Mechanisms of stress resilience

71 stress - CopyLast year a new peer-reviewed journal called Neurobiology of Stress made its debut. The journal is published by Elsevier, who, in an uncharacteristic move, has provided Open Access for its first three issues. So hurry up and download the papers.

The very first issue is centered around the idea of resilience. That is, exposed to the same stressors, some people are more likely to develop stress-induced diseases, whereas others seem to be immune to the serious effects of stress.

Much research has been carried out to uncover the effects of chronic stress or of an exposure to a single severe stressor, which vary from cardiovascular disorders, obesity, irritable bowel syndrome, immune system dysfunctions to posttraumatic stress disorder, generalized anxiety, specific phobias, or depression. By comparison, there is little, but significant data on resilience: the ability to NOT develop those nasty stress-induced disorders. Without doubt, one reason for this scarcity is the difficulty in finding such rare subjects in our extremely stressful society. Therefore most of the papers in this issue focus on animal models.

Nevertheless, there is enough data on resilience to lead to no less that twenty reviews on the subject. It was difficult to choose one as most are very interesting, tackling various aspects of resilience, from sex differences to prenatal exposure to stress, from epigenetic to neurochemical modifications, from social inequalities to neurogenesis and so on.

So I chose for today a more general review of Pfau & Russo (2015), entitled ” Peripheral and central mechanisms of stress resilience”. After it introduces the reader to four animal models of resilience, the paper looks at the neruoendocrine responses to stress and identifies some possible chemical mediators of resilience (like certain hormones), then at the immune responses to stress (bad, bad cytokines), and finally at the brain responses to stress (surprisingly, not focusing on amygdala, hypothalamus or hippocampus, but on the dopamine system originating from ventral tegmental area).

I catalogue the review as a medium difficulty read because it requires a certain amount of knowledge of the stress field beforehand. But do check out the other ones in the issue, too!

Reference: Pfau ML & Russo SJ (1 Jan 2015). Peripheral and central mechanisms of stress resilience. Neurobiology of Stress, 1:66-79. Article | FREE PDF

By Neuronicus, 24 January 2016


Dopamine role still not settled

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No idea why the prefrontal cortex neuron is Australian, but here you go. Cartoon made by me with free (to the best of my knowledge) clipart elements. Feel free to use to your heart’s content.

There have been literally thousands of pages published about the dopamine function(s). Dopamine, which made its stage debut as the “pleasure molecule”, is a chemical produced by some neurons in your brain that is vital to its functioning. It has been involved in virtually all types of behavior and most diseases, from pain to pleasure, from mating to addiction, from working-memory to decision-making, from autism to Parkinson’s, from depression to schizophrenia.

Here is another account about what dopamine really does in the brain. Schwartenbeck et al. (2015) trained 26 young adults to play a game in which they had to decide whether to accept an initial offer of small change or to wait for a more substantial offer. If they waited too long, they would lose everything. After that, the subjects played the game in the fMRI. The authors argue that their clever game allows segregation between previously known roles of dopamine, like salience or reward prediction.

As expected with most fMRI studies, a brain salad lit up (that is, your task activated many other structures in addition to your region of interest), which the authors address only very briefly. Instead, they focus on the timing of activation of their near and dear midbrain dopamine neurons, which they cannot detect directly in the scanner because their cluster is too small, so they infer their location by proxy. Anyway, after some glorious mental (and mathematical) gymnastics Schwartenbeck et al. (2015) conclude that

1) “humans perform hierarchical probabilistic Bayesian inference” (p. 3434) (i.e. “I don’t have a clue what’s going on here, so I’ll go with my gut instinct on this one”) and

2) dopamine discharges reflect the confidence in those inferences (i.e. “how sure am I that doing this is going to bring me goodies?”)

With the obvious caveat that the MRI doesn’t have the resolution to isolate the midbrain dopamine clusters and that these clusters refer to two very distinct population of dopamine neurons (ventral tegmental area and substantia nigra) with different physiological, topographical, and anatomical properties, and distinct connections, the study adds to the body of knowledge of “for the love of Berridge and Schultz, what the hell are you DOIN’, dopamine neuron?”.

Reference: Schwartenbeck, P., FitzGerald, T. H., Mathys, C., Dolan, R., & Friston K. (October 2015, Epub 23 July 2014). The Dopaminergic Midbrain Encodes the Expected Certainty about Desired Outcomes. Cerebral Cortex, 25:3434–3445, doi:10.1093/cercor/bhu159. Article + FREE PDF

By Neuronicus, 8 October 2015