The antinociceptive effects of alpha7 nicotinic agonists in an acute pain model

{alpha}7-nAChR-mediated suppression of hyperexcitability of colonic dorsal root ganglia neurons in experimental colitis

Controlled clinical trials of nicotine transdermal patch for treatment of ulcerative colitis have been shown to improve histological and global clinical scores of colitis. Here we report that nicotine (1 microM) suppresses in vitro hyperexcitability of colonic dorsal root ganglia (DRG) (L(1)-L(2)) neurons in the dextran sodium sulfate (DSS)-induced mouse model of acute colonic inflammation. Nicotine gradually reduced regenerative multiple-spike action potentials in colitis mice to a single action potential. Nicotine's effect on hyperexcitability of inflamed neurons was blocked in the presence of an alpha(7)-nicotinic acetylcholine receptor (nAChR) antagonist, methyllicaconitine, while choline, the alpha(7)-nAChR agonist, induced a similar effect to that of nicotine. Consistent with these findings, nicotine failed to suppress hyperexcitability in colonic DRG neurons from DSS-treated alpha(7) knockout mice. Furthermore, colonic DRG neurons from DSS-treated alpha(7) knockout mice were characterized by lower rheobase (10 +/- 5 vs. 77 +/- 13 pA, respectively) and current threshold (28 +/- 4 vs. 103 +/- 8 pA, respectively) levels than DSS-treated C57BL/J6 mice. An interesting observation of this study is that 8 of 12 colonic DRG (L(1)-L(2)) neurons from control alpha(7) knockout mice exhibited multiple-spike action potential firing while no wild-type neurons did. Overall, our findings suggest that nicotine at low 1 microM concentration suppresses in vitro hyperexcitability of inflamed colonic DRG neurons in a mouse model of acute colonic inflammation via activation of alpha(7)-nAChRs.

Intrathecal injection of an alpha seven nicotinic acetylcholine receptor agonist attenuates gp120-induced mechanical allodynia and spinal pro-inflammatory cytokine profiles in rats

Nicotinic acetylcholine receptors (nAchRs) are not only key receptors in the autonomic nervous system, but also are present on immune cells. The alpha seven subunit of nAchR (alpha7nAchR) suppresses pro-inflammation in peripheral monocytes by decreasing pro-inflammatory cytokine production. In spinal cord, alpha7nAchRs are found on microglia, which are known to induce and maintain pain. We predicted that alpha7nAchR agonists might attenuate intrathecal HIV-1 gp120-induced, pro-inflammatory cytokine- and microglia-dependent mechanical allodynia. Choline, a precursor for acetylcholine and selective agonist for alpha7nAchR, was administered intrathecally either with, or 30 min after, intrathecal gp120. Choline significantly blocked and reversed gp120-induced mechanical allodynia for at least 4 h after drug administration. In addition, intrathecal choline, delivered either with or 30 min after gp120, reduced gp120-induced IL-1beta protein and pro-inflammatory cytokine mRNAs within the lumbar spinal cord. A second alpha7nAchR agonist, GTS-21, also significantly reversed gp120-induced mechanical allodynia and lumbar spinal cord levels of pro-inflammatory cytokine mRNAs and IL-1beta protein. A role of microglia is suggested by the observation that intrathecal choline suppressed the gp120-induced expression of, cd11b, a macrophage/microglial activation marker. Taken together, the data support that alpha7nAchR may be a novel target for treating pain where microglia maintain the pro-inflammatory state within the spinal cord.

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.

Nicotine addiction and nicotinic receptors: lessons from genetically modified mice

The past decades have seen a revolution in our understanding of brain diseases and in particular of drug addiction. This has been largely due to the identification of neurotransmitter receptors and the development of animal models, which together have enabled the investigation of brain functions from the molecular to the cognitive level. Tobacco smoking, the principal - yet avoidable - cause of lung cancer is associated with nicotine addiction. Recent studies in mice involving deletion and replacement of nicotinic acetylcholine receptor subunits have begun to identify the molecular mechanisms underlying nicotine addiction and might offer new therapeutic strategies to treat this addiction.

Antinociceptive and anti-inflammatory effects of choline in a mouse model of postoperative pain

BACKGROUND

Choline is a dietary supplement that activates alpha7 nicotinic receptors. alpha7 nicotinic activation reduces cytokine production by macrophages and has antinociceptive activity in inflammatory pain models. We hypothesized that systemic administration of choline would reduce the inflammatory response from macrophages and have antinociceptive efficacy in a murine model of postoperative pain.

METHODS

We studied the response of wild-type and alpha7 nicotinic knockout mice to heat and punctate pressure after a model surgical procedure. We investigated the effect of genotype and choline treatment on alpha-bungarotoxin binding to, and their production of tumour necrosis factor (TNF) from, macrophages.

RESULTS

Choline provided moderate antinociception. The ED(50) for choline inhibition of heat-induced allodynia was 1.7 mg kg(-1) h(-1). The ED(50) for punctate pressure threshold was 4.7 mg kg(-1) h(-1) choline. alpha7 nicotinic knockout mice had no change in hypersensitivity to heat or pressure and were significantly different from littermate controls when treated with choline 5 mg kg(-1) h(-1) (P<0.05, 0.01). Choline 100 mM reduced binding of alpha-bungarotoxin to macrophages by 72% and decreased their release of TNF by up to 51 (sd 11)%. There was no difference by genotype in the inhibition of TNF release by choline.

CONCLUSIONS

Systemic choline is a moderately effective analgesic via activation of alpha7 nicotinic acetylcholine receptors. The antinocicepive effect may not be mediated by a reduction of TNF pathway cytokine release from macrophages. Although choline at millimolar concentrations clearly inhibits the release of TNF, this effect is not alpha7 subunit-dependent and occurs at concentrations likely higher than reached systemically in vivo.

Beta2* nicotinic acetylcholine receptors modulate pain sensitivity in acutely abstinent tobacco smokers

INTRODUCTION

Nicotine and tobacco smoking administration have demonstrated antinociceptive effects that are mediated by the nicotinic acetylcholine receptor containing the beta2* subunit (beta(2)*-nAChR). In this study, we examined the relationship between beta(2)*-nAChR availability and nociception during acute withdrawal in human tobacco smokers using [(123)I]5-IA-85380 ([(123)I]5-IA) and single photon emission computed tomography (SPECT) brain imaging.

METHODS

Tobacco smokers (n = 24, aged 34 +/- 11 years) participated in the cold pressor task during acute withdrawal (up to 3 hr) and a second cold pressor task following 7-13 days of smoking abstinence on the day they were imaged with [(123)I]5-IA SPECT. The cold pressor task is used to measure pain sensitivity (when subjects first feel pain) and pain tolerance (when subjects cannot withstand pain).

RESULTS

Following 7-13 days of tobacco smoking abstinence, increased pain sensitivity, for example, shorter time to first feel pain, was significantly associated with higher beta(2)*-nAChR availability in the thalamus (r = -.43), parietal (r = -.50), frontal (r = -.55), anterior cingulate (r = -.44), temporal (r = -.43), and occipital (r = -.48) cortices. The percent change in pain sensitivity from the first to second cold pressor task was significantly correlated with beta(2)*-nAChR availability in the thalamus (r = -.57), cerebellum (r = -.50), striatum (r = -.057), parietal (r = -.46), anterior cingulate (r = -.48), temporal (r = -.55), and occipital (r = -.57) cortices. Similar associations were not observed with pain tolerance.

DISCUSSION

This suggests that beta(2)*-nAChRs play a role in pain sensitivity but not pain tolerance during tobacco smoking withdrawal. If individuals are more likely to relapse in response to painful stimuli, lower beta(2)*-nAChR availability during acute abstinence may be protective.

