Increases in alpha4* but not alpha3*/alpha6* nicotinic receptor sites and function in the primate striatum following chronic oral nicotine treatment

Independent and Combined Effects of Nicotine or Chronic Tobacco Smoking and HIV on the Brain: A Review of Preclinical and Clinical Studies

Tobacco smoking is highly prevalent among HIV-infected individuals. Chronic smokers with HIV showed greater cognitive deficits and impulsivity, and had more psychopathological symptoms and greater neuroinflammation than HIV non-smokers or smokers without HIV infection. However, preclinical studies that evaluated the combined effects of HIV-infection and tobacco smoking are scare. The preclinical models typically used cell cultures or animal models that involved specific HIV viral proteins or the administration of nicotine to rodents. These preclinical models consistently demonstrated that nicotine had neuroprotective and anti-inflammatory effects, leading to cognitive enhancement. Although the major addictive ingredient in tobacco smoking is nicotine, chronic smoking does not lead to improved cognitive function in humans. Therefore, preclinical studies designed to unravel the interactive effects of chronic tobacco smoking and HIV infection are needed. In this review, we summarized the preclinical studies that demonstrated the neuroprotective effects of nicotine, the neurotoxic effects of the HIV viral proteins, and the scant literature on nicotine or tobacco smoke in HIV transgenic rat models. We also reviewed the clinical studies that evaluated the neurotoxic effects of tobacco smoking, HIV infection and their combined effects on the brain, including studies that evaluated the cognitive and behavioral assessments, as well as neuroimaging measures. Lastly, we compared the different approaches between preclinical and clinical studies, identified some gaps and proposed some future directions. Graphical abstract Independent and combined effects of HIV and tobacco/nicotine. Left top and bottom panels: Both clinical studies of HIV infected persons and preclinical studies using viral proteins in vitro or in vivo in animal models showed that HIV infection could lead to neurotoxicity and neuroinflammation. Right top and bottom panels: While clinical studies of tobacco smoking consistently showed deleterious effects of smoking, clinical and preclinical studies that used nicotine show mild cognitive enhancement, neuroprotective and possibly anti-inflammatory effects. In the developing brain, however, nicotine is neurotoxic. Middle overlapping panels: Clinical studies of persons with HIV who were smokers typically showed additive deleterious effects of HIV and tobacco smoking. However, in the preclinical studies, when nicotine was administered to the HIV-1 Tg rats, the neurotoxic effects of HIV were attenuated, but tobacco smoke worsened the inflammatory cascade.

Nicotine Self-Administration Induces Plastic Changes to Nicotinic Receptors in Medial Habenula

Chronic nicotine upregulates nicotinic acetylcholine receptors (nAChRs) throughout the brain, and reducing their activity may promote somatic and affective states that lead to nicotine seeking. nAChRs are functionally upregulated in animal models using passive nicotine administration, but whether/how it occurs in response to volitional nicotine intake is unknown. The distinction is critical, as drug self-administration (SA) can induce neurotransmission and cellular excitability changes that passive drug administration does not. In this study, we probed the question of whether medial habenula (MHb) nAChRs are functionally augmented by nicotine SA. Male rats were implanted with an indwelling jugular catheter and trained to nose poke for nicotine infusions. A saline SA group controlled for non-specific responding and nicotine-associated visual cues. Using patch-clamp whole-cell recordings and local application of acetylcholine, we observed robust functional enhancement of nAChRs in MHb neurons from rats with a history of nicotine SA. To determine whether upregulated receptors are generally enhanced or directed to specific cellular compartments, we imaged neurons during recordings using two-photon laser scanning microscopy (2PLSM). nAChR activity at the cell soma and on proximal and distal dendrites was examined by local nicotine uncaging using a photoactivatable nicotine (PA-Nic) probe and focal laser flash photolysis. Results from this experiment revealed strong nAChR enhancement at all examined cellular locations. Our study demonstrates nAChR functional enhancement by nicotine SA, confirming that volitional nicotine intake sensitizes cholinergic systems in the brain. This may be a critical plasticity change supporting nicotine addiction.

Chronic Menthol Does Not Change Stoichiometry or Functional Plasma Membrane Levels of Mouse α3β4-Containing Nicotinic Acetylcholine Receptors

Heteromeric α3β4 nicotinic acetylcholine (ACh) receptors (nAChRs) are pentameric ligand-gated cation channels that include at least two α3 and two β4 subunits. They have functions in peripheral tissue and peripheral and central nervous systems. We examined the effects of chronic treatment with menthol, a major flavor additive in tobacco cigarettes and electronic nicotine delivery systems, on mouse α3β4 nAChRs transiently transfected into neuroblastoma-2a cells. Chronic menthol treatment at 500 nM, near the estimated menthol concentration in the brain following cigarette smoking, altered neither the [ACh]-response relationship nor Zn2+ sensitivity of ACh-evoked currents, suggesting that menthol does not change α3β4 nAChR subunit stoichiometry. Chronic menthol treatment failed to change the current density (peak current amplitude/cell capacitance) of 100 μM ACh-evoked currents. Chronic menthol treatment accelerated desensitization of 100 and 200 μM ACh-evoked currents. Chronic nicotine treatment (250 μM) decreased ACh-induced currents, and we found no additional effect of including chronic menthol. These data contrast with previously reported, marked effects of chronic menthol on β2* nAChRs studied in the same expression system. Mechanistically, the data support the emerging interpretation that both chronic menthol and chronic nicotine act on nAChRs in the early exocytotic pathway, and that this pathway does not present a rate-limiting step to the export of α3β4 nAChRs; these nAChRs include endoplasmic reticulum (ER) export motifs but not ER retention motifs. Previous reports show that smoking mentholated cigarettes enhances tobacco addiction; but our results show that this effect is unlikely to arise via menthol actions on α3β4 nAChRs.

