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

Stimulating oxytocin receptors in the basolateral amygdala enhances stimulus processing: Differential and consistent effects for stimuli paired with fear versus sucrose in extinction and reversal learning

Oxytocin (OT) influences a range of social behaviors by enhancing the salience of social cues and regulating the expression of specific social behaviors (e.g., maternal care versus defensive aggression). We previously showed that stimulating OT receptors in the basolateral amygdala of rats also enhanced the salience of fear conditioned stimuli: relative to rats given vehicle infusions, rats infused with [Thr4,Gly7]-oxytocin (TGOT), a selective OT receptor agonist, showed greater discrimination between a cue predictive of danger, and one that signaled safety. In the present series of experiments using male rats, the effects of OT receptor activation in the basolateral amygdala on stimulus processing were examined further using conditioning protocols that consist of changes in stimulus-outcome contingencies (i.e., extinction and reversal), and with stimuli paired with aversive (i.e., foot shock) and appetitive (i.e., sucrose) outcomes. It was revealed that the effects of OTR stimulation diverge for aversive and appetitive learning - enhancing the former but not the latter. However, across both types of learning, OTR stimulation enhanced the detection of conditioned stimuli. Overall, these results are consistent with an emerging view of OT's effects on stimulus salience; facilitating the detection of meaningful stimuli while reducing responding to those that are irrelevant.

Detection, processing and reinforcement of social cues: regulation by the oxytocin system

Many social behaviours are evolutionarily conserved and are essential for the healthy development of an individual. The neuropeptide oxytocin (OXT) is crucial for the fine-tuned regulation of social interactions in mammals. The advent and application of state-of-the-art methodological approaches that allow the activity of neuronal circuits involving OXT to be monitored and functionally manipulated in laboratory mammals have deepened our understanding of the roles of OXT in these behaviours. In this Review, we discuss how OXT promotes the sensory detection and evaluation of social cues, the subsequent approach and display of social behaviour, and the rewarding consequences of social interactions in selected reproductive and non-reproductive social behaviours. Social stressors - such as social isolation, exposure to social defeat or social trauma, and partner loss - are often paralleled by maladaptations of the OXT system, and restoring OXT system functioning can reinstate socio-emotional allostasis. Thus, the OXT system acts as a dynamic mediator of appropriate behavioural adaptations to environmental challenges by enhancing and reinforcing social salience and buffering social stress.

Photoswitchable Probes of Oxytocin and Vasopressin

Oxytocin (OT) and vasopressin (VP) are related neuropeptides that regulate many biological processes. In humans, OT and VP act via four G protein-coupled receptors, OTR, V1aR, V1bR, and V2R (VPRs), which are associated with several disorders. To investigate the therapeutic potential of these receptors, particularly in the receptor-dense areas of the brain, molecular probes with a high temporal and spatial resolution are required. Such a spatiotemporal resolution can be achieved by incorporating photochromic moieties into OT and VP. Here, we report the design, synthesis, and (photo)pharmacological characterization of 12 OT- and VP-derived photoprobes using different modification strategies. Despite OT's and VP's sensitivity toward structural changes, we identified two photoprobes with good potency and photoswitch window for investigating the OTR and V1bR. These photoprobes should be of high value for producing cutting-edge photocontrollable peptide probes for the study of dynamic and kinetic receptor activation processes in specific regions of the brain.

Oxytocin increases itch sensitivity of mice in late pregnancy and its peripheral receptor mechanism of itch-promoting effect

Oxytocin (OXT) is secreted in a large amount during the middle and late pregnancy. Except for the regulation of functions related to childbirth, OXT is involved in the regulation of cognition, social behavior, addiction, pain and so on. Our aim is to confirm the increase of OXT content in mice in late pregnancy is the main cause of itch during pregnancy and observe whether exogenously administered OXT can induce or increase itch sensitivity. The research shows that itch sensitivity of mice increased significantly in late pregnancy and basically returned to normal one day after delivery. The number of OXT-positive neurons in paraventricular nucleus (PVN) and the content of OXT in serum of the late pregnant mice increased significantly, and decreased sharply after delivery. Intradermal injection of low concentration of OXT (0.2 nmol/L) could not induce scratching behavior in mice, but high concentration of OXT (5 nmol/L, 10 nmol/L) could do this in a dose-dependent manner. Low concentration of OXT significantly increased the itch sensitivity to histamine. Intradermal injection of oxytocin receptor (OXTR) or arginine vasopressin-1a receptor (AVPR1A) antagonist did not affect histamine-induced scratching behavior, but both reversed the increase of itch sensitivity in late pregnant mice or the facilitated itch sensitivity by OXT. Study suggests that both endogenous and exogenous increases in OXT can increase the body's sensitivity to itch, and even induce itch directly. Pruritus during pregnancy is closely related to the increase of OXT content in vivo. In the periphery, the itch-promoting effect of OXT is mediated by OXTR and AVPR1A.

Arginine vasopressin potentiates inspiratory bursting in hypoglossal motoneurons of neonatal mice

Vasopressin (AVP) acts as a neurotransmitter and its activity can potentiate respiratory activity. Hypoglossal (XII) motoneurons that innervate the tongue express V1a vasopressin receptors, which are excitatory. Therefore, we hypothesized that V1a receptor activation at XII motoneurons would potentiate inspiratory bursting. We developed this study to determine whether AVP can potentiate inspiratory bursting in rhythmic medullary slice preparations in neonatal (postnatal, P0-5) mice. Bath or local application of AVP potentiated inspiratory bursting compared to baseline XII inspiratory burst amplitude. Antagonizing V1a receptors revealed significant attenuation of the AVP-mediated potentiation of inspiratory bursting, while antagonism of oxytocin receptors (at which AVP has similar binding affinity) revealed a trend to attenuate AVP-mediated potentiation of inspiratory bursting. Finally, we discovered that the AVP-mediated potentiation of inspiratory bursting increases significantly with postnatal maturation from P0-5. Overall, these data support that AVP potentiates inspiratory bursting directly at XII motoneurons.

Vasopressin V1a receptor and oxytocin receptor regulate murine sperm motility differently

Specific receptors for the neurohypophyseal hormones, arginine vasopressin (AVP) and oxytocin, are present in the male reproductive organs. However, their exact roles remain unknown. To elucidate the physiological functions of pituitary hormones in male reproduction, this study first focused on the distribution and function of one of the AVP receptors, V1a. In situ hybridization analysis revealed high expression of the Avpr1a in Leydig cells of the testes and narrow/clear cells in the epididymis, with the expression pattern differing from that of the oxytocin receptor (OTR). Notably, persistent motility and highly proportional hyperactivation were observed in spermatozoa from V1a receptor-deficient mice. In contrast, OTR blocking by antagonist atosiban decreased hyperactivation rate. Furthermore, AVP stimulation could alter the extracellular pH mediated by the V1a receptor. The results highlight the crucial role of neurohypophyseal hormones in male reproductive physiology, with potential contradicting roles of V1a and OTR in sperm maturation. Our findings suggest that V1a receptor antagonists are potential therapeutic drugs for male infertility.

Oxytocin receptor is not required for social attachment in prairie voles

Prairie voles are among a small group of mammals that display long-term social attachment between mating partners. Many pharmacological studies show that signaling via the oxytocin receptor (Oxtr) is critical for the display of social monogamy in these animals. We used CRISPR mutagenesis to generate three different Oxtr-null mutant prairie vole lines. Oxtr mutants displayed social attachment such that males and females showed a behavioral preference for their mating partners over a stranger of the opposite sex, even when assayed using different experimental setups. Mothers lacking Oxtr delivered viable pups, and parents displayed care for their young and raised them to the weanling stage. Together, our studies unexpectedly reveal that social attachment, parturition, and parental behavior can occur in the absence of Oxtr signaling in prairie voles.

