Plasma catecholamine concentrations in normotensive rats of different strains and in spontaneously hypertensive rats under basal conditions and during cold exposure

Blood collection in unstressed, conscious, and freely moving mice through implantation of catheters in the jugular vein: a new simplified protocol

The mouse has become the most common mammalian animal model used in biomedical research. However, laboratory techniques used previously in rats and other larger animals to sample blood had to be adapted in mice due to their lower mouse plasma volume. Sampling is further confounded by the variability in plasma hormone and metabolite concentrations that can occur from the stress or the anesthesia that accompanies the collection. In this article, we describe in detail a protocol we developed for blood sampling in conscious, unrestrained mice. Our protocol implements the use of chronic indwelling catheters in the right external jugular vein, allowing the mice to recover fully in their home cages, untethered until the time of blood sampling. This protocol employs catheters that remain patent for days and does not require the purchase of expensive equipment. We validated this protocol by measuring the time course of plasma norepinephrine (NE) concentration during and after the relief of acute immobilization stress in wild type (WT) and pendrin knockout (KO) mice and compared these results with our previously published values. We found that following relief from immobilization stress, it takes longer for plasma NE concentration to return to basal levels in the pendrin KO than in the wild type mice. These results highlight the potential utility of this protocol and the potential role of pendrin in the neuroendocrine response to acute stress.

Rat strain-related differences in myocardial adrenergic tone and the impact on cardiac fibrosis, adrenergic responsiveness and myocardial structure and function

Sprague-Dawley (SD) rats have been reported to have a higher sympathetic activity than Wistar-Kyoto (WKY) rats. In the present study we sought to determine if these rat strain-related differences in sympathetic activity exist at a myocardial level and whether they translate into changes in cardiac fibrosis, contractile responsiveness to adrenergic agonists, and cardiac structure and function. Coronary effluent noradrenaline concentrations, as determined in isolated, perfused heart preparations, were higher in 5-month-old SD as compared to age-matched WKY male rats. This difference was accompanied by higher resting heart rates in SD rats as assessed in vivo. However, increases in myocardial noradrenaline release in SD rats did not translate into enhanced myocardial fibrosis, cardiac hypertrophy or remodeling, changes in basal ventricular systolic and diastolic function, or to down-regulation of inotropic responses to the beta-adrenoreceptor agonists, noradrenaline, isoproterenol and dobutamine. Although age-matched male SD rats were heavier, no differences in absolute heart weights were noted between rat strains. Moreover, left ventricular (LV) posterior wall thickness as assessed by echocardiography, as well as cardiac myocyte dimensions as determined by laser scanning confocal microscopy were similar between rat strains. Furthermore, LV internal diameters as determined in vivo, as well as LV diastolic volume intercept determined in isolated, perfused heart preparations were similar between rat strains. Increases in myocardial noradrenaline release in SD rats also did not translate into differences in LV systolic chamber and myocardial function as assessed in vivo (LV endocardial and midwall fractional shortening) and at controlled loads and heart rates ex vivo (the slope of the LV developed pressure-volume relation determined). Likewise, neither myocardial hydroxyproline content nor LV chamber stiffness as assessed by the slope of the LV end-diastolic pressure-volume relation were different in SD and WKY rats. In conclusion, rat strain-related differences in cardiac adrenergic tone do indeed exist, but in young animals these differences do not translate into cardiac phenotypes known to contribute to progressive cardiac dysfunction.

Effects of L-arginine oral supplements in pregnant spontaneously hypertensive rats

PURPOSE: To evaluate the effects of L-arginine oral supplementation in spontaneously hypertensive pregnant rats (SHR). METHODS: Thirty SHR and ten Wistar-EPM-1 virgin female rats were used in the study. Before randomization, females were caged with males of the same strain (3:1). Pregnancy was confirmed by sperm-positive vaginal smear (Day 0). Wistar-EPM-1 rats served as counterpart control (C-1). SHR rats were randomized in 4 groups (n=10): Group Control 2, non-treated rats; Group L-Arginine treated with L-arginine 2%; Group Alpha-methyldopa treated with Alpha-methyldopa 33mg/Kg; Group L-Arginine+Alpha-methyldopa treated with L-arginine 2%+Alpha-methyldopa 33mg/Kg. L-arginine 2% solution was offered ad libitum in drinking water and Alpha-methyldopa was administered by gavage twice a day during the length of pregnancy (20 days). Blood pressure was measured by tailcuff plethysmography on days 0 and 20. Body weight was measured on days 0, 10 and 20. Results were expressed as mean ± SD (Standard Deviation). One-Way ANOVA/Tukey (or Kruskal-Wallis/Dunn, as appropriate) was used for group comparisons. Statistical significance was accepted as p<0.05. RESULTS: There was no significant weight gain in isolated L-arginine treated SHR. Mean blood pressure decreased in L-arginine-treated SLR compared with untreated-SHR rats. CONCLUSION: L-arginine oral supplementation reduces blood pressure in spontaneously hypertensive rats during pregnancy.

