Electrically evoked calcium responses in rods of the frog retina

In the rod of the isolated frog retina perfused with normal Ringer solution, a transretinal current pulse flowing from the receptor side to the vitreous side evoked a response consisting of a transient depolarization and a subsequent damped oscillation. The transient depolarization was not an all-or-none response, but was a graded one which depended on the intensity and duration of the electrical stimulation. The amplitude of the initial depolarization varied with Ca2+ concentrations of the perfusate, being enhanced in the high Ca2+ and reduced in the low Ca2+ medium. Furthermore, it was enhanced by perfusion with a Ba2+-containing solution, and suppressed by exposure to Co2+. These observations suggest that the initial depolarization can be attributed to an increase of Ca2+ conductance. The late hyperpolarization which followed the initial depolarization was suppressed by membrane depolarization and was enhanced by membrane hyperpolarization. In addition, the evoked responses were not affected by application of either L-aspartate or L-glutamate, both of which blocked the synaptic transmission from the photoreceptors to the second order neurons. Thus it is unlikely that the late hyperpolarization is an IPSP-like response evoked by the negative feedback input from horizontal cells. Rather the rod membrane itself seems to have an oscillatory property.