Antipruritic Effects of Kappa Opioid Receptor Agonists: Evidence from Rodents to Humans.

Centrally administered bombesin induces scratching and grooming in rats. These behaviors were blocked by early benzomorphan kappa opioid receptor (KOR) agonists as reported by Gmerek and Cowan in 1984. This was the first evidence that KORs may be involved in the sensation of itch-like behaviors. Subsequent development of additional animal models for acute and chronic itch has led to important discoveries since then. For example, it was found that (a) gastrin-releasing peptide (GRP), natriuretic polypeptide b and their cognate receptors are keys for the transmission of itch sensation at the spinal cord level, (b) dynorphins (Dyns), the endogenous KOR agonists, work as inhibitory neuromodulators of itch at the spinal cord level, (c) in a mouse model for acute itch, certain KOR antagonists elicit scratching, (d) in mouse models of acute or chronic itch, KOR agonists (e.g., U50,488, nalfurafine, CR 845, nalbuphine) suppress scratching induced by different pruritogens, and (e) nalfurafine, CR 845, and nalbuphine are in the clinic or in clinical trials for pruritus associated with chronic kidney disease and chronic liver disease, as well as pruritus in chronic skin diseases.

Chemokine receptor antagonists enhance morphine's antinociceptive effect but not respiratory depression.

AIMS: We have shown that chemokines injected into the periaqueductal gray region of the brain blocks opioid-induced analgesia in the rat cold-water tail flick test (CWTF). The present experiments tested whether chemokine receptor antagonists (CRAs), in combination with sub-analgesic doses of morphine, would provide maximal analgesia in the CWTF test and the mouse formalin pain assay. The effect of CRAs on respiratory depression was also evaluated. MAIN METHODS: One, two or four CRAs (AMD3100/CXCR4, maraviroc/CCR5, RS504393/CCR2 orAZD8797/CX3CR1) were used in combination with sub-analgesic doses of morphine, all given systemically. Pain was assessed using the rat CWTF test or formalin injection into the paw of mice scored by licking. Respiration and oxygen saturation were measured in rats using a MouseOX® Plus - pulse oximeter. KEY FINDINGS: In the CWTF test, a sub-maximal dose of morphine in combination with maraviroc alone, maraviroc plus AMD3100, or with the four chemokine receptor antagonists, produced synergistic increases in antinociception. In the formalin test, the combination of four CRAs plus a sub-maximal dose of morphine resulted in increased antinociception in both male and female mice. AMD3100 had an additive effect with morphine in both sexes. Coadministration of CRAs with morphine did not potentiate the opioid respiratory depressive effect. SIGNIFICANCE: These results support the conclusion that combinations of CRAs can increase the potency of sub-analgesic doses of morphine analgesia without increasing respiratory depression. The results support an "opioid sparing" strategy for alleviation of pain using reduced doses of opioids in combination with CRAs to achieve maximal analgesia.

Antipruritic Effect of Nalbuphine, a Kappa Opioid Receptor Agonist, in Mice: A Pan Antipruritic.

Antipruritic effects of kappa opioid receptor (KOR) agonists have been shown in rodent models of acute and chronic scratching (itchlike behavior). Three KOR agonists, nalfurafine, difelikefalin, and nalbuphine, are in clinical studies for antipruritic effects in chronic itch of systemic and skin diseases. Nalfurafine (in Japan) and difelikefalin (in the USA) were approved to be used in the treatment of chronic itch in hemodialysis patients. The FDA-approved nalbuphine has been used in clinic for over 40 years, and it is the only narcotic agonist that is not scheduled. We aimed to study (a) antiscratch activity of nalbuphine against TAT-HIV-1 protein (controls HIV transcription)-, deoxycholic acid (DCA, bile acid)-, and chloroquine (CQ)-induced scratching in a mouse model of acute itch; and (b) whether the effect of nalbuphine is produced via KORs. First, dose-responses were developed for pruritogens. Mice were pretreated with nalbuphine (0.3-10 mg/kg) and then a submaximal dose of pruritogens were administered and the number of scratching bouts was counted. To study if the antiscratch effect of nalbuphine is produced via KOR, we used KOR knock out mice and pharmacologic inhibition of KORs using nor-binaltorphimine, a KOR antagonist. For this aim, we used CQ as a pruritogen. We found that: (a) TAT-HIV-1 protein elicits scratching in a dose-dependent manner; (b) nalbuphine inhibits scratching induced by TAT-HIV-1, DCA, and CQ dose-dependently; and (c) nalbuphine inhibits scratching induced by CQ through KORs. In conclusion, nalbuphine inhibits scratching elicited by multiple pruritogens.

