Photoreceptor-bipolar cell transmission in the perfused retina eyecup of the mudpuppy.

The hypothesis that a synaptic transmitter is released by photoreceptors in the dark is supported by experiments in which cobalt was used as a synaptic blocking agent, while intracellular recordings of receptors and neurons that are directly postsynaptic to receptors were maintained. In the dark the depolarizing bipolars are hyperpolarized, whereas the hyperpolarizing bipolars are depolarized.