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

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.

Changes in visual-evoked potential habituation induced by hyperventilation in migraine

Hyperventilation is often associated with stress, an established trigger factor for migraine. Between attacks, migraine is associated with a deficit in habituation to visual-evoked potentials (VEP) that worsens just before the attack. Hyperventilation slows electroencephalographic (EEG) activity and decreases the functional response in the occipital cortex during visual stimulation. The neural mechanisms underlying deficient-evoked potential habituation in migraineurs remain unclear. To find out whether hyperventilation alters VEP habituation, we recorded VEPs before and after experimentally induced hyperventilation lasting 3 min in 18 healthy subjects and 18 migraine patients between attacks. We measured VEP P100 amplitudes in six sequential blocks of 100 sweeps and habituation as the change in amplitude over the six blocks. In healthy subjects, hyperventilation decreased VEP amplitude in block 1 and abolished the normal VEP habituation. In migraine patients, hyperventilation further decreased the already low block 1 amplitude and worsened the interictal habituation deficit. Hyperventilation worsens the habituation deficit in migraineurs possibly by increasing dysrhythmia in the brainstem-thalamo-cortical network.

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.

Effect of medetomidine on respiration and minimum alveolar concentration in halothane- and isoflurane-anesthetized dogs

OBJECTIVE

To evaluate the effect of medetomidine on minimum alveolar concentration (MAC), respiratory rate, tidal volume, minute volume (V(M)), and maximum inspiratory occlusion pressure (IOCP(max)) in halothane- and isoflurane-anesthetized dogs.

ANIMALS

6 healthy adult dogs (3 males and 3 females).

PROCEDURE

The MAC of both inhalants was determined before and 5, 30, and 60 minutes after administration of medetomidine (5 microg/kg, IV). Dogs were subsequently anesthetized by administration of halothane or isoflurane and administered saline (0.9% NaCl) solution IV or medetomidine (5 microg/kg, IV). Respiratory variables and IOCP(max) were measured at specific MAC values 15 minutes before and 5, 30, and 60 minutes after IV administration of medetomidine while dogs breathed 0% and 10% fractional inspired carbon dioxide (FICO2). Slopes of the lines for VM/FICO2 and IOCP(max)/FICO2 were then calculated.

RESULTS

Administration of medetomidine decreased MAC of both inhalants. Slope of V(M)/FICO2 increased in dogs anesthetized with halothane after administration of medetomidine, compared with corresponding values in dogs anesthetized with isoflurane. Administration of medetomidine with a simultaneous decrease in inhalant concentration significantly increased the slope for V(M)/FICO2, compared with values after administration of saline solution in dogs anesthetized with halothane but not isoflurane. Values for IOCP(max) did not differ significantly between groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Equipotent doses of halothane and isoflurane have differing effects on respiration that are most likely attributable to differences in drug effects on central respiratory centers. Relatively low doses of medetomidine decrease the MAC of halothane and isoflurane in dogs.

Intrathecal Neostigmine Prevents Intrathecal Clonidine from Attenuating Hypercapnic Cerebral Vasodilation in Rabbits

We previously demonstrated that lumbar intrathecal alpha(2) agonists attenuate hypercapnia-induced cerebral vasodilation. The combination of intrathecal clonidine and neostigmine is being investigated as pain therapy. The effects of their combination on cerebrovascular reactivity are unknown. We allocated rabbits anesthetized with pentobarbital to two groups: (a) clonidine (normal saline followed 30 min later by clonidine 2 microg/kg, both into the lumbar intrathecal space; n = 6), and (b) neostigmine-pretreatment (neostigmine 2 microg/kg followed 30 min later by clonidine 2 microg/kg, both into the lumbar intrathecal space; n = 6). We then evaluated the hypercapnia-induced changes in pial arteriolar diameter in these two groups using the closed cranial window preparation. The pial arteriolar dilator response to hypercapnia was significantly attenuated in the clonidine group (14% +/- 4%, 4% +/- 4%, 6% +/- 6%, and 5% +/- 7% for before and 30, 60, and 90 min, respectively). Neither normal saline nor neostigmine alone induced any change in the cerebral reactivity to hypercapnia. Pretreatment with neostigmine completely prevented the clonidine-induced attenuation of the hypercapnic cerebral vasodilation attenuated by intrathecal clonidine (16% +/- 7%, 15% +/- 6%, 12% +/- 6%, and 16% +/- 8%, respectively).

