Hindbrain oxytocin receptors contribute to the effects of circulating oxytocin on food intake in male rats

Understanding oxytocin in human physiology and pathophysiology: A path towards therapeutics

•Oxytocin is a multifaceted hypothalamic-pituitary hormone involved in energy homeostasis, mental health, and bone metabolism.•Oxytocin deficiency in energy deficit states and in hypopituitarism is associated with worse mental health and bone health.•Oxytocin modulates appetitive neurocircuitry, improves impulse control, and reduces food intake in humans.•Defining the oxytocin system in human physiology and pathophysiology could lead to novel therapeutic strategies.

Oxytocin and corticotropin-releasing hormone exaggerate nucleus tractus solitarii neuronal and synaptic activity following chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) in rodents mimics the hypoxia-induced elevation of blood pressure seen in individuals experiencing episodic breathing. The brainstem nucleus tractus solitarii (nTS) is the first site of visceral sensory afferent integration, and thus is critical for cardiorespiratory homeostasis and its adaptation during a variety of stressors. In addition, the paraventricular nucleus of the hypothalamus (PVN), in part through its nTS projections that contain oxytocin (OT) and/or corticotropin-releasing hormone (CRH), contributes to cardiorespiratory regulation. Within the nTS, these PVN-derived neuropeptides alter nTS activity and the cardiorespiratory response to hypoxia. Nevertheless, their contribution to nTS activity after CIH is not fully understood. We hypothesized that OT and CRH would increase nTS activity to a greater extent following CIH, and co-activation of OT+CRH receptors would further magnify nTS activity. Our data show that compared to their normoxic controls, 10 days' CIH exaggerated nTS discharge, excitatory synaptic currents and Ca2+ influx in response to CRH, which were further enhanced by the addition of OT. CIH increased the tonic functional contribution of CRH receptors, which occurred with elevation of mRNA and protein. Together, our data demonstrate that intermittent hypoxia exaggerates the expression and function of neuropeptides on nTS activity. KEY POINTS: Episodic breathing and chronic intermittent hypoxia (CIH) are associated with autonomic dysregulation, including elevated sympathetic nervous system activity. Altered nucleus tractus solitarii (nTS) activity contributes to this response. Neurons originating in the paraventricular nucleus (PVN), including those containing oxytocin (OT) and corticotropin-releasing hormone (CRH), project to the nTS, and modulate the cardiorespiratory system. Their role in CIH is unknown. In this study, we focused on OT and CRH individually and together on nTS activity from rats exposed to either CIH or normoxia control. We show that after CIH, CRH alone and with OT increased to a greater extent overall nTS discharge, neuronal calcium influx, synaptic transmission to second-order nTS neurons, and OT and CRH receptor expression. These results provide insights into the underlying circuits and mechanisms contributing to autonomic dysfunction during periods of episodic breathing.

Sympathetic innervation of interscapular brown adipose tissue is not a predominant mediator of oxytocin-elicited reductions of body weight and adiposity in male diet-induced obese mice

Previous studies indicate that CNS administration of oxytocin (OT) reduces body weight in high fat diet-induced obese (DIO) rodents by reducing food intake and increasing energy expenditure (EE). We recently demonstrated that hindbrain (fourth ventricular [4V]) administration of OT elicits weight loss and elevates interscapular brown adipose tissue temperature (T IBAT , a surrogate measure of increased EE) in DIO mice. What remains unclear is whether OT-elicited weight loss requires increased sympathetic nervous system (SNS) outflow to IBAT. We hypothesized that OT-induced stimulation of SNS outflow to IBAT contributes to its ability to activate BAT and elicit weight loss in DIO mice. To test this hypothesis, we determined the effect of disrupting SNS activation of IBAT on the ability of 4V OT administration to increase T IBAT and elicit weight loss in DIO mice. We first determined whether bilateral surgical SNS denervation to IBAT was successful as noted by ≥ 60% reduction in IBAT norepinephrine (NE) content in DIO mice. NE content was selectively reduced in IBAT at 1-, 6- and 7-weeks post-denervation by 95.9±2.0, 77.4±12.7 and 93.6±4.6% ( P <0.05), respectively and was unchanged in inguinal white adipose tissue, pancreas or liver. We subsequently measured the effects of acute 4V OT (1, 5 µg ≈ 0.99, 4.96 nmol) on T IBAT in DIO mice following sham or bilateral surgical SNS denervation to IBAT. We found that the high dose of 4V OT (5 µg ≈ 4.96 nmol) elevated T IBAT similarly in sham mice as in denervated mice. We subsequently measured the effects of chronic 4V OT (16 nmol/day over 29 days) or vehicle infusions on body weight, adiposity and food intake in DIO mice following sham or bilateral surgical denervation of IBAT. Chronic 4V OT reduced body weight by 5.7±2.23% and 6.6±1.4% in sham and denervated mice ( P <0.05), respectively, and this effect was similar between groups ( P =NS). OT produced corresponding reductions in whole body fat mass ( P <0.05). Together, these findings support the hypothesis that sympathetic innervation of IBAT is not necessary for OT-elicited increases in BAT thermogenesis and reductions of body weight and adiposity in male DIO mice.

The neurohypophyseal hormone oxytocin and eating behaviors: a narrative review

BACKGROUND

The neuropeptide oxytocin (OT) is crucial in several conditions, such as lactation, parturition, mother-infant interaction, and psychosocial function. Moreover, OT may be involved in the regulation of eating behaviors.

METHODS

This review briefly summarizes data concerning the role of OT in eating behaviors. Appropriate keywords and medical subject headings were identified and searched for in PubMed/MEDLINE. References of original articles and reviews were screened, examined, and selected.

RESULTS

Hypothalamic OT-secreting neurons project to different cerebral areas controlling eating behaviors, such as the amygdala, area postrema, nucleus of the solitary tract, and dorsal motor nucleus of the vagus nerve. Intracerebral/ventricular OT administration decreases food intake and body weight in wild and genetically obese rats. OT may alter food intake and the quality of meals, especially carbohydrates and sweets, in humans.

DISCUSSION

OT may play a role in the pathophysiology of eating disorders with potential therapeutic perspectives. In obese patients and those with certain eating disorders, such as bulimia nervosa or binge/compulsive eating, OT may reduce appetite and caloric consumption. Conversely, OT administered to patients with anorexia nervosa may paradoxically stimulate appetite, possibly by lowering anxiety which usually complicates the management of these patients. Nevertheless, OT administration (e.g., intranasal route) is not always associated with clinical benefit, probably because intranasally administered OT fails to achieve therapeutic intracerebral levels of the hormone.

CONCLUSION

OT administration could play a therapeutic role in managing eating disorders and disordered eating. However, specific studies are needed to clarify this issue with regard to dose-finding and route and administration time.

