Modification of antiallodynic and antinociceptive effects of morphine by peripheral and central action of fluoxetine in a neuropathic mice model

We have previously reported that serotonin concentration was reduced in the brain of mice with neuropathic pain and that it may be related to reduction of morphine analgesic effects. To further prove this pharmacological action, we applied fluoxetine, a selective serotonin reuptake inhibitor, to determine whether it suppressed neuropathic pain and examined how its different administration routes would affect antinociceptive and antiallodynic effects of morphine in diabetic (DM) and sciatic nerve ligation (SL) mice, as models of neuropathic pain. Antiallodynia and antinociceptive effect of drugs were measured by using von Frey filament and tail pinch tests, respectively. Fluoxetine given alone, intracerebroventicularly (i.c.v., 15 microg/mouse) or intraperitoneally (i.p., 5 and 10 mg/kg) did not produce any effect in either model. However, fluoxetine given i.p. enhanced both antiallodynic and antinociceptive effects of morphine. Administration of fluoxetine i.c.v., slightly enhanced only the antiallodynic effect of morphine in SL mice. Ketanserine, a serotonin 2A receptor antagonist (i.p., 1 mg/kg) and naloxone, an opioid receptor antagonist (i.p., 3 mg/kg), blocked the combined antinociceptive effect of fluoxetine and morphine. Our data show that fluoxetine itself lacks antinociceptive properties in the two neuropathy models, but it enhances the analgesic effect of morphine in the periphery and suggests that co-administration of morphine with fluoxetine may have therapeutic potential in treatment of neuropathic pain.     

Molecular basis for the dosing time-dependency of anti-allodynic effects of gabapentin in a mouse model of neuropathic pain

Background

Ecto-5'-nucleotidase (NT5E, also known as CD73) hydrolyzes extracellular adenosine 5'-monophosphate (AMP) to adenosine in nociceptive circuits. Since adenosine has antinociceptive effects in rodents and humans, we hypothesized that NT5E, an enzyme that generates adenosine, might also have antinociceptive effects in vivo.

Results

To test this hypothesis, we purified a soluble version of mouse NT5E (mNT5E) using the baculovirus expression system. Recombinant mNT5E hydrolyzed AMP in biochemical assays and was inhibited by α,β-methylene-adenosine 5'-diphosphate (α,β-me-ADP; IC₅₀ = 0.43 μM), a selective inhibitor of NT5E. mNT5E exhibited a dose-dependent thermal antinociceptive effect that lasted for two days when injected intrathecally in wild-type mice. In addition, mNT5E had thermal antihyperalgesic and mechanical antiallodynic effects that lasted for two days in the complete Freund's adjuvant (CFA) model of inflammatory pain and the spared nerve injury (SNI) model of neuropathic pain. In contrast, mNT5E had no antinociceptive effects when injected intrathecally into adenosine A₁ receptor (A ₁ R, Adora1) knockout mice.

Conclusion

Our data indicate that the long lasting antinociceptive effects of mNT5E are due to hydrolysis of AMP followed by activation of A₁R. Moreover, our data suggest recombinant NT5E could be used to treat chronic pain and to study many other physiological processes that are regulated by NT5E.     

Comparison of the antinociceptive effects of ibuprofen arginate and ibuprofen in rat models of inflammatory and neuropathic pain

