An important factor contributing to the high relapse rates among smokers is nicotine withdrawal symptoms. Multiple studies suggest that decreased dopamine release in nucleus accumbens plays a key role in withdrawal. However, recent reports showed that long-term nicotine exposure itself also decreases accumbal dopamine release, suggesting that additional mechanisms are involved in withdrawal. Here, we used real-time cyclic voltammetry in brain slices containing the nucleus accumbens to further elucidate the changes in dopamine release linked to nicotine withdrawal. Rats received vehicle or nicotine via the drinking water for 2–3 months. Studies assessing the expression of somatic signs in vehicle-treated, nicotine-treated, and 24-h nicotine withdrawn rats showed that nicotine withdrawal led to a significant increase in somatic signs. Subsequent voltammetry studies showed that long-term nicotine decreased single-pulse-stimulated dopamine release via an interaction at α6β2* receptors. Nicotine withdrawal led to a partial recovery in α6β2* receptor-mediated release. In addition, long-term nicotine treatment alone increased dopamine release paired-pulse ratios and this was partially reversed with nicotine removal. We then evaluated the effect of bath-applied nicotine and varenicline on dopamine release. Nicotine and varenicline both decreased single-pulse-stimulated release in vehicle-treated, nicotine-treated, and nicotine withdrawn rats. However, bath-applied varenicline increased paired-pulse ratios to a greater extent than nicotine during long-term nicotine treatment and after its withdrawal. Altogether these data suggest that nicotine withdrawal is associated with a partial restoration of dopamine release measures to control levels and that varenicline's differential modulation of dopamine release may contribute to its mechanism of action.
Acetylcholine; addiction; nicotinic receptor; voltammetry
Long-term nicotine exposure induces alterations in dopamine transmission in nucleus accumbens (NAcc) that sustain the reinforcing effects of smoking. One approach to understand the adaptive changes that arise involves measurement of endogenous dopamine release using voltammetry. We therefore treated rats for 2-3 months with nicotine and examined alterations in nAChR subtype expression and electrically-evoked dopamine release in rat NAcc shell, a region key in addiction. Long-term nicotine treatment selectively decreased stimulated α6β2* nAChR-mediated dopamine release compared to vehicle-treated rats. It also reduced α6β2* nAChRs, suggesting the receptor decline may contribute to the functional loss. This decreased response in release after chronic nicotine treatment was still partially sensitive to the agonist nicotine. Studies with an acetylcholinesterase inhibitor demonstrated that the response was also sensitive to increased endogenous acetylcholine. However, unlike the agonists, nAChR antagonists decreased dopamine release only in vehicle- but not nicotine-treated rats. Since antagonists function by blocking the action of acetylcholine, their ineffectiveness suggests that reduced acetylcholine levels partly underlie the dampened α6β2* nAChR-mediated function in nicotine-treated rats. Since long-term nicotine modifies dopamine release by decreasing α6β2* nAChRs and their function, these data suggest that interventions that target this subtype may be useful for treating nicotine dependence.
addiction; nicotine; nicotinic receptors; nucleus accumbens; voltammetry
Accumulating evidence supports the idea that drugs acting at nicotinic acetylcholine receptors (nAChRs) may be beneficial for Parkinson's disease, a neurodegenerative movement disorder characterized by a loss of nigrostriatal dopaminergic neurons. Nicotine administration to parkinsonian animals protects against nigrostriatal damage. In addition, nicotine and nAChR drugs improve L-dopa-induced dyskinesias, a debilitating side effect of L-dopa therapy which remains the gold-standard treatment for Parkinson's disease. Nicotine exerts its antidyskinetic effect by interacting with multiple nAChRs. One approach to identify the subtypes specifically involved in L-dopa-induced dyskinesias is through the use of nAChR subunit null mutant mice. Previous work with β2 and α6 nAChR knockout mice has shown that α6β2* nAChRs were necessary for the development/maintenance of L-dopa-induced abnormal involuntary movements (AIMs). The present results in parkinsonian α4 nAChR knockout mice indicate that α4β2* nAChRs also play an essential role since nicotine did not reduce L-dopa-induced AIMs in such mice. Combined analyses of the data from α4 and α6 knockout mice suggest that the α6α4β2β3 subtype may be critical. In contrast to the studies with α4 and α6 knockout mice, nicotine treatment did reduce L-dopa-induced AIMs in parkinsonian α7 nAChR knockout mice. However, α7 nAChR subunit deletion alone increased baseline AIMs, suggesting that α7 receptors exert an inhibitory influence on L-dopa-induced AIMs. In conclusion, α6β2*, α4β2* and α7 nAChRs all modulate L-dopa-induced AIMs, although their mode of regulation varies. Thus drugs targeting one or multiple nAChRs may be optimal for reducing L-dopa-induced dyskinesias in Parkinson's disease.
