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

Method of measuring effects of study procedures in single and pair housed New Zealand White rabbits (Oryctolagus cuniculus)

Social housing of laboratory rabbits is encouraged and thought to improve animal welfare due to the social nature of this species. However, there is limited published information comparing the physiologic and cardiovascular (CV) effects of paired and single housed adult female rabbits in commonly used laboratory caging. This study describes measurement of heart rate, systolic blood pressure, activity level, body temperature and pairing methods in four female New Zealand White rabbits that were previously implanted with M10 cardiovascular telemetry devices. Data was collected in single housed rabbits having no history of social housing while they were undisturbed in the home cage, during restraint, intramuscular injections and intravenous blood collection. The same animals were then placed in compatible pairs and housed in conventional Allentown caging. As expected, we found increased activity in paired rabbits but no significant differences in body temperatures, and CV parameters in single and paired rabbits undergoing the same procedures. These data suggest that paired rabbits can be used for safety pharmacology studies with minimal impact to data, while supporting improved animal welfare.

Centrally acting antihypertensives change the psychogenic cardiovascular reactivity

Clonidine (CL) and Rilmenidine (RI) are among the most frequently prescribed centrally acting antihypertensives. Here, we compared CL and RI effects on psychogenic cardiovascular reactivity to sonant, luminous, motosensory, and vibrotactile stimuli during neurogenic hypertension. The femoral artery and vein of Wistar (WT - normotensive) and spontaneously hypertensive rats (SHR) were catheterized before (24 h interval) i.p. injection of vehicle (NaCl 0.9%, control - CT group), CL (10 µg/kg), or RI (10 µg/kg) and acute exposure to luminous (5000 lm), sonant (75 dB sudden tap), motor (180° cage twist), and air-jet (10 L/min - restraint and vibrotactile). Findings showed that: (i) CL or RI reduced the arterial pressure of SHR, without affecting basal heart rate in WT and SHR; (ii) different stimuli evoked pressor and tachycardic responses; (iii) CL and RI reduced pressor response to sound; (iv) CL or RI reduced pressor responses to luminous stimulus without a change in peak tachycardia in SHR; (v) cage twist increased blood pressure in SHR, which was attenuated by CL or RI; (vi) air-jet increased pressure and heart rate; (vii) CL or RI attenuated the pressor responses to air-jet in SHR while RI reduced the chronotropic reactivity in both strains. Altogether, both antihypertensives relieved the psychogenic cardiovascular responses to different stimuli. The RI elicited higher cardioprotective effects through a reduction in air-jet-induced tachycardia.

A simple approach to studying cerebral blood flow during psychological stress

The purpose of this work was to determine, in the Wistar male rats, the role of the stress system in the control of cerebral hemodynamics induced by emotional stress (air jet). Blood flow rates in the internal carotid arteries have been obtained using chronically implanted Doppler probes. In this investigation, air-jet stress provokes a rapid and substantial elevation of blood pressure combined with hyperemia and vasodilation. Spectral analysis reveals an increase in the variability of carotid blood flow and carotid vascular conductance during stress compared to the baseline state. The coherence between the two carotid blood flow, which provides a linear correlation index in the frequency domain, was calculated before (ordinary coherence) and after mathematical elimination of the influence of blood pressure (partial coherence). The main advantages of the technique are as follows:• It allows the measurement of cerebral blood flow while comparing them with the physiological parameters of the animal such as blood pressure.• Air-jet stress is a psychological rather than physical stress that generates hyperemia in a non-pharmacological way (no side effects) without injuring the animal or damaging the experimental installation during the recording sessions.• A simple method for tracing a kinetic path before and at the moment of stress initiation.• The presence of a catheter facilitates the administration of drug substances if required.• Laboratory-developed software, based on the Lab VIEW 5.1, allows real-time monitoring of blood pressure and carotid flow rates signals using fast Fourier transform.

