Responses of regional cerebral blood flow following focal electrical stimulation of the nucleus basalis of Meynert and the medial septum using the [14C]iodoantipyrine method in rats

The role of the nucleus basalis of Meynert in neuromodulation therapy: a systematic review from the perspective of neural network oscillations

As a crucial component of the cerebral cholinergic system and the Papez circuit in the basal forebrain, dysfunction of the nucleus basalis of Meynert (NBM) is associated with various neurodegenerative disorders. However, no drugs, including existing cholinesterase inhibitors, have been shown to reverse this dysfunction. Due to advancements in neuromodulation technology, researchers are exploring the use of deep brain stimulation (DBS) therapy targeting the NBM (NBM-DBS) to treat mental and neurological disorders as well as the related mechanisms. Herein, we provided an update on the research progress on cognition-related neural network oscillations and complex anatomical and projective relationships between the NBM and other cognitive structures and circuits. Furthermore, we reviewed previous animal studies of NBM lesions, NBM-DBS models, and clinical case studies to summarize the important functions of the NBM in neuromodulation. In addition to elucidating the mechanism of the NBM neural network, future research should focus on to other types of neurons in the NBM, despite the fact that cholinergic neurons are still the key target for cell type-specific activation by DBS.

The Nucleus Basalis of Meynert and Its Role in Deep Brain Stimulation for Cognitive Disorders: A Historical Perspective

The nucleus basalis of Meynert (nbM) was first described at the end of the 19th century and named after its discoverer, Theodor Meynert. The nbM contains a large population of cholinergic neurons that project their axons to the entire cortical mantle, the olfactory tubercle, and the amygdala. It has been functionally associated with the control of attention and maintenance of arousal, both key functions for appropriate learning and memory formation. This structure is well-conserved across vertebrates, although its degree of organization varies between species. Since early in the investigation of its functional and pathological significance, its degeneration has been linked to various major neuropsychiatric disorders. For instance, Lewy bodies, a hallmark in the diagnosis of Parkinson's disease, were originally described in the nbM. Since then, its involvement in other Lewy body and dementia-related disorders has been recognized. In the context of recent positive outcomes following nbM deep brain stimulation in subjects with dementia-associated disorders, we review the literature from an historical perspective focusing on how the nbM came into focus as a promising therapeutic option for patients with Alzheimer's disease. Moreover, we will discuss what is needed to further develop and widely implement this approach as well as examine novel medical indications for which nbM deep brain stimulation may prove beneficial.

Effect of basal forebrain stimulation on extracellular acetylcholine release and blood flow in the olfactory bulb

The olfactory bulb receives cholinergic basal forebrain input, as does the neocortex; however, the in vivo physiological functions regarding the release of extracellular acetylcholine and regulation of regional blood flow in the olfactory bulb are unclear. We used in vivo microdialysis to measure the extracellular acetylcholine levels in the olfactory bulb of urethane-anesthetized rats. Focal chemical stimulation by microinjection of L-glutamate into the horizontal limb of the diagonal band of Broca (HDB) in the basal forebrain, which is the main source of cholinergic input to the olfactory bulb, increased extracellular acetylcholine release in the ipsilateral olfactory bulb. When the regional cerebral blood flow was measured using laser speckle contrast imaging, the focal chemical stimulation of the HDB did not significantly alter the blood flow in the olfactory bulb, while increases were observed in the neocortex. Our results suggest a functional difference between the olfactory bulb and neocortex regarding cerebral blood flow regulation through the release of acetylcholine by cholinergic basal forebrain input.

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.

