Nicotine and Parkinson's disease: implications for therapy

Enhanced synthesis and release of dopamine in transgenic mice with gain-of-function α6* nAChRs

α6β2* nicotinic acetylcholine receptors (nAChRs)s in the ventral tegmental area to nucleus accumbens (NAc) pathway are implicated in the response to nicotine, and recent work suggests these receptors play a role in the rewarding action of ethanol. Here, we studied mice expressing gain-of-function α6β2* nAChRs (α6L9'S mice) that are hypersensitive to nicotine and endogenous acetylcholine. Evoked extracellular dopamine (DA) levels were enhanced in α6L9'S NAc slices compared to control, non-transgenic (non-Tg) slices. Extracellular DA levels in both non-Tg and α6L9'S slices were further enhanced in the presence of GBR12909, suggesting intact DA transporter function in both mouse strains. Ongoing α6β2* nAChR activation by acetylcholine plays a role in enhancing DA levels, as α-conotoxin MII completely abolished evoked DA release in α6L9'S slices and decreased spontaneous DA release from striatal synaptosomes. In HPLC experiments, α6L9'S NAc tissue contained significantly more DA, 3,4-dihydroxyphenylacetic acid, and homovanillic acid compared to non-Tg NAc tissue. Serotonin (5-HT), 5-hydroxyindoleacetic acid, and norepinephrine (NE) were unchanged in α6L9'S compared to non-Tg tissue. Western blot analysis revealed increased tyrosine hydroxylase expression in α6L9'S NAc. Overall, these results show that enhanced α6β2* nAChR activity in NAc can stimulate DA production and lead to increased extracellular DA levels.

A genetic basis for the variable effect of smoking/nicotine on Parkinson's disease

Prior studies have established an inverse association between cigarette smoking and the risk of developing Parkinson's disease (PD), and currently, the disease-modifying potential of the nicotine patch is being tested in clinical trials. To identify genes that interact with the effect of smoking/nicotine, we conducted genome-wide interaction studies in humans and in Drosophila. We identified SV2C, which encodes a synaptic-vesicle protein in PD-vulnerable substantia nigra (P=1 × 10(-7) for gene-smoking interaction on PD risk), and CG14691, which is predicted to encode a synaptic-vesicle protein in Drosophila (P=2 × 10(-11) for nicotine-paraquat interaction on gene expression). SV2C is biologically plausible because nicotine enhances the release of dopamine through synaptic vesicles, and PD is caused by the depletion of dopamine. Effect of smoking on PD varied by SV2C genotype from protective to neutral to harmful (P=5 × 10(-10)). Taken together, cross-validating evidence from humans and Drosophila suggests SV2C is involved in PD pathogenesis and it might be a useful marker for pharmacogenomics studies involving nicotine.

Nicotine reduces established levodopa-induced dyskinesias in a monkey model of Parkinson's disease

Although 3,4-dihydroxyphenylalanine (levodopa) is the gold-standard treatment for Parkinson's disease, it can lead to disabling dyskinesias. Previous work demonstrated that nicotine reduces levodopa-induced dyskinesias (LIDs) in several parkinsonian animal models. The goal of this study was to determine whether the duration of nicotine administration affects its ability to reduce LIDs in levodopa-primed and levodopa-naíve monkeys and also to test whether tolerance develops to the beneficial effects of nicotine. Monkeys were injected with MPTP (1.9-2.0 mg/kg subcutaneously) over 3 to 5 months until parkinsonism developed. Nicotine (300 μg/mL) was administered in drinking water (over 4-6 months) to levodopa-primed or levodopa-naíve monkeys, with levodopa/carbidopa (10/2.5 mg/kg) gavaged twice daily. One set of MPTP-lesioned monkeys (n = 23) was first gavaged with levodopa and subsequently received nicotine 4 weeks later, when dyskinesias plateaued, or 8 weeks later, when dyskinesias were established. A 60% to 70% decrease in LIDs was observed after several weeks of nicotine treatment in both groups. A second set of monkeys (n = 26) received nicotine 8 or 2 weeks before levodopa. In the 8-week nicotine pretreatment group, there was an immediate reduction in LIDs, which plateaued at 60% to 70%. In the 2-week nicotine pretreatment group, there were initial small decreases in LIDs, which plateaued at 60% to 70% several weeks later. Thus, nicotine pretreatment and nicotine post-treatment were similarly efficacious in reducing LIDs. The beneficial effect of nicotine persisted throughout the study (17-23 weeks). Nicotine did not worsen parkinsonism. These data suggest that nicotine treatment has potential as a successful antidyskinetic therapy for patients with Parkinson's disease.

