Noradrenaline action on cat retinal ganglion cells is mediated by dopamine (D2) receptors

Effects of iontophoretically applied noradrenaline, dopamine and their receptor antagonists on the retinal ganglion cells, were studied in optically intact eyes of barbiturate-anaesthetized cats. Noradrenaline inhibited visually evoked and spontaneous firing of all classes of retinal ganglion cells: the effect being greater on ON- than on OFF-cells and slightly more potent than dopamine on a given cell. All alpha- and beta-adrenoreceptor blockers tested tended to change spikes, but were generally ineffective in blocking the noradrenaline-induced inhibition, when not affecting spikes. The noradrenaline-induced inhibition was, however, effectively blocked by dopamine D2-receptor antagonists. The alpha- and beta-adrenoreceptor antagonists applied alone had no effect, suggesting the absence of endogenous noradrenergic antagonism, although alpha-type adrenergic antagonism was suggestive on a very small number of cells. These results suggest that: (1) noradrenaline action on cat retinal ganglion cells is mediated via dopamine D2-receptors; (2) noradrenaline is not generally released on them, except there may be physiologically active alpha-receptors on a few cells; and (3) many of the adrenoreceptor blockers affect membrane properties of the retinal ganglion cells, in a similar manner to local anaesthetics.