Clonidine inhibits itch-related response through stimulation of α2-adrenoceptors in the spinal cord in mice

Differential Contribution of 5-HT4, 5-HT5, and 5-HT6 Receptors to Acute Pruriceptive Processing Induced by Chloroquine and Histamine in Mice

The involvement of serotonin (5-HT) and/or noradrenaline in acute pruriceptive processing in the central nervous system (CNS) has been reported using antidepressants, such as milnacipran, a serotonin and noradrenaline reuptake inhibitor, and mirtazapine, a noradrenergic and specific serotonergic antidepressant; however, the roles of 5-HT receptor family in acute pruriceptive processing have not been fully elucidated in the CNS. In the present study, scratching behavior induced by chloroquine (CQ) was ameliorated by milnacipran or mirtazapine, and these effects were reversed by SB207266, a 5-HT4 antagonist, or SB258585, a 5-HT6 antagonist, but not by SB258585, a 5-HT5 antagonist. Moreover, CQ-induced scratches were mitigated by intrathecal injection of 5-HT4 agonists, such as BIMU8 and ML10302, and the 5-HT6 agonist, WAY208466. Conversely, histamine-induced scratches were not affected by the 5-HT4 agonists or a 5-HT6 agonist. Similarly, the amelioration of histamine-induced scratches by these antidepressants was not reversed by the 5-HT4, 5-HT5, or 5-HT6 receptor antagonist. Therefore, 5-HT is involved in the amelioration of CQ-induced scratches by milnacipran and mirtazapine, and 5-HT4, 5-HT5, and 5-HT6 receptors play differential roles in acute pruriceptive processing after administration of CQ or histamine.

Roles of 5-HT3 and 5-HT7 receptors in acute pruriceptive processing in mice

The roles of serotonin (5-HT) and/or noradrenaline in acute pruriceptive processing have been demonstrated using antidepressants, such as milnacipran, a serotonin and noradrenaline reuptake inhibitor, and mirtazapine, a noradrenergic and specific serotonergic antidepressant; however, the involvement of 5-HT in acute pruriceptive processing has not yet been elucidated in detail. Scratching events induced by chloroquine (CQ) were attenuated by the administration of milnacipran or mirtazapine, and these effects were reversed by a treatment with ondansetron, a 5-HT₃ antagonist, or SB26970, a 5-HT₇ antagonist. CQ-induced scratching events were also ameliorated by the intrathecal administration of 5-HT, SR572227A and RS56812 (5-HT₃ agonists), and LP211 and LP44 (5-HT₇ agonists), indicating the modulation of CQ-induced scratching events by 5-HT and noradrenaline. By contrast, histamine-induced scratching events were not markedly affected by the administration of 5-HT and 5-HT₇ agonists, whereas 5-HT₃ agonists exerted attenuating effects. Similarly, they were not clearly reversed by the administration of the 5-HT₇ antagonist, unlike a 5-HT₃ antagonist. Therefore, 5-HT is involved in the attenuating effects of milnacipran and mirtazapine on CQ- and histamine-induced scratching events, and 5-HT₃ and 5-HT₇ receptors play different roles in pruriceptive processing induced by histamine or CQ.

Pharmacological Characterization of a Novel Mouse Model of Cholestatic Pruritus

Patients with cholestatic liver diseases, such as primary biliary cirrhosis, usually suffer from pruritus. However, the pathogenesis of cholestatic pruritus is unclear, and there is no current effective treatment for it. In order to find a treatment for the condition, an appropriate mouse model should be developed. Therefore, here, we established a surgically-induced mouse model of cholestatic pruritus. The bile duct was ligated in order to block bile secretion from the anterior, right, and left lobes, with the exception of the caudate lobe. Serum levels of total bile acid increased after bile duct ligation (BDL). The spontaneous hind paw scratching was also increased in BDL mice. Spontaneous scratching was reduced in BDL mice by naloxone (µ-opioid receptor antagonist), U-50,488H (κ-opioid receptor agonist), and clonidine (α2-adrenoceptor agonist). Azelastine (H₁ receptor antagonist with membrane-stabilizing activity) slightly reduced scratching. However, terfenadine (H₁ receptor antagonist), methysergide (serotonin (5-HT)₂ receptor antagonist), ondansetron (5-HT₃ receptor antagonist), proteinase-activated receptor 2-neutralizing antibody, fluvoxamine (selective serotonin reuptake inhibitor), milnacipran (serotonin-noradrenalin reuptake inhibitor), and cyproheptadine (H₁ and 5-HT₂ receptor antagonist) did not affect scratching. These results suggested that partial obstruction of bile secretion in mice induced anti-histamine-resistant itching and that central opioid system is involved in cholestatic itching.

