Daily exercise normalizes the number of diaphorase (NOS) positive neurons in the hypothalamus of hypertensive rats

A Critical Role for the Paraventricular Nucleus of the Hypothalamus in the Regulation of the Volume Reflex in Normal and Various Cardiovascular Disease States

PURPOSE OF REVIEW

This review focuses on studies implicating forebrain neural pathways and neuromodulator systems, particularly, the nitric oxide system within the paraventricular nucleus of the hypothalamus in regulating neurohumoral drive, autonomic pathways, and fluid balance.

RECENT FINDINGS

Accumulating evidence from animals with experimental models of hypertension and heart failure as well as humans with hypertension suggests that alterations in central neural pathways, particularly, within the PVN neuromodulated by neuronal nitric oxide, are involved in regulating sympathetic outflow particularly to the kidney resulting in alterations in fluid balance commonly observed in hypertension and heart failure states. The characteristics of the hypertensive and heart failure states include alterations in neuronal nitric oxide within the PVN to cause an increase in renal sympathetic nerve activity to result in sodium and fluid retention in these diseases. A comprehensive understanding of these mechanisms will enhance our ability to treat hypertensive and heart failure conditions and their cardiovascular complications more efficiently.

Age-dependent alterations to paraventricular nucleus insulin-like growth factor 1 receptor as a possible link between sympathoexcitation and inflammation

Modifications to neural circuits of the paraventricular hypothalamic nucleus (PVN) have been implicated in sympathoexcitation and systemic cardiovascular dysfunction. However, to date, the role of insulin-like growth factor 1 receptor (IGF-1R) expression on PVN pathophysiology is unknown. Using confocal immunofluorescence quantification and electrophysiological recordings from acute PVN slices, we investigated the mechanism through which age-dependent IGF-1R depletion contributes to the progression of inflammation and sympathoexcitation in the PVN of spontaneously hypertensive rats (SHR). Four and twenty weeks old SHR and Wistar Kyoto (WKY) rats were used for this study. Our data showed that angiotensin I/II and pro-inflammatory high mobility box group protein 1 (HMGB1) exhibited increased expression in the PVN of SHR versus WKY at 4 weeks (p < 0.01), and were even more highly expressed with age in SHR (p < 0.001). This correlated with a significant decrease in IGF-1R expression, with age, in the PVN of SHR when compared with WKY (p < 0.001) and were accompanied by related changes in astrocytes and microglia. In subsequent analyses, we found an age-dependent change in the expression of proteins associated with IGF-1R signaling pathways involved in inflammatory responses and synaptic function in the PVN. MAPK/ErK was more highly expressed in the PVN of SHR by the fourth week (p < 0.001; vs. WKY), while expression of neuronal nitric oxide synthase (p < 0.001) and calcium-calmodulin-dependent kinase II alpha (CamKIIα; p < 0.001) were significantly decreased by the 4th and 20th week, respectively. Age-dependent changes in MAPK/ErK expression in the PVN correlated with an increase in the expression of vesicular glutamate transporter (p < 0.001 vs. WKY), while decreased levels of CamKIIα was associated with a decreased expression of tyrosine hydroxylase (p < 0.001) by the 20th week. In addition, reduced labeling for ϒ-aminobutyric acid in the PVN of SHR (p < 0.001) correlated with a decrease in neuronal nitric oxide synthase labeling (p < 0.001) when compared with the WKY by the 20th week. Electrophysiological recordings from neurons in acute slice preparations of the PVN of 4 weeks old SHR revealed spontaneous post-synaptic currents of higher frequency when compared with neurons from WKY PNV slices of the same age (p < 0.001; n = 14 cells). This also correlated with an increase in PSD-95 in the PVN of SHR when compared with the WKY (p < 0.001). Overall, we found an age-dependent reduction of IGF-1R, and related altered expression of associated downstream signaling molecules that may represent a link between the concurrent progression of synaptic dysfunction and inflammation in the PVN of SHR.

Aerobic physical training increases contractile response and reduces cardiac fibrosis in rats subjected to early ovarian hormone deprivation

We investigated the effects of early ovarian hormone deprivation on the heart and the role of physical training in this condition using different approaches: cardiac autonomic tone, contractility, morphology and function, and cardiac fibrosis. Female Wistar rats ( n = 48) were assigned into two groups: ovariectomized (Ovx; 10-wk-old) and control rats (Sham; 10-wk-old). Each group was further divided into two subgroups, sedentary and trained (aerobic training by swimming for 10 wk). The sedentary groups showed similar cardiac autonomic tone values; however, only the Sham group had an increase in vagal participation for the determination of the basal heart rate after physical training. The contractile responses to cardiac β-agonists of the sedentary groups were similar, including an increased response to a β1-agonist (dobutamine) observed after physical training. The Ovx sedentary group presented changes in cardiac morphology, which resulted in decreases in the ejection fraction, fractional shortening, and cardiac index compared with the Sham sedentary group. Physical training did little to alter these findings. Moreover, histology analysis showed a significant increase in cardiac fibrosis in the sedentary Ovx group, which was not observed in the trained Ovx group. We conclude that early ovarian hormone deprivation in rats impairs autonomic control, cardiac morphology, and cardiac function and increases cardiac fibrosis; however, it does not affect the contractility induced by dobutamine and salbutamol. Furthermore, this model of physical training prevented an increase in fibrosis and promoted an increase in the cardiac contractile response but had little effect on cardiac autonomic control or morphological and functional parameters.

Modulation of angiotensin II signaling following exercise training in heart failure

Sympathetic activation is a consistent finding in the chronic heart failure (CHF) state. Current therapy for CHF targets the renin-angiotensin II (ANG II) and adrenergic systems. Angiotensin converting enzyme (ACE) inhibitors and ANG II receptor blockers are standard treatments along with β-adrenergic blockade. However, the mortality and morbidity of this disease is still extremely high, even with good medical management. Exercise training (ExT) is currently being used in many centers as an adjunctive therapy for CHF. Clinical studies have shown that ExT is a safe, effective, and inexpensive way to improve quality of life, work capacity, and longevity in patients with CHF. This review discusses the potential neural interactions between ANG II and sympatho-excitation in CHF and the modulation of this interaction by ExT. We briefly review the current understanding of the modulation of the angiotensin type 1 receptor in sympatho-excitatory areas of the brain and in the periphery (i.e., in the carotid body and skeletal muscle). We discuss possible cellular mechanisms by which ExT may impact the sympatho-excitatory process by reducing oxidative stress, increasing nitric oxide. and reducing ANG II. We also discuss the potential role of ACE2 and Ang 1-7 in the sympathetic response to ExT. Fruitful areas of further investigation are the role and mechanisms by which pre-sympathetic neuronal metabolic activity in response to individual bouts of exercise regulate redox mechanisms and discharge at rest in CHF and other sympatho-excitatory states.

