Interaction of the nicotinic agonist (R,S)-3-pyridyl-1-methyl-2-(3-pyridyl)-azetidine (MPA) with nicotinic acetylcholine receptor subtypes expressed in cell lines and rat cortex

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).

Nicotine enhances intracellular nicotinic receptor maturation: A novel mechanism of neural plasticity?

Nicotine addiction, the primary cause of tobacco consumption, is mediated through nicotine binding to brain nicotinic acetylcholine receptor (nAChRs). Upon chronic exposure, nicotine elicits a cascade of events, starting with nAChR activation and desensitization, followed by a long term up-regulation that corresponds to an increase in the number of the high affinity nAChRs, a paradoxical process that occurs in the brain of smokers. Recent investigation of the maturation and trafficking of the major brain alpha4beta2 nAChR demonstrates that up-regulation is initiated in the endoplasmic reticulum soon after protein translation. The data thus far accumulated provide evidence that nicotine elicits up-regulation by promoting maturation of nAChR precursors that would otherwise be degraded. This "maturational enhancer" action of nicotine probably contributes to the long term effect of chronic nicotine, and suggests a novel mechanism of neuronal plasticity through an yet unknown endogenous substance which would modulate the receptor expression under physiological conditions.

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.

Modulation of dopamine release by the nicotinic agonist epibatidine in the frontal cortex and the nucleus accumbens of naive and chronic nicotine treated rats

The effect of the nicotinic acetylcholine receptors (nAChRs) agonist (+/-)epibatidine on the modulation of dopamine (DA) release was investigated by microdialysis in vivo in the frontal cortex and the nucleus accumbens of naive and chronic nicotine-treated awake rats. (+/-)Epibatidine (2.5 microg/kg, s.c.), contrary to (-)nicotine (0.5 mg/kg, s.c.), decreased the extracellular concentrations of DA in the brain of naive rats. Subchronic nicotine treatment (0.45 mg/kg, s.c., twice daily for 7 days) attenuated the (+/-)epibatidine induced decrease in the DA level. The extracellular concentrations of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were elevated by (+/-)epibatidine administration in both naïve and subchronic treated rats. The findings suggest that the decrease in DA extracellular concentrations induced by the high affinity nAChRs agonist (+/-)epibatidine might be due to inactivation of nAChRs, which can be overcome by subchronic treatment with nicotine. Different mechanisms in modulation of DA release appears to be involved in the rat brain by (+/-)epibatidine compare to (-)nicotine.

Cholinergic nicotinic receptors: competitive ligands, allosteric modulators, and their potential applications

Discovery of the important role played by nicotinic acetylcholine receptors (nAChRs) in several CNS disorders has called attention to these membrane proteins and to ligands able to modulate their functions. The existence of different subtypes at multiple levels has complicated the understanding of this receptor's physiological role, but at the same time has increased the efforts to discover selective compounds in order to improve the pharmacological characterization of this kind of receptor and to make the possible therapeutical use of its modulators safer. This review focuses on the structure of new ligands for nAChRs, agonists, antagonists and allosteric modulators, and on their possible applications.

Ligands for in vivo imaging of nicotinic receptor subtypes in Alzheimer brain

The neuronal nicotinic acetylcholine receptors (nAChR) are involved in functional processes in brain including cognitive function and memory. A severe loss of the nAChRs has been detected in brain of patients with Alzheimer's disease (AD). There is a great interest to image nAChRs noninvasive for detection of receptor impairments even at a presymptomatic stage of AD as well for monitoring outcome of drug treatment. (S) [11C]Nicotine, has so far been the only nAChR ligand used in positron emission tomography (PET) studies for visualizing nAChRs in human brain. In order to develop PET/SPECT nAChRs ligands for detection of subtypes of nAChRs nicotine analogues, epibatidine and A-85380 compounds have been characterized in vitro and investigated in vivo. Epibatidine and A-85380 have been found to have higher specific signals and more favorable kinetic parameters than nicotine and its analogues. The epibatidine and A-85380 compounds can also be radiolabeled with high specific radioactivity, show affinities for the nAChRs in the pM range and readily cross the blood-brain barrier. In addition they reversibly bind to the nAChRs and show low non-specific binding and moderately fast metabolism. Due to a probably high alpha4beta2 nAChR selectivity combined with low toxicity, the A-85380 analogs presently seem to be the most promising nAChR ligand imaging of subtypes of nAChRs in human brain.

