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

Noradrenaline released from locus coeruleus axons contracts cerebral capillary pericytes via 2 adrenergic receptors

Noradrenaline (NA) release from locus coeruleus axons generates vascular contractile tone in arteriolar smooth muscle and contractile capillary pericytes. This tone allows neuronal activity to evoke vasodilation that increases local cerebral blood flow (CBF). Much of the vascular resistance within the brain is located in capillaries and locus coeruleus axons have NA release sites closer to pericytes than to arterioles. In acute brain slices, NA contracted pericytes but did not raise the pericyte cytoplasmic Ca2+ concentration, while the α1 agonist phenylephrine did not evoke contraction. Blocking α2 adrenergic receptors (α2Rs, which induce contraction by inhibiting cAMP production), greatly reduced the NA-evoked pericyte contraction, whereas stimulating α2Rs using xylazine (a sedative) or clonidine (an anti-hypertensive drug) evoked pericyte contraction. Noradrenaline-evoked pericyte contraction and capillary constriction are thus mediated via α2Rs. Consequently, α2Rs may not only modulate CBF in health and pathological conditions, but also contribute to CBF changes evoked by α2R ligands administered in research, veterinary and clinical settings.

Locus Coeruleus and neurovascular unit: From its role in physiology to its potential role in Alzheimer's disease pathogenesis

Locus coeruleus (LC) is the main noradrenergic (NA) nucleus of the central nervous system. LC degenerates early during Alzheimer's disease (AD) and NA loss might concur to AD pathogenesis. Aside from neurons, LC terminals provide dense innervation of brain intraparenchymal arterioles/capillaries, and NA modulates astrocyte functions. The term neurovascular unit (NVU) defines the strict anatomical/functional interaction occurring between neurons, glial cells, and brain vessels. NVU plays a fundamental role in coupling the energy demand of activated brain regions with regional cerebral blood flow, it includes the blood-brain barrier (BBB), plays an active role in neuroinflammation, and participates also to the glymphatic system. NVU alteration is involved in AD pathophysiology through several mechanisms, mainly related to a relative oligoemia in activated brain regions and impairment of structural and functional BBB integrity, which contributes also to the intracerebral accumulation of insoluble amyloid. We review the existing data on the morphological features of LC-NA innervation of the NVU, as well as its contribution to neurovascular coupling and BBB proper functioning. After introducing the main experimental data linking LC with AD, which have repeatedly shown a key role of neuroinflammation and increased amyloid plaque formation, we discuss the potential mechanisms by which the loss of NVU modulation by LC might contribute to AD pathogenesis. Surprisingly, thus far not so many studies have tested directly these mechanisms in models of AD in which LC has been lesioned experimentally. Clarifying the interaction of LC with NVU in AD pathogenesis may disclose potential therapeutic targets for AD.

Topographic Distribution of Contractile Protein in the Human Macular Microvasculature

Purpose

We studied the topographic distribution of contractile protein in different orders of the human macular microvasculature to further understanding of the sites for capillary blood flow regulation.

Methods

Nine donor eyes from eight donors were cannulated at the central retinal artery and perfusion labeled for alpha smooth muscle actin (αSMA) and filamentous actin (F-actin). Confocal images were collected from the macula region, viewed, projected, and converted to a 255 grayscale for measurements. The mean intensity was measured for macular arterioles, venules, and capillary segments. The diameter of each vessel segment measured was recorded. The normalized mean intensity values from all images were ranked according to vessel types and size with a total of nine categories.

Results

F-actin was present throughout the macular microvasculature whereas αSMA labeling showed variations. Overall, αSMA has a more prominent presence in the macular arterioles than in the macular capillaries and venules, and αSMA strongly labeled the smaller macular arterioles. Some capillaries also labeled positive for αSMA, including some of the capillaries in the innermost capillary ring surrounding the foveola. It was weakly present in the capillaries on the venous side and larger venules. In the larger macular arterioles closer to 100 μm in diameter, αSMA labeling was weakly present and not as ubiquitous as in the smaller arterioles.

Conclusions

Nonuniform distribution of contractile proteins in the different types, orders, and sizes of macular microvasculature indicates that these vessels may have different contractile capability and roles in macular flow regulation.

