A specific deficiency in paraventricular vasopressin and oxytocin in the spontaneously hypertensive rat

Vasopressinergic hypothalamic neurons are recruited during the audiogenic seizure of WARs

The Wistar Audiogenic Rat (WAR) is a genetic model of reflex epilepsy with seizures induced by high-intensity sound stimulation (120 dB SPL). In spite of the known neural substrates involved in WAR seizure phenotype, neuroendocrine hypothalamic neurons were never investigated. In this work, AVP immunohistochemistry in the hypothalamus and radioimmunoassay (RIA) in plasma and in hypothalamic and hypophysial tissues were performed on both controls and WARs in order to evaluate the dynamics of AVP release due to seizure induction. Susceptible animals (WARs) displayed at least tonic-clonic convulsions followed by clonic spasms, while resistant Wistar rats (R) had no convulsive behavior. Animals were sacrificed at 3 instances: basal condition (without stimulus) and at 3 and 10 min after sound stimulation. For the immunohistochemistry AVP study, brains were harvested and processed by the avidin-biotin-peroxidase detection method. Optic densitometry was used for quantifying AVP labeling in supraoptic (SON) and paraventricular (PVN) hypothalamic nuclei. SON presented higher densitometry levels (%D--relative to background) for both WARs and R when compared to PVN. Nevertheless, both nuclei presented a marked decrease, referenced to basal levels, in %D for WARs at 3 min (approximately 35%) against a discrete change for R (approximately 90%). RIA results were significantly higher in the hypophysis of WARs when compared to R rats, at 3 min. Also, at 3 min, plasma AVP in WARs (89.32 +/- 24.81 pg/mL) were higher than in R (12.01 +/- 2.39 pg/mL). We conclude, based on the AVP releasing profiles, that vasopressinergic hypothalamic neurons are recruited during the audiogenic seizure of WARs.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Increased pressor function of central vasopressinergic system in hypertensive renin transgenic rats

OBJECTIVE

Renin transgenic hypertensive rats [TGR(mRen2)27] have increased contents of angiotensin II and arginine vasopressin (AVP) in the cardiovascular brain regions. The aim of the present study was to evaluate the effects of centrally released AVP on the regulation of baseline blood pressure in TGR(mRen2)27 rats and to determine the interaction between AVP and angiotensin II in the central control of blood pressure in this model of hypertension.

DESIGN

Three basic series of experiments were performed on 20 TGR(mRen2)27 and 20 Hannover Sprague-Dawley conscious rats, chronically instrumented with lateral cerebral ventricle (LCV) cannulae and femoral artery catheters. In series 1, blood pressure and heart rate were recorded during an LCV infusion of artificial cerebrospinal fluid before and after LCV administration of angiotensin II. In series 2, the effects of an LCV administration of angiotensin 11 (100 ng) on mean arterial pressure and the heart rate were determined during LCV infusion of a selective AVP receptor (V1) antagonist [1-(1-mercapto-4-methylcyclohexaneacetic acid)-8-arginine vasopressin (MeCAAVP) and d(CH2)5[Tyr(Me)2,Ala-NH2(9)]AVP] or a selective angiotensin II type 1 (AT1) receptor antagonist (losartan) or both. In series 3, mean arterial pressure and the heart rate were determined after an LCV injection of either AVP (10 ng) or AVP together with angiotensin II.

RESULTS

The LCV infusions of antagonists to V1 and AT1 receptors caused significant comparable decreases in baseline MAP in TGR(mRen2)27 but not in Sprague-Dawley rats. Angiotensin II elicited significant pressor responses, both in TGR(mRen2)27 and in Sprague-Dawley rats. Blockade of V1 receptors significantly reduced the duration and the maximum amplitude of the central pressor response to angiotensin II in TGR(mRen2)27 rats, whereas in Sprague-Dawley rats the maximum pressor effect was not significantly altered. In both strains, the pressor response to angiotensin II was abolished by blockade of AT1 receptors.

