The identification of novel structural compound classes exhibiting high affinity for neuronal nicotinic acetylcholine receptors and analgesic efficacy in preclinical models of pain

An integrated overview on pyrrolizines as potential anti-inflammatory, analgesic and antipyretic agents

Despite the existence of huge number of NSAIDs, the quest for safer drugs is still in the focus of several drug discovery programs. Pyrrolizine heterocyclic system is among the privileged scaffolds utilized in this regard. At least one of these pyrrolizines, ketorolac, has reached the market. The current review represents a collective effort to highlight the reported pyrrolizines with anti-inflammatory and analgesic potential and categorize them into eight different classes. Furthermore, the various synthetic approaches, structure-activity relationship as well as metabolic pathways have been discussed. Taken together, this review sets a base for researchers to design and synthesize novel pyrrolizine-based libraries for further development into safer and efficient anti-inflammatory and analgesic agents.

Contemporary and emergent pharmacological therapies for chronic pain: nonopioid analgesia

The American Pain Society estimates that 50 million Americans are partially or totally disabled by pain. This striking statistic is certain to increase as our population continues to age. In order to combat this growing problem, healthcare professionals must arm themselves with information. By developing the appropriate pain assessment skills and by staying abreast of the rapidly-changing therapies used in pain management, clinicians can dramatically impact the quality of life of those living with pain.

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).

Haeme oxygenase-1 overexpression via nAChRs and the transcription factor Nrf2 has antinociceptive effects in the formalin test

Epibatidine has shown antinociceptive effects in various pain models, being 200-fold more potent than morphine. Previous results from our laboratory demonstrated that HO-1 overexpression has an antinociceptive effect in the formalin test. Furthermore, epibatidine was able to induce haeme oxygenase-1 (HO-1). So, the aim of this study was to investigate the effect of HO-1 overexpression induced by epibatidine in nociception elicited by formalin injection in the mice hindpaw. Administration of epibatidine (4 microg/kg) 24h before the test reduced the nociceptive response during the first phase and second phase of the formalin test. This effect was prevented by treatment with tin protoporphyrin (SnPP, an inhibitor of HO-1 activity) administered via intraplantar 5min before the test, suggesting a main role of HO-1. Western blot analysis revealed that epibatidine treatment increased by 2-fold HO-1 expression in the paw; this effect was lost in knockout mice for nuclear factor-erythroid 2-related factor 2 (Nrf2) and was accompanied by the loss of its antinociceptive effect. Furthermore, the antinociceptive effect of epibatidine was related to the activation of alpha7 and/or alpha9 nAChRs since methyllycaconitine (MLA) and mecamylamine but not dihydro-beta-erythroidine (DHbetaE) reverted this effect. Finally, we showed by flow cytometry and by immunofluorescence that white blood cells of the animals injected with epibatidine expressed more HO-1 than control animals, and this expression was also reverted by MLA pre-treatment. These findings demonstrate that HO-1 induction by epibatidine has antinociceptive and anti-inflammatory effects by the activation of MLA-sensitive nAChRs.

Central Nicotinic Receptors: Structure, Function, Ligands, and Therapeutic Potential

The growing interest in nicotinic receptors, because of their wide expression in neuronal and non-neuronal tissues and their involvement in several important CNS pathologies, has stimulated the synthesis of a high number of ligands able to modulate their function. These membrane proteins appear to be highly heterogeneous, and still only incomplete information is available on their structure, subunit composition, and stoichiometry. This is due to the lack of selective ligands to study the role of nAChR under physiological or pathological conditions; so far, only compounds showing selectivity between alpha4beta2 and alpha7 receptors have been obtained. The nicotinic receptor ligands have been designed starting from lead compounds from natural sources such as nicotine, cytisine, or epibatidine, and, more recently, through the high-throughput screening of chemical libraries. This review focuses on the structure of the new agonists, antagonists, and allosteric ligands of nicotinic receptors, it highlights the current knowledge on the binding site models as a molecular modeling approach to design new compounds, and it discusses the nAChR modulators which have entered clinical trials.

