Nitric oxide mediates Fos expression in the spinal cord induced by mechanical noxious stimulation.

Immunocytochemical localization of Fos protein was used to analyze the involvement of nitric oxide (NO) in the expression of Fos in the spinal cord, induced by mechanical noxious stimulation (NS). Mechanical NS was applied to the left hindpaw 30 minutes after intrathecal administration of the NO synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME) and the resulting Fos expression in the spinal cord dorsal horn was compared with that obtained in rats exposed only to the mechanical NS. Pretreatment with L-NAME but not its stereoisomer N omega-nitro-D-arginine methyl ester (D-NAME), produced a dose-dependent suppression of Fos expression induced by mechanical noxious stimulation. These results indicate that NO modulates the expression of Fos in the dorsal horn induced by mechanical noxious stimulation and further support the hypothesis that NO is involved in nociceptive events occurring in the spinal cord in response to a peripheral noxious stimulus.     

Release of substance P into the superficial dorsal horn following nociceptive activation of the hindpaw of the rat.

Substance P (SP) has been widely proposed as being involved in the transmission of nociceptive information in the dorsal horn of the spinal cord. Formalin injected into the hindpaw as a nociceptive stimulus has been shown to increase the amount of immunoreactive SP in the dorsal horn, perhaps by decreasing SP release from primary afferent neurons. Much is known concerning the release of SP from tissue slices or from the entire spinal cord in vivo. However, less is known about the release patterns of SP in the superficial dorsal horn during the activation of peripheral nociceptors. In this study, noxious pinch applied to and formalin injection into the hindpaw were used as nociceptive stimuli while a stereotaxic push-pull cannula was used to perfuse the L5 dorsal horn. Experiments were conducted in unanesthetized decerebrate/spinal rats, and radioimmunoassay was used to determine the SP-like immunoreactivity (SPLI) content of collected perfusates. Results demonstrate that graded intensities of noxious mechanical pinch produced progressively increased release of SPLI into the dorsal horn; SPLI release returned to baseline rates following termination of the stimulus. The injection of 100 microliters of 5% formalin into the hindpaw produced a biphasic inhibition of SPLI release 0-40 min and greater than 60 min after formalin injection. The application of a noxious pinch following formalin injection produced an increase in SPLI release which did not return to baseline rates; this may be indicative of production of a hyperalgesic state caused by formalin injection. The results of this study support the concept that formalin injected into the hindpaw activates segmental antinociceptive systems which block SP release and limit nociceptive transmission.(ABSTRACT TRUNCATED AT 250 WORDS)     

Alterations in spinal cord gene expression after hindpaw formalin injection

Heme oxygenase type 2 (HO-2) is an enzyme that uses heme as a substrate to produce iron, biliverdin, and carbon monoxide (CO). This enzyme participates in regulation of nociceptive signal transmission in spinal cord tissue. We set out to identify genes undergoing alterations in expression in a model of inflammatory pain and to determine whether HO-2 participates in that regulation. After the hindpaw injection of formalin in mice, we measured changes in expression of immediate early genes including c-fos, c-jun, jun B, nerve growth factor induced genes (NGFI-A and NGFI-B) and activity-related cytoskeletal protein (ARC) using real-time PCR. The mRNA corresponding to these genes increased in abundance in the first hour after formalin injection and then slowly declined. Changes in the abundance of prodynorphin, extracellular signal related kinases (ERK1 and ERK2) and N-methyl-D-aspartate (NMDA) receptor R1 subunit mRNA generally peaked between 8 and 12 hr after formalin injection. In HO-2 null mutant mice, the enhancement of expression was less for all genes studied. We went on to quantify gene expression in superficial dorsal horn tissue using laser capture microdissection followed by RNA amplification and real-time PCR. The results confirmed that the changes in gene expression were occurring in regions of the spinal cord involved in nociceptive processing. We conclude that the hindpaw injection of formalin leads to enhanced early and late expression of many genes in spinal cord dorsal horn tissue, and that this enhancement of expression relies to a degree on the presence of HO-2.     

