Dissociation of oxytocin effects on body weight in two variants of female Sprague-Dawley rats

Chronic oxytocin administration as a tool for investigation and treatment: A cross-disciplinary systematic review

Oxytocin (OT) subserves various physiological, behavioral, and cognitive processes. This paired with the ability to administer OT with minimal and inconsistent side effects has spurred research to explore its therapeutic potential. Findings from single-dose studies indicate that OT administration may be beneficial, at least under certain circumstances. The state of the field, however, is less clear regarding effects from chronic OT administration, which more closely resembles long-term treatment. To address this gap, this review synthesizes existing findings on the use of chronic OT administration in animal and human work. In addition to detailing the effects of chronic OT administration across different functional domains, this review highlights factors that have contributed to mixed findings. Based on this review, a basic framework of interrelated regulatory functions sensitive to chronic OT administration is offered. The paper also identifies future research directions across different contexts, populations, and outcomes, specifically calling for more systematic and standardized research on chronic OT administration in humans to supplement and expand what is currently known from preclinical work.

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.

Oxytocin effects on experimental skin wound healing

OBJECTIVE

Oxytocin (OXY) has significant effects on mammalian behavior. Next to its role in lactation and social interactions, it is described to support better wound healing as well. However, direct OXY effects on wound healing and the regeneration of the microvascular network are still not clarified. We therefore examined the effects of OXY and an OXY receptor antagonist [atosiban (ATO)] on skin wound healing, focusing on epithelialization and neovascularization.

METHODS

Skin wound healing has been assessed using intravital fluorescence microscopy in a model of full dermal thickness wounds in the dorsal skin fold chamber of hairless mice. Animals received repetitive low or high doses of OXY or ATO. Morphological and cellular characterization of skin tissue repair was performed by histology and in vitro cell assays.

RESULTS

The assessment of skin tissue repair using this therapy regimen showed that OXY and ATO had no major influence on epithelialization, neovascularization, wound cellularity, or inflammation. Moreover, OXY and ATO did neither stimulate nor deteriorate keratinocyte or fibroblast migration and proliferation.

CONCLUSION

In summary, this study is the first to demonstrate that OXY application does not impair skin wound healing or cell behavior. However, until now, the used transmitter system seems not to be clarified in detail, and it might be proposed that it is associated with the stress response of the organism to various stimuli.

C-tactile afferents: Cutaneous mediators of oxytocin release during affiliative tactile interactions?

Low intensity, non-noxious, stimulation of cutaneous somatosensory nerves has been shown to trigger oxytocin release and is associated with increased social motivation, plus reduced physiological and behavioural reactivity to stressors. However, to date, little attention has been paid to the specific nature of the mechanosensory nerves which mediate these effects. In recent years, the neuroscientific study of human skin nerves (microneurography studies on single peripheral nerve fibres) has led to the identification and characterisation of a class of touch sensitive nerve fibres named C-tactile afferents. Neither itch nor pain receptive, these unmyelinated, low threshold mechanoreceptors, found only in hairy skin, respond optimally to low force/velocity stroking touch. Notably, the speed of stroking which C-tactile afferents fire most strongly to is also that which people perceive to be most pleasant. The social touch hypothesis posits that this system of nerves has evolved in mammals to signal the rewarding value of physical contact in nurturing and social interactions. In support of this hypothesis, we review the evidence that cutaneous stimulation directly targeted to optimally activate C-tactile afferents reduces physiological arousal, carries a positive affective value and, under healthy conditions, inhibits responses to painful stimuli. These effects mirror those, we also review, which have been reported following endogenous release and exogenous administration of oxytocin. Taken together this suggests C-tactile afferent stimulation may mediate oxytocin release during affiliative tactile interactions.

Effects of Oxytocin Administration on the Response of Piglets to Weaning

Simple Summary

Weaning is a stressful milestone for domestic animals. It is often performed at an early age and as an abrupt change in comparison to the transitional period seen in feral or wild animals. Oxytocin, a hormone associated with attachment, could improve the response of piglets to weaning. Piglets were either given oxytocin intranasally, subcutaneously, or handled as controls. Oxytocin had no effect on the physiological response to weaning. However, oxytocin increased the frequency of mild aggressive social behaviors between OT-administered and control pigs. Hence, the use of a single administration of oxytocin prior to weaning in pigs is not recommended.

Abstract

Weaning is often an abrupt and stressful process. We studied the effects of administering oxytocin, subcutaneously or intranasally, on the ability of pigs to cope with weaning. On a commercial farm 144, 30 day-old pigs from 24 litters were used. On the day of weaning, one male and one female in each litter were administered one of three treatments: intranasal oxytocin (24 International Unit), subcutaneous oxytocin (10 International Unit per kg of body weight), or handled as a control. The pigs were placed in one of eight weaner pens, split by sex and with an equal representation of treatments. Data included body weight and growth, physiology (neutrophil:lymphocyte ratio, plasma cortisol, C-reactive protein and Tumor Necrosis Factor-α concentrations), and behavior (feeding, drinking, social behavior). Both oxytocin treatments tended to result in higher levels of mild aggression within groups (p = 0.08), specifically between oxytocin-administered and control pigs (subcutaneous to control p = 0.03; intranasal to control p = 0.10). Subcutaneously-administered pigs tended to frequent the feeder more often than intranasally-administered pigs (p < 0.10), with the latter having slightly lower body weight 38 days post-weaning (p = 0.03). However, acute oxytocin administration did not result in any noticeable physiological changes 4 or 28 h post-weaning. Hence, the use of a single administration of oxytocin prior to weaning in pigs is not recommended, at least not in the conditions studied here.

