Cholinergic neurons of the nucleus basalis of Meynert receive cholinergic, catecholaminergic and GABAergic synapses: an electron microscopic investigation in the monkey

The Neuroanatomy of Induced Pluripotent Stem Cells: In Vitro Models of Subcortical Nuclei in Neurodegenerative Disorders

Neuromodulatory subcortical systems (NSSs) are monoaminergic and cholinergic neuronal groups that are markedly and precociously involved in the pathogenesis of many neurodegenerative disorders (NDDs), including Parkinson's and Alzheimer's diseases. In humans, although many tools have been developed to infer information on these nuclei, encompassing neuroimaging and neurophysiological methods, a detailed and specific direct evaluation of their cellular features in vivo has been difficult to obtain until recent years. The development of induced pluripotent stem cell (iPSC) models has allowed research to deeply delve into the cellular and molecular biology of NSS neurons. In fact, iPSCs can be produced easily and non-invasively from patients' fibroblasts or circulating blood monocytes, by de-differentiating those cells using specific protocols, and then be re-differentiated towards neural phenotypes, which may reproduce the specific features of the correspondent brain neurons (including NSS ones) from the same patient. In this review, we summarized findings obtained in the field of NDDs using iPSCs, with the aim to understand how reliably these might represent in vitro models of NSS. We found that most of the current literature in the field of iPSCs and NSSs in NDDs has focused on midbrain dopaminergic neurons in Parkinson's disease, providing interesting results on cellular pathophysiology and even leading to the first human autologous transplantation. Differentiation protocols for noradrenergic, cholinergic, and serotoninergic neurons have also been recently defined and published. Thus, it might be expected that in the near future, this approach could extend to other NSSs and other NDDs.

Differential cholinergic innervation of lemniscal versus non-lemniscal regions of the inferior colliculus

The inferior colliculus (IC), a midbrain hub for integration of auditory information, receives dense cholinergic input that could modulate nearly all aspects of hearing. A key step in understanding cholinergic modulation is to identify the source(s) and termination patterns of cholinergic input. These issues have not been addressed for the IC in mice, an increasingly important model for study of hearing. We examined cholinergic inputs to the IC in adult male and female mice. We used retrograde tracing and immunochemistry to identify three sources of cholinergic innervation of the mouse IC: the pedunculopontine tegmental nucleus (PPT), the laterodorsal tegmental nucleus (LDT) and the lateral paragigantocellular nucleus (LPGi). We then used Cre-dependent labeling of cholinergic neurons in normal-hearing ChAT-Cre mice to selectively label the cholinergic projections to the IC from each of the cholinergic sources. Labeling of cholinergic projections from the PPT and LDT revealed cholinergic axons and boutons terminating throughout the IC, with the ipsilateral projection being denser. Electron microscopic examination showed that these cholinergic axons can form traditional synaptic junctions with IC neurons. In separate experiments, selective labeling of cholinergic projections from the LPGi revealed bilateral projections to the IC. The LPGi axons exhibited relatively equal densities on ipsilateral and contralateral sides, but on both sides the terminations were largely restricted to the non-lemniscal regions of the IC (i.e., the dorsal cortex, lateral cortex and intercollicular tegmentum). We conclude first that cholinergic axons can form traditional synapses in the IC. In addition, lemniscal and non-lemniscal regions of the IC receive different patterns of cholinergic innervation. The lemniscal IC (IC central nucleus) is innervated by cholinergic neurons in the PPT and the LDT whereas the non-lemniscal "shell" areas of the IC are innervated by the PPT and LDT and by cholinergic neurons in the LPGi. DATA AVAILABILITY: Data will be made available on request.

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.

Cognitive Aging and the Primate Basal Forebrain Revisited: Disproportionate GABAergic Vulnerability Revealed

Basal forebrain (BF) projections to the hippocampus and cortex are anatomically positioned to influence a broad range of cognitive capacities that are known to decline in normal aging, including executive function and memory. Although a long history of research on neurocognitive aging has focused on the role of the cholinergic basal forebrain system, intermingled GABAergic cells are numerically as prominent and well positioned to regulate the activity of their cortical projection targets, including the hippocampus and prefrontal cortex. The effects of aging on noncholinergic BF neurons in primates, however, are largely unknown. In this study, we conducted quantitative morphometric analyses in brains from young adult (6 females, 2 males) and aged (11 females, 5 males) rhesus monkeys (Macaca mulatta) that displayed significant impairment on standard tests that require the prefrontal cortex and hippocampus. Cholinergic (ChAT+) and GABAergic (GAD67+) neurons were quantified through the full rostrocaudal extent of the BF. Total BF immunopositive neuron number (ChAT+ plus GAD67+) was significantly lower in aged monkeys compared with young, largely because of fewer GAD67+ cells. Additionally, GAD67+ neuron volume was greater selectively in aged monkeys without cognitive impairment compared with young monkeys. These findings indicate that the GABAergic component of the primate BF is disproportionally vulnerable to aging, implying a loss of inhibitory drive to cortical circuitry. Moreover, adaptive reorganization of the GABAergic circuitry may contribute to successful neurocognitive outcomes.SIGNIFICANCE STATEMENT A long history of research has confirmed the role of the basal forebrain in cognitive aging. The majority of that work has focused on BF cholinergic neurons that innervate the cortical mantle. Codistributed BF GABAergic populations are also well positioned to influence cognitive function, yet little is known about this prominent neuronal population in the aged brain. In this unprecedented quantitative comparison of both cholinergic and GABAergic BF neurons in young and aged rhesus macaques, we found that neuron number is significantly reduced in the aged BF compared with young, and that this reduction is disproportionately because of a loss of GABAergic neurons. Together, our findings encourage a new perspective on the functional organization of the primate BF in neurocognitive aging.

Corticotropin releasing factor in the nucleus basalis of Meynert impairs attentional performance and reduces levels of glutamate and taurine in male and female rats

Psychiatric disorders that are characterized by impairments in sustained attention, including attention deficit hyperactivity disorder (ADHD), post-traumatic stress disorder (PTSD), and major depression are also sensitive to exacerbation by stress. Sustained attention relies on cholinergic and non-cholinergic projections from the nucleus basalis of Meynert (NBM) in the basal forebrain to the medial prefrontal cortex (mPFC). We have previously shown that central administration of the stress neuropeptide corticotropin releasing factor (CRF) impairs performance on the sustained attention task (SAT) in adult male and female rats. The present study investigated whether this effect was mediated by CRF's action in the NBM. Rats were administered CRF in the NBM and subsequent SAT performance was measured. A high dose of CRF (100 ng) significantly impaired performance on non-signaled events across sex. Because performance on non-signaled events is believed to depend on non-cholinergic (i.e., GABA and glutamate) signaling, high performance liquid chromatography was used to quantify amino acid levels in the NBM and mPFC. We found females have higher levels of glutamate, glutamine, GABA glycine, and alanine in the NBM than males. Importantly, CRF in the NBM led to a local decrease of taurine and several amino acids involved in glutamate synthesis in males and females, changes which may mediate the CRF-induced SAT performance deficit. Together these studies suggest that CRF regulation of amino acids in the NMB contributes to stress-induced attention deficits.

