRPR system to regulate itch in the mouse spinal cord, and 2) both NPRA and GRPR antagonists may have antipruritic efficacy against centrally, but not peripherally, elicited itch.

Hydrogen sulfide-induced itch requires activation of Cav3.2 T-type calcium channel in mice

The contributions of gasotransmitters to itch sensation are largely unknown. In this study, we aimed to investigate the roles of hydrogen sulfide (H₂S), a ubiquitous gasotransmitter, in itch signaling. We found that intradermal injection of H₂S donors NaHS or Na₂S, but not GYY4137 (a slow-releasing H₂S donor), dose-dependently induced scratching behavior in a μ-opioid receptor-dependent and histamine-independent manner in mice. Interestingly, NaHS induced itch via unique mechanisms that involved capsaicin-insensitive A-fibers, but not TRPV1-expressing C-fibers that are traditionally considered for mediating itch, revealed by depletion of TRPV1-expressing C-fibers by systemic resiniferatoxin treatment. Moreover, local application of capsaizapine (TRPV1 blocker) or HC-030031 (TRPA1 blocker) had no effects on NaHS-evoked scratching. Strikingly, pharmacological blockade and silencing of Ca_v3.2 T-type calcium channel by mibefradil, ascorbic acid, zinc chloride or Ca_v3.2 siRNA dramatically decreased NaHS-evoked scratching. NaHS induced robust alloknesis (touch-evoked itch), which was inhibited by T-type calcium channels blocker mibefradil. Compound 48/80-induced itch was enhanced by an endogenous precursor of H₂S (L-cysteine) but attenuated by inhibitors of H₂S-producing enzymes cystathionine γ-lyase and cystathionine β-synthase. These results indicated that H₂S, as a novel nonhistaminergic itch mediator, may activates Ca_v3.2 T-type calcium channel, probably located at A-fibers, to induce scratching and alloknesis in mice.

Anatomical evidence of pruriceptive trigeminothalamic and trigeminoparabrachial projection neurons in mice

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. Histamine, chloroquine, and capsaicin intradermally 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. Among neurons retrogradely labeled from the thalamus, 43%, 8%, and 22% were Fos-positive following id histamine, chloroquine, or capsaicin. Among 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. Among the neurons retrogradely labeled from LPb, 36%, 29%, and 33% were Fos positive following id injection of histamine, chloroquine, and capsaicin, respectively. Among 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.

Neurophysiology and itch pathways

As we all can easily differentiate the sensations of itch and pain, the most straightforward neurophysiologic concept would consist of two specific pathways that independently encode itch and pain. Indeed, a neuronal pathway for histamine-induced itch in the peripheral and central nervous system has been described in animals and humans, and recently several non-histaminergic pathways for itch have been discovered in rodents that support a dichotomous concept differentiated into a pain and an itch pathway, with both pathways being composed of different "flavors." Numerous markers and mediators have been found that are linked to itch processing pathways. Thus, the delineation of neuronal pathways for itch from pain pathways seemingly proves that all sensory aspects of itch are based on an itch-specific neuronal pathway. However, such a concept is incomplete as itch can also be induced by the activation of the pain pathway in particular when the stimulus is applied in a highly localized spatial pattern. These opposite views reflect the old dispute between specificity and pattern theories of itch. Rather than only being of theoretic interest, this conceptual problem has key implication for the strategy to treat chronic itch as key therapeutic targets would be either itch-specific pathways or unspecific nociceptive pathways.

The effect of propofol on intrathecal morphine-induced pruritus and its mechanism

BACKGROUND

Previous studies have shown that a low dose of propofol IV bolus had a beneficial effect on intrathecal morphine-induced pruritus in humans. However, its exact mechanism has not been fully understood. In this study, we hypothesized that propofol relieved intrathecal morphine-induced pruritus in rats by upregulating the expression of cannabinoid-1 (CB[1]) receptors in anterior cingulate cortex (ACC).

METHODS

Twenty-four Sprague-Dawley rats were divided into a control group and 20, 40, 80 μg/kg morphine groups to create an intrathecal morphine-induced scratching model. The effects of propofol on intrathecal 40 μg/kg morphine-induced scratching responses were then evaluated. Sixty rats were randomly assigned to control, normal saline, intralipid, and propofol groups, with pruritus behavior observation or killed 8 minutes after venous injection of normal saline, intralipid, or propofol, and brain tissues were then collected for assay. Immunohistochemistry was then performed to identify the expression of CB (1) receptor in ACC, and the concentration of CB(1) receptor in ACC was determined by Western blot analysis.

