1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Abstract 16 LRRK2 mutations

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12. 6Dept. Medical Genetics, University of British Columbia, Vancouver, Canada. 13. 14. 15. Abstract. 16. LRRK2 mutations produce end-stage Parkinson's ...

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Initial elevations in glutamate and dopamine neurotransmission decline with age, as does exploratory behavior, in LRRK2 G2019S knock-in mice

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Mattia Volta2*, Dayne A. Beccano-Kelly3*, Sarah A. Paschall4*, Stefano Cataldi4+, Sarah MacIsaac4+,

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Naila Kuhlmann4, Chelsie A. Kadgien4, Igor Tatarnikov4, Jesse Fox4, Jaskaran Khinda4, Emma Mitchell4,

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Sabrina Bergeron4, Heather Melrose5, Matthew J. Farrer6$ & Austen J. Milnerwood1$

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+ $

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Dept. Neurology & Neurosurgery, Montreal Neurological Institute, McGill University, Montreal, Canada

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Institute for Biomedicine, EURAC, Bolzano, Italy

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Department of Physiology, Anatomy & Genetics, University of Oxford, Oxford, U.K.

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Graduate Program in Neurosciences, University of British Columbia, Vancouver, Canada

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Mayo Clinic, Jacksonville, Florida, U.S.A.

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Dept. Medical Genetics, University of British Columbia, Vancouver, Canada

* equal contributions

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Abstract

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LRRK2 mutations produce end-stage Parkinson’s disease (PD) with reduced nigrostriatal

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dopamine. Conversely, asymptomatic carriers have increased dopamine turnover and altered

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brain connectivity. LRRK2 pathophysiology remains unclear, but reduced dopamine and

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mitochondrial abnormalities occur in aged mutant knock-in (GKI) mice. Conversely, cultured

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GKI neurons exhibit increased synaptic transmission. We assessed behavior and synaptic

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glutamate and dopamine function across ages. Young GKI exhibit more vertical exploration,

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elevated glutamate and dopamine transmission, and aberrant D2-receptor responses. These

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phenomena decline with age, but are stable in littermates. In young GKI, dopamine transients are

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slower, independent of DAT, increasing dopamine extracellular lifetime. Slowing of dopamine

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transients is observed with age in littermates, suggesting premature ageing of dopamine synapses

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in GKI. Thus, GKI mice exhibit early, but declining, synaptic and behavioral phenotypes,

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making them amenable to investigation of early pathophysiological, and later parkinsonian-like,

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alterations. This model will prove valuable in efforts to develop neuroprotection for PD.

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Introduction

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Parkinson’s disease (PD) is clinically diagnosed when patients present with characteristic

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progressive motor symptoms, although post-mortem detection of Lewy pathology and nigral cell

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loss are currently required for confirmation. A recent study suggests nigral cell death may be as

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low as 0-10% 1-3 years from diagnosis, whereas dopamine functional markers such as tyrosine

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hydroxylase (TH) and dopamine transporter (DAT) are profoundly reduced at the earliest points

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assessed 1. The rapid and near complete loss of dopamine functional markers at (or within a few

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years of) diagnosis argues that ongoing clinical deterioration over several years is due to loss of

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compensatory mechanisms and / or dysfunction of non-dopaminergic neurons.

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Although motor symptoms respond well to current therapy (e.g., dopamine replacement

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by L-DOPA or deep brain stimulation; DBS), PD is a multisystem disorder with a host of L-

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DOPA unresponsive features. All patients suffer a range of non-motor symptoms, many of which

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appear to precede motor onset by years or decades 2–8. Cognitive dysfunction is seen in ~40% of

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newly diagnosed PD cases 9–11 in the form of deficits in attention, executive function, verbal

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fluency and visuospatial processing, rather than memory per se (although memory is often also

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impaired) 6,12. This dysexecutive / subcortical syndrome is thought to be due to impaired cortico-

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striatal basal ganglia processing for action selection 2,13,14. There are no effective symptomatic

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treatments for many of these non-motor issues, nor are there currently any disease-modifying

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(neuroprotective) interventions.

