Inhibition of centrally-evoked pressor responses by neurohypophyseal peptides and their fragments

Vasopressin V1a and V1b Receptors: From Molecules to Physiological Systems

The neurohypophysial hormone arginine vasopressin (AVP) is essential for a wide range of physiological functions, including water reabsorption, cardiovascular homeostasis, hormone secretion, and social behavior. These and other actions of AVP are mediated by at least three distinct receptor subtypes: V1a, V1b, and V2. Although the antidiuretic action of AVP and V2 receptor in renal distal tubules and collecting ducts is relatively well understood, recent years have seen an increasing understanding of the physiological roles of V1a and V1b receptors. The V1a receptor is originally found in the vascular smooth muscle and the V1b receptor in the anterior pituitary. Deletion of V1a or V1b receptor genes in mice revealed that the contributions of these receptors extend far beyond cardiovascular or hormone-secreting functions. Together with extensively developed pharmacological tools, genetically altered rodent models have advanced the understanding of a variety of AVP systems. Our report reviews the findings in this important field by covering a wide range of research, from the molecular physiology of V1a and V1b receptors to studies on whole animals, including gene knockout/knockdown studies.

Cardiovascular effects of oxytocin

The well known effects of oxytocin on uterine contraction and milk ejection were found as early as the beginning of the 20th century. Since then many other effects of oxytocin have been found and among them a great number of effects on the cardiovascular system. Oxytocin is released from the neurohypophysis into the circulation and from parvocellular neurons within the paraventricular nucleus (PVN) to many areas within the central nervous system (CNS). Indeed, oxytocin may modify blood pressure as well as heart rate both through effects within the CNS and through effects in other organs, such as the heart, blood vessels and kidney. Oxytocin may also cause cardiovascular effects by affecting other mediators, such as atrial natriuretic peptide (ANP), nitric oxide (NO) and alpha 2-adrenoreceptors.

Supramedullary modulation of sympathetic vasomotor function

1. Supramedullary structures including the ventral medial prefrontal cortex (MPFC) and the midbrain cuneiform nucleus (CnF) project directly and indirectly to premotor sympatho-excitatory neurons of the rostral ventrolateral medulla (RVLM) that are critically involved in the generation of sympathetic vasomotor tone. 2. Electrophysiological studies have demonstrated that activation of depressor sites within the MPFC is associated with splanchnic sympathetic vasomotor inhibition and inhibition of the activity of RVLM sympathoexcitatory neurons. 3. Antidromic mapping and anatomical studies support the notion that a relay in the nucleus tractus solitarius is involved in the cardiovascular response to MPFC stimulation. 4. The midbrain CnF, which lies adjacent to the midbrain periaqueductal grey, is a sympathoexcitatory region of the midbrain reticular formation. Sympathoexcitatory responses evoked from the CnF are associated with short-latency excitation of RVLM neurons. 5. Cuneiform nucleus stimulation induces the expression of mRNA for the immediate early genes c-fos and NGFI-A in mid-brain, pontine and hypothalamic structures. 6. The MPFC and CnF are supramedullary structures with opposing modulatory influences on sympathetic vasomotor drive, whose roles in cardiovascular control mechanisms warrant further investigation.

Central vasopressin is required for the complete development of deoxycorticosterone-salt hypertension in rats with hereditary diabetes insipidus

It has been shown that vasopressin receptors are upregulated in the brain and that the central vasopressin pathway is involved in the development of deoxycorticosterone acetate (DOCA)-salt hypertension. However, it is unclear whether central vasopressin, in itself, is essential for this type of hypertension. To clarify this issue, the effect of centrally administered vasopressin on the development of DOCA-salt hypertension was studied in homozygous Brattleboro rats which genetically lack vasopressin. In homozygous Brattleboro rats, treatment with intracerebroventricular infusion of vasopressin (1 ng/h) alone or DOCA-salt (weekly subcutaneous injection of 30 mg/kg deoxycorticosterone acetate and 0.3% NaCl to drink) alone had no effect on systolic blood pressure (SBP). On the other hand, hypertension was partially restored in homozygous Brattleboro rats treated with intracerebroventricular infusion of vasopressin and DOCA-salt (SBP: 175 +/- 4 mmHg), although the magnitude of elevation of SBP was one-third of that in Long Evans rats treated with DOCA-salt (278 +/- 15 mmHg). These hypertensive homozygous Brattleboro rats showed an increase in fluid intake and urinary sodium excretion, as observed in DOCA-salt hypertensive Long Evans rats. These results suggest that central vasopressin is required for the complete development of DOCA-salt hypertension and the mechanism is, in part, due to enhanced sodium intake through the additive effect of central vasopressin and DOCA-salt.