CONCLUSIONS

The results indicate that the elevated blood pressure in TGR(mRen2)27 rats is partly caused by increased function of the brain angiotensinergic AT1 and vasopressinergic V1 systems. Centrally released AVP is involved in mediation of the pressor effect exerted by centrally applied angiotensin II in TGR(mRen2)27 rats.

Upregulation of renal and vascular nitric oxide synthase in young spontaneously hypertensive rats

The available data on the role of the L-arginine/nitric oxide (NO) pathway in the genesis of hypertension in spontaneously hypertensive rats (SHR) are limited and contradictory. In an attempt to address this issue, male SHR were studied during the early phase of evolution of hypertension (age 8 to 12 weeks) to distinguish the primary changes of NO metabolism from those caused by advanced hypertension, vasculopathy, and aging late in the course of the disease. A group of age-matched male Wistar-Kyoto rats (WKY) served as controls. The SHR exhibited a marked rise in arterial blood pressure and a significant increase in urinary excretion and plasma concentration of NO metabolites (nitrite/nitrate [NOx]). Likewise, the SHR showed a significant elevation of thoracic aorta NO synthase (NOS) activity coupled with significant increases of kidney, aorta, inducible NOS (iNOS), and endothelial NOS (eNOS) proteins. In an attempt to determine whether the enhanced L-arginine/NO pathway is a consequence of hypertension, studies were repeated using 3-week-old animals before the onset of hypertension. The study revealed significant increases in urinary NOx excretion as well as vascular eNOS and renal iNOS proteins. In conclusion, the L-arginine/NO pathway is upregulated in young SHR both before and after the onset of hypertension. Thus, development of hypertension is not due to a primary impairment of NO production in SHR. On the contrary, NO production is increased in young SHR both before and after the onset of hypertension.

Correction of hypertension by normalization of endothelial levels of fibroblast growth factor and nitric oxide synthase in spontaneously hypertensive rats

Acidic and basic fibroblast growth factors (FGFs) share a wide range of diverse biological activities. To date, low levels of FGF have not been correlated with a pathophysiologic state. We report that blood vessels of spontaneously hypertensive rats are shown to be associated with a marked decrement in endothelial basic FGF content. This decrement correlates both with hypertension and with a decrease in the endothelial content of nitric oxide synthase. Restoration of FGF to physiological levels in the vascular wall, either by systemic administration or by in vivo gene transfer, significantly augmented the number of endothelial cells with positive immunostaining for nitric oxide synthase, corrected hypertension, and ameliorated endothelial-dependent responses to vasoconstrictors. These results suggest an important role for FGFs in blood pressure homeostasis and open new avenues for the understanding of the etiology and treatment of hypertension.

Effects of beta 1- and beta 2-adrenoceptor agonists applied into the hypothalamic paraventricular nuclei of spontaneously hypertensive rats on urine production

We investigated effects of beta-adrenoceptor agonists (beta 1-selective: T-1583 and dobutamine, beta 2-selective: fenoterol, non-selective: isoproterenol) on urine outflow rate, blood pressure, heart rate, respiratory rate and rectal temperature. The drugs were applied into the paraventricular nuclei (PVN) of spontaneously hypertensive (SHR), Wistar-Kyoto (WKY) and Wistar rats. Fenoterol and isoproterenol markedly decreased the urine outflow rate, compared with T-1583 and dobutamine in the rats. There was no marked difference among the three strains in responsiveness to fenoterol and isoproterenol. The antidiuretic effects of fenoterol were inhibited by a beta 2-selective antagonist, butoxamine, more markedly than a beta 1-selective antagonist, atenolol, in SHR; and the inhibitory effects of these drugs were partial in WKY. In Wistar rats, the effect of fenoterol was inhibited by a non-selective beta-antagonist, timolol, but not by atenolol or butoxamine. A vasopressin antagonist (i.v.) did not diminish the antidiuretic effect of fenoterol. Fenoterol reduced the blood pressure in SHR and WKY, but not in Wistar rats. It was suggested that there were predominantly beta 2-adrenoceptors mediating antidiuresis in SHR. In WKY and Wistar rats, however, the beta-adrenoceptor subtypes mediating antidiuresis have yet to be determined. The ability of beta-adrenoceptor agonists to decrease urine outflow rates in SHR was not altered as compared to that in the control rats. beta-Adrenoceptor-mediated antidiuresis was not due to vasopressin release.

