Nicotinic receptors as CNS targets for Parkinson’s disease

Abstract

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.

1. Parkinson’s disease and the nicotinic cholinergic system

The pathological hallmarks of Parkinson’s disease are the presence of intracellular Lewy bodies and an extensive degeneration of the nigrostriatal dopaminergic system. [1–4]. There is a ≥70% decline in striatal dopamine and ≥50% loss of nigral dopaminergic neurons with the onset of clinical symptoms, which include bradykinesia, rigidity, and tremor [1–4].

Although Parkinson’s disease has primarily been considered a dopaminergic disorder, it is becoming increasingly clear that multiple CNS systems are involved in its pathogenesis [5–7]. Braak and coworkers have also identified Lewy bodies in numerous non-dopaminergic brain regions including the locus coeruleus, raphe nuclei, thalamus, amygdala, olfactory nuclei, pedunculopontine nucleus, and cerebral cortex [5, 6]. These observations are in agreement with much earlier studies, which indicated that multiple CNS neuronal systems are affected in Parkinson’s disease [8, 9]. Significant declines have been observed in molecular markers of many neurotransmitters and neuromodulators, such as enkephalin, somatostatin, cholecystokinin, and substance P, as well as changes in the noradrenergic, glutamatergic, serotonergic, and cholinergic systems [8, 9].

This review focuses on the involvement of the nicotinic cholinergic system in Parkinson’s disease for the following reasons. First, its close association and functional interaction with the nigrostriatal dopaminergic system suggests that nicotine or nicotinic agonists have potential in treatment of the motor symptoms of the disease [10, 11]. In addition, the finding that stimulation of the nicotinic cholinergic system ameliorates cognitive declines [12] suggests that nicotinic acetylcholine receptor (nAChR) drugs may be useful in treating some of the nondopaminergic deficits associated with Parkinson’s disease, such as dementia that develops in 50% of Parkinson’s disease patients with disease progression [1–4]. Lastly, there is very compelling epidemiological evidence demonstrating a negative association between smoking and Parkinson’s disease that has been attributed to the nicotine in tobacco, at least in part [11, 13–17].

Altogether these observations suggest that the nicotinic cholinergic system represents a promising direction for much-needed therapies for Parkinson’s disease. Moreover, the identification of unique nAChR subtypes in the nigrostriatal system (as discussed later) may allow for the development of drugs that selectively target this debilitating neurological disorder.

2. Nicotine and nicotinic agonists for neuroprotection

A wealth of epidemiological studies has demonstrated an inverse correlation between smoking and development of Parkinson’s disease. The decreased incidence of Parkinson’s disease in smokers has consistently been demonstrated in both prospective and retrospective studies, is dose- and time-dependent, and does not appear to be due to increased mortality [11, 13–17]. Although the component in cigarette smoke that confers this apparent neuroprotective action remains to be identified, numerous studies using both culture systems and experimental animal models suggest that nicotine may play an important role, as detailed below.

2.1. Nicotine-mediated protection against toxic insults in culture models

Extensive evidence using culture systems has shown that nicotine protects against toxicity mediated by β-amyloid, glutamate, ethanol, trophic factor deprivation, arachidonic acid and others [11, 18–20]. Of more direct relevance are studies using cultured ventral mesencephalic dopaminergic neurons which show that nicotine pre-treatment attenuates toxicity-induced by MPP⁺, the metabolite of MPTP that selectively destroys dopaminergic terminals [21]. There is thus a firm basis for the hypothesis that nicotine has neuroprotective properties using in vitro systems.

2.2. Nicotine-mediated protection against toxic insults in experimental animal models

There is also a large body of literature demonstrating nicotine-mediated protection against dopaminergic damage in parkinsonian animal models [22]. In rats, nicotine pre-treatment protects against nigrostriatal degeneration induced by the dopaminergic neurotoxin 6-hydroxydopamine or by mechanical lesions [23–28]. A neuroprotective effect of nicotine is also observed in striatum of mice in which nigrostriatal damage is induced by the selective dopaminergic neurotoxin MPTP [24, 29–39]. It should be noted, however, that results are somewhat inconsistent in this model with protection in some but not all studies. Although the nature of this variability is uncertain, it may be due to the relatively large lesion generated with MPTP. This possibility stems from findings that neuroprotection is observed in mice treated with methamphetamine and paraquat, both of which result in milder damage to the nigrostriatal dopaminergic system [24, 40].

