The nucleus basalis magnocellularis: the origin of a cholinergic projection to the neocortex of the rat

Distinct cholinergic circuits underlie discrete effects of reward on attention

Attention and reward are functions that are critical for the control of behavior, and massive multi-region neural systems have evolved to support the discrete computations associated with each. Previous research has also identified that attention and reward interact, though our understanding of the neural mechanisms that mediate this interplay is incomplete. Here, we review the basic neuroanatomy of attention, reward, and cholinergic systems. We then examine specific contexts in which attention and reward computations interact. Building on this work, we propose two discrete neural circuits whereby acetylcholine, released from cell groups located in different parts of the brain, mediates the impact of stimulus-reward associations as well as motivation on attentional control. We conclude by examining these circuits as a potential shared loci of dysfunction across diseases states associated with deficits in attention and reward.

Parvalbumin-expressing basal forebrain neurons mediate learning from negative experience

Parvalbumin (PV)-expressing GABAergic neurons of the basal forebrain (BFPVNs) were proposed to serve as a rapid and transient arousal system, yet their exact role in awake behaviors remains unclear. We performed bulk calcium measurements and electrophysiology with optogenetic tagging from the horizontal limb of the diagonal band of Broca (HDB) while male mice were performing an associative learning task. BFPVNs responded with a distinctive, phasic activation to punishment, but showed slower and delayed responses to reward and outcome-predicting stimuli. Optogenetic inhibition during punishment impaired the formation of cue-outcome associations, suggesting a causal role of BFPVNs in associative learning. BFPVNs received strong inputs from the hypothalamus, the septal complex and the median raphe region, while they synapsed on diverse cell types in key limbic structures, where they broadcasted information about aversive stimuli. We propose that the arousing effect of BFPVNs is recruited by aversive stimuli to serve crucial associative learning functions.

Functional neuroanatomy of basal forebrain projections to the basolateral amygdala: Transmitters, receptors, and neuronal subpopulations

The projections of the basal forebrain (BF) to the hippocampus and neocortex have been extensively studied and shown to be important for higher cognitive functions, including attention, learning, and memory. Much less is known about the BF projections to the basolateral nuclear complex of the amygdala (BNC), although the cholinergic innervation of this region by the BF is actually far more robust than that of cortical areas. This review will focus on light and electron microscopic tract-tracing and immunohistochemical (IHC) studies, many of which were published in the last decade, that have analyzed the relationship of BF inputs and their receptors to specific neuronal subtypes in the BNC in order to better understand the anatomical substrates of BF-BNC circuitry. The results indicate that BF inputs to the BNC mainly target the basolateral nucleus of the BNC (BL) and arise from cholinergic, GABAergic, and perhaps glutamatergic BF neurons. Cholinergic inputs mainly target dendrites and spines of pyramidal neurons (PNs) that express muscarinic receptors (MRs). MRs are also expressed by cholinergic axons, as well as cortical and thalamic axons that synapse with PN dendrites and spines. BF GABAergic axons to the BL also express MRs and mainly target BL interneurons that contain parvalbumin. It is suggested that BF-BL circuitry could be very important for generating rhythmic oscillations known to be critical for emotional learning. BF cholinergic inputs to the BNC might also contribute to memory formation by activating M1 receptors located on PN dendritic shafts and spines that also express NMDA receptors.

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.

Ovariectomy-induced hormone deprivation aggravates Aβ1-42 deposition in the basolateral amygdala and cholinergic fiber loss in the cortex but not cognitive behavioral symptoms in a triple transgenic mouse model of Alzheimer's disease

Alzheimer's disease is the most common type of dementia, being highly prevalent in elderly women. The advanced progression may be due to decreased hormone synthesis during post-menopause as estradiol and progesterone both have neuroprotective potentials. We aimed to confirm that female hormone depletion aggravates the progression of dementia in a triple transgenic mouse model of Alzheimer's disease (3xTg-AD). As pathological hallmarks are known to appear in 6-month-old animals, we expected to see disease-like changes in the 4-month-old 3xTg-AD mice only after hormone depletion. Three-month-old female 3xTg-AD mice were compared with their age-matched controls. As a menopause model, ovaries were removed (OVX or Sham surgery). After 1-month recovery, the body composition of the animals was measured by an MRI scan. The cognitive and anxiety parameters were evaluated by different behavioral tests, modeling different aspects (Y-maze, Morris water maze, open-field, social discrimination, elevated plus maze, light-dark box, fox odor, operant conditioning, and conditioned fear test). At the end of the experiment, uterus was collected, amyloid-β accumulation, and the cholinergic system in the brain was examined by immunohistochemistry. The uterus weight decreased, and the body weight increased significantly in the OVX animals. The MRI data showed that the body weight change can be due to fat accumulation. Moreover, OVX increased anxiety in control, but decreased in 3xTg-AD animals, the later genotype being more anxious by default based on the anxiety z-score. In general, 3xTg-AD mice moved less. In relation to cognition, neither the 3xTg-AD genotype nor OVX surgery impaired learning and memory in general. Despite no progression of dementia-like behavior after OVX, at the histological level, OVX aggravated the amyloid-β plaque deposition in the basolateral amygdala and induced early cholinergic neuronal fiber loss in the somatosensory cortex of the transgenic animals. We confirmed that OVX induced menopausal symptoms. Removal of the sexual steroids aggravated the appearance of AD-related alterations in the brain without significantly affecting the behavior. Thus, the OVX in young, 3-month-old 3xTg-AD mice might be a suitable model for testing the effect of new treatment options on structural changes; however, to reveal any beneficial effect on behavior, a later time point might be needed.