AimsIbuprofen arginate is a highly soluble salt formed by combining racemic ibuprofen with the amino acid l-arginine. This formulation is absorbed faster, and it is safe and effective in treating many forms of mild to moderate pain. We compared the analgesic effect of ibuprofen arginate and conventional ibuprofen in rat models of pain.Main methodsMechanical and cold allodynia were assessed in the chronic constriction injury (CCI) model of neuropathic pain, and mechanical allodynia was also examined in capsaicin-injected rats (a model of central sensitization). Inflammatory hypersensitivity was assessed with the formalin test. Ibuprofen-l-arginine, ibuprofen, l-arginine or saline was administered orally on a daily basis after CCI or capsaicin injection, and the von Frey and cold plate tests were performed on days 1, 3 and 7 after CCI or capsaicin administration. In the formalin-induced inflammatory pain test, the drugs were administered 30 min before formalin injection.Key findingsIbuprofen only exerts an antinociceptive effect in the formalin model whereas ibuprofen-l-arginine exerts antinociceptive effects on both mechanical and cold allodynia induced by CCI, mechanical allodynia induced by capsaicin injection, and in phase 2 of the formalin test, exhibiting superior antinociceptive activity to ibuprofen in all these tests. l-Arginine only exerted antinociceptive effects on cold allodynia in CCI.SignificanceThese results demonstrate that ibuprofen arginate has stronger antinociceptive effects than ibuprofen in all the models used, suggesting it might improve the therapeutic management of neuropathic and inflammatory pain.     

Spinal mechanisms of NPY analgesia

We review previously published data, and present some new data, indicating that spinal application of neuropeptide Y (NPY) reduces behavioral and neurophysiological signs of acute and chronic pain. In models of acute pain, early behavioral studies showed that spinal (intrathecal) administration of NPY and Y2 receptor agonists decrease thermal nociception. Subsequent neurophysiological studies indicated that Y2-mediated inhibition of excitatory neurotransmitter release from primary afferent terminals in the substantia gelatinosa may contribute to the antinociceptive actions of NPY. As with acute pain, NPY reduced behavioral signs of inflammatory pain such as mechanical allodynia and thermal hyperalgesia; however, receptor antagonist studies indicate an important contribution of spinal Y1 rather than Y2 receptors. Interestingly, Y1 agonists suppress inhibitory synaptic events in dorsal horn neurons (indeed, well known mu-opioid analgesic drugs produce similar cellular actions). To resolve the behavioral and neurophysiological data, we propose that NPY/Y1 inhibits the spinal release of inhibitory neurotransmitters (GABA and glycine) onto inhibitory neurons, e.g. disinhibition of pain inhibition, resulting in hyporeflexia. The above mechanisms of Y1- and Y2-mediated analgesia may also operate in the setting of peripheral nerve injury, and new data indicate that NPY dose-dependently inhibits behavioral signs of neuropathic pain. Indeed, neurophysiological studies indicate that Y2-mediated inhibition of Ca(2+) channel currents in dorsal root ganglion neurons is actually increased after axotomy. We conclude that spinal delivery of Y1 agonists may be of use in the treatment of chronic inflammatory pain, and that the use of Y1 and Y2 agonists in neuropathic pain warrants further consideration.     

Cannabinoid-mediated modulation of neuropathic pain and microglial accumulation in a model of murine type I diabetic peripheral neuropathic pain

Background

Despite the frequency of diabetes mellitus and its relationship to diabetic peripheral neuropathy (DPN) and neuropathic pain (NeP), our understanding of underlying mechanisms leading to chronic pain in diabetes remains poor. Recent evidence has demonstated a prominent role of microglial cells in neuropathic pain states. One potential therapeutic option gaining clinical acceptance is the cannabinoids, for which cannabinoid receptors (CB) are expressed on neurons and microglia. We studied the accumulation and activation of spinal and thalamic microglia in streptozotocin (STZ)-diabetic CD1 mice and the impact of cannabinoid receptor agonism/antagonism during the development of a chronic NeP state. We provided either intranasal or intraperitoneal cannabinoid agonists/antagonists at multiple doses both at the initiation of diabetes as well as after establishment of diabetes and its related NeP state.

Results

Tactile allodynia and thermal hypersensitivity were observed over 8 months in diabetic mice without intervention. Microglial density increases were seen in the dorsal spinal cord and in thalamic nuclei and were accompanied by elevation of phosphorylated p38 MAPK, a marker of microglial activation. When initiated coincidentally with diabetes, moderate-high doses of intranasal cannabidiol (cannaboid receptor 2 agonist) and intraperitoneal cannabidiol attenuated the development of an NeP state, even after their discontinuation and without modification of the diabetic state. Cannabidiol was also associated with restriction in elevation of microglial density in the dorsal spinal cord and elevation in phosphorylated p38 MAPK. When initiated in an established DPN NeP state, both CB1 and CB2 agonists demonstrated an antinociceptive effect until their discontinuation. There were no pronociceptive effects demonstated for either CB1 or CB2 antagonists.

