Electron microscopic demonstration of tyrosine hydroxylase-immunoreactive interplexiform cells in the lamprey retina

To clarify the controversies about the existence (or not) of dopaminergic interplexiform cells in the lamprey retina, we have performed an immunocytochemical electron microscopic study of the retina of the river lamprey Lampetra fluviatilis, using anti-tyrosine hydroxylase antibody. We demonstrate the presence of immunoreactive processes in both inner (IPL) and outer (OPL) plexiform layers. The external processes are in close contact with horizontal cell processes and photoreceptor terminals in the OPL, but do not make classical synapses. Some of them are ensheathed within Müller cell cytoplasm. Thus, dopaminergic interplexiform cells do exist in Lampetra fluviatilis, but the absence of synapses rather supports a paracrine action of dopamine in the outer retina.