Chemokine Receptor Antagonists in Combination with Morphine as a Novel Strategy for Opioid Dose Reduction in Pain Management.

INTRODUCTION: Although opioids are widely prescribed for pain, in many circumstances, they have only modest efficacy. Preclinical studies have shown that chemokines, immune mediators released during tissue injury and inflammation, can desensitize opioid receptors and block opioid analgesia by a process termed "heterologous desensitization." The present studies tested the hypothesis that in evoked pain, certain chemokine receptor antagonists (CRAs), given with a submaximal dose of morphine, would result in enhanced morphine potency. METHODS: Three rodent pain assays were used: incisional pain in rats, the cold-water tail flick test in rats, and the formalin test in mice. The FDA-approved, commercially available CRAs, maraviroc and AMD3100, were used. They block the chemokine receptors and ligands, CCR5/CCL5 (RANTES) and CXCR4/CXCL4 (SDF-1α), respectively. RESULTS: In the incisional pain assay, it was found that the combination of a single CRA, or of both CRAs, with morphine significantly shifted the morphine dose-response curve to the left, as much as 3.3-fold. In the cold-water tail flick and formalin tests, significant increases of the antinociceptive effects of morphine were also observed when combined with CRAs. CONCLUSIONS: These results support the potential of a new "opioid-sparing" approach for pain treatment, which combines CRAs with reduced doses of morphine.

Nalbuphine, a kappa opioid receptor agonist and mu opioid receptor antagonist attenuates pruritus, decreases IL-31, and increases IL-10 in mice with contact dermatitis.

Chronic itch is one of the disturbing symptoms of inflammatory skin diseases. Kappa opioid receptor agonists are effective in suppressing scratching in mice against different pruritogens. Nalbuphine, a nonscheduled kappa opioid receptor agonist and mu opioid receptor antagonist, has been in clinical use for post-operative pain management since the 1980s and recently has been in clinical trials for chronic itch of prurigo nodularis (https://www.trevitherapeutics.com/nalbuphine). We studied whether nalbuphine is effective against chronic scratching induced by rostral neck application of 1-fluoro-2,4-dinitrobenzene (DNFB), an accepted mouse model of contact dermatitis to study pruritoceptive itch. Mice were treated once a week with either saline or nalbuphine 20 min before the third, fifth, seventh, and ninth sensitizations with DNFB and the number of scratching bouts was counted for 30 min. Skin samples from the neck of mice at week 4 were used to measure protein levels and mRNA expressions of chemokines and cytokines. Different sets of mice were used to study sedation and anhedonic-like behavior of nalbuphine. We found that: nalbuphine (a) antagonized scratching in a dose- and time-dependent manner without affecting locomotion, b) decreased IL-31, and increased anti-inflammatory IL-10, and c) induced more elevations in the levels of CCL2, CCL3, CCL12, CXCL1, CXCL2, CXCL9, CXCL10, IL-1ß, IL-16, TIMP-1, M-CSF, TREM-1 and M1-type macrophages compared to saline. Increases in chemokines and cytokines and M1 macrophages by nalbuphine suggest an inflammatory phase of healing in damaged skin due to scratching. Our data indicate that nalbuphine is an effective antipruritic in murine model of pruritoceptive itch.

Opioid-sparing effects of cannabinoids on morphine analgesia: participation of CB1 and CB2 receptors.

