Neurogenic hypertension after depletion of norepinephrine in anterior hypothalamus induced by 6-hydroxydopamine administration into the ventral pons: role of serotonin

The influence of respiration on brainstem and cardiovagal response to auricular vagus nerve stimulation: A multimodal ultrahigh-field (7T) fMRI study

BACKGROUND

Brainstem-focused mechanisms supporting transcutaneous auricular VNS (taVNS) effects are not well understood, particularly in humans. We employed ultrahigh field (7T) fMRI and evaluated the influence of respiratory phase for optimal targeting, applying our respiratory-gated auricular vagal afferent nerve stimulation (RAVANS) technique.

HYPOTHESIS

We proposed that targeting of nucleus tractus solitarii (NTS) and cardiovagal modulation in response to taVNS stimuli would be enhanced when stimulation is delivered during a more receptive state, i.e. exhalation.

METHODS

Brainstem fMRI response to auricular taVNS (cymba conchae) was assessed for stimulation delivered during exhalation (eRAVANS) or inhalation (iRAVANS), while exhalation-gated stimulation over the greater auricular nerve (GANctrl, i.e. earlobe) was included as control. Furthermore, we evaluated cardiovagal response to stimulation by calculating instantaneous HF-HRV from cardiac data recorded during fMRI.

RESULTS

Our findings demonstrated that eRAVANS evoked fMRI signal increase in ipsilateral pontomedullary junction in a cluster including purported NTS. Brainstem response to GANctrl localized a partially-overlapping cluster, more ventrolateral, consistent with spinal trigeminal nucleus. A region-of-interest analysis also found eRAVANS activation in monoaminergic source nuclei including locus coeruleus (LC, noradrenergic) and both dorsal and median raphe (serotonergic) nuclei. Response to eRAVANS was significantly greater than iRAVANS for all nuclei, and greater than GANctrl in LC and raphe nuclei. Furthermore, eRAVANS, but not iRAVANS, enhanced cardiovagal modulation, confirming enhanced eRAVANS response on both central and peripheral neurophysiological levels.

CONCLUSION

7T fMRI localized brainstem response to taVNS, linked such response with autonomic outflow, and demonstrated that taVNS applied during exhalation enhanced NTS targeting.

Genome-wide transcriptome analysis of hypothalamus in rats with inherited stress-induced arterial hypertension

Background

The hypothalamus has an important role in the onset and maintenance of hypertension and stress responses. Rats with inherited stress-induced arterial hypertension (ISIAH), reproducing the human stress-sensitive hypertensive state with predominant involvement of the neuroendocrine hypothalamic-pituitary-adrenal and sympathoadrenal axes, were used for analysis of the hypothalamus transcriptome.

Results

RNA-seq analysis revealed 139 genes differentially expressed in the hypothalami of hypertensive ISIAH and normotensive Wistar Albino Glaxo (WAG) rats. According to the annotation in databases, 18 of the differentially expressed genes (DEGs) were associated with arterial hypertension. The Gene Ontology (GO) functional annotation showed that these genes were related to different biological processes that may contribute to the hypertension development in the ISIAH rats. The most significantly affected processes were the following: regulation of hormone levels, immune system process, regulation of response to stimulus, blood circulation, response to stress, response to hormone stimulus, transport, metabolic processes, and endocrine system development. The most significantly affected metabolic pathways were those associated with the function of the immune system and cell adhesion molecules and the metabolism of retinol and arachidonic acid. Of the top 40 DEGs making the greatest contribution to the interstrain differences, there were 3 genes (Ephx2, Cst3 and Ltbp2) associated with hypertension that were considered to be suitable for further studies as potential targets for the stress-sensitive hypertension therapy. Seven DEGs were found to be common between hypothalamic transcriptomes of ISIAH rats and Schlager mice with established neurogenic hypertension.

Conclusions

The results of this study revealed multiple DEGs and possible mechanisms specifying the hypothalamic function in the hypertensive ISIAH rats. These results provide a basis for further investigation of the signalling mechanisms that affect hypothalamic output related to stress-sensitive hypertension development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-015-0307-8) contains supplementary material, which is available to authorized users.

Acute effects of tianeptine on circulating neurotransmitters and cardiovascular parameters

Tianeptine is a serotonin-uptake enhancer drug whose antidepressant effectiveness is based on its ability to reduce rather than increase serotonin availability at the synaptic cleft. This paradoxical neuropharmacological mechanism has raised doubt among neuropharmacologists and psychiatrists as to the role of tianeptine as a trusty-reliable antidepressant drug. This controversial issue led us to investigate the acute effects of a single, oral dose (12.5 mg) of this drug on circulating neurotransmitters and cardiovascular parameters in 50 healthy subjects. The drug provoked a striking and significant reduction of plasma noradrenaline (NA) and plasma serotonin (f-5-HT) while it increased plasma dopamine (DA) and platelet serotonin (p-5-HT) concentrations within the 4-h study period. No adrenaline (Ad) changes were registered. The NA/Ad ratio and the f-5-HT/p-5-HT ratio showed significant reduction throughout the test. Finally, although diastolic blood pressure (DBP) showed significant decrease, neither systolic blood pressure (SBP) nor heart rate (HR) showed significant change. These findings are consistent with the postulation that tianeptine reduces both neural sympathetic activity and parasympathetic activity without affecting adrenal sympathetic activity, enabling us to discuss the possible mechanisms involved in the antidepressant effects of tianeptine. The well-known fact that major depressed patients always show raised NA plus lower than normal p-5-HT levels, both disorders which are normalized by tianeptine, gives neurochemical support to the clinical improvement triggered by the drug in these patients. Summarizing, the results presented in this study demonstrate that tianeptine triggers significant reduction of circulating noradrenaline and plasma serotonin while increasing circulating dopamine and platelet serotonin. Other possible neuropharmacological effects are also discussed.

