Cerebral blood flow in rats during physiological and humoral stimuli

Sympathetic influence on cerebral blood flow and metabolism during exercise in humans

This review focuses on the possibility that autonomic activity influences cerebral blood flow (CBF) and metabolism during exercise in humans. Apart from cerebral autoregulation, the arterial carbon dioxide tension, and neuronal activation, it may be that the autonomic nervous system influences CBF as evidenced by pharmacological manipulation of adrenergic and cholinergic receptors. Cholinergic blockade by glycopyrrolate blocks the exercise-induced increase in the transcranial Doppler determined mean flow velocity (MCA Vmean). Conversely, alpha-adrenergic activation increases that expression of cerebral perfusion and reduces the near-infrared determined cerebral oxygenation at rest, but not during exercise associated with an increased cerebral metabolic rate for oxygen (CMRO(2)), suggesting competition between CMRO(2) and sympathetic control of CBF. CMRO(2) does not change during even intense handgrip, but increases during cycling exercise. The increase in CMRO(2) is unaffected by beta-adrenergic blockade even though CBF is reduced suggesting that cerebral oxygenation becomes critical and a limited cerebral mitochondrial oxygen tension may induce fatigue. Also, sympathetic activity may drive cerebral non-oxidative carbohydrate uptake during exercise. Adrenaline appears to accelerate cerebral glycolysis through a beta2-adrenergic receptor mechanism since noradrenaline is without such an effect. In addition, the exercise-induced cerebral non-oxidative carbohydrate uptake is blocked by combined beta 1/2-adrenergic blockade, but not by beta1-adrenergic blockade. Furthermore, endurance training appears to lower the cerebral non-oxidative carbohydrate uptake and preserve cerebral oxygenation during submaximal exercise. This is possibly related to an attenuated catecholamine response. Finally, exercise promotes brain health as evidenced by increased release of brain-derived neurotrophic factor (BDNF) from the brain.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Continuous monitoring of regional cerebral blood flow: experimental and clinical validation of a novel thermal diffusion microprobe

OBJECT

Current clinical neuromonitoring techniques lack adequate surveillance of cerebral perfusion. In this article, a novel thermal diffusion (TD) microprobe is evaluated for the continuous and quantitative assessment of intraparenchymal regional cerebral blood flow (rCBF).

METHODS

To characterize the temporal resolution of this new technique, rCBF measured using the TD microprobe (TD-rCBF) was compared with rCBF levels measured by laser Doppler (LD) flowmetry during standardized variations of CBF in a sheep model. For validation of absolute values, the microprobe was implanted subcortically (20 mm below the level of dura) into 16 brain-injured patients, and TD-rCBF was compared with simultaneous rCBF measurements obtained using stable xenon-enhanced computerized tomography scanning (sXe-rCBF). The two techniques were compared using linear regression analysis as well as the Bland and Altman method. Stable TD-rCBF measurements could be obtained throughout all 3- to 5-hour sheep experiments. During hypercapnia, TD-rCBF increased from 49.3+/-15.8 ml/100 g/min (mean +/- standard deviation) to 119.6+/-47.3 ml/100 g/ min, whereas hypocapnia produced a decline in TD-rCBF from 51.2+/-12.8 ml/100 g/min to 39.3+/-5.6 m/100 g/min. Variations in mean arterial blood pressure revealed an intact autoregulation with pressure limits of approximately 65 mm Hg and approximately 170 mm Hg. After cardiac arrest TD-rCBF declined rapidly to 0 ml/100 g/min. The dynamics of changes in TD-rCBF corresponded well to the dynamics of the LD readings. A comparison of TD-rCBF and sXe-rCBF revealed a good correlation (r = 0.89; p < 0.0001) and a mean difference of 1.1+/-5.2 ml/100 g/min between the two techniques.

