N-methyl-D-aspartate receptor-mediated changes in thermal nociception: allosteric modulation at glycine and polyamine recognition sites

A randomized, double-blind, placebo-controlled study of daily cannabidiol for the treatment of canine osteoarthritis pain

ABSTRACT

Over the last 2 decades, affirmative diagnoses of osteoarthritis (OA) in the United States have tripled due to increasing rates of obesity and an aging population. Hemp-derived cannabidiol (CBD) is the major nontetrahydrocannabinol component of cannabis and has been promoted as a potential treatment for a wide variety of disparate inflammatory conditions. Here, we evaluated CBD for its ability to modulate the production of proinflammatory cytokines in vitro and in murine models of induced inflammation and further validated the ability of a liposomal formulation to increase bioavailability in mice and in humans. Subsequently, the therapeutic potential of both naked and liposomally encapsulated CBD was explored in a 4-week, randomized placebo-controlled, double-blinded study in a spontaneous canine model of OA. In vitro and in mouse models, CBD significantly attenuated the production of proinflammatory cytokines IL-6 and TNF-α while elevating levels of anti-inflammatory IL-10. In the veterinary study, CBD significantly decreased pain and increased mobility in a dose-dependent fashion among animals with an affirmative diagnosis of OA. Liposomal CBD (20 mg/day) was as effective as the highest dose of nonliposomal CBD (50 mg/day) in improving clinical outcomes. Hematocrit, comprehensive metabolic profile, and clinical chemistry indicated no significant detrimental impact of CBD administration over the 4-week analysis period. This study supports the safety and therapeutic potential of hemp-derived CBD for relieving arthritic pain and suggests follow-up investigations in humans are warranted.

Anti-GD2 induced allodynia in rats can be reduced by pretreatment with DFMO

Background

Anti-GD2 therapy with dinutuximab is effective in improving the survival of high-risk neuroblastoma patients in remission and after relapse. However, allodynia is the major dose-limiting side effect, hindering its use for neuroblastoma patients at higher doses and for other GD2-expressing malignancies. As polyamines can enhance neuronal sensitization, including development of allodynia and other forms of pathological pain, we hypothesized that polyamine depletion might prove an effective strategy for relief of anti-GD2 induced allodynia.

Method

Sprague-Dawley rats were allowed to drink water containing various concentrations of difluoromethylornithine (DFMO) for several days prior to behavioral testing. Anti-GD2 (14G2a) was injected into the tail vein of lightly sedated animals and basal mechanical hindpaw withdrawal threshold assessed by von Frey filaments. Endpoint serum DFMO and polyamines, assessed 24h after 14G2a injection, were measured by HPLC and mass spectrometry.

Results

An i.v. injection of 14G2a causes increased paw sensitivity to light touch in this model, a response that closely mimics patient allodynia. Animals allowed to drink water containing 1% DFMO exhibited a significant reduction of 14G2a-induced pain sensitivity (allodynia). Increasing the dosage of the immunotherapeutic increased the magnitude (intensity and duration) of the pain behavior. Administration of DFMO attenuated the enhanced sensitivity. Consistent with the known actions of DFMO on ornithine decarboxylase (ODC), serum putrescene and spermidine levels were significantly reduced by DFMO, though the decrease in endpoint polyamine levels did not directly correlate with the behavioral changes.

Conclusions

Our results demonstrate that DFMO is an effective agent for reducing anti-GD2 -induced allodynia. Using DFMO in conjunction with dinutuximab may allow for dose escalation in neuroblastoma patients. The reduction in pain may be sufficient to allow new patient populations to utilize this therapy given the more acceptable side effect profile. Thus, DFMO may be an important adjunct to anti-GD2 immunotherapy in addition to a role as a potential anti-cancer therapeutic.

Characterization of the Effects of L-4-Chlorokynurenine on Nociception in Rodents

Upon systemic administration in rats, the prodrug L-4-chlorokynurenine (4-Cl-KYN; AV-101; VistaGen Therapeutics, Inc, South San Francisco, CA) is rapidly absorbed, actively transported across the blood-brain barrier, and converted in astrocytes to 7-chlorokynurenic acid (7-Cl-KYNA), a potent and specific antagonist of the glycine B coagonist site of the N-methyl-D-aspartate (NMDA) receptor. We examined the effects of 4-Cl-KYN in several rat models of hyperalgesia and allodynia and determined the concentrations of 4-Cl-KYN and newly produced 7-Cl-KYNA in serum, brain, and spinal cord. Adult male rats were given 4-Cl-KYN (56, 167, 500 mg/kg), the NMDA receptor antagonist MK-801 (.1, .3, 1.0 mg/kg), or gabapentin (33, 100, 300 mg/kg) intraperitoneally, and were then examined on rotarod, intraplantar formalin-evoked flinching, thermal escape in the normal and carrageenan-inflamed paw, and allodynia after sciatic nerve ligation. Our conclusions show that after systemic delivery, the highest 2 doses (167 and 500 mg/kg) of 4-Cl-KYN yielded brain concentrations of 7-Cl-KYNA exceeding its half maximal inhibitory concentration (IC₅₀) at the glycine B site and resulted in dose-dependent antihyperalgesia in the 4 models of facilitated processing associated with tissue inflammation and nerve injury. On the basis of the relative dose requirements for analgesic actions and side effect profiles from these experiments, 4-Cl-KYN is predicted to have antihyperalgesic efficacy and a therapeutic ratio equal to gabapentin and superior to MK-801.

PERSPECTIVE

These studies show that systemic administration of the prodrug 4-Cl-KYN produces high central nervous system levels of 7-Cl-KYNA, a potent and highly selective antagonist of the NMDA receptor. Compared with other drugs tested, 4-Cl-KYN has robust antinociceptive effects with a better side effect profile, highlighting its potential for treating hyperpathic pain states.

Current and Future Issues in the Development of Spinal Agents for the Management of Pain

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.

Paeoniflorin inhibits excitatory amino acid agonist-and high-dose morphine-induced nociceptive behavior in mice via modulation of N-methyl-D-aspartate receptors

BACKGROUND

Pain, the most common reasons for physician consultation, is a major symptom in many medical conditions that can significantly interfere with a person's life quality and general functioning. Almost all painkillers have its untoward effects. Therefore, seeking for a safe medication for pain relieve is notable nowadays. Paeonia lactiflora is a well-known traditional Chinese medicine. Paeoniflorin is an active component found in Paeonia lactiflora, which has been reported to inhibit formalin-induced nociceptive behavior in mice. Aims of this present study were to investigate effects of paeoniflorin on excitatory amino acid agonist- or high-dose morphine-induced nociceptive behaviors in mice.

