Otx2 expression is restricted to dopaminergic neurons of the ventral tegmental area in the adult brain

Mesencephalic-diencephalic dopaminergic (mdDA) neurons control motor, sensorimotor and motivated behaviour and their degeneration or abnormal functioning is associated with important pathologies, such as Parkinson's disease and psychiatric disorders. Despite great efforts, the molecular basis and the genetic factors differentially controlling identity, survival and vulnerability to neurodegeneration of mdDA neurons of the substantia nigra (SN) and ventral tegmental area (VTA) are poorly understood. We have previously shown that the transcription factor Otx2 is required for identity, fate and proliferation of mesencephalic DA (mesDA) progenitors. By using mouse models and immunohistochemistry, we have investigated whether Otx2 is expressed also in post-mitotic mdDA neurons. Our data reveal that Otx2 is expressed in post-mitotic mesDA neurons during mid-late gestation and in the adult brain. Remarkably, Otx2 expression is sharply excluded from mdDA neurons of the SN and is restricted to a relevant fraction of VTA neurons. Otx2(+)-TH+ neurons are concentrated to the ventral part of the VTA. Combined expression with other regionalized VTA markers shows that Otx2+-TH+ neurons are prevalently Girk2(-) and Calb(+) and among these, those located in the medial and ventralmost portion of the VTA are also Ahd2(+). These findings indicate that Otx2 represents the first transcription factor with a proven role in mdDA neurogenesis whose expression discriminates between SN and a relevant proportion of VTA neurons. This supports the possibility that Otx2 may act as a post-mitotic selector controlling functional features (e.g. identity and/or survival) of a relevant fraction of VTA neurons in the adult.