Effect of sympathetic nerve stimulation on cerebral and cephalic blood flow in dogs

Effects of anesthesia on cerebral blood flow, metabolism, and neuroprotection

Administration of anesthetic agents fundamentally shifts the responsibility for maintenance of homeostasis from the patient and their intrinsic physiological regulatory mechanisms to the anesthesiologist. Continuous delivery of oxygen and nutrients to the brain is necessary to prevent irreversible injury and arises from a complex series of regulatory mechanisms that ensure uninterrupted cerebral blood flow. Our understanding of these regulatory mechanisms and the effects of anesthetics on them has been driven by the tireless work of pioneers in the field. It is of paramount importance that the anesthesiologist shares this understanding. Herein, we will review the physiological determinants of cerebral blood flow and how delivery of anesthesia impacts these processes.

Blood oxygenation level-dependent (BOLD)-based techniques for the quantification of brain hemodynamic and metabolic properties - theoretical models and experimental approaches

The quantitative evaluation of brain hemodynamics and metabolism, particularly the relationship between brain function and oxygen utilization, is important for the understanding of normal human brain operation, as well as the pathophysiology of neurological disorders. It can also be of great importance for the evaluation of hypoxia within tumors of the brain and other organs. A fundamental discovery by Ogawa and coworkers of the blood oxygenation level-dependent (BOLD) contrast opened up the possibility to use this effect to study brain hemodynamic and metabolic properties by means of MRI measurements. Such measurements require the development of theoretical models connecting the MRI signal to brain structure and function, and the design of experimental techniques allowing MR measurements to be made of the salient features of theoretical models. In this review, we discuss several such theoretical models and experimental methods for the quantification of brain hemodynamic and metabolic properties. The review's main focus is on methods for the evaluation of the oxygen extraction fraction (OEF) based on the measurement of the blood oxygenation level. A combination of the measurement of OEF and the cerebral blood flow (CBF) allows an evaluation to be made of the cerebral metabolic rate of oxygen consumption (CMRO2 ). We first consider in detail the magnetic properties of blood - magnetic susceptibility, MR relaxation and theoretical models of the intravascular contribution to the MR signal under different experimental conditions. We then describe a 'through-space' effect - the influence of inhomogeneous magnetic fields, created in the extravascular space by intravascular deoxygenated blood, on the formation of the MR signal. Further, we describe several experimental techniques taking advantage of these theoretical models. Some of these techniques - MR susceptometry and T2 -based quantification of OEF - utilize the intravascular MR signal. Another technique - quantitative BOLD - evaluates OEF by making use of through-space effects. In this review, we target both scientists just entering the MR field and more experienced MR researchers interested in the application of advanced BOLD-based techniques to the study of the brain in health and disease.

Endothelium-dependent control of cerebrovascular functions through age: exercise for healthy cerebrovascular aging

Cognitive performances are tightly associated with the maximal aerobic exercise capacity, both of which decline with age. The benefits on mental health of regular exercise, which slows the age-dependent decline in maximal aerobic exercise capacity, have been established for centuries. In addition, the maintenance of an optimal cerebrovascular endothelial function through regular exercise, part of a healthy lifestyle, emerges as one of the key and primary elements of successful brain aging. Physical exercise requires the activation of specific brain areas that trigger a local increase in cerebral blood flow to match neuronal metabolic needs. In this review, we propose three ways by which exercise could maintain the cerebrovascular endothelial function, a premise to a healthy cerebrovascular function and an optimal regulation of cerebral blood flow. First, exercise increases blood flow locally and increases shear stress temporarily, a known stimulus for endothelial cell maintenance of Akt-dependent expression of endothelial nitric oxide synthase, nitric oxide generation, and the expression of antioxidant defenses. Second, the rise in circulating catecholamines during exercise not only facilitates adequate blood and nutrient delivery by stimulating heart function and mobilizing energy supplies but also enhances endothelial repair mechanisms and angiogenesis. Third, in the long term, regular exercise sustains a low resting heart rate that reduces the mechanical stress imposed to the endothelium of cerebral arteries by the cardiac cycle. Any chronic variation from a healthy environment will perturb metabolism and thus hasten endothelial damage, favoring hypoperfusion and neuronal stress.

