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L M Proenza

Publications and source records attributed to L M Proenza.

13 recordsLinked to original sources
PubMed1987

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.

Animals↗
PubMed1985

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.

Animals↗
PubMed1985

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.

Animals↗
PubMed1978

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.

Animals↗
PubMed1978

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).

Animals↗
PubMed1978

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.

Animals↗
PubMed1977

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.

Animals↗