Development of ligands for in vivo imaging of cerebral nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) mediate a variety of brain functions. Findings from postmortem studies and clinical investigations have implicated them in the pathophysiology and treatment of Alzheimer's and Parkinson's diseases and other CNS disorders (e.g. Tourette's syndrome, epilepsy, nicotine dependence). Therefore, it ultimately might be useful to image nAChRs noninvasively for diagnosis, for studies on how changes in nAChRs might contribute to cerebral disorders, for development of therapies targeted at nAChRs, and to monitor the effects of such treatments. To date, only (S)-(-)-nicotine, radiolabeled with 11C, has been used for external imaging of nAChRs in human subjects. Since this radiotracer presents drawbacks, new ligands, with more favorable properties, have been synthesized and tested. Three general classes of compounds, namely, nicotine and its analogs, epibatidine and related compounds, and 3-pyridyl ether compounds, including A-85380, have been evaluated. Analogs of A-85380 appear to be the most promising candidates because of their low toxicity and high selectivity for the alpha4beta2 subtype of nAChRs.

Neuronal nicotinic receptors in the human brain

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of ligand gated ion channels which are widely distributed in the human brain. Multiple subtypes of these receptors exist, each with individual pharmacological and functional profiles. They mediate the effects of nicotine, a widely used drug of abuse, are involved in a number of physiological and behavioural processes and are additionally implicated in a number of pathological conditions such as Alzheimer's disease, Parkinson's disease and schizophrenia. The nAChRs have a pentameric structure composed of five membrane spanning subunits, of which nine different types have thus far been identified and cloned. The multiple subunits identified provide the basis for the heterogeneity of structure and function observed in the nAChR subtypes and are responsible for the individual characteristics of each. A substantial amount of information on human nAChR structure and function has come from studies on neuroblastoma cell lines which naturally express nAChRs and from recombinant nAChRs expressed in Xenopus oocytes. In vitro brain nAChR distribution can be mapped with a number of appropriate agonist and antagonist radioligands and subunit distribution may be mapped by in situ hybridization using subunit specific mRNA probes. Receptor distribution in the living human brain can be studied with noninvasive imaging techniques such as PET and SPECT, with a significant reduction in nAChRs in the brains of Alzheimer's patients having been identified with [11C] nicotine in PET studies. Despite the significant body of knowledge now accumulated about nAChRs, much remains to be elucidated. This review will attempt to describe the current knowledge on the nAChR subtypes in the human brain, their functional roles and neuropathological involvement.

Modulatory role of presynaptic nicotinic receptors in synaptic and non-synaptic chemical communication in the central nervous system

Neuronal nicotinic acetylcholine receptors (nAChRs) belong to a family of ligand-gated channels closely related to but distinct from the muscle nAChRs. Recent progress in neurochemical and pharmacological methods supports the hypothesis of presynaptically located nAChRs on axon terminals and indicates that the major effect of nAChR is the modulation rather than processing of fast synaptic transmission. Strong neurochemical evidence indicate that the most important function of presynaptic nAChRs in either synaptic or non-synaptic localization is to increase transmitter release initiated by axonal firing, or directly induce Na(+) and Ca(2+) influx followed by a depolarization sufficient to activate local voltage-sensitive Ca(2+) channels, as a result transmitter of vesicular origin will be released. Therefore, it is somewhat expected that nicotine-induced transmitter release of different monoamines including norepinephrine (NE), dopamine (DA), serotonin (5-HT) can be tetrodotoxin (TTX)- and [Ca(2+)](o)-sensitive. However, some of the nAChR agonists at higher concentrations (1, 1-dimethyl-4-phenylpiperazinium (DMPP) and lobeline), besides their effects on presynaptic nAChRs, are able to inhibit the uptake of NE and 5-HT into nerve terminals, thereby their transmitter releasing effects are extended in time and space. The effect on the uptake process is different from classical nicotinic actions, not being sensitive to nAChR antagonism, but can be prevented by selective uptake blockers or reduced temperature. Considering neurochemical, pharmacological and electrophysiological evidence it seems likely that presynaptic nAChRs on monoaminergic fibers are composed of alpha3 or alpha4 subunits in combination with the beta2 subunit. This is supported by the observation that nicotinic agonists have no presynaptic effect on transmitter release in knockout mice lacking the beta2 nAChR subunit gene. The essential brain function lies not only in impulse transmission within a hard-wired neuronal circuitry but also within synaptic and non-synaptic communication subjected to presynaptic modulation. Since the varicose noradrenergic, dopaminergic, serotonergic, glutamatergic and cholinergic axon terminals mainly do not make synaptic contact, but their varicosities are equipped with nAChRs and these non-synaptically localized receptors are of high affinity, it is suggested that nicotine inhaled during smoking might exert its behavioral, psychological, neurological and neuroendocrinological effects via these receptors.