Conclusions

The prevention of microglial accumulation and activation in the dorsal spinal cord was associated with limited development of a neuropathic pain state. Cannabinoids demonstrated antinociceptive effects in this mouse model of DPN. These results suggest that such interventions may also benefit humans with DPN, and their early introduction may also modify the development of the NeP state.     

Spinal and peripheral mechanisms of cannabinoid antinociception: behavioral,neurophysiological and neuroanatomical perspectives

A large body of literature indicates that cannabinoids suppress behavioral responses to acute and persistent noxious stimulation. This review examines behavioral, neurophysiological and neuroanatomical evidence supporting a role for cannabinoids in suppressing nociceptive transmission at spinal and peripheral levels. The development of subtype-selective competitive antagonists and high-affinity agonists provides the pharmacological tools required to study cannabinoid antinociceptive mechanisms. These studies provide insight into the functional roles of cannabinoid receptor subtypes, CB1 and CB2, in cannabinoid antinociceptive mechanisms as revealed in animal models of acute and persistent (somatic inflammatory, visceral inflammatory, neuropathic) pain. Localization studies employing receptor binding and quantitative autoradiography, immunocytochemistry and in situ hybridization are reviewed to examine the distribution of cannabinoid receptors at these levels and provide a neuroanatomical framework with which to understand the roles of endogenous cannabinoids in sensory processing.     

Involvement of adenosine A1 and A2A receptors in (-)-linalool-induced antinociception

In recent studies performed in our laboratory we have shown that acute administration of (-)-linalool, the natural occurring enantiomer in essential oils, possesses anti-inflammatory, antihyperalgesic and antinociceptive effects in different animal models. The antihyperalgesic and antinociceptive effects of (-)-linalool have been ascribed to its capacity in stimulating the opioidergic, cholinergic and dopaminergic systems, as well as to its interaction with K+ channels, or to its local anaesthetic activity and/or to the negative modulation of glutamate transmission. Activation of A1 or A2A receptors has been shown to induce antinociceptive effects, and the possible involvement of adenosine in (-)-linalool antinociceptive effect, has not been elucidated yet. Therefore, in the present study, we have investigated the effects of 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a selective adenosine A1 receptor antagonist and the effects of 3,7-dimethyl-1-propargilxanthine (DMPX), a selective adenosine A2A receptor antagonist on the antinociception of (-)-linalool in mice, measured in the hot-plate test. Both DPCPX (0.1 mg/kg; i.p.) and DMPX (0.1 mg/kg; i.p.) pre-treatment significantly depressed the antinociceptive effect of (-)-linalool at the highest doses tested. These findings demonstrated that the effect of (-)-linalool on pain responses is, at least partially, mediated by the activity of adenosine A1 and A2A receptors.     

GABA(B) receptor agonist baclofen has non-specific antinociceptive effect in the model of peripheral neuropathy in the rat

Baclofen, which is a specific agonist of the metabotropic GABA(B) receptor, is used in clinical practice for the treatment of spasticity of skeletal muscles. It also exerts an analgesic effect, but this effect is still not clear and especially controversial in neuropathic pain. In this work, we studied the antinociceptive effects of baclofen in a model of chronic peripheral neuropathic pain - loose ligation of the sciatic nerve (chronic constriction injury, CCI). As controls we used sham-operated animals. The changes of thermal pain threshold were measured using the plantar test 15-25 days after the operation. The obtained results suggest that baclofen increases pain threshold in both groups. The antinociceptive effect of baclofen was dose-dependent and the maximum response without motor deficits was observed at a dose of 15 mg/kg s.c. In the rats with CCI, significant differences between affected (ipsilateral) and contralateral hind paw were present. This difference was dose-dependent, the highest value (6.2+/-1.37 s) was found at the dose of 20 mg/kg. Based on our results and previous findings it could be summarized that baclofen has antinociceptive action, which is attenuated in the model of chronic neuropathic pain probably due to the degeneration of GABA interneurons after chronic constriction injury.     

