Effects of permeability of midtectal barriers in goldfish on compression of the visuotectal projection rostrally and regenerative escape caudally.

Physiological mapping and anatomical methods were used to evaluate changes in the retinotectal projection of goldfish 16-200 days after insertion of permeable or impermeable barriers that bisected the tectum into rostral and caudal halves. The projection to rostral tectum was left intact. Barriers composed of Gelfilm or impermeable Nucleopore material induced within 2-3 months an orderly compression of the visual field representation in rostral tectum only slightly less complete than that observed in animals with caudal half-tectal ablation. In contrast, Nucleopore filter barriers with 0.1-micron or 8-micron holes did not cause significant compression. According to both mapping and autoradiographic tracing, reinnervation of tectum behind the barriers occurred among all groups within 1-2 months. Physiologically, the projection caudal to permeable barriers was typically complete and appropriate, whereas the caudal projection in fish with impermeable barriers eventually consisted of a greatly expanded representation of the extreme temporal visual field. Autoradiography, normal fiber impregnations, and the orthograde horseradish peroxidase method revealed that regeneration past the barriers involved the formation of large bundles passing vertically along the cut tectal margin and through the underlying valvula cerebelli or lateral tegmentum. The simultaneous rostral compression and caudal expansion in the visual representation formed when more impermeable barriers were used provides evidence that, in addition to the influence of position-dependent properties, axonal competition for target territory contributes to the control of the distribution of optic arbors. Further research is required to determine why reinnervation of tectum caudal to the more permeable barriers was more complete with respect to visual representation.