Neonatal oxytocin administration and supplemental milk ameliorate the weaning transition and alter hormonal expression in the gastrointestinal tract in pigs

The aim of this study was to investigate the influences of milk supplementation during lactation, over 1 wk after weaning, and oxytocin administration for the first 14 d of life on the pigs' response to weaning. Pigs from 20 litters were allocated to each of these 3 treatments in a randomized factorial design. Oxytocin was administered subcutaneously daily from 0 to 14 d of age at a rate of 10 I.U. per kg. The milk supplement consisted of a mixture of 25% skim milk powder offered either during lactation between 10 and 20 d of age or for the first week after weaning as a transitional diet along with dry pellets. Pigs were weaned at 21 d of age. Growth rate was measured from birth to slaughter at 140 d of age and feed intake of supplemental milk or feed from 10 to 56 d of age. Organ weights (heart, liver, stomach, and kidneys) and the gene expression of ghrelin, leptin, and glucagon-like peptides (glucagon-like peptide-1 and glucagon-like peptide-2) were measured in the stomach, ileum, and duodenum at 10, 21, and 28 d of age. Milk supplementation after weaning resulted in immediate feed intake and partially alleviated the depression in growth rate over the first 7 d postweaning (P < 0.001), but milk supplementation during lactation had no effects (P > 0.1). However, effects were only transient and disappeared once the milk liquid diet was removed. Neonatal oxytocin administration reduced weight loss over the first 2 d after weaning (P = 0.03), without affecting feed intake (P > 0.1), hence possibly reducing weaning stress. Seven days after weaning, oxytocin-treated pigs had greater stomach ghrelin and leptin expression (both P = 0.02), and pigs supplemented with milk after weaning had greater stomach leptin and glucagon-like peptide-2 expression (P = 0.02 and P = 0.05, respectively). Hence, neonatal oxytocin administration or postweaning milk supplementation are both effective means of enhancing gastric leptin expression and reducing weight loss at weaning, likely improving gut health during this critical period.

Oxytocin secretion is associated with severity of disordered eating psychopathology and insular cortex hypoactivation in anorexia nervosa

CONTEXT

Animal data suggest that oxytocin is a satiety hormone. We have demonstrated that anorexia nervosa (anorexia), a disorder characterized by food restriction, low weight, and hypoleptinemia, is associated with decreased nocturnal oxytocin secretion. We have also reported functional magnetic resonance imaging (fMRI) hypoactivation in anorexia in brain regions involved in food motivation. The relationships between oxytocin, food-motivation neurocircuitry, and disordered eating psychopathology have not been investigated in humans.

OBJECTIVE

The objective of the study was to determine whether the oxytocin response to feeding in anorexia differs from healthy women and to establish the relationship between oxytocin secretion and disordered eating psychopathology and food-motivation neurocircuitry.

DESIGN

This was a cross-sectional study.

SETTING

The study was conducted at a clinical research center.

PARTICIPANTS

Participants included 35 women: 13 anorexia (AN), nine weight-recovered anorexia (ANWR), and 13 healthy controls (HC).

MEASURES

Peripheral oxytocin and leptin levels were measured fasting and 30, 60, and 120 min after a standardized mixed meal. The Eating Disorder Examination-Questionnaire was used to assess disordered eating psychopathology. fMRI was performed during visual processing of food and nonfood stimuli to measure brain activation before and after the meal.

RESULTS

Mean oxytocin levels were higher in AN than HC at 60 and 120 min and lower in ANWR than HC at 0, 30, and 120 min and AN at all time points. Mean oxytocin area under the curve (AUC) was highest in AN, intermediate in HC, and lowest in ANWR. Mean leptin levels at all time points and AUC were lower in AN than HC and ANWR. Oxytocin AUC was associated with leptin AUC in ANWR and HC but not in AN. Oxytocin AUC was associated with the severity of disordered eating psychopathology in AN and ANWR, independent of leptin secretion, and was associated with between-group variance in fMRI activation in food motivation brain regions, including the hypothalamus, amygdala, hippocampus, orbitofrontal cortex, and insula.

CONCLUSIONS

Oxytocin may be involved in the pathophysiology of anorexia.

Adolescent oxytocin exposure causes persistent reductions in anxiety and alcohol consumption and enhances sociability in rats

Previous studies have suggested that administration of oxytocin (OT) can have modulatory effects on social and anxiety-like behavior in mammals that may endure beyond the time of acute OT administration. The current study examined whether repeated administration of OT to male Wistar rats (n = 48) during a key developmental epoch (early adolescence) altered their physiology and behavior in later-life. Group housed rats were given intraperitoneal injections of either 1 mg/kg OT or vehicle during early adolescence (post natal-days [PND] 33–42). OT treatment caused a transient inhibition of body weight gain that recovered quickly after the cessation of treatment. At PND 50, the rats pre-treated with OT displayed less anxiety-like behavior on the emergence test, while at PND 55 they showed greater levels of social interaction. A subgroup of OT pre-treated rats examined at PND 63 showed a strong trend towards increased plasma OT levels, and also displayed significantly increased OT receptor mRNA in the hypothalamus. Rats pre-treated with OT and their controls showed similar induction of beer intake in daily 70 min test sessions (PND 63 onwards) in which the alcohol concentration of beer was gradually increased across days from 0.44% to 4.44%. However, when given ad libitum access to beer in their home cages from PND 72 onwards (early adulthood), consumption of beer but not water was significantly less in the OT pre-treated rats. A “booster” shot of OT (1 mg/kg) given after 25 days of ad libitum access to beer had a strong acute inhibitory effect on beer intake without affecting water intake. Overall these results suggest that exogenous OT administered during adolescence can have subtle yet enduring effects on anxiety, sociability and the motivation to consume alcohol. Such effects may reflect the inherent neuroplasticity of brain OT systems and a feed-forward effect whereby exogenous OT upregulates endogenous OT systems.