RESULTS

Compared with the control group, rats in the 20, 40, 80 μg/kg morphine groups had higher mean scratching response rates after intrathecal morphine injection (P =0.020, 0.005, and 0.002, respectively). There was a statistical difference between 20 and 40 μg/kg morphine groups at 10 to 15 and 15 to 20 timepoints after intrathecal morphine injection (P = 0.049 and 0.017, respectively). Propofol almost abolished the scratching response that was induced by 40 μg/kg intrathecal morphine injection (F[2, 15] = 46.87, P < 0.001; F[22, 165] = 2.37, P = 0.001). Compared with the intralipid and normal saline groups, the scratching behavior was significantly attenuated in the propofol group (P < 0.001). Compared with control, normal saline, and intralipid groups, the protein expression of CB(1) receptor in ACC (Western blot) in the propofol group increased (0.86 ± 0.21, 0.94 ± 0.18, 0.86 ± 0.13, and 1.34 ± 0.32, respectively, P < 0.001). There was no significant difference among control, normal saline, and intralipid groups. Compared with the control, normal saline, and intralipid groups, the average number of neurons of CB(1) receptor in the ACC area were higher in the propofol group (21.0 ± 1.4, 19.3 ± 1.8, 24.8 ± 7.7, and 37.2 ± 3.3, respectively, P < 0.001).

CONCLUSIONS

Morphine elicits dose-independent scratching responses after intrathecal injection in rats. Morphine 40 μg/kg intrathecal injection-induced scratching responses can be prevented by propofol. Increased protein expression of CB(1) receptors in ACC may contribute to the reversal of intrathecal morphine-induced scratching.

Scratching inhibits serotonin-evoked responses of rat dorsal horn neurons in a site- and state-dependent manner

Scratching inhibits pruritogen-evoked responses of neurons in the superficial dorsal horn, implicating a spinal site for scratch inhibition of itch. We investigated if scratching differentially affects neurons depending on whether they are activated by itchy vs. painful stimuli, and if the degree of inhibition depends on the relative location of scratching. We recorded from rat lumbar dorsal horn neurons responsive to intradermal (id) microinjection of serotonin (5-hydroxytryptamine, 5-HT). During the response to 5-HT, scratch stimuli (3mm, 300 mN, 2 Hz, 20s) were delivered at the injection site within the mechanosensitive receptive field (on-site), or 4-30 mm away, outside of the receptive field (off-site). During off-site scratching, 5-HT-evoked firing was significantly attenuated followed by recovery. On-site scratching excited neurons, followed by a significant post-scratch decrease in 5-HT-evoked firing. Most neurons additionally responded to mustard oil (allyl isothiocyanate). Off-site scratching had no effect, while on-site scratching excited the neurons. These results indicate that scratching exerts a state-dependent inhibitory effect on responses of spinal neurons to pruritic but not algesic stimuli. Moreover, on-site scratching first excited neurons followed by inhibition, while off-site scratching immediately evoked the inhibition of pruritogen-evoked activity. This accounts for the suppression of itch by scratching at a distance from the site of the itchy stimulus.

Electroacupuncture Attenuates 5'-Guanidinonaltrindole-Evoked Scratching and Spinal c-Fos Expression in the Mouse

The present study was undertaken to investigate the influence of electroacupuncture (EA) on compulsive scratching in mice and c-Fos expression elicited by subcutaneous (s.c.) administration of a known puritogen, 5'-guanidinonaltrindole (GNTI) to the neck. Application of EA to Hegu (LI4) and Quchi (LI11) acupoints at 2 Hz, but not 100 Hz, attenuated GNTI-evoked scratching. In mice pretreated with the µ opioid receptor antagonist naloxone, EA 2 Hz did not attenuate GNTI-evoked scratching, whereas EA at 2 Hz did attenuate GNTI-evoked scratching in mice pretreated with the κ opioid receptor antagonist nor-binaltorphimine. Moreover, intradermal (i.d.) administration of the selective µ opioid receptor agonist [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin acetate (DAMGO) attenuated GNTI-evoked scratching behavior, while s.c. administration of DAMGO was ineffective. GNTI provoked c-Fos expression on the lateral side of the superficial layer of the dorsal horn of the cervical spinal cord. Application of 2 Hz EA to LI4 and LI11 decreased the number of c-Fos positive nuclei induced by GNTI. It may be concluded that application of 2 Hz EA to LI4 and LI11 attenuates scratching behavior induced by GNTI in mice and that the peripheral µ opioid system is involved, at least in part, in the anti-pruritic effects of EA.

