l-DOPA-induced dyskinesia (LID), a detrimental consequence of dopamine replacement therapy for Parkinson's disease, is associated with an alteration in dopamine D1 receptor (D1R) and glutamate receptor interactions. We hypothesized that the synaptic scaffolding protein PSD-95 plays a pivotal role in this process, as it interacts with D1R, regulates its trafficking and function, and is overexpressed in LID. Here, we demonstrate in rat and macaque models that disrupting the interaction between D1R and PSD-95 in the striatum reduces LID development and severity. Single quantum dot imaging revealed that this benefit was achieved primarily by destabilizing D1R localization, via increased lateral diffusion followed by increased internalization and diminished surface expression. These findings indicate that altering D1R trafficking via synapse-associated scaffolding proteins may be useful in the treatment of dyskinesia in Parkinson's patients. IntroductionIn the striatum, dopamine (DA) terminals from the substantia nigra pars compacta (SNc) converge with glutamatergic signals from the cortex on dendritic spines of striatal medium spiny projecting GABAergic neurons (1, 2). The degeneration of the nigrostriatal pathway in Parkinson's disease (PD) induces complex modifications in both DA and glutamate signaling, leading to significant morphological and functional modifications in the striatal neuronal circuitry (3-5). Chronic DA replacement therapy with l-3,4-dihydroxyphenylalanine (l-DOPA) superimposes upon these DA depletion-induced changes, resulting in debilitating motor complications known as l-DOPAinduced dyskinesia (LID) (6-8). At the molecular level, the subcellular organization of and functional interactions between glutamate and DA receptors within the striatum are crucial both in the pathogenesis of PD (9) and in the development of LID (10, 11). LID has indeed been associated with plastic changes in postsynaptic neuronal targets in the striatum, including elevated extracellular levels of glutamate (12) and DA (13) and abnormal trafficking of DA D1 receptor (D1R) (14, 15) and of NMDA and AMPA glutamate receptor subunits (5,10,16,17). Such exaggerated DA and glutamate receptor expression at the plasma membrane results in abnormal activation of key signaling kinases (18)(19)(20)(21)(22). All these changes point to dysfunctional interactions between DA and glutamate neurotransmission in LID (5,23,24), although the molecular mechanisms remain elusive, despite recent progress (14, 25).The membrane-associated guanylate kinase (MAGUK) proteins, such as postsynaptic density 95 (PSD-95), organize ionotropic glutamate receptors and their associated signaling proteins, regulating the strength of synaptic activity. Interestingly, PSD-95 might also interact with DA D1R (26), thereby potentially regulating DA

P2X receptors (P2XRs) are ATP-gated cation channels widely expressed in the brain where they mediate action of extracellular ATP released by neurons or glia. Although purinergic signaling has multiple effects on synaptic transmission and plasticity, P2XR function at brain synapses remains to be established. Here, we show that activation of postsynaptic P2XRs by exogenous ATP or noradrenaline-dependent glial release of endogenous ATP decreases the amplitude of miniature excitatory postsynaptic currents and AMPA-evoked currents in cultured hippocampal neurons. We also observed a P2X-mediated depression of field potentials recorded in CA1 region from brain slices. P2X2Rs trigger dynamin-dependent internalization of AMPA receptors (AMPARs), leading to reduced surface AMPARs in dendrites and at synapses. AMPAR alteration required calcium influx through opened ATP-gated channels and phosphatase or CamKII activities. These findings indicate that postsynaptic P2XRs play a critical role in regulating the surface expression of AMPARs and thereby regulate the synaptic strength.

