Suppressive effect of vasopressin on the hyperglycemic response to intracranial injection of 2-deoxy-D-glucose

Interplay of Angiotensin Peptides, Vasopressin, and Insulin in the Heart: Experimental and Clinical Evidence of Altered Interactions in Obesity and Diabetes Mellitus

The present review draws attention to the specific role of angiotensin peptides [angiotensin II (Ang II), angiotensin-(1-7) (Ang-(1-7)], vasopressin (AVP), and insulin in the regulation of the coronary blood flow and cardiac contractions. The interactions of angiotensin peptides, AVP, and insulin in the heart and in the brain are also discussed. The intracardiac production and the supply of angiotensin peptides and AVP from the systemic circulation enable their easy access to the coronary vessels and the cardiomyocytes. Coronary vessels and cardiomyocytes are furnished with AT1 receptors, AT2 receptors, Ang (1-7) receptors, vasopressin V1 receptors, and insulin receptor substrates. The presence of some of these molecules in the same cells creates good conditions for their interaction at the signaling level. The broad spectrum of actions allows for the engagement of angiotensin peptides, AVP, and insulin in the regulation of the most vital cardiac processes, including (1) cardiac tissue oxygenation, energy production, and metabolism; (2) the generation of the other cardiovascular compounds, such as nitric oxide, bradykinin (Bk), and endothelin; and (3) the regulation of cardiac work by the autonomic nervous system and the cardiovascular neurons of the brain. Multiple experimental studies and clinical observations show that the interactions of Ang II, Ang(1-7), AVP, and insulin in the heart and in the brain are markedly altered during heart failure, hypertension, obesity, and diabetes mellitus, especially when these diseases coexist. A survey of the literature presented in the review provides evidence for the belief that very individualized treatment, including interactions of angiotensins and vasopressin with insulin, should be applied in patients suffering from both the cardiovascular and metabolic diseases.

Suprachiasmatic Nucleus Neuropeptides and Their Control of Endogenous Glucose Production

Defective control of endogenous glucose production is an important factor responsible for hyperglycaemia in the diabetic individual. During the past decade, progressively more evidence has appeared indicating a strong and potentially causal relationship between disturbances of the circadian system and defects of metabolic regulation, including glucose metabolism. The detrimental effects of disturbed circadian rhythms may have their origin in disturbances of the molecular clock mechanisms in peripheral organs, such as the pancreas and liver, or in the central brain clock in the hypothalamic suprachiasmatic nuclei (SCN). To assess the role of SCN output per se on glucose metabolism, we investigated (i) the effect of several SCN neurotransmitters on endogenous glucose production and (ii) the effect of SCN neuronal activity on hepatic and systemic insulin sensitivity. We show that silencing of SCN neuronal activity results in decreased hepatic insulin sensitivity and increased peripheral insulin sensitivity. Furthermore, both oxytocin neurones in the paraventricular nucleus of the hypothalamus (PVN) and orexin neurones in the lateral hypothalamus may be important targets for the SCN control of glucose metabolism. These data further highlight the role of the central clock in the pathophysiology of insulin resistance.

Carotid Chemoreceptor Reflex Modulation by Arginine-Vasopressin Microinjected into the Nucleus Tractus Solitarius in Rats

BACKGROUND

In addition to their role of sensing O2, pH, CO2, osmolarity and temperature, carotid body receptors (CBR) were proposed by us and others to have a glucose-sensing role in the blood entering the brain, integrating information about blood glucose and O2 levels essential for central nervous system (CNS) metabolism. The nucleus tractus solitarius (NTS) is an important relay station in central metabolic control and receives signals from peripheral glucose-sensitive hepatoportal afferences, from central glucose-responsive neurons in the brainstem and from CBR and arginine-vasopressin (AVP)-containing axons from hypothalamic nuclei.

METHODS

In normal Wistar rats anesthetized with pentobarbital, permanent cannulas were placed stereotaxically in the NTS. Glucose changes were induced in vivo after CBR stimulation with sodium cyanide (NaCN-5 microg/100 g), preceded by an infusion of AVP [(10 or 40 pmol/100 nL of artificial cerebrospinal fluid) aCSF] or an antagonist for V1a receptors (anti-glycogenolytic vasopressin analogue-VP1-A) (100 pmol/100 nL of aCSF) into the NTS.

RESULTS

CBR stimulation after an AVP infusion (larger dose) into the NTS resulted in a significantly higher arterial glucose and lower brain arterial-venous glucose difference. In the same way, VP1-A administration in the NTS significantly decreased the effects observed after AVP priming before CBR stimulation or preceding the CBR stimulation, alone.

CONCLUSIONS

We propose that AVP in the NTS could participate in glucose homeostasis, modulating the information arising in CBR after histotoxic-anoxia stimulation.

