METHODS We used goldfish (Carassius auratus) for our experiments. The goldfish were 7∼9 cm in length and weighed 10∼16 g. We purchased the fish at a fish shop and maintained them at room temperature in an aquarium under a 12 hour light/dark daily cycle. The fish were fed twice daily in the morning and evening. Electroretinography Fish were fully anaesthetized in a water bath containing 0.02% MS-222 (tricaine methanesulfonate) and placed on a wet plaster fish bed with one eye in the upward direction. During recording, aerated water containing 0.005% MS-222 continuously flowed though the mouth and over the gills to maintain anesthesia. Ag-AgCl pallets were used as a ref- erence in the mouth, and ground electrodes were placed be- hind the fish body. A brush made with pig hair was mount- ed on a microelectrode holder (MEH6SF, WPI, Sarasota, FL, USA) for the recording electrode and contained Ringer’s solution inside for electrical conduction. A hydraulic micro- manipulator (MO-10, Narishige, Tokyo, Japan) system was used to move the tip of the brush in microscale to contact the center of the cornea for corneal ERG recordings. The recording electrode was connected to positive inputs of a differential amplifier (gain of 10000, bandpass filter 0.1∼ 100 Hz: DAM50, WPI). Amplified signals were acquired us- ing Axoscope software (Axon Instrument, Union City, CA, DY Kim and CS Jung Rod and cone signals both converge on bipolar cells in the goldfish retina [19,20]. Using the separation processing [21,22], only cone signals contributed to the ERG b-wave activated by ON-bipolar cells in the cone system, as we light adapted the fish with background illumination to saturate the rods. We used steady background illumination of 10 μW/ 2 cm , which lead to compression of rod responses, and light beam stimulation of 980 μW/cm2 to evoke the b-wave res- ponse. For photopic ambient light condition, the background illumination of 10 μW/cm2 is sufficient illumination level based on results of Hood and Finkelstein [23]. We have al- ready identified that 0.4 μW/cm2 in light intensity is suffi- cient for photopic ambient light level in behavioral experi- ments with goldfish [24]. The light source was a white light beam from a beam projector (BX324, LG Corp., Seoul, Korea). The optical densities of the light beam were meas- ured with a radiometer (IL1400A, International Light, Inc., Newburyport, MA, USA). Intravitreal injection All drugs were diluted in a control goldfish Ringer’s sol- ution (in mM: 125 NaCl, 2.6 KCl, 2.5 CaCl2, 1 MgCl2, 10 glucose, and 10 HEPES; pH 7.8). All drug injections were performed on fully anaesthetized fish. After obtaining con- trol ERG data for each goldfish, the ERG measurements were obtained as soon as 2 μl of Ringer solution as a control condition and an agent solution for the drug treatment con- dition were injected sequentially every hour into one eye of the goldfish using an intraocular kit (IO-KIT, WPI). All chemicals were obtained from Sigma Chemical Company (St. Louis, MO, USA). 220 Animals demonstrate that the central receptive response of ON-bi- polar cells can be altered by uncoupling the gap junction in the retinal neurons related to NO and DA. USA). Light stimulation Fig. 1. A schematic drawing, as seen from the side, of the overall process of the light stimulus for electroretinogram (ERG) measurements and consequent responses. (A) The dashed and dotted lines show a sketch of the ray paths of the smaller and larger white circle beams emitted from the projector, respectively. a, computer; b, beam projector; c, surface mirror; d, eye ball; e, iris; f, retina (shaded area); g, recording electrode; h, differential amplifier. (B) The ERG responses were evoked by small and large sizes of circle light beam before (upper and down, left) and after (upper and down, right) intravitreal injection of 200 μM MFA. The small and large sizes of the light beam imaged on the surface of the mirror were 6 mm and 60 mm in diameter corresponding to the values of 0.012o and 0.12o in the field of view. Light stimulus was a 4-s presented by the square wave. The components of ERG a-, b-, c-, and d-waves are denoted as a, b, c, and d.