Electroacupuncture reduces cocaine-induced seizures and mortality in mice

The aims of this study were to characterize the protective profile of electroacupuncture (EA) on cocaine-induced seizures and mortality in mice. Mice were treated with EA (2 Hz, 50 Hz, and 100 Hz), or they underwent needle insertion without anesthesia at the Dazhui (GV14) and Baihui (GV20) acupoints before cocaine administration. EA at 50 Hz applied to GV14 and GV20 significantly reduced the seizure severity induced by a single dose of cocaine (75 mg/kg; i.p.). Furthermore, needle insertion into GV14 and GV20 and EA at 2 Hz and 50 Hz at both acupoints significantly reduced the mortality rate induced by a single lethal dose of cocaine (125 mg/kg; i.p.). In the sham control group, EA at 50 Hz applied to bilateral Tianzong (SI11) acupoints had no protective effects against cocaine. In addition, EA at 50 Hz applied to GV14 and GV20 failed to reduce the incidence of seizures and mortality induced by the local anesthetic procaine. In an immunohistochemistry study, EA (50 Hz) pretreatment at GV14 and GV20 decreased cocaine (75 mg/kg; i.p.)-induced c-Fos expression in the paraventricular thalamus. While the dopamine D₃ receptor antagonist, SB-277011-A (30 mg/kg; s.c), did not by itself affect cocaine-induced seizure severity, it prevented the effects of EA on cocaine-induced seizures. These results suggest that EA alleviates cocaine-induced seizures and mortality and that the dopamine D₃ receptor is involved, at least in part, in the anticonvulsant effects of EA in mice.

Oxidative stress induces itch via activation of transient receptor potential subtype ankyrin 1 in mice

OBJECTIVE

To investigate the role of oxidative stress in itch-indicative scratching behavior in mice, and furthermore, to define the cellular and molecular mechanisms underlying oxidative stress-mediated itch.

METHODS

Scratching behavior was induced by intradermal injection of the oxidants hydrogen peroxide (H₂O₂) or tert-butylhydroperoxide (tBHP) into the nape of the neck in mice. The mice were observed for 30 min.

RESULTS

Intradermal H₂O₂ (0.03%-1%) or tBHP (1-30 μmol) elicited robust scratching behavior, displaying an inverted U-shaped dose-response curve. Naloxone, an opioid receptor antagonist, but not morphine, largely suppressed the oxidant-induced scratching. Chlorpheniramine, a histamine H1 receptor antagonist, blocked histamine- but not oxidant-induced scratching, indicating the involvement of a histamine-independent mechanism in oxidant-evoked itch. Further, resiniferatoxin treatment abolished oxidant-induced scratching, suggesting an essential role of C-fibers. Notably, blockade of transient receptor potential subtype ankyrin 1 (TRPA1) with the selective TRPA1 antagonist HC-030031, or genetic deletion of Trpa1 but not Trpv1 (subfamily V, member 1) resulted in a profound reduction in H₂O₂-evoked scratching. Finally, systemic administration of the antioxidant N-acetyl-L-cysteine or trolox (a water-soluble vitamin E analog) attenuated scratching induced by the oxidants.

CONCLUSION

Oxidative stress by different oxidants induces profound scratching behavior, which is largely histamine- and TRPV1-independent but TRPA1-dependent. Antioxidants and TRPA1 antagonists may be used to treat human itch conditions associated with oxidative stress.

