Oxytocin antisense reduces salt intake in the baroreceptor-denervated rat

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.

Blood pressure regulation in stress: focus on nitric oxide-dependent mechanisms

Stress is considered a risk factor associated with the development of various civilization diseases including cardiovascular diseases, malignant tumors and mental disorders. Research investigating mechanisms involved in stress-induced hypertension have attracted much attention of physicians and researchers, however, there are still ambiguous results concerning a causal relationship between stress and long-term elevation of blood pressure (BP). Several studies have observed that mechanisms involved in the development of stress-induced hypertension include increased activity of sympathetic nervous system (SNS), glucocorticoid (GC) overload and altered endothelial function including decreased nitric oxide (NO) bioavailability. Nitric oxide is well known neurotransmitter, neuromodulator and vasodilator involved in regulation of neuroendocrine mechanisms and cardiovascular responses to stressors. Thus NO plays a crucial role in the regulation of the stress systems and thereby in the BP regulation in stress. Elevated NO synthesis, especially in the initial phase of stress, may be considered a stress-limiting mechanism, facilitating the recovery from stress to the resting levels via attenuation of both GC release and SNS activity as well as by increased NO-dependent vasorelaxation. On the other hand, reduced levels of NO were observed in the later phases of stress and in subjects with genetic predisposition to hypertension, irrespectively, in which reduced NO bioavailability may account for disruption of NO-mediated BP regulatory mechanisms and accentuated SNS and GC effects. This review summarizes current knowledge on the role of stress in development of hypertension with a special focus on the interactions among NO and other biological systems affecting blood pressure and vascular function.

Evidence for an interaction between CB1 cannabinoid and oxytocin receptors in food and water intake

Oxytocin and CB(1) cannabinoid receptors independently modulate food intake. Although an interaction between oxytocin and cannabinoid systems has been demonstrated with respect to the cannabinoid withdrawal syndrome, the interaction between these systems in modulating food intake has not yet been examined. The present study had three primary purposes: (1) to determine whether oxytocin and a CB(1) receptor antagonist block food and fluid intake in a supra-additive manner, (2) to determine the relative position of the CB(1) receptors in the chain of control of food intake in relation to the oxytocin system, and (3) to determine whether the increase in fluid intake induced by an oxytocin antagonist is mediated via cannabinoid receptors. Rats were habituated to the test environment and injection procedure, and then received intracerebroventricular (ICV) injections of various combinations of the oxytocin receptor antagonist tocinoic acid, the cannabionid receptor agonist delta(9)-tetrahydrocannabinol (THC), oxytocin, or the cannabinoid receptor antagonist SR 141716. Food and water intake and locomotor activity were then measured for 120 min. When administrated alone, SR 141716 and oxytocin dose-dependently attenuated baseline food intake, while oxytocin but not SR 141716 reduced water intake. Sub-anorectic doses of SR 141716 and oxytocin attenuated baseline feeding beyond what would be expected by the sum of the individual drug effects without affecting baseline water intake. THC stimulated feeding but not water intake. THC-induced feeding was not blocked by oxytocin, however, the oxytocin did attenuate water intake during such feeding. SR 141716 dose-dependently reduced tocinoic-acid-stimulated food intake and partially attenuated water intake. Locomotor activity was not significantly affected by any drug treatments, suggesting that effects on feeding were not due to a non-specific reduction in motivated behaviour. These findings reveal an interaction between cannabinoid and oxytocin systems in food intake. Results further reveal that the oxytocin system effects on water intake are partially mediated via CB(1) receptors, CB(1) receptors are located downstream from oxytocin receptors, and CB(1) receptor signalling is necessary to prevent oxytocin from altering food intake.

Stress-induced pressor and corticosterone responses in oxytocin-deficient mice

We used oxytocin knockout (OTKO) mice to investigate the role of oxytocin in regulation of blood pressure, heart rate and stress reactivity (pressure reactivity and plasma corticosterone). Male OTKO and control wild-type mice with carotid arterial catheters were exposed to intermittent shaker stress for 7 days (2 min stressors, 45 times per day). Mean arterial pressure (MAP) and heart rate (HR) were recorded continuously (24 h) before stress (basal), on stress days 1, 3 and 7 (S1, S3 and S7) and 1 day poststress (recovery). Plasma corticosterone (Cort) was measured before stress and 30 min after the last stress on day 7. Twenty-four hour averages of MAP and HR were lower in OTKO mice than in controls (P < 0.0001 and P < 0.005, respectively) with a significant diurnal rhythm. Chronic stress (S1 and S3) produced an increase in 24 h average MAP in OTKO mice, but not in controls. There were no stress-related changes in 24 h average HR values between control and OTKO mice. The immediate pressor responses were analysed during the dark and light periods (19.00 and 08.00 h). During the dark period, stress-induced pressor responses were observed only in OTKO mice (S1 and S3). In the light period, stress-induced MAP increases were seen on all days in OTKO mice and on days S1 and S3 in controls. There were no differences in baseline Cort between the groups; however, OTKO mice showed a reduced response to chronic stress (+298 versus+411%, OTKO mice versus controls, P < 0.005). In conclusion, oxytocin deficiency alters the endocrine and pressor responses to chronic stress, suggesting that the endogenous oxytocin system is important in regulating the stress-induced pressor response.

