Modulation of pain by [1DMe]NPYF, a stable analogue of neuropeptide FF, in neuropathic rats

Spinal DN-9, a Peptidic Multifunctional Opioid/Neuropeptide FF Agonist Produced Potent Nontolerance Forming Analgesia With Limited Side Effects

The development of multitarget opioid drugs has emerged as an attractive therapeutic strategy to eliminate opioid-related side effects. Our previous study developed a series of opioid and neuropeptide FF pharmacophore-containing chimeric peptides, including DN-9 (Tyr-D.Ala-Gly-NMe.Phe-Gly-Pro-Gln-Arg-Phe-NH₂), which produced potent nontolerance forming analgesia at the supraspinal level. In the present study, the antinociceptive effects of DN-9 in a series of preclinical pain models and the potential side-effects were investigated at the spinal level in mice. In the tail-flick test, intrathecal injection of DN-9 produced potent analgesia with an ED₅₀ value at 1.33 pmol, and the spinal antinociception of DN-9 was mainly mediated by μ- and κ-opioid receptors. In addition, DN-9-induced spinal antinociception was augmented by the neuropeptide FF receptors antagonist. Furthermore, DN-9 could decrease both the frequency and amplitude of sEPSCs in lamina IIo neurons of the spinal cord, which were mediated by opioid receptors. In contrast to morphine, chronic intrathecal treatments with DN-9 did not induce analgesic tolerance, c-Fos expression or microglial activation. Intrathecal injection of DN-9 showed potent analgesia with antinociceptive ED₅₀ values between .66 and 55.04 pmol in different pain models, including the formalin test, acetic acid-induced writhing test, carrageenan-induced inflammatory pain and neuropathic pain. Moreover, DN-9 did not show side effects in locomotor function and coordination, gastrointestinal transit inhibition, the cardiovascular system, and body temperature regulation at antinociceptive doses. Taken together, the present study showed DN-9 produced effective, nontolerance forming analgesia with reduced side effects at the spinal level. DN-9 might be a promising compound for developing multifunctional opioid analgesics with limited adverse effects. PERSPECTIVE: This article presents the potent and nontolerance forming analgesia effects of DN-9 in a series of preclinical pain models with less opioid related adverse effects at the spinal level in mice. This study also demonstrates that DN-9 has translational potential into an intrathecal analgesic.

The role of orphan G protein-coupled receptors in the modulation of pain: A review

G protein-coupled receptors (GPCRs) comprise a large number of receptors. Orphan GPCRs are divided into six families. These groups contain orphan receptors for which the endogenous ligands are unclear. They have various physiological effects in the body and have the potential to be used in the treatment of different diseases. Considering their important role in the central and peripheral nervous system, their role in the treatment of pain has been the subject of some recent studies. At present, there are effective therapeutics for the treatment of pain including opioid medications and non-steroidal anti-inflammatory drugs. However, the side effects of these drugs and the risks of tolerance and dependence remain a major problem. In addition, neuropathic pain is a condition that does not respond to currently available analgesic medications well. In the present review article, we aimed to review the most recent findings regarding the role of orphan GPCRs in the treatment of pain. Accordingly, based on the preclinical findings, the role of GPR3, GPR7, GPR8, GPR18, GPR30, GPR35, GPR40, GPR55, GPR74, and GPR147 in the treatment of pain was discussed. The present study highlights the role of orphan GPCRs in the modulation of pain and implies that these receptors are potential new targets for finding better and more efficient therapeutics for the management of pain particularly neuropathic pain.

Opioid-modulating properties of the neuropeptide FF system

Opioid receptors are involved in the control of pain perception in the central nervous system together with endogenous neuropeptides, termed opioid-modulating peptides, participating in a homeostatic system. Neuropeptide FF (NPFF) and related peptides possess anti-opioid properties, the cellular mechanisms of which are still unclear. The purpose of this review is to detail the phenomenon of cross-talk taking place between opioid and NPFF systems at the in vivo pharmacological level and to propose cellular and molecular models of functioning. A better knowledge of the mechanisms underlying opioid-modulating properties of NPFF has potential therapeutic interest for the control of opioid functions, notably for alleviating pain and/or for the treatment of opioid abuse.