Acetylcholine from the nucleus basalis magnocellularis facilitates the retrieval of well-established memory

Retrieval deficit of long-term memory is a cardinal symptom of dementia and has been proposed to associate with abnormalities in the central cholinergic system. Difficulty in the retrieval of memory is experienced by healthy individuals and not limited to patients with neurological disorders that result in forgetfulness. The difficulty of retrieving memories is associated with various factors, such as how often the event was experienced or remembered, but it is unclear how the cholinergic system plays a role in the retrieval of memory formed by a daily routine (accumulated experience). To investigate this point, we trained rats moderately (for a week) or extensively (for a month) to detect a visual cue in a two-alternative forced-choice task. First, we confirmed the well-established memory in the extensively trained group was more resistant to the retrieval problem than recently acquired memory in the moderately trained group. Next, we tested the effect of a cholinesterase inhibitor, donepezil, on the retrieval of memory after a long no-task period in extensively trained rats. Pre-administration of donepezil improved performance and reduced the latency of task initiation compared to the saline-treated group. Finally, we lesioned cholinergic neurons of the nucleus basalis magnocellularis (NBM), which project to the entire neocortex, by injecting the cholinergic toxin 192 IgG-saporin. NBM-lesioned rats showed severely impaired task initiation and performance. These abilities recovered as the trials progressed, though they never reached the level observed in rats with intact NBM. These results suggest that acetylcholine released from the NBM contributes to the retrieval of well-established memory developed by a daily routine.

Structure and function of dual-source cholinergic modulation in early vision

Behavioral states such as arousal and attention have profound effects on sensory processing, determining how-even whether-a stimulus is perceived. This state-dependence is believed to arise, at least in part, in response to inputs from subcortical structures that release neuromodulators such as acetylcholine, often nonsynaptically. The mechanisms that underlie the interaction between these nonsynaptic signals and the more point-to-point synaptic cortical circuitry are not well understood. This review highlights the state of the field, with a focus on cholinergic action in early visual processing. Key anatomical and physiological features of both the cholinergic and the visual systems are discussed. Furthermore, presenting evidence of cholinergic modulation in visual thalamus and primary visual cortex, we explore potential functional roles of acetylcholine and its effects on the processing of visual input over the sleep-wake cycle, sensory gain control during wakefulness, and consider evidence for cholinergic support of visual attention.

Alteration of the nucleus basalis of Meynert afferents to vibrissae-related sensory cortex in de-whiskered adolescent congenital hypothyroid rats

INTRODUCTION

Thyroid hypofunction during early development results in anatomical alterations in the cerebellum, cerebrum, hippocampus and other brain structures. The plastic organization of the nucleus basalis of Meynert (nBM) projections to the whiskers-related somatosensory (wS1) cortex in adolescent pups with maternal thyroid hypofunction and sensory deprivation was assessed through retrograde WGA-HRP labeling.

METHODS

Congenital hypothyroidism induced by adding PTU (25 ppm) to the drinking water from embryonic day 16 to postnatal day (PND) 60. Pregnant rats were divided to intact and congenital hypothyroid groups. In each group, the total whiskers of pups (4 of 8) were trimmed continuously from PND 0 to PND 60.

RESULTS

Following separately WGA-HRP injections into wS1, retrogradely labeled neurons were observed in nBM. The number of labeled neurons in nBM were higher in the congenital hypothyroid and whisker deprived groups compared to their controls (P < 0.05).

CONCLUSION

Based on our results both congenital hypothyroidism and sensory deprivation may disturb normal development of cortical circuits in of nBM afferents to the wS1 cortex.

How Movement Modulates Hearing

Hearing is often viewed as a passive process: Sound enters the ear, triggers a cascade of activity through the auditory system, and culminates in an auditory percept. In contrast to a passive process, motor-related signals strongly modulate the auditory system from the eardrum to the cortex. The motor modulation of auditory activity is most well documented during speech and other vocalizations but also can be detected during a wide variety of other sound-generating behaviors. An influential idea is that these motor-related signals suppress neural responses to predictable movement-generated sounds, thereby enhancing sensitivity to environmental sounds during movement while helping to detect errors in learned acoustic behaviors, including speech and musicianship. Findings in humans, monkeys, songbirds, and mice provide new insights into the circuits that convey motor-related signals to the auditory system, while lending support to the idea that these signals function predictively to facilitate hearing and vocal learning.

Selective Activation of Basal Forebrain Cholinergic Neurons Attenuates Polymicrobial Sepsis-Induced Inflammation via the Cholinergic Anti-Inflammatory Pathway

Supplemental Digital Content is available in the text.

Objectives:

Basal forebrain cholinergic neurons are proposed as a major neuromodulatory system in inflammatory modulation. However, the function of basal forebrain cholinergic neurons in sepsis is unknown, and the neural pathways underlying cholinergic anti-inflammation remain unexplored.

Design:

Animal research.

Setting:

University research laboratory.

Subjects:

Male wild-type C57BL/6 mice and ChAT-ChR2-EYFP (ChAT) transgenic mice.

Interventions:

The cholinergic neuronal activity of the basal forebrain was manipulated optogenetically. Cecal ligation and puncture was produced to induce sepsis. Left cervical vagotomy and 6-hydroxydopamine injection to the spleen were used.

