Intrathecal alpha2 adrenoceptor agonist clonidine inhibits mechanical transmission in mouse spinal cord via activation of muscarinic M1 receptors

Novel artificial nerve transplantation of human iPSC-derived neurite bundles enhanced nerve regeneration after peripheral nerve injury

BACKGROUND

Severe peripheral nerve damage always requires surgical treatment. Autologous nerve transplantation is a standard treatment, but it is not sufficient due to length limitations and extended surgical time. Even with the available artificial nerves, there is still large room for improvement in their therapeutic effects. Novel treatments for peripheral nerve injury are greatly expected.

METHODS

Using a specialized microfluidic device, we generated artificial neurite bundles from human iPSC-derived motor and sensory nerve organoids. We developed a new technology to isolate cell-free neurite bundles from spheroids. Transplantation therapy was carried out for large nerve defects in rat sciatic nerve with novel artificial nerve conduit filled with lineally assembled sets of human neurite bundles. Quantitative comparisons were performed over time to search for the artificial nerve with the therapeutic effect, evaluating the recovery of motor and sensory functions and histological regeneration. In addition, a multidimensional unbiased gene expression profiling was carried out by using next-generation sequencing.

RESULT

After transplantation, the neurite bundle-derived artificial nerves exerted significant therapeutic effects, both functionally and histologically. Remarkably, therapeutic efficacy was achieved without immunosuppression, even in xenotransplantation. Transplanted neurite bundles fully dissolved after several weeks, with no tumor formation or cell proliferation, confirming their biosafety. Posttransplant gene expression analysis highlighted the immune system's role in recovery.

CONCLUSION

The combination of newly developed microfluidic devices and iPSC technology enables the preparation of artificial nerves from organoid-derived neurite bundles in advance for future treatment of peripheral nerve injury patients. A promising, safe, and effective peripheral nerve treatment is now ready for clinical application.

Enhanced analgesic cholinergic tone in the spinal cord in a mouse model of neuropathic pain

Endogenous acetylcholine (ACh) is an important modulator of nociceptive sensory processing in the spinal cord. An increased level of spinal ACh induces analgesia both in humans and rodents while interfering with cholinergic signaling is allodynic, demonstrating that a basal tone of spinal ACh modulates nociceptive responses in naïve animals. The plasticity undergone by this cholinergic system in chronic pain situation is unknown, and the mere presence of this tone in neuropathic animals is controversial. We have addressed these issues in mice through behavioral experiments, histology, electrophysiology and molecular biology, in the cuff model of peripheral neuropathy. Our behavior experiments demonstrate the persistence, and even increased impact of the analgesic cholinergic tone acting through nicotinic receptors in cuff animals. The neuropathy does not affect the number or membrane properties of dorsal horn cholinergic neurons, nor specifically the frequency of their synaptic inputs. The alterations thus appear to be in the neurons receiving the cholinergic signaling, which is confirmed by the fact that subthreshold doses of acetylcholinesterase (AChE) inhibitors in sham animals become anti-allodynic in cuff mice and by the altered expression of the β2 nicotinic receptor subunit. Our results demonstrate that endogenous cholinergic signaling can be manipulated to relieve mechanical allodynia in animal models of peripheral neuropathy. Until now, AChE inhibitors have mainly been used in the clinics in situations of acute pain (parturition, post-operative). The fact that lower doses (thus with fewer side effects) could be efficient in chronic pain conditions opens new avenues for the treatment of neuropathic pain. SIGNIFICANCE STATEMENT: Chronic pain continues to be the most common cause of disability that impairs the quality of life, accruing enormous and escalating socio-economic costs. A better understanding of the plasticity of spinal neuronal networks, crucially involved in nociceptive processing, could help designing new therapeutic avenues. We here demonstrate that chronic pain modifies the spinal nociceptive network in such a way that it becomes more sensitive to cholinergic modulations. The spinal cholinergic system is responsible for an analgesic tone that can be exacerbated by acetylcholinesterase inhibitors, a property used in the clinic to relief acute pain (child birth, post-op). Our results suggest that lower doses of acetylcholinesterases, with even fewer side effects, could be efficient to relieve chronic pain.