CONCLUSIONS

The novel TD microprobe provides a sensitive, continuous, and real-time assessment of intraparenchymal rCBF in absolute flow values that are in good agreement with sXe-rCBF measurements. This study provides the basis for the integration of TD-rCBF into multimodal monitoring of patients who are at risk for secondary brain injury.

Cortical NOS inhibition raises the lower limit of cerebral blood flow-arterial pressure autoregulation

The maintenance of constant cerebral blood flow (CBF) as arterial blood pressure is reduced, commonly referred to as CBF-pressure autoregulation, is typically characterized by a plateau until the vasodilatory capacity is exhausted at the lower limit, after which flow falls linearly with pressure. We investigated the effect of cortical, as opposed to systemic, nitric oxide synthase (NOS) inhibition on the lower limit of CBF-pressure autoregulation. Forty-four Sprague-Dawley rats were anesthetized with halothane and N2O in O2. With a closed cranial window placed the previous day in a ventilated and physiologically stable preparation, we determined the CBF using laser-Doppler flowmetry. Animals with low reactivity to inhaled CO2 and suffused ADP or ACh were excluded. Five arterial pressures from 100 to 40 mmHg were obtained with controlled hemorrhagic hypotension under cortical suffusion with artificial cerebrospinal fluid (aCSF) and then again after suffusion for 35 (n = 5) and 105 min (n = 10) with aCSF, 10(-3) M Nomega-nitro-L-arginine (L-NNA; n = 12), or 10(-3) M Nomega-nitro-D-arginine (D-NNA; n = 5). An additional group (n = 7) was studied after a 105-min suffusion of L-NNA followed by a single blood withdrawal procedure. The lower limit of autoregulation was identified visually by four blinded reviewers as a change in the slope of the five-point plot of CBF vs. mean arterial blood pressure. The lower limit of 90 +/- 4.3 mmHg after 105 min of 1 mM L-NNA suffusion was increased compared with the value in the time-control group of 75 +/- 5.3 mmHg (P < 0.01; ANOVA) and the initial value of 67 +/- 3.7 mmHg (P < 0.001). The lower limit of 84 +/- 5.9 mmHg in seven animals with 105 min of suffusion of 1 mM L-NNA without previous blood withdrawal was significantly increased (P < 0.01) in comparison with 70 +/- 1.9 mmHg from those with just aCSF suffusion (n = 37). No changes in lower limit for the other agents or conditions, including 105 or 35 min of aCSF or 35 min of L-NNA suffusion, were detected. The lack of effect on the lower limit with D-NNA suffusion suggests an enzymatic mechanism, and the lengthy L-NNA exposure of 105 min, but not 35 min, suggests inhibition of a diffusionally distant NOS source that mediates autoregulation. Thus cortical suffusion of L-NNA raises the lower limit of autoregulation, strongly suggesting that nitric oxide is at least one of the vasodilators active during hypotension as arterial pressure is reduced from normal.

Differential effects of migraine drugs on cerebral blood flow autoregulation

The effect of the migraine drugs ergotamine and sumatriptan on the cerebral blood flow (CBF) autoregulation was studied in halothane/nitrous oxide-anesthetized normotensive Wistar Kyoto rats. Ergotamine, an ergot alkaloid affecting 5HT, norepinephrine, and dopamine receptors, was administered intravenously as a single dose of 25 microg/kg. Sumatriptan, a selective 5HT1-like receptor agonist, was administered by intravenous infusion of 300 microg/kg/h. CBF was measured with the intracarotid 133Xe-injection method. The blood pressure limits of CBF autoregulation were determined by computerized least sum of square analysis. CBF autoregulation was preserved after both ergotamine and sumatriptan. Ergotamine shifted the lower blood pressure limit of CBF autoregulation towards higher blood pressures from 60 +/- 3 mmHg to 82 +/- 4 mmHg (p<0.01), but did not significantly affect the upper blood pressure limit of CBF autoregulation. Sumatriptan had no significant effects on the blood pressure limits of CBF autoregulation.

