Topographic analysis of the innervation of the rat neocortex and hippocampus by the basal forebrain cholinergic system

Ultrastructural organization of choline-acetyltransferase-immunoreactive fibres innervating the neocortex from embryonic ventral forebrain grafts

Suspension grafts of foetal tissue rich in cholinergic neurones were transplanted into the frontoparietal cortex of rats that had previously undergone deafferentation of the extrinsic cholinergic innervation of the cortex by injection of ibotenic acid into the nucleus basalis magnocellularis. The cortical tissue containing the graft was processed for electron microscopic immunocytochemistry by using a monoclonal antibody to choline acetyltransferase (ChAT) in order to examine the contacts established between cholinergic fibres from the graft and the host neocortex. The density, distribution, and targets of this graft-host innervation were compared with those seen in the intact and deafferented cortex. ChAT-positive fibres in both grafted and control animals formed extensive synaptic connections with various cortical neural elements--those of graft origin being of similar morphology to those in the intact cortex. However, the distribution of postsynaptic cortical targets of the graft-derived ChAT-immunoreactive boutons was abnormal, such that a greater percentage of such terminals formed synaptic contacts with neuronal perikarya, especially layer V pyramidal neurones, than was seen in control brains. It is possible that the formation of new synaptic contacts between the embryonic graft and host frontoparietal cortex may, in part, be necessary for the restoration of functional activity that has been previously reported in these grafted animals.

Memory related role of the posterior cholinergic system

In order to establish a model for the possible neuropathology of patients with Alzheimer's disease, various behaviors of rats with different chemical lesions in cholinergic regions were studied and compared with those of sham-operated control rats. A battery of neurological tests was used as well as activity measurements and two learning tasks: a positively reinforced place-learning task with delay periods of Os, 1 min, 15 min, and 2h, and a shock-motivated two-way active avoidance task. While in general no intergroup differences were obtained in performance on the neurological test battery or the rats' activity in an open field, there were marked impairments in the three lesioned groups compared to the control group in the two learning tasks. These deficits were less severe in the two groups with lesions of the medial septal/vertical diagonal band of Broca region and the nucleus basalis of Meynert region, but rather marked in the group with lesions of neurons situated in the pontomesencephalic region, although the amount of ibotenic acid injected had been the same for all groups. We conclude from these data that changes in mesencephalic cholinergic regions might play a significant role in the pathology of Alzheimer's disease. The existence of such regions was recently established for primates as well, thus providing a basis for justifying an animal model of this human disease.

Medial septal and nucleus basalis magnocellularis lesions produce order memory deficits in rats which mimic symptomatology of Alzheimer's disease

Rats with electrolytic lesions of the medial septum or ibotenic acid lesions of the nucleus basalis magnocellularis (NBM) were tested in an order memory task for an 8-item list of varying spatial locations within an 8-arm radial maze. Results indicated that rats with small medial septal lesions resulting in small AchE depletion of dorsal hippocampal formation were impaired only for the first, but not the last choice orders of the list. Animals with large medial septal lesions resulting in large AchE depletion of the dorsal hippocampal formation displayed an order memory deficit for all the choice orders of the list. In contrast, rats with small NBM lesions resulting in small AchE depletion of parietal and part of frontal cortex were impaired only for the last, but not the first choice orders of the list. Animals with large NBM lesions resulting in large AchE depletion of parietal and part of frontal cortex displayed an order memory deficit for all the choice orders of the list. The relationship between these findings and mnemonic symptomatology of Alzheimer's disease was discussed, as was the possible meaning of these results in providing an animal model for studying certain aspects of the disease.

Axonal branching of basal forebrain projections to the neocortex: a double-labeling study in the cat

Double-labeling of basal forebrain neurons by retrograde axonal transport demonstrates divergent collateralization among undecussated axonal projections to the neocortex. These branched fibers originate from a considerable complement of large polymorphic cell bodies located mainly in the basal nucleus of Meynert. They terminate in multiple neocortical sites including the precruciate, postcruciate and/or cingulate gyri. This extensive intra- and intergyral axonal branching indicates that neurons in the basal forebrain of the cat have extensive axonal fields innervating adjacent neocortical gyri.

The neurobiologic consequences of Down syndrome

Trisomy of the whole or distal part of human chromosome 21 (HSA 21) (Ts21) results in Down Syndrome (DS), which is characterized in part by mental retardation and associated neurological abnormalities. Structural abnormalities observed frequently include reduced brain weight, decreased number and depth of sulci in the cerebral cortices, neuronal heterotopias, and reduced numbers of specific populations of neurons, such as granule cells, in the cerebral cortices. Abnormalities in the structure of cells, primarily of the dendrites, are observed in portions of the neuraxis, such as the hippocampus, cerebellum, and cerebral cortices. Functional abnormalities in membrane properties in peripheral structures and in neurotransmitter enzyme systems in both peripheral and central structures are observed also. Brains of DS individuals over the age of 40 exhibit the characteristic neuropathologic and neurochemical stigmata of Alzheimer's disease (AD). The cholinergic and noradrenergic systems appear to be particularly vulnerable. To elucidate the mechanisms responsible for these abnormalities, identification of the genes located in the distal part of HSA 21 and the systematic study of animal model systems with close genetic homology are essential.

Cholinergic neurons and fibres in the rat visual cortex

Choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, was localized immunocytochemically in neurons and fibres in the rat visual cortex using a monoclonal antibody. ChAT-labelled cells were non-pyramidal neurons, primarily of the bipolar form, distributed in layers II through VI but concentrated in layers II & III. Their perikarya contained a large nucleus and a small amount of perinuclear cytoplasm. The somata and dendrites of all labelled cells received Gray's type I and type II synapses. ChAT-stained axons formed a dense and diffuse network throughout the visual cortex and particularly in layer V. Electron microscopy revealed that the great majority formed type II synaptic contacts with dendrites of various sizes, unlabelled non-pyramidal somata and, on a few occasions, with ChAT-labelled cells. However, a very small number of terminals appeared to form type I synaptic contacts. This study describes the morphological organization of the cholinergic system in the visual cortex, the function of which has been under extensive investigation.

Benzodiazepine binding sites: localization and characterization in the limbic system of the rat brain

The distribution of benzodiazepine binding sites was analysed in limbic structures of rat brain by quantitative radioautography of brain sections incubated with 3H-flunitrazepam (3H-FLU). Quantitative estimation of the binding parameters was made in each range of postero-anterior sections taken. Distribution of 3H-FLU binding sites was found to be rather homogeneous in most of the structures examined but there were regional differences which resulted from variations in the densities of sites rather than in their affinities. A particular distribution pattern of 3H-FLU binding sites was observed in the cingulate cortex contrasting with the homogeneous postero-anterior distribution measured in other cortical areas in the same slices. A significantly greater density of sites was found in the anterior part of the structure as compared to the posterior part. This difference, which corresponds to a change in the density of sites without alteration of their apparent affinity and occurs at a precise anatomical level, is discussed with reference to the anatomical organization of this brain structure and to its possible functional implications.