Chronic Nicotine Exposure Attenuates Methamphetamine-Induced Dopaminergic Deficits

Repeated methamphetamine (METH) administrations cause persistent dopaminergic deficits resembling aspects of Parkinson's disease. Many METH abusers smoke cigarettes and thus self-administer nicotine; yet few studies have investigated the effects of nicotine on METH-induced dopaminergic deficits. This interaction is of interest because preclinical studies demonstrate that nicotine can be neuroprotective, perhaps owing to effects involving α4β2 and α6β2 nicotinic acetylcholine receptors (nAChRs). This study revealed that oral nicotine exposure beginning in adolescence [postnatal day (PND) 40] through adulthood [PND 96] attenuated METH-induced striatal dopaminergic deficits when METH was administered at PND 89. This protection did not appear to be due to nicotine-induced alterations in METH pharmacokinetics. Short-term (i.e., 21-day) high-dose nicotine exposure also protected when administered from PND 40 to PND 61 (with METH at PND 54), but this protective effect did not persist. Short-term (i.e., 21-day) high-dose nicotine exposure did not protect when administered postadolescence (i.e., beginning at PND 61, with METH at PND 75). However, protection was engendered if the duration of nicotine exposure was extended to 39 days (with METH at PND 93). Autoradiographic analysis revealed that nicotine increased striatal α4β2 expression, as assessed using [(125)I]epibatidine. Both METH and nicotine decreased striatal α6β2 expression, as assessed using [(125)I]α-conotoxin MII. These findings indicate that nicotine protects against METH-induced striatal dopaminergic deficits, perhaps by affecting α4β2 and/or α6β2 expression, and that both age of onset and duration of nicotine exposure affect this protection.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Contributions of β2 subunit-containing nAChRs to chronic nicotine-induced alterations in cognitive flexibility in mice

RATIONALE

Deficits in executive functions underlie compulsive drug use, and understanding how nicotine influences these cognitive processes may provide important information on neurobiological substrates of nicotine addiction. Accumulating evidence suggests that β2 subunit-containing nicotinic receptors (nAChRs) are involved in the reinforcing process of nicotine addiction. Whether these nAChRs also contributes to the detrimental effects of chronic nicotine on flexible decision-making is not known.

OBJECTIVES

In the present study, the effects of chronic nicotine were assessed in mice with partial or complete deletion of the β2 subunit-containing nAChR gene (β2+/- or β2-/-) performing an operant cognitive flexibility task.

RESULTS

Visual discrimination learning was not affected in saline-treated β2 nAChR mutants as compared to the wild-type (β2+/+) mice; yet, chronic nicotine facilitated acquisition of visual discrimination in all genotypes. The acquisition of new egocentric response strategy set-shifting remained similar in all genotypes, and there was no effect of treatment. Chronic nicotine treatment impaired reversal learning in β2+/+ mice by increasing response perseveration to the previously rewarded stimulus. Moreover, the acquisition of inverted stimulus-reward contingencies did not differ between β2+/+ and β2-/- mice exposed to chronic nicotine. Interestingly, nicotine-induced reversal learning deficits were not observed in β2+/- mice.

CONCLUSIONS

Collectively, these findings suggest that β2 subunit-containing nAChRs are not critical for visual discrimination learning and extra dimensional rule shift. However, sustained activation of these nAChRs with nicotine may interfere with inhibitory control processes influencing affective shifts in stimulus-reward contingencies.

Nicotinic receptors in non-human primates: Analysis of genetic and functional conservation with humans

Nicotinic acetylcholine receptors (nAChRs) are highly conserved between humans and non-human primates. Conservation exists at the level of genomic structure, protein structure and epigenetics. Overall homology of nAChRs at the protein level is 98% in macaques versus 89% in mice, which is highly relevant for evaluating subtype-specific ligands that have different affinities in humans versus rodents. In addition to conservation at the protein level, there is high conservation of genomic structure in terms of intron and exon size and placement of CpG sites that play a key role in epigenetic regulation. Analysis of single nucleotide polymorphisms (SNPs) shows that while the majority of SNPs are not conserved between humans and macaques, some functional polymorphisms are. Most significantly, cynomolgus monkeys express a similar α5 nAChR Asp398Asn polymorphism to the human α5 Asp398Asn polymorphism that has been linked to greater nicotine addiction and smoking related disease. Monkeys can be trained to readily self-administer nicotine, and in an initial study we have demonstrated that cynomolgus monkeys bearing the α5 D398N polymorphism show a reduced behavioral sensitivity to oral nicotine and tend to consume it in a different pattern when compared to wild-type monkeys. Thus the combination of highly homologous nAChR, higher cortical functions and capacity for complex training makes non-human primates a unique model to study in vivo functions of nicotinic receptors. In particular, primate studies on nicotine addiction and evaluation of therapies to prevent or overcome nicotine addiction are likely to be highly predictive of treatment outcomes in humans.

Inside-out neuropharmacology of nicotinic drugs

Upregulation of neuronal nicotinic acetylcholine receptors (AChRs) is a venerable result of chronic exposure to nicotine; but it is one of several consequences of pharmacological chaperoning by nicotine and by some other nicotinic ligands, especially agonists. Nicotinic ligands permeate through cell membranes, bind to immature AChR oligomers, elicit incompletely understood conformational reorganizations, increase the interaction between adjacent AChR subunits, and enhance the maturation process toward stable AChR pentamers. These changes and stabilizations in turn lead to increases in both anterograde and retrograde traffic within the early secretory pathway. In addition to the eventual upregulation of AChRs at the plasma membrane, other effects of pharmacological chaperoning include modifications to endoplasmic reticulum stress and to the unfolded protein response. Because these processes depend on pharmacological chaperoning within intracellular organelles, we group them as "inside-out pharmacology". This term contrasts with the better-known, acute, "outside-in" effects of activating and desensitizing plasma membrane AChRs. We review current knowledge concerning the mechanisms and consequences of inside-out pharmacology. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Expression of cloned α6* nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (AChRs) are ACh-gated ion channels formed from five homologous subunits in subtypes defined by their subunit composition and stoichiometry. Some subtypes readily produce functional AChRs in Xenopus oocytes and transfected cell lines. α6β2β3* AChRs (subtypes formed from these subunits and perhaps others) are not easily expressed. This may be because the types of neurons in which they are expressed (typically dopaminergic neurons) have unique chaperones for assembling α6β2β3* AChRs, especially in the presence of the other AChR subtypes. Because these relatively minor brain AChR subtypes are of major importance in addiction to nicotine, it is important for drug development as well as investigation of their functional properties to be able to efficiently express human α6β2β3* AChRs. We review the issues and progress in expressing α6* AChRs. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Lynx1 supports neuronal health in the mouse dorsal striatum during aging: an ultrastructural investigation

Nicotinic acetylcholine receptors have been shown to participate in neuroprotection in the aging brain. Lynx protein modulators dampen the activity of the cholinergic system through direct interaction with nicotinic receptors. Although lynx1 null mutant mice exhibit augmented learning and plasticity, they also exhibit macroscopic vacuolation in the dorsal striatum as they age, detectable at the optical microscope level. Despite the relevance of the lynx1 gene to brain function, little is known about the cellular ultrastructure of these age-related changes. In this study, we assessed degeneration in the dorsal striatum in 1-, 3-, 7-, and 13-month-old mice, using optical and transmission electron microscopy. We observed a loss of nerve fibers, a breakdown in nerve fiber bundles, and a loss of neuronal nuclei in the 13-month-old lynx1 null striatum. At higher magnification, these nerve fibers displayed intracellular vacuoles and disordered myelin sheaths. Few or none of these morphological alterations were present in younger lynx1 null mutant mice or in heterozygous lynx1 null mutant mice at any age. These data indicate that neuronal health can be maintained by titrating lynx1 dosage and that the lynx1 gene may participate in a trade-off between neuroprotection and augmented learning.

