Purpose: Glutamate has been suggested to regulate the development of retinal neurons, but ontogenic expression of ionotropic glutamate receptors has only recently been characterized in the rat retina. The purpose of this study was to examine the expression patterns of AMPA receptors and to functionally map glutamatergic drive in the developing rabbit retina.
Methods: The retinas from New Zealand White rabbits of different developmental stages (embryonic days [E]21 and 26, postnatal days [P]0-10, and adult) were isolated and cryosectioned into vertical slices. Antibodies against GluR1, -R2/3, and -R4 were used to examine the postnatal expression patterns of the AMPA receptor subunits. To further map the glutamatergic drive in the developing rabbit retina, an agmatine (AGB)-activation assay was also used.
Results: All AMPA receptor subunits, including GluR1, -R2/3, and -R4, were expressed in the inner plexiform layer as early as E26 and were convincingly labeled in the outer plexiform layer at P2. These AMPA subunits showed different spatial distribution and temporal expression patterns across the postnatal stages examined. The immunoreactivity of the AMPA subunits was weak at P0 to P2 and then showed a striking increase at P4 to P6. The AGB activation assay revealed that some amacrine and ganglion cells were activated with 2 microM AMPA as early as E26 and, in the presence of an increased concentration of AMPA (20 microM), some potential horizontal cells were activated at the same stage.
Conclusions: AMPA glutamate receptors express and function during the early stages of the developing rabbit retina, indicating that AMPA receptors are functional before synapse formation. The period of increasing expression pattern of AMPA subunits also coincides with the switch of the glutamatergic drive of the retinal wave and thus may contribute to the synaptic maturation in the retinal circuits.