Changes in vasopressin concentration in plasma and cerebrospinal fluid in response to hemorrhage in anesthetized dogs

Chronic Cardiovascular Effects of Central Vasopressin in Conscious Rats

To clarify the cardiovascular effects of central vasopressin (AVP), a chronic intracerebroventricular (ICV) infusion of AVP was performed in conscious Wistar normotensive rats. Animals were divided into 3 groups: 1) AVP 1 ng/hr (Low), 2) AVP 100 ng/hr (High), and 3) saline (control) ICV infusion. After a 6 day control period, AVP or saline was continuously infused into the lateral cerebroventricle at a rate of 1 microliter/hr using osmotic minipump for 7 days. As a result, a dose-related elevation of AVP concentration in CSF was achieved. Systolic blood pressure in both Low and High AVP infusion was slightly (7-12 mmHg) but significantly higher than that in control. ICV infusion of AVP did not alter urine volume, electrolytes excretion or osmolality, and AVP vascular antagonist injected intravenously failed to affect mean arterial pressure. Furthermore, plasma catecholamines and renin activity did not differ significantly among the groups. Thus, chronic ICV infusion of AVP induced the elevation of blood pressure, which is due to centrally mediated effect of AVP.

Vasopressin receptor subtypes on mesenteric and cremasteric arterioles in rat

We studied the effects of a selective vasopressin V(1A) receptor antagonist [1-(1-(4-(3-acetylaminopropoxy)benzoyl)-4-piperidyl)-3, 4-dihydro-2(1H)-quinolinone (OPC-21268)] and a selective vasopressin V(2) receptor antagonist [5-dimethylamino-1(4-(2-methylbenzoylamino)benzoyl)-2,3,4, 5-tetrahydro-1H-benzazepine (OPC-31260)] on vasopressin-induced contraction of mesenteric and cremasteric arterioles in urethane-anaesthetized rats. Vasopressin was infused intravenously for 60 min or applied topically to arterioles directly. Vasopressin infusion (50, 100 or 500 ng/kg/min) decreased the diameter of both mesenteric and cremasteric arterioles. Vasopressin (500 ng/kg/min)-induced vasoconstriction was antagonized by OPC-21268 (0. 2, 1.0 and 5.0 mg/kg, i.v.), dose-dependently, but not by OPC-31260. Topically applied vasopressin (4.6x10(-10)-4.6x10(-8) M) dose-dependently constricted both microvessels. Pre-administration of OPC-21268 (5.0 mg/kg, i.v.) completely inhibited topically applied vasopressin-induced vasoconstriction in both microvessels, and OPC-31260 partially inhibited it in cremasteric arterioles. These results suggest that vasopressin induces vasoconstriction in rat mesenteric and cremasteric arterioles mainly by stimulating vasopressin V(1A) receptors, while vasoconstriction in cremasteric arterioles is partly associated with stimulation of vasopressin V(2) receptors.

Role of brain vasopressin in regulation of blood pressure

Using recent advances in brain physiological, neurohistochemical, and molecular biological techniques, it could be demonstrated that the central action of vasopressin (VP) is important in cardiovascular regulation and in the pathogenesis of hypertension. VP is now known to be located in the area of the brain involved in cardiovascular regulation. Furthermore, in various pathophysiological states, brain VP secretion is regulated separately from the peripheral VP secretion system. The role of brain VP in the regulation of the circadian rhythm of blood pressure is becoming a topic of major interest.

Repeated stress, like vasopressin, sensitizes the excitatory effects of corticotropin releasing factor on the acoustic startle reflex

In the rat, evidence now suggests a neurotransmitter function for the neuropeptides arginine vasopressin (AVP) and corticotropin releasing factor (CRF), implicating them in various autonomic, behavioral, and neuroendocrine responses to stress. Repeated AVP/CRF release in the pituitary portal circulation, due to stress, sensitizes and potentiates the release of ACTH from the anterior pituitary. Using a neuroanatomically well-defined behavior, the acoustic startle reflex in the rat, we sought to determine whether an interaction between AVP, CRF and stress might also occur centrally as measured by increased behavioral sensitivity to AVP or CRF given directly into the brain. The first experiment tested whether repeated intraventricular (i.c.v.) infusion of AVP would lead to an increase in the excitatory effect of a subthreshold dose of AVP on the acoustic startle reflex when infused 48 h later. Different groups of rats were infused with various doses of AVP (0.3, 3, or 30 ng) or vehicle on Day 1 and tested for startle over the next 60 min. On Day 2, 48 h later, all animals were infused with a single dose of AVP (300 pg) and tested for startle. Infusion of AVP on Day 1 did not increase startle consistently at any dose, but did lead to a sensitized excitatory effect of AVP on startle on Day 2 which was non-monotonically related to the dose of AVP given on Day 1. Experiment 2 tested whether AVP on Day 1 would sensitize the excitatory effects on startle of CRF given i.c.v. on Day 2. Different groups of rats were infused i.c.v. with various doses of AVP (10, 30, 100, 300 pg) or vehicle on Day 1. On Day 2, 48 h later, all rats were infused with a subthreshold dose of CRF (0.25 microg). Infusion of AVP on Day 1 led to a sensitized excitatory effect of CRF on startle on Day 2 which was non-monotonically related to the dose of AVP given on Day 1. In experiment 3, we tested whether footshocks given on Day 1 would sensitize the excitatory effect of CRF on startle tested 48 h later. Different groups were given footshocks (0.2, 0.4, 0.8, 1.6 mA) on Day 1. On Day 2, 48 h later, all rats were infused with a subthreshold dose of CRF (0.25 microg). Footshocks given on Day 1 led to a sensitized excitatory effect of CRF on startle on Day 2 which was non-monotonically related to the intensity of footshock on Day 1. Taken together, these results suggest that an interaction between AVP, CRF and stress may occur centrally, consistent with other studies showing similar interactions peripherally. This may provide a model system for analyzing how prior stress leads to enhanced behavioral reactions to subsequent stressors and a mechanism to explain dysregulation of the stress response.

