Cell type-specific plasticity of striatal projection neurons in parkinsonism and L-DOPA-induced dyskinesia

The striatum is widely viewed as the fulcrum of pathophysiology in Parkinson’s disease (PD) and L-DOPA-induced dyskinesia (LID). In these disease states, the balance in activity of striatal direct pathway spiny projection neurons (dSPNs) and indirect pathway spiny projection neurons (iSPNs) is disrupted, leading to aberrant action selection. However, it is unclear whether countervailing mechanisms are engaged in these states. Here we report that iSPN intrinsic excitability and excitatory corticostriatal synaptic connectivity were lower in PD models than normal; ​L-DOPA treatment restored these properties. Conversely, dSPN intrinsic excitability was elevated in tissue from PD models and suppressed in LID models. Although the synaptic connectivity of dSPNs did not change in PD models, it fell with ​L-DOPA treatment. In neither case, however, was the strength of corticostriatal connections globally scaled. Thus, SPNs manifested homeostatic adaptations in intrinsic excitability and in the number but not strength of excitatory corticostriatal synapses

Striatal mRNA expression patterns underlying peak dose L-DOPA-induced dyskinesia in the 6-OHDA hemiparkinsonian rat

L-DOPA is the primary pharmacological treatment for relief of the motor symptoms of Parkinson’s disease (PD). With prolonged treatment (⩾5 years) the majority of patients will develop abnormal involuntary movements as a result of L-DOPA treatment, known as L-DOPA-induced dyskinesia. Understanding the underlying mechanisms of dyskinesia is a crucial step toward developing treatments for this debilitating side effect. We used the 6-hydroxydopamine (6-OHDA) rat model of PD treated with a three-week dosing regimen of L-DOPA plus the dopa decarboxylase inhibitor benserazide (4 mg/kg and 7.5 mg/kg s.c., respectively) to induce dyskinesia in 50% of individuals. We then used RNA-seq to investigate the differences in mRNA expression in the striatum of dyskinetic animals, non-dyskinetic animals, and untreated parkinsonian controls at the peak of dyskinesia expression, 60 min after L-DOPA administration. Overall, 255 genes were differentially expressed; with significant differences in mRNA expression observed between all three groups. In dyskinetic animals 129 genes were more highly expressed and 14 less highly expressed when compared with non-dyskinetic and untreated parkinsonian controls. In L-DOPA treated animals 42 genes were more highly expressed and 95 less highly expressed when compared with untreated parkinsonian controls. Gene set cluster analysis revealed an increase in expression of genes associated with the cytoskeleton and phosphoproteins in dyskinetic animals compared with non-dyskinetic animals, which is consistent with recent studies documenting an increase in synapses in dyskinetic animals. These genes may be potential targets for drugs to ameliorate L-DOPA-induced dyskinesia or as an adjunct treatment to prevent their occurrence

Levodopa-induced dyskinesia in Parkinson disease: Current and Evolving Concepts.

Levodopa‐induced dyskinesia is a common complication in Parkinson disease. Pathogenic mechanisms include phasic stimulation of dopamine receptors, nonphysiological levodopa‐to‐dopamine conversion in serotonergic neurons, hyperactivity of corticostriatal glutamatergic transmission, and overstimulation of nicotinic acetylcholine receptors on dopamine‐releasing axons. Delay in initiating levodopa is no longer recommended, as dyskinesia development is a function of disease duration rather than cumulative levodopa exposure. We review current and in‐development treatments for peak‐dose dyskinesia but suggest that improvements in levodopa delivery alone may reduce its future prevalence

Optostimulation of striatonigral terminals in substantia nigra induces dyskinesia that increases after L‐DOPA in a mouse model of Parkinson's disease

