The antinociceptive effect of intrathecal administration of epibatidine with clonidine or neostigmine in the formalin test in rats

Plasticity in ventral pallidal cholinergic neuron-derived circuits contributes to comorbid chronic pain-like and depression-like behaviour in male mice

Nucleus- and cell-specific interrogation of individual basal forebrain (BF) cholinergic circuits is crucial for refining targets to treat comorbid chronic pain-like and depression-like behaviour. As the ventral pallidum (VP) in the BF regulates pain perception and emotions, we aim to address the role of VP-derived cholinergic circuits in hyperalgesia and depression-like behaviour in chronic pain mouse model. In male mice, VP cholinergic neurons innervate local non-cholinergic neurons and modulate downstream basolateral amygdala (BLA) neurons through nicotinic acetylcholine receptors. These cholinergic circuits are mobilized by pain-like stimuli and become hyperactive during persistent pain. Acute stimulation of VP cholinergic neurons and the VP-BLA cholinergic projection reduces pain threshold in naïve mice whereas inhibition of the circuits elevated pain threshold in pain-like states. Multi-day repetitive modulation of the VP-BLA cholinergic pathway regulates depression-like behaviour in persistent pain. Therefore, VP-derived cholinergic circuits are implicated in comorbid hyperalgesia and depression-like behaviour in chronic pain mouse model.

The nAChR Chaperone TMEM35a (NACHO) Contributes to the Development of Hyperalgesia in Mice

Pain is a major health problem, affecting over fifty million adults in the US alone, with significant economic cost in medical care and lost productivity. Despite evidence implicating nicotinic acetylcholine receptors (nAChRs) in pathological pain, their specific contribution to pain processing in the spinal cord remains unclear given their presence in both neuronal and non-neuronal cell types. Here we investigated if loss of neuronal-specific TMEM35a (NACHO), a novel chaperone for functional expression of the homomeric α7 and assembly of the heteromeric α3, α4, and α6-containing nAChRs, modulates pain in mice. Mice with tmem35a deletion exhibited thermal hyperalgesia and mechanical allodynia. Intrathecal administration of nicotine and the α7-specific agonist, PHA543613, produced analgesic responses to noxious heat and mechanical stimuli in tmem35a KO mice, respectively, suggesting residual expression of these receptors or off-target effects. Since NACHO is expressed only in neurons, these findings indicate that neuronal α7 nAChR in the spinal cord contributes to heat nociception. To further determine the molecular basis underlying the pain phenotype, we analyzed the spinal cord transcriptome. Compared to WT control, the spinal cord of tmem35a KO mice exhibited 72 differentially-expressed genes (DEGs). These DEGs were mapped onto functional gene networks using the knowledge-based database, Ingenuity Pathway Analysis, and suggests increased neuroinflammation as a potential contributing factor for the hyperalgesia in tmem35a KO mice. Collectively, these findings implicate a heightened inflammatory response in the absence of neuronal NACHO activity. Additional studies are needed to determine the precise mechanism by which NACHO in the spinal cord modulates pain.

Antinociceptive effect of tebanicline for various noxious stimuli-induced behaviours in mice

Tebanicline (ABT-594), an analogue of epibatidine, exhibits potent antinociceptive effects and high affinity for the nicotinic acetylcholine receptor in the central nervous system. We assessed whether tebanicline exerts an effect on various noxious stimuli and mediates the nicotine receptor or opioid receptor through stimulation. The antinociceptive effects of tebanicline were determined by noxious chemical, thermal and mechanical stimuli-induced behaviours in mice. Tebanicline had dose-dependent analgesic effects in formalin, hot-plate and tail-pressure tests. By contrast, the antinociceptive effect of tebanicline was not demonstrated in the tail-flick assay. Pre-treatment with mecamylamine, a nicotinic acetylcholine receptor antagonist, blocked the effects of tebanicline in formalin, tail-pressure and hot-plate tests. Moreover, pre-treatment with naloxone, an opioid receptor antagonist, only partially inhibited the effects of tebanicline in formalin and tail-pressure tests. Tebanicline produced antinociception in persistent chemical (formalin), acute thermal (hot-plate, but not tail-flick) and mechanical (tail-pressure) pain states. Moreover, tebanicline stimulated the nicotinic acetylcholine receptor and opioid receptor.