Oxytocin accelerates tight junction formation and impairs cellular migration in 3D spheroids: evidence from Gapmer-induced exon skipping

Oxytocin (OXT) is a neuropeptide that has been associated with neurological diseases like autism, a strong regulating activity on anxiety and stress-related behavior, physiological effects during pregnancy and parenting, and various cellular effects in neoplastic tissue. In this study, we aimed to unravel the underlying mechanism that OXT employs to regulate cell-cell contacts, spheroid formation, and cellular migration in a 3D culture model of human MLS-402 cells. We have generated a labeled OXT receptor (OXTR) overexpressing cell line cultivated in spheroids that were treated with the OXTR agonists OXT, Atosiban, and Thr4-Gly7-oxytocin (TGOT); with or without a pre-treatment of antisense oligos (Gapmers) that induce exon skipping in the human OXTR gene. This exon skipping leads to the exclusion of exon 4 and therefore a receptor that lost its intracellular G-protein-binding domain. Sensitive digital PCR (dPCR) provided us with the means to differentiate between wild type and truncated OXTR in our cellular model. OXTR truncation differentially activated intracellular signaling cascades related to cell-cell attachment and proliferation like Akt, ERK1/2-RSK1/2, HSP27, STAT1/5, and CREB, as assessed by a Kinase Profiler Assay. Digital and transmission electron microscopy revealed increased tight junction formation and well-organized cellular protrusions into an enlarged extracellular space after OXT treatment, resulting in increased cellular survival. In summary, OXT decreases cellular migration but increases cell-cell contacts and therefore improves nutrient supply. These data reveal a novel cellular effect of OXT that might have implications for degenerating CNS diseases and tumor formation in various tissues.

dDAVP Downregulates the AQP3-Mediated Glycerol Transport via V1aR in Human Colon HCT8 Cells

Vasopressin (AVP) plays a key function in controlling body water and salt balance through the activation of the vasopressin receptors V1aR and V2R. Abnormal secretion of AVP can cause the syndrome of inappropriate antidiuresis that leads to hyponatremia, which is an electrolyte disorder often observed in the elderly hospitalized and oncologic patients. Beyond kidneys, the colonic epithelium modulates water and salt homeostasis. The water channel AQP3, expressed in villus epithelial cells is implicated in water absorption across human colonic surface cells. Here, the action of dDAVP, a stable vasopressin analog, was evaluated on the AQP3 expression and function using human colon HCT8 cells as an experimental model. Confocal and Western Blotting analysis revealed that HCT8 cells express both V1aR and V2R. Long-term (72 h) treatment with dDAVP reduced glycerol uptake and cell viability. These effects were prevented by SR49059, a synthetic antagonist of V1aR, but not by tolvaptan, a specific V2R antagonist. Of note, the SR49059 action was impaired by DFP00173, a selective inhibitor of AQP3. Interestingly, compared to the normal colonic mucosa, in the colon of patients with adenocarcinoma, the expression of V1aR was significantly decreased. These findings were confirmed by gene expression analysis with RNA-Seq data. Overall, data suggest that dDAVP, through the V1aR dependent pathway, reduces AQP3 mediated glycerol uptake, a process that is reversed in adenocarcinoma, suggesting that the AVP-dependent AQP3 pathway may represent a novel target in colon diseases associated with abnormal cell growth.

Species differences in the effect of oxytocin on maternal behavior: A model incorporating the potential for allomaternal contributions

Oxytocin has historically been linked to processes involved with maternal behavior. However, the relative importance of oxytocin for maternal behavior widely varies among mammalian species, from indispensable to apparently nonessential. This review proposes a new model in which the relative importance of oxytocin for mothering across species is explained by an evolutionary pressure which we term "allomaternal potential", or the degree to which other conspecifics are capable and likely to assist with caregiving. It is notable that in animals where allomaternal potential is high (i.e., many quality helpers are available), oxytocin is decoupled from mothering. However, in animals where allomaternal potential is low (i.e., conspecifics refuse to, or do not provide, quality help), oxytocin is crucial for mothering. We posit that this relationship is a form of kin selection, whereby oxytocin is a signal that leads mothers to preferentially dispense resources to their own young when quality helpers are unlikely.

Oxytocin and Related Peptide Hormones: Candidate Anti-Inflammatory Therapy in Early Stages of Sepsis

Sepsis is a potentially life-threatening systemic inflammatory syndrome characterized by dysregulated host immunological responses to infection. Uncontrolled immune cell activation and exponential elevation in circulating cytokines can lead to sepsis, septic shock, multiple organ dysfunction syndrome, and death. Sepsis is associated with high re-hospitalization and recovery may be incomplete, with long term sequelae including post-sepsis syndrome. Consequently, sepsis continues to be a leading cause of morbidity and mortality across the world. In our recent review of human chorionic gonadotropin (hCG), we noted that its major properties including promotion of fertility, parturition, and lactation were described over a century ago. By contrast, the anti-inflammatory properties of this hormone have been recognized only more recently. Vasopressin, a hormone best known for its anti-diuretic effect, also has anti-inflammatory actions. Surprisingly, vasopressin's close cousin, oxytocin, has broader and more potent anti-inflammatory effects than vasopressin and a larger number of pre-clinical studies supporting its potential role in limiting sepsis-associated organ damage. This review explores possible links between oxytocin and related octapeptide hormones and sepsis-related modulation of pro-inflammatory and anti-inflammatory activities.

Oxytocin stimulates lipolysis, prostaglandin E2 synthesis, and leptin secretion in 3T3-L1 adipocytes

A model of oxytocin in the regulation of metabolic status has described one of oxytocin synthesis and release from the neurohypophysis in response to leptin, to suppress further leptin release. In addition, a lipogenic role for oxytocin has been suggested, consistent with an insulinergic action. This model, however, may be incorrect. Oxytocin reduces fat mass in the absence of either leptin or leptin receptor signalling, thereby challenging the interdependence between leptin and oxytocin. An oxytocin induced production of the anti-lipolytic prostaglandin E₂ (PGE₂) might account for this. Media from 3T3-L1 differentiated adipocytes treated with oxytocin (0-50 nmol.L^-1) for 24 hrs were assayed for PGE₂, leptin, adiponectin, and glycerol. Harvested cells were analysed for lipid droplet triglyceride and cytosolic free fatty acid (FFA) by flow cytometry, and for altered expression of lipolytic and lipogenic associated gene ontology transcripts by cDNA array. Both PGE₂ and leptin secretion were significantly increased by oxytocin treatment whilst adiponectin secretion was not. A significant increase in cytosolic FFA was detected following oxytocin treatment, similar to that determined following treatment with isoproterenol (positive control). A significant increase in glycerol release to the culture media confirmed a lipolytic effect. No enrichment of lipolytic and lipogenic associated gene ontology transcripts was determined, but significant overrepresentation of chemosensory olfactory transcripts was. In conclusion, oxytocin stimulates lipolysis in 3T3-L1 adipocytes, mediated by autocrine/paracrine actions of PGE₂ and leptin. To confirm that this response is mediated solely by the oxytocin receptor, further experiments would require those effects being blocked by a specific oxytocin antagonist.

Co-Stimulation of Oxytocin and Arginine-Vasopressin Receptors Affect Hypothalamic Neurospheroid Size

Oxytocin (OXT) is a neuropeptide involved in a plethora of behavioral and physiological processes. However, there is a prominent lack of 3D cell culture models that investigate the effects of OXT on a cellular/molecular level. In this study, we established a hypothalamic neuronal spheroid model to investigate the cellular response in a more realistic 3D setting. Our data indicate that the formation of spheroids itself does not alter the basic characteristics of the cell line and that markers of cellular morphology and connectivity are stably expressed. We found that both OXT and arginine vasopressin (AVP) treatment increase spheroid size (surface area and volume), as well as individual nucleus size, which serves as an indicator for cellular proliferation. The cellular response to both OXT and AVP seems mainly to be mediated by the AVP receptor 1a (V1aR); however, the OXT receptor (OXTR) contributes significantly to the observed proliferative effect. When we blocked the OXTR pharmacologically or knocked down the OXTR by siRNA, the OXT- or AVP-induced cellular proliferation decreased. In summary, we established a 3D cell culture model of the neuronal response to OXT and AVP and found that spheroids react to the treatment via their respective receptors but also via cross-talk between the two receptor types.