Relationships among cardiovascular, muscular, and oxytocin responses during human sexual activity

To determine the psychophysiological correlates of hormonal response during sexual activity, systolic blood pressure (SBP), anal electromyography (EMG), and anal photoplethysmography (APG) were monitored continuously throughout testing in 13 women and 10 men. Each subject completed two or more tests of self-stimulation to 5 min beyond orgasm. Blood samples were obtained continuously for measurement of oxytocin (OT) levels. In both men and women, very high positive correlations were observed between the percentage change in levels from baseline through orgasm of: OT and SBP; OT and EMG intensity prior to and during orgasm; APG and EMG. The number of anal contractions and duration of orgasm were also highly correlated. Two patterns of orgasm were defined by the presence or absence of a quiescent period between orgasmic contractions. EMG and APG amplitudes correlated with the pattern of orgasm. Subjective orgasm intensity correlated significantly with increased levels of OT in multiorgasmic women only. The positive correlations between measures are consistent with a possible functional role for OT in human sexual response.

A rapid and sensitive radioimmunohistochemical assay for quantitation of vasopressin in discrete brain regions with an anatomical resolution

Radioimmunoassay has become a widely used method to study different neuroactive substances from brain tissue extracts, but cannot provide anatomical resolution. Here we describe a simple and sensitive radioimmunohistochemical assay (RIHA) to quantify a peptide, vasopressin (VP), in discrete brain regions of rats with 3-day water deprivation. After decapitation, brains were removed, frozen with dry ice and cut into 14-microns cryostat sections which were then fixed with 4% paraformaldehyde in phosphate-buffered saline. After rinses, tissue sections were stored in a freezer until use. For RIHA, brain tissue sections were pre-incubated, and then incubated with rabbit vasopressin antibody (1:2000 dilution) for 24 h at room temperature. After rinses, sections were incubated with 125I-labeled goat antirabbit IgG (1:200 dilution) for 1 h. Specimens were processed for quantitative autoradiography after rinses and drying. RIHA with aid of a computer-assisted image analysis system revealed that the VP content was significantly reduced in the paraventricular hypothalamic nucleus (PVN) and supraoptic nucleus (SON) of rats with 3-day water deprivation, whereas a parallel in situ hybridization study further demonstrated that VP mRNAs in the PVN and SON were greatly increased. In summary, this experiment demonstrates that RIHA is a simple and powerful tool which is able to detect changes of VP in the hypothalamus of dehydrated rats. Combining this method with in situ hybridization to assess mRNA expression allows assessment of the functional significance of the peptide changes. In this case, dehydration depletes vasopressin and upregulates its synthesis. Therefore, the combined use of RIHA and in situ hybridization should have general applicability to evaluate the functional role of a peptide or neurotransmitter system in response to stimuli in a quantitative way with anatomical resolution.