RESULTS

Paeoniflorin (100, 200, 500 nmol, i.c.v.) alone and combined with glutamatergic antagonists (MK-801 14.8 pmol, or NBQX 5 nmol, i.t.) inhibited nociception. Those agents also inhibited the clonic seizure-like excitation induced by high-dose morphine (250 nmol, i.t) in mice. Antisense oligodeoxynucleotides of NMDA receptor subunits NR1, NR2A, NR2B significantly enhanced the inhibition of paeoniflorin on excitatory amino acid-and high-dose morphine-induced nociception. Docking energy data revealed that paeoniflorin had stronger binding activity in NR2A and NR2B than NR2C of NMDA receptors.

CONCLUSIONS

Results of this study indicate that paeoniflorin-induced inhibition of excitatory amino acid agonist- and high-dose morphine-induced nociceptive behaviors might be due to modulation of NMDA receptors, specifically the NR2B subunit.

The role of glia in stress: polyamines and brain disorders

This review focuses on the roles of glia and polyamines (PAs) in brain function and dysfunction, highlighting how PAs are one of the principal differences between glia and neurons. The novel role of PAs, such as putrescine, spermidine, and spermine and their precursors and derivatives, is discussed. However, PAs have not yet been a focus of much glial research. They affect many neuronal and glial receptors, channels, and transporters. They are therefore key elements in the development of many diseases and syndromes, thus forming the rationale for PA-focused and glia-focused therapy for these conditions.

Increased sensitivity to inflammatory pain induced by subcutaneous formalin injection in serine racemase knock-out mice

D-Serine, an endogenous coagonist of the N-methyl-D-aspartate receptor (NMDAR), is widely distributed in the central nervous system and is synthesized from L-serine by serine racemase (SR). NMDAR plays an important role in pain processing including central sensitization that eventually causes hyperalgesia. To elucidate the roles of D-serine and SR in pain transmission, we evaluated the behavioral changes and spinal nociceptive processing induced by formalin using SR knock-out (KO) mice. We found that SR is mainly distributed in lamina II of the dorsal horn of the spinal cord in wild-type (WT) mice. Although the formalin injected subcutaneously induced the biphasic pain response of licking in SR-KO and WT mice, the time spent on licking was significantly longer in the SR-KO mice during the second phase of the formalin test. The number of neurons immunopositive for c-Fos and phosphorylated extracellular signal-regulated kinase (p-ERK), which are molecular pain markers, in laminae I-II of the ipsilateral dorsal horn was significantly larger in the SR-KO mice. Immunohistochemical staining revealed that the distribution of SR changed from being broad to being concentrated in cell bodies after the formalin injection. On the other hand, the expression level of the cytosolic SR in the ipsilateral dorsal horn significantly decreased. Oral administration of 10 mM D-serine in drinking water for one week cancelled the difference in pain behaviors between WT and SR-KO mice in phase 2 of the formalin test. These findings demonstrate that the SR-KO mice showed increased sensitivity to inflammatory pain and the WT mice showed translocation of SR and decreased SR expression levels after the formalin injection, which suggest a novel antinociceptive mechanism via SR indicating an important role of D-serine in pain transmission.

Possible role of spinal astrocytes in maintaining chronic pain sensitization: review of current evidence with focus on bFGF/JNK pathway

Although pain is regarded traditionally as neuronally mediated, recent progress shows an important role of spinal glial cells in persistent pain sensitization. Mounting evidence has implicated spinal microglia in the development of chronic pain (e.g. neuropathic pain after peripheral nerve injury). Less is known about the role of astrocytes in pain regulation. However, astrocytes have very close contact with synapses and maintain homeostasis in the extracellular environment. In this review, we provide evidence to support a role of spinal astrocytes in maintaining chronic pain. In particular, c-Jun N-terminal kinase (JNK) is activated persistently in spinal astrocytes in a neuropathic pain condition produced by spinal nerve ligation. This activation is required for the maintenance of neuropathic pain because spinal infusion of JNK inhibitors can reverse mechanical allodynia, a major symptom of neuropathic pain. Further study reveals that JNK is activated strongly in astrocytes by basic fibroblast growth factor (bFGF), an astroglial activator. Intrathecal infusion of bFGF also produces persistent mechanical allodynia. After peripheral nerve injury, bFGF might be produced by primary sensory neurons and spinal astrocytes because nerve injury produces robust bFGF upregulation in both cell types. Therefore, the bFGF/JNK pathway is an important signalling pathway in spinal astrocytes for chronic pain sensitization. Investigation of signaling mechanisms in spinal astrocytes will identify new molecular targets for the management of chronic pain.

Contribution of peripheral vanilloid receptor to the nociception induced by injection of spermine in mice

Polyamines (putrescine, spermidine and spermine) are important endogenous regulators of ion channels, such as vanilloid (TRPV1), glutamatergic (NMDA or AMPA/kainate) and acid-sensitive (ASIC) receptors. In the present study, we have investigated the possible nociceptive effect induced by polyamines and the mechanisms involved in this nociception in vivo. The subcutaneous (s.c.) injection of capsaicin (as positive control), spermine, spermidine or putrescine produced nociception with ED(50) of 0.16 (0.07-0.39)nmol/paw, 0.4 (0.2-0.7) μmol/paw, 0.3 (0.1-0.9) μmol/paw and 3.2 (0.9-11.5) μmol/paw, respectively. The antagonists of NMDA (MK801, 1 nmol/paw), AMPA/kainate (DNQX, 1 nmol/paw) or ASIC receptors (amiloride, 100 nmol/paw) failed to reduce the spermine-trigged nociception. However, the TRPV1 antagonists capsazepine or SB366791 (1 nmol/paw) reduced spermine-induced nociception, with inhibition of 81 ± 10 and 68 ± 9%, respectively. The previous desensitization with resiniferatoxin (RTX) largely reduced the spermine-induced nociception and TRPV1 expression in the sciatic nerve, with reductions of 82 ± 9% and 67 ± 11%, respectively. Furthermore, the combination of spermine (100 nmol/paw) and RTX (0.005 fmol/paw), in doses which alone were not capable of inducing nociception, produced nociceptive behaviors. Moreover, different concentrations of spermine (3-300 μM) enhanced the specific binding of [(3)H]-RTX to TRPV1 receptor. Altogether, polyamines produce spontaneous nociceptive effect through the stimulation of TRPV1, but not of ionotropic glutamate or ASIC receptors.

