Role of endogenous opiates and extracellular K+ accumulation in the inhibition of frog spinal reflexes by electrical skin stimulation

Electrical skin stimulation of the hind limb (10-100 Hz, 30 s-5 min) at the intensity which leads only to the excitation of low threshold afferents depressed (for 1-30 min) the flexor reflex evoked in spinal frogs by nociceptive stimuli. The inhibition, which lasted for longer than 5 min was blocked by naloxone. Short-term poststimulation effects were associated with an increase of extracellular K+ concentration (delta [K]e) and were not blocked by naloxone. Enkephalins or morphine applied to the spinal cord surface increased the threshold for flexor reflexes while naloxone decrease their threshold. The stimulation was followed by short-term hyperpolarization of primary afferents (PAH; 1-5 min) and by depression of dorsal root potentials (DPRs) which had a similar time course to the delta [K]e, and were not blocked by naloxone. This period was frequently followed by longlasting PAH and enhancement of DRPs (5-30 min), which were abolished by naloxone. Superfusion of the isolated spinal cord with opioids produced PAH and enhanced DRPs evoked by nociceptive stimuli, while naloxone or increase of [K] in Ringer solution depolarized primary afferents and depressed DRPs. It is suggested that the antinociceptive effects of electrical stimulation of low threshold cutaneous afferents in spinal frogs involves at least two mechanisms. The short-term effect may result from delta [K]e, especially at high stimulus strength and is equally effective against noxious and non-noxious stimuli. The longlasting effects selectively affecting nociceptive transmission appear to be produced by endogenous opioids.     

Inhibition of p38 mitogen-activated protein kinase attenuates interleukin-1beta-induced thermal hyperalgesia and inducible nitric oxide synthase expression in the spinal cord

We have reported recently that intrathecal (i.t.) injection of interleukin-1beta (IL-1beta), at a dose of 100 ng, induces inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) production in the spinal cord and results in thermal hyperalgesia in rats. This study further examines the role of mitogen-activated protein kinase (MAPK) in i.t. IL-1beta-mediated iNOS-NO cascade in spinal nociceptive signal transduction. All rats were implanted with an i.t. catheter either with or without an additional microdialysis probe. Paw withdrawal latency to radiant heat is used to assess thermal hyperalgesia. The iNOS and MAPK protein expression in the spinal cord dorsal horn were examined by western blot. The [NO] in CSF dialysates were also measured. Intrathecal IL-1beta leads to a time-dependent up-regulation of phosphorylated p38 (p-p38) MAPK protein expression in the spinal cord 30-240 min following IL-1beta injection (i.t.). However, neither the phosphorylated extracellular signal-regulated kinase (p-ERK) nor phosphorylated c-Jun NH2-terminal kinase (p-JNK) was affected. The total amount of p38, ERK, and JNK MAPK proteins were not affected following IL-1beta injection. Intrathecal administration of either selective p38 MAPK, or JNK, or ERK inhibitor alone did not affect the thermal nociceptive threshold or iNOS protein expression in the spinal cord. However, pretreatment with a p38 MAPK inhibitor significantly reduced the IL-1beta-induced p-p38 MAPK expression by 38-49%, and nearly completely blocked the subsequent iNOS expression (reduction by 86.6%), NO production, and thermal hyperalgesia. In contrast, both ERK and JNK inhibitor pretreatments only partially (approximately 50%) inhibited the IL-1beta-induced iNOS expression in the spinal cord. Our results suggest that p38 MAPK plays a pivotal role in i.t. IL-1beta-induced spinal sensitization and nociceptive signal transduction.     

