Dynorphin-(1-8) inhibits the release of substance P-like immunoreactivity in the spinal cord of rats following a noxious mechanical stimulus

Lumbar endplate microfracture injury induces Modic-like changes, intervertebral disc degeneration and spinal cord sensitization - an in vivo rat model

BACKGROUND CONTEXT

Endplate (EP) injury plays critical roles in painful IVD degeneration since Modic changes (MCs) are highly associated with pain. Models of EP microfracture that progress to painful conditions are needed to better understand pathophysiological mechanisms and screen therapeutics.

PURPOSE

Establish in vivo rat lumbar EP microfracture model and assess crosstalk between IVD, vertebra and spinal cord.

STUDY DESIGN/SETTING

In vivo rat EP microfracture injury model with characterization of IVD degeneration, vertebral remodeling, spinal cord substance P (SubP), and pain-related behaviors.

METHODS

EP-injury was induced in 5 month-old male Sprague-Dawley rats L4-5 and L5-6 IVDs by puncturing through the cephalad vertebral body and EP into the NP of the IVDs followed by intradiscal injections of TNFα (n=7) or PBS (n=6), compared with Sham (surgery without EP-injury, n=6). The EP-injury model was assessed for IVD height, histological degeneration, pain-like behaviors (hindpaw von Frey and forepaw grip test), lumbar spine MRI and μCT, and spinal cord SubP.

RESULTS

Surgically-induced EP microfracture with PBS and TNFα injection induced IVD degeneration with decreased IVD height and MRI T2 signal, vertebral remodeling, and secondary damage to cartilage EP adjacent to the injury. Both EP injury groups showed MC-like changes around defects with hypointensity on T1-weighted and hyperintensity on T2-weighted MRI, suggestive of MC type 1. EP injuries caused significantly decreased paw withdrawal threshold, reduced axial grip, and increased spinal cord SubP, suggesting axial spinal discomfort and mechanical hypersensitivity and with spinal cord sensitization.

CONCLUSIONS

Surgically-induced EP microfracture can cause crosstalk between IVD, vertebra, and spinal cord with chronic pain-like conditions.

CLINICAL SIGNIFICANCE

This rat EP microfracture model was validated to induce broad spinal degenerative changes that may be useful to improve understanding of MC-like changes and for therapeutic screening.

Lumbar endplate microfracture injury induces Modic-like changes, intervertebral disc degeneration and spinal cord sensitization - An In Vivo Rat Model

BACKGROUND CONTEXT : Endplate (EP) injury plays critical roles in painful IVD degeneration since Modic changes (MCs) are highly associated with pain. Models of EP microfracture that progress to painful conditions are needed to better understand pathophysiological mechanisms and screen therapeutics. PURPOSE : Establish in vivo rat lumbar EP microfracture model with painful phenotype. STUDY DESIGN/SETTING : In vivo rat study to characterize EP-injury model with characterization of IVD degeneration, vertebral bone marrow remodeling, spinal cord sensitization, and pain-related behaviors. METHODS : EP-driven degeneration was induced in 5-month-old male Sprague-Dawley rats L4-5 and L5-6 IVDs through the proximal vertebral body injury with intradiscal injections of TNFα (n=7) or PBS (n=6), compared to Sham (surgery without EP-injury, n=6). The EP-driven model was assessed for IVD height, histological degeneration, pain-like behaviors (hindpaw von Frey and forepaw grip test), lumbar spine MRI and μCT analyses, and spinal cord substance P (SubP). RESULTS : EP injuries induced IVD degeneration with decreased IVD height and MRI T2 values. EP injury with PBS and TNFα both showed MC type1-like changes on T1 and T2-weighted MRI, trabecular bone remodeling on μCT, and damage in cartilage EP adjacent to the injury. EP injuries caused significantly decreased paw withdrawal threshold and reduced grip forces, suggesting increased pain sensitivity and axial spinal discomfort. Spinal cord dorsal horn SubP was significantly increased, indicating spinal cord sensitization. CONCLUSIONS : EP microfracture can induce crosstalk between vertebral bone marrow, IVD and spinal cord with chronic pain-like conditions. CLINICAL SIGNIFICANCE : This rat EP microfracture model of IVD degeneration was validated to induce MC-like changes and pain-like behaviors that we hope will be useful to screen therapies and improve treatment for EP-drive pain.

