Sites and mechanisms for nicotine's action in the brain

The involvement of mesolimbic dopamine system in cotinine self-administration in rats

Cotinine is the major metabolite of nicotine and has recently been shown to be self-administered intravenously by rats. However, mechanisms underlying cotinine self-administration remained unknown. Mesolimbic dopamine system projecting from the ventral tegmental area (VTA) to nucleus accumbens (NAC) is closely implicated in drug reinforcement, including nicotine. The objective of the current study was to determine potential involvement of mesolimbic dopamine system in cotinine self-administration. An intracranial self-administration experiment demonstrates that cotinine at 0.88 and 1.76 ng/100 nl/infusion was self-infused into the VTA by rats. Rats produced more infusions of cotinine than vehicle and responded more on active than inactive lever during acquisition, reduced responding when cotinine was replaced by vehicle, and resumed responding during re-exposure to cotinine. Microinjection of cotinine at 1.76 ng/100 nl/infusion into the VTA increased extracellular dopamine levels within the NAC. Subcutaneous injection of cotinine at 1 mg/kg also increased extracellular dopamine levels within the NAC. Administration of the D1-like receptor antagonist SCH 23390 attenuated intravenous cotinine self-administration. On the other hand, bupropion, a catecholamine uptake inhibitor, did not significantly alter intravenous cotinine self-administration. These results suggest that activation of mesolimbic dopamine system may represent one cellular mechanism underlying cotinine self-administration. This shared mechanism between cotinine and nicotine suggests that cotinine may play a role in nicotine reinforcement.

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.

Cotinine antagonizes the behavioral effects of nicotine exposure in the planarian Girardia tigrina

Nicotine is one of the most addictive drugs abused by humans. Our laboratory and others have demonstrated that nicotine decreases motility and induces seizure-like behavior in planarians (pSLM, which are vigorous writhing and bending of the body) in a concentration-dependent manner. Nicotine also induces withdrawal-like behaviors in these worms. Cotinine is the major nicotine metabolite in humans, although it is not the final product of nicotine metabolism. Cotinine is mostly inactive in vertebrate nervous systems and is currently being explored as a molecule which possess most of nicotine's beneficial effects and few of its undesirable ones. It is not known whether cotinine is a product of nicotine metabolism in planarians. We found that cotinine by itself does not seem to elicit any behavioral effects in planarians up to a concentration of 1mM. We also show that cotinine antagonizes the aforementioned nicotine-induced motility decrease and also decreases the expression of nicotine-induced pSLMs in a concentration-dependent manner. Also cotinine prevents the manifestation of some of the withdrawal-like behaviors induced by nicotine in our experimental organism. Thus, we obtained evidence supporting that cotinine antagonizes nicotine in this planarian species. Possible explanations include competitive binding of both compounds at overlapping binding sites, at different nicotinic receptor subtypes, or maybe allosteric interactions.

R-(+) and S-(-) isomers of cotinine augment cholinergic responses in vitro and in vivo

The nicotine metabolite cotinine (1-methyl-5-[3-pyridynl]-2-pyrrolidinone), like its precursor, has been found to exhibit procognitive and neuroprotective effects in some model systems; however, the mechanism of these effects is unknown. In this study, both the R-(+) and S-(-) isomers of cotinine were initially evaluated in an extensive profiling screen and found to be relatively inactive across a wide range of potential pharmacologic targets. Electrophysiological studies on human α4β2 and α7 nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes confirmed the absence of agonistic activity of cotinine at α4β2 or α7 nAChRs. However, a significant increase in the current evoked by a low concentration of acetylcholine was observed at α7 nAChRs exposed to 1.0 μM R-(+)- or S-(-)-cotinine. Based on these results, we used a spontaneous novel object recognition (NOR) procedure for rodents to test the hypothesis that R-(+)- or S-(-)-cotinine might improve recognition memory when administered alone or in combination with the Alzheimer's disease (AD) therapeutic agent donepezil. Although both isomers enhanced NOR performance when they were coadministered with donepezil, neither isomer was active alone. Moreover, the procognitive effects of the drug combinations were blocked by methyllycaconitine and dihydro-β-erythroidine, indicating that both α7 and α4β2 nAChRs contribute to the response. These results indicate that cotinine may sensitize α7 nAChRs to low levels of acetylcholine (a previously uncharacterized mechanism), and that cotinine could be used as an adjunctive agent to improve the effective dose range of cholinergic compounds (e.g., donepezil) in the treatment of AD and other memory disorders.

Evaluation of nicotine and cotinine analogs as potential neuroprotective agents for Alzheimer's disease

The currently available therapies for Alzheimer's disease (AD) and related forms of dementia are limited by modest efficacy, adverse side effects, and the fact that they do not prevent the relentless progression of the illness. The purpose of the studies described here was to investigate the neuroprotective effects of the nicotine metabolite cotinine as well as a small series of cotinine and nicotine analogs (including stereoisomers) and to compare their effects to the four clinically prescribed AD therapies.

Cellular, molecular, and genetic substrates underlying the impact of nicotine on learning

Addiction is a chronic disorder marked by long-lasting maladaptive changes in behavior and in reward system function. However, the factors that contribute to the behavioral and biological changes that occur with addiction are complex and go beyond reward. Addiction involves changes in cognitive control and the development of disruptive drug-stimuli associations that can drive behavior. A reason for the strong influence drugs of abuse can exert on cognition may be the striking overlap between the neurobiological substrates of addiction and of learning and memory, especially areas involved in declarative memory. Declarative memories are critically involved in the formation of autobiographical memories, and the ability of drugs of abuse to alter these memories could be particularly detrimental. A key structure in this memory system is the hippocampus, which is critically involved in binding multimodal stimuli together to form complex long-term memories. While all drugs of abuse can alter hippocampal function, this review focuses on nicotine. Addiction to tobacco products is insidious, with the majority of smokers wanting to quit; yet the majority of those that attempt to quit fail. Nicotine addiction is associated with the presence of drug-context and drug-cue associations that trigger drug seeking behavior and altered cognition during periods of abstinence, which contributes to relapse. This suggests that understanding the effects of nicotine on learning and memory will advance understanding and potentially facilitate treating nicotine addiction. The following sections examine: (1) how the effects of nicotine on hippocampus-dependent learning change as nicotine administration transitions from acute to chronic and then to withdrawal from chronic treatment and the potential impact of these changes on addiction, (2) how nicotine usurps the cellular mechanisms of synaptic plasticity, (3) the physiological changes in the hippocampus that may contribute to nicotine withdrawal deficits in learning, and (4) the role of genetics and developmental stage (i.e., adolescence) in these effects.

