CALCIUM ACTION-POTENTIALS IN RETINAL BIPOLAR NEURONS

Patch-clamp and calcium-indicator measurements were used to examine th e electrical excitability of large-terminal bipolar neurons from goldf ish retina. Large, transient increases in intracellular calcium occurr ed spontaneously in the synaptic terminal but not in the soma of bipol ar neurons. Calcium transients were blocked by hyperpolarization, by e xternal application of calcium-channel blockers, and by the neurotrans mitters GABA and glutamate. These observations suggest that calcium ac tion potentials are responsible for the spontaneous increases in intra terminal calcium, which was directly confirmed by electrical recording s of calcium-dependent action potentials in both whole-cell and perfor ated-patch recordings. We suggest that regenerative depolarization pro duced by the opening of calcium channels in the synaptic terminal of o n-type bipolar neurons represents an amplification mechanism in the hi gh-sensitivity ON pathway in the dark-adapted fish retina.