Role of peripheral polyamines in the development of inflammatory pain

Polyamines (putrescine, spermidine and spermine) are aliphatic amines that are produced by the action of ornithine decarboxylase (ODC) in a rate-limiting and protein kinase C (PKC)-regulated step. Because high levels of polyamines are found in the synovial fluid of arthritic patients, the aim of the present study was to identify the role of peripherally produced polyamines in a model of inflammatory pain induced by adjuvant. The subcutaneous injection of Complete Freund's adjuvant (CFA, 50 μL/paw) caused the development of mechanical allodynia and edema. Moreover, it increased ODC expression and activity and PKC activation. Administration of the selective ODC inhibitor DFMO (10 μmol/paw) attenuated the development of allodynia and edema and decreased ODC activity in both control and CFA-treated animals. Furthermore, administration of the PKC inhibitor GF109203X (1 nmol/paw) reduced allodynia and ODC activity in animals injected with CFA. A subcutaneous injection of putrescine (10 μmol/paw), spermidine (3-10 μmol/paw) or spermine (0.3-3 μmol/paw) into the rat paw also caused mechanical allodynia and edema. The present results suggest that endogenously synthesized polyamines are involved in the development of nociception and edema caused by an adjuvant. Moreover, polyamine production in inflammatory sites seems to be related to an increase in ODC activity stimulated by PKC activation. Thus, controlling polyamine synthesis and action could be a method of controlling inflammatory pain.

The Therapeutic Potential of D-Amino Acid Oxidase (DAAO) Inhibitors

D-amino acid oxidase (DAAO) is a flavoenzyme that degrades D-amino acids through the process of oxidative deamination. DAAO regulation of D-amino acid levels has been associated with several physiological processes ranging from hormone secretion to synaptic transmission and cognition. Recent genetic studies have identified a mutation on chromosome 13 in schizophrenia patients that encodes two gene products (G30 and G72) that are associated with DAAO. Furthermore, DAAO expression and enzyme activity has been reported to be increased in post mortem brain tissue samples from patients with schizophrenia compared to healthy controls. D-serine, a D-amino acid that is regulated by DAAO, is a potent, endogenous co-agonist of the N-methyl-D-aspartic acid (NMDA) receptor. Because NMDA receptor dysfunction is thought to be involved in the positive (psychotic), negative and cognitive symptoms in schizophrenia, there has been much interest in developing potent and selective DAAO inhibitors for the treatment of this disease. Several research reports have been published that describe the synthesis and biological effects of novel, selective, small molecule inhibitors of DAAO. Many of these compounds have been shown, when given systemically, to increase D-serine concentrations in the blood and brain. However, the efficacy of these compounds in behavioral assays that measure antipsychotic potential and pro-cognitive effects in laboratory animals has been inconsistent. This article highlights and reviews research advances for DAAO inhibitors published in peer reviewed journals.

Endogenous kappa-opioid receptor systems inhibit hyperalgesia associated with localized peripheral inflammation

Peripheral inflammation evokes functional and biochemical changes in the periphery and spinal cord which result in central sensitization and hypersensitivity. Inhibitory control systems from the rostral ventromedial medulla (RVM) are also activated. The present study investigates whether endogenous kappa-opioid receptor (KOPr) systems contribute to these neuroadaptations. Inflammation was induced by intraplantar injection of complete Freund's adjuvant (CFA) into one hindpaw. Mechanical and thermal thresholds were determined using the Von Frey and radiant heat tests, respectively. KOPr gene deletion in mice or systemic administration of the long-acting KOPr antagonist, norbinaltorphimine (norBNI) significantly exacerbated mechanical and thermal hypersensitivity of the ipsilateral, inflamed paw. Thermal and mechanical thresholds of the non-inflamed, contralateral hindpaw were unaffected by CFA treatment. However, gene deletion as well as norBNI treatment resulted in mechanical, but not thermal hypersensitivity of the non-inflamed paw. Similar results were obtained when norBNI was administered intrathecally or into the RVM in rats. These data demonstrate a previously unrecognized role of endogenous KOPr systems in inhibiting hyperalgesia during inflammation. Furthermore, they demonstrate that decreased KOPr activity in either the spinal cord or RVM not only enhances mechanical and thermal hyperalgesia of the inflamed limb but also leads to an unmasking of mechanical hyperalgesia at a site remote from inflammation. The differential effects of KOPr antagonism on mechanical versus thermal thresholds for the non-inflamed paw support the notion that distinct neuroanatomical or neurochemical mechanisms modulate the processing of thermal versus mechanical stimuli.

Intrathecally administered D-cycloserine produces nociceptive behavior through the activation of N-methyl-D-aspartate receptor ion-channel complex acting on the glycine recognition site

Intrathecal (i.t.) administration of D-cycloserine (100 and 300 fmol), a partial agonist of the glycine recognition site on the N-methyl-D-aspartate (NMDA) receptor ion-channel complex, produced a behavioral response mainly consisting of biting and/or licking of the hindpaw and the tail along with slight hindlimb scratching directed toward the flank in mice, which peaked at 5 - 10 min and almost disappeared at 15 min after the injection. The behavior induced by D-cycloserine (300 fmol) was dose-dependently inhibited by an intraperitoneal injection of morphine (0.5-2 mg/kg), suggesting that the behavioral response is related to nociception. The nociceptive behavior was also dose-dependently inhibited by i.t. co-administration of 7-chlorokynurenic acid (0.25-4 nmol), a competitive antagonist of the glycine recognition site on the NMDA receptor ion-channel complex; D-(-)-2-amino-5-phosphonovaleric acid (62.5-500 pmol), a competitive NMDA receptor antagonist; MK-801 (62.5-500 pmol), an NMDA ion-channel blocker; ifenprodil (0.5-8 nmol); arcaine (31-125 pmol); and agmatine (0.1-10 pmol), all being antagonists of the polyamine recognition site on the NMDA receptor ion-channel complex. However, [D-Phe7,D-His9]-substance P(6-11), a specific antagonist for substance P (NK1) receptors, and MEN-10,376, a tachykinin NK2-receptor antagonist, had no effect on D-cycloserine-induced nociceptive behavior. These results in the mouse spinal cord suggest that D-cycloserine-induced nociceptive behavior is mediated through the activation of the NMDA receptor ion-channel complex by acting on the glycine recognition site and that it does not involve the tachykinin receptor mechanism.