Oxytocin Receptor Activation Rescues Opioid-Induced Respiratory Depression by Systemic Fentanyl in the Rat

Opioid overdose intervention by naloxone, a high affinity receptor antagonist, reverses opioid-induced respiratory depression (OIRD) and analgesia by displacing opioids. Systemic naloxone stimulates release of the hypothalamic neuropeptide oxytocin, which has analgesic properties and participates in cardiorespiratory homeostasis. To test the hypothesis that oxytocin can reverse OIRD, we assessed the rescue potential of graded doses (0, 0.1, 2, 5, 10, 50 nmol/kg, i.v.) of oxytocin to counter fentanyl (60 nmol/kg, i.v.)-induced depression of neural inspiration indexed by recording phrenic nerve activity (PNA) in anesthetized (urethane/α-chloralose), vagotomized, and artificially ventilated rats. Oxytocin dose-dependently rescued fentanyl OIRD by almost immediately reversing PNA burst arrest (P = 0.0057) and restoring baseline burst frequency (P = 0.0016) and amplitude (P = 0.0025) at low but not high doses, resulting in inverted bell-shaped dose-response curves. Oxytocin receptor antagonism (40 nmol/kg, i.v.) prevented oxytocin reversal of OIRD (arrest: P = 0.0066, frequency: P = 0.0207, amplitude: P = 0.0022). Vasopressin 1A receptor (V1aR) antagonism restored high-dose oxytocin efficacy to rescue OIRD (P = 0.0170 to P < 0.0001), resulting in classic sigmoidal dose-response curves, and prevented (P = 0.0135) transient hypertension from V1aR cross-activation (P = 0.0275). Alone, vasopressin (5 nmol/kg, i.v.) failed to reverse fentanyl respiratory arrest (P = 0.6184). The nonpeptide oxytocin receptor agonist WAY-267464 (75 nmol/kg, i.v.), which has V1aR antagonist properties, quickly reversed fentanyl OIRD (P < 0.0001), with rapid recovery of PNA frequency (P = 0.0011) and amplitude (P = 0.0044) without adverse hemodynamic consequences (P = 0.9991). Findings indicate that peptide and nonpeptide agonist activation of oxytocin receptors without V1aR cross-activation rescues fentanyl OIRD. Oxytocin receptor agonists could be lifesaving resuscitation agents that enhance rather than interrupt opioid analgesia. SIGNIFICANCE STATEMENT: Oxytocin receptor activation produces analgesia. Here, we demonstrate that activation by the US Food and Drug Administration-approved agonist oxytocin and the nonpeptide partial agonist WAY-267464 can each reverse fentanyl cardiorespiratory depression. Selective targeting of oxytocin receptors for resuscitation from opioid overdose, alone or in combination with an opioid antagonist, could eliminate or attenuate negative side effects associated with traditional opioid receptor antagonism.

Oxytocin and vasopressin: Signalling, behavioural modulation and potential therapeutic effects

Oxytocin (OT) and vasopressin (AVP) are endogenous ligands for OT and AVP receptors in the brain and in the peripheral system. Several studies demonstrate that OT and AVP have opposite roles in modulating stress, anxiety and social behaviours. Interestingly, both peptides and their receptors exhibit high sequence homology which could account for the biased signalling interaction of the peptides with OT and AVP receptors. However, how and under which conditions this crosstalk occurs in vivo remains unclear. In this review we shed light on the complexity of the roles of OT and AVP, by focusing on their signalling and behavioural differences and exploring the crosstalk between the receptor systems. Moreover, we discuss the potential of OT and AVP receptors as therapeutic targets to treat human disorders, such as autism, schizophrenia and drug abuse.

Nature-inspired dimerization as a strategy to modulate neuropeptide pharmacology exemplified with vasopressin and oxytocin

Vasopressin (VP) and oxytocin (OT) are cyclic neuropeptides that regulate fundamental physiological functions via four G protein-coupled receptors, V_1aR, V_1bR, V₂R, and OTR. Ligand development remains challenging for these receptors due to complex structure–activity relationships. Here, we investigated dimerization as a strategy for developing ligands with novel pharmacology. We regioselectively synthesised and systematically studied parallel, antiparallel and N- to C-terminal cyclized homo- and heterodimer constructs of VP, OT and dVDAVP (1-deamino-4-valine-8-d-arginine-VP). All disulfide-linked dimers, except for the head-to-tail cyclized constructs, retained nanomolar potency despite the structural implications of dimerization. Our results support a single chain interaction for receptor activation. Dimer orientation had little impact on activity, except for the dVDAVP homodimers, where an antagonist to agonist switch was observed at the V_1aR. This study provides novel insights into the structural requirements of VP/OT receptor activation and spotlights dimerization as a strategy to modulate pharmacology, a concept also frequently observed in nature.

Structural and pharmacological study of parallel, antiparallel and N- to C-terminal cyclized homo- and heterodimers of vasopressin and oxytocin. This study spotlights dimerization as a strategy to modulate the pharmacology of neuropeptides.

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.

Oxytocin Promotes Accurate Fear Discrimination and Adaptive Defensive Behaviors

The nonapeptide, oxytocin (OT), known for its role in social bonding and attachment formation, has demonstrated anxiolytic properties in animal models and human studies. However, its role in the regulation of fear responses appears more complex, brain site-specific, sex-specific, and dependent on a prior stress history. Studies have shown that OT neurons in the hypothalamus are activated during cued and contextual fear conditioning and during fear recall, highlighting the recruitment of endogenous oxytocin system in fear learning. OT is released into the extended amygdala, which contains the central nucleus of the amygdala (CeA) and the bed nucleus of the stria terminalis (BNST), both critical for the regulation of fear and anxiety-like behaviors. Behavioral studies report that OT in the CeA reduces contextual fear responses; whereas in the BNST, OT receptor (OTR) neurotransmission facilitates cued fear and reduces fear responses to un-signaled, diffuse threats. These ostensibly contrasting behavioral effects support growing evidence that OT works to promote fear discrimination by reducing contextual fear or fear of diffuse threats, yet strengthening fear responses to imminent and predictable threats. Recent studies from the basolateral nucleus of the amygdala (BLA) support this notion and show that activation of OTR in the BLA facilitates fear discrimination by increasing fear responses to discrete cues. Also, OTR transmission in the CeA has been shown to mediate a switch from passive freezing to active escape behaviors in confrontation with an imminent, yet escapable threat but reduce reactivity to distant threats. Therefore, OT appears to increase the salience of relevant threat-signaling cues yet reduce fear responses to un-signaled, distant, or diffuse threats. Lastly, OTR signaling has been shown to underlie emotional discrimination between conspecifics during time of distress, social transmission of fear, and social buffering of fear. As OT has been shown to enhance salience of both positive and negative social experiences, it can also serve as a warning system against potential threats in social networks. Here, we extend the social salience hypothesis by proposing that OT enhances the salience of relevant environmental cues also in non-social contexts, and as such promotes active defensive behaviors.

