Bicuculline produces reversible red-green color blindness in goldfish, as revealed by monocular behavioral testing

Systemic thyroid hormone is necessary and sufficient to induce ultraviolet-sensitive cone loss in the juvenile rainbow trout retina

Rainbow trout possess ultraviolet-sensitive (UVS) cones in their retina that degenerate naturally during development. This phenomenon can be induced with exogenous thyroxine [T4, a thyroid hormone (TH)] treatment. However, the previous T4 exposure experiments employed static water immersion; a method that could introduce confounding stress effects on the fish. Because of this, it was uncertain if T4 alone was sufficient to induce retinal changes or if stress-related hormones were also necessary to initiate this process. Furthermore, it was unclear whether endogenous T4 was the factor responsible for initiating natural UVS cone loss during development. The current study examined the role of systemic T4 on the juvenile rainbow trout retina using a slow-release implant. Exogenous T4 treatment resulted in SWS1 opsin downregulation and UVS cone loss after four weeks of exposure, signifying that T4 is sufficient to induce this process. Blocking endogenous T4 production with propylthiouracil (PTU, an anti-thyroid agent) attenuated SWS1 downregulation and UVS cone loss in the retina of naturally developing rainbow trout, suggesting that endogenous T4 is necessary to initiate retinal remodelling during development. Quantitative real-time RT-PCR analysis demonstrated that several TH-regulating components are expressed in the trout retina, and that expression levels of the TH receptor isoform TRbeta and the type 2 deiodinase (D2) change with T4 treatment. This suggests that T4 may act directly on the retina to induce UVS cone loss. Taken together, these results demonstrate that systemic TH is necessary and sufficient to induce SWS1 opsin downregulation and UVS cone loss in the retina of juvenile rainbow trout.

Temporal resolution and temporal transfer properties: Gabaergic and cholinergic mechanisms

Temporal resolution is a basic property of the visual system and critically depends upon retinal temporal coding properties which are also of importance for directional coding. Whether the temporal coding properties for directional coding derive form inherent properties or critically depend upon the temporal coding mechanisms is unclear. Here, the influence of acetylcholine and GABA upon photopic temporal coding was investigated in goldfish, using flicker stimuli, in a behavioral and an electrophysiological (ERG) approach. The goldfish temporal resolution ability decreased from more than 90% correct choices at 20 Hz flicker frequency to about 65% at 45 Hz flicker frequency with a flicker fusion frequency of approximately 39 Hz. Blockade of GABAa-receptors reduced the flicker fusion frequency to about 23 Hz, not affecting temporal resolution below 20 Hz flicker frequency. Partial blockade of nicotinic acetylcholine receptors reduced the flicker fusion frequency slightly and lowered the temporal resolution ability in the 25–30 Hz range. Blockade of muscarinic acetylcholine receptors had a smaller effect than the partial blockade of nicotinic acetylcholine receptors. In ERG-recordings, blocking GABAa-receptors increased the a- and b-wave amplitude, induced a delay, an increase and a slow fall-off of the d-wave. Blocking GABAc-receptors had little effect. Blocking GABAa- or GABAa/c-receptors changed the temporal resolution, when expressed as a linear filter, from a 3rd degree filter with resonance to a low order low-pass filter with a low upper limit frequency. The temporal transfer properties were barely changed by blocking either nicotinic or muscarinic acteylcholine receptors, although ERG-components increased in amplitude to varying degrees. The behavioral and electrophysiological data indicate the important role of GABA for temporal processing but little involvement of the cholinergic system. It is proposed that the interaction of the GABAergic amacrine cell network and bipolar cells determines the gain of the retinal temporal coding in the upper frequency range.

Small field motion detection in goldfish is red-green color blind and mediated by the M-cone type

Large field motion detection in goldfish, measured in the optomotor response, is based on the L-cone type, and is therefore color-blind (Schaerer & Neumeyer, 1996). In experiments using a two-choice training procedure, we investigated now whether the same holds for the detection of a small moving object (size: 8 mm diameter; velocity: 7 cm/s). In initial experiments, we found that goldfish did not discriminate between a moving and a stationary stimulus, obviously not taking attention to the cue “moving.” Therefore, random dot patterns were used in which the stimulus was visible only when moving. Using black and white random dot patterns with variable contrast between 0.2 and 1, we found that the fish could see motion only with high (0.8) contrast. In the decisive experiment, a red-green random dot pattern was used. By keeping the intensity of the red dots constant and reducing the intensity of the green dots, a narrow intensity range was found in which goldfish could no longer discriminate between the moving random dot stimulus in random dot surround and the stationary random dot pattern. The same was the case when a red moving disk was presented in green surround. This is the evidence that object motion is red-green color blind, i.e., color information cannot be used to detect the moving object. Calculations of the cone excitation values revealed that the M-cone type is decisive, as this cone type (and not the L-cone type) is not modulated by that particular red-green pattern in which the moving stimulus was invisible.