Involvement of serine protease and proteinase-activated receptor 2 in dermatophyte-associated itch in mice

We investigated the involvement of serine protease and proteinase-activated receptor 2 (PAR(2)) in dermatophyte-induced itch in mice. An intradermal injection of an extract of the dermatophyte Arthroderma vanbreuseghemii (ADV) induced hind-paw scratching, an itch-related behavior. ADV extract-induced scratching was inhibited by the opioid receptor antagonists naloxone and naltrexone, the serine protease inhibitor nafamostat mesylate, and the PAR(2) receptor antagonist FSLLRY-NH(2). ADV extract-induced scratching was not inhibited by the H(1) histamine receptor antagonist terfenadine or by mast cell deficiency. Heat pretreatment of the ADV extract markedly reduced the scratch-inducing and serine protease activities. Proteolytic cleavage within the extracellular N terminus of the PAR(2) receptor exposes a sequence that serves as a tethered ligand for the receptor. The ADV extract as well as tryptase and trypsin cleaved a synthetic N-terminal peptide of the PAR(2) receptor. The present results suggest that serine protease secreted by dermatophytes causes itching through activation of the PAR(2) receptors, which may be a causal mechanism of dernatophytosis itch.

Genetic enhancement of behavioral itch responses in mice lacking phosphoinositide 3-kinase-γ (PI3Kγ)

Phosphoinositide 3-kinases (PI3Ks) are important for synaptic plasticity and various brain functions. The only class IB isoform of PI3K, PI3Kγ, has received the most attention due to its unique roles in synaptic plasticity and cognition. However, the potential role of PI3Kγ in sensory transmission, such as pain and itch has not been examined. In this study, we present the evidence for the first time, that genetic deletion of PI3Kγ enhanced scratching behaviours in histamine-dependent and protease-activated receptor 2 (PAR-2)-dependent itch. In contrast, PI3Kγ-deficient mice did not exhibit enhanced scratching in chloroquine-induced itch, suggesting that PI3Kγ selectively contributes to certain types of behavioal itch response. Furthermore, PI3Kγ-deficient mice exhibited normal acute nociceptive responses to thermal and mechanical noxious stimuli. Behavioral licking responses to intraplantar injections of formalin and mechanical allodynia in a chronic inflammatory pain model (CFA) were also not affected by PI3Kγ gene deletion. Our findings indicate that PI3Kγ selectively contributes to behavioral itching induced by histamine and PAR-2 agonist, but not chloroquine agonist.

Gastrin-releasing peptide induces itch-related responses through mast cell degranulation in mice

Gastrin-releasing peptide (GRP), secreted from the central terminals of primary afferents, is involved in the transmission of itch signals in the spinal dorsal horn. Although primary afferents containing GRP are distributed throughout the skin, the role of peripherally released GRP in the itch response is unknown. We investigated whether GRP acts on the skin to induce an itch response in mice. Intradermal injections of GRP(18-27) (1-300 nmol/site) elicited scratching. GRP(18-27)-induced scratching was inhibited by the μ-opioid receptor antagonist naltrexone hydrochloride, the BB(2) bombesin receptor antagonist RC-3095, the H(1) histamine receptor antagonists fexofenadine hydrochloride and chlorpheniramine maleate, and the PAR(2) proteinase-activated receptor antagonist FSLLRY-NH(2). Mast cell deficiency significantly, but not completely, reduced the GRP(18-27)-induced scratching. BB(2) bombesin receptors are present in mast cells in the skin, and intradermal injection of GRP(18-27), not only induced scratching, but also led to mast cell degranulation. GRP(18-27)-induced mast cell degranulation was inhibited by the BB(2) bombesin receptor antagonist RC-3095. These results suggest that peripherally released GRP can induce an itch response, at least partly, through activation of BB(2) receptors present in the mast cells, triggering their degradation and the release of histamine and the serine proteinase, tryptase.