Systematic Administration of B Vitamins Alleviates Diabetic Pain and Inhibits Associated Expression of P2X3 and TRPV1 in Dorsal Root Ganglion Neurons and Proinflammatory Cytokines in Spinal Cord in Rats

Background

Treatment of diabetic neuropathic pain (DNP) continues to be a major challenge, and underlying mechanisms of DNP remain elusive. We investigated treatment effects of B vitamins on DPN- and DNP-associated alterations of neurochemical signaling in the nociceptive dorsal root ganglion (DRG) neurons and the spinal cord in rats.

Methods

DNP was produced in male, adult, Sprague Dawley rats by single i.p. streptozotocin (STZ). Western blot analysis and immunohistochemistry were used to analyze protein expressions in DRG and ELISA to measure the proinflammatory cytokines in the spinal cord. Behaviorally expressed DNP was determined by measuring the sensitivity of hindpaw skin to mechanical and thermal stimulation.

Results

There were 87.5% (77/88) rats which developed high blood glucose within 1-2 weeks following STZ injection. Of which, 70.13% (n = 54/77) animals exhibited DNP manifested as mechanical allodynia and/or thermal hyperalgesia. Intraperitoneal administration of vitamins B1/B6/B12 (100/100/2 mg/kg, one or multiple doses) significantly attenuated DNP without affecting the blood glucose. Expressions of P2X3 and TRPV1 in CGRP-positive and IB4-positive DRG neurons as well as the interleukin-1β, tumor necrosis factor-α, and nerve growth factor in the lumbar spinal cord were greatly increased in DNP rats. Such DNP-associated neurochemical alterations were also greatly suppressed by the B-vitamin treatment.

Conclusions

B-vitamin treatment can greatly suppress chronic DNP and DNP-associated increased activities of P2X3 and TRPV1 in DRG and the spinal proinflammatory cytokines, which may contribute to the pathogenesis of DNP. Systematic administration of B vitamins can be a strategy for DNP management in clinic.

Activations of muscarinic M1 receptors in the anterior cingulate cortex contribute to the antinociceptive effect via GABAergic transmission

Background Cholinergic systems regulate the synaptic transmission resulting in the contribution of the nociceptive behaviors. Anterior cingulate cortex is a key cortical area to play roles in nociception and chronic pain. However, the effect of the activation of cholinergic system for nociception is still unknown in the cortical area. Here, we tested whether the activation of cholinergic receptors can regulate nociceptive behaviors in adult rat anterior cingulate cortex by integrative methods including behavior, immunohistochemical, and electrophysiological methods. Results We found that muscarinic M₁ receptors were clearly expressed in the anterior cingulate cortex. Using behavioral tests, we identified that microinjection of a selective muscarinic M₁ receptors agonist McN-A-343 into the anterior cingulate cortex dose dependently increased the mechanical threshold. In contrast, the local injection of McN-A-343 into the anterior cingulate cortex showed normal motor function. The microinjection of a selective M₁ receptors antagonist pirenzepine blocked the McN-A-343-induced antinociceptive effect. Pirenzepine alone into the anterior cingulate cortex decreased the mechanical thresholds. The local injection of the GABA_A receptors antagonist bicuculline into the anterior cingulate cortex also inhibited the McN-A-343-induced antinociceptive effect and decreased the mechanical threshold. Finally, we further tested whether the activation of M₁ receptors could regulate GABAergic transmission using whole-cell patch-clamp recordings. The activation of M₁ receptors enhanced the frequency of spontaneous and miniature inhibitory postsynaptic currents as well as the amplitude of spontaneous inhibitory postsynaptic currents in the anterior cingulate cortex. Conclusions These results suggest that the activation of muscarinic M₁ receptors in part increased the mechanical threshold by increasing GABAergic transmitter release and facilitating GABAergic transmission in the anterior cingulate cortex.

