Circadian intervention of obesity development via resting-stage feeding manipulation or oxytocin treatment

The obesity pandemic can be viewed as a result of an imbalanced reaction to changing environmental factors. Recent research has linked circadian arrhythmicity to obesity and related diseases; however, the underlying mechanisms are still unclear. In this study, we found that high-fat diet (HFD) feeding strikingly promoted daytime rather than nighttime caloric intake in mice, leading to feeding circadian arrhythmicity. Using scheduled feeding with a defined amount of daily HFD intake, we found that an increase in the ratio of daytime to nighttime feeding promoted weight gain, whereas a decrease of this ratio rebalanced energy expenditure to counteract obesity. In identifying the underlying mechanism, we found that hypothalamic release of anorexigenic neuropeptide oxytocin displayed a diurnal rhythm of daytime rise and nighttime decline, which negatively correlated with the diurnal feeding activities of normal chow-fed mice. In contrast, chronic HFD feeding abrogated oxytocin diurnal rhythmicity, primarily by suppressing daytime oxytocin rise. Using pharmacological experiments with hypothalamic injection of oxytocin or oxytocin antagonist, we showed that daytime manipulation of oxytocin can change feeding circadian patterns to reprogram energy expenditure, leading to attenuation or induction of obesity independently of 24-h caloric intake. Also importantly, we found that peripheral injection of oxytocin activated hypothalamic oxytocin neurons to release oxytocin, and exerted metabolic effects similar to central oxytocin injection, thus offering a practical clinical avenue to use oxytocin in obesity control. In conclusion, resting-stage oxytocin release and feeding activity represent a critical circadian mechanism and therapeutic target for obesity.     

Pharmacological chaperones for the oxytocin receptor increase oxytocin responsiveness in myometrial cells

Oxytocin is a potent uterotonic agent administered to nearly all patients during childbirth in the United States. Inadequate oxytocin response can necessitate Cesarean delivery or lead to uterine atony and postpartum hemorrhage. Thus, it may be clinically useful to identify patients at risk for poor oxytocin response and develop strategies to sensitize the uterus to oxytocin. Previously, we showed that the V281M variant in the oxytocin receptor (OXTR) gene impairs OXTR trafficking to the cell surface, leading to a decreased oxytocin response in cells. Here, we sought to identify pharmacological chaperones that increased oxytocin response in cells expressing WT or V281M OXTR. We screened nine small-molecule agonists and antagonists of the oxytocin/vasopressin receptor family and identified two, SR49059 and L371,257, that restored both OXTR trafficking and oxytocin response in HEK293T cells transfected with V281M OXTR. In hTERT-immortalized human myometrial cells, which endogenously express WT OXTR, treatment with SR49059 and L371,257 increased the amount of OXTR on the cell surface by two- to fourfold. Furthermore, SR49059 and L371,257 increased the endogenous oxytocin response in hTERT-immortalized human myometrial cells by 35% and induced robust oxytocin responses in primary myometrial cells obtained from patients at the time of Cesarean section. If future studies demonstrate that these pharmacological chaperones or related compounds function similarly in vivo, we propose that they could potentially be used to enhance clinical response to oxytocin.     

Development and clinical application of a bioluminescence enzyme immunoassay for oxytocin

The development of a highly sensitive analytical method for oxytocin could be useful in the diagnosis and treatment of autistic spectrum disorder. We previously developed a colorimetric enzyme immunoassay (EIA) for plasma oxytocin measurement. In this study, we developed a method to measure oxytocin concentrations using a higher sensitivity bioluminescent EIA. Biotinylated oxytocin bridged with five lysine residues was used in a competitive format. The standard curve range for oxytocin was 1.0 to 1000 pg/assay. In addition, there was good correlation between the colorimetric and bioluminescent immunoassays in terms of measured oxytocin concentration (r = 0.9665, n = 48). The bioluminescent EIA for plasma oxytocin was more rapid and provided higher sensitivity than the colorimetric immunoassay, making it suitable for clinical application.     