High-calorie diets uncouple hypothalamic oxytocin neurons from a gut-to-brain satiation pathway via κ-opioid signaling

Oxytocin-expressing paraventricular hypothalamic neurons (PVNOT neurons) integrate afferent signals from the gut, including cholecystokinin (CCK), to adjust whole-body energy homeostasis. However, the molecular underpinnings by which PVNOT neurons orchestrate gut-to-brain feeding control remain unclear. Here, we show that mice undergoing selective ablation of PVNOT neurons fail to reduce food intake in response to CCK and develop hyperphagic obesity on a chow diet. Notably, exposing wild-type mice to a high-fat/high-sugar (HFHS) diet recapitulates this insensitivity toward CCK, which is linked to diet-induced transcriptional and electrophysiological aberrations specifically in PVNOT neurons. Restoring OT pathways in diet-induced obese (DIO) mice via chemogenetics or polypharmacology sufficiently re-establishes CCK's anorexigenic effects. Last, by single-cell profiling, we identify a specialized PVNOT neuronal subpopulation with increased κ-opioid signaling under an HFHS diet, which restrains their CCK-evoked activation. In sum, we document a (patho)mechanism by which PVNOT signaling uncouples a gut-brain satiation pathway under obesogenic conditions.

Theranostic in GLP-1R molecular imaging: challenges and emerging opportunities

Theranostic in nuclear medicine combines diagnostic imaging and internal irradiation therapy using different therapeutic nuclear probes for visual diagnosis and precise treatment. GLP-1R is a popular receptor target in endocrine diseases, non-alcoholic steatohepatitis, tumors, and other areas. Likewise, it has also made breakthroughs in the development of molecular imaging. It was recognized that GLP-1R imaging originated from the study of insulinoma and afterwards was expanded in application including islet transplantation, pancreatic β-cell mass measurement, and ATP-dependent potassium channel-related endocrine diseases. Fortunately, GLP-1R molecular imaging has been involved in ischemic cardiomyocytes and neurodegenerative diseases. These signs illustrate the power of GLP-1R molecular imaging in the development of medicine. However, it is still limited to imaging diagnosis research in the current molecular imaging environment. The lack of molecular-targeted therapeutics related report hinders its radiology theranostic. In this article, the current research status, challenges, and emerging opportunities for GLP-1R molecular imaging are discussed in order to open a new path for theranostics and to promote the evolution of molecular medicine.

Microbes, oxytocin and stress: Converging players regulating eating behavior

Oxytocin is a peptide-hormone extensively studied for its multifaceted biological functions and has recently gained attention for its role in eating behavior, through its action as an anorexigenic neuropeptide. Moreover, the gut microbiota is involved in oxytocinergic signaling through the brain-gut axis, specifically in the regulation of social behavior. The gut microbiota is also implicated in appetite regulation and is postulated to play a role in central regulation of hedonic eating. In this review, we provide an overview on oxytocin and its individual links with the microbiome, the homeostatic and non-homeostatic regulation of eating behavior as well as social behavior and stress.

Novel Long-Acting Oxytocin Analog with Increased Efficacy in Reducing Food Intake and Body Weight

Oxytocin (OXT) analogues have been designed to overcome the limitation of the short half-life of the native OXT peptide. Here, we tested ASK2131 on obesity related outcomes in diet-induced obese (DIO) Sprague Dawley rats. In vitro function assays were conducted. The effects of daily subcutaneous injections of ASK2131 vs. OXT and pair-feeding were assessed on food intake and body weight in vivo. ASK2131 is a longer-lasting OXT analog with improved pharmacokinetics compared to OXT (T_1/2: 2.3 vs. 0.12 h). In chronic 22-day administration, ASK2131 was administered at 50 nmol/kg, while OXT doses were titrated up to 600 nmol/kg because OXT appeared to be less effective at reducing energy intake relative to ASK2131 at equimolar doses. After 22 days, vehicle-treated animals gained 4.5% body weight, OXT rats maintained their body weight, while those treated with ASK2131 declined in weight continuously over the 22-day period, leading to a 6.6 ± 1.3% reduction (mean ± standard error) compared to baseline. Compared to their pair-fed counterparts, ASK2131-treated rats showed a more pronounced reduction in body weight through most of the study. In summary, ASK2131 is a promising OXT-based therapeutic, with extended in vivo stability and improved potency leading to a profound reduction in body weight partly explained by reduced food intake.

Behavioral plasticity: Role of neuropeptides in shaping feeding responses

Behavioral plasticity refers to changes occurring due to external influences on an organism, including adaptation, learning, memory and enduring influences from early life experience. There are 2 types of behavioral plasticity: "developmental", which refers to gene/environment interactions affecting a phenotype, and "activational" which refers to innate physiology and can involve structural physiological changes of the body. In this review, we focus on feeding behavior, and studies involving neuropeptides that influence behavioral plasticity - primarily opioids, orexin, neuropeptide Y, and oxytocin. In each section of the review, we include examples of behavioral plasticity as it relates to actions of these neuropeptides. It can be concluded from this review that eating behavior is influenced by a number of external factors, including time of day, type of food available, energy balance state, and stressors. The reviewed work underscores that environmental factors play a critical role in feeding behavior and energy balance, but changes in eating behavior also result from a multitude of non-environmental factors, such that there can be no single mechanism or variable that can explain ingestive behavior.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Sucrose intake by rats affected by both intraperitoneal oxytocin administration and time of day

RATIONALE

Daily limited access to palatable food or drink at a fixed time is commonly used in rodent models of bingeing. Under these conditions, entrainment may modulate intake patterns. Oxytocin is involved in circadian patterns of intake and, when administered peripherally, reduces sucrose intake. However, oxytocin's effects on intake under limited-access conditions and its potential interaction with entrainment have not been explored.

OBJECTIVES

This study examined the role of entrainment on intake patterns, oxytocin's effects on sucrose intakes and locomotor activity and whether oxytocin's effects were mediated by its actions at the oxytocin receptor.

METHODS

Sated rats received daily 1-h access to 10% sucrose solution either at a fixed or varied time of day. Rats received intraperitoneal oxytocin (0 mg/kg, 0.3 mg/kg, 1 mg/kg, 3 mg/kg) prior to sucrose access, and spontaneous locomotor activity was assessed in an open-field test. Rats were then pre-treated with an oxytocin receptor antagonist, L368,899, prior to oxytocin before sucrose access.

RESULTS

Intake patterns did not differ between fixed- or varied-time presentations; rats consumed more sucrose solution in the middle as opposed to the early-dark phase. Oxytocin dose-dependently reduced sucrose intakes, but also reduced locomotor activity. There was some evidence of partial blockade of oxytocin-induced sucrose intake reductions by L368,899, but the results were unclear.

CONCLUSIONS

Time of day and oxytocin impact sucrose solution intake under daily limited access in rats and the sedative-like effects of oxytocin should be considered in future studies on oxytocin and food intake.