Yohimbine as a Starting Point to Access Diverse Natural Product-Like Agents with Re-programmed Activities against Cancer-Relevant GPCR Targets

G protein-coupled receptors (GPCRs) constitute the largest protein superfamily in the human genome. GPCRs play key roles in mediating a wide variety of physiological events including proliferation and cancer metastasis. Given the major roles that GPCRs play in mediating cancer growth, they present promising targets for small molecule therapeutics. One of the principal goals of our lab is to identify complex natural products (NPs) suitable for ring distortion, or the dramatic altering of the inherently complex architectures of NPs, to rapidly generate an array of compounds with diverse molecular skeletal systems. The overarching goal of our ring distortion approach is to re-program the biological activity of select natural products and identify new compounds of importance to the treatment of disease, such as cancer. Described herein are the results from biological screens of diverse small molecules derived from the indole alkaloid yohimbine against a panel of GPCRs involved in various diseases. Several analogues displayed highly differential antagonistic activities across the GPCRs tested. We highlight the re-programmed profile of one analogue, Y7g, which exhibited selective antagonistic activities against AVPR2 (IC₅₀ = 459 nM) and OXTR (IC₅₀ = 1.16 µM). The activity profile of Y7g could correlate its HIF-dependent anti-cancer activity to its GPCR antagonism since these receptors are known to be upregulated in hypoxic cellular environments. Our findings demonstrate that the ring distortion of yohimbine can lead to the identification of new compounds capable of interacting with distinct cancer-relevant targets.

Critical role of V1a vasopressin receptor in murine parturition†

The precise mechanisms of the reproductive physiological processes, such as labor initiation, are poorly understood. Oxytocin (OT) is one of the well-known uterotonics and is clinically adopted as a medication to facilitate childbirth. Vasopressin (VP), a posterior pituitary hormone similar to OT, has also been proposed to be involved in the reproductive physiology. In this study, we found that a total deficiency of V1a receptor subtype (V1aR) in mice resulted in a reduced number of pups, delayed labor initiation, and increased post-delivery hemorrhage compared with those in wild-type mice. Among the VP receptor subtypes, only V1aR was found to be expressed in the murine uterus, and its distribution pattern was different from that of the oxytocin receptor (OTR); V1aR expression was mainly distributed in the circular myometrium, whereas OTR was strongly expressed in both the circular and longitudinal myometrium. The maximum contractile force of the circular myometrium, induced by VP or OT, was attenuated in the pregnant uterus of Avpr1a-deficient mice. Contrarily, while OT expression was decreased in the Avpr1a-deficient uterus, OTR expression was significantly increased. These results suggest that V1aR deficiency not only reduces the uterine contractile force but also perturbs the expression of genes responsible for the reproductive physiology. Therefore, V1aR is necessary to exert the maximum contraction of the circular myometrium to deliver pups. This study revealed an important role of V1aR in physiological contraction and term parturition in mice.

Comparison of the pharmacological profiles of arginine vasopressin and oxytocin analogs at marmoset, macaque, and human vasopressin 1a receptor

Arginine vasopressin (AVP) and oxytocin (OT) are nonapeptides that bind to G-protein coupled receptors and influence social behaviors. Consensus mammalian AVP and OT (Leu⁸-OT) sequences are highly conserved. In marmosets, an amino acid change in the 8th position of the peptide (Pro⁸-OT) exhibits unique structural and functional properties. There is ∼85 % structural homology between the OT receptor (OTR) and vasopressin 1a receptor (V1aR) resulting in significant cross-reactivity between the ligands and receptors. Chinese hamster ovary (CHO) cells expressing marmoset (mV1aR), macaque (qV1aR), or human vasopressin receptor 1a (hV1aR) were used to assess AVP, Leu⁸-OT and Pro⁸-OT pharmacological profiles. To assess activation of Gq, functional assays were performed using Fluo-3 to measure ligand-induced Ca²⁺ mobilization. In all three V1aR-expressing cell lines, AVP was more potent than the OT ligands. To assess ligand-induced hyperpolarization, FLIPR Membrane Potential (FMP) assays were performed. In all three V1aR lines, AVP was more potent than the OT analogs. The distinctive U-shaped concentration-response curve displayed by AVP may reflect enhanced desensitization of the mV1aR and hV1aR, which is not observed with qV1aR. Evaluation of Ca²⁺-activated potassium (K⁺) channels using the inhibitors apamin, paxilline, and TRAM-34 demonstrated that both intermediate and large conductance Ca²⁺-activated K⁺ channels contributed to membrane hyperpolarization, with different pharmacological profiles identified for distinct ligand-receptor combinations. Taken together, these data suggest differences in ligand-receptor signaling that may underlie differences in social behavior. Integrative studies of behavior, genetics and ligand-receptor interaction will help elucidate the connection between receptor pharmacology and social behaviors.

Effects of route of administration on oxytocin-induced changes in regional cerebral blood flow in humans

Could nose-to-brain pathways mediate the effects of peptides such as oxytocin (OT) on brain physiology when delivered intranasally? We address this question by contrasting two methods of intranasal administration (a standard nasal spray, and a nebulizer expected to improve OT deposition in nasal areas putatively involved in direct nose-to-brain transport) to intravenous administration in terms of effects on regional cerebral blood flow during two hours post-dosing. We demonstrate that OT-induced decreases in amygdala perfusion, a key hub of the OT central circuitry, are explained entirely by OT increases in systemic circulation following both intranasal and intravenous OT administration. Yet we also provide robust evidence confirming the validity of the intranasal route to target specific brain regions. Our work has important translational implications and demonstrates the need to carefully consider the method of administration in our efforts to engage specific central oxytocinergic targets for the treatment of neuropsychiatric disorders.

Comparison of the pharmacologic profiles of arginine vasopressin and oxytocin analogs at marmoset, titi monkey, macaque, and human oxytocin receptors

The oxytocin-arginine vasopressin (OT-AVP) ligand-receptor family influences a variety of physiological, behavioral, and social behavioral processes in the brain and periphery. The OT-AVP family is highly conserved in mammals, but recent discoveries have revealed remarkable diversity in OT ligands and receptors in New World Monkeys (NWMs) providing a unique opportunity to assess the effects of genetic variation on pharmacological signatures of peptide ligands. The consensus mammalian OT sequence has leucine in the 8^th position (Leu⁸-OT), whereas a number of NWMs, including the marmoset, have proline in the 8^th position (Pro⁸-OT) resulting in a more rigid tail structure. OT and AVP bind to OT’s cognate G-protein coupled receptor (OTR), which couples to various G-proteins (G_i/o, G_q, G_s) to stimulate diverse signaling pathways. CHO cells expressing marmoset (mOTR), titi monkey (tOTR), macaque (qOTR), or human (hOTR) OT receptors were used to compare AVP and OT analog-induced signaling. Assessment of G_q-mediated increase in intracellular calcium (Ca²⁺) demonstrated that AVP was less potent than OT analogs at OTRs from species whose endogenous ligand is Leu⁸-OT (tOTR, qOTR, hOTR), relative to Pro⁸-OT. Likewise, AVP-induced membrane hyperpolarization was less potent at these same OTRs. Evaluation of (Ca²⁺)-activated potassium (K⁺) channels using the inhibitors apamin, paxilline, and TRAM-34 demonstrated that both intermediate and large conductance Ca²⁺-activated K⁺ channels contributed to membrane hyperpolarization, with different pharmacological profiles identified for distinct ligand-receptor combinations. Understanding more fully the contributions of structure activity relationships for these peptide ligands at vasopressin and OT receptors will help guide the development of OT-mediated therapeutics.