The Neurodegenerative Elderly Syndrome (NES) hypothesis: Alzheimer and Parkinson are two faces of the same disease

Increasing evidence suggests that Alzheimer's disease (AD) and Parkinson's disease (PD) share monoamine and alpha-synuclein (αSyn) dysfunctions, often beginning years before clinical manifestations onset. The triggers for these impairments and the causes leading these early neurodegenerative processes to become AD or PD remain unclear. We address these issues by proposing a radically new perspective to frame AD and PD: they are different manifestations of one only disease we call "Neurodegenerative Elderly Syndrome (NES)". NES goes through three phases. The seeding stage, which starts years before clinical signs, and where the part of the brain-body affected by the initial αSyn and monoamine dysfunctions, influences the future possible progression of NES towards PD or AD. The compensatory stage, where the clinical symptoms are still silent thanks to compensatory mechanisms keeping monoamine concentrations homeostasis. The bifurcation stage, where NES becomes AD or PD. We present recent literature supporting NES and discuss how this hypothesis could radically change the comprehension of AD and PD comorbidities and the design of novel system-level diagnostic and therapeutic actions.

The Neuromodulatory Role of the Noradrenergic and Cholinergic Systems and Their Interplay in Cognitive Functions: A Focused Review

The noradrenergic and cholinergic modulation of functionally distinct regions of the brain has become one of the primary organizational principles behind understanding the contribution of each system to the diversity of neural computation in the central nervous system. Decades of work has shown that a diverse family of receptors, stratified across different brain regions, and circuit-specific afferent and efferent projections play a critical role in helping such widespread neuromodulatory systems obtain substantial heterogeneity in neural information processing. This review briefly discusses the anatomical layout of both the noradrenergic and cholinergic systems, as well as the types and distributions of relevant receptors for each system. Previous work characterizing the direct and indirect interaction between these two systems is discussed, especially in the context of higher order cognitive functions such as attention, learning, and the decision-making process. Though a substantial amount of work has been done to characterize the role of each neuromodulator, a cohesive understanding of the region-specific cooperation of these two systems is not yet fully realized. For the field to progress, new experiments will need to be conducted that capitalize on the modular subdivisions of the brain and systematically explore the role of norepinephrine and acetylcholine in each of these subunits and across the full range of receptors expressed in different cell types in these regions.

Computational Modeling of Catecholamines Dysfunction in Alzheimer's Disease at Pre-Plaque Stage

Background:

Alzheimer’s disease (AD) etiopathogenesis remains partially unexplained. The main conceptual framework used to study AD is the Amyloid Cascade Hypothesis, although the failure of recent clinical experimentation seems to reduce its potential in AD research.

Objective:

A possible explanation for the failure of clinical trials is that they are set too late in AD progression. Recent studies suggest that the ventral tegmental area (VTA) degeneration could be one of the first events occurring in AD progression (pre-plaque stage).

Methods:

Here we investigate this hypothesis through a computational model and computer simulations validated with behavioral and neural data from patients.

Results:

We show that VTA degeneration might lead to system-level adjustments of catecholamine release, triggering a sequence of events leading to relevant clinical and pathological signs of AD. These changes consist first in a midfrontal-driven compensatory hyperactivation of both VTA and locus coeruleus (norepinephrine) followed, with the progression of the VTA impairment, by a downregulation of catecholamine release. These processes could then trigger the neural degeneration at the cortical and hippocampal levels, due to the chronic loss of the neuroprotective role of norepinephrine.

Conclusion:

Our novel hypothesis might contribute to the formulation of a wider system-level view of AD which might help to devise early diagnostic and therapeutic interventions.

In vivo MRI Structural and PET Metabolic Connectivity Study of Dopamine Pathways in Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is characterized by an involvement of brain dopamine (DA) circuitry, the presence of which has been associated with emergence of both neuropsychiatric symptoms and cognitive deficits.

OBJECTIVE

In order to investigate whether and how the DA pathways are involved in the pathophysiology of AD, we assessed by in vivo neuroimaging the structural and metabolic alterations of subcortical and cortical DA pathways and targets.

METHODS

We included 54 healthy control participants, 53 amyloid-positive subjects with mild cognitive impairment due to AD (MCI-AD), and 60 amyloid-positive patients with probable dementia due to AD (ADD), all with structural 3T MRI and 18F-FDG-PET scans. We assessed MRI-based gray matter reductions in the MCI-AD and ADD groups within an anatomical a priori-defined Nigrostriatal and Mesocorticolimbic DA pathways, followed by 18F-FDG-PET metabolic connectivity analyses to evaluate network-level metabolic connectivity changes.

RESULTS

We found significant tissue loss in the Mesocorticolimbic over the Nigrostriatal pathway. Atrophy was evident in the ventral striatum, orbitofrontal cortex, and medial temporal lobe structures, and already plateaued in the MCI-AD stage. Degree of atrophy in Mesocorticolimbic regions positively correlated with the severity of depression, anxiety, and apathy in MCI-AD and ADD subgroups. Additionally, we observed significant alterations of metabolic connectivity between the ventral striatum and fronto-cingulate regions in ADD, but not in MCI-AD. There were no metabolic connectivity changes within the Nigrostriatal pathway.

CONCLUSION

Our cross-sectional data support a clinically-meaningful, yet stage-dependent, involvement of the Mesocorticolimbic system in AD. Longitudinal and clinical correlation studies are needed to further establish the relevance of DA system involvement in AD.