Hemorrhage-induced vasopressin release in the paraventricular nucleus measured by in vivo microdialysis

Experiments were carried out, using the technique of in vivo microdialysis in conscious rats, to determine whether hemorrhage, a potent stimulus for the release of vasopressin from the posterior pituitary into the circulation, would also result in a local release of vasopressin from the paraventricular nucleus (PVN), and whether this release is affected by gender. Male and non-estrous female rats were prepared with a microdialysis probe adjacent to the PVN and femoral arterial and venous catheters the day before the experiment. On the day of the experiment, rats was bled either 20% or 30% of blood volume. The concentration of vasopressin in the dialysate increased significantly in the males following both hemorrhages and in the females following the 30% hemorrhage. There were no statistically significant differences in the post-hemorrhage dialysate vasopressin concentration with respect to either gender or magnitude of the hemorrhage. The plasma vasopressin concentration increased markedly in response to the hemorrhage and this response was greater in females following the 30% hemorrhage. There were no gender differences in the reduction in arterial pressure following either hemorrhage. It is concluded that physiological stimuli for the release of vasopressin into the circulation also result in intrahypothalamic release of this hormone.

Does endogenous peripheral arginine vasopressin have a role in the febrile responses of conscious rabbits?

1. The actions of peripheral arginine vasopressin (AVP) on the febrile responses of conscious rabbits induced by peripherally administered polyinosinic:polycytidylic acid (poly(I).poly(C)) have been studied using an AVP V1 receptor antagonist ([deamino-Pen1, O-Me-Tyr2, Arg8]-vasopressin). 2. Temperature responses were monitored continuously using rectal thermistor probes. Test substances were administered intravenously (i.v.). Blood samples were taken at timed intervals from a marginal ear vein and plasma PGE2 and PGF2 alpha levels determined by radioimmunoassay. 3. Poly(I).poly(C) (2.5 micrograms/kg) stimulated a reproducible biphasic rise in body temperature with a lag phase of 45-60 min and peaks at 90 and 225 min. The febrile response was accompanied by a 5-fold rise in circulating immunoreactive (ir) PGE2, which peaked after 90 min and remained elevated up to 300 min. Poly(I).poly(C) also stimulated a 2.5-fold rise in circulating irPGF2 alpha, which peaked after 150 min and was followed by a return to basal levels after 300 min. 4. The overall magnitude of the febrile response to poly(I).poly(C) (2.5 micrograms/kg, i.v.) was significantly antagonized by the AVP V1 receptor antagonist (250 micrograms/kg, i.v.) administered 5 min prior to the pyrogen. 5. The irPGE2 response to poly(I).poly(C) (2.5 micrograms/kg, i.v.) was significantly antagonized by the AVP V1 receptor antagonist (250 micrograms/kg, i.v.) administered 5 min prior to the pyrogen. The irPGF2 alpha response was only reduced at the peak 150 min time point measurement. 6. In conclusion, these results show a modulatory role for a peripherally administered AVP V1 antagonist in the febrile responses to poly(I).poly(C), suggesting a possible propyretic role for endogenous peripheral AVP. This modulatory role appears to be mediated via actions on prostaglandin E2.

Triphasic response of rat intracerebral arterioles to increasing concentrations of vasopressin in vitro

To determine how vasopressin affects the vascular tone of the smaller cerebral arterioles, we carried out an in vitro study of isolated and cannulated intracerebral arterioles of rats. We found that increasing concentrations of vasopressin induced a triphasic response of vasodilation (10(-12)-10(-11) M), vasoconstriction (10(-10)-10(-8) M), and vasodilation stabilizing to control diameter (10(-7)-10(-6) M) and that the maximum constriction was twice the maximum dilation in these smaller arterioles [21.2 +/- 13.1% (mean +/- SD) decrease in diameter vs. 11.2 +/- 5.7% increased]. Pretreatment of the arterioles with NG-monomethyl-L-arginine (10(-4) M), a specific inhibitor of endothelium-derived relaxing factor, abolished the vasopressin-induced vasodilation and significantly increased the vasoconstriction. These results suggest that these arterioles were maintained in a dilated state by an endothelium-derived relaxing factor activated by vasopressin. Both vasodilation and vasoconstriction were found to be mediated through vasopressin V1 receptors in a study of arterioles pretreated with d(CH2)5Tyr(Me)arginine vasopressin (10(-6) M), a vasopressin V1 receptor antagonist. These results support the hypothesis that vasopressin may constrict smaller cerebral arterioles while simultaneously dilating larger cerebral arteries. Our results also suggest that vasopressin may aggravate cerebral ischemia in pathological conditions, such as subarachnoid hemorrhage, when the arteriolar response to vasopressin shifts from vasodilation to vasoconstriction due to increased vasopressin levels in plasma and CSF and impaired endothelium-derived relaxation.

