The cholinergic pathway to cerebral blood vessels. II. Physiological studies

Integrative regulation of human brain blood flow

Herein, we review mechanisms regulating cerebral blood flow (CBF), with specific focus on humans. We revisit important concepts from the older literature and describe the interaction of various mechanisms of cerebrovascular control. We amalgamate this broad scope of information into a brief review, rather than detailing any one mechanism or area of research. The relationship between regulatory mechanisms is emphasized, but the following three broad categories of control are explicated: (1) the effect of blood gases and neuronal metabolism on CBF; (2) buffering of CBF with changes in blood pressure, termed cerebral autoregulation; and (3) the role of the autonomic nervous system in CBF regulation. With respect to these control mechanisms, we provide evidence against several canonized paradigms of CBF control. Specifically, we corroborate the following four key theses: (1) that cerebral autoregulation does not maintain constant perfusion through a mean arterial pressure range of 60-150 mmHg; (2) that there is important stimulatory synergism and regulatory interdependence of arterial blood gases and blood pressure on CBF regulation; (3) that cerebral autoregulation and cerebrovascular sensitivity to changes in arterial blood gases are not modulated solely at the pial arterioles; and (4) that neurogenic control of the cerebral vasculature is an important player in autoregulatory function and, crucially, acts to buffer surges in perfusion pressure. Finally, we summarize the state of our knowledge with respect to these areas, outline important gaps in the literature and suggest avenues for future research.

Inhibition of acetylcholinesterase modulates the autoregulation of cerebral blood flow and attenuates ischemic brain metabolism in hypertensive rats

We designed the present study to examine whether or not the inhibition of acetylcholinesterase modulates cerebral microcirculation in hypotension and improves brain metabolism in ischemia induced by bilateral carotid artery occlusion in hypertensive rats. Blood flow to the parietal cortex was determined by the H2 clearance method. Lactate, pyruvate, and ATP were estimated by enzymatic methods. Acetylcholinesterase inhibitor (AChEI, ENA-713), at 0.05, 0.1, or 0.5 mg/kg, was intravenously injected 10 min before either hemorrhagic hypotension or cerebral ischemia. The levels of acetylcholine in the control were 29.3 +/- 8.1 (mean +/- SD) and 39.5 +/- 8.1 pmol/mg in the cortex and hippocampus, respectively, and they were significantly decreased by 15-19% after 60 min of ischemia in the vehicle-treated rats. AChEI preserved the levels to 93-98% of the control (p < 0.05 versus vehicle). The lower limit of autoregulation was 74 +/- 9% of the resting values. The administration of AChEI helped preserve blood flow and lowered the limit to 64 +/- 6% (p < 0.05 versus control). After 60 min of ischemia, lactate increased 6.5-fold and ATP decreased to 64% of the control value. The administration of AChEI dose-dependently reduced the lactate level 1.9- to 3.9-fold and well preserved the ATP level to 94-97% of the control. The inhibition of acetylcholinesterase activity may preserve cerebral autoregulation during hypotension and protect cerebral metabolism against ischemic insult.

Changes in intra- and extracranial tissue blood flow upon stimulation of a reticular area dorsal to the facial nucleus in cats

1. A small area in the dorsal part of the lateral tegmental field specifically responsible for the increase of blood flow in the common carotid artery (CCA) without accompanying change in the resting blood pressure was first identified in our laboratory. Since the area is located just dorsal to the facial nucleus, we named it the dorsal facial area (DFA; Kuo et al. 1987). 2. The purpose of this study was to clarify whether an increase of blood flow in intra- and/or extracranial tissues was responsible for the increase in CCA blood flow upon DFA stimulation, and to determine the role of cholinergic transmission in this response. 3. In 20 cats under chloralose and urethane anaesthesia, microsphere reference flow technique was used to measure the regional blood flow of intra- and extracranial tissues. 4. Electrical stimulation of the DFA appeared to increase the regional blood flow of both cerebral hemispheres (intracranial tissues) and to increase predominantly the regional blood flow of extracranial tissues on the side ipsilateral to stimulation. Increases in the regional blood flow of intracranial tissues were enhanced after i.v. administration of atropine but reduced with physostigmine. In contrast, increases in the regional blood flow of extracranial tissues were reduced after i.v. atropine but enhanced after physostigmine. 5. These findings suggest that DFA stimulation may promote the release of ACh in intra- and extracranial vessels. The muscarinic action may restrict the DFA-induced increase in blood flow of intracranial tissues, but enhance that of extracranial tissues.

Cholinergic and vasoactive intestinal polypeptidergic innervation of the cerebral arteries

Acetylcholine and vasoactive intestinal polypeptide are not only two vasoactive agonists that predominantly induce a vasodilatation of the cerebral arteries, but also correspond to neurotransmitters that innervate the various anatomical segments of the cerebral vasculature. The distinct patterns of the cerebrovascular cholinergic and vasoactive intestinal polypeptidergic innervation, their neurochemistry, in vitro and in vivo pharmacology, as well as the putative pathophysiological implications of these neurotransmission systems are critically summarized on the basis of the most recently published literature.

