Evidence that inhibition of spinal nitric oxide production contributes to the antinociceptive effects of emulsified isoflurane on formalin-induced pain in rats

The aim of the present study was to investigate the contribution of spinal nitric oxide (NO) to the antinociceptive effects of emulsified isofluane in rats. The formalin test was used to assess nociceptive responses. Immunocytochemistry and histochemistry were performed to determine the effects of emulsified isoflurane on formalin-induced changes in Fos-like immunoreactive (Fos-LI)- and nicotinamide adenine dinucleotide phosphatediaphorase (NADPH-d)-positive neurons, respectively. The results showed that emulsified isofluane, administered intraperitoneally, significantly decreased the formalin-induced paw licking time and that this was attenuated by pretreatment with intrathecal injection of the NO precursor L-arginine. Furthermore, Fos-LI- and NADPH-d-positive neurons were mainly found in the ipsilateral dorsal horn after injection of formalin, some of which were Fos-LI/NADPH-d double-labelled neurons. Administration of emulsified isofluane significantly decreased Fos-LI- and NADPH-d-positive, as well as Fos-LI/NADPH-d double-labelled, neurons. Finally, emulsified isofluane produced a significant reduction of NOS activity and a decrease of NO production in the spinal cord of formalin-treated rats. In conclusion, the results suggest that inhibition of spinal NO production contributes to the antinociceptive effects of emulsified isofluane on formalin-induced pain in rats.     

鞘内注射促皮质素抑制甲醛痛敏大鼠脊髓NOS阳性神经元增多

目的：研究鞘内注射促皮质素（Ｃｏｒ）对甲醛痛敏大鼠脊髓北角一氧化氮合酶（ＮＯＳ）阳性神经元增多的影响。方法：采用痛经均数（ＰＩＲ）测定、ＮＡＤＰＨ－ｄ组织化学法、Ｆｏｓ免痛组织化学法梁 色,观察鞘内注射９ｉｔｈ）Ｃｏｒ对甲醛痛敏大鼠脊髓北角ＮＯＳ阳性神经元、Ｆｏｓ免疫反应神经元、ＮＯＳ／Ｆｏｓ双标记神经元及痛敏的影响。结果：ｉｔｈ Ｃｏｒ（０．５～１．５Ｕ）均能显著抑制甲醛引起的大鼠脊髓北角ＮＯＳ     

预先应用咪哒唑仑抑制小鼠和大鼠吗啡戒断反应

目的:研究咪哒唑仑对小鼠和大鼠吗啡戒断反应的影响.方法:实验中采用急性和慢性吗啡依赖和纳洛酮催促戒断模型.使用放免法测定cAMP含量,免疫组织化学方法观察Fos蛋白表达变化.结果:合用咪哒唑仑和吗啡可抑制小鼠急性和慢性吗啡依赖的发展.在急性吗啡依赖小鼠,咪哒唑仑-吗啡组纳络酮催促跳跃的ED_(50)(10.4,8.5-12.3 mg/kg)明显大于生理盐水-吗啡组(3.0,1.9-4.3 mg/kg)(P<0.01).在慢性吗啡依赖小鼠,咪哒唑仑-吗啡组纳络酮催促跳跃的发生率和跳跃次数明显低于生理盐水-吗啡组(P<0.01).预先使用咪哒唑仑抑制吗啡戒断大鼠脊髓Fos蛋白表达,但不能抑制脊髓cAMP含量的增加.结论:咪哒唑仑通过抑制脊髓神经元敏感化减轻吗啡戒断反应,cAMP信号转导通路不参与介导这一效应.     

