FACTORS AFFECTING THE CEREBROVASCULAR RESPONSE TO NORADRENALINE IN THE DOG

The cerebrovascular response to norepinephrine: A scoping systematic review of the animal and human literature

Intravenous norepinephrine (NE) is utilized commonly in critical care for cardiovascular support. NE's impact on cerebrovasculature is unclear and may carry important implications during states of critical neurological illness. The aim of the study was to perform a scoping review of the literature on the cerebrovascular/cerebral blood flow (CBF) effects of NE. A search of MEDLINE, BIOSIS, EMBASE, Global Health, SCOPUS, and Cochrane Library from inception to December 2019 was performed. All manuscripts pertaining to the administration of NE, in which the impact on CBF/cerebral vasculature was recorded, were included. We identified 62 animal studies and 26 human studies. Overall, there was a trend to a direct vasoconstriction effect of NE on the cerebral vasculature, with conflicting studies having demonstrated both increases and decreases in regional CBF (rCBF) or global CBF. Healthy animals and those undergoing cardiopulmonary resuscitation demonstrated a dose-dependent increase in CBF with NE administration. However, animal models and human patients with acquired brain injury had varied responses in CBF to NE administration. The animal models indicate an increase in cerebral vasoconstriction with NE administration through the alpha receptors in vessels. Global and rCBF during the injection of NE displays a wide variation depending on treatment and model/patient.

Adaptative response of antioxidant enzymes in different areas of rat brain after repeated d-amphetamine administration

d-Amphetamine has been shown to be a potential brain neurotoxic agent, particularly to dopaminergic neurones. Reactive oxygen species indirectly generated by this drug have been indicated as an important factor in the appearance of neuronal damage but little is known about the adaptations of brain antioxidant systems to its chronic administration. In this study, the activities of several antioxidant enzymes in different areas of rat brain were measured after repeated administration of d-amphetamine sulphate (sc, 20 mg/kg/day, for 14 days), namely glutathione-S-transferase (GST), glutathione peroxidase (GPx), glutathione reductase (GRed), catalase, and superoxide dismutase (SOD). When compared to a pair-fed control group, d-amphetamine treatment enhanced the activity of GST in hypothalamus to 167%, GPx in striatum to 127%, in nucleus accumbens to 192%, and in medial prefrontal cortex to 139%, GRed in hypothalamus to 139%, as well as catalase in medial prefrontal cortex to 153%. However, the same comparison revealed a decrease in the activity of GRed in medial pre-frontal cortex by 35%. Food restriction itself reduced GRed activity by 49% and enhanced catalase activity to 271% in nucleus accumbens. The modifications observed for the measured antioxidant enzymes reveal that oxidative stress probably plays a role in the deleterious effects of this drug in CNS and that, in general, the brain areas studied underwent adaptations which provided protection against the continuous administration of the drug.

In vivo modulation of norepinephrine-induced cerebral oxygenation states by hypoxia and hyperoxia

The effect of intravenous norepinephrine (NE) administration on three O2-dependent parameters of cerebral oxygenation was studied in the parietal cortex of skull intact anesthetized rats. Reflectance spectrophotometry was used to measure in vivo changes in cortical hemoglobin saturation (Hb/HbO2), blood volume (BV), and cytochrome c oxidase (cyt. a,a3) oxidation-reduction state. The influence of arterial pressure of oxygen (paO2) on norepinephrine-induced changes in cortical microcirculatory O2 delivery and cyt. a,a3 redox state was tested under conditions of normoxia, hypoxia, and hyperoxia. Norepinephrine produced cyt. a,a3 redox changes which were independent of compensatory alterations in cortical blood volume and changes in systemic blood pressure at the tested physiological extremes. During normoxia, NE caused dose-dependent systemic pressure-related increases in the oxidation level of cyt. a,a3. Conversely, in hypoxia NE caused a reduction. Microcirculatory and cyt. a,a3 redox responses to low doses of NE during hyperoxia were similar to those obtained at high doses during normoxia. The kinetic pattern of changes in hemoglobin saturation, cyt. a,a3 redox state, and cortical blood volume during normoxia and hypoxia was consistent with direct alteration in oxygen delivery to the respiratory chain and possible modification of cerebral oxidative metabolism. Blood-brain barrier alterations and vascular smooth muscle resistance changes to NE under tested conditions of oxygenation are postulated to be responsible for the observed results.

Monoamine oxidase activity in the cerebral vasculature: comparison between fresh microvessels from different structures and cell cultures derived from microvessels

Monoamine oxidase (MAO) activity was studied in various preparations of porcine brain microvessels to explore further the role of this enzyme in the blood-brain barrier to catecholamines. No difference was noted (Vm and Km) between microvessels isolated from three structures (caudate nucleus, thalamus, and cerebral cortex) in which the responses to circulating catecholamines in vivo are markedly different. Large and small microvessels from the caudate nucleus and the thalamus presented the same specific activity. Cell cultures obtained from small microvessels were rich in endothelial cells as identified by the presence of Factor VIII-related antigen. These preparations displayed an MAO activity about ninefold less than freshly isolated microvessels, although their prostaglandin synthetase activity appeared normal. These results suggest that MAO activity is not the main factor determining the regional differences in the cerebrovascular reactions to catecholamines, that MAO is not specifically localized in the endothelium but must be also present in the smooth muscle, and that the MAO activity is greatly decreased during cell culture.

