Oxytocin and vasopressin agonists and antagonists as research tools and potential therapeutics

Nonapeptides mediate trade-offs in parental care strategy

Parental care represents a suite of distinct behaviors performed by parents to maximize fitness. Dynamic shifts in parental care behaviors, such as between nest defense and direct provisioning of the offspring, are required in response to environmental variation. However, the neural mechanisms which mediate such behavioral shifts remain a mystery. The anemonefish, Amphiprion ocellaris, represents an experimentally valuable model in social neuroscience which is conducive to manipulating the environment while simultaneously measuring parental care. The goal of this study was to determine the extent to which arginine vasotocin (AVT) and isotocin (IT) signaling are necessary for males to shift between direct egg care and aggressive nest defense in the presence of intruders, Domino damselfish (Dascyllus trimaculatus). The IT receptor antagonist desGly-NH₂-d(CH₂)₅[D-Tyr²,Thr⁴]OVT, significantly reduced direct egg care, while at the same time increased levels of aggressive nest defense relative to vehicle. Conversely, blockade of AVT using the antagonist d(CH₂)₅[Tyr(Me)²]AVP, reduced aggression and tended to increase egg care. Results demonstrate that male anemonefish alter their parental strategy in response to allospecific intruders, and that IT and AVT signaling oppositely regulate parental care displays of aggression versus egg care.

A Self-Assembling Lipidic Peptide and Selective Partial V2 Receptor Agonist Inhibits Urine Production

Lipidised analgesic peptide prodrugs self-assemble into peptide nanofibers; with the nanofiber morphology protecting the peptide from plasma degradation and improving therapeutic efficacy. Extending this learning, we hypothesised that a self-assembling lipidized peptide arginine vasopressin (AVP) receptor agonist, that had not been designed as a prodrug, could prove pharmacologically active and control urine production. The only approved AVP receptor agonist, desmopressin is indicated for the treatment of central diabetes insipidus (DI), bedwetting, haemophilia A and von Willebrand disease. Desmopressin is well tolerated by most patients, however adverse effects, such as hyponatraemia and water intoxication necessitate a strict fluid intake, thus motivating the search for alternative DI treatments. Selective V2 receptor agonism is required for anti-DI activity and we hypothesised that our new lipidized peptide (METx) would lead to selective AVP receptor agonism. METx was synthesised and characterised and then tested for activity against the V2, V1a and OT uterine receptors and not tested against the V1b receptor as METx was not expected to cross the blood brain barrier. METx was also tested in vivo in a healthy rat model. METx forms nanofibers and is a partial V2 receptor agonist (determined by measuring MDCK cell line cAMP accumulation), producing 57% of AVP's maximal activity (EC50 = 2.7 nM) and is not a V1a agonist up to a concentration of 1 μM (determined by measuring A7r5 cell line D-myo-inositol-1-phosphate accumulation). METx is a weak OT receptor antagonist, reducing the frequency of OT induced contractions (EC50 = 350 nM) and increasing the OT EC50 from 0.081 nM to 21 nM at a concentration of 600 nM. METx (41 nM) had no effect on spontaneous uterine contractions and METx (100 nM) had no effect on OT induced uterine contractions. Simulated binding studies show that binding avidity to the receptors follows the trend: V2 > OT > V1a. On intravenous injection, a nanoparticle formulation of METx reduced urine production in a healthy rat model in a dose responsive manner, with 40 mg kg^-1 METx resulting in no urine production over 4 hours. The lipidized self-assembling peptide - METx - is a selective competitive V2 receptor agonist and an anti-diuretic.

An Allostatic Theory of Oxytocin

Oxytocin has garnered considerable interest for its role in social behavior, as well as for the potential of intranasal administration to treat social difficulties. However, current theoretical models for the role of oxytocin in social behavior pay little consideration to its evolutionary and developmental history. This article aims to broaden our understanding of the role of oxytocin in social behavior by adopting an ethological approach through the lens of Nikolaas Tinbergen's 'four questions' - how does oxytocin work; how does the role of oxytocin change during development; how does oxytocin enhance survival; and how did the oxytocin system evolve? We argue that oxytocin is most accurately described as an allostatic hormone that modulates both social and non-social behavior by maintaining stability through changing environments.

Oxytocin receptor antagonist reverses the blunting effect of pair bonding on fear learning in monogamous prairie voles

Social relationships among spouses, family members, and friends are known to affect physical and mental health. In particular, long-lasting bonds between socio-sexual partners have profound effects on cognitive, social, emotional, and physical well-being. We have previously reported that pair bonding in monogamous prairie voles (Microtus ochrogaster) is prevented by a single prolonged stress (SPS) paradigm, which causes behavioral and endocrine symptoms resembling post-traumatic stress disorder (PTSD) patients in rats (Arai et al., 2016). Since fear memory function is crucial for anxiety-related disorders such as PTSD, we investigated the effects of pair bonding on fear learning in prairie voles. We applied an SPS paradigm to male prairie voles after the cohabitation with a male (cage-mate group) or female (pair-bonded group). The cage-mate group, but not the pair-bonded group, showed enhanced fear response in a contextual fear conditioning test following the SPS treatment. Immunohistochemical analyses revealed that cFos-positive cells in the central amygdala were increased in the pair-bonded group after the contextual fear conditioning test and that oxytocin immunoreactivity in the paraventricular nucleus of the hypothalamus was significantly higher in the pair-bonded group than the cage-mate group. This pair-bonding dependent blunting of fear memory response was confirmed by a passive avoidance test, another fear-based learning test. Interestingly, intracerebroventricular injection of an oxytocin receptor antagonist 30 min before the passive avoidance test blocked the blunting effect of pair bonding on fear learning. Thus, pair bonding between socio-sexual partners results in social buffering in the absence of the partner, blunting fear learning, which may be mediated by oxytocin signaling.

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.

Amygdala, neuropeptides, and chronic pain-related affective behaviors

Neuropeptides play important modulatory roles throughout the nervous system, functioning as direct effectors or as interacting partners with other neuropeptide and neurotransmitter systems. Limbic brain areas involved in learning, memory and emotions are particularly rich in neuropeptides. This review will focus on the amygdala, a limbic region that plays a key role in emotional-affective behaviors and pain modulation. The amygdala is comprised of different nuclei; the basolateral (BLA) and central (CeA) nuclei and in between, the intercalated cells (ITC), have been linked to pain-related functions. A wide range of neuropeptides are found in the amygdala, particularly in the CeA, but this review will discuss those neuropeptides that have been explored for their role in pain modulation. Calcitonin gene-related peptide (CGRP) is a key peptide in the afferent nociceptive pathway from the parabrachial area and mediates excitatory drive of CeA neurons. CeA neurons containing corticotropin releasing factor (CRF) and/or somatostatin (SOM) are a source of long-range projections and serve major output functions, but CRF also acts locally to excite neurons in the CeA and BLA. Neuropeptide S (NPS) is associated with inhibitory ITC neurons that gate amygdala output. Oxytocin and vasopressin exert opposite (inhibitory and excitatory, respectively) effects on amygdala output. The opioid system of mu, delta and kappa receptors (MOR, DOR, KOR) and their peptide ligands (β-endorphin, enkephalin, dynorphin) have complex and partially opposing effects on amygdala function. Neuropeptides therefore serve as valuable targets to regulate amygdala function in pain conditions. This article is part of the special issue on Neuropeptides.

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.

