Effects of cotinine on information processing in nonsmokers

Dopaminergic and norepinephrinergic modulation of endogenous event-related potentials: A systematic review and meta-analysis

Event-related potentials (ERPs) represent the cortical processing of sensory, motor or cognitive functions invoked by particular events or stimuli. A current theory posits that the catecholaminergic neurotransmitters dopamine (DA) and norepinephrine (NE) modulate a number of endogenous ERPs during various cognitive processes. This manuscript aims to evaluate a leading neurotransmitter hypothesis with a systematic overview and meta-analysis of pharmacologic DA and NE manipulation of specific ERPs in healthy subjects during executive function. Specifically, the frontally-distributed P3a, N2, and Ne/ERN (or error-related negativity) are supposedly modulated primarily by DA, whereas the parietally-distributed P3b is thought to be modulated by NE. Based on preceding research, we refer to this distinction between frontally-distributed DA-sensitive and parietally-distributed NE-sensitive ERP components as the Extended Neurobiological Polich (ENP) hypothesis. Our systematic review and meta-analysis indicate that this distinction is too simplistic and many factors interact with DA and NE to influence these specific ERPs. These may include genetic factors, the specific cognitive processes engaged, or elements of study design, i.e. session or sequence effects or data-analysis strategies.

Tobacco smoke exposure and mitochondrial DNA copy number on neurobehavioural performance: A community study

The influence of tobacco smoke has been a controversial and very questionable subject within the field of neurological behaviours. To examine the dose-response relationships between tobacco smoke and neurological performance, we investigated whether mitochondrial DNA copy number (mtDNAcn) mediates these relationships. We used restricted cubic spline models to estimate the dose-response relationships. A mediation model was also used to detect the mediating effect. Increased cotinine was negatively associated with auditory memory scores and a 0.51 decrease in mtDNAcn. MtDNAcn acts as a mediator between cotinine and auditory memory. Tobacco smoke levels were inversely associated with mtDNAcn and neurobehavioural changes, and there was a mediation effect between cotinine levels and auditory memory by mtDNAcn.

Cotinine: Pharmacologically Active Metabolite of Nicotine and Neural Mechanisms for Its Actions

Tobacco use disorder continues to be a leading public health issue and cause of premature death in the United States. Nicotine is considered as the major tobacco alkaloid causing addiction through its actions on nicotinic acetylcholine receptors (nAChRs). Current pharmacotherapies targeting nicotine's effects produce only modest effectiveness in promoting cessation, highlighting the critical need for a better understanding of mechanisms of nicotine addiction to inform future treatments. There is growing interest in identifying potential contributions of non-nicotine components to tobacco reinforcement. Cotinine is a minor alkaloid, but the major metabolite of nicotine that can act as a weak agonist of nAChRs. Accumulating evidence indicates that cotinine produces diverse effects and may contribute to effects of nicotine. In this review, we summarize findings implicating cotinine as a neuroactive metabolite of nicotine and discuss available evidence regarding potential mechanisms underlying its effects. Preclinical findings reveal that cotinine crosses the blood brain barrier and interacts with both nAChRs and non-nAChRs in the nervous system, and produces neuropharmacological and behavioral effects. Clinical studies suggest that cotinine is psychoactive in humans. However, reviewing evidence regarding mechanisms underlying effects of cotinine provides a mixed picture with a lack of consensus. Therefore, more research is warranted in order to provide better insight into the actions of cotinine and its contribution to tobacco addiction.

Effects of transdermal nicotine delivery on cognitive outcomes: A meta-analysis

OBJECTIVE

By the association of nicotinic acetylcholine receptors in the brain, nicotine in the therapeutic window lowers neuronal damage and raises protective factors. These data, however, are contradicted by other findings. Here, we assessed the effects of transdermal nicotine administration on cognitive functions in healthy non-smoker adults by systematic review and meta-analysis of clinical trials.

METHODS

We included reports of clinical trials comparing the effects of nicotine patches with placebo in healthy non-smoking adults. The main outcome was the impact of nicotine patches on overall cognitive function with a focus on attention and memory. Standard meta-analytic and statistical methods measured the effect of transdermal nicotine compared with placebo patches.

