Lesion of the nucleus basalis magnocellularis does not affect cerebral cortical blood flow in rats

Modulation of somatosensory-evoked cortical blood flow changes by GABAergic inhibition of the nucleus basalis of Meynert in urethane-anaesthetized rats

Vascular changes associated with brain functions are thought to be tightly coupled with neuronal activity through neuronal glucose consumption or the local release of vasoactive agents. In contrast, another view suggests that cortical blood flow is strongly regulated by the nucleus basalis of Meynert (NBM), independently of regional metabolism. Thus, although cortical regional cerebral blood flow (rCBF) variations induced by somatosensory stimulation are strongly linked to neuronal activity, they may also be partly controlled by the NBM. In the present study, cortical rCBF alterations in response to innocuous brushing of the hindlimb (HL) were investigated by laser speckle contrast imaging. The contribution of NBM to these changes was examined after injection of the GABAergic agonist muscimol into the right NBM, allowing comparison of somatosensory-evoked cortical rCBF modifications before and after NBM inactivation. As expected, HL brushing elicited a robust rCBF increase in the contralateral parietal cortex (PC), over the representation of the HL. However, these alterations were decreased, by approximately 40%, in the hemisphere ipsilateral to muscimol inactivation of NBM, whereas vehicle injection did not produce any significant variation. The results demonstrate that cortical rCBF changes induced by somatosensory stimulation are partly regulated by NBM.

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.

Selective immunotoxin-induced cholinergic deafferentation alters blood flow distribution in the cerebral cortex

Adult rats received intracerebroventricular (i.c.v.) administration of either phosphate buffer (PBS) or 192 IgG-saporin (Toxin), 3.6 micrograms rat-1, a cholinergic immunotoxin. Six to eight weeks later, the animals received a continuous intravenous (i.v.) infusion of either physostigmine (4.2 micrograms kg-1 min-1) or saline, followed by measurement of cerebral cortical blood flow (CBF) with the autoradiographic Iodo-14C-antipyrine methodology in four groups of animals: Toxin i.c.v.+saline i.v. (n=9), Toxin i.c.v.+physostigmine i.v. (n=6), PBS i.c.v.+saline i.v. (n=6) and PBS i.c.v. +physostigmine i.v. (n=6). Choline acetyltransferase activity (ChAT) was assessed with Fonnum's method in samples of cortical tissue adjacent to the sites of CBF measurement. ChAT decreased in all regions of the Toxin groups when compared to PBS (% decrease: hippocampus=93%, neocortex=80-84%, entorhinal-piriform cortex=42%, amygdala=28%). CBF decreased globally in Toxin+SAL, most severely in posterior parietal and temporal regions (24-40% decrease from PBS+saline). Physostigmine enhanced CBF predominantly in these same areas both in PBS and Toxin animals although to a lesser extent in the latter. Our results demonstrate the importance of cholinergic mechanisms in the control of CBF. The similarity between the topography of CBF decrease following administration of the immunotoxin to that observed in Alzheimer's disease suggests that the CBF pattern observed in this disease may be the result of cholinergic deafferentation.

Iodo-QNB cortical binding and brain perfusion: effects of a cholinergic basal forebrain lesion in the rat

This study deals with the question of whether in vivo application of [125I]iodo-quinuclidinyl-benzilate (QNB) is able to demonstrate changes in cortical muscarinic receptor density induced by a cholinergic immunolesion of the rat basal forebrain cholinergic system, and whether the potential effects on IQNB distribution in vivo are also associated with effects on regional cerebral perfusion. Immunolesioned and control animals were injected with (R,S) [125]iodo-QNB and with [99mTc]-d,l-hexamethylpropyleneamine oxime (HMPAO). The cerebral distribution of both tracers was imaged using double tracer autoradiography. Impaired cholinergic transmission was paralleled by a 10-15% increase of [125I]iodo-QNB binding in the regions of cortex and hippocampus. The local cerebral blood flow remained unchanged after cholinergic lesion.

Glucose metabolism in cholinoceptive cortical rat brain regions after basal forebrain cholinergic lesion

To address the question whether the changes in cortical glucose metabolism observed in patients with Alzheimer's disease are interrelated with, or consequences of, basal forebrain cholinergic cell loss, an experimental approach was employed to produce cortical cholinergic dysfunction in rat brain by administration of the cholinergic immunotoxin 192IgG-saporin. [14C]D-glucose utilization in brain homogenates, D-glucose-displaceable [3H]cytochalasin B binding to glucose transporters (GLUT). Northern and Western analyses, as well as in vivo [14C]2-deoxyglucose autoradiography were used to quantify the regional glucose metabolism. Basal forebrain cholinergic lesion resulted in transient increases in glucose transporter binding in cortical regions displaying reduced acetylcholinesterase activity, already detectable seven days after lesion with peak values around 30 days post lesion. Western analysis revealed that the changes in total glucose transporter binding are mainly due to changes in the GLUT3 subtype only, while the levels of GLUT1 and GLUT3 mRNA (Northern analysis) were not affected by cholinergic lesion. Both immunocytochemistry and in situ hybridization demonstrated preferential localizations of GLUT1 on brain capillaries and GLUT3 on neurons, respectively. A lesion-induced transient decrease in [14C]D-glucose utilization seven days post lesion was detected in the lesion site, whereas cholinoceptive cortical regions were not affected. In vivo [14C]deoxyglucose uptake was transiently increased in cholinoceptive cortical regions and in the lesion site being highest between three to seven days after lesion. The cholinergic lesion-induced transient up-regulation of cortical glucose transporters and deoxyglucose uptake reflects an increased glucose demand in regions depleted by acetylcholine suggesting functional links between cortical cholinergic activity and glucose metabolism in cholinoceptive target regions.

Cholinergic projection from the basal forebrain and cerebral glucose metabolism in rats: a dynamic PET study

To investigate the influence of cholinergic projections from the basal forebrain on cerebral cortex metabolism, we evaluated the cerebral metabolic rate of glucose (CMRGlu) after selective inhibition of cholinergic neurons in the rat basal forebrain using the pyruvate dehydrogenase complex inhibitor 3-bromopyruvic acid (BPA), and compared the results with those obtained after lesioning the basal forebrain with ibotenic acid, as well as with those from a sham-operated control group. CMRGlu was measured using positron emission tomography (PET) with [18F]-2-fluoro-2-deoxy-D-glucose (FDG). Three days after surgery, CMRGlu and k3 (phosphorylation of FDG) were reduced similarly in the frontal cortex on the BPA-injected side and in the ibotenic acid-treated group, whereas K1 (transport rate of FDG from the plasma to brain) showed no marked changes. At 3 weeks postoperatively, the CMRGlu and k3 of the frontal cortex in both groups recovered to levels similar to those of the sham-operated group. The main difference between the BPA and ibotenic acid groups was that CMRGlu showed mild reduction on the side contralateral to the operation in the former, while such reduction was confined to the ipsilateral hemisphere in the latter. The present results indicate that the cholinergic system in the basal forebrain regulates cerebral cortex glucose metabolism through direct excitation of cortical neurons.

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.

In vivo measurement of acetylcholinesterase activity in the brain with a radioactive acetylcholine analog

A novel method for visualization of brain acetylcholinesterase (AChE) in vivo has been developed. Following intravenous administration of a radiolabelled acetylcholine analog, N-methyl-3-piperidyl acetate, there was very good agreement between the distribution of radioactivity and AChE activity in the brain of rat and monkey. The method would be applicable for in vivo studies of human brain AChE activity in disorders of central cholinergic systems such as Alzheimer's disease.

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.