The potential application of nicotinic acetylcholine receptor agonists for the treatment of rheumatoid arthritis

Although the disease-modifying anti-rheumatic drugs (DMARD) have been widely used in clinics, rheumatoid arthritis (RA) is not completely curable so far. Hence, to seek new drugs for the treatment of RA has become a pursuing goal of rheumatologists and orthopedic surgeons. Since the major pathological characteristic of RA is inflammation, the exploration of anti-inflammatory drugs has become a hotspot. Moreover, the anti-inflammatory and anti-nociceptive functions of nicotinic acetylcholine receptors as well as the related mechanisms have been recently discovered. Based on the above-mentioned, it is promising that the nicotinic acetylcholine receptor agonists will be applied for the treatment of RA. Following in vitro, in vivo experiments as well as clinical trials will even strengthen this viewpoint and more RA patients will benefit from the application of such agents.

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.

A transdermal nicotine patch is not effective for postoperative pain management in smokers: a pilot dose-ranging study

BACKGROUND

Nicotine has an antinociceptive effect in animal models. The analgesic effect in humans has been examined, but studies have had mixed results. A proposed etiology is variability in chronic nicotine exposure because of differences in tobacco smoking rates and second-hand smoke exposure. In this study, we examined the postoperative analgesic effect of a transdermal nicotine patch in smokers in a parallel design to a previous study in nonsmokers.

METHODS

We conducted a randomized, double-blind, prospective, placebo-controlled trial of 28 patients undergoing abdominal or pelvic surgery who required patient-controlled analgesia and an overnight hospital stay. Before anesthetic induction, a transdermal nicotine patch was applied (0, 5, 10, or 15 mg). The primary outcome variable was postoperative pain reported over the first hour and over the next 5 days using a standard numerical rating scale. Secondary outcome variables were pain medication use, hemodynamic values, nausea, and sedation.

RESULTS

Patients treated with nicotine reported higher pain scores than those treated with placebo over the first hour after surgery (P < 0.01, average numerical rating scale increase = 0.67) and there was no difference between groups in the subsequent 5 days (P > 0.05). There was no significant dose effect. Diastolic blood pressure in the first hour was higher in the placebo group compared with the nicotine-treated group (P < 0.01, average increase = 11 mm Hg). There was no difference in nausea or sedation.

CONCLUSIONS

Transdermal nicotine, 5-15 mg, failed to relieve postoperative pain or reduce opioid use in smokers.

Hypothesis of snake and insect venoms against Human Immunodeficiency Virus: a review

Background

Snake and insect venoms have been demonstrated to have beneficial effects in the treatment of certain diseases including drug resistant human immunodeficiency virus (HIV) infection. We evaluated and hypothesized the probable mechanisms of venoms against HIV.

Methods

Previous literatures published over a period of 30 years (1979-2009) were searched using the key words snake venom, insect venom, mechanisms and HIV. Mechanisms were identified and discussed.

Results & Conclusion

With reference to mechanisms of action, properties and components of snake venom such as sequence homology and enzymes (protease or L- amino acid oxidase) may have an effect on membrane protein and/or act against HIV at multiple levels or cells carrying HIV virus resulting in enhanced effect of anti-retroviral therapy (ART). This may cause a decrease in viral load and improvement in clinical as well as immunological status. Insect venom and human Phospholipase A₂ (PLA₂) have potential anti-viral activity through inhibition of virion entry into the cells. However, all these require further evaluation in order to establish its role against HIV as an independent one or as a supplement.

The analgesic and toxic effects of nornicotine enantiomers alone and in interaction with morphine in rodent models of acute and persistent pain

Neuronal nicotinic acetylcholinic receptors (nAChR) are promising targets for the development of novel analgesics. Nicotine and other nAChR-agonists produce profound analgesia in rodent models of acute and persistent pain. However, significant side-effects are of concern. Nornicotine (N-desmethyl-nicotine) appears to activate different nAChR subtypes, has a better pharmacokinetic profile, and produces less toxicity than nicotine. Little is known about its analgesic properties. In the present study, the S(-)- and R(+)-enantiomers of nornicotine were characterized with regard to analgesia and side-effects profile. Efficacy was demonstrated in rat models of pain where central sensitization is involved: i.e. the chronic constriction nerve injury model of peripheral neuropathy and the formalin model of tonic inflammatory pain. The desirable (analgesic) properties resided predominantly in the S(-)- rather than the R(+)-enantiomer. In contrast, undesirable effects (motor in-coordination, reduced locomotor activity, ataxia) were more pronounced with the R(+)-enantiomer. This is an interesting finding, which may suggest separation of toxicity from analgesia by utilization of S(-)-enantiomer of nornicotine. Maximum analgesic effectiveness without significant side-effects was achieved when S(-)-nornicotine (sub-analgesic dose) was combined with a low-dose of the micro-opioid, morphine. These preclinical data suggest that S(-)-nornicotine may be of value, either alone or in combination with an opioid, for treatment of a broad-spectrum of pain (i.e. nociceptive, neuropathic, and mixed pain).

The effect of peripherally administered CDP-choline in an acute inflammatory pain model: the role of alpha7 nicotinic acetylcholine receptor

BACKGROUND

CDP-choline (citicholine; cytidine-5'-diphosphate choline) is an endogenously produced nucleotide which, when injected intracerebroventricularly, exerts an antinociceptive effect in acute pain models mediated by central cholinergic mechanisms and alpha7 nicotinic acetylcholine receptors (alpha7nAChR). Previous reports also suggest that the peripheral cholinergic system has an antiinflammatory role mediated by alpha7nAChRs on macrophages.

METHODS

We used male Sprague-Dawley rats to assess the antihypersensitivity and antiinflammatory effect of CDP-choline after intraplantar injection of carrageenan (100 microL, 2%). Mechanical paw withdrawal thresholds and paw thickness were measured by Randall-Selitto testing and microcallipers, respectively. All drugs were administered intraplantarly in a volume 50 microL.

RESULTS

CDP-choline (1, 2.5, 5 micromol; intraplantar) increased the mechanical paw withdrawal threshold and decreased paw edema in a dose- and time-dependent manner in the carrageenan-injected hindpaw. CDP-choline administration to the noninflamed contralateral hindpaw did not alter ipsilateral inflammation. Methyllycaconitine (100 nmol), a selective alpha7nAChR antagonist, completely blocked the effects of CDP-choline when administered to the inflamed hindpaw. However, the administration of methyllycaconitine to the contralateral hindpaw did not block the effects of CDP-choline in the ipsilateral paw. The administration of CDP-choline (5 micromol) 10 min after carrageenan administration to the ipsilateral hindpaw did not reduce swelling and edema but did significantly reduce hypersensitivity. Treatment with CDP-choline decreased tumor necrosis factor-alpha production in the rat paw tissue after carrageenan.

CONCLUSIONS

The results of this study suggest that intraplantar CDP-choline has antihypersensitivity and antiinflammatory effects mediated via alpha7nAChRs in the carrageenan-induced inflammatory pain model.