Noradrenergic inhibition of spinal hyperexcitation elicited by cutaneous cold stimuli in rats with oxaliplatin-induced allodynia: electrophysiological and behavioral assessments

We investigated the spinal action of noradrenaline on cold-elicited hyperexcitation detected in dorsal horn neurons of rats with allodynia induced by an oxaliplatin (6 mg/kg, i.p.) injection. In vivo extracellular recordings from the spinal dorsal horn showed that wide dynamic range neurons responded to cutaneous acetone (10 μl) stimulation in normal rats, and cold-elicited firings in oxaliplatin-administered rats were increased with a longer duration, correlated with behavioral responses. These responses were significantly attenuated by spinal administration (50 μM) of noradrenaline or its agonists, clonidine (α2), phenylephrine (α1) and isoprenaline (β), in descending order of efficacy. Thus, the inhibitory effect of noradrenaline on spinal oxaliplatin-induced cold hyperexcitation is mediated mainly by activation of α2- and/or α1-adrenoceptors.

[Easing human pain: challenges to a novel therapeutic drug in neuropathic pain]

Pain control is very important since, even today, 20 million people suffer from neuropathic pain. Although many basic science and clinical researchers have made efforts to control pain, the mechanism of neuropathic pain is unfortunately still not fully understood. Morphine, a prototypical opioid, is a useful medicine to relieve severe pain. However, repeated or continuous use of morphine and other opioids are associated with a potential risk of analgesic tolerance, which requires an increase in dosage to maintain the same efficacy. In addition, morphine is not always effective in neuropathic pain. In this review we focus on: (1) the role of muscarinic receptors in the spinal cord and anterior cingulate cortex (ACC) in neuropathic pain, (2) how chemokine (C-C motif) ligand 1 (CCL-1) is involved in neuropathic pain, and (3) the novel mechanism of morphine tolerance. The findings in this study may cast new light on novel mechanism of neuropathic pain and development of novel clinical medicines in the future.

Mechanisms of electroacupuncture-induced analgesia on neuropathic pain in animal model

Neuropathic pain remains as one of the most difficult clinical pain syndromes to treat. Electroacupuncture (EA), involving endogenous opioids and neurotransmitters in the central nervous system (CNS), is reported to be clinically efficacious in various fields of pain. Although multiple experimental articles were conducted to assess the effect of EA-induced analgesia, no review has been published to assess the efficacy and clarify the mechanism of EA on neuropathic pain. To this aim, this study was firstly designed to evaluate the EA-induced analgesic effect on neuropathic pain and secondly to guide and help future efforts to advance the neuropathic pain treatment. For this purpose, articles referring to the analgesic effect of acupuncture on neuropathic pain and particularly the work performed in our own laboratory were analyzed. Based on the articles reviewed, the role of spinal opioidergic, adrenergic, serotonergic, cholinergic, and GABAergic receptors in the mechanism of EA-induced analgesia was studied. The results of this research demonstrate that μ and δ opioid receptors, α₂-adrenoreceptors, 5-HT_1A and 5-HT₃ serotonergic receptors, M₁ muscarinic receptors, and GABA_A and GABA_B GABAergic receptors are involved in the mechanisms of EA-induced analgesia on neuropathic pain.

CCL-1 in the spinal cord contributes to neuropathic pain induced by nerve injury

Cytokines such as interleukins are known to be involved in the development of neuropathic pain through activation of neuroglia. However, the role of chemokine (C-C motif) ligand 1 (CCL-1), a well-characterized chemokine secreted by activated T cells, in the nociceptive transmission remains unclear. We found that CCL-1 was upregulated in the spinal dorsal horn after partial sciatic nerve ligation. Therefore, we examined actions of recombinant CCL-1 on behavioural pain score, synaptic transmission, glial cell function and cytokine production in the spinal dorsal horn. Here we show that CCL-1 is one of the key mediators involved in the development of neuropathic pain. Expression of CCL-1 mRNA was mainly detected in the ipsilateral dorsal root ganglion, and the expression of specific CCL-1 receptor CCR-8 was upregulated in the superficial dorsal horn. Increased expression of CCR-8 was observed not only in neurons but also in microglia and astrocytes in the ipsilateral side. Recombinant CCL-1 injected intrathecally (i.t.) to naive mice induced allodynia, which was prevented by the supplemental addition of N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801. Patch-clamp recordings from spinal cord slices revealed that application of CCL-1 transiently enhanced excitatory synaptic transmission in the substantia gelatinosa (lamina II). In the long term, i.t. injection of CCL-1 induced phosphorylation of NMDA receptor subunit, NR1 and NR2B, in the spinal cord. Injection of CCL-1 also upregulated mRNA level of glial cell markers and proinflammatory cytokines (IL-1β, TNF-α and IL-6). The tactile allodynia induced by nerve ligation was attenuated by prophylactic and chronic administration of neutralizing antibody against CCL-1 and by knocking down of CCR-8. Our results indicate that CCL-1 is one of the key molecules in pathogenesis, and CCL-1/CCR-8 signaling system can be a potential target for drug development in the treatment for neuropathic pain.