Putrescine produces antidepressant-like effects in the forced swimming test and in the tail suspension test in mice

Putrescine, a polyamine present at high concentrations in the mammalian brain, was suggested to play a role in the modulation of depression. Thus, this study investigated the effect of putrescine in the mouse forced swimming test (FST) and in the tail suspension test (TST), two models predictive of antidepressant activity. Putrescine significantly reduced the immobility time both in the FST and in the TST (dose range of 1-10 mg/kg, i.p.), without changing locomotion in an open-field. I.c.v. injection of putrescine (0.1-10 nmol/site) also reduced the immobility time in the FST and in the TST. The pretreatment of mice with arcaine (1 mg/kg, i.p., an antagonist of the polyamine-site of NMDA receptor) completely blocked the anti-immobility effect of putrescine (10 mg/kg, i.p.). A subeffective dose of putrescine (0.1 mg/kg, i.p.) produced a synergistic antidepressant-like effect with agmatine (0.001 mg/kg, i.p.) in the FST. Moreover, a subeffective dose of putrescine (0.01 nmol/site, i.c.v.) produced a synergistic antidepressant-like effect with arcaine (50 microg/site, i.c.v.). The results indicate that putrescine produces antidepressant-like effects in the FST that seems to be mediated through its interaction with the polyamine-site of NMDA receptors.

Distinct effects of D-serine on spinal nociceptive responses in normal and carrageenan-injected rats

Single unit extracellular recordings from dorsal horn neurons were performed with glass micropipettes in pentobarbital-anesthetized rats. A total of 60 wide dynamic range (WDR) neurons were obtained from 34 rats. In normal rats (20/34), spinally administered D-serine (10 nmol), a putative endogenous agonist of glycine site of NMDA receptors, significantly enhanced the C- but not Abeta-, and Adelta-fiber responses of WDR neurons in the spinal dorsal horn. When 1 nmol of the glycine site antagonist 7-chlorokynurenic acid (7-CK) was co-administered with 10 nmol D-serine, the facilitation of D-serine on C-fiber response was completely blocked. 7-CK (1 nmol) alone failed to influence Abeta-, Adelta-, and C-fiber responses of WDR neurons. In contrast, in carrageenan-injected rats (14/34), 10 nmol D-serine had no effect on C-fiber response, while 1 nmol 7-CK per se markedly depressed C-fiber response of WDR neurons. These findings suggest that under physiological conditions, glycine sites in the spinal cord were available but became saturated following peripheral inflammation. Thus, increased endogenous d-serine or glycine may be involved in nociceptive transmission by modulating NMDA receptor activities. The glycine site of NMDA receptors may become a target for the prevention of inflammatory pain.

A pilot study of nitric oxide blood levels in patients with chronic orofacial pain

BACKGROUND

Control of pain is the major goal in the management of chronic orofacial pain (COP) patients. The pathogenesis of COP is currently not well understood. Consequently, the treatment of COP may be suboptimal or even harmful. Based on independent observations, we propose that local elevated levels of nitric oxide (NO) may have a central role in the pathogenesis of COP.

HYPOTHESIS

NO level in the orofacial region of COP patients is elevated. A regional increased level of NO causes excessive vasodilatation. This hyperperfusion is manifested by hyperthermia of the overlying skin, while NO enhances nociception, aggravating orofacial pain. An alternative mechanism involving NO as a neurotransmitter at the CNS level may contribute to orofacial pain, but seems not to account for all the known clinical observations.

METHODS

Two groups of subjects were studied: 5 patients with COP and 59 control subjects. For each subject we collected blood samples for analysis of nitrite\nitrate (or NOx).

RESULTS

(1) NOx blood levels for 5 patients diagnosed with COP was 65.9 microM (SD of 10.4) verses 42.7 microM (SD of 24.2) for 59 control subjects, the difference being statistically significant, t-statistic = -2.12 (P > .05). (2) No statistical difference was found for NOx blood levels for 59 control subjects divided by gender (male vs female), with 23 female controls having NOx blood levels of 42.6 microM (SD of 25.2) and male controls having NOx blood levels of 42.8 microM (SD of 24.0), t-statistic = -0.03, P = .98.

CONCLUSION

This pilot study suggests that NO blood levels may have an association with COP. A better understanding of the mechanism of chronic orofacial pain is expected to lead to more precise diagnostic staging and management of this disorder.

Spinal CK2 regulates nociceptive signaling in models of inflammatory pain

Casein kinase 2 (CK2) is a widely expressed protein kinase. Over the last several years a long list of protein substrates has evolved, many of which have proven or hypothesized roles in nociceptive signal transmission. However, CK2 has not itself been demonstrated to participate in nociception prior to this time. We set out to test the hypothesis that spinal CK2 regulates nociception using several pain models. Our first studies focused on the ability of the selective CK2 inhibitors 4,5,6,7-tetrabromobenzotriazole (TBBT) and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) to reduce formalin-stimulated pain behaviors in mice. Both phases of the response to subcutaneous formalin were strongly inhibited by intrathecal administration of TBBT or DRB in dose-dependent fashion. Likewise, using the complete Freund's adjuvant (CFA) model of chronic inflammatory pain, TBBT was observed to strongly reduce mechanical allodynia. The inhibition of spinal CK2 with either inhibitor did not, however, alter withdrawal latencies in the hotplate thermal pain model while intrathecal morphine was very effective. Immunohistochemical studies demonstrated all three known CK2 subunits, alpha, alpha' and beta to be expressed in spinal cord tissue as did real-time PCR experiments. While mRNA levels for each of the subunits was transiently enhanced after formalin or CFA hindpaw injection, overall spinal cord protein levels were not elevated in a sustained fashion. Our results indicate that CK2 participates in inflammatory nociception both in the acute and chronic phases. Simple changes in the abundance of spinal CK2 subunits do not likely underlie these phenomena, however.

Antinociceptive interactions between intrathecal gabapentin and MK801 or NBQX in rat formalin test

Antagonists for spinal N-methyl-D-aspartate (NMDA) and amino-hydroxy-methtyl-isoxazolepropionate (AMPA) receptors are effective in attenuating acute nociception or injury-induced hyperalgesia. The antinociception of spinal gabapentin is developed in injury-induced hyperalgesia without affecting acute nociception. The authors evaluated the effects of intrathecal gabapentin, NMDA antagonist (MK801) and AMPA antagonist (NBQX) in the formalin test which shows injury-induced hyperalgesia as well as acute pain. We further assessed the interactions between gabapentin and either MK801 or NBQX. Male Sprague-Dawley rats were implanted with intrathecal catheters. To evoke pain, 50 microL of 5% formalin solution was injected into the hindpaw. The interaction was investigated by a fixed dose analysis or an isobolographic analysis. MK801 and NBQX suppressed flinching responses during phase 1 of the formalin test, while gabapentin had little effect on phase 1. All three agents decreased the phase 2 flinching response. A fixed dose analysis in phase 1 showed that gabapentin potentiated the antinociceptive effect of MK801 and NBQX. Isobolographic analysis in phase 2 revealed a synergistic interaction after coadministration of gabapentin-MK801 or gabapentin-NBQX. Correspondingly, spinal gabapentin with NMDA or AMPA antagonist may be useful in managing acute pain and injury-induced hyperalgesia.