鞘内注射促皮质素抑制甲醛痛敏大鼠脊髓NOS阳性神经元增多

目的:研究鞘内注射促皮质素(Cor)对甲醛痛敏大鼠脊髓背角一氧化氮合酶(NOS)阳性神经元增多的影响。方法:采用痛级均数(PIR)测定、NADPH-d组织化学法、Fos免疫组织化学法染色,观察鞘内注射(ith)Cor对甲醛痛敏大鼠脊髓背角NOS阳性神经元、Fos免疫反应神经元、NOS/Fos双标记神经元及痛敏的影响。结果:ith Cor(0.5-1.5U)均能显著抑制甲醛引起的大鼠脊髓背角NOS、Fos、NOS/Fos阳性神经元的增多和痛敏反应,其作用为ith NOS底物左旋精氨酸(Arg,5-15nmol)部分翻转。结论:Cor通过抑制大鼠脊髓背角NOS阳性神经元的增多抑制痛敏。     

NO介导吗啡戒断大鼠脊髓Fos蛋白和NMDA_(1A)受体mRNA表达的增加

目的:观察NO在纳洛酮催促吗啡戒断大鼠脊髓神经元活动变化中的作用。方法:采用Fos免疫组织化学、NADPH-d组织化学、Fos/NADPH-d双标、鞘内注射、反义寡核苷酸和RT-RCR技术。结果:急性应用纳洛酮和慢性应用吗啡对大鼠脊髓Fos蛋白及NADPH-d阳性神经元表达无明显影响,二者也无Fos/NADPH-d双标神经元表达;纳洛酮催促吗啡戒断大鼠脊髓Fos蛋白、NADPH-d阳性神经元、纤维和终末表达明显增加,且出现Fos/NADPH-d双标神经元表达。预先鞘内注射nNOS反义寡核苷酸明显降低吗啡戒断症状评分,减少吗啡戒断大鼠脊髓Fos蛋白及NMDA_(1A)R mRNA表达。结论:NO介导吗啡戒断大鼠脊髓Fos和NMD_(1A)R mRNA表达的增加。     

吗啡依赖及戒断大鼠脊髓和脑干中一氧化氮合酶基因的表达

目的　观察吗啡依赖或吗啡戒断大鼠脊髓和脑干中一氧化氮合酶 (NOS)基因表达的变化。方法　以 β actin为内参照 ,用逆转录聚合酶链反应 (RT PCR)测定NOSmRNA的表达水平。结果　吗啡依赖大鼠脊髓和脑干NOS表达水平较正常对照大鼠降低 ,纳洛酮 ( 4mg·kg-1,ip)激发大鼠吗啡戒断症状 1h后脊髓和脑干中NOS表达水平明显升高 ,戒断 2h和 4h后NOS基因表达较 1h组减少。NOS抑制剂L N 硝基精氨酸甲酯 (L NAME ,10mg·kg-1)处理后大鼠吗啡戒断症状减少 ,同时脊髓和脑干的NOS基因表达水平较戒断 1h组明显降低。甲基东莨菪碱 ( 0 5mg·kg-1)处理组脊髓和脑干中NOS表达水平较戒断 1h组明显降低 ;选择性毒蕈碱受体M1拮抗剂 pirenzepine( 10mg·kg-1)处理组动物脊髓中NOS表达水平较戒断 1h组降低 ,而脑干中NOS表达水平没有改变 ;NMDA受体拮抗剂MK 80 1( 0 12 5mg·kg-1)处理后脊髓和脑干中NOS表达水平较戒断 1h组没有差异。结论　吗啡慢性处理后脊髓和脑干中NOSmR NA水平降低 ,抑制内源性NO生成和阻断毒蕈碱受体可以减少吗啡戒断所引起脊髓和脑干中NOS基因的表达     

Increased analgesic tolerance to acute morphine in fosB knock-out mice: a gender study

The proteins of Fos family are a potential candidate to link molecular mechanisms of morphine action with behavioural effects such as morphine-induced reward, dependence and tolerance. We used both male and female mice lacking fosB gene to study its contribution to morphine effects. Morphine analgesia (tail-flick test) and hypothermia were studied using morphine at cumulative doses in morphine-naïve and morphine-tolerant (tolerance induced by 24 h prior 100 mg/kg morphine administration) mice. FosB −/− mice, as compared to fosB +/+ mice, developed enhanced tolerance to morphine-induced analgesia. No effects of genotype or gender on tolerance to morphine-induced hypothermia were observed. These results suggest that fosB may be involved in the development of tolerance to morphine analgesia but not hypothermia. The gender study implicates that lack of FosB proteins in female fosB −/− mice enhanced morphine analgesic potency. In conclusion, we show that fosB gene is important to analgesia but not hypothermia phenotype indicating its role in morphine effects.     

