Otx1 promotes basal dendritic growth and regulates intrinsic electrophysiological and synaptic properties of layer V pyramidal neurons in mouse motor cortex

The transcription factor Otx1 is specifically expressed in layer V pyramidal cells (L5PCs) in the cerebral cortex. Otx1 null mutant mice have a defect in the developmental axon pruning of L5PCs and show epileptic seizures. However, the role of Otx1 in electrophysiology, morphology and synaptology of the cortical neurons has not been fully investigated. This study examines the influences of Otx1 on neuronal properties of L5PCs by loss- and gain-of-function approaches. Mice with an Otx1-null mutation had decreased structural measurements of basal dendrites in L5PCs. In contrast, the size of basal dendrites was increased in the Otx1-over-expressed pyramidal cells (PCs) in L2/3 where the gene normally does not express. PCs showed burst and non-burst firing patterns of action potentials. The proportion of burst firing neurons was reduced in the Otx1 mutant but increased in the neurons over-expressing Otx1. Although the burst firing population decreased, the proportion of those bursting neurons with a low threshold increased in the Otx1 mutant mice. Moreover, excitatory facilitating synaptic connections formed between L5PCs were predominant in the Otx1 mutant mice, which greatly contrasted with the predominant depressing synaptic connections in the controls. Taken together, it suggests an enhanced activity of neuronal network in the cortex of Otx1 mutant mice. These data indicate that the Otx1 expression is essential for the normal development of dendritic morphology, intrinsic electrophysiology and synaptic dynamics of L5PCs. This study provides new insights into molecular mechanisms underlying the spatial and temporal regulation of neuronal and synaptic properties of L5PCs, and improves our understanding on the generation of epileptic seizures.