The effect of the cold pressor test on a visually evoked cerebral blood flow velocity response

We investigated the hypothesis that during tonic pain stimulus, neurovascular coupling (NVC) decreases, measuring visually evoked cerebral blood flow velocity response (VEFR) during cold pressor test (CPT) in healthy human subjects as a test. VEFR was calculated as a relative increase in blood flow velocity in the posterior cerebral artery from average values during the last 5 s of the stimulus-OFF period to average values during the last 10 s of the stimulus-ON period. Three consecutive experimental phases were compared: basal, CPT and recovery. During CPT, end-diastolic and mean VEFR increased from 20.2 to 23.6% (p < 0.05) and from 17.5 to 20.0% (p < 0.05), respectively. In recovery phase, end-diastolic and mean VEFR decreased to 17.7% and 15.5%, respectively. Both values were statistically significantly different from CPT phase (p < 0.05). Compared with the basal phase, only end-diastolic VEFR was statistically significantly different in the recovery phase (p < 0.05). Our results are consistent with the assumption that there is a change in the activity of NVC during CPT because of the modulatory influence of subcortical structures activated during tonic pain. Contrary to our expectations, the combined effect of such influences increases rather than decreases NVC.

Cholinergic modulation of cognition: insights from human pharmacological functional neuroimaging

Evidence from lesion and cortical-slice studies implicate the neocortical cholinergic system in the modulation of sensory, attentional and memory processing. In this review we consider findings from sixty-three healthy human cholinergic functional neuroimaging studies that probe interactions of cholinergic drugs with brain activation profiles, and relate these to contemporary neurobiological models. Consistent patterns that emerge are: (1) the direction of cholinergic modulation of sensory cortex activations depends upon top-down influences; (2) cholinergic hyperstimulation reduces top-down selective modulation of sensory cortices; (3) cholinergic hyperstimulation interacts with task-specific frontoparietal activations according to one of several patterns, including: suppression of parietal-mediated reorienting; decreasing ‘effort’-associated activations in prefrontal regions; and deactivation of a ‘resting-state network’ in medial cortex, with reciprocal recruitment of dorsolateral frontoparietal regions during performance-challenging conditions; (4) encoding-related activations in both neocortical and hippocampal regions are disrupted by cholinergic blockade, or enhanced with cholinergic stimulation, while the opposite profile is observed during retrieval; (5) many examples exist of an ‘inverted-U shaped’ pattern of cholinergic influences by which the direction of functional neural activation (and performance) depends upon both task (e.g. relative difficulty) and subject (e.g. age) factors. Overall, human cholinergic functional neuroimaging studies both corroborate and extend physiological accounts of cholinergic function arising from other experimental contexts, while providing mechanistic insights into cholinergic-acting drugs and their potential clinical applications.

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.

Specific mechanism for blood inflow stimulation in brain area prone to Alzheimer's disease lesions

The present study describes the specific two-stage mechanism that intensifies blood supply to the brain area comprising amygdala, hippocampus, olfactory bulb, entorhinal cortex, and neocortex (AHBC). Cholinergic neurons from the nuclei of basal forebrain induce vasodilatory effect through release of acetylcholine. In physiological aging the efficacy of this neuronal system declines, while intensive formation of amyloidogenic peptides starts. These peptides at low, picomolar concentrations activate alpha7 nicotinic acetylcholine receptors, thus enhancing angiogenesis and in so doing restoring blood supply to the AHBC area.

Responses of Acetylcholine Release and Regional Blood Flow in the Hippocampus during Walking in Aged Rats

Walking produced increases in both the extracellular acetylcholine level and regional blood flow in the hippocampus in aged rats 26-29 months old. The present results in aged rats were compared with our previous data in young adult rats (Nakajima et al., 2003), and it was found that both the acetylcholine and blood flow responses in the hippocampus were well maintained in aged rats.

Developmental origins of the age-related decline in cortical cholinergic function and associated cognitive abilities

Ontogenetic abnormalities in the regulation of the cortical cholinergic input system are hypothesized to mediate early-life cognitive limitations (ECL) that later escalate, based on reciprocal interactions between a dysregulated cholinergic system and age-related neuronal and vascular processes, to mild cognitive impairment (MCI) and, subsequently, for a majority of subjects, senile dementia. This process is speculated to begin with the disruption of trophic factor support of the basal forebrain ascending cholinergic system early in life, leading to dysregulation of cortical cholinergic transmission during the initial decades of life and associated limitations in cognitive capacities. Results from neurochemical and behavioral experiments support the possibility that aging reveals the vulnerability of an abnormally regulated cortical cholinergic input system. The decline of the cholinergic system is further accelerated as a result of interactions with amyloid precursor protein metabolism and processing, and with cerebral microvascular abnormalities. The determination of the developmental variables that render the cortical cholinergic input system vulnerable to age-related processes represents an important step toward the understanding of the role of this neuronal system in the age-related decline in cognitive functions.

