Involvement of the basal nucleus of Meynert on regional cerebral cortical vasodilation associated with masticatory muscle activity in rats.

We examined the neural mechanisms for increases in regional cerebral blood flow (rCBF) in the neocortex associated with mastication, focusing on the cortical vasodilative system derived from the nucleus basalis of Meynert (NBM). In pentobarbital-anesthetized rats, parietal cortical rCBF was recorded simultaneously with electromyogram (EMG) of jaw muscles, local field potentials of frontal cortex, multi-unit activity of NBM neurons, and systemic mean arterial pressure (MAP). When spontaneous rhythmic EMG activity was observed with cortical desynchronization, an increase in NBM activity and a marked rCBF increase independent of MAP changes were observed. A similar rCBF increase was elicited by repetitive electrical stimulation of unilateral cortical masticatory areas. The magnitude of rCBF increase was partially attenuated by administration of the GABAergic agonist muscimol into the NBM. The rCBF increase persisted after immobilization with systemic muscle relaxant (vecuronium). rCBF did not change when jaw muscle activity was induced by electrical stimulation of the pyramidal tract. The results suggest that activation of NBM vasodilator neurons contributes at least in part to the rCBF increase associated with masticatory muscle activity, and that the NBM activation is induced by central commands from the motor cortex, independently of feedback from brainstem central pattern generator or contracting muscles.

Layer-specific dilation of penetrating arteries induced by stimulation of the nucleus basalis of Meynert in the mouse frontal cortex.

To clarify mechanisms through which activation of the nucleus basalis of Meynert (NBM) increases cerebral cortical blood flow, we examined whether cortical parenchymal arteries dilate during NBM stimulation in anesthetized mice. We used two-photon microscopy to measure the diameter of single penetrating arteries at different depths (~800 µm, layers I to V) of the frontal cortex, and examined changes in the diameter during focal electrical stimulation of the NBM (0.5 ms at 30 to 50 µA and 50 Hz) and hypercapnia (3% CO2 inhalation). Stimulation of the NBM caused diameter of penetrating arteries to increase by 9% to 13% of the prestimulus diameter throughout the different layers of the cortex, except at the cortical surface and upper part of layer V, where the diameter of penetrating arteries increased only slightly during NBM stimulation. Hypercapnia caused obvious dilation of the penetrating arteries in all cortical layers, including the surface arteries. The diameters began to increase within 1 second after the onset of NBM stimulation in the upper cortical layers, and later in lower layers. Our results indicate that activation of the NBM dilates cortical penetrating arteries in a layer-specific manner in magnitude and latency, presumably related to the density of cholinergic nerve terminals from the NBM.

Control of cerebral cortical blood flow by stimulation of basal forebrain cholinergic areas in mice.

We examined whether activity of the nucleus basalis of Meynert (NBM) regulates regional cerebral cortical blood flow (rCBF) in mice, using laser speckle and laser Doppler flowmetry. In anesthetized mice, unilateral focal stimulation, either electrical or chemical, of the NBM increased rCBF of the ipsilateral cerebral cortex in the frontal, parietal and occipital lobes, independent of changes in systemic blood pressure. Most of vasodilative responses to low intensity stimuli (2 times threshold intensity: 2T) were abolished by atropine (a muscarinic cholinergic blocker), whereas responses to higher intensity stimuli (3T) were abolished by atropine and mecamylamine (a nicotinic cholinergic blocker). Blood flow changes were largest when the tip of the electrode was located within the area containing cholinergic neurons shown by choline acetyltransferase-immunocytochemistry. These results suggest that cholinergic projections from basal forebrain neurons in mice cause vasodilation in the ipsilateral cerebral cortex by a combination of muscarinic and nicotinic mechanisms, as previously found in rats and cats.

Reality confusion in spontaneous confabulation.

A woman produced spontaneous confabulations after rupture of an anterior communicating artery aneurysm. She confused currently irrelevant with currently relevant information in implicit memory; confabulations about people concerned only new acquaintances; false reality could be induced by an intensive 5-minute discussion; and in a recognition task, she confused false repetitions in another modality with real item repetitions. The findings support the theory that the defect causing spontaneous confabulation precedes conscious memory processing.