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While the etiology for most Parkinson patients remains unknown, asides age, gene

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mutations contribute most risk 15. Pathogenic mutations in leucine-rich repeat kinase 2 (LRRK2)

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account for ~1% of all PD, of which LRRK2 G2019S is most frequent; identified in ~30% of

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cases in some ethnicities 16,17. Affected LRRK2 individuals develop late-onset, L-DOPA-

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responsive motor parkinsonism that is clinically and often pathologically indistinguishable from

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idiopathic PD 18,19. Dopamine PET imaging of affected LRRK2 mutation carriers reveals

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progressive neurochemical alterations similar to those of sporadic PD, namely impaired

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presynaptic dopamine function 20,21. Further study in LRRK2 families reveals surprising

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alterations in clinically asymptomatic, otherwise healthy, mutation carriers, including: i) early

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increases in dopamine turnover by PET 22, ii) changes in cortical connectivity by resting state

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MRI and neurochemical changes 20,23,24, and iii) alterations in cognitive tests of executive

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function 25. Advances in our understanding of PD argue cell death and overt motor dysfunction

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are late occurrences, preceded by dysfunction of dopaminergic and non-dopaminergic systems.

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In this light, in model systems modelling PD etiology, the underlying pathophysiology and

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phenotypes should also be expected to be initially subtle, progressive, and include dysfunction of

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multiple neuronal systems, prior to cell loss.

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We engineered the LRRK2 G2019S substitution into the endogenous mouse gene

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(G2019S knock-in mice; GKI) which in vivo produced reductions in basal and pharmacologically

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evoked nigrostriatal dopamine release in mice aged >12 months by microdialysis 26. This

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Parkinson’s-like deficit was not observed at 6 months, and occurred despite a normal

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compliment of nigral neurons and nigrostriatal dopamine markers (TH). Contrastingly, in

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cortical neurons cultured from the same GKI mice, we observed increases in glutamatergic and

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GABAergic synaptic transmission at 21 days in vitro 27.

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To investigate this disparity, we probed dopamine and glutamate release, neuronal

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morphology, synaptic proteins and behavior in young and aged GKI mice. Young animals

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exhibit increased exploratory rearing behavior and increases in striatal glutamate and dopamine

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transmission. As GKI mice age, they exhibit less exploratory rearing and reductions in both

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glutamate and dopamine transmission. However, extracellular lifetime of single dopamine

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release events is enhanced in young GKI animals, and maintained in aged animals, at which

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point wild-type littermate values have increased to parity. Several measures demonstrate the

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LRRK2 G2019S mutation produces alterations in young adult mice, most of which decline with

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age, prior to ages where hypodopaminergia, mitochondrial and tau pathology are observed. We

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provide further evidence that GKI mice provide a valuable model in which to probe early

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pathophysiological effects of mutant LRRK2, and later classically PD-like deficits. We conclude

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that understanding the early pathophysiological changes in etiological models may offer the best

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hope for development of neuroprotection in PD and related diseases.

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Materials & Methods

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G2019S knock-in mice and behavioral testing

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C57Bl/6J wild-type (WT) and Lrrk2 G2019S knock-in heterozygous (GKI) mice 26,27 were

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maintained according to Canadian Council on Animal Care regulations. To avoid confounds of

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oestrus cycle upon behavior and neural connectivity, only male animals were used in this work.

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Mice undergoing surgery were weighed at age of use, and all other mice in the colony were

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weighed at a single time point to produce an age vs. weight plot. Separate cohorts of adult

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animals were tested (once only) at 1-6 and 12-18 months of age. After familiarization to handling

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over three days with the operator, mice underwent the following behavioral paradigms and

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videos were analyzed post-hoc using ANY-maze (Stoelting) behavioral tracking software, as

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previously 28,29. Open field (OF) test: mice explored an arena (48cm x 48cm) for 15 min.

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Cylinder test: mice were placed in a 1l beaker and video recorded for 5min. The number of

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rearings and forelimb wall contacts were scored manually off-line. All testing and analysis was

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performed experimenter blind.

