K.H. Ruddock

Depolarization of retinal horizontal cells by excitatory amino acid neurotransmitter agonists

We have recorded the intracellular membrane potential of horizontal cells, second-order interneurones of the vertebrate retina, from fish retinae perfused with Ringer solution containing agonist drugs of the excitatory neurotransmitters, L-glutamate and L-aspartate. We show that at concentrations greater than about 10 microM, kainate and quisqualate have a potent depolarizing effect on horizontal cells and suppress their light evoked electrical responses (S-potentials). In contrast, a third agonist, N-methyl-D-aspartate, hyperpolarizes horizontal cells. The depolarizing action of kainate and quisqualate persists in the presence of the synaptic blocker, cobalt chloride, which implies that they bind directly on the horizontal cell membranes. Two kainate-related drugs, which are active on invertebrate neurones, were also examined, one, alpha-ketokainate, simulates the action of kainate, but the other, dihydrokainate, is ineffective on horizontal cells. L-Glutamate binds with high affinity at quisqualate sites, whereas L-aspartate binds with high affinity at NMDA sites, thus we conclude that L-glutamate is the likely neurotransmitter at the photoreceptor-horizontal cell synapse.

Effects of picrotoxin and strychine on fish retinal S-potentials: Evidence for inhibitory control of depolarizing responses

Simultaneous recording of the light evoked electrical signals from different classes of slow(S)-potential unit in the isolated fish retina established that picrotoxin, an antagonist of GABA, selectively suppresses the depolarizing component of C-type S-potentials. In contrast, equivalent concentrations of strychnine have no effect on S-potentials, but suppress a component of transient amacrine cell responses. On the basis of these and other experimental data [3,4] we propose a network diagram for the C-type S-potential units in the cyprinid fish retina. This network includes a GABA-ergic feed-back loop and is essentially similar to that proposed by Fuortes and Simon [5].

Changes in structure and electrophysiological function of retinal neurones induced by lase irradiation

Abstract Transmission and scanning electron microscopic studies of fish retinae irradiated by a laser beam establish that the rod photoreceptors are subject to extensive mechanical disruption by such beams. Correspondingly, the electrophysiological responses to light recorded from rod-driven horizontal cells are suppressed following laser irradiation. It is shown that by correct choice of laser beam wavelength, the signals arising in different classes of cone-driven horizontal cells can be selectively suppressed. The spatial localization of the electrophysiological effects associated with laser irradiation is illustrated and it is concluded that the method provides a useful tool for the analysis of light-evoked retinal signals.

Electrophysiological effects of GABA on fish retinal horizontal cells are blocked by bicuculline but not by picrotoxin

We have studied electrophysiologically the actions of gamma-aminobutyric acid (GABA) and related pharmacological agents on fish retinal horizontal cells by recording intracellularly from isolated retinae perfused with Ringer containing the various drugs. We show that although GABA usually hyperpolarizes the membrane potential relative to its dark level, it sometimes and particularly at higher (greater than or equal to 5 mM) concentration produces membrane depolarization, with reduction in the light evoked responses (S-potentials) in both cases. These effects are reversed by bicuculline but not by picrotoxin, although both agents antagonize GABA in many other preparations [5, 25]. The GABA uptake blocker nipecotic acid [15] hyperpolarizes horizontal cells and reduces their light evoked responses, and again these effects are reversed by bicuculline but not by picrotoxin. beta-Alanine, which blocks glial GABA transport [29], and diaminobutyric acid (DABA), which blocks neuronal GABA transport [14, 29, 31], have effects similar to those of nipecotic acid. We discuss these actions of GABA and of the other related drugs and their differential sensitivity to bicuculline and picrotoxin.

Selective loss of function associated with a central visual defect

Abstract In this paper, the highly abnormal visual responses of a male subject are described and the organisation of the central visual pathways in man implied by these responses is discussed. The subject does not perceive images formed in light of wavelength λ ⩾ 610 nm, his visual acuity under such illumination being at least some 1° of visual angle. Nonetheless, he is able to construct stereo-images formed by fusion of a red and a green array of random elements [7], although he cannot perceive the elements of the red pattern. Other functions of central vision, such as binocular rivalry and contrast threshold elevation for grating stimulus patterns, are also elicited by unperceived red patterns. The results are examined in relation to the electrophysiological organisation of the central visual pathways.

The spatial and temporal organisation of motion perception units in human vision

Measurements of threshold illumination levels for detection of retinally non-localised moving targets show that detection of a moving target is influenced by both the spatial and the temporal modulation of the background field. The temporal response characteristics obtained from these measurements are similar to those obtained from experiments on detection of temporal flicker. Experiments with spatially modulated background fields reveal visual mechanisms with spatial properties which are essentially independent of many stimulus parameters. The response amplitude of the spatial filter which characterises these mechanisms increases linearly as a function of background contrast and is independent of the relative orientation between the background structure and the direction of target movement. These properties are used to compute the two-dimensional spatial characteristics of mechanisms involved in the detection of moving targets.

Properties of amacrine cell responses recorded from isolated fish retinae

Abstract Amacrine cell responses have been recorded from the inner nuclear layer of fish retinae. Both transient and sustained type responses have been observed and rod and cone components of these responses were identified. It is shown that laser beam irradiation transforms sustained type to transient type responses, and that it also suppresses the sustained component of the transient responses. Rod-driven amacrine cell responses were also suppressed following laser irradiation.

Effects of local cobalt chloride injection on lateral spread of signals in fish (roach) retina.

There is extensive spatial summation in electrical responses of both horizontal and amacrine cells of the fish retina. The effect of locally perfused cobalt chloride on the lateral spread of signals along the layers of these interneurones has been studied in the roach (Rutilus rutilis) retina. It is shown that lateral transmission is blocked in amacrine but not in horizontal cells and this result is examined in relation to the synaptic coupling of the interneurones.

A colour-dependent abnormality in human visual detection of stimulus motion and spatial structure

We have studied the wavelength dependence of visual responses in a single human subject, M.W., who has an unusual visual defect. It is shown that for red stimuli, both spatial resolution for grating patterns and detection of target movement are grossly abnormal. On the basis of these and other [8] experimental data, we argue that M.W.s response pattern implies interdependent central visual processing of colour, movement and spatial structure. We examine this conclusion in relation to recent anatomical and electrophysiological findings.

Terbium simulates the action of calcium on electrophysiological activity in the isolated fish retina.

It has been suggested that terbium (Tb) could be substituted for calcium (Ca) in biological tissue and, because of its useful spectroscopic properties, provide an assessment of calcium distribution in the tissue. In order to assess their physiological effects, we have examined the action of free terbium ions on the membrane potential and light-evoked responses of horizontal cells, and on the massed light evoked response (ERG) of the neural retina. Measurements were performed by intracellular and extracellular recording from the isolated fish retina, superfused with Ringers of various ionic compositions, and certain technical difficulties which occurred in the use of terbium are discussed. We show that Tb ions mimic Ca ions in suppression of the ERG and in hyperpolarization of the horizontal cell membrane potential. The effects of Tb persist after its removal from the superfusate for a longer period than do those of calcium, which indicates that terbium binds more firmly at its site(s) activity. Tb and Ca both suppress the light-evoked S-potentials from the horizontal cells, although some differences were observed in the waveform of the S-potentials during the onset of suppression. We conclude that the physiological effects of Tb on the retina are essentially similar to those of Ca, and that Tb could, therefore, prove a useful marker of Ca distribution in neural tissue.