Parkinson's disease is associated with a preferential degeneration of the nigrostriatal DA pathway (Hornykiewicz,
1975). The loss of DA modulation triggers a complex series of neurochemical, anatomical, and electrophysiological alterations that lead to persistent changes in striatal neurons and their signaling pathways. For instance, striatonigral MSNs develop D1 receptor supersensitivity and reduced expression of D1 receptors (Gerfen et al.,
1990). Elevations in D2 receptor protein and mRNA are also observed together with increases in enkephalin expression in striatopallidal MSNs (Gerfen et al.,
1990). A substantial population of corticostriatal terminals also expresses D2 receptors (Wang and Pickel,
2002) which become hypersensitive in the absence of DA innervation (Calabresi et al.,
1993; Bamford et al.,
2004; Picconi et al.,
2004). The loss of DA tone on these D2 heteroreceptors is likely to result in enhanced glutamatergic transmission and altered NMDA receptor function (Meshul et al.,
1999; Nash et al.,
1999; Betarbet et al.,
2004). Alterations in dendritic spine morphology and complexity have also been described in both DA-depleted rats and patients with PD (Ingham et al.,
1989; Stephens et al.,
2005) which may occur preferentially in striatopallidal MSNs (Day et al.,
2006).
Most pathophysiological models and metabolic studies of PD predict that the net effect of these alterations is an imbalance in striatal output in which the indirect pathway becomes functionally hyperactive and the direct pathway is hypoactive (Marsden,
1982; Albin et al.,
1989; Alexander et al.,
1990; Hirsch et al.,
2000; see Figure C). Recent studies using optogenetic control of striatonigral and striatopallidal MSNs have provided substantial evidence supporting the validity of this pioneering basal ganglia model (Kravitz et al.,
2010). These studies by Kravitz and colleagues showed that bilateral activation of striatopallidal MSNs elicits a parkinsonian state characterized by freezing, bradykinesia, and decreased locomotion. Stimulation of striatonigral MSNs reduced freezing and facilitated locomotion. Additionally, stimulation of striatonigral MSNs reversed motor deficits observed in parkinsonian mice (Kravitz et al.,
2010).
The above model has also served as the framework for recent studies aimed at understanding how changes in striatal DA transmission impact the temporal dynamics and plasticity of cortico-basal ganglia transmission. For instance, recent studies have suggested that striatopallidal neurons in DA-depleted animals become more responsive to corticostriatal inputs and as a result, exhibit bursts of spike activity which correlates with cortical oscillations (Tseng et al.,
2001; Mallet et al.,
2006; Walters et al.,
2007). Thus, loss of D2 receptor-mediated inhibition of striatopallidal neurons and their corticostriatal inputs results in the unfiltered spreading of cortical rhythms to the GP and other components of the basal ganglia and some of the modifications in neuron activity that may underlie the pathophysiology of PD (Murer et al.,
2002). In addition to increased spike activity (Mallet et al.,
2006), striatopallidal neurons recorded in DA-depleted mice also exhibit a selective loss of endocannabinoid-dependent LTD (Kreitzer and Malenka,
2007). Further studies by Surmeier et al. (
2009) have shown that following DA depletion, the pairing of presynaptic and postsynaptic activity, in any order, induced LTD in D1 receptor-expressing MSNs and LTP in D2 receptor-expressing MSNs in an exclusive manner which prohibited bidirectional plasticity (Shen et al.,
2008). These observations indicate that, following striatal DA depletion, activity-dependent changes in the strength of corticostriatal synaptic transmission occur in parallel with alterations in intrinsic excitability (Surmeier et al.,
2009).
While the role of NO–sGC–cGMP signaling in the above pathological changes induced in the parkinsonian striatum remains to be fully characterized, it is clear that disruption of striatal NO–sGC–cGMP signaling cascades results in profound changes in behavioral, electrophysiological, and molecular responses to pharmacological manipulations of DA and glutamate transmission (Morris,
1995; Greengard,
2001; West et al.,
2002; Del Bel et al.,
2005; Ondracek et al.,
2008; Threlfell et al.,
2009; Sammut et al.,
2010; West et al., 2009). For example, studies by Del Bel et al. (
2005) have shown that striatal nNOS interneurons play a critical role in the generation of motor behavior. Systemic and intrastriatal exposure to NOS and sGC inhibitors has been shown to depress basal locomotion and induce catalepsy (Stewart et al.,
1994; Del Bel et al.,
2004; Echeverry et al.,
2007). Pharmacological disruption of NO function also potentiates catalepsy induced via D2-class receptor antagonists (Del Bel and Guimaraes,
2000; Cavas and Navarro,
2002). Furthermore, motor activation stimulated by substance P (Mancuso et al.,
1994), NMDA receptor antagonists (Deutsch et al.,
1996), and DA receptor agonists (Starr and Starr,
1995) is suppressed following systemic administration of NOS inhibitors.
