Adaptation-dependent changes of bipolar cell terminals in fish retina: Effects on overall morphology and spinule formation in Ma and Mb cells

We have investigated the effects of light and dark adaptation on the overall morphology of bipolar cell (BC) terminals in sublaminae a and b of the inner plexiform layer after labelling with Lucifer Yellow (LY) and PKC immunostaining using confocal laser scanning microscopy and serially sectioned material for electron microscopy. Three-dimensional reconstructed terminals showed marked adaptation-dependent changes of their morphology. Terminals of mixed rod-cone BCs in sublamina a (Ma BC) were irregular and scalloped in light adapted, but smooth and regular in dark-adapted specimens. Terminals from mixed rod-cone BCs in sublamina b (Mb BCs) exhibited an opposite behaviour. At the ultrastructural level, bipolar terminals in both sublaminae showed fingerlike extensions (spinules) invaginating presynaptic amacrine cell (AC) processes. Sixty-two percent of the dark-adapted Mb terminals in sublamina b showed spinules, whereas 21% of the light-adapted terminals had spinules. By contrast, 50.6% of the light-adapted Ma terminals in sublamina a formed spinules, compared to 17.8% of the dark-adapted Ma terminals in this sublamina. These observations reflect the functional subdivision of the inner plexiform layer in an inner ON-and an outer OFF-centre lamina. Our findings suggest that the synaptic plasticity of BC axon terminals may be due to differences of BC membrane potential, or the activity of AC input onto bipolar terminals. They may contribute to processes of fine tuning regulating the efficiency of AC-BC interaction under varying adaptation conditions.