Neuropeptide Y produces anxiolytic effects in spontaneously hypertensive rats

The sedative and anxiolytic effects of intracerebroventricular administration of neuropeptide Y (NPY) were studied in spontaneously hypertensive rats (SHR) and in normotensive Wistar-Kyoto (WKy) rats using the two-compartment exploratory test, and in the open-field test after habituation. In the two-compartment tests, NPY produced anxiolytic effects by increasing the exploratory activity in SHR at a dose (0.25 nmol) lower than the minimal effective dose in WKy rats (1.25 nmol). In SHR, anxiolytic effects were observed for the whole NPY dose range (0.25-5.0 nmol), whereas in normotensive WKy rats the highest dose (5.0 nmol) failed to increase exploratory activity. The open-field test showed reduced locomotor activity and rearings in WKy rats when injected with 5.0 nmol NPY. These effects were not observed in SHR. The absence of sedative effects and the higher sensitivity to the anxiolytic effects of NPY in SHR are suggestive of a genetically determined difference in central NPY systems involved in behavioral adaptation that may be relevant for the development of hypertension.

Kindling in spontaneous hypertensive rats

Vasopressin is a neurohormone and neuromodulator with many effects on behavior. Rats lacking vasopressin have been found to develop kindled seizures more slowly with amygdala stimulation. In the present study the spontaneous hypertensive (SH) rat and rats from the parent strain, the Wistar-Kyoto (WKY) rat received amygdala and pyriform kindling. The SH rat has been reported to have increased plasma vasopressin and increased brain vasopressin release. Plasma vasopressin, osmolality and hematocrit were also measured in blood samples obtained through chronic, indwelling catheters implanted in SH, WKY normal and Sprague-Dawley rats. SH rats were found to kindle with fewer afterdischarges than WKY normal rats with both amygdala and pyriform cortex stimulation. The total afterdischarge duration required to reach each kindling stage was significantly shorter in the SH rat. Plasma osmolality and vasopressin were significantly higher in the SH rats compared to WKY normal rats and Sprague-Dawley rats. These findings provide additional evidence that vasopressin may influence the establishment of enduring behaviors such as kindled seizures.

Antihypertensive effect of vasopressin withdrawal in young and adult spontaneously hypertensive rats

The extent to which age influences the effect of prolonged intravenous infusion and withdrawal of arginine vasopressin (AVP) on blood pressure was investigated in spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto controls (WKY) of 6-, 10-, 14-, 18-, and 22-week age groups. The pressor response to AVP (20 ng/kg/min for 3 hours) was relatively well maintained in WKY but showed an age-dependent tachyphylaxis in SHR. After cessation of the infusion, arterial pressure of SHR fell in all age groups. In contrast, withdrawal of AVP in WKY resulted in little or no hypotensive response. Thus, a withdrawal-induced antihypertensive phenomenon (WAP) to AVP was specific to SHR. The magnitude of the WAP was significantly correlated with the level of initial blood pressure in SHR (r = -0.81, p less than 0.001). The magnitude of the tachyphylaxis during the AVP infusion was also correlated with the level of initial blood pressure in SHR (r = -0.66, p less than 0.001). Accordingly, a significant correlation was found between the magnitude of the WAP and the degree of tachyphylaxis to the pressor activity of AVP in SHR (r = 0.69, p less than 0.001). The significance of this is unknown, but it might mean that a common underlying mechanism existed in the expression of the tachyphylactic phenomenon and the WAP in SHR. Finally, an apparent enhancement in the baroreceptor reflex sensitivity was observed in both SHR and WKY during the infusion of AVP, but the magnitude of this enhancement appeared to be greater in SHR than in WKY.(ABSTRACT TRUNCATED AT 250 WORDS)

In situ hybridization demonstrates vasopressin gene transcription in hypothalamic neurons of crossbred hypertensive x diabetes insipidus rats