Exercise training lowers the enhanced tonically active glutamatergic input to the rostral ventrolateral medulla in hypertensive rats

AIMS

It is well known that low-intensity exercise training (ExT) is beneficial to cardiovascular dysfunction in hypertension. The tonically active glutamatergic input to the rostral ventrolateral medulla (RVLM), a key region for control of blood pressure and sympathetic tone, has been demonstrated to be increased in hypertensive rats. The aim of this study was to determine the effect of ExT on the increased glutamatergic input to the RVLM in spontaneously hypertensive rat (SHR).

METHODS

Normotensive rats Wistar-Kyoto (WKY) and SHR were treadmill trained or remained sedentary (Sed) for 12 weeks and classed into four groups (WKY-Sed, WKY-ExT, SHR-Sed, and SHR-ExT). The release of glutamate in the RVLM and its contribution to cardiovascular activity were determined in WKY and SHR after treatment of ExT.

RESULTS

Blood pressure and sympathetic tone were significantly reduced in SHR after treatment with ExT. Bilateral microinjection of the glutamate receptor antagonist kynurenic acid (2.7 nmol in 100 nL) into the RVLM significantly decreased resting blood pressure, heart rate, and renal sympathetic nerve activity in SHR-Sed but not in WKY groups (WKY-Sed and WKY-ExT). However, the degree of reduction in these cardiovascular parameters evoked by KYN was significantly blunted in SHR-ExT compared with SHR-Sed group. The concentration of glutamate and the protein expression of vesicular glutamate transporter 2 in the RVLM were significantly increased in SHR-Sed compared with WKY-Sed, whereas they were reduced after treatment with ExT.

CONCLUSION

Our findings suggest that ExT attenuates the enhancement in the tonically acting glutamatergic input to the RVLM of hypertensive rats, thereby reducing the sympathetic hyperactivity and blood pressure.

Paraventricular nucleus control of blood pressure in two-kidney, one-clip rats: effects of exercise training and resting blood pressure

Exercise-induced changes in γ-aminobutyric acid (GABA) or nitric oxide signaling within the paraventricular nucleus (PVN) have not been studied in renovascular hypertension. We tested whether exercise training decreases mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) in two-kidney, one-clip (2K-1C) hypertensive rats due to enhanced nitric oxide or GABA signaling within PVN. Conscious, unrestrained male Sprague-Dawley rats with either sham (Sham) or right renal artery clipping (2K-1C) were assigned to sedentary (SED) or voluntary wheel running (ExT) for 6 or 12 wk. MAP and angiotensin II (ANG II) were elevated in 2K-1C SED rats. The 2K-1C ExT rats displayed lower MAP at 6 wk that did not decline further by 12 wk. Plasma ANG II was lower in 2K-1C ExT rats. Increases in MAP, heart rate, and RSNA to blockade of PVN nitric oxide in 2K-1C SED rats were attenuated compared with either Sham group. Exercise training restored the responses in 2K-1C ExT rats. The increase in MAP in response to bicuculline was inversely correlated with baseline MAP. The rise in MAP was lower in 2K-1C SED vs. either Sham group and was normalized in the 2K-1C ExT rats. Paradoxically, heart rate and RSNA responses were not diminished in 2K-1C SED rats but were significantly lower in the 2K-1C ExT rats. Thus the decrease in arterial pressure in 2K-1C hypertension associated with exercise training is likely due to diminished excitatory inputs to PVN because of lower ANG II and higher nitritergic tone rather than enhanced GABA inhibition of sympathetic output.

Regulation of arterial pressure by the paraventricular nucleus in conscious rats: interactions among glutamate, GABA, and nitric oxide

The paraventricular nucleus (PVN) of the hypothalamus is an important site for autonomic and neuroendocrine regulation. Experiments in anesthetized animals and in vitro indicate an interaction among gamma-aminobutyric acid (GABA), nitric oxide (NO), and glutamate in the PVN. The cardiovascular role of the PVN and interactions of these neurotransmitters in conscious animals have not been evaluated fully. In chronically instrumented conscious rats, mean arterial pressure (MAP) and heart rate (HR) responses to microinjections (100 nl) in the region of the PVN were tested. Bilateral blockade of ionotropic excitatory amino acid (EAA) receptors (kynurenic acid, Kyn) in the PVN produced small but significant decreases in MAP and HR. GABA_A receptor blockade (bicuculline, Bic), and inhibition of NO synthase [(NOS), N-(G)-monomethyl-L-arginine, L-NMMA] each increased MAP and HR. The NO donor sodium nitroprusside (SNP) produced depressor responses that were attenuated by Bic. NOS inhibition potentiated both pressor responses to the selective EAA agonist, N-methyl-D-aspartic acid (NMDA), and depressor responses to Kyn. Increases in MAP and HR due to Bic were blunted by prior blockade of EAA receptors. Thus, pressor responses to GABA blockade require EAA receptors and GABA neurotransmission contributes to NO inhibition. Tonic excitatory effects of glutamate in the PVN are tonically attenuated by NO. These data demonstrate that, in the PVN of conscious rats, GABA, glutamate, and NO interact in a complex fashion to regulate arterial pressure and HR under normal conditions.

Construction and validation of lentiviral vector carrying rat neuronal nitric oxide synthase in vitro and in vivo

In the present study, we developed a lentiviral vector with human cytomegalovirus promoter permitting high-level of nNOS expression. Neuronal cell line NG108 was used as an in vitro model to check the validity of gene transfer. The cells were infected with lenti-EGFP or lenti-nNOS particles for 24h. Lenti-nNOS infection in the NG108 cells induced dose dependent increase in mRNA and protein for nNOS; with a dose of 2.5 × 10⁴ pfu/ml, nNOS mRNA expression increased by 40-fold while protein expression was increased by 2.5-fold compared to lenti-EGFP. Moreover, lenti-nNOS infection caused a greater increase in nNOS immunoreactivity in NG108 cells compared to lenti-EGFP as shown by immonocytochemistry. nNOS expression showed time dependent increases with lenti-nNOS infection with maximum up-regulation observed after two weeks of infection. Moreover, in vivo, unilateral injection of lenti-nNOS into the paraventricular nucleus (PVN) of rats induced a 27-fold increase of nNOS protein level in the injected side compared to non-injected side and this escalation was sustained up to three weeks. Overall, lenti-EGFP injection in the PVN did not show any significant change in nNOS expression. Furthermore, NADPH-diaphorase staining of nNOS in the PVN infected with lenti-nNOS induced a visible increase in nNOS expression compared with contralateral non-injected side up to three weeks. These results indicate that this approach of lentiviral mediated gene transfer of nNOS may provide a new means to up-regulate the nNOS expression for longer periods of time compared to adenoviral transfection and can be used as a research tool and potentially a therapy for chronic diseases involving impaired nNOS expression.

Chronic treadmill running in normotensive rats resets the resting blood pressure to lower levels by upregulating the hypothalamic GABAergic system

OBJECTIVE

The cardiovascular integration center not only sends out signals to offset the stimulus-induced responses but also resets the resting blood pressure. We hypothesize that GABAergic adaptations in the hypothalamus participate in the chronic exercise-induced cardiovascular resetting effects in conscious normotensive animals.