Retinal capillary perfusion: Spatial and temporal heterogeneity

The central role of the cardiovascular system is to maintain adequate capillary perfusion. The spatially and temporally heterogeneous nature of capillary perfusion has been reported in some organs. However, such heterogeneous perfusion properties have not been sufficiently explored in the retina. Arguably, spatial and temporal heterogeneity of capillary perfusion could be more predominant in the retina than that in other organs. This is because the retina is one of the highest metabolic demand neural tissues yet it has a limited blood supply due to optical requirements. In addition, the unique heterogeneous distribution of retinal neural cells within different layers and regions, and the significant heterogeneity of intraretinal oxygen distribution and consumption add to the complexity. Retinal blood flow distribution must match consumption of nutrients such as oxygen and glucose within the retina at the cellular level in order to effectively maintain cell survival and function. Sophisticated local blood flow control in the microcirculation is likely required to control the retinal capillary perfusion to supply local retinal tissue and accommodate temporal and spatial variations in metabolic supply and demand. The authors would like to update the knowledge of the retinal microvessel and capillary network and retinal oxidative metabolism from their own studies and the work of others. The coupling between blood supply and energy demands in the retina is particularly interesting. We will mostly describe information regarding the retinal microvessel network and retinal oxidative metabolism relevant to the spatial and temporal heterogeneity of capillary perfusion. We believe that there is significant and necessary spatial and temporal heterogeneity and active regulation of retinal blood flow in the retina, particularly in the macular region. Recently, retinal optical coherence tomography angiography (OCTA) has been widely used in ophthalmology, both experimentally and clinically. OCTA could be a valuable tool for examining retinal microvessel and capillary network structurally and has potential for determining retinal capillary perfusion and its control. We have demonstrated spatial and temporal heterogeneity of capillary perfusion in the retina both experimentally and clinically. We have also found close relationships between the smallest arterioles and capillaries within paired arterioles and venules and determined the distribution of smooth muscle cell contraction proteins in these vessels. Spatial and temporal heterogeneity of retinal capillary perfusion could be a useful parameter to determine retinal microvessel regulatory capability as an early assay for retinal vascular diseases. This topic will be of great interest, not only for the eye but also other organs. The retina could be the best model for such investigations. Unlike cerebral vessels, retinal vessels can be seen even at the capillary level. The purpose of this manuscript is to share our current understanding with the readers and encourage more researchers and clinicians to investigate this field. We begin by reviewing the general principles of microcirculation properties and the spatial and temporal heterogeneity of the capillary perfusion in other organs, before considering the special requirements of the retina. The local heterogeneity of oxygen supply and demand in the retina and the need to have a limited and well-regulated retinal circulation to preserve the transparency of the retina is discussed. We then consider how such a delicate balance of metabolic supply and consumption is achieved. Finally we discuss how new imaging methodologies such as optical coherence tomography angiography may be able to detect the presence of spatial and temporal heterogeneity of capillary perfusion in a clinical setting. We also provide some new information of the control role of very small arterioles in the modulation of retinal capillary perfusion which could be an interesting topic for further investigation.

Evidence for a predominant intrinsic sympathetic control of cerebral blood flow alterations in an animal model of cerebral arteriovenous malformation

In terms of neurogenic cerebral blood flow (CBF) control, the activity of the sympathetic nervous system (SNS) has a regulating effect. The impact of a manipulation of both the peripheral (via the perivascular sympathetic net) and central components (via the intracortical noradrenergic terminals originating from the locus coeruleus) on CBF-and especially on hyperperfusion syndromes-is unclear. To test the specific patterns following such alterations, cortical oxygen saturation (rSO2), regional CBF (rCBF), and cortical interstitial norepinephrine (NE) concentrations were measured. Twelve weeks after either the creation of an extracranial AV fistula or sham operation, 80 male Sprague-Dawley rats underwent one of the following procedures: (1) no SNS manipulation, (2) peripheral SNS inhibition via bilateral sympathectomy, (3) central SNS inhibition via the neurotoxin DSP-4, or (4) complete SNS inhibition. Norepinephrine concentrations were lowest after complete inhibition (NE [nmol]: pre, 1.8 ± 1.2; post, 2.4 ± 1.8) and highest following peripheral inhibition (NE [nmol]: pre, 3.6 ± 1.9; post, 6.6 ± 4.4). Following fistula occlusion, rCBF (laser Doppler unit [LDU]) and rSO2 (%SO2) increases were highest after complete inhibition (pre: 204 ± 14 LDU, 34 ± 3%SO2; post: 228 ± 18 LDU, 39 ± 3%SO2) and lowest after peripheral inhibition (pre: 221 ± 18 LDU, 41 ± 2%SO2; post: 226 ± 14 LDU, 47 ± 2%SO2). Thus, a complete inhibition down-regulates SNS activity and provokes a cortical hyperperfusion condition. With this, the hitherto unknown predominant role of the intrinsic component could be demonstrated for the first time in vivo.

Functional neuroimaging of normal human sleep by positron emission tomography

Functional neuroimaging using positron emission tomography has recently yielded original data on the functional neuroanatomy of human sleep. This paper attempts to describe the possibilities and limitations of the technique and clarify its usefulness in sleep research. A short overview of the methods of acquisition and statistical analysis (statistical parametric mapping, SPM) is presented before the results of PET sleep studies are reviewed. The discussion attempts to integrate the functional neuroimaging data into the body of knowledge already acquired on sleep in animals and humans using various other techniques (intracellular recordings, in situ neurophysiology, lesional and pharmacological trials, scalp EEG recordings, behavioural or psychological description). The published PET data describe a very reproducible functional neuroanatomy in sleep. The core characteristics of this 'canonical' sleep may be summarized as follows. In slow-wave sleep, most deactivated areas are located in the dorsal pons and mesencephalon, cerebellum, thalami, basal ganglia, basal forebrain/hypothalamus, prefrontal cortex, anterior cingulate cortex, precuneus and in the mesial aspect of the temporal lobe. During rapid-eye movement sleep, significant activations were found in the pontine tegmentum, thalamic nuclei, limbic areas (amygdaloid complexes, hippocampal formation, anterior cingulate cortex) and in the posterior cortices (temporo-occipital areas). In contrast, the dorso-lateral prefrontal cortex, parietal cortex, as well as the posterior cingulate cortex and precuneus, were the least active brain regions. These preliminary studies open up a whole field in sleep research. More detailed explorations of sleep in humans are now accessible to experimental challenges using PET and other neuroimaging techniques. These new methods will contribute to a better understanding of sleep functions.