In situ hybridization, Northern blotting, and solution hybridization assay were used to examine vasopressin-gene messenger ribonucleic acid (mRNA) transcripts in hypothalamic tissue from five strains of rats: Long-Evans, Wistar-Kyoto, and diabetes insipidus (Brattleboro) rats, spontaneously hypertensive-stroke-prone rats, and cross-bred diabetes insipidus x spontaneously hypertensive-stroke-prone rats. A 290 base-pair, single-stranded RNA probe, with 221 bases complementary to exon C of the vasopressin gene was synthesized by the SP6 transcription vector system. This probe labeled appropriate neurohypophysial hypothalamic neurons, as well as suprachiasmatic nucleus cells, in tissue samples from all five strains of rats. Confirming other recent hybridization results with diabetes insipidus rat brain tissue, diabetic animals were found to transcribe their mutated vasopressin gene. In addition, this investigation found that the cross-bred diabetic-hypertensive rat also synthesizes a vasopressin-gene messenger ribonucleic acid transcript. Quantitative analyses of solution and in situ hybridization results suggested the cross-bred diabetic-hypertensive rat exhibits a level of vasopressin-gene messenger ribonucleic acid similar to diabetes insipidus rats. This observation is consistent with previous data on the cross-bred diabetic-hypertensive rat which suggests they inherit the mutant vasopressin gene from Brattleboro rats.

Abnormalities in hypothalamic and neurohypophysial vasopressin content are not a consequence of hypertension in the spontaneously hypertensive rat

In order to determine if the decreased hypothalamic and increased posterior pituitary content of vasopressin (VP) observed previously in spontaneously hypertensive rats (SHR) were a secondary consequence of the hypertension, the effect of preventing the development of hypertension on VP content of the hypothalamoneurohypophyseal system was evaluated. Two methods for preventing the hypertension were used: (1) chronic angiotensin-converting enzyme inhibition (oral captopril, 100 mg/kg/day at 4-12 weeks of age); and (2) intraventricular 6-hydroxydopamine (6-OHDA, 200 micrograms at 4 and 5 weeks of age). Both of these treatments markedly attenuated the increase in systolic blood pressure in SHRs at 5-11 weeks of age. The captopril-treated rats had a significant elevation in serum renin activity at 12 weeks of age indicating the presence of chronic converting enzyme inhibition, and the 6-OHDA-treatment resulted in a depletion of hypothalamic (86%) and brainstem (76%) norepinephrine content. Hypothalamic VP content was reduced in untreated SHRs compared to normotensive Wistar-Kyoto rats (WKYs, P = 0.0015). It was not significantly altered in either strain by the 6-OHDA treatment. Captopril caused a reduction in hypothalamic VP content in both SHRs and WKYs (P less than 0.01). Posterior pituitary VP content was elevated in untreated SHRs compared to WKYs (P less than 0.001), and remained elevated with captopril and 6-OHDA treatments. These data indicate that the abnormalities in VP content in the hypothalamus and posterior pituitary of SHRs are not a response to the hypertension. Therefore, they may represent primary abnormalities in the SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin gene expression in hypertensive, normotensive, and diabetes insipidus rats

This investigation examined the presence and abundance of vasopressin-gene messenger ribonucleic acid (mRNA) transcripts in hypothalamic tissue from five strains of rats: Long Evans, Wistar-Kyoto, and diabetes insipidus (Brattleboro) rats, stroke-prone spontaneously hypertensive rats, and cross-bred diabetes insipidus x stroke-prone spontaneously hypertensive rats. A single-stranded RNA probe complementary to exon C of the vasopressin gene was utilized for in situ hybridization and identified hypothalamic 'vasopressinergic' neurons in tissue from all five strains of rats. The results obtained by solution and in situ hybridization suggested the cross-bred diabetic-hypertensive rat exhibits a level of vasopressin-gene messenger ribonucleic acid similar to diabetes insipidus rats. This observation is consistent with previous physiological data which suggests cross-bred diabetic-hypertensive rats inherit the mutated vasopressin gene of the Brattleboro rat.

