Morphine In Neuropathic Pain Research Articles

Telmisartan Is a Promising Agent for Managing Neuropathic Pain and Delaying Opioid Analgesic Tolerance in Rats.

Despite the large arsenal of analgesic medications, neuropathic pain (NP) management is not solved yet. Angiotensin II receptor type 1 (AT1) has been identified as a potential target in NP therapy. Here, we investigate the antiallodynic effect of AT1 blockers telmisartan and losartan, and particularly their combination with morphine on rat mononeuropathic pain following acute or chronic oral administration. The impact of telmisartan on morphine analgesic tolerance was also assessed using the rat tail-flick assay. Morphine potency and efficacy in spinal cord samples of treated neuropathic animals were assessed by [35S]GTPγS-binding assay. Finally, the glutamate content of the cerebrospinal fluid (CSF) was measured by capillary electrophoresis. Oral telmisartan or losartan in higher doses showed an acute antiallodynic effect. In the chronic treatment study, the combination of subanalgesic doses of telmisartan and morphine ameliorated allodynia and resulted in a leftward shift in the dose-response curve of morphine in the [35S]GTPγS binding assay and increased CSF glutamate content. Telmisartan delayed morphine analgesic-tolerance development. Our study has identified a promising combination therapy composed of telmisartan and morphine for NP and opioid tolerance. Since telmisartan is an inhibitor of AT1 and activator of PPAR-γ, future studies are needed to analyze the effect of each component.

Effect of Exosomal Protein Expression and Electrochemical Nano Interface in Morphine Analgesia Tolerance.

The current understanding of neurons cannot well explain the phenomenon of morphine tolerance, and even if some neuronal drugs are used in combination with morphine, they cannot achieve good results. In recent years, exosomal proteins have been the role of morphine tolerance and morphine-induced hyperalgesia is becoming more and more important. In clinical application, a larger dose is needed to achieve the desired analgesic effect, and large doses of morphine will aggravate the adverse drug reactions. The purpose of this experiment is to explore the role and mechanism of exosomal protein expression and electrochemical nanointerface in morphine analgesic tolerance and provide a theoretical basis for the application of exosomal protein in the clinical prevention and treatment of morphine tolerance. In this paper, adult male rats were randomly divided into groups and analyzed from four aspects: enhancing the analgesic effect of high-dose morphine, preventing morphine tolerance, the effect of morphine-induced hyperalgesia, and changes in mRNA levels of related genes. Experiments have shown that the mRNA levels of exosomal protein-related genes in the spinal cord of morphine-tolerant rats are significantly increased, and the rats given exosome protein inhibitors can prolong morphine analgesia and relieve morphine tolerance and hyperalgesia in behavior. It can inhibit the increase of exosomal protein-related genes. It shows that low-dose long-term morphine treatment can cause spinal endoplasmic reticulum stress, and the exosomal protein inhibitor 4-PBA or TUDCA can enhance the analgesic effect of morphine in neuropathic pain.

Effect of Exosomal Protein Expression and Electrochemical Nano Interface in Morphine Analgesia Tolerance.

The current understanding of neurons cannot well explain the phenomenon of morphine tolerance, and even if some neuronal drugs are used in combination with morphine, they cannot achieve good results. In recent years, exosomal proteins have been the role of morphine tolerance and morphine-induced hyperalgesia is becoming more and more important. In clinical application, a larger dose is needed to achieve the desired analgesic effect, and large doses of morphine will aggravate the adverse drug reactions. The purpose of this experiment is to explore the role and mechanism of exosomal protein expression and electrochemical nanointerface in morphine analgesic tolerance and provide a theoretical basis for the application of exosomal protein in the clinical prevention and treatment of morphine tolerance. In this paper, adult male rats were randomly divided into groups and analyzed from four aspects: enhancing the analgesic effect of high-dose morphine, preventing morphine tolerance, the effect of morphine-induced hyperalgesia, and changes in mRNA levels of related genes. Experiments have shown that the mRNA levels of exosomal protein-related genes in the spinal cord of morphine-tolerant rats are significantly increased, and the rats given exosome protein inhibitors can prolong morphine analgesia and relieve morphine tolerance and hyperalgesia in behavior. It can inhibit the increase of exosomal protein-related genes. It shows that low-dose long-term morphine treatment can cause spinal endoplasmic reticulum stress, and the exosomal protein inhibitor 4-PBA or TUDCA can enhance the analgesic effect of morphine in neuropathic pain.