Bicuculline antagonizes 5-HT(3A) and alpha2 glycine receptors expressed in Xenopus oocytes

The present study examined the effects of bicuculline on the mouse 5-hydroxytryptamine(3A) receptor (5-HT(3A) receptor and the human alpha2 subunit of the glycine receptor. Bicuculline antagonized both the 5-HT(3A) receptor (IC(50)=20.12+/-0.39 microM) and the alpha2 glycine receptor (IC(50)=169.40+/-1.73 microM). A competitive form of antagonism by bicuculline was suggested by experiments in which the EC(50)s for 5-HT and glycine were increased in the 5-HT(3A) and alpha2 glycine receptors, respectively, as bicuculline concentrations were increased. A competitive nature of antagonism by bicuculline at the 5-HT(3A) receptor was also suggested by displacement of the competitive antagonist, [3H]GR65630 in SF21 insect cells expressing the 5-HT(3A) receptor (K(i)=19.01+/-0.71 microM). Our data and that of others reveal that bicuculline, a purported selective antagonist of the GABA(A) receptor, antagonizes at least one receptor subclass in every member of the superfamily of ligand-gated ion channels.

Light-modulated release of RFamide-like neuropeptides from nervus terminalis axon terminals in the retina of goldfish

The nervus terminalis of teleosts, a cranial nerve anatomically associated with the olfactory system, projects to visual system targets including retina and optic tectum. It is known to contain gonadotropin-releasing hormone and RFamide-like peptides, but its function remains unknown. We have probed nervus terminalis function in goldfish by measuring peptide content in retina and tectum with a radioimmunoassay for A18Famide (neuropeptide AF; bovine morphine-modulating peptide). We found that retinal peptide content increased in the dark and decreased in the light, whereas tectal peptide content decreased in the dark and increased in the light. In addition, RFamide-like peptide content in the retina was transiently decreased by severing both olfactory tracts, increased in light-adapted eyes treated with a GABAergic agonist (isoguvacine), and decreased in dark-adapted eyes treated with GABAergic antagonists (bicuculline and picrotoxin). We also found that RFamide-like peptide release could be induced in dark-adapted isolated-superfused retinas by exposure to light or a high concentration (102.5 mM) of potassium ions. We interpret the increase and decrease in peptide content as reflecting a decrease and increase, respectively, in rate of peptide release. We propose that the release and accumulation of RFamide-like peptides in axon terminals of nervus terminalis processes in the retina are modulated primarily by neurons intrinsic to the retina and regulated by light. Peptide release appears to be inhibited tonically in the dark by GABA acting through GABAA receptors; light facilitates peptide release by disinhibition due to a reduction in GABA release. In addition, we propose that electrical signals originating outside the retina can override these intrinsic release-modulating influences.

Reduction of red-green discrimination by dopamine D1 receptor antagonists and retinal dopamine depletion

Reduction of wavelength discrimination ability in the 560-640 nm range, but not in the 404-540 nm range, has been demonstrated in goldfish after intravitreal injection of D1-dopamine receptor antagonists. Intravitreal injection of the dopaminergic neurotoxin 6-OH-dopamine severely reduced wavelength discrimination ability in the 540-661 nm range within 3 days. Discrimination ability could be reconstituted by the D1-agonist SKF 38393. Animals recovered from injection of 6-OH-dopamine within 14-16 days. No change of wavelength discrimination was induced by 6-OH-dopamine in the 461-540 nm range. We conclude that under photopic conditions dopamine modulates retinal mechanisms involved in red-green colour coding via D1-dopamine receptor-like binding sites.

Modulating wavelength discrimination in goldfish with ethambutol and stimulus intensity

Wavelength discrimination in goldfish was measured behaviourally. Both acute application of ethambutol injected into the eye and chronic application by feeding the animals daily 25 mg ethambutol for 1 month had the same effect on wavelength discrimination in the range of 560-640 nm. This means that: (1) electrophysiological experiments, in which drug application is primarily acute, reflect the same disturbance as behavioural experiments, in which drug application is chronic; and that (2) the origin of the color vision defect must be retinal. Furthermore reduction in stimulus intensity by 2 log units caused, in control fish, a similar disturbance in wavelength discrimination as induced by ethambutol, whereas an increase of stimulus intensity by 2 log units abolished in ethambutol-fed fish the discrimination disturbance. These results indicate that ethambutol shifts the threshold for wavelength discrimination without changing the absolute sensitivity of the cone systems.