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Electrophysiology

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Whole-cell patch clamp recording was conducted in striatal projection neurons (SPNs) in 300μm

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thick coronal slices from 1-18 month-old male mice, as in 30,31. To help preserve cell viability,

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slices from >12 month old animals were pre-incubated in recovery solution containing (in mM:

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93 NMDG, 93 HCl, 2.5 KCl, 1.2 NaH2PO4, 30 NaHCO3, 20 HEPES, 25 Glucose, 5 Sodium

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Ascorbate, 2 Thiourea, 3 Sodium Pyruvate, 10 MgSO4.7H2O, 0.5 CaCl2.2H2O, pH 7.3-7.4,

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300-310 mOsm) for 15min at 34˚C prior to transfer to a holding chamber. Slices were held and

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perfused at RT with artificial cerebrospinal fluid (ACSF) containing (in mM): 125 NaCl, 2.5

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KCl, 25 NaHCO3, 1.25 NaH2PO4, 2 MgCl2, 2 CaCl2, 10 glucose, pH 7.3–7.4, 300–310 mOsm).

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Cells were visualized by IR-DIC on an Olympus BX51 microscope (20x + 4x magnifier) and

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SPNs visually identified by somatic size (8–20 μm), morphology and location within the

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dorsolateral striatum, 50–150 μm beneath the slice surface. Data were acquired by Multiclamp

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700B amplifier digitized at 10 kHz, filtered at 2 kHz and analyzed in Clampfit10 (Molecular

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Devices). Pipette resistance (Rp) was 5–8 MΩ when filled with (in mM): 130 Cs

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methanesulfonate, 5 CsCl, 4 NaCl, 1 MgCl2, 5 EGTA, 10 HEPES, 5 QX-314, 0.5 GTP, 10 Na2-

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phosphocreatine, and 5 MgATP, 0.1 spermine, pH 7.2, 290 mOsm. Tolerance for series

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resistance (Rs) was 22 months 26).

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Longo et al., (2014) found homozygous GKI mice exhibited hyperactivity at all ages

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tested 35; although such a result might be impacted by lower body weight in mutants. In

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agreement, we previously observed an increase (~10%) in open field activity in a small cohort of

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homozygous GKI mice at 6 (but not 12) months in the founding colony 26; whereas hyperactivity

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was not observed at any age in heterozygous mice 26. Here we tested large cohorts in the open

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field exploration task and, in agreement with our previous report, found no significant effects

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(Figure 1.B.i, WT n=66 & 34, GKI n=72 & 44, for 12 months, respectively; 2-way

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ANOVA: F(1,212)=0.09, 0.002, 0.007 and p=0.8, 0.9 & 0.9 for age, genotype and interaction,

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respectively). We similarly found no evidence for thigmotaxis at either age point, suggesting a

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lack of anxiety- or anxiolytic-like phenotypes, which may have altered open field exploration

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(center path ratios were not significantly different).

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Contrastingly, in the cylinder exploration test conducted in a smaller environment that

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may stimulate mice or relieve some stress of open field testing, we found significant age and

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age-genotype interaction effects (Figure 1.B.ii, WT n=50 & 33, GKI n=59 & 49, for 12

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months, respectively; 2-way ANOVA: F(1,187)=23.6, 7.3, and p=0.0001, 0.008 for age and

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genotype-age interaction, respectively). Post-test analysis demonstrated a significant ~25%

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increase in rearing events in GKI mice aged 35% increase in miniature excitatory postsynaptic current (mEPSC) frequency at 3 weeks

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in vitro 27. To determine whether similar alterations to cortical/thalamic glutamate release occur

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in GKI mouse brain, we conducted whole-cell patch-clamp recording of dorsolateral striatal

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medium-sized spiny projection neurons (MSNs or SPNs; referred to herein as SPNs) in slices

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acutely prepared from young and aged GKI and WT littermate mice (Figure 2). Intrinsic

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membrane properties were as predicted for SPNs 30,33,38, and although there were statistically

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significant age effects upon membrane capacitance (Cm), membrane resistance (Rm) and decay

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time constants (Tau m), there were no genotype or genotype-age interaction effects (see Figure

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2.Supplement 1; 2-way ANOVA values included). Thus, intrinsic membrane properties of SPNs

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are not altered by the presence of G2019S LRRK2.

