Nitric oxide mediates the thermal hyperalgesia produced in a model of neuropathic pain in the rat

Neuropeptides, nitric oxide synthase and GAP-43 in B4-binding and RT97 immunoreactive primary sensory neurons: normal distribution pattern and changes after peripheral nerve transection and aging

We have here sought to cross-correlate the expression of immunoreactivities for several neuropeptides, nitric oxide synthase (NOS) and the growth associated protein GAP-43 in subpopulations of dorsal root ganglion (DRG) neurons tagged by the selective markers isolectin B4 and the neurofilament antibody RT97, selective for, respectively, subpopulations of small and large DRG neurons. By use of double- and triple-labeling immunohistochemistry, non-manipulated and sciatic nerve transected young adult rats as well as aged (30-months-old) rats were examined using a confocal microscope equipped with enhanced spectral separation. In young adult rats, the DRG neuron profiles could be divided into three subpopulations (B4 binding (B4+) approximately 50%; RT97-immunoreactive (RT97+) approximately 35%; B4-/RT97- approximately 15%). Calcitonin gene-related peptide (CGRP) is expressed in all three subpopulations. Galanin message-associated peptide (GMAP) colocalize with CGRP (100%) but is not expressed in RT97+ profiles. NOS is present in the RT97- subpopulations and frequently colocalize with CGRP (92%). GAP-43 is expressed in all three DRG subpopulations and colocalize with CGRP (88%), GMAP (38%) and/or NOS (22%). Only very small differences were seen among the young adult rats, implicating that the size of respective subpopulation as well as the expression pattern for neuropeptides, NOS and GAP-43 are fairly stable. Sciatic nerve transection reduced B4-binding but not RT97-like immunoreactivity. Distinct changes in the expression of neuropeptides, NOS and GAP-43 were evident in the DRG subpopulations and, furthermore, the regulatory changes were very similar among the lesioned animals. The relative size of the DRG subpopulations was unaffected by aging, while the expression of neuropeptides was altered showing similarities with the changes induced by axotomy in young adult rats.

Pharmacological modulation of secondary mediator systems--cyclic AMP and cyclic GMP--on inflammatory hyperalgesia

1. The objective of the present paper was to evaluate the relevance of neuronal balance of cyclic AMP and cyclic GMP concentration for functional regulation of nociceptor sensitivity during inflammation. 2. Injection of PGE2 (10-100 ng paw-1) evoked a dose-dependent hyperalgesic effect which was mediated via a cyclic AMP-activated protein kinase (PKA) inasmuch as hyperalgesia was blocked by the PKA inhibitor H89. 3. The PDE4 inhibitor rolipram and RP73401, but not PDE3 and PDE5 inhibitors potentiated the hyperalgesic effects of PGE2. The hyperalgesic effect of dopamine was also enhanced by rolipram. Moreover, rolipram significantly potentiated hyperalgesia induced by carrageenan, bradykinin, TNF alpha, IL-1 beta, IL-6 and IL-8. This suggests that neuronal cyclic AMP mediates the prostanoid and sympathetic components of mechanical hyperalgesia. Moreover, in the neuron cyclic AMP is mainly metabolized by PDE4. 4. To examine the role of the NO/cyclic GMP pathway in modulating mechanical hyperalgesia, we tested the effects of the soluble guanylate cyclase inhibitor, ODQ. This substance counteracts the inhibitory effects of the NO donor, SNAP, on the hyperalgesia induced by PGE2. 5. The ODQ potentiated hyperalgesia induced by carrageenan, bradykinin, TNF alpha, IL-1 beta, IL-6 and IL-8. In contrast, ODQ had no significant effect on the hyperalgesia induced by PGE2 and dopamine. This indicates that the hyperalgesic cytokines may activate soluble guanylate cyclase, which down-regulate the ability of these substances to cause hyperalgesia. This event appears not to be mediated by prostaglandin or dopamine. 6. In conclusion, the results presented in this paper confirm an association between (i) hyperalgesia and elevated levels of cyclic AMP as well as (ii) antinociception and elevated levels of cyclic GMP. The intracellular levels of cyclic AMP that enhance hyperalgesia are controlled by the PDE4 isoform and appear to result in activation of protein kinase A whereas the intracellular levels of cyclic GMP results from activation of a soluble guanylate cyclase.

Pressor reflex evoked by static muscle contraction: role of nitric oxide in the dorsal horn

In this study, we tested the hypothesis that nitric oxide (NO) production in the dorsal horn is involved in producing the pressor reflex elicited by static contraction of skeletal muscle. 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 and static contraction of the ipsilateral triceps surae muscle was evoked by electrically stimulating the peripheral ends of the L7 and S1 ventral roots. Dialysis of the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 50 mmol/l syringe concentration, based upon dose-response data) into the dorsal horn at L6 and S1 failed to attenuate the peak change in mean arterial pressure (MAP) evoked by static contraction (DeltaMAP in mmHg: 57 +/- 5 before and 50 +/- 6 after 2 h of L-NAME). However, this dialysis of L-NAME reduced the magnitude of the initial pressor response as the MAP at 10 s of the contraction fell from 27 +/- 4 to 17 +/- 4 mmHg. On the other hand, 2 h of L-arginine dialysis (50 mmol/l) shifted the curve representing the time course of the pressor response upward and increased the peak pressor response to static contraction from 51 +/- 9 to 68 +/- 9 mmHg. A 2-h dialysis of D-NAME (50 mmol/l), the inactive enantiomer of L-NAME, had no effect on the time course or the peak pressor response (DeltaMAP in mmHg: 78 +/- 12 before and 72 +/- 15 after). These data suggest that NO production in the dorsal horn has a modulatory influence on the pressor reflex evoked by static contraction of skeletal muscle and that increasing the level of NO in the dorsal horn enhances the excitability of dorsal horn cells to muscle afferent input.

The role of systemic, spinal and supraspinal L-arginine-nitric oxide-cGMP pathway in thermal hyperalgesia caused by intrathecal injection of glutamate in mice

The intrathecal (i.t.) administration of glutamate (10-100 nmol) caused dose-related hyperalgesia (mean ED50 of 35 nmol) when assessed in the thermal behaviour model of nociception, the hot-plate test maintained at 50 degrees C. The i.p., i.t. or intracerebroventricular (i.c.v.) injection of the nitric oxide synthase inhibitors, L-NOARG and L-NAME, did not induce any detectable effect per se, but instead, produced dose-related inhibition of glutamate-induced hyperalgesia. D-NAME, the inactive enantiomer of L-NAME, had no effect. The i.c.v. or i.t. administration of L-NIO caused graded attenuation of glutamate-induced hyperalgesia. L-arginine (3.4 mmol kg(-1), i.p.), but not D-arginine (3.4 mmol kg(-1), i.p.) significantly potentiated glutamate (10 nmol)-induced hyperalgesia, an action that was prevented by L-NOARG (137 nmol kg(-1)). The co-injection of S-nitroso-N-acetyl-D,L-penicillamine (SNAP) (0.22 micromol) or 8-bromo-cGMP (22.5 nmol) with glutamate (10 nmol), via either i.t. or i.c.v. routes, also significantly enhanced glutamate-induced hyperalgesia. The guanylate cyclase inhibitors LY 83583 (0.1-1.0 nmol) or ODQ (30-300 pmol) co-administered with glutamate, dose-dependently antagonised the glutamate-induced hyperalgesia. Collectively, these results demonstrate that the i.t. injection of glutamate into the spinal cord of mice produces dose-related hyperalgesia an effect that was largely mediated by the L-arginine-nitric oxide-cGMP pathway from both spinal and supraspinal sites.

Neuronal nitric oxide synthase in the neural pathways of the urinary bladder

Nitric oxide (NO) is a unique biological messenger molecule. It serves, in part, as a neurotransmitter in the central and peripheral nervous systems. Neurons containing NO have been identified histochemically by the presence of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) reactivity or immunohistochemically by the antibody for neuronal NO synthase (n-NOS). Previous histochemical or pharmacological studies have raised the possibility that NO may play an important role in the neural pathways of the lower urinary tract. There is also considerable evidence to suggest that n-NOS is plastic and could be upregulated following certain lesions in the lower urinary tract. The present review summarises the distribution of n-NOS containing neurons innervating the urinary bladder and the changes of the enzyme expression in some experimentally induced pathological conditions.

