Intracisternal injection of opioids induces itch-associated response through mu-opioid receptors in mice

Effects of Intrathecal κ-Opioid Receptor Agonist on Morphine-Induced Itch and Antinociception in Mice

The μ-opioid receptor (MOR) agonist-induced itch is a significant issue associated with analgesic therapies. Research suggested that systemically administered κ-opioid receptor (KOR) agonists inhibit intrathecal morphine-induced itch in primates. However, serious adverse effects induced by systemically administered KOR agonists may restrict their usefulness in humans. We investigated the effects of intrathecal KOR agonists on intrathecal morphine-mediated itch and antinociception in mice.Mice received intrathecal injections of one of the following drugs: morphine (0.1-1.0 nmol), the selective KOR agonist TRK-820 100 pmol, the combination of morphine 0.3 nmol + TRK-820 (10-100 pmol), and 5 μL of saline. One hour after intraperitoneal administration of the selective KOR antagonist nor-binaltorphimine 1.0 μmol, the effect of TRK-820 100 pmol on intrathecal morphine 0.3 nmol-induced scratching was tested. Total numbers of scratches after intrathecal injection were analyzed. After observing scratching behavior, sedation level was evaluated subjectively. Nociceptive threshold was determined by tail immersion test with intrathecal injections of the following agents: morphine (0.1-1.0 nmol), TRK-820 (10-100 pmol), morphine 0.1 nmol + TRK-820 10 pmol, and 5 μL of saline.Intrathecal TRK-820 dose-dependently inhibited intrathecal morphine-induced scratching compared with that in the saline group. Intraperitoneal nor-binaltorphimine completely inhibited the antiscratching effect of intrathecal TRK-820 100 pmol. The combination of morphine 0.3 nmol and TRK-820 did not alter the sedation score compared with that in the morphine 0.3 nmol group. Morphine 0.1 nmol + TRK-820 10 pmol significantly produced greater thermal antinociceptive effects than morphine 0.1 nmol.We demonstrated that intrathecal KOR agonists exert antipruritic effects on intrathecal morphine-induced itch without affecting sedation. The combination of intrathecal morphine and intrathecal KOR agonists produces more potent antinociceptive effects against a thermal stimulus compared with morphine alone.

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.

Opioids in preclinical and clinical trials

Since 1952, when Gates determined the stereo structure of morphine, numerous groups have focused on discovering a nonnarcotic opioid drug. Although several natural, semisynthetic, and synthetic opioid ligands (alkaloids and peptides) have been developed in clinical studies, very few were nonnarcotic opioid drugs. One of the most important studies in the opioid field appeared in 1976, when Martin and colleagues established types of opioid receptors (these are now classified into mu, delta, and kappa types). Later, Portoghese discovered a highly selective mu type opioid receptor antagonist, beta-funaltrexamine. This led to the finding that the mu type opioid receptor was correlated to drug dependence. Consequently, delta, and particularly kappa, opioid agonists were expected to lead to ideal opioid drugs. Moreover, opioid antagonists were evaluated for the treatment of symptoms related to undesirable opioid system activation. In this chapter, we provide a short survey of opioid ligands in development and describe the discovery of the two most promising drugs, TRK-851 and TRK-820 (nalfurafine hydrochloride).

Determination Of The Efficacy And Side-effect Profile Of Lower Doses Of Intrathecal Morphine In Patients Undergoing Total Knee Arthroplasty

BACKGROUND

Intrathecal (IT) morphine provides excellent post-operative analgesia, but causes multiple side effects including nausea and vomiting (PONV), pruritus and respiratory depression, particularly at higher doses. The lowest effective dose of spinal morphine in patients undergoing total knee arthroplasty is not known.

METHODS

We evaluated the analgesic efficacy and side effect profile of 100 - 300 μg IT morphine in patients undergoing elective total knee replacement in this prospective, randomized, controlled, double-blind study. Sixty patients over the age of 60 undergoing elective knee arthroplasty were enrolled. Patients were randomized to receive spinal anaesthesia with 15 mg Bupivacaine and IT morphine in three groups: (i) 100 μg; (ii) 200 μg; and (iii) 300 μg.

