Angiotensin II in dorsomedial hypothalamus modulates cardiovascular arousal caused by stress but not feeding in rabbits

The dorsomedial hypothalamus (DMH) is critically implicated in the cardiovascular response to emotional stress. This study aimed to determine whether the DMH is also important in cardiovascular arousal associated with appetitive feeding behavior and, if so, whether locally released angiotensin II and glutamate are important in this arousal. Emotional (air-jet) stress and feeding elicited similar tachycardic (+51 and +45 beats/min, respectively) and pressor (+16 and +9 mmHg, respectively) responses in conscious rabbits. Bilateral microinjection of GABAA agonist muscimol (500 pmol) into the DMH, but not nearby hypothalamic regions, attenuated pressor and tachycardic responses to air-jet by 56–63% and evoked anorexia. Conversely, stimulation of the DMH with the glutamate analog kainic acid (250 pmol) elicited hypertension (+25 mmHg) and tachycardia (+114 beats/min) and activated feeding behavior. Local microinjection of a glutamate receptor antagonist, kynurenic acid (10 nmol), decreased pressor responses to stress and eating by 46 and 72%, respectively, without affecting feeding behavior. Bilateral microinjection of a selective AT1-receptor antagonist, candesartan (500 pmol), into the DMH, but not nearby sites, attenuated pressor and tachycardic stress responses by 31 and 33%, respectively. Candesartan did not alter feeding behavior or circulatory response to feeding. These results indicate that, in addition to its role in mediating stress responses, the DMH may be important in regulating cardiovascular arousal associated with feeding. Local glutamatergic inputs appear to regulate cardiovascular response to both stress and feeding. Conversely, angiotensin II, acting via AT1 receptors, may selectively modulate, in the DMH, cardiovascular response to stress, but not feeding.

Imidazoline receptors associated with noradrenergic terminals in the rostral ventrolateral medulla mediate the hypotensive responses of moxonidine but not clonidine

We determined whether the cardiovascular actions of central anti-hypertensive agents clonidine and moxonidine are dependent on noradrenergic or serotonergic innervation of the rostral ventrolateral medulla (RVLM) in conscious rabbits. 6-Hydroxydopamine (6-OHDA) or 5,6-dihydroxytriptamine (5,6-DHT) was injected into the RVLM to deplete noradrenergic and serotonergic terminals respectively. One, 2 and 4 weeks later, responses to fourth ventricular (4V) clonidine (0.65 microg/kg) and moxonidine (0.44 microg/kg) were examined. Destruction of noradrenergic pathways in the RVLM by 6-OHDA reduced the hypotensive response to 4V moxonidine to 62%, 47% and 60% of that observed in vehicle treated rabbits at weeks 1, 2 and 4 respectively. The moxonidine induced bradycardia was similarly attenuated (to 46% of vehicle). Conversely, 6-OHDA had no effect on the hypotensive or bradycardic effects of 4V clonidine. Efaroxan (I(1)-imidazoline receptor/alpha(2)-adrenoceptor antagonist; 3.5, 11, 35 microg/kg) and 2-methoxyidazoxan (alpha(2)-adrenoceptor antagonist; 0.3, 0.9, 3 microg/kg) equally reversed the hypotension to 4V clonidine, suggesting a mainly alpha(2)-adrenoceptor mechanism. Efaroxan preferentially reversed responses to moxonidine in both vehicle and 5,6-DHT groups and in the 1st week after 6-OHDA, suggesting a mechanism involving mainly I(1)-imidazoline receptors. This selectivity was subsequently lost in the 2nd and 4th weeks when the remaining hypotension was mainly mediated by alpha(2)-adrenoceptors. Depletion of serotonergic terminals did not alter the responses to either agonist nor did it change the relative effectiveness of the antagonists. Western blots of RVLM tissues probed with imidazoline and alpha(2)-adrenoceptor antisera showed a pattern of bands close to that reported in other species. The main effect of 6-OHDA was an 18% lower level of the 42 kDa imidazoline protein (P<0.05). We conclude that the hypotensive and bradycardic actions of moxonidine but not clonidine are mediated through imidazoline receptors and are dependent on intact noradrenergic pathways within the RVLM. Furthermore, the noradrenergic innervation may be associated with a 42 kDa imidazoline receptor protein.

Nonsymmetrical double logistic analysis of ambulatory blood pressure recordings

We developed an asymmetric double logistic curve-fitting procedure for circadian analysis that can determine the rate of change in variables during the day-to-night separately from the night-to-day transition for use in animal studies. We now have applied this procedure to 24-h systolic (SAP) and diastolic arterial pressure (DAP) and heart rate ambulatory recordings from 302 patients. In 292 cases, all parameters showed a pattern of higher day and lower night values. In men there was a similar rate of transition between day and night or from night to day for both SAP and DAP that lasted 3–4 h, indicating a symmetrical diurnal pattern. By contrast, women showed a faster rate of decrease in mean arterial pressure in the evening compared with men ( P < 0.05) and therefore showed an asymmetric diurnal SAP pattern. For both men and women, there was a markedly greater rate of morning increase in heart rate compared with the rate of evening decrease (2.2- and 1.9-fold, respectively, P < 0.001). The logistic method provided a better fit than the square-wave or the cosinor method ( P < 0.001) and more appropriately detected nondippers. We conclude that analysis of ambulatory recordings by a new logistic curve-fitting method reveals more rapid reductions in evening SAP in women than men but both have two- to threefold more rapid morning rates of tachycardia. The ability of the double logistic method to determine the diurnal blood pressure rates of change independently is key to determining new markers for cardiovascular risk.

Non-symmetrical double-logistic analysis of 24-h blood pressure recordings in normotensive and hypertensive rats

OBJECTIVE

To determine the suitability of a new logistic curve fitting procedure to measure the diurnal rates of transition from the active to the asleep periods separately.

METHOD

We applied this method to 24-h telemetry recordings of systolic, mean, diastolic arterial pressure (SAP, MAP, DAP, respectively), heart rate (HR) and locomotor activity of normotensive Sprague-Dawley rats (SDR) and spontaneously hypertensive rats (SHR).

RESULTS

There was a similar pattern of higher awake and lower sleep values (16 +/- 1 mmHg SAP, 77 +/- 2 bpm HR and 40 +/- 2 units activity) in SHR. In SDR, awake-asleep differences were less for SAP (9 +/- 1 mmHg) but similar for HR (83 +/- 2 bpm). In SHR, while the blood pressure patterns were symmetrical, the rate of rise in activity and HR during arousal was more rapid than the rate of decline during the dark to light transition. By contrast in SDR, the arousal rate of increase in blood pressure and HR was much less than the rate of decline. Thus SHR have an exaggerated arousal surge in DAP compared with SDR. Double logistic provides a better fit than Cosinor or square wave and better estimates of day-night differences than partial Fourier.

CONCLUSIONS

Analysis of 24-h recordings by a new logistic curve method reveals distinct asymmetric circadian patterns of cardiovascular and activity changes in rats. The greater surge in arousal blood pressure in SHR is not associated with differences in HR or activity changes and may be inherent to the underlying mechanisms contributing to the hypertension in SHR.

Aromatase-deficient (ArKO) mice have reduced blood pressure and baroreflex sensitivity

Aromatase-deficient (ArKO) mice are deficient in estrogens due to deletion of the aromatase gene. We hypothesized that there may be changes in the cardiovascular system of ArKO mice because of evidence linking estrogens with improved cardiovascular outcomes and the induction of the glucocorticoid-metabolizing enzyme, 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2), gene in the kidney, which is important for the regulation of blood pressure (BP). BP and baroreflex sensitivity (BRS) in female conscious ArKO mice were compared with those in age- and weight-matched wild-type (WT) mice. Power spectral analysis was used to determine cardiovascular variability and BRS. Although systolic BP was similar in the two groups, diastolic and mean BPs were lower in the ArKO mice (-6.3 +/- 1.9 and -4.6 +/- 2.1 mm Hg, respectively). Heart rate (HR) was greater in the ArKO mice (+36 +/- 6 beats/min). The mean BP in WT mice was 105 mm Hg, and the HR was 481 beats/min. In the autonomic frequency range, BP variability was 74% greater, and HR variability was only 26% that in WT mice. The BRS of ArKO mice was 46% of the value observed in WT mice. 11betaHSD2 levels were unaltered in ArKO mice, except in the kidney, where they were only 10% of WT levels. Estradiol administration to ArKO mice restored renal 11betaHSD2 to WT levels. The results show that ArKO mice have lower diastolic BP, but increased BP variability, perhaps due to an impaired BRS. Thus, aromatase activity is critical for normal autonomic control of the heart and, hence, for reducing the deleterious effects of high BP variability.

