Increased Expression of Macrophage Migration Inhibitory Factor in the Nucleus of the Solitary Tract Attenuates Renovascular Hypertension in Rats

BACKGROUND

Macrophage migration inhibitory factor (MIF) is an intracellular inhibitory regulator of the actions of angiotensin II in the central nervous system. Renovascular hypertensive 2-kidney, 1-clip (2K1C) rats have an increased activity of the renin-angiotensin system and a decrease in baroreflex function compared to normotensive (NT) rats. In the present study, we tested the effects of MIF overexpression within the nucleus of the solitary tract (NTS), a key brainstem region for cardiovascular regulation, on the development of hypertension, on baroreflex function, and on water and food intake in 2K1C rats.

METHODS

Holtzman NT rats received a silver clip around the left renal artery to induce 2K1C hypertension. Three weeks later, rats were microinjected in the NTS with AAV2-CBA-MIF, to increase the expression of MIF, or with the control vector AAV2-CBA-enhanced green fluorescent protein. Mean arterial pressure (MAP) and heart rate were recorded by telemetry. Baroreflex function was tested, and water and food intake were also measured.

RESULTS

Increasing MIF expression in the NTS of 2K1C rats attenuated the development of hypertension, reversed the impairment of baroreflex function, and reduced the increase in water intake. In contrast to 2K1C rats, similar increases in MIF expression in the NTS of NT rats produced no changes in baseline MAP, baroreflex function, or water intake.

CONCLUSIONS

These results indicate that an increased expression of MIF within the NTS attenuates the development of hypertension and restores the baroreflex function in 2K1C rats.

Overexpression of AT2R in the solitary-vagal complex improves baroreflex in the spontaneously hypertensive rat

The aim of this study was to investigate the physiological effects of increased angiotensin II type 2 receptor (AT2R) expression in the solitary-vagal complex (nucleus of the solitary tract/dorsal motor nucleus of the vagus; NTS/DVM) on baroreflex function in non-anaesthetised normotensive (NT) and spontaneously hypertensive rats (SHR). Ten week old NT Holtzman and SHR were microinjected with either an adeno-associated virus expressing AT2R (AAV2-CBA-AT2R) or enhanced green fluorescent protein (control; AAV2-CBA-eGFP) into the NTS/DVM. Baroreflex and telemetry recordings were performed on four experimental groups: 1) NTeGFP, 2) NTAT2R, 3) SHReGFP and 4) SHRAT2R (n=4-7/group). Following in-vivo experimental procedures, brains were harvested for gene expression analysis. Impaired bradycardia in SHReGFP was restored in SHR rats overexpressing AT2R in the NTS/DMV. mRNA levels of angiotensin converting enzyme decreased and angiotensin converting enzyme 2 increased in the NTS/DMV of SHRAT2R compared to SHReGFP. Increased levels of pro-inflammatory cytokine mRNA levels in the SHReGFP group also decreased in the SHRAT2R group. AT2R overexpression did not elicit any significant change in mean arterial pressure (MAP) in all groups from baseline to 4weeks post viral transfection. Both SHReGFP and SHRAT2R showed a significant elevation in MAP compared to the NTeGFP and NTAT2R groups. Increased AT2R expression within the NTS/DMV of SHR was effective at improving baroreflex function but not MAP. We propose possible mediators involved in improving baroreflex are in the ANG II/ACE2 axis, suggesting a potential beneficial modulatory effect of AT2R overexpression in the NTS/DMV of neurogenic hypertensive rats.

Impaired blood pressure compensation following hemorrhage in conscious obese Zucker rats

AIMS

Hemorrhagic shock leads to a higher risk of mortality and morbidity in obese patients, however the mechanisms for these outcomes are unclear. We hypothesized that following severe hemorrhage, blood pressure control in conscious obese Zucker rats (OZ) is impaired.

MAIN METHODS

Experiments were performed in conscious lean Zucker rats (LZ) and OZ. Blood pressure, heart rate, cardiac output, total peripheral resistance (TPR), plasma renin activity (PRA), plasma antidiuretic hormone (ADH), and blood gasses were measured before and after severe hemorrhage (35% of the total blood volume).

KEY FINDINGS

Basal blood pressure, cardiac output, TPR, PRA, and ADH levels were not different between LZ and OZ. Compared to LZ, OZ exhibited impaired baroreflex control of heart rate and showed higher levels of vascular adrenergic tone. One hour after the hemorrhage, LZ and OZ exhibited similar decreases in cardiac output. However, blood pressure, heart rate, TPR, PRA, and ADH levels were lower in OZ than in LZ.

SIGNIFICANCE

These results indicate that conscious OZ has impaired blood pressure compensation after hemorrhage due to a blunted increase in TPR. This is due at least in part to an impaired regulation of vasoconstrictor hormones. To our knowledge, the current study is the first to demonstrate that hemodynamic responses and associated hormone secretion are impaired in a conscious obese model.

The hypothalamic-neurohypophyseal system: from genome to physiology

The elucidation of the genomes of a large number of mammalian species has produced a huge amount of data on which to base physiological studies. These endeavours have also produced surprises, not least of which has been the revelation that the number of protein coding genes needed to make a mammal is only 22 333 (give or take). However, this small number belies an unanticipated complexity that has only recently been revealed as a result of genomic studies. This complexity is evident at a number of levels: (i) cis-regulatory sequences; (ii) noncoding and antisense mRNAs, most of which have no known function; (iii) alternative splicing that results in the generation of multiple, subtly different mature mRNAs from the precursor transcript encoded by a single gene; and (iv) post-translational processing and modification. In this review, we examine the steps being taken to decipher genome complexity in the context of gene expression, regulation and function in the hypothalamic-neurohypophyseal system (HNS). Five unique stories explain: (i) the use of transcriptomics to identify genes involved in the response to physiological (dehydration) and pathological (hypertension) cues; (ii) the use of mass spectrometry for single-cell level identification of biological active peptides in the HNS, and to measure in vitro release; (iii) the use of transgenic lines that express fusion transgenes enabling (by cross-breeding) the generation of double transgenic lines that can be used to study vasopressin (AVP) and oxytocin (OXT) neurones in the HNS, as well as their neuroanatomy, electrophysiology and activation upon exposure to any given stimulus; (iv) the use of viral vectors to demonstrate that somato-dendritically released AVP plays an important role in cardiovascular homeostasis by binding to V1a receptors on local somata and dendrites; and (v) the use of virally-mediated optogenetics to dissect the role of OXT and AVP in the modulation of a wide variety of behaviours.