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.

Recent advances in gene manipulation and nicotinic acetylcholine receptor biology

Pharmacological and immunological methods have been valuable for both identifying some native nicotinic acetylcholine receptor (nAChR) subtypes that exist in vivo and determining the neurobiological and behavioral role of certain nAChR subtypes. However, these approaches suffer from shortage of subtype specific ligands and reliable immunological reagents. Consequently, genetic approaches have been developed to complement earlier approaches to identify native nAChR subtypes and to assess the contribution of nAChRs to brain function and behavior. In this review we describe how assembly partners, knock-in mice and targeted lentiviral re-expression of genes have been utilized to improve our understanding of nAChR neurobiology. In addition, we summarize emerging genetic tools in nAChR research.

Nicotine regulates multiple synaptic proteins by inhibiting proteasomal activity

Ubiquitination regulates the degradation, membrane trafficking, and transcription of proteins. At mammalian synapses, the ubiquitin-proteasome system (UPS) influences synaptic transmission and plasticity. Nicotine also has the ability to affect synaptic function via mechanisms that remain partially unknown. We found that nicotine, at concentrations achieved by smokers, reduced proteasomal activity, produced accumulation of ubiquitinated synaptic proteins, and increased total protein levels. In particular, a 24 h exposure to nicotine decreased proteasome-dependent degradation of the alpha7 nicotinic acetylcholine receptor (nAChR) subunit, as indicated by the accumulation of ubiquitinated alpha7. The same nicotine treatment increased the levels of the AMPA glutamate receptor subunit GluR1, the NMDA receptor subunit NR2A, the metabotropic receptor mGluR1alpha, the plasticity factor Homer-1A, and the scaffolding postsynaptic density protein PSD-95, whereas the levels of another scaffolding protein, Shank, were reduced. These changes were associated with an inhibition of proteasomal chymotrypsin-like activity by nicotine. The nAChR antagonist mecamylamine was only partially able to block the effects of nicotine on the UPS, indicating that nAChR activation does not completely explain nicotine-induced inhibition of proteasomal catalytic activity. A competition binding assay suggested a direct interaction between nicotine and the 20S proteasome. These results suggest that the UPS might participate in nicotine-dependent synaptic plasticity.

Effects of nicotine on memory impairment induced by blockade of muscarinic, nicotinic and dopamine D2 receptors in rats

Scopolamine dose-dependently inhibits passive avoidance latency and decreases spontaneous alternation in the Y-maze, suggesting effects on long-term and short-term memory, respectively. Chlorisondamine (10 mg/kg), a compound which produces a long-lasting central nicotinic receptor blockade, did not affect short-term and long-term memory performance. In normal rats, nicotine at the doses of 0.3, 1.0, and 3.0 mg/kg administered once had a facilitating effect on short-term memory; a higher dose (3.0 mg/kg) did not show a more pronounced effect than a lower one (0.3 mg/kg). Nicotine, by activating the nicotinic acetylcholine receptors, attenuated the impairment of short-term memory induced by muscarinic or dopamine D2 receptor blockade. On long-term memory, a single dose of nicotine (0.3, 1.0, 3.0 mg/kg) did not affect memory performance, but improved it after chronic (10 consecutive days, 0.3 mg/kg) administration. The antiamnesic effect of nicotine administered once was observed in scopolamine-, scopolamine+chlorisondamine- or sulpiride-treated rats. These results suggest that the antiamnesic effect of nicotine can result from an action at nicotinic receptors subtypes not blocked by chlorisondamine or at nonnicotinic receptors.

Lack of effect of 5HT3 antagonist in mediating subjective and behavioral responses to cotinine

Previous studies have shown that cotinine, a metabolite of nicotine, may antagonize some of the therapeutic effects of nicotine. The mechanisms underlying cotinine's effects are unclear, but cotinine has been observed to increase serotonin levels in the brain. Thus, it is possible that blocking serotonin effects may antagonize the actions of cotinine, thereby reducing its impact on responses to nicotine. This study determined whether granisetron, a 5HT(3) receptor antagonist, would enhance the efficacy of the nicotine patch. Subjects were randomly assigned to one of the three granisetron conditions (N=43 for 2 mg/day; N=43 for 1 mg/day; N=42 for 0 mg/day) and asked to take the assigned medication daily during 15 days of tobacco abstinence. Because we were interested in interactions between cotinine and serotonin, all groups were also treated with a 21-mg nicotine patch. Assessments of withdrawal symptoms were made for 1 week during baseline smoking and several times during the experimental period. There was a near but nonsignificant difference among groups on a measure of tobacco withdrawal and no significant differences on global measures of drug effects or physiological measures. The data do not strongly support the hypothesis that 5HT(3) agonism is the mechanism by which cotinine offsets the effects of nicotine.