In inflammatory reactive astrocytes co-cultured with brain endothelial cells nicotine-evoked Ca(2+) transients are attenuated due to interleukin-1beta release and rearrangement of actin filaments

The aim of this study was to investigate whether nicotine acetylcholine receptors (nAChRs) are expressed in a more pronounced way in astrocytes co-cultured with microvascular endothelial cells from adult rat brain, compared with monocultured astrocytes, as a sign of a more developed signal transduction system. Also investigated was whether nicotine plays a role in the control of neuroinflammatory reactivity in astrocytes. Ca(2+) imaging experiments were performed using cells loaded with the Ca(2+) indicator Fura-2/AM. Co-cultured astrocytes responded to lower concentrations of nicotine than did monocultured astrocytes, indicating that they are more sensitive to nicotine. Co-cultured astrocytes also expressed a higher selectivity for alpha7nAChR and alpha4/beta2 subunits and evoked higher Ca(2+) transients compared with monocultured astrocytes. The Ca(2+) transients referred to are activators of Ca(2+)-induced Ca(2+) release from intracellular stores, both IP(3) and ryanodine, triggered by influx through receptor channels. The nicotine-induced Ca(2+) transients were attenuated after incubation with the inflammatory mediator lipopolysaccharide (LPS), but were not attenuated after incubation with the pain-transmitting peptides substance P and calcitonin-gene-related peptide, nor with the infection and inflammation stress mediator, leptin. Furthermore, LPS-induced release of interleukin-1beta (IL-1beta) measured by enzyme-linked immunosorbent assay (ELISA) was more pronounced in co-cultured versus monocultured astrocytes. Incubation with both LPS and IL-1beta further attenuated nicotine-induced Ca(2+) response. We also found that LPS and IL-1beta induced rearrangement of the F-actin filaments, as measured with an Alexa488-conjugated phalloidin probe. The rearrangements consisted of increases in ring formations and a more dispersed appearance of the filaments. These results indicate that there is a connection between a dysfunction of nicotine Ca(2+) signaling in inflammatory reactive astrocytes and upregulation of IL-1beta and the rearrangements of actin filaments in the cells.

Prenatal exposure to nicotine affects substance p and preprotachykinin-A mRNA levels in newborn rat

Prenatal nicotine exposure influences neuronal development including effects on several neurotransmitter systems. It also attenuates the ventilatory response to hypoxia, known to require a functional substance P-ergic system. Previous studies have shown that nicotine increases the risk for sudden infant death syndrome (SIDS) by 4-fold, and that SIDS-victims have elevated brainstem levels of substance P. We, therefore, studied the effect of prenatal nicotine exposure on the levels of substance P-like immunoreactivity by RIA in the brain in newborn rat pups. The expression of the substance P precursor preprotachykinin A mRNA was also determined by real-time reverse transcriptase-polymerase chain reaction in carotid body, in petrosal/jugular and trigeminal ganglia, in cervical and lumbar dorsal root ganglia, and in the brainstem. We found that prenatal nicotine exposure increased levels of substance P-like immunoreactivity in the brainstem without changing levels in other parts of the brain or in the adrenals. Furthermore, mRNA levels were increased in the carotid bodies and in the petrosal ganglia, in contrast to the decreased levels in the cervical dorsal root ganglia. We conclude that nicotine causes alterations in the substance P-ergic system in the brainstem, possibly linked to the increased risk for SIDS after prenatal nicotine exposure.

Isoflurane prevents nicotine-evoked norepinephrine release from the mouse spinal cord at low clinical concentrations

BACKGROUND

Volatile anesthetics inhibit nicotinic acetylcholine receptors at subanesthetic concentrations. In both animal and human studies, similar concentrations of volatile anesthetics have been associated with increased sensitivity to pain. Nicotinic analgesia is thought to involve the enhanced release of norepinephrine. These studies are intended as a "proof of concept" that alteration of the nicotinic facilitation of norepinephrine release is a potential mechanism for isoflurane-induced pronociception.

METHODS

We conducted our study using a murine lumbar spinal cord slice model. We evoked norepinephrine release with nicotine in the presence and absence of isoflurane. To identify the type of nicotinic receptor involved, we studied the effect of receptor and subtype-specific ligands and genetically engineered mice, which lacked the gene expression for the nicotinic beta2 subunit. The amount of [(3)H]-norepinephrine released was measured under the different conditions.

RESULTS

Nicotine-facilitated norepinephrine release was significantly and maximally inhibited by isoflurane at concentrations that enhance pain sensitivity in vivo (0.38%). Facilitation of norepinephrine release was mimicked by the alpha 7 selective agonist choline and inhibited in the presence of alpha-bungarotoxin, an alpha 7-nicotinic selective antagonist. Facilitation of norepinephrine release was not different in animals lacking beta2 subunits compared with matched controls.

CONCLUSIONS

Nicotinic facilitation of norepinephrine release in the spinal cord is inhibited by isoflurane at low clinically relevant concentrations. Because the net effect of noradrenergic tone in the spinal cord is inhibitory, the removal of this mechanism might be responsible for the enhanced pain sensitivity seen at these concentrations of isoflurane.

Activation of the alpha7-nicotinic acetylcholine receptor reverses complete freund adjuvant-induced mechanical hyperalgesia in the rat via a central site of action

The role of specific nicotinic receptor (nAChR) subtypes in antinociception has not been fully elucidated because of the lack, until recently, of selective tool compounds. (R)-N-(1-azabicyclo[2.2.2]oct-3-yl)(5-(2-pyridyl)thiopene-2-carboxamide) (compound B) is reported to be an agonist selective for the alpha(7)nAChR and in the present study was found to be efficacious in inflammatory pain models in 2 species. Compound B reversed complete Freund adjuvant-induced reductions in paw withdrawal thresholds in rat and mouse in a dose-related manner, producing maximum reversals of 65% +/- 4% at 10 mg/kg and 87% +/- 15% at 20 mg/kg. When rats and mice were predosed with the centrally penetrant, broad-spectrum nicotinic receptor antagonist mecamylamine, the efficacy of the agonist was significantly inhibited, producing reversals of only 11% +/- 5% at 10 mg/kg and 5% +/- 13% at 20 mg/kg, confirming activity via nicotinic receptors. Rats were also predosed systemically with the selective low-brain penetrant alpha(7)-antagonist methyllycaconitine, which had no effect on agonist activity (90% +/- 18% at 10 mg/kg), suggesting a central involvement. This hypothesis was further established with methyllycaconitine completely inhibited the agonist effect when dosed intrathecally (1% +/- 7%).

PERSPECTIVE

These studies provide good rationale for the utility of selective, central nervous system penetrant agonists at the alpha(7)-nicotinic receptor for the treatment of inflammatory pain.

The selective nicotinic acetylcholine receptor alpha7 agonist JN403 is active in animal models of cognition, sensory gating, epilepsy and pain

Several lines of evidence suggest that the nicotinic acetylcholine receptor alpha7 (nAChR alpha7) is involved in central nervous system disorders like schizophrenia and Alzheimer's disease as well as in inflammatory disorders like sepsis and pancreatitis. The present article describes the in vivo effects of JN403, a compound recently characterized to be a potent and selective partial nAChR alpha7 agonist. JN403 rapidly penetrates into the brain after i.v. and after p.o. administration in mice and rats. In the social recognition test in mice JN403 facilitates learning/memory performance over a broad dose range. JN403 shows anxiolytic-like properties in the social exploration model in rats and the effects are retained after a 6h pre-treatment period and after subchronic administration. The effect on sensory inhibition was investigated in DBA/2 mice, a strain with reduced sensory inhibition under standard experimental conditions. Systemic administration of JN403 restores sensory gating in DBA/2 mice, both in anaesthetized and awake animals. Furthermore, JN403 shows anticonvulsant potential in the audiogenic seizure paradigm in DBA/2 mice. In the two models of permanent pain tested, JN403 produces a significant reversal of mechanical hyperalgesia. The onset was fast and the duration lasted for about 6h. Altogether, the present set of data suggests that nAChR alpha7 agonists, like JN403 may be beneficial for improving learning/memory performance, restoring sensory gating deficits, and alleviating pain, epileptic seizures and conditions of anxiety.