Dorsal horn administration of L-arginine accentuates the pressor response evoked by activation of muscle mechanoreceptors

Recent work from our laboratory suggests that nitric oxide production in the dorsal horn has a modulatory influence on the pressor reflex evoked by static contraction of skeletal muscle. In this study, we tested the hypothesis that nitric oxide production in the dorsal horn is involved in producing the pressor reflex elicited by activation of skeletal muscle mechanoreceptors. Cats were anesthetized with alpha-chloralose (80 mg/kg) and urethane (100 mg/kg) and a laminectomy was performed. With the exception of the L7 dorsal root, the dorsal and ventral roots from L5 to S2 were sectioned on one side. Muscle mechanoreceptors were activated by manually stretching the ipsilateral triceps surae muscle 1.5 cm. To block nitric oxide synthase, a 50 mM solution of nt-nitro-L-argenine methyl ester (L-NAME) (a dose that altered the pressor reflex to static contraction) was microdialyzed into the dorsal horn at L6 and S1. Dialysis of L-NAME failed to attenuate the peak change in mean arterial pressure evoked by muscle stretch (45 +/- 6 mmHg before and 44 +/- 9 mmHg after 2 h of L-NAME dialysis). On the other hand, 2 h of L-arginine dialysis (50 mM) increased the peak pressor response to muscle stretch from 43 +/- 3 to 57 +/- 5 mmHg. These data suggest that administration of L-arginine enhances the excitability of dorsal horn cells receiving input from muscle mechanoreceptors, thus increasing the pressor response evoked by activation of this type of muscle afferent neuron.     

Microinjection of the nitric oxide synthase inhibitor L-NAME into the lateral basal forebrain alters the sleep/wake cycle of the rat

The effects of N(G)-nitro-L-arginine methyl ester (L-NAME) (0.19-0.74 micromol), a competitive inhibitor of the enzyme nitric oxide synthase (NOS); L-arginine (48.0-191.0 nmol), a nitric oxide (NO) precursor; and molsidomine (0.06-0.24 nmol), an NO donor, on spontaneous sleep were studied in adult rats implanted for chronic sleep recordings. Direct bilateral application of L-NAME into the nucleus of the horizontal limb of the diagonal band of Broca (HDB) increased waking (W) and reduced slow wave sleep (SWS). On the other hand, intra-HDB injection of L-arginine or molsidomine induced slight but inconsistent changes of sleep variables that did not attain significance. Pretreatment with L-arginine (191.0 nmol) or molsidomine (0.24 nmol) prevented the increase of W and the reduction of SWS induced by L-NAME (0.37 micromol), thus indicating that a decrease in the availability of NO may be involved in the effects of L-NAME on sleep variables. An increase in the release of acetylcholine (ACh) and/or a reduction in the output of gamma-aminobutyric acid (GABA) and adenosine could tentatively explain the effects of L-NAME on SWS and W.     

Effects of multiple intrathecal administration of L-arginine with different doses on formalin-induced nociceptive behavioral responses in rats

Objective

To investigate the effects of nitric oxide (NO) with different doses on modulation of inflammatory pain, and its possible mechanisms.

Methods

NO precursor L-arginine (L-Arg) was intrathecally administered in rats at a dose of 10 μg per day (low dose group) or 250 μg per day (high dose group) for a succession of 4 d. Normal saline was applied as a control. Then the rats were subcutaneously injected with formalin (100 μL, 2%) into the right hindpaw, and the nociceptive behavioral responses within 1 h were observed. At 4 h after formalin injection, neuronal NO synthase (nNOS) and c-Fos expression in spinal dorsal horn was examined with immunocytochemistry method.

Results

The subcutaneous injection of formalin evoked biphasic behaviors of licking or biting the injected paw. There was no difference in acute phase of formalin test among the 3 groups, while in tonic phase, the licking and biting time, and the protein levels of nNOS and c-Fos in spinal dorsal horn were significantly decreased in low dose group and increased in high dose group, compared with those in control group.