Effects of Prolonged Seizures on Basal Forebrain Cholinergic Neurons: Evidence and Potential Clinical Relevance

Seizures originating from limbic structures, especially when prolonged for several minutes/hours up to status epilepticus (SE), can cause specific neurodegenerative phenomena in limbic and subcortical structures. The cholinergic nuclei belonging to the basal forebrain (BF) (namely, medial septal nucleus (MSN), diagonal band of Broca (DBB), and nucleus basalis of Meynert (NBM)) belong to the limbic system, while playing a pivotal role in cognition and sleep-waking cycle. Given the strong interconnections linking these limbic nuclei with limbic cortical structures, a persistent effect of SE originating from limbic structures on cBF morphology is plausible. Nonetheless, only a few experimental studies have addressed this issue. In this review, we describe available data and discuss their significance in the scenario of seizure-induced brain damage. In detail, the manuscript moves from a recent study in a model of focally induced limbic SE, in which the pure effects of seizure spreading through the natural anatomical pathways towards the cholinergic nuclei of BF were tracked by neuronal degeneration. In this experimental setting, a loss of cholinergic neurons was measured in all BF nuclei, to various extents depending on the specific nucleus. These findings are discussed in the light of the effects on the very same nuclei following SE induced by systemic injections of kainate or pilocarpine. The various effects including discrepancies among different studies are discussed. Potential implications for human diseases are included.

A circuit view of deep brain stimulation in Alzheimer's disease and the possible mechanisms

Alzheimer's disease (AD) is characterized by chronic progressive cognitive deterioration frequently accompanied by psychopathological symptoms, including changes in personality and social isolation, which severely reduce quality of life. Currently, no viable therapies or present-day drugs developed for the treatment of AD symptoms are able to slow or reverse AD progression or prevent the advance of neurodegeneration. As such, non-drug alternatives are currently being tested, including deep brain stimulation (DBS). DBS is an established therapy for several neurological and psychiatric indications, such as movement disorders. Studies assessing DBS for other disorders have also found improvements in cognitive function, providing the impetus for clinical trials on DBS for AD. Targets of DBS in AD clinical trials and animal model studies include the fornix, entorhinal cortex (EC), nucleus basalis of Meynert (NBM), and vertical limb of diagonal band (VDB). However, there is still no comprehensive theory explaining the effects of DBS on AD symptoms or a consensus on which targets provide optimal benefits. This article reviews the anatomy of memory circuits related to AD, as well as studies on DBS rescue of AD in these circuits and the possible therapeutic mechanisms.

Neuroimmune nexus of depression and dementia: Shared mechanisms and therapeutic targets

Dysfunctional immune activity is a physiological component of both Alzheimer's disease (AD) and major depressive disorder (MDD). The extent to which altered immune activity influences the development of their respective cognitive symptoms and neuropathologies remains under investigation. It is evident, however, that immune activity affects neuronal function and circuit integrity. In both disorders, alterations are present in similar immune networks and neuroendocrine signalling pathways, immune responses persist in overlapping neuroanatomical locations, and morphological and structural irregularities are noted in similar domains. Epidemiological studies have also linked the two disorders, and their genetic and environmental risk factors intersect along immune-activating pathways and can be synonymous with one another. While each of these disorders individually contains a large degree of heterogeneity, their shared immunological components may link distinct phenotypes within each disorder. This review will therefore highlight the shared immune pathways of AD and MDD, their overlapping neuroanatomical features, and previously applied, as well as novel, approaches to pharmacologically manipulate immune pathways, in each neurological condition. LINKED ARTICLES: This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

Pretangle pathology within cholinergic nucleus basalis neurons coincides with neurotrophic and neurotransmitter receptor gene dysregulation during the progression of Alzheimer's disease

Cholinergic basal forebrain neurons of the nucleus basalis of Meynert (nbM) regulate attentional and memory function and are exquisitely prone to tau pathology and neurofibrillary tangle (NFT) formation during the progression of Alzheimer's disease (AD). nbM neurons require the neurotrophin nerve growth factor (NGF), its cognate receptor TrkA, and the pan-neurotrophin receptor p75NTR for their maintenance and survival. Additionally, nbM neuronal activity and cholinergic tone are regulated by the expression of nicotinic (nAChR) and muscarinic (mAChR) acetylcholine receptors as well as receptors modulating glutamatergic and catecholaminergic afferent signaling. To date, the molecular and cellular relationships between the evolution of tau pathology and nbM neuronal survival remain unknown. To address this knowledge gap, we profiled cholinotrophic pathway genes within nbM neurons immunostained for pS422, a pretangle phosphorylation event preceding tau C-terminal truncation at D421, or dual-labeled for pS422 and TauC3, a later stage tau neo-epitope revealed by this same C-terminal truncation event, via single-population custom microarray analysis. nbM neurons were obtained from postmortem tissues from subjects who died with an antemortem clinical diagnosis of no cognitive impairment (NCI), mild cognitive impairment (MCI), or mild/moderate AD. Quantitative analysis revealed significant downregulation of mRNAs encoding TrkA as well as TrkB, TrkC, and the Trk-mediated downstream pro-survival kinase Akt in pS422+ compared to unlabeled, pS422-negative nbM neurons. In addition, pS422+ neurons displayed a downregulation of transcripts encoding NMDA receptor subunit 2B, metabotropic glutamate receptor 2, D2 dopamine receptor, and β1 adrenoceptor. By contrast, transcripts encoding p75NTR were downregulated in dual-labeled pS422+/TauC3+ neurons. Appearance of the TauC3 epitope was also associated with an upregulation of the α7 nAChR subunit and differential downregulation of the β2 nAChR subunit. Notably, we found that gene expression patterns for each cell phenotype did not differ with clinical diagnosis. However, linear regression revealed that global cognition and Braak stage were predictors of select transcript changes within both unlabeled and pS422+/TauC3- neurons. Taken together, these cell phenotype-specific gene expression profiling data suggest that dysregulation of neurotrophic and neurotransmitter signaling is an early pathogenic mechanism associated with NFT formation in vulnerable nbM neurons and cognitive decline in AD, which may be amenable to therapeutic intervention early in the disease process.

Striatal adenosine A2A receptor neurons control active-period sleep via parvalbumin neurons in external globus pallidus

Dysfunction of the striatum is frequently associated with sleep disturbances. However, its role in sleep-wake regulation has been paid little attention even though the striatum densely expresses adenosine A_2A receptors (A_2ARs), which are essential for adenosine-induced sleep. Here we showed that chemogenetic activation of A_2AR neurons in specific subregions of the striatum induced a remarkable increase in non-rapid eye movement (NREM) sleep. Anatomical mapping and immunoelectron microscopy revealed that striatal A_2AR neurons innervated the external globus pallidus (GPe) in a topographically organized manner and preferentially formed inhibitory synapses with GPe parvalbumin (PV) neurons. Moreover, lesions of GPe PV neurons abolished the sleep-promoting effect of striatal A_2AR neurons. In addition, chemogenetic inhibition of striatal A_2AR neurons led to a significant decrease of NREM sleep at active period, but not inactive period of mice. These findings reveal a prominent contribution of striatal A_2AR neuron/GPe PV neuron circuit in sleep control.