Intrathecal cholecystokinin interacts with morphine but not substance P in modulating the nociceptive flexion reflex in the rat

The effect of intrathecal (IT) cholecystokinin (CCK), substance P (SP) and morphine (MO) on spinal cord excitability was studied in decerebrate, spinalized rats. CCK had a weaker facilitatory effect on the nociceptive flexion reflex than SP. The possible functional significance of the coexistence of CCK and SP in neurons projecting to the spinal cord was tested by coadministration of the two peptides. At the doses tested no synergistic interaction on the reflex was found with CCK and SP. IT MO caused a brief enhancement followed by a prolonged depression of the reflex. A high dose of CCK injected prior to MO increased the facilitatory effect and decreased the depressive effect of the opiate on the reflex. The effect of desulfated (D) CCK was similar to CCK but at a higher dose. Naloxone (NAL) had a similar effect as CCK when administered prior to MO. The MO-induced depression of the reflex was readily reversed by NAL, but not by CCK. The results indicate that CCK may prevent the inhibitory effect of MO on spinal cord excitability to nociceptive stimulation, but does not reverse it. CCK may alter the balance of excitation-inhibition between various types of dorsal horn interneurons that are involved in the transmission of nociceptive information.     

Modulatory roles of the NPFF system in pain mechanisms at the spinal level

The possible roles of the NPFF system in pain processing are summarized from the viewpoints of (1) biological activities of NPFF, (2) anatomical distribution of NPFF and its receptor(s) and (3) the regulation of NPFF and receptor(s) in animal models of pain. NPFF and NPFF analogues were found to have analgesic, pronociceptive and morphine modulating activities. Since the isolation of NPFF, several other RF-NH2 peptides have been identified and some of them were found to have nociceptive or morphine modulating activity. Depending on the pharmacological doses and locations of administration, NPFF may exhibit the biological activities of other structurally related RF-NH2 peptides thus complicating NPFF bioactivity studies and their interpretation. Acid sensing ion channels were found to respond to RF-NH2 peptides including NPFF, raising the possibility that interaction of NPFF and acid sensing ion channels can modulate nociceptive activity. NPFF and NPFF receptor mRNAs are highly expressed and localized in the superficial layers of the dorsal cord, the two genes are also in dorsal root ganglia though at much lower level. The spinal NPFF system is up-regulated by peripheral inflammation in the rat. Furthermore, immunohistochemically, NPFF receptor 2-protein was demonstrated to be increased in the primary afferents in the spinal cord of rats with peripheral inflammation. Regulation and localization of spinal NPFF systems, taken together with the analgesic bioactivity of intrathecally administered NPFF, strongly suggest involvement of spinal NPFF system in pain processing.     

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.     

Intradermal hypertonic saline-induced behavior as a nociceptive test in mice

We report a nociceptive test involving peripheral irritation which produces behavior similar to that elicited by intrathecally injected substance P. Intradermal hypertonic saline injected to the lower abdominal area produced quantifiable behavior in mice. The behavior consisted of licking, biting and scratching directed to the location of i.d. injection, and was dose-dependent with respect to the concentration and volume of saline. Intrathecally administered (D-Pro2, D-Trp7,9)-SP, a substance P antagonist, dose-dependently blocked the behaviors induced by intrathecally administered substance P as well as those induced by intradermally injected hypertonic saline, indicating a possibly common final pathway at the spinal cord level for the manifestation of both behaviors. Hypertonic saline-induced behavior was blocked completely by morphine and a partial opiate agonist (pentazocine) in a dose-dependent manner, but was not blocked by another partial opiate agonist (nalorphine). The behavior was not blocked by non-steroidal anti-inflammatory agents. This nociceptive test, in conjunction with the substance P-induced behavior test, may allow discrimination between agents acting pre- or post-synaptically in the spinal cord. Baclofen, a GABAB agonist thought to act presynaptically, changed substance P-induced behavior and hypertonic saline-induced behavior in opposite directions.     

Chronic opioid potentiates presynaptic but impairs postsynaptic N-methyl-D-aspartic acid receptor activity in spinal cords: implications for opioid hyperalgesia and tolerance