Topical nitroglycerin and pain in purpura fulminans

Topical nitroglycerin has been previously described as an adjunctive therapy to increase perfusion to areas of purpura affected in purpura fulminans. We report a case of purpura fulminans in which topical nitroglycerin was found to provide analgesia after its application to purpuric lesions. The broader role for the use of topical nitroglycerin in pain management deserves further study and evaluation.

Effects of intrathecal administration of nitric oxide synthase inhibitors on carrageenan-induced thermal hyperalgesia

1. We examined the effects of various nitric oxide synthase (NOS) inhibitors on carrageenan-induced thermal hyperalgesia. 2. First, we determined the time point at which a subcutaneous plantar injection of carrageenan into the rat hindpaw produced maximum thermal hyperalgesia. Subsequently, we demonstrated that intrathecal administration of the non-selective NOS inhibitor L-N(G)-nitro-arginine methyl ester (L-NAME) produces a dose-dependent reduction of carrageenan-induced thermal hyperalgesia. 3. Four relatively selective NOS inhibitors were then tested for their efficacy at reducing carrageenan-induced thermal hyperalgesia. Initially, the effects of prolonged treatment with inhibitors of neuronal [7-nitroindazole (7-NI) and 3-bromo-7-nitroindazole (3-Br)] and inducible [aminoguanidine (AG) and 2-amino-5,6-dihydro-methylthiazine (AMT)] NOS were examined. All agents were injected three times intrathecally during the course of inflammation caused by the plantar injection of carrageenan, and thermal hyperalgesia was measured at 6 h post-carrageenan using a plantar apparatus. 4. All inhibitors, except for 7-NI, were effective at attenuating the carrageenan-induced thermal hyperalgesia when compared with vehicle treatment. 5. Finally, the effects of early versus late administration of neuronal and inducible NOS inhibitors on carrageenan-induced thermal hyperalgesia were examined. We found that neither 3-Br nor AG significantly affected thermal hyperalgesia when administered during the early phase of carrageenan inflammation, while only AG was able to reduce thermal hyperalgesia when administered during the late phase of the injury. 6. Our results suggest that inducible NOS contributes to thermal hyperalgesia in only the late stages of the carrageenan-induced inflammatory response, while neuronal NOS likely plays a role throughout the entire time course of the injury.

Axotomy increases NADPH-diaphorase activity in the dorsal root ganglia and lumbar spinal cord of the turtle Trachemys dorbigni

Seven days after transection of the sciatic nerve NADPH-diaphorase activity increased in the small and medium neurons of the dorsal root ganglia of the turtle. However, this increase was observed only in medium neurons for up to 90 days. At this time a bilateral increase of NADPH-diaphorase staining was observed in all areas and neuronal types of the dorsal horn, and in positive motoneurons in the lumbar spinal cord, ipsilateral to the lesion. A similar increase was also demonstrable in spinal glial and endothelial cells. These findings are discussed in relation to the role of nitric oxide in hyperalgesia and neuronal regeneration or degeneration.

Nitric oxide mediates the central sensitization of primate spinothalamic tract neurons

Nitric oxide (NO) has been proposed to contribute to the development of hyperalgesia by activating the NO/guanosine 3',5'-cyclic monophosphate (cGMP) signal transduction pathway in the spinal cord. We have examined the effects of NO on the responses of primate spinothalamic tract (STT) neurons to peripheral cutaneous stimuli and on the sensitization of STT cells following intradermal injection of capsaicin. The NO level within the spinal dorsal horn was increased by microdialysis of a NO donor, 3-morpholinosydnonimine (SIN-1). SIN-1 enhanced the responses of STT cells to both weak and strong mechanical stimulation of the skin. This effect was preferentially on deep wide dynamic range STT neurons. The responses of none of the neurons tested to noxious heat stimuli were significantly changed when SIN-1 was administered. Intradermal injection of capsaicin increased dramatically the content of NO metabolites, NO-2/NO-3, within the dorsal horn. This effect was attenuated by pretreatment of the spinal cord with a nitric oxide synthase (NOS) inhibitor, NG-nitro-L-arginine methyl ester (L-NAME). Sensitization of STT cells induced by intradermal injection of capsaicin was also prevented by pretreatment of the dorsal horn with the NOS inhibitors, L-NAME or 7-nitroindazole. Blockade of NOS did not significantly affect the responses of STT cells to peripheral stimulation in the absence of capsaicin injection. The data suggest that NO contributes to the development and maintenance of central sensitization of STT cells and the resultant mechanical hyperalgesia and allodynia after peripheral tissue damage or inflammation. NO seems to play little role in signaling peripheral stimuli under physiological conditions.

Nitric oxide-mediated spinal disinhibition contributes to the sensitization of primate spinothalamic tract neurons

This study concentrated on whether an increase in spinal nitric oxide (NO) diminishes inhibition of spinothalamic tract (STT) cells induced by activating the periaqueductal gray (PAG) or spinal glycinergic and GABAergic receptors, thus contributing to the sensitization of STT neurons. A reduction in inhibition of the responses to cutaneous mechanical stimuli induced by PAG stimulation was seen in wide dynamic range (WDR) STT cells located in the deep layers of the dorsal horn when these neurons were sensitized during administration of a NO donor, 3-morpholinosydnonimine (SIN-1), into the dorsal horn by microdialysis. In contrast, PAG-induced inhibition of the responses of high-threshold (HT) and superficial WDR STT cells was not significantly changed by spinal infusion of SIN-1. A reduction in PAG inhibition when STT cells were sensitized after intradermal injection of capsaicin could be nearly completely blocked by pretreatment of the dorsal horn with a NO synthase inhibitor, 7-nitroindazole. Moreover, spinal inhibition of nociceptive activity of deep WDR STT neurons elicited by iontophoretic release of glycine and GABA agonists was attenuated by administration of SIN-1. This change paralleled the change in PAG-induced inhibition. However, the inhibition of HT and superficial WDR cells induced by glycine and GABA release did not show a significant change when SIN-1 was administered spinally. Combined with our recent results, these data show that the effectiveness of spinal inhibition can be reduced by the NO/cGMP pathway. Thus disinhibition may constitute one mechanism underlying central sensitization.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Correlation between autotomy-behavior and current theories of neuropathic pain

The past 10 years have brought several new experimental models with which to study chronic neuropathic pain in animals. Consequently, our knowledge about the mechanisms subserving neuropathic pain in humans has improved. However, the first animal model that was used for studying this type of chronic pain was the autotomy-model which can still be considered as a useful tool for pain studies. The present review assesses some of the similarities and differences between autotomy-model and more recent models of experimental traumatic mononeuropathy. In addition, it considers some of the similarities between the results obtained in clinical studies and in autotomy studies.

Recent advances in the pharmacology of nerve-injury pain

Nerve injury pain remains a complex clinical challenge. Although the development of animal models of nerve injury pain has aided our understanding of potential pathophysiologic mechanisms for this condition, effective treatment still remains beyond our reach. Several classes of agents appear to block pain behavior in these animal models and humans, but they are often limited in their use by low efficacy, or undesirable side-effects. A prerequisite for the improvement of nerve injury pain includes the development of clinically-relevant animal models in which therapeutic targets can be identified.

Rapid development of nitric oxide-induced hyperalgesia depends on an alternate to the cGMP-mediated pathway in the rat neuropathic pain model

Intrathecal injection of a nitric oxide releasing compound, NOC-18, was used to define the role of nitric oxide (NO) in the spinal mechanism of neuropathic pain caused by unilateral chronic constriction injury to rat sciatic nerves. Paw withdrawal latency was used to evaluate nociception induced by thermal stimuli before surgery and afterwards at 1, 3, and 6 h, and on days 1, 2, 3, 4, 5, 8, and 12 after the nerve ligature. In the sham-surgery control groups, intrathecal injection of 10 or 100 microg of NOC-18 did not produce any change in withdrawal latencies. In rats with unilateral nerve ligation, however, administration of 1 or 10 microg, but not 0.1 microg, of NOC-18 significantly shortened the time in which thermal hyperalgesia developed after nerve injury. Injection of 1 microg of NOC-18 decreased the onset time of thermal hyperalgesia from 2 days to 3 h and with 10 microg hyperalgesia developed within 1 h after the nerve injury. The effects of intrathecal injection of MK-801, a N-methyl-D-aspartate (NMDA) receptor antagonist, N-nitro-L-arginine methyl ester (L-NAME), a NO synthase inhibitor, methylene blue (MB), a soluble guanylate cyclase inhibitor, and hemoglobin (Hb), a NO scavenger, on the development of thermal hyperalgesia after the sciatic nerve ligature were examined in the presence and absence of 1 and 10 microg of NOC-18. Acceleration of the development of thermal hyperalgesia induced by 1 and 10 microg NOC-18 was completely inhibited by Hb, but was not affected by either MK-801, L-NAME or MB. These findings indicate that NO plays an important role in the rapid development of thermal hyperalgesia after the nerve injury, but that facilitation of nociceptive processing in the spinal cord may entail an alternate to the NO-cyclic guanosine 3',5'-monophosphate (cGMP) pathway.