Emerging drugs for neuropathic pain

Although there are many analgesics on the market for the treatment of nociceptive pain, there are none with FDA approval for the treatment of neuropathic pain. With a better understanding of the anatomy and physiology of pain, there is a significant effort in developing new drugs that interact specifically with pain pathways. This higher drug specificity is likely to result in drugs that are more efficacious with fewer side effects. This has led to the development of many drugs for the treatment of neuropathic pain. These drugs are divided into the following therapeutic classes: 1) N-methyl-D-aspartate (NMDA) receptor antagonists, 2) ion channel antagonists, 3) alpha2-agonists, 4) nicotinic receptor agonists, 5) prostaglandin receptor antagonists, 6) adenosine agonists and adenosine kinase inhibitors, 7) neuropeptide antagonists, and 8) prosaposins. The results of preclinical and clinical trials are promising for these new agents. Whether these agents will be efficacious as single agents is yet to be determined; however, preliminary results show that combination therapy may be more beneficial with fewer side effects.

Effect of the synthetic polyamine N,N'-bis-(3-aminopropyl) cyclohexane-1,4-diamine (DCD) on rat spinal cord nociceptive transmission

In rats submitted to a C-fiber reflex response paradigm, intravenous (i.v.) administration of 2.5, 5 and 10 mg/kg of the synthetic polyamine N,N'-bis-(3-aminopropyl) cyclohexane-1,4-diamine (DCD) dose-dependently reduced both the integrated C reflex responses and wind-up activity. Inhibitory effects of the polyamine on spinal cord nociceptive transmission are likely to be consequence of blockade by extracellular DCD of NMDA receptor channels localized in dorsal horn neurons, although modulatory actions at supraspinal level and at other ion channels could also be possible.

Positive N-methyl-d-aspartate receptor modulation by selective glycine transporter-1 inhibition in the rat dorsal spinal cord in vivo

In this study we have employed the selective glycine transporter-1 (GlyT-1) and GlyT-2 transporter inhibitors R-(-)-N-methyl-N-[3-[(4-trifluoromethyl)phenoxy]-3-phenyl-propyl]glycine (1:1) lithium salt (Org 24598) and 4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopently)methyl]benzamide (Org 25543), respectively, and microdialysis perfusion to determine the effect of GlyT transporter inhibition on extracellular amino acid concentrations in the lumbar dorsal spinal cord of the halothane-anaesthetised rat. Reverse dialysis of Org 24598 (0.1-10 microM) induced a concentration-related increase in extracellular glycine accompanied by a progressive increase in citrulline, but not aspartate, glutamate or GABA, efflux. Org 25543 (10 microM) by the same route induced a similar increase in glycine levels without affecting the efflux of other amino acids quantified. To test the hypothesis that the increase in citrulline efflux resulted from activation of the N-methyl-D-aspartate receptor (NMDA-R)/nitric oxide synthase (NOS) signalling cascade, the sensitivity was determined of GlyT-1 inhibition-induced effects to NMDA-R antagonism or NOS inhibition. Co-administration by reverse dialysis of the selective NMDA-R channel blocker MK-801 (0.5 mM) or the selective antagonist of the strychnine-insensitive glycine site, 7-chlorokynurenic acid (1 mM), with Org 24598 (10 microM) did not affect the uptake inhibition-induced increase in glycine efflux, but did significantly attenuate the increase in extracellular citrulline. Similarly, co-administration with Org 24598 of the isoform non-selective and selective neuronal NOS inhibitors Nomega-nitro-L-arginine methyl ester (1 mM) or 1-(2-trifluoromethylphenyl)imidazole (0.2 mM), respectively, prevented Org 24598-induced citrulline efflux with no effect on increased glycine efflux. These data provide evidence that the observed increased in extracellular citrulline is a consequence of positive modulation of NMDA-R, secondary to increased extracellular glycine and support a protective role for GlyT-1 against fluctuations in extracellular glycine uptake at glutamatergic synapses in the dorsal spinal cord. Such a mechanism could be important to NMDA-R-mediated synaptic plasticity in the spinal cord and be of relevance to the clinical usage of GlyT-1 inhibitors.

Spinal antinociceptive effect of agmatine and tentative analysis of involved receptors: study in an electrophysiological model of rats

The present study was designed to evaluate the antinociceptive effect of agmatine and the receptors involved at the spinal level by using an experimental model in which agmatine was intrathecally (i.t.) administered while the changes of nociceptively-evoked discharges in thalamic parafascicular (PF) neurons were monitored in anesthetized Wistar rats. The results showed that: (1) i.t. agmatine dose-dependently suppressed the nociceptive discharges of PF neurons induced by the tail pinch; (2) i.t. yohimbine did not block the agmatine-induced suppressive effect of nociceptive discharges in these neurons; and (3) the agmatine-induced suppression could be blocked significantly by i.t. idazoxan. The results suggest that agmatine suppresses the transmission of nociceptive inputs at the spinal level mainly through the activation of imidazoline receptors other than alpha(2)-adrenoceptors.

Vocalization responses after intrathecal administration of ionotropic glutamate receptor agonists in rats

Inotropic glutamate receptors in the spinal cord (N-methyl-D-aspartic acid [NMDA], alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA], and kainate receptors) seem to play a key role in acute pain transmission and the neuronal plasticity in chronic pain states. Vocalization responses produced by activation of these receptors on the pain pathways can be quantified semiautomatically and thus could be used as a research tool. We studied vocalization responses induced by intrathecal administration of various agonists acting at the glutamate receptors in normal rats and in the presence of peripheral inflammation and a chronic constriction injury model of neuropathic pain. The nonselective endogenous agonist, glutamate, and the NMDA receptor glycine site agonist D-serine did not produce vocalization, whereas selective agonists acting at AMPA, NMDA, and kainate receptors produced dose-related vocalization responses. The vocalization response evoked by the administration of AMPA was significantly increased in the neuropathic pain model. In conclusion, spinal administration of ionotropic glutamate receptor agonists produce short-lasting, dose-related vocalization responses that can be used as a basic research and screening tool for analgesic studies. However, peripheral inflammation or nerve injury did not substantially alter vocalization responses overall, possibly indicating that the vocalization test is not a good tool for studying the role of excitatory amino acids in these pathological pain conditions.

