Regulation of early spontaneous network activity and GABAergic neurons development by thyroid hormone

Early in development spontaneous activity modulates survival and connectivity of neurons and thus plays a crucial role in the formation of neural networks. The emergence of synchronous activity in cultured neocortical networks initially is driven by large GABAergic interneurons. Here we studied the impact of thyroid hormone on early network development and especially on the development of large GABAergic neurons. Triiodothyronine enhances the frequency of early spontaneous synchronous network activity and an overall increase in network connectivity is indicated by the increased density of glutamatergic and GABAergic synapses. The hormone-induced increase of activity parallels cell type–specific changes in neuronal soma size and cell density, with strong effects on somatic and axonal growth of large GABAergic interneurons. Interestingly, large GABAergic neuron growth is both activity- and hormone-regulated. Blocking neuronal activity by tetrodotoxin or the glutamate receptor blockers D-2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitroquinoxaline-2,3-dione disodium reveals a direct contribution of triiodothyronine to somatic growth, which also precedes the formation of synchronous network activity. The hormone-mediated effects on spontaneous activity and on large GABAergic neurons growth can be blocked by the nuclear thyroid hormone receptors antagonist 1-850. Thus, our data suggest that triiodothyronine actions result in functional maturation of early cortical networks and cell type–specific structural alterations. The increase in spontaneous activity might initially follow the growth of the large GABAergic neurons, which show an exquisite sensitivity to the presence of thyroid hormones. For the most part, however, the hormone-mediated growth of the GABAergic neurons relies strongly on the maturation of glutamatergic synaptic activity.