Mechanisms involved in the cardiovascular effects caused by acute osmotic stimulation in conscious rats

Both the autonomic nervous system and the neuroendocrine system are activated by osmotic stimulation (OS) evoking cardiovascular effects. The current study investigated the mechanisms involved in the cardiovascular responses evoked by an acute osmotic stimulus with intraperitoneal (i.p.) injection of either isotonic (0.15 M NaCl) or hypertonic saline (0.6 M NaCl) in conscious rats. Hypertonic saline increased mean arterial pressure (MAP) and heart rate (HR) for 30 min, as well as plasma osmolality and sodium content. Urinary sodium and urinary volume were also increased. Pretreatment with the ganglion blocker pentolinium (i.v.) did not affect the pressor response, but significantly decreased the tachycardic response caused by OS. Pretreatment with the V₁-vasopressin receptor antagonist dTyr(CH₂)₅(Me)AVP (i.v.) reduced the pressor response, without affecting the tachycardic response evoked by the hypertonic OS. Neither the pressor nor the tachycardic response to OS was affected by pretreatment with either the oxytocin receptor antagonist atosiban or the α1-antagonist prazosin. Pretreatment with the β1-antagonist atenolol had no effect on the pressor response, but markedly decreased the tachycardic response evoked by OS. Results indicate that i.p. hypertonic OS-evoked pressor response is mediated by the release of vasopressin, with a minor influence of the vascular sympathetic input.LAY SUMMARYIncreased plasma osmolality, such as that observed during dehydration or salt intake, is a potent stimulus yielding to marked cardiovascular and neuroendocrine responses. The intraperitoneal (i.p.) injection of hypertonic saline solution is a commonly used animal model to cause a sustained increase in plasma osmolality, leading to a cardiovascular response characterized by sustained blood pressure and heart increases, whose systemic mechanisms were presently studied. Our findings indicate that the pressor response to the i.p. osmotic stimulus (OS) is mediated mainly by the release of vasopressin into the blood circulation with a minor or even the noninvolvement of the vascular sympathetic nervous system, whereas activation of the sympathetic-cardiac system mediates the tachycardic response to OS.

Targetable cellular signaling events mediate vascular pathology in vascular Ehlers-Danlos syndrome

Vascular Ehlers-Danlos syndrome (vEDS) is an autosomal-dominant connective tissue disorder caused by heterozygous mutations in the COL3A1 gene, which encodes the pro-α 1 chain of collagen III. Loss of structural integrity of the extracellular matrix is believed to drive the signs and symptoms of this condition, including spontaneous arterial dissection and/or rupture, the major cause of mortality. We created 2 mouse models of vEDS that carry heterozygous mutations in Col3a1 that encode glycine substitutions analogous to those found in patients, and we showed that signaling abnormalities in the PLC/IP3/PKC/ERK pathway (phospholipase C/inositol 1,4,5-triphosphate/protein kinase C/extracellular signal-regulated kinase) are major mediators of vascular pathology. Treatment with pharmacologic inhibitors of ERK1/2 or PKCβ prevented death due to spontaneous aortic rupture. Additionally, we found that pregnancy- and puberty-associated accentuation of vascular risk, also seen in vEDS patients, was rescued by attenuation of oxytocin and androgen signaling, respectively. Taken together, our results provide evidence that targetable signaling abnormalities contribute to the pathogenesis of vEDS, highlighting unanticipated therapeutic opportunities.

Functionally Selective Inhibition of the Oxytocin Receptor by Retosiban in Human Myometrial Smooth Muscle

Novel small molecule inhibitors of the oxytocin receptor (OTR) may have distinct pharmacology and mode of action when compared with first-generation oxytocin antagonists when used for the prevention of preterm birth. The aim was to determine the mechanism of action of small molecule OTR antagonists retosiban and epelsiban compared with the currently used peptide-based compound atosiban. Human myometrial samples were obtained at cesarean section and subjected to pharmacological manipulations to establish the effect of antagonist binding to OTR on downstream signaling. Retosiban antagonism of oxytocin action in human myometrium was potent, rapid, and reversible. Inhibition of inositol 1,4,5-trisphosphate (IP3) production followed single-site competitive binding kinetics for epelsiban, retosiban, and atosiban. Retosiban inhibited basal production of IP3 in the absence of oxytocin. Oxytocin and atosiban but not retosiban inhibited forskolin, and calcitonin stimulated 3',5'-cyclic adenosine 5'-mono-phosphate (cAMP) production. Inhibition of cAMP was reversed by pertussis toxin. Oxytocin and atosiban, but not retosiban and epelsiban, stimulated extracellular regulated kinase (ERK)1/2 activity in a time- and concentration-dependent manner. Oxytocin and atosiban stimulated cyclo-oxygenase 2 activity and subsequent production of prostaglandin E2 and F2α. Prostaglandin production was inhibited by rofecoxib, pertussin toxin, and ERK inhibitor U0126. Oxytocin but not retosiban or atosiban stimulated coupling of the OTR to Gα q G-proteins. Oxytocin and atosiban but not retosiban stimulated coupling of the OTR to Gα i G-proteins. Retosiban and epelsiban demonstrate distinct pharmacology when compared with atosiban in human myometrial smooth muscle. Atosiban displays agonist activity at micromolar concentrations leading to stimulation of prostaglandin production.

Orally administered oxytocin alters brain activation and behaviors of pre-weaning mice

Oxytocin (OXT) regulates adult social behavior and has been implicated in its development. Because mammalian milk contains OXT and we have recently identified OXT receptors (OXTR) in the face and oronasal cavity of pre-weaning mice, we hypothesize that orally applied OXT may impact brain activity and acute behavior in developing mice. Oral OXT may have effects in the absence of sensory stimulation or perhaps by modulating sensory input, such as whisker stimulation. The present study investigates the acute c-Fos response in the paraventricular nucleus of the hypothalamus (PVN) and along whisker sensory processing brain regions (trigeminothalamocortical circuit) to orally applied OXT, compared to saline, with and without whisker stimulation in postnatal day (P) 14 and P21 male and female mice. Acute behavioral responses were also quantified after oral OXT with whisker stimulation in a non-social context. Oral OXT with and without whisker stimulation increased c-Fos activity in the PVN of males and decreased c-Fos in the ventroposterior medial thalamus in both males and females compared to saline. Additionally, oral OXT with whisker stimulation decreased c-Fos activity across whisker sensory processing brain regions in males and females and decreased c-Fos activity in the trigeminal motor nucleus of females. Lastly, oral OXT with whisker stimulation increased males' locomotor behavior and decreased females' oromotor behavior compared to saline-treated controls. These data indicate that orally applied OXT has acute brain and behavioral effects on developing mice. OXT-modulated sensory signals may bias brain and behavior development toward the social world.

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.

Development and therapeutic potential of vasopressin synthetic analog [V4Q5]dDAVP as a novel anticancer agent

Since its discovery, arginine vasopressin (AVP) was subjected to several modifications with the aim of obtaining novel derivatives with increased potency and selectivity for biomedical use. Desmopressin (dDAVP) is a first generation synthetic analog of AVP with hemostatic and antimetastatic activity. dDAVP acts as a selective agonist of the arginine vasopressin type 2 receptor (AVPR2) present in microvascular endothelium and cancer cells. Considering its selective effects on AVPR2-expressing malignant and vascular tissue, and interesting antitumor profile, dDAVP was used as a lead compound for the development of novel peptide analogs with enhanced anticancer efficacy. After conducting different structure-activity relationship studies to determine key aminoacidic positions for its antitumor activity against AVPR2-expressing malignant cells, dDAVP was rationally modified and a wide panel of synthetic analogs with different sequence and structural modifications was assessed. As a result of this structure-based drug derivatization novel AVP analog [V⁴Q⁵]dDAVP (1-deamino-4-valine-5-glutamine-8-d-arginine vasopressin) was selected as the most active candidate and further developed. [V⁴Q⁵]dDAVP was evaluated in highly aggressive and metastatic cancer preclinical models deploying enhanced cytostatic, antimetastatic and angiostatic effects in comparison to parental peptide dDAVP. In addition, novel compound demonstrated good tolerability as evaluated in several toxicological studies, and cooperative therapeutic effects after combination with standard-of-care chemotherapy. In summary, due to its ability to inhibit growth and tumor-associated angiogenesis, as well as impairing progression of metastatic disease, AVP analogs such as novel [V⁴Q⁵]dDAVP are promising compounds for further development as coadjuvant agents for the management of advance or recurrent cancers.