Tempol Attenuates Excitatory Actions of Angiotensin II in the Rostral Ventrolateral Medulla During Emotional Stress

Superoxide has been shown to be an important intracellular mediator of actions of angiotensin II. Recently, we found that blockade of angiotensin II type-1 receptors in the rostral ventrolateral medulla (RVLM) abrogated the pressor effect of emotional stress in rabbits. In the present study, we examined the influence of superoxide dismutase mimetics, tempol and tiron, in RVLM on cardiovascular stress response in conscious rabbits. Air-jet stress evoked a sustained increase in blood pressure (+14±2 mm Hg), tachycardia (+52±7 bpm), and renal sympathoactivation (+58±8%). Bilateral microinjections of tempol or tiron (20 nmol) into RVLM did not alter resting cardiovascular parameters, but attenuated the pressor, sympathetic, and tachycardiac response to stress by 40% to 55%. By contrast, 3-carbamoylproxyl, which is structurally close to tempol but has a lower superoxide scavenging activity, did not alter the stress response. Neither tempol nor tiron altered the sympathoexcitatory response to glutamate microinjections into RVLM or to baroreceptor unloading. Microinjections of nitric oxide synthase inhibitor N G -nitro- l -arginine methyl ester ( l -NAME; 10 nmol) into RVLM did not affect the stress response. Coinjections of tempol and l -NAME decreased the pressor response to stress by 35±3%. Tempol attenuated the pressor response to microinjection of angiotensin II into RVLM by 59±15%, whereas l -NAME did not alter this response. These results suggest that superoxide dismutase mimetics in RVLM attenuate, partially via a nitric oxide-independent mechanism, the pressor effect of emotional stress in rabbits. Together with our previous studies, these results also indicate that superoxide is a key mediator of excitatory actions of angiotensin II in RVLM during acute stress.

Are centrally acting imidazoline agents appropriate therapy for renovascular hypertension?

An increased role of the sympathetic nervous system has been suggested to be a major contributor to the chronic elevation of BP in renovascular hypertension. We assessed the effects of rilmenidine, a centrally acting antihypertensive imidazoline agent, on BP and renal sympathetic nerve activity (RSNA) in 2K1C renovascular hypertensive conscious rabbits. Rabbits were made hypertensive with a renal clip or were sham-operated and were studied 3 and 6 weeks later. Acute treatment with rilmenidine reduced BP to a greater extent in the hypertensive rabbits. Although rilmenidine reduced the heart rate by the same extent in all groups, rilmenidine produced much less inhibition of RSNA in the hypertensive animals. These studies suggest that the contribution of the sympathetic nervous system is greater in 2K1C hypertension and that imidazoline agents may be beneficial in treating renovascular hypertension.

Sympathetic Responses to Stress and Rilmenidine in 2K1C Rabbits

We determined whether the sympathetic excitatory responses to environmental stressors and the sympathoinhibitory responses to rilmenidine are altered by renovascular hypertension. Rabbits were made hypertensive with a clip on the right renal artery, and a left renal nerve recording electrode was implanted. After 3 or 6 weeks, the animals were given air-jet stress and loud noise stress before and after intravenous rilmenidine. Three and 6 weeks after renal clipping, mean arterial pressure was 28% and 36% greater than preclip values. Air-jet stress elicited a marked increase in renal sympathetic nerve activity, mean arterial pressure, and heart rate. Renal sympathetic nerve activity responses were much greater in hypertensive rabbits, but the pressor responses were similar to those observed in normotensive animals. Acute administration of rilmenidine decreased blood pressure more in hypertensive animals but with a much lesser inhibition of sympathetic activity. Rilmenidine markedly reduced increased sympathetic activity during air-jet stress in 3-week clipped rabbits but to a lesser extent in the other groups. These studies show that while sympathetic responses to stress were markedly enhanced in renal clip hypertensive rabbits, they did not result in greater pressor responses, thus suggesting that vascular neuroeffector mechanisms were not altered. By contrast, the increased effects of rilmenidine suggest a much greater contribution to the hypertension by the sympathetic nervous system, but one that is caused by an enhanced "nonvascular" neuroeffector mechanism. As such, sympathoinhibitory agents such as rilmenidine are very suitable and very effective agents for the treatment of renovascular hypertension.

The rostral ventrolateral medulla mediates sympathetic baroreflex responses to intraventricular angiotensin II in rabbits

The present study examined the role of the rostral ventrolateral medulla (RVLM) in mediating the pressor and renal sympathetic baroreflex effects of intraventricularly administered angiotensin II (Ang II) in urethane anaesthetised rabbits. Microinjection of Ang II over a wide range of medullary sites showed that pressor responses were observed only in the RVLM. Ang II was particularly potent in producing a transient pressor response at this site with a half maximal dose of 9 fmol. The administration of the Ang II antagonist Sar(1)-Ile(8)-Ang II (10 pmol) bilaterally into the RVLM inhibited the pressor response to local and fourth ventricular Ang II, but not the pressor response to RVLM applied glutamate. To determine the contribution of the RVLM to the renal sympathetic baroreflex effects of Ang II, blood pressure-renal sympathetic nerve activity (RSNA) curves were constructed with intravenous infusion of phenylephrine or nitroprusside before and after Ang II, vehicle or glutamate infusions into the RVLM. Ang II infusion of 4 pmol/min into the RVLM increased blood pressure by 8+/-3 mm Hg and shifted the renal sympathetic baroreflex curve to the right. The maximum RSNA evoked by lowering blood pressure increased by 36+/-6%, similar to the effect seen with fourth ventricular Ang II and RVLM glutamate. These studies suggest that the major medullary pressor site of action of Ang II when injected into the hindbrain cerebro-spinal fluid of anaesthetized rabbits is the RVLM where it facilitates baroreflex control of RSNA.

Method for in vivo calibration of renal sympathetic nerve activity in rabbits

A major difficulty of recording from peripheral sympathetic nerves is that microvolt values reflect characteristics of the recording conditions and limit comparisons between different experimental groups. In this study we assessed methods of calibrating renal sympathetic nerve activity (RSNA) in conscious rabbits. Calibration values were obtained from maximum RSNA responses to nasopharyngeal stimulation, airjet stress or unloading baroreceptors. Curves relating RSNA to blood pressure were produced by raising and lowering blood pressure with vasoactive drugs. To assess whether normalization would eliminate differences between RSNA curves which were most likely due to recording conditions, rabbits were first divided into two groups with high or low basal microvolt levels of RSNA, then again into two groups with high or low heart rate. In both cases, curves were similar if values were normalized by nasopharyngeal stimulation or by the upper plateau value. In hypertensive rabbits, where the baroreflex is suppressed, only the nasopharyngeal method showed this attenuated pattern. This method also eliminated the 50% decay in basal RSNA measured over 5 weeks. We conclude that expressing RSNA in terms of the maximum response to nasopharyngeal stimulation provides a calibration method suitable for comparing nerve activity over the long term as well as showing valid differences in baroreflex curves between different experimental groups.

AT1 receptors in the RVLM mediate pressor responses to emotional stress in rabbits

In this study, we examined the role of angiotensin type 1 (AT1) receptors in the rostral ventrolateral medulla (RVLM) in mediating the pressor action of emotional stress in conscious rabbits. Rabbits were chronically instrumented with guide cannulas for bilateral microinjections into the RVLM and an electrode for measuring renal sympathetic nerve activity (RSNA). Airjet stress evoked increases in arterial pressure, heart rate, and RSNA, which reached a maximum (+9+/-1 mm Hg, +20+/-5 beats/min, and +93+/-17%, respectively) in the first 2 minutes of stress exposure. Then RSNA rapidly returned to prestress values, while arterial pressure and heart rate remained close to the maximal level until the conclusion of the 7-minute airjet exposure. Microinjections of the nonselective angiotensin receptor antagonist sarile (0.5 nmol, n=8) or AT1 receptor antagonists losartan (2 nmol, n=6) or candesartan (0.2 nmol, n=6) into the RVLM did not alter resting cardiovascular parameters. By contrast, the antagonists attenuated the sustained phase (4 to 7 minutes) of the pressor stress response by 55% to 89%. However, only sarile decreased the onset of this response. The antagonists affected neither the stress-induced tachycardia nor the pressor response to glutamate microinjections. Microinfusion of angiotensin II (4 pmol/min, n=8) into the RVLM did not change the pressor response to airjet stress but attenuated tachycardic response by 47%. Microinjections of vehicle did not alter the cardiovascular stress response. Sarile, losartan, and angiotensin II did not affect the sympathoexcitatory response to baroreceptor unloading. These results suggest that AT1 receptors in the RVLM are important in mediating the pressor effects of emotional stress in conscious rabbits.