Pharmacological chaperoning of nAChRs: a therapeutic target for Parkinson's disease

Chronic exposure to nicotine results in an upregulation of neuronal nicotinic acetylcholine receptors (nAChRs) at the cellular plasma membrane. nAChR upregulation occurs via nicotine-mediated pharmacological receptor chaperoning and is thought to contribute to the addictive properties of tobacco as well as relapse following smoking cessation. At the subcellular level, pharmacological chaperoning by nicotine and nicotinic ligands causes profound changes in the structure and function of the endoplasmic reticulum (ER), ER exit sites, the Golgi apparatus and secretory vesicles of cells. Chaperoning-induced changes in cell physiology exert an overall inhibitory effect on the ER stress/unfolded protein response. Cell autonomous factors such as the repertoire of nAChR subtypes expressed by neurons and the pharmacological properties of nicotinic ligands (full or partial agonist versus competitive antagonist) govern the efficiency of receptor chaperoning and upregulation. Together, these findings are beginning to pave the way for developing pharmacological chaperones to treat Parkinson's disease and nicotine addiction.

Nicotinic receptors regulate the dynamic range of dopamine release in vivo

Nicotinic acetylcholine receptors (nAChRs) are expressed presynaptically on dopamine axon terminals, and their activation by endogenous acetylcholine from striatal cholinergic interneurons enhances dopamine release both independently of and in concert with dopamine neuron activity. Acute nAChR inactivation is believed to enhance the contrast between low- and high-frequency dopamine cell activity. Although these studies reveal a key role for acute activation and inactivation of nAChRs in striatal microcircuitry, it remains unknown if chronic inactivation/desensitization of nAChRs can alter dopamine release dynamics. Using in vivo cyclic voltammetry in anaesthetized mice, we examined whether chronic inactivation of nAChRs modulates dopamine release across a parametric range of stimulation, varying both frequency and pulse number. Deletion of β2*nAChRs and chronic nicotine exposure greatly diminished dopamine release across the entire range of stimulation parameters. In addition, we observed a facilitation of dopamine release at low frequency and pulse number in wild-type mice that is absent in the β2* knockout and chronic nicotine mice. These data suggest that deletion or chronic desensitization of nAChRs reduces the dynamic range of dopamine release in response to dopamine cell activity, decreasing rather than increasing contrast between high and low dopamine activity.

Acute nicotine differentially impacts anticipatory valence- and magnitude-related striatal activity

BACKGROUND

Dopaminergic activity plays a role in mediating the rewarding aspects of abused drugs, including nicotine. Nicotine modulates the reinforcing properties of other motivational stimuli, yet the mechanisms of this interaction are poorly understood. This study aimed to ascertain the impact of nicotine exposure on neuronal activity associated with reinforcing outcomes in dependent smokers.

METHODS

Smokers (n = 28) and control subjects (n = 28) underwent functional imaging during performance of a monetary incentive delay task. Using a randomized, counterbalanced design, smokers completed scanning after placement of a nicotine or placebo patch; nonsmokers were scanned twice without nicotine manipulation. In regions along dopaminergic pathway trajectories, we considered event-related activity for valence (reward/gain vs. punishment/loss), magnitude (small, medium, large), and outcome (successful vs. unsuccessful).

RESULTS

Both nicotine and placebo patch conditions were associated with reduced activity in regions supporting anticipatory valence, including ventral striatum. In contrast, relative to controls, acute nicotine increased activity in dorsal striatum for anticipated magnitude. Across conditions, anticipatory valence-related activity in the striatum was negatively associated with plasma nicotine concentration, whereas the number of cigarettes daily correlated negatively with loss anticipation activity in the medial prefrontal cortex only during abstinence.

CONCLUSIONS

These data suggest a partial dissociation in the state- and trait-specific effects of smoking and nicotine exposure on magnitude- and valence-dependent anticipatory activity within discrete reward processing brain regions. Such variability may help explain, in part, nicotine's impact on the reinforcing properties of nondrug stimuli and speak to the continued motivation to smoke and cessation difficulty.

α6β2* and α4β2* nicotinic acetylcholine receptors as drug targets for Parkinson's disease

Parkinson's disease is a debilitating movement disorder characterized by a generalized dysfunction of the nervous system, with a particularly prominent decline in the nigrostriatal dopaminergic pathway. Although there is currently no cure, drugs targeting the dopaminergic system provide major symptomatic relief. As well, agents directed to other neurotransmitter systems are of therapeutic benefit. Such drugs may act by directly improving functional deficits in these other systems, or they may restore aberrant motor activity that arises as a result of a dopaminergic imbalance. Recent research attention has focused on a role for drugs targeting the nicotinic cholinergic systems. The rationale for such work stems from basic research findings that there is an extensive overlap in the organization and function of the nicotinic cholinergic and dopaminergic systems in the basal ganglia. In addition, nicotinic acetylcholine receptor (nAChR) drugs could have clinical potential for Parkinson's disease. Evidence for this proposition stems from studies with experimental animal models showing that nicotine protects against neurotoxin-induced nigrostriatal damage and improves motor complications associated with l-DOPA, the "gold standard" for Parkinson's disease treatment. Nicotine interacts with multiple central nervous system receptors to generate therapeutic responses but also produces side effects. It is important therefore to identify the nAChR subtypes most beneficial for treating Parkinson's disease. Here we review nAChRs with particular emphasis on the subtypes that contribute to basal ganglia function. Accumulating evidence suggests that drugs targeting α6β2* and α4β2* nAChR may prove useful in the management of Parkinson's disease.

