Interactions between enkephalin and GABA in avian retina

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Abstract
In addition to conventional neurotransmitters such as acetylcholine, dopamine, glycine and γ-aminobutyric acid (GABA)1,2, a number of peptide-immunoreactive substances have recently been localized in the vertebrate retina3,4. The functional roles of these retinal peptides and their interactions with conventional neurotransmitters are largely unknown. We have previously shown that exogenous opiates affect both the release of GABA and the firing patterns of ganglion cells in the goldfish retina5, and we have now begun a systematic characterization of the opioid pathways in the chicken retina, because, among vertebrate retinas, avian retinas contain the highest concentration of enkephalins6. Monoclonal antibodies specific for enkephalin have been used to demonstrate that a subpopulation of enkephalin-containing amacrine cells exists in the chicken retina6,7. This retina also synthesizes Met5-enkephalin and releases it on cell depolarization6,8,9. The enkephalin-induced inhibition of GABA release in goldfish retina5 led us to examine whether similar interactions occur in chicken, and if so, whether enkephalins and GABA coexist in the same amacrine cells. Our results, presented here, indicate that exogenous enkephalins do indeed inhibit GABA release in the chicken retina. Surprisingly, we found that although some amacrine cells contain both enkephalin and GABA, others contain only one or the other.