METHODS

Male Wistar rats were subjected to chronic moderate exercise (CME, 8-week treadmill running at moderate intensity). A biotelemetry system was used to measure blood pressure, heart rate, autonomic nervous activities, baroreflex sensitivity and endogenous GABAergic activities in the paraventricular nucleus and the posterior hypothalamic area. Hypothalamic specimens were collected for quantifying GABA-related proteins and GABAergic neurons.

RESULTS

CME reduced resting blood pressure, heart rate, sympathetic activity and enhanced parasympathetic activity and baroreflex sensitivity. Additionally, CME elevated the resting level of hypothalamic GABAergic activities, increased the percentage of GABAergic neurons in the hypothalamus and upregulated the hypothalamic protein levels of neuronal nitric oxide synthase, GAD67 and gephyrin, but not GABAA receptor. Moreover, a single bout of moderate exercise transiently elevated blood pressure and heart rate with prolonged high levels of neural controls (sympathetic activity, baroreflex sensitivity and hypothalamic GABAergic activities). CME accelerated the postexercise recovery in cardiovascular parameters and neural control alterations.

CONCLUSION

Chronic treadmill running in normotensive rats augmented the GABAergic system in both paraventricular nucleus and posterior hypothalamic area, resulting in lower resting blood pressure, heart rate and sympathetic tone under conscious unrestraint conditions. This study provides insight into mechanisms important for explaining how chronic exercise resets the resting blood pressure.

Chronic intermittent hypoxia induces NMDA receptor-dependent plasticity and suppresses nitric oxide signaling in the mouse hypothalamic paraventricular nucleus

Chronic intermittent hypoxia (CIH) is a concomitant of sleep apnea that produces a slowly developing chemosensory-dependent blood pressure elevation ascribed in part to NMDA receptor-dependent plasticity and reduced nitric oxide (NO) signaling in the carotid body. The hypothalamic paraventricular nucleus (PVN) is responsive to hypoxic stress and also contains neurons that express NMDA receptors and neuronal nitric oxide synthase (nNOS). We tested the hypothesis that extended (35 d) CIH results in a decrease in the surface/synaptic availability of the essential NMDA NR1 subunit in nNOS-containing neurons and NMDA-induced NO production in the PVN of mice. As compared with controls, the 35 d CIH-exposed mice showed a significant increase in blood pressure and an increased density of NR1 immunogold particles located in the cytoplasm of nNOS-containing dendrites. Neither of these between-group differences was seen after 14 d, even though there was already a reduction in the NR1 plasmalemmal density at this time point. Patch-clamp recording of PVN neurons in slices showed a significant reduction in NMDA currents after either 14 or 35 d exposure to CIH compared with sham controls. In contrast, NO production, as measured by the NO-sensitive fluorescent dye 4-amino-5-methylamino-2',7'-difluorofluorescein, was suppressed only in the 35 d CIH group. We conclude that CIH produces a reduction in the surface/synaptic targeting of NR1 in nNOS neurons and decreases NMDA receptor-mediated currents in the PVN before the emergence of hypertension, the development of which may be enabled by suppression of NO signaling in this brain region.

Immunohistochemical expression of Bcl-2, p53 and caspase-9 in hypothalamus magnocellular centers of nNOS knockout mice following water deprivation

We studied the interactions between apoptosis regulator proteins (Bcl-2, p53 and caspase-9) and neuronal nitric oxide in vasopressinergic magnocellular centers of the hypothalamus using neuronal nitric oxide synthase (nNOS) gene knockout mice. nNOS gene deletion resulted in accumulation of Bcl-2, p53 and caspase-9 in the paraventricular (PVN) and supraoptic (SON) nuclei in controls. Dehydration increased the levels of all three apoptosis regulator proteins studied in nuclei of wild type mice. In the hypothalamus magnocellular centers of nNOS knockout mice, however, expression of Bcl-2, p53 and caspase-9 was unchanged after dehydration. The number of magnocellular neurons did not change in the SON and PVN of nNOS deficient mice compared to wild type, and after dehydration, cell death was not observed in either nucleus of wild type or knockout mice despite activation of apoptosis regulator protein expression. Thus, we demonstrated that gene disruption of nNOS prevents activation of Bcl-2, p53 and caspase-9 expression during water deprivation, and that nNOS deficiency did not affect survival of magnocellular neurons of the hypothalamus.

Neuronal nitric oxide synthase within paraventricular nucleus: blood pressure and baroreflex in two-kidney, one-clip hypertensive rats

The renin-angiotensin system is activated in the early phase of two-kidney, one-clip (2K-1C) hypertension. The paraventricular nucleus (PVN) integrates inputs regulating sympathetic outflow. The PVN receives inputs from plasma angiotensin II via projections from circumventricular organs and from renal afferent nerves transmitted via the nucleus tractus solitarii. Nitric oxide within the PVN may exert a sympathoinhibitory effect. These studies tested whether decreasing endogenous nitric oxide by introducing dominant negative (DN) constructs for neuronal nitric oxide synthase (nNOS) into PVN chronically augments hypertension and/or modulates baroreflex function. Male 6-week-old Sprague-Dawley rats underwent sham surgery or right renal artery clipping and placement of radiotelemetry transmitters. One week later, the PVN was injected bilaterally with 250 nl artificial cerebrospinal fluid containing 250 ng microl(-1) of RSV beta-galactosidase (beta-Gal), cytomegalovirus (CMV) wild-type (WT nNOS), or respiratory syncytial virus (RSV) haeme domain or RSV haemeRedF (DN nNOS). Haemodynamics were monitored for 5 weeks. Then left renal nerve electrodes were placed, and 2 days later the rats underwent baroreflex testing in the conscious state. The rise in mean arterial pressure (MAP) was significantly potentiated in the DN nNOS 2K-1C group beyond 15 days after PVN injection. By day 35, MAP in the 2K-1C groups was 152 +/- 6.3 (beta-Gal), 155.1 +/- 6.6 (WT nNOS) and 179 +/- 5.4 mmHg (DN nNOS; P < 0.01 versus all other groups). Sham-clipped rats remained normotensive. All groups displayed progressive bradycardia over time that was attenuated in the DN nNOS 2K-1C group. Baroreflex curves shifted to higher pressures, and baroreflex sensitivity of heart rate was diminished to a similar extent in all groups of 2K-1C rats. The baroreflex response of renal sympathetic nerve activity was preserved. The PVN tissue from DN nNOS rats had decreased dimerization of nNOS and generation of total nitric oxide. These findings indicate that chronic interference of nNOS dimerization required for generation of nitric oxide within the PVN potentiates the increase of blood pressure by modulating the sympathoexcitation that accompanies renovascular hypertension.

The effect of aerobic physical training on cardiac autonomic control of rats submitted to ovariectomy

OBJECTIVE

To investigate the effect of aerobic physical training on cardiovascular autonomic control in ovariectomized rats using different approaches.

