Presence Of Oxytocin Receptors Research Articles

A functional selective effect of oxytocin secreted under restraint stress in rats

Restraint stress (RS) is an unavoidable stress model that triggers activation of the autonomic nervous system, endocrine activity, and behavioral changes in rodents. Furthermore, RS induces secretion of oxytocin into the bloodstream, indicating a possible physiological role in the stress response in this model. The presence of oxytocin receptors in vessels and heart favors this possible idea. However, the role of oxytocin secreted in RS and effects on the cardiovascular system are still unclear. The aim of this study was to analyze the influence of oxytocin on cardiovascular effects during RS sessions. Rats were subjected to pharmacological (blockade of either oxytocin, vasopressin, or muscarinic receptors) or surgical (hypophysectomy or sinoaortic denervation) approaches to study the functional role of oxytocin and its receptor during RS. Plasma levels of oxytocin and vasopressin were measured after RS. RS increased arterial pressure, heart rate, and plasma oxytocin content, but not vasopressin. Treatment with atosiban (a Gi biased agonist) inhibited restraint-evoked tachycardia without affecting blood pressure. However, this effect was no longer observed after sinoaortic denervation, homatropine (M2 muscarinic antagonist) treatment or hypophysectomy, indicating that parasympathetic activation mediated by oxytocin secreted to the periphery is responsible for blocking the increase in tachycardic responses observed in the atosiban-treated group. Corroborating this, L-368,899 (oxytocin antagonist) treatment showed an opposite effect to atosiban, increasing tachycardic responses to restraint. Thus, this provides evidence that oxytocin secreted to the periphery attenuates tachycardic responses evoked by restraint via increased parasympathetic activity, promoting cardioprotection by reducing the stress-evoked heart rate increase.

The oxytocin receptor, luteolysis and the maintenance of pregnancy

During luteal regression episodic pulses of oxytocin secretion become coupled to the release of prostaglandin F2 alpha (PGF2 alpha) following synthesis of endometrial oxytocin receptors, but in early pregnancy the inhibition of oxytocin receptor formation by the conceptus prevents the development of the pulsatile pattern of PGF2 alpha release needed to achieve luteolysis. Oxytocin receptors are present on the luminal epithelium in ovariectomized and anoestrous ewes, in pregnant animals throughout most of gestation (day 21 to term) and in explants of endometrial tissue cultured in vitro. These receptors can be downregulated for a brief period by progesterone (10-12 days in sheep, 12-14 days in cattle). This period of inhibition can be extended by infusion of interferon tau (IFN-tau) (which probably inhibits oxytocin receptor gene transcription) or of oxytocin into the systemic circulation (which may act further downstream, possibly at the level of translation). Oxytocin receptors also develop on the caruncular stroma and deep glands at oestrus, but these need positive upregulation and appear dependent on an oestrogenic environment. Only epithelial receptors are needed to achieve a maximal PGF2 alpha response to an oxytocin challenge, but the presence of oxytocin receptors does not necessarily confer responsiveness as other factors may influence intracellular coupling mechanisms and precursor availability. The duration of the luteal phase is regulated by the time of the initial post-ovulatory rise in progesterone and the duration of exposure to progesterone.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular Mechanisms of Oxytocin Signaling at the Synaptic Connection.

Aberrant regulation of oxytocin signaling is associated with the etiology of neurodevelopmental disorders. Synaptic dysfunctions in neurodevelopmental disorders are becoming increasingly known, and their pathogenic mechanisms could be a target of potential therapeutic intervention. Therefore, it is important to pay attention to the role of oxytocin and its receptor in synapse structure, function, and neuron connectivity. An early alteration in oxytocin signaling may disturb neuronal maturation and may have short-term and long-term pathological consequences. At the molecular level, neurodevelopmental disorders include alterations in cytoskeletal rearrangement and neuritogenesis resulting in a diversity of synaptopathies. The presence of oxytocin receptors in the presynaptic and postsynaptic membranes and the direct effects of oxytocin on neuronal excitability by regulating the activity of ion channels in the cell membrane implicate that alterations in oxytocin signaling could be involved in synaptopathies. The ability of oxytocin to modulate neurogenesis, synaptic plasticity, and certain parameters of cytoskeletal arrangement is discussed in the present review.