Effects of yohimbine on cerebral blood flow, symptoms, and physiological functions in humans

OBJECTIVE

Increases in adrenergic activity are associated with stress, anxiety, and other psychiatric, neurological, and medical disorders. To improve understanding of normal CNS adrenergic function, CBF responses to adrenergic stimulation were determined.

METHODS

Using PET, the CBF changes after intravenous yohimbine, an alpha2-adrenoreceptor antagonist that produces adrenergic activation, were compared with placebo in nine healthy humans. Heart rate, blood pressure, Paco2, plasma catecholamines, and symptom responses were also determined.

RESULTS

Among nonscan variables, yohimbine produced significant symptom increases (including a panic attack in one subject), a decrease in Paco2 due to hyperventilation, increases in systolic and diastolic blood pressure, and a trend toward a significant norepinephrine increase. Among scan results, yohimbine produced a significant decrease in whole-brain absolute CBF; regional decreases were greatest in cortical areas. Medial frontal cortex, thalamus, insular cortex, and cerebellum showed significant increases after normalization to whole brain. Medial frontal CBF change was correlated with increases in anxiety. A panic attack produced an increase instead of a decrease in whole-brain CBF. Factors potentially contributing to the observed CBF changes were critically reviewed. Specific regional increases were most likely due in large part to activation produced by adrenergically induced anxiety and visceral symptoms.

CONCLUSIONS

This study supports the relationship of anxiety and interoceptive processes with medial frontal, insular, and thalamic activation and provides a baseline for comparison of normal yohimbine-induced CNS adrenergic activation, adrenergically-based symptoms, and other markers of adrenergic function to stress, emotion, and the adrenergic pathophysiologies of various CNS-related disorders.

Regulation of regional cerebral blood flow by cholinergic fibers originating in the basal forebrain

We review mainly recent studies on vasodilative regulation of cortex and hippocampus by central cholinergic nerves originating in the basal forebrain. We also briefly review the influence of other central noradrenergic fibers originating in the locus ceruleus, serotonergic fibers originating in the dorsal raphe nucleus, dopaminergic fibers originating in the substantia nigra, and peripheral sympathetic and parasympathetic nerve fibers upon regulation of regional cerebral blood flow. Local metabolites have long been considered to play an important physiological role in regulating regional cerebral blood flow. However, the evidence reviewed here emphasizes that the regulation of regional cerebral blood flow by these central cholinergic nerves is independent of regional metabolism. We propose through this review that although studies investigating neural regulation of cortical and hippocampal blood flow by cholinergic fibers originating in the basal forebrain have added much to the understanding of regulation of regional cerebral blood flow further studies are needed to determine the physiological relevance of regional cerebral blood flow in relation to higher nervous functions such as memory, learning, and personality, and changes in these cognitive functions with aging and pathology such as Alzheimer's disease.

Effect of chemical sympathectomy on cerebral blood flow in rats

Thirty male Wistar rats, weighing 350 to 400 gm each, received stereotactic injections of 6-hydroxydopamine (300 micrograms/kg) into the left lateral ventricle. The same amount of saline was injected into a control group of 15 rats. Seven days after this procedure, cerebral blood flow (CBF) was measured by the hydrogen clearance method. A hypertensive condition at a mean arterial pressure of about 160 mm Hg was maintained for 1 hour by intravenous infusion of phenylephrine. In the 6-hydroxydopamine-treated group, CBF increased significantly after the elevation of systemic blood pressure compared with that in the control group, and cerebral autoregulation was impaired. After a 1-hour study, the specific gravity of the cerebral tissue in the treated group significantly decreased; electron microscopic studies at that time revealed brain edema. It is suggested that depletion of brain noradrenaline levels causes a disturbance in cerebral microvascular tone and renders the cerebral blood vessels more vulnerable to hypertension.