Nicotine-induced protection against striatal damage is also observed in MPTP-treated nonhuman primates, a model that may more closely resemble the human disease [41–43]. A chronic nicotine treatment regimen improved a variety of neurochemical markers in MPTP-lesioned nicotine treated animals, including tyrosine hydroxylase, the dopamine and vesicular monoamine transporters, dopamine levels and nAChR expression [42, 43]. In addition, nicotine treatment normalized lesion-induced overactivity of the nigrostriatal pathway and preserved synaptic plasticity lost with nigrostriatal damage [41].

These combined results from different experimental animal models support the idea that the reduced incidence of Parkinson’s disease in smokers may relate to the nicotine in tobacco.

3. Nicotine and nicotinic agonists for the symptomatic treatment of Parkinson’s disease

Results from animal studies also lend support to the idea that nicotine and/or nAChR agonists may prove useful for treatment of Parkinson’s disease symptoms. NAChR stimulation is well known to modulate locomotor activity in unlesioned animals [44], as well as ameliorate motor behaviors in animals with nigrostriatal damage. In rats with a unilateral 6-hydroxydopamine lesion, nicotine or nicotinic agonist treatment reduces apomorphine-induced turning behavior [45, 46]. Treatment with nicotine or the nicotinic agonist SIB-1508Y also improves the antiparkinsonian action of levodopa in MPTP-lesioned monkeys [47, 48]. Interestingly, nAChR activation also ameliorated deficits associated with nigrostriatal damage in tasks that evaluated aspects of attentional and executive cognitive functions [49].

The effect of smoking, and nicotine or nicotinic agonist treatment on Parkinson’s disease symptoms has also been investigated (Table 1). Smoking (possibly acting via nicotine) reduced the tremor, rigidity, bradykinesia and gait disturbances observed in Parkinson’s disease, although effects were transient and short lasting [50]. In another report, improvement was noted in two elderly patients with Parkinson’s disease treated with the nicotine gum and patch for several months [51]. Variable results have been obtained in more recent small-scale clinical trials. Kelton and co-workers [52] observed improvements in areas of cognitive performance and motor measures after treatment with intravenous nicotine and the nicotine patch, with effects sustained for up to 1 month after the drug. However, Vieregge et al. observed no improvements in patients administered the patch for 3 weeks [53], although the lack of efficacy may have related to the fact that Parkinsonism was only assessed 3 weeks after patch cessation. On the other hand, Lemay et al. [54] also obtained no improvement in motor or cognitive deficits with immediate testing of Parkinson’s disease symptoms after 3 to 4 weeks of treatment. Several studies also investigated the effect of acute exposure (1 day) to the nicotine patch or gum, with improvement in one but a worsening of motor performance in Parkinson’s disease patients in two other studies [55, 56]. There was also no observable effect of the nicotinic agonist SIB-1508Y on parkinsonism after 4 weeks of treatment [57]. Possible reasons for these varying effects on Parkinson’s disease symptoms among the different studies may include differences in the severity of parkinsonism, the end-points under study, the study design (open-label versus blinded) and/or the dose, timing and mode of administration of nicotine.

Table 1.