Effect of chronic administration of L-arginine, NG-nitro-L-arginine or their combination on morphine concentration in peripheral tissues and urine of the mouse

1. Chronic administration of L-arginine (200 mg/kg, i.p) twice a day for 4 days decreased the antinociceptive response to subcutaneously, but not to intracerebroventricularly, administered morphine in male Swiss-Webster mice, as measured by the tail-flick test. 2. The decreased antinociceptive response to morphine was reversed by concurrent administration of NG-nitro-L-arginine (L-NNA) (5 mg/kg, IP), an inhibitor of nitric oxide synthase. 3. The concentrations of morphine in mice treated chronically with L-arginine and then given morphine (10 mg/kg, SC) were determined in the peripheral tissues. L-Arginine treatment significantly increased the concentration of morphine in spleen and lungs, did not modify it in liver, kidneys and urine. L-NNA by itself had no effect on the distribution of morphine in peripheral tissues but reversed the changes induced by chronic treatment with L-arginine. 4. Acute administration of L-arginine (200 mg/kg, IP) did not modify either the morphine antinociception or the morphine distribution in peripheral tissues. 5. Previous studies from this laboratory indicated that chronic treatment with L-arginine decreases the concentration of morphine in several brain regions and spinal cord of mice. 6. The facts that chronic treatment with L-arginine does not alter antinociception induced by ICV administered morphine and it increases the concentration of morphine in peripheral tissues while decreasing it in brain regions after peripheral administration of morphine suggest that the decreased antinociception induced by subcutaneously administered morphine may be related to its decreased entry into the brain.

Local nitric oxide synthase activity in a model of neuropathic pain

A local inflammatory reaction may play an important role in the development of neuropathic pain following peripheral nerve injury. One important participant in the inflammatory response of injured peripheral nerve may be nitric oxide (NO). In this work, we examined physiological and morphological evidence for nitric oxide synthase (NOS) activation in the chronic constriction injury model of neuropathic pain in rats. Physiological evidence of local NO action was provided by studying NO-mediated changes in local blood flow associated with the injury site. Immunohistochemistry was used to localize isoforms of NOS that might generate NO. Sciatic nerve injury associated with behavioural evidence of neuropathic pain had substantial rises in local blood flow. The NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME), but not NG-nitro-D-arginine methyl ester (D-NAME), reversed the hyperaemia in a dose-dependent fashion proximal to the constriction at 48 h and distally at 14 days post-operation when applied systemically or topically. Aminoguanidine, a NOS inhibitor with relatively greater selectivity for the inducible NOS (iNOS) isoform, reversed nerve hyperaemia distal to the constriction only at 14 days. NOS-like immunoreactivity of the neuronal and endothelial isoforms was identified just proximal to the constriction at 48 h. iNOS-like immunoreactivity was observed at 7 and 14 days at the constriction and distal sites, respectively. This work provides evidence for local NOS expression and NO action in the chronic constriction injury model of neuropathic pain. NO has local physiological actions that include vasodilatation of microvessels and that may be important in the development of pain sensitivity.

Stimulation of nitric oxide release from rat spinal cord by prostaglandin E2

1. We recently demonstrated that intrathecal administration of prostaglandin E2 (PGE2) and PGF2alpha induced allodynia through a pathway that includes the glutamate receptor and nitric oxide (NO)-generating systems from pharmacological studies. In order to clarify the involvement of NO in prostaglandin-induced allodynia, we measured NO released from rat spinal cord slices by a chemiluminescence method. 2. PGE2 stimulated NO release from both dorsal and ventral regions all along the spinal cord. PGE2 stimulated the release within 10 min and increased it in a time-dependent manner. 3. The PGE2-induced NO release was observed at 100 nM-10 microM. PGF2alpha stimulated the release at concentrations higher than 1 microM, but PGD2 (up to 10 microM) did not enhance it. 4. 17-Phenyl-omega-trinor PGE2 (EP1 > EP3) and sulprostone (EP1 < EP3) were as potent as PGE2, but PGE1 was less potent, in stimulating NO release. While M&B 28767 (EP3) did not enhance the release, butaprost (EP2) stimulated it at 1 microM. The PGE2-evoked release was blocked by ONO-NT-012, a bifunctional EP1 antagonist/EP3 agonist. 5. The PGE2-evoked release was Ca2+-dependent and blocked by MK-801 (NMDA receptor antagonist) and L-NAME (NO synthase inhibitor). The release was also inhibited by PGD2 and dibutyryl-cyclic AMP. 6. The present study demonstrated that PGE2 stimulates NO release in the rat spinal cord by activation of NMDA receptors through the EP1 receptor, and supports our previous findings that the NO-generating system is involved in the PGE2-induced allodynia.

Central terminals of orofacial primary afferents and NADPH-diaphorase activity in the trigemino-solitary complex of rats

Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) activity and the central terminal fields of branches of the mandibular and chorda tympani nerves were visualized histochemically at the same time using transganglionic transport of wheat germ agglutinin conjugated with horseradish peroxidase. The blue NADPH-d-positive neurons comprised a sparse network in the dorsomedial spinal trigeminal subnucleus oralis and a dense one in the rostral lateral division of the nucleus of the solitary tract. In the subnucleus caudalis, most labeled neurons were in the superficial zone, and smaller numbers were in the magnocellular zone. The NADPH-d-positive neurons in the subnucleus oralis and the nucleus of the solitary tract overlapped mostly with the transganglionically labeled terminal field from the lingual nerve, partly with the terminal field from the inferior alveolar and chorda tympani nerves, and rarely with the terminal field from the mental nerve. The NADPH-d-positive neurons in the dorsomedial paratrigeminal nucleus and subnucleus caudalis overlapped mostly with the terminal field from the lingual nerve, partly with the terminal field from the inferior alveolar and mental nerves and never with the terminal field from the chorda tympani. A statistically significant reduction in the number of NADPH-d-positive neurons was seen bilaterally in subnucleus oralis and the nucleus of the solitary tract when the lingual nerve was transected. Inflammatory insults to the lingual nerve or tooth pulps significantly increased the number of NADPH-d-positive neurons in subnucleus oralis, the nucleus of the solitary tract, and subnucleus caudalis. These results show that the NO/cyclic GMP system in the trigeminal and solitary nuclei is differentially regulated trans-synaptically by trigeminal afferents depending on the nucleus and sensory modality.

