Cardiovascular effects of centrally administered endothelin-1 and its relationship to changes in cerebral blood flow

Sovateltide Mediated Endothelin B Receptors Agonism and Curbing Neurological Disorders

Neurological/neurovascular disorders constitute the leading cause of disability and the second leading cause of death globally. Major neurological/neurovascular disorders or diseases include cerebral stroke, Alzheimer’s disease, spinal cord injury, neonatal hypoxic-ischemic encephalopathy, and others. Their pathophysiology is considered highly complex and is the main obstacle in developing any drugs for these diseases. In this review, we have described the endothelin system, its involvement in neurovascular disorders, the importance of endothelin B receptors (ETBRs) as a novel potential drug target, and its agonism by IRL-1620 (INN—sovateltide), which we are developing as a drug candidate for treating the above-mentioned neurological disorders/diseases. In addition, we have highlighted the results of our preclinical and clinical studies related to these diseases. The phase I safety and tolerability study of sovateltide has shown it as a safe and tolerable compound at therapeutic dosages. Furthermore, preclinical and clinical phase II studies have demonstrated the efficacy of sovateltide in treating acute ischemic stroke. It is under development as a first-in-class drug. In addition, efficacy studies in Alzheimer’s disease (AD), acute spinal cord injury, and neonatal hypoxic-ischemic encephalopathy (HIE) are ongoing. Successful completion of these studies will validate that ETBRs signaling can be an important target in developing drugs to treat neurological/neurovascular diseases.

Novel Approaches for the Treatment of Alzheimer's and Parkinson's Disease

Neurodegenerative disorders affect around one billion people worldwide. They can arise from a combination of genomic, epigenomic, metabolic, and environmental factors. Aging is the leading risk factor for most chronic illnesses of old age, including Alzheimer’s and Parkinson’s diseases. A progressive neurodegenerative process and neuroinflammation occur, and no current therapies can prevent, slow, or halt disease progression. To date, no novel disease-modifying therapies have been shown to provide significant benefit for patients who suffer from these devastating disorders. Therefore, early diagnosis and the discovery of new targets and novel therapies are of upmost importance. Neurodegenerative diseases, like in other age-related disorders, the progression of pathology begins many years before the onset of symptoms. Many efforts in this field have led to the conclusion that exits some similar events among these diseases that can explain why the aging brain is so vulnerable to suffer neurodegenerative diseases. This article reviews the current knowledge about these diseases by summarizing the most common features of major neurodegenerative disorders, their causes and consequences, and the proposed novel therapeutic approaches.

A novel neuroregenerative approach using ET(B) receptor agonist, IRL-1620, to treat CNS disorders

Endothelin B (ET(B)) receptors present in abundance the central nervous system (CNS) have been shown to have significant implications in its development and neurogenesis. We have targeted ET(B) receptors stimulation using a highly specific agonist, IRL-1620, to treat CNS disorders. In a rat model of cerebral ischemia intravenous administration IRL-1620 significantly reduced infarct volume and improved neurological and motor functions compared to control. This improvement, in part, is due to an increase in neuroregeneration. We also investigated the role of IRL-1620 in animal models of Alzheimer's disease (AD). IRL-1620 improved learning and memory, reduced oxidative stress and increased VEGF and NGF in Abeta treated rats. IRL-1620 also improved learning and memory in an aged APP/PS1 transgenic mouse model of AD. These promising findings prompted us to initiate human studies. Successful chemistry, manufacturing and control along with mice, rat and dog toxicological studies led to completion of a human Phase I study in healthy volunteers. We found that a dose of 0.6 microg/kg of IRL-1620 can be safely administered, three times every four hours, without any adverse effect. A Phase II clinical study with IRL-1620 has been initiated in patients with cerebral ischemia and mild to moderate AD.

Stimulation of endothelin B receptors by IRL-1620 decreases the progression of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by severe cognitive impairment that ultimately leads to death. Endothelin (ET) and its receptors have been considered as therapeutic targets for AD. Recent studies in our lab have shown that stimulation of ETB receptors provide significant neuroprotection following Aβ1-40 administration. It is possible that IRL-1620 may be neuroprotective due to angiogenesis. However, the effect of IRL-1620 on neurovascular remodeling following Aβ1-40 administration has not been established. The purpose of this study was to determine the effect of stimulation of ETB receptors by IRL-1620 on vascular and neuronal growth factors after Aβ1-40 administration. Rats were treated with Aβ1-40 (day 1, 7 and 14) in the lateral cerebral ventricles using stereotaxically implanted cannula and received three intravenous injections of IRL-1620 (an ETB agonist), and/or BQ788 (an ETB antagonist) at 2-h interval on day 8; experiments were performed on day 15. Rats were sacrificed for estimation of brain ETB receptors, vascular endothelial growth factor (VEGF) and nerve growth factor (NGF) expression using immunofluorescence and Western blot. In the Morris swim task, amyloid-β (Aβ)-treated rats showed a significant (p<0.0001) impairment in spatial memory. Rats treated with IRL-1620 significantly (p<0.001) reduced the cognitive impairment induced by Aβ. BQ788 treatment completely blocked IRL-1620-induced improvement in cognitive impairment. IRL-1620 treatment enhanced the number of blood vessels labeled with VEGF compared to vehicle treatment. Additionally, cells showed increased (p<0.001) positive staining for NGF in IRL-1620-treated animals. ETB, VEGF and NGF protein expression significantly (p<0.001) increased in the brain of IRL-1620-treated rats as compared to vehicle. Pretreatment with BQ788 blocked the effects of IRL-1620, thus confirming the role of ETB receptors in the neurovascular remodeling actions of IRL-1620. Results of the present study demonstrate that IRL-1620 improves both acquisition (learning) and retention (memory) on the water maze task and enhances angiogenic and neurogenic remodeling. These findings indicate that the ETB receptor may be a novel therapeutic target for AD and other neurovascular degenerative disorders.