Summary of the studies testing nicotine or a nicotinic agonist in Parkinson’s disease patients

Study	Test agent	Type of study	# subjects	Duration		Final dose/day	Anti-parkinsonian effect
				Dose titration	Dose maintenance
Ishikawa & Miyatake, 1993	Smoking and nicotine gum	Open-label	6	Chronic smoker		NA	Yes
Fagerstrom et al., 1994	Nicotine gum and patch	Open-label	2	≥ 7 mo		15 mg patch + gum	Yes
Clemens et al., 1995	Nicotine gum	Double-blinded placebo-controlled	48	ND	1 day	3 × 2 mg	No
Ebersbach et al., 1999	Nicotine patch	Double-blinded crossover	16	ND	12 hours	7 mg#	No
Kelton et al., 2000	Nicotine iv and patch	Open-label	15	2 wk	1 wk	14 mg	Yes
Vieregge et al., 2001	Nicotine patch	Double-blinded placebo-controlled	32	1 wk	2 wk	14 mg	No
Mitsuoka et al., 2002	Nicotine gum	Open-label	8	ND	1 day	NA	Yes
Lemay et al., 2004	Nicotine patch	Open-label	22	22 days	3 days	21 mg	No
Shoulson et al., 2006	SIB-1508Y	Double-blinded placebo controlled	77	2 wk	2 wk	10 mg	No

^#Used the 35 mg patch, of which 14 mg is absorbed over 24 h, or 7 mg over 12 h. ND – not done; NA, not available.

Similarly variable results have been obtained in nonhuman primates. In our results in nonhuman primates, nicotine administration via the drinking water did not modify Parkinsonism over an 8-week period [58], although Domino et al. [47] did obtain small effects on motor activity using a different short term injection treatment protocol. On the other hand, although nicotine treatment had no effect in the short term in our monkey studies, longer term administration via the drinking water (12 months) did improve striatal dopaminergic measures decreased with nigrostriatal damage [41–43]. These latter findings may suggest that nicotine treatment primarily has a protective and/or restorative effect with chronic use, but no acute symptomatic effects on Parkinsonism, at least at the doses tested to date.

4. Striatal cholinergic system

In order to understand how nAChR activation is involved in neuroprotection and/or symptomatic improvements in Parkinson’s disease, knowledge of the anatomical relationship between the striatal cholinergic and dopaminergic systems is critical. Two major neuronal cell types are present in the striatum. These include the GABAergic medium spiny projection neurons that comprise the greater majority (~95%) of neurons, as well as a much smaller population of interneurons (5%) of which the majority are GABAergic and about a third cholinergic. These larger cholinergic interneurons form a dense network in striatum that closely overlaps with dopaminergic terminals [10]. Thus high levels of acetylcholine, choline acetyltransferase and acetylcholinesterase [59] coincide with expression of dopamine, tyrosine hydroxylase and other dopaminergic markers [60]. Under physiological conditions, the cholinergic interneurons are tonically active [10]. They release acetylcholine, which subsequently interacts with nAChRs on striatal cell bodies and nerve terminals, including dopaminergic projections from the substantia nigra and glutamatergic afferents from the cortex [61–64]. Stimulation of nAChRs at these sites results in dopamine release mediated through various nAChR subtypes [65–67], as discussed in the following section.

5. Nicotinic receptors in the nigrostriatal system

Identification of the nAChRs that modulate dopaminergic function and characterization of changes in their expression with nigrostriatal damage is essential for the development of Parkinson’s disease therapies using nAChR ligands. NAChRs are pentameric ligand gated ion channels comprised of different combinations of α and β subunits to form heteromeric receptors, and select α subunits to form homomeric receptors [67–71]. There is a requirement for α subunits in the receptor complex, as these possess the recognition site for acetylcholine. The β subunit does not bind acetylcholine, however it modulates the interaction of the ligand with the α subunit, thereby affecting the physiological and pharmacological properties of the receptor [67–71]. Multiple nAChR subtypes (Fig. 1) have been identified in the nigrostriatal system using a variety of complementary experimental approaches.

Fig. 1.

Putative subunit composition of nAChR subtypes in rodent, monkey and human striatum. Note expression of the α4β2, α6α4β2β3, α6β2β3 and α7 nAChR subtypes across the three species. In contrast, the α4α5β2 and α6(nonα4)β2* nAChR subtypes appear to be expressed only in rodent striatum, while the α4α2β2 and α3β2* subtypes are expressed in monkey striatum. The * indicates the possible presence of other nAChR subunits in the receptor complex.