Neuropeptide FF receptors have opposing modulatory effects on nociception

The role of neuropeptide FF (NPFF) and its analogs in pain modulation is ambiguous. Although NPFF was first characterized as an antiopioid peptide, both antinociceptive and pronociceptive effects have been reported, depending on the route of administration. Currently, two NPFF receptors, termed FF1 and FF2, have been identified and cloned, but their roles in pain modulation remain elusive because of the lack of availability of selective compounds suitable for systemic administration in in vivo models. Ligand-binding studies confirm ubiquitous expression of both subtypes in brain, whereas only FF2 receptors are expressed spinally. This disparity in localization has served as the foundation of the hypothesis that FF1 receptors mediate the pronociceptive actions of NPFF. We have identified novel small molecule NPFF receptor agonists and antagonists with varying degrees of FF2/FF1 functional selectivity. Using these pharmacological tools in vivo has allowed us to define the roles of NPFF receptor subtypes as pertains to the modulation of nociception. We demonstrate that selective FF2 agonism does not modulate acute pain but instead ameliorates inflammatory and neuropathic pains. Treatment with a nonselective FF1/FF2 agonist potentiates allodynia in neuropathic rats and increases sensitivity to noxious thermal and to non-noxious mechanical stimuli in normal rats in an FF1 antagonist-reversible manner. Treatment with FF1 antagonists reversed established mechanical allodynia, indicating the possibility of increased NPFF tone through FF1 receptors. In conclusion, we provide evidence for the opposing roles of NPFF receptors and highlight selective FF2 agonism and/or selective FF1 antagonism as potential targets warranting further investigation.

Facilitation of spinal morphine analgesia in normal and morphine tolerant animals by neuropeptide SF and related peptides

Neuropeptide FF and related synthetic amidated peptides have been shown to elicit sustained anti-nociceptive responses and potently augment spinal anti-nociceptive actions of spinal morphine in tests of thermal and mechanical nociception. Recent studies have described the occurrence of another octapeptide, neuropeptide SF (NPSF) in the spinal cord and the cerebrospinal fluid and demonstrated its affinity for the NPFF receptors. This study examined the effects of NPSF and two putative precursor peptides, EFW-NPSF and NPAF, on the spinal actions of morphine in normal and opioid tolerant rats using the tailflick and pawpressure tests. In normal rats, NPSF demonstrated weak intrinsic activity but sub-effective doses of the peptide significantly increased the magnitude and duration of spinal morphine anti-nociception in both tests. A low-dose of NPSF also augmented the spinal actions of a delta receptor agonist, deltorphin. The morphine-potentiating effect of NPSF was shared by EFW-NPSF and the octadecapeptide NPAF. In animal rendered tolerant by continuous intrathecal infusion of morphine for 6 days, low dose NPSF itself elicited a significant anti-nociceptive response and potently increased morphine-induced response in both tests. In animals made tolerant by repeated injections of intrathecal morphine, administration of NPSF, EFW-NPSF, and NPAF with morphine reversed the loss of the anti-nociceptive effect and restored the agonist potency. The results demonstrate that in normal animals NPSF and related peptides exert strong potentiating effect on morphine anti-nociception at the spinal level and in tolerant animals these agents can reverse the loss of morphine potency.

RFamide-related peptides signal through the neuropeptide FF receptor and regulate pain-related responses in the rat

The mammalian RFamide-related peptide RFRP1 was found to signal through the neuropeptide FF 2 receptor expressed in Xenopus oocytes. The peptide induced a dose-dependent outward current, which was dependent on the simultaneous expression of GIRK1 and GIRK4 potassium channels. In neuropathic rats, RFRP1 administered intrathecally induced tactile antiallodynia and thermal antinociception, whereas in the solitary tract nucleus it produced only mechanical antihyperalgesia. Expression of the RFamide-related peptide mRNA in the rat CNS was distinctly different from that of neuropeptide FF. Most notably, the gene was not expressed in the hindbrain or spinal cord at detectable levels. However, there was a prominent group of RFamide-related peptide mRNA-expressing neurons in the central hypothalamus, in the area in and between the dorsomedial and ventromedial nuclei. The results suggest that RFamide-related peptides are potentially involved in pain regulation through a hypothalamo-medullary projection system, and possibly via action on neuropeptide FF 2 receptors. In neuropathic animals, the pain suppressive effect of RFamide-related peptide varies depending on the submodality of noxious test stimulation and the site of RFamide-related peptide administration.

Prolactin-releasing peptide affects pain, allodynia and autonomic reflexes through medullary mechanisms

Prolactin-releasing peptide (PrRP) and neuropeptide FF (NPFF) are RF-amide peptides expressed in brain areas involved in pain modulation. NPFF displays multiple effects on acute, inflammatory and neuropathic pain. The potential role of PrRP in pain was addressed by intrathecal and intracerebral injections of PrRP on pain-related responses in both neuropathic and normal rats. Particularly in the dorsal medulla, PrRP produced significant antinociception in normal rats and an antiallodynic effect in neuropathic rats. To understand the basis of PrRP-induced pain modulation, distributions of PrRP, PrRP receptor, and NPFF were compared in the rat central nervous system. PrRP and NPFF mRNA were expressed in different parts of the nucleus of the solitary tract. In the medulla, PrRP receptor mRNA expression was abundant only in area postrema. Of the peptides studied, only NPFF mRNA was found in the dorsal horn of the spinal cord and spinal nucleus of the trigeminal nerve. PrRP-immunoreactivity corresponded to the mRNA distribution. Even if the neuronal groups producing NPFF and PrRP were distinct, the fiber networks immunoreactive for PrRP and NPFF overlapped. The results show that PrRP modulates nociception due to supraspinal rather than spinal action, and that its antinociceptive mechanism differs from that previously characterized for NPFF.