Glutamate acts as a neurotransmitter for gastrin releasing peptide-sensitive and insensitive itch-related synaptic transmission in mammalian spinal cord

Itch sensation is one of the major sensory experiences of human and animals. Recent studies have proposed that gastrin releasing peptide (GRP) is a key neurotransmitter for itch in spinal cord. However, no direct evidence is available to indicate that GRP actually mediate responses between primary afferent fibers and dorsal horn neurons. Here we performed integrative neurobiological experiments to test this question. We found that a small population of rat dorsal horn neurons responded to GRP application with increases in calcium signaling. Whole-cell patch-clamp recordings revealed that a part of superficial dorsal horn neurons responded to GRP application with the increase of action potential firing in adult rats and mice, and these dorsal horn neurons received exclusively primary afferent C-fiber inputs. On the other hands, few A_δ inputs receiving cells were found to be GRP positive. Finally, we found that evoked sensory responses between primary afferent C fibers and GRP positive superficial dorsal horn neurons are mediated by glutamate but not GRP. CNQX, a blocker of AMPA and kainate (KA) receptors, completely inhibited evoked EPSCs, including in those Fos-GFP positive dorsal horn cells activated by itching. Our findings provide the direct evidence that glutamate is the principal excitatory transmitter between C fibers and GRP positive dorsal horn neurons. Our results will help to understand the neuronal mechanism of itch and aid future treatment for patients with pruritic disease.

Facial injections of pruritogens or algogens elicit distinct behavior responses in rats and excite overlapping populations of primary sensory and trigeminal subnucleus caudalis neurons

In the present study, we investigated whether intradermal cheek injection of pruritogens or algogens differentially elicits hindlimb scratches or forelimb wipes in Sprague-Dawley rats, as recently reported in mice. We also investigated responses of primary sensory trigeminal ganglion (TG) and dorsal root ganglion (DRG) cells, as well as second-order neurons in trigeminal subnucleus caudalis (Vc), to pruritic and algesic stimuli. 5-HT was the most effective chemical to elicit dose-dependent bouts of hindlimb scratches directed to the cheek, with significantly less forelimb wiping, consistent with itch. Chloroquine also elicited significant scratching but not wiping. Allyl isothiocyanate (AITC; mustard oil) elicited dose-dependent wiping with no significant scratching. Capsaicin elicited equivalent numbers of scratch bouts and wipes, suggesting a mixed itch and pain sensation. By calcium imaging, ∼ 6% of cultured TG and DRG cells responded to 5-HT. The majority of 5-HT-sensitive cells also responded to chloroquine, AITC, and/or capsaicin, and one-third responded to histamine. Using a chemical search strategy, we identified single units in Vc that responded to intradermal cheek injection of 5-HT. Most were wide dynamic range (WDR) or nociceptive specific (NS), and a few were mechanically insensitive. The large majority additionally responded to AITC and/or capsaicin and thus were not pruritogen selective. These results suggest that primary and second-order neurons responsive to pruritogens and algogens may utilize a population coding mechanism to distinguish between itch and pain, sensations that are behaviorally manifested by distinct hindlimb scratching and forelimb wiping responses.

Responsiveness of C neurons in rat dorsal root ganglion to 5-hydroxytryptamine-induced pruritic stimuli in vivo

Itching is a common symptom in dermatologic diseases and causes restless scratching of the skin, which aggravates the condition. The mechanism of the itch sensation, however, is enigmatic. The present study included behavioral tests and electrophysiological recordings from rat dorsal root ganglion (DRG) neurons in vivo to analyze the response to pruritic stimuli induced by topical application of 5-hydroxytryptamine (5-HT) to the skin. Topically applied 5-HT to the rostral back evoked scratching, whereas application of the vehicle did not. Following subcutaneous injection of the opioid receptor antagonist naloxone, the number of scratches decreased, suggesting that the scratching was preferentially mediated by itch but not pain sensation. To elucidate the firing properties of DRG neurons in response to topically applied 5-HT, intracellular recordings were made from DRG neurons in vivo. None of the Abeta and Adelta neurons responded to 5-HT; in contrast, 25 of 91 C neurons (27%) exhibited repetitive firing in response to 5-HT, which could be classified into two firing patterns: one was a transient type, characterized by low firing frequency that decreased within 5 min; the other was a long-lasting type, having high firing frequency that continued increasing after 5 min. The time course of the firing pattern of long-lasting C neurons was comparable to the scratching behavior. Intriguingly, the long-lasting-type neurons had a significantly smaller fast afterhyperpolarization than that of the 5-HT-insensitive neurons. These observations suggest that the long-lasting-firing C neurons in rat DRG sensitive to 5-HT are responsible for conveying pruritic information to the spinal cord.