The Neuroanatomy of the Reticular Nucleus Locus Coeruleus in Alzheimer's Disease

Alzheimer’s Disease (AD) features the accumulation of β-amyloid and Tau aggregates, which deposit as extracellular plaques and intracellular neurofibrillary tangles (NFTs), respectively. Neuronal Tau aggregates may appear early in life, in the absence of clinical symptoms. This occurs in the brainstem reticular formation and mostly within Locus Coeruleus (LC), which is consistently affected during AD. LC is the main source of forebrain norepinephrine (NE) and it modulates a variety of functions including sleep-waking cycle, alertness, synaptic plasticity, and memory. The iso-dendritic nature of LC neurons allows their axons to spread NE throughout the whole forebrain. Likewise, a prion-like hypothesis suggests that Tau aggregates may travel along LC axons to reach out cortical neurons. Despite this timing is compatible with cross-sectional studies, there is no actual evidence for a causal relationship between these events. In the present mini-review, we dedicate special emphasis to those various mechanisms that may link degeneration of LC neurons to the onset of AD pathology. This includes the hypothesis that a damage to LC neurons contributes to the onset of dementia due to a loss of neuroprotective effects or, even the chance that, LC degenerates independently from cortical pathology. At the same time, since LC neurons are lost in a variety of neuropsychiatric disorders we considered which molecular mechanism may render these brainstem neurons so vulnerable.

Cholinergic Circuit Control of Postnatal Neurogenesis

New neuron addition via continued neurogenesis in the postnatal/adult mammalian brain presents a distinct form of nervous system plasticity. During embryonic development, precise temporal and spatial patterns of neurogenesis are necessary to create the nervous system architecture. Similar between embryonic and postnatal stages, neurogenic proliferation is regulated by neural stem cell (NSC)-intrinsic mechanisms layered upon cues from their local microenvironmental niche. Following developmental assembly, it remains relatively unclear what may be the key driving forces that sustain continued production of neurons in the postnatal/adult brain. Recent experimental evidence suggests that patterned activity from specific neural circuits can also directly govern postnatal/adult neurogenesis. Here, we review experimental findings that revealed cholinergic modulation, and how patterns of neuronal activity and acetylcholine release may differentially or synergistically activate downstream signaling in NSCs. Higher-order excitatory and inhibitory inputs regulating cholinergic neuron firing, and their implications in neurogenesis control are also considered.

Basal Forebrain Cholinergic System and Memory

Basal forebrain cholinergic neurons constitute a way station for many ascending and descending pathways. These cholinergic neurons have a role in eliciting cortical activation and arousal. It is well established that they are mainly involved in cognitive processes requiring increased levels of arousal, attentive states and/or cortical activation with desynchronized activity in the EEG. These cholinergic neurons are modulated by several afferents of different neurotransmitter systems. Of particular importance within the cortical targets of basal forebrain neurons is the hippocampal cortex. The septohippocampal pathway is a bidirectional pathway constituting the main septal efferent system, which is widely known to be implicated in every memory process investigated. The present work aims to review the main neurotransmitter systems involved in modulating cognitive processes related to learning and memory through modulation of basal forebrain neurons.

Cholinergic circuitry of the human nucleus basalis and its fate in Alzheimer's disease

The nucleus basalis is located at the confluence of the limbic and reticular activating systems. It receives dopaminergic input from the ventral tegmental area/substantia nigra, serotonergic input from the raphe nuclei, and noradrenergic input from the nucleus locus coeruleus. Its cholinergic contingent, known as Ch4, provides the principal source of acetylcholine for the cerebral cortex and amygdala. More than half of presynaptic varicosities along its cholinergic axons make traditional synaptic contacts with cortical neurons. Limbic and paralimbic cortices of the brain receive the heaviest cholinergic input from Ch4 and are also the principal sources of reciprocal cortical projections back to the nucleus basalis. This limbic affiliation explains the role of the nucleus basalis in modulating the impact and memorability of incoming sensory information. The anatomical continuity of the nucleus basalis with other basomedial limbic structures may underlie its early and high vulnerability to the tauopathy and neurofibrillary degeneration of Alzheimer's disease. The tauopathy in Ch4 eventually leads to the degeneration of the cholinergic axons that it sends to the cerebral cortex. The early involvement of Ch4 has a magnifying effect on Alzheimer's pathology, because neurofibrillary degeneration in a small number of neurons can perturb neurotransmission in all cortical areas. Although the exact contribution of the Ch4 lesion to the cognitive changes of Alzheimer's disease remains poorly understood, the cholinergic circuitry of the nucleus basalis is emerging as one of the most strategically positioned and behaviorally consequential modulatory systems of the human cerebral cortex. J. Comp. Neurol. 521:4124-4144, 2013. © 2013 Wiley Periodicals, Inc.

Neuropathological changes in the nucleus basalis in schizophrenia

The nucleus basalis has not been examined in detail in severe mental illness. Several studies have demonstrated decreases in glia and glial markers in the cerebral cortex in schizophrenia, familial bipolar disorder and recurrent depression. Changes in neocortical neuron size and shape have also been reported. The nucleus basalis is a collection of large cholinergic neurons in the basal forebrain receiving information from the midbrain and limbic system, projecting to the cortex and involved with attention, learning and memory, and receives regulation from serotonergic inputs. Forty-one cases aged 41-60 years with schizophrenia or major depressive disorder with age-matched controls were collected. Formalin-fixed paraffin-embedded coronal nucleus basalis sections were histologically stained for oligodendrocyte identification with cresyl-haematoxylin counterstain, for neuroarchitecture with differentiated cresyl violet stain and astrocytes were detected by glial fibrillary acid protein immunohistochemistry. Cell density and neuroarchitecture were measured using Image Pro Plus. There were larger NB oval neuron soma in the combined schizophrenia and major depression disorder groups (p = 0.038), with no significant change between controls and schizophrenia and major depression disorder separately. There is a significant reduction in oligodendrocyte density (p = 0.038) in the nucleus basalis in schizophrenia. The ratio of gemistocytic to fibrillary astrocytes showed a greater proportion of the former in schizophrenia (18.1 %) and major depressive disorder (39.9 %) than in controls (7.9 %). These results suggest glial cell abnormalities in the nucleus basalis in schizophrenia possibly leading to cortical-limbic disturbance and subcortical dysfunction.

