Plasticity in the ipsilateral visuotectal projection persists after lesions of one nucleus isthmi in Xenopus

Visual input has a profound effect on the development of binocular maps in the tectum of the frog Xenopus laevis. Input from the ipsilateral eye, which is relayed to the tectum via the opposite nucleus isthmi, is normally in register with the retinotectal map from the contralateral eye. However, if one eye is rotated during larval stages while the other eye is left in normal orientation, then the resulting mismatched visual input induces the crossed isthmotectal axons to change their trajectories and to establish a reoriented ipsilateral visuotectal map in register with the contralateral retinotectal map. The major cue which aligns the two maps is the correlation of visually-evoked activity from the two eyes. This experiment was designed to determine whether the uncrossed isthmotectal projection is necessary to organize the map transmitted by the crossed isthmotectal axons. Each NI receives a topographic map from the tectum on the same side of the brain and therefore carries the same topographic information as the retinotectal projection, and each NI transmits that map not only to the opposite tectum but also back to the same tectum from which it received its input. Thus, the uncrossed isthmotectal axons provide each tectum with a map which is essentially topographically identical to the retinotectal map but which is slightly delayed temporally. The uncrossed isthmotectal axons therefore could provide topographic cues to the guide the alignment of the crossed isthmotectal axons as they establish the ipsilateral visuotectal map. In order to determine whether the uncrossed isthmotectal projection is an important source of topographic cues for the crossed isthmotectal axons, the right nucleus isthmi was ablated and one eye was rotated by 90°-150° in midlarval tadpoles. At 23-63 weeks post-metamorphosis, the binocular projections to the right tectum were mapped electrophysiologically. In 7 of the 8 frogs with lesions of at least 44% of the nucleus isthmi, the ipsilateral map was aligned with the contralateral map. Thus, the crossed isthmotectal fibers, which relay the ipsilateral visuotectal projection, do not require the presence of the uncrossed isthmotectal axons in order to form a map in alignment with the contralateral retinotectal projection.