Mechanisms of μ-opioid receptor inhibition of NMDA receptor-induced substance P release in the rat spinal cord

The interaction between NMDA receptors and μ-opioid receptors in primary afferent terminals was studied by using NMDA to induce substance P release, measured as neurokinin 1 receptor internalization. In rat spinal cord slices, the μ-opioid receptor agonists morphine, DAMGO and endomorphin-2 inhibited NMDA-induced substance P release, whereas the antagonist CTAP right-shifted the concentration response of DAMGO. In vivo, substance P release induced by intrathecal NMDA after priming with BDNF was inhibited by DAMGO. ω-Conotoxins MVIIC and GVIA inhibited about half of the NMDA-induced substance P release, showing that it was partially mediated by the opening of voltage-gated calcium (Cav) channels. In contrast, DAMGO or ω-conotoxins did not inhibit capsaicin-induced substance P release. In cultured DRG neurons, DAMGO but not ω-conotoxin inhibited NMDA-induced increases in intracellular calcium, indicating that μ-opioid receptors can inhibit NMDA receptor function by mechanisms other than inactivation of Cav channels. Moreover, DAMGO decreased the ω-conotoxin-insensitive component of the substance P release. Potent inhibition by ifenprodil showed that these NMDA receptors have the NR2B subunit. Activators of adenylyl cyclase and protein kinase A (PKA) induced substance P release and this was decreased by the NMDA receptor blocker MK-801 and by DAMGO. Conversely, inhibitors of adenylyl cyclase and PKA, but not of protein kinase C, decreased NMDA-induced substance P release. Hence, these NMDA receptors are positively modulated by the adenylyl cyclase-PKA pathway, which is inhibited by μ-opioid receptors. In conclusion, μ-opioid receptors inhibit NMDA receptor-induced substance P release through Cav channel inactivation and adenylyl cyclase inhibition.

Characterization of endoproteolytic processing of dynorphins by proprotein convertases using mouse spinal cord S9 fractions and mass spectrometry

Dynorphins are important neuropeptides with a central role in nociception and pain alleviation. Many mechanisms regulate endogenous dynorphin concentrations, including proteolysis. Proprotein convertases (PCs) are widely expressed in the central nervous system and specifically cleave at C-terminal of either a pair of basic amino acids, or a single basic residue. The proteolysis control of endogenous big dynorphin (BDyn) and dynorphin A (Dyn A) levels has a profound impact on pain perception and the role of PCs remain unclear. The objective of this study was to decipher the role of PC1 and PC2 in the proteolysis control of BDyn and Dyn A levels using cellular fractions of spinal cords from wild-type (WT), PC1(-/+) and PC2(-/+) animals and mass spectrometry. Our results clearly demonstrate that both PC1 and PC2 are involved in the proteolysis regulation of BDyn and Dyn A with a more important role for PC1. C-terminal processing of BDyn generates specific peptide fragments dynorphin 1-19, dynorphin 1-13, dynorphin 1-11 and dynorphin 1-7, and C-terminal processing of Dyn A generates dynorphin 1-13, dynorphin 1-11 and dynorphin 1-7, all these peptide fragments are associated with PC1 or PC2 processing. Moreover, the proteolysis of BDyn leads to the formation of Dyn A and Leu-Enk, two important opioid peptides. The rate of formation of both is significantly reduced in cellular fractions of spinal cord mutant mice. As a consequence, even the partial inhibition of PC1 or PC2 may impair the endogenous opioid system.

