Ontogeny of Thyroid Hormone-Processing Systems in Rat Brain

Abstract

Recent morphologic, biochemical and functional evidence supports a direct role for thyroid hormones in adult brain (1–4). Nevertheless, the concept that adult brain is unresponsive to thyroid hormones continues, at least in some quarters, to prevail. By contrast, a role for the hormone during brain development has, for some time, been assumed, even though observations suggesting a cause and effect relationship between triiodothyronine (T3) in the developing brain and a T3-dependent response have been presented only recently. Evidence that T3 nuclear receptors are homologous to the products of the C-erb A protooncogene family provides a compelling rationale for involvement of these receptors in early events associated with blast cell replication and specification. This rationale is now coupled with evidence that a high degree of T3 nuclear receptor occupancy coincides in time with the period of active neurogenesis in the fetal lamb (5). However, there is as yet no demonstrated link between T3 nuclear receptor complex formation and the somewhat later effects of the hormone on growth of nerve cell processes, synaptogenesis and myelin formation, and, as yet, no evidence for participation of the T3 nuclear receptor in adult brain activities.