Dyskinesias; Nicotine; Nicotinic receptors; Parkinson’s disease; Striatum
Although 3,4-dihydroxyphenylalanine (L-dopa) is the gold-standard treatment for Parkinson’s disease, it can lead to disabling dyskinesias. Previous work showed that nicotine reduces L-dopa-induced dyskinesias (LIDs) in several parkinsonian animal models. The goal of this study was to determine if the time of nicotine administration affected its ability to reduce LIDs in L-dopa-primed and L-dopa naïve monkeys, and also to test if tolerance developed to nicotine’s beneficial effects.
Monkeys were injected with MPTP (1.9–2.0 mg/kg sc) over 3–5 months until parkinsonian. Nicotine (300 μg/ml) was administered in drinking water (4–6 months) to L-dopa-primed or L-dopa-naïve monkeys, with L-dopa/carbidopa (10/2.5 mg/kg) gavaged twice daily.
One set of MPTP-lesioned monkeys (n=23) was first gavaged with L-dopa and subsequently given nicotine 4 wk later when dyskinesias have plateaud or 8 wk later when dyskinesias are established. A 60–70% decrease in LIDs was observed after several weeks of nicotine treatment in both groups. A second set of monkeys (n=26) was given nicotine 8 or 2 weeks before L-dopa. In the 8 week nicotine pretreatment group, there was an immediate reduction in LIDs, which plateaud at 60–70%. In the 2 week nicotine pretreatment group, there were initial small decreases in LIDs, which plateaud at 60–70% several weeks later. Thus, nicotine pre- and post-treatment was 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 Parkinson’s disease patients.
dyskinesia; L-dopa; nicotine; nicotinic; nonhuman primate; Parkinson’s disease
L-dopa-induced dyskinesias are a serious long-term side effect of dopamine replacement therapy for Parkinson’s disease for which there are few treatment options. Our previous studies showed that nicotine decreased L-dopa-induced abnormal involuntary movements (AIMs). Subsequent work with knockout mice demonstrated that α6β2* nicotinic receptors (nAChRs) play a key role. The present experiments were done to determine if α4β2* nAChRs are also involved in L-dopa-induced dyskinesias. To approach this, we took advantage of the finding that α6β2* nAChRs are predominantly present on striatal dopaminergic nerve terminals, while a significant population of α4β2* nAChRs are located on other neurons. Thus, a severe dopaminergic lesion would cause a major loss in α6β2*, but not α4β2* nAChRs. Experiments were therefore done in which rats were unilaterally lesioned with 6-hydroxydopamine, at a dose that lead to severe nigrostriatal damage. The dopamine transporter, a dopamine nerve terminal marker, was decreased by >99%. This lesion also decreased striatal α6β2* nAChRs by 97%, while α4β2* nAChRs were reduced by only 12% compared to control. A series of β2* nAChR compounds, including TC-2696, TI-10165, TC-8831, TC-10600 and sazetidine reduced L-dopa-induced AIMs in these rats by 23–32%. TC-2696, TI-10165, TC-8831 were also tested for parkinsonism, with no effect on this behavior. Tolerance did not develop with up to 3 months of treatment. Since α4a5β2 nAChRs are also predominantly on striatal dopamine terminals, these data suggest that drugs targeting α4β2 nAChRs may reduce L-dopa-induced dyskinesias in late stage Parkinson’s disease.
