Oxytocin and Human Relations

Oxytocin is a peptide that behaves like a hormone and a neurotransmitter in the body. In each of these versions it takes on different, but not entirely separate, functions.

• As a hormone, it is segregated in the hypothalamus, stored in the neurohypophysis, passes into the bloodstream and reaches its target organs. In the uterus, it stimulates contractions during childbirth, and in the mammary gland it facilitates the ejection of milk during lactation.
• As a neurotransmitter, it is synthesized by central oxytocin neurons that have receptors in multiple areas of the brain associated with the limbic system, such as the amygdala, nucleus accumbens and hypothalamus.

Since oxytocin does not cross the blood-brain barrier, the activity of this molecule as a hormone and its action as a neurotransmitter are expected to be independent.

Many experiments have been performed to try to clarify the role of oxytocin as a neurotransmitter. Below we have five of the more representative examples.

Quails establish their hierarchical relationships through pecking. Those at the highest level peck those below them in the pecking order. A group of quail chicks were injected with intracerebral oxytocin during their rearing phase. While they were chicks, they barely called. When they were adults, they showed a significant tendency towards submission and assumed the lowest positions in the social hierarchy.

Prairie voles (Microtus ochrogaster) are monogamous and gregarious, while montane voles lead a solitary life. Prairie voles have been found to have more oxytocin receptors than montane voles, and their distribution in the brain is also different. These results have been replicated in certain primate species.

The sex hormones, oestrogens and progesterone, regulate the sexual behaviour and reproductive function of mammals. In female rats, these hormones facilitate the synthesis of oxytocin and increase the density of its receptors. As a result, females present lordosis, a characteristic behaviour indicative of sexual receptivity. In males, the administration of oxytocin causes erections. On the other hand, oxytocin levels in blood increase during orgasm.

Ewes recognise their lambs for the first two hours after birth through their odour. If the ewe is prevented from coming into contact with her lambs during this “window”, she no longer recognises them, even if they are brought back into contact. There are two ways to reopen this “window”. One is to artificially dilate the ewe’s vagina, simulating a birth, and bring the mother and the lamb back into contact with each other. The other is to inject oxytocin intravenously into the mother. In fact, the dilation of the vagina during labour triggers the release of oxytocin.

Some experiments have been performed in humans with intranasal oxytocin inhalation to avoid the side effects caused by the intravenous administration of oxytocin. In couples, the inhalation of oxytocin has been shown to foster an attitude of tolerance that allows them to more harmoniously resolve conflict. In groups taking part in a game based on making risky investments, after oxytocin is inhaled, players exhibited behaviours consistent with a greater predisposition to self-confidence and diminished fear.

The interpretation of these experiments is not always clear due to the complex network of neural connections and different neuromodulators involved in a single process. For example, oxytocin may contribute to triggering the release of endogenous opioids, such as endorphin, which would be responsible for providing that reassuring and pleasant effect.

It seems that, throughout evolution, the same substance, oxytocin, has been used in different types of relationships as a generator of trust and emotional intimacy. At the same time, it would inhibit, or ease, feelings of threat and rivalry in relationships. It could be present in the sense of well-being associated with a loved one’s embrace or the intimacy that tends to develop after sexual relations. Therefore, it would facilitate relationships, such as parenting, bonding with and nurturing a baby, a partner, as well as foster social bonds.

Oxytocin would not directly generate this feeling of closeness and intimacy, but it would be part of the complex network of interactions between different neurotransmitters. Endogenous opioids, such as endorphins, would likely form part of this system. As such, their reassuring and pleasant effect would be enhanced by oxytocin.

This wide range of oxytocin functions can already be found in reptiles. In this group of animals, sexual and reproductive behaviour is regulated by vasotocin. Vasotocin is the precursor of oxytocin and vasopressin in mammals. In fact, these molecules share all but one amino acid, and the genes responsible for their synthesis are very close on the same chromosome. It is conceivable that a precursor of these molecules could already be found in the first vertebrates, some 500 million years ago.

Thus, injecting vasotocin into the brain of frogs and lizards triggers behaviours such as the production of courtship “calls” and, if given the opportunity, mating with females. On the other hand, the blood vasotocin concentration in turtles returning to the beach for egg laying begins to increase once they reach the beach, continues to rise when digging the hole, and peaks during spawning. It then decreases when the turtle covers the hole and is back to normal once she is back in the sea.

In an attempt to rewrite history, the vasotocin present in the reptiles regulated sexual and reproductive behaviour. Mammals, which descended from reptiles, “took advantage” of the successor to vasotocin, oxytocin, to maintain some of its functions and acquire new ones (this process is called “exaptation”).

Its involvement in sexual behaviour, e.g. sexual arousal and orgasm, was maintained. Also in reproduction, it still stimulates contractions during labour (which may, in a way, equate to the turtle’s spawning) and the ejection of milk when breastfeeding.

The new functions would fall under the category of relationships. Probably, initially associated with the oxytocin peak occurring at the end of pregnancy, this molecule, now as a neurotransmitter, facilitated the creation of the bond between the baby and mother (more precisely, the caregiver figure, whoever that may be).

This very particular intimate relationship, that of mother and child, would be the one that would later be extrapolated to other types of relationships. In adolescence, and again, in the context of changes in sex hormones, it would facilitate the emergence and maintenance of romantic relationships and social relationships.

This way of looking at it, from the neurobiological standpoint, may support certain psychoanalytic concepts, like the importance of early relationships in childhood as the foundations of future adult relationships or the common aspects between different types of relationships, such as the mother-child relationship and the couple relationship.

Vasopressin, the other “daughter” of vasotocin, also presents this hormone-neurotransmitter duality. As a hormone, again, like oxytocin, it is synthesised in the hypothalamus and stored in the neurohypophysis and, in this case, its function is antidiuretic. As a neurotransmitter, its functions are reminiscent of oxytocin, but with certain nuances.

Vasopressin is more present in males, while oxytocin predominates in females, though both are present in all individuals. Both are involved in the regulation of relationships. However, vasopressin is associated with a greater component of aggressiveness and rivalry, while oxytocin is associated with the aspect of care.

François Jacob, awarded the 1965 Nobel Prize, compared evolution with a “handyman” capable of transforming anything within reach into something with a new function (for example, an arm into a wing).

Originally, oxytocin was associated with reproduction, and, subsequently, its functions were extended to interpersonal relationships. Initially, to the relationship between mother and child, and later, to the emotional aspects of romantic relationships and to social relationships in general.

References

Eibl-Eibesfeldt, “Human Ethology” Ed. Routledge. 2007

Holmes, J. “John Bowlby and attachment theory”. Ed. Routledge. 1993.

Jacob, F. “Evolution and tinkering” Science 196: 1161-6. 1977.

Freud, S. “Three essays on the theory of sexuality”. Standard edition 7. Ed. Hogarth press. 1905/1968.

Pankseep, J. “Affective neuroscience”. Ed. Oxford University Press. 1998

Pankseep, J. & Biven, L. “The archeology of mind”. Ed. W. W. Norton and Company.  2012.

Web pages

https://es.wikipedia.org/wiki/Oxitocina

https://www.quimica.es/enciclopedia/Oxitocina.html#:~:text=La%20oxitocina%20es%20un%20p%C3%A9ptido,masa%20molecular%20de%201007%20daltons.