Cerebral circulation and norepinephrine: relevance of the blood-brain barrier

Regional blood flow in canine brain during nicotine infusion: pentobarbital vs. chloralose anesthesia

We compared vasoactive effects of intravenous nicotine (36 micrograms/kg/min) in regional cerebral circulations under pentobarbital and chloralose anesthesia. Experiments were conducted in three groups of dogs: Group I, pentobarbital anesthesia with fixed ventilation; Group II, chloralose anesthesia with fixed ventilation; Group III, chloralose anesthesia with free breathing. Values for regional cerebral blood flow measured with 15 mu radioactive microspheres were used to compute regional cerebral vascular resistance (rCBR). In Group I, nicotine had no effect on rCVR in cerebral cortex, and it increased significantly rCVR in cerebellum (+17%), pons (+13%), medulla (+23%), and spinal cord (+19%). Using chloralose instead of pentobarbital in dogs with fixed ventilation (Group II), caused a significant reduction in rCVR in the cerebral cortex during nicotine, although it did not alter significantly nicotine-induced changes in rCVR in other regions of the brain. Hypocapnic alkalosis during nicotine-induced hyperventilation (Group III) resulted in significant increases in rCVR in all regions of the brain; however, the increases in rCVR in non-cortical regions more than doubled those in the cerebral cortex. The present results indicate: Nicotine-induced vasodilation in cerebral cortex was blunted by pentobarbital anesthesia. Nicotine-induced vasodilation in cerebral cortex under chloralose anesthesia was sufficient to nullify in part the potent vasoconstrictor effect of hypocapnic alkalosis.

beta-Adrenergic receptors in brain microvessels of diabetic rats

A significant decrease in the number of beta-adrenergic receptors was observed in cerebral microvessels of fatty (fa/fa) and streptozotocin-induced diabetic rats, without receptor affinity changes. These results suggest that alterations of central adrenergic regulation of small vessels may be involved in brain microvasculature disturbances that occur with diabetes.

Regional cerebral blood flow and oxygen consumption of the canine brain during hemorrhagic hypotension

The sequential changes in systemic and cerebral hemodynamics, systemic and cerebral oxygen transport and consumption rates, and the regional blood flows (measured with 15 micron microspheres) to the cortical and subcortical brain tissues were determined in nine dogs subjected to graded hemorrhage (10 ml/kg X 4 at 15 min intervals). As hemorrhage progressed, both mean arterial pressure and cardiac output decreased progressively. In contrast to the systemic circulation, the mean cerebral blood flow (mCBF) was well maintained by cerebral vasodilation and the cerebral O2 consumption rate (CMRO2) increased during the first three stages of hemorrhage. At 40 ml/kg of hemorrhage, there were significant reduction in mCBF and CMRO2 despite the increase in O2 extraction, suggesting the occurrence of cerebral hypoxia and decompensation of the cerebral circulation. There were remarkable regional variations in the responses of regional cerebral blood flows (rCBF) to hypovolemia, resulting in a significant redistribution of cerebral blood flow. The fractions of cardiac output supplying the diencephalon (thalamus and hypothalamus), the brain stem (pons and medulla oblongata) and the cervical spinal cord increased after hemorrhage up to 40 ml/kg. The redistribution of rCBF favors those areas where neurons related to cardiovascular control are located. These findings have significant implications relating to hemodynamic regulation during hemorrhagic hypotension.

Functional consequences of unilateral lesion of the locus coeruleus: a quantitative [14C]2-deoxyglucose investigation

The functional consequences, as reflected in local rates of glucose utilization, of ablation of the locus coeruleus (the nucleus from which a major portion of the ascending noradrenergic fibres arise) have been examined in conscious rats with the quantitative autoradiographic [14C]2-deoxyglucose technique. Measurements of glucose utilization were made 72 h after histologically verified unilateral electrolytic lesions of the locus coeruleus. In the overwhelming majority of the 35 grey matter regions examined, the rate of glucose utilization was unaltered by lesions of the locus coeruleus, and in the limited number of CNS regions in which significant alterations were observed, the magnitude of the changes was invariably modest (less than 20% different from sham-operated control animals). Reductions in glucose use were observed in ipsilateral ventral (by 14%) and lateral thalamic nuclei (by 17%), and rates of glucose utilization in most regions of cerebral cortex were significantly lower (about 10%) in the ipsilateral hemisphere relative to the hemisphere contralateral to the lesion. In one region, the median raphe nucleus, glucose utilization was significantly elevated (by 19%) following lesions of the locus coeruleus. Attempts to accentuate the effects of locus coeruleus lesions by pharmacological manipulation of CNS adrenoreceptors by means of the systemic administration of phenoxybenzamine (30 mg/kg, 40 min prior to measurement of glucose use) in animals bearing unilateral locus coeruleus lesions were unsuccessful; the modest alterations in glucose utilization observed following locus coeruleus lesion alone were even less pronounced in lesioned animals receiving phenoxybenzamine. The alterations in local glucose utilization provoked by phenoxybenzamine were similar in sham-lesioned and locus coeruleus-lesioned animals. It would appear that the functional consequences, in terms of glucose utilization, are much less pronounced when a single neurotransmitter system (in the present studies, noradrenergic neurones) is lesioned than when a multiple neurotransmitter, functionally integrated pathway (such as the visual system) is disrupted.