Reorganization of the spinal dorsal horn in models of chronic pain: correlation with behaviour

Central reorganization is known to occur in chronic pain models resulting from peripheral injury. Systematic analysis of anatomical and behavioural changes and a comparison of these changes between different models over an extended time course has not been reported. We address this issue by quantifying alterations in markers known to be associated with central reorganization in three models of peripheral injury: complete Freund's adjuvant induced inflammation of the hindpaw, chronic constriction of the sciatic nerve, and tight ligation of the sciatic nerve. Hyperalgesic behaviour to thermal and mechanical stimuli was quantified at four, seven, 14, 28 days post-injury. Distribution and immunodensity changes of the mu-opioid receptor, the neurokinin-1 receptor, and brain nitric oxide synthase distribution were assessed in the superficial dorsal horn, laminae I-II, of the lumbar spinal cord of the rat. Reorganization and behavioural changes were quantified as a per cent change (ipsilateral versus contralateral) and examined together over the duration of the experiment. Chronic constriction injury and inflammation both produced hyperalgesic behaviour in the hindpaw ipsilateral to injury. Decreases in thermal and mechanical withdrawal latencies were maximal at day 4. Complete Freund's adjuvant-treated animals displayed a 25.5%+/-3.8% decline in thermal withdrawal latency and 84.1%+/-8.0% decline in mechanical withdrawal latency. Chronic constriction of the sciatic nerve resulted in an decrease in thermal and mechanical withdrawal latencies, 27.9%+/-3.3%, 90.5%+/-4.4%, respectively. Tight ligation of the sciatic nerve resulted in early increases in the latency of withdrawal that were maximal at seven days 40.7%+/-8.4% for thermal stimulus and at four days 417%+/-5.8% for mechanical stimulus, consistent with deafferentation. The greatest changes in immunolabelling were always found at L4-L5 spinal level, corresponding to the entry zone of sciatic afferents. Mu-opioid receptor immunodensities increased in the dorsal horn ipsilateral to the treated side up to a maximum of 38.3%+/-5.6% at day 7 with inflammation and up to 26.3%+/-3.2%, at day 14 with chronic constriction injury. Mu-opioid receptor immunodensities decreased maximally by 20.0%+/-2.1% at day 4 in the tight ligature model. Significant differences in mu-opioid receptor immunolabelling persisted at day 28 for neuropathic models, at which time there was an absence of significant hyperalgesic behaviour in any group. The number of brain nitric oxide synthase-positive cells decreased at seven days by a maximum of 45.3%+/-5.1% and 59.0%+/-5.2%, respectively, in animals with chronic constriction injury or tight ligature. This decline in immunolabelled brain nitric oxide synthase cells in the dorsal horn ipsilateral to injury persisted at day 28. No significant alteration in brain nitric oxide synthase immunolabelling was found in association with inflammation of the hindpaw. Inducible nitric oxide synthase was not detected in the dorsal horn at any time during the experiment in either tissue of treated or control rats. Neurokinin-1 receptor immunodensity consistently increased ipsilateral to injury irrespective of the type of injury, and, of the three markers, paralleled behaviour most closely. Changes were maximal for inflammation at four days (75.2%+/-9.3%), for chronic constriction injury at four days (85.1%+/-14.6%) and for tight ligature at 14 days (85.7%+/-11.4%). Comparison of behavioural and anatomical data demonstrates that the peak hyperalgesia is concomitant with the greatest increase in neurokinin-1 receptor immunodensity ipsilateral to the injury. The increase in mu-opioid receptor immunodensity parallels behaviour but with a delayed time course, peaking as hyperalgesia abates, except in the case of tight ligature animals where the decrease in immunolabelling appears permanent. (ABSTRACT TRUNCATED)

Intrathecal administration of a new nitric oxide donor, NOC-18, produces acute thermal hyperalgesia in the rat

A nitric oxide releasing compound, NOC-18, was injected intrathecally in order to determine the role of NO in spinal nociceptive mechanisms in rats. The nociceptive threshold was evaluated by the radiant heat tail-flick test. The effects of intrathecal injection of N-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor; methylene blue (MB), a soluble guanylate cyclase inhibitor and hemoglobin (Hb), an NO scavenger, on the nociceptive threshold were measured in the presence and absence of 0.1, 1 and 10 microg of NOC-18. The results were compared with a control group of rats which were injected with the same volume of normal saline. NOC-18 caused a dose-dependent curtailment of the tail-flick latency during the period from 15 to 150 min. L-NAME, MB and Hb all produced prolongation of the tail-flick latency during the same time period. The hyperalgesia induced by this concentration range of NOC-18 was completely blocked by Hb, but was not affected by either L-NAME or MB. These findings indicate that NO plays a direct role in thermal hyperalgesia in the spinal cord, and that an another pathway in addition to the NO-cGMP pathway may be involved.

Nitric oxide in the rat spinal cord in Freund's adjuvant-induced hyperalgesia

To elucidate the involvement of nitric oxide in spinal nociceptive processing, the correlation of thermal withdrawal latency with nitric oxide synthase-stained neurons in the rat lumbar dorsal horn was analyzed after adjuvant-induced inflammation. From 4 hr through 5 days after subcutaneous injection of complete Freund's adjuvant into the hind paw, a marked thermal hyperalgesia was observed for heat stimulus applied to the affected region. NADPH-diaphorase- and nitric oxide synthase-positive neurons increased significantly in the superficial layers of the dorsal horn ipsilateral to the inflamed hind paw at day 3 of adjuvant-induced inflammation. No change in NADPH-diaphorase-positive neurons was observed at 1 hr and 1 day of adjuvant-induced inflammation. The intravenous administration of N omega-nitro-L-arginine methyl ester (L-NAME, 50 mg/kg), an inhibitor of nitric oxide synthase, significantly blocked the adjuvant-induced thermal hyperalgesia at day 3 of inflammation, but not at day 1; and it had no effect in non-inflamed rats. This anti-hyperalgesic effect of L-NAME at day 3 of inflammation was reversed by the prior administration of L-arginine (600 mg/kg, i.p.), a substrate of nitric oxide synthase. These data suggest that nitric oxide producing neurons in the spinal dorsal horn are involved in maintaining and facilitating the hyperalgesia associated with chronic nociception.

Ultrastructural study of NADPH-d positive neurons in laminae I and II of the rat caudal spinal trigeminal nucleus

The present study investigated the ultrastructure of neurons in the caudal spinal trigeminal nucleus. These neurons which are believed to function as interneurons in the transmission of orofacial nonreflexive nociceptive information, measured 20 microns x 11 microns, and were nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) positive. The reaction product, formazan, was localized in the nuclear envelope, mitochondria, rough endoplasmic reticulum, and multivesicular bodies of these neurons. It was also localized in the membrane of the smooth endoplasmic reticulum at the axon terminal. The neurons were contacted by both axosomatic and axodendritic synapses formed by both NADPH-d positive and NADPH-d negative axon terminals. Two types of NADPH-d positive axon terminals could be recognized. The first was a large terminal containing many stained mitochondria and unstained small round agranular vesicles mixed with some slightly flattened ones. It formed asymmetrical axodendritic synapse. The second type of axon terminals contained pleomorphic synaptic vesicles and formed asymmetrical synapses upon both dendrites and soma. The sources of NADPH-d positive axon terminals were discussed. Most of the unstained axon terminals forming axosomatic and axodendritic synapses with stained cell bodies and dendrites contained flattened vesicles. In addition to the above, complicated synaptic configurations showing NADPH-d positive axoaxonic synapses in relation to NADPH-d negative dendritic spines were also seen in which a NADPH-d negative dendritic spine was completely contacted by a NADPH-d positive bouton which was in turn contacted by another NADPH-d positive bouton.

Gabapentin (neurontin) and S-(+)-3-isobutylgaba represent a novel class of selective antihyperalgesic agents

1. Gabapentin (neurontin) is a novel antiepileptic agent that binds to the alpha 2 delta subunit of voltage-dependent calcium channels. The only other compound known to possess affinity for this recognition site is the (S)-(+)-enantiomer of 3-isobutylgaba. However, the corresponding (R)-(-)-enantiomer is 10 fold weaker. The present study evaluates the activity of gabapentin and the two enantiomers of 3-isobutylgaba in formalin and carrageenan-induced inflammatory pain models. 2. In the rat formalin test, S-(+)-3-isobutylgaba (1-100 mg kg-1) and gabapentin (10-300 mg kg-1) dose-dependently inhibited the late phase of the nociceptive response with respective minimum effective doses (MED) of 10 and 30 mg kg-1, s.c. This antihyperalgesic action of gabapentin was insensitive to naloxone (0.1-10.0 mg kg-1, s.c.). In contrast, the R-(-)-enantiomer of 3-isobutylgaba (1-100 mg kg-1) produced a modest inhibition of the late phase at the highest dose of 100 mg kg-1. However, none of the compounds showed any effect during the early phase of the response. 3. The s.c. administration of either S-(+)-3-isobutylgaba (1-30 mg kg-1) or gabapentin (10-100 mg kg-1), after the development of peak carrageenan-induced thermal hyperalgesia, dose-dependently antagonized the maintenance of this response with MED of 3 and 30 mg kg-1, respectively. Similar administration of the two compounds also blocked maintenance of carrageenan-induced mechanical hyperalgesia with MED of 3 and 10 mg kg-1, respectively. In contrast, R-(-)-3-isobutylgaba failed to show any effect in the two hyperalgesia models. 4. The intrathecal administration of gabapentin dose-dependently (1-100 micrograms/animal) blocked carrageenan-induced mechanical hyperalgesia. In contrast, administration of similar doses of gabapentin into the inflamed paw was ineffective at blocking this response. 5. Unlike morphine, the repeated administration of gabapentin (100 mg kg-1 at start and culminating to 400 mg kg-1) over 6 days did not lead to the induction of tolerance to its antihyperalgesic action in the formalin test. Furthermore, the morphine tolerance did not cross generalize to gabapentin. The s.c. administration of gabapentin (10-300 mg kg-1), R-(-) (3-100 mg kg-1) or S-(+)-3-isobutylgaba (3-100 mg kg-1) failed to inhibit gastrointestinal motility, as measured by the charcoal meal test in the rat. Moreover, the three compounds (1-100 mg kg-1, s.c.) did not generalize to the morphine discriminative stimulus. Gabapentin (30-300 mg kg-1) and S-(+)-isobutylgaba (1-100 mg kg-1) showed sedative/ataxic properties only at the highest dose tested in the rota-rod apparatus. 6. Gabapentin (30-300 mg kg-1, s.c.) failed to show an antinociceptive action in transient pain models. It is concluded that gabapentin represents a novel class of antihyperalgesic agents.