Conclusion

These results suggest that multiple administration of NO with different doses may produce different effects. On one hand, the low dose of NO can induce antinociception. On the other hand, the high dose of NO can induce pronociception.     

Peripheral NMDA and non-NMDA receptors contribute to nociception: an electrophysiological study

The present study investigated the effects of peripheral administration of N-methy-D-aspartate (NMDA) and non-NMDA receptor antagonists on C-fiber evoked responses of the spinal dorsal horn neurons in the spinalized rats. When DL-2-amino-5-phosphonovaleric acid (AP5) (10 mM, 1 mM, 0.1 mM, 20 microl) or 6, 7-dinitroquinoxaline-2, 3-dione (DNQX) (1 mM, 0.1 mM, 0.01 mM, 20 microl) was subcutaneously injected into the receptive field on the hindplantar region, C-fiber evoked responses of the dorsal horn neurons were profoundly inhibited in a dose-dependent manner. Three hours after subcutaneous injection of carrageenan into the ipsilateral hindpaw, NMDA and non-NMDA antagonist-induced inhibition of C-fiber evoked responses was more potent than that in the normal rat (Student's t-test, p < 0.05). In the carragenan-treated rats, DNQX-induced inhibition was stronger than AP-5-induced one (Student's t-test, p < 0.05). The results suggest that peripheral NMDA and non-NMDA receptors are involved in mediating excitation of nociceptors.     

Systemic morphine suppresses noxious stimulus-evoked Fos protein-like immunoreactivity in the rat spinal cord.

Previous experiments have shown that noxious stimulation increases expression of the c-fos proto-oncogene in subpopulations of spinal cord neurons. c-fos expression was assessed by immunostaining for Fos, the nuclear phosphoprotein product of the c-fos gene. In this study, we examined the effect of systemic morphine on Fos-like immunoreactivity (FLI) evoked in the formalin test, a widely used model of persistent pain. Awake rats received a subcutaneous 150 microliters injection of 5% formalin into the plantar aspect of the right hindpaw. The pattern of nuclear FLI was consistent with the known nociceptive primary afferent input from the hindpaw. Dense labeling was recorded in the superficial dorsal horn (laminae I and IIo) and in the neck of the dorsal horn (laminae V and VI), areas that contain large populations of nociceptive neurons. Sparse labeling was noted in lamina IIi and in the nucleus proprius (laminae III and IV), generally considered to be nonnociceptive areas of the cord. Fos immunoreactivity was also evoked in the ventromedial gray, including laminae VII, VIII, and X. There was no labeling in lamina IX of the ventral horn. Since FLI was time dependent and distributed over several spinal segments, we focused our analysis where maximal staining was found (L3-L5) and at the earliest time point of the peak Fos immunoreactivity (2 hr). Twenty minutes prior to the formalin injection, the rats received morphine (1.0, 2.5, 5.0, or 10 mg/kg, s.c.) or saline vehicle. Two hours later, the rats were killed, their spinal cords removed, and 50 microns transverse sections of the lumbar enlargement were immunostained with a rabbit polyclonal antiserum directed against Fos. Prior treatment with morphine sulfate profoundly suppressed formalin-evoked FLI in a dose-dependent and naloxone-reversible manner. The dose-response relationship of morphine-induced suppression of FLI varied in different laminae. To quantify the effect of morphine on FLI, labeled neurons in sections taken from the L4/5 level of each rat were plotted with a camera lucida and counted. Staining in the neck of the dorsal horn (laminae V and VI) and in more ventral laminae VII, VIII, and X, was profoundly suppressed by doses of morphine which also suppress formalin-evoked behavior. Although the labeling was also significantly reduced in laminae I and II, at the highest doses of morphine there was substantial residual labeling in the superficial dorsal horn. These data indicate that analgesia from systemic opiates involves differential regulation of nociceptive processing in subpopulations of spinal nociceptive neurons.     

