It’s one of the guarantees of life: stress. At its core, it’s a perception of a physical or psychological threat and is designed to help us survive. But the triggers are varied and as such, there is no single way to deal with the impending sensation of harm.
For years, researchers have studied the stress spectrum and identified numerous behavioural changes. Most are relatively simple to understand such as heightened awareness, risk avoidance, and the fight or flight response. Yet some actions are self-directed and may appear to be independent of normal coping strategies. These latter actions may be indicative of pathologies requiring medical attention.
Identifying behaviours may be easy through simple observation yet this only provides a visual confirmation of symptoms. What is missing is an understanding of the mechanism behind the development of irregular responses. Although the brain is without a doubt the source, finding the right location as well as the responsible cell types has remained a mystery. But there now may be some answers thanks to the laboratory of Jaideep Bains at the University of Calgary.
The team recently published an article in Nature Communications (1) in which they examined complex behaviours in mice after a stressful event. The group combined visual observations of behaviour with manipulations to either activate or silence one group of cells in an area of the brain known as the paraventricular nucleus of the hypothalamus, PVN for short. Based on the results, they uncovered brain circuit which may be responsible for controlling behaviours after stress and, surprisingly, may be involved in altered behaviours associated with autism spectrum disorder.
To initiate stress in the mice, the group used a well-known method known as the footshock test. Although the procedure is best known for its ability to help cement a particular type of behavioural response (2) akin to Pavlovian conditioning, it also elicits an immediate stress response. This was the perfect way to assess the mice.
Before the stress test, the mice were observed in their normal habitat and the researchers identified a palette of eight distinct behaviours that the mice displayed in a random fashion. After the stress event was concluded, the team expected to see changes in these actions. If they were correct, there would be a shift in the frequency of the behaviours.
When the mice were returned to their home cages, the group noted immediate changes in the way the animals behaved. All the individual actions observed prior to the stress were still evident, but now they appeared ordered with the mice using a very specific sequence in which they progressed from highly vigilant state in which they walked and explored their environment to a state in which they exhibited long bouts of grooming. The authors hypothesized that this switch from behaviour that is focused on the outside world to one that is self-referential or inwardly focused is an important part of recovering from a stressful event.
Next, the authors focused on the brain circuit that may be involved in controlling this shift from behaviour that is outward focused to one that is inward focused. They turned their attention to cells in the PVN that manufacture a well-known stress chemical signal called corticotropin-releasing hormone. or CRH (3).
To determine the role of CRH neurons in the stress response, the team used an optogenetic method (4) in order to either turn on or turn off CRH neurons. Immediately after the mice were stressed, the brains were exposed to either yellow (off) or blue (on) light and behaviour was monitored. If the team was right, the yellow light would interfere with the shift from exploratory our outward focused behaviours to inward focused grooming behaviour while blue light would enhance the inward focused behaviour.
When the tests came back, the results revealed that yellow light did reduce grooming, but increased rearing and walking. This confirmed one part of the hypothesis that these CRH neurons were necessary for the mice to shift from behaviour that was focused on the outside world to behaviour that was focused exclusively on the mice themselves.
Next, using blue light, the authors could increase the amount of time spent on inward focused grooming behaviour which was accompanied by a decrease in exploratory behaviour. The information suggested the CRH neurons were sufficient to drive the switch from external-focused to internal-focused behaviour.
The only question left to answer was the strength of influence of CRH neurons on overall behaviour. To do this, they exposed the mice brains to blue light while undergoing different types of stress including the introduction of new environments and new objects. As expected, the overstimulation of these CRH neurons reduced interest in the environment and again led to an increase in grooming and self-directed behaviours.
The results of the study reveal the importance of the CRH neurons in dealing with stress. These neurons help to dampen the impact of environmental cues to help cope with different stimuli. In addition, there appears to be natural sequence of behaviours that promote the return to a baseline state.
Yet, as seen in the last experiment, if the activity from these CRH neurons is uncontrolled, the brain may not have a natural baseline state as it inadvertently avoids the environment and focuses on self-directed behaviour. This complex reaction in which some individuals tune out the outside world and exhibit repetitive, self-directed and stereotyped behaviours is seen in some people with anxiety or autism spectrum disorder. The similarity may not be indicative of cause or association, yet the results of this study suggest there may indeed be a previously unknown role for CRH neurons in some autistic-like behaviours.
1. Fuzesi T, Daviu N, Wamsteeker Cusulin JI, Bonin RP, Bains JS. Hypothalamic CRH neurons orchestrate complex behaviours after stress. Nat Commun. 2016;7:11937.
2. Sprott RL, Waller MB. The effects of electroconvulsive shock on the action of a reinforcing stimulus. Journal of the Experimental Analysis of Behavior. 1966;9(6):663-9.
3. Stenzel-Poore MP, Heinrichs SC, Rivest S, Koob GF, Vale WW. Overproduction of corticotropin-releasing factor in transgenic mice: a genetic model of anxiogenic behavior. J Neurosci. 1994;14(5 Pt 1):2579-84. http://www.jneurosci.org/content/14/5/2579.full.pdf
4. Wamsteeker Cusulin JI, Fuzesi T, Watts AG, Bains JS. Characterization of corticotropin-releasing hormone neurons in the paraventricular nucleus of the hypothalamus of Crh-IRES-Cre mutant mice. PLoS One. 2013;8(5):e64943. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0064943
Text by Jason Tetro, for CAN-ACN