Dyskinesia; L-dopa; nicotine; nicotinic; Parkinson’s disease; sazetidine
L-dopa-induced dyskinesias (LIDs) are a side effect of Parkinson’s disease therapy that is thought to arise, at least in part, because of excessive dopaminergic activity. Thus, drugs that regulate dopaminergic tone may provide an approach to manage LIDs. Our previous studies showed that nicotine treatment reduced LIDs in parkinsonian animal models. The present work investigates whether nicotine may exert its beneficial effects by modulating presynaptic dopaminergic function. Rats were unilaterally lesioned by injection of 6-OHDA (2 × 3 ug per site) into the medial forebrain bundle to yield moderate parkinsonism. They were then implanted with minipumps containing vehicle or nicotine (2.0 mg/kg/d) and rendered dyskinetic with L-dopa (8 mg/kg plus 15 mg/kg benserazide). Lesioning alone decreased the striatal dopamine transporter, nicotinic receptor (nAChR) levels and nAChR-mediated 3H-dopamine release, consistent with previous results. Nicotine administration reduced L-dopa induced abnormal involuntary movements throughout the course of the study (4 months). Nicotine treatment led to declines in the striatal dopamine transporter, α6β2* nAChRs and various components of α6β2* and α4β2* nAChR-mediated release. L-dopa treatment had no effect. These data suggest that nicotine may improve LIDs in parkinsonian animal models by dampening striatal dopaminergic activity.
LIDs; dopamine; nicotine; nicotinic receptors; nigrostriatal lesion
L-Dopa-induced dyskinesias (LIDs) are abnormal involuntary movements that develop with long term L-dopa therapy for Parkinson’s disease. Studies show that nicotine administration reduced LIDs in several parkinsonian animal models. The present work was done to understand the factors that regulate the nicotine-mediated reduction in LIDs in MPTP-lesioned nonhuman primates. To approach this, we used two groups of monkeys, one with mild-moderate and the other with more severe parkinsonism rendered dyskinetic using L-dopa. In mild-moderately parkinsonian monkeys, nicotine pretreatment (300 μg/ml via drinking water) prevented the development of LIDs by ~75%. This improvement was maintained when the nicotine dose was lowered to 50 μg/ml but was lost with nicotine removal. Nicotine re-exposure again decreased LIDs. By contrast, nicotine treatment did not reduce LIDs in monkeys with more severe parkinsonism. We next determined how nicotine’s ability to reduce LIDs correlated with lesion-induced changes in the striatal dopamine transporter and 3H-dopamine release in these two groups of monkeys. The striatal dopamine transporter was reduced to 54% and 28% of control in mild-moderately and more severely parkinsonian monkeys, respectively. However, basal, K+, α4β2* and α6β2* nAChR-evoked 3H-dopamine release were near control levels in striatum of mild-moderately parkinsonian monkeys. By contrast, these same release measures were reduced to a significantly greater extent in striatum of more severely parkinsonian monkeys. Thus, nicotine best improves LIDs in lesioned monkeys in which striatal dopamine transmission is still relatively intact. These data suggest that nicotine treatment would most effectively reduce LIDs in patients with mild to moderate Parkinson’s disease.