Opioids are the most effective analgesics for the treatment of moderate to severe pain. However, chronic opioid treatment can cause both hyperalgesia and analgesic tolerance, which limit their clinical efficacy. In this study, we determined the role of pre- and postsynaptic NMDA receptors (NMDARs) in controlling increased glutamatergic input in the spinal cord induced by chronic systemic morphine administration. Whole-cell voltage clamp recordings of excitatory postsynaptic currents (EPSCs) were performed on dorsal horn neurons in rat spinal cord slices. Chronic morphine significantly increased the amplitude of monosynaptic EPSCs evoked from the dorsal root and the frequency of spontaneous EPSCs, and these changes were largely attenuated by blocking NMDARs and by inhibiting PKC, but not PKA. Also, blocking NR2A- or NR2B-containing NMDARs significantly reduced the frequency of spontaneous EPSCs and the amplitude of evoked EPSCs in morphine-treated rats. Strikingly, morphine treatment largely decreased the amplitude of evoked NMDAR-EPSCs and NMDAR currents of dorsal horn neurons elicited by puff NMDA application. The reduction in postsynaptic NMDAR currents caused by morphine was prevented by resiniferatoxin pretreatment to ablate TRPV1-expressing primary afferents. Furthermore, intrathecal injection of the NMDAR antagonist significantly attenuated the development of analgesic tolerance and the reduction in nociceptive thresholds induced by chronic morphine. Collectively, our findings indicate that chronic opioid treatment potentiates presynaptic, but impairs postsynaptic, NMDAR activity in the spinal cord. PKC-mediated increases in NMDAR activity at nociceptive primary afferent terminals in the spinal cord contribute critically to the development of opioid hyperalgesia and analgesic tolerance.     

Morphine tolerance in arthritic rats and serotonergic system

To understand whether chronic inflammation alters the development of morphine tolerance, the tail-flick test was used to evaluate the analgesic effect of morphine (75 mg tablet, s.c.) in the arthritic rats at the day 9-12 after the inoculation with Freund's adjuvant. Spinal cord monoamines and amino acid neurotransmitters were concomitantly measured. Chronic inflammation attenuated the antinociceptive effect of morphine as tolerance developed faster in the arthritic rats compared to the vehicle-treated controls. In addition, ratio of 5-hydroxyindole-3-acetic acid/5-hydroxytryptamine (5-HIAA/5-HT) increased in the lumbar spinal cord of arthritic rats without any change in the concentrations of norepinephrine, glutamate, aspartate or GABA. Interestingly, increased serotonin turnover in the spinal cord was observed in both control and arthritic rats 24 hours after morphine treatment. Overall, the results suggest a significant role of serotonin up-regulation in the spinal cord during chronic pain and the development of morphine tolerance.     

脊髓水平细胞外信号调节激酶激活参与吗啡依赖的形成及戒断反应的表达

在大鼠吗啡依赖和戒断模型上,采用行为学、免疫组织化学和Western blot方法观察吗啡依赖及戒断大鼠脊髓神经元磷酸化细胞外信号调节激酶（phospho-extracellular signal-regulated kinase,pERK）表达的变化,及鞘内注射促分裂原活化蛋白激酶激酶（mitogen-activated protein kinase kinase,MEK）抑制剂U0126或ERK反义寡核苷酸对吗啡依赖大鼠纳洛酮催促戒断反应、触诱发痛及脊髓神经元pERK表达的影响,探讨脊髓水平pERK在介导吗啡依赖和戒断过程中的作用。结果显示：（1）在吗啡依赖形成过程中,大鼠脊髓胞浆与胞核非磷酸化ERK表达没有改变,但pERK表达逐渐增加,纳洛酮催促戒断后,仍有进一步增加的趋势,戒断1h后,其表达量明显下降,但仍高于对照组。（2）鞘内预先注射MEK抑制剂U0126或ERK反义寡核苷酸能明显抑制吗啡戒断反应和戒断引起的痛觉异常;与行为学结果一致,脊髓背角pERK阳性神经元表达与脊髓胞浆和胞核pERK表达也明显降低。上述结果提示,脊髓水平ERK激活和核转位参与吗啡依赖的形成及戒断反应的表达。     

Induction of nociceptive responses by intrathecal injection of interleukin-1 in mice.

