Chimeric peptide of Met-enkephalin and FMRFa induces antinociception and attenuates development of tolerance to morphine antinociception.

A synthetic chimeric peptide of Met-enkephalin and FMRFamide (YGGFMKKKFMRFa), based on MERF was synthesized. This peptide was tested for possible antinociceptive effects using the tail flick test in mice. The effect of the chimeric peptide on morphine antinociception and development of tolerance to the antinociceptive action of morphine was also investigated. The chimeric peptide produced significant, dose-dependent antinociception (40, 60 and 90 mg/kg) in the tail flick test. Pretreatment with naloxone (5 mg/kg, IP) significantly attenuated the antinociceptive effect induced by the chimeric peptide (90 mg/kg, IP), indicating involvement of an opioidergic mechanism. In combination experiments with morphine, the antinociceptive dose of the chimeric peptide (60 mg/kg, IP) potentiated morphine (7 mg/kg, IP) antinociception. A low dose of the chimeric peptide (10 mg/kg, IP), that did not produce significant antinociception on its own, also potentiated morphine antinociception. In the tolerance studies, male albino mice received twice daily injections of morphine (20 mg/kg, IP) followed by either saline (0.1 ml) or chimeric peptide (80 mg/kg, IP) for a period of 4 days. A control group received twice daily injections of saline (0.1 ml) for the same period. When tested on Day 5, tolerance to antinociceptive action of morphine (15 mg/kg, IP) was evidenced by decreased response in chronic morphine plus saline treated mice compared to control group. Concurrent administration of chimeric peptide (80 mg/kg, IP) with morphine significantly attenuated the development of tolerance to the antinociceptive action of morphine. The preliminary results of this study demonstrate that peripherally administered chimeric peptide can produce dose dependent, naloxone reversible, antinociception; potentiate morphine antinociception and attenuate morphine tolerance, indicating a possible role of these type of amphiactive sequences in antinociception and its modulation. These chimeric peptides may also prove to be useful tools for further ascertaining the role of FMRFa family of peptides in mechanisms leading to opiate tolerance and dependence.     

Characterization of the effects produced by neurokinins and three agonists selective for neurokinin receptor subtypes in a spinal nociceptive reflex of the rat

In the awake restrained rat the intrathecal (i.th.) administration of 6.5 pmol-40 nmol of substance P (SP), neurokinin A (NKA) or one of two selective NK-1 receptor agonists [Pro9, Met(O2)11]SP, denoted ana1 and [beta-Ala4, Sar9, Met(O2)11]SP , denoted ana2 decreased reaction time (RT) to a noxious radiant heat stimulus in a dose-related manner. The following rank order of potency was observed in relation to this response: ana1 = ana2 greater than SP much greater than NKA. The decrement of tail-flick latency was greatest at 1 min and RT returned to the basal level within 6-11 min post-administration. However, in some rats SP produced a small increase in RT (anti-nociception) at 6-11 min post-administration. The i.th. administration of neurokinin B (NKB) or a selective NK-3 receptor agonist [beta-Asp4, MePhe7]NKB), denoted ana3 induced an antinociceptive effect which was greatest at 1 min and lasted less than 11 min after NKB or more than 30 min after ana3 administration. The magnitude of the increase in RT produced by 65 pmol-40 nmol doses of these peptides is ana3 much greater than NKB much greater than SP. The effect of NKB (8.0 nmol) was significantly blocked (P less than 0.005) by prior i.th. administration of naloxone (opioid antagonist) but not by idazoxan (alpha 2-adrenoceptor antagonist), [Thi5,8, D-Phe7]BK (kinin antagonist), or following bilateral adrenalectomy. From these results, we conclude that NKB-induced antinociception is mediated by the spinal release of an opioid and not through a BK or NA mechanism. The results also suggest that the nociceptive and antinociceptive effects of neuro-kinins are mediated by the activation of NK-1 and NK-3 receptor subtypes respectively, in the rat spinal cord.     