Combinations of haloperidol and morphine produce maximal levels of anti‐hyperalgesic or potentiation effects in rats with neuropathic pain

Neuropathic pain (NP) is a chronic disorder that result from an injury or dysfunction at central or peripheral nervous system, promoting alterations in control and modulation of nociceptive signalling pathways. The haloperidol is a butyrophenone prototype, that recently some authors have described as anti‐nociceptive drug mediated by sigma‐1 receptors antagonism. Besides, morphine is a prototype analgesic opioid drug currently used in the NP treatment.ObjectiveTo detect the optimal combinations generated by therapy of haloperidol with morphine in NP induced by chronic constriction injury (CCI).MethodsThe anti‐hyperalgesic effects of haloperidol (0.017–0.100 mg/Kg, s.c.) and morphine (0.01–3.16 mg/Kg, s.c.) were determined after single‐doses, both in monotherapy and combined, using the von Frey test. Evaluations were done until 10 days post‐surgery at 30, 60, 90, 120 and 180 minutes after drugs administration and data were processed using Surface of Synergic Interaction analysis (SSI).ResultsHaloperidol and morphine showed a dose‐dependent anti‐hyperalgesic effects. From 16 combinations studied, six were potentiation, while remaining 10 combinations resulted in an additive effect or limited by the maximum effect of the individual doses. The combination morphine (0.0316 mg/Kg) and haloperidol (0.01778 mg/Kg) showed the maximum potentiation effect (p<0.001). In turn, the combination of maximum anti‐hyperalgesic efficacy (p<0.01) was morphine (1.0 mg/Kg) and haloperidol (0.1 mg/Kg). Interestingly, was detected that is required a haloperidol single dose tenfold higher than haloperidol dose employed by the combination of maximum potentiation for reach similar antinociceptive effect.ConclusionsThis study showed that haloperidol and morphine in combination can produce addition and potentiation effects in neuropathic pain, and the SSI analysis allowed to detect the optimal combinations to produce maximum efficacy or maximum potentiation of anti‐hyperalgesic effects.Support or Funding InformationDepartment of Pharmacobiology, Cinvestav‐Unidad Coapa, C.P. 14330, Tlálpan, Mexico City, Mexico.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

A Novel Toxin from Haplopelma lividum Selectively Inhibits the NaV1.8 Channel and Possesses Potent Analgesic Efficacy.

Spider venoms are a complex mixture of peptides with a large number of neurotoxins targeting ion channels. Although thousands of peptide toxins have been identified from venoms of numerous species of spiders, many unknown species urgently need to be investigated. In this study, a novel sodium channel inhibitor, µ-TRTX-Hl1a, was identified from the venom of Haplopelma lividum. It contained eight cysteines and formed a conserved cysteine pattern of ICK motif. µ-TRTX-Hl1a inhibited the TTX-resistant (TTX-r) sodium channel current rather than the TTX-sensitive (TTX-s) sodium channel current. Meanwhile, µ-TRTX-Hl1a selectively inhibited NaV1.8 with an IC50 value of 2.19 μM. Intraperitoneal injection of µ-TRTX-Hl1a dose-dependently reduced inflammatory and neuropathic pain in rodent models of formalin-induced paw licking, tail-flicking, acetic acid-induced writhing, and hot plate test. It showed a better analgesic effect than morphine in inflammatory pain and equipotent effect to morphine in neuropathic pain. These findings demonstrate that µ-TRTX-Hl1a might be a valuable tool for physiology studies on NaV1.8 and a promising lead molecule for pain therapeutics.