Synthesis and characterization of binding of 5-[76Br]bromo-3-[[2(S)-azetidinyl]methoxy]pyridine, a novel nicotinic acetylcholine receptor ligand, in rat brain

5-[76Br]Bromo-3-[[2(S)-azetidinyl]methoxy]pyridine ([76Br]BAP), a novel nicotinic acetylcholine receptor ligand, was synthesized using [76Br]bromide in an oxidative bromodestannylation of the corresponding trimethylstannyl compound. The radiochemical yield was 25%, and the specific radioactivity was on the order of 1 Ci/micromol. The binding properties of [76Br]BAP were characterized in vitro and in vivo in rat brain, and positron emission tomography (PET) experiments were performed in two rhesus monkeys. In association experiments on membranes of the cortex and thalamus, >90% of maximal specific [76Br]BAP binding was obtained after 60 min. The dissociation half-life of [76Br]BAP was 51 +/- 6 min in cortical membranes and 56 +/- 3 min in thalamic membranes. Saturation experiments with [76Br]BAP revealed one population of binding sites with dissociation constant (K(D)) values of 36 +/- 9 and 30 +/- 9 pM in membranes of cortex and thalamus, respectively. The maximal binding site density (Bmax) values were 90 +/- 17 and 207 +/- 33 fmol/mg in membranes of cortex and thalamus, respectively. Scatchard plots were nonlinear, and the Hill coefficients were <1, suggesting the presence of a lower-affinity binding site. In vitro autoradiography studies showed that binding of [76Br]BAP was high in the thalamus and presubiculum, moderate in the cortex and striatum, and low in the cerebellum and hippocampus. A similar pattern of [76Br]BAP accumulation was observed by ex vivo autoradiography. In vivo, binding of [76Br]BAP in whole rat brain was blocked by preinjection of (S)(-)-nicotine (0.3 mg/kg) by 27, 52, 68, and 91% at survival times of 10, 25, 40, 120, and 300 min, respectively. In a preliminary PET study in rhesus monkeys, the highest [76Br]BAP uptake was found in the thalamus, and radioactivity was displaceable by approximately 60% with cytisine and by 50% with (S)(-)-nicotine. The data of this study indicate that [76Br]BAP is a promising radioligand for the characterization of nicotinic acetylcholine receptors in vivo.

In vivo positron emission tomography studies on the novel nicotinic receptor agonist [11C]MPA compared with [11C]ABT-418 and (S)(-)[11C]nicotine in rhesus monkeys