Discovery of {1-[4-(2-{hexahydropyrrolo[3,4-c]pyrrol-2(1H)-yl}-1H-benzimidazol-1-yl)piperidin-1-yl]cyclooctyl}methanol,systemically potent novel non-peptide agonist of nociceptin/orphanin FQ receptor as analgesic for the treatment of neuropathic pain: Design,synthesis, and structure–activity relationships

Neuropathic pain is a serious chronic disorder caused by lesion or dysfunction in the nervous systems. Endogenous nociceptin/orphanin FQ (N/OFQ) peptide and N/OFQ peptide (NOP) receptor [or opioid-receptor-like-1 (ORL1) receptor] are located in the central and peripheral nervous systems, the immune systems, and peripheral organs, and have a crucial role in the pain sensory system. Indeed, peripheral or intrathecal N/OFQ has displayed antinociceptive activities in neuropathic pain models, and inhibitory effects on pain-related neurotransmitter releases and on synaptic transmissions of C- and Aδ-fibers. In this study, design, synthesis, and structure–activity relationships of peripheral/spinal cord-targeting non-peptide NOP receptor agonist were investigated for the treatment of neuropathic pain, which resulted in the discovery of highly selective and potent novel NOP receptor full agonist {1-[4-(2-{hexahydropyrrolo[3,4-c]pyrrol-2(1H)-yl}-1H-benzimidazol-1-yl)piperidin-1-yl]cyclooctyl}methanol 1 (HPCOM) as systemically (subcutaneously) potent new-class analgesic. Thus, 1 demonstrates dose-dependent inhibitory effect against mechanical allodynia in chronic constriction injury-induced neuropathic pain model rats, robust metabolic stability and little hERG potassium ion channel binding affinity, with its unique and potentially safe profiles and mechanisms, which were distinctive from those of N/OFQ in terms of site-differential effects.     

Antinociceptive effect of topiramate in models of acute pain and diabetic neuropathy in rodents

This study assesses the antinociceptive effect induced by different dosages of topiramate (TP), an anticonvulsant drug that is orally administered in models of neuropathic pain and acute pain in rats and mice, respectively. Orally administered TP (80 mg/Kg) in mice causes antinociception in the first and second phases of a formalin test, while in doses of 20 and 40 mg/Kg it was only effective in the second phase. TP (80 mg/Kg, p.o) also exhibited antinociceptive action in the hot plate test, however, it did not have an effect in the capsaicin test in mice, nor in the model of neuropathic pain in diabetic rats. The antinociceptive effect caused by TP (80 mg/Kg, p.o) in the formalin test was reversed by prior treatment with naloxone (opioid antagonist), but not with glibenclamide (antagonist of the potassium channel), ondansetron (antagonist of the serotonin 5HT3 receptor) or cyproheptadine (antagonist of the serotonin 5HT2A receptor).The data show that TP has an important antinociceptive effect in the models of nociception induced by chemical (formalin) or thermal (hot plate) stimuli, and that the opioid system plays a part in the antinociceptive effect, as shown by formalin.     