An inhibitor of cerebral uptake of noradrenaline in jaundiced blood plasma

The permeability of the blood-brain barrier to noradrenaline was estimated in rats with bile duct ligation by intracarotid injection of [14C]-L-noradrenaline, 3H2O, and [113mIn]ethylene diamine tetraacetate (EDTA) under pentobarbitone anaesthesia. Brain uptake of [14C]noradrenaline was expressed as a percentage of that of 3H2O (brain uptake index, BUI) and corrected for the "blood background" by the 113mIn. The BUI of noradrenaline (1.20 +/- 0.19) was not increased in jaundice (0.78 +/- 0.18). The capacity of oxygenated homogenates of rat brain to remove noradrenaline was measured. The presence of jaundiced plasma always caused a substantial suppression of noradrenaline removal. No effect of jaundice on specific radioactive assays for catechol-O-methyl transferase (COMT) or monoamine oxidase (MAO) could be demonstrated.

Regional blood flow in canine brain during nicotine infusion: effect of autonomic blocking drugs

Radioactive microspheres (15 mu) were used to measure regional cerebral blood flow during intravenous infusion of nicotine (36 micrograms/kg/min) in anesthetized, open chest dogs. Experiments were conducted with uncontrolled mean aortic pressure and intact autonomic receptors (Series I; n = 9), and in four groups of dogs with mean aortic pressure held constant (Series II); 1) with intact autonomic receptors (n = 6), 2) after beta adrenergic blockade (n = 8), 3) after alpha and beta adrenergic blockade (n = 6), 4) after alpha and beta adrenergic and cholinergic blockade (n = 4). In Series I, nicotine raised mean aortic pressure (+ 72%) and increased flow in cerebral cortex (+ 67%), cerebellum (+ 38%), pons (+ 46%), medulla (+ 39%), and spinal cord (+ 48%). In all regions, but cortex, increases in vascular resistance limited nicotine-induced increases in flow. In Series II, nicotine changed flow only in cortex. Without blockade, nicotine increased cortical flow (+ 38%); but beta blockade abolished this increase in flow. After alpha and beta blockade nicotine again raised cortical flow (+ 29%), and additional cholinergic blockade had no effect on this response. It is concluded that nicotine causes predominant beta receptor mediated vasodilation in cerebral cortex, although it also activates alpha (vasoconstrictor) receptors and a non-adrenergic, non-cholinergic vasodilator mechanism in this region of brain.

The effects of intravenous norepinephrine on the local coupling between glucose utilization and blood flow in the rat brain

Norepinephrine was infused intravenously in two groups of normal, awake rats. In one group local cerebral glucose utilization (LCGU) was measured by the deoxyglucose method (Sokoloff et al. 1977b); in the other group local cerebral blood flow (LCBF) was determined by the iodoantipyrine method (Sakurada et al. 1978). The experiments were performed during a stable state in which the heart rate was reduced between 36% (LCGU experiments) and 27% (LCBF experiments). Norepinephrine infusion reduced LCGU in all 39 structures measured between - 18 and - 37% from control values obtained in a group of normal non-infused rats. The decrease in LCGU was significant (P less than 0.05) in 38 of the 39 structures tested. LCBF was increased but not statistically significantly in most of the structures examined. When the LCGU values of the various structures during norepinephrine infusion were correlated with their corresponding LCBF values, a tight correlation (r = 0.94) was found indicating a close coupling between LCGU and LCBF during norepinephrine infusion. When compared to the relationship between LCGU and LCBF in a normal, non-infused control group, the slope of the regression line was increased significantly (P less than 0.01) by the norepinephrine infusion, indicating a resetting of the coupling mechanism. This means that, at a given metabolic rate, a higher blood flow is needed to perfuse a brain structure during norepinephrine infusion than during control conditions.

Neurogenic modification of the vulnerability of the blood-brain barrier during acute hypertension in conscious rats

To study the possible influence of sympathetic adrenergic tone on the blood-brain barrier function during acute hypertension in conscious unrestrained rats with indwelling catheters in the aorta and a jugular vein the blood pressure was increase by noradrenaline, 6-hydroxydopamine (6-OHDA) or baclofen. One or 60 min later the rats were sacrificed and the extravasation of 125I labelled albumin determined in the brain. After i.v. injection of noradrenaline the baroreceptor reflex will decrease the sympathetic tone whereas the blood pressure increased induced by the other two drugs is accompanied by an increased sympathetic activity. One minute after a corresponding rise in blood pressure the albumin content in the brain was considerably lower in rats given 6-OHDA than in those given noradrenaline. 60 min after the injection of 6-OHDA or baclofen the extravasation in the brain did not differ despite a considerably more rapid increase in pressure after 6-OHDA. Pretreatment with clonidine increased the blood-brain barrier dysfunction in rats given 6-OHDA but not in those given baclofen, probably because the slower rise in pressure facilitates myogenic autoregulation. It is concluded that neurogenic influences on vessel tone can modify the response of the blood-brain barrier during acute hypertension in conscious rats.

Hypertension reduces the number of beta-adrenergic receptors in rat brain microvessels

Beta-adrenergic receptor function was measured in cerebral microvessels of spontaneously and DOCA-salt hypertensive rats using 125I-iodohydroxybenzylpindolol (IHYP). Both in genetic and in experimental hypertension, a significant decrease in the number of beta-receptor sites was observed, without receptor affinity changes. These results suggest that alterations of central adrenergic regulation of small vessels may participate in the pathogenetic mechanisms leading to the development of the central hypertensive disease.