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Analysis of spontaneous EPSCs (sEPSCs, Figure 2.A) revealed a significant main age

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effect upon event amplitude (Figure 2.A.i. 2-way ANOVA; F(1,165)=3.92, p=0.049, WT n=40(17)

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& 31(12), GKI n=53(20) & 44(15), for 1-3 and >12 months, respectively), but no genotype or

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genotype-age interaction effects (F(1,165)=2.1, 1.8, p=0.15 & 0.18, respectively). Contrastingly,

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analysis of sEPSC frequencies revealed significant main genotype and age effects (Figure 2.A.ii.

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2-way ANOVA; F(1,165)=6.3, 17.3, p=0.013, 0.0001, respectively), and a significant interaction

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between genotype and age (F(1,165)=7.8, p=0.006). Post-tests demonstrated a significant ~47%

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increase in sEPSC frequency in GKI SPNs aged 1-3m, relative to WT littermates (p=0.0004).

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This phenomenon was significantly reduced with age in GKI mice (p=0.0001), but not WT

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animals (p=0.6). To more discretely interrogate the age-dependency of this large increase in

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sEPSC event frequency, comparisons were made between GKI and WT data sets in approximate

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1, 3, 12 and 18 month groupings (Figure 1. Supplement 2, RM-ANOVA values included); inter-

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event interval cumulative probabilities demonstrated that frequency is higher in GKI SPNs at 1

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and 3, but not 12 or 18 months of age.

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A recent report by Matikainen-Ankney et al., (2016) also found a similar increase in SPN

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event frequency in slices from similar GKI mice (aged 12 months, no such genotype interaction was

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observed (Figure 3.B.i). The data demonstrate that early hypersensitivity of dopamine release to

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chronic D2 activation in GKI slices is not present in slices from older animals.

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Striatal dopamine transmission is tightly regulated by dopamine transporter (DAT)-

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mediated reuptake; DAT levels, location and activity dictate the sphere of influence and,

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dominating over diffusion, are the major determinant of released dopamine’s extracellular

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lifetime in the striatum53–55. There was a significant main effect of genotype (Figure 3.B.ii. No

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Drug; 2-way ANOVA; F(1,81)=9.73, p=0.003) upon single response decay times (Tau); decays

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were significantly slower in slices from GKI mice aged 3m than WT controls (post-test p=0.01),

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but similar in older slices. Slower DA transients might be expected to result from reduced DAT

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levels or activity, but we found no genotype, age or interaction effects upon DAT (or TH)

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staining (Figure 3. Supplement 1.A. ANOVA values included). To further probe DAT we also

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assessed protein levels biochemically and found a significant main effect of genotype, due to

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increased DAT protein (relative to GAPDH) in GKI brains (Figure 3. Supplement 1.B. ANOVA

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values included).

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Although increased DAT protein suggests slower transients are not due to a paucity of

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DAT, it is possible that slowing of DA transients in GKI is due to a functional impairment in

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DAT clearance after release. Therefore, in a separate set of experiments we evoked release in a

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high concentration of the DAT blocker GBR12909 (1uM, IC50=6.63nM; Figure 3.B.ii). In the

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absence of DAT re-uptake, as expected, WT decays were approximately twice as long (No Drug

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442.1±38.9ms, +GBR12909 788.2±ms); however, there remained significant main genotype and

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age effects (Figure 3.B.ii. +GBR12909; 2-way ANOVA; F(1,97)=8.55 & 15.35, p=0.004 &

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0.0002, respectively). Consistent with results in the presence of DAT reuptake, when DAT was

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blocked decays were significantly longer in slices from GKI mice than WT controls aged 3m

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(post-test p=0.014). Decays in WT animals slowed with age (post-test p=0.002) to an extent

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similar to both young old GKI decay times.

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The data demonstrate augmented dopamine transmission in young GKI mice revealed by

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repeated stimuli and extracellular lifetime of individual pulses, which is not due to a deficit in

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DAT-dependent clearance. Moreover, an age-dependent increase in stimulated striatal dopamine

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signalling (increased extracellular lifetime) appears to occur in GKI mice at an earlier time point.

