Multiple output pathways of the basal forebrain: organization, chemical heterogeneity, and roles in vigilance

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.

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.

Somatostatin neurons in the central amygdala mediate anxiety by disinhibition of the central sublenticular extended amygdala

Fear and anxiety are two defensive emotional states evoked by threats in the environment. Fear can be initiated by either imminent or future threats, but experimentally, it is typically studied as a phasic response initiated by imminent danger that subsides when the threats is removed. In contrast, anxiety is a sustained response, initiated by imagined or potential threats. The central amygdala (CeA) is a key structure active during both fear and anxiety but thought to engage different neural systems. Fear responses are triggered by activation of somatostatin (SOM) expressing neurons in the lateral division of the CeA (CeL), and downstream projections from the medial division. Anxiety responses engage the central extended amygdala that includes the CeA, central sublenticular extended amygdala (SLEAc) and bed nucleus of the stria terminalis, but the nature of connections between these regions is not understood. Here using a combination of tract tracing, electrophysiology, and behavioral analysis in mice, we show that a population of SOM+ neurons in the CeL project to the SLEAc where they inhibit local GABAergic interneurons. Optogenetic activation of this input to the SLEAc has no effect on movement, but is anxiogenic in both open field and elevated plus maze. Our results define the inhibitory connections between CeL and SLEAc and establish a specific CeL to SLEAc projection as a circuit element in mediating anxiety.

Chronic basal forebrain activation improves spatial memory, boosts neurotrophin receptor expression, and lowers BACE1 and Aβ42 levels in the cerebral cortex in mice

The etiology of Alzheimer's dementia has been hypothesized in terms of basal forebrain cholinergic decline, and in terms of reflecting beta-amyloid neuropathology. To study these different biological elements, we activated the basal forebrain in 5xFAD Alzheimer's model mice and littermates. Mice received 5 months of 1 h per day intermittent stimulation of the basal forebrain, which includes cholinergic projections to the cortical mantle. Then, mice were behaviorally tested followed by tissue analysis. The 5xFAD mice performed worse in water-maze testing than littermates. Stimulated groups learned the water maze better than unstimulated groups. Stimulated groups had 2-3-fold increases in frontal cortex immunoblot measures of the neurotrophin receptors for nerve growth factor and brain-derived neurotrophic factor, and a more than 50% decrease in the expression of amyloid cleavage enzyme BACE1. Stimulation also led to lower Aβ42 in 5xFAD mice. These data support a causal relationship between basal forebrain activation and both neurotrophin activation and reduced Aβ42 generation and accumulation. The observation that basal forebrain activation suppresses Aβ42 accumulation, combined with the known high-affinity antagonism of nicotinic receptors by Aβ42, documents bidirectional antagonism between acetylcholine and Aβ42.

Improving Visual Working Memory with Cholinergic Deep Brain Stimulation

Acetylcholine is a critical modulatory neurotransmitter for cognitive function. Cholinergic drugs improve cognitive performance and enhance neuronal activity in the sensory and association cortices. An alternative means of improving cognitive function is through the use of deep brain stimulation. Prior animal studies have demonstrated that stimulation of the nucleus basalis of Meynert through DBS improves cognitive performance on a visual working memory task to the same degree as cholinesterase inhibitors. Additionally, unlike current pharmacological treatments for neurocognitive disorders, DBS does not lose efficacy over time and adverse effects are rare. These findings suggest that DBS may be a promising alternative for treating cognitive impairments in neurodegenerative disorders such as Alzheimer's disease. Thus, further research and human trials should be considered to assess the potential of DBS as a therapeutic treatment for these disorders.