3'-Fluoro substitution in the pyridine ring of epibatidine improves selectivity and efficacy for alpha4beta2 versus alpha3beta4 nAChRs

The analog of epibatidine having a fluoro substituent at the 3' position of the pyridine ring has been recently developed and shown to possess binding affinity in the pM range to alpha4beta2 nAChRs and in the nM range to alpha7 nAChRs and to exhibit potent agonist activity in nicotine-induced analgesia tests. Here we used patch-clamp technique in a whole-cell configuration to compare functional activity of 3'-fluoroepibatidine to that of epibatidine by itself on recombinant alpha4beta2, alpha7 and alpha3beta4 neuronal nAChRs. The agonist effect of (+/-)-epibatidine was partial and yielded comparable EC50s of 0.012 microM (72% efficacy) and 0.027 microM (81% efficacy) at alpha4beta2 and alpha3beta4 nAChRs, respectively, but was full at alpha7 nAChRs with an EC50 of 4.8 muM. Testing of the analog at different concentrations revealed that it acts as a full agonist with an EC50 of 0.36 microM at alpha4beta2 nAChRs and induces partial agonist effect (66% efficacy) at alpha7 nAChRs with an EC50 of 9.8 microM and an IC50 corresponding to 225 microM. In contrast, the analog caused only 24% maximal activation at the range of concentrations from 0.1 to 100 microM and, in addition, induced an inhibition of alpha3beta4 nAChR function with an IC50 of 8.3 microM. Our functional data, which are in agreement with previous binding and behavioral findings, demonstrate that 3'-fluoro substitution in the pyridine ring of epibatidine results in an improved pharmacological profile as observed by an increased efficacy and selectivity for alpha4beta2 versus alpha3beta4 nAChRs.

Peripheral nerve injury alters spinal nicotinic acetylcholine receptor pharmacology

Nicotinic acetylcholine receptors are widely expressed in the rat spinal cord and modulate innocuous and nociceptive transmission. The present studies were designed to investigate the plasticity of spinal nicotinic acetylcholine receptors modulating mechanosensitive information following spinal nerve ligation. A tonic inhibitory cholinergic tone mediated by dihydro-beta-erythroidine- (DHbetaE) and methyllycaconitine- (MLA) sensitive nicotinic acetylcholine receptors was identified in the normal rat spinal cord and cholinergic tone at both populations of nicotinic acetylcholine receptors was lost ipsilateral to spinal nerve ligation. The administration of intrathecal nicotinic acetylcholine receptor agonists reduced mechanical paw pressure thresholds with a potency of epibatidine=A-85380>>nicotine>choline in the normal rat. Following spinal nerve ligation, intrathecal epibatidine and nicotine produced an ipsilateral antinociception, but intrathecal A-85380 and choline did not. The antinociceptive response to intrathecal nicotine was blocked with the alpha7 and alpha9alpha10-selective nicotinic acetylcholine receptor antagonist, MLA, and the alphabeta heteromeric nicotinic acetylcholine receptor antagonist, DHbetaE. The antinociceptive effects of both intrathecal nicotine and epibatidine were mediated by GABA(A) receptors. Spinal [(3)H]epibatidine saturation binding was unchanged in spinal nerve-ligated rats, but spinal nerve ligation did increase the ability of nicotine to displace [(3)H]epibatidine from spinal cord membranes. Spinal nerve ligation altered the expression of nicotinic acetylcholine receptor subunits ipsilaterally, with a large increase in the modulatory alpha5 subunit. Taken together these results suggest that pro- and antinociceptive populations of spinal nicotinic acetylcholine receptors modulate the transmission of mechanosensitive information and that spinal nerve ligation-induced changes in spinal nicotinic acetylcholine receptors likely result from a change in subunit composition rather than overt loss of nicotinic acetylcholine receptor subtypes.

The activation of nicotinic acetylcholine receptors enhances the inhibitory synaptic transmission in the deep dorsal horn neurons of the adult rat spinal cord

Somatosensory information can be modulated by nicotinic acetylcholine receptors (nAChRs) in the superficial dorsal horn of the spinal cord. Nonetheless, the functional significance of nAChRs in the deep dorsal horn of adult animals remains unclear. Using whole-cell patch-clamp recordings from lamina V neurons in the adult rat spinal cord, we investigated whether the activation of nAChRs could modulate the inhibitory synaptic transmission in the deep dorsal horn. In the presence of CNQX and APV to block excitatory glutamatergic synaptic transmission, bath applications of nicotine (100 microM) significantly increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in almost all neurons tested. The effect of nicotine was mimicked by N-methyl-4-(3-pyridinyl)-3-butene-1-amine (RJR-2403, 100 microM), an alpha 4 beta 2-nAChR agonist, and was also mimicked by choline (10 mM), an alpha 7-nAChR agonist. The effect of nicotine was completely blocked by the nAChR antagonist mecamylamine (5 microM). In the presence of tetrodotoxin (0.5 microM), nicotine (100 microM) significantly increased the miniature IPSC frequency. On the other hand, RJR-2403 (100 microM) or choline (10 mM) did not affect miniature IPSCs. The application of nicotine (100 microM) also evoked a large inward current in all lamina V neurons tested when cells were held at -60 mV. Similarly, RJR-2403 (100 microM) induced inward currents in the majority of lamina V neurons examined. On the other hand, choline (10 mM) did not elicit any detectable whole-cell currents. These results suggest that several nAChR subtypes are expressed on the presynaptic terminals, preterminals, and neuronal cell bodies within lamina V and that these nAChRs are involved in the modulation of inhibitory synaptic activity in the deep dorsal horn of the spinal cord.

Targeting the cholinergic system as a therapeutic strategy for the treatment of pain

Acetylcholine mediates its effects through both the nicotinic acetylcholine receptors (ligand-gated ion channels) and the G protein-coupled muscarinic receptors. It plays pivotal roles in a diverse array of physiological processes and its activity is controlled through enzymatic degradation by acetylcholinesterase. The effects of receptor agonists and enzyme inhibitors, collectively termed cholinomimetics, in antinociception/analgesia are well established. These compounds successfully inhibit pain signaling in both humans and animals and are efficacious in a number of different preclinical and clinical pain models, suggesting a broad therapeutic potential. In this review we examine and discuss the evidence for the therapeutic exploitation of the cholinergic system as an approach to treat pain.

Genetic Approaches Identify Differential Roles for α4β2*Nicotinic Receptors in Acute Models of Antinociception in Mice

The effects of nicotine on the tail-flick and hot-plate tests were determined to identify nicotinic receptor subtypes responsible for spinally and supraspinally mediated nicotine analgesia in knockin mice expressing hypersensitive alpha(4) nicotinic receptors (L9'S), in seven inbred mouse strains (C57BL/6, DBA/2, A/2, CBA/2, BALB/cByJ, C3H/HeJ, and 129/SvEv), and in two F1 hybrids (B6CBAF1 and B6D2F1). L9'S heterozygotes were approximately 6-fold more sensitive to the antinociceptive effects of nicotine than the wild-type controls in the hot-plate test but not in the tail-flick assay. Large differences in the effects of nicotine were also observed with both tests for the seven mouse strains. A/J and 129 mice were 6- to 8-fold more sensitive than CBA and BALB mice. In addition, B6CBAF1 hybrid mice were even less sensitive than CBA mice. Nicotinic binding sites were measured in three spinal cord regions and the hindbrain of the inbred strains. Significant differences in cytisine-sensitive, high affinity [(125)I]epibatidine binding site levels (alpha(4)beta(2)(*) subtypes), but not in (125)I-alpha-bungarotoxin binding (alpha(7)(*) subtypes), were observed. Significant negative correlations between cytisine-sensitive [(125)I]epibatidine binding and nicotine ED(50) for both tests were noted. Our results indicate that alpha(4)beta(2)(*) acetylcholine nicotinic receptors (nAChR) are important in mediating nicotine analgesia in supraspinal responses, while also showing that alpha(4)beta(2)(*)-nAChR and at least one other nAChR subtype appear to modulate spinal actions.