Convolutamydine A and synthetic analogues have antinociceptive properties in mice

Convolutamydine A, an oxindole that originated from a marine bryozoan, has several biological effects. In this study, we aimed to investigate the antinociceptive effects of convolutamydine A and two new synthetic analogues. Convolutamydine A and the two analogues were given orally to assess their ability to induce antinociceptive effects. Formalin-induced licking response, acetic acid-induced contortions, and hot plate models were used to characterize the effects of convolutamydine A and its analogues. Convolutamydine A (4,6-bromo-3-(2-oxopropyl)-3-hydroxy-2-oxindole), compound 1 (3-(2-oxopropyl)-3-hydroxy-2-oxindole), and compound 2 (5-bromo-3-(2-oxopropyl)-3-hydroxy-2-oxindole) caused peripheral antinociceptive and anti-inflammatory effects in the acetic acid-induced contortions and the formalin-induced licking models. Supraspinal effects were also observed in the hot plate model and were similar to those obtained with morphine. The peripheral effects were not mediated by the cholinergic or opioid systems. The antinociceptive effects of convolutamydine A seem to be mediated by all three systems (cholinergic, opioid, and nitric oxide systems), and the mechanism of action of compounds 1 and 2 involved cholinergic and nitric oxide-mediated mechanisms. Convolutamydine A and its analogues (compounds 1 and 2) showed good antinociceptive ability after systemic administration in acute pain models. The antinociceptive action mediated by cholinergic, opioid, and nitric oxide systems could explain why convolutamydine A, compound 1, and compound 2 retained their antinociceptive effects. The doses used were similar to the doses of morphine and were much lower than that of acetylsalicylic acid, the classical analgesic and anti-inflammatory drug. In conclusion, convolutamydine A and the two analogues demonstrated antinociceptive effects comparable to morphine's effects.

Phenotypes of pain behavior in phospholipase C-related but catalytically inactive protein type 1 knockout mice

Phospholipase C-related inactive protein (PRIP) plays important roles in trafficking to the plasma membrane of GABA(A) receptor, which is involved in the dominant inhibitory neurotransmission in the spinal cord and plays an important role in nociceptive transmission. However, the role of PRIP in pain sensation remains unknown. In this study, we investigated the phenotypes of pain behaviors in PRIP type 1 knockout (PRIP-1 (-/-)) mice. The mutant mice showed hyperalgesic responses in the second phase of the formalin test and the von Frey test as compared with those in wild-type mice. In situ hybridization studies of GABA(A) receptors revealed significantly decreased expression of γ2 subunit mRNA in the dorsal and ventral horns of the spinal cord in PRIP-1 (-/-) mice, but no difference in α1 subunit mRNA expression. β2 subunit mRNA expression was significantly higher in PRIP-1 (-/-) mice than in wild-type mice in all areas of the spinal cord. On the other hand, the slow decay time constant for the spontaneous inhibitory current was significantly increased by treatment with diazepam in wild-type mice, but not in PRIP-1 (-/-) mice. These results suggest that PRIP-1 (-/-) mice exhibit the changes of the function and subunits expression of GABA(A) receptor in the spinal cord, which may be responsible for abnormal pain sensation in these mice.

Antagonism by haloperidol and its metabolites of mechanical hypersensitivity induced by intraplantar capsaicin in mice: role of sigma-1 receptors

RATIONALE

We evaluated the effects of haloperidol and its metabolites on capsaicin-induced mechanical hypersensitivity (allodynia) and on nociceptive pain induced by punctate mechanical stimuli in mice.

