Separate agonist and peptide antagonist binding sites of the oxytocin receptor defined by their transfer into the V2 vasopressin receptor

Near-infrared nanosensors enable optical imaging of oxytocin with selectivity over vasopressin in acute mouse brain slices

Oxytocin plays a critical role in regulating social behaviors, yet our understanding of its function in both neurological health and disease remains incomplete. Real-time oxytocin imaging probes with spatiotemporal resolution relevant to its endogenous signaling are required to fully elucidate oxytocin's role in the brain. Herein, we describe a near-infrared oxytocin nanosensor (nIROXT), a synthetic probe capable of imaging oxytocin in the brain without interference from its structural analogue, vasopressin. nIROXT leverages the inherent tissue-transparent fluorescence of single-walled carbon nanotubes (SWCNT) and the molecular recognition capacity of an oxytocin receptor peptide fragment to selectively and reversibly image oxytocin. We employ these nanosensors to monitor electrically stimulated oxytocin release in brain tissue, revealing oxytocin release sites with a median size of 3 µm in the paraventricular nucleus of C57BL/6 mice, which putatively represents the spatial diffusion of oxytocin from its point of release. These data demonstrate that covalent SWCNT constructs, such as nIROXT, are powerful optical tools that can be leveraged to measure neuropeptide release in brain tissue.

Oxytocin receptors in the prefrontal cortex play important roles in short-term social recognition in mice

We examined the roles of oxytocin (OT) receptors in the prefrontal cortex (PFC) in short- and long-term social recognition and anxiety-related behaviors in mice. Mice injected with high or low doses of an OT receptor antagonist (OTA) or vehicle performed the social recognition test, the open-field test, and the light-dark transition test. In the social recognition test, with three daily trials over three consecutive days, control mice showed short-term recognition of a conspecific on all three days. In contrast, a high-dose injection of OTA impaired short-term social recognition on the second and third days, and it was impaired by a low-dose injection of OTA on the third day. These results suggested that OTA injection into the PFC dose-dependently inhibited short-term social recognition within each day. All three groups did not show any long-term social recognition across three days. OTA injection did not affect anxiety related behavior in the open-field and light-dark transition tests. Our findings demonstrated that OT receptors in the PFC played important roles in short-term social recognition.

Lack of evidence for coevolution between oxytocin receptor N-terminal variants and monogamy in placental mammals

Oxytocin (OXT) is a neurohypophyseal hormone that influences a wide range of affiliative behaviors, such as pair-bonding and infant care, across mammals. The effects of OXT depend significantly on an adequate interaction with its receptor, OXTR. OXTR belongs to the G-protein coupled receptor family. The extracellular N-terminal domain of OXTR interacts with the linear C-terminal tail of OXT and is required for OXT binding. Across mammalian species there is a genetic diversity in OXTR terminal sequence. Previous work on primates has shown an association between OXTR phylogeny and monogamy. However, it is not clear whether this variation coevolved with either mating system (monogamy) or infant care behaviors (such as allomaternal care). Here, we take a phylogenetic comparative and evolutionary modeling approach across a wide range of placental mammals (n = 60) to test whether OXTR N-terminal variants co-evolved with either monogamy or allomaternal care behaviors. Our results indicate that the diversity in OXTR N-terminal region is unlikely to provide the underlying genetic bases for variation in mating system and/or allomaternal behavior as we find no evidence for co-evolution between protein sequence and affiliative behaviors. Hence, the role played by OXT in influencing affiliative behaviors is unlikely to be mediated by the genetic diversity of its receptor.

Circuits underlying social function and dysfunction

Substantial advances have been made toward understanding the genetic and environmental risk factors for autism, a neurodevelopmental disorder with social impairment as a core feature. In combination with optogenetic and chemogenetic tools to manipulate neural circuits in vivo, it is now possible to use model systems to test how specific neural circuits underlie social function and dysfunction. Here, we review the literature that has identified circuits associated with social interest (sociability), social reward, social memory, dominance, and aggression, and we outline a preliminary roadmap of the neural circuits driving these social behaviors. We highlight the neural circuitry underlying each behavioral domain, as well as develop an interactive map of how these circuits overlap across domains. We find that some of the circuits underlying social behavior are general and are involved in the control of multiple behavioral aspects, whereas other circuits appear to be specialized for specific aspects of social behavior. Our overlapping circuit map therefore helps to delineate the circuits involved in the various domains of social behavior and to identify gaps in knowledge.

A genetically encoded sensor measures temporal oxytocin release from different neuronal compartments

Oxytocin (OT), a peptide hormone and neuromodulator, is involved in diverse physiological and pathophysiological processes in the central nervous system and the periphery. However, the regulation and functional sequences of spatial OT release in the brain remain poorly understood. We describe a genetically encoded G-protein-coupled receptor activation-based (GRAB) OT sensor called GRABOT1.0. In contrast to previous methods, GRABOT1.0 enables imaging of OT release ex vivo and in vivo with suitable sensitivity, specificity and spatiotemporal resolution. Using this sensor, we visualize stimulation-induced OT release from specific neuronal compartments in mouse brain slices and discover that N-type calcium channels predominantly mediate axonal OT release, whereas L-type calcium channels mediate somatodendritic OT release. We identify differences in the fusion machinery of OT release for axon terminals versus somata and dendrites. Finally, we measure OT dynamics in various brain regions in mice during male courtship behavior. Thus, GRABOT1.0 provides insights into the role of compartmental OT release in physiological and behavioral functions.

Oxytocin and vasopressin in the hippocampus

Oxytocin (OXT) and vasopressin (AVP) are related neuropeptides that exert a wide range of effects on general health, homeostasis, development, reproduction, adaptability, cognition, social and nonsocial behaviors. The two peptides are mainly of hypothalamic origin and execute their peripheral and central physiological roles via OXT and AVP receptors, which are members of the G protein-coupled receptor family. These receptors, largely distributed in the body, are abundantly expressed in the hippocampus, a brain region particularly vulnerable to stress exposure and various lesions. OXT and AVP have important roles in the hippocampus, by modulating important processes like neuronal excitability, network oscillatory activity, synaptic plasticity, and social recognition memory. This chapter includes an overview regarding OXT and AVP structure, synthesis, receptor distribution, and functions, focusing on their relationship with the hippocampus and mechanisms by which they influence hippocampal activity. Brief information regarding hippocampal structure and susceptibility to lesions is also provided. The roles of OXT and AVP in neurodevelopment and adult central nervous system function and disorders are highlighted, discussing their potential use as targeted therapeutic tools in neuropsychiatric diseases.