Dopamine; L-dopa-induced dyskinesias; Nicotine; Nicotinic receptors; Parkinson’s disease
L-Dopa-induced dyskinesias are a serious side effect that develops in most Parkinson’s disease patients on dopamine replacement therapy. Few treatment options are available to manage dyskinesias; however, recent studies show that nicotine reduces these abnormal involuntary movements (AIMs) in parkinsonian animals by acting at nicotinic acetylcholine receptors (nAChRs). Identification of the nAChR subtypes that mediate this reduction in AIMs is important as it will help in the development of nAChR subtype selective drugs for their treatment. Here we investigate the role of α6β2* nAChRs, a subtype selectively present in the nigrostriatal pathway, using α6 nAChR subunit null mutant (α6(-/-)) mice. Wildtype and α6(-/-) mice were lesioned by unilateral injection of 6-hydroxydopamine (3 μg/μl) into the medial forebrain bundle. They were then given L-dopa (3 mg/kg) plus benserazide (15 mg/kg) 2-3 wk later. L-dopa-induced AIMs developed to a similar extent in α6(-/-) and wildtype mice. However, AIMs in α6(-/-) mice declined to ~50% of that in wildtype mice with continued L-dopa treatment. Nicotine treatment also decreased AIMs by ~50% in wildtype mice, although not in α6(-/-) mice. There were no effects on parkinsonism under any experimental condition. To conclude, the similar declines in L-dopa-induced AIMs in nicotine-treated wildtype mice and in α6(-/-) mice treated with and without nicotine indicate an essential role for α6β2* nAChRs in the maintenance of L-dopa-induced AIMs. These findings suggest that α6β2* nAChR drugs have potential for reducing L-dopa-induced dyskinesias in Parkinson’s disease.
alpha6; dyskinesia; L-dopa; nicotine; 6-hydroxydopamine; Parkinson’s disease
A promising target for improved therapeutics in Parkinson's disease is the nicotinic acetylcholine receptor (nAChR). nAChRs are widely distributed throughout the brain, including the nigrostriatal system, and exert important modulatory effects on numerous behaviors. Accumulating evidence suggests that drugs such as nicotine that act at these sites may be of benefit for Parkinson's disease treatment. Recent work indicates that a potential novel therapeutic application is the use of nicotine to reduce levodopa-induced dyskinesias, a side effect of dopamine replacement therapy for Parkinson's disease. Several clinical trials also report that nicotine may diminish disease symptoms. Not only may nAChR drugs provide symptomatic improvement, but they may also attenuate the neurodegenerative process itself. This latter idea is supported by epidemiological studies which consistently demonstrate a ~50% reduced incidence of Parkinson's disease in smokers. Experimental work in parkinsonian animal models suggests that nicotine in tobacco may contribute to this protection. These combined findings suggest that nicotine and nAChR drugs offer the possibility of improved therapeutics for Parkinson's disease.
Nicotine; nicotinic receptors; levodopa; dyskinesias; neuroprotection; parkinsonian; 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 which demonstrate 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, including α4β2, α6β2 and/or α7 containing nAChRs, 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 anti-depressant 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.
Neuroprotection; Nicotine; Nicotinic; Nigrostriatal damage; Parkinson’s disease
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.
Addiction; Cyclic voltammetry; Nucleus accumbens; Parkinson’s disease; Striatum
L-dopa therapy for Parkinson's disease leads to dyskinesias or abnormal involuntary movement (AIMs) for which there are few treatment options. Our previous data showed that nicotine administration reduced L-dopa-induced AIMs in parkinsonian monkeys and rats. To further understand how nicotine mediates its antidyskinetic action, we investigated the effect of nicotinic receptor (nAChR) agonists in unilateral 6-OHDA-lesioned rats with varying striatal damage. We first tested the drugs in L-dopa-treated rats with a near-complete striatal dopamine lesion (>99%), the standard rodent dyskinesia model. Varenicline, an agonist that interacts with multiple nAChRs, did not significantly reduce L-dopa-induced AIMs, while 5-iodo-A-85380 (A-85380), which acts selectively at α4β2* and α6β2* subtypes, reduced AIMs by 20%. By contrast, both varenicline and A-85380 reduced L-dopa-induced AIMs by 40–50% in rats with a partial striatal dopamine lesion. Neither drug worsened the antiparkinsonian action of L-dopa. The results show that selective nicotinic agonists reduce dyskinesias, and that they are optimally effective in animals with partial striatal dopamine damage. These findings suggest that presynaptic dopamine terminal α4β2* and α6β2* nAChRs are critical for nicotine’s antidyskinetic action. The current data have important implications for the use of nicotinic receptor-directed drugs for L-dopa-induced dyskinesias, a debilitating motor complication of dopamine replacement therapy for Parkinson’s disease.
