Morpho‐Functional Changes of Nigral Dopamine Neurons in an α‐Synuclein Model of Parkinson's Disease

Abstract

Background: The accumulation of α-synuclein (α-syn) fibrils in intraneuronal inclusions called Lewy bodies and Lewy neurites is a pathological signature of Parkinson's disease (PD). Although several aspects linked to α-syn-dependent pathology (concerning its spreading, aggregation, and activation of inflammatory and neurodegenerative processes) have been under intense investigation, less attention has been devoted to the real impact of α-syn overexpression on structural and functional properties of substantia nigra pars compacta (SNpc) dopamine (DA) neurons, particularly at tardive stages of α-syn buildup, despite this has obvious relevance to comprehending mechanisms beyond PD progression. Objectives: We aimed to determine the consequences of a prolonged α-syn overexpression on somatodendritic morphology and functions of SNpc DA neurons. Methods: We performed immunohistochemistry, stereological DA cell counts, analyses of dendritic arborization, ex vivo patch-clamp recordings, and in vivo DA microdialysis measurements in a 12-to 13-month-old transgenic rat model overexpressing the full-length human α-syn (Snca +/+ ) and age-matched wild-type rats.Results: Aged Snca +/+ rats have mild loss of SNpc DA neurons and decreased basal DA levels in the SN. Residual nigral DA neurons display smaller soma and compromised dendritic arborization and, in parallel, increased firing activity, switch in firing mode, and hyperexcitability associated with hypofunction of fast activating/ inactivating voltage-gated K + channels and Ca 2+ -and voltage-activated large conductance K + channels. These intrinsic currents underlie the repolarization/ afterhyperpolarization phase of action potentials, thus affecting neuronal excitability. Conclusions: Besides clarifying α-syn-induced pathological landmarks, such evidence reveals compensatory functional mechanisms that nigral DA neurons could adopt during PD progression to counteract neurodegeneration.

Systemic inflammation triggers long-lasting neuroinflammation and accelerates neurodegeneration in a rat model of Parkinson’s disease overexpressing human α-synuclein

“…Another important feature highlighted in the present study is the dystrophic TH + dendritic arborizations, branching from SNpc surviving DAergic neurons towards the SNpr in LPS Snca +/+ rats, anticipating by almost seven months similar alterations in the naϊve Snca +/+ animals 21 . The loss of neuronal complexity and the decreased dendritic arborization have been linked to α-synuclein overexpression in virally infected DAergic neurons in vitro and in vivo , preceding their eventual death 34,35 and resembling PD patient’s post- mortem alterations 36 .…”

“…All procedures follow the guidelines on the ethical use of animals from the Council Directiveof the European Communities (2010/63/EU) and were approved by the Italian Ministry of Health (Authorization N°617-2019PR). Experimental animals were obtained by crossing heterozygous males with heterozygous female rats and were confirmed as WT or Snca +/+ following genotyping with quantitative PCRusing DNA from ear biopsies and the primers for copy numbers of the α-synuclein transgene: SynProm-F: 5′-cgctcgagcggtaggaccgcttgttttagac-3′ and LC- SynPromR: 5′-cctctttc cacgccactatc-3′, normalized to the rat β-actin reference gene with primers: β-actin- F: 5′- agccatgtacgtagccatcca-3′ and β-actin-R: 5′-tctccggagtccatcacaatg-3′ 18,19,21 .…”

“…In line with this, TH protein levels decreased in the SN of Wistar rats In the present investigation, we tested the hypothesis that PD development occurs through a doublehit mechanism involving the combined interplay of elevated endotoxin and aggregated α-synuclein, with the outcome of neuronal degeneration in the SNpc. To this aim, we used rats overexpressing human α-synuclein (Snca +/+ ) characterized by a prodromal asymptomatic phase, with the first functional signs of DAergic pathology starting at four months of age, later resulting in a 25-27% depletion of TH and soma-dendritic neuronal modification at 12 months, accompanied by altered firing properties of DAergic neurons [18][19][20][21] . In this animal model, we induced systemic inflammation at a presymptomatic age (2 mont hs) via a single intraperitoneal injection of LPS, and we evaluated the effects on neuroinflammation and the DAergic system three months later.…”

ErbB inhibition rescues nigral dopamine neuron hyperactivity and repetitive behaviors in a mouse model of fragile X syndrome

“…Patch-clamp recordings of SNpc DA neurons were performed with a setup equipped with an upright microscope (Nikon Eclipse FN1) and an infrared video-camera (CoolSNAP Photometrics). SNpc DA neurons were identified based on their morphology (fusiform, tightly packed, and medium to large-sized cell bodies), localization (in the medial SNpc, close to the medial terminal nucleus of the accessory optic tract, MT), and their typical electrophysiological features, as slow spontaneous firing (1-8 Hz) in cell-attached configuration, large action potential (>2ms) with a prominent afterhyperpolarization, or the presence of a prominent hyperpolarization-activated inward current (I h ) in response to hyperpolarizing voltage steps 33,85 . 15 min), that unmask pure mGluR5-or mGluR1-mediated currents respectively 33 .…”

Molecular and Epigenetic Aspects of Opioid Receptors in Drug Addiction and Pain Management in Sport

“…DA neurons located in the SNpc and VTA are characterized by spontaneous action potential firing, either as the tonic regular pacemaker (single spike) or as phasic burst firing (as recently reviewed by [75]). The latter has been mainly described in vivo, as it depends on glutamatergic circuitry impinging onto DA neurons, whereas primary cultured DA neurons or those maintained in ex vivo brain slice preparations mainly fire as regular pacemakers [76,77]. Bursting activity, consisting of clusters of two-to-ten action potentials, causes DA release either at the distal sites [78][79][80] or locally, within the midbrain [81][82][83].…”