The potential of subtype-selective neuronal nicotinic acetylcholine receptor agonists as therapeutic agents

Secondhand Smoke Exposure and Low Blood Lead Levels in Association With Attention-Deficit Hyperactivity Disorder and Its Symptom Domain in Children: A Community-Based Case-Control Study

AIM

Secondhand smoke (SHS) is a major indoor pollutant. We examined the possible association between exposure to both SHS and low levels of lead and attention-deficit-hyperactivity disorder (ADHD) and its symptom domain in children.

METHODS

This case-control study was based on the results of a community survey using the ADHD rating scale conducted in 49 elementary schools. Both cases and control subjects were confirmed by a child psychiatrist. Each case was matched with one control subject according to gender, school, and grade in school. Using a multivariate conditional logistic regression model, we analyzed 214 case-control pairs of children who ranged in age from 6 to 10 years. Urine and blood levels of cotinine and of lead were determined, and information pertaining to SHS exposure was obtained by means of a questionnaire.

RESULTS

Exposure to low levels of lead (geometric mean = 1.65 µg/dL) was related to ADHD, particularly inattention (odds ratio [OR] = 1.67, 95% confidence interval [CI] = 1.07-2.59), whereas SHS exposure was associated mainly with hyperactivity/impulsivity (OR = 3.85, 95% CI = 1.55-9.56). In the pathway from blood lead to hyperactivity/impulsivity, children's SHS exposure mediated and indirectly accounted for about 73% of this relationship. The combined exposure to lead and SHS synergistically increased the risk of ADHD, evident as both inattention and hyperactivity/impulsivity.

CONCLUSION

SHS, which is associated with hyperactivity/impulsivity in particular, combined with exposure to low blood levels of lead synergistically increased the risk of ADHD. Therefore, the exposure of children to both SHS and lead needs to be reduced.

IMPLICATIONS

Although exposure to low levels of lead has been shown to be associated with ADHD, there is little evidence of symptom domain specificity. In our study, low blood lead levels were related to inattention. In addition, prenatal or postnatal exposure to SHS increased the risk of ADHD, particularly hyperactivity/impulsivity. Combined exposure to lead and SHS synergistically increased the risk for both these ADHD symptom domains. To protect children from environmental risk factors related to ADHD, it is necessary to further reduce children's exposure to SHS and lead, even in those with low blood lead levels.

Discovery of benzamide analogs as negative allosteric modulators of human neuronal nicotinic receptors: pharmacophore modeling and structure-activity relationship studies

The present study describes our ongoing efforts toward the discovery of drugs that selectively target nAChR subtypes. We exploited knowledge on nAChR ligands and their binding site that were previously identified by our laboratory through virtual screenings and identified benzamide analogs as a novel chemical class of neuronal nicotinic receptor (nAChR) ligands. The lead molecule, compound 1 (4-(allyloxy)-N-(6-methylpyridin-2-yl)benzamide) inhibits nAChR activity with an IC₅₀ value of 6.0 (3.4-10.6) μM on human α4β2 nAChRs with a ∼5-fold preference against human α3β4 nAChRs. Twenty-six analogs of compound 1 were also either synthesized or purchased for structure-activity relationship (SAR) studies and provided information relating the chemical/structural properties of the molecules to their ability to inhibit nAChR activity. The discovery of subtype-selective ligands of nAChRs described here should contribute significantly to our understanding of the involvement of specific nAChR subtypes in normal and pathophysiological states.

The first crystal structure of a free neutral form of a nicotine derivative

The single crystal X-ray diffraction analysis of a racemic form of a nicotine derivative, 5-(2-hydroxy-2-methylbut-3-ynyl)-3-(1-methyl-2-pyrrolidinyl)pyridine (2), provides structural information on molecular geometry and intermolecular interactions that could be relevant for the transport and docking of nicotine ligands to nicotinic acetylcholine receptors (nAChRs), since it is the first crystal structure of a free neutral form of a nicotine derivative. Each enantiomer of the two independent molecules of the asymmetric unit is characterized by (i) an anti orientation of the N2 methyl substituent with respect to the pyridine ring (ii) an envelope form of the pyrrolidine ring (iii) an orientation roughly perpendicular of the pyridine and pyrrolidine rings. The crystal packing is characterized by the formation of OH...N1 hydrogen-bond cyclic dimers that are packed through van der Waals interactions. This selectivity of hydrogen-bonding site in favor of the N1 pyridine nitrogen found in the solid state for (2) agrees with the greater hydrogen-bond acceptor strength of the N1 pyridine nitrogen compared to the N2 pyrrolidine one previously found in solution for nicotine (1).

Pharmacological characterization of (S)-(2)-5-ethynyl-3-(1-methyl-2-pyrrolidinyl)pyridine HCl (SIB-1508Y, Altinicline), a novel nicotinic acetylcholine receptor agonist

(S)-(2)-5-ethynyl-3-(1-methyl-2-pyrrolidinyl)pyridine HCl (SIB-1508Y, Altinicline), is a subtype-selective neuronal nicotinic acetylcholine receptor (nAChR) agonist. In rodents, SIB-1508Y exhibited antidepressant activity, reversed age-related decrements in vigilance, and improved motor and cognitive function in primate models of Parkinson's disease. The goal of the study was to explore neurochemical effects of SIB-1508Y and its isomer, SIB-1680WD. In vitro, SIB-1508Y increased dopamine (DA) release from slices of rat striatum, nucleus accumbens (NAc), olfactory tubercles (OT) and prefrontal cortices (PFC) in a concentration-dependent manner. Relative to its robust effects on DA release from various brain regions, SIB-1508Y was minimally effective at increasing NE release from hippocampus or PFC, and 5-HT release from PFC. SIB-1680WD was less potent and efficacious than SIB-1508Y, but did not act as a partial agonist. Subcutaneous injection of SIB-1508Y (10 mg/kg) increased striatal DA release and this release was sensitive to blockade by the non-competitive nAChR antagonist, mecamylamine (Mec). SIB-1508Y also increased hippocampal ACh release selectively without affecting striatal ACh release. Hippocampal ACh release evoked by SIB-1508Y was attenuated by nAChR antagonists Mec and Dihydro-beta-erythroidine (DHbetaE), and also by the DA D1 receptor antagonist, SCH-23390. These results are consistent with previously established pharmacology of nAChR regulation of hippocampal ACh release. Repeated administration of SIB-1508Y did not result in an enhanced striatal DA release or hippocampal ACh release. In summary, the abilities of SIB-1508Y to release multiple neurotransmitters in distinct brain regions may contribute to its behavioral profile.