Glutamate receptors in RVLM modulate sympathetic baroreflex in conscious rabbits

In this study, we examined the effect of excitatory amino acid (EAA) receptor blockade in the rostral ventrolateral medulla (RVLM) on the renal sympathetic baroreflex in conscious rabbits. Rabbits were implanted with guide cannulas for bilateral microinjections into the RVLM (+2 to +3 mm from the obex, n = 8) or into the intermediate ventrolateral medulla (IVLM; 0 to +1 mm from the obex, n = 5) and with an electrode for measuring renal sympathetic nerve activity (RSNA). After 7 days of recovery, microinjection of the EAA receptor antagonist kynurenate (10 nmol) into the RVLM did not affect resting RSNA or arterial pressure. Kynurenate decreased the gain of the RSNA baroreflex by 53% but did not change the reflex range. By contrast, injection of kynurenate into the IVLM increased resting arterial pressure and RSNA by 27 mmHg and 88%, respectively, but did not alter the RSNA baroreflex gain or range. Pentobarbital sodium anesthesia attenuated the gain and range of the RSNA baroreflex by 78 and 40%, respectively. Under these conditions, microinjection of kynurenate into the RVLM did not cause any further change in the gain of this reflex. These results suggest that endogenous EAA neurotransmitters in the RVLM are important in modulating the sympathetic baroreflex in conscious rabbits. Anesthesia can mask the functional significance of EAAs in the RVLM in modulating the baroreflexes, which may explain why previous studies in anesthetized animals found no effect of blocking EAA receptors in the RVLM on sympathetic baroreflexes.

Importance of imidazoline-preferring receptors in the cardiovascular actions of chronically administered moxonidine, rilmenidine and clonidine in conscious rabbits

OBJECTIVE

To determine the involvement of central imidazoline receptors in the cardiovascular actions of the chronically administered antihypertensive agents moxonidine, rilmenidine and clonidine.

DESIGN AND METHODS

In 21 rabbits with implanted fourth-ventricular catheters, we investigated the central effects of three cumulative doses of an I(1)-imidazoline/alpha(2)-adrenoceptor antagonist, efaroxan, and of an alpha(2)-adrenoceptor antagonist, 2-methoxyidazoxan (2-MI), on the changes in blood pressure and heart rate (HR) elicited by chronic subcutaneous administration of moxonidine, rilmenidine and clonidine, after 1 and 3 weeks of treatment. A low, medium and high dose of 2-MI was matched to three doses of efaroxan, such that each produced equal reversal of the hypotension induced by fourth-ventricular alpha-methyldopa and hence produced a similar degree of alpha(2)-adrenoceptor blockade.

RESULTS

Clonidine and moxonidine, at doses of 1 mg/kg per day, and rilmenidine at 5 mg/kg per day, produced sustained reductions in mean arterial pressure of 13 +/- 3, 15 +/- 2 and 13 +/- 2 mmHg, respectively over the 3-week treatment period, but did not alter HR. Central administration of efaroxan on day 9 and day 23 of treatment produced a greater increase in blood pressure than did 2-MI with all three antihypertensive agents. Blood pressure reached levels that were significantly above the original control values. By contrast, the alpha(2)-adrenoceptor antagonist 2-MI only induced a rebound blood pressure effect in clonidine- and to a lesser extent in rilmenidine-treated rabbits. Both efaroxan and 2-MI produced a similar degree of tachycardia in moxonidine-, rilmenidine- and clonidine-treated animals.(2)

CONCLUSIONS

The greater effect of efaroxan compared to the alpha(2)-adrenoceptor antagonist 2-MI suggests that the hypotension induced by chronic subcutaneous administration of moxonidine, rilmenidine and clonidine is mediated predominantly via an action on central imidazoline receptors. Furthermore, all agents showed a propensity to produce rebound hypertension with imidazoline receptor blockade. However, only clonidine showed a rebound phenomenon when challenged by acute central alpha(2)-adrenoceptor blockade

Impaired central stress-induced release of noradrenaline in rats with heart failure: a microdialysis study

Sympathetic hyperactivity in rats with heart failure is associated with increased extracellular noradrenaline in the hypothalamic paraventricular nucleus at rest. However, it is unknown how this nucleus responds to stressful stimuli. In the present study we therefore examined the basal and stress-induced release of noradrenaline in the paraventricular nucleus of conscious Sprague-Dawley rats with heart failure measured by in vivo microdialysis. Basal noradrenaline concentration in the paraventricular nucleus of rats with heart failure was more than double that in sham-operated controls. Immobilization stress decreases noradrenaline levels in the paraventricular nucleus of rats with heart failure to 57% of baseline, while it increased in sham-operated controls to 228%. However, serum corticosterone was similarly elevated at 30 and 90 min post-stress in both experimental groups. We have shown that heart failure causes an impairment of the central noradrenergic system's response to acute sympatho-excitation but does not affect the hypothalamo-pituitary-adrenocortical response.

Angiotensin and baroreflex control of the circulation

There is a close association between the location of angiotensin (Ang) receptors and many important brain nuclei involved in the regulation of the cardiovascular system. The present review encompasses the physiological role of Ang II in the brainstem, particularly in relation to its influence on baroreflex control of the heart and kidney. Activation of AT1 receptors in the brainstem by fourth ventricle (4V) administration to conscious rabbits or local administration of Ang II into the rostral ventrolateral medulla (RVLM) of anesthetized rabbits acutely increases renal sympathetic nerve activity (RSNA) and RSNA baroreflex responses. Administration of the Ang antagonist Sarile into the RVLM of anesthetized rabbits blocked the effects of Ang II on the RSNA baroreflex, indicating that the RVLM is the major site of sympathoexcitatory action of Ang II given into the cerebrospinal fluid surrounding the brainstem. However, in conscious animals, blockade of endogenous Ang receptors in the brainstem by the 4V AT1 receptor antagonist losartan resulted in sympathoexcitation, suggesting an overall greater activity of endogenous Ang II within the sympathoinhibitory pathways. However, the RSNA response to airjet stress in conscious rabbits was markedly attenuated. While we found no effect of acute central Ang on heart rate baroreflexes, chronic 4V infusion inhibited the baroreflex and chronic losartan increased baroreflex gain. Thus, brainstem Ang II acutely alters sympathetic responses to specific afferent inputs thus forming part of a potentially important mechanism for the integration of autonomic response patterns. The sympathoexcitatory AT1 receptors appear to be activated during stress, surgery and anesthesia.

A real-time algorithm for the quantification of blood pressure waveforms

A real-time algorithm for quantification of biological oscillatory signals, such as arterial blood pressure (BP) is proposed which does not require user intervention and works on waveforms complicated by rapid changes in the mean level, frequency, or by the presence of arrhythmia. The algorithm is based on the continous independent assessment of the refractory period (RP). In the first stage, a sample of the signal is band-pass filtered. During the next stage: 1) the local maxima in the filtered signal are identified and their pulse amplitudes (PA) measured on the side opposite to the possible notch position and 2) those maxima whose PA exceeds some threshold are selected and an array of RP values is formed as a fraction of the moving estimate of the interval between successive selected peaks. Finally, the original signal is analyzed by means of two moving averages (MAs) with short and long averaging time intervals. The true peaks are determined as the maxima between intersections of MAs if the peak-to-peak or the intersection-to-intersection intervals since the previous peak and the previous intersection exceed the RP. The algorithm proved to be superior against three commercially available heartbeat detectors yielding an error rate of 0.09%.

Ionotropic glutamate receptors in the rostral ventrolateral medulla mediate sympathetic responses to acute stress in conscious rabbits

In conscious, chronically instrumented rabbits (n = 7), airjet stress evoked increases in arterial pressure (AP) and renal sympathetic nerve activity (RSNA), which were greatest in the first 2 min (+ 10 mm Hg and + 127%, respectively), but then rapidly reached a stable level (+ 7 mm Hg and + 37%, respectively). Bilateral microinjection into the rostral ventrolateral medulla (RVLM) of an ionotropic excitatory amino acid (EAA) receptor antagonist kynurenate (10 nmol/200 nl) did not affect resting AP and RSNA, but reduced their initial peak responses to airjet by 80% and 52%, respectively, without altering the stable levels of these responses. By contrast, bilateral microinfusion of glutamate (2 nmol/20 nl/min) into the RVLM increased resting AP by 13 mm Hg, but did not alter the RSNA and AP responses to airjet stress. These results suggest that the RVLM is an essential site for conveying excitatory environmental influences to the sympathetic nervous system in conscious rabbits. The EAA receptors are critically important in initiating the pressor and sympathoexcitatory responses to acute emotional stress, but play relatively little role in the maintenance of these responses.