Central carbachol stimulates vasopressin release into interstitial fluid adjacent to the paraventricular nucleus

We have used an in vivo double microdialysis probe technique in conscious rats to determine whether the application of carbachol to one paraventricular nucleus (PVN) can result in increased local release of vasopressin from that PVN. Experiments were carried out 24 h after placement of microdialysis probes lateral to each PVN. When both probes were perfused initially with 0.9% NaCl, vasopressin was detected in the outflow (dialysate) from both probes. When carbachol (100 micrograms/ml) was included in the perfusate of one probe for the first 10 min of a 30-min collection period, while the other probe continued to be perfused with saline alone, there was a seven-fold increase in the concentration of vasopressin in the dialysate from the carbachol-perfused probe; the vasopressin concentration in the dialysate from the contralateral probe increased only slightly. The plasma vasopressin concentration was also elevated. When one of the paired probes was perfused with carbachol (100 micrograms/ml) for 30 min, there were similar increases in the concentration of vasopressin in the dialysate from both probes and a sustained increase in the plasma vasopressin concentration. Thus, vasopressin is released into the interstitial fluid adjacent to the PVN under basal conditions, and this release can be substantially increased when vasopressin secretion to the periphery is stimulated.

Effects of vasopressin and oxytocin on canine cerebral circulation in vivo

In vivo experiments on the vasoactive effects of vasopressin and oxytocin on cerebral circulation were carried out in anesthetized dogs, using an electromagnetic flowmeter to measure vertebral blood flow and angiography to measure the internal diameter of the basilar artery. Direct bolus infusion of 1 pmol to 1 nmol of vasopressin or 10 pmol to 10 nmol of oxytocin into a femoral-vertebral artery shunt produced a dose-dependent decrease in vertebral artery blood flow without significantly affecting mean arterial blood pressure. Vasopressin was more potent than endothelin and neuropeptide Y, which have also been demonstrated to induce long-lasting decreases in vertebral artery blood flow. However, direct bolus infusion of vasopressin (100 pmol and 1 nmol) or oxytocin (1 nmol and 10 nmol) into the vertebral artery dilated major vessels including the vertebral, anterior spinal, and basilar arteries, as well as the circle of Willis and its main branches, while endothelin (1 nmol) and neuropeptide Y (5 nmol) caused no change in the diameters of major cerebral arteries. The V1 antagonist d(CH2)5tyrosine(methyl) arginine vasopressin suppressed the effects of both vasopressin and oxytocin. Vasopressin was over 10 times as potent as oxytocin in both assays. The vasodilatory effect of vasopressin, which may be mediated by an endothelium-dependent mechanism, was functionally damaged in dogs after experimental subarachnoid hemorrhage. These data suggest regional differences in the sensitivity and responsiveness of vasculature to vasopressin and oxytocin, and specifically that both peptides act through V1 receptors to decrease the resistance of large vessels and increase the resistance of small vessels.

Third ventricular subependymal oxytocin-like immunoreactive neuronal plexus in the rat

In a previous retrograde tracing study, a dense subependymal neuronal plexus was found along the anterior ventral third ventricle that projects to the posterior pituitary. In the work reported here, the oxytocin-like immunoreactive neurons of this plexus were studied in detail. It has a population of about 650 cells with a great wealth of dendrites. The neurons are of magnocellular neurosecretory type with long straight dendrites running parallel to the ependyma. The plexus is composed of a dorsal and a ventral part. The dorsal part consists of about 75% of the whole population and is most dense at the levels of the anterior and medial magnocellular paraventricular nuclei. Their dendrites appear vacuous in immunohistochemically stained sections and have a tendency to form fascicles. The ventral part is more sparse. The dendrites of the subependymal plexus are well organized so the anteriorly located ones tend to be directed rostrally and the posteriorly ones caudally. The functional significance of the plexus is discussed.

Criteria for establishing a physiological role for brain peptides. A case in point: the role of vasopressin in thermoregulation during fever and antipyresis

This paper has attempted to present and discuss the criteria necessary for the evaluation of a specific physiological role for a peptide in the CNS. These criteria are based on many experimental approaches to the problem and conclusions must be supported by the weight of the evidence. These criteria were illustrated by examining the hypothesis that AVP is an antipyretic neurotransmitter involved in regulating febrile increases in Tb by release and action in the VSA of the brain. The weight of the evidence in this case implies that this hypothesis is essentially correct. The only serious conflicting evidence comes from the work with Brattleboro rats. It is hoped that further research will resolve these discrepancies or result in a suitably modified hypothesis.

Vasopressin reduces release from vasopressin-neurons and oxytocin-neurons by acting on V2-like receptors

The effects of arginine vasopressin (AVP), and of the V2-AVP receptor agonist 1-deamino[8-D-arginine] vasopressin (DDAVP) on release from the vasopressin-neurons and oxytocin-neurons of Long-Evans rats were evaluated using specific radioimmunoassays for rat neurophysins. AVP (1 microgram, 1 nmol) or DDAVP (25 ng, 25 pmol) was administered i.p. to animals 1 h before they received an i.v. infusion of 18% saline at 10 microliters/100 g b. wt./min for 60 min. Both AVP and DDAVP decreased the responsiveness (slope) but not the sensitivity threshold of vasopressin-neurons to acute changes in plasma osmolality. Since the amounts of the peptides giving comparable decreases in responsiveness were directly related to their antidiuretic potencies, it is most probable that this influence is mediated through V2-like receptors. However, while ruling out a significant contribution of V1-type receptors, the data do not exclude involvement of other vasopressin receptors (e.g. V3-type receptors). Both AVP and DDAVP also appeared to have an inhibitory effect on release from oxytocin-neurons, but in this case they significantly altered sensitivity threshold but not responsiveness to acute changes in plasma osmolality. Because AVP produced a shift in sensitivity threshold larger than that by DDAVP when the peptides were used in amounts related to their antidiuretic potencies, our results suggest that the feedback influence of AVP on oxytocin-neurons is largely, although not entirely, exercised through V2-like receptors.