Nornicotine, a nicotine metabolite and tobacco alkaloid: desensitization of nicotinic receptor-stimulated dopamine release from rat striatum

Nornicotine, a major tobacco alkaloid and nicotine metabolite, accumulates in rat brain in pharmacologically relevant concentrations following repeated nicotine administration. Nornicotine-evoked striatal dopamine release is Ca2+-dependent, stereoselective and sensitive to nicotinic receptor antagonists, indicating nicotinic receptor-mediation. The present study determined if S-(-)-nornicotine desensitizes nicotinic receptors and if cross-desensitization to S-(-)-nicotine occurs. S-(-)-Nicotine (10 and 100 nM) diminished [3H]overflow from [3H]dopamine-preloaded rat striatal slices following subsequent superfusion with 10 microM S-(-)-nicotine (46% and 74%, respectively) or 10 microM S-(-)-nornicotine (59% and 81%, respectively). S-(-)-Nornicotine (1 and 10 microM) diminished the response to subsequent superfusion with 10 microM S-(-)-nornicotine (85% and 97%, respectively) or 10 microM S-(-)-nicotine (82% and 88%, respectively). Thus, similar to S-(-)-nicotine, S-(-)-nornicotine desensitizes nicotinic receptors. but with approximately 12-fold lower potency. Cross-desensitization suggests involvement of common nicotinic receptor subtypes. Therefore, S-(-)-nicotine metabolites, such as nornicotine, have neuropharmacologically relevant effects.

Characterization of ethyl (3-quinuclidinyl) acetate (EQA) as a ligand for acetylcholine receptors

Recent studies suggest that neuronal nicotinic acetylcholine receptors (nAChRs) may play a role in several CNS disorders and that subtype selective nicotinic ligands may be useful in the treatment of these disorders. Ethyl (3-quinuclidinyl)acetate (EQA) is a bulky, reverse-ester analog of ACh, that produces signs of cholinergic stimulation that may be nicotinic in origin The objective of the present study was to further evaluate EQA as a potential cholinergic ligand. Behavioral studies, smooth muscle assays, and radioligand binding assays were performed on this novel ligand. The effects of EQA on blood pressure and acetylcholinesterase activity were also evaluated. The results of the study suggest that EQA is an agonist at peripheral nicotinic acetylcholine receptors and may have antagonist properties at central nicotinic receptors.

Contribution of CNS nicotine metabolites to the neuropharmacological effects of nicotine and tobacco smoking

Nicotine, the principal alkaloid in tobacco products, is generally accepted to be the active pharmacological agent responsible for CNS effects resulting from tobacco use. Arguments are presented in this commentary which take issue with this popular dogma, by providing evidence that nicotine metabolites may also be responsible for the CNS effects commonly attributed to nicotine. CNS effects attributed to nicotine include reinforcing effects, mood elevation, arousal, locomotor stimulant effects, and learning and memory enhancement. The reinforcing and locomotor stimulant effects of nicotine have been suggested to be the result of activation of CNS dopaminergic systems, and nicotine-induced modulation of dopaminergic neurotransmission has been studied in detail. Nicotine acts at a family of nicotinic receptor subtypes composed of multiple subunits; however, the exact composition of the subunits in native nicotinic receptors and the functional significance of the receptor subtype diversity are currently unknown. This nicotinic subtype diversity increases the complexity of the potential mechanisms of action of nicotine and its metabolites. Although peripheral metabolism of nicotine has been studied extensively, metabolism in the CNS has not been investigated to any great extent. Recently, studies from our laboratory have demonstrated that several nicotine metabolites are present in the CNS after acute nicotine administration. Moreover, nicotine metabolites are pharmacologically active in neurochemical and behavioral assays. Thus, CNS effects resulting from nicotine exposure may not be due solely to nicotine, but may result, at least in part, from the actions of nicotine metabolites.

Alzheimer's drug design based upon an invertebrate toxin (anabaseine) which is a potent nicotinic receptor agonist

Naturally occurring toxins can often serve as useful chemical tools for investigating signalling processes in nervous and other systems. Tetrodotoxin and alpha-bungarotoxin are prime examples of toxins which are widely used in neurobiological research. Some toxins may also become molecular models for designing new drugs. Usually drugs are small, non-peptide molecules, as these display better bioavailability, longer durations of action and are less likely to generate immune responses. The relatively large size and conformational flexibility of peptides and protein toxins makes them more challenging molecular models for rational drug design. This article considers a marine invertebrate toxin, anabaseine, and describes how manipulation of the structure of this alkaloid has provided a drug candidate which selectively stimulates mammalian brain alpha7 nicotinic receptors. Numerous anabaseine analogs were synthesized and subjected to a variety of pharmacological, behavioral and toxcicological tests. This led to the choice of GTS-21 (also known as 3-(2,4-dimethoxybenzylidene)-anabaseine or DMXBA), as a drug candidate for the treatment of Alzheimer's dementia. The chemical and pharmacological properties of GTS-21 are compared with those of the initial lead compound, anabaseine.

[3H]benzylpempidine, a new radioligand for probing a putative channel site on nicotinic cholinergic receptors

A study was undertaken to assess the receptor binding characteristics of [3H]4-benzylpempidine to an allosteric site on calf brain membranes associated with nicotinic cholinergic receptors and to compare the binding affinity of novel arylpempidine analogs with their ability to antagonize the behavioral effects of nicotine in mice. Scatchard analysis of the binding yielded a K(d) of 20 nM and a B(max) of 330 fmols/mg membrane protein. [3H]4-benzylpempidine appears to be a more satisfactory ligand than [3H]mecamylamine, since it possessed a 50-fold greater affinity and its binding was far less sensitive to inorganic ions and Tris. Among the arylpempidine analogs 4-m-chlorobenzylidenepempidine and 4-benzylidenepempidine had the lowest K(i) values (1.4 nM and 5.0 nM, respectively) and were the most potent in antagonizing nicotine-induced seizures in mice. Although the K(i) values for pempidine and mecamylamine were 1-2 orders of magnitude greater than any of the arylpempidines, the dose required to antagonize nicotine-induced seizures in mice was comparable to the arylpempidines. One explanation for this apparent discrepancy in the correlation of binding affinity and nicotine antagonism is the lower brain penetration of arylpempidines compared to mecamylamine, following their systemic administration to mice.

