Neurogenic control of cerebral circulation

Regulation of Cerebral Blood Flow: Response to Cytochrome P450 Lipid Metabolites

There have been numerous reviews related to the cerebral circulation. Most of these reviews are similar in many ways. In the present review, we thought it important to provide an overview of function with specific attention to details of cerebral arterial control related to brain homeostasis, maintenance of neuronal energy demands, and a unique perspective related to the role of astrocytes. A coming review in this series will discuss cerebral vascular development and unique properties of the neonatal circulation and developing brain, thus, many aspects of development are missing here. Similarly, a review of the response of the brain and cerebral circulation to heat stress has recently appeared in this series (8). By trying to make this review unique, some obvious topics were not discussed in lieu of others, which are from recent and provocative research such as endothelium-derived hyperpolarizing factor, circadian regulation of proteins effecting cerebral blood flow, and unique properties of the neurovascular unit. © 2018 American Physiological Society. Compr Physiol 8:801-821, 2018.

Sympathetic mechanisms in cerebral blood flow alterations induced by spinal cord stimulation

OBJECT

Cervical spinal cord stimulation (SCS) has been found to augment cerebral blood flow (CBF) in a number of animal models, although the mechanisms underlying the cerebrovascular effects of SCS are poorly described. In this study, the authors examined the role of sympathetic tone in CBF alterations induced by SCS in rats.

METHODS

Spinal cord stimulation was performed at three intervals while CBF was monitored with laser Doppler flowmetry (LDF). Either hexamethonium (5, 10, or 20 mg/kg), prazosin (0.25, 0.5, or 1 mg/kg), idazoxan (0.5, 1, or 2 mg/kg), propranolol (1, 2, or 4 mg/kg), or vehicle was administered intravenously before the second stimulation. Changes in LDF values due to SCS were recorded as the percentage of change from baseline values and were analyzed. In vehicle-treated animals, SCS increased LDF values by 60.5 +/- 1.8% over baseline, whereas both high-dose hexamethonium and prazosin completely abolished the SCS-induced increases in LDF values. On the other hand, LDF values increased by 50.9 +/- 4% and 61.4 +/- 4% after SCS in the presence of idazoxan or propranolol, respectively. Administration of sympathetic nervous system blockers resulted in a variable degree of systemic hypotension as well. Nevertheless, induced hypotension without sympathetic blockade had only a minimal effect on SCS-induced augmentation of LDF values (48 +/- 1.4% over baseline).

CONCLUSIONS

Sympathetic tone plays a major role in SCS-induced increases in CBF. This effect seems to be mediated primarily by alpha1-adrenergic receptors. Systemic hypotension alone cannot explain the effects of sympathetic blockade on the SCS response. Clinical use of SCS in the treatment of cerebral ischemia should take alpha1-adrenergic receptor sympathetic tone into account.

Isolation of arteriolar microvessels and culture of smooth muscle cells from cerebral cortex of guinea pig

Published methods for the isolation of cerebral microvessels primarily yield terminal resistance vessels and capillary networks, not the more proximal, subpial penetrating arterioles desired for certain studies. We report a novel method for isolating microvessels from the cerebral cortex of a single guinea-pig brain that yields large arteriolar complexes that are up to 50% intact. Instead of using homogenization to disperse brain parenchyma, we digested cortical fragments with trypsin, gently dispersed the parenchyma mechanically, and recovered microvascular complexes by sieving. Phase-contrast and electron microscopy showed primary (penetrating) arterioles, secondary arterioles, and capillary networks that frequently were in continuity as intact microvascular units. Culture of microvascular cells was carried out by enzymatic dissociation followed by an overnight incubation in a recovery medium at 4 degrees C before plating onto fibronectin-modified surfaces. Viability of isolated cells was demonstrated by good cell attachment and prompt proliferation that resulted in confluent cultures after 10 days. Confluent secondary cultures demonstrated characteristic features of smooth muscle cells, including a "hill-and-valley" growth pattern and expression of alpha-actin. Less than 1% of cells were endothelial or astrocytic cells by immunocytochemical and morphologic criteria. Ultrastructural studies demonstrated evidence of a synthetic phenotype of smooth muscle cell and absence of a significant number of fibroblasts. This method demonstrates that viable smooth muscle cells from the cerebral parenchymal microvasculature can be isolated in bulk quantities for study in vitro.