Cognitive Deficits in Aging Related to Changes in Basal Forebrain Neuronal Activity

Aging is a physiological process accompanied by a decline in cognitive performance. The cholinergic neurons of the basal forebrain provide projections to the cortex that are directly engaged in many cognitive processes in mammals. In addition, basal forebrain neurons contribute to the generation of different rhythms in the EEG along the sleep/wakefulness cycle. The aim of this review is to provide an overview of recent advances grouped around the changes in basal forebrain activity during healthy aging. Elucidating the underlying mechanisms of brain function and their decline is especially relevant in today's society as an increasingly aged population faces higher risks of developing neurodegenerative diseases such as Alzheimer's disease. The profound age-related cognitive deficits and neurodegenerative diseases associated with basal forebrain dysfunction highlight the importance of investigating the aging of this brain region.

Polypyrimidine tract binding protein knockdown reverses depression-like behaviors and cognition impairment in mice with lesioned cholinergic neurons

Background and objectives

Depression is a common comorbidity of dementia and may be a risk factor for dementia. Accumulating evidence has suggested that the cholinergic system plays a central role in dementia and depression, and the loss of cholinergic neurons is associated with memory decline in aging and Alzheimer's patients. A specific loss of cholinergic neurons in the horizontal limb of the diagonal band of Broca (HDB) is correlated with depression and dysfunction of cognition in mice. In this study, we examined the potential regenerative mechanisms of knockdown the RNA-binding protein polypyrimidine tract binding protein (PTB) in reversing depression-like behaviors and cognition impairment in mice with lesioned cholinergic neurons.

Methods

We lesioned cholinergic neurons in mice induced by injection of 192 IgG-saporin into HDB; then, we injected either antisense oligonucleotides or adeno-associated virus-shRNA (GFAP promoter) into the injured area of HDB to deplete PTB followed by a broad range of methodologies including behavioral examinations, Western blot, RT-qPCR and immunofluorescence.

Results

We found that the conversion of astrocytes to newborn neurons by using antisense oligonucleotides on PTB in vitro, and depletion of PTB using either antisense oligonucleotides or adeno-associated virus-shRNA into the injured area of HDB could specifically transform astrocytes into cholinergic neurons. Meanwhile, knockdown of PTB by both approaches could relieve the depression-like behaviors shown by sucrose preference, forced swimming or tail-suspension tests, and alleviate cognitive impairment such as fear conditioning and novel object recognition in mice with lesioned cholinergic neurons.

Conclusion

These findings suggest that supplementing cholinergic neurons after PTB knockdown may be a promising therapeutic strategy to revert depression-like behaviors and cognitive impairment.

Reward ameliorates depressive-like behaviors via inhibition of the substantia innominata to the lateral habenula projection

The lateral habenula (LHb) is implicated in emotional processing, especially depression. Recent studies indicate that the basal forebrain (BF) transmits reward or aversive signals to the LHb. However, the contribution of the BF-LHb circuit to the pathophysiology of depression still needs to be determined. Here, we find that the excitatory projection to the LHb from the substantia innominata (SI), a BF subregion, is activated by aversive stimuli and inhibited by reward stimuli. Furthermore, chronic activation of the SI-LHb circuit is sufficient to induce depressive-like behaviors, whereas inhibition of the circuit alleviates chronic stress-induced depressive-like phenotype. We also find that reward consumption buffers depressive-like behaviors induced by chronic activation of the SI-LHb circuit. In summary, we systematically define the function and mechanism of the SI-LHb circuit in modulating depressive-like behaviors, thus providing important insights to better decipher LHb processing in the pathophysiology of depression.

Morphological, Structural, and Functional Networks Highlight the Role of the Cortical-Subcortical Circuit in Individuals With Subjective Cognitive Decline