BACKGROUND AND PURPOSE: Much of the opioid epidemic arose from abuse of prescription opioid drugs. This study sought to determine if the combination of a cannabinoid with an opioid could produce additive or synergistic effects on pain, allowing reduction in the opioid dose needed for maximal analgesia. EXPERIMENTAL APPROACH: Pain was assayed using the formalin test in mice and the carrageenan assay in rats. Morphine and two synthetic cannabinoids were tested: WIN55,212-2 (WIN), which binds to both CB1 and CB2 receptors, and possibly TRPV1 channels; and GP1a, which has activity at CB2 receptors and is reported to inhibit fatty acid amide hydrolase, thus raising levels of endogenous cannabinoids. KEY RESULTS: Morphine in combination with WIN in the formalin test gave synergistic analgesia. Studies with selective antagonists showed that WIN was acting through CB1 receptors. Morphine in combination with GP1a in the formalin test was sub-additive. In the carrageenan test, WIN had no added effect when combined with morphine, but GP1a with morphine showed enhanced analgesia. Both WIN and Gp1a used alone had analgesic activity in the formalin pain test, but not in the carrageenan pain test. CONCLUSIONS AND IMPLICATIONS: The ability of a cannabinoid to produce an additive or synergistic effect on analgesia when combined with morphine varies with the pain assay and may be mediated by CB1 or CB2 receptors. These results hold the promise of using cannabinoids to reduce the dose of opioids for analgesia in certain pain conditions.

Phosphoproteomic approach for agonist-specific signaling in mouse brains: mTOR pathway is involved in κ opioid aversion.

Kappa opioid receptor (KOR) agonists produce analgesic and anti-pruritic effects, but their clinical application was limited by dysphoria and hallucinations. Nalfurafine, a clinically used KOR agonist, does not cause dysphoria or hallucinations at therapeutic doses in humans. We found that in CD-1 mice nalfurafine produced analgesic and anti-scratch effects dose-dependently, like the prototypic KOR agonist U50,488H. In contrast, unlike U50,488H, nalfurafine caused no aversion, anhedonia, or sedation or and a low level of motor incoordination at the effective analgesia and anti-scratch doses. Thus, we established a mouse model that recapitulated important aspects of the clinical observations. We then employed a phosphoproteomics approach to investigate mechanisms underlying differential KOR-mediated effects. A large-scale mass spectrometry (MS)-based analysis on brains revealed that nalfurafine perturbed phosphoproteomes differently from U50,488H in a brain-region specific manner after 30-min treatment. In particular, U50,488H and nalfurafine imparted phosphorylation changes to proteins found in different cellular components or signaling pathways in different brain regions. Notably, we observed that U50,488H, but not nalfurafine, activated the mammalian target of rapamycin (mTOR) pathway in the striatum and cortex. Inhibition of the mTOR pathway by rapamycin abolished U50,488H-induced aversion, without affecting analgesic, anti-scratch, and sedative effects and motor incoordination. The results indicate that the mTOR pathway is involved in KOR agonist-induced aversion. This is the first demonstration that phosphoproteomics can be applied to agonist-specific signaling of G protein-coupled receptors (GPCRs) in mouse brains to unravel pharmacologically important pathways. Furthermore, this is one of the first two reports that the mTOR pathway mediates aversion caused by KOR activation.

Coadministration of Chemokine Receptor Antagonists with Morphine Potentiates Morphine's Analgesic Effect on Incisional Pain in Rats.