CO2 central chemosensitivity: why are there so many sensing molecules?

CO2 central chemoreceptors (CCRs) play a critical role in respiratory and cardiovascular controls. Although the primary sensory cells and their neuronal networks remain elusive, recent studies have begun to shed insight into the molecular mechanisms of several pH sensitive proteins. These putative CO2/pH-sensing molecules are expressed in the brainstem, detect P(CO2) at physiological levels, and couple the P(CO2) to membrane excitability. Functional analysis suggests that multiple CO2/pH-sensing molecules are needed to achieve high sensitivity and broad bandwidth of the CCRs. In contrast to the diversity of pH sensitive molecules, molecular mechanisms for CO2 sensing are rather general. The sensing molecules detect pH changes rather than molecular CO2. One or a few titratable amino acid residues in these proteins are usually involved. Protonation of these residues may lead to a change in protein conformation that is coupled to a change in channel activity. Depending on the location of the protonation sites, a membrane protein can detect extra- and/or intracellular pH.

[Clonidine combined with flunitrazepam before carotid endarterectomy decreases cerebrovascular CO2 reactivity]

OBJECTIVE

Assess cerebrovascular CO2 reactivity changes using transcranial Doppler sonography (TCD) after oral premedication associating clonidine (2 micrograms.kg-1) and flunitrazepam (70 micrograms.kg-1) in patients scheduled for carotid stenosis surgery.

STUDY DESIGN

Prospective study, not randomized, the patient being his own "control".

PATIENTS AND METHODS

Thirteen patients undergoing carotid endarterectomy under cervical plexus block were included. The monitoring included: automated arterial pressure cuff, ECG, radial artery catheter, TCD with probe secured in temporal window. The study of the cerebrovascular CO2 reactivity was performed with TCD recording on the side of operation, on the day before, and on the day of carotid endarterectomy, 90 min after the premedication, immediately before surgery. To change PaCO2, four ventilatory states were successively performed: (1) normoventilation, (2) hyperventilation, (3) hypoventilation, (4) "breath-holding test". At each state, it was noted: HR, MAP, PaCO2, mean blood flow velocity in the middle cerebral artery (Vm-MCA), resistance index of Pourcelot (RI), cerebrovascular reactivity (slope Vm-MCA/PaCO2). The results (+/- SEM) were analyzed by Wilcoxon test or t test.

RESULTS

After premedication, cerebrovascular CO2 reactivity decreased (0.043 +/- 0.019 vs 0.034 +/- 0.013; p < 0.05) without modification of RI (0.578 +/- 0.291 vs 0.612 +/- 0.025; NS). No complication during carotid clamping was reported.

CONCLUSION

Inclusion of clonidine in premedication before carotid stenosis surgery must be questioned because a decrease of cerebrovascular CO2 reactivity could be deleterious in case of intraoperative stroke.

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.

Carbonic anhydrase I inhibition by nitric oxide: implications for mediation of the hypercapnia-induced vasodilator response