Subjective cognitive decline (SCD) is considered the earliest stage of the clinical manifestations of the continuous progression of Alzheimer’s Disease (AD). Previous studies have suggested that multimodal brain networks play an important role in the early diagnosis and mechanisms underlying SCD. However, most of the previous studies focused on a single modality, and lacked correlation analysis between different modal biomarkers and brain regions. In order to further explore the specific characteristic of the multimodal brain networks in the stage of SCD, 22 individuals with SCD and 20 matched healthy controls (HCs) were recruited in the present study. We constructed the individual morphological, structural and functional brain networks based on 3D-T1 structural magnetic resonance imaging (sMRI), diffusion tensor imaging (DTI) and resting-state functional magnetic resonance imaging (rs-fMRI), respectively. A t-test was used to select the connections with significant difference, and a multi-kernel support vector machine (MK-SVM) was applied to combine the selected multimodal connections to distinguish SCD from HCs. Moreover, we further identified the consensus connections of brain networks as the most discriminative features to explore the pathological mechanisms and potential biomarkers associated with SCD. Our results shown that the combination of three modal connections using MK-SVM achieved the best classification performance, with an accuracy of 92.68%, sensitivity of 95.00%, and specificity of 90.48%. Furthermore, the consensus connections and hub nodes based on the morphological, structural, and functional networks identified in our study exhibited abnormal cortical-subcortical connections in individuals with SCD. In addition, the functional networks presented more discriminative connections and hubs in the cortical-subcortical regions, and were found to perform better in distinguishing SCD from HCs. Therefore, our findings highlight the role of the cortical-subcortical circuit in individuals with SCD from the perspective of a multimodal brain network, providing potential biomarkers for the diagnosis and prediction of the preclinical stage of AD.

The Medial Septum as a Potential Target for Treating Brain Disorders Associated With Oscillopathies

The medial septum (MS), as part of the basal forebrain, supports many physiological functions, from sensorimotor integration to cognition. With often reciprocal connections with a broad set of peers at all major divisions of the brain, the MS orchestrates oscillatory neuronal activities throughout the brain. These oscillations are critical in generating sensory and emotional salience, locomotion, maintaining mood, supporting innate anxiety, and governing learning and memory. Accumulating evidence points out that the physiological oscillations under septal influence are frequently disrupted or altered in pathological conditions. Therefore, the MS may be a potential target for treating neurological and psychiatric disorders with abnormal oscillations (oscillopathies) to restore healthy patterns or erase undesired ones. Recent studies have revealed that the patterned stimulation of the MS alleviates symptoms of epilepsy. We discuss here that stimulus timing is a critical determinant of treatment efficacy on multiple time scales. On-demand stimulation may dramatically reduce side effects by not interfering with normal physiological functions. A precise pattern-matched stimulation through adaptive timing governed by the ongoing oscillations is essential to effectively terminate pathological oscillations. The time-targeted strategy for the MS stimulation may provide an effective way of treating multiple disorders including Alzheimer's disease, anxiety/fear, schizophrenia, and depression, as well as pain.

Forebrain Acetylcholine Modulates Isoflurane and Ketamine Anesthesia in Adult Mice

BACKGROUND

Cholinergic drugs are known to modulate general anesthesia, but anesthesia responses in acetylcholine-deficient mice have not been studied. It was hypothesized that mice with genetic deficiency of forebrain acetylcholine show increased anesthetic sensitivity to isoflurane and ketamine and decreased gamma-frequency brain activity.

METHODS

Male adult mice with heterozygous knockdown of vesicular acetylcholine transporter in the brain or homozygous knockout of the transporter in the basal forebrain were compared with wild-type mice. Hippocampal and frontal cortical electrographic activity and righting reflex were studied in response to isoflurane and ketamine doses.