Key points• We used optogenetics approach to characterize the short-term plasticity of striato-pallidal (STR-GP) and pallido-pallidal (GP-GP) GABAergic synapses in rat brain slices.• We show that only GP-GP (and not STR-GP) transmission is augmented by chronic dopamine depletion.• Finally, we report that altered GP-GP synaptic transmission promotes neuronal synchronization and rebound bursting in globus pallidus neurons.• Our results support the conclusion that maladaptive GP-GP GABAergic transmission is likely to be a key underlying factor of the pathological activity in the globus pallidus observed in Parkinson's disease. AbstractThe pattern of activity of globus pallidus (GP) neurons is tightly regulated by GABAergic inhibition. In addition to extrinsic inputs from the striatum (STR-GP) the other source of GABA to GP neurons arises from intrinsic intranuclear axon collaterals (GP-GP). While the contribution of striatal inputs has been studied, notably its hyperactivity in Parkinson's disease (PD), the properties and function of intranuclear inhibition remain poorly understood. Our objective was therefore to test the impact of chronic dopamine depletion on pallido-pallidal transmission. Using patch-clamp whole-cell recordings in rat brain slices, we combined electrical and optogenetic stimulations with pharmacology to differentiate basic synaptic properties of STR-GP and GP-GP GABAergic synapses. GP-GP synapses were characterized by activity-dependent depression and insensitivity to the D 2 receptor specific agonist quinpirole and STR-GP synapses by frequency-dependent facilitation and quinpirole modulation. Chronic dopamine deprivation obtained in 6-OHDA lesioned animals boosted the amplitude of GP-GP IPSCs but did not modify STR-GP transmission and increased the amplitude of miniature IPSCs. Replacement of calcium by strontium confirmed that the quantal amplitude was increased at GP-GP synapses. Finally, we demonstrated that boosted GP-GP transmission promotes resetting of autonomous activity and rebound-burst firing after dopamine depletion. These results suggest that GP-GP synaptic transmission (but not STR-GP) is augmented by chronic dopamine depletion which could contribute to the aberrant GP neuronal activity observed in PD.

The essence of neuronal function is to generate outputs in response to synaptic potentials. Synaptic integration at postsynaptic sites determines neuronal outputs in the CNS. Using immunohistochemical and electrophysiological approaches, we first reveal that steroidogenic factor 1 (SF-1) green fluorescent protein (GFP)-positive neurons in the ventromedial nucleus of the hypothalamus express P2X4 subunits that are activated by exogenous ATP. Increased membrane expression of P2X4 channels by using a peptide competing with P2X4 intracellular endocytosis motif enhances neuronal excitability of SF-1 GFP-positive neurons. This increased excitability is inhibited by a P2X receptor antagonist. Furthermore, increased surface P2X4 receptor expression significantly decreases the frequency and the amplitude of GABAergic postsynaptic currents of SF-1 GFPpositive neurons. Co-immunopurification and pulldown assays reveal that P2X4 receptors complex with aminobutyric acid, type A (GABA A ) receptors and demonstrate that two amino acids in the carboxyl tail of the P2X4 subunit are crucial for its physical association with GABA A receptors. Mutation of these two residues prevents the physical association, thereby blocking cross-inhibition between P2X4 and GABA A receptors. Moreover, disruption of the physical coupling using competitive peptides containing the identified motif abolishes current inhibition between P2X4 and GABA A receptors in recombinant system and P2X4 receptor-mediated GABAergic depression in SF-1 GFP-positive neurons. Our present work thus provides evidence for cross-talk between excitatory and inhibitory receptors that appears to be crucial in determining GABAergic synaptic strength at a central synapse.ATP acts at cell surface receptors of two fundamentally distinct types: ligand-gated ion channels (P2X receptors) and G-protein-coupled receptors (P2Y receptors). ATP P2X receptors are widely distributed in excitable and non-excitable cells of vertebrates and are nonselective cation channels (1). Their activation mediates membrane depolarization and calcium influx (2). Among the seven P2X subunits, P2X4 receptors are most widely distributed in the CNS (3). It appears that P2X4-containing receptors are involved in purinergic synaptic transmission at central synapses since they are located at postsynaptic sites in the brain, including the hippocampus and the cerebellum (4). Although there is increasing evidence for their implications in various physiological and pathological conditions (5), the physiological role of P2X receptors at the synaptic level has been poorly defined, at least in part, due to the paucity of purinergic synaptic transmission in the CNS.In the nervous system, ATP appears to be primarily a cotransmitter rather than a principal transmitter (6). ATP is released either with an inhibitory neurotransmitter, GABA 4 (7-10) or with an excitatory neurotransmitter, glutamate in the CNS (4, 11). Recent studies have clearly demonstrated the interactions of P2X receptors with other ligand-gated channels, i...