Enhanced serotonin-mediated responses in the nucleus tractus solitarius of spontaneously hypertensive rats

Transcriptome of the NTS in exercise-trained spontaneously hypertensive rats: implications for NTS function and plasticity in regulating blood pressure

The nucleus tractus solitarii (NTS) controls the cardiovascular system during exercise, and alteration of its function may underlie exercise-induced cardiovascular adaptation. To understand the molecular basis of the NTS's plasticity in regulating blood pressure (BP) and its potential contribution to the antihypertensive effects, we characterized the gene expression profiles at the level of the NTS after long-term daily wheel running in spontaneously hypertensive rats (SHRs). Genome-wide microarray analysis was performed to screen for differentially expressed genes in the NTS between exercise-trained (12 wk) and control SHRs. Pathway analysis using the Kyoto Encyclopedia of Genes and Genomes database revealed that daily exercise altered the expression levels of NTS genes that are functionally associated with metabolic pathways (5 genes), neuroactive ligand-receptor interactions (4 genes), cell adhesion molecules (3 genes), and cytokine-cytokine receptor interactions (3 genes). One of the genes that belonged to the neuroactive ligand-receptor interactions category was histamine receptor H(1). Since we confirmed that the pressor response induced by activation of this receptor is increased after long-term daily exercise, it is suggested that functional plasticity in the histaminergic system may mediate the facilitation of blood pressure control in response to exercise but may not be involved in the lowered basal BP level found in exercise-trained SHRs. Since abnormal inflammatory states in the NTS are known to be prohypertensive in SHRs, altered gene expression of the inflammatory molecules identified in this study may be related to the antihypertensive effects in exercise-trained SHRs, although such speculation awaits functional validation.

Spontaneously hypertensive rat as a model of vascular brain disorder: microanatomy, neurochemistry and behavior

Arterial hypertension is the main risk factor for stroke and plays a role in the development of vascular cognitive impairment (VCI) and vascular dementia (VaD). An association between hypertension and reduced cerebral blood flow and VCI is documented and arterial hypertension in midlife is associated with a higher probability of cognitive impairment. These findings suggest that arterial hypertension is a main cause of vascular brain disorder (VBD). Spontaneously hypertensive rat (SHR) is the rat strain most extensively investigated and used for assessing hypertensive brain damage and treatment of it. They are normotensive at birth and at 6months they have a sustained hypertension. Time-dependent rise of arterial blood pressure, the occurrence of brain atrophy, loss of nerve cells and glial reaction are phenomena shared to some extent with hypertensive brain damage in humans. SHR present changes of some neurotransmitter systems that may have functional and behavioral relevance. An impaired cholinergic neurotransmission characterizes SHR, similarly as reported in patients affected by VaD. SHR are also characterized by a dopaminergic hypofunction and noradrenergic hyperactivity similarly as occurs in attention-deficit with hyperactivity disorder (ADHD). Microanatomical, neurochemical and behavioral data on SHR are in favor of the hypothesis that this strain is a suitable model of VBD. Changes in catecholaminergic transmission put forward SHR as a possible model of ADHD as well. Hence SHR could represent a multi-faced model of two important groups of pathologies, VBD and ADHD. As for most models, researchers should always consider that SHR offer some similarities with corresponding human pathologies, but they do not suffer from the same disease. This paper reviews the main microanatomical, neurochemical and behavioral characteristics of SHR with particular reference as an animal model of brain vascular injury.

Hypoalgesia associated with elevated resting blood pressure: evidence for endogenous opioid involvement

This study used a placebo-controlled, between-subjects opioid blockade design to evaluate endogenous opioid involvement in the hypoalgesia associated with elevated resting blood pressure (BP) in 163 healthy individuals. Participants were randomly assigned to Drug condition (placebo, naltrexone) and Task Order (computerized maze task with harassment followed by an ischemic pain task or vice versa). Resting BP was assessed, followed by drug administration, and then the pain and maze tasks. A significant Drug x Systolic BP (SBP) interaction was observed on McGill Pain Questionnaire-Affective pain ratings (P < .01), indicating that BP-related hypoalgesia observed under placebo was absent under opioid blockade. A significant Gender x Drug x SBP x Task Order interaction was observed for VAS pain intensity (P < .02). Examination of simple effects comprising this interaction suggested that BP-related hypoalgesia occurred only in male participants who experienced the pain task in the absence of emotional arousal, and indicated that this hypoalgesia occurred under placebo but not under opioid-blockade. Results suggest that under some circumstance, BP-related hypoalgesia may have an endogenous opioid-mediated component in healthy individuals, particularly men.