RESULTS

The loss-of-righting-reflex dose for isoflurane was lower in knockout (mean ± SD, 0.76 ± 0.08%, n = 18, P = 0.005) but not knockdown (0.78 ± 0.07%, n = 24, P = 0.021), as compared to wild-type mice (0.83 ± 0.07%, n = 23), using a significance criterion of P = 0.017 for three planned comparisons. Loss-of-righting-reflex dose for ketamine was lower in knockout (144 ± 39 mg/kg, n = 14, P = 0.006) but not knockdown (162 ± 32 mg/kg, n = 20, P = 0.602) as compared to wild-type mice (168 ± 24 mg/kg, n = 21). Hippocampal high-gamma (63 to 100 Hz) power after isoflurane was significantly lower in knockout and knockdown mice compared to wild-type mice (isoflurane-dose and mouse-group interaction effect, F[8,56] = 2.87, P = 0.010; n = 5 to 6 mice per group). Hippocampal high-gamma power after ketamine was significantly lower in both knockout and knockdown mice when compared to wild-type mice (interaction effect F[2,13] = 6.06, P = 0.014). The change in frontal cortical gamma power with isoflurane or ketamine was not statistically different among knockout, knockdown, and wild-type mice.

CONCLUSIONS

These findings suggest that forebrain cholinergic neurons modulate behavioral sensitivity and hippocampal gamma activity during isoflurane and ketamine anesthesia.

EDITOR’S PERSPECTIVE

Sleep-Wake Neurobiology

Sleep and wakefulness are complex, tightly regulated behaviors that occur in virtually all animals. With recent exciting developments in neuroscience methodologies such as optogenetics, chemogenetics, and cell-specific calcium imaging technology, researchers can advance our understanding of how discrete neuronal groups precisely modulate states of sleep and wakefulness. In this chapter, we provide an overview of key neurotransmitter systems, neurons, and circuits that regulate states of sleep and wakefulness. We also describe long-standing models for the regulation of sleep/wake and non-rapid eye movement/rapid eye movement cycling. We contrast previous knowledge derived from classic approaches such as brain stimulation, lesions, cFos expression, and single-unit recordings, with emerging data using the newest technologies. Our understanding of neural circuits underlying the regulation of sleep and wakefulness is rapidly evolving, and this knowledge is critical for our field to elucidate the enigmatic function(s) of sleep.

GABAA receptors in the basal forebrain mediates emergence from propofol anaesthesia in rats

PURPOSE

The aim of the current study was to explore the role of the basal forebrain (BF) in propofol anaesthesia.

METHODS

In the present study, we observed the neural activities of the BF during propofol anaesthesia using calcium fibre photometry recording. Subsequently, ibotenic acid was injected into the BF to verify the role of the BF in propofol anaesthesia. Finally, to test whether GABA_A receptors in the BF were involved in modulating propofol anaesthesia, muscimol (GABA_A receptor agonist) and gabazine (GABA_A receptor antagonist) were microinjected into the BF. Cortical electroencephalogram (EEG), time to loss of righting reflex (LORR), and recovery of righting reflex (RORR) under propofol anaesthesia were recorded and analysed.

RESULTS

The activity of BF neurons was inhibited during induction of propofol anaesthesia and activated during emergence from propofol anaesthesia. In addition, non-specifical lesion of BF neurons significantly prolonged the time to RORR and increased delta power in the frontal cortex under propofol anaesthesia. Next, microinjection of muscimol into the BF delayed emergence from propofol anaesthesia, increased delta power of the frontal cortex, and decreased gamma power under propofol anaesthesia. Conversely, infusion of gabazine accelerated emergence times and decreased EEG delta power.

CONCLUSIONS

The basal forebrain is involved in modulating frontal cortex delta activity and emergence from propofol anaesthesia. Additionally, the GABA_A receptors in the basal forebrain are involved in regulating emergence propofol anaesthesia.

Effects of basal forebrain stimulation on the vibrotactile responses of neurons from the hindpaw representation in the rat SI cortex