Liquid chromatography-electrospray linear ion trap mass spectrometry analysis of targeted neuropeptides in Tac1(-/-) mouse spinal cords reveals significant lower concentration of opioid peptides

Tachykinin and opioid peptides play a central role in pain transmission, modulation and inhibition. The treatment of pain is very important in medicine and many studies using NK1 receptor antagonists failed to show significant analgesic effects in humans. Recent investigations suggest that both pronociceptive tachykinins and the analgesic opioid systems are important for normal pain sensation. The analysis of opioid peptides in Tac1(-/-) spinal cord tissues offers a great opportunity to verify the influence of the tachykinin system on specific opioid peptides. The objectives of this study were to develop an HPLC-MS/MRM assay to quantify targeted peptides in spinal cord tissues. Secondly, we wanted to verify if the Tac1(-/-) mouse endogenous opioid system is hampered and therefore affects significantly the pain modulatory pathways. Targeted neuropeptides were analyzed by high performance liquid chromatography linear ion trap mass spectrometry. Our results reveal that EM-2, Leu-Enk and Dyn A were down-regulated in Tac1(-/-) spinal cord tissues. Interestingly, Dyn A was almost 3 fold down-regulated (p<0.0001). No significant concentration differences were observed in mouse Tac1(-/-) spinal cords for Met-Enk and CGRP. The analysis of Tac1(-/-) mouse spinal cords revealed noteworthy decreases of EM-2, Leu-Enk and Dyn A concentrations which strongly suggest a significant impact on the endogenous pain-relieving mechanisms. These observations may have insightful impact on future analgesic drug developments and therapeutic strategies.

Determination of specific neuropeptides modulation time course in a rat model of osteoarthritis pain by liquid chromatography ion trap mass spectrometry

Animal models are useful to evaluate pharmacological therapies to alleviate joint pain. The present study characterized central neuropeptides modulation in the monoiodoacetate (MIA) rat model. Animals receiving a single 3mg MIA injection were euthanized at 3, 7, 14, 21 and 28 days post injection. Spinal cords were analyzed by liquid chromatography ion trap mass spectrometry. Up-regulations of the calcitonin gene-related peptide and substance P were observed starting on days 7 and 28 respectively, whereas big dynorphin(₁₋₃₂) content decreased significantly on day 14 in comparison to control animals (P<0.05). Preclinical drug evaluations using this model should be conducted between 7 and 21 days post injection when the lesions resemble most to human osteoarthritis.

Functional blockage of the cannabinoid receptor type 1 evokes a kappa-opiate-dependent analgesia

Progress in the control and treatment of pain may be facilitated by a better understanding of mechanisms underlying nociceptive processing. Cannabinoids and opioids are endogenous modulator of pain sensation, but therapies based in these compounds are not completely exploited because of their side effects. To test the role of cannabinoid receptor type 1 (CB1-R) inhibition in nociception, we performed a subchronic administration of the CB1-R antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM281) in mice. This treatment provoked analgesia in peripheral thermal and visceral models of pain. Analysis of genes encoded for the opioid system in the spinal cord showed an increase in the expression of genes encoded for the κ-opioid system in AM281-injected mice compared with vehicle-injected ones. Furthermore, systemic administration of nor-binaltorphimine, a κ-opioid receptor antagonist, blocked AM281-induced analgesia. Finally, c-fos expression in the dorsal spinal cord and higher centers of pain processing after noxious stimulation were significantly lower in AM281-injected mice than in vehicle-injected animals, indicating that dynorphin could block nociceptive information transmission at the spinal cord level. These results indicate the existence of a cross-talk between opioid and cannabinoid systems in nociception. Furthermore, the results suggest that CB1-R antagonists could be useful as a new therapeutic approach for pain relief.

