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

Hypothalamic response with PKA/CREB signaling is associated with direct cerebroventricular administration of bombesin-induced scratching

Bombesin (BN) is an itch-specific mediator that causes intense itch-scratching activity in mammals. Although most examinations of BN-induced itch processing have focused on the spinal cord, the involvement of central nervous system mechanisms remains unclear. Here, we investigated how relationships among hypothalamic regions regulate BN-mediated itch-scratch processes. We found that intracerebroventricular (i.c.v.) administration of BN (0.04-4 μg) elicited intense itch scratching in mice, whereas BN (0.4-400 μg) administered via intravenous tail injection failed to evoke a scratching response. Additionally, nalfurafine had no significant effects on BN-induced scratching behavior, indicating that central modulation of BN is distinct from histamine-mediated histaminergic itch and chloroquine-mediated non-histaminergic itch signaling pathways. We labeled BN with a fluorescent tag, 7-nitrobenz-2-oxa-1 (NBD), and traced its fluorescence in the hypothalamus for 30 min following i.c.v. NBD-BN administration. Accordingly, we confirmed that i.c.v. administration of BN enhanced c-Fos expression in the dorsal medial nucleus of the hypothalamus, where neuromedin B receptors and gastrin-releasing peptide receptors are highly expressed. Interestingly, in situ injection of BN into the hypothalamus immediately and robustly induced itch-scratching behavior. Moreover, gene transcripts and western blot assay revealed that BN receptor-dependent PKA/CREB signaling was upregulated in the hypothalamus after i.c.v. administration of BN. Consistently, pretreatment with a PKA inhibitor, Rp-cAMP, significantly reduced BN-induced scratching behavior. Our results indicate that the dorsal medial nucleus of the hypothalamus may be a key nucleus in mediating BN-mediated itch and hypothalamic PKA/CREB signaling is involved in regulating BN-mediated itch.

New pharmacological perspectives and therapeutic options for opioids: Differences matter

Opioids remain the major drug class for the treatment of acute, chronic and cancer pain, but have major harmful effects such as dependence and opioid-induced ventilatory impairment. Although no new typical opioids have come onto the market in the past almost 50 years, a plethora of new innovative formulations has been developed to meet the clinical need. This review is intended to shed light on new understanding of the molecular pharmacology of opioids, which has arisen largely due to the genomic revolution, and what new drugs may become available in the coming years. Atypical opioids have and are being developed which not only target the mu opioid receptor but other targets in the pain pathway. Biased mu agonists have been developed but remain 'unbiased' clinically. The contribution of drugs targeting non-mu opioid receptors either alone or as heterodimers shows potential promise but remains understudied. That gene splice variants of the mu opioid receptor produce multiple receptor isoforms in different brain regions, and may change with pain chronicity and phenotype, presents new challenges but also opportunities for precision pain medicine. Finally, that opioids also have pro-inflammatory effects not aligned with mu opioid receptor binding affinity implicates a fresh understanding of their role in chronic pain, whether cancer or non-cancer. Hopefully, a new understanding of opioid analgesic drug action may lead to new drug development and better precision medicine in acute and chronic pain relief with less patient harm.

Antinociceptive, reinforcing, and pruritic effects of the G-protein signalling-biased mu opioid receptor agonist PZM21 in non-human primates

BACKGROUND

A novel G-protein signalling-biased mu opioid peptide (MOP) receptor agonist, PZM21, was recently developed with a distinct chemical structure. It is a potent G_i/o activator with minimal β-arrestin-2 recruitment. Despite intriguing activity in rodent models, PZM21 function in non-human primates is unknown. The aim of this study was to investigate PZM21 actions after systemic or intrathecal administration in primates.

METHODS

Antinociceptive, reinforcing, and pruritic effects of PZM21 were compared with those of the clinically used MOP receptor agonists oxycodone and morphine in assays of acute thermal nociception, capsaicin-induced thermal allodynia, itch scratching responses, and drug self-administration in gonadally intact, adult rhesus macaques (10 males, six females).