Activation of the intracerebral cholinergic nerve fibers originating in the basal forebrain increases regional cerebral blood flow in the rat's cortex and hippocampus

In the rat, activation of the intracerebral cholinergic system originating in the basal forebrain and projecting to the cortex and hippocampus releases acetylcholine in the cortex and hippocampus, which results in vasodilation and an increase in regional cerebral blood flow (rCBF) in the cortex and hippocampus. The augmentation of rCBF is independent of both systemic blood pressure and regional metabolism. The intracerebral cholinergic fibers are able to act as autonomic nerve fibers for the regulation of cortical and hippocampal blood flow.

Stimulation of the nucleus basalis of Meynert increases diameter of the parenchymal blood vessels in the rat cerebral cortex

To verify the hypothesis that stimulation of the nucleus basalis of Meynert (NBM) induces vasodilation in the cerebral cortical parenchyma, we investigated whether the diameter of parenchymal blood vessels of rat parietal cortex is increased during stimulation of NBM using histological techniques. The parietal cortex was fixed by immersion fixation in situ during focal electrical stimulation of the NBM, which increased cortical blood flow. Cortical tissues were sectioned horizontally to the cortical surface, and the parenchymal blood vessels were morphometrically analyzed using electron microscopy. Mean inner diameter of the parenchymal blood vessels in NBM stimulated rats (5.51+/-0.33 microm) was significantly larger than that in non-stimulated control rats (4.93+/-0.23 microm). The result suggests that functional vasodilation in the cortical parenchyma during NBM stimulation correlates with histologically observed vasodilation in the cortical parenchyma.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

The effect of walking on regional blood flow and acetylcholine in the hippocampus in conscious rats

Recent studies in our laboratory have demonstrated that stimulation of the septal complex (i.e., the medial septal nucleus and the nucleus of the diagonal band) increases extracellular acetylcholine (ACh) release and, consequently, results in an increase in regional cerebral blood flow in the hippocampus (Hpc CBF) via activation of the nicotinic ACh receptors (nAChRs) [Neurosci. Lett. 107 (1989) 135; Neurosci. Lett. 112 (1990a) 263]. The present study aimed to examine the effects of walking on Hpc CBF, measured by laser Doppler flowmetry, in conscious rats. Walking at a moderate speed (4 cm/s) on a treadmill for 30 s produced increases in Hpc CBF and mean arterial pressure (MAP), reaching 107 +/- 1% and 105 +/- 1% of the prewalking control values, respectively. Walking for 3 min produced an increase in ACh release in the extracellular space of the hippocampus. The increase in Hpc CBF during walking was attenuated by mecamylamine (abbreviated as MEC here; 2 mg/kg, i.v.), a nAChR antagonist permeable to the blood-brain barrier (BBB), but not by hexamethonium (denoted as C6 here; 20 mg/kg, i.v.), a nAChR antagonist impermeable to the BBB, while the walking-induced increase in MAP was abolished by either agent. The response of Hpc CBF and MAP were not altered by atropine (abbreviated as ATR here; 0.5 mg/kg, i.v.), a muscarinic AChR antagonist permeable to the BBB. The increase in Hpc CBF during walking was attenuated by N(omega)-nitro-L-arginine methyl ester (L-NAME, 3 and 30 mg/kg, i.v.), a nitric oxide synthase (NOS) inhibitor, and the reduced responses were reversed following the intravenous (i.v.) administration of a physiological precursor of NO, L-arginine (600 mg/kg). The results suggest that the increase in Hpc CBF during walking is independent of MAP and attributable at least to activation of the nAChRs by the cholinergic vasodilator nerves projecting to the hippocampus and to production of NO in the hippocampus.

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.

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.