Neuronal nicotinic receptors as analgesic targets: it's a winding road

Along with their well known role in nicotine addiction and autonomic physiology, neuronal nicotinic receptors (nAChRs) also have profound analgesic effects in animal models and humans. This is not a new idea, even in the early 1500s, soon after tobacco was introduced to the new world, its proponents listed pain relief among the beneficial properties of smoking. In recent years, analgesics that target specific nAChR subtypes have shown highly efficacious antinociceptive properties in acute and chronic pain models. To date, the side effects of these drugs have precluded their advancement to the clinic. This review summarizes the recent efforts to identify novel analgesics that target nAChRs, and outlines some of the key neural substrates that contribute to these physiological effects. There remain many unanswered mechanistic questions in this field, and there are still compelling reasons to explore neuronal nAChRs as targets for the relief of pain.

Combination Drug Therapy for Pain following Chronic Spinal Cord Injury

A number of mechanisms have been elucidated that maintain neuropathic pain due to spinal cord injury (SCI). While target-based therapeutics are being developed based on elucidation of these mechanisms, treatment for neuropathic SCI pain has not been entirely satisfactory due in part to the significant convergence of neurological and inflammatory processes that maintain the neuropathic pain state. Thus, a combination drug treatment strategy, wherein several pain-related mechanism are simultaneously engaged, could be more efficacious than treatment against individual mechanisms alone. Also, by engaging several targets at once, it may be possible to reduce the doses of the individual drugs, thereby minimizing the potential for adverse side effects. Positive preclinical and clinical studies have demonstrated improved efficacy of combination drug treatment over single drug treatment in neuropathic pain of peripheral origin, and perhaps such combinations could be utilized for neuropathic SCI pain. At the same time, there are mechanisms that distinguish SCI from peripheral neuropathic pain, so novel combination therapies will be needed.

MD-354 selectively antagonizes the antinociceptive effects of (-)nicotine in the mouse tail-flick assay

RATIONALE

(-)Nicotine produces antinociceptive effects in rodents. meta-Chlorophenylguanidine (MD-354), an analgesia-enhancing agent, binds at 5-HT(3) and alpha(2)-adrenoceptors and potentiates the antinociceptive effects of an "inactive" dose of clonidine. The present study examined the actions of MD-354 on (-)nicotine-induced antinociception.

MATERIALS AND METHODS

Mouse tail-flick and other assays were employed.

RESULTS

In the tail-flick assay, (-)nicotine (ED(50) = 1.66 mg/kg) but not MD-354 produced dose-related antinociceptive effects. Administered in combination with (-)nicotine (2.5 mg/kg), MD-354 (AD(50) = 3.4 mg/kg) did not potentiate, but effectively antagonized the antinociceptive actions of (-)nicotine. In a mouse hot-plate assay, MD-354 failed to modify (-)nicotine responses. In combination with a locomotor activity-suppressing dose of (-)nicotine, MD-354 (up to 17 mg/kg) failed to antagonize (-)nicotine-induced hypolocomotion. In a rat drug discrimination paradigm using (-)nicotine as training drug, MD-354 produced saline-appropriate responding; in combination with the training dose of (-)nicotine, MD-354 failed to antagonize the nicotine cue.

CONCLUSIONS

MD-354 selectively antagonizes the antinociceptive actions of (-)nicotine in the tail-flick, but not in the hot-plate assay, or either the motor effects, or discriminative stimulus effects of (-)nicotine. The most parsimonious explanation is that MD-354 might act as a negative allosteric modulator of alpha 7 nACh receptors, and radioligand binding and functional data are provided to support this conclusion.