Osmotic regulation of vasopressin and renin in spontaneously hypertensive rats

Abnormalities in the vasopressin and renin systems have been reported in spontaneously hypertensive rats (SHR). Therefore, studies were performed to evaluate the responsiveness of these systems to changes in plasma osmolality and sodium concentration. These variables were manipulated in vivo by intraperitoneal administration of distilled water, isotonic saline, or hypertonic saline to 8- and 18-week-old SHR and normotensive Wistar-Kyoto rats (WKY). Animals were decapitated 30 minutes later, and trunk blood was collected. The hypertonic saline injections resulted in an increase in plasma osmolality and serum sodium at both ages (p less than 0.001). Serum vasopressin was higher in all groups of animals receiving hypertonic saline (1200 mosm/kg H2O; p less than 0.05), but the magnitude of increase was not significantly different in the SHR and WKY at either age. Serum renin activity was lower in SHR than in WKY following acute decreases in serum sodium at 8 weeks, but it was the same for both strains at 18 weeks. Both kidney renin content and concentration were lower in SHR than in WKY at 18 weeks but not at 8 weeks. Therefore, the suppressed renin response to acute osmotic challenge in 8-week-old SHR is not the consequence of reduced kidney renin content. The vasopressin response to osmotic stimulation also was evaluated in vitro using hypothalamoneurohypophyseal explants obtained from 5-, 8-, and 18-week-old SHR and WKY.(ABSTRACT TRUNCATED AT 250 WORDS)

A tissue culture model for the study of peptide synthesis and secretion from microdissected hypothalamic explants

Using established tissue culture methodology, a protocol has been developed for the culture of specific hypothalamic regions. A microdissection technique is used to remove the paraventricular and supraoptic nuclear regions from fresh tissue. These explants are maintained in culture and appear to be viable on the basis of several criteria: the continuing presence of peptides in both tissue and media, the ability to incorporate a labeled amino acid precursor into proteins and peptides, the histological appearance using phase contrast microscopy, and the immunocytochemical identification of peptide neurons. This tissue culture model may prove useful in the study of neuroendocrine mechanisms in specific brain regions.

Chronic arthritis in the rat: differential changes in discrete brain pools of vasopressin as compared to oxytocin

Rats inoculated in the tail-base with killed Mycobacterium butyricum developed an arthritic swelling and inflammation of the limbs accompanied by a hyperalgesia to noxious pressure applied thereto. These changes were maximal at 3 weeks and had subsided by 10 weeks post-inoculation. At 3 weeks, arthritic rats manifested an elevation in levels of immunoreactive (ir)-vasopressin (VP) but not ir-oxytocin (OT) in the midbrain. In contrast, ir-OT was increased in the medulla-pons while ir-VP was unaltered therein. These changes had disappeared by 10 weeks. No other brain region displayed changes. Thus, chronic arthritis is associated with selective and reversible effects upon discrete brain pools of ir-VP and ir-OT. The data clearly demonstrate that pools of ir-VP and ir-OT can be modulated independently of each other in particular brain tissues. Whether the changes are produced by, or reflect a functional response to, the pain rather than other characteristic of the arthritis, remains to be determined.

Reduction in brain immunoreactive corticotropin-releasing factor (CRF) in spontaneously hypertensive rats

The brain CRF concentration of spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) was examined by rat CRF radioimmunoassay. Anti-CRF serum was developed by immunizing rabbits with synthetic rat CRF. Synthetic rat CRF was also used as tracer and standard. The displacement of 125I-rat CRF by serially diluted extracts of male Wistar rats hypothalamus, thalamus, midbrain, pons, medulla oblongata, cerebral cortex, cerebellum and neurointermediate lobe was parallel to the displacement of synthetic rat CRF. In both WKY and SHR the highest levels of CRF immunoreactivity were shown by the hypothalamus and neuro-intermediate lobe, and considerable CRF immunoreactivity was also detected in other brain regions. The CRF immunoreactivity in the hypothalamus, neurointermediate lobe, midbrain, medulla oblongata and cerebral cortex was significantly reduced in SHR and it may suggest that CRF abnormality may be implicated in the reported abnormalities in the pituitary-adrenal axis, autonomic response and behavior of SHR.