Chronic exposure to nicotine is associated with reduced reward-related activity in the striatum but not the midbrain

BACKGROUND

The reinforcing effects of nicotine are mediated by brain regions that also support temporal difference error (TDE) processing; yet, the impact of nicotine on TDE is undetermined.

METHODS

Dependent smokers (n = 21) and matched control subjects (n = 21) were trained to associate a juice reward with a visual cue in a classical conditioning paradigm. Subjects subsequently underwent functional magnetic resonance imaging sessions in which they were exposed to trials where they either received juice as temporally predicted or where the juice was withheld (negative TDE) and later received unexpectedly (positive TDE). Subjects were scanned in two sessions that were identical, except that smokers had a transdermal nicotine (21 mg) or placebo patch placed before scanning. Analysis focused on regions along the trajectory of mesocorticolimbic and nigrostriatal dopaminergic pathways.

RESULTS

There was a reduction in TDE-related function in smokers in the striatum, which did not differ as a function of patch manipulation but was predicted by the duration (years) of smoking. Activation in midbrain regions was not impacted by group or drug condition.

CONCLUSIONS

These data suggest a differential effect of smoking status on the neural substrates of reward in distinct dopaminergic pathway regions, which may be partially attributable to chronic nicotine exposure. The failure of transdermal nicotine to alter reward-related functional processes, either within smokers or between smokers and control subjects, implies that acute nicotine patch administration is insufficient to modify reward processing, which has been linked to abstinence-induced anhedonia in smokers and may play a critical role in smoking relapse.

Role of α6 nicotinic receptors in CNS dopaminergic function: relevance to addiction and neurological disorders

Although a relative newcomer to the nicotinic acetylcholine receptor (nAChR) family, substantial evidence suggests that α6 containing nAChRs play a key role in CNS function. This subtype is unique in its relatively restricted localization to the visual system and catecholaminergic pathways. These latter include the mesolimbic and nigrostriatal dopaminergic systems, which may account for the involvement of α6 containing nAChRs in the rewarding properties of nicotine and in movement. Here, we review the literature on the role of α6 containing nAChRs with a focus on the striatum and nucleus accumbens. This includes molecular, electrophysiological and behavioral studies in control and lesioned animal models, as well as in different genetic models. Converging evidence suggest that the major α6 containing nAChRs subtypes in the nigrostriatal and mesolimbic dopamine system are the α6β2β3 and α6α4β2β3 nAChR populations. They appear to have a dominant role in regulating dopamine release, with consequent effects on nAChR-modulated dopaminergic functions such as reinforcement and motor behavior. Altogether these data suggest that drugs directed to α6 containing nAChRs may be of benefit for the treatment of addiction and for neurological disorders with locomotor deficits such as Parkinson's disease.

Nicotine reduces L-DOPA-induced dyskinesias by acting at beta2* nicotinic receptors

L-DOPA-induced dyskinesias or abnormal involuntary movements (AIMs) are a debilitating adverse complication associated with prolonged L-DOPA administration for Parkinson's disease. Few treatments are currently available for dyskinesias. Our recent data showed that nicotine reduced L-DOPA-induced AIMs in parkinsonian animal models. An important question is the nicotinic acetylcholine receptor (nAChR) subtypes through which nicotine exerts this beneficial effect, because such knowledge would allow for the development of drugs that target the relevant receptor population(s). To address this, we used β2 nAChR subunit knockout [β2(-/-)] mice because β2-containing nAChRs are key regulators of nigrostriatal dopaminergic function. All of the mice were lesioned by intracranial injection of 6-hydroxydopamine into the right medial forebrain bundle. Lesioning resulted in a similar degree of nigrostriatal damage and parkinsonism in β2(-/-) and wild-type mice. All of the mice then were injected with L-DOPA (3 mg/kg) plus benserazide (15 mg/kg) once daily for 4 weeks until AIMs were fully developed. L-DOPA-induced AIMs were approximately 40% less in the β2(-/-) mice compared with the wild-type mice. It is interesting to note that nicotine (300 μg/ml in drinking water) reduced L-DOPA-induced AIMs by 40% in wild-type mice but had no effect in β2(-/-) mice with partial nigrostriatal damage. The nicotine-mediated decline in AIMs was much less pronounced in wild-type mice with near-complete degeneration, suggesting that presynaptic nAChRs on dopaminergic terminals have a major influence. These data demonstrate an essential role for β2* nAChRs in the antidyskinetic effect of nicotine and suggest that drugs targeting these subtypes may be useful for the management of L-DOPA-induced dyskinesias in Parkinson's disease.

Neural systems governed by nicotinic acetylcholine receptors: emerging hypotheses

Cholinergic neurons and nicotinic acetylcholine receptors (nAChRs) in the brain participate in diverse functions: reward, learning and memory, mood, sensory processing, pain, and neuroprotection. Nicotinic systems also have well-known roles in drug abuse. Here, we review recent insights into nicotinic function, linking exogenous and endogenous manipulations of nAChRs to alterations in synapses, circuits, and behavior. We also discuss how these contemporary advances can motivate attempts to exploit nicotinic systems therapeutically in Parkinson's disease, cognitive decline, epilepsy, and schizophrenia.

Nicotinic receptors and stages of nicotine dependence

Smoking is one of the leading causes of preventable death, where nicotine has been identified as the primary addictive constituent of tobacco. Consequently, there have been extensive investigations into the neuroadaptations that occur as nicotine dependence develops, where numerous neurological systems have been implicated. The focus of this review was on nicotinic acetylcholine receptor neuroadaptations that occur during the development of nicotine dependence. This focus was selected because (1) the nicotinic receptors are the primary binding sites for both nicotine and the most efficacious pharmacological smoking cessation treatments and (2) the receptors are located throughout the brain with considerable neuromodulatory ability. However, there was difficulty associated in outlining the role of nicotinic receptors in the development of nicotine dependence because it comprises a series of stages involving different neurological systems rather than a single state. To address this issue, the review adopts a novel approach and considers the role of nicotinic receptor subtypes at separate stages of the nicotine dependence cycle. This information was then used to examine the nicotinic receptor-related therapeutic mechanisms of three main pharmacological smoking cessation treatments.