Intrathecal (i.t.) injection (between lumbar vertebrae 5 and 6) into mice of a markedly low dose of IL-1alpha (3x10(-4) fmol or 5.4 fg in 5 microl per mouse) induced behaviors involving scratching, biting, and licking of non-stimulated hindpaws. The IL-1-induced behaviors appeared within 10 min of the injection of IL-1alpha, peaked at 20-40 min, and had disappeared 60 min after the injection. The IL-1-induced behaviors were similar to the nociceptive responses induced in mice by i.t. injection of substance P (SP) or subcutaneous (s.c.) injection of formalin into the footpad. The IL-1-induced behaviors were suppressed by intraperitoneal morphine, indicating that they are nociceptive responses. The nociceptive responses induced by 3x10(-4) (5.4 fg) of IL-1alpha were almost completely suppressed by co-injection of 0.3 fmol (7.2 pg) of an IL-1 receptor antagonist (IL-1ra). An antiserum against substance P, but not an antiserum against somatostatin, suppressed the IL-1-induced nociceptive responses. The nociceptive responses induced by s.c. injection of 2% formalin into the footpad were also inhibited by i.t. injection of 30 pmol (720 ng) of IL-1ra. These results suggest that IL-1 may play a role in hyperalgesia in mice by acting as a factor augmenting pain transmission in the spinal cord at least in part by either directly or indirectly releasing substance P.     

Spinal cord thyrotropin releasing hormone receptors of morphine tolerant-dependent and abstinent rats

The effect of chronic administration of morphine and its withdrawal on the binding of 3H-[3-MeHis2]thyrotropin releasing hormone (3H-MeTRH) to membranes of the spinal cord of the rat was determined. Male Sprague-Dawley rats were implanted with either 6 placebo or 6 morphine pellets (each containing 75-mg morphine base) during a 7-day period. Two sets of animals were used. In one, the pellets were left intact at the time of sacrificing (tolerant-dependent) and in the other, the pellets were removed 16 hours prior to sacrificing (abstinent rats). In placebo-pellet-implanted rats, 3H-MeTRH bound to the spinal cord membranes at a single high affinity binding site with a Bmax of 21.3 +/- 1.6 fmol/mg protein, and an apparent dissociation constant Kd of 4.7 +/- 0.8 nM. In morphine tolerant-dependent or abstinent rats, the binding constants of 3H-MeTRH to spinal cord membranes were unaffected. Previous studies from this laboratory indicate that TRH can inhibit morphine tolerance-dependence and abstinence processes without modifying brain TRH receptors. Together with the present results, it appears that the inhibitory effect of TRH on morphine tolerance-dependence and abstinence is probably not mediated via central TRH receptors but may be due to its interaction with other neurotransmitter systems.     

Biological time-dependent difference in effect of peroxynitrite demonstrated by the mouse hot plate pain model

We previously demonstrated the rhythmic pattern of L-arginine/nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) cascade in nociceptive processes. The coupled production of excess NO and superoxide leads to the formation of an unstable intermediate peroxynitrite, which is primarily responsible for NO-mediated toxicity. In the present study, we evaluated the biological time-dependent effects of exogenously administered peroxynitrite on nociceptive processes and peroxynitrite-induced changes in the analgesic effect of morphine using the mouse hot-plate pain model. Experiments were performed at four different times of day (1, 7, 13, and 19 hours after lights on, i.e., HALO) in mice of both sexes synchronized to a 12 h:12 h light-dark cycle. Animals were injected intraperitoneally (i.p.) with saline or 10 mg/kg morphine 30 min before and 0.001 mg/kg peroxynitrite 30 sec before hot-plate testing, respectively. The analgesic effect of morphine exhibited significant biological time-dependent differences in the thermally-induced algesia; whereas, administration of peroxynitrite alone exhibited either significant algesic or analgesic effect, depending on the circadian time of its injection. Concomitant administration of peroxynitrite and morphine reduced morphine-induced analgesia at three of the four different study time points. In conclusion, peroxynitrite displayed nociceptive and antinociceptive when administered alone according to the circadian time of treatment, while it diminished analgesic activity when administered in combination with morphine at certain biological times.     