Cytochrome oxidase activity in the hypothalamus of SHR and normotensive rats before and after fasting

The metabolic activity in the brains of adult spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY), and Wistar (W) rats was assessed before and after fasting using cytochrome oxidase (COX) histochemistry. Before fasting, metabolic activity in the paraventricular (PVH) and supraoptic (SON) nuclei of the hypothalamus in SHR was greater than in control rats. Fasting elicited a decrease in arterial pressure (AP) in SHR and WKY; in SHR the decrease in AP was accompanied by a decrease of metabolic activity in the PVH and SON. The findings of this study support the hypothesis that the PVH and SON are involved in the hypertension and in the increased levels of sympathetic nervous activity and vasopressin production known to occur in SHR. In addition, the PVH and possibly the SON may be involved in the suppression of sympathetic nervous system activity and the lowering of arterial pressure which are associated with fasting.

Cholinergic stimulation of vasopressin release in spontaneously hypertensive rats

Plasma vasopressin (VP) concentration is elevated in spontaneously hypertensive rats (SHRs) relative to their normotensive Wistar-Kyoto (WKY) controls. The possibility that this reflects altered responsiveness of the hypothalamo-neurohypophyseal system (HNS) in SHRs was examined by comparing VP release in response to acetylcholine from organ cultured HNS explants obtained from SHR and WKY donors. Explants were prepared from 5-, 8-, and 18-week-old animals. Blood pressure was significantly elevated in the 8- and 18-week-old SHR donors relative to their age-matched WKY donors. VP release was assessed on the 4th day of culture during a control hour and during the subsequent hour in the presence of acetylcholine. Acetylcholine caused a concentration-dependent stimulation of VP release from both types of explants, but the response was significantly greater in the explants from 5- and 8-week-old SHRs than in explants from age-matched WKYs. The explants from 18-week-old SHRs and WKYs demonstrated comparable sensitivity to acetylcholine. Basal VP release was not significantly different in explants from age-matched SHRs and WKYs, but it did increase with donor age in both strains. These studies indicate potential hyperresponsiveness of the HNS to excitatory stimuli in SHRs during the developmental phase of hypertension. The hyperresponsiveness disappears in the chronically hypertensive phase. Thus, increased sensitivity of the HNS during the development of hypertension may contribute to the elevation of plasma VP concentration in SHRs.

Vasopressin as a putative mediator in SHR hypoalgesia

The purpose of this note is to show that vasopressin might be involved in the hypoalgesia of the spontaneously hypertensive rat. This proposal rests upon published data.

Changes in paraventricular vasopressin and oxytocin during the development of spontaneous hypertension

The potential role of central neuroendocrine changes in the development of spontaneous hypertension was evaluated. The developmental changes in blood pressure and hypothalamic and plasma levels of vasopressin (AVP) and oxytocin (OT) were determined in groups of SHR and WKY animals from 3 to 24 weeks of age. Hypothalamic OT content was significantly lower in 3-, 6-, and 12-week-old SHR rats compared to age-matched WKY animals. Hypothalamic AVP content was not different at 3 weeks of age, but was lower in the SHRs at 6 and 12 weeks. To localize strain differences in AVP and OT, specific hypothalamic nuclei were removed from 300 microns frozen brain sections, and hormone content measured. Paraventricular AVP and OT content was lower in the SHRs which had increased blood pressure (6, 12, and 24 weeks of age) but not in the prehypertensive groups (3 weeks of age). Neuropeptide content was unchanged in the supraoptic nucleus or median eminence. Plasma levels of AVP were increased in the SHR, while OT was unchanged. Thus, genetic hypertension is associated with specific and localized changes in hypothalamic AVP and OT. The fact that the peptide deficit occurred in the paraventricular nucleus, a region thought to be involved in the control of autonomic function, may have important implications in terms of the pathogenesis of hypertension.