RESULTS

We included 31 publications involving 978 subjects. Nicotine patches boosted cognitive function in healthy adults (0.233 SMD, 95%CI, 0.111-0.355, p < .001). Overall heterogeneity of the studies was found to be modest (ϰ²  = 68.24, T²  = 0.07, I²  = 50.17%, p < .001). Also, nicotine patches improved attention (0.231 SMD, 95%CI, 0.106-0.356, p < .001). We found the inter-study heterogeneity to be low (ϰ²  = 40.95, T²  = 0.03, I²  = 34.07%, p = .042). Further, the enhancement of memory by transdermal nicotine did not reach statistical significance in normal subjects (0.270 SMD, 95% CI, -0.293-0.833, p = .347). Also, high inter-study heterogeneity was found among studies (ϰ²  = 27.25, T²  = 0.43, I²  = 77.98%, p < .001).

CONCLUSION

The meta-analysis showed that transdermal nicotine had statistically significant positive effects on attention, and non-significant effects on memory, in healthy non-smoking adults. The results encourage further studies of the therapeutic potential of nicotine patches in disorders of cognition.

Molecular Insights Into Memory-Enhancing Metabolites of Nicotine in Brain: A Systematic Review

Background: The alleged procognitive effects of nicotine and its metabolites in brain are controversial. Objective: Here, we review the pharmacologically active metabolites of nicotine in brain and their effects on neuronal mechanisms involving two main cognitive domains, i.e., learning and memory. Methods: We searched Embase, Medline via PubMed, Scopus, and Web of Science databases for entries no later than May 2018, and restricted the search to articles about nicotine metabolites and cognitive behavior or cognitive mechanisms. Results: The initial search yielded 425 articles, of which 17 were eligible for inclusion after application of exclusion criteria. Of these, 13 were experimental, two were clinical, and two were conference papers. Conclusions: The results revealed three pharmacologically active biotransformations of nicotine in the brain, including cotinine, norcotinine, and nornicotine, among which cotinine and nornicotine both had a procognitive impact without adverse effects. The observed effect was significant only for cotinine.

Cotinine impacts sensory processing in DBA/2 mice through changes in the conditioning amplitude

Cotinine, a major metabolite of nicotine, has produced improved learning and memory in rodents and non-human primates and corrects apomorphine-induced loss of pre-pulse startle inhibition in rats. The present study assessed cotinine, both acute and chronic (7-day), in the sensory inhibition paradigm in DBA/2 mice. These mice spontaneously show a deficit in hippocampal sensory inhibition, as assessed by the P20-N40 EEG paradigm, which models the deficit observed in schizophrenia patients. Anesthetized DBA/2 mice were recorded in the CA3 region of hippocampus for inhibition of paired, identical auditory stimuli, then administered cotinine (0.33, 0.1, 0.33, 1.0 or 3.3 mg/kg SQ) and recorded for 90 min. At doses of 0.1, 0.33 and 1.0 mg/kg, there were significant increases in conditioning amplitude, with no changes in test amplitude or TC ratio. Blockade of α4β2 nicotinic receptors with central administration of DHΒE blocked the increase in the conditioning amplitude induced by the 1.0 mg/kg dose of cotinine, as did blockade of α7 nicotinic receptors with α-bungarotoxin. Daily injections of 0.33, 1.0 or 3.3 mg/kg for 7 days produced similar increases in the conditioning amplitude on the 7th day, but only at the 0.33 and 3.3 mg/kg doses. Determination of the "carry over" effect of the previous 6 daily doses of cotinine, prior to the 7th dose, showed that there was a significant increase in the conditioning amplitude as compared to the baseline data for mice receiving the equivalent acute dose. There were no significant effects on test amplitude or TC ratio for any of the chronic doses. These data suggest that cotinine modulates the conditioning amplitude in the sensory inhibition paradigm through the α4β2 nicotinic receptor and possibly also through the α7 nicotinic receptor, as well. However the data do not suggest that cotinine is a potential therapeutic for the treatment of sensory inhibition deficits in schizophrenia.