IMPLICATIONS

Vocalization responses evoked by spinal administration of ionotropic glutamate receptor agonists can be used for experimental analgesic studies. However, pathological pain models did not substantially alter vocalization responses, possibly indicating that this test is not suitable for studying the role of spinal excitatory amino acids in central sensitization.

Neuroplasticity Induced by Tooth Pulp Stimulation in Trigeminal Subnucleus Oralis Involves NMDA Receptor Mechanisms

We have recently demonstrated that application of the mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces significant increases in jaw muscle electromyographic (EMG) activity and neuroplastic changes in trigeminal (V) subnucleus caudalis. Since subnucleus oralis (Vo) as well as caudalis receives projections from molar pulp afferents and is also an integral brain stem relay of afferent input from orofacial structures, we tested whether MO application to the exposed pulp induces neuroplastic changes in oralis neurons and whether microinjection of MK-801, a noncompetitive NMDA antagonist, into the Vo influences the pulp/MO-induced neuroplastic changes in chloralose/urethan-anesthetized rats. Single neuronal activity was recorded in Vo, and neurons classified as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR), nociceptive-specific (NS), deep (D), or skin/mucosa and deep (S + D). The spontaneous activity, mechanoreceptive field (RF) size, mechanical threshold, and response to suprathreshold mechanical stimuli applied to the neuronal RF were assessed prior to and throughout a 40- to 60-min period after MO application to the maxillary molar pulp. In animals pretreated with saline microinjection (0.3 μl) into the Vo, MO application to the pulp produced a significant increase in spontaneous activity, expansion of the pinch or deep RF, decrease in the mechanical threshold, and increase in response to suprathreshold mechanical stimuli of the nociceptive (WDR, NS, and S + D) neurons except for those nociceptive neurons having their RF only in the intraoral region. The pulpal application of MO did not produce any significant neuroplastic changes in LTM neurons. Furthermore, in animals pretreated with MK-801 microinjection (3 μg/0.3 μl) into the Vo, MO application to the pulp did not produce any significant changes in the RF and response properties of nociceptive neurons. In other animals pretreated with saline (0.3 μl) or MK-801 (3 μg/0.3 μl) microinjected into the Vo, mineral oil application to the pulp did not produce any significant changes in RF and response properties of nociceptive neurons. These findings indicate that the application of MO to the tooth pulp can induce significant neuroplastic changes in oralis nociceptive neurons and that central NMDA receptor mechanisms may be involved in these neuroplastic changes.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Interaction among NMDA receptor-, NMDA glycine site- and AMPA receptor antagonists in spinally mediated analgesia

PURPOSE

The NMDA (N-methyl-D-aspartate) receptor antagonists and the NMDA glycine site antagonists given alone have minimal effects on acute nociception. In contrast, the AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor antagonists have a major role in acute nociception. We investigated the interactions among these three antagonists in acute nociception.

METHODS

Sprague-Dawley rats (250-300 g) were implanted with chronic lumbar intrathecal catheters and were tested for their thermal withdrawal response using the hot plate test after intrathecal administration of AP-5 (NMDA receptor antagonist), ACEA 1021 (NMDA glycine site antagonist), or ACEA 2085 (AMPA receptor antagonist). The combinations of these three agents were also tested.

RESULTS

Intrathecal administration of ACEA 2085 had a dose dependent analgesic effect while intrathecal AP-5 or ACEA 1021 could not induce dose dependent effect. Co-administration of AP-5 10 microg and ACEA 2085 intrathecally showed no changes in the thermal response latency compared with ACEA 2085 alone. ACEA 1021, 12 microg, and AP-5 showed left-ward shift of the dose effect curve only with small doses of AP-5 (1 microg, 3 microg). Only the smallest dose of ACEA 2085 (0.1 ng) with ACEA 1021 12 microg induced antinociception compared with that of ACEA 2085 alone.

CONCLUSIONS

The combination of the NMDA glycine site antagonist and low doses of the NMDA receptor antagonist or the AMPA receptor antagonist increased the analgesic effect on acute thermal nociception with increased side effects, while the NMDA receptor antagonist and the AMPA receptor antagonist had no such interaction.

Intrathecally administered spermine produces the scratching, biting and licking behaviour in mice

Intrathecal (i.t.) administration of spermine (0.1-10000 fmol), an endogenous polyamine, produced the behavioural response mainly consisting of biting and/or licking of the hindpaw along with a slight hindlimb scratching directed toward the flank in mice, which peaked at 5-15 min and almost disappeared at 30 min after an injection. The behaviour induced by spermine (10 pmol) was dose-dependently inhibited by intraperitoneal injection of morphine (0.125-0.5 mg/kg). The characteristic behaviour was also inhibited dose-dependently by i.t. co-administration of ifenprodil (62.5-4000 pmol), a competitive antagonist of the polyamine recognition site on N-methyl-D-aspartate (NMDA) receptor ion-channel complex, and D(-)-2-amino-5-phosphonovaleric acid (D-APV) (0.5-2 nmol) and 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) (7. 8-500 pmol), the competitive NMDA receptor antagonists, and (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,b]cycloheptene-5, 10-imine hydrogen maleate (MK-801) (0.5-4 nmol), an NMDA ion-channel blocker, but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA receptor antagonist. Both (2S, 3S)-[cis-2-(diphenylmethyl)-N-[(2-methoxyphenyl)-methyl]-1-azabicy clo [2.2.2]octane-3-amine] (CP-96,345), a non-peptidic neurokinin-1 (NK-1) receptor antagonist, and CP-96,344, its inactive 2R,3R enantiomer, inhibited spermine-induced behavioural response in a dose-dependent manner. However, [Tyr(6), D-Phe(7), D-His(9)]-substance P(6-11) (sendide) and [D-Phe(7), D-His(9)]-substance P(6-11), the selective antagonists for NK-1 receptors, were without affecting spermine-induced behaviour. These results indicate that spermine-induced behaviour is mediated through the polyamine recognition site on NMDA receptor ion-channel complex without the involvement of substance P system in the mouse spinal cord.