Recording sympathetic nerve activity in conscious humans and other mammals: guidelines and the road to standardization

Over the past several decades, studies of the sympathetic nervous system in humans, sheep, rabbits, rats, and mice have substantially increased mechanistic understanding of cardiovascular function and dysfunction. Recently, interest in sympathetic neural mechanisms contributing to blood pressure control has grown, in part because of the development of devices or surgical procedures that treat hypertension by manipulating sympathetic outflow. Studies in animal models have provided important insights into physiological and pathophysiological mechanisms that are not accessible in human studies. Across species and among laboratories, various approaches have been developed to record, quantify, analyze, and interpret sympathetic nerve activity (SNA). In general, SNA demonstrates "bursting" behavior, where groups of action potentials are synchronized and linked to the cardiac cycle via the arterial baroreflex. In humans, it is common to quantify SNA as bursts per minute or bursts per 100 heart beats. This type of quantification can be done in other species but is only commonly reported in sheep, which have heart rates similar to humans. In rabbits, rats, and mice, SNA is often recorded relative to a maximal level elicited in the laboratory to control for differences in electrode position among animals or on different study days. SNA in humans can also be presented as total activity, where normalization to the largest burst is a common approach. The goal of the present paper is to put together a summary of "best practices" in several of the most common experimental models and to discuss opportunities and challenges relative to the optimal measurement of SNA across species.Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/guidelines-for-measuring-sympathetic-nerve-activity/.

The Effects of Rilmenidine and Perindopril on Arousal Blood Pressure during 24 Hour Recordings in SHR

The surge in arterial pressure during arousal in the waking period is thought to be largely due to activation of the sympathetic nervous system. In this study we compared in SHR the effects of chronic administration of the centrally acting sympatholytic agent rilmenidine with an angiotensin converting enzyme inhibitor perindopril on the rate of rise and power of the surge in mean arterial pressure (MAP) that occurs with arousal associated with the onset of night. Recordings were made using radiotelemetry in 17 adult SHR before and after treatment with rilmenidine (2mg/kg/day), perindopril (1mg/kg/day) or vehicle in the drinking water for 2 weeks. Rilmenidine reduced MAP by 7.2 ± 1.7mmHg while perindopril reduced MAP by 19 ± 3mmHg. Double logistic curve fit analysis showed that the rate and power of increase in systolic pressure during the transition from light to dark was reduced by 50% and 65%, respectively, but had no effect on diastolic pressure. Rilmenidine also reduced blood pressure variability in the autonomic frequency in the active period as assessed by spectral analysis which is consistent with reduction in sympathetic nervous system activity. Perindopril had no effect on the rate or power of the arousal surge in either systolic or diastolic pressure. These results suggest that the arousal induced surge in blood pressure can largely be reduced by an antihypertensive agent that inhibits the sympathetic nervous system and that angiotensin converting enzyme inhibition, while effective in reducing blood pressure, does not alter the rate or power of the surge associated with arousal.

Differential activation of renal sympathetic burst amplitude and frequency during hypoxia, stress and baroreflexes with chronic angiotensin treatment

NEW FINDINGS

What is the central question of this study? Is the elevated tonic renal nerve activity induced by chronic angiotensin administration mediated by recruitment or increased firing frequency and does this occur via stress, chemoreflex or baroreflex pathways? What is the main finding and its importance? Long-term angiotensin treatment in rabbits elevates renal sympathetic nerve activity by recruitment of previously silent fibres. This was similar to the effect of chemoreflex stimulation, but not to stress or baroreceptor activation, suggesting that presympathetic pathways activated by angiotensin may be common to those activated by chemoreceptors. Modulation of sympathetic nerve activity involves control by the CNS of the amplitude of neural discharges, reflecting recruitment of neurons and their firing frequency. We tested whether elevated tonic renal sympathetic nerve activity (RSNA) induced by chronic angiotensin administration is mediated by recruitment or increased firing frequency and whether this is characteristic of the pattern observed with activation of stress, chemoreflex or baroreflex pathways. Conscious rabbits treated with angiotensin II for 12 weeks to increase blood pressure by 10-30% were subjected to stress (air jet), hypoxia (10% O2 + 3% CO2) and drug-induced changes in blood pressure to produce baroreflexes. Total RSNA and RSNA burst amplitude were scaled to 100 normalized units (n.u.) by the maximal response to smoke. After 12 weeks of treatment, blood pressure was 17% higher than baseline 68 ± 1 mmHg (P = 0.02). Compared with sham treatment, total RSNA and burst amplitude were +82% (P < 0.001) and 39% (P = 0.04) greater, but burst frequency was similar. Total RSNA increased during hypoxia (+38% from 4.9 ± 0.7 n.u.), owing to greater amplitude, but not frequency. Air-jet stress increased total RSNA (+44% from 4.3 ± 0.5 n.u.) and burst frequency (+21% from 5.4 ± 0.7 bursts s(-1) ), but not amplitude. Angiotensin enhanced total RSNA responses to both air jet (+33%) and hypoxia (+58%), but only increased the amplitude response to air jet. The RSNA baroreflexes reset to the higher blood pressure, but amplitude or frequency was not differentially altered. Chronic angiotensin treatment elevated RSNA by recruitment of neurons, which is similar to chemoreflex stimulation, but not to stress or baroreceptor activation, suggesting that presympathetic pathways activated by angiotensin may be common to those activated by chemoreceptors.