RESULTS

Both 200 μg and 300 μg IT morphine provided comparable levels of postoperative analgesia. However, patients that received 100 μg had greater pain postoperatively, with higher pain scores and a greater requirement for supplemental morphine. There were no differences between groups with regard to PONV, pruritus, sedation, respiratory depression or urinary retention.

CONCLUSION

Both 200 μg and 300 μg provided comparable postoperative analgesia, which was superior to that provided by 100 mug IT morphine in patients undergoing total knee arthroplasty. Based on these findings, we recommend that 200 μg IT morphine be used in these patients.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier NCT00695045.

Deletion of μ- and κ-Opioid Receptors in Mice Changes Epidermal Hypertrophy, Density of Peripheral Nerve Endings, and Itch Behavior

The mu- (MOR) and kappa- (KOR) opioid receptors have been implicated in the regulation of homeostasis of non-neuronal cells, such as keratinocytes, and sensations like pain and chronic pruritus. Therefore, we have studied the phenotype of skin after deletion of MOR and KOR. In addition, we applied a dry skin model in these knockout mice and compared the different mice before and after induction of the dermatitis in terms of epidermal thickness, epidermal peripheral nerve ending distribution, dermal inflammatory infiltrate (mast cells, CD4 positive lymphocytes), and scratching behavior. MOR knockout mice reveal as phenotype a significantly thinner epidermis and a higher density of epidermal fiber staining by protein gene product 9.5 than the wild-type counterparts. Epidermal hypertrophy, induced by the dry skin dermatitis, was significantly less developed in MOR knockout than in wild-type mice. Neither mast cells nor CD4 T(h)-lymphocytes are involved in the changes of epidermal nerve endings and epidermal homeostasis. Finally, behavior experiments revealed that MOR and KOR knockout mice scratch less after induction of dry skin dermatitis than wild-type mice. These results indicate that MOR and KOR are important in skin homeostasis, epidermal nerve fiber regulation, and pathophysiology of itching.

Pharmacological characterization of a chronic pruritus model induced by multiple application of 2,4,6-trinitrochlorobenzene in NC mice

Female NC/Jic mice were sensitized and challenged repeatedly at 48 h intervals for 10 and 30 days by painting 1% 2,4,6-trinitrochlorobenzene (TNCB) on both ears. Mice challenged with TNCB for 30 days developed an inflammatory dermatitis with high immunoglobulin E (IgE) titer. Histological analysis with acidic Toluidine Blue staining revealed that dermal mast cells markedly differentiated and intensely degranulated, consistent with a dramatic increase in scratching behavior. A significant increase in total scratching events could be observed in mice treated with TNCB for a short period of 10 days. Extending the term of TNCB application to 30 days, the IgE titer and number of mast cells elevated significantly, and thus various drugs were evaluated pharmacologically by using the mice treated with TNCB for 30 days. Terfenadine and cyproheptadine attenuated the chronic scratching behavior. Tacrolimus and dexamethasone were less effective and cromolyn showed no effect. In addition, terfenadine and tacrolimus suppressed the degranulation of mast cells. The present chronic scratching model could be suitable to evaluate drugs effective for suppression of mast cell differentiation and degranulation by irritation, and may represent a promising tool to develop new drugs for inflammatory pruritus associated with, for example, atopic dermatitis.

Involvement of central mu-opioid system in the scratching behavior in mice, and the suppression of it by the activation of kappa-opioid system

The role of central mu- and kappa-opioid receptors in the regulation of itch sensation was examined using pruritogen-induced mouse scratching behavior model. Intracerebroventricular administration of beta-funaltrexamine, a selective mu-opioid receptor antagonist, inhibited the scratching behavior induced by intradermal substance P, but subcutaneous administration of beta-funaltrexamine did not. Similarly, the scratching inhibitory activity of subcutaneously administered TRK-820, (-)-17-(cyclopropylmethyl)-3, 14beta-dihydroxy-4, 5alpha-epoxy-6beta-[N-methyl-trans-3-(3-furyl) acrylamido] morphinan hydrochloride, a kappa-opioid receptor agonist, was antagonized by intracerebroventricular administration of nor-binaltorphimine (10 microg/site), a kappa-opioid receptor antagonist, but was not by subcutaneous administration of nor-binaltorphimine. In addition, the scratching induced by the direct activation of central mu-opioid receptor by intracisternal morphine was significantly and dose-dependently inhibited by subcutaneous administration of TRK-820. Taken all together, it is suggested that the central mu-opioid receptors play a role in the processing of itch sensation, and the activation of central kappa-opioid receptors antagonize the central mu-opioid receptor mediated itch processing, thereby suppressing itch sensation.