Neurocirculatory and nigrostriatal abnormalities in Parkinson disease from LRRK2 mutation

BACKGROUND

Patients with Parkinson disease (PD) often have cardiac sympathetic denervation and failure of neurocirculatory regulation by baroreflexes. Familial PD caused by mutation of the gene encoding alpha-synuclein or by alpha-synuclein gene triplication also features cardiac sympathetic denervation and baroreflex failure.

METHODS

Here we report results of cardiac sympathetic neuroimaging by 6-[(18)F]fluorodopamine PET, baroreflex testing based on beat-to-beat hemodynamic responses to the Valsalva maneuver, and nigrostriatal neuroimaging using 6-[(18)F] fluorodopa PET in a proband with mutation of the gene encoding leucine-rich repeat kinase 2 (LRRK2), the most common genetic abnormality identified so far in familial PD.

RESULTS

The patient had no detectable 6-[(18)F] fluorodopamine-derived radioactivity in the left ventricular myocardium, a progressive fall in blood pressure during the Valsalva maneuver and no pressure overshoot after release of the maneuver, and decreased 6-[(18)F] fluorodopa-derived radioactivity bilaterally in the putamen and substantia nigra.

CONCLUSION

This patient with Parkinson disease (PD) caused by LRRK2 mutation had evidence of cardiac sympathetic denervation, baroreflex-sympathoneural and baroreflex-cardiovagal failure, and nigrostriatal dopamine deficiency, a pattern resembling that in the sporadic disease. The results fit with the concept that in LRRK2 PD, parkinsonism, cardiac sympathetic denervation, and baroreflex failure can result from a common pathogenetic process.

Alpha- and beta-adrenergic receptor polymorphisms in hypertensive and normotensive offspring

BACKGROUND

The offspring of hypertensive families are characterized by higher arterial blood pressure values and a depressed autonomic control of heart rate. The present study aimed to verify whether these differences are associated with a different genotype distribution of functionally relevant polymorphisms of the alpha- and beta-adrenergic receptor (AR) genes.

METHODS

We selected 109 age- and sex-matched young normotensive subjects with (FH+, n = 56) and without (FH-, n = 53) a family history of hypertension who underwent evaluation of arterial pressure; 24-h electrocardiogram monitoring to assess time-domain parameters of autonomic heart rate control [i.e. mean RR interval (NN), SD of RR intervals (SDNN) and mean square root of the differences of consecutive RR intervals (rMSSD)]; spectral baroreflex sensitivity measurement; and echo-Doppler to assess diastolic function and left ventricular mass. They were also characterized for the following polymorphisms by means of polymerase chain reaction-restriction fragment polymorphism analysis: Arg492Cys in the alpha1a-AR; Del301-303 in the alpha2b-AR; Ser49Gly and Arg389Gly in the beta1-AR; and the 5' leader cistron Arg19Cys, Arg16Gly and Gln27Glu in the beta2-AR.

RESULTS

FH+ individuals showed a higher systolic pressure, a lower SDNN and a greater isovolumic relaxation time compared to normotensive offspring. No differences were found between the two groups when genotype distribution of the studied polymorphisms was considered. Subjects carrying alpha1a-AR Cys492 allelic variant showed lower values of NN, SDNN and rMSSD, independent of age, gender and body mass index.

CONCLUSIONS

The functionally relevant polymorphisms of alpha2b-, beta1- and beta2-AR genes are not associated with a family history of essential hypertension. The Arg492Cys polymorphism of the alpha1a-AR gene, although not associated with a family history of hypertension, was strongly related to autonomic control of heart rate.

Baroreflex gain in normotensive and GH hypertensive rats before and after early gonadectomy

The authors have assessed arterial baroreflex gain in urethane-anesthetized normotensive and New Zealand genetically hypertensive (GH) rats and investigated the effect of gonadectomy in adult animals at 3 weeks of age postnatally. No gender differences in resting blood pressures existed for either normotensive or GH strains. In normotensive animals, bradycardic gain was greater than tachycardic gain and was lower in females than in males. Tachycardic gain was similar in GH and normotensive rats of either sex, but bradycardic gain was lower in GH. Gonadectomy had no effect on baroreflex gain in male or female animals of either strain.

Baroreflex responses in neuronal nitric oxide synthase knoukout mice (nNOS)

The present study sought to determine the influence of the neuronal isoform of nitric oxide synthase (nNOS) on the baroreflex gain (mean index: bpm/mm Hg) and on the respiratory response to baroreflex activation by using nNOS knockout (nNOS-/-) mice. Experiments were performed with nNOS+/+ wild type (WT; n=12) and nNOS-/- mice (n=11), both of the C57Bl6 strain, randomly assigned to the two groups. Carotid artery and external jugular vein were cannulated under halothane, and after recuperation, urethane was intravenously injected. Baroreflex gain was evaluated by intravenous injection of phenylephrine (PE: 15-18 microg/kg) and sodium nitroprusside (SNP: 46-50 microg/kg) in WT and nNOS-/- anesthetized mice. At baseline, nNOS-/- mice had a comparable heart rate (604.5+/-23.6 vs. 618.7+/-11.2 bpm) but higher mean arterial pressure (112.4+/-6.3 vs. 94.8+/-3.9 mm Hg, P<0.05) than WT mice. Heart rate reflex was significantly reduced (P<0.05) in nNOS-/- mice, tachycardic and bradycardic responses were -1.04+/-0.3 and -2.03+/-0.5 in nNOS-/- mice vs. -4+/-0.7 and -4.52+/-0.2 in WT mice, respectively. To characterize the effect of arterial pressure changes on respiratory output, PE was injected in a separate group of WT (n=7) and nNOS-/- (n=6) mice. Tidal diaphragm activity decreased in WT mice (P<0.05); however, diaphragm minute respiration and respiratory frequency were not different between the strains of mice. In nNOS-/- group, heart rate reflex by PE injection was significantly reduced (P<0.05). These findings suggest that the absence of nNOS activity leads to an elevation of the baseline blood pressure and also shows the importance of this enzyme on the transmission of baroreflex signals.

A polymorphism in the endothelin-A receptor gene is linked to baroreflex sensitivity

BACKGROUND

The baroreflex plays an essential role in regulating the cardiovascular system. However, very few studies have focused on the links between genetic polymorphisms and baroreflex sensitivity (BRS).

