Oxytocin

Structure

Oxytocin is a new polypeptide comprising amino-acid, whose two groupings cystein are connected by a Pont disulfide (Cys¹ - Cys⁶). The sequence of the amino-acids is presented Ci-dessous.

H─Cys─Tyr─Ile─Glu─Asn─Cys─Pro─Leu─Gly─NH₂

Although oxytocin and the Vasopressine have close structures (seven amino-acids jointly), these two hormones have very different effects.

Effects

The first recognized effects of oxytocin were its faculty to accelerate the Accouchement at the Mammifères. Oxytocin indeed causes the contraction of the smooth muscles of the uterus and accelerates work. This hormone also makes it possible the uterus to retract after expulsion, so that it finds its initial position.

From the point of view of the evolution, oxytocin and the Vasopressine are old substances whose actions strongly contributed to the survival of the species, although according to two opposite strategies: the system fight or escape for the Vasopressine and the calm system and contact for oxytocin. The chemical composition of oxytocin is the same one at all the Mammifères, while the molecular structure of the Vasopressine differs slightly at certain species.

The oxytocin injection in the brain of a mammal produces significant modifications of its behavior: reduction in aggressiveness, increase in sociability, greater resistance to the pain, lowers blood-pressure, increase in the appetite and maternal behavior in the females. These effects persist on average twice longer in the females than in the males. It is interesting to note that they are transmitted, although with a less amount, other live animals in the same cage and not having undergone the treatment. The experiment showed that this transmission was carried out by the olfactive way, probably by the Phéromones which touch an antiquated part of the system Olfactif: the voméro-nasal Body.

The natural oxytocin produced in the neuronal circuit plays a crucial role in the attachment between the female Mammifère and its newborn. Studies on ewes non-gestantes showed that the oxytocin injection per way Intraventriculaire (in the brain) makes it possible to produce maternal reflexes artificially. Estrogen administration of S and Progesterone more one vagino-cervical stimulation (sexual) produced the same effect. On the other hand, this effect is cancelled if the ewe is under Péridurale. (Source: KEVERNE EB, KENDRICK KM, 1994. Maternal-behavior in sheep and its neuroendocrine regulation. ACTA PAEDIATRICA, 83, p.47-56 Suppl. 397)

Oxytocin in the blood circuit is essential to the reflex of ejection of the placenta. However, the phase which follows the birth of the human baby immediately corresponds for the mother to a natural oxytocin peak ever equalized, under condition which it did not receive from artificial oxytocin (Syntocinon ™), that it does not have cold, that it is not subjected to an intense light, and that its intimacy is respected. In the contrary case, it is often necessary to manage artificial oxytocin (or Misoprostol) to facilitate the ejection of the placenta and to prevent a Hémorragie of the delivery, first cause of Maternal death in France.

During tétée, oxytocin stimulates the ejection of milk by supporting the contraction of the cells myoépithéliales which surround the cells (acini) of the glands mammaires. Oxytocin does not have control on the production of the milk, which is dependant on the estrogen Prolactine and the S.

In number 7042 of the Natural review (June 2005), a research team of the University of Zurich publishes the results of an experiment tending to prove that oxytocin places that which inhales some in a state of confidence. The experiment consisted of a roleplay whose participants were to decide or not to entrust money to a person not inspiring confidence but promising important benefit. About half of the guinea-pigs having inhaled oxytocin decided to entrust their money to the doubtful investor, while 1/5 only of the guinea-pigs having inhaled a placebo agreed to do it.

Regulation

The regulation of the production of oxytocin is done by nervous way. During the Childbirth, the stimulus is the dilation of the uterine collar which is detected by mechanoreceptors present on the wall of the uterus. These receivers send information to the central nervous system which starts the production of oxytocin.

In the gland mammaire, the suction of the Mamelon in the same way is detected by receivers connected to the central nervous system: this stimulation involves the production of oxytocin by the hypothalamus.

Contrary to other hormones, oxytocin does not profit from a regulating device ( feedback ) which would make it possible to limit its production. It seems on the contrary that oxytocin activates processes which function in “cascade”. This is why one can observe of it important peaks and a production on a “pulsatile” mode. A recent study undertaken by the team of Kervin Uvnäs Moberg, in Sweden, watch that the intermediate duration of breast feeding to the center is higher among women who profited from pulsatile production of natural oxytocin during their Accouchement.

Mode of action

Oxytocin is fixed on the receivers of the muscular cells of the uterus and the glands mammaires. These receivers, coupled to a Protein G, activate the phospholipases C leading to the increase the intracellular calcium concentration. The Ion S Ca²⁺ thus released support the interactions between proteins of Actine and Myosine, at the base of the muscular contraction.

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