Gap Junction Contributions to the Goldfish Electroretinogram at the Photopic Illumination Level 223 [31-33]. As the b-wave results were only related to MFA, it can be construed that both NO and MFA affected b-wave amplitude by the same mechanism associated with gap junctions. We believe that DA may act on the gap-junction coupling of ON-bipolar cells either directly or indirectly. D1-like re- ceptors are expressed by horizontal and ON-bipolar cells, and D2-like receptors are expressed by rod, cones, and IPCs in the goldfish retina [34-37]. In our experiments, the D1-like receptor agonist SKF 38393 produced a significant increase in b-wave amplitude. Decreased b-wave amplitude by a D1-like receptor antagonist such as SCH23390 was enhanced by MFA. A different modulation of the ERG was reported by Huppe-Gourgues et al. (2005), who showed that a D1 ago- nist provokes a reduction in the ERG b-wave components in the rabbit retina [38]. They also used D1- and D2-like DA receptor agonists and antagonists. Their results with DA-related agents conflict with our data. We think that these differences are attributed to differences in experimen- tal methods such as light stimulation and species. While we used a small-sized beam that was reflected by a mirror and a wide projected background light, they used a flash lamp placed in front of the animal’s head for light stimu- lation. We think this stimulation works as a condition of the wide full field. When we used the large size light beam corresponding to the value of 0.12o in the field of view for light stimulation, the amplification by MFA on b-wave ap- peared weak. However, the light stimulus with small sized light beam in our experiment could be considered a small spot light. Hedden and Dowling (1978) reported that DA enhances the center response of bipolar cells to small spots of light in isolated goldfish retinas [39], whereas it reduces the surround response to an annual light. Furthermore, while DA-IPCs occur in the goldfish retina, the DA retinal cells are amacrine cells in the rabbit [40-45]. D2-like receptors are expressed by rod, cones, and DA-IPCs in the goldfish retina [34-37]. In our experiment, almost no changes in ERG a-waves were observed to be caused by D2-like receptor activation. If the D2 receptors between rods and cones are blocked with an antagonist, rod-cone gap-junction coupling increases [46], but DA on photoreceptor cells is involved in mediating nonelectrophy- siological events (e.g., retinomotor moments) and has no di- rect effect on electrophysiological activities of photorecep- tors [44]. D2-like receptor agonists and antagonists produced oppo- site responses of the ERG b-wave; a slight decrease com- pared to the control and an increase, respectively. These effects of D2 agents are assumed to have resulted from ac- tivity of the DA-IPCs, which form output connections di- rected to horizontal cells, bipolar cell dendrites, and the ter- minals of rods and cones in the outer plexiform layer. Although Harsanyi and Mangel (1992) have shown that DA activation of D2 autoreceptors on IPCs inhibits DA release onto horizontal cells and causes increased electrical cou- pling between them in the goldfish retina [35], uncoupling of horizontal cells alters the receptive fields of retinal bipo- lar cells in the carp retina [47]. We do not rule out whether the effects of D2 agents on bipolar cells were indirectly mediated by gap junction channels of horizontal cells or/and directly of bipolar cells, as it is likely that both are included. The enhanced b-wave amplitude by DA remained un- changed after further application of a DA and SNP mixture. Single-channel data show that NO reduces gap junction channel open probability by reducing channel opening fre- quency, and that DA reduces gap junction channel opening by reducing both channel opening duration and opening fre- quency in dissociated horizontal cells of the teleost retina [12,17]. Although these results offer the possibility of a syn- ergistic interaction between DA and NO for junction con- ductance, we did not observe any synergistic interactions between them on the ERG b-waves. Our results suggest that DA and NO are not independent but interact. ACKNOW LEDGEMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0012276). 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