Angiotensin II enhances GABA(B) receptor-mediated responses and expression in nucleus tractus solitarii of rats

Angiotensin II (ANG II) increases GABA(B) receptor expression in neuronal cultures from the nucleus tractus solitarii (NTS). In the present study, the chronic effects of ANG II on GABA(B) receptor expression and activity were examined in the NTS of Sprague-Dawley rats. Intracerebroventricular infusion of ANG II caused a significant elevation in blood pressure (BP) and an increase in GABA(B) receptor expression in the NTS. Conversely, chronic N(G)-nitro-l-arginine methyl ester (l-NAME) treatment also increased BP, but had no effect on GABA(B) receptor expression in the NTS. Next, we examined the BP response to the GABA(B) receptor agonist baclofen microinjected into the NTS of ANG II- or artificial cerebrospinal fluid (aCSF)-infused rats. NTS microinjection of baclofen increased BP in both groups of rats. However, the pressor response to baclofen was enhanced in ANG II-infused rats compared with aCSF-infused rats. In addition, bilateral microinjection of the GABA(B) receptor antagonist CGP-35348 into the NTS evoked a decrease in BP in both group of rats, and the depressor responses to CGP-35348 were enhanced in the ANG II-infused rats. In contrast, the pressor responses to the GABA(A) receptor agonist muscimol and the depressor responses to the GABA(A) receptor antagonist bicuculline were comparable between aCSF- and ANG II-infused rats. These results indicate that chronic ANG II infusion stimulates GABA(B) receptor expression and augments GABA(B) receptor-mediated responses in the NTS. This effect could contribute to the central nervous system actions of ANG II that result in dampening of baroreflexes and elevation in arterial BP.

Characterization of contractile 5-hydroxytryptamine receptor subtypes in the in situ autoperfused kidney in the anaesthetized rat

Using several 5-hydroxytryptamine (5-HT) agonists and antagonists, we attempted to characterize the receptor subtypes involved in the contractile response to 5-HT in the in situ autoperfused rat kidney. An intra-arterial (i.a.) bolus injection of 5-HT (0.00000125 to 0.1 microg/kg) increased renal perfusion pressure in a dose-dependent way but did not affect the systemic blood pressure. The selective 5-HT2 receptor agonist alpha-methyl-5-HT (alpha-methyl-5-hydroxytryptamine) and the non-selective 5-HT2C receptor agonist (1-(3-chlorophenyl)piperazine), m-CPP, caused a local vasoconstrictor effect in the autoperfused rat kidney, whereas BW723C86, a selective 5-HT2B receptor agonist, the 5-HT1 receptor agonist 5-carboxamidotryptamine, 5-CT, and the selective 5-HT3 receptor agonist m-CPBG (1-(m-chlorophenyl)-biguanide) did not modify the renal perfusion pressure. The vasoconstrictor effect elicited by alpha-methyl-5-HT and m-CPP was significantly decreased by ritanserin (a 5-HT2 receptor antagonist), SB 206553 (3,5-Dihydro-5-methyl-N-3pyridinylbenzo[1,2.b:4,5-b']dipyrrole(1H)-carboxamide hydrochloride), a selective 5-HT2B/2C receptor antagonist and enalapril, but was not modified by pretreatment with spiperone (a 5-HT2A receptor antagonist). The results of protein expression analyses allow us to postulate that 5HT-SRC (a 5-HT2C receptor protein) is expressed in renal tissue and differentially expressed in renal artery. Our data suggest also that the serotonergic vasoconstrictor response induced in the in situ autoperfused rat kidney would be mediated by local 5-HT2C receptor activation.

Role of the serotonin system in ADHD: treatment implications

A limited number of studies have considered whether the activity of serotonin (5-hydroxytryptamine [5-HT]) contributes to the problems experienced by youngsters with attention-deficit/hyperactivity disorder (ADHD). The aim of this article is to review this work and propose interpretations. Peripheral measures of 5-HT and its metabolite do not point to a widespread association with the diagnosis. However, separate consideration of the major domains of dysfunction (motor activity, inattention and impulsivity) support a more differentiated assessment. The marked innervation of motor regions of the brain by 5-HT projections and the clear involvement of 5-HT systems in the control of locomotion in animals suggests a likely node for dysfunction in ADHD. The few relevant studies do not show evidence of this, but more attention should be accorded to the issue. The situation is different for attention-related processes; here, there are deficiencies in perceptual sensitivity and the appropriate designation of saliency to stimulation. These are attributable, in part, to altered 5-HT activity. Marked and opposite changes of 5-HT responsivity are associated with behavioral and cognitive impulsivity. There is also a growing series of studies demonstrating preferential transmission of various genetic markers for 5-HT receptors that are expressed in ADHD. Currently, the heterogeneity of methods in this young discipline restricts the possibilities of definition of these markers and the types of ADHD in which they are expressed.