Brain sites mediating the discriminative stimulus effects of nicotine in rats

Pharmacological studies suggest that the discriminative stimulus (DS) produced by nicotine is mediated centrally. The aim of the present study was to identify neuroanatomical substrates that mediate the DS properties of nicotine. Specifically, the nucleus accumbens, a brain region known to mediate the DS effects of amphetamine and cocaine, was investigated using a two-lever operant drug discrimination paradigm. Male hooded rats were trained to discriminate nicotine (0.2 mg/kg s.c.) from saline with a tandem schedule of food reinforcement. Once stimulus control was attained, a randomised sequence of nicotine microinjections (2-8 micrograms) was tested for generalisation during brief extinction tests. It was confirmed that the stimulus produced by the systemic administration of nicotine generalized to nicotine administered bilaterally into the dorsal hippocampus, with significant decreases in overall response rates. Microinjections of nicotine (1-8 micrograms) into the nucleus accumbens failed to produce any dose-related increases in responding on the nicotine-appropriate lever although these microinjections also produced significant decreases in response rates. Smaller doses (1-4 micrograms) of nicotine administered into the fourth ventricle produced characteristic prostration responses but these microinjections failed to produce generalization in tests carried out 20 min later, when the disabling effects of prostration had dissipated. These results suggest that the DS effects of nicotine may be mediated, at least in part, through the dorsal hippocampus. Results from intra-accumbens and intraventricular injections suggest that these regions may not be important in mediating the DS effects of nicotine.

The pharmacology of (-)-nicotine and novel cholinergic channel modulators

Advances in the understanding of the molecular biology and pharmacology of nAChRs may provide targets for the development of novel and selective modulators of nAChRs in the brain. This contention is supported by the dissimilar behavioral effects observed following systemic administration of currently available nicotinic ligands. The concept of multiple subtypes of nAChRs is not unique, as evidenced by the pharmacology of other ligand-gated ion channels, such as GABA-A receptor, which also exist in multiple subtypes. At present, with respect to the nAChRs, relatively few of the subtypes identified have been cloned from human tissue and pharmacologically evaluated, but several groups are focusing their research efforts in this direction. With a thorough understanding of the pharmacological and functional characteristics of more of the putative human nAChR subtypes, this knowledge will facilitate the discovery of more efficacious and less toxic ChCMs that may provide potential novel therapeutic agents for a variety of CNS conditions.

Nicotine blocks angiotensin II inhibition of LTP in the dentate gyrus

Angiotensin (ANG)-containing axons, terminals, and receptors have been found in the hippocampus. When angiotensin II (ANG II) is administered to the dentate gyrus, long-term potentiation (LTP) induction, in response to medial perforant path stimulation, is inhibited and it can be blocked by losartan, an ANG II AT1 receptor antagonist. ANG II has been shown to mediate impairment of the retention of an inhibitory shock avoidance response and to be involved in ethanol and diazepam inhibition of dentate gyrus LTP, all of which can be blocked by losartan. Nicotine acetylcholine receptors are found in the hippocampus and nicotine is involved in the enhancement of complex and important psychological functions that are mediated by the hippocampus; therefore, the possibility that nicotine prevents the ANG II inhibition of dentate granule cell LTP was examined. Nicotine pretreatment reduced ANG II inhibition of LTP induction in a dose-dependent manner. Mecamylamine blocked the nicotine antagonism of ANG II-induced LTP inhibition and normal LTP occurred, whereas hexamethonium was ineffective in blocking these central effects of nicotine. Nicotine by itself did not affect normal LTP under these conditions. Nicotinic blocking of the ANG II inhibition of a frequency dependent type of synaptic plasticity provides a function for central nicotinic receptors and a possible mechanism of action a) to explain the enhancement of learning and memory by nicotine, b) an explanation for tobacco smoking while drinking alcohol, and c) a possible basis for the excessive use of tobacco in depression and schizophrenia that supports a possible therapeutic use of nicotine in some mental disorders.

In vivo effects of (-)-nicotine on ethanol-induced increase in glucose utilization in the mouse cerebellum

The purpose of this study was to investigate the possible in vivo effects of (-)-nicotine, ethanol, and an adenosine agonist N6-cyclohexyladenosine (CHA) when injected individually as well as in various combinations on glucose utilization in the fresh cerebellar slices of mice. Mice received ICV (-)-nicotine or CHA followed 5 min later by a test dose of ethanol (2 g/kg; IP). Animals were killed 20 min postethanol treatment and fresh slices (300 microns) of cerebellum were incubated in a glucose medium in Warburg flasks using 14C-glucose as a tracer. Trapped 14CO2 was counted to estimate glucose utilization. Ethanol treatment markedly accentuated glucose utilization, whereas the pretreatment with (-)-nicotine (125 and 250 ng, ICV) resulted in a significant attenuation in the ethanol-induced increase in glucose utilization. However, ICV (-)-nicotine (125 ng) alone did not produce any change in the cerebellar glucose utilization. The attenuation of ethanol-induced increase in glucose utilization by (-)-nicotine was nearly totally blocked by ICV hexamethonium, a purported nicotinic antagonist, suggesting participation of cholinergic-nicotinic receptors. The (-)-nicotine pretreatment also significantly attenuated both the ICV CHA (25 ng)-induced increase in glucose utilization and the accentuation of ethanol-induced increase in glucose utilization by CHA. The antagonistic effect of (-)-nicotine on CHA- and ethanol-induced increase in glucose utilization indicating an interaction between (-)-nicotine and ethanol and between (-)-nicotine and adenosine may suggest involvement of postreceptor (nicotinic and adenosine) mechanisms including ionic channels.