Decreased nocturnal oxytocin levels in anorexia nervosa are associated with low bone mineral density and fat mass

OBJECTIVE

Anorexia nervosa is characterized by self-induced starvation and associated with severe bone and fat loss. Oxytocin is a peptide hormone involved in appetite and energy homeostasis. Recent data show that oxytocin has an anabolic effect on bone and stimulates osteoblast function. There is limited information about oxytocin levels or their relationship to decreased bone mineral density in anorexia nervosa. Our objective was to investigate the relationship between oxytocin levels, bone mineral density, and body composition in women with anorexia nervosa.

METHOD

We studied 36 women, mean ± SEM age 27.6 ± 1.3 years: 17 with DSM-IV anorexia nervosa and 19 healthy controls in a cross-sectional study. Oxytocin levels were determined from pooled serum samples obtained every 20 minutes from 8 pm to 8 am during an inpatient overnight visit. Fasting leptin levels were measured. Bone mineral density at the anterior-posterior and lateral spine and hip and body composition were assessed by dual energy x-ray absorptiometry. The study was conducted from September 2004 to June 2008.

RESULTS

Subjects with anorexia nervosa versus healthy controls had lower mean ± SEM oxytocin levels (14.3 ± 1.5 vs 31.8 ± 5.1 pg/mL, P = .003), leptin levels (2.7 ± 0.5 vs 11.4 ± 1.1 ng/mL, P < .0001), bone mineral density (anterior-posterior spine: 0.83 ± 0.02 vs 1.04 ± 0.03; lateral spine: 0.63 ± 0.02 vs 0.81 ± 0.02; total hip: 0.79 ± 0.03 vs 0.97 ± 0.03 g/cm², P < .0001), and fat mass (8.8 ± 0.6 vs 19.7 ± 0.9 kg, P < .0001). Oxytocin levels were associated with bone mineral density at the anterior-posterior (r = 0.40, P = .02) and lateral (r = 0.36, P = .04) spine, fat mass (r = 0.42, P = .01), and leptin levels (r = 0.55, P = .001).

CONCLUSIONS

Overnight secretion of oxytocin in women with anorexia nervosa is decreased compared with healthy women. Low oxytocin levels are associated with decreased bone mineral density and body fat and may contribute to anorexia nervosa-induced bone loss.

Oxytocin polyuria and polydipsia is blocked by NaCl administration in food-deprived male rats

We examined the effects of NaCl injections on the polydipsia and polyuria induced by subcutaneous oxytocin (OT) administration in food-deprived male rats. During the first 12 h of the treatment day, both food deprivation and OT administration increased urine excretion but reduced water intake, water balance (fluid intake minus urine volume) and body weight. OT treatment enhanced urine excretion and the reduction in water balance and body weight without reducing the water intake of food-deprived animals. Analysis of the physiological effects of OT administration showed increases in urinary sodium concentration, sodium excretion and a reduced plasma sodium concentration. During the second 12 h, OT increased both urine excretion and water intake in food-deprived but not in ad lib.-fed rats. However, hypertonic NaCl administration at the start of this second 12-h period blocked the polyuric and polydipsic responses observed in the OT/deprived group but increased the water intake of the ad lib. groups. After the whole 24-h period, animals treated with OT showed a water balance and body weight change matching those observed in Control animals. Although the recording time period is a critical factor to demonstrate the effect of peripheral OT administration on water intake, the results obtained suggest that the polyuric and polydipsic responses observed in food-deprived animals depend on the negative sodium and water balance induced by the natriuretic effect of OT and the unavailability of sodium. These OT-induced deficits can be counteracted by the administration of hypertonic NaCl solutions or simply by the intake of standard food.

Potentiated effect of systemic administration of oxytocin on hypertonic NaCl intake in food-deprived male rats

Subcutaneous administration of oxytocin (OT) increases water intake and sodium/urine excretion in food-deprived male rats. This study analyzes the effect of OT administration (at 0830 and 1430h) on the consumption of water and hypertonic NaCl (1.5%). In the first experiment, injections of OT increased the intake of hypertonic NaCl (but not of water) in food-deprived rats but not in ad lib-fed animals during the second 12 h (2030 to 0830) of the treatment day. The net concentration of the fluid consumed by OT/deprived animals was close to isotonic. In the second experiment, the initial effect of OT administration was an increase in urine volume and urinary sodium excretion and concentration by food-deprived animals during the first 12 h (0830 to 2030). These findings suggest that in food-deprived animals, systemic administration of OT induces NaCl intake as a consequence of previous urine loss and urinary sodium excretion.

Postnatal oxytocin treatment of spontaneously hypertensive male rats decreases blood pressure and body weight in adulthood

Postnatal oxytocin treatment decreases blood pressure and increases body weight in adult normotensive rats. The aim of the present study was to investigate the effect of postnatally administered oxytocin on blood pressure, heart rate and body weight in spontaneously hypertensive rats (SHR). For this purpose SHR male pups were given oxytocin (1 mg/kg) or saline subcutaneously once a day on days 10-14 after birth. Blood pressure and heart rate were measured at the age of 2 months. Weight was registered continuously. The postnatally oxytocin-treated male SHR had significantly lower systolic blood pressure as adults compared to the controls (158 vs. 169; p<0.05). They also had a tendency to lower diastolic blood pressure (119 vs. 128; p=0.10). Heart rate was equal in the two groups. The postnatally oxytocin-treated male SHR had a significantly lower body weight at the age of 5-8 weeks compared to the controls (ANOVA p=0.014).