Effects of CDP-choline on acetylcholine-induced relaxation of the perfused carotid vascular beds of the rat

1. The effects of an infusion of cytidine-5'-diphosphocholine (CDP-choline) on the relaxation induced by exogenous acetylcholine (ACh) was studied in the isolated and perfused external (ECB) and internal (ICB) carotid vascular beds of the rat. Changes in perfusion pressure were recorded during a dose-response curve to ACh and after a 30 min perfusion with CDP-choline (1 mg/min). 2. ACh induced a dose-dependent relaxation in both vascular beds, indicating the presence of muscarinic receptors. The affinity of the receptors for ACh in the ICB was significantly lower than in the ECB (ED50: 120 +/- 21.4 ng and 69 +/- 10.3 ng, respectively). 3. In the ICB the infusion of CDP-choline for 30 min significantly shifted the dose-response curve to ACh to the left, potentiating the relaxation. This effect was not seen in the ECB. 4. The infusion of hemicholinium (4 microM) for 30 min together with CDP-choline completely prevented the potentiation of exogenous ACh-induced relaxation in the ICB. 5. The results of the present work suggest that CDP-choline is acting by increasing choline levels in the cholinergic nerve terminals of the ICB, increasing the synthesis and/or release of ACh.

Cerebral blood flow is not coupled to neuronal activity during stimulation of the facial nerve vasodilator system

There is a considerable body of evidence to suggest that activation of vasodilator fibers in the parasympathetic facial (VIIn) nerve can increase cerebral blood flow. The changes seen with VIIn stimulation raise the question as to whether they occur independent or in parallel with changes in cerebral metabolism. In these studies cerebral cortical perfusion was monitored continuously using laser Doppler flowmetry (CBFLDF) in the alpha-chloralose anesthetised cat. Cell firing in the region underlying the laser Doppler probe was monitored using tungsten-in-glass microelectrodes whose signals were amplified and filtered, and then monitored on-line by a microcomputer. Thus measures of both blood flow and local functional activity could be obtained that were continuous and contemporaneous. The VIIn was electrically stimulated through a craniotomy after isolation from the brainstem. CBFLDF and cell firing were monitored during several physiological manoeuvres. Hypercapnia produced the expected increase in CBFLDF that was brisk and stimulus locked. Cell firing did not alter except for a brief increase that was seen at the initiation of the hypercapnia and not maintained. The CBFLDF signal autoregulated to a level of 50-60 mmHg with no change in cellular activity. To determine if classical dynamic flow/metabolism coupling was present bicuculline, a GABAA receptor antagonist was superfused over the cortex. This led to increases in both CBFLDF and cell firing that were tightly and clearly linked. Stimulation of the VIIn led to a marked increase in the CBFLDF signal (47 +/- 7%) that was not accompanied by changes in cell firing.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of Cl area neurons globally increases regional cerebral blood flow but not metabolism

We examined the effects of electrical and chemical stimulation of the Cl area of the rostral ventrolateral medulla (RVL) on regional cerebral blood flow (rCBF) and regional cerebral glucose utilization (rCGU) in anesthetized (chloralose), paralyzed (curare) and ventilated rats. rCBF and rCGU were measured using 14C-iodoantipyrine (IAP) and 14C-deoxyglucose (2-DG), respectively, as indicators, with bilateral regional dissection of 11 brain regions. Electrical stimulation of the RVL elicited increases in arterial pressure (AP), heart rate (HR) and plasma concentration of epinephrine (EPI) and norepinephrine (NE). In addition, stimulation of the RVL, but not the adjacent medial longitudinal fasciculus, with AP maintained, increased rCBF (p less than 0.05, n = 6), but not rCGU, bilaterally and symmetrically (134-169% of control) throughout the brain. Bilateral adrenalectomy abolished the increase in plasma EPI elicited by stimulation of the RVL but did not affect resting rCBF (n = 5) or the elevation in rCBF elicited by RVL stimulation (n = 5). Increases in rCBF elicited by RVL stimulation were also unaffected by acute transection of the superior cervical ganglion (p greater than 0.05). Kainic acid (KA) microinjected into the RVL unilaterally (n = 6) at a dose producing sustained elevation in AP (5 nmol in 100 nl), elicited changes in rCBF similar to those elicited by electrical stimulation. We conclude that neurons within the RVL, possibly those of the adrenergic Cl group, can initiate a global cerebrovasodilation, but not an increase in rCGU, largely through neural pathways intrinsic to the brain. The responses may represent activation of networks in RVL mediating circulatory adjustments to hypoxia.

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.