Single-unit responses of serotonergic dorsal raphe neurons to specific motor challenges in freely moving cats

Serotonin has been hypothesized to play an important role in the central control of motor function. Consistent with this hypothesis, virtually all serotonergic neurons within the medullary nuclei raphe obscurus and raphe pallidus in cats are activated in response to specific motor challenges. To determine whether the response profile of serotonergic neurons in the midbrain is similar to that observed in the medulla, the single-unit activity of serotonergic dorsal raphe nucleus cells was studied during three specific motor activities: treadmill-induced locomotion, hypercarbia-induced ventilatory response and spontaneous feeding. In contrast to the results obtained for medullary raphe cells, none of the serotonergic dorsal raphe cells studied (n=26) demonstrated increased firing during treadmill-induced locomotion. A subset of serotonergic dorsal raphe cells (8/36) responded to the hypercarbic ventilatory challenge with increased firing rates that were directly related to the fraction of inspired carbon dioxide, and a non-overlapping subset of cells (6/31) was activated during feeding. All feeding-on cells demonstrated a rapid activation and de-activation coincident with feeding onset and offset, respectively. Although the proportions of serotonergic cells activated by hypercarbia or feeding in the dorsal raphe nucleus were similar to those found in the medullary raphe, there were several major distinctions in the response characteristics for the two cell groups. In contrast to the medullary serotonergic neurons, only a minority of dorsal raphe nucleus serotonergic neurons responded to a motor challenge. Overall, the above results suggest very different roles for the midbrain and medullary serotonergic neurons in response to motor activities.

Differential effects of increasing doses of alpha-trinositol on cerebral blood flow autoregulation

The effect of neuropeptide Y inhibition with alpha-trinositol on the cerebral blood flow autoregulation was studied in Wistar Kyoto rats. alpha-Trinositol was tested in two doses: one dose (5 mg kg-1 hr-1) selectively affecting neuropeptide Y and one higher dose (50 mg kg-1 hr-1) affecting both neuropeptide Y and the adrenergic response. The cerebral blood flow was measured with the intracarotid 133xenon injection method in halothane nitrous oxide-anaesthetized animals. Blood pressure was raised by norepinephrine infusion and lowered by controlled haemorrhage in separate groups of rats. In addition we examined the effect of alpha-trinositol on neuropeptide Y-induced contraction of cerebral vessels in vitro. The in vitro study demonstrated inhibition of neuropeptide Y-induced contraction with a alpha-trinositol dose selective of neuropeptide Y. The in vivo study demonstrated that cerebral blood flow autoregulation was preserved after both doses of alpha-trinositol. alpha-Trinositol in the low neuropeptide Y-selective dose (5 mg kg-1 hr-1) did not affect the blood pressure limits of cerebral blood flow autoregulation, but the higher dose (50 mg kg-1 hr-1) of alpha-trinositol shifted the upper blood pressure limit of cerebral blood flow autoregulation towards lower blood pressures, an effect probably due to inhibition of both the adrenergic and neuropeptide Y systems.

Cerebral blood flow autoregulation during hypobaric hypotension assessed by laser Doppler scanning

Hypobaric hypotension was used to reduce systemic blood pressure in rats below the lower threshold of CBF autoregulation to evaluate a new laser Doppler (LD) "scanning" technique. Spontaneously breathing male Wistar Kyoto rats (n = 8) were anesthetized with chloral hydrate and the head fixed in a stereotaxic head holder. A cranial window with intact dura mater was introduced to assess local CBF (lCBF) by LD. One stationary probe served to detect rapid flow changes, whereas the second probe was used to sample lCBF recordings from many cortical locations by means of a stepping motor-controlled micromanipulator to obtain lCBF frequency histograms. Advantages are an improved spatial resolution together with the easy detection of low-flow areas and a better comparison of data from individual experiments. Arterial blood pressure was stepwise reduced by exposing the lower body portions to subatmospheric pressures (hypobaric hypotension), thus avoiding the use of drugs or heparinization. The lower threshold of CBF autoregulation was detected by "scanning" at arterial pressures between 50 and 46 mm Hg, with low-flow spots occurring immediately. The data suggest LD scanning as a method suited particularly for studies where lCBF inhomogeneities are expected, e.g., the ischemic penumbra or sinus vein thrombosis.