METHODS

A total of 146 hypertensive individuals who had never been treated, and 105 healthy individuals (controls) were included in the study. The genotypes of 17 polymorphisms of 11 genes involved in the regulation of the cardiovascular system were studied. BRS was measured using a sequence method: BRS was evaluated as the slope of spontaneous increases [systolic blood pressure (SBP)+/reflex response (RR)+] or decreases (SBP-/RR-) in SBP and pulse interval by recording blood pressure (BP) continuously for 20 min.

RESULTS

Following univariate analysis, the genetic polymorphism of endothelin receptor A EDNRA/C+1222T was found to be significantly correlated with the BRS (SBP-/RR-) level in both populations. In normotensive subjects, mean BRS values (SBP-/RR-) were 11.93 +/- 3.69 ms/mmHg in EDNRA CC homozygotes, 9.94 +/- 2.97 ms/mmHg in CT heterozygotes and 9.51 +/- 3.16 ms/mmHg in TT homozygotes (P = 0.01). In hypertensive subjects, mean BRS values (SBP-/RR-) were 9.26 +/- 3.59 ms/mmHg in EDNRA CC homozygotes, 9.03 +/- 4.14 ms/mmHg in CT heterozygotes and 6.60 +/- 2.42 ms/mmHg in TT homozygotes (P = 0.01). After adjustment for age, sex, SBP and diastolic blood pressure and body mass index, the EDNRA/C+1222T polymorphism remained significantly correlated with BRS in both normotensive (P = 0.01) and hypertensive (P = 0.01) subjects.

CONCLUSIONS

These results suggest that the endothelin system may be involved in the regulation of BRS in humans. In particular, the T allele of the EDNRA/C+1222T polymorphism is associated with a reduction in BRS in both healthy and hypertensive subjects.

Blood pressure, baroreflex sensitivity, and end organ damage in hybrid offspring of spontaneously hypertensive rats and Sprague-Dawley rats

AIM

To investigate the blood pressure (BP), baroreflex sensitivity (BRS), and organ damage in hybrids of spontaneously hypertensive rats and Sprague-Dawley rats.

METHODS

Spontaneously hypertensive rats and Sprague-Dawley rats were crossbred, and the F1 hybrids were inbred randomly to produce an F2 generation. At the age of 52 weeks, the F1 and F2 hybrids were tested to determine BP and BRS in a conscious state. Histopathological examinations were carried out after BP recording and BRS studies.

RESULTS

BP and BRS were not different in F1 and F2 hybrids. BRS was inversely related to systolic BP (SBP) in male, female, or whole populations of hybrids. Quantitatively, BRS values were one-third determined by SBP level (the determinant coefficient was 0.326). The indexes for left ventricular hypertrophy, aortic hypertrophy, and renal damage were all positively related to BP, and negatively related to BRS. In multiple-regression analysis, left ventricular and aortic hypertrophy and glomerulosclerosis score were all most significantly associated with lower BRS and higher systolic BP. The contribution of BRS to left ventricular and aortic hypertrophy and glomerulosclerosis was greater than that of SBP.

CONCLUSION

The present work with hybrid rats demonstrated quantitatively that the BRS value was one-third determined by SBP level. Both BP level and BRS value contributed greatly to the hypertensive organ damage. However, the contribution of BRS to the hypertensive organ damage was greater than that of BP level in these rats.

Bradykinin B2 receptor gene (???58T/C) polymorphism influences baroreflex sensitivity in never-treated hypertensive patients

BACKGROUND

Most evidence currently favours a fundamental role of the autonomic nervous system in the pathogenesis of essential hypertension. Recent studies suggest that about 40% of baroreflex variation, an index of cardiac autonomic control, is influenced by genetic factors.

METHODS AND RESULTS

The aim of this study was to investigate the effect of a common polymorphic variant of the bradykinin B2 receptor gene (B2R; -58T/C) on the autonomic regulation of baroreflex sensitivity (BRS) in 129 mild-moderate never-treated hypertensive patients. No significant differences were found for clinical and biochemical parameters among genotypes. BRS increased with the number of B2R T alleles. B2R genotype was a strong independent predictor of BRS, accounting for 12% of its variation. We suggest that a decrease in the transcription of the bradykinin B2R gene in the presence of the B2R -58C allele could reduce BRS via the diminished effect of bradykinin.

CONCLUSIONS

B2R genotype can explain part of the BRS variation that is unaccounted for by simple anthropometric variables and common risk factors.

Autonomic nervous system activity in normotensive subjects with a family history of hypertension

This study was designed to address alterations in autonomic nervous system activity in normotensive subjects with a family history of hypertension. We compared the autonomic nervous system activity in 59 normotensives with a family history of hypertension and 46 normotensives with no family history of hypertension. Skin blood flow was measured using laser-Doppler method on the nailfold skin in the resting condition, during systemic cooling and during upright tilting. Finger blood pressure, pulse and ECG were monitored by a finapres device. Heart rate, systolic pressure and microvascular flow power spectral analyses were performed using fast Fourier transformation. Baroreflex sensitivity was estimated with the sequence method. Compared to the control group, normotensives with a family history of hypertension showed significantly higher systolic pressure, decreased proportion and area of the high-frequency band of the heart rate variability power spectrum and reduced baroreflex sensitivity in the resting condition as well as a decreased proportion and area of the high-frequency band of the heart rate variability power spectrum during systemic cooling. We also proved a different time course of baroreflex sensitivity during upright tilting in the two groups. In contrast, we did not find any significant differences in the parameters of systolic pressure and microvascular variability power spectra between the two groups. Our results indicate that even normotensives with a family history of hypertension exhibit an increased ratio of sympathetic to parasympathetic activity at the cardiac level; however, they do not show any alteration of the vascular sympathetic reactivity.

Functional organisation of central cardiovascular pathways: studies using c-fos gene expression

Until about 10 years ago, knowledge of the functional organisation of the central pathways that subserve cardiovascular responses to homeostatic challenges and other stressors was based almost entirely on studies in anaesthetised animals. More recently, however, many studies have used the method of the expression of immediate early genes, particularly the c-fos gene, to identify populations of central neurons that are activated by such challenges in conscious animals. In this review we first consider the advantages and limitations of this method. Then, we discuss how the application of the method of immediate early gene expression, when used alone or in combination with other methods, has contributed to our understanding of the central mechanisms that regulate the autonomic and neuroendocrine response to various cardiovascular challenges (e.g., hypotension, hypoxia, hypovolemia, and other stressors) as they operate in the conscious state. In general, the results of studies of central cardiovascular pathways using immediate early gene expression are consistent with previous studies in anaesthetised animals, but in addition have revealed other previously unrecognised pathways that also contribute to cardiovascular regulation. Finally, we briefly consider recent evidence indicating that immediate early gene expression can modify the functional properties of central cardiovascular neurons, and the possible significance of this in producing long-term changes in the regulation of the cardiovascular system both in normal and pathological conditions.