Characterization of scratching responses in rats following centrally administered morphine or bombesin

The aim of this study was to characterize scratching behavior elicited by central administration of morphine or bombesin in rats, and to determine the role of opioid receptors in scratching induced by both pruritogenic agents. Central administration included intracisternal (i.c.), intrathecal (i.t.), and intracerebroventricular (i.c.v.) routes. Scratching events made with hind paws were counted by observers blinded to treatment conditions. Intracisternal morphine (0.01-0.1 microg) produced dose-dependent increases in scratching; the maximum response to i.c. morphine 0.1 microg was approximately 500 scratches within a 1-hour period. Neither i.t. nor i.c.v. morphine significantly increased scratching. Bombesin (0.01-0.32 microg) elicited robust scratching following i.c. administration. The maximum response to i.c. bombesin 0.32 microg was approximately 4000 scratches within a 1-hour period. Both i.t. and i.c.v. bombesin produced profound scratching at similar doses. Antagonist studies confirmed that mu-opioid receptors selectively mediate i.c. morphine-induced scratching. However, selective mu-, kappa-, and delta-opioid antagonists did not attenuate i.c. bombesin-induced scratching. These results demonstrate that morphine and bombesin elicit scratching through different receptor mechanisms, at different central sites, and to different degrees.

Optimizing the dose of intrathecal morphine in older patients undergoing hip arthroplasty

Intrathecal (IT) morphine provides excellent postoperative analgesia but may result in many side effects, including postoperative nausea and vomiting, pruritus, and respiratory depression, particularly at larger doses. Older patients may be at particular risk. The optimal dose of spinal morphine in older patients undergoing hip arthroplasty is not known. We designed this prospective, randomized, controlled, double-blinded study to evaluate the analgesic efficacy and side effect profile of 50-200 microg of IT morphine in older patients undergoing elective hip arthroplasty. Sixty patients older than 65 years undergoing elective hip arthroplasty were enrolled. Patients were randomized to receive spinal anesthesia with 15 mg of bupivacaine and IT morphine in four groups: 1). 0 microg, 2). 50 microg, 3). 100 microg, and 4). 200 microg. IT morphine 100 and 200 microg produced effective pain relief and decreased the postoperative requirement for morphine compared with control. IT morphine 50 microg did not provide effective pain relief. Both 100 and 200 microg of IT morphine provided comparable levels of postoperative analgesia. There were no between-group differences in postoperative nausea and vomiting, sedation, respiratory depression, or urinary retention. Pruritus was significantly more frequent with 200 microg of IT morphine. In conclusion, 100 microg of IT morphine provided the best balance between analgesic efficacy and side effect profile in older patients undergoing hip arthroplasty.

IMPLICATIONS

The dosage of intrathecal morphine that provides the best balance between analgesic efficacy and side effect profile in the older patient undergoing hip arthroplasty is not known. This prospective, randomized, controlled, double-blinded clinical trial demonstrates that a dose of 100 microg of intrathecal morphine provides the best balance between efficacy and side effects, compared with doses of 0, 50, and 200 microg of morphine, in this patient population.

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

We investigated a spinal site for opioid modulation of itch-related scratching behavior in rats. Intradermal 5-HT (2%, 10 microl) elicited intermittent bouts of hindlimb scratching directed toward the injection site (nape of neck) beginning within minutes and lasting >1 hr. 5-HT-evoked scratching was significantly reduced by systemic administration of the opiate antagonist naltrexone but was not affected by systemic morphine at a dosage (3 mg/kg) that induces analgesia. Intradermal 5-HT elicited a significant increase in c-fos-like immunoreactivity (FLI) in superficial laminas I-III at the lateral aspect of the cervical C3-C6 dorsal horn compared with controls receiving intradermal saline. Neither systemic morphine nor naltrexone significantly affected counts of 5-HT-evoked FLI. The lack of effect of morphine suggests that intradermal 5-HT activates dorsal horn neurons, signaling itch but not pain. Attenuation of 5-HT-evoked scratching but not spinal FLI by naltrexone suggests a supraspinal site for its antipruritic action. In contrast, morphine significantly attenuated FLI elicited by intradermal capsaicin, a chemical that induces pain but not scratching.