Oxytocin as an Anti-obesity Treatment

Obesity is a growing health concern, as it increases risk for heart disease, hypertension, type 2 diabetes, cancer, COVID-19 related hospitalizations and mortality. However, current weight loss therapies are often associated with psychiatric or cardiovascular side effects or poor tolerability that limit their long-term use. The hypothalamic neuropeptide, oxytocin (OT), mediates a wide range of physiologic actions, which include reproductive behavior, formation of prosocial behaviors and control of body weight. We and others have shown that OT circumvents leptin resistance and elicits weight loss in diet-induced obese rodents and non-human primates by reducing both food intake and increasing energy expenditure (EE). Chronic intranasal OT also elicits promising effects on weight loss in obese humans. This review evaluates the potential use of OT as a therapeutic strategy to treat obesity in rodents, non-human primates, and humans, and identifies potential mechanisms that mediate this effect.

Hindbrain Administration of Oxytocin Reduces Food Intake, Weight Gain and Activates Catecholamine Neurons in the Hindbrain Nucleus of the Solitary Tract in Rats

Existing studies show that CNS oxytocin (OT) signaling is important in the control of energy balance, but it is unclear which neurons may contribute to these effects. Our goals were to examine (1) the dose-response effects of acute OT administration into the third (3V; forebrain) and fourth (4V; hindbrain) ventricles to assess sensitivity to OT in forebrain and hindbrain sites, (2) the extent to which chronic 4V administration of OT reduces weight gain associated with the progression of diet-induced obesity, and (3) whether nucleus tractus solitarius (NTS) catecholamine neurons are downstream targets of 4V OT. Initially, we examined the dose-response effects of 3V and 4V OT (0.04, 0.2, 1, or 5 μg). 3V and 4V OT (5 μg) suppressed 0.5-h food intake by 71.7 ± 6.0% and 60 ± 12.9%, respectively. 4V OT (0.04, 0.2, 1 μg) reduced food intake by 30.9 ± 12.9, 42.1 ± 9.4, and 56.4 ± 9.0%, respectively, whereas 3V administration of OT (1 μg) was only effective at reducing 0.5-h food intake by 38.3 ± 10.9%. We subsequently found that chronic 4V OT infusion, as with chronic 3V infusion, reduced body weight gain (specific to fat mass) and tended to reduce plasma leptin in high-fat diet (HFD)-fed rats, in part, through a reduction in energy intake. Lastly, we determined that 4V OT increased the number of hindbrain caudal NTS Fos (+) neurons (156 ± 25) relative to vehicle (12 ± 3). The 4V OT also induced Fos in tyrosine hydroxylase (TH; marker of catecholamine neurons) (+) neurons (25 ± 7%) relative to vehicle (0.8 ± 0.3%). Collectively, these findings support the hypothesis that OT within the hindbrain is effective at reducing food intake, weight gain, and adiposity and that NTS catecholamine neurons in addition to non-catecholaminergic neurons are downstream targets of CNS OT.

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

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

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

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

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

Effects of Combined Oxytocin and Beta-3 Receptor Agonist (CL 316243) Treatment on Body Weight and Adiposity in Male Diet-Induced Obese Rats

Previous studies have indicated that oxytocin (OT) reduces body weight in diet-induced obese (DIO) rodents through reductions in energy intake and increases in energy expenditure. We recently demonstrated that hindbrain [fourth ventricular (4V)] administration of OT evokes weight loss and elevates interscapular brown adipose tissue temperature (T IBAT ) in DIO rats. What remains unclear is whether OT can be used as an adjunct with other drugs that directly target beta-3 receptors in IBAT to promote BAT thermogenesis and reduce body weight in DIO rats. We hypothesized that the combined treatment of OT and the beta-3 agonist, CL 316243, would produce an additive effect to decrease body weight and adiposity in DIO rats by reducing energy intake and increasing BAT thermogenesis. We assessed the effects of 4V infusions of OT (16 nmol/day) or vehicle (VEH) in combination with daily intraperitoneal injections of CL 316243 (0.5 mg/kg) or VEH on food intake, T IBAT , body weight and body composition. OT and CL 316243 alone reduced body weight by 7.8 ± 1.3% (P < 0.05) and 9.1 ± 2.1% (P < 0.05), respectively, but the combined treatment produced more pronounced weight loss (15.5 ± 1.2%; P < 0.05) than either treatment alone. These effects were associated with decreased adiposity, adipocyte size, energy intake and increased uncoupling protein 1 (UCP-1) content in epididymal white adipose tissue (EWAT) (P < 0.05). In addition, CL 316243 alone (P < 0.05) and in combination with OT (P < 0.05) elevated T IBAT and IBAT UCP-1 content and IBAT thermogenic gene expression. These findings are consistent with the hypothesis that the combined treatment of OT and the beta-3 agonist, CL 316243, produces an additive effect to decrease body weight. The findings from the current study suggest that the effects of the combined treatment on energy intake, fat mass, adipocyte size and browning of EWAT were not additive and appear to be driven, in part, by transient changes in energy intake in response to OT or CL 316243 alone as well as CL 316243-elicited reduction of fat mass and adipocyte size and induction of browning of EWAT.

Nasal oxytocin for the treatment of psychiatric disorders and pain: achieving meaningful brain concentrations

There is evidence of the therapeutic potential of intranasal oxytocin for the treatment of pain and various psychiatric disorders, however, there is scant evidence that oxytocin reaches the brain. We quantified the concentration and distribution pattern of [¹²⁵I]-radiolabeled oxytocin in the brains and peripheral tissues of rats after intranasal delivery using gamma counting and autoradiography, respectively. Radiolabel was detected in high concentrations in the trigeminal and olfactory nerves as well as in brain regions along their trajectories. Considerable concentrations were observed in the blood, however, relatively low levels of radiolabel were measured in peripheral tissues. The addition of a mucoadhesive did not enhance brain concentrations. These results provide support for intranasal OT reaching the brain via the olfactory and trigeminal neural pathways. These findings will inform the design and interpretation of clinical studies with intranasal oxytocin.

The role of nucleus of the solitary tract glucagon-like peptide-1 and prolactin-releasing peptide neurons in stress: anatomy, physiology and cellular interactions

Neuroendocrine, behavioural and autonomic responses to stressful stimuli are orchestrated by complex neural circuits. The caudal nucleus of the solitary tract (cNTS) in the dorsomedial hindbrain is uniquely positioned to integrate signals of both interoceptive and psychogenic stress. Within the cNTS, glucagon‐like peptide‐1 (GLP‐1) and prolactin‐releasing peptide (PrRP) neurons play crucial roles in organising neural responses to a broad range of stressors. In this review we discuss the anatomical and functional overlap between PrRP and GLP‐1 neurons. We outline their co‐activation in response to stressful stimuli and their importance as mediators of behavioural and physiological stress responses. Finally, we review evidence that PrRP neurons are downstream of GLP‐1 neurons and outline unexplored areas of the research field. Based on the current state‐of‐knowledge, PrRP and GLP‐1 neurons may be compelling targets in the treatment of stress‐related disorders.

Oxytocin: A Potential Therapeutic for Obesity

Oxytocin is a neuropeptide involved in the homeostasis of food consumption and energy; it affects hedonic eating. Studies in obese or binge-eating patients reported the hypophagic effect of oxytocin, which reduced caloric intake after administration. Several studies have demonstrated the effect of oxytocin’s increasing energy intake, decreasing food consumption, and contributing to weight loss. Oxytocin’s effects on food intake and metabolism suggest its therapeutic potential for treating obesity and binge eating.

