Long-term in-vivo analysis of habenular neurocircuit development identifies a thalamic bilateral cluster of early projecting cells (ThEPC) critical for interhemispheric communication and ventral habenula (vHb) formation

In the dorsal diencephalon of all vertebrates, the habenular neurocircuit transfers cognitive information from the forebrain into the ventral mid- and hindbrain via long axon fibers in the stria medullaris (SM) and fasciculus retroflexus (FR) on both sides of the brain. How these axons navigate through the brain and whether communication between brain hemispheres is required during the formation of this neuronal network is still an open question. The bilaterally formed habenulae in the dorsal diencephalon in zebrafish consist of the asymmetrically formed dorsal habenula nucleus (dHb) and the symmetric ventral habenula nucleus (vHb). While development of the dHb has been well described, the origin of the vHb and the genetic cascades underlying its development are not known. We use the habenular network as a model to investigate how axon elongation is coordinated during embryonic development. This can best be done by recording its development in-vivo. As this neural circuit takes at least 4 days to develop and spans about 300 µm in anterior-posterior and dorso-ventral direction, we needed to develop a novel assay to investigate its development in the living zebrafish embryo. In our studies, we identified a transgenic line of zebrafish expressing GFP throughout the habenular neurocircuit development in all subnuclei and their efferent projections. Combining optimised in-vivo 2-photon (2-PM) long-term image recording and colour code analysis with focal laser ablation of neurons, we discovered a neuronal network essential for dorsal habenular axon elongation and pathfinding. We present evidence that a bilateral cluster of early projecting neurons in the thalamus (ThEPC) functions as intermediate target for dHb axonal elongation via ipsilateral short- and contralateral long-range axonal communication between the two brain hemispheres. Moreover, we show that a subset of ThEPC neurons contributes to the forming ventral habenula, which development is controlled by tcf7l2 mediated Wnt/beta-catenin signalling