Persistent exercise attenuates nicotine- but not clonidine-induced antinociception in female rats

Exercise decreases the antinociceptive effects of opiate drugs. It has been hypothesized that the exercise-induced attenuation of opiate drug action is the result of the development of cross-tolerance between endogenous opioids released during exercise and exogenous opiates. The present study was designed to evaluate the role of exercise on non-opiate antinociception. Female Long-Evans rats were allowed ad lib access to running wheels. After 3 weeks, antinociceptive responses of animals were measured using the tail flick test following the administration of clonidine or nicotine. Nicotine and clonidine both produced dose-dependent increases in antinociceptive responses. Active animals were significantly less sensitive to nicotine-induced antinociception than inactive animals. There was no difference between the two groups in clonidine-induced antinociception. The results of these experiments suggest that exercise does not attenuate non-opioid, clonidine-induced antinociception. However, exercise does attenuate nicotine-induced antinociception. Therefore, the effect of persistent exercise on analgesic drugs is not specific to opiates.

Analgesic action of nicotine on tibial nerve transection (TNT)-induced mechanical allodynia through enhancement of the glycinergic inhibitory system in spinal cord

The activation of cholinergic pathways by nicotine elicits various physiological and pharmacological effects in mammals. For example, the stimulation of nicotinic acetylcholine receptors (nAChRs) leads to an antinociceptive effect. However, it remains to be elucidated which subtypes of nAChR are involved in the antinociceptive effect of nicotine on nerve injury-induced allodynia and the underlying cascades of the nAChR-mediated antiallodynic effect. In this study, we attempted to characterize the actions of nicotine at the spinal level against mechanical allodynia in an animal model of neuropathic pain, tibial nerve transection (TNT) in rats. It was found that the intrathecal injection of nicotine, RJR-2403, a selective alpha4beta2 nAChR agonist, and choline, a selective alpha7 nAChR agonist, produced an antinociceptive effect on the TNT-induced allodynia. The actions of nicotine were almost completely suppressed by pretreatment with mecamylamine, a non-selective nicotinic antagonist, or dihydro-beta-erythroidine, a selective alpha4beta2 nAChR antagonist, and partially reversed by pretreatment with methyllycaconitine, a selective alpha7 nAChR antagonist. Furthermore, pretreatment with strychnine, a glycine receptor antagonist, blocked the antinociception induced by nicotine, RJR-2403, and choline. On the other hand, the GABAA antagonist bicuculline did not reverse the antiallodynic effect of nicotine. Together, these results indicate that the alpha4beta2 and alpha7 nAChR system, by enhancing the activities of glycinergic neurons at the spinal level, exerts a suppressive effect on the nociceptive transduction in neuropathic pain.

The antinociceptive effects of centrally administered CDP-choline on acute pain models in rats: the involvement of cholinergic system

This study investigates the antinociceptive effect of intracerebroventricular (i.c.v.) injection of cytidine-5'-diphosphate choline (CDP-choline; citicoline) and the involvement of cholinergic mechanisms in rats. Three different pain models were utilized: thermal paw withdrawal test, mechanical paw pressure test and acetic acid writhing test. The i.c.v. administration of CDP-choline (0.5, 1.0 and 2.0 micromol) produced dose and time-dependent antinociception. Equimolar dose of choline (1 micromol; i.c.v.) produced antinociceptive response similar to the one observed in CDP-choline given animals. On the other hand, cytidine (1 micromol; i.c.v.) failed to produce response in the thermal paw withdrawal test and the mechanical paw pressure test but in the writhing test in which it produced significant antinociceptive effect. CDP-choline-induced antinociception was prevented by the neuronal high affinity choline uptake inhibitor HC-3 (1 microg; i.c.v.), the nonselective nicotinic receptor antagonist mecamylamine (50 microg; i.c.v.) and by the alpha(7)-selective nicotinic receptor antagonist, MLA (25 microg; i.c.v.). However, it was not changed by the nonselective muscarinic receptor antagonist atropine (10 microg; i.c.v.) in the thermal paw withdrawal test and mechanical paw pressure test. In the writhing test, all antagonist pretreatments produced blockade similar to that obtained from CDP-choline injected animals. CDP-choline did not impair the motor performance of rats as evaluated by a rota-rod test. Therefore, it can be postulated that CDP-choline exerts an antinociceptive effect mediated by a central cholinergic mechanism. Activation of specific alpha(7)-nicotinic cholinergic receptors through the activation of presynaptic cholinergic mechanisms appears to be involved in the antinociceptive effect of this drug.

Tonic inhibitory role of alpha4beta2 subtype of nicotinic acetylcholine receptors on nociceptive transmission in the spinal cord in mice

In the spinal dorsal horn, activation of the nicotinic acetylcholine receptors (nAChR) by exogenously applied agonists is known to enhance inhibitory synaptic transmission, and to produce analgesia. However, it is still unknown whether endogenously released acetylcholine exerts a tonic inhibition on nociceptive transmission through the nAChRs in the spinal dorsal horn. Here, we report the presence of such a tonic inhibitory mechanism in the spinal dorsal horn in mice. In behavioral experiments, intrathecal (i.t.) injection of non-selective nAChR antagonist mecamylamine and alpha4beta2 subtype-selective antagonist dihydro-beta-erythroidine (DHbetaE) dose-dependently induced thermal and mechanical hyperalgesia in mice while the alpha7-selective antagonist methyllycaconitine (MLA) had no effect. Similarly, antisense knock-down of alpha4 subunit of nAChR, but not alpha7 subunit, in spinal cord induced thermal and mechanical hyperalgesia. In whole-cell patch-clamp experiments in spinal cord slice preparation from adult mice, the frequency of miniature inhibitory postsynaptic currents (mIPSCs) observed in substantia gelatinosa (SG) neurons was decreased by mecamylamine and DHbetaE, but not by MLA. The amplitudes of the mIPSCs were not affected. The nicotinic antagonists decreased the frequency of both GABAergic and glycinergic IPSCs. On the other hand, the nicotinic antagonists had no effect on the excitatory postsynaptic currents (EPSCs). Finally, acetylcholine-esterase inhibitor neostigmine-induced facilitation of IPSC frequencies in SG neurons was inhibited by mecamylamine and DHbetaE. Altogether these findings suggest that nicotinic cholinergic system in the spinal dorsal horn can tonically inhibit nociceptive transmission through presynaptic facilitation of inhibitory neurotransmission in SG via the alpha4beta2 subtype of nAChR.

An autoradiographic analysis of [125I]alpha-bungarotoxin binding in rat brain after chronic nicotine exposure

Chronic exposure to nicotine has been shown to increase binding to high affinity nicotinic cholinergic receptors in rat brain, but the effect of this treatment on the low affinity alpha7 nicotinic receptors has been less well characterized. Male Sprague-Dawley rats were treated with saline or nicotine (6 mg/kg/day, by osmotic minipump) for 14 days. Frozen brain sections were then prepared and processed for quantitative autoradiography using [(125)I]alpha-bungarotoxin to measure the effect of this treatment on low affinity nicotinic receptors. Nicotine exposure increased [(125)I]alpha-bungarotoxin binding in 26 of 52 brain regions analyzed; increases ranged from 10 to 70% over saline controls. Increases were seen in all areas of the brain, but were more prominent in forebrain areas, and especially in cerebral cortex. These data demonstrate that low affinity alpha7 nicotinic receptors are also up-regulated by chronic nicotine. This phenomenon may be relevant to the heavy use of tobacco products in diseases like schizophrenia, and needs to be considered in the design of pharmaceuticals directed at this receptor system.