Treatment of acute focal cerebral ischemia with propranolol

Propranolol's potential as a protective agent against tissue injury has been noted in experimental myocardial, renal and early acute focal cerebral ischemia. The purpose of the present investigation was to study further the effects of racemic (d,l) propranolol on blood-brain barrier permeability, morphological changes, cortical electrical activity, and regional cerebral blood flow (rCBF) in experimental focal cerebral ischemia. Thirty adult cats, anesthetized with nitrous oxide, underwent 6 hours of right middle cerebral artery (MCA) occlusion. Fifteen cats were untreated. Fifteen cats were given a continuous infusion of racemic propranolol (1 mg/kg/hr) for 7 hours beginning 1 hour before MCA occlusion and a 4 mg/kg bolus immediately before occlusion, both directly into the right carotid artery. Right Sylvian rCBF did not significantly differ in the treated and untreated groups. Carbon filling defects and vital dye (i.e., Evans blue and fluorescein) extravasation were less severe in the propranolol treated animals. Light microscopic findings demonstrated no difference in infarct size between the two groups. The findings suggest that at doses given, racemic propranolol does not exert a protective effect upon cerebral tissue subjected to 6 hours of incomplete ischemia.

A possible role of the carotid body in the pathogenesis of cluster headache

An hypothesis regarding the possible role of the carotid body in the pathogenesis of cluster headache is presented. It states: 1. The pathways concerned with cyclic cluster periods may begin centrally involving specific areas in the hypothalamus. The major influence of this physiological change is proposed to be an inhibition of the sympathetic and disinhibition of parasympathetic supplies to the carotid body. The result, whether due to increased vasomotor tonus or interruption of intrinsic sympathetic stimulation, is suggested to cause diminished peripheral chemoreceptor activity. 2. The pathway concerned with onset of spontaneous or induced attacks begins, as proposed, with oxygen desaturation--which, upon reaching threshold levels may induce a hyperactive chemoreceptor response, and stimulate through afferent pathways the nuclei of the 7th and 10th cranial nerves and respiratory centers, via the nucleus solitarius. 3. The consequence of this excitation may involve the third suggested pathway resulting in stimulation of peripheral secretory and other receptors innervated by the cranial nerves.

Monoamine oxidase activity in brain microvessels determined using natural and artificial substrates: relevance to the blood-brain barrier

The possible contribution of cerebrovascular monoamine oxidase (MAO) to the blood-brain barrier to catecholamines was studied in isolated porcine and rat microvessels by determining its activity with various substrates. Michaelis-Menten kinetic constants, Km and Vmax, were determined using noradrenaline (NA) as substrate in a Tris medium. Km values were 0.25 +/- 0.05 mM in control and 0.16 +/- 0.09 mM in ultrasonically disintegrated (USD) preparations (difference not significant); Vmax in USD preparations (1.83 +/- 0.20 n.atoms O2 min-1 mg protein-1) was slightly higher (p less than 0.05) than in control preparations (1.35 +/- 0.11 n.atoms O2 min-1 mg protein-1), suggesting a certain restriction by the plasma membrane of substrate access to the enzyme. This phenomenon was confirmed in a more physiological, ionic medium; the activity was then approximately doubled for 1 mM NA, whereas that for 1 mM beta-phenylethylamine (beta-PEA), a lipid-soluble substrate, tended to decrease with USD treatment. These results show that this highly active form of MAO is unlikely to be saturated by physiological concentrations of catecholamine. It can be estimated that, for a plasma concentration of NA of 1 microM, a facilitated diffusion accelerating the entry of the catecholamine into the cells by at least 15-fold would be necessary in order to exceed the catabolic capacity of MAO. It is concluded that circulating catecholamines are not likely to cross the endothelial barrier of cerebral microvessels intact, and that the small quantities of radioactivity detected in the parenchyma in measurements of the brain uptake index essentially represent metabolites due to MAO activity.