Measurements and Main Results:

Photostimulation of basal forebrain cholinergic neurons induced a significant decrease in the levels of tumor necrosis factor-α and interleukin-6 in the serum and spleen. When cecal ligation and puncture was combined with left cervical vagotomy in photostimulated ChAT mice, these reductions in tumor necrosis factor-α and interleukin-6 were partly reversed. Furthermore, photostimulating basal forebrain cholinergic neurons induced a large increase in c-Fos expression in the basal forebrain, the dorsal motor nucleus of the vagus, and the ventral part of the solitary nucleus. Among them, 35.2% were tyrosine hydroxylase positive neurons. Furthermore, chemical denervation showed that dopaminergic neurotransmission to the spleen is indispensable for the anti-inflammation.

Conclusions:

These results are the first to demonstrate that selectively activating basal forebrain cholinergic neurons is sufficient to attenuate systemic inflammation in sepsis. Specifically, photostimulation of basal forebrain cholinergic neurons activated dopaminergic neurons in dorsal motor nucleus of the vagus/ventral part of the solitary nucleus, and this dopaminergic efferent signal was further transmitted by the vagus nerve to the spleen. This cholinergic-to-dopaminergic neural circuitry, connecting central cholinergic neurons to the peripheral organ, might have mediated the anti-inflammatory effect in sepsis.

Cell-Specific Cholinergic Modulation of Excitability of Layer 5B Principal Neurons in Mouse Auditory Cortex

The neuromodulator acetylcholine (ACh) is crucial for several cognitive functions, such as perception, attention, and learning and memory. Whereas, in most cases, the cellular circuits or the specific neurons via which ACh exerts its cognitive effects remain unknown, it is known that auditory cortex (AC) neurons projecting from layer 5B (L5B) to the inferior colliculus, corticocollicular neurons, are required for cholinergic-mediated relearning of sound localization after occlusion of one ear. Therefore, elucidation of the effects of ACh on the excitability of corticocollicular neurons will bridge the cell-specific and cognitive properties of ACh. Because AC L5B contains another class of neurons that project to the contralateral cortex, corticocallosal neurons, to identify the cell-specific mechanisms that enable corticocollicular neurons to participate in sound localization relearning, we investigated the effects of ACh release on both L5B corticocallosal and corticocollicular neurons. Using in vitro electrophysiology and optogenetics in mouse brain slices, we found that ACh generated nicotinic ACh receptor (nAChR)-mediated depolarizing potentials and muscarinic ACh receptor (mAChR)-mediated hyperpolarizing potentials in AC L5B corticocallosal neurons. In corticocollicular neurons, ACh release also generated nAChR-mediated depolarizing potentials. However, in contrast to the mAChR-mediated hyperpolarizing potentials in corticocallosal neurons, ACh generated prolonged mAChR-mediated depolarizing potentials in corticocollicular neurons. These prolonged depolarizing potentials generated persistent firing in corticocollicular neurons, whereas corticocallosal neurons lacking mAChR-mediated depolarizing potentials did not show persistent firing. We propose that ACh-mediated persistent firing in corticocollicular neurons may represent a critical mechanism required for learning-induced plasticity in AC.

SIGNIFICANCE STATEMENT

Acetylcholine (ACh) is crucial for cognitive functions. Whereas in most cases the cellular circuits or the specific neurons via which ACh exerts its cognitive effects remain unknown, it is known that auditory cortex (AC) corticocollicular neurons projecting from layer 5B to the inferior colliculus are required for cholinergic-mediated relearning of sound localization after occlusion of one ear. Therefore, elucidation of the effects of ACh on the excitability of corticocollicular neurons will bridge the cell-specific and cognitive properties of ACh. Our results suggest that cell-specific ACh-mediated persistent firing in corticocollicular neurons may represent a critical mechanism required for learning-induced plasticity in AC. Moreover, our results provide synaptic mechanisms via which ACh may mediate its effects on AC receptive fields.

Behavioral Effects of Neural Transplantation

Considerable evidence suggests that transplantation of fetal neural tissue ameliorates the behavioral deficits observed in a variety of animal models of CNS disorders. However, it is also becoming increasingly clear that neural transplants do not necessarily produce behavioral recovery, and in some cases have either no beneficial effects, magnify existing behavioral abnormalities, or even produce a unique constellation of deficits. Regardless, studies demonstrating the successful use of neural transplants in reducing or eliminating behavioral deficits in these animal models has led directly to their clinical application in human neurodegenerative disorders such as Parkinson's disease. This review examines the beneficial and deleterious behavioral consequences of neural transplants in different animal models of human diseases, and discusses the possible mechanisms by which neural transplants might produce behavior recovery.

Central Cholinergic Neurons Are Rapidly Recruited by Reinforcement Feedback

Basal forebrain cholinergic neurons constitute a major neuromodulatory system implicated in normal cognition and neurodegenerative dementias. Cholinergic projections densely innervate neocortex, releasing acetylcholine to regulate arousal, attention, and learning. However, their precise behavioral function is poorly understood because identified cholinergic neurons have never been recorded during behavior. To determine which aspects of cognition their activity might support, we recorded cholinergic neurons using optogenetic identification in mice performing an auditory detection task requiring sustained attention. We found that a non-cholinergic basal forebrain population-but not cholinergic neurons-were correlated with trial-to-trial measures of attention. Surprisingly, cholinergic neurons responded to reward and punishment with unusual speed and precision (18 ± 3 ms). Cholinergic responses were scaled by the unexpectedness of reinforcement and were highly similar across neurons and two nuclei innervating distinct cortical areas. These results reveal that the cholinergic system broadcasts a rapid and precisely timed reinforcement signal, supporting fast cortical activation and plasticity.