A dissociative change in the efficacy of supraspinal versus spinal morphine in the neuropathic rat

The efficacy of spinally versus supraspinally administered morphine was studied in rats with a spinal nerve ligation-induced neuropathy. Behavioural assessment indicated that the effect of intrathecally administered morphine on pain-related responses was attenuated when compared with unoperated controls. The decreased efficacy of spinal morphine was associated with neuropathic symptoms, since sham ligation or nerve ligation without accompanying tactile allodynia did not lead to spinal inefficacy of morphine. In contrast, the pain attenuating effect of morphine in the periaqueductal gray (PAG) was enhanced in neuropathic animals. The effect of systemically administered morphine on pain-related behavior of neuropathic rats was in the same range as in controls or decreased, depending on the test. Coadministration of lidocaine or MK-801, a N-methyl-D-aspartate (NMDA) receptor antagonist, into the rostroventromedial medulla enhanced the tactile antiallodynic but not the thermal antinociceptive effect of intrathecally administered morphine in neuropathic animals. Supraspinal administration of MK-801 or lidocaine did not influence efficacy of spinal morphine in sham-operated animals. Electrophysiological recordings of nociceptive wide-dynamic range (WDR) neurons in the deep spinal dorsal horn of pentobarbitone-anesthetized animals corresponded to a large extent with behavioral results. The inhibitory effect of spinally and systemically administered morphine on WDR neuron responses was attenuated whereas that induced by morphine in the PAG was enhanced in neuropathic animals. The results indicate that in spinal nerve ligation-induced neuropathy the efficacy of spinal morphine is decreased whereas that of supraspinal morphine is increased. Descending influence from brainstem-spinal pathways, involving NMDA receptors in the rostroventromedial medulla, may contribute to the selective reduction in tactile antiallodynic efficacy of spinal morphine.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (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).

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Response properties of neurons in the rostroventromedial medulla of neuropathic rats: attempted modulation of responses by [1DMe]NPYF, a neuropeptide FF analogue

We determined whether chronic neuropathy changes response properties of neurons in the rostroventromedial medulla of rats, and whether (d-Tyr)L(Me-Phe)QPQRF-amide, a neuropeptide FF analogue, in the periaqueductal gray produces changes in responses of rostroventromedial medullary neurons that might underlie its antiallodynic effect described earlier. Single unit recordings of medullary neurons were performed in lightly anesthetized neuropathic and control animals. Spontaneous activity and the responses to noxious thermal and mechanical stimulation of the hind paw were determined with and without administration of (d-Tyr)L(Me-Phe)QPQRF-amide. The neurons were classified into three groups: ON-neurons increased, OFF-neurons decreased, and NEUTRAL-neurons did not change their discharge rate prior to a limb withdrawal induced by noxious stimulation of the skin. Spontaneous activity and heat-evoked responses of ON-neurons were not different between neuropathic and control animals, whereas their mechanically evoked responses were reduced in neuropathy. Response properties of OFF-neurons were not different between neuropathic and control animals. Spontaneous activity of NEUTRAL-neurons was not different between neuropathic and control animals. (d-Tyr)L(Me-Phe)QPQRF-amide in the periaqueductal gray had no significant effect on evoked responses or spontaneous activity of ON- or OFF-neurons, independent of the experimental group. However, (d-Tyr)L(Me-Phe)QPQRF-amide produced a significant attenuation of spontaneous activity of NEUTRAL-neurons in neuropathic animals. In a behavioral study performed in unanesthetized animals it was found that intrathecal administration of methysergide, a serotonin antagonist, selectively attenuated neuropathic symptoms. Also, light pentobarbitone anesthesia markedly attenuated, but did not abolish, behaviorally determined neuropathic symptoms. From these results we suggest that NEUTRAL-neurons of the rostroventromedial medulla may have a role in neuropathy and they may be involved in attenuation of mechanical hypersensitivity by (d-Tyr)L(Me-Phe)QPQRF-amide in the periaqueductal gray. It is proposed that in neuropathy the synaptic effects of descending impulses from medullary NEUTRAL-neurons on their axonal targets in the spinal cord are changed so that this contributes to mechanical hypersensitivity, due to mechanisms that are at least partly serotoninergic.