乳铁蛋白对福尔马林致痛大鼠行为学及一氧化氮合酶表达的影响

目的通过观察鞘内注射乳铁蛋白对福尔马林致痛大鼠行为学及一氧化氮合酶（NOS）表达的影响，探讨乳铁蛋白可能的抗伤害机制。方法雄性SD大鼠80只，随机分为生理盐水组（NS组）和福尔马林组（F组）；实验组分为乳铁蛋白1ug-福尔马林组（F-R1组），乳铁蛋白10ug-福尔马林组（F—R10组）和乳铁蛋白100ug-福尔马林组（F—R100组）。五组鞘内分别给予NS 20ul，NS 20ul，乳铁蛋白1ug，乳铁蛋白10ug，乳铁蛋白100ug；10min后，除NS组大鼠组底注射NS 100ul外，其余各组均给予5％福尔马林100ul。经以上处理8只动物在福尔马林处理后30min取脊髓观察NOS的表达，另8只动物在福尔马林处理后0～1h观察行为学。结果F组的缩腿舔爪的时间和脊髓NOS表达显著长于，强于NS组；预先给予乳铁蛋白能抑制以上的作用，且呈剂量依赖性。结论乳铁蛋白明显抑制福尔马林致痛大鼠痛行为及NOS表达，可能是其产生抗伤害作用机制之一。     

Effect of kappa-opioid receptor agonists on morphine analgesia in morphine-naive and morphine-tolerant rats

The effect of i.p. administration of kappa-opioid receptor agonists, bremazocine, tifluadom and U-50,488H on morphine (8 mg/kg i.p.)-induced analgesia in morphine-naive and morphine tolerant male Sprague-Dawley rats was determined using the tail-flick test. The tolerance to morphine in the rats was induced by s.c., implantation of six morphine pellets during a 7-day period. Implantation of morphine pellets resulted in the development of tolerance as evidenced by the decrease in the analgesic response to morphine when compared to placebo pellets implanted rats. Bremazocine (0.3, 1.0 and 3.0 mg/kg) and U-50,488H (16 mg/kg) antagonized morphine-induced analgesia in morphine-naive rats while tifluadom (8 and 16 mg/kg) potentiated the effect. In morphine-tolerant rats, bremazocine (3 mg/kg) and U-50,488H (16 mg/kg) potentiated morphine-induced analgesia. Tifluadom at any of the doses had no effect on morphine-induced analgesia in morphine-tolerant rats. These results provide evidence that different kappa-opioid agonists modify morphine-induced analgesia differentially in morphine-naive and morphine-tolerant rats.     

Alterations in brain Protein Kinase A activity and reversal of morphine tolerance by two fragments of native Protein Kinase A inhibitor peptide (PKI)

Two peptide fragments of native Protein Kinase A inhibitor (PKI), PKI-(6-22)-amide and PKI-(Myr-14-22)-amide, significantly reversed low-level morphine antinociceptive tolerance in mice. The inhibition of Protein Kinase A (PKA) activity by both peptide fragments was then measured in specific brain regions (thalamus, periaqueductal gray (PAG), and medulla) and in lumbar spinal cord (LSC), which in previous studies have been shown to play a role in morphine-induced analgesia. In drug naive animals, cytosolic PKA activity was greater than particulate PKA activity in each region, while cytosolic and particulate PKA activities were greater in thalamus and PAG compared to medulla and LSC. The addition of both peptides to homogenates from each region completely abolished cytosolic and particulate PKA activities in vitro. Following injection into the lateral ventricle of the brain of drug naive mice and morphine-tolerant mice, both peptides inhibited PKA activity in the cytosolic, but not the particulate fraction of LSC. In addition, cytosolic and particulate PKA activities were inhibited by both peptides in thalamus. These results demonstrate that the inhibition of PKA reverses morphine tolerance. Moreover, the inhibition of PKA activity in specific brain regions and LSC from morphine-tolerant mice by PKI analogs administered i.c.v. is evidence that PKA plays a role in morphine tolerance.     