Comparison of sympathetic nerve activity normalization procedures in conscious rabbits

One of the main constraints associated with recording sympathetic nerve activity (SNA) in both humans and experimental animals is that microvolt values reflect characteristics of the recording conditions and limit comparisons between different experimental groups. The nasopharyngeal response has been validated for normalizing renal SNA (RSNA) in conscious rabbits, and in humans muscle SNA is normalized to the maximum burst in the resting period. We compared these two methods of normalization to determine whether either could detect elevated RSNA in hypertensive rabbits compared with normotensive controls. We also tested whether either method eliminated differences based only on different recording conditions by separating RSNA of control (sham) rabbits into two groups with low or high microvolts. Hypertension was induced by 5 wk of renal clipping (2K1C), 3 wk of high-fat diet (HFD), or 3 mo infusion of a low dose of angiotensin (ANG II). Normalization to the nasopharyngeal response revealed RSNA that was 88, 51, and 34% greater in 2K1C, HFD, and ANG II rabbits, respectively, than shams (P < 0.05), but normalization to the maximum burst showed no differences. The RSNA baroreflex followed a similar pattern whether RSNA was expressed in microvolts or normalized. Both methods abolished the difference between low and high microvolt RSNA. These results suggest that maximum burst amplitude is a useful technique for minimizing differences between recording conditions but is unable to detect real differences between groups. We conclude that the nasopharyngeal reflex is the superior method for normalizing sympathetic recordings in conscious rabbits.

Comparison of blood pressure and sympathetic activity of rabbits in their home cage and the laboratory environment

Methodological improvements in measuring cardiovascular parameters have meant that data can be collected from freely moving animals in their home cage. However, experiments in rabbits still often require them to be restrained in a laboratory setting. The aim of this study was to determine whether measurements collected when rabbits were placed in a holding box in the laboratory are representative of values obtained in freely moving conscious rabbits. Nine New Zealand White rabbits received two radiotelemetry implants to monitor mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA). The MAP measured in the laboratory (71 ± 1 mmHg) was similar to that in the home cage (69 ± 1 mmHg), but there was less MAP variability. The RSNA was also similar in both environments. In contrast, laboratory heart rate (HR) was 7% lower than home cage HR (181 ± 4 beats min(-1), P < 0.001), but HR variability was similar. Baroreflex gain, assessed by spectral analysis, was 19% higher in the laboratory than in the home cage due to lower MAP mid-frequency variability in the laboratory. Home cage circadian patterns of MAP and HR were strongly influenced by feeding and activity. Nevertheless, MAP and RSNA laboratory measurements were the same as average 24 h values and remained similar over several weeks. We conclude that while HR is generally lower in the laboratory, a valid representation of MAP and RSNA can be given by laboratory measurements.

Role of the bed nucleus of the stria terminalis in the cardiovascular responses to acute restraint stress in rats

The aim of this work was to test the hypothesis that the bed nucleus of the stria terminalis (BST) and noradrenergic neurotransmission therein mediate cardiovascular responses to acute restraint stress in rats. Bilateral microinjection of the non-specific synaptic blocker CoCl(2) (0.1 nmol/100 nl) into the BST enhanced the heart rate (HR) increase associated with acute restraint without affecting the blood pressure increase, indicating that synapses within the BST influence restraint-evoked HR changes. BST pretreatment with the selective alpha(1)-adrenoceptor antagonist WB4101 (15 nmol/100 nl) caused similar effects to cobalt, indicating that local noradrenergic neurotransmission mediates the BST inhibitory influence on restraint-related HR responses. BST treatment with equimolar doses of the alpha(2)-adrenoceptor antagonist RX821002 or the beta-adrenoceptor antagonist propranolol did not affect restraint-related cardiovascular responses, reinforcing the inference that alpha(1)-adrenoceptors mediate the BST-related inhibitory influence on HR responses. Microinjection of WB4101 into the BST of rats pretreated intravenously with the anticholinergic drug homatropine methyl bromide (0.2 mg/kg) did not affect restraint-related cardiovascular responses, indicating that the inhibitory influence of the BST on the restraint-evoked HR increase could be related to an increase in parasympathetic activity. Thus, our results suggest an inhibitory influence of the BST on the HR increase evoked by restraint stress, and that this is mediated by local alpha(1)-adrenoceptors. The results also indicate that such an inhibitory influence is a result of parasympathetic activation.

