Basal forebrain control of cortical blood flow and tissue gases in conscious aged rats

Regulation of cerebral cortical blood flow by the basal forebrain cholinergic fibers and aging

This article reviews the study of neural vasodilator mechanisms of the cerebral cortex by basal forebrain cholinergic nerve fibers and their age-related function in rats. During the last decade, we have demonstrated a neural regulatory system of cerebral blood flow in rats involving intracerebral cholinergic vasodilator nerve fibers originating in the basal forebrain and projecting to the cerebral cortex. Activation of these cholinergic vasodilator fibers results in the release of acetylcholine (ACh) within the cortex, activation of both nicotinic and muscarinic ACh receptors, and vasodilatation without coupling to glucose metabolic rates. This cholinergic vasodilator system has been shown to decline with age in rats mainly due to age-related declines of nicotinic ACh receptor activity. However, muscarinic ACh receptor activity and the release of ACh into the extracellular space in the cortex are well maintained during aging. The present age-related decline of the intracerebral cholinergic vasodilator system found in rats seems to affect cognitive function during aging, although this cholinergic vasodilator system has not yet been demonstrated in humans.

Effects of age on cholinergic vasodilation of cortical cerebral blood vessels in rats

The present study examined the age-related changes in the cholinergic vasodilative system originating in the nucleus basalis of Meynert (NBM) and projecting to the cerebral cortex using Wistar rats of three different ages; young adult (4-7 months), old (24-25 months), and very old (32-42 months) rats. The vasodilative responses in frontal and parietal cortices, measured by laser Doppler flowmetry, induced by electrical stimulation of NBM without blood pressure response were well maintained in old rats, but declined significantly in very old rats. Extracellular acethylcholine (ACh) release in both cortices collected by a microdialysis technique showed both basal levels and response to NBM stimulation to be well maintained in both old and very old rats. The vasodilative cerebral blood flow response elicited by stimulation of the muscarinic ACh receptors, using their agonist, arecoline, was also well maintained in old and very old rats. Considering the present data and our previous finding that the cerebral cortical vasodilative response to activation of the nicotinic ACh receptors using their agonist, nicotine, was markedly reduced in very old rats (Neurosci. Lett., 228 (1997) 203), it was concluded that the age-related decline of nicotinic ACh receptor activity was a cause of the decline of the vasodilative responses elicited by NBM stimulation in very old rats. This result suggests that a reduction of the cholinergic vasodilative system in very old rats due to decreased activity of the nicotinic ACh receptor may cause insufficient blood flow in the cortex when the cortical neurons require.

Differentiated cerebrovascular effects of physostigmine and tacrine in cortical areas deafferented from the nucleus basalis magnocellularis suggest involvement of basalocortical projections to microvessels

Cholinesterase inhibitors used to treat Alzheimer's disease according to the principle of cholinergic replacement therapy have proved to be less beneficial than expected. The present study was designed to investigate the cerebrovascular response to physostigmine and tacrine in the experimental model of lesioning of the nucleus basalis magnocellularis (NBM), a model involving a cholinergic deficit. Regional cerebral blood flow was measured by the [14C]iodoantipyrine tissue sampling technique in conscious rats infused with i.v. physostigmine (0.2 mg/kg/h), tacrine (8 mg/kg/h), or saline, 3-5 weeks after unilateral lesion of the NBM with ibotenic acid. Physostigmine and tacrine dose-dependently increased blood flow in most cortical and subcortical regions compared to the control group. However, physostigmine caused smaller blood flow increases in several areas, mostly cortical, of the lesioned compared to the intact hemisphere. The converse was observed with tacrine. A facilitated circulatory response appeared in cortical areas deafferented from the NBM, especially in the frontal cortex. These results provide evidence for distinct NBM-dependent components of the cortical cerebrovascular effects of physostigmine and tacrine. They suggest the involvement of different cellular postsynaptic targets of the NBM. The physostigmine-type effects could involve direct projects onto an inhibitory cortical interneuron supersensitized by deafferentation. This arrangement may explain why physostigmine and perhaps other cholinergic agonists are unable to specifically compensate for a deficit in NBM functioning. The tacrine-type effects presumably involve projections to the microvasculature, including perivascular astrocytes. The neurovascular junction would be sensitized by deafferentation from the NBM. Our data suggest that the regulatory mechanisms of blood flow originating in the NBM might constitute a target of neurodegenerative processes of Alzheimer's disease.