Effect of ambient temperature on cardiovascular parameters in rats and mice: a comparative approach

Ambient air temperatures (T(a)) of <6 degrees C or >29 degrees C have been shown to induce large changes in arterial blood pressure and heart rate in homeotherms. The present study was designed to investigate whether small incremental changes in T(a), such as those found in typical laboratory settings, would have an impact on blood pressure and other cardiovascular parameters in mice and rats. We predicted that small decreases in T(a) would impact the cardiovascular parameters of mice more than rats due to the increased thermogenic demands resulting from a greater surface area-to-volume ratio in mice relative to rats. Cardiovascular parameters were measured with radiotelemetry in mice and rats that were housed in temperature-controlled environments. The animals were exposed to different T(a) every 72 h, beginning at 30 degrees C and incrementally decreasing by 4 degrees C at each time interval to 18 degrees C and then incrementally increasing back up to 30 degrees C. As T(a) decreased, mean blood pressure, heart rate, and pulse pressure increased significantly for both mice (1.6 mmHg/ degrees C, 14.4 beats.min(-1). degrees C(-1), and 0.8 mmHg/ degrees C, respectively) and rats (1.2 mmHg/ degrees C, 8.1 beats.min(-1). degrees C(-1), and 0.8 mmHg/ degrees C, respectively). Thus small changes in T(a) significantly impact the cardiovascular parameters of both rats and mice, with mice demonstrating a greater sensitivity to these T(a) changes.

Molecular genetic approaches to investigate individual variations in behavioral and neuroendocrine stress responses

A large response range can be observed in both behavioral and neuroendocrine responses to environmental challenges. This variation can arise from central mechanisms such as those involved in the shaping of general response tendencies (temperaments) or involves only one or the other output system (behavioral vs. endocrine response). The participation of genetic factors in this variability is demonstrated by family and twin studies in humans, the comparison of inbred strains and selection experiments in animals. Those inbred strains diverging for specific traits of stress reactivity are invaluable tools for the study of the molecular bases of this genetic variability. Until recently, it was only possible to study biological differences between contrasting strains, such as neurotransmitter pathways in the brain or hormone receptor properties, in order to suggest structural differences in candidate genes. The increase of the power of molecular biology tools allows the systematic screening of significant genes for the search of molecular variants. More recently, it was possible to search for genes without any preliminary functional hypothesis (mRNA differential expression, nucleic acid arrays, QTL search). The approach known as quantitative trait loci (QTL) analysis is based on the association between polymorphic anonymous markers and the phenotypical value of the trait under study in a segregating population (such as F2 or backcross). It allows the location of chromosomal regions involved in trait variability and ultimately the identification of the mutated gene(s). Therefore, in a first step, those studies skip the 'black box' of intermediate mechanisms, but the knowledge of the gene(s) responsible for trait variability will point out to the pathway responsible for the phenotypical differences. Since variations in stress-related responses may be related to numerous pathological conditions such as behavioral and mood disorders, drug abuse, cardiovascular diseases or obesity, and production traits in farm animals, these studies can be expected to bring significant knowledge for new therapeutic approaches in humans and improved efficiency of selection in farm animals.

Effects of hypertension on maternal adaptations to pregnancy: experimental study on spontaneously hypertensive rats

CONTEXT

Animal models for essential hypertension have been used for understanding the human pathological conditions observed in pregnant hypertensive women.

OBJECTIVE

To study the possible effects of pregnancy on hypertension and of hypertension on pregnancy in spontaneously hypertensive rats (SHR), and in their normotensive Wistar-Kyoto (WKY) counterparts.

TYPE OF STUDY

Comparative study using laboratory animals.

SETTING

Animal Research Laboratory of Clinical Medicine at the Medical School of Botucatu, São Paulo State University, Brazil.

SAMPLE

Ten to twelve-week-old virgin female normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). The animals were separated into four groups: 15 pregnant spontaneously hypertensive rats (SHR-P), 10 non-pregnant spontaneously hypertensive rats (SHR-NP), 15 pregnant normotensive rats (WKY-P), and 10 non-pregnant normotensive rats (WKY-NP).