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.

Stress versus depression

1. Exhaustive evidence is quoted showing that uncontrollable (uncoping) stress provoked in experimental mammals leads to depletion of central noradrenergic activity+ adrenomedullary-cortical gland hyperactivity. These physiological disorders cause the typical neuroendocrine peripheral profile: a) raised catecholamines (CA) in plasma [noradrenaline (NA)+adrenaline (Ad)+dopamine (DA), b) reduced NA/Ad ratio in plasma and c) raised plasma cortisol. 2. Exhaustive evidence is quoted which indicates that severely ill humans show peripheral neuroendocrine profile similar to that found in mammals submitted to uncontrollable stress situation. Further, the NA/Ad ratio does not increase but decreases during orthostasis and exercise stress challenges, as well as oral glucose stress (tolerance) test. 3. Exhaustive evidence is quoted which indicates that endogenous depressed subjects show a neuroendocrine profile opposite to that observed in stressed mammals and severely ill humans. This profile consists of central NA (neural sympathetic) hyperactivity+ adrenomedullary glands hyporresponsivity. These disorders are reflected in a three to ten fold increase of the NA/Ad ratio in plasma. 4. Exhaustive evidence is also quoted showing that dysthymic depressed patients show low plasma catecholamines+low NA/Ad plasma ratio (< 2) during supine-resting condition, it is normalized at orthostasis and exercise periods. 5. It is quoted evidence showing that whereas platelet serotonin is increased in dysthymics, the same is reduced in both endogenous depressed and stressed mammals as well as severely ill humans. 6. It is quoted evidence showing that free serotonin in plasma is greatly raised in uncoping stressed mammals and severely ill humans. The same parameter is normal or slightly increased in dysthymic and endogenous depressed humans. These findings are consistent with the increased platelet aggregability observed in "uncontrollable" stressed mammals and in severely ill, but not depressed patients. 7. It is also quoted evidence showing that whereas parasympathetic activity is absent in uncontrollable stressed mammals and severely ill humans, the same is increased in both types of depressed humans. 8. According to the above, the authors postulate the existence of 3 distinct central+ peripheral neuroendocrine profiles for endogenous depression, dysthymic depression and maladaptation to stress syndrome. These different profiles should lead researchers to attempt different therapeutical approach. 9. In view of the fact that the authors found much clinical overlap among the three syndromes (endogenous depression, dysthymic depression and severely ill patients), they believe that a differential diagnosis should be based on neurochemical, neuroendocrine, physiologic, metabolic and neuropharmacological grounds. 10. The experimentally induced uncontrollable stress (behavioral despair) syndrome in mammals should not be used as a valid model of human depressive syndrome.

Norepinephrine levels in discrete brain nuclei in borderline hypertensive rats exposed to compound stressors

The borderline hypertensive rat (BHR) appears to be an appropriate model for investigating the role of the environment in producing hypertension. Previous studies have demonstrated that the BHR shows chronic blood pressure elevations to both stress and high salt intake. Other studies suggest that interactions between the brain and kidney play an important role in initiating this hypertension. The central noradrenergic system has been implicated in these effects, especially in the hypothalamus. Because exercise has been found to attenuate stress-induced hypertension in the BHR, the current study sought to examine the impact of stressors paired with exercise (salt intake or stress) with those combining stress and high salt. Male BHR were exposed to either control, salt plus stress, salt plus exercise, or stress plus exercise conditions for either 2 or 6 months, beginning at 2 months of age. Following sacrifice, brain nuclei in the brain stem and hypothalamus were removed using the Palkovits micropunch technique. Punches were analyzed for NE content via liquid chromatography with electrochemical detection. Compared with the control condition, chronic salt plus stress led to reductions in NE content, especially in the hypothalamus. Compared with salt plus stress, the exercise conditions were associated with elevated NE levels, especially in the early phases of exposure to the treatment. The possible role of exercise training in preventing a central nervous system trigger from inducing hypertension in the BHR is discussed.

Central catecholamine, sympathetic nerve and vascular protein in the acute phase of two-kidney, one-clip renovascular hypertension in rats

Incorporation of 3H-proline into the non-collagenous protein in mesenteric arteries in two-kidney, one-clip hypertensive rats was greater than that in normotensive rats. Splanchnicotomy predominantly over the root of mesenteric arteries or intracranioventricular injection of 6-hydroxydopamine prevented the development of hypertension in 2K-1C rats concomitant with the reduction of incorporation of 3H-proline into the non-collagenous protein in mesenteric arteries. The content of norepinephrine in the hypothalamus in 2K-1C rats was lower than that in normotensive control rats. These findings indicate that increased non-collagenous protein synthesis in mesenteric arteries or low level of hypothalamic norepinephrine has facilitative effects on the development of 2K-1C hypertension.