Retrograde transport of fluorescent tracers reveals extensive ipsi- and contralateral claustrocortical connections in the rat

The projections from the claustrum to the cerebral cortex in the rat were examined by means of retrogradely transported fluorescent tracers Fast Blue (FB) and Diamidino Yellow dihydrochloride (DY), injected in the prefrontal, motor, somatosensory, auditory, and visual fields. In all cases, substantial numbers of retrogradely labeled neurons were observed in the ipsilateral and moderate to scant numbers in the contralateral claustrum insulare. Symmetrical bilateral injections of FB and DY as well as simultaneous injections of the tracers in the motor and visual cortex of the same hemisphere revealed no double-labeled neurons in the claustrum. The following conclusions may be drawn: The claustral projections to the motor, somatosensory, and visual cortex are prominent. The projection to the prefrontal cortex is less substantial and that to the auditory cortex is relatively modest. The claustrocortical connections lack the clear-cut topographic pattern of the thalamic nuclei but are, to some degree, preferentially arranged, albeit with considerable overlapping of the subpopulations of corticopetal neurons, a coarse anteroposterior topographic distribution appears to exist also in rodents. Neurons contributing to the claustrocortical connection project either ipsilaterally or contralaterally but not bilaterally. Projections to different cortical fields of one hemisphere also originate from separate claustral neurons.

Prenatal development of nucleus basalis complex and related fiber systems in man: a histochemical study

To provide parameters for study of the "cholinergic" innervation of a human fetal cerebrum, we have analyzed the prenatal development of histochemical reactivity in the nucleus basalis complex (a magnocellular complex known to contain a high concentration of cholinergic perikarya). Brains from fetuses and premature infants ranging between 8 and 35 weeks of gestation were frozen cut and processed by the thiocholine method for the demonstration of acetylcholinesterase activity. Since no consistent results were obtained with inhibitors on the material younger than 15 weeks, the histochemical reactivity for early stages was expressed as the total cholinesterase reactivity. The first sign of histochemical differentiation of the basal telencephalon is the appearance of a dark cholinesterase reactive "spot" situated between the developing lenticular nucleus and basal telencephalon surface as early as 9 weeks of gestation. The first cholinesterase reactive bundle connects this reactive area (nucleus basalis complex anlage) with the strongly reactive fiber system situated along the dorsal side of the optic tract. During the next "stage" (10.5 weeks), there is a significant increase in the size of the nucleus basalis complex and strongly cholinesterase reactive neuropil occupies the sublenticular, diagonal and septal areas. At this stage we have seen two new cholinesterase-reactive bundles: one well developed cholinesterase reactive fiber stratum approaching (but not penetrating) the neocortical anlage through the external capsule and another minute bundle running towards the medial limbic cortex through the precommissural septum. The supraoptic fiber system can be traced now to the pregeniculate area and the tegmentum. At 15 weeks, the first acetylcholinesterase reactive perikarya appear and the nucleus basalis complex anlage becomes segregated into several strongly reactive territories, corresponding in position to the medial septal, diagonal and basal nuclei as defined on adjacent Nissl stained sections. At this stage, fibers from the nucleus basalis complex enter the "white" matter of frontal, temporal, parietal and occipital parts of the cerebral hemisphere via the external capsule. Between 15 and 18 weeks, acetylcholinesterase fibers spread throughout the "white" matter of the cerebral hemisphere. In the next "stage" (18-22 weeks), strongly reactive fibers can be followed from the nucleus basalis below the putamen and through the external capsule to the transient, synapse-rich subplate zone of frontal, temporal, parietal and occipital cortices.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholinergic limbic projections and behavioral role of basal forebrain nuclei in the rat

The purposes of the present study were to identify cholinergic non-neocortical projections of the basal forebrain and to determine the role of this region in the regulation of estrogen-dependent reproductive behaviors in the rat. Bilateral electrolytic lesions were placed in an area encompassing the horizontal limb of the diagonal band, as well as portions of the substantia innominata and magnocellular preoptic nucleus, and choline acetyltransferase (CAT) activity was assayed in microdissected brain areas seven days after lesion. Compared to sham surgery, lesions of this region significantly reduced CAT activity in the basal amygdala (34%), dorsal hippocampus (14%), cingulate cortex (25%), piriform cortex (36%), and entorhinal cortex (34%). Other limbic and midbrain structures do not appear to receive significant cholinergic innervation from this locus since no reductions in CAT were detected after bilateral lesions. These included the anterior hypothalamus, ventromedial hypothalamus, mammillary nucleus, habenula, subiculum, ventral hippocampus, insular cortex, central gray, and interpeduncular nucleus. Behaviorally, female rats with bilateral lesions of the basal forebrain displayed an unusually high incidence of rejection behavior in response to attempted mounts by stimulus male rats in sexual behavior tests. There was no effect of basal forebrain lesions on the incidence of lordosis exhibited by these females. The dissociation of rejection and lordosis suggests that distinct neural pathways mediate the occurrence of these reproductive behaviors and that rejection behavior may be regulated by basal forebrain pathways.

N-methylaspartate: an effective tool for lesioning basal forebrain cholinergic neurons of the rat

The ability of the excitotoxin, N-methyl-D,L-aspartic acid (NMA), to destroy basal forebrain cholinergic (BFC) neurons was evaluated. NMA (100 nmol) was directly injected into the peripallidum, a region containing a proportionately large number of cortically-projecting BFC neurons. Cholineacetyltransferase (ChAT) activity 10 days later was markedly and significantly reduced (up to 62%) in the cortex ipsilateral to the lesion. NMA induced a focal lesion affecting BFC neurons without damaging axons of passage or causing lesions distant from the site of injection. ChAT immunohistochemistry (IHC) was used to directly demonstrate loss of ChAT-positive neurons from the lesion site. This loss persisted at all survival times examined, from 2 days to 7.5 months post-injection.

