Introduction
Hormones are chemical substances released from the posterior pituitary gland and are responsible for many important functions of the body. Oxytocin and vasopressin are hormones secreted from the same brain region called the hypothalamus. Though they are similar in their structure, they exhibit different actions in stressful situations.
What Are Oxytocin and Vasopressin?
Oxytocin and vasopressins are the neuropeptides that can affect social processes in mammals. Recent advances have found that these hormones and their receptors can treat social deficits in neurodevelopmental disorders. Many studies have found an association between these peptides and the social abilities of individuals.
Vasopressin is also called arginine vasopressin (AVP). Oxytocin carries out functions like stimulating uterine contractions and milk production after birth, whereas AVP regulates water by regulating urine concentration in the kidney. Apart from these regular functions, it has been noticed that these two hormones regulate many social behaviors in mammals.
Oxytocin and vasopressin are neuropeptides consisting of nine amino acids. These molecules differ in structure at the 3rd and 8th positions. Both of these are present and coded on chromosome 20.
These are produced from magnocellular neurons situated in supraoptic and paraventricular nuclei. The neurons project their axons into the posterior pituitary gland. The peptides are stored in vesicles until their release is triggered.
Oxytocin and Stress:
Stressful stimuli produce responses in the neuroendocrine, immune, and autonomic nervous systems and behaviors. These responses influence human health conditions. Stress stimuli like noxious stimuli, conditioned fear stimuli, stress due to immobilization, social defeat, etc., activate neurons that synthesize oxytocin in the hypothalamus.
This causes the release of oxytocin into the plasma or the brain. Exercise can also release oxytocin. During forced swimming and shaker stress, both within the hypothalamus and peripheral release of oxytocin can be seen.
In humans, it is observed that there is an increase in levels of oxytocin in plasma and saliva after exposure to stresses like physical stress, public speaking, in front of unknown jury members, and other types of psychological stress. Oxytocin neurons are activated due to fear or other stress stimuli that play a role in releasing oxytocin.
A study has reported that oxytocin reduces the activity of the hypothalamic-pituitary-adrenal axis. This action causes decreased inflammation and reduced anxiety-related behaviours.
Another study has shown that when decreased levels of oxytocin are present, it increases the levels of corticosterone in response to shaker stress, which results in increased temperature and anxiety-related behaviours. This helps determine that oxytocin has a role in stressful situations and helps reduce stress.
Social interactions can help with stress management through visceral, tactile, olfactory, auditory, and visual sensations. Oxytocin acts as a buffer in social situations. Oxytocin has a role in sexual interactions. During sexual interactions, oxytocin neurines get activated in the hypothalamus. Studies have reported that oxytocin helps reduce anxiety-related behaviours during non-mating social interactions. Activating oxytocin neurons through social support may help reduce stress responses.
Oxytocin has an analgesic effect. Oxytocin neurons get activated in response to noxious stimuli, act on multiple sites like the spinal cord and dorsal root ganglion, and help produce analgesic effects.
In some conditions, oxytocin, when administered, may show anxiogenic effects. Some studies have reported that oxytocin receptors play a major role in social avoidance.
Vasopressin and Stress:
Vasopressin is a neuropeptide and has a role in fluid balancing in the body. It is also called arginine vasopressin (AVP). This is produced in the nerve cell bodies in the hypothalamic magnocellular paraventricular (PVN), supraoptic (SON), accessary nucleus, and other brain parts. The two pathways involved in stress response are the hypothalami-hypophysial (HHS) and hypothalamo-infundibular (HIS) tracts.
Studies have reported that the HHS tract is responsible for situations of stress causing a response. They thought the HHS tract would activate as a response to physical stresses like immobilization, hemorrhage, etc. When HHS is activated, this causes an increase in the levels of AVP. This, in turn, decreases respiratory infection, myocardial infarction, stroke, and metabolic syndrome among individuals with septic shock. There was an increase in the levels of copeptin among those exposed to emotional stressors.
Vasopressin receptors are V1a, V1b, and V2. Among these, V1b receptors are responsible for social recognition memory, and AVP also plays a role in this process. AVP has a role in stress, pain, and inflammation. In stressful situations, glucocorticoid levels are increased. AVP has a role in controlling the levels of glucocorticoids. Adrenocorticotrophic hormone (ACTH) stimulates aldosterone and vasopressin. During physical stress, these two hormones are released into circulation, and sympathetic activation increases angiotensin II. These mechanisms help fight stress.
When the levels of AVP increase, it becomes a stress hormone. AVP plays a role in adaptation and water balance in difficult situations. It helps with sympathetic activation. In extreme cases of metabolically conservative and shutdown responses. Hence, during extreme stress, AVP takes the lead over oxytocin.
A centrally acting AVP regulates emotional states. AVP and corticotrophin-releasing hormone (CRH) both increase the effects of aggression and anxiety. These play a role in situations of intense challenge. AVP plays a role in the circadian rhythm, regulating the sleep cycle and increasing blood pressure. These two factors are related to stress.
AVP is mainly related to male-typical social behaviours like aggression, pair bond formation, and stress response. A study has shown a relationship between increased levels of AVP in cerebrospinal fluid and aggression. Another study found a relationship between AVP and social communication. Among men, increased AVP decreased the friendliness of faces, whereas among women, increased AVP increased the friendliness of faces. AVP can affect the autonomic response to threatening faces, increasing anxiety.
Conclusion
Vasopressin and oxytocin are hormones that regulate different functions in the body. Apart from their role, they also take part in managing stressful situations. They regulate social behaviours and help in stressful situations. Their neurobiology and neurogenetics must be understood to understand their role in stress. Hence, knowing about these hormones and their function in stressful situations is important.