5.1. Rodents

Robust nAChR expression has been identified in the rodent nigrostriatal system using radiolabeled epibatidine, a ligand that labels multiple receptor subtypes. The use of other ligands such as radiolabeled nicotine, α-bungarotoxin and α-conotoxinMII has helped identify nAChR subtypes, including those containing the α4, α6 and α7 subunits [67, 69, 71]. These findings from receptor binding studies are in agreement with the results of in situ hybridization and RT-PCR work, which demonstrate the presence of α2 through α7 and β2 through β4 nAChR subunit transcripts in the striatum and/or substantia nigra [72–77]. The results of immunoprecipitation studies using subunit-selective nAChR antibodies suggest that the greater majority of these transcripts are translated into protein to form receptors [64, 78]. These show that all but the α2, α3 and β4 subunits are present in the striatum and/or substantia nigra of mice and rats. These data, coupled with the results of functional studies (³H-dopamine release from striatal synaptosomes) using wild-type and nAChR null mutant mice, indicate that the major nAChR subtypes in rodent striatum are α6α4β2β3, α6β2β3 and α4β2* (*indicates the possible presence of other subunits in the receptor complex) nAChRs (Fig. 1) [63, 64, 79, 80]. In contrast, α7 nAChRs are present at a much lower density, particularly in rats [81].

5.2. Monkeys

Multiple nAChRs have also been identified in striatum of nonhuman primates. nAChR subunit mRNAs in the primate nigrostriatal system include the α2, α4 through α7, and β2 through β4 transcripts [82, 83]. Receptor binding studies with ¹²⁵I-epibatidine demonstrate that receptors are expressed in both the striatum and substantia nigra [84–86]. Subsequent studies with more selective ligands, such as ¹²⁵I-α-conotoxinMII indicated the presence of α6β2* nAChR subtypes, in the nigrostriatal system [84, 87], while the use of ¹²⁵I-epibatidine in the presence of unlabelled α-conotoxinMII suggested the presence of α4β2* receptors. Immunoprecipitation studies with receptor subunit-directed antibodies show that all subunits except α5 and β4 are expressed in the primate striatum and substantia nigra. The major nAChR subtypes appear similar to those in the rodent and include α6α4β2β3, α6β2β3 and α4β2 (Fig. 1), with only very low α7 nAChR expression [87, 88]. There are some variations in the minor subtypes, with expression of α4α2β2 and α3β2* nAChRs in nonhuman primates, but not rodent striatum [87].

5.3. Humans

Although studies in human striatum are more limited, the nAChR subtypes appear to be similar to those in the rodent and monkey nigrostriatal systems [89, 90]. Evidence for this stems from the results of in situ hybridization and RT-PCR studies which demonstrate the presence of multiple nAChR transcripts including α3, α4, α7, and β2 mRNAs, with the others not yet investigated [91–94]. Radioligand binding studies show that ¹²⁵I-epibatidine, ³H-nicotine, ¹²⁵I-A85380, ¹²⁵I-α-bungarotoxin and ¹²⁵I-α-conotoxinMII all bind to human striatum suggesting the presence of nAChRs containing the α4, α3 and/or α6, α7, and β2 [89, 90, 95–104]. The α4, α6, β2 and β3 nAChR subunits have also been identified in human striatum using immunoprecipitation studies with subunit specific human nAChR antibodies [105]. These combined data suggest the presence of α4β2*, α6β2* and α7 nAChRs in human striatum (Fig. 1).

6. Alterations in striatal nAChRs with nigrostriatal damage

A key question is whether nAChRs are altered with nigrostriatal damage in experimental models and in Parkinson’s disease. Such knowledge is critical for the development of effective targeted therapies in Parkinson’s disease. To approach this, work has been done in animal models with nigrostriatal damage, as well as with tissue from Parkinson’s disease brains.