Structural and Functional Diversity of Animal Toxins Interacting With GPCRs

Peptide toxins from venoms have undergone a long evolutionary process allowing host defense or prey capture and making them highly selective and potent for their target. This has resulted in the emergence of a large panel of toxins from a wide diversity of species, with varied structures and multiple associated biological functions. In this way, animal toxins constitute an inexhaustible reservoir of druggable molecules due to their interesting pharmacological properties. One of the most interesting classes of therapeutic targets is the G-protein coupled receptors (GPCRs). GPCRs represent the largest family of membrane receptors in mammals with approximately 800 different members. They are involved in almost all biological functions and are the target of almost 30% of drugs currently on the market. Given the interest of GPCRs in the therapeutic field, the study of toxins that can interact with and modulate their activity with the purpose of drug development is of particular importance. The present review focuses on toxins targeting GPCRs, including peptide-interacting receptors or aminergic receptors, with a particular focus on structural aspects and, when relevant, on potential medical applications. The toxins described here exhibit a great diversity in size, from 10 to 80 amino acids long, in disulfide bridges, from none to five, and belong to a large panel of structural scaffolds. Particular toxin structures developed here include inhibitory cystine knot (ICK), three-finger fold, and Kunitz-type toxins. We summarize current knowledge on the structural and functional diversity of toxins interacting with GPCRs, concerning first the agonist-mimicking toxins that act as endogenous agonists targeting the corresponding receptor, and second the toxins that differ structurally from natural agonists and which display agonist, antagonist, or allosteric properties.

Robust Reductions of Body Weight and Food Intake by an Oxytocin Analog in Rats

Background: Oxytocin is a hypothalamic neuropeptide that participates in the network of appetite regulation. Recently the oxytocin signaling pathway has emerged as an attractive target for treating obesity. However, the short half-life limits its development as a clinical therapeutic. Here we provide results from testing a long-lasting, potent and selective oxytocin analog ASK1476 on its efficacy to reduce food intake and body weight in comparison to the native oxytocin peptide.

Methods: ASK1476 features two specific amino acid substitutions in positions 7 and 8 combined with a short polyethylene glycol spacer. Short time dose escalation experiments testing increasing doses of 3 days each were performed in diet-induced overweight (DIO) male rats assessing effects on body weight as well as changes in food intake. Furthermore, DIO rats were tested for changes in body weight, food intake, temperature, and locomotor activity over 28 days of treatment (oxytocin, ASK1476, or vehicle).

Results: In dose escalation experiments, significant reductions in food intake relative to baseline were detected beginning with doses of 15 nmol/kg ASK1476 (−15.2 ± 2.3 kcal/d, p = 0.0017) and 20 nmol/kg oxytocin (−11.2.9 ± 2.4 kcal/d, p = 0.0106) with corresponding significant changes in body weight (ASK1476: −5.2 ± 0.8 g, p = 0.0016; oxytocin: −2.6 ± 0.7 g, p = 0.0326). In long-term experiments, there was no difference on body weight change between 120 nmol/kg/d ASK1476 (−71.4 ± 34.2 g, p = 0.039) and 600 nmol/kg/d oxytocin (−91.8 ± 32.2 g, p = 0.035) relative to vehicle (706.9 ± 28.3 g), indicating a stronger dose response for ASK1476. Likewise, both ASK1476 and oxytocin at these doses resulted in similar reductions in 28-day cumulative food intake (ASK1476: −562.7 ± 115.0 kcal, p = 0.0001; oxytocin: −557.1 ± 101.3 kcal, p = 0.0001) relative to vehicle treatment (2716 ± 75.4 kcal), while no effects were detected on locomotor activity or body temperature.

Conclusion: This study provides proof-of-concept data demonstrating an oxytocin analog with extended in vivo stability and improved potency to reduce food intake and body weight in DIO animals which could mark a new avenue in anti-obesity drug interventions.

Oxytocin in the Male Reproductive Tract; The Therapeutic Potential of Oxytocin-Agonists and-Antagonists

In this review, the role of oxytocin and oxytocin-like agents (acting via the oxytocin receptor and belonging to the oxytocin-family) in the male reproductive tract is considered. Previous research (dating back over 60 years) is revised and connected with recently found aspects of the role oxytocin plays in male reproductive health. The local expression of oxytocin and its receptor in the male reproductive tract of different species is summarized. Colocalization and possible crosstalk to other agents and receptors and their resulting effects are discussed. The role of the newly reported oxytocin focused signaling pathways in the male reproductive tract, other than mediating contractility, is critically examined. The structure and effect of the most promising oxytocin-agonists and -antagonists are reviewed for their potential in treating male disorders with origins in the male reproductive tract such as prostate diseases and ejaculatory disorders.

Oxytocin Receptor Signaling in Vascular Function and Stroke

The oxytocin receptor (OXTR) is a G protein-coupled receptor with a diverse repertoire of intracellular signaling pathways, which are activated in response to binding oxytocin (OXT) and a similar nonapeptide, vasopressin. This review summarizes the cell and molecular biology of the OXTR and its downstream signaling cascades, particularly focusing on the vasoactive functions of OXTR signaling in humans and animal models, as well as the clinical applications of OXTR targeting cerebrovascular accidents.

Crystal structure of the human oxytocin receptor

The peptide hormone oxytocin modulates socioemotional behavior and sexual reproduction via the centrally expressed oxytocin receptor (OTR) across several species. Here, we report the crystal structure of human OTR in complex with retosiban, a nonpeptidic antagonist developed as an oral drug for the prevention of preterm labor. Our structure reveals insights into the detailed interactions between the G protein-coupled receptor (GPCR) and an OTR-selective antagonist. The observation of an extrahelical cholesterol molecule, binding in an unexpected location between helices IV and V, provides a structural rationale for its allosteric effect and critical influence on OTR function. Furthermore, our structure in combination with experimental data allows the identification of a conserved neurohypophyseal receptor-specific coordination site for Mg²⁺ that acts as potent, positive allosteric modulator for agonist binding. Together, these results further our molecular understanding of the oxytocin/vasopressin receptor family and will facilitate structure-guided development of new therapeutics.