Involvement of cGMP in nociceptive processing by and sensitization of spinothalamic neurons in primates

Central sensitization of spinothalamic tract (STT) neurons in anesthetized monkeys after intradermal injection of capsaicin depends in part on disinhibition. Protein kinase C is suggested to participate in this process. The present study shows that the nitric oxide-cGMP (NO-cGMP) signal transduction system also contributes to sensitization of wide dynamic range (WDR) STT neurons located in the deep dorsal horn. The NO-cGMP system was activated by microdialysis administration into the dorsal horn of 8-bromo-cGMP, an analog of cGMP. Sensitization of STT cells by 8-bromo-cGMP increased the responses of deep WDR STT cells to both weak and strong mechanical stimulation of the skin and simultaneously attenuated the inhibition of the same neurons produced by stimulation in the periaqueductal gray (PAG). In contrast, WDR STT cells in the superficial dorsal horn and high-threshold (HT) STT cells in superficial or deep layers showed reduced responses to mechanical stimulation of the skin after infusion of 8-bromo-cGMP, and PAG inhibition of these neurons was unaffected. Sensitization of STT cells and the attenuation of PAG inhibition induced by intradermal injection of capsaicin were prevented by preteatment of the dorsal horn with a guanylate cyclase inhibitor, 1 H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. The results support the hypothesis that activation of the NO-cGMP signal transduction system contributes to the sensitization of WDR STT neurons in the deep dorsal horn and helps explain why intradermal capsaicin injections often fail to sensitize superficial and HT STT cells. The results also support the idea that sensitization of STT cells is produced in part by disinhibition.

The role of phospholipase A2 and nitric oxide in pain-related behavior produced by an allograft of intervertebral disc material to the sciatic nerve of the rat

STUDY DESIGN

To elucidate the pathomechanisms of radicular pain secondary to lumbar disc herniation.

OBJECTIVES

To evaluate whether intervertebral disc material applied to the sciatic nerve produces hyperalgesia, and if the hyperalgesia in influenced by inhibitors of phospholipase A2 and nitric oxide synthase.

SUMMARY OF BACKGROUND DATA

Previously, the authors reported that application of nucleus pulposus and anulus fibrosus material to the lumbar epidural space produces different forms of hyperalgesia (mechanical versus thermal), with different and distinct histologic changes. Additional pharmacologic studies showed that phospholipase A2 and nitric oxide are involved in the mechanisms that produce the mechanical and thermal hyperalgesia, respectively, N omega-nitro-L-arginine methyl ester and mepacrine are relatively selective inhibitors of nitric oxide synthase and phospholipase A2, respectively. However, it is not known what the relation is between the hyperalgesia produced and the activation and involvement of phospholipase A2 and production of nitric oxide, or why the application of nucleus pulposus and nucleus pulposus with anulus fibrosus produces different types of hyperalgesia.

METHODS

Experiments were performed in five groups of rats: The control group (no treatment), the sham group (exposure of the sciatic nerve only), the fat group (allografted fat on the sciatic nerve), the nucleus pulposus group (allografted nucleus pulposus) and the nucleus pulposus + anulus fibrosus group (allografted nucleus pulposus and anulus fibrosus). Withdrawal threshold and latency from mechanical pressure and a radiant heat to hind paws were measured preoperatively and postoperatively. After local sciatic nerve administration of N theta-nitro-L-arginine methyl ester or mepacrine into the operated site, sensitivities to noxious stimuli were reevaluated after treatment.

RESULTS

Only rats in the nucleus pulposus group showed evidence of mechanical hyperalgesia. However, injection of N theta-nitro-L-arginine methyl ester resulted in evidence of mechanical hyperalgesia in the nucleus pulposus + anulus fibrosus group. Mechanical hyperalgesia was produced in the nucleus pulposus group and after injection of N theta-nitro-L-arginine methyl ester in the nucleus pulposus+anulus fibrosus group, both of which returned to normal after mepacrine injection. There were no significant changes in sensitivity to thermal stimuli in any of the experimental groups.

CONCLUSION

It appears that phospholipase A2 and nitric oxide play important but different roles in pathomechanisms of radicular pain in lumbar disc herniation.

Histochemical demonstration of nitric oxide in herniated lumbar discs. A clinical and animal model study

STUDY DESIGN

This study was designed to localize the cells that produce nitric oxide in a lumbar disc herniation by histochemical method, including in situ hybridization.

OBJECTIVE

To clarify which cells in herniated lumbar discs produce nitric oxide.

SUMMARY OF BACKGROUND DATA

It was reported that herniated lumbar intervertebral disc specimens in culture are capable of producing nitric oxide.

METHODS

Surgical specimens from lumbar disc herniation were examined to determine nitric oxide synthase histologically using nicotinamide adenine dinucleotide phosphate diaphorase histochemistry. Allografts of intervertebral disc materials were placed on the epidural space at L6 level in the rat. Nitric oxide synthase was examined in the applied tissues using nicotinamide adenine dinucleotide phosphate diaphorase histochemistry and in situ hybridization histochemistry.

RESULTS

Nicotinamide adenine dinucleotide phosphate diaphorase (nitric oxide synthase) positive cells were observed in 2 (40%) of 5 herniated disc materials in patients. The positive cells were mainly in granulation tissue around intervertebral disc materials. In animal models, nitric oxide synthase-positive cells were observed in all specimens at 1 and 2 weeks postoperatively. Newly formed vessels and small round cells in granulation tissue around the grafted intervertebral disc showed positive reaction. In situ hybridization demonstrated the expression of inducible isoform of nitric oxide synthase messenger RNA (mRNA) identical to small round cells around the applied intervertebral disc.

CONCLUSION

Nitric oxide in a lumbar disc herniation is mainly produced by cells in granulation tissue around the herniated intervertebral disc.

NG-nitro-L-arginine methyl ester (L-NAME) prevents tachyphylaxis to local anesthetics in a dose-dependent manner

A model of local anesthetic tachyphylaxis was developed in our group previously using repeated sciatic nerve blocks in rats. In this model, thermal hyperalgesia accelerated tachyphylaxis, and the noncompetitive N-methyl-D-aspartic acid (NMDA) receptor antagonist, MK-801, prevented both hyperalgesia and tachyphylaxis. Nitric oxide is thought to be a second messenger for NMDA pathways in the spinal cord, and appears to be involved in spinal mechanisms of hyperalgesia. We hypothesized that nitric oxide synthase inhibitors would also inhibit the development of tachyphylaxis. Repeated rat sciatic nerve blocks were placed by percutaneous injection of 2-chloroprocaine. Block duration was tested by measuring hot-plate latency at 56 degrees C. Two hours before the first nerve block, rats received intraperitoneal injections with saline or one of six concentrations of NG-nitro-L-arginine methyl ester (L-NAME) in a randomized, blinded pattern. Control rats developed tachyphylaxis as seen previously: the duration of the third block was 30% that of the first. L-NAME inhibited the development of tachyphylaxis in a dose-dependent manner; tachyphylaxis was inhibited by 50% using L-NAME at 0.2mg/kg and completely abolished by 50 mg/kg. Nitric oxide pathways may be involved in the development of tachyphylaxis to local anesthetic nerve block.