Antinociceptive mechanism of the spirocyclopiperazinium compound LXM-10 in mice and rats

Drugs typically used to treat pain are limited by their undesirable side effects, which has prompted a search for mechanistically different analgesic agents. We report the antinociception effect of the spirocyclopiperazinium compound LXM-10 via activation of peripheral alpha7 nicotinic and muscarinic acetylcholine receptors in mice. This effect was attenuated by hexamethonium, atropine methylnitrate, methyllycaconitine citrate, tropicamide, bicuculline, and phaclofen. Competition receptor-binding assays in vitro showed that LXM-10 binds with high affinity alpha7 nAchR and with low affinity M4 receptors. Our findings show that the antinociception signaling pathway of LXM-10 underlies activation of peripheral alpha7 nicotinic and possibly of M4 muscarinic receptors, which activate GABA(A) and GABA(B) receptors, resulting in antinociceptive effects without obvious side effects.

Cannabinoid receptor-mediated antinociception with acetaminophen drug combinations in rats with neuropathic spinal cord injury pain

Pre-clinical evidence demonstrates that neuropathic spinal cord injury (SCI) pain is maintained by a number of neurobiological mechanisms, suggesting that treatments directed at several pain-related targets may be more advantageous compared to a treatment focused on a single target. The current study evaluated the efficacy of the non-opiate analgesic acetaminophen, which has several putative analgesic mechanisms, combined with analgesic drugs used to treat neuropathic pain in a rat model of below-level neuropathic SCI pain. Following an acute compression of the mid-thoracic spinal cord, rats exhibited robust hind paw hypersensitivity to innocuous mechanical stimulation. Fifty percent antinociceptive doses of gabapentin, morphine, tramadol or memantine were combined with an ineffective dose of acetaminophen; acetaminophen alone was not antinociceptive. The combination of acetaminophen with either tramadol or memantine resulted in an additive antinociceptive effect. Acetaminophen combined with either morphine or gabapentin, however, resulted in supra-additive (synergistic) efficacy. One of the analgesic mechanisms of acetaminophen is inhibiting the uptake of endocannabinoids from the extracellular space. Pre-treatment with AM251, a cannabinoid-1 receptor (CB1) antagonist, significantly diminished the antinociceptive effect of the acetaminophen + gabapentin combination. Pre-treatment with AM630, a cannabinoid-2 receptor (CB2) antagonist, did not have an effect on this combination. By contrast, both AM251 and AM630 reduced the efficacy of the acetaminophen + morphine combination. None of the active drugs alone were affected by either CB receptor antagonist. The results imply that modulation of the endocannabinoid system in addition to other mechanisms mediate the synergistic antinociceptive effects of acetaminophen combinations. Despite the presence of a cannabinoid mechanism, synergism was not present in all acetaminophen combinations. The combination of currently available drugs may be an appropriate option in ameliorating neuropathic SCI pain if single drug therapy is ineffective.

Application of nicotine enantiomers, derivatives and analogues in therapy of neurodegenerative disorders

This review gives a brief overview over the major aspects of application of the nicotine alkaloid and its close derivatives in the therapy of some neurodegenerative disorders and diseases (e.g. Alzheimer's disease, Parkinson's disease, Tourette's syndrome, schizophrenia etc.). The issues concerning methods of nicotine analysis and isolation, and some molecular aspects of nicotine pharmacology are included. The natural and synthetic analogues of nicotine that are considered for medical practice are also mentioned. The molecular properties of two naturally occurring nicotine enantiomers are compared--the less-common but less-toxic (R)-nicotine is suggested as a natural compound that may find its place in pharmaceutical practice.

Intrathecal analgesic drug therapy

Intrathecal analgesic infusion therapy through an implantable pump system is a sophisticated medical therapy for those who suffer from chronic, severe pain who are unresponsive to traditional medical therapy. This article describes the patient selection and monitoring process and reviews available intrathecal medications and combinations.