Could chronic pain and spread of pain sensation be induced and maintained by glial activation?

An injury often starts with acute physiological pain, which becomes inflammatory or neuropathic, and may sometimes become chronic. It has been proposed recently that activated glial cells, astrocytes and microglia within the central nervous system could maintain the pain sensation even after the original injury or inflammation has healed, and convert it into chronic by altering neuronal excitability. Glial cell activation has also been proposed to be involved in the phenomenon of spread of pain sensation ipsilaterally or to the contralateral side (i.e. mirror image pain). Substance P and calcitonin gene-related peptide, released due to an inflammatory process, interact with the endothelial cells of the blood-spinal cord and blood-brain barriers. The barriers open partially and substances may influence adjacent glial cells. Such substances are also released from neurones carrying the 'pain message' all the way from the injury to the cerebral cortex. Pro-inflammatory cytokines may be released from the microglial cells, and astroglial Ca2+-transients or oscillations may spread within the astroglial networks. One theory is that Ca2+-oscillations could facilitate the formation of new synapses. These new synapses could establish neuronal contacts for maintaining and spreading the pain sensation. If this theory holds true, it is possible that Ca2+ waves, production of cytokines and growth factors could be modified by selective anti-inflammatory drugs to achieve a balance in the activities of the different intercellular and intracellular processes. This paper reviews current knowledge about glial mechanisms underlying the phenomena of chronic pain and spread of the pain sensation.

A long-form alpha-neurotoxin from cobra venom produces potent opioid-independent analgesia

AIM

In light of the antinociceptive activity of the short-chain neurotoxin, cobrotoxin, and other acetylcholine antagonists, the antinociceptive activity and mechanisms of cobratoxin (CTX), a long-chain postsynaptic alpha-neurotoxin, was investigated in rodent pain models.

METHODS

CTX was administered intraperitoneally (30, 45, 68 microg/kg), intra-cerebral ventricularly (4.5 microg/kg) or microinjected into periaqueductal gray (PAG; 4.5 microg/kg). The antinociceptive action was tested using the hot-plate and acetic acid writhing tests in mice and rats. The involvement of the cholinergic system and opioid system in CTX-induced analgesia was examined by pretreatment of animals with atropine (0.5 mg/kg, im; or 10 mg/kg, ip) or naloxone (1 and 5 mg/kg, ip). The effect of CTX on motor activity was tested using the Animex test.

RESULTS

CTX exhibited a dose-dependent analgesic action in mice as determined by both the hot-plate and acetic acid writhing tests. The peak effect of analgesia was seen 3 h after administration. In the mouse acetic acid writhing test, the intra-cerebral ventricular administration of CTX at 4.5 microg/kg (1/12th of a systemic dose) produced marked analgesic effects. Microinjection of CTX (4.5 microg/kg) into the PAG region did not elicit an analgesic action in rats in the hot-plate test. Atropine at 0.5 mg/kg (im) and naloxone at 1 and 5 mg/kg (ip) both failed to block the analgesic effects of CTX, but atropine at 10 mg/kg (ip) did antagonize the analgesia mediated by CTX in the mouse acetic acid writhing test. Acetylsalicylic acid (300 mg/kg) did not enhance the analgesic effects of CTX. At the highest effective dose of 68 microg/kg the neurotoxin did not change the spontaneous mobility of mice.

CONCLUSION

CTX has analgesic effects, which are mediated in the central nervous system though not through the PAG. The central cholinergic system but not opioid system appears to be involved in the antinociceptive action of CTX.

Methadone increases intracellular calcium in SH-SY5Y and SH-EP1-hα7 cells by activating neuronal nicotinic acetylcholine receptors

(-)-Methadone acts as an agonist at opioid receptors. Both (+)- and (-)-enantiomers of methadone have been suggested to be potent non-competitive antagonists of alpha3beta4 neuronal nicotinic acetylcholine receptors (nAChRs). In the present study, we have examined interactions of methadone with nAChRs by using receptor binding assays, patch-clamp recording and calcium fluorometry imaging with SH-SY5Y cells naturally expressing alpha7 and alpha3* nAChR subtypes and SH-EP1-halpha7 cells heterologously expressing human alpha7 nAChRs. Methadone potently inhibited binding of [3H]methyllycaconitine to alpha7 nAChRs and that of [3H]epibatidine to alpha3* nAChRs. Methadone pretreatment induced up-regulation of epibatidine binding sites in SH-SY5Y cells. Using whole-cell patch-clamp recording, both isomers of methadone activated cation currents via mecamylamine-sensitive nAChRs in SH-SY5Y cells. Nicotine and both (+)- and (-)-methadone evoked increases in [Ca2+]i in both fluo-3AM loaded cell lines, and these effects were blocked by mecamylamine and by the alpha7 selective antagonist methyllycaconitine, suggesting effects of methadone as alpha7-nAChR agonist. Sensitivity of sustained nicotine and methadone effects to blockade by CdCl2, ryanodine and xestospongin-c implicates voltage-operated Ca2+ channels and intracellular Ca2+ stores as downstream modulators of elevated [Ca2+]i. Collectively, our results suggest that methadone engages in complex and potentially pharmacologically significant interactions with nAChRs.

Antinociceptive effects of choline against acute and inflammatory pain

We used the hot plate test and the formalin test to evaluate the antinociception of choline after i.c.v. or i.v. administration. The analgesic mechanism of choline was also studied. The response latency of mice was significantly prolonged in the hot plate test after choline (90-120 mug/animals) i.c.v. administration in a dose-dependent manner. Pretreatment with methyllycaconitine citrate (MLA), alpha-bungarotoxin, or atropine blocked the antinociception of choline in the hot plate test. In contrast, mecamylamine and naloxone had no effect. No antinociceptive action of choline was found in the hot plate test, but it did have an effect in the late phase of the formalin test after i.v. administration. The effect of choline on anti-inflammatory pain was blocked by MLA, but not by mecamylamine, naloxone and atropine, which is indicative of the involvement of alpha7 receptors in peripheral sites. When choline (2 mg/kg) was coadministered with aspirin (9.4 mg/kg), the licking/biting times in the late phase significantly decreased, although no effects were shown when these doses of drugs were used alone. Similarly, coadministration of choline (2 mg/kg) with morphine (0.165 mg/kg) significantly increased the antinociception of morphine in the late phase, but had no effect in the early phase. These results demonstrate that activation of alpha7 nicotinic receptors by choline elicits antinociceptive effects both in an acute thermal pain model and in an inflammatory pain model. Choline holds promise for development as a non-addictive analgesic drug and in reducing the regular dose of aspirin or morphine in inflammatory pain.

