Different appearance of actions of excitatory amino acids on turtle horizontal cells in different preparations; eyecup and isolated retina.
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Amino Acids↗
Biomedical subjects
E Miyachi
Publications and source records attributed to E Miyachi.
Excitatory amino acids have different effects on horizontal cells in eyecup and isolated retina.
Horizontal cells in the mudpuppy eyecup responded to continuous superfusion with L-glutamate, L-aspartate, kainate and quisqualate with a transient depolarization and reduction of the light evoked responses. However, in isolated retina preparations, in which these substances were applied to the photoreceptor side of the retina, the effects were sustained as long as the agonists were present. These results suggest that the transient action of these agonists in eyecup preparations was due to the rapid development of an intraretinal diffusion barrier, and are consistent with the hypothesis that photoreceptors release an excitatory amino acid transmitter.
Amino Acids↗
Push-pull effect of surround illumination on excitatory and inhibitory inputs to mudpuppy retinal ganglion cells.
1. Changes in membrane potential and conductance were measured in on-centre and off-centre ganglion cells during the responses to illumination of different portions of the receptive field. 2. In on-centre ganglion cells the sustained depolarizing response to steady illumination of the receptive field centre was associated with a net increase in conductance. In the presence of centre illumination, stimulation of the surround with an annulus of light caused a hyperpolarization and a net decrease in conductance, and the reversal potential of the light-evoked response was shifted in a negative direction. In the absence of centre illumination the same annular stimulus caused a hyperpolarization and a net increase in conductance. 3. In off-centre ganglion cells the sustained hyperpolarizing response to centre illumination was associated with a net increase in conductance. In the presence of centre illumination, stimulation of the surround with an annulus caused a depolarization and a net decrease in conductance, and the reversal potential of the light-evoked response was shifted in a positive direction. In the absence of centre illumination the same annulus caused a depolarization and a net increase in conductance. 4. The results indicate that illumination of the receptive field surround can affect both the excitatory and inhibitory sustained inputs to a given ganglion cell in a 'push-pull' manner, by decreasing the synaptic input that was increased by centre illumination and increasing the synaptic input of opposite sign. The relative effect of a given surround illumination on these two inputs, and hence the sign and magnitude of the net conductance change, varied with the amount of centre illumination.
Action Potentials↗
The effect of cholinergic agonists and antagonists on ganglion cells in the mudpuppy retina.
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Animals↗
Electrically evoked calcium responses in rods of the frog retina.
In the rod of the isolated frog retina perfused with normal Ringer solution, a transretinal current pulse flowing from the receptor side to the vitreous side evoked a response consisting of a transient depolarization and a subsequent damped oscillation. The transient depolarization was not an all-or-none response, but was a graded one which depended on the intensity and duration of the electrical stimulation. The amplitude of the initial depolarization varied with Ca2+ concentrations of the perfusate, being enhanced in the high Ca2+ and reduced in the low Ca2+ medium. Furthermore, it was enhanced by perfusion with a Ba2+-containing solution, and suppressed by exposure to Co2+. These observations suggest that the initial depolarization can be attributed to an increase of Ca2+ conductance. The late hyperpolarization which followed the initial depolarization was suppressed by membrane depolarization and was enhanced by membrane hyperpolarization. In addition, the evoked responses were not affected by application of either L-aspartate or L-glutamate, both of which blocked the synaptic transmission from the photoreceptors to the second order neurons. Thus it is unlikely that the late hyperpolarization is an IPSP-like response evoked by the negative feedback input from horizontal cells. Rather the rod membrane itself seems to have an oscillatory property.
Animals↗
GABA-mediated negative feedback from horizontal cells to cones in carp retina.
In the isolated, perfused retina of the carp, properties of the feedback pathway from horizontal cells to cones were investigated by means of electrophysiological and neuropharmacological methods. When horizontal cells were hyperpolarized by illumination with an annulus, a depolarizing synaptic potential was produced in cones at the center of the annulus, suggesting that horizontal cells receive inputs from cones and exert a negative feedback to cones. On the other hand, a hyperpolarization (EPSP) was induced in horizontal cells by application of a transretinal current pulse, which activated the release of transmitter from the photoreceptor terminals. The IPSP was abolished when the retina was perfused with a GABA-containing Ringer solution, because of desensitization of the feedback synapse. GABA also hyperpolarized the cone membrane, indicating the presence of a GABA-sensitive site in the cone. These results suggest that the GABA-mediated negative feedback operates from horizontal cells to cones in the dark, and ceases its function in the light.
Animals↗
GABA-mediated negative feedback and color opponency in carp retina.
In the isolated, perfused retina of the carp, the effects of GABA and its antagonists, picrotoxin and bicuculline, were investigated on spectral responses of horizontal cells. When the GABA-mediated feedback from horizontal cells to cones was desensitized or blocked by these chemicals, remarkable changes were observed in spectral responses, due to selective suppression of responses to long wavelengths. In particular, the depolarizing responses in the biphasic C-type (H2) cell were abolished, while the hyperpolarizing responses to short wavelengths were retained, resulting in a monophasic spectral response curve. From these results, it was concluded that the GABA-mediated negative feedback plays an essential role in neural mechanisms which convert the trichromatic process at the level of cones into the opponent color process in horizontal cells.
Animals↗