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O. Biehlmaier, S. Kustermann, S.C. Neuhauss, K. Kohler; Onset and Maturation of Synaptic Structures and Functionality at the Cone Terminal of the Zebrafish Retina . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1648.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: So far, only little has been reported on the morphology of the synaptic connections between photoreceptors and their second order neurons during zebrafish retinal development. Here, we describe the appearance and differentiation of synaptic structures at the cone terminal of the developing zebrafish retina together with the maturation of directly connecting second order neurons and a dopaminergic third order feedback-neuron from the inner retina. Methods: Zebrafish larvae were examined at developmental stages from 2 to 7dpf (days post fertilization) and in the adult. Synaptic maturation at the photoreceptor terminals was examined with antibodies against synapse associated proteins (SNAP-25, Syntaxin3, SV2). The appearance of synaptic plasticity at the so called spinule-type synapses between cones and horizontal cells was assessed by electron microscopy and the maturation of photoreceptor downstream connections was identified by immunocytochemistry for GluR4 (AMPA-type glutamate receptor), protein kinase ß1 (mixed rod-cone bipolar cells), and tyrosine hydroxylase (dopaminergic interplexiform cells). Results: First transmitter-filled synaptic vesicles (SV2-labeling) and SNARE-complex proteins (SNAP25-, Syntaxin3-labeling) were found in the OPL and IPL at 2.5dpf. Additionally, we found that the classical ultrastructural components at the cone terminal, namely cone synaptic ribbons, bipolar cell dendrites, and horizontal cell spinules form very early in zebrafish retinal development, at 3.5dpf. Spinule ultrastructure and plasticity reaches morphological maturity until 5dpf, and, therefore, enables adaptational processes triggered by the ambient light such as spinule formation and retraction already by that time. Furthermore, at least one major population of the vertical signal pathway, the mixed rod-cone bipolar cells (PKCß1-labeling), are fully developed by 5dpf, and AMPA-type glutamate receptors (GluR4-labeling) at the apical dendrites of OFF-BCs in the OPL are also present at this developmental stage. Conclusion: Our data suggest that the onset of signal transmission from the cones to their second order neurons and their putative neuroreceptors becomes possible at 3.5dpf, and is virtually mature at 5dpf. However, the final step in development takes place between 7dpf and the adult stage. So far, we do not know which signals or substances mediate this final step in synaptogenesis.
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