Cerebral blood flow, sympathetic nerve activity and stroke risk in obstructive sleep apnoea. Is there a direct link?

Obstructive sleep apnoea (OSA) is significantly associated with the risk of stroke, and this association is independent of other risk factors, including hypertension, atrial fibrillation and diabetes mellitus. Therefore, additional pathogenic mechanisms may exist, which contribute to the increased risk of stroke. OSA is characterized by prolonged sympathetic overactivity; however the role of the sympathetic nervous system in regulating cerebral circulation remains a matter of controversy. Converging data indicate that brain perfusion is significantly distorted in OSA, with reported decreases in cerebral blood flow as well as intermittent surges in blood pressure and cerebral blood flow velocity. Based on recent research, there is accumulating evidence that sympathetic nerve activity is an important element in brain protection against excessive increases in perfusion pressure during blood pressure surges and flow during rapid eye movement sleep. The aim of this article was to review: (i) the current physiological knowledge related to the role of the sympathetic system in the regulation of cerebral blood flow, (ii) how the influence of the sympathetic system on cerebral vessels is affected by apnoea (increased PaCO(2)) and (iii) the potential significance of the pathological sympathetic system/PaCO(2) interplay in OSA. Sympathetic system seems to be at least partially involved in pathogenesis of distorted haemodynamics and stroke in OSA patients. However, there are still several open questions that need to be addressed before the effective therapeutic strategies can be implemented.

Validation of oxygen extraction fraction measurement by qBOLD technique

Measurement of brain tissue oxygen extraction fraction (OEF) in both baseline and functionally activated states can provide important information on brain functioning in health and disease. The recently proposed quantitative BOLD (qBOLD) technique is MRI-based and provides a regional in vivo OEF measurement (He and Yablonskiy, MRM 2007, 57:115-126). It is based on a previously developed analytical BOLD model and incorporates prior knowledge about the brain tissue composition including the contributions from grey matter, white matter, cerebrospinal fluid, interstitial fluid and intravascular blood. The qBOLD model also allows for the separation of contributions to the BOLD signal from OEF and the deoxyhemoglobin containing blood volume (DBV). The objective of this study is to validate OEF measurements provided by the qBOLD approach. To this end we use a rat model and compare qBOLD OEF measurements against direct measurements of the blood oxygenation level obtained from venous blood drawn directly from the superior sagittal sinus. The cerebral venous oxygenation level of the rat was manipulated by utilizing different anestheisa methods. The study demonstrates a very good agreement between qBOLD approach and direct measurements.

Measurement of cerebral blood flow in dogs with near infrared spectroscopy in the reflectance mode is invalid

Near infrared spectroscopy (NIRS) is used to measure CBF (CBFNIRS) in humans, based on Fick's principle, using oxygen as an intravascular tracer. We compared CBFNIRS with CBF measured by microspheres (CBF mu) and the venous outflow technique (CBFv) in 15 dogs, altering CBF with ventilation-induced changes in PaCO2. Five hundred forty-nine CBFNIRS measurements were attempted using an integration time of 2.5 s on the saturation signal from the tongue. One hundred ninety-eight (36.1%) of the measurements fulfilled predefined criteria. The coefficient of variation (CV) for six measurements under stable conditions was 29.1%. The CBFNIRS measurements correlated best with microsphere-measured blood flows in the cortical gray matter (median 0.43, range 0.16-0.93); the contributions of the skull and dura were variable. The CBFv varied by a médian of 12% (range 0-67%) during the CBFNIRS measurements. The percentage of acceptable CBFNIRS measurements, the CV, and the correlation coefficients of the CBFNIRS were improved by using saturation signal directly from the artery and varying the integration time with an estimate of the minimum transit time. The current method of measuring CBFNIRS in the reflectance mode is in-accurate when compared with other accepted techniques.