Catecholamine metabolism in the vas deferens and the adrenal gland with special reference to the central catecholamine-depleted state

Experiments were carried out to elucidate the role of central catecholamines in regulating catecholamine metabolism in the vas deferens and adrenal gland of the rat. Rats were injected intracerebroventricularly (i.c.v.) with either vehicle or 6-hydroxydopamine (6-OHDA). Groups of animals pretreated with vehicle or 6-OHDA (i.c.v.) were injected intraperitoneally (i.p.) with alpha-methyl-para-tyrosine (AMT), a tyrosine hydroxylase inhibitor. Catecholamine turnover rates were estimated by determining norepinephrine or epinephrine content after administering AMT. Central norepinephrine and dopamine contents decreased significantly (p less than 0.05) after treatment with 6-OHDA and AMT. The norepinephrine content of the vas deferens of rats pretreated with 6-OHDA was markedly reduced (p less than 0.001) after administration of AMT, whereas that of the vehicle-treated rats remained unchanged. Administration of 6-OHDA had no effect on the norepinephrine or epinephrine content of the adrenal gland. The present results indicate that central monoaminergic neurons have an inhibitory effect on the adrenergic neurons of the vas deferens. In contrast, this inhibitory regulation does not appear to be exerted on the adrenal glands.

Decreased norepinephrine release in anterior hypothalamus of NaCl-sensitive spontaneously hypertensive rats during high NaCl intake

We previously demonstrated that dietary NaCl supplementation reduces endogenous norepinephrine stores and turnover in the anterior hypothalamic area (AHA) of male NaCl sensitive spontaneously hypertensive rats (SHR-S) but not in NaCl resistant control rats and have implicated this mechanism in the pathogenesis of NaCl sensitive hypertension. In the current study, we tested directly the hypothesis that dietary NaCl supplementation decreases the release of norepinephrine from nerve terminals in the AHA of SHR-S using the push-pull perfusion technique. Conscious, freely moving SHR-S and control Wistar-Kyoto (WKY) rats were studied after 2-3 weeks of 8% or 1% NaCl feeding. In the 1% NaCl fed SHR-S, 3-methoxy-4-hydroxyphenylglycol (MOPEG, the major metabolite of norepinephrine in brain) levels averaged 272 +/- 32 pg/10 min; norepinephrine levels, 17 +/- 2 pg/10 min; in the 8% NaCl fed SHR-S, MOPEG levels averaged 72 +/- 7 pg/10 min; norepinephrine levels were 6 +/- 1 pg/10 min. There was a positive linear correlation (r = 0.777; P less than 0.01) between MOPEG and norepinephrine levels in AHA perfusates, indicating that perfusate MOPEG levels provide a useful index of norepinephrine release from AHA nerve terminals. In contrast, MOPEG levels in AHA perfusates were not affected by dietary NaCl intake in control WKY, and in control posterior hypothalamic perfusates, were not affected by dietary NaCl intake in SHR-S.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential actions of the medial region of caudal medulla on autonomic nerve activities

1. The inhibitory effects produced by activation of the medial region of caudal medulla on activities of the left and right cardiac sympathetic, vagus and greater splanchnic nerves were studied in chloralose-urethane anaesthetized cats. 2. Electrical stimulation of the medial region produced an 80-92% inhibition of the sympathetic nerve activities, and a 45% and 58% inhibition of the left and right cardiac vagal nerve activities, respectively. There were no significant differences between effects elicited in the left and right autonomic nerves. Similar but smaller inhibitory effects were produced by micro-injection of sodium glutamate (0.5 mol/L) or DL-homocysteic acid (50 mmol/L) to the same medullary sites. 3. These data suggest that neurons residing in the medial medullary region exert strong inhibitory effects on autonomic nerve activities. Since the vasculature is principally innervated by sympathetic nerves, inhibition of sympathetic nerve activities might be the principal factor responsible for the depressor effects caused by activation of the medial region of caudal medulla. The heart is innervated both by sympathetic and parasympathetic nerves. Thus, their simultaneous inhibition during activation of the medial region elicits only a weak and variable inhibition of the heart.

Intrahypothalamic clonidine infusion prevents NaCl-sensitive hypertension

We have previously shown that dietary NaCl supplementation increases blood pressure and sympathetic nervous system activity in association with decreased norepinephrine release and increased alpha 2-adrenergic receptor number in the anterior hypothalamic area of salt-sensitive spontaneously hypertensive rats (SHR-S) but not in salt-resistant spontaneously hypertensive rats (SHR-R) or Wistar-Kyoto (WKY) rats. Further, acute microinjection of clonidine into the anterior hypothalamic area produced depressor responses that were augmented by high salt feeding in SHR-S but not in SHR-R or WKY rats. The current study tested the hypothesis that chronic infusion of clonidine into the anterior hypothalamic area prevents salt-sensitive hypertension in SHR-S. Beginning at age 7 weeks, immediately before initiation of 1% or 8% salt diets, clonidine (2 ng/min) or saline vehicle was infused into the anterior hypothalamic area or femoral vein of male SHR-S via osmotic minipump for 20 days. In SHR-S fed an 8% salt diet, chronic microinfusion of clonidine into the anterior hypothalamic area offset the hypertensive effect of the dietary salt supplementation and reduced the enhancing effects of dietary salt on left ventricular weight and plasma norepinephrine levels. In contrast, chronic microinfusion of clonidine into the anterior hypothalamic area did not significantly affect any of these measures in 1% salt-fed SHR-S. Intravenous infusion of clonidine at the rate used for the anterior hypothalamic area infusion did not alter any of these measures in 8% salt-fed SHR-S.(ABSTRACT TRUNCATED AT 250 WORDS)