Effect of peripheral and central alpha-adrenoceptor blockade on cerebral microvascular and blood flow responses to hypoxia

The purpose of this study was to evaluate the effects of vascular and central alpha-adrenoceptor blockade on cerebral blood flow (CBF) and utilization of brain arteriolar and capillary reserve in conscious rats during normoxia and hypoxia (8% O2 in N2). Animals were divided into three groups and administered either saline, N-methyl chlorpromazine (does not cross the blood-brain barrier), or phenoxybenzamine (crosses the blood-brain barrier) in equipotent doses. Neither agent affected regional CBF and the utilization of brain microvascular reserve during normoxia. CBF increased from 70.9 +/- 2.9 (SEM) ml/min/100 g in the control normoxic group to 123.8 +/- 4.2 ml/min/100 g in control hypoxic animals. In control, hypoxic flow to pons and medulla of the brain was higher than to cortex, hypothalamus or thalamus. The percent of arterioles/mm2 perfused increased from 49.6 +/- 2.0% during control normoxia to 65.6 +/- 3.0% during control hypoxia. The percentage of capillaries/mm2 perfused changed similarly. Hypoxic CBF was increased similarly after administration of N-methyl chlorpromazine or phenoxybenzamine. Administration of N-methyl chlorpromazine or phenoxybenzamine eliminated regional differences in hypoxic CBF and the utilization of arterioles, and did not affect capillary response. There was no difference between the effect of N-methyl chlorpromazine and phenoxybenzamine on cerebral microvascular and blood flow responses to hypoxia. It was concluded that peripheral alpha-adrenoceptors affect the distribution of regional microvascular and blood flow responses to hypoxia, and central alpha-adrenoceptors probably do not participate in this effect.

Protection against ischemia-induced neuronal damage by the alpha 2-adrenoceptor antagonist idazoxan: influence of time of administration and possible mechanisms of action

The protective effect of the alpha 2-receptor antagonist idazoxan against neuronal damage in the neocortex and in the hippocampal CA1 region was studied in rats exposed to 10 min of incomplete forebrain ischemia. When administered i.v. immediately after ischemia (0.1 mg/kg) and subsequently for 6 h (10 micrograms/kg/min), idazoxan significantly reduced neuronal damage in the hippocampus (from 84 to 26%) and in the vulnerable parts of the neocortex (from 15 to 1%). The bolus dose alone provided no significant protection. When idazoxan administration was delayed for 30 min, no significant protection was noticed in the neocortex, and the effect in the hippocampus was ambiguous. A transient elevation of plasma corticosterone levels was induced during ischemia. Idazoxan administration for 2 h did not affect postischemic changes in corticosterone levels compared with saline infusion. Idazoxan (10(-7)-10(-4) M) did not influence the in vitro binding to glutamate receptors in brain slices. Thus, the protective effect of idazoxan cannot be explained by suppression of the plasma corticosteroid levels or via an antagonistic effect on glutamate receptors. Idazoxan apparently protects neurons when given during the first hours of postischemic reperfusion, while histopathological necrosis of neurons becomes visible 48-72 h after ischemia. Detrimental processes causing delayed neuronal death occur in the early postischemic phase and can be influenced by adrenoceptor ligands. Idazoxan may protect by several mechanisms but probably exerts its protective postischemic effect mainly through an increased noradrenergic neuronal activity and an elevation of extracellular noradrenaline (NA) levels in the brain. The favorable effects of NA may either be due to inhibition of excitotoxic neurotransmission or activation of survival-promoting and trophic processes.

Low frequency stimulation of the locus coeruleus reduces regional cerebral blood flow in the spinalized cat

Regional cerebral blood flow (RCBF) was studied during low frequency (15/s) and high frequency (50/s) electrical stimulation of the locus coeruleus (LC) in the alpha-chloralose-anesthetized cat using the freely diffusible tracer [14C]iodoantipyrine and regional brain dissection. The responses were determined in animals spinalized at the C1/C2 level to eliminate systemic effects of pontine stimulation such as alterations in blood pressure and heart rate. The spinalization, itself, did not alter resting RCBF or reactivity to hypercapnia. Low frequency stimulation reduced regional cerebral blood flow in the cortex, basal ganglia and white matter of the corpus callosum. The reductions in RCBF were maximal (35%) in the occipital cortex whereas no changes were seen in the colliculi. No changes were seen in any brain areas with high frequency stimulation. The relevance of this brainstem effect on cerebral blood flow to pathological states such as stroke and migraine is discussed.

Locus ceruleus morphometry in aging and schizophrenia

The locus ceruleus (LC), a pigmented brainstem nucleus rich in noradrenergic neurons, has been proposed to be involved in the pathophysiology of aging and schizophrenia. We undertook a quantitative neuropathological study of the LC in these two conditions. A computing planimeter was employed to count the total number of neurons and measure the volume of the LC, neuronal cross-sectional area, and percent of neuronal area occupied by neuromelanin in the brains of 39 subjects; 13 "normative", 15 leucotomized schizophrenic (most had died in the preneuroleptic era), and 11 leucotomized non-schizophrenic control patients, ranging in age from 11 to 94 years. There was a significant inverse correlation between age and total number of LC neurons, neuronal size, and LC volume, and a significant positive correlation between age and the percentage of neuronal area occupied by neuromelanin. Although schizophrenics did not differ significantly from control groups on any of the parameters of LC morphology, there was a trend for reduced LC volume in schizophrenic brains. Also, the LC of leucotomized patients tended to have increased neuromelanin content and slightly increased cell counts compared to normals, although the importance of this finding is not clear.