Mesencephalic cuneiform nucleus and its ascending and descending projections serve stress-related cardiovascular responses in the rat

The aim of the present study was to explore the neuroanatomic network that underlies the cardiovascular responses of reticular formation origin in the region of the cuneiform nucleus (CNF). The study was performed in urethane anesthetized male Wistar rats. The left iliac artery was supplied with a catheter for the measurement of systemic blood pressure. Low intensity electrical stimulation of the mesencephalic reticular formation (MRF) in the vicinity of the CNF always resulted in pressor and bradycardiac responses, whereas stimulation in the parabrachial nucleus (PB) and Kölliker-Fuse nucleus (KF) led to a pressor response and a small tachycardiac response. The cuneiform area may be placed in the center of a circuit that serves a specific autonomic response pattern to stress: parallel activation of the sympathetic (pressor response) and parasympathetic limb (bradycardia). The efferent connections of the effective stimulation sites in the MRF and the CNF area, were investigated by anterograde tracing with the lectin Phaseolus vulgaris leucoagglutine (PHA-L). The CNF sends descending fibers to the gigantocellular reticular nuclei (GI), the motor nucleus of the vagus (DMNV) and nucleus tractus solitarius (NTS). These projections are probably involved in the bradycardiac response to stimulation. The descending pathway to the NTS/DMNV and GI may therefore be the parasympathetic limb of the circuit. Furthermore, the CNF sends ascending fibers to limbic forebrain areas and descending fibers to the PB-KF complex. The KF in its turn projects to the rostroventrolateral medullary nucleus (RVLM) and the intermediolateral cell column (IML). These latter projections are partly involved in producing the pressor response and thereby represent the sympathetic limb of the circuit. Accordingly, the transection of the descending fibers from the CNF to the PB-KF complex resulted in a decreased pressor and an increased bradycardiac response. This suggests that a baroreceptor reflex-induced bradycardia which results from blood pressure increase can be excluded as the origin of the stimulation-induced bradycardia, and that the pressor and bradycardiac responses are two independent moieties. It cannot be excluded that ascending fibers from the CNF are also involved in producing the pressor response. On the basis of the present physiological and neuroanatomical study, a brain circuit has been proposed in which the cuneiform nucleus has a central position. The described brain circuit may serve a passive coping strategy to novel, painful or threatening stimuli during which the animals show orientation/attention or freezing behavior accompanied by a bradycardiac and pressor response.

Vasopressin and oxytocin gene expression in the supraoptic and paraventricular nucleus of the spontaneously hypertensive rat (SHR) during development of hypertension

To study the regulation of hypothalamic vasopressin (VP) and oxytocin (OT) gene expression in relation to the development of hypertension, levels of VP mRNA and OT mRNA were determined in spontaneously hypertensive rats (SHR). Differences in VP and OT mRNA content of the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of 4- and 10-week-old SHR and Wistar-Kyoto controls (WKY) were quantitated by dot-blot and Northern blot analysis. VP and OT pituitary content and VP plasma levels were measured by radioimmunoassays. VP mRNA levels were approximately 2-fold and 3-fold higher in the SON and PVN of 4-week-old SHR, respectively, as compared to controls. The OT mRNA levels were approximately 35% lower in both nuclei of the SHR. There was no difference in VP and OT pituitary content between 4-week-old SHR and WKY, but VP plasma levels were higher in SHR. In the 10-week-old SHR VP mRNA levels were still approximately 30-40% higher and the OT mRNA levels were approximately 40% lower in both nuclei when compared to age-matched WKY. Pituitary VP and OT contents were respectively 1.5-fold higher and 20% lower in the 10-week-old SHR than in 10-week-old WKY. VP plasma levels were still elevated in the SHR. The data indicate that in the hypothalamo-neurohypophyseal system of the SHR the VP system is in a higher state of activity, while the OT system is lower in activity.