Oxytocinergic regulation of cardiovascular function: studies in oxytocin-deficient mice

Oxytocin (OT) has been implicated in the cardiovascular responses to exercise, stress, and baroreflex adjustments. Studies were conducted to determine the effect of genetic manipulation of the OT gene on blood pressure (BP), heart rate (HR), and autonomic/baroreflex function. OT knockout (OTKO -/-) and control +/+ mice were prepared with chronic arterial catheters. OTKO -/- mice exhibited a mild hypotension (102 +/- 3 vs. 110 +/- 3 mmHg). Sympathetic and vagal tone were tested using beta(1)-adrenergic and cholinergic blockade (atenolol and atropine). Magnitude of sympathetic and vagal tone to the heart and periphery was not significantly different between groups. However, there was an upward shift of sympathetic tone to higher HR values in OTKO -/- mice. This displacement combined with unchanged basal HR led to larger responses to cholinergic blockade (+77 +/- 25 vs. +5 +/- 15 beats/min, OTKO -/- vs. control +/+ group). There was also an increase in baroreflex gain (-13.1 +/- 2.5 vs. -4.1 +/- 1.2 beats x min(-1) x mmHg(-1), OTKO -/- vs. control +/+ group) over a smaller BP range. Results show that OTKO -/- mice are characterized by 1) hypotension, suggesting that OT is involved in tonic BP maintenance; 2) enhanced baroreflex gain over a small BP range, suggesting that OT extends the functional range of arterial baroreceptor reflex; and 3) shift in autonomic balance, indicating that OT reduces the sympathetic reserve.

In vivo gene transfer to dissect neuronal mechanisms regulating cardiorespiratory function

This lecture reviews recent information from our laboratory regarding brainstem mechanisms regulating the arterial baroreceptor reflex. Our long-term goal is to understand some of the mechanisms involved in the etiology of essential hypertension. Our hypothesis is that this problem may arise, in part, because of changes within brainstem circuits controlling arterial pressure, and in particular to occlusion of baroreceptive information at the level of the primary afferent relay within the brainstem. Although it is established that baroreceptors provide a mechanism for short-term regulation of arterial pressure, there is convincing evidence that they also play a role in its long-term control (see Thrasher 2002, for an example). It follows that dysfunction of this reflex circuit could contribute to high blood pressure levels. Here, we discuss the central actions of angiotensin II on the baroreceptor reflex circuitry within the nucleus of the solitary tract (NTS) for arterial pressure control. Our findings have led us to hypothesize a novel form of intercellular communication within the NTS, one of vascular-neuronal signaling.

Enhanced baroreflex sensitivity in free-moving calponin knockout mice

Calponin is an actin binding protein in vascular smooth muscle that modifies contractile responses. However, its role in mean arterial pressure (MAP) regulation has not been clarified. To assess this, MAP and heart rate (HR) were measured in calponin knockout (KO) mice, and the results were compared with those in wild-type (WT) mice. The measurements were performed every 100 ms during a 60-min free-moving state each day for 3 days. Mice in both groups rested during approximately 70% of the total measuring period. The mean HR during rest was significantly lower in KO mice than in WT mice but with no significant difference in MAP between the groups. The change in HR response (deltaHR) to spontaneous change in MAP (deltaMAP) varied in a wider range in KO mice with an 80% increase in the coefficient of variation for HR (P < 0.05), whereas MAP in KO mice was controlled in a narrow range similar to that in WT mice. The baroreflex sensitivity (deltaHR/deltaMAP), determined from the change in HR to the spontaneous change in MAP, was twofold higher in KO mice than that in WT mice (P < 0.01), whereas there were no significant differences in the baroreflex sensitivity determined by intravascular administration of phenylephrine and sodium nitroprusside between the two groups (P > 0.1). The MAP response to the administrated doses of phenylephrine in KO mice was reduced to one-half of that in WT mice (P < 0.01) but with no significant difference in the response to sodium nitroprusside between the groups. The differences in HR variability and the spontaneous baroreflex sensitivity between the two groups completely disappeared after carotid sinus denervation. These results suggest that the higher variability in HR for KO mice was caused by the increased spontaneous arterial baroreflex sensitivity, though not detected by the intra-arterial administration of the drug, and that the higher variability of HR may be a compensatory adaptation to the blunted alpha-adrenergic response of peripheral vessels to sympathetic nervous activity.

First model of spontaneous vagal hyperreactivity and its mode of genetic transmission

BACKGROUND

The main purpose of our study was to define an animal model of vagal hyperreactivity and its genetic transmission.

METHODS AND RESULTS

We first investigated the vagal reactivity with phenylephrine in conscious rabbits. Barosensitivity and the maximal bradycardic response were measured at the upper mean blood pressure plateau. Hyperreactive (H) animals were selected and crossbred with normal (N) ones. Results showed no significant difference between calculated barosensitivity values after the different doses of phenylephrine. In contrast, an increase of the values and a great dispersion appeared 1 to 5 beats after the end of the ramp. Marked pauses (6000 to 20 000 ms) were obtained with some rabbits, which were blocked by atropine. A significant excess of hyperreactive offspring was observed in HxH crossings compared with NxN ones (39.4% male and 42.3% female offspring versus 14.4% and 4.4%, respectively). Few female offspring were hyperreactive compared with males in NxH and NxN crossings (4.1% versus 23.4% and 4.4% versus 14.4%, respectively).

CONCLUSIONS

This study describes the first model of spontaneous vagal pauses. The inheritance could be polygenic with a partial sex-limited character.