Opposite effects of oxytocin on proliferation of osteosarcoma cell lines

Oxytocin stimulates proliferation of human osteoblast-like (hOB) cells and human osteosarcoma cells (SaOS-2). In contrast, oxytocin has also been shown to inhibit proliferation of other cell lines such as breast cancer cells. The aim of the present study was to investigate the effects of different concentrations of oxytocin on cell proliferation in osteosarcoma cell lines of different stages of differentiation: SaOS-2, TE-85, and UMR-106. For this purpose cells were incubated with oxytocin (1-1000 pmol/l). Cell proliferation was measured by [(3)H]thymidine incorporation and a commercially available kit (EZ4U). Incubation with oxytocin during 24 h increased proliferation of SaOS-2 cells significantly (100 pmol/l: p<0.01). In contrast, 24 h of incubation with oxytocin decreased proliferation of TE-85 (100 pmol/l: p<0.01) and UMR-106 cells significantly (100 pmol/l: p<0.01). The effects of oxytocin in SaOS-2 and TE-85, but not in UMR-106 cells, were abolished when the cells were incubated with both oxytocin and an oxytocin antagonist (1-deamino-2-D-Tyr-(Oet)-4-Thr-8-Orn-oxytocin). Instead incubation with the oxytocin antagonist alone decreased proliferation of UMR-106 cells significantly (p<0.001). Thus oxytocin has the capacity to both stimulate and inhibit cell proliferation of osteosarcoma cells. This effect might be dependent on the stage of differentiation of the cancer cells.

Oxytocin, water intake, and food sodium availability in male rats

This study examined the effect of subcutaneous administration of the neurohormone oxytocin on water intake of ad lib-fed (with or without sodium availability in the diet) and food-deprived animals. Results of the first experiment showed that oxytocin increased water intake and urine excretion in food-deprived but not in ad lib-fed animals. However, oxytocin treatment did not modify the reduced water "balance" (fluid intake minus urine volume) resulting from food deprivation or the daily food intake (Experiment 1). The dose-dependent polydipsic effect of oxytocin on food-deprived rats was always preceded by an increase in sodium and fluid urine excretion (Experiment 2). Oxytocin also increased the water intake of animals fed ad lib with a low sodium diet (Experiment 3). These results suggest that the effect of oxytocin on water intake is dependent on the presence or absence of sodium in the diet and that the excretion of sodium is the main mechanism of oxytocinergic polydipsia in food-deprived male rats.

Oxytocin decreases corticosterone and nociception and increases motor activity in OVX rats

OBJECTIVES

In the present study the effects of oxytocin administered subcutaneously (s.c.) or intravaginally (i.vag.) on spontaneous motor activity, nociceptive thresholds and plasma corticosterone levels were examined in female ovariectomized (OVX) rats.

METHODS

Oxytocin (1 mg/kg s.c. or 100 microg i.vag.) was administered once a day for 10 days to OVX rats. Controls received saline s.c. or cellulose gel i.vag. Spontaneous motor activity was observed in an open-field arena, nociceptive thresholds were investigated by the tail-flick test, and corticosterone and oxytocin plasma levels were measured by radioimmunassay, 3, 4 and 5 days respectively, after the end of the treatment period.

RESULTS

Both oxytocin administered s.c. and i.vag. increased forward locomotion (p<0.05) and nociceptive thresholds (p<0.05) significantly. In addition, oxytocin s.c. increased the amount of locomotor activity (p<0.05). Plasma corticosterone levels were decreased (p<0.05) and oxytocin levels were unchanged when measured 5 days after the last administration of oxytocin s.c. or i.vag.

CONCLUSION

The present data indicate that oxytocin induces a spectrum of long-lasting effects in OVX rats, including an increase in spontaneous motor activity, elevation of nociceptive thresholds and decrease of corticosterone levels. Similar effects may be induced by estrogens. In addition, these data indicate that i.vag. administration of oxytocin may be used to induce oxytocin-mediated effects.

Oxytocin increases the density of high affinity α2-adrenoceptors within the hypothalamus, the amygdala and the nucleus of the solitary tract in ovariectomized rats

Oxytocin induces long-term changes in, for example, blood pressure, spontaneous motor activity and corticosterone levels in rats. Previous studies in male rats have suggested a role for alpha(2)-adrenoceptors within the central nervous system in these effects. The aim of the present study was to investigate if oxytocin treatment in female rats would influence alpha(2)-adrenoceptors within the hypothalamus, the amygdala and the nucleus of the solitary tract (NTS). For this purpose, female ovariectomized (OVX) rats were treated with oxytocin (1 mg/kg s.c.) or saline once a day for 10 days. Rats were decapitated 5 days after the last injection, and brains and plasma were collected. Quantitative receptor autoradiography for characterization of high affinity alpha(2)-adrenoceptor agonist binding and radioimmunoassay for corticosterone were performed. Oxytocin increased the B(max) values of the alpha(2)-adrenoceptor agonist [3H]UK14.304 binding sites significantly in all the analyzed areas (P<0.05). K(d) values were unchanged. Plasma levels of corticosterone were significantly decreased in the oxytocin-treated rats (P<0.05). These findings are in further support of an interaction between oxytocin receptors and alpha(2)-adrenoceptors and show that oxytocin treatment may increase alpha(2)-adrenoceptor recognition probably leading to an increase in alpha(2)-adrenoceptor signaling in several parts of the brain.

