The scotopic threshold response (STR) and slow negative response are negative-going potentials in the dark-adapted electroretinogram (ERG) of the cat eye that originate proximal to the photoreceptors and are present at threshold and with dim illuminations. The present paper examines the hypothesis that these events are associated with Müller cell responses to potassium released by proximal retinal neurons by measuring light-dependent changes in extracellular K+ concentration [( K+]o, in proximal retina. 2. Extracellular field potentials and changes in [K+]o, evoked in response to diffuse illumination of the dark-adapted retina, were recorded with a double-barreled K+-sensitive microelectrode placed in the retina at different depths. The vitreal ERG was recorded at the same time. 3. The dynamic range of the light-evoked increases in [K+]o, recorded in proximal retina from threshold to saturation, was strikingly similar to that of the STR and slow negative response. The threshold for the [K+]o increase was near that of the most sensitive ganglion cells, and the response saturated approximately 2.4 log units below rod saturation. Also, onset latencies and rates of rise for the increases in [K+]o and the field potentials in proximal retina followed similar functions of intensity. 4. The depth distribution of the light-evoked [K+]o increases resembled that of the STR. The increases in [K+]o were largest and fastest in proximal retina where the STR was largest. This was true both for very low intensity stimuli, below the threshold for PII (DC-component and b-wave) in distal retina, and for stimulus intensities near STR saturation, where PII (DC-component) was present. For stimuli of intensities near rod saturation, when the b-wave was present in distal retina, there was a small fast increase in [K+]o at light onset in distal retina and a slower increase in proximal retina. Both increases were truncated by spread of the large light-evoked decrease in [K+]o in subretinal space that is known to cause the c-wave in the ERG and slow PIII in neural retina. 5. The duration of the increases in [K+]o in proximal retina was more sustained than the proximal field potentials. In response to stimuli of 2-4 s, the field potential began to recover toward base line after about 300 ms, whereas the increases in [K+]o remained at maximal levels until stimulus offset.(ABSTRACT TRUNCATED AT 400 WORDS)