Light microscopic evidence of striatal input to intrapallidal neurons of cholinergic cell group Ch4 in the rat: a study employing the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L)

Injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) were placed in various striatal loci in the rat. Within the globus pallidus, PHA-L-filled striatofugal axons were seen to approach cholinergic neurons, identified with either acetylcholinesterase histochemistry or choline acetyltransferase immunohistochemistry, and, apparently, to contact the surface of such cells with axonal varicosities. Since these varicosities are thought to mark the sites of synaptic terminals, such juxtapositions provide strong light-microscopic evidence that intrapallidal cholinergic neurons in the rat receive a direct innervation from the striatum and are integrated into the circuitry of the basal ganglia.

A correlated light and electron microscopic immunocytochemical study of cholinergic terminals and neurons in the rat amygdaloid body with special emphasis on the basolateral amygdaloid nucleus

The cholinergic innervation of the rat basolateral amygdaloid nucleus (BL) was determined by the immunocytochemical localization of the acetylcholine biosynthetic enzyme, choline acetyltransferase (ChAT). ChAT-immunoreactive (ChAT-IR) elements were observed throughout the BL in the form of fine puncta and varicose fibers. Electron microscopy revealed that the immunoreactive puncta represented small terminals (0.3-1.2 micron), most of which formed synaptic contacts with unlabeled dendritic shafts or spines. Less frequently, ChAT-IR terminals established synaptic contacts with large neuronal cell bodies, which had all the characteristics of projection neurons as defined on the basis of axonal projections to the ventral striatum. ChAT-IR terminals were sometimes seen to form synaptic contacts with small neuronal cell bodies, including those of ChAT-IR neurons. The ChAT-IR boutons contained pleomorphic clear vesicles of varying size, and the large majority of the synapses were of the symmetric type. Small ChAT-IR neurons were observed in all parts of the BL. Although the ChAT-IR cell bodies varied widely in shape from typical fusiform to round, most had a more or less oval shape with a major diameter of 10-14 micron. Most of the ChAT-IR neurons seemed to display a radial bipolar dendritic pattern, but multipolar cells were also observed. The ChAT-IR neurons contained an indented nucleus, which was often eccentrically located and surrounded by a thin or moderately thin rim of cytoplasm. The results obtained are discussed in relation to a quasi-cortical organization of the BL.

Cholinergic and GABAergic afferents to the olfactory bulb in the rat with special emphasis on the projection neurons in the nucleus of the horizontal limb of the diagonal band

We have examined the location of cholinergic and GABAergic neurons that project to the rat main olfactory bulb by combining choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) immunohistochemistry with retrograde fluorescent tracing. Since many of the projection neurons are located in subcortical basal forebrain structures, where the delineation of individual regions is difficult, particular care was taken to localize projection neurons with respect to such landmarks as the ventral pallidum (identified on the basis of GAD immunoreactivity), the diagonal band, and medial forebrain bundle. In addition, sections with fluorescent tracers or immunofluorescence were counterstained for Nissl substance in order to correlate tracer or immunopositive neurons with the cytoarchitecture of the basal forebrain. The majority of the cholinergic bulbopetal neurons are located in the medial half of the nucleus of the horizontal limb of the diagonal band (HDB), whereas only a few are located in its lateral half. A substantial number of cholinergic bulbopetal cells are also found in the sublenticular substantia innominata. A small number of cholinergic bulbopetal neurons, finally, are located in the ventrolateral portion of the nucleus of the vertical limb of the diagonal band. At the level of the crossing of the anterior commissure, approximately 17% of the bulbopetal neurons in the HDB are ChAT-positive. The noncholinergic bulbopetal cells are located mainly in the lateral half of the HDB. GAD-containing bulbopetal neurons are primarily located in the caudal part of the HDB, especially in its lateral part. About 30% of the bulbopetal projection neurons in the HDB are GAD-positive. A few GAD-positive bulbopetal cells, furthermore, are located in the ventral pallidum, anterior amygdaloid area, deep olfactory cortex, nucleus of the lateral olfactory tract, lateral hypothalamic area, and tuberomamillary nucleus. The topography of bulbopetal neurons was compared to other projection neurons in the HDB. After multiple injections of fluorescent tracer in the neocortex, retrogradely labeled neurons were concentrated in the most medial part of the HDB, while neurons projecting to the olfactory and entorhinal cortices were located in the ventral part of the HDB. These results show that the cells of the HDB can be divided into subpopulations based upon projection target as well as transmitter content. Furthermore, these subpopulations correspond, at least to a considerable extent, to areas that can be defined on cyto- and fibroarchitectural grounds.

Early cytoarchitectonic development of the anlage of the basal nucleus of Meynert in the human fetus

The present study describes a short period of cytoarchitectonic development of the anlage of the basal nucleus of Meynert between 9 and 15 weeks of gestation (crown-rump length 42-120 mm). On the basis of temporo-spatial reconstruction of the cytoarchitectonic pattern within the basal telencephalon it was evident that magnocellular aggregations of the basal telencephalon contain the most differentiated cells in the whole prosencephalon of the 15-week-old human fetus. At this stage development many magnocellular islands can be observed in the sublenticular region. However, it seems that they are in antero-posterioral continuation and the real number of magnocellular islands is much smaller than observed in a single section. The most voluminous magnocellular aggregations are situated around the temporal limb of the anterior commissure and below the ventral pallidal surface in the 15th week of gestation. Between 12 and 15 weeks of gestation, at the most rostral levels, the distinct cell group appeared with unique cytoarchitectonic properties ('albino cell group'). This cell group is situated close to the ventral putaminal surface within the capsula externa fibres and it corresponds to the subputaminal nucleus of Ayala.

Somatostatin in the central nervous system: physiology and pathological modifications

Since its discovery, at the beginning of 1973, somatostatin's multiple actions, in relation to its wide anatomical distribution have been widely documented. Its biochemical pathways have been elucidated with the discovery of other molecular forms as well as the mechanisms of its neuronal release. However, no definite proof is available concerning a neurotransmitter role for any peptide of the somatostatin family other than somatostatin-14. The precise determination of the roles of somatostatin in brain are still hampered by the poor pharmacology of the peptide. New tools are badly needed and in particular a true antagonist at the receptor site. The mechanisms of action of somatostatin are now well under way at least in the pituitary model. More information should come from this model and be applied to brain cells in vitro. The greatest challenge of somatostatin brain function lies in its role in the pathophysiology of neurological diseases such as Alzheimer's dementia and Huntington's disease. Nature has been using somatostatin-related molecules since inhibitory control was first needed in cell functions. Time will tell us if somatostatin is really an old peptide involved in senile dementia.

