Frog tectal efferent axons fail to regenerate within the CNS but grow within peripheral nerve implants

Abstract

Tectal efferent axons, located adjacent to the optic tract, fail to regenerate past diencephalic lesions in Rana pipiens even though optic axons regenerate after the same injury (M. J. Lyon and D. J. Stelzner, J. Comp. Neurol. 255: 511–525). We tested the possibility that tectal efferent axons can regenerate within peripheral nerve implants. A 6- to 8-mm segment of autologous sciatic nerve was implanted into the anterolateral ( N = 23) or centrolateral ( N = 22) portion of the dorsal surface of the tectum. Frogs survived for 6 ( N = 16) or 12 weeks ( N = 29) before the free end of the nerve was recut and HRP applied. A control group had the nerve crushed prior to the HRP application. Neurons within the tectum, near and medial to the implant site, were retrogradely labeled from the nerve graft in most experimental operates but no neurons were labeled in controls. In addition, neurons were also labeled in nuclei which projected to the tectum in a number of cases. Three times as many neurons were labeled in 12-week operates (42 ± 46) as in 6-week operates (15 ± 12). The morphology and location of labeled neurons in the tectum was similar to tectal efferent neurons except that the somal area of neurons labeled from the graft was significantly larger (41%) than normal tectal efferent neurons. The basic finding is similar to experiments using the same paradigm in the mammalian central nervous system (CNS). One difference is the minimal glial reaction at the graft insertion site. Thus, amphibian tectal efferent neurons are similar to mammalian CNS neurons in their response to axonal injury.