THYROID-HORMONE AFFECTS SCHWANN-CELL AND OLIGODENDROCYTE GENE-EXPRESSION AT THE GLIAL TRANSITION ZONE OF THE VIIITH NERVE PRIOR TO COCHLEA FUNCTION

All cranial nerves, as well as the VIIIth nerve which invades the coch lea, have a proximal end in which myelin is formed by Schwann cells an d a distal end which is surrounded by oligodendrocytes. The question w hich arises in this context is whether peripheral and central parts of these nerves myelinate simultaneously or subsequently and whether the myelination of either of the parts occurs simultaneously at the onset of the cochlea function and under the control of neuronal activity. I n the present paper, we examined the relative time course of the myeli nogenesis of the distal part of the VIIIth nerve by analyzing the expr ession of peripheral protein P0, proteolipid protein and myelin basic protein. To our surprise, we observed that the expression of myelin ma rkers in the peripheral and central part of the intradural part of the VIIIth nerve started simultaneously, from postnatal day 2 onwards, lo ng before the onset of cochlea function, The expression rapidly achiev ed saturation levels on the approach to postnatal day 12, the day on w hich the cochlea function commenced, Because of its importance for the neuronal and morphological maturation of the cochlea during this time , an additional role of thyroid hormone in cochlear myelinogenesis was considered. Indeed, it transpires that this hormone ensures the rapid accomplishment of glial gene expression, not only in the central but also in the peripheral part of the cochlea, Furthermore, an analysis o f the thyroid hormone receptors, TR alpha and TR beta, indicates that TR beta is necessary for myelinogenesis of the VIIIth nerve. Rapid thy roid hormone-dependent saturation of myelin marker gene expression in Schwann cells and oligodendrocytes of the VIIIth nerve may guarantee n erve conduction and synchronized impulse transmission at the onset of hearing. The thyroid hormone-dependent commencement of nerve conductio n is discussed in connection with the patterning refinement of central auditory pathways and the acquisition of deafness.