TC-8831, a nicotinic acetylcholine receptor agonist, reduces L-DOPA-induced dyskinesia in the MPTP macaque

Long-term L-DOPA treatment for Parkinson's disease (PD) is limited by motor complications, particularly L-DOPA-induced dyskinesia (LID). A therapy with the ability to ameliorate LID without reducing anti-parkinsonian benefit would be of great value. We assessed the ability of TC-8831, an agonist at nicotinic acetylcholine receptors (nAChR) containing α6β2/α4β2 subunit combinations, to provide such benefits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine- (MPTP) lesioned macaques with established LID. Animals were treated orally for consecutive 14-day periods with twice-daily vehicle (weeks 1-2) or TC-8831 (0.03, 0.1 or 0.3 mg/kg, weeks 3-8). L-DOPA was also administered, once-daily, (weeks 1-12, median-dose 30 mg/kg, p.o.). For the following two-weeks (weeks 9-10), TC-8831 was washed out, while once-daily L-DOPA treatment was maintained. The effects of once-daily amantadine (3 mg/kg, p.o.) were then assessed over weeks 11-12. LID, parkinsonism, duration and quality of ON-time were assessed weekly by a neurologist blinded to treatment. TC-8831 reduced the duration of 'bad' ON-time (ON-time with disabling dyskinesia) by up to 62% and decreased LID severity (median score 18 cf. 34 (vehicle), 0.1 mg/kg, 1-3 h period). TC-8831 also significantly reduced choreiform and dystonic dyskinesia (median scores 6 and 31 cf. 19 and 31 respectively (vehicle), both 0.03 mg/kg, 1-3 h). At no time did TC-8831 treatment result in a reduction in anti-parkinsonian benefit of L-DOPA. By comparison, amantadine also significantly reduced dyskinesia and decreased 'bad' ON-time (up to 61%) but at the expense of total ON-time (reduced by up to 23%). TC-8831 displayed robust anti-dyskinetic actions and improved the quality of ON-time evoked by L-DOPA without any reduction in anti-parkinsonian benefit.

Nicotine promotes survival of cells expressing amyloid precursor protein and presenilin: implication for Alzheimer's disease

Amyloid-β protein (Aβ) accumulation is one of the major hallmarks of Alzheimer's disease (AD) and plays a crucial role in its pathogenesis. Cellular models whereby amyloid precursor protein (APP) is highly expressed are commonly used to test the efficacy of novel neuroprotective compounds. In addition to Aβ, it is known that mutation in the protein presenilin contributes to early onset AD. Recently, a cellular neuroblastoma model where both APP and presenilin are expressed has become available. Since protective effects of nicotine against various neurotoxins have been observed, this study was designed to determine whether nicotine would also protect against cellular damage induced by APP or APP and presenilin. Wild type neuroblastoma (N2a) cell line, and those transfected with amyloid precursor protein (APP), and the combination of APP and presenilin were pretreated with various concentrations of nicotine and the survivability of the cells were determined by MTT assay. Nicotine dose dependently provided protection against cellular loss in all cell lines, with highest protection in the double transfected (44%) followed by single transfected (30%), and wild type (21%). The effects of nicotine in turn were blocked by mecamylamine, a non-selective nicotinic antagonist. These results suggest differential sensitivity of cell lines representing AD pathology to the protective effects of nicotine and provide further support of therapeutic potential of nicotinic agonists in at least a subtype of AD patients.

Caffeine drinking, cigarette smoking, and dopaminergic replacement therapy dose in Parkinson's disease

The objective of this study is to assess the effect of smoking and caffeine intake in the dosage of dopaminergic replacement therapy. Patients were recruited from the movement disorders clinic of the National Institute of Neurology and Neurosurgery in Mexico City. An interviewer-administered structured questionnaire was given to all subjects regarding their smoking and caffeine drinking habits. Dopaminergic replacement therapy information was collected and levodopa, dopamine agonists, and levodopa equivalent daily doses were calculated. 146 Parkinson's disease patients (50 % female) were included. All patients were on antiparkinsonian treatment, with a mean levodopa equivalent daily dose (LEDD) of 550.2 ± 408. Patients were stratified according to smoking and caffeine drinking status. 104 (71.2 %) of the patients were "never smokers", 33 (22.6 %) were "former smokers" and 9 (6.2 %) were "current smokers". 40 (27.4 %) patients reported no history of caffeine intake, 36 (24.7 %) were former consumers and 70 (47.9 %) were current caffeine drinkers. No association between LEDD and smoking or caffeine intake was found. A weak positive correlation (r = 0.22, p < 0.04) was found between the daily dose of pramipexole and the daily intake of caffeine. LEDD, levodopa daily dose and dopamine agonist daily dose were not related to smoking or caffeine intake status. We found a weak correlation between caffeine daily intake and pramipexole dose. Further prospective exploration is needed to address the interaction of concomitant A2A antagonism induced by caffeine intake and dopaminergic replacement therapy.

