Sources and sinks of light-evoked delta [K+]o in the vertebrate retina.

In the vertebrate retina, recordings of light-evoked changes in extracellular K+ concentration delta [K+]o are of particular interest because this tissue is complex and multilayered, yet can be activated routinely with its "natural" stimulus (i.e., light). This review identifies the components of the spatiotemporal profile of retinal light-evoked delta [K+]o and then presents evidence concerning the specific neural origins of these components as well as the mechanisms by which these delta [K+]o are dispersed from extracellular space. Finally, to gain improved resolution of K+ sources and sinks, the technique of ion source density is introduced and applied to both model and real spatiotemporal distributions of delta [K+]o.

Laminar profile of resistivity in frog retina.

Measurements of absolute transretinal resistance and of the relative resistance of the various retinal layers were obtained in the frog. The resistance of clamped sections of isolated retina was 66 omega . cm2, which results in an average resistivity (rho) between inner and outer limiting membranes of 5,050 omega . cm. In eyecups, relative resistances (obtained by passing constant currents across the retina) were assigned to specific layers of the retina with the aid of physiological criteria (e.g., depths of light-evoked field potentials, changes in extracellular K+ concentration, base-line noise level, and resistance). These relative resistances were then converted to absolute values, a calculation feasible because the region between inner and outer limiting membranes, which has the same structure in both isolated and eyecup retinas, could be specified during experiments. Resistivities (in omega . cm) for the retinal layers include 1) subretinal space, 970; 2) inner and outer nuclear layers, 6,800; and 3) inner plexiform layer, 1,750. The ganglion cell and optic nerve fiber layers were too thin to resolve individually, but rho of the two layers combined was 7,900. The outer plexiform layer was also too thin to reliably resolve, but its rho is likely the same as the inner plexiform layer. The extracellular space volume fraction (alpha) of the retinal layers was estimated from these rho s, and the following values were obtained: 1) subretinal space, 0.12; 2) outer and inner nuclear layers, 0.03; 3) inner and outer plexiform layers, 0.11; and 4) ganglion cell and optic nerve fiber layers, 0.02. The decreased rho and increased alpha of the inner plexiform layer and the subretinal space, compared with that of the nuclear layers, are expected from their anatomy. A consideration of these inhomogeneities is required in analyses of field potentials and of changes in extracellular ionic concentrations.

Light-evoked increases in extracellular K+ in the plexiform layers of amphibian retinas.

Recordings of light-evoked changes in extracellular K+ concentration (delta[K+]o) were obtained in the retinas of frog and mudpuppy. In eyecup preparations, various recording approaches were used and provided evidence for a K increase near the outer plexiform layer (distal K increase). This distal K increase could be pharmacologically dissociated from the well-known, large K increase in the proximal retina by the application of ethanol and gamma-aminobutyric acid. The distal K increase also often showed surround antagonism. A retinal slice preparation was used to permit electrode placement into the desired retinal layers under direct visual control and without the risk of electrode damage to adjacent layers. In the slice, a distinct distal K increase was found in the outer plexiform layer, in addition to the prominent K increase in the inner plexiform layer. Compared with eyecups, only weak K increases were found in the nuclear layers of the slice. This suggests that the K responses observed in the nuclear layers of eyecups may be generated by K+ diffusing along the electrode track from the plexiform layers. In the context of current models of ERG b-wave generation, the magnitude of the recorded distal K increase, compared with the proximal K increase, seems too small to give rise to the b-wave. However, the distal K increase may be differentially depressed by electrode dead space. It is also possible that if certain aspects of the models of b-wave generation were modified, then the observed distal K increase could give rise to the b-wave.

Aspartate-induced dissociation of proximal from distal retinal activity in the mudpuppy.

The effects of aspartate (Asp) on the ERG and on neuronal, glial, and K+ responses were monitored continuously in the superfused mudpuppy eyecup. Asp induced a time-dependent sequence of events which may be divided into three stages: Stage 1, initially, light-evoked responses throughout the retina are depressed; Stage 2, distal responses (horizontal, bipolar, and K+ responses) return to near pre-drug amplitudes and there is a simultaneous ERG enhancement, but responses in the proximal retina remain suppressed; Stage 3, a second depression of retinal responses leads to a-wave isolation. The dissociation of distal from proximal responses observed during Stage 2 strongly supports the hypothesis that the ERG b-wave results from events arising in the distal retinal network.

Endogenous circadian rhythm in electroretinogram of free-moving lizards.

An endogenous circadian rhythm in the electroretinogram (ERG) of free-moving lizards, Anolis carolinensis, can be demonstrated in experiments lasting up to eight days. The rhythm does not appear to arise from processes that could modulate the amount of light reaching the photoreceptors. Moreover, since the rhythm is well-developed in the b-wave, but not a-wave, the direct modulation of neural processes proximal to the photoreceptors may be involved. Lastly, this rhythm may have ecological significance for this diurnally active species, since the largest responses occur at projected midday.

Transient adaptation and sensitization in the retina of Necturus.