MAIN MEASUREMENTS

The blood pressure was evaluated by the tail cuff method, in rats either with or without prior training for the handling necessary for tail cuff measurements. The maternal volume expansion was indirectly evaluated by weight gain, and by systemic parameters as hematocrit, hemoglobin, total protein, albumin and sodium retention. The perinatal outcome of pregnancy was evaluated by analysis of resorptions, litter size, rate of low weight and number of stillbirths.

RESULTS

The late fall in blood pressure in the pregnant SHR strain and in the normotensive WKY strain can only be detected in rats previously trained to accept the handling necessary for the tail cuff measurement. During pregnancy the body weight gain was significantly higher in WKY than in SHR rats. Systemic parameters were significantly lower in pregnant WKY rats than in non-pregnant WKY rats, while no differences were observed between pregnant and non-pregnant SHR groups. In pregnant WKY rats the sodium retention was higher from the 13th day onwards, while in SHR rats this occurred only on the 21st day. The characteristics of reproductive function such as number and weight of fetus, perinatal mortality and the resorption rate were significantly affected in the SHR strain.

CONCLUSION

The SHR strain may be considered as a model for chronic hypovolemic maternal hypertension, with the fetal growth retardation being determined by this hypovolemic state.

Cardiovascular and metabolic responses of hypertensive and normotensive rats to one week of cold exposure

Challenges to energy homeostasis, such as cold exposure, can have consequences for both metabolic and cardiovascular functioning. We hypothesized that 1-wk cold exposure (4 degrees C) would produce concurrent increases in metabolic rate (VO(2); indirect calorimetry), heart rate (HR), and mean arterial blood pressure (MAP) measured by telemetry. In the initial hours of change in ambient temperature (T(a)), both spontaneously hypertensive rats (SHRs) and normotensive Sprague-Dawley rats showed rapid increases (in cold) or decreases (in rewarming) of VO(2), HR, and MAP, although the initial changes in MAP and HR were more exaggerated in SHRs. Throughout cold exposure, HR, VO(2), food intake, and locomotor activity remained elevated but MAP decreased in both strains, particularly in the SHR. During rewarming, all measures normalized quickly in both strains except MAP, which fell below baseline (hypotension) for the first few days. The results indicate that variations of T(a) produce rapid changes in a suite of cardiovascular and behavioral responses that have many similarities in hypertensive and normotensive strains of rats. The findings are consistent with the general concept that the cardiovascular responses to cold exposure in rats are closely related to and perhaps a secondary consequence of the mechanisms responsible for increasing heat production.

Behavioral effects of acute and chronic fluoxetine in Wistar-Kyoto rats

It has been previously reported that Wistar-Kyoto (WKY) rats may be useful in the study of the biological mechanisms involved in stress-related disorders. In the present study, WKY were treated acutely or chronically (one daily i.p. injection for 22-24 days) with the selective 5-HT reuptake inhibitor and clinically effective antidepressant and anxiolytic fluoxetine (5 and 20 mg/kg) and exposed to the forced swimming test (FST) and to the elevated plus-maze (EPM) at different times postinjection (30, 60, min or 24 h). In the FST, WKY failed to respond to fluoxetine, regardless of treatment. In the EPM, acute fluoxetine (20 mg/kg) produced anxiolytic-like effects when animals were tested 24 h, but not 30 min after drug administration. Positive effects in the EPM were evident on both conventional (open-arm activity) and ethological (risk assessment) measures in the absence of effect on activity measures (total and closed-arm entries). No evidence for anxiolytic-like activity was observed following chronic fluoxetine. These results indicate that WKY rats are resistant to fluoxetine treatment in the FST, while their behavior may be modified in the EPM when animals received a single fluoxetine challenge 24 h before testing. Overall, these findings provided little evidence that WKY rats may represent a valid model of stress-related disorders.

Stress and emotionality: a multidimensional and genetic approach

The use of behavioural tests aiming to assess the psychological components of stress in animals has led to divergent and sometimes arbitrary interpretations of animal behaviour. This paper presents a critical evaluation of behavioural methods currently used to investigate stress and emotionality. One of its main goals is to demonstrate, through experimental evidence, that emotionality may no longer be seen as a unidimensional construct. Accordingly, following a discussion about concepts, we propose a multiple-testing approach, paralleled by factor analyses, as a tool to dissociate and study the different dimensions of emotionality. Within this multidimensional context, genetic studies (illustrated here by different rat models) are shown to be particularly useful to investigate the neurobiology of stress/emotionality. A genetic approach can be used (i) to broaden and dissect the variability of responses within and between populations and (ii) to search for the molecular bases (i.e. genes and gene products) which underlie such a variability.