Insights into the neurobiology of the nicotinic cholinergic system and nicotine addiction from mice expressing nicotinic receptors harboring gain-of-function mutations

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated, cation-selective ion channels expressed throughout the brain. Although these channels have been investigated for several decades, it is still challenging 1) to identify the important nAChR subunits in cholinergic transmission and nicotine dependence and 2) to develop nAChR subtype-specific ligands. To overcome these challenges, we and others have studied mice expressing mutant, gain-of-function nAChR subunits. In this review, we discuss this research approach and the results it has yielded to date. Gain-of-function mutations, including those in nAChR subunits, provide an approach that is complementary to loss-of-function studies such as gene knockouts; the former allows one to answer questions of sufficiency and the latter addresses questions of necessity. Mutant mice expressing gain-of-function nAChR subunits are commonly produced using traditional gene targeting in embryonic stem cells, but novel approaches such as bacterial artificial chromosome transgenesis have yielded important insights as well. α7 nAChRs were the first nAChRs to be targeted with a gain-of-function mutation, followed by a pair of α4 nAChR gain-of-function mutant mice. These α4 nAChR gain-of-function mice (α4 L9'S mice, followed by α4 L9'A mice) provided an important system to probe α4 nAChR function in vivo, particularly in the dopamine reward system. α6 nAChR gain-of-function mice provided the first robust system allowing specific manipulation of this receptor subtype. Other targeted mutations in various nAChR subunits have also been produced and have yielded important insights into nicotinic cholinergic biology. As nAChR research advances and more details associated with nAChR expression and function emerge, we expect that existing and new mouse lines expressing gain-of-function nAChR subunits will continue to provide new insights.

Nicotine as a potential neuroprotective agent for Parkinson's disease

Converging research efforts suggest that nicotine and other drugs that act at nicotinic acetylcholine receptors (nAChRs) may be beneficial in the management of Parkinson's disease. This idea initially stemmed from the results of epidemiological studies that demonstrated that smoking is associated with a decreased incidence of Parkinson's disease. The subsequent finding that nicotine administration protected against nigrostriatal damage in parkinsonian animal models led to the idea that nicotine in tobacco products may contribute to this apparent protective action. Nicotine most likely exerts its effects by interacting at nAChRs. Accumulating research indicates that multiple subtypes containing nAChRs, including α4β2, α6β2, and/or α7, may be involved. Stimulation of nAChRs initially activates various intracellular transduction pathways primarily via alterations in calcium signaling. Consequent adaptations in immune responsiveness and trophic factors may ultimately mediate nicotine's ability to reduce/halt the neuronal damage that arises in Parkinson's disease. In addition to a potential neuroprotective action, nicotine also has antidepressant properties and improves attention/cognition. Altogether, these findings suggest that nicotine and nAChR drugs represent promising therapeutic agents for the management of Parkinson's disease.

Nicotine as a potential neuroprotective agent for Parkinson's disease

Converging research efforts suggest that nicotine and other drugs that act at nicotinic acetylcholine receptors (nAChRs) may be beneficial in the management of Parkinson's disease. This idea initially stemmed from the results of epidemiological studies that demonstrated that smoking is associated with a decreased incidence of Parkinson's disease. The subsequent finding that nicotine administration protected against nigrostriatal damage in parkinsonian animal models led to the idea that nicotine in tobacco products may contribute to this apparent protective action. Nicotine most likely exerts its effects by interacting at nAChRs. Accumulating research indicates that multiple subtypes containing nAChRs, including α4β2, α6β2, and/or α7, may be involved. Stimulation of nAChRs initially activates various intracellular transduction pathways primarily via alterations in calcium signaling. Consequent adaptations in immune responsiveness and trophic factors may ultimately mediate nicotine's ability to reduce/halt the neuronal damage that arises in Parkinson's disease. In addition to a potential neuroprotective action, nicotine also has antidepressant properties and improves attention/cognition. Altogether, these findings suggest that nicotine and nAChR drugs represent promising therapeutic agents for the management of Parkinson's disease.

Protective effects of nicotine against aminochrome-induced toxicity in substantia nigra derived cells: implications for Parkinson's disease

Parkinson's disease is a debilitating progressive neurodegenerative disorder that results from the loss of or damage to dopaminergic cells containing neuromelanin in the substantia nigra (SN). The underlying neurodegenerative mechanism(s), however, remain elusive. Aminochrome, the precursor of neuromelanin is an endogenous substance capable of inducing selective neurotoxicity to dopaminergic neurons in SN. Nicotine, on the other hand, may offer protective effects against dopaminergic cell damage induced by various neurotoxins including MPTP and salsolinol. In this study, we sought to determine whether nicotine may also protect against aminochrome-induced toxicity in SN derived RCSN-3 cells. Exposure of RCSN-3 cells to a combination of aminochrome (50 μM) and dicoumarol (50 μM) for 48 h induced approximately 70 % cell death. Pretreatment with nicotine, dose-dependently blocked this toxicity. The effects of nicotine in turn were dose-dependently blocked by mecamylamine, a non-selective nicotinic receptor antagonist. These results suggest involvement of nicotinic receptors in protective effects of nicotine against aminochrome-induced toxicity and provide further evidence for possible therapeutic effects of nicotine or nicotinic agonists in Parkinson's disease.