The role of oxytocin in regulation of appetitive behaviour, body weight and glucose homeostasis

Obesity and its associated complications have reached epidemic proportions in the USA and also worldwide, highlighting the need for new and more effective treatments. Although the neuropeptide oxytocin (OXT) is well recognised for its peripheral effects on reproductive behaviour, the release of OXT from somatodendrites and axonal terminals within the central nervous system (CNS) is also implicated in the control of energy balance. In this review, we summarise historical data highlighting the effects of exogenous OXT as a short-term regulator of food intake in a context-specific manner and the receptor populations that may mediate these effects. We also describe what is known about the physiological role of endogenous OXT in the control of energy balance and whether serum and brain levels of OXT relate to obesity on a consistent basis across animal models and humans with obesity. We describe recent data on the effectiveness of chronic CNS administration of OXT to decrease food intake and weight gain or to elicit weight loss in diet-induced obese (DIO) and genetically obese mice and rats. Of clinical importance is the finding that chronic central and peripheral OXT treatments both evoke weight loss in obese animal models with impaired leptin signalling at doses that are not associated with visceral illness, tachyphylaxis or adverse cardiovascular effects. Moreover, these results have been largely recapitulated following chronic s.c. or intranasal treatment in DIO non-human primates (rhesus monkeys) and obese humans, respectively. We also identify plausible mechanisms that contribute to the effects of OXT on body weight and glucose homeostasis in rodents, non-human primates and humans. We conclude by describing the ongoing challenges that remain before OXT-based therapeutics can be used as a long-term strategy to treat obesity in humans.

Synthesis, Pharmacological and Structural Characterization of Novel Conopressins from Conus miliaris

Cone snails produce a fast-acting and often paralyzing venom, largely dominated by disulfide-rich conotoxins targeting ion channels. Although disulfide-poor conopeptides are usually minor components of cone snail venoms, their ability to target key membrane receptors such as GPCRs make them highly valuable as drug lead compounds. From the venom gland transcriptome of Conus miliaris, we report here on the discovery and characterization of two conopressins, which are nonapeptide ligands of the vasopressin/oxytocin receptor family. These novel sequence variants show unusual features, including a charge inversion at the critical position 8, with an aspartate instead of a highly conserved lysine or arginine residue. Both the amidated and acid C-terminal analogues were synthesized, followed by pharmacological characterization on human and zebrafish receptors and structural investigation by NMR. Whereas conopressin-M1 showed weak and only partial agonist activity at hV1bR (amidated form only) and ZFV1a1R (both amidated and acid form), both conopressin-M2 analogues acted as full agonists at the ZFV2 receptor with low micromolar affinity. Together with the NMR structures of amidated conopressins-M1, -M2 and -G, this study provides novel structure-activity relationship information that may help in the design of more selective ligands.

Leu8 and Pro8 oxytocin agonism differs across human, macaque, and marmoset vasopressin 1a receptors

Oxytocin (OXT) is an important neuromodulator of social behaviors via activation of both oxytocin receptors (OXTR) and vasopressin (AVP) 1a receptors (AVPR1a). Marmosets are neotropical primates with a modified OXT ligand (Pro⁸-OXT), and this ligand shows significant coevolution with traits including social monogamy and litter size. Pro⁸-OXT produces more potent and efficacious responses at primate OXTR and stronger behavioral effects than the consensus mammalian OXT ligand (Leu⁸-OXT). Here, we tested whether OXT/AVP ligands show differential levels of crosstalk at primate AVPR1a. We measured binding affinities and Ca²⁺ signaling responses of AVP, Pro⁸-OXT and Leu⁸-OXT at human, macaque, and marmoset AVPR1a. We found that AVP binds with higher affinity than OXT across AVPR1a, and marmoset AVPR1a show a 10-fold lower OXT binding affinity compared to human and macaque AVPR1a. Both Leu⁸-OXT and Pro⁸-OXT produce a less efficacious response than AVP at human AVPR1a and higher efficacious response than AVP at marmoset AVPR1a. These data suggest that OXT might partially antagonize endogenous human AVPR1a signaling and enhance marmoset AVPR1a signaling. These findings aid in further understanding inconsistencies observed following systemic intranasal administration of OXT and provide important insights into taxon-specific differences in nonapeptide ligand/receptor coevolution and behavior.

Binding Characteristics of Two Oxytocin Variants and Vasopressin at Oxytocin Receptors from Four Primate Species with Different Social Behavior Patterns

A clade of New World monkeys (NWMs) exhibits considerable diversity in both oxytocin (OT) ligand and oxytocin receptor (OTR) structure. Most notable is the variant Pro⁸-OT, with proline instead of leucine at the eighth position, resulting in a rigid bend in the peptide backbone. A higher proportion of species that express Pro⁸-OT also engage in biparental care and social monogamy. When marmosets (genus Callithrix), a biparental and monogamous Pro⁸-OT NWM species, are administered the ancestral Leu⁸-OT, there is no change in social behavior compared with saline treatment. However, when Pro⁸-OT is administered, marmosets' sociosexual and prosocial behaviors are altered. The studies here tested the hypothesis that OTR binding affinities and OT-induced intracellular Ca²⁺ potencies would favor the native OT ligand in OTRs from four primate species, each representing a unique combination of ancestral lineage, breeding system, and native OT ligand: humans (Leu⁸-OT, monogamous, apes), macaques (Leu⁸-OT, nonmonogamous, Old World monkey), marmosets (Pro⁸-OT, monogamous, NWM), and titi monkeys (Leu⁸-OT, monogamous, NWM). OTRs were expressed in immortalized Chinese hamster ovary cells and tested for intact-cell binding affinities for Pro⁸-OT, Leu⁸-OT, and arginine vasopressin (AVP), as well as intracellular Ca²⁺ signaling after stimulation with Pro⁸-OT, Leu⁸-OT, and AVP. Contrary to our hypothesis, Pro⁸-OT bound at modestly higher affinities and stimulated calcium signaling at modestly higher potencies compared with Leu⁸-OT in all four primate OTRs. Thus, differences downstream from a ligand-receptor binding event are more likely to explain the different behavioral responses to these two ligands.

Effects of Chronic Oxytocin Administration and Diet Composition on Oxytocin and Vasopressin 1a Receptor Binding in the Rat Brain

Oxytocin (OT) elicits weight loss in diet-induced obese (DIO) rodents, nonhuman primates, and humans, in part, by reducing food intake. Chronic OT administration produces more sustained weight loss in high-fat diet (HFD)-fed DIO rodents relative to chow-fed controls, but the reasons for this effect remain unclear. We hypothesized that HFD-induced obesity is associated with elevated OT receptor (OXTR) binding in brain regions where OT is known to cause decreased food intake and that this sensitized neural system is one mechanism by which OT preferentially elicits weight loss in DIO rodents. We therefore determined the impact of diet (HFD vs chow) and drug treatment (chronic OT infusion vs vehicle) on (1) OXTR binding in hindbrain and forebrain sites where OT suppresses food intake relative to control sites that express OXTR and (2) forebrain vasopressin 1a receptor (AVPR1a) density to evaluate the specificity of any OT effects. Using quantitative receptor autoradiography, we found that (1) diet composition failed to alter OXTR or AVPR1a binding; (2) chronic OT treatment produced largely global reductions in forebrain OXTR and AVPR1a binding without significantly altering hindbrain OXTR binding. These findings suggest that forebrain OXTR and AVPR1a are down-regulated in response to chronic OT treatment. Given that chronic intranasal OT may be used as a therapeutic strategy to treat obesity, future studies should consider the potential downregulatory effect that chronic treatment can have across forebrain and hindbrain nonapeptide receptors and assess the potential contribution of both receptor subtypes to the outcome measures.

