Evidence that different regional sympathetic outflows vary in their sensitivity to the sympathoinhibitory actions of putative 5-HT1A and alpha 2-adrenoceptor agonists in anaesthetized cats

Facilitation of central imidazoline I(1)-site/extracellular signal-regulated kinase/p38 mitogen-activated protein kinase signalling mediates the hypotensive effect of ethanol in rats with acute renal failure

BACKGROUND AND PURPOSE

This study investigated the role of central sympathetic activity and related mitogen-activated protein kinase (MAPK) signalling in the cardiovascular effects of ethanol in a model of acute renal failure (ARF).

EXPERIMENTAL APPROACH

The effects of pharmacological interventions that inhibit peripheral or central sympathetic activity or MAPK on the cardiovascular actions of ethanol in rats with ARF induced by glycerol were evaluated.

KEY RESULTS

Glycerol (50%, 10 mL.kg(-1), i.m.) caused progressive increases and decreases in blood pressure (BP) and heart rate (HR) respectively. Subsequent i.v. ethanol (0.25 or 1 g.kg(-1)) elicited dose-related changes in BP (decreases) and HR (increases). These effects were replicated after intracisternal (i.c.) administration of ethanol. Blockade of nicotinic cholinoceptors (nAChR, hexamethonium, 20 mg.kg(-1)) or alpha(1)-adrenoceptors (prazosin, 1 mg.kg(-1)) attenuated cardiovascular effects of ethanol. Ethanol hypotension was also attenuated after the centrally acting sympatholytic drug moxonidine (selective I(1)-site agonist, 100 microg.kg(-1) i.v.), but not guanabenz (selective alpha(2)-receptor agonist, 30 microg.kg(-1), i.v.), suggesting involvement of central circuits of I(1) sites in ethanol-evoked hypotension. Selective blockade I(1) sites (efaroxan) but not alpha(2) (yohimbine) adrenoceptors abolished the hypotensive response to ethanol. Intracisternal administration of PD98059 or SB203580, inhibitors of extracellular signal-regulated kinase (ERK 1/2) and p38 MAPK, respectively, reduced the hypotensive action of moxonidine or ethanol. When used simultaneously, the two MAPK inhibitors produced additive attenuation of ethanol hypotension.

CONCLUSIONS AND IMPLICATIONS

Sympathoinhibitory pathways of central I(1)-sites and downstream ERK/p38 MAPK signalling were involved in the hypotensive action of ethanol in ARF.

The 5-hydroxytryptamine1A receptor agonist, (+)-8-hydroxy-2-(di-n-propylamino)-tetralin, increases cardiac output and renal perfusion in rats subjected to hypovolemic shock

The 5-hydroxytryptamine(1A) receptor agonist, (+)-8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), raises blood pressure (BP) and venous tone in rats subjected to hemorrhagic shock. Here, BP, ascending aortic blood flow [i.e., estimate of cardiac output (CO)] and venous blood gases were measured to determine the hemodynamic effects of 8-OH-DPAT (30 nmol/kg i.v., n = 10), saline (n = 10), or an equipressor infusion of epinephrine (n = 10) in unanesthetized rats subjected to hemorrhagic shock (25 min of hypotensive hemorrhage, approximately 50 mm Hg). Renal and iliac blood flow were measured in separate groups of similarly hemorrhaged rats given the same dose of 8-OH-DPAT (n = 7) or saline (n = 6). Compared with saline treatment, 8-OH-DPAT produced a sustained rise in BP (+32 +/- 4 versus +9 +/- 2 mm Hg, 15 min after injection, P < 0.01) and CO (+27 +/- 5 versus +4 +/- 6 ml/min/kg, P < 0.01) but did not affect total peripheral resistance (TPR). Infusion of epinephrine reduced CO (-12 +/- 6 ml/min/kg, P < 0.01) and dramatically increased TPR [+0.37 +/- 0.11 versus +0.05 +/- 0.05 log (mm Hg/ml/min/kg), P < 0.01]. 8-OH-DPAT increased renal conductance (+7 +/- 1 versus +4 +/- 1 microl/min/mm Hg, P < 0.01) but did not significantly affect iliac conductance. 8-OH-DPAT attenuated further development of acidosis compared with either saline or epinephrine (-5.6 +/- 1.6 versus -13.0 +/- 2.0 versus -11.3 +/- 2.6 mmol/liter base excess 45 min after start of hemorrhage, both P < 0.01 versus 8-OH-DPAT). These data demonstrate that 8-OH-DPAT improves hemodynamics during circulatory shock, in part, through renal vasodilation and mobilizing of blood stores.

The control of adrenergic function in heart failure: therapeutic intervention

Chronic heart failure is characterised by excess adrenergic activity that augurs a poor prognosis. The reasons for increased adrenergic activity are complex and incompletely understood. The circumstantial evidence relating increased activity to adverse outcome is powerful, but not yet conclusive. In normal subjects, autonomic control of the circulation is predominantly under the control of sympatho-inhibitory inputs from the arterial and cardiopulmonary baroreceptors, with a small input from the excitatory ergo- and chemo-receptors. In heart failure, the situation is reversed, with loss of the restraining input from the baroreceptors and an increase in the excitatory inputs, resulting in excessive adrenergic activity. The circumstantial evidence linking neuroendocrine activation with poor outcome coupled with the clinical success of inhibition of the renin-angiotensin-aldosterone system has long suggested that inhibition of adrenergic activity might be beneficial in heart failure. There is a number of potential ways of achieving this. Improved treatment of heart failure itself may reduce sympathetic drive. There is an interplay between angiotensin II, aldosterone and the sympathetic nervous system, and thus RAAS antagonists, such as angiotensin converting enzyme inhibitors and spironolactone could directly reduce sympathetic activation. Exercise rehabilitation may similarly reduce sympathetic activity.Recently, beta-adrenergic receptor antagonists have been conclusively shown to improve symptoms, reduce hospitalisations and increase survival. However, the demonstration that central reduction of sympathetic activity with agents such as moxonidine increases morbidity and mortality suggests that we do not properly understand the role of sympathetic activation in the pathophysiology of heart failure.

A study of VitalView for behavioural and physiological monitoring in laboratory rats

We describe the use of a commercially available telemetry and data acquisition system to record heart rate, body temperature and activity of freely behaving rats with transmitters that operate without batteries (transponders). The system uses PDT 4000HR E-Mitters (Mini Mitter, OR, USA) to acquire animal temperature, heart rate and motor activity data. E-Mitters obtain power from a radiofrequency field produced by an ER-4000 energizer/receiver so that transponders can collect data on heart rate, body temperature and gross motor activity. ER-4000 energizers/receivers are designed to be placed below the animals' cage. Data output from receivers is managed by a Windows PC-based data acquisition system, VitalView. In this study, we report that a good correlation exists between VitalView and Powerlab for the determination of heart rate and between intra-abdominal (telemetric) and colonic body temperature (rectal digital thermometer) in rats. Assessment of this system by using agents that have well-documented effects on heart rate, body temperature and locomotor activity have also been determined. An additional feature of VitalView is the incorporation of behavioural inputs (feeding monitors to monitor duration and frequency of feeding and a lickometer to monitor drinking bouts) into the data acquisition system designed primarily to acquire data from the implanted transponders. Circadian rhythms for all parameters were established in rats with E-Mitters implanted. VitalView may be used for the determination of multiple parameters in freely behaving animals using transponders, which operate without batteries. This capability is unique in its field and represents a recent advance in biotelemetric monitoring of laboratory animals.