Interactions of galanin and opioids in nociceptive modulation in the arcuate nucleus of hypothalamus in rats

The fact that galanin, beta-endorphin and their receptors are present in the arcuate nucleus of hypothalamus (ARC), coupled with our previous observation that both beta-endorphin and galanin play antinociceptive roles in pain modulation in the ARC, made it of interest to study their interactions. The hindpaw withdrawal latency (HWL) in response to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. We showed that the antinociceptive effect induced by intra-ARC injection of galanin was dose-dependently attenuated by the following intra-ARC injection of naloxone. Furthermore, intra-ARC administration of the selective mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) attenuated the increased HWL induced by intra-ARC injection of galanin in a dose-dependent manner, while the delta-opioid receptor antagonist naltrindole or the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) did not. Moreover, intra-ARC injection of a galanin receptor antagonist galantide attenuated intraperitoneal morphine-induced increases in HWLs. These results demonstrate that the antinociceptive effect of galanin was related to the opioid system, especially mu-opioid receptor was involved in, and that systemic morphine induced antinociception involves galanin in the ARC.     

Synthesis and antinociceptive effects of endomorphin-1 analogs with C-terminal linked by oligoarginine

Endomorphins (EMs) cannot be delivered into the central nervous system (CNS) in sufficient quantity to elicit antinociception when given systemically because they are severely restricted by the blood-brain barrier (BBB). In the present study, we investigated herein a series of EM-1 analogs with C-terminal linked by oligoarginine in order to improve the brain delivery and antinociception after systemic administration. Indeed, all these analogs decreased the opioid receptor affinity and in vitro pharmacological activity. Moreover, analogs 4, 7-9 produced a less potent antinociceptive activity after intracerebroventricular (i.c.v.) administration, with the ED₅₀ values about 11- to 13-fold lower potencies than that of EM-1. Nevertheless, our results revealed that EM-1 failed to induce any significant antinociception at a dose of 50 μmol/kg after subcutaneous (s.c.) administration, whereas equimolar dose of these four analogs produced a little low but significant antinociceptive effects. Naloxone (10 nmol/kg, i.c.v.) significantly blocked the antinociceptive effects, indicating an opioid and central mechanism. These results demonstrated that C-terminal of EM-1 linked to oligoarginine improved the brain delivery, eliciting potent antinociception following peripheral administration.     

Identification and synthesis of norhydromorphone, and determination of antinociceptive activities in the rat formalin test

The main objective of this paper is to report the identification and synthesis of norhydromorphone, a novel metabolite of hydromorphone, and its antinociceptive activities when tested in the formalin test as compared to other known analgesics. In addition, we are reporting for the first time the lack of antinociceptive activities of hydromorphone-3-glucuronide, dihydromorphine-3-glucuronide and dihydroisomorphine-3-glucuronide in the rat formalin test. Norhydromorphone was isolated and identified as a metabolite of hydromorphone in a cancer patient's urine. An authentic standard of norhydromorphone was synthesized. The identity of norhydromorphone in the urine sample was confirmed by comparing the LC retention time and MS ion fragmentation with the synthetic standard using a liquid chromatographic-mass spectrometric-mass spectrometric (LC-MS-MS) assay. Norhydromorphone was found to be a minor metabolite of hydromorphone in the urine. Additionally, the antinociceptive activities of norhydromorphone, hydromorphone, morphine, dihydromorphine, dihydroisomorphine, hydromorphone-3-glucuronide, dihydromorphine-3-glucuronide and dihydroisomorphine-3-glucuronide were determined in the rat formalin test following intraperitoneal (i.p.) administration. Only limited antinociception was observed and no significant increase in antinociception was detected at the three doses tested. The increased polarity of norhydromorphone as compared to hydromorphone due to the primary piperidine nitrogen may make it less favorable to cross the blood-brain-barrier (BBB), which may be partly responsible. In addition, lower intrinsic antinociceptive activity, which remains to be determined, could also contribute to the low antinociception. Our results also show that hydromorphone was five times as potent as morphine in the formalin test, while dihydromorphine and dihydroisomorphine were equipotent to and 36% as potent as morphine, respectively. Hydromorphone-3-glucuronide, dihydromorphine-3-glucuronide and dihydroisomorphine-3-glucuronide did not exhibit any antinociceptive effect at the doses tested. The results further underscore the importance of a free C3-OH to the analgesic effect of morphine alkaloids.     