Pituitary blood flow

The direction of pituitary blood flow, the amount of pituitary blood flow, its regional control, and the role of the median eminence microcirculation are the subjects of this review. Present concepts of pituitary blood flow are focused almost entirely on its direction and arouse from studies of pituitary vascular anatomy performed almost 50 years ago. The development of new anatomic techniques has led to a reappraisal of pituitary angioarchitecture, stimulated physiological studies to clarify the pattern of blood flow within the entire gland, and led to a reappraisal of accepted concepts of directional pituitary blood flow. The availability of techniques to accurately measure organ blood flow has permitted study of pituitary blood flow; and, when combined with knowledge of pituitary anatomy, the application of these techniques promises to provide a means to develop insight into control of the mechanisms by which chemical messengers are delivered to the pituitary to control its function. New anatomic techniques promise to develop new understanding of the three-dimensional arrangement of median eminence microvasculature and yield new concepts of blood flow regulation within the median eminence that can be tested by physiological means.

Cerebral hemodynamics, oxygen uptake and cerebral arteriovenous differences of catecholamines following E. coli endotoxin in dogs

In experimental endotoxic shock in dogs, a decrease in cerebral blood flow and an increase in cerebral metabolic rate of oxygen (CMRO2) have been shown to occur. In this situation the blood levels of adrenaline and noradrenaline are markedly elevated. The aim of the present study was to investigate whether a cerebral uptake of circulating catecholamines with a possible influence on CMRO2 takes place in the brain. In eight anaesthetized dogs, arterial blood, superior sagittal sinus blood and cerebrospinal fluid were analyzed for the concentrations of adrenaline, noradrenaline and dopamine before and up to 4 h after an injection of E. coli endotoxin 1.0-1.5 mg . kg-1. The blood levels of adrenaline and noradrenaline, but not dopamine, increased in response to the endotoxin. From about 30 min after the endotoxin injection, arteriovenous adrenaline and noradrenaline differences indicating a cerebral uptake were most often seen. Increased concentrations of noradrenaline, adrenaline and dopamine in cerebrospinal fluid were observed. Noradrenaline gave the highest concentrations and these were correlated to the CMRO2. In some animals the blood and cerebrospinal fluid concentrations of adrenaline seemed to be related. These results indicate that catecholamines might be of importance for the development of an increased CMRO2 in endotoxic shock.

The influence of antihypertensive therapy on cerebral autoregulation in aged hypertensive rats

Cerebrovascular responses to hemorrhage induced hypotension were tested in aged (24 month) spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) after ten weeks of antihypertensive drug or sham treatment. Antihypertensive drug treatment consisted of 1 mg/kg/day minoxidil, 4 mg/kg/day hydralazine and 4 mg/kg/day propranolol given in the drinking water. Antihypertensive therapy produced a 30% decrease in systolic blood pressure in aged SHR to levels not significantly different from sham treated WKY. Cerebral blood flow (CBF), measured under control conditions using ketamine anesthesia, was not significantly different between sham and drug treated SHR and WKY. Cerebral oxygen metabolic rate (CMRO2) was significantly decreased in drug treated SHR and WKY compared to sham treated rats. Lowering blood pressure to a level between 80 and 95 torr produced no significant change in CBF or CMRO2 in sham or drug treated WKY or antihypertensive treated SHR, but a produced a significant decrease it both CBF and CMRO2 in sham treated SHR. Decreasing mean blood pressure to 50-60 torr produced a significant decrease in CBF but not CMRO2 in both WKY treatment groups and in hypertensive treated SHR, but again in sham treated SHR both CBF ana CMRO2 were significantly decreased. These results indicate that aged hypertensive rats are unable to maintain CBF or CMRO2 under even moderate hypotensive test conditions, and that cerebral autoregulation can be improved with antihypertensive therapy.

Response of local blood flow in the caudate nucleus of the cat to intraventricular administration of histamine

The effect of intraventricular histamine on blood flow in the caudate nucleus of the cat was studied by means of the hydrogen clearance technique. Bilateral ventriculo-cisternal perfusion was installed. After a control period during which both lateral ventricles were perfused with mock CSF with the same composition, the drug under study was added to one side (experimental side) while the other side was perfused further with the control mock SCF (control side). At each point in time, blood flow at the experimental side was compared to that at the control side. Histamine (10(-3) M) caused a severe vasodilatation and this effect was completely antagonised by the H2-receptor blocker cimetidine (10(-2) M). Cimetidine had no vasoactive effects of itself in the concentration used. The H2-receptor agonist Dimaprit (10(-3) M) had a vasodilator effect although less important than histamine. Indirect evidence was gained that H1-receptors are not active in the vascular bed under study.

Brain hypoxia: the turning-point in the genesis of the migraine attack?