Modulation of stress by imidazoline binding sites: implications for psychiatric disorders

In this review, we present evidence for the involvement of imidazoline binding sites (IBS) in modulating responses to stress, through central control of monoaminergic and hypothalamo-pituitary-adrenal (HPA) axis activity. Pharmacological and physiological evidence is presented for differential effects of different IBS subtypes on serotoninergic and catecholaminergic pathways involved in control of basal and stress-stimulated HPA axis activity. IBS ligands can modulate behavioural and neuroendocrine responses in animal models of stress, depression and anxiety, and a body of evidence exists for alterations in central IBS expression in psychiatric patients, which can be normalised partially or fully by treatment with antidepressants. Dysfunction in monoaminergic systems and the HPA axis under basal and stress-induced activation has been extensively reported in psychiatric illnesses. On the basis of the literature, we suggest a potential therapeutic role for selective IBS ligands in the treatment of depression and anxiety disorders.

Cardiac and renal baroreflex control during stress in conscious renovascular hypertensive rabbits: effect of rilmenidine

OBJECTIVE

We examined whether renal sympathetic nerve activity (RSNA) and heart rate (HR) baroreflexes in conscious rabbits were altered by exposure to a combination of stress and hypertension and determined how this was modified by acute and chronic treatment with the sympathoinhibitory agent rilmenidine.

METHODS

Rabbits were made hypertensive with a renal-artery clip and a renal nerve recording electrode was implanted 4-5 weeks later. After recovery, baroreflexes were measured before and during airjet stress and again after receiving rilmenidine (either acutely or by infusion for 3 weeks).

RESULTS

Renal clipping increased mean arterial pressure (MAP) and shifted baroreflex RSNA and HR curves rightward. The HR and RSNA upper plateaus were similar to those of normotensive animals but HR baroreflex sensitivity was reduced in the hypertensive group. Airjet stress lowered HR baroreflex sensitivity in sham but not in hypertensive rabbits. By contrast, stress increased the baroreflex-induced maximum RSNA in hypertensive animals but not in normotensive rabbits. MAP variability was greater in the hypertensive group but was unaffected by airjet stress. Acute and chronic rilmenidine lowered MAP to close to normotensive levels, markedly reduced MAP variability and RSNA but did not prevent the RSNA baroreflex facilitation produced by airjet stress.

CONCLUSION

Baroreflex control of HR was diminished by either hypertension or acute airjet stress but the effects were not additive. Although the baroreflex-induced RSNA maximum was increased by stress only in hypertensive animals, rilmenidine was effective in minimizing the reflex autonomic disturbances produced by hypertension and stress.

Effects of rilmenidine on 24-h rhythmicity of blood pressure and spontaneous baroreflex sensitivity in essential hypertensive subjects

OBJECTIVE

To study the effects of the centrally acting imidazoline-like compound rilmenidine on the circadian and short-term cardiovascular rhythms derived from continuous blood pressure (BP) recordings in patients with mild essential hypertension.

METHODS

This was a single-center, open study. Recordings were obtained from eight subjects, using a Portapres during two 24-h hospitalizations: the first after the inclusion visit and the second 4 weeks after starting rilmenidine treatment (1 or 2 mg/day). For circadian analysis of cardiovascular variables, 10 min were selected every hour to obtain 24 periods per subject for each session. Spontaneous baroreflex sensitivity (BRS) was estimated using the sequence technique and the cross-spectral analysis between systolic BP and interbeat intervals.

RESULTS

Rilmenidine significantly reduced the overall systolic and diastolic BP and heart rate (P < 0.001). The effects of rilmenidine on BP and heart rhythm were marked during the daytime. Rilmenidine reduced the low-frequency (LF) component of systolic BP variability throughout the 24 h. The highest values of spontaneous BRS were observed at night. Rilmenidine increased the BRS obtained by the slope of the sequence method throughout the 24-h period (P < 0.001). The LF gain was significantly increased with rilmenidine during the day and the night.