RESULTS

Subcutaneous administration of haloperidol or its metabolites I or II (reduced haloperidol) dose-dependently reversed capsaicin-induced (1 microg, intraplantar) mechanical hypersensitivity of the hind paw (stimulated with a nonpainful, 0.5-g force, punctate stimulus). The order of potency of these drugs to induce antiallodynic effects was the order of their affinity for brain sigma-1 (sigma(1)) receptor ([(3)H](+)-pentazocine-labeled). Antiallodynic activity of haloperidol and its metabolites was dose-dependently prevented by the selective sigma(1) receptor agonist PRE-084, but not by naloxone. These results suggest the involvement of sigma(1) receptors, but discard any role of the endogenous opioid system, on the antiallodynic effects. Dopamine receptor antagonism also appears unlikely to be involved in these effects, since the D(2)/D(3) receptor antagonist (-)-sulpiride, which had no affinity for sigma(1) receptors, showed no antiallodynic effect. None of these drugs modified hind-paw withdrawal after a painful (4 g force) punctate mechanical stimulus in noncapsaicin-sensitized animals. As expected, the control drug gabapentin showed antiallodynic but not antinociceptive activity, whereas clonidine exhibited both activities and rofecoxib, used as negative control, showed neither.

CONCLUSION

These results show that haloperidol and its metabolites I and II produce antiallodynic but not antinociceptive effects against punctate mechanical stimuli and suggest that their antiallodynic effect may be due to blockade of sigma(1) receptors but not to dopamine receptor antagonism.

Spinal cholinergic mechanism of the relieving effects of electroacupuncture on cold and warm allodynia in a rat model of neuropathic pain

This study was performed to determine whether spinal cholinergic systems mediate the relieving effects of electroacupuncture (EA) on cold and warm allodynia in a rat model of neuropathic pain. For neuropathic surgery, the right superior caudal trunk was resected at the level between the S1 and S2 spinal nerves innervating the tail. Two weeks after the injury, the intrathecal (i.t.) catheter was implanted. Five days after the catheterization, the rats were injected with atropine (non-selective muscarinic antagonist, 30 microg), mecamylamine (non-selective nicotinic antagonist, 50 microg), pirenzepine (M(1) muscarinic antagonist, 10 microg), methoctramine (M(2) antagonist, 10 microg) or 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) (M(3) antagonist, 10 microg). Ten minutes after the injection, EA was applied to the ST36 acupoint for 30 min. The cold and warm allodynia were assessed by the tail immersion test [i.e., immersing the tail in cold (4 degrees C) or warm (40 degrees C) water and measuring the latency of an abrupt tail movement] before and after the treatments. The i.t. atropine, but not mecamylamine, blocked the relieving effects of EA on cold and warm allodynia. Furthermore, i.t. pirenzepine attenuated the antiallodynic effects of EA, whereas methoctramine and 4-DAMP did not. These results suggest that spinal muscarinic receptors, especially M(1) subtype, mediate the EA-induced antiallodynia in neuropathic rats.

Modulation of P2X receptors in dorsal root ganglion neurons of streptozotocin-induced diabetic neuropathy

Painful diabetic neuropathy causes hyperalgesia and does not respond to commonly used analgesics such as non-steroidal anti-inflammatory drugs or opioids at doses below those producing disruptive side effects. In the present study, we examined the effect of P2X receptor antagonists, which are known to modulate the pain pathway, on mechanical hyperalgesia in streptozotocin (STZ)-induced diabetic mice. The paw withdrawal frequency measured by von Frey filaments, began to significantly increase 5 days after STZ injection and was maintained for more than 14 days. Intrathecal administration of P2X receptor antagonists (PPADS and TNP-ATP) inhibited the mechanical allodynia in diabetic mice. The levels of P2X(2) and P2X(3) receptors mRNA were significantly increased in diabetic mice at 14 days after the intravenous injection of STZ. These results suggest that the upregulation of P2X(2), P2X(3) and/or P2X(2/3) receptor in DRG neurons is associated with mechanical allodynia in STZ-induced diabetic mice.