The role of neuroeffector mechanisms in cerebral hyperperfusion syndromes

Cerebral hyperperfusion, a state in which blood flow exceeds the metabolic needs of brain, may complicate a number of neurological and neurosurgical conditions. It may account for the propensity with which hemorrhage, cerebral edema, or seizures follow embolic stroke, carotid endarterectomy, or the excision of large arteriovenous malformations, and for some of the morbidity that accompanies acute severe head injury, prolonged seizures, and acute severe hypertension. Hyperperfusion syndromes have in common acute increases in blood pressure, vasodilatation, breakdown of the blood-brain barrier, and the development of cerebral edema. These common features suggest the possibility that they share the same pathogenic mechanisms. It was believed until recently that reactive hyperemia was caused primarily by the generation of vasoactive metabolites, which induced vasodilatation through relaxation of vascular smooth muscle. However, the authors have recently established that the release of vasoactive neuropeptides from perivascular sensory nerves via axon reflex-like mechanisms has a significant bearing upon a number of hyperperfusion syndromes. In this article, the authors summarize their data and discuss possible therapeutic implications for blockade of these nerves or their constituent neuropeptides.

A mathematical model of overall cerebral blood flow regulation in the rat

In the present work a mathematical model of the cerebrovascular regulatory system in the rat is presented. The model, a generalization of our previous one, includes the reactivity of proximal segments of the cerebrovascular bed and the neurogenic and myogenic feedback regulatory mechanisms besides the action of chemical regulatory factors. The model is then used to analyze the interaction of mechanisms regulating cerebral blood flow in several conditions of physiological importance. In the first stage of the work we simulated experiments in which the neural fibers are cut and artificially stimulated with external means. According to experimental evidence, simulation results point out the existence of an escape of blood flow from stimulation. The model imputes this escape phenomenon to the antagonistic action of chemical factors working on the distal segments of the cerebrovascular bed. In a second stage, we studied the neurogenic mechanism action in a physiological closed-loop condition. With this general model, autoregulation to arterial pressure changes and postischemic reactive hyperemia have been analyzed. A comparison of simulation results with recent experimental data shows that the model is able to produce 60-70% of the experimental regulatory capacity of the cerebrovascular bed. However, some relevant discrepancies still exist between the model and the experimental results, especially as regards the dilatory capacity of small cerebral arterioles. These discrepancies underline the existence of further regulatory mechanisms working on the cerebrovascular bed, the nature of which must still be clarified.

Hyperperfusion and enhanced metabolic state of the brain during sudden lowering of the cutaneous temperature in rats

Forty-eight anesthetized and artificially ventilated rats were pretreated with the ganglion blocking agent, pentolinium (5 mg/kg, i.v.), followed by continuous phenylephrine infusion for maintenance of normotension. The rats were subjected to mock treatment (16 rats), to sudden exposure to thermoneutral water (34-36 degrees C, 16 rats), or to cold water (16 degrees C, 16 rats). The latter treatment lowered the cutaneous temperature to 25-30 degrees C. Neither one of these treatments produced any significant changes in arterial pressure or heart rate. Exposure to cold elicited hyperperfusion (microsphere method) in each of the brain regions studied. The increase in cerebral cortical blood flow (by 59 +/- 8%, mean +/- SEM) was most significant. The cerebral cortical metabolic rate for oxygen was increased by 46 +/- 10%. During the cold exposure, excellent tight coupling between the metabolic rate and blood flow was observed in the cerebral cortex. Mock treatment and exposure to thermoneutral water resulted in only trivial increases in blood flow and in metabolic rate. Thus, during cold exposure, an enhanced metabolic state of the brain represented the primary determinant increasing the brain blood flow, provided that normotension was maintained.

Parasympathetic denervation of rat pial vessels significantly increases infarction volume following middle cerebral artery occlusion

Studies were undertaken in Long Evans rats to examine the hypothesis that chronic unilateral sectioning of vasodilating nerve fibers (parasympathetic and/or sensory) innervating the circle of Willis increases infarction volume following unilateral branch occlusion of the middle cerebral artery (MCA) combined with temporary (45 min) bilateral common carotid occlusion. Infarct size was measured 24 h after surgical occlusion from seven coronal slices. Infarction volume (mean +/- SD) in sham animals (group A) and surgically naive animals (group B) measured 153 +/- 43 and 131 +/- 38 mm3, respectively. After lesions of both sensory (nasociliary nerve) and parasympathetic efferents at the ethmoidal foramen (group C, combined lesion) or selective lesions of parasympathetic efferents (group D), infarction volume increased [214 +/- 47 mm3 (p less than 0.01) and 209 +/- 46 mm3 (p less than 0.05), respectively]. No increases were detected after cutting the nasociliary nerve alone (group E) or occluding the external ethmoidal artery (group F) [145 +/- 39 mm3 (p greater than 0.05) and 124 +/- 63 mm3 (p greater than 0.05), respectively]. The infarct was predominantly located within cortical gray matter and became enlarged on its superior and inferior aspects after parasympathectomy. Large infarcts were noted whether animals breathed spontaneously (all of the above) or were artificially respired or whether animals were anesthetized with xylazine and ketamine or chloral hydrate. Taken together, these studies suggest a previously unrecognized protective role for autonomic parasympathetic fibers in the pathophysiology of focal cerebral ischemia that is not shared by sensory fibers. The importance of autonomic vasodilating fibers to blood flow in ischemic brain merits further study.