DESIGN

Female Wistar rats were divided into four groups: sedentary sham rats (group SSR), trained sham rats(group TSR), sedentary ovariectomized rats (group SOR), and trained ovariectomized rats (group TOR). Animals from the trained groups were submitted to a physical training protocol (swimming) for 12 weeks.

RESULTS

Pharmacological evaluation showed that animals from group TSR had an increase in their cardiac vagal tonus compared with the animals from groups SSR and SOR. The analysis of heart rate variability (HRV) showed that groups TSR and SOR had fewer low-frequency oscillations (0.20-0.75 Hz) compared with groups SSR and TOR.When groups TSR and SOR were compared, the former was found to have fewer oscillations. With regard to high frequency oscillations (0.75-2.5 Hz), group SSR had a reduction compared with the other groups, whereas group TSR had the greatest oscillation compared with groups SOR and TOR, with all values expressed in normalized units.Analysis of HRV was performed after pharmacological blockade, and low-frequency oscillations were found to be predominantly sympathetic in sedentary animals, whereas there was no predominance in trained animals.

CONCLUSION

Ovariectomy did not change the tonic autonomic control of the heart and, in addition, reduced the participation of sympathetic component in cardiac modulation. Physical training, on the other hand, increased the participation of parasympathetic modulation on the HRV, including ovariectomized rats.

Nitric oxide synthase activity and expression are decreased in the paraventricular nucleus of pregnant rats

Pregnancy is characterized by elevated heart rate and decreased total peripheral resistance and arterial blood pressure. Plasma volume is expanded and plasma osmolality is decreased, yet vasopressin secretion in pregnant animals, including humans, is no different than levels in the nonpregnant state. Although reflex compensatory sympathoexcitation is suppressed, baseline sympathetic nerve activity to the heart and vasculature is well maintained or slightly elevated in pregnancy. Clearly there are central nervous system (CNS) adaptations in systems for regulation of cardiovascular and body fluid homeostasis in pregnant animals. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important CNS sites for control of sympathetic nerve activity and vasopressin secretion. Nitric oxide (NO), an important neuromodulator in these hypothalamic nuclei, contributes to tonic inhibition of neurosecretory and pre-autonomic neurons. Alterations in NO within the PVN and SON could contribute to changes in regulation of vasopressin and sympathetic nerve activity in pregnancy. In the present study, nitric oxide synthase (NOS) activity (NADPH-diaphorase staining), neuronal NOS (nNOS) protein, and nNOS mRNA were assessed in nonpregnant estrus stage and near-term pregnant rats. nNOS mRNA, protein, and activity were greater in the PVN than in the SON. In the PVN only, pregnancy was associated with significant decreases in all three measurements for assessment of nNOS. Thus decreased NO production and relative disinhibition of the PVN may contribute to maintenance of baseline vasopressin secretion and baseline sympathetic nerve activity in the pregnant state.

Voluntary wheel running and pacing-induced dysfunction in hypertension

PURPOSE

We examined how voluntary wheel running in the female, spontaneously hypertensive rat (SHR) impacts myocardial tolerance to pacing stress and determined whether direct adenylyl cyclase agonism via forskolin infusion improved myocardial performance during pacing.

METHODS

Twenty-five 16-week-old female Wistar Kyoto (WKY, n = 8) and SHR (n = 17) were utilized. Animals within the SHR group were randomly assigned to a sedentary (SHR-SED, n = 8) or a voluntary wheel running (SHR-WHL, n = 9) group. The SHR-WHL had free access to a running wheel 24 h/day. Resting heart rates and blood pressures were collected immediately prior to sacrifice utilizing a tail cuff apparatus. Left ventricular (LV) function was measured in a Langendorff, isovolumic preparation during pacing stress (8.5 Hz) and during pacing stress + forskolin (5 micromol/L).

RESULTS

SHR-WHL showed cardiac enlargement without alterations in heart rate, systolic blood pressure, or rate-pressure product. Pacing stress impaired inotropic and lusitropic performance to a similar extent in all groups (p < 0.05), while forskolin infusion improved LV function to a similar extent in all groups (p < 0.05).

CONCLUSIONS

These data suggest that voluntary wheel running in SHR does not protect from pacing-induced myocardial dysfunction, and adenylyl cyclase agonism during pacing stress can functionally protect the heart. These data reiterate the importance of a competent myocardial beta-adrenergic signaling cascade.

Heterogeneous distribution of basal cyclic guanosine monophosphate within distinct neuronal populations in the hypothalamic paraventricular nucleus

The supraoptic (SON) and the paraventricular (PVN) hypothalamic nuclei constitute major neuronal substrates underlying nitric oxide (NO) effects on autonomic and neuroendocrine control. Within these nuclei, constitutively produced NO restrains the firing activity of magnocellular neurosecretory and preautonomic neurons, actions thought to be mediated by a cGMP-dependent enhancement of GABAergic inhibitory transmission. In the present study, we expanded on this knowledge by performing a detailed anatomical characterization of constitutive NO-receptive, cGMP-producing neurons within the PVN. To this end, we combined tract-tracing techniques and immunohistochemistry to visualize cGMP immunoreactivity within functionally, neurochemically, and topographically discrete PVN neuronal populations in Wistar rats. Basal cGMP immunoreactivity was readily observed in the PVN, both in neuronal and vascular profiles. The incidence of cGMP immunoreactivity was significantly higher in magnocellular (69%) compared with preautonomic ( approximately 10%) neuronal populations (P < 0.01). No differences were observed between oxytocin (OT) and vasopressin (VP) magnocellular neurons. In preautonomic neurons, the incidence of cGMP was independent of their subnuclei distribution, innervated target (i.e., intermediolateral cell column, nucleus tractus solitarii, or rostral ventrolateral medulla) or their neurochemical phenotype (i.e., OT or VP). Finally, high levels of cGMP immunoreactivity were observed in GABAergic somata and terminals within the PVN of eGFP-GAD67 transgenic mice. Altogether, these data support a highly heterogeneous distribution of basal cGMP levels within the PVN and further support the notion that constitutive NO actions in the PVN involve intricate cell-cell interactions, as well as heterogeneous signaling modalities.