Quantitative assessment of directed hind limb scratching behavior as a rodent itch model

Hind limb scratching is used increasingly as an itch model in rodents. Scratching is usually quantified as the number of scratching bouts over a 60 min period. Since the antipruritic effect of scratching should depend on the total time of skin contact, then the duration of scratching bouts and within-bout scratching frequency may also be important factors. Therefore, we measured these parameters during episodes of scratching directed toward the site (nape of neck) of intradermal injection of serotonin in Sprague-Dawley rats. Serotonin elicited significantly more scratching bouts than saline. There was a biphasic pattern of scratching over time, with peaks at 10-20 and 40-50 min. Although cumulative bout duration (2-min intervals) had a similar biphasic distribution, the mean individual bout duration (2.1 s) did not change significantly over time. Within-bout scratching frequency remained constant over time at 8 Hz. The number of scratching bouts was suppressed in a dose dependent manner by naltrexone (3 and 5 mg/kg), while the individual bout duration and the within-bout frequency were not significantly different compared with serotonin-evoked scratching without naltrexone. These results validate the total number of scratching bouts as an indicator of the magnitude of itch-related scratching.

Activation of kappa-opioid receptors inhibits pruritus evoked by subcutaneous or intrathecal administration of morphine in monkeys

Pruritus (itch sensation) is the most common side effect associated with spinal administration of morphine given to humans for analgesia. A variety of agents have been proposed as antipruritics with poorly understood mechanisms and they are effective with variable success. kappa-Opioid agonists possess several actions that are opposite to micro -opioid agonists. We proposed to investigate the role of kappa-opioid receptors (KORs) in morphine-induced scratching and antinociception in monkeys. Scratching responses were counted by observers blinded to treatment. Antinociception was measured by a warm water (50 degrees C) tail-withdrawal assay. Pretreatment with low doses of trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)-benzeneacetamide (U-50488H) (0.032-0.18 mg/kg s.c.), a selective KOR agonist, dose dependently suppressed the s.c. morphine dose-effect curve for scratching and potentiated s.c. morphine-induced antinociception. In addition, s.c. U-50488H attenuated i.t. morphine (10 and 32 micro g)-induced scratching while maintaining or enhancing i.t. morphine-induced antinociception. The combination of s.c. or i.t. morphine with low doses of U-50488H did not cause sedation. More importantly, pretreatment with 3.2 mg/kg nor-binaltorphimine, a selective KOR antagonist, blocked the effects of s.c. U-50488H on both s.c. and i.t. morphine-induced scratching. These results indicate that activation of KOR attenuates morphine-induced scratching without interfering with antinociception in monkeys. This mechanism-based finding provides functional evidence in support of the clinical potential of KOR agonists as antipruritics in the presence of MOR agonist-induced pruritus.

Endomorphins and related opioid peptides

Opioid peptides and their G-protein-coupled receptors (delta, kappa, mu) are located in the central nervous system and peripheral tissues. The opioid system has been studied to determine the intrinsic mechanism of modulation of pain and to develop uniquely effective pain-control substances with minimal abuse potential and side effects. Two types of endogenous opioid peptides exist, one containing Try-Gly-Gly-Phe as the message domain (enkephalins, endorphins, dynorphins) and the other containing the Tyr-Pro-Phe/Trp sequence (endomorphins-1 and -2). Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2), which has high mu receptor affinity (Ki = 0.36 nM) and remarkable selectivity (4000- and 15,000-fold preference over the delta and kappa receptors, respectively), was isolated from bovine and human brain. In addition, endomorphin-2 (Tyr-Pro-Phe-Phe-NH2), isolated from the same sources, exhibited high mu receptor affinity (Ki = 0.69 nM) and very high selectivity (13,000- and 7500-fold preference relative to delta and kappa receptors, respectively). Both opioids bind to mu-opioid receptors, thereby activating G-proteins, resulting in regulation of gastrointestinal motility, manifestation of antinociception, and effects on the vascular systems and memory. To develop novel analgesics with less addictive properties, evaluation of the structure-activity relationships of the endomorphins led to the design of more potent and stable analgesics. Opioidmimetics and opioid peptides containing the amino acid sequence of the message domain of endomorphins, Tyr-Pro-Phe/Trp, could exhibit unique binding activity and lead to the development of new therapeutic drugs for controlling pain.