Crossdesensitization between opioid and chemokine receptors and involvement of chemokines in pain modulation are well established. We investigated if coadministration of chemokine receptor antagonists (CRAs) with morphine would enhance the analgesic potency of morphine on incisional pain in rats. Animals underwent incisional surgery on the left hind paw and pain responses were evaluated using von Frey filaments at various time points postsurgery between 15 and 360 minutes and daily between 24 and 72 hours. Dose-response curves for morphine, maraviroc (a CCR5 antagonist), and AMD3100 (a CXCR4 antagonist) alone were established. While morphine significantly reduced pain in a time- and dose-dependent manner, maraviroc and AMD3100 had no effect by themselves. Coadministration of either maraviroc or AMD3100 with morphine significantly increased morphine's analgesic effect on incisional pain, shifting the dose-response curve to the left 2.3- and 1.8-fold, respectively. Coadministration of both CRAs with morphine significantly shifted further the morphine dose-response curve to the left 3.3-fold. The effect of treatments on mRNA levels in the draining popliteal lymph node for a panel of chemokines and cytokines showed that message for many of these mediators was upregulated by the incision, and the combination of morphine with the CRAs markedly downregulated them. The data show that combining morphine with CRAs potentiates morphine's analgesic effect on incisional pain. Thus, the same analgesic effect of morphine alone can be achieved with lower doses of morphine when combined with CRAs. Using morphine in lower doses could reduce unwanted side effects and possibly block development of tolerance and dependence.

Phoenixin: a novel brain-gut-skin peptide with multiple bioactivity.

In this brief review we summarize the current fndings relative to the discovery of a small peptide ligand, phoenixin (PNX). Using a bioinformatic approach, two novel peptides PNX-14 and PNX-20 containing 14 and 20 amino acids, respectively, were isolated from diverse tissues including the brain, heart, lung and stomach. Mass spectrometry analysis identified a major and minor peak corresponding to PNX-14 and PNX-20, in rat or mouse spinal cord extracts. With the use of a rabbit polyclonal antiserum, phoenixin immunoreactivity (irPNX) was detected in discrete areas of the rodent brain including several hypothalamic subnuclei and dorsal motor nucleus of the vagus. In addition, irPNX was detected in a population of sensory ganglion cells including dorsal root ganglion, nodose ganglion and trigeminal ganglion, and in cell processes densely distributed to the superficial layers of the dorsal horn, nucleus of the solitary tract and spinal trigeminal tract. irPNX cell processes were also detected in the skin and myenteric plexus, suggesting a brain-gut and/or brain-skin connection. Pharmacological studies show that PNX-14 injected subcutaneously to the nape of the neck of mice provoked dose-dependent repetitive scratching bouts directed to the back of the neck with the hindpaws. Our result suggests that the peptide PNX-14 and/or PNX-20, may serve as one of the endogenous signal molecules transducing itch sensation. Additionally, results from other laboratories show that exogenous PNX may affect a number of diverse behaviors such as memory formation, depression, reproduction, food-intake and anxiolytic-like behaviors.

Local, Controlled Delivery of Local Anesthetics In Vivo from Polymer - Xerogel Composites.

PURPOSE: Polymer-xerogel composite materials have been introduced to better optimize local anesthetics release kinetics for the pain management. In a previous study, it was shown that by adjusting various compositional and nano-structural properties of both inorganic xerogels and polymers, zero-order release kinetics over 7 days can be achieved in vitro. In this study, in vitro release properties are confirmed in vivo using a model that tests for actual functionality of the released local anesthetics. METHODS: Composite materials made with tyrosine-polyethylene glycol(PEG)-derived poly(ether carbonate) copolymers and silica-based sol-gel (xerogel) were synthesized. The in vivo release from the composite controlled release materials was demonstrated by local anesthetics delivery in a rat incisional pain model. RESULTS: The tactile allodynia resulting from incision was significantly attenuated in rats receiving drug-containing composites compared with the control and sham groups for the duration during which natural healing had not yet taken place. The concentration of drug (bupivacaine) in blood is dose dependent and maintained stable up to 120 h post-surgery, the longest time point measured. CONCLUSIONS: These in vivo studies show that polymer-xerogel composite materials with controlled release properties represent a promising class of controlled release materials for pain management.

Lack of adverse effects during a target animal safety trial of extended-release buprenorphine in Fischer 344 rats.