Basal forebrain (BF) cholinergic system is important for attention and modulates sensory processing. We focused on the hindpaw representation in rat primary somatosensory cortex (S1), which receives inputs related to mechanoreceptors identical to those in human glabrous skin. Spike data were recorded from S1 tactile neurons (n = 87) with (ON condition: 0.5-ms bipolar current pulses at 100 Hz; amplitude 50 μA, duration 0.5 s at each trial) and without (OFF condition) electrical stimulation of BF in anesthetized rats. We expected that prior activation of BF would induce changes in the vibrotactile responses of neurons during sinusoidal (5, 40, and 250 Hz) mechanical stimulation of the glabrous skin. The experiment consisted of sequential OFF-ON conditions in two-time blocks separated by 30 min to test possible remaining effects. Average firing rates (AFRs) and vector strengths of spike phases (VS) were analyzed for different neuron types [regular spiking (RS) and fast spiking (FS)] in different cortical layers (III-VI). Immediate effect of BF activation was only significant by increasing synchronization to 5-Hz vibrotactile stimulus within the second block. Regardless of frequency, ON-OFF paired VS differences were significantly higher in the second block compared to the first, more prominent for RS neurons, and in general for neurons in layers III and VI. No such effects could be found on AFRs. The results suggest that cholinergic activation induces some changes in the hindpaw area, enabling relatively higher increases in synchronization to vibrotactile inputs with subsequent BF modulation. In addition, this modulation depends on neuron type and layer, which may be related to detailed projection pattern from BF.

Rabies Virus Pseudotyped with CVS-N2C Glycoprotein as a Powerful Tool for Retrograde Neuronal Network Tracing

Efficient viral vectors for mapping and manipulating long-projection neuronal circuits are crucial in structural and functional studies of the brain. The SAD strain rabies virus with the glycoprotein gene deleted pseudotyped with the N2C glycoprotein (SAD-RV(ΔG)-N2C(G)) shows strong neuro-tropism in cell culture, but its in vivo efficiency for retrograde gene transduction and neuro-tropism have not been systematically characterized. We compared these features in different mouse brain regions for SAD-RV-N2C(G) and two other widely-used retrograde tracers, SAD-RV(ΔG)-B19(G) and rAAV2-retro. We found that SAD-RV(ΔG)-N2C(G) enhanced the infection efficiency of long-projecting neurons by ~10 times but with very similar neuro-tropism, compared with SAD-RV(ΔG)-B19(G). On the other hand, SAD-RV(ΔG)-N2C(G) had an infection efficiency comparable with rAAV2-retro, but a more restricted diffusion range, and broader tropism to different types and regions of long-projecting neuronal populations. These results demonstrate that SAD-RV(ΔG)-N2C(G) can serve as an effective retrograde vector for studying neuronal circuits.

Basal forebrain subcortical projections

The basal forebrain (BF) contains at least three distinct populations of neurons (cholinergic, glutamatergic, and GABA-ergic) across its different regions (medial septum, diagonal band, magnocellular preoptic area, and substantia innominata). Much attention has focused on the BF's ascending projections to cortex, but less is known about descending projections to subcortical regions. Given the neurochemical and anatomical heterogeneity of the BF, we used conditional anterograde tracing to map the patterns of subcortical projections from multiple BF regions and neurochemical cell types using mice that express Cre recombinase only in cholinergic, glutamatergic, or GABAergic neurons. We confirmed that different BF regions innervate distinct subcortical targets, with more subcortical projections arising from neurons in the caudal and lateral BF (substantia innominata and magnocellular preoptic area). Additionally, glutamatergic and GABAergic BF neurons have distinct patterns of descending projections, while cholinergic descending projections are sparse. Considering the intensity of glutamatergic and GABAergic descending projections, the BF likely acts through subcortical targets to promote arousal, motivation, and other behaviors.

Nitric oxide donor molsidomine promotes retrieval of object recognition memory in a model of cognitive deficit induced by 192 IgG-saporin