Oxytocin/vasopressin-like neuropeptide signaling in insects

The origin of the oxytocin (OT)/vasopressin (VP) signaling system is thought to date back more than 600million years. OT/VP-like peptides have been identified in numerous invertebrate phyla including molluscs, annelids, nematodes and insects. However, to date we only have a limited understanding of the biological role(s) of this GPCR-mediated signaling system in insects. This chapter presents the current knowledge of OT/VP-like neuropeptide signaling in insects by providing a brief overview of insect OT/VP-like neuropeptides, their genetic and structural commonalities, and their experimentally tested and proposed functions. Despite their widespread occurrence across insect orders these peptides (and their endogenous receptors) appear to be absent in common insect model species, such as flies and bees. We therefore explain the known functionalities of this signaling system in three different insect model systems: beetles, locusts, and ants. Additionally, we review the phylogenetic distribution of the OT/VP signaling system in arthropods as obtained from extensive genome/transcriptome mining. Finally, we discuss the unique challenges in the development of selective OT/VP ligands for human receptors and share our perspective on the possible application of insect- and other non-mammalian-derived OT/VP-like peptide ligands in pharmacology.

Impaired hypotensive effects of centrally acting oxytocin in SHR and WKY rats exposed to chronic mild stress

The present study was designed to determine the role of centrally acting oxytocin (OT) in the regulation of blood pressure during chronic mild stress (CMS) in spontaneously hypertensive (SHR; n = 36) and normotensive Wistar-Kyoto (WKY; n = 38) rats. The rats were implanted with osmotic minipumps for intracerebroventricular infusions of 0.9% NaCl, OT, and oxytocin receptor antagonist (OTANT) and divided into two groups: SHR and WKY 1) exposed to 4-wk CMS and 2) not exposed to stress (controls). After 4 wk, hemodynamic parameters were recorded at rest and after an application of acute stressor [air-jet stress (AJS)]. Resting mean arterial blood pressure (MAP) was significantly lower in CMS-exposed SHR and WKY infused with OT than in the corresponding groups receiving saline. Exposure to CMS exaggerated the AJS-dependent pressor response in WKY receiving saline but not in the corresponding group of SHR. OT infusion reduced the AJS-dependent pressor response in both CMS-exposed and not exposed SHR and in CMS-exposed WKY. Intracerebroventricular infusion of OTANT potentiated the AJS-dependent pressor response in both stressed and not stressed WKY rats but not in SHR. The results show that centrally delivered OT decreases resting MAP during CMS in both SHR and WKY rats and that in SHR it reduces pressor responses to AJS under control and CMS conditions, whereas in WKY this effect is significant only after CMS exposure. The study indicates that endogenous centrally acting OT may play an essential role in buffering pressor responses to AJS in CMS-exposed and not exposed WKY rats and that this function is significantly impaired in SHR.

I8-arachnotocin-an arthropod-derived G protein-biased ligand of the human vasopressin V2 receptor

The neuropeptides oxytocin (OT) and vasopressin (VP) and their G protein-coupled receptors OTR, V_1aR, V_1bR, and V₂R form an important and widely-distributed neuroendocrine signaling system. In mammals, this signaling system regulates water homeostasis, blood pressure, reproduction, as well as social behaviors such as pair bonding, trust and aggression. There exists high demand for ligands with differing pharmacological profiles to study the physiological and pathological functions of the individual receptor subtypes. Here, we present the pharmacological characterization of an arthropod (Metaseiulus occidentalis) OT/VP-like nonapeptide across the human OT/VP receptors. I8-arachnotocin is a full agonist with respect to second messenger signaling at human V₂R (EC₅₀ 34 nM) and V_1bR (EC₅₀ 1.2 µM), a partial agonist at OTR (EC₅₀ 790 nM), and a competitive antagonist at V_1aR [pA₂ 6.25 (558 nM)]. Intriguingly, I8-arachnotocin activated the Gα_s pathway of V₂R without recruiting either β-arrestin-1 or β-arrestin-2. I8-arachnotocin might thus be a novel pharmacological tool to study the (patho)physiological relevance of β-arrestin-1 or -2 recruitment to the V₂R. These findings furthermore highlight arthropods as a novel, vast and untapped source for the discovery of novel pharmacological probes and potential drug leads targeting neurohormone receptors.

Peptides, Antibodies, Peptide Antibodies and More

The applications of peptides and antibodies to multiple targets have emerged as powerful tools in research, diagnostics, vaccine development, and therapeutics. Antibodies are unique since they, in theory, can be directed to any desired target, which illustrates their versatile nature and broad spectrum of use as illustrated by numerous applications of peptide antibodies. In recent years, due to the inherent limitations such as size and physical properties of antibodies, it has been attempted to generate new molecular compounds with equally high specificity and affinity, albeit with relatively low success. Based on this, peptides, antibodies, and peptide antibodies have established their importance and remain crucial reagents in molecular biology.

Effects of optogenetic stimulation of vasopressinergic retinal afferents on suprachiasmatic neurones

Physiological circadian rhythms are orchestrated by the hypothalamic suprachiasmatic nucleus (SCN). The activity of SCN cells is synchronised by environmental signals, including light information from retinal ganglion cells (RGCs). We recently described a population of vasopressin-expressing RGCs (VP-RGC) that send axonal projections to the SCN. To determine how these VP-RGCs influence the activity of cells in the SCN, we used optogenetic tools to specifically activate their axon terminals within the SCN. Rats were intravitreally injected with a recombinant adeno-associated virus to express the channelrhodopsin-2 and the red fluorescent protein mCherry under the vasopressin promoter (VP-ChR2mCherry). In vitro recordings in acute brain slices showed that approximately 30% of ventrolateral SCN cells responded to optogenetic stimulation with an increase in firing rate that progressively increased during the first 200 seconds of stimulation and which persisted after the end of stimulation. Finally, application of a vasopressin V1A receptor antagonist dampened the response to optogenetic stimulation. Our data suggest that optogenetic stimulation of VP-RGC axons within the SCN influences the activity of SCN cells in a vasopressin-dependent manner.

Exogenous vasopressin dose-dependently modulates gastric microcirculatory oxygenation in dogs via V1A receptor

BACKGROUND

Hypercapnia improves gastric microcirculatory oxygenation (μHbO₂) and increases vasopressin plasma levels, whereas V1A receptor blockade abolishes the increase of μHbO₂. The aim of this study was to evaluate the effect of exogenous vasopressin (AVP) in increasing doses on microcirculatory perfusion and oxygenation and systemic hemodynamic variables. Furthermore, we evaluated the role of the vasopressin V1A receptor in mediating the effects.

METHODS

In repetitive experiments, six anesthetized dogs received a selective vasopressin V1A receptor inhibitor ([Pmp¹, Tyr (Me)²]-Arg⁸-Vasopressin) or sodium chloride (control groups). Thereafter, a continuous infusion of AVP was started with dose escalation every 30 min (0.001 ng/kg/min-1 ng/kg/min). Microcirculatory variables of the oral and gastric mucosa were measured with reflectance spectrometry, laser Doppler flowmetry, and incident dark field imaging. Transpulmonary thermodilution was used to measure systemic hemodynamic variables. AVP plasma concentrations were measured during baseline conditions and 30 min after each dose escalation.