Evidence for in vivo cerebrovascular neurogenic vasodilatation in the rat

To determine the function of cerebrovascular parasympathetic nerves, the calibre of rat pial arteries was continuously measured when the nerves (the postganglionic fibres originating from the sphenopalatine ganglion) were electrically stimulated in vivo. The pial arteries (72.3 +/- 2.8 microns) dilated immediately after electrical stimulation (5 V, 10 Hz, 0.5 ms, 1 min duration). Their diameter increased 4.7 +/- 0.1% (p less than 0.01), 6.3 +/- 1.7%, 5.1 +/- 0.3% (p less than 0.05), 6.3 +/- 1.4%, at 15, 30, 45 and 60 s after initiation of stimulation, respectively. No significant change was observed in systemic arterial blood pressure or the expiratory carbon dioxide content during stimulation. This is the first direct demonstration of in vivo cerebrovascular neurogenic vasodilatation in the rat.

Role of nucleus basalis in cholinergic control of cortical blood flow

The present investigation was designed to determine the effect of lesions localized to the nucleus basalis/substantia innominata (NB) on resting and cholinergically activated regional cerebral cortical blood flow (rCBF). Ibotenic acid (10 micrograms) was infused locally at 1 mm caudal to bregma, 3 mm lateral to the midline, and 8 mm below the cortical surface. Effectiveness of lesions was demonstrated by histological verification of lesion sites and determination of choline acetyltransferase activity in cerebral cortex homogenates. rCBF was measured with the autoradiographic iodo-14C-antipyrine technique. Resting rCBF was similar in the hemisphere that received the NB lesion and in the contralateral (intact) side in all regions examined. Physostigmine intravenous infusion (3.3 micrograms.kg-1.min-1) enhanced rCBF in frontal, parietal, occipital, and temporal cortex. The increase was symmetrical, however, indicating inability of NB lesion to affect this phenomenon. It is concluded that the cortical cholinergic afferents originating in the NB are not involved in the control of rCBF.

High-frequency stimulation of the facial nerve results in local cortical release of vasoactive intestinal polypeptide in the anesthetised cat

Local cortical release of vasoactive intestinal polypeptide (VIP) was measured using a sensitive radioimmunoassay following direct electrical stimulation of the facial nerve in the anaesthetised cat. During activation of the facial nerve dilator pathway VIP was released at the cortex and collected into a physiological superfusate, its concentration increasing from 4.2 +/- 1.2 to 15.5 +/- 2.4 pmol/l. Administration of the nicotinic ganglion blocking agent hexamethonium (10 mg/kg i.v.) eliminated this response demonstrating that the release is mediated via an autonomic ganglion. Given previous experiments that have demonstrated that stimulation of the facial nerve leads to a neurogenically mediated dilatation of the cerebral vasculature, these data further implicate VIP as the transmitter in this pathway.

Selective electrical stimulation of postganglionic cerebrovascular parasympathetic nerve fibers originating from the sphenopalatine ganglion enhances cortical blood flow in the rat

Recently, the origins and pathways of cerebrovascular acetylcholine- and vasoactive intestinal polypeptide-containing nerves have been elucidated in detail in the rat: The sphenopalatine ganglion is the major source for postganglionic parasympathetic fibers to the vascular beds of the cerebral hemispheres. To clarify the functional role of the nerves on cerebral blood vessels in vivo, brain cortical microvascular blood flow was measured in rats during electrical stimulation of these particular postganglionic fibers. Animals were subjected to transection of the right nasociliary nerve 2 weeks before the flow measurements to eliminate activation of peptidergic sensory fibers. Relative change in microvascular blood flow was continuously recorded by a laser-Doppler flowmeter system under alpha-chloralose anesthesia. The postganglionic fibers were electrically stimulated just proximal to the ethmoidal foramen by a bipolar platinum electrode (5 V; 0.5 ms; 3, 10, 30, 60 Hz; as a continuous stimulation for 90 s). Stimulation at 10 Hz induced a marked increase of the cortical blood flow (CoBF) on the ipsilateral side, whereas no change was observed on the contralateral side. It reached a maximum mean value of 42.5% at 46 s, and then slightly declined during the remaining stimulation period. No significant changes were observed in the mean arterial blood pressure or blood gases during or after stimulation. Both atropine and scopolamine failed to alter this flow increase. Electrical stimulation of the postganglionic fibers at different frequencies revealed a maximal increase in the CoBF at 30 Hz in the control situation (47.2%), but at 10 Hz after scopolamine administration (51.6%).(ABSTRACT TRUNCATED AT 250 WORDS)

Origins and pathways of choline acetyltransferase-positive parasympathetic nerve fibers to cerebral vessels in rat

