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L-DOPA-induced dyskinesia (LID) remains a major complication of Parkinson's disease management for which better therapies are necessary. The contribution of the striatonigral direct pathway to LID is widely acknowledged but whether the striatopallidal pathway is involved remains debated. Selective optogenetic stimulation of striatonigral axon terminals induces dyskinesia in mice rendered hemiparkinsonian with the toxin 6-hydroxydopamine (6-OHDA). Here we show that optogenetically-induced dyskinesia is increased by the D2-type dopamine receptor agonist quinpirole. Although the quinpirole effect may be mediated by D2 receptor stimulation in striatopallidal neurons, alternative mechanisms may be responsible as well. To selectively modulate the striatopallidal pathway, we selectively expressed channelrhodopsin-2 (ChR2) in D2 receptor expressing neurons by crossing D2-Cre and ChR2-flox mice. The animals were rendered hemiparkinsonian and implanted with an optic fiber at the ipsilateral external globus pallidus (GPe). Stimulation of ChR2 at striatopallidal axon terminals reduced LID and also general motility during the off L-DOPA state, without modifying the pro-motor effect of low doses of L-DOPA producing mild or no dyskinesia. Overall, the present study shows that D2-type dopamine receptors and the striatopallidal pathway modulate dyskinesia and suggest that targeting striatopallidal axon terminals at the GPe may have therapeutic potential in the management of LID.

La dopamina (DA) es un neurotransmisor ampliamente distribuido en el cerebro que interviene en el control del movimiento, en el sistema de recompensa, sueño, atención y procesos de memoria y aprendizaje. Además, regula estados emocionales, motivación y toma de decisiones. La DA es el neurotransmisor que controla las neuronas estriatales de proyección (NEP), que son el principal tipo de neuronas del estriado y del núcleo accumbens (NAc). Estudios previos han mostrado el papel clave de la DA en la regulación de la plasticidad estructural y sináptica de las NEP. Los astrocitos, establecen una comunicación bidireccional con las neuronas, conocida como Sinapsis Tripartita. Sin embargo, debido a la complejidad de las interacciones de las NEP y los astrocitos en múltiples circuitos, muchos de estos mecanismos aún siguen siendo desconocidos. En esta tesis, se han establecido algunos de estos mecanismos mediante el estudio de los siguientes objetivos: I) relación entre la concentración de DA liberada en el estriado y la densidad de espinas dendríticas, II) papel de los receptores dopaminérgicos D1 y D2 en la regulación estructural y sináptica de las neuronas estriatales de proyección, III), relación entre la liberación de DA y la activación de diferentes grupos de astrocitos, IV), modulación de la plasticidad sináptica de las NEP por los astrocitos. Para el abordaje de estos objetivos se emplearon: registros de patch-clamp en la configuración célula entera in vitro en secciones coronales de estriado y NAc, estudios de optogenética, registros electroquímicos de medición en la liberación de DA y estudios morfológicos mediante microinyecciones por iontoforesis de las NEP. Nuestros resultados demuestran que la densidad de espinas dendríticas en las NEP está regulada por los niveles de DA en función del grado de inervación proveniente de la sustancia negra. Además, demostramos que el uso de drogas psicoestimulantes modifica la plasticidad sináptica de la vía dopaminérgica ATV-NAc

Dopamine receptors play an important role in motivational, emotional, and motor responses. In addition, growing evidence suggests a key role of hippocampal dopamine receptors in learning and memory. It is well known that associative learning and synaptic plasticity of CA3-CA1 requires the dopamine D1 receptor (D1R). However, the specific role of the dopamine D2 receptor (D2R) on memory-related neuroplasticity processes is still undefined. Here, by using two models of D2R loss, D2R knockout mice (Drd2-/-) and mice with intrahippocampal injections of Drd2-small interfering RNA (Drd2-siRNA), we aimed to investigate how D2R is involved in learning and memory as well as in long-term potentiation of the hippocampus. Our studies revealed that the genetic inactivation of D2R impaired the spatial memory, associative learning, and the classical conditioning of eyelid responses. Similarly, deletion of D2R reduced the activity-dependent synaptic plasticity in the hippocampal CA1-CA3 synapse. Our results demonstrate the first direct evidence that D2R is essential in behaving mice for trace eye blink conditioning and associated changes in hippocampal synaptic strength. Taken together, these results indicate a key role of D2R in regulating hippocampal plasticity changes and, in consequence, acquisition and consolidation of spatial and associative forms of memory.

Background: Dopamine transmission is involved in the maintenance of the structural plasticity of direct-pathway and indirect-pathway striatal projection neurons (d-SPNs and i-SPNs, respectively). The lack of dopamine in Parkinson's disease produces synaptic remodeling in both types of SPNs, reducing the length of the dendritic arbor and spine density and increasing the intrinsic excitability. Meanwhile, the elevation of dopamine levels by levodopa recovers these alterations selectively in i-SPNs. However, little is known about the specific role of the D1 receptor (D1R) in these alterations. Methods: To explore the specific role of D1R in the synaptic remodeling of SPNs, we used knockout D1R mice (D1R) and wild-type mice crossed with drd2-enhanced green fluorescent protein (eGFP) to identify d-SPNs and i-SPNs. Corticostriatal slices were used for reconstruction of the dendritic arbors after Lucifer yellow intracellular injection and for whole-cell recordings in naïve and parkinsonian mice treated with saline or levodopa. Results: The genetic inactivation of D1R reduces the length of the dendritic tree and the spine density in all SPNs, although more so in d-SPNs, which also increases their spiking. In parkinsonian D1R mice, the spine density decreases in i-SPNs, and this spine loss recovers after chronic levodopa. Conclusions: D1R is essential for the maintenance of spine plasticity in d-SPNs but also affects i-SPNs, indicating an important crosstalk between these 2 types of neurons. © 2020 International Parkinson and Movement Disorder Society.