Sympathetic modulation of renal blood flow by rilmenidine and captopril: central vs. peripheral effects

Renal blood flow (RBF) is modulated by renal sympathetic nerve activity (RSNA). However, agents that are supposed to reduce sympathetic tone, such as rilmenidine and captopril, influence RBF also by direct arteriolar effects. The present study was designed to test to what extent the renal nerves contribute to the renal hemodynamic response to rilmenidine and captopril. We used a technique that allows simultaneous recording of RBF and RSNA to the same kidney in conscious rabbits. We compared the dose-dependent effects of rilmenidine (0.01-1 mg/kg) and captopril (0.03-3 mg/kg) on RBF and RSNA in intact and renal denervated (RNX) rabbits. Because rilmenidine and captopril lower blood pressure, studies were also performed in sinoaortically denervated (SAD) rabbits to determine the role of the baroreflex in the renal hemodynamic response. Rilmenidine reduced arterial pressure, RBF, and RSNA dose dependently. In intact rabbits (n = 10), renal conductance (RC) remained unaltered (3 +/- 5%), even after the 1-mg/kg dose, which completely abolished RSNA. In RNX rabbits (n = 6), RC fell by 18 +/- 5%, whereas in SAD rabbits (n = 7) RC increased by 30 +/- 20% after rilmenidine. In intact rabbits, captopril increased RSNA maximally by 64 +/- 8%. RSNA did not rise in SAD rabbits. Despite the differential response or absence of RSNA, captopril increased RC to a comparable degree (maximally 40-50%) in all three groups. Using spectral analysis techniques, we found that in all groups, independently of ongoing RSNA, captopril, but not rilmenidine, attenuated both myogenic (0.07-0.25 Hz) and tubuloglomerular feedback (0.01-0.07 Hz) related fluctuations in RC. We conclude that, in conscious rabbits, the renal vasodilator effect of rilmenidine depends on the level of ongoing RSNA. Its sympatholytic effect is, however, blunted by a direct arteriolar vasoconstrictor effect. In contrast, the renal vasodilator effect of captopril is not modulated by ongoing RSNA and is associated with impairment of autoregulation of RBF.

Renal and cardiac sympathetic baroreflexes in hypertensive rabbits

1. The purpose of the present study was to assess the changes to renal sympathetic nerve activity (RSNA) baroreflexes during the development of hypertension after renal clipping in conscious rabbits. 2. Rabbits were fitted with a clip on the right renal artery or underwent a sham operation under halothane anaesthesia. A recording electrode was implanted on the left renal nerve 1 week before the experiment, 3 or 6 weeks after the initial operation. During the experiment, drug-induced ramp rises and falls in mean arterial pressure (MAP) were used to produce RSNA and heart rate (HR) baroreflex curves. The RSNA for each experiment was calibrated against maximum RSNA evoked by stimulation of baroreceptor-independent trigeminal afferents. 3. Mean arterial pressure was 20 and 36% higher 3 and 6 weeks after clip implantation, respectively. Renal sympathetic nerve activity baroreflex curves were reset rightwards accordingly, but the shape of the RSNA curves was differentially affected. 4. At both hypertensive periods, MAP-HR baroreflex gain was markedly reduced due to a reduction in curvature. The HR baroreflex range was increased. The RSNA baroreflex gain was reduced at 3 weeks, which was due to a 35% lower RSNA baroreflex range, but was similar to sham animals at 6 weeks. 5. The results show that, in established two kidney, one clip hypertension in rabbits, the sympathetic baroreflex is relatively well preserved but sensitivity of cardiac baroreflexes is attenuated. Therefore, the short-term inhibition of RSNA baroreflexes is not related to the level of blood pressure or the development of secondary changes, such as cardiac or vascular hypertrophy, but may be related to circulating angiotensin, which is known to increase at this time.

Involvement of imidazoline receptors in the baroreflex effects of rilmenidine in conscious rabbits

OBJECTIVE

It has been suggested that imidazoline receptors rather than alpha2-adrenoceptors are involved in the sympathoinhibitory action of centrally acting antihypertensive drugs such as rilmenidine. In the present study, we examined the relative importance of alpha2-adrenoceptors and imidazoline receptors in modulating the renal sympathetic and heart rate (HR) baroreflex in response to central administration of rilmenidine in conscious normotensive rabbits.

METHODS

In seven conscious rabbits, chronically instrumented with a fourth ventricular (4V) catheter, aortic and vena caval cuff occluders and a renal nerve electrode, we continuously recorded renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP) and HR and assessed baroreflex MAP-RSNA and MAP-HR relationships with balloon-induced ramp rises and falls in MAP. Rabbits were treated with 4V rilmenidine (22 microg/kg) followed by 4V idazoxan (30 microg/kg; a mixed alpha2-adrenoceptor and imidazoline receptor antagonist) or 4V 2-methoxy-idazoxan (1 microg/kg; an alpha2-adrenoceptor antagonist with little affinity for imidazoline receptors).

RESULTS

Rilmenidine lowered blood pressure by 24% and reduced both upper and lower plateaus of the renal sympathetic baroreflex curve, such that the RSNA range (difference between plateaus) was reduced by 40% (-32 +/- 10 normalized units). Curves were shifted to the left with the fall in MAP. Idazoxan restored MAP, maximum RSNA and the RSNA baroreflex range. By contrast the alpha2-adrenoceptor antagonist 2-methoxy-idazoxan caused only a partial recovery of MAP and RSNA baroreflex upper plateau and range (-9 +/- 2 mmHg, 29 and 33% lower than control). Both antagonists partially restored the HR baroreflex.

CONCLUSION

These findings suggest that in conscious rabbits, both imidazoline receptors and alpha2-adrenoceptors are involved in the central antihypertensive and baroreflex actions of rilmenidine, but that activation of imidazoline receptors is more important for its renal sympathoinhibitory action.

Central angiotensin and baroreceptor control of circulation

Angiotensin (Ang) receptors are located in many important central nuclei involved in the regulation of the cardiovascular system. While most interest has focused on forebrain circumventricular actions, areas of the brainstem such as the nucleus of the solitary tract and the ventrolateral medulla contain high concentrations of AT1 receptors. The present review encompasses the physiological role of Ang II in the hindbrain, particularly in relation to its influence on baroreflex control mechanisms. In rabbits there are sympatho-excitatory AT1 receptors in the rostral ventrolateral medulla (RVLM), accessible to Ang II from the cerebrospinal fluid. Activation of these receptors acutely increases renal sympathetic nerve activity (RSNA) and RSNA baroreflex responses. However, blockade of endogenous Ang receptors in the brainstem also shows sympathoexcitation, suggesting there is greater endogenous activity of a sympathoinhibitory Ang II action. Microinjections of angiotensin antagonists into the RVLM showed relatively little tonic activity of endogenous Ang II influencing sympathetic activity in conscious rabbits. However, Ang II receptors in the RVLM mediate sympathetic responses to airjet stress in conscious rabbits. Similarly with respect to heart rate baroreflexes, there appears to be little tonic effect of angiotensin in the brainstem in normal conscious animals. Chronic infusion of Ang II for two weeks into the fourth ventricle of conscious rabbits inhibits the cardiac baroreflex while infusion of losartan increases the gain of the reflex. These actions suggest that Ang II in the brainstem modulates sympathetic responses depending on specific afferent and synaptic inputs in both the short term but importantly also in the long term, thus forming an important mechanism for increasing the range of adaptive response patterns.

Relative importance of rostral ventrolateral medulla in sympathoinhibitory action of rilmenidine in conscious and anesthetized rabbits

The pressor region of the rostral ventrolateral medulla (RVLM) is a critical site in the sympathoinhibitory action of imidazoline receptor agonists as shown by studies in anesthetized animals. The aim of this study was to compare the importance of the RVLM in mediating the inhibitory action of rilmenidine on renal sympathetic nerve activity (RSNA) and arterial pressure in urethane-anesthetized rabbits (n = 11) and in conscious, chronically instrumented rabbits (n = 6). Bilateral microinjection of rilmenidine (4 nmol in 100 nl) into the RVLM caused a greater decrease in resting arterial pressure in anesthetized animals (-19 mm Hg) than in conscious animals (-8 mm Hg). By contrast, the decrease in resting RSNA evoked by rilmenidine was similar in conscious (-27%) and anesthetized (-36%) rabbits. Furthermore, rilmenidine microinjection into the RVLM was equally effective in inhibiting the RSNA baroreflex in both groups of animals. The upper plateau of the RSNA baroreflex decreased by 37% and 42%, and gain decreased by 41% and 44% after rilmenidine treatments in conscious and anesthetized rabbits, respectively. We conclude that the RVLM plays an equally important role in the inhibitory action of rilmenidine on RSNA in conscious and anesthetized rabbits either at rest or during baroreflex responses. A relatively moderate effect of rilmenidine on arterial pressure in conscious, chronically instrumented rabbits may relate to a lower level of sympathetic drive compared with anesthetized animals.