Interstitial fluid adenosine and sagittal sinus blood flow during bicuculline-seizures in newborn piglets

Changes of interstitial fluid adenosine concentrations and effects of O2 supply on interstitial fluid adenosine were studied by the brain dialysis technique in the frontal cortex of newborn piglets subjected to bicuculline-induced seizures. The O2 supply was changed globally by changing MABP and locally by varying PO2 in the artificial CSF perfusing the dialysis cannula. Sagittal sinus blood flow (SSBF), cerebrovascular resistance (CVR), and CMRO2 were also examined in the same animals. Seizures increased interstitial fluid adenosine 7.9-fold (p less than 0.05) when ictal MABP was maintained at preictal level and perfusate PO2 was 24 mm Hg (group 1, n = 6). Interstitial fluid adenosine increased 11.8-fold (p less than 0.05) during seizures associated with moderate systemic hypotension and the low perfusate PO2 (group 2, n = 6). By contrast, seizures increased interstitial fluid adenosine three-fold (p less than 0.05) when perfusate PO2 was increased to 182 mm Hg and ictal MABP was maintained at preictal level (group 3, n = 8). When ictal MABP was elevated from the preictal level and the perfusate was rich in oxygen, seizures failed to increase interstitial fluid adenosine (group 4, n = 7). In groups 1 and 3, the increase in interstitial fluid adenosine during seizures was associated with significant increases in SSBF and CMRO2, as well as significant decreases in CVR. These data suggest that the increase in O2 supply during seizures in piglets did not match completely the increase in O2 demand and resulted in enhanced release of adenosine into the interstitial space.

Clinical applications of mechanical ventilation

This paper reviews recent applications of mechanical ventilation such as controlled hypoventilation in acute asthma, domiciliary nocturnal ventilation in chronic respiratory failure due to neuromuscular disease and improvement of left ventricular performance by raised intrathoracic pressure. Established uses of mechanical ventilation include control of respiratory failure, intracranial pressure and pulmonary hypertension while other uses such as internal splinting of flail chest, simultaneous ventilation-compression cardiopulmonary resuscitation and prophylactic postoperative ventilation are more controversial.

Relationship of somatosensory evoked potentials and cerebral oxygen consumption during hypoxic hypoxia in dogs

The effects of hypoxic hypoxia on cerebral hemodynamics and somatosensory evoked potential (SEP) were studied in 10 pentobarbital anestheteized dogs. Cerebral blood flow (CBF) was measured using the venous outflow technique and cerebral oxygen consumption (CMRO2) was calculated from the arterio-cerebro-venous oxygen difference times CBF. SEP was evaluated by percutaneous stimulation of an upper extremity nerve and was recorded over the contralateral somatosensory cortex. The latencies of the initial negative wave (N1), second positive wave (P2) and the amplitude of the primary complex (P1N1) were measured. Animals were breathed sequentially with oxygen concentrations of 21, 10, 6, 5, and 4.5% for five minutes each. Animals were returned to room air breathing when the amplitude of the SEP decreased to less than 20% of control and were observed for 30 minutes following reoxygenation. Severe hypoxia (4.5% O2) increased CBF to 200% of control, decreased CMRO2 to 45% of control, decreased amplitude and increased latency of SEP. Following reoxygenation, as CMRO2 increased toward control, latency of SEP decreased and amplitude increased and CBF returned to baseline within 30 min. During hypoxia and reoxygenation, the latencies of N1 and P2 and the amplitude of P1N1 were correlated with CMRO2 in individual animals. We conclude that changes in SEP amplitude and latency reflect changes in CMRO2 despite high CBF during rapidly progressive hypoxic hypoxia and following reoxygenation.

