Cerebellar astrocytes treated by thyroid hormone modulate neuronal proliferation

Thyroid hormones are important for neurogenesis and gliogenesis during brai n development. We have previously demonstrated that triiodothyronine (T-3) treatment induced proliferation in primary culture astrocytes derived from the cerebellum of neonatal rats. Conditioned medium obtained from those T-3 -treated astrocytes (T3CM) mimicked the effect of hormonal treatment on the se cells. Because neuron-glia interaction plays an important role in brain development, we tested the ability of such T-3-glial CM to influence neuron al physiology. With that aim, neurons from 19-day embryonic cerebella were cultivated for 24 h in the presence of CM obtained from T-3-treated cerebel lar astrocytes. Interestingly, the cerebellar neuronal population increased by 60-80% in T3CM. Addition of 5 mM forskolin enhanced the responsiveness of cerebellar neurons to astrocytes T3CM, but it did not interfere with neu ronal survival in control medium. Conversely, inhibition of adenylate cycla se by its specific inhibitor, SQ22536, reversed the T3CM effect on neurons. These data strongly suggest that cAMP signal transduction pathways might b e implicated in such an event. Analysis of bromodeoxyuridil incorporation r evealed that the increase in neuron number in T3CM was partially due to neu ron proliferation, because the proliferation index was three times higher i n T3CM than in control medium. Neutralizing antibody assays demonstrated th at T3CM effects on neurons are due, at least in part, to the presence of tu mor necrosis factor-beta and epidermal growth factor. Thus, we report here a novel molecular mechanism of action of thyroid hormone on cerebellar neur onal cells: Thyroid hormone induces astrocytes to secrete growth factors th at can interfere with neuronal proliferation via a paracrine pathway. (C) 1 999 Wiley-Liss, Inc.