Silencing of Cholinergic Basal Forebrain Neurons Using Archaerhodopsin Prolongs Slow-Wave Sleep in Mice

The basal forebrain (BF) plays a crucial role in cortical activation. Our previous study showed that activation of cholinergic BF neurons alone is sufficient to suppress slow-wave sleep (SWS) and promote wakefulness and rapid-eye-movement (REM) sleep. However, the exact role of silencing cholinergic BF neurons in the sleep-wake cycle remains unclear. We inhibitied the cholinergic BF neurons genetically targeted with archaerhodopsin (Arch) with yellow light to clarify the role of cholinergic BF neurons in the sleep-wake cycle. Bilateral inactivation of cholinergic BF neurons genetically targeted with archaerhodopsin prolonged SWS and decreased the probability of awakening from SWS in mice. However, silencing these neurons changed neither the duration of wakefulness or REM sleep, nor the probability of transitions to other sleep-wake episodes from wakefulness or REM sleep. Furthermore, silencing these neurons for 6 h within the inactive or active period increased the duration of SWS at the expense of the duration of wakefulness, as well as increasing the number of prolonged SWS episodes (120-240 s). The lost wakefulness was compensated by a delayed increase of wakefulness, so the total duration of SWS and wakefulness during 24 h was kept stable. Our results indicate that the main effect of these neurons is to terminate SWS, whereas wakefulness or REM sleep may be determined by co-operation of the cholinergic BF neurons with other arousal-sleep control systems.

Developmental alterations of the septohippocampal cholinergic projection in a lissencephalic mouse model

LIS1 is one of principal genes related with Type I lissencephaly, a severe human brain malformation characterized by abnormal neuronal migration in the cortex. The LIS1 gene encodes a brain-specific 45kDa non-catalytic subunit of platelet-activating factor (PAF) acetylhydrolase-1b (PAFAH1b), an enzyme that inactivates the PAF. We have studied the role of Lis1 using a Lis1/sLis1 murine model, which has deleted the first coding exon from Lis1 gene. Homozygous mice are not viable but heterozygous have shown a delayed corticogenesis and neuronal dysplasia, with enhanced cortical excitability. Lis1/sLis1 embryos also exhibited a delay of cortical innervation by the thalamocortical fibers. We have explored in Lis1/sLis1 mice anomalies in forebrain cholinergic neuron development, which migrate from pallium to subpallium, and functionally represent the main cholinergic input to the cerebral cortex, modulating cortical activity and facilitating attention, learning, and memory. We hypothesized that primary migration anomalies and/or disorganized cortex could affect cholinergic projections from the basal forebrain and septum in Lis1/sLis1 mouse. To accomplish our objective we have first studied basal forebrain neurons in Lis1/sLis1 mice during development, and described structural and hodological differences between wild-type and Lis1/sLis1 embryos. In addition, septohippocampal projections showed altered development in mutant embryos. Basal forebrain abnormalities could contribute to hippocampal excitability anomalies secondary to Lis1 mutations and may explain the cognitive symptoms associated to cortical displasia-related mental diseases and epileptogenic syndromes.

Substitution of natural sensory input by artificial neurostimulation of an amputated trigeminal nerve does not prevent the degeneration of basal forebrain cholinergic circuits projecting to the somatosensory cortex

Peripheral deafferentation downregulates acetylcholine (ACh) synthesis in sensory cortices. However, the responsible neural circuits and processes are not known. We irreversibly transected the rat infraorbital nerve and implanted neuroprosthetic microdevices for proximal stump stimulation, and assessed cytochrome-oxidase and choline- acetyl-transferase (ChAT) in somatosensory, auditory and visual cortices; estimated the number and density of ACh-neurons in the magnocellular basal nucleus (MBN); and localized down-regulated ACh-neurons in basal forebrain using retrograde labeling from deafferented cortices. Here we show that nerve transection, causes down regulation of MBN cholinergic neurons. Stimulation of the cut nerve reverses the metabolic decline but does not affect the decrease in cholinergic fibers in cortex or cholinergic neurons in basal forebrain. Artifical stimulation of the nerve also has no affect of ACh-innervation of other cortices. Cortical ChAT depletion is due to loss of corticopetal MBN ChAT-expressing neurons. MBN ChAT downregulation is not due to a decrease of afferent activity or to a failure of trophic support. Basalocortical ACh circuits are sensory specific, ACh is provided to each sensory cortex "on demand" by dedicated circuits. Our data support the existence of a modality-specific cortex-MBN-cortex circuit for cognitive information processing.

Curtailing effect of awakening on visual responses of cortical neurons by cholinergic activation of inhibitory circuits

Visual responsiveness of cortical neurons changes depending on the brain state. Neural circuit mechanism underlying this change is unclear. By applying the method of in vivo two-photon functional calcium imaging to transgenic rats in which GABAergic neurons express fluorescent protein, we analyzed changes in visual response properties of cortical neurons when animals became awakened from anesthesia. In the awake state, the magnitude and reliability of visual responses of GABAergic neurons increased whereas the decay of responses of excitatory neurons became faster. To test whether the basal forebrain (BF) cholinergic projection is involved in these changes, we analyzed effects of electrical and optogenetic activation of BF on visual responses of mouse cortical neurons with in vivo imaging and whole-cell recordings. Electrical BF stimulation in anesthetized animals induced the same direction of changes in visual responses of both groups of neurons as awakening. Optogenetic activation increased the frequency of visually evoked action potentials in GABAergic neurons but induced the delayed hyperpolarization that ceased the late generation of action potentials in excitatory neurons. Pharmacological analysis in slice preparations revealed that photoactivation-induced depolarization of layer 1 GABAergic neurons was blocked by a nicotinic receptor antagonist, whereas non-fast-spiking layer 2/3 GABAergic neurons was blocked only by the application of both nicotinic and muscarinic receptor antagonists. These results suggest that the effect of awakening is mediated mainly through nicotinic activation of layer 1 GABAergic neurons and mixed nicotinic/muscarinic activation of layer 2/3 non-fast-spiking GABAergic neurons, which together curtails the visual responses of excitatory neurons.

