Filippo del Bene
Date: February 08, 2018. 12:00
Location: CCU Seminar Room
Title: Deciphering anatomy and function of an inter-hemispheric neural circuit in the zebrafish optic tectum.
Affiliation: Institut Curie, France.
Larval zebrafish show complex goal-directed hunting behaviour that seems to be mainly guided by vision and requires sensory integration to detect the prey and a series of specific locomotor manoeuvres to track it. During hunting, zebrafish larvae converge their eyes thereby considerably increasing the overlapping field of view. Furthermore, the final capture swim is being stereotypically initiated when the larva is at a distance of about 0.5mm to the prey. These evidences suggest that larva might be able to estimate object distance by using binocular visual information. However, given that the larva has entirely crossed projections from the eyes to the visual brain, the interpretation of binocular information on this connectional level, like e.g. found in mammals, is unlikely. Thus the neural substrate for such a mechanism in zebrafish is as of yet unknown.
We recently identified a zebrafish line that expresses Gal4 in a previously undescribed commissural neuron population in the brain. We found these neurons to be bilateral-symmetrically distributed adjacent to the ventral tectum. Furthermore, they connect the tectal halves, form synapses there and were thus termed intertectal neurons (ITNs). We reasoned that ITNs might be good candidates for the potential transfer and/or the integration of binocular visual signals and that they thus might have a role during larval prey capture.
We next unilaterally ablated ITNs with 2p imaging and subsequently examined free-swimming behaviour of ablated vs. ctrl fish during an assay of prey capture. Ablated fish hunted less efficiently than wild-type but basic motor parameters were not affected. However, ablated fish showed a drastically diminished probability of initiating capture swims when close to the prey, arguing that ITNs might indeed play a role in the last step of the prey hunting sequence, the initiation of the capture swim.
Using 2p Ca imaging, we established that ITNs respond to moving bars and small dots simulating moving prey. Furthermore, after unilateral eye-ablation, we observed Ca transients in the tectal hemisphere not receiving any retinal input that were co-localized with the trajectories of the contralateral ITN arbours suggesting that ITNs might transfer prey-specific information to the ipsilateral tectum with respect to the prey.
In summary, we anatomically describe novel, previously unknown, inter-hemispheric neural connections in the zebrafish visual system, establish their role in inter-tectal visual signal transfer and show that they are important for the successful completion of the hunting behaviour sequence.