Synthesis of 2-(pyridin-3-yl)-1-azabicyclo[3.2.2]nonane, 2-(pyridin-3-yl)-1-azabicyclo[2.2.2]octane, and 2-(pyridin-3-yl)-1-azabicyclo[3.2.1]octane, a class of potent nicotinic acetylcholine receptor-ligands

In an attempt to generate nicotinic acetylcholine receptor (nAChR) ligands selective for the alpha4beta2 and alpha7 subtype receptors we designed and synthesized constrained versions of anabasine, a naturally occurring nAChR ligand. 2-(Pyridin-3-yl)-1-azabicyclo[2.2.2]octane, 2-(pyridin-3-yl)-1-azabicyclo[3.2.2]nonane, and several of their derivatives have been synthesized in both an enantioselective and a racemic manner utilizing the same basic synthetic approach. For the racemic synthesis, alkylation of N-(diphenylmethylene)-1-(pyridin-3-yl)methanamine with the appropriate bromoalkyltetrahydropyran gave intermediates which were readily elaborated into 2-(pyridin-3-yl)-1-azabicyclo[2.2.2]octane and 2-(pyridin-3-yl)-1-azabicyclo[3.2.2]nonane via a ring opening/aminocyclization sequence. An alternate synthesis of 2-(pyridin-3-yl)-1-azabicyclo[3.2.2]nonane via the alkylation of N-(1-(pyridin-3-ylethylidene)propan-2-amine has also been achieved. The enantioselective syntheses followed the same general scheme, but utilized imines derived from (+)- and (-)-2-hydroxy-3-pinanone. Chiral HPLC shows that the desired compounds were synthesized in >99.5% ee. X-ray crystallography was subsequently used to unambiguously characterize these stereochemically pure nAChR ligands. All compounds synthesized exhibited high affinity for the alpha4beta2 nAChR subtype ( K i < or = 0.5-15 nM), a subset bound with high affinity for the alpha7 receptor subtype ( K i < or = 110 nM), selectivity over the alpha3beta4 (ganglion) receptor subtype was seen within the 2-(pyridin-3-yl)-1-azabicyclo[2.2.2]octane series and for the muscle (alpha1betagammadelta) subtype in the 2-(pyridin-3-yl)-1-azabicyclo[3.2.2]nonane series.

Radiosynthesis and ex vivo evaluation of [11C]-SIB-1553A as a PET radiotracer for beta4 selective subtype nicotinic acetylcholine receptor

[11C]-SIB-1553A ((+/-)-4-[2-((N-[11C]-methyl)-2-pyrrolidinyl)ethyl]thiophenol) was labelled with carbon-11 (t1/2=20.4 min) and evaluated in vivo as potential radiotracer for noninvasive assessment of the beta4 subunit nicotinic acetylcholine neurotransmission system with positron emission tomography (PET). The labelling precursor was obtained within five steps from N-Boc-prolinal in 45-56% overall yields. The radiosynthesis of [11C]-SIB-1553A was achieved by a selective N-[11C]-methylation in 32 min with a radiochemical purity greater than 97%, 7.5-30 GBq/micromol of specific radioactivity and 55-65% radiochemical yield (decay corrected, based on [11C]methyl iodide). The ex vivo pharmacological profile of [11C]-SIB-1553A was evaluated in rats with biodistribution studies in organs and in brain structures by autoradiography. The radiotracer uptake in the brain reached 0.49 %ID/g at 10 min and no brain radiometabolite was detected 40 min after intravenous injection. The quantification of radioactivity in various cerebral structures indicated a significantly higher radioactivity level at 15 min than at 30 min. Among the beta4 nAChR subunit-rich structures studied in the rat brain, only the thalamus at 15 and 30 min and the hippocampus at 30 min showed significantly higher uptake. Moreover, competition studies performed with SIB-1553A (15 min before the radiotracer injection) revealed only a low specific binding estimated to 7% of the total binding at 15 min and 13% at 30 min.

Presynaptic lonotropic receptors

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.

Profile of nicotinic acetylcholine receptor agonists ABT-594 and A-582941, with differential subtype selectivity, on delayed matching accuracy by young monkeys

ABT-594 and A-582941 are high affinity neuronal nicotinic acetylcholine receptor agonists with differential selectivity for the alpha4beta2 and the alpha7 subtypes, respectively. This study was designed to determine whether either compound, like nicotine also possesses cognitive-enhancing ability. The compounds were administered by intramuscular injection to young adult Rhesus monkeys trained to perform two versions of a computer-assisted delayed matching-to-sample (DMTS) task. ABT-594 (0.115-3.7 microg/kg) significantly improved DMTS accuracies, shifting the retention curve (accuracy-delay relationship) to the right in a parallel fashion. DMTS accuracy also was maintained during the sessions initiated 24h after compound administration. Because task accuracy was improved during short delay trials, a separate study was performed in which non-predictable distractors were inserted within the DMTS format to impair accuracy. The 0.115 microg/kg dose of ABT-594 almost completely reversed distractor-impaired performance associated with short delay trials. The alpha7 nAChR agonist, A-582941 (1.14-38 microg/kg) also significantly improved DMTS accuracies. The compound produced a significant improvement during long delay trials. The effect was twice as robust for long delay as compared with short delay trials and A-582941 was not as effective as ABT-594 in improving short delay trial accuracy. A-582941 also failed to sustain task improvement during sessions run 24h after dosing. These data are consistent with the ability of subtype-preferring nicotinic receptor agonists to enhance specific components of working memory and cognitive function, and they suggest that differential subtype selectivity could result in varied pharmacological response profiles.

Molecular targets for treating cognitive dysfunction in schizophrenia

Cognitive impairment is a core feature of schizophrenia as deficits are present in the majority of patients, frequently precede the onset of other positive symptoms, persist even with successful treatment of positive symptoms, and account for a significant portion of functional impairment in schizophrenia. While the atypical antipsychotics have produced incremental improvements in the cognitive function of patients with schizophrenia, overall treatment remains inadequate. In recent years, there has been an increased interest in developing novel strategies for treating the cognitive deficits in schizophrenia, focusing on ameliorating impairments in working memory, attention, and social cognition. Here we review various molecular targets that are actively being explored for potential drug discovery efforts in schizophrenia and cognition. These molecular targets include dopamine receptors in the prefrontal cortex, nicotinic and muscarinic acetylcholine receptors, the glutamatergic excitatory synapse, various serotonin receptors, and the gamma-aminobutyric acid (GABA) system.

N, N -disubstituted piperazines and homopiperazines: synthesis and affinities at alpha4beta2* and alpha7* neuronal nicotinic acetylcholine receptors

A series of N, N- disubstituted piperazines and homopiperazines were prepared and evaluated for binding to natural alpha4beta2* and alpha7* neuronal nicotinic acetylcholine receptors (nAChRs) using whole brain membrane. Some compounds exhibited good selectivity for alpha4beta2* nAChRs and did not interact with the alpha7* nAChRs subtype. The most potent analogs were compounds 8-19 (K(i) = 10.4 microM), 8-13 (K(i) = 12.0 microM), and 8-24 (K(i) = 12.8 microM). Thus, linking together a pyridine pi-system and a cyclic amine moiety via a homopiperazine ring affords compounds with low affinity but with good selectivity for alpha4beta2* nAChRs.

The therapeutic potential of nicotine and nicotinic agonists for weight control

Transdermal nicotine patches have been successfully introduced as a safe and powerful aid to smoking cessation; this has contributed to the rising interest in additional therapeutic applications for nicotine and synthetic nicotinic agonists. Nicotine and nicotinic agonists may have a therapeutic potential for a variety of disorders, including Alzheimer's and Parkinson's diseases, depression, attention deficit disorder, Tourette's syndrome and ulcerative colitis. These interests are partially fuelled by the urgent need of the tobacco industry to find new niches for nicotine in a world bound eventually to retire from cigarette smoking. At the same time, there is an increased interest in developing drugs for fighting obesity, a growing affliction of industrialised nations. This review presents data on the potential of nicotine, and in particular synthetic nicotinic agonists, for controlling body weight. Nicotinic agonists may become relatively safe, effective and inexpensive alternatives for several optional drugs currently being developed for treating human obesity, including beta-3-adrenergic agonists, leptin and its agonists, and neuropeptide Y antagonists.

Treatments for schizophrenia: a critical review of pharmacology and mechanisms of action of antipsychotic drugs

The treatment of schizophrenia has evolved over the past half century primarily in the context of antipsychotic drug development. Although there has been significant progress resulting in the availability and use of numerous medications, these reflect three basic classes of medications (conventional (typical), atypical and dopamine partial agonist antipsychotics) all of which, despite working by varying mechanisms of actions, act principally on dopamine systems. Many of the second-generation (atypical and dopamine partial agonist) antipsychotics are believed to offer advantages over first-generation agents in the treatment for schizophrenia. However, the pharmacological properties that confer the different therapeutic effects of the new generation of antipsychotic drugs have remained elusive, and certain side effects can still impact patient health and quality of life. Moreover, the efficacy of antipsychotic drugs is limited prompting the clinical use of adjunctive pharmacy to augment the effects of treatment. In addition, the search for novel and nondopaminergic antipsychotic drugs has not been successful to date, though numerous development strategies continue to be pursued, guided by various pathophysiologic hypotheses. This article provides a brief review and critique of the current therapeutic armamentarium for treating schizophrenia and drug development strategies and theories of mechanisms of action of antipsychotics, and focuses on novel targets for therapeutic agents for future drug development.

Nicotine and epibatidine triggered prolonged rise in calcium and TH gene transcription in PC12 cells

The effect of epibatidine on regulation of [Ca2+]i and tyrosine hydroxylase (TH) transcription was examined. Epibatidine triggers a biphasic rise in [Ca2+]i in PC12 cells similar to that observed with nicotine. There was an immediate transient increase in [Ca2+]i and a subsequent sustained second elevation. In contrast to nicotine, the epibatidine-triggered increase in [Ca2+]i was independent of activation of alpha7 nicotinic acetylcholine receptors, as it was not altered by either methyllycaconitine or alpha-bungarotoxin. The second [Ca2+]i elevation involves calcium release from intracellular stores and is inhibited by dantrolene or xestospongin C. Epibatidine, like nicotine, elevated TH promoter driven reporter transcription, mostly mediated by the cyclic-AMP responsive motifs. Elevation in TH promoter activity requires Ca2+ and cAMP since it is inhibited by 1,2-bis(o-Aminophenoxy)ethane-N,N,N',N'-tetraacetic Acid Tetra (acetoxymethyl ester) (BAPTA-AM) or 2',5'-dideoxyadenosine (DDA). The results reveal that epibatidine can elevate [Ca2+]i in an alpha7 independent manner and nevertheless induce TH transcription.

In vitro inhibition of recombinant ligand-gated ion channels by high concentrations of milnacipran

RATIONALE

Tricyclic antidepressants are known to inhibit various ligand-gated ion-channel (LGIC) receptors, and some of their clinical features may be associated with this activity. The effects of milnacipran, a selective inhibitor of the reuptake of serotonin and noradrenaline, on LGIC receptors have not yet been investigated on such ion-channel receptors.

OBJECTIVES

To determine the in vitro effect of milnacipran on four recombinant LGIC receptors, nicotinic acetylcholine, N-methyl-D-aspartate, gamma-amino butyric acid (GABA) and 5-hydroxytryptamine3A receptors.

METHODS

Receptors were expressed in Xenopus oocytes and LGIC activity measured using a two-voltage clamp technique.

RESULTS

There was no interaction at the GABA receptor. The results at the other LGIC receptors showed that they could be inhibited by high concentrations of milnacipran.

CONCLUSIONS

At high concentrations, milnacipran can inhibit certain LGICs. It is, however, unlikely that these interactions have any clinical consequence under normal therapeutic conditions, since the concentrations required are considerably higher than those achieved in plasma of treated patients.