Enhanced antinociception by nicotinic receptor agonist epibatidine and adrenal medullary transplants in the spinal subarachnoid space

Adrenal medullary transplants in the spinal subarachnoid space can reduce nociception, via the release of catecholamines and other analgesic substances, and this may be enhanced by stimulation of transplanted chromaffin cell surface nicotinic acetylcholine receptors (nAChRs). In addition, spinal nAChRs have been implicated in modulating nociception and can interact synergistically with alpha-adrenergic agents. Thus, enhanced antinociception by potent nAChR agonists such as frog skin derivative epibatidine in adrenal-transplanted animals could potentially occur via multiple mechanisms, including nicotinic-alpha-adrenergic synergy and stimulation of chromaffin cell nicotinic receptors. In order to test this, male Sprague-Dawley rats were implanted with intrathecal catheters and either adrenal medullary or control striated muscle transplants in the spinal subarachnoid space at the lumbar enlargement. Animals were tested for nociceptive responses before and after intrathecal injection of several doses of epibatidine using acute analgesiometric tests (tail flick, paw pressure) and the formalin test. After adrenal medullary, but not control, transplantation, nociceptive thresholds to acute noxious stimuli were slightly but consistently elevated, and phase 2 formalin responses decreased. Following intrathecal injection of epibatidine, acute nociceptive response latencies were modestly elevated and phase 2 formalin flinches modestly suppressed in control animals, but only at the highest dose test, with some attendant motor side-effects. In contrast, in adrenal medullary-transplanted animals, epibatidine elevated responses to acute noxious stimuli and markedly suppressed phase 2 formalin responses in a dose-related fashion. The enhanced antinociceptive effect following epibatidine was attenuated with either nAChR antagonist mecamylamine or alpha-adrenergic receptor antagonist phentolamine. The current results demonstrate that intrathecal injection of the nAChR ligand epibatidine can produce significant antinociception in adrenal-transplanted rats in both acute and tonic nociceptive tests and suggest that the use of nicotinic agents in combination with adrenal medullary transplantation could provide maximal therapeutic benefit by synergistically improving antinociception while avoiding the detrimental side-effects of these agents.

(±) Epibatidine Increases Acetylcholine Release Partly through an Action on Muscarinic Receptors

Epibatidine has been used in a wide dose range and was found to produce both nociceptive and antinociceptive effects. The different effects were partly explained by an action on multiple nicotinic receptor systems. The present study investigated the possibility that part of the action of intraspinally or subcutaneously administered (+/-) epibatidine, is mediated through an action on muscarinic receptors. Radioligand receptor assays were performed using homogenates of rat spinal cord and muscarinic M1-M5 receptors expressed in Sf9 cells. The intraspinal acetylcholine releasing effect of intraspinally and subcutaneously administered (+/-) epibatidine was studied with and without with atropine pretreatment. (+/-) Epibatidine has affinity for muscarinic receptors both in spinal cord tissue and expressed in Sf9 cells. The intraspinal administration of 160 microM (+/-) epibatidine produced an increase in acetylcholine release that was reduced by pretreatment with 100 microM atropine. Subcutaneous administration of 30 microg/kg (+/-) epibatidine produced an increase in intraspinal acetylcholine release that was not inhibited by 5 mg/kg subcutaneous atropine pretreatment. We conclude that (+/-) epibatidine, in microM concentrations, is a partial muscarinic receptor agonist that may interact with spinal muscarinic receptors to increase acetylcholine release. Epibatidine induced spinal acetylcholine release observed after subcutaneous administration appears not to be mediated via muscarinic receptor. The dual action on both nicotinic receptors and muscarinic receptors may explain the potent analgesic effect observed after epibatidine administration.