Nitric oxide (NO) plays a leading role in learning and memory processes. Previously, we showed its ability to modify the deleterious effect of immunotoxin 192 IgG-saporin (192-IgG-SAP) in the cholinergic system. The aim of this study was to analyze the potential of a NO donor (molsidomine, MOLS) to prevent the recognition memory deficits resulting from the septal cholinergic denervation by 192 IgG-SAP in rats. Quantification of neuronal and endothelial nitric oxide synthase (nNOS and eNOS, respectively) expression was evaluated in striatum, prefrontal cortex, and hippocampus. In addition, a choline acetyltransferase immunohistochemical analysis was performed in medial septum and assessed the effect of MOLS treatment on the spatial working memory of rats through a recognition memory test. Results showed that 192-IgG-SAP reduced the immunoreactivity of cholinergic septal neurons (41%), compared with PBS-receiving control rats (p < 0.05). Treatment with MOLS alone failed to antagonize the septal neuron population loss but prevented the progressive abnormal morphological changes of neurons. Those animals exposed to 192-IgG-SAP immunotoxin exhibited a reduction of cortical nNOS expression against the control group, whereas expression was enhanced in the 192-IgG-SAP + MOLS group. The most relevant finding was the recovering of the discrimination index exhibited by the 192-IgG-SAP + MOLS group. When compared with the rats exposed to the 192-IgG-SAP immunotoxin, they reached values similar to those observed in the PBS group. Our results show that although MOLS failed to block the cholinergic neurons loss induced by 192-IgG-SAP, it avoided the neuronal damage progression.

Different Subgroups of Cholinergic Neurons in the Basal Forebrain Are Distinctly Innervated by the Olfactory Regions and Activated Differentially in Olfactory Memory Retrieval

The mammalian basal forebrain (BF), a heterogenous structure providing the primary cholinergic inputs to cortical and limbic structures, plays a crucial role in various physiological processes such as learning/memory and attention. Despite the involvement of the BF cholinergic neurons (BFCNs) in olfaction related memory has been reported, the underlying neural circuits remain poorly understood. Here, we combined viral trans-synaptic tracing systems and ChAT-cre transgenic mice to systematically reveal the relationship between the olfactory system and the different subsets of BFCNs. The retrograde adeno-associated virus and rabies virus (AAV-RV) tracing showed that different subregional BFCNs received diverse inputs from multiple olfactory cortices. The cholinergic neurons in medial and caudal horizontal diagonal band Broca (HDB), magnocellular preoptic area (MCPO) and ventral substantia innominate (SI; hereafter HMS complex, HMSc) received the inputs from the entire olfactory system such as the olfactory bulb (OB), anterior olfactory nucleus (AON), entorhinal cortex (ENT), basolateral amygdala and especially the piriform cortex (PC) and hippocampus (HIP); while medial septum (MS/DB) and a part of rostral HDB (hereafter MS/DB complex, MS/DBc), predominantly from HIP; and nucleus basalis Meynert (NBM) and dorsal SI (hereafter NBM complex, NBMc), mainly from the central amygdala. The anterograde vesicular stomatitis virus (VSV) tracing further validated that the major target of the OB to the BF is HMSc. To correlate these structural relations between the BFCNs and olfactory functions, the neurons activated in the BF during olfaction related task were mapped with c-fos immunostaining. It was found that some of the BFCNs were activated in go/no-go olfactory discrimination task, but with different activated patterns. Interestingly, the BFCNs in HMSc were more significantly activated than the other subregions. Therefore, our data have demonstrated that among the different subgroups of BFCNs, HMSc is more closely related to the olfactory system, both structurally and functionally. This work provides the evidence for distinct roles of different subsets of BFNCs in olfaction associated memory.

Basal Forebrain Nuclei Display Distinct Projecting Pathways and Functional Circuits to Sensory Primary and Prefrontal Cortices in the Rat

