Oxytocin inhibits ACTH and peripheral catecholamine secretion in the urethane-anesthetized rat

Intertwined associations between oxytocin, immune system and major depressive disorder

Major depressive disorder (MDD) is a prominent psychiatric disorder with a high prevalence rate. The recent COVID-19 pandemic has exacerbated the already high prevalence of MDD. Unfortunately, a significant proportion of patients are unresponsive to conventional treatments, necessitating the exploration of novel therapeutic strategies. Oxytocin, an endogenous neuropeptide, has emerged as a promising candidate with anxiolytic and antidepressant properties. Oxytocin has been shown to alleviate emotional disorders by modulating the hypothalamic-pituitary-adrenal (HPA) axis and the central immune system. The dysfunction of the immune system has been strongly linked to the onset and progression of depression. The central immune system is believed to be a key target of oxytocin in ameliorating emotional disorders. In this review, we examine the evidence regarding the interactions between oxytocin, the immune system, and depressive disorder. Moreover, we summarize and speculate on the potential roles of the intertwined association between oxytocin and the central immune system in treating emotional disorders.

The interplay between oxytocin and the CRF system: regulation of the stress response

Oxytocin (OT) has drawn the attention of researchers since 1930. Since then, many aspects of oxytocin have been uncovered, such as reproductive functions, dampening anxiety, enhancing socioemotional behavior, or regulating genomic effects on a cellular level. Here, we want to focus on the interaction between the OT system and the stress/corticotropin-releasing factor (CRF)-system of the brain. Depending on the nature of the stressor, OT is released simultaneously or directly after the stress from the neurohypophysis into the periphery and/or via somato-dendritic release in stress-sensitive brain areas. This stress-induced OT release might serve to modulate or dampen the stress response; however, the functional relevance is not yet fully understood. In this review, we will describe the effects of OT and discuss the interplay between OT and CRF on a cellular, physiological, and behavioral level.

Oxytocin in corticosterone-induced chronic stress model: Focus on adrenal gland function

Chronic stress conditions can lead to considerable and extensible changes in physiological and psychological performances, and in emergence of risk for various somatic diseases. On the other hand, the neuropeptide oxytocin is reported to increase the resistance of the organism to stress and modulate activity of autonomic nervous system. Chronic corticosterone administration is used as a rat model for a state observed in terms of chronic stress exposure, when negative feedback mechanism of hypothalamus-pituitary-adrenal axis activity is disrupted. In our study, we aimed to investigate whether chronic administration of oxytocin (10 IU/400μL/day for 14days, s.c.) influenced adrenal gland morphology and activity in adult male Wistar rats during long-term corticosterone administration via drinking water (100mg/L for 21days). We examined the influence of treatments on the levels of adrenal gland hormones, corticosterone, adrenaline and noradrenaline, as well as their response to an acute stress challenge evoked by 15-min forced swimming. In addition, the expression of two main monoamine transporters, the noradrenaline transporter (NAT) and vesicular monoamine transporter 2 (VMAT2) in adrenal medulla was measured in the rats exposed to acute stress. Our results showed that oxytocin treatment prevented corticosterone-induced decrease in body weight gain, attenuated adrenal gland atrophy by increasing glandular weight, and the area of the zona fasciculate and reticularis. Chronic corticosterone intake blunted the response of all measured hormones to acute stress, whereas concomitant oxytocin treatment reversed adrenaline and noradrenaline response to acute stress. Furthermore, in adrenal medulla, oxytocin produced significant vasodilatation and stimulated expression of both catecholamine transporters detected both on mRNA and protein level. Our data suggest that oxytocin, by reducing atrophy of adrenal gland, and by increasing catecholamine storage capacity, may be beneficial in conditions accompanied with high glucocorticoid levels, such as chronic stress exposure.

Effects of oxytocin on adreno-medullary catecholamine synthesis, uptake and storage in rats exposed to chronic isolation stress

PURPOSE/AIM

The adreno-medullar system represents one of the main systems involved in the response to stressful events. The neuropeptide oxytocin, is highly sensitive to the social environment, and regulates autonomic function. Adreno-medullary activity is dependent on the synthesis of catecholamine, its reuptake, release, degradation and vesicular transport. A direct influence of oxytocin on catecholamine synthesizing enzyme and transports in animals exposed to chronic social isolation stress has not been studied yet.