Heart rate variability and baroreflex function in AT2 receptor-disrupted mice

We adapted telemetry and sequence analysis employed in humans to mice and measured heart rate variability and the spontaneous baroreflex sensitivity in angiotensin II type 2 (AT2) receptor-deleted (AT2 -/-) and wild-type (AT2 +/+) mice with either deoxycorticosterone acetate (DOCA)-salt hypertension or N(omega)-nitro-L-arginine methylester hydrochloride (L-NAME) hypertension. Mean arterial pressure leveled during the day at 101+/-1 mm Hg and during the night at 109+/-1 mm Hg in AT2 receptor-deleted mice, compared with 98+/-2 mm Hg (day) and 104+/-2 mm Hg (night) in wild-type mice. Mean arterial pressure increased in AT2 receptor-deleted mice with L-NAME to 114+/-1 mm Hg (day) and 121+/-1 mm Hg (night), compared with 105+/-2 mm Hg (day) and 111+/-2 mm Hg (night), respectively. DOCA-salt also increased day and night blood pressures in AT2 receptor-deleted mice to a greater degree than in wild-type mice. Heart rate variability in the time and frequency domain was not different between AT2 receptor-deleted mice and AT2 receptor-deleted mice at baseline. Systolic blood pressure variability in the low frequency band was lower in AT2 receptor-deleted mice (0.6+/-0.1 ms2 versus 3.9+/-1.3 ms2) than in wild-type mice. Baroreceptor-heart rate reflex sensitivity was significantly increased in AT2 receptor-deleted mice compared with wild-type mice (3.4+/-0.6 versus 2.1+/-0.5 ms/mm Hg). These differences remained after DOCA-salt and L-NAME treatments. We conclude that activation of the AT2 receptor impairs arterial baroreceptor reflex function, probably by a central action. These data support the existence of an inhibitory central effect of the AT2 receptor on baroreflex function.

The BK channel beta1 subunit gene is associated with human baroreflex and blood pressure regulation

BACKGROUND

The baroreflex, which is important for the minute-to-minute regulation of blood pressure and heart rate, is influenced by genetic variance. Ion channels are important to baroreflex afferent and efferent function. Mice missing the beta1 subunit of the Ca2+-sensitive potassium channel (BK) are hypertensive and have a reset baroreflex. We tested the hypothesis that variants in the gene (KCNMB1) coding for the BK beta1 subunit are associated with baroreflex function.

METHODS

We studied six single-nucleotide polymorphisms (SNPs) in KCNMB1.

RESULTS

Four SNPs in intron 3, exon 4a, exon 4b and exon 4c gave significant results. For instance, exon 4b SNP AA individuals had higher heart rate variability, compared to CA, or CC persons, in particular in the high-frequency range. The low-frequency range showed no association. Consistent with the heart rate variability data, homozygous AA persons had greater baroreflex slopes than CA or CC persons, also in the high-frequency range. These associations could not be shown in the low-frequency range for heart rate variability and baroreflex slopes.

CONCLUSIONS

These data support the notion that variants in channel genes may be responsible for the great range in heart rate variability and baroreflex function observed in humans. Such variation may also play a role in the development of hypertension.

Elastin mutation is associated with a reduced gain of the baroreceptor--heart rate reflex in patients with Williams syndrome

Microdeletion of chromosome 7q, encompassing the elastin locus, has been identified in patients with Williams syndrome (WS). This study tested the hypothesis that loss of medial elastin affects the discharge of baroreceptors and consequently the baroreflex sensitivity (BRS). Eight untreated patients with WS (14.8 +/- 2.4 y, m +/- SEM) were compared to 8 healthy subjects (15.1 +/- 2.3 y). Blood pressure (BP) was recorded using a Finapres monitor in the supine position. Systolic BP (SBP) levels were 117.8 +/- 4.4 mmHg in WS compared to 110.9 +/- 5.7 in controls (ns). Pulse rate (PR, taken as a surrogate for heart rate) was higher in the WS (89.6 +/- 1.0 vs 74.1 +/- 2.3 beats/min in controls, P < 0.01). The variance (total power) of PI variability was reduced in WS subjects. The amplitudes of the low frequency (LF, 0.1 Hz) and high frequency (HF, respiratory) PI component (modulus) were reduced in WS (210.5 +/- 4.3 vs 34.6 +/- 2.6 ms, P = 0.02 for LF, for HF). The gain of the SBP-PI transfer function was diminished in the low frequency (LF, 0.1 Hz) and the HF range as well (5.8 +/- 0.7 vs 12.1 +/- 1.8 ms/mmHg for LF, P < 0.01 and 6.2 +/- 1.0 vs 21.7 +/- 4.6 ms/mmHg for HF, P < 0.01). The BRS obtained with the sequence technique was also reduced in WS (8.2 +/- 0.9 vs 21.5 +/- 2.9 ms/mm Hg in controls, P < 0.001). The percent of beats involved in baroreflex sequences observed in WS was also diminished to 20% compared to 48% in controls (P < 0.001). In conclusion a BRS reduction associated with a PR elevation was observed in normotensive WS subjects. It is likely abnormal elastic fiber assembly at the arterial level alters baroreceptor discharges.

Genetic influences on cardiovascular responses to an acoustic startle stimulus in rats

1. The aim of the present study was to assess the cardiovascular differences among five inbred rat strains (n=16 per strain), including spontaneously hypertensive rats (SHR), Wistar Kyoto (WKY) rats, Wistar Furth (WF) rats, Fischer (F344) rats and Lewis (Lew) rats and the usual outbred Wistar (W) rat strain (n=25). 2. These strains were compared under resting conditions for blood pressure (BP) and heart rate (HR) levels and for their baroreceptor-HR reflex sensitivity. In addition, their responses to an acoustic startle stimulus were measured. 3. A consistent rise in BP was observed among the groups as a result of the noise stimulus. This rise in systolic BP (SBP) averaged (+/-SEM) 37 +/- 2 mmHg in the SHR and 34 +/- 4 mmHg in F344 rats, while the response was only 23 +/- 3 mmHg in WKY rats. Pulse pressure (PP) was increased following noise in all groups. The delay for the BP response for all groups combined was 1.6 +/- 0.1 s. 4. Most animals had minimal HR variations, except F344 rats, responding with a 42 +/- 13 b.p.m. decrease 3.0 s after the stimulus (i.e. 1.3 s after the maximal 34 +/- 4 mmHg SBP rise). 5. The highest SBP (160 +/- 3 mmHg) and diastolic BP (104 +/- 3 mmHg) were observed in inbred SHR. Other groups were normotensive. Resting PP was elevated for SHR (56 +/- 2 mmHg) compared with the other groups (40 +/- 2 mmHg). The highest HR was found in F344 and WF rats, with 389 +/- 11 and 372 +/- 7 b.p.m., respectively. The lowest HR was observed in SHR and Lewis rats, with 335 +/- 7 and 323 +/- 7 b.p.m., respectively. The least sensitive baroreflex function was observed in SHR (0.8 +/- 0.1 b.p.m./mmHg) compared with the other strains (1.4 +/- 0.2 b.p.m./mmHg). 6. The present study confirms the importance of genetic factors on the cardiovascular responses of rats to a noise startle stimulus. Two inbred normotensive rat strains, namely F344 and WKY rats, which exhibit a substantial difference in pressor response to noise, may be used to unravel the mechanisms of sympathetic activation.