Nicotine facilitates memory consolidation in perceptual learning

Perceptual learning is a special type of non-declarative learning that involves experience-dependent plasticity in sensory cortices. The cholinergic system is known to modulate declarative learning. In particular, reduced levels or efficacy of the neurotransmitter acetylcholine were found to facilitate declarative memory consolidation. However, little is known about the role of the cholinergic system in memory consolidation of non-declarative learning. Here we compared two groups of non-smoking men who learned a visual texture discrimination task (TDT). One group received chewing tobacco containing nicotine for 1 h directly following the TDT training. The other group received a similar tasting control substance without nicotine. Electroencephalographic recordings during substance consumption showed reduced alpha activity and P300 latencies in the nicotine group compared to the control group. When re-tested on the TDT the following day, both groups responded more accurately and more rapidly than during training. These improvements were specific to the retinal location and orientation of the texture elements of the TDT suggesting that learning involved early visual cortex. A group comparison showed that learning effects were more pronounced in the nicotine group than in the control group. These findings suggest that oral consumption of nicotine enhances the efficacy of nicotinic acetylcholine receptors. Our findings further suggest that enhanced efficacy of the cholinergic system facilitates memory consolidation in perceptual learning (and possibly other types of non-declarative learning). In that regard acetylcholine seems to affect consolidation processes in perceptual learning in a different manner than in declarative learning. Alternatively, our findings might reflect dose-dependent cholinergic modulation of memory consolidation. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

The nicotine metabolite, cotinine, attenuates glutamate (NMDA) antagonist-related effects on the performance of the five choice serial reaction time task (5C-SRTT) in rats

Cotinine, the most predominant metabolite of nicotine in mammalian species, has a pharmacological half-life that greatly exceeds its precursor. However, until recently, relatively few studies had been conducted to systematically characterize the behavioral pharmacology of cotinine. Our previous work indicated that cotinine improves prepulse inhibition of the auditory startle response in rats in pharmacological impairment models and that it improves working memory in non-human primates. Here we tested the hypothesis that cotinine improves sustained attention in rats and attenuates behavioral alterations induced by the glutamate (NMDA) antagonist MK-801. The effects of acute subcutaneous (dose range 0.03-10.0 mg/kg) and chronic oral administration (2.0 mg/kg/day in drinking water) of cotinine were evaluated in fixed and variable stimulus duration (VSD) as well as variable intertrial interval (VITI) versions of a five choice serial reaction time task (5C-SRTT). The results indicated only subtle effects of acute cotinine (administered alone) on performance of the 5C-SRTT (e.g., decreases in timeout responses). However, depending on dose, acute treatment with cotinine attenuated MK-801-related impairments in accuracy and elevations in timeout responses, and it increased the number of completed trials. Moreover, chronic cotinine attenuated MK-801-related impairments in accuracy and it reduced premature and timeout responses when the demands of the task were increased (i.e., by presenting VSDs or VITIs in addition to administering MK-801). These data suggest that cotinine may represent a prototype for compounds that have therapeutic potential for neuropsychiatric disorders (i.e., by improving sustained attention and decreasing impulsive and compulsive behaviors), especially those characterized by glutamate receptor alterations.

A reversible model of the cognitive impairment associated with schizophrenia in monkeys: potential therapeutic effects of two nicotinic acetylcholine receptor agonists

In monkeys proficient in the performance of a computer-assisted delayed response task, administration of sub-sedative doses of ketamine significantly impaired task performance after the 2mg/kg dose, producing a decrease in accuracies across all four delay intervals. Ketamine elicited occasional and inconsistent increases in task latencies. But in general processing speed was not dramatically affected by the test dose. Pretreatment with the alpha7 nicotinic receptor agonist GTS-21 (DMXB-A) [3-[(3E)-3-[(2,4-dimethoxyphenyl) methylidene]-5,6-dihydro-4H-pyridin-2-yl]pyridine] produced a dose-dependent attenuation of ketamine-induced decreases in task accuracies. In fact, the best dose of GTS-21 completely reversed the effects of ketamine. The nicotine metabolite cotinine is a cognitive-enhancer, and active in models predictive of antipsychotic activity. Pretreatment with cotinine did not reverse the task deficits produced by ketamine, and selection of a best dose was necessary to show the activity of cotinine. However, the best dose of cotinine, like GTS-21, completely reversed the ketamine-induced task deficits. Task accuracies were increased relative to their non-ketamine baselines during sessions run 24h later. The cotinine-ketamine order of administration was reversed to provide a more clinically relevant model, and cotinine post-treatment regimen produced a clear reversal of the ketamine-induced task deficits. The protracted task improvement also was still evident. The DMTS task impairment induced by ketamine was capable of being completely reversed by two compounds that are known to improve working memory and cognition. The model could provide a means of late stage preclinical evaluation of new compounds that address the cognitive impairment associated with major psychotic disease.