Extended-release buprenorphine is an effective analgesic in laboratory animals, and its safety has been established in mice but not in rats. The authors used a target animal safety trial to evaluate the safety of extended-release buprenorphine in rats. Fischer 344 rats received post-surgical subcutaneous injections of 1.3 mg, 3.9 mg or 6.5 mg buprenorphine per kg body weight (two times, six times or ten times the intended dose, respectively), and their body weight, clinical signs and symptoms, clinical pathology and histopathology were monitored for 4 d. Body weight was not significantly different in rats that received buprenorphine compared with control rats. Signs of nausea-related behavior were observed in 25% of the rats treated with buprenorphine. Clinical pathology results for all rats were normal, and gross and microscopic histopathology examinations identified no substantial abnormalities, suggesting that this behavior was of minor consequence. Other adverse events previously reported to occur with opiate therapy, including weight loss and dermal lesions at drug injection sites, were not observed in this study. The results of this study show that post-surgical administration of an extended-release buprenorphine product is safe in Fischer 344 rats and does not necessarily cause substantial adverse effects, confirming that opiate therapy is a viable choice in laboratory animal medicine.

Targeting Itch with Ligands Selective for κ Opioid Receptors.

Several chemically diverse pruritogens, including bombesin, compound 48/80, norbinaltorphimine, and 5'-GNTI, cause rodents to scratch excessively in a stable, uniform manner and consequently provide convenient animal models of itch against which potential antipruritics may be evaluated, structure-activity relationships established, and the nature of spontaneous, repetitive behavior itself analyzed. Decreasing the number of scratching bouts in these apparently simple models has been the requisite first step in the progress of kappa opioid agonists such as nalbuphine, asimadoline, and CR845 toward clinical testing as antipruritics. Nalfurafine is the prime example of a kappa agonist spanning the developmental divide between scratching mice models and commercialization within 10 years. Patients undergoing hemodialysis and suffering from the itching associated with uremic pruritus, and potentially those inflicted with atopic dermatitis, are the beneficiaries.

Scratching activates microglia in the mouse spinal cord.

This study tested the hypothesis that repetitive scratching provoked by two known pruritogens, compound 48/80 and 5'-guanidinonaltrindole (GNTI), is accompanied by activation of microglial cells in the mouse spinal cord. Immunohistochemical studies revealed that the complement receptor 3, also known as cluster determinant 11b (CD11b), a cell surface marker of microglial cells, was upregulated in the spinal cord 10-30 min after a subcutaneous (s.c.) injection of compound 48/80 (50 µg/100 µl) or GNTI (0.3 mg/kg) to the back of the mouse neck. Numerous intensely labeled CD11b-immunoreactive (CD11b-ir) cells, with the appearance of hypertrophic reactive microglia, were distributed throughout the gray and white matter. In contrast, weakly labeled CD11b-ir cells were distributed in the spinal cord from mice injected with saline. Western blots showed that CD11b expression levels were significantly increased in spinal cords of mice injected s.c. with either pruritogen, reached a peak response in about 30 min, and declined to about the basal level in the ensuing 60 min. In addition, phospho-p38 (p-p38) but not p38 levels were upregulated in spinal cords from mice injected with compound 48/80 or GNTI, with a time course parallel to that of CD11b expression. Pretreatment of the mice with nalfurafine (20 µg/kg; s.c.), a κ-opioid receptor agonist that has been shown to suppress scratching, reduced CD11b and p-p38 expression induced by either pruritogen. The results demonstrate, for the first time, that scratch behavior induced by the pruritogens GNTI and compound 48/80 is accompanied by a parallel activation of microglial cells in the spinal cord.

Sex difference in κ-opioid receptor (KOPR)-mediated behaviors, brain region KOPR level and KOPR-mediated guanosine 5'-O-(3-[35S]thiotriphosphate) binding in the guinea pig.