CONCLUSIONS

Rilmenidine may differentially affect the baroreflex-dependent (phasic or reflex) and the baroreflex-independent (tonic) autonomic outflow. The 24-h approach reinforced this concept, since indexes of BRS were increased throughout the 24-h period while BP was reduced during the daytime.

Effects of early weaning on anxiety and autonomic responses to stress in rats

Environmental stimuli affect various aspects of the early physical and behavioral development in rats. One of the most important events in the early stage of life is weaning, and we recently reported that precocious weaning augments anxiety and aggressiveness in rats and mice. In the present study, we investigated the autonomic responses to stress in two groups of rats: the early-weaned group (weaned at 16 days of age), and the normally weaned group (weaned at 30 days) as a control. First, the early and normally weaned rats were subjected to an elevated plus-maze test to assess their anxiety levels. It was confirmed that early-weaned male rats, but not the females, showed a lower frequency of entry into and shorter duration of stay in the open arms of the maze compared to the normally weaned rats. Subsequently, the two groups were either placed in a novel clean cage or exposed to an unfamiliar conspecific, and their heart rates and core body temperatures were monitored to evaluate their autonomic stress responses. There was an exacerbation of autonomic responses, such as stress-induced hyperthermia and tachycardia, and an alternation of behavioral responses, including increased sniffing, and decreased grooming and resting. These effects of early weaning were significant only in males. In contrast, when rats encountered an unfamiliar individual, no significant differences were observed between the two groups in either sex. This suggests that stimuli emanating from an unfamiliar intruder were too intense to detect the augmentation of stress responses in the early-weaned rat. The results of the present study demonstrate that precocious weaning augments, not only behavioral but also autonomic responses, to stressful conditions with sexually dimorphic patterns, i.e. more profoundly in males than in females.

Central autonomic integration of psychological stressors: Focus on cardiovascular modulation

During stress the sympathoadrenal system and the hypothalamo-pituitary-adrenal axis act in a coordinated manner to force changes within an animal's current physiological and behavioral state. Such changes have been described as 'fight flight' or stress responses. The central nervous system may generate a stress response by different neural circuits, this being dependent upon the type of stressor presented. For instance, the central control of the autonomic function during physical stress would seem to be based on existing homeostatic mechanisms. In contrast, with exposure to psychological stress the means by which autonomic outflow is regulated has not been fully established. This review discusses recent observations of autonomic flow, cardiovascular components in particular, during psychological stress and the possible implications these may have for our understanding of the central nervous system. In addition, an update of recent findings concerning several regions thought to be important to the regulation of autonomic function during psychological stress exposure is provided.

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.

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.

A thermodynamic model of the sympathetic and parasympathetic nervous systems

In light of the nonequilibrium thermodynamics by I. Prigogine, the autonomic nervous system as a whole may be viewed as a dissipative structure progressively assembled in the course of evolution, plastically and rhythmically interfaced between forebrain, internal and external environments, to regulate energy, matter and information exchanges. In the present paper, this hypothesis is further pursued to verify whether the two main divisions of the autonomic nervous system, the sympathetic and parasympathetic systems, may support different types of exchange with the external environment. Previous data from hypothalamic stimulation experiments, studies of locus coeruleus function and available data on behavioral functional organization indicate that (1) tight engagement with the external environment, (2) high level of energy mobilization and utilization and (3) information mainly related to exteroceptive sensory stimulation characterize a behavioral prevalence of sympathoadrenal activation. On the other hand, (1) disengagement from the external environment, (2) low levels of internal energy and (3) dominance of proprioceptive information characterize a behavioral prevalence of vagal tone. Behavioral matter exchanges such as feeding, drinking, micturition and defecation are equally absent at the extreme of sympathoadrenal and vagally driven behaviors. The autonomic nervous system as a whole is genetically determined, but the sympathoadrenal system has been mainly designed to organize the visceral apparatus for an action to be performed by the biological system in the external environment and to deal with the novelty of task and of the environment, while the functional role of the parasympathetic is to prepare the visceral apparatus for an action to be performed by the biological system on itself, for recovery and self-protection (homeostasis), and is reinforced by repetition of phylo- and ontogenetically determined patterns. The available clinical data further support this interpretation indicating that an increased sympathetic and a decreased vagal tone may represent a consistent risk factor for cardiovascular diseases.

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.

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.

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.