Brain capillaries and cholinergic neurons persist in organotypic brain slices in the absence of blood flow.

Angiogenesis plays an important role during development of the brain and under pathological conditions. The aim of the present study was to observe interaction of brain capillaries and cholinergic neurons in organotypic brain slices. Immunohistochemistry was used to visualize brain capillary-like structures (RECA-1 antigen) and cholinergic neurons (choline acetyltransferase). Under normal culture conditions, a very low number of brain capillaries was found in 2- and 4-week-old cortex brain slices. Treatment of slices with acidic medium (pH 6) or hyperthermia (42 degrees C) markedly enhanced the number of brain capillaries. Incubation with 10 ng/mL vascular endothelial growth factor only enhanced angiogenesis in more developed slices. Cholinergic neurons survived in slices of the basal nucleus of Meynert; however, hyperthermia but not acidosis markedly decreased their number. In coslices of the basal nucleus of Meynert and cortex (pretreated with acidic medium), a high number of RECA-1-positive capillaries and cholinergic neurons persisted and displayed strong nerve fibre growth of cholinergic fibres into the cortex. In conclusion, we have demonstrated that RECA-1-positive capillaries and cholinergic neurons can be studied in slice cultures in the absence of blood perfusion, and that this model could provide a system to study mechanisms involved in vascular dementia.

Effects of stimulating the nucleus basalis of Meynert on blood flow and delayed neuronal death following transient ischemia in the rat cerebral cortex.

An increase in cortical cerebral blood flow (CBF), independent of metabolic vasodilation, via the activation of cholinergic neurons originating in the nucleus basalis of Meynert (NBM) in the basal forebrain and projecting to the widespread cortices was recently demonstrated. In the present study, we aimed to clarify whether the increase in CBF following a stimulation of the NBM can improve delayed death of the cortical neurons following transient ischemia in rats. CBF was measured with a laser Doppler flowmeter, and the delayed neuronal death of the cerebral cortex produced by intermittent (every 5 s) occlusions of the unilateral common carotid artery for 60 min was measured histologically in the cortical hemisphere at 3 different coronal levels (6 microm thickness). In control rats without occlusion there were 6000-8000 intact neurons and 9-19 damaged neurons in the cortical hemisphere at each coronal level. During the occlusions, CBF ipsilateral to the occluded artery decreased by 13-32% of the preocclusion level. Five days after the occlusions, the numbers of damaged neurons were increased to 75-181. Repetitive electrical stimulation was delivered to the NBM, ipsilateral to the occluded artery, starting 5 min before the occlusions and finishing around the end of them. The increase in CBF induced by NBM stimulation prevented the occlusion-induced decrease in CBF in all 3 of the cortices. The delayed death of the cortical neurons previously observed after the occlusions was scarcely observable in all the cortices when NBM was stimulated. The present results suggest that NBM-originating vasodilative activation can protect the ischemia-induced delayed death of cortical neurons by preventing a blood flow decrease in widespread cortices.

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

The intracranial neural vasodilative system of cholinergic fibers projecting from the basal forebrain to the cortex was discovered by Biesold, Inanami, Sato and Sato (Biesold, D., Inanami, O., Sato, A., Sato, Y., 1989. Stimulation of the nucleus basalis of Meynert increases cerebral cortical blood flow in rats. Neurosci. Lett. 98, 39-44) using laser Doppler flowmetry in anesthetized rats. This cholinergic vasodilative system, which operates by increasing extracellular ACh release, relies upon activation of both muscarinic and nicotinic cholinergic receptors in the parenchyma of the cortex. Further, the involvement of nitric oxide in this cholinergic vasodilation, indicates the necessity to this system of neurons, which contain nitric oxide synthase. The increase in cortical blood flow elicited by this cholinergic vasodilative system is independent of systemic blood pressure and is not coupled to cortical metabolic rates. This cholinergic vasodilative system may be activated by somatic afferent stimulation. Most of the data presented here were obtained in anesthetized animals.