RESULTS

After subcutaneous administration, PZM21 (1.0-6.0 mg kg^-1) and oxycodone (0.1-0.6 mg kg^-1) induced dose-dependent thermal antinociceptive effects (P<0.05); PZM21 was 10 times less potent than oxycodone. PZM21 exerted oxycodone-like reinforcing effects and strength as determined by two operant schedules of reinforcement in the intravenous drug self-administration assay. After intrathecal administration, PZM21 (0.03-0.3 mg) dose-dependently attenuated capsaicin-induced thermal allodynia (P<0.05). Although intrathecal PZM21 and morphine induced MOP receptor-mediated antiallodynic effects, both compounds induced robust, long-lasting itch scratching.

CONCLUSIONS

PZM21 induced antinociceptive, reinforcing, and pruritic effects similar to clinically used MOP receptor agonists in primates. Although structure-based discovery of PZM21 identified a novel avenue for studying G-protein signalling-biased ligands, biasing an agonist towards G-protein signalling pathways did not determine or alter reinforcing (i.e. abuse potential) or pruritic effects of MOP receptor agonists in a translationally relevant non-human primate model.

Effects of NOP-Related Ligands in Nonhuman Primates

The nociceptin/orphanin FQ peptide (NOP) receptor-related ligands have been demonstrated in preclinical studies for several therapeutic applications. This article highlights (1) how nonhuman primates (NHP) were used to facilitate the development and application of positron emission tomography tracers in humans; (2) effects of an endogenous NOP ligand, nociceptin/orphanin FQ, and its interaction with mu opioid peptide (MOP) receptor agonists; and (3) promising functional profiles of NOP-related agonists in NHP as analgesics and treatment for substance use disorders. NHP models offer the most phylogenetically appropriate evaluation of opioid and non-opioid receptor functions and drug effects. Based on preclinical and clinical data of ligands with mixed NOP/MOP receptor agonist activity, several factors including their intrinsic efficacies for activating NOP versus MOP receptors and different study endpoints in NHP could contribute to different pharmacological profiles. Ample evidence from NHP studies indicates that bifunctional NOP/MOP receptor agonists have opened an exciting avenue for developing safe, effective medications with fewer side effects for treating pain and drug addiction. In particular, bifunctional NOP/MOP partial agonists hold a great potential as (1) effective spinal analgesics without itch side effects; (2) safe, nonaddictive analgesics without opioid side effects such as respiratory depression; and (3) effective medications for substance use disorders.

Spinal Mechanisms of Itch Transmission

Peripheral itch stimuli are transmitted by sensory neurons to the spinal cord dorsal horn, which then transmits the information to the brain. The molecular and cellular mechanisms within the dorsal horn for itch transmission have only been investigated and identified during the past ten years. This review covers the progress that has been made in identifying the peptide families in sensory neurons and the receptor families in dorsal horn neurons as putative itch transmitters, with a focus on gastrin-releasing peptide (GRP)-GRP receptor signaling. Also discussed are the signaling mechanisms, including opioids, by which various types of itch are transmitted and modulated, as well as the many conflicting results arising from recent studies.

The peptidergic control circuit for sighing

Sighs are long, deep breaths expressing sadness, relief or exhaustion. Sighs also occur spontaneously every few minutes to reinflate alveoli, and sighing increases under hypoxia, stress, and certain psychiatric conditions. Here we use molecular, genetic, and pharmacologic approaches to identify a peptidergic sigh control circuit in murine brain. Small neural subpopulations in a key breathing control centre, the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG), express bombesin-like neuropeptide genes neuromedin B (Nmb) or gastrin-releasing peptide (Grp). These project to the preBötzinger Complex (preBötC), the respiratory rhythm generator, which expresses NMB and GRP receptors in overlapping subsets of ~200 neurons. Introducing either neuropeptide into preBötC or onto preBötC slices, induced sighing or in vitro sigh activity, whereas elimination or inhibition of either receptor reduced basal sighing, and inhibition of both abolished it. Ablating receptor-expressing neurons eliminated basal and hypoxia-induced sighing, but left breathing otherwise intact initially. We propose that these overlapping peptidergic pathways comprise the core of a sigh control circuit that integrates physiological and perhaps emotional input to transform normal breaths into sighs.

Supraspinal actions of nociceptin/orphanin FQ, morphine and substance P in regulating pain and itch in non-human primates

BACKGROUND AND PURPOSE

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor agonists display a promising analgesic profile in preclinical studies. However, supraspinal N/OFQ produced hyperalgesia in rodents and such effects have not been addressed in primates. Thus, the aim of this study was to investigate the effects of centrally administered ligands on regulating pain and itch in non-human primates. In particular, nociceptive thresholds affected by intracisternal N/OFQ were compared with those of morphine and substance P, known to provide analgesia and mediate hyperalgesia, respectively, in humans.