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.

Novel oxytocin receptor variants in laboring women requiring high doses of oxytocin

BACKGROUND

Although oxytocin commonly is used to augment or induce labor, it is difficult to predict its effectiveness because oxytocin dose requirements vary significantly among women. One possibility is that women requiring high or low doses of oxytocin have variations in the oxytocin receptor gene.

OBJECTIVES

To identify oxytocin receptor gene variants in laboring women with low and high oxytocin dosage requirements.

STUDY DESIGN

Term, nulliparous women requiring oxytocin doses of ≤4 mU/min (low-dose-requiring, n = 83) or ≥20 mU/min (high-dose-requiring, n = 104) for labor augmentation or induction provided consent to a postpartum blood draw as a source of genomic DNA. Targeted-amplicon sequencing (coverage >30×) with MiSeq (Illumina) was performed to discover variants in the coding exons of the oxytocin receptor gene. Baseline relevant clinical history, outcomes, demographics, and oxytocin receptor gene sequence variants and their allele frequencies were compared between low-dose-requiring and high-dose-requiring women. The Scale-Invariant Feature Transform algorithm was used to predict the effect of variants on oxytocin receptor function. The Fisher exact or χ² tests were used for categorical variables, and Student t tests or Wilcoxon rank sum tests were used for continuous variables. A P value < .05 was considered statistically significant.

RESULTS

The high-dose-requiring women had greater rates of obesity and diabetes and were more likely to have undergone labor induction and required prostaglandins. High-dose-requiring women were more likely to undergo cesarean delivery for first-stage arrest and less likely to undergo cesarean delivery for nonreassuring fetal status. Targeted sequencing of the oxytocin receptor gene in the total cohort (n = 187) revealed 30 distinct coding variants: 17 nonsynonymous, 11 synonymous, and 2 small structural variants. One novel variant (A243T) was found in both the low- and high-dose-requiring groups. Three novel variants (Y106H, A240_A249del, and P197delfs*206) resulting in an amino acid substitution, loss of 9 amino acids, and a frameshift stop mutation, respectively, were identified only in low-dose-requiring women. Nine nonsynonymous variants were unique to the high-dose-requiring group. These included 3 known variants (R151C, G221S, and W228C) and 6 novel variants (M133V, R150L, H173R, A248V, G253R, and I266V). Of these, R150L, R151C, and H173R were predicted by Scale-Invariant Feature Transform algorithm to damage oxytocin receptor function. There was no statistically significant association between the numbers of synonymous and nonsynonymous substitutions in the patient groups.

CONCLUSION

Obesity, diabetes, and labor induction were associated with the requirement for high doses of oxytocin. We did not identify significant differences in the prevalence of oxytocin receptor variants between low-dose-requiring and high-dose-requiring women, but novel oxytocin receptor variants were enriched in the high-dose-requiring women. We also found 3 oxytocin receptor variants (2 novel, 1 known) that were predicted to damage oxytocin receptor function and would likely increase an individual's risk for requiring a high oxytocin dose. Further investigation of oxytocin receptor variants and their effects on protein function will inform precision medicine in pregnant women.

A Distributed Network for Social Cognition Enriched for Oxytocin Receptors

Oxytocin is a neuropeptide important for social behaviors such as maternal care and parent-infant bonding. It is believed that oxytocin receptor signaling in the brain is critical for these behaviors, but it is unknown precisely when and where oxytocin receptors are expressed or which neural circuits are directly sensitive to oxytocin. To overcome this challenge, we generated specific antibodies to the mouse oxytocin receptor and examined receptor expression throughout the brain. We identified a distributed network of female mouse brain regions for maternal behaviors that are especially enriched for oxytocin receptors, including the piriform cortex, the left auditory cortex, and CA2 of the hippocampus. Electron microscopic analysis of the cerebral cortex revealed that oxytocin receptors were mainly expressed at synapses, as well as on axons and glial processes. Functionally, oxytocin transiently reduced synaptic inhibition in multiple brain regions and enabled long-term synaptic plasticity in the auditory cortex. Thus modulation of inhibition may be a general mechanism by which oxytocin can act throughout the brain to regulate parental behaviors and social cognition.

Evidence for a PGF2α auto-amplification system in the endometrium in mares

In mares, prostaglandin F2α (PGF2α) secreted from the endometrium is a major luteolysin. Some domestic animals have an auto-amplification system in which PGF2α can stimulate its own production. Here, we investigated whether this is also the case in mares. In an in vivo study, mares at the mid-luteal phase (days 6-8 of estrous cycle) were injected i.m. with cloprostenol (250 µg) and blood samples were collected at fixed intervals until 72 h after treatment. Progesterone (P4) concentrations started decreasing 45 min after the injection and continued to decrease up to 24 h (P < 0.05). In turn, 13,14-dihydro-15-keto-PGF2α (PGFM) metabolite started to increase 4h after an injection and continued to increase up to 72 h (P < 0.05). PGF receptor (PTGFR) mRNA expression in the endometrium was significantly higher in the late luteal phase than in the early and regressed luteal phases (P < 0.05). In vitro, PGF2α significantly stimulated (P < 0.05) PGF2α production by endometrial tissues and endometrial epithelial and stromal cells and significantly increased (P < 0.05) the mRNA expression of prostaglandin-endoperoxide synthase-2 (PTGS2), an enzyme involved in PGF2α synthesis in endometrial cell. These findings strongly suggest the existence of an endometrial PGF2α auto-amplification system in mares.

