Alpha-adrenoreceptors and muscarine receptors in human pial arteries and microvessels: a receptor binding study

High-dose phenylephrine increases meningeal blood flow through TRPV1 receptor activation and release of calcitonin gene-related peptide

BACKGROUND

The α₁ -adrenoceptor agonist, phenylephrine, is used at high concentrations as a mydriatic agent and for the treatment of nasal congestion. Among its adverse side-effects transient burning sensations are reported indicating activation of the trigeminal nociceptive system.

METHODS

Neuropeptide release, calcium imaging and meningeal blood flow recordings were applied in rodent models of meningeal nociception to clarify possible receptor mechanisms underlying these pain phenomena.

RESULTS

Phenylephrine above 10 mM dose-dependently released calcitonin gene-related peptide (CGRP) from the dura mater and isolated trigeminal ganglia, whereas hyperosmotic mannitol at 90 mM was ineffective. The phenylephrine-evoked release was blocked by the transient receptor potential vanilloid 1 (TRPV1) antagonist BCTC and did not occur in trigeminal ganglia of TRPV1-deficient mice. Phenylephrine at 30 mM caused calcium transients in cultured trigeminal ganglion neurons responding to the TRPV1 agonist capsaicin and in HEK293T cells expressing human TRPV1. Local application of phenylephrine at micromolar concentrations to the exposed rat dura mater reduced meningeal blood flow, whereas concentrations above 10 mM caused increased meningeal blood flow. The flow increase was abolished by pre-application of the CGRP receptor antagonist CGRP_8-37 or the TRPV1 antagonist BCTC.

CONCLUSIONS

Phenylephrine at high millimolar concentrations activates TRPV1 receptor channels of perivascular afferents and, upon calcium inflow, releases CGRP, which increases meningeal blood flow. Activation of TRPV1 receptors may underlie trigeminal nociception leading to cranial pain such as local burning sensations or headaches caused by administration of high doses of phenylephrine.

SIGNIFICANCE

Phenylephrine is used at high concentrations as a mydriaticum and for treating nasal congestion. As adverse side-effects burning sensations and headaches have been described. Phenylephrine at high concentrations causes calcium transients in trigeminal afferents, CGRP release and increased meningeal blood flow upon activation of TRPV1 receptor channels, which is likely underlying the reported pain phenomena.

Vascular dysfunction in the pathogenesis of Alzheimer's disease--A review of endothelium-mediated mechanisms and ensuing vicious circles

Late-onset dementia is a major health concern in the ageing population. Alzheimer's disease (AD) accounts for the largest proportion (65-70%) of dementia cases in the older population. Despite considerable research effort, the pathogenesis of late-onset AD remains unclear. Substantial evidence suggests that the neurodegenerative process is initiated by chronic cerebral hypoperfusion (CCH) caused by ageing and cardiovascular conditions. CCH causes reduced oxygen, glucose and other nutrient supply to the brain, with direct damage not only to the parenchymal cells, but also to the blood-brain barrier (BBB), a key mediator of cerebral homeostasis. BBB dysfunction mediates the indirect neurotoxic effects of CCH by promoting oxidative stress, inflammation, paracellular permeability, and dysregulation of nitric oxide, a key regulator of regional blood flow. As such, BBB dysfunction mediates a vicious circle in which cerebral perfusion is reduced further and the neurodegenerative process is accelerated. Endothelial interaction with pericytes and astrocytes could also play a role in the process. Reciprocal interactions between vascular dysfunction and neurodegeneration could further contribute to the development of the disease. A comprehensive overview of the complex scenario of interacting endothelium-mediated processes is currently lacking, and could prospectively contribute to the identification of adequate therapeutic interventions. This study reviews the current literature of in vitro and ex vivo studies on endothelium-mediated mechanisms underlying vascular dysfunction in AD pathogenesis, with the aim of presenting a comprehensive overview of the complex network of causative relationships. Particular emphasis is given to vicious circles which can accelerate the process of neurovascular degeneration.

