GABA(A) receptor immunoreactivity is transiently expressed in the developing outer retina

Extensive evidence has suggested a trophic role of gamma-aminobutyric acid (GABA) on developing cone photoreceptors in postnatal retina. In a previous study, we showed that GABA raises intracellular calcium levels in the developing cones via activation of GABA(A) receptors. Using confocal microscopy in conjunction with immunocytochemistry, we have now demonstrated that (1) GABA(A) receptor subunits are localized on cone cell bodies as well as on cone pedicles, indicating that GABA has a direct, rather than indirect, effect on cones and (2) the temporal expression of GABA(A) receptor subunits coincides with the developmental effects of GABA on cone synaptogenesis. An antibody against the beta 2/3 subunits of the GABA(A) receptor and a specific cone marker peanut-agglutinin lectin (PNA) were used to double-label wholemount neonatal retinal preparations. Results show that GABA(A) receptors are transiently expressed on cone photoreceptors in the early stages of postnatal retinal development. GABA (A)receptor immunoreactivity is clearly present on cone cell bodies and their processes and on other--as yet unidentified--elements (horizontal cells?) in the outer plexiform layer. Immunoreactivity decreases within cone photoreceptor somata after postnatal day 5, but persists in the processes of the outer plexiform layer until day 7. Our results provide support for the hypothesis that GABA acts as an important developmental regulator of cone photoreceptor maturation.