Oxytocin and vasopressin: linking pituitary neuropeptides and their receptors to social neurocircuits

Oxytocin and vasopressin are pituitary neuropeptides that have been shown to affect social processes in mammals. There is growing interest in these molecules and their receptors as potential precipitants of, and/or treatments for, social deficits in neurodevelopmental disorders, including autism spectrum disorder. Numerous behavioral-genetic studies suggest that there is an association between these peptides and individual social abilities; however, an explanatory model that links hormonal activity at the receptor level to complex human behavior remains elusive. The following review summarizes the known associations between the oxytocin and vasopressin neuropeptide systems and social neurocircuits in the brain. Following a micro- to macro- level trajectory, current literature on the synthesis and secretion of these peptides, and the structure, function and distribution of their respective receptors is first surveyed. Next, current models regarding the mechanism of action of these peptides on microcircuitry and other neurotransmitter systems are discussed. Functional neuroimaging evidence on the acute effects of exogenous administration of these peptides on brain activity is then reviewed. Overall, a model in which the local neuromodulatory effects of pituitary neuropeptides on brainstem and basal forebrain regions strengthen signaling within social neurocircuits proves appealing. However, these findings are derived from animal models; more research is needed to clarify the relevance of these mechanisms to human behavior and treatment of social deficits in neuropsychiatric disorders.

Molecular mechanisms underlying oxytocin-induced cardiomyocyte protection from simulated ischemia-reperfusion

Oxytocin (OT) stimulates cardioprotection. Here we investigated heart-derived H9c2 cells in simulated ischemia-reperfusion (I-R) experiments in order to examine the mechanism of OT protection. I-R was induced in an anoxic chamber for 2 hours and followed by 2 h of reperfusion. In comparison to normoxia, I-R resulted in decrease of formazan production by H9c2 cells to 63.5 ± 1.7% (MTT assay) and in enhanced apoptosis from 1.7 ± 0.3% to 2.8 ± 0.4% (Tunel test). Using these assays it was observed that treatment with OT (1-500 nM) exerted significant protection during I-R, especially when OT was added at the time of ischemia or reperfusion. Using the CM-H2DCFDA probe we found that OT triggers a short-lived burst in reactive oxygen species (ROS) production in cells but reduces ROS production evoked by I-R. In cells treated with OT, Western-blot revealed the phosphorylation of Akt (Thr 308, p-Akt), eNOS and ERK 1/2. Microscopy showed translocation of p-Akt and eNOS into the nuclear and perinuclear area and NO production in cells treated with OT. The OT-induced protection against I-R was abrogated by an OT antagonist, the Pi3K inhibitor Wortmannin, the cGMP-dependent protein kinase (PKG) inhibitor, KT5823, as well as soluble guanylate cyclase (GC) inhibitor, ODQ, and particulate GC antagonist, A71915. In conditions of I-R, the cells with siRNA-mediated reduction in OT receptor (OTR) expression responded to OT treatment by enhanced apoptosis. In conclusion, the OTR protected H9c2 cells against I-R, especially if activated at the onset of ischemia or reperfusion. The OTR-transduced signals include pro-survival kinases, such as Akt and PKG.

Arginine-, D-arginine-vasopressin, and their inverso analogues in micellar and liposomic models of cell membrane: CD, NMR, and molecular dynamics studies

We describe the synthesis, pharmacological properties, and structures of antidiuretic agonists, arginine vasopressin (AVP) and [d-Arg⁸]-vasopressin (DAVP), and their inverso analogues. The structures of the peptides are studied based on micellar and liposomic models of cell membranes using CD spectroscopy. Additionally, three-dimensional structures in mixed anionic–zwitterionic micelles are obtained using NMR spectroscopy and molecular dynamics simulations. NMR data have shown that AVP and DAVP tend to adopt typical of vasopressin-like peptides β-turns: in the 2–5 and 3–6 fragments. The inverso-analogues also adopt β-turns in the 3–6 fragments. For this reason, their inactivity seems to be due to the difference in side chains orientations of Tyr², Phe³, and Arg⁸, important for interactions with the receptors. Again, the potent antidiuretic activity of DAVP can be explained by CD data suggesting differences in mutual arrangement of the aromatic side chains of Tyr² and Phe³ in this peptide in liposomes rather than of native AVP. In the presence of liposomes, the smallest conformational changes of the peptides are noticed with DPPC and the largest with DPPG liposomes. This suggests that electrostatic interactions are crucial for the peptide–membrane interactions. We obtained similar, probably active, conformations of the antidiuretic agonists in the mixed DPC/SDS micelles (5:1) and in the mixed DPPC/DPPG (7:3) liposomes. Thus it can be speculated that the anionic–zwitterionic liposomes as well as the anionic–zwitterionic micelles, mimicking the eukaryotic cell membrane environment, partially restrict conformational freedom of the peptides and probably induce conformations resembling those of biologically relevant ones.

Genetic diversity in oxytocin ligands and receptors in New World monkeys

Oxytocin (OXT) is an important neurohypophyseal hormone that influences wide spectrum of reproductive and social processes. Eutherian mammals possess a highly conserved sequence of OXT (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly). However, in this study, we sequenced the coding region for OXT in 22 species covering all New World monkeys (NWM) genera and clades, and characterize five OXT variants, including consensus mammalian Leu⁸-OXT, major variant Pro⁸-OXT, and three previously unreported variants: Ala⁸-OXT, Thr⁸-OXT, and Phe²-OXT. Pro⁸-OXT shows clear structural and physicochemical differences from Leu⁸-OXT. We report multiple predicted amino acid substitutions in the G protein-coupled OXT receptor (OXTR), especially in the critical N-terminus, which is crucial for OXT recognition and binding. Genera with same Pro⁸-OXT tend to cluster together on a phylogenetic tree based on OXTR sequence, and we demonstrate significant coevolution between OXT and OXTR. NWM species are characterized by high incidence of social monogamy, and we document an association between OXTR phylogeny and social monogamy. Our results demonstrate remarkable genetic diversity in the NWM OXT/OXTR system, which can provide a foundation for molecular, pharmacological, and behavioral studies of the role of OXT signaling in regulating complex social phenotypes.

