Infralimbic cortical glutamate output is necessary for the neural and behavioral consequences of chronic stress
Authors: Pace Sebastian A. , Christensen Connor , Schackmuth Morgan K. , Wallace Tyler , McKlveen Jessica M. , Beischel Will , Morano Rachel , Scheimann Jessie R. , Wilson Steven P. , Herman James P. , Myers Brent 
Published: July 2020

Exposure to prolonged stress is a major risk-factor for psychiatric disorders such as generalized anxiety and major depressive disorder (MDD). Human imaging studies have identified structural and functional abnormalities in the prefrontal cortex of MDD patients, particularly Brodmann’s area 25 (BA25). Further, deep brain stimulation of BA25 reduces symptoms of treatment-resistant depression. The rat homolog of BA25 is the infralimbic cortex (IL), which is critical for cognitive appraisal, executive function, and physiological stress reactivity. Previous studies indicate that the IL undergoes stress-induced changes in excitatory/inhibitory balance culminating in reduced activity of glutamate output neurons. However, the regulatory role of IL glutamate output in mood-related behaviors after chronic variable stress (CVS) is unknown. Here, we utilized a lentiviral-packaged small-interfering RNA to reduce translation of vesicular glutamate transporter 1 (vGluT1 siRNA), thereby constraining IL glutamate output. This viral-mediated gene transfer was used in conjunction with a quantitative anatomical analysis of cells expressing the stable immediate-early gene product ΔFosB, which accumulates in response to repeated neural activation. Through assessment of ΔFosB-expressing neurons across the frontal lobe in adult male rats, we mapped regions altered by chronic stress and determined the coordinating role of the IL in frontal cortical plasticity. Specifically, CVS-exposed rats had increased density of ΔFosB-expressing cells in the IL and decreased density in the anterior insula. The latter effect was dependent on IL glutamate output. Next, we examined the interaction of CVS and reduced IL glutamate output in behavioral assays examining coping, anxiety-like behavior, associative learning, and nociception. IL glutamate knockdown decreased immobility during the forced swim test compared to GFP controls, both in rats exposed to CVS as well as rats without previous stress exposure. Further, vGluT1 siRNA prevented CVS-induced avoidance behaviors, while also reducing risk aversion and passive coping. Ultimately, this study identifies the necessity of IL glutamatergic output for regulating frontal cortical neural activity and behavior following chronic stress. These findings also highlight how disruption of excitatory/inhibitory balance within specific frontal cortical cell populations may impact neurobehavioral adaptation and lead to stress-related disorders.

Highlights

Chronic stress increased ΔFosB in the infralimbic cortex and decreased insular ΔFosB

Decreased insular ΔFosB was dependent on infralimbic glutamate output

Knockdown of infralimbic glutamate release reduced passive coping

Avoidance behaviors after chronic stress were dependent on infralimbic glutamate

Infralimbic projections innervated excitatory and inhibitory neurons in the insula