The psychobiology of emotion: the role of the oxytocinergic system

A necessary condition for the individual's survival is the capacity for mental, behavioral, and physiological adaptation to external and internal conditions. Consequently, the integrated organism strives to maintain a dynamic, functional balance and integrity under varying conditions. Effective individual adaptation processes are basically dependent on the functioning of the integrated psychophysiological system. In humans, the brain plays a fundamental role in these processes. It serves the adaptation of individuals to current and anticipated conditions by selecting, interpreting, and transforming information into mental, behavioral, and physiological responses. In doing so, the incoming information is linked to existing structures of emotions, values, and goals. Consequently, the interpretation of external information may vary and become subjective depending on an individual's present and past experiences (see e.g., Magnusson, 2003). Hitherto, empirical research has been mainly concerned with the aspect of the psychophysiological system, which is activated in situations that are perceived by the individual as threatening, harmful, or demanding and in which the fight-flight and stress responses described by Cannon (1929) and Selye (1976) play an important role. The aim of this article is to draw attention to a component of the psychophysiological system, the calm and connection system, underlying well-being and socialization. By including this new system, the model of the integrated individual becomes more complete and it enriches the understanding of emotional aspects of brain functioning.

Neuropeptides in hypothalamic neuronal disorders

A few examples of hypothalamic, peptidergic disorders leading to clinical signs and symptoms are presented in this review. Increased activity of corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus (PVN) and decreased activity of the vasopressin neurons in the biological clock and of the thyroxine-releasing hormone (TRH) neurons in the PVN contribute to the signs and symptoms of depression. In men, the central nucleus of the bed nucleus of the stria terminalis (BSTc) is about twice as large and contains twice as many somatostatin neurons as in women. In transsexuals this sex difference is reversed, pointing to a role of this structure in gender. Luteinizing hormone-releasing hormone (LHRH) neurons are formed in the fetal olfactory placade and migrate along the terminal nerve fibers into the hypothalamus. In Kallmann's syndrome the migration process of the LHRH (gonadotropin-releasing hormone) neurons is aborted, which explains the joint occurrence of hypogonadotropic hypogonadism and anosmia in this syndrome. In postmenopausal women, the neurons of the infundibular nucleus hypertrophy and become hyperactive because of the disappearance of the estrogen feedback and contain hyperactive peptidergic neurons. Climacteric flushes may be caused by hyperactivity of the neurokinin-B or LHRH neurons in this nucleus. The hypocretin (orexin) neurons in the perifornical area are involved in sleep. In narcolepsy with cataplexy, a loss of these neurons, probably due to an autoimmune process, is found. Obese subjects with a mutation in the gene that encodes for leptin, the preproghrelin gene, or the alpha-melanocyte-stimulating hormone (alpha-MSH) gene have been described. Decreased numbers and activity of the oxytocin neurons in the PVN may be responsible for the absence of satiety in Prader-Willi syndrome. Moreover, a glucocorticoid receptor polymorphism is associated with obesitas and dysregulation of the hypothalamus-pituitary-adrenal axis. In contrast, two single nucleotide polymorphisms (SNPs) of the AGRP gene have been associated with anorexia nervosa.

Postnatal oxytocin alleviates adverse effects in adult rat offspring caused by maternal malnutrition

Repeated oxytocin administration to adult rats causes a long-term decrease of plasma levels of corticosterone and blood pressure and stimulates growth and fat retention. Maternal undernutrition increases blood pressure and plasma corticosterone in adult offspring. We hypothesized that oxytocin treatment early in life would alleviate adverse effects of intrauterine food restriction. Male pups from ad libitum-fed and food-restricted (fed 60% of ad libitum intake) dams were injected with oxytocin or saline in days 1-14 after birth. At 4 mo, blood pressure, plasma levels of corticosterone, and adiposity were assessed. Oxytocin treatment decreased blood pressure independently of nutrition, whereas the increased plasma levels of corticosterone were lowered to normal levels in food-restricted offspring. Blood pressure and adiposity were not affected by in utero food restriction, whereas birth and adult weight were. In conclusion, postnatal events may alleviate adverse effects caused by in utero food restriction. In contrast to more severe food restriction, a moderate general food restriction during gestation had no effect on blood pressure in the offspring.

Oxytocin decreases plasma levels of thyroid-stimulating hormone and thyroid hormones in rats

Oxytocin (OXT) administered to rats induces several long-lasting physiological and metabolic effects. The aim of the present study was to investigate the effects of oxytocin treatment on plasma levels of thyroid-stimulating hormone (TSH), free thyroxine (fT4) and free triiodothyronine (fT3). For this purpose, oxytocin or NaCl was administered intracerebroventricularly (i.c.v.) (0.3 micro g) or subcutaneously (s.c.) (1 mg/kg) once a day for 5 days to male rats. Five or ten days after the last injection, rats were decapitated, blood was collected and hormone levels were analyzed by fluoroimmunoassay. The oxytocin treatment i.c.v. decreased plasma levels of TSH (p<0.05), fT3 (p<0.01) and fT4 (p<0.05) when measured at day 5 after oxytocin treatment, whereas the effect was abolished when measured at day 10. Oxytocin treatment s.c. did not affect plasma levels of TSH, fT3 or fT4. Thus, the effect seems to have been mediated within the central nervous system, and TSH and the thyroid hormones may be involved in some of the metabolic effects in response to oxytocin.

Postnatal oxytocin treatment and postnatal stroking of rats reduce blood pressure in adulthood

The aim of the present study was to investigate the effects of postnatal oxytocin (OT) treatment and postnatal stroking on blood pressure and heart rate in adult rats. For this purpose, rats were treated subcutaneously with OT (1 mg/kg) once a day on days 1-14 after birth, or exposed to stroking on the ventral side of the abdomen for 5 min once a day on days 1-7 after birth. Blood pressure and heart rate were measured at the age of 7-8 months. The OT-treated male rats had a significantly reduced diastolic blood pressure in adulthood (p < 0.001), and in the female rats, both systolic (p < 0.01) and diastolic blood pressures (p < 0.001) were significantly lower compared to controls given saline postnatally. OT reduced blood pressure also in prenatally stressed female rats, which had a significantly higher blood pressure in adulthood compared to control rats that had not been exposed to prenatal stress. Also, the postnatal stroking reduced diastolic blood pressure in adulthood (p < 0.05). No changes in heart rate were found. In conclusion, both postnatal OT treatment and postnatal stroking reduced blood pressure in adulthood. In addition, in female rats, OT reduced the increase in blood pressure caused by prenatal stress.

