The obvious question is whether the increase in 5-HT might directly influence GR gene expression. This issue is remarkably difficult to address with in vivo studies, in which pharmacological manipulations targeting a specific neurotransmitter system inevitably alter other systems, as well as systems in other brain regions. This issue begs an in vitro approach in which the relevant system, the hippocampal neurons, can be examined in a cell culture system. In vitro, the treatment of primary hippocampal cell cultures with 5-HT increases GR expression and this effect is mediated by 5-HT7 receptor activation.75-77 The 5-HT7 receptor is positively coupled to cyclic adenosine monophosphate (cAMP) and GR expression in cultured hippocampal neurons is also significantly increased after treatment with 8-bromo-cAMP (a stable cAMP analog) or with various doses of the specific 5-HT7 receptor agonists, such as 5-carboxamidotryptamine (5-CT). For all conditions, the effect on GR expression is apparent only after 4 days of treatment, a seemingly obscure fact whose importance will later become evidence. The effect of 5-CT on GR expression is blocked by methiothepin. Likewise, 5-CT produces a significant increase
