Retrograde and anterograde-transneuronal degeneration in the parabigeminal nucleus following tectal lesions in developing rats.

Degenerative effects in the parabigeminal nucleus were studied in adult rats that had received lesions of the superior colliculus at different postnatal ages. Unilateral lesions lead to complete degeneration of the dorsal and ventral divisions of the parabigeminal nucleus, which connect reciprocally with the damaged tectum, and produce effects on the middle division which receives afferents from the ipsilateral and projects to the contralateral tectum. On the side contralateral to the lesion there was a reduction in the cross-sectional area of the division as a consequence of retrograde degeneration, except in rats operated on at birth or when adults; ipsilateral to the lesion there was a reduction in the cross-sectional area of the division as a result of anterograde-transneuronal degeneration, except in animals operated on when adults. The reduction in cross-sectional area is partly due to a decrease in the number of neurones in the middle division. The maximal effects in both cases occurred in animals operated on at 10 days after birth, and the sensitivity to retrograde degeneration follows a U-shaped curve. Bilateral tectal lesions performed at birth or 5 days after birth produce virtually complete degeneration of the whole parabigeminal nuclei. Anterograde radioautographic tracing showed that all parabigeminal divisions received topographically organized projections from the ipsilateral superior colliculus. Ipsilateral projections from the dorsal and ventral divisions and crossed projections from the middle division were identified in the tectum of newborn rats with fluorescent retrograde tracers. Possible explanations for the cases in which tectal lesions failed to produce an effect on the architecture of the middle division include increased resistance to axonal damage and regulative processes over natural neuronal death. The results also indicate that the two forms of degeneration are additive with regard to the whole population of developing parabigeminal neurones.