Peripheral nerve injury reduces analgesic effects of systemic morphine via spinal 5-hydroxytryptamine 3 receptors.

Morphine produces powerful analgesic effects against acute pain, but it is not effective against neuropathic pain, and the mechanisms underlying this reduced efficacy remain unclear. Here, the authors compared the efficacy of systemic morphine between normal rats and rats with peripheral nerve injury, with a specific focus on descending serotonergic mechanisms. After L5 spinal nerve ligation injury, male Sprague-Dawley rats were subjected to behavioral testing, in vivo microdialysis of the spinal dorsal horn to determine serotonin (5-hydroxytryptamine [5-HT]) and noradrenaline release, and immunohistochemistry (n = 6 in each group). Intraperitoneal administration of morphine (1, 3, or 10 mg/kg) produced analgesic effects in normal and spinal nerve ligation rats, but the effects were greater in normal rats (P < 0.001). Morphine increased 5-HT release (450 to 500% of the baseline), but not noradrenaline release, in the spinal dorsal horn via activation of serotonergic neurons in the rostral ventromedial medulla. Intrathecal pretreatment with ondansetron (3 μg), a 5-HT3 receptor antagonist, or 5,7-dihydroxytryptamine creatinine sulfate (100 μg), a selective neurotoxin for serotonergic terminals, attenuated the analgesic effect of morphine (10 mg/kg) in normal rats but increased the analgesic effect of morphine in spinal nerve ligation rats (both P < 0.05). Systemic administration of morphine increases 5-HT levels in the spinal cord, and the increase in 5-HT contributes to morphine-induced analgesia in the normal state but attenuates that in neuropathic pain through spinal 5-HT3 receptors. The plasticity of the descending serotonergic system may contribute to the reduced efficacy of systemic morphine in neuropathic pain.

Effects of selective and non-selective inhibitors of nitric oxide synthase on morphine- and endomorphin-1-induced analgesia in acute and neuropathic pain in rats

Nitric oxide (NO) has been reported to be involved in the mechanisms of pain generation throughout the nervous system. We examined the effects of intrathecally (i.t.) administered nitric oxide synthase (NOS) inhibitors on the antinociceptive effects of morphine and endomorphin-1 during acute pain and in chronic constriction injury (CCI)-exposed rats. We used NG-nitro-l-arginine methyl ester (l-NAME), a non-selective NOS inhibitor; 7-nitroindazole (7-NI) or 1-(2-trifluoromethyl-phenyl)-imidazole (TRIM), selective inhibitors of neuronal NOS (NOS1); and 1400W dihydrochloride, a selective inhibitor of inducible NOS (NOS2). Morphine (0.5–2.5 μg) and endomorphin-1 (2.5–20 μg) in acute pain and morphine (10–40 μg) and endomorphin-1 (5–20 μg) after CCI-injury were combined with NOS inhibitors. For acute pain, the ED50 for endomorphin-1 (7.1 μg) was higher than that of morphine (1.3 μg) in the tail-flick test. For neuropathic pain, the ED50 value for morphine was much higher (43.2 μg) than that of endomorphin-1 (9.2 μg) in von Frey test. NOS inhibitors slightly influenced pain thresholds in both pain models. Moreover, in neuropathic pain, the effects of morphine were more potentiated by l-NAME, TRIM, 7-NI and 1400W (12×, 8.6×, 4.1× and 5.3×, respectively) than were the effects of endomorphin-1 (2.7×, 4.3×, 3.4× and 2.1×, respectively) in the von Frey test. Minocycline which is known to enhance the efficiency of morphine in neuropathic pain, decreased the mRNA expression of NOS1 in the DRG and NOS2 and C1q in the spinal cord after CCI. Both NOS2 and IBA-1 protein levels in the spinal cord and NOS1, NOS2 and IBA1 protein levels in DRG decreased after minocycline administration. In conclusion, our results provide evidence that both neuronal and non-neuronal NOS/NO pathways contribute to the behavioural pain responses evoked by nerve injury. The NOS inhibitors regardless of the type of pain enhanced morphine antinociception and, to a lesser extent, altered the effects of endomorphin-1, an opioid ligand with a peptidergic structure.