Long-term ouabain administration does not alter blood pressure in conscious Sprague-Dawley rats

1. We tested the ability of ouabain to cause chronic hypertension by continuously infusing ouabain for 28 days (miniosmotic pump implantation; i.p.). The blood pressure and metabolic effects of sham (150 mmol/L NaCl; n = 12) or ouabain infusion (10 micrograms/kg per day; n = 14; 100 micrograms/kg per day; n = 14) were examined in conscious Sprague-Dawley rats. 2. Plasma ouabain concentrations measured after 28 days of ouabain infusion were as follows: sham, not detectable (n = 11); ouabain 10 micrograms/kg per day, 0.60 +/- 0.07 nmol/L (n = 14); and ouabain 100 micrograms/kg per day, 7.17 +/- 0.57 nmol/L (n = 14; P < 0.001). 3. Sham or ouabain infusion did not alter food intake, bodyweight, water intake or urine output in conscious rats. 4. Blood pressure was not altered by sham treatment. Ouabain at 10 micrograms/kg per day or 100 micrograms/kg per day did not produce consistent rises in blood pressure. Ouabain at 10 micrograms/kg per day increased blood pressure on treatment day 12 only (+6 mmHg; P < 0.05), while at 100 micrograms/kg per day blood pressure increased on treatment days 16 (+9 mmHg; P < 0.05) and day 18 (+8 mmHg; P < 0.05) only. There was no significant difference in blood pressure between sham and ouabain groups. 5. Renal blood flow was decreased in rats infused with ouabain at 10 micrograms/kg per day (2.0 +/- 0.3 mL/min per 100 g bodyweight; n = 5; P < 0.01) and 100 micrograms/kg per day (2.2 +/- 0.4 mL/min per 100 g bodyweight; n = 7; P < 0.05) compared with sham treatment (3.5 +/- 0.2 mL/min per 100 g bodyweight; n = 6). Renal vascular resistance was increased in rats treated with ouabain at 10 micrograms/kg per day (65.5 +/- 12.6 mmHg/mL per min per 100 g bodyweight; n = 5; P < 0.01) and 100 micrograms/kg per day (66.0 +/- 15.6 mmHg/mL per min per 100 g bodyweight; n = 7; P < 0.05) compared with sham treatment (32.6 +/- 2.5 mmHg/mL per min per 100 g bodyweight; n = 6). 6. High plasma concentrations of ouabain do not cause consistent increases in blood pressure in conscious Sprague-Dawley rats.

Basal and reactive plasma catecholamine levels under stress and anesthesia in rabbits

The present study was undertaken to determine if electrical stimulation of the hypothalamic defense area (HDA) and baroreflex activation elicited by head up body tilt produced changes in plasma catecholamine (CA) levels in anesthetized rabbits. We also compared the effects of two anesthetics, isoflurane and sodium pentobarbital, upon basal and reactive CA levels, and upon autonomic reactivity. HDA stimulation was found to produce significant increases in plasma norepinephrine (NE) levels but not epinephrine (E) levels. Passive tilt was found to produce statistically significant increases in NE levels for both anesthetics used and a significant increase in E levels for animals anesthetized with isoflurane. Basal and reactive measurements provided evidence that pentobarbital has a more suppressive effect upon the autonomic nervous system than isoflurane: (a) Basal NE levels were significantly lower in pentobarbital anesthetized animals than in isoflurane-anesthetized animals; and (b) Baroreceptor sensitivity to a passive tilt stressor was significantly higher for animals anesthetized with isoflurane than for animals anesthetized with pentobarbital.

Stress-induced changes of circulating neuropeptide Y in the rat: comparison with catecholamines

Circulating concentrations of neuropeptide Y-like immunoreactivity (NPY), noradrenaline (NA) and adrenaline (AD) were measured in conscious, chronically catheterized rats submitted to various stress protocols. Basal plasma levels of NPY, NA and AD (194 +/- 52 fmol/ml, 0.90 +/- 0.11 pmol/ml and 0.52 +/- 0.07 pmol/ml) were increased by handling (+132%, +76% and +629%, respectively) and rose further during electric shock treatment. Adrenalectomy resulted in the complete disappearance of circulating adrenaline but did not alter either control or stress values of noradrenaline. In comparison circulating levels of NPY were reduced, but not significantly in adrenalectomized animals. Insulin stress induced a large increase in plasma AD levels and cold stress induced an increase in plasma NA levels, without any parallel change in NPY concentrations. These results demonstrate that NPY, which is colocalized with catecholamines in the peripheral nervous systems, is also released during stress responses and that its release parallels more closely changes in circulating NA than AD. Furthermore, stress-induced changes in circulating NPY-like immunoreactivity do not originate from the adrenal gland but mainly from the peripheral nervous system, and the release of NPY is dependent upon the nature of the stimulus.