Variation in oxytocin is related to variation in affiliative behavior in monogamous, pairbonded tamarins

Oxytocin plays an important role in monogamous pairbonded female voles, but not in polygamous voles. Here we examined a socially monogamous cooperatively breeding primate where both sexes share in parental care and territory defense for within species variation in behavior and female and male oxytocin levels in 14 pairs of cotton-top tamarins (Saguinus oedipus). In order to obtain a stable chronic assessment of hormones and behavior, we observed behavior and collected urinary hormonal samples across the tamarins’ 3-week ovulatory cycle. We found similar levels of urinary oxytocin in both sexes. However, basal urinary oxytocin levels varied 10-fold across pairs and pair-mates displayed similar oxytocin levels. Affiliative behavior (contact, grooming, sex) also varied greatly across the sample and explained more than half the variance in pair oxytocin levels. The variables accounting for variation in oxytocin levels differed by sex. Mutual contact and grooming explained most of the variance in female oxytocin levels, whereas sexual behavior explained most of the variance in male oxytocin levels. The initiation of contact by males and solicitation of sex by females were related to increased levels of oxytocin in both. This study demonstrates within-species variation in oxytocin that is directly related to levels of affiliative and sexual behavior. However, different behavioral mechanisms influence oxytocin levels in males and females and a strong pair relationship (as indexed by high levels of oxytocin) may require the activation of appropriate mechanisms for both sexes.     

Peripheral oxytocin suppresses food intake and causes weight loss in diet-induced obese rats

Growing evidence suggests that oxytocin plays an important role in the regulation of energy balance and that central oxytocin administration induces weight loss in diet-induced obese (DIO) animals. To gain a better understanding of how oxytocin mediates these effects, we examined feeding and neuronal responses to oxytocin in animals rendered obese following exposure to either a high-fat (HFD) or low-fat diet (LFD). Our findings demonstrate that peripheral administration of oxytocin dose-dependently reduces food intake and body weight to a similar extent in rats maintained on either diet. Moreover, the effect of oxytocin to induce weight loss remained intact in leptin receptor-deficient Koletsky (fa(k)/fa(k)) rats relative to their lean littermates. To determine whether systemically administered oxytocin activates hindbrain areas that regulate meal size, we measured neuronal c-Fos induction in the nucleus of the solitary tract (NTS) and area postrema (AP). We observed a robust neuronal response to oxytocin in these hindbrain areas that was unexpectedly increased in rats rendered obese on a HFD relative to lean, LFD-fed controls. Finally, we report that repeated daily peripheral administration of oxytocin in DIO animals elicited a sustained reduction of food intake and body weight while preventing the reduction of energy expenditure characteristic of weight-reduced animals. These findings extend recent evidence suggesting that oxytocin circumvents leptin resistance and induces weight-loss in DIO animals through a mechanism involving activation of neurons in the NTS and AP, key hindbrain areas for processing satiety-related inputs.     

Sexual dimorphism in oxytocin responses to health perception and disgust,with implications for theories on pathogen detection

In response to a recent hypothesis that the neuropeptide oxytocin might be involved in human pathogen avoidance mechanisms, we report the results of a study in which we investigate the effect of intranasal oxytocin on two behaviors serving as proxies for pathogen detection. Participants received either oxytocin or a placebo and were asked to evaluate (1) the health of Caucasian male computer-generated pictures that varied in facial redness (an indicator of hemoglobin perfusion) and (2) a series of pictures depicting disgusting scenarios. Men, but not women, evaluated all faces, regardless of color, as less healthy when given oxytocin compared to a placebo. Women, on the other hand, expressed decreased disgust when given oxytocin compared to a placebo. These results suggest that intranasal oxytocin administration does not facilitate pathogen detection based on visual cues, but instead reveal clear sex differences in the perception of health and sickness cues.     

Molecular modeling of the neurohypophyseal receptor/atosiban complexes

The neurohypophyseal nonapeptide hormone oxytocin (OT) is the strongest uterotonic substance known and is responsible for the initiation of labor. Conversely, oxytocin antagonists blocking uterine OT receptor can suppress uterus contraction. In this paper we describe a computer simulated docking pertinent to affinity of an oxytocin antagonist atosiban towards OT receptor, versus vasopressin V1a and V2 receptors.     