Post-ischemic hypermetabolism in cat brain

Delayed postischemic brain hypoperfusion and hypermetabolism are likely detrimental factors to neurologic recovery after transient global brain ischemia and may be mediated by catecholamines acting via adrenergic receptors. We evaluated the effects of alpha and beta receptor blockade on cerebral blood flow (CBF) and metabolism after 16 min transient global brain ischemia. Ischemia was induced by arterial hypotension and a high pressure neck tourniquet in 13 anesthetized cats. Six cats were untreated, 4 received propranolol 1 mg/kg, IV and 3 a combination of propranolol and phentolamine, one mg/kg injected one min before recirculation. Total CBF was measured by continuous monitoring of cerebral venous 133Xe clearance after bolus intra-arterial injection. Arterial and cerebral venous oxygen, glucose and lactate were measured. Cerebral cortex glucose and lactate were measured 3 hours post-ischemia after in situ freezing with liquid N2. The cerebral cortex of 3 cats anesthetized, but not subjected to ischemia, was similarly frozen and analyzed for glucose and lactate. Total CBF was relatively constant for up to 3 h post-ischemia in all groups, but significant changes in fast and slow-flow rates and compartment sizes were observed. In untreated cats, the normal 60/40 percent relative weight of the fast and slow-flow compartments was reversed to 30/70 percent by 1 hr post-ischemia. Propranolol attenuated the size of the fast-flow compartment in the first 30 min post-ischemia which was partially restored by phentolamine. Brain oxygen consumption increased 2 to 3-fold by 1 h post-ischemia in all groups. Propranolol compromised CBF and impaired glucose and lactate oxidation which was partly reversed by phentolamine. We concluded that within the first 30 min post-ischemia, beta, and to a lesser extent, alpha receptors predominate in the modulation of cerebrovascular tone. By 1 h post-ischemia, however, adrenergic modulation of cerebrovascular tone is lost. Delayed post-ischemic hypermetabolism unlike stress-induced, but like hypoxia-induced hypermetabolism is only partially affected by beta blockade. Propranolol apparently compromises brain oxygen consumption secondary to a reduction in brain O2 supply while phentolamine improves perfusion and oxygen consumption.

Validity of cerebral blood flow measurements obtained with quantitative tracer techniques

A great number of results for the cerebral blood flow obtained in the animal with quantitative tracer techniques have been collected from the literature. They are exposed in order to compare both normal flow values in different laboratory species, and the characteristics, accuracy and sensitivity of each technique. A dramatic overall dispersion of flow values is observed, allowing neither the flow level particular to each species to be estimated, nor the average value provided by a given technique to be found. The physiological and technological causes of such a dispersion are discussed. Several techniques seem to have limitations which even alter the interpretation of their results, and especially the origin of the local or regional blood flow results. Other techniques may be criticized from the quantitative standpoint, but give more reliable results.

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.

The effect of beta-adrenergic receptor blocking drugs on cerebral blood flow

1 Cerebral blood flow (CBF) was measured by the 133xenon inhalation method in 33 newly-diagnosed hypertensive patients prior to commencing therapy. 2 Blood pressure was treated by using a varying sequence of four different drugs, namely labetalol, metoprolol, oxprenolol and sotalol, each of which is a beta-adrenergic receptor blocking agent, but with differing additional properties. 3 CBF measurements were repeated when blood pressure was controlled. No significant change in CBF was found with any of the four drugs, in contrast to the fall which has been reported when drugs of this type are administered acutely.

Methods for assessment of the effect of drugs on cerebral blood flow in man

The cerebral circulation is not an unimportant vascular bed but it is difficult one to investigate. Methodological difficulties are the main reasons for the paucity of clinical pharmacology studies to date. Of the methods currently available those of the McHenry (1964) and Wyper et al. (1976) appear to be the most useful to the clinical investigator. Increasing use of multiple detectors even with inhalation techniques may give evidence of regional drug effects upon the cerebral circulation. Improvements in methodology in the last few years, particularly the development of atraumatic methods are likely to act as an important spur to research in this field.

Intracerebral gas partial pressure changes under vasoactive drugs. A mass spectrometry study

An original technique based on mass spectrometry has been used to measure simultaneously in undrugged, freely-breathing animals local PO2 and PCO2 in the brain (caudate nucleus) and arterial PO2 and PCO2 in the aorta. Injection of vasoactive drugs brought about significant modifications of the partial pressures of the physiological gases in the brain. The origin of these variations was discussed, based on the combination of 3 possible factors: cerebral vascular changes, systemic metabolic changes (PaO2 and PaCO2) and local metabolic changes. It was demonstrated that local variations of PCO2 cannot be held to be responsible for the modifications of cerebral blood flow induced by papaverine or the sympathomimetic drugs.

A catecholamine-mediated increase in cerebral oxygen uptake during immobilisation stress in rats

Anxiety and grave apprehension have been supposed to increase cerebral metabolism, and it has earlier been suggested that intravenous infusion of adrenaline may increase cerebral blood flow (CBF) and cerebral oxygen consumption (CMR02). In an experimental model on rats, it could be shown that immobilisation stress increased CBF and CMR02 after 5 min (about 150% of control values) and 30 min (about 190% of control values). By previous adrenalectomy or by administration of a beta-receptor blocker (propranolol, 1.4 mg/kg) the changes in CBF and CMR02 could be prevented. It is concluded that the excessive increase in CBF and CMR02 was mediated via release of catecholamines from the adrenal glands.