Nicotinic modulation of GABAergic synaptic transmission in the spinal cord dorsal horn

While the mechanisms underlying nicotinic acetylcholine receptor (nAChR)-mediated analgesia remain unresolved, one process that is almost certainly involved is the recently-described nicotinic enhancement of inhibitory synaptic transmission in the spinal cord dorsal horn. Despite these observations, the prototypical nicotinic analgesic (epibatidine) has not yet been shown to modulate inhibitory transmission in the spinal cord. Furthermore, while nAChRs have been implicated in short-term modulation, no studies have investigated the role of nAChRs in the modulation of long-term synaptic plasticity of inhibitory transmission in dorsal horn. Whole-cell patch clamp recordings from dorsal horn neurons of neonatal rat spinal cord slices were therefore conducted to investigate the short- and long-term effects of nicotinic agonists on GABAergic transmission. GABAergic synaptic transmission was enhanced in 86% of neurons during applications of 1 microM nicotine (mean increased spontaneous GABAergic inhibitory postsynaptic current (sIPSC) frequency was approximately 500% of baseline). Epibatidine (100 nM) induced an increase to an average of approximately 3000% of baseline, and this effect was concentration dependent (EC50=43 nM). Nicotinic enhancement was inhibited by mecamylamine and DHbetaE, suggesting an important role for non-alpha7 nAChRs. Tetrodotoxin (TTX) did not alter the prevalence or magnitude of the effect of nicotine, but the responses had a shorter duration. Nicotine did not alter evoked GABAergic IPSC amplitude, yet the long-term depression (LTD) induced by strong stimulation of inhibitory inputs was reduced when paired with nicotine. These results provide support for a mechanism of nicotinic analgesia dependent on both short and long-term modulation of GABAergic synaptic transmission in the spinal cord dorsal horn.

Cholinergic stimulation blocks endothelial cell activation and leukocyte recruitment during inflammation

Endothelial cell activation plays a critical role in regulating leukocyte recruitment during inflammation and infection. Based on recent studies showing that acetylcholine and other cholinergic mediators suppress the production of proinflammatory cytokines via the α7 nicotinic acetylcholine receptor (α7 nAChR) expressed by macrophages and our observations that human microvascular endothelial cells express the α7 nAChR, we examined the effect of cholinergic stimulation on endothelial cell activation in vitro and in vivo. Using the Shwartzman reaction, we observed that nicotine (2 mg/kg) and the novel cholinergic agent CAP55 (12 mg/kg) inhibit endothelial cell adhesion molecule expression. Using endothelial cell cultures, we observed the direct inhibitory effects of acetylcholine and cholinergic agents on tumor necrosis factor (TNF)-induced endothelial cell activation. Mecamylamine, an nAChR antagonist, reversed the inhibition of endothelial cell activation by both cholinergic agonists, confirming the antiinflammatory role of the nAChR cholinergic pathway. In vitro mechanistic studies revealed that nicotine blocked TNF-induced nuclear factor–κB nuclear entry in an inhibitor κB (IκB)α- and IκBɛ-dependent manner. Finally, with the carrageenan air pouch model, both vagus nerve stimulation and cholinergic agonists significantly blocked leukocyte migration in vivo. These findings identify the endothelium, a key regulator of leukocyte trafficking during inflammation, as a target of anti-inflammatory cholinergic mediators.

Loss of functional neuronal nicotinic receptors in dorsal root ganglion neurons in a rat model of neuropathic pain

Recent evidence has suggested that the anti-allodynic effect of neuronal acetylcholine receptor (nAChR) agonists may have a peripheral component [L.E. Rueter, K.L. Kohlhaas, P. Curzon, C.S. Surowy, M.D. Meyer, Peripheral and central sites of action for A-85380 in the spinal nerve ligation model of neuropathic pain, Pain 103 (2003) 269-276]. In further studies of the peripheral anti-allodynic mechanisms of nAChR agonists, we investigated the function of nAChRs in acutely isolated dorsal root ganglion (DRG) neurons from allodynic [L5-L6 spinal nerve ligation (SNL)] and naive adult rats. Following determination of cell diameter and membrane capacitance, responses to rapid applications of nAChR agonists were recorded under whole cell patch clamp. nAChR inward currents were observed in approximately 60% of naive neurons, across small, medium, and large diameter cells. Evoked nAChR currents could be clustered into three broad classes: fast transient, biphasic, and slow desensitizing currents, consistent with multiple subtypes of nAChR expressed in DRG [J.R. Genzen, W. Van Cleve, D.S. McGehee, Dorsal root ganglion neurons express multiple nicotinic acetylcholine receptor subtypes, J. Neurophysiol. 86 (2001) 1773-1782]. In contrast, in neurons from allodynic animals, the occurrence and amplitude of responses to nAChR agonists were significantly reduced. Reduced responsiveness to nAChR agonists covered the range of DRG neuron sizes. The decrease in the responsiveness to nAChR agonists was not seen in neighboring uninjured L4 neurons. The significant decrease in the number of cells with nAChR agonist responses, compounded with the significant decrease in response amplitude, indicates that there is a marked down regulation of functional nAChRs in DRG somata associated with SNL.

Synthesis and nicotinic acetylcholine receptor binding affinities of 2- and 3-isoxazolyl-8-azabicyclo[3.2.1]octanes

A series of epiboxidine homologues, 2- and 3-isoxazole substituted 8-azabicyclo[3.2.1]octane derivatives was synthesized and evaluated as potential ligands for neuronal nicotinic acetylcholine receptors in [(3)H]cytisine labeled rat brain. The 2beta-isoxazolyl-8-azabicyclo[3.2.1]octane 9b (K(i)=3 nM) was the most potent compound of the series with a binding affinity twice that of nicotine. The 3beta-isoxazolyl-8-azabicyclo[3.2.1]octane 15b (K(i)=148 nM) exhibited moderate affinity while the corresponding 2alpha- and 3alpha-isomers exhibited micromolar binding affinity.

Antinociception induced by chronic exposure of rats to cigarette smoke

To investigate if chronic exposure to cigarette smoke induces analgesia, rats were exposed to concentrated cigarette smoke in an environmental chamber over four successive 5-day blocks (6 h/day), with 2 smoke-free days between blocks. A control group was exposed to room air. Tail flick latencies increased significantly (analgesia) during each smoke exposure block, with a relative decline in analgesia across blocks (tolerance) and a return to control levels during the first three smoke-free interludes while remaining higher after the conclusion of the 4-week exposure period. Mechanical (von Frey) withdrawal thresholds declined over time in smoke-exposed and control groups, with the smoke-exposed group showing significantly lower thresholds. Plasma nicotine reached 95.4 +/- 32 (S.D.) ng/ml at the end of weekly smoke exposure and declined to 44.9 +/- 10.6 ng/ml 24 h after withdrawal. Rats lost weight during smoke exposure and quickly regained weight during smoke-free interludes and at the cessation of smoke exposure. Analgesia may contribute to the initiation of smoking, and rapid reversal of the analgesic effect following acute exposure may contribute to the difficulty in quitting smoking.

The solubilizing detergents, Tween 80 and Triton X-100 non-competitively inhibit α7-nicotinic acetylcholine receptor function in Xenopus oocytes

Because many studies rely upon detergents to solubilize lipophilic agents such as cannabinoid drugs, we examined the effect of commonly employed detergents on the function of the cloned alpha(7) subunit of the nicotinic ACh receptor. Homomeric alpha(7) receptors were expressed in Xenopus oocytes and the two-microelectrode voltage-clamp technique was used to assess their electrophysiological properties. The detergents Tween 80 and Triton X-100 reversibly inhibited ACh (100 microM)-induced inward currents in a concentration-dependent manner, with IC(50) values of 610 nM and 1.4 microM, respectively. The effects of these detergents were independent of membrane potential, and they were not mediated by endogenous Ca(2+)-dependent Cl(-) channels, since they were unaffected by intracellularly injected BAPTA, and recorded in Ca(2+)-free bathing solution containing 2 mM Ba(2+). Both detergents also decreased the maximal effect of ACh, without significantly affecting its EC(50), indicating a non-competitive interaction with the nACh alpha(7) receptors. In contrast to the effects of these detergents, we found that cholic acid (10 microM), DMSO (10 microM) and Tocrisol (0.01% v/v) did not cause a significant effect on nicotinic responses. In conclusion, we demonstrate that the detergents Tween 80 and Triton X-100 are potent inhibitors of neuronal nACh alpha(7) receptors expressed in Xenopus oocytes, and we suggest that studies utilizing these detergents to solubilize lipophilic drugs should be scrutinized for such effects.