Parkinson's disease

Parkinson's disease (PD) is the most common age-related motoric neurodegenerative disease initially described in the 1800's by James Parkinson as the 'Shaking Palsy'. Loss of the neurotransmitter dopamine was recognized as underlying the pathophysiology of the motor dysfunction; subsequently discovery of dopamine replacement therapies brought substantial symptomatic benefit to PD patients. However, these therapies do not fully treat the clinical syndrome nor do they alter the natural history of this disorder motivating clinicians and researchers to further investigate the clinical phenotype, pathophysiology/pathobiology and etiology of this devastating disease. Although the exact cause of sporadic PD remains enigmatic studies of familial and rare toxicant forms of this disorder have laid the foundation for genome wide explorations and environmental studies. The combination of methodical clinical evaluation, systematic pathological studies and detailed genetic analyses have revealed that PD is a multifaceted disorder with a wide-range of clinical symptoms and pathology that include regions outside the dopamine system. One common thread in PD is the presence of intracytoplasmic inclusions that contain the protein, α-synuclein. The presence of toxic aggregated forms of α-synuclein (e.g., amyloid structures) are purported to be a harbinger of subsequent pathology. In fact, PD is both a cerebral amyloid disease and the most common synucleinopathy, that is, diseases that display accumulations of α-synuclein. Here we present our current understanding of PD etiology, pathology, clinical symptoms and therapeutic approaches with an emphasis on misfolded α-synuclein.

Positive and negative effects of alcohol and nicotine and their interactions: a mechanistic review

Nicotine and alcohol are two of the most commonly abused legal substances. Heavy use of one drug can often lead to, or is predictive of, heavy use of the other drug in adolescents and adults. Heavy drinking and smoking alone are of significant health hazard. The combination of the two, however, can result in synergistic adverse effects particularly in incidences of various cancers (e.g., esophagus). Although detrimental consequences of smoking are well established, nicotine by itself might possess positive and even therapeutic potential. Similarly, alcohol at low or moderated doses may confer beneficial health effects. These opposing findings have generated considerable interest in how these drugs act. Here we will briefly review the negative impact of drinking-smoking co-morbidity followed by factors that appear to contribute to the high rate of co-use of alcohol and nicotine. Our main focus will be on what research is telling us about the central actions and interactions of these drugs, and what has been elucidated about the mechanisms of their positive and negative effects. We will conclude by making suggestions for future research in this area.

Toxic effects of low alcohol and nicotine combinations in SH-SY5Y cells are apoptotically mediated

It is well established that combination of heavy drinking and smoking has severe health consequences. However, at relatively low concentrations, both alcohol and nicotine may have beneficial effects including neuroprotection. Thus, protective effects of low alcohol concentration against beta-amyloid-induced toxicity in organotypic hippocampal slices and protective effects of nicotine against salsolinol-induced toxicity in human-derived neuroblastoma cells (SH-SY5Y) have been reported. In this study, we sought to determine whether alcohol might also be protective against salsolinol-induced toxicity in SH-SY5Y cells and whether the combination of low doses of alcohol and nicotine might have an additive or synergistic effect. Pre-exposure of SH-SY5Y cells to either ethanol (1 or 10 mM) or nicotine (20 or 50 μM) significantly attenuated salsolinol-induced toxicity. However, contrary to the expectation the combination of low doses of alcohol and nicotine not only did not provide any synergistic or additive protective effect, but exacerbated salsolinol-induced toxicity. Indeed, simple combination of low alcohol and nicotine resulted in significant toxicity in SH-SY5Y cells. This toxicity, reflected in a reduction in cell viability was associated with an increase in apoptosis as determined by caspase-3 measurement. These in vitro results suggest that combination of even low concentrations of alcohol and nicotine may activate apoptotic mechanisms that can lead to cell toxicity and detrimental consequences.

Drosophila models of Parkinson's disease

Parkinson's disease (PD) is the second most prevalent neurodegenerative disorder principally affecting the dopaminergic neurons of the substantia nigra. The pathogenic mechanisms are unknown and there are currently no cure or disease-modifying therapies. Recent genetic linkage studies have begun to identify single-gene mutations responsible for rare heritable forms of PD and define genetic risk factors contributing to disease prevalence in sporadic cases. These findings provide an opportunity to gain insight into the molecular mechanisms of this disorder through the creation and analysis of appropriate genetic models. One model system that has proven surprisingly tractable for these studies is the fruit fly, Drosophila melanogaster. Analysis of a number of Drosophila models of PD has revealed some profound and sometimes surprising insights into PD pathogenesis. Moreover, these models can be used to investigate potential therapeutic strategies that may be effective in vivo, and tests have highlighted the efficacy of a number of neuroprotective compounds. Here, I review the methodologies employed in developing the various Drosophila models, and the recent advances that these models in particular have contributed to our understanding of the mechanisms that underlie PD pathogenesis and possible treatment strategies.

Alpha7 nicotinic receptors as novel therapeutic targets for inflammation-based diseases

In recent years the etiopathology of a number of debilitating diseases such as type 2 diabetes, arthritis, atherosclerosis, psoriasis, asthma, cystic fibrosis, sepsis, and ulcerative colitis has increasingly been linked to runaway cytokine-mediated inflammation. Cytokine-based therapeutic agents play a major role in the treatment of these diseases. However, the temporospatial changes in various cytokines are still poorly understood and attempts to date have focused on the inhibition of specific cytokines such as TNF-α. As an alternative approach, a number of preclinical studies have confirmed the therapeutic potential of targeting alpha7 nicotinic acetylcholine receptor-mediated anti-inflammatory effects through modulation of proinflammatory cytokines. This "cholinergic anti-inflammatory pathway" modulates the immune system through cholinergic mechanisms that act on alpha7 receptors expressed on macrophages and immune cells. If the preclinical findings translate into human efficacy this approach could potentially provide new therapies for treating a broad array of intractable diseases and conditions with inflammatory components.