Post-injury repeated administrations of minocycline improve the antinociceptive effect of morphine in chronic constriction injury model of neuropathic pain in rat

It is confirmed that pharmacological attenuation of glial cells can alleviate neuropathic pain by lowering proinflammatory cytokine expression. The present study tries to confirm that post-injury administration of glia inhibitor, minocycline, can attenuate the neuropathic pain symptoms and improves the efficacy of morphine anti-nociception in chronic constriction injury (CCI). Male Wistar rats (230–270g) underwent surgery for induction CCI model of neuropathy. For assessment of the thermal hyperalgesia and mechanical allodynia after CCI induction, morphine (2.5, 5, 7.5, 10 and 15mg/kg; s.c.) and saline were administered on post-operative days (PODs) 0, 6 and 14. Hargreaves and Von–Frey tests were performed before and 30min after morphine administration, respectively. The results showed significant decrease in antinociceptive effect of morphine on POD 6 compared to POD 0 only at the dose of 5mg/kg. On the other hand, on POD 14 the antinociceptive effect of morphine (5, 7.5, 10 and 15mg/kg) significantly decreased in comparison with POD 0. In another set of experiments, animals received minocycline (10, 20 and 40mg/kg; i.p.) for eight days from POD 6 to 13 and then the antinociceptive effect of single dose of morphine 5mg/kg was tested on POD 14. Behavioral tests showed that minocycline (40mg/kg) could effectively attenuate the thermal hyperalgesia and mechanical allodynia on POD 13. Moreover, minocycline (40, 20mg/kg) improved the anti-hyperalgesic, and minocycline (40mg/kg) improved the anti-allodynic effects of morphine 5mg/kg on POD 14. It seems that the reduction of antinociceptive effect of morphine after CCI may be mediated through glia activation. Modulation of glial activity by minocycline can attenuate CCI-induced neuropathic pain. It is also shown that repeated post-injury administration of minocycline improves the antinociceptive effect of morphine in neuropathic pain.

Agonist-dependent attenuation of μ-opioid receptor-mediated G-protein activation in the dorsal root ganglia of neuropathic rats

Besides generally accepted lower analgesic potencies of opioids in neuropathic pain, our recent pharmacological reports have demonstrated that the effectiveness of the micro-opioid receptor (MOR) agonists in neuropathy might depends upon the chemical/structural property of these compounds (alkaloid vs. peptides). Such findings prompted us to investigate the changes in MOR mRNA expression (estimated by PCR) as well as MOR functional activity (examined by [(35)S]GTPgammaS binding) in the dorsal horn of the spinal cord and the dorsal root ganglia (DRG) of neuropathic rats at different time points after sciatic nerve ligation. We found that the spinal MOR mRNA level and agonist-stimulated [(35)S]GTPgammaS binding were not affected by nerve injury. In contrast, down-regulation of MOR mRNA in the ipsilateral side of DRG developed 3 (approximately 63% reduction) and 14 (approximately 89% reduction) days after the ligation. The decrease was paralleled with pronounced reduction in the stimulation of [(35)S]GTPgammaS binding by morphine (approximately 37-39%). Thus, neuropathy-induced specific dysfunction of MOR to activate G-protein together with changes in the MOR synthesis might be related, at least in part, to diminish analgesic efficacy of morphine in neuropathic pain. Interesting observations from current studies are linked to endomorphins (EMs), which do not affect the G protein stimulation of MOR after nerve ligation. This intriguing property of EMs, together with previously reported high analgesic efficacy of these compounds indicate that chemically/structurally different MOR agonists, particularly morphine versus EMs, may differentially interact with receptors causing distinct pharmacological effects in chronic pain.