Nociceptive behavior induced by the endogenous opioid peptides dynorphins in uninjured mice: evidence with intrathecal N-ethylmaleimide inhibiting dynorphin degradation

Dynorphins, the endogenous opioid peptides derived from prodynorphin may participate not only in the inhibition, but also in facilitation of spinal nociceptive transmission. However, the mechanism of pronociceptive dynorphin actions, and the comparative potential of prodynorphin processing products to induce these actions were not fully elucidated. In our studies, we examined pronociceptive effects of prodynorphin fragments dynorphins A and B and big dynorphin consisting of dynorphins A and B, and focused on the mechanisms underlying these effects. Our principal finding was that big dynorphin was the most potent pronociceptive dynorphin; when administered intrathecally into mice at extremely low doses (1-10fmol), big dynorphin produced nociceptive behavior through the activation of the NMDA receptor ion-channel complex by acting on the polyamine recognition site. We next examined whether the endogenous dynorphins participate in the spinal nociceptive transmission using N-ethylmaleimide (NEM) that blocks dynorphin degradation by inhibiting cysteine proteases. Similar to big dynorphin and dynorphin A, NEM produced nociceptive behavior mediated through inhibition of the degradation of endogenous dynorphins, presumably big dynorphin that in turn activates the NMDA receptor ion-channel complex by acting on the polyamine recognition site. Our findings support the notion that endogenous dynorphins are critical neurochemical mediators of spinal nociceptive transmission in uninjured animals. This chapter will review above-described phenomena and their mechanism.

Molecular bases of caloric restriction regulation of neuronal synaptic plasticity

Aging is associated with the decline of cognitive properties. This situation is magnified when neurodegenerative processes associated with aging appear in human patients. Neuronal synaptic plasticity events underlie cognitive properties in the central nervous system. Caloric restriction (CR; either a decrease in food intake or an intermittent fasting diet) can extend life span and increase disease resistance. Recent studies have shown that CR can have profound effects on brain function and vulnerability to injury and disease. Moreover, CR can stimulate the production of new neurons from stem cells (neurogenesis) and can enhance synaptic plasticity, which modulate pain sensation, enhance cognitive function, and may increase the ability of the brain to resist aging. The beneficial effects of CR appear to be the result of a cellular stress response stimulating the production of proteins that enhance neuronal plasticity and resistance to oxidative and metabolic insults; they include neurotrophic factors, neurotransmitter receptors, protein chaperones, and mitochondrial biosynthesis regulators. In this review, we will present and discuss the effect of CR in synaptic processes underlying analgesia and cognitive improvement in healthy, sick, and aging animals. We will also discuss the possible role of mitochondrial biogenesis induced by CR in regulation of neuronal synaptic plasticity.

C2 spinal cord stimulation induces dynorphin release from rat T4 spinal cord: potential modulation of myocardial ischemia-sensitive neurons

During myocardial ischemia, the cranial cervical spinal cord (C1-C2) modulates the central processing of the cardiac nociceptive signal. This study was done to determine 1) whether C2 SCS-induced release of an analgesic neuropeptide in the dorsal horn of the thoracic (T4) spinal cord; 2) if one of the sources of this analgesic peptide was cervical propriospinal neurons, and 3) if chemical inactivation of C2 neurons altered local T4 substance P (SP) release during concurrent C2 SCS and cardiac ischemia. Ischemia was induced by intermittent occlusion of the left anterior descending coronary artery (CoAO) in urethane-anesthetized Sprague-Dawley rats. Release of dynorphin A (1-13), (DYN) and SP was determined using antibody-coated microprobes inserted into T4. SCS alone induced DYN release from laminae I-V in T4, and this release was maintained during CoAO. C2 injection of the excitotoxin, ibotenic acid, prior to SCS, inhibited T4 DYN release during SCS and ischemia; it also reversed the inhibition of SP release from T4 dorsal laminae during C2 SCS and CoAO. Injection of the kappa-opioid antagonist, nor-binaltorphimine, into T4 also allowed an increased SP release during SCS and CoAO. CoAO increased the number of Fos-positive neurons in T4 dorsal horns but not in the intermediolateral columns (IML), while SCS (either alone or during CoAO) minimized this dorsal horn response to CoAO alone, while inducing T4 IML neuronal recruitment. These results suggest that activation of cervical propriospinal pathways induces DYN release in the thoracic spinal cord, thereby modulating nociceptive signals from the ischemic heart.