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As stimulated release is not impaired in older GKI slices, we conclude that the latent reduction in

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extracellular dopamine levels observed at 12 (but not 6) months by microdialysis 26 is a result of

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changes to the endogenous regulation of nigrostriatal dopamine release, rather than impaired

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vesicular release per se. Interestingly, in agreement with our data here, another recent report in

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similar knock-in mice by Longo and colleagues 56 found that DAT levels and activity are

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increased in GKI brains from animals aged 12 (but not 3) months. In this light, synaptic DA

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release seen here may appear similar to WT in aged GKI slices (or even reduced by

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microdialysis 26) due to increased clearance through DAT, masking a persisting increase in DA

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release.

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Summary & Conclusions

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Familial and idiopathic PD are now accepted as complex motor and non-motor syndromes

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resulting from dysfunction to multiple neurotransmitter systems and cell types throughout the

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peripheral and central nervous system 15,57. Although some treatments are effective for motor

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dysfunction, none slow or prevent the neurodegenerative process. Many non-motor symptoms

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antedate motor manifestations (and clinical diagnosis) by years to decades; demonstrating

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protracted pathological processes. In this light, investigations of PD etiology in model systems,

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especially short-lived rodents, should expect initially subtle, potentially progressive, dysfunction

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of multiple neuronal systems, prior to (or without) overt motor dysfunction and cell loss 15,57.

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GKI mice harbor a single point mutation, which confers the greatest genetic risk for PD

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in humans. We find that spontaneous exploration is significantly elevated in certain tasks in

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young mice, in broad agreement with work by other labs in similar animals 35. This exploratory

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hyperactivity is seemingly context dependent, observed in cylinder exploration but not evident in

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the open field; as such we predict this reflects changes in motivation, rather than motor function

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per se. We expect ongoing investigations of these animals in more complex environments and

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tasks will uncover other phenotypes. We previously observed that presynaptic glutamatergic

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release is elevated in cortical cultures from GKI mice 27, suggesting altered synapse development

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and / or mature function. Here we find that similar increases are observed in glutamate release

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onto striatal neurons in brain slices from young GKI animals, without changes in synapse

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number. This increase is not preserved as animals age, with GKI mice exhibiting a pronounced

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down-regulation of spontaneous glutamate release. A recent independent study of similar GKI

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animals drew similar conclusions 39, and the weight of evidence demonstrates early alterations to

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synaptic function produced by endogenous G2019S mutations.

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Our previous study revealed reduced extracellular dopamine levels in the striata of aged

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(12 months), but not young (6 month) GKI mice 26. Our analyses of dopamine transmission here

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demonstrates that reductions in vivo are unlikely to be due to impaired release or DAT reuptake,

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and are likely a consequence of altered regulation of dopamine release and/or nigral burst firing

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patterns in vivo. In young GKI mouse slices, dopamine release was elevated upon repeated

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stimulation, and the extracellular lifetime of dopamine was consistently augmented. This

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suggests that, similar to glutamate synapses, dopamine transmission is elevated in young animals

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by the G2019S mutation, an elevation which is lost with ageing. Recent brain imaging studies

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demonstrate that clinically manifest LRRK2 mutation carriers develop deterioration of the

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dopamine and serotonin systems, similarly to sporadic PD patients 22,58; however, non-manifest

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mutation carriers exhibit increased striatal dopamine turnover 22 and early increases in serotonin

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transporter binding 58. We conclude that the GKI mouse is a valuable model in which to probe

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the etiology and early pathophysiology of LRRK2, and potentially sporadic, parkinsonism.

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Interventions against such pathophysiological processes in these models may provide the

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functional neuroprotection, so desperately lacking for Parkinson’s disease and related disorders.