The presence of cholinergic nerve fibers in the brain vasculature has been a matter of controversy, partly due to the lack of a reliable histochemical marker. Accordingly, no distinct information about the origin and pathways for such fibers has been available. In the present study on the rat pial vasculature, utilizing a choline acetyltransferase (ChAT) antibody, which is able to demonstrate this enzyme in peripheral nervous tissue, evidence was obtained for an innervation by cholinergic fibers of large pial arteries. Vasoactive intestinal polypeptide (VIP) was present in or in close association with these fibers. By the aid of the retrograde axonal tracer True Blue (TB) applied to the middle cerebral arterial wall, such fibers were shown to originate in a subgroup of ChAT-positive cells in the sphenopalatine, otic, and internal carotid ganglia, which, in addition, contained VIP. The ChAT-positive pial nerve fibers were few in relation to the VIP-immunoreactive fibers, as was also illustrated by the few TB-positive cells in the ganglia that were ChAT positive as compared with the number of cells that were VIP positive. Only a small population of ChAT-containing neurons in these ganglia appeared to project to the pial vessels. The pathway from the sphenopalatine ganglion is via a membranous structure on the medial orbital wall, through the ethmoidal foramen, and along the internal ethmoidal artery to reach the circle of Willis. The fibers from the internal carotid and otic ganglia probably bridge to the internal carotid artery in the carotid canal, those from the otic ganglion after an initial course in the lesser superficial petrosal nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of stimulation of the sphenopalatine ganglion on cortical blood flow in the rat

The effects of electrical stimulation of the sphenopalatine ganglion on cortical blood flow and gas partial pressures (PO2 and PCO2) were studied in the anesthetized rat. Tissue PO2, PCO2, and local CBF were measured simultaneously in both parietal cortices by means of mass spectrometry. Stimulation of the sphenopalatine ganglion increased CBF and tissue PO2 by approximately 50 and 20%, respectively, in the ipsilateral parietal cortex. Smaller but significant increases in CBF and tissue PO2 were simultaneously seen in the contralateral parietal cortex. These variations were also accompanied by small decreases in PCO2 in both parietal cortices and a 5% increase in mean arterial pressure, whereas cortical electrical activity did not change. We conclude that the cholinergic (and vasoactive intestinal polypeptidergic) innervation of the cerebral blood vessels, arising from the sphenopalatine ganglion has significant vasomotor potential and that this system may be of functional importance.

Neuronal and endothelial sites of acetylcholine synthesis and release associated with microvessels in rat cerebral cortex: ultrastructural and neurochemical studies

We sought to establish what proportion of the cholinergic innervation of the cerebral cortex (CX) is associated with intraparenchymal blood vessels by using immunocytochemical and neurochemical techniques, and whether [3H]acetylcholine ([3H]ACh) is synthesized and released by elements associated with cortical microvessels (MV). MVs and, for comparison, tissue homogenates were prepared using sucrose gradient/differential ultracentrifugation methods. Efficacy of the separation technique was indicated by the activity of gamma-glutamyltranspeptidase (up to 29.2-fold enrichment), an endothelial cell marker enzyme, in the MV fraction and microscopy. The size of isolated microvessels ranged from 5 to 40 micron (o.d.) with 67.7% of the vessels less than 10 micron and 32.2% between 11 and 40 micron (690 vessels measured from 4 animals). By electron microscopy immunoreactive choline acetyltransferase (ChAT), the biosynthetic enzyme for ACh, was localized to: (a) axons and axon terminals opposed to the basal laminae of capillaries and small arterioles, and (b) capillary endothelial cells. ChAT-labeled elements associated with MVs were most prominent in layers I, III and V of the CX consistent with the local pattern of cholinergic innervation. The absolute amount of ACh synthesized (pmol Ach/100 mg wet wt.) by elements associated with cortical MVs was relatively small (2.3% total cortical homogenate activity). Inhibition of MV ChAT activity to 5% of control by the specific ChAT inhibitor, 4-naphthylvinylpyridine, and HPLC analysis of the product, indicated that authentic ACh was measured. Other tissues similarly synthesized small amounts of ACh relative to the CX, caudate nucleus (CN, 2.4%), cerebellum (CRB, 1.4%) and liver (LIV, 3.9%). Consistent with the known extent of the cholinergic innervation of the tissues examined, the rank order of ChAT associated for both MVs and homogenate were: CN greater than CX much greater than CRB greater than LIV. However, based on the specific activities of ChAT, cortical MVs have the remarkable capacity to synthesize ACh at rates 95% greater than cortical (S1 fraction) homogenate (59.0 +/- 3.5 nmol/mg protein/40 min; n = 7), which is enriched in nerve terminals. Except for LV (+11%), other tissues also had remarkably high ChAT activity in MV (% above corresponding homogenate; P less than 0.05, n = 5): CN (+269) and CRB (+313). Release of [3H]ACh from MVs and, for comparison, nerve terminals were graded to K+ depolarization stimulus (5-55 mM), maximal with 55 mM K+ and Ca2+ dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo 31P nuclear magnetic resonance (NMR) study of cerebral metabolism during histotoxic hypoxia in mice