Neuronal control of brain microvessel function

Cerebral capillary endothelium forms a barrier limiting and controlling the movement of ions and solutes between blood and brain. Recent anatomical, physiological and biochemical studies have suggested the possibility that capillary function may be directly controlled by neuronal structures. Alterations in neuronal systems involved in the regulation of microcirculation may account for microvascular dysfunctions which occur in different pathologic conditions.

Innervation of brain intraparenchymal vessels in subhuman primates: ultrastructural observations

Sympathetic innervation of intraparenchymal blood vessels in the basal ganglia was demonstrated by transmission electron microscopy in arteries, arterioles, and capillaries of the subhuman primate brain. Small arteries (40-120 micron) and some arterioles (12-40 micron) are innervated only at branching sites. However, arterioles occasionally may be innervated at points distal to their origin. Capillary innervation was very infrequently observed.

Cerebral infarction due to carotid occlusion and carbon monoxide exposure. II. Influence of preganglionic cervical sympathectomy

Unilateral cerebral infarcts were produced in the rat by ligation of one common carotid artery and subsequent exposure to carbon monoxide. The incidence and extension of brain infarcts was increased in animals with additional ipsilateral cervical preganglionic sympathectomy. Sympathectomy did not affect markedly the respiration and systemic circulation. The effect of sympathectomy was attributed to a cutaneous vasodilation, leading to an extracranial steal phenomenon.

Acute cerebral revascularization: Part 3. Cerebral blood flow

The efficacy of cerebral revascularization by a superficial temporal-middle cerebral artery anastomosis done 4 and 24 hours following combined occlusion of the middle cerebral and internal carotid arteries (MCA/ICA) was tested in 15 dogs. An untreated and a sham-operated group of 8 and 5 dogs, respectively, controlled possible intercurrent variables. Regional cerebral blood flow (rCBF) was measured by the hydrogen clearance method. Values of normal and preocclusion blood flow under basal conditions and following inhalation of 5% CO2 were similar to values reported by other researchers. Combined MCA/ICA occlusion produced a significant drop in rCBF in the ipsilateral hemisphere (p less than 0.01). The rCBF returned to normal following revascularization in 4 and 24 hours, and after the sham procedure. The rCBF response to inhalation of 5% CO2 was normal in the revascularized group, but was reversed (p less than 0.01) in the sham group. The reversal of flow in the sham group suggested that loss of cerebral autoregulation and steal of blood flow in the infarcted area occurred.

Prostaglandin levels in isolated brain microvessels and in normal and norepinephrine-stimulated cat brain homogenates

The levels of PGD2, PGE2, PGF2 alpha and 6-keto-PGF1 alpha (6KF1 alpha) produced from endogenous arachidonic acid (AA) were quantitated in cat cerebral cortical homogenates and microvessels isolated from cat cerebral cortex using gas chromatography/mass spectrometry (GC/MS). There was a six-fold enrichment of 6KF1 alpha levels in isolated microvessels, compared to homogenates, suggesting that 6KF1 alpha is of vascular, rather than neuronal origin. In order to further understand any possible role that norepinephrine (NE) might have on modulation of PG synthesis, we studied the effects of 0.5 mM NE on PG synthesis from endogenous AA and from 3H-PGG2, the endoperoxide precursor of PGs. In cat cortical homogenates NE induced a 74% increase in PGD2 and PGF2 alpha, a 62% increase in PGE2, and a 36% increase in 6KF1 alpha, as measured by GC/MS. NE caused a twofold increase in the conversion of 3H-PGG2 to 3H-PGF2 alpha, with a concomitant decrease in 3H-PGE2 and 3H-6KF1 alpha formation, and no change in 3H-PGD2 synthesis. NE had no effect on the total conversion of 3H-PGG2 to 3H-PGs, nor on the breakdown of 3H-PGG2 in the absence of brain tissue. We conclude that NE stimulates extravascular synthesis of PGD2, PGE2 and PGF2 alpha by stimulation of the prostaglandin synthetase complex, in addition to NE's stimulatory effect on the conversion of PGG2 to PGF2 alpha, and that the lack of effect on NE on 6KF1 alpha synthesis reflects either a failure to achieve an adequate concentration at the vascular tissue, or an absence of the mechanism whereby NE stimulates PG synthetase.