Sustained cerebral vasoconstriction during bilateral sympathetic stimulation in anesthetized rabbits

Temporal aspects of bilateral sympathetic nerve stimulation on cerebral blood flow (CBF) were examined in anesthetized rabbits (n = 7). CBF ranged from 32 to 50 ml/min per 100 g. Bilateral stimulation reduced blood flow by 17-31% to cerebrum, diencephalon-mesencephalon and cerebellum, and responses were constant between 2 and 6 min of stimulation. Sustained cerebral vasoconstriction is consistent with an important role for sympathetic nerves in the regulation of CBF.

Transient analysis of the canine cerebrovascular response to carbon dioxide

Cerebral venous outflow and carbon dioxide transients were studied during five different transitional states: (1) on and off 10% carbon dioxide breathing, (2) on and off hyperventilation, (3) on 7% carbon dioxide breathing, (4) on 10% carbon dioxide breathing initiated from 7% carbon dioxide breathing, and (5) on 10% carbon dioxide breathing initiated during intracarotid papaverine infusion, in pentobarbital anesthetized, paralyzed, mechanically ventilated dogs. Plots of the temporal relationships between these variables indicated that cerebral blood flow is closely related with cerebral venous carbon dioxide tension but not arterial carbon dioxide tension. The rate at which flow changed upon transition from one steady state to another was phase dependent, in that longer times were required to establish stable conditions in the on phase than in the off phase. The magnitude of the maximum rates of change in cerebral blood flow achieved during transition was influenced both by the size of the forcing function and the level of flow present at the time the response was initiated. Directional changes had no effect upon the maximum rate of the flow change as long as equivalent-sized forcing functions were employed and the initial blood flow levels were similar between responses. However, faster flow transients could be produced by increasing either of the latter two factors. These findings are consistent with the hypothesis that it is either tissue carbon dioxide tension or cerebral venous carbon dioxide tension that is the important variable regulated by cerebral blood flow. The rate-limiting factor in the response appears to be carbon dioxide delivery rate and not the rate of carbon dioxide diffusion.

Retroglenoid venoconstriction and its influence on canine intracranial venous pressures

Using standard in vitro techniques, we found that the canine retroglenoid vein, a vessel that drains a significant fraction of canine cerebral venous effluent, demonstrated the following: an average wall thickness of approximately 240 microns; a norepinephrine (NE) content of approximately 3 micrograms/g tissue; a NE uptake capacity (uptake 1) of approximately 8 nmol/g tissue; an ED50 for NE of 1.9 X 10(-8) M; and a phentolamine-sensitive constriction during electric transmural stimulation that had a median effective frequency of approximately 3 Hz and a maximum response that was approximately 84% of the maximum response to exogenous NE. In a separate series of in vivo experiments conducted in six alpha-chloralose-anesthetized dogs, we found that electrical stimulation of the left superior cervical ganglion produced a phentolamine-sensitive, frequency-dependent increase in cerebral venous pressure (CVP) of up to 19 mm Hg when all cerebral venous effluent was diverted through the left retroglenoid vein. Taken together, our findings suggest that the canine retroglenoid vein undergoes a marked vasoconstriction during physiological frequencies of electric sympathetic nerve stimulation in vivo. Although our data further suggest that the retroglenoid is not a dominant influence on CVP in the intact dog, they do encourage a cautious interpretation of cerebral venous outflow data obtained with techniques in which cerebral effluent is drained primarily by extracranial veins.