Cardiovascular effects of endothelin-1₁₋₃₁ microinjected into the nucleus tractus solitarius of anesthetized rats

Endothelin-1₁₋₃₁ (ET-1₁₋₃₁) is a 31-amino-acid vasoactive peptide that plays an important role in the regulation of cardiovascular function. However, the cardiovascular effects of central ET-1₁₋₃₁ are still not fully understood. In this study, we assess the effects of ET-1₁₋₃₁ within the nucleus tractus solitarius (NTS) of anesthetized rats and explore the underlying mechanisms of these effects. Bilateral microinjections of ET-1₁₋₃₁ into the NTS produced dose-dependent hypotension and bradycardia, very similar to the effects of a unilateral microinjection of ET-1₁₋₃₁ into the NTS. Bilateral microinjections of ET-1₁₋₃₁ into the NTS significantly decreased baroreflex function in a time-dependent manner. The hypotensive and bradycardic effects induced by the microinjection of ET-1₁₋₃₁ into the NTS were significantly decreased by the ETA receptor antagonist BQ123 and by kynurenic acid, but not by the ETB receptor antagonist BQ788. These results show that ET-1₁₋₃₁ injected into the NTS produces hypotension and bradycardia, mediated by ETA receptors and, at least partly, by the glutamate receptor.

Cardiovascular effects of centrally applied endothelin-11–31 and its relationship to endothelin-11–21 in rats

Endothelin-1(1-31) (ET-1(1-31)) is a novel member of the endothelin family, which comprises 31 amino acids and derived from the selective hydrolysis of big ET-1 by chymase. Although ET-1(1-31) has been reported to be involved in biological effects via direct or indirect (converting to ET-1(1-21)) mechanisms, the cardiovascular effects of central ET-1(1-31) are not fully identified. The present study was designed to comparatively investigate the cardiovascular effects of intracerebroventricular (icv) application of ET-1(1-31) or ET-1(1-21) in anesthetized rats. Injection (icv) of ET-1(1-31) (500 pmol) produced a biphasic blood pressure response: an initial increase (from 118+/-8 to 138+/-14 mmHg, P<0.05) followed by a sustained decrease in BP (from 118+/-8 to 58+/-9 mmHg, P<0.05), which was very similar to BP response to icv injection of big ET-1 (500 pmol) or ET-1(1-21) (25 pmol)(.) The cardiovascular effects of icv injection of ET-1(1-31) or ET-1(1-21) were completely antagonized by ET(A) receptor antagonist BQ123 but not ET(B) receptor antagonist BQ788. Furthermore, pretreatment with ET converting enzyme inhibitor phosphoramidon (10 nmol) abolished the cardiovascular effects evoked by icv injection of ET-1(1-31) or big ET-1. In conclusion, the current data showed that central ET-1(1-31) produced the similar cardiovascular effects as those of central ET-1(1-21), and suggesting that the central cardiovascular effects of ET-1(1-31) resulted from it converting to ET-1(1-21) and then activating ET(A) receptors.

Vagally mediated cholestatic and choleretic effects of centrally applied Endothelin-1 through ETA receptors

The role of Endothelin-1 (ET-1) in the central nervous system is not fully understood yet although several studies strongly support its neuromodulatory role. A high density of endothelin receptors is present in the dorsal vagal complex that is the major site for the regulation of the digestive function. Therefore in the present study we sought to establish the role of ET-1 in the central regulation of bile secretion in the rat. Intracerebroventricular ET-1 injection exhibited opposite behaviors on spontaneous bile secretion according to the dose administered. Lower doses of ET-1 (1 fM) increased bile flow and bicarbonate excretion whereas higher doses (1 nM) decreased bile flow and bile acid output. Both the choleretic and the cholestatic effects of ET-1 were abolished in animals pretreated with icv BQ-610 (selective ETA antagonist) but not with BQ-788 (selective ETB antagonist). In addition, truncal vagotomy but not adrenergic blockade abolished ET-1 effects on bile secretion. Brain nitric oxide was not involved in ET-1 response since L-NAME pretreatment failed to affect ET-1 actions on the liver. Portal venous pressure was increased by centrally administered ET-1 being the magnitude of the increase similar with low and high doses of the peptide. These results show that centrally applied ET-1 modified different bile flow fractions independent of hemodynamic changes. Lower doses of ET-1 increased bile acid independent flow whereas higher doses decreased bile acid dependent flow. Vagal pathways through the activation of apparently distinct ETA receptors mediated the cholestatic as well as the choleretic effects induced by ET-1. Present findings show that ET-1 participates in the central regulation of bile secretion in the rat and give further insights into the complexity of brain-liver interaction.

