Biochemical, immunocytochemical and morphological evidence for an interaction between thyroid hormone and nerve growth factor in the developing cerebellum of normal and hypothyroid rats

The effects of treatment with L-thyroxine (T4), 2.5 S nerve growth factor (NGF), monoclonal anti-NGF and monoclonal anti-NGF receptor antibodies, separately or together, on the two main processes of cerebellar histogenesis, the disappearance of the external granular layer (egl) and Purkinje cell (PC) morphogenesis, were studied in 10-day-old (1 day after the last injection) and 15-day-old normal and hypothyroid rats. The results provide the following information. (1) Anti-NGF antibodies given to normal rats alter more markedly the growth of PC soma and dendrite than the developmental pattern of egl. In contrast, anti-NGF receptor antibodies mainly delay the disappearance of egl, with minor changes in PC morphogenesis. This is the first evidence for a physiological role of NGF in neuronal maturation in both pre- and postmigratory phases. (2) The delays in the disappearance of egl and hypotrophy of PC due to hypothyroidism are greater than those induced in normal rats by anti-NGF antibodies, and T4 therapy in hypothyroid rats is more effective than that with NGF. The effects of combined T4/NGF treatment on PC size (including soma and dendrite) were approximately the sum of individual effects, with no apparent positive cooperation. Moreover, the effects of NGF treatment, but not those of T4, disappear over the long term. (3) Thyroid deficiency strongly reduces NGF receptor immunoreactivity. Anti-NGF antibodies given to thyroid-deficient rats partly counteract T4 therapy on the cerebellar growth and cortex layering, whereas they potentiate the action of the T4 on the growth of PC nuclei. The PC somas of thyroid-deficient rats assume a normal shape only after T4/NGF treatment. The perisomatic processes of immature PC in thyroid-deficient rats disappear after T4 therapy whereas they grow after NGF treatment. These results strongly suggest that NGF is complementary to thyroid hormone, and that the T4 action is partly mediated and regulated by NGF. Finally, thyroid hormone appears to have a long-term permissive role, while NGF may be a local and short-term limiting neurotrophic factor. Such a balance is essential for ensuring a normal time course of cerebellar histogenesis.