MATERIALS AND METHODS

In the present study, we examined the effect of chronic oxytocin treatment on the level of plasma catecholamine and its content, mRNA and protein levels of tyrosine hydroxylase (TH), noradrenaline transporter (NET) as well as vesicular monoamine transporter 2 (VMAT2) in the adrenal medulla of socially isolated rats.

RESULTS

Our results show that, by the end of 12 weeks, social isolation did not produce any significant changes in catecholamine content but increased plasma catecholamine level and synthesis in the adrenal medulla. Oxytocin treatment had no further effect either on catecholamine synthesis or content in socially stressed animals whereas a significant elevation of plasma norepinephrine and epinephrine were reduced. On the other hand, chronic isolation caused a significant increase in VMAT2 and decrease in NET protein levels. Oxytocin treatment brought about an increase in protein levels of NET and its return to the levels of control group. Besides, it further increases VMAT2 protein levels in the adrenal medulla of individually housed rats.

CONCLUSION

The present results show that peripheral oxytocin treatment enhances catecholamine uptake and storage in the adrenal medulla of chronically isolated animals.

Peripheral oxytocin treatment affects the rat adreno-medullary catecholamine content modulating expression of vesicular monoamine transporter 2

The neuropeptide oxytocin has been shown to influence on neuroendocrine function. The aim of the present study was to investigate the effect of peripheral oxytocin treatment on the synthesis, uptake and content of adreno-medullary catecholamine. For this purpose oxytocin (3.6μg/100g body weight, s.c) was administrated to male rats once a day over 14 days. In order to assess the effect of peripheral oxytocin treatment on adreno-medullary catecholamine we measured epinephrine and norepinephrine content and gene expression of tyrosine hydroxylase (TH), norepinephrine transporter (NET) and vesicular monoamine transporter 2 (VMAT2) in the adrenal medulla. Our results show a significant increase of epinephrine (1.7-fold, p<0.05) and norepinephrine (1.5-fold, p<0.05) content in oxytocin treated animals compared to saline treated ones. Oxytocin treatment had no effect either on mRNA or protein level of TH and NET. Under oxytocin treatment the increase in VMAT2 mRNA level was not statistically significant, but it caused a significant increase in protein level of VMAT2 (3.7-fold, p<0.001). These findings indicate that oxytocin treatment increases catecholamine content in the rat adrenal medulla modulating VMAT2 expression.

Role of central oxytocin in stress-induced cardioprotection in ischemic-reperfused heart model

BACKGROUND AND PURPOSE

There is growing evidence that stress contributes to cardiovascular disease and triggers the release of oxytocin. Moreover previous studies confirmed oxytocin mimics the protection associated with ischemic preconditioning. The present study was aimed to assess the possible cardioprotective effects of the centrally released oxytocin in response to stress and intracerebroventricular (i.c.v.) administration of exogenous oxytocin in ischemic-reperfused isolated rat heart.

METHODS AND SUBJECTS

Rats were divided in two main groups and all of them were subjected to i.c.v. infusion of vehicle or drugs: unstressed rats [control: vehicle, oxytocin (OT; 100 ng/5 μl), atosiban (ATO; 4.3 μg/5 μl) as oxytocin antagonist, ATO+OT] and stressed rats [St: stress, OT+St, ATO+St]. After anesthesia, hearts were isolated and subjected to 30 min regional ischemia and 60 min reperfusion (IR). Acute stress protocol included swimming for 10 min before anesthesia. Myocardial function, infarct size, coronary flow, ventricular arrhythmia, and biochemical parameters such as creatine kinase and lactate dehydrogenase were measured. Ischemia-induced ventricular arrhythmias were counted during the occlusion period.

RESULTS

The plasma levels of oxytocin and corticosterone were significantly elevated by stress. Unexpectedly hearts of stressed rats showed a marked depression of IR injury compared to control group. I.c.v. infusion of oxytocin mimicked the cardioprotective effects of stress, yet did not elevate plasma oxytocin level. The protective effects of both stress and i.c.v. oxytocin were blocked by i.c.v. oxytocin antagonist.

CONCLUSIONS

These findings suggest that i.c.v. infusion of exogenous oxytocin and centrally released endogenous oxytocin in response to stress could play a role in induction of a preconditioning effect in ischemic-reperfused rat heart via brain receptors.

