Xiomara A. Perez

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 Parkinsons disease. This idea initially stemmed from the results of epidemiological studies that demonstrated that smoking is associated with a decreased incidence of Parkinsons 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 nicotines ability to reduce/halt the neuronal damage that arises in Parkinsons 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 Parkinsons disease.

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 Parkinsons 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 Parkinsons disease. Extensive studies in parkinsonian animals show that nicotine protects against nigrostriatal damage, findings that may explain the well-established decline in Parkinsons 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 Parkinsons disease. These combined observations suggest that nAChR stimulation may represent a useful treatment strategy for Parkinsons 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 Parkinsons disease, with a minimum of adverse side effects.

Long-term nicotine treatment differentially regulates striatal α6α4β2* and α6(nonα4)β2* nAChR expression and function

Nicotine treatment has long been associated with alterations in α4β2* nicotinic acetylcholine receptor (nAChR) expression that modify dopaminergic function. However, the influence of long-term 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. Long-term nicotine treatment via drinking water enhanced nonburst and burst endogenous dopamine release from rat striatal slices. In control animals, α6β2* nAChR blockade with α-conotoxin MII (α-CtxMII) decreased release with nonburst 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 nonburst and burst firing. To elucidate the α6β2* nAChR subtypes altered with long-term nicotine treatment, we used the novel α-CtxMII analog 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 Parkinsons disease.

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 Parkinsons disease.

Pre-synaptic dopaminergic compensation after moderate nigrostriatal damage in non-human primates.

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.

α6β2* and α4β2* nicotinic receptors both regulate dopamine signaling with increased nigrostriatal damage; relevance to Parkinson's disease

Nicotinic receptors (nAChRs) are important modulators of dopaminergic transmission in striatum, a region critical to Parkinsons disease. The nAChRs mainly involved are the α6β2* and α4β2* subtypes. Lesion studies show that the α6β2* receptor is decreased to a much greater extent with nigrostriatal damage than the α4β2* subtype raising the question whether this latter nAChR population is more important with increased nigrostriatal damage. To address this, we investigated the effect of varying nigrostriatal damage on α6β2* and α4β2* receptor-modulated dopamine signaling using cyclic voltammetry. This approach offers the advantage that changes in dopamine release can be observed under different neuronal firing conditions. Total single-pulse-evoked dopamine release decreased in direct proportion to declines in the dopamine transporter and dopamine uptake. We next used α-conotoxinMII and mecamylamine to understand the role of the α4β2* and α6β2* subtypes in release. Single-pulse–stimulated α6β2* and α4β2* receptor dopamine release decreased to a similar extent with increasing nigrostriatal damage, indicating that both subtypes contribute to the control of dopaminergic transmission with lesioning. Total burst-stimulated dopamine release also decreased proportionately with nigrostriatal damage. However, the role of the α4β2* and α6β2* nAChRs varied with different degrees of lesioning, suggesting that the two subtypes play a unique function with burst firing, with a somewhat more prominent and possibly more selective role for the α6β2* subtype. These data have important therapeutic implications because they suggest that drugs directed to both α4β2* and α6β2* nAChRs may be useful in the treatment of neurological disorders such as Parkinsons disease.

Prominent Role of α3/α6β2* 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 Parkinsons 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, α3/α6β2* nAChRs regulated ≥80% of non-burst- (single pulse) nAChR-modulated dopamine release, and α4β2* 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 α3/α6β2* 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.

Compensation in pre-synaptic dopaminergic function following nigrostriatal damage in primates

Clinical symptoms of Parkinsons disease only become evident after 70–80% reductions in striatal dopamine. To investigate the importance of pre‐synaptic dopaminergic mechanisms in this compensation, we determined the effect of nigrostriatal damage on dopaminergic markers and function in primates. MPTP treatment resulted in a graded dopamine loss with moderate to severe declines in ventromedial striatum (approximately 60–95%) and the greatest reductions (approximately 95–99%) in dorsolateral striatum. A somewhat less severe pattern of loss was observed for striatal nicotinic receptor, tyrosine hydroxylase and vesicular monoamine transporter expression. Declines in striatal dopamine uptake and transporter sites were also less severe than the reduction in dopamine levels, with enhanced dopamine turnover in the dorsolateral striatum after lesioning. The greatest degree of adaptation occurred for nicotine‐evoked [3H]dopamine release from striatal synaptosomes, which was relatively intact in ventromedial striatum after lesioning, despite > 50% declines in dopamine. This maintenance of evoked release was not due to compensatory alterations in nicotinic receptor characteristics. Rather, there appeared to be a generalized preservation of release processes in ventromedial striatum, with K+‐evoked release also near control levels after lesioning. These combined compensatory mechanisms help explain the finding that Parkinsons disease symptomatology develops only with major losses of striatal dopamine.

Role for the nicotinic cholinergic system in movement disorders; therapeutic implications

A large body of evidence using experimental animal models shows that the nicotinic cholinergic system is involved in the control of movement under physiological conditions. This work raised the question whether dysregulation of this system may contribute to motor dysfunction and whether drugs targeting nicotinic acetylcholine receptors (nAChRs) may be of therapeutic benefit in movement disorders. Accumulating preclinical studies now show that drugs acting at nAChRs improve drug-induced dyskinesias. The general nAChR agonist nicotine, as well as several nAChR agonists (varenicline, ABT-089 and ABT-894), reduces l-dopa-induced abnormal involuntary movements or dyskinesias up to 60% in parkinsonian nonhuman primates and rodents. These dyskinesias are potentially debilitating abnormal involuntary movements that arise as a complication of l-dopa therapy for Parkinsons disease. In addition, nicotine and varenicline decrease antipsychotic-induced abnormal involuntary movements in rodent models of tardive dyskinesia. Antipsychotic-induced dyskinesias frequently arise as a side effect of chronic drug treatment for schizophrenia, psychosis and other psychiatric disorders. Preclinical and clinical studies also show that the nAChR agonist varenicline improves balance and coordination in various ataxias. Lastly, nicotine has been reported to attenuate the dyskinetic symptoms of Tourettes disorder. Several nAChR subtypes appear to be involved in these beneficial effects of nicotine and nAChR drugs including α4β2*, α6β2* and α7 nAChRs (the asterisk indicates the possible presence of other subunits in the receptor). Overall, the above findings, coupled with nicotines neuroprotective effects, suggest that nAChR drugs have potential for future drug development for movement disorders.

Nicotine-mediated improvement in L-dopa-induced dyskinesias in MPTP-lesioned monkeys is dependent on dopamine nerve terminal function.

L-dopa-induced dyskinesias (LIDs) are abnormal involuntary movements that develop with long term L-dopa therapy for Parkinsons 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 nicotines ability to reduce LIDs correlated with lesion-induced changes in the striatal dopamine transporter and (3)H-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 (3)H-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 Parkinsons disease.