A histochemical and ultrastructural study of serotonin-containing nerves in cerebral blood vessels of the lamprey

Lamprey, Entosphenus japonicus, cerebral blood vessel autonomic nerve supply was studied with fluorescence and cholinesterase histochemistry and electron microscopy. Nerve fibers emitting a yellow fluorescence characteristic of serotonin (Exc./Em. max.; 380/530 nm) were found on the major cerebral and pial arteries, but not acetylcholinesterase (AChE)-positive ones. Single ganglion cells also emitting a strong yellow fluorescence were seen in the artery adventitia. On rare occasions these cells were observed in pairs. Terminal varicosities of central catecholamine-containing nerves (Exc./Em. max.; 410/475 nm) were observed on parenchymal capillaries, but not central AChE-positive nerve terminals. In ganglion cells, dense cored vesicles (ca. 130 nm in average diameter; DCV) were abundant in the Golgi area, suggesting their formation at this site. Two types of DCV were observed; one with a homogeneous dense core and the other with a granular core. DCV were numerous in axons as well, axons in which many small clear vesicles (40--60 nm in diameter) as well as DCV were occasionally observed. The question of whether the small clear vesicles or the DCV contained serotonin could not be resolved.

Adrenergic control of cerebral blood flow and energy metabolism in the rat

Studies in rats were designed to separate and define the roles of the intrinsic and extrinsic adrenergic neurons in the control of cerebral blood flow (CBF) and cerebral energy metabolism. The data suggest several conclusions: 1. Arterial sympathetic innervation plays a role in the autoregulation of cerebral circulation. 2. The central adrenergic neurons have several functions: a) they enhance cerebral vascular tone by action on alpha receptor sites. b) They play an important role in the metabolic control of CBF. The proton-sensitive receptor sites on blood vessel walls require beta-adrenergic input in order to function. c) They influence metabolic rate of brain tissue by acting on beta-receptor sites on the cell membrane.

A histochemical study of the innervation of cerebral blood vessels in the turtle

Specific histochemical techniques for the demonstration of noradrenaline and of acetylcholinesterase have been used to study the distribution of adrenergic and cholinergic nerves to the cerebral blood vessels of turtle, Geoclemys reevesii. The major and medium-sized cerebral arteries were supplied with dense adrenergic nerve plexuses, the plexuses were particularly dense in the medium-sized pial arteries of very thick vascular wall, indicating the functional significance of these arteries in the cerebral circulation. The parenchymal arterioles and capillaries were also supplied with adrenergic nerves. On the other hand, the cholinergic innervation was less dense than the adrenergic one and the acetylcholinesterase activity of the nerve fibres was remarkably weak. By contrast, the parenchymal small arterioles and capillaries exhibited heavy acetylcholinesterase activity on the vascular wall and, in addition, the capillaries were supplied with the well-stained acetylcholinesterase-positive nerve fibres. These fibres and also the adrenergic fibres associating with the capillaries appear to be of central origin. It is suggested that the cholinergic mechanisms in the parenchymal small vessels also play an important role in the cerebral circulation. The basophil leucocytes observed abundantly in the blood of turtle emitted an intensive greenish yellow fluorescence after formaldehyde gas-treatment.

Relationships among intracranial pressure, blood pressure, and superficial cerebral vasculature after experimental occlusion of one middle cerebral artery

A cranial window conforming to the contours of the underlying cerebral cortical surface was implanted successfully in 18 cats. Subsequently the left middle cerebral artery (MCA) was occluded inside the sealed cranium and changes in the superficial cortical vasculature were related to measurements of intracranial pressure (ICP), measured extradurally, and to the resulting infarcts. Vascular changes early after MCA occlusion were not predictive of the outcome of the occlusion, except for aggregation of formed elements of the blood in arterioles, which was a bad prognostic sign. Secondary reactive hyperemia was not beneficial; increases of ICP suggested that hyperemia led to increased cerebral edema as well as to swelling.

Comparison of nerves to cerebral and extracerebral blood vessels: a differential effect of alpha methyl tyrosine on norepinephrine content

Alpha methyl tryosine (AMT), and inhibitor of norepinephrine (NOR) synthesis, was injected intraperitoneally (200 mg/kg) in Sprague Dawley rats, kept in a cold room, or at room temperature for 16 hours. Using formaldehyde induced NOR fluorescence, nerve counts were made on whole mounts of cerebral and femoral arterioles 14-300 micronm in diameter, utilizing a grid superimposed on the vessels. Cold had no effect on the number of visible (i.e. fluorescing) nerves. AMT had an appreciable effect but only on nerves to femoral arterioles, where a significant reduction in nerve count was observed in both cold stressed and non stressed rates, when compared with animals not given AMT. Since the counting technique is sensitive only to large depletions of NOR, we cannot conclude that AMT failed to affect NOR content in cerebrovascular nerves. However, if such an effect was present, it was much less than the effect of AMT on nerves to femoral vessels. We suggest that the differential effect of AMT on these 2 vascular beds may indicate a lower basal level of NOR release from cerebrovascular nerves, which would correlate with the difficulty of demonstrating basal sympathetic tone in this vascular bed.

