The effects of tectal lesion on the survival of isthmic neurones in Xenopus

Combined effects of deafferentation and de-efferentation on isthmo-optic neurons during the period of their naturally occurring cell death

We have studied the effects on the chick embryo's isthmo-optic nucleus of de-efferentation alone or in combination with deafferentation. De-efferentation was achieved by pharmacological destruction of the axonal target cells in the retina at E13, or by colchicine-blockade of axoplasmic transport in the intraocular parts of the isthmo-optic axons at E13; deafferentation was by a tectal lesion at E11 or E12. De-efferentation alone causes all the isthmo-optic neurons to die, and mostly by the "endocytic-autophagic" mode of cell death, which is characterized by pronounced endocytosis (of an intravascularly injected label) and by intense, clumped activity of two lysosomal enzymes (acid phosphatase and N-acetyl-beta-glucosaminidase). Deafferentation plus de-efferentation caused there to be less endocytic-autophagic dying cells in the isthmo-optic nucleus than after de-efferentation alone, but all the neurons still died. Our interpretation is that deafferentation switched many of the isthmo-optic neurons to a completely different (nonendocytic, nonautophagic) mode of cell death.

Lack of axon regeneration of isthmic neurons in juvenile Xenopus

In young adult Xenopus laevis frogs the axons of isthmic neurons projecting to the contralateral tectum were severed at the postoptic commissure and the survival of such neurons was studied between 2 and 26 weeks after the operation. Pyknotic neurons were first observed in the isthmic nucleus 2 weeks after axotomy. The continued neuron loss resulted in the decrease from 2920 in intact animals to 1520 surviving isthmic neurons in animals 26 weeks after the operation. These findings suggest that (i) severed axons of the isthmic neurons are not able to regenerate to their natural target, and (ii) isthmic neurons require continuous contact with appropriate tectal target for their survival.

The ultrastructural organization of the isthmic nucleus in Xenopus

The isthmic nucleus (IN) of the frog brain forms a linkage, relaying visual information from one tectum to the other. It receives afferent input from the tectum of the same side and projects bilaterally to both tecta. The ultrastructural features of the tecto-isthmic synaptic connections were studied in young postmetamorphic Xenopus frogs. Most synaptic profiles in the isthmic nucleus have spheroidal vesicles and an asymmetric zone of apposition. Frequently, synaptic glomeruli consisting of up to 8 terminal boutons surrounding a shaft dendrite were observed. The synaptic density in the rostral IN was slightly higher than in the middle or caudal portions. Partial deafferentation by transection of the tecto-isthmic pathway or total deafferentation by removal of the tectum was followed by a widespread degeneration of terminals in the ipsilateral IN. In the former case, the density of synapses in the IN decreased initially by about 64%, and then increased by 30 days after operation to about 50% of the normal synaptic density. After tectal removal, all the terminal boutons in the isthmic neuropil degenerated by 3 days after operation. These studies, along with recent findings, indicate that most, if not all, of the afferent fibres to IN are of tectal origin.