Pathomechanism of pain-related behavior produced by allografts of intervertebral disc in the rat

STUDY DESIGN

This study was designed to evaluate whether allografts of intervertebral disc materials produce hyperalgesia in the rat and whether an immune response, pH, or chemicals correlate with the induced hyperalgesia.

OBJECTIVE

To elucidate the pathomechanisms of radicular pain secondary to lumbar disc herniation.

SUMMARY OF BACKGROUND DATA

It has been reported that a low pH, an autoimmune reaction, or chemical radiculitis is likely responsible for radicular pain associated with lumbar disc herniation. In animal studies, it has been shown that hyperalgesia (an increased sensitivity to painful stimuli) involves activation of phospholipase A2 and nitric oxide synthase.

METHODS

Fat, nucleus pulposus, and anulus fibrosus were allografted into the epidural space at L6 in the rat. Withdrawal response thresholds to mechanical stimuli and withdrawal response latencies to thermal stimuli on the tail and pH in the applied tissues were measured after surgery. Interleukin-1, phospholipase A2, and nitric oxide synthase were examined in the applied tissues using immunohistochemistry, nicotineamide adenine dinucleotide phosphate-diaphorase histochemistry, and in situ hybridization.

RESULTS

Allografted fat did not produce hyperalgesia. Allografts of nucleus pulposus and nucleus pulposus plus anulus fibrosis showed evidence of mechanical and thermal hyperalgesia, respectively. There were no observed changes in pH over time. Although interleukin-1 was demonstrated in all applied tissues, phospholipase A2 was only observed around the applied nucleus A2 was only observed around the applied nucleus pulposus and nucleus pulposus plus anulus fibrosus. Nitric oxide synthase was only markedly increased around the applied tissues.

CONCLUSION

The nucleus pulposus and anulus fibrosus produce different forms of hyperalgesia (mechanical vs. thermal) associated with different and distinct immunohistochemical changes. It is possible that radicular pain of a lumbar disc herniation results from chemicals, such as phospholipase A2 and nitric oxide.

Antinociceptive synergistic interaction between morphine and n omega-nitro 1-arginine methyl ester on thermal nociceptive tests in the rats

PURPOSE

This study was conducted to demonstrate if subeffective dose of N omega Nitro L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, interacts with morphine when given intrathecally (IT), epidurally (EP), and intravenously (IV) to produce a synergistic antinociceptive effect in normal rats.

METHODS

Chronically catheterized 72 male Wistar rats were used in this study. We measured the tail flick latency in response to thermal stimulation of the tail on a hot plate (53 degrees C), and determined dose-response functions of IT, EP and IV morphine, L-NAME, and morphine co-administered with subeffective doses of L-NAME. The antinociceptive effect was evaluated using the doses for 50% maximum probable effect (ED50). The interaction between morphine and L-NAME was evaluated using an isobolographic approach. ANOVA was used for the statistical analysis. Significance was taken at P < 0.05.

RESULTS

Morphine and L-NAME produced dose-related antinociceptive effects in the IT (ED50 = 1.23 +/- 0.18 micrograms (Mean +/- SEM) and 76.0 +/- 14.5 micrograms), EP (ED50 = 32.6 +/- 2.4 micrograms and 560 +/- 97 micrograms), and IV (ED50 = 563 +/- 71.8 micrograms and 16.0 +/- 4.0 mg) groups, respectively. Co-administration of small doses of L-NAME and morphine produced reductions in the ED50 values for morphine (0.16 +/- 0.03 microgram in IT, 1.18 +/- 0.32 micrograms in EP, and 50.5 +/- 11.4 micrograms in IV groups) (P < 0.01), suggesting a multiplicative interaction of L-NAME with morphine.

CONCLUSION

L-NAME has a synergistic antinociceptive interaction with morphine in response to thermal stimulation when given intrathecally, epidurally or intravenously in rats.

Alterations in rat spinal cord cGMP by peripheral nerve injury and adrenal medullary transplantation

Adrenal medullary chromaffin cells implanted into the spinal subarachnoid space can reduce abnormal pain-related responses in chronic pain models. Persistent pain is thought to involve the activation of N-methyl-D-aspartate (NMDA) receptors and subsequent production of nitric oxide (NO) and cyclic guanosine 3',5'-monophosphate (cGMP). Changes in dorsal horn levels of cGMP in the rat were determined in conjunction with alterations in pain behaviors following peripheral nerve injury and adrenal medullary transplantation. Results indicated increased spinal cGMP levels in parallel with thermal and mechanical hyperalgesia and tactile allodynia consequent to chronic constriction injury of the sciatic nerve in rats. Adrenal medullary, but not control transplants, attenuated the hyperalgesia and allodynia and decreased spinal cGMP content. These results suggest that adrenal medullary transplants may reduce abnormal pain by intervention in the spinal NMDA-NO cascade.

Calmodulin inhibitors induce spinal analgesia in rats

Calcium is an important intracellular messenger that interacts with Ca(2+)-binding proteins, such as calmodulin (CaM), to activate several intracellular enzymes. The involvement of Ca2+ in the transmission of nociceptive signals has been demonstrated at the spina level. Specifically, spinal sensitization induced by persistent nociceptive stimulation seems to be related to an increase of cytosolic calcium and the subsequent activation of several enzymes, some of which are Ca2+/CaM dependent. In order to elucidate the possible implication of calmodulin in these pain processes, we have studied the effect of two calmodulin inhibitors (W-7 and calmidazolium) or the formalin and tail-flick tests in rats after their intrathecal administration. Antinociceptive effects were observed in both tests by injecting 0.12-1 mumol/rat of calmidazolium and 0.25-2 mumol/rat of W-7. Calmidazolium was more potent than W-7 in inhibiting both phases of the formalin test, whereas lower doses of W-7 in comparison to calmidazolium affected the tail-flick latencies. In addition, both drugs induced, at high doses, a muscular flaccidity of the hindlimbs that impaired normal walking in the rats. This effect caused; significant reduction of the rotarod performance when 1 mumol/rat of calmidazolium or 2 mumol/rat of W-7 were injected. Overall, our results show that calmodulin inhibitors are capable of producing spinal analgesia on phasic and tonic noxious stimuli in rats, thus rendering them a promising potential as analgesics.

The excitatory and inhibitory modulation of primary afferent fibre-evoked responses of ventral roots in the neonatal rat spinal cord exerted by nitric oxide