RESULTS

During control conditions, gastric μHbO₂ did not change during the course of experiments. Infusion of 0.001 ng/kg/min and 0.01 ng/kg/min AVP increased gastric μHbO₂ to 87 ± 4% and 87 ± 6%, respectively, compared to baseline values (80 ± 7%), whereas application of 1 ng/kg/min AVP strongly reduced gastric μHbO₂ (59 ± 16%). V1A receptor blockade prior to AVP treatment abolished these effects on μHbO₂. AVP dose-dependently enhanced systemic vascular resistance (SVR) and decreased cardiac output (CO). After prior V1A receptor blockade, SVR was reduced and CO increased (0.1 ng/kg/min + 1 ng/kg/min AVP).

CONCLUSIONS

Exogenous AVP dose-dependently modulates gastric μHbO₂, with an increased μHbO₂ with ultra-low dose AVP. The effects of AVP on μHbO₂ are abolished by V1A receptor inhibition. These effects are independent of a modulation of systemic hemodynamic variables.

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.

Central oxytocin alters cortisol and behavioral responses of guinea pig pups during isolation in a novel environment

The neuropeptide oxytocin plays key roles in social bonding and stress reduction, and thus appears to be a likely mediator of maternal buffering of infant stress responses. In the guinea pig, the presence of the mother in a threatening environment buffers cortisol elevations as well as active (vocalizing) and passive (e.g. crouching) responses typical of isolation in this species; yet, effects of OT in guinea pig pups under any conditions have not been reported. Here, we examined the ability of intracerebroventricular (ICV) OT to moderate plasma cortisol levels and behavior in guinea pig pups isolated in a brightly lit, novel environment, and the ability of a highly selective OT antagonist (OTA) to reduce buffering by the mother. We found that ICV OT moderated cortisol levels and vocalizations, but increased time spent in the crouched stance, particularly in females. In addition, OT modulated other ongoing behaviors in a sex-dependent fashion. In females, OT reduced duration of walking and rearing, and increased time spent quiet, while in males OT increased duration of rearing. OTA, however, was without effect on cortisol levels or behavior. These findings, including sex differences in response, extend results from other species to the guinea pig. Further, while demonstrating that exogenous OT is sufficient to reduce biobehavioral stress responses typical of isolated guinea pig infants, the results suggest that endogenous OT is not necessary for maternal buffering of infant responses in this species.

The effects of extended nitric oxide release on responses of the human non-pregnant myometrium to endothelin-1 or vasopressin

BACKGROUND

Uterotonic mediators: endothelin-1 (ET-1), arginine vasopressin (AVP), and nitric oxide (NO) play important roles in the regulation of uterine contractility. We hypothesize that NO affects both ET-1 or AVP. Therefore, this study investigated the involvement of extended exogenous NO release in the regulation of responses of the human non-pregnant myometrium to ET-1 and AVP.

METHODS

Specimens were obtained from 10 premenopausal women, undergoing hysterectomy for benign gynecological disorders. Responses of the myometrial strips to ET-1 or AVP in the absence and presence of an exogenous NO donor (diethylenetriamine; DETA/NO; 10-4 mol/L) were recorded under isometric conditions. To inhibit endogenous NO, a competitive inhibitor of NO synthase, L-NG-nitroarginine (L-NNA) was added to the organ bath.

RESULTS

ET-1 enhanced the spontaneous contractile activity of the myometrium more powerfully (p < 0.01) than AVP. Preincubation with exogenous NO weakened ET-1- or AVP-induced increases in this contractile activity (p < 0.05). However, unexpected results were obtained after preincubation with L-NNA and with DETA/NO then added. Both ET-1 and AVP induced augmented contractile effects in almost all concentrations compared with the responses to these peptides alone or after NOS synthase inhibition (both p < 0.01).

CONCLUSIONS

This study demonstrated for the first time that extended incubation with a NO donor influences the uterine muscle response evoked by ET-1 and AVP. Both endogenous and exogenous NO is involved in the control of the uterine responses to ET-1 or AVP of non-pregnant myometrium. Furthermore, both peptides stimulate increased uterine contractility when the local imbalance between the constrictive and relaxing mediators takes place.

Intracerebroventricular administration of arginine vasotocin (AVT) induces anorexigenesis and anxiety-like behavior in goldfish

Arginine vasotocin (AVT) is known as a neurohypophyseal hormone that regulates water- and mineral-balance in non-mammalian vertebrates. Recent studies revealed that AVT also exerts central effects on behavior. The goldfish has several merits for evaluation of behavioral changes. However, there is few information on the behavioral action of AVT in this species. Here we examined the effects of AVT on food intake and psychomotor activity. AVT was administered intracerebroventricularly at 1, 5 and 10 pmol g^-1 body weight (BW). Intracerebroventricular (ICV) administration of AVT at 5 and 10 pmol g^-1 BW significantly decreased food intake during 30 min after injection and recovery from anesthesia. The AVT-induced anorexigenic action was attenuated by treatment with the AVT receptor V1aR antagonist Manning compound (MC) at 50 pmol g^-1 BW. As the goldfish tends to prefer the lower to the upper area of a tank, we used this preference behavior for assessing psychomotor activity during a 30-min observation period. ICV administration of AVT at 1, 5 and 10 pmol g^-1 BW significantly prolonged the time spent in the lower area, but did not affect locomotor activity in the tank at any dose. The action of AVT was similar to that of the central-type benzodiazepine receptor inverse agonist FG-7142 at 10 pmol g^-1 BW. AVT-induced anxiety-like behavior was blocked by treatment with MC at 50 pmol g^-1 BW. These results indicate that AVT affects food intake and psychophysiological status, and also induces anorexigenic- and anxiogenic-like actions via the V1aR-signaling pathway in the goldfish brain.

Disulfide engineering on temporin-SHf: Stabilizing the bioactive conformation of an ultra-short antimicrobial peptide

In Silico searching for short antimicrobial peptides has revealed temporin-SHf as the short (8AA), hydrophobic, broad spectrum, and natural antimicrobial peptide. Important drawback associated with temporin-SHf is the susceptibility of its bioactive conformation for denaturation and proteolytic degradation. In the current report, disulfide engineering strategy has been adopted to improve the stability of bioactive conformation of temporin-SHf. The functionally non-critical Leu4 and Ile7 residues at i and i + 3 position of helical conformation of temporin-SHf were mutated with cysteine disulfide. Designed [L4C, I7C]temporin-SHf was synthesized, characterized using NMR spectroscopy, and accessed for antimicrobial activity. [L4C, I7C]Temporin-SHf adopts helical conformation from Phe3 to Phe8 in the absence of membrane-mimetic environment and retains broad spectrum antimicrobial activity. The reduction potential of cysteine disulfide of [L4C, I7C]temporin-SHf is -289 mV. Trypsin-induced digestion and serum-induced digestion have confirmed the advantage of cysteine disulfide in imparting proteolytic stability to temporin-SHf. Disulfide-stabilized temporin-SHf may serve as a good model for the rational design of temporin-SHf based antibiotics for treatment of infectious diseases.