Effects of sympathetic stimulation on cerebral and ocular blood flow. Modification by hypertension, hypercapnia, acetazolamide, PGI2 and papaverine

The effect of unilateral, electrical stimulation of the cervical sympathetic chain in rabbits anesthetized with pentobarbital sodium and vasodilated by hypercapnia, acetazolamide, papaverine or PGI2 was investigated to determine to what extent the sympathetic nerves to the brain and the eye cause vasoconstriction and prevent overperfusion in previously vasodilated animals. Evans blue was given as a tracer for protein leakage. Blood flow determinations were made with the labelled microsphere method during normotension and acute arterial hypertension. Hypertension was induced by ligation of the thoracic aorta and in some animals metaraminol or angiotensin was also used. Acetazolamide caused a two to threefold increase in cerebral blood flow (CBF) and hypercapnia resulted in a fivefold increase. CBF was not markedly affected by papaverine or PGI2. In the choroid plexus, the ciliary body and choroid, papaverine and hypercapnia caused significant blood flow increases on the control side. Sympathetic stimulation induced a 12% blood flow reduction in the brain in normotensive, hypercapnic animals. Marked effects of sympathetic stimulation at normotension were obtained under all conditions in the eye. In the hypertensive state the CBF reduction during sympathetic stimulation was moderate, but highly significant in hypercapnic or papaverine-treated animals as well as in controls. Leakage of Evans blue was more frequently seen on the nonstimulated side of the brain. In the eye there was leakage only on the control side except in PGI2-treated animals where 2 rabbits had bilateral leakage. The effect of sympathetic stimulation on the blood flow in the cerebrum and cerebellum in vasodilated animals seems to be small or absent if the blood pressure is normal. In the eye pronounced vasoconstriction occurs under these conditions. In acute arterial hypertension sympathetic stimulation protects both the cerebral and ocular barriers even under conditions of marked vasodilation.

Effects of facial nerve section and stimulation on cerebral and ocular blood flow in hemorrhagic hypotension

Albino rabbits were anesthetized and artificially ventilated. In two groups of animals the facial nerve on one side was sectioned and/or electrically stimulated at the internal acoustic pore. Hemorrhagic hypotension was induced to reveal a possible parasympathetic vasodilator mechanism, normally masked. The labelled microsphere method was used for flow determination. Section of the facial nerve did not produce any difference between sectioned and intact side, concerning cerebral, ocular and mandibular gland blood flows at normal or low blood pressures. Stimulation of the facial nerve at arterial hypotension produced significant ipsilateral increases in the choroidal and the mandibular gland blood flows. Regional and total cerebral blood flow remained unaffected. The results indicate no or only minimal contribution of the facial nerve to the cerebral vascular tone under conditions of general anesthesia. Further evidence is given in this study for a vasodilator pathway to the eye via the facial nerve, but the resting vasodilator tone under general anesthesia seems to be very low at normotensive as well as hypotensive states.

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.

Early effects of cervical sympathetic stimulation on cerebral, ocular and cochlear blood flow

Autoregulatory mechanisms may be expected to modify effects of vasomotor nerve stimulation in many tissues. Attempts were made to reveal a distinct early, but transient effect of cervical sympathetic stimulation on cerebral, retinal and cochlear blood flow. The labelled microsphere method was used to determine regional blood flow during electrical stimulation of the cervical sympathetic chain for 15-25 s and 5 min. At a frequency of 6 Hz there was 5% reduction in cerebral flow at 15-25 s and 7% at 5 min. In the choroid plexus the mean reduction was 22% at 15-25 s but decreased 10% after 5 min. In the cerebellum, optic nerve and retina, sympathetic stimulation had no appreciable effect on the blood flow. In the cochlea and iris, the blood flow reductions were 25 and 32%, respectively, on both occasions. In the choroid, vasoconstriction increased with time, whereas in the masseter muscle there was a decrease. Thus in the present experiments no indication was found of an autoregulatory escape phenomenon in the brain, the eye or the cochlea. Some escape was noted in the masseter muscle.