Autonomic nervous system function in patients with monogenic hypertension and brachydactyly: a field study in north-eastern Turkey

Laboratory studies in patients with autosomal-dominant hypertension and brachydactyly showed increased sensitivity to sympathetic stimuli and severe abnormalities in baroreflex buffering. To further elucidate the mechanisms by which impaired baroreflex sensitivity could influence blood pressure (BP), we conducted autonomic testing under field conditions. We studied 17 hypertensive affected (13 to 48 years, BMI 22.7 +/- 6.5 kg/m(2), 160 +/- 23/98 +/- 15 mm Hg) and 12 normotensive non-affected (9 to 60 years, BMI 24.0 +/- 4.7 kg/m(2), 120 +/- 16/70 +/- 10 mm Hg) family members. Pulse intervals and finger BP were measured using the Portapres device. Valsalva ratio, the blood pressure overshoot during phase IV of the Valsalva manoeuver, the Ewing coefficient (RR30/15 ratio), and heart rate and BP variability were similar in affected and non-affected family members. Overall, baroreflex sensitivity calculated using the cross-spectral (BRSLF, BRSHF) and sequence techniques (BRS+, BRS-) was not different between the groups. However, in younger family members, BRS+ was 12 +/- 3.7 and 22 +/- 13 msec/mm Hg in affected and in non-affected family members, respectively. The decline in BRS with age and with increasing blood pressure was absent in affected family members. We conclude that autonomic reflex testing conducted under field conditions is not impaired in patients with monogenic hypertension and brachydactyly. However, noninvasive testing showed impaired baroreflex control of heart rate at a young age. The reduced BRS in young family members with moderate arterial hypertension may suggest that the impaired baroreflex function is not secondary to the hypertension but rather a primary abnormality, which aggravates the progression of hypertension.

Reduced baroreflex sensitivity and blunted endogenous nitric oxide synthesis precede the development of hypertension in TGR(mREN2)27 rats

Transgenic TGR(mREN2)27 (TGR) rats are an animal model of fulminant hypertension characterized by an inverse circadian blood pressure profile. The present study addressed the contribution of nitric oxide (NO) synthesis and baroreflex function to hypertension and the inverse blood pressure pattern. NO synthesis was measured at four different times of day indirectly by excretion of NO metabolites (NOx: NO2- and NO3-) in the urine of 5- and 11-week-old TGR and Sprague-Dawley (SPRD) controls. Blood pressure, heart rate, and motor activity were recorded in age-matched rats of both strains using an implantable telemetry system. Beat-to-beat recording of blood pressure and pulse interval was performed hourly in 6-week-old animals over 24 h. From these data, baroreflex sensitivity (BRS) was calculated by linear regression of spontaneous fluctuations of blood pressure and corresponding changes of pulse interval. Baroreflex sensitivity was lower in pre-hypertensive TGR rats than in SPRD rats, and the reduction was restricted to the daily resting period. In both strains, NOx excretion showed circadian rhythmicity, with peak values during the activity period at night. Interestingly, excretion of NOx was reduced during the resting period in 5-week-old TGR rats prior to the development of hypertension. Impairment of NO synthesis and baroreflex function precede the development of hypertension in TGR rats. The reduction of both parameters was restricted to the resting period and, therefore, could be involved in the development of the inverse circadian blood pressure profile of TGR rats.

Genetic influences on baroreflex function in normal twins

Blood pressure and heart rate are strongly influenced by genetic factors; however, despite the pivotal role of genetics in short-term cardiovascular regulation, little is known about the genetic contribution to baroreflex function. We assessed genetic influence on baroreflex sensitivity (BRS) in 149 twin pairs (88 monozygotic of age 33+/-13 years and BMI 23+/-4 kg/m(2) and 61 dizygotic of age 33+/-11 years and BMI 24+/-4 kg/m(2)). ECG and finger arterial blood pressures were measured continuously under resting conditions. BRS values were calculated by use of cross-spectral analysis (baroreflex slope calculated as mean value of transfer function between systolic blood pressure and the R-R interval in the low-frequency band [BRSLF] and baroreflex slope calculated as the mean value of transfer function between systolic blood pressure and R-R interval in the respiratory frequency band [BRSHF]) and the sequence technique (BRS+, BRS-). Heritability (h(2)) was estimated with a path-modeling approach. BRS values did not differ significantly between groups (monozygotic, BRSLF, 17+/-13; BRSHF, 21+/-18; BRS+, 19+/-16; and BRS-, 21+/-15, and dizygotic, BRSLF, 16+/-9; BRSHF, 20+/-14; BRS+, 18+/-10; and BRS-, 20+/-11 ms/mm Hg), and were significantly correlated (P:<0.001). When variances and covariances for monozygotic and dizygotic twins were compared, significant correlations were found for BRS in monozygotic (range, r=0.38 to 0.48) but not in dizygotic twin pairs (r=-0.03 to 0.09). Thus, BRS is heritable; the variability can be explained by genetic influences (P:<0.01; h(2) range, 0.36 to 0.44). The genetic influence on BRS remained strong after correction for BMI and blood pressure. Therefore, BRS is strongly genetically determined, probably by different genes than are resting blood pressure and BMI.

Determinants of spontaneous baroreflex sensitivity in a healthy working population

Baroreflex sensitivity (BRS) by the spontaneous sequence technique has been widely used as a cardiac autonomic index for a variety of pathological conditions. However, little information is available on determinants of the variability of spontaneous BRS and on age-related reference values of this measurement in a healthy population. We evaluated BRS as the slope of spontaneous changes in systolic blood pressure (BP) and pulse interval from 10 minutes BP (Finapres) and ECG recordings in 1134 healthy volunteers 18 to 60 years of age. Measurement of BRS could be obtained in 90% of subjects. Those with unmeasurable spontaneous BRS had a slightly lower heart rate but were otherwise not different from the rest of the population. BRS was inversely related to age (lnBRS, 3.24-0.03xage; r(2)=0.23; P:<0.0001) in both genders. In addition, univariate analysis revealed a significant inverse correlation between BRS and heart rate, body mass index, and BP. Sedentary lifestyle and regular alcohol consumption were also associated with lower BRS. However, only age, heart rate, systolic and diastolic BP, body mass index, smoking, and gender were independent predictors of BRS in a multivariate model, accounting for 47% of the variance of BRS. The present study provides reference values for spontaneous BRS in a healthy white population. Only approximately half of the variability of BRS could be explained by anthropometric variables and common risk factors, which suggests that a significant proportion of interindividual differences may reflect genetic heterogeneity.