Neuroendocrine and cardiovascular parameters during simulation of stress-induced rise in circulating oxytocin in the rat

Physiological functions of oxytocin released during stress are not well understood. We have (1) investigated the release of oxytocin during chronic stress using two long-term stress models and (2) simulated stress-induced oxytocin secretion by chronic treatment with oxytocin via osmotic minipumps. Plasma oxytocin levels were significantly elevated in rats subjected to acute immobilization stress for 120 min, to repeated immobilization for 7 days and to combined chronic cold stress exposure for 28 days with 7 days immobilization. To simulate elevation of oxytocin during chronic stress, rats were implanted with osmotic minipumps subcutaneously and treated with oxytocin (3.6 microg/100 g body weight/day) or vehicle for 2 weeks. Chronic subcutaneous oxytocin infusion led to an increase in plasma oxytocin, adrenocorticotropic hormone, corticosterone, adrenal weights and heart/body weight ratio. Oxytocin treatment had no effect on the incorporation of 5-bromo-2-deoxyuridine into DNA in the heart ventricle. Mean arterial pressure response to intravenous phenylephrine was reduced in oxytocin-treated animals. Decrease in adrenal tyrosin hydroxylase mRNA following oxytocin treatment was not statistically significant. Oxytocin treatment failed to modify food intake and slightly increased water consumption. These data provide evidence on increased concentrations of oxytocin during chronic stress. It is possible that the role of oxytocin released during stress is in modulating hypothalamic-pituitary-adrenocortical axis and selected sympathetic functions.

Increase in plasma ACTH induced by urethane is not a consequence of hyperosmolality

Although anesthetic doses of urethane increase plasma levels of ACTH, the exact mechanism through which this occurs is unclear. We theorized that these increases might be a consequence of an increased systemic osmolality owing to the large doses of urethane usually employed. To evaluate this possibility, we measured plasma osmolality and ACTH in a total of six rats after graded infusions of urethane (N=3 rats) or equimolar amounts of mannitol (N=3 rats). Rats received infusions at 15 min intervals up to a cumulative dose equivalent to an anesthetic dose for urethane (1.4 g/kg). Blood samples (0.35 ml) were withdrawn at baseline and 10 min after each infusion. Urethane and mannitol produced significant and equivalent increases in plasma osmolality. However, only urethane evoked increases in plasma ACTH which were maximal (252+/-55 pg/ml from a baseline of 27+/-7 pg/ml) after a cumulative dose of 1 g/kg. Thus, increases in plasma ACTH seen after anesthetic doses of urethane are unlikely to be a consequence of its effect on plasma osmolality.

Oxytocin: the great facilitator of life

Oxytocin (Oxt) is a nonapeptide hormone best known for its role in lactation and parturition. Since 1906 when its uterine-contracting properties were described until 50 years later when its sequence was elucidated, research has focused on its peripheral roles in reproduction. Only over the past several decades have researchers focused on what functions Oxt might have in the brain, the subject of this review. Immunohistochemical studies revealed that magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei are the neurons of origin for the Oxt released from the posterior pituitary. Smaller cells in various parts of the brain, as well as release from magnocellular dendrites, provide the Oxt responsible for modulating various behaviors at its only identified receptor. Although Oxt is implicated in a variety of "non-social" behaviors, such as learning, anxiety, feeding and pain perception, it is Oxt's roles in various social behaviors that have come to the fore recently. Oxt is important for social memory and attachment, sexual and maternal behavior, and aggression. Recent work implicates Oxt in human bonding and trust as well. Human disorders characterized by aberrant social interactions, such as autism and schizophrenia, may also involve Oxt expression. Many, if not most, of Oxt's functions, from social interactions (affiliation, aggression) and sexual behavior to eventual parturition, lactation and maternal behavior, may be viewed as specifically facilitating species propagation.

