Abstract. The common adverse effect of centrally-injected µ-opioid receptor (µ-OR) agonists is pruritus. This study was conducted using mice to examine whether different subtypes of µ-OR would be responsible for pruritus and analgesia. Intracisternal injections of morphine and morphine-6β-glucronide (M6G), but not M3G, produced an antinociceptive effect. Morphine, but neither M6G nor M3G, induced facial scratching, a pruritus-related response. Facial scratching following morphine was not affected by the µ 1 -OR antagonist naloxonazine at doses that inhibited the antinociceptive effects. The results suggest that different subtype and/ or splice variants of µ-OR are separately involved in pruritus and antinociception of opioids.

Itch, a skin sensation that provokes the desire to scratch, accompanies various skin diseases such as atopic dermatitis. Although H 1 histamine-receptor antagonists are the drugs of first choice for the treatment of itch, many pruritic diseases except acute urticaria do not show a good response.1) Therefore, it is important to develop new drugs that are effective against antihistamine-resistant pruritus.The fruiting body of Ganoderma lucidum (G. lucidum) KARST has been used as a crude drug in China, Japan, and Korea for the treatment of hypertension, chronic hepatitis, hyperglycemia, cancer, and chronic bronchitis.2,3) The extract of G. lucidum has been shown to have an antiallergic activity, which is mainly due to its inhibitory effect on the histamine release from mast cells. 4,5) Many antiallergic drugs generally show antipruritic activity, 6) but the antipruritic activity of G. lucidum has not yet been elucidated.Scratching, an itch-related response, can be elicited by intradermal injections of various substances, including histamine, 7) substance P, 8,9) 5-hydroxytryptamine (5-HT), 10) proteinase-activated receptor-2 (PAR 2 )-activating peptide, 11) and compound 48/80. 8,12) Scratching induced by these substances may not be mediated by the same primary afferents. 9,[13][14][15] Therefore, in the present study, we examined whether G. lucidum can ameliorate the scratching elicited by the pruritogens mentioned above in order to determine the antipruritic efficacy of G. lucidum and its possible mechanisms. MATERIALS AND METHODS AnimalsMale ICR mice (Japan SLC, Ltd., Shizuoka, Japan) aged 5-8 weeks and weighing 24-26 g were used. They were housed under controlled temperature (23°CϮ 1°C), humidity (60%Ϯ5%), and light (lights on from 8:00-20:00 h). Food and water were freely available. Procedures for animal experiments were approved by the Committee for Animal Experiments at the University of Toyama and were conducted in accordance with the guidelines of the Japanese Pharmacological Society.Agents 5-HT, a-methyl-5-HT, and compound 48/80were purchased from Sigma (St. Louis, MO, U.S.A.). Substance P and histamine were purchased from Peptide Institute, Inc. (Osaka, Japan) and Wako Pure Chemical Industries, Ltd. (Osaka, Japan), respectively. Ser-Leu-Ile-Gly-Arg-Leu-NH 2 (SLIGRL-NH 2 ) was synthesized and identified by JBL with a peptide synthesizer PSSM-8 (Shimadzu Co., Kyoto, Japan) and a matrix assisted laser desorption/ionization timeof-flight (MALDI TOF)-MS Autoflex T1 (Bruker Daltonics, Billerica, MA, U.S.A.), respectively. These agents were dissolved in physiological saline and injected intradermally in a volume of 50 ml into the rostral skin of mice, the hair of which was clipped a day before the experiment was conducted. Preparation of the Methanol Extract of G. lucidum (MEGL)The dried powder (1 kg) of G. lucidum (Koshiro Co., Ltd., Kyoto, Japan) was extracted three times with methanol for 3 h and the methanol extract was then dried; the yield of MEGL was 17.8%. MEGL was suspended in 5% gum arabic before use a...

Mu-opioid receptor antagonists have been claimed to produce inhibitory effects on experimentally induced itch in normal subjects 1) and pruritus in patients with dermatoses and visceral diseases. [2][3][4] In animal experiments, mu-opioid receptor antagonists suppress itch-related behaviors in several kinds of itch models. [5][6][7][8][9][10] The mu-opioid receptor antagonist naltrexone suppresses itch-related behaviors without effects on the increased activity of primary afferents, suggesting that it inhibits itch through central action. 6)There are at least two possible sites of pruritogenic action of opioids, the spinal dorsal horn and the lower brainstem. An intradermal injection of histamine induces hyperalgesia (enhanced prickling pain) and hyperknesis (enhanced pricking-evoked itch), and intradermal pretreatment with the local anesthetic chloroprocaine produces the inhibition of pain and the enhancement of itch.11) In contrast, itch sensation is generally reduced by pain produced by scratching. Therefore, it was suggested that itch produced by intrathecal and epidural injection of mu-opioid receptor agonists in humans is due to the inhibition of pain input by opioids in the dorsal horn. 12)Regarding the lower brainstem, injections of mu-opioid receptor agonists into the cisterna magna and medullary dorsal horn evoked facial scratching in animals. [13][14][15][16] Since naltrexone inhibits scratching, but not Fos expression in the spinal dorsal horn, evoked by an intradermal injection of serotonin, it is suggested that mu-opioid receptor in the brain, rather than the spinal dorsal horn, is involved in itch.17) Thus, the present study was conducted to determine whether cutaneous itch involves mu-opioid receptors in either of the spinal dorsal horn or lower brainstem or in both regions.Serotonin is a potent pruritogen in mice. 18) Scratching behavior and increase in cutaneous nerve firing following an intradermal injection of serotonin are similar to each other in dose-response relationship and time-course, suggesting the peripheral pruritogenic action of serotonin.18) An intradermal injection of serotonin causes scratching through 5-HT 2 serotonin receptor in the skin.10) An injection of serotonin into the hind paw elicits biting and licking of the treated paw, and an injection of dilute formalin licking alone.5) Systemic injections of the opioid antagonist naloxone and the 5-HT 1/2 serotonin receptor antagonist methysergide suppress biting, but not licking, induced by serotonin.5) These findings suggest that serotonin-induced biting behavior is an itch-related response. Thus, in the present experiments, we compared the effects of intrathecal injection via a lumbar puncture and intracisternal injection of naloxone on serotonin-induced biting of the hind paw. We also tested whether intracisternal injection of naloxone would affect serotonin-induced scratching of the rostral back, another itch-related response. MATERIALS AND METHODS AnimalsMale ICR mice (Japan SLC, Shizuoka) of 5-7 weeks of age were used. The...

κ-Opioid receptor agonists with high selectivity over the μ-opioid receptor and peripheral selectivity are attractive targets in the development of drugs for pain. We have previously attempted to create novel analgesics with peripheral selective κ-opioid receptor agonist on the basis of TRK-820. In this study, we elucidated the biological properties of 17-hydroxy-cyclopropylmethyl and 10α-hydroxy derivatives. These compounds were found to have better κ-opioid receptor selectivity and peripheral selectivity than TRK-820.

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 (H 1 receptor antagonist with membrane-stabilizing activity) slightly reduced scratching. However, terfenadine (H 1 receptor antagonist), methysergide (serotonin (5-HT) 2 receptor antagonist), ondansetron (5-HT 3 receptor antagonist), proteinase-activated receptor 2-neutralizing antibody, fluvoxamine (selective serotonin reuptake inhibitor), milnacipran (serotonin-noradrenalin reuptake inhibitor), and cyproheptadine (H 1 and 5-HT 2 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.