Gene-environment interactions in Parkinson's disease: the importance of animal modeling

Parkinson's disease (PD), a late-onset neurodegenerative disorder, occurs most commonly in a "sporadic" (idiopathic) form, without a clearly defined genetic basis and only a vaguely delineated pathogenesis. Together, the various monogenic forms of PD (i.e., those arising from mutations in single genes) account for a minority of PD cases but have provided crucial insights into disease mechanisms. Although it is commonly believed that sporadic PD is caused by a lifetime of environmental exposures that are superimposed on an individual's composite genetic susceptibility, this hypothesis has not been tested adequately. This article reviews genetic and environmental factors that have been associated with PD and attempts to put these into a pathogenic framework. We argue that animal modeling will become increasingly important in attempting to elucidate gene-environment interactions, to define pathogenic mechanisms, and to provide a platform for testing of targeted therapeutic interventions.

Cholinergic modulation of locomotion and striatal dopamine release is mediated by alpha6alpha4* nicotinic acetylcholine receptors

Dopamine (DA) release in striatum is governed by firing rates of midbrain DA neurons, striatal cholinergic tone, and nicotinic ACh receptors (nAChRs) on DA presynaptic terminals. DA neurons selectively express alpha6* nAChRs, which show high ACh and nicotine sensitivity. To help identify nAChR subtypes that control DA transmission, we studied transgenic mice expressing hypersensitive alpha6(L9'S)* receptors. alpha6(L9'S) mice are hyperactive, travel greater distance, exhibit increased ambulatory behaviors such as walking, turning, and rearing, and show decreased pausing, hanging, drinking, and grooming. These effects were mediated by alpha6alpha4* pentamers, as alpha6(L9'S) mice lacking alpha4 subunits displayed essentially normal behavior. In alpha6(L9'S) mice, receptor numbers are normal, but loss of alpha4 subunits leads to fewer and less sensitive alpha6* receptors. Gain-of-function nicotine-stimulated DA release from striatal synaptosomes requires alpha4 subunits, implicating alpha6alpha4beta2* nAChRs in alpha6(L9'S) mouse behaviors. In brain slices, we applied electrochemical measurements to study control of DA release by alpha6(L9'S) nAChRs. Burst stimulation of DA fibers elicited increased DA release relative to single action potentials selectively in alpha6(L9'S), but not WT or alpha4KO/alpha6(L9'S), mice. Thus, increased nAChR activity, like decreased activity, leads to enhanced extracellular DA release during phasic firing. Bursts may directly enhance DA release from alpha6(L9'S) presynaptic terminals, as there was no difference in striatal DA receptor numbers or DA transporter levels or function in vitro. These results implicate alpha6alpha4beta2* nAChRs in cholinergic control of DA transmission, and strongly suggest that these receptors are candidate drug targets for disorders involving the DA system.

Decaffeinated coffee and nicotine-free tobacco provide neuroprotection in Drosophila models of Parkinson's disease through an NRF2-dependent mechanism

Epidemiological studies have revealed a significantly reduced risk of Parkinson's disease (PD) among coffee and tobacco users, although it is unclear whether these correlations reflect neuroprotective/symptomatic effects of these agents or preexisting differences in the brains of tobacco and coffee users. Here, we report that coffee and tobacco, but not caffeine or nicotine, are neuroprotective in fly PD models. We further report that decaffeinated coffee and nicotine-free tobacco are as neuroprotective as their caffeine and nicotine-containing counterparts and that the neuroprotective effects of decaffeinated coffee and nicotine-free tobacco are also evident in Drosophila models of Alzheimer's disease and polyglutamine disease. Finally, we report that the neuroprotective effects of decaffeinated coffee and nicotine-free tobacco require the cytoprotective transcription factor Nrf2 and that a known Nrf2 activator in coffee, cafestol, is also able to confer neuroprotection in our fly models of PD. Our findings indicate that coffee and tobacco contain Nrf2-activating compounds that may account for the reduced risk of PD among coffee and tobacco users. These compounds represent attractive candidates for therapeutic intervention in PD and perhaps other neurodegenerative diseases.

