Chang-Sub Jung

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.

Inhibition of nitric oxide synthase induces increased production of growth-associated protein 43 in the developing retina of the postnatal rat

We investigated the effects of N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, on retinal development in the postnatal rat by immunocytochemistry and immunoblotting using antisera against neuronal nitric oxide synthase (nNOS) or growth-associated protein 43 (GAP-43). An nNOS-immunoreactive band of 155 kDa and a GAP-43-immunoreactive band of 48 kDa were present in the extracts of both control and L-NAME-treated rat retinas. The intensity of the nNOS-immunoreactive band was much weaker in the treated rats, whereas the intensity of the GAP-43-immunoreactive band of 48 kDa was much stronger in the treated rats. Much stronger GAP-43 immunoreactivity was visible in the inner plexiform layer (IPL) of the treated retinas at P10, P14 and P21. Our findings suggest that NO may play an important role in the maturation of the IPL in the developing rat retina.

Run-up of γ-aminobutyric acidC responses in catfish retinal cone-horizontal cell axon-terminals is modulated by protein kinase A and C

Using whole-cell voltage-clamp techniques, we investigated the protein kinase modulation of gamma-aminobutyric acid(C) (GABA(C))-activated currents relating to run-up regulation in dissociated cone-horizontal cell (HC) axon-terminals from catfish retina. GABA induced an inward chloride current in cells voltage-clamped at -70 mV. With repetitive applications of 10 microM GABA, the peaks of the GABA responses increased up to approximately 135% of the control responses during a period of 10 min. Intracellular application of forskolin, an adenylate cyclase activator, decreased the run-up of GABA(C) responses. H8 dihydrochloride, a cAMP inhibitor, enhanced this run-up to 190% of the control responses. 1-oleoyl-2-acetyl-sn-glycerol, a protein kinase C activator, accelerated the run-up of GABA(C) responses. GF 109203X, a PKC inhibitor, decreased the run-up. These results suggest that retinal GABA(C) responses in cone-HC axon-terminals are modulated by both protein kinase A and C.

GABA receptors on horizontal cells in the goldfish retina

We investigated the localization of GABA(A) and GABA(C) receptors in horizontal cells (HCs) and HC axon terminals (ATs) dissociated from goldfish retina, using whole-cell patch-clamping recordings. Applications of GABA on HCs induced two groups with inward currents at the holding potential of -50 mV: One was a sustained inward current in the H1 cell, with one type of HCAT (AT1), and the other was a transient inward current in other HC soma and HCAT (AT2). Co-application of GABA with bicuculline or SR95531, GABA(A) receptor antagonists, showed a non-blocking effect in the sustained current, but a blocking effect in the transient current. The sustained current was evoked by cis-4-aminocrotonic acid (CACA), a GABA(C) receptor agonist, while the transient current was not induced by CACA, but mimicked by muscimol, a GABA(A) receptor agonist. Both the sustained and transient currents were completely blocked by picrotoxin and not mimicked by baclofen, a GABA(B) receptor agonist. Thus H1 cell and AT1 have GABA(C) receptors, while H2, H3 cells and AT2 have GABA(A) receptors.

The GABAC receptor is present in cone-horizontal cell axon terminals isolated from catfish retina

Whole cell voltage-clamp recordings were performed on isolated terminals and somata from catfish retina to compare the distribution of excitatory and inhibitory receptors in both structures. Saturating concentrations of glutamate or kainate produced small currents in axon terminals, averaging less than 8% of the current evoked in the soma. In contrast, application of high concentrations of gamma-aminobutyric acid (GABA) produced approximately similar current amplitudes in both structures. Based on estimates of membrane surface area, GABA-induced current densities were around 0.05 pA/microm2 for both structures. The GABA-activated current in the axon terminal was not blocked by bicuculline or SR95531, but was completely inhibited by picrotoxin. Baclofen did not mimic the GABA effect, but trans-4-aminocrotonic acid (TACA, 300 microM) and muscimol (1 mM) elicited currents of 100 and 40 pA, respectively. These results suggest that the axon terminals of cone-horizontal cells possess GABA(C) receptors at a high density, do not possess GABA(A) or GABA(B) receptors, and have few glutamate receptors. The GABA(C) receptors could function as postsynaptic receptors in the inner plexiform layer or as autoreceptors.

Intraocular Injection of Muscimol Induces Illusory Motion Reversal in Goldfish

Induced activation of the gamma-aminobutyric acid(A) (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 microM), coexistence of retrograde rotation and decreased anterograde rotation (0.1~30 microM) and only retrograde rotation (100 microM~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.

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 OMRs and ETRs 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.

The functional roles of chemical coupling and gap junction between horizontal cells in the catfish retina

Abstract For the investigation of the roles of chemical coupling and gap junction between horizontal cells, the spatio-temporal properties of horizontal cell network were studied using conventional physiological recording techniques and computer simulations. Our results show that the spatial integration of light signal may be mainly performed by electrical junctions, and the temporal integration may be regulated by chemical coupling between horizontal cells. This means that visual information can be modulated spatio-temporally by pathways involving both electrical coupling composed of gap junctions and chemical coupling composed of GABAergic synapses in outer retina of the low vertebrate.