Mechanisms of hypoglycemic brain damage. Evidence against a significant role of the noradrenergic locus coeruleus system

Selective lesions of the noradrenergic locus coeruleus (LC) system have recently been shown to aggravate both ischemic and epileptic brain damage. This study explores the possibility that the LC system also influences hypoglycemic brain injury. Bilateral 6-hydroxydopamine lesions of the LC projection to the forebrain were found to cause no change in the degree of neuronal necrosis in the neocortex, hippocampal formation and caudate-putamen following 30 min of reversible insulin-induced hypoglycemic coma. We propose that selective neuronal necrosis in ischemia and status epilepticus is due to the action of excitatory amino acids at synaptic sites, which can be partly counteracted by noradrenaline release from inhibitory LC terminals. In hypoglycemia, excitatory amino acids probably cause brain damage via a local and more diffuse toxic effect which is not significantly influenced by the activation of the LC system.

Increased cerebral blood flow during hypercapnia is not affected by lesion of the nucleus locus ceruleus

To test the hypothesis that the putative noradrenergic innervation of intraparenchymal cerebral blood vessels from the nucleus locus ceruleus mediates the vasodilatory response to hypercapnia, regional cerebral blood flow was measured by iodo-[14C]antipyrine autoradiography in awake and restrained rats with unilateral 6-hydroxydopamine lesion of the nucleus locus ceruleus and in unlesioned control rats. Hypercapnia, induced by the inhalation of 5% or 8% CO2 in air for 8 minutes caused a 2 to 5-fold increase in regional cerebral blood flow. However, despite a marked reduction of about 90% in cortical norepinephrine levels ipsilateral to the lesion, blood flow to the frontal and parietal cortex, hippocampus, striatum and cerebellum increased to the same extent in ipsilateral and contralateral regions. Thus, lesion of the locus ceruleus and the resultant depletion of endogenous cortical and hippocampal norepinephrine, does not influence the cerebrovascular response to hypercapnia.

Subarachnoid hemorrhage in the rat: cerebral blood flow and glucose metabolism after selective lesions of the catecholamine systems in the brainstem

A double-isotope autoradiographic technique was used to evaluate CBF and glucose metabolism 2 days after a subarachnoid hemorrhage (SAH) in rats with lesions in the lower brainstem. Lesioning in the mesencephalon of the ascending catecholamine pathways from locus ceruleus and from the A1 and A2 nuclei, or lesioning in the medulla oblongata of the ascending fibers from A1 and A2, prevents the development of the global changes in flow and metabolism seen in normal animals post SAH. Also the focal low-flow areas with markedly elevated deoxyglucose uptake, which can develop in normal animals 2 days post SAH, were not seen in the lesioned animals after the SAH. The findings indicate that the A1 and A2 nuclei, which project to the hypothalamus-pituitary, are essential for the flow and metabolic changes after an SAH. The lesions per se did not change baseline flow and metabolism as compared with sham-lesioned animals.

A role of the central catecholamine neuron in cerebral circulation

The effect of the central catecholaminergic neurons on the cerebral microcirculation was investigated by means of a unilateral intracerebral injection of 6-hydroxydopamine (6-OHDA) which produced the degeneration of catecholamine (CA) nerve terminals. Subsequent observation with CA histofluorescence revealed an absence of CA fibers in the vicinity of the 6-OHDA injection site. A significant increase in regional cerebral blood flow (rCBF), measured by the hydrogen clearance method, was demonstrated in the CA-depleted cortex under normocapnia as compared with rCBF in the control cortex (CA-depleted cortex 47.0 +/- 2.8 ml/100 gm/min; control cortex 38.5 +/- 3.5 ml/100 gm/min; p less than 0.005). The increased rCBF in the cortex treated with 6-OHDA was suppressed by the iontophoretic replacement of noradrenaline (NA) to the CA-depleted cortex. An iontophoretic replacement of 10(-5) M dopamine (DA) mildly suppressed the increased rCBF in the 6-OHDA-treated cortex. The CO2 reactivity in the CA-depleted cortex was significantly lower than that of the control cortex (CA-depleted cortex 2.13% +/- 0.6%/mm Hg; control cortex 3.53% +/- 0.70%/mm Hg). No change was noticeable in the cerebral glucose metabolism in the CA-depleted cortex in an investigation based on tritiated (3H)-deoxyglucose uptake. It is suggested that the 6-OHDA-induced change in cerebral blood flow (CBF) is not secondary to alterations in cerebral metabolic rate, and that the central NA neuron system innervating intraparenchymal blood vessels regulates CBF through a direct vasoconstrictive effect on the cerebral blood vessels. The central DA neuron system may modulate the cerebral circulation as a mild vasoconstrictor.