Pharmacological evaluation of methylcarbamylcholine-induced drinking behavior in rats

Methylcarbamylcholine (MCC), a structural analog of carbachol (an acetylcholine agonist), has been reported to be a specific nicotinic cholinergic receptor ligand. MCC produces a robust polydipsic response shortly following central administration. The purpose of the present study was to pharmacologically characterize this increase in drinking behavior. Male Wistar rats were implanted with intracerebroventricular (ICV) cannula guides directed at the left lateral ventricle. Following a recovery period, animals were injected ICV with saline or various doses of MCC (3-60 micrograms) and water consumption was quantified. MCC produced a dose-related, transient increase in water consumption that peaked at a dose of 30 micrograms. In contrast, nicotine, a potent nicotinic cholinergic receptor agonist, did not produce changes in drinking following ICV administration. MCC-induced increases in drinking were not blocked by pretreatment with several selective nicotinic receptor antagonists including dihydro-beta-erythriodine (DHBE), hexamethonium, and mecamylamine. However, pretreatment with the muscarinic antagonist atropine (0.01 or 1.0 microgram) completely abolished MCC-induced polydipsia. Following a chronic treatment regimen (MCC injected ICV twice daily for 10 days), no tolerance to MCC-induced changes in water consumption was observed. Previous studies have demonstrated that tolerance develops to nicotinic-receptor mediated responses following the identical chronic treatment paradigm. These results suggest that MCC-induced polydipsia is mediated through stimulation of muscarinic rather than nicotinic receptors.

Locomotor activity after nicotine infusions into the fourth ventricle of rats

Microinjections of nicotine into the fourth ventricle of rats were reported previously to produce a characteristic prostration syndrome; similar microinjections have been investigated for effects on locomotor activity. It was confirmed that nicotine (4 micrograms) administered into the fourth ventricle of rats produced prostration which was also manifested on a second challenge with the drug. Increasing doses of nicotine produced increasing magnitudes of prostration and dose-related decreases in locomotor activity. In rats pretreated with nicotine (0.4 mg/kg SC) for 10 days, no tolerance was seen to either the prostration response or the locomotor depression. Mecamylamine (1.0 mg/kg SC) completely prevented the prostration response produced by 4 micrograms of nicotine, but the locomotor depression was still evident. The locomotor changes following intraventricular administration of nicotine appeared to be different from the locomotor depression seen following systemic administration because the posture of the animals was different and the latter effects showed tolerance with repeated exposures to nicotine and were fully blocked by mecamylamine. These findings suggested that the prostration response and the locomotor depression were mediated by different mechanisms.

Intracerebellar nicotinic-cholinergic participation in the cerebellar adenosinergic modulation of ethanol-induced motor incoordination in mice

Many epidemiological studies have suggested a high correlation between the use of tobacco and ethanol, the two most frequently abused psychoactive drugs. Recently, we reported behavioral interactions between (-)-nicotine, (-)-cotinine and ethanol within the CNS. The present report is a confirmation and an extension of that study. Using a 2 g/kg ethanol-induced motor incoordination (EIMI) as the test response, possible behavioral interactions between (-)-nicotine, (-)-cotinine and ethanol and between (-)-nicotine, (-)-cotinine and adenosine agonist + ethanol in the cerebellum were investigated. (-)-Nicotine, 0.625, 1.25 and 5 ng intracerebellarly (ICB) significantly attenuated EIMI in a dose-related manner. Likewise, ICB injection of 1.25, 2.5, and 5 ng (-)-cotinine, a major metabolite of nicotine, significantly attenuated EIMI after the same i.p. dose of ethanol as in case of (-)-nicotine but less markedly compared to (-)-nicotine. No change in normal motor coordination was observed when the highest dose of (-)-nicotine or (-)-cotinine was injected ICB followed by saline control, suggesting selectivity of their behavioral interactions with ethanol. The attenuation of EIMI by (-)-nicotine and (-)-cotinine was blocked by ICB hexamethonium (1 microgram) and trimethaphan (100 ng), the purported nicotinic-cholinergic antagonists. Finally, the ICB injection of adenosine agonists, N6-cyclohexyladenosine (CHA) or 5'-N-ethylcarboxamidoadenosine (NECA), produced marked accentuation of EIMI which was significantly antagonized by ICB (-)-nicotine and (-)-cotinine. The data obtained in the present study suggested, for the first time, a cerebellar adenosinergic-nicotinic cholinergic interaction and modulation of EIMI. The data also suggested participation of cerebellar nicotinic-cholinergic receptors in EIMI.

Effects of central nicotinic cholinergic receptor blockade produced by chlorisondamine on learning and memory performance in rats

The effects of chronic nicotinic receptor blockade on the performance of learning and memory tasks were determined using chlorisondamine, a compound which produces central nicotinic cholinergic receptor blockade that lasts for several weeks after a single icv administration. Chlorisondamine treatment did not affect the acquisition of spatial information in the Morris water maze or in the radial arm maze, tasks in which performance is reportedly disrupted by acute administration of the nicotinic antagonist, mecamylamine. Chlorisondamine also did not affect performance in the inhibitory avoidance task and did not alter the memory enhancement found in this task after post-training administration of nicotine. Mecamylamine, however, completely blocked the memory-enhancing effects of nicotine. In contrast to the differential ability to chlorisondamine and mecamylamine to block nicotine's memory-enhancing effects, these antagonists produced comparable blockade of nicotine's effects on open field behavior. It is unlikely that the different effects of systemically administered mecamylamine and centrally administered chlorisondamine on nicotine-induced memory enhancement are due to mecamylamine's peripheral effects, since hexamethonium, a peripherally active nicotinic antagonist, did not block nicotine-induced memory enhancement. The different pattern of effects of mecamylamine and chlorisondamine may be related to compensatory mechanisms being selectively induced by chronic blockade produced by chlorisondamine and not by acute blockade produced by mecamylamine. Alternatively, different effects of these two nicotinic cholinergic antagonists on the performance of learning and memory tasks might be related to selective actions of these compounds at nicotinic receptor subtypes or at nonnicotinic receptors.

