The oxytocin receptor

Role of the Oxytocin Receptor Expressed in the Rostral Medullary Raphe in Thermoregulation During Cold Conditions

Recent papers have reported that oxytocin (Oxt) and the oxytocin receptor (Oxtr) may be involved in the regulation of food intake in mammals. We therefore suspected the Oxt/Oxtr system to be involved in energy homeostasis. In previous studies, we found a tendency toward obesity in Oxtr-deficient (Oxtr (-/-)) mice, as well as impaired thermoregulation when these mice were exposed to cold conditions. In the present study, we observed the expression of Oxtr in the rostral medullary raphe (RMR), the brain region known to control thermogenesis in brown adipose tissue (BAT). Through immunohistochemistry, we detected neurons expressing Oxtr and c-Fos in the RMR of mice exposed to cold conditions. Up to 40% of Oxtr-positive neurons in RMR were classified as glutamatergic neurons, as shown by immunostaining using anti-VGLUT3 antibody. In addition, mice with exclusive expression of Oxtr in the RMR were generated by injecting an AAV-Oxtr vector into the RMR region of Oxtr (-/-) mice. We confirmed the recovery of thermoregulatory ability in the manipulated mice during exposure to cold conditions. Moreover, mice with RMR-specific expression of Oxtr lost the typical morphological change in BAT observed in Oxtr (-/-) mice. Additionally, increased expression of the β3-adrenergic receptor gene, Adrb3, was observed in BAT. These results are the first to show the critical role of RMR Oxtr expression in thermoregulation during cold conditions.

Oxytocin receptor in the hypothalamus is sufficient to rescue normal thermoregulatory function in male oxytocin receptor knockout mice

Oxytocin (OXT) and OXT receptor (OXTR) have been implicated in the regulation of energy homeostasis, but the detailed mechanism is still unclear. We recently showed late-onset obesity and impaired cold-induced thermogenesis in male OXTR knockout (Oxtr(-/-)) mice. Here we demonstrate that the OXTR in the hypothalamus has important functions in thermoregulation. Male Oxtr(-/-) mice failed to maintain their body temperatures during exposure to a cold environment. Oxtr(-/-) mice also showed decreased neuronal activation in the thermoregulatory hypothalamic region during cold exposure. Normal cold-induced thermogenesis was recovered in Oxtr(-/-) mice by restoring OXTR to the hypothalamus with an adeno-associated virus-Oxtr vector. In addition, brown adipose tissue (BAT) in Oxtr(-/-) mice contained larger lipid droplets in both 10- and 20-week-old compared with BAT from age-matched Oxtr(+/+) control mice. In BAT, the expression level of β3-adrenergic receptor at normal temperature was lower in Oxtr(-/-) mice than that in control mice. In contrast, α2A-adrenergic receptor expression level was higher in BAT from Oxtr(-/-) mice in both normal and cold temperatures. Because β3- and α2A-adrenergic receptors are known to have opposite effects on the thermoregulation, the imbalance of adrenergic receptors is suspected to affect this dysfunction in the thermoregulation. Our study is the first to demonstrate that the central OXT/OXTR system plays important roles in the regulation of body temperature homeostasis.

Oxytocin and social motivation

Humans are fundamentally social creatures who are ‘motivated’ to be with others. In this review we examine the role of oxytocin (OT) as it relates to social motivation. OT is synthesized in the brain and throughout the body, including in the heart, thymus, gastrointestinal tract, as well as reproductive organs. The distribution of the OT receptor (OTR) system in both the brain and periphery is even more far-reaching and its expression is subject to changes over the course of development. OTR expression is also sensitive to changes in the external environment and the internal somatic world. The OT system functions as an important element within a complex, developmentally sensitive biobehavioral system. Other elements include sensory inputs, the salience, reward, and threat detection pathways, the hypothalamic-pituitary-gonadal axis, and the hypothalamic-pituitary-adrenal stress response axis. Despite an ever expanding scientific literature, key unresolved questions remain concerning the interplay of the central and peripheral components of this complex biobehavioral system that dynamically engages the brain and the body as humans interact with social partners over the course of development.

Oxytocin and the neural mechanisms regulating social cognition and affiliative behavior

Oxytocin is produced in the hypothalamus and released into the circulation through the neurohypophyseal system. Peripherally released oxytocin facilitates parturition and milk ejection during nursing. Centrally released oxytocin coordinates the onset of maternal nurturing behavior at parturition and plays a role in mother-infant bonding. More recent studies have revealed a more general role for oxytocin in modulating affiliative behavior in both sexes. Oxytocin regulates alloparental care and pair bonding in female monogamous prairie voles. Social recognition in male and female mice is also modulated by oxytocin. In humans, oxytocin increases gaze to the eye region of human faces and enhances interpersonal trust and the ability to infer the emotions of others from facial cues. While the neurohypopheseal oxytocin system has been well characterized, less is known regarding the nature of oxytocin release within the brain. Here we review the role of oxytocin in the regulation of prosocial interactions, and discuss the neuroanatomy of the central oxytocin system.

