Increased dopaminergic innervation in the brain of conditional mutant mice overexpressing Otx2: effects on locomotor behavior and seizure susceptibility.

The homeobox-containing transcription factor Otx2 controls the identity, fate and proliferation of mesencephalic dopaminergic (mesDA) neurons. Transgenic mice, in which Otx2 was conditionally overexpressed by a Cre recombinase expressed under the transcriptional control of the Engrailed1 gene (En1(Cre/+); tOtx2(ov/+)), show an increased number of mesDA neurons during development. In adult mice, Otx2 is expressed in a subset of neurons in the ventral tegmental area (VTA) and its overexpression renders mesDA more resistant to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-HCl (MPTP) neurotoxin. Here we further investigated the neurological consequences of the increased number of mesDA neurons in En1(Cre/+); tOtx2(ov/+) adult mice. Immunohistochemistry for the active, glycosylated form of the dopamine transporter (glyco-Dat) showed that En1(Cre/+); tOtx2(ov/+) adult mice display an increased density of mesocortical DAergic fibers, as compared to control animals. Increased glyco-Dat staining was accompanied by a marked hypolocomotion in En1(Cre/+); tOtx2(ov/+) mice, as detected in the open field test. Since conditional knockout mice lacking Otx2 in mesDA precursors (En1(Cre/+); Otx2(floxv/flox) mice) show a marked resistance to kainic acid (KA)-induced seizures, we investigated the behavioral response to KA in En1(Cre/+); tOtx2(ov/+) and control mice. No difference was observed between mutant and control mice, but En1(Cre/+); tOtx2(ov/+) mice showed a markedly different c-fos mRNA induction profile in the cerebral cortex and hippocampus after KA seizures, as compared to controls. Accordingly, an increased density of parvalbumin (PV)-positive inhibitory interneurons was detected in the deep layers of the frontal cortex of naïve En1(Cre/+); tOtx2(ov/+) mice, as compared to controls. These data indicate that Otx2 overexpression results in increased DAergic innervation and PV cell density in the fronto-parietal cortex, with important consequences on spontaneous locomotor activity and seizure-induced gene expression. Our results strengthen the notion that Otx2 mutant mouse models are a powerful genetic tool to unravel the molecular and behavioral consequences of altered development of the DAergic system.