Inhibitory control of synaptic activity in goldfish Mb bipolar cell terminals visualized by FM1-43

To investigate the physiology and plasticity of mixed rod-cone ON-bipolar c ells (Mb) in the goldfish retina; we established a slice preparation which allows us to optically monitor the synaptic activity of bipolar cell axon t erminals. We used the styryl dye FM1-43 which is incorporated into active a xon terminals due to synaptic vesicle cycling and thus reflects synaptic ac tivity. Different activity states of the axon terminals were revealed when slices prepared from light-adapted retinae were incubated in the presence o f FM1-43 under various conditions. Depolarizing high K+ Ringer (50 mM) and the gamma-butyric acid (GABA) antagonist bicuculline (100 muM) resulted in more than two-fold increase in the number of stained terminals compared to slices stained in normal Ringer. In contrast, GABA treatment (0.5 mM) reduc ed the frequency of stained terminals. Thus, in light-adapted retinal slice s the synaptic activity of Mb axon terminals can be modulated towards highe r and lower activity states. The fact that the GABA antagonist bicuculline had similar effects as stimulatory high K+ Ringer suggests that inhibitory control is an important component in the regulation of synaptic activity an d transmitter release in Mb terminals.