Methylation in Syn and Psd95 genes underlie the inhibitory effect of oxytocin on oxycodone-induced conditioned place preference

Oxycodone (Oxy) is one of the most effective analgesics in medicine, but is associated with the development of dependence. Recent studies demonstrating epigenetic changes in the brain after exposure to opiates have provided an insight into possible mechanisms underlying addiction. Oxytocin (OT), an endogenous neuropeptide well known for preventing drug abuse, is a promising pharmacotherapy to counteract addiction. Therefore, we explored the mechanism of Oxy addiction and the role of OT in Oxy-induced epigenetic alterations. In this study, drug-induced changes in conditioned place preference (CPP), i.e. the expression of synaptic proteins and synaptic density in the ventral tegmental area (VTA) were measured. We also sought to identify DNA methyltransferases (DNMTs), ten-eleven translocations (TETs), global 5-methylcytosine (5-mC), and DNA methylation of two genes implicated in plasticity (Synaptophysin, Syn; Post-synaptic density protein 95, Psd95). Oxy (3.0 mg/kg, i.p.) induced CPP acquisition in Sprague-Dawley rats. Oxy down-regulated DNMT1 and up-regulated TET1-3, leading to a decrease in global 5-mC levels and differential demethylation at exon 1 of Syn and exon 2 of Psd95. These changes in DNA methylation of Syn and Psd95 elevated the expression of synaptic proteins (SYN, PSD95) and synaptic density in the VTA. Pretreatment with OT (2.5 µg, i.c.v.) via its receptor specifically blocked Oxy CPP, normalized synaptic density, and regulated DNMT1 and TET2-3 causing reverse of DNA demethylation of Syn and Psd95. DNA methylation is an important gene regulation mechanism underlying Oxy CPP, and OT - via its receptor - could specifically inhibit Oxy addiction.

Intradermal Injection of Oxytocin Aggravates Chloroquine-Induced Itch Responses via Activating the Vasopressin-1a Receptor/Nitric Oxide Pathway in Mice

Oxytocin (OT), a hormone synthesized within the paraventricular nucleus and supraoptic nucleus of the hypothalamus, when given intracerebroventricularly, induces strong scratching behaviors. However, it is not clear whether intradermal injection (ID) of OT elicits itch sensation. Herein, we found that OT (0.02 mg/ml) did not elicit an itch-scratching response in mice but aggravated chloroquine (CQ, 3 mmol/L)-elicited scratching behavior. Similar to OT, arginine vasopressin (AVP, 0.02 mg/ml), which is structurally related to OT, also enhanced CQ-induced scratching behavior but did not directly induce scratching behavior in mice. Mechanistically, OT-mediated enhancement of CQ-induced scratching behavior was significantly suppressed by conivaptan (0.05 mg/ml), a vasopressin-1a receptor (V1AR) antagonist and 1,400 W (3 mg/kg), inhibitor of inducible nitric oxide synthase (iNOS), but not OT receptor (OTR) antagonist L-368,899 (0.05 mg/ml). Notably, conivaptan also directly decreased CQ-induced scratching. In conclusion, OT plays a role in CQ-induced scratching behavior via V1AR binding events. V1AR antagonists could be used as possible treatments for CQ-induced itch.

Oxytocin increases eye gaze in schizophrenia

Abnormal eye gaze is common in schizophrenia and linked to functional impairment. The hypothalamic neuropeptide oxytocin modulates visual attention to social stimuli, but its effects on eye gaze in schizophrenia are unknown. We examined visual scanning of faces in men with schizophrenia and neurotypical controls to quantify oxytocin effects on eye gaze. In a randomized, double-blind, crossover study, 33 men with schizophrenia and 39 matched controls received one dose of intranasal oxytocin (40 IU) and placebo on separate testing days. Participants viewed 20 color photographs of faces while their gaze patterns were recorded. We tested for differences in fixation time on the eyes between patients and controls as well as oxytocin effects using linear mixed-effects models. We also tested whether attachment style, symptom severity, and anti-dopaminergic medication dosage moderated oxytocin effects. In the placebo condition, patients showed reduced fixation time on the eyes compared to controls. Oxytocin was associated with an increase in fixation time among patients, but a decrease among controls. Higher attachment anxiety and greater symptom severity predicted increased fixation time on the eyes on oxytocin versus placebo. Anti-dopaminergic medication dosage and attachment avoidance did not impact response to oxytocin. Consistent with findings that oxytocin optimizes processing of social stimuli, intranasal oxytocin enhanced eye gaze in men with schizophrenia. Further work is needed to determine whether changes in eye gaze impact social cognition and functional outcomes. Both attachment anxiety and symptom severity predicted oxytocin response, highlighting the importance of examining potential moderators of oxytocin effects in future studies.

Attenuation of Oxytocin and Serotonin 2A Receptor Signaling through Novel Heteroreceptor Formation

The oxytocin receptor (OTR) and the 5-hydroxytryptamine 2A receptor (5-HTR_2A) are expressed in similar brain regions modulating central pathways critical for social and cognition-related behaviors. Signaling crosstalk between their endogenous ligands, oxytocin (OT) and serotonin (5-hydroxytryptamine, 5-HT), highlights the complex interplay between these two neurotransmitter systems and may be indicative of the formation of heteroreceptor complexes with subsequent downstream signaling changes. In this study, we assess the possible formation of OTR-5HTR_2A heteromers in living cells and the functional downstream consequences of this receptor-receptor interaction. First, we demonstrated the existence of a physical interaction between the OTR and 5-HTR_2A in vitro, using a flow cytometry-based FRET approach and confocal microscopy. Furthermore, we investigated the formation of this specific heteroreceptor complex ex vivo in the brain sections using the Proximity Ligation Assay (PLA). The OTR-5HTR_2A heteroreceptor complexes were identified in limbic regions (including hippocampus, cingulate cortex, and nucleus accumbens), key regions associated with cognition and social-related behaviors. Next, functional cellular-based assays to assess the OTR-5HTR_2A downstream signaling crosstalk showed a reduction in potency and efficacy of OT and OTR synthetic agonists, carbetocin and WAY267464, on OTR-mediated Gαq signaling. Similarly, the activation of 5-HTR_2A by the endogenous agonist, 5-HT, also revealed attenuation in Gαq-mediated signaling. Finally, altered receptor trafficking within the cell was demonstrated, indicative of cotrafficking of the OTR/5-HTR_2A pair. Overall, these results constitute a novel mechanism of specific interaction between the OT and 5-HT neurotransmitters via OTR-5HTR_2A heteroreceptor formation and provide potential new therapeutic strategies in the treatment of social and cognition-related diseases.

Increased anxiety and decreased sociability induced by paternal deprivation involve the PVN-PrL OTergic pathway

Early adverse experiences often have devastating consequences. However, whether preweaning paternal deprivation (PD) affects emotional and social behaviors and their underlying neural mechanisms remain unexplored. Using monogamous mandarin voles, we found that PD increased anxiety-like behavior and attenuated social preference in adulthood. PD also decreased the number of oxytocin (OT)-positive neurons projecting from the paraventricular nucleus (PVN) and reduced the levels of the medial prefrontal cortex OT receptor protein in females and of the OT receptor and V1a receptor proteins in males. Intra-prelimbic cortical OT injections reversed the PD-induced changes in anxiety-like behavior and social preferences. Optogenetic activation of the prelimbic cortex OT terminals from PVN OT neurons reversed the PD-induced changes in emotion and social preference behaviors, whereas optogenetic inhibition was anxiogenic and impaired social preference in naive voles. These findings demonstrate that PD increases anxiety-like behavior and attenuates social preferences through the involvement of PVN OT neuron projections to the prelimbic cortex.

Parental care early in life is essential for normal development of the brain in humans and some other animals. It also lays the ground work for healthy behaviors later in life. Many studies have looked at the importance of a mother’s care, but less attention has been paid to the role played by fathers. Research shows that children who grow up without a father are at risk of emotional and behavioral problems later in life. But it is not clear how missing a father’s care affects brain development.

Oxytocin, a chemical produced by a part of the brain called the paraventricular nucleus, plays a key role in parental bonding. Another part of the brain called the prelimbic cortex regulates many emotions and many complex behaviors. Studying animals, like the mandarin vole, that form strong bonds with both parents is one way to learn more about how the loss of paternal care affects oxytocin or emotional and behavioral health.