Reduced cortical vasodilatory response to stimulation of the nucleus basalis of Meynert in the aged rat and evidence for a control of the cerebral circulation

In earlier studies we showed that electrical stimulation of the rat nucleus basalis of Meynert (NBM) induces large increases in cerebral blood flow, mainly through cholinergic mechanisms. We then investigated the effect of aging on this influence by measuring cortical blood flow (CoBF) and tissue gas partial pressures (PtO2, PtCO2) in the conscious young adult and aged rat. NBM stimulation increased frontal (+101%) and parietal (+29%) CoBF in young rats. The effects were halved in aged rats. Moreover, PtO2 was significantly increased in young but not in aged rats. By contrast, the corticovascular reactivity to hypercapnia did not differ between young and aged rats, nor did the potentiating vasodilator effect of physostigmine. In combined autoradiographic measurements of cerebral blood flow and cerebral glucose utilization, we recently found that the cortical circulatory response to NBM stimulation was not accompanied by significant metabolic change. Thus, the blood flow changes observed in the cortex cannot be ascribed to increased metabolic activity. The distribution of this uncoupling coincides with that of cholinergic NBM projections directly impinging on cortical microvessels. These data support the cortical microcirculation and suggest the possible involvement of NBM dysfunction in the pathology of cortical microcirculation.

Autoradiographic evidence for flow-metabolism uncoupling during stimulation of the nucleus basalis of Meynert in the conscious rat

We earlier reported that electrical stimulation of the rat nucleus basalis of Meynert (NBM) induces large cerebral blood flow increases, particularly in frontal cortical areas but also in some subcortical regions. The present study was designed to address the issue of blood flow control exerted by NBM projections. To this aim, we have determined whether these flow increases were associated with proportionate changes in metabolic activity as evaluated by cerebral glucose utilization (CGU) strictly under the same experimental conditions in the conscious rat. An electrode was chronically implanted in a reactive site of the NBM as determined by laser-Doppler flowmetry (LDF) of the cortical circulation. One to two weeks later, while the cortical blood flow was monitored by LDF, we measured CGU using the [14C]2-deoxyglucose autoradiographic technique during unilateral electrical stimulation of the NBM, and analyzed the local flow-metabolism relationship. The large increases in cortical blood flow induced by NBM stimulation, exceeding 300% in various frontal areas, were associated with at most 24% increases in CGU (as compared with the control group) in one frontal area. By contrast, strong increases in CGU exceeding 150% were observed in subcortical regions ipsilateral to the stimulation, especially in extrapyramidal structures, associated with proportionate CBF changes. Thus, none of the blood flow changes observed in the cortex can be ascribed to an increased metabolic activity, whereas CBF and CGU were coupled in many subcortical areas. This result indicates that different mechanisms, which do not necessarily involve any metabolic factor, contribute to the regulation of the cerebral circulation at the cortical and subcortical level. Because the distribution of the uncoupling is coincident with that of cholinergic NBM projections directly reaching cortical microvessels, these data strongly support the hypothesis that NBM neurons are capable of exerting a neurogenic control of the cortical microcirculation.

Autoradiographic distribution of cerebral blood flow increases elicited by stimulation of the nucleus basalis magnocellularis in the unanesthetized rat

The nucleus basalis magnocellularis (NBM) of the rat, equivalent of Meynert's nucleus in the primate, is the origin of the main cholinergic innervation of the cerebral cortex. Stimulation of this area has been previously shown to induced marked, cholinergically mediated, blood flow increases in the frontal and parietal cortices. However, the complete distribution of the cerebrovascular effects of NBM stimulation within the whole brain has not been determined. In the present study, we used the [14C]iodoantipyrine autoradiographic method to measure local cerebral blood flow (CBF) in the unanesthetized rat, chronically implanted with a stimulation electrode. We performed unilateral electrical stimulation of the NBM in order to compare both the interhemispheric differences in blood flow and the differences with a group of sham-stimulated rats. Considerable blood flow increases were found in most neocortical areas, exceeding 400% in the frontal area, compared to the control group. Marked responses also appeared in discrete subcortical regions such as the zona incerta, some thalamic nuclei and structures of the extrapyramidal system. These responses were mostly ipsilateral to the stimulation. The significance and the distribution of these blood flow increases are related first, to anatomical and functional data on mainly the cholinergic projections from the NBM, but also non-cholinergic pathways connected with the NBM, second, to biochemical data on the basalocortical system, and third, to the limited ultrastructural data on the innervation of microvascular elements. This cerebrovascular study represents a step in the elucidation of the function of the basalocortical system and provides data which may be related to certain deficits of degenerative disorders such as Alzheimer's disease in which this system is consistently affected.