A-85380; dyskinesia; L-dopa; nicotine; nicotinic; varenicline
Despite a dramatic loss of nigrostriatal dopaminergic neurons in Parkinson’s disease, clinical symptoms only arise with 70–80% reduction of striatal dopamine. The mechanisms responsible for this functional compensation are currently under debate. Although initial studies showed an enhanced pre-synaptic dopaminergic function with nigrostriatal degeneration, more recent work suggests that functional compensation is not dopamine-mediated. To address this issue, we used cyclic voltammetry to directly measure endogenous dopamine release from striatal slices of control monkeys and animals with a moderate or severe MPTP-induced dopaminergic lesion. The moderately lesioned monkeys were asymptomatic, while the severely lesioned animals were parkinsonian. In monkeys with a moderate lesion, a 300% increase was obtained in endogenous striatal dopamine release. In contrast, in striatal slices from severely lesioned animals, a small % of evoked dopamine signals were similar in amplitude to control while the greater majority were undetectable. These findings suggest that pre-synaptic dopaminergic compensation develops in residual dopaminergic terminals with moderate lesioning, but that this response is lost with severe nigrostriatal damage. Such an interpretation is supported by the results of dopamine turnover studies. This enhanced pre-synaptic dopaminergic activity may be important in maintaining normal motor function during the initial stages of Parkinson’s disease.
compensation; dopamine release; MPTP; non-human primate; Parkinson’s disease; voltammetry
We have previously demonstrated that Type I neuronal nitric oxide synthase (nNOS)-expressing neurons are sleep-active in the cortex of mice, rats, and hamsters. These neurons are known to be GABAergic, to express Neuropeptide Y (NPY) and, in rats, to co-express the Substance P (SP) receptor NK1, suggesting a possible role for SP in sleep/wake regulation. To evaluate the degree of co-expression of nNOS and NK1 in the cortex among mammals, we used double immunofluorescence for nNOS and NK1 and determined the anatomical distribution in mouse, rat, and squirrel monkey cortex. Type I nNOS neurons co-expressed NK1 in all three species although the anatomical distribution within the cortex was species-specific. We then performed in vitro patch clamp recordings in cortical neurons in mouse and rat slices using the SP conjugate tetramethylrhodamine-SP (TMR-SP) to identify NK1-expressing cells and evaluated the effects of SP on these neurons. Bath application of SP (0.03–1 μM) resulted in a sustained increase in firing rate of these neurons; depolarization persisted in the presence of tetrodotoxin. These results suggest a conserved role for SP in the regulation of cortical sleep-active neurons in mammals.
nitric oxide; NOS-1; bNOS; sleep homeostasis; cerebral cortex; neurogliaform; tac1; tac1r
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 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 α3/α6β2* and α4β2* 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.
