Postnatal Changes of HRP-Labeled Corticospinal Neurons in Congenital Hydrocephalic Rats (HTX)

Summary

In the present study we examined the influence of hydrocephalic changes on the growth of corticospinal neurons and the formation of their neuronal connection with the spinal cord in the congenital hydrocephalic rat (HTX) during the postnatal period, using the horseradish peroxidase (HRP)-labeling method.

We injected 0.1–2.0 μl of 20%–50% HRP solution into the cervical cords of HTX rats which had either high or slight dilatation of the lateral ventricles, at postnatal days 1, 7, 14, and 21, and observed the HRP-labeled corticospinal neurons in the cerebral cortex and the structure of the brain stem each day.

At postnatal day 1, HRP-labeled neurons were found in layer V, which was in the dorsal and lateral areas of the cerebral cortex in both types of hydrocephalic rats. From this finding these neurons could be identified as corticospinal neurons and their axons seemed to reach into the spinal cord. By postnatal day 21, HRP-labeled corticospinal neurons were found in the area from the dorsal to the lateral part of the cerebral cortex in the hydrocephalic rats with slight dilatation of the lateral ventricles. However, in the hydrocephalic rats with high dilatation of the lateral ventricles, HRP-labeled corticospinal neurons gradually decreased and a few neurons were found in the lateral part of the cerebral cortex. At postnatal day 21, also, degenerative changes appeared in the corticospinal tract through the pyramis of the medulla oblongata.

These findings indicate that the neuronal connection between the cerebral cortex and the spinal cord, made by the corticospinal axons, gradually disappears when cortical neurons, including corticospinal neurons are destroyed, such destruction being caused by the progresive dilatation of the lateral ventricles and subsequently by the secondary degeneration of the corticospinal axons.

Kohn et al. (1981) reported that, in the congenital hydrocephalic rat (HTX), dilatation of the lateral ventricles appeared at birth and that the thickness of the cerebral cortex gradually decreased, depending on the advancement of lateral ventricular dilatation. These changes seem to influence the differentiation and growth of neurons and neuroglial cells in the cerebral cortex.

Some of the descending projection neurons in the cerebral cortex are corticospinal neurons, which connect the cerebral cortex and the spinal cord. In rats the corticospinal axons are already distributed in the spinal cord at postnatal day 1 (Joosten et al. 1987; Ohtani and Shirai 1988). Moreover, the synaptic formation of the corticospinal neurons advances on their axon terminals in the spinal cord and on their somas through postnatal day 14 (Ohtani et al. 1987; Miyabayashi and Shirai 1988).

In the present study the postnatal development of the corticospinal neurons in the congenital hydrocephalic rat (HTX) was investigated, using the HRP-labeling method, in order to define the effect of hydrocephalic changes on the formation of neuronal connections by cerebral cortical neurons.