Oxytocin—its role in male reproduction and new potential therapeutic uses

Oxytocin (OT) is traditionally thought of as a "female" neurohypophysis hormone due to its role in parturition and milk ejection. However, OT is recognized as having endocrine and paracrine roles in male reproduction. At ejaculation, a burst of OT is released from the neurohypophysis into the systemic circulation and stimulates contractions of the reproductive tract aiding sperm release. There is conclusive evidence that OT is synthesized within the mammalian testis, epididymis and prostate and the presence of OT receptors (OTRs) through the reproductive tract supports a local action for this peptide. OT has a paracrine role in stimulating contractility of the seminiferous tubules, epididymis and the prostate gland. Interestingly, OT has also been shown to modulate androgen levels in these tissues via stimulation of the conversion of testosterone to dihydrotestostone (DHT) by 5alpha-reductase. The elucidation of OT's role in male reproduction has suggested a number of potential therapeutic uses for this hormone. Exogenous administration of OT has, in some cases, been shown to increase the numbers of ejaculated sperm, possibly by stimulating contractions of the reproductive tract and thus aiding sperm passage. Within the prostate, OT has been shown to affect gland growth both directly and via its interaction with androgen metabolism. Prostate pathologies due to unregulated cell proliferation/growth, such as benign prostatic hyperplasia and cancer, are unfortunately very common and few effective treatments are available. Greater understanding of paracrine growth mediators, such as OT, is likely to provide new mechanisms for treating such pathologies.

Oxytocin stimulates secretory processes in lactating rabbit mammary epithelial cells

Oxytocin plays a major role in lactation mainly by its action on milk ejection via the contraction of myoepithelial cells. The effect of oxytocin on milk production and the presence of oxytocin receptors on different epithelial cells suggest that this hormone may play a role in mammary epithelial cells. To determine precisely the various roles of oxytocin, we studied localization of oxytocin receptors in lactating rabbit and rat mammary tissue and the influence of oxytocin on secretory processes in lactating rabbit mammary epithelial cells. Immunolocalization of oxytocin receptors on mammary epithelial cells by immunofluorescence and in mammary tissue by immunogold in addition to in situ hybridization showed that lactating rat and rabbit mammary epithelial cells expressed oxytocin receptors. Moreover, oxytocin bound specifically to epithelial cells. To determine whether oxytocin had an effect on lactating rabbit mammary epithelial cells, isolated mammary fragments were incubated in the presence or absence of 10(-6) i.u. ml(-1) of oxytocin. After 1 min of incubation with oxytocin, the morphology of epithelial cells and the localization of caseins and proteins associated with the secretory traffic suggested a striking acceleration of the transport leading to exocytosis, whereas the contraction of myoepithelial cells was only detectable after 7 min. Addition of 10(-8) g ml(-1) of atosiban before the addition of oxytocin prevented the oxytocin effect on secretory processes and on myoepithelial cell contraction. Addition of 10(-6) i.u. ml(-1) of vasopressin to the incubation medium did not mimic the stimulating effect of oxytocin on secretory traffic. These results show that lactating rabbit and rat mammary epithelial cells express oxytocin receptors and that oxytocin binds to these receptors. They strongly suggest that oxytocin has a dual effect on lactating mammary tissue: an acceleration of the intracellular transfer of caseins in mammary epithelial cells followed by the contraction of myoepithelial cells.

The concentrations of catecholamines and oxytocin receptors in the oviduct and its contractile activity in cows during the estrous cycle