Chronic hindbrain administration of oxytocin elicits weight loss in male diet-induced obese mice

Previous studies indicate that oxytocin (OT) administration reduces body weight in high-fat diet (HFD)-induced obese (DIO) rodents through both reductions in food intake and increases in energy expenditure. We recently demonstrated that chronic hindbrain [fourth ventricular (4V)] infusions of OT evoke weight loss in DIO rats. Based on these findings, we hypothesized that chronic 4V OT would elicit weight loss in DIO mice. We assessed the effects of 4V infusions of OT (16 nmol/day) or vehicle over 28 days on body weight, food intake, and body composition. OT reduced body weight by approximately 4.5% ± 1.4% in DIO mice relative to OT pretreatment body weight (P < 0.05). These effects were associated with reduced adiposity and adipocyte size [inguinal white adipose tissue (IWAT)] (P < 0.05) and attributed, in part, to reduced energy intake (P < 0.05) at a dose that did not increase kaolin intake (P = NS). OT tended to increase uncoupling protein-1 expression in IWAT (0.05 < P < 0.1) suggesting that OT stimulates browning of WAT. To assess OT-elicited changes in brown adipose tissue (BAT) thermogenesis, we examined the effects of 4V OT on interscapular BAT temperature (TIBAT). 4V OT (1 µg) elevated TIBAT at 0.75 (P = 0.08), 1, and 1.25 h (P < 0.05) postinjection; a higher dose (5 µg) elevated TIBAT at 0.75-, 1-, 1.25-, 1.5-, 1.75- (P < 0.05), and 2-h (0.05 < P < 0.1) postinjection. Together, these findings support the hypothesis that chronic hindbrain OT treatment evokes sustained weight loss in DIO mice by reducing energy intake and increasing BAT thermogenesis at a dose that is not associated with evidence of visceral illness.

Central and peripheral release of oxytocin: Relevance of neuroendocrine and neurotransmitter actions for physiology and behavior

The hypothalamic neuropeptide oxytocin (OT) is critically involved in the modulation of socio-emotional behavior, sexual competence, and pain perception and anticipation. While intracellular signaling of OT and its receptor (OTR), as well as the functional connectivity of hypothalamic and extra-hypothalamic OT projections, have been recently explored, it remains elusive how one single molecule has pleotropic effects from cell proliferation all the way to modulation of complex cognitive processes. Moreover, there are astonishing species-dependent differences in the way OT regulates various sensory modalities such as touch, olfaction, and vision, which can be explained by differences in OTR expression in brain regions processing sensory information. Recent research highlights a small subpopulation of OT-synthesizing cells, namely, parvocellular cells, which merely constitute 1% of the total number of OT cells but act as "master cells' that regulate the activity of the entire OT system. In this chapter, we summarize the latest advances in the field of OT research with a particular focus on differences between rodents, monkeys and humans and highlight the main differences between OT and its "sister" peptide arginine-vasopressin, which often exerts opposite effects on physiology and behavior.

Central oxytocin signaling inhibits food reward-motivated behaviors and VTA dopamine responses to food-predictive cues in male rats

Oxytocin potently reduces food intake and is a potential target system for obesity treatment. A better understanding of the behavioral and neurobiological mechanisms mediating oxytocin's anorexigenic effects may guide more effective obesity pharmacotherapy development. The present study examined the effects of central (lateral intracerebroventricular [ICV]) administration of oxytocin in rats on motivated responding for palatable food. Various conditioning procedures were employed to measure distinct appetitive behavioral domains, including food seeking in the absence of consumption (conditioned place preference expression), impulsive responding for food (differential reinforcement of low rates of responding), effort-based appetitive decision making (high-effort palatable vs. low-effort bland food), and sucrose reward value encoding following a motivational shift (incentive learning). Results reveal that ICV oxytocin potently reduces food-seeking behavior, impulsivity, and effort-based palatable food choice, yet does not influence encoding of sucrose reward value in the incentive learning task. To investigate a potential neurobiological mechanism mediating these behavioral outcomes, we utilized in vivo fiber photometry in ventral tegmental area (VTA) dopamine neurons to examine oxytocin's effect on phasic dopamine neuron responses to sucrose-predictive Pavlovian cues. Results reveal that ICV oxytocin significantly reduced food cue-evoked dopamine neuron activity. Collectively, these data reveal that central oxytocin signaling inhibits various obesity-relevant conditioned appetitive behaviors, potentially via reductions in food cue-driven phasic dopamine neural responses in the VTA.

NTS and VTA oxytocin reduces food motivation and food seeking

Oxytocin (OT) is a neuropeptide whose central receptor-mediated actions include reducing food intake. One mechanism of its behavioral action is the amplification of the feeding inhibitory effects of gastrointestinal (GI) satiation signals processed by hindbrain neurons. OT treatment also reduces carbohydrate intake in humans and rodents, and correspondingly, deficits in central OT receptor (OT-R) signaling increase sucrose self-administration. This suggests that additional processes contribute to central OT effects on feeding. This study investigated the hypothesis that central OT reduces food intake by decreasing food seeking and food motivation. As central OT-Rs are expressed widely, a related focus was to assess the role of one or more OT-R-expressing nuclei in food motivation and food-seeking behavior. OT was delivered to the lateral ventricle (LV), nucleus tractus solitarius (NTS), or ventral tegmental area (VTA), and a progressive ratio (PR) schedule of operant reinforcement and an operant reinstatement paradigm were used to measure motivated feeding behavior and food-seeking behavior, respectively. OT delivered to the LV, NTS, or VTA reduced 1) motivation to work for food and 2) reinstatement of food-seeking behavior. Results provide a novel and additional interpretation for central OT-driven food intake inhibition to include the reduction of food motivation and food seeking.

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.

The hypothalamus to brainstem circuit suppresses late-onset body weight gain

Body weight (BW) is regulated in age-dependent manner; it continues to increase during growth period, and reaches a plateau once reaching adulthood. However, its underlying mechanism remains unknown. Regarding such mechanisms in the brain, we here report that neural circuits from the hypothalamus (paraventricular nucleus: PVN) to the brainstem (dorsal vagal complex: DVC) suppress late-onset BW gain without affecting food intake. The genetic suppression of the PVN-DVC circuit induced BW increase only in aged rats, indicating that this circuit contributes to suppress the BW at a fixed level after reaching adulthood. PVN neurons in the hypothalamus were inactive in younger rats but active in aged rats. The density of neuropeptide Y (NPY) terminal/fiber is reduced in the aged rat PVN area. The differences in neuronal activity, including oxytocin neurons in the PVN, were affected by the application of NPY or its receptor inhibitor, indicating that NPY is a possible regulator of this pathway. Our data provide new insights into understanding age-dependent BW regulation.