Effects of nicotine on blood flow and delayed neuronal death following intermittent transient ischemia in rat hippocampus

A cholinergic neural vasodilative response in the cerebral cortex and hippocampus, independent of metabolic vasodilation, was recently demonstrated by activating the nicotinic acetylcholine receptors (nAChRs) via activation of cholinergic neurons originating in the nucleus basalis of Meynert and septal complex in the basal forebrain and projecting to the cortex and hippocampus (see reviews by Sato A and Sato Y: Neurosci Res 14: 242--274, 1992; Sato A and Sato Y: Alzheimer Dis Assoc Disord 9: 28--38, 1995). In the present study, we aimed to examine whether an increase in regional blood flow in the hippocampus (Hpc-BF) following stimulation of the nAChRs by i.v. injection of nicotine could improve the delayed death of the hippocampal neurons following transient ischemia in rats. Hpc-BF was measured by using a laser Doppler flowmeter. During intermittent (every 2 min) transient occlusion for a total of 6 min of bilateral carotid arteries besides permanent ligation of bilateral vertebral arteries, Hpc-BF decreased to about 16% of the preocclusion level, and 5 or 7 d later, after the occlusion, delayed neuronal death occurred in approximately 70% of the CA1 hippocampal neurons. Hpc-BF was increased dose-dependently by injection of nicotine (30--100 microg/kg, i.v.), independent of mean arterial pressure. Nicotine (30--100 microg/kg) administered 5 min before occlusion slightly but significantly attenuated the occlusion-induced decrease in Hpc-BF. The delayed death of the CA1 hippocampal neurons occurring after transient occlusion was attenuated by pretreatment with nicotine (30--100 microg/kg) to approximately 50% of the total neurons. The results indicate that nAChR stimulation-induced increases in Hpc-BF can protect against ischemia-induced delayed death of hippocampal neurons.

Autoradiographic mapping of cerebral blood flow responses to cholinergic stimulation of the rat substantia innominata: modulatory effect of galanin

In order to analyze the precise cerebrovascular effects of a specific cholinergic stimulation of the rat substantia innominata and their modulation by galanin, cerebral blood flow was measured by the [14C]-iodoantipyrine autoradiographic method in anesthetized (urethane and alpha-chloralose), artificially ventilated male Sprague-Dawley rats that received a microinjection into the substantia innominata of saline (n = 7), or 63 pmol of galanin (n = 8), or 50 nmoles of carbachol (n = 6) or a coinjection of carbachol and galanin (n = 8). Significant carbachol-induced cerebral blood flow increases were noted in ipsilateral cortices (+36%, p < 0.01 in the cingulate to +82%, p < 0.01 in the parietal somatosensory cortices), but also in ipsilateral hippocampus and ipsilateral thalamus. These cerebral blood flow increases were abolished by the coinjection of carbachol and galanin, while infusions of galanin alone failed to affect cerebral blood flow. Cholinergic stimulation of the substantia innominata represents thus a good model for the analysis of the detailed pharmacological properties of the cholinergic vasodilatatory basalocortical system. The existence of an inhibitory galaninergic modulation of this system could be of particular interest, in terms of cerebrovascular reactivity, in various neurodegenerative states.

Regional cerebral blood flow responses to neurochemical stimulation of the substantia innominata in the anaesthetized rat

Since electrical stimulation of neurones may activate not only cell bodies but also neuronal fibres, this study aimed to test a selectively cholinergic neurochemical stimulation of the rat substantia innominata (SI) by the local microinjection of carbachol; the effects of this acetylcholine agonist were compared with glutamate. Cortical and subcortical cerebral blood flow (CBF) were measured in anaesthetized rats with the [14C]iodoantipyrine method by the tissue sampling technique immediately following the intracerebral (SI) microinjection of saline, 50 nmol of carbachol or glutamate. Carbachol microinjection into the SI induced a transient but significant vasodilation in frontoparietal motor (+28%) and temporal (+41%) cortices, that lasted for less than 10 min. Glutamate did not elicit any significant CBF modifications when compared to control rats although a significant interhemispheric asymmetry after microinjection was observed in the frontoparietal motor cortex. This latter observation would suggest that the glutamate-induced cortical response is less pronounced than that elicited by carbachol. Overall, these results demonstrate that a selective cholinergic stimulation of the SI can induce a transient cortical vasodilation and further confirms the hypothesis of a muscarinic modulation of CBF via this basal structure.

