Focus on α4β2* and α6β2* nAChRs for Parkinson's Disease Therapeutics

Abstract

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.

Conflict of interest statement

Conflicts of Interest

No potential conflicts of interest to disclose.

Figures

Figure 1. Progressive decrease in nAChR-mediated dopamine release with increasing nigrostriatal damage

A. Electrically stimulated dopamine release was determined using cyclic voltammetry in coronal rat brain slices containing the striatum. Release was measured in the dorsolateral striatum as this area is most affected with Parkinson’s disease. Release was elicited through a bipolar stimulating electrode and detected by a carbon fiber recording microelectrode manufactured in house as previously described (Perez et al., 2008). B. Sprague Dawley rats were lesioned with varying doses of 6-OHDA to achieve different degrees of striatal dopamine denervation. Representative traces of single-pulse stimulated total dopamine release show that release decreased proportionately with increasing nigrostriatal damage. C. Dopamine release was measured in the absence and presence of the α6β2* nAChR antagonist α-CtxMII (100nM) or the general nAChR blocker mecamylamine (100µM). α6β2* nAChR-mediated release was determined by subtracting release in the presence of α-CtxMII from total release. α4β2* mediated release was determined by subtracting release in the presence of mecamylamine from that in the presence of α-CtxMII. There were significant decreases in α6β2* and α4β2* nAChR-mediated release with nigrostriatal damage. Values represent the mean ± SEM of 4–9 rats. *p < 0.05; **p < 0.01; ***p < 0.001 indicate significance of difference from control using a Newman-Keuls multiple comparisons post hoc test. Taken in modified form with permission from Perez et al., 2010.

Figure 2. Schematic of the putative changes in neuronal nAChR expression occurring with moderate and near-complete nigrostriatal damage

Representative striatal dopaminergic (DA), GABAergic (GABA) and glutamatergic (GLU) neuronal populations are depicted. In the intact striatum (left panel), five different nAChR subtypes are expressed presynaptically on dopaminergic terminals: α6 α4β2β3, α6β2β3, α6β2, α4 α5β2 and α4β2 nAChRs. α4β2* and α7 nAChRs are also expressed in post-synaptic GABAergic and glutamatergic neurons, respectively. Declines in DA terminal density are illustrated with a decrease in the color intensity. Partial lesions (middle panel) associated with 60–80% decreases in dopamine transporter result in a complete loss of dopamine terminals expressing α6α4β2β3 nAChRs (shown in gray). This receptor subtype is preferentially decreased with nigrostriatal damage and thus is hypothesized to be localized to a selectively vulnerable population of dopamine terminals (61). In addition, smaller declines in dopamine terminals expressing other nAChRs are also observed (shown in lighter blue). This terminal loss is associated with a partial decrease in α6β2β3, α6β2, α4 α5β2 and α4β2 nAChR expression (as depicted by their lighter color and dashed outlines). It is currently unknown whether α6β2* and α4β2* nAChRs are co-expressed in the same dopamine terminals thus α4 α5β2 and α4β2 nAChRs have been placed in all dopamine neuron populations. All presynaptic α6β2* and α4β2* nAChRs are essentially lost with a near-complete loss of dopamine terminals (right panel) as illustrated by their white color and dashed outlines. By contrast, postsynaptic α4β2* nAChR are unaffected by nigrostriatal damage, as are α7 receptors.

Figure 3. NAChR agonists decrease L-dopa induced dyskinesias

Lesioned rats were administered varenicline (0.5 mg/kg/d) or A-85380 (0.37 umol/kg/d) twice daily at 8-h intervals for 4 consecutive days. L-dopa methyl ester (8 mg/kg s.c.) plus benserazide (15 mg/kg s.c.) was administered 10 min after the first dose of either agonist. After 4 days of treatment, total AIMS were evaluated as a sum of oral, forelimb and axial AIMS. Values represent the mean ± SEM of 8 animals per treatment group. Significance of difference from vehicle using a Mann-Whitney test or one-way repeated ANOVA followed by a Bonferroni post hoc test (for time course), *p < 0.05; **p < 0.01. Significant main effect of nicotine with time; #p < 0.05. Taken in modified form with permission from Huang et al., 2010.