The hypothesis postulates that a brief episode of focal cerebral hypoxia occurs in every attack of migraine. Clinical biochemical and technical (EEG and CT scans) evidence is summarized suggesting that cerebral hypoxia is seen as the turning-point in the pathogenesis of the attack. It may be provoked by different mechanisms in different patients; the potential role of decreased oxygen supply and of increased oxygen need are reviewed and excess sympathetic drive is considered a potential key mechanism in a majority of patients. Whether or not focal hypoxia leads to a genuine migraine attack, depends largely upon the quality of the whirlpool of biochemical, vascular and hematological changes that follow the hypoxic episode. These changes are discussed and it is concluded that those which have been reported to occur during migraine attacks could be due to a preceding hypoxic event. Finally, the hypoxia viewpoint is confronted with some popular theories about the pathogenesis of migraine. It is found that the other points of view are compatible with the hypoxia hypothesis.

Cerebral blood flow and oxygen uptake in endotoxic shock. An experimental study in dogs

Cerebral blood flow (CBF), cerebral oxygen uptake (CMRO2) and central haemodynamics in anaesthetized dogs with controlled ventilation were studied at intervals for 2 h following an intravenous injection of E. coli endotoxin, 1.0-1.5 mg/kg. CBF showed a 30% decrease within 15 min after the endotoxin administration, while the arterial blood pressure was still not markedly depressed. Autoregulation to arterial blood pressure changes was maintained during endotoxinaemia and the cerebrovascular reaction to changes in arterial carbon dioxide tension (PaCO2) depressed. Normocapnic animals (PaCO2) greater than or equal to 4.0 kPa) showed an increase in CMRO2 of over 40%, that was obvious 1 h after the administration of endotoxin. The intracranial pressure was decreased with 5 min of the administration of endotoxin irrespective of the prevailing arterial blood pressure. Thereafter, it was raised above the control level. Two hours after endotoxin increased protein concentrations in cerebrospinal fluid were seen, results compatible with blood-brain barrier damage and penetration of other substances; e.g. monoamines released during endotoxinaemia could thus be expected to have a direct influence on both cerebral blood flow and metabolism.

The effects of high dose methylprednisolone or fluid volume expansion on cerebral haemodynamics and oxygen uptake in endotoxic shock. An experimental study in dogs

E coli endotoxin has been shown to decrease cerebral blood flow (CBF) and increase cerebral metabolic rate of oxygen (CMRO2) in normocapnic dogs. The aim of this study was to investigate the effects of different clinical treatments on the cerebral and central circulation already under the influence of endotoxin. Thus the animals were treated with either methylprednisolone or a lactated Ringer's solution. CBF, CMRO2 and intracranial pressure were followed. Central haemodynamic parameters, i.e. cardiac output, aortic pressure and pulmonary artery pressure, were also measured. Five dogs were given methylprednisolone (Solu-Medrol) 30 mg/kg 90 min after the endotoxin injection. Following this drug there were no significant changes in CBF when compared to controls. The primarily increased CMRO2 did, however, show a transient decrease. Five animals were treated with a lactated Ringer's solution. (Ringerdex), 30 mg/kg, given intravenously over 20 min starting 90 min after the endotoxin injection. In these animals, the cardiac output as well as CMRO2 returned to the values before endotoxin. CBF did not increase significantly.

Effect of diazepam on cerebral monoamine synthesis during hypoxia and hypercapnia in the rat

In view of the fact that diazepam has been shown to prevent an increase in catecholamine synthesis and/or turnover rates in stressful situations, and to modify the cerebral metabolic (and circulatory) response to hypoxia and hypercapnia, the influence of the drug on synthesis rates of DOPA and 5-HTP in three regions of the rat brain were studied under normoxic-normocapnic conditions, as well as in hypoxia and hypercapnia. In order to exclude a modifying influence of variations in tissue pO2 during hypercapnia, cerebral venous pO2 was kept at control values by moderate arterial hypoxia. When compared to the control state (paralyzed animals maintained on 70% N2O) normoxic and normocapnic animals given diazepam (in the absence of N2O) showed a slightly enhanced DOPA synthesis in limbic structures and reduced 5-HTP synthesis in limbic structures and striatum. In hypoxia, the drug considerably curtailed DOPA synthesis in limbic structures and striatum but had no effect on synthesis rate in cortex. The drug also appeared to exaggerate the generalized reduction in 5-HTP synthesis observed under 70% N2O. In hypercapnia, diazepam reduced the enhanced rate of DOPA synthesis (observed under 70% N2O) in striatum but left that in the cortex unchanged. The drug prevented the hypercapnia-induced increase in 5-HTP synthesis, observed under 70% N2O. It is concluded that diazepam significantly alters dopamine and serotonin synthesis in hypoxia and hypercapnia. Probably an indirect action, perhaps related to the stress-alleviating effect of diazepam, is involved. The results suggest that the effect of the drug on cerebral metabolic rate and blood flow in hypoxia and hypercapnia is unrelated to changes in noradrenaline synthesis or turnover. Furthermore, although the results demonstrate that diazepam modulates dopamine metabolism in hypoxia and hypercapnia it seems questionable that this influence can explain the metabolic and circulatory effects of diazepam in these conditions.