A histochemical study of the innervation of cerebral blood vessels in the snake

Dual innervation of snake cerebral blood vessels by adrenergic and cholinergic fibres was demonstrated with the use of histochemical methods. Although the nerve plexuses are somewhat less dense, the essential features of innervation of the blood vessels are similar to those of mammals with the exception that the adrenergic plexuses are more prominent than the cholinergic plexuses. The major arteries of the cerebral carotid system have a rich nerve supply. However, the innervation is less rich in the basilar and poor in the spinal (vertebral) arteries. Although the arteries supplying the right side of head are poorly developed, three pairs of arteries, cerebral carotids, ophthalmics and spinals, supply the snake brain. The carotids and ophthalmics are densely innervated and are accompanied by thick nerve bundles, suggesting that the nerves preferentially enter the skull along those arteries. Some parenchymal arterioles are also dually innervated. Connection between the brain parenchyma and intracerebral capillaries via both cholinergic and adrenergic fibres was observed. In addition cholinergic nerve fibers, connecting capillaries and the intramedullary nerve fibre bundles, were noticed. Capillary blood flow may be influenced by both adrenergic and cholinergic central neurons. The walls of capillaries also exhibit heavy acetylcholinesterase activity. This may indicate an important role for the capillary in the regulation of intracerebral blood flow.

Sympathetic regulation of cephalic blood flow

Blood flow to bilateral tissues (cranial and extracranial) was studied by means of the particle distribution method in two groups of anesthetized dogs (five using 25-mu radioactive microspheres, six using 15-mu microspheres) and five anesthetized stumptail Macaques monkeys (8-mj spheres) during unilateral sympathetic stimulation. The stimulatory parameters were adjusted to produce maximum pupillary dilatation. In the five dogs hemispheric and regional cerebral blood flow decreased but not significantly. Flow to the extracranial tissues decreased 82%. Hemispheric brain blood flow averaged 0.70 ml/min/gm for Paco2 of 40 mm Hg. In the six dogs sympathetic stimulation did not significantly decrease cerebral blood flow but decreased flow to extracranial tissues (72.3%). At an average Paco2 of 33.2 mm Hg, hemispheric blood flow to the unstimulated side averaged 0.51 ml/min/gm. In the five monkeys findings were essentially the same as those observed in the dogs. The hemispheric blood flow averaged 0.36 ml/min/gm on the nonstimulated side for an average Paco2 of 36.6 mm Hg. Under the conditions studied, electrical stimulation of the cervical sympathetic nerves does not appear to modify regional or total brain blood flow in dogs and Macaques monkeys. The vascular response in oral and other extracranial tissues is very dramatic, however.

Direct evidence for absence of beta-adrenergic receptors in rat cerebral vessels histochemical study with a fluorescent beta-blocker

A fluorescent marker for beta-adrenergic receptor sites, 9-amino-acridin propranolol (9-AAP), was administered intravenously to rats. In contrast to other tissues which are known to contain beta-adrenergic receptors, 9-AAP fluorescence was not observed in the walls of the pial as well as parenchymal cerebral vessels. These negative findings strongly suggest that in the rat, beta-adrenergic receptors are not present in the cerebral vasculature. The role of the alpha-adrenergic receptors needs more study.

The relevance of peripheral baroreceptors and chemoreceptors to regulation of cerebral blood flow in the cat

The contribution of neural vasomotor reflexes to the control of cerebral blood flow (CBF) was investigated in 30 cats lightly anesthetized with pentobarbital. CBF was measured both by kinetic analysis and by the intial slope technique of the washout curve of a bolus of 133Xe. Autoregulation (10 cats) and responsiveness to alteration in arterial PCO2 (PaCO2) (10 cats) and arterial PO2 (Pao2) (five cats) were assessed both before and after bilateral intracranial division of the 9th and 10th nerves. In an additional group (five cats), related changes in CBF to alteration of PaCO2 were recorded before and after unilateral section of the 7th and 8th nerves. Autoregulation was preserve after division of the 9th and 10th nerves and there was no significant change in the PaCO2 response curves. Section of the 7th and 8th cranial nerves did not produce conclusive results in the small number of cats studied. A conclusion that the facial nerves are not dominant in responses to hypercapnia seems justified, but a modulating role for these nerves is possible. These studies do not exclude a physiological role for these nerves in the autoregulation of CBF, but do indicate that the cerebral vascular bed apparently is capable of functioning normally after their division.