1. At present, CO2 is considered to be the most important factor in regulating cerebral blood flow by modification of the interstitial fluid and extracellular pH, but the mechanism by which hypercapnia produces vasodilation is still controversial. In the present paper we investigated the effect of hypercapnia on carbonic anhydrase (CA) activity. We also studied the combined effects of CO2 with either indomethacin or an L-arginine analogue on CA activity. 2. Nine groups of 12 rabbits each were established. Groups 1-4 were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 20, 60, 120 and 180 min. Group 5 rabbits received 15 mg/kg bodyweight, i.v., indomethacin and, after 1 h, were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 2 h. Group 6 animals were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 2 h and then received indomethacin. Group 7 rabbits received 100 mg/kg bodyweight, i.v., NG-monomethyl-L-arginine (L-NMMA) and, after 1 h, were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 2 h. Group 8 rabbits were ventilated for 2 h with a mixture of 10% CO2, 21% O2 and 69% N2 and were then administered L-NMMA. Group 9 rabbits received L-NMMA treatment concomitant with ventilation for 2 h with a mixture of 10% CO2, 21% O2 and 69% N2. In all groups, the erythrocyte CA activity was measured, as well as PaCO2 before and after ventilation or treatment. 3. The present study shows that CO2 reduces CA I activity down to complete inhibition and antagonizes the activating effects of indomethacin and L-NMMA on this isozyme. Our data prove that nitric oxide- and prostaglandin-induced CA I inhibition is involved in the vasodilation produced by hypercapnia. These results suggest that, due to subsequent pH changes, CA I is directly implicated in the modulation of vascular processes in the organism.

Role of nitric oxide in regulating cerebrocortical oxygen consumption and blood flow during hypercapnia

The effect of the nitric oxide (NO) synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) on the response of cerebrocortical oxygen consumption (CMRO2) and blood flow (CBF) to two levels of hypercapnia (PaCO2 approximately 60 mm Hg and PaCO2 approximately 90 mm Hg) was investigated in ketamine-anesthetized rats. CBF was calculated using the Kety-Schmidt approach and CMRO2 was calculated from the product of CBF and the arteriovenous (superior sagittal sinus) difference for oxygen. L-NAME treatment did not have a significant effect on either CMRO2 or CBF under normocapnic conditions but inhibited the hypercapnic increase of CMRO2 and the hypercapnic increase in CBF. These results suggest that NO plays a role in the response of CMRO2 and CBF during hypercapnia and are consistent with the suggestion that at least part of the increase in CBF observed during hypercapnia is coupled to an increase in CMRO2.

The effect of carbon dioxide on cerebral arteries

The constancy of cerebral blood flow and volume relies heavily upon the cerebral arteries' intrinsic ability to respond to changes in the partial pressure of arterial CO2. The physiologic mechanisms underlying these responses have not been determined, although changes in extracellular and intracellular pH, mediation by prostanoids and neural activity have been suggested. CO2 reactivity can be influenced by oxygen status and blood pressure and can vary according to age and brain region. In certain pathological conditions or diseases, it can be severely altered. Modern techniques, which measure CBF in cases of cerebral hemodynamic insufficiency, head injury or tumor, rely on the inherent ability of the cerebral circulation to respond to changing levels of CO2.

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.

Effects of carbon dioxide inhalation on cerebral blood flow and oxygen tissue level in spontaneously hypertensive rabbits

BACKGROUND AND PURPOSE

Because previous studies have yielded conflicting results, this study was designed to investigate the efficiency of cerebrovascular reactivity to carbon dioxide in hypertension associated with moderate diffuse cerebral ischemic lesions.

METHODS

The effects of carbon dioxide inhalation on mean arterial blood pressure, heart and respiration rates, cerebral cortical blood flow, polarographically detected oxygen currents (oxygen availability), and cerebral electrical activity were compared in 14 spontaneously hypertensive and 16 normotensive rabbits anesthetized with urethane and alpha-chloralose. Blood flow was measured with the hydrogen clearance and thermal clearance methods.

RESULTS

In the resting state the frequency of electrical activity shifted to slower components, the levels of oxygen availability and cerebral blood flow were lower (p less than 0.01), and the ratio of the two latter parameters was greater (p less than 0.01) in hypertensive rabbits than in normotensive animals. Carbon dioxide inhalation induced more marked increases in cerebral blood flow, respiration rate, and oxygen availability in hypertensive (p less than 0.01) than in normotensive (p less than 0.05) rabbits. The ratio of oxygen availability to cerebral blood flow decreased (p less than 0.01) in the former and did not change significantly in the latter group. The carbon dioxide-induced rise in blood flow was also slower and more protracted in hypertensive rabbits (p less than 0.01). Histological investigation revealed groups of neurons with ischemic changes in the cortex of the hypertensive rabbits.