Blockade of endogenous anterior hypothalamic atrial natriuretic peptide with monoclonal antibody lowers blood pressure in spontaneously hypertensive rats

We have previously shown that the atrial natriuretic peptide (ANP) content of the anterior hypothalamic region of NaCl-sensitive spontaneously hypertensive rats (SHR-S) is higher than that of Wistar-Kyoto (WKY) rats. ANP has been shown to inhibit neuronal norepinephrine release and to reduce the excitability of hypothalamic neurons. This study tested the hypothesis that blockade of endogenous ANP in the anterior hypothalamus by local microinjection of a monoclonal antibody to ANP (MAb KY-ANP-II) lowers blood pressure in SHR-S. Purified MAb KY-ANP-II (0.055 and 0.55 micrograms) or control mouse IgG in 200 nl saline was microinjected into the anterior hypothalamic area (AHA) of conscious SHR-S and control WKY rats. As a further control, Mab KY-ANP-II (0.55 microgram) was microinjected into the posterior hypothalamic area (PHA) of SHR-S. Anterior hypothalamic microinjection of MAb KY-ANP-II caused significant dose-related decreases in mean arterial pressure (MAP) and heart rate (HR) in SHR-S but not in WKY rats. Control injections of equal volumes of IgG had no effect on MAP or HR. Microinjection of Mab KY-ANP-II into PHA produced no significant alteration in MAP or HR in SHR-S. These data provide the first demonstration that endogenous ANP in a region of brain known to influence cardiovascular function mediates BP and HR control in the rat. These findings suggest that the increased endogenous ANP in the anterior hypothalamus of SHR-S may be involved in the central regulation of BP in the model.

Lesions of the anterior hypothalamic area increase arterial pressure in NaCl-sensitive spontaneously hypertensive rats

In NaCl-sensitive spontaneously hypertensive rats, diets high in NaCl increase arterial pressure and peripheral sympathetic nervous system activity and decrease the sympatho-inhibition mediated by the anterior hypothalamic area. To test the importance of the defect in anterior hypothalamic area-mediated sympatho-inhibition in the pathogenesis of NaCl-sensitive hypertension, bilateral ibotenic acid lesions of the anterior hypothalamic area were made in NaCl-sensitive spontaneous hypertensive rats, in NaCl-resistant spontaneously hypertensive rats and in normotensive, NaCl-resistant Wistar Kyoto rats. In NaCl-sensitive spontaneous hypertensive rats on a basal NaCl diet, the anterior hypothalamic area lesions caused a rapid rise in arterial pressure within the first week after surgery; by 21 days after surgery, mean systolic arterial pressure of the lesion group was 24 mmHg higher than that of the sham-operated group. In a second experiment, NaCl-sensitive spontaneous hypertensive rats were placed on an 8% NaCl diet 1 day after the lesion of the anterior hypothalamic area. 5 days after the operation, the lesion group of NaCl-sensitive spontaneous hypertensive rats on the 8% NaCl diet had a significantly higher arterial pressure than the sham-operated group, but by 1 week after the lesion, arterial pressures were not significantly different between the lesion and sham-operated NaCl-sensitive spontaneous hypertensive rats on the high NaCl diet. In Wistar Kyoto rats on a basal NaCl diet, lesions of the anterior hypothalamic area resulted in a small, transient elevation of arterial pressure, but no sustained effect. In NaCl-resistant spontaneous hypertensive rats, the anterior hypothalamic area lesions did not affect arterial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

NaCl does not affect hypothalamic noradrenergic input in deoxycorticosterone acetate/NaCl and Dahl salt-sensitive rats

Previous studies from our laboratories demonstrated that dietary NaCl supplementation in NaCl-sensitive spontaneously hypertensive rats elevates blood pressure, increases peripheral sympathetic nervous system activity, and depresses endogenous norepinephrine stores and turnover in the anterior hypothalamus. These findings suggest that reduced noradrenergic input to sympathoinhibitory neurons in anterior hypothalamus contributes to NaCl-sensitive hypertension in spontaneously hypertensive rats. The current study tested the hypothesis that dietary NaCl supplementation depresses endogenous norepinephrine stores and turnover in anterior hypothalamus of two other NaCl-sensitive models of hypertension, the Dahl salt-sensitive rat and the deoxycorticosterone acetate/NaCl hypertensive rat, thus increasing blood pressure by reducing noradrenergic input to the anterior hypothalamus. Dahl salt-sensitive rats were fed a high (8%) NaCl diet, and deoxycorticosterone acetate/NaCl rats rats drank 1% NaCl solution ad libitum for 2 or 4 weeks. Age-matched Dahl salt-sensitive rats fed a basal 1% NaCl diet and uninephrectomized Sprague-Dawley rats drinking tap water were controls. Regional brain catecholamines were determined by high-performance liquid chromatography with electrochemical detection. Norepinephrine turnover in hypothalamus (anterior, posterior, and ventral regions) and brain stem (pons and medulla) was assessed using the dopamine beta-hydroxylase inhibitor 1-cyclohexyl-2-mercapto-imidazole. High NaCl treatment caused significant elevations in blood pressure in Dahl salt-sensitive and deoxycorticosterone acetate/NaCl rats, but endogenous norepinephrine levels and turnover rates were not significantly different in anterior hypothalamus or any other brain region studied between the NaCl-supplemented and control groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Central noradrenergic neurons and vascular non-collagen protein in the initial phase of two-kidney, one-clip renovascular hypertension