Inhibition of noxious stimulus-induced spinal prostaglandin E2 release by flurbiprofen enantiomers: a microdialysis study

Peripheral noxious stimuli have been shown to induce prostaglandin (PG) E2 release at the site of inflammation and in the spinal cord. The antiinflammatory and antinociceptive effects of cyclooxygenase-inhibiting drugs are thought to depend on the inhibition of PG synthesis. R-Flurbiprofen, however, does not inhibit cyclooxygenase activity in vitro but still produces antinociceptive effects. To find out whether R-flurbiprofen acts via inhibition of spinal PG release, concentrations of PGE2 and flurbiprofen in spinal cord tissue were assessed by microdialysis. The catheter was transversally implanted through the dorsal horns of the spinal cord at level L4. R- and S-flurbiprofen (9 and 27 mg kg(-1), respectively) were administered intravenously 10-15 min before subcutaneous injection of formalin into the dorsal surface of one hindpaw. Flurbiprofen was rapidly distributed into the spinal cord with maximal concentrations after 30-45 min. Baseline PGE2 dialysate concentrations were 100.6 +/- 6.4 pg ml(-1) (mean +/- SEM). After formalin injection they rose about threefold with a maximum of 299.4 +/- 68.4 pg ml(-1) at 7.5 min. After approximately 1 h PGE2 levels returned to baseline. Both flurbiprofen enantiomers completely prevented the formalin-induced increase of spinal PGE2 release and reduced PGE2 concentrations below basal levels. S- and R-flurbiprofen at 9 mg kg(-1) produced a minimum of 15.8 +/- 5.2 and 27.7 +/- 14.9 pg ml(-1), respectively, and 27 mg kg(-1) S- and R-flurbiprofen resulted in 11.7 +/- 1.7 and 9.3 +/- 4.7 pg ml(-1), respectively. PGE2 levels remained at the minimum up to the end of the observation period at 5 h. When 27 mg kg(-1) R-flurbiprofen was injected intravenously without subsequent formalin challenge, baseline immunoreactive PGE2 concentrations were not affected. S-Flurbiprofen (27 mg kg(-1)), however, led to a moderate reduction (approximately 40%). The data suggest that antinociception produced by R-flurbiprofen is mediated at least in part by inhibition of stimulated spinal PGE2 release and support the current view that increased spinal PGE2 release significantly contributes to nociceptive processing.     

Spinal Morphine Administration Reduces the Fatty Acid Contents in Spinal Cord and Brain by Increasing Oxidative Stress

It is well known that oxidative stress damages bimolecules such as DNA and lipids. No study is available on the morphine-induced oxidative damage and fatty acids changes in brain and spinal tissues. The aim of this work was to determine the effects of morphine on the concentrations and compositions of fatty acid in spinal cord segments and brain tissues in rabbits as well as lipid peroxidation (LP) and glutathione (GSH) levels in cortex brain. Twelve New Zealand albino rabbits were used and they were randomly assigned to two groups of 6 rabbits each. First group used as control although morphine administrated to rats in second group. Cortex brain and (cervical, thoracic, lumbar) samples were taken. The fatty acids between n:18.0 and 21.0 were present in spinal cord sections and n:10 fatty acids in control animals were present in the brain tissues. Compared to n:20.0–24.0 fatty acids in spinal cord sections and 8.0 fatty acids in the brain tissues of drug administered animals. The concentration and composition of the fatty acid methyl esters in spinal cord and brain tissues was decreased by morphine treatments. LP levels in the cortex brain were increased although GSH levels were decreased by the morphine administration. In conclusion, unsaturated fatty acids contents in brain and spinal cord sections and GSH were reduced by administrating spinal morphine although oxidative stress as LP increased. The inhibition oxidative damage may be a useful strategy for the development of a new protection for morphine administration as well as opiate abuse.     