Neuromelanin; Nicotine; Nicotinic receptor; Substantia nigra; Parkinson’s disease
The nicotine metabolite cotinine is an abundant long-lived bio-active compound that may contribute to the overall physiological effects of tobacco use. Although its mechanism of action in the central nervous system has not been extensively investigated, cotinine is known to evoke dopamine release in the nigrostriatal pathway through an interaction at nicotinic receptors (nAChRs). Because considerable evidence now demonstrates the presence of multiple nAChRs in the striatum, the present experiments were done to determine the subtypes through which cotinine exerts its effects in monkeys, a species that expresses similar densities of striatal α4β2* (nAChR containing the α4 and β2 subunits, but not α3 or α6) and α3/α6β2* (nAChR composed of the α3 or α6 subunits and β2) nAChRs. Competition binding studies showed that cotinine interacts with both α4β2* and α3/α6β2* nAChR subtypes in the caudate, with cotinine IC50 values for inhibition of 5-[125I]iodo-3-[2(S)-azetinylmethoxy]pyridine-2HCl ([125I]A-85380) and 125I-α-conotoxinMII binding in the micromolar range. This interaction at the receptor level is of functional significance because cotinine stimulated both α4β2* and α3/α6β2* nAChR [3H]dopamine release from caudate synaptosomes. Our results unexpectedly showed that nicotine evokes [3H]dopamine release from two α3/α6β2* nAChR populations, one of which was sensitive to cotinine and the other was not. This cotinine-insensitive subtype was only present in the medial caudate and was preferentially lost with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced nigrostriatal damage. In contrast, cotinine and nicotine elicited equivalent levels of α4β2* nAChR-mediated dopamine release. These data demonstrate that cotinine functionally discriminates between two α3/α6β2* nAChRs in monkey striatum, with the cotinine-insensitive α3/α6β2* nAChR preferentially vulnerable to nigrostriatal damage.
L-dopa is one of the best treatments for the motor symptoms of Parkinson’s disease. However, its use is limited by the fact that it provides only symptomatic relief and chronic therapy leads to dyskinesias. There is therefore a continual search for novel therapeutic approaches. Nicotine, a drug that acts at nicotinic acetylcholine receptors (nAChRs), has been shown to protect against nigrostriatal damage and reduce L-dopa-induced dyskinesias. NAChRs may therefore represent novel targets for Parkinson's disease management. Since there are multiple nAChRs throughout the body, it is important to understand the subtypes involved in striatal function to allow for the development of drugs with optimal beneficial effects. Here we discuss recent work from our laboratory which indicates that α6β2* and α4β2* nAChRs are key in regulating striatal dopaminergic function. Experiments in parkinsonian rats using cyclic voltammetry showed that both α6β2* and α4β2* nAChR-mediated evoked-dopamine release in striatal slices is affected by nigrostriatal damage. These subtypes also appear to be important for neuroprotection against nigrostriatal damage and the nicotine-mediated reduction in L-dopa-induced dyskinesias in parkinsonian animal models. Our combined findings indicate that α4β2* and α6β2* nAChRs may represent useful therapeutic targets for Parkinson’s disease.
Dopamine; Dyskinesias; Neuroprotection; Nicotinic receptors; Parkinson's; disease nigrostriatal damage
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 α4β2*, α6β2* and α7 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.
L-Dopa-induced dyskinesias; Neuroprotection; Nicotine; Nicotinic; Nigrostriatal; Parkinson’s disease
Nicotine reduces dopaminergic deficits in parkinsonian animals when administered before nigrostriatal damage. Here we tested whether nicotine is also beneficial when given to rats and monkeys with pre-existing nigrostriatal damage. Rats were administered nicotine before and after a unilateral 6-hydroxydopamine (6-OHDA) lesion of the medial forebrain bundle, and the results compared to those in which rats received nicotine only after lesioning. Nicotine pretreatment attenuated behavioral deficits and lessened lesion-induced losses of the striatal dopamine transporter, and α6β2* and α4β2* nicotinic receptors (nAChRs). In contrast, nicotine administered two weeks after lesioning, when 6-OHDA-induced neurodegenerative effects are essentially complete, did not improve these same measures. Similar results were observed in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys. Nicotine did not enhance striatal markers when administered to monkeys with pre-existing nigrostriatal damage, in contrast to previous data that showed improvements when nicotine was given to monkeys before lesioning. These combined findings in two animal models suggest that nicotine is neuroprotective rather than neurorestorative against nigrostriatal damage. Receptor studies with 125I-α-conotoxinMII (α-CtxMII) and the α-CtxMII analog E11A were next done to determine whether nicotine treatment pre- or post-lesioning differentially affected expression of α6α4β2* and α6(nonα4)β2* nAChR subtypes in striatum. The observations suggest that protection against nigrostriatal damage may be linked to striatal α6α4β2* nAChRs.