Rats had been given intraventricular injection of 6-hydroxydopamine (6-OHDA) before clipping the unilateral renal artery (2K-1C) that caused selective ablation of the central noradrenergic neurons. Central catecholamines and the in vivo incorporation of 3H-proline into vascular non-collagen protein were determined in 2K-1C rats in the acute hypertensive stage. It is suggested that increased non-collagen protein synthesis in the mesenteric artery and the low level of hypothalamic norepinephrine concentration may participate in the development of 2K-1C hypertension in rats.

High NaCl diet increases anterior hypothalamic alpha 2-adrenoceptors in SHR

Previous studies carried out in our laboratories demonstrated that dietary NaCl supplementation induced an increase in blood pressure associated with a reduction in noradrenaline release in the anterior hypothalamic region of NaCl-sensitive spontaneously hypertensive rats (SHR). The present study tested the hypothesis that this reduction in noradrenaline release induces a compensatory increase in alpha 2-adrenoceptor number in the anterior hypothalamic region of SHR. Low affinity p-[3H]aminoclonidine (alpha 2-adrenoceptor agonist) binding in the anterior hypothalamic region was increased significantly in SHR, but not in normotensive Wistar-Kyoto (WKY) rats on a high (compared to a basal) NaCl diet. Increased p-[3H]aminoclonidine binding was present at 1 and 2 weeks following initiation of the diets. In contrast, in the posterior hypothalamic region of SHR, alpha 2-adrenoceptors were significantly reduced following 1 week on the high (compared to basal) NaCl diet, and no difference between groups was observed following 2 weeks on the diets. The high NaCl diet did not alter alpha 2-adrenoceptors in the medulla, nor did it affect alpha 1- or beta-adrenoceptors in any brain region studied in either SHR or WKY. In SHR, dietary Ca2+ supplementation diminished the blood pressure elevation associated with high NaCl diets, and simultaneously prevented the NaCl induced increase in alpha 2-adrenoceptors in the anterior hypothalamic region. These data support the hypothesis that, in susceptible individuals, changes in noradrenergic transmission in the anterior hypothalamic region contribute to the cardiovascular effects of dietary NaCl loading and Ca2+ supplementation.

The neural basis of salt sensitivity in the rat: altered hypothalamic function

Dietary NaCl supplementation in NaCl-sensitive spontaneously hypertensive rats (SHR-S) elevates blood pressure, increases peripheral sympathetic nervous system activity and depresses endogenous noradrenaline stores and noradrenaline release in the anterior hypothalamus. NaCl-resistant spontaneously hypertensive rats (SHR-R) and normotensive Wistar Kyoto (WKY) rats are resistant to the NaCl-induced alterations in blood pressure and central and peripheral noradrenergic activity, suggesting that the alterations observed in the SHR-S during NaCl loading are genetically mediated. The anterior hypothalamus is a major cardiovascular regulatory region, and depressor responses elicited by pharmacologic (alpha 2 adrenoceptor) stimulation of this area are exaggerated in SHR-S fed a high NaCl diet compared with SHR-S fed a basal diet and compared with SHR-R and WKY fed a high or basal NaCl diet. Membrane-binding techniques confirm that alpha 2 adrenoceptors in the anterior hypothalamic area are increased in number in SHR-S fed a high NaCl diet, presumably reflecting upregulation in response to reduced local noradrenaline release. These findings are consistent with the hypothesis that decreased noradrenergic activity of sympathoinhibitory neurons in the anterior hypothalamic area may mediate the exacerbation in hypertension that occurs in SHR-S during dietary NaCl supplementation.

High NaCl diet reduces hypothalamic norepinephrine turnover in hypertensive rats

The current study tested the hypothesis that high NaCl diets elevate blood pressure in NaCl-sensitive spontaneously hypertensive rats (SHR-S) by reducing noradrenergic input to depressor neurons in the anterior hypothalamus. SHR-S were studied at 7 weeks of age, and age-matched salt resistant SHR (SHR-R) and normotensive Wistar-Kyoto rats (WKY) were controls. Rats were fed either high (8%) NaCl or control (1% NaCl) diets for 2 weeks, following which norepinephrine turnover in hypothalamus (anterior, posterior, and ventral regions), brainstem (pons and medulla), and thoracic spinal cord was assessed using the dopamine beta-hydroxylase inhibitor 1-cyclohexyl-2-mercapto-imidazole (CHMI). Regional brain catecholamines were measured by high performance liquid chromatography with electrochemical detection following intraperitoneal injection of CHMI or vehicle. Disappearance of norepinephrine following CHMI was used as an index of noradrenergic neuronal activity. The 8% NaCl diet caused a significant elevation in blood pressure in SHR-S but not in SHR-R or WKY. Endogenous norepinephrine levels and turnover were lower in the anterior hypothalamus of SHR-S fed 8% NaCl than in those fed 1% NaCl but were not significantly different in other groups. Endogenous norepinephrine levels and turnover were greater in pons of 8% NaCl--fed SHR-S than in those fed 1% NaCl but were not significantly different in other groups. These observations support the hypothesis that reduced noradrenergic input to depressor neurons in the anterior hypothalamus and increased noradrenergic input to neurons in the pons are related to NaCl sensitivity in the SHR-S.