Mapping brain activity following administration of a nicotinic acetylcholine receptor agonist, ABT-594, using functional magnetic resonance imaging in awake rats

Administration of ABT-594, a potent agonist for nicotinic acetylcholine receptors with selectivity for the alpha4beta2 receptor subtype, is known to modulate a diverse array of behaviors including those associated with nociception, anxiety and motor function. In this study, we sought to gain insight into the neural actions of ABT-594, in vivo, by conducting functional magnetic resonance imaging in awake and anesthetized rats. Using T(2)*-weighted gradient echo imaging and an ultrasmall superparamagnetic iron oxide contrast agent, functional imaging was conducted on a 4.7 T magnet to measure changes in relative cerebral blood volume. In awake, restrained, male Sprague-Dawley rats that were acclimated to the imaging environment, injection of ABT-594 (0.03-0.3 micromol/kg, i.v.) evoked changes to relative cerebral blood volume in several neural regions including the cingulate, somatosensory, motor, auditory, and pre-frontal cortices as well as the thalamus and the periaqueductal gray/dorsal raphe. These effects were typically bimodal with significant decreases in relative cerebral blood volume at the 0.03 micromol/kg dose and increases at the higher doses (0.1 and 0.3 micromol/kg). The decreases and increases in relative cerebral blood volume were often observed within the same region, but triggered by different doses. Both increases and decreases in relative cerebral blood volume were blocked by pretreatment with the noncompetitive nicotinic acetylcholine receptor antagonist, mecamylamine (5 micromol/kg, i.p.) in awake rats. Administration of ABT-594 (0.1 micromol/kg, i.v.) to alpha-chloralose-anesthetized rats did not significantly alter relative cerebral blood volume in any brain region suggesting an anesthetic-related interference with the effects of ABT-594. The neural regions affected by administration of ABT-594 corresponded well to the known pre-clinical behavioral profile for this compound, and demonstrate the utility of using functional magnetic resonance imaging in awake animals to study pharmacological action.

Involvement of spinal GABA receptors in the regulation of intraspinal acetylcholine release

It has been shown that analgesics such as morphine, lidocaine and clonidine increase the release of spinal acetylcholine. Acetylcholine may therefore play an important role in the regulation of spinal pain threshold. Since behavioral as well as in vitro studies have shown a clear involvement of GABA (gamma-amino butyric acid) receptors in the regulation of spinal nociceptive mechanisms, the present study focused on the role of GABA receptors for spinal acetylcholine release control. GABA receptor agonists and antagonists were infused via a spinal microdialysis probe and acetylcholine release was measured. The GABA(A) receptor agonist muscimol decreased acetylcholine release and the antagonist bicuculline increased acetylcholine release. The GABA(B) receptor agonist baclofen decreased acetylcholine release whereas the antagonist saclofen did not change acetylcholine release. The results suggest that both GABA receptor subtypes have an inhibitory role on spinal dorsal horn acetylcholine release and that the GABA(A) receptors are tonically regulating acetylcholine release.

Expression of functional nicotinic acetylcholine receptors in rat urinary bladder epithelial cells

Although nicotinic acetylcholine receptors in both the central and peripheral nervous systems play a prominent role in the control of urinary bladder function, little is known regarding expression or function of nicotinic receptors in the bladder epithelium, or urothelium. Nicotinic receptors have been described in epithelial cells lining the upper gastrointestinal tract, respiratory tract, and the skin. Thus the present study examined the expression and functionality of nicotinic receptors in the urothelium, as well as the effects of stimulation of nicotinic receptors on the micturition reflex. mRNA for the alpha3, alpha5, alpha7, beta3, and beta4 nicotinic subunits was identified in rat urothelial cells using RT-PCR. Western blotting also confirmed urothelial expression of the alpha3- and alpha7-subunits. Application of nicotine (50 nM) to cultured rat urothelial cells elicited an increase in intracellular Ca2+ concentration, indicating that at least some of the subunits form functional channels. These effects were blocked by the application of the nicotinic antagonist hexamethonium. During in vivo bladder cystometrograms in urethane-anesthetized rats, intravesical administration of nicotine, choline, or the antagonists methyllycaconitine citrate and hexamethonium elicited changes in voiding parameters. Intravesical nicotine (50 nM, 1 microM) increased the intercontraction interval. Intravesical choline (1-100 microM) also affected bladder reflexes similarly, suggesting that alpha7 nicotinic receptors mediate this effect. Intravesical administration of hexamethonium (1-100 microM) potentiated the nicotine-induced changes in bladder reflexes. Methyllycaconitine citrate, a specific alpha7-receptor antagonist, prevented nicotine-, choline-, and hexamethonium-induced bladder inhibition. These results are the first indication that stimulation of nonneuronal nicotinic receptors in the bladder can affect micturition.