MPTP; nicotine; neuroprotection; neurorestoration; 6-OHDA; Parkinson's disease
Nicotine treatment has long been associated with alterations in α4β2* nicotinic acetylcholine receptor (nAChR) expression that modify dopaminergic function. However, the influence of chronic nicotine treatment on the α6β2* 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 α6β2* nAChR function and expression. Chronic nicotine via drinking water enhanced non-burst and burst endogenous dopamine release from rat striatal slices. In control animals, α6β2* nAChR blockade with α-conotoxinMII (α-CtxMII) decreased release with non-burst stimulation but not with burst firing. These data in control animals suggest that varying stimulus frequencies differentially regulate α6β2* nAChR-evoked dopamine release. In contrast, in nicotine-treated rats, α6β2* nAChR blockade elicited a similar pattern of dopamine release with non-burst and burst firing. To elucidate the α6β2* nAChR subtypes altered with chronic nicotine treatment, we used the novel α-CtxMII analogue E11A, in combination with α4 nAChR knockout mice. 125I-α-CtxMII competition studies in striatum of knockout mice showed that nicotine treatment decreased the α6α4β2* subtype, but increased the α6(nonα4)β2* nAChR population. These data indicate that α6β2* nAChR-evoked dopamine release in nicotine-treated rats is mediated by the α6(nonα4)β2* nAChR subtype, and suggest that the α6α4β2* nAChR and/or α4β2* nAChR contribute to the differential effect of higher frequency stimulation on dopamine release under control conditions. Thus, α6β2* nAChR subtypes may represent important targets for smoking cessation therapies and neurological disorders involving these receptors such as Parkinson's disease.
Paraquat, an herbicide widely used in the agricultural industry, has been associated with lung, liver, and kidney toxicity in humans. In addition, it is linked to an increased risk of Parkinson’s disease. For this reason, we had previously investigated the effects of paraquat in mice and showed that it influenced striatal nicotinic receptor (nAChR) expression but not nAChR-mediated dopaminergic function. Since non-human primates are evolutionarily closer to humans and may better model the effects of pesticide exposure in man, we examined the effects of paraquat on striatal nAChR function and expression in monkeys. Monkeys were administered saline or paraquat once weekly for six weeks, after which nAChR levels and receptor-evoked 3H-dopamine (3H-DA) release were measured in striatum. The functional studies showed that paraquat exposure attenuated dopamine (DA) release evoked by α3/α6β2* nAChRs, a subtype present only on striatal dopaminergic terminals, with no decline in release mediated by α4β2* nAChRs, present on both DA terminals and striatal neurons. Paraquat treatment decreased α4β2* but not α3/α6β2* nAChR expression. The differential effects of paraquat on nAChR expression and receptor-evoked 3H-DA release emphasize the importance of evaluating changes in functional measures. The finding that paraquat treatment has a negative impact on striatal nAChR-mediated dopaminergic activity in monkeys but not mice indicates the need for determining the effects of pesticides in higher species.
Epidemiological studies consistently demonstrate a reduced incidence of Parkinson's disease in smokers. As an approach to evaluate whether nicotine in tobacco may be involved in this apparent protective effect, we compared the effect of mainstream 1R4F cigarette smoke solutions, which contain chemicals inhaled by active smokers, and nicotine against 6-hydroxydopamine (6-OHDA)-induced toxicity in an in vitro cell culture system. For this purpose we used terminally differentiated SH-SY5Y neuroblastoma cells that exhibit a catecholaminergic phenotype and express nicotinic receptors. Cells were pre-incubated for 24 h in mainstream-cigarette smoke solutions (0.06, 0.2, or 0.6 cigarette puffs/ml) made from University of Kentucky 1R4F research brand cigarettes, followed by the addition of 6-OHDA for another 24–48 h. The 0.2, but not 0.06, puffs/ml dose, significantly protected against 6-OHDA-induced toxicity in SH-SY5Y cells. This dose yielded final nicotine concentrations of ~5 × 10−7 M, which is similar to plasma smoking levels. Although the 0.6 puffs/ml dose caused significant toxicity on its own, it also appeared to protect against 6-OHDA-induced damage. We next tested the effect of nicotine, as well as its metabolite cotinine. These agents protected against the toxic effects of 6-OHDA in SH-SY5Y cells at concentrations ranging from 10−7 to 10−5 M. These combined results support the idea that nicotine is one of the components in cigarette smoke that has a protective effect against neurotoxic insults. These data suggest that nicotine may be of potential therapeutic value for Parkinson's disease.