Role of sympathetic nervous system in hypotensive action of taurine in DOCA-salt rats

We tested the hypothesis that the antihypertensive effects of dietary taurine supplementation in deoxycorticosterone acetate (DOCA)-salt rats may be attributed to the suppression of sympathetic nervous system activity. In uninephrectomized rats treated with DOCA while receiving 1% NaCl solution for 2 weeks, systolic blood pressure was significantly increased as compared with that in control rats treated with vehicle suspension and tap water. Sympathetic nervous system activity was assessed by tissue norepinephrine turnover, which was determined from the rate of decline of tissue norepinephrine concentration after the administration of alpha-methyl-p-tyrosine, a potent inhibitor of the rate-limiting step of catecholamine synthesis. Cardiac and splenic norepinephrine turnover during either normal conditions or cold exposure (4 degrees C, 8 hours) were markedly increased in DOCA-salt rats as compared with control rats. Also, DOCA-salt rats had increased depressor response to hexamethonium bromide, a ganglion blocker. In contrast, supplementation of 1% taurine in DOCA-salt rats attenuated the development of the hypertension associated with the normalization of both the increased depressor response to ganglionic blockade and the accelerated cardiac and splenic norepinephrine turnover during either normal conditions or cold exposure. Taurine supplementation in control rats, however, had no effect on blood pressure or norepinephrine turnover during cold exposure. These results suggest that taurine supplementation suppresses sympathetic overactivity in DOCA-salt rats, thus leading to inhibition of the development of hypertension.

Plasma catecholamine measurements in resting and stressed conscious rats, using high performance liquid chromatography with electrochemical detection

Plasma epinephrine (EPI) and norepinephrine (NE) were measured in conscious, unrestrained rats at rest, and following exercise, cold stress and hemorrhage. Heart rate and mean arterial pressure were monitored at all points. Minor modifications of a standard, commercially available methodology achieved good chromatographic separation of both EPI and NE. Sensitivity was sufficient for all NE measurements and for EPI measurements in the stress conditions. Estimates of EPI by this procedure were somewhat higher than those in other reports. Absolute values of NE, as well as the selective, qualitative changes found in both EPI and NE under each stress condition, were consistent with those found in other studies which used the radioenzymatic method. Blood withdrawal of two 1.5 ml samples, 3 hours apart, had no detectable effect on EPI, NE, mean arterial pressure or heart rate in otherwise non-stressed rats.

Stress, behavior and experimental hypertension

Over the past twenty-five years, several animal models of human essential hypertension have been produced through the development of inbred strains or lines of laboratory rats. The general availability of laboratory rats with genetically determined increases in arterial blood pressure has stimulated an impressive volume of research in the pathophysiology of experimental hypertension. In contrast, relatively little attention has been devoted to the study of behavioral correlates of experimental hypertension. In this review, I will evaluate the advantages and limitations of studying animal models of essential hypertension. Emphasis will then be placed on the relationship between stressful stimulation and behavioral and physiological responsiveness in two animal models of essential hypertension. Specifically, studies from my laboratory have examined sympathetic nervous system activity and behaviors of rats under basal conditions and following acute or chronic exposure to stressful stimulation. These findings indicate that the spontaneously hypertensive (SHR) strain is excessively responsive behaviorally and physiologically to a variety of stressful stimuli when compared to its Wistar-Kyoto (WKY) normotensive control strain. In contrast, the behavioral and physiological responses of New Zealand genetically hypertensive (GH) and normotensive (N) rats do not differ following acute exposure to stress. Thus, the hyperreactivity of SHR rats to stressful stimulation is not necessarily related to the development of hypertension but may be a valuable marker of the predisposition to develop high blood pressure in rats of the SHR strain. An experimental approach is outlined for examining the causal relationship between a genetically determined physiological or behavioral marker and the development of hypertension.

Effects of neurochemical lesions restricted to spinal cord monoaminergic neurons on blood pressure and sympathetic activity of spontaneously hypertensive rats

Intraspinal (i.s.) injection of 6-hydroxydopamine or 5,7-dihydroxytryptamine in newborn spontaneously hypertensive rats (SHR) resulted, in the adult animal (30-week-old), in a marked decrease of spinal cord noradrenaline (NA) or 5-hydroxytryptamine (5-HT) levels, respectively. Since both neurotoxin- and vehicle-injected rats developed full hypertension and had similar plasma catecholamine concentrations, it is concluded that in SHR neither spinal cord NA nor 5-HT play a major role in development and maintenance of hypertension.