Non-noxious skin stimulation activates the nucleus basalis of Meynert and promotes NGF secretion in the parietal cortex via nicotinic ACh receptors

The effects of non-noxious skin stimulation on nerve growth factor (NGF) secretion in the parietal cortex were examined in anesthetized rats. Innocuous skin stimulation was delivered to the left hindlimb with a soft-hair brush. Extracellular NGF in the right parietal cortex was collected by microdialysis methods using a protein-permeable probe and was measured using an enzyme-linked immune-sorbent assay. Brushing produced a significant increase in extracellular NGF levels. This NGF response was not observed in rats pretreated with a nicotinic ACh receptor (nAChR) antagonist mecamylamine. We further examined whether brushing could activate the basal forebrain nucleus (nucleus basalis of Meynert, NBM), which is the main source of cholinergic fibers in the cerebral cortex, by means of functional MRI. The blood oxygen level-dependent signal in the right NBM was significantly higher during brushing compared to baseline. The results suggest that non-noxious skin stimulation activates NBM and promotes NGF secretion in the parietal cortex via nAChRs.

Cholinergic contributions to supramodal attentional processes in rats

Cholinergic neurotransmission has been shown to play an important role in modulating attentional processing of visual stimuli. However, it is not yet clear whether the neurochemical acetylcholine (ACh) is necessary exclusively for visual attention, or if it also contributes to attentional functions through some modality-independent (supramodal) mechanism. To answer this question, we examined the effects of reduced cortical cholinergic afferentation on both a traditional visual and a novel olfactory five-choice serial reaction time task (5-CSRTT), the benchmark rodent test of sustained attention in rats. Following the successful acquisition of both modalities of the task, the rats underwent either a cholinergic immunotoxic- or sham-lesion surgery of the nucleus basalis magnocellularis (NBM), the basal forebrain nuclei that provide the majority of neocortical ACh. Reduced cholinergic afferentation to the neocortex was induced by bilaterally infusing the cholinergic immunotoxin 192 IgG-saporin into the NBM. After surgery, ACh-NBM-lesioned rats performed comparably to sham-lesioned rats under the conditions of low attentional demand, but displayed behavioral decrements relative to the sham-lesioned rats when the attentional demands of the task were increased. Moreover, this decrement in attentional functioning correlated significantly with the number of choline acetyltransferase-immunoreactive cells in the NBM. Importantly, the nature of this behavioral decrement was identical in the visual and olfactory 5-CSRTTs. Together, these data suggest the presence of a supramodal attentional modulatory cortical network whose activity is dependent on cholinergic innervation from the NBM.

Modulation of high- and low-frequency components of the cortical local field potential via nicotinic and muscarinic acetylcholine receptors in anesthetized mice

Release of acetylcholine (ACh) in neocortex is important for learning, memory and attention tasks. The primary source of ACh in neocortex is axons ascending from the basal forebrain. Release of ACh from these axons evokes changes in the cortical local field potential (LFP), including a decline in low-frequency spectral power that is often referred to as desynchronization of the LFP and is thought to result from the activation of muscarinic ACh receptors. Using channelrhodopsin-2, we selectively stimulated the axons of only cholinergic basal forebrain neurons in primary somatosensory cortex of the urethane-anesthetized mouse while monitoring the LFP. Cholinergic stimulation caused desynchronization and two brief increases in higher-frequency power at stimulus onset and offset. Desynchronization (1-6 Hz) was localized, extending ≤ 1 mm from the edge of stimulation, and consisted of both nicotinic and muscarinic receptor-mediated components that were inhibited by mecamylamine and atropine, respectively. Hence we have identified a nicotinic receptor-mediated component to desynchronization. The increase in higher-frequency power (>10 Hz) at stimulus onset was also mediated by activation of nicotinic and muscarinic receptors. However, the increase in higher-frequency power (10-20 Hz) at stimulus offset was evoked by activation of muscarinic receptors and inhibited by activation of nicotinic receptors. We conclude that the activation of nicotinic and muscarinic ACh receptors in neocortex exerts several effects that are reflected in distinct frequency bands of the cortical LFP in urethane-anesthetized mice.

Modulation-specific and laminar-dependent effects of acetylcholine on visual responses in the rat primary visual cortex

Acetylcholine (ACh) is secreted from cholinergic neurons in the basal forebrain to regions throughout the cerebral cortex, including the primary visual cortex (V1), and influences neuronal activities across all six layers via a form of diffuse extrasynaptic modulation termed volume transmission. To understand this effect in V1, we performed extracellular multi-point recordings of neuronal responses to drifting sinusoidal grating stimuli from the cortical layers of V1 in anesthetized rats and examined the modulatory effects of topically administered ACh. ACh facilitated or suppressed the visual responses of individual cells with a laminar bias: response suppression prevailed in layers 2/3, whereas response facilitation prevailed in layer 5. ACh effects on the stimulus contrast-response function showed that ACh changes the response gain upward or downward in facilitated or suppressed cells, respectively. Next, ACh effects on the signal-to-noise (S/N) ratio and the grating-phase information were tested. The grating-phase information was calculated as the F1/F0 ratio, which represents the amount of temporal response modulation at the fundamental frequency (F1) of a drifting grating relative to the mean evoked response (F0). In facilitated cells, ACh improved the S/N ratio, while in suppressed cells it enhanced the F1/F0 ratio without any concurrent reduction in the S/N ratio. These effects were predominantly observed in regular-spiking cells, but not in fast-spiking cells. Electrophysiological and histological findings suggest that ACh promotes the signaling of grating-phase information to higher-order areas by a suppressive effect on supragranular layers and enhances feedback signals with a high S/N ratio to subcortical areas by a facilitatory effect on infragranular layers. Thus, ACh distinctly and finely controls visual information processing in a manner that is specific for the modulation and cell type and is also laminar dependent.

