Gap Junction Contributions to the Goldfish Electroretinogram at the Photopic Illumination Level

Understanding how the b-wave of the electroretinogram (ERG) is generated by full-field light stimulation is still a challenge in visual neuroscience. To understand more about the origin of the b-wave, we studied the contributions of gap junctions to the ERG b-wave. Many types of retinal neurons are connected to similar and different neighboring neurons through gap junctions. The photopic (cone-dominated) ERG, stimulated by a small light beam, was recorded from goldfish (Carassius auratus) using a corneal electrode. Data were obtained before and after intravitreal injection of agents into the eye under a photopic illumination level. Several agents were used to affect gap junctions, such as dopamine D1 and D2 receptor agonists and antagonists, a nitric oxide (NO) donor, a nitric oxide synthase (NOS) inhibitor, the gap junction blocker meclofenamic acid (MFA), and mixtures of these agents. The ERG b-waves, which were enhanced by MFA, sodium nitroprusside (SNP), SKF 38393, and sulpiride, remained following application of a further injection of a mixture with MFA. The ERG b-waves decreased following NG-nitro-L-arginine methyl ester (L-NAME), SCH 23390, and quinpirole administration but were enhanced by further injection of a mixture with MFA. These results indicate that gap junction activity influences b-waves of the ERG related to NO and dopamine actions.

The Role of the Pattern Edge in Goldfish Visual Motion Detection

To understand the function of edges in perception of moving objects, we defined four questions to answer. Is the focus point in visual motion detection of a moving object: (1) the body or the edge of the object, (2) the leading edge or trailing edge of the object, (3) different in scotopic, mesopic and photopic luminance levels, or (4) different for colored objects? We measured the Optomotor Response (OMR) and Edge Triggering Response (ETR) of goldfish. We used a square and sine wave patterns with black and red stripes and a square wave pattern with black and grey stripes to generate OMR's and ETR's in the goldfish. When we used black and red stripes, the black leading edges stimulated an ETR under scotopic conditions, red leading edges stimulated an ETR under photopic conditions, and both black and red leading edges stimulated an ETR under mesopic luminance levels. For black and gray stripes, only black leading edges stimulated an ETR in all three light illumination levels. We observed less OMR and ETR results using the sine wave pattern compared to using the square wave pattern. From these results, we deduced that the goldfish tend to prefer tracking the leading edge of the pattern. The goldfish can also detect the color of the moving pattern under photopic luminance conditions. We decided that ETR is an intriguing factor in OMR, and is suitable as a method of behavioral measurement in visual system research.

Intraocular Injection of Muscimol Induces Illusory Motion Reversal in Goldfish

Induced activation of the gamma-aminobutyric acidA (GABA(A)) receptor in the retina of goldfish caused the fish to rotate in the opposite direction to that of the spinning pattern during an optomotor response (OMR) measurement. Muscimol, a GABA(A) receptor agonist, modified OMR in a concentration-dependent manner. The GABA(B) receptor agonist baclofen and GABA(C) receptor agonist CACA did not affect OMR. The observed modifications in OMR included decreased anterograde rotation (0.01~0.03 micrometer), coexistence of retrograde rotation and decreased anterograde rotation (0.1~30 micrometer) and only retrograde rotation (100 micrometer~1 mM). In contrast, the GABA(A) receptor antagonist bicuculline blocked muscimolinduced retrograde rotation. Based on these results, we inferred that the coding inducing retrograde movement of the goldfish retina is essentially associated with the GABA(A) receptor-related visual pathway. Furthermore, from our novel approach using observations of goldfish behavior the induced discrete snapshot duration was approximately 573 ms when the fish were under the influence of muscimol.

Two Types of Voltage-activated Calcium Currents in Goldfish Horizontal Cells

In horizontal cells (HCs) that were freshly dissociated from goldfish retina, two types of voltage- dependent calcium currents (ICa) were recorded using a patch-clamping configuration: a transient type current and a sustained type current. The cell was held at -40 mV, and the prepulse step of -90 mV was applied before command pulse between -65 and +55 mV. The transient Ca2+ current was activated by depolarization to around -50 mV from a prepulse voltage of -90 mV lasting at least 400 ms and reached a maximal value near -25 mV. On the other hand, the sustained Ca2+ current was induced by pre-inactivation for less than 10 ms duration. Its activation started near -10 mV and peaked at +20 mV. Co2+ (2 mM) suppressed both of these two components, but nifedipine (20microM), L-type Ca2+ channel antagonist, blocked only the sustained current. Based on the activation voltage and the pharmacological specificity, the sustained current appears to be similar to L-type ICa and the transient type to T-type ICa. This study is the first to confirm that transient type ICa together with the sustained one is present in HCs dissociated from goldfish retina.

