Graziella Madeo

Centrality of striatal cholinergic transmission in Basal Ganglia function

Work over the past two decades revealed a previously unexpected role for striatal cholinergic interneurons in the context of basal ganglia function. The recognition that these interneurons are essential in synaptic plasticity and motor learning represents a significant step ahead in deciphering how the striatum processes cortical inputs, and why pathological circumstances cause motor dysfunction. Loss of the reciprocal modulation between dopaminergic inputs and the intrinsic cholinergic innervation within the striatum appears to be the trigger for pathophysiological changes occurring in basal ganglia disorders. Accordingly, there is now compelling evidence showing profound changes in cholinergic markers in these disorders, in particular Parkinsons disease and dystonia. Based on converging experimental and clinical evidence, we provide an overview of the role of striatal cholinergic transmission in physiological and pathological conditions, in the context of the pathogenesis of movement disorders.

Cholinergic dysfunction alters synaptic integration between thalamostriatal and corticostriatal inputs in DYT1 dystonia

Projections from thalamic intralaminar nuclei convey sensory signals to striatal cholinergic interneurons. These neurons respond with a pause in their pacemaking activity, enabling synaptic integration with cortical inputs to medium spiny neurons (MSNs), thus playing a crucial role in motor function. In mice with the DYT1 dystonia mutation, stimulation of thalamostriatal axons, mimicking a response to salient events, evoked a shortened pause and triggered an abnormal spiking activity in interneurons. This altered pattern caused a significant rearrangement of the temporal sequence of synaptic activity mediated by M1 and M2 muscarinic receptors in MSNs, consisting of an increase in postsynaptic currents and a decrease of presynaptic inhibition, respectively. Consistent with a major role of acetylcholine, either lowering cholinergic tone or antagonizing postsynaptic M1 muscarinic receptors normalized synaptic activity. Our data demonstrate an abnormal time window for synaptic integration between thalamostriatal and corticostriatal inputs, which might alter the action selection process, thereby predisposing DYT1 gene mutation carriers to develop dystonic movements.

Metabotropic glutamate receptor subtype 4 selectively modulates both glutamate and GABA transmission in the striatum: implications for Parkinson’s disease treatment

Alterations of striatal synaptic transmission have been associated with several motor disorders involving the basal ganglia, such as Parkinson’s disease. For this reason, we investigated the role of group‐III metabotropic glutamate (mGlu) receptors in regulating synaptic transmission in the striatum by electrophysiological recordings and by using our novel orthosteric agonist (3S)‐3‐[(3‐amino‐3‐carboxypropyl(hydroxy)phosphinyl)‐hydroxymethyl]‐5‐nitrothiophene (LSP1‐3081) and l‐2‐amino‐4‐phosphonobutanoate (L‐AP4). Here, we show that both drugs dose‐dependently reduced glutamate‐ and GABA‐mediated post‐synaptic potentials, and increased the paired‐pulse ratio. Moreover, they decreased the frequency, but not the amplitude, of glutamate and GABA spontaneous and miniature post‐synaptic currents. Their inhibitory effect was abolished by (RS)‐α‐cyclopropyl‐4‐phosphonophenylglycine and was lost in slices from mGlu4 knock‐out mice. Furthermore, (S)‐3,4‐dicarboxyphenylglycine did not affect glutamate and GABA transmission. Finally, intrastriatal LSP1‐3081 or L‐AP4 injection improved akinesia measured by the cylinder test. These results demonstrate that mGlu4 receptor selectively modulates striatal glutamate and GABA synaptic transmission, suggesting that it could represent an interesting target for selective pharmacological intervention in movement disorders involving basal ganglia circuitry.

Early synaptic dysfunction in Parkinson's disease: Insights from animal models

The appearance of motor manifestations in Parkinsons disease (PD) is invariably linked to degeneration of nigral dopaminergic neurons of the substantia nigra pars compacta. Traditional views on PD neuropathology have been grounded in the assumption that the prime event of neurodegeneration involves neuronal cell bodies with the accumulation of metabolic products. However, this view has recently been challenged by both clinical and experimental evidence. Neuropathological studies in human brain samples and both in vivo and in vitro models support the hypothesis that nigrostriatal synapses may indeed be affected at the earliest stages of the neurodegenerative process. The mechanisms leading to either structural or functional synaptic dysfunction are starting to be elucidated and include dysregulation of axonal transport, impairment of the exocytosis and endocytosis machinery, altered intracellular trafficking, and loss of corticostriatal synaptic plasticity. The aim of this review is to try to integrate different lines of evidence from both pathogenic and genetic animal models that, to different extents, suggest that early synaptic impairment may represent the key event in PD pathogenesis. Understanding the molecular and cellular events underlying such synaptopathy is a fundamental step toward developing specific biomarkers of early dopaminergic dysfunction and, more importantly, designing novel therapies targeting the synaptic apparatus of selective, vulnerable synapses.