AVPR1b variation and the emergence of adaptive phenotypes in Platyrrhini primates

Platyrrhini (New World monkeys, NWm) are a group of primates characterized by behavioral and reproductive traits that are otherwise uncommon among primates, including social monogamy, direct paternal care, and twin births. As a consequence, the study of Platyrrhine primates is an invaluable tool for the discovery of the genetic repertoire underlying these taxon-specific traits. Recently, high conservation of vasopressin (AVP) sequence, in contrast with high variability of oxytocin (OXT), has been described in NWm. AVP and OXT functions are possible due to interaction with their receptors: AVPR1a, AVPR1b, AVPR2, and OXTR; and the variability in this system is associated with the traits mentioned above. Understanding the variability in the receptors is thus fundamental to understand the function and evolution of the system as a whole. Here we describe the variability of AVPR1b coding region in 20 NWm species, which is well-known to influence behavioral traits such as aggression, anxiety, and stress control in placental mammals. Our results indicate that 4% of AVPR1b sites may be under positive selection and a significant number of sites under relaxed selective constraint. Considering the known role of AVPR1b, we suggest that some of the changes described here for the Platyrrhini may be a part of the genetic repertoire connected with the complex network of neuroendocrine mechanisms of AVP-OXT system in the modulation of the HPA axis. Thus, these changes may have promoted the emergence of social behaviors such as direct paternal care in socially monogamous species that are also characterized by small body size and twin births.

Vasopressin Cells in the Rodent Olfactory Bulb Resemble Non-Bursting Superficial Tufted Cells and Are Primarily Inhibited upon Olfactory Nerve Stimulation

The intrinsic vasopressin system of the olfactory bulb is involved in social odor processing and consists of glutamatergic vasopressin cells (VPCs) located at the medial border of the glomerular layer. To characterize VPCs in detail, we combined various electrophysiological, neuroanatomical, and two-photon Ca2+ imaging techniques in acute bulb slices from juvenile transgenic rats with eGFP-labeled VPCs. VPCs showed regular non-bursting firing patterns, and displayed slower membrane time constants and higher input resistances versus other glutamatergic tufted cell types. VPC axons spread deeply into the external plexiform and superficial granule cell layer (GCL). Axonal projections fell into two subclasses, with either denser local columnar collaterals or longer-ranging single projections running laterally within the internal plexiform layer and deeper within the granule cell layer. VPCs always featured lateral dendrites and a tortuous apical dendrite that innervated a single glomerulus with a homogenously branching tuft. These tufts lacked Ca2+ transients in response to single somatically-evoked action potentials and showed a moderate Ca2+ increase upon prolonged action potential trains.Notably, electrical olfactory nerve stimulation did not result in synaptic excitation of VPCs, but triggered substantial GABAA receptor-mediated IPSPs that masked excitatory barrages with yet longer latency. Exogenous vasopressin application reduced those IPSPs, as well as olfactory nerve-evoked EPSPs recorded from external tufted cells. In summary, VPCs can be classified as non-bursting, vertical superficial tufted cells. Moreover, our findings imply that sensory input alone cannot trigger excitation of VPCs, arguing for specific additional pathways for excitation or disinhibition in social contexts.

Dissociation of natriuresis and diuresis by oxytocin molecular forms in rats

In the rat, oxytocin (OT) produces dose-dependent diuretic and natriuretic responses. Post-translational enzymatic conversion of the OT biosynthetic precursor forms both mature and C-terminally extended peptides. The plasma concentrations of these C-terminally extended peptides (OT-G; OT-GK and OT-GKR) are elevated in newborns and pregnant rats. Intravenous injection of OT-GKR to rats inhibits diuresis, whereas injection of amidated OT stimulates diuresis. Since OT and OT-GKR show different effects on the urine flow, we investigated whether OT-GKR modulates renal action by inhibition of the arginine-vasopressin (AVP) receptor V2 (V2R), the receptor involved in renal water reabsorption. Experiments were carried out in the 8-week-old Wistar rats receiving intravenous (iv) injections of vehicle, OT, OT-GKR or OT+OT-GKR combination. OT (10 μmol/kg) increased urine outflow by 40% (P<0.01) and sodium excretion by 47% (P<0.01). Treatment with OT-GKR (10 μmol/kg) decreased diuresis by 50% (P<0.001), decreased sodium excretion by 50% (P<0.05) and lowered potassium by 42% (P<0.05). OT antagonist (OTA) reduced diuresis and natriuresis exerted by OT, whereas the anti-diuretic effect of OT-GKR was unaffected by OTA. The treatment with V2R antagonist (V2A) in the presence and absence of OT induced diuresis, sodium and potassium outflow. V2A in the presence of OT-GKR only partially increased diuresis and natriuresis. Autoradiography and molecular docking analysis showed potent binding of OT-GKR to V2R. Finally, the release of cAMP from CHO cells overexpressing V2 receptor was induced by low concentration of AVP (EC50:4.2e-011), at higher concentrations of OT (EC50:3.2e-010) and by the highest concentrations of OT-GKR (EC50:1.1e-006). OT-GKR potentiated cAMP release when combined with AVP, but blocked cAMP release when combined with OT. These results suggest that OT-GKR by competing for the OT renal receptor (OTR) and binding to V2R in the kidney, induces anti-diuretic, anti-natriuretic, and anti-kaliuretic effects.

Long-Acting and Selective Oxytocin Peptide Analogs Show Antidiabetic and Antiobesity Effects in Male Mice

Oxytocin (OXT) has been shown to suppress appetite, induce weight loss, and improve glycemic control and lipid metabolism in several species, including humans, monkeys, and rodents. However, OXT's short half-life in circulation and lack of receptor selectivity limit its application and efficacy. In this study, we report an OXT peptide analog (OXTGly) that is potent and selective for the OXT receptor (OXTR). OXT, but not OXTGly, activated vasopressin receptors in vitro and acutely increased blood pressure in vivo when administered IP. OXT suppressed food intake in mice, whereas OXTGly had a moderate effect on food intake when administered IP or intracerebroventricularly. Both OXT (IP) and OXTGly (IP) improved glycemic control in glucose tolerance tests. Additionally, both OXT (IP) and OXTGly (IP) stimulated insulin, glucagon-like peptide 1, and glucagon secretion in mice. We generated lipid-conjugated OXT (acylated-OXT) and OXTGly (acylated-OXTGly) and demonstrated that these molecules have significantly extended half-lives in vivo. Compared with OXT, 2-week treatment of diet-induced obese mice with acylated-OXT [subcutaneous(ly) (SC)] resulted in enhanced body weight reduction, an improved lipid profile, and gene expression changes consistent with increased lipolysis and decreased gluconeogenesis. Treatment with acylated-OXTGly (SC) also resulted in a statistically significant weight loss, albeit to a lesser degree compared with acylated-OXT treatment. In conclusion, we demonstrate that selective activation of the OXTR pathway results in both acute and chronic metabolic benefits, whereas potential activation of vasopressin receptors by nonselective OXT analogs causes physiological stress that contributes to additional weight loss.

The effects of combined oxytocin and cognitive behavioral social skills training on social cognition in schizophrenia

BACKGROUND

Individuals with schizophrenia have deficits in social cognition that are associated with poor functional outcome. Unfortunately, current treatments result in only modest improvement in social cognition. Oxytocin, a neuropeptide with pro-social effects, has significant benefits for social cognition in the general population. However, studies examining the efficacy of oxytocin in schizophrenia have yielded inconsistent results. One reason for inconsistency may be that oxytocin has typically not been combined with psychosocial interventions. It may be necessary for individuals with schizophrenia to receive concurrent psychosocial treatment while taking oxytocin to have the context needed to make gains in social cognitive skills.

METHODS

The current study tested this hypothesis in a 24-week (48 session) double-blind, placebo-controlled trial that combined oxytocin and Cognitive-Behavioral Social Skills Training (CBSST), which included elements from Social Cognition and Interaction Training (SCIT). Participants included 62 outpatients diagnosed with schizophrenia (placebo n = 31; oxytocin n = 31) who received 36 IU BID, with supervised administration 45 min prior to sessions on CBSST group therapy days. Participants completed a battery of measures administered at 0, 12, and 24 weeks that assessed social cognition.

RESULTS

CBSST generally failed to enhance social cognition from baseline to end of study, and there was no additive benefit of oxytocin beyond the effects of CBSST alone.