Responses to repetitive stimulation were monitored at several retinal levels in the eyecup of the mudpuppy Necturus maculosus. When alternating sequences of low-intensity small and large spots were presented, two effects were found, which could be localized to the proximal retina: (a) response decrement (RD), in which, after the first small spot response, subsequent small spot responses are decreased in amplitude and (b) transient response enhancement (TRE), in which the first small spot response after a large spot sequence is larger than preceding or subsequent small spot responses. RD and TRE are absent or weak in sustained on or off responses (horizontal and bipolar cells, and ON and OFF ganglion cell post-stimulus time histograms (PSTH) but are particularly well developed in the on/off responses of the proximal retina (proximal negative response, M-wave, PSTHs of ON/OFF ganglion cells, and intracellular responses from on/off neurons and Müller cells). RD and TRE appear to arise from a stimulus-evoked slow depolarization in on/off neurons that interacts with the amplitude of succeeding responses. We conclude that RD and TRE are a form of neural adaptation that is largely specific to the on/off channels of the proximal retina.

Neurons, potassium, and glia in proximal retina of Necturus.

Light-evoked K+ flux and intracellular Müller (glial) cell and on/off-neuron responses were recorded from the proximal retina of Necturus in eyecups from which the vitreous was not drained. On/off-responses, probably arising from amacrine cells, showed an initial transient and a sustained component that always exhibited surround antagonism. Müller cell responses were small but otherwise similar to those recorded in eyecups drained of vitreous. The proximal K+ increase and Müller cell responses had identical decay times, and on some occasions the latency and rise time of the K+ increase nearly matched Müller cell responses, indicating that the recorded K+ responses were not always appreciably degraded by electrode "dead space." The spatiotemporal distribution of the K+ increase showed that both diffusion and active reuptake play important roles in K+ clearance. The relationship between on/off-neuron responses and the K+ increase was modelled by assuming that (a) K+ release is positively related to the instantaneous amplitude of the neural response, and (b) K+ accumulating in extracellular space is cleared via mechanisms with approximately exponential time-courses. These two processes were approximated by low-pass filtering the on/off-neuron responses, resulting in modelled responses that match the wave form and time-course of the K+ increase and behave quantitatively like the K+ increase to changes in stimulus intensity and diameter. Thus, on/off-neurons are probably a primary source of the proximal light-evoked K+ increase that depolarizes glial cells to generate the M-wave.

Laminar separation of light-evoked K+ flux and field potentials in frog retina.

Light-evoked changes in [K+] 0 and field potentials were recorded from the retinas of grass frogs. In the proximal retina, light induced an increase in [K+]0. This increase had components at light onset and offset, was maximal with small spot stimulation, and reached greatest amplitude at the same depth as the proximal negative response (PNR). Extracellular dye marking revealed that this depth was within the inner plexiform layer. The off-components of both the K+ increase and PNR occurred distal to the on-components, thus supporting recent proposal that "off" synapases lie distal to "on" synapses. Since a well-developed M-wave, having a time course nearly identical to the K+ increase, was also seen in the proximal retina, this field potential appears to be a normal component of the intraretinal electroretinogram.

Light-evoked changes in extracellular potassium concentration in munpuppy retina.

Light-evoked changes in extracellular potassium concentration ([K+]0) and field potentials were recorded simultaneously in response to a wide variety of stimuli and at various depths within the retina of Necturus. At both light onset and offset, small diameter flashed stimuli elicit a large increase in [K+]0 in the proximal retina. The depth profile of this K+ increase is nearly identical to that of the proximal negative response (PNR), and both responses exhibit similar behavior to a number of other stimulus parameters. This suggests that the same neurons which generate the PNR may be the source of the observed K+ flux. Large diameter flashed stimuli elicit a slow decrease in [K+]0 in the distal retina and a small increase proximally. The K+ increase occurs at a depth where the b-wave of the electroretinogram is positive going, and where its current source lies. Increasing background light intensity decreases [K+]0 in the distal retina, and generally increases [K+]0 proximally.

A comparison of the proximal negative response and ganglion cell responses to sinusoidal flicker.

In the mudpuppy retina, sinusoidal light stimulation of the proximal negative response (PNR) demonstrates two main components whose phases are essentially the same as those of the spike discharges of ON/OFF ganglion cells. Oscillations superimposed upon these components are synchronized with the spike discharges of ON/OFF cells, but not of ON or OFF cells. Intracellular recordings from ON/OFF ganglion cells reveal slow potentials with nearly identical waveforms and phases to those of the PNR. These results, together with previously published comparisons between the flesh-evoked PNR and amacrine cell responses, support the suggestion that the PNR arises from activity in the on/off system (amacrine and/or ganglion cells).

Relationship between Müller cell responses, a local transretinal potential, and potassium flux.

1. In the Necturus retina, light-evoked field potentials, Müller (glial) cell responses, and extracellular potassium ion concentration ([K+]0) were recorded and found to exhibit the three-way correlation characteristic of these variables elsewhere in the nervous system. 2. Müller cell responses to a flashed spot or annulus consist primarily of slow depolarizations at both light onset and offset. The responses are maximum to 0.5-mm-diameter spots and decrease with larger diameters. Responses to stimulus intensity and flicker were also used to characterize Müller cell behavior. 3. In response to long-duration stimuli, the initial Müller cell depolarization is followed by a very slow hyperpolarization, which is likely the origin of slow PIII. 4. A new extracellular potential is described, the M-wave, the basic properties of which suggest that it is generated by Müller cells. Moreover, the M-wave and Müller cells show remarkably similar behavior to a wide variety of stimulus parameters. 5. In the proximal retina, [K+]0 increases at both light onset and offset with a time course similar to that of Müller cell depolarizing responses. This K+ increase also behaves similarly to the Müller cell depolarization in response to changes in stimulus parameters. 6. It is concluded that light stimulation leads to an increase in [K+]0 in the proximal retina and that this increase depolarizes Müller cells whose associated currents, in turn, generate the M-wave.