Differential Effects of Endogenous and Synthetic Cannabinoids on α7-Nicotinic Acetylcholine Receptor-Mediated Responses in Xenopus Oocytes

The effects of endogenous and synthetic cannabinoid receptor agonists, including 2-arachidonoylglycerol (2-AG), R-methanandamide, WIN55,212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenylcarbonyl)-6H-pyrrolo[3,2,1ij]quinolin-6-one], and CP 55,940 [1alpha,2beta-(R)-5alpha]-(-)-5-(1,1-dimethyl)-2-[5-hydroxy-2-(3-hydroxypropyl) cyclohexyl-phenol], and the psychoactive constituent of marijuana, Delta9-tetrahydrocannabinol (Delta9-THC), on the function of homomeric alpha7-nicotinic acetylcholine (nACh) receptors expressed in Xenopus oocytes was investigated using the two-electrode voltage-clamp technique. The endogenous cannabinoid receptor ligands 2-AG and the metabolically stable analog of anandamide (arachidonylethanolamide), R-methanandamide, reversibly inhibited currents evoked with ACh (100 microM) in a concentration-dependent manner (IC50 values of 168 and 183 nM, respectively). In contrast, the synthetic cannabinoid receptor agonists CP 55,940, WIN55,212-2, and the phytochemical Delta9-THC did not alter alpha7-nACh receptor function. The inhibition of alpha7-mediated currents by 2-AG was found to be non-competitive and voltage-independent. Additional experiments using endocannabinoid metabolites suggested that arachidonic acid, but not ethanolamine or glycerol, could also inhibit the alpha7-nACh receptor function. Whereas the effects of arachidonic acid were also noncompetitive and voltage-independent, its potency was much lower than 2-AG and anandamide. Results of studies with chimeric alpha7-nACh-5-hydroxytryptamine (5-HT)3 receptors comprised of the amino-terminal domain of the alpha7-nACh receptor and the transmembrane and carboxyl-terminal domains of 5-HT3 receptors indicated that the site of interaction of the endocannabinoids with the alpha7-nAChR was not located on the N-terminal region of the receptor. These data indicate that cannabinoid receptor ligands that are produced in situ potently inhibit alpha7-nACh receptor function, whereas the synthetic cannabinoid ligands, and Delta9-THC, are without effect, or are relatively ineffective at inhibiting these receptors.

Peripheral nerve injury induces trans-synaptic modification of channels, receptors and signal pathways in rat dorsal spinal cord

Peripheral tissue injury-induced central sensitization may result from the altered biochemical properties of spinal dorsal horn. However, peripheral nerve injury-induced modification of genes in the dorsal horn remains largely unknown. Here we identified strong changes of 14 channels, 25 receptors and 42 signal transduction related molecules in Sprague-Dawley rat dorsal spinal cord 14 days after peripheral axotomy by cDNA microarray. Twenty-nine genes were further confirmed by semiquantitative RT-PCR, Northern blotting, in situ hybridization and immunohistochemistry. These regulated genes included Ca2+ channel alpha1E and alpha2/delta1 subunits, alpha subunits for Na+ channel 1 and 6, Na+ channel beta subunit, AMAP receptor GluR3 and 4, GABAA receptor alpha5 subunit, nicotinic acetylcholine receptor alpha5 and beta2 subunits, PKC alpha, betaI and delta isozymes, JNK1-3, ERK2-3, p38 MAPK and BatK and Lyn tyrosine-protein kinases, indicating that several signal transduction pathways were activated in dorsal spinal cord by peripheral nerve injury. These results demonstrate that peripheral nerve injury causes phenotypic changes in spinal dorsal horn. Increases in Ca2+ channel alpha2/delta1 subunit, GABAA receptor alpha5 subunit, Na+ channels and nicotinic acetylcholine receptors in both dorsal spinal cord and dorsal root ganglia indicate their potential roles in neuropathic pain control.

Alpha 4 nicotinic acetylcholine receptor subunit links cholinergic to brainstem monoaminergic neurotransmission

Agonists of nicotinic receptors containing the alpha4-subunit produce antinociception accompanied by several adverse side effects. The purpose of this study was to determine the distribution of the alpha4-subunit of nicotinic acetylcholine receptors (nAChR) in brainstem monoaminergic nuclei that may contribute to these effects using dual labeling immunofluorescence methods. The alpha4-subunit immunoreactivity was enriched in serotonergic (nucleus raphe magnus, pallidus, obscurus, and dorsalis) and noradrenergic (A5, locus coeruleus (LC), A7) areas associated with antinociception, where it was commonly colocalized with serotonin (5-HT) or tyrosine hydroxylase (TH) immunoreactivity. However, it was also noted that alpha4 was present in all other brainstem monoaminergic nuclei examined (adrenergic C1-C3, noradrenergic A1-alpha4, dopamine A9 and A10, nucleus raphe medianus). To determine if alpha4 agonists could impact neural activity in brainstem, monoaminergic nuclei that are associated with antinociception, the expression of c-Fos in response to the systemic administration of epibatidine (2.5, 5, or 10 microg/kg) was examined. Epibatidine produced a robust (2-5-fold) increase in c-Fos expression, which was not dose dependent, in all of these areas examined except the nucleus raphe magnus. These results suggest that the alpha4 subunit is positioned to mediate the effects of acetylcholine widely across many, if not all, monoaminergic neurons in the brainstem. These observations emphasize the potential involvement of noradrenergic, as well as serotonergic mechanisms in epibatidine's analgesic effects, and they also suggest that even selective alpha4 ligand may have widespread effects on brain monoamine neurotransmission.

The endogenous cannabinoid anandamide inhibits alpha7 nicotinic acetylcholine receptor-mediated responses in Xenopus oocytes

The effect of the endogenous cannabinoid ligand anandamide on the function of the cloned alpha7 subunit of the nicotinic acetylcholine (ACh) receptor expressed in Xenopus oocytes was investigated by using the two-electrode voltage-clamp technique. Anandamide reversibly inhibited nicotine (10 microM) induced-currents in a concentration-dependent manner (10 nM to 30 microM), with an IC50 value of 229.7 +/- 20.4 nM. The effect of anandamide was neither dependent on the membrane potential nor meditated by endogenous Ca2+ dependent Cl- channels since it was unaffected by intracellularly injected BAPTA and perfusion with Ca2+-free bathing solution containing 2 mM Ba2+. Anandamide decreased the maximal nicotine-induced responses without significantly affecting its potency, indicating that it acts as a noncompetitive antagonist on nicotinic acetylcholine (nACh) alpha7 receptors. This effect was not mediated by CB1 or CB2 receptors, as neither the selective CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboximide hydrochloride (SR 141716A) nor CB2 receptor antagonist N-((1S)-endo-1,3,3-trimethyl-bicyclo-heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR 144528) reduced the inhibition by anandamide. In addition, inhibition of nicotinic responses by anandamide was not sensitive to either pertussis toxin treatment or to the membrane permeable cAMP analog 8-Br-cAMP (0.2 mM). Inhibitors of enzymes involved in anandamide metabolism including phenylmethylsulfonyl fluoride, superoxide dismutase, and indomethacin, or the anandamide transport inhibitor AM404 did not prevent anandamide inhibition of nicotinic responses, suggesting that anandamide itself acted on nicotinic receptors. In conclusion, these results demonstrate that the endogenous cannabinoid anandamide inhibits the function of nACh alpha7 receptors expressed in Xenopus oocytes in a cannabinoid receptor-independent and noncompetitive manner.