Molecular dynamics simulation of human neurohypophyseal hormone receptors complexed with oxytocin-modeling of an activated state

The neurohypophyseal hormone oxytocin (CYIQNCPLG-NH(2), OT) is involved in the control of labor, secretion of milk and many social and behavioral functions via interaction with its receptors (OTR) located in the uterus, mammary glands and peripheral tissues, respectively. In this paper we propose the interactions responsible for OT binding and selectivity to OTR versus vasopressin ([F3,R8]OT, AVP) receptors: V1aR and V2R, all three belonging to the Class A G protein-coupled receptors (GPCRs). Three-dimensional models of the activated receptors were constructed using a multiple sequence alignment and the activated rhodopsin-transducin (MII-Gt) prototype [Slusarz and Ciarkowski, 2004] as a template. The 1 ns unconstrained molecular dynamics (MD) of three pairs of receptor-OT complexes (two complexes per each receptor) immersed in the fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) lipid bilayer was conducted in the AMBER 7.0 force field. The relaxed models of ligand-receptor complexes were used to identify the putative binding sites of OT. The stabilizing interactions with conserved Gln residues in all complexes were identified. The nonconserved hydrophobic residues were proposed as responsible for OTR-OT selectivity and ligand recognition. These results provide guidelines for experimental site-directed mutagenesis and if confirmed, they may be helpful in designing new selective OT analogs with both agonistic or antagonistic properties.

Pervasive social deficits, but normal parturition, in oxytocin receptor-deficient mice

The oxytocin receptor (OXTR) and its ligand, oxytocin (OXT), regulate reproductive physiology (i.e., parturition and lactation) and sociosexual behaviors. To define the essential functions of OXTR, we generated mice with a null mutation in the Oxtr gene (Oxtr(-/-)) and compared them with OXT-deficient (Oxt(-/-)) mice. Oxtr(-/-) mice were viable and had no obvious deficits in fertility or reproductive behavior. Oxtr(-/-) dams exhibited normal parturition but demonstrated defects in lactation and maternal nurturing. Infant Oxtr(-/-) males emitted fewer ultrasonic vocalizations than wild-type littermates in response to social isolation. Adult Oxtr(-/-) males also showed deficits in social discrimination and elevated aggressive behavior. Ligand Oxt(-/-) males from Oxt(-/-) dams, but not from Oxt(+/-) dams, showed similar high levels of aggression. These data suggest a developmental role for the OXT/OXTR system in shaping adult aggressive behavior. Our studies demonstrate that OXTR plays a critical role in regulating several aspects of social behavior and may have important implications for developmental psychiatric disorders characterized by deficits in social behavior.

Differential expression of mesotocin receptors in the uterus and ovary of the pregnant tammar wallaby

Mesotocin, an oxytocin-like peptide, is released in highest concentrations during parturition in macropodid marsupials. In late pregnant wallabies, uterine sensitivity to mesotocin increases markedly in the myometrium of the gravid uterus. This coincides with a significant increase in myometrial mesotocin receptor concentrations 3–4 days before term. To date, there is no information on mesotocin receptor gene expression in female wallaby reproductive tissues. This study aimed to examine mesotocin receptor gene expression in the uterus and ovaries of pregnant tammar wallabies, and to localise mesotocin receptors within the uterus. An RT-PCR strategy produced a consensus nucleotide sequence of 834 bp, which encoded 278 amino acids of transmembrane domains I to VI. This protein sequence has approximately 80% homology with the bovine and rat oxytocin receptor exon 2 region. Only one mesotocin receptor was detected in the tammar genome. The myometrium and mammary gland both expressed a 4.1 kb mesotocin receptor gene transcript. Myometrial mesotocin receptor gene expression increased on day 22 of the 26-day gestation and was significantly higher in the gravid than the non-gravid uterus in late pregnancy. This pattern of mesotocin receptor gene expression paralleled mesotocin receptor concentrations. Mesotocin binding sites were localised only to the myometrium, the highest densities being observed in the gravid uterus. Finally, this study showed high expression of mesotocin receptors in the corpus luteum. The pattern of luteal mesotocin receptor expression differed from the myometrium, with a decrease in mesotocin receptors occurring on the day of expected births.

The differential coupling of oxytocin receptors to uterine contractions in murine estrous cycle

Oxytocin (OT)-induced uterine contractility in different ovulatory phases has not been characterized in mouse. While the uterus was refractory to OT at diestrous phase, numerous contractions were observed at estrous phase. The magnitude of uterine contractile response to OT at estrous phase was larger than that at diestrous phase. There was also significant difference in the pD(2) values for OT between the samples from the two phases. However, the level of OT receptor (OTR) transcripts and proteins was equivalent at the two phases. These results suggest that OT-induced uterine contraction is not only regulated by the quantity of OTR in the non-pregnant mouse uterus.

The oxytocin receptor system: structure, function, and regulation

The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.