Effects of Nitric Oxide on Inhibitory Receptors of Rod Bipolar Cells of Rat Retina

The effects of nitric oxide (NO) on inhibitory neurotransmitter receptors and some types of inhibitory receptors in dissociated rod bipolar cell (RBC) were investigated. In the whole cell voltage-clamping mode, the gamma-aminobutyric acid (GABA) activated current showed both sustained and transient components. GABA activated transient current was fully blocked by bicuculine, a GABAA receptor antagonist. The cis-4-aminocrotonic acid (CACA), a GABAC receptor agonist, evoked the sustained current that was not blocked by bicuculline (BIC). Glycine activated the transient current. These results indicate that the RBCs possess GABAA, GABAC, and glycine inhibitory receptors. Sodium nitroprusside (SNP), a NO analogue, reduced the currents activated by GABAA receptor only, however, did not reduce the currents activated by either GABAC or glycine receptors. This study signifies further that only NO depresses the fast inhibitory response activated by GABAA receptor in RBC. We, therefore, postulate that NO might depress the light-on/off transient inhibitory responses in RBCs in the rat retina.

Effects of the Center-Surround Light Stimuli on the Catfish Retianl Neurons

For the investigation of the receptive field of the retinal neurons, the spatial and temporal properties of horizontal cells and the center-surround antagonisms of bipolar cells and the third-order neurons were studied using conventional intracellular recording techniques. Horizontal cells were hyperpolarized by the large annulus light stimuli (id: 3.5mm) and required at least 0.2 seconds of dark periods to respond enough. The amplitudes of the response of the ON-bipolar cell were decreased as the diameter of the spot stimuli was increased to 2.6mm and the responses were inverted at 2.6mm of inner diameter of the spreading annulus stimuli with fixed width. ON-sustained cell with spikes generated ON-sustained light responses by small annulus (1400-2100micrometer), but elicited OFF-sustained responses by large annulus (2100-2800micrometer). ON-sustained cell without spikes also showed surround antagonism. The spikes were generated at the spot of 490micrometer diameter and they were disappeared at the spot of 5600micrometer diameter. And, OFF-transient component of the ON-OFF transient cell was enhanced as annulus stimuli spread. The results suggest that the horizontal cells have large and monotonic receptive fields while the bipolar cells and the ON-sustained cells have large biphasic receptive fields in the catfish retina.

Chemical coupling between horizontal cells in the catfish retina

The effects of GABA and glutamate on the horizontal cells were explored by an intracellular recording method to discern the mechanisms of receptive field formation by chemical coupling in the catfish outer retina. The results suggest that the horizontal cells of the catfish retina might use GABA as their transmitters and that the GABAergic system contributes to the formation of receptive fields of the horizontal cells. GABA|c receptors may be involved in a chemical coupling between horizontal cells and concerned with the depolarizing actions by GABA on horizontal cells in the catfish retina. Since the chloride equilibrium potential is more positive than the dark membrane potential in horizontal cells, GABA released from a horizontal cell may depolarize the neighboring horizontal cells. Thus a chemical coupling between horizontal cells may be formed. GABA|A receptors also may be involved in the negative feedback mechanism between photoreceptor and horizontal cell. And glutamate may be involved in connecting positive and negative feedback systems since it potentiated the GABA's actions. Therefore, it is presumed that large receptive fields in the catfish retina are formed not only by electrical coupling but also by chemical coupling between horizontal cells. And information travels laterally by pathways involving both electrical coupling composed of gap junctions and chemical coupling in the retinal network.

Preferential Suppression of the On Pathway by r-Aminobutyric Acid in the Catfish Retina

The effects of r-aminobutyric acid(GABA) agonsits and antagonists were explored by the intracellular recording method to discern the preferential suppression of the ON component by GABA on the ON-OFF transient cell in the catfish (Ictalurus punctatus) retina. Experiments were performed in the superfused eyecup preparation. The animals were decapitated and pited before the eye, and the surrounding tissue was removed from the skull. The retina was exposed by excising the cornea, iris, and vitreous. This preparation rested on a wad of Ringer`s soaked cotton in contact with an Ag/Agcl reference electrode. Solutions were delivered through a manifold system that was connected to a pipette located near the absorbent wick. Electro-physiological recordings were made using standard intracellular electrodes filled with 2 M potassium acetate. The electrical signal was recorded with an amplifierand a penwriter, viewed on an oscilloscope, and stored on a data recorder. The light sources were red light-emitting-diode (LED) and the stimuli were full field illumination covering the cntire retina. GABA preferentially reduced ON light responses in ON-OFF transient cell. and GABA hyperpolarized bipolar cells, but the effects on ON bipolar cells were more sensitive than OFF bipolar cells. CACA and TACA, GABAc receptor agonist, did not act on bipolar cells. CACA and TACA, GABAc receptor agonists, hyperpolarized bipolar cells but the sensitivity deferences between ON and OFF bipolar cell were not observed. These results suggest that the preferential suppression of the ON component of the ON-OFF transient cell by GABA was resulted from the presynaptic mechanism that reduced bipolar cell input.