Nicotinic receptor-mediated reduction in L-DOPA-induced dyskinesias may occur via desensitization

L-DOPA-induced dyskinesias in Parkinson's disease are a significant clinical problem for which few therapies are available. We recently showed that nicotine reduces L-DOPA-induced abnormal involuntary movements (AIMs) in parkinsonian animals, suggesting it may be useful for the treatment of L-DOPA-induced dyskinesias. The present experiments were performed to understand the mechanisms whereby nicotine reduces L-DOPA-induced AIMs. We used a well established model of dyskinesias, L-DOPA-treated unilateral 6-hydroxydopamine-lesioned rats. Dose-ranging studies showed that injection of 0.1 mg/kg nicotine once or twice daily for 4 or 10 days most effectively reduced AIMs, with no worsening of parkinsonism. Importantly, a single nicotine injection did not reduce AIMs, indicating that nicotine's effect is caused by long-term rather than short-term molecular changes. Administration of the metabolite cotinine did not reduce AIMs, suggesting a direct effect of nicotine. Experiments with the nicotinic receptor (nAChR) antagonist mecamylamine were done to determine whether nicotine acted via a receptor-mediated mechanism. Unexpectedly, several days of mecamylamine injection (1.0 mg/kg) alone significantly ameliorated dyskinesias to a comparable extent as nicotine. The decline in AIMs with combined nicotine and mecamylamine treatment was not additive, suggesting that nicotine exerts its effects via a nAChR interaction. This latter finding, combined with data showing that mecamylamine reduced AIMs to a similar extent as nicotine, and that nicotine or mecamylamine treatment both decreased alpha6beta2* and increased alpha4beta2* nAChR expression, suggests that the nicotine-mediated improvement in L-DOPA-induced AIMs may involve a desensitization block. These data have important implications for the treatment of L-DOPA-induced dyskinesias in Parkinson's disease.

Effects of nicotine on depressive-like behavior and hippocampal volume of female WKY rats

The observed high incidence of smoking amongst depressed individuals has led to the hypothesis of 'self medication" with nicotine in some of these patients. The inbred Wistar-Kyoto (WKY) rats exhibit depressive-like characteristics as evidenced by exaggerated immobility in the forced swim test (FST). One aim of this study was to investigate whether nicotine may have an antidepressant-like effect in these animals. Moreover, because of human postmortem studies indicating a reduction of the hippocampus volume in depressed patients, it was of interest to determine whether such an anatomical anomaly may also be manifested in WKY rats and whether it would be affected by chronic nicotine treatment. Adult female WKY and their control Wistar rats were administered nicotine consecutively (0.2 mg/kg, i.p., once or twice daily for 14 days) and their activity in an open field, as well as their immobility in FST were assessed either 15 min or 18 h after the last injection. Another set of animals was treated twice daily with 0.2 mg/kg nicotine for 14 days and sacrificed on day 15 for stereological evaluation of the hippocampal volume. When tested 15 min after the last injection, once or twice daily nicotine exacerbated the immobility in the FST in WKY rats only. When tested 18 h after the last injection, only twice daily nicotine treatment resulted in less immobility in the FST in WKY rats. Open field locomotor activity was not affected by any nicotine regimen. WKY rats had significantly less hippocampal volume (approximately 20%) than Wistar rats which was not altered by nicotine. These findings further validate the use of WKY rats as an animal model of human depression and signify the importance of inherent genetic differences in final behavioral outcome of nicotine.

Modeling complex genetic and environmental influences on comorbid bipolar disorder with tobacco use disorder

Background

Comorbidity of psychiatric and substance use disorders represents a significant complication in the clinical course of both disorders. Bipolar Disorder (BD) is a psychiatric disorder characterized by severe mood swings, ranging from mania to depression, and up to a 70% rate of comorbid Tobacco Use Disorder (TUD). We found epidemiological evidence consistent with a common underlying etiology for BD and TUD, as well as evidence of both genetic and environmental influences on BD and TUD. Therefore, we hypothesized a common underlying genetic etiology, interacting with nicotine exposure, influencing susceptibility to both BD and TUD.

Methods

Using meta-analysis, we compared TUD rates for BD patients and the general population. We identified candidate genes showing statistically significant, replicated, evidence of association with both BD and TUD. We assessed commonality among these candidate genes and hypothesized broader, multi-gene network influences on the comorbidity. Using Fisher Exact tests we tested our hypothesized genetic networks for association with the comorbidity, then compared the inferences drawn with those derived from the commonality assessment. Finally, we prioritized candidate SNPs for validation.

Results

We estimate risk for TUD among BD patients at 2.4 times that of the general population. We found three candidate genes associated with both BD and TUD (COMT, SLC6A3, and SLC6A4) and commonality analysis suggests that these genes interact in predisposing psychiatric and substance use disorders. We identified a 69 gene network that influences neurotransmitter signaling and shows significant over-representation of genes associated with BD and TUD, as well as genes differentially expressed with exposure to tobacco smoke. Twenty four of these genes are known drug targets.

Conclusions

This work highlights novel bioinformatics resources and demonstrates the effectiveness of using an integrated bioinformatics approach to improve our understanding of complex disease etiology. We illustrate the development and testing of hypotheses for a comorbidity predisposed by both genetic and environmental influences. Consistent with our hypothesis, the selected network models multiple interacting genetic influences on comorbid BD with TUD, as well as the environmental influence of nicotine. This network nominates candidate genes for validation and drug testing, and we offer a panel of SNPs prioritized for follow-up.

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.