Affinity ligands and related agents for brain muscarinic and nicotinic cholinergic receptors

This study describes the chemical synthesis and receptor binding characteristics of various affinity ligands and related ligands for brain muscarinic and nicotinic cholinergic receptors, including the 4-bromoacetamidobenzoic acid esters of dimethylaminoethanol (DMBAB) and choline (BABC) and 4-iodoacetamidobenzoylcholine (IABC). The reversible binding of [3H]3-quinuclidinylbenzilate ([3H]QNB) to calf brain membranes was inhibited in a concentration-dependent and saturable manner by DMBAB, BABC, and IABC with Ki values of 8 x 10(-7), 3 x 10(-7) and 8 x 10(-7) M, respectively; the Ki values for inhibition of reversible binding of the nicotinic ligand, [3H]methylcarbamylcholine ([3H]-MCC), were 1 x 10(-6), 6 x 10(-8), and 1 x 10(-6) M, respectively. The Ki values for irreversible inhibition of [3H]QNB binding were 8 x 10(-7), 1 x 10(-7), and 2 x 10(-7) M for DMBAB, BABC, and IABC, respectively, and for [3H]MCC binding, 8 x 10(-5), 1 x 10(-5), and 2 x 10(-5) M, respectively. Although DMBAB was found to inhibit the QNB-induced hyperactivity in mice, it did not antagonize the toxic or other pharmacologic effects of oxotremorine. Structure-activity studies with various non-affinity analogues of the 4-aminobenzoate ester of dimethylaminoethanol and choline revealed that removal of the NH2 moiety from the phenyl group increased affinity for the muscarinic but not the nicotinic cholinergic site, and quaternization of the ester side chain greatly increased affinity for the muscarinic site. Dimethylation of NH2 in 4-aminobenzoylcholine decreased the affinity for both cholinergic sites. Replacement of NH2 by NO2 increased affinity for the muscarinic but not the nicotinic site, whereas quaternization of the 4-nitrobenzoyl ester markedly increased affinity for the nicotinic site while diminishing affinity for the muscarinic site. The findings indicate that DMBAB and its analogues are useful affinity ligands for examining the biochemical and functional characteristics of brain cholinergic receptors, particularly the muscarinic which has an affinity near the nanomolar concentration range.

Nicotinic receptor agonists facilitate retention of avoidance training: participation of dopaminergic mechanisms

The effect of nicotinic receptor agonists on retention of the inhibitory avoidance (IA) response were investigated in mice. Animals received intraperitoneal drug injections before training, and retention was evaluated 24 h later. Nicotine and cytisine, but not lobeline, significantly increased retention of the IA training. Cotinine, the main metabolite of nicotine, was inactive in the same test. Retention was not affected by the injection of the D1-D2 receptor antagonist cis-flupentixol, but the preadministration of cis-flupentixol significantly blocked the facilitatory effect of nicotine and cytisine on memory. These results demonstrate that the nicotinic receptor agonists nicotine and cytisine facilitate the retention of avoidance responses and suggest that this effect is mediated through central dopaminergic pathways.

A role for the basal ganglia in nicotinic modulation of the blink reflex

In humans and rats we found that nicotine transiently modifies the blink reflex. For blinks elicited by stimulation of the supraorbital branch of the trigeminal nerve, nicotine decreased the magnitude of the orbicularis oculi electromyogram (OOemg) and increased the latency of only the long-latency (R2) component. For blinks elicited by electrical stimulation of the cornea, nicotine decreased the magnitude and increased the latency of the single component of OOemg response. Since nicotine modified only one component of the supraorbitally elicited blink reflex, nicotine must act primarily on the central nervous system rather than at the muscle. The effects of nicotine could be caused by direct action on lower brainstem interneurons or indirectly by modulating descending systems impinging on blink interneurons. Since precollicular decerebration eliminated nicotine's effects on the blink reflex, nicotine must act through descending systems. Three lines of evidence suggest that nicotine affects the blink reflex through the basal ganglia by causing dopamine release in the striatum. First, stimulation of the substantia nigra mimicked the effects of nicotine on the blink reflex. Second, haloperidol, a dopamine (D2) receptor antagonist, blocked the effect of nicotine on the blink reflex. Third, apomorphine, a D2 receptor agonist, mimicked the effects of nicotine on the blink reflex.

Tolerance to the convulsions induced by daily nicotine treatment in rats

Development of tolerance to the nicotine-induced convulsions in rats was examined. Acute intraperitoneal (i.p.) administration of nicotine (2.5, 3.75 and 5 mg/kg) produced convulsions in a dose-dependent manner. Mecamylamine (1 mg/kg, i.p.) antagonized the convulsions, but hexamethonium (5 mg/kg, i.p.) did not modify them. Daily nicotine administration (2.5, 3.75 and 5 mg/kg, i.p.) once a day for 6 days developed tolerance to the convulsions induced by nicotine. After the daily administrations of nicotine for 6 days, the effects of a challenge administration of nicotine (2 mg/kg) on the nicotine-induced convulsions were tested on the 7th-day. Further tolerances were also developed by the 7th-day challenge administration. After the 7th-day test, nicotine levels of the brain and blood 15 min after the challenge injection were measured. With nicotine (5 mg/kg once a day)-treatment, nicotine levels of all the brain regions were increased. In contrast, a similar challenge injection had no effect on blood nicotine level. These results indicate that the development of tolerance to the nicotine-induced convulsions is produced relatively earlier and day by day by daily administrations to rats, which is closely related with the increase in brain nicotine level.