Differential cerebrovascular and metabolic responses in specific neural systems elicited from the centromedian-parafascicular complex

The effect of electrical stimulation of the centromedian-parafascicular complex on local cerebral blood flow and local cerebral glucose utilization was investigated in anesthetized, paralysed and ventilated rats. Local cerebral blood flow and local cerebral glucose utilization were measured in separate groups of animals using the autoradiographic (14C)iodoantipyrine and (14C)2-deoxyglucose methods, respectively. Because of the well-established centromedian-parafascicular complex neuroanatomical connections, three functional neuronal systems were analysed and compared: the extrapyramidal motor system the limbic system and the reticular formation, also known as the ascending activating system. Cortical regions not included in the limbic system were considered separately. The validity of comparisons between changes in local cerebral blood flow and local cerebral glucose utilization across the brain was verified by assessing the reactivity and stability of the cortical blood flow during long-term centromedian-parafascicular complex stimulation. Centromedian-parafascicular complex stimulation elicited a marked but heterogeneous increase in local cerebral blood flow in 50 of the 52 cerebral structures measured. The most pronounced increases were seen in the lateral habenular nucleus (331 +/- 30% of control), the zona incerta (400 +/- 55%), the mesencephalic reticular formation (415 +/- 122%) and the parietal cortex (211 +/- 35%). In contrast, local cerebral glucose utilization remained statistically unchanged (P greater than 0.05) in 28 of these 50 individual brain regions during centromedian-parafascicular complex stimulation. The most pronounced increases in local cerebral glucose utilization were seen in the zona incerta (123 +/- 28%) and the mesencephalic reticular formation (193 +/- 26%). Local cerebral blood flow and local cerebral glucose utilization were linearly related in unstimulated controls, considering either all brain regions taken as a whole or the three systems separately. The significant increase in the slopes of the regression line between local cerebral blood flow and local cerebral glucose utilization for the reticular formation and the limbic system during centromedian-parafascicular complex stimulation indicates, however, that the coupling mechanisms for these systems, but not for the extrapyramidal motor system, were reset. The local cerebral blood flow to local cerebral glucose utilization ratio was heterogeneous in controls and differentially increased during centromedian-parafascicular complex stimulation, being markedly pronounced in the parietal cortex and in the reticular formation. We conclude that these results, for the first time, provide evidence that, the functionally well-defined neural networks may have different mechanisms whereby changes in vascular and metabolic demands are regulated.

Efferent microvascular responses to electrical stimulation of the area postrema in rats

As part of its role to transduce blood-borne and afferent neural stimuli to the brain, the area postrema conducts efferent projections monosynaptically to individual nuclei of the medulla oblongata and pons. We hypothesized that electrical activation of the area postrema would mimic this transduction process and couple microvascular responses in efferent sites to local increases in tissue metabolism reported previously. We used quantitative autoradiographic techniques and image analysis to measure capillary transfer constants for [14C]alpha-aminoisobutyric acid (AIB, a small, neutral amino acid) and blood flow (iodo[14C]antipyrine) in individual brainstem structures of anesthetized rats. The area postrema was stimulated electrically by means of a monopolar microelectrode positioned stereotaxically 100 microns deep in the dorsocentral aspect of the organ. There were no significant effects of stimulation on [14C]AIB influx or blood flow in control hindbrain structures where postremal projections are sparse or absent--the spinal trigeminal nucleus, reticular formation, or cerebellar vermis. Stimulation of the area postrema produced equivalent increases in transcapillary influx of [14C]AIB and capillary blood flow in the nucleus of the solitary tract, dorsal motor nuclei of the vagus nerves, ventrolateral medullary C1 region, locus coeruleus, dorsal tegmental nuclei, and lateral parabrachial nuclei. Formation of ratios interrelating rates of [14C]AIB influx and blood flow with previously assessed values of tissue glucose metabolism indicated that these measures increased proportionately during postremal stimulation. Such proportional increases in capillary [14C]AIB transfer and blood flow during tissue activation by area postrema stimulation are consistent with interpretation that the increase in blood flow resulted from recruitment of unused surface area in the capillary networks of individual efferent nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute sympathetic denervation does not eliminate the effect of angiotensin converting enzyme inhibition on CBF autoregulation in spontaneously hypertensive rats