Antinociceptive effect of (S)-N-desmethyl trimebutine against a mechanical stimulus in a rat model of peripheral neuropathy

Trimebutine (2-dimethylamino-2-phenylbutyl 3,4,5-trimethoxybenzoate, hydrogen maleate) relieves abdominal pain in humans. In the present study, the antinociceptive action of systemic (S)-N-desmethyl trimebutine, a stereoisomer of N-monodesmethyl trimebutine, the main metabolite of trimebutine in humans, was studied in a rat model of neuropathic pain produced by chronic constriction injury to the sciatic nerve. Mechanical (vocalization threshold to hindpaw pressure) stimulus was used. Experiments were performed two weeks after surgery when the pain-related behaviour has fully developed. (S)-N-desmethyl trimebutine (1, 3, 10 mg/kg s.c.) produced dose-dependent antinociceptive effects on the nerve-injured and the contralateral hindpaw. The effect of the lowest dose (1 mg/kg s.c.) of (S)-N-desmethyl trimebutine on the nerve-injured paw was equal to that seen after a ten time stronger dose on the contralateral paw. The effect of (S)-N-desmethyl trimebutine (1 mg/kg) was not naloxone reversible. The results suggest that systemic (S)-N-desmethyl trimebutine may be useful in the treatment of some aspects of neuropathic pain.     

Effects of (1DMe)NPYF, a synthetic neuropeptide FF analogue, in different pain models.

The antinociceptive effects of intrathecal (IT) (1DMe)NPYF were studied in adult Sprague-Dawley rats. (1DMe)NPYF produced dose-dependent antinociception that was reduced by subcutaneous injection of naloxone. (1DMe)NPYF (0.5 nmol) also potentiated the antinociceptive effects of intrathecal morphine 7.8 nmol. This suggests that the antinociceptive effects of (1DMe)NPYF are partially mediated by opioid receptor activation. In carrageenan inflammation, 5-10 nmol of (1DMe)NPYF was effective against both thermal hyperalgesia and mechanical allodynia. In the neuropathic pain model, the lowest dose tested (0.5 nmol) showed antiallodynic effects against cold allodynia. The results suggest a potential role for (1DMe)NPYF in the treatment of pain including neuropathic pain.     

Inhibition of pain responses by activation of CB(2) cannabinoid receptors

Cannabinoid receptor agonists diminish responses to painful stimuli. Extensive evidence demonstrates that CB(1) cannabinoid receptor activation inhibits pain responses. Recently, the synthesis of CB(2) cannabinoid receptor-selective agonists has allowed testing whether CB(2) receptor activation inhibits pain. CB(2) receptor activation is sufficient to inhibit acute nociception, inflammatory hyperalgesia, and the allodynia and hyperalgesia produced in a neuropathic pain model. Studies using site-specific administration of agonist and antagonist have suggested that CB(2) receptor agonists inhibit pain responses by acting at peripheral sites. CB(2) receptor activation also inhibits edema and plasma extravasation produced by inflammation. CB(2) receptor-selective agonists do not produce central nervous system (CNS) effects typical of cannabinoids retaining agonist activity at the CB(1) receptor. Peripheral antinociception without CNS effects is consistent with the peripheral distribution of CB(2) receptors. CB(2) receptor agonists may have promise for the treatment of pain and inflammation without CNS side effects.     

Prostatic acid phosphatase is an ectonucleotidase and suppresses pain by generating adenosine

Thiamine monophosphatase (TMPase, also known as fluoride-resistant acid phosphatase) is a classic histochemical marker of small-diameter dorsal root ganglia neurons. The molecular identity of TMPase is currently unknown. We found that TMPase is identical to the transmembrane isoform of prostatic acid phosphatase (PAP), an enzyme with unknown molecular and physiological functions. We then found that PAP knockout mice have normal acute pain sensitivity but enhanced sensitivity in chronic inflammatory and neuropathic pain models. In gain-of-function studies, intraspinal injection of PAP protein has potent antinociceptive, antihyperalgesic, and antiallodynic effects that last longer than the opioid analgesic morphine. PAP suppresses pain by functioning as an ecto-5'-nucleotidase. Specifically, PAP dephosphorylates extracellular adenosine monophosphate (AMP) to adenosine and activates A1-adenosine receptors in dorsal spinal cord. Our studies reveal molecular and physiological functions for PAP in purine nucleotide metabolism and nociception and suggest a novel use for PAP in the treatment of chronic pain.     