Presynaptic actions of endocannabinoids mediate alpha-MSH-induced inhibition of oxytocin cells

We recently showed that central injections of alpha-melanocyte-stimulating hormone (alpha-MSH) inhibits oxytocin cells and reduces peripheral release of oxytocin, but induces oxytocin release from dendrites. Dendritic oxytocin release can be triggered by agents that mobilize intracellular calcium. Oxytocin, like alpha-MSH, mobilizes intracellular calcium stores in oxytocin cells and triggers presynaptic inhibition of afferent inputs that is mediated by cannabinoids. We hypothesized that this mechanism might underlie the inhibitory effects of alpha-MSH. To test this, we recorded extracellularly from identified oxytocin and vasopressin cells in the anesthetized rat supraoptic nucleus (SON). Retrodialysis of a CB1 cannabinoid receptor antagonist to the SON blocked the inhibitory effects of intracerebroventricular injections of alpha-MSH on the spontaneous activity of oxytocin cells. We then monitored synaptically mediated responses of SON cells to stimulation of the organum vasculosum of the lamina terminalis (OVLT); this evoked a mixed response comprising an inhibitory component mediated by GABA and an excitatory component mediated by glutamate, as identified by the effects of bicuculline and 6-cyano-7-nitroquinoxaline-2,3-dione applied to the SON by retrodialysis. Application of CB1 receptor agonists to the SON attenuated the excitatory effects of OVLT stimulation in both oxytocin and vasopressin cells, whereas alpha-MSH attenuated the responses of oxytocin cells only. Thus alpha-MSH can act as a "switch"; it triggers oxytocin release centrally, but at the same time through initiating endocannabinoid production in oxytocin cells inhibits their electrical activity and hence, peripheral secretion.     

Effect of pharmacological doses of oxytocin on insulin response to glucose in normal man

In this study we have examined the effect of the administration of oxytocin on basal blood concentrations of insulin, glucagon, cortisol, growth hormone, and on the dynamic secretory response of these hormones to intravenous glucose administration (0.33 g/kg) in basal condition and after the injection of 3 IU (1 plus 2 IU/1 h) or 6 IU (2 plus 4 IU/1 h) of oxytocin (6 subjects for each group). The highest dose of oxytocin (6 IU) used significantly increased insulin secretion in response to intravenously administered glucose. No significant change of insulin secretion was observed with 3 IU of oxytocin. Glucagon, cortisol, and growth hormone response to intravenous injection of glucose was not affected by oxytocin (3 or 6 IU) administration. These results suggest that high doses of oxytocin affect beta-cell function in normal man.     

Effects of inactivation of oxytocin receptor and inhibition of prostaglandin synthesis on uterine oxytocin receptor and gap junction formation and labor in the rat.

Normal term labor is associated with a surge in myometrial oxytocin receptor formation and gap junction development. We have previously shown that inhibition of prostaglandin synthesis by naproxen sodium, 2.0 mg/day, suppressed oxytocin receptor formation but not gap junction formation and prolonged gestation. In this study, we investigated the effects of a specific oxytocin antagonist on oxytocin receptor formation, gap junction formation, and labor in the rat. [Pen1,Phe(Me)2,Thr4,Orn8]oxytocin, a specific oxytocin antagonist, was infused subcutaneously during the last 3 days of pregnancy at 300 micrograms/day. Measurements of myometrial oxytocin receptor concentrations and gap junction formation on days 21 and 22 and days 22-23 (in labor) pregnant uteri showed no significant differences in the Bmax and Kd values between the control and the treated group. Gestation period was not prolonged by the oxytocin antagonist. However, in a separate group of day 23 pregnant rats, the uterine contractile response to 60 mU of oxytocin i.v. was found completely blocked by 10 micrograms of the oxytocin antagonist. These findings suggest that although functional oxytocin receptors did not appear to be essential for the initiation of labor, oxytocin antagonists may still be effective in the prevention of premature contractions. We also examined the effects of a higher dose of naproxen sodium, 5.0 mg/day, on gap junction formation. At this dose, naproxen sodium suppressed both oxytocin receptor and gap junction formation, prolonged gestation, and delayed parturition by 24 h or longer. Prostaglandin appears to be an important regulator or mediator of oxytocin receptor and gap junction formation and plays a critical role in the initiation of labor.     