The effect of angiotensin converting enzyme inhibition with captopril (10 mg/kg i.v.) on CBF autoregulation was studied in 16 spontaneously hypertensive rats (8 control and 8 treated with captopril) subjected to acute cervical sympathectomy. CBF was measured repetitively by the intra-arterial 133Xe injection method, during the manipulation of MABP by norepinephrine or hemorrhagic hypotension. Prior to the administration of drugs, baseline MABP was 112 +/- 10 mm Hg in the control group and 119 +/- 11 mm Hg in the captopril group. Baseline CBF was 99 +/- 19 ml/100 g/min, with no difference in the two groups. In agreement with previous findings in rats with intact sympathetic nerves, the lower limit of CBF autoregulation was reduced from the MABP interval of 70-89 to 50-69 mm Hg by captopril.

Influence of ascending serotonergic pathways on glucose use in the conscious rat brain. II. Effects of electrical stimulation of the rostral raphé nuclei

Although lesions of the rostral raphé nuclei have minimal effects on integrated functional activity, as studied by the 2-deoxyglucose technique, the repercussions of activating the ascending serotonergic pathways have yet to be reported in the literature. To examine this question, we studied the consequences of the electrical stimulation of the rostral (median or dorsal) raphé nuclei on local cerebral glucose use in the conscious rat. Glucose use was determined by quantitative autoradiography in 105 defined brain structures. Raphé stimulation increased glucose utilization in a number of well-defined structures and pathways, dorsal raphé stimulation being systematically more effective than median raphé stimulation. Of all the neocortical regions studied, only the somatosensory cortex displayed a columnar and laminar pattern of increased glucose use that was restricted to the somatotopic delineation of the rat's head and face. Increased glucose use was seen in almost all key elements of the extrapyramidal system with the notable exception of the caudate-putamen. The thalamic nuclei that were activated by rostral raphé stimulation included those that subserve the processing of somesthetic, accessory visual and limbic information. Raphé stimulation-induced decreases in local cerebral glucose use were never observed. Almost all of the induced changes could be prevented or obtunded by prior intraventricular administration of the serotonergic neurotoxin 5,7-dihydroxytryptamine, suggesting that the majority of the raphé-induced changes in integrated functional activity were mediated via the activation of serotonergic neurones. The magnitude and pattern of the increases in glucose use could not always be correlated with the regional density of serotonergic innervation nor with the distribution of 5-hydroxytryptamine receptor subtypes in the adult brain. However, the pattern of increased cortical glucose use closely matches the selective serotonergic innervation of the somatosensory cortex found in early postnatal development. Thus, it would appear that the 2-deoxyglucose technique reveals functional units in the cortex that are innervated at an early ontogenic stage. We postulate that the discrete and highly organized changes in integrated functional activity that follow raphé stimulation are due to serotonin acting in a phasic manner on restricted, possibly specialized, postsynaptic structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Local cholinergic mechanisms participate in the increase in cortical cerebral blood flow elicited by electrical stimulation of the fastigial nucleus in rat