Neuroprotective strategies in Parkinson's disease : an update on progress

In spite of the extensive studies performed on postmortem substantia nigra from Parkinson's disease patients, the aetiology of the disease has not yet been established. Nevertheless, these studies have demonstrated that, at the time of death, a cascade of events had been initiated that may contribute to the demise of the melanin-containing nigro-striatal dopamine neurons. These events include increased levels of iron and monoamine oxidase (MAO)-B activity, oxidative stress, inflammatory processes, glutamatergic excitotoxicity, nitric oxide synthesis, abnormal protein folding and aggregation, reduced expression of trophic factors, depletion of endogenous antioxidants such as reduced glutathione, and altered calcium homeostasis. To a large extent, the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA) animal models of Parkinson's disease confirm these findings. Furthermore, neuroprotection can be afforded in these models with iron chelators, radical scavenger antioxidants, MAO-B inhibitors, glutamate antagonists, nitric oxide synthase inhibitors, calcium channel antagonists and trophic factors. Despite the success obtained with animal models, clinical neuroprotection is much more difficult to accomplish. Although the negative studies obtained with the MAO-B inhibitor selegiline (deprenyl) and the antioxidant tocopherol (vitamin E) may have resulted from an inappropriate choice of drug (selegiline) or an inadequate dose (tocopherol), the niggling problem that still remains is why these drugs, and others, do work in animals while they fail in the clinic. One reason for this may be related to the fact that in normal human brains the number of dopaminergic neurons falls by around 3-5% every decade, while in Parkinson's disease this decline is greater. Brain autopsy studies have shown that by the time the disease is identified, some 70-75% of the dopamine-containing neurons have been lost. More sensitive reliable methods and clinical correlative markers are required to discern between confoundable symptomatic effects versus a possible neuroprotective action of drugs, namely, the ability to delay or forestall disease progression by protecting or rescuing the remaining dopamine neurons or even restoring those that have been lost.A number of other possibilities for the clinical failure of potential neuroprotectants also exist. First, the animal models of Parkinson's disease may not be totally reflective of the disease and, therefore, the chemical pathologies established in the animal models may not cause, or contribute to, the progression of the disease clinically. Second, because of the series of events occurring in neurodegeneration and our ignorance about which of these factors constitutes the primary event in the pathogenic process, a single drug may not be adequate to induce neuroprotection and, as a consequence, use of a cocktail of drugs may be more appropriate. The latter concept receives support from recent complementary DNA (cDNA) microarray gene expression studies, which show the existence of a gene cascade of events occurring in the nigrostriatal pathway of MPTP, 6-OHDA and methamphetamine animal models of Parkinson's disease. Even with the advent of powerful new tools such as genomics, proteomics, brain imaging, gene replacement therapy and knockout animal models, the desired end result of neuroprotection is still beyond our current capability.

Co-expression of alpha7 and beta2 nicotinic acetylcholine receptor subunit mRNAs within rat brain cholinergic neurons

Nicotine enhances cognitive and attentional processes through stimulation of the basal forebrain cholinergic system. Although muscarinic cholinergic autoreceptors have been well characterized, pharmacological characterization of nicotinic autoreceptors has proven more difficult. The present study used double-labeling in situ hybridization to determine expression of nicotinic acetylcholine receptor (nAChR) subunit mRNAs within basal forebrain cholinergic neurons in order to gain information about possible nAChR autoreceptor properties. Cholinergic cells of the mesopontine tegmentum and striatal interneurons were also examined, as were septohippocampal GABAergic neurons that interact with cholinergic neurons to regulate hippocampal activity. alpha7 and beta2 nAChR mRNAs were found to be co-expressed in almost all cholinergic cells and in the majority of GABAergic neurons examined. alpha4 nAChR mRNA expression was restricted to cholinergic cells of the nucleus basalis magnocellularis, and to non-cholinergic cells of the medial septum and mesopontine tegmentum. These data suggest possible regional differences in the pharmacological properties of nicotinic autoreceptors on cholinergic cells. Whereas most cholinergic cells express rapidly desensitizing alpha7 homomers or alpha7beta2 heteromers, cortical projection neurons may also express a pharmacologically distinct alpha4beta2 nAChR subtype. There may also be differential nAChR regulation of cholinergic and non-cholinergic cells within the mesopontine tegmentum that are implicated in acquisition of nicotine self-administration.

Chronic nicotine administration increases NGF-like immunoreactivity in frontoparietal cerebral cortex

Nicotine/nicotine agonists, which have been proposed as therapeutic agents for the treatment of Alzheimer's disease and other neurodegenerative disorders, produce a wide variety of effects on the nervous system. Some mechanisms involved remain poorly understood. In this work, immunohistochemical techniques were used to determine the effect of nicotine on nerve growth factor (NGF) in the frontoparietal (motor, somatosensory) brain cortex of the albino rat. Nicotine was chronically administered intraperitoneally using osmotic pumps (0.35 mg nicotine base/kg body weight/day for 14 days). An increase in the number and the immunoreaction intensity of NGF-like positive pyramidal and nonpyramidal neurons of these cortical areas was observed after treatment. Immunopositive astroglial cells were always seen in sections of treated animals but not in controls. The neuropil of control animals was, in general, devoid of reaction, but in treated animals, immunopositive prolongations were located randomly, some in close association with capillaries. At the electron microscopic level, these prolongations were demonstrated as belonging to neurons (dendrites and axons) and astroglial cells. Nicotinic activation of selected neurons and glial cells seems to trigger NGF/neurotrophic mechanisms, suggesting their use may be of benefit in prevention and treatment of neurodegenerative diseases.

Cognitive enhancing properties and tolerability of cholinergic agents in mice: a comparative study of nicotine, donepezil, and SIB-1553A, a subtype-selective ligand for nicotinic acetylcholine receptors

Several studies have demonstrated the importance of nicotinic mechanisms in the pathophysiology of neurodegenerative and cognitive disorders, warranting the search and development of novel nicotinic ligands as potential therapeutic agents. The present study was designed to assess whether the subtype-selective nicotinic acetylcholine receptor (nAChR) ligand SIB-1553A [(+/-)-4-([2-(1-methyl-2-pyrrolidinyl)ethyl]thio)phenol hydrochloride], with predominant agonist activity at beta4 subunit-containing human nAChRs, and no activity at muscle nAChR subtypes, could enhance cognitive performance in rodents with a more desirable safety/tolerability profile as compared to the nonselective prototypic nAChR ligand nicotine. SIB-1553A was equi-efficacious to nicotine in improving working memory performance in scopolamine-treated mice as measured by increased alternation in a T-maze, and was more efficacious than nicotine in improving the baseline cognitive performance of aged mice. This effect on working memory was confirmed in a delayed nonmatching to place task using the eight-arm radial maze. SIB-1553A produced dose-dependent side effects (ie motor deficits and seizures), although these effects were observed at doses 12 to 640-fold above those required to increase cognitive performance. Overall, SIB-1553A was significantly less potent than nicotine in eliciting these undesirable effects. Thus, the subtype-selective profile of SIB-1553A appears to translate into a more efficacious and better tolerated nAChR ligand as compared to nicotine. In the present studies, cognitive enhancement induced by SIB-1553A was similar in magnitude to that produced by the clinically efficacious acetylcholinesterase inhibitor donepezil. Taken together, the present data confirm the importance of nAChR subtypes in modulating cognitive processes, and suggest that activation of nAChR subtypes by selective nAChR ligands may be a viable approach to enhance cognitive performance.

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.