Blood pressure responses of endothelin-1 1-31 within the rostral ventrolateral medulla through conversion to endothelin-1 1-21

Endothelin-1 1-31 (ET-1 1-31), a novel member of the endothelin family comprising 31 amino acids and derived from the selective hydrolysis of big ET-1 by chymase, directly activates endothelin receptors or converts to ET-1 1-21 by ET converting enzyme (ECE). The cardiovascular effects of central ET-1 1-31 are not identified. The present study was designed to investigate the cardiovascular actions of ET-1 1-31 within the rostral ventrolateral medulla (RVLM) in anesthetized rats. Bilateral injection of ET-1 1-31 (0.5, 1, and 2 pmol for each side) into the rostral ventrolateral medulla produced an initial pressor and/or a long-lasting hypotensive action but did not affect HR. Unilateral microinjection of 2 and 4 pmol of ET-1 1-31 into the rostral ventrolateral medulla only produced a significant (P < 0.05) transient increase in blood pressure by an average of 13 and 12 mm Hg, respectively, whereas unilateral microinjection of 8 pmol of ET-1 1-31 produced a sustained fall in blood pressure (from 92 +/- 6 to 69 +/- 8 mm Hg, P < 0.05). The transient pressor effect of unilaterally injecting ET-1 1-31 (4 pmol) into the rostral ventrolateral medulla was completely abolished by pretreatment with either ETA receptor antagonist BQ123 (83 +/- 2 versus 84 +/- 5 mm Hg, P > 0.05) or ET converting enzyme inhibitor phosphoramidon (99 +/- 5 versus 99 +/- 7 mm Hg, P > 0.05) but not ETB receptor antagonist IRL1038 (89 +/- 6 versus 96 +/- 7 mm Hg, P < 0.05). In addition, prior injection of phosphoramidon also completely abolished the long-lasting hypotension of intra-RVLM ET-1 1-31 (8 pmol) but did not modify the depressor action of intra-RVLM ET-1 1-21 (from 100 +/- 6 to 76 +/- 8 mm Hg, P < 0.05). In conclusion, the current results suggest that the cardiovascular effects of intra-RVLM ET-1 1-31 might be the result of conversion of ET-1 1-31 to ET-1 1-21 through activation of ETA receptors.

Role of prostaglandin, endothelin and sympathetic nervous system on the L-NAME-induced pressor responses in spontaneously hypertensive rats

We tested the hypothesis that in spontaneously hypertensive rat (SHR) NO produced centrally influences the resting arterial blood pressure by attenuating mechanisms involving prostaglandins, angiotensin II, endothelin and sympathetic nervous system. L-NAME (200 micro g/5 micro l), an inhibitor of NO synthase, administered intracerebroventricularly (i.c.v.) to awake and freely moving rats increased mean arterial blood pressure (MABP) in a biphasic pattern: an early transient increase within 1 min and a late prolonged response starting at 45 min and persisting for the duration of experiment (180 min). The two pressor responses involve different neurochemical mechanisms and, based on their latencies, they appear to reflect different anatomical sites of action of L-NAME. The late, but not the early pressor response, was prevented by pretreatment with chlorisondamine (2.5 mg/kg, i.v.), a ganglionic blocker, indicating its dependence on the sympathetic nervous system. Both pressor responses were abolished by i.c.v. pretreatment with indomethacin (200 micro g/5 micro l, i.c.v.), an inhibitor of cyclo-oxygenase, showing that they are mediated by prostaglandin(s). In contrast, losartan (25 micro g/5 micro l), an angiotensin II AT(1) receptor antagonist, had no effect. The initial pressor response was also attenuated by pretreatment with the endothelin ET(A)/ET(B) receptor antagonist, PD 145065 (48 micro g/2 micro l, i.c.v.). Intravenous pretreatment with another ET(A)/ET(B) receptor antagonist, L-754,142 (15 mg/kg as a bolus+15 mg/kg/h for 180 min), however, attenuated both responses to L-NAME. It is possible that L-754,142 crossed the blood-brain barrier and blocked, in addition, central ET(A)/ET(B) receptors. These studies show that NO synthesized in the brain attenuates pressor mechanisms involving prostaglandin, endothelin and sympathetic nervous system, but not angiotensin II, to modulate resting arterial blood pressure.

Endothelin-A receptors and NO mediate decrease in arterial pressure during recovery from restraint

We investigated the role of central endothelin-A (ET(A)) receptors and nitric oxide (NO) in regulating arterial pressure during restraint stress and recovery from stress. Rats received intracerebroventricular (icv) injections of the ET(A) receptor antagonist BQ123 (24 microg/kg) and were then subjected to two restraint-rest cycles (1 h of restraint and 1 h of rest/cycle). Although mean arterial pressure (MAP) values in BQ123-treated and control rats increased at the onset of restraint and remained elevated during restraint, MAP values in BQ123-treated rats were consistently greater than in control rats. During rest periods, MAP values in control rats decreased to below baseline levels, whereas those in BQ123-treated rats remained significantly higher. NO content was decreased in the brain stems of BQ123-treated compared with control rats after the 4-h protocol. Injections (icv) of the NO synthase (NOS) inhibitor N(G)-nitro-L-arginine (L-NNA) eliminated the decreases in MAP values during rest periods in both BQ123-treated and control rats. Inhibition of neuronal NOS with icv injection of 7-nitroindazole sodium salt resulted in MAP values intermediate between control rats and rats receiving L-NNA. These results support the hypothesis that endothelin acts through ET(A) receptors in the brain, possibly via release of NO, to decrease arterial pressure during restraint and recovery from restraint.