4-Bromoacetamidoprocaine: an affinity ligand for brain muscarinic and nicotinic cholinergic receptors

This study describes the synthesis, receptor binding characteristics, and some behavioral effects of p-bromoacetamidoprocaine (BAP), a new affinity ligand for brain muscarinic and nicotinic cholinergic receptors. The reversible binding of [3H]QNB to rat brain membranes was inhibited in a concentration dependent and saturable manner by both procaine and BAP, with Ki values of 4 x 10(-6) and 3 x 10(-7) M, respectively, and complete inhibition at 1 x 10(-5) M. Both procaine and BAP, although at much concentrations, inhibited the binding of [3H]methylcarbamylcholine in a concentration dependent manner, with Ki values of 5 x 10(-5) and 1 x 10(-5) M, respectively, and complete inhibition for both at 1 x 10(-3) M. Plots of the % irreversible inhibition of [3H]QNB, [3H]nicotine, and [3H]MCC vs [BAP] yielded Ki values of 7 x 10(-8), 1 x 10(-4), and 6 x 10(-5) M, respectively. In behavioral studies BAP was able to antagonize the QNB-induced hyperactivity in mice; however, BAP did not appear to alter nicotine-induced seizure activity or other behavioral effects in mice. A plot of the time course of inhibition by BAP for [3H]QNB binding revealed that the inhibition was almost complete within 10 min exposure at 37 degrees. The findings indicate that BAP is a useful affinity ligand for examining the biochemical and functional characteristics of brain cholinergic receptors, particularly the muscarinic which has an affinity near the nM concentration range.

Nicotine-induced alteration in Tyr-Gly-Gly and Met-enkephalin in discrete brain nuclei reflects altered enkephalin neuron activity

Nicotine acts in CNS, but the pathways and mechanisms of its actions are poorly understood. Recent studies suggest an interaction between brain nicotinic receptors and endogenous opioid peptides. Acute administration of nicotine may alter enkephalin release without affecting brain enkephalin level. Tyr-Gly-Gly has been shown previously to be an extraneuronal metabolite of opioid peptides derived from proenkephalin A. Concentrations of Tyr-Gly-Gly in brain were used to provide an index of enkephalin release in vivo. Thus we examined the thesis that nicotine alters brain neuronal enkephalin release, by measuring Tyr-Gly-Gly levels in specific brain nuclei from rats treated with nicotine 0.3 mg/kg SC 10 min before decapitation. Of 30 brain regions investigated, acute nicotine increased Tyr-Gly-Gly immunoreactivity in nucleus accumbens and in lower brain stem areas including dorsal raphe, pontine reticular formation, gigantocellular reticular formation, locus coeruleus, sensory trigeminal nucleus and the caudal part of ventrolateral medulla oblongata. Concomitantly, nicotine produced a significant decrease in native Met-enkephalin in central amygdala, flocculo-nodular lobe of cerebellum, caudal part of the ventrolateral medulla and intermediolateral cell column of the spinal cord. It is probable that the effects of nicotine to increase Tyr-Gly-Gly and alter Met-enkephalin concentration are mediated by nicotine-induced release of enkephalin at these brain sites. Furthermore, some of the physiologic and pharmacologic effects of nicotine may be mediated by such enkephalin release.

[3H]mecamylamine binding to rat brain membranes. Studies with mecamylamine and nicotine analogues

Mecamylamine, an antagonist to nicotine, does not compete at the nicotinic recognition site, but is believed to block the ion channel of the nicotinic receptor. The present study demonstrates specific, saturable [3H]mecamylamine binding in rat brain membranes. [3H]Mecamylamine binding was destroyed by heating at 100 degrees and trypsin. Scatchard analysis revealed the presence of two sites with Kd values of 9.6 x 10(-8) and 1.1 x 10(-6) M and Bmax values of 7 x 10(-12) and 3 x 10(-11) mol/mg protein respectively. A good correlation was observed between the Ki values for [3H]mecamylamine binding of a number of mecamylamine and related analogues and their ability to block nicotine-induced prostration in rats and seizures in mice. Inorganic cations, particularly divalent, and various ion channel blockers, such as phencyclidine and verapamil, exhibited a high affinity for the [3H]mecamylamine site. Although mecamylamine did not block nicotine binding, nicotine and its analogues exhibited a high affinity for the [3H]mecamylamine site, a finding which suggests that nicotine acts directly on ion channels as well as the nicotinic cholinergic recognition sites. The data are consistent with the notion that mecamylamine interacts with the open ion channel of the nicotinic receptor.

Mechanisms involved in the respiratory depressant actions of nicotine in anesthetized rats

In the urethane-pentobarbital anesthetized rat, the respiratory depressant and lethal effects of intravenously infused (-)-nicotine (120 micrograms/kg/min) or (+)-nicotine (600 micrograms/kg/min) were effectively prevented by pretreatment with the opioid antagonist, naltrexone, whereas the lethal effect of (-)-nicotine (120 micrograms/kg/min) was not altered by bilateral adrenalectomy. Further, pretreatment with either the nicotinic ganglion-blocker, mecamylamine, a secondary amine, or the quarternary nicotinic ganglion-blocker, hexamethonium, completely prevented the lethal effects of (-)-nicotine (120 micrograms/kg/min). These data suggest that central opioidergic and nicotinic processes are involved in nicotine's respiratory depressant and lethal effects.