Depletion of retinal dopamine increases brightness perception in goldfish

The effect of unilateral depletion of retinal dopamine on goldfish visual behavior was studied using a behavioral reflex, the dorsal light reaction (DLR). Retinal dopamine was depleted by intraocular injections of 6-hydroxydopamine (6-OHDA) on two successive days. By 2 weeks postinjection, dopamine interplexiform cells (DA-IPC) were not detected using tyrosine-hydroxylase immunoreactivity (TH-IR). By 6 weeks postinjection, generation of DA-IPC was observed at the marginal zone and by 9 months postinjection, 2-3 rows of DA-IPC were present at the marginal zone. Neurites extended several hundred micrometers toward the central retina. By 2 weeks postinjection, all 6-OHDA lesioned fish tilted 7-15 deg toward the injected eye under uniform overhead illumination. The tilting did not occur under scotopic illumination and reappeared within 1 min of light adaptation. Quantitation of the DLR showed that 6-OHDA lesioned fish behaved as if light were 2.4 log units more intense to the injected eye. Partial recovery was observed by 9 months postinjection, paralleling the reappearance of DA-IPC at the marginal zone. Tilting also was induced by unilateral intraocular injection with D1 and D2 dopamine receptor antagonists, SCH 23390 and S(-)-sulpiride, respectively. Fish did not tilt if they were light adapted at the time of injection. Tilting was observed if the animals were dark-adapted for 3 h and left in the dark for 1 h postinjection. Fish tilted toward the drug-injected eye within 2 min of light adaptation and gradually returned to vertical within 2 h. The tilting response to S(-)-sulpiride was stronger (approximately 20 deg vs. approximately 5 deg) and occurred at lower concentration (1 microM vs. 10 microM) compared to SCH 23390. We conclude that dopamine depletion mimics the dorsal light reaction by increasing the luminosity output of the eye and that dopamine is directly involved in photopic luminosity function.

APB alters photopic spectral sensitivity of the goldfish retina

Because the glutamate analog 2-amino-4-phosphonobutyric acid (APB) alters synaptic transmission at the outer plexiform layer in goldfish we asked whether intraocular injection of ABP would alter the spectral sensitivity of the retina. The spectral sensitivity of the ON and OFF components of the optic nerve response (ONR) in goldfish was measured in the presence and absence of APB, under four chromatic adaptation condition. APB decreased absolute sensitivity and altered spectral sensitivity for both ON and OFF responses under each adaptation condition. The spectral sensitivity of the OFF response was altered most at short wavelengths, in a manner consistent with a change in the balance of additive cone inputs. For the ON response, the effects of APB were consistent with a change in spectral antagonism, particularly between M- and L-cones. These results suggest that the activity in the retinal cone pathways in goldfish can be influenced by a mechanism incorporating an APB-sensitive receptor, and that this receptor may be intimately involved with setting the balance of cone inputs to spectrally-opponent neurons.

GABA-ergic control of visual perception in healthy volunteers: effects of midazolam, a benzodiazepine, on spatio-temporal contrast sensitivity

1. We studied the effects of midazolam (MDZ), a benzodiazepine, on spatio-temporal contrast sensitivity, choice reaction time, and mood visual analogue scales in healthy volunteers. 2. Eight extensively trained, healthy volunteers were included in a placebo-controlled cross-over double-blind trial of MDZ (0.15 mg kg-1). Treatments were injected intramuscularly and evaluations were performed before and 0.5, 1, 2, 3 and 4 h after drug administration. Spatio-temporal contrast sensitivity was measured using a micro-computer with appropriate software. Stimuli were vertical gratings with adjustable contrast, with spatial frequencies of 0.25, 1 and 4 cpd. Four conditions of temporal modulation were used: the grating was either static or drifting laterally with temporal frequencies of 1, 3 and 9 Hz. 3. An analysis of variance was performed on the data. As compared with placebo, MDZ induced an increase in choice reaction time and sedation (as assessed on visual analogue scales). From 0.5-4 h after the injection, MDZ produced an overall decrease in visual sensitivity, as compared with placebo. More specifically, MDZ preferentially affected medium to high spatial frequencies and low temporal frequencies. Several non-exclusive hypotheses may account for the results: 1) an increase in the size of the receptive fields, 2) a preferential effect on the visual parvocellular pathways which mediate the sensitivity to high spatial and low temporal frequencies.