EXPERIMENTAL APPROACH

Intrathecal catheters were installed to allow intracisternal and lumbar intrathecal administration in awake and unanaesthetized rhesus monkeys. Nociceptive responses were measured using the warm water tail-withdrawal assay. Itch scratching responses were scored from videotapes recording behavioural activities of monkeys in their home cages. Antagonist studies were conducted to validate the receptor mechanisms underlying intracisternally elicited behavioural responses.

KEY RESULTS

Intracisternal morphine (100 nmol) elicited more head scratches than those after intrathecal morphine. Distinct dermatomal scratching locations between the two routes suggest a corresponding activation of supraspinal and spinal μ receptors. Unlike intracisternal substance P, which induced hyperalgesia, intracisternal N/OFQ (100 nmol) produced antinociceptive effects mediated by NOP receptors. Neither peptide increased scratching responses.

CONCLUSIONS AND IMPLICATIONS

Taken together, these results demonstrated differential actions of ligands in the primate supraspinal region in regulating pain and itch. This study not only improves scientific understanding of the N/OFQ-NOP receptor system in pain processing but also supports the therapeutic potential of NOP-related ligands as analgesics.

Distinct functions of opioid-related peptides and gastrin-releasing peptide in regulating itch and pain in the spinal cord of primates

How neuropeptides in the primate spinal cord regulate itch and pain is largely unknown. Here we elucidate the sensory functions of spinal opioid-related peptides and gastrin-releasing peptide (GRP) in awake, behaving monkeys. Following intrathecal administration, β-endorphin (10-100 nmol) and GRP (1-10 nmol) dose-dependently elicit the same degree of robust itch scratching, which can be inhibited by mu-opioid peptide (MOP) receptor and GRP receptor (BB2) antagonists, respectively. Unlike β-endorphin, which produces itch and attenuates inflammatory pain, GRP only elicits itch without affecting pain. In contrast, enkephalins (100-1000 nmol) and nociceptin-orphanin FQ (3-30 nmol) only inhibit pain without eliciting itch. More intriguingly, dynorphin A(1-17) (10-100 nmol) dose-dependently attenuates both β-endorphin- and GRP-elicited robust scratching without affecting pain processing. The anti-itch effects of dynorphin A can be reversed by a kappa-opioid peptide (KOP) receptor antagonist nor-binaltorphimine. These nonhuman primate behavioral models with spinal delivery of ligands advance our understanding of distinct functions of neuropeptides for modulating itch and pain. In particular, we demonstrate causal links for itch-eliciting effects by β-endorphin-MOP receptor and GRP-BB2 receptor systems and itch-inhibiting effects by the dynorphin A-KOP receptor system. These studies will facilitate transforming discoveries of novel ligand-receptor systems into future therapies as antipruritics and/or analgesics in humans.

Effects of the NOP agonist SCH221510 on producing and attenuating reinforcing effects as measured by drug self-administration in rats

Nociceptin/orphanin FQ peptide (NOP) receptor agonists attenuate morphine-induced conditioned place preference in rodents. However, it is not known whether NOP agonists have reinforcing properties or can inhibit mu opioid receptor (MOP)-mediated reinforcement as measured by drug self-administration in rodents. Further understanding the behavioral effects of NOP agonists could suggest them as having potential in attenuating reinforcing effects of opioids. In the first part of the study, reinforcing properties of selective NOP agonist SCH221510 were determined and compared with the full MOP agonist remifentanil under fixed-ratio 5 (FR5) and progressive-ratio (PR) schedules of drug self-administration. In the second part, effects of systemic and intracisternal pretreatment of SCH221510 were determined and compared with MOP antagonist naltrexone in attenuating reinforcing effects of remifentanil and a non-drug reinforcer (sucrose pellets). Remifentanil self-administration (0.3-10 µg/kg/infusion) generated a biphasic dose-response curve, characteristic of drugs with reinforcing properties. SCH221510 (3-300 µg/kg/infusion) self-administration resulted in flat dose-response curves and early break-points under the PR, indicative of drugs lacking reinforcing value. Intracisternally, but not systemically, administered SCH221510 (0.3-3 µg) attenuated remifentanil self-administration, comparable with systemic naltrexone (0.03-0.3 mg/kg). SCH221510 (1-3 µg), unlike naltrexone (0.03-1 mg/kg), attenuated responding for sucrose pellets. Both effects of SCH221510 were reversed by the NOP antagonist J-113397 (0.3-3 µg). These results suggest that SCH221510 does not function as a reinforcer in rats, and that it can attenuate the reinforcing value of MOP agonists; therefore, the potential utility of NOP agonists for the treatment of drug addiction warrants further evaluation.