6.1. Rodents

The most commonly used parkinsonian rat model involves unilateral 6-hydroxydopamine injection into the striatum, substantia nigra or medial forebrain bundle. Nigrostriatal damage decreases high affinity nicotine binding sites that are thought to represent α4β2* nAChRs, while α7 nAChRs are unaffected [106, 107]. Nigrostriatal damage also leads to dramatic declines in binding of ¹²⁵I-α-conotoxinMII [108], a ligand that identifies α6β2* in rodent striatum [80, 109]. The loss of α6β2* receptors correlates well with declines in the striatal dopamine transporter, a dopaminergic neuronal marker [64, 78, 108], suggesting that this receptor subtype is located primarily on striatal dopaminergic nerve terminals. The results of immunoprecipitation studies using nAChR subunit-targeted antibodies support these findings and show that the largest reductions are in α6 and β3 subunit proteins, followed by smaller declines in α4, α5 and β2 subunits [64, 78]. There was no change in α7 nAChRs, while the α2, α3 and β4 subunit proteins are not expressed in rodent striatum.

These combined results indicate that there is a loss of both α4β2* and α6β2* nAChRs with nigrostriatal damage in rodents. The larger decrease in the latter population most likely occurs because α6β2* nAChRs are localized primarily on striatal dopaminergic terminals, while α4β2* receptors are expressed on both dopaminergic terminals and other striatal neurons.

6.2. Monkeys

These data in rodents correspond well with those in nonhuman primates with MPTP-induced nigrostriatal damage (Table 2). A moderate lesion led to declines predominately in α6β2* nAChRs that closely paralleled reductions in the dopamine transporter [84]. With severe nigrostriatal damage (≥90%), decreases were also evident in α4β2* nAChR although they were of smaller magnitude compared to those in α6β2* nAChRs [85, 110]. Immunoprecipitation studies with antibodies to human nAChR subunits showed that the major reductions with severe nigrostriatal damage were in the α6 and β3 subunits, followed by decreases in α4, β2, and α3 subunits [87]. There was no change in α7 nAChRs, while the α5 and β4 subunit proteins are not expressed in monkey striatum [87].

Table 2.

Summary of the declines in different nAChR subtypes in monkey striatum with nigrostriatal damage

nAChR subtype	Present on striatal dopamine terminals	Severity of lesion
		Moderate	Severe
α6α4β2β3	Yes	↓↓	↓↓
α6β2β3	Yes	-	↓
α4β2	Yes	-	↓
α4β2*	No	-	-
α7	No	-	-

The α3β2* nAChR subtype is also expressed in monkey striatum and decreased with lesioning. However, the effect of moderate versus severe nigrostriatal damage remains to be determined [87]. The α4β2* receptors may contain either α4β2 or α4α2β2 subunits. Moderate lesion, ≥ 50% decline in the dopamine transporter; severe lesion, ≥ 90% decline in the dopamine transporter; ↓ indicates a decline; - indicates no change.

In summary, α6β2* nAChRs appear particularly vulnerable to nigrostriatal damage as they are selectively decreased with a moderate lesion. In contrast, α4β2* subtypes are affected only with severe degeneration, while α7 nAChRs are unchanged (Table 2).

6.3. Humans

Results in brains of Parkinson’s disease cases were similar to those in parkinsonian animal models. Declines were observed in radiolabeled epibatidine, which targets most nAChR subtypes and ¹²⁵I-A85380, which identifies β2-containing nAChRs [89, 90, 111]. Receptor studies with ³H-nicotine also demonstrate ~50% decline in striatum, as well as in other areas including substantia nigra, cortex and hippocampus [98–101]. These latter findings support the contention that there is a generalized loss of neuronal integrity in Parkinson’s disease [5, 6]. As in the rodent and monkey studies, declines in α6β2* nAChRs in Parkinson’s disease brains are significantly greater than in α4β2* receptors in some striatal regions, as evaluated using receptor binding assays [103, 104]. This observation is supported by results from immunoprecipitation studies which showed that the α6 subunit is decreased by ≥80%, whereas the α4 and β2 subunits were decreased by ~50% [105].