Effects of clonidine on cerebral blood flow and the response to arterial CO2

CBF, as measured by the clearance of 133Xe or 85Kr in the pentobarbital-anesthetized cat, displays a monotonic increase as the PaCO2 is elevated over a range of 20-60 mm Hg (slope Xe, 1.65 +/- 0.14 ml/100g/min/mm Hg; slope Kr, 1.40 +/- 0.11 ml/100 g/min/mm Hg). Clonidine (20 micrograms/kg i.v.), a centrally acting, alpha 2-preferring agonist, reduced the slope of the PaCO2-CBF response functions for Xe and Kr by 70 and 64%, respectively. Clonidine reduced normocarbic CBF-Xe by 36%, but had no effect on normocarbic CBF-Kr. ST-91, a polar structural analog of clonidine that does not cross the blood-brain barrier, did not reproduce the effects of clonidine when administered at an equivalent dose. This indicates that the effects of clonidine observed were secondary to its action on central rather than peripheral sites. In addition to the effects on the clearance of CBF markers, clonidine reduced the increased MABP otherwise evoked by elevated PaCO2. Reduction in the MABP response to PaCO2 did not account for the lowering of CBF during hypercarbia. In separate experiments where MABP was elevated to correspond with the PaCO2-MABP response observed in the absence of clonidine, a comparable reduction in the slope of the PaCO2 response was also observed. In addition, the pressure autoregulatory response was unaltered after clonidine treatment. These observations suggest that the central action of alpha 2-receptors on the CBF-CO2 response cannot be attributed to an altered perfusion pressure.

Effect in cat of locus coeruleus lesions on the response of cerebral blood flow and cardiac output to altered paCO2

In pentobarbital-anesthetized cats, over arterial paCO2 values of 20-60 mm Hg, cerebral blood flow (CBF, Xenon) and cardiac output (CO, thermal dilution) show positively inflicted curves with slopes significantly greater than zero. To examine the role of the locus coeruleus (LC) in these responses, bilateral stereotactic thermo-coagulation lesions of the LC were made. The effect of lesions confirmed to involve the LC bilaterally (n = 10), were compared with the effects of misdirected lesions placed in the cerebellum and lateral to the LC (n = 10) and sham lesions (n = 10). Ten days after the lesioning procedure, the animals were re-anesthetized with pentobarbital and paCO2 response curves were determined for CBF and CO prior to and following intravenous administration of propranolol (1 mg/kg, i.v.). The results obtained with the sham-operated animals and the animals with lesions outside of the LC were indistinguishable. Bilateral LC lesions had no significant effect on normocapnic CBF as compared to control animals. They did, however, significantly reduce the slope of the CBF paCO2 response curve. Propranolol produced a significant reduction in CBF in lesioned and non-lesioned animals measured at all levels of pCO2 and did not alter the slope of the pCO2 response curve for any group as compared to predrug values. Bilateral lesions of the LC did not significantly alter either normocapnic CO or the slope of the CO-paCO2 relationship, but did reduce the elevation in mean arterial blood pressure otherwise observed during hypercarbia. Measurement of norepinephrine levels in cortex indicate a close correlation between the ability of the lesion to reduce norepinephrine content and produce the observed physiological effects. The results of these experiments suggest that the hypercapnic response of CBF, but not CO to arterial paCO2 is modulated by systems which traverse the dorsal brainstem. The role of the locus coeruleus-catecholamine cell bodies in this effect, however, must be considered speculative until further transmitter-selective interventions are carried out.

Convulsive and non-convulsive ethanol withdrawal behaviour in rats with lesions of the noradrenergic locus coeruleus system

The ascending noradrenergic pathways from the locus coeruleus were lesioned bilaterally in 10 rats by intracerebral 6-hydroxydopamine injections. Ten rats were sham-operated. All animals were subjected to a 4-day ethanol intoxication period using intragastric intubation. Intoxication and withdrawal assessments were performed blindly. The 6-hydroxydopamine lesions did not appear to affect tolerance to ethanol. During withdrawal, however, lesioned animals showed minor, but statistically significant changes in scores of certain non-convulsive withdrawal signs, but incidence and intensity of spontaneous and audiogenic convulsive seizures were not different between the groups.

Lesions of the locus coeruleus system aggravate ischemic damage in the rat brain

The possibility that the noradrenergic locus coeruleus system influences brain damage following ischemia was explored in rats. Bilateral lesions of the locus coeruleus projections to the forebrain aggravated the neuronal necrosis in the hippocampal CA1 region and neocortex following complete cerebral ischemia induced by transient cardiac arrest. These findings provide evidence that the postischemic activation of the inhibitory locus coeruleus system could counteract a possible detrimental neuronal hyperexcitation, thereby limiting neuronal necrosis.

Subarachnoid haemorrhage in the rat: effect on the development of vasospasm of selective lesions of the catecholamine systems in the lower brain stem

Intracisternal injection of blood in the rat produces an angiographically demonstrable biphasic vasospasm. Lesioning at the level of the mesencephalon of the ascending catecholamine pathways from locus coeruleus in the pons and the A1 and A2 nuclei in the medulla oblongata prior to cisternal blood injection prevents the development of both acute and late spasm. Selective lesioning in the medulla oblongata of ascending fibres from A1 and A2 also prevents development of spasm, indicating that these nuclei, which project to the hypothalamus-pituitary, are essential for the spasm syndrome. It is suggested that a substance vasospasm is produced by a substance liberated either by the hypothalamus or by the pituitary is involved in the occurrence of spasm.