Central cardiovascular regulation and 5-hydroxytryptamine receptors

Evidence is provided to support the view that central 5-HT(1A) and 5-HT(2) receptors are the major receptor subtypes important in cardiovascular regulation. Data are also provided to implicate 5-HT(1B/1D/1F) receptors in central cardiovascular regulation. Activation of 5-HT(2) receptors generally causes sympathoexcitation and a rise in blood pressure and this is mainly mediated by 5-HT(2A) receptors. However, presympathetic vasomotor neurones located in the hindbrain (RVLM), controlling sympathetic outflow to the heart, are not activated in the same way as other presympathetic vasomotor neurones, although activation of 5-HT(2) receptors located in the midbrain can activate sympathetic outflow to the heart. Furthermore, at least in the rat, these midbrain 5-HT(2A) receptors are also responsible for the release of vasopressin by activation of a central angiotensinergic pathway. The ability of vasopressin directly and/or indirectly to modify renal sympathetic outflow involves the activation of central 5-HT(2B) receptors, which in turn, when activated via the i.c.v. route, can cause selective renal sympathoexcitation. Evidence is also provided which indicates that the reflex control of parasympathetic outflow to the heart and to other organs involves central 5-HT(1A) receptors located in the vicinity of these preganglionic vagal neurones. Finally, 5-HT(3) receptors are implicated in the afferent regulation of central sympathetic and parasympathetic tone.

Moxonidine: some controversy

Initially it was considered that moxonidine, like clonidine, acted at central (2)-adrenoceptors to reduce blood pressure. With the characterisation of imidazoline binding sites distinct from (2)-adrenoceptors, the consensus became that moxonidine was acting predominantly at imidazoline I(1) receptors in the rostral ventrolateral medulla to lower blood pressure. Moxonidine acts at prejunctional (2)-adrenoceptors on sympathetic nerve endings to decrease noradrenaline release and this may contribute to its ability to lower blood pressure. The predominant site of action of moxonidine may also depend on route of administration, with imidazoline I(1) receptors being predominant after central, and (2)-adrenoceptors predominant after systemic administration. The controversy over the mechanism and site of action with moxonidine is ongoing. In animal models, moxonidine lowers blood pressure, reduces cardiac hypertrophy and remodelling, reduces cardiac arrhythmias and increases blood flow in cerebral ischaemia. Moxonidine also has beneficial effects in animal models of diabetes and kidney disease. Moxonidine increases sodium and water excretion in rats, but not humans. Animal studies indicate that moxonidine may be useful in the treatment of glaucoma by reducing intra-ocular pressure. Animal studies show that moxonidine may also be effective in pain and in ethanol withdrawal. In humans, the pharmacokinetics of moxonidine are of the one-compartment model with first-order absorption. Renal elimination is the major route of elimination and individual titration of moxonidine is needed in patients with renal impairment. There is overwhelming evidence that moxonidine is a safe and effective antihypertensive. A large clinical trial of moxonidine in heart failure, MOXCON, was stopped because of excessive deaths in the moxonidine group. Moxonidine should not be used in patients with heart failure, but there are no obvious reasons to stop its use as an antihypertensive, or its development for other clinical uses.

Chronic I(1)-imidazoline agonism : sympathetic mechanisms in hypertension

Evidence exists for a state of sympathetic hyperactivity in essential hypertension, and moxonidine, a new central sympathetic inhibitor, has been introduced for its treatment. Acute administration of moxonidine lowers peripheral sympathetic neural output. This study examined the effect of chronic moxonidine therapy, at increasing therapeutic doses, on resting peripheral sympathetic activity and vascular resistance and their responses to physiological reflex maneuvers. Twelve newly diagnosed patients with essential hypertension were studied sequentially at least 1 month apart, initially on no therapy, then on 200 microg, and finally on 400 microg of oral moxonidine daily. Changes in heart rate, arterial blood pressure, calf vascular resistance, and peripheral sympathetic drive were assessed at rest and during reflex maneuvers. Peroneal microneurography was used to quantify peripheral sympathetic vasoconstrictor activity by single-unit and multiunit techniques. Moxonidine therapy progressively reduced resting mean arterial pressure (P<0.0001) without affecting heart rate. At 200 microg daily, there was a significant reduction in sympathetic nerve activity (P<0.001) and calf vascular resistance (P<0.01). At 400 microg daily, further reductions were smaller and insignificant. Responses to cold stimulus and isometric handgrip exercise showed a similar pattern, with the greatest magnitude of change at 200 microg daily. In patients with essential hypertension, chronic moxonidine therapy inhibited resting sympathetic vasoconstrictor drive and also its reflex responses. The magnitude of inhibition became less as the therapeutic dose was increased, suggesting that moxonidine may be more effective under conditions of high sympathetic activity.

Effect of moxonidine on blood pressure and sympathetic tone in conscious spontaneously hypertensive rats

The effects of moxonidine on blood pressure, heart rate and sympathetic tone were studied in conscious spontaneously hypertensive rats. Intravenous moxonidine (80 nmol) transiently increased blood pressure without affecting heart rate or splanchnic nerve activity. Moxonidine (20-80 nmol) given into the fourth cerebral ventricle dose-dependently lowered mean arterial pressure, heart rate and sympathetic outflow (maximally by 60 +/- 3 mm Hg, 148 +/- 10 beats min(-1) and 15 +/- 3 microV). Moxonidine was more effective by this route than after the injection into the lateral ventricle. Clonidine (20-80 nmol) produced an initial pressor response after both intracerebroventricular routes of administration. A decrease in blood pressure was observed only when clonidine was given into the fourth ventricle. Clonidine decreased heart rate and splanchnic nerve activity similarly like moxonidine when the substances were given into the fourth ventricle. The data imply that the hypotensive effect of moxonidine is related to central sympathoinhibition. The main site of this action appears to be in the brainstem region.