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.

6'-Methylpyrido[3,4-b]norhomotropane: synthesis and outstanding potency in relation to the alpha4beta2 nicotinic receptor pharmacophore model

6'-Methylpyrido[3,4-b]norhomotropane [synthesis as the racemate reported here] is more potent at the alpha4beta2 nicotinic receptor than any previous bridged nicotinoid. The two nitrogens and 6'-methyl substituent are superimposable on the two nitrogens and 6-chloro substituent of epibatidine, with the best fit on comparing the chair conformer of the (1R)-pyridonorhomotropane with natural (1R)-epibatidine. In this pharmacophore model, the 6'-methyl substituent may be equivalent to the acetyl methyl of acetylcholine.

Nicotinic Agonists, Antagonists, and Modulators From Natural Sources

1. Acetylcholine receptors were initially defined as nicotinic or muscarinic, based on selective activation by two natural products, nicotine and muscarine. Several further nicotinic agonists have been discovered from natural sources, including cytisine, anatoxin, ferruginine, anabaseine, epibatidine, and epiquinamide. These have provided lead structures for the design of a wide range of synthetic agents. 2. Natural sources have also provided competitive nicotinic antagonists, such as the Erythrina alkaloids, the tubocurarines, and methyllycaconitine. Noncompetitive antagonists, such as the histrionicotoxins, various izidines, decahydroquinolines, spiropyrrolizidine oximes, pseudophrynamines, ibogaine, strychnine, cocaine, and sparteine have come from natural sources. Finally, galanthamine, codeine, and ivermectin represent positive modulators of nicotinic function, derived from natural sources. 3. Clearly, research on acetylcholine receptors and functions has been dependent on key natural products and the synthetic agents that they inspired.

Neonicotinoid insecticide toxicology: mechanisms of selective action

The neonicotinoids, the newest major class of insecticides, have outstanding potency and systemic action for crop protection against piercing-sucking pests, and they are highly effective for flea control on cats and dogs. Their common names are acetamiprid, clothianidin, dinotefuran, imidacloprid, nitenpyram, thiacloprid, and thiamethoxam. They generally have low toxicity to mammals (acute and chronic), birds, and fish. Biotransformations involve some activation reactions but largely detoxification mechanisms. In contrast to nicotine, epibatidine, and other ammonium or iminium nicotinoids, which are mostly protonated at physiological pH, the neonicotinoids are not protonated and have an electronegative nitro or cyano pharmacophore. Agonist recognition by the nicotinic receptor involves cation-pi interaction for nicotinoids in mammals and possibly a cationic subsite for interaction with the nitro or cyano substituent of neonicotinoids in insects. The low affinity of neonicotinoids for vertebrate relative to insect nicotinic receptors is a major factor in their favorable toxicological profile.

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.

Toxins in anti-nociception and anti-inflammation

The use of toxins as novel molecular probes to study the structure-function relationship of ion-channels and receptors as well as potential therapeutics in the treatment of wide variety of diseases is well documented. The high specificity and selectivity of these toxins have attracted a great deal of interest as candidates for drug development. This review highlights the involvement of the proteins and peptide toxins as well as non-proteinaceous compounds derived from both venomous and non-venomous animals, in anti-nociception and anti-inflammation. The possible mechanisms of these potential therapeutic agents and possible clinical applications in the treatment of pain and inflammation are also summarized.

α4β2 nACh Receptor pharmacophore models

Progress towards the development of alpha4beta2 nicotinic acetylcholinergic receptor pharmacophores is reviewed from the early Beers and Riech model to the newer vector models.