Anorexigenic action of nitric oxide synthase inhibition in the raphe nuclei

The present report examined the effects of midbrain raphe nuclei injections of nitric oxide synthase inhibitors NG-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI) on eating behavior. L-NAME (5-500 pmol) and 7-NI (2-200 pmol) were administered into either the dorsal or median raphe nucleus. Both nitric oxide synthase inhibitors decreased food intake in adult male Sprague-Dawley rats when injected into either raphe site. Further, eating elicited by dorsal and median raphe injections of the 5-HT1A agonist 8-OH-DPAT (0.8 nmol) was attenuated by L-NAME or 7-NI pretreatment. Our findings indicate that nitric oxide acts within the raphe to alter food intake. The inhibitory effects of L-NAME and 7-NI on eating elicited by 8-OH-DPAT further suggest that nitric oxide and 5-HT systems interact in the control of food intake.     

Defensive-like behaviors and antinociception induced by NMDA injection into the periaqueductal gray of mice depend on nitric oxide synthesis

Glutamate NMDA receptor activation within the periaqueductal gray (PAG) leads to antinociceptive, autonomic and behavioral responses characterized as the fear reaction. Considering that NMDA receptor triggers activation of neuronal nitric oxide synthase (nNOS), enzyme that produces nitric oxide (NO), this study investigated the effects of intra-PAG infusions of NPLA (Nomega-propyl-L-arginine), an nNOS inhibitor, on behavioral and antinociceptive responses induced by local injection of NMDA receptor agonist in mice. The behaviors measured were frequency of jumping and rearing as well as duration (in seconds) of running and freezing. Nociception was assessed during the second phase of the formalin test (injection of 50 microl of formalin 2.5% into the dorsal surface of the right hind paw). Five to seven days after stereotaxic surgery for intracerebral cannula implantation, mice were injected with formalin into the paw, and 10 min later, they received intra-dPAG injection of NPLA (0, 0.2, or 0.4 nmol/0.1 microl). Ten minutes later, they were injected with NMDA (N-methyl-D-aspartate: 0 or 0.04 nmol/0.1 microl) into the same midbrain site and were immediately placed in glass holding cage for recording the defensive behavior and the time spent on licking the injected paw with formalin during a period of 10 min. Microinjections of NMDA significantly decreased nociception response and produced jumping, running, and freezing reactions. Intra-dPAG injections of NPLA (0.4 nmol) completely blocked the NMDA effects without affecting either behavioral or nociceptive responses in intra-dPAG saline-injected animals, except for the rearing frequency that was increased by the nNOS inhibitor. These results strongly suggest the involvement of NO within the PAG in the antinociceptive and defensive reactions induced by local glutamate NMDA receptor activation in this midbrain structure.     

L-NAME减少三叉神经脊束核尾侧段和上颈髓后角c-fos和nNOS的表达

目的:进一步研究NO在偏头痛发病机制中的作用,探讨三叉神经血管反射的中枢机制。方法:应用电刺激猫上矢状窦区硬脑膜动物模型,冰冻组织连续切片后行免疫组化染色,分别观察了NOS抑制剂Nω-nitro-L-argi-nine methylester(L-NAME,100mg/kg)对下延髓、上颈髓c-fos蛋白和神经元型NOS(nNOS)表达的影响。结果:c-fos和nNOS阳性神经元主要位于延髓三叉神经脊束核尾侧段和C1后角的浅层(Ⅰ、Ⅱ层),其深层、孤束缝核和中央核、中央导水管两侧可见数个直径较大、轴突较长的nNOS阳性神经元;而nNOS阳性神经纤维遍布后角及中央导水管两侧,并似向前角和后角浅层方向投射,其中后角浅层着色明显比其它部位深。电刺激后可见较长的纤维呈串珠样改变。刺激组动物的c-fos及nNOS阳性神经元数均比假手术对照组明显增加(P<0.01),L-NAME组比刺激组明显减少(P<0.01),而刺激组动物的c-fos及nNOS阳性神经元数与生理盐水组相比均无显著差异(P>0.05)。结论:刺激猫上矢状窦区硬脑膜可激活三叉神经脑干中枢神经元,L-NAME可能通过抑制NO的生成阻断此部位神经元的激活,从而终止偏头痛发作。     