Hemodynamic effects of cannabinoids: coronary and cerebral vasodilation mediated by cannabinoid CB(1) receptors

Activation of peripheral cannabinoid CB(1) receptors elicits hypotension. Using the radioactive microsphere technique, we examined the effects of cannabinoids on systemic hemodynamics in anesthetized rats. The potent cannabinoid CB(1) receptor agonist HU-210 ([-]-11-OH-Delta(9) tetrahydrocannabinol dimethylheptyl, 10 microg/kg i.v.) reduced mean blood pressure by 57+/-5 mm Hg by decreasing cardiac index from 37+/-1 to 23+/-2 ml/min/100 g (P<0.05) without significantly affecting systemic vascular resistance index. HU-210 elicited a similar decrease in blood pressure following ganglionic blockade and vasopressin infusion. The endogenous cannabinoid anandamide (arachidonyl ethanolamide, 4 mg/kg i.v.) decreased blood pressure by 40+/-7 mm Hg by reducing systemic vascular resistance index from 3.3+/-0.1 to 2.3+/-0.1 mm Hg min/ml/100 g (P<0.05), leaving cardiac index and stroke volume index unchanged. HU-210, anandamide, and its metabolically stable analog, R-methanandamide, lowered vascular resistance primarily in the coronaries and the brain. These vasodilator effects remained unchanged when autoregulation was prevented by maintaining blood pressure through volume replacement, but were prevented by pretreatment with the cannabinoid CB(1) receptor antagonist SR141716A (N-[piperidin-1-yl]-5-[4-chlorophenyl]-1-[2,4-dichlorophenyl]-4-methyl-1H-pyrazole-3-carboxamide HCl; 3 mg/kg i.v.). Only anandamide and R-methanandamide were vasodilators in the mesentery. We conclude that cannabinoids elicit profound coronary and cerebral vasodilation in vivo by direct activation of vascular cannabinoid CB(1) receptors, rather than via autoregulation, a decrease in sympathetic tone or, in the case of anandamide, the action of a non-cannabinoid metabolite. Differences between the hemodynamic profile of various cannabinoids may reflect quantitative differences in cannabinoid CB(1) receptor expression in different tissues and/or the involvement of as-yet-unidentified receptors.

Endothelin system: the double-edged sword in health and disease

The endothelin system consists of two G-protein-coupled receptors, three peptide ligands, and two activating peptidases. Its pharmacological complexity is reflected by the diverse expression pattern of endothelin system components, which have a variety of physiological and pathophysiological roles. In the vessels, the endothelin system has a basal vasoconstricting role and participates in the development of diseases such as hypertension, atherosclerosis, and vasospasm after subarachnoid hemorrhage. In the heart, the endothelin system affects inotropy and chronotropy, and it mediates cardiac hypertrophy and remodeling in congestive heart failure. In the lungs, the endothelin system regulates the tone of airways and blood vessels, and it is involved in the development of pulmonary hypertension. In the kidney, it controls water and sodium excretion and acid-base balance, and it participates in acute and chronic renal failure. In the brain, the endothelin system modulates cardiorespiratory centers and the release of hormones. More advanced functional analysis of the endothelin system awaits not only additional pharmacological studies using highly specific endothelin antagonists but also the generation of genetically altered rodent models with conditional loss-of-function and gain-of-function manipulations.

Treatment with an endothelin type A receptor-antagonist after cardiac arrest and resuscitation improves cerebral hemodynamic and functional recovery in rats

OBJECTIVE

Successful resuscitation of the brain after cardiac arrest requires unimpaired microcirculatory reperfusion. Postischemic cerebral hypoperfusion presumably is mediated through activation of endothelin type A receptors (ET(A)). The effect of the selective ET(A) antagonist BQ123 on cerebral blood flow and function was studied in a rat model of cardiac arrest.

DESIGN

Prospective, randomized trial.

SETTING

Experimental animal laboratory.

SUBJECTS

Twelve male Sprague-Dawley rats (290-350 g).

INTERVENTIONS

Cardiac arrest for 12 mins was induced by electrical fibrillation of the heart, followed by standardized cardiopulmonary resuscitation. BQ123 (0.8 mg/kg; n = 6) or its vehicle (saline; n = 6) was injected intravenously at 15 mins after the return of spontaneous circulation.

MEASUREMENTS

Cortical blood flow was measured by laser-Doppler flowmetry, electrophysiological function by recording the amplitude of somatosensory evoked potentials, vascular reactivity by ventilation with 6% CO2, and the functional coupling of blood flow by recording the laser-Doppler flow (LDF) changes during somatosensory stimulation. Hemodynamic and functional cerebral recovery was monitored for 3 hrs after the return of spontaneous circulation.