Toll-like Receptor 2 Mediates Peripheral Nerve Injury-induced NADPH Oxidase 2 Expression in Spinal Cord Microglia

We have previously reported that NADPH oxidase 2 (Nox2) is up-regulated in spinal cord microglia after spinal nerve injury, demonstrating that it is critical for microglia activation and subsequent pain hypersensitivity. However, the mechanisms and molecules involved in Nox2 induction have not been elucidated. Previous studies have shown that Toll-like receptors (TLRs) are involved in nerve injury-induced spinal cord microglia activation. In this study, we investigated the role of TLR in Nox2 expression in spinal cord microglia after peripheral nerve injury. Studies using TLR knock-out mice have shown that nerve injury-induced microglial Nox2 up-regulation is abrogated in TLR2 but not in TLR3 or -4 knock-out mice. Intrathecal injection of lipoteichoic acid, a TLR2 agonist, induced Nox2 expression in spinal cord microglia both at the mRNA and protein levels. Similarly, lipoteichoic acid stimulation induced Nox2 expression and reactive oxygen species production in primary spinal cord glial cells in vitro. Studies on intracellular signaling pathways indicate that NF-κB and p38 MAP kinase activation is required for TLR2-induced Nox2 expression in glial cells. Conclusively, our data show that TLR2 mediates nerve injury-induced Nox2 gene expression in spinal cord microglia via NF-κB and p38 activation and thereby may contribute to spinal cord microglia activation.     

脊髓水平nNOS和iNOS在吗啡戒断反应中的作用差异

目的:观察鞘内注射选择性一氧化氮合酶(nNOS)和诱导型一氧化氮合酶(iNOS)抑制剂对吗啡依赖大鼠纳洛酮催促戒断反应、脊髓Fos蛋白表达和脊髓神经元nNOS和iNOS表达的影响,以探讨nNOS和iNOS在吗啡依赖和戒断反应中的作用。方法:在大鼠吗啡依赖和戒断模型上,采用行为学、免疫组织化学和Western blot方法观察鞘内应用nNOS抑制剂7-硝基吲哚(7-Ni)和iNOS抑制剂氨基胍(AG)对吗啡依赖大鼠纳洛酮催促戒断反应、脊髓Fos蛋白表达和脊髓神经元nNOS和iNOS表达的影响。结果:①鞘内注射7-Ni、AG可明显减轻吗啡依赖大鼠戒断症状,戒断组戒断症状评分为28.6±4.89,7-Ni组为16.2±3.99(P<0.01),AG组为22.94±4.0(P<0.05);戒断组TEA评分为13.5±2.55,7-Ni、AG组分别为7.5±2.56、10.5±2.71(P<0.05);②鞘内注射7-Ni、AG可减少脊髓背角Fos阳性神经元的数目,7-Ni、AG组为228.2±49.5、296.8±50.6,低于戒断组(380±71,P<0.05);③7-Ni、AG组nNOS和iNOS阳性神经元的数目分别为169±32、10.2±2.85,均低于戒断组(239±45,16.8±5.1,P<0.05),两给药组脊髓NOS蛋白的表达也显著减少。结论:nNOS和iNOS抑制剂能减轻吗啡依赖及戒断大鼠的戒断症状和在脊髓水平抑制nNOS和iNOS的表达,nNOS起主要作用而iNOS可能起辅助作用。     

The pharmacological profile of delta opioid receptor ligands, (+) and (-) TAN-67 on pain modulation

We designed the nonpeptidic highly selective delta opioid receptor agonist on the basis of message address concept and the accessory site theory and synthesized (+/-) TAN-67. In spite of highly potent agonistic activity in in vitro assay, (+/-) TAN-67 (racemate) afforded a weak antinociceptive effect in the mouse tail-flick test. This result led us to separate (+/-) TAN-67 to optical pure compounds, (+) and (-) TAN-67. An i.t.-treatment with (-) TAN-67 produced profound antinociceptive effects through specifically acting on delta1 receptors. Unlike (-) TAN-67, i.t.-administered (+) TAN-67 displayed dose-related nociceptive behaviors such as scratching, biting and licking. The effect of (+) TAN-67 was blocked by i.t.-treatment with NTI (delta receptor antagonist) and (-) TAN-67 (delta1 receptor agonist), but not by morphine (mu receptor agonist). The mechanisms involved in spinal pain modulation induced by (+) and (-) TAN-67 were also described.     