N-methyl-D-aspartate receptor antagonist MK-801 suppresses glial pro-inflammatory cytokine expression in morphine-tolerant rats

Chronic opioid therapy induces tolerance and hyperalgesia, which hinders the efficacy of opioid treatment. Previous studies have shown that inhibition of neuroinflammation and glutamatergic receptor activation prevents the development of morphine tolerance. The aim of the present study was to examine whether N-Methyl-d-aspartate receptors are involved in the regulation of chronic morphine-induced neuroinflammation in morphine-tolerant rats.Morphine tolerance was induced in male Wistar rats by intrathecal infusion of morphine (15 μg/h) for 5 days. Tail-flick latency was measured to estimate the antinociceptive effect of morphine. Morphine challenge (15 μg, intrathecally) on day 5 at 3 h after discontinuation of morphine infusion produced a significant antinociceptive effect in saline-infused rats, but not in morphine-tolerant rats. Pretreatment with MK-801 (20 μg, intrathecally) 30 min before morphine challenge preserved its antinociceptive effect in morphine-tolerant rats. Morphine-tolerant rats expressed high levels of the pro-inflammatory cytokines interleukin-1β, interleukin-6, and tumor necrosis factor-α and the increase in interleukin-1β and interleukin-6, and tumor necrosis factor-α levels was prevented by MK-801 pre-treatment at both the protein and mRNA levels.The results show that a single dose of MK-801 reduces the increase in pro-inflammatory cytokines in the spinal cord, thus re-sensitizing neurons to the antinociceptive effect of morphine in morphine-tolerant rats. This study provides a piece of theoretical evidence that NMDA antagonist can be a therapeutic adjuvant in treating morphine tolerant patients for pain relief.     

Antinociceptive effects of St. John's wort, Harpagophytum procumbens extract and Grape seed proanthocyanidins extract in mice

Hypericum perforatum extract (St. John's wort, SJW), Harpagophytum procumbens extract (HPE) and Grape seed proanthocyanidin extract (GSPE) have a broad spectrum of biological activities including antidepressant, anti-inflammatory or anti-oxidant effects. The aim of this study was to clarify antinociceptive properties of SJW, HPE and GSPE in mice with mechanisms that might potentially underlie these activities. Also, the effects of these herbal extracts on the antinociception and plasma and brain concentrations of morphine were examined. Oral pretreatment with SJW (100-1000 mg/kg) and HPE (30-300 mg/kg) attenuated significantly times of licking/biting both first and second phases of formalin injection in mice in the dose-dependent manner, and GSPE (10-300 mg/kg) suppressed second phase. Naloxone (5 mg/kg, s.c.) significantly attenuated antinociceptive effect of HPE but not SJW and GSPE. Formalin injection resulted in significant increase in the content of nitrites/nitrates (NO(x)) in mouse spinal cord. The rise of spinal NO(x) content by formalin was significantly attenuated by HPE and SJW. The pretreatment with SJW significantly potentiated an antinociceptive effect of morphine (0.3 mg/kg, s.c.), although concentrations of morphine in plasma and brain were not significantly changed by these herbal extracts. In conclusion, the present study has shown that SJW, HPE and GSPE exert significant antinociceptive effects in the formalin test of mice. In addition, opioidergic system seems to be involved in the antinociceptive effect of HPE but not SJW and GSPE. Furthermore, SJW potentiates morphine-induced antinociception possibly by pharmacodynamic interaction.     

