Chronic melatonin and binocular plasticity in Xenopus frogs

The topographic binocular maps in the optic tectum of Xenopus frogs are notable both for their dramatic plasticity during development and for the high expression of melatonin receptors in the circuitry contributing to those binocular maps. The goal of this study was to determine whether melatonin contributes to the control of binocular tectal plasticity. During development, rotation of one eye leads to compensatory rewiring of ipsilateral maps. The effect of 3-4 months of chronic 20 or 200 nM melatonin on this rewiring was tested by electrophysiological mapping. No decrease in plasticity was observed. In adult Xenopus, rotation of one eye normally does not lead to rewiring of the ipsilateral projection, although adults can exhibit plasticity if they have been dark-reared or have been treated as adults with NMDA. We tested whether exposure to 20-200 nM melatonin during and after the normal critical period would similarly extend plasticity. Eye rotation in adults that had been treated with melatonin did not demonstrate retained plasticity. These results show that melatonin does not reduce the normally high plasticity characteristic of young Xenopus nor does it increase the normally low plasticity of adult Xenopus.