Norepinephrine in cerebrospinal fluid of patients with cerebral vasospasm

The content of norepinephrine (NE) in the ventricular, basal cisternal, and lumbar cerebrospinal fluid (CSF) was determined in 19 patients with ruptured cerebral aneurysms at different intervals according to the presence or absence of vasospasm. Twelve were operated on within 3 days after subarachnoid hemorrhage (SAH), prior to the occurrence of vasospasm. Postoperatively, CSF was continuously drained from a basal cistern or lateral ventricle. Norepinephrine was assayed by the highly sensitive automated fluorometric method. The concentration of NE increased in all sites of CSF sampling along with the appearance of vasospasm. Above all, the cisternal CSF of patients with vasospasm contained significantly higher NE (0.246 +/- 0.049 ng/ml, mean +/- SEM) compared to those without vasospasm (0.075 +/- 0.001 ng/ml) (p less than 0.001). However, since this increase cannot be considered to be high enough locally to constrict cerebral arteries, this might be only a secondary phenomenon due to release of NE into CSF from various sources in the brain.

Cortical injury without ischemia produced by topical monoamines

Responses to monoamines perfused over the cortical surface through modified pial windows were monitored in 106 cats. Norepinephrine (NE) and serotonin (5-HT) were diluted in mock CSF to concentrations of 50 and 500 ng/ml respectively, levels at or near the maximum concentrations to which the cortical surface might be exposed in subarachnoid hemorrhage or damage to nearby neurons. Each cat had simultaneous one-hour perfusions of monoamine solution over one hemisphere and a control solution over the other hemisphere thus serving as its own control. The perfusion solutions were observed to be restricted to the area of the pial window, and minimal histological damage was seen with the perfusion technique. The 5-HT perfusions were associated with an almost 20% narrowing of small pial arteries and arterioles but no significant effect on regional cerebral blood flow (rCBF), cortical water content or cortical function as monitored by EEG and somatosensory evoked potentials (SEP). In contrast, NE caused cortical edema and changes in the EEG and SEP's without significant vascular effect. These results suggest a non-ischemic toxicity of NE released by subarachnoid hemorrhage or cerebral damage.

Anesthetics affect the cerebral metabolic response to circulatory catecholamines

This study examined whether the effect of intravenous infusions of either epinephrine or norepinephrine on cerebral metabolic rate for oxygen (CMRO2) in the dog was modified by different anesthetics. Infusions of either epinephrine or norepinephrine at rates of 0.1-0.25 mu.kg-1.min-1 reversibly increased the CMRO2 by 17-23% during anesthesia with cyclopropane 20% and nitrous oxide 50% in oxygen, whereas infusions at rates of 0.1-25.0 micrograms.kg-1.min-1 had no effect in dogs anesthetized with other inhalational or intravenous agents. Cyclopropane/nitrous oxide also increased permeability of the blood-brain barrier to Evan's blue dye whereas the other anesthetics tested did not. It is concluded that epinephrine and norepinephrine crossed the blood-brain barrier during cyclopropane anesthesia, accounting for the increase in CMRO2. The authors speculate that cyclopropane may have increased blood-brain barrier permeability by a direct effect on endothelial cells or by affecting central adrenergic systems and that epinephrine or norepinephrine may increase CMRO2 either by a direct action on neuronal receptors or via metabolically coupled synaptic events.

Cerebral blood flow in rats during physiological and humoral stimuli

The technique for estimating cerebral blood flow (CBF) in anesthetized rats by injecting 133Xe into the internal carotid artery represents a potentially useful and inexpensive model for screening cerebral vascular responses to pathophysiological and pharmacological stimuli. We have examined associated neuropathology, the validity and the reproducibility of the method, and made comparisons of initial slope estimates of CBF with those obtained by stochastic analysis. Initial slope estimates (CBF = 1.62 +/- 0.04 ml min-1g-1, X +/- SE, N = 38) were linearly related to stochastic measurements (CBF = 1.42 +/- 0.09 ml min-1g-1, N = 6), and overestimated mean CBF by about 15%. A reactivity to CO2 of 0.05 ml min-1g-1 per mm Hg, and an auto-regulation range of 70 to 180 mm Hg were found. CBF responses to the intra-arterial infusion of aminergic drugs were determined before and after opening of the blood-brain barrier with hypertonic urea. Serotonin reduced CBF after, but not before, the administration of urea. Acetylcholine increased CBF when the barrier was intact, the effect being augmented when the barrier was disrupted; these responses were reduced by atropine. Histamine increased CBF only after barrier opening, and this response was attenuated by the H2-receptor antagonist, metiamide. These studies indicate that initial slope estimates of CBF derived in rats from intracarotid 133Xe injection, which represents an inexpensive and simplified approach for screening cerebral circulatory adjustments, may facilitate the characterization of stimuli affecting CBF.

Cerebral blood flow and oxygen consumption in the rat brain after lesions of the noradrenergic locus coeruleus system

The effect of lesions of the locus coeruleus neuron system on cerebral metabolic rate for oxygen (CMRO2) and blood flow (CBF) was evaluated in paralyzed and mechanically ventilated rats, using a 133xenon modification of the Kety-Schmidt inert gas technique. Bilateral electrothermic lesions of its ascending bundle caused no significant change in CBF or CMRO2. The 6-hydroxydopamine lesions did not influence the CBF and CMRO2 responses to hypercapnia and hypoxia. It is concluded that the locus coeruleus does not exert any resting tone on CBF and CMRO2 and that no influence on the CBF and CMRO2 responses to hypercapnia and hypoxia is mediated via its ascending projections.