We sought to determine whether the increase in regional cerebral blood flow (rCBF) elicited within the cerebral cortex (CX) by electrical stimulation of the fastigial nucleus (FN) of the cerebellum is: prevented by local application of the muscarinic cholinergic receptor antagonist, atropine and temporally correlated with a stimulus-locked release of acetylcholine (ACh) from the cortical surface. Rats were anesthetized, paralyzed, ventilated, with arterial blood gases controlled and arterial pressure maintained within the autoregulated range. Bilateral craniotomies were performed over a standardized region of the sensory motor CX and superfusion devices stereotaxically positioned on the cortical surface. Cortical surface temperature, as well as pH, pCO2 and pO2 of the solutions applied to the cortex were also carefully controlled. rCBF was measured in dissected regions of frontal (FCX), parietal (PCX), and occipital cortices (OCX), caudate nucleus (CN), and hippocampus (HIPP) by the Kety principle using [14C]iodoantipyrine as indicator. Resting rCBF (ml/100 g/min) in unoperated control animals ranged from 70 +/- 5 in HIPP to 95 +/- 7 in PCX and was unaffected by bilateral craniotomies and placement of superfusion devices containing Kreb's bicarbonate buffer (vehicle) on the cortical surface. Local application of atropine (ATR, 100 microM) to the right PCX via the superfusion device did not affect resting rCBF. With FN stimulation rCBF increased bilaterally and symmetrically in all areas up to 227% in PCX. ATR application attenuated by 59% the FN-elicited increase in rCBF on the ipsilateral frontoparietal CX, without affecting blood flow in adjacent structures. ATR did not affect cortical cerebrovasodilation produced by hypercarbia (arterial pCO2 = 59.0 +/- 1.4 mm Hg). FN-stimulation resulted in a small (22%) but significant (P less than 0.05, n = 9) reduction in the release of [3H]ACh from the cortical surface, while supramaximal depolarization with 55 mM K+ increased [3H]ACh release by 251%. These studies indicate that: increases in cortical rCBF elicited by FN stimulation, but not hypercarbia, are in large part mediated by local muscarinic cholinergic receptors; resting rCBF is not tonically affected by muscarinic receptor activation; and the release of ACh from the cortical surface is, in general, reduced during FN-stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Carotenoid compound crocetin improves cerebral oxygenation in hemorrhaged rats

The carotenoid compound crocetin has been shown to increase oxygen diffusivity in vitro. In the present study the effect of crocetin on tissue oxygenation was examined in the cerebral cortex of rats subjected to hemorrhage. Twelve male Sprague-Dawley rats were anesthetized with pentobarbital and ventilation was controlled (PaCO2 = 33 mm Hg). A craniotomy was performed and the animals were hemorrhaged (20% of estimated total blood volume). Six of 12 animals then received a bolus of crocetin (2 U in 0.1 ml saline); the remaining animals received saline (0.1 ml i.v.) only. Values for mean arterial pressure. PO2, PCO2, pH, and hematocrit did not differ in rats that received either saline or crocetin. Tissue oxygen tension (PtO2) was measured at approximately 170 locations in the parietal cerebral cortex of each rat by a platinum-oxygen microelectrode technique. Results were compared by PtO2 frequency histograms. Crocetin as compared with saline treatment resulted in a right shift of the PtO2 frequency distribution and a significant decrease in the frequency of occurrence of low PtO2 values. The average of individual median PtO2 values was significantly greater in crocetin-treated animals as compared with those receiving saline (7.6 +/- 1.7 vs. 3.2 +/- 1.2 mm Hg, respectively). The results suggest that the carotenoid compound crocetin improves tissue oxygenation in the cerebral cortex of hemorrhaged rats.