Chronic nicotine selectively enhances alpha4beta2* nicotinic acetylcholine receptors in the nigrostriatal dopamine pathway

These electrophysiological experiments, in slices and intact animals, study the effects of in vivo chronic exposure to nicotine on functional alpha4beta2* nAChRs in the nigrostriatal dopaminergic (DA) pathway. Recordings were made in wild-type and alpha4 nicotinic acetylcholine receptor (nAChR) subunit knock-out mice. Chronic nicotine enhanced methyllycaconitine citrate hydrate-resistant, dihydro-beta-erythroidine hydrobromide-sensitive nicotinic currents elicited by 3-1000 mum ACh in GABAergic neurons of the substantia nigra pars reticulata (SNr), but not in DA neurons of the substantia nigra pars compacta (SNc). This enhancement leads to higher firing rates of SNr GABAergic neurons and consequently to increased GABAergic inhibition of the SNc DA neurons. In the dorsal striatum, functional alpha4* nAChRs were not found on the neuronal somata; however, nicotine acts via alpha4beta2* nAChRs in the DA terminals to modulate glutamate release onto the medium spiny neurons. Chronic nicotine also increased the number and/or function of these alpha4beta2* nAChRs. These data suggest that in nigrostriatal DA pathway, chronic nicotine enhancement of alpha4beta2* nAChRs displays selectivity in cell type and in nAChR subtype as well as in cellular compartment. These selective events augment inhibition of SNc DA neurons by SNr GABAergic neurons and also temper the release of glutamate in the dorsal striatum. The effects may reduce the risk of excitotoxicity in SNc DA neurons and may also counteract the increased effectiveness of corticostriatal glutamatergic inputs during degeneration of the DA system. These processes may contribute to the inverse correlation between tobacco use and Parkinson's disease.

The expression of CYP2D22, an ortholog of human CYP2D6, in mouse striatum and its modulation in 1-methyl 4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinson's disease phenotype and nicotine-mediated neuroprotection

The main contributory factors of Parkinson's disease (PD) are aging, genetic factors, and environmental exposure to pesticides and heavy metals. CYP2D22, a mouse ortholog of human CYP2D6, is expected to participate in a chemically induced PD phenotype due to its structural resemblance with CYP2D6. Despite its expected participation in PD, its expression in the nigrostriatal tissues and modulation by the chemicals that induce PD or offer neuroprotection have not yet been investigated. The present study was undertaken to investigate CYP2D22 expression in mouse striatum and to assess its involvement in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD phenotype and nicotine-mediated neuroprotection. The animals were treated intraperitoneally daily with nicotine (1 mg/kg) for 8 weeks, followed by MPTP (20 mg/kg) + nicotine (1 mg/kg) for 2-4 weeks and vice versa, along with respective controls. In some sets of experiments, the animals were treated with ketoconazole (300 mg/kg), a CYP3AH/CYP2D22 inhibitor, along with nicotine and/or MPTP. Tyrosine hydroxylase (TH)-immunoreactivity in the substantia nigra, the expression of nicotinic acetylcholine receptors (nAChRs) alpha6 and alpha4, dopamine content, and 1-methyl-4-phenylpyridinium ion (MPP(+)) level in the striatum were measured to confirm the MPTP-induced PD phenotype and nicotine-mediated neuroprotection. CYP2D22 and nAChRs expressions were measured in the striatum by RT-PCR/western blotting and dopamine level; CYP2D22 catalytic activity and MPP(+) content were determined by high-performance liquid chromatography (HPLC). MPTP increased dopaminergic neuronal degeneration and the striatal MPP(+) level and reduced striatal dopamine content; it attenuated expression/activity of CYP2D22 and nAChRs that were significantly restored in nicotine-treated animals. Ketoconazole reduced the nicotine-mediated increase in CYP2D22 expression and activity, dopamine content, and TH-immunoreactivity. The results indicate the expression of CYP2D22 in mouse striatum and its possible role in the MPTP-induced PD phenotype and nicotine-mediated neuroprotection.

Chronic nicotine treatment increases nAChRs and microglial expression in monkey substantia nigra after nigrostriatal damage

Our previous work had shown that long-term nicotine administration improved dopaminergic markers and nicotinic receptors (nAChRs) in the striatum of monkeys with nigrostriatal damage. The present experiments were done to determine whether nicotine treatment also led to changes in the substantia nigra, the region containing dopaminergic cell bodies. Monkeys were chronically treated with nicotine in the drinking water for 6 months after which they were injected with low dose of 1-methyl-4-phenyl-1,2,3,6-tetrahydrophridine (MPTP) for a further 6-month period. Nicotine was administered until the monkeys were euthanized 2 months after the last MPTP injection. Nicotine treatment did not affect the dopamine transporter or the number of tyrosine hydroxylase positive cells in the substantia nigra of lesioned monkeys. However, nicotine administration did lead to a greater increase in alpha3/alpha6beta2* and alpha4beta2* nAChRs in lesioned monkeys compared to controls. Nicotine also significantly elevated microglia and reduced the number of extracellular neuromelanin deposits in the substantia nigra of MPTP-lesioned monkeys. These findings indicate that long-term nicotine treatment modulates expression of several molecular measures in monkey substantia nigra that may result in an improvement in nigral integrity and/or function. These observations may have therapeutic implications for Parkinson's disease.

beta2-Nicotinic acetylcholine receptor availability during acute and prolonged abstinence from tobacco smoking

CONTEXT

Available levels of nicotinic acetylcholine receptors containing the beta(2) subunit (beta(2)*-nAChR) are higher in recently abstinent tobacco smokers compared with participants who never smoked. Variations in beta(2)*-nAChR availability during the course of abstinence may be related to the urge to smoke, the extent of nicotine withdrawal, and successful abstinence.

OBJECTIVE

To examine changes in beta(2)*-nAChR availability during acute and prolonged abstinence from tobacco smoking and to determine how changes in beta(2)*-nAChR availability were related to clinical features of tobacco smoking.

DESIGN

Tobacco smokers participated in up to 4 iodide 123-labeled 5-iodo-A-85380 ([(123)I]5-IA) single-photon emission computed tomography (SPECT) scans during abstinence at 1 day (n = 7) and 1 (n = 17), 2 (n = 7), 4 (n = 11), and 6 to 12 (n = 6) weeks. Age-matched nonsmokers participated in a single [(123)I]5-IA SPECT scan. All participants completed 1 magnetic resonance imaging study.

SETTING

Academic imaging center.

PARTICIPANTS

Tobacco smokers (n = 19) and an age-matched nonsmoker comparison group (n = 20). Main Outcome Measure The [(123)I]5-IA SPECT images were converted to distribution volume and were analyzed using regions of interest.