Neuroregulatory effects of nicotine

The impact of nicotine on the central nervous system is, in an important sense, neuroregulatory, with cascading effects on physiological and biochemical function as well as on behavioral activity. Accordingly, the neurotransmitter and neuroendocrine effects of nicotine constitute a critical part of its biological action, which includes reinforcing as well as pathophysiological consequences. This review focuses on nicotine's effects on cholinergic and non-cholinergic nicotine receptors and on the responses of catecholamines, monoamines, hypophyseal hormones, and cortisol. The contribution of critical variables, such as timing and duration of neuroregulator release and the patterns that make up the total response, is still largely unknown, particularly with regard to the effects of environmental context, history of nicotine use, and mode of administration. The evidence suggests that by altering the bioavailability of the above-listed neuroregulators, nicotine serves as a pharmacological "coping response", providing immediate though temporary improvement in affect or performance in response to environmental demands. Much of what is known to date is based on studies involving the administration of agonists and antagonists under different environmental conditions. Newer technological approaches such as autoradiography and positron emission tomography show potential for determining the neuroregulatory patterns involved and specifying nicotine's locus of action relevant to its behavioral and physiological effects.

Properties of putative nicotine receptors identified on cultured cortical neurons

Cultured cortical neurons derived from fetal rat brain contain putative high-affinity nicotinic receptors. Cortical glial cells do not contain any high-affinity binding sites for nicotine. The affinity, kinetic binding properties and pharmacological specificity of neuronal sites are the same as those determined for adult brain tissue. However, the number of sites in cultured neurons is lower than in adult cortex. Cultured cortical neurons appear to be a suitable model system for defining the location(s) and functional properties of high-affinity nicotinic receptors in mammalian brain.

Structure-activity studies of carbamate and other esters: agonists and antagonists to nicotine

A number of aromatic, cycloalkyl, and heterocyclic carbamic acid esters, thiocarbamic acid esters, and carboxylic acid esters of di- and trial-kylaminoalkyl and heterocyclic amino alcohols have been synthesized and tested for their pharmacologic and receptor binding characteristics at the nicotine receptor. Receptor binding was measured in rat brain membranes using (-)-3H-nicotine or 3H-methylcarbamylcholine as radioligands. The compounds were tested for their ability to produce seizures and prostration and to antagonize the nicotine-induced prostration and seizures as well as the hypertensive action of nicotine in rats. Among the potent agonists were the N-methylcarbamyl and N-methylthiocarbamyl esters of choline (trimethylaminoethanol), with the tertiary amino derivatives between considerably less potent than the quaternary. Potent antagonists included trimethylaminoethyl benzoate, 3-quinuclidinyl benzoate, and trimethylaminoethyl esters of phenyl and phenylthiocarbamic acids. One of the most potent antagonists to nicotine was alpha-lobeline.

Pharmacologic determinants of tobacco self-administration by humans

Tobacco is a naturally occurring source of nicotine, which is a chemical of demonstrable abuse liability and dependence potential. All commonly used forms of tobacco result in the delivery of nicotine to the central nervous system (CNS), where its actions affect the probability of subsequent use. The role of nicotine as a determinant of patterns of tobacco self-administration and other tobacco-associated responses has frequently been confounded by the complexity of this form of drug self-administration, since the amount of nicotine delivered to the CNS is not a simple function of the amount of tobacco consumed. The present paper is a summary of data which indicate that nicotine administration and withdrawal are determinants of tobacco ingestion. Recent data that are reviewed include those which indicate that the following effects of nicotine bear an orderly relation to the dose administered: (1) reduction of cigarette smoking, (2) production of discriminative effects, and (3) blockade of tobacco withdrawal symptoms. A secondary intent of the present paper is to describe aspects of tobacco dependence which are relevant to the appreciation of the subsequent papers appearing in this series of eight.

Progress in understanding the relationship between the pharmacological effects of nicotine and human tobacco dependence

The present paper is intended to serve as an introduction to the series of eight papers which follow in this issue of Pharmacology Biochemistry and Behavior. A brief historical review of research that is at the root of much recent progress is provided in the present paper. In addition, we provide some data which illustrates the scope of tobacco-related research, world wide, in an effort to provide a perspective as to the vast amount of research activity that is currently in progress. Seven of the papers which follow were presented at a symposium held under the auspices of the American Society for Pharmacology and Therapeutics in 1986. Taken together, these papers are intended to provide new data and to review major areas of pharmacologic research relevant to the understanding and treatment of tobacco dependence. The topics include the behavioral and physiologic mechanisms by which the effects of nicotine are mediated, metabolic aspects of nicotine kinetics, and genetic determinants of responses to nicotine. The final paper is a discussion of the implications of these recent data for the pharmacologic treatment of tobacco dependence.

Nicotinic agonists regulate alpha-bungarotoxin binding sites of TE671 human medulloblastoma cells

The TE671 human medulloblastoma cell line expresses a variety of characteristics of human neurons. Among these characteristics is the expression of membrane-bound high-affinity binding sites for alpha-bungarotoxin, which is a potent antagonist of functional nicotinic acetylcholine receptors on these cells. These toxin binding sites represent a class of nicotinic receptor isotypes present in mammalian brain. Treatment of TE671 cells during proliferative growth phase with nicotine or carbamylcholine, but not with muscarine or d-tubocurarine, induced up to a five-fold increase in the density of radiolabeled toxin binding sites in crude membrane fractions. This effect was blocked by co-incubation with the nicotinic antagonists d-tubocurarine and decamethonium, but not by mecamylamine or by muscarinic antagonists. Following a 10-13 h lag phase upon removal of agonist, recovery of the up-regulated sites to control values occurred within an additional 10-20 h. These studies indicate that the expression of functional nicotinic acetylcholine receptors on TE671 cells is subject to regulation by nicotinic agonists. Studies of the murine CNS have consistently indicated nicotine-induced up-regulation of nicotinic acetylcholine receptors, thereby supporting the identification of the toxin binding site on these cells as the functional nicotinic receptor. Although a mechanism for this effect is not apparent, nicotine-induced receptor blockade does not appear to be involved.