Water-deprivation-induced expression of neuronal nitric oxide synthase in the hypothalamic paraventricular nucleus of rat

This study was conducted to define the molecular mechanism by which dehydration induces expression of neuronal nitric oxide synthase (nNOS) in the hypothalamic paraventricular nucleus (PVN). Rats were deprived from water for 48 hr and then sacrificed immediately or 1 hr after ad libitum access to water. Another group of rats had free access to food and water and was included as euhydrate control group. The PVN sections fixed with 4% paraformaldehyde were processed for nNOS immunohistochemistry and NADPH-diaphorase (NADPH-d)/pCREB or NADPH-d/c-Fos double staining. nNOS-ir neurons significantly increased with water deprivation and decreased with rehydration, both in the posterior magnocellular (pM)- and the medial parvocellular (mP)-PVN. Most NADPH-d histostained neurons in the PVN appeared to exhibit pCREB-ir as well. Water deprivation markedly increased, and rehydration decreased, NADPH-d/pCREB neurons both in the pM- and in the mP-PVN. Gel shift assay demonstrated that dehydration may promote CREB binding to nNOS promoter in the PVN neurons. Significant amounts of NADPH-d-stained neurons in the PVN of water-deprived rats (67-68% in both the mP and the pM) exhibited c-Fos-ir. NADPH-d/c-Fos neurons in the pM-PVN were increased by water deprivation but not changed by rehydration. NADPH-d/c-Fos double-stained neurons in the mP-PVN did not significantly change depending on different water conditions. These results suggest that pCREB may play a role in dehydration-induced nNOS gene expression in the PVN neurons, and c-Fos might not be implicated in the regulatory pathway.

Diminished A-type potassium current and altered firing properties in presympathetic PVN neurones in renovascular hypertensive rats

Accumulating evidence supports a contribution of the hypothalamic paraventricular nucleus (PVN) to sympathoexcitation and elevated blood pressure in renovascular hypertension. However, the underlying mechanisms resulting in altered neuronal function in hypertensive rats remain largely unknown. Here, we aimed to address whether the transient outward potassium current (I(A)) in identified rostral ventrolateral medulla (RVLM)-projecting PVN neurones is altered in hypertensive rats, and whether such changes affected single and repetitive action potential properties and associated changes in intracellular Ca(2+) levels. Patch-clamp recordings obtained from PVN-RVLM neurons showed a reduction in I(A) current magnitude and single channel conductance, and an enhanced steady-state current inactivation in hypertensive rats. Morphometric reconstructions of intracellularly labelled PVN-RVLM neurons showed a diminished dendritic surface area in hypertensive rats. Consistent with a diminished I(A) availability, action potentials in PVN-RVLM neurons in hypertensive rats were broader, decayed more slowly, and were less sensitive to the K(+) channel blocker 4-aminopyridine. Simultaneous patch clamp recordings and confocal Ca(2+) imaging demonstrated enhanced action potential-evoked intracellular Ca(2+) transients in hypertensive rats. Finally, spike broadening during repetitive firing discharge was enhanced in PVN-RVLM neurons from hypertensive rats. Altogether, our results indicate that diminished I(A) availability constitutes a contributing mechanism underlying aberrant central neuronal function in renovascular hypertension.

Exercise training and sympathetic nervous system activity: evidence for physical activity dependent neural plasticity

1. It has been generally accepted that regular physical activity is associated with beneficial effects on the cardiovascular system. In fact, the idea that exercise maintains cardiovascular health is evident by the direct links between a sedentary lifestyle and the risk of cardiovascular and other disease states. 2. Cardiovascular diseases, such as hypertension and heart failure, are often associated with sympathetic nervous system (SNS) overactivity. Conversely, exercise has been shown to reduce hypertension and decrease elevated SNS activity. In addition, there is evidence that exercise may reduce resting blood pressure and sympathetic outflow in normal individuals. 3. Although somewhat controversial in humans, evidence from animal studies also indicates that exercise training reduces baroreflex-mediated and other forms of sympathoexcitation in normal individuals. Collectively, these data are consistent with the hypothesis that physical activity may decrease, and physical inactivity may increase, the incidence of cardiovascular disease via alterations in SNS activity. Despite the important clinical implications of this possibility, the mechanisms by which exercise alters control of SNS activity remain to be fully elucidated. 4. Recent evidence suggests that central nervous system (CNS) plasticity occurs under a variety of conditions, including varying levels of physical activity. The purpose of the present brief review is to provide evidence that changes within the CNS contribute importantly to altered regulation of the SNS observed following exercise training. The primary hypothesis is that physical activity versus inactivity produces plasticity within neural networks that regulate SNS activity. This hypothesis is supported by published and preliminary data that suggest that exercise training may reduce sympathoexcitation by reducing activation of neurons within cardiovascular regions of the brain. These mechanisms are likely to be important in disease states of sympathetic overactivity and in normal healthy individuals whose risk of cardiovascular disease is reduced by leading an active versus sedentary lifestyle.

Exercise training-induced remodeling of paraventricular nucleus (nor)adrenergic innervation in normotensive and hypertensive rats

Activation of oxytocin (OT)ergic projections from the hypothalamic paraventricular nucleus (PVN) to the nucleus tractus solitarii contributes to cardiovascular adjustments during exercise training (EXT). Moreover, a deficit in this central OTergic pathway is associated with altered cardiovascular function in hypertension. Since PVN catecholaminergic inputs, known to be activated during EXT, modulate PVN cardiovascular-related functions, we aimed here to determine whether remodeling of PVN (nor)adrenergic innervation occurs during EXT and whether this phenomenon is affected by hypertension. Confocal immunofluorescence microscopy and tract tracing were used to quantify changes in (nor)adrenergic innervation density in PVN subnuclei and in identified dorsal vagal complex (DVC) projecting neurons (PVN-DVC) in EXT normotensive [Wistar-Kyoto rat (WKY)] and hypertensive [spontaneously hypertensive rat (SHR)] rats. In WKY, EXT increased the density of PVN dopamine β-hydroxylase immunoreactivity (DBHir) (160%). Furthermore, the number and density of DBHir boutons overlapping PVN-DVC OTergic neurons were also increased during EXT (130%), effects that were blunted in SHR. Conversely, while DBHir in the medial parvocellular subnucleus (an area enriched in corticotropin-releasing hormone neurons) was not changed by EXT in WKY, a diminished DBHir was observed in trained SHR. Overall, these data support the concept that the PVN (nor)adrenergic innervation undergoes plastic remodeling during EXT, an effect that is differentially affected during hypertension. The functional implications of PVN (nor)adrenergic remodeling in relation to the central peptidergic control of cardiovascular function during EXT are discussed.

Exercise training prevents arterial baroreflex dysfunction in rats treated with central angiotensin II

Angiotensin II (Ang II)-induced arterial baroreflex dysfunction is associated with superoxide generation in the brain. Exercise training (EX) improves baroreflex function and decreases oxidative stress in cardiovascular diseases linked to elevated central Ang II. The aim of this study was to determine whether previous EX prevents baroreflex impairment caused by central administration of exogenous Ang II via an Ang II-superoxide mechanism. Four groups of rats were used: non-EX artificial cerebrospinal fluid infused, non-EX Ang II infused, EX artificial cerebrospinal fluid infused, and EX Ang II infused. Rats were treadmill trained for 3 to 4 weeks and subjected to intracerebroventricular infusion of Ang II over the last 3 days of EX. Twenty-four hours after the end of EX, the arterial baroreflex was assessed in anesthetized rats. Compared with non-EX artificial cerebrospinal fluid-infused rats, Ang II significantly decreased baroreflex sensitivity (maximum gain: 3.0+/-0.2% of maximum per millimeter of mercury versus 1.6+/-0.1% of maximum per millimeter of mercury; P<0.01), which was abolished by acute intracerebroventricular infusion of the Ang II type 1 receptor antagonist losartan and the reduced nicotinamide-adenine dinucleotide phosphate oxidase inhibitor apocynin. EX prevented the decrease in baroreflex sensitivity and downregulated Ang II type 1 receptor and NADPH oxidase subunit protein expression in the paraventricular nucleus of Ang II-infused rats. Finally, EX decreased superoxide production in the paraventricular nucleus of Ang II-infused rats. These results indicate that EX improves arterial baroreflex function in conditions of high brain Ang II, which is mediated by the central Ang II type 1 receptor and associated with a reduction in central oxidative stress.