1. We investigated the role of nitric oxide (NO) in modulating spinal synaptic responses evoked by electrical and noxious sensory stimuli in the neonatal rat spinal cord in vitro. 2. Potentials were recorded extracellularly from a ventral root (L3-L5) of the isolated spinal cord preparation or spinal cord-saphenous nerve-skin preparation of 0- to 2-day-old rats. Spinal reflexes were elicited by electrical stimulation of the ipsilateral dorsal root or by noxious skin stimulation. 3. In the spinal cord preparation, single shock stimulation of a dorsal root at C-fibre strength induced mono-synaptic reflex followed by a slow depolarizing response lasting about 30 s (slow ventral root potential; slow VRP) in the ipsilateral ventral root of the same segment. Bath-application of NO gas-containing medium (10(-4)- 10(-2) dilution of saturated medium) and NO donors, 1-hydroxy-2-oxo-3-(N-ethyl-2-aminoethyl)-3-ethyl-1-triazene (NOC12, 3-300 microM), S-nitroso-N-acetyl-D,L-penicillamine (SNAP, 3-300 microM) and S-nitroso-L-glutathione (GSNO, 3-300 microM), produced an inhibition of the slow VRP and a depolarization of ventral roots. Another NO donor, 3-morpholinosydononimine (SIN-1, 30-300 microM), also depressed the slow VRP but did not depolarize ventral roots. These agents did not affect the mono-synaptic reflex. 4. In the spinal cord-saphenous nerve-skin preparation, application of capsaicin (0.1-0.2 microM) to skin evoked a slow depolarizing response of the L3 ventral root. This slow VRP was depressed by NOC12 (10-300 microM) and SIN-1 (100-300 microM). When the concentration of NOC12 was increased to 1 mM, spontaneous synaptic activities were augmented and the depressant effect of NOC12 on the slow VRP became less pronounced. 5. A NO-scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide( carboxy- PTIO, 100-300 microM) prevented the depressant effect on the dorsal root-evoked slow VRP and ventral root depolarizing effects of NO donors. Carboxy-PTIO increased spontaneous synaptic activities and markedly potentiated the slow VRP. A NO synthase (NOS) inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME, 0.03-1 microM), but not D-NAME (0.03-1 microM), also markedly potentiated the slow VRP and this effect was reversed by L-arginine (300 microM). 6. 8-Bromo-cyclic guanosine 3': 5'-monophosphate (8-Br-cyclic GMP, 100-300 microM) produced both an inhibition of the slow VRP and a depolarization of ventral roots. A cyclic GMP-dependent protein kinase inhibitor, KT5823 (0.3 microM), partly inhibited the depressant effects of NO donors and 8-Br-cyclic GMP on the dorsal root-evoked slow VRP. In contrast, KT5823 did not inhibit the depolarizing effects of NO donors. 7. Perfusion of the spinal cord with medium containing tetrodotoxin (0.3 microM) and/or low Ca2+ (0.1 mM)-high Mg2+ (10 mM) markedly potentiated the depolarizing effect of NO donors. The SNAP-evoked depolarization in the tetrodotoxin-containing low Ca(2+)-high Mg2+ medium was significantly inhibited by excitatory amino acid receptor antagonists D-(-)-2-amino-5-phosphonovaleric acid (30 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). 8. The present study suggests that inhibitory and excitatory mechanisms meditated by the NO-cyclic GMP cascade are involved in the primary afferent fibre-evoked nociceptive transmission in the neonatal rat spinal cord. The inhibitory mechanism, but not the excitatory mechanism, appears to be partly mediated by cyclic GMP-dependent protein kinase. It is also suggested that Ca(2+)-independent release of excitatory amino acid neurotransmitters contributes to the depolarizing response to NO of ventral roots.

A nitric oxide synthesis inhibitor (L-NAME) reduces licking behavior and Fos-labeling in the spinal cord of rats during formalin-induced inflammation

Formalin injected subcutaneously into the hindpaw of the rat produces an animal model of inflammation that exhibits a phasic component and a tonic component of pain. We evaluated the effects of a nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME), on a formalin-induced behavior, hindpaw licking, and on Fos-labeling of nuclei in the fifth lumbar spinal segment. Our results demonstrated that pretreatment with intrathecal doses of 0.3 and 1.0 mg of L-NAME significantly reduced licking behavior associated with injection of formalin into the left hindpaw of the rat. In addition, these same doses of L-NAME reduced formalin-induced Fos-labeling in the ipsilateral dorsal gray matter (as compared to the contralateral gray matter). Qualitative assessment suggested that the reduction in labeling occurred primarily in the superficial dorsal horn. The stereoisomer, D-NAME, administered at the same doses had little to no effect on either formalin-induced licking or Fos-labeling. Finally, our results revealed that total licking time was related to Fos-labeling. Rats that spent less time licking the hindpaw exhibited a smaller increase in Fos-labeling. Our results suggest that the production of nitric oxide is associated with licking behavior resulting from formalin injection into the hindpaw of rats. Our results also suggest that the production of nitric oxide and Fos are associated. Indeed, these substances may be involved in spinal pathways associated with nociception.

Treatment of a chronic allodynia-like response in spinally injured rats: effects of systemically administered nitric oxide synthase inhibitors

We have previously reported that we have observed chronic pain-like response to light mechanical stimuli (allodynia) in rats after severe spinal cord ischemia, which resembles some painful conditions in chronic spinally injured patients and is not relieved by a number of conventional analgesics used for treating chronic neuropathic pain. In the present study, we tested the effects of the non-selective nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) and the selective neuronal NOS inhibitor 7-nitro indazole (7-NI) and 6-nitro indazole (6-NI) on the chronic allodynia-like behavior. Systemic L-NAME dose-dependently relieved mechanical allodynia-like response in a stereo-specific and L-arginine-reversible manner without causing sedation or motor deficits. However, L-NAME significantly elevated systemic blood pressure. Systemic 7-NI relieved chronic allodynia in a L-arginine reversible manner, did not increase blood pressure or induce sedation, but caused motor deficits at a high dose, which was not reversed by L-arginine. Systemic 6-NI also relieved the chronic allodynia, which was however associated with severe sedation. In order to exclude the possibility that the effect of L-NAME on blood pressure was involved in the analgesic effect observed, the effect of systemically applied adrenaline was examined. Adrenaline increased the systemic blood pressure to a similar extent as L-NAME, but did not relieve allodynia. It is suggested that blockade of NOS by L-NAME relieved the chronic allodynia-like behavior in spinally injured rats. This effect was likely to be mediated by a blockade of neuronal isoforms of NOS, as 7-NI relieved the allodynia in a L-arginine-reversible manner. Consequently, generation of NO by neuronal NOS may be critically involved in the maintenance of this abnormal pain-related sensation. The possibility of using NOS inhibitors as potential novel analgesics is discussed.

The nitric oxide-cyclic GMP pathway is required for nociceptive signalling at specific loci within the somatosensory pathway

The involvement of nitric oxide in nociceptive processing was examined at the main loci of synaptic transmission within the rat somatosensory pathway from the caudal sural cutaneous nerve. Intrathecal (lumbar 1-3) administration of the nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (30 micrograms), inhibited nitric oxide synthase in this region of the spinal cord by greater than 80% but had no significant effect on nitric oxide synthase in parietal cerebral cortex, thalamus or medulla/pons. In a rat model of peripheral neuropathy (one to two week ligation of the caudal sural cutaneous nerve), intrathecal administration of the same dose of N omega-nitro-L-arginine methyl ester prevented the hyperalgesic response to thermal stimuli. Administration of 30 micrograms N omega-nitro-L-arginine methyl ester into the lateral ventricle had no effect on nitric oxide synthase in the lumbar 1-3 region of the spinal cord but gave substantial inhibition in higher areas of the somatosensory pathway (parietal cerebral cortex, thalamus and medulla/pons). Nitric oxide synthase in the parietal cerebral cortex (but not thalamus) was inhibited to a greater extent in the hemisphere ipsilateral to the site of administration. Administration of 30 micrograms N omega-nitro-L-arginine methyl ester into the lateral ventricle decreased thermal hyperalgesia, but only when N omega-nitro-L-arginine methyl ester was administered contralateral to the ligated caudal sural cutaneous nerve and therefore ipsilateral to the cortical nociceptive processing from this nerve. Intrathecal and intracerebroventricular administration of the selective inhibitor of nitric oxide-sensitive guanylyl cyclase, 1-H-[1,2,4]oxadiazalo[4,3-a]quinoxalin-1-one, also decreased the hyperalgesic response to thermal stimuli. These data demonstrate that, in a model of neuropathic pain, nitric oxide is involved in nociceptive processing at spinal and cerebrocortical synaptic loci of the somatosensory pathway and that its actions appear to be mediated through guanylyl cyclase.

Effect of spinal nitric oxide inhibition on capsaicin-induced nociceptive response

Pretreatment with the nitric oxide synthase (NOS) inhibitor L-NG-nitro arginine methyl ester (L-NAME), injected intraperitoneally (i.p.) or intrathecally (i.t.), produced a significant antinociception in the mouse assessed by the capsaicin-induced paw licking procedure. Varying the administration time of an effective dose of L-NAME (160nmol, i.t.) resulted in a significant decrease of the brief nociceptive behavioral response induced by capsaicin, even when L-NAME was given 2 hr before capsaicin. L-NAME, injected i.p. or i.t., produced a dose-related reduction in paw licking in the second phase of the formalin (2.0%) response without affecting the first phase. L-Arginine (600 mg/kg, i.p.) but not D-arginine (600 mg/kg, i.p.) reversed the antinociceptive effect of L-NAME in the capsaicin test. Antinociceptive effect of L-NAME, injected i.p. or i.t., was more potent in the second phase response of formalin-induced paw licking than in capsaicin-induced nociceptive response. The inhibitory action of L-NAME was reversed by L-arginine but not D-arginine in the second phase response. L-Arginine alone was without affecting capsaicin- and formalin-induced nociceptive responses. These results suggest that spinal nitric oxide (NO) may be involved in the mechanisms of capsaicin-induced brief nociceptive stimuli, but not in the first, acute phase of the formalin-induced response in mice.