Time course of the enhancement and restoration of the analgesic efficacy of codeine and morphine by Δ9-tetrahydrocannabinol

Delta9-tetrahydrocannabinol (delta9-THC) synergizes with morphine and codeine by releasing endogenous opioids. These studies determined 1) the duration of enhancement of morphine and codeine by delta9-THC, 2) the effect of (delta9-THC on the time course of fully efficacious doses of the opioids, 3) restoration of efficacy of morphine and codeine by delta9-THC, and 4) duration of restoration. Sub-active combination doses of delta9-THC/morphine or delta9-THC/codeine are equivalent in duration of action and efficacy to high-dose opioids alone. Delta9-THC (20 mg/kg p.o.) significantly restores the antinociceptive effects of both high-dose morphine and codeine (100 and 200 mg/kg p.o., respectively) at later time points at which morphine or codeine was no longer active (360- and 120-min post-administration, respectively). Thus, the cannabinoid/opioid combination might be useful in therapeutics to enhance opioid activity, as well as to restore the efficacy of opioids.

Possible involvement of dynorphin A-(1-17) release via mu1-opioid receptors in spinal antinociception by endomorphin-2

The antinociception induced by i.t. or i.c.v. administration of endomorphins is mediated via mu-opioid receptors. However, although endomorphins do not have an appreciable affinity for kappa-opioid receptors, pretreatment with the kappa-opioid receptor antagonist norbinaltorphimine markedly reduces the antinociceptive response to i.c.v. or i.t. administered endomorphin-2 but not endomorphin-1. These results suggest that endomorphin-2 initially stimulates mu-opioid receptors, which subsequently induce the release of dynorphins that act on kappa-opioid receptors to produce antinociception. The present study was performed in mice to determine whether the release of dynorphins by i.t. administered endomorphin-2 is mediated through mu-opioid receptors to produce antinociception. Intrathecal pretreatment with an antiserum against dynorphin A-(1-17), but not against dynorphin B-(1-13) or alpha-neoendorphin, dose-dependently prevented the paw-withdrawal inhibition by endomorphin-2. The pretreatments with these antisera did not affect the endomorphin-1- or [D-Ala(2),MePhe(4),Gly(ol)(5)]enkephalin-induced paw-withdrawal inhibition. The attenuation of endomorphin-2-induced antinociception by i.t. pretreatment with an antiserum against dynorphin A-(1-17) or s.c. pretreatment with norbinaltorphimine was blocked dose-dependently by s.c. pretreatment with the mu-opioid receptor antagonist beta-funaltrexamine or the mu(1)-opioid receptor antagonist naloxonazine at ultra-low doses that are ineffective against mu-opioid receptor agonists. These results suggest that the spinal antinociception induced by endomorphin-2 is mediated through the stimulation of a distinct subtype of mu(1)-opioid receptor that induces the release of the endogenous kappa-opioid peptide dynorphin A-(1-17) in the spinal cord.