Anticholinergic drugs rescue synaptic plasticity in DYT1 dystonia: Role of M1 muscarinic receptors

Broad‐spectrum muscarinic receptor antagonists have represented the first available treatment for different movement disorders such as dystonia. However, the specificity of these drugs and their mechanism of action is not entirely clear. We performed a systematic analysis of the effects of anticholinergic drugs on short‐ and long‐term plasticity recorded from striatal medium spiny neurons from DYT1 dystonia knock‐in (Tor1a+/Δgag) mice heterozygous for ΔE‐torsinA and their controls (Tor1a+/+ mice). Antagonists were chosen that had previously been proposed to be selective for muscarinic receptor subtypes and included pirenzepine, trihexyphenydil, biperiden, orphenadrine, and a novel selective M1 antagonist, VU0255035. Tor1a+/Δgag mice exhibited a significant impairment of corticostriatal synaptic plasticity. Anticholinergics had no significant effects on intrinsic membrane properties and on short‐term plasticity of striatal neurons. However, they exhibited a differential ability to restore the corticostriatal plasticity deficits. A complete rescue of both long‐term depression (LTD) and synaptic depotentiation (SD) was obtained by applying the M1‐preferring antagonists pirenzepine and trihexyphenidyl as well as VU0255035. Conversely, the nonselective antagonist orphenadrine produced only a partial rescue of synaptic plasticity, whereas biperiden and ethopropazine failed to restore plasticity. The selectivity for M1 receptors was further demonstrated by their ability to counteract the M1‐dependent potentiation of N‐methyl‐d‐aspartate (NMDA) current recorded from striatal neurons. Our study demonstrates that selective M1 muscarinic receptor antagonism offsets synaptic plasticity deficits in the striatum of mice with the DYT1 dystonia mutation, providing a potential mechanistic rationale for the development of improved antimuscarinic therapies for this movement disorder.

Homeostatic changes of the endocannabinoid system in Parkinson's disease

Endocannabinoids (eCBs) are endogenous lipids that bind principally type‐1 and type‐2 cannabinoid (CB1 and CB2) receptors. N‐Arachidonoylethanolamine (AEA, anandamide) and 2‐arachidonoylglycerol (2‐AG) are the best characterized eCBs that are released from membrane phospholipid precursors through multiple biosynthetic pathways. Together with their receptors and metabolic enzymes, eCBs form the so‐called “eCB system”. The later has been involved in a wide variety of actions, including modulation of basal ganglia function. Consistently, both eCB levels and CB1 receptor expression are high in several basal ganglia regions, and more specifically in the striatum and in its target projection areas. In these regions, the eCB system establishes a close functional interaction with dopaminergic neurotransmission, supporting a relevant role for eCBs in the control of voluntary movements. Accordingly, compelling experimental and clinical evidence suggests that a profound rearrangement of the eCB system in the basal ganglia follows dopamine depletion, as it occurs in Parkinsons disease (PD).

Distinct roles of group I mGlu receptors in striatal function.

In the recent past, evidence accumulated in favour of a central role of group I metabotropic glutamate (mGlu) receptors, mGlu1 and mGlu5, in the modulation of cell excitability both of striatal medium spiny projection neurons (MSNs) and interneuronal population. Electrophysiological and pharmacological studies have clearly shown that activation of mGlu1 and mGlu5 receptors exerts distinct actions, depending on the neuronal subtype involved. MGlu5 receptor activation mediates the potentiation of NMDA responses in MSNs, and underlies the retrograde inhibitory signaling by endocannabinoids at corticostriatal synapses. Conversely, both group I mGlu receptors are involved in long-term synaptic plasticity of MSNs. Likewise, either mGlu1 or mGlu5 receptors are engaged in shaping the excitability of large cholinergic interneurons, playing different roles in the membrane responses. Differently, although GABAergic parvalbumin-positive, fast-spiking interneurons have been shown to express both group I receptors, only mGlu1 receptor seems to mediate membrane and synaptic responses. This review provides a brief survey of the cellular and synaptic actions of group I mGlu receptors, and discusses the potential relevance of these findings in neostriatal function and motor control.