Antisense knockdown of spinal-mGluR1 reduces the sustained phase of formalin-induced nociceptive responses

To examine the role of mGluR1 (a subunit of the group I metabotropic glutamate receptor) in the nociceptive responses of rats following a subcutaneous injection of formalin into the plantar surface of the hind paw, we delivered antisense oligonucleotides (ODNs) against mGluR1 into the rat lumbar spinal cord (L3-L5) intrathecally using an HVJ-liposome-mediated gene transfer method. Rats treated with a single injection of mGluR1 antisense ODNs into the intrathecal space of the lumbar spinal cord showed a marked reduction of the early-sustained phase of formalin-induced nociceptive responses, but not of their acute phase. The reduction of nociceptive behavioral responses became apparent at day 2 after the antisense treatment and lasted for 2 days. This corresponded to a long-lasting down-regulation (46%) of mGluR1 expression in the lumbar cord. This down-regulated mGluR1 was observed at day 2 and persisted until day 4 after the intrathecal infusion of mGluR1 antisense ODN. In contrast, rats treated with mGluR1 sense or mismatch ODNs showed none of these changes. These results suggest that mGluR1 may play a crucial role in the sustained nociception of formalin-induced behavioral responses.

Nicotinic Acetylcholinergic Receptors Regulate the Intraspinal Release of Acetylcholine in Male Rats

Activation of cholinergic receptors in the spinal cord increases the intraspinal release of acetylcholine (ACh) and produces potent analgesia. The mechanisms that regulate the release of spinal ACh are not fully known. In the present study, we investigated the role of nicotinic ACh receptors in the regulation of intraspinal ACh release. Using an in vivo intraspinal microdialysis technique, nicotine was administered alone and in combination with the nicotinic antagonists mecamylamine (50 microM), dihydro-beta-erythroidine (DbetaE) (500 microM) and methyllycaconitine (MLA) (40 nM). Administration of nicotine (1 microM-1 mM) produced a dose dependent increase of intraspinal ACh release, while 10 mM nicotine resulted in dramatic increase in ACh release followed by a decrease to baseline. Administration of mecamylamine or DbetaE also induced an increased ACh release while MLA caused a decreased release. Mecamylamine and DbetaE, but not MLA pretreatment attenuated the stimulatory effect of 100 microM nicotine on intraspinal ACh release. It is suggested that spinal ACh release is regulated by different nicotinic ACh receptors. These receptors may tonically regulate spinal ACh release either directly or indirectly via inhibitory interneurones. Some of these receptors may be desensitised by high nicotine concentrations leading to a reduction of ACh release.

Antinociception of intrathecal cholinesterase inhibitors and cholinergic receptors in rats

BACKGROUND

Intrathecal cholinesterase inhibitors have been shown to have an antinociceptive effect which is mediated through the spinal cholinergic receptors, mainly muscarinic receptor. Spinal nicotinic receptor also has been involved in the control of nociception. Authors characterized the respective role of muscarinic or nicotinic receptor for the antinociception of cholinesterase inhibitors and further determined the antinociceptive potency of them.

METHODS

Rats were prepared with intrathecal catheters. Formalin-induced flinching response was regarded as a nociceptive behavior.

RESULTS

Intrathecal neostigmine, physostigmine and edrophonium produced a dose-dependent suppression of flinching in both phases. Atropine and the M1 selective antagonist attenuated the effect of them, while the M2 selective antagonist did not affect. M3, M4 selective, and nicotinic receptor antagonists reversed the antinociception induced by edrophonium, but by neither neostigmine nor physostigmine. The ordering of potency was neostigmine > physostigmine > > edrophonium.

CONCLUSION

These data indicate that the nicotinic receptor may be involved, at least in part, in the antinociceptive action of cholinesterase inhibitor at the spinal level, and M1 receptor subtype may be a common pharmacologic site of action. Moreover, neostigmine is more potent than physostigmine and edrophonium.

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.

