What Is the Hypothalamic-Pituitary-Adrenal Axis?

The hypothalamic-pituitary-adrenal axis (HPA or HTPA axis ), also known as the limbic system-hypothalamus-pituitary-adrenal axis (LHPA axis), is a complex collection of direct actions and feedback interactions It includes the hypothalamus (a hollow funnel-shaped area in the brain), the pituitary (a pea-like structure in the lower part of the hypothalamus), and the adrenal gland (a small circular vertebral organ in the upper part of the kidney). The interaction between these three forms the HPA axis. The HPA axis is an important part of the neuroendocrine system and is involved in controlling stress responses and regulates many physical activities such as digestion, the immune system, mood and mood, sexual behavior, and energy storage and consumption. Many species, from the most primitive organisms to humans, have HPA axes. It is a mechanism that coordinates the interaction of glands, hormones, and parts of the midbrain, particularly those involved in mediating the general adaptation syndrome (GAS).

Hypothalamic-pituitary-adrenal axis

The HPA axis mainly includes the following three parts:

The release of CRH from the hypothalamus is affected by a number of factors, including stress stimuli-the effect of nerve impulses on the hypothalamus, cortisol levels in the blood, and circadian rhythms. For healthy people, cortisol levels rise rapidly after waking up, and peak blood concentrations can be reached within 30-45 minutes. Then, the cortisol content gradually decreased during the day, and increased again in the evening. In the evening, cortisol levels fell again, reaching a minimum around midnight. Studies have found that abnormal cyclic cortisol fluctuations are associated with various diseases, such as chronic fatigue syndrome (MacHale, 1998), insomnia (Backhaus, 2004), and burnout (Pruessner, 1999).

From the anatomical structure, the brain's amygdala, hippocampus and other nuclear groups have a physical connection with the hypothalamus, this connection allows the brain nucleus to stimulate the HPA axis. The nerve impulses from the receptors reach the lateral area of the amygdala via the afferent nerve, and are processed and summarized into the cerebral cortex together with other information. The central system can project impulses such as fear to different regions of the brain. In the hypothalamus, neural impulses of fear can both activate the sympathetic nervous system and regulate the hypothalamus-pituitary-adrenal axis.

After the body is stimulated by stress, cortisol synthesis increases. This increase in hormone levels can cause a state of readiness. Some of the body's "alertness" reactions, such as immune responses, will temporarily weaken, making the body ready for potential dangers. ^[2]

Through experiments, scientists have studied a variety of different stressors and their effects on the HPA axis in different environments. There can be many different types of stressors-in experiments with rats, two types of stressors are often used: community stress and physical stress. Both stressors are able to activate the function of the HPA axis through different pathway mechanisms. Many monoamine neurotransmitters play an important role in the regulation of the HPA axis, especially dopamine, serotonin, and norepinephrine. The adaptor drugs in traditional Chinese medicine (such as ginseng, ganoderma, etc.) can work by regulating the HPA axis.

Mammals and other vertebrates also have HPA axes. For example, biologists have found that lower social status can cause chronic tension through studies of fish, manifested by a lack of aggressive behavior, lack of dominance, and long-term threats from dominant fish. Serotonin (5HT) may be an activating neurotransmitter that regulates the stress response. Elevated serotonin levels can increase the concentration of -melanocyte stimulating hormone in the cytoplasm and darken the skin (this is one of the salmon populations. Social signals), activate the HPA axis, and peers inhibit offensive behavior. The addition of the amino acid L-tryptophan (the precursor of serotonin) to the diet of rainbow trout weakened salmon's aggressive behavior and response to stimuli. However, the results also point out that cortisol levels in the cytoplasm are not affected by this food-borne L-tryptophan.