Now, He et al. show that mandarin voles raised without a father are more anxious and socialize less with other voles than those raised with a father. The voles deprived of paternal care also have fewer oxytocin-producing cells in the paraventricular nucleus and fewer receptors for oxytocin in the prelimbic cortex. Injecting oxytocin into the prelimbic cortex eliminated the anxious and antisocial behavior seen in the voles lacking paternal care. Using a technique called optogenetics to restore the release of oxytocin in the prelimbic cortex reduced anxious behavior and restored normal social interactions. Using the same approach to interfere with communication between the paraventricular nucleus and prelimbic cortex in voles raised with a father also triggered anxious and antisocial behavior.

The experiments reveal that fathers play an important role in brain and behavioral development in mandarin voles. He et al. show that a lack of paternal care leads to deficits in oxytocin and a poor communication between the paraventricular nucleus and prelimbic cortex that contribute to emotional and social abnormalities in the voles. More studies are needed to determine father’s care has similar effects in humans. But if this relationship is confirmed, it might lead scientists to develop new strategies for treating psychiatric disorders in people deprived of paternal care.

The impact of early life stress on the central oxytocin system and susceptibility for drug addiction: Applicability of oxytocin as a pharmacotherapy

Early life trauma is strongly associated with an increased vulnerability to abuse illicit drugs and the impairment of neural development. This includes alterations to the development of the oxytocin system, which plays a pivotal role in the regulation of social behaviours and emotion. Dysregulation of this important system also contributes to increased susceptibility to develop drug addiction. In this review, we provide an overview of the animal models of early life stress that are widely used, and discuss the impact that early life stress has on drug-taking behaviour in adolescence and adulthood in both sexes. We link this to the changes that early life stress has on the endogenous oxytocin system, and how exogenously administered oxytocin may help to re-establish functioning of the system, and in turn, reduce drug-taking behaviour.

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.

Neural control of parental behaviors

Parenting is a multicomponent social behavior that is essential for the survival of offspring in many species. Despite extensive characterization of individual brain areas involved in parental care, we do not fully understand how discrete aspects of this behavior are orchestrated at the neural circuit level. Recent progress in identifying genetically specified neuronal populations critical for parenting, and the use of genetic and viral tools for circuit-cracking now allow us to deconstruct the underlying circuitry and, thus, to elucidate how different aspects of parental care are controlled. Here we review the latest advances, outline possible organizational principles of parental circuits and discuss future challenges.

Oxytocin produces thermal analgesia via vasopressin-1a receptor by modulating TRPV1 and potassium conductance in the dorsal root ganglion neurons

Recent studies have provided several lines of evidence that peripheral administration of oxytocin induces analgesia in human and rodents. However, the exact underlying mechanism of analgesia still remains elusive. In the present study, we aimed to identify which receptor could mediate the analgesic effect of intraperitoneal injection of oxytocin and its cellular mechanisms in thermal pain behavior. We found that oxytocin-induced analgesia could be reversed by d(CH₂)₅[Tyr(Me)²,Dab⁵] AVP, a vasopressin-1a (V1a) receptor antagonist, but not by desGly-NH₂-d(CH₂)₅[DTyr², Thr⁴]OVT, an oxytocin receptor antagonist. Single cell RT-PCR analysis revealed that V1a receptor, compared to oxytocin, vasopressin-1b and vasopressin-2 receptors, was more profoundly expressed in dorsal root ganglion (DRG) neurons and the expression of V1a receptor was predominant in transient receptor potential vanilloid 1 (TRPV1)-expressing DRG neurons. Fura-2 based calcium imaging experiments showed that capsaicin-induced calcium transient was significantly inhibited by oxytocin and that such inhibition was reversed by V1a receptor antagonist. Additionally, whole cell patch clamp recording demonstrated that oxytocin significantly increased potassium conductance via V1a receptor in DRG neurons. Taken together, our findings suggest that analgesic effects produced by peripheral administration of oxytocin were attributable to the activation of V1a receptor, resulting in reduction of TRPV1 activity and enhancement of potassium conductance in DRG neurons.

Subtle modifications to oxytocin produce ligands that retain potency and improved selectivity across species

Oxytocin and vasopressin mediate various physiological functions that are important for osmoregulation, reproduction, cardiovascular function, social behavior, memory, and learning through four G protein-coupled receptors that are also implicated in high-profile disorders. Targeting these receptors is challenging because of the difficulty in obtaining ligands that retain selectivity across rodents and humans for translational studies. We identified a selective and more stable oxytocin receptor (OTR) agonist by subtly modifying the pharmacophore framework of human oxytocin and vasopressin. [Se-Se]-oxytocin-OH displayed similar potency to oxytocin but improved selectivity for OTR, an effect that was retained in mice. Centrally infused [Se-Se]-oxytocin-OH potently reversed social fear in mice, confirming that this action was mediated by OTR and not by V1a or V1b vasopressin receptors. In addition, [Se-Se]-oxytocin-OH produced a more regular contraction pattern than did oxytocin in a preclinical labor induction and augmentation model using myometrial strips from cesarean sections. [Se-Se]-oxytocin-OH had no activity in human cardiomyocytes, indicating a potentially improved safety profile and therapeutic window compared to those of clinically used oxytocin. In conclusion, [Se-Se]-oxytocin-OH is a novel probe for validating OTR as a therapeutic target in various biological systems and is a promising new lead for therapeutic development. Our medicinal chemistry approach may also be applicable to other peptidergic signaling systems with similar selectivity issues.

The role of peripheral vasopressin 1A and oxytocin receptors on the subcutaneous vasopressin antinociceptive effects

BACKGROUND

Vasopressin (AVP) seems to play a role as an antinociceptive neurohormone, but little is known about the peripheral site of action of its antinociceptive effects. Moreover, AVP can produce motor impairment that could be confused with behavioural antinociception. Finally, it is not clear which receptor is involved in the peripheral antinociceptive AVP effects.

METHODS

In anaesthetized rats with end-tidal CO₂ monitoring, extracellular unitary recordings were performed, measuring the evoked activity mediated by Aβ-, Aδ-, C-fibres and post-discharge. Behavioural nociception and motor impairment were evaluated under subcutaneous AVP (0.1-10 μg) using formalin and rotarod tests. Selective antagonists to vasopressin (V_1A R) or oxytocin receptors (OTR) were used. Additionally, vasopressin and oxytocin receptors were explored immunohistochemically in skin tissues.

RESULTS

Subcutaneous AVP (1 and 10 μg/paw) induced antinociception and a transitory reduction of the end-tidal CO₂ . The neuronal activity associated with Aδ- and C-fibre activation was diminished, but no effect was observed on Aβ-fibres. AVP also reduced paw flinches in the formalin test and a transitory locomotor impairment was also found. The AVP-induced antinociception was blocked by the selective antagonist to V_1A R (SR49059) or OTR (L368,899). Immunohistochemical evidence of skin VP and OT receptors is given.

CONCLUSIONS

Subcutaneous AVP produces antinociception and behavioural analgesia. Both V1a and OTR participate in those effects. Our findings suggest that antinociception could be produced in a local manner using a novel vasopressin receptor located in cutaneous sensorial fibres. Additionally, subcutaneous AVP also produces important systemic effects such as respiratory and locomotor impairment.

SIGNIFICANCE

Our findings support that AVP produces peripheral antinociception and behavioural analgesia in a local manner; nevertheless, systemic effects are also presented. Additionally, this is the first detailed electrophysiological analysis of AVP antinociceptive action after subcutaneous administration. The results are reasonably explained by the demonstration of V_1A R and OTR in cutaneous fibres.