Vascular responses to vasopressin are tone-dependent in the cerebral circulation of the newborn pig

The effects of lysine vasopressin (LVP) on pial arteriolar diameter and cortical periarachnoid fluid prostanoid concentrations were investigated in newborn pigs. Chloralose-anesthetized piglets were equipped with closed cranial windows over the parietal cortex for observation of pial arterioles and collection of cerebrospinal fluid (CSF) passing over the cerebral surface. Prostanoids in the CSF were determined by radioimmunoassay. LVP (10-1,000 microU/ml) elicited concentration-dependent increases in pial arteriolar diameter associated with increased levels of 6-keto-prostaglandin (PG)F1 alpha, PGE2, thromboxane B2, and PGF2 alpha. LVP-induced pial arteriolar dilation was unchanged after intravenous indomethacin (5 mg/kg). Conversely, LVP constricts pial arterioles previously dilated by physiological (hemorrhagic hypotension) and pharmacological (topically applied PGE2 or isoproterenol) intervention. This constriction is potentiated by indomethacin. Vascular and biochemical changes elicited by LVP were blocked by intravenous [1-(beta-mercapto-beta beta-cyclopentamethylene propionic acid),2,(O-methyl)-Tyr-AVP] (5 micrograms/kg), a putative V1 receptor antagonist, whereas vascular effects of norepinephrine and U46619, a thromboxane A2 mimic, were unchanged. Therefore, the degree of vascular tone appears to influence responses of the newborn pig cerebral circulation to LVP.

Humoral regulation of blood flow to choroid plexus: role of arginine vasopressin

The goal of this study was to examine humoral mechanisms that regulate blood flow to the choroid plexus. We determined the effects of arginine vasopressin on blood flow (microspheres) to the choroid plexus in anesthetized and awake rabbits. In anesthetized rabbits, blood flow to the choroid plexus was 342 +/- 31 (mean +/- SEM) ml/min/100 g under control conditions. Intravenous infusion of vasopressin at 4 and 40 mU/kg increased plasma vasopressin levels from 11 +/- 1 to 55 +/- 15 and 441 +/- 120 pg/ml, respectively, and blood flow to the choroid plexus decreased by 48 +/- 6% and 70 +/- 4%. Cerebral blood flow was not affected by infusion of vasopressin. Similar responses to infusion of vasopressin were observed in awake rabbits. The V1 antagonist [d(CH2)5Tyr(Me)AVP] (10 micrograms/kg i.v.) had no effect on resting blood flow, but abolished the effect of vasopressin on blood flow to the choroid plexus. Vasoconstrictor responses of the choroid plexus to intravenous infusion of phenylephrine were not attenuated by the V1 antagonist. Thus, circulating vasopressin, at plasma levels that are observed under physiological and pathophysiological conditions, has marked effects on blood flow to the choroid plexus. These effects appear to be mediated through a V1 receptor. We speculate that vasopressin may play an important role in regulation of blood flow to the choroid plexus and perhaps in the regulation of cerebrospinal fluid production.

Gradient of arginine vasopressin concentration but not angiotensin II concentration between cerebrospinal fluid of anterior 3rd ventricle and cisterna magna in dogs

Dogs were chronically implanted with two devices for cerebrospinal fluid (CSF) sampling from (a) the anterior part of the 3rd ventricle and (b) the cisterna magna. In conscious dogs arginine vasopressin (AVP) concentration of CSF samples collected at different occasions were 2-3 times higher in the CSF of the 3rd ventricle as compared to the AVP concentration of the cisterna magna. Inhalation anesthesia stimulated AVP release into the CSF at both sites by a factor of about 2, the gradient between 3rd ventricle and cisterna magna CSF of 2-3 remained for AVP in simultaneously collected samples. In contrast, angiotensin II-like immunoreactivity of CSF was not significantly different at both sites, neither in the conscious dogs nor during anesthesia. It is concluded that the main amount of AVP enters the CSF at the 3rd ventricular level.

Interaction of changes in the third ventricular CSF tonicity, central and systemic AVP concentrations and water intake

Arginine vasopressin (AVP) is assumed to be involved as a central transmitter or modulator in the control of autonomic functions including thirst. In conscious dogs AVP concentration in cerebrospinal fluid (CSF) from the anterior part of the third ventricle (A3V) was analysed before and after local elevation of CSF osmolality by intracerebroventricular (i.c.v.) infusion of 0.35 M NaCl and after i.c.v. AVP infusion at 46 and 138 fmol ml-1 for 10 min. In addition, the effects of these i.c.v. infusions on water intake, plasma AVP concentration and blood pressure were investigated. In euhydrated dogs 0.35 M NaCl i.c.v. did not alter AVP concentration in the CSF during the subsequent 2 h. In contrast, plasma AVP concentration had increased significantly from 3.4 +/- 0.3 (control) to 6.4 +/- 0.7 and 4.7 +/- 0.3 fmol ml-1, 4 and 16 min, respectively, after the hypertonic stimulus. Drinking was stimulated with an average water intake of 14.5 +/- 3.7 ml kg-1 body wt. However, AVP infusion into the A3V did not elicit water intake despite increases of AVP concentration in the A3V by factors up to 40 above control. The same animals responded with spontaneous drinking to 0.35 M NaCl i.c.v. administered 160 min after the end of AVP infusions. Exogenously administered AVP disappeared from the A3V with a time constant of 13.8 min. The results do not support the view that AVP in the A3V CSF per se stimulates drinking.