Dorsal horn neurons expressing NK-1 receptors mediate scratching in rats

Itch is thought to be signaled by pruritogen-responsive neurons in the superficial spinal dorsal horn. Many neurons here express the substance P NK-1 receptor. We investigated whether neurotoxic destruction of spinal NK-1-expressing neurons affected itch-related scratching behavior. Rats received intracisternal substance P conjugated to saporin (SP-SAP), or saporin (SAP) only (controls), and were subsequently tested for scratching behavior elicited by intradermal 5-hydroxytryptamine. SAP controls exhibited dose-related hindlimb scratching, which was significantly attenuated in SP-SAP-treated rats. There was a virtual absence of NK-1 immunoreactive neurons in superficial laminae of the upper cervical and medullary dorsal horn in SP-SAP-treated rats. These results indicate that superficial dorsal horn neurons expressing NK-1 receptors play a key role in spinal itch transmission.

Nalfurafine prevents 5'-guanidinonaltrindole- and compound 48/80-induced spinal c-fos expression and attenuates 5'-guanidinonaltrindole-elicited scratching behavior in mice

The aims of the present study were to establish if nalfurafine, a kappa opioid agonist, inhibits compulsive scratching in mice elicited by the s.c. administration (behind the neck) of 5'-guanidinonaltrindole (GNTI), a kappa opioid antagonist; to assess if nalfurafine prevents c-fos expression provoked by GNTI or compound 48/80, two chemically diverse pruritogens; and to distinguish on the basis of neuroanatomy, those neurons in the brainstem activated by either GNTI-induced itch or formalin-induced pain (both compounds given s.c. to the right cheek). Pretreatment of mice with nalfurafine (0.001-0.03 mg/kg s.c.) attenuated GNTI (0.3 mg/kg)-evoked scratching dose-dependently. A standard antiscratch dose of nalfurafine (0.02 mg/kg) had no marked effect on the spontaneous locomotion of mice. Tolerance did not develop to the antiscratch activity of nalfurafine. Both GNTI and compound 48/80 provoked c-fos expression on the lateral side of the superficial layer of the dorsal horn of the cervical spinal cord and pretreating mice with nalfurafine inhibited c-fos expression induced by both pruritogens. In contrast to formalin, GNTI did not induce c-fos expression in the trigeminal nucleus suggesting that pain and itch sensations are projected differently along the sensory trigeminal pathway. Our data indicate that the kappa opioid system is involved, at least in part, in the pathogenesis of itch; and that nalfurafine attenuates excessive scratching and prevents scratch-induced neuronal activity at the spinal level. On the basis of our results, nalfurafine holds promise as a potentially useful antipruritic in human conditions involving itch.

Itch-related responses of dorsal horn neurons to cutaneous allergic stimulation in mice

This study was conducted to identify the mechanosensitive dorsal horn neurons involved in allergic itch. We examined 98 units responsive to cutaneous allergy; 90 showed only immediate responses, which subsided before the onset of itch-related behavior and eight showed immediate and sustained responses, the latter of which was similar in duration to itch-related behavior, suggesting the involvement of sustained units in itch signaling. Sustained units were localized in the superficial, but not deep, layers of the dorsal horn. They were wide dynamic range or nociceptive specific, but not low threshold and four of eight were noxious heat sensitive. The results suggest that a small minority of neurons in the superficial dorsal horn are involved in allergic itch signals.

Scratching behavior and Fos expression in superficial dorsal horn elicited by protease-activated receptor agonists and other itch mediators in mice

Protease-activated receptor (PAR)-2 and PAR-4 are implicated in nonhistaminergic itch. We investigated dose dependence, tachyphylaxis, and cross-tachyphylaxis of itch-associated scratching elicited by intradermal injections of PAR-2 and PAR-4 agonists, serotonin (5-hydroxytryptamine, 5-HT), and histamine in ICR mice, as well as mu-opioid modulation of PAR-2 agonist-evoked scratching. Each agent elicited dose-related increases in scratch bouts. Scratching elicited by the PAR-4 agonist and histamine both exhibited significant tachyphylaxis but no cross-tachyphylaxis with each other. Scratching evoked by 5-HT did not exhibit significant tachyphylaxis but did exhibit significant cross-tachyphylaxis to scratching evoked by the PAR-2 and PAR-4 agonists and histamine. Naltrexone and high-dose morphine (10 mg/kg) attenuated PAR-2 agonist-evoked scratching, whereas lower dose morphine (1 mg/kg) had no effect. High-dose morphine also significantly increased circling behavior, which may have interfered with scratching. The PAR-2 agonist and 5-HT produced overlapping distributions of Fos-like immunoreactivity in the superficial dorsal horn. These results indicate that PAR-2 and PAR-4 agonists, histamine, and 5-HT elicit itch-related scratching and activate superficial dorsal horn neurons that may participate in scratch reflex and ascending itch signaling pathways.

