Dendritic complexity in the rat secondary motor cortex is not affected by chronic variable stress

Hendrix College, Conway, Arkansas, 72032; Arizona State University, Tempe, Arizona, 85287

Chronic stress is a well-established risk factor for developing a substance use disorder. Some research suggests that stress may facilitate impaired behavioral control, resulting in an increased vulnerability to addiction. However, the specific mechanisms through which these two phenomena interact are not fully understood. Recent research supports the hypothesis that addiction is the result of a transition from voluntary substance use to compulsive, uncontrolled use. This is also often described as an increasing bias towards habitual action strategies over flexible, goal-directed ones. Because of this, several brain regions thought to be involved in either of these decisionmaking processes have been analyzed for morphological changes in dendritic complexity (a measure of dendritic branching) resulting from chronic stress exposure. The secondary motor cortex (M2) is implicated in flexible, goal-directed behavior with extensive connections to other regions involved in addiction (e.g., the striatum, ventral tegmental area, basolateral amygdala, and orbitofrontal cortex) but has not yet been investigated in the context of stress. Therefore, we examined the effects of chronic variable stress (CVS) on dendritic complexity in the rat M2 with the prediction that rats exposed to CVS would exhibit decreased dendritic complexity in this region compared to those in the control group. Pyramidal neurons in Layer 3 of the M2 were identified and manually reconstructed, and the dendritic complexity of each neuron was quantified using Sholl ring analysis. We found no significant difference in dendritic complexity between neurons from CVS and control rats, suggesting that cortical regions associated with goal-directed action strategies may not be as susceptible to chronic stress-induced changes in structural plasticity as observed in other regions.

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