Richard Shiells

Retinal on-bipolar cells contain a nitric oxide-sensitive guanylate cyclase.

Retinal on-bipolar cells possess specialized glutamate receptors which are coupled via a G-protein to the control of a cyclic GMP (cGMP) cascade. Whole-cell voltage clamp recordings were obtained from light-responsive on-bipolar cells in retinal slices of the dogfish. Inclusion of nitroprusside in the patch-pipette solution induced effects in on-bipolar cells which were consistent with a rise in intracellular cGMP and thus stimulation of guanylate cyclase (GC) activity. Conversely, the soluble GC inhibitors, methylene blue and ferricyanide, induced effects consistent with a fall in intracellular cGMP. Activators of particulate GC had no effect. We conclude that cGMP synthesis in on-bipolar cells is catalysed by a NO-sensitive cyclase.

Iontophoretic study of the action of excitatory amino acids on rod horizontal cells of the dogfish retina

Much interest has been focused on the amino acids, L-glutamate and L-aspartate, as possible neurotransmitters of vertebrate photoreceptors. These amino acids and a number of their analogues were applied iontophoretically to rod horizontal cells on the surface of dark-adapted dogfish retinal slices under visual control. L-glutamate and kainate were found to be of approximately equal potency in depolarizing rod horizontal cells, while L-aspartate was about one tenth as potent. Simultaneous iontophoretic pulses applied to two barrels, each containing either L-glutamate or kainate, produced a larger depolarization than expected for linear summation. Potentiation was most prominent when synaptic transmitter release was reduced by light, demonstrating that these agonists interact with the same postsynaptic receptors as those acted upon by the rod neurotransmitter. Analysis of dose—response curves indicated that at least two molecules of agonist were required to open a cationic channel, presumably the basis for the depolarization. The cells did not become desensitized to long or repeated exposures of the agonists.

Blocking AMPA receptor desensitization prolongs spontaneous EPSC decay times and depolarizes H1 horizontal cells in carp retinal slices

Desensitization of H1 horizontal cell (H1 HC) glutamate receptors was investigated in carp retinal slices using cyclothiazide (CTZ), an inhibitor of AMPA receptor desensitization. 100 microM CTZ depolarized H1 HCs and increased the amplitude of light responses, without any prominent changes in their kinetics. Spontaneous EPSCs (sEPSCs) in H1 HCs were observed in the presence of 2.5 mM heptanol, an uncoupling agent of gap junctions. 20 microM GYKI52466 (an AMPA receptor antagonist) blocked the sEPSCs, consistent with the sEPSCs being mediated by AMPA receptors. 100 microM cobalt suppressed the frequency of sEPSCs without changing their mean peak amplitude, suggesting that calcium-dependent transmitter release from cones was not affected by heptanol. CTZ increased the total inward charge transferred per sEPSC by increasing the sEPSC decay time constant twofold, without any significant change in their frequency and mean peak amplitude. This suggests that the depolarizing effect of CTZ on H1 HCs was due to blocking desensitization of AMPA receptors, increasing the inward current induced by glutamate released from cone synaptic terminals. The desensitization of glutamate receptors may function to extend the dynamic range of H1 HC light responses.

Analysis of spontaneous EPSCs in retinal horizontal cells of the carp

Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded under Whole-cell voltage clamp from carp type 1 horizontal cells (H1 cells) uncoupled by dopamine in retinal slices. Red light steps, which hyperpolarise cones and reduce glutamate release, induced outward current responses accompanied by a suppression of sEPSCs. sEPSCs decayed exponentially with a mean time constant of 0.71+/-0.07 ms and had a reversal potential near 0 mV. Power spectral analysis of sEPSCs revealed a similar decay time constant. They were suppressed by a non-NMDA receptor antagonist, CNQX at 10 microM, and a relatively specific AMPA receptor antagonist, GYKI52466 at 20 microM. The presence of sEPSCs suggests that the release of glutamate from cone synaptic terminals is vesicular. The reduction in mean sEPSC frequency with red light was not accompanied by a significant change in the mean sEPSC conductance increase (482+/-59 pS), suggesting that a decrease in the vesicular release rate from cones does not alter the vesicular glutamate concentration (quantal contents). The results suggest that the spontaneous events in H1 cells were contributed by non-NMDA (possibly AMPA) type glutamate receptors modulated by the red cone input.

Retinal synapses. Glutamate receptors for signal amplification.

The localization of metabotropic glutamate receptors in retinal bipolar cells supports the view that these receptors are involved in the synaptic amplification that allows the detection of dim-light stimuli.

On-Bipolar Cells, Visual Sensitivity and the b-Wave

It has been known for some time from psychophysical observations that the visual system can perform as a near-ideal detector of light, capable of detecting the absorption of a few photons. This high sensitivity is generated in the retina; rods respond to single photons and there is a large amplification at the synapse with some bipolar cells (Ashmore & Falk 1976; 1980). The absorption of a photon within the receptive field of a ganglion cell of a dark-adapted retina has a high probability of eliciting extra impulses in that ganglion cell (Barlow, Levick & Yoon 1971).

A Metabotropic Glutamate Receptor Mediates Synaptic Transmission from Blue-Sensitive Cones to ON-Bipolar Cells in Carp Retina

Glutamate receptors mediate synaptic transmission from photoreceptors to bipolar cells in the vertebrate retina. L-2-amino-4-phosphonobutyrate (L-APB), a group III metabotropic glutamate receptor (mGluR) agonist, was used to identify mGluR6, which mediates rod synaptic input to ON-bipolar cells. A presynaptic mGluR has been similarly identified on red-sensitive cone synaptic terminals [1]. Whether a post-synaptic mGluR is involved in signaling from red-, green-, or blue-sensitive cones to ON-bipolar cells has not, however, been determined in the teleost retina. To characterize the postsynaptic glutamate receptor, electroretinogram (ERG) b-waves (a measure of ON-bipolar cell activity) and intracellular responses from cones to trichromatic stimuli (blue, 455 nm; green, 549 nm; and red, 694 nm) were recorded from light-adapted isolated carp retina.