Horizontal cells form an electrically coupled network for the transmission of inhibitory signals in the outer retina. In teleosts, horizontal cell coupling is modulated by the neurotransmitter dopamine. Using voltage-clamped pairs of teleost horizontal cells, we have examined the effects of dopamine on the conductance and gating properties of the cell-to-cell channels that mediate electrical synaptic transmission. Variance analysis of the junctional current noise showed that dopamine substantially reduced the open probability of gap junction channels, from 0.75 to 0.14. Direct observation of unitary junctional gating events in poorly coupled cell pairs indicated that these channels have a unitary conductance of 50-60 pS. The elementary conductance of channels in cell pairs treated with dopamine (48.7 +/- 6.6 pS) was statistically indistinguishable from channels in untreated cells (53.2 +/- 7.2 pS). Uncoupling with octanol also yielded a similar unitary conductance (61.1 +/- 11.1 pS). Our results suggest that dopamine reduces the open probability of gap junctional channels by decreasing their open duration.