MAIN RESULTS

Forty-five minutes after the return of spontaneous circulation, postischemic hypoperfusion developed in both groups, as reflected by a decrease of the LDF signal to about 60% of the preischemic level. In untreated animals, hypoperfusion persisted throughout the observation time, but in animals receiving BQ123, LDF gradually returned to normal. CO2 reactivity in untreated animals was severely reduced for 2-3 hrs after the onset of recirculation, whereas after BQ123 treatment it returned to normal and after 2 hrs even above normal. The ET(A) antagonist also induced a more rapid recovery of the somatosensory evoked potentials amplitude and of the functional blood flow response to somatosensory stimulation, but these parameters did not recover completely within the observation period.

CONCLUSIONS

Application of the ET(A) antagonist BQ123 during the early reperfusion period after cardiac arrest shortens postischemic cerebral hypoperfusion and accelerates the restoration of the cerebrovascular CO2 reactivity and the recovery of electrophysiologic function.

Endothelin type A-antagonist improves long-term neurological recovery after cardiac arrest in rats

OBJECTIVE

Antagonists of endothelin (ET(A)) receptors improve postischemic hypoperfusion. In this study we investigated whether the selective ET(A)-antagonist BQ123 also improves postischemic functional recovery.

STUDY DESIGN

Cardiac arrest of 12 mins duration was induced in rats by electrical fibrillation of the heart, followed by advanced cardiopulmonary resuscitation. BQ123 (0.8 mg/kg; n = 9) or its vehicle (saline; n = 9) was injected intravenously at 15 mins after the return of spontaneous circulation. The neurologic deficit was scored daily for 7 days after resuscitation by rating consciousness, various sensory and motor functions, and coordination tests. On day 7, we measured functional coupling of cerebral blood flow under halothane anesthesia by recording laser-Doppler flow during electrical forepaw stimulation, and we measured vascular reactivity to CO2 by measuring the laser-Doppler flow change during ventilation with 6% CO2. The brains were perfusion-fixated with 4% paraformaldehyde, and the histopathologic damage was evaluated in the CA1 sector of hippocampus, in the motor cortex, and in the cerebellum.

RESULTS

Treatment with BQ123 had no effect on histopathologic damage, but it significantly improved neurologic recovery. In all nine treated rats, neurologic performance returned to near normal within 2 days whereas four of nine untreated animals developed spastic paralysis of the hind limbs and severe coordination deficits. BQ123 also normalized CO2 reactivity and improved the functional cerebral blood flow response to somatosensory stimulation.

CONCLUSIONS

The ET(A)-antagonist BQ123 significantly improves neurologic outcome after 12 mins of cardiac arrest. The apparent restoration of vascular reactivity demonstrates a correlation between hemodynamic factors and functional recovery.

Role of endothelin receptor subtypes in the behavioral effects of the intracerebroventricular administration of endothelin-1 in conscious rats

The role of endothelin receptor subtypes, i.e., ET(A) and ET(B) receptors, in the behavioral effects of the intracerebroventricular (ICV) administration of endothelin-1 were examined in conscious rats. ICV administration of endothelin-1 (1-9 pmol/rat) dose dependently produced barrel rolling and other convulsive behaviors including bodily twitching, rigidity, back crawling, fore/hindlimb dystonia, fore/hindlimb clonus, tail extension, and facial clonus. Moreover, a marked increase in spontaneous locomotor activity was observed in animals that were treated with a low dose of endothelin-1 (1 pmol/rat, ICV). Endothelin-1 (9 pmol/rat, ICV)-induced barrel rolling and other convulsive behaviors were completely suppressed by the coadministration of BQ-123 (15 nmol, ICV), a specific endothelin ET(A) receptor antagonist, but not of BQ-788 (15 nmol/rat, ICV), a specific endothelin ET(B) receptor antagonist. In contrast, increased locomotor activity produced by treatment with a low dose of endothelin-1 (1 pmol/rat, ICV) was antagonized by coadministration of BQ-788, but not of BQ123. These results indicate that endothelin-1, which has affinity for both endothelin ET(A) and ET(B) receptors, most likely acts on central ET(A) receptors to evoke barrel rolling and other convulsive behaviors. In addition, activation of central ET(B) receptors may be involved in the increase in spontaneous locomotor activity. These results suggest that brain endothelin receptor subtypes may be involved in the regulation of various physiological functions.