Circulating Oxytocin Is Genetically Determined and Associated With Obesity and Impaired Glucose Tolerance

CONTEXT

Despite the emerging evidence on the role of oxytocin (OXT) in metabolic diseases, there is a lack of well-powered studies addressing the relationship of circulating OXT with obesity and diabetes.

OBJECTIVES AND DESIGN

Here, we measured OXT in a study cohort (n = 721; 396 women, 325 men; mean age ± SD, 47.7 ± 15.2 years) with subphenotypes related to obesity, including anthropometric traits such as body mass index [BMI (mean ± SD), 26.8 ± 4.6 kg/m2], waist-to-hip ratio (WHR; 0.88 ± 0.09), blood parameters (glucose, 5.32 ± 0.50 mmol/L; insulin, 5.3 ± 3.3 µU/mL), and oral glucose tolerance test to clarify the association with OXT. We also tested in a genome-wide association study (GWAS) whether the interindividual variation in OXT serum levels might be explained by genetic variation.

RESULTS

The OXT concentration was increased in subjects with elevated BMI and positively correlated with WHR, waist circumference, and triglyceride levels. The OXT concentration in subjects with BMI <25 kg/m2 was significantly lower (n = 256; 78.6 pg/mL) than in subjects with a BMI between 25 and 30 kg/m2 (n = 314; 98.5 pg/mL, P = 6 × 10-6) and with BMI >30 kg/m2 (n = 137; 106.4 pg/mL, P = 8 × 10-6). OXT levels were also positively correlated with plasma glucose and insulin and were elevated in subjects with impaired glucose tolerance (P = 4.6 × 10-3). Heritability of OXT was estimated at 12.8%. In a GWAS, two hits in linkage disequilibrium close (19 kb) to the OXT reached genome-wide significant association (top-hit rs12625893, P = 3.1 × 10-8, explained variance 3%).

CONCLUSIONS

Our data show that OXT is genetically affected by a variant near OXT and is associated with obesity and impaired glucose tolerance.

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.

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.

Endogenous Oxytocin Levels in Relation to Food Intake, Menstrual Phase, and Age in Females

CONTEXT

Oxytocin regulates a range of physiological processes including eating behavior and oxytocin administration reduces caloric intake in males. There are few data on oxytocin and eating behavior in healthy females or on the response of endogenous oxytocin to food intake and its relationship to appetite in humans.

OBJECTIVES

To determine the postprandial pattern of oxytocin levels, the relationship between oxytocin and appetite, and the impact of menstrual cycle phase and age on oxytocin levels in females.

DESIGN

Cross-sectional.

SETTING

Clinical research center.

PARTICIPANTS

Fifty-five healthy females (age 10 to 45 years).

INTERVENTIONS

A standardized mixed meal was administered.

MAIN OUTCOME MEASUREMENTS

Blood sampling for oxytocin occurred at fasting and at 30, 60, and 120 minutes postmeal. Appetite was assessed using Visual Analogue Scales pre- and postmeal.

RESULTS

Mean fasting oxytocin levels were 1011.2 ± 52.3 pg/mL (SEM) and decreased at 30 and 60 minutes postmeal (P = 0.001 and P = 0.003, respectively). Mean oxytocin levels decreased19.6% ± 3.0% from baseline to nadir. Oxytocin area under the curve was lower in the early to midfollicular menstrual cycle phase (P = 0.0003) and higher in younger females (P = 0.002). The percent change in oxytocin (baseline to nadir) was associated with postprandial hunger (rs = -0.291, P = 0.03) and fullness (rs = 0.345, P = 0.009). These relations remained significant after controlling for calories consumed, menstrual cycle status, and age (P = 0.023 and P = 0.0001, respectively).

CONCLUSIONS

Peripheral oxytocin levels in females decrease after a mixed meal and are associated with appetite independent of menstrual phase, age, and caloric intake, suggesting that endogenous oxytocin levels may play a role in perceived hunger and satiety.

Effect of Oxytocin on Hunger Discrimination

Centrally and peripherally administered oxytocin (OT) decreases food intake and activation of the endogenous OT systems, which is associated with termination of feeding. Evidence gathered thus far points to OT as a facilitator of early satiation, a peptide that reduces the need for a meal that has already begun. It is not known, however, whether OT can diminish a feeling of hunger, thereby decreasing a perceived need to seek calories. Therefore, in the current project, we first confirmed that intraperitoneal (i.p.) OT at 0.3-1 mg/kg reduces food intake in deprived and non-deprived rats. We then used those OT doses in a unique hunger discrimination protocol. First, rats were trained to discriminate between 22- and 2-h food deprivation (hungry vs. sated state) in a two-lever operant procedure. After rats acquired the discrimination, they were food-restricted for 22 h and given i.p. OT before a generalization test session. OT did not decrease 22-h deprivation-appropriate responding to match that following 2-h food deprivation, thus, it did not reduce the perceived level of hunger. In order to better understand the mechanisms behind this ineffectiveness of OT, we used c-Fos immunohistochemistry to determine whether i.p. OT activates a different subset of feeding-related brain sites under 22- vs. 2-h deprivation. We found that in sated animals, OT induces c-Fos changes in a broader network of hypothalamic and brain stem sites compared to those affected in the hungry state. Finally, by employing qPCR analysis, we asked whether food deprivation vs. sated state have an impact on OT receptor expression in the brain stem, a CNS "entry" region for peripheral OT. Fasted animals had significantly lower OT receptor mRNA levels than their ad libitum-fed counterparts. We conclude that OT does not diminish a feeling of hunger before a start of a meal. Instead OT's anorexigenic properties are manifested once consumption has already begun which is-at least to some extent-driven by changes in brain responsiveness to OT treatment in the hungry vs. fed state. OT should be viewed as a mediator of early satiation rather than as a molecule that diminishes perceived hunger.

The relationship between oxytocin, dietary intake and feeding: A systematic review and meta-analysis of studies in mice and rats

The neuropeptide oxytocin has been associated with food intake and feeding behaviour. This systematic review aimed to investigate the impact of oxytocin on dietary intake and feeding behaviour in rodent studies. Six electronic databases were searched to identify published studies to April 2018. Preclinical studies in mice and rats were included if they reported: (1) a dietary measure (i.e. food or nutrient and/or behaviour (2) an oxytocin measure, and (3) relationship between the two measures. A total of 75 articles (n = 246 experiments) were included, and study quality appraised. The majority of studies were carried out in males (87%). The top three oxytocin outcomes assessed were: exogenous oxytocin administration (n = 126), oxytocin-receptor antagonist administration (n = 46) and oxytocin gene deletion (n = 29). Meta-analysis of exogenous studies in mice (3 studies, n = 43 comparisons) and rats (n = 8 studies, n = 82 comparisons) showed an overall decrease in food intake with maximum effect shown at 2 h post-administration.