Angiotensin converting enzyme activity in the amygdaloid complex in a neurogenic hypertensive model

The bilateral destruction of the ventral noradrenergic pathway induced by 6-hydroxydopamine (6-OHDA) administration into the ventral pons led to an increase in arterial blood pressure (ABP) and norepinephrine depletion in the amygdaloid complex, nucleus accumbens, septal area and olfactory bulb. Specific angiotensin converting enzyme (ACE) activity was significantly increased only in the amygdaloid complex (Control: 4.56 +/- 0.95; Vehicle: 4.08 +/- 1.07; 6-OHDA: 11.76 +/- 1.84). A significant correlation between arterial blood pressure and specific ACE activity levels in the amygdaloid complex was observed (r: 0.775; p less than 0.002). These results suggest that an increase in specific ACE activity of the amygdaloid complex after norepinephrine depletion could play a role in the development of hypertension in this model.

Role of central serotonergic (5-HT2) receptor in blood pressure regulation in rats

To investigate the role of the serotonergic nervous system in blood pressure regulation, 5 micrograms of 5-hydroxytryptamine (5-HT) was given i.c.v. before and after 1 microgram of i.c.v. xylamidine or 200 micrograms of i.c.v. ketanserin or 200 micrograms of i.v. ketanserin in conscious Wistar Kyoto rats. Also i.v. (0.5, 1, 2 micrograms) or i.c.v. (1 microgram) phenylephrine (PHE) were given before and after 1 microgram of i.c.v. xylamidine. I.c.v. 5-HT elicited a consistent pressor response of approximately 27mmHg and slight decrease in heart rate. MAP and heart rate did not change after xylamidine or ketanserin. Whereas pressor response to i.c.v. 5-HT after i.c.v. ketanserin or i.c.v. xylamidine was suppressed, it did not change after i.v. ketanserin. Neither i.c.v. nor i.v. PHE-induced pressor response was influenced by i.c.v. xylamidine pretreatment. These data suggest that the central 5-HT2 receptor may subserve pressor function in rats.

Low TRH-TSH responses in human essential hypertension

Thirty female and male essential hypertensive patients and eighteen normotensive controls were submitted to the TRH-TSH conventional test (200 ug intravenously, bolus injection of TRH). Supramaximal doses of 400 and 600 ug were repeated with a week interval to each subject. Hypertensives showed a significant lower response to both conventional and supramaximal TRH doses. Hypothalamic-pituitary-thyroid axis abnormalities, secondary to TRH-receptor alterations, could account for this result.

Spontaneously hypertensive rats exhibit reduced hypothalamic noradrenergic input after NaCl loading

Spontaneously hypertensive rats (SHR) of the Okamoto strain exhibit a significant exacerbation in severity of hypertension when fed diets high in NaCl. To examine the hypothesis that abnormalities in the monoaminergic innervation of the hypothalamus and brainstem contribute to the NaCl-induced exacerbation of hypertension, the monoamine and monoamine metabolite contents of specific hypothalamic and brainstem regions thought to be involved in the pathogenesis of hypertension were determined in SHR fed a diet containing 8% or 1% NaCl for either 2 or 6 weeks beginning at age 8 weeks. SHR maintained on the 8% NaCl diet for 2 weeks displayed significant decreases in norepinephrine in both the anterior and posterior hypothalamic regions but not in other brainstem or hypothalamic regions, as compared with animals consuming 1% NaCl. In addition, stores of the principal terminal norepinephrine metabolite 3-methoxy-4-hydroxyphenylglycol were reduced in the anterior hypothalamic region of SHR fed an 8% NaCl diet for 2 weeks. After 6 weeks on the diets, SHR fed 8% NaCl showed small but statistically nonsignificant reductions in norepinephrine stores of the anterior hypothalamic region as compared with SHR fed a basal diet, while WKY fed 8% NaCl had significantly elevated norepinephrine stores in the anterior hypothalamic region as compared with WKY fed a basal diet. There was a significant group X diet interaction (p less than 0.05). After 6 weeks on the 8% NaCl diet, SHR (but not WKY) displayed a significant reduction in norepinephrine content of the posterior hypothalamic region. No NaCl-induced differences in norepinephrine stores were found in the pons or medulla of either strain.(ABSTRACT TRUNCATED AT 250 WORDS)

Exacerbation of hypertension by high chloride, moderate sodium diet in the salt-sensitive spontaneously hypertensive rat