The novel 11C-labeled nicotinic agonist (R,S)-1-[11C]methyl-2(3-pyridyl)azetidine ([11C]MPA) was evaluated as a positron emission tomography (PET) ligand for in vivo characterization of nicotinic acetylcholine receptors in the brain of Rhesus monkeys in comparison with the nicotinic ligands (S)-3-methyl-5-(1-[11C]methyl-2-pyrrolidinyl)isoxazol ([11C]ABT-418) and (S)(-)[11C]nicotine. The nicotinic receptor agonist [11C]MPA demonstrated rapid uptake into the brain to a similar extent as (S)(-) [11C]nicotine and [11C]ABT-418. When unlabeled (S)(-)nicotine (0.02 mg/kg) was administered 5 min before the radioactive tracers, the uptake of [11C]MPA was decreased by 25% in the thalamus, 19% in the temporal cortex, and 11% in the cerebellum, whereas an increase was found for the uptake of (S)(-)[11C]nicotine and [11C]ABT-418. This finding indicates specific binding of [11C]MPA to nicotinic receptors in the brain in a simple classical displacement study. [11C]MPA seems to be a more promising radiotracer than (S)(-)[11C]nicotine or [11C]ABT-418 for PET studies to characterize nicotinic receptors in the brain.

Differential changes of nicotinic receptors in the rat brain following ibotenic acid and 192-IgG saporin lesions of the nucleus basalis magnocellularis

The basal forebrain cholinergic neurons are implicated in the pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD). The nicotinic acetylcholine receptors (nAChRs) have been found to be significantly afflicted in AD. To study the underlying mechanisms for dysfunction of the basal forebrain cholinergic neurons development of suitable animal models is warranted. In this study we investigated the effects of bilateral lesions of the nucleus basalis magnocellularis on nAChRs in the rat brain using the cholinergic system selective immunotoxin 192-IgG saporin and non-selective excitotoxin ibotenic acid. Changes in nAChRs were measured by 3H-cytisine and 3H-epibatidine, two ligands with different selectivity for nAChRs subtypes. In the parietal cortex of ibotenic acid lesioned rates, the choline acetyltransferase activity (ChAT) was decreased by 24% while no changes were detected in the frontal cortex or hippocampus. Similarly, a 40% decrease was observed in the number of nAChRs labelled by 3H-cytisine, but not by 3H-epibatidine, in the parietal cortex, while no changes were found in the frontal cortex or hippocampus. Although the 192-IgG saporin induced lesions reduced the ChAT activity in the frontal cortex, parietal cortex and hippocampus by 77, 50 and 21%, respectively, no changes were observed in the number of nAChRs as studied by 3H-cytisine or 3H-epibatidine. The results indicate a difference in vulnerability of the cortical nAChR subtypes to experimental lesions of the nucleus basalis magnocellularis. The findings in this study suggest that a major portion of the nAChRs might be located on non-cholinergic neurons in the brain.

Regional distribution of subtypes of nicotinic receptors in human brain and effect of aging studied by (+/-)-[3H]epibatidine

Epibatidine, a potent nicotinic agonist, was used to study the regional distribution of nicotinic acetylcholine receptor binding sites in the human brain. Saturation studies performed in the human temporal cortex with (+/-)-[3H]epibatidine revealed binding to two binding sites with Kd and Bmax values of 0.018 and 4.2 nM, 12.7 and 15.4 fmol/mg protein, respectively. Competition studies with (+/-)-[3H]epibatidine/unlabelled nicotine or [3H]nicotine/unlabelled (+/-)-epibatidine showed binding to two binding sites in the human temporal cortex (Ki=0.16 and 12.6 nM; 0.007 and 0.3 nM, respectively). Similarly, when unlabelled nicotine was used to displace (+/-)-[3H]epibatidine, two binding sites were also revealed in the thalamus and the cerebellum of human brain (Ki=0.065 and 7.7 nM; 0.07 and 12.5 nM, respectively). The regional binding of (+/-)-[3H]epibatidine binding in human brain was somewhat different from that of [3H]nicotine. A proportionally higher binding was observed for (+/-)-[3H]epibatidine in the cerebellum and the thalamus compared to [3H]nicotine, probably reflecting different selectivity to nicotinic receptor subtypes. A marked significant age-related decrease in (+/-)-[3H]epibatidine binding was observed in the frontal and the temporal cortices (-79%, -84%, respectively) of human subjects between 56-85 years of age, which was similar to that of [3H]nicotine (-82%, -79%, respectively). The (+/-)-[3H]epibatidine binding in the cerebellum decreased significantly with age (-77%), while [3H]nicotine binding showed no significant age-related changes in this brain region. The findings indicate that a specifically modulate regional nicotinic receptors in human brain.