Influence of rostral ventrolateral medulla on renal sympathetic baroreflex in conscious rabbits

Previous studies with anesthetized animals have shown that the pressor region of the rostral ventrolateral medulla (RVLM) is a critical site in vasomotor control. The aim of this study was to develop, in conscious rabbits, a technique for microinjecting into the RVLM and to determine the influence of this area on renal sympathetic nerve activity (RSNA) and arterial pressure (AP) using local injections of glutamate, rilmenidine, ANG II and sarile. Rabbits were implanted with guide cannulas for bilateral microinjections into the RVLM (n = 7) or into the intermediate ventrolateral medulla (IVLM, n = 6) and an electrode for measuring RSNA. After 7 days of recovery, injections of glutamate (10 and 20 nmol) into the RVLM increased RSNA by 81 and 88% and AP by 17 and 25 mmHg, respectively. Infusion of glutamate (2 nmol/min) into the RVLM increased AP by 15 mmHg and the RSNA baroreflex range by 38%. By contrast, injection of the imidazoline receptor agonist rilmenidine (4 nmol) into the RVLM decreased AP by 8 mmHg and the RSNA baroreflex range by 37%. Injections of rilmenidine into the IVLM did not alter AP or RSNA. Surprisingly, treatments with ANG II (4 pmol/min) or the ANG II receptor antagonist sarile (500 pmol) into the RVLM did not affect the resting or baroreflex parameters. Infusion of ANG II (4 pmol/min) into the fourth ventricle increased AP and facilitated the RSNA baroreflex. Our results show that agents administered via a novel microinjecting system for conscious rabbits can selectively modulate neuronal activity in circumscribed regions of the ventrolateral medulla. We conclude that the RVLM plays a key role in circulatory control in conscious rabbits. However, we find no evidence for the role of ANG II receptors in the RVLM in the moment-to-moment regulation of AP and RSNA.

Comparison of renal sympathetic baroreflex effects of rilmenidine and alpha-methylnoradrenaline in the ventrolateral medulla of the rabbit

OBJECTIVE

To determine the influence of imidazoline receptors and alpha2-adrenoceptors in the rostral ventrolateral medulla (RVLM) on the renal sympathetic baroreflex.

METHODS

The effects of rilmenidine (4 nmol) and alpha-methylnoradrenaline (alpha-MNA, 80 nmol) micro-injected into the RVLM of urethane-anaesthetized rabbits previously implanted with renal nerve recording electrodes were examined before and after micro-injection of the imidazoline receptor/alpha2-adrenoceptor antagonist idazoxan and the alpha2-adrenoceptor antagonist 2-methoxyidazoxan (2-MI).

RESULTS

Rilmenidine and alpha-MNA both lowered mean arterial pressure (MAP) by 28% and renal sympathetic nerve activity (RSNA) by 35%, and reduced RSNA upper plateaus and ranges by 30-70%. Rilmenidine decreased both sympathetic burst frequency and amplitude while alpha-MNA reduced amplitude only. Rilmenidine shifted the RSNA baroreflex curve to the left while alpha-MNA shifted the curve to the right Idazoxan (13 nmol) reversed the hypotension and all RSNA effects of rilmenidine, while 2-MI (4 nmol) increased MAP 18% above the control and also reversed all RSNA parameters. By contrast, 2-MI reversed the alpha-MNA-induced hypotension and partially restored RSNA and the upper plateau of the RSNA baroreflex curve. Idazoxan treatment only partially reversed the hypotension after alpha-MNA and had no effect on any of the baroreflex curves.

CONCLUSION

Both alpha-MNA and rilmenidine injected into the RVLM of rabbits produce renal sympathetic inhibition, but differences in the location of the baroreflex curve and the pattern of effects on burst amplitude and frequency suggest different mechanisms of action. The effects of idazoxan suggest that rilmenidine acts via imidazoline receptors. Since 2-MI reversed the actions of alpha-MNA and also rilmenidine, this suggests that alpha2-adrenoceptor hypotension can be produced in the rabbit RVLM and that rilmenidine may activate alpha2-adrenoceptors, possibly as a result of activating imidazoline receptors.

Cardiac vagal responsiveness during development in spontaneously hypertensive rats

In the present study we sought to determine the effect of age, hypertension and endogenous angiotensin on the chronotropic responses to vagal stimulation in urethane anesthetized-normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). After beta-adrenoceptor blockade with atenolol, the right and left vagal nerves were stimulated with graded frequencies between 1 and 32 Hz in 5-, 8- and 22-week-old animals. At all ages and in both strains, there was a strong linear relationship between the degree of bradycardia and the log of the stimulation frequency. At the age of 5 weeks, the bradycardia to stimulation of the right vagus was greater in SHR than that observed in WKY (P<0.05). However, in 8- and 22 week-old animals, no differences were observed between the response to vagal stimulation in WKY and SHR. Thus, there was an age-dependent increase in the response to right vagal stimulation in WKY, but no such trend in SHR. No significant age-dependent changes in left vagal responses were observed in either strain. Left vagal responses were approximately half of the response to right vagal stimulation at all ages in SHR and in 8-22 week WKY, but similar to right vagal responses in 5 week WKY. Administration of the angiotensin converting enzyme inhibitor perindopril, which effectively blocked the formation of endogenous angiotensin, did not affect responses to vagal stimulation at any age, in either strain. These results suggest that the baroreflex vagal deficit observed in adult SHR compared to WKY is not due to a difference in the responsiveness of the cardiac vagal neuroeffector mechanism nor due to an effect of circulating angiotensin II. Furthermore, the enhanced vagal bradycardia observed in very young SHR which was due primarily to the earlier establishment of the adult vagal response pattern may indicate accelerated vagal development in this strain compared to WKY.

I1 imidazoline receptors in cardiovascular regulation: the place of rilmenidine

From the very earliest suggestion of a distinction between imidazoline receptors and alpha2-adrenoceptors, there has been much debate as to their contribution to the antihypertensive actions of clonidine-like agents. However, with the development of drugs such as rilmenidine that are more selective for I1 imidazoline receptors, their role and also their close relationship with alpha2-adrenoceptors has become clearer. We have examined this question using a range of imidazoline and alpha2-adrenoceptor antagonists given centrally and peripherally to conscious rabbits. We found that second-generation agents such as rilmenidine preferentially act via imidazoline receptors but that alpha2-adrenoceptors are important for the hypotension produced by the first-generation agents clonidine and alpha-methyldopa. In addition to the hypotension, rilmenidine facilitates cardiac vagal baroreflexes and inhibits cardiac sympathetic baroreflexes and diminishes the increase in renal sympathetic activity produced by environmental stress. In other studies using anesthetized rabbits and direct measures of sympathetic nerve activity, we confirmed that the major site of sympathoinhibitory actions and sympathetic baroreflex effects of rilmenidine is the rostral ventrolateral medulla. Our results also suggest that alpha2-adrenoceptors are activated as a consequence of imidazoline receptor activation by rilmenidine. Thus, though imidazoline receptors appear to be the primary target of rilmenidine, "downstream" alpha2-adrenoceptors within the brainstem are also involved and need to be considered in developing pharmacologic strategies for antihypertensive treatment involving imidazoline agents.

Influence of pontine A5 region on renal sympathetic nerve activity in conscious rabbits

The effects of inhibiting the neural activity in the pontine A5 region on renal sympathetic responses to baroreflex and/or chemoreflex activation were examined in conscious rabbits. Eight rabbits were chronically instrumented with guide cannulas for bilateral microinjections into the A5 area and an electrode for measuring renal sympathetic nerve activity (RSNA). Baroreflex curves were obtained under conditions of normoxia and hypoxia (10% O(2) + 3% CO(2)) after injections into the A5 region of the GABA receptor agonist muscimol or vehicle solution. Under normoxia, injections of muscimol did not affect resting RSNA or blood pressure but increased the range of the RSNA baroreflex by 24 and 33% at doses of 175 or 875 pmol, respectively, without affecting the reflex gain. Hypoxia alone increased resting RSNA by 63%, as well as the range and gain of the RSNA baroreflex by 53 and 89%, respectively, without affecting blood pressure. However, under hypoxia, muscimol increased resting RSNA by 37 and 47% but decreased the gain of the RSNA baroreflex by 19 and 34% at doses of 175 or 875 pmol, respectively, without affecting the reflex range. The effects of muscimol on RSNA were mediated via changes in the amplitude of the sympathetic bursts, whereas burst frequency remained unaffected. These data suggest that the A5 region has a little tonic influence on RSNA in conscious rabbits but serves to limit the renal sympathetic responses to baroreceptor unloading or chemoreceptor stimulation. The different changes in the baroreflex range and gain evoked by muscimol under normoxia and hypoxia indicate that the A5 modulatory action may depend on the activity of the afferent inputs to this region.

