The effects of glycogen synthase kinase-3beta in serotonin neurons.

Glycogen synthase kinase-3 (GSK3) is a constitutively active protein kinase in brain. Increasing evidence has shown that GSK3 acts as a modulator in the serotonin neurotransmission system, including direct interaction with serotonin 1B (5-HT1B) receptors in a highly selective manner and prominent modulating effect on 5-HT1B receptor activity. In this study, we utilized the serotonin neuron-selective GSK3ß knockout (snGSK3ß-KO) mice to test if GSK3ß in serotonin neurons selectively modulates 5-HT1B autoreceptor activity and function. The snGSK3ß-KO mice were generated by crossbreeding GSK3ß-floxed mice and ePet1-Cre mice. These mice had normal growth and physiological characteristics, similar numbers of tryptophan hydroxylase-2 (TpH2)-expressing serotonin neurons, and the same brain serotonin content as in littermate wild type mice. However, the expression of GSK3ß in snGSK3ß-KO mice was diminished in TpH2-expressing serotonin neurons. Compared to littermate wild type mice, snGSK3ß-KO mice had a reduced response to the 5-HT1B receptor agonist anpirtoline in the regulation of serotonergic neuron firing, cAMP production, and serotonin release, whereas these animals displayed a normal response to the 5-HT1A receptor agonist 8-OH-DPAT. The effect of anpirtoline on the horizontal, center, and vertical activities in the open field test was differentially affected by GSK3ß depletion in serotonin neurons, wherein vertical activity, but not horizontal activity, was significantly altered in snGSK3ß-KO mice. In addition, there was an enhanced anti-immobility response to anpirtoline in the tail suspension test in snGSK3ß-KO mice. Therefore, results of this study demonstrated a serotonin neuron-targeting function of GSK3ß by regulating 5-HT1B autoreceptors, which impacts serotonergic neuron firing, serotonin release, and serotonin-regulated behaviors.

Behavioral stress-induced activation of FoxO3a in the cerebral cortex of mice.

BACKGROUND: The transcription factor FoxO3a is highly expressed in brain, but little is known about the response of FoxO3a to behavioral stress and its impact in the associated behavioral changes. METHODS: We tested the response of brain FoxO3a in the learned helplessness (LH) paradigm and tested signaling pathways that mediate the response of FoxO3a. RESULTS: A single session of inescapable shocks (IES) in mice reduced FoxO3a phosphorylation at the Akt-regulating serine/threonine residues and induced prolonged nuclear accumulation of FoxO3a in the cerebral cortex, both indicating activation of FoxO3a in brain. The response of FoxO3a is accompanied by a transient inactivation of Akt and a prolonged activation of glycogen synthase kinase-3beta (GSK3ß). Noticeably, FoxO3a formed a protein complex with GSK3ß in the cerebral cortex, and the interaction between the two proteins was stronger in IES-treated mice. Inhibition of glycogen synthase kinase-3 was able to abolish IES-induced LH behavior, disrupt IES-induced GSK3ß-FoxO3a interaction, and reduce nuclear FoxO3a accumulation. In vitro approaches further revealed that the interaction between GSK3ß and FoxO3a was strongest when both were active; FoxO3a was phosphorylated by recombinant GSK3ß; and glycogen synthase kinase-3 inhibitors effectively reduced FoxO3a transcriptional activity. Importantly, IES-induced LH behavior was markedly diminished in FoxO3a-deficient mice that had minimal FoxO3a expression and reduced levels of FoxO3a-inducible genes. CONCLUSIONS: FoxO3a is activated in response to IES by interacting with GSK3ß, and inhibition of GSK3ß or reducing FoxO3a expression promotes resistance to stress-induced behavioral disturbance by disrupting this signaling mechanism.