Vasoactive intestinal polypeptide and the canine cerebral circulation

A potential role for cerebrovascular nerves containing vasoactive intestinal polypeptide (VIP) was examined in 24 anesthetized, ventilated dogs. Cerebral blood flow (CBF) was measured by either the cerebral venous outflow or microsphere method. Plasma VIP concentration was measured by radioimmunoassay. Hypercapnia (5% and 10% CO2) and hypoxia (7% O2) produced significant increases in cerebral venous outflow, but had no affect on arterial or cerebral venous VIP concentrations. Measurements of VIP in cerebrospinal fluid (CSF) made during 5% and 8% CO2 breathing also were not different from control values. VIP produced large dose-dependent increases in common carotid artery and temporalis muscle blood flow when injected or infused intraarterially; however, VIP had no effect on total or regional cerebral blood flow (rCBF) within the brain when administered in a similar manner. Unilateral perfusion of the cerebral ventricles with VIP produced significant increases (range: 11-80%) in rCBF. These data are consistent with the possibility that local release of VIP from perivascular nerve endings could affect CBF. The unresponsiveness of canine cerebral vessels to blood-borne VIP may be due to the blood-brain barrier, since VIP dilates cerebral vessels when the barrier is bypassed by intraventricular infusion. These studies do not support the hypothesis that CBF changes induced during hypercapnia or hypoxia are mediated by VIP.

Canine cerebral metabolism and blood flow during hypoxemia and normoxic recovery from hypoxemia

There are conflicting reports regarding the effects of hypoxemia on the cerebral metabolic rate for oxygen (CMRO2). Accordingly, we examined the changes in CMRO2 during normoxia, progressive hypoxia (PaO2 of 37, 27, and 23 mm Hg), and normoxic recovery from hypoxia. Measurements were made in dogs anesthetized with nitrous oxide (60-70%) and halothane (less than 0.1%) in oxygen. Arterial-cerebral venous blood oxygen content differences and cerebral blood flow (CBF) were measured simultaneously, the latter by a technique (collection of sagittal sinus outflow) previously validated for conditions of near-maximal CBF. The duration of each of the three hypoxic exposures was approximately 10 min. CMRO2 was significantly decreased (14%) only when the arterial blood oxygen tension was reduced to 23 mm Hg. CBF increased progressively to a maximum of 153% of control. Posthypoxemic brain biopsy values for cerebral metabolites obtained 40 min after normoxemia had been restored were normal. These results, in conjunction with an unchanged CMRO2 at 40 min normoxic recovery, suggest that no gross irreversible brain cell damage occurred. We conclude that with progressive hypoxemia, CMRO2 remains stable until oxygen demand exceeds oxygen delivery, resulting thereafter in a progressive reduction in CMRO2.

Enzymes related to monoamine transmitter metabolism in brain microvessels

The activities of tyrosine hydroxylase, aromatic L-aminoacid decarboxylase, monoamine oxidase, and catechol-O-methyltransferase were measured in microvessel (capillaries and venules), parenchymal arterioles, and pial vessels from rat brains, and the decarboxylase activity was compared in brain microvessels from rabbit, cat, dog, pig, cow, baboon, and man. Cranial sympathectomy was performed to estimate the neuronal contribution to the enzyme activities. All vascular regions had substantial activities of the various enzymes studied. The activity of aromatic L-aminoacid decarboxylase in cerebral microvessels was high in rat, dog, pig, cow, and man; intermediate in rabbit and cat; and low in baboon. In addition to this enzyme, cerebral microvessels also contained tyrosine hydroxylase and monoamine oxidase. Aromatic aminoacid decarboxylase and monoamine oxidase serve an enzymatic barrier function at the microvascular level, whereas the main function of tyrosine hydroxylase is probably to synthesize monoamines within nerve terminals that remain in close association with microvessels under the conditions used for preparation of the microvascular fraction. In larger intracerebral and pial vessels monoamine oxidase was present both in the wall itself and in perivascular sympathetic nerves; the remaining two enzymes had a primarily neuronal localization. The latter types of vessels also contained catechol-O-methyltransferase in their walls.

Effect of chronic hypertension on the blood-brain barrier

Disruption of the blood-brain barrier (BBB) during acute hypertension may contribute to hypertensive encephalopathy. In this study we tested the hypothesis that, in chronic hypertension, vascular changes might influence the susceptibility of the BBB to disruption. Spontaneously hypertensive rats (SHR) and normotensive rats (WKY), 3-4 months of age, were anesthetized and acute hypertension was produced by infusing phenylephrine intravenously (i.v.). Permeability of the BBB was studied with radioactive iodine serum albumin (RISA) injected i.v. The ratio of brain-to-blood RISA was used as an index of permeability of the BBB expressed as protein transfer. In both SHR and WKY at resting arterial pressure, the protein transfer was less than 0.10%. In WKY exposed to acute hypertension (mean arterial pressure increased by 87 +/- 7 mm Hg), the protein transfer was 2.77 +/- 0.60%. In SHR with acute hypertension superimposed on chronic hypertension (arterial pressure increased by 80 +/- 7 mm Hg), the protein transfer was 1.16 +/- 0.45% (p less than 0.05, SHR vs WKY). These data suggest that cerebral vessels are less susceptible to disruption of the BBB by acute hypertension in SHR than in WKY. We speculate that the finding of reduced susceptibility to BBB disruption in chronic hypertension may explain, in part, the apparent susceptibility of previously normotensive patients to acute hypertensive encephalopathy.