Contractile effects of perivascularly applied vasopressin on the pial artery of the cat brain

1. The effects of perivascularly applied vasopressin on the diameter of pial arteries (control 298 +/- 14 S.E. micron) of the brain were examined after chronic implantation of a cranial window in fifteen anaesthetized cats. 2. Application of vasopressin resulted in a dose-dependent contraction. The threshold concentration for contraction was 3 X 10(-10) M, the half-maximal effective concentration (ED50) (1.6 +/- 0.2) X 10(-9) M, and the maximum reduction in artery diameter 37 +/- 2%. 3. The contraction was powerfully inhibited by perivascular application of a 10(-7) M solution of the vasopressin antagonist, [1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid),2-(O-methyl)tyrosine]arginine vasopressin. 4. Perivascular application of noradrenaline induced a dose-dependent contraction of the pial artery. The ED50 was (8.9 +/- 2.5) X 10(-7) M, and the maximum reduction in artery diameter was 33 +/- 2%. 5. Such noradrenaline-induced contraction was not modified at all in the presence of a subthreshold dose (2 X 10(-10) M) of vasopressin (P greater than 0.05, for the over-all difference in size of the contraction, ED50 and maximum contraction). 6. In another experimental setting it was also found that neither the subthreshold nor a suprathreshold (10(-9) M) dose of vasopressin modified the contraction induced by 10(-6) M-noradrenaline (P greater than 0.05, compared to the contraction in the absence of vasopressin). 7. Thus a powerful and sensitive contractile response of the pial arteries to perivascularly applied vasopressin was demonstrated. However, the modifying effect of vasopressin on the contraction induced by perivascularly applied noradrenaline was minimal.

Arginine vasopressin causes morphological changes suggestive of fluid transport in rat choroid plexus epithelium

The experiments described herein use an in vitro preparation of choroid plexus to demonstrate that it is a vasopressin-responsive organ by morphologic criteria. Choroid plexus from rats was incubated for one hour in graded concentrations of arginine vasopressin (AVP). Within physiologic range of molar concentration, incubation in vasopressin induced a decrease in basal and lateral spaces in choroid plexus epithelial cells as well as an increase in number of dark cells. The number of cells with basal spaces decreased significantly from 82.7 +/- 9.2 in control tissue to 19 +/- 18 in tissue incubated in 10(-12) M AVP; similarly, the number with lateral cellular spaces decreased from 20 +/- 8.8 to 7.6 +/- 2.2 cells in 10(-10) M AVP. Dark cells increased in number from 3.8 +/- 2.6 in control conditions to 49 +/- 4 with 10(-9) M vasopressin. These data suggest important effects of arginine vasopressin in cerebrospinal fluid (CSF) on choroid plexus, compatible with enhanced fluid transport across choroid epithelial cells.

Potent stimuli for vasopressin release, hypertonic saline and hemorrhage, cause antipyresis in the rat

Two potent stimuli for AVP release into the blood, hemorrhage and hypertonic saline, were evaluated for their antipyretic effects in the rat. Hemorrhage of 20% of estimated blood volume reduced brain temperature of febrile but not afebrile rats confirming earlier research in the sheep. Hypertonic saline was also antipyretic in the rat. Hypertonic urea was somewhat less antipyretic whereas hypertonic glucose had no effect on febrile temperatures. AVP release into the peripheral circulation showed the relationship saline greater than urea greater than glucose and parallelled the antipyretic effectiveness of these solutes. The antipyresis caused by hypertonic saline was not significantly different in rats passively immunized intravenously with AVP antiserum than in rats which received hypertonic saline alone. These results provide indirect evidence that endogenous AVP is released in the brain following hemorrhage or hypertonic challenge and that this endogenous AVP can affect central febrile pathways.

Septal release of vasopressin in response to osmotic, hypovolemic and electrical stimulation in rats

The central release of vasopressin was studied in anesthetized rats using push-pull perfusions and radioimmunoassay of the hormone. A basal release was observed in the lateral septum and in the lateral ventricle, whereas no vasopressin was detected in the perfusates from the caudate nucleus. Under osmotic stimulation, vasopressin release increased up to 12 and 60 times basal levels following i.p. injections of 5 ml and 10 ml/kg b.wt. of 2 M NaCl, respectively. This increase was blocked by using a calcium-free perfusion medium containing 0.1 mM EGTA. In the lateral ventricle, osmotic stimulation (5 ml/kg of 2 M NaCl i.p.) had the same effect as in the septum. In the caudate nucleus, no release was observed. Hemorrhage also increased the septal release of vasopressin in 5 out of 6 animals tested. Electrical stimulation of the pituitary stalk and of the supraoptic nucleus was used to evoke the release of vasopressin into the bloodstream. Septal release slightly decreased during pituitary stalk stimulation, whereas it did increase during stimulation of the supraoptic region. Our results show that systemic stimuli for vasopressin release evoke both a peripheral and a septal release of the hormone. The dissociation of the effects of electrical stimulation of the pituitary stalk and of the supraoptic nucleus suggests, however, that the vasopressinergic neurones responsible for septal release are distinct from those which project to the neurohypophysis.