Endothelin converting enzyme-1-, endothelin-1-, and endothelin-3-like immunoreactivity in the rat brain

Neurons likely to use endothelin as a neurotransmitter/neurohormone were mapped in the rat brain using polyclonal antibodies directed against endothelin-converting enzyme-1, endothelin-1, and endothelin-3. Anti-endothelin-converting enzyme-1 antibodies produced the most robust staining, permitting the best visualization of the distribution and morphology of neurons. Labeled neurons were found in the dorsal thalamic nuclei and reticular thalamic nuclei, medial preoptic area, pontine nucleus, and locus coeruleus. Localization of endothelin-converting enzyme-like immunoreactivity in the locus coeruleus and in the reticular nucleus of the thalamus suggests that endothelin is co-localized with norepinephrine and GABA, respectively. Additionally, endothelin-converting enzyme-like immunoreactivity was found in the globus pallidus, septal nuclei, and in both the vertical and horizontal limbs of the nucleus of the diagonal band of Broca, and the ventrolateral area of the caudate-putamen. Strong endothelin-converting enzyme-like immunoreactivity was found in a continuous band of pyramidal neurons throughout the neocortex primarily in layer V, extending into the cingulate gyrus and piriform cortex. Motor nuclei, including oculomotor, facial, and trigeminal nuclei, were also endothelin-converting enzyme-immunoreactive. In the cerebellum, Purkinje cells were stained. Non-neuronal cells such as oligodendroglia, microglia, and astrocytes generally were not endothelin-converting enzyme-immunoreactive, although astrocytes were rarely stained. Endothelin-converting enzyme-, endothelin-1-, and endothelin-3-like immunoreactivities were generally found co-existing in given nuclei. The diversity of neurons immunostained for endothelin suggests multiple roles of endothelin in the CNS.

Central effects of endothelin and its antagonists on sympathetic and cardiovascular regulation in SHR-SP

Intracerebroventricular injections of endothelin-1 (ET-1) are reported to cause dose-related increases in sympathetic nerve activity and blood pressure in anesthetized normotensive rats. These studies were performed to determine the following: which endothelin receptor, A or B, is involved in mediating sympathetic and cardiovascular effects of ET-1 injected centrally; whether central endothelin tonically participates in blood pressure regulation in normotensive rats; and whether the altered endothelin system in the central nervous system contributes to blood pressure elevation in hypertensive rats. ET-1, ET-A antagonist (BQ-123), or ET-B antagonist (RES-701-1) was injected into the lateral cerebral ventricle (i.c.v.) of urethane-anesthetized normotensive Wistar and Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHRs), and stroke-prone SHRs (SHR-SPs). In Wistar rats, i.c.v. injections of ET-1 (1, 5, 10 pmol) consistently increased sympathetic nerve activity, thereby elevating blood pressure in a dose-related manner. The pressor responses induced by i.c.v. ET-1 were abolished after intravenous pretreatment with phentolamine. Neither ET-A nor ET-B antagonist, when injected centrally, altered basal levels of sympathetic nerve activity, heart rate, or blood pressure in Wistar rats. However, sympathetic activation and pressor responses induced by i.c.v. injection of endothelin were completely abolished after i.c.v. pretreatment with ET-A antagonist but were unaffected after pretreatment with ET-B antagonist. Although i.c.v. injections of ET-1 increased sympathetic nerve activity and blood pressure in WKY rats, SHRs, and SHR-SPs, the magnitudes of these responses did not differ among these three groups. In contrast, i.c.v. injections of ET-A antagonist decreased sympathetic nerve activity, blood pressure, and heart rate only in SHR-SPs, but not in WKY rats and SHRs. In addition, the depressor effects of i.c.v. ET-A antagonist in SHR-SPs were ascertained while these rats were awake. In summary, i.c.v. injections of ET-1 increased sympathetic nerve activity and blood pressure via ET-A receptors but not via ET-B receptors. Central ET might tonically activate sympathetic nerve activity to thereby contribute to blood pressure elevation in SHR-SPs, but not in WKY rats and SHRs.

Central and peripheral administration of endothelin-1 induces an increase in blood pressure in conscious trout

The central and peripheral cardiovascular effects of endothelin (ET)-1 and ET-3 were investigated in conscious rainbow trout. Both intracerebroventricular and intra-arterial injections of ET-1 (6. 25-25 pmol) but not ET-3 (25 pmol) caused a dose-dependent increase in mean dorsal aortic blood pressure and a concomitant decrease in heart rate. The hypertensive effects induced by intra-arterial and intracerebroventricular injection of ET-1 were associated with a significant (P < 0.05) increase in systemic vascular resistance. Intracerebroventricular injection of ET-1 induced a twofold higher pressor response than that caused by intra-arterial injection of ET-1 and provoked a barostatic gain that was reduced by 2.5- to 3-fold compared with that calculated after intra-arterial administration of the peptide. The ET receptor antagonist bosentan significantly (P < 0.05) attenuated these responses regardless of the route of administration. Finally, intra-arterial injection of ET-1 did not significantly modify plasma cortisol level. The present data demonstrate that intracerebroventricular and intra-arterial administration of very low doses of ET-1 produces hypertension in conscious trout. The lack of effect of ET-3 indicates that the hemodynamic actions of ET-1 are mediated both centrally and peripherally through ETA receptors.