Recent evidence supports that specific projections between different basal forebrain (BF) nuclei and their cortical targets are necessary to modulate cognitive functions in the cortex. We tested the hypothesis of the existence of specific neuronal populations in the BF linking with specific sensory, motor, and prefrontal cortices in rats. Neuronal tracing techniques were performed using retrograde tracers injected in the primary somatosensory (S1), auditory (A1), and visual (V1) cortical areas, in the medial prefrontal cortex (mPFC) as well as in BF nuclei. Results indicate that the vertical and horizontal diagonal band of Broca (VDB/HDB) nuclei target specific sensory cortical areas and maintains reciprocal projections with the prelimbic/infralimbic (PL/IL) area of the mPFC. The basal magnocellular nucleus (B nucleus) has more widespread targets in the sensory-motor cortex and does not project to the PL/IL cortex. Optogenetic stimulation was used to establish if BF neurons modulate whisker responses recorded in S1 and PL/IL cortices. We drove the expression of high levels of channelrhodopsin-2, tagged with a fluorescent protein (ChR2-eYFP) by injection of a virus in HDB or B nuclei. Blue-light pulses were delivered to the BF through a thin optic fiber to stimulate these neurons. Blue-light stimulation directed toward the HDB facilitated whisker responses in S1 cortex through activation of muscarinic receptors. The same optogenetic stimulation of HDB induced an inhibition of whisker responses in mPFC by activation of nicotinic receptors. Blue-light stimulation directed toward the B nucleus had lower effects than HDB stimulation. Our findings pointed the presence of specific neuronal networks between the BF and the cortex that may play different roles in the control of cortical activity.

Distribution of choline acetyltransferase (ChAT) immunoreactivity in the brain of the teleost Cyprinus carpio

Cholinergic systems play a role in basic cerebral functions and its dysfunction is associated with deficit in neurodegenerative disease. Mechanisms involved in human brain diseases, are often approached by using fish models, especially cyprinids, given basic similarities of the fish brain to that of mammals. In the present paper, the organization of central cholinergic systems have been described in the cyprinid Cyprinus carpio, the common carp, by using specific polyclonal antibodies against ChAT, the synthetic enzyme of acetylcholine, that is currently used as a specific marker for cholinergic neurons in all vertebrates.  In this work, serial transverse sections of the brain and the spinal cord were immunostained for ChAT. Results showed that positive neurons are present in several nuclei of the forebrain, the midbrain, the hindbrain and the spinal cord. Moreover, ChAT-positive neurons were detected in the synencephalon and in the cerebellum. In addition to neuronal bodies, afferent varicose fibers were stained for ChAT in the ventral telencephalon, the preoptic area, the hypothalamus and the posterior tuberculum. No neuronal cell bodies were present in the telencephalon. The comparison of cholinergic distribution pattern in the Cyprinus carpio central nervous system has revealed similarities but also some interesting differences with other cyprinids. Our results provide additional information on the cholinergic system from a phylogenetic point of view and may add new perspectives to physiological roles of cholinergic system during evolution and the neuroanatomical basis of neurological diseases.

Somatostatin Neurons in the Basal Forebrain Promote High-Calorie Food Intake

Obesity has become a global issue, and the overconsumption of food is thought to be a major contributor. However, the regulatory neural circuits that regulate palatable food consumption remain unclear. Here, we report that somatostatin (SOM) neurons and GABAergic (VGAT) neurons in the basal forebrain (BF) play specific roles in regulating feeding. Optogenetic stimulation of BF SOM neurons increased fat and sucrose intake within minutes and promoted anxiety-like behaviors. Furthermore, optogenetic stimulation of projections from BF SOM neurons to the lateral hypothalamic area (LHA) selectively resulted in fat intake. In addition, activation of BF VGAT neurons rapidly induced general food intake and gnawing behaviors. Whole-brain mapping of inputs and outputs showed that BF SOM neurons form bidirectional connections with several brain areas important in feeding and regulation of emotion. Collectively, these results suggest that BF SOM neurons play a selective role in hedonic feeding.

Multimodal Encoding of Novelty, Reward, and Learning in the Primate Nucleus Basalis of Meynert

Associative learning is crucial for daily function, involving a complex network of brain regions. One region, the nucleus basalis of Meynert (NBM), is a highly interconnected, largely cholinergic structure implicated in multiple aspects of learning. We show that single neurons in the NBM of nonhuman primates (NHