Sun-Ho Bai

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.

Effects of baclofen on the directional selectivity of transient neurons in the catfish retina

Abstract Effects of baclofen on the directional selectivity of the inner retinal neurons were studied using conventional intracellular recording techniques and computer simulations. Baclofen suppressed sustained components, but enhanced transient components of third-order neurons in the catfish retina. In addition, directional selectivity of transient neurons was enhanced by the application of baclofen. The simplified network model was tested to account for the details of directional selectivity observed in the inner retina. Results suggest that the directional selectivity in fish retina could be evoked by the asymmetric distribution of GABAergic third-order neurons.

The effects of low chloride fluids on light responses of the catfish retinal neurons

Abstract Perfusion experiments were performed to analyze functional roles of the chloride channels in the retinal neurons. The network model was tested for its ability to account for the details of light responses observed in retinal neurons perfused with low chloride fluids. Light responses of the second-order neurons were visibly enhanced. In addition, the wave shapes and the firing rates of the third-order neurons were evidently varied by the low chloride fluids. Results suggest that the sensitivity of the retinal neurons to light stimuli could be changed by extracellular concentration of chloride ion.

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.