Pronociceptive role of dynorphins in uninjured animals: N -ethylmaleimide-induced nociceptive behavior mediated through inhibition of dynorphin degradation

Intrathecal (i.t.) administration into mice of N-ethylmaleimide (NEM), a cysteine protease inhibitor, produced a characteristic behavioral response, the biting and/or licking of the hindpaw and the tail along with slight hindlimb scratching directed toward the flank. The behavior induced by NEM was inhibited by the intraperitoneal injection of morphine. We have recently reported that dynorphin A and, more potently big dynorphin, consisting of dynorphins A and B, produce the same type of nociceptive response whereas dynorphin B does not [Tan-No K, Esashi A, Nakagawasai O, Niijima F, Tadano T, Sakurada C, Sakurada T, Bakalkin G, Terenius L, Kisara K. Intrathecally administered big dynorphin, a prodynorphin-derived peptide, produces nociceptive behavior through an N-methyl-d-aspartate receptor mechanism. Brain Res 2002;952:7-14]. The NEM-induced nociceptive behavior was inhibited by pretreatment with dynorphin A- or dynorphin B-antiserum and each antiserum also reduced the nociceptive effects of i.t.-injected synthetic big dynorphin. The characteristic NEM-evoked response was not observed in prodynorphin knockout mice. Naloxone, an opioid receptor antagonist, had no effects on the NEM-induced behavior. Ifenprodil, arcaine and agmatine, antagonists at the polyamine recognition site on the N-methyl-D-aspartate (NMDA) receptor ion-channel complex, and MK-801, an NMDA ion-channel blocker inhibited the NEM-induced effects. Ro25-6981, an antagonist of the NMDA receptor subtype containing NR2B subunit was not active. NEM completely inhibited degradation of dynorphin A by soluble and particulate fractions of mouse spinal cord. Collectively, the results demonstrate that endogenous prodynorphin-derived peptides are pronociceptive in uninjured animals, and required for the NEM-induced behavior. The NEM effects may be mediated through inhibition of the degradation of endogenous dynorphins, presumably big dynorphin that in turn activates the NMDA receptor ion-channel complex by acting on the polyamine recognition site.

Left vagal stimulation induces dynorphin release and suppresses substance P release from the rat thoracic spinal cord during cardiac ischemia

Electrostimulatory forms of therapy can reduce angina that arises from activation of cardiac nociceptive afferent fibers during transient ischemia. This study sought to determine the effects of electrical stimulation of left thoracic vagal afferents (C(8)-T(1) level) on the release of putative nociceptive [substance P (SP)] and analgesic [dynorphin (Dyn)] peptides in the dorsal horn at the T(4) spinal level during coronary artery occlusion in urethane-anesthetized Sprague-Dawley rats. Release of Dyn and SP was measured by using antibody-coated microprobes. While Dyn and SP had a basal release, occlusion of the left anterior descending coronary artery only affected SP release, causing an increase from lamina I-VII. Left vagal stimulation increased Dyn release, inhibited basal SP release, and blunted the coronary artery occlusion-induced release of SP. Dyn release reflected activation of descending pathways in the thoracic spinal cord, because vagal afferent stimulation still increased the release of Dyn after bilateral dorsal rhizotomy of T(2)-T(5). These results indicate that electrostimulatory therapy, using vagal afferent excitation, may induce analgesia, in part, via inhibition of the release of SP in the spinal cord, possibly through a Dyn-mediated neuronal interaction.

Analgesia induced by dietary restriction is mediated by the kappa-opioid system

Progress in the control and treatment of pain may be facilitated by a better understanding of mechanisms underlying nociceptive processing. Here we show that mice subjected to an intermittent fasting diet (IFD) display markedly reduced responses in models of thermal and visceral pain compared with mice fed ad libitum (AL). Pharmacological analyses suggest that a change in the endogenous kappa-opioid system underlies IFD-induced analgesia. The levels of prodynorphin mRNA and kappa-opioid receptors in the spinal cord are higher in IFD than in AL mice. Furthermore, in spinal cord nuclear protein extracts, the activity of the transcriptional repressor DREAM (downstream regulatory element antagonist modulator), the main regulator of prodynorphin expression, is lower in IFD than in AL mice. Finally, c-Fos expression in dorsal spinal cord after noxious stimulation is significantly lower in IFD than in AL animals, indicating that dynorphin could block nociceptive information at the spinal cord. These results suggest that dietary restriction together with administration of kappa-opioid agonists could be useful as a new therapeutic approach for pain relief.