CONCLUSIONS

Findings suggest that combined CBSST and oxytocin had minimal benefit for social cognition, adding to the growing literature indicating null effects of oxytocin in multi-dose trials. Methodological and biological factors may contribute to inconsistent results across studies.

Discovery of Potent, Selective, and Short-Acting Peptidic V2 Receptor Agonists

The vasopressin analogue desmopressin (desamino-d-arginine⁸ vasopressin, dDAVP, 1) is a potent vasopressin 2 (V₂) receptor (V₂R) agonist approved in many countries for the treatment of diabetes insipidus, primary nocturnal enuresis, nocturia, and coagulation disorders. Since 1 is primarily excreted via the kidneys, an age-related decline in kidney function leads to slower elimination, prolonged antidiuresis, and hyponatremia. In search of novel, potent, selective, and short-acting peptidic V₂R agonists, we synthesized a series of C-terminally truncated analogues of [Val⁴]dDAVP, 2, modified in positions 2, 3, and 7 and/or at the disulfide bridge. The peptides were evaluated for in vitro potency at the human V₂ receptor, selectivity versus the related receptors (human vasopressin 1a receptor, human vasopressin 1b receptor, and human oxytocin receptor), and pharmacokinetic profiles in rodents and other higher species. The truncated analogues show excellent potency at the V₂R, increased systemic clearance, and shorter half-life in rats. Two compounds 19 (c(Bua-Cpa-Thi-Val-Asn-Cys)-Pro-Agm) and 38 (c(Bua-Cpa-Thi-Val-Asn-Cys)-Pro-d-Arg-NEt₂) have been selected for clinical development for nocturia.

Involvement of oxytocin and GABA in consolation behavior elicited by socially defeated individuals in mandarin voles

Consolation, which entails comforting contact directed toward a distressed party, is a common empathetic response in humans and other species with advanced cognition. Here, using the social defeat paradigm, we provide empirical evidence that highly social and monogamous mandarin voles (Microtus mandarinus) increased grooming toward a socially defeated partner but not toward a partner who underwent only separation. This selective behavioral response existed in both males and females. Accompanied with these behavioral changes, c-Fos expression was elevated in many of the brain regions relevant for emotional processing, such as the anterior cingulate cortex (ACC), bed nucleus of the stria terminalis, paraventricular nucleus (PVN), basal/basolateral and central nucleus of the amygdala, and lateral habenular nucleus in both sexes; in the medial preoptic area, the increase in c-Fos expression was found only in females, whereas in the medial nucleus of the amygdala, this increase was found only in males. In particular, the GAD67/c-Fos and oxytocin (OT)/c-Fos colocalization rates were elevated in the ACC and PVN, indicating selective activation of GABA and OT neurons in these regions. The "stressed" pairs matched their anxiety-like behaviors in the open-field test, and their plasma corticosterone levels correlated well with each other, suggesting an empathy-based mechanism. This partner-directed grooming was blocked by pretreatment with an OT receptor antagonist or a GABA_A receptor antagonist in the ACC but not by a V1a subtype vasopressin receptor antagonist. We conclude that consolation behavior can be elicited by the social defeat paradigm in mandarin voles, and this behavior may be involved in a coordinated network of emotion-related brain structures, which differs slightly between the sexes. We also found that the endogenous OT and the GABA systems within the ACC are essential for consolation behavior in mandarin voles.

Oxytocin receptors in the dorsolateral bed nucleus of the stria terminalis (BNST) bias fear learning toward temporally predictable cued fear

The inability to discriminate between threat and safety is a hallmark of stress-induced psychiatric disorders, including post-traumatic stress disorder. Dorsolateral bed nucleus of the stria terminalis (BNST_dl) is critically involved in the modulation of fear and anxiety, and has been proposed to regulate discrimination between signaled (cued, predictable) and unsignaled (unpredictable) threats. We recently showed that oxytocin receptors (OTRs) in the BNST_dl facilitate acquisition of cued fear measured in a fear-potentiated startle (FPS). In the current study, using in vivo microdialysis in awake male Sprague-Dawley rats, a double immunofluorescence approach with confocal microscopy, as well as retrograde tracing of hypothalamic BNST-projecting OT neurons, we investigated whether fear conditioning activates OT system and modulates OT release. To determine the role of OTR in fear memory formation, we also infused OTR antagonist or OT into the BNST_dl before fear conditioning and measured rats' ability to discriminate between cued (signaled) and non-cued (unsignaled) fear using FPS. In contrast to acute stress (exposure to forced swim stress or foot shocks alone), cued fear conditioning increases OT content in BNST_dl microdialysates. In addition, fear conditioning induces moderate activation of OT neurons in the paraventricular nucleus of the hypothalamus and robust activation in the supraoptic and accessory nuclei of the hypothalamus. Application of OT into the BNST_dl facilitates fear learning toward signaled, predictable threats, whereas blocking OTR attenuates this effect. We conclude that OTR neurotransmission in the BNST_dl plays a pivotal role in strengthening fear learning of temporally predictable, signaled threats.

Oxytocin blocks enhanced motivation for alcohol in alcohol dependence and blocks alcohol effects on GABAergic transmission in the central amygdala

Oxytocin administration has been reported to decrease consumption, withdrawal, and drug-seeking associated with several drugs of abuse and thus represents a promising pharmacological approach to treat drug addiction. We used an established rat model of alcohol dependence to investigate oxytocin's effects on dependence-induced alcohol drinking, enhanced motivation for alcohol, and altered GABAergic transmission in the central nucleus of the amygdala (CeA). Intraperitoneal oxytocin administration blocked escalated alcohol drinking and the enhanced motivation for alcohol in alcohol-dependent but not nondependent rats. Intranasal oxytocin delivery fully replicated these effects. Intraperitoneal administration had minor but significant effects of reducing locomotion and intake of non-alcoholic palatable solutions, whereas intranasal oxytocin administration did not. In dependent rats, intracerebroventricular administration of oxytocin or the oxytocin receptor agonist PF-06655075, which does not cross the blood-brain barrier (i.e., it would not diffuse to the periphery), but not systemic administration of PF-06655075 (i.e., it would not reach the brain), decreased alcohol drinking. Administration of a peripherally restricted oxytocin receptor antagonist did not reverse the effect of intranasal oxytocin on alcohol drinking. Ex vivo electrophysiological recordings from CeA neurons indicated that oxytocin decreases evoked GABA transmission in nondependent but not in dependent rats, whereas oxytocin decreased the amplitude of spontaneous GABAergic responses in both groups. Oxytocin blocked the facilitatory effects of acute alcohol on GABA release in the CeA of dependent but not nondependent rats. Together, these results provide converging evidence that oxytocin specifically and selectively blocks the enhanced motivation for alcohol drinking that develops in alcohol dependence likely via a central mechanism that may result from altered oxytocin effects on CeA GABA transmission in alcohol dependence. Neuroadaptations in endogenous oxytocin signaling may provide a mechanism to further our understanding of alcohol use disorder.

Comparing vasopressin and oxytocin fiber and receptor density patterns in the social behavior neural network: Implications for cross-system signaling

Vasopressin (AVP) and oxytocin (OXT) regulate social behavior by binding to their canonical receptors, the vasopressin V1a receptor (V1aR) and oxytocin receptor (OTR), respectively. Recent studies suggest that these neuropeptides may also signal via each other's receptors. The extent to which such cross-system signaling occurs likely depends on anatomical overlap between AVP/OXT fibers and V1aR/OTR expression. By comparing AVP/OXT fiber densities with V1aR/OTR binding densities throughout the rat social behavior neural network (SBNN), we propose the potential for cross-system signaling in four regions: the medial amygdala (MeA), bed nucleus of the stria terminalis (BNSTp), medial preoptic area, and periaqueductal grey. We also discuss possible implications of corresponding sex (higher in males versus females) and age (higher in adults versus juveniles) differences in AVP fiber and OTR binding densities in the MeA and BNSTp. Overall, this review reveals the need to unravel the consequences of potential cross-system signaling between AVP and OXT systems in the SBNN for the regulation of social behavior.