Inhibition of Rho-Kinase in the Nucleus Tractus Solitarius Enhances Glutamate Sensitivity in Rats

The Rho/Rho–kinase pathway in the central nervous system is involved in the maintenance of dendritic spines, which form the postsynaptic contact sites of excitatory synapses. Inhibition of the Rho–kinase pathway in neuron promotes dendritic spines or branches. In contrast, activation of the Rho/Rho–kinase pathway reduces dendritic spines or branches. Recent studies suggest that morphological changes of dendritic spines occur rapidly, and spine morphology is associated with glutamate sensitivity. The aim of the present study was to determine whether Rho-kinase activity affects glutamate sensitivity in the nucleus tractus solitarii (NTS) of Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). We first examined the effects of unilateral glutamate injection in the NTS. There was a significantly smaller decrease in arterial pressure in SHR than in WKY. We then examined the depressor responses evoked by unilateral glutamate injection into the NTS after preinjection of Y-27632, a specific Rho-kinase inhibitor. Preinjection of Y-27632 enhanced the glutamate response in both strains. However, the magnitude of the augmentation was significantly greater in SHR than in WKY. Furthermore, we recorded single-unit activity of NTS neurons from medulla brain slice preparations. N -methyl- d -aspartate (NMDA) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) was applied iontophoretically to the recorded neurons, and neuronal activity was recorded before and after Y-27632 perfusion. Y-27632 perfusion increased the response to NMDA and AMPA. These results suggest that inhibition of Rho-kinase activity in the NTS enhances glutamate sensitivity in WKY and SHR and might improve impaired glutamate sensitivity in SHR.

High salt diet enhances cardiovascular responses from the nucleus tractus solitarius and ventrolateral medulla of Sprague-Dawley rats

High salt intake has been shown to augment the sensitivity of rostral ventrolateral medulla (RVLM) sympathoexcitatory neurons. We examined the effects of 4 weeks of high dietary salt (8%) on the sensitivity of nucleus tractus solitarius (NTS) and caudal ventrolateral medulla (CVLM) in controlling RVLM. In chloralose-anesthetized Sprague-Dawley rats, high salt intake did not elevate baseline arterial pressure or heart rate (HR). In high-salt group, NTS, CVLM, and RVLM responses to glutamate were greater. NTS responses to acetylcholine or serotonin, which is independent of baroreflex, also were greater. Phenylephrine or nitroprusside (i.v.) elicited similar changes in arterial pressure and heart rate, the baroreflex sensitivity also was similar in both groups of rats. These results suggest that high salt intake augments the sensitivity of NTS and CVLM sending inhibitory input to RVLM. This presumably may inhibit the RVLM, thereby inhibiting the elevation of arterial pressure.

Effect of the activation of central 5-HT2C receptors by the 5-HT2C agonist mCPP on blood pressure and heart rate in rats

In the present study we investigated the role of central 5-HT2C receptors in the control of blood pressure and heart rate in non-stressed and stressed, adult, male, Wistar rats. Third ventricle injections of the 5-HT2C agonist mCPP elicited a significant increase in blood pressure in non-stressed animals. The initial period of this hypertensive response (10-30 min after mCPP administration) was accompanied by baroreflex-mediated bradycardia, while after this period the coexistence of hypertension and tachycardia was observed. These cardiovascular effects promoted by the central administration of mCPP were blocked by pretreatment with the 5-HT2C antagonist, SDZ SER 082. The administration of SDZ SER 082 alone induced no significant changes in blood pressure or heart rate. The pharmacological stimulation of central 5-HT2C receptors by mCPP did not change the hypertensive or tachycardic responses induced by restraint stress. Conversely, the blockade of central 5-HT2C receptors by SDZ SER 082 blunted stress-induced hypertension without modifying stress-induced tachycardia. It is concluded that the activation of central 5-HT2C receptors induces hypertension in non-stressed rats and that the normal function of these receptors is essential for the rise in blood pressure that occurs in the course of restraint stress.

