Influence of triiodothyronine ( l-T3) on the morphological and biochemical development of fetal brain acetylcholinesterase-positive neurons cultured in a chemically defined medium

Abstract

In cerebral hemisphere cultures initiated from 15-day-old rat embryos, the number of acetylcholinesterase-positive (AChE+) cells increased from 6.8 ± 1.6 cells/well on day 3 to 112 ± 16 cells/well on day 15. With time in culture, AChE+ cells increased both in size of the perikarya and neurite length. The addition of l-triiodothyronine ( l-T3) at a concentration of 3 × 10 −8 M at the initiation of the culture had no effect on the number of AChE+ cells but significantly increased the size and neurite length of AChE+ neurons after 5 days in vitro. These morphological effects are associated with biochemical effects. l-T3 increased AChE activity in both a dose- and time-dependent manner (the stimulatory effect of l-T3 becomes significant between day 8 and day 15). Since a major part of AChE+ cells may be cholinergic neurons, we have also measured the effect of l-T3 on ChAT activity. l-T3 also increased ChAT activity in a dose and time dependent manner. Furthermore, treatment of cultures with l-T3 at different times in culture demonstrated the presence of a critical period which occurs in vitro around day 20, since the stimulatory effect of l-T3 on ChAT activity is lost after 20 days in vitro. Studies of the time necessary for l-T3 to increase both ChAT and AChE activities show that 2 days and 15 days, respectively, are required for l-T3 to significantly stimulate both enzyme activities. This in vitro analysis demonstrated the morphological effect of l-T3 on the size and the neurite length of AChE+ cells. These effects are associated with biochemical effects on ChAT and AChE activities. Thus, it appears that thyroid hormones regulate several steps of neuronal maturation.