Changes in morphine-induced activation of cerebral Na(+),K(+)-ATPase during morphine tolerance: biochemical and behavioral consequences

There is ample evidence of the biological changes produced by the sustained activation of opioid receptors. We evaluated the adaptive changes of cerebral Na(+),K(+)-ATPase in response to the sustained administration of morphine (minipumps, 45mg/kg/day, 6 days) in CD-1 mice and the functional role of these changes in opioid antinociception. The antinociceptive effect of morphine as determined with tail-flick tests was reduced in morphine-tolerant mice. There were no significant changes in the density of high-affinity Na(+),K(+)-ATPase α subunits labeled with [(3)H]ouabain in forebrain membranes from morphine-tolerant compared to those of morphine-naive animals. Western blot analysis showed that there were no significant differences between groups in the changes in relative abundance of α(1) and α(3) subunits of Na(+),K(+)-ATPase in the spinal cord or forebrain. However, the morphine-induced stimulation of Na(+),K(+)-ATPase activity was significantly lower in brain synaptosomes from morphine-tolerant mice (EC(50)=1.79±0.10μM) than in synaptosomes from morphine-naive mice (EC(50)=0.69±0.12μM). Furthermore, adaptive alterations in the time-course of basal Na(+),K(+)-ATPase activity were observed after sustained morphine treatment, with a change from a bi-exponential decay model (morphine-naive mice) to a mono-exponential model (morphine-tolerant mice). In behavioral studies the antinociceptive effects of morphine (s.c.) in the tail-flick test were dose-dependently antagonized by ouabain (1 and 10ng/mouse, i.c.v.) in morphine-naive mice, but not in morphine-tolerant mice. These findings suggest that during morphine tolerance, adaptive cellular changes take place in cerebral Na(+),K(+)-ATPase activity which are of functional relevance for morphine-induced antinociception.     

芋螺毒素ω-SO3单次及连续给药对福尔马林致大鼠炎性疼痛的镇痛作用

目的评价单次及连续鞘内给药ω-SO3对福尔马林致大鼠炎性疼痛的镇痛作用。方法采用福尔马林致大鼠炎性疼痛模型,通过大鼠鞘内置管术分别单次及多次鞘内给药,观察ω-SO3对炎性疼痛急性期和持续期的镇痛强度和有效时间以及连续给药对其镇痛作用可能的影响;通过大鼠自发活动实验评价ω-SO3单次鞘内给药可能引起的中枢副反应。结果在大鼠福尔马林炎性疼痛模型上,单次鞘内给药ω-SO3产生剂量及时间依赖性的镇痛作用,其抑制炎性疼痛急性期和持续期的ED50值分别为1.79和0.41ng.g-1,比吗啡的镇痛作用强并且有效时间长;以ED80剂量每日鞘内给药2次连续5d后,ω-SO3仍然产生与单次给药类似的镇痛强度,而吗啡的镇痛作用降低,说明产生镇痛耐受,更有意义的是ω-SO3对吗啡镇痛耐受大鼠仍具有镇痛作用。在上述镇痛剂量范围内,ω-SO3单次鞘内给药对大鼠自发活动没有明显的影响。结论ω-SO3对福尔马林致炎性疼痛的急性期和持续期均具良好的镇痛作用,其镇痛作用强,有效时间长,连续给药不产生自身镇痛耐受,并且对吗啡不产生交叉耐受。     

Alteration of spinal protein kinase C expression and kinetics in morphine, but not clonidine, tolerance.