Cotinine, a neuroactive metabolite of nicotine: potential for treating disorders of impaired cognition

The pharmacological effects of the tobacco-derived alkaloid nicotine have been widely studied in humans and animals for decades. However, relatively little attention has been given to the potential actions of its major metabolite, cotinine. After nicotine consumption the duration of cotinine's presence in blood and brain greatly exceeds that of nicotine. Therefore, cotinine could mediate the more protracted pharmacological effects of nicotine. The studies described in this report were thus designed to further investigate certain neuropharmacological actions of cotinine. Behavioral tests (e.g., delayed matching-to-sample) were conducted in aged rhesus monkeys to assess the effects of cotinine on working memory and attention. In rats a prepulse inhibition (PPI) procedure was used to assess the effects of the compound on auditory gating - a method for predicting the potential antipsychotic properties of drugs. Cotinine exhibited significant effectiveness in these tasks. The drug was also cytoprotective in differentiated PC-12 cells with a potency equivalent to that of nicotine. The effects of chronic cotinine treatment on the expression of nicotinic and muscarinic acetylcholine receptors in rat brain were measured by [125I]epibatidine, [125I]alpha-bungarotoxin ([125I]BTX), [3H]pirenzepine ([3H]PRZ), and [3H]AFDX-384 ([3H]AFX) autoradiography. Unlike nicotine, cotinine failed to upregulate the expression of brain nicotinic receptors. Based on its relative safety in man, cotinine should prove useful in the treatment of diseases of impaired cognition and behavior without exhibiting the toxicity usually attributed to nicotine.

Simultaneous determination of nicotine and its metabolite, cotinine, in rat blood and brain tissue using microdialysis coupled with liquid chromatography: Pharmacokinetic application

To elucidate the disposition of nicotine in the brain is important because the neuropharmacological effects from nicotine exposure are centrally predominated. The aim of the present study was to develop a rapid and simple method for the simultaneous determination of unbound nicotine and its main metabolite, cotinine, in rat blood and brain tissue. We coupled a multiple sites microdialysis sampling technique with HPLC-UV system to characterize the pharmacokinetics of both nicotine and cotinine. Microdialysis probes were inserted into the jugular vein/right atrium and brain striatum of Sprague-Dawley rats, and nicotine (2 mg/kg, i.v.) was administered via the femoral vein. Dialysates were collected every 10 min and injected directly into a HPLC system. Both nicotine and cotinine were separated by a phenyl-hexyl column (150 mm x 4.6 mm) from dialysates within 12 min. The mobile phase consisted of an acetonitrile-methanol-20 mM monosodium phosphate buffer (55:45:900, v/v/v, pH adjusted to 5.1) with a flow-rate of 1 ml/min. The wavelength of the UV detector was set at 260 nm. The limit of quantification for nicotine and cotinine were 0.25 microg/ml and 0.05 microg/ml, respectively. Intra- and inter-day precision and accuracy of both measurements fell well within the predefined limits of acceptability. The blood and brain concentration-time profile of nicotine and cotinine suggests that nicotine is easily to get into the central nervous system and cotinine exhibits a long retention time and accumulates in blood.