The selective 5-HT(1A) receptor antagonist NAD-299 increases acetylcholine release but not extracellular glutamate levels in the frontal cortex and hippocampus of awake rat

The effects of the HT(1A) receptor antagonist NAD-299 on extracellular acetylcholine (ACh) and glutamate (Glu) levels in the frontal cortex (FC) and ventral hippocampus (HPC) of the awake rats were investigated by the use of in vivo microdialysis. Systemic administration of NAD-299 (0.3; 1 and 3micromol/kg s.c.) caused a dose-dependent increase in ACh levels in FC and HPC (peak value of 209% and 221%, respectively) and this effect was comparable to that induced by donepezil (2.63micromol/kg s.c.). Moreover, the ACh levels in the FC increased even after repeated (14days) treatment with NAD-299 and when NAD-299 was injected locally into the nucleus basalis magnocellularis or perfused through the microdialysis probe implanted in the cortex. In contrast, NAD-299 failed to alter the extracellular levels of glutamate after systemic (3micromol/kg s.c.) or local (100microM) administration. The present data support the hypothesis that cholinergic transmission in cortico-limbic regions can be enhanced via blockade of postsynaptic 5-HT(1A) receptors, which may underlie the proposed cognitive enhancing properties of NAD-299 in models characterized by cholinergic deficit.

Regulation of cortical acetylcholine release: insights from in vivo microdialysis studies

Acetylcholine release links the activity of presynaptic neurons with their postsynaptic targets and thus represents the intercellular correlate of cholinergic neurotransmission. Here, we review the regulation and functional significance of acetylcholine release in the mammalian cerebral cortex, with a particular emphasis on information derived from in vivo microdialysis studies over the past three decades. This information is integrated with anatomical and behavioral data to derive conclusions regarding the role of cortical cholinergic transmission in normal behavioral and how its dysregulation may contribute to cognitive correlates of several neuropsychiatric conditions. Some unresolved issues regarding the regulation and significance of cortical acetylcholine release and the promise of new methodology for advancing our knowledge in this area are also briefly discussed.

The role of 5-HT(1A) receptors in learning and memory

The ascending serotonin (5-HT) neurons innervate the cerebral cortex, hippocampus, septum and amygdala, all representing brain regions associated with various domains of cognition. The 5-HT innervation is diffuse and extensively arborized with few synaptic contacts, which indicates that 5-HT can affect a large number of neurons in a paracrine mode. Serotonin signaling is mediated by 14 receptor subtypes with different functional and transductional properties. The 5-HT(1A) subtype is of particular interest, since it is one of the main mediators of the action of 5-HT. Moreover, the 5-HT(1A) receptor regulates the activity of 5-HT neurons via autoreceptors, and it regulates the function of several neurotransmitter systems via postsynaptic receptors (heteroreceptors). This review assesses the pharmacological and genetic evidence that implicates the 5-HT(1A) receptor in learning and memory. The 5-HT(1A) receptors are in the position to influence the activity of glutamatergic, cholinergic and possibly GABAergic neurons in the cerebral cortex, hippocampus and in the septohippocampal projection, thereby affecting declarative and non-declarative memory functions. Moreover, the 5-HT(1A) receptor regulates several transduction mechanisms such as kinases and immediate early genes implicated in memory formation. Based on studies in rodents the stimulation of 5-HT(1A) receptors generally produces learning impairments by interfering with memory-encoding mechanisms. In contrast, antagonists of 5-HT(1A) receptors facilitate certain types of memory by enhancing hippocampal/cortical cholinergic and/or glutamatergic neurotransmission. Some data also support a potential role for the 5-HT(1A) receptor in memory consolidation. Available results also implicate the 5-HT(1A) receptor in the retrieval of aversive or emotional memories, supporting an involvement in reconsolidation. The contribution of 5-HT(1A) receptors in cognitive impairments in various psychiatric disorders is still unclear. However, there is evidence that 5-HT(1A) receptors may play differential roles in normal brain function and in psychopathological states. Taken together, the evidence indicates that the 5-HT(1A) receptor is a target for novel therapeutic advances in several neuropsychiatric disorders characterized by various cognitive deficits.

Synaptologic and fine structural features distinguishing a subset of basal forebrain cholinergic neurons embedded in the dense intrinsic fiber network of the caudal extended amygdala

Cholinergic basal forebrain neurons confined within the intrinsic connections of the extended amygdala in the caudal sublenticular region and anterior amygdaloid area (cSLR/AAA) differ from other basal forebrain cholinergic neurons in several morphological and neurochemical respects. These cSLR/AAA cholinergic neurons have been subjected to additional investigations described in this report. First, fibers traced anterogradely following injections of Phaseolus vulgaris-leucoagglutinin in the central amygdaloid nucleus were shown to contact cSLR/AAA cholinergic neurons and dendrites. Second, these neurons were shown to be contacted by numerous GABAergic boutons with symmetric synaptic specializations. Third, the numbers of synaptic densities of morphologically characterized symmetric contacts on the somata and proximal dendrites of cSLR/AAA cholinergic neurons were shown to significantly exceed those of extra-cSLR/AAA cholinergic neurons. Fourth, fine structural features distinguishing cSLR/AAA cholinergic neurons from other basal forebrain cholinergic neurons were revealed. Specifically, cSLR/AAA cholinergic neurons have less abundant cytoplasm and a less well-organized system of rough endoplasmic reticulum than their counterparts in other parts of the basal forebrain. Thus, morphologically and neurochemically distinct cSLR/AAA cholinergic neurons exhibit robust proximal inhibitory inputs, of which a significant number originate in the extended amygdala, while cholinergic neurons outside this region lack a substrate for strong proximal inhibitory input. The implications of these findings for interaction of fear, anxiety, and attention are considered.

The evolving theory of basal forebrain functional-anatomical 'macrosystems'

The conceptual basis and continuing development of Alheid and Heimer's [Alheid, G.F., Heimer, L., 1988. New perspectives in basal forebrain organization of special relevance for neuropsychiatric disorders: the striatopallidal, amygdaloid and corticopetal components of substantia innominata. Neuroscience 27, 1-39] theory of basal forebrain organization based on the description of basal forebrain functional-anatomical 'macrosytems' is reviewed. It is posed that the macrosystem theory leads to a hypothesis that different macrosystems cooperate and compete to exert distinct influences on motor and cognitive function. Emergent corollaries include, e.g. that the organization of the outputs of different macrosystems should differ. Consistent with these considerations, extant literature and some unpublished data indicate that the input nuclei of macrosystems are not abundantly interconnected and macrosystems systems have distinct neuroanatomical relationships with basal forebrain and brainstem cholinergic and dopaminergic ascending modulatory systems. Furthermore, macrosystem outputs appear to be directed almost exclusively at the reticular formation or structures intimately associated with it. The relative merits of the theory of functional-anatomical macrosystems are discussed in relation to Swanson's model of cerebral hemisphere control of motivated behavior.