Pial arteriolar responses in the mouse brain revisited

Arteriolar responses were measured on the cerebral surface of the mouse using an image splitter and TV monitor. The response to locally applied norepinephrine (NOR) was significantly more frequent for vessels greater than 30 mu I.D. than for smaller vessels. However, even the smaller vessels were frequently constricted by NOR in doses of 5 mug per milliliter. Reserpine (5 mg per kilogram) failed to alter the response to NOR at either 24 or 72 hours after reserpinization. At 48 hours the threshold dose of NOR was reduced, but the effect was slight (two-tailed, P = 0.08). Both propranolol (10(-6) M3 and phentolamine (10(-5M) blocked responses to 5 mug per milliliter of NOR, but neither agent altered resting arteriolar diameter. Isoproterenol, tyramine, and histamine had no effect. Serotonin (5HT) constricted the arterioles but did not potentiate the response to NOR. Additive or potentiated effects were not observed with NOR 5HT or histamine in any combination. These data indicate the presence of alpha-adrenergic receptors in murine cerebral surface arterioles, but do not establish a significant tonic effect of norepinephrine. The existence or role of a beta-receptor in these murine cerebral surface arterioles remains an unsettled question.

The "richness" of sympathetic innervation. A comparison of cerebral and extracerebral blood vessels

The number of adrengeric nerves was quantified, on both cerebral and femoral blood vessels. No difference was found between the two vascular beds. The data failed to establish a "richer" innervation of cerebral vessels. This is in agreement with my previous, extensive, subjective, and unpublished impression. Consequently, the suggestion of others, which ascribes certain features of cerebrovascular behavior to an unusually rich vascular innervation, remained unproved.

XIII. Cerebral circulation and metabolism in stroke. Cerebral circulation and metabolism in stroke study group

An understanding of the cerebral circulation is so fundamental to comprehension of the pathogenesis of stroke that cerebral blood flow and metabolism merit review in this series of reports. The authors recognize that the research described here is very technical in nature and may appear to have little practical application to clinical medicine. Nevertheless, these matters are basic to the development of precise methods for the measurement of regional cerebral blood flow in man which could be used to monitor the therapy of stroke with greater success than is possible at present.

Determinants of response of pial arteries to norepinephrine and sympathetic nerve stimulation

Feline pial arteries larger than 100 mu in diameter constricted in response to cervical sympathetic nerve stimulation suggests or in response to topical application of norepinephrine. Smaller pial arteries were unresponsive to norepinephrine. This unresponsiveness persisted when norepinephrine was dissolved in CSF with high calcium ion concentration, or in CSF with both high calcium ion and zero magnesium ion concentration, or when it was dissolved in the acid fluid used by Wahl et al. and applied by constant infusion or by intermittent application. Comparison of the responses of the larger pial vessels to norepinephrine and to sympathetic nerve stimulation that maximal activation of sympathetic nerves achieves a concentration of released norepinephrine equal to 5.9 x 10(-6) M. The constriction of the larger pial vessels in response to sympathetic nerve stimulation could account for modest reductions in cerebral blood flow.

Anatomical mapping of the cerebral nervi vasorum in the human brain

The authors present an anatomical study of the vascular nerve fiber distribution to the human intracranial arteries. These fibers are abundant in the circle of Willis and its neighboring arteries, but there are no vascular nerves in small pial arteries over the convexity. These nerve fibers are provided with a cluster of synaptic vesicles as they approach the smooth muscle cells of the media. Adrenergic axons are also found in these fibers.