Cholinergic mechanisms involved in the pain relieving effect of spinal cord stimulation in a model of neuropathy

The mechanisms underlying the pain relieving effect of spinal cord stimulation (SCS) on neuropathic pain remain unclear. We have previously demonstrated that suppression of tactile hypersensitivity produced by SCS may be potentiated by i.t. clonidine in a rat model of mononeuropathy. Since the analgesic effect of this drug is mediated mainly via cholinergic mechanisms, a study exploring the possible involvement of the spinal cholinergic system in SCS was undertaken. The effect of SCS was assessed with von Frey filaments in rats displaying tactile hypersensitivity after partial ligation of the sciatic nerve and both SCS-responding and non-responding as well as normal rats were subjected to microdialysis in the dorsal horn. Acetylcholine (ACh) was analyzed with HPLC before, during and after SCS. SCS produced significantly increased release of ACh in the dorsal horn in rats responding to SCS whereas the release was unaffected in the non-responding animals. Furthermore, the basal release of ACh was significantly lower in nerve lesioned than in normal rats. In another group of rats it was found that the response to SCS was completely eliminated by i.t. atropine and a muscarinic M(4) receptor antagonist while a partial attenuation was produced by M(1) and M(2) antagonists. Blocking of nicotinic receptors did not influence the SCS effect. In conclusion, the attenuating effect of SCS on pain related behavior is associated with the activation of the cholinergic system in the dorsal horn and mediated via muscarinic receptors, particularly M(4,) while nicotinic receptors appear not to be involved.

Antinociceptive Synergistic Interaction Between Clonidine and Ouabain on Thermal Nociceptive Tests in the Rat

The antinociceptive effect produced by spinal injection of clonidine (an alpha(2)-adrenergic agonist) is mediated by a cholinergic mechanism. We aimed in the current study to evaluate the antinociceptive interaction between intrathecally administered ouabain, an inhibitor of Na(+), K(+)-ATPase, and clonidine. We used rats chronically implanted with lumbar intrathecal catheters to examine the ability of intrathecal clonidine and ouabain and the mixtures of clonidine-ouabain to alter tail-flick latency. To characterize the interaction, isobolographic analysis was performed. Intrathecal clonidine (0.5-10 microg) and ouabain (0.1-5 microg) produced significant dose- and time-dependent antinociception in the tail-flick tests. The median effective dose (ED(50)) values for intrathecally administered ouabain and clonidine were 2.3 microg and 4.7 microg, respectively. The experimental point for the ouabain-clonidine combination decreased significantly (P < .05) below the lines of additivity. Isobolographic analysis exhibited a synergistic interaction after the coadministration of ouabain and clonidine. No motor impairment was observed in the animals after intrathecal administration of the combination of ouabain and clonidine or clonidine alone. Intrathecal pretreatment with atropine but not yohimbine blocked the antinociceptive effect of ouabain and attenuated its interaction with spinal clonidine. These results suggest that the synergistic interaction of ouabain and clonidine were probably mediated, at least in part, via an enhancement of cholinergic transmission in the spinal nociceptive processing system.

PERSPECTIVE

Although intrathecal clonidine produces pronounced analgesia, antinociceptive doses of intrathecal clonidine produce several side effects, including hypotension, bradycardia, and sedation. This article presents antinociceptive synergistic interaction between clonidine and ouabain on thermal nociceptive tests in the rat.

The anti-inflammatory effect of peripheral bee venom stimulation is mediated by central muscarinic type 2 receptors and activation of sympathetic preganglionic neurons

The anti-inflammatory effect (AI) induced by peripheral injection of diluted bee venom (dBV) involves activation of spinal cord circuits and is mediated by catecholamine release from adrenal medulla, but the precise neuronal mechanisms involved are not fully understood. In a recent study, we demonstrated that an increase in spinal acetylcholine is involved in mediating the anti-inflammatory effect of dBV and that this mediation also involves adrenomedullary activation. The present study utilized the mouse air pouch inflammation model to evaluate the involvement of spinal acetylcholine receptors and sympathetic preganglionic neurons (SPNs) in dBV's anti-inflammatory effect (dBVAI). Intrathecal (IT) pretreatment with atropine (muscarinic cholinergic antagonist) but not hexamethonium (nicotinic cholinergic antagonist) significantly suppressed dBVAI on zymosan-evoked leukocyte migration. Subsequent experiments showed that IT pretreatment with methoctramine (a muscarinic receptor type 2; M(2) antagonist), but not pirenzepine (an M(1) antagonist) or 4-DAMP (an M(3) antagonist), suppressed the dBVAI. In addition, dBV stimulation specifically increased Fos expression in SPNs of the T7-T11, but not the T1-T6 or T12-L2 spinal cord segments, in animals with zymosan-induced inflammation. Moreover, IT methoctramine pretreatment suppressed this dBV-induced Fos expression specifically in SPNs of T7-T11 level. Peripheral sympathetic denervation using 6-hydroxydopamine (6-OHDA) treatment (which spares sympathetic adrenal medullary innervation) did not alter dBVAI. Collectively these results indicate that dBV stimulation leads to spinal cord acetylcholine release that in turn acts on spinal M(2) receptors, which via a hypothesized disinhibition mechanism activates SPNs that project to the adrenal medulla. This activation ultimately leads to the release of adrenal catecholamines that contribute to dBVAI.