Centrally-mediated antinociceptive action of RWJ-22757 (formerly McN-5195): involvement of spinal descending inhibitory pathways (an hypothesis)

The present studies were an attempt to examine the mechanism of action of the novel antinociceptive compound RWJ-22757, (+/-)-trans-3-(2-bromophenyl)-octahydroindolizine (McN-5195). Intracerebroventricular (i.c.v.) administration of RWJ-22757 produced dose-related antinociception in the mouse tail-flick (48 degrees C) and rat hot-plate (51 degrees C) tests (ED50 = 243.3 and 261.3 micrograms, respectively). In contrast, intrathecal (i.t.) administration was without effect. The antinociception produced by peripherally (i.p.) or centrally (i.c.v.) administered RWJ-22757 was attenuated by i.t. administration of 2 micrograms phentolamine, 5 micrograms yohimbine, or 10 micrograms methysergide. I.t. administration of naloxone, at a dose (0.5 micrograms) that significantly attenuated the antinociceptive effects of peripherally or centrally administered morphine, had no effect on RWJ-22757-induced antinociception. We conclude from these results, coupled with the overall pharmacological and neurochemical profile of RWJ-22757, that the data are consistent with the hypothesis that RWJ-22757 produces antinociception predominantly at a site or sites located supraspinally with little or no activity at the spinal level and that RWJ-22757 activates adrenergic and serotonergic descending inhibitory pathways, increasing the tonic activity of endogenous antinociceptive systems.     

Gender differences in the antinociceptive effect of tramadol,alone or in combination with gabapentin,in mice

Gender difference in the antinociceptive effect of tramadol and gabapentin (alone or in combination) were investigated in mice. For investigation of acute antinociceptive effect, tramadol and gabapentin were administered to mice by intraperitoneal injection and per os, respectively, and antinociceptive activity was measured by the tail-flick test 30 min after drug administration. For investigation of the development of antinociceptive tolerance to analgesics, mice were injected with tramadol (60 mg/kg), alone or in combination with gabapentin (75 mg/kg), twice daily for seven consecutive days and the tail-flicks were tested on experimental days 1, 3, 5 and 7. Results showed there was a lower ED₅₀ value of tramadol antinociception in males than in females, indicating that females were less sensitive to the drug. Gabapentin produces a limited antinociception in both males and females. The combination of gabapentin and tramadol produced synergistic effect without gender difference. Repeated administration of tramadol produced antinociceptive tolerance in both genders. Gabapentin produced synergistic effect in tramadol-tolerant mice and repeated administration of gabapentin did not alter the synergistic effect in tramadol-tolerant mice. Because females show a higher overall prevalence of pain and less sensitivity to opioids, our finding may suggest a clinical significance of combined use of the two drugs.     