Effects of naltrexone on spontaneous itch-associated responses in NC mice with chronic dermatitis

The effects of the opioid antagonist naltrexone on spontaneous itch-associated behaviors and cutaneous nerve activities were examined to determine whether it inhibits pruritus through peripheral action in NC mice with chronic dermatitis. Their rostral-back scratching and caudal-back biting were 19 and 3.4 times more, respectively, than those of control mice. The activities of cutaneous nerves innervating the rostral and caudal back were 9.5 and 5.4 times more, respectively, in affected mice than in control mice. Subcutaneous injections of naltrexone significantly inhibited the scratching and biting, without effects on the nerve activities. The results suggest that the peripheral action does not play a central role in inhibiting chronic itch-associated behaviors.

Itch-associated response induced by experimental dry skin in mice

The present study was conducted to establish a new mouse model of dry skin pruritus. The rostral back was treated daily with cutaneous application of acetone/ether (1:1) mixture (AE), water following AE (AEW), 1% sodium lauryl sulfate (SLS) or tape stripping (TS). On the day after 5-day treatment, although all four treatments significantly decreased stratum corneum (SC) hydration and increased transepidermal water loss (TEWL), only AEW treatment significantly increased spontaneous scratching. An increase in the frequency of TS produced the marked increase of TEWL, without significant effects on SC hydration and spontaneous scratching. In AEW-treated mice, changes in SC hydration and TEWL were marked in the initial 2-day period, while spontaneous scratching increased gradually from 3 days after starting the treatment. The degranulation of cutaneous mast cells was increased by SLS treatment but not by other treatments. There was no apparent difference in AEW-induced spontaneous scratching between mast cell-deficient mice (WBB6F1-W/Wv) and normal littermates (WBB6F1-+/+). Opioid antagonists, naloxone and naltrexone, (1 mg/kg, subcutaneously) significantly suppressed spontaneous scratching in AEW-treated mice. It is suggested that spontaneous scratching of AEW-treated mice is an itch-related response and a useful model for studying the mechanisms of dry skin pruritus.

Vasopressin into the preoptic area increases grooming behavior in mice

In mice, the neuropeptide arginine-8-vasopressin (AVP) induces excessive grooming, scratching, and hyperactivity when administered intracerebroventricularly. In hamsters, AVP infusion into the medial preoptic area/anterior hypothalamus (MPOA/AH) increases flank marking and flank mark grooming. We measured the behavioral effects of administration of AVP (0, 1, and 10 ng/250 nl) into the preoptic area (POA) of male C57BL/6 mice. Administration of AVP into the POA induced robust effects on grooming, including increased hindleg scratching and face washing. Rearing and olfactory investigation were inhibited by AVP into the POA. These findings indicate that the POA is one site in which AVP induces grooming behavior in mice.

Roles of mast cells and histamine in mosquito bite-induced allergic itch-associated responses in mice

We investigated itch-associated responses (scratching) to mosquito bites and the role of histamine and mast cells in mosquito-induced itching in mice. Although the first bites of mosquito Aedes albopictus did not increase scratching, repeated bites increased scratching. The response was not diminished even after an interval of 2 months. Similarly, repeated intradermal (i.d.) injections of salivary gland extract (SGE) from Aedes albopictus increased scratching after SGE injection itself and mosquito bites. The scratching peaked within 10 min and almost subsided by 60 min. The opioid antagonist naloxone (1 mg/kg, s.c.) inhibited scratching following SGE injection. Although the non-sedative H1-histamine-receptor antagonist terfenadine (30 mg/kg, p.o.) significantly suppressed scratching induced by histamine (100 nmol/site, i.d.) in either naive or mosquito-sensitized mice, it did not affect mosquito-induced scratching in mosquito-sensitized mice. Repeated injections of SGE increased scratching in mast cell-deficient (WBB6F1-W/Wv) mice as well as in normal (WBB6F1-+/+) littermates. Repeated exposure to mosquito bites roughly doubled serum concentrations of total IgE and IgG1, but not IgG2a. Repeated injections of SGE markedly increased plasma extravasation induced by mosquito bites and such an increase was almost completely suppressed by terfenadine (30 mg/kg, p.o.). The results show the presence of histamine-mediated and histamine-independent mechanisms in cutaneous itching and suggest that histamine probably released from mast cells does not play an important role in itching in immediate allergic reaction. Our murine model of mosquito itching may be useful for studying the mechanisms of immediate allergic itching.