A pharmacological validation of radiotelemetry in conscious, freely moving rats

Two reference substances were used in the present study. d-Amphetamine is a direct catecholamine-releasing agent which has a marked stimulant effect upon locomotor activity at low to moderate doses and induces stereotypy at higher doses. (+/-)8-Hydroxy-2-(di-n-propylamino)-tetraline [(+/-)8-OH-DPAT] is a selective 5-HT1A receptor agonist which produces a well-defined behavioral syndrome and a dose-dependent hypothermia. The first aim of this study was to validate that the d-amphetamine-induced activity monitored by telemetry correlated to that concomitantly measured in automated cages and complement these measures with an ethologically based direct observational technique. d-Amphetamine (2.5 and 5.0 micromol/kg s.c.) stimulated locomotion as assessed with radiotelemetry, in automatic cages and by observation. Accompanying these behavioral effects was a concurrent increase (assessed by radiotelemetry) in heart rate but not in blood pressure. The second part of this study examined the pharmacological effects of (+/-)8-OH-DPAT (0.09-6.1 micromol/kg s.c.) on behavior (observation and activity) and temperature and on the cardiovascular system. (+/-)8-OH-DPAT induced the classical serotonergic syndrome of lower lip retraction, forepaw treading, and flattened body posture (observation), and this was accompanied by a concomitant hypothermia (radiotelemetry). (+/-)8-OH-DPAT also induced a dose-dependent and significant decrease in heart rate for 50 min of the 1-h long observation period. This was not accompanied by an increase in blood pressure in spite of the increased activity as seen with all three methods. These results show that radiotelemetry can be used as a tool to measure activity, core temperature, and the cardiovascular parameters in animals that are less stressed than those that are restrained for similar more invasive measurements, and that this technique can be used in combination with others to produce a more complete ethogram of the animal's responses to pharmacological challenges.

Role of cardiac output in ethanol-evoked attenuation of centrally mediated hypotension in conscious rats

Our previous studies have shown that ethanol selectively counteracts centrally mediated hypotensive responses. This study investigated the role of cardiac output and peripheral resistance in the antagonistic interaction between ethanol and antihypertensive drugs. Changes in blood pressure, heart rate, cardiac index, stroke volume, and peripheral resistance elicited by clonidine and subsequent ethanol or saline administration were evaluated in conscious rats. The aortic barodenervated rat was employed because it exhibits greater hypotensive responses to clonidine compared with the intact rat. Aortic barodenervation elicited acute rises in blood pressure, heart rate, and peripheral resistance, whereas cardiac index and stroke volume were not altered. The blood pressure of conscious aortic barodenervated rats returned to sham-operated levels by 48 hours due to concomitant reductions in cardiac index and stroke volume; the peripheral resistance, however, remained significantly elevated. Clonidine (30 microg/kg, I.V.) elicited greater decreases in blood pressure in aortic barodenervated compared with sham-operated rats. The hypotension was caused by decreases in cardiac index and stroke volume because peripheral resistance did not change. Ethanol (1 g/kg, I.V.) counteracted the hypotensive effect of clonidine and raised blood pressure to levels higher than preclonidine values. Significant (P<.05) increases in cardiac index and stroke volume and decreases in peripheral resistance accompanied the pressor effect of ethanol. Additional control groups were included in the study to determine the selectivity of the interaction. A dose of hydralazine (0.5 mg/kg, I.V.) was used that produced similar hypotension to that evoked by clonidine in aortic barodenervated rats. Hydralazine-evoked hypotension was similar in denervated and control rats and resulted from significant reductions in peripheral resistance. Reflex increases in heart rate and stroke volume and hence cardiac output were observed. Ethanol given after hydralazine produced a short-lived pressor effect (<5 minutes versus 40 minutes in the case of clonidine) and counteracted the sympathetically mediated increases in cardiac output, stroke volume, and heart rate. These findings support our hypothesis that ethanol selectively counteracts hypotensive responses of central origin by reversing the reduction in cardiac output elicited by clonidine.

Pretreatment with quinpirole inhibits the central antihypertensive effects of rilmenidine and alpha-methyldopa in conscious rats

Treatment of conscious spontaneously hypertensive rats (SHR) with the dopamine D2 receptor agonist quinpirole causes a short-lasting pressor response and apparent desensitisation to the effects of subsequent injections of quinpirole or central antihypertensives such as clonidine. In the present study, a number of aspects of this apparent desensitisation were investigated. Thirty minutes after intravenous injection of quinpirole into spontaneously hypertensive rats, treatment with the dopamine D2 receptors antagonist raclopride caused a significant fall in blood pressure. At this time point, circulating levels of vasopressin were not significantly different compared to controls. In Brattleboro rats, the pressor response to quinpirole was reduced in the first 15 min after injection, but not difference in blood pressure was observed at later time points. In SHR which had been treated with quinpirole, the central antihypertensive effects of rilmenidine or alpha-methyldopa were significantly inhibited. By contrast, the bradycardia induced by these drugs was similar in quinpirole-treated rats and controls. Quinpirole pretreatment caused an enhancement of the hypotension but a reduction of the reflex tachycardia after intravenous treatment with hydralazine. In SHR treated with methylatropine and quinpirole, the upper plateau of the sympathetic baroreceptor-heart rate reflex curve was reduced. These results show that treatment with quinpirole has marked effects on central sympathetic vasomotor mechanisms which are the target of antihypertensive drugs such as rilmenidine and alpha-methyldopa. At least some of these effects may occur at the level of the sympathetic baroreflex. Moreover, while the effects of quinpirole on sympathetic regulation are prolonged, the initial pressor response is counteracted by an as yet unidentified compensatory mechanism which can be unmasked when quinpirole is displaced from its receptor by dopamine D2 receptor antagonist treatment.

The cardiovascular and renal functional responses to the 5-HT1A receptor agonist flesinoxan in two rat models of hypertension

1. This study investigated the importance of renal sympathetic nerves in regulating sodium and water excretion from the kidneys of stroke prone spontaneously hypertensive and 2K1C Goldblatt hypertensive rats anaesthetized with chloralose/urethane (17.5/300 mg initially and supplemented at regular intervals), and prepared for measurement of renal function. 2. In stroke prone spontaneously hypertensive rats, flesinoxan, 30-1000 micrograms kg-1, i.v., caused graded reductions in blood pressure and heart rate of 74 +/- 5 mmHg and 63 +/- 9 beats min-1, respectively at the highest dose (P < 0.001). Renal blood flow did not change at any dose of drug while glomerular filtration rate fell by some 20% (P < 0.001) at the highest dose of drug, absolute and fractional sodium excretions, approximately doubled at 100 micrograms kg-1, and thereafter fell to below the baseline level at 1000 micrograms kg-1. 3. This pattern of excretory response was abolished following acute renal denervation when flesinoxan caused dose-related reductions in urine flow and sodium excretion, similar to that obtained by a mechanical reduction of renal perfusion pressure. 4. Flesinoxan administration (30-1000 micrograms kg-1, i.v.) into 2K1C Goldblatt hypertensive rats caused a maximum decrease in blood pressure and heart rate (both P < 0.001) of 34 +/- 3 mmHg and 20 +/- 6 beats min-1 and while renal blood flow and glomerular filtration rate were autoregulated, from 160 to 125 mmHg, there were dose-related decreases in urine volume and sodium excretion from the clipped and non-clipped kidneys of approximately 50-60% at the highest dose. 5. These findings suggest that in the stroke prone spontaneously hypertensive rat the renal nerves importantly control sodium and water reabsorption at the level of the tubules, whereas in 2K1C Goldblatt hypertensive rats, they play a minor role.