Oxytocin stimulates proliferation of human osteoblast-like cells

Oxytocin receptors have recently been demonstrated in human osteoblast-like (hOB) cells. In this study, oxytocin 100-1000 pmol/l increased cell proliferation of primary cultures of hOB cells, measured by [3H]thymidine incorporation, (P<0.01). In human osteosarcoma cell-line (SaOS-2), oxytocin 100 pmol/l increased cell proliferation (measured by [3H]thymidine incorporation and a commercially available kit) and protein synthesis ([3H]proline incorporation) (P<0.05). The increase in cell proliferation was abolished when SaOS-2 cells were incubated with an oxytocin antagonist and oxytocin. Oxytocin 100 pmol/l decreased interleukin-6 (IL-6) production of the hOB cells (23.4+/-1.96 versus 33.4+/-2.65 pg/well; P<0.001). These findings indicate that oxytocin may affect bone metabolism in humans.

Oxytocin decreases carrageenan induced inflammation in rats

The effects of oxytocin on carrageenan-induced inflammation in rat hindpaw was examined. Oxytocin at 100 (P < 0.05) and 1000 microg/kg s.c. (P < 0.05), but not at 1 and 10 microg/kg s.c., reduced the edema of the paw when measured up to 10 h after the injection. An additional experiment showed that the effect was comparable to the effect of the glucocorticoid dexamethasone. No effect was found by oxytocin i.c.v. In addition, rats with carrageenan-induced inflammation given oxytocin (1000 microg/kg s.c.) responded differently to nociceptive mechanical stimulation (P < 0.05) and had a reduced amount of myeloperoxidase (marker for neutrophil recruitment) in the paw (P < 0.01).

Oxytocin facilitates behavioural, metabolic and physiological adaptations during lactation

The aim of this article is to propose that oxytocin not only stimulates milk let down, but also adapts behaviour and physiology to facilitate lactation in mammals including dairy cattle. Circulating oxytocin as well as neurogenic oxytocin participates in these regulatory processes. In short, oxytocin stimulates maternal interaction and attachment between mother and young. It also participates in the metabolic prerequisites for milk production by e.g. stimulating glucagon release and thereby, mobilisation of glucose. Digestive and anabolic aspects of metabolism are also stimulated, e.g. by increased vagal nerve activity. Adaptations consistent with an antistress like pattern are also induced. Cortisol levels are decreased as well as blood pressure, and behaviours characterised by calm, reduced levels of anxiety and more social activity are promoted. These effects seem to be present in monogastric animals as well as in ruminants. The expression of various aspects of these adaptations vary according to the special needs and living environmental circumstances of different species. The mechanisms behind the effect spectrum of oxytocin are being explored in other experimental models. A second aim of this paper is to suggest that efficiency of lactation can be promoted by facilitating oxytocin release in connection with milking by enhancing the amount of sensory stimulation.

Effects of oxytocin treatment early in pregnancy on fetal growth in ad libitum-fed and food-restricted rats

The effects of oxytocin on fetal and placental growth and on maternal weight gain and accumulation of body fat were studied in ad libitum-fed and food-restricted (receiving 70% of the food intake of the ad libitum-fed group) pregnant rats. Further, a possible role of the IGF axis in mediating oxytocin-induced changes was assessed. Pregnant rats were injected subcutaneously once a day during gestational d 1-5 with saline or oxytocin (1 mg/kg). Ad libitum-fed oxytocin-treated pregnant rats had higher circulating levels of IGF-I, larger placentas, fetuses, and newborn pups and contained less body fat at the end of pregnancy. In food-restricted dams, oxytocin-treatment had no effect on fetal and placental growth. Additionally, food restriction attenuated the normal increase in IGF binding protein-3 protease proteolysis during pregnancy. The results show that oxytocin may affect maternal adaptations to pregnancy and stimulate fetal growth. We suggest that this effect may be mediated by increased IGF-I in ad libitum-fed animals, whereas food restriction may block this effect by resulting in low levels of circulating IGF-I and by attenuating the pregnancy-associated increase in IGF binding protein-3 protease activity and, thereby, further compromise IGF bioavailability.

Catecholaminergic neurotransmission: flagship of all neurobiology

The paper describes, with focus on the first half of the century, the roles played by study of the sympathoadrenal system for developing modern neurobiology. Adrenaline isolated from extracts of adrenal medulla was the first intercellular messenger to be chemically identified and synthesized. Similarities between effects of adrenaline and sympathetic nerve stimulation led to the first concrete proposal of chemical neurotransmission. That effluent from a sympathetically or parasympathetically stimulated frog heart induced acceleration or slowing of an unstimulated recipient heart was the first conclusive proof of chemical neurotransmission. Acetylcholine (in parasympathetic or somatomotor) and noradrenaline (in sympathetic nerves) are the first identified mammalian neurotransmitters. The existence of a'receptive substance for adrenaline' represents the first proposal that target cells recognize and react to the released transmitter. Deviations for the '-ergic concept, in which sympathetic and parasympathetic nerves are termed 'adrenergic' and 'cholinergic', led to discovery of 'non-adrenergic, non-cholinergic' nerves and a range of other transmitters. That some effects of e.g. sympathetic nerve stimulation are not blocked by any noradrenaline antagonist led to the recognition that some nerves utilize more than one transmitter. Noradrenaline in sympathetic nerves was the first neurotransmitter to be visualized in the light microscope. catecholamines in adrenal medulla and sympathetic nerves were the first messengers to be shown to be stored in vesicles.

