[Modulation of electrical coupling of horizontal cells of the retina]

Horizontal cells are electrically coupled in the teleost retina. This coupling is modulated by dopamine, which is in turn liberated from interplexiform amacrine cells. The present review includes studies carried out to elucidate the intracellular mechanisms responsible for such modulation of the resistance of intercellular unions, that results from the action of dopamine on receptors located on the cell membrane. In order to study the effects of possible second messengers, hydrogen ions (H+), cyclic adenosine monophosphate (cAMP) or calcium ions (Ca2+) were injected electrophoretically from multiple microelectrodes that simultaneously allow for voltage recording in the injected cells and in neighboring ones. A resistive hexagonal network model was used to calculate the cell membrane resistance (Rm) and the coupling resistance (Ra) between the two impaled cells. Intracellular injections of Lucifer yellow were used to microscopically verify the changes produced in Ra. The microinjection of either H+ or cAMP led to reduction in coupling ratio and increase in Ra, while Rm was reduced. These changes are similar to those produced by dopamine perfusion of the retina and, equally, are corroborated by the hindrance of diffusion to neighboring cells of the Lucifer yellow injected simultaneously. No changes were detected upon Ca2+ injection, but the chelator EGTA resulted in an increased electrical coupling and Rm reduction. The results reported are consistent with the idea that both H+ and cAMP are second messengers involved in dopamine's modulation of intercellular coupling, which is produced by changes in the resistance of intercellular unions, while Ca2+ would act as a modulator through its effect on the cell membrane resistance.