Muscarinic cholinergic receptors mediate the cerebrovasodilation elicited by stimulation of the cerebellar fastigial nucleus in rat

We have investigated the effects of systemic administration of atropine sulfate on the global cerebrovascular vasodilation elicited by electrical stimulation of the cerebellar fastigial nucleus (FN) in chloralose-anesthetized rat. Atropine (0.3 mg/kg, i.v.) abolished the cerebrovasodilation elicited from FN but did not affect the concomitant elevation in arterial pressure and the EEG changes. We conclude that the cerebrovascular effect of FN stimulation, but not the peripheral cardiovascular or EEG changes, are mediated by cholinergic muscarinic receptors associated with cerebral vessels and/or intrinsic neural pathways.

Regional cerebral perfusion during hypertension depends on the hypertensive agent

Cerebral blood flow (CBF) was measured in 40 regions of the rat central nervous system by the [14C]iodoantipyrine autoradiographic technique during a moderate elevation in mean arterial blood pressure (to ca. 150 mmHg), induced by i.v. infusion of either dopamine (DA) or noradrenaline (NA). Hypertension induced by DA resulted in significant increases (median = 44%) in local CBF in 38 of the 40 brain regions investigated. In contrast, during NA infusion, CBF was elevated only slightly (median = 15%) in a few (8 of 40) brain regions (P less than 0.05). The cerebrovascular response to hypertension appears to be dependent upon the catecholamine which is employed to elicit the elevation in arterial blood pressure.

Functional bases for a central serotonergic involvement in classic migraine: a speculative view

The rôle of serotonin (5-HT) in the cerebrovascular bed is the subject of the following review. Cerebral blood vessels are supplied with 5-HT-containing fibres which originate in the raphé nuclei in the brainstem. The activation of this system may result in a constriction of large arteries and a dilatation of arterioles. Intra-arterial administration of 5-HT causes reduction in cerebral blood flow and metabolism provided it bypasses the blood-brain barrier. The findings, marked changes in plasma levels of 5-HT and in cerebral blood flow during a classic migraine attack, are suggestive of an involvement of the 5-HT system.

Influence of transmural pressure of myogenic responses of isolated cerebral arteries of the rat

We examined the diameter responses of isolated and pressurized posterior cerebral artery branches to various static and dynamic pressure alterations. These vessels, dissected from an anatomically identifiable location in the rat brain, developed tone when placed in a normal calcium physiological salt solution (1.6 mM Ca-PSS). Following a series of transmural pressure steps (delta p) of 25 or 50 mm Hg completed in 1-2 s and made every 5 min, they attained additional tone resulting in a mean luminal diameter of 139 micron at 100 mm Hg which was 35% less than their relaxed size measured in 1 mM EGTA-PSS. Continuous measurements of wall thickness and lumen diameter were obtained using a video electronic system in 1-2 mm long arterial segments, and autoregulatory gain factors calculated. Myogenic responses were obtained from each of 6 vessels taken from 6 WKY rats. Diameters following the step pressure changes were usually stable within 2-4 min. The data defined a myogenic regulatory pressure range from 49-145 mm Hg. Gain values averaged about 17% of that necessary for these arteries to maintain perfect flow autoregulation. Our results for myogenicity are comparable with the pressure range for blood flow autoregulation reported by others for the rat. We conclude that myogenic mechanisms, at least in this size artery, are partly responsible for flow autoregulation, and that they are supplemented by metabolic mechanisms operative in the intact rat brain.

Regional cerebral blood flow decreases during hyperglycemia

The presence of hyperglycemia before brain ischemia increases stroke-related morbidity and mortality in experimental animals and humans. However, little is known of the effect of hyperglycemia on regional cerebral blood flow (rCBF). Acute hyperglycemia was induced in awake but restrained rats by intraperitoneal injection of 50% D-glucose. Regional flow was determined using [14C]iodoantipyrine and quantitative autoradiography. Elevation of plasma glucose from 11 to 39 mM was associated with a 24% reduction in rCBF when compared with controls that received normal saline. Intraperitoneal D-mannitol produced an elevation of plasma osmolality equivalent to that observed with glucose. However, rCBF was only reduced by 10%. Hyperglycemia appears to produce a global decrease in rCBF in awake rats that cannot be completely explained by the attendant increase in plasma osmolality. If a similar influence is present during brain ischemia, hyperglycemia could extend areas of critical flow limitation.