Functional activity of the noradrenergic innervation of large cerebral arteries

1 The role of the sympathetic innervation of cerebral arteries remains controversial. Therefore, the functional activity of the adrenergic innervation of the rabbit basilar artery was characterized and compared to that of a peripheral artery, the ear artery. 2 Both the ear artery and basilar artery have similar endogenous noradrenaline (NA) contents but accumulation of [3H]-NA was considerably greater in the basilar artery. 3 Studies of tritium efflux after loading with [3H]-NA demonstrated a considerable non-neuronal component since neither guanethidine nor tetrodotoxin completely blocked tritium efflux during nerve stimulation. Pretreatment with blockers of uptake2 did not eliminate this problem. 4 Comparison of methods for estimating the functional activity of adrenergic nerves showed that, for the vessels studied, NA content and [3H]-NA accumulation gave markedly different answers. Fractional release of [3H]-NA did not correspond to fractional release of endogenous NA. 5 Adrenergic nerves innervating cerebral arteries are shown to have a high activity relative to a peripheral artery. While cerebrovascular sympathetic innervation may not play an important role in normal circumstances, its influence may be seen in pathological conditions.

Sympathetic modulation of hypercapnic cerebral vasodilation in dogs

We measured cerebral blood flow using both the radioactive microsphere technique and the cerebral venous outflow technique in dogs anesthetized with chloralase. The effect of sympathetic stimulation on cerebral blood flow was observed during both normocapnia and prolonged hypercapnia using both blood flow techniques. The increase in blood flow with hypercapnia was the same with both methods. During hypercapnia the venous outflow method showed a 38% decrease and microspheres an 18% decrease in cerebral blood flow with sympathetic stimulation. At normal CO2, stimulation caused a decrease in cerebral venous flow: no change was observed with the microsphere method. Analysis of the blood flow patterns to extracerebral tissues and evaluation of extracerebral arterial reference samples failed to prove the existence of axial streaming and subsequent skimming of microspheres within the cephalic circulation. It is concluded that direct electrical stimulation of the sympathetic innervation of the cerebral vessels is capable of reducing cerebral blood flow even during a profound hypercapnic vasodilation.

Is there an active mechanism limiting the influence of the sympathetic system on the cerebral vascular bed? Evidence for vasomotor escape from sympathetic stimulation in the rabbit

The influence of the cervical sympathetic chain on cerebral circulation in the rabbit was studied by means of 3 complementary techniques. Two dynamic techniques involving chronically implanted probes were used: blood flow in the caudate nucleus (CN) was measured by thermal clearance; tissue PO2 and PCO2 in the same structure were measured by mass spectrometry. Other variables measured continuously and simultaneously included arterial blood pressure (BP), PaO2 and PaCO2. The third technique was a tissue sampling method based on the Fick principle and using 14C1 ethanol as tracer. Blood flow in 7 regions was measured at stable BP, PaO2 and PaCO2. Stimulation of the sympathetic chain at 15 Hz induced mean maximal decreases in CN blood flow of 23.9% (thermal clearance) and 24.4% (ethanol technique). Mean decrease of PO2 in the CN at 15 Hz was 16.6%. Significant falls in blood flow were observed with the ethanol technique in all 7 structures measured. During prolonged stimulation (greater than 1 min) CN blood flow and PO2 were found to escape towards the baseline level, which was sometimes even exceeded during the stimulation (blood flow). Stimulation frequency had only a very moderate influence on the rate of escape, and no evidence of a metabolic mechanism was found, although injection of barbiturate decreased the escape. These results are discussed with respect to the conflicting evidence on the effects of sympathetic stimulation in the brain, and to possible mechanisms for the escape phenomenon.

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.

Analysis of the effect of bilateral sympathetic stimulation of cerebral and cephalic blood flow in the dog

Unilateral stimulation of the cervical sympathetic in dogs had no effect on cerebral blood flow (CBF) measured by the venous outflow technique. Since this technique measured CBF from both cerebral hemispheres, small changes induced by unilateral stimulation could have been masked by a large constant CBF measured from the contralteral hemisphere. To test this possibility the effect of simultaneous bilateral sympathetic stimulation was studied when the dog was breathing either normal air or a gas mixture of 10%CO2. During normocapnia, no changes in CBF occurred; during hypercapnia CBF increased 19% following passively the increase in blood pressure. These data indicate that bilateral stimulation of extracranial sympathetic nerves does not exert a significant effect on CBF. We show mathematically and experimentally that unoccluded anastomses will cause CBF to appear to decrease in response to sympathetic stimulation. This may explain why others have observed changes in CBF during sympathetic stimulation.