Effects of an acute stressor on blood pressure and heart rate in rats pretreated with intracerebroventricular oxytocin injections

Oxytocin induces a long-lasting reduction of blood pressure in rats. The aim of the present study was to investigate the effects of an acute stressor on blood pressure and heart rate in rats previously exposed to repeated administration of intracerebroventricular (ICV) oxytocin. For this purpose oxytocin (0.3 microg, ICV) was administered to male rats once a day during 5 days. Blood pressure and heart rate were measured before and after treatment. In addition, blood pressure and heart rate were measured during 30 min after exposure to 10s of noise from an alarm clock. The oxytocin treatment reduced blood pressure significantly (systolic: 108+/-4.6 vs. 121+/-1.8, p<0.01, diastolic: 96+/-5.1 vs. 108+/-3.0, p<0.01), whereas heart rate remained unchanged. In contrast, systolic and diastolic blood pressure increased significantly after the exposure to the ringing alarm clock in the oxytocin-treated rats (p<0.05), and became equal to the blood pressure in controls. In addition, heart rate increased and stayed significantly higher in the oxytocin-treated rats compared to the controls during the 30 min observation period (ANOVA p<0.01). Twenty-four hours later, blood pressure was again significantly lower in the oxytocin-treated rats compared to controls (p<0.01). In conclusion, oxytocin decreased blood pressure without changing pulse rate. However, when the oxytocin-treated rats were subjected to the unexpected noise from a ringing alarm clock blood pressure and heart rate increased significantly. No such effect was observed in the control group. Thus repeated oxytocin treatment can, in spite of decreasing blood pressure during basal conditions, increase cardiovascular reactivity to some types of stressors.

Modification of social memory, hypothalamic-pituitary-adrenal axis, and brain asymmetry by neonatal novelty exposure

Although corticosterone (a stress hormone) is known to influence social behavior and memory processes, little has been explored concerning its modulatory role in social recognition. In rats, social recognition memory for conspecifics typically lasts <2 hr when evaluated using a habituation paradigm. Using neonatal novelty exposure, a brief and transient early life stimulation method known to produce long-lasting changes in the hypothalamic-pituitary-adrenal axis, we found that social recognition memory was prolonged to at least 24 hr during adulthood. This prolonged social memory was paralleled by a reduction in the basal blood concentration of corticosterone. The same neonatal stimulation also resulted in a functional asymmetry expressed as a greater right-turn preference in a novel environment. Rats that preferred to turn right showed better social recognition memory. These inter-related changes in basal blood corticosterone concentration, turning asymmetry, and social recognition memory suggest that stress hormones and brain asymmetry are likely candidates for modulating social memory. Furthermore, given that neonatal stimulation has been shown to improve learning and memory performance primarily under aversive learning situations, the neonatal novelty exposure-induced enhancement in social recognition broadens the impact of early life stimulation to include the social domain.

Inhibitory effect of oxytocin on corticotrope function in humans: are vasopressin and oxytocin ying-yang neurohormones?

Oxytocin (OT) and vasopressin (VP) are very similar neurohypophyseal peptides. While VP is known as an ACTH stimulating factor synergistic to CRF since two decades, the inhibiting activity of OT, first demonstrated in the human, is now confirmed in various species including mouse and rat!It is likely that endogenous oxytocinergic system which can be activated by physiological and/or pharmacological manipulation can "buffer" the stress activated vasopressin-ACTH-cortisol action. Since VP and OT share also opposite action on cognitive function, those two "sister" neuropeptides might be considered as "ago-antagonist" or "ying-yang" neurohormones!

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.

Mitotic activity and cell deletion in ventral prostate epithelium of intact and castrated oxytocin-treated rats

Previous study has shown that oxytocin (OT) attenuated the postcastration regression of ventral prostate epithelium in rats. To decide if this effect is associated with stimulation of cell division or improvement of cell survival, metaphase index of secretory cells and frequency of apoptotic bodies in the epithelium, combined with stereological parameters, were determined in the present study. OT was injected subcutaneously in a dose of 0.25 IU/100 g/d during 5 postorchiectomal days and the same treatment was applied to intact rats. Only the prostate of castrated animals responded to OT. They had greater total volume of the epithelium, total number and metaphase index of secretory cells, but lower frequency of apoptotic bodies. These findings demonstrate the ability of OT to stimulate mitotic activity and to diminish mortality of secretory cells caused by orchiectomy.

Neural influence on oxytocin-induced changes of adrenomedullary catecholamines in the pigeon

Oxytocin (0.25 IU/100 g body wt) was injected intraperitoneally only once to unilaterally splanchnic denervated pigeons. The effects were investigated after 0.5, 4, 12, 24, 72, 144 and 216 h of administration. The findings revealed that oxytocin caused the release of more norepinephrine (NE) from the denervated glands up to 24 h after treatment. In contrast, oxytocin-induced release of epinephrine (E) showed no significant difference in between the innervated and the denervated glands after 0.5 h of treatment. Strikingly, oxytocin brought about release of more E from the denervated glands after 24 h of treatment. This indicates that the first phase of NE release and the second phase of E release are neurally regulated. The findings further revealed that the resynthesis of both NE and E was faster in the innervated glands after 216 h of oxytocin treatment. This clearly points out that the splanchnic nerve accelerates resynthesis of both NE and E induced by oxytocin.