Anatomy and physiology of a nociceptive modulatory system

Although efferent control of sensory transmission is a well-established concept, a specific network for nociceptive modulation has only recently been discovered. This network includes interconnected components at midbrain, medullary and spinal levels. At the midbrain level, electrical stimulation of the periaqueductal grey (p.a.g.) inhibits spinal neurons that respond to noxious stimuli as well as nociceptor-induced reflexes and escape behaviour in a variety of species. Midbrain stimulation also produces analgesia in patients with clinically significant pain. The rostral ventral medulla (r.v.m.) has similar behavioural and physiological effects and mediates midbrain antinociceptive actions at the level of the spinal cord. Endorphins are present at all levels of this nociceptive modulating network. Opiate microinjections at p.a.g., r.v.m. or spinal levels produce analgesia, presumably by mimicking the actions of the endorphins. The nociceptive modulatory system is diffusely organized, highly interconnected and appears to act as a unit whether activated by opiates or electrical stimulation. There are two classes of r.v.m. neurons the activity of which is correlated with the occurrence of reflexes induced by noxious stimulation. One class (the on-cell) accelerates, the other class (the off-cell) pauses just before tail flick. Both classes project to the spinal cord and are excited by electrical stimulation of the midbrain. However, when morphine is injected either systemically or into the p.a.g., the off-cell is excited and the on-cell stops firing. The off-cell is probably the r.v.m. output cell that inhibits nociceptive transmission at the level of the spinal cord. The function of the on-cell is not clear. The nociceptive modulatory system can be activated by a variety of stressful environmental factors, which are often, but not necessarily, noxious. The idea that the system acts as a simple negative feedback circuit is not consistent with its known properties.     

鞘内注射神经激肽-1受体激动剂Sar-SP增强大鼠脊髓一氧化氮合酶表达和一氧化氮生成

探讨P物质(substance P,SP)对脊髓一氧化氮合酶(nitric oxide synthase,NOS)表达和一氧化氮(nitric oxide,NO)生成的影响。实验用热甩尾法测定大鼠痛阈的变化,分别应用NADPH-d组织化学法和硝酸还原法测定大鼠脊髓内NOS表达和NO生成的变化。结果显示,鞘内注射神经激肽-1受体(neurokinin-1 receptor,NK-1)激动剂[Sar^9,Met(O2)^11]-substance P(Sar-SP)可使大鼠痛阈降低,脊髓后角浅层和中央管周围灰质内NOS表达增强,脊髓腰膨大部位NO生成增多;预先鞘内注射非选择性NK-1受体拮抗剂[D—Arg^1,D-Trp^7,9,Leu^11]-substance P(spantide)可抑制上述变化。结果表明,SP可促进脊髓内NOS表达和NO生成。     

Interleukin-1β Plays Key Roles in LPA-Induced Amplification of LPA Production in Neuropathic Pain Model

Lysophosphatidic acid (LPA) is a bioactive lipid mediator that exerts a wide range of biological actions. In recent decades, LPA has been demonstrated as an important initiator of neuropathic pain based on the mechanisms of LPA-induced feed-forward LPA amplification. In this study, we examined the possible involvement of interleukin (IL)-1β in such LPA production. Intrathecal (i.t.) LPA injection rapidly increased the expression of IL-1β mRNA in the spinal dorsal horn as early as 0.5 h after injection, and the level reached peak at 2 h. Through a developed quantitative mass spectrometry for detecting LPA species, the elevated levels of 18:1, 16:0, and 18:0 LPA in the spinal dorsal horn were observed at 3 h after 18:1 LPA injection and this elevation was completely blocked by the pretreatment of IL-1β-neutralizing antibody. Moreover, enzyme assay experiments showed that LPA (i.t.) significantly activated calcium-independent phospholipase A₂ (iPLA₂) and cytosolic phospholipase A₂ (cPLA₂) in the spinal dorsal horn at 1 and 2 h, respectively, and these biochemical changes were also significantly inhibited by IL-1β-neutralizing antibody. Similarly, IL-1β-neutralizing antibody reversed LPA-induced neuropathic pain-like behavior. These findings suggest that the early release of IL-1β is involved in LPA-induced amplification of LPA production, which underlies the initial mechanisms of LPA-induced neuropathic pain.