Changes in lamination and neuronal survival in the isthmo-optic nucleus following the intraocular injection of tetrodotoxin in chick embryos.

We have studied how the development of the isthmo-optic nucleus (ION) is affected by electrical activity in the ION's axonal target territory, the contralateral retina. Electrical activity was blocked or reduced in the retina for various periods by tetrodotoxin injected intraocularly in different doses. The effects on the morphology of the retina appear to have been minor. During the ION's period of naturally occurring neuronal death (embryonic days 12 to 17), the injections substantially reduced this neuronal death and disrupted the development of lamination in the contralateral ION; there was also a lesser reduction in neuronal death in the ipsilateral ION. The dose of tetrodotoxin required to affect lamination was lower than that affecting neuronal death. Thus, the effects on neuronal death and on lamination were independent, since either could occur without the other. These effects were mediated by retrograde signals (probably two or more) from the eye; they occurred too early for the alternative anterograde route via the optic tectum (which projects to the ION) to be responsible. After embryonic day 17, the ION's response to intraocular tetrodotoxin changes abruptly from increased survival to total and rapid degeneration.

Maintenance of targeting errors by isthmo-optic axons following the intraocular injection of tetrodotoxin in chick embryos.

We have studied the role of electrical activity in the elimination of axonal targeting errors, which is a normal process in brain development. The experiments were focused on the isthmo-optic nucleus (ION), which, in adults, projects in topographical order on the contralateral retina. During embryogenesis, however, a few isthmo-optic neurons project to the ipsilateral retina, and many project to topographically inappropriate parts of the contralateral one; both kinds of targeting error are known to be eliminated by the deaths of the parent neurons. We injected tetrodotoxin (TTX) intraocularly at embryonic days 13 and 15 and, on the latter, applied a retrograde label to the retina of the same eye. Embryos were fixed at embryonic day 17. In some embryos, the label was a peripherally placed fleck of the carbocyanine dye "diI"; the resulting retrogradely labeled neurons in the contralateral ION were much more widely scattered in the TTX-injected embryos than in controls (errors in topography). In other embryos, the label was a solution of rhodamine-B-isothiocyanate (RITC) injected into the vitreous body; this yielded several ipsilaterally labeled isthmo-optic neurons in the TTX-injected embryos, but virtually none in the controls. The numbers of both kinds of aberrantly projecting neuron approached those previously reported near the beginning of the ION's period of neuronal death. We conclude that electrical activity plays an important role in the elimination of axonal targeting errors in the chick embryo's isthmo-optic system.

Reduction in the death and cytolamination of developing neurons by tetrodotoxin in the axonal target region.

Tetrodotoxin was injected into one eye of chick embryos so as to block activity in the target territory of the isthmo-optic nucleus (ION) during its period of neuronal death. This markedly reduced the neuronal death and thereby prolonged the survival of some 'aberrantly' projecting neurons which would normally all die. In addition, the cytoarchitecture of the ION developed abnormally. Since these two effects differ markedly in their dose-dependence and in other ways, they cannot both be explained by changes in the strength of a single retrograde signal.

Rapid onset of neuronal death induced by blockade of either axoplasmic transport or action potentials in afferent fibers during brain development.

We have investigated how neurons in the optic tecta of embryonic day 16 chick embryos depend for survival on their afferents from the retina. To distinguish between activity-mediated effects and other, "trophic," ones, we compared the effects on the tectal neurons of blocking intraocular axoplasmic transport (with colchicine) or action potentials (by means of TTX). Both interventions rapidly induced the appearance of dying (pyknotic) neurons in the tectum, with major increases in their number occurring within 13 hr post-colchicine and within 9 hr post-TTX. Following both drugs, the dying neurons were morphologically similar, and in both cases the cell death depended on protein synthesis. However, the effects of colchicine and of TTX could be dissociated, since the most superficial tectal neurons became pyknotic only in response to colchicine, and, with a sufficiently short survival time (9 hr), the deep cells of the stratum griseum centrale became pyknotic only in response to TTX. We hence argue that the survival of the tectal neurons depends on their ongoing maintenance by substances released from retinotectal axon terminals, the release being activity dependent in the case of the deep neurons but independent of activity in the case of the superficial ones.