Molecular mechanisms in low-serotonin induced mood and anxiety disorders.

The amygdala has been shown to be involved in both epilepsy and emotional disturbances such as in mood and anxiety disorders. It is suggested that anxiety and mood disorders may be the result of sub-seizure hyperexcitability in limbic areas such as the amygdala. Epilepsy-like mechanisms involve increased glutamatergic activity, whereas low serotonin (5-hydroxytryptamine, 5-HT) is associated with abnormal emotion. Although much evidence suggests low 5-HT increases excitability, the molecular mechanisms underlying this process are not known. Here we explored the ability of low serotonin to increase glutamate receptor (GluR) expression resulting in increased anxiety-like behavior. Using qRT-PCR, we found that individually-housed rats treated with p-chlorophenylalanine (300 mg/kg i.p.), an inhibitor of tyrosine hydroxylase, resulted in 21.8 fold higher GluR1 mRNA expression in amygdala neurons. Similar results were found in rats treated with bilateral infusions of 5’7-dihydroxytryptamine (DHT, 8 μg/side) into the lateral nucleus of the amygdala (LA) resulting in a 10³ fold increase in GluR1 mRNA. Further, Western blot analysis confirmed an overall 50.0% increase in GluR1 protein expression without any significant increase in other GluR subunits. These results suggested that low serotonin induces hyperexcitability of LA neurons by increasing GluR1 mRNA, and led to increased expression of GluR1. Additionally, we showed that these molecular changes resulted in behavioral differences in the open field maze, but not the plus maze. Rats treated with 5,7-DHT had a higher degree of center avoidance. The signaling pathway involved was also of interest. We found that the increase in GluRs may be mediated by CaMKII as shown by a ~60% increase in CaMKII phosphorylation. In order to further investigate, RNAi delivered by an AAV vector was used to knock down CaMKII expression and the effects on GluR expression was assessed. We showed that knockdown of CaMKII resulted in up to 37% decrease in GluR1 mRNA. CaMKII knockdown was also able to reverse the anxiogenic effects of decreased 5-HT, increasing center entries up to 30% and center duration up to 40% compared to controls. These results suggested that low serotonin induced hyperexcitability in LA neurons by increasing GluR1 possibly through a CaMKII mediated pathway, and led to increased open-field anxiety. This mechanism could be important in the pathophysiology of mood and anxiety disorders.