Antinociceptive synergism between spinally administered morphine and clonidine decreases to an additive interaction in morphine- and clonidine-tolerant mice. Spinally administered protein kinase C (PKC) inhibitors also decrease the synergism to addition. To determine whether chronic morphine or clonidine treatment alters spinal PKC activity, the present studies measured PKC activity and expression of PKC isoform proteins in spinal cord cytosol and membrane fractions. Mice were treated for 4 days with either placebo pellets, morphine pellets, s.c. saline, or s.c. clonidine. Morphine pellet-implanted mice were tolerant to morphine-induced tail flick antinociception, but not cross-tolerant to clonidine. Clonidine-pretreated mice were tolerant to clonidine, but not cross-tolerant to morphine. Induction of morphine tolerance produced a 2-fold lower Km value for PKC (8.24 +/- 1.67 microM in placebo pellet vs 4.43 +/- 1.24 microM in morphine pellet) in cytosol, but not membrane fractions from spinal cord. Vmax values were not different. No difference in Km or Vmax values was found between proteins from saline- and clonidine-pretreated animals. Immunoreactive cPKCalpha, betaI, and gamma isoforms decreased 14, 26, and 17%, respectively, in cytosol from morphine-tolerant animals. No difference in PKC isoforms was found in the membranes or in fractions from clonidine-tolerant mice. Morphine tolerance, but not clonidine tolerance, enhanced PKC activity while decreasing protein expression.     

Cross talk between nitric oxide and ERK1/2 signaling pathway in the spinal cord mediates naloxone-precipitated withdrawal in morphine-dependent rats

Our recent study has shown activation of spinal extracellular signal-regulated kinase-1 and -2 (ERK1/2), a member of the mitogen-activated protein kinase (MAPK) family, contributes to naloxone-precipitated withdrawal and withdrawal-induced spinal neuronal sensitization in morphine-dependent rats. However, the mechanism and significance of the spinal ERK1/2 activation during morphine dependence and withdrawal remain unknown. In this study, we reported that intrathecal (i.t.) pretreatment with either the non-selective nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), neuronal NOS (nNOS) inhibitor 7-nitro indazole (7-NI), or the inducible NOS (iNOS) inhibitor aminoguanidine (AG), could reduce morphine withdrawal-induced increase of phospho-ERK1/2 (pERK1/2) expression in the rat spinal cord. On the other hand, attenuation of the spinal ERK phosphorylation by the MAPK kinase (MEK) inhibitor U0126 also could inhibit the increase of nNOS and iNOS expression in the spinal cord of morphine withdrawal rats. Inhibitory expression of pERK1/2 by i.t. NOS inhibitor L-NAME, 7-NI or AG and of nNOS and iNOS by i.t. U0126 in the spinal cord were accompanied by decreased scores of morphine withdrawal and the inhibited spinal Fos protein (a maker for neuronal excitation or activation) expression induced by morphine withdrawal. These findings suggest cross talk between nitric oxide (NO) and the ERK1/2 signaling pathway mediates morphine withdrawal and withdrawal-induced spinal neuronal sensitization in morphine-dependent rats.     

鞘内注射大麻酚类抑制大鼠脊髓内有毒刺激引起的Fos蛋白样免疫反应及其与吗

AIM: To determine whether cannabinoids suppress noxious stimulus-evoked Fos protein-like immunoreactivity (FLI) through direct actions at the spinal level. METHODS: Rats were implanted with intrathecal (ith) catheters at least one week prior to evaluation in the formalin test. Effects of the cannabinoid agonist, CP55,940 (80 micrograms ith) on formalin pain and FLI in rat spinal cord were compared with that of the prototypic narcotic analgesic, morphine (20 micrograms ith). CP55,940 suppressed pain behavior and FLI induced by intraplantar formalin. The cannabinoid suppressed Fos in the neck region of the dorsal horn and in the ventral horn, but not in the nucleus proprius. The efficacy of the cannabinoid in suppressing FLI in these laminae and pain behavior was comparable to morphine administered via the same route. However, only morphine suppressed FLI in the superficial dorsal horn relative to vehicle treatment. CONCLUSION: Cannabinoids suppress nociceptive processing, in part, through actions at the spinal level. However, morphine showed greater potency and efficacy than CP55,940 in suppressing formalin-induced FLI following spinal administration.     

