An analysis of the alpha-adrenoceptor modulation of vasomotor tone at the level of lateral medullary pressor area (LMPA)

Imidazoline antihypertensive drugs: a critical review on their mechanism of action

It was long thought that the prototypical centrally acting antihypertensive drug clonidine lowers sympathetic tone by activating alpha(2)-adrenoceptors in the brain stem. Supported by the development of two new centrally acting drugs, rilmenidine and moxonidine, the imidazoline hypothesis evolved recently. It assumes the existence of a new group of receptors, the imidazoline receptors, and attributes the sympathoinhibition to activation of I(1) imidazoline receptors in the medulla oblongata. This review analyzes the mechanism of action of clonidine-like drugs, with special attention given to the imidazoline hypothesis. Two conclusions are drawn. The first is that the arguments against the imidazoline hypothesis outweigh the observations that support it and that the sympathoinhibitory effects of clonidine-like drugs are best explained by activation of alpha(2)-adrenoceptors. The second conclusion is that this class of drugs lowers sympathetic tone not only by a primary action in cardiovascular regulatory centres in the medulla oblongata. Peripheral presynaptic inhibition of transmitter release from postganglionic sympathetic neurons contributes to the overall sympathoinhibition.

Ethanol selectively counteracts hypotension evoked by central I(1)-imidazoline but not alpha2-adrenergic receptor activation in spontaneously hypertensive rats

Previous studies from our laboratory showed that ethanol counteracts hypotensive responses to clonidine in spontaneously hypertensive rats. This study investigated whether this effect of ethanol involves interaction with central alpha2-adrenoceptors or I(1)-imidazoline receptors or both. The effects of ethanol (0.5 or 1 g/kg, i.v.) or an equal volume of saline on hypotensive and bradycardic responses to clonidine (mixed alpha2-adrenoceptor/I(1)-imidazoline receptor agonist), rilmenidine (selective I(1)-imidazoline receptor agonist), or alpha-methylnorepinephrine (selective alpha2-adrenoceptor agonist) were studied in conscious spontaneously hypertensive rats. Intracisternal administration of clonidine (0.5 microg), rilmenidine (25 microg), or alpha-methylnorepinephrine (4 microg) elicited similar decreases in mean arterial pressure (MAP; 25-30 mm Hg) that lasted > or =60 min. Subsequent administration of ethanol (0.5 and 1 g/kg, i.v.) counteracted the hypotensive effect of clonidine in a dose-related manner. Ethanol (1 g/kg) increased the blood pressure to levels similar to baseline (preclonidine) levels, and blood pressure remained significantly (p < 0.05) higher compared with the corresponding values in saline-treated rats. Similarly, ethanol (0.5 and 1 g/kg, i.v.) dose-dependently counteracted the hypotensive effect of rilmenidine. The antagonizing effects of ethanol on hypotension evoked by clonidine and rilmenidine were comparable in terms of both magnitude and duration. In contrast, ethanol (0.5 or 1 g/kg) had no effect on hypotension evoked by alpha-methylnorepinephrine. Except for a brief increase in blood pressure by ethanol (1 g/kg) at 5 min, blood pressure values obtained in alpha-methylnorepinephrine-treated rats receiving any of the two doses of ethanol were similar to postsaline values. Ethanol had no effect on bradycardic responses to any of the three hypotensive agents. Blood ethanol concentrations were similar regardless of the antihypertensive drug used. We concluded that the adverse hemodynamic effect of ethanol on centrally mediated hypotensive responses depends on the types of receptors involved in the elicitation of this response. That ethanol counteracts decreases in blood pressure evoked by clonidine and rilmenidine but not by alpha-methylnorepinephrine suggests an interaction between ethanol and central pathways involved in I(1)-imidazoline receptor-mediated hypotension.