Activity of new NOP receptor ligands in a rat peripheral mononeuropathy model: Potentiation of morphine anti-allodynic activity by NOP receptor antagonists

The effect of new NOP receptor agonists and antagonists in the rat chronic constriction injury model was investigated. Intraperitoneally administered NOP receptor agonist SR14150 and antagonists SR16430 and SR14148, had no effect on mechanical allodynia when given alone. The nonselective NOP/mu-opioid receptor agonist SR16435, however, produced an anti-allodynic response, similar to morphine and reversible by naloxone. Notably, co-administration of the NOP receptor antagonists potentiated the anti-allodynic activity of both morphine and SR16435. Increased levels of the NOP receptor are implicated in the reduced efficacy of morphine in neuropathic pain. Our results suggest the utility of NOP receptor antagonists for potentiating opioid efficacy in chronic pain.

Morphine and endomorphin-1 differently influence pronociceptin/orphanin FQ system in neuropathic rats

In the present study, we investigated the influence of intrathecal (i.t.) administration of morphine and endomorphin-1 on the level of pronociceptin/orphanin FQ and opioid receptor-like 1 (ORL1) receptor mRNAs in the lumbar part of the spinal cord in the rat model of neuropathic pain. The ligation of the sciatic nerve did not change the levels of pronociceptin/orphanin FQ and ORL1 receptor mRNAs in laminae I–VI of the dorsal horn when measured by in situ hybridisation 2 and 7 days after the nerve injury, but ORL1 receptor mRNA level in the ventral horn was significantly increased. Two μ-opioid receptor agonists, morphine and endomorphin-1, whose effectiveness in neuropathic pain is different, also disparately influenced nociceptin/orphanin FQ system in this pain model, inasmuch as an increase in pronociceptin/orphanin FQ and ORL1 receptor mRNAs was observed in laminae I–VI after morphine administration (5 μg i.t.) but not after endomorphin-1 treatment (5 μg i.t.). Moreover, the injection of ORL1 receptor antagonists (PheΨ; 30 μg i.t.) before morphine potentiated the effect of morphine in neuropathic pain model. Therefore, the activation of the endogenous nociceptin/orphanin FQ system, which is known to exhibit antiopioidergic activity, apart from its analgesic action, could be the reason for lower responsiveness to morphine in neuropathic pain.

Loss of peripheral morphine analgesia contributes to the reduced effectiveness of systemic morphine in neuropathic pain.

It is well known that the analgesic potency of morphine is reduced in neuropathic pain. In this study, we demonstrate that the decreased effectiveness of systemic morphine in neuropathic pain might be caused by the loss of morphine analgesia at the periphery. When given s.c. or i.t., the dose-response curves for morphine analgesia in Hargreaves thermal test were shifted rightward in partial sciatic nerve-injured mice compared with control sham-operated mice. The dose-response curves for i.c.v. morphine analgesia, however, were unchanged in nerve-injured mice, indicating no decrease in morphine potency at the supraspinal level. On the other hand, the dose-dependent analgesia produced by intraplantar (i.pl.) morphine in sham-operated mice almost completely disappeared in nerve-injured mice. With the more sensitive algogenic-induced nociceptive flexion test, significant reduction in the analgesic potency of systemic morphine was observed for bradykinin (BK) nociception in nerve-injured mice, and the analgesic effect of i.pl. morphine against BK nociception in sham-operated mice disappeared in nerve-injured mice. In immunohistochemical experiments, we found that, under normal state, mu-opioid receptors (MOPs) were mainly expressed in small-diameter unmyelinated dorsal root ganglion (DRG) neurons and colocalized with bradykinin B2 receptors. When we examined MOP expression in the DRG of nerve-injured mice, we observed a drastic decrease in MOP expression. Altogether, these data suggest that the lower potency of systemic morphine in neuropathic pain could be at least partly caused by the decreased MOP expression in DRG and subsequent loss of peripheral morphine analgesia in such a condition.