N,N-disubstituted piperazines: synthesis and affinities at alpha4beta2(*) and alpha7(*) neuronal nicotinic acetylcholine receptors

A series of N,N-disubstituted piperazines were prepared and evaluated for binding to alpha4beta2(*) and alpha7(*) neuronal nicotinic acetylcholine receptors using rat striatum and whole brain membrane preparations, respectively. This series of compounds exhibited selectivity for alpha4beta2(*) nAChRs and did not interact with the alpha7(*) nAChRs subtype. The most potent analogues were compounds 8b and 8f (K(i)=32 microM). Thus, linking together a pyridine pi-system and a cyclic amine moiety via a piperazine ring affords compounds with low affinity, but good selectivity for alpha4beta2(*) nicotinic receptors.

Alzheimer's disease and the basal forebrain cholinergic system: relations to beta-amyloid peptides, cognition, and treatment strategies

Alzheimer's disease (AD) is the most common form of degenerative dementia and is characterized by progressive impairment in cognitive function during mid- to late-adult life. Brains from AD patients show several distinct neuropathological features, including extracellular beta-amyloid-containing plaques, intracellular neurofibrillary tangles composed of abnormally phosphorylated tau, and degeneration of cholinergic neurons of the basal forebrain. In this review, we will present evidence implicating involvement of the basal forebrain cholinergic system in AD pathogenesis and its accompanying cognitive deficits. We will initially discuss recent results indicating a link between cholinergic mechanisms and the pathogenic events that characterize AD, notably amyloid-beta peptides. Following this, animal models of dementia will be discussed in light of the relationship between basal forebrain cholinergic hypofunction and cognitive impairments in AD. Finally, past, present, and future treatment strategies aimed at alleviating the cognitive symptomatology of AD by improving basal forebrain cholinergic function will be addressed.

Cholinergic nicotinic systems in Alzheimer's disease: prospects for pharmacological intervention

Within the last few years, research into the cause and progression of Alzheimer's disease has made significant advances. Although there is still no preventative treatment or cure for this neurodegenerative illness, the development of drugs that may alleviate some of the cognitive symptoms associated with it is advancing. Cholinesterase inhibitors are at present the most effective form of treatment and have shown significant overall response rates in clinical trials. However, although some patients show substantial improvement when treated with this class of drugs, there is considerable variability in the amount of benefit gained in different individuals in terms of their cognitive and behavioural functioning. Furthermore, unfortunately some patients gain little or no benefit from these drugs. It would therefore be of great advantage to explore alternative therapeutic possibilities. This article reviews the potential involvement of the nicotinic cholinergic system in Alzheimer's disease and discusses the possibility of nicotinic pharmacotherapy. Substantial evidence indicates the involvement of the nicotinic cholinergic system in the pathology of Alzheimer's disease. Drugs targeting these sites may not only have a positive effect on cognitive function, but also have additional therapeutic benefits in terms of restoring the hypoactivity in the excitatory amino acid pyramidal system and even slowing the emergence of Alzheimer's disease pathology. The conclusion of this review is that nicotinic treatments are an important potential source of new therapeutic interventions in Alzheimer's disease.

Nicotine produces selective degeneration in the medial habenula and fasciculus retroflexus

Nicotine's neurotoxic properties in rats were investigated by administering (-)-nicotine tartrate for 5 days either continuously in doses of 5.01, 5.72, 6.44, 7.13, 20.41 and 43.1 mg/kg/day via osmotic minipump or intermittently at 11.32 mg/kg/day via one daily subcutaneous injection. As assessed by silver staining, neurotoxicity was seen almost exclusively in the axons of the medial habenula and its output tract, the fasciculus retroflexus, in all treatment groups except the lowest dose. Within the habenula, the damage was noted in the ventral-medial-most portion of the nucleus which is thought to be dense with the alpha 4 beta 2 and/or alpha 3 beta 4 receptor subtypes. Past research has shown the medial habenula to be highly sensitive to the effects of nicotine, and these findings, in conjunction with related research using dopaminergic stimulants, indicate that the habenula may be a weak link in the neurotoxicity seen following stimulant drugs of abuse.

Histochemical study of acute and chronic intraperitoneal nicotine effects on several glycolytic and Krebs cycle dehydrogenase activities in the frontoparietal cortex and subcortical nuclei of the rat brain

The effects of nicotine on the activity of different dehydrogenases in frontoparietal regions and subcortical nuclei of the rat brain have been studied using histochemical methods. Nicotine sulphate was intraperitoneally administered in acute (4 mg/kg/day x 3 days) or chronic (ALZET osmotic pump providing 2 mg/kg/day x 15 days) doses. The enzymes analyzed were glyceraldehyde-3-phosphate, lactate, malate and succinate dehydrogenases (gly3PDH, LDH, MDH, and SDH, respectively). The results demonstrate that chronic as well as acute administration of nicotine produced strong increases in all these enzymatic activities in the superior layers (I, II and III) of the frontoparietal cortex (cingulate, motor and somatosensory regions); but high increases were not seen in the deeper layers of the cortex or in the subcortical nuclei (substantia nigra, caudate-putamen, nucleus accumbens or nucleus basalis magnocellularis). These hyperactivities were produced in brain regions with normally low enzymatic activity (cortex), but not in those with great intensity (subcortical nuclei). The results are in rough agreement with previous reports on nicotine-induced increases in glucose utilization, gly3PDH genic expression and neuronal hyperactivity in the brain cortex; but significant discrepancies between the cortical enzymatic maps and those obtained both in these studies and others on nicotine(N)-receptor localization have been appreciated. The results support the hypothesis that nicotinic cholinergic drugs can have metabolic, long-lasting stimulant effects on cortical neurons at specific points (probably layer III pyramidal cells and structures with alpha7-N-receptors) of the cortical circuits that could be of great interest in improving altered cognitive functions that are present in Alzheimer disease, as well as in other less severe mental disturbances. Mitochondrial hyperfunction should also be evaluated as a possible side-effect (as an oxidative stress inductor) of these kinds of drugs.

Expression of nicotinic acetylcholine receptors in Alzheimer's disease: postmortem investigations and experimental approaches

Nicotinic ligand binding studies have shown rather early that the cholinoceptive system is affected in Alzheimer's disease (AD). Today, molecular histochemistry enables one to study the nicotinic acetylcholine receptor (nAChR) subunit expression on the cellular level in human autopsy brains, in animal models and in in vitro approaches, thus deciphering the distribution of nAChRs and their role as potential therapeutic targets. The studies on the nAChR expression in the frontal and temporal cortex of AD patients and age-matched controls could demonstrate that both, the numbers of alpha4- and alpha7-immunoreactive neurons and the quantitative amount, in particular of the alpha4 protein, were markedly decreased in AD. Because the number of the corresponding mRNA expressing neurons was unchanged these findings point to a translational/posttranslational rather than a transcriptional event as an underlying cause. This assumption is supported by direct mutation screening of the CHRNA4 gene which showed no functionally important mutations. To get more insight into the underlying mechanisms, two model systems organotypic culture and primary hippocampal culture - have been established, both allowing to mimic nAChR expression in vitro. In ongoing studies the possible impact of beta-amyloid (Abeta) on nAChR expression is tested. Preliminary results obtained from primary cultures point to an impaired nAChR expression following Abeta exposure.