Aortic barodenervation up-regulates alpha2-adrenoceptors in the nucleus tractus solitarius and rostral ventrolateral medulla: an autoradiographic study

Earlier findings have shown that alpha2-adrenoceptors in the nucleus tractus solitarius and rostral ventrolateral medulla modulate baroreflexes. The present study investigated whether attenuation of baroreflexes induced by surgical interruption of aortic baroafferents is related to an alteration of alpha2-adrenoceptor binding in these regions of the brainstem. In vitro autoradiography was utilized to assess the density and binding dissociation constant (affinity) of alpha2-adrenoceptors in the rostral ventrolateral medulla and in the middle and rostral portions of the nucleus tractus solitarius of aortic-barodenervated and sham-operated rats. Compared to sham operation, aortic barodenervation caused an acute rise in mean arterial pressure and heart rate and a significant reduction in baroreflex sensitivity. Two days later, mean arterial pressure and heart rate of conscious aortic-barodenervated rats subsided to sham-operated levels, whereas the baroreflex sensitivity remained significantly (P<0.05) reduced when measured by phenylephrine (0.55+/-0.08 vs 1.26+/-0.07 ms/mmHg) or nitroprusside (0.43+/-0.06 vs 1.01+/-0.09ms/mmHg). Examination of brainstem coronal sections obtained from separate groups of rats 48 h after surgery and preincubated with [3H]rauwolscine (0.5-16 nM) revealed that labeling of alpha2 binding sites was saturable and of high affinity. Scatchard analysis of the saturation isotherms obtained from the three brain areas of sham-operated rats showed an uneven distribution of alpha2 binding sites; the rostral nucleus tractus solitarius exhibited the highest density and lowest affinity. Aortic barodenervation caused region-dependent changes in the binding activity of alpha2-adrenoceptors. These changes comprised significant (P<0.05) increases in the density of alpha2-adrenoceptors in the middle nucleus tractus solitarius (436+/-60 vs 240+/-50 fmol/mg protein) and rostral ventrolateral medulla (350+/-67 vs 194+/-35 fmol/mg protein) compared with sham-operated rats; no significant changes occurred in the rostral nucleus tractus solitarius. The affinity of alpha2 binding sites was not changed by aortic barodenervation in any of the three brain regions. These findings suggest that attenuation of baroreflexes produced by aortic barodenervation coincides with up-regulation of alpha2-adrenoceptors in brainstem areas that play critical roles in the control of cardiovascular functions.

The role of the peripheral sympathetic nervous system in the natriuresis following central administration of an I1 imidazoline agonist, moxonidine

1. Central administration of the I1-imidazoline receptor agonist moxonidine increases sodium excretion without alteration of blood pressure. In the present study we determined whether this natriuretic action was mediated through a decrease in activity of the sympathetic nervous system, as has been reported for the antihypertensive action of this compound. Interruption of the sympathetic nervous system was achieved with prazosin (alpha 1-adrenoceptor antagonist) and renal denervation. 2. In pentobarbitone-anaesthetized Sprague-Dawley rats, intracerebroventricular (i.c.v.) injection of moxonidine alone increased urine volume and sodium excretion. Prazosin (0.15 mg kg-1, i.v.) alone decreased urine flow rate and sodium excretion as compared to the vehicle controls. In the presence of prazosin, i.c.v. injection of moxonidine failed to increase sodium excretion or urine volume as compared to animals which received the prazosin alone. 3. The administration of moxonide (i.c.v.) to sham renal-denervated animals caused an increase in urine flow rate, urine sodium excretion, osmolar clearance and free water clearance. The increase in sodium excretion and osmolar clearance were completely attenuated in renal denervated rats, however, urine flow rate was still increased and this was secondary to the increase in free water clearance which remained intact. 4. These results indicate the importance of an intact sympathetic nervous system in the renal response to i.c.v. moxonidine. Moreover, the differential antagonism of these interventions on solute and water excretion indicate that they may be mediated at two separate sites and/or receptors following i.c.v. moxonidine.

Sodium excretion following central administration of an I1 imidazoline receptor agonist, moxonidine