Itch elicited by intradermal injection of serotonin, intracisternal injection of morphine, and their synergistic interactions in rats

We used the cheek model of itch and pain in rats to determine the dose-response relationships for intradermal injection of serotonin and α methylserotonin on scratching behavior. We also determined the dose-related effects of intracisternally injected morphine on scratching, effects that were greatly reduced by administration of the opiate antagonist naloxone. We then examined the interactions of intradermal injection of serotonin and intracisternal injection of morphine on scratching and found that the two procedures act synergistically to increase itch. These results suggest that morphine applied to the CNS is capable of producing itch and greatly increasing itch originating in the skin (hyperknesis).

Characterization of pruriceptive trigeminothalamic tract neurons in rats

Rodent models of facial itch and pain provide a valuable tool for distinguishing between behaviors related to each sensation. In rats, pruritogens applied to the face elicit scratching using the hindlimb while algogens elicit wiping using the forelimb. We wished to determine the role of trigeminothalamic tract (VTT) neurons in carrying information regarding facial itch and pain to the forebrain. We have characterized responses to facially applied pruritogens (serotonin, BAM8-22, chloroquine, histamine, capsaicin, and cowhage) and noxious stimuli in 104 VTT neurons recorded from anesthetized rats. Each VTT neuron had a mechanically sensitive cutaneous receptive field on the ipsilateral face. All pruriceptive VTT neurons also responded to noxious mechanical and/or thermal stimulation. Over half of VTT neurons responsive to noxious stimuli also responded to at least one pruritogen. Each tested pruritogen, with the exception of cowhage, produced an increase in discharge rate in a subset of VTT neurons. The response to each pruritogen was characterized, including maximum discharge rate, response duration, and spike timing dynamics. Pruriceptive VTT neurons were recorded from throughout superficial and deep layers of the spinal trigeminal nucleus and were shown to project via antidromic mapping to the ventroposterior medial nucleus or posterior thalamic nuclei. These results indicate that pruriceptive VTT neurons are a subset of polymodal nociceptive VTT neurons and characterize a system conducive to future experiments regarding the similarities and differences between facial itch and pain.

Physiological function of gastrin-releasing peptide and neuromedin B receptors in regulating itch scratching behavior in the spinal cord of mice

Pruritus (itch) is a severe side effect associated with the use of drugs as well as hepatic and hematological disorders. Previous studies in rodents suggest that bombesin receptor subtypes i.e. receptors for gastrin-releasing peptide (GRPr) and neuromedin B (NMBr) differentially regulate itch scratching. However, to what degree spinal GRPr and NMBr regulate scratching evoked by intrathecally administered bombesin-related peptides is not known. The first aim of this study was to pharmacologically compare the dose-response curves for scratching induced by intrathecally administered bombesin-related peptides versus morphine, which is known to elicit itch in humans. The second aim was to determine if spinal GRPr and NMBr selectively or generally mediate scratching behavior. Mice received intrathecal injection of bombesin (0.01-0.3 nmol), GRP (0.01-0.3 nmol), NMB (0.1-1 nmol) or morphine (0.3-3 nmol) and were observed for one hour for scratching activity. Bombesin elicited most profound scratching over one hour followed by GRP and NMB, whereas morphine failed to evoke scratching response indicating the insensitivity of mouse models to intrathecal opioid-induced itch. Intrathecal pretreatment with GRPr antagonist RC-3095 (0.03-0.1 nmol) produced a parallel rightward shift in the dose response curve of GRP-induced scratching but not NMB-induced scratching. Similarly, PD168368 (1-3 nmol) only attenuated NMB but not GRP-induced scratching. Individual or co-administration of RC-3095 and PD168368 failed to alter bombesin-evoked scratching. A higher dose of RC-3095 (0.3 nmol) generally suppressed scratching induced by all three peptides but also compromised motor function in the rotarod test. Together, these data indicate that spinal GRPr and NMBr independently drive itch neurotransmission in mice and may not mediate bombesin-induced scratching. GRPr antagonists at functionally receptor-selective doses only block spinal GRP-elicited scratching but the suppression of scratching at higher doses is confounded by motor impairment.

