In-vivo Light and Dark Adaptation and In-vitro K+ Stimulation Influence the Morphology of Synapses at Mb Bipolar Cell Terminals in Goldfish Retinae

Abstract

Abstract: : Purpose:To study the impact of neural activity on the morphology of distinct populations of synapses at bipolar cell axon terminals (BCTs) in goldfish retinae. We investigated morphological changes of two populations of synapses in Mb BCTs at ribbon synapses (BCT presynaptic) and non-ribbon associated chemical synapses (NRACS) (BCT postsynaptic). Methods:Retinae from light and dark adapted animals and retinae treated in-vitro with K+- and bicuculline-supplemented Ringer were used for ultrastructural analysis. In ribbon synapses, we measured the angle (α) between the presynaptic membranes flanking the synaptic ribbon. In NRACSs we determined the curvature of synaptic membranes and counted the numbers of convex, flat and concave configurations. FM1-43 staining of slice preparations was used to monitor the activity of Mb BCTs during K+- and bicuculline treatment. Results:Light adapted Mb BCTs showed a ribbon synaptic angle (α) of 115±21° (mean±SEM), whereas dark adapted BCTs had smaller values of α (61±17°) (serial section analysis, n=25 ribbon synapses per group). K+ Ringer treatment of dark adapted retinae lead to larger values of α compared to controls. Light and dark adaptation also influenced the shape of NRACSs. At noon, 56 % of NRACSs were convex, whereas 36 % were flat and 6.9 % were concave. Mb BCTs from midnight animals had 4.3 % convex NRACSs, 32 % flat and 64 % concave synapses. The percentage of concave NRACSs was significantly reduced after 30 min incubation in K+ Ringer. Bicuculline treatment had no significant influence on α and on NRACSs shape. K+-Ringer and bicuculline incubation in the presence of FM1-43 lead to the specific labelling of Mb BCTs. Conclusion:In-vitro experiments with dark adapted retinae reveal that depolarisation of Mb terminals with K+ leads to morphological changes which are typical of light adapted terminals. Our results indicate that increased activity of the Mb terminals results in conspicuous changes of two populations of synapses at MB BCTs.

Keywords: 330 bipolar cells • 594 synapse • 315 anatomy