Polyamine deprivation alters formalin-induced hyperalgesia and decreases morphine efficacy

Although the exact functions of polyamines in the nervous system remain still unclear, they are thought to have a physiological role in intracellular signal processing and neurotransmission. Polyamine deprivation which consists in the reduction of both the endogenous and exogenous sources of polyamines is a promising treatment for cancer. In a previous study we have shown that this treatment provokes an analgesic effect in rats submitted to brief phasic nociceptive tests. The present study examined the effect of polyamine deprivation on pain-related behaviors and spinal c-fos expression evoked in the formalin test presumed to better reflect clinical pain, using morphine as analgesia control. Polyamine deprivation per se altered the characteristic pain-related behaviors, reducing the interphase depression of pain, without inducing changes in the spinal Fos staining. In addition this treatment prevented the antinociceptive effect of morphine both on behavioral responses and on spinal c-fos expression. In polyamine-deprived rats, despite morphine injection, nociceptive scores remained dramatically high during the intermediate and the late phases of the response and the number of Fos immunoreactive neurons remained largely higher in deeper layers than in morphine control rats. Altogether these data support a modulatory role of polyamines both on the neuronal circuitry mediating sensory information, and on mechanisms underlying morphine analgesia.

NMDA receptor mechanisms contribute to neuroplasticity induced in caudalis nociceptive neurons by tooth pulp stimulation

We recently demonstrated that application of mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces significant and prolonged increases in jaw muscle electromyographic (EMG) activity that are suggestive of central neuroplasticity. Because small-fiber afferents, including pulp afferents, access nociceptive neurons in trigeminal (V) subnucleus caudalis, this study examined whether pulpal application of MO induces neuroplastic changes in caudalis nociceptive neurons (wide dynamic range and nociceptive specific) and whether central N-methyl--aspartate (NMDA) receptor mechanisms are involved in these MO-induced neuroplastic changes. After pretreatment with vehicle (saline, 10 microliter i.t.) to the surface of the medulla, the pulpal application of MO to the maxillary molar tooth pulp produced a significant increase in neuronal spontaneous activity, a significant expansion of the pinch and/or tactile mechanoreceptive field (RF), a significant decrease in mechanical threshold, and significant increases in neuronal responses to graded pinch stimuli. Compared with vehicle-treated rats, pretreatment with the NMDA receptor antagonist MK-801 (10 microgram/10 microliter i.t.) followed by MO application to the pulp in another group of rats significantly reduced or abolished these MO-induced neuroplastic changes in nociceptive neurons. In another group of rats pretreated with saline (intrathecally), mineral oil application to the pulp did not show any significant changes in spontaneous activity or RF properties over the 40-min observation period. The pulpal application of MO in other rats (pretreated with saline, intrathecally) did not produce any significant neuroplastic changes in caudalis low-threshold mechanoreceptive neurons. These results indicate that the MO-induced activation of molar pulpal afferents can produce profound NMDA receptor-related neuroplastic changes in caudalis nociceptive neurons. Such neuroplastic changes may contribute to the hyperalgesia and spread of pain that can be associated with pulpal inflammation.

The possible role of nitric oxide in the physiopathology of pain associated with temporomandibular joint disorders

Temporomandibular joint disorders (TMD) pose a significant challenge to the practice of oral and maxillofacial surgery. When painful, TMD are generally associated with hyperthermia of the overlying skin. It is hypothesized that this skin hyperthermia, caused by regional vasodilation, is induced by nitric oxide (NO) produced in the extravascular space of the joint. Extravascular NO can be produced by osteoblasts, chondrocytes and macrophages, or by stimulated neurons. It is suggested that this kind of pain is associated with NO-enhanced sensitivity of the peripheral nociceptors. Verification and clinical implications of the proposed mechanism are discussed.

The effect of intrathecal gabapentin and 3-isobutyl gamma-aminobutyric acid on the hyperalgesia observed after thermal injury in the rat

Gabapentin is an anticonvulsant that may represent a novel class of drugs, which has novel spinal antihyperalgesic activity. We sought to characterize this spinal action in a model of hyperalgesia that involves a mild thermal injury to the hind paw of the rat. Rats were prepared with chronic spinal catheters. Under brief halothane anesthesia, a thermal injury was induced by applying the left hind paw to a thermal surface (52.5 degrees C) for 45 s. This exposure results in mild erythema but no blistering. Thermal escape latency of the hind paw was determined using an underglass thermal stimulus with which response latencies of the injured and uninjured (normal) paw could be obtained. Thirty minutes after thermal injury, the response latency in all groups decreased from 10-12 s to 5-7 s. Uninjured paw withdrawal latency was unaltered. The intrathecal injection of gabapentin (30-300 microg) produced a dose-dependent reversal of the hyperalgesia but had no effect on the response latency of the normal hind paw, even at the largest doses. A similar reversal was observed after intrathecal delivery of the structural analog S(+)-3-isobutyl gamma-aminobutyric acid (GABA) (30-300 microg), but not after the largest dose of its stereoisomer R(-)-3-isobutyl GABA (300 microg). The effects of both intrathecal gabapentin and S(+)-3-isobutyl GABA were reversed by intrathecal D-serine, but not L-serine. All effects were observed at doses that had no significant effect on motor function. These observations, in conjunction with the accumulating data on binding and transmitter release, emphasize that these gabapentinoids can selectively modulate the facilitation of spinal nociceptive processing otherwise generated by persistent small afferent input generated by tissue injury.

IMPLICATIONS

Gabapentin and its analog, 3-isobutyl gamma-aminobutyric acid, given spinally, produce a dose-dependent, D-serine-sensitive reversal of the thermal hyperalgesia evoked by mild thermal injury.

A double-blind provocative study of chocolate as a trigger of headache

A provocative double-blind study of headache was performed using chocolate as the active agent and carob as the placebo. The chocolate and carob samples were formulated to duplicate products used in an earlier study (1) in which strong differential effects between the ability of chocolate and carob to trigger headache in migraine were shown. Sixty-three women with chronic headache (50% migraine, 37.5% tension-type, 12.5% combined migraine and tension-type) participated in the study. After 2 weeks of following a diet that restricted vasoactive amine-rich foods, each subject underwent double-blinded provocative trials with two samples of chocolate and two of carob presented in random order. Diaries were maintained by the subjects throughout the study, monitoring diet and headache. The results demonstrated that chocolate was not more likely to provoke headache than was carob in any of the headache diagnostic groups (chi2(2)=0.36, p=0.83). Interestingly, these results were independent of subjects' beliefs regarding the role of chocolate in the instigation of headache (chi2(1)=0.73, p=0.39). Headache diagnosis and the concomitant use of additional vasoactive amine-containing foods were also not associated with chocolate acting as a headache trigger. Thus, contrary to the commonly held belief of patients and physicians, chocolate does not appear to play a significant role in triggering headaches in typical migraine, tension-type, or combined headache sufferers.