Serotonin and noradrenaline modulate chronic itch processing in mice

The roles of serotonin and noradrenaline in the modulation of chronic pruriceptive processing currently remain unclear. To clarify the contribution of serotonin and noradrenaline to chronic itch, the effects of the administration of antidepressants or noradrenaline reuptake inhibitors were evaluated in the present study. A pretreatment with milnacipran, a serotonin and noradrenaline reuptake inhibitor, and mirtazapine, a noradrenergic and specific serotonergic antidepressant, attenuated the induction of spontaneous scratching behavior in mice with chronic itch. The administration of a serotonin reuptake inhibitor, such as fluvoxamine and paroxetine, but not escitalopram, or a noradrenaline reuptake inhibitor, such as atomoxetine and nisoxetine, ameliorated the induction of spontaneous scratching behavior in mice with chronic itch. Furthermore, this attenuation was reversed by the administration of yohimbine, a selective α₂-adrenoceptor antagonist, or methysergide, a non-selective serotonin receptor antagonist. These results suggest that elevated serotonin and noradrenaline levels are involved in the attenuation of scratching behavior induced by chronic itch, and serotonin receptors and an α₂-adrenoceptor play a crucial role in chronic pruriceptive processing.

Role of serotonin and noradrenaline in the acute itch processing in mice

The contribution of serotonin and noradrenaline to the modulation of pruriceptive processing was evaluated by administrating antidepressants or noradrenaline reuptake inhibitors. The pretreatment with milnacipran, a serotonin and noradrenaline reuptake inhibitor, and mirtazapine, a noradrenergic and specific serotonergic antidepressant, attenuated the induction of scratching behavior by chloroquine, a representative pruritogen, indicating the involvement of serotonin and/or noradrenaline in the modulation of pruriceptive processing. By contrast, the single administration of noradrenaline reuptake inhibitor such as atomoxetine and nisoxetine or serotonin reuptake inhibitor such as fluvoxamine and escitalopram had little effect on chloroquine-induced scratching, whereas the induction of scratching behavior by chloroquine was significantly ameliorated by co-administration of serotonin reuptake inhibitors and noradrenaline reuptake inhibitors. These results indicate that the simultaneous increases of serotonin and noradrenaline elicit the attenuating effect on pruriceptive processing induced by acute itch, and may also play a crucial role in the descending itch inhibitory system.

Peripheral gabapentin regulates mosquito allergy-induced itch in mice

The antipruritic activity of gabapentin, an anticonvulsant, was studied in a mouse model of allergic itch. In mice sensitized by an extract of the salivary glands of the mosquito (ESGM), an intradermal injection of ESGM elicited scratching and increased peripheral nerve firing. Oral or intradermal administration of gabapentin at the ESGM injection site inhibited ESGM-induced scratching and peripheral nerve firing. However, gabapentin did not affect histamine-induced scratching. The distributions of immunoreactivity to the voltage-dependent calcium channel α₂δ-1 subunit, a site of gabapentin action, and the histamine H₁ receptor differed in the mouse dorsal root ganglia. The α₂δ-1 subunit was mainly found in neurons that were 15-20 µm in diameter, whereas the H₁ receptor was mainly in 20-30 µm neurons. In addition, α₂δ-1 subunit immunoreactivity co-localized with that of transient receptor potential vanilloid 1 (TRPV1). These results suggest that gabapentin regulates allergic itch by acting on the calcium channel α₂δ-1 subunit in peripheral TRPV1-positive neurons.

Descending serotonergic and noradrenergic systems do not regulate the antipruritic effects of cannabinoids

BACKGROUND

For centuries, cannabinoids have been known to be effective in pain states. Itch and pain are two sensations sharing a lot in common.

OBJECTIVE

The goal of this research was to observe whether the cannabinoid agonist WIN 55,212-2 reduces serotonin-induced scratching behaviour and whether neurotoxic destruction of descending serotonergic and noradrenergic pathways mediate the antipruritic effect of WIN 55,212-2. Material and methods Scratching behaviour was induced by intradermal injection of serotonin (50 µg/50 µl/mouse) to Balb/c mice. The neurotoxins 5,7-dihydroxytryptamine (5,7-DHT, 50 μg/mouse) and 6-hydroxydopamine (6-OHDA, 20 μg/mouse) are applied intrathecally to deplete serotonin and noradrenaline in the spinal cord. WIN 55,212-2 (1, 3, 10 mg/kg, i.p.) dose-dependently attenuated serotonin-induced scratches. Neurotoxic destruction of neither the serotonergic nor the noradrenergic systems by 5,7-DHT and 6-OHDA, respectively, had any effect on the antipruritic action of WIN 55,212-2.

CONCLUSION

Our findings indicate that cannabinoids dose-dependently reduce serotonin-induced scratching behaviour and neurotoxic destruction of descending inhibitory pathways does not mediate this antipruritic effect.

Methods for preclinical assessment of antipruritic agents and itch mechanisms independent of mast-cell histamine

Itch is a sensation that provokes a desire to scratch. Mast-cell histamine was thought to be a key itch mediator. However, histamine and mast-cell degranulation were reported not to elicit scratching in animals. It was difficult to investigate the pathophysiology of itching and to evaluate the antipruritic efficacy of chemical agents in the early 1990 s. We showed that hind-paw scratching and biting were elicited by stimulation with pruritogenic agents in mice. Those results demonstrated for the first time that cutaneous itching could be evaluated behaviorally in animals. We established various animal models of pathological itch of the skin (dry skin, mosquito allergy, surfactant-induced pruritus, and herpes zoster) and mucus membranes (pollen allergy). Mast-cell histamine did not play a key role in itching in any animal model examined except for the pollen allergy model. Histamine is not an exclusive itch mediator of mast cells; tryptase and leukotriene B4 released from mast cells also act as itch mediators. Epidermal keratinocytes release several itch mediators, such as leukotriene B4, sphingosylphosphorylcholine, thromboxane A2, nociceptin, nitric oxide, and histamine, which may play important roles in pathological itching. Appropriate animal models of pathological itching are needed for pharmacological evaluation of the antipruritic efficacy of chemical agents.