Effects of central infusion of ANG II and losartan on the cardiac baroreflex in rabbits

The effect of chronic activation or inhibition of central ANG II receptors on cardiac baroreflex function in conscious normotensive rabbits was examined. Animals received a fourth ventricular (4V) infusion of ANG II (30 and 100 ng/h), losartan (3 and 30 microg/h), or Ringer solution (2 microl/h) for 2 wk. After 1 and 2 wk, ANG II (100 ng/h) decreased cardiac baroreflex gain by 20 and 37%, respectively (P = 0.015), whereas losartan (30 microg/h) increased baroreflex gain by 24 and 58%, respectively (P = 0.02). Within 1 wk of the end of the infusions, cardiac baroreflex gain had returned to control. Ringer solution or the lower doses of ANG II or losartan did not modify the cardiac baroreflex function. Blood pressure and heart rate were not altered by any treatment, nor was their variability affected. These data demonstrate a novel long-term modulation of cardiac baroreflexes by endogenous ANG II that is independent of blood pressure level.

Quinpirole treatment increases renal sympathetic nerve activity and baroreflex gain in conscious rabbits: a spectral study

Intravenous administration of 0.3 mg/kg of quinpirole to conscious rabbits that had been pretreated with domperidone caused a marked increase in blood pressure and renal sympathetic nerve activity with a peak at 5-10 min after injection (25% and 3-fold increase, respectively). Spectral analysis of the blood pressure-renal sympathetic nerve activity relationship in the 0.2-0.4 Hz domain showed that baroreflex gain was markedly increased at 5-10 min (4-fold) and at 20-25 min after injection (3.7-fold). These results show that administration of the dopamine D(2)/D(3) receptor agonist quinpirole causes profound and long-lasting changes in the central integration of the sympathetic baroreceptor-vasomotor reflex.

A five-parameter logistic equation for investigating asymmetry of curvature in baroreflex studies

Baroreceptor reflex curves are usually analyzed using a symmetric four-parameter function. We wished to ascertain the validity of assuming symmetry in the baroreflex curve and also of constraining the curves to pass through the resting blood pressure and heart rate (HR) values. Therefore, we have investigated the suitability of a new five-parameter asymmetric logistic model for analysis of baroreflex curves from rabbits and dogs. The five-parameter model is an extension of the usual four-parameter model and reduces to that model when the fitted data are symmetrical. Using 30 data sets of blood pressure versus renal sympathetic nerve activity (RSNA) and HR from six conscious rabbits, we compared the five-parameter curves with the four-parameter model. We also tested the effect of forcing these baroreflex curves through the resting point. We found that the five-parameter model reduced the unexplained variation and gave small but important improvements to the estimates of plateaus for RSNA and HR and the HR gain. Although forcing the HR curves through the resting values had little effect, this procedure, when applied to RSNA, produced a worse curve fit by increasing the unexplained variation with alteration to most of the estimated curve parameters. The mean arterial pressure-HR baroreflex relationship from six conscious dogs was also analyzed and showed clear evidence of systematic asymmetry. We conclude that the asymmetric model is a valuable extension to the symmetric logistic model when examining baroreceptor reflexes, giving improved estimates of the parameters and a new approach to examining the mechanisms contributing to baroreflex curve asymmetry. Furthermore, forcing the curves through the resting value is a statistically questionable practice when analyzing RSNA, because it affects the parameter estimates.

Central imidazoline- and alpha 2-receptors involved in the cardiovascular actions of centrally acting antihypertensive agents

There has been a continuing and yet unresolved debate concerning the existence and contribution of imidazoline receptors to the antihypertensive actions of clonidine-like agents. Studies from our laboratory have examined the importance of imidazoline receptors and alpha 2-adrenoceptors in the mechanism of action of centrally acting antihypertensive drugs. We used conscious rabbits and imidazoline and specific alpha 2-adrenoceptor antagonists to show that second-generation agents rilmenidine and moxonidine act preferentially through imidazoline receptors but that alpha 2-adrenoceptors are important for the hypotension produced by clonidine and alpha-methyldopa. Using microinjections of the imidazoline antagonists into the rostral ventrolateral medulla (RVLM) of anesthetized rabbits we confirmed the generally held view that this is the major site of sympathoinhibitory actions of centrally acting antihypertensive agents. However, we also found that alpha 2-adrenoceptors are present in this nucleus and appear to be activated as a consequence of imidazoline receptor activation. In recent studies using a noradrenergic neurotoxin microinjected into the RVLM we found that this treatment selectively blocked the actions of moxonidine but did not affect the level of imidazole proteins, suggesting that I1-imidazoline receptors may be located presynaptic to the noradrenergic terminal. By contrast, clonidine acts directly on the alpha 2-adrenoceptors perhaps located on cell bodies in the nucleus. In conclusion, our studies suggest that imidazoline receptors and alpha 2-adrenoceptors within the RVLM are important for the antihypertensive actions of clonidine-like drugs.

Similar baroreflex bradycardic actions of atrial natriuretic peptide and B and C types of natriuretic peptides in conscious rats

OBJECTIVE

We have previously shown that atrial natriuretic peptide (ANP) modulates cardiac barosensitive afferent pathways to enhance reflex bradycardia in rats. The present study examined whether B-type natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) also modulate heart rate reflex function.

DESIGN

Baroreflex bradycardia was evoked by rapid (over 4-6 s) intravenous (i.v.) infusions of methoxamine (100 microg/kg; 'ramp' baroreflex technique) in the presence of infused i.v. natriuretic peptide and of vehicle (0.9% saline, 270 microl/h) in conscious adult Munich-Wistar rats. Initially a dose-response study to ANP (infused at 25, 50 and 100 pmol/kg per min i.v.) was performed in 10 rats to determine an appropriate dose for subsequent experiments with the other peptides. In a separate group of 11 animals, rat BNP-32 and rat CNP-22 were infused at 50 pmol/kg per min i.v.

RESULTS

Reflex responses to ANP were dose-related, with a significant increase in baroreflex sensitivity of 50+/-15% at the 25 pmol dose, 102+/-10% at the 50 pmol dose and 117+/-11% at 100 pmol dose (all P<0.05). BNP and CNP (50 pmol/kg/min i.v.) substantially increased baroreflex bradycardia (by 115+/-17% and 62+/-15%, respectively; P<0.05) compared to vehicle infusion.

CONCLUSIONS

Both BNP and CNP augmented baroreflex slowing of heart rate in response to rapid increases in blood pressure in rats. Whereas other reports have shown marked differences in cardiovascular responses between the natriuretic peptides, particularly with CNP, our findings demonstrate an important common action of ANP, BNP and CNP to facilitate vagal heart rate baroreflexes.

Sympathetic response to stimulation of the pontine A5 region in conscious rabbits

Studies in anaesthetized animals have shown that the pontine A5 noradrenergic region plays an important role in the sympathetic control of arterial pressure (AP). The aim of this study was to develop, in conscious rabbits, a technique for microinjections into the A5 region and examine the effects of stimulation of this region on renal sympathetic nerve activity (RSNA). In preliminary mapping experiments on four anaesthetized rabbits, electrical stimulation of the A5 region induced a pressor response ranging between 25 and 75 mmHg while unilateral injection of glutamate (100 nmol) did not change AP. The mapping experiments were used to enable guide cannulae implantation for subsequent microinjections into the A5 region. In six conscious rabbits, unilateral injection of glutamate (100 nmol) caused a consistent increase in RSNA (+45%) but did not change AP. In another eight rabbits, bilateral injection of glutamate (0.3, 3, 30 nmol) into the A5 region dose-dependently increased RSNA by 13%, 30% and 40%, respectively. In four rabbits, angiotensin II (0.3, 3, 30 pmol) injected bilaterally into the A5 region increased RSNA by 5%, 22% and 28%, respectively. In all animals the increase in RSNA was mainly mediated by increasing amplitude of sympathetic synchronized bursts while their frequency remained unchanged. However, both glutamate and angiotensin II did not change AP indicating that the sympathoexcitatory response to the A5 stimulation might be relatively confined to the renal bed. Using a novel microinjection technique developed for conscious rabbits, we found that the A5 region may provide an important excitatory and possibly selective input to the renal sympathetic preganglionic neurons.

Comparison of the baroreceptor-heart rate reflex effects of moxonidine, rilmenidine and clonidine in conscious rabbits

In 10 conscious rabbits, the baroreceptor-heart rate (HR) reflex effects of centrally acting antihypertensive agents with high affinity for imidazoline receptors (IRs), moxonidine and rilmenidine, were compared with clonidine which acts predominantly via central alpha2-adrenoceptors. Dose regimens were chosen to give similar hypotension (-17+/-1 mm Hg) and bradycardia (-27+/-2 b/min) for all three agents given into the fourth ventricle. Baroreceptor-HR reflex curves were assessed by i.v. drug induced changes in blood pressure. With all treatments, the baroreflex curves with both vagal and sympathetic effectors intact were shifted to the left, corresponding to the hypotension, and the bradycardia plateau was reduced. Rilmenidine and moxonidine also reduced the upper plateau such that the curves were shifted parallel down the HR scale with no change in the HR range. By contrast, clonidine only decreased the lower plateau, and thus increased HR range (+19+/-6%). Moxonidine, but not rilmenidine, reduced the baroreflex gain by reducing the curvature. Clonidine also decreased curvature but this did not result in a reduction in gain as it was offset by the increase in HR range. The gain and range of the cardiac sympathetic component, as assessed after vagal blockade, was reduced by rilmenidine by 53 and 40% respectively, but was not affected by the other agents. The calculated vagal component of the curves showed that all agents produced a greater vagal bradycardia in response to a rise in pressure and that both rilmenidine and clonidine increased vagal HR range. The present study results show that many of the baroreflex effects of clonidine, such as facilitating cardiac vagal responses, are shared by the second generation agent rilmenidine, suggesting that they are primarily due to alpha2-adrenoceptor activation. In addition, the inhibition of the sympathetic component of the baroreflex, observed with rilmenidine, and not clonidine suggests that this effect may involve IRs. By contrast moxonidine, the most specific agent for I1 receptors, produces mainly a baroreflex independent inhibition of cardiac sympathetic activity with little effect on vagal activity.

Relationship between imidazoline and alpha2-adrenoceptors involved in the sympatho-inhibitory actions of centrally acting antihypertensive agents

Since the first suggestion of the existence of imidazoline receptors, there has been a continuing and yet unresolved debate as to their contribution to the antihypertensive actions of clonidine-like agents. In this review we bring together a number of studies from our laboratory which have examined the importance and interdependence of imidazoline receptors and alpha2-adrenoceptors in the mechanism of action of centrally acting antihypertensive drugs. Using conscious rabbits and a range of imidazoline and specific alpha2-adrenoceptor antagonists we have consistently found that second generation agents rilmenidine and moxonidine preferentially act via imidazoline receptors but that alpha2-adrenoceptors are important for the hypotension produced by clonidine and alpha-methyldopa. Despite this difference in receptor mechanism, the hypotension produced by all these drugs is dependent on central noradrenergic pathways. In other studies using anaesthetised rabbits and direct measures of sympathetic nerve activity we confirmed the generally held view that the major site of sympatho-inhibitory actions and sympathetic baroreflex effects of centrally acting antihypertensive agents is the rostral ventrolateral medulla (RVLM). We also found, using microinjection of specific antagonists, that alpha2-adrenoceptors in this nucleus appear to be activated as a consequence of imidazoline receptor activation. Thus, there appears to be a close relationship between imidazoline receptors and alpha2-adrenoceptors located in the RVLM in mediating the hypotension and inhibition of renal sympathetic nerve activity. Furthermore in recent studies using a noradrenergic neurotoxin microinjected into the RVLM we found that this treatment selectively blocked the actions of moxonidine but not clonidine, suggesting that I1-imidazoline receptors may be located on adrenergic terminals in situ. By contrast, clonidine acts predominantly via alpha2-adrenoceptors, perhaps located on cell bodies in the nucleus. We conclude that there is indeed a close nexus between 'presynaptic' imidazoline receptors on noradrenergic terminals and 'downstream' alpha2-adrenoceptors within the RVLM. Our hypothesis brings together opposing points of view that the mechanism for hypotension must involve either the imidazoline receptor or the alpha2-adrenoceptor. Clearly both are important.

Effect of rilmenidine on the cadiovascular responses to stress in the conscious rabbit

Environmental stress can cause an increase in sympathetic nerve activity both in humans and animals. While centrally acting antihypertensive drugs such as rilmenidine are known to reduce sympathetic tone, it is not clear whether they also influence the cardiovascular responses to acute stress. In the present study we examined the effects of systemic treatment with rilmenidine on the sympathetic and haemodynamic responses to air jet or noise stress. Twelve conscious rabbits previously implanted with a renal nerve recording electrode were subjected to an 8 l/min stream of air directed at their face for 10 min or exposure to 10 min of white noise (approximately 85 dB). Both air jet and noise stress elicited increases in renal sympathetic nerve activity (RSNA) which were greatest in the first minute (+55+/-9% and +40+/-6%, respectively), but which quickly reached a stable level over the subsequent 9 min (+24+/-6% and +9+/-5%, respectively). This was accompanied by a small increase in heart rate (HR) and mean arterial pressure (MAP). Intravenous rilmenidine (273 microg/kg) reduced MAP from 85+/-3 mm Hg to 68+/-2 mm Hg and HR from 203+/-10 b/min to 188+/-10 b/min and lowered basal RSNA by 54%. Rilmenidine reduced the increase in RSNA seen during the first minute of air jet stress by 35% and reduced the average increase over the next 9 min by 68%. However, rilmenidine had little effect on either the initial or stable RSNA responses to noise stress. Saline treatment did not alter the RSNA responses to either air jet or noise stress. The results show that centrally-acting antihypertensive agents not only lower basal RSNA, but can differentially influence environmentally induced sympathetic responses. In addition, the differential effect of rilmenidine on noise and air jet stress suggests that they may involve quite different central processing.

Interaction of the dopamine D2 receptor agonist quinpirole with sympathetic vasomotor tone and the central action of rilmenidine in conscious rabbits

Previous studies in conscious rats have shown that systemic administration of the dopamine D2 receptor agonist quinpirole causes a centrally-mediated increase in blood pressure which is associated with increased plasma levels of noradrenaline and adrenaline. In addition, treatment with quinpirole caused a marked inhibition of the antihypertensive effect of centrally-acting sympatho-inhibitory drugs such as clonidine, rilmenidine and alpha-methyldopa, suggesting an interaction at the level of sympathetic vasomotor tone. The main aim of the present study was investigate in conscious rabbits the effect of quinpirole on renal sympathetic nerve activity. In addition, we studied the effect of pretreatment with quinpirole on responses to additional quinpirole injections or rilmenidine treatment. Quinpirole treatment caused a prolonged dose-dependent increase in blood pressure and heart rate. Additional injection of quinpirole, 30 min after the first treatment, caused a significantly smaller pressor response (7+/-2 vs. 17+/-2 mm Hg). Injection of rilmenidine caused a larger decrease in blood pressure in rabbits which had been pretreated with quinpirole than in controls (-28+/-3 vs. -14+/-3 mm Hg). Total renal sympathetic nerve activity was markedly increased by quinpirole treatment (3.5-fold), an effect which could be attributed to both increased amplitude and increased frequency of the renal nerve signal. After a second injection of quinpirole, 30 min after the first treatment, only total renal sympathetic nerve activity and amplitude were increased and the effects were reduced. These results show marked actions of quinpirole on renal sympathetic nerve activity in conscious rabbits. However, the previously described apparent desensitisation to the antihypertensive effect of rilmenidine could not be observed in rabbits, suggesting marked species differences in the mechanism and site of action of rilmenidine.

Recent advances in imidazoline receptor research: ligands--localization and isolation--signaling--functional and clinical studies

In this article we outline the highlights of this special issue of the journal containing a series of articles covering many aspects of current interest in the field of imidazoline receptor research. This volume is the result of an international symposium held in September 1997 in Melbourne as an official satellite of the inaugural meeting of the International Society of Autonomic Neurosciences held in Cairns, Australia. A wide range of topics relating to imidazoline receptors were canvassed, including endogenous and synthetic ligands, identification and localisation of binding sites, putative transduction mechanisms and experimental and clinical functional studies.

Role of central catecholaminergic pathways in the actions of endogenous ANG II on sympathetic reflexes

In the present study, we examined the effect of blockade of the brain stem renin-angiotensin system on renal sympathetic baroreflexes and chemoreflexes in conscious rabbits and examined the role of central catecholaminergic pathways in these responses. Eleven rabbits underwent preliminary surgical instrumentation and pretreatment with central 6-hydroxydopamine (6-OHDA, 500 micrograms/kg) or ascorbic acid 6 wk before the commencement of the experiments. Baroreflex curves were determined under conditions of normoxia and hypoxia (10% O2 + 3% CO2) before and after central administration of either Ringer solution, the ANG II receptor antagonist losartan (10 micrograms), or the angiotensin-converting enzyme inhibitor enalaprilat (500 ng) on separate days. Losartan increased the upper plateau and the range of the mean arterial pressure (MAP)-renal sympathetic nerve activity (RSNA) curve (79 and 78%, respectively) in intact rabbits, whereas this effect was not observed in 6-OHDA-pretreated rabbits. Hypoxia elicited an increase in resting RSNA (111% in intact rabbits and 74% in 6-OHDA-injected rabbits) and elevated the upper plateau of the RSNA-MAP curve in both groups (89% in intact rabbits and 114% in 6-OHDA-injected rabbits). During hypoxia, losartan and enalaprilat increased the RSNA upper plateau in intact rabbits but had no effect in 6-OHDA-pretreated rabbits. No effects on the MAP-heart rate baroreflex curves were observed. Thus the effect of losartan to increase RSNA, particularly during hypoxia and baroreceptor unloading, being abolished by central noradrenergic depletion suggests that the endogenous ANG II which normally causes an inhibition of renal sympathetic motoneurons is dependent on the integrity of central catecholaminergic pathways.