Thus, both α4β2* and α6β2* nAChRs were decreased in Parkinson’s disease brains, with somewhat greater declines in this latter receptor population. There appeared to be little change in the α7 nAChR population, although declines have been observed in some studies [90, 111].

7. Receptor studies with the α-conotoxinMII analog E11A

7.1. Identification of multiple α6β2* nAChR subtypes in control striatum across species

As already mentioned, the α6β2* nAChR population may be of particular relevance to nigrostriatal function and to Parkinson’s disease [71]. This idea is based on results showing that its localization is fairly restricted in the CNS to the dopaminergic nigrostriatal and a few other catecholaminergic systems. In addition, declines in α6β2* nAChRs closely parallel the loss of the striatal dopamine transporter. Previous rodents studies have shown that mouse striatum expresses two α6β2* nAChR subtypes, those containing the α6α4β2β3 and α6β2β3 subunits [63, 79]. Similarly, our studies in monkeys suggest the presence of at least two striatal α6β2* nAChRs [87, 112]. In a continued effort to more precisely determine the subunit composition of these receptors, experiments were recently performed with α-conotoxinMII analogs to determine whether these might discriminate between α6β2* subtypes. One such analog α-conotoxinMII E11A, in which the glutamic acid at position 11 is replaced with alanine [113], inhibited ¹²⁵I-α-conotoxinMII binding to striatum from mice, monkeys and humans in a biphasic manner (Fig. 2) [88]. These data indicate that α-conotoxinMII E11A distinguishes between a very high (fM) and high affinity (pM) α6β2* subtype. Subsequent receptor studies showed that only the very high affinity site was lost in striatum of α4 null mutant mice (Fig. 2) [88]. Altogether, these data suggest that the very high affinity site is the α6α4β2β3 receptor and the high affinity site the α6β2β3 subtype.

Fig. 2.

Preferential decline in the striatal α6α4β2β3 nAChR subtype in animal models (mouse and monkey) of nigrostriatal damage and Parkinson’s disease. ¹²⁵I-α-conotoxinMII (0.5 nM) competition assays were performed using varying concentrations of the α-conotoxinMII analog E11A. In the upper left panel, biphasic inhibition curves (data fit best to a two site model) were obtained with E11A in the striatum of wild type mice suggesting the presence of at least two E11A-sensitive ¹²⁵I-α-conotoxinMII binding sites. In contrast monophasic inhibition (data fit best to a one site model) was obtained using striatal sections from α4 nAChR null mutant mice. Since these mice do not express the α4 subunit, the remaining high affinity-binding site is most likely the α6β2β3 nAChR, while the very high affinity-binding site that is lost may represent the α6α4β2β3 subtype. The three remaining panels depict the effect of nigrostriatal damage on the nAChR subtypes in mouse, monkey and human striatum. Biphasic inhibition curves (data fit best to two site model) were obtained with E11A under control conditions. These data suggest that E11A discriminates between at least two α6β2-containing nAChR populations, a very high α6α4β2β3 and a high affinity α6β2β3 nAChR subtype. Similar competition analyses using sections from lesioned animals and Parkinson’s disease striatum yielded monophasic curves, suggesting the loss of the very high affinity E11A-sensitive α6α4β2β3 nAChR subtype. Symbols represent means ± S.E.M. of 4–8 mice, 4–6 monkeys and 5 control and 4 Parkinson’s disease cases. Reproduced in modified form with permission from reference [88]