Antinociceptive activity of the S1P-receptor agonist FTY720

FTY720 is a novel immunosuppressive drug that inhibits the egress of lymphocytes from secondary lymphoid tissues and thymus. In its phosphorylated form FTY720 is a potent S1P receptor agonist. Recently it was also shown that FTY720 can reduce prostaglandin synthesis through the direct inhibition of the cytosolic phospholipase A2 (cPLA2). Since prostaglandins are important mediators of nociception, we studied the effects of FTY720 in different models of nociception. We found that intraperitoneal administration of FTY720 reduced dose-dependently the nociceptive behaviour of rats in the formalin assay. Although the antinociceptive doses of FTY720 were too low to alter the lymphocyte count, prostanoid concentrations in the plasma were dramatically reduced. Surprisingly, intrathecally administered FTY720 reduced the nociceptive behaviour in the formalin assay without altering spinal prostaglandin synthesis, indicating that additional antinociceptive mechanisms beside the inhibition of prostaglandin synthesis are involved. Accordingly, FTY720 reduced also the nociceptive behaviour in the spared nerve injury model for neuropathic pain which does not depend on prostaglandin synthesis. In this model the antinociceptive effect of FTY720 was similar to gabapentin, a commonly used drug to treat neuropathic pain. Taken together we show for the first time that FTY720 possesses antinociceptive properties and that FTY720 reduces nociceptive behaviour during neuropathic pain.     

Recombinant Mouse PAP Has pH-Dependent Ectonucleotidase Activity and Acts through A1-Adenosine Receptors to Mediate Antinociception

Prostatic acid phosphatase (PAP) is expressed in nociceptive neurons and functions as an ectonucleotidase. When injected intraspinally, the secretory isoforms of human and bovine PAP protein have potent and long-lasting antinociceptive effects that are dependent on A₁-adenosine receptor (A₁R) activation. In this study, we purified the secretory isoform of mouse (m)PAP using the baculovirus expression system to determine if recombinant mPAP also had antinociceptive properties. We found that mPAP dephosphorylated AMP, and to a much lesser extent, ADP at neutral pH (pH 7.0). In contrast, mPAP dephosphorylated all purine nucleotides (AMP, ADP, ATP) at an acidic pH (pH 5.6). The transmembrane isoform of mPAP had similar pH-dependent ectonucleotidase activity. A single intraspinal injection of mPAP protein had long-lasting (three day) antinociceptive properties, including antihyperalgesic and antiallodynic effects in the Complete Freund''s Adjuvant (CFA) inflammatory pain model. These antinociceptive effects were transiently blocked by the A₁R antagonist 8-cyclopentyl-1, 3-dipropylxanthine (CPX), suggesting mPAP dephosphorylates nucleotides to adenosine to mediate antinociception just like human and bovine PAP. Our studies indicate that PAP has species-conserved antinociceptive effects and has pH-dependent ectonucleotidase activity. The ability to metabolize nucleotides in a pH-dependent manner could be relevant to conditions like inflammation where tissue acidosis and nucleotide release occur. Lastly, our studies demonstrate that recombinant PAP protein can be used to treat chronic pain in animal models.     

Antinociceptive and neurochemical effects of a single dose of IB-MECA in chronic pain rat models

This study aimed to evaluate the effect of a single administration of IB-MECA, an A3 adenosine receptor agonist, upon the nociceptive response and central biomarkers of rats submitted to chronic pain models. A total of 136 adult male Wistar rats were divided into two protocols: (1) chronic inflammatory pain (CIP) using complete Freund’s adjuvant and (2) neuropathic pain (NP) by chronic constriction injury of the sciatic nerve. Thermal and mechanical hyperalgesia was measured using von Frey (VF), Randal-Selitto (RS), and hot plate (HP) tests. Rats were treated with a single dose of IB-MECA (0.5 μmol/kg i.p.), a vehicle (dimethyl sulfoxide—DMSO), or positive control (morphine, 5 mg/kg i.p.). Interleukin 1β (IL-1β), brain-derived neurotrophic factor (BDNF), and nerve growth factor (NGF) levels were measured in the brainstem and spinal cord using enzyme-linked immunosorbent assay (ELISA). The establishment of the chronic pain (CIP or NP) model was observed 14 days after induction by a decreased nociceptive threshold in all three tests (GEE, P < 0.05). The antinociceptive effect of a single dose of IB-MECA was observed in both chronic pain models, but this was more effective in NP model. There was an increase in IL-1β levels promoted by CIP. NP model promoted increase in the brainstem BDNF levels, which was reversed by IB-MECA     