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.

Autoradiographic study of the cerebrovascular effects of stimulation of the substantia innominata: convenient stimulation paradigm

The aim of this study was to determine the distribution within the whole brain of the vascular effects of stimulation of the substantia innominata. This basal forebrain nucleus is the major cholinergic input in the neocortex in the rodent. The local cerebral blood flow was measured by the autoradiographic [14C]iodoantipyrine technique in a group of control and a group of stimulated unanesthetized rats. The substantia innominata was electrically stimulated through a chronically implanted electrode. The stimulation induced blood flow increases exceeding 200% in the hemisphere ipsilateral to the stimulation and 100% in the contralateral hemisphere compared to the control group. The ipsilateral vasodilations were observed not only in the cortical areas but also in some subcortical structures. Comparison with previous data suggests that part of the effects is due to cholinergic neurons of the substantia innominata and part to non-cholinergic neurons and indirect effects. However, only two out of eight stimulated rats displayed this response. The low reproducibility of the results is discussed, considering the stimulation paradigm which has been developed for future measurements of the cerebral glucose utilization which requires a long duration stimulation period.

The septo-hippocampal pathways and their relevance to human memory: a case report

The interaction between the septal region and the hippocampal formation appears indispensable for the maintenance of normal memory and learning mechanisms in humans. The disruption of some combination of septo-hippocampal pathways, especially the disruption of the "dorsal route", deteriorates explicit memory functions. The case of a 25-year-old male patient is presented who developed anterograde and to a certain extent retrograde amnesia following rupture and repair of an arteriovenous malformation in the atrium of the left ventricle. A left-sided lesion of the dorsal route involving the posterior cingulate bundle, the longitudinal striae (as part of the supracommissural hippocampus) and the fornix appeared responsible for his mnemonic deficits. The implications of these findings for the understanding of other clinical cases, particularly those with lesions of the septal region, the anterior and posterior singular gyrus/cingulate bundle and the fornix are discussed.

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.

Responses of regional cerebral blood flow to intravenous administration of thyrotropin releasing hormone in aged rats

The effects of i.v. administration of thyrotropin releasing hormone (TRH) on regional cerebral blood flow (rCBF) were examined in both healthy adult (3-5 months old) and healthy aged (24-25 months old) male Wistar rats under halothane anesthesia. The rCBFs in 9 different brain regions-cerebral cortex, caudate putamen, hippocampus, thalamus + hypothalamus, superior colliculus, inferior colliculus, cerebellum, pons, and medulla-were measured by [14C]iodoantipyrine method. In the adult rats, i.v. administration of TRH (300 micrograms/kg) produced significant increases in rCBFs in cerebral cortex, caudate putamen, hippocampus, thalamus + hypothalamus and superior colliculus. In the aged rats, the rCBFs in all brain regions measured did not change significantly by TRH administration. From these results, it is suggested that the system involved in TRH-induced vasodilatation of cerebral blood vessels was impaired with aging.

Stimulation of the nucleus basalis of Meynert does not influence glucose utilization of the cerebral cortex in anesthetized rats

The effect of focal electrical stimulation of the nucleus basalis of Meynert (NBM) on the regional cerebral metabolic rate for glucose (rCMRglc) was examined in halothane-anesthetized rats using the quantitative [14C]2-deoxy-D-glucose method. The stimulation of the unilateral NBM (with parameters of 200 microA, 0.5 ms, 50 Hz for 45 min) did not influence rCMRglc in any brain regions except for the stimulated NBM itself. In cerebral cortex ipsilateral to the NBM stimulated, the regional cerebral blood flow (rCBF) was increased by the NBM stimulation through the stimulated period, while rCMRglc was not increased. These results suggest that the increase in rCBF in the cortex following focal electrical stimulation of the NBM is not a consequence of an increase in rCMRglc in the cerebral cortex.