Essential role for endothelin ET(B) receptors in fever induced by LPS (E. coli) in rats

1. The influence of endothelin receptor antagonists on febrile responses to E. coli lipopolysaccharide (LPS), interleukin-1beta (IL-1beta), tumour necrosis factor-alpha (TNF-alpha) and endothelin-1 (ET-1) was assessed in conscious rats. 2. Intravenous (i.v.) LPS (5.0 microg kg(-1)) markedly increased rectal temperature to a peak of 1.30 degrees C over baseline at 2.5 h. Pretreatment with the mixed endothelin ET(A)/ET(B) receptor antagonist bosentan (10 mg kg(-1), i.v.) or the selective endothelin ET(B) receptor antagonist BQ-788 (N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D -1-methoxycarboyl-D-norleucine; 3 pmol, into a lateral cerebral ventricle-i.c.v.) reduced the peak response to LPS to 0.90 and 0.75 degrees C, respectively. The selective endothelin ET(A) receptor antagonist BQ-123 (cyclo[D-Trp-D-Asp-Pro-D-Val-Leu]; 3 pmol, i.c.v.) was ineffective. 3. Increases in temperature caused by IL-1beta (180 fmol, i.c.v.), TNF-alpha (14.4 pmol, i.c.v.) or IL-1beta (150 pmol kg(-1), i.v.) were unaffected by BQ-788 (3 pmol, i.c.v.). 4. Central injection of endothelin-1 (0.1 to 3 fmol, i.c.v.) caused slowly-developing and long-lasting increases in rectal temperature (starting 2 h after administration and peaking at 4-6 h between 0.90 and 1.15 degrees C) which were not clearly dose-dependent. The response to endothelin-1 (1 fmol, i.c.v.) was prevented by BQ-788, but not by BQ-123 (each at 3 pmol, i.c.v.). Intraperitoneal pretreatment with the cyclo-oxygenase inhibitor indomethacin (2 mg kg(-1)), which partially reduced LPS-induced fever, did not modify the hyperthermic response to endothelin-1 (3 fmol, i.c.v.). 5. Therefore, central endothelin(s) participates importantly in the development of LPS-induced fever, via activation of a prostanoid-independent endothelin ET(B) receptor-mediated mechanism possibly not situated downstream from IL-1beta or TNF-alpha in the fever cascade.

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

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

In vivo and in vitro action of endothelin-1 on goat cerebrovascular bed

This study concerned the effects and mechanisms of action of endothelin-1 on the cerebral circulation. Cerebral blood flow was electromagnetically measured in awake goats. Endothelin-1 (0.01-0.3 nmol) produced dose-dependent decreases in this flow (maximal reduction = 34%) and increases in cerebrovascular resistance (maximal increase = 74%) (P < 0.01). IRL 1620 (Suc-[Glu9, Ala11,15]endothelin-1-(8-21), agonist for endothelin ET(B) receptors, 0.01-0.3 nmol) slightly decreased cerebral blood flow. The effects of endothelin-1, but not those of IRL 1620, on cerebral blood flow were diminished by 50% during infusion of the antagonist for endothelin ET(A) receptors, BQ-123 (cyclo-(D-Asp-Pro-D-Val-Leu-Trp), 2 nmol min(-1)), but not affected during infusion of the antagonist for endothelin ET(B) receptors, BQ-788 (N-[N-[N-[(2,6-dimethyl-1-piperidinyl)carbonyl]-4-methyl-L-Leucyl-1-(met hoxycarbonyl)-D-tryptophyl]-Dnorleucine monosodium), 2 nmol min(-1)). Intravenous administration of NW-nitro-L-arginine methyl ester (L-NAME, 47 mg kg(-1)) or NW-nitro-L-arginine (L-NNA, 47 mg kg(-1)) reduced basal cerebral blood flow by 39 and 33%, increased cerebrovascular resistance by 108 and 98% and mean arterial pressure by 23 and 17%, and decreased heart rate by 27 and 25%, respectively (all at least P < 0.05). The increases in cerebrovascular resistance (as absolute values) induced by endothelin-1 were not affected during either L-NAME or L-NNA (as absolute values and percentages). Intravenous administration of meclofenamate (5 mg kg(-1)) did not change the cerebrovascular effects of endothelin-1 and IRL 1620. In isolated goat cerebral arteries under control, resting conditions, endothelin-1 (10(-11)-10(-7) M) induced concentration-dependent contractions (EC50 = 4.78 X 10(-9) M; maximal contraction = 3177+/-129 mg), whereas IRL 1620 (10(-11)-10(-7) M) produced no effect. This contraction produced by endothelin-1 was competitively blocked by BQ-123 (10(-7)-3 X 10(-6) M), and was not affected by BQ-788 (10(-6) and 10(-5) M). L-NAME (10(-4) M), meclofenamate (10(-5) M), indomethacin (10(-5) M), L-NAME (10(-4) M) plus meclofenamate (10(-5) M) and phosphoramidon (10(-4) M) did not affect the contraction in response to endothelin-1. Endothelium removal increased the response to endothelin-1, as well as the BQ-123 antagonism against endothelin-1 (pA2 values, 7.62 vs. 6.88; P < 0.01). In both intact and de-endothelized arteries precontracted with prostaglandin F2alpha endothelin-1 induced a further contraction, and IRL 1620 caused no effect. These results suggest that: (1) endothelin-1 produces cerebral vasoconstriction by activating endothelin ET(A) receptors probably located in smooth muscle; (2) endothelin ET(B) receptors, nitric oxide and prostanoids might be not involved in the cerebrovascular action of endothelin-1, and (3) endothelium removal may increase cerebrovascular reactivity by increasing sensitivity of endothelin ET(A) receptors to endothelin-1.