The therapeutic potential of nociceptin/orphanin FQ receptor agonists as analgesics without abuse liability

Although mu opioid (MOP) receptor agonists are the most commonly used analgesics for the treatment of moderate to severe pain in the clinic, the side effects of MOP agonists such as abuse liability limit their value as a medication. Research to identify novel analgesics without adverse effects is pivotal to advance the health care of humans. The nociceptin/orphanin FQ peptide (NOP) receptor, the fourth opioid receptor subtype, mediates distinctive actions in nonhuman primates which suggests the possibility that activity at this receptor may result in strong analgesia in the absence of virtually all of the side effects associated with MOP agonists. The present review highlights the recent progress of pharmacological studies of NOP-related ligands in primates. Selective NOP agonists, either peptidic or nonpeptidic, produce full analgesia in various assays in primates, when delivered systemically or intrathecally. Yet small molecule NOP agonists do not serve as reinforcers, indicating a lack of abuse liability. Given that NOP agonists have low abuse liability and that coactivation of NOP and MOP receptors produces synergistic antinociception, it is worth developing bifunctional NOP/MOP ligands. The outcomes of these studies and recent developments provide new perspectives to establish a translational bridge for understanding the biobehavioral functions of NOP receptors in primates and for facilitating the development of NOP-related ligands as a new generation of analgesics without abuse liability in humans.

Amyloid Beta Peptide slows down sensory-induced hippocampal oscillations

Alzheimer's disease (AD) progresses with a deterioration of hippocampal function that is likely induced by amyloid beta (Aβ) oligomers. Hippocampal function is strongly dependent on theta rhythm, and disruptions in this rhythm have been related to the reduction of cognitive performance in AD. Accordingly, both AD patients and AD-transgenic mice show an increase in theta rhythm at rest but a reduction in cognitive-induced theta rhythm. We have previously found that monomers of the short sequence of Aβ (peptide 25-35) reduce sensory-induced theta oscillations. However, considering on the one hand that different Aβ sequences differentially affect hippocampal oscillations and on the other hand that Aβ oligomers seem to be responsible for the cognitive decline observed in AD, here we aimed to explore the effect of Aβ oligomers on sensory-induced theta rhythm. Our results show that intracisternal injection of Aβ1-42 oligomers, which has no significant effect on spontaneous hippocampal activity, disrupts the induction of theta rhythm upon sensory stimulation. Instead of increasing the power in the theta band, the hippocampus of Aβ-treated animals responds to sensory stimulation (tail pinch) with an increase in lower frequencies. These findings demonstrate that Aβ alters induced theta rhythm, providing an in vivo model to test for therapeutic approaches to overcome Aβ-induced hippocampal and cognitive dysfunctions.

The role of central gastrin-releasing peptide and neuromedin B receptors in the modulation of scratching behavior in rats

Bombesin is a pruritogenic agent that causes intense itch-scratching activity in rodents. Bombesin has high affinity for the gastrin-releasing peptide (GRP) receptor (GRPr) and the neuromedin B (NMB) receptor (NMBr). The aim of this study was to investigate pharmacologically the ability of GRPr and NMBr to elicit scratching behavior in rats. The intracerebroventricular route was selected for drug delivery because the study focused on supraspinal sites of action. The magnitude and duration of scratching produced by the naturally occurring peptides GRP and NMB were characterized. Antagonists selective for GRPr [(d-Tpi6, Leu13Ψ(CH2-NH)-Leu14)Bombesin(6-14) (RC-3095)] and NMBr [(S)-α-methyl-α-[[[(4-nitrophenyl)amino]carbonyl]amino]-N-[[1-(2-pyridinyl)cyclohexyl]methyl]-1H-indole-3-propanamide (PD168368)] were used to define the role of GRPr and NMBr in the scratching response. After intracerebroventricular administration, GRP (0.03-0.3 nmol) and NMB (0.1-1 nmol) dose-dependently elicited marked scratching. There was a tolerance to scratching elicited by daily repeated administration of bombesin, GRP, or NMB. Presession administration of RC-3095 (0.1-1 nmol) and PD168368 (0.3-3 nmol) dose-dependently antagonized scratching elicited by GRP and NMB, respectively. More importantly, 1 nmol of RC-3095 failed to block NMB-elicited scratching, and 3 nmol of PD168368 failed to block GRP-elicited scratching. In addition, pretreatment with effective doses of RC-3095 or PD168368 alone or in combination did not block bombesin-elicited scratching. Through the use of the selective antagonists RC-3095 and PD168368, this study demonstrates that central GRPr and NMBr act independently to elicit scratching behavior and there is an additional, unidentified receptor mechanism underlying bombesin-elicited scratching.

Long-lasting antinociceptive spinal effects in primates of the novel nociceptin/orphanin FQ receptor agonist UFP-112

Chemical modifications of nociceptin/orphanin FQ (N/OFQ) peptide that result in increased potency and resistance to degradation have recently lead to the discovery of [(pF)Phe(4)Aib(7)Arg(14)Lys(15)]N/OFQ-NH(2) (UFP-112), a novel N/OFQ peptide (NOP) receptor agonist. The aim of this study was to investigate the pharmacological profile of intrathecally administered UFP-112 in monkeys under different behavioral assays. Intrathecal UFP-112 (1-10 nmol) dose-dependently produced antinociception against an acute noxious stimulus (50 degrees C water) and capsaicin-induced thermal hyperalgesia. Intrathecal UFP-112-induced antinociception could be reversed by a NOP receptor antagonist, J-113397 (0.1mg/kg), but not by a classic opioid receptor antagonist, naltrexone (0.03 mg/kg). Like intrathecal morphine, UFP-112 produced antinociception in two primate pain models with a similar magnitude of effectiveness and a similar duration of action that last for 4-5h. Unlike intrathecal morphine, UFP-112 did not produce itch/scratching responses. In addition, intrathecal inactive doses of UFP-112 and morphine produced significant antinociceptive effects when given in combination without increasing scratching responses. These results demonstrated that intrathecal UFP-112 produced long-lasting morphine-comparable antinociceptive effects without potential itch side effect. This study is the first to provide functional evidence that selective NOP receptor agonists such as UFP-112 alone or in conjunction with morphine may improve the quality of spinal analgesia.

A typology of heroin-dependent patients based on their history of self-injurious behaviours

Self-injurious behaviours (SIB) can provide useful criteria for subtyping heroin-dependent patients, since SIB have been related to an opioid system dysfunction and they hinder patient management. The frequency of nine varieties of moderate/superficial SIB during active heroin use was assessed retrospectively in 164 heroin-dependent patients. A principal component analysis of SIB episodes revealed a four-component solution which accounted for 69.3% of the variance. The components were named as follows (percentage of variance explained by each component is enclosed in parentheses): 'SIB with objects' (27.3%), 'SIB by biting/scratching/hair-pulling' (18.2%), 'SIB by hitting' (12.3%), and 'SIB by picking scabs' (11.5%). A cluster analysis using the results of the principal component analysis enabled us to define three types of heroin-dependent patients, labelled: 'low-occurrence SIB cluster' (59.8%), 'high-occurrence scab-picking cluster' (31.7%) and 'high-occurrence hitting and cutting cluster' (8.5%). SIB by hitting was the most discriminatory component among clusters: its frequency was at a minimum in the low-occurrence SIB cluster, and attained a maximum in the high-occurrence hitting and cutting cluster. However, there were no differences among clusters regarding heroin-use variables. Patients from the low-occurrence SIB cluster, compared with those from the other two clusters, reported fewer episodes of SIB or suicide attempts and were diagnosed less frequently with bulimia. Patients from the high-occurrence scab-picking cluster had a very frequent history of these SIB, while the opposite was true in patients from the high-occurrence hitting and cutting cluster. Patients from this cluster probably presented staff members with the main management problems.

Antinociceptive effects of nociceptin/orphanin FQ administered intrathecally in monkeys

Nociceptin/orphanin FQ (N/OFQ) is the endogenous peptide for the NOP receptors. Depending on the doses, intrathecal administration of N/OFQ has dual actions (ie, hyperalgesia and antinociception) in rodents. However, the pharmacological profile of intrathecal N/OFQ is not fully known in primates. The aim of this study was to investigate behavioral effects of intrathecal N/OFQ over a wide dose range and to compare its effects with ligands known to produce hyperalgesia or antinociception in monkeys. Intrathecal N/OFQ from 1 fmol to 1 nmol did not produce any hyperalgesic or scratching responses. In contrast, intrathecal substance P 100 nmol produced hyperalgesia, and intrathecal DAMGO 10 nmol produced antinociception. At the dose range between 10 nmol and 1 micromol, intrathecal N/OFQ dose-dependently produced thermal antinociception against a noxious stimulus in 2 intensities. More importantly, N/OFQ in combined with intrathecal morphine dose-dependently potentiated morphine-induced antinociception without inhibiting morphine-induced itch/scratching. Taken together, this study is the first to provide a unique functional profile of intrathecal N/OFQ over a wide dose range in primates. Intrathecal N/OFQ produces thermal antinociception without anti-morphine actions or scratching responses, indicating that N/OFQ or NOP receptor agonists represent a promising target as spinal analgesics.

PERSPECTIVE

Intrathecal administration of N/OFQ only produced thermal antinociception, not hyperalgesia, in monkeys. In addition, intrathecal N/OFQ does not have anti-morphine actions or itch/scratching responses. This study strongly supports the therapeutic potential of N/OFQ or NOP receptor agonists as spinal analgesics for clinical trials.

Effects of atypical kappa-opioid receptor agonists on intrathecal morphine-induced itch and analgesia in primates

Itch/pruritus is the most common side effect associated with spinal administration of morphine given to humans for analgesia. The aim of this study was to investigate the effectiveness of kappa-opioid receptor (KOR) agonists with diverse chemical structures as antipruritics and to elucidate the receptor mechanism underlying the antipruritic effect in monkeys. In particular, previously proposed non-KOR-1 agonists, including nalfurafine [TRK-820, 17-cyclopropylmethyl-3,14 beta-dihydroxy-4,5 alpha-epoxy-6 beta-[N-methyl-trans-3-(3-furyl)acrylamido]morphinan], bremazocine [(+/-)-6-ethyl-1,2,3,4,5,6-hexahydro-3-[(1-hydroxycyclopropy)-methyl]-11,11-dimethyl-2,6-methano-3-benzazocin-8-ol], and GR 89696 [4-[(3,4-dichlorophenyl)acetyl]-3-(1-pyrrolidinylmethyl)-1-piperazinecarboxylic acid methyl ester] were studied in various behavioral assays for measuring itch/scratching, analgesia, and respiratory depression. Systemic administration of nalfurafine (0.1-1 microg/kg), bremazocine (0.1-1 microg/kg), or GR 89696 (0.01-0.1 microg/kg) dose-dependently attenuated intrathecal morphine (0.03 mg)-induced scratching responses without affecting morphine antinociception. The combination of intrathecal morphine with these KOR agonists did not cause sedation. In addition, pretreatment with effective antiscratching doses of nalfurafine, bremazocine, or GR 89696 did not antagonize systemic morphine-induced antinociception and respiratory depression. The dose-addition analysis revealed that there is no subadditivity for nalfurafine in combination with morphine in the antinociceptive effect. Furthermore, the KOR antagonist study revealed that antiscratching effects of both nalfurafine and a prototypical KOR-1 agonist, U-50488H [trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)-benzeneacetamide], could be blocked completely by a selective KOR antagonist, nor-binaltorphimine (3 mg/kg). These findings suggest that the agonist action on KOR mainly contributes to the effectiveness of these atypical KOR agonists as antipruritics, and there is no evidence for KOR subtypes or mu-opioid antagonist action underlying the effects of these KOR agonists. This mechanism-based study further supports the clinical potential of KOR agonists as antipruritics under the context of spinal opioid analgesia.

The spinal antinociceptive effects of endomorphins in rats: behavioral and G protein functional studies

BACKGROUND

Endomorphin-1 and endomorphin-2 are endogenous peptides that are highly selective for mu-opioid receptors. However, studies of their functional efficacy and selectivity are controversial. In this study, we systematically compared the effects of intrathecal (i.t.) administration of endomorphin-1 and -2 on nociception assays and G protein activation with those of [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), a highly effective peptidic mu-opioid receptor agonist.