The effect of lesions in the locus coeruleus on the physiological responses of the cerebral blood vessels in cats

The effects of cerebral blood flow (CBF) of lesions placed stereotactically in or near the locus coeruleus were studied in 15 lightly anesthetized cats; 5 control cats in which the electrode was placed but no lesion created, and 10 experimental cats in which a lesion was created. The response of CBF to changes in Paco2 and in mean arterial blood pressure was determined by 133Xe-washout studies 10 days after the stereotactic procedures. The sites of the lesions were studied histologically, and their effects on catecholamine concentrations in the paraventricular hypothalamic nucleus, anterior ventral nucleus of the thalamus, and parietal cortex were determined by radio-chemical assay. Control animals and those with lesions near, but not in, the locus coeruleus had normal Paco2--CBF response curves and normal catecholamine concentrations in the areas of biopsy. Bilateral destruction of the locus coeruleus was confirmed in 3 animals on histological examination and in these animals there were decreased levels of catecholamines in the areas of assay, higher resting CBFs at normocapnia, and significantly abnormal CBF--Paco2 response curves. The autoregulatory response to changes in perfusion pressure was preserved. Thus, noradrenergic neurons originating in the locus coeruleus may contribute to the control of intraparenchymal cerebral vessels and disturbance of this control may be important in the pathology of cerebral ischemia.

Quantitative multiregional blood flow measurements during cervical sympathetic stimulation

The ethanol tissue sampling method for rCBF measurement was used to obtain information on the effects of cervical sympathetic stimulation in 8 cerebral structures in the non-anaesthetized rabbit. Sympathetic stimulation induced flow decreases of 12-29% according to structure, confirming the capability of this nerve to significantly reduce rCBF. Furthermore, a regional differentiation of cerebral structures into an 'anterior' group (mean decrease 22%) and a 'posterior' group (mean decrease 12%) with different reactivity to stimulation was established, thus confirming previous work in this laboratory with a local thermoclearance technique, and histochemical studies on sympathetic innervation to cerebral arteries and arterioles. These results provide strong evidence of the functional nature of the sympathetic nervous system in CBF regulation. The complementary nature of the ethanol technique (quantitative, multiregional measurement) and the local thermoclearance technique (continuous, semiquantitative measurements in 2-3 regions), and the absence of anaesthesia and significant trauma, means that combination of these two techniques offers considerable advantages in research on dynamic phenomena of the kind studied here.

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.

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.

Sympathetic control of cerebral blood flow in acute arterial hypertension

The cervical sympathetic chain on one side was stimulated electrically at 10-20 Hz and an acute rise in arterial blood pressure was produced by: intravenous injection of angiotensin, ligation of the thoracic aorta, or ligation of the aorta combined with injection of metaraminol. The blood flow through the cerebrum and the cerebellum was determined by using labelled microspheres. At high blood pressures there was multifocal breakdown of the blood-brain barrier in the cerebrum as indicated by leakage of Evans blue. The breakdown was restricted to the control side or much more marked on that side than on the stimulated side. Sympathetic stimulation prevented also breakdown of the blood-aqueous barrier. The blood flow through the cerebrum on the control side was higher than that on the stimulated side in all experiments. Regions with breakdown of the blood-brain barrier had flow rates which were about 10 times normal values. Cerebellar blood flow was less affected by the hypertension and did not react significantly to sympathetic stimulation. The results indicate that stimulation of the sympathetic nerves to the brain tends to prevent forced dilatation of the arterioles with a resulting regional overperfusion with blood and breakdown of the blood-brain barrier. It is concluded that one role of the sympathetic nerves supplying the brain is to extend the pressure region with autoregulation in its upper part under conditions of a general increase in sympathetic vasomotor activity.