The Cholinergic Lesion of Alzheimer's Disease: Pivotal Factor or Side Show?

A profound loss of cortical cholinergic innervation is a nearly invariant feature of advanced Alzheimer's disease (AD). The temporal course of this lesion and its relationship to other aspects of the disease have not yet been fully clarified. Despite assertions to the contrary, a review of the evidence suggests that a perturbation of cholinergic innervation is likely to be present even in the very early stages of AD. This cholinergic lesion is unlikely to be a major determinant of the clinical symptoms or of the neuropathological lesions. Nonetheless, it almost certainly contributes to the severity of the cognitive and behavioral deficits, especially in the areas of memory and attention. The cholinergic lesion may also influence the progression of the neuropathological process through complex interactions with amyloidogenesis, tau phosphorylation and neuroplasticity.

Electrophysiological evidence for the existence of a posterior cortical-prefrontal-basal forebrain circuitry in modulating sensory responses in visual and somatosensory rat cortical areas

The prefrontal cortex (PFC) receives input from sensory neocortical regions and sends projections to the basal forebrain (BF). The present study tested the possibility that pathways from sensory cortical regions via the PFC-BF and from the BF back to specific sensory cortical areas could modulate sensory responses. Two prefrontal areas that responded to stimulation of the primary somatosensory and visual cortices were delineated: an area encompassing the rostral part of the cingulate cortex that responded to visual cortex stimulation, and a region dorso-lateral to the first in the precentral-motor association area that reacted to somatosensory cortex stimulation. Moreover, BF neurons responded to PFC electrical stimulation. They were located in the ventral pallidum, substantia innominata and the horizontal limb of the diagonal-band areas. Of the responsive BF neurons 42% reacted only to stimulation of 'visually-responsive,' 33% responded only to the 'somatosensory-responsive' prefrontal sites and the remaining neurons reacted to both prefrontal cortical areas. The effect of BF and PFC stimulations on somatosensory and visual-evoked potentials was tested. BF stimulation increased the amplitude of both sensory-evoked potentials. However, stimulation of the 'somatosensory-responsive' prefrontal area increased only somatosensory-evoked potentials while 'visually-responsive' prefrontal-area stimulation increased only visual-evoked potentials. Atropine blocked both facilitatory effects. The proposed cortico-prefronto-basalo-cortical circuitry may have an important role in cortical plasticity and selective attention.

Trafficking of the muscarinic m2 autoreceptor in cholinergic basalocortical neurons in vivo: differential regulation of plasma membrane receptor availability and intraneuronal localization in acetylcholinesterase-deficient and -inhibited mice

In vivo, the abundance of receptors at the neuronal plasma membrane may be critical in the mediation of pre- and postsynaptic responses. Thus, we have studied the membrane availability and intraneuronal distribution of the m2 muscarinic autoreceptor (m2R) in cholinergic neurons of the nucleus basalis magnocellularis (NBM) projecting to the frontal cortex (FC). We have studied the subcellular compartmentalization of m2R at somatodendritic postsynaptic and axonal presynaptic sites in control animals (AChE +/+) and in two animal models: mice displaying acute acetylcholinesterase (AChE) inhibition by treatment with metrifonate, and AChE-deficient mice (AChE -/-). In control animals, m2R was mainly located at the plasma membrane in the somatodendritic field of NBM and in cortical varicosities. Acute AChE inhibition and chronic AChE deficiency induced a dramatic decrease of cell surface m2R in the somatodendritic compartment. This finding was associated with two different intracytoplasmic events: (1). internalization of m2R in endosomes after acute AChE inhibition, (2). exaggerated storage of m2R in the endoplasmic reticulum and Golgi complex in AChE -/- mice. In contrast, the m2R density was higher at the membrane of cortical varicosities in AChE -/- mice but unchanged in acutely AChE-inhibited mice. Our data demonstrate that acute and chronic stimulation provoke, in vivo, depletion of the membrane store of somatodendritic m2R through different intracellular mechanisms: endocytosis of receptors from the plasma membrane to the cytoplasm (acute) or regulation of their delivery from intracytoplasmic stores to the plasma membrane (chronic). The increase of m2R at the membrane of axonal varicosities after chronic stimulation suggest modulation of presynaptic cholinergic activity, including neurotransmitter release.

Direct catecholaminergic-cholinergic interactions in the basal forebrain. III. Adrenergic innervation of choline acetyltransferase-containing neurons in the rat

The central adrenergic neurons have been suggested to play a role in the regulation of arousal and in the neuronal control of the cardiovascular system. To provide morphological evidence that these functions could be mediated via the basal forebrain, we performed correlated light and electron microscopic double-immunolabeling experiments using antibodies against phenylethanolamine N-methyltransferase (PNMT) and choline acetyltransferase, the synthesizing enzymes for adrenaline and acetylcholine, respectively. Most adrenergic/cholinergic appositions were located in the horizontal limb of diagonal band of Broca, within the substantia innominata, and in a narrow band bordering the substantia innominata and the globus pallidus. Quantitative analysis indicated that cholinergic neurons of the substantia innominata receive significantly higher numbers of adrenergic appositions than cholinergic cells in the rest of the basal forebrain. In the majority of cases, the ultrastructural analysis revealed axodendritic asymmetric synapses. By comparing the number and distribution of dopamine beta-hydroxylase (DBH)/cholinergic appositions, described earlier, with those of PNMT/cholinergic interactions in the basal forebrain, it can be concluded that a significant proportion of putative DBH/cholinergic contacts may represent adrenergic input. Our results support the hypothesis that the adrenergic/cholinergic link in the basal forebrain may represent a critical component of a central network coordinating autonomic regulation with cortical activation.

Projections from the amygdaloid complex to the magnocellular cholinergic basal forebrain in rat

The amygdaloid complex has a key role in the modulation of behavioral responses in life-threatening situations, including the direction of attentional responses to sensory stimuli. The pathways from the amygdala to the basal forebrain cholinergic system, which projects to the cortex, are proposed to contribute to the modulation. To further explore the topography and postsynaptic targets of these pathways, we investigated the projections from the different divisions of the lateral, basal, accessory basal, and central nuclei of the amygdala to the cholinergic basal forebrain in rat using a sensitive anterograde tracer, Phaseolus vulgaris leucoagglutinin. The most substantial projections from the amygdala to the basal forebrain are directed to the ventrolateral and dorsomedial aspects of the substantia innominata and the fundus of the striatum. The heaviest projections originate in the capsular, lateral, and intermediate divisions of the central nucleus as well as in the magnocellular and parvicellular divisions of the basal nucleus. Light microscopic analysis of double-stained preparations revealed that the distribution of amygdaloid efferents and cholinergic neurons overlaps most prominently in the ventrolateral substantia innominata. Despite the fact that the central nucleus efferents and cholinergic elements overlap in the ventrolateral substantia innominata, electron microscopic analysis revealed, first, that the postsynaptic targets of the central nucleus efferents are non-cholinergic, probably GABAergic, neurons. Second, 80% of the synaptic contacts were symmetric. The present data extend previous observations showing that the different amygdaloid nuclei provide projections to the selective basal forebrain areas. Further, the central nucleus efferents modulate cholinergic neurons in the basal forebrain indirectly via the GABAergic interneurons.