Cerebral circulatory and metabolic effects of piribedil

Two aspects of the cerebrovascular action of the putative dopaminergic agonist, piribedil, have been examined. The vasomotor responses of isolated feline middle cerebral artery to piribedil and its metabolite, S584, were first examined and the effects of piribedil upon cerebral blood flow, cerebral oxygen consumption and the electroencephalogram (EEG) were then investigated in anaesthetised baboons. Neither piribedil nor S584 displayed any marked vasomotor efficacy in vitro, with small changes in tension being observed only with large concentrations (greater than 10(-4) M). In the anaesthetised baboons, the administration of piribedil (0.1 and 1 mg/kg, i.v.) resulted in significant increases in cerebral blood flow (40 +/- 10% and 49 +/- 14%, respectively) (mean +/- S.E.M.) and cerebral oxygen consumption (13 +/- 10% and 17 +/- 6%) which were accompanied by an increase in low voltage fast activity of the EEG. Prior administration of the putative dopaminergic antagonist, pimozide (0.5 mg/kg), which itself was without significant effect upon cerebral blood flow and oxygen consumption, prevented the cerebral circulatory, metabolic and EEG alterations induced by piribedil (1 mg/kg). It would appear likely that the action of piribedil upon cerebral metabolic activity was principally responsible for the increases in cerebral tissue perfusion which followed its administration.

6-hydroxydopamine and the blood-brain barrier in adult conscious rats

6-hydroxydopamine (6-OHDA), 15 or 50 mg . kg-1 given as bolus i.v. injection to adult conscious rats with aortic catheter, rapidly increased mean arterial pressure by 70-78 mmHg. The pressure returned to normal within 40-60 min. The cerebrovascular permeability in rats given 6-OHDA and sacrificed 10 or 60 min later was enhanced as indicated by extravasation of Evans blue albumin and significant increase of 125I human serum albumin content in brain tissue compared to control rats. When the increase in blood pressure was diminished by i.v. phentolamine, 6-OHDA treated rats did not differ from controls. It is concluded that the blood pressure elevation induced by i.v. 6-OHDA facilitates the entry of the drug into the brain parenchyma.

Barrier mechanisms for neurotransmitter monoamines and their precursors at the blood-brain interface

The integrity of the endothelial cell lining of the cerebrovascular bed constitutes a morphological blood-brain barrier mechanism to neurotransmitter monoamines. Circulating monoamines are prevented from entering the brain primarily at the luminal membrane of the endothelial ling. The small percentage of amines that may pass this membrane is deaminated within the endothelial cells and pericytes of brain microvessels (capillaries, venules, and small veins) and, in the case of large parenchymal and pial vessels, in the smooth muscle layers, where O-methylation also takes place. In the choroid plexus a corresponding deamination and O-methylation takes place in the epithelial cells. The presence of these enzymes constitutes a further, enzymatic, blood-brain barrier in the brain vessels for these monoamines. The monoamine precursors L-3,4-dihydroxyphenylalanine (L-dopa) and L-5-hydroxytryptophan readily pass from the luminal endothelial cell membrane but are trapped by another enzymatic barrier mechanism. Within the endothelial cells and pericytes of the microvasculature, these compounds are decarboxylated to their corresponding amines and then immediately deaminated. One clinical implication of these enzymatic barrier mechanisms is the use of decarboxylase and monoamine oxidase inhibitors as adjuncts to L-dopa treatment of Parkinson disease; these substances facilitate the entry of L-dopa into brain and thus increase the amount of dopamine available at receptor sites. A brief hypertensive or hypertonic stimulus can transiently open the blood-brain barrier through an effect on endothelial cell linings. High circulating concentrations of monoamines can also open the morphological barrier, but probably only indirectly by inducing an acute rise in systemic blood pressure. Once the barrier is open, systemically administered monoamines enter the brain parenchyma, where they can induce pronounced changes in cerebral blood flow and metabolism.

Relationship of cerebral blood flow to cardiac output, mean arterial pressure, blood volume, and alpha and beta blockade in cats

The relationship among cerebral blood flow (CBF), blood volume, cardiac output (CO), and mean arterial blood pressure (MABP) at varying levels of arterial CO2 tensions (PaCO2) were studied in 70 normal cats. The CBF was measured from the clearance curve of xenon-133 and CO with a thermal dilution catheter placed in the pulmonary artery. The CBF, CO, and MABP values varied appropriately with changes in PaCO2, confirming the reliability of the preparations and the presence of normal autoregulatory responses. Moderate hypovolemia that did not change MABP did, nevertheless, significantly decrease CO and CBF. In an effort to determine if this decrease in CO and CBF were coupled responses, the effects of beta stimulation, hypervolemia, and alpha and beta blockade were investigated. Propranolol, in a dosage insufficient to change MABP, decreased both CO and CBF. This agent abolished the CO response to elevations in PaCO2 but not the CBF response, making it unlikely that this CBF reduction resulted from impaired cerebral autoregulation. Isoproterenol, which, in contrast to propranolol, does not cross the normal blood-brain barrier, alone or in combination with phenoxybenzamine, produced a 38% and 72% increase in CO, respectively, without a change in CBF. Alpha blockade (no major change in CO) and beta blockage (major decrease in CO) did not significantly effect cerebral autoregulation to changes in MABP from angiotensin. The ability of the brain to resist increases in MABP and CO and maintain normal CBF is explained by normal cerebral autoregulation. However, its vulnerability to modest decreases in blood volume, which cannot be attributed to variations in perfusion pressure, is unexplained but obviously has important therapeutic implications. This may be related to reduction in CO, changes in autonomic activity, or a decrease in the size of the perfused capillary bed.