Supraspinal antinociceptive effect of apelin-13 in a mouse visceral pain model

Apelin, as the endogenous ligand of the APJ receptor, is a novel identified neuropeptide whose biological functions are not fully understood. APJ receptor mRNA was found in several brain regions related to descending control system of pain, such as amygdala, hypothalamus and dorsal raphe nucleus (DRN). The present study was designed to determine whether supraspinal apelin-13 may produce antinociceptive effect observed in the acetic acid-induced writhing test, a model of visceral pain. Apelin-13 not only significantly produced preemptive antinociception at the dose of 0.3, 0.5, 1 and 3μg/mouse when injected intracerebroventricularly (i.c.v.) before acetic acid, but also significantly induced antinociception at a dose of 0.5, 1 and 3μg/mouse when injected i.c.v. after acetic acid. And i.c.v. apelin-13 did not influence 30-min locomotor activity counts in mice. Intrathecal (i.t.) administration of apelin-13 (1 and 3μg/mouse) significantly decreased the number of writhes, however, intraperitoneal (i.p.) injection of apelin-13 (10-100μg/mouse) had no effect on the number of writhes in the writhing test. The specific APJ receptor antagonist apelin-13(F13A), no-specific opioid receptor antagonist naloxone and μ-opioid receptor antagonist β-funaltrexamine hydrochloride (β-FNA) could significantly antagonize the antinociceptive effect of i.c.v. apelin-13, suggesting APJ receptor and μ-opioid receptor are involved in this process. Central low dose of apelin-13 (0.3μg/mouse, i.c.v.) could significantly potentiate the analgesic potencies of modest and even relatively ineffective doses of morphine administrated at supraspinal level. This enhanced antinociceptive effect was reversed by naloxone, suggesting that the potentiated analgesic response is mediated by opioid-responsive neurons.     

Antinociceptive role of galanin in the spinal cord of rats with inflammation, an involvement of opioid systems

The present study investigated the role of galanin in the transmission of nociceptive information in the spinal cord of rats with inflammation. Bilateral decreases in hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation were observed after acute inflammation induced by injection of carrageenan into the plantar region of the rat left hindpaw. Intrathecal injection of galanin induced significant increases in the HWLs to thermal and mechanical stimulation in rats with inflammation. The galanin-induced antinociceptive effect was more pronounced in rats with inflammation than that in intact rats. The antinociceptive effect of galanin was partly inhibited by intrathecal injection of naloxone. Furthermore, intrathecal administration of galantide, an antagonist of galanin receptor, could attenuate the antinociceptive effect induced by intraperitoneal injection of morphine, suggesting an involvement of opioid systems in the galanin-induced antinociception. The results indicate that galanin plays an important role in the transmission of nociceptive information in the spinal cord of rats with inflammation, and opioid systems are involved in the galanin-induced antinociception.     

Antinociceptive activity of a 3(2H)-pyridazinone derivative in mice.

The antinociceptive activity of a 3(2H)-pyridazinone derivative (18a) was investigated in mice. 18a administered at doses which did not change either motor coordination or locomotor activity was able to induce antinociceptive effects in four nociceptive tests, the hot plate test, the tail flick test, the writhing test, and the formalin test. In the hot plate and tail flick test, 18a-induced antinociception was observed both after intraperitoneal administration and after intracerebroventricular injection thus indicating 18a has a central site of action. The pretreatment with the opioid antagonist naloxone, the alpha2-antagonist yohimbine or the GABA(B) antagonist CGP 35348 did not change 18a-induced antinociception in the hot plate test and in the tail flick test. Pretreatment with nicotinic antagonist mecamylamine did not change 18a effects either. A reversion of the 18a effects was observed after pretreatment with the muscarinic antagonists atropine and pirenzepine. Binding experiments revealed that 18a binds to muscarinic receptors, suggesting that 18a antinociception is mediated by central muscarinic receptors. The above findings together with the lack of parasympathomimetic cholinergic side effects indicate useful clinical application for this compound.     

The antinociceptive effect and pharmacokinetics of olvanil following oral and subcutaneous dosing in the mouse

Mice were tested for response latency on a 55 degrees C hot plate after subcutaneous (S.C.) or oral administration of olvanil (dose level 200 and 300 mg/kg, respectively). Only the S.C. injection of olvanil produced antinociception. A pharmacokinetics experiment with radiolabeled olvanil (200 mg/kg) was conducted to determine whether this antinociception difference was related to a difference in plasma concentration of olvanil following the two routes of administration. The results indicate that concentrations of radioactivity (olvanil plus metabolites) in plasma reach a peak higher and faster after oral dosing than after S.C. injection. However, the area under the concentration-time curve (AUC) for recovery of radioactivity was slightly higher after the S.C. injection than after the oral dose of olvanil. In contrast, intact olvanil is barely measurable (10 to 30 ng/g) in plasma following an oral dose but is present in high concentration (100 to 2000 ng/g) following S.C. injection. The AUC for olvanil was also higher following a S.C. dose. These data indicate that olvanil fails to produce antinociception after oral dosing in mice not due to lack of absorption, but because it undergoes first pass metabolism.     

Comparative measurement of spinal CSF microdialysate concentrations and concomitant antinociception of morphine and morphine-6beta-glucuronide in rats

Morphine-6beta-glucuronide (M6G) is well known as a potent active metabolite in humans. To clarify concentration-antinociceptive effect relationships for morphine and M6G, we evaluated comparatively the pharmacokinetics and antinociceptive effects of morphine and M6G. The spinal CSF concentration and antinociception were simultaneously measured by using the combination of a microdialysis method and the formalin test in conscious rats after the s.c. administration of morphine (0.3-3 mg/kg) and M6G (0.1-3 mg/kg). The plasma concentration of M6G after s.c. administration was higher than that of morphine, as shown by the 2.1 times greater value of area under the concentration-time curve (AUC(plasma)). The spinal CSF concentrations of morphine and M6G increased dose-dependently. The AUC(CSF) of M6G was 1.6-1.8 times higher than that of morphine at each dose. Administration of morphine and M6G dose-dependently suppressed the flinching behavior induced by formalin injection. The ED(50) values for M6G were 3 times lower than those of morphine, although the spinal CSF concentration versus antinociceptive effect curves of morphine and M6G were very similar, with similar EC(50) values. These results suggest that the antinociceptive potencies of morphine and M6G, evaluated by simultaneous measurements of spinal CSF drug concentration and antinociception, are equivalent. Simultaneous measurement of spinal CSF concentration and antinociception by using microdialysis should be useful for elucidating the relationship between pharmacokinetics and pharmacodynamics of various opioids.     

Long-lasting antinociceptive effect of DAMGO chloromethyl ketone in rats

Previously, the opioid peptide Tyr-D-Ala-Gly-(NMe)Phe-CH2Cl (DAMCK) has been shown to bind irreversibly to mu opioid receptors in vitro. In the present work, the antinociceptive effect of DAMCK has been evaluated. Rats treated systemically with DAMCK (1-100 pg/kg) displayed a dose-dependent increase in tail-flick analgesia that peaked by 15 min, then stayed about the same until 60 min, followed by some decrease over time. Higher doses of DAMCK (10 ng/kg-100 microg/kg) produced a near-maximal antinociceptive effect that remained stable for 4 h. Significant antinociception was still detected 8 h, but not 24 h postinjection. In comparison, the parent peptide DAMGO (Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol) reached maximal effect by about 30 min, followed by a rapid cessation of its antinociceptive response. Naloxone administered before DAMCK antagonized the antinociceptive response of DAMCK, indicating that it was mediated via opioid receptors. Naloxone administered 45 min after DAMCK attenuated the tail-flick response to some extent, but a substantial part (40-60% depending on the peptide concentration and evaluation time) remained unaffected. Central administration of DAMCK also elicited time- and concentration-dependent, profound, opioid receptor mediated, apparently irreversible antinociception.     

Antinociceptive action of aqueous extract of the leaves of Ixora coccinea

The aim of this study was to examine the antinociceptive potential of leaves of Ixora coccinea (family: Rubeaceae). One of four doses (500, 750, 1000 or 1500 mg/kg, n = 8/dose) of aqueous leaf extract (ALE) or 1 ml of distilled water was orally administered to male rats and antinociceptive activity was ascertained using three models ofnociception (tail flick, hot plate and formalin tests). The results showed that ALE possesses considerable antinociceptive activity (when evaluated in hot plate and formalin test but not in tail flick test). The antinociceptive activity of the ALE had a rapid onset (within 1h) and a fairly long duration of action (up to 5 h) with a peak effect at 3 h. Further, the antinociceptive activity was dose-dependent and was not associated with harmful side-effects or toxicity even following subchronic administration. The antinociceptive action was mediated centrally at the supraspinal level mainly via dopaminergic mechanism. In addition, it is likely that antioxidant activity of the ALE could have played an auxiliary role in inducing antinociception. Dopaminergic and antioxidative activities of ALE could arise, respectively, from its quaternary base alkaloid and flavonoid constituents.     