The alterations of cerebral energetic metabolism and intracellular brain pH that occur during histotoxic hypoxia were estimated in mice by in vivo 31P nuclear magnetic resonance (NMR) spectrometry. The brain spectra obtained by means of chronically implanted surface coils connected to a special designed probe were recorded sequentially and continuously before, during, and after histotoxic hypoxia induced by an injection of potassium cyanide. The levels of PCr, ATP, inorganic phosphate, and intracellular pH estimated from the records of the 31P cerebral spectra and the cerebral cortical activity allow noninvasive monitoring of both the energetic metabolism and the functional state of the brain in unanesthetized animals. The time courses of these different parameters are largely the same as those obtained previously by invasive methods, however, the simultaneous and continuous monitoring performed in this study exhibits several unexpected dissociations between, respectively, onset of coma, decrease in PCr level and intracellular pHi, and recovery of normal levels of PCr and intracellular pH. These dissociations indicate that tissue acidosis plays a minor role in the changes in PCr levels, compared with ATP, and they confirm that the thresholds of oxidative metabolism required for functional tissue activity and a normal rate of ATP are clearly different.

Continuous determination of the cerebrovascular changes induced by bicuculline and kainic acid in unanaesthetized spontaneously breathing rats

Cerebral blood flow was sequentially determined (every 2-3 min) with helium clearance in two "vulnerable" structures: the hippocampus and the frontoparietal cortex during bicuculline (n = 11) and kainic acid (n = 9)-induced seizures in unanaesthetized, spontaneously breathing rats. Tissue partial pressures of oxygen and carbon dioxide were continuously and simultaneously evaluated in the same brain areas. All these variables were measured by mass spectrometry with a single gas sampling cannula previously implanted in each structure. The systemic variables, arterial blood pressure, arterial partial pressures of oxygen and carbon dioxide, pH, and bicarbonate concentration were also determined. Arterial and venous catheters were chronically implanted several days prior to the definitive experiments. Bicuculline induced short (about 15 min), recurrent, generalized seizures, with an abrupt rise in arterial blood pressure, an arterial metabolic acidosis and comparable blood flow increases (4-fold) in the hippocampus and the neocortex. A marked increase in tissue partial pressure of oxygen was always preceded by an increase in tissue partial pressure of carbon dioxide. After the seizures, in the 5 rats that survived, cerebral blood flow was significantly lowered; tissue partial pressure of oxygen and partial pressure of carbon dioxide also decreased, but to a lesser extent. Histological examination revealed two types of lesions: predominantly selective chromatolysis but also ischaemic cell change. Kainic acid first induced a decrease in arterial pressure and then hypertension during status epilepticus, with a return of arterial pressure towards basal levels during the recovery period (4 h after the injection). Respiratory alkalosis occurred throughout the experiment. Cerebral blood flow increased progressively to become maximal during status epilepticus. This vasodilatation was greater in the hippocampus (x 8) than in the neocortex (x 4). During recovery, cerebral blood flow tended to decrease but remained significantly elevated. In both structures, tissue partial pressure of oxygen was first lowered while tissue partial pressure of carbon dioxide was elevated; with the occurrence of the wet dog shakes, tissue partial pressure of O2 increased and tissue partial pressure of CO2 decreased. The changes in tissue gases were maximal during status epilepticus and tended to return to their basal levels thereafter, but no decrease in tissue partial pressure of O2 was observed, even 4 h after kainic acid administration. Histological analysis demonstrated ischaemic cell changes, particularly in the limbic system.(ABSTRACT TRUNCATED AT 400 WORDS)

Vasoactive-intestinal-polypeptide (VIP)-like immunoreactive cells in the skull base of rats. A combined study using acetylcholinesterase histochemistry

We investigated the distribution of vasoactive intestinal polypeptide (VIP)-like immunoreactive cell bodies in relation to the major cerebral and internal carotid arteries at the skull base in rats. Acetylcholinesterase (AChE) histochemistry was also applied to investigate the localization of this enzyme. VIP staining revealed a few positive cell bodies in nerves close to the internal carotid artery at the base of the skull as well as in the cerebral arterial wall. Ganglion-like cell bodies were detectable within the greater superficial petrosal (GSP) nerve. AChE activity was observed in VIP-like immunoreactive cell bodies along the whole of the GSP nerve. These cell bodies in the GSP nerve may give rise to at least some of the perivascular VIP- and AChE-containing nerves of the internal carotid arteries at the base of the skull.