RESULTS

Compared with nonsmokers, beta(2)*-nAChR availability in the striatum, cortex, and cerebellum of smokers was not different at 1 day of abstinence, was significantly higher at 1 week of abstinence, and was not different at 4 or at 6 to 12 weeks of abstinence. In smokers, beta(2)*-nAChR availability was significantly lower in the cortex and cerebellum at 6 to 12 weeks compared with 1 week of abstinence. In addition, cerebellar beta(2)*-nAChR availability at 4 weeks of abstinence was positively correlated with craving on the day of the SPECT scan.

CONCLUSIONS

These data suggest that higher beta(2)*-nAChR availability persists up to 1 month of abstinence and normalizes to nonsmoker levels by 6 to 12 weeks of abstinence from tobacco smoking. These marked and persistent changes in beta(2)*-nAChR availability may contribute to difficulties with tobacco cessation.

Nicotine sensitization of monkey striatal dopamine release

This study with monkeys was designed to answer two questions. 1) Does acute nicotine preferentially release more dopamine in the striatum than in the prefrontal cortex? 2) Do repeated doses of nicotine produce sensitization of striatal dopamine release? Microdialysis techniques were used to measure extracellular dopamine in both brain regions in two separate groups of conscious animals. The acute nicotine i.v. dose schedule was a bolus of 32 microg/kg plus an infusion of +/-0.8 microg/kg/min and a 100 microg/kg bolus plus an infusion of +/-2.53 microg/kg/min for 30 min to mimic human tobacco smoking arterial plasma nicotine concentrations. Acute nicotine given i.v. released more dopamine in the striatum than in the prefrontal cortex. In the second experiment, for convenience, daily nicotine was given i.m. and not i.v. bid in doses of 32 or 100 microg/kg for nine days. Dopamine release was measured after overnight nicotine abstinence using the i.v. dose schedule from the first experiment. Baseline dopamine release was significantly reduced (77.6% of control, P<0.05). With a lowered baseline, a greater facilitation of dopamine release was produced by nicotine compared to that obtained under control conditions when the baseline was higher. The impaired dopamine release with overnight nicotine abstinence was transiently enhanced in a dose dependent manner. These data regarding the striatum are consistent with previous findings in rodents of nicotine sensitization of dopamine release especially in nucleus accumbens following repeated administration.

Cellular events in nicotine addiction

In the 25 years since the observation that chronic exposure to nicotine could regulate the number and function of high affinity nicotine binding sites in the brain there has been a major effort to link alterations in nicotinic acetylcholine receptors (nAChRs) to nicotine-induced behaviors that drive the addiction to tobacco products. Here we review the proposed roles of various nAChR subtypes in the addiction process, with emphasis on how they are regulated by nicotine and the implications for understanding the cellular neurobiology of addiction to this drug.

Prominent role of alpha3/alpha6beta2* nAChRs in regulating evoked dopamine release in primate putamen: effect of long-term nicotine treatment

Brain dopaminergic systems are critical in motor control as evidenced by findings that their disruption results in movement disorders such as Parkinson's disease. Nicotinic acetylcholine receptor (nAChR) activation plays an important role in regulating striatal dopaminergic function. Rodent studies show that short-term nicotine exposure influences stimulated striatal dopamine release with responsiveness dependent on neuronal activity. However, studies have not yet been done in nonhuman primates, nor has work been done to evaluate the effect of long-term nicotine exposure, which is relevant for therapies for chronic neurological disorders. Here, we used voltammetry to assess the role of nAChRs on evoked dopamine release from monkey putamen slices. In both ventral and dorsal putamen, alpha3/alpha6beta2(*) nAChRs regulated > or =80% of non-burst- (single pulse) nAChR-modulated dopamine release, and alpha4beta2(*) nAChRs regulated the remainder. Similar results were observed with burst-firing in ventral but not dorsal putamen, indicating that nAChR-modulated effects on release depend on the subregion and firing frequency. Next, we investigated the consequence of long-term nicotine exposure via the drinking water on nAChR-modulated responsiveness. Nicotine treatment altered both non-burst- and burst-stimulated dopamine release in ventral but not dorsal putamen. Altogether, these data support a predominant role for alpha3/alpha6beta2(*) nAChRs in the regulation of evoked dopamine release in nonhuman primate putamen. They also show that long-term nicotine treatment selectively modifies nAChR-modulated release in distinct striatal subregions. These findings have implications for the development of treatments for addiction and neurological disorders with nAChR dysfunction.

Prenatal exposure of rats to nicotine causes persistent alterations of nicotinic cholinergic receptors

We examined for immediate and persistent changes in nAChRs in cerebral cortex, thalamus and striatum of male rats caused by prenatal exposure to nicotine from gestational day 3 to postnatal day 10 (PN10), and how such exposure affected the responses of adolescents to subsequent nicotine challenge. Receptor numbers were assessed by [(3)H]epibatidine binding and receptor function was measured by acetylcholine-stimulated (86)Rb efflux (cerebral cortex and thalamus) and nicotine-stimulated dopamine release (striatum). Immediate effects of prenatal nicotine, assessed in PN10 animals, were not detected for any parameter. A subsequent 14 day nicotine exposure in adolescence revealed persistent changes caused by prenatal nicotine exposure. Nicotine exposure in adolescents caused up-regulation of binding in all three regions; however, this up-regulation was lost in thalamus from animals prenatally exposed to nicotine. Nicotine exposure in adolescents caused decreased nicotine-stimulated dopamine release in striatum; this effect was lost in animals prenatally exposed to nicotine. Comparison of parameters in PN10 and PN42 rats revealed developmental changes in the CNS cholinergic system. In thalamus, binding increased with age, as did the proportion of (86)Rb efflux with high sensitivity to acetylcholine. In cortex, binding also increased with age, but there was no change in total (86)Rb efflux, and the proportion of high to low sensitivity efflux declined with age. Nicotine-stimulated striatal dopamine release (both total and alpha-conotoxin MII-resistant release) increased with age in naïve animals, but not in those prenatally exposed to nicotine. These findings demonstrate that prenatal exposure to nicotine causes alterations in nAChRs and in their regulation by nicotine that persist into adolescence. These changes may play a role in the increased risk for nicotine addiction observed in adolescent offspring of smoking mothers.