Effect of acupuncture-like stimulation on cortical cerebral blood flow in anesthetized rats.

The effect of acupuncture-like stimulation of various areas (cheek, forepaw, upper arm, chest, back, lower leg, hindpaw, perineum) on cortical cerebral blood flow (CBF) was examined in anesthetized rats. An acupuncture needle (diameter, 340 microm) was inserted into the skin and underlying muscles at a depth of about 5 mm and twisted to the right and left once a second for 1 min. CBF of the cortex was measured using a laser Doppler flowmeter. Stimulation of the cheek, forepaw, upper arm and hindpaw produced significant increases in CBF, but stimulation of the chest, back, lower leg and perineum did not produce significant responses. Stimulation of the cheek, forepaw, and hindpaw produced an increase in mean arterial pressure (MAP), while stimulation of the back produced a decrease in MAP. Stimulation of the upper arm, chest, lower leg and perineum did not produce a significant MAP response. After spinal transection at the 1st to 2nd thoracic level, the blood pressure response to stimulation of the cheek and forepaw was suppressed, whereas an increase in CBF still took place. The increase in CBF induced by forepaw stimulation was abolished by severance of the somatic nerves at the brachial plexus. Forepaw stimulation enhanced the activity of the radial, ulnar and median nerves. Furthermore, in the present study, passing of an electric current through acupuncture needles showed that excitation of group III (Adelta) and group IV (C) afferent fibers in the somatic nerve was capable of producing an increase in CBF, whereas excitation of group I (Aalpha) and group II (Abeta) fibers was ineffective. The increase in CBF induced by forepaw stimulation was almost abolished by intravenous administration of muscarinic and nicotinic cholinergic blocking agents (atropine 5 mg/kg and mecamylamine 20 mg/kg), and by bilateral lesions in the nucleus basalis of Meynert. Acupuncture-like stimulation of a forepaw increased acetylcholine release in the cerebral cortex. We concluded that the increase in CBF, independent of systemic blood pressure, elicited by acupuncture stimulation is a reflex response in which the afferent nerve pathway is composed of somatic group III and IV afferent nerves, and efferent nerve pathway includes intrinsic cholinergic vasodilators originating in the nucleus basalis of Meynert.

Basal forebrain nitric oxide synthase (NOS)-containing neurons project to microvessels and NOS neurons in the rat neocortex: cellular basis for cortical blood flow regulation.

Stimulation of basal forebrain neurons results in local increases in cortical cerebral blood flow that are dependent upon cholinergic and nitrergic mechanisms. In the present study, we investigated the possibility that basal forebrain nitric oxide synthase (NOS)-containing neurons project to microvessels and NOS interneurons in the rat cerebral cortex. We performed quisqualic (QUIS) acid lesions of the basal forebrain and evaluated their effects on cortical NOS immunostained nerve terminals, with emphasis on those associated with microvessels and NOS interneurons, both at the light and/or electron microscopic levels. The results show that basal forebrain NOS neurons provide about one third of the overall cortical NOS innervation. Further, the data indicate that basalocortical NOS fibres establish privileged associations with microvessels and NOS neurons, as respective denervations of 60 and 45% were observed following lesion. At the electron microscopic level, most perivascular NOS neuronal elements corresponded to nerve terminals and a majority ( approximately 25%) of these were located in the immediate vicinity of the blood vessels, similar to the perivascular distribution reported previously for classic neurotransmitters/neuromediators. NOS terminals abutting on cortical NOS neurons were primarily nonjunctional. Altogether, these results raise the possibility that not only cholinergic but also nitrergic basal forebrain neurons are involved in the flow response observed following stimulation of the basal forebrain. Further, they suggest interactions between basalocortical and intracortical NOS neurons. We conclude that these interactions are involved in the spatial and temporal regulation of cortical perfusion following basal forebrain activation, and that they may become dysfunctional in pathologies such as Alzheimer's disease which affects both the basal forebrain and the cortical NOS neurons.

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.