METHODS

Male Sprague-Dawley rats were used. Acute and inflammatory pain models were used to compare the duration and magnitude of antinociception. Agonist-stimulated [(35)S]GTP gamma S binding was used to observe the functional activity at the level of the receptor-G protein in both spinal cord and thalamic membranes. In addition, antagonists selective for each receptor type were used to verify the functional selectivity of endomorphins in the rat spinal cord.

RESULTS

After i.t. administration, endomorphin-1 and -2 produced less antinociceptive effects than DAMGO in the model of acute pain. Concentration-response curves for DAMGO-, endomorphin-1-, and endomorphin-2-stimulated [(35)S]GTP gamma S binding revealed that both endomorphin-1 and -2 produced less G protein activation (i.e., approximately 50%-60%) than DAMGO did in the membranes of spinal cord and thalamus. In addition, i.t. endomorphin-induced antinociception was blocked by mu-opioid receptor selective dose of naltrexone (P < 0.05), but not by delta- and kappa-opioid receptor antagonists, naltrindole and nor-binaltorphimine (P > 0.05).

CONCLUSIONS

Endomorphins are partial agonists for G protein activation at spinal and thalamic mu-opioid receptors. Both in vivo and in vitro measurements together suggest that DAMGO is more effective than endomorphins. Spinal endomorphins' antinociceptive efficacy may range between 53% and 84% depending on the intensity and modality of the nociceptive stimulus.

Kappa opioid agonists suppress chloroquine-induced scratching in mice

Chemotherapy of malaria fever with chloroquine is often associated with generalized pruritus of unknown pathogenesis. This adverse side effect leads to diminished compliance. We report that chloroquine (1.25-40 mg/kg, s.c.) elicits dose-related, compulsive, and vigorous scratching in mice. This frenzied behavior is essentially abolished when the mice are pretreated s.c. or orally with nalfurafine (TRK-820), a centrally penetrating kappa opioid agonist. Peripheral kappa receptors are involved because chloroquine-induced scratching is also antagonized by the peripherally restricted kappa agonist, ICI 204,448: R,S-N-[2-(N-methyl-3,4-dichlorophenylacetamido)-2-(3-carboxyphenyl) ethyl]pyrrolidine. We propose that combination therapy for malaria with chloroquine and a kappa agonist (probably one targeting peripheral receptors) will lead to better treatment compliance because of a reduced incidence of pruritus.

Endogenous opiates and behavior: 2003

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

The Role of Central μ Opioid Receptors in Opioid-Induced Itch in Primates

Pruritus (itch sensation) is a significant clinical problem. The aim of this study was to elucidate the roles of opioid receptor types and the site of action in opioid-induced itch in monkeys. Observers who were blinded to the conditions counted scratching after administration of various drugs. Intravenous (i.v.) administration of mu opioid receptor (MOR) agonists (fentanyl, alfentanil, remifentanil, and morphine) evoked scratching in a dose- and time-dependent manner. However, the kappa opioid agonist U-50488H [trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)-benzeneacetamide] and delta opioid agonist SNC80 [(+)-4-[(alphaR)-alpha-[2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl]-3-methoxybenzyl]-N,N-diethylbenzamide] did not increase scratching. Intrathecal (i.t.) administration of peptidic MOR agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO, 0.00032-0.01 mg) evoked scratching, but i.v. DAMGO (0.01-1 mg/kg) did not increase scratching. A similar difference between i.t. and i.v. effectiveness was seen with morphine. Antagonist studies revealed that i.v. administration of an opioid receptor antagonist (naltrexone, 0.0032-0.1 mg/kg) dose dependently attenuated scratching induced by i.v. fentanyl (0.018 mg/kg) or morphine (1 mg/kg). However, a peripherally selective opioid antagonist (quaternary naltrexone, 0.0032-0.32 mg/kg) did not block i.v. fentanyl- or morphine-induced scratching. Moreover, a histamine antagonist (diphenhydramine, 0.1-10 mg/kg), failed to attenuate scratching induced by i.t. morphine (0.032 mg) or i.v. morphine (1 mg/kg). Pretreatment with a selective MOR antagonist (clocinnamox, 0.1 mg/kg), but not kappa or delta opioid antagonists (nor-binaltorphimine or naltrindole), blocked i.t. morphine-induced scratching. Together, these data suggest that MOR, not other opioid receptor types or histamine, mediates scratching evoked by opioid analgesics. More important, this study provides in vivo pharmacological evidence that activation of central MOR plays an important role in opioid-induced itch in primates.