The in vitro hippocampal cell culture system mimics the in vivo world with surprising authenticity. The increase in GR levels in cultured hippocampal neurons following 5-HT treatment persists following 5-HT removal from the medium; for as long as the cultures can be maintained, there is a sustained increase in GR levels as long as 50 days beyond the removal of 5-HT from the medium. Thus, 5-HT can act directly on hippocampal neurons to increase GR expression, and the effect of 5-HT on GR expression is observed in hippocampal culture cells mimics the long-term effects of early environmental events. These findings provide an in vitro “programming” model. Activation of cAMP pathways can regulate gene transcription through effects on a number of transcription factors, including, of course, the cAMP-response element binding protein (CREB) via an enhanced phosphorylation of CREB. In this instance, the second-messenger system alters the activity of the transcription factor, through enzymatic modification and phosphorylation, rather than production. CREB regulates gene transcription through pathways that involve the transcriptional cofactor, CREB -binding protein (CBP). Primary hippocampal cell cultures treated with 8-bromo-cAMP, 5-CT, or 5-HT show a significant increase in CBP expression.