In salt-sensitive spontaneously hypertensive rats (SHR-S) of the Okamoto strain, dietary salt loading causes an exacerbation of hypertension that is associated with a decrease in noradrenergic input to the depressor neurons in the anterior hypothalamus. In the present study, the contribution of chloride to the salt-induced hypertensive response was examined in the SHR-S, in order to test the hypothesis that diets high in chloride but moderate in sodium elevate blood pressure in genetically predisposed subjects. SHR-S were fed diets high in NaCl (1.97% Na+, 2.93% Cl-; 5% NaCl), high in chloride (2.93%) but moderate in sodium (0.39%) or moderate in NaCl (0.39% Na+, 0.61% Cl-; 1% NaCl). After 2 weeks, rats on the high (5%) NaCl diet exhibited a significant elevation in blood pressure compared to rats on the moderate (1%) NaCl diet, and this elevation was maintained throughout the next 3 weeks. SHR-S on the high chloride diet were not significantly more hypertensive than 1% NaCl-fed SHR-S during the first 3 weeks, but during the fourth and fifth weeks, SHR-S on the high chloride diet displayed a significant exacerbation of hypertension. The diet-induced elevation in blood pressure in groups fed either the 5% NaCl or high chloride (compared to 1% NaCl) diets was associated with significant decreases in norepinephrine stores in the anterior hypothalamic region, but no other changes in monoamines or monoamine metabolites in this region or in the posterior hypothalamic region. The high chloride diet did not increase blood pressure in normotensive Wistar-Kyoto rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Organization of central adrenergic pathways: I. Relationships of ventrolateral medullary projections to the hypothalamus and spinal cord

We studied the organization of projections from the C1 adrenergic and A1 noradrenergic cell groups in the ventrolateral medulla (VLM) to the hypothalamus and the spinal cord by using a combination of retrograde transport of fluorescent tracers and immunocytochemistry. Three issues were addressed. Neurons in the VLM that stain immunohistochemically for phenylethanolamine N-methyltransferase (PNMT) have been assumed to be adrenergic. However, the presence of PNMT-immunoreactive neurons in the hypothalamus that do not stain for tyrosine hydroxylase (TH) prompted us to re-evaluate the VLM by an elution-restaining immunohistochemical procedure. We confirmed that nearly all of the rostral medullary PNMT-immunoreactive neurons also stained for TH. By contrast, in the caudal medulla, very few TH-positive neurons stained for PNMT. Neurons of the C1 group in the rostral VLM project both to the thoracic spinal cord and to the hypothalamus. To determine whether individual C1 neurons send collaterals to the hypothalamus and spinal cord, we injected different-colored fluorescent dyes (diamidino yellow or fast blue) into the thoracic spinal gray matter and either the median preoptic (MnPO) or paraventricular (PVH) nuclei of the hypothalamus. Very few double-labeled neurons were found in the VLM, indicating that hypothalamic and spinal cord projections arise from almost completely independent populations of cells. Approximately half of the neurons projecting to the spinal cord from rostral VLM were not immunoreactive for TH or PNMT, indicating that a substantial part of this projection is noncatecholaminergic. The MnPO and the PVH both receive extensive catecholaminergic inputs from the VLM. We also used fluorescent retrograde tracers to determine whether individual VLM neurons send collaterals to both hypothalamic sites. Approximately 20% of neurons projecting to the MnPO in the rostral two thirds of the VLM also sent collaterials to the PVH, nearly all of these neurons being TH-positive. The collateralization of the VLM catecholaminergic projection to the hypothalamus may provide an anatomical substrate for integration of fore-brain participation in cardiovascular regulation. In contrast, the adrenergic projection from the VLM to the intermediolateral column of the spinal cord arises from a separate population of neurons.

Role of stress in the exacerbation of chronic illness: effects of clonidine administration on blood pressure and plasma norepinephrine, cortisol, growth hormone and prolactin concentrations

Systolic blood pressure (SBP), diastolic blood pressure (DBP), and plasma norepinephrine (NE), cortisol (CRT), growth hormone (GH) and prolactin (PRL) were studied before and after clonidine (2.5 micrograms/kg i.m.) administration in 193 chronic severely ill patients and 193 normal subjects matched by age and sex. During exacerbation periods (positive manifestations of impairment and progressive disease), the patients showed higher NE, CRT and DBP than the normals or when they were investigated during non-exacerbation periods (92 of the 193). Clonidine induced sharp, marked reductions of NE, CRT and DBP, plus a sudden increase of GH, in all the patients during exacerbation periods. Non-significant reductions of NE, CRT and DBP were observed in normals and in patients during non-exacerbation periods. On the other hand, the GH increase registered during exacerbation periods was of an order of magnitude higher than that registered in normals and in patients during non-exacerbation periods. Significant reduction of SBP was registered both in normals and patients (exacerbation and non-exacerbation periods). Some tendency to PRL lowering was observed during exacerbation periods only. A high positive correlation between NE and DBP (pre- and post-clonidine values) was obtained during exacerbation periods in patients, but not in normals or during non-exacerbation periods in the patients. Similarly, a close negative correlation was obtained between CRT and GH (postclonidine values) during exacerbation periods, but not in normals or during non-exacerbation periods. No significant correlation was found between NE and SBP in any group of subjects. The clonidine-induced changes in GH and CRT observed in the patients during exacerbation periods were in striking contrast to the absence of these changes in depressed patients. This finding is consistent with the low rate of depression (6.7%) registered among our patients during exacerbation periods. The high plasma NE and CRT levels registered in chronic severely ill patients during exacerbation periods reflect a central and peripheral sympathetic hyperactivity, accompanied by an overactivity of the pituitary--adrenocortical axis. The strong reduction of DBP, NE and CRT, along with the sharp and great increase of GH, might be useful as indicators in assessing the exacerbation and progression of severe chronic illnesses.