CONCLUSIONS

We suggest that in hypertensive rabbits the mild multiple ischemic lesions are the basis of functional disturbances, including reduced resting cerebral blood flow, greater oxygen tissue level, slower response to carbon dioxide, and greater vasodilatory capacity.

Significance of an opiate mechanism in the adjustment of cerebrocortical oxygen consumption and blood flow during hypercapnic stress

The role of adrenal medulla-derived enkephalins in the control of hypercapnic cerebrocortical blood flow (CBF) and oxygen consumption (CMRO2) was investigated in the ketamine anesthetized rat. Three experimental interventions were utilized: inhibition of opioid receptors with naloxone, decrease of adrenal enkephalin production with chronic adrenal medullectomy, and treatment of adrenal demedullated animals with the synthetic enkephalin analog, D-Ala2, N-Me-Phe4, Gly5-ol-enkephalin (DAGO). In intact, untreated animals hypercapnia increased CBF and CMRO2 by approximately 300 and 35%, respectively. Naloxone reduced the hypercapnic increase of CBF, and transformed the hypercapnic increase of CMRO2 into a decrease. The mid-points of the dose-response curves for (1)-naloxone and (d)-naloxone were 10 micrograms/kg and 100 micrograms/kg, respectively. Adrenal demedullation and treatment with (1)-naloxone (0.2 mg/kg) decreased the hypercapnic CBF and CMRO2 by approximately 50%. DAGO treatment of adrenal demedullated animals restored the hypercapnic CBF and CMRO2 to values similar to those found in intact animals. These observations suggest that opioid peptides (most likely adrenal medulla-derived enkephalins) play a significant role in the regulation of CMRO2 and CBF during moderate hypercapnia.

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.

Cerebral blood flow responses to inhaled carbon dioxide in schizophrenia

Carbon dioxide (CO2) is a potent cerebrovasodilator; even mild changes in CO2 are associated with marked changes in cerebral blood flow (CBF). We measured CBF before and after 5% CO2 inhalation in 19 medicated patients with schizophrenia and 16 normal volunteers. Another group of 16 volunteers had 2 CBF measurements under resting conditions. Although both patients and controls showed marked CBF increase during CO2 inhalation, the CBF response was significantly less in the patients. Change in CBF per mm of CO2 was lower in the patients. The second group of controls did not show significant differences between the 2 resting CBF measurements.

Systematic studies on the effects of the NMDA receptor antagonist MK-801 on cerebral blood flow and responsivity, EEG, and blood-brain barrier following complete reversible cerebral ischemia

The dose-dependent effects of MK-801, a glutamate receptor antagonist, on changes in CBF, CBF-PaCO2 responsiveness (133Xe clearance), EEG, and blood-brain barrier (methylene blue) were examined after a 15-min period of reversible complete global ischemia induced in halothane-anesthetized cats by occlusion of the vertebral and carotid arteries. Pretreatment with doses of MK-801 of greater than or equal to 0.5 mg/kg had no effect on resting CBF measures and produced a dose-dependent slowing of the dominant EEG frequency. In animals receiving this agent, there was an almost immediate return of baseline EEG patterns upon reinstitution of flow, no hypoperfusion after 2 h of reflow, preservation of CBF and CBF-PaCO2 responsiveness, and maintenance of blood-brain barrier integrity. In contrast, parallel control animals and animals treated with MK-801 at a dose of 0.1 mg/kg exhibited poor recovery based on the above parameters. MK-801 also diminished in a dose-dependent manner the CSF levels of 6-keto-prostaglandin (PG) F1 alpha (stable metabolite of PGI2) and thromboxane (Tx) B2 (stable metabolite of TxA2), which were otherwise elevated in vehicle-treated animals 2 h after reflow. Of particular interest, the CSF TxB2/6-keto-PGF1 alpha ratio in vehicle-treated animals was near 2. In animals pretreated with MK-801, at doses of greater than or equal to 0.5 mg/kg, this ratio was nearly 1. These observations are consistent with a possible triggering role of glutamate release in initiating at least part of the acute sequelae of ischemia. Such release in an electrically silent cell would increase Ca2+ influx and activate free fatty acid metabolism, leading to probable changes in vascular function and changes in blood-brain barrier permeability.

