Stimulus-evoked slow potential shifts and changes in [K+]0 of the frog optic tectum

In 17 frogs (Rana esculenta var ridibunda) immobilised with succinyl choline the optic tectal surface was stimulated by trains of electrical pulses or by a flash to the contralateral eye. Sustained potential shifts (SPSs) and changes in extracellular potassium concentration (delta[K+]0) were simultaneously recorded. In response to electrical stimulation SPSs of maximal amplitudes (1.19 +/- 0.1 mV) were recorded between 50 and 200 microns in depth and maximal delta[K+]0 (0.69 +/- 0.08 mM) between 100 and 350 microns. The changes of SPS and delta[K+]0 showed a close similarity in experiments with changes in voltage, pulse duration and frequency of stimuli within a train. The induced SPS had a duration of 28 +/- 1.54 s, the delta[K+]0 of 32 +/- 1.23 s. The flash stimulus induced an SPS and delta[K+]0 of maximal amplitudes between 50 and 200 microns in depth with values of 0.57 +/- 0.1 mV and 0.29 +/- 0.03 mM respectively. An additional wave with a latency of ca 1 s and a duration of ca 3 s arose on the background of the SPS to a flash stimulus, associated with an additional increase in [K+]0. It is considered that the accumulation of K+ in extracellular space, with neuronal activity, results in depolarization of radial processes of ependymal glia. This is reflected in the neuropil of the upper layers of the optic tectum as an SPS.