The spinal muscarinic receptor subtypes contribute to the morphine-induced antinociceptive effects in thermal stimulation in mice

The present study was undertaken to clarify how spinal muscarinic receptors can be involved in the antinociceptive effects induced by morphine in thermal stimulation. The morphine-induced antinociceptive effects (26.6 micromol/kg, s.c.) was inhibited by an intrathecal (i.t.) injection of the muscarinic antagonist (M) atropine and the M(1)/M(4) antagonist pirenzepine in a dose-dependent manner. In contrast, the M(2) antagonist methoctramine and the M(3) antagonist 4-DAMP did not inhibit the morphine-induced antinociceptive effects. Injection (i.t.) of the putative M(1) agonist McN-A-343 resulted in dose-dependent antinociceptive effects in thermal stimuli. In addition, antinociceptive effects induced by the i.t. injection of morphine were not inhibited by the M(1)/M(4) antagonist pirenzepine, although pirenzepine did inhibit the intracerebroventricular (i.c.v.) injection of morphine-induced antinociceptive effects. These results suggest that the morphine-induced antinociceptive effects in thermal stimuli are regulated by the M(1) or M(4) receptor in the spinal cord.

Stability of admixture containing morphine sulfate, bupivacaine hydrochloride, and clonidine hydrochloride in an implantable infusion system

Intrathecal infusion is often performed using drug combinations. This study was conducted to evaluate the stability of the admixture of morphine sulfate, bupivacaine hydrochloride, and clonidine hydrochloride when used in an implantable pump under simulated clinical use conditions. SynchroMed implantable pumps were filled with an admixture and incubated at 37 degrees C for a period of 90 days. Drug admixture stored in glass vials at 4 degrees C and at 37 degrees C served as controls. Samples which included pump reservoir and catheter delivered aliquots were collected every 30 days and analyzed for drug concentrations using a stability-indicating HPLC method. All drugs contained in the admixture were stable and the original concentrations remained greater than 96%. Over 90 days, and with the pump at the simulated body temperature of 37 degrees C, there were no evident heat catalyzed or device catalyzed reactions.

Intrathecal clonidine inhibits mechanical allodynia via activation of the spinal muscarinic M1 receptor in streptozotocin-induced diabetic mice

We examined the involvement of the spinal muscarinic receptors in the clonidine-induced antiallodynic effects. Mechanical sensitivity was assessed by stimulating the hind paw with von Frey filaments. In streptozotocin-treated (200 mg/kg, i.v.) diabetic mice, hypersensitivity to mechanical stimulation appeared 3 days after streptozotocin administration, and persisted for 11 days. This mechanical hypersensitivity (allodynia) was inhibited by the intrathecal (i.t.) injection of clonidine. The muscarinic receptor antagonist atropine (i.t.) and alpha2-adrenoreceptor antagonist yohimbine (i.t. or subcutaneous injection) abolished the antiallodynic effect of clonidine. The effect was mimicked by the muscarinic M1 receptor antagonist pirenzepine, but not by the muscarinic M2 receptor antagonist methoctoramine or the muscarinic M3 receptor antagonist 4-DAMP (4-diphenyl-acetoxy-N-methylpiperidine methiodide). In addition, the mechanical hypersensitivity in diabetic mice was reduced by the selective muscarinic M1 receptor agonist McN-A-343 (4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium chloride) (i.t.). These results suggest that spinal muscarinic M1 receptors participate in the antiallodynic effect of clonidine in diabetic mice.

