GABA-INDUCED INCREASES IN [CA(2+)]I IN RETINAL NEURONS OF POSTNATAL RABBITS

Previous studies have indicated that gamma-aminobutyric acid (GABA) pl ays an important trophic role in the synapse formation between horizon tal cells and photoreceptors in postnatal rabbit retina. However, the mechanism of the GABA effect has not been identified. Using fluo-3 Ca2 + imaging and confocal laser scanning microscopy we examined the effec t of GABA on [Ca2+](i) during postnatal retinal development. GABA (100 mu M) evoked a fast and transient increase of [Ca2+](i) in selected p opulations of freshly dissociated retinal cells from postnatal rabbits . This increase was apparent on postnatal day 1 and reached a maximum on day 5. Little increase in [Ca2+](i) was observed in retinal cells i solated from adult rabbits. GABA receptor antagonists, picrotoxin and bicuculline, significantly reduced the response. The GABA(B) agonist, baclofen, did not evoke any [CA(2+)](i) changes. The GABA-induced incr ease in [Ca2+](i) was observed in all retinal layers in neonatal retin al whole-mount explants. In the outer retina, the increase was seen in cone photoreceptors which were specifically labeled with peanut agglu tinin (PNS). The Gaba-induced increase in [Ca2+](i) may provide an imp ortant mechanism for regulating cone synaptogenesis in the outer plexi form layer of the postnatal retina.