Consumption of solutions containing sodium chloride is enhanced in female oxytocin-deficient mice

Intact and ovariectomized oxytocin (OT)-deficient (OT-/-) and wild-type (OT+/+) mice were tested for consumption of 0.5 M NaCl solution or tap water in a 2-bottle choice test. During 3 days of acclimation, voluntary ingestion of NaCl was equal between genotypes. After overnight fluid deprivation, intact OT-/- mice ingested 2 times more NaCl solution than OT+/+ mice in the 6th hr, but not the 1st hr, after reintroduction of fluid. Ovariectomized mice consumed less than intact mice after overnight fluid deprivation. When a 0.2 M NaCl solution was administered for 6 days in ovariectomized mice, OT-/- mice voluntarily consumed greater amounts than OT+/+ mice. After overnight fluid deprivation, consumption by OT-/- mice was 3 times that of OT+/+ mice at 1 hr and 2-fold greater after 6 hr. Enhanced intake of NaCl-containing solutions in female OT-/- mice suggests that central OT may be an important inhibitor of sodium consumption.

Enhanced salt intake in oxytocin deficient mice

The maternal roles of oxytocin (OT) are well known, but recent work suggests that OT is also a vital component in fluid balance regulation. To explore the role of OT in salt/volume regulation, we studied NaCl intake in a genetically modified mouse strain lacking OT. Using male control and OT knockout mice (OTKO), we determined the circadian pattern of salt and water intake under need-free conditions. For the study of intake, a two-bottle choice system was used to provide access to water and 2% NaCl with computerized monitoring of licking activity. Salt licking activity (licks/24 h) for controls was 59 +/- 22 vs. 380 +/- 105 in OTKO (P < 0.05). The volume of salt consumed (ml/24 h) was 0.4 +/- 0.1 in controls vs. 1.8 +/- 0.4 in OTKO (P < 0.01). There was no statistical difference in the consumption of water between the groups. However, the initiation of water intake was shifted, with an advancement of almost 3 h in OTKO (P < 0.01). Differences in the timing of salt intake could not be determined due to the low volume of salt consumed by controls. Taken together, these data show that removal of OT amplifies the salt-seeking behavior associated with normal daily fluid fluctuations. The fact that OTKO voluntarily consume a normally aversive salt solution further implies that OT is a powerful regulator of circadian salt appetite.

Brain as a unique antisense environment

During the last few years, antisense oligodeoxyribonucleotides (asODN) have become a commonly used tool for blocking of gene expression in the mammalian central nervous system. Successful gene inhibition has been reported for such diverse targets as those encoding neurotransmitter receptors, neuropeptides, trophic factors, transcription factors, cytokines, transporters, ion channels, and others. This review presents a discussion of recent studies on ODN in the brain, with a focus on specific approaches taken by the researchers in this field and especially on peculiar features of this organ as a milieu for asODN action. It is concluded that from the presented literature survey no coherent view on how to rationally design ODN for brain studies has emerged.

Stress increases oxytocin release within the hypothalamic paraventricular nucleus

Evidence indicates that the hypothalamic paraventricular nucleus (PVN) and oxytocin (OT) neurons in particular play a role in the physiological response to stress. Microdialysis (MD) experiments were performed to determine whether OT is released into the PVN during shaker stress. Male rats were prepared with venous catheters and PVN guide cannulae. OT and vasopressin (VP) release into PVN and peripheral blood were measured under basal conditions and during and after shaker stress (10 min at 110 cycles/min). Stress produced a specific increase in PVN and plasma OT. Dialysate OT levels were 0.3+/-0.1, 2.8+/-1.2 and 1.3+/-0.6 pg/sample (control, stress and recovery, respectively). Plasma OT was significantly increased during stress (3.7+/-1.2 vs. 11.7+/-2.3 pg/ml, basal vs. stress, respectively). When MD probes were located outside the PVN, there was no increase in OT release, demonstrating site specificity. Stress produced no change in VP levels, either in dialysate or plasma. These results show that OT, but not VP, is released into the PVN and peripheral blood in response to shaker stress. The data raise the possibility that local release of OT into the PVN plays a role in the neuroendocrine stress cascade.

Baroreceptor regulation of salt intake

Baroreceptor input plays a critical role in body fluid balance and the endocrine response to NaCl consumption [25]. Experiments were performed to characterize the alterations in salt intake that are seen after baroreceptor denervation. Using chronically baroreceptor denervated (SAD) or control (CON) male Sprague-Dawley rats, we determined: 1) concentration-dependent consumption of NaCl, 2) time course of saline intake, 3) effect of food access on saline intake, 4) intake of sucrose vs. saline, and 5) water vs. saline intake using a choice paradigm. In protocols 1-4 the rats were given a single bottle containing saline or sucrose for a 2-h period during the early dark period. A comparison of the intake of varying concentrations of NaCl (0.3 to 2.0% NaCl, six concentrations) demonstrated that the SAD consumed significantly less NaCl than the CON (from 0.9 to 2% NaCl), Saline Intake in SAD was 14-56% of the CON (significant group, salt concentration and interaction effects). Regression analysis demonstrated that in the SAD there was an inverse relationship between concentration and the amount of NaCl consumed (p < 0.02), an effect not seen in the CON. There were also differences in the pattern of saline intake with the CON showing the highest consumption in the early dark period with a gradual decrease as compared to the SAD, which demonstrated a uniformly lower pattern of consumption. The reduction in intake in the SAD appeared to be specific for NaCl because there was no difference in water or sucrose intake. The deficit could not be attributed to alterations in food intake, nor was there any difference in the amount of water consumed after the saline challenge.