Evidence for a role of arginine vasotocin receptors in the gill during salinity acclimation by a euryhaline teleost fish

The nonapeptide arginine vasotocin (AVT) regulates osmotic balance in teleost fishes, but its mechanisms of action are not fully understood. Recently, it was discovered that nonapeptide receptors in teleost fishes are differentiated into two V1a-type, several V2-type, and two isotocin (IT) receptors, but it remains unclear which receptors mediate AVT's effects on gill osmoregulation. Here, we examined the role of nonapeptide receptors in the gill of the euryhaline Amargosa pupfish (Cyprinodon nevadensis amargosae) during osmotic acclimation. Transcripts for the teleost V1a-type receptor v1a2 were upregulated over fourfold in gill 24 h after transferring pupfish from 7.5 ppt to seawater (35 ppt) or hypersaline (55 ppt) conditions and downregulated after transfer to freshwater (0.3 ppt). Gill transcripts for the nonapeptide degradation enzyme leucyl-cystinyl aminopeptidase (LNPEP) also increased in fish acclimating to 35 ppt. To test whether the effects of AVT on the gill might be mediated by a V1a-type receptor, we administered AVT or a V1-type receptor antagonist (Manning compound) intraperitoneally to pupfish before transfer to 0.4 ppt or 35 ppt. Pupfish transferred to 35 ppt exhibited elevated gill mRNA abundance for cystic fibrosis transmembrane conductance regulator (cftr), but that upregulation diminished under V1-receptor inhibition. AVT inhibited the increase in gill Na⁺/Cl^- cotransporter 2 (ncc2) transcript abundance that occurs following transfer to hypoosmotic environments, whereas V1-type receptor antagonism increased ncc2 mRNAs even without a change in salinity. These findings indicate that AVT acts via a V1-type receptor to regulate gill Cl^- transport by inhibiting Cl^- uptake and facilitating Cl^- secretion during seawater acclimation.

Altered Behaviors and Impaired Synaptic Function in a Novel Rat Model With a Complete Shank3 Deletion

Mutations within the Shank3 gene, which encodes a key postsynaptic density (PSD) protein at glutamatergic synapses, contribute to the genetic etiology of defined autism spectrum disorders (ASDs), including Phelan-McDermid syndrome (PMS) and intellectual disabilities (ID). Although there are a series of genetic mouse models to study Shank3 gene in ASDs, there are few rat models with species-specific advantages. In this study, we established and characterized a novel rat model with a deletion spanning exons 11–21 of Shank3, leading to a complete loss of the major SHANK3 isoforms. Synaptic function and plasticity of Shank3-deficient rats were impaired detected by biochemical and electrophysiological analyses. Shank3-depleted rats showed impaired social memory but not impaired social interaction behaviors. In addition, impaired learning and memory, increased anxiety-like behavior, increased mechanical pain threshold and decreased thermal sensation were observed in Shank3-deficient rats. It is worth to note that Shank3-deficient rats had nearly normal levels of the endogenous social neurohormones oxytocin (OXT) and arginine-vasopressin (AVP). This new rat model will help to further investigate the etiology and assess potential therapeutic target and strategy for Shank3-related neurodevelopmental disorders.

Oxytocin and vasopressin inhibit hyper-aggressive behaviour in socially isolated mice

Despite the high prevalence of aggression across a wide range of disorders, there is a severe lack of pharmacological treatments. Recent rodent studies have shown both centrally and peripherally administered oxytocin is effective in reducing territorial aggression, an adaptive form of aggression not reflective of pathological hyper-aggression. The current study tested i.p. administered oxytocin and vasopressin in a model of non-territorial hyper-aggression and examined the involvement of oxytocin receptors (OXTR) and vasopressin V1a receptors (V1aR). Male Swiss mice (N = 160) were either socially isolated or group housed for 6 weeks prior to the commencement of testing; wherein two unfamiliar weight and condition matched mice were placed into a neutral context for 10 min. Socially isolated mice exhibited heightened aggression that was powerfully and dose-dependently inhibited by oxytocin and vasopressin and that was accompanied by dose-dependent increases in close social contact (huddling) and grooming. These anti-aggressive effects of oxytocin were blocked by pre-treatment with a higher dose of selective V1aR antagonist SR49059 (20 mg/kg i.p.), but not a lower dose of SR49059 (5 mg/kg i.p.) or selective OXTR antagonist L-368,899 (10 mg/kg i.p.). This is consistent with a growing number of studies linking a range of effects of exogenous oxytocin to actions at the V1a receptor. Interestingly, the highest dose of the OXTR agonist TGOT (10 mg/kg) also reduced isolation-induced aggression. These results suggest that while activation of the V1a receptor appears critical for the anti-aggressive effects of oxytocin, activation of the oxytocin receptor cannot be excluded. This article is part of the Special Issue entitled 'Current status of the neurobiology of aggression and impulsivity.'

Oxytocin/vasopressin-like peptide inotocin regulates cuticular hydrocarbon synthesis and water balancing in ants

Inotocin, the oxytocin/vasopressin-like peptide, is widely conserved in arthropods; however, little is known about its molecular function. Here, we show that, in ants, the expression levels of inotocin and its receptor are correlated with the age of workers and their behavior. We also demonstrate that inotocin signaling is involved in desiccation resistance by regulating the synthesis of cuticular hydrocarbons. We propose that the up-regulation of inotocin and its receptor as workers age and switch tasks from nursing to foraging is a key physiological adaption to survive drier environments outside of the nest.

Oxytocin/vasopressin-like peptides are important regulators of physiology and social behavior in vertebrates. However, the function of inotocin, the homologous peptide in arthropods, remains largely unknown. Here, we show that the level of expression of inotocin and inotocin receptor are correlated with task allocation in the ant Camponotus fellah. Both genes are up-regulated when workers age and switch tasks from nursing to foraging. in situ hybridization revealed that inotocin receptor is specifically expressed in oenocytes, which are specialized cells synthesizing cuticular hydrocarbons which function as desiccation barriers in insects and for social recognition in ants. dsRNA injection targeting inotocin receptor, together with pharmacological treatments using three identified antagonists blocking inotocin signaling, revealed that inotocin signaling regulates the expression of cytochrome P450 4G1 (CYP4G1) and the synthesis of cuticular hydrocarbons, which play an important role in desiccation resistance once workers initiate foraging.

NMDA receptors potentiate activity-dependent dendritic release of neuropeptides from hypothalamic neurons

KEY POINTS

Using 'sniffer' cell biosensors, we evaluated the effects of specific firing patterns and frequencies on activity-dependent somatodendritic release of vasopressin from paraventricular nucleus neurones. Somatodendritic release of vasopressin was rarely observed during continuous firing but was strengthened by clustered activity. Moreover, release evoked at any given frequency was robustly potentiated by NMDA receptor (NMDAR)-mediated firing. Differently from axonal release, NMDAR activation was necessary for somatodendritic release to occur at physiological firing frequencies, acting thus as a gating mechanism by which activity-dependent release from these two neuronal compartments could be independently regulated. The NMDAR-mediated potentiation was independent of a specific firing pattern and was not accompanied by increased spike broadening, but correlated with higher dendritic Ca²⁺ levels. Our studies provide fundamental novel information regarding stimulus-secretion coupling at somatodendritic compartments, and shed light into mechanisms by which activity-dependent release of neuronal signals from axonal terminals and dendrites could be regulated in a spatially compartmentalized manner.

