The response of rat adrenal medulla to oxytocin

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.

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.

Oxytocin-induced renin secretion by denervated kidney in anaesthetized rat

The effects of oxytocin on renin secretion by denervated kidney were investigated in vivo, by infusing the peptide directly into the renal artery of anaesthetized rats. Renin secretion was calculated by the renal veno-arterial difference in plasma renin activity multiplied by renal plasma flow. The intra-renal arterial (i.r.a.) infusion of oxytocin (1.5 or 15 ng/kg/min, 10 min) induced a sixfold increase in renin secretion as compared to vehicle-treated controls, without effects on renal blood flow, mean arterial blood pressure, glomerular filtration rate or natriuresis. The effect of oxytocin (1.5 ng/kg/min) was prevented by pretreatment with an oxytocin receptor antagonist, desGly-NH(2),d(CH(2))(5)[D-Tyr(2),Thr(4),Orn(8)]vasotocin] (5.6 microg/kg bolus i.v. 20 min before oxytocin infusion, followed by 2.8 microg/kg/min i.r.a.). Nadolol (2.5 mg/kg i.v.), a beta-adrenoceptor antagonist, also blocked the oxytocin-induced increase in renin secretion. These results show that oxytocin is able to stimulate renin release by activating oxytocin receptors but that beta-adrenoceptors also seem to be involved.