Effects of common medications on cerebral vasospasm after subarachnoid haemorrhage

Cerebral vasospasm is a common and serious complication of aneurysmal subarachnoid haemorrhage (SAH). At present, no consistently effective preventative and therapeutic measures are available, perhaps because of incomplete understanding of the pathogenesis of vasospasm. Experimental studies provide evidence that the incidence and severity of vasospasm after SAH can be modulated by drugs that affect neurotransmitter levels, intracellular signalling mechanisms, vascular smooth muscle function, inflammation and cellular proliferation, and the concentration of 'spasmogenic' factors. Preliminary clinical studies indicate that some illicit drugs and common prescription medications can have similar effects in humans. Recognition of these pharmacological effects is important because medications that can worsen or alleviate vasospasm are frequently administered to SAH patients to treat coincident medical problems.

Head position modifies cerebrovascular response to orthostatic stress

Previous experiments have shown that the vestibular system participates in cardiovascular control. However, the effects of vestibular activation on cerebrovascular regulation are not known. Therefore, the present experiment tested the hypothesis that specific vestibular activations may be beneficial to cerebral circulation during simulated orthostatic stress. Middle cerebral artery blood flow velocity (CBV; Doppler ultrasound) was measured to examine the effects of head-down neck flexion (HDNF) compared to head-down neck extension (HDNE) with and without lower body negative pressure (LBNP; -40 mmHg) (n=9). The change in CBV (DeltaCBV) during HDNF and HDNE were not different during baseline conditions, however, during LBNP, DeltaCBV was greater in HDNE compared to HDNF (-5.5+/-3.2 cm/s, -11+/-4.6%) vs. (-0.7+/-1.0 cm/s, -1.9+/-1.9%), respectively (P<0.05). Concomitantly, the change in cerebrovascular resistance (DeltaCVR) between rest and LBNP was also greater during HDNE (0.48+/-0.08 mmHg/cm per s, 42.8+/-10.8%) compared with HDNF (0.26+/-0.05 mmHg/cm per s, 22+/-4.1%) (P<0.05). P(ET)CO(2) was greater in HDNE (45+/-2 mmHg) compared to HDNF (42+/-2 mmHg; P<0.05) during LBNP. These results suggest that the vestibular system may affect cerebrovascular tone during simulated postural stress by either constriction or dilation, depending on the vestibular stimulus.

Effects of induced hypertension on intracranial pressure and flow velocities of the middle cerebral arteries in patients with large hemispheric stroke

BACKGROUND AND PURPOSE

Our aim was to prospectively evaluate the effects of induced arterial hypertension in patients with large ischemic stroke.

METHODS

A total of 47 monitoring sessions in 19 patients with acute, complete, or subtotal middle cerebral artery (MCA) territory stroke were performed. Intracranial pressure (ICP) was monitored using a parenchymal catheter. Mean arterial blood pressure (MAP), ICP, and peak mean flow velocity of the middle cerebral arteries (V(m)MCA) were continuously recorded. Patients with acute ICP crises were excluded. After obtaining baseline values, MAP was raised by an infusion of norepinephrine to reach an MAP increase of at least 10 mm Hg. After MAP had reached a peak plateau level, the norepinephrine infusion was stopped.

RESULTS

Baseline MAP was 83.6+/-1.6 mm Hg and rose to 108.9+/-2.0 mm Hg after infusion of norepinephrine. ICP slightly increased from 11.6+/-0.9 mm Hg to 11.8+/-0.9 mm Hg (P<0.05). Cerebral perfusion pressure rose from baseline 72.2+/-2 mm Hg to 97+/-1 mm Hg (P<0.0001). V(m)MCA was already higher on the affected side during baseline measurements. At maximum MAP levels, V(m)MCA rose by 25.5+/-5.5 cm/s on the affected side and by 8.6+/-1.6 cm/s on the contralateral side.

CONCLUSIONS

In patients with large hemispheric stroke without an acute ICP crisis, induced hypertension enhances cerebral perfusion pressure and augments the V(m)MCA(s), more so on the affected side. The ICP slightly increases; however, this is probably not clinically significant.