Oxytocin causes a sustained decrease in plasma levels of corticosterone in rats

The aim of this study was to investigate how oxytocin (OXT) influences plasma levels of ACTH and corticosterone in rats. A single injection of OXT (1 mg/kg s.c.) caused a transient increase in ACTH and corticosterone. In contrast, 1 mg/kg OXT (but not 10-100 microg/kg) decreased corticosterone, but not ACTH levels, 6 h after the injection. OXT (1 mg/kg s.c.) administered once a day for 5 days, decreased cortiocosterone for 10 days after the last injection. An acute challenge with ACTH increased corticosterone to the same level in rats pretreated with OXT and controls. Dexamethasone decreased corticosterone to equal levels in both groups. Thus, OXT seems to be able to stimulate as well as to inhibit the activity within the HPA-axis within a short- and a long-term perspective, respectively.

Endocrine and behavioral traits in low-avoidance Sprague-Dawley rats

In the present series of experiments, we have examined the endocrine profile of two stable colonies of Sprague-Dawley rats, here labeled Stock A, and Stock B, differing markedly in their ability to acquire a conditioned avoidance response. On separate occasions, the animals were subjected to five daily sessions (approximately 20 trials per 15 min session) of conditioned avoidance training, measurements of startle reactivity to an auditory stimulation and open-field spontaneous locomotor activity observations. The experiments were concluded by taking blood samples for later analysis of plasma glucose and plasma levels of the following hormones: insulin, gastrin, CCK, glucagon, somatostatin, oxytocin and corticosterone. The low-performing Stock B animals were characterized by [1] being more reactive to sensory stimulation: higher startle amplitude and shorter startle latency; [2] having higher plasma insulin and corticosterone levels, whereas plasma gastrin and oxytocin were significantly lowered and a strong tendency for a decrease also in plasma CCK. There were no differences in spontaneous locomotor activity between the two substrains. Taking total variability in avoidance performance into account, there was a statistically significant positive correlation between plasma oxytocin, as well as gastrin, levels and avoidance performance. The evidence obtained here, and in other laboratories, suggests that the Stock B animals display hormonal changes indicative of a submissive-defensive reaction pattern. Thus, the avoidance acquisition deficits displayed by the present Sprague-Dawley stocks A and B, are in all probability caused by emotional reactions when challenged with external stimuli requiring active responding.

Short-term increase and long-term decrease of blood pressure in response to oxytocin-potentiating effect of female steroid hormones

To investigate how the effects of oxytocin on blood pressure are influenced by female sex hormones, oxytocin (1 mg/kg, s.c.) was given to intact cycling and ovariectomized (OVX) female rats. Oxytocin caused a transient increase in blood pressure, most pronounced during proestrus (p < 0.01) and estrus (p < 0.01). This increase was partially antagonized by an oxytocin antagonist. When oxytocin was given for 5 days, blood pressure decreased (intact rats: 123+/-1.5 vs. 130+/-1.3 mm Hg; p < 0.001, OVX rats: 120+/-3.0 vs. 129+/-1.1 mm Hg; p < 0.001). This decrease, not abolished by the oxytocin antagonist, persisted for 3 weeks in intact rats and for 8 days in OVX rats. If oxytocin treatment of OVX rats continued, a nadir of 12 mm Hg (118+/-1.7 mm Hg; p < 0.001) was reached after 8 days. Thereafter heart rate decreased significantly (p < 0.05). One daily oxytocin injection for 12 days to OVX rats decreased blood pressure for 3 weeks, as in intact rats. These results show that acute and chronic oxytocin treatment cause opposite effects on blood pressure, and that these effects are modified by female sex hormones.

Oxytocin may mediate the benefits of positive social interaction and emotions

During breastfeeding or suckling, maternal oxytocin levels are raised by somatosensory stimulation. Oxytocin may, however, also be released by nonnoxious stimuli such as touch, warm temperature etc. in plasma and in cerebrospinal fluid. Consequently, oxytocin may be involved in physiological and behavioral effects induced by social interaction in a more general context. In both male and female rats oxytocin exerts potent physiological antistress effects. If daily oxytocin injections are repeated over a 5-day period, blood pressure is decreased by 10-20 mmHg, the withdrawal latency to heat stimuli is prolonged, cortisol levels are decreased and insulin and cholecystokinin levels are increased. These effects last from 1 to several weeks after the last injection. After repeated oxytocin treatment weight gain may be promoted and the healing rate of wounds increased. Most behavioral and physiological effects induced by oxytocin can be blocked by oxytocin antagonists. In contrast, the antistress effects can not, suggesting that unidentified oxytocin receptors may exist. The prolonged latency in the tail-flick test can be temporarily reversed by administration of naloxone, suggesting that endogenous opioid activity has been increased by the oxytocin injections. In contrast, the long-term lowering of blood pressure and of cortisol levels as well as the sedative effects of oxytocin have been found to be related to an increased activity of central alpha 2-adrenoceptors. Positive social interactions have been related to health-promoting effects. Oxytocin released in response to social stimuli may be part of a neuroendocrine substrate which underlies the benefits of positive social experiences. Such processes may in addition explain the health-promoting effects of certain alternative therapies. Because of the special properties of oxytocin, including the fact that it can become conditioned to psychological state or imagery, oxytocin may also mediate the benefits attributed to therapies such as hypnosis or meditation.