The above behavioral studies demonstrate that in animals with an intact DA system, striatal NO–sGC–cGMP transmission is likely to play an important role in facilitating locomotor activity. It is less clear how this role for NO–sGC–cGMP signaling may change in the parkinsonian striatum. Various measures of striatal NOS activity have indicated that NO signaling may be disrupted in patients with PD (Bockelmann et al.,
1994; Eve et al.,
1998) and DA-depleted rats (De Vente et al.,
2000; Sahach et al.,
2000; Barthwal et al.,
2001; Sancesario et al.,
2004). With regard to NOS activity measured in DA-depleted rodents, however, conflicting outcomes have been reported between the above studies performed in rats and studies of mouse models (see Chalimoniuk and Langfort,
2007; Chalimoniuk et al.,
2004). Moreover, conflicting results have been reported in studies measuring cGMP levels in striatal tissue from DA-depleted rodents (De Vente et al.,
2000; Chalimoniuk et al.,
2004; Sancesario et al.,
2004; Chalimoniuk and Langfort,
2007; Giorgi et al.,
2008). Interestingly, PDE mRNA, protein, and activity are elevated in DA-depleted rats (Sancesario et al.,
2004; Giorgi et al.,
2008), indicating that cyclic nucleotide metabolism is elevated in PD. While speculative, this may be a compensatory homeostatic change induced in MSNs in response to overactivity resulting from striatal DA depletion (i.e., an attempt by the MSNs to reverse cyclic nucleotide over-production). In support of this, a subpopulation of MSNs (40%) has been reported to be hyper-responsive to NO generators (Galati et al.,
2008). Therefore, upregulation of PDE activity would be expected to compensate for this abnormal responsivity of these MSNs to NO. A parallel down-regulation of nNOS activity and NO signaling would also facilitate the normalization of cyclic nucleotide levels following DA depletion. As indicated above, there is strong evidence for this in both DA-depleted rats (De Vente et al.,
2000; Sahach et al.,
2000; Barthwal et al.,
2001; Sancesario et al.,
2004) and post-mortem tissue from patients with PD (Bockelmann et al.,
1994; Eve et al.,
1998). However, studies examining how parallel measures of nNOS, sGC, cGMP, PKG, and various PDEs change across time following DA depletion are needed to clarify the complex pathophysiological and homeostatic changes in this signaling pathway in PD.
Given the above findings, it is likely that striatal DA denervation results in transient and dynamic alterations in the synthesis (NOS and sGC dependent) and degradation (PDE dependent) of striatal cyclic nucleotides. These complex changes are likely to be complicated further by
l-DOPA treatment. Indeed, in patients with PD, serum levels of cGMP are reportedly increased following
l-DOPA therapy (Chalimoniuk and Stepien,
2004). However, studies in DA-depleted rats have shown that
l-DOPA-induced dyskinesias are associated with decreased striatal cyclic nucleotide levels (Giorgi et al.,
2008; Picconi et al.,
2011). Moreover, the non-selective PDE inhibitor zaprinast was shown to reverse decreases in striatal cyclic nucleotide levels and abnormal involuntary movements induced by
l-DOPA administration (Giorgi et al.,
2008; Picconi et al.,
2011). Calabresi and colleagues also showed that PDE inhibition can rescue abnormal synaptic plasticity observed in dyskinetic rats (Picconi et al.,
2011). On the other hand, studies aimed at decreasing NO signaling (and presumably cGMP levels) have shown that co-administration of NOS inhibitors with
l-DOPA attenuates
l-DOPA-induced dyskinesias (Padovan-Neto et al.,
2009). NOS inhibition also improved the motor performance of the same animals on a rotorod test (Padovan-Neto et al.,
2009). Taken together, the above studies suggest that under some circumstances, drugs with opposite pharmacological profiles (PDE inhibitors increase cGMP, nNOS inhibitors decrease cGMP) may both be beneficial for reversing
l-DOPA-induced dyskinesias. Further studies determining the time course of
l-DOPA-induced changes in cyclic nucleotide levels in the absence and presence of these inhibitors should open new avenues which will be essential for understanding and treating PD and side effects associated with
l-DOPA therapy.