Exercise training attenuates increases in lumbar sympathetic nerve activity produced by stimulation of the rostral ventrolateral medulla

Exercise training (ExTr) has been associated with blunted activation of the sympathetic nervous system in several animal models and in some human studies. Although these data are consistent with the hypothesis that ExTr reduces the incidence of cardiovascular diseases via reduced sympathoexcitation, the mechanisms are unknown. The rostral ventrolateral medulla (RVLM) is important in control of sympathetic nervous system activity in both physiological and pathophysiological states. The purpose of the present study was to test the hypothesis that ExTr results in reduced sympathoexcitation mediated at the level of the RVLM. Male Sprague-Dawley rats were treadmill trained or remained sedentary for 8-10 wk. RVLM microinjections were performed under Inactin anesthesia while mean arterial pressure, heart rate, and lumbar sympathetic nerve activity (LSNA) were recorded. Bilateral microinjections of the GABA(A) antagonist bicuculline (5 mM, 90 nl) into the RVLM increased LSNA in sedentary animals (169 +/- 33%), which was blunted in ExTr animals (100 +/- 22%, P < 0.05). Activation of the RVLM with unilateral microinjections of glutamate (10 mM, 30 nl) increased LSNA in sedentary animals (76 +/- 13%), which was also attenuated by training (26 +/- 2%, P < 0.05). Bilateral microinjections of the ionotropic glutamate receptor antagonist kynurenate (40 mM, 90 nl) produced small increases in mean arterial pressure and LSNA that were similar between groups. Results suggest that ExTr may reduce increases in LSNA due to reduced activation of the RVLM. Conversely, we speculate that the relatively enhanced activation of LSNA in sedentary animals may be related to the increased incidence of cardiovascular disease associated with a sedentary lifestyle.

Increased nitric oxide synthase activity and expression in the hypothalamus of hindlimb unloaded rats

Upon return from spaceflight or resumption of normal posture after bed rest, individuals often exhibit cardiovascular deconditioning. Although the mechanisms responsible for cardiovascular deconditioning have yet to be fully elucidated, alterations within the central nervous system have been postulated to be involved. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important brain regions in control of sympathetic outflow and body fluid homeostasis. Nitric oxide (NO) modulates the activity of PVN and SON neurons, and alterations in NO transmission within these brain regions may contribute to symptoms of cardiovascular deconditioning. The purpose of the present study was to examine nitric oxide synthase (NOS) activity and expression in the PVN and SON of control and hindlimb unloaded (HU) rats, an animal model of cardiovascular deconditioning. The number of neurons exhibiting NOS activity as assessed by NADPH-diaphorase staining was significantly greater in the PVN but not SON of HU rats. Western blot analysis revealed that neuronal NOS (nNOS) but not endothelial NOS (eNOS) protein expression was higher in the PVN of HU rats. In the SON, there was a strong trend for an increase in nNOS (p=0.052) and a significant increase in eNOS expression in HU rats. Our results suggest that increased nNOS in the PVN contributes to autonomic and humoral alterations following cardiovascular deconditioning. In contrast, the functional significance of increases in nNOS and eNOS protein in the SON may be related to alterations in vasopressin release observed previously in HU rats.

Distinct subpopulations of cyclic guanosine monophosphate (cGMP) and neuronal nitric oxide synthase (nNOS) containing sympathetic preganglionic neurons in spontaneously hypertensive and Wistar-Kyoto rats

The sympathetic preganglionic neurons (SPN) of the intermediolateral cell column (IML) play a critical role in the maintenance of vascular tone. We undertook a comparative neuroanatomical analysis of neuronal nitric oxide synthase (nNOS) expression in the SPN of the mature normotensive Wistar Kyoto (WKY) and spontaneously hypertensive rat (SHR). The anatomical relationship between nNOS and the NO signaling molecule cyclic guanosine monophosphate (cGMP) was also determined. All animals were male, age > 6 months. Fluorogold (FG) retrograde labeling of SPN (detected with immunohistochemistry) was combined with NADPH-diaphorase histochemistry for NOS in the thoracic spinal cord (T1-11, n = 5 WKY, 5 SHR). There was no difference in the total number of FG-labeled SPN (WKY 6,542 +/- 828, SHR 6,091 +/- 820), but the proportion of FG-labeled cells expressing NOS was significantly less in the SHR (WKY 64.4 +/- 5.1 vs. SHR 55.6 +/- 2.1, P < 0.05). Fluorescence immunohistochemistry for nNOS/cGMP (n = 4 WKY, 4 SHR) was also performed. Confocal microscopy revealed that all nNOS-positive SPN contain cGMP and confirmed a strain-specific anatomical arrangement of SPN cell clusters. A novel subpopulation of cGMP-only cells were also identified. Double labeling for cGMP and choline acetyltransferase (n = 3 WKY, 3 SHR), confirmed these cells as SPN in both WKY and SHR. These results suggest that cGMP is a key signaling molecule in SPN, and that a reduced number of NOS neurons in the SHR may play a role in the increase in sympathetic tone associated with hypertension in these animals.

Exercise training improves renal excretory responses to acute volume expansion in rats with heart failure

Experiments were performed to test the postulate that exercise training (ExT) improves the blunted renal excretory response to acute volume expansion (VE), in part, by normalizing the neural component of the volume reflex typically observed in chronic heart failure (HF). Diuretic and natriuretic responses to acute VE were examined in sedentary and ExT groups of rats with either HF or sham-operated controls. Experiments were performed in anesthetized (Inactin) rats 6 wk after coronary ligation surgery. Histological data indicated that there was a 34.9 +/- 3.0% outer and 42.5 +/- 3.2% inner infarct of the myocardium in the HF group. Sham rats had no observable damage to the myocardium. In sedentary rats with HF, VE produced a blunted diuresis (46% of sham) and natriuresis (35% of sham) compared with sham-operated control rats. However, acute VE-induced diuresis and natriuresis in ExT rats with HF were comparable to sham rats and significantly higher than sedentary HF rats. Renal denervation abolished the salutary effects of ExT on renal excretory response to acute VE in HF. Since glomerular filtration rates were not significantly different between the groups, renal hemodynamic changes may not account for the blunted renal responses in rats with HF. Additional experiments confirmed that renal sympathetic nerve activity responses to acute VE were blunted in sedentary HF rats; however, ExT normalized the renal sympathoinhibition in HF rats. These results confirm an impairment of neurally mediated excretory responses to acute VE in rats with HF. ExT restored the blunted excretory responses as well as the renal sympathoinhibitory response to acute VE in HF rats. Thus the beneficial effects of ExT on cardiovascular regulation in HF may be partly due to improvement of the neural component of volume reflex.