We examined whether sex differences in κ-opioid receptor (KOPR) pharmacology exist in guinea pigs, which are more similar to humans in the expression level and distribution of KOPR in the brain than rats and mice. The KOPR agonist trans-(±)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)benzeneacetamide methanesulfonate (U50,488H) produced a dose-dependent increase in abnormal postures and immobility with more effects in males than females. Males also showed more U50,488H-induced antinociception in the paw pressure test than females. Pretreatment with the KOPR antagonist norbinaltorphimine blocked U50,488H-induced abnormal body postures and antinociception. In contrast, inhibition of cocaine-induced hyperambulation by U50,488H was more effective in females than males. Thus, sex differences in the effects of U50,488H are endpoint-dependent. We then examined whether sex differences in KOPR levels and KOPR-mediated G protein activation in brain regions may contribute to the observed differences using quantitative in vitro autoradiography of [(3)H](5a,7a,8b)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)1-oxaspiro(4,5)dec-8-yl)benzeacetamide ([(3)H]U69,593) binding to the KOPR and U50,488H-stimulated guanosine 5'-O-(3-[(35)S]thiotriphosphate ([(35)S]GTPγS) binding. Compared with females, males exhibited more [(3)H]U69,593 binding in the deep layers of somatosensory and insular cortices, claustrum, endopiriform nucleus, periaqueductal gray, and substantial nigra. Concomitantly, U50,488H-stimulated [(35)S]GTPγS binding was greater in males than females in the superficial and deep layers of somatosensory and insular cortices, caudate putamen, claustrum, medial geniculate nucleus, and cerebellum. In contrast, compared with males, females showed more U50,488H-stimulated [(35)S]GTPγS binding in the dentate gyrus and a trend of higher [(35)S]GTPγS binding in the hypothalamus. These data demonstrate that males and females differ in KOPR expression and KOPR-mediated G protein activation in distinct brain regions, which may contribute to the observed sex differences in KOPR-mediated pharmacology.

Differential antinociceptive effects of buprenorphine and methadone in the presence of HIV-gp120.

BACKGROUND: We showed recently that elevated brain levels of the chemokine stromal cell-derived growth factor-1α (SDF-1α/CXCL12, a ligand for the human immunodeficiency virus [HIV] co-receptor CXCR4) diminish the antinociceptive effect of morphine, but failed to influence buprenorphine-induced antinociception. AIMS: Because the HIV-1 coat protein, glycoprotein 120 (gp120) T-tropic strain, binds to the same receptor as SDF-1α/CXCL12, the present experiments were designed to investigate the consequence of administering gp120 to rat brain on buprenorphine-induced antinociception in the 54°C hot plate test. For comparative purposes, the effect of gp120 on an equi-antinociceptive dose of methadone was also examined. METHODS: A sterilized stainless-steel C313G guide cannula was implanted into the periaqueductal grey (PAG), a brain region critical for the processing of pain signals, and a primary site of action of many analgesics. Rats were pretreated with gp120, administered into the PAG. RESULTS: The subsequent antinociception associated with methadone was diminished whereas buprenorphine-induced antinociception was unaffected. Buprenorphine thus appears to be a more effective analgesic than methadone in the presence of gp120 in the brain, a condition that is associated with HIV-related pain and infection.

Icilin-evoked behavioral stimulation is attenuated by alpha2-adrenoceptor activation.

Icilin is a transient receptor potential cation channel subfamily M (TRPM8) agonist that produces behavioral activation in rats and mice. Its hallmark overt pharmacological effect is wet-dog shakes (WDS) in rats. The vigorous shaking associated with icilin is dependent on NMDA receptor activation and nitric oxide production, but little else is known about the biological systems that modulate the behavioral phenomenon. The present study investigated the hypothesis that alpha(2)-adrenoceptor activation inhibits icilin-induced WDS. Rats injected with icilin (0.5, 1, 2.5, 5mg/kg, i.p.) displayed dose-related WDS that were inhibited by pretreatment with a fixed dose of clonidine (0.15 mg/kg, s.c.). Shaking behavior caused by a fixed dose (2.5mg/kg) of icilin was also inhibited in a dose-related manner by clonidine pretreatment (0.03-0.15 mg/kg, s.c.) and reduced by clonidine posttreatment (0.15 mg/kg, s.c.). Pretreatment with a peripherally restricted alpha(2)-adrenoceptor agonist, ST91 (0.075, 0.15 mg/kg), also decreased the incidence of shaking elicited by 2.5mg/kg of icilin. Pretreatment with yohimbine (2mg/kg, i.p.) enhanced the shaking induced by a low dose of icilin (0.5mg/kg). The imidazoline site agonists, agmatine (150mg/kg, i.p.) and 2-BFI (7 mg/kg, i.p.), did not affect icilin-evoked shaking. These results suggest that alpha(2)-adrenoceptor activation inhibits shaking induced by icilin and that increases in peripheral, as well as central, alpha(2)-adrenoceptor signaling oppose the behavioral stimulant effect of icilin.

