Knowledge of the mechanism(s) whereby nicotine results in its beneficial effects in Parkinson’s disease is very important because this would allow for the development of selective therapies, with a minimum of adverse side effects. Nicotine generally exerts its effects in the peripheral and central nervous system by stimulating nAChRs. These are pentameric ligand-gated ion channels composed of α and β subunits or only of α subunits [
20,
59-
63]. Importantly, the nAChRs in the peripheral nervous system and the brain are distinct from each other, providing an opportunity of selective targeting of the desired receptors. The nAChRs of most relevance to the pathophysiology of Parkinson’s disease are most likely the ones in the nigrostriatal tract, although receptors in other brain regions may also play critical roles. Identification of these nAChRs has proved quite challenging because multiple subtypes are expressed with six different α (α2, α3, α4, α5, α6, α7) and three different β (β2, β3, β4) subunits in the nigrostriatal pathway. Accumulating studies indicate that the primary subtypes in the striatum are the α4β2* and α6β2* receptors, together with a smaller population of α7 nAChRs [
19,
20,
59,
60,
63] (the asterisk indicates the possible presence of other subunits in the receptor complex). Evidence for their presence stems from a wide variety of approaches, including work with nAChR null mutant mice, nAChR subtype selective antibodies and lesion studies [
20,
59,
60,
64]. Acetylcholine, released from tonically active striatal cholinergic interneurons, interacts with the α4β2* and α6β2* nAChRs to modulate dopamine release and consequently dopaminergic function [
65]. Recent studies in striatal slices using fast scan cyclic voltammetry, show that the α6β2* nAChR population may play a particularly prominent role in modulating evoked dopamine release, with a considerable proportion of electrically stimulated dopamine release regulated by this subtype in both rat and monkey striatum [
19,
66-
69] ().
Work now indicates that the α6β2* nAChRs are composed of several subtypes, including the α6α4β2* and α6(nonα4)β2* populations [
70,
71]. Interestingly, these two α6β2* subtypes appear to be differentially regulated with chronic nicotine dosing [
69]. Evidence for this stems from recent results using cyclic voltammetry, which show a differential regulation of α6β2* nAChR mediated dopamine release from striatal slices under non-burst (1 pulse) and burst (4-pulse) stimulus conditions in control rats. This is no longer observed after long-term nicotine treatment, suggesting that the normal regulation of burst-stimulated dopamine release is disrupted [
69]. Further work with nAChR null mutant mice indicate that the α6α4β2* nAChR population may be important for this differential effect of higher frequency stimulation on dopamine release [
69].
Not only are the α6α4β2* and α6(nonα4)β2* nAChRs subtypes variably affected by nicotine exposure, but they also appear to be differentially affected by nigrostriatal damage. For instance, the α6α4β2* nAChR subtype is more susceptible to lesioning than the α6(nonα4)β2* or α4β2* nAChR populations in parkinsonian rodents and monkeys [
72]. Similar findings were also observed in the brains of Parkinson’s disease patients [
72] (). These data may indicate that the α6α4β2* nAChR subtype is expressed on dopaminergic terminals that are more susceptible to nigrostriatal degeneration. Such a possibility is consistent with studies showing that some dopaminergic neuron populations are more sensitive to neurodegenerative insults than others, which may relate to the presence of specific molecular markers on certain dopaminergic neurons but not others. Examples include calbindin, whose presence is associated with nigrostriatal dopamine neuron survival (Gerfen et al., 1987a; German et al., 1992; Liang et al., 1996). By contrast, neuromelanin has been negatively linked to dopaminergic neuron survival in some studies (Herrero et al., 1993; Hirsch et al., 1988; McCormack et al., 2004; Zecca et al., 2003). The preferential loss of the α6α4β2* subtype with initial nigrostriatal damage, subsequently followed by the α6(nonα4)β2* and α4β2* nAChR subtypes, may suggest that these different nAChR subtypes are expressed on dopaminergic neurons with varying repertoires of molecular markers.
Nicotine-mediated neuroprotection against nigrostriatal damage also appears to involve various nAChR subtypes. That protection is receptor-mediated is apparent from work showing a decline with nAChR blockade [
73]. Evidence for an involvement of the α4β2* nAChR subtype is based on studies showing a loss of nicotine-mediated protection in α4β2* nAChR knockout mice with nigrostriatal damage [
74]. A role for select α6β2* nAChR subtypes in neuroprotection stems from work in lesioned rats [
41], using the neurotoxin α-CtxMII E11A to differentiate between α6α4β2* and α6(nonα4)β2* subtypes [
72]. Receptor competition studies in control and lesioned rats treated with and without nicotine showed that the α6α4β2* subtype is present only when nicotine-mediated protection is observed. This suggests that this latter subtype may be necessary for neuroprotection and a target for the development of protective strategies against Parkinson’s disease. The mechanisms whereby an interaction at α6α4β2* nAChR results in protection against nigrostriatal damage are not yet known as the intracellular signaling pathways linked to this specific nAChR subtype remain to be elucidated. However, based on knowledge of other nAChR subtypes, one might anticipate that changes in multiple downstream pathways are involved, such as alterations in various kinases, including phosphatidylinositol 3-kinase (PI3K), Akt, mitogen-activated protein kinase (MAPK) and jnk kinase, caspases 3, 8 and 9, nitric oxide synthase, the cell survival protein Bcl-2, and other intracellular events [
25,
60,
75]. Activation of these intracellular messengers may modulate immune responsiveness to enhance neuronal function/integrity [
76,
77]. Alternatively, or as well, they may activate different trophic factors, such as brain-derived neurotrophic factor (BDNF) and/or basic fibroblast growth factor-2 (FGF-2) levels, which are implicated in neuroprotection against nigrostriatal damage [
78-
81].
The ability of nicotine to reduce L-dopa-induced dyskinetic-like movements most likely also occurs via an interaction at nAChRs since improvement is not observed in the presence of the nAChR blocker mecamylamine [
82]. It is currently not known which striatal nAChRs populations are involved in the nicotine-mediated decline in L-dopa-induced dyskinesias. However, since the primary receptors in the brain, and specifically the nigrostriatal pathway, are the α4β2*, α6β2* and α7 subtypes, we anticipate these may be important [
19,
20,
59,
60,
63]. With respect to brain region, we expect that the receptors present in the nigrostriatal pathway play a role; however, it is also possible that nAChRs in other brain regions are key. This idea is based on extensive evidence that multiple neurotransmitter systems are implicated in the development of L-dopa-induced dyskinetic-like movements [
17,
56]. Such a possibility is not unexpected since the striatum functions in an integrated fashion with the globus pallidus, thalamus, various cortical areas, subthalamic nucleus, cerebellum, and other areas [
17,
56].
Altogether these findings suggest a role for α4β2*, α6β2* and/or α7 nAChR-directed ligands for Parkinson’s disease therapeutics. Continued work is critical to identify the specific nAChR subtypes involved in neuroprotection and for improvement in L-dopa-induced dyskinesias. Such knowledge will allow for the development of nAChR drugs with optimal beneficial and a minimum of adverse effects for Parkinson’s disease management.