Angiotensin-mediated increase in renal sympathetic nerve discharge within the PVN: role of nitric oxide

The paraventricular nucleus (PVN) of the hypothalamus is known to be an important site of integration in the central nervous system for sympathetic outflow. ANG II and nitric oxide (NO) play an important role in regulation of sympathetic nerve activity. The purpose of the present study was to examine how the interaction between NO and ANG II within the PVN affects sympathetic outflow in rats. Renal sympathetic nerve discharge (RSND), arterial blood pressure (AP), and heart rate (HR) were measured in response to administration of ANG II and N(G)-monomethyl-l-arginine (L-NMMA) into the PVN. Microinjection of ANG II (0.05, 0.5, and 1.0 nmol) into the PVN increased RSND, AP, and HR in a dose-dependent manner, resulting in increases of 53 +/- 9%, 19 +/- 3 mmHg, and 32 +/- 12 beats/min from baseline, respectively, at the highest dose. These responses were significantly enhanced by prior microinjection of L-NMMA and were blocked by losartan, an ANG II type 1 receptor antagonist. Similarly, administration of antisense to neuronal NO synthase within the PVN also potentiated the ANG II responses. Conversely, overexpression of neuronal NOS within the PVN with adenoviral gene transfer significantly attenuated ANG II responses. Push-pull administration of ANG II (1 nmol) into the PVN induced an increase in NO release. Our data indicate that ANG II type 1 receptors within the PVN mediate an excitatory effect on RSND, AP, and HR. NO in the PVN, which can be induced by ANG II stimulation, in turn inhibits the ANG II-mediated increase in sympathetic nerve activity. This negative-feedback mechanism within the PVN may play an important role in maintaining the overall balance and tone of sympathetic outflow.

Exercise Training Differentially Affects Intrinsic Excitability of Autonomic and Neuroendocrine Neurons in the Hypothalamic Paraventricular Nucleus

Oxytocinergic and vasopressinergic brain stem projections have been shown to play an important role in mediating cardiovascular adjustments during exercise training (ET). The aim of the present work was to determine whether the intrinsic excitability of hypothalamic neurons giving rise to brain stem peptidergic projections is altered as a consequence of ET. Whole cell patch-clamp recordings were obtained from nucleus of the solitarii tract (NTS)-projecting paraventricular nucleus of the hypothalamus (PVN) neurons and from supraoptic nucleus (SON) and PVN magnocellular cells (MNCs), in hypothalamic slices obtained from sedentary (S) and ET rats. Our results indicate that intrinsic excitability of PVN neurons that innervate the NTS (PVN-NTS) is enhanced by ET, resulting in a more efficient input-output function (increase number of evoked actions potentials, steeper frequency/current relationships and slower decaying frequency/time relationships). Changes in input-output function were accompanied by smaller hyperpolarizing afterpotentials (HAPs) and afterhyperpolarizing potentials (AHPs), during and after trains of spikes, respectively. On the other hand, a decreased efficacy in the input-output function was observed in SON/PVN MNCs during ET. Altogether, our results indicate that ET differentially affects the intrinsic excitability of autonomic and neurosecretory SON and PVN neurons. Increased excitability in PVN-NTS neurons may contribute to enhanced release of OT and VP peptides in the dorsal brain stem, and cardiovascular fine-tuning during exercise training.

Putative role of the NTS in alterations in neural control of the circulation following exercise training in rats

Exercise training (ExTr) has been associated with alterations in neural control of the circulation, including effects on arterial baroreflex function. The nucleus tractus solitarius (NTS) is the primary termination site of cardiovascular afferents and critical in the regulation of baroreflex-mediated changes in heart rate (HR) and sympathetic nervous system outflow. The purpose of the present study was to determine whether ExTr is associated with alterations in neurotransmitter regulation of neurons involved in control of cardiovascular function at the level of the NTS. We hypothesized that ExTr would increase glutamatergic and reduce GABAergic transmission in the NTS and that, collectively, these changes would result in a greater overall sympathoinhibitory drive from the NTS in ExTr animals. To test these hypotheses, male Sprague-Dawley rats were treadmill trained or maintained under sedentary conditions for 8-10 wk. NTS microinjections were performed in Inactin-anesthetized animals instrumented to record mean arterial pressure (MAP), HR, and lumbar sympathetic nerve activity (LSNA). Generalized activation of the NTS with unilateral microinjections of glutamate (1-10 mM, 30 nl) produced dose-dependent decreases in MAP, HR, and LSNA that were unaffected by ExTr. Bilateral inhibition of NTS with the GABAA agonist muscimol (1 mM, 90 nl) produced increases in MAP and LSNA that were blunted by ExTr. In contrast, pressor and sympathoexcitatory responses to bilateral microinjections of the ionotropic glutamate receptor antagonist, kynurenate (40 mM, 90 nl), were similar between groups. Bradycardic responses to bilateral microinjections of the GABAA antagonist bicuculline (0.1 mM, 90 nl) were attenuated by ExTr. These data indicate that alterations in neurotransmission at the level of the NTS contribute importantly to regulation of HR and LSNA in ExTr animals. In addition to alterations at NTS, these experiments suggest indirectly that changes in other cardiovascular nuclei contribute to the observed alterations in neural control of the circulation following ExTr.

Daily exercise-induced cardioprotection is associated with changes in calcium regulatory proteins in hypertensive rats

Epidemiological data document that regular exercise protects against the morbidity and mortality associated with ischemic heart disease. Therefore, we tested the hypothesis that daily exercise (DE) increases the ventricular arrhythmia threshold (VAT) induced by coronary artery occlusion and alters the expression of calcium regulatory proteins. The VAT was defined as the time from coronary occlusion to sustained ventricular tachycardia resulting in a reduction in arterial pressure. To test this hypothesis, we recorded the VAT in conscious sedentary normotensive, sedentary hypertensive, and DE hypertensive rats, and we associated these thresholds with the protein expression of the L-type calcium channel, Na+/Ca2+ exchanger, phospholamban, and sarco(endo)plasmic reticulum Ca2+-ATPase. Results document a significantly reduced time to ventricular arrhythmias (sedentary hypertensive, 3.7 ± 0.3 min vs. sedentary normotensive, 4.8 ± 0.3 min), an increased Na+/Ca2+ exchanger protein expression (47%), and a decreased phospholamban protein expression (−34%) in conscious hypertensive rats. DE increased the VAT (5.9 ± 0.2 min), decreased the protein expression of the Na+/Ca2+ exchanger, and normalized the protein expression of phospholamban in the hypertensive rats. Thus DE may be a primary prevention approach for reducing the incidence of arrhythmias by altering calcium regulatory proteins in hypertensive rats.