7.2. Preferential loss of the α6α4β2β3 nAChR subtype with nigrostriatal damage in mice, monkeys and humans

We subsequently investigated the effect of nigrostriatal lesioning on these two striatal α6β2* populations in two animal models and in Parkinson’s disease (Fig. 2). There was a preferential loss of the very high affinity E11A binding site (α6α4β2β3) in striatum of MPTP-treated mice, MPTP-treated monkeys and Parkinson’s disease cases [88]. This selective loss of the α6α4β2β3 binding site correlated well with reductions in striatal dopamine transporter [88]. In monkeys with moderate nigrostriatal damage (~50% reduction in dopamine transporter) there was a preferential reduction in the α6α4β2β3 subtype (Fig. 2 and 3), while the α6β2β3 and α4β2 nAChR populations were reduced primarily with severe lesions (≥ 90%) (Fig. 3; Table 2) [88]. The α6α4β2β3 subtype may thus provide a unique marker for dopamine nerve terminals that are particularly sensitive to nigrostriatal degeneration. This possibility is consistent with previous work indicating that dopaminergic afferents from the nigra to the striatum are variably susceptible to nigrostriatal insults [114, 115]. Studies to understand the molecular basis for this enhanced vulnerability have linked calbindin to nigrostriatal dopamine neuron survival [116–118] while neuromelanin has been negatively associated [119–122]. However, a definitive connection of either of these markers with nigrostriatal damage remains to be established. On the other hand, there was a fairly clear-cut relationship between the loss of the α6α4β2β3 nAChR and nigrostriatal damage, possibly indicating that this subtype identifies selectively vulnerable dopaminergic terminals. Studies to investigate this possibility may also provide insight about the mechanisms responsible for the nigrostriatal neurodegeneration.

Fig. 3.

Schematic representation of nAChR localization within the nigrostriatal pathway based on receptor binding, antibody immunoprecipitation and functional studies. The nigrostriatal dopaminergic terminals express three subtypes, the α4β2*, α6β2β3 and α6α4β2β3 nAChR populations. The results of lesion studies in animal models suggest that α6α4β2β3 subtype is localized to a select population of dopaminergic terminals that are particularly vulnerable to nigrostriatal insults (black “X”). The α4β2* and α6β2β3 subtypes are most likely present on terminals other than those expressing α6α4β2β3 nAChRs since they are reduced only with more severe dopaminergic lesions (grey “X”). It is not known whether these latter two subtypes are on the same or separate dopaminergic terminals. The precise postsynaptic localization of the α4β2* nAChR subtype in striatum is presently uncertain, but they may be present on cholinergic and/or GABAergic neurons. α7 nAChRs are located on glutamatergic afferent terminals from the cortex. ACh, acetylcholine; DA, dopamine; Glu, glutamate; SN, substantia nigra. The * indicates the possible presence of other nAChR subunits in the receptor complex.

8. Concluding remarks

Accumulating evidence suggests that nigrostriatal nAChRs represent potential targets for the treatment of Parkinson’s disease for symptomatic improvements and/or for long-term neuroprotection. Several nAChR populations have been identified in the nigrostriatal system. These include α4β2* and α6β2β3 subtypes that are decreased primarily with severe nigrostriatal degeneration, and the α6α4β2β3 subtype that is significantly reduced even with only moderate damage (Fig. 3). These latter findings suggest that the α6α4β2β3 nAChR subtype represents a marker for neurons particularly vulnerable to nigrostriatal damage. In addition, these data suggest that this receptor subtype may represent a particularly relevant target for Parkinson’s disease therapeutics as stimulation of the remaining α6α4β2β3 nAChRs may enhance function lost with nigrostriatal damage. This possibility stems from results showing that chronic nicotine treatment improved striatal integrity and function. On the other hand, nicotine is well known to desensitize or block nAChRs raising the question whether nicotine may induce its beneficial effects by blocking aberrant nAChR-mediated activity. This latter interpretation would suggest that an α6α4β2β3 nAChR antagonist (and not an agonist) may be the drug of choice to counteract the effects of nigrostriatal injury and/or provide symptomatic improvements. Continued studies to elucidate the role of α6β2* nAChRs in nigrostriatal function should help resolve what type of α6β2* nAChR ligand would prove most beneficial against the effects of nigrostriatal damage in neurological disorders such as Parkinson’s disease.

Abbreviations

nAChR

nicotinic acetylcholine receptor