Striatal Dopamine Transporter Function Is Facilitated by Converging Biology of α-Synuclein and Cholesterol

Striatal dopamine transporters (DAT) powerfully regulate dopamine signaling, and can contribute risk to degeneration in Parkinson’s disease (PD). DATs can interact with the neuronal protein α-synuclein, which is associated with the etiology and molecular pathology of idiopathic and familial PD. Here, we tested whether DAT function in governing dopamine (DA) uptake and release is modified in a human-α-synuclein-overexpressing (SNCA-OVX) transgenic mouse model of early PD. Using fast-scan cyclic voltammetry (FCV) in ex vivo acute striatal slices to detect DA release, and biochemical assays, we show that several aspects of DAT function are promoted in SNCA-OVX mice. Compared to background control α-synuclein-null mice (Snca-null), the SNCA-OVX mice have elevated DA uptake rates, and more pronounced effects of DAT inhibitors on evoked extracellular DA concentrations ([DA] ) and on short-term plasticity (STP) in DA release, indicating DATs play a greater role in limiting DA release and in driving STP. We found that DAT membrane levels and radioligand binding sites correlated with α-synuclein level. Furthermore, DAT function in Snca-null and SNCA-OVX mice could also be promoted by applying cholesterol, and using Tof-SIMS we found genotype-differences in striatal lipids, with lower striatal cholesterol in SNCA-OVX mice. An inhibitor of cholesterol efflux transporter ABCA1 or a cholesterol chelator in SNCA-OVX mice reduced the effects of DAT-inhibitors on evoked [DA] . Together these data indicate that human α-synuclein in a mouse model of PD promotes striatal DAT function, in a manner supported by extracellular cholesterol, suggesting converging biology of α-synuclein and cholesterol that regulates DAT function and could impact DA function and PD pathophysiology

GABA uptake transporters support dopamine release in dorsal striatum with maladaptive downregulation in a parkinsonism model

Striatal dopamine (DA) is critical for action and learning. Recent data show that DA release is under tonic inhibition by striatal GABA. Ambient striatal GABA tone on striatal projection neurons can be determined by plasma membrane GABA uptake transporters (GATs) located on astrocytes and neurons. However, whether striatal GATs and astrocytes determine DA output are unknown. We reveal that DA release in mouse dorsolateral striatum, but not nucleus accumbens core, is governed by GAT-1 and GAT-3. These GATs are partly localized to astrocytes, and are enriched in dorsolateral striatum compared to accumbens core. In a mouse model of early parkinsonism, GATs are downregulated, tonic GABAergic inhibition of DA release augmented, and nigrostriatal GABA co-release attenuated. These data define previously unappreciated and important roles for GATs and astrocytes in supporting DA release in striatum, and reveal a maladaptive plasticity in early parkinsonism that impairs DA output in vulnerable striatal regions

Investigating the control of striatal dopamine neurotransmission by axonal calcium channels and by striatal neuromodulators: Insights for Parkinson's disease

Dopamine (DA) is a key striatal neuromodulator which is central to processes including action selection and reward-related learning. DA dysfunction is associated with a number of psychomotor disorders, most notably of which is Parkinson's disease (PD). This thesis uses fast-scan cyclic voltammetry in acute mouse striatal slices to detect DA release at carbon fibre microelectrodes with subsecond temporal resolution, to investigate factors affecting the presynaptic control of DA release. In this thesis, I have investigated the roles of voltage dependent calcium channels (VDCCs) and the neuromodulator, substance P (SP), in the presynaptic control of DA, in the presence of nicotinic acetylcholine receptor (nAChR) blockade. This is because ACh has profound modulatory and driving effects on DA release, via activation of nAChRs on DA terminals. In CPu, blockers for N-, P/Q-, T- or L-type VDCCs (ω-Conotoxin GVIA, ω-Agatoxin IVA, NNC 55-0396, isradipine) reduced DA release to varying degrees (N&gt;P/Q&gt;T&gt;L). Furthermore, L-type function was eliminated by α-synuclein knockout. In NAc, only N and P/Q-blockers modified DA release (N&gt;P/Q) and more weakly than in CPu. Frequency-specific effects of some VGCCs were reproduced by changes to extracellular Ca2+ or release probability, consistent with Ca2+ entry governing the relationship between DA release probability and its short-term plasticity. Finally I have shown that SP can directly modulate striatal DA release in a manner that depends on striosome-matrix location. To date there is much conflict in the literature on the role of SP in a number of striatal processes, this finding may help to resolve these conflicts and shed light on the little understood role of the striosome-matrix division of the striatum.</p

Investigating the control of striatal dopamine neurotransmission by axonal calcium channels and by striatal neuromodulators: Insights for Parkinson's disease

Dopamine (DA) is a key striatal neuromodulator which is central to processes including action selection and reward-related learning. DA dysfunction is associated with a number of psychomotor disorders, most notably of which is Parkinson's disease (PD). This thesis uses fast-scan cyclic voltammetry in acute mouse striatal slices to detect DA release at carbon fibre microelectrodes with subsecond temporal resolution, to investigate factors affecting the presynaptic control of DA release. In this thesis, I have investigated the roles of voltage dependent calcium channels (VDCCs) and the neuromodulator, substance P (SP), in the presynaptic control of DA, in the presence of nicotinic acetylcholine receptor (nAChR) blockade. This is because ACh has profound modulatory and driving effects on DA release, via activation of nAChRs on DA terminals. In CPu, blockers for N-, P/Q-, T- or L-type VDCCs (&omega;-Conotoxin GVIA, &omega;-Agatoxin IVA, NNC 55-0396, isradipine) reduced DA release to varying degrees (N>P/Q>T>L). Furthermore, L-type function was eliminated by &alpha;-synuclein knockout. In NAc, only N and P/Q-blockers modified DA release (N>P/Q) and more weakly than in CPu. Frequency-specific effects of some VGCCs were reproduced by changes to extracellular Ca2+ or release probability, consistent with Ca2+ entry governing the relationship between DA release probability and its short-term plasticity. Finally I have shown that SP can directly modulate striatal DA release in a manner that depends on striosome-matrix location. To date there is much conflict in the literature on the role of SP in a number of striatal processes, this finding may help to resolve these conflicts and shed light on the little understood role of the striosome-matrix division of the striatum.</p

Transcription factors FOXA1 and FOXA2 maintain dopaminergic neuronal properties and control feeding behavior in adult mice

Midbrain dopaminergic (mDA) neurons are implicated in cognitive functions, neuropsychiatric disorders, and pathological conditions; hence understanding genes regulating their homeostasis has medical relevance. Transcription factors FOXA1 and FOXA2 (FOXA1/2) are key determinants of mDA neuronal identity during development, but their roles in adult mDA neurons are unknown. We used a conditional knockout strategy to specifically ablate FOXA1/2 in mDA neurons of adult mice. We show that deletion of Foxa1/2 results in down-regulation of tyrosine hydroxylase, the rate-limiting enzyme of dopamine (DA) biosynthesis, specifically in dopaminergic neurons of the substantia nigra pars compacta (SNc). In addition, DA synthesis and striatal DA transmission were reduced after Foxa1/2 deletion. Furthermore, the burst-firing activity characteristic of SNc mDA neurons was drastically reduced in the absence of FOXA1/2. These molecular and functional alterations lead to a severe feeding deficit in adult Foxa1/2 mutant mice, independently of motor control, which could be rescued by L-DOPA treatment. FOXA1/2 therefore control the maintenance of molecular and physiological properties of SNc mDA neurons and impact on feeding behavior in adult mice