The effect of alpha-adrenergic innervation on caudate blood flow

A thermal diffusion probe, with cannulas for intracerebral microinfusion of drugs and an electrode to monitor electroencephalographic (EEG) activity, was used to examine the local effect of vasoactive amines in a 4 to 5 mm sphere of caudate nucleus in cats. The results demonstrated that it is possible to alter local cerebral blood flow (CBF) without causing any change in systemic blood pressure or heart rate, or in CBF and the EEG in the opposite caudate. One-microliter intracerebral injections, containing varying amounts of phenylephrine, increased local CBF in proportion to dose. The effect was blocked by intracerebral infusion of phentolamine. However, local alpha-adrenergic blockade did not inhibit vascular responses to blood pressure elevation or to metabolic influences on local blood flow.

Quantitative changes in regional cerebral blood flow of rats induced by alpha- and beta-adrenergic stimulants

Cerebral blood flow was measured with the 14C-ethanol technique in 8 regions (frontal, parieto-temporal and occipital cortex, caudate nucleus, thalamus, cerebellum, mesencephalon, and pons) of rats. The highest flow values (83-89.5 ml/100 g/min) were found in cortical areas, whereas pons had the lowest flow (48 ml/100 g/min). Intravenous infusion of noradrenaline or adrenaline markedly reduced rCBF (by 22-48% of control levels) in all regions except thalamus, mesencephalon, and pons. The noradrenaline-induced reduction was blocked, and the effect of adrenaline reversed, after pretreatment with the alpha-receptor antagonist, phentolamine. Isoprenaline infusion markedly augmented rCBF in thalamus, mesencephalon, pons, and also in the caudate nucleus. The response was reduced by the beta-receptor antagonist, propranolol. The experiments show the presence and heterogenous distribution in the cerebrovascular bed of slpha- and beta-adrenoceptors that can be activated by sympathomimetics given systematically. If noradrenaline was allowed to pass the blood-brain barrier after osmotic opening with urea, an increased regional flow was obtained, probably due to a mechanism where the vasodilator effect secondary to activation of cerebral metabolism predominated over the direct vasoconstrictor effect of the amine.

Correlated electrical and mechanical responses of isolated rabbit pial arteries to some vasoactive drugs

Simultaneous measurements were made of spike activity and perfusion pressure (PA) in intact segments of rabbit middle cerebral artery in vitro. The segments were mounted on a Teflon tube designed so that the perfusing solution flowed in the annular space between the tube and the artery wall, thus magnifying the PA changes occurring when the artery constricted or dilated. A widened portion of the Teflon tube immobilized 1--2 mm of the artery segment for electrical recording with fine glass microelectrodes. Spontaneous spike activity (extra- and intracellular) was regularly observed. When a steady PA and spike discharge was obtained, tests were performed by substituting for the normal perfusion liquid, solutions containing 5 microgram/ml norepinephrine, 5 microgram/ml angiotensin II or 7.5 microgram/ml isoproterenol. Norepinephrine and angiotensin each increased spike frequency (+ 293 and + 126%) and PA (+ 6.6 and + 7.9 mm Hg) whereas isoproterenol decreased spike frequency (-89%) and PA (-22.9 mm Hg). These results a) confirm the presence of receptors to these agents in pial arteries, and b) demonstrate a high degree of correlation between membrane electrical events and mechanical activity of these spontaneously-active myovascular cells.

Increase in local cerebral blood flow induced by circulating adrenaline: involvement of blood-brain barrier dysfunction

The influence of intravenous infusion of adrenaline (8 micrograms.kg-1.min-1) upon local cerebral blood flow (CBF) in paralyzed and artifically ventilated rats was measured autoradiographically with 14C-iodoantipyrine as the diffusible tracer. At this dose, adrenaline invariably increased local CBF even though blood pressure was close to normal at the time of the CBF measurement. In average, local CBF increased to 400% of control. In 6 of 9 animals the increase in flow was inhomogenous with randomingly distributed areas of very high flow rates. Experiments with i.v. administration of Evans blue prior to infusion of adrenaline showed that areas of Evans blue extravasation appeared in 3 of 4 animals. Although areas of extravasation often corresponded to areas of high flow rates the former were much more circumscribed. Furthermore, very high flow rates were found in areas showing no sign of blood-brain barrier dysfunction. It is concluded that the increase in CBF was at least partly due to a pressure-mediated passage of adrenaline across the blood-brain barrier but that such a passage can occur in the absence of macroscopically visible extravasation of protein.

Blood-brain barrier to albumin in awake rats in acute hypertension induced by adrenaline, noradrenaline or angiotensin

Acute hypertension was induced by adrenaline, noradrenaline or angiotensin in awake unrestrained rats with chronic indwelling catheters in a jugular vein and in the aorta. The leakage of 125IHSA (human serum albumin) into the brains from rats given adrenaline was significantly larger than in the brains from rats given noradrenaline or angiotensin. It is likely that the enhanced vulnerability of the blood-brain barrier to an adrenaline-induced increase in blood pressure is due to the beta-adrenergic stimulating effect of adrenaline.