1. Previously we have shown that an intrarenal infusion of moxonidine, an I1-imidazoline receptor agonist, resulted in a natriuresis which was inhibited by intravenous idazoxan, a selective imidazoline receptor antagonist. Therefore we examined the effects of renal function of intracerebroventricular (i.c.v.) administration of moxonidine with or without i.c.v. idazoxan. 2. Seven days after unilateral nephrectomy, Sprague-Dawley rats had i.c.v. cannulae implanted. Three days later the rats were anaesthetized (pentobarbitone), followed by cannulation of the jugular vein (fluid and drug administration), carotid artery (blood pressure) and the ureter (urine collection). 3. After a 45 min stabilization period, the effect of moxonidine was investigated by the i.c.v. administration of either isotonic saline or moxonidine (0.1, 0.3 or 1 nmol in isotonic saline) administered in 5 microliters over 1 min. All doses of moxonidine resulted in an increase in urine flow with a concomitant increase in sodium excretion without affecting blood pressure. The highest dose of moxonidine (1 nmol) also increased free water clearance. 4. In a second series of experiments, the effects of idazoxan on the natriuretic response to i.c.v. moxonidine were determined. Moxonidine (0.3 nmol) again increased sodium and water excretion as compared to the i.c.v. saline control animals. Pretreatment with i.c.v. idazoxan (0.3 nmol), at a dose which alone failed to alter sodium and water excretion, completely attenuated the renal response to moxonidine. These results are consistent with central I1-imidazoline receptors mediating a moxonidine-induced increase in sodium and water excretion at doses that do not alter blood pressure.

Suppression of desynchronized sleep through microinjection of the alpha 2-adrenergic agonist clonidine in the dorsal pontine tegmentum of the cat

The relationships between sleep-waking states and the activity of the noradrenergic system are controversial. In particular, according to an influential model of desynchronized sleep (DS) generation, the arrest of firing of noradrenergic neurons in the locus coeruleus should enhance DS, due to the release from inhibition of executive neurons located in the nearby pontine tegmentum. Since locus coeruleus neurons are strongly inhibited by alpha 2-adrenergic agonists like clonidine, this agent would be expected to increase DS. Yet clonidine powerfully decreases DS when injected systemically in several species. In this study, clonidine was microinjected locally into the dorsal pontine tegmentum of the cat, a region which comprises anatomically the whole locus coeruleus complex and which plays a key role in the generation of DS. In accord with the results of systemic experiments, bilateral injections of clonidine almost suppressed DS and unilateral injections consistently reduced it. The effects were dose dependent and site specific. It is suggested that clonidine may suppress DS by acting additionally on non-noradrenergic cell groups located in the dorsal pontine tegmentum.

Evidence for clonidine presynaptically modulating amino acid release in the rostral ventral medulla: role in hypertension

Reports suggested that the predominant site of action for the antihypertensive effects of clonidine is the rostral ventrolateral medulla (RVL), the presumed tonic vasomotor center. This study examined whether clonidine directly interacts with nerve terminal alpha 2-adrenergic receptors in the RVL to inhibit the release of sympathoexcitatory transmitters like glutamate (Glu) and aspartate (Asp), and/or facilitate the release of sympathoinhibitory transmitters like gamma-aminobutyric acid (GABA). Release of GABA and Glu was measured from synaptosomes prepared from the rostral ventral medulla of spontaneously hypertensive rats (SHR), a genetic model of hypertension, and normotensive Wistar-Kyoto rats (WKY). Quantification of neurotransmitter release was performed by high-performance liquid chromatography. Depolarization with 35 mM K+ significantly increased by 58-110% the release of GABA, Glu and Asp; however, no strain differences were observed. In contrast, spontaneous release of GABA and Asp was significantly lower in SHR than that of WKY (-36 and -41%, respectively); this effect was not observed for Glu. Clonidine (1 and 10 microM) enhanced the spontaneous release of GABA (+44%), Asp (+50%) and Glu (+70%) in SHR, but not WKY; this effect was prevented by yohimbine (1 microM). These data, together with previous findings, support the presence of facilitory alpha 2-adrenergic receptors on nerve terminals of GABAergic, glutamatergic and aspartatergic neurons in the rostral ventral medulla. These findings also suggest the existence of another inhibitory transmitter that may mediate the actions of clonidine to decrease sympathetic outflow from the RVL.

