Abstract
The ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) of the midbrain are associated with Parkinson’s disease (PD), schizophrenia, mood disorders and addiction. Based on the recently unraveled heterogeneity within the VTA and SNc, where glutamate, GABA and co-releasing neurons have been found to co-exist with the classical dopamine neurons, there is a compelling need for identification of gene expression patterns that represent this heterogeneity and that are of value for development of human therapies. Here, several unique gene expression patterns were identified in the mouse midbrain of which NeuroD6 and Grp were expressed within different dopaminergic subpopulations of the VTA, and TrpV1 within a small heterogeneous population. Optogenetics-coupled in vivo amperometry revealed a previously unknown glutamatergic mesoaccumbal pathway characterized by TrpV1-Cre-expression. Human GRP was strongly detected in non-melanized dopaminergic neurons within the SNc of both control and PD brains, suggesting GRP as a marker for neuroprotected neurons in PD. This study thus unravels markers for distinct subpopulations of neurons within the mouse and human midbrain, defines unique anatomical subregions within the VTA and exposes an entirely new glutamatergic pathway. Finally, both TRPV1 and GRP are implied in midbrain physiology of importance to neurological and neuropsychiatric disorders.
Highlights
Have demonstrated that ventral tegmental area (VTA)/substantia nigra pars compacta (SNc)-derived glutamate and GABA interact with various aspects of DA neurotransmission, and that these newly discovered VTA/SNc populations are involved in similar brain processes as the DA neurons
To compare gene expression profiles between the VTA and the SNc in the newborn mouse, the direct red fluorescence obtained from the reporter protein tdTomato was used to grossly outline the outer borders of the VTA and SNc which were dissected from DA transporter (Dat)-CretdTom pups at postnatal day 3 (P3) (Fig. 1a top)
These 15 genes were: Gastrin-releasing peptide (Grp), Nicotinic acetylcholine receptor subunit alpha-2 (Chrna2), Protein phosphatase-1 subunit-17 (Gsbs), Transient receptor potential cation channel subtype V1 (TrpV1), Melanocortin-3-receptor (Mc3r), Neuropilin-2 (Nrp2), Cysteine-rich secretory protein-1 (Crisp1), G-protein-coupled receptor-83 (Gpr83), Neurotrophin-3 (Ntf3), Tachykinin-receptor-3 (Tacr3), Neuromedin-U (Nmu) and Follistatin (Fst) which all showed higher expression in the VTA, and the Zinc-finger-protein-of-the-cerebellum-2 (Zic2), Serine peptidase inhibitor subtype-f1 (Serpinf1) and Sine-oculis-related homeobox-3 (Six3) genes that were higher in the SNc samples
Summary
Have demonstrated that VTA/SNc-derived glutamate and GABA interact with various aspects of DA neurotransmission, and that these newly discovered VTA/SNc populations are involved in similar brain processes as the DA neurons It is far from resolved how this occurs and what impact it may have on cognitive, affective and motoric function. In light of the recent realization of the multitude of different neuronal cell types that co-exist in the ventral midbrain, there is a compelling need for identification of gene expression patterns that represent this anatomical and functional heterogeneity. Such patterns should prove useful both for improved resolution in animal models and for clinical purposes. TrpV1-Cre transgenic mice were selected for functional optogenetics within the midbrain, while the human counterparts of the genes identified in the mouse were assessed in human brain material, including post-mortem tissue derived from PD patients
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