In vitro experiments on oviducts of cyclic cows were undertaken to study: (1) the content of dopamine (DA), noradrenaline (NA) and adrenaline (A) in infundibulum, ampulla and isthmus, (2) the concentration of oxytocin receptors (OTR) in oviductal tissues and (3) the motility of ampulla and isthmus. Changes of DA content were observed in the infundibulum and the ampulla with maximal values occurring on Days 6–10 of the estrous cycle. The mean NA content was greatest in infundibulum<ampulla<isthmus. NA concentrations were the highest in the isthmus on Days 1–5 and 16–21, whereas in infundibulum NA levels were low during the whole estrous cycle. Mean A content was the highest ( P<0.001) in the isthmus and in the studied regions of the oviduct A content decreased from Days 1–5 to Days 16–21 ( P<0.01). Oxytocin receptors densities were measured in oviducts collected from cows on Days 16–21 (277.1±151.4 fmol/mg protein; K d 20.5±10.9 nM). Oxytocin (10 −7 M) increased the area under the contractile curve (AUC) of the ampulla and the isthmus on Days 16–21 and of the isthmus on Days 1–5 ( P<0.01). Acetylcholine (Ach) (10 −7 M) stimulated the ampulla and the isthmus contractions on Days 1–5 ( P<0.05) and ampulla contractions on Days 6–10 and 16–21 ( P<0.01). NA (10 −5 M) relaxed the ampulla and the isthmus during most of the studied days but was most effective on Days 1–5 and 16–21 ( P<0.01). In conclusion: (a) catecholamine content in the bovine oviduct varies by region and phase of the estrous cycle, (b) the presence of OTR and the stimulatory effect of oxytocin on oviduct motility are evident in the follicular-phase cows, (c) Ach and NA modify contractile activity in the oviduct of cows during follicular and early-luteal phases and (d) the studied oviductal parameters did not differ by ipsilateral and contralateral relationships to the active ovary in cow.

Vasopressin and oxytocin receptors

The oxytocin and the vasopressin V1a, V1b and V2 receptors have recently been cloned and shown to form a sub-family within the large superfamily of G-protein-linked receptors. Renal V2 receptors mediate vasopressin-induced water reabsorption via induction of intracellular cAMP production in collecting duct cells. Most remaining actions of vasopressin on blood vessel constriction, liver glycogenolysis, platelet adhesion, adrenal angiotensin II secretion and certain brain functions are mediated via v1a-type receptors that are coupled to a Gq/11 protein. V1 receptor activation leads to stimulation of phospholipases C, D and A2 and an increase in intracellular calcium. Vasopressin stimulates pituitary corticotrophin release via a third vasopressin receptor type (V1b) which is present on corticotrophs. Oxytocin induces myometrial contraction, endometrial prostaglandin F2 alpha production, mammary gland milk ejection, renal natriuresis and specific sexual, affiliative and maternal behaviours via oxytocin receptors which are also coupled to a Gq/11 protein. Although only one oxytocin receptor type has been cloned so far, recent binding studies indicate that uterine endometrial oxytocin receptors may constitute a distinct receptor subtype. In contrast to most other membrane receptors, the expression of oxytocin receptors undergoes very rapid and physiologically relevant up-and-down-regulation. A > 100-fold up-regulation of uterine oxytocin receptors occurs during gestation and may represent the trigger for parturition. Indeed, oxytocin receptor antagonists are able to counteract preterm labour and may soon be available for clinical use. The presence of oxytocin receptors on breast cancer cells and the growth-inhibitory effects of OT suggest a potential use of oxytocin analogues for breast cancer treatment. Whereas no mutations of the oxytocin or V1a or V1b receptors have been found, over 60 different genetic mutations of the (renal) V2 receptor have been described which represent the cause for congenital nephrogenic diabetes insipidus.

Effects of prostaglandins and oestradiol-17 beta on oxytocin binding in cultured bovine luteal cells.

The aim of the investigation was to evaluate the possible action of prostaglandins (PGs) and oestradiol-17 beta (oestradiol) on the specific binding for oxytocin in bovine luteal cells. Cultured cells of bovine corpora lutea at the mid-luteal stage (Day 8-12 of the oestrous cycle) were examined for the presence of oxytocin receptors by a radioreceptor assay using the 125I-labelled oxytocin antagonist [d(CH2)5,Tyr(Me)2,Thr4,Tyr-NH29]-vasotocin (125I-OVT) as a ligand. The cells were cultured for 48 h in total. In the final 15 h of culture, the luteal cells were exposed to varying concentrations of PGF2 alpha, PGE2 and/or oestradiol. After culture, the cells were incubated with 37,000 dpm (0.5 nM) 125I-OVT with or without 100 nM of unlabelled oxytocin. PGF2 alpha, at 10(-8) M and 10(-7) M, stimulated the specific binding for oxytocin to levels as high as 128% of controls (P < 0.01); by contrast, PGE2, PGI2 or oestradiol had no effect on oxytocin binding. Scatchard analysis revealed that the concentration of oxytocin receptors was increased (P < 0.05) from 6.7 fmol micrograms-1 DNA to 8.4 fmol micrograms-1 DNA by stimulation with 10(-7) M of PGF2 alpha without changing the binding affinity. No further increase in the specific binding was observed when PGF2 alpha was used in combination with PGE2, PGI2 or oestradiol at a concentration of 10(-7) M. Addition of indomethacin (28 microM) resulted in the inhibition of PGF2 alpha secretion, coinciding with a significant decrease in oxytocin binding (P < 0.01). However, addition of arachidonic acid (100 microM) caused a significant increase in the secretion of PGF2 alpha and the specific binding for oxytocin concomitantly (P < 0.05). When the protein kinase C (PKC) activity of the luteal cells was inactivated by preincubating cells for 13 h with 1 microM phorbol 12-myristate 13-acetate before PGF2 alpha stimulation, the specific binding for oxytocin was not affected by PGF2 alpha stimulation (10(-7) M) in the final 15 h of culture. These data suggest that PGF2 alpha may be one of the potent regulators for luteal oxytocin receptors in a paracrine and/or autocrine manner, and that its action is mediated by PKC.

