A. The Oxytocin Receptor

The biological actions of oxytocin depend on its high affinity for its receptor, relative to those for vasopressin, V1a, V1b, and V2; the tissue specific distribution of these receptors; and the regulation of its secretion. Comparison of the oxytocin receptor (OTR), depicted in Figure 1, with that of V2 shown in Figure 2 reveals that the two are closely related structurally, particularly in the membrane-spanning regions. Like the vasopressin receptor V1a, the oxytocin receptor is coupled to Gq/11 through which it activates phospholipase C-β. Downstream events include Ca2+ release from intracellular stores, PKC activation, and activation of the MAP kinase cascade. Some oxytocin signaling also involves increasing intracellular calcium through influx from extracellular sources. Finally both within the brain and peripheral tissues oxytocin can, through the activation of PLA2, stimulate the production of prostaglandins.

Fig1. The structure of the oxytocin receptor. Residues shown in black are conserved in the entire GPCR family (as are the seven trans membrane regions) and those in purple are conserved among the oxytocin and vasopressin receptors. The extracellular domains are labeled E1–E4 and the solid lines show binding sites for oxytocin and other agonists. The dashed rectangles indicate residues involved in G-protein binding and the palmitoylation sites are shown with orange hydrocarbon chains.

Fig2. The secondary structure of the V2 receptor for arginine vasopressin. Residues shown in red are positions of mutations leading to diabetes insipidus, the primary clinical manifestation of lack of AVP.

B. Oxytocin and Parturition

In many mammals oxytocin is an important player in the initiation of parturition (the process of the birth of the young at the end of pregnancy). In others (mice) the lack of oxytocin through genetic manipulation does not impair the birth process, suggesting either that it is not required or that redundant processes are available for this crucial function. In humans the maternal and fetal blood levels of oxytocin are low at the onset of labor and rise progressively during its course. In all species studied, the number of uterine oxytocin receptors rises during pregnancy, increasing the sensitivity of the myometrium to the hormone. Through its effects on intracellular calcium concentration and prostaglandin production, oxytocin stimulates and coordinates the contractions of the myometrium. As will be discussed in more detail in Chapter 14 the signals for oxytocin release come from the uterus, but the increase in its secretion as labor progresses is due in part to local positive feedback effects at the hypothalamic neuron.

C. Oxytocin and Lactation

 In mammals, suckling and other stimuli (auditory, visual) related to feeding the infant are transmitted rapidly over a spinal reflex arc to the paraventricular nucleus of the hypothalamus. This stimulus appears to be cholinergic and is transmitted to nerve endings in the posterior pituitary, resulting in Ca2+ uptake, depolarization, and exocytosis of oxytocin into the circulation. Negative emotions, such as fear and stress, can inhibit oxytocin release. Only a few seconds are required for oxytocin to reach its receptors in the myoepithelial cells of the mammary gland. Through the activation of phospholipase C and the resulting changes in intracellular Ca2+, the mammary ductules and ducts contract and milk is ejected from the gland through the nipple into the infant’s mouth.

D. Oxytocin and the Brain

The oxytocin receptor is expressed in the brain in a region-, sex-, and species-specific manner. This suggests that the biological actions of oxytocin are quite likely to vary by species and by gender and indeed behavioral and neurobiological studies in many mammalian species, including humans, confirm that this is the case. Table 1 lists some of the locations of the oxytocin receptor in the central nervous system in the human and, for comparison, in the adult rat. The species differences in OT receptor localization are clear from this table. For example, several areas show abundant OT binding in the human and negligible binding in the rat. In most cases, the association between binding and physiological activity has not yet been established.

Table1. Oxytocin Receptors in the Central Nervous System

1. Maternal Behavior

 In several species, oxytocin availability is associated with maternal behavior characteristic for the species. For example in rats, pup-grooming and hovering are increased by oxytocin as are reciprocal mother–pup behaviors. In mice lacking the oxytocin gene, some maternal behaviors, such as pup retrieval and licking, are decreased compared to wild type mice. In humans, several studies suggest that there is a similar positive relationship between oxytocin levels and positive inter actions between human mothers and their infants. In particular, higher levels of oxytocin measured during pregnancy are associated with positive maternal behavior (gaze, touch, vocalization) and other indices of maternal adaptation. The role of oxytocin, which is dependent on a number of other physiological and psychosocial factors, in human parental behavior will become better understood as larger and more detailed studies, as well as those extended to fathers, are carried out.

2. Social Behavior: Trust and Fear

Building trust is a necessary step in relationships and social bonding. Studies in humans suggest that oxytocin increases trust of strangers (and others). Although the role of oxytocin in everyday life is not known, much attention is being paid to its possible involvement in social disorders, particularly isolating ones, such as phobias and autism. Related to its possible effects on social interaction through trust is the role that oxytocin has in modulating the hypothalamic pituitary-adrenal axis and its stress response.

اخر الاخبار

اخبار العتبة العباسية المقدسة

بالتعاون مع جامعة ستانفورد.. جامعة الكفيل تعتمد منهجًا متطورًا لطب الطوارئ

سماحة السيد الصافي يستقبل وفد شركة (سيتي فارما) الباكستانية المختصة في الأدوية

شعبة فاطمة بنت أسد تعلن أسماء الفائزات في مسابقة ميثاق العدالة

المجمع العلمي يطلق البرنامج التطويري الربيعي لطلبة الحفظ بنسخته الخامسة في كربلاء

المزيد

الآخبار الصحية

اللحظات الأخيرة قبل الموت.. هذا هو تسلسل "فقدان الحواس"

وخز الأطراف وضعف الذاكرة.. علامات على نقص فيتامين هام

عادة صباحية بسيطة.. تأثيرها قد يحسن ضغط الدم

عادات يومية تدمر قلبك بصمت.. خبراء يطلقون التحذير

المزيد

الآخبار التكنلوجية

الذكاء الاصطناعي يفك لغز آثار أقدام الديناصورات

القمر أولا.."سبيس إكس" تؤجل خططها الخاصة بالرحلات إلى المريخ

الذكور أم الإناث؟ دراسة "تنسف الشائع" بشأن التوحد

خبير يكشف عن اقتراب العلماء من فك شفرات "لغات الحيوانات" !

المزيد

مواضيع ذات صلة

Biological Actions of T3

Adrenal Replacement in ACTH Deficiency

Rathke cleft cysts

GHD with Accompanying Features

Biological Actions of Arginine Vasopressin (AVP)

Structures of Oxytocin and Vasopressin

Syndrome of Inappropriate Antidiuresis

Hypothalamic Diabetes Insipidus

The Physiology of oxytocin (OT)

Regulation and Biological Actions of Growth Hormone and Prolactin

Hypothalamic Control of Growth Hormone Secretion

Corticotropin-Releasing Hormone