Cortical cholinergic input is required for normal auditory perception and experience-dependent plasticity in adult ferrets

The nucleus basalis (NB) in the basal forebrain provides most of the cholinergic input to the neocortex and has been implicated in a variety of cognitive functions related to the processing of sensory stimuli. However, the role that cortical acetylcholine release plays in perception remains unclear. Here we show that selective loss of cholinergic NB neurons that project to the cortex reduces the accuracy with which ferrets localize brief sounds and prevents them from adaptively reweighting auditory localization cues in response to chronic occlusion of one ear. Cholinergic input to the cortex was disrupted by making bilateral injections of the immunotoxin ME20.4-SAP into the NB. This produced a substantial loss of both p75 neurotrophin receptor (p75(NTR))-positive and choline acetyltransferase-positive cells in this region and of acetylcholinesterase-positive fibers throughout the auditory cortex. These animals were significantly impaired in their ability to localize short broadband sounds (40-500 ms in duration) in the horizontal plane, with larger cholinergic cell lesions producing greater performance impairments. Although they localized longer sounds with normal accuracy, their response times were significantly longer than controls. Ferrets with cholinergic forebrain lesions were also less able to relearn to localize sound after plugging one ear. In contrast to controls, they exhibited little recovery of localization performance after behavioral training. Together, these results show that cortical cholinergic inputs contribute to the perception of sound source location under normal hearing conditions and play a critical role in allowing the auditory system to adapt to changes in the spatial cues available.

Basal forebrain activation controls contrast sensitivity in primary visual cortex

BACKGROUND

The basal forebrain (BF) regulates cortical activity by the action of cholinergic projections to the cortex. At the same time, it also sends substantial GABAergic projections to both cortex and thalamus, whose functional role has received far less attention. We used deep brain stimulation (DBS) in the BF, which is thought to activate both types of projections, to investigate the impact of BF activation on V1 neural activity.

RESULTS

BF stimulation robustly increased V1 single and multi-unit activity, led to moderate decreases in orientation selectivity and a remarkable increase in contrast sensitivity as demonstrated by a reduced semi-saturation contrast. The spontaneous V1 local field potential often exhibited spectral peaks centered at 40 and 70 Hz as well as reliably showed a broad γ-band (30-90 Hz) increase following BF stimulation, whereas effects in a low frequency band (1-10 Hz) were less consistent. The broad γ-band, rather than low frequency activity or spectral peaks was the best predictor of both the firing rate increase and contrast sensitivity increase of V1 unit activity.

CONCLUSIONS

We conclude that BF activation has a strong influence on contrast sensitivity in V1. We suggest that, in addition to cholinergic modulation, the BF GABAergic projections play a crucial role in the impact of BF DBS on cortical activity.

Identification and distribution of projections from monoaminergic and cholinergic nuclei to functionally differentiated subregions of prefrontal cortex

The prefrontal cortex (PFC) is implicated in a variety of cognitive and executive functions and is composed of several distinct networks, including anterior cingulate cortex (ACC), medial prefrontal cortex (mPFC), and orbitofrontal cortex (OFC). These regions serve dissociable cognitive functions, and are heavily innervated by acetylcholine, dopamine, serotonin and norepinephrine systems. In this study, fluorescently labeled retrograde tracers were injected into the ACC, mPFC, and OFC, and labeled cells were identified in the nucleus basalis (NB), ventral tegmental area (VTA), dorsal raphe nucleus (DRN) and locus coeruleus (LC). DRN and LC showed similar distributions of retrogradely labeled neurons such that most were single labeled and the largest population projected to mPFC. VTA showed a slightly greater proportion of double and triple labeled neurons, with the largest population projecting to OFC. NB, on the other hand, showed mostly double and triple labeled neurons projecting to multiple subregions. Therefore, subsets of VTA, DRN and LC neurons may be capable of modulating individual prefrontal subregions independently, whereas NB cells may exert a more unified influence on the three areas simultaneously. These findings emphasize the unique aspects of the cholinergic and monoaminergic projections to functionally and anatomically distinct subregions of PFC.

Cholinergic modulation of response gain in the rat primary visual cortex

Acetylcholine (ACh) is known to modulate neuronal activity in the rodent primary visual cortex (V1). Although cholinergic modulation has been extensively examined in vitro, far less is understood regarding how ACh modulates visual information processing in vivo. We therefore extracellularly recorded visual responses to drifting sinusoidal grating stimuli from V1 of anesthetized rats and tested the effects of ACh administered locally by microiontophoresis. ACh exerted response facilitation or suppression in individual neurons across all cortical layers without any laminar bias. We assessed ACh effects on the stimulus contrast-response function, finding that ACh increased or decreased the response to varying stimulus contrasts in proportion to the magnitude of the control response without changing the shape of the original contrast-response function, which describes response gain control but not contrast gain control. Our results indicate that ACh serves as a gain controller in the visual cortex of rodents.