Oxytocin increases locus coeruleus alpha 2-adrenoreceptor responsiveness in rats

Oxytocin induces several antistress-like effects in rats. As the central noradrenergic nucleus locus coeruleus (LC) participates in arousal reactions and cardiovascular control, we used extracellular single-cell recording techniques to investigate a potential influence of oxytocin on LC neuronal activity. Oxytocin (1 mg/kg s.c.) given once a day for 5 days to male rats, increased the responsiveness of noradrenergic LC neurons to intravenous (i.v.) administration of clonidine, when compared to controls treated with saline for 5 days (P < 0.01). The spontaneous firing rate was not affected by oxytocin treatment. This study indicates that oxytocin treatment increases the responsiveness of LC alpha 2-adrenoreceptors.

Steroid dependent effects of oxytocin on spontaneous motor activity in female rats

In the present study, dose relationships for effects of oxytocin (OXY) on spontaneous motor activity in female rats were investigated. Ovariectomized (OVX) and cycling female Sprague-Dawley rats were given OXY 10-1000 microg/kg s.c. or saline, 10 min before registration of motor activity in an open-field arena. In the OVX rats, 100 microg/kg of OXY increased the activity in the center of the arena, whereas 1000 microg/kg decreased locomotor activity (LA). In the cycling rats, OXY 100-1000 microg/kg decreased LA during diestrus, while 1000 microg/kg also decreased LA during metestrus. The latter dose also reduced the exploratory behavior during estrus. In a second experiment, OVX rats were pretreated with estradiol benzoate (EB) and progesterone (P). When P levels were predominant, OXY 10-1000 microg/kg decreased LA. Oxytocin 10-100 microg/kg given after pretreatment with EB increased the activity in the center of the arena, whereas 1000 micro/kg given in the presence of both EB and P increased peripheral activity (PA). These results show that the effects of OXY on motor activity in female rats are modified by female sex steroid hormones.

Postnatal oxytocin injections cause sustained weight gain and increased nociceptive thresholds in male and female rats

The aim of the present study was to investigate possible long-term effects of postnatally administered oxytocin on weight gain, gastrointestinal hormone levels, and nociceptive thresholds in rats. For this purpose, s.c. daily injections of oxytocin (1 mg/kg) or saline (NaCl, 0.9%) were given to male and female rat pups on d 10-14 after birth. The animals were killed at the age of 60 or 94 d. Treatment with oxytocin resulted in higher body weight in males, 60 d after birth, and in females from d 60 and throughout the rest of the experiment, compared with controls. The higher body weight was due to an increased weight gain in oxytocin-treated rats, compared with controls, which was most pronounced between 40 and 60 d after birth. Oxytocin-treated male rats had increased circulating levels of cholecystokinin, a tendency to increased plasma levels of insulin (p = 0.066), and relatively more adipose tissue in the thigh and interscapular region, compared with controls. At the age of 60 d, oxytocin-treated female and male rats had a prolonged withdrawal latency when measured in the tail-flick test, compared with controls. This study shows that oxytocin can induce long-lasting changes in weight gain, hormone levels, and nociceptive thresholds, when administered postnatally, in female and male rats.

Antistress Pattern Induced by Oxytocin

Repeated oxytocin injections cause lowered blood pressure, decreased cortisol levels, increased withdrawal latency, increased release of vagally controlled gastrointestinal hormones, and increased weight gain. Together, these effects form an antistress pattern. Nonnoxious sensory stimuli release oxytocin and induce an effect spectrum similar to the one caused by oxytocin injections.

Oxytocin increases nociceptive thresholds in a long-term perspective in female and male rats

Oxytocin (0.1 and 1.0 mg/kg s.c.) given to male rats during 5 days, increased tail-flick latency when measured 1 (P < 0.05) and 7 days (0.1 mg/kg, P < 0.05; 1.0 mg/kg, P < 0.01) after the last injection. The effect was gone 2 weeks after the end of the treatment. If an additional injection of oxytocin was given 10 days after a previous 5 day treatment period, the significant difference persisted after 3 weeks (P < 0.05). Tail-flick latency was significantly delayed also in oxytocin-treated females when measured 1 week after the treatment period (P < 0.05). Naloxone, but not an oxytocin antagonist, temporarily antagonised the oxytocin induced delay in withdrawal latency. This indicates that oxytocin may act by increasing the activity of opioid mechanisms.

The oxytocin receptor antagonist 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin inhibits effects of the 5-HT1A receptor agonist 8-OH-DPAT on plasma levels of insulin, cholecystokinin and somatostatin

The aim of the present study was to investigate whether the 5-HT1A receptor agonist 8-OH-DPAT, which previously has been shown to release oxytocin, also influences plasma levels of gastrointestinal and pancreatic hormones, and if so, whether such an effect is mediated by an oxytocinergic mechanism. For this purpose 8-OH-DPAT (0.5 mg/kg s.c.) was injected to male rats pretreated with the oxytocin receptor antagonist 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin (1 mg/kg s.c.), or vehicle. Thirty min after injection of 8-OH-DPAT, plasma levels of oxytocin were significantly increased. 8-OH-DPAT also increased insulin and decreased CCK and somatostatin levels, effects that were blocked by pretreatment with the oxytocin antagonist. Taken together, these data suggest that the effect of 8-OH-DPAT on plasma levels of insulin, somatostatin and CCK may be mediated by oxytocin. In previous experiments, we have shown that following i.c.v. application of oxytocin, plasma levels of insulin are increased through a cholinergic mechanism. In this study, 2 ng of oxytocin decreased plasma levels of CCK, gastrin and somatostatin, effects that were blocked by pretreatment with atropine. Since oxytocinergic fibers which originate in the PVN project to the DMX, we suggest that the effect on the release of insulin, CCK and somatostatin induced by the 5 HT1A receptor agonist 8-OH-DPAT may be mediated by an oxytocinergic activation of a vagal mechanism.