Intracisternal, but not intrathecal, injection of naloxone inhibits cutaneous itch-related response in mice

The present study was conducted to determine whether cutaneous itch involves mu-opioid receptors in either of the spinal cord or lower brainstem or in both regions in mice. An intraplantar injection of serotonin hydrochloride (100 nmol/site) induced biting, an itch-related behavior. The behavior was inhibited by subcutaneous (0.3-1 mg/kg) and intracisternal (1--10 nmol/site), but not intrathecal (1--10 nmol/site), injections of naloxone hydrochloride. An intradermal injection of serotonin (100 nmol/site) to the rostral back induced scratching, an itch-related behavior, which was inhibited by subcutaneous (1 mg/kg) and intracisternal (10 nmol/site) injections of naloxone. These results suggest that mu-opioid receptor in the lower brainstem, but not spinal cord, is a site of central pruritogenic action of opioids and is involved in the facilitatory regulation of itch signaling.

A unique therapeutic approach to emesis and itch with a proanthocyanidin-rich genonutrient

Background

We examined the therapeutic potential of a proprietary Croton palanostigma extract (Zangrado^®) in the management of emesis and itch.

Methods

Emesis was induced in ferrets with morphine-6-glucuronide (0.05 mg/kg sc) in the presence of Zangrado (3 mg/kg, ip) and the cannabinoid receptor 1 antagonist, AM 251 (5 mg/kg, ip). Topical Zangrado (1%) was assessed for anti-pruretic actions in the 5-HT-induced scratching model in rats and evaluated in capsaicin-induced gastric hyperemia as measured by laser doppler flow. In the Apc^Minmouse model of precancerous adenomatosis polyposis, mice received Zangrado (100 μg/ml in drinking water) from the age of 6 – 16 weeks for effects on polyp number. In RAW 264.7 cells Zangrado was examined for effects on lipopolysaccharide-induced nitrite production.

Results

Zangrado was a highly effective anti-emetic, reducing morphine-induced vomiting and retching by 77%. These benefits were not associated with sedation or hypothermia and were not reversed by cannabinoid receptor antagonism. Itch responses were blocked in both the morphine and 5-HT models. Zangrado did not exacerbate the Apc^Mincondition rather health was improved. Capsaicin-induced hyperemia was blocked by Zangrado, which also attenuated the production of nitric oxide by activated macrophages.

Conclusion

Zangrado is an effective anti-emetic and anti-itch therapy that is devoid of common side-effects, cannabinoid-independent and broadly suppresses sensory afferent nerve activation. This complementary medicine represents a promising new approach to the management of nausea, itch and irritable bowel syndrome.

The skin as a mirror of the soul: exploring the possible roles of serotonin

Serotonin (5-hydroxytryptamine; 5-HT) is an important mediator of bidirectional interactions between the neuroendocrine system and the skin. The rate of synthesis of 5-HT from l-tryptophan can be enhanced by brain-derived neuronal growth factor, cytokines, exposure to ultraviolet light and steroids. The major source of 5-HT in the skin are platelets, which, upon aggregation, release this biogenic amine. Moreover, the epidermal and dermal skin express the enzymes required for the transformation of tryptophan to 5-HT, and certain skin cells, such as melanocytes, have been demonstrated to produce 5-HT. In addition, rodent mast cells produce 5-HT, but human mast cells have not yet been fully examined in this respect. Skin cells express functionally active, membrane-bound receptors for 5-HT, as well as proteins that transport 5-HT. The interactions of 5-HT with these various proteins determines the nature, magnitude and duration of serotonergic responses. The immune and vasculature systems in the skin are traditional targets for bioregulation by 5-HT. Moreover, recent findings indicate that keratinocytes, melanocytes and dermal fibroblasts also respond to this amine in various ways. Thus, mammalian skin is both a site for the production of and a target for bioregulation by 5-HT. This indicates that agonists and antagonists directed towards specific 5-HT receptors could be useful in connection with treatment of skin diseases. Based on our increasing knowledge concerning these receptors and their plasticity, future research will focus on the development of serotonergic drugs that exert metabotrophic effects on the cells of the skin without affecting the central nervous system.