Endothelin induces dopamine release from rat striatum via endothelin-B receptors

The aim of the present study was to determine whether local administration of endothelin induces the release of dopamine in the rat striatum and to characterize and localize endothelin receptors in this brain region. Local injection of endothelin-1 (10 pmol) into the ventral striatum of urethane-anaesthetized rats caused an increase of 8 microM in the extracellular concentration of dopamine as measured by in vivo chronoamperometry. The peak increase in dopamine concentration occurred within 5 min of endothelin injection. Injection of the selective endothelin-B receptor agonist [Ala1.3,11.15]endothelin-1 (10 pmol) also caused an increase in extracellular dopamine concentration, suggesting that endothelin is acting at the endothelin-B receptor to elicit its effect. In rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway, the response to local injection of endothelin-1 (10 pmol) was significantly inhibited on the lesioned side as compared to the non-lesioned side. In contrast, pretreatment of the rats with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate (5 mg/kg, i.p.) or the nitric oxide synthase inhibitor NG-nitro-L-arginine (3 mg/kg, i.p.) did not alter the endothelin-induced release of dopamine. In binding studies, addition of endothelin-1 displaced [125I]endothelin-1 with a Ki of 220 pM. The endothelin-B receptor antagonist BQ788 displaced [125I]endothelin-1 with a Ki of 120 nM, whereas the endothelin-A receptor antagonist BQ123 produced only a 25% displacement at 10 microM, suggesting that endothelin receptors in the striatum are of the endothelin-B subtype. In rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal dopamine system, [125I]endothelin-1 binding was reduced by 53% in lesioned striatum compared to non-lesioned striatum, with no difference in the Kd. These data provide evidence that endothelin acts on a homogeneous population of endothelin-B receptors within the striatum to cause the release of dopamine and that a significant proportion of these receptors is located on dopaminergic neurons.

Imidazole receptors. 22-24 July 1998, Bonn, Germany

This two-day symposium was the third international meeting on imidazoline receptors and an official satellite of the XIIIth International Congress of Pharmacology, held the following week in Munich. The meeting was very well-organized by the local hosts, Manfred Gothert and Gert Molderings (Pharmacology Department, Bonn University, Germany). With approximately 100 participants from 13 countries, representing Europe, North and South America, and Australia, the symposium was slightly smaller than the 1994 meeting in New York, but similar in size to the 1992 inaugural meeting in Paris. What stood out from this meeting was the considerable maturation in the confidence that imidazoline-binding sites are functional receptors. Also, the therapeutic implications have dramatically diversified in the last four years. It is now thought that these receptors play a role in hypertension, pancreatic function, cell proliferation, regulation of body fat, neuroprotection, inflammation and some psychiatric disorders, such as depression. This may partly explain the reluctance to further classify imidazoline subtypes, while the previous I1 and I2 classification is now clearly inadequate to satisfy the four to five subtypes that were described. The format of the meeting was similar to those held previously with 27 invited state of the art presentations divided into seven sessions covering topics from molecular and cellular biology, endogenous ligands, receptor identification and classification through functional studies and pathophysiology and their relevance. Over 40 submitted posters were displayed for the entire meeting and a new addition to the format was an evening set aside for three concurrent poster discussion sessions, which provided each participant with the opportunity to present work to a wider audience. This review will focus on the major developments presented at the symposium following the thematic outline of the program.

ANP and bradycardic reflexes in hypertensive rats: influence of cardiac hypertrophy

In previous studies we demonstrated that in normotensive rats, but not in spontaneously hypertensive rats (SHR), atrial natriuretic peptide (ANP) enhances bradycardic reflexes through an action on cardiac vagal afferent pathways. The present study aimed to determine whether cardiac hypertrophy, hypertension, or a nonreversible genetic factor accounted for the insensitivity of SHR to ANP action on cardiac reflex pathways. SHR were treated with the angiotensin-converting enzyme (ACE) inhibitor perindopril (3 mg/kg per day) for 6 weeks from 4 to 9 weeks of age (SHR-S, n=10) or for 9 weeks from 4 to 12 weeks of age (SHR-L, n=10) or were untreated (SHR, n=10) to produce differential effects on blood pressure and left ventricle/body weight ratio (LV/BW). Untreated normotensive Wistar-Kyoto rats (WKY, n=10) were also studied. At 13 weeks of age, all rats were instrumented with aortic and jugular catheters, and at 14 weeks we measured heart rate reflexes to rapid intravenous infusions of methoxamine (100 microg/kg, cardiac baroreflex) and serotonin (5 to 60 microg/kg, von Bezold-Jarisch cardiac chemosensitive reflex), with either alpha-rat ANP (150 ng/kg per minute IV) or saline vehicle (270 microL/h IV) infusion. Perindopril treatment for 6-week (SHR-S) and 9-week (SHR-L) durations maintained blood pressure at normotensive levels in both groups. SHR-S exhibited a small degree of cardiac hypertrophy (LV/BW was 8% higher than in WKY but 11% less than in untreated SHR), but LV/BW was normalized in SHR-L (to within 1% of WKY LV/BW). In WKY, ANP significantly (P<0.05) enhanced bradycardic responses to both the cardiac baroreflex (by 42+/-10%) and von Bezold-Jarisch chemosensitive reflex (by 17+/-5%) activation but had no effect in SHR. The cardiac reflex action of ANP was restored in SHR-L (ANP enhanced reflex bradycardia by 28+/-12% and 36+/-8%, baroreflex and von Bezold-Jarisch reflex, respectively; P<0.05), but SHR-S, which developed some cardiac hypertrophy, remained unresponsive to ANP. Our results suggest that the inability of ANP to sensitize cardiac vagal (nonarterial) afferents in SHR was not due to an inherited irreversible component, or the hypertension per se, but was associated with the presence of cardiac hypertrophy. A functional consequence of hypertension-induced cardiac hypertrophy may be the inhibition of the cardioprotective action of ANP through cardiac vagal reflexes.

Site and receptors involved in the sympathoinhibitory actions of rilmenidine

INTRODUCTION

It is well accepted that centrally acting antihypertensive agents such as rilmenidine reduce blood pressure through inhibition of the sympathetic nervous system. The central receptor involved, be it a central imidazoline receptor or an alpha2-adrenoceptor and its location, is less certain.

METHODS AND RESULTS

The present paper reviews studies from our laboratory examining these questions by using antagonists with differing affinity for imidazoline receptors and alpha2-adrenoceptors. In addition, we have used various routes of administration in conscious and anaesthetized normotensive rabbits. We found that rilmenidine was more potent in its hypotensive action when administered into the fourth ventricle than when given intravenously and considerably more potent when injected into the rostroventrolateral medulla (RVLM) compared to the nucleus of the solitary tract (NTS). By contrast, alpha-methylnoradrenaline, which acts solely through alpha2-adrenoceptors to produce hypotension, was similarly potent in both the NTS and RVLM. Injections of rilmenidine into the RVLM reduced renal sympathetic tone and produced a marked inhibition of renal sympathetic baroreflex responses. The pattern of renal sympathetic baroreflex effects of rilmenidine administered into the RVLM was similar to the effects of the fourth ventricular or intravenous administration. These studies together support the view that the RVLM is the major site of action. We have determined the relative importance of imidazoline receptors and alpha2-adrenoceptors in the inhibition of renal sympathetic nerve activity produced by rilmenidine administered into the RVLM, the fourth ventricle or intravenously. Initial studies in conscious rabbits showed that intravenous or fourth ventricular administration of rilmenidine induced renal sympatho-inhibition which was preferentially reversed by idazoxan or efaroxan (imidazoline/alpha2-adrenoceptor antagonist) compared to 2-methoxyidazoxan (alpha2-adrenoceptor antagonist). In anaesthetized rabbits, administration of idazoxan into the RVLM reversed the inhibition of the renal sympathetic activity induced by RVLM or intravenous administration of rilmenidine. In contrast, idazoxan had no effect on the responses produced by the alpha2-adrenoceptor agonist alpha-methylnoradrenaline.

CONCLUSION

Our studies suggest that rilmenidine given systemically acts primarily on imidazoline receptors in the RVLM to reduce sympathetic tone and to modulate sympathetic baroreflexes.