Correlation between regional cerebral blood flow and EEG frequency in the contralateral hemisphere in acute cerebral infarction

The relationship between the rCBF and the electroencephalographic (EEG) frequency was investigated in the contralateral hemisphere of 22 patients with acute cerebral infarction. Reduced rCBF was observed in all patients studied. The degree of rCBF reduction was mild, moderate, or severe and ranged between 6 and 80% from the lowest age-matched normal values obtained in our laboratory. The frequency indices remained within normal limits (mean - 10.4 Hz) in 16 patients. Slower frequencies (mean - 6.3 Hz) were recorded in 6 patients. No correlation was found between the two parameters (P = 0.89). Both the EEG frequency and the rCBF are known to be closely related to the cerebral metabolic rate. The observed rCBF depression without concomitant changes in the EEG frequency raises the question of the role of globally-reduced cerebral metabolism as the cause of rCBF reduction in the noninfarcted hemisphere in stroke patients. Our findings constitute additional evidence that the contralateral hemisphere is involved in the haemodynamic changes occurring in acute cerebral infarction.

Pial microcirculation in subarachnoid hemorrhage

Microsurgical and microscopic methods were employed in guinea pigs to expose, observe, and measure response characteristics of cerebral cortical pial microvessels and microcirculation to traumatic and nontraumatic experimental subarachnoid hemorrhage. Bleeding produced by vascular micropuncture was associated with a 44.3% arteriolar constriction. Topical application of homologous blood alone produced a 33.2% vasoconstriction. Observed microcirculatory flow characteristics subsequent to such microvascular changes were consistent with those known to be associated with cerebral cortical infarction. These changes could be prevented or reversed by topical application of the alpha adrenergic blocker, phenoxybenzamine. Topical pretreatment with the beta adrenergic blocker, propranolol, prevented blood-induced spasm, but did not reverse such spasm once it had been established. A chemo-mechanical mechanism is suggested as underlying the vasoconstriction association with rupture of pial microvessels. It is thought that consideration of such microvascular characteristics, in conjunction with those known to be associated with larger intracranial vessels, adds to current knowledge of the pathophysiology of subarachnoid hemorrhage and may be extrapolated to bear future clinical import.

The importance and relevance of studies of the pial microcirculation

Micropuncture data suggest that pial arterioles contribute only slightly less than parenchymal cerebral arterioles to total resting cerebrovascular resistance. Care must be taken in interpreting the micropuncture data, or the contribution of pial arterioles to cerebrovascular resistance may be erroneously underestimated. These considerations and the fact that pial arterioles have been shown to be highly reactive to a variety of physiological and abnormal stimuli suggest strongly that changes in pial arteriolar diameter should contribute importantly to control of flow to the underlying brain. In fact, parallels between changes in pial vascular diameter and regional blood flow have been observed. Moreover, since the responses of pial vessels to important vasoactive stimuli are qualitatively similar to those of the cerebral circulation as a whole when the latter are inferred from measurements of flow, the directly observable pial vessels may provide a model for the responses of the unseen parenchymal segments of the cerebrovascular bed. Such a model would be essential to our understanding of the control of cerebral blood flow, even if pial vessels themselves did not participate in the control of flow. Thus there are multiple reasons for continued study of the pial vessels, particularly with modern techniques developed for microcirculatory investigations.

Dynamics and control mechanisms in maintenance of regional cerebral blood flow

Experiments with rabbits, cats, and monkeys during recording of complex physiological processes (LEPG, ThG, Po 2 , Pco 2 , and ECoG) in functionally discrete brain regions of awake animals have shown that functional changes, expressed as desynchronization effects on ECoG, are followed by an increase of local blood flow (LCBF) in regional brain cortex up to 0.3 to 0.4 ml per minute per 1 gm brain tissue or an increase of 35% to 45% of resting levels of LCBF. Under normal physiological conditions LCBF and Po 2 change periodically without any external interference at frequency ranges 0.005 to 0.2 cps. This is characteristic of all brain regions and all species of animals investigated. These variations range in amplitude as much as 28% of the mean level of LCBF.

Changes of LCBF have no correlation with changes of systemic blood pressure (SAP). Local control mechanisms appear to be responsible for them. The interrelationships of changes of functional activity and CBF in local regions of awake brain are characterized by a very short time delay (less than one second), but under narcosis it increased up to ten seconds. All these facts taken together suggest that control mechanisms responsible for local brain vascular reactions have two components; the first is metabolic and the second is neurogenic in nature.

The role of mechanical factors in the pathogenesis of short-term and prolonged spasm of the cerebral arteries

✓ The authors describe fibrous structures they name “chordae” that stabilize the position of arteries in cerebral subarachnoid spaces. The innervation of these structures and their relation to the innervation apparatus of the arterial wall is discussed. Animal experiments and human autopsy material were used to study the role of the stabilizing structures in the pathogenesis of arterial spasm following the rupture of saccular aneurysms. Mechanical stimuli produced short-term but not prolonged arterial spasm.