Altered baroreflex control of heart rate in bradykinin B2-receptor knockout mice

Recently, we have shown that a knockout mouse strain lacking the bradykinin B2-receptor gene exhibits an accelerated heart rate (HR) under basal conditions, this alteration being associated with mildly elevated blood pressure (BP) levels and ultimately with the development of cardiomyopathy. The goal of the present study was to determine whether genetic disruption of the B2-receptor alters autonomic cardiovascular reflexes to acute or chronic changes in BP. The direct mean BP and HR levels of unrestrained B2 knockout mice (B2-/-) were higher than those of wild type (B2+/+) controls (131 +/- 2 vs. 105 +/- 2 mm Hg and 480 +/- 5 vs. 414 +/- 8 beats/min, P < 0.01 for both comparisons). The difference in HR observed between groups under basal conditions was nullified by the acute administration of propranolol and atropine as well as by hexamethonium; it was attenuated by long-term blockade of angiotensin AT1 receptors. In B2-/- mice, the presence of an alteration in baroreceptor regulation of HR was supported by a reduced gain in the HR responses to acute nitroprusside-induced hypotension or phenylephrine-induced hypertension (slope of the regression line: 0.82 +/- 0.07 vs. 5.58 +/- 0.08 beats/min per mmHg in B2+/+, P < 0.01), as well as by an exaggerated tachycardic response to chronic hypertension induced by clipping of the left renal artery (60 +/- 3 vs. 15 +/- 3 beats/min in B2+/+, P < 0.01). Our findings indicate that disruption of the bradykinin B2-receptor gene is associated with an impaired baroreflex control of HR. The combination of chronically elevated resting HR and impaired baroreflex control could contribute to the development of cardiomyopathy in these animals.

Joint effects of an aldosterone synthase (CYP11B2) gene polymorphism and classic risk factors on risk of myocardial infarction

BACKGROUND

The -344C allele of a 2-allele (C or T) polymorphism in the promoter of the gene encoding aldosterone synthase (CYP11B2) is associated with increased left ventricular size and mass and with decreased baroreflex sensitivity, known risk factors for morbidity and mortality associated with myocardial infarction (MI). We hypothesized that this polymorphism was a risk factor for MI.

METHODS AND RESULTS

We used a nested case-control design to investigate the relationships between this polymorphism and the risk of nonfatal MI in 141 cases and 270 matched controls from the Helsinki Heart Study, a coronary primary prevention trial in dyslipidemic, middle-aged men. There was a nonsignificant trend of increasing risk of MI with number of copies of the -344C allele. However, this allele was associated in a gene dosage-dependent manner with markedly increased MI risk conferred by classic risk factors. Whereas smoking conferred a relative risk of MI of 2.50 (P=0.0001) compared with nonsmokers in the entire study population, the relative risk increased to 4.67 in -344CC homozygous smokers (relative to nonsmokers with the same genotype, P=0.003) and decreased to 1.09 in -344TT homozygotes relative to nonsmokers with this genotype. Similar joint effects were noted with genotype and decreased HDL cholesterol level as combined risk factors.

CONCLUSIONS

Smoking and dyslipidemia are more potent risk factors for nonfatal MI in males who have the -344C allele of CYP11B2.

Aldosterone synthase (CYP11B2) polymorphisms and cardiovascular function

In addition to regulating renal sodium resorption and, thus, intravascular volume, aldosterone may have direct effects on the cardiovascular system. We previously identified a polymorphism (-344C/T) in the promoter of the aldosterone synthase (CYP11B2) gene that affects binding of the SF-1 transcription factor and thus might influence gene expression. We found that, whereas this polymorphism has inconsistent associations with levels of aldosterone secretion and blood pressure, the -344C allele is strongly associated with increased left ventricular size and decreased baroreflex sensitivity in healthy individuals. These physiological parameters are cardiovascular risk factors. Indeed, preliminary studies suggest that the -344C allele is also associated with increased risk of myocardial infarction in high risk dyslipidemic males.

Impaired cerebral cortex development and blood pressure regulation in FGF-2-deficient mice

Fibroblast growth factor-2 (FGF-2) has been implicated in various signaling processes which control embryonic growth and differentiation, adult physiology and pathology. To analyze the in vivo functions of this signaling molecule, the FGF-2 gene was inactivated by homologous recombination in mouse embryonic stem cells. FGF-2-deficient mice are viable, but display cerebral cortex defects at birth. Bromodeoxyuridine pulse labeling of embryos showed that proliferation of neuronal progenitors is normal, whereas a fraction of them fail to colonize their target layers in the cerebral cortex. A corresponding reduction in parvalbumin-positive neurons is observed in adult cortical layers. Neuronal defects are not limited to the cerebral cortex, as ectopic parvalbumin-positive neurons are present in the hippocampal commissure and neuronal deficiencies are observed in the cervical spinal cord. Physiological studies showed that FGF-2-deficient adult mice are hypotensive. They respond normally to angiotensin II-induced hypertension, whereas neural regulation of blood pressure by the baroreceptor reflex is impaired. The present genetic study establishes that FGF-2 participates in controlling fates, migration and differentiation of neuronal cells, whereas it is not essential for their proliferation. The observed autonomic dysfunction in FGF-2-deficient adult mice uncovers more general roles in neural development and function.

C-fos expression in central neurons mediating the arterial baroreceptor reflex

The immediate early gene c-fos is a transcription regulating factor that is widely employed as a marker of neuronal activation. In this study we have used c-fos expression to identify vasomotor neurons in the brainstem and spinal cord that are activated after interventions that alter blood pressure. These neurons are likely to be those that subserve the arterial baroreceptor reflex and maintain blood pressure within a defined range. With the combination of Fos expression and neuronal tracing, we describe the location and central connections of these neurons. The differential expression of Fos in neurons in separate regions of the brainstem and spinal cord, after either hypotensive or hypertensive stimuli in conscious rats, supports current opinion about baroreflex circuitry. The central processes of baroafferent neurons synapse with second order baroreflex neurons in the nucleus tractus solitarius. From this region baroreceptor information is transmitted to neurons in the caudal ventrolateral medulla and then to neurons in the rostral ventrolateral medulla. The sympathetic preganglionic neurons in the intermediolateral column of the thoracolumbar spinal cord are the final crucial site involved in the arterial baroreflex.