Role of neurotransmitters in the central regulation of the cardiovascular system

The last decade has seen tremendous progress in determining the nature of the neurotransmitters which regulate central nervous system pathways involved in the regulation of blood pressure. Investigations are now pursuing the identity and functional importance of neurotransmitters contained within pathways shown to be important in cardiovascular regulation. In addition, several key components of the brain stem networks involved in the control of sympathetic activity have been identified. For example, numerous studies indicate the importance of neurons located in the rostral ventrolateral medulla in the regulation of SPN. Indeed, this area contains medullospinal sympathoexcitatory neurons which represent the final site of integration of many brain stem and reflex pathways involved in the regulation of sympathetic nerve activity. The neurotransmitter which is utilized by this medullospinal pathway remains unknown. Epinephrine, substance P and glutamate have all been hypothesized as primary chemical mediators in the descending pathway from the brain stem to SPN. Interestingly, lesions of, or antagonists to, epinephrine, substance P, glutamate and 5-HT neurons all abolish sympathetic activity and reduce blood pressure to a level similar to that in a spinal animal. Clearly, not all these transmitters are primary mediators of sympathetic information carried from the brain stem to the spinal cord. It is likely that monoamines and neuropeptides act in the IML, as in other area of the central nervous system, as neuromodulators to set the level of excitability of SPN rather than relaying sympathetic information over a functionally specific medullospinal pathway. This conclusion is supported by the observation that midline medullary 5-HT neurons provide a tonic excitatory input to SPN, but receive no afferent inputs from other central sympathetic or baroreceptor pathways. However, the firing of 5-HT neurons appears to relate to the state of vigilance of the animal. This suggests that 5-HT neurons may lower the threshold of SPN to sympathetic inputs during states of wakefulness. In addition, the time course of the norepinephrine-mediated slow EPSPs and IPSPs in SPN is consistent with a gain-setting function. By analogy, epinephrine is likely to act as a neuromodulator in the IML rather than to serve as the primary mediator of sympathetic information descending from the rostral ventrolateral medulla.(ABSTRACT TRUNCATED AT 400 WORDS)

Prazosin produces a sustained and reflex-mediated increase in renal sympathetic nerve activity in anesthetized dogs

Prazosin (1 mg/kg i.v.) produced a decrease in blood pressure associated with an increase in renal sympathetic nerve activity in anesthetized dogs. The sympathetic baroreflex curve was shifted to the left. Prazosin (10 micrograms/kg into the vertebral artery) did not change the blood pressure but increased renal sympathetic discharges. The baroreflex curve was not altered. Prazosin (100 micrograms/kg into the vertebral artery) induced a decrease in blood pressure and an increase in sympathetic discharges. Prazosin (1 mg/kg i.v. or 100 micrograms/kg i.c.) induced a fall in blood pressure without any change in sympathetic nerve activity in barodenervated dogs. Restoration of the resting blood pressure by angiotensin II infusion (10-20 ng/kg per min) restored the baroreflex curve in anesthetized dogs given prazosin (1 mg/kg i.v.) to close to the initial position. Prazosin, (1 mg/kg i.v.) did not change the sympathoinhibitory effect of clonidine (injected into the vertebral artery) and the reversal effect of piperoxan in barodenervated dogs. In conclusion, prazosin reduces blood pressure by blockade of peripheral alpha 1-adrenoceptors. The shift to the left of the sympathetic baroreflex curve is due to the hypotensive effect of prazosin. No evidence was found for a central sympathoinhibitory effect of prazosin.

Involvement of central alpha- and beta-adrenoceptors in the pressor response to electrical stimulation of the rostral ventrolateral medulla in rats

1. Electrical stimulation of the C1 area of the rostral ventrolateral medulla in rats elicits an increase in mean arterial pressure (MAP). 2. This increase in MAP is attenuated by intra-hypothalamic and intracisternal administration of the non-selective beta-adrenoceptor antagonist (+/-)-propranolol and the beta 2-selective antagonist ICI 118551. 3. The selective beta 1-antagonist atenolol and (+)-propranolol, which is inactive on beta-adrenoceptors, did not alter the increase in MAP produced by stimulation of the C1 area. 4. The alpha 2-adrenoceptor antagonist idazoxan enhanced the effects of C1 stimulation on MAP when administered either into the posterior hypothalamus or intracisternally. 5. The results indicate that beta 2- and alpha 2-receptors both have a role in the mediation of the rise in MAP during stimulation of the C1 area and that the receptors involved are located both within the hypothalamus and the spinal cord.