Effect of melatonin injection into the periaqueductal gray on antinociception and tonic immobility in male rats

Melatonin (MLT) is a neurohormone with significant involvement in several biological functions, of which antinociception and tonic immobility (TI) may be the key neurobehavioral components to survive in adverse conditions such as a predator attack. TI-induced antinociception can be elicited, facilitated, or increased through opioid and γ-aminobutyric acid (GABA) among other chemical mediators at several levels of the central nervous system, mainly in the periaqueductal gray (PAG). The aim of this study was to assess the effect of the microinjection of MLT into the main PAG regions that are related to different integrated defensive responses, namely dorsal (D) and ventrolateral (VL), on both antinociception through the tail-flick (TF) test and TI duration as single behavioral response and on combined behavioral responses (TF/TI). We found that the microinjection of MLT into the main PAG areas produced antinociception but did not affect the TI duration. The microinjection of MLT into the D-PAG decreased TF latency during TI in the combined trial (TF/TI), which implies that TI-induced antinociception was blocked. The microinjection of MLT into the VL-PAG maintained the antinociceptive capability of the TI without addition or increase in the antinociceptive effects, implying a permissive effect by MLT on the TI-induced antinociception. MLT administration into the D-PAG decreased the TI duration on the TF/TI, whereas MLT administration into the VL-PAG had the opposite effect of significantly increasing TI duration with the TF/TI trial.     

Supraspinal circuitry mediating opioid antinociception: Antagonist and synergy studies in multiple sites

Supraspinal opioid antinociception is mediated by sensitive brain sites capable of supporting this response following microinjection of opioid agonists. These sites include the ventrolateral periaqueductal gray (vIPAG), the rostral ventromedial medulla (RVM), the locus coeruleus and the amygdala. Each of these sites comprise an interconnected anatomical and physiologically relevant system mediating antinociceptive responses through regional interactions. Such interactions have been identified using two pharmacological approaches: (1) the ability of selective antagonists delivered to one site to block antinociception elicited by opioid agonists in a second site, and (2) the presence of synergistic antinociceptive interactions following simultaneous administration of subthreshold doses of opioid agonists into pairs of sites. Thus, the RVM has essential serotonergic, opioid, cholinergic and NMDA synapses that are necessary for the full expression of morphine antinociception elicited from the vIPAG, and the vIPAG has essential opioid synapses that are necessary for the full expression of opioid antinociception elicited from the amygdala. Further, the vIPAG, RVM, locus coeruleus and amygdala interact with each other in synergistically supporting opioid antinociception.     

Stress-Induced Antinociception in Fish Reversed by Naloxone

Pain perception in non-mammalian vertebrates such as fish is a controversial issue. We demonstrate that, in the fish Leporinus macrocephalus, an imposed restraint can modulate the behavioral response to a noxious stimulus, specifically the subcutaneous injection of 3% formaldehyde. In the first experiment, formaldehyde was applied immediately after 3 or 5 min of the restraint. Inhibition of the increase in locomotor activity in response to formaldehyde was observed, which suggests a possible restraint-induced antinociception. In the second experiment, the noxious stimulus was applied 0, 5, 10 and 15 min after the restraint, and both 3 and 5 min of restraint promoted short-term antinociception of approximately 5 min. In experiments 3 and 4, an intraperitoneal injection of naloxone (30 mg.kg⁻¹) was administered 30 min prior to the restraint. The 3- minute restraint-induced antinociception was blocked by pretreatment with naloxone, but the corresponding 5-minute response was not. One possible explanation for this result is that an opioid and a non-preferential μ–opioid and/or non-opioid mechanism participate in this response modulation. Furthermore, we observed that both the 3- and 5- minutes restraint were severely stressful events for the organism, promoting marked increases in serum cortisol levels. These data indicate that the response to a noxious stimulus can be modulated by an environmental stressor in fish, as is the case in mammals. To our knowledge, this study is the first evidence for the existence of an endogenous antinociceptive system that is activated by an acute standardized stress in fish. Additionally, it characterizes the antinociceptive response induced by stress in terms of its time course and the opioid mediation, providing information for understanding the evolution of nociception modulation.     

