Regulation of transmitter release from retinal neurons with synaptic ribbons

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Bipolar cells, the second order neurons in the retina, respond to light stimulation with graded potentials. In their axon terminals there are synaptic ribbons, to which synaptic vesicles are attached. To elucidate the regulatory mechanisms of transmitter (glutamate) release, the patch-clamp technique was applied to the axon terminals of Mb1 bipolar cells that were enzymatically isolated from the goldfish retina. We measured simultaneously the Ca current and membrane capacitance changes associated with fusion of synaptic vesicles. At the same time, the released glutamate was monitored electrophysiologically with glutamate receptor-rich neurons. The following conclusions were obtained. 1) Glutamate is released with a minimal delay of-1 ms after the onset of the Ca current. 2) Transmitter release is stopped within 300 ms after the termination of the Ca current. 3) There are at least two kinds of synaptic vesicle pools ; an immediately releasable pool and a reserve pool. 4) Activation of protein kinase C in bipolar cells potentiates transmitter release by increasing the size of the reserve pool. 5) The immediately releasable pool and the reservoir pool may correspond to synaptic vesicles attached to the bottom row of the synaptic ribbons and those to the upper rows, respectively. Furthermore, we investigated properties of glutamatergic synaptic transmission by simultaneously voltage-clamping a pair of connected bipolar cells and their post-synaptic neurons (amacrine or ganglion cells) in the newt retinal slice preparation. The following conclusions were obtained. 6) Only non-NMDA receptors may be expressed in the postsynaptic regions immediately beneath the release sites of bipolar cells, whereas NMDA receptors slightly away from the release sites. 7) Both non-NMDA and NMDA receptors cooperate to transfer the graded photoresponses of bipolar cells proportionally to post-synaptic neurons.