Differential involvement of D1 and D2 dopamine receptors in L-DOPA-induced angiogenic activity in a rat model of Parkinson's disease

Angiogenesis occurs in the brains of Parkinson's disease patients, but the effects of dopamine replacement therapy on this process have not been examined. Using rats with 6-hydroxydopamine lesions, we have compared angiogenic responses induced in the basal ganglia by chronic treatment with either L-DOPA, or bromocriptine, or a selective D1 receptor agonist (SKF38393). Moreover, we have asked whether L-DOPA-induced angiogenesis can be blocked by co-treatment with either a D1- or a D2 receptor antagonist (SCH23390 and eticlopride, respectively), or by an inhibitor of extracellular signal-regulated kinases 1 and 2 (ERK1/2) (SL327). L-DOPA, but not bromocriptine, induced dyskinesia, which was associated with endothelial proliferation, upregulation of immature endothelial markers (nestin) and downregulation of endothelial barrier antigen in the striatum and its output structures. At a dose inducing dyskinesia (1.5 mg/kg/day), SKF38393 elicited angiogenic changes similar to L-DOPA. Antagonism of D1- but not D2 class receptors completely suppressed both the development of dyskinesia and the upregulation of angiogenesis markers. In fact, L-DOPA-induced endothelial proliferation was markedly exacerbated by low-dose D2 antagonism (0.01 mg/kg eticlopride). Inhibition of ERK1/2 by SL327 attenuated L-DOPA-induced dyskinesia and completely inhibited all markers of angiogenesis. These results highlight the specific link between treatment-induced dyskinesias and microvascular remodeling in the dopamine-denervated brain. L-DOPA-induced angiogenesis requires stimulation of D1 receptors and activation of ERK1/2, whereas the stimulation of D2 receptors seems to oppose this response.

Novel therapeutic strategies in Parkinson's disease

PD is a neurodegenerative disorder triggered by genetic and/or environmental factors and the pathological processes begin many years before motor symptom manifestation. Several drugs are available to treat PD, but there are still many aspects of the disease that have not been well addressed. These include nonmotor symptoms, disease progression, and preventing levodopa-induced motor complications. Besides the concept of continuous dopaminergic stimulation, the benefit of which has not been proved in clinical settings (see the STRIDE-PD trial), the nondopaminergic drugs offer promising alternatives to dopaminergic medication. However, modification and further development of dopaminergic molecules may provide significant symptomatic improvement and improved quality of life. In this review, we summarized new treatments that are in the pipeline, with patients being recruited for clinical trials. Among the compounds being studied are well-known ones (e.g., folic acid or methylphenidate), which have been used in other diseases, and newly developed drugs with known mode of action (e.g., the dopamine agonist aplindore) or compounds with completely new mechanisms, which have not yet been used in clinical settings (e.g., levetiracetam or Neu 120). A major breakthrough in treating Parkinson's disease cannot be expected with molecules from the first two cases, whereas significant clinical benefits can be predicted with the drugs in the last group. However, without these large-scale, well-designed, multicenter trials, the promising preclinical results cannot be directly adopted in patient care. Hopefully, some of these trials will end soon with positive results, and certain drugs will become available with which to treat PD patients, although still many aspects (e.g., the most problematic one, the question of neuroprotection) still need to be addressed.

Potential influences of complementary therapy on motor and non-motor complications in Parkinson's disease

Nearly two-thirds of patients with Parkinson's disease (PD) use vitamins or nutritional supplements, and many more may use other complementary therapies, yet <50% of patients have discussed the use of these complementary therapies with a healthcare professional. Physicians should be aware of the complementary therapies their patients with PD are using, and the possible effects of these therapies on motor and non-motor symptoms. Complementary therapies, such as altered diet, dietary supplements, vitamin therapy, herbal supplements, caffeine, nicotine, exercise, physical therapy, massage therapy, melatonin, bright-light therapy and acupuncture, may all influence the symptoms of PD and/or the effectiveness of dopaminergic therapy. Preliminary evidence suggests complementary therapy also may influence non-motor symptoms of PD, such as respiratory disorders, gastrointestinal disorders, mood disorders, sleep and orthostatic hypotension. Whenever possible, clinicians should ensure that complementary therapy is used appropriately in PD patients without reducing the benefits of dopaminergic therapy.

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.

Smoking, nicotine and neuropsychiatric disorders

Tobacco smoking is an extremely addictive and harmful form of nicotine (NIC) consumption, but unfortunately also the most prevalent. Although disproportionately high frequencies of smoking and its health consequences among psychiatric patients are widely known, the neurobiological background of this epidemiological association is still obscure. The diverse neuroactive effects of NIC and some other major tobacco smoke constituents in the central nervous system may underlie this association. This present paper summarizes the pharmacology of NIC and its receptors (nAChR) based on a systematic review of the literature. The role of the brain's reward system(s) in NIC addiction and the results of functional and structural neuroimaging studies on smoking-related states and behaviors (i.e. dependence, craving, withdrawal) are also discussed. In addition, the epidemiological, neurobiological, and genetic aspects of smoking in several specific neuropsychiatric disorders are reviewed and the clinical relevance of smoking in these disease states addressed.

