Retinal horizontal cell gap junctional conductance is modulated by dopamine through a cyclic AMP-dependent protein kinase.

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Abstract
The action of many neuromodulators is mediated by intracellular second messengers such as cyclic AMP. In the retina, exogenously applied dopamine alters the conductance of gap junctions between cultured horizontal cells and this effects is mediated by cyclic AMP. However, it is not known how cyclic AMP modulates horizontal cell gap junctions function. Here I report that cyclic AMP works by way of a cyclic AMP-dependent protein kinase. Cyclic AMP-dependent protein kinase injected into coupled horizontal cells from white bass (Roccus chrysops) rapidly and reversibly uncoupled the cells, mimicking the actions of dopamine. The threshold for the effect was between 0.06 and 0.03 .mu.M. Injection of Walsh inhibitor of protein kinase [Walsh, D.A., Ashby, C. D., Gonzalez, C., Calkins, D., Fischer, E. H. and Krebs, E. G. (1971) J. Biol. Chem. 246, 1977-1985] blocked the effect of dopamine. Thus, the action of dopamine is to raise intracellular levels of cyclic AMP, which then activates a cyclic AMP-dependent protein kinase. Although not tested, it is likely that the cyclic AMP-dependent protein kinase phosphorylates a protein, possibly a gap junction protein, to alter conductance.