The localization of central cholinergic neurons

Over the past decade our understanding of the localization of central cholinergic neurons has greatly increased. Interest in these systems has also intensified due to the involvement of cholinergic mechanisms in Alzheimer's disease. The distribution of central cholinergic neurons is reviewed, focusing on recent work in experimental animals. The pharmacohistochemical procedure for acetylcholinesterase and the development of antibodies to choline acetyltransferase are two of the major technical advances that have shaped our knowledge of the distribution of central cholinergic neurons. The results, advantages and limitations of both techniques are discussed. A discussion of the phenomenon of coexistence of acetylcholine with neuroactive peptides in central neurons is also included.

Basal forebrain neurons projecting to the rat frontoparietal cortex: electrophysiological and pharmacological properties

Neurons located in the ventromedial globus pallidus (nucleus basalis) and substantia innominata, that were antidromically driven by electrical stimulation of the frontoparietal cortex, were recorded in the urethane anesthetized rat. The basalocortical neurons (BCNs) were antidromically driven with latencies of 1.1-13.5 ms, giving conduction velocities of 0.6-6.8 m/s. Many BCNs had regular patterns of spontaneous discharge (mean spontaneous activity: 20 impulses/s). Most BCNs were not responsive to non-noxious peripheral somatic stimulation. BCNs were readily excited by the iontophoretic application of glutamate and strongly inhibited by GABA. Eighty-five percent of the BCNs could be excited by acetylcholine. They could also be excited by cholinergic agonists. Muscarinic agonists excited a higher proportion of BCNs than nicotinic agonists. Excitatory responses to acetylcholine, carbachol and muscarinic agonists were abolished by atropine.

Galanin-like immunoreactivity in cholinergic neurons of the septum-basal forebrain complex projecting to the hippocampus of the rat

It is now well recognized that there are several groups of cholinergic neurons in the basal forebrain with direct projections to various cortical regions. Immunohistochemical investigations of the distribution of the neuropeptide galanin (GAL) have shown that two of these brain areas, the medial septum and diagonal band, contained large numbers of GAL-immunoreactive neurons. In the present study, double staining techniques using antibodies raised against choline acetyltransferase (ChAT) revealed that GAL- and ChAT-like immunoreactivities are colocalized within a subpopulation of the cholinergic neurons within the medial septum and diagonal band. This colocalization of GAL- and ChAT-immunoreactivities was not seen to occur within other groups of forebrain cholinergic neurons. Immunohistochemistry carried out subsequent to injections of fluorescent retrograde tracers into the hippocampal formation revealed that both ChAT/GAL- and ChAT-containing neurons project to the hippocampal formation. The question of GAL as a modulator of cholinergic transmission in this projection is discussed.

Distribution of cholinergic muscarinic binding sites in guinea-pig brain as determined by in vitro autoradiography of [3H]N-methyl scopolamine binding

The distribution of muscarinic binding sites was analyzed in regions of the guinea-pig brain with semi-quantitative densitometry of [3H]N-methyl scopolamine binding, a muscarinic antagonist. In the rostral forebrain, high levels of binding were detected in the caudate putamen, nucleus accumbens and olfactory tubercle while intermediate levels of binding were observed in the medial and lateral septum, bed nucleus, and vertical and horizontal limbs of the diagonal band. The hypothalamus displayed binding that ranged from low levels in the preoptic area to intermediate levels in the mammillary nucleus. In limbic areas such as the thalamus, amygdala and hippocampus, a heterogeneous pattern of binding was evident in various subregions which tended to correspond with known innervation by cholinergic afferents. In the midbrain, binding was high in the superficial layer of the superior colliculus and the medial geniculate while intermediate binding was recorded in the lateral geniculate and the lateral aspect of the central gray. The pattern of muscarinic binding observed in the brain of the guinea-pig is similar to distributions of this binding site previously reported in the rat brain and the human brain.

Topographical projection of cholinergic neurons in the basal forebrain to the cingulate cortex in the rat

The cholinergic innervation of the rat's posterior cingulate cortex (Brodmann's area 29) was studied using acetylcholinesterase (AChE) histochemistry. Electrolytic lesion of the ipsilateral medial septum and diagonal band region (MS-DB) reduced the diffuse AChE staining in layers I, II, III and V of the cingulate cortex. Kainic acid lesion of the ipsilateral globus pallidus and substantia innominata area (GP-SI) abolished the dense band of AChE stain in layer IV, with small reductions of AChE stain in other layers. The results indicate that the medial cholinergic pathway from MS-DB terminates diffusely in layers I, II, III and V while the lateral cholinergic pathway from the GP-SI predominantly ends in layer IV of the posterior cingulate cortex.

Age-impaired impulse flow from nucleus basalis to cortex

Recent studies have renewed interest in the role of acetylcholine (ACh) in the cognitive changes associated with ageing and dementia. Deficits in cortical choline acetyltransferase (ChAT) in Alzheimer's disease have been consistently demonstrated, while other research has suggested a connection between deterioration of cortical ACh fibres and dementia. However, despite clear biochemical and anatomical evidence for a fall in ACh in dementia, results of therapeutic trials with cholinergic agonists, precursors and cholinesterase inhibitors have been inconsistent. Such findings suggest that cortical cholinergic disorders are not wholly a function of simple biochemical change; alterations of impulse flow along cholinergic fibres could well be as debilitating. An important extrinsic source of cortical ACh innervation derives from neurones diffusely located in rat basal forebrain, denoted the nucleus basalis (NB). We have now investigated the impulse conduction properties of cortically projecting, putatively cholinergic NB axons in adult and aged rats and have found that conduction latencies from NB to frontal cortex are significantly longer (by 51%) in aged animals. In addition, systematic analysis varying cortical stimulation depth revealed that these longer latencies are due entirely to decreased conduction velocities in the subcortical fibre projections. Indeed, intracortical velocities were virtually identical in the two groups. Our results indicate that ageing occasions a decrease in the temporal fidelity of impulse flow in the cholinergic input to the cortex from the NB, a previously overlooked but potentially important element in cognitive deficits that occur with age.

Transplantation of embryonic ventral forebrain neurons to the neocortex of rats with lesions of nucleus basalis magnocellularis--I. Biochemical and anatomical observations

Unilateral ibotenic acid lesions of the rat nucleus basalis magnocellularis produce approximately 60% depletion of choline acetyltransferase activity in ipsilateral frontal and frontoparietal neocortex. This depletion, which represents the loss of most of the extrinsic neocortical cholinergic input, is stable for at least 6 months. Embryonic ventral forebrain neurons survive transplantation to such cholinergically denervated neocortex. Cholinergic cells abound within these transplants and appear able to reinnervate the cholinergically depleted host cortex, as assessed histochemically and by measurement of choline acetyltransferase activity. Outgrowing fibres may extend beyond 2 mm from the grafts and often appear to be organized in an appropriate laminar pattern within the host cortex. Peptidergic neurons are sparse within the grafts and their fibres frequently appear unable to grow into the host tissue. Control grafts of non-cholinergic embryonic hippocampal cells survive well but have no effect on cortical depletions of acetylcholinesterase or choline acetyltransferase activity. Reconstruction of the extrinsic cholinergic input to the cortex by transplantation provides a useful tool for understanding the functions of this pathway.