Neuronal nicotinic receptors in synaptic functions in humans and rats: physiological and clinical relevance

The present report describes the participation of nicotinic receptors (nAChRs) in controlling the excitability of local neuronal circuitries in the rat hippocampus and in the human cerebral cortex. The patch-clamp technique was used to record responses triggered by the non-selective agonist ACh and the alpha7-nAChR-selective agonist choline in interneurons of human cerebral cortical and rat hippocampal slices. Evidence is provided that functional alpha7- and alpha4beta2-like nAChRs are present on somatodendritic and/or preterminal/terminal regions of interneurons in the CA1 field of the rat hippocampus and in the human cerebral cortex and that activation of the different nAChR subtypes present in the preterminal/terminal areas of the interneurons triggers the tetrodotoxin-sensitive release of GABA. Modulation by nAChRs of GABAergic transmission, which can result either in inhibition or disinhibition of pyramidal neurons, depends both on the receptor subtype present in the interneurons and on the agonist acting upon these receptors. Not only do alpha7 nAChRs desensitize faster than alpha4beta2 nAChRs, but also alpha7 nAChR desensitization induced by ACh lasts longer than that induced by choline. These mechanisms, which appear to be retained across species, might explain the involvement of nAChRs in cognitive functions and in such neurological disorders as Alzheimer's disease and schizophrenia.

Central nicotinic receptors, neurotrophic factors and neuroprotection

The multiple combinations of nAChR subunits identified in central nervous structures possess distinct pharmacological and physiological properties. A growing number of data have shown that compounds interacting with neuronal nAChRs have, both in vivo and in vitro, the potential to be neuroprotective and that treatment with nAChR agonists elicit long-lasting improving of cognitive performance in a variety of behavioural tests in rats, monkeys and humans. Epidemiological and clinical studies suggested also a potential neuroprotective/trophic role of (-)-nicotine in neurodegenerative disease, such as Alzheimer's and Parkinson's disease. Taken together experimental and clinical data largely indicate a neuroprotective/trophic role of nAChR activation involving mainly alpha7 and alpha4beta2 nAChR subtypes, as evidenced using selective nAChR antagonists, and by potent nAChR agonists recently found displaying efficacy and/or larger selective affinities than (-)-nicotine for neuronal nAChR subtypes. A neurotrophic factor gene regulation by nAChR signalling has been taken into consideration as possible mechanism involved in neuroprotective/trophic effects by nAChR activation and has evidenced an involvement of the fibroblast growth factor (FGF-2) gene as a target of nAChR signalling. These findings suggested that FGF-2 could be involved, according to the FGF-2 neurotrophic functions, in nAChR mechanisms mediating the neuronal survival, trophism and plasticity.

Nicotinic treatment for degenerative neuropsychiatric disorders such as Alzheimer's disease and Parkinson's disease

Nicotinic systems play an important role in the neural basis of working memory and attention. Recent progress in understanding of the structure, function, and distribution of central nervous system (CNS) nicotinic receptors and their pharmacology has opened up new possibilities for novel CNS therapeutics with nicotinic agents. In this paper, we review the theoretical justification and the experimental evidence supporting these developments. We focus on the applications of nicotinic agonists in CNS disorders that are degenerative in nature, namely Parkinson's disease and Alzheimer's disease. We suggest that there is considerable potential for therapeutic applications in the near future. Clinically, two major issues remain: (a) the selectivity of effects, that is, developing compounds which are selective in producing improvement in cognition, motor function, attention, or pain without significant side-effects; and (b) the realistic likelihood of long-term improvements in everyday functioning in people who have degenerative diseases.

Drugs selective for nicotinic receptor subtypes: a real possibility or a dream?

Nicotine exerts a number of different effects on the nervous system by interacting with neuronal nicotinic acetylcholine receptors (nAChRs). These effects are mediated by its interaction with different nAChR subtypes, and this has led to the finding of subtype specific agonists and antagonists. In the search for subtype-selective drugs, we have synthesized some compounds derived from 4-oxystilbene, two of which (MG624 and F3) are selective ligands for the chick neuronal alphaBgtx receptors containing the alpha7 and/or alpha8 subunits. They have an antagonist action on oocyte-expressed chick and rat alpha7 subtypes. These compounds are selective toward the alpha7-containing receptors in chick, but, in mammals, although they still retain their potency toward alpha7-containing receptors, they are also active in non-alpha7-containing receptors.

The activation and inhibition of human nicotinic acetylcholine receptor by RJR-2403 indicate a selectivity for the alpha4beta2 receptor subtype

Human nicotinic acetylcholine (ACh) receptor subtypes expressed in Xenopus oocytes were characterized in terms of their activation by the experimental agonist RJR-2403. Responses to RJR-2403 were compared with those evoked by ACh and nicotine. These agonists were also characterized in terms of whether application of the drugs had the effect of producing a residual inhibition that was manifest as a decrease in subsequent control responses to ACh measured 5 min after the washout of the drug. For the activation of alpha4beta2 receptors, RJR-2403 had an efficacy equivalent to that of ACh and was more potent than ACh. RJR-2403 was less efficacious than ACh for other human receptor subtypes, suggesting that it is a partial agonist for all these receptors. Nicotine activated peak currents in human alpha4beta2 and alpha3beta2 receptors that were 85 and 50% of the respective ACh maximum responses. Nicotine was an efficacious activator of human alpha7 receptors, with a potency similar to ACh, whereas RJR-2403 had very low potency and efficacy for these receptors. At concentrations of <1 mM, RJR-2403 did not produce any residual inhibition of subsequent ACh responses for any receptor subtype. In contrast, nicotine produced profound residual inhibition of human alpha4beta2, alpha3beta2, and alpha7 receptors with IC(50) values of 150, 200, and 150 microM, respectively. Co-expression of the human alpha5 subunit with alpha3 and beta2 subunits had the effect of producing protracted responses to ACh and increasing residual inhibition by ACh and nicotine but not RJR-2403. In conclusion, our results, presented in the context of the complex pharmacology of nicotine for both activating and inhibiting neuronal nicotinic receptor subtypes, suggest that RJR-2403 will be a potent and relatively selective activator of human alpha4beta2 receptors.