Microdialysis in pain research

In vivo microdialysis has been used in preclinical pain research for more than a decade. This valuable tool allows correlations between nociceptive behavior and neurotransmitter release in pain-related CNS sites. However, several methodological issues must be considered to adequately interpret microdialysis data. Thus, the aim of this review is to describe key considerations, potential pitfalls, and important control experiments. We focus on animal experiments which evaluate the effects of noxious stimulation on CNS neurotransmitter release, particularly those that address clinically relevant problems in patients with long-lasting painful conditions.

Endogenous opioid peptides contribute to antinociceptive potency of intrathecal [Dmt1]DALDA

[Dmt(1)]DALDA (H-Dmt-d-Arg-Phe-Lys-NH(2); Dmt = 2',6'-dimethyltyrosine) is a dermorphin analog that shows high affinity and selectivity for the mu opioid receptor. The intrathecal potency of [Dmt(1)]DALDA far exceeded its affinity at mu receptors and suggests that other mechanisms must be involved in its action in the spinal cord. The affinity and selectivity of [Dmt(1)]DALDA was determined using cell membranes expressing cloned human mu, delta, and kappa opioid receptors. Competitive displacement binding with [(3)H][Dmt(1)]DALDA, [(3)H]DPDPE (H-Tyr-d-Pen-Gly-Phe-d-Pen), and [(3)H]U69,593 [(5alpha,7alpha,8beta)-(+)-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro[4.5]dec-8-yl)-benzeneacetamide] revealed K(i) of 156 +/- 26 pM for mu opioid receptor (MOR), 1.67 +/- 0.04 microM for delta opioid receptor (DOR), and K(i) of 4.4 +/- 1.7 nM for kappa opioid receptor (KOR), respectively. [Dmt(1)]DALDA increased guanosine 5'-O-(3-[(35)S]thiotriphosphate) binding in MOR, DOR, and KOR membranes, with EC(50) being 17 (8.8-33) nM, 2 (1.2-3.2) microM, and 124 (15-1000) nM, respectively. Intrathecal [Dmt(1)]DALDA inhibited the tail-flick response in mice with ED(50) = 1.22 (0.59-2.34) pmol. Intrathecal administration of an antiserum against dynorphin A(1-17) or [Met(5)]enkephalin significantly attenuated the response to i.t. [Dmt(1)]DALDA, resulting in ED(50) of 6.2 (3.6-12.6) pmol and 6.6 (3.5-19.6) pmol, respectively. Neither antisera had any effect on the response to i.t. morphine. Intracerebroventricular (i.c.v.) [Dmt(1)]DALDA was not affected by previous i.c.v. administration of anti-Dyn or anti-ME. Pretreatment with norbinaltorphimine or naltriben also attenuated the antinociceptive response to i.t., but not i.c.v., [Dmt(1)]DALDA. These data suggest that i.t. [Dmt(1)]DALDA causes the release of dynorphin and [Met(5)]enkephalin-like substances that act at kappa and delta receptors, respectively, to contribute to the extraordinary potency of [Dmt(1)]DALDA.

DREAM is a critical transcriptional repressor for pain modulation

Control and treatment of chronic pain remain major clinical challenges. Progress may be facilitated by a greater understanding of the mechanisms underlying pain processing. Here we show that the calcium-sensing protein DREAM is a transcriptional repressor involved in modulating pain. dream(-/-) mice displayed markedly reduced responses in models of acute thermal, mechanical, and visceral pain. dream(-/-) mice also exhibited reduced pain behaviors in models of chronic neuropathic and inflammatory pain. However, dream(-/-) mice showed no major defects in motor function or learning and memory. Mice lacking DREAM had elevated levels of prodynorphin mRNA and dynorphin A peptides in the spinal cord, and the reduction of pain behaviors in dream(-/-) mice was mediated through dynorphin-selective kappa (kappa)-opiate receptors. Thus, DREAM appears to be a critical transcriptional repressor in pain processing.