Oxytocin receptors in the mammary gland and reproductive tract of a marsupial, the brushtail possum (Trichosurus vulpecula).

Previous studies of marsupial lactation have shown that the milk-ejection reflex changes in sensitivity, being greater in small mammary glands sucked by small pouch young and lesser in larger glands supplying milk to larger young. The involvement of oxytocin receptors in these changes was examined in the brushtail possum Trichosurus vulpecula. Oxytocin receptors were measured in the mammary glands, uterus, and medial vaginal sacs by radioreceptor assay, using [3H]oxytocin as radioligand. In the mammary gland, a single oxytocin binding site was found with an affinity and receptor concentration of 0.81 +/- 0.41 l/nmol and 10.2 +/- 4.8 pmol/g tissue respectively (SD, 10 possums). Competitive displacement curves with related peptides and analogs showed the following order of specificity: d(CH2)5[Tyr(Me)2,Thr4,Tyr9-NH2]-vasotocin much greater than vasotocin greater than oxytocin = Arg-vasopressin greater than mesotocin greater than [Thr4,Gly7]-oxytocin = Lys-vasopressin greater than [deamino-Pen1, O-methyl-Tyr2, Arg8]-vasopressin greater than isotocin much greater than [d(CH2)5, D-Phe2, Ile4, Ala9-NH2]-AVP. [3H]Oxytocin did not bind to vasopressin receptors in the thoracic aorta. The concentration of oxytocin receptors was very high in small mammary glands (18.6 pmol/g tissue in a 2-g gland) and decreased logarithmically as the size of the mammary gland increased. It is suggested that the changes in the sensitivity of milk ejection to oxytocin is related to the concentration of mammary oxytocin receptors. The presence of oxytocin receptors in both uterus and median vaginal sacs extends previous observations and supports the hypothesis that in marsupial parturition, the uterus and medial vaginal sacs respond as a single functional unit to oxytocin.

Oxytocin receptor in human fetal membranes at term and during labor

Human fetal membranes, taken from 30 patients submitted to caesarean section during the final stages of gestation and labor, were examined in order to evaluate the presence and characteristics of the oxytocin receptor. The presence of oxytocin receptors in human fetal membranes, both in the amnion and in the chorion-decidua, was demonstrated in this study. The receptor binding to oxytocin showed a significant increase during early and advanced labor compared with before the onset of labor. When the pre-labor level was taken as the normalized form (control = 100) the increase with respect to the control ( 10 cases) for the amnion in early labor (2.27 times ± 0.11, mean ± SEM, P < 0.001, 10 cases) and in advanced labor (2.53 times ± 0.15, 10 cases, P < 0.001) was highly significant. In the chorion-decidua the increase was 1.61 times ± 0.09, P < 0.001 in early labor and 1.66 times ± 0.19, P < 0.001 in advanced labor. Scatchard analysis showed a single receptor site for oxytocin in amnion and chorion-decidua. The dissociation constant ( K d ) did not change during the various stages of labor: the mean values found were 0.228 ± 0.02 (mean ± SEM) nM in the amnion and 0.193 ± 0.03 nM in the chorion-decidua respectively. These findings suggest that human fetal membranes are target organs for oxytocin and that they might play a role in the onset of labor through an increase of receptor binding.