Intravenous dexmedetomidine inhibits cerebrovascular dilation induced by isoflurane and sevoflurane in dogs

Our aim in this study, performed using a closed cranial window preparation, was to investigate the effect of systemic pretreatment with dexmedetomidine on cerebrovascular response to isoflurane or sevoflurane. After instrumentation under pentobarbital anesthesia, 48 dogs were assigned to one of two groups: the isoflurane group or the sevoflurane group (n = 24 each). Twenty-four dogs received saline (n = 6) or one of three different doses of dexmedetomidine (0.5, 1.0, or 2.0 micrograms/kg) (n = 6 each) i.v. Animals were then exposed to three different minimum alveolar anesthetic concentrations (MACs; 0.5, 1.0, and 1.5) of either isoflurane or sevoflurane. Cerebrovascular diameters were measured at each stage. Pretreatment with dexmedetomidine decreased pial vessel diameters. Both isoflurane and sevoflurane significantly dilated both arterioles and venules in a concentration-dependent manner. Isoflurane- and sevoflurane-induced dilation of cerebral arterioles was significantly attenuated in the presence of dexmedetomidine. The dexmedetomidine-induced attenuation of the vascular responses was not dependent on the dose of dexmedetomidine and was not different between isoflurane and sevoflurane. The vasodilation of cerebral pial vessels induced by isoflurane and sevoflurane could be attenuated by the systemic administration of dexmedetomidine, and this interaction between dexmedetomidine and volatile anesthetics showed no evidence of dose-dependency.

IMPLICATIONS

The systemic administration of dexmedetomidine attenuates the dilation of cerebral vessels induced by isoflurane and sevoflurane in pentobarbital-anesthetized dogs. This interaction was not dependent on the clinical (0.5-2.0 micrograms/kg) dose of dexmedetomidine and was not different between isoflurane and sevoflurane anesthesia.

Pharmacological elevation of blood pressure in acute stroke. Clinical effects and safety

BACKGROUND AND PURPOSE

Lowering of blood pressure can adversely affect ischemic symptoms in acute stroke. The aim of our study was to determine whether induced hypertension in stroke is safe and to examine its effects on neurological deficits in patients presenting with acute cerebral ischemia.

METHODS

We retrospectively reviewed all patients admitted to our neurological intensive care unit with the diagnosis of ischemic stroke over a 2.5-year period. Thirty-three patients were not given a pressor agent (Ph- group), while 30 were treated with phenylephrine (Ph+ group) in an attempt to improve cerebral perfusion.

RESULTS

Baseline characteristics showed few differences between the Ph+ and Ph- groups. Intracerebral hemorrhage, brain edema, cardiac morbidity, and mortality were not increased in the Ph+ group. In 10 of 30 Ph+ patients, a systolic blood pressure threshold was identified below which ischemic deficits worsened and above which deficits improved. The mean threshold was 156 mm Hg (range, 120 to 190 mm Hg). The mean number of stenotic/occluded cerebral arteries was greater in those Ph+ patients with an identified clinical blood pressure threshold (mean, 2.1 per patient) than in Ph+ patients without a threshold (mean, 1.2 per patient; P < .05).

CONCLUSIONS

The results suggest that careful use of phenylephrine induced hypertension is not associated with an increase in morbidity or mortality in acute stroke. Although based on a retrospective analysis of clinical practice, this report suggests that a subset of patients, particularly those with multiple stenosis of cerebral arteries, may improve neurologically upon elevation of the blood pressure.

Safety of hypertensive hypervolemic therapy with phenylephrine in the treatment of delayed ischemic deficits after subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Hypertensive hypervolemic therapy has been shown to reverse delayed ischemic deficits after aneurysmal subarachnoid hemorrhage. Concern has been raised about systemic complications of therapy, including pulmonary edema and myocardial ischemia, especially when high doses of vasopressors are used. Patients in whom delayed ischemic deficits were treated with hypervolemia and phenylephrine were prospectively evaluated for signs of systemic toxicity.

METHODS

Twenty-four consecutive patients treated with hypertensive hypervolemic therapy after aneurysmal subarachnoid hemorrhage were studied. Sixty-seven percent had underlying cardiac disease, vascular disease, or hypertension. No patient was excluded because of age or preexisting cardiac disease. Patients were closely monitored for signs of congestive heart failure (physical examination, chest x-ray films, arterial blood gases, cardiac index, pulmonary artery wedge pressure, and oxygen requirement). Indicators of cardiac ischemia and other extracerebral toxicity that were monitored included cardiac enzymes, electrocardiograms, serum creatinine, electrolyte and lactic acid levels, gastrointestinal motility, and urine output.