Norbinaltorphimine, a selective kappa-opioid receptor antagonist, induces an itch-associated response in mice

We examined the possibility that scratching induced by norbinaltorphimine, a selective kappa-opioid receptor antagonist, is due to an itch sensation, using compound 48/80 as control pruritogenic agent. When norbinaltorphimine was injected s.c. into the rostral back, mice scratched the skin around the injection site with their hind paws. Although the intensity of the scratching could not be compared because the dose and injection route were different, the character and time course of the scratching behavior induced by compound 48/80 injected i.d. were similar to those with norbinaltorphimine. The scratching behavior induced by norbinaltorphimine was dose-dependently and significantly inhibited by pretreatment with chlorpheniramine. Compound 48/80-induced scratching was also dose-dependently and significantly inhibited by p.o. pretreatment with chlorpheniramine. The scratching behavior induced by norbinaltorphimine was dose-dependently and significantly inhibited by pretreatment with U-50,488H (trans-(+/-)-2-(3,4-dichlorophenyl)-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] acetamide methansulfonate), a kappa-opioid receptor agonist. Unexpectedly, the scratching behavior induced by compound 48/80 was also dose-dependently and significantly reduced by pretreatment with U-50,488H. These results suggest that the injection of norbinaltorphimine into the rostral back of the mouse elicited scratching, which may be an itch-associated response. Furthermore, the scratching behavior produced by norbinaltorphimine may be due in part to the release of histamine followed by antagonism of kappa-opioid receptors.

Understanding pruritus in systemic disease

Many pruritic conditions do not originate in the skin, but are the result of systemic abnormality. Among the diseases that can cause pruritus are renal insufficiency, cholestasis, Hodgkin's lymphoma, polycythemia vera, solid tumors, and many others. Other pruritic conditions appear to be iatrogenic; opioid-induced pruritus may be the most important in palliative medicine. Successful treatment of the underlying condition usually relieves itch. But, with time, many diseases progress and treatment of the cause will be impossible. Topical treatments may be of limited value. Strategies involving systemic treatments include use of antidepressants, oral opioid antagonists, or cholestyramine. There is no one cure for all pruritic symptoms. Better understanding of mechanisms of pruritus may help develop better treatments.

Superficial dorsal horn neurons identified by intracutaneous histamine: chemonociceptive responses and modulation by morphine

We have investigated whether neurons in superficial laminae of the spinal dorsal horn respond to intracutaneous (ic) delivery of histamine and other irritant chemicals, and thus might be involved in signaling sensations of itch or chemogenic pain. Single-unit recordings were made from superficial lumbar dorsal horn neurons in pentobarbital sodium-anesthetized rats. Chemoresponsive units were identified using ic microinjection of histamine (3%, 1 microl) into the hindpaw as a search stimulus. All superficial units so identified [9 nociceptive-specific (NS), 26 wide-dynamic-range (WDR)] responded to subsequent ic histamine. A comparison group of histamine-responsive deep dorsal horn neurons (n = 16) was similarly identified. The mean histamine-evoked discharge decayed to 50% of the maximal rate significantly more slowly for the superficial (92.2 s +/- 65.5, mean +/- SD) compared with deep dorsal horn neurons (28. 2 s +/- 11.6). In addition to responding to histamine, most superficial dorsal horn neurons were also excited by ic nicotine (22/25 units), capsaicin (21/22), topical mustard oil (5/6), noxious heat (26/30), and noxious and/or innocuous mechanical stimuli (except for 1 unit that did not have a mechanosensitive receptive field). Application of a brief noxious heat stimulus during the response to ic histamine evoked an additive response in all but two cases, followed by transient depression of firing in 11/20 units. Intrathecal (IT) administration of morphine had mixed effects on superficial dorsal horn neuronal responses to ic histamine and noxious heat. Low morphine concentrations (100 nM to 1 microM) facilitated histamine-evoked responses (to >130% of control) in 9/24 units, depressed the responses (by >70%) in 11/24, and had no effect in 4. Naloxone reversed morphine-induced effects in some but not all cases. A higher morphine concentration (10 microM) had a largely depressant, naloxone-reversible effect on histamine responses. Responses of the same superficial neurons to noxious heat were facilitated (15/25), reduced (8/25), or unaffected (2/25) by low morphine concentrations and were depressed by the higher morphine concentration. In contrast, deep dorsal horn neuronal responses to both histamine and noxious heat were primarily depressed by low concentrations of morphine in a naloxone-reversible manner. These results indicate that superficial dorsal horn neurons respond to both pruritic and algesic chemical stimuli and thus might participate in transmitting sensations of itch and/or chemogenic pain. The facilitation of superficial neuronal responses to histamine by low concentrations of morphine, coupled with inhibition of deep dorsal horn neurons, might underlie the development of pruritus that is often observed after epidural morphine.