Effects of acute and chronic nicotine on catecholamine neurons of the nucleus of the solitary tract

Nicotine is an addictive drug that has broad effects throughout the brain. One site of action is the nucleus of the solitary tract (NTS), where nicotine initiates a stress response and modulates cardiovascular and gastric function through nicotinic acetylcholine receptors (nAChRs). Catecholamine (CA) neurons in the NTS influence stress and gastric and cardiovascular reflexes, making them potential mediators of nicotine's effects; however nicotine's effect on these neurons is unknown. Here, we determined nicotine's actions on NTS-CA neurons by use of patch-clamp techniques in brain slices from transgenic mice expressing enhanced green fluorescent protein driven by the tyrosine hydroxylase promoter (TH-EGFP). Picospritzing nicotine both induced a direct inward current and increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in NTS-CA neurons, effects blocked by nonselective nAChR antagonists TMPH and MLA. The increase in sEPSC frequency was mimicked by nAChRα7 agonist AR-R17779 and blocked by nAChRα7 antagonist MG624. AR-R17779 also increased the firing of TH-EGFP neurons, an effect dependent on glutamate inputs, as it was blocked by the glutamate antagonist NBQX. In contrast, the nicotine-induced current was mimicked by nAChRα4β2 agonist RJR2403 and blocked by nAChRα4β2 antagonist DHβE. RJR2403 also increased the firing rate of TH-EGFP neurons independently of glutamate. Finally, both somatodendritic and sEPSC nicotine responses from NTS-CA neurons were larger in nicotine-dependent mice that had under gone spontaneous nicotine withdrawal. These results demonstrate that 1) nicotine activates NTS-CA neurons both directly, by inducing a direct current, and indirectly, by increasing glutamate inputs, and 2) NTS-CA nicotine responsiveness is altered during nicotine withdrawal.

Intranasal oxytocin reduces weight gain in diet-induced obese prairie voles

Oxytocin (OT) elicits weight loss in diet-induced obese (DIO) rodents, nonhuman primates and humans by reducing food intake and increasing energy expenditure. In addition to being important in the regulation of energy balance, OT is involved in social behaviors including parent-infant bonds, friendships, and pair bonds. However, the impact of social context on susceptibility to diet-induced obesity (DIO) and feeding behavior (including food sharing) has not been investigated in a rodent model that forms strong social bonds (i.e. prairie vole). Our goals were to determine in Prairie voles (Microtus ochrogaster) whether i) social context impacts susceptibility to DIO and ii) chronic intranasal OT reverses DIO. Voles were housed in divided cages with holes in the divider and paired with a same-sex animal with either the same food [high fat diet (HFD)/HFD, [low fat diet (LFD; chow)/chow], or the opposite food (HFD/chow or chow/HFD) for 19 weeks. HFD-fed voles pair-housed with voles maintained on the HFD demonstrated increased weight relative to pair-housed voles that were both maintained on chow. The study was repeated to determine the impact of social context on DIO susceptibility and body composition when animals are maintained on purified sugar-sweetened HFD and LFD to enhance palatability. As before, we found that voles demonstrated higher weight gain on the HFD/HFD housing paradigm, in part, through increased energy intake and the weight gain was a consequence of an increase in fat mass. However, HFD-fed animals housed with LFD-fed animals (and vice versa) showed intermediate patterns of weight gain and evidence of food sharing. Of translational importance is the finding that chronic intranasal OT appeared to reduce weight gain in DIO voles through a decrease in fat mass with no reduction in lean body mass. These effects were associated with transient reductions in food intake and increased food sharing. These findings identify a role of social context in the pathogenesis of DIO and indicate that chronic intranasal OT treatment reduces weight gain and body fat mass in DIO prairie voles, in part, by reducing food intake.

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

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

A Systematic Review and Quantitative Meta-Analysis of Oxytocin's Effects on Feeding

PURPOSE

Oxytocin's anorexigenic effects have been widely documented and accepted; however, no paper has yet used the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines to compile previous findings in a single systematic review and quantitative meta-analysis. The current paper aimed to identify published and unpublished studies examining the effects of oxytocin on energy intake in animals and humans, and the factors that moderate this effect.

METHODS

Web of Science, Pub Med, and Ovid were searched for published and unpublished studies reporting the effects of oxytocin on energy intake in wild-type animals and in humans, when administered in the absence of other active drugs or surgery.

RESULTS

2049 articles were identified through the original systematic literature search, from which 54 articles were identified as relevant for inclusion in this review. An additional 3 relevant articles were identified in a later update of the literature search. Overall, a single-dose of oxytocin was found to reduce feeding in animals. Despite several individual studies which found that this effect persists to the end of the third week of chronic administration in rodent models, overall, this anorexigenic effect did not hold in the meta-analyses testing the effects of chronic administration. There was no overall effect of oxytocin on energy intake in humans, although a trend was identified for oxytocin to reduce consumption of solid foods.

CONCLUSIONS

Oxytocin reduces energy intake when administered as a single dose. Oxytocin can inhibit feeding over two- to three-week periods in rodent models. These effects typically do not persist beyond the third week of treatment. The anorexigenic effect of oxytocin is moderated by pregnant status, dose, method of administration, and diet composition. This article is protected by copyright. All rights reserved.

Oxytocin curbs calorie intake via food-specific increases in the activity of brain areas that process reward and establish cognitive control

The hypothalamic neurohormone oxytocin decreases food intake via largely unexplored mechanisms. We investigated the central nervous mediation of oxytocin's hypophagic effect in comparison to its impact on the processing of generalized rewards. Fifteen fasted normal-weight, young men received intranasal oxytocin (24 IU) or placebo before functional magnetic resonance imaging (fMRI) measurements of brain activity during exposure to food stimuli and a monetary incentive delay task (MID). Subsequently, ad-libitum breakfast intake was assessed. Oxytocin compared to placebo increased activity in the ventromedial prefrontal cortex, supplementary motor area, anterior cingulate, and ventrolateral prefrontal cortices in response to high- vs. low-calorie food images in the fasted state, and reduced calorie intake by 12%. During anticipation of monetary rewards, oxytocin compared to placebo augmented striatal, orbitofrontal and insular activity without altering MID performance. We conclude that during the anticipation of generalized rewards, oxytocin stimulates dopaminergic reward-processing circuits. In contrast, oxytocin restrains food intake by enhancing the activity of brain regions that exert cognitive control, while concomitantly increasing the activity of structures that process food reward value. This pattern points towards a specific role of oxytocin in the regulation of eating behaviour in humans that might be of relevance for potential clinical applications.

Beneficial effects of chronic oxytocin administration and social co-housing in a rodent model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is in part due to a deficit in memory consolidation and extinction. Oxytocin (OXT) has anxiolytic effects and promotes prosocial behaviors in both rodents and humans, and evidence suggests that it plays a role in memory consolidation. We studied the effects of administered OXT and social co-housing in a rodent model of PTSD. Acute OXT yielded a short-term increase in the recall of the traumatic memory if administered immediately after trauma. Low doses of OXT delivered chronically had a cumulating anxiolytic effect that became apparent after 4 days and persisted. Repeated injections of OXT after short re-exposures to the trauma apparatus yielded a long-term reduction in anxiety. Co-housing with naive nonshocked animals decreased the memory of the traumatic context compared with single-housed animals. In the long term, these animals showed less thigmotaxis and increased interest in novel objects, and a low OXT plasma level. Co-housed PTSD animals showed an increase in risk-taking behavior. These results suggest beneficial effects of OXT if administered chronically through increases in memory consolidation after re-exposure to a safe trauma context. We also show differences between the benefits of social co-housing with naive rats and co-housing with other shocked animals on trauma-induced long-term anxiety.