Synthesis and nicotinic binding of novel phenyl derivatives of UB-165. Identifying factors associated with alpha7 selectivity

Four racemic phenyl-substituted analogues 3-6 of the potent nicotinic agonist UB-165 1 have been synthesised and evaluated against the alpha(4)beta(2), alpha(3)beta(4), and alpha(7) neuronal nicotinic receptors. The 2'-phenyl derivative 3 shows no activity at these major receptor subtypes, while the 4'-phenyl analogue 4 shows an enhanced level of alpha(7) selectivity as compared to UB-165 and deschloro UB-165 2. These results are discussed within the context of recent pharmacophore models.

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.

Epibatidine: impact on nicotinic receptor research

Epibatidine is a natural product that was isolated and identified by Daly and coworkers in 1992. Since that time, it has had a profound influence on the investigation of alpha4beta2 nicotinic cholinergic (nACh) receptor pharmacophore models, and has inspired the development of novel agents with therapeutic potential in CNS disorders. Apart from acetylcholine and nicotine, probably no other agent has had as much recent impact on nACh research as has epibatidine.

3D QSAR analyses-guided rational design of novel ligands for the (alpha4)2(beta2)3 nicotinic acetylcholine receptor

Three-dimensional quantitative structure-activity relationship methods, the comparative molecular field analysis (CoMFA) and the comparative molecular similarity indices analysis (CoMSIA), were applied using a training set of 45 ligands of the (alpha4)2(beta2)3 nicotinic acetylcholine receptor (nAChR). All compounds are related to (-)-epibatidine, (-)-cytisine, (+)-anatoxin-a, and (-)-ferruginine, and additionally, novel diazabicyclo[4.2.1]nonane- and quinuclidin-2-ene-based structures were included. Their biological data have been determined by utilizing the same experimental protocol. Statistically reliable models of good predictive power (CoMFA r2 = 0.928, q2 = 0.692, no. of components = 3; CoMSIA r2 = 0.899, q2 = 0.701, no. of components = 3) were achieved. The results obtained were graphically interpreted in terms of field contribution maps. Hence, physicochemical determinants of binding, such as steric and electrostatic and, for the first time, hydrophobic, hydrogen bond donor, and hydrogen bond acceptor properties, were mapped back onto the molecular structures of a set of nAChR modulators. In particular, changes in the binding affinity of the modulators as a result of modifications in the aromatic ring systems could be rationalized by the steric, electrostatic, hydrophobic, and hydrogen bond acceptor properties. These results were used to guide the rational design of new nAChR ligands such as 48-52 and 54, which were subsequently synthesized for the first time and tested. Key steps of our synthetic approaches were successfully applied Stille and Suzuki cross-coupling reactions. Predictive r2 values of 0.614 and 0.660 for CoMFA and CoMSIA, respectively, obtained for 22 in part previously unknown ligands for the (alpha4)2(beta2)3 subtype, demonstrate the high quality of the 3D QSAR models.

Binding of nicotine and homoazanicotine analogues at neuronal nicotinic acetylcholinergic (nACh) receptors

A total of 20 substituted analogues of nicotine (1a) and homoazanicotine (3a) were examined in order to determine whether or not they might bind in a similar manner at alpha4beta2 nicotinic acetylcholinergic (nACh) receptors. It was found that parallel structural changes in the two series resulted in parallel shifts in affinity. Evidence suggests that the two series are binding in a comparable fashion.

Conformationally constrained nicotines: polycyclic, bridged, and spiro-annulated analogues as novel ligands for the nicotinic acetylcholine receptor

A set of novel nicotine-related, conformationally constrained compounds, including tetracyclic, bridged (4), and tricyclic, spiro-annulated (5) structures, were synthesized in a straightforward manner and optically resolved in a convenient fashion with (+)- and (-)-O,O'-di-p-toluoyltartaric acids. Absolute configurations were determined by X-ray crystallography. These compounds were evaluated for their ability to displace [(3)H]cytisine in a rat forebrain preparation and compared to (-)-nicotine. Three substances emerged with high affinity in the low nanomolar range. Moreover, one of these compounds ((+)-5b) showed not only high binding affinity (K(i) = 4.79 nM) but also significant enantioselectivity over its antipode (K(i) = 148 nM), supporting the hypothesis that conformational restraint can lead to high-affinity ligands, which are stereochemically discriminated by the nicotinic acetylcholine receptor and may feature optimum locations of the active sites of the pharmacophore.

