A new type of interplexiform cell in the goldfish retina is PNMT-immunoreactive

A new cell type immunoreactive for phenylethanolamine N-methyltransferase, the synthesizing enzyme for epinephrine, has been identified in the goldfish retina. The somata of the immunoreactive cells were located in the proximal inner nuclear layer and immunoreactive processes were located in both the inner and outer plexiform layers, suggesting that this cell may be an interplexiform cell. Confocal microscopy was used to establish that the putative adrenergic neurites in the outer plexiform layer were located between the somata of horizontal cells and photoreceptor cell synaptic terminals. Intracellular recordings from horizontal cells demonstrated that epinephrine had an effect on horizontal cells, but that the action of epinephrine was consistent with an effect on dopamine receptors, rather than adrenergic receptors. The function of the putative 'I3' interplexiform cell therefore remains unclear, possibly modulating horizontal cell function with a transmitter other than epinephrine or affecting photoreceptors or the retinal pigment epithelium rather than horizontal cells.