The Anorexigenic Neural Pathways of Oxytocin and Their Clinical Implication

Oxytocin was discovered in 1906 as a peptide that promotes delivery and milk ejection; however, its additional physiological functions were determined 100 years later. Many recent articles have reported newly discovered effects of oxytocin on social communication, bonding, reward-related behavior, adipose tissue, and muscle and food intake regulation. Because oxytocin neurons project to various regions in the brain that contribute to both feeding reward (hedonic feeding) and the regulation of energy balance (homeostatic feeding), the mechanisms of oxytocin on food intake regulation are complicated and largely unknown. Oxytocin neurons in the paraventricular nucleus (PVN) receive neural projections from the arcuate nucleus (ARC), which is an important center for feeding regulation. On the other hand, these neurons in the PVN and supraoptic nucleus project to the ARC. PVN oxytocin neurons also project to the brain stem and the reward-related limbic system. In addition to this, oxytocin induces lipolysis and decreases fat mass. However, these effects in feeding and adipose tissue are known to be dependent on body weight (BW). Oxytocin treatment is more effective in food intake regulation and fat mass decline for individuals with leptin resistance and higher BW, but is known to be less effective in individuals with normal BW. In this review, we present in detail the recent findings on the physiological role of oxytocin in feeding regulation and the anorexigenic neural pathway of oxytocin neurons, as well as the advantage of oxytocin usage for anti-obesity treatment.

The effects of oxytocin on eating behaviour and metabolism in humans

Oxytocin, a hypothalamic hormone that is secreted directly into the brain and enters the peripheral circulation through the posterior pituitary gland, regulates a range of physiologic processes, including eating behaviour and metabolism. In rodents and nonhuman primates, chronic oxytocin administration leads to sustained weight reduction by reducing food intake, increasing energy expenditure and inducing lipolysis. Oxytocin might improve glucose homeostasis, independently of its effects on weight. Clinical studies are beginning to translate these important preclinical findings to humans. This Review describes key data linking oxytocin to eating behaviour and metabolism in humans. For example, a single intranasal dose of oxytocin can reduce caloric intake, increase fat oxidation and improve insulin sensitivity in men. Furthermore, a pilot study of 8 weeks of oxytocin treatment in adults with obesity or overweight led to substantial weight loss. Together, these data support further investigation of interventions that target pathways involving oxytocin as potential therapeutics in metabolic disorders, including obesity and diabetes mellitus. Therapeutic considerations and areas for further research are also discussed.

Diversity of oxytocin neurons: beyond magno- and parvocellular cell types?

The hypothalamic neuropeptide oxytocin (OT), which is evolutionarily conserved among different species throughout the animal kingdom, is a key modulator of a variety of socio-emotional behaviors such as fear, trust and empathy. OT cells in the mammalian hypothalamus have been traditionally divided into two distinct types - magnocellular (magnOT) and parvocellular (parvOT) or preautonomic neurons. This distinction is based on OT cell sizes and shapes, projections, electrophysiological activity and functions. Indeed, while neuroendocrine magnOT neurons are known to primarily project their axons to the posterior pituitary and to a number of forebrain regions, non-neuroendocrine parvOT neurons have been seen as the main source of OT innervation of the brainstem and spinal cord to control autonomic functions and pain perception. However, very recent findings demonstrated distinct genetic profiles in OT neurons, allowing discrimination of at least four types of cells expressing OT. Furthermore, unexpected axonal projections of parvOT neurons to the forebrain and magnOT neurons to the midbrain have been newly reported. In this review, we focus on the detailed analysis of methods of distinction between OT cell types, in- and output sites, morphology as well as on the direct connectivity between OT neurons and its physiological significance. At the end, we propose a hypothesis that the central OT system is composed of more than just two OT cell types, which should be further verified by the application of available genetic and anatomical techniques. This article is protected by copyright. All rights reserved.

Chronic hindbrain administration of oxytocin is sufficient to elicit weight loss in diet-induced obese rats

Oxytocin (OT) administration elicits weight loss in diet-induced obese (DIO) rodents, nonhuman primates, and humans by reducing energy intake and increasing energy expenditure. Although the neurocircuitry underlying these effects remains uncertain, OT neurons in the paraventricular nucleus are positioned to control both energy intake and sympathetic nervous system outflow to interscapular brown adipose tissue (BAT) through projections to the hindbrain nucleus of the solitary tract and spinal cord. The current work was undertaken to examine whether central OT increases BAT thermogenesis, whether this effect involves hindbrain OT receptors (OTRs), and whether such effects are associated with sustained weight loss following chronic administration. To assess OT-elicited changes in BAT thermogenesis, we measured the effects of intracerebroventricular administration of OT on interscapular BAT temperature in rats and mice. Because fourth ventricular (4V) infusion targets hindbrain OTRs, whereas third ventricular (3V) administration targets both forebrain and hindbrain OTRs, we compared responses to OT following chronic 3V infusion in DIO rats and mice and chronic 4V infusion in DIO rats. We report that chronic 4V infusion of OT into two distinct rat models recapitulates the effects of 3V OT to ameliorate DIO by reducing fat mass. While reduced food intake contributes to this effect, our finding that 4V OT also increases BAT thermogenesis suggests that increased energy expenditure may contribute as well. Collectively, these findings support the hypothesis that, in DIO rats, OT action in the hindbrain evokes sustained weight loss by reducing energy intake and increasing BAT thermogenesis.

Intravenous administration of oxytocin in rats acutely decreases deprivation-induced chow intake, but it fails to affect consumption of palatable solutions

Despite its limited ability to cross the blood-brain barrier, peripherally administered oxytocin (OT) acutely decreases food intake, most likely via the brainstem and hypothalamic mechanisms. Studies performed to date have focused mainly on the effects of subcutaneous or intraperitoneal OT on the consumption of only solid calorie-dense diets (either standard or high-fat), whereas it is unknown whether, similarly to central OT, peripherally administered peptide reduces intake of calorie-dilute and non-caloric palatable solutions. In this project, we established that 0.1μg/kg intravenous (IV) OT is the lowest anorexigenic dose, decreasing deprivation-induced standard chow intake by ca. 40% in rats and its effect does not stem from aversion. We then used this dose in paradigms in which effects of centrally acting OT ligands on consumption of palatable solutions had been previously reported. We found that IV OT did not change episodic intake of individually presented palatable solutions containing 10% sucrose, 0.1% saccharin, combined 10% sucrose-0.1% saccharin or 4.1%. Intralipid and it failed to affect daily scheduled consumption of a sucrose solution in non-deprived rats. In a two-bottle choice test, IV OT did not shift animals' preference from sucrose to Intralipid. Finally, OT injected IV prior to the simultaneous presentation chow and a sucrose solution in food-deprived rats significantly decreased chow intake, whereas sugar water consumption remained unchanged. We conclude that IV OT reduces deprivation-induced chow intake without causing aversion, but the dose effective in decreasing energy-driven consumption of high-calorie food fails to affect consumption of palatable calorie-dilute solutions.