Ongoing trials in Alzheimer's disease

Researchers have sought to understand the underlying pathophysiology of Alzheimer's disease (AD) ever since Dr A Alzheimer first described the condition in 1907. Unfortunately however, until recently, they have done so with limited success. This lack of clarity has deterred advancements in therapeutic drug research beyond all but the purely symptomatic treatment relief currently available. However, through spatio-temporal analysis of the two types of cerebral lesions that characterise the disorder (senile plaques and neurofibrillary tangles) and the compilation of genetic data concerning familial AD, there now exists the foundation for a more comprehensive understanding of the disease. Although symptomatic cholinergic strategies have beneficial effects, their benefits are modest and current research has turned to the development of other promising strategies, including oestrogen replacement, anti-inflammatory agents, free radical scavengers, anti-oxidants and monoamine oxidase-B (MAO-B) inhibitors. Many of these strategies may have some merit, however further analysis and structured research are necessary before a definitive decision can be made about their efficacy and possible role in AD therapy. Strategies that are directed at halting the underlying biochemical changes in AD are nearing clinical testing and offer the promise for meaningful therapeutic outcomes.

Neuronal nicotinic receptors, important new players in brain function

Acetylcholine receptors are cationic channels whose opening is controlled by acetylcholine. They are key molecules in the cholinergic nicotinic transmission in a number of areas of the central and peripheral nervous system. Because of the structural complexity, given by the numerous subunits that forms these receptors, they have different pharmacological and biophysical properties. Here we give a brief account of the known and consolidated data regarding neuronal nicotinic receptors, as as an introduction to the articles reported in this issue, in order to allow readers who are not familiar with the field to place the detailed information in the right context.

Nicotine, brain nicotinic receptors, and neuropsychiatric disorders

Neuronal nicotinic acetylcholine receptors (nAChRs) represent a large family of ligand-gated cation channels with diverse structures and properties. In contrast to the muscular nAChRs, the physiological functions of neuronal nAChRs are not well defined to date. Behavioral studies indicate that brain nAChRs participate in complex functions such as attention, memory, and cognition, whereas clinical data suggest their involvement in the pathogenesis of certain neuropsychiatric disorders (Alzheimer's and Parkinson's diseases, Tourette's syndrome, schizophrenia, depression, etc.). For the majority of these disorders, the use of nAChRs' agonists may represent either a prophylactic (especially for Alzheimer's and Parkinson's diseases) or a symptomatic treatment. The possible mechanisms underlying these beneficial effects as well as the characteristics and potential therapeutic use of new, subtype-selective nAChRs agonists are presented.

Recombinant human receptors and functional assays in the discovery of altinicline (SIB-1508Y), a novel acetylcholine-gated ion channel (nAChR) agonist

Neuronal nicotinic acetylcholine receptors (nAChRs) are a class of ion channels with significant potential as molecular targets for the design of drugs to treat a variety of CNS disorders. The discovery that neuronal nAChRs are further subdivided into multiple subtypes suggests that drugs which act selectively at specific nAChR subtypes might effectively treat Parkinson's disease (PD), Alzheimer's disease (AD), schizophrenia, ADHD, depression, anxiety or pain without the accompanying adverse side effects associated with non-selective agents such as nicotine (1) and epibatidine. Altinicline (SIB-1508Y) is a novel, small molecule designed to selectively activate neuronal nAChRs and is undergoing clinical evaluation for the treatment of PD. It was selected from a series of compounds primarily on the basis of results from functional assays, including (a) measurement of Ca2+ flux in stable cell lines expressing specific recombinant human neuronal nAChR subtypes; (b) determination of in vitro and in vivo neurotransmitter release; (c) in vivo models of PD. Biological data on both altinicline and the series of compounds from which it was selected are reported.

Nicotinic systems in central nervous systems disease: degenerative disorders and beyond

Advances in the understanding of the structure, function, and distribution of central nervous system (CNS) nicotinic receptors has provided the impetus for new studies examining the role(s) that these receptors and associated processes may play in CNS functions. Further motivation has come from the realization that such receptors are changed in degenerative neurologic diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). Ongoing investigations of the molecular substructure of CNS nicotinic receptors and their pharmacology have begun to open up new possibilities for novel CNS therapeutics with nicotinic agents. Exploiting these possibilities will require understanding of the role(s) that these receptor systems play in human cognitive, behavioral, motor, and sensory functioning. Clues from careful studies of human cognition and behavior are beginning to emerge and will provide direction for studies of potentially therapeutic novel nicotinic agents. Modulation of these receptors with the ultimate goal of producing therapeutic benefits is the goal of these investigations and drug development. This paper will review studies from our laboratory and others that point to the importance of CNS nicotinic mechanisms in normal human cognitive and behavioral functioning as well as their role in disease states. In addition, this paper will examine potential clinical applications of nicotine and/or nicotinic agonists in a variety of CNS disorders with particular emphasis on structural brain disease including: movement disorders such as Parkinson's disease and Tourette's syndrome, cognitive/behavioral disorders such as Alzheimer's disease, attention deficit/hyperactivity disorder, and schizophrenia, and other more speculative applications. Important results from early therapeutic studies of nicotine and/or nicotinic agonists in these disease states are presented. For example, recent studies with nicotine and novel nicotinic agonists such as ABT-418 by our group in AD patients suggest that nicotinic stimulation can improve the acquisition and retention of verbal information and decrease errors. Preliminary results from a series of studies examining the acute and subchronic quantitative effects of nicotine on cognitive and motor functioning in Parkinson's disease suggest that acute nicotine administration and stimulation improves some aspects of cognitive and motor performance and may improve the processing speed of more complex tasks. The most likely near-term applications of novel nicotinic agonists in CNS disorders are likely to be in those disorders that are degenerative in nature, e.g. Parkinson's disease and Alzheimer's disease, or other movement disorders such as Tourette's syndrome. The most likely direct therapeutic role for nicotinic agonists is as augmentation therapy in combination with other agents rather than as monotherapy, except early in disease states or as a prophylactic or preventative treatment.