The Effects of the alpha2-Adrenergic Receptor Agonists Clonidine and Rilmenidine, and Antagonists Yohimbine and Efaroxan, on the Spinal Cholinergic Receptor System in the Rat

Cholinergic agonists produce spinal antinociception via mechanisms involving an increased release of intraspinal acetylcholine. The cholinergic receptor system interacts with several other receptor types, such as alpha2-adrenergic receptors. To fully understand these interactions, the effects of various receptor ligands on the cholinergic system must be investigated in detail. This study was initiated to investigate the effects of the alpha2-adrenergic receptor agonists clonidine and rilmenidine and the alpha2-adrenergic receptor antagonists yohimbine and efaroxan on spinal cholinergic receptors in the rat. Spinal microdialysis was used to measure in vivo changes of acetylcholine after administration of the ligands, with or without nicotinic receptor blockade. In addition, in vitro binding properties of the ligands on muscarinic and nicotinic receptors were investigated. It was found that clonidine and rilmenidine increased, while yohimbine decreased spinal acetylcholine release. Efaroxan affected acetylcholine release differently depending on concentration. Nicotinic receptor blockade attenuated the effect of all ligands. All ligands showed poor binding affinity for muscarinic receptors. On the other hand, all ligands possessed affinity for nicotinic receptors. Clonidine and yohimbine binding was best fit to a one site binding curve and rilmenidine and efaroxan to a two site binding curve. The present study demonstrates that the tested alpha2-adrenergic receptor ligands affect intraspinal acetylcholine release in the rat evoked by nicotinic receptor mechanisms in vivo, and that they possess binding affinity to nicotinic receptors in vitro. The binding of alpha2-adrenergic receptor ligands to nicotinic receptors might affect the intraspinal release of acetylcholine.

Spinal muscarinic receptors are activated during low or high frequency TENS-induced antihyperalgesia in rats

Transcutaneous electrical nerve stimulation (TENS) is a non-pharmacological modality used clinically to relieve pain. Central involvement of serotonin and endogenous opioids are implicated in TENS-induced analgesia. Activation of spinal cholinergic receptors is antinociceptive and these receptors interact with opioid and serotonin receptors. In the current study, the possible involvement of spinal cholinergic receptors in TENS analgesia was investigated in rats. Hyperalgesia was induced by inflaming one knee joint with 3% kaolin-carrageenan and assessed by measuring paw withdrawal latency (PWL) to heat before and 4 h after injection. The non-selective nicotinic antagonist mecamylamine (50 microg), non-selective muscarinic antagonist atropine (30 microg) or one of the muscarinic subtype antagonists: pirenzepine (M1, 10 microg), methoctramine (M2, 10 microg), 4-DAMP (M3, 10 microg), or saline was administered intrathecally just prior to TENS treatment. Low or high frequency TENS was then applied to the inflamed knee and PWL was determined again. Atropine, pirenzepine and 4-DAMP significantly attenuated the antihyperalgesic effects of low and high frequency TENS while mecamylamine and methoctramine had no effects, compared to saline control. The results show that TENS-induced antihyperalgesia is mediated partially by activation of spinal muscarinic receptors but not spinal nicotinic receptors. Further, the results also indicate that spinal M1 and M3 muscarinic receptor subtypes mediate the muscarinic component of TENS antihyperalgesia.

The role of spinal muscarinic acetylcholine receptors in clonidine-induced anti-nociceptive effects in rats

We have examined the effects of intrathecal (i.t.) injection of the muscarinic acetylcholine receptor antagonist atropine on the clonidine-induced nociceptive effect in formalin-induced nociception in rats. The injection of 5% formalin into the hind paw caused biphasic nociceptive responses, and i.t. injection of clonidine inhibited both phases of the nociceptive response in a dose-dependent manner. Pretreatment with atropine (i.t.) only partially inhibited the nociceptive effect of clonidine. These results suggest that the nociceptive effect of clonidine in the rat formalin model may be at least partly mediated by muscarinic acetylcholine receptors in the spinal cord.