High-Sugar, but Not High-Fat, Food Activates Supraoptic Nucleus Neurons in the Male Rat

Oxytocin is a potent anorexigen and is believed to have a role in satiety signaling. We developed rat models to study the activity of oxytocin neurons in response to voluntary consumption or oral gavage of foods using c-Fos immunohistochemistry and in vivo electrophysiology. Using c-Fos expression as an indirect marker of neural activation, we showed that the percentage of magnocellular oxytocin neurons expressing c-Fos increased with voluntary consumption of sweetened condensed milk (SCM). To model the effect of food in the stomach, we gavaged anesthetized rats with SCM. The percentage of supraoptic nucleus and paraventricular nucleus magnocellular oxytocin-immunoreactive neurons expressing c-Fos increased with SCM gavage but not with gastric distention. To further examine the activity of the supraoptic nucleus, we made in vivo electrophysiological recordings from SON neurons, where anesthetized rats were gavaged with SCM or single cream. Pharmacologically identified oxytocin neurons responded to SCM gavage with a linear, proportional, and sustained increase in firing rate, but cream gavage resulted in a transient reduction in firing rate. Blood glucose increased after SCM gavage but not cream gavage. Plasma osmolarity and plasma sodium were unchanged throughout. We show that in response to high-sugar, but not high-fat, food in the stomach, there is an increase in the activity of oxytocin neurons. This does not appear to be a consequence of stomach distention or changes in osmotic pressure. Our data suggest that the presence of specific foods with different macronutrient profiles in the stomach differentially regulates the activity of oxytocin neurons.

High glucose increases action potential firing of catecholamine neurons in the nucleus of the solitary tract by increasing spontaneous glutamate inputs

Glucose is a crucial substrate essential for cell survival and function. Changes in glucose levels impact neuronal activity and glucose deprivation increases feeding. Several brain regions have been shown to respond to glucoprivation, including the nucleus of the solitary tract (NTS) in the brain stem. The NTS is the primary site in the brain that receives visceral afferent information from the gastrointestinal tract. The catecholaminergic (CA) subpopulation within the NTS modulates many homeostatic functions including cardiovascular reflexes, respiration, food intake, arousal, and stress. However, it is not known if they respond to changes in glucose. Here we determined whether NTS-CA neurons respond to changes in glucose concentration and the mechanism involved. We found that decreasing glucose concentrations from 5 mM to 2 mM to 1 mM, significantly decreased action potential firing in a cell-attached preparation, whereas increasing it back to 5 mM increased the firing rate. This effect was dependent on glutamate release from afferent terminals and required presynaptic 5-HT₃Rs. Decreasing the glucose concentration also decreased both basal and 5-HT₃R agonist-induced increase in the frequency of spontaneous glutamate inputs onto NTS-CA neurons. Low glucose also blunted 5-HT-induced inward currents in nodose ganglia neurons, which are the cell bodies of vagal afferents. The effect of low glucose in both nodose ganglia cells and in NTS slices was mimicked by the glucokinase inhibitor glucosamine. This study suggests that NTS-CA neurons are glucosensing through a presynaptic mechanism that is dependent on vagal glutamate release, 5-HT₃R activity, and glucokinase.

Hormonal systems, human social bonding, and affiliation

Which hormones are implicated in human social bonding and affiliation? And how does field research speak to this issue? We begin by laying out a broad view of how endocrine hormones in general modulate life history allocations of energy and other resources, and the ways in which their neuromodulatory functions must be understood within a broader conceptualization of how they have been shaped to affect allocations. We then turn to four specific hormones or hormone families that have received much attention: oxytocin, opioids, prolactin, and progesterone. Each plays a role in regulating psychological capacities and propensities that underlie individuals' interactions with important social targets. Yet in no case is it clear exactly what regulatory roles these hormones play. We suggest several directions for future research.

Attenuated hypothalamic responses to α-melanocyte stimulating hormone during pregnancy in the rat

KEY POINTS

Increased appetite and weight gain occurs during pregnancy, associated with development of leptin resistance, and satiety responses to the anorectic peptide α-melanocyte stimulating hormone (α-MSH) are suppressed. This study investigated hypothalamic responses to α-MSH during pregnancy, using c-fos expression in specific hypothalamic nuclei as a marker of neuronal signalling, and in vivo electrophysiology in supraoptic nucleus (SON) oxytocin neurons, as a representative α-MSH-responsive neuronal population that shows a well-characterised α-MSH-induced inhibition of firing. While icv injection of α-MSH significantly increased the number of c-fos-positive cells in the paraventricular, supraoptic, arcuate and ventromedial hypothalamic nuclei in non-pregnant rats, this response was suppressed in pregnant rats. Similarly, SON oxytocin neurons in pregnant rats did not demonstrate characteristic α-MSH-induced inhibition of firing that was observed in non-pregnant animals. Given the known functions of α-MSH in the hypothalamus, the attenuated responses are likely to facilitate adaptive changes in appetite regulation and oxytocin secretion during pregnancy.

ABSTRACT

During pregnancy, a state of positive energy balance develops to support the growing fetus and to deposit fat in preparation for the subsequent metabolic demands of lactation. As part of this maternal adaptation, the satiety response to the anorectic peptide α-melanocyte stimulating hormone (α-MSH) is suppressed. To investigate whether pregnancy is associated with changes in the response of hypothalamic α-MSH target neurons, non-pregnant and pregnant rats were treated with α-MSH or vehicle and c-fos expression in hypothalamic nuclei was then examined. Furthermore, the firing rate of supraoptic nucleus (SON) oxytocin neurons, a known α-MSH responsive neuronal population, was examined in non-pregnant and pregnant rats following α-MSH treatment. Intracerebroventricular injection of α-MSH significantly increased the number of c-fos-positive cells in the paraventricular, arcuate and ventromedial hypothalamic nuclei in non-pregnant rats, but no significant increase was observed in any of these regions in pregnant rats. In the SON, α-MSH did induce expression of c-fos during pregnancy, but this was significantly reduced compared to that observed in the non-pregnant group. Furthermore, during pregnancy, SON oxytocin neurons did not demonstrate the characteristic α-MSH-induced inhibition of firing rate that was observed in non-pregnant animals. Melanocortin receptor mRNA levels during pregnancy were similar to non-pregnant animals, suggesting that receptor down-regulation is unlikely to be a mechanism underlying the attenuated responses to α-MSH during pregnancy. Given the known functions of α-MSH in the hypothalamus, the attenuated responses will facilitate adaptive changes in appetite regulation and oxytocin secretion during pregnancy.