Studies of vasopressin in the human cerebrospinal fluid

The development of sensitive radioimmunoassays has permitted measurement of the low concentration of vasopressin in the human cerebrospinal fluid. There is accumulating evidence to suggest that vasopressin is involved in a variety of brain functions. As an effective blood-cerebrospinal fluid barrier to vasopressin has been demonstrated, the concentration of vasopressin in the cerebrospinal fluid probably reflects the release of vasopressin within the brain. In human subjects without intracranial disease, the concentration of vasopressin in the cerebrospinal fluid is in the range 0.5-2.0 pg/ml with only little diurnal variation. Intracranial disorders associated with increased intracranial pressure may cause increased cerebrospinal fluid vasopressin concentrations, whereas degenerative brain diseases are associated with low concentrations. Only little is known about the physiologic stimuli which alter the concentration of vasopressin in cerebrospinal fluid. The concentration in cerebrospinal fluid is not influenced by a number of stimuli that cause release of vasopressin into the blood, i.e. changes in plasma osmolality, postural changes, and nausea. Elevation of the intracranial pressure, changes in the composition of the cerebrospinal fluid, electrical stimulation of the hypothalamus, and severe hemorrhage provoke an increase in cerebrospinal fluid vasopressin level.

Hypothalamic knife cuts alter vasopressin induced recovery of blood pressure following hemorrhage

Effects of knife cuts posterior to the paraventricular nucleus (PVN) alone or to both the PVN and the supraoptic nucleus (SON) on vasopressin dependent restoration and maintenance of blood pressure following hemorrhage were tested in the rat. Conscious, unrestrained animals were hemorrhaged a volume equivalent to 1.8% of body weight from a femoral arterial catheter. Blood pressure was monitored for 30 min with no treatment, 30 min following iv injection of a specific antagonist to the pressor action of vasopressin, and 15 min during iv infusion of the competitive blocker of angiotensin II, saralasin. Restoration of blood pressure and the decrease in blood pressure with vasopressin blockade in rats with knife cuts posterior to the PVN alone were similar to that of control-operated animals. However, if knife cuts extended to the level of the SON, blood pressure was not restored, and vasopressin blockade did not result in a reduction of blood pressure. Saralasin infusion produced a similar decrease in blood pressure in all groups of animals. These data show that when knife cuts are confined to the area posterior to the PVN, vasopressin contributes to the restoration of blood pressure following hemorrhage. However, when cuts extend into the ventral hypothalamus, the contribution of vasopressin is eliminated.

CSF vasopressin rhythm is effectively insulated from osmotic regulation of plasma vasopressin

By using our method for continuous removal of cisternal cerebrospinal fluid (CSF) and intermittent sampling of blood from unanesthetized freely moving cats, we investigated the effect of osmotic-induced changes in plasma vasopressin on the daily rhythm of CSF vasopressin. Examination of the daily profiles of vasopressin and osmolality in the CSF and plasma of six euhydrated animals showed that CSF vasopressin concentrations exhibit a clear daily rhythm, whereas CSF osmolality and plasma vasopressin and osmolality do not exhibit such daily variation. A 48-h period of water deprivation caused marked sustained elevations in plasma vasopressin concentrations, which returned to basal levels on rehydration. In contrast, water deprivation had only a small effect on the CSF vasopressin rhythm. Although there was a significant elevation of the normally low nighttime CSF vasopressin levels during water deprivation in three of the four animals studied, high daytime vasopressin levels were unaltered and the daily rhythm was clearly evident before, during, and after the period of water removal in all animals. Changes between plasma vasopressin and osmolality were significantly correlated in all animals. Changes between plasma and CSF osmolality were significantly correlated in three of the four animals. The data indicate that the circadian regulation of the CSF vasopressin rhythm is effectively insulated from the osmotic regulation of plasma vasopressin.

Vasopressin causes endothelium-dependent relaxations of the canine basilar artery

The effect of synthetic 8-arginine vasopressin (vasopressin) was studied in isolated canine basilar, left circumflex coronary, and femoral arteries of the dog. Vascular rings with and without endothelium were suspended for isometric tension recording in physiological salt solution. The removal of the endothelium was confirmed by the absence of relaxations induced by either thrombin (basilar arteries) or acetylcholine (coronary and femoral arteries). In the basilar artery, vasopressin induced concentration-dependent inhibition of myogenic tone. In basilar and coronary arteries, the hormone caused concentration-dependent relaxations during contractions evoked by prostaglandin F2 alpha. In femoral arteries, vasopressin caused contraction. After removal of the endothelium, the inhibitory responses to vasopressin were abolished in basilar arteries and significantly reduced in left circumflex coronary arteries. The contractions of femoral arteries were not affected by endothelium removal. The V1-vasopressinergic antagonist d(CH2)5Tyr(Me)AVP prevented the inhibitory response to vasopressin, but did not alter endothelium-dependent relaxations of basilar arteries caused by adenosine diphosphate. These results demonstrate that the endothelial cells mediate relaxation induced by vasopressin via specific V1-vasopressinergic receptors.