Exercise training improves endogenous nitric oxide mechanisms within the paraventricular nucleus in rats with heart failure

Previously, we have demonstrated that an altered endogenous nitric oxide (NO) mechanism within the paraventricular nucleus (PVN) contributes to increased renal sympathetic nerve activity (RSNA) in heart failure (HF) rats. The goal of this study was to examine the effect of exercise training (ExT) in improving the endogenous NO mechanism within the PVN involved in the regulation of RSNA in rats with HF. ExT significantly restored the decreased number of neuronal NO synthase (nNOS)-positive neurons in the PVN (129 +/- 17 vs. 99 +/- 6). nNOS mRNA expression and protein levels in the PVN were also significantly increased in HF-ExT rats compared with HF-sedentary rats. To examine the functional role of NO within the PVN, an inhibitor of NOS, N(G)-monomethyl-L-arginine, was microinjected into the PVN. Dose-dependent increases in RSNA, arterial blood pressure (BP), and heart rate (HR) were produced in all rats. There was a blunted increase in these parameters in HF rats compared with the sham-operated rats. ExT significantly augmented RSNA responses in rats with HF (33% vs. 20% at the highest dose), thus normalizing the responses. The NO donor sodium nitroprusside, microinjected into the PVN, produced dose-dependent decreases in RSNA, BP, and HR in both sham and HF rats. ExT significantly improved the blunted decrease in RSNA in HF rats (36% vs. 17% at the highest dose). In conclusion, our data indicate that ExT improves the altered NO mechanism within the PVN and restores NO-mediated changes in RSNA in rats with HF.

Exercise and endothelial function: role of endothelium-derived nitric oxide and oxidative stress in healthy subjects and hypertensive patients

Recent epidemiologic studies have shown that aerobic exercise, one of lifestyle modifications, reduces cardiovascular morbidity and mortality in the general population. However, the mechanisms underlying the anti-atherogenic and anti-hypertensive effects of exercise remain unclear. Hypertension is associated with alteration in endothelial function mediated through reduced nitric oxide (NO) bioavailability. Endothelial dysfunction is an early feature of atherosclerosis and vascular diseases in humans. Exercise training has been shown to improve endothelial function in animal models of hypertension and in patients with essential hypertension. These findings suggest that endothelial dysfunction in hypertension is reversible. Lifestyle modifications including exercise are expected to prevent cardiovascular complications through an augmentation of endothelial function in hypertensive patients. It is thought that exercise increases NO production and decreases NO inactivation, leading to an increase in NO bioavailability. In this review, we will focus on recent findings and on possible mechanisms underlying the beneficial effects of exercise on endothelial function in patients with hypertension.

Differential regulation of hippocampal progenitor proliferation by opioid receptor antagonists in running and non-running spontaneously hypertensive rats

Voluntary running in mice and forced treadmill running in rats have been shown to increase the amount of proliferating cells in the hippocampus. Little is known as yet about the mechanisms involved in these processes. It is well known that the endogenous opioid system is affected during running and other forms of physical exercise. In this study, we evaluated the involvement of the endogenous opioids in the regulation of hippocampal proliferation in non-running and voluntary running rats. Nine days of wheel running was compared with non-running in spontaneously hypertensive rats (SHR), a rat strain known to run voluntarily. On the last 2 days of the experimental period all rats received two daily injections of the opioid receptor antagonists naltrexone or naltrindole together with injections of bromodeoxyuridine to label dividing cells. Brain sections from the running rats showed approximately a five-fold increase in newly generated cells in the hippocampus, and this increase was partly reduced by naltrexone but not by naltrindole. By contrast, both naltrexone and naltrindole increased hippocampal proliferation in non-running rats. In non-running rats the administration of naltrexone decreased corticosterone levels and adrenal gland weights, whereas no significant effects on these parameters could be detected for naltrindole. However, adrenal gland weights were increased in naltrexone- but not in naltrindole-administered running rats. In addition, in voluntary running rats there was a three-fold increase in the hippocampal levels of Met-enkephalin-Arg-Phe compared with non-runners, indicating an increase in opioid activity in the hippocampus during running. These data suggest an involvement of endogenous opioids in the regulation of hippocampal proliferation in non-running rats, probably through hypothalamic-pituitary-adrenal axis modulation. During voluntary running in SHR naltrexone altered hippocampal proliferation via as yet unknown mechanisms.

Amelioration of depressed cardiopulmonary reflex control of sympathetic nerve activity by short-term exercise training in male rabbits with heart failure

The reflex regulation of sympathetic nerve activity has been demonstrated to be impaired in the chronic heart failure (CHF) state compared with the normal condition (Liu JL, Murakami H, and Zucker IH. Circ Res 82: 496-502, 1998). Exercise training (Ex) appears to be beneficial to patients with CHF and has been shown to reduce sympathetic outflow in this disease state (Hambrecht R, Hilbrich L, Erbs S, Gielen S, Fiehn E, Schoene N, and Schuler G. J Am Coll Cardiol 35: 706-713, 2000). We tested the hypothesis that Ex corrects the reduced cardiopulmonary (CP) reflex response to volume expansion in the CHF state. Normal, normal with Ex, CHF, and CHF with Ex (CHF-Ex) groups (n = 10-21) of male New Zealand White rabbits were studied. CHF was induced by chronic ventricular pacing. Rabbits were instrumented to record left ventricular end-diastolic pressure (LVEDP), left ventricular end-diastolic diameter (LVEDD), and renal sympathetic nerve activity (RSNA). Experiments were carried out with the animals in the conscious state. Volume expansion was performed with 6% dextran in normal saline at a rate of 5 ml/min to approximately 20% of estimated plasma volume without any significant effect on mean arterial pressure being exhibited. The relationships between RSNA and LVEDP and between RSNA and LVEDD were determined by linear regression; the slopes served as an index of CP reflex sensitivity. Normal rabbits exhibited a CP reflex sensitivity of -8.4 +/- 1.5%delta RSNA/mmHg. This value fell to 0.0 +/- 1.3%delta RSNA/mmHg in CHF rabbits (P < 0.001). Ex increased CP reflex sensitivity to -5.0 +/- 0.7%delta RSNA/mmHg in CHF-Ex rabbits (P < 0.05 compared with CHF). A similar trend was seen when related to the change in LVEDD. Furthermore, resting RSNA expressed as a percentage of maximum RSNA in response to cigarette smoke was also normalized by Ex in rabbits with CHF. Ex had no effect on these parameters in normal rabbits. These data confirm an impairment of CP reflex sensitivity and sympathoexcitation in CHF vs. normal animals. Ex substantially restored both CP reflex sensitivity and baseline RSNA in CHF animals. Thus Ex beneficially affects reflex regulation in CHF, thereby lowering resting sympathetic nerve activity.