Nitric oxide and c-Jun N-terminal kinase are involved in the development of dark neurons induced by inflammatory pain

Dark neurons, whose morphological characteristics are consistent with those of cells undergoing apoptosis, are generated in vivo as an acute or delayed consequence of several pathological situations and lesions. The present study was designed to evaluate whether inflammatory pain induced by injection of formalin to the rat hind paw lead to the formation of dark neurons in the dorsal horn of the lumbar spinal cord in rat. Since nitric oxide (NO) and c-Jun N-terminal Kinase (JNK) pathway are involved in the mechanisms of pain generation and degenerative neuronal alteration, their roles were also considered. The methods used spectrophotometrical analysis of the serum nitrite (metabolite of NO) and histological procedures for detection of dark neurons, following induction of inflammatory pain. According to the results, injection of formalin led to an increase of the serum nitrite level in both concentration and time-dependent manners. Visual inspections of the lumbar spinal cord sections showed that, on day 5, following chronic injections of 5% formalin, numbers of dark neurons were significantly increased. Acute and chronic administration of 1% or 2.5% formalin did not induce any remarkable neuronal alterations in the dorsal horn of the lumbar spinal cord. Daily intrathecal administration of quercetin (inhibitor of JNK pathway) 100 microg/rat, or 2-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (PTIO; NO scavenger) 30 mug/rat before injection of 5% formalin led to a reliable reduction in the number of dark neurons. These results indicate that induction of inflammatory pain for longer periods may result in a serious central disorder. Pretreatment with neutralizers or inhibitors of NO and JNK may exert a neuroprotective effect in this regard.     

EFFECTS OF NITRIC OXIDE ON PASSIVE AVOIDANCE LEARNING IN RATS

To investigate the effects of nitric oxide (NO) on passive avoidance learning, L-NAME, D-NAME, and L-arginine were administered i.p. 30 min prior to learning trial; the effects of these substances were tested 24 h later using a passive avoidance apparatus in rats. To reveal the effect of NO on consolidation of acquired memory, L-NAME, D-NAME, and L-arginine were administered i.p. immediately after learning trials and animals were tested 24 h later. Effect of NO on retention was also investigated by injecting L-NAME, D-NAME, and L-arginine (same dosages) 30 min prior to 24 h testing (retrieval). L-NAME administered 30 min before and 24 h after learning trial significantly decreased the avoidance latency but there was no significant effect on consolidation. L-Arginine appeared to enhance the retention of acquired memory significantly, whereas D-NAME had no effect on any testing regime. The results suggest that NO may be involved in learning and retention of passive avoidance     

Effects of nitric oxide on passive avoidance learning in rats

To investigate the effects of nitric oxide (NO) on passive avoidance learning, L-NAME, D-NAME, and L-arginine were administered i.p. 30 min prior to learning trial; the effects of these substances were tested 24 h later using a passive avoidance apparatus in rats. To reveal the effect of NO on consolidation of acquired memory, L-NAME, D-NAME, and L-arginine were administered i.p. immediately after learning trials and animals were tested 24 h later. Effect of NO on retention was also investigated by injecting L-NAME, D-NAME, and L-arginine (same dosages) 30 min prior to 24 h testing (retrieval). L-NAME administered 30 min before and 24 h after learning trial significantly decreased the avoidance latency but there was no significant effect on consolidation. L-Arginine appeared to enhance the retention of acquired memory significantly, whereas D-NAME had no effect on any testing regime. The results suggest that NO may be involved in learning and retention of passive avoidance.     