Conformations of a model cyclic hexapeptide, CYIQNC: (1)H-NMR and molecular dynamics studies

Solution conformation of the cyclic hexapeptide sequence, [cyclo-S-Cys-Tyr-Ile-Gln-Asn-Cys-S] (CYIQNC) - a disulfide-linked fragment of a neurohypophyseal peptide hormone oxytocin (OT) - has been investigated by high-field one-dimensional (1D) and two-dimensional (2D) NMR spectroscopic methods and compared with the results obtained from computer simulation studies. (1)H-NMR results based on temperature dependence of amide proton chemical shifts and nuclear Overhauser effect indicate that peptide in solution populates different conformations, characterized by two fused β-turns. The segment Ile(3)-Gln(4)-Asn(5)-Cys(6) yields a preferred type-III β-turn at residues 4, 5 (HB, 3HN → 6CO), while the segment Cys(6), Cys(1)-Tyr(2)-Ile(3) exhibits inherently weaker, flexible β-turn either of type I/II'/III/half-turn at residues 1, 2 (HB, 6HN → 3CO). The computer simulation studies using a mixed protocol of distance geometry-simulated annealing followed by constrained minimization, restrained molecular dynamics, and energy minimization showed the possibility of existence of additional conformations with the hydrogen bonds, (a) 5HN → 3CO and (b) 2HN → 6CO. These results, therefore, indicate that the additional conformations obtained from both NMR and simulation studies can also be possible to the peptide. These additional conformations might have very small population in the solution and did not show their signatures in these conditions. These findings will be helpful in designing more analogs with modifications in the cyclic moiety of OT.

Thermodynamics of the interaction between oxytocin and its myometrial receptor in sheep: a stepwise binding mechanism

Entropy (ΔS), enthalpy (ΔH) and heat capacity (ΔCp) changes attending the oxytocin interaction with its two binding sites on myometrial cell membranes in sheep were derived from the temperature dependence of Kd values. The high affinity oxytocin site (Kd on the order of 10(-9)mol l(-1), 25 °C), ascribed to the oxytocin receptor (OXTR), is entropy-driven in the temperature range 0-37 °C. Enthalpy component prevails as a driving force in the binding to the low affinity site (Kd ≈ 10(-7)) within the higher temperature range. ΔCp values in both cases do not differ significantly from zero but become highly relevant in the presence of a GTP analog (10(-4)M GTP-γS). Under these conditions, ΔCp in the low site interaction becomes negative and ΔS is shifted toward negative values (enthalpy drift); ΔCp of the high affinity site rises to a high positive value and the interaction is even more strongly entropy driven. Atosiban, a competitive antagonist of oxytocin at OXTR displays a single significant binding site on myometrial cells (Kd about 10(-7)mol l(-1)). Thermodynamic profiles of atosiban and the low affinity oxytocin site show conspicuous similarities, indicating that the inhibitor is bound to the low affinity site, and not, with a lower affinity, to the putative receptor protein. It is suggested that the interaction of oxytocin with its responding system on myometrial membranes follows in two distinct steps that are likely to be associated with several independent binding domains in the GPCR receptor.

Sequence variants in oxytocin pathway genes and preterm birth: a candidate gene association study

Background

Preterm birth (PTB) is a complex disorder associated with significant neonatal mortality and morbidity and long-term adverse health consequences. Multiple lines of evidence suggest that genetic factors play an important role in its etiology. This study was designed to identify genetic variation associated with PTB in oxytocin pathway genes whose role in parturition is well known.

Methods

To identify common genetic variants predisposing to PTB, we genotyped 16 single nucleotide polymorphisms (SNPs) in the oxytocin (OXT), oxytocin receptor (OXTR), and leucyl/cystinyl aminopeptidase (LNPEP) genes in 651 case infants from the U.S. and one or both of their parents. In addition, we examined the role of rare genetic variation in susceptibility to PTB by conducting direct sequence analysis of OXTR in 1394 cases and 1112 controls from the U.S., Argentina, Denmark, and Finland. This study was further extended to maternal triads (maternal grandparents-mother of a case infant, N=309). We also performed in vitro analysis of selected rare OXTR missense variants to evaluate their functional importance.

Results

Maternal genetic effect analysis of the SNP genotype data revealed four SNPs in LNPEP that show significant association with prematurity. In our case–control sequence analysis, we detected fourteen coding variants in exon 3 of OXTR, all but four of which were found in cases only. Of the fourteen variants, three were previously unreported novel rare variants. When the sequence data from the maternal triads were analyzed using the transmission disequilibrium test, two common missense SNPs (rs4686302 and rs237902) in OXTR showed suggestive association for three gestational age subgroups. In vitro functional assays showed a significant difference in ligand binding between wild-type and two mutant receptors.

Conclusions

Our study suggests an association between maternal common polymorphisms in LNPEP and susceptibility to PTB. Maternal OXTR missense SNPs rs4686302 and rs237902 may have gestational age-dependent effects on prematurity. Most of the OXTR rare variants identified do not appear to significantly contribute to the risk of PTB, but those shown to affect receptor function in our in vitro study warrant further investigation. Future studies with larger sample sizes are needed to confirm the findings of this study.

Molecular evolution of the neuropeptide S receptor

The neuropeptide S receptor (NPSR) is a recently deorphanized member of the G protein-coupled receptor (GPCR) superfamily and is activated by the neuropeptide S (NPS). NPSR and NPS are widely expressed in central nervous system and are known to have crucial roles in asthma pathogenesis, locomotor activity, wakefulness, anxiety and food intake. The NPS-NPSR system was previously thought to have first evolved in the tetrapods. Here we examine the origin and the molecular evolution of the NPSR using in-silico comparative analyses and document the molecular basis of divergence of the NPSR from its closest vertebrate paralogs. In this study, NPSR-like sequences have been identified in a hemichordate and a cephalochordate, suggesting an earlier emergence of a NPSR-like sequence in the metazoan lineage. Phylogenetic analyses revealed that the NPSR is most closely related to the invertebrate cardioacceleratory peptide receptor (CCAPR) and the group of vasopressin-like receptors. Gene structure features were congruent with the phylogenetic clustering and supported the orthology of NPSR to the invertebrate NPSR-like and CCAPR. A site-specific analysis between the vertebrate NPSR and the well studied paralogous vasopressin-like receptor subtypes revealed several putative amino acid sites that may account for the observed functional divergence between them. The data can facilitate experimental studies aiming at deciphering the common features as well as those related to ligand binding and signal transduction processes specific to the NPSR.

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-Gly-Lys-Arg: a novel cardiomyogenic peptide

Background

Oxytocin (OT), synthesized in the heart, has the ability to heal injured hearts and to promote cardiomyogenesis from stem cells. Recently, we reported that the OT-GKR molecule, a processing intermediate of OT, potently increased the spontaneous formation of cardiomyocytes (CM) in embryonic stem D3 cells and augmented glucose uptake in newborn rat CM above the level stimulated by OT. In the present experiments, we investigated whether OT-GKR exists in fetal and newborn rodent hearts, interacts with the OT receptors (OTR) and primes the generation of contracting cells expressing CM markers in P19 cells, a model for the study of early heart differentiation.