Vasopressin in plasma and cerebrospinal fluid of dogs during hypoxia or acidosis

Hypoxia and hypercapnia have been shown to cause an increase in the concentration of vasopressin in plasma, but their effects on vasopressin in cerebrospinal fluid (CSF) are not known. In addition, the effect of metabolic acidosis on plasma and CSF vasopressin has not been reported. In this study, plasma and CSF vasopressin levels were measured in anesthetized dogs subjected to either hypoxia, hypercapnia, or metabolic acidosis. Rate and depth of respiration were closely regulated with the aid of muscle paralysis and mechanical ventilation. Vasopressin increased markedly in both plasma and CSF during severe hypoxia (10% O2) and during hypercapnia (10% CO2) but did not change during either mild (15% O2) or moderate (12.5% O2) hypoxia. Although mild hypoxia by itself did not affect either plasma or CSF vasopressin, it did potentiate the increase in plasma and CSF vasopressin that was induced by severe hypercapnia, thus suggesting that hypoxia and hypercapnia may exert synergistic effects on vasopressin secretion. Metabolic acidosis produced by slow intravenous infusion of 1 N hydrochloric acid decreased arterial pH to values comparable to those induced by hypercapnia and increased vasopressin in plasma; CSF vasopressin was unchanged. These results are consistent with the concept that the source of vasopressin secreted into plasma may be different from that secreted into CSF.

Plasma and cerebrospinal fluid vasopressin and osmolality in relation to thirst

Conscious dogs chronically implanted with a device for cerebrospinal fluid (CSF) sampling from the anterior 3rd ventricle were submitted to 24 h dehydration. During rehydration by drinking the total water intake ( TWI ) after 16 min was determined in 8 and after 90 min in 14 experiments. Samples were simultaneously drawn to determine the osmolalities (Posm, CSFosm ) and AVP concentrations (PAVP, CSFavp ) of plasma and CSF. After 24 h dehydration all of these parameters were significantly elevated in comparison to euhydrated dogs investigated on 19 occasions. In 8 experiments 60% of the final TWI had been ingested within the first 16 min with no changes of Posm, CSFosm and CSFAVP , but a significant decrease of PAVP at this time. TWI per kg body weight ( TWI X kg-1) after 90 min was significantly correlated with the osmolalities and AVP levels in plasma and CSF prior to rehydration. The decreases of Posm, CSFosm and PAVP, but not of CSFAVP , were significantly correlated with TWI X kg-1. The results indicate that PAVP and CSFAVP are subject to long term control by body fluid tonicity exhibiting a feedback relationship to water intake. In addition, PAVP but not CSFAVP seems to be under short term, possibly nonosmotic, control during water intake.

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.

Increased motor disturbances in response to arginine vasopressin following hemorrhage or hypertonic saline: evidence for central AVP release in rats

The effects of hemorrhage and parenteral hypertonic saline on the behavioural responses to centrally-administered arginine vasopressin (AVP) were examined in rats. Both hemorrhage and hypertonic saline act as potent stimuli for neurohypophysial vasopressin release, and may serve as potential stimuli for cerebral AVP release. When administered into a lateral cerebral ventricle of the rat brain, AVP has a potent convulsant action; this effect increases in severity upon subsequent administration. Removal of 15% of the estimated blood volume from the conscious rat or infusion of 1.0 ml of 1.5 M sodium chloride solution into the peritoneal cavity can mimic the effect of a central injection of AVP in 'sensitizing' the brain to the behavioural effects of subsequent injections of AVP. This suggests that these stimuli which are known to activate posterior pituitary secretion of AVP also induce the release of AVP (or a closely related molecule), from neuronal fibres within the brain.

Vasopressin is important for restoring cardiovascular homeostasis in fetal lambs subjected to hemorrhage

To determine if the posterior pituitary hormone vasopressin is important for maintaining fetal cardiovascular homeostasis during hypovolemic stress, in seven chronically catheterized fetal lambs we induced hemorrhage of 20% of estimated blood volume in the presence and in the absence of a potent antagonist to the pressor effects of vasopressin. The study was a paired crossover design with at least 48 hours separating experiments in the same animal. Injection of the vasopressin antagonist did not alter basal fetal heart rate or arterial blood pressure, but hemorrhage of 2% of estimated fetal blood volume per minute for 10 minutes produced a greater fall in blood pressure (13 +/- 2 versus 10 +/- 2 torr, p less than 0.05) when the blocker was present than when it was absent. Arterial blood pressure remained below control levels longer following hemorrhage when the fetuses were pretreated with the antagonist (49.7 +/- 6 versus 26.6 +/- 6 minutes, p less than 0.01), and the integrated fall in arterial blood pressure with hemorrhage was greatest (283 +/- 53 versus 169 +/- 57 mm Hg . min p less than 0.01) when the blocker was used. The fall in heart rate following hemorrhage was similar with and without blocker pretreatment. These results indicate that vasopressin plays a physiologic role in blood pressure regulation in fetal lambs during periods of hypovolemia.

Centrally mediated effects of neurohypophyseal hormones

The neurohypophyseal hormones oxytocin and vasopressin cause a variety of biological effects in animals which are mediated by central nervous system mechanisms. Among the best studied of these effects is the modulation of both memory processes and the development of drug tolerance and dependence. Neurohypophyseal hormones have also been shown to alter various physiological parameters such as heart rate and body temperature following central administration. In addition, these peptides can profoundly alter spontaneous, unlearned behavior in several rodent species. Many of the centrally mediated effects of neurohypophyseal hormones have been shown to be elicited at sites within the brain stem and the limbic system where vasopressin and oxytocin occur in cell bodies, axons and nerve terminals, suggesting a physiological role for these peptide effects. The various central effects of neurohypophyseal hormones involve different mechanisms which can be distinguished from one another on the basis of required dose, time-course of action, and structure-activity relationships. Thus, alterations of spontaneous behavior are mediated by putative receptors closely related to vasopressin receptors in blood vessels responsible for the peripheral pressor response while the effects on memory processes are mediated by a mechanism which is not closely related to those involved in the peripheral hormonal effects of the peptides. The influence of neurohypophyseal hormones on memory and attention may be useful clinically. A potential role for these peptides in mental disorders is discussed.