RESULTS

Volume expansion and phenylephrine infusion produced an increase in several hemodynamic parameters including pulmonary artery wedge pressure, which rose 28% (13 +/- 3.6 to 16 +/- 1.9 mm Hg), mean arterial blood pressure, which rose 25% (99 +/- 12.5 to 123 +/- 11.4 mm Hg), and systemic vascular resistance, which rose 46% (1234 +/- 294 to 1739 +/- 315 dyne.s-1.cm-5); however, there was no change in cardiac index (3.9 +/- 0.9 to 4.0 +/- 0.6 L.min-1.m-2). There were no clinically significant episodes of pulmonary edema requiring a change in vasopressor therapy and no myocardial infarctions. Phenylephrine was stopped in only one patient (incidence, 4%; 95% confidence interval, 0% to 12%), who developed an exacerbation of his preexisting bradycardia. There was no evidence of noncardiac organ system toxicity. Eighty-eight percent of the patients exhibited neurological improvement.

CONCLUSIONS

Hypertensive hypervolemic therapy with the use of high-dose phenylephrine can be administered with acceptable systemic toxicity, even in patients with previous cardiac disease, provided that close monitoring is performed. To minimize risk, aggressive treatment should probably be reserved for patients with signs of delayed ischemia rather than administered prophylactically.

Cholinergic and vasoactive intestinal polypeptidergic innervation of the cerebral arteries

Acetylcholine and vasoactive intestinal polypeptide are not only two vasoactive agonists that predominantly induce a vasodilatation of the cerebral arteries, but also correspond to neurotransmitters that innervate the various anatomical segments of the cerebral vasculature. The distinct patterns of the cerebrovascular cholinergic and vasoactive intestinal polypeptidergic innervation, their neurochemistry, in vitro and in vivo pharmacology, as well as the putative pathophysiological implications of these neurotransmission systems are critically summarized on the basis of the most recently published literature.

Innervation of cerebral arteries by nerves containing 5-hydroxytryptamine and noradrenaline

Noradrenaline (NA)-containing nerves, mainly originating in the sympathetic superior cervical ganglia, supply large and small cerebral arteries. In large cerebral arteries, nerves containing serotonin (5-hydroxytryptamine, 5-HT) may represent neuronal uptake of circulating 5-HT by sympathetic nerves. 5-HT-containing nerves supplying small pial vessels probably have a central origin in the dorsal raphe nucleus. In most species, NA is a weak vasoconstrictor (alpha 1- or alpha 2-adrenoceptors), while 5-HT is a potent vasoconstrictor (5-HT2 or 5-HT1-like receptors) of large cerebral arteries. In contrast, both NA and 5-HT tend to cause vasodilatation in small pial vessels and arterioles. Adrenergic and serotonergic transmission can be modulated by pH, a range of putative neurotransmitters and neuromodulators, and by the endothelium. Sumatriptan, a 5-HT1-like receptor agonist, has been shown to be effective in the treatment of migraine. Changes in NA- or 5-HT-containing nerves and/or in the responses of cerebral vessels to NA and 5-HT have been observed in a variety of vascular disorders, including cerebral vasospasm following subarachnoid haemorrhage, hypertension, and atherosclerosis.

Adrenergic receptors coupled to adenylate cyclase in human cerebromicrovascular endothelium

Cultured endothelium derived from three microvascular fractions of human brain was used to characterize adrenergic receptors coupled to adenylate cyclase activity. Catecholamines (norepinephrine, epinephrine) and their analogs (isoproterenol, phenylephrine, 6-fluoronorepinephrine) dose-dependently stimulated endothelial production of cAMP. Antagonists for beta 1 and beta 2 receptors (propranolol, atenolol, and butoxamine) and for alpha 1-receptors (prazosin) dose-dependently blocked cAMP formation induced by the tested adrenergic agonists. Clonidine, an alpha 2 > alpha 1-agonist, also inhibited isoproterenol-stimulated production of cAMP while yohimbine (alpha 2 > alpha 1 antagonist) augmented the norepinephrine or epinephrine-induced accumulation of cAMP. Cholera toxin-induced ADP ribosylation of the stimulatory guanine nucleotide binding protein (Gs) abolished the stimulatory effect of norepinephrine, epinephrine, phenylephrine or 6-fluoronorepinephrine on cAMP formation. ADP ribosylation of the inhibitory guanine nucleotide binding protein (Gi) by pertussis toxin had no effect on either phenylephrine- or 6-fluoronorepinephrine-induced production of cAMP while it increased the norepinephrine and epinephrine-induced accumulation of cAMP. These findings represent the first documentation of beta 1-, beta 2-, alpha 1 and alpha 2-adrenergic receptors linked to adenylate cyclase in endothelium derived from human brain microvasculature. These data also indicate that activation of endothelial alpha 1 -adrenergic receptors is mediated by a signal transduction mechanism associated with Gs protein. The results strongly support the presence of various receptor-controlled adrenergic regulatory mechanisms on human cerebromicrovascular endothelium.