Repeated nicotine exposure in rats: effects on memory function, cholinergic markers and nerve growth factor

A decrease in the number of nicotinic-acetylcholine receptors (nAChRs) in the brain is thought to contribute to the cognitive dysfunction associated with diseases as diverse as Alzheimer's disease and schizophrenia. Interestingly, nicotine and similar compounds have been shown to enhance memory function and increase the expression of nAChRs and therefore, could have a therapeutic role in the aforementioned diseases. Nicotine has also been shown to exert positive effects on certain neurotrophins such as nerve growth factor (NGF), and therefore could play a role beyond mere symptomatic therapy. However, to date, comprehensive studies of nicotine's effects on the expression of specific acetylcholine (ACh) receptor subtypes, key cholinergic proteins (that are regulated by NGF) such as choline acetyltransferase (ChAT) and the vesicular ACh transporter (VAChT) are lacking. Studies to further investigate the effects of nicotine on NGF especially its high- and low-affinity receptors are also needed. In the present study, male Wistar rats exposed a relatively low dosage of nicotine (0.35 mg/kg every 12 h) for 14 days demonstrated improved memory performance (assessed in two separate water maze testing methods) when compared with controls. Autoradiographic experiments indicated that nicotine increased [3H]-epibatidine, [125I]-alpha-bungarotoxin and [3H]-AFDX384, but not [3H]-pirenzepine binding sites in several learning- and memory-related brain areas. The expression of ChAT, VAChT, as well as tropomyosin-receptor kinase A (TrkA) NGF receptors and phospho-TrK receptors was increased by nicotine in the hippocampus. No changes were observed in the levels of the NGF peptide or low affinity p75 neurotrophin receptors (p75NTR), however. These results suggest that repeated exposure to nicotine results in positive effects on central cholinergic markers and memory function, which may be mediated via effects on high-affinity NGF receptors.

Nicotinic receptor mechanisms and cognition in normal states and neuropsychiatric disorders

Cigarette smoking rates in the American population are approximately 23%, whereas rates of smoking in clinical and population studies of individuals with neuropsychiatric disorders are typically two- to four-fold higher. Studies conducted in a variety of neuropsychiatric populations [e.g. attention-deficit hyperactivity disorder (ADHD), Alzheimer's disease, schizophrenia] have collectively suggested that nicotine may be efficacious in remediating selected cognitive deficits associated with these disorders, thus providing a framework for understanding the specific vulnerability of these patients to smoking initiation and maintenance. However, the specific gain in cognitive performance produced by nicotine administration in healthy subjects with normal cognitive function is less clear. This article reviews our current understanding of central nicotinic acetylcholine receptor (nAChRs) systems in normal and neuropsychiatric disease states and, specifically, their role with respect to cognitive dysfunction and clinical symptoms in several specific neuropsychiatric populations, including ADHD, Alzheimer's disease, Parkinson's disease, Tourette's Disorder, schizophrenia and affective disorders. The potential benefits of nicotinic agents for therapeutic use in neuropsychiatric disorders is discussed, as well as directions for further research in this area.

Metabolism and Disposition Kinetics of Nicotine

Nicotine is of importance as the addictive chemical in tobacco, pharmacotherapy for smoking cessation, a potential medication for several diseases, and a useful probe drug for phenotyping cytochrome P450 2A6 (CYP2A6). We review current knowledge about the metabolism and disposition kinetics of nicotine, some other naturally occurring tobacco alkaloids, and nicotine analogs that are under development as potential therapeutic agents. The focus is on studies in humans, but animal data are mentioned when relevant to the interpretation of human data. The pathways of nicotine metabolism are described in detail. Absorption, distribution, metabolism, and excretion of nicotine and related compounds are reviewed. Enzymes involved in nicotine metabolism including cytochrome P450 enzymes, aldehyde oxidase, flavin-containing monooxygenase 3, amine N-methyltransferase, and UDP-glucuronosyltransferases are represented, as well as factors affecting metabolism, such as genetic variations in metabolic enzymes, effects of diet, age, gender, pregnancy, liver and kidney diseases, and racial and ethnic differences. Also effects of smoking and various inhibitors and inducers, including oral contraceptives, on nicotine metabolism are discussed. Due to the significance of the CYP2A6 enzyme in nicotine clearance, special emphasis is given to the effects and population distributions of CYP2A6 alleles and the regulation of CYP2A6 enzyme.