Effects of TRH on cerebral and peripheral blood flows; role of submesencephalic brain stem centres

The localization of the origin of the cardiovascular effects elicited by thyrotropin-releasing hormone (TRH) was attempted in this study. The radioactively labelled microsphere method was employed for measurement of regional cerebral (rCBF) and peripheral blood flow in albino rabbits anaesthetized with urethane. The effect of 50 micrograms and 2 mg kg-1 TRH (administered i.v.) on rCBF and peripheral blood flow was evaluated in animals with the brain stem sectioned (BSS) at the level of pons-mesencephalon. The cerebral vasodilating effect of TRH was abolished or attenuated, while the peripheral vasoconstriction and increase in mean arterial blood pressure (MAP) was unaffected. Cordotomy at the CI level caused a marked fall in MAP and abolished the pressor response to TRH. In animals infused with angiotensin II, in order to normalize the decreased MAP after cordotomy, TRH caused a marked increase in rCBF. Administration of 50 ng and 5 micrograms TRH into the fourth ventricle caused a marked peripheral vasoconstriction and pressor response. The same amounts of TRH administered into the mesencephalic aqueduct caused a marked increase in rCBF and peripheral vasoconstriction. The results indicate that TRH elicits the pressor and peripheral vasoconstrictor responses from a submesencephalic brain stem region. The increase in rCBF caused by TRH is probably mediated by a somewhat higher submesencephalic level.

Intrinsic and extrinsic mechanisms involved in the cerebrovascular reaction elicited by immobilization stress in rabbits

Variations in cerebral blood flow and partial pressures of oxygen and carbon dioxide (pO2, pCO2) were studied in rabbits during short-duration (1 min) immobilization stress. The techniques used enabled us to determine these variables locally in the caudate nucleus in a continuous, simultaneous and quantitative fashion. It could be shown that cerebral blood flow and arterial blood pressure increased in parallel immediately after inducing the stress reaction, and that pO2 increased further, indicating that cerebral oxygen supply is maintained by the hyperaemia. Previous administration of a beta-receptor blocker or of a cholinergic receptor blocker significantly diminished the cerebrovascular reaction to stress, inducing a decrease in pO2 during the reaction. Administration of both blockers nearly abolished the cerebral vasodilation studied. Previous administration of an alpha-receptor blocker enhanced the reactive hyperaemia. No disturbance of the blood-brain barrier could be observed in rabbits subjected to stress. Intravenous injection of adrenaline, as well as angiotensin II inducing similar increases in blood pressure, had no comparable effect on the blood flow. The conclusion is that in this model of anxiety, neurogenic mechanisms are involved in the provision of a sufficient oxygen supply to the brain.

Response of local blood flow in the caudate nucleus of the cat to intraventricular administration of carbachol

The effect of perfusion of the cerebral ventricles with artificial cerebrospinal fluid containing carbachol on the blood flow in the caudate nucleus of the cat and the possibility to inhibit this effect by anticholinergic drugs was studied by means of the hydrogen clearance technique. After a control period during which both lateral ventricles were perfused with artificial CSF of identical composition, the drug under study was added on one side (experimental side) while the other side continued to be perfused with the control artificial CSF (control side). The blood flow on the experimental side and on the control side were compared. A dose dependent response to carbachol was observed. Lower concentrations of carbachol (10(-6) up to 10(-4)M) caused vasodilatation whereas high concentrations (10(-3)M) caused local vasoconstriction. The increase in the local blood flow caused by the low carbachol concentrations was reduced by both atropine (10(-5)M) and hexamethonium (10(-3)M). The fall in CBF observed with the high carbachol concentration was prevented by atropine (10(-5)M). It may be concluded that low, physiologically more meaningful, carbachol concentrations cause a local vasodilatation due to interaction with both muscarinic and nicotinic receptors.

Global cerebral vasodilatation elicited by focal electrical stimulation within the dorsal medullary reticular formation in anesthetized rat

We examined the effects of electrical stimulation of a restricted area of the dorsal medullary reticular formation (DMRF) on regional cerebral blood flow (CBF) in anesthetized (by chloralose), paralyzed (by curare) rats. CBF was measured in tissue samples by the Kety principle, with 14C-iodoantipyrine as indicator. Stimulation of DMRF elicited a widespread, significant increase in CBF in 12 of 13 areas. The increase in flow was greatest in cerebral cortex, up to 240% of control. However, it was also substantially increased in selected regions of telencephalon, diencephalon, mesencephalon, and lower brainstem, but not cerebellum. In contrast, electrical stimulation of the midline (interstitial nucleus of the medial longitudinal fasciculus) 1 mm medial to the DMRF did not change CBF. The increase in CBF evoked by DMRF stimulation persisted after transection of the spinal cord at C1 or cervical sympathetic trunk. We conclude that excitation of neurons originating in or passing through the DMRF can elicit a potent and virtually global increase of CBF. The effect appears to be mediated by intrinsic pathways of the central nervous system.

