Evolution Of The Stress Response
The idea of natural selection is that genes that confer a fitness advantage increase in the population while ones that decrease fitness become less common. This concept can help us gain insight to important physiological concepts such as stress and its response. Understanding the evolutionary history of the stress response and its signaling pathways can guide us into gaining knowledge on how stress hormones like epinephrine (adrenaline) evolved and why it prevailed in ancestral environments and continues to have such important physiological functions.
Adrenaline is the immediate hormone that is involved in the fight or flight response. Adrenaline works by binding to its alpha-1 adrenergic receptor which is a G-coupled protein receptor that triggers a number of transduction pathways and results in many physiological changes like increased heart rate, increased glucose synthesis to provide energy, shunting of blood from gut and skin to muscles, heightened awareness, and noticeable increase in strength and performance. These physiological changes are an important adaptation that equip the animal to be ready for danger or flee from it. Pertaining to the molecular evolution of the stress hormone, studies have shown that there is possible coevolution between adrenaline and its receptor. The receptor responsible for binding adrenaline and leading to the stress response is the alpha-1 adrenergic receptor.
These are also responsible for binding other hormones such as dopamine and norepinephrine. In a study done by Malhotra et al. , the correlation coefficient of adrenergic receptors with their interacting partners were calculated by comparing evolutionary distances of hormone with their interacting receptors. If the receptor responsible for a specific signaling hormone had a positive correlation, then they were likely to coevolve. It was demonstrated that the values of evolutionary distances of 10 homologous species (5 aves and 5 mammals) of epinephrine with the same 10 homologous species of alpha-1 adrenergic receptors was highly correlated – thus confirming that the signaling molecule indeed co-evolved with its receptor.
The significance of the co-evolution between signaling molecule and receptor is that it can show networks of interaction important for cellular function and allows us to understand the how and why of certain signal-receptor interactions. The stress response and its hormones not only allows an animal to reach its peak performance level during stress, but it also has some significant effects on immune function. A paper by Shelley Adamo explained the stress response’s effect on immune function in invertebrates. In the study, they used a comparative method to assess the stress response between vertebrates and invertebrates. It was found that stress induced changes on vertebrates are immunosuppressive but the existence of some specialized receptors on immune cells suggest its adaptive function. As of invertebrates, the stress response is just as complex but not always immunosuppressive. It is also demonstrated that stress hormones in invertebrate immune function may help to realign resources during the fight-or-flight behavior.
The importance of this is that it shows the wide array of variety the stress response has in both vertebrates and invertebrates. It not only has functions allowing an animal to be physiologically equipped for stress, but also has counter effects on the immune system as well. In ancestral environments, stressors pertaining to survival were a daily effort. From the difficulty to acquire food to fleeing away from a predator to rampant infections, stress could be present any time. For an organism to have evolved, they had to develop traits that conferred a fitness advantage. Stress hormones allow an organism to respond to stressful situations in external environment by modifying its physiological functions and thus being ready for whatever the environment throws at them.