The production of intracranial vascular spasm by hypothalamic extract

✓ This study reports a new method for producing acute intracranial vascular spasm and compares it with the commonly used technique of injecting blood into the subarachnoid space. The data from this investigation provide angiographic evidence that extracts of hypothalamic tissue when injected into the cisterna magna of mongrel dogs will produce a high incidence (86%) of diffuse spasm in most intracranial vessels. Extracts of cerebral cortical tissue produced no constriction. The onset of spasm initiated by hypothalamic extract was later (1½ hours postinjection) and the duration of spasm longer (up to 5 or 6 hours) than that produced by the presence of blood. This indicates, then, that there are substances in certain brain tissues that could also be released into the cerebrospinal fluid and add to spasm produced by blood in the subarachnoid space.

[Neurological complications of heart surgery. Review of their pathogenesis and bases for their treatment]

São relatadas as complicações neurológicas assinaladas em 320 pacientes submetidos a cirurgias cardíacas, com uma incidência de 7,8%. As etiologias mais comuns encontradas foram a embolia aérea (4,0%) e isquemia cerebral após hipotensão sistêmica (2,4%). Os autores analisam a patogenia das complicações cerebrais e tentam correlacioná-las com o fluxo sangüíneo cerebral, com o metabolismo cerebral e com a dinâmica dos pequenos vasos cerebrais. Uma revisão a respeito dos métodos para tratamento da isquemia cerebral é apresentada.

Cerebrovascular response to intracarotid injection of serotonin before and after middle cerebral artery occlusion

The effect of intracarotid injection of serotonin (5-HT) on internal carotid artery flow and oxygen availability (O(2)a) of the cerebral cortex was studied in 10 baboons. Vasoconstriction occurred in the vascular bed of the territory supplied by the injected artery. After one middle cerebral artery was occluded the vasoconstrictor effect of 5-HT was more pronounced, particularly in the non-ischaemic hemisphere. The capacity of the cerebral vessels to provide collateral blood flow was reduced in both ischaemic and non-ischaemic areas of brain. As a result of focal cerebral ischaemia, 5-HT may accumulate in the brain and contribute to the progression of infarction.

Treatment of cerebral vasospasm from subarachnoid hemorrhage with isoproterenol and lidocaine hydrochloride

✓ Initial experience with intravenously administered isoproterenol and lidocaine hydrochloride in 14 patients with severe spasm from subarachnoid hemorrhage is summarized. All patients were actively deteriorating from progressive spasm without other major complications; 12 of 14 improved, and two died. The method of treatment, results, and rationale for this method of therapy are discussed.

Regional cerebral blood flow during stimulation of seventh cranial nerve

The right seventh cranial nerve (n. VII) was exposed intracranially via a suboccipital approach in each of eight cats. Measurements of cerebral blood flow (CBF) were made from superficial cortex, sulcal cortex, basal cortex, the basal ganglia, and the centrum semiovale of each cerebral hemisphere of each cat by autoradiography using 14 Cantipyrine. In six cats, measurements of CBF were made during electric stimulation of n. VII; in two of these, the nerve was sectioned before stimulation. In the remaining two cats, measurements were made after exposure but without stimulation of n. VII; in these cats, there were no side-to-side differences of CBF. In each of the four cats with n. VII intact, CBF values were lower on the stimulated side. In each of the two cats with n. VII sectioned, CBF values were higher on the stimulated side. Thus, stimulation of n. VII causes regional increases of CBF only when centripetal effects of stimulation are prevented. Cerebral vasodilatation has been observed by others during stimulation of a sectioned or an intact n. VII. When an intact nerve is stimulated, vasodilatation apparently is unable to compensate for decreases of CBF mediated centripetally through brain stem structures and extracranial or basal cerebral vessels; moreover, such vasodilatation may be due in part to regulatory responses of vessels to decreases of perfusion pressure.