Background and Purpose: L-DOPA-induced dyskinesia (LID) remains a major complication of L-DOPA therapy in Parkinson's disease. LID is believed to result from inhibition of substantia nigra reticulata (SNr) neurons by GABAergic striatal projection neurons that become supersensitive to dopamine receptor stimulation after severe nigrostriatal degeneration. Here, we asked if stimulation of direct medium spiny neuron (dMSN) GABAergic terminals at the SNr can produce a full dyskinetic state similar to that induced by L-DOPA. Experimental Approach: Adult C57BL6 mice were lesioned with 6-hydroxydopamine in the medial forebrain bundle. Channel rhodopsin was expressed in striatonigral terminals by ipsilateral striatal injection of adeno-associated viral particles under the CaMKII promoter. Optic fibres were implanted on the ipsilateral SNr. Optical stimulation was performed before and 24 hr after three daily doses of L-DOPA at subthreshold and suprathreshold dyskinetic doses. We also examined the combined effect of light stimulation and an acute L-DOPA challenge. Key Results: Optostimulation of striatonigral terminals inhibited SNr neurons and induced all dyskinesia subtypes (optostimulation-induced dyskinesia [OID]) in 6-hydroxydopamine animals, but not in sham-lesioned animals. Additionally, chronic L-DOPA administration sensitised dyskinetic responses to striatonigral terminal optostimulation, as OIDs were more severe 24 hr after L-DOPA administration. Furthermore, L-DOPA combined with light stimulation did not result in higher dyskinesia scores than OID alone, suggesting that optostimulation has a masking effect on LID. Conclusion and Implications: This work suggests that striatonigral inhibition of basal ganglia output (SNr) is a decisive mechanism mediating LID and identifies the SNr as a target for managing LID.Fil: Keifman, Ettel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Fisiología y Biofísica Bernardo Houssay. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Fisiología y Biofísica Bernardo Houssay; Argentina. Consejo Superior de Investigaciones Científicas; EspañaFil: Ruiz De Diego, Irene. Consejo Superior de Investigaciones Científicas; EspañaFil: Pafundo, Diego Esteban. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Fisiología y Biofísica Bernardo Houssay. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Fisiología y Biofísica Bernardo Houssay; ArgentinaFil: Paz, Rodrigo Manuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Fisiología y Biofísica Bernardo Houssay. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Fisiología y Biofísica Bernardo Houssay; ArgentinaFil: Solís, Oscar. Consejo Superior de Investigaciones Científicas; EspañaFil: Murer, Mario Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Fisiología y Biofísica Bernardo Houssay. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Fisiología y Biofísica Bernardo Houssay; ArgentinaFil: Moratalla, Rosario. Consejo Superior de Investigaciones Científicas; Españ

Striatal adaptations in experimental parkinsonism and L-DOPA-induced dyskinesia

Parkinson’s disease (PD) is a neurodegenerative disorder, characterized by the loss of dopamine (DA) producing neurons in the substantia nigra pars compacta (SNc), resulting in typical motor symptoms. DA replacement with L-DOPA is the standard therapy for PD. However, with treatment duration many patients face the severe treatment complication of L-DOPA-induced dyskinesia (LID), constituting in abnormal involuntary movements (AIMs). The etiology of PD and LID is largely unknown, but both pathophysiological states are linked to DA. How neurons in a DA-receptive brain region adapt to the pathophysiological states of PD and LID is the topic of this thesis’ work. The striatum is the “hub” into the basal ganglia network and implicated in movement control. Striatal spiny projection neurons (SPNs) divide into two subpopulations, forming the so-called direct and indirect pathway of the basal ganglia. Due to the expression of different DA receptors, direct and indirect pathway SPNs (dSPNs and iSPNs, respectively) are oppositely modulated by DA. D1 receptor (D1R) stimulation in the DA-denervated, parkinsonian striatum leads to a supersensitive activation of ERK1/2 in dSPNs. This aberrant signaling activation is widely believed to be a core mechanism leading to the development of LID. In the first study we investigated which signaling pathways participate in this D1R-induced ERK1/2 activation. We found a distinct and complex interaction between PKA- and Ca2+-dependent pathways, which is critically modulated by mGluR5. In the second study we further investigated the antidsykinetic profile of mGluR5 antagonist treatment, finding that the choice of animal model influences the outcome of antidyskinetic therapy testing. Striatal adaptations, sensitive to beneficial mGluR5 inhibition, appear not to be represented in only partially DA-denervated animals. In the last study we investigated possible homeostatic mechanisms in SPNs during PD and LID. We found that both iSPNs and dSPNs display potential homeostatic adaptations of excitability that are likely to counteract the loss of DA signaling and balance perturbations in firing activity. The changes were oppositely directed in iSPNs and dSPNs, reflecting the bidirectional modulation by DA. In contrast, PD-associated dendritic atrophy was found in both subpopulations and is independent of DAergic signaling. Synaptic adaptations in SPNs in PD and LID appeared not to follow homeostatic ruling. Specifically, we found that SPNs do not exhibit synaptic scaling, but rather selective elimination of spines. The failure to preserve the pattern of weighted synaptic inputs suggests that SPNs may not be able to appropriately regulate basal ganglia related behavior in PD and LID. Taken together, the results of this thesis reveal new molecular and physiological adaptations of SPNs in experimental models of PD and LID. Identifying if they are compensatory or maladaptive is difficult, but the more our understanding proceeds the better we can refine preclinical animal models and define potential treatment options for PD and LID