Stability of numbers but not size of mouse forebrain cholinergic neurons to 53 months

In normal mammalian aging there is a reduction of cholinergic markers in a variety of regions. To determine whether this reduction is related to reduced numbers of basal forebrain cholinergic neurons, we counted the number and measured the sizes of the magnocellular acetylcholinesterase-positive neurons in this region of 7, 15, and 53-month-old C57Bl/6NNIA mice. Data were collected from coded slides containing the medial septum, nucleus of the diagonal band, magnocellular preoptic nucleus, and nucleus basalis magnocellularis. There was no decline in numbers of basal forebrain acetylcholinesterase-positive neurons in any of the regions studied. However, cell sizes showed a progressive age-related decline which was greatest in the nucleus basalis magnocellularis.

Development of cholinergic markers in mouse forebrain. I. Choline acetyltransferase enzyme activity and acetylcholinesterase histochemistry

Measurements of choline acetyltransferase (ChAT) activity were made during the development of the neocortical cholinergic innervation, and correlated with the development of the acetylcholinesterase (AChE) staining pattern in mouse cerebral cortex and several other areas of the forebrain between the time of initial onset and maturity ChAT activity can first be measured on postnatal day 6 (P6). The enzyme reaches 40% of adult activity by P18 and adult values by 7 weeks postnatal. The onset of AChE staining varies for different regions of the forebrain and for various areas within the cerebral cortex. The earliest appearance of AChE is seen in several basal forebrain nuclei including the striatum, the ventromedial region of the globus pallidus and the hypothalamus on embryonic day 18 (E18). In neocortex and olfactory cortex, AChE-stained axons are seen in the white matter before birth, but do not enter cingulate cortex and hippocampus until P2. By P2. almost all areas of the basal forebrain and diencephalon have acquired some AChE staining pattern. The adult distribution of AChE staining is reached by 3 weeks postnatal in all areas of the forebrain. Adult cerebral cortex shows a characteristic pattern of alternating AChE dense and AChE sparse bands which vary in depth depending on the cortical area. The cortical banding pattern develops in an 'inside-out' fashion, starting in layer VI and gradually entering more superficial layers. In parallel with the AChE pattern of development in cortex, transient AChE staining can be observed in some thalamic nuclei and in some forebrain fiber systems. In the neostriatum patches of intense AChE staining first develop along the ventrolateral border, then spread throughout the whole nucleus and finally coalesce to a uniform high density over the entire neostriatum. We discuss the close spatial and temporal correspondence between AChE pattern development and reported data on synapse formation, and speculate on the role of the cortical cholinergic system in development.

Catecholaminergic innervation of the septal area in man: immunocytochemical study using TH and DBH antibodies

The catecholaminergic innervation of the human septal area and closely related structures has been visualized by using tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) as immunocytochemical markers. TH-like immunoreactivity with no corresponding DBH labelling was considered to be indicative of dopaminergic fibers. Catecholaminergic innervation offered the following similarities to that of rodents: moderate innervation in the medial septal division, with predominant DBH immunolabelling; dense dopaminergic innervation in the lateral septal nuclei, organized in a laminar pattern; presence of dopaminergic pericellular arrangements in the dorsal septum and bed n. of the stria terminalis; clustering of dopaminergic terminals in n. accumbens associated with a medioventral zone of DBH-like immunoreactive fibers; close overlap between dopaminergic fields and acetylcholinesterase-reactive zones in both the lateral septum and the n. of the stria terminalis. Differences with the catecholaminergic septal innervation of rodents consisted of general caudal extension of the dopaminergic fields, possibly accounted for by the vertical stretching and caudal displacement of the septal nuclei in man; complementary lateromedial topography of dopaminergic and DBH-immunoreactive inputs in the n. of the stria terminalis as opposed to their dorsoventral organization in rodents; presence of TH-immunolabelled cell group in the anterior olfactory nucleus and parolfactory cortex, which seems specific for primates. Precise topographical mapping of the catecholaminergic structures in this central region of the limbic forebrain seems to be a prerequisite for accurate tissue sampling in the biochemical investigations of pathological cases and should help in the interpretation of aminergic dysfunction in a variety of human diseases.

Memory impairments following basal forebrain lesions

The functional contribution of the nucleus basalis magnocellularis (NBM) and medial septal area (MSA) to memory was evaluated in 4 behavioral tasks. The tasks were postoperative acquisition of a win-stay spatial discrimination in a T-maze, a win-shift spatial discrimination on a radial arm maze, active avoidance in a shuttle box, and passive avoidance in a shuttle box. Bilateral lesions were made by injecting ibotenic acid (IBO) into the NBM or MSA. Control rats received operations in which no neurotoxin was injected. When compared to controls, rats with lesions in either the NBM or MSA had significantly impaired choice accuracy in the T-maze and radial maze tasks, took significantly fewer trials to reach criterion in the acquisition, but not the retention of an active avoidance task, and significantly more trials to reach criterion in the passive avoidance task. The results show that equivalent behavioral changes are obtained from lesions in the NBM and MSA in tasks that vary in their type of motivation, reinforcement, response-reinforcement contingency, and response. These behavioral changes suggest that the NBM and MSA may both be involved in memory.

Cholinergic modulation of mediodorsal thalamic input into cingulate cortex

Field potentials in cingulate cortex (area 24) produced by electrical stimulation of the mediodorsal thalamic nucleus were diminished by iontophoretic ejection of the cholinergic agonist, carbachol. The effect was frequency dependent: field potentials produced by 7.0 Hz stimulation were reduced by 34%. Potentials produced by 0.5 Hz stimulation were not significantly changed. This reduction was blocked by muscarinic but not nicotinic antagonists.