ABSTRACT

Dendrites are now recognized to be active transmitting neuronal compartments subserving complex brain functions, including motor behaviours and homeostatic neurohumoral responses. Still, the precise mechanisms underlying activity-dependent release of dendritic signals, and how dendritic release is regulated independently from axonal release, remains largely unknown. We used 'sniffer' biosensor cells to enable the measurement and study of activity-dependent dendritic release of vasopressin (VP) from hypothalamic neurones in brain slices. Sniffer_VP responses were dose-dependent, with a threshold detection level of 0.5 nM for VP, being thus a highly sensitive tool to detect endogenous physiological levels of the neuropeptide. Somatodendritic release of VP was rarely observed in response to a burst of action potentials fired in continuous mode, but was strengthened by clustered firing activity. Moreover, release evoked at any given frequency was robustly potentiated when firing was triggered by NMDA receptor (NMDAR) activation. Differently from axonal release, NMDAR activation was necessary for dendritic release to occur at physiological firing frequencies. Thus, we propose that NMDARs may act as a gating mechanism by which activity-dependent release from these two neuronal compartments can be independently regulated. The NMDAR-mediated potentiation of dendritic release was independent of a particular action potential waveform, firing pattern evoked, or a more pronounced spiked broadening, but correlated with higher dendritic Ca²⁺ levels. Overall, our studies provide fundamental novel information regarding stimulus-secretion coupling at neuronal dendrites, and shed light into mechanisms by which activity-dependent release of neuronal signals from axonal terminals and dendrites can be regulated in a spatially compartmentalized manner.

The effect of the release of exogenous nitric oxide on the responses of the pregnant human myometrium to oxytocin

OBJECTIVE

Introduction: Currently there is insufficient evidence to support the routine administration of nitric oxide donors in the treatment of threatened preterm labor. An understanding of the role that nitric oxide plays in the management of threatened preterm labor may lead to more effective treatment and prevention. The aim of our study was to examine the involvement of exogenous nitric oxide release in regulating responses of the human pregnant myometrium to oxytocin.

PATIENTS AND METHODS

Material and methods: Biopsies of human myometrial tissue during pregnancy were obtained from 8 pregnant women, aged 21-35 years. The responses of the specimens to oxytocin in the absence and presence of a DETA/NO were recorded under isometric conditions. Preincubation with exogenous nitric oxide significantly (p<0.001) attenuated the contractile response of the uterine strips to oxytocin in concentrations higher than 10-8 mol/L.

RESULTS

Results: The inhibition of nitric oxide synthesis alone or in combination with DETA/NO incubation did not significantly change the oxytocin contractile effect in the concentration-response curve. Moreover, there was no significant variation in the mean value for log EC50 for oxytocin between the group with oxytocin alone and other groups.

CONCLUSION

Conclusions: We present evidence in support of the hypothesis that continuous nitric oxide supply to the human pregnant myometrium environment attenuates its response to oxytocin but only when endogenous production of nitric oxide is not impaired.

Design of Oxytocin Analogs

The neurohypophyseal hormone oxytocin (OT) and related modulators of the oxytocin receptor (OTR) have been the subject of intensive research for nearly seven decades. Despite having rather poor drug-like properties, OT is used as a treatment for labor induction, postpartum hemorrhage, and lactation support. The potential use of OT in the treatment of central nervous system (CNS)-related diseases has recently renewed interest in the pharmacology of OT. Oxytocin is one of the most extensively studied cyclic peptides and since the elucidation of its structure in 1953 thousands of peptidic OT analogs with antagonistic and agonistic properties have been synthesized and biologically evaluated. Among them are atosiban, a mixed oxytocin receptor (OTR)/vasopressin 1a receptor (V_1aR) antagonist used as a tocolytic agent approved (in certain countries), and carbetocin, a longer acting OTR agonist on the market for the treatment of postpartum hemorrhage. Many other OT analogs with improved pharmacological properties (e.g., barusiban, Antag III) have been identified. These peptides have been tested in clinical trials and/or used as pharmacological tools. In this chapter, the modifications of the OT molecule that led to the discovery of these compounds are reviewed.

Oxytocin attenuates phencyclidine hyperactivity and increases social interaction and nucleus accumben dopamine release in rats

The pituitary neuropeptide oxytocin promotes social behavior, and is a potential adjunct therapy for social deficits in schizophrenia and autism. Oxytocin may mediate pro-social effects by modulating monoamine release in limbic and cortical areas, which was investigated herein using in vivo microdialysis, after establishing a dose that did not produce accompanying sedative or thermoregulatory effects that could concomitantly influence behavior. The effects of oxytocin (0.03-0.3 mg/kg subcutaneous) on locomotor activity, core body temperature, and social behavior (social interaction and ultrasonic vocalizations) were examined in adult male Lister-hooded rats, using selective antagonists to determine the role of oxytocin and vasopressin V_1a receptors. Dopamine and serotonin efflux in the prefrontal cortex and nucleus accumbens of conscious rats were assessed using microdialysis. 0.3 mg/kg oxytocin modestly reduced activity and caused hypothermia but only the latter was attenuated by the V_1a receptor antagonist, SR49059 (1 mg/kg intraperitoneal). Oxytocin at 0.1 mg/kg, which did not alter activity and had little effect on temperature, significantly attenuated phencyclidine-induced hyperactivity and increased social interaction between unfamiliar rats without altering the number or pattern of ultrasonic vocalizations. In the same rats, oxytocin (0.1 mg/kg) selectively elevated dopamine overflow in the nucleus accumbens, but not prefrontal cortex, without influencing serotonin efflux. Systemic oxytocin administration attenuated phencyclidine-induced hyperactivity and increased pro-social behavior without decreasing core body temperature and selectively enhanced nucleus accumbens dopamine release, consistent with activation of mesocorticolimbic circuits regulating associative/reward behavior being involved. This highlights the therapeutic potential of oxytocin to treat social behavioral deficits seen in psychiatric disorders such as schizophrenia.

Oxytocin and vasopressin increase male-directed threats and vocalizations in female macaques

In a previous study, we reported that intranasal delivery of both oxytocin (OT) and arginine vasopressin (AVP) to male macaques relaxes spontaneous social interactions, flattens the existing dominance hierarchy, and increases behavioral synchrony with other monkeys. Here we report that intranasal OT and AVP administration modulates the behaviors of female macaque monkeys, but in robustly different ways from males. Most notably, both neuropeptides increase threatening and vocalization behaviors of females when they encounter males, and these behaviors effectively increase the social status of females over males. While OT and AVP heighten the confrontational nature of intersexual encounters, both peptides relax interactions between females. Finally, as previously reported for males, treating an individual female monkey with OT or AVP significantly modulates the behavior of her non-treated partner. Together, these findings show that OT and AVP can either inhibit or promote aggression, depending on sex and behavioral context, and call for a more careful, systematic examination of the functions of these neuropeptides in both sexes, especially in the context of therapeutics for human social disorders.

Preventive effects of oxytocin and oxytocin receptor in breast cancer pathogenesis

AIM

Modifications of oxytocin (OT) concentration and OT receptor (OXTR) expression level have different effects on breast cancer-derived cells. This study was conducted to evaluate OT variation in breast cancer patients and to evaluate OXTR expression changes in breast cancer tissues.

METHODS

The plasma concentrations of OT in both breast cancer patients and healthy individuals' samples were assessed. OXTR variations were then assessed in both cancerous and noncancerous breast tissues.

RESULTS

OT had an increase in breast cancer patients and expression of OXTR in contralateral breast was more than cancerous tissues.

CONCLUSION

Despite the high levels of OT concentration in breast cancer patients, it seems that a lower expression of OXTR in cancerous tissues can be effective in the breast cancer progression.