The development of non-retinal afferent projections to the frog optic tectum and the substance P immunoreactivity of tectal connections

Accessibility in early development and the presence of a retinotopic map have made the amphibian optic tectum a popular system for exploration of the role of synaptic function in central map refinement. Although a great deal is known about the development of retinal innervation of the tectum, little information exists about the development of non-retinal tectal inputs. Since these other afferent systems may contribute to the synaptic drive of developing tectal cells and thereby be involved in the activity-dependent refinement of the retinotectal map, we sought to determine whether these inputs are present at the early tadpole stages when the first retinal axons refine their synaptic order within the tectal neuropil. Rhodamine-labelled latex beads, retrogradely transported from injection sites in the optic tecta, were used to identify tectal afferent projections. Projection patterns in very young tadpoles were identical to those found in juvenile frogs and heavily labelled regions included areas of the posteroventral tegmental field, the posterior tuberculum, the ventromedial thalamic nucleus, the ventral part of the ventrolateral thalamic nucleus, the suprachiasmatic nucleus and discrete regions within the central and anterior thalamic nuclei. Ipsilateral nucleus isthmi cells were also labelled, indicating the existence of an isthmo-tectal projection in even the youngest animals examined. Additionally, substance P-like immunoreactive tecto-isthmal fibers were traced from the optic tectum to the nucleus isthmi. The presence of these connections suggests that feedback from the nucleus isthmi and/or input from the other brain areas projecting to the tectum may play a role in modulating the cellular mechanisms that underlie the formation of the visual map.