Neuronal conditions of spinal cord in dermatitis are improved by olopatadine

Intense pruritus and cutaneous reactivity represent cardinal features of eczema. The resulting scratching behaviors alter neuronal conditions of the spinal dorsal horn where the primary sensory afferent fibers transmit cutaneous stimulation and deteriorate eczematous skin lesions. We investigated the effects of olopatadine hydrochloride (olopatadine) on alteration of neuronal conditions of the spinal dorsal horn and eczematous skin lesions induced by contact dermatitis. Eczematous lesions were induced by repeated application of diphenylcyclopropenone (DCP) in BALB/c mice. Olopatadine suppressed scratching behavior caused by repeated application of DCP in mice. Increased expressions of c-Fos and substance P in the spinal dorsal horn following DCP application were improved by olopatadine. Furthermore, olopatadine diminished the number of infiltrating cells and levels of cytokines in eczematous skin lesions resulting from DCP application. Olopatadine improves neurological conditions in the spinal cord and eczematous skin lesions in a murine contact dermatitis model.

Bradykinin is a potent pruritogen in atopic dermatitis: a switch from pain to itch

Histamine, substance P, serotonin and bradykinin were applied by iontophoresis to lesional and visually non-lesional skin of 14 patients with atopic dermatitis, and normal skin of 15 healthy volunteers. Itch could be evoked by light stroking of skin with a cotton swab (alloknesis) in all lesional skin sites, but not in non-lesional or normal skin. Substances were applied in the same skin area before and 3 h after administration of placebo or antihistamine (olopatadine hydrochloride: H1-receptor-blocker). Intensities of itch and pain sensation and areas of flare and wheal were measured. All the substances induced significantly more intense itch in lesional skin than in non-lesional skin of patients. Even bradykinin, which evoked only weak itch and pain of similar intensities in non-lesional skin of patients and in healthy volunteers, induced intense itch in lesional skin, while the simultaneously increased pain did not suppress the itch sensation, indicating central sensitization. Histamine- and substance P-induced itch was almost completely suppressed by antihistamines, whereas bradykinin- and serotonin-induced itch was not. This suggests that substance P is a histamine-dependent pruritogen also in lesional skin under sensitized conditions but that bradykinin and serotonin are histamine-independent pruritogens in lesional skin. It is concluded that serotonin and bradykinin, classic endogenous algogens, can turn into potent histamine-independent pruritogens in lesional skin of atopic dermatitis.

The neurobiology of itch

The neurobiology of itch, which is formally known as pruritus, and its interaction with pain have been illustrated by the complexity of specific mediators, itch-related neuronal pathways and the central processing of itch. Scratch-induced pain can abolish itch, and analgesic opioids can generate itch, which indicates an antagonistic interaction. However, recent data suggest that there is a broad overlap between pain- and itch-related peripheral mediators and/or receptors, and there are astonishingly similar mechanisms of neuronal sensitization in the PNS and the CNS. The antagonistic interaction between pain and itch is already exploited in pruritus therapy, and current research concentrates on the identification of common targets for future analgesic and antipruritic therapy.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Spinal c-fos expression associated with spontaneous biting in a mouse model of dry skin pruritus

As a model of dry skin pruritus in mice, one hind paw was treated twice daily with a mixture of acetone/diethylether/water (AEW); controls received water only. A protective collar prevented the animals from accessing the treatment area. At 16 days, the collar was removed and AEW-treated mice exhibited marked biting of the treated paw; the number and cumulative duration of bites was significantly greater than in controls. After 3 additional treatment days (collars intact), animals were perfused for c-fos immunohistochemistry. There was significantly more fos-like immunoreactivity in the ipsilateral lumbar spinal cord of AEW-treated animals, with the majority in superficial laminae. It is proposed that biting of the dry skin reflects pruritus, and that neurons predominantly in superficial laminae of the dorsal horn may signal itch sensation.