Reduction of vagal pressor reflexes by neurohypophyseal peptides and related compounds

The effects of intracisternal injections of [Lys8]vasopressin and [Arg8]vasopressin (25, 50, 100, 200 mU/kg) and related compounds oxytocin (25, 50, 100, 200 mU/kg), felypressin (25, 50, 100, 200 mU/kg) and vasotocin (100, 200, 400, 800 ng/kg) on the acute neurogenic pressor responses to afferent vagal stimulation (5, 10, 20 and 30 Hz) were studied in urethane-anaesthetized dogs. [Arg8]vasopressin and [Lys8]vasopressin (50, 100, 200 mU/kg) elicited a significant decrease in the pressor responses. The depressor effects of oxytocin (50, 100, 200 mU/kg) were less marked and felypressin or vasotocin remained inactive. These results suggest that vasopressin, and oxytocin, containing neurons may be modulators involved in central cardiovascular regulation. Since these cardiovascular reflexes are related to an increase in sympathetic tone, the present work suggests that neurohypophyseal peptides could play an inhibitory role in the regulation of blood pressure through a central inhibition of sympathetic tone. In term of structure-activity relationships the present work suggests that under our experimental conditions the sequence 1-5 of amino acids in vasopressin is important for the depressor action.

[Cardiovascular effect of the antidiuretic hormone arginine vasopressin]

The two major biological actions of vasopressin are antidiuresis and vasoconstriction. The antidiuretic action of low concentrations of vasopressin is well established and concentrations 10 to 100 times above those required for antidiuresis elevate arterial blood pressure. Antidiuresis is mediated by V2-receptors at the kidney, whereas vasopressin constricts arterioles by binding at V1-receptors. Pharmacological effects of specific antagonists of the vasoconstrictor activity of vasopressin (vascular or V1-receptor antagonists) are presented. Low concentrations of vasopressin do have significant hemodynamic effects. Physiological concentrations of vasopressin cause vasoconstriction and elevate systemic vascular resistance. In subjects with intact cardiovascular reflex activity, however, cardiac output falls concomitantly and blood pressure therefore does not change. In animals with baroreceptor deafferentation or in patients with blunted baroreceptor reflexes (autonomic insufficiency) a rise in plasma vasopressin causes vasoconstriction and an increase in blood pressure, because cardiac output does not fall under these conditions. Vasopressin contributes substantially via increase in systemic vascular resistance to maintain blood pressure during water deprivation. During hemorrhage and hypotension vasopressin has a major role to restore blood pressure. In experimental hypertension vasopressin contributes to the development and maintenance of high blood pressure in DOCA, but not in genetic hypertensive rats. The role of vasopressin in human hypertension is not yet clear. Vasopressin in extrahypothalamic areas of the brain affects circulatory regulation by interaction with central cardiovascular control centers. The exact mechanism of how vasopressin is involved in central regulation of blood pressure remains to be established. In contrast to our previous opinion vasopressin is a vasoactive hormone also at low plasma concentrations. Its cardiovascular action is more complex than previously assumed.

The cardiovascular effects of oxytocin in conscious male rats

In conscious, chronically instrumented normotensive male Wistar rats intravenous (i.v.) administration of oxytocin (OXT) (greater than or equal to 100 ng) induced a dose-related biphasic change in mean arterial pressure (MAP). This consisted of an initial pressor effect accompanied by bradycardia and a decrease in cardiac output (CO), followed by a more prolonged fall in MAP which reached a maximum 30 min after injection and was accompanied by an increase in CO. The more specific (Thr4,Gly7]OXT analogue (0.01-10 micrograms i.v.) caused a dose-related fall in MAP and a rise in CO which reached a maximum after 15-30 min. Similarly in spontaneously hypertensive rats of the stroke prone strain (SHRSP) an initial pressor effect and delayed fall in MAP were apparent after OXT (0.1 and 10 micrograms i.v.) only the decrease in MAP being evident with the [Thr4,Gly7]OXT analogue. These responses were significantly larger than those observed in Wistar Kyoto controls. The pressor effects are therefore interpreted to be due to vasopressin receptor activation while the depressor effects appear to be oxytocin specific. In sinoaortic denervated rats, OXT (0.1 and 10 micrograms i.v.) induced an enhanced initial pressor effect with a much reduced reflex bradycardia and fall in CO. A larger and more prolonged delayed fall in MAP was apparent with both OXT and [Thr4,Gly7]OXT accompanied by a decrease in CO when compared to sham-operated controls. Intracisternally (i.c.) administered OXT (0.05-10 ng) had no effect on MAP or heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Pro-Leu-GlyNH2 affects dopamine and noradrenaline utilization in rat limbic-forebrain nuclei

The effects of Pro-Leu-GlyNH2 (PLG), administered i.c.v. in doses of 3.5, 35, 350 and 3500 pmol, were studied on the alpha-MPT-induced disappearance of catecholamines in microdissected rat brain nuclei. PLG, dose-dependently, increased dopamine disappearance in the nucleus caudatus and globus pallidus, whereas a decrease in dopamine disappearance was observed in the nucleus dorsomedialis. Noradrenaline disappearance was decreased in the medial septal nucleus, anterior hypothalamic area and lateral amygdala. A tendency towards an increase in noradrenaline disappearance was observed in the nucl. supraopticus. These data show that PLG has a central site of action. The effects of PLG on dopamine disappearance are comparable to those previously found with vasopressin, while the effects of PLG on noradrenaline utilization show a striking similarity with those previously obtained with oxytocin.