A daily vasopressin rhythm in rat cerebrospinal fluid

Cerebrospinal fluid (CSF) was serially withdrawn in individual, unanesthetized, unrestrained rats and assayed for vasopressin using a sensitive and specific radioimmunoassay. A prominent daily rhythm in the CSF concentrations of this peptide was found under diurnal lighting conditions. Low levels during the dark period alternated with high values during the light period; the rhythm appeared to anticipate the artificial 'dawn' and 'dusk' by a few hours. An 8-h phase shift in diurnal lighting caused a corresponding phase shift in the CSF rhythm. In addition, the rhythm persisted for at least 10 days in the absence of periodic environmental lighting cues in animals blinded by bilateral orbital enucleation; the rhythm was disrupted after 10 days of constant light. Blood vasopressin concentrations did not show a daily rhythm. Our results indicate that the daily vasopressin rhythm in rodent CSF is endogenously generated and that its phase is synchronized to the environmental light-dark cycle.

Response of brain and pituitary pools of dynorphin as compared to vasopressin to acute stress in the rat

5 min foot-shock resulted in an elevation in levels of immunoreactive (ir)-dynorphin (DYN) and ir-vasopressin (VP) in the hypothalamus whereas their levels were not significantly modified in the neurointermediate pituitary. In the anterior pituitary, however, a fall in ir-DYN in the absence of any change in ir-VP was detected. Further, in the thalamus and medulla/pons, respectively, a decrease and increase in levels of ir-VP was observed whilst the content of ir-DYN therein was not significantly affected. These findings support the concept of a common origin and co-modulation of DYN and VP in the hypothalamic-neural lobe axis and that, extrinsic to this, DYN and VP are localized and modulated independently of each other.

Neurohypophysial hormones and brain function: the neurophysiological effects of oxytocin and vasopressin

An increasing body of evidence suggests that the neurohypophysial hormones, in addition to their classical actions, may also act as neurotransmitters. They have a widespread but discontinuous distribution in the CNS; apart from their presence in the magnocellular nuclei they may be found in the hippocampus, amygdala, septum, substantia nigra, brainstem and spinal cord. They exert profound effects on behavior, particularly on memory, a function frequently ascribed to the hippocampus, amygdala and septum; on memory consolidation, internal reward and self stimulation functions frequently ascribed to brainstem and diencephalic aminergic systems including the substantia nigra and on sensory and autonomic responses which involve the medulla and spinal cord. When applied to the CNS they alter multiple unit activity in certain regions, particularly the hippocampus and cells which contain neural lobe hormones appear to be able to drive other cells synaptically. Finally application of the hormones can profoundly affect the activity of single nerve cells in just those parts of the CNS where, on the basis of their behavioral actions, they might be expected to act.

Central infusion of vasopressin decreased plasma vasopressin concentration in dogs

The effects of increasing the cerebrospinal fluid (CSF) vasopressin concentration (CSFADH) by intracerebroventricular infusion of vasopressin on the plasma vasopressin concentration (PADH) were studied in four groups of anesthetized dogs. One group received an intracerebroventricular infusion of artificial CSF (ACSF) alone for 90 min; the other groups were infused intracerebroventricularly with vasopressin at rates of 10, 20, or 50 microunits/min for 90 min. Arterial blood and CSF samples were taken just before infusion and at 30-min intervals for 210 min. Vasopressin infused intracerebroventricularly at 10, 20, and 50 microunits/min resulted in peak CSFADH of 32.2 +/- 5.3, 82.6 +/- 4.5, and 131.4 +/- 12.5 microunits/ml and reductions in PADH of 32, 47, and 51%, respectively. Only the latter two responses were significant (P less than 0.5-0.01). Because the peak increases in CSFADH after intracerebroventricular infusion of vasopressin ranged from values that were similar to or five times higher than those seen after severe hemorrhage or intracerebroventricular hypertonic saline infusion, we suggest that centrally acting vasopressin may play a physiological role in control of vasopressin secretion.

Central neurohypophyseal peptide pathways: interactions with endocrine and other autonomic functions

Electrophysiological and pharmacological studies have been carried out in rats and rabbits to attempt to identify possible functional roles for neurohypophyseal peptides in brain. In anesthetized rats, single unit recordings and antidromic activation criteria were utilized to identify projections of the paraventricular nucleus (PVN) to neurohypophysis and to extrahypothalamic areas (amygdala or nucleus tractus solitarius). None of the cells tested innervated more than one of these areas and, when tested for their responses to haemorrhage, increased body osmolarity, or suckling of pups, only the osmotic stimulus caused increased activity in some cells projecting to amygdala or nucleus tractus solitarius. Indirect evidence as well as direct measurement by radioimmunoassay of arginine vasopressin (AVP) in brain perfusates revealed probable central release of AVP in response to stimuli known to activate pituitary secretion of this peptide. These observations raise the possibility that certain brain and pituitary peptidergic systems may function in a co-ordinated manner.

Cerebrospinal fluid vasopressin and oxytocin: evidence for an osmotic response

A push-pull perfusion technique was used to sample third ventricular cerebrospinal fluid (CSF) in the conscious rat. Ventricular perfusate contained significant quantities of vasopressin and oxytocin, 13.1 and 4.5 pg/ml, respectively. Vasopressin and oxytocin levels were stable over a 3 h perfusion period. High sodium artificial CSF elicited a 4-5-fold stimulation of perfusate concentrations of both peptides. These findings suggest that the CSF represents a dynamic compartment in terms of neuroendocrine function.