Role of nitric oxide in the physiopathology of pain

Many painful disorders, including joint dysfunctions such as rheumatoid arthritis (RA) or temporomandibular joint disorders (TMD), are associated with hyperthermia of the overlying skin. The same is true of certain intractable chronic pain conditions, such as chronic orofacial pain, which may be associated with TMD. We suggest that this skin hyperthermia, caused by regional vasodilation, is induced by extravascular nitric oxide (NO). Extravascular NO can be produced in the affected joint by osteoblasts, chondrocytes, and macrophages, by mechanical stimulation of endothelial cells, or by stimulated neurons. In view of a strong correlation between pain and skin hyperthermia in these disorders, and the evidence that NO enhances the sensitivity of peripheral nociceptors, we also suggest that at least this kind of pain is associated with excessive local level of NO. This hypothesis can be verified by dynamic area telethermometry, assessing the effect of NO on the sympathetic nervous function. This mechanism, which is in line with the general role of NO as a mediator between different organ systems, also may be relevant to any pain associated with enhanced immune response. Clinical implications of the proposed mechanism are discussed.

Free D-aspartate and D-serine in the mammalian brain and periphery

It has long been assumed that L-forms of amino acids exclusively constitute free amino acid pools in mammals. However, a variety of studies in the last decade has demonstrated that free D-aspartate and D-serine occur in mammals and may have important physiological function in mammals. Free D-serine is confined predominantly to the forebrain structure, and the distribution and development of D-serine correspond well with those of the N-methyl-D-aspartate (NMDA)-type excitatory amino acid receptor. As D-serine acts as a potent and selective agonist for the strychnine-insensitive glycine site of the NMDA receptor, it is proposed that D-serine is a potential candidate for an NMDA receptor-related glycine site agonist in mammalian brain. In contrast, widespread and transient emergence of a high concentration of free D-aspartate is observed in the brain and periphery. Since the periods of maximal emergence of D-aspartate in the brain and periphery occur during critical periods of morphological and functional maturation of the organs, D-aspartate could participate in the regulation of these regulation of these developmental processes of the organs. This review deals with the recent advances in the studies of presence of free D-aspartate and D-serine and their metabolic systems in mammals. Since D-aspartate and D-serine have been shown to potentiate NMDA receptor-mediated transmission through the glutamate binding site and the strychnine-insensitive glycine binding site, respectively, and have been utilized extensively as potent and selective tools to study the excitatory amino acid system in the brain, we shall discuss also the NMDA receptor and uptake system of D-amino acids.

The glycine site of the NMDA receptor contributes to neurokinin1 receptor agonist facilitation of NMDA receptor agonist-evoked activity in rat dorsal horn neurons

We have investigated the role of the glycine recognition site of the N-methyl-D-aspartate receptor (the GlyNMDA site) in the facilitation of NMDA receptor agonist-evoked activity in rat dorsal horn neurons that is brought about by neurokinin1 (NK1) receptor agonist and the contribution of protein kinase C (PKC) activation to this phenomenon. Ionophoresis of the selective NMDA receptor agonist 1-aminocyclobutane-cis-1,3-dicarboxylic acid (ACBD) produced a sustained increase in the firing rate of single laminae III-V neurons recorded extracellularly using multibarrelled glass electrodes. The highly selective NK1 receptor agonist acetyl-[Arg6,Sar9,Met(O2)11]-SP6-11 (Sar9-SP) greatly facilitated this response, but under the present conditions had no effect when applied alone or with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor agonist) at the same current. In the presence of the GLyNMDA site antagonists 2-carboxy-4,6-dichloro-(1H)-indole-3-propanoic acid (MDL 29951), 7-chloro-3-(cyclopropylcarbonyl)-4-hydroxy-2(1H)-quinoline (L701,252), 5,7-dinitroquinaxoline-2,3-dione (MNQX) or 7-chlorothiokynurenic acid (7-CTK), or the PKC inhibitors, chelerythrine or GF109203X, the Sar9-SP-induced facilitation of ACBD-evoked activity was prevented, generally restoring activity to a level similar to that in the presence of ACBD alone, whilst an AMPA receptor antagonist, 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (NBQX) did not inhibit the facilitation. At the same ionophoretic currents these compounds had no effect on ACBD-evoked activity in the absence of Sar9-SP but were inhibitory at significantly greater currents. To further substantiate the importance of the GlyNMDA site in the interaction, the effects of NMDA receptor antagonists selective for alternative recognition sites on the NMDA receptor were investigated. MK-801, a non-competitive NMDA receptor antagonist and arcaine, a competitive inhibitor at the polyamine site, were applied to the facilitated activity seen in the presence of Sar9-SP and ACBD, and to ACBD-evoked activity alone. Unlike the GlyNMDA site antagonists and PKC inhibitors, these compounds reduced both facilitated and ACBD-evoked activity at similar currents. Furthermore, like the NK1 receptor agonist, a selective GlyNMDA site agonist 1-aminocyclopropane carboxylic acid (ACPC) caused facilitation of ACBD-evoked activity which was also blocked by currents of L701,252 that did not alter activity evoked by ACBD alone. These data suggest that activation of the GlyNMDA site (perhaps as a consequence of glycine release or modification of its influence by intracellular signalling cascades) is an essential component of the means by which NK1 receptor activation results in facilitated responsiveness of dorsal horn neurons towards NMDA receptor agonists.

Polyamine deprivation provokes an antalgic effect

It is well established that inhibition of putrescine formation using D,L-2-(difluoromethyl)ornithine and feeding a polyamine-deficient diet together with non-absorbable antibiotics (neomycin and metronidazole), prevent almost completely the growth of tumors in rats. A similar regimen given to patients with prostate cancer not only reduced the titer of prostate specific antigen in serum, but surprisingly provoked at the same time an antalgic effect. This observation led us to study the potentiation effect of polyamine deprivation on pain threshold in healthy rats. Animals were fed for 2 weeks with an artificial diet of known polyamine content, in combination with antibiotics and 2-(difluoromethyl)ornithine, and were then submitted to pain stimuli using two models, the Randall-Selitto test and the Tail-Flick test. Polyamine deprivation produced in these models an increase in the latency of the response, even under conditions which did not produce significant changes of the polyamine concentrations in blood and brain. From these observations, we may conclude that the polyamines play a role in the perception of nociceptive stimuli under physiological conditions.