Methodology/Principal Findings

High performance liquid chromatography of newborn rat heart extracts indicated that OT-GKR was a dominant form of OT. Immunocytochemistry of mouse embryos (embryonic day 15) showed cardiac OT-GKR accumulation and OTR expression. Computerized molecular modeling revealed OT-GKR docking to active OTR sites and to V1a receptor of vasopressin. In embryonic P19 cells, OT-GKR induced contracting cell colonies and ventricular CM markers more potently than OT, an effect being suppressed by OT antagonists and OTR-specific small interfering (si) RNA. The V1a receptor antagonist and specific si-RNA also significantly reduced OT-GKR-stimulated P19 contracting cells. In comparison to OT, OT-GKR induced in P19 cells less α-actinin, myogenin and MyoD mRNA, skeletal muscle markers.

Conclusions/Significance

These results raise the possibility that C-terminally extended OT molecules stimulate CM differentiation and contribute to heart growth during fetal life.

Oxytocin as feeding inhibitor: maintaining homeostasis in consummatory behavior

Initial studies showed that the anorexigenic peptide oxytocin (OT) regulates gastric motility, responds to stomach distention and to elevated osmolality, and blocks consumption of toxic foods. Most recently, it has been proposed to act as a mediator of general and carbohydrate-specific satiety and regulator of body weight. In the current review, we discuss the function of OT as a homeostatic inhibitor of consumption, capable of mitigating multiple aspects of ingestive behavior and energy metabolism.

Fluorescent agonists and antagonists for vasopressin/oxytocin G protein-coupled receptors: usefulness in ligand screening assays and receptor studies

Different series of fluorescent agonists and antagonists have been developed and characterized for arginine-vasopressin and oxytocin G protein-coupled receptors. Both cyclic and linear peptide analogs of the neurohypophysial hormones are useful tools for investigating receptor localization and trafficking, analysing receptor structural organization, and developing new receptor-selective high-throughput ligand screening assays.

Extracellular loop 3 (ECL3) and ECL3-proximal transmembrane domains VI and VII of the mesotocin and vasotocin receptors confer differential ligand selectivity and signaling activity

Mesotocin (MT) and vasotocin (VT) are the nonmammalian orthologs of mammalian oxytocin (OT) and arginine vasopressin (AVP), respectively. The OT/AVP family of peptides has arisen from gene duplication but has evolved to possess high selectivity toward their cognate receptors. The process of molecular evolution of receptors to confer high selectivity to their cognate ligands, however, is poorly understood. We constructed a series of reciprocal chimeras using a pair of bullfrog MT receptor (MTR) and VT1 receptor (VT1R) DNA fragments. Among the MTR/VT1R chimeras, the MTR chimera containing a region from transmembrane domain (TMD) VI to the carboxyl-terminal tail (C-tail) of VT1R showed an increased sensitivity to VT, while a chimeric VT1R containing TMD VI to C-tail of MTR showed an increased sensitivity to MT. Further dissection of domains using additional chimeras demonstrated that the receptor with the fragment containing extracellular loop 3 (ECL3) and ECL3-proximal TMDs VI and VII of MTR increased MT selectivity. This fragment is also important for receptor conformation that permits the signaling ability of the receptor. Particularly, the amino acids Val/Ile(6.54) in TMD VI and Pro/Glu(7.29) in ECL3 appear to be involved in this activity, since double mutation of these amino acids completely blocked signaling activity while maintaining ligand binding activity. Mutations at these residues in human OT and AVP 1a receptors markedly decreased receptor signaling activity. This study provides clues for understanding molecular coevolution of the OT/AVP peptides and their receptors with regard to receptor-ligand binding and receptor signaling activity.

Modeling the human oxytocin receptor for drug discovery efforts

The oxytocin receptor belongs to class A receptors within the great family of G protein-coupled receptors. The endogenous ligand oxytocin is a nonapeptide hormone that induces the uterine contractions at parturition and is used to induce the labor. The peptide oxytocin and, even more, its non-peptide antagonist, could be valuable tools in tocolysis. The knowledge of the three-dimensional structure of the oxytocin receptor and the determination of the main interaction points between the receptor and the ligands may help to develop selective oxytocin agonists and antagonist. This review summarizes the knowledge about the mapping of the binding domain of the oxytocin receptor and the efforts in the field of molecular modeling studies related to oxytocin receptor-ligand interactions.

Agonist selectivity in the oxytocin/vasopressin receptor family: new insights and challenges

The design and development of selective agonists acting at the OT (oxytocin)/AVP (vasopressin) receptors has been and continues to be a difficult task because of the great similarity among the different receptor subtypes as well as the high degree of chemical similarity between the active ligands. In recent decades, at least a thousand synthetic peptides have been synthesized and examined for their ability to bind to and activate the different OT/AVP receptors; an effort that has led to the identification of several receptor subtype-selective agonists in the rat. However, owing to species differences between rat and human AVP/OT receptors, these peptides do not exhibit the same selectivities in human receptor assays. Furthermore, the discovery of receptor promiscuity, which is the ability of a single receptor subtype to couple to several different G-proteins, has led to the definition of a completely new class of compounds, referred to here as coupling-selective ligands, which may activate, within a single receptor subtype, only a specific signalling pathway. Finally, the accumulating evidence that GPCRs (G-protein-coupled receptors) do not function as monomers, but as dimers/oligomers, opens up the design of another class of specific ligands, bivalent ligands, in which two agonist and/or antagonist moieties are joined by a spacer of the appropriate length to allow the simultaneous binding at the two subunits within the dimer. The pharmacological properties and selectivity profiles of these bivalent ligands, which remain to be investigated, could lead to highly novel research tools and potential therapeutic agents.

Receptor-based QSAR studies of non-peptide human oxytocin receptor antagonists

In the present study, QSAR calculations were performed on the receptor-based alignment of 58 non-peptide human oxytocin receptor antagonists. With the aid of different scoring functions (AutoDock 3.05 built-in and X-Score 1.2) the evolved receptor-ligand complexes were characterized. By means of various datasets it was confirmed that the scoring functions were not capable to predict the biological activity correctly in compounds containing a rigid derivative in the variable region. To improve the pKi prediction 3D-QSAR calculation was performed. The regions related to the biological activity were determined by using cross-validated r2(q2)-guided region selection (q2-GRS) method. The predictive power of the CoMFA model [r(pred)2=0.89, q2(LMO, five groups)=0.695+/-0.034] allowed prediction of the biological activities of newly synthesized compounds and confirmed the receptor-based alignment.

