ADAPTATION-DEPENDENT PLASTICITY OF ROD BIPOLAR CELL AXON TERMINAL MORPHOLOGY IN THE RAT RETINA

We chose synaptic terminals of rat rod bipolar cells as a model system to study activity-related changes in the overall morphology and the f ine structure of synaptic sites. Using confocal laser scanning microsc opy in conjunction with three-dimensional reconstruction and electron microscopy, we examined the effect of light and dark adaptation on axo n terminals identified by protein kinase C (PKC) immunoreactivity. Rod bipolar cell axon terminals consisted of 2-3 polymorphic boutons situ ated close to the ganglion cell layer and a single ovoid swelling loca ted more distally. Both components of the terminal complex showed adap tation-dependent differences in the distribution of PKC immunoreactivi ty and in their morphology. In light-adapted rod bipolar cell axon ter minals, PKC immunoreactivity was homogeneously distributed throughout the cytoplasm, whereas terminals from dark-adapted animals showed PKC immunoreactivity preferentially localised in the submembrane compartme nt and a reduced staining of the more central cytoplasm. In three-dime nsional reconstructions of optical sections and at the ultrastructural level, the shape of light-adapted axon terminals was round and smooth and exhibited more convexly curved synaptic membranes. In contrast, d ark-adapted terminals had irregular contours, numerous dimples and a c oncave synaptic curvature. No spinules of bipolar cell terminals were observed in dark-adapted material. These observations are discussed in the context of activity-related morphological. plasticity of central nervous system synapses and of the functions of PKC in the cycle of ve sicle fusion and retrieval at the tonically active ribbon synapses of the rod bipolar axon terminal.