GABA as a developmental neurotransmitter in the outer plexiform layer of the vertebrate retina.

GABA is present in certain retinal neurons before synapses are formed, and it has a variety of effects on the developing retina. Its exact role in retinal maturation is not clear; however, there is growing evidence from rabbit retina that neonatal horizontal cells produce GABA within the outer retina, which in turn is necessary for normal synapse formation of photoreceptors. In addition to its classic role as an inhibitory transmitter, GABA in the neonate increases intracellular calcium levels in selected cells. This latter property is lost during postnatal development, along with a general decrease in the level of expression of both pre- and postsynaptic GABAergic markers and a decrease in the number of GABAergic neurons. The adult GABAergic circuitry in adult retina may represent a restricted, perhaps simplified version of the more complex and diversified interactions of the GABA system during development.