Presynaptic inhibition of GABAergic miniature currents by metabotropic glutamate receptor in the rat CNS

The modulation of spontaneous miniature GABAergic inhibitory postsynaptic currents (mIPSC) by the metabotropic glutamate receptors was investigated in the mechanically dissociated rat nucleus basalis of Meynert neurons using the conventional whole-cell patch recording configuration. An application of (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (tACPD) reversibly reduced the frequency of mIPSC without affecting the current amplitude distribution. The application of K+ channel blockers such as 4-aminopyridine, Cs+, Ba2+ or tetraethylammonium increased the mIPSC frequency, but failed to inhibit the tACPD action on mIPSC. Although the removal of Ca2+ from the extracellular solution reduced the mIPSC frequency, the inhibitory effect of tACPD on mIPSC was unaltered. These results suggested that neither voltage-dependent K+ or Ca2+ channels are involved in the inhibitory effect of tACPD on mIPSC frequency. Forskolin, an activator of adenylate cyclase, facilitated the mIPSC frequency in a concentration-dependent manner and inhibited the tACPD-induced suppression of mIPSC frequency. 8-Br-cAMP, a membrane permeable analog of cAMP, also prevented the inhibitory action of tACPD. However, Sp-cAMP, an activator of protein kinase A, could not prevent the inhibitory action of tACPD. L-CCG-I and (2R,4R)-APDC, group II mGluR agonists, mimicked the tACPD action on mIPSC frequency, but L-AP4, a group III mGluR agonist, had no such effect. MCCG, a group II mGluR antagonist, fully blocked the tACPD action. It was concluded that the activation of group II mGluR on the GABAergic presynaptic nerve terminals projecting to the rat nucleus basalis of Meynert neurons therefore inhibits the GABA release by reducing the activity of the cAMP-dependent pathway.

Synapses on GABAergic neurons in the basolateral nucleus of the rat amygdala: double-labeling immunoelectron microscopy

Although the basolateral nucleus (BL) of the amygdala is known to contain an abundance of gamma-aminobutyric acid (GABA)ergic neurons that regulate the amygdaloid projection neurons and influence storage and consolidation of memory, it remains to be determined what type of neuronal input controls GABAergic neurons in the BL. We examined the synapses that GABAergic neurons form with GABAergic and noradrenergic neurons and terminals with unknown transmitters by double-labeling immunoelectron microscopy using anti-GABA and dopamine-beta-hydroxylase (DBH) antisera. The medium and small dendrites of the GABAergic neurons were shown to receive symmetric, inhibitory-type synapses from GABAergic axon terminals and asymmetric, excitatory-type synapses from noradrenergic axon terminals. Each segment of the GABAergic neurons from perikarya to dendritic spines received both symmetric and asymmetric synapses from unlabeled axon terminals of various forms and sizes. The incidence rates of the two types of synapses were almost identical. Our results suggest that GABAergic neurons in the BL of the rat amygdala might be affected by the excitatory influence of the noradrenergic system and the inhibitory influence of the GABAergic system. Furthermore, these neurons are also strongly influenced by both excitatory and inhibitory-type synapses from neuronal systems other than the GABAergic and noradrenergic systems.

Long-term treatment with antidepressant drugs reduces the sensitivity of cortical cholinergic neurons to the activating actions of stress and the anxiogenic drug FG 7142

Certain antidepressant drugs exert an anxiolytic action in both humans and rodents. The effects of long-term treatment with imipramine or mirtazapine, two antidepressant drugs with different mechanisms of action, on the response of cortical cholinergic neurons to foot-shock stress or to the anxiogenic drug FG 7142 were investigated in freely moving rats. Chronic treatment with imipramine or mirtazapine reduced the increase in cortical acetylcholine output induced by foot-shock stress by approximately 50%. The same treatment also reduced the sensitivity of cortical cholinergic neurons to the stimulatory effect of acute administration of FG 7142. In contrast, the administration of a single dose of either antidepressant 40 min before foot shock or FG 7142 injection failed to increase the threshold of excitability of cortical cholinergic neurons. These results demonstrate that long-term treatment with either imipramine or mirtazapine reduces the sensitivity of cortical cholinergic neurons to stress or to an anxiogenic drug with an efficacy similar to that of acute administration of benzodiazepines. The neurochemical mechanism responsible for regulation of cholinergic neuron sensitivity might contribute to the modulation of cognitive function associated with emotional and affective disorders.

Cholinergic systems and schizophrenia: primary pathology or epiphenomena?

Post mortem schizophrenia research has been driven first by the dopamine and then the glutamate hypotheses. These hypotheses posit primary pathology in pathways dependent upon dopamine or glutamate neurotransmission. Although the dopamine and glutamate hypotheses retain considerable theoretical strength, neurobiological findings of altered dopamine or glutamate activity in schizophrenia do not explain all features of this disorder. A more synthetic approach would suggest that focal pathological change in either the prefrontal cortex or mesial temporal lobe leads to neurochemical changes in multiple neurotransmitter systems. Despite the limited experimental evidence for abnormal cholinergic neurotransmission in psychiatric disorders, increased understanding of the role of acetylcholine in the human brain and its relationship to other neurotransmitter systems has led to a rapidly growing interest in the cholinergic system in schizophrenia. This review focuses on the basic anatomy of the mammalian cholinergic system, and its possible involvement in the neurobiology of schizophrenia. Summaries of cholinergic cell groups, projection pathways, and receptor systems, in the primate and human brain, are followed by a brief discussion of the functional correlations between aberrant cholinergic neurotransmission and the signs and symptoms of schizophrenia.