Influence of gamma-hydroxybutyrate on the relationship between local cerebral glucose utilization and local cerebral blood flow in the rat brain

The relationship between local cerebral glucose utilization (LCGU) and local CBF (LCBF) was examined during the action of gamma-hydroxybutyrate (GHB) (900 mg/kg i.v.) in conscious rats. GHB induced discrepant effects on blood flow and metabolism. LCGU was markedly depressed in all structures examined, whereas LCBF was differently affected in that no related changes were observed. Global glucose utilization was markedly depressed (-51%), whereas global blood flow was not significantly altered. The marked dissociation between the changes in global glucose utilization and global blood flow induced by GHB is reflected only to a minor degree in the local values inasmuch as the correlation between LCGU and LCBF was only slightly weakened and its heterogeneity was increased.

Long-lasting reduction of cortical blood flow of the brain after spreading depression with preserved autoregulation and impaired CO2 response

The purpose of the present study was to examine the cerebral perfusion after spreading depression with special emphasis on its relation to the perfusion changes in migraine. The cerebrovascular reactivity to changes of arterial PCO2 tension in the range of 24-70 mm Hg and to changes of MABP in the range of 55-150 mm Hg was studied in the rat brain after one episode of cortical spreading depression. Regional CBF (rCBF) was measured in the frontal, parietal, and occipital cortex, the basal ganglia, and the cerebellum of both hemispheres after the intravenous bolus injection of [14C]iodoantipyrine. rCBF decreased in the cortical regions to 69-73% of control values for 1 h after spreading depression in every single rat, but remained unchanged in the basal ganglia and cerebellum. CBF changed with alterations of arterial carbon dioxide tension in all brain regions. In control regions, CBF changed by 2.8-3.9% per mm Hg PaCO2 change, whereas in the spreading depression cortex, CBF changed by only 1.3-1.5%. The difference between control and spreading depression cortex was significant at p less than 0.001, indicating a reduced CO2 sensitivity of the cortical regions invaded by spreading depression. rCBF remained unchanged in all brain regions in the MABP range of 80-150 mm Hg, reflecting the intact autoregulatory function of the two sides, and at a MABP of less than 80 mm Hg, rCBF decreased in parallel in symmetrical regions. It is concluded that the hypoperfusion after spreading depression is due to a vasoconstrictor stimulus overriding the vasodilatory effect of acid pH.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral blood flow in rats with renal and spontaneous hypertension: resetting of the lower limit of autoregulation

The effect of chronic hypertension on cerebral blood flow (CBF) was studied in anaesthetised rats. CBF was measured with the intracarotid 133Xe injection method. Rats with spontaneous and renal hypertension were compared with normotensive controls. The lower limit of autoregulation was determined during controlled haemorrhage. In the normotensive rats, CBF remained constant until mean arterial pressure (MAP) had decreased to the range of 50-69 mm Hg. Thereafter, CBF decreased with each further decrease in MAP. In both types of hypertensive rats, CBF remained constant until MAP had decreased to the range of 70-89 mm Hg. Thus, a 20-mm Hg shift of the lower limit of CBF autoregulation was found in both spontaneous and renal hypertensive rats. A neuropathological study revealed ischaemic brains lesions in half of the hypertensive rats following hypotension, whereas only a single lesion was found in one of six normotensive rats. No ischaemic brain lesions were found in a control study in which CBF was shown to be stable over a 21/2-h period. In conclusion, hypertensive rats showed a shift of the lower limit of CBF autoregulation as well as an increased susceptibility to ischaemic brain damage during hypotension. These findings presumably reflect hypertensive structural changes in the cerebral circulation.