A comparison of the effects of doxazosin and terazosin on the spontaneous sympathetic drive to the bladder and related organs in anaesthetized cats

The effects of i.v. infusion of the alpha1-adrenoceptor antagonists doxazosin and terazosin (2 mg kg-1 h-1) on spontaneous hypogastric, renal and inferior cardiac nerve activity, spontaneous bladder contractions, blood pressure, heart rate and femoral arterial flow were investigated separately in alpha-chloralose-anaesthetized cats. Both drugs caused a reduction in hypogastric nerve activity associated with no overt changes in spontaneous bladder contractions. Doxazosin was more potent than terazosin, in that there was a significant reduction in hypogastric nerve activity after 20 min (0.67 mg kg-1) of infusion, while for terazosin this occurred after 40 min (1.33 mg kg-1). Both drugs also caused significant falls in blood pressure of 34 +/- 3 mm Hg and 33 +/- 4 mm Hg after 60 min. This was associated with no change in heart rate for doxazosin while terazosin caused an initial and significant increase in heart rate of 20 +/- 3 beats min-1 by 5 min, declining by 30 min to 1 +/- 5 beats min-1. This terazosin-induced tachycardia was associated with a significant increase in cardiac nerve activity of 128 +/- 22%. Both drugs caused increases in renal nerve activity however only for doxazosin was this increase significant. Femoral arterial conductance was also increased by both drugs, however, for doxazosin this increase was immediate and larger over the infusion period. These results demonstrate that alpha1-adrenoceptor antagonists can reduce sympathetic drive to the bladder and related organs.

Involvement of alpha 2-adrenoceptors in the cardiovascular effects of moxonidine

The central sympathoinhibition caused by moxonidine has been explained by activation of alpha 2-adrenoceptors on the one hand, and by an action at imidazoline I1 receptors on the other hand. In order to examine these possibilities, effects of moxonidine were compared with those of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304), an alpha 2-adrenoceptor agonist with very low affinity for I1 receptors, in conscious rabbits. The interaction with yohimbine, an alpha 2-adrenoceptor antagonist with very low affinity for imidazoline I1 receptors, was also studied. Moxonidine 3-100 micrograms kg-1 and UK 14304 1-30 micrograms kg-1 i.v. elicited similar effects: they decreased arterial blood pressure after a transient increase, decreased renal sympathetic nerve activity (recorded with chronically implanted electrodes), decreased heart rate and decreased the plasma noradrenaline concentration. Yohimbine given i.v. antagonized the effects of moxonidine and of UK 14304 in a similar manner. Yohimbine injected into the cisterna magna (i.c.) prevented the hypotension but did not change the decrease in plasma noradrenaline and heart rate, again in the case of both moxonidine and UK 14304. The agreement of the effect patterns of moxonidine and UK 14304, and the similar antagonism of yohimbine against either drug, demonstrate involvement of alpha 2-adrenoceptors in their central sympathoinhibitory action. The resistance of the bradycardia and the plasma noradrenaline fall against yohimbine i.c. indicates a contribution of presynaptic alpha 2-adrenergic inhibition of transmitter release from postganglionic sympathetic neurons to the overall reduction of sympathetic tone.

Involvement of peripheral presynaptic inhibition in the reduction of sympathetic tone by moxonidine, rilmenidine and UK 14304

We studied the possibility that presynaptic inhibition of transmitter release from postganglionic sympathetic neurons contributes to the overall reduction of sympathetic tone produced by moxonidine, rilmenidine and 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK 14304). In pithed rabbits without electric stimulation, moxonidine, rilmenidine and UK 14304 caused a long-lasting, > 10 min, increase in arterial pressure. Heart rate was not changed. In pithed rabbits in which sympathetic tone was created by electric stimulation through the pithing rod (2 Hz), the same doses of moxonidine, rilmenidine and UK 14304 caused only a brief, < 10 min, blood pressure rise. Heart rate was decreased, as were the plasma concentrations of noradrenaline and adrenaline. Dose-response curves for the effects on the plasma noradrenaline concentration (stimulated pithed rabbits) were compared with previously obtained dose-response curves for depression of renal sympathetic nerve activity (conscious rabbits). For each drug, the curve describing peripheral presynaptic inhibition and the curve describing central sympathoinhibition were very similar. Both the power and the dose dependence of the peripheral inhibitory effect support its contribution to the overall decrease in sympathetic tone produced by clonidine-like drugs in intact animals. The peripheral effect in all likelihood consists in activation of presynaptic alpha 2-autoreceptors. The agreement of the dose-response curves for the peripheral and for the central effect supports the view that the central effect, like the peripheral one, is mediated through alpha 2-adrenoceptors.

Mechanism of sympathoinhibition by imidazolines

The aim of the present study is to characterize the cardiovascular effects of rilmenidine and moxonidine, two recently developed centrally acting antihypertensive drugs. Rilmenidine and moxonidine are alpha 2-adrenoceptor agonists and, in addition, possess affinity for imidazoline (I1)-receptors. To determine if alpha 2- or I1-receptors are involved in sympathoinhibition, rilmenidine and moxonidine were compared with UK 14304, an alpha 2-agonist devoid of affinity for I1-receptors, and antagonism by the "pure" alpha 2-adrenoceptor antagonists yohimbine, SK&F86466, and RX821002 was studied. When injected intravenously into conscious rabbits, rilmenidine and moxonidine, on the one hand, and UK 14304, on the other, elicited a similar pattern of effects. Thus, transient hypertension was followed by long-lasting hypotension accompanied by a decrease in heart rate, renal sympathetic nerve firing rate, and plasma norepinephrine concentration. The effects of rilmenidine, moxonidine, and UK 14304 were antagonized by intravenously administered yohimbine, SK&F86466, and RX821002. The effects of moxonidine and UK 14304 were also prevented by yohimbine injected into the cisterna magna. Altogether, the degree of antagonism of the effects of rilmenidine and moxonidine did not differ from the degree of antagonism of the effects of UK 14304. Rilmenidine, moxonidine, and UK 14304 were also given to pithed rabbits in which a constant sympathetic tone was maintained by electrical stimulation of the sympathetic nerves. At doses that caused sympathoinhibition in conscious rabbits, they lowered the plasma norepinephrine concentration markedly. Our experiments show by direct measurement of sympathetic nerve activity and plasma norepepinephrine concentration that rilmenidine, moxonidine, and UK 14304 cause sympathoinhibition. As a consequence, blood pressure and heart rate decrease. The simplest interpretation of the blockade of central sympathoinhibition by the selective alpha 2-adrenoceptor antagonists is that rilmenidine, moxonidine, and UK 14304 primarily activated alpha 2-adrenoceptors. The decrease in plasma norepinephrine in pithed rabbits indicates peripheral presynaptic alpha 2-adrenoceptor-mediated inhibition of norepinephrine release per action potential from postganglionic sympathetic axons and suggests a contribution of this mechanism to the overall reduction in sympathetic tone.