Molecular docking-based study of vasopressin analogues modified at positions 2 and 3 with N-methylphenylalanine: influence on receptor-bound conformations and interactions with vasopressin and oxytocin receptors

In this study, four cyclic vasopressin (CYFQNCPRG-NH(2), AVP) analogues substituted at positions 2 and 3 with four combinations of enantiomers of N-methylphenylalanine have been investigated. Three-dimensional structures of analogues have been formerly determined using NMR spectroscopy in dimethyl sulfoxide. Three-dimensional models of the vasopressin and oxytocin receptors were constructed by combining the multiple sequence alignment and the RD crystal structure as a template. The analogues have been docked into the receptor using the AutoDock program. The relaxation of the receptor-ligand complexes using energy minimization, followed by the constrained simulated annealing protocols (CSA), has been performed. The receptor-bound conformations of the investigated analogues have been proposed. We concluded that the N-methylated residues at positions 2 and 3 act as a structural restraint, determining the conformation of analogues, their location inside the receptor cavity, and mutual arrangement of the aromatic side chains. The conserved polar residues constitute the handles keeping the biologically active analogues inside the binding cavity. The Arg(8)-D(2.65) salt bridge might be responsible for analogue-selective binding in OTR and V1aR versus V2R, where the positively charged K(2.65) 100 is present at the equivalent position.

Analysis of interactions responsible for vasopressin binding to human neurohypophyseal hormone receptors-molecular dynamics study of the activated receptor-vasopressin-G(alpha) systems

Vasopressin (CYFQNCPRG-NH(2), AVP) is a semicyclic endogenous peptide, which exerts a variety of biological effects in mammals. The main physiological roles of AVP are the regulation of water balance and the control of blood pressure and adrenocorticotropin hormone (ACTH) secretion, mediated via three different subtypes of vasopressin receptors: V1a, V1b and V2 receptors (V1aR, V1bR and V2R, respectively). They are the members of the class A, G-protein-coupled receptors (GPCRs). AVP also modulates several behavioral and social functions. In this study, the interactions responsible for AVP binding to vasopressin V1a and V2 receptors versus the closely related oxytocin ([I3,L8]AVP, OT) receptor (OTR) have been investigated. Three-dimensional models of the activated receptors were constructed using multiple sequence alignment, followed by homology modeling using the complex of activated rhodopsin with Gt(alpha) C-terminal peptide of transducin MII-Gt(338-350) prototype as a template. AVP was docked into the receptor-G(alpha) systems. The three lowest-energy pairs of receptor-AVP-G(alpha) (two complexes per each receptor) were selected. The 1-ns unconstrained molecular dynamics (MD) of complexes embedded into the fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) lipid bilayer was conducted in the AMBER 7.0 force field. Six relaxed receptor-AVP-G(alpha) models were obtained. The residues responsible for AVP binding to vasopressin receptors have been identified and a different mechanism of AVP binding to V2R than to V1aR has been proposed.

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.

Selectivity of d[Cha4]AVP and SSR149415 at human vasopressin and oxytocin receptors: evidence that SSR149415 is a mixed vasopressin V1b/oxytocin receptor antagonist

1 A possible role of arginine vasopressin (AVP) V(1b) receptor subtype in stress-related disorders has been recently highlighted by the discovery of the agonist [1-deamino-4-cyclohexylalanine] AVP (d[Cha(4)]AVP) and the antagonist SSR149415. Both compounds have been proposed to target specifically V(1b) receptors, since the reported affinities for the related V(1a), V(2) and oxytocin receptors are in the micromolar or submicromolar range. In the present study, we further investigated the binding affinities of d[Cha(4)]AVP and SSR149415 at recombinant human vasopressin V(1b) (hV(1b)) and oxytocin (hOT) receptors expressed in Chinese hamster ovary (CHO) cells and functional properties of both compounds at hV(1b), hV(1a), hV(2) and hOT receptors. 2 d[Cha(4)]AVP bound to hV(1b) receptors and hOT receptors with pK(i) values of 9.68+/-0.06 and 7.68+/-0.09, respectively. SSR149415 showed pK(i) values of 9.34+/-0.06 at hV(1b) and 8.82+/-0.16 at hOT receptors. 3 d[Cha(4)]AVP stimulated [Ca(2+)](i) increase in hV(1b)-CHO cells with a pEC(50) value of 10.05+/-0.15. It showed pEC(50) values of 6.53+/-0.17 and 5.92+/-0.02 at hV(1a) and hV(2) receptors, respectively, and behaved as a weak antagonist at hOT receptors (pK(B)=6.31+/-0.12). SSR149415 inhibited the agonist-induced [Ca(2+)](i) increase with pK(B) values of 9.19+/-0.07 in hV(1b)-CHO and 8.72+/-0.15 in hOT-CHO cells. A functional pK(i) value of 7.23+/-0.10 was found for SSR1494151 at hV(1a) receptors, whereas it did not inhibit 20 nM AVP response at hV(2) receptors up to 3 microM. 4 Data obtained confirmed the high potency and selectivity of d[Cha(4)]AVP at hV(1b) receptors, but revealed that SSR149415, in addition to the high potency at hV(1b) receptors, displays a significant antagonism at hOT receptors.

Investigation of mechanism of desmopressin binding in vasopressin V2 receptor versus vasopressin V1a and oxytocin receptors: Molecular dynamics simulation of the agonist-bound state in the membrane–aqueous system

The vasopressin V2 receptor (V2R) belongs to the Class A G protein-coupled receptors (GPCRs). V2R is expressed in the renal collecting duct (CD), where it mediates the antidiuretic action of the neurohypophyseal hormone arginine vasopressin (CYFQNCPRG-NH2, AVP). Desmopressin ([1-deamino, 8-D]AVP, dDAVP) is strong selective V2R agonist with negligible pressor and uterotonic activity. In this paper, the interactions responsible for binding of dDAVP to vasopressin V2 receptor versus vasopressin V1a and oxytocin receptors has been examined. Three-dimensional activated models of the receptors were constructed using the multiple sequence alignment and the complex of activated rhodopsin with Gt(alpha) C-terminal peptide of transducin MII-Gt(alpha) (338-350) prototype (Slusarz, R.; Ciarkowski, J. Acta Biochim Pol 2004 51, 129-136) as a template. The 1-ns unconstrained molecular dynamics (MD) of receptor-dDAVP complexes immersed in the fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) membrane model was conducted in an Amber 7.0 force field. Highly conserved transmembrane residues have been proposed as being responsible for V2R activation and G protein coupling. Molecular mechanism of the dDAVP binding has been suggested. The internal water molecules involved in an intricate network of the hydrogen bonds inside the receptor cavity have been identified and their role in the stabilization of the agonist-bound state proposed.