Itch-scratch responses induced by opioids through central mu opioid receptors in mice

We examined scratch-inducing effects of intracisternal, intrathecal and intradermal injections of morphine and some opioid agonists in mice. Intracisternal injection of morphine (3 nmol/animal) and the mu-receptor agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]enkephalin (DAMGO; 0.2 nmol/animal) elicited scratching of the face, with little effect on scratching of the trunk. Intracisternal injection of the delta-receptor agonist [D-Pen(2,5)]enkephalin (DPDPE) and the kappa-receptor agonist U50488 were without effects. Intrathecal injection of morphine (0.1-3 nmol/animal) produced a dose-dependent increase in body scratching, with little effects on face scratching. Face scratching induced by intrathecal morphine (3 nmol/animal) was almost abolished by subcutaneous pretreatment with naloxone (1 mg/kg). Intradermal injections of morphine (3-100 nmol/site), DAMGO (1-100 nmol/site), DPDPE (10 and 100 nmol/site) and U50488 (10-100 nmol/site) did not elicit scratching of the site of injection. Intradermal injection of histamine (100 nmol/site) induced the scratching in ICR, but not ddY, mice and serotonin (30 and 50 nmol/site) elicited the scratching in either strain of mice. The results suggest that opioids induce scratching, and probably itching, through central mu-opioid receptors in the mouse.

An experimental itch model in monkeys: characterization of intrathecal morphine-induced scratching and antinociception

BACKGROUND

The most common side effect of spinal opioid administration is pruritus, which has been treated with a variety of agents with variable success. Currently, there are few animal models developed to study this side effect. The aim of this study was to establish a nonhuman primate model to pharmacologically characterize the effects of intrathecal administration of morphine.

METHODS

Eight adult rhesus monkeys were used. Scratching responses were videotaped and counted by observers who were blinded to experimental conditions. Antinociception was measured by a warm-water (50 degrees C) tail-withdrawal assay. The dose-response of intrathecal morphine (1-320 microg) for both scratching and antinociception in all subjects was established. An opioid antagonist, nalmefene, was administered either intravenously or subcutaneously to assess its efficacy against intrathecal morphine.

RESULTS

Intrathecal morphine (1-32 microg) increased scratching in a dose-dependent manner. Higher doses of intrathecal morphine (10-100 microg) produced thermal antinociception in a dose-dependent manner. On the other hand, nalmefene (10-32 microg/kg intravenously) attenuated maximum scratching responses among subjects. Pretreatment with nalmefene (32 microg/kg subcutaneously) produced approximately 10-fold rightward shifts of intrathecal morphine dose-response curves for both behavioral effects.

CONCLUSIONS

These data indicate that intrathecal morphine-induced scratching and antinociception are mediated by opioid receptors. The magnitude of nalmefene antagonism of intrathecal morphine is consistent with microL opioid receptor mediation. This experimental itch model is useful for evaluating different agents that may suppress scratching without interfering with antinociception. It may also facilitate the clarification of mechanisms underlying these phenomena.