Evidence that locus coeruleus is the site where clonidine and drugs acting at alpha 1- and alpha 2-adrenoceptors affect sleep and arousal mechanisms

The behavioural and electrocortical (ECoG) effects of clonidine were studied after microinjection into the third cerebral ventricle, or microinfusion into some specific areas of the rat brain rich in noradrenaline-containing cell bodies (locus coeruleus) or into areas receiving noradrenergic terminals (dorsal hippocampus, amygdaloid complex, thalamus, frontal and sensimotor cortex). The ECoG effects were continuously analysed and quantified by means of a Berg-Fourier analyser as total power and as power in preselected bands of frequency. Clonidine (9.4 to 75 nmol) given into the third cerebral ventricle produced behavioural sedation and sleep and a dose-dependent increase in ECoG total voltage power as well as in the lower frequency bands. Much lower doses were required to produce similar behavioural and ECoG spectrum power effects after either unilateral or bilateral microinfusion of clonidine into the locus coeruleus. Doses of clonidine equimolar to those given into the third cerebral ventricle, were almost ineffective in inducing behavioural and ECoG sleep after their microinfusion into the dorsal hippocampus. In addition, a dose (0.56 nmol) of clonidine which, given into the locus coeruleus, produced marked behavioural sleep and ECoG synchronization, lacked effects when given into the ventral or anterior thalamus, into the amygdaloid complex or onto the frontal and sensimotor cortex. The behavioural and ECoG spectrum power effects of clonidine given into the third cerebral ventricle or into the locus coeruleus were prevented by antagonists of alpha 2-adrenoceptors but not by alpha 1-adrenoceptor antagonists. Intraventricular microinjection, or microinfusion into the locus coeruleus, of yohimbine, a selective alpha 2-adrenoceptor antagonist, produced behavioural arousal, increase in locomotor and exploratory activity, tachypnoea and ECoG desynchronization with a significant reduction in total voltage power. Similar stimulatory effects were also observed after microinjection of phentolamine into the same sites. No significant effects on behaviour and ECoG activity were evoked after intraventricular injection or microinfusion into the locus coeruleus of prazosin or methoxamine.

The blood pressure effects of alpha-adrenoceptor antagonists injected in the medullary site of action of clonidine: the nucleus reticularis lateralis

We administered a series of alpha-blocking drugs to the nucleus reticularis lateralis (NRL) of the medulla oblongata, the main site for the hypotensive action of clonidine. These experiments were performed on pentobarbital anaesthetized cats. Drugs were injected through a needle which was stereotaxically inserted. Prazosin (6 nmol) was hypertensive (MBP = +25 +/- 8%), corynanthine had no effect and AR-C239 (7 nmol), another alpha 1-blocker, was hypotensive (MBP = -16 +/- 3.5%). The alpha 2-blockers, yohimbine and idaxozan, were hypotensive. The blood pressure effects of alpha-blocking drugs directly microinjected in the nucleus reticularis lateralis cannot be simply related to their selectivity for a particular subtype of alpha-receptors.

An endogenous clonidine-displacing substance from bovine brain: receptor binding and hypotensive actions in the ventrolateral medulla

A substance has been isolated from bovine brain which displaces 3H-clonidine binding to rat brain membranes (clonidine-displacing substance; CDS). To determine whether CDS is similar to the antihypertensive agent clonidine, the in vitro binding properties of partially-purified CDS and its physiological action in the rostral ventrolateral medulla were examined. Like clonidine, CDS potently inhibited 3H-para-aminoclonidine binding to receptors in bovine ventrolateral medulla membranes (clonidine, IC50 = 24 +/- 8nM; CDS, IC50 = 0.30 +/- .10 Units), with highest affinity for non-adrenergic sites (clonidine, IC50 = 6 +/- 1nM; CDS, IC50 = 0.12 +/- .07 Units). CDS had no effect at beta-adrenergic or muscarinic cholinergic receptors. Like clonidine, CDS elicited a potent, reversible (less than 10 min) dose-dependent fall in arterial pressure (AP) and heart rate when microinjected specifically into the C1 area of the rostral ventrolateral medulla in the rat (maximum delta AP, -65 +/- 7 mm Hg). CDS represents an as-yet-uncharacterized endogenous, physiologically-active agent in brain which may participate in cardiovascular control via non-adrenergic receptors in the rostral ventrolateral medulla.