Systemic nitroglycerin increases nNOS levels in rat trigeminal nucleus caudalis

Systemic administration of nitroglycerin, a nitric oxide donor, triggers in migraineurs a delayed attack of unknown mechanisms. Subcutaneous nitroglycerin (10 mg/kg) produced a significant increase of nitric oxide synthase (NOS)- and c-fos-immunoreactive neurons in the cervical part of trigeminal nucleus caudalis in rats after 4 h. This effect was not observed in the thoracic dorsal horn. Similar increase of NOS and c-fos was obtained in the brain stem after a somatic nociceptive stimulus, i.e. on the side of the formalin injection in the lip. Nitric oxide is thus able to increase NOS availability in second order nociceptive trigeminal neurons, which may be relevant for central sensitization and the understanding of its effect in migraine.     

Electroacupuncture induces the expression of Fos in rat dorsal horn via capsaicin-insensitive afferents

Fos is expressed in rat dorsal horn neurons after electroacupuncture (E-acupuncture), but it is unclear which types of afferent fibers are involved in the expression. It is thought that the Fos expression is induced via Adelta afferents rather than C afferents, since the threshold of Adelta afferents to electrical stimulation is much lower than that of unmyelinated ones. Therefore, neonatally capsaicin treated rats lacking many C afferents were examined to clarify this. Fos expression in the dorsal horn after injection of formalin into the hindpaw was severely attenuated by neonatal capsaicin treatment. However, Fos expression after E-acupuncture to the pads of the hindpaw was unaffected by the same treatment. These results suggest that E-acupuncture induces the expression of Fos in the dorsal horn neurons via capsaicin-insensitive afferents, presumably Adelta afferents.     

Activity-dependent neuronal plasticity following tissue injury and inflammation.

Increases in neuronal activity in response to tissue injury lead to changes in gene expression and prolonged changes in the nervous system. These functional changes appear to contribute to the hyperalgesia and spontaneous pain associated with tissue injury. This activity-dependent plasticity involves neuropeptides, such as dynorphin, substance P and calcitonin gene-related peptide, and excitatory amino acids, such as NMDA, which are chemical mediators involved in nociceptive processing. Unilateral inflammation in the hindpaw of the rat results in an increase in the expression of preprodynorphin and preproenkephalin mRNA in the spinal cord, which parallels the behavioral hyperalgesia associated with the inflammation. Cellular intermediate-early genes, such as c-fos, are also expressed in spinal cord neurons following inflammation and activation of nociceptors. Peripheral inflammation results in an enlargement of the receptive fields of many of these neurons. Dynorphin applied to the spinal cord also induces an enlargement of receptive fields. NMDA antagonists block the hyperexcitability produced by inflammation. A model has been proposed in which dynorphin, substance P and calcitonin gene-related peptide enhance excitability at NMDA receptor sites, leading first to dorsal horn hyperexcitability and then to excessive depolarization and excitotoxicity.     

Excitotoxic action of NMDA agonists on nigrostriatal dopaminergic neurons: modulation by inhibition of nitric oxide synthesis

Focal infusions of N-methyl-d-aspartate (NMDA) or an endogenous NMDA agonist, quinolinic acid (QUIN), into the substantia nigra pars compacta (SNc) of adult Sprague-Dawley rats resulted in a dose-dependent depletion of ipsilateral striatal tyrosine hydroxylase (TH) activity, a biochemical marker for dopaminergic neurons. To assess the intermediary role of nitric oxide in the neurotoxicity elicited by these toxins, their action was tested in animals treated with N^ω-nitro-l-arginine methyl ester (L-NAME). Systemic injections (2 injections; 8 h apart) of L-NAME (100, 150 and 250 mg/kg) produced a dose-related inhibition of cerebellar nitric oxide synthase (NOS) activity. The time-course of cerebellar NOS inhibition following L-NAME (250 mg/kg) was rapid in onset and lasted for at least 24 h following the second injection. An L-NAME treatment regimen of 250 mg/kg, with the second injection given 24 h prior to assessment of NOS activity, produced an 87 and 91% inhibition of cerebellar and nigral NOS activity, respectively. Intranigral infusion of 40 and 60 nmol QUIN reduced ipsilateral striatal TH activity by 62 and 75%, respectively. However, 40 and 60 nmol QUIN infusions into animals pretreated with L-NAME (250 mg/kg) reduced striatal TH activity by 83 and 96%, respectively. Intranigral infusion of 15 and 30 nmol NMDA produced a 48 and 77% decrease in striatal TH activity, respectively, whereas the same doses of NMDA given to animals pretreated with L-NAME (250 mg/kg) resulted in a 59 and 88% decrease in TH activity. Thus, both QUIN and NMDA toxicity was enhanced following L-NAME pretreatment. The destruction of the nigrostriatal pathway was verified using TH immunocytochemistry of the SNc. It was also observed that a low dose of L-NAME (1.5 mg/kg), previously shown to be neuroprotective in cerebral ischemic damage, did not influence NMDA (15 nmol) neurotoxicity. The results of this study show that extensive inhibition of NOS activity enhances NMDA receptor-mediated excitoxicity.     