Cyclic AMP concentrations in rat neocortex and hippocampus during and following incomplete ischemia: effects of central noradrenergic neurons, prostaglandins, and adenosine

The concentrations of cyclic AMP, noradrenaline, glycogen, glucose, lactate, pyruvate, labile phosphate compounds, and free fatty acids were investigated in the rat neocortex and hippocampus during and following cerebral ischemia. An incomplete ischemia of 5 and 15 min duration was induced by bilateral carotid clamping combined with hypotension. The postischemic events were studied after 5, 15, and 60 min of recirculation. Five minutes of ischemia did not significantly alter the neocortical or hippocampal concentrations of cyclic AMP. After 15 min of ischemia the neocortical levels decreased significantly below control values. In the recirculation period following ischemia a significant elevation of the cyclic AMP concentrations was observed. Following 5 min of recirculation after 5 min of ischemia the levels increased from 2.53 +/- 0.21 nmol X g-1 to 5.18 +/- 0.09 nmol X g-1 in the neocortex and from 2.14 +/- 0.16 nmol X g-1 to 3.52 +/- 0.35 nmol X g-1 in the hippocampus. Five minutes of recirculation following 15 min of ischemia led to a significant increase in the levels of cyclic AMP, to 12.86 +/- 1.43 nmol X g-1 in the neocortex to 5.58 +/- 0.57 nmol X g-1 in the hippocampus. With longer recirculation periods the cyclic AMP levels progressively decreased and were similar to control values after 60 min. Depletion of cortical noradrenaline by at least 95% was performed by injections of 6-hydroxydopamine into the ascending axon bundles from the locus ceruleus. The lesion did not significantly change the ischemic or post-ischemic neocortical and hippocampal levels of cyclic AMP, glycogen, or free fatty acids including arachidonic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of locus coeruleus stimulation on regional cerebral oxygen consumption in the cat

Regional cerebral oxygen consumption was determined during stimulation of the intra-axial noradrenergic pathway to quantitate the metabolic effects of this manipulation on cerebral oxygen extraction, cerebral blood flow (CBF) and its regional distribution. Regional arterial and venous oxygen saturation were examined microspectrophotometrically. Regional CBF was examined using radioactively tagged microspheres (15 +/- 3 microns in diameter). Oxygen consumption was calculated as the regional product of CBF and oxygen extraction. Bipolar concentric electrodes were stereotaxically implanted bilaterally in the locus coeruleus of alpha-chloralose anesthetized, artificially respired adult mongrel cats. The control group was killed after hemodynamic and CBF measurements were taken. The experimental group was sacrificed after these same measurements were taken before and during 10 min of bilateral locus coeruleus stimulation. The cats' heads were simultaneously sawed in 3 places and quickly frozen in liquid nitrogen-cooled propane. Systolic blood pressure was significantly increased during treatment. The heterogeneity of venous oxygen saturation was significantly reduced by stimulation. Average CBF and oxygen consumption were significantly decreased to 57% and 59% of control, respectively. Oxygen consumption was significantly reduced in the hypothalamus from 1.5 +/- 0.3 to 0.9 +/- 0.3 ml O2/min/100 g and from 3.5 +/- 0.9 to 1.2 +/- 0.4 ml O2/min/100 g in the cerebellum by treatment. Changes in the neuronal and/or synthetic cerebral activity produced regional decreases in cerebral oxygen consumption and secondarily altered CBF. These changes are probably due to interaction of the intraparenchymal noradrenergic pathways with other systems or processes in the brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Delayed postischemic hypoperfusion: evidence against involvement of the noradrenergic locus ceruleus system

This study explores the possibility that the delayed hypoperfusion observed after an ischemic insult might be due to vasoconstriction induced by the release of noradrenaline from nerves originating in the locus ceruleus. Bilateral 6-hydroxydopamine lesions of the ascending bundles from the locus ceruleus were carried out in the caudal mesencephalon of rats. Local CBF was measured with an autoradiographic technique 60 min following the start of recirculation after incomplete forebrain ischemia. No significant differences in CBF between nonoperated, sham-operated, and noradrenaline-depleted animals were observed in any structure of the forebrain. It is concluded that the noradrenergic locus ceruleus system does not contribute to the development of delayed postischemic hypoperfusion.

