Electrical coupling of retinal horizontal cells mediated by distinct voltage-independent junctions

Electrical coupling between H2 horizontal cell pairs isolated from the hybr id bass retina was studied using dual whole-cell, voltage-clamp technique. Voltage-dependent inactivation of junctional currents in response to steps in transjunctional voltage (Vj) over a range of +/-100 mV was characterized for 89 cell pairs. Approximately one-quarter of the pairs exhibited strong ly voltage-dependent junctions (>50% reduction in junctional current at +/- 100 mV), another quarter of the pairs exhibited voltage-independent junctio nal current (<5% reduction at +/-100 mV), and the remainder of the pairs ex hibited intermediate values for voltage inactivation. We focused on further characterizing the Vj-independent junctions of horizontal cells, which hav e not been described previously in detail. When Lucifer Yellow dye was incl uded in one recording pipette. pairs exhibiting Vj-independent coupling sho wed no (9/12), or limited (3/12), passage of dye. Vj-independent coupling w as markedly less sensitive to the modulators SNP (100-300 mu M, -9% reducti on in coupling) and dopamine (100-300 mu M, -6%) than were Vj-dependent jun ctions (-45% and -44%). However, simultaneous application of both SNP and d opamine significantly reduced Vj-independent coupling (-56%). Both Vj-indep endent and Vj-dependent junctions were blocked by DMSO (1-2%), but Vj-indep endent junctions were not blocked by heptanol. Single-channel junctional co nductances of Vj-independent junctions range from 112-180 pS, versus 50-60 pS for Vj-dependent junctions. The results reveal chat Vj-independent coupl ing in a subpopulation of horizontal cells from the hybrid bass retina is m ediated by cellular junctions with physiological and pharmacological charac teristics distinct from those previously described in fish horizontal cells .