Muscarinic binding of pial vessels and arachnoid membrane

The muscarinic sites in arachnoid and pial vessels were compared by analysis of the binding of quinuclidinyl benzilate (QNB) to membrane preparations. Saturation analysis indicated that the process was saturable, high affinity, and related to protein concentration in both structures. Although the affinities in the two structures [KD = 0.039 (arachnoid) and 0.097 nM (pial vessels)] were similar, the arachnoid had approximately 10-fold more binding sites (Bmax = 2,100 fmol/mg of protein) than the pial vessels (Bmax = 250 fmol/mg of protein). This difference was found in both bovine and porcine fractions. Pharmacological analysis of [3H]QNB displacement by muscarinic and nonmuscarinic ligands gave the typical pattern of muscarinic receptors in both structures. Inhibition of binding to pial vessels by the M1 antagonist pirenzepine revealed only one low-affinity site (Ki = 7.8 x 10(-7) M), whereas, the arachnoid had a small proportion (21%) of high-affinity sites (Ki = 2.2 x 10(-9) M) associated with low-affinity sites (Ki = 5.50 x 10(-7) M). It is concluded that muscarinic-mediated effects that do not involve the M1 subtype are induced in bovine pial vessels by a relatively low concentration of binding sites. The high content of muscarinic binding sites and their diversity in the arachnoid suggest a functional role for muscarinic cholinergic receptors in this structure.

Muscarinic cholinergic receptors on the endothelium of human cerebral arteries

To characterize the muscarinic cholinergic receptors on the endothelium of human cerebral arteries, isometric tension measurement and receptor autoradiographic studies were performed. Acetylcholine (ACh) induced dose-dependent relaxation of human cerebral arteries precontracted by 10(-5) M serotonin, with an EC50 of 1.9 +/- 0.6 X 10(-6) M (n = 7). The relaxation was abolished by 10(-5) M hemoglobin. Autoradiography, using the muscarinic antagonist [3H]propylbenzilycholine mustard, demonstrated the high density of muscarinic cholinergic receptors on the endothelial cells of human cerebral arteries, especially on the luminal surface of the endothelium. These findings suggest that ACh-induced relaxation mediated by muscarinic cholinergic receptors on the endothelium has a physiological function in human cerebral arteries.

Increased alpha 2- and beta 2-adrenergic receptors in cerebral microvessels in Alzheimer disease

Adrenergic receptors exist in brain microvessels which are innervated by noradrenergic locus ceruleus neurons. Biochemical and pathological studies indicate locus ceruleus degeneration in Alzheimer disease (AD), which can cause adrenergic receptor alterations in brain microvessels. To assess this, we studied adrenergic receptors in human brain microvessels from AD subjects and age-matched controls by ligand binding methods. Total beta receptors of cerebral microvessels and beta 2 receptors, the type which predominates in microvessels, were significantly increased in AD. Compared to the cerebral cortex, there was a paucity of alpha 1-adrenergic receptors in cerebral microvessels, and they did not change in AD. Binding to alpha 2 receptors in cerebral microvessels was approximately 50% of that in the cortex, and these receptors increased by approximately 60% in cerebral microvessels of AD subjects. These findings suggest adrenergic receptor 'upregulation' in response to noradrenergic deafferentation in AD, which may have functional consequences at the blood-brain barrier.