Treatment of acute focal cerebral ischemia with propranolol

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

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

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

Influence of lesions of the noradrenergic locus coeruleus system on the cerebral metabolic response to bicuculline-induced seizures

The objective of the present study was to explore if lesions of the ascending noradrenergic pathways, originating in the locus coeruleus, modulate the cerebral metabolic response to bicuculline-induced seizures in rats. Bilateral noradrenergic lesions were performed by 6-hydroxydopamine injections in the caudal mesencephalon, 12-22 days before seizures were induced in animals ventilated on N2O:O2 (75:25). After 5 min of seizures the brain was frozen in situ and cerebral cortex and hippocampus were sampled for analysis. Labile phosphates, glycolytic metabolites, cyclic nucleotides, and free fatty acids were measured. In another series, lesioned animals were used for measurements of cerebral oxygen consumption. The noradrenergic lesions neither modified the electroencephalographically recorded seizure discharge, nor did they alter cerebral oxygen consumption or cerebral energy state. However, when compared to sham-operated animals, those with noradrenergic lesions had significantly higher (115% and 68%) glycogen concentrations and lower (50% and 52%) cyclic AMP concentrations in cerebral cortex and hippocampus, respectively, demonstrating the marked influence of noradrenergic activity on adenylate cyclase activity and glycogenolysis. The lesions failed to modulate the rise in free fatty acids in the cerebral cortex, or the cyclic GMP concentrations in the cerebral cortex and hippocampus. Thus, increased noradrenergic activity during status epilepticus does not seem responsible for lipolysis or for activation of guanylate cyclase.

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

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

Cerebral circulatory response to hypercapnia: effects of lesions of central dopaminergic and serotoninergic neuron systems

The present study explores the possibility that the central dopaminergic and serotoninergic neuron systems influence CBF under normocapnic and hypercapnic conditions. In the first part of the study the effect of unilateral 6-hydroxydopamine lesion of the nigrostriatal dopamine pathway on local cerebral blood flow (1-CBF) was measured autoradiographically with [14C]iodoantipyrine as the diffusible tracer. The lesion caused no major effect on CBF under normocapnic or hypercapnic conditions. However, the circulatory response to hypercapnia was slightly enhanced (about 10%) in the denervated caudate-putamen. It is suggested that under hypercapnic conditions the pronounced increase in blood flow in the caudate-putamen is normally modulated by a slight vasoconstriction caused by dopamine release from the nigrostriatal system. In the second part of the study the effect of intraventricular 5,7-dihydroxytryptamine on cerebral metabolic rate for oxygen (CMRO2) and CBF was evaluated using a 133xenon modification of the Kety-Schmidt inert gas technique. The lesion, which removed about 90% of cortical 5-hydroxytryptamine, had no effect on the circulatory response to hypercapnia, not did it alter CMRO2. Under normocapnic conditions, though, the lesion seemed to induced a minor increase in CMRO2, which indicates that the serotoninergic system exerts a depressant resting tone on metabolic rate in the brain.

Cerebral blood flow and oxygen consumption in normocapnia and hypercapnia: modulating influence of paravertebral sympathetic blockade at the low thoracic level

The objective of the present study was to explore whether the systemic consequences of sympathoadrenal activation influence the cerebral circulatory and metabolic effects of hypercapnia in the rat. To that end, a bilateral blockade of the sympathetic chain was performed at the low thoracic level by paravertebral injection of local anaesthetic. The injection was followed by a reduction in blood pressure and, in comparison to animals injected with local anaesthetic intramuscularly, those with paravertebral blockade showed lower blood and tissue concentrations of glucose and lactate. Overall ("cortical") CBF and CMRO2 were measured with a 133xenon modification of the Kety-Schmidt technique, and local CBF was estimated autoradiographically with 14C-iodoantipyrine as the diffusible tracer. Paravertebral blockade failed to modify the circulatory response to hypercapnia, nor did it prevent the increase in CMRO2d previously noted in this preparation. In animals maintained ventilated on 70% N2O, paravertebral blockade reduced overall CBF by 30% and local CBF by 30-40%, with a suggested but statistically nonsignificant reduction in CMRO2. In unparalysed, awake animals the blockade failed to affect local CBF. It is concluded, therefore, that blockade of the sympathetic chain causes a reduction of CBF only in the stressful conditions prevailing in paralysed and ventilated animals.

Effect of propranolol on local cerebral blood flow under normocapnic and hypercapnic conditions

The effect of propranolol (2.5 mg kg-1, i.v.) on local cerebral blood flow (CBF) in normocapnia was studied in rats maintained artificially ventilated on 70% N2O and 30% O2. The method used was autoradiography with [14C]iodoantipyrine. Although a single dose of propranolol, given 30 min prior to CBF measurements, somewhat reduced mean CBF values in all of the 22 structures analysed, none of the changes were significant. The results confirm previous ones, in which overall CBF was measured, in showing that beta-adrenergic mechanisms have little effect on normal cerebrovascular tone. Following a single dose of propranolol, results obtained in hypercapnia were equally negative; neither did CBF fall significantly when propranolol was given by constant infusion during 15 min. Furthermore, local CBF did not differ between animals infused with dl-propranolol and d-propranolol. It is concluded that in the rat, propranolol has but small effects on the CBF response to hypercapnia, if any. The results reveal that local CO2 responsiveness, calculated as delta CBF/delta PCO2, varies with normocapnic flow rates.