Mechanisms of chloroquine-induced body-scratching behavior in rats: evidence of involvement of endogenous opioid peptides

Chloroquine is commonly used in the chemotherapy of malaria fever, and as an antiinflammatory disease-modifying agent in patients with rheumatoid arthritis or systemic lupus erythematosus. Administration of chloroquine (20.0 mg/kg IP) significantly (p < 0.05) increased the frequency of body scratching in rats to 29.5+/-9 in 30 min, compared to saline control animals (6.5+/-2/30 min). Morphine, a mu-opiate receptor agonist (1.0 mg/kg IP), potentiated the chloroquine-induced rat body scratching to 40+/-6.6, while the mu-opiate receptor antagonist, naltrexone (0.25 mg/kg, IP, given 15 min prior) blocked the chloroquine induced body scratching to 4.5+/-2 (p < 0.05 ANOVA). In addition, the frequency of chloroquine (20.0 mg/kg IP)-induced body scratching was significantly reduced to 9.1+/-3 in 30 min in rats rendered tolerant to morphine (p < 0.05 ANOVA) compared to the scratching frequency of 40+/-6.6 in morphine-naive rats. These suggests an involvement of mu-opioid receptors and/or endogenous opioid peptides in chloroquine induced body scratching in rats. Promethazine, a histamine-receptor antagonist (1.0 mg/kg IP, given 15 min prior to chloroquine) and the corticosteroid, dexamethasone (1.0 mg/kg, IP, given 15 min prior) separately and significantly (p < 0.01) inhibited the chloroquine-induced scratching in rats, in a similar manner to clinical studies in malaria. Collectively, the novel results implicate opioidergic mechanisms, and confirm the efficacy of antihistamine and corticosteroids in chloroquine body scratching in rats. It also strongly suggests that the chloroquine-induced body-scratching behavior in the rat may be a useful experimental model for chloroquine-induced pruritus in humans.

Itch-associated response induced by intradermal serotonin through 5-HT2 receptors in mice

Serotonin (5-HT) is pruritogenic in humans and suggested to be involved in some pruritic diseases. Our experiments were carried out to determine whether an intradermal injection of 5-HT would elicit itch-associated response in mice and to elucidate the 5-HT receptor subtypes involved in this 5-HT action. 5-HT (14.1-235 nmol site(-1)) injected intradermally into the rostral back elicited scratching of the injected site, with bell-shaped dose-response relationship. The scratching induced by 5-HT (100 nmol site(-1), peak effective dose) was suppressed by capsaicin (repeated administration) and the opioid antagonist naloxone, features being similar to human itching. Scratching was also elicited by the 5-HT2 receptor agonist alpha-methylserotonin, but not by the 5-HT1A receptor agonist R(+)-8-hydroxy-N,N-dipropyl-2-aminotetralin nor the 5-HT3 receptor agonists 2-methylserotonin and 1-phenylbiganide. Scratching induced by 5-HT and alpha-methylserotonin was inhibited by peroral pretreatment with 5-HT1/2 receptor antagonists methysergide and cyproheptadine. 5-HT-induced scratching was also inhibited by intradermal injection of methysergide. Peroral pretreatment with 5-HT3 receptor antagonists ondansetron and 3-tropanyl-3, 5-dichrobenzoate did not significantly suppress 5-HT-induced scratching. The results suggest that scratching induced by intradermal injection of 5-HT is itch-associated response. The 5-HT action may be mediated at least partly by cutaneous 5-HT2 receptors.

Itch-associated response and antinociception induced by intracisternal endomorphins in mice

Endomorphin-1 and endomorphin-2 are newly identified endogenous peptides and have high affinity and selectivity for mu-opioid receptors. The present experiments were conducted to determine whether intracisternal injection of these peptides would produce an itch-associated response and antinociception and to compare their effects to that of morphine. Endomorphin-1 and endomorphin-2 (0.3-3 nmol/mouse) elicited facial scratching characterized by bell-shaped dose-response curves with a peak effect at endomorphin-1 at 0.3 nmol/mouse and endomorphin-2 at 1 nmol/mouse. Their peak effects were inhibited by subcutaneous pretreatment with naloxone (1 mg/kg). Morphine (0.3-30 nmol/mouse) produced facial scratching, and its dose-response curve was also bell-shaped. Scratching of the body trunk, head and ears were not elicited by these doses of endomorphins and morphine. Endomorphin-1 and -2 at doses of 0.3-3 nmol/mouse produced dose-dependent antinociception, as measured with the tail-pressure test. The potency and duration of actions of these peptides were comparable to those of morphine. The results suggest that endomorphin-1 and endomorphin-2 are involved in itch-signaling and pain-inhibiting functions of the brain.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.