Differential effects of clonidine on renal sympathetic nerve activity and heart rate at onset of static exercise

The effects of the alpha 2-adrenergic agonist clonidine on the increase in renal sympathetic nerve activity (RSNA) and heart rate (HR) at the onset of voluntary static exercise were studied using conscious cats. Five cats were trained to press a bar with one forelimb. A total of 60 exercise trials were performed before and after injection of clonidine (5-10 micrograms/kg i.v.). Before clonidine, RSNA and HR increased immediately before or at the onset of exercise, which was followed by a rise in arterial blood pressure (AP). The initial increases in RSNA and HR are likely to be caused by descending input from higher brain centers. After clonidine, baseline RSNA was decreased to 21 +/- 5% of the control before clonidine, probably due to a central action of clonidine. HR and AP were decreased from 221 +/- 4 to 178 +/- 5 bpm and from 108 +/- 2 to 82 +/- 4 mmHg, respectively. The increase in RSNA at the onset of exercise and the rise in AP during exercise were blunted to 56-57% of the responses before clonidine injection. In contrast, the increase in HR at the onset of exercise was not altered by clonidine. Thus, it is suggested that clonidine, administered intravenously, attenuates the centrally-induced increase in RSNA at the onset of static exercise but does not affect the increase in HR.

The renal functional responses to 5-HT1A receptor agonist, flesinoxan, in anaesthetized, normotensive rat

1. The present study was designed to examine the effects of a centrally acting 5-HT1A receptor agonist, flesinoxan, on the cardiovascular system and renal haemodynamics and excretory function. 2. In chloralose-urethane anaesthetized Wistar rats, i.v. administration of bolus doses of flesinoxan, at 30, 100, 300 and 1000 micrograms kg-1, caused significant, dose-dependent decreases in mean arterial pressure, of 33 +/- 2 mmHg (P < 0.001) and heart rate of 57 +/- 9 beats min-1 (P < 0.001) at the highest dose used. Despite this substantial fall in perfusion pressure there were no meaningful changes in the renal excretion of water and sodium. In a second group of rats, reduction of renal perfusion pressure mechanically to the same values as observed in rats given flesinoxan (i.e. 100, 92, 84 and 76 mmHg) produced reductions in urine flow, absolute and fractional sodium excretions reaching a maximum of 74, 86 and 84% respectively (all P < 0.001) at the lowest pressure. These reductions were significantly larger than those seen in the previous group of animals. 3. In the group of rats subjected to renal denervation, flesinoxan produced changes in blood pressure and heart rate which were not different from those observed in intact animals. However, the reduction in pressure was accompanied by significant decreases in urine flow of 71%, absolute sodium excretion of 68% and fractional sodium excretion of 67% (all P < 0.001) at the highest dose, which were all significantly greater than the changes seen in the innervated animals but were not different from those observed when renal perfusion pressure was reduced mechanically. 4. The findings of this investigation showed that flesinoxan was effective in lowering blood pressure and heart rate in the anaesthetized rat, which was probably due to decreased sympathetic nerve activity.Renal excretion of water and sodium was well preserved in the face of the flesinoxan-induced hypotension.The maintenance of fluid excretion with flesinoxan appeared to be mediated via changes in renal nerve activity, since it did not occur when the kidney was denervated.

Clinical experience with moxonidine

Moxonidine is an imidazoline receptor modulator, specific for the I1-imidazoline receptor. The stimulation of imidazoline receptors represents a new mode of antihypertensive action to inhibit peripheral alpha-adrenergic tone by a central mechanism. Acute hemodynamic studies reveal moxonidine produces an acute fall of blood pressure and systemic vascular resistance. Heart rate, cardiac output, stroke volume, and pulmonary artery pressures are not affected. Left ventricular end-systolic and diastolic volumes are reduced. Ejection fraction is not significantly affected but 6-month studies showed a regression of left ventricular hypertrophy. After oral administration the maximum concentration of moxonidine is reached in about 1 hour, and elimination half-life is 2.5 hours, prolonged by renal insufficiency. The antihypertensive effect lasts longer than would be expected from the half-life. Open studies with moxonidine have revealed falls between 20 and 29 mmHg systolic, and between 10 and 19 mmHg diastolic blood pressure. In the largest study, over 12 months in 141 patients, most patients were controlled by 0.2 mg daily (58%) or 0.2 mg b.i.d. (38%). Moxonidine has been compared with representatives from each important class of antihypertensive drugs. In a crossover trial of clonidine in 20 patients, blood pressure control was similar, but the incidence of tiredness and dry mouth was less on moxonidine, as was the total number of patients experiencing side effects, 85% versus 30% (p < 0.01). In a larger parallel group study of moxonidine (n = 122) and clonidine (n = 30), blood pressure control was similar, but the overall incidence of side effects was less on moxonidine. In comparative studies of moxonidine with atenolol, ACE inhibitors, dihydropyridine calcium antagonists, hydrochlorothiazide, and alpha 1 blockade, the blood pressure control with representatives of these various classes of drugs was similar to moxonidine.

Antiemetic effects of flesinoxan in cats: comparisons with 8-hydroxy-2-(di-n-propylamino)tetralin

The antiemetic effects of flesinoxan were evaluated following s.c. administration in cats. Flesinoxan produced a dose-dependent suppression of motion sickness and also reduced xylazine-induced emesis at higher doses. Flesinoxan had a short latency to onset and may have a brief duration of action. It was slightly more potent that 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), in contrast to their relative potencies on most other in vivo measures. High doses of both agonists produced defensive behavior as a result of 5-HT1A receptor stimulation. (-)-Propranolol, which previously reduced 8-OH-DPAT suppression of feline motion sickness, failed to reduce the antiemetic effect of flesinoxan. The dose of 3 mg/kg of NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine) produced a slight decrease in motion sickness and added to the suppression of motion sickness by low doses of flesinoxan via an uncertain mechanism. It also reduced the antiemetic effect of higher doses of flesinoxan. In contrast, NAN-190 produced additive antiemetic effects when combined with 8-OH-DPAT and little if any reduction. NAN-190 reduced the defensiveness produced by both flesinoxan and 8-OH-DPAT. Phentolamine and sulpiride reduced neither the antiemetic effect nor the defensive behavior produced by flesinoxan, thus ruling out a role for alpha-adrenoceptors and dopamine D2 receptors. Flesinoxan exerted a broad spectrum antiemetic effect by an action at 5-HT1A receptors as does 8-OH-DPAT, but differed in its response to putative 5-HT1A receptor antagonists.