Effect of centrally administered endothelin agonists on systemic and regional blood circulation in the rat: role of sympathetic nervous system

The aims of the present study were to determine (1) the hypotensive and regional circulatory effects of centrally administered endothelin (ET) ETA and ETB agonists, and (2) the role of the sympathetic nervous system in the mediation of hypotensive effects due to centrally administered ET-1. The systemic haemodynamics and regional blood circulation in urethane anaesthetized rats following intracerebroventricular (i.c.v.) administration of ET-1, ET-2, SRT6b, ET-3 and SRT6c (10, 30 and 90 ng) were determined by a radioactive microsphere technique. The effect of centrally administered ET-1 on sympathetic nerve activity was also analysed. Systemic haemodynamics and regional blood circulation were determined before (baseline) and 30 min after administration of ET agonists. Cumulative administration of three doses of saline (5 microliters, i.c.v. at 30 min intervals) did not produce any significant cardiovascular effects. ET-1, ET-2 and SRT6b produced a decrease in blood pressure (51%, 47% and 41%, respectively) along with a decrease in cardiac output (58%, 60% and 45%, respectively) and stroke volume. Heart rate and total peripheral resistance were not affected. ET-1, ET-2 and SRT6b also produced a significant reduction in blood flow to the brain, kidneys, heart, portal, mesentery and pancreas, gastrointestinal tract (GIT) and musculoskeletal system. The effect of ET-2 on the cardiovascular system was less intense in comparison with ET-1 and SRT6b. Centrally administered specific ETB receptor agonists ET-3 and SRT6c did not produce any change in systemic haemodynamics and regional blood flow. Centrally administered ET-1 (90 ng) produced a significant decrease (61%) in sympathetic nerve activity 30 min after drug administration, along with a fall in blood pressure. It is concluded that centrally administered ETA agonists produce significant cardiovascular effects mediating through the sympathetic nervous system.

Physiological role of brain endothelin in the central autonomic control: from neuron to knockout mouse

Although endothelin (ET) was discovered as a potent vascular endothelium-derived constricting peptide, its presumed physiological and pathophysiological roles are now considered much more diverse than originally though. Endothelin in the brain is thought to be deeply involved in the central autonomic control and consequent cardiorespiratory homeostasis, possibly as a neuromodulator or a hormone that functions locally in an autocrine/paracrine manner or widely through delivery by the cerebrospinal fluid (CSF). This notion is based on the following lines of evidence. (1) Mature ET, its precursors, converting enzymes, and receptors all are detected at strategic sites in the central nervous system (CNS), especially those controlling the autonomic functions. (2) The ET is present in the CSF at concentrations higher than in the plasma. (3) There is a topographical correspondence of ET and its receptors in the CNS. (4) The ET is released by primary cultures of hypothalamic neurons. (5) When ET binds to its receptors, intracellular calcium channels. (6) An intracerebroventricular or topical application of ET to CNS sites elicits a pattern of cardiorespiratory changes accompanied by responses of vasomotor and respiratory neurons. (7) Recently generated knockout mice with disrupted genes encoding ET-1 exhibited, along with malformations in a subset of the tissues of neural crest cell lineage, cardiorespiratory abnormalities including elevation of arterial pressure, sympathetic overactivity, and impairment of the respiratory reflex. Definitive evidence is expected from thorough analyses of knockout mice by applying conventional experimental methods.

Effect of ETA receptor antagonists on cardiovascular responses induced by centrally administered sarafotoxin 6b: role of sympathetic nervous system

The present study was carried out to investigate the cardiovascular effects of centrally administered SRT6b in saline, BQ123 and BMS182874 pretreated male Sprague-Dawley rats, using a radioactive microsphere technique. SRT6b (100 ng, ICV) produced a transient increase (40%) in blood pressure at 5 min followed by a sustained decrease (-42%) at 30 and 60 min in control rats. Total peripheral resistance and heart rate were not significantly altered. Cardiac output increased (16%) at 5 min and decreased 30 and 60 min following SRT6b administration. Central venous pressure was not affected by SRT6b. Regional blood flow and vascular resistance did not change at 5 min following administration of SRT6b. However, a significant decrease in blood flow to the brain, heart, kidneys, liver, spleen, gastrointestinal tract and mesentery and pancreas was observed 30 and 60 min following administration of SRT6b in control (saline treated) rats. Pretreatment with ETA selective receptor antagonists, BQ123 (10 micrograms, ICV) or BMS182874 (50 micrograms, ICV) significantly attenuated the pressor and depressor effects of centrally administered SRT6b. SRT6b induced decrease in blood flow was completely blocked by pretreatment with BQ123 or BMS182874. ET-1 (100 ng, ICV) produced an increase followed by a decrease similar to SRT6b. Reserpine (5 mg/kg, IP) pretreatment attenuated the cardiovascular effects of ET-1. Role of sympathetic nervous system was determined by measuring splanchnic nerve activity. SRT6b when administered in the lateral cerebral ventricle did not produce any significant effect at 5 min, however, a significant decrease in sympathetic nerve activity was observed 30 min after its administration. It is concluded that centrally administered SRT6b produces significant changes in systematic and regional blood circulation which can be completely blocked by ETA receptor antagonist. The cardiovascular effects of centrally administered SRT6b appear to be mediated through the sympathetic nervous system.