Amphetamine-stimulated cortical acetylcholine release: role of the basal forebrain

Systemic administration of amphetamine results in increases in the release of acetylcholine in the cortex. Basal forebrain mediation of this effect was examined in three experiments using microdialysis in freely-moving rats. Experiment 1 examined whether dopamine receptor activity within the basal forebrain was necessary for amphetamine-induced increase in cortical acetylcholine by examining whether intra-basalis perfusion of dopamine antagonists attenuates this increase. Systemic administration of 2.0 mg/kg amphetamine increased dopamine efflux within the basal forebrain nearly 700% above basal levels. However, the increase in cortical acetylcholine efflux following amphetamine administration was unaffected by intra-basalis perfusions of high concentrations of D1- (100 microM SCH 23390) or D2-like (100 microM sulpiride) dopamine receptor antagonists. Experiments 2 and 3 determined whether glutamatergic or GABAergic local modulation of the excitability of the basal forebrain cholinergic neurons influences the ability of systemic amphetamine to increase cortical acetylcholine efflux. In Experiment 2, perfusion of kynurenate (1.0 mM), a non-selective glutamate receptor antagonist, into the basal forebrain attenuated the increase in cortical acetylcholine produced by amphetamine. Experiment 3 revealed that positive modulation of GABAergic transmission by bilateral intra-basalis infusion of the benzodiazepine receptor agonist chlordiazepoxide (40 microg/hemisphere) also attenuated the amphetamine-stimulated increase in cortical acetylcholine efflux. These data suggest that amphetamine increases cortical acetylcholine release via a complex neuronal network rather than simply increasing basal forebrain D1 or D2 receptor activity.

Local synaptic connections of basal forebrain neurons

Single, biocytin filled neurons in combination with immunocytochemistry and retrograde tracing as well as material with traditional double-immunolabeling were used at the light and electron microscopic levels to study the neural circuitry within the basal forebrain. Cholinergic neurons projecting to the frontal cortex exhibited extensive local collaterals terminating on non-cholinergic, (possible GABAergic) neurons within the basal forebrain. Elaborate axon arbors confined to the basal forebrain region also originated from NPY, somatostatin and other non-cholinergic interneurons. It is proposed that putative interneurons together with local collaterals from projection neurons contribute to regional integrative processing in the basal forebrain that may participate in more selective functions, such as attention and cortical plasticity.

Cortical cholinergic inputs mediating arousal, attentional processing and dreaming: differential afferent regulation of the basal forebrain by telencephalic and brainstem afferents

Basal forebrain corticopetal neurons participate in the mediation of arousal, specific attentional functions and rapid eye movement sleep-associated dreaming. Recent studies on the afferent regulation of basal forebrain neurons by telencephalic and brainstem inputs have provided the basis for hypotheses which, collectively, propose that the involvement of basal forebrain corticopetal projections in arousal, attention and dreaming can be dissociated on the basis of their regulation via major afferent projections. While the processing underlying sustained, selective and divided attention performance depends on the integrity of the telencephalic afferent regulation of basal forebrain corticopetal neurons, arousal-induced attentional processing (i.e. stimulus detection, selection and processing as a result of a novel, highly salient, aversive or incentive stimuli) is mediated via the ability of brainstem ascending noradrenergic projections to the basal forebrain to activate or "recruit" these telencephalic afferent circuits of the basal forebrain. In rapid eye movement sleep, both the basal forebrain and thalamic cortiocopetal projections are stimulated by cholinergic afferents originating mainly from the pedunculopontine and laterodorsal tegmenta in the brainstem. Rapid eye movement sleep-associated dreaming is described as a form of hyperattentional processing, mediated by increased activity of cortical cholinergic inputs and their cortical interactions with activated thalamic efferents. In this context, long-standing speculations about the similarities between dreaming and psychotic cognition are substantiated by describing the role of an over(re)active cortical cholinergic input system in either condition. Finally, while determination of the afferent regulation of basal forebrain corticopetal neurons in different behavioral/cognitive states assists in defining the general cognitive functions of cortical acetylcholine, this research requires a specification of the precise anatomical organization of basal forebrain afferents and their interactions in the basal forebrain. Furthermore, the present hypotheses remain incomplete because of the paucity of data concerning the regulation and role of basal forebrain non-cholinergic, particularly GABAergic, efferents.

Monoaminergic-cholinergic interactions in the primate basal forebrain

Anatomical studies in the rat have shown that the cholinergic cells of the nucleus basalis receive synapses from monoamine axons, but similar evidence is lacking in primates. We used single- and double-labeling immunocytochemistry to visualize monoamine axons and their relationship with the cholinergic cells of the basal forebrain of the monkey. Norepinephrine axons, labeled with dopamine-beta-hydroxylase antibodies, formed a bed of fine varicose axons that co-distributed with the cholinergic cells. Tyrosine hydroxylase-immunoreactive axons, presumed to be mainly dopaminergic, were 10-20 times more abundant than dopamine-beta-hydroxylase axons throughout the basal forebrain, except in the medial septal area, where their density was lower. Serotonin-immunoreactive axons formed a dense axon plexus throughout the basal forebrain. Double-labeling light microscopy demonstrated that each of the three types of monoamine axons formed frequent direct contacts with the cholinergic cells. Electron microscopy showed that the noradrenergic and the putative dopaminergic axons synapsed on the cholinergic cells. In the human brain, immunolabeling with antibodies to dopamine-beta-hydroxylase, tyrosine hydroxylase and tryptophan hydroxylase (for serotonin axons) showed axon densities in the nucleus basalis comparable to those of the monkey brain. The data demonstrate that all three of these monoamine systems innervate the cholinergic and possibly also the non-cholinergic cells of the nucleus basalis, and therefore affect the release of acetylcholine in the cerebral cortex.

The basal forebrain corticopetal system revisited

The medial septum, diagonal bands, ventral pallidum, substantia innominata, globus pallidus, and internal capsule contain a heterogeneous population of neurons, including cholinergic and noncholinergic (mostly GABA containing), corticopetal projection neurons, and interneurons. This highly complex brain region, which constitutes a significant part of the basal forebrain has been implicated in attention, motivation, learning, as well as in a number of neuropsychiatric disorders, such as Alzheimer's disease, Parkinson's disease, and schizophrenia. Part of the difficulty in understanding the functions of the basal forebrain, as well as the aberrant information-processing characteristics of these disease states lies in the fact that the organizational principles of this brain area remained largely elusive. On the basis of new anatomical data, it is proposed that a large part of the basal forebrain corticopetal system be organized into longitudinal bands. Considering the topographic organization of cortical afferents to different divisions of the prefrontal cortex and a similar topographic projection of these prefrontal areas to basal forebrain regions, it is suggested that several functionally segregated cortico-prefronto-basal forebrain-cortical circuits exist. It is envisaged that such specific "triangular" circuits could amplify selective attentional processing in posterior sensory cortical areas.