Light/dark cycles modulate asymmetric parapineal connectivity to the left habenula in zebrafish larvae

Abstract

Introduction: Parapineal (PpO) neurons develop asymmetric connectivity to the left habenula (lHb) in the dorsal diencephalon of embryonic and larval zebrafish. Recent studies relate this asymmetric connectivity with sensory responses to light. However, the structural and functional bases of this behaviour are still unknown. To begin addressing this issue we investigated the axonal structural configuration and distribution of pre-synaptic proteins in PpO-lHb projections of zebrafish larvae searching for plastic changes modulated by light. Material and Methods: PpO-lHb connectivity was visualised by confocal microscopy in dissected brains of Tg(foxd3:GFP) zebrafish larvae maintained under different Light (L) and Dark (D) conditions: (a) cycles of 14L and 10D hours [L:D], (b) continuous dark [D:D], and (c) continuous light [L:L]. After fixation, anti-GFP and anti-SNAP-25 inmunofluorescence were performed in peeled brains and confocal images acquired. 3D image analysis included manual/automatic segmentation, computing shape descriptors of PpO projections, and pre-synaptic puncta quantification. Results: PpO projections show a characteristic morphology that undergo rhythmic changes in larvae subject to L:D cycles, particularly during the L-D and D-L transitions. Also, the number and composition of pre-synaptic protein expression in the PpO-lHb circuit show rhythmic remodelling two hours before the L-D and D-L transitions. This features were not found in the D:D and L:L conditions. Discussion: These results suggest that the PpO-lHb circuit in zebrafish larvae responds to Light:Dark rhythmical changes through a mechanism of plasticity, generating structural and synaptic remodelling of PpO projections. Funding: ICM P09-015-F, FONDECYT (3160421, 1161274, 1151029), CONICYT PIA ACT1402, FONDAP 15150012.