Arginine-vasopressin inhibits centrally induced pressor responses by involving hippocampal mechanisms

Administration of arginine-vasopressin (AVP) or prolyl-leucyl-glycinamide (PLG) into a lateral cerebral ventricle reduced the magnitude of systolic blood pressure increase (pressor response) induced by electrical stimulation of the mesencephalic reticular formation (MRF) in urethane-anesthetized rats. Bilateral destruction of the dorsal hippocampus prevented the action of AVP on the pressor response. However, the effect of PLG was only slightly reduced by hippocampal lesion. Microinjection of AVP in the dentate area of the dorsal hippocampus mimicked the action of intracerebroventricularly administered peptides. The effect of a single injection of AVP lasted at least for 60 min. Neither hippocampal damage nor peptide administrations resulted in changes in mean arterial blood pressure (basal BP). Bradycardiac response accompanied the BP increase during MRF stimulation. Hippocampal damage or intracerebroventricular administration of AVP and PLG failed to affect the cardiac response. Injection of AVP into the hippocampus tended to reduce the magnitude of cardiac responses caused by MRF stimulation. It is suggested that the inhibition by AVP of a pressor response produced by MRF stimulation involves the dorsal hippocampus. The action of PLG or related peptides seems to be, at least in part, through mechanisms not involving the hippocampus.

Central cardiovascular regulation and the role of vasopressin: a review

This paper will review the current state of knowledge concerning interactions between vasopressin and central neural mechanisms of cardiovascular regulation. The development of information concerning systemic cardiovascular effects of vasopressin and interactions between vasopressin and the peripheral autonomic system is outlined to provide an introduction to the topic. Major themes discussed in the rest of the paper include a survey of information suggesting direct central effects of vasopressin on autonomic control of blood pressure and heart rate and the possible localization of the central site of effect. Evidence that circulating vasopressin may act on central cardiovascular control, especially baroreflex function, is reviewed, as is the possibility of vasopressin effects on baroreflex control independent of circulating vasopressin. A survey of central pathways containing vasopressin which may be relevant to central cardiovascular actions of vasopressin is presented along with a discussion of possible regulation of activity in these pathways. Some evidence of an association between alterations in brain vasopressin levels and hypertension in experimental animals is also introduced.

Inhibition of pressor responses induced by electrical stimulation of the mesencephalon by vasopressin and oxytocin

Receptors for vasopressin are present in blood vessels, kidney and the brain. Earlier studies indicated that the memory and learning effects of vasopressin are exerted via receptor sites in the brain and that the classical hormonal effects in the periphery can be dissociated in the molecule from the central action. Vasopressin also affects blood pressure regulation. Arg8-vasopressin (AVP) and oxytocin upon intracerebroventricular administration reduced the pressor response elicited by electrical stimulation of the rat mesencephalic reticular formation. Desglycinamide-AVP also exerted this effect. Lesion and microinjection studies revealed that the dentate gyrus of the hippocampus may be a site of action of AVP or its active fragments, inhibiting central pressor responses. Oxytocin appeared to act on structures in the vicinity of the fourth cerebral ventricle.

Reduction of a centrally induced pressor response by neurohypophyseal peptides: the involvement of lower brainstem mechanisms

Pressor and bradycardiac responses induced by electrical stimulation of the mesencephalic reticular formation in urethane-anesthetized rats were used as model of neurogenic hypertension. Oxytocin (OXT) and prolyl-leucyl-glycinamide (OXT-(7-9] administered into the fourth cerebral ventricle markedly attenuated the magnitude of the pressor response. OXT-(7-9) was somewhat more potent than OXT and its effect was dose-dependent. Microinjection of OXT-(7-9) into the dorsal raphe nucleus reduced the pressor response as well. [Arg8]vasopressin (AVP) did not affect the pressor response when administered via this route, while prolyl-arginyl-glycinamide (AVP-(7-9] had an action that was similar to that of OXT-(7-9). None of these peptides affected the magnitude of the bradycardiac response. It is suggested that OXT and related fragments modulate neurogenic hypertensive responses through lower brainstem mechanisms.