A randomised, double blind, placebo controlled crossover study of the cholecystokinin 2 antagonist L-365,260 as an adjunct to strong opioids in chronic human neuropathic pain

The aim of this study was to establish if the cholecystokinin (CCK) 2 antagonist L-365,260 augments the analgesic effect of morphine in human subjects with chronic neuropathic pain. This is a randomised, double blind, placebo controlled study of 40 adult subjects taking morphine for neuropathic pain. Each received placebo, L-365,260 30 mg and L-365,260 120 mg in three divided doses daily separated by a washout period in random order. Pain, activity, sedation, sleep and side effects were recorded along with 12 lead ECGs, renal and liver function tests and full blood pictures. L-365,260 failed to augment the analgesic effect of morphine at any of the dose levels used. Side effects were minor. There were no changes in ECGs and biochemical indices were unaltered with its use. The CCK 2 antagonist L-365,260 does not augment the analgesic effect of morphine in subjects with chronic neuropathic pain. L-365,260 was well tolerated and side effects from its use were minor.

The Mechanism of Antiallodynic Effect of Intrathecal Morphine in Neuropathic Pain Induced by Spinal Nerve Ligation: The Effect of Methysergide and Theophylline

Background: Although the efficacy of morphine in a neuropathic pain state is somewhat controversial, spinally administered morphine reversed the tactile allodynia in a previous animal study. Using a behaviorial test, we examined the involvement of serotonergic and adenosine receptors in the mechanism of the antiallodynic action of the intrathecal morphine by injection of serotonergic and adenosine antagonist in a rat model of neuropathic pain induced by a spinal nerve ligation. Methods: Male Sprague-Dawley rats were prepared with a tight ligation of the left lumbar 5th and 6th spinal nerve and a chronic lumbar intrathecal catheter implantation. Morphine 1 was administered intrathecally to attenuate the tactile allodynia. Methysergide 10 and 30, theophylline 20 was administered intrathecally before and after the injection of morphine in order to investigate the reversal of an increased allodynic threshold by morphine. The allodynic thresholds for the left hindpaw withdrawl to von Frey hairs were assessed and converted to %MPE. Results: The tactile allodynic threshold was significantly increased by 1 of intrathecal morphine (P and 30, but not theophylline 20, reversed significantly the antiallodynic effect of intrathecal morphine in pre- and post-treatment (P

Functional mu opioid receptors are reduced in the spinal cord dorsal horn of diabetic rats.

The mechanisms of decreased spinal analgesic potency of morphine in neuropathic pain are not fully known. Agonist-stimulated [35S]GTPgammaS receptor autoradiography has been used to measure receptor activation of G proteins in vitro. Using this technique, we determined changes in the functional mu opioid receptors in the spinal dorsal horn in diabetic rats. Rats were rendered diabetic with an intraperitoneal injection of streptozotocin. The lumbar spinal cord was obtained from age-matched normal and diabetic rats 4 weeks after streptozotocin treatment. [D-Ala2,N-MePhe4,Gly5-ol]-enkephalin (DAMGO, 10 microm)-stimulated [35S]GTPgammaS binding was performed in both tissue sections and isolated membranes. The DAMGO-stimulated [35S]GTPgammaS binding in the spinal dorsal horn was significantly reduced (approximately 37%) in diabetic rats compared with normal rats. However, [35S]GTPgammaS bindings in the spinal dorsal horn stimulated by other G protein-coupled receptor agonists, including [D-Pen2,D-Pen5]-enkephalin, R(-)N6-(2-phenylisopropyl)-adenosine, and WIN-55212, were not significantly altered in diabetic rats. The basal [35S]GTPgammaS binding in the spinal dorsal horn was slightly (approximately 13%) but significantly increased in diabetic rats. Western blot analysis revealed no significant difference in the expression of the alpha subunits of G(i) and G(o) proteins in the dorsal spinal cord between normal and diabetic rats. These data suggest that the functional mu opioid receptors in the spinal cord dorsal horn of diabetic rats are reduced. The impaired functional mu opioid receptors in the spinal cord may constitute one of the mechanisms underlying the reduced spinal analgesic effect of mu opioids in diabetic neuropathic pain.