The response of rat adrenal medulla to oxytocin

Effects of oxytocin (OT) on the adrenal chromaffin tissue of male rats were examined by coupled morphometric and biochemical techniques. Synthetic OT was administered in doses of 0.14 and 0.25 IU/100 g/d during 7 or 10 consecutive days and the effects were followed 1, 24, 72 and 168 hours after the last injection. The function and structure of chromaffin cells were affected by the higher dose of OT only. They caused divergent responses on their amine contents. Adrenaline, noradrenaline and dopamine contents were increased, while serotonin content was decreased. These changes were different in duration and time of incidence. Stereological analysis showed an enhanced number of chromaffin cells and an increase in their total volume. The parallelism between the changes in chromaffin cell number and the catecholamine content strongly suggests a mitogenic effect of the applied OT.

Effect of arginine vasopressin and oxytocin on acetylcholine-stimulation of corticosteroid and catecholamine secretion from the rat adrenal gland perfused in situ

The effects of acetylcholine, arginine vasopressin (AVP) and oxytocin (OXT) on both catecholamine and steroid secretion have been investigated using the isolated rat adrenal gland perfused in situ. Significant stimulation of steroid (aldosterone and corticosterone) secretion occurred with 1 mumol/l acetylcholine; the ED50 was approximately 20-fold higher (circa 20 mumol/l) than that for catecholamine secretion. The highest concentration of acetylcholine used (100 mumol/l) stimulated aldosterone secretion eight-fold; corticosterone secretion four-fold; noradrenaline and adrenaline secretion three-fold. AVP at 100 nmol/l but not at 1 nmol/l significantly stimulated the secretion of both steroids and catecholamines. OXT had no significant effect on corticosteroid or catecholamine secretion at either concentration. The effects on aldosterone secretion of simultaneous administration of acetylcholine and AVP were additive. No similar effect was seen on corticosterone or catecholamine secretion where the degree of stimulation was the same as for acetylcholine alone. OXT (100 nmol/l) inhibited acetylcholine-stimulated aldosterone secretion but had no effect on acetylcholine-stimulated catecholamine secretion. Carbachol was equipotent with acetylcholine in stimulating steroid secretion from the perfused gland. Our results support the hypothesis that acetylcholine may play a role in the control of steroid secretion by the rat adrenal cortex. They fail to support a role for AVP and OXT in modulating catecholamine secretion by the adrenal medulla except at high concentrations.

Vasopressin and 1-deamino-8-D-arginine-vasopressin (DDAVP) reduce elevated plasma catecholamine levels in rats with hypothalamic deafferentation

1. Anterolateral cut (ALC) of the medial basal hypothalamus (MBH) in rats produces an elevation of plasma catecholamine levels, especially of norepinephrine (NE), in unstressed animals and a more pronounced rise of plasma NE levels in response to immobilization (IMO). Animals with ALC have a destroyed corresponding vasopressin (AVP) and other peptides containing innervation of the median eminence and the posterior pituitary, resulting in the prevention of increased AVP secretion during the early intervals of IMO. 2. The administration of AVP (Pitressin, 7 days, 1 IU per rat i.m.) or of 1-deamino-8-D-arginine-vasopressin (DDAVP), an AVP analogue without pressoric activity, taken in drinking water (about 100 micrograms per day) was almost equally potent in decreasing the elevated water consumption and plasma NE levels in unstressed rats with ALC. However, the stress-induced potentiation of plasma NE levels in rats with ALC was not influenced by AVP substitution and only partly reduced by DDAVP in the late IMO intervals. 3. The lack of circulating vasopressin is the main factor in the mechanism of increased activity of the sympathoadrenal system induced by ALC in unstressed rats. 4. The regulation of sympathoadrenal activity by vasopressin and DDAVP in rats with ALC seems to be mediated predominantly by V2-subtype receptors. 5. In stressed rats with ALC the potentiation of plasma NE levels was not reduced after AVP or DDAVP administration, suggesting that some addition regulatory mechanisms were involved.