Renal denervation prevents sodium retention and hypertension in salt-sensitive rabbits with genetic baroreflex impairment

1. Rabbits with a genetic impairment in baroreflex control of heart rate become hypertensive on a high salt diet. The present study determined the effect of bilateral renal denervation on blood pressure and sodium balance after salt loading (four times normal intake; 28-36 mEq NaCl/day) in normotensive rabbits with high (Group I) and low (Group II) baroreflex sensitivity, respectively. 2. Eight rabbits in each group were denervated or sham-denervated 1 week before commencement of the high salt diet. Before operation, the two groups differed only in the gain of their cardiac baroreflex (Group I, -6.4 +/- 0.4 beats min-1 mmHg-1; Group II, -3.2 +/- 0.15 beats min-1 mmHg-1). 3. In Group I sham-denervated rabbits, mean arterial pressure remained unchanged, and plasma renin activity and heart rate fell significantly in response to the high salt. In Group II sham-denervated rabbits, mean arterial pressure increased by 10.6 +/- 1.2 mmHg, and heart rate and plasma renin activity remained unchanged. Their cumulative Na+ retention and weight gain was more than twice that of Group I sham-denervated rabbits. 4. Renal denervation decreased plasma renin activity in both groups to < 1 pmol Ang I h-1 ml-1, lowered cumulative Na+ retention from 102 +/- 4 to 35 +/- 5 mEq (P < 0.01) and completely prevented the increase in mean arterial pressure in response to high salt in Group II. 5. The results suggest that Group II rabbits retain salt and fluid in response to their diet because of an abnormality in their control of renal nerve activity, possibly via vagal afferents. This results in blood pressure elevation because of an inability to lower peripheral resistance and heart rate in response to the increase in cardiac output. 6. Since they display several of the characteristics of salt-sensitive hypertensive humans, i.e. salt retention, normal plasma renin activity, but abnormal regulation of plasma renin activity and blood flow in response to salt loading, Group II are an appropriate model of human salt-induced hypertension.

c-Fos expression in brain in response to hypotension and hypertension in conscious rats

Hypotension- and hypertension-evoked expression of the protein product, Fos, of the immediate early gene c-fos was assessed throughout the rat brain as an approach for describing the neuronal populations that respond to alterations in arterial blood pressure. Conscious, chronically catheterized rats were treated with the vasoconstricting drug phenylephrine or the vasodilatating drug hydralazine to increase or decrease, respectively, arterial pressure by approx. 40 mm Hg for 90 min. Rats were then anesthetized, fixed by vascular perfusion, and sections representing the entire brain were processed for the immunocytochemical localization of Fos. In control rats treated with isotonic saline, few Fos-positive neurons were observed. In contrast, phenylephrine and hydralazine treatments resulted in different, yet reproducible, patterns of Fos expression in the brain, with hydralazine evoking Fos expression in more brain regions than phenylephrine. Brain regions containing Fos-positive neurons in rats treated with hydralazine included nucleus tractus solitarius, area postrema, caudal ventrolateral medulla, rostral ventrolateral medulla, bed nucleus of the stria terminalis, amygdala, paraventricular nucleus, supraoptic nucleus, subfornical organ and the Islands of Calleja. The nucleus tractus solitarius, paraventricular nucleus and the amygdala also contained Fos-positive neurons in phenylephrine-treated rats, although the number of Fos-positive neurons was always less than that noted in the hydralazine-treated rats and the location of Fos-positive neurons within these regions tended to differ between treatments. These results generally fit within an emerging understanding of brain circuitry underlying cardiovascular regulation.

Comparison of the effects of angiotensin II, losartan, and enalapril on baroreflex control of heart rate in conscious rabbits

We compared the effects of angiotensin II (AII), enalapril, and losartan given by acute intravenous (i.v.) injection, on cardiac baroreflex sensitivity in two groups of conscious normotensive rabbits bred for high gain [> 5 beats/min/mm Hg] (group I) and low gain [< 4 beats/min/mm Hg] (group II) of the mean arterial blood pressure-heart rate (MAP/HR) relationship, respectively. Full sigmoid barocurves were produced in 62 rabbits by i.v. injection of phenylephrine (PE 1-15 micrograms/kg) and nitroglycerin (NTG 1-20 micrograms/kg) after pretreatment with saline or with AII, enalapril, or losartan. In group II, AII had a biphasic effect on baroreflex sensitivity; at 10 and 50 ng/kg/min, it increased the gain from 3.47 +/- 0.21 to 4.75 +/- 0.44 and 5.13 +/- 0.28 beats/min/mm Hg (p < 0.05 and p < 0.025, respectively), but had no effect at 100 ng/kg/min. BP increased in these rabbits by 11.5, 10, and 23 mm Hg after these three doses. In group I, AII 50 ng/kg/min increased BP by 8 mm Hg, (p < 0.05) and decreased gain from 6.13 +/- 0.32 to 4.75 +/- 0.44 beats/min/mm Hg (p < 0.01). Therefore, AII 50 ng/kg/min equalized baroreflex sensitivity in the two groups. In group II, both losartan (2.5 mg/kg) and enalapril (1 mg/kg) decreased BP by 8.6 +/- 1.0 1.0 and 10.2 +/- 2.1 mm Hg, (p < 0.01), respectively, and increased the gain by 1-2 beats/min/mm Hg; the drugs did not influence baroreflex sensitivity to any significant extent in group I.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of c-fos functional mapping to identify the central baroreceptor reflex pathway: advantages and limitations

Prolonged stimulation of many neurons results in the expression of the immediate early gene c-fos, which in turn cause the production of the protein Fos, whose presence in a cell can be detected by immunocytochemistry. This method has been used in both conscious and anaesthetized animals to identify central neurons involved in the baroreceptor reflex. In this paper we review the factors that can influence c-fos expression, with particular emphasis on the effects of different anaesthetic agents. We conclude that the c-fos method of functional mapping, when applied carefully and critically, is a very useful method of identifying central neurons that are activated by cardiovascular stimuli in conscious animals. Anaesthetic agents can significantly alter c-fos expression, and this effect differs greatly according to the type of anaesthetic used.