Knockdown of spinal opioid receptors by antisense targeting β-arrestin reduces morphine tolerance and allodynia in rat

The development of morphine tolerance and sciatic nerve injury-induced allodynia after functional knockdown of spinal opioid receptors using antisense oligonucleotides targeting β-arrestin was investigated. Ineffectiveness of morphine in neuropathic pain suggests an implication of the same mechanism in these two processes. The development of morphine tolerance (10 μg intrathecally (i.th.), every 12 h) was significantly inhibited in rats, which received i.th. β-arrestin antisenses (2 nM). Acute and chronic (6 days) i.th. administration of antisenses antagonized the allodynia in the rat model of neuropathic pain. Our results demonstrated that i.th. administration of β-arrestin antisenses delayed development of tolerance to morphine and nerve injury-induced cold allodynia, which suggest that both of the investigated phenomena may be mediated by a similar mechanism, e.g. receptor desensitization. Moreover, the antisense oligonucleotides targeting β-arrestin may constitute a new approach to the therapy of neuropathic pain.

Resolution of (RS)-Proglumide using Lipase from Candida cylindraceae

Proglumide is used in the treatment of neuropathic pain. It acts by inhibiting peptide cholecystokinin (CCK). Neural injury produces an elevation in plasma CCK. Proglumide has been also shown to augment the analgesic effect of sustained release morphine in neuropathic pain. Currently proglumide is administered as a racemic mixture. In the present study, an attempt is made to separate the racemic mixture of the drug using lipase obtained from Candida cylindracea by stereoselective esterification. Enzymatic stereoselective esterification was carried out in organic solvents. The resolution was studied using a chromatographic column with a chiral support and mass spectrometry. The reaction conditions for stereoselective esterification including amount of substrate, amount of enzyme, alcohol, solvent and temperature were optimised during the present investigation. Butanol and hexanol were found to be suitable for formation of S and R esters, respectively. Hexane was the best solvent for esterification and the optimum temperature was found to be 30°C.

The Mechanism of Antiallodynic Effect of Intrathecal Morphine in Neuropathic Pain Induced by Spinal Nerve Ligation

Background: Although the efficacy of morphine in a neuropathic pain state is somewhat controversial, intrathecally administered morphine reversed the mechanical allodynia in a previous animal study. Using a behavioral test, we investigated the mechanism of the antiallodynic action of intrathecal morphine by administering opioids, α2 adrenergic and cholinergic receptor antagonists in a rat model of neuropathic pain induced by a spinal nerve ligation injury. Methods: Male Sprague Dawley rats were prepared with a tigh1 ligation of the left lumbar 5th and 6th spinal nerves and insertion of a chronic lumbar intrathecal catheter. Morphine 1 μg was administered intrathecally to attenuate the mechanical allodynic. Naloxane 10 μg, yohimbine 30 μg, prazosin 30 μg, atropine 10 μg, pirenzepine 10 μg, and methoctramine 10 μg was administered intrathecally before and after the injection of morphine in order to investigate the reversal of an increased allodynic threshold by morphine. The allodynic thresholds for the left hindpaw withdrawal to von Frey hairs were assessed and converted to %MPE. Results: A reduction of mechanical alladynia by intrathecal morphine was produced. Naloxone pretreatment, but not posttreatment, reversed the antiallodynic effect of intrathecal morphine (P < 0.01). All α2 adrenergic and cholinergic receptor antagonists used here did not reverse it. Conclusions: The results suggest that the reversal mechanism of mechanical allodynia by intrathecal morphine appears to be mediated mostly by the opioid receptor system, but not the α2 adrcenergic and cholinergic receptor systems, at the spinal level in a rat model of e spinal nerve ligation injury.

The Mechanism of Action of Intrathecal Morphine in Neuropathic Pain Induced by Nerve Ligation: The Effect of Naloxone

The Mechanism of Action of Intrathecal Morphine in Neuropathic Pain Induced by Nerve Ligation: The Effect of Naloxone