Effects of interleukins on plasma arginine vasopressin and oxytocin levels in conscious, freely moving rats.

To elucidate whether interleukins are involved in vasopressin or oxytocin release during cytokine-related stressful conditions, we examined the effects of human interleukin-1 beta and interleukin-6 on plasma vasopressin and oxytocin levels in rats. Interleukin-1 beta administrated intravenously stimulated both the vasopressin and oxytocin secretion in dose-dependent manners. Neither hormone release was observed following interleukin-6 administration. Pretreatment with aspirin significantly attenuated the effects of interleukin-1 beta on both the vasopressin and oxytocin levels. SC-19220, a prostaglandin E2 receptor antagonist, did not affect the interleukin-1 beta-induced increase of plasma oxytocin levels, but almost completely abolished its effect on plasma vasopressin levels. These results suggest that under certain stressful conditions which accompany the stimulation of cytokine production, interleukin-1 is involved in the increase of plasma vasopressin and oxytocin levels and, moreover, different kinds of prostaglandins are suggested to participate in these interleukin-1-induced hormone release.     

Behavioral stress decreases plasma oxytocin concentrations in primates

Using rhesus monkeys, we studied the effects of a behavioral stress on plasma concentrations of adrenocorticotropin, oxytocin, and vasopressin. The stress resulted in significant increases in adrenocorticotropin and significant decreases in oxytocin concentrations. No significant changes were seen in vasopressin concentrations. To further explore the relationship between plasma oxytocin and pituitary-adrenal function, dexamethasone was administered to rhesus monkeys. This resulted in significant increases in plasma oxytocin concentrations, while adrenocorticotropin decreased.     

Intracerebral oxytocin modulates sleep-wake behaviour in male rats

Oxytocin released within the brain under basal conditions and in response to stress is differentially involved in the regulation of the hypothalamo-pituitary-adrenal (HPA) axis. Because the HPA axis plays an important role in the regulation of wakefulness, central oxytocin may modulate sleep-wake behaviour. In the present vehicle-controlled study, we assessed the influence of a selective oxytocin receptor antagonist (des-Gly-NH2d(CH2)5 [Tyr(Me)2,Thr4] OVT; 0.75 microg/5 microl) or of synthetic oxytocin (0.1 microg and 1 microg/5 microl), infused into the lateral ventricle (i.c.v.), on the sleep pattern in male Wistar rats (n=7). Compared to vehicle, the oxytocin antagonist slightly but persistently increased wakefulness at the expense of all sleep states. This finding indicates that endogenous brain oxytocin promotes sleep. However, acute icv infusion of oxytocin delayed sleep onset latency, which resulted in a transient reduction of non-REMS and REMS, and augmented high-frequency activity in the electroencephalogram (EEG) within non-REMS. These observations agree with previous reports that icv oxytocin induces a state of arousal. Based on these findings, we postulate that oxytocin has a dual mechanism of action in dependence of the physiological state. Under basal, stress-free conditions, endogenous oxytocin may promote sleep. Conversely, the high brain levels of oxytocin after central oxytocin infusion may reflect a condition of stress accompanied by behavioural arousal and, possibly via an excitatory action on the CRH system, increase vigilance.     

The role of oxytocin in relationships between dogs and humans and potential applications for the treatment of separation anxiety in dogs

The hormone oxytocin plays an important role in attachment formation and bonding between humans and domestic dogs. Recent research has led to increased interest in potential applications for intranasal oxytocin to aid with the treatment of psychological disorders in humans. While a few studies have explored the effects of intranasally administered oxytocin on social cognition and social bonding in dogs, alternative applications have not yet been explored for the treatment of behavioural problems in this species. One potentially important application for intranasal oxytocin in dogs could be the treatment of separation anxiety, a common attachment disorder in dogs. Here we provide an overview of what is known about the role of oxytocin in the human–dog bond and canine separation anxiety, and discuss considerations for future research looking to integrate oxytocin into behavioural treatment based on recent findings from both the human and dog literature.     