Oxytocin and vasopressin neurones in principal and accessory hypothalamic magnocellular structures express Fos-immunoreactivity in response to acute glucose deprivation

Neurohypophyseal secretion of oxytocin and vasopressin is elevated in response to decreased systemic glucose availability. In these studies, dual-label immunocytochemistry was used to identify hypothalamic neuropeptidergic magnocellular neurones that are transcriptionally activated in response to glucose substrate imbalance. Two h after i.p. injection of the glucose antimetabolite, 2-deoxy-D-glucose (2DG), or the vehicle, saline, groups of adult male rats were anaesthetized by i.p. injection with sodium pentobarbital and killed by transcardial perfusion. Sections (25 microm) through anterior and tuberal levels of the hypothalamus were processed for nuclear Fos- and cytoplasmic neuropeptide immunoreactivity (-ir). A high proportion of oxytocin-ir neurones in the supraoptic, paraventricular, and adjunct structures, including the anterior commissural, periventricular magnocellular, posterior perifornical, recurrent supraoptic, medial forebrain, and circular nuclei, were colabelled for nuclear Fos-ir following administration of 2DG. Large numbers of vasopressin neurones in the supraoptic, circular, posterior perifornical, and medial forebrain nuclei, and posterior magnocellular division and posterior subnucleus of the paraventricular nucleus were also immunostained for Fos in rats injected with the antimetabolite. These results show that decreased glucose metabolism is a stimulus for activation of the Fos stimulus-transcription cascade within oxytocin-and vasopressin-immunopositive neurones in several hypothalamic loci, findings that reflect activation of the Fos-stimulus transcription cascade within large proportions of these cell populations during this metabolic challenge. These data suggest that both peripheral hormonal and central modulatory functions of these neuropeptidergic neurones may be influenced by cellular glucose availability.

Pharmacological manipulation of central nitric oxide/guanylate cyclase activity alters Fos expression by rat hypothalamic vasopressinergic neurons during acute glucose deprivation

Neurohypophyseal secretion of arginine vasopressin is stimulated by decreased systemic glucose availability. Nitric oxide is produced by paraventricular and supraoptic magnocellular neurons, and is implicated in central mechanisms controlling plasma sasopressin and glucose levels. The current studies investigated the role of this neurotransmitter in glucoprivic induction of AP-1 transcriptional activity in hypothalamic vasopressinergic neurons by examining whether pharmacological manipulation of central nitric oxide/guanylate cyclase/cGMP signaling alters nuclear accumulation of Fos immunoreactivity in these cells. Adult male rats pretreated by intraventricular administration of saline exhibited extensive colabeling of vasopressinergic neurons in both brain sites for Fos following systemic injection of the glucose antimetabolite, 2-deoxy-D-glucose. Pretreatment with the nitric oxide donor. SIN1, resulted in decreased numbers of paraventricular and supraoptic Fos-positive vasopressinergic neurons during glucoprivation. In other animals. coadministration of SIN1 and the nitric-oxide sensitive guanylate cyclase inhibitor, ODQ, prior to the antimetabolite reversed these inhibitory effects of SIN1 on Fos expression by these cells. Intracerebral administration of ODQ alone did not significantly enhance expression of Fos by vasopressinergic neurons in either site. The present studies demonstrate that exogenous activation of the nitric oxide/guanylate cyclase/cGMP pathway in the brain inhibits nuclear accumulation of the AP-1 transcription factor, Fos, in vasopressinergic neurons during cellular glucopenia, and suggest that this neurotransmitter is critical for transactivational effects of glucoprivation on these neuropeptidergic neurons.

Circadian rhythm of Arg-vasopressin contents in the suprachiasmatic nucleus in relation to corticosterone

Circadian rhythms of locomotor activity and adrenal glucocorticoid are controlled by the suprachiasmatic nucleus (SCN), the center of a biological clock, in mammals. Arg-vasopressin (AVP) contents in the SCN play a role in endogenous circadian rhythm during the absence of time cues. The AVP-containing neurons in the SCN are considered to transmit a circadian signal to the other parts of the brain. The circadian rhythms of AVP in the SCN in relation to the plasma corticosterone and locomotor activity were investigated. Under the light-dark cycle, plasma corticosterone levels were reciprocally correlated with the AVP content in the SCN. Under free-running conditions with constant dim light, AVP rhythms were reciprocally synchronized with the locomotor activity. The correlation of AVP with plasma corticosterone is different at different times of the day both under the LD cycle and constant dim light. Dexamethasone (i. p., 0.1 mg/100) increased the AVP contents, and this tendency was significantly greater during the dark period. These results indicate that corticosterone in the blood may regulate the circadian rhythm through AVP variation in the SCN.

A diurnal rhythm of stimulatory input to the hypothalamo-pituitary-adrenal system as revealed by timed intrahypothalamic administration of the vasopressin V1 antagonist

The mammalian suprachiasmatic nuclei (SCN) contain an endogenous pacemaker that generates daily rhythms in behavior and secretion of hormones. We hypothesized that the SCN imposes its circadian rhythm on the rest of the brain via a rhythmic release of its transmitters in its target areas. Previously, we demonstrated a pronounced inhibitory effect of vasopressin (VP), released from SCN terminals in the dorsomedial hypothalamus, on the release of the adrenal hormone corticosterone. In the present study, microdialysis-mediated intracerebral administration of the VP V1-receptor antagonist was used to pursue the study of the mechanisms underlying the circadian control of basal corticosterone release. Using timed administrations of the VP antagonist divided equally over the day/night cycle, we were able to uncover the existence of an additional stimulatory input from the SCN to the hypothalamopituitary-adrenal (HPA) axis. Peak activity of this stimulatory SCN input takes place during the second half of the light period, after the daily peak of VP secretion, with a delay of approximately 4-6 hr. In all likelihood, the inhibitory and stimulatory circadian input via separate mechanisms affects corticosterone release. Together, these two opposing circadian control mechanisms of the HPA axis enable a precise timing of the circadian peak in corticosterone release.