Neuroprotection; Cigarette smoke; Nicotine; Cotinine; 6-Hydroxydopamine; Parkinson’s disease
Parkinson’s disease is a debilitating neurodegenerative movement disorder characterized by damage to the nigrostriatal dopaminergic system. Current therapies are symptomatic only and may be accompanied by serious side effects. There is therefore a continual search for novel compounds for the treatment of Parkinson’s disease symptoms, as well as to reduce or halt disease progression. Nicotine administration has been reported to improve motor deficits that arise with nigrostriatal damage in parkinsonian animals and in Parkinson’s disease. In addition, nicotine protects against nigrostriatal damage in experimental models, findings that have led to the suggestion that the reduced incidence of Parkinson’s disease in smokers may be due to the nicotine in tobacco. Altogether, these observations suggest that nicotine treatment may be beneficial in Parkinson’s disease. Nicotine interacts with multiple nicotinic receptor (nAChR) subtypes in the peripheral and central nervous system, as well as in skeletal muscle. Work to identify the subtypes affected in Parkinson’s disease is therefore critical for the development of targeted therapies. Results show that striatal α6β2-containing nAChRs are particularly susceptible to nigrostriatal damage, with a decline in receptor levels that closely parallels losses in striatal dopamine. In contrast, α4β2-containing nAChRs are decreased to a much smaller extent under the same conditions. These observations suggest that development of nAChR agonists or antagonists targeted to α6β2-containing nAChRs may represent a particularly relevant target for Parkinson’s disease therapeutics.
α-ConotoxinMII; Nicotine; Nicotinic; Parkinson’s disease; Nigrostriatal; Striatum
Several lines of evidence demonstrate that the striatal enkephalinergic system may be involved in the development of LIDs. Preproenkephalin-B (PPE-B) transcript levels are elevated with LIDs and there are also declines in κ- and other opioid receptors in different regions of the basal ganglia. If reduced κ-opioid receptors are linked to LIDs, it is possible that drugs that stimulate this subtype may decrease dyskinesias. We therefore initiated experiments to investigate the effect of κ-opioid receptor activation on LIDs. We first tested the selective κ-agonist U50,488 in rats with unilateral lesions of the nigrostriatal pathway. Chronic L-dopa treatment induced abnormal involuntary movements, including axial, orolingual and forelimb dyskinesias contralateral to the lesion. U50,488 administration prior to L-dopa treatment reduced these movements by 70%, suggesting that U50,488 has potential as an anti-dyskinetic treatment. We next tested its effect in a parkinsonian nonhuman primate model, which offers the advantage that parkinsonism and LIDs can clearly be differentiated and that the dyskinesias are similar to those in parkinsonian patients. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys were treated with L-dopa (5 mg/kg, p.o.) twice daily for 3 weeks to induce dyskinesias. As in the rodent model, U50,488 (0.1–1.0 mg/kg, i.m.) decreased LIDs in a dose-dependent fashion. However the anti-parkinsonian effect of L-dopa was similarly reduced, and side effects developed, including sedation and vomiting. These data suggest that κ-opioid agonists such as U50,488 may not be clinically useful antidyskinetic agents because they also reverse the anti-parkinsonian effect of L-dopa.
dyskinesia; rat; monkey; U50; 488; Parkinson’s disease