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Figures

Figure 1. Increased exploratory behavior in young GKI mice declines with age. A) Animal body weight over time. There was no main effect of genotype upon mouse weight, but there was a significant interaction between weight and age (see text and Figure 1 supplement 1). Weights were not significantly at any age up to 500 days (i; animal n shown in ii); however, in the oldest age-group there here was a statistically significant increase in GKI mean weight (post-test *p=0.018). B) Spontaneous exploratory behavior in young and old mice. Analysis of open field exploration showed no genotype (p=0.9) or interaction (p=0.9) effects on total distance travelled (i). In the cylinder test for vertical exploration (ii) there was a significant interaction between genotype and age upon rearing events (p=0.008). At 12m old mice (ii). Upon repeated stimulation there were significant (post-test *p=0.05, **p=0.01), elevations in dopamine peak release in GKI mice, relative to WT controls at 3 months (ii). In WT and GKI slices from >12 month mice, peak dopamine release was more variable, there a was no modulation induced by repeated stimulation, as in younger slices, nor were there genotype differences. There was a significant age-dependent increase in D2 autoreceptor mediated paired-pulse depression (iii; 3m n=20(5) & 13(5), >12m n=18(6) & 30(11) for WT and GKI, respectively, post-test *** p=0.001), reflected by reduced paired-pule ratios (PPR), but there was no main effect of genotype (p=0.7) or genotype-age interaction (p=0.99). B) The D2 agonist quinpirole (10uM) equally suppressed peak evoked dopamine release in WT and GKI slices from 3m mice after a 10-minute wash-in; however, continued exposure

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revealed a trend toward a main effect of genotype (p=0.07) on repeated release, and a significant genotype-time interaction (p=0.039, post-test *p=0.049 at 18min post application). This effect was not observed in >12 month slices (i). Evoked dopamine transient decay time revealed a significant main genotype effect (ii. No drug, 3m n=20(5) & 13(5), >12m n=19(6) & 33(11) for WT and GKI, respectively p=0.0025) due to significantly longer decay times (slower transients) in 3m GKI slices (post-test *p=0.027). In >12m slices, WT decay times had increased to a value similar to 3 month GKI (ii. No drug). The presence of the presynaptic dopamine transporter (DAT) blocker GBR (10uM, ii. +GBR12909) increased all transient decay times, as expected. There were significant main effects of age and genotype (3m n =17(6) & 23(8), >12m n=31(9) & 30(11), p=0.0002 & 0.0043 for WT and GKI, respectively) upon transient lifetime; in the absence of dopamine re-uptake, 3 GKI transients were still significantly slower than WT controls (post-test *p=0.014), and significant age-dependent increases in dopamine decay times were observed in WT slices (post-test *p=0.0024).

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Figure 1. Supplement 1. Weights of WT, GKI and GKI homozygous animals are similar until advanced ages. Animal body weight over time. There was no main effect of genotype upon mouse weight, but there was a significant interaction between weight and age (2-way ANOVA, genotype p=0.17, interaction p=0.049). Weights were not significantly at any age up to 500 days; however, in the oldest age-group there was a statistically significant increase in GKI (Holm-Sidak post-test *p=0.018) and GKI Homo (post-test ##p=0.009) mean weight, relative to WT animals.

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Figure 2. Supplement 1. There are no genotype-dependent alterations to SPN intrinsic membrane properties. Whole-cell recording of medium-sized spiny striatal projection neurons (SPNs) in membrane test mode and analysis of intrinsic membrane properties. While significant age-dependent changes in membrane capacitance (Cm), resistance (Rm) and decay time (Taum) were observed, there were no genotype or interaction effects. 2-way ANOVA detailed below.

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Figure 2. Supplement 2. GKI SPN sEPSC frequency is similarly increased in 1 and 3 month slices, and similar to WT in 12 and 18 month slices. Whole-cell recording in dorsolateral striatal SPNs and analysis of sEPSCs recorded at -70mV. Higher sEPSC frequencies (as reflected by decreased inter-event intervals) underlie significant main genotype effects to similar extents in slices from 1 and 3 month animals, but there were no significant effects of genotype in 12 or 18 month slices. Details of 2-way RM-ANOVA and Holm-Sidak post-tests are shown below.

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Figure 2. Supplement 3. Spontaneous EPSCs recorded in dorsolateral striatum in the coronal slice preparation are TTX insensitive. A.i) Example traces of spontaneous EPSCs (sEPSCs) in whole-cell voltage-clamp (WCVC) recordings in striatal projection neurons (SPN) of the dorsolateral striatum, prior to, and in the presence of, tetrodotoxin (TTX; 1uM) rendering any remaining responses miniature EPSCs (mEPSCs). ii) There was no significant effect of TTX upon mean sEPSC amplitude (paired t-test p=0.82) or frequency (paired t-test p=0.29) within each cell. Expressed as a percentage of the initial sEPSC mean value, neither mEPSC amplitudes nor frequencies were significantly altered by TTX application (onesample t-test against 100, p=0.7 & 0.2 respectively). The weak trend in frequency was towards increases in GKI, rather than the decreases predicted if action potential-dependent events were contributing to mean sEPSC frequency.