Alternative Forms of Interaction of Substance P and Opioids in Nociceptive Transmission

Many years preclinical and clinical anatomic, pharmacologic, and physiologic studies suggest that SP- and opioid-expressing neurons produce opposite biological effects at the spinal level, i.e., nociception and antinociception, respectively. However, in certain circumstances intrathecally administered SP is capable of reinforcing of spinal morphine analgesia and may therefore function as an opioid adjuvant in vivo. The SP dose-response curve of spinally administered SP follows a bell-shaped or inverted-U configuration, permitting pharmacological dissociation of opioid-potentiating and analgesic properties of SP from traditional hyperalgesic effects seen at significantly higher concentrations. This analgesic effect is blocked by naloxone but unaffected by transection of the spinal cord, thus demonstrating the lack of supraspinal modulation. The present report briefly describes both reinforcing and opposing interactions between multiple opioid systems and substance P at the spinal level. We propose that a likely mechanism underlying SP-mediated enhancement of opioid analgesia is the ability of SP to release endogenous opioid peptides within the local spinal cord environment.     

Alternative Forms of Interaction of Substance P and Opioids in Nociceptive Transmission

Summary Many years preclinical and clinical anatomic, pharmacologic, and physiologic studies suggest that SP- and opioid-expressing neurons produce opposite biological effects at the spinal level, i.e., nociception and antinociception, respectively. However, in certain circumstances intrathecally administered SP is capable of reinforcing of spinal morphine analgesia and may therefore function as an opioid adjuvantin vivo. The SP dose-response curve of spinally administered SP follows a bell-shaped or inverted-U configuration, permitting pharmacological dissociation of opioid-potentiating and analgesic properties of SP from traditional hyperalgesic effects seen at significantly higher concentrations. This analgesic effect is blocked by naloxone but unaffected by transection of the spinal cord, thus demonstrating the lack of supraspinal modulation. The present report briefly describes both reinforcing and opposing interactions between multiple opioid systems and substance P at the spinal level. We propose that a likely mechanism underlying SP-mediated enhancement of opioid analgesia is the ability of SP to release endogenous opioid peptides within the local spinal cord environment.     

Neutrophilic cell-free exudate induces antinociception mediate by the protein S100A9

Calcium-binding protein S100A9 (MRP-14) induces antinociceptive effect in an experimental model of painful sensibility and participates of antinociception observed during neutrophilic peritonitis induced by glycogen or carrageenan in mice. In this study, the direct antinociceptive role of the protein S100A9 in neutrophilic cell-free exudates obtained of mice injected with glycogen was investigated. Mice were intraperitoneally injected with a glycogen solution, and after 4, 8, 24, and 48 hours, either the pattern of cell migration of the peritoneal exudate or the nociceptive response of animals was evaluated. The glycogen-induced neutrophilic peritonitis evoked antinociception 4 and 8 hours after inoculation of the irritant. Peritoneal cell-free exudates, collected in different times after the irritant injection, were transferred to naive animals which were submitted to the nociceptive test. The transference of exudates also induced antinociceptive effect, and neutralization of S100A9 activity by anti-S100A9 monoclonal antibody totally reverted this response. This effect was not observed when experiments were made 24 or 48 hours after glycogen injection. These results clearly indicate that S100A9 is secreted during glycogen-induced neutrophilic peritonitis, and that this protein is responsible by antinociception observed in the initial phase of inflammatory reaction. Thus, these data reinforce the hypothesis that the calcium-binding protein S100A9 participates of the endogenous control of inflammatory pain.