Conformationally rigid derivatives of WAY-267,464: Synthesis and pharmacology at the human oxytocin and vasopressin-1a receptors

WAY-267,464 (1) and twelve conformationally rigid analogues (3a-f-4a-f) were synthesised, characterised and evaluated in cellular assays with the aim of systematically exploring interactions with the oxytocin receptor (OTR). Each analogue was evaluated in radioligand binding displacement assays at both human OTR and arginine vasopressin 1a receptors (V_1aR). Physiological characterisation was determined by whole cell IP1 accumulation assays on stably transfected human embryonic kidney (HEK) cells. Incorporation of the rigid, optionally substituted benzene ring abolished OTR activity and diminished V_1aR pharmacology when compared to 1. A general trend was observed in V_1aR affinity for the propyl analogues (3d-3f) which identified the ortho-substituted analogue as the best in series (Ki = 251 nM) followed by a decrease in affinity through the meta and para-derivatives (3e; Ki = 874 nM and 3f; Ki = 1756 nM respectively). This study confirms the importance of the central pharmacophoric motifs of WAY-267,464 and illuminates the differences in the binding pocket of the highly conserved OTR and V_1aR.

Pharmacoperone rescue of vasopressin 2 receptor mutants reveals unexpected constitutive activity and coupling bias

Pharmacoperones are small molecules that diffuse into cells and rescue misfolded, mistrafficked protein mutants, restoring their function. These substances act with high target specificity, serving as templates to fold (or refold) receptors, enzymes, ion channels or other proteins and enable them to pass the scrutiny of the cellular quality control system ("rescue"). In the present study we demonstrate that a rescued mutant (L83Q) of the vasopressin type 2 receptor (V2R), shows a strong bias for Gs coupling unlike the WT V2 receptor, which couples to both Gs and Gq/11. Failure of the mutant to couple to Gq/11 was not due to a limiting quantity of G-proteins since other Gq/11-coupled receptors (WT V2R, histamine receptor and muscarinic receptor) responded appropriately to their ligands. Transfection with DNA encoding Gq enabled the V2 receptor mutant to couple to this G protein, but only modestly compared with the WT receptor. Fourteen V2R mutant pharmacoperones, of multiple chemical classes, obtained from a high throughput screen of a 660,000 structure library, and one V2R peptidomimetic antagonist rescues L83Q. The rescued mutant shows similar bias with all pharmacoperones identified, suggesting that the bias is intrinsic to the mutant protein's structure, rather than due to the chemical class of the pharmacoperone. In the case of V2R mutant Y128S, rescue with a pharmacoperone revealed constitutive activity, also with bias for Gs, although both IP and cAMP were produced in response to agonist. These results suggest that particular rescued receptor mutants show functional characteristics that differ from the WT receptor; a finding that may be important to consider as pharmacoperones are developed as therapeutic agents.

Vasopressin V1a receptors mediate the hypertensive effects of [Pyr1 ]apelin-13 in the rat rostral ventrolateral medulla

Key points

Dysfunctions in CNS regulation of arterial blood pressure lead to an increase in sympathetic nerve activity that participates in the pathogenesis of hypertension.

The apelin‐apelin receptor system affects arterial blood pressure homeostasis; however, the central mechanisms underlying apelin‐mediated changes in sympathetic nerve activity and blood pressure have not been clarified.

We explored the mechanisms involved in the regulation of [Pyr¹]apelin‐13‐mediated cardiovascular control within the rostral ventrolateral medulla (RVLM) using selective receptor antagonists.

We show that [Pyr¹]apelin‐13 acts as a modulating neurotransmitter in the normotensive RVLM to affect vascular tone through interaction with the vasopressin V1a receptor but that [Pyr¹]apelin‐13‐induced sympathoexcitation is independent of angiotensin II receptor type 1, oxytocin, ionotropic glutamate and GABA_A receptors.

Our data confirm a role for the apelin peptide system in cardiovascular regulation at the level of the RVLM and highlight that this system is a possible potential therapeutic target for the treatment of hypertension.

Abstract

Apelin is a ubiquitous peptide that can elevate arterial blood pressure (ABP) yet understanding of the mechanisms involved remain incomplete. Bilateral microinjection of [Pyr¹]apelin‐13 into the rostral ventrolateral medulla (RVLM), a major source of sympathoexcitatory neurones, increases ABP and sympathetic nerve activity. We aimed to investigate the potential involvement of neurotransmitter systems through which the apelin pressor response may occur within the RVLM. Adult male Wistar rats were anaesthetized and ABP was monitored via a femoral arterial catheter. Bilateral RVLM microinjection of [Pyr¹]apelin‐13 significantly increased ABP (9 ± 1 mmHg) compared to saline (−1 ± 2mmHg; P < 0.001), which was blocked by pretreatment with the apelin receptor antagonist, F13A (0 ± 1 mmHg; P < 0.01). The rise in ABP was associated with an increase in the low frequency spectra of systolic BP (13.9 ± 4.3% total power; P < 0.001), indicative of sympathetic vasomotor activation. The [Pyr¹]apelin‐13‐mediated pressor response and the increased low frequency spectra of systolic BP response were fully maintained despite RVLM pretreatment with the angiotensin II type 1 receptor antagonist losartan, the oxytocin receptor antagonist desGly‐NH₂, d(CH₂)₅[D‐Tyr²,Thr⁴]OVT, the ionotropic glutamate receptor antagonist kynurenate or the GABA_A antagonist bicuculline (P > 0.05). By contrast, the [Pyr¹]apelin‐13 induced pressor and sympathoexcitatory effects were abolished by pretreatment of the RVLM with the vasopressin V1a receptor antagonist, SR 49059 (−1 ± 1 mmHg; 1.1 ± 1.1% total power, respectively; P < 0.001). These findings suggest that the pressor action of [Pyr¹]apelin‐13 in the RVLM of normotensive rats is not mediated via angiotensin II type 1 receptor, oxytocin, ionotropic glutamate or GABA_A receptors but instead involves a close relationship with the neuropeptide modulator vasopressin.

Dysfunctions in CNS regulation of arterial blood pressure lead to an increase in sympathetic nerve activity that participates in the pathogenesis of hypertension.

The apelin‐apelin receptor system affects arterial blood pressure homeostasis; however, the central mechanisms underlying apelin‐mediated changes in sympathetic nerve activity and blood pressure have not been clarified.

We explored the mechanisms involved in the regulation of [Pyr¹]apelin‐13‐mediated cardiovascular control within the rostral ventrolateral medulla (RVLM) using selective receptor antagonists.

We show that [Pyr¹]apelin‐13 acts as a modulating neurotransmitter in the normotensive RVLM to affect vascular tone through interaction with the vasopressin V1a receptor but that [Pyr¹]apelin‐13‐induced sympathoexcitation is independent of angiotensin II receptor type 1, oxytocin, ionotropic glutamate and GABA_A receptors.

Our data confirm a role for the apelin peptide system in cardiovascular regulation at the level of the RVLM and highlight that this system is a possible potential therapeutic target for the treatment of hypertension.

Development of a human vasopressin V1a-receptor antagonist from an evolutionary-related insect neuropeptide

Characterisation of G protein-coupled receptors (GPCR) relies on the availability of a toolbox of ligands that selectively modulate different functional states of the receptors. To uncover such molecules, we explored a unique strategy for ligand discovery that takes advantage of the evolutionary conservation of the 600-million-year-old oxytocin/vasopressin signalling system. We isolated the insect oxytocin/vasopressin orthologue inotocin from the black garden ant (Lasius niger), identified and cloned its cognate receptor and determined its pharmacological properties on the insect and human oxytocin/vasopressin receptors. Subsequently, we identified a functional dichotomy: inotocin activated the insect inotocin and the human vasopressin V1b receptors, but inhibited the human V1aR. Replacement of Arg8 of inotocin by D-Arg8 led to a potent, stable and competitive V1aR-antagonist ([D-Arg8]-inotocin) with a 3,000-fold binding selectivity for the human V1aR over the other three subtypes, OTR, V1bR and V2R. The Arg8/D-Arg8 ligand-pair was further investigated to gain novel insights into the oxytocin/vasopressin peptide-receptor interaction, which led to the identification of key residues of the receptors that are important for ligand functionality and selectivity. These observations could play an important role for development of oxytocin/vasopressin receptor modulators that would enable clear distinction of the physiological and pathological responses of the individual receptor subtypes.