Cardiovascular effects of perfusion of the rostral rat hypothalamus with clonidine: differential interactions with prazosin and yohimbine

The present studies examined the role of alpha 1- versus alpha 2-adrenoceptors in the cardiovascular effects of clonidine administered into the anterior hypothalamic/pre-optic (AH/PO) region of the forebrain by the push-pull perfusion technique. Push-pull cannulas were placed bilaterally into the AH/PO region of anesthetized, paralyzed and ventilated rats. Perfusion of this area with artificial CSF (0.015 ml/min), yohimbine (5 or 50 microM) for 30 min did not affect mean arterial blood pressure or heart rate. Perfusion of the AH/PO region with clonidine (0.55-5.50 mM) resulted in a concentration-dependent reduction of mean arterial pressure and heart rate. The hypotensive effects of clonidine were found to be greater than the bradycardic effects, when expressed as a percent of pre-infusion baseline values. Co-perfusion with yohimbine (5, 50 microM) significantly attenuated the hypotensive, but not the bradycardic, effects of a single concentration (1.75 mM) of clonidine; this selective antagonism of the hypotensive effect of clonidine by yohimbine was concentration dependent. In contrast to yohimbine, co-perfusion with 5 microM prazosin did not significantly affect either the clonidine-induced hypotension or bradycardia. Co-perfusion with the higher concentration (50 microM) of prazosin significantly reversed the bradycardic, but not the hypotensive, effects of 1.75 mM clonidine. These results suggest that AH/PO clonidine perfusion depresses both mean arterial pressure and heart rate, and that the clonidine-induced hypotension is due to alpha 2-adrenoceptor activation, while the clonidine-induced bradycardia is due to alpha 1-adrenoceptor activation.

Participation of central norepinephrine in the development of two-kidney, one clip Goldblatt hypertension

Two series of experiments were conducted to ascertain whether the central catecholamines would participate in the elevation of blood pressure in two-kidney, one clip (2K-1C) Goldblatt hypertensive rats. The effect of 6-hydroxydopamine (6-OHDA) injected intraventricularly (250 micrograms/kg.) before the induction of hypertension (acute phase) or after the establishment of hypertension (chronic phase) was examined in 2K-1C rats and sham rats. The evidence has been presented: 1) that the hypothalamic norepinephrine content of 2K-1C rats in acute phase is 50 per cent lower (p less than 0.001) than that of sham rats, but norepinephrine content in the pons is similar in these animals, 2) that the content of dopamine in hypothalamus or pons is similar in 2K-1C rats and normotensive age-matched sham rats in acute phase, 3) that the content of these monoamines either in hypothalamus or pons in 2K-1C rats in chronic phase is almost identical with that in sham rats, 4) that administration of 6-OHDA markedly reduced the development of hypertension in 2K-1C rats in the acute phase, but did not affect their sustained hypertension in the chronic phase. These findings suggest that hypothalamic norepinephrine participates in the development of hypertension in 2K-1C rats in the acute phase but is not connected with the maintenance of their established hypertension in the chronic phase.

Comparison of norepinephrine release in hypertensive rats: I. Hypothalamic and brainstem tissues

Stimulation induced 3H-norepinephrine release was measured in hypothalamus and brainstem of spontaneously hypertensive (SHR) and normotensive (WKY) rats. Age dependent changes in 3H-norepinephrine release were shown to occur in the anterior and posterior hypothalamus and the A2 region of the nucleus tractus solitarius (NTS). In an attempt to determine whether these changes in 3H-transmitter release were causal or merely secondary to the increase in blood pressure, similar release studies were carried out in DOCA-salt and one kidney-one clip hypertensive animals with similar levels of systolic blood pressure. The changes in stimulus-induced 3H-norepinephrine release seen in the SHR were not observed in the other two models of hypertension, suggesting that one: they were not secondary to an increase in systolic blood pressure; and two that the changes observed in the SHR may possibly play a role in the development and/or maintenance of the hypertension.

Distribution of alpha 2 agonist binding sites in the rat and human central nervous system: analysis of some functional, anatomic correlates of the pharmacologic effects of clonidine and related adrenergic agents

Using [3H]para-aminoclonidine, alpha 2 adrenergic binding sites have been mapped in the rat and human CNS using in vitro labeling autoradiographic techniques. In both the rat and human thoracic spinal cord, high densities of alpha 2 binding sites were associated with the substantia gelatinosa and the intermediolateral cell column. In the rat medulla, high binding site density was observed in the medial nucleus of the solitary tract, dorsal motor nucleus of the vagus, raphe pallidus and the substantia gelatinosa of the trigeminal nucleus, while lower levels of specific binding were found in the lateral and ventrolateral medulla. In the human, a similar distribution was observed. However, significantly lower levels of specific binding were seen in the medial nts as opposed to the dmv. In the rat, high levels of specific binding were seen at pontine and midbrain levels in the locus coeruleus, parabrachial nucleus and periaqueductal gray. In the forebrain, several hypothalmic and limbic regions, including the paraventricular and arcuate nuclei of the hypothalamus, the central, medial and basal nuclei of the amygdala, lateral septum and bed nucleus of the stria terminalis and pyriform, entorhinal and insular cortex were labeled. Each of these regions are involved in either modulating autonomic functions directly or integrating somatosensory and/or affective function with autonomic mechanisms. Further, these regions are interrelated by reciprocal connections, and neurons that utilize noradrenaline or adrenaline as their neurotransmitter form a vital part of these connections. Thus, these functional, anatomical and neurochemical correlates of the alpha 2 binding site distribution establish a neurological basis for the complex pharmacological effects of centrally acting alpha 2 agonists.