RESUMO
The topographic positioning of retinal axons in the optic tectum is regulated, at least in part, by ephrinA/EphA repulsive interactions. Temporal axons, expressing high levels of EphA receptors, project to the ephrinA5-poor anterior tectum and avoid the ephrinA5-rich posterior tectum. To examine the dynamic behavior of temporal growth cones when they first encounter ephrinA, we manipulated ephrinA-coated beads with a laser tweezer into desired positions around the growth cones of chick retinal axons in culture. At high concentrations of ephrinA5 on the beads, growth cones typically collapsed on contacting the bead. At low concentrations, however, growth cones showed heterogeneous responses with some growth cones showing repulsive turning and others showing attractive turning after contacting the bead. Experiments with two beads indicate that retinal axons integrate guidance information that is provided simultaneously at two discrete locations. When a time-delay was introduced between exposure to the first and the second bead, individual axons exhibited a stereotyped response to the repeated stimuli, either responding with attraction followed by attraction, or showing repulsion followed by repulsion or collapse. Our results suggest the existence of at least two retinal subpopulations from the temporal retina, one being attracted, another being repelled by low levels of ephrinA5. These findings demonstrate that temporal retinal axons are not universally repelled by ephrinA5 and suggest that their ability to respond differentially to low concentrations may help them to map in a continuous manner over the surface of the anterior tectum.