Fluoxetine prevents development of an early stress-related molecular signature in the rat infralimbic medial prefrontal cortex. Implications for depression?

Abstract

Background: Psychological stress, particularly in chronic form, can lead to mood and cognitive dysfunction and is a major risk factor in the development of depressive states. How stress affects the brain to cause psychopathologies is incompletely understood. We sought to characterise potential depression related mechanisms by analysing gene expression and molecular pathways in the infralimbic medial prefrontal cortex (ILmPFC), following a repeated psychological stress paradigm. The ILmPFC is thought to be involved in the processing of emotionally contextual information and in orchestrating the related autonomic responses, and it is one of the brain regions implicated in both stress responses and depression.

Results: Genome-wide microarray analysis of gene expression showed sub-chronic restraint stress resulted predominantly in a reduction in transcripts 24 hours after the last stress episode, with 239 genes significantly decreased, while just 24 genes had increased transcript abundance. Molecular pathway analysis using DAVID identified 8 pathways that were significantly enriched in the differentially expressed gene list, with genes belonging to the brain-derived neurotrophic factor - neurotrophin receptor tyrosine kinase 2 (BDNF-Ntrk2) pathway most enriched. Of the three intracellular signalling pathways that are downstream of Ntrk2, real-time quantitative PCR confirmed that only the PI3K-AKT-GSK3B and MAPK/ERK pathways were affected by sub-chronic stress, with the PLCγ pathway unaffected. Interestingly, chronic antidepressant treatment with the selective serotonin reuptake inhibitor, fluoxetine, prevented the stress-induced Ntrk2 and PI3K pathway changes, but it had no effect on the MAPK/ERK pathway.

Conclusions: These findings indicate that abnormal BDNF-Ntrk2 signalling may manifest at a relatively early time point, and is consistent with a molecular signature of depression developing well before depression-like behaviours occur. Targeting this pathway prophylactically, particularly in depression-susceptible individuals, may be of therapeutic benefit.

Figure 1

A-D Graphs depicting the effects of sub-chronic restraint stress (RST), fluoxetine treatment without stress (FLX), and fluoxetine treatment with stress (RST+FLX) on gene expression in the ILmPFC. Values are percentage means (±SEM) relative to unhandled controls (dashed horizontal line at 100%). Sub-chronic restraint stress reduced the transcript levels for the plasma membrane neurotrophin receptor genes ntrk2 and ntrk3 (A). Note, chronic fluoxetine treatment prevented the stress-induced ntrk2 but not ntrk3 transcript reduction. Sub-chronic restraint stress reduced transcript levels for the PI3K-AKT1-GSK3B (B) and NTRK2-MAPK/ERK (C) but not the PLCγ1 (D) intracellular signalling pathways. Interestingly, fluoxetine treatment prevented the sub-chronic stress-induced reduction in PI3K-AKT1-GSK3B signalling pathway, but not the NTRK2-MAPK/ERK (B-Raf and MAPK1 genes) pathway. Gene expression of the serine-threonine kinase, mTOR, a downstream target of the PI3K-AKT1 pathway, was reduced by sub-chronic stress, an effect not prevented by fluoxetine (D). * denotes p < 0.05.