Comparison of the effects of IVth ventricular administration of some tryptamine analogues with those of 8-OH-DPAT on autonomic outflow in the anaesthetized cat

1 The present study compares the effects on representative autonomic outflows of IVth ventricular application of tryptamine analogues which act at 5-HT1 receptors with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). 2 Cumulative doses of 8-OH-DPAT, N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT) and 5-carboxamidotryptamine (5-CT, 2.5-40 nmol kg-1), sumatriptan (10-160 nmol kg-1), indorenate (100-800 nmol kg-1), 5-hydroxytryptamine (5-HT, 20-640 nmol kg-1) both alone and in the presence of cinanserin (0.1 mg kg-1) were given into the IVth ventricle of cats which were anaesthetized with a mixture of alpha-chloralose and pentobarbitone sodium, neuromuscularly blocked and artificially ventilated. Recordings were made of arterial blood pressure, heart rate, renal, cardiac, splanchnic and phrenic nerve activities, femoral arterial flow, tracheal and intragastric pressures. 3 Central application of each of the agonists evoked significant falls in arterial blood pressure. In addition 8-OH-DPAT, DP-5-CT, 5-CT and 5-HT all evoked a differential inhibition of sympathetic nerve activities, with renal nerve activity being the most sensitive and cardiac nerve activity the least sensitive. In the dose-ranges used, administration of sumatriptan evoked reductions only in renal and splanchnic nerve activities whilst indorenate reduced activity in all three sympathetic nerves to a similar extent. 4. The effect of the agonists on heart rate was more inconsistent than the effects on sympathetic outflow.IVth ventricular application of 5-CT and sumatriptan were without effect on heart rate whilst 8-OH-DPAT, DP-5-CT, indorenate and 5-HT alone and in the presence of cinanserin all evoked significant bradycardias. However, whilst atropine partially reversed the bradycardias evoked by 8-OHDPAT and only slightly reversed those caused by indorenate, atropine was without effect on those evoked by DP-5-CT or 5-HT.5. None of the analogues tested had significant effects on gut motility, phrenic nerve discharge or tracheal pressure. 8-OH-DPAT, DP-5-CT, indorenate and 5-HT were without effect on femoral arterial conductance. However, following pretreatment with cinanserin, 5-HT evoked a significant reduction in femoral arterial conductance. At its highest dose, sumatriptan evoked a significant increase in femoral arterial conductance as did 5-CT at the 20 nmol kg-1 dose.6. It is concluded that the present data support the view that 5-HT1A receptors at the level of the brainstem are involved in the central sympathoinhibitory effects caused by intravenous administration of 5-HT1A agonists. Further, brainstem 5-HT1A receptors play an important role in the control of renal sympathetic outflow while brainstem 5-HT2 receptors are involved in the control of skeletal muscle and/or skin blood flow. Selective tryptamine agonists for 5-HT1A receptors differ from non-tryptamine agonists in that they do not cause an increase in central cardiac vagal tone.

Differential cardiovascular effects of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), flesinoxan, 5-methyl-urapidil and MDL 75,608A in conscious spontaneously hypertensive rats

The effects of intravenous (i.v.) administration of four agonists at central 5-HT1A receptors were investigated and compared. Acute iv injection of 0.1 mg/kg of 8-OH-DPAT induced a decrease in blood pressure both in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The maximal hypotensive effect was observed 15 and 10 min after injection, respectively, but the effect was greater and longer-lasting in the SHR. 8-OH-DPAT significantly decreased heart rate in WKY and, to a lesser extent, in SHR. The i.v. injection of 1 mg/kg of flesinoxan caused a similar fall in blood pressure and heart rate in SHR and WKY. The i.v. administration of 1 mg/kg of 5-methyl-urapidil or MDL 75,608A caused a fall in blood pressure which was significantly more pronounced in SHR than in WKY. 5-methyl-urapidil induced a significant tachycardia in WKY, but had little effect on heart rate in SHR. MDL 75,608A caused a short-lasting tachycardia in SHR and WKY. In conscious SHR, the intracerebroventricular (icv) injection of 10 micrograms of 8-OH-DPAT or 100 micrograms of either flesinoxan or MDL 75,608A caused a decrease in blood pressure and heart rate. The icv injection of 100 micrograms of 5-methyl-urapidil caused only a decrease in blood pressure. Chronic pre-treatment with these compounds, by daily i.v. injection, did not significantly influence the hypotensive or bradycardic effects in an acute experiment. The involvement of alpha 1-adrenoceptors in the effects of these compounds was studied by administering phenylephrine (1 microgram/i.v.) at 5-min intervals before and after the i.v. injection of the experimental compounds. The injection of phenylephrine reproducibly increased blood pressure by 35-40 mm Hg after saline pre-treatment, and these responses were not affected by the i.v. injection of 0.1 mg/kg of either 8-OH-DPAT or 1 mg/kg of flesinoxan. In contrast, the phenylephrine-induced pressor responses were markedly diminished at 5 min after treatment with 1 mg/kg of either 5-methyl-urapidil or MDL 75,608A, but slowly recovered thereafter. These results show that the 5-HT1A receptor agonists 8-OH-DPAT, flesinoxan, 5-methyl-urapidil and MDL 75,608A show antihypertensive properties in conscious SHR after iv or icv injection. However, the mechanism of action of the compounds differs: 8-OH-DPAT and flesinoxan may act predominantly as 5-HT1A receptor agonists, where as 5-methyl-urapidil and MDL 75,608A also seem to have an effect on peripheral alpha 1-adrenoceptors.

Neural mediation of the cardiovascular responses to intrathecal administration of substance P in the rat: slowing of the cardioacceleration by an adrenal opioid factor

Substance P, given intrathecally at the second (T2) or ninth (T9) thoracic level in the anesthetized rat, increased heart rate, arterial pressure and circulating catecholamines. At T9 in adrenalectomized animals and at T2 in intact animals, the cardioacceleration was more abrupt than in intact animals injected at T9 suggesting that the adrenals are not necessary for the cardiovascular responses and that the adrenals may have released a factor which slows the neurally mediated cardioacceleration. As opioids are co-released with catecholamines from the adrenals, naloxone (10 mg/kg i.v.) or nalorphine HCl (which does not cross the blood-brain barrier; 10 mg/kg s.c.) was given 5 min before administration of substance P at T9 in intact rats. In both groups the cardioacceleration was similar to that elicited in adrenalectomized animals, indicating that the adrenal factor was opioid and that its action was peripheral rather than central. When propranolol (10 mg/kg i.v.) was given 3 or 15 min before, substance P increased arterial pressure but heart rate was unchanged, indicating that the opioid factor was not slowing the cardioacceleration by a direct effect on the heart. The results indicate that intrathecal administration of substance P produces a neurally mediated increase in arterial pressure and heart rate and induces the release of an adrenal opioid factor which slows the neurally-mediated cardioacceleration by an action in the periphery. This indicates a functional interaction between humoral and neural sympathetic mechanisms regulating the cardiovascular system.

Can the 5-HT2/1c agonist DOI cause differential sympatho-excitation in nerves supplying the heart in anaesthetized cats?

