Axon-target interactions maintain synaptic gene expression in retinae transplanted to intracranial regions of the rat

In the present study we examined the effects of optic axon-CNS target interactions on gene expression in the rat retina. These studies took advantage of a transplantation paradigm that allowed us to assay gene expression in retinae transplanted to different intracranial locations in the neonatal rat that either promoted (dorsal midbrain) or precluded (cerebral cortex) the formation of retino-collicular connections. Using in situ hybridization experiments, we observed that transplantation to the dorsal midbrain resulted in a relatively normal pattern of nicotinic acetylcholine receptor (nAChR) beta-3 subunit and glutamate receptor 3 (GluR3) gene expression. In contrast, retinae transplanted to the cerebral cortex (which did not result in normal retino-collicular interactions) showed a dramatic reduction in nAChR beta-3 subunit and GluR3 gene expression. These results agree with those obtained in the adult goldfish retina, where it has been demonstrated that an optic nerve-optic tectum interaction is responsible for the re-induction nAChR and NMDA receptor gene expression during optic nerve regeneration. Taken together, these results support the hypothesis that proper axon-target interactions are required for maintenance of nAChR and glutamate receptor gene expression in the mature vertebrate retina.