Long-term nicotine treatment differentially regulates striatal alpha6alpha4beta2* and alpha6(nonalpha4)beta2* nAChR expression and function

Nicotine treatment has long been associated with alterations in alpha4beta2(*) nicotinic acetylcholine receptor (nAChR) expression that modify dopaminergic function. However, the influence of long-term nicotine treatment on the alpha6beta2(*) nAChR, a subtype specifically localized on dopaminergic neurons, is less clear. Here we used voltammetry, as well as receptor binding studies, to identify the effects of nicotine on striatal alpha6beta2(*) nAChR function and expression. Long-term nicotine treatment via drinking water enhanced nonburst and burst endogenous dopamine release from rat striatal slices. In control animals, alpha6beta2(*) nAChR blockade with alpha-conotoxin MII (alpha-CtxMII) decreased release with nonburst stimulation but not with burst firing. These data in control animals suggest that varying stimulus frequencies differentially regulate alpha6beta2(*) nAChR-evoked dopamine release. In contrast, in nicotine-treated rats, alpha6beta2(*) nAChR blockade elicited a similar pattern of dopamine release with nonburst and burst firing. To elucidate the alpha6beta2(*) nAChR subtypes altered with long-term nicotine treatment, we used the novel alpha-CtxMII analog E11A in combination with alpha4 nAChR knockout mice. (125)I-alpha-CtxMII competition studies in striatum of knockout mice showed that nicotine treatment decreased the alpha6alpha4beta2(*) subtype but increased the alpha6(nonalpha4)beta2(*) nAChR population. These data indicate that alpha6beta2(*) nAChR-evoked dopamine release in nicotine-treated rats is mediated by the alpha6(nonalpha4)beta2(*) nAChR subtype and suggest that the alpha6alpha4beta2(*) nAChR and/or alpha4beta2(*) nAChR contribute to the differential effect of higher frequency stimulation on dopamine release under control conditions. Thus, alpha6beta2(*) nAChR subtypes may represent important targets for smoking cessation therapies and neurological disorders involving these receptors such as Parkinson's disease.

Presynaptic nicotinic receptors: a dynamic and diverse cholinergic filter of striatal dopamine neurotransmission

The effects of nicotine on dopamine transmission from mesostriatal dopamine neurons are central to its reinforcing properties. Only recently however, has the influence of presynaptic nicotinic receptors (nAChRs) on dopaminergic axon terminals within striatum begun to be understood. Here, rather than simply enhancing (or inhibiting) dopamine release, nAChRs perform the role of a presynaptic filter, whose influence on dopamine release probability depends on presynaptic activity in dopaminergic as well as cholinergic neurons. Both mesostriatal dopaminergic neurons and striatal cholinergic interneurons play key roles in motivational and sensorimotor processing by the basal ganglia. Moreover, it appears that the striatal influence of dopamine and ACh cannot be fully appreciated without an understanding of their reciprocal interactions. We will review the powerful filtering by nAChRs of striatal dopamine release and discuss its dependence on activity in dopaminergic and cholinergic neurons. We will also review how nicotine, acting via nAChR desensitization, promotes the sensitivity of dopamine synapses to activity. This filtering action might provide a mechanism through which nicotine promotes how burst activity in dopamine neurons facilitates goal-directed behaviour and reinforcement processing. More generally, it indicates that we should not restrict our view of presynaptic nAChRs to simply enhancing neurotransmitter release. We will also summarize current understanding of the forms and functions of the diverse nAChRs purported to exist on dopaminergic axons. A greater understanding of nAChR form and function is imperative to guide the design of ligands with subtype-selective efficacy for improved therapeutic interventions in nicotine addiction as well as Parkinson's disease.

Discovery of a novel nicotinic receptor antagonist for the treatment of nicotine addiction: 1-(3-Picolinium)-12-triethylammonium-dodecane dibromide (TMPD)

Limitations in efficacy and high relapse rates of currently available smoking cessation agents reveal the need for more efficacious pharmacotherapies. One strategy is to develop subtype-selective nicotinic receptor (nAChR) antagonists that inhibit nicotine-evoked dopamine (DA) release, the primary neurotransmitter involved in nicotine reward. Simple alkylation of the pyridino N-atom converts nicotine from a potent agonist into a potent antagonist. The classical antagonists, hexamethonium and decamethonium, differentiate between peripheral nAChR subtypes. Using a similar approach, we interconnected varying quaternary ammonium moieties with a lipophilic linker to provide N,N'-bis-nicotinium analogs, affording a lead compound, N,N'-dodecyl-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), which inhibited nicotine-evoked DA release and decreased nicotine self-administration. The current work describes a novel compound, 1-(3-picolinium)-12-triethylammonium-dodecane dibromide (TMPD), a hybrid of bPiDDB and decamethonium. TMPD completely inhibited (IC(50)=500 nM) nicotine-evoked DA release from superfused rat striatal slices, suggesting that TMPD acts as a nAChR antagonist at more than one subtype. TMPD (1 microM) inhibited the response to acetylcholine at alpha3beta4, alpha4beta4, alpha4beta2, and alpha1beta1varepsilondelta receptors expressed in Xenopus oocytes. TMPD had a 2-fold higher affinity than choline for the blood-brain barrier choline transporter, suggesting brain bioavailability. TMPD did not inhibit hyperactivity in nicotine sensitized rats, but significantly and specifically decreased nicotine self-administration. Together, the results suggest that TMPD may have the ability to reduce the rewarding effect of nicotine with minimal side effects, a pharmacological profile indicative of potential clinical utility for the treatment of tobacco dependence.

Partial recovery of striatal nicotinic receptors in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys with chronic oral nicotine