Side-effects of postoperative epidural analgesia in children: a randomized study comparing morphine and clonidine

BACKGROUND

Morphine is widely used in association with local anaesthetics for postoperative epidural analgesia. There are no data on the prolonged use of clonidine for postoperative analgesia in children. The primary outcome of this randomized, double-blind trial was to compare the incidence of side-effects after epidural infusion of clonidine or morphine, in association with ropivacaine in children.

METHODS

After institutional approval, 26 children, aged 3-12 years, who were scheduled for abdominal surgery, had an epidural catheter placed after induction of general anaesthesia. Patients were then randomized to two different groups. After an initial bolus of 2.5 mg x kg-1 0.25% ropivacaine with either 40 micro g x kg-1 morphine (group M, n = 14) or 1 micro g x kg-1 clonidine (group C, n = 12), an epidural infusion was started at a rate of 0.4 ml x kg-1 x h-1. The patients in the M group received an infusion of 0.08% ropivacaine with 10 micro g.ml-1 morphine, those in the group C an infusion of 0.08% ropivacaine with 0.6 micro g.ml-1 clonidine.

RESULTS

The two groups were similar with respect to age, sex and weight. One patient in the C group was excluded for misplacement of the epidural catheter. The incidence of vomiting and pruritus was significantly higher in the M group compared with the C group (64% and 85% versus 0%, respectively). The incidence of pain was significantly higher in the C group compared with the M group (73% versus 29%) as well as the need for rescue analgesia medications.

CONCLUSIONS

Epidural clonidine is followed by a significantly lower incidence of side-effects. However, its analgesic effects, at least at the doses used in this study, are less potent than those of epidural morphine.

Intrathecal transplantation of neuroblastoma cells decreases heat hyperalgesia and cold allodynia in a rat model of neuropathic pain

Intrathecal grafting of cells as biological pumps to deliver monoamines, endorphins, and/or trophic factors, has been shown to be effective in treating chronic pain both in experimental animals and in clinical trials. We have tested whether intrathecal implantation of neuroblastoma cells reduces heat hyperalgesia and cold allodynia in a rat model of neuropathic pain induced by chronic constriction injury (CCI) of the sciatic nerve. Behavioral tests and cerebrospinal fluid (CSF) collection were performed before CCI, 1 week later (after which, vehicle or NB69 cells were intrathecally injected) and at 4, 7, and 14 days post-injection. Both CSF sampling and injection of the cells were performed by direct lumbar puncture. Intrathecal grafting of 4 x 10(6) NB69 neuroblastoma cells reduced to basal levels the nociceptive response to heat in nerve-injured hindpaws, while the response of control limbs remained unchanged. Similarly, the allodynic response to cold elicited by acetone evaporation decreased in the animals implanted with NB69 cells. An increase in the concentrations of dopamine and serotonin metabolites of around 150% was observed in the CSF of animals that received grafts of NB69 cells. These data suggest that the monoamines released by NB69 cells in the intrathecal space produce analgesia to neuropathic pain in rats.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Alpha2 receptors and agonists in pain management

Alpha(2) agonists have been in clinical use for decades, primarily in the treatment of hypertension. In recent years, alpha(2) agonists have found wider application, particularly in the fields of anesthesia and pain management. It has been noted that these agents can enhance analgesia provided by traditional analgesics, such as opiates, and may result in opiate-sparing effects. This has important implications for the management of acute postoperative pain and chronic pain states, including disorders involving spasticity or myofascial pain, neuropathic pain, and chronic daily headaches. The clinical utility of these agents is ever expanding, as they are gaining broader use in neuraxial analgesia, and new applications are continuously under investigation. The alpha(2) agonists that are currently employed in anesthesia and pain management include clonidine, tizanidine, and dexmedetomidine. Moxonidine and radolmidine, which are not currently in clinical use in humans, may offer favorable side-effect profiles when compared with traditional alpha(2) agonists, and may thereby allow for more widespread pain management applications.