An Obligate Role of Oxytocin Neurons in Diet Induced Energy Expenditure

Oxytocin neurons represent one of the major subsets of neurons in the paraventricular hypothalamus (PVH), a critical brain region for energy homeostasis. Despite substantial evidence supporting a role of oxytocin in body weight regulation, it remains controversial whether oxytocin neurons directly regulate body weight homeostasis, feeding or energy expenditure. Pharmacologic doses of oxytocin suppress feeding through a proposed melanocortin responsive projection from the PVH to the hindbrain. In contrast, deficiency in oxytocin or its receptor leads to reduced energy expenditure without feeding abnormalities. To test the physiological function of oxytocin neurons, we specifically ablated oxytocin neurons in adult mice. Our results show that oxytocin neuron ablation in adult animals has no effect on body weight, food intake or energy expenditure on a regular diet. Interestingly, male mice lacking oxytocin neurons are more sensitive to high fat diet-induced obesity due solely to reduced energy expenditure. In addition, despite a normal food intake, these mice exhibit a blunted food intake response to leptin administration. Thus, our study suggests that oxytocin neurons are required to resist the obesity associated with a high fat diet; but their role in feeding is permissive and can be compensated for by redundant pathways.     

Regulation of neural oxytocin gene expression by gonadal steroids in pubertal rats

We have previously demonstrated that neuronal oxytocin mRNA increases during the pubertal development of female rats. In this paper we have examined the factors that regulate this developmental increase in both male and female rats. Northern blot analysis demonstrated that neural oxytocin mRNA increased 5- to 10-fold from postnatal day 20 (P20) to P60 in animals of both sexes, coincident with puberty. Mature male rats and females at all stages of the estrous cycle expressed similar levels of neural oxytocin mRNA. Pubertal up-regulation of oxytocin mRNA was largely, but not completely, inhibited by prepubescent gonadectomy, indicating a requirement for intact gonads as well as some other as yet undefined factor(s). Pubertal treatment of gonadectomized animals with estradiol or testosterone abolished the effects of gonadectomy; treated animals expressed levels of neural oxytocin mRNA similar to those in controls. However, treatment of prepubertal animals with estradiol or testosterone from P10 to P20 had no effect on oxytocin mRNA levels, suggesting that neural maturation or other factors are necessary requisites for steroid sensitivity. To determine whether neural activin played any role in regulating oxytocin mRNA during puberty, we examined levels of inhibin/activin beta A-chain mRNA. This mRNA was expressed at similar levels in all brain regions and did not vary as a function of gonadectomy or steroid treatment, making it unlikely that activin mediates the observed changes. Together, these data indicate that neural oxytocin mRNA is induced by gonadal steroids during puberty, and suggest a mechanism for coordinating development of reproductive functions with other pubertal changes.     

Disentangling the link between depressive symptoms and plasma oxytocin in men: The role of brooding rumination

The links between plasma oxytocin and depression are controversial, ranging from negative to positive associations. The present study was conducted to reconcile those conflicting findings; amongst the features of depression, we considered rumination and hypothesised that rumination would function as moderator between depressive symptoms and oxytocin. Seventy five clinically normal adult male volunteers were assessed for depressive characteristics by means of the Ruminative Responses Scale and Beck’s Depression Inventory-II; plasma oxytocin was measured by means of competitive enzyme immunoassay. The results demonstrate that high depressive symptoms were negatively associated with oxytocin concentrations at high rumination levels while such an association did not exist at low levels of rumination. The present findings suggest there are complex associations between oxytocin and brooding rumination, the latter being an important feature among depressive symptoms observed in clinically normal individuals. This complexity can underlie the current lack of consensus on the role of oxytocin in depression.     

Developmental experiences and the oxytocin receptor system

The long-term effects of developmental experiences on social behavior, and the neuropeptide systems such as oxytocin which subserve the behavior, are still little understood. In this article, we review various types of early experience, including normal development, knockout models, pharmacological exposures, and early social experiences. We consider the processes by which experience can affect oxytocin receptor binding, and what is known about the directionality of experience effects on oxytocin receptors. Finally, we attempt to synthesize the literature into a predictive model as to the direction of early experience effects on oxytocin receptor binding potential, and whether these changes have functional significance. These predictions are relevant to current human health practice, given proposals to use chronic intranasal oxytocin to treat developmental disorders including autism and schizophrenia. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.