A correlated light and electron microscopic study of identified cholinergic basal forebrain neurons that project to the cortex in the rat

Cholinergic neurons in the basal forebrain which project to the frontal cortex were studied by combining the retrograde transport of a conjugate of horseradish peroxidase and wheat germ agglutinin with choline acetyltransferase immunohistochemistry. Neurons that were both retrogradely labelled and immunoreactive were found on the medial, lateral, and ventral borders of the globus pallidus, within the globus pallidus, as well as in the substantia innominata and ventral pallidum region. The cell bodies averaged 31 by 19 micron in size and had sparsely branching dendrites. Cells which were labelled by both techniques were first characterised in the light microscope and then studied in the electron microscope. The perikarya had large amounts of cytoplasm with abundant organelles. The nuclei were indented, were usually eccentrically placed, and contained prominent nucleoli. The synaptic input onto the cell bodies and their dendrites was studied in serial sections. The synaptic input onto the perikarya and proximal dendrites was sparse but the density increased on more distal regions of the dendrites. Subjunctional bodies were associated with the postsynaptic membrane in 20-30% of the synaptic contacts and these were classified as asymmetrical; the remaining contacts could not be classified because of an association of the immunoreaction product with the postsynaptic membrane. The synaptic input to these cells was distinctly different from that onto typical globus pallidus cells, the perikarya and dendrites of which were characteristically ensheathed in synaptic boutons.

Noncollateral projections of basal forebrain neurons to frontal and parietal neocortex in primates

To test the hypothesis that axons of the basal forebrain cholinergic system collateralize to innervate widely separated areas of cortex, two distinct, retrogradely transported fluorescent dyes were injected into discrete neocortical regions of three macaques. In two monkeys, True Blue was injected into parietal cortex and Nuclear Yellow into frontal cortex; in a third monkey, placement of the dyes was reversed. Following these large (3-10 microliters total) injections, neurons single labeled with either Nuclear Yellow or True Blue were seen throughout most of the ipsilateral nucleus basalis of Meynert and nucleus of the diagonal band of Broca. Neurons projecting to either frontal or parietal cortex were most heavily concentrated in the anteromedial aspect of the basal forebrain. A small number of labeled neurons was also seen in the contralateral basal forebrain. Cells single labeled with either True Blue or Nuclear Yellow were frequently adjacent to one another, but in no case was a neuron labeled with both dyes. Thus, individual neurons of the basal forebrain complex do not appear to innervate both frontal and parietal lobes of monkeys. This finding is consistent with recent studies in rodents which suggest that basal forebrain neurons innervate relatively small, restricted cortical fields.

Profound disturbances of spontaneous and learned behaviors following lesions of the nucleus basalis magnocellularis in the rat

It has been shown that a marked decline in the cortical activity of the cholinergic synthesizing enzyme choline-acetyltransferase (ChAT), accompanied by a severe neuronal loss in the nucleus basalis magnocellularis of Meynert occurs in the brains of patients with senile dementia of the Alzheimer type. However, the functional role of these neurons is largely unknown. In fact, very few studies have been done in animals. In this paper we report the behavioral effects of the lesion of the nucleus basalis magnocellularis in the rat either by radiofrequency current or by ibotenic acid injection at the level of the cell bodies. The two kinds of lesion lead to a profound disturbance of spontaneous and learned behaviors. There is a complete disorganization of behavior which is evidenced by an enhanced locomotor activity, an alteration in alimentary and hoarding behavior. In addition, we observed a deterioration of spatial memory and an incapacity to reverse a previously learned response. Biochemical assay showed that radiofrequency and ibotenic acid lesions produced a decrease of ChAT activity in the prefrontal and sensorimotor cortices and in amygdala without affecting the hippocampus or striatum. Ibotenic acid lesions seem to specifically destroy the cell bodies of the nucleus basalis magnocellularis since the dopaminergic and noradrenergic fibers of passage remained intact as measured by the unchanged level of endogenous catecholamine concentration in the terminal region in the prefrontal cortex. Presently, it cannot be said that the behavioral syndrome results solely from the lesion of the cholinergic neurons. Also, it is likely that the lesion of the nucleus basalis magnocellularis in the rat does not exactly reproduce the behavioral syndrome observed in Alzheimer's disease in man. However, this experimental approach in leading to a better knowledge of the functioning of these neurones could improve our understanding of this disease.

Organization of cerebral cortical afferent systems in the rat. II. Hypothalamocortical projections

The organization of hypothalamic projections to the cerebral cortex in the rat has been studied using retrograde and anterograde tracer methods. Four separate populations of hypothalamic neurons, which constitute a major source of diffuse cortical innervation, were identified: Tuberal lateral hypothalamic (LHAt) neurons which innervate the cerebral cortex tend to cluster in the perifornical region, in the zona incerta, and along the medial edge of the cerebral peduncle, at levels roughly coextensive with the ventromedial hypothalamic nucleus. Most of these neurons project to the ipsilateral cortex; a small percentage innervate the contralateral cortex, but this varies among cortical terminal fields. The perifornical neurons are organized in a roughly topographic medial-to-lateral relationship with respect to their cortical terminal fields. Field of Forel (FF) neurons, which project primarily to the frontal cortex of the ipsilateral hemisphere, are located just ventral to the medial edge of the medial lemniscus, at the level of the ventromedial basal thalamic nucleus. The more laterally placed neurons innervate the lateral frontal, insular and perirhinal cortex; the more medial neurons, around the mammillothalamic tract, innervate the medial frontopolar, prelimbic, infralimbic, and anterior cingulate cortex. Posterior lateral hypothalamic (LHAp) neurons form a dense cluster spanning the lateral hypothalamus, from the cerebral peduncle to the posterior hypothalamic area at premammillary levels, and extending into the supramammillary nucleus and the adjacent ventral tegmental area. LHAp neurons innervate the entire cerebral cortex, predominantly on the ipsilateral side. Populations of LHAp neurons projecting to different cortical target areas show considerable spatial overlap, but computer plots of the centers of these populations demonstrate a strict topographic relationship with respect to the cerebral cortex. Tuberomammillary (TMN) neurons form a sheet along the ventrolateral surface of the premammillary hypothalamus. About twice as many TMN neurons innervate the ipsilateral, as compared to the contralateral hemisphere; it is not known whether single neurons project to both hemispheres. No topographic organization of the TMN cortical projection is apparent. Injections of different-colored fluorescent dyes into various cortical areas demonstrate that hypothalamic neurons in general have rather restricted cortical terminal fields. Only occasional neurons are found, primarily in LHAt, which are double labeled by injections into different cytoarchitectonic areas.(ABSTRACT TRUNCATED AT 400 WORDS)

Basal forebrain innervation of rodent neocortex: studies using acetylcholinesterase histochemistry, Golgi and lesion strategies