Mysterious alpha6-containing nAChRs: function, pharmacology, and pathophysiology

Neuronal nicotinic acetylcholine receptors (nAChRs) are the superfamily of ligand-gated ion channels and widely expressed throughout the central and peripheral nervous systems. nAChRs play crucial roles in modulating a wide range of higher cognitive functions by mediating presynaptic, postsynaptic, and extrasynaptic signaling. Thus far, nine alpha (alpha2-alpha10) and three beta (beta2, beta3, and beta4) subunits have been identified in the CNS, and these subunits assemble to form a diversity of functional nAChRs. Although alpha4beta2- and alpha7-nAChRs are the two major functional nAChR types in the CNS, alpha6*-nAChRs are abundantly expressed in the midbrain dopaminergic (DAergic) system, including mesocorticolimbic and nigrostriatal pathways, and particularly present in presynaptic nerve terminals. Recently, functional and pharmacological profiles of alpha6*-nAChRs have been assessed with the use of alpha6 subunit blockers such as alpha-conotoxin MII and PIA, and also by using alpha6 subunit knockout mice. By modulating DA release in the nucleus accumbens (NAc) and modulating GABA release onto DAergic neurons in the ventral tegmental area (VTA), alpha6*-nAChRs may play important roles in the mediation of nicotine reward and addiction. Furthermore, alpha6*-nAChRs in the nigrostriatal DAergic system may be promising targets for selective preventative treatment of Parkinson's disease (PD). Thus, alpha6*-nAChRs may hold promise for future clinical treatment of human disorders, such as nicotine addiction and PD. In this review, we mainly focus on the recent advances in the understanding of alpha6*-nAChR function, pharmacology and pathophysiology.

Multiple roles for nicotine in Parkinson's disease

There exists a remarkable diversity of neurotransmitter compounds in the striatum, a pivotal brain region in the pathology of Parkinson's disease, a movement disorder characterized by rigidity, tremor and bradykinesia. The striatal dopaminergic system, which is particularly vulnerable to neurodegeneration in this disorder, appears to be the major contributor to these motor problems. However, numerous other neurotransmitter systems in the striatum most likely also play a significant role, including the nicotinic cholinergic system. Indeed, there is an extensive anatomical overlap between dopaminergic and cholinergic neurons, and acetylcholine is well known to modulate striatal dopamine release both in vitro and in vivo. Nicotine, a drug that stimulates nicotinic acetylcholine receptors (nAChRs), influences several functions relevant to Parkinson's disease. Extensive studies in parkinsonian animals show that nicotine protects against nigrostriatal damage, findings that may explain the well-established decline in Parkinson's disease incidence with tobacco use. In addition, recent work shows that nicotine reduces l-dopa-induced abnormal involuntary movements, a debilitating complication of l-dopa therapy for Parkinson's disease. These combined observations suggest that nAChR stimulation may represent a useful treatment strategy for Parkinson's disease for neuroprotection and symptomatic treatment. Importantly, only selective nAChR subtypes are present in the striatum including the alpha4beta2*, alpha6beta2* and alpha7 nAChR populations. Treatment with nAChR ligands directed to these subtypes may thus yield optimal therapeutic benefit for Parkinson's disease, with a minimum of adverse side effects.

Additive protective effects of donepezil and nicotine against salsolinol-induced cytotoxicity in SH-SY5Y cells

Although the etiology of Parkinson's disease (PD) remains elusive, a number of toxins including elevated salsolinol, an endogenous metabolite of dopamine may contribute to its pathology. It was reported recently that nicotine may have protective effects against salsolinol-induced toxicity in human neuroblastoma derived SH-SY5Y cells and that these effects of nicotine are mediated by nicotinic receptors. Donepezil (Aricept) is a reversible non-competitive acetylcholinesterase inhibitor that is approved for use in mild to moderate Alzheimer's disease. The increase in acetylcholine concentrations is believed to be the major contributory factor in donepezil's therapeutic efficacy. However, cholinesterase inhibitors may also directly interact with nicotinic receptors and possess neuroprotective properties. In this study, we sought to determine whether donepezil may have protective effects against salsolinol-induced toxicity in SH-SY5Y cells and whether the combination of donepezil and nicotine may result in additive protection. Moreover, it was of interest to elucidate the role of nicotinic receptors as well as cell cycle and apoptosis in mechanism of action of these compounds. SH-SY5Y cells were exposed to 0.6 mM salsolinol with and without various drug pretreatments for 48 h. Nicotine (50 muM) resulted in approximately 54% protection and donepezil (5 muM) resulted in approximately 40% protection, and the combination of the two resulted in an additive (approximately 93%) protection against salsolinol-induced toxicity. Salsolinol caused an arrest of the cells in G(1)-phase of cell cycle and an increase in apoptotic indices that were blocked by the combination of donepezil and nicotine. Mecamylamine, a non-selective nicotinic receptor antagonist completely blocked the effects of nicotine and partially attenuated the effects of donepezil. A combination of atropine, a muscarinic receptor antagonist and mecamylamine completely blocked the effects of donepezil, indicating involvement of both nicotinic and muscarinic receptors in donepezil's actions. The findings suggest a therapeutic potential for the combination of donepezil and nicotine in PD.

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.