Recent studies in nonhuman primates show that chronic nicotine treatment protects against nigrostriatal degeneration, with a partial restoration of neurochemical and functional measures in the striatum. The present studies were done to determine whether long-term nicotine treatment also protected against striatal nicotinic receptor (nAChR) losses after nigrostriatal damage. Monkeys were administered nicotine in the drinking water for 6 months and subsequently lesioned with the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) over several months while nicotine was continued. (125)I-Epibatidine, [(125)I]5-[(125)I]iodo-3(2(S)-azetidinylmethoxy)-pyridine (A85380), and (125)I-alpha-conotoxinMII autoradiography was performed to evaluate changes in alpha4beta2* and alpha3/alpha6beta2* nAChRs, the major striatal subtypes. Nicotine treatment increased alpha4beta2* nAChRs by > or =50% in striatum of both unlesioned and lesioned animals. This increase in alpha4beta2* nAChRs was significantly greater in lesioned compared with unlesioned monkey striatum. Chronic nicotine treatment led to a small decrease in alpha3/alpha6beta2* nAChR subtypes. The decline in alpha3/alpha6beta2* subtypes, defined using alpha-conotoxinMII-sensitive (125)I-epibatidine or [(125)I]A85380 binding, was significantly smaller in striatum of nicotine-treated lesioned monkeys compared with unlesioned monkeys. This difference was not observed for alpha3/alpha6beta2* nAChRs identified using (125)I-alpha-conotoxinMII. These data suggest that there are at least two striatal alpha3/alpha6beta2* subtypes that are differentially affected by chronic nicotine treatment in lesioned animals. In addition, the results showing an improvement in striatal alpha4beta2* and select alpha3/alpha6beta2* nAChR subtypes, combined with previous work, demonstrate that chronic nicotine treatment restores and/or protects against the loss of multiple molecular markers after nigrostriatal damage. Such findings suggest that nicotine or nicotinic agonists may be of therapeutic value in Parkinson's disease.

Beta3 subunits promote expression and nicotine-induced up-regulation of human nicotinic alpha6* nicotinic acetylcholine receptors expressed in transfected cell lines

Nicotinic acetylcholine receptors (AChRs) containing alpha6 subunits are typically found at aminergic nerve endings where they play important roles in nicotine addiction and Parkinson's disease. alpha6* AChRs usually contain beta3 subunits. beta3 subunits are presumed to assemble only in the accessory subunit position within AChRs where they do not participate in forming acetylcholine binding sites. Assembly of subunits in the accessory position may be a critical final step in assembly of mature AChRs. Human alpha6 AChRs subtypes were permanently transfected into human tsA201 human embryonic kidney (HEK) cell lines. alpha6beta2beta3 and alpha6beta4beta3 cell lines were found to express much larger amounts of AChRs and were more sensitive to nicotine-induced increase in the amount of AChRs than were alpha6beta2 or alpha6beta4 cell lines. The increased sensitivity to nicotine-induced up-regulation was due not to a beta3-induced increase in affinity for nicotine but probably to a direct effect on assembly of AChR subunits. HEK cells express only a small amount of mature alpha6beta2 AChRs, but many of these subunits are on the cell surface. This contrasts with Xenopus laevis oocytes, which express a large amount of incorrectly assembled alpha6beta2 subunits that bind cholinergic ligands but form large amorphous intracellular aggregates. Monoclonal antibodies (mAbs) were made to the alpha6 and beta3 subunits to aid in the characterization of these AChRs. The alpha6 mAbs bind to epitopes C-terminal of the extracellular domain. These data demonstrate that both cell type and the accessory subunit beta3 can play important roles in alpha6* AChR expression, stability, and up-regulation by nicotine.

Chronic oral nicotine normalizes dopaminergic function and synaptic plasticity in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned primates

Our recent studies show that chronic oral nicotine partially protects against striatal damage in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated nonhuman primates. To identify the cellular changes associated with this protective action, we investigated the effects of nicotine treatment on stimulus-evoked dopamine release, dopamine turnover, and synaptic plasticity in striatum from lesioned and unlesioned animals. Monkeys were chronically (6 months) treated with nicotine in the drinking water and subsequently lesioned with the dopaminergic neurotoxin MPTP (6 months) while nicotine was continued. Nigrostriatal damage increased nicotinic acetylcholine receptor (nAChR)-mediated fractional dopamine release from residual terminals, primarily through changes in alpha3*/alpha6* nAChRs. In contrast, fractional receptor-evoked dopamine release was similar to control in unlesioned and lesioned animals with chronic oral nicotine. Long-term nicotine administration also attenuated the enhanced K(+)-evoked fractional dopamine release from synaptosomes of MPTP-lesioned animals, suggesting that nicotine treatment had a generalized effect on dopaminergic function. This premise was further supported by experiments showing that nicotine dosing decreased the elevated dopamine turnover that occurs after nigrostriatal damage. We next investigated changes in synaptic plasticity with lesioning and nicotine treatment. Nicotine treatment alone enhanced synaptic plasticity by lowering the threshold for long-term depression (LTD) in the corticostriatal pathway. MPTP lesioning led to a loss of LTD, a measure of short-term synaptic plasticity. In contrast, LTD was preserved in nicotine-treated lesioned animals. Thus, the present data show that the disruptions in striatal dopaminergic function after nigrostriatal damage were attenuated with chronic nicotine administration. These cellular alterations may underlie the ability of nicotine to maintain/restore normal function with nigrostriatal damage.

Differential regulation of mesolimbic alpha 3/alpha 6 beta 2 and alpha 4 beta 2 nicotinic acetylcholine receptor sites and function after long-term oral nicotine to monkeys

Because the mesolimbic dopamine system plays a critical role in nicotine addiction/reinforcement and because nicotinic receptors regulate dopamine release, we initiated a study to evaluate the long-term effects of nicotine (>6 months at the final dose) on nicotinic acetylcholine receptor (nAChR) sites and function in the nucleus accumbens of nonhuman primates. Nicotine was given in the drinking water as this mode of administration is long-term but intermittent, thus resembling smoking in this aspect. We determined the effects of nicotine treatment on function and binding of the alpha3/alpha6beta2* and alpha4beta2* nAChRs subtypes in nucleus accumbens, a region directly implicated in the addictive effects of nicotine. To evaluate function, we measured nicotine and K+-evoked [3H]dopamine release from nucleus accumbens synaptosomes. Changes in alpha4beta2* and alpha3/alpha6beta2* nAChRs were measured using 125I-epibatidine, [125I]A85380 [5-[125I]iodo-3(2(S)-azetidinylmethoxy) pyridine] and 125I-alpha-conotoxin MII autoradiography. Chronic nicotine treatment, which led to plasma nicotine levels in the range of smokers, significantly increased nucleus accumbens alpha4beta2* nAChR sites and function compared with control. By contrast, this treatment did not significantly change alpha3/alpha6beta2* nAChR sites or evoked dopamine release in this region compared with control. Thus, these data are distinct from previous results in striatum in which the same nicotine treatment paradigm decreased striatal alpha3/alpha6beta2* nAChR sites and function. The finding that long-term nicotine treatment selectively modulates alpha4beta2* and not alpha3/alpha6beta2* nAChR expression in primate nucleus accumbens is consistent with the results of studies in nicotinic receptor mutant mice implicating the alpha4beta2* nAChR subtype in nicotine-mediated addiction.