Neuropathic pain in rats is associated with altered nitric oxide synthase activity in neural tissue

Peripheral nerve injury may lead to a chronic neuropathic pain state that results from an increase in excitability of central neurons. This central sensitization is mediated via an N-methyl-D-aspartic acid (NMDA) receptor and may involve the production of nitric oxide (NO). As NO is suggested to play a role in nociceptive transmission following nerve injury, we examined for altered NO synthase activity at multiple levels of peripheral and spinal neural tissue in a rat model of neuropathic pain. Peripheral neuropathy was induced in rats (N = 12) by ligation of the left L5 and L6 nerve roots. Six other rats had sham surgery. An ipsilateral decrease in paw withdrawal threshold to mechanical stimuli confirmed the presence of a neuropathic pain state. Samples of the lumbar and thoracic spinal cords, L4, L5, and L6 dorsal root ganglia (DRGs), and the sciatic nerves were obtained from the lesioned and contralateral sides at 2 and 4 weeks after neuropathic surgery (N = 6 per group). In the lumbar spinal cord, a bilateral decrease in nitric oxide synthase (NOS) activity was observed 2 and 4 weeks after neuropathic surgery. NOS activity was increased in the ipsilateral L5 and 6 DRGs 2 weeks following neuropathic surgery. An increase in NOS activity in the DRG may be an early mechanism for inducing more central changes. The bilaterally decreased NOS activity in the lumbar spinal cord may be secondary to a negative feedback mechanism resulting from increased NO production in the spinal dorsal root ganglia. Multiple alterations in expression of NOS activity that occur in both peripheral and central processing may play a role in the pain behavior resulting from peripheral nerve injury. (Preliminary results of these studies have been presented in abstract form at the annual meetings of the Society for Neuroscience, 1994, and the American Society of Anesthesiologists, 1994).

Expression of nitric oxide synthase type II in the spinal cord under conditions producing thermal hyperalgesia

There is evidence supporting spinal cord nitric oxide (NO) production in the mechanisms underlying hyperalgesia, presumed to arise from the activity of neuronal nitric oxide synthase type I (NOS I). Intrathecal administration of interleukin-1 beta and interferon-gamma to rats results in a thermal hyperalgesia which peaks at 2 h post-injection but which is undetectable 8 h post-injection. Expression of mRNA for nitric oxide synthase type II (NOS II) was detected by reverse transcription-polymerase chain reaction followed by Southern hybridization utilizing specific oligonucleotides in spinal cord tissue from animals 4 h and 8 h after cytokine injection, but not at longer time points. NOS II protein was detected in soluble fractions of spinal cords from animals 4 h and 8 h after cytokine injection. In situ hybridization for NOS II mRNA revealed positive cells bilaterally in the spinal cord 4 h after cytokine injection in a perivascular distribution and scattered throughout the gray and white matter. Immunohistochemistry for NOS II showed a similar distribution which could only be partially accounted for by macrophages/microglia. These results provide evidence for induction of NOS II expression under conditions producing thermal hyperalgesia and suggest a possible role in this behavior for the production of NO by a variety of cell types in the CNS.

Application of nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester, on injured nerve attenuates neuropathy-induced thermal hyperalgesia in rats

This study tested the ability of a nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME), to attenuate behavioral hyperalgesia in a rat model of neuropathic pain [Bennett, G.J. and Xie, Y.-K., Pain, 33 (1988) 87-107]. A mononeuropathy was produced by chronic constriction injury (CCI) of the sciatic nerve. Thermal hyperalgesia was assessed by a reduction of paw withdrawal latency to a noxious heat source. Following CCI, there was significant hyperalgesia in groups of rats treated with D-NAME (n = 7), an inactive isomer of L-NAME, saline (n = 7) or systemic L-NAME (n = 10). In contrast, when L-NAME was applied directly and continuously to the site of CCI (5.0 micrograms/microliter per h for up to 2 weeks) via an osmotic pump implanted at the time of the injury, no significant thermal hyperalgesia was observed (n = 8). The results suggest the involvement of nitric oxide in the development and maintenance of thermal hyperalgesia in a rat model of neuropathy. The blockade of nitric oxide production at the site of injury may provide a new approach for treatment of neuropathic pain.

Effect of a nitric oxide donor (glyceryl trinitrate) on nociceptive thresholds in man

Several animal studies suggest that nitric oxide (NO) plays a role in central and peripheral modulation of nociception. Glyceryl trinitrate (GTN) exerts its physiological actions via donation of NO. The purpose of the present study was to examine the effect of this NO donor on nociceptive thresholds in man. On two different study days separated by at least a week 12 healthy subjects received a staircase infusion of GTN (0.015, 0.25, 1.0, 2.0 micrograms/kg/min, 20 min each dose) or placebo in a randomized double-blind crossover design. Before the infusion and after 15 min of infusion on each dose, pressure pain detection and tolerance thresholds were determined by pressure algometry (Somomedic AB, Sweden) in three different anatomic regions (finger, a temporal region with interposed myofascial tissue and a temporal region without interposed myofascial tissue). Relative to placebo, the three higher GTN doses induced a decrease in both detection and tolerance thresholds in the temporal region with interposed myofascial tissue (p = 0.003 detection and p = 0.002 tolerance thresholds, Friedman). No such changes were observed in the other two stimulated regions. These results could reflect central facilitation of nociception by NO. However, we regard convergence of nociceptive input from pericranial myofascial tissue and from cephalic blood vessels dilated by NO as a more likely explanation of our findings.

Adrenalectomy increases reduced nicotinamide adenine dinucleotide phosphate-diaphorase activity in the rat spinal trigeminal subnucleus caudalis

Neurons exhibiting reduced nicotinamide adenine dinucleotide phosphate-diaphorase activity (NADPHd) were quantified at 500 microns rostrocaudal intervals in spinal trigeminal nucleus (Vsp) of adenalectomized (ADX), ADX + corticosterone, and sham-ADX rats 6-12 days after surgery. NADPHd neurons were found predominantly in Vsp subnucleus caudalis (Vc) and in dorsomedial subnucleus oralis. ADX significantly increased the number of NADPHd neurons in superficial laminae of Vc, an effect reversed by chronic corticosterone replacement. ADX effects on NADPHd in superficial laminae of Vc but not in deep laminae of Vc or in the periobex region of Vsp paralleled previously observed sites of ADX enhancement of noxious stimulus-induced Fos-like immunoreactivity. The results indicate that chronic changes in adrenal steroid status regulate NADPHd, a mechanism that may both derive from changes in nitric oxide synthase expression and influence the processing of nociceptive information by central trigeminal neurons.

Increased NADPH-diaphorase reactivity and cytokine expression in dorsal root ganglia in acquired immunodeficiency syndrome

We studied lumbosacral dorsal root ganglia (DRGs) from 10 patients with acquired immunodeficiency syndrome (AIDS) and five controls using immunocytochemistry, in situ hybridization and NADPH-diaphorase (NADPHd) histochemistry. Human immunodeficiency virus (HIV)-1 RNA was detected in five AIDS cases, and HIV-1 p24 antigen was found in four of these patients. The densities of nodules of Nageotte (nN), macrophages and major histocompatibility complex-class II-positive cells were significantly increased in the DRGs of AIDS patients compared to controls. Cytomegalovirus antigen was observed in the DRGs of four AIDS cases and one control, but without its presence being related to neuronal degeneration. Furthermore, we detected tumor necrosis factor, interferon-gamma, interleukin (IL)-1 beta, and IL-6 in the DRGs from AIDS patients. Using NADPHd histochemistry, we showed that the number of NADPHd-positive neurons was significantly increased in the DRGs of AIDS patients compared to controls, implying upregulation of nitric-oxide (NO) production in AIDS DRGs. Generally, there were increased numbers of nN in DRGs which contained more NADPHd-positive neurons. Additionally, immunoreactivity for an inducible form of NO synthase was detected in interstitial cells in AIDS DRGs. These results suggest that reactive inflammation, including the production of cytokines, occurs in the DRGs of AIDS patients and that excessive production of NO may be related to neuronal degeneration in AIDS DRGs.