Acetylcholinesterase (AChE)-rich projections from basal forebrain to neocortex cerebri were characterized in the present study. The purpose was to investigate 3 aspects of these projections in rats and mice that have been incompletely described in previous work: intracortical organization of the fibers, subcortical pathways and axonal branching patterns of individual basal forebrain neurons. AChE histochemistry, lesions and Golgi impregnations were the principal strategies employed in this light microscopic study. The moderately dense, AChE-stained innervation of neocortex can be altered by intracortical lesions. The results depended on the region involved and the orientation of the lesion. Sagittal knife cuts had barely detectable effects, regardless of sites. Coronal knife cut lesions in medial cortex resulted in substantial loss of staining in cingulate and medial occipital fields. In contrast, coronal lesions of lateral or anterior cortex produce only small zonal reductions in staining. The interpretation of the latter findings that we favor is that AChE-rich basal forebrain fibers enter lateral/anterior cortex and branch densely there, but in tangentially limited and overlapping terminal domains. Observations on the topography and targets of AChE-rich basal forebrain cortical afferents revealed that the fibers could be grouped based on certain characteristics. Three sets of fibers were distinguishable: anterior pathway innervating cortex of the frontal pole. These fibers were traceable to the region of the substantia innominata/nucleus basalis. They crossed the neostriatum and external capsule in the sagittal plane, forming in 3 dimensions an orderly sheet-like array of fibers bridging the anteroventral surface of the neostriatum with nearby polar cortex medial pathway innervating cingulate and medial occipital cortex. Emerging predominantly from the region of the diagonal band, the fibers run caudally as a triangular bundle in deep layer VI of cingulate cortex. lateral pathway innervating most of remaining lateral neocortex. The fibers radiate out from substantia innominata/nucleus basalis with a complex 3-dimensional organization. In all pathways, fibers enter and initially run within layer VI before ascending pialward, although the intracortical course in layer VI differs between pathways. These fibers primarily terminate in layer V with a secondary concentration in layer I. However, the latter appears to receive substantial AChE-stained inputs from other sources, possibly intracortical, as well. The pathways overlap at their respective boundary zones. This system is comparably organized in rats and mice.(ABSTRACT TRUNCATED AT 400 WORDS)

The pattern of cortical projections from the intermediate parts of the magnocellular nucleus basalis in the rat demonstrated by tracing with Phaseolus vulgaris-leucoagglutinin

The pattern and distribution of the cortical projections from intermediate parts of the cholinergic basal magnocellular nucleus were studied by anterogradely transported Phaseolus vulgaris-leucoagglutinin. This immunocytochemical tracing technique reveals the detailed morphology and distribution of efferents from this intermediate area in the nucleus basalis to the various areas and layers of cortex and amygdala. Major projections with a relatively high density of terminal boutons were found in layers I, II and VI of the frontal cortex, in layers V and VI of parietal and temporal areas, in the entire perirhinal and entorhinal cortices, and in the basolateral nucleus of the amygdaloid body. From the nucleus basalis area studied, few if any projections could be demonstrated to cingulate and occipital cortical regions.

Distribution of central cholinergic neurons in the baboon (Papio papio). I. General morphology

The morphological characteristics of cholinergic neurons in the central nervous system (CNS) of the baboon (Papio papio) were studied by choline acetyltransferase (ChAT) immunohistochemistry and acetylcholinesterase (AChE) pharmacohistochemistry. The distributions of central cholinergic neurons as visualized by these two histochemical techniques were similar in most, but not all regions of the brain and spinal cord. Based upon these observations, central cholinergic neurons that are immunoreactive to ChAT and intensely stained for AChE by the pharmacohistochemical procedure can be divided into four major groups: (1) those in the caudate nucleus, putamen, nucleus accumbens and anterior perforated substance. These ChAT-containing and AChE-intense neurons are large and multipolar, and are scattered throughout these structures. (2) The rostral cholinergic column, which consists of a continuous mass of cholinergic perikarya situated in the medial septal nucleus, nucleus of the diagonal band, and nucleus basalis (Meynert). The ChAT-immunoreactive and AChE-intense cell bodies of the nucleus basalis are a prominent feature in the basal forebrain of the baboon. The labeled neurons are large, multipolar, and hyperchromic and show a tendency to aggregate in cell clusters. These cells are distributed within the full extent of the substantia innominata, often being associated with subcortical fiber networks such as the medullary laminae of the globus pallidus. (3) The caudal cholinergic column, which consists of a continuous group of cholinergic neurons in the caudal midbrain and pontine tegmentum. The rostral component of this group of cells is the nucleus tegmenti pedunculopontinus (subnucleus compacta) and it extends caudally to include the laterodorsal tegmental nucleus. Compared to that in other species the nucleus tegmenti pedunculopontinus in the baboon appears to occupy a relatively greater volume and is composed of a greater number of cholinergic neurons. The cells of the caudal column are large and hyperchromic. (4) Nuclei of origin of somatic and visceral efferents of the cranial nerves (III, IV, V, VI, VII, IX, X, XI, XII) and spinal nerves. In addition to these major cholinergic cell groups, a small population of ChAT-positive and AChE-intense cell bodies can be observed at the floor of the fourth ventricle and in lamina VII and X of the cervical cord. The present findings indicate that although some differences exist, the overall distribution and morphological features of cholinergic cell bodies identified in the baboon brain and spinal cord are similar to those demonstrated previously in investigations of the rhesus monkey and nonprimates.

Efferent connections of the ventral pallidum: evidence of a dual striato pallidofugal pathway

Previous histological and histochemical studies have provided evidence that the globus pallidus (external pallidal segment) as conventionally delineated in the rat extends ventrally and rostrally beneath the transverse limb of the anterior commissure, invading the olfactory tubercle with its most ventral ramifications. This infracommissural subdivision of the globus pallidus or ventral pallidum (VP) is most selectively identified by being pervaded by a dense plexus of substance-P-positive striatofugal fibers; the extent of this plexus indicates that the VP behind the anterior commissure continues dorsally over some distance into the anteroventromedial part of the generally recognized (supracommissural) globus pallidus; the adjoining anterodorsolateral pallidal region, here named dorsal pallidum (DP), receives only few substance-P-positive fibers, but contains a dense plexus of enkephalin-positive striatal afferents that also pervades VP. Available autoradiographic data indicate that VP and DP receive their striatal innervation from two different subdivisions of the striatum: whereas VP is innervated by a large, anteroventromedial striatal region receiving substantial inputs from a variety of limbic and limbic-system-associated structures (and therefore called "limbic striatum"), DP receives its striatal input from an anterodorsolateral striatal sector receiving only sparse limbic afferents ("nonlimbic" striatum) but instead heavily innervated by the sensorimotor cortex. The present autoradiographic study has produced evidence that this dichotomy in the striatopallidal projection is to a large extent continued beyond the globus pallidus: whereas the efferents of DP were traced to the subthalamic nucleus and substantia nigra, those of VP were found to involve not only the subthalamic nucleus and substantia nigra but also the frontocingulate (and adjoining medial sensorimotor) cortex, the amygdala, lateral habenular and mediodorsal thalamic nucleus, hypothalamus, ventral tegmental area, and tegmental regions farther caudal and dorsal in the midbrain. These findings indicate that the ventral pallidum can convey striatopallidal outflow of limbic antecedents not only into extrapyramidal circuits but also back into the circuitry of the limbic system.

