A new research paper published in the journal JNeurosci studied how chronic stress affects brains differently. In studying the stress and reward centers of the brain, researchers identified their impact on mood. Prakash et al. found that the number of serotonin neurons in the area of the brain responsible for regulating stress and reward - the ventral part of their dorsal raphe nucleus, altered the ability of rats to experience feelings of pleasure. Anhedonia is a core symptom of low mood, which is the inability to feel enjoyment when experiencing normally pleasurable activities.
Prakash et al. worked with rats that were susceptible to anhedonia and trained them to trigger reward circuits in their brains via an electrode. He looked at how they responded when given access to self-stimulation after experiencing stress, and compared their response to rats that were resilient to stress that did not suffer from anhedonia.
The rats were exposed to social stress and then given access to self-stimulation 15 minutes afterward. The researchers found that stress significantly increased the amount and intensity of self-stimulation required to feel pleasure in the anhedonia prone rats. While stress had little effect on the amount of self-stimulation required to feel pleasure for the resilient rats.
When looking at their brains the researchers found that the resilient rats had fewer serotonin receptors. This was due to the anhedonia rats recruiting non-serotonin receptors. Prakash then activated neurons in the amygdala and the anhedonia prone rats became more resilient to social stress.
This research provides important information on the underlying mechanisms whereby stress can cause a depressed mood in some individuals, but not others.
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