Molecular Basis of Oxytocin Receptor Signalling in the Brain: What We Know and What We Need to Know

Oxytocin (OT), a hypothalamic neuropeptide involved in regulating the social behaviour of all vertebrates, has been proposed as a treatment for a number of neuropsychiatric disorders characterised by deficits in the social domain. Over the last few decades, advances focused on understanding the social effects of OT and its role in physiological conditions and brain diseases, but much less has been done to clarify the molecular cascade of events involved in mediating such effects and in particular the cellular and molecular pharmacology of OT and its target receptor (OTR) in neuronal and glial cells.The entity and persistence of OT activity in the brain is closely related to the expression and regulation of the OTR expressed on the cell surface, which transmits the signal intracellularly and permits OT to affect cell function. Understanding the various signalling mechanisms mediating OTR-induced cell responses is crucial to determine the different responses in different cells and brain regions, and the success of OT and OT-derived analogues in the treatment of neurodevelopmental and psychiatric diseases depends on how well we can control such responses. In this review, we will consider the most important aspects of OT/OTR signalling by focusing on the molecular events involved in OT binding and coupling, on the main signalling pathways activated by the OTR in neuronal cells and on intracellular and plasma membrane OTR trafficking, all of which contribute to the quantitative and qualitative features of OT responses in the brain.

The neurocircuitry involved in oxytocin modulation of methamphetamine addiction

The role of oxytocin in attenuating the abuse of licit and illicit drugs, including the psychostimulant methamphetamine, has been examined with increased ferocity in recent years. This is largely driven by the potential application of oxytocin as a pharmacotherapy. However, the neural mechanisms by which oxytocin modulates methamphetamine abuse are not well understood. Recent research identified an important role for the accumbens core and subthalamic nucleus in this process, which likely involves an interaction with dopamine, glutamate, GABA, and vasopressin. In addition to providing an overview of methamphetamine, the endogenous oxytocin system, and the effects of exogenous oxytocin on drug abuse, we propose a neural circuit through which exogenous oxytocin modulates methamphetamine abuse, focusing on its interaction with neurochemicals within the accumbens core and subthalamic nucleus. A growing understanding of exogenous oxytocin effects at a neurochemical and neurobiological level will assist in its evaluation as a pharmacotherapy for drug addiction.

Differential Role of Neurohypophysial Hormones in Hypotension and Nitric Oxide Production During Endotoxaemia

Besides their well-established endocrine roles, vasopressin and oxytocin are also important regulators of immune function, participating in a complex neuroendocrine-immune network. In the present study, we investigated whether and how vasopressin and oxytocin could modulate lipopolysaccharide (LPS)-induced nitric oxide (NO) production in a well-established model of experimental endotoxaemia. Male Wistar rats were previously treated i.v. with vasopressin V1 or oxytocin receptor antagonists and then received either an i.v. LPS injection to induce endotoxaemia or a saline imjection as a control. The animals were divided into two groups: in the first group, blood was collected at 2, 4 and 6 h after LPS injection; in the second group, mean arterial blood pressure (MABP) and heart rate (HR) were recorded over 6 h. Plasma vasopressin and oxytocin values were higher in LPS- compared to saline-injected animals at 2 and 4 h but returned to basal levels at 6 h. NO levels exhibited an opposite pattern, showing a progressive increase over the entire period. The previous administration of a vasopressin V1 receptor antagonist significantly reduced NO plasma concentrations at 2 and 4 h but not at 6 h. By contrast, oxytocin receptor agonist pre-treatment had no effect on the NO plasma concentration. In relation to MABP, previous treatment with vasopressin V1 receptor antagonist reversed the LPS-induced hypotension at 4 h, although this was not the case for oxytocin antagonist-treated animals. None of the antagonists affected HR. Our findings indicate that vasopressin (but not oxytocin) has effects on NO production during endotoxaemia in rats, although they do not lend support to the proposed anti-inflammatory actions of vasopressin during endotoxaemia.

Possible Involvement of the Rat Hypothalamo-Neurohypophysial/-Spinal Oxytocinergic Pathways in Acute Nociceptive Responses

Oxytocin (OXT)-containing neurosecretory cells in the parvocellular divisions of the paraventricular nucleus (PVN), which project to the medulla and spinal cord, are involved in various physiological functions, such as sensory modulation and autonomic processes. In the present study, we examined OXT expression in the hypothalamo-spinal pathway, as well as the hypothalamo-neurohypophysial system, which includes the magnocellular neurosecretory cells in the PVN and the supraoptic nucleus (SON), after s.c. injection of saline or formalin into the hindpaws of transgenic rats that express the OXT and monomeric red fluorescent protein 1 (mRFP1) fusion gene. (i) The numbers of OXT-mRFP1 neurones that expressed Fos-like immunoreactivity (-IR) and OXT-mRFP1 intensity were increased significantly in the magnocellular/parvocellular PVN and SON after s.c. injection of formalin. (ii) OXT-mRFP1 neurones in the anterior parvocellular PVN, which may project to the dorsal horn of the spinal cord, were activated by s.c. injection of formalin, as indicated by a significant increases of Fos-IR and mRFP1 intensity intensity. (iii) Formalin injection caused a significant transient increase in plasma OXT. (iv) OXT, mRFP1 and corticotrophin-releasing hormone mRNAs in the PVN were significantly increased after s.c. injection of formalin. (v) An intrathecal injection of OXT-saporin induced hypersensitivity in conscious rats. Taken together, these results suggest that the hypothalamo-neurohypophysial/-spinal OXTergic pathways may be involved in acute nociceptive responses in rats.

Effects of intranasal oxytocin on steroid hormones in men and women

BACKGROUND

Recent interest in the social and cognitive effects of intranasal oxytocin prompts a need for understanding its physiological effects in humans. Few studies have examined the effects of intranasal oxytocin on steroid hormones. Filling this gap is especially important given the evidence that steroid hormones participate in some of the same behavioral functions as oxytocin, e.g. in stress, processing of emotional stimuli, aggression, trust, empathy, and parental care.

METHODS

In randomized, double-blind experiments, we administered oxytocin (24 IU) or saline placebo to 97 healthy participants. Saliva samples were collected before and at several time points after the oxytocin/placebo administration to assess the levels of cortisol, progesterone, and testosterone.

RESULTS

Oxytocin had no effects on testosterone, progesterone, or cortisol in women or men.

CONCLUSION

Acute intranasal oxytocin does not affect the levels of cortisol, testosterone or progesterone in humans, at least in the absence of a stressful context. These data suggest that acute oxytocin does not have a direct impact on the human hypothalamic-pituitary-adrenal or hypothalamic-pituitary-gonadal axes under nonstressful circumstances. This knowledge helps rule out potential mechanisms for some of the effects of oxytocin in humans and adds to the generally limited body of knowledge on the basic physiological or psychological effects of intranasal oxytocin in human beings.

Oxytocin in General Anxiety and Social Fear: A Translational Approach

The neuropeptide oxytocin (OXT) has been revealed as a profound anxiolytic and antistress factor of the brain, besides its many prosocial and reproductive effects. Therefore, there is substantial scientific and medical interest in its potential therapeutic use for the treatment of psychopathologies associated with anxiety, fear, and social dysfunctions, such as generalized anxiety disorder, posttraumatic stress disorder, and social anxiety disorder, as well as autism and schizophrenia, among others. Focusing on preclinical studies, we review the existing evidence for the regulatory capacity of OXT to fine-tune general and social anxiety-related behaviors, as well as cued and social fear conditioning from a translational perspective. The available evidence from animal and human studies substantiates the hypothesis of an imbalance of the endogenous brain OXT system in the etiology of anxiety disorders, particularly those with a social component such as social anxiety disorder. In addition, such an imbalance of the OXT system is also likely to be the consequence of chronic OXT treatment resulting in a dose-dependent reduction in OXT receptor availability and increased anxiety.