Glutamic acid decarboxylase activity decreases in mouse neocortex after lesions of the basal forebrain

Glutamic acid decarboxylase (GAD) activity was measured in the cerebral cortex of animals after acute and chronic lesions to basal forebrain cholinergic nuclei. Such lesions were shown to result in an extensive depletion of cholinergic markers in parietal cerebral cortex. A statistically significant 30% decrease in GAD activity was first detected at 6 weeks postlesion and was still measurable 8 months after the lesion. These results suggest that cholinergic inputs to cortex indirectly or directly influence GABAergic transmission in cortex.

Cholinergic projections from the basal forebrain to the basolateral amygdaloid complex: a combined retrograde fluorescent and immunohistochemical study

We have examined the location of cholinergic and non-cholinergic neurons that project to the rat basolateral amygdaloid nucleus by using choline acetyltransferase (ChAT) immunohistochemistry in combination with retrograde fluorescent tracing on the same tissue section. Since many tracer-and ChAT-positive neurons were identified in basal forebrain areas, including the ventral pallidum, we also stained many of the sections for glutamate decarboxylase, a suitable marker for the delineation of pallidal areas. Cholinergic neurons projecting to the basolateral amygdaloid nucleus were observed in a continuous territory stretching from the dorsal part of ventral pallidum, through sublenticular substantia innominata to ventral parts of globus pallidus and peripallidal areas. Non-cholinergic neurons projecting to the basolateral amygdaloid nucleus were found intermixed within the same structures and constitute approximately 25% of the amygdalopetal projection neurons in these ventral forebrain structures. Since amygdalopetal cholinergic neurons were demonstrated in areas generally recognized as giving rise to cholinergic projections to cerebral cortex, several retrograde double-labeling experiments with two different fluorescent tracers were performed for the purpose of detecting the possible existence of collateral projections. The results obtained showed that the cholinergic basal forebrain neurons in general project to only one forebrain region, and, furthermore, that the cholinergic system consists of partially overlapping subsets of neurons that project to various neocortical and allocortical areas and to the amygdaloid body.

Immunocytochemical localization of choline acetyltransferase in rat cerebral cortex: a study of cholinergic neurons and synapses

Choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme and a definitive marker for cholinergic neurons, was localized immunocytochemically in the motor and somatic sensory regions of rat cerebral cortex with monoclonal antibodies. ChAT-positive (ChAT+) varicose fibers and terminal-like structures were distributed in a loose network throughout the cortex. Some immunoreactive cortical fibers were continuous with those in the white matter underlying the cortex, and many of these fibers presumably originated from subcortical cholinergic neurons. ChAT+ fibers appeared to be rather evenly distributed throughout all layers of the motor cortex, but a subtle laminar pattern was evident in the somatic sensory cortex, where lower concentrations of fibers in layer IV contrasted with higher concentrations in layer V. Electron microscopy demonstrated that immunoreaction product was concentrated in synaptic vesicle-filled profiles and that many of these structures formed synaptic contacts. ChAT+ synapses were present in all cortical layers, and the majority were of the symmetric type, although a few asymmetric ones were also observed. The most common postsynaptic elements were small to medium-sized dendritic shafts of unidentified origin. In addition, ChAT+ terminals formed synaptic contacts with apical and, probably, basilar dendrites of pyramidal neurons, as well as with the somata of ChAT-negative nonpyramidal neurons. ChAT+ cell bodies were present throughout cortical layers II-VI, but were most concentrated in layers II-III. The somata were small in size, and the majority of ChAT+ neurons were bipolar in form, displaying vertically oriented dendrites that often extended across several cortical layers. Electron microscopy confirmed the presence of immunoreaction product within the cytoplasm of small neurons and revealed that they received both symmetric and asymmetric synapses on their somata and proximal dendrites. These observations support an identification of ChAT+ cells as nonpyramidal intrinsic neurons and thus indicate that there is an intrinsic source of cholinergic innervation of the rat cerebral cortex, as well as the previously described extrinsic sources.

GABAergic control of the cholinergic projections to the frontal cortex is not tonic

The turnover rate of acetylcholine was measured in the frontal cortex of rats after either microinjection of bicuculline into the substantia inominata (the source of the cortical cholinergic innervation) or kainic acid lesioning of the nucleus accumbens (the source of GABAergic innervation of the substantia inominata). Neither treatment affected cortical acetylcholine metabolism, suggesting that the GABAergic inhibition of the substantia inominata-cortical cholinergic pathway is not tonic.

Ultrastructural evidence of amygdalofugal axons terminating on cholinergic cells of the rostral forebrain

In the present study a double-label ultrastructural procedure was used to study amygdalofugal fibers contacting cholinergic cells of the rostral forebrain. Following horseradish peroxidase (HRP) injections into the basolateral amygdala, anterogradely transported HRP was detected in axon terminals contacting the dendrites of choline acetyltransferase-containing cells in the ventral pallidum.

A cytoarchitectonic and histochemical study of nucleus basalis and associated cell groups in the normal human brain

Several recent studies have reported loss of neurons in the nucleus basalis in Alzheimer's disease. However, few detailed studies of the normal distribution of these neurons in the human brain have appeared. We have used Nissl staining and acetylcholinesterase histochemical staining of the human basal forebrain, alone or in combination to identify the organization of the nucleus basalis and associated cell groups, (or collectively, the magnocellular basal nucleus) in the normal human brain. The magnocellular basal nucleus includes a series of clusters of neurons and scattered perikarya extending from the medial septum and diagonal band nucleus rostrally, through the substantia innominata to the furthest caudal extent of the globus pallidus. This distribution is similar to that which has been described in the monkey. Furthermore, acetylcholinesterase-positive fibers in the human brain are seen in the two major pathways that have been identified as carrying magnocellular basal nucleus axons to the cerebral cortex in other species. These observations suggest that the topographic organization of the magnocellular basal projection to cerebral cortex in other species probably exists in man as well. It will therefore be important in future studies of the fate of these neurons in neurological degenerative diseases to assess the loss of neurons in the different components of the magnocellular basal nucleus in relation to the clinical evidence for dysfunction in the cortical areas which they innervate.