Nitric oxide modulates articular sensory discharge and responsiveness to bradykinin in normal and arthritic rats in vivo

Nitric oxide is implicated in peripheral nociceptive processing. This study determined the effects of the nitric oxide synthase inhibitor, L-NAME, on neural discharge from articular C-fibre afferents innervating normal and arthritic ankle joints in anaesthetized rats. Intra-arterial injection of L-NAME (10-20 mg kg(-1)) increased neural discharge in normal and arthritic ankle joints, whereas D-NAME (30 mg kg(-1)) had no effect. The excitation induced by L-NAME (20 mg kg(-1)) was reduced by co-injecting the nitric oxide precursor, L-arginine (50 mg kg(-1)). L-NAME (20 mg kg(-1)) also enhanced responsiveness to bradykinin (10 microg) but only in arthritic rats, whereas L-arginine (50 mg kg(-1)) reduced the excitation by bradykinin (30 microg) in both groups. These results provide evidence that nitric oxide modulates articular C-fibre activity and reduces responsiveness to bradykinin.     

Sulpiride injections into the medial septum reverse the influence of intra-medial septum injection of L-arginine on expression of place conditioning-induced by morphine in rats

Effects of intra-medial septum injections of L-arginine, a precursor of nitric oxide, N(G)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase, and sulpiride, a selective antagonist of dopamine D2 receptor on morphine-induced conditioned place preference (CPP) in male Wistar rats were examined. Using a 3-day schedule of conditioning, morphine (0.5-7.5 mg/kg, s.c.) produced a significant place preference in a dose-dependent manner. The maximum response was observed with 5.0 mg/kg of opioid. Sulpiride (0.3, 1.0 and 3.0 microg/rat), but not L-arginine (0.3, 1.0 and 3.0 microg/rat) or L-NAME (0.3, 1.0 and 3.0 microg/rat), in combination with morphine (5.0 mg/kg), during conditioning, significantly altered morphine-induced CPP. Single doses (0.3, 1.0 and 3.0 microg/rat) of either L-arginine or L-NAME, during conditioning, did not induce CPP. Sulpiride at 0.3-3.0 microg/rat, intra-medial septum, during conditioning, produced a significant conditioned place aversion. Intra-medial septum injections of L-arginine but not L-NAME or sulpiride, 1-2 min before testing, increased the expression of morphine-induced CPP. The administration of sulpiride (0.3, 1.0 and 3.0 microg/rat), but not L-NAME (0.3, 1.0 and 3.0 microg/rat), 1-2 min before the injection of L-arginine (0.3 microg/rat) on day of test, significantly attenuated the response to L-arginine. L-Arginine (0.3-3.0 microg/rat), during conditioning, showed a statistically significant increase in locomotor activity compared with that to control group. Moreover, sulpiride decreased locomotion by itself or in combination with morphine during conditioning and on the test day of morphine CPP. It can be concluded that L-arginine, a precursor of nitric oxide, in the rat median septum may play a role in expression of morphine conditioning due to dopamine release in this area.