A comparison of the effects on sympathetic nerve activity to the heart of intravenous administration of the selective 5-HT2/1c agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) alone and in the presence of the peripherally acting 5-HT2/1c antagonist BW501C67 were made in alpha-chloralose anaesthetized cats. Activity in both cardiac sympathetic nerves running in the vagus and in both inferior cardiac nerves was simultaneously recorded. In addition renal and phrenic nerve activity, heart rate, arterial blood pressure, femoral arterial flow and tracheal pressure were also recorded. DOI evoked a rise in blood pressure and increased femoral arterial resistance in both groups of animals. In the BW501C67 pretreated animals, DOI had no effect on heart rate but caused a significant increase in all sympathetic nerve activities. In non-pretreated animals, however, the rise in blood pressure was associated with variable effects on sympathetic nerve activity, a significant rise only occurring in renal nerve activity. In these experiments DOI evoked a bradycardia. The variability in sympathetic nerve activity in the non-pretreated animals may have resulted from the rise in blood pressure evoking a baroreceptor-mediated central sympathoinhibition which would interfere with the central sympathoexcitatory effects of DOI. It is concluded that centrally, DOI will cause sympathoexcitation. In addition, intravenous DOI acting on 5-HT2 receptors on bronchial smooth muscle evokes bronchoconstriction as indicated by the very large rise in tracheal pressure in non-BW501C67-pretreated animals. If not controlled this severely compromises respiration leading to a large overestimate of the sympathoexcitatory effects of stimulation of central 5-HT2/1c receptors.

Moxonidine. A review of its pharmacology, and therapeutic use in essential hypertension

Interest in centrally acting antihypertensive agents has recently been renewed with the development of drugs that are associated with fewer central adverse effects (e.g. sedation, dry mouth) than the older drugs in this class. Moxonidine reduces sympathetic outflow and hence lowers blood pressure through stimulation of central imidazoline receptors. Blood pressure is decreased by 10 to 20% during moxonidine treatment, with about 70% of patients with mild to moderate hypertension achieving a diastolic pressure of < 90mm Hg. The relatively few published comparative studies demonstrate that moxonidine has efficacy comparable with that of clonidine, prazosin, atenolol, nifedipine, captopril and hydrochlorothiazide. It is at least as well tolerated as these agents in trials and, importantly, appears to cause less sedation and dry mouth than clonidine. Compliance may be aided by the once- or twice-daily administration schedule with moxonidine, and dosage adjustment is only necessary in patients with moderate renal impairment. While its published clinical data base needs further expanding, moxonidine thus appears to be a more attractive option than oral clonidine, and may be considered along with the other classes of drug used to treat patients with mild to moderate hypertension.

Central administration of 5-HT activates 5-HT1A receptors to cause sympathoexcitation and 5-HT2/5-HT1C receptors to release vasopressin in anaesthetized rats

1. The effects of intracerebroventricular injections to the right lateral ventricle (i.c.v.) of 5-hydroxytryptamine (5-HT, 40 and 120 nmol kg-1), N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT; 3 nmol kg-1), 5-carboxamidotryptamine (5-CT; 3 nmol kg-1), 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT; 3, 40 and 120 nmol kg-1) and 1-(2,5-di-methoxy-4-iodophenyl)-2-aminopropane (DOI; 40 and 120 nmol kg-1) on renal sympathetic nerve activity, blood pressure, heart rate and phrenic nerve activity were investigated in normotensive rats anaesthetized with alpha-chloralose. 2. 5-HT caused a long lasting pressor response which was associated with an initial bradycardia and renal sympathoinhibition followed by a tachycardia and renal sympathoexcitation. Pretreatment with the 5-HT2/5-HT1C receptor antagonists, cinanserin (300 nmol kg-1, i.c.v.) or LY 53857 (300 nmol kg-1, i.c.v.) reversed the initial bradycardia and sympathoinhibition to tachycardia and sympathoexcitation. Combined pretreatment with LY 53857 (300 nmol kg-1, i.c.v.) and the 5-HT1A antagonist, spiroxatrine (300 nmol kg-1, i.c.v.), blocked the effects of 5-HT on all the above variables. 3. Pretreatment with the vasopressin V1-receptor antagonist, beta-mercapto-beta,beta-cyclopentamethylene-propionyl1, O-Me-Tyr2, Arg8-vasopressin [(d(CH2)5Tyr(Me)AVP, 10 micrograms kg-1, i.v.] did not affect the magnitude but reduced the duration of the pressor response produced by i.c.v. 5-HT and reversed the initial bradycardia and renal sympathoinhibition to tachycardia and sympathoexcitation. 4. 1-(2,5-Di-methoxy-4-iodophenyl)-2-aminopropane (DOI) caused a pressor effect which was associated with a bradycardia and sympathoinhibition. These effects were blocked by pretreatment with BWSOlC67 (0.1 mg kg-', i.v.), a peripherally acting 5-HT2/5-HTc receptor antagonist. However,BWSOlC67 (0.1 mg kg-', i.v.) failed to block the effects of i.c.v. 5-HT.5. DP-5-CT, 5-CT and 8-OH-DPAT (3 nmol kg-', i.c.v.) caused sympathoexcitation, tachycardia and a rise in blood pressure. Pretreatment with methiothepin (1 mg kg-', i.v.) or spiroxatrine (300 nmol kg-',i.c.v.) attenuated the response to i.c.v. DP-5-CT.6. It is concluded that i.c.v. administration of 5-HT activates 5-HTlA receptors to cause sympathoexcitation and 5-HT2 or 5-HT1c receptors to cause the release of vasopressin.

8-OH-DPAT-induced inhibition of renal sympathetic nerve activity and serotonin neuronal firing

The effects of the 5-HT1A receptor 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) administered i.v. on medullary 5-HT neuronal firing and renal sympathetic nerve activity were determined in spontaneously breathing anaesthetized cats. Low doses of 8-OH-DPAT (1-3 micrograms/kg) caused a similar reduction in 5-HT neuronal firing and renal nerve activity while high doses (10-30 micrograms/kg) completely inhibited neuronal firing but caused only 80% inhibition of renal nerve activity. These data are discussed in relationship to the mechanism of sympatholytic action of 8-OH-DPAT and the serotonergic regulation of sympathetic nerve activity.

Changes in the tail surface temperature of the rat following injection of 5-hydroxytryptamine into the ventrolateral medulla

5-Hydroxytryptamine (5-HT) was injected into the rostral ventrolateral medulla (RVLM) in urethane-anaesthetized rats and its effect assessed on thermoregulatory and non-thermoregulatory cutaneous circulations by the measurement of skin surface temperatures. 5-Hydroxytryptamine (5-50 nmol) produced a dose-related fall in blood pressure (5-20 mmHg) and an increase in tail and plantar foot surface temperatures, indicative of dilatation in the underlying cutaneous circulations. If heat was not applied to the animal, the body temperature fell by 1-2 degrees C within 15-25 min. The decrease in tail and foot temperatures, produced by low frequency (25 Hz, 5 min) electrical stimulation, was antagonized by the injection of 5-HT at the site of stimulation. 5-Carboxyamidotryptamine (2.5-20 nmol) and flesinoxan (5-25 nmol) produced responses similar to 5-HT. The 5-HT2 receptor agonist, alpha-methyl 5-hydroxytryptamine (alpha-methyl 5-HT, 5.5-100 nmol) was only effective in increasing tail and plantar foot temperatures, at dose levels above 25 nmol. However, in a few sites restricted to the anterior region of the RVLM, alpha-methyl 5-HT (11 nmol) evoked a small decrease in tail and foot temperatures, indicative of a constrictor effect, without influencing resting cardiovascular parameters. The results are discussed in relation to the central mechanisms which underly the hypothermia and hyperthermia produced by 5-HT1A and 5-HT2 receptor agonists.