Cell-type-specific modulation of neocortical activity by basal forebrain input

Activation of the cholinergic neurons in the basal forebrain (BF) desynchronizes cortical activity and enhances sensory processing during arousal and attention. How the cholinergic input modulates the activity of different subtypes of cortical neurons remains unclear. Using in vivo two-photon calcium imaging of neurons in layers 1 and 2/3 of mouse visual cortex, we show that electrical stimulation of the BF bi-directionally modulates the activity of excitatory neurons as well as several subtypes of inhibitory interneurons. While glutamatergic activity contributed to the activation of both excitatory and inhibitory neurons, the contribution of acetylcholine (ACh) was more complex. Excitatory and parvalbumin-positive (PV+) neurons were activated through muscarinic ACh receptors (mAChRs) at low levels of cortical desynchronization and suppressed through nicotinic ACh receptors (nAChRs) when cortical desynchronization was strong. In contrast, vasoactive intestinal peptide-positive (VIP+) and layer 1 interneurons were preferentially activated through nAChRs during strong cortical desynchronization. Thus, cholinergic input from the BF causes a significant shift in the relative activity levels of different subtypes of cortical neurons at increasing levels of cortical desynchronization.

Detection of an inhibitory cortical gradient underlying peak shift in learning: a neural basis for a false memory

Experience often does not produce veridical memory. Understanding false attribution of events constitutes an important problem in memory research. "Peak shift" is a well-characterized, controllable phenomenon in which human and animal subjects that receive reinforcement associated with one sensory stimulus later respond maximally to another stimulus in post-training stimulus generalization tests. Peak shift ordinarily develops in discrimination learning (reinforced CS+, unreinforced CS-) and has long been attributed to the interaction of an excitatory gradient centered on the CS+ and an inhibitory gradient centered on the CS-; the shift is away from the CS-. In contrast, we have obtained peak shifts during single tone frequency training, using stimulation of the cholinergic nucleus basalis (NB) to implant behavioral memory into the rat. As we also recorded cortical activity, we took the opportunity to investigate the possible existence of a neural frequency gradient that could account for behavioral peak shift. Behavioral frequency generalization gradients (FGGs, interruption of ongoing respiration) were determined twice before training while evoked potentials were recorded from the primary auditory cortex (A1), to obtain a baseline gradient of "habituatory" neural decrement. A post-training behavioral FGG obtained 24h after three daily sessions of a single tone paired with NB stimulation (200 trials/day) revealed a peak shift. The peak of the FGG was at a frequency lower than the CS while the cortical inhibitory gradient was at a frequency higher than the CS frequency. Further analysis indicated that the frequency location and magnitude of the gradient could account for the behavioral peak shift. These results provide a neural basis for a systematic case of memory misattribution and may provide an animal model for the study of the neural bases of a type of "false memory".

Evidence for broad versus segregated projections from cholinergic and noradrenergic nuclei to functionally and anatomically discrete subregions of prefrontal cortex

The prefrontal cortex (PFC) is implicated in a variety of cognitive and executive operations. However, this region is not a single functional unit; rather, it is composed of several functionally and anatomically distinct networks, including anterior cingulate cortex (ACC), medial prefrontal cortex (mPFC), and orbitofrontal cortex (OFC). These prefrontal subregions serve dissociable behavioral functions, and are unique in their afferent and efferent connections. Each of these subregions is innervated by ascending cholinergic and noradrenergic systems, each of which likewise has a distinct role in cognitive function; yet the distribution and projection patterns of cells in the source nuclei for these pathways have not been examined in great detail. In this study, fluorescent retrograde tracers were injected into ACC, mPFC, and OFC, and labeled cells were identified in the cholinergic nucleus basalis of Meynert (NBM) and noradrenergic nucleus locus coeruleus (LC). Injections into all three cortical regions consistently labeled cells primarily ipsilateral to the injection site with a minimal contralateral component. In NBM, retrogradely labeled neurons were scattered throughout the rostral half of the nucleus, whereas those in LC tended to cluster in the core of the nucleus, and were rarely localized within the rostral or caudal poles. In NBM, more than half of all retrogradely labeled cells possessed axon collaterals projecting two or more PFC subregions. In LC, however, only 4.3% of retrogradely labeled neurons possessed collaterals targeting any two prefrontal subregions simultaneously, and no cells were identified that projected to all three regions. Of all labeled LC neurons, 49.3% projected only to mPFC, 28.5% projected only to OFC, and 18.0% projected only to ACC. These findings suggest that subsets of LC neurons may be capable of modulating neuronal activity in individual prefrontal subregions independently, whereas assemblies of NBM cells may exert a more unified influence on the three areas, simultaneously. This work emphasizes unique aspects of the cholinergic and noradrenergic projections to functionally and anatomically distinct subregions of PFC and provides insights regarding global versus segregated regulation of prefrontal operations by these neuromodulatory pathways.

Galantamine improves olfactory learning in the Ts65Dn mouse model of Down syndrome

Down syndrome (DS) is the most common form of congenital intellectual disability. Although DS involves multiple disturbances in various tissues, there is little doubt that in terms of quality of life cognitive impairment is the most serious facet and there is no effective treatment for this aspect of the syndrome. The Ts65Dn mouse model of DS recapitulates multiple aspects of DS including cognitive impairment. Here the Ts65Dn mouse model of DS was evaluated in an associative learning paradigm based on olfactory cues. In contrast to disomic controls, trisomic mice exhibited significant deficits in olfactory learning. Treatment of trisomic mice with the acetylcholinesterase inhibitor galantamine resulted in a significant improvement in olfactory learning. Collectively, our study indicates that olfactory learning can be a sensitive tool for evaluating deficits in associative learning in mouse models of DS and that galantamine has therapeutic potential for improving cognitive abilities.