Nicotinic receptor mechanisms and cognition in normal states and neuropsychiatric disorders

Cigarette smoking rates in the American population are approximately 23%, whereas rates of smoking in clinical and population studies of individuals with neuropsychiatric disorders are typically two- to four-fold higher. Studies conducted in a variety of neuropsychiatric populations [e.g. attention-deficit hyperactivity disorder (ADHD), Alzheimer's disease, schizophrenia] have collectively suggested that nicotine may be efficacious in remediating selected cognitive deficits associated with these disorders, thus providing a framework for understanding the specific vulnerability of these patients to smoking initiation and maintenance. However, the specific gain in cognitive performance produced by nicotine administration in healthy subjects with normal cognitive function is less clear. This article reviews our current understanding of central nicotinic acetylcholine receptor (nAChRs) systems in normal and neuropsychiatric disease states and, specifically, their role with respect to cognitive dysfunction and clinical symptoms in several specific neuropsychiatric populations, including ADHD, Alzheimer's disease, Parkinson's disease, Tourette's Disorder, schizophrenia and affective disorders. The potential benefits of nicotinic agents for therapeutic use in neuropsychiatric disorders is discussed, as well as directions for further research in this area.

The potential role of cotinine in the cognitive and neuroprotective actions of nicotine

Cotinine is a primary metabolite of nicotine that has been suggested in many studies in animals and in humans to exert measurable effects on aspects of on-going behavior or on cognitive function. Much of the interest in cotinine derives from its long pharmacological half-life (15-19 hours) relative to nicotine (2-3 hours). Despite decades of study focusing on nicotine as the predominant behaviorally active component of tobacco, there continue to be aspects of the pharmacology of the drug that have yet to be explained. For example, nicotine can evoke a protracted behavioral response, i.e., in great excess of the presence of the drug in the plasma. Also, there is often a striking differential between the potency for nicotine-induced behavioral responses in humans and animals, and its potency as a cholinergic agonist, neurochemically. One possibility that may explain one or more of these properties of nicotine is the presence of a long-lived bioactive metabolite or breakdown product of nicotine such as cotinine. Preliminary data in support of this hypothesis are consistent with the ability of cotinine to improve performance accuracy on delayed matching task by macaque monkeys, and in reversing apomorphine-induced deficits in prepulse inhibition of acoustic startle in rats. The drug also was shown to be as potent as nicotine in the ability to act as a cytoprotective agent in cells that express a neuronal cholinergic phenotype. This new appreciation for the role of cotinine in nicotine's actions, and as a pharmacological agent in its own right, particularly in aspects of cognitive function and for neuroprotection, ultimately may be applied towards the treatment of Alzheimer's disease and related disorders, and for various psychiatric syndromes.

Genetic variation in CYP2A6-mediated nicotine metabolism alters smoking behavior

Approximately 50% of the initiation of tobacco dependence is genetically influenced, whereas maintenance of dependent smoking behavior and amount smoked have approximately 70% genetic contribution (1-5). Determining the variation in nicotine's inactivation is important because of nicotine's role in producing tobacco dependence and regulating smoking patterns (6-11). The genetically polymorphic CYP2A6 enzyme is responsible for the majority of the metabolic inactivation of nicotine to cotinine (12-14). Both in vitro and in vivo studies have demonstrated considerable interindividual variation in CYP2A6 activity (15-17). CYP2A6 is genetically polymorphic, individuals carrying inactive CYP2A6 alleles have decreased nicotine metabolism, are less likely to become smokers and if they do, they smoke fewer cigarettes per day (13,18,19). The decrease in smoking behavior was confirmed by measuring carbon monoxide (CO, a measure of smoke inhalation) levels, plasma and urine nicotine and cotinine levels, and cigarette counts (13,18,19). A duplication variant in the CYP2A6 gene locus has been identified which increases nicotine inactivation and increases smoking (19). CYP2A6 can also activate tobacco smoke procarcinogens (e.g. NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone); current studies are investigating the role of CYP2A6 in risk for lung cancer. Based on these epidemiologic data it was postulated that inhibition of CYP2A6 activity might be useful in a therapeutic context. Kinetic studies in humans indicated that selective CYP2A6 inhibitors decrease the metabolic removal of nicotine. It was also shown that inhibiting CYP2A6 in vivo (phenocopying, or mimicking the genetic defect) in smokers results in decreased smoking, making nicotine orally bioavailable, and the rerouting of procarcinogens to detoxifying pathways (20-22).