Involvement of kappa opioid receptors in formalin-induced inhibition of analgesic tolerance to morphine in mice

This study examined the role of kappa opioid receptors (KOR) in the mechanism underlying tolerance to the analgesic effects of morphine induced by chronic pain. The analgesic effect of morphine (10 mg kg(-1)), estimated by the tail-flick test in mice, gradually decreased during repeated daily morphine treatment. A significant decrease in the analgesic effect of morphine was seen on the fifth day of repeated morphine treatment compared with the first day. Chronic pain was induced by subcutaneous administration of 2% formalin into the dorsal part of the left hind paw, which significantly inhibited development of tolerance to morphine analgesia. The effect of formalin-induced pain on inhibition of morphine tolerance was reversed by the KOR antagonist nor-binaltorphimine. Furthermore, an antisense oligodeoxynucleotide, but not a missense oligodeoxynucleotide, against KOR completely suppressed the inhibitory effect of formalin-induced pain on morphine tolerance. Naltrindole, an antagonist of delta opioid receptor, did not affect chronic-pain-induced tolerance to morphine. Our findings show that the inhibitory effect of chronic pain on analgesic tolerance to morphine is mediated by KOR rather than delta opioid receptors.     

Direct evidence for the involvement of the mesolimbic kappa-opioid system in the morphine-induced rewarding effect under an inflammatory pain-like state.

Recent clinical studies have demonstrated that when morphine is used to control pain in cancer patients, psychological dependence is not a major concern. The present study was undertaken to ascertain the modulation of psychological dependence on morphine under a chronic pain-like state in rats. The prototypical mu-opioid receptor agonist morphine (8 mg/kg, i.p.) induced a dose-dependent place preference. In the present study, we found that an inflammatory pain-like state following formalin injection significantly suppressed the morphine-induced rewarding effect. This effect was almost reversed by s.c. pretreatment with the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI, 5 mg/kg). Furthermore, the morphine-induced increase in dopamine (DA) turnover in the limbic forebrain was significantly inhibited by treatment with formalin. This inhibition was also suppressed by pretreatment with nor-BNI. In addition, in vivo microdialysis studies clearly showed that the morphine-induced increase in the extracellular levels of DA and its metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, in the nucleus accumbens (N.Acc.) was significantly decreased in rats that had been pretreated with formalin. This effect was in turn reversed by the microinjection of a specific dynorphin A antibody into the N.Acc. These findings suggest that the inflammatory pain-like state induced by formalin injection may have caused a sustained activation of the kappa-opioidergic system within the N.Acc., resulting in suppression of the morphine-induced rewarding effect in rats. The present study provides further evidence of the clinical usefulness of morphine in patients suffering from severe pain.     

Inhibitory effects of glycine on morphine-induced hyperactivity,reverse tolerance and postsynaptic dopamine receptor supersensitivity in mice

The effects of glycine on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity in mice was examined. A single administration of morphine (10 mg/kg, s.c.) induced hyperactivity as measured in mice. The morphine-induced hyperactivity was inhibited by pretreatment with glycine (100, 200 and 400 mg/kg, i.p.). In addition, it was found repeated administration of morphine (10 mg/kg, s.c.) to mice daily for 6 days caused an increase in motor activity which could be induced by a subsequent morphine dose, an effect known as reverse tolerance or sensitization. Glycine (100, 200 and 400 mg/kg, i.p.) also inhibited morphine-induced reverse tolerance. Mice that had received 7 daily repeated administrations of morphine also developed postsynaptic dopamine receptor supersensitivity, as shown by enhanced ambulatory activity after administration of apomorphine (2 mg/kg, s.c.). Glycine inhibited the development of postsynaptic dopamine receptor supersensitivity induced by repeated administration of morphine. It is suggested that the inhibitory effects of glycine might be mediated by dopaminergic (DAergic) transmission. Accordingly, the inhibition by glycine of the morphine-induced hyperactivity, reverse tolerance and dopamine receptor supersensitivity suggests that glycine might be useful for the treatment of morphine addiction.