Analgesic efficacy of buprenorphine in the presence of high levels of SDF-1α/CXCL12 in the brain.

Although morphine is often the best option for treating acute and chronic severe pain, its analgesic activity can be blocked in situations in which there are elevated levels of chemokines. Indeed, recently we have shown that elevated brain levels of the chemokine stromal cell-derived growth factor-1alpha (SDF-1α/CXCL12, the ligand of the HIV co-receptor CXCR4) diminish the antinociceptive effect of morphine. The purpose of the present study was to investigate whether such an effect is restricted to morphine or extends to other opioid medications such as buprenorphine. A sterilized stainless-steel C313G guide cannula was implanted into the periaqueductal grey (PAG), a brain region critical to the processing of pain signals, and a primary site of action of many analgesic compounds. The cold-water (-3°C) tail-flick test (CWT) was used to measure antinociception. Rats were pretreated with SDF-1α/CXCL12 administered into the PAG, and the antinociceptive actions of buprenorphine were measured. Direct infusion of SDF-1α/CXCL12 into the PAG failed to alter the antinociceptive action of buprenorphine. The presence of SDF-1α/CXCL12 in the PAG differentially alters the antinociceptive function of opioid medications. While it was able to diminish the antinociception induced by morphine (Adler et al., 2006), SDF-1α/CXCL12 did not affect the buprenorphine-induced antinociception. Buprenorphine appears to be more effective in the presence of high levels of SDF-1α/CXCL12 in the brain (which frequently occurs during neuroinflammatory conditions).

Investigation of gastrin-releasing peptide as a mediator for 5'-guanidinonaltrindole-induced compulsive scratching in mice.

Gastrin-releasing peptide (GRP) has been implicated in the itch-scratch cycle. We investigated if this gut-brain-skin peptide plays a role in the compulsive, hindleg scratching of the neck of mice by 5'-guanidinonaltrindole (GNTI), the kappa opioid receptor antagonist, and in the antipruritic activity of nalfurafine, the kappa opioid agonist. Previously, we showed that GNTI (0.03-1mg/kg, s.c.) elicits dose-related scratching and that nalfurafine (0.001-0.02mg/kg, s.c.) inhibits this behavior in mice. Utilizing immunohistochemistry, GRP positive nerve fibers were detected in mouse skin and superficial layer of the dorsal horn of the spinal cord as well as GRP positive cells in the dorsal root ganglion. Pretreating mice with either a pseudopeptide GRP receptor antagonist, RC-3095 (10-30mg/kg, s.c. at -15min), or a peptide GRP receptor antagonist, [d-Phe(6)]bombesin(6-13) methyl ester (2-100nmol, i.t. at -10min), did not suppress GNTI-induced scratching. However, pretreating mice with either antagonist inhibited scratching precipitated by the GRP receptor agonist, GRP(18-27) (2nmol, i.t.). Pretreating mice with a muscarinic M(1) receptor agonist, McN-A-343 (1.5-15µg/5µl, i.t. at -10min) antagonized GNTI-induced scratching. Norbinaltorphimine (20mg/kg, i.p. at -18 to -20h), a kappa opioid antagonist, countered the antiscratch activity of nalfurafine. We conclude that (a) the GRP receptor system does not mediate GNTI-induced scratching and (b) the kappa opioid system is involved, at least in part, in the scratch suppressing activity of nalfurafine.