Regulation of nigrostriatal dopamine neurotransmission by gamma-synuclein: down- and over-expression mouse models

Abstract

Synucleins are small, highly conserved proteins in vertebrates, especially abundant in neurons and typically enriched in presynaptic terminals. Mutations in ¿-synuclein gene have been reported in families susceptible to inherited forms of Parkinson¿s disease, but far less is known about other members of the synuclein family (ß- and ¿-synucleins), which have been implicated in the pathophysiology of neurodegenerative diseases and several synucleinopathies. Here, we examined the distribution and cellular localization of ¿-synuclein in the mouse brain using in situ hybridization and immunohistochemistry procedures, and we evaluated the functional role of ¿-synuclein in nigrostriatal dopamine (DA) neurotransmission in vivo. We used two different models with: 1) down-regulated ¿-synuclein expression specifically in DA neurons of substantia nigra compacta/ventral tegmental area (SNc/VTA) by small interfering RNA molecules (siRNA), or 2) over-expressed ¿-synuclein in SNc/VTA DA neurons using AAV10-CMV-mouse-¿-synuclein vector. In C57BL/6J control mice, abundant levels of ¿-synuclein mRNA were found in the monoaminergic nuclei including SNc, VTA, raphe nuclei and locus coeruleus, as well as in the habenular nuclei. Double in situ hybridization and immunohistochemistry showed a specific colocalization of ¿-synuclein in TH-positive DA neurons. Unilateral infusion of three siRNA sequences targeting ¿-synuclein (4.2 nmol) decreased ¿-synuclein mRNA levels in DA neurons of SNpc (50% and 80% of aCSF-treated mice 24h and 72h post-infusion, respectively). Furthermore, ¿-synuclein suppression displayed an enhanced striatal DA tone using intracerebral microdialysis, which is reflected in higher extracellular DA levels in siRNA-treated mice during local veratridine (50 ¿M), nomifensine (1-10-50 ¿M) and amphetamine (1-10-100 ¿M) administrations and lower levels with quinpirole (10 ¿M). Meanwhile, ¿-synuclein over-expression led to lower extracellular DA levels during nomifensine (1-10-50 ¿M) administration. In conclusion, these results confirm that ¿- synuclein is a negative regulator of DA neurotransmission and suggest that siRNA-induced ¿-synuclein suppression in midbrain dopaminergic neurons may lead to new therapies for synucleinopathies.