The effects of norepinephrine depletion on cerebral blood flow in the rat

Cerebral blood flow (CBF) was measured by [14C]butanol indicator fractionation in 10 rats given intraventricular injections of 6-hydroxydopamine (6-OHDA) compared to 8 saline-injected controls. Rats treated with 6-OHDA displayed an 83% reduction in cortical norepinephrine (NE) levels. CBF was significantly increased in 6-OHDA-treated rats compared to controls (average whole brain blood flow of 126.0 +/- 8.3 and 97.1 +/- 10.6 ml.min-1.10(-2)g-1 respectively, P less than 0.05). These studies suggest that noradrenergic innervation of the brain and cerebral microvasculature exerts a moderating effect on resting CBF.

The effects of carbon dioxide inhalation on plasma MHPG, plasma hormones respiratory rate, and behavior in the rhesus monkey

The effects of inhalation of air and 3 concentrations of carbon dioxide (CO2) on plasma levels of the norepinephrine metabolite, MHPG, plasma hormones, and behavioral activation were assessed in eight chair-adapted Rhesus monkeys (Macaca mulatta). In comparison to air, inhalation of 5%, 7.5% and 10% CO2 for 180 minutes produced significant dose-dependent increases in respiratory rate, plasma MHPG, cortisol, growth hormone and prolactin. CO2 at the 7.5% concentration produced peak changes in behavior at 15, growth hormone at 30, and cortisol and MHPG at 180 minutes without producing changes in prolactin. The lack of previously reported CO2 induced changes in MHPG, growth hormone and prolactin in humans exposed to 7.5% CO2 for only 15 minutes, may therefore relate to the relatively short duration of CO2 exposure.

Cerebral blood flow changes induced by CO2 in anxiety

Cerebral blood flow (CBF) was measured twice with the 133Xenon inhalation technique, under resting conditions and during 5% CO2 inhalation, in 13 patients with generalized anxiety disorder and a group of normal volunteers of comparable age and sex distribution. CO2 inhalation was associated with similar increases in end-tidal CO2 (PECO2) and CBF. Neither group showed statistically significant increases in state anxiety. However, when subjects (both patients and controls) who became anxious during CO2 inhalation were compared with those who did not, on associated CBF changes, significant differences emerged. The former showed less marked CBF increase as compared to the latter in the absence of any significant differences between the two groups on PECO2 during the second measurement. Changes in state anxiety and CBF showed a statistically significant inverse correlation for the entire group.

Effect of preischemia cyclooxygenase inhibition by zomepirac sodium on reflow, cerebral autoregulation, and EEG recovery in the cat after global ischemia

Zomepirac sodium (ZS) (5 mg/kg i.v.) was used to evaluate the effects of preischemia cyclooxygenase inhibition on CBF (as assessed by 133Xe clearance), CBF-PaCO2 responsiveness, and electrophysiologic (EEG) parameters before and after a 15-min period of complete global ischemia produced by four-vessel occlusion and mild hypotension. During the 15-min period of ischemia, CBF was essentially zero. Following reflow all groups displayed an initial hyperemia as compared with control (92 +/- 11 vs. 141-146 ml/100 g/min). Saline-treated animals during reflow displayed a delayed hypoperfusion (26 +/- 3 ml/100 g/min), which showed no improvement during the 2-h reflow period prior to death. In contrast, ZS-treated animals during reflow displayed significantly higher flows during the hypoperfusion phase (72 +/- 9 ml/100 g/min). The CBF-PaCO2 response displayed an approximately sevenfold reduction in slope at 2 h after reflow in saline-treated animals. This decrease in PaCO2 reactivity was not observed in the ZS-pretreated animals. With regard to EEG, all animals showed a total flattening during the 15 min of ischemia. In saline-treated animals only one of seven showed any sign of even marginal recovery. In ZS-treated animals EEG activity showed prominent recovery in seven of seven. Brainstem auditory evoked potentials were monitored and showed prominent recovery of amplitude and latency in ZS but not saline-treated animals during reflow.

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.