[Background]: Numerous studies indicate an association between neurodegenerative and metabolic diseases. Although still a matter of debate, growing evidence from epidemiological and animal studies indicate that preexisting diabetes increases the risk to develop Parkinson's disease. However, the mechanisms of such an association are unknown., [Objectives]: We investigated whether diabetes alters striatal dopamine neurotransmission and assessed the vulnerability of nigrostriatal neurons to neurodegeneration., [Methods]: We used streptozotocin‐treated and genetically diabetic db/db mice. Expression of oxidative stress and nigrostriatal neuronal markers and levels of dopamine and its metabolites were monitored. Dopamine release and uptake were assessed using fast‐scan cyclic voltammetry. 6‐Hydroxydopamine was unilaterally injected into the striatum using stereotaxic surgery. Motor performance was scored using specific tests., [Results]: Diabetes resulted in oxidative stress and decreased levels of dopamine and its metabolites in the striatum. Levels of proteins regulating dopamine release and uptake, including the dopamine transporter, the Girk2 potassium channel, the vesicular monoamine transporter 2, and the presynaptic vesicle protein synaptobrevin‐2, were decreased in diabetic mice. Electrically evoked levels of extracellular dopamine in the striatum were enhanced, and altered dopamine uptake was observed. Striatal microinjections of a subthreshold dose of the neurotoxin 6‐hydroxydopamine in diabetic mice, insufficient to cause motor alterations in nondiabetic animals, resulted in motor impairment, higher loss of striatal dopaminergic axons, and decreased neuronal cell bodies in the substantia nigra., [Conclusions]: Our results indicate that diabetes promotes striatal oxidative stress, alters dopamine neurotransmission, and increases vulnerability to neurodegenerative damage leading to motor impairment.

Dopaminergic afferents innervate spiny projection neurons (SPNs) in the striatum, maintaining basal gangliaactivity. The loss of striatal innervation is the hallmark of Parkinson's disease (PD), which is characterized bydopaminergic denervation. A lack of dopamine in the dorsal striatum induces plasticity changes in SPNs.However, PD-associated denervation is progressive, and how plasticity is modified in partially innervated areasis poorly understood. The most studied models of PD are based on the use of neurotoxins that induce an almostcomplete striatal denervation. To investigate the impact of partial dopamine (DA) innervation in striatal plas-ticity, we use a genetic model of PD, Aphakia (Ak) mice, whose striatum presents an increasing dorso-ventralgradient of dopamine innervation. We studied SPNs in three different areas (dorsal, middle and ventral, withlow, moderate and high innervation by tyrosine hydroxylase TH-positive axons, respectively) using fast scancyclic voltammetry, microiontophoresis, immunohistochemistry and patch clamp techniques. Our data show anincreasing dorso-ventral gradient of extracellular DA levels, overlapping with the gradient of TH innervation.Interestingly, spine loss in both direct (d-SPN) and indirect SPNs (i-SPN) decreases from dorsal to ventral in theparkinsonian striatum of Ak mice, following the decrease in DA levels. However, their dendritic trees and thenumber of nodes are only reduced in the poorly innervated dorsal areas and remain unaltered in moderate andhighly innervated areas. Thefiring rate of direct SPNs does not change in either moderate or highly innervatedareas, but increases in poorly innervated areas. In contrast, action potential frequency of indirect SPNs does notchange along the dorso-ventral innervation gradient. Ourfindings indicate that spine density in d-SPNs and i-SPNs varies in a dopamine concentration-dependent manner, indicating that both d- and i-SPN are similarlyinnervated by DA.

In toxin-based models of Parkinson's disease (PD), striatal projection neurons (SPNs) exhibit dendritic atrophy and spine loss concurrent with an increase in excitability. Chronic l-DOPA treatment that induces dyskinesia selectively restores spine density and excitability in indirect pathway SPNs (iSPNs), whereas spine loss and hyperexcitability persist in direct pathway SPNs (dSPNs). These alterations have only been characterized in toxin-based models of PD, raising the possibility that they are an artifact of exposure to the toxin, which may engage compensatory mechanisms independent of the PD-like pathology or due to the loss of dopaminergic afferents. To test all these, we studied the synaptic remodeling in Pitx3-/- or aphakia mice, a genetic model of PD, in which most of the dopamine neurons in the substantia nigra fail to fully differentiate and to innervate the striatum. We made 3D reconstructions of the dendritic arbor and measured excitability in identified SPNs located in dorsal striatum of BAC-Pitx3-/- mice treated with saline or l-DOPA. Both dSPNs and iSPNs from BAC-Pitx3-/- mice had shorter dendritic trees, lower spine density, and more action potentials than their counterparts from WT mice. Chronic l-DOPA treatment restored spine density and firing rate in iSPNs. By contrast, in dSPNs, spine loss and hyperexcitability persisted following l-DOPA treatment, which is similar to what happens in 6-OHDA WT mice. This indicates that dopamine-mediated synaptic remodeling and plasticity is independent of dopamine innervation during SPN development and that Pitx3-/- mice are a good model because they develop the same pathology described in the toxins-based models and in human postmortem studies of advanced PD.SIGNIFICANCE STATEMENT As the only genetic model of Parkinson's disease (PD) that develops dyskinesia, Pitx3-/- mice reproduce the behavioral effects seen in humans and are a good system for studying dopamine-induced synaptic remodeling. The studies we pres