Noradrenergic modulation of itch transmission in the spinal cord

Inhibition of both itching and scratching is important in the treatment of chronic pruritic diseases, because itching has a negative impact on quality of life and vigorous scratching worsens skin conditions. Pharmacological modulation of itch transmission in the dorsal horn is an effective way to inhibit both itching and scratching in pruritic diseases. Pruriceptive transmission in the spinal dorsal horn undergoes inhibitory modulation by the descending noradrenergic system. The noradrenergic inhibition is mediated by excitatory α₁-adrenoceptors located on inhibitory interneurons and inhibitory α₂-adrenoceptors located on central terminals of primary sensory neurons. The descending noradrenergic system and α-adrenoceptors in the dorsal horn are potential targets for antipruritic drugs.

Case report of atopic dermatitis with refractory pruritus markedly improved with the novel use of clonidine and trimeprazine

We report a 6-year-old boy with severe atopic dermatitis and refractory pruritus. The novel use of clonidine, an adrenergic agonist, along with trimeprazine, led to dramatic improvement. This represents the first case report of clonidine's effect in relieving pruritus in atopic dermatitis.

Milnacipran inhibits itch-related responses in mice through the enhancement of noradrenergic transmission in the spinal cord

We investigated whether milnacipran, a serotonin-noradrenaline reuptake inhibitor, exhibits an antipruritic effect through the spinal action in mice. Intrathecal injections of milnacipran (0.1 - 10 μg/site) significantly suppressed serotonin-induced biting, which is an itch-related response. However, such an effect was not observed with fluvoxamine (10 μg/site), which is a selective serotonin reuptake inhibitor. Furthermore, an intraperitoneal injection of milnacipran (10 mg/kg) inhibited serotonin-induced biting. When phentolamine (1.0 μg/site), a non-selective α-adrenoceptor antagonist, was intrathecally injected, it inhibited the above response of milnacipran. These results suggest that milnacipran suppresses itching through the inhibition of noradrenaline reuptake in the spinal cord.

Effects of acute stressors on itch- and pain-related behaviors in rats

Many acute stressors reduce pain, a phenomenon called stress-induced antinociception (SIA). Stress also is associated with increased scratching in chronic itch conditions. We investigated effects of acute stressors on facial itch and pain using a recently introduced rat model. Under baseline (no-swim) conditions, intradermal (id) cheek microinjection of the pruritogen serotonin (5-HT) selectively elicited hindlimb scratch bouts, whereas the algogen mustard oil (allyl isothiocyanate [AITC]) selectively elicited ipsilateral forepaw swipes, directed to the cheek injection site. To test effects of swim stress, rats received id cheek microinjection of 5-HT (1%), AITC (10%), or vehicle, and were then subjected to one of the following swim conditions: (1) weak SIA (W-SIA), (2) naltrexone-sensitive SIA (intermediate or I-SIA), or (3) naltrexone-insensitive SIA (strong or S-SIA). After the swim, we recorded the number of hindlimb scratch bouts and forelimb swipes directed to the cheek injection site, as well as facial grooming by both forepaws. Under S-SIA, AITC-evoked swiping and 5-HT-evoked scratching were both reduced. I-SIA reduced AITC-evoked swiping with no effect on 5-HT-evoked scratching. Facial grooming immediately post-swim was suppressed by S-SIA, but not I- or W-SIA. W-SIA tended to equalize scratching and swiping elicited by 5-HT and AITC compared with no-swim controls, suggesting altered itch and pain processing. Exercise (wheel-running), novelty, cold exposure, and fear (shaker table), key components of swim stress, differentially affected tail-flick latencies and 5-HT-evoked swiping and scratching behavior. Thus, itch and pain can be simultaneously suppressed by a combination of acute stress-related factors via an opioid-independent mechanism.

Tonic inhibition of allergic itch signaling by the descending noradrenergic system in mice

We investigated whether the descending noradrenergic system regulates allergic itch. Mosquito allergy of the hind paw elicited biting, an itch-related response, in sensitized mice. The biting was inhibited by intrathecal clonidine and reversed by yohimbine, an α(2)-adrenoceptor antagonist. The biting was increased by intrathecal pretreatment with the catecholaminergic neurotoxin 6-hydroxydopamine and the α-adrenoceptor antagonist phentolamine but not the serotonergic neurotoxin 5,7-dihydroxytryptamine. We propose that α(2)-adrenoceptors are involved in the inhibition of allergic itch in the spinal cord and that the descending noradrenergic system exerts a tonic inhibition on the itch signaling. The serotonergic system may not be involved.