Influence of α1-adrenoceptor blockade and/or 5-HT1Aagonism on blood pressure and heart rate at rest and during exercise in hypertensive dogs

1. Antihypertensive effects resulting from alpha 1-adrenoceptor blockade and stimulation of central nervous 5-HT1A receptors were compared with the effects arising from stimulation of 5-HT1A receptors alone during arterial hypertension. 2. Urapidil and 5-methyl-urapidil were less effective in decreasing arterial blood pressure than the lowest dose of the selective 5-HT1A receptor agonist, flesinoxan. After the higher dose of urapidil, a certain dampening of barareceptor reflex was found which was also seen with flesinoxan. 3. Flesinoxan was the only drug which did not reduce the exercise-induced increase in systolic arterial blood pressure. 4. Stimulation of 5-HT1A receptors alone, which is assumed to occur with flesinoxan, exerted antihypertensive activity only at low doses, without inducing reflex tachycardia at rest. 5. Only the combined effects of alpha 1-adrenoceptor blockade and 5-HT1A receptor stimulation, as assumed to occur with urapidil and 5-methyl-urapidil, lead to both a decrease in arterial blood pressure at rest and during exercise.

Investigation of the effects of IVth ventricular administration of the 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), on autonomic outflow in the anaesthetized cat

1. The effects of IVth ventricular injections of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on renal, splanchnic and cardiac sympathetic nerve activities, phrenic nerve activity, arterial blood pressure, heart rate, femoral arterial conductance, tracheal and intragastric pressures were investigated in alpha-chloralose anaesthetized, neuromuscular blocked and artificially ventilated cats. 2. Cumulative doses of DOI (80, 160 and 320 nmol kg-1) injected into the IVth ventricle caused an increase in mean arterial blood pressure, a fall in femoral arterial conductance, an increase in tracheal pressure and a decrease in the rate of phrenic nerve bursts but did not affect any of the other variables recorded. 3. Even after i.v. administration of the peripheral 5-HT2 antagonist BW501C67 (2 mg kg-1) following the highest dose of DOI there was still a significant pressor response, a fall in femoral arterial conductance and small increase in tracheal pressure. 4. In control experiments, intravenous infusion of noradrenaline to raise blood pressure to the levels obtained during the cumulative doses of DOI caused large falls in renal, splanchnic and cardiac nerve activities which were all significantly lower than those recorded during the cumulative doses of DOI. 5. The results of this study provide evidence for a brainstem site of action of DOI in producing hypertension and further support the hypothesis that central 5-HT2 receptors are involved in the control of skeletal muscle and skin vascular beds.

BMY 7378 is an agonist at 5-HT1A receptors mediating hypotension and renal sympatho-inhibition in anaesthetised cats

The putative 5-HT1A receptor antagonist BMY 7378 (3-100 micrograms.kg-1 i.v.) caused reductions in blood pressure, heart rate and efferent renal nerve activity in anaesthetised cats. Similar effects were produced by the selective 5-HT1A receptor agonist, 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT 1-10 micrograms.kg-1 i.v.). The sympatho-inhibitory effects of BMY 7378 and 8-OH-DPAT, but not those of clonidine were reversed by the non-selective 5-HT1A receptor antagonist, spipirone (1 mg.kg-1 i.v.). It is concluded that BMY 7378 is an agonist at 5-HT1A receptors mediating hypotension and renal sympatho-inhibition in anaesthetised cats.

The mechanism of the sympathoinhibitory action of urapidil: role of 5-HT1A receptors

1. An investigation was carried out to determine if the sympathoinhibition caused by urapidil is due to activation of 5-HT1A receptors by investigating whether it could be reversed by the non-selective 5-HT1A receptor antagonist spiperone. To control for the possibility of functional antagonism by spiperone, the ability of spiperone to reverse the sympathoinhibition caused by clonidine was also investigated. These experiments were carried out in anaesthetized prazosin-pretreated cats to prevent the alpha 1-adrenoceptor antagonist action of urapidil and spiperone from masking any effects observed. 2. Cats were anaesthetized with alpha-chloralose and simultaneous recordings were made of whole cardiac, splanchnic and renal nerve activities, blood pressure, heart rate and femoral arterial flow (from which conductance was derived). All animals were initially pretreated with prazosin (1 mg kg-1, i.v.) given in divided doses (0.75 followed 10 min later by 0.25 mg kg-1), then either urapidil (0.75 mg kg-1, i.v.) or clonidine (10 micrograms kg-1, i.v. in two divided doses) followed by 3 separate injections of spiperone (1 mg kg-1, i.v.). In another set of experiments urapidil was given followed by injections of the appropriate vehicle for spiperone, while in another set urapidil was replaced with an injection of the appropriate vehicle followed by injections of spiperone. In the experiments with clonidine, the alpha 2-adrenoceptor antagonist Wy 26392 (0.3 mg kg-1) was given after the last injection of spiperone. 3. The prazosin pretreatment caused a fall in blood pressure associated with femoral vasodilatation, a small bradycardia and little change in cardiac, splanchnic or renal nerve activities. Urapidil or clonidine injection after prazosin caused sympathoinhibition associated with an additional bradycardia. However, only urapidil caused an additional fall in blood pressure. Spiperone injections reversed the sympathoinhibition caused by urapidil but not that caused by clonidine. The sympathoinhibition caused by clonidine was reversed by the alpha 2-adrenoceptor antagonist Wy 26392. 4. These results show that the sympathoinhibition caused by urapidil in prazosin-pretreated cats can be reversed by spiperone. The reversal of this sympathoinhibition is not due to functional antagonism. It is concluded that urapidil can cause sympathoinhibition by activation of 5-HTlA receptors.

Actions of 8-OH-DPAT on sympathetic and respiratory drives, blood pressure and heart rate in the rabbit

The effects of i.v. 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on renal and cardiac sympathetic nerve activity, phrenic nerve activity, blood pressure and heart rate were investigated in urethane and pentobarbitone anaesthetised rabbits. Cumulative doses of 8-OH-DPAT (0.5-512 micrograms kg-1) in urethane anaesthetised rabbits, in which simultaneous recordings of renal and phrenic nerve activity were made, caused dose related decreases in mean blood pressure which were maximal after 32 micrograms kg-1 and dose related decreases in heart rate. Effects on renal and phrenic nerve activity were variable. A single dose of 8-OH-DPAT (32 micrograms kg-1) in either urethane or pentobarbitone anaesthetised rabbits, in which simultaneous recordings of cardiac as well as renal and phrenic nerve activity were made, caused significant falls in blood pressure, heart rate and cardiac nerve activity. Renal nerve activity showed no significant change whilst phrenic nerve activity tended to increase but this was only significant in the pentobarbitone anaesthetised rabbits. In all experiments the bradycardia produced by 8-OH-DPAT was unaltered by atropine. These results show that in the anaesthetised rabbit the hypotension and bradycardia caused by 8-OH-DPAT were associated with a decrease in cardiac nerve activity, increased central respiratory drive, and no change in renal nerve activity or vagal tone.