Inhibitory retinal efferents from dopaminergic cells in the optic lobe of the octopus

The effect of dopamine (DA) and its related compounds upon retinal activities of the octopus was compared with that of repetitive stimulation of the optic nerve, while recording ERGs and light-elicited spike discharges. Retinal perfusion with DA or its agonist, apomorphine, markedly enhanced ERGs. The effect appeared at about 10(-8) M and saturated at 10(-6) M. Norepinephrine and L-DOPA had very little effect. Intraretinal injections of DA at different depths revealed that the site of DA action was in the plexiform layer. ERGs were also enhanced by optic nerve stimulation, which inhibited spike discharges. This inhibitory effect was particularly evident when discharges were weak. Consequently the inhibitory effect of optic nerve stimulation was exerted predominantly to the peripheral areas of the receptive field of a nerve bundle, thus resulting in a considerable reduction in the size of the receptive field. These observations suggest that the spatial resolution of this animal may be improved by the efferent activity.