Isolation and microsequencing of a novel cotinine receptor

1. Nicotine and its main derivative. cotinine, are reported to have distinct central activities in mammals. In this study, the cotinine receptor was separated by biochemical procedures including radio receptor, affinity-chromatography, SDS-PAGE, and N-terminal sequencing assays. 2. Consistently, the results showed that distinctive cotinine receptors exist in different tissues of mammals. In rat brain, the affinity chromatography and [125I]cotinine receptor essays were used to isolate a 40-kDa protein (p40) with higher affinity for cotinine than alpha-bungarotoxin and nicotine. The N-terminus amino acid sequences of the p40 and its internal tryptic peptides showed no identity to recently described protein sequences, with the exception of homology to the human p205 synovial fluid protein. 3. These results, in agreement with other behavioral studies, are the first molecular evidence for distinctive nicotine and cotinine receptors in mammals.

Effects of cotinine at cholinergic nicotinic receptors of the sympathetic superior cervical ganglion of the mouse

Nicotine, the principal alkaloid in tobacco, is generally accepted to be responsible for most neuropharmacological effects due to tobacco use. Little is known about the action of cotinine, the major metabolite from nicotine, at neuronal structures. To evaluate the mode of action of cotinine at neuronal receptors, its effect on the surface compound potential of the sympathetic superior cervical ganglion (SCG) of the mouse was studied. The modulation of nicotine induced surface potentials by cotinine was tested. It was found that 2-min applications of cotinine (0.1-30 mmol/l) induced concentration dependent depolarizations at the SCG (EC(50)=1.7 mmol/l) which were followed by hyperpolarizations and weak afterdepolarizations. The intrinsic activity of cotinine compares to that induced by much lower concentrations of nicotine (EC(50)=21 micromol/l). These cotinine effects may be mediated at least in part by nicotine impurities which were found by capillary electrophoresis to be 0.1 and 0.8% in different batches of cotinine. Continuous application of 300 micromol/l cotinine shifted the concentration-response curve of nicotine to the right and reduced (IC(50)=302 micromol/l) the effects of submaximal nicotine concentrations (30 micromol/l). This effect could not be mimicked by continuous application of a nicotine concentration (0.3 micromol/l) equivalent to the lower impurity in cotinine. Therefore, the antagonistic action of cotinine at peripheral neuronal nicotinic receptors is at least in part independent of nicotine impurity. The observed antagonistic effect of cotinine at nicotinic receptors likely contributes to the neuropharmacological effects of smoking.

Activation and inhibition of the human alpha7 nicotinic acetylcholine receptor by agonists

To better understand the effects of weak as well as strong agonists at the human alpha7 nicotinic acetylcholine receptor (human alpha7 nAChR), the abilities of several classic nAChR agonists to both activate and inhibit (desensitize) the human alpha7 nAChR expressed in Xenopus oocytes were quantified and compared. Activation was measured during 0.2-20 s agonist application, as required to elicit a peak response. Inhibition was measured as the reduction in the agonist response to 200 microM ACh in the presence of inhibitor during a 5-20 min incubation. Acetylcholine (ACh), (-)-nicotine, (+)-nicotine, and 1,1-dimethyl-4-phenylpiperazinium (DMPP) were 62- to 130-fold more potent as inhibitors than as activators, with excellent correlation between the IC50 and EC50 values (r2 = 0.924). Agonist concentrations that elicited only 0.6-1.2% nAChR activation were sufficient to inhibit the response to ACh by 50%. Thus, even a very weak agonist could appear to be a potent and effective inhibitor through receptor desensitization. (-)-Lobeline, in contrast, acted as an antagonist at the human alpha7 nAChR, eliciting no detectable agonist-like response at concentrations up to 1 mM, but inhibiting the response to ACh with an IC50 value of 8.5 microM. (-)-Cotinine and the novel ligand ABT-089 [2-methyl-3-(2-(S)-pyrrolidinylmethoxy)pyridine] acted as weak agonists at the human alpha7 nAChR (1 and 1.5% response at 1 mM, respectively) and inhibited the response to ACh with IC50) values of 175 and 48 microM, respectively. These effects could be explained by receptor desensitization, at least in part.