Increased binding of norepinephrine by nerves to cerebral blood vessels: evidence from the effects of reserpine on nerves to cerebral and extracerebral blood vessels

It has been proposed by others that adrenergic nerves to cerebral blood vessels bind norepinephrine more avidly than do nerves to vessels outside the brain. This suggestion is supported by the present data which show that in the rat intraperitoneal reserpine depletes norepinephrine less readily from nerves to cerebral blood vessels than from nerves to extracerebral blood vessels. Alternate hypotheses to explain our data are contradicted by available evidence, except for the hypothesis of unequal distribution of reserpine between perivascular nerves in various locations. No evidence has been located to favor the latter hypothesis. The postulate of increased norepinephrine binding by nerves to cerebral vessels not only explains the present data, but also can account for the surprisingly small responses of cerebral vessels to exogenous norepinephrine or to sympathetic stimulation.

Evidence for the direct effect of adrenergic drugs on the cerebral vascular bed of the unanesthetized goat

Despite considerable research, the question of whether adrenergic drugs exert direct effects on the cerebral circulation has remained unresolved. With the development of a method for monitoring continuously the entire blood flow to one hemisphere in the unanesthetized goat, we have been able to study this problem directly. The effects of epinephrine, norepinephrine, and isoproterenol administered by close intra-arterial injection were investigated in 15 goats in which an electromagnetic flowmeter had been implanted previously on the internal maxillary artery, which, in this animal, provides the sole blood supply to a hemisphere. Both epinephrine and norepinephrine (0.1 to 5.0 µg) produced dose-dependent reductions in cerebral blood flow, a decrease of 55 ± 3% (SEM) occurring with the highest dose. Alpha receptor blockade of the ipsilateral hemisphere with phenoxybenzamine totally or partially abolished this cerebral vasoconstriction. Isoproterenol (0.01 to 1.0 µg) produced dose-dependent increases in cerebral blood flow, an increment of 75 ± 6% occurring with the highest dose. Beta blockade with propranolol totally or partially abolished the cerebral vasodilation induced by isoproterenol. Thus, epinephrine, norepinephrine, and isoproterenol exert powerful direct effects on the cerebral circulation of the unanesthetized goat, and these effects appear to be mediated by alpha and beta receptors.

Humoral responses of smooth muscle from rabbit subarachnoid artery compared to kidney, mesentery, lung, heart, and skin vascular smooth muscle

Smooth muscle strips from rabbit subarachnoid arteries did not respond to catecholamines in concentrations which caused strips from skin, mesentery, and kidney vessels to contract. The strips from subarachnoid arteries responded to serotonin, histamine, and angiotensin II but not to bradykinin, acetylcholine, methacholine, and adenosine phosphate compounds. Heart and lung vascular strips did not respond to catecholamines but responded to acetylcholine; cardiac strips responded to dilute methacholine. Lung strips often failed to respond to histamine in usual concentrations and did respond to bradykinin. The subarachnoid strips had a distinactive pattern of respone as compared to vascular tissue from other organs. Results of other isolated vessel studies are reviewed along with pertinent investigations of cerebral autoregulation, vasospasm, and autonomic nerves. The possible significance of these findings is discussed.

The sympathetic nervous system and the regulation of cerebral blood flow in man

Pharmacological blockade of the α adrenergic system by 20 mg phentolemine I.V. was shown not to influence CBF in 14 patients with an intact cerebral autoregulation. If the autoregulation--tested with angiotensin--was impaired for some reason, CBF was found to follow proportionally the blood pressure changes provoked by the blockade. The physiological sympathetic stimulation provoked by distention of the bladder did not alter the CBF (seven patients). The CO 2 reactivity was unaffected by the sympathetic α blockade (six patients).

It appears from these studies that the sympathetic system does not play any substantial role in regulation of CBF in man, and thus it would seem useless to try to influence the cerebral circulation for therapeutic purposes by systemic application of drugs, which directly or indirectly work via the sympathetic system.

Unresponsiveness of pial precapillary vessels to catecholamines and sympathetic nerve stimulation

A systematic analysis of the possible existence of neurogenic control of precapillary pial vessels was made in three species (cat, dog, and rabbit). In all of these animals, pial vessels failed to respond to externally applied isoproterenol or norepinephrine in high concentrations (up to 100 µg/ml), although the vessels did dilate in response to externally applied histamine. Adrenergic nerve endings on the pial vessels were demonstrated by fluorescent histochemical techniques specific for catecholamines. However, in the absence of changes in arterial blood pressure and arterial blood carbon dioxide tension, pial precapillary vessels showed no change in caliber in response to stimulation of the ipsilateral superior cervical ganglion. These results show that pial precapillary vessels are not subject to vasoconstriction probably because they lack sufficient receptors for the catecholamine neurotransmitter.