After learning about PTSD last week, I found myself thinking about how people with PTSD react to stressful and traumatic situations if they experience these events again, or just how they deal with stress if they are already in a depression. So I found this article that by Izquierdo, Wellman, & Holmes (2006) on the effects of stress on dendritic retraction in mice after they have gone fear conditioning and fear extinction training. The fear conditioning/extinction is supposed to mimic the traumatic event/therapy. Previous research has shown that the medial prefrontal cortex (mPFC) is implicated in fear conditioning, and that it is activated in both humans and rats during fear extinction. Previous research has also shown that lesions to the mPFC cause a deficit in extinction learning, and that stimulation of the mPFC mimics the effects of extinction learning through neural outputs to the amygdala. As such, the researchers wer looked to see if stress-induced changes in mPFC function would impair fear extinction. In relation to PTSD, patients have been found to have smaller mPFC volumes as well as a mPFC that is less activated during fear extinction than normal controls.
First they performed an uncontrollable stress paradigm, forced swim test for 10 minutes either 3 consecutive days, 1 day, or no days (control). Then, 24 hours after the force swim, they were subjected to the fear conditioning paradigm, using footshock. Then, 24 hours later, mice were subjected to fear extinction, in which they are placed in a novel context compared to the conditioning paradigm. The extinction process was 40 footshocks (the conditioned stimulus). Another 24 hours later they were tested for context recall, and then killed for brain retrieval.
They found that there was no effect on the fear conditioning no matter what type of stress they had undergone (0,1, or 3 days). However they found that freezing behavior during extinction was significantly effected by stress. It also took mice who had 3 days of stress significantly longer to reach criterion of extinction conditioning then the other 2 conditions. They also found in the infralimbic cortex that stress significantly decreased the length in the apical branches of the pyramidal neurons in that area.
this is a golgi staining, and the thick vertical dendrite that extends vertically upwards is the apical branch (thought I’d give you a visual). This study showed that exposure to uncontrollable stress before fear conditioning impaired fear extinction in mice, meaning that stress produces a selective deficit in fear extinction instead of effecting fear conditioning (aka impairing learning and memory). They further suggested that:
the finding that even brief exposure to a sufficiently stressful event can cause resistance to fear extinction provides a novel model of stress-induced psychopathology in neuropsychiatric conditions characterized by impaired extinction, such as PTSD.