Regional specificity of synaptic plasticity deficits in a knock-in mouse model of DYT1 dystonia

DYT1 dystonia is a movement disorder caused by a deletion in the C-terminal of the protein torsinA. It is unclear how torsinA mutation might disrupt cellular processes encoding motor activity, and whether this impairment occurs in specific brain regions. Here, we report a selective impairment of corticostriatal synaptic plasticity in knock-in mice heterozygous for Δ-torsinA (Tor1a(+/Δgag) mice) as compared to controls (Tor1a(+/+) mice). In striatal spiny neurons from Tor1a(+/Δgag) mice, high-frequency stimulation failed to induce long-term depression (LTD), whereas long-term potentiation (LTP) exhibited increased amplitude. Of interest, blockade of D2 dopamine receptors (D2Rs) increased LTP in Tor1a(+/+) mice to a level comparable to that measured in Tor1a(+/Δgag) mice and normalized the levels of potentiation across mouse groups. A low-frequency stimulation (LFS) protocol was unable to depotentiate corticostriatal synapses in Tor1a(+/Δgag) mice. Muscarinic M1 acetylcholine receptor (mAChR) blockade rescued plasticity deficits. Additionally, we found an abnormal responsiveness of cholinergic interneurons to D2R activation, consisting in an excitatory response rather than the expected inhibition, further confirming an imbalance between dopaminergic and cholinergic signaling in the striatum. Conversely, synaptic activity and plasticity in the CA1 hippocampal region were unaltered in Tor1a(+/Δgag) mice. Importantly, the M1 mAChR-dependent enhancement of hippocampal LTP was unaffected in both genotypes. Similarly, both basic properties of dopaminergic nigral neurons and their responses to D2R activation were normal. These results provide evidence for a regional specificity of the electrophysiological abnormalities observed and demonstrate the reproducibility of such alterations in distinct models of DYT1 dystonia.

Activation of 5-HT6 receptors inhibits corticostriatal glutamatergic transmission

We investigated the effect of 5-HT6 receptor subtype activation on glutamatergic transmission by means of whole-cell patch-clamp electrophysiological recordings from medium spiny neurons of the striatum and layer V pyramidal neurons of the prefrontal cortex. To this aim, we took advantage of a novel ligand, ST1936, showing nM affinity and agonist activity at the 5-HT6 receptor subtype. Our data show that 5-HT6 receptor activation by ST1936 reduces the frequency of spontaneous excitatory postsynaptic currents, with an IC50 of 1.3 μM. Moreover, 5-HT6 receptor activation also reduced the amplitude of spontaneous excitatory postsynaptic currents recorded from medium spiny neurons, suggesting a mechanism of action involving postsynaptic 5-HT6 receptors, as further confirmed by the paired-pulse analysis on evoked excitatory postsynaptic currents and by recordings of miniature glutamatergic events. The inhibitory effect of ST1936 on glutamatergic transmission was prevented by the selective 5-HT6 receptor antagonist SB258585 and mimicked by a different agonist, WAY-181187. Conversely, in the cortex ST1936 reduced the frequency, but not the amplitude, of spontaneous excitatory postsynaptic currents suggesting a presynaptic or indirect effect of the 5-HT6 receptor.

PINK1 heterozygous mutations induce subtle alterations in dopamine-dependent synaptic plasticity

Homozygous or compound heterozygous mutations in the phosphatase and tensin homolog‐induced putative kinase 1 (PINK1) gene are causative of autosomal recessive, early onset Parkinsons disease. Single heterozygous mutations have been detected repeatedly both in a subset of patients and in unaffected individuals, and the significance of these mutations has long been debated. Several neurophysiological studies from non‐manifesting PINK1 heterozygotes have demonstrated the existence of neural plasticity abnormalities, indicating the presence of specific endophenotypic traits in the heterozygous state. We performed a functional analysis of corticostriatal synaptic plasticity in heterozygous PINK1 knockout (PINK1+/−) mice using a multidisciplinary approach and observed that, despite normal motor behavior, repetitive activation of cortical inputs to striatal neurons failed to induce long‐term potentiation (LTP), whereas long‐term depression was normal. Although nigral dopaminergic neurons exhibited normal morphological and electrophysiological properties with normal responses to dopamine receptor activation, a significantly lower dopamine release was measured in the striatum of PINK1+/− mice compared with control mice, suggesting that a decrease in stimulus‐evoked dopamine overflow acts as a major determinant for the LTP deficit. Accordingly, pharmacological agents capable of increasing the availability of dopamine in the synaptic cleft restored normal LTP in heterozygous mice. Moreover, monoamine oxidase B inhibitors rescued physiological LTP and normal dopamine release. Our results provide novel evidence for striatal plasticity abnormalities, even in the heterozygous disease state. These alterations might be considered an endophenotype to this monogenic form of Parkinsons disease and a valid tool with which to characterize early disease stage and design possible disease‐modifying therapies.