Mechanisms of Transmitter Release : Ca-Dependent Release and Ca-Independent Release.

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The purpose of this project was to investigate how chemical transmitters are released from retinal cells. Cells were isolated from the vertebrate retinae and voltage-clamped using a patch pipette in the whole-cell clamp configuration. The following results were obtained.1. The Ca current of mouse bipolar cells was of T type. The activation threshold was low and the current was not blocked by dihydropyridines. On the other hand, the Ca current of goldfish bipolar cells was of L type ; activation threshold was high, and the current was sensitive to dihydropyridines. In both species, Ca current was induced at the synaptic terminal as well as at the cell body. This observation suggests that the Ca current may play a key role in the transmitter release.2. Since glutamate is a leading candidate for the transmitter of bipolar cells, release of glutamate was examined by using bipolar cells isolated from the goldfish retina. Solitary catfish horizontal cells were used as a glutamate probe. It was demonstrated that gultamate (or closely related substance) was released from bipolar cells when the Ca current of synaptic terminal was activated by depolarization.3. Uptake mechanisms of transmitter were investigated electrophysiologically. Application of glutamate induced an inward current in salamander Muller cells (retinal glia). This current was due to the activation of electrogenic glutamate uptake carriers ; one molecule of glutamate was cotransported with three Na ions. The carrier showed a strong voltage-dependence and Ca ions were not required for its activation. Efflux of glutamate was observed but it was not electrogenic. This carrier seems to contribute to the Ca-independent release of transmitters.4. Turtle photoreceptors responded to glutamate with depolarization. Some response properties were similar to those of the glutamate carrier of salamander Muller cells. This suggests that turtle photoreceptors may have glutamate carriers.