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Figure 2. Supplement 4. There are no differences in the number of postsynaptic specializations, or presynaptic excitatory nerve terminal markers in the striatum of young or aged GKI mice. A) Representative bright-field images of Golgi-impregnated medium-sized spiny projection neurons (SPNs) of the dorsolateral striatum from WT and GKI mice aged 1-3m (i), with inserts showing zoom of areas marked in main image by dashed lines. There were significant increases in the density of postsynaptic protrusions as animals aged between 1-3 and >12 months, but there were no significant differences between WT and GKI dendritic protrusions at either age (WT n=57(9) & 18(3), GKI n=56(9) & 16(3), for 1-3 & >12, respectively). B) Representative confocal micrograph of VGluT1 and VGluT2 immunofluorescence staining in 2 month-old WT dorsolateral striatum, demonstrating the expected pattern of a higher density of VGluT1 than VGluT2 glutamatergic terminals and very little overlap between VGluT1 and VGluT2 (i). There were no significant effects of age or genotype upon VGluT1 or VGluT2 cluster densities (ii; WT n=8(4) & 9(4), GKI n=8(4) & 7(4), for 2m & >12m, respectively). Details of 2-way ANOVA and Holm-Sidak post-tests are shown below.

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Figure 2. Supplement 5. Paired-pulse facilitation profiles in the striatum of aged GKI mice are similar & GKI mutant PPRs are insensitive to dopamine agonism and antagonism. Whole-cell patch clamp recording and eEPSC paired-pulse experiments. i) There were no significant differences in glutamatergic paired-pulse ratio at >12 months, similarly to the 1-3m age point in the absence of quinpirole (Fig.2.C). At 1m of age, when WT and GKI PPRs differ significantly in the presence of quinpirole (Fig.2.C), GKI PPRs were insensitive to both dopamine agonism (ii. 10uM quinpirole) and antagonism (iii. 10uM remoxipride). Details of 2-way ANOVA and Holm-Sidak post-tests are shown below.

Figure 2. Supplement 6. Spontaneous EPSCs recorded in GKI dorsolateral striatum SNPs are remoxipride insensitive. Spontaneous EPSCs (sEPSCs) were recorded prior to, and in the presence of, remoxipride (Remox; 10uM). A) There was no significant effect of remoxipride upon mean sEPSC amplitude (paired t-test p=0.86) or frequency (paired t-test p=0.80) within each cell. Expressed as a percentage of the initial sEPSC mean value, neither amplitude nor frequency was significantly altered by remoxipride application (one-sample t-test against 100, p=0.84 & 0.74, respectively).

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Figure 2. Supplement 7. There were no differences in GKI sEPSC frequency between D1- and D2dopamine receptor expressing SPNs, and no consistent effect of quinpirole upon eEPSC peak or PPR in either cell type. Whole-cell patch clamp recording in D1 vs. D2-type SPNs in GKI slices. A) There were no differences in sEPSC mean event frequency in either cell type at young (1-3m) or old (>12m) age points. B) The D2 agonist quinpirole had no consistent modulatory effect upon D1 or D2 eEPSC peak amplitudes, or Paired-pulse ratios expressed as post-application values over pre-application (C). 2-way ANOVA and unpaired Student’s t-test values are shown.

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Figure 3. Supplement 1. Early alterations in GKI dopamine release are not associated with reductions in nigrostriatal dopamine markers and persist despite genotype-dependent increases in DAT protein levels by western blot. A) Staining for presynaptic dopamine markers DAT and tyrosine hydroxylase (TH; i) showed highly specific enrichment in the striatum; however, there were no age or genotype differences (ii). Although variable, the data suggest there is no loss of either terminal marker in GKI striatum. B) Striatal DAT levels were also assessed by western blot (i); although variable results in young GKI suggest no loss of DAT. ii) A significant main genotype effect (due to increased DAT levels in aged GKI) revealed increases in striatal DAT levels in the presence of endogenous G2019S. 2-way values are shown.

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