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 c holine the optic tectal surface was stimulated by trains of electrical pulses or by a flash to the contralateral eye. Sustained potential sh ifts (SPSs) and changes in extracellular potassium concentration (DELT A-[K+]0) were simultaneously recorded. In response to electrical stimu lation SPSs of maximal amplitudes (1.19 +/- 0.1 mV) were recorded betw een 50 and 200-mu-m in depth and maximal DELTA-[K+]0 (0.69 +/- 0.08 mM ) between 100 and 350-mu-m. The changes of SPS and DELTA-[K+]0 showed a close similarity in experiments with changes in voltage, pulse durat ion and frequency of stimuli within a train. The induced SPS had a dur ation of 28 +/- 1.54 s, the DELTA-[K+]0 of 32 +/- 1.23 s. The flash st imulus induced an SPS and DELTA-[K+]0 of maximal amplitudes between 50 and 200-mu-m in depth with values of 0.57 +/- 0.1 mV and 0.29 +/- 0.0 3 mM respectively. An additional wave with a latency of ca 1 s and a d uration of ca 3 s arose on the background of the SPS to a flash stimul us, associated with an additional increase in [K+]0. It is considered that the accumulation of K+ in extracellular space, with neuronal acti vity, results in depolarization of radial processes of ependymal glia. This is reflected in the neuropil of the upper layers of the optic te ctum as an SPS.