Cholinergic cerebral vasodilatation: lack of involvement of cranial parasympathetic nerves

Cerebral blood flow (CBF) was estimated by measurement of internal carotid blood flow (ICBF) and sagittal sinus blood flow (SSBF) in mechanically ventilated rabbits under 70% N2O/30% O2. Electrical stimulation of cranial nerves III, VII, IX, or X, with stimulus parameters adequate to excite other visceromotor outflows of these nerves, failed to elicit change in CBF. Combined bilateral section of nerves VII, VIII, IX, X, and XI had no effect on the reactivity of CBF to CO2, nor did the sectioning of these nerves affect the increases in CBF induced by physostigmine. Division of the sinus and aortic nerves and of the vagi in the neck failed to change CO2 reactivity, even though normocapnic CBF was reduced. Pentobarbital blocked the increase in CBF produced by physostigmine, but had no effect on that produced by pilocarpine. The results indicate that cranial parasympathetic nerves do not contain cerebral vasodilator fibers, and that they are not the source of acetylcholine which is presumably involved in CBF regulation.

Cholinergic mechanism in the large cat cerebral artery

The isolated cat cerebral arteries (basilar, middle cerebral, anterior cerebral, and internal carotid) were studied in vitro. ACh at low concentration (3 x 10(-8) to 3 x 10(-6) M) induced relaxation, and at high concentration (10(-5) to 3 x 10(-3) M) induced constriction of the arteries with endothelial cells. In contrast, concentration of any magnitude (10(-6) to 3 x 10(-3) M) induced constriction exclusively in arteries without endothelium. Atropine (3 x 10(-6) to 3 x 10(-5) M) blocked and physostigmine (3 x 10(-6) M) potentiated both ACh-induced relaxation and constriction. These results suggest that the relaxation induced by exogenous ACh is solely dependent on the endothelial cells and that the primary effect of the direct action of ACh on the smooth muscle cells is constriction. Transmural nerve stimulation (TNS) induced a frequency-dependent relaxation in the arteries with or without endothelium. Neither atropine nor physostigmine affected the TNS-induced dilator response in either preparation. This, together with the wide separation between the nerve and endothelium in the vessel wall, suggests that ACh is not involved in TNS-induced vasodilation. Furthermore, the TNS-induced relaxation at any frequency is not smaller but greater in the arteries without endothelial cells than in those with endothelial cells. Blockade of the TNS-induced vasodilation by tetrodotoxin (TTX) or cold storage denervation did not prevent the arteries from relaxing in response to ACh or methacholine (MCh). It is suggested that the TNS-induced vasodilation is independent of the endothelial cells and that the vasodilation is due to the direct action of a yet-to-be identified dilator transmitter on the smooth muscle cells. Results of the present study support our previous finding that, in the cat cerebral artery. ACh is more likely to be a constrictor transmitter than a dilator transmitter.

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

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

Effects of cholinergic nerves on cerebral blood flow in cats

We studied the effects of parasympathetic nerves on cerebral blood flow (CBF). The greater superficial petrosal nerve, which apparently supplies cholinergic fibers to cerebral vessels and the lacrimal gland, was sectioned on one side at the internal auditory meatus in anesthetized cats. CBF was measured with 15-microns microspheres. Section of the petrosal nerve did not alter resting CBF. In addition, electrical stimulation of the distal cut end of the petrosal nerve had no effect on total CBF. In one area of the brain, the caudate nucleus, stimulation increased blood flow from 29 +/- 2 to 36 +/- 2 (mean +/- SE) ml/min per 100 g. Lacrimal gland blood flow increased from 42 +/- 7 to 198 +/- 32 ml/min per 100 g during petrosal stimulation, which indicates that the stimulus was potent. In the same experiments, CBF increased 3- to 4-fold during hypercapnia; thus, cerebral vessels were responsive to another dilator stimulus. In other experiments, petrosal nerve section did not alter the response of cerebral vessels to hypercapnia (PCO2 > 50 mm Hg) or hypoxia (PO2 < 34 mm Hg). We conclude: (1) there is little or no resting vasodilator tone provided to cerebral vessels by the petrosal nerve; (2) petrosal nerve stimulation has a major effect on blood flow to the lacrimal gland but does not increase CBF; and (3) petrosal nerve section has little effect on the response of cerebral vessels to hypercapnia or hypoxia.

The cholinergic pathway to cerebral blood vessels. I. Morphological studies

The application of cobalt chloride to the peripheral cut end of the greater superficial petrosal nerve (g.s.p.n.) in rats revealed that only a few fibers in the plexus of nerves on the adventitial surface of the internal carotid artery were in axonal continuity with the g.s.p.n. A similarly small contribution of cholinergic fibers to cerebral blood vessels from this nerve was suggested by the observation that section of the g.s.p.n. resulted in an insignificant reduction in the density of the AChE-staining plexus in the internal carotid and cerebral arteries and in the incidence of at most 2% degenerate terminals of those observed on the middle cerebral artery. Alternative explanations of the results are discussed: that the AChE-staining fibers are postganglionic, that the time course for degeneration is unusually slow and that non-cholinergic fibers stain non-specifically for AChE. It is concluded that a cholinergic dilator pathway is most probably carried by the g.s.p.n. but that it is not unique.