Cholinergic modulation of response gain in the primary visual cortex of the macaque

ACh modulates neuronal activity throughout the cerebral cortex, including the primary visual cortex (V1). However, a number of issues regarding this modulation remain unknown, such as the effect and its function and the receptor subtypes involved. To address these issues, we combined extracellular single-unit recordings and microiontophoretic administration of ACh and measured V1 neuronal responses to drifting sinusoidal grating stimuli in anesthetized macaque monkeys. ACh was found to have mostly facilitatory effects on the visual responses, although some cases of suppressive effects were also seen. To assess the functional role of ACh, we further examined how ACh modulates the stimulus contrast-response function, finding that the response gain increased with the facilitatory effect. The response facilitation was completely or strongly blocked by atropine (At), a muscarinic ACh receptor (mAChR) antagonist, in almost all neurons (96% of cells), whereas any residual effect after At administration was fully removed by mecamylamine, a nicotinic AChR (nAChR) antagonist, suggesting a predominant role for mAChRs in this mechanism. Furthermore, we found no laminar distribution bias for the facilitatory modulation, although the relative contribution of mAChRs was smaller in layer 4C than in other layers. The suppressive effect was blocked completely by At. These results demonstrate that ACh plays an important role in visual information processing in V1 by controlling the response gain via mAChRs across all cortical layers and via nAChRs, mainly in layer 4C.

Allosteric activators of muscarinic receptors as novel approaches for treatment of CNS disorders

Muscarinic acetylcholine receptors (mAChRs) represent exciting therapeutic targets for the treatment of multiple CNS disorders. The high degree of conservation of amino acids comprising the orthosteric acetylcholine (ACh) binding site between individual mAChR subtypes has hindered the development of subtype-selective compounds that bind to this site. As a result, many academic and industry researchers are now focusing on developing allosteric activators of mAChRs including both positive allosteric modulators (PAMs) and allosteric agonists. In the past 10 years major advances have been achieved in the discovery of allosteric ligands that possess much greater selectivity for individual mAChR subtypes when compared to previously developed orthosteric agents. These novel allosteric modulators of mAChRs may provide therapeutic potential for treatment of a number of CNS disorders such as Alzheimer's disease and schizophrenia.

Evolving concepts of sleep cycle generation: From brain centers to neuronal populations

As neurophysiological investigations of sleep cycle control have provided an increasingly detailed picture of events at the cellular level, the concept that the sleep cycle is generated by the interaction of multiple, anatomically distributed sets of neurons has gradually replaced the hypothesis that sleep is generated by a single, highly localized neuronal oscillator.

Cell groups that discharge during rapid-eye-movement (REM) sleep (REM-on) and neurons that slow or cease firing during REM sleep (REM-off) have long been thought to comprise at least two neurochemically distinct populations. The fact that putatively cholinoceptive and/or cholinergic (REM-on) and putatively aminergic (REM-off) cell populations discharge reciprocally over the sleep cycle suggests a causal interdependence.

In some brain stem areas these cell groups are not anatomically segregated and may instead be neurochemically mixed (interpenetrated). This finding raises important theoretical and practical issues not anticipated in the original reciprocal-interaction model. The electrophysiological evidence concerning the REM-on and REM-off cell groups suggests a gradient of sleep-dependent membrane excitability changes that may be a function of the connectivity strength within an anatomically distributed neuronal network. The connectivity strength may be influenced by the degree of neurochemical interpenetration between the REM-on and REM-offcells. Recognition of these complexities forces us to revise the reciprocal-interaction model and to seek new methods to test its tenets.

Cholinergic microinjection experiments indicate that some populations of REM-on cells can execute specific portions of the REM sleep syndrome or block the generation of REM sleep. This observation suggests that the order of activation within the anatomically distributed generator populations may be critical in determining behavioral outcome. Support for the cholinergic tenets of the reciprocal-interaction model has been reinforced by observations from sleep-disorders medicine.

Specific predictions of the reciprocal-interaction model and suggestions for testing these predictions are enumerated for future experimental programs that aim to understand the cellular and molecular basis of the mammalian sleep cycle.

Reticulo-cortical activity and behavior: A critique of the arousal theory and a new synthesis

It is traditionally believed that cerebral activation (the presence of low voltage fast electrical activity in the neocortex and rhythmical slow activity in the hippocampus) is correlated with arousal, while deactivation (the presence of large amplitude irregular slow waves or spindles in both the neocortex and the hippocampus) is correlated with sleep or coma. However, since there are many exceptions, these generalizations have only limited validity. Activated patterns occur in normal sleep (active or paradoxical sleep) and during states of anesthesia and coma. Deactivated patterns occur, at times, during normal waking, or during behavior in awake animals treated with atropinic drugs. Also, the fact that patterns characteristic of sleep, arousal, and waking behavior continue in decorticate animals indicates that reticulo-cortical mechanisms are not essential for these aspects of behavior.

These puzzles have been largely resolved by recent research indicating that there are two different kinds of input from the reticular activating system to the hippocampus and neocortex. One input is probably cholinergic; it may play a role in stimulus control of behavior. The second input is noncholinergic and appears to be related to motor activity; movement-related input to the neocortex may be dependent on a trace amine.

Reticulo-cortical systems are not related to arousal in the traditional sense, but may play a role in the control of adaptive behavior by influencing the activity of the cerebral cortex, which in turn exerts control over subcortical circuits that co-ordinate muscle activity to produce behavior.