Cannabinoid CB<Subscript>2</Subscript> Receptor-Mediated Anti-nociception in Models of Acute and Chronic Pain

The endocannabinoid system consists of cannabinoid CB₁ and CB₂ receptors, endogenous ligands and their synthesising/metabolising enzymes. Cannabinoid receptors are present at key sites involved in the relay and modulation of nociceptive information. The analgesic effects of cannabinoids have been well documented. The usefulness of nonselective cannabinoid agonists can, however, be limited by psychoactive side effects associated with activation of CB₁ receptors. Following the recent evidence for CB₂ receptors existing in the nervous system and reports of their up-regulation in chronic pain states and neurodegenerative diseases, much research is now aimed at shedding light on the role of the CB₂ receptor in human disease. Recent studies have demonstrated anti-nociceptive effects of selective CB₂ receptor agonists in animal models of pain in the absence of CNS side effects. This review focuses on the analgesic potential of CB₂ receptor agonists for inflammatory, post-operative and neuropathic pain states and discusses their possible sites and mechanisms of action. Jhaveri and Sagar joint first author.     

cDNA microarray analysis of the differential gene expression in the neuropathic pain and electroacupuncture treatment models

Partial nerve injury is the main cause of neuropathic pain disorders in humans. Acupuncture has long been used to relieve pain. It is known to relieve pain by controlling the activities of the autonomic nervous system. Although the mechanism of neuropathic pain and analgesic effects of electroacupuncture (EA) have been studied in a rat model system, its detailed mechanism at the molecular level remains unclear. To identify genes that might serve as either markers or explain these distinct biological functions, a cDNA microarray analysis was used to compare the expression of 8,400 genes among three sample groups. Messenger RNAs that were pooled from the spinal nerves of 7 normal, 7 neuropathic pain, and 7 EA treatment rat models were compared. Sixty-eight genes were differentially expressed more than 2-fold in the neuropathic rat model when compared to the normal, and restored to the normal expression level after the EA treatment. These genes are involved in a number of biological processes, including the signal transduction, gene expression, and nociceptive pathways. Confirmation of the differential gene expression was performed by a dotblot analysis. Dot-blotting results showed that the opioid receptor sigma was among those genes. This indicates that opioid-signaling events are involved in neuropathic pain and the analgesic effects of EA. The potential application of these data include the identification and characterization of signaling pathways that are involved in the EA treatment, studies on the role of the opioid receptor in neuropathic pain, and further exploration on the role of selected identified genes in animal models.     

In vivo trafficking of endogenous opioid receptors

Several approaches have been taken for these in vivo studies. In many studies, the use of semi-quantitative immuno-electron microscopy is the approach of choice. Endogenous opioid receptors display differential subcellular distributions with mu opioid receptor (MOPR) being mostly present on the plasma membrane and delta-opioid receptor (DOPR) and kappa-opioid receptor (KOPR) having a significant intracellular pool. Etorphine and DAMGO cause endocytosis of the MOPR, but morphine does not, except in some dendrites. Interestingly, chronic inflammatory pain and morphine treatment promote trafficking of intracellular DOPR to the cell surface which may account for the enhanced antinociceptive effects of DOPR agonists. KOPR has been reported to be associated with secretory vesicles in the posterior pituitary and translocated to the cell surface upon salt loading along with the release of vasopressin. The study of endogenous opioid receptors using in vivo models has produced some interesting results that could not have been anticipated in vitro. In vivo studies, therefore, are essential to provide insight into the mechanisms underlying opioid receptor regulation.