Role of central 5-HT3 receptors in the control of blood pressure in stressed and non-stressed rats

The aim of the present study was to investigate the participation of central 5-HT(3) receptors in the control of blood pressure and heart rate (HR) of non-stressed and stressed rats. The pharmacological stimulation of brain 5-HT(3) receptors by third ventricle injections of the selective 5-HT(3) receptor agonist m-CPBG induced a significant decrease in blood pressure in non-stressed rats and impaired the hypertensive response induced by restraint stress. The blockade of brain 5-HT(3) receptors by the central administration of the selective 5-HT(3) antagonist ondansetron elicited a significant increase in blood pressure in non-stressed rats. Conversely, the hypertensive response induced by restraint stress was not affected by central administration of ondansetron. Additionally, baroreflex-mediated bradycardia during phenylephrine-induced hypertensive response was preserved in non-stressed animals receiving third ventricle injections of m-CPBG, while the baroreflex-mediated tachycardia that occurs during the hypotensive response induced by the administration of sodium nitroprusside was impaired. It is concluded that the serotoninergic component represented by the brain 5-HT(3) receptors exerts a tonic inhibitory influence on the central control of blood pressure in non-stressed rats, probably by a sympathoinhibitory-related mechanism. On the other hand, during stress, this central 5-HT(3)-dependent inhibitory drive is overwhelmed by the different neurochemical systems that harmonically trigger and sustain the hypertensive response.

Regional brain variations of cytochrome oxidase activity in spontaneously hypertensive mice

To explore the central disturbances resulting from blood pressure changes, spontaneously hypertensive mice (SHM) were compared to normotensive controls for cytochrome oxidase (CO) activity, an index of oxidative capacity in the central nervous system and a marker of long-term regional brain metabolism and neuronal activity. In all brain areas presenting significant enzymatic variations, only increases in CO activity were found in SHM, particularly the central autonomic network. However, only specific regions were affected, namely the insular cortex and the hypothalamic nuclei principally involved in high-order autonomic control. Altered limbic structures included the lateral septum, various hippocampal subregions, as well as prelimbic cortex. CO activity was also elevated in several forebrain regions, including those directly connected to the limbic system, such as the nucleus accumbens, the claustrum, and dorsomedial and reticular thalamic nuclei, as well as subthalamic and ventrolateral thalamic nuclei. In the brainstem, the only regions affected were the locus coeruleus, site of noradrenergic cell bodies, the trigeminal system, and four interconnected regions: the inferior colliculus, the paramedial reticular formation, the medial vestibular, and the cerebellar fastigial nuclei. These data show that specific regions modulating sympathetic nerve discharge are activated in young adult SHM, possibly due to mitochondrial dysfunction and excitotoxicity.

Blunted fenfluramine-evoked prolactin secretion in hypertensive rats

Plasma prolactin (PRL) levels after acute administration of fenfluramine (FEN) have been used as a probe of brain serotonin activity. FEN-evoked increases in PRL levels inversely correlate with arterial blood pressure (ABP) in humans (Muldoon et al. Hypertension. 1998;32:972-975), thereby suggesting that brain serotonin activity may be reduced in hypertension. The present study sought to determine whether the relation between FEN-evoked PRL levels and ABP was present in two rat models of hypertension. Experiments were performed in awake male rats that were instrumented with femoral arterial and venous catheters 2 days before experiments. FEN (3.0 mg/kg IV) significantly increased plasma PRL levels in both spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY); however, FEN-evoked PRL levels were significantly lower in SHR compared with WKY, though baseline levels were similar between strains. Similar results were obtained in rats with chronic hypertension produced by figure-8 renal wrap plus contralateral nephrectomy. In contrast, the increase in PRL levels evoked by the serotonin receptor agonist m-CPP or the dopamine receptor antagonist eticlopride did not differ between SHR and WKY, indicating that PRL secretion is not generally blunted in chronic hypertensive rats. Furthermore, FEN-evoked PRL levels were not attenuated in rats made acutely hypertensive by an infusion of the alpha-adrenergic agonist phenylephrine. Thus, the present findings are consistent with the human data and suggest that chronic hypertension is associated with a presynaptic alteration in brain serotonin function.

