Cloning and expression of the rhesus monkey oxytocin receptor

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

The neuroanatomical distribution of oxytocin receptor binding and mRNA in the male rhesus macaque (Macaca mulatta)

The rhesus macaque (Macaca mulatta) is an important primate model for social cognition, and recent studies have begun to explore the impact of oxytocin on social cognition and behavior. Macaques have great potential for elucidating the neural mechanisms by which oxytocin modulates social cognition, which has implications for oxytocin-based pharmacotherapies for psychiatric disorders such as autism and schizophrenia. Previous attempts to localize oxytocin receptors (OXTR) in the rhesus macaque brain have failed due to reduced selectivity of radioligands, which in primates bind to both OXTR and the structurally similar vasopressin 1a receptor (AVPR1A). We have developed a pharmacologically-informed competitive binding autoradiography protocol that selectively reveals OXTR and AVPR1A binding sites in primate brain sections. Using this protocol, we describe the neuroanatomical distribution of OXTR in the macaque. Finally, we use in situ hybridization to localize OXTR mRNA. Our results demonstrate that OXTR expression in the macaque brain is much more restricted than AVPR1A. OXTR is largely limited to the nucleus basalis of Meynert, pedunculopontine tegmental nucleus, the superficial gray layer of the superior colliculus, the trapezoid body, and the ventromedial hypothalamus. These regions are involved in a variety of functions relevant to social cognition, including modulating visual attention, processing auditory and multimodal sensory stimuli, and controlling orienting responses to visual stimuli. These results provide insights into the neural mechanisms by which oxytocin modulates social cognition and behavior in this species, which, like humans, uses vision and audition as the primary modalities for social communication.

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.

Neuroendocrine control of body fluid metabolism

Mammals control the volume and osmolality of their body fluids from stimuli that arise from both the intracellular and extracellular fluid compartments. These stimuli are sensed by two kinds of receptors: osmoreceptor-Na+ receptors and volume or pressure receptors. This information is conveyed to specific areas of the central nervous system responsible for an integrated response, which depends on the integrity of the anteroventral region of the third ventricle, e.g., organum vasculosum of the lamina terminalis, median preoptic nucleus, and subfornical organ. The hypothalamo-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting vasopressin and oxytocin in response to osmotic and nonosmotic stimuli. Since the discovery of the atrial natriuretic peptide (ANP), a large number of publications have demonstrated that this peptide provides a potent defense mechanism against volume overload in mammals, including humans. ANP is mostly localized in the heart, but ANP and its receptor are also found in hypothalamic and brain stem areas involved in body fluid volume and blood pressure regulation. Blood volume expansion acts not only directly on the heart, by stretch of atrial myocytes to increase the release of ANP, but also on the brain ANPergic neurons through afferent inputs from baroreceptors. Angiotensin II also plays an important role in the regulation of body fluids, being a potent inducer of thirst and, in general, antagonizes the actions of ANP. This review emphasizes the role played by brain ANP and its interaction with neurohypophysial hormones in the control of body fluid homeostasis.

A nonpeptide oxytocin receptor antagonist radioligand highly selective for human receptors

A novel, potent nonpeptide oxytocin receptor antagonist (1-(1-(2-(2,2,2-trifluoroethoxy)-4-(1-methylsulfonyl-4-piperidinyloxy) phenylacetyl)-4-piperidinyl)-3,4-dihydro-2(1H)-quinolinone) has been identified that can be labeled to high specific activity with [35S]. In binding studies, this compound exhibits sub-nanomolar affinity and a high degree of selectivity (900-1800-fold) for human oxytocin receptors compared to human vasopressin receptors. This compound appears suitable for studying the pharmacology of oxytocin receptors in human and nonhuman primate tissues, for which there is currently a paucity of highly selective tools. It may also be useful as a nonlabeled competitor or as a radioligand in autoradiographic studies of oxytocin receptor localization in these tissues.

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.

Receptor blockers - general aspects with respect to their use in domestic animal reproduction

Receptor blockers compete with the respective agonist for binding to a given receptor without inducing complete signal transduction. In recent years, major interest has focused on sex-steroid hormone receptor blockers (antagonists). Indications have been obtained that inadequate changes in receptor conformation and subsequent failure of transcriptional activation are major events preventing hormonal activity. However, various subtypes and variants of receptors and receptor mutations have also been identified. Expression of antihormonal effects may vary depending on the type of receptor the blocker is bound to. Hence, receptor blockers may also have an inherent agonistic activity. Aglepristone is the first antiprogestin registered for veterinary use with the indication "interruption or prevention of pregnancy"; similarly, these types of compounds were successfully used for induction of parturition in the dog and cat and for conservative treatment of pyometra in the dog. Moreover, application of antiprogestins has clearly demonstrated the role of progesterone as a major factor controlling overt pseudopregnancy in dogs. With respect to farm animals, parturition was induced in cows without an increased incidence of retained fetal membranes. Other than antiprogestins, antioestrogens and antiandrogens are still in a more experimental phase. In particular for use in humans, high-affinity blockers binding to the oxytocin/vasopressin receptor are in development; they exert distinct tocolytic activities. Also, the release of GnRH can be inhibited by respective antagonists; however, their use in reproduction is still hampered by the high dose requirement and the side effects observed.