Characterization of [125I]HEAT binding to alpha 1-receptors in human brain: assessment in aging and Alzheimer's disease

The binding of [125I]2-(beta-4-hydroxyphenylethylamino-ethyltetralone ([ 125I]HEAT), an alpha 1-adrenergic receptor antagonist, to human brain membranes was characterized and the binding assessed in tissue from subjects with Alzheimer's disease (AD) and aging controls. Under Na+-K+ phosphate buffer conditions, [125I]HEAT bound to a single class of binding sites in prefrontal cortex (Brodmann area 10) with a Kd of about 120 pM. High binding capacities of [125I]HEAT were evident in the hippocampus and neocortex but were low in subcortical areas and cerebral microvessels comparable to the regional distribution of [3H]prazosin binding reported previously. Displacement of [125I]HEAT by various adrenergic drugs was consistent with its binding to alpha 1-adrenergic receptors. The specific binding was not affected by postmortem delay between death and freezing of tissue at autopsy. There was no correlation of [125I]HEAT binding with age of subjects. In AD subjects, the binding was significantly decreased in prefrontal cortex by about 25% but not changed in hippocampus, putamen or cerebellum compared to age-matched controls. The reduced binding of [125I]HEAT in prefrontal cortex may reflect a region-specific change in alpha 1-adrenergic receptors associated with neuronal loss in AD.

Acetylcholinesterase-containing fibers and choline acetyltransferase activity in isolated cerebral microvessels from goats

Microvessels have been isolated from goat cerebral cortex and caudate nucleus. The purity of the preparations was assessed by light microscopy and by the high enrichment in the marker enzymes alkaline phosphatase and gamma-glutamyltransferase. Choline acetyltransferase activity was detected in the vascular fractions, being significantly higher in capillaries than in larger vessels. Acetylcholinesterase (AChE)-containing fibers were visualized in vessels of different caliber. Vessels with diameters larger than 70-90 microns showed a network pattern of fibers similar to that of pial arteries. In small vessels (10-70 microns) longitudinal or helical fibers were observed with occasional side-branches that surround the vessel. No AChE staining was visualized in isolated capillaries under light microscopy. This study shows that isolated intracerebral microvessels are suitable preparations for histochemical studies of perivascular nerves. Taken together, the biochemical and histological results are in accordance with a cholinergic innervation of the goat intracerebral vasculature.

Impairment of cerebellar blood flow autoregulation during cerebral ischemia in spontaneously hypertensive rats

Participation of the autonomic nervous system in cerebellar autoregulation during supratentorial cerebral ischemia induced by bilateral carotid ligation was studied using 23 spontaneously hypertensive rats. Cerebral and cerebellar blood flows measured by a hydrogen clearance method were evaluated under stepwise hemorrhagic hypotension before and 30 minutes after ligation and after a 30-minute recirculation period following 1 hour of ligation. alpha-Adrenergic blockade with phenoxybenzamine, beta-adrenergic blockade with propranolol, and muscarinic cholinergic blockade with atropine were selectively administered before ligation for inhibition of sympathetic and parasympathetic tone. Cerebral blood flow autoregulation was severely impaired during and after cerebral ischemia in each treatment group. During cerebral ischemia, cerebellar blood flow autoregulation was also significantly impaired in both the propranolol and atropine groups although it was better preserved in the phenoxybenzamine group. After recirculation, cerebellar blood flow autoregulation recovered almost to the normal range in the phenoxybenzamine and atropine groups but remained impaired in the propranolol group. Our results suggest that impaired cerebellar blood flow autoregulation in supratentorial cerebral ischemia is partly modulated by the alpha-adrenoceptor system, which is activated by hypertensive stimuli and cerebral ischemia, leading to vasoconstriction in the cerebellum.

A micromethod for the isolation of large and small microvessels from frozen autopsied human brain

Microvessels were isolated from autopsied human brain using a simple procedure involving disruption, sieving, and centrifugation on a sucrose density gradient. The present procedure is characterized by isolation, from frozen autopsied brain, of materials either from the cerebral cortex or white matter, and subsequent separation of the capillary fraction from the large vessel fraction. The preparation appears highly purified under phase-contrast microscopic examination. The purity was also established by the enrichment of gamma-glutamyl transpeptidase activity and by the nearly negligible cerebroside content in the vessel fractions as compared to the brain homogenate.