A comparison of the binding of three series of nicotinic ligands

A total of 24 aryl-substituted analogues of nicotine (1a) and two related series of nicotinic ligands, aminomethylpyridines 3 and ether analogues 8, were examined to determine if they bind at alpha4beta2 nACh receptors in a common manner. A modest correlation (r=0.785) was found between the affinities of the nicotine analogues and derivatives of 3, but little correlation (r=0.348) was found with analogues 8. However, a modest correlation (r=0.742) exists between the binding of analogues 3 and 8. It seems that 1-series and 8-series compounds bind differently but that the 3-series compounds share some intermediate binding similarity with both.

The role of nicotinic acetylcholine receptors in the mechanisms of anesthesia

Nicotinic acetylcholine receptors are members of the ligand-gated ion channel superfamily, that includes also gamma-amino-butiric-acid(A), glycine, and 5-hydroxytryptamine(3) receptors. Functional nicotinic acetylcholine receptors result from the association of five subunits each contributing to the pore lining. The major neuronal nicotinic acetylcholine receptors are heterologous pentamers of alpha4beta2 subunits (brain), or alpha3beta4 subunits (autonomic ganglia). Another class of neuronal receptors that are found both in the central and peripheral nervous system is the homomeric alpha7 receptor. The muscle receptor subtypes comprise of alphabetadeltagamma (embryonal) or alphabetadeltaepsilon (adult) subunits. Although nicotinic acetylcholine receptors are not directly involved in the hypnotic component of anesthesia, it is possible that modulation of central nicotinic transmission by volatile agents contributes to analgesia. The main effect of anesthetic agents on nicotinic acetylcholine receptors is inhibitory. Volatile anesthetics and ketamine are the most potent inhibitors both at alpha4beta2 and alpha3beta4 receptors with clinically relevant IC(50) values. Neuronal nicotinic acetylcholine receptors are more sensitive to anesthetics than their muscle counterparts, with the exception of the alpha7 receptor. Several intravenous anesthetics such as barbiturates, etomidate, and propofol exert also an inhibitory effect on the nicotinic acetylcholine receptors, but only at concentrations higher than those necessary for anesthesia. Usual clinical concentrations of curare cause competitive inhibition of muscle nicotinic acetylcholine receptors while higher concentrations may induce open channel blockade. Neuronal nAChRs like alpha4beta2 and alpha3beta4 are inhibited by atracurium, a curare derivative, but at low concentrations the alpha4beta2 receptor is activated. Inhibition of sympathetic transmission by clinically relevant concentrations of some anesthetic agents is probably one of the factors involved in arterial hypotension during anesthesia.

The therapeutic potential of nicotinic acetylcholine receptor agonists for pain control

Due to the limitations of currently available analgesics, a number of novel alternatives are currently under investigation, including neuronal nicotinic acetylcholine receptor (nAChR) agonists. During the 1990s, the discovery of the antinociceptive properties of the potent nAChR agonist epibatidine in rodents sparked interest in the analgesic potential of this class of compounds. Although epibatidine also has several mechanism-related toxicities, the identification of considerable nAChR diversity suggested that the toxicities and therapeutic actions of the compound might be mediated by distinct receptor subtypes. Consistent with this view, a number of novel nAChR agonists with antinociceptive activity and improved safety profiles in preclinical models have now been identified, including A-85380, ABT-594, DBO-83, SIB-1663 and RJR-2403. Of these, ABT-594 is the most advanced and is currently in Phase II clinical evaluation. Nicotinically-mediated antinociception has been demonstrated in a variety of rodent pain models and is likely mediated by the activation of descending inhibitory pathways originating in the brainstem with the predominant high-affinity nicotine site in brain, the alpha4beta2 subtype, playing a critical role. Thus, preclinical findings suggest that nAChR agonists have the potential to be highly efficacious treatments in a variety of pain states. However, clinical proof-of-principle studies will be required to determine if nAChR agonists are active in pathological pain.