The relationship between resting blood pressure and acute pain sensitivity in healthy normotensives and chronic back pain sufferers: the effects of opioid blockade

Resting blood pressure is inversely correlated with acute pain sensitivity in healthy normotensives. This study tested: (1) whether endogenous opioid activity is necessary for this adaptive relationship to occur, (2) whether this relationship is altered in chronic low back pain (LBP), and (3) whether endogenous opioid dysfunction underlies any such alterations. Fifty-one pain-free normotensives and 44 normotensive chronic LBP sufferers received opioid blockade (8 mg naloxone i.v.) or placebo blockade (saline) in randomized, counterbalanced order in separate sessions. During each session, subjects participated in a 1-min finger pressure (FP) pain task followed by an ischemic (ISC) forearm pain task. Among pain-free normotensives, elevated resting systolic (SBP) and diastolic (DBP) blood pressure were associated with significantly higher ISC pain thresholds (P values <0.05). Elevated SBP was also associated with significantly lower FP pain ratings (P<0.05). Opioid blockade had no significant effect on the BP-pain relationships detected (P values >0.10). In combined groups analyses, a significant subject typexSBP interaction (P<0.005) was found on ISC pain threshold: elevated SBP was associated with higher pain threshold in pain-free controls, but with lower pain threshold in LBP subjects. Although subject typexBP interactions on FP and ISC pain ratings were not significant, inclusion of LBP subjects in these analyses resulted in the overall relationship between BP and pain sensitivity becoming positive (P values <0.05). Opioid blockade exerted no significant main or interaction effects in these combined groups analyses (p values >0.10). Higher DBP was associated with greater clinical pain intensity among the LBP subjects (P<0.001). Overall, these results suggest: (1) endogenous opioids do not mediate the inverse relationship between resting blood pressure and acute pain sensitivity in pain-free normotensives; (2) the BP-pain sensitivity relationship is altered in chronic pain, suggesting dysfunction in pain regulatory systems, and (3) these alterations are not related to opioid dysfunction.

Effect of phenylephrine injected into the nucleus tractus solitarius of Sprague-Dawley rats and spontaneously hypertensive rats

In alpha-chloralose-anesthetized Sprague-Dawley (SD) rats with unilateral nucleus tractus solitarius (NTS) lesions, injection of the alpha(1)-adrenergic receptor agonist phenylephrine into the contralateral NTS dose-dependently increased arterial pressure (AP). Bunazosin (0.1 nmol) or prazosin (0.36 nmol), an alpha(1)-adrenergic receptor antagonist, also increased AP. When injected into the NTS, pre-treatment with phenylephrine (10 nmol) or both antagonists abolished the cardiovascular effects of glutamate and acetylcholine. In contrast, pre-treatment with prazosin or methylatropine did not alter the effect of phenylephrine. Phenylephrine (30 nmol) injected into the NTS abolished aortic depressor nerve (ADN) evoked-responses. The pressor effect of phenylephrine in the NTS was exaggerated in spontaneously hypertensive rats (SHR). These results suggest that when injected into the NTS, the effect of phenylephrine may be due to a baroreflex blockade resulting from direct modulatory actions or non-specific neuronal alterations rather than stimulating the alpha(1)-adrenergic receptor. Additionally, this effect is enhanced in SHR.

Increased baroreceptor response in mice deficient in monoamine oxidase A and B

The recent development of mice doubly deficient for monoamine oxidase A and B (MAO-A/B, respectively) has raised questions about the impact of these mutations on cardiovascular function, in so far as these animals demonstrate increased tissue levels of the vasoactive amines serotonin, norepinephrine, dopamine, and phenylethylamine. We recorded femoral arterial pressures and electrocardiograms in adult MAO-A/B-deficient mice during halothane-nitrous oxide anesthesia as well as 30 min postoperatively. During both anesthesia and recovery, systolic, diastolic, and mean arterial pressures were 10-15 mmHg lower in MAO-A/B-deficient mice compared with normal controls (P < 0.01). Mutants also showed a greater baroreceptor-mediated reduction in heart rate in response to hypertension after intravenous pulses of phenylephrine or angiotensin II. Tachycardia elicited in response to hypotension after nitroprusside was greater in mutants than in controls. Heart rate responsiveness to changes in arterial pressure was abolished after administration of glycopyrrolate, with no differences in this phenomenon noted between genotypes. These data suggest that prevention of hypertension may occur in chronic states of catecholaminergic/indoleaminergic excess by increased gain of the baroreflex.