Dynorphin A elicits an increase in intracellular calcium in cultured neurons via a non-opioid, non-NMDA mechanism

The opioid peptide dynorphin A is known to elicit a number of pathological effects that may result from neuronal excitotoxicity. An up-regulation of this peptide has also been causally related to the dysesthesia associated with inflammation and nerve injury. These effects of dynorphin A are not mediated through opioid receptor activation but can be effectively blocked by pretreatment with N-methyl-D-aspartate (NMDA) receptor antagonists, thus implicating the excitatory amino acid system as a mediator of the actions of dynorphin A and/or its fragments. A direct interaction between dynorphin A and the NMDA receptors has been well established; however the physiological relevance of this interaction remains equivocal. This study examined whether dynorphin A elicits a neuronal excitatory effect that may underlie its activation of the NMDA receptors. Calcium imaging of individual cultured cortical neurons showed that the nonopioid peptide dynorphin A(2-17) induced a time- and dose-dependent increase in intracellular calcium. This excitatory effect of dynorphin A(2-17) was insensitive to (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine (MK-801) pretreatment in NMDA-responsive cells. Thus dynorphin A stimulates neuronal cells via a nonopioid, non-NMDA mechanism. This excitatory action of dynorphin A could modulate NMDA receptor activity in vivo by enhancing excitatory neurotransmitter release or by potentiating NMDA receptor function in a calcium-dependent manner. Further characterization of this novel site of action of dynorphin A may provide new insight into the underlying mechanisms of dynorphin excitotoxicity and its pathological role in neuropathy.

Cannabinoid modulation of dynorphin A: correlation to cannabinoid-induced antinociception

Intrathecal administration of anandamide, delta9-tetrahydrocannabinol (THC) and (-)-3-[2-hydroxy-4-(1,1-dimethyheptyl)ptyl)phenyl]-4-(3-hydr oxypropyl)-cicloexan-1-ol (CP55,940) induced spinal antinociception accompanied by differential kappa-opioid receptor involvement and dynorphin A peptide release. Antinociception using the tail-flick test was induced by the classical cannabinoid THC and was blocked totally by 17,17'-bis(cyclopropylmethyl)-6',6,7,7'-tetrahydro-4,5,4'5'-diepoxy++ +-6,6'-(imino)[7,7'-bimorphinan]-3,3',14,14'-tetrol (norbinaltorphimine) indicating a significant and critical kappa-opioid receptor component. The endogenous cannabinoid, anandamide and the non-classical bicyclic cannabinoid, CP55,940, induced non-nor-BNI-sensitive effects. The N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazo le-carboxamide (SR141716A)-mediated attenuation of spinal antinociception imparted by the various cannabinoids indicates cannabinoid CB1 receptor involvement. THC-induced an enhancement of immunoreactive dynorphin A release which coincided with the onset, but not duration antinociception. The release of dynorphin A was also attenuated by SR141716A suggesting it is cannabinoid CB1 receptor-mediated. These data indicate a critical role for dynorphin A release in the initiation of the antinociceptive effects of the cannabinoids at the spinal level.

Advances in fibromyalgia: possible role for central neurochemicals

The neurophysiologic term allodynia has been applied to fibromyalgia because people with that disorder experience pain from pressure stimuli which are not normally painful. The nociceptive neurotransmitters of animal studies are now relevant to this human model of chronic, widespread pain. Evidence is presented to implicate several chemical pain mediators (including serotonin, substance P, nerve growth factor, and dynorphin A) in the pathogenesis of fibromyalgia. This perception is hopeful because it offers many new options for the development of innovative therapy.