Neurotrophic effects of central nicotinic receptor activation

A growing number of data have shown that compounds interacting with neuronal nicotinic acetylcholine receptors (nAChRs) have, both in vivo and in vitro, the potential to be neuroprotective and that treatment with nAChR agonists elicit long-lasting improvement of cognitive performance in a variety of behavioural tests in rats, monkeys and humans. Epidemiological and clinical studies suggested also a potential neuroprotective/trophic role of (-)-nicotine in neurodegenerative disease, such as Alzheimer's disease and Parkinson's disease. This neuroprotective/trophic role of nAChR activation has been mainly mediated by alpha7 and alpha4beta2 nAChR subtypes, as evidenced using selective nAChR antagonists, and by potent nAChR agonists recently found displaying efficacy and/or larger selective affinities than (-)-nicotine for neuronal nAChR subtypes. A neurotrophic factor gene regulation by nAChR signalling has been taken into consideration as a possible mechanism involved in neuroprotective/trophic effects of nAChR activation and has given evidence that the fibroblast growth factor (FGF-2) gene is a target for nAChR signalling. These findings suggested that FGF-2 could be involved, in view of its neurotrophic functions, in nAChR mechanisms mediating neuronal survival, trophism and plasticity.

Structure of acetylcholinesterase complexed with (-)-galanthamine at 2.3 A resolution

(-)-Galanthamine (GAL), an alkaloid from the flower, the common snowdrop (Galanthus nivalis), shows anticholinesterase activity. This property has made GAL the target of research as to its effectiveness in the treatment of Alzheimer's disease. We have solved the X-ray crystal structure of GAL bound in the active site of Torpedo californica acetylcholinesterase (TcAChE) to 2.3 A resolution. The inhibitor binds at the base of the active site gorge of TcAChE, interacting with both the choline-binding site (Trp-84) and the acyl-binding pocket (Phe-288, Phe-290). The tertiary amine group of GAL does not interact closely with Trp-84; rather, the double bond of its cyclohexene ring stacks against the indole ring. The tertiary amine appears to make a non-conventional hydrogen bond, via its N-methyl group, to Asp-72, near the top of the gorge. The hydroxyl group of the inhibitor makes a strong hydrogen bond (2.7 A) with Glu-199. The relatively tight binding of GAL to TcAChE appears to arise from a number of moderate to weak interactions with the protein, coupled to a low entropy cost for binding due to the rigid nature of the inhibitor.

The nicotinic acetylcholine receptor agonist (+/-)-epibatidine increases FGF-2 mRNA and protein levels in the rat brain

In a previous work, we showed that acute intermittent nicotine treatment up-regulates the level of fibroblast growth factor-2 (FGF-2) mRNA in brain regions of tel- and mesencephalon of rats suggesting that neuroprotective effect of (-)nicotine may, at least in part, involve an activation of the neuronal FGF-2 signalling. The present experiments were designed to extend the study on the nicotinic receptor mediated up-regulation of FGF-2 mRNA levels to the use of the potent nicotinic acetylcholine receptor (nAChR) agonist (+/-)-epibatidine. The (+/-)-epibatidine treatment led to a strong and long lasting up-regulation of FGF-2 mRNA expression in the cerebral cortex, in the hippocampal formation, in the striatum and in the substantia nigra. This FGF-2 mRNA induction, already statistically significant at 4 h, peaked at 12 h from treatment and was only partially returned towards normal levels at 48 h, the last time point examined. Using Western blot analysis it was found that the epibatidine-induced upregulation of FGF-mRNA is accompaned by an increase of FGF-2 protein level at the 20-h time-interval. These (+/-)-epibatidine effects on FGF-2 expression were antagonized by the non-competitive nAChR antagonist mecamylamine, indicating an involvement of nicotinic receptors. In the same brain areas examined, no changes were observed in the fibroblast growth factor receptor-1 (FGFR-1) mRNA levels, in brain-derived neurotrophic factor (BDNF) and in glial cell line-derived neurotrophic factor (GDNF) mRNA levels. In view of the neurotrophic function of FGF-2, these results, together with previous ones, could further help to understand the molecular mechanisms mediating the previously observed neuroprotective effects of (-)nicotine.

Therapeutic potential of neuronal nicotinic acetylcholine receptor agonists as novel analgesics

Pharmacological treatments for pain have come largely from two classes of compounds--the opioids and the nonsteroidal anti-inflammatory drugs (NSAIDs). Because of deficiencies associated with these two classes of compounds, exploration of novel approaches to pain relief has intensified of late. Nicotine, a neuronal nicotinic acetylcholine receptor (nAChR) agonist, has long been known to have antinociceptive effects in both experimental animals and humans. The relatively modest antinociceptive effects and the toxicities associated with nicotine preclude its development as an analgesic agent. However, recent discoveries in the nAChR field have stimulated interest in nAChR-targeted compounds as potential analgesic agents. Epibatidine, a potent nAChR agonist, was found to have full efficacy relative to opioids in preclinical pain models. Although epibatidine is toxic, these observations demonstrated that modest efficacy is not a general limitation of nAChR agonists. Moreover, exploration of the molecular biology of nAChRs revealed evidence of receptor diversity, suggesting that nAChR subtype-selective agents less toxic than nicotine might be discovered; and early medicinal chemistry efforts already have resulted in compounds with improved safety profiles. For example, ABT-594 is a nAChR agonist with the antinociceptive efficacy of epibatidine, but with an improved safety profile. This commentary reviews recent findings with nAChR-targeted compounds, explores potential mechanisms responsible for nAChR-mediated antinociception, and raises issues that must be addressed in developing compounds of this class as analgesics.

Presynaptic ionotropic receptors and the control of transmitter release

The quantity of neurotransmitter released into the synaptic cleft, the reliability with which it is released, and the response of the postsynaptic cell to that transmitter all contribute to the strength of a synaptic connection. The presynaptic nerve terminal is a major regulatory site for activity-dependent changes in synaptic function. Ionotropic receptors for the inhibitory amino acid GABA, expressed on the presynaptic terminals of crustacean motor axons and vertebrate sensory neurons, were the first well-defined mechanism for the heterosynaptic transmitter-mediated regulation of transmitter release. Recently, presynaptic ionotropic receptors for a large range of transmitters have been found to be widespread throughout the central and peripheral nervous systems. In this review, we first consider some general theoretical issues regarding whether and how presynaptic ionotropic receptors are important regulators of presynaptic function. We consider the criteria that should be met to identify a presynaptic ionotropic receptor and its regulatory function and review several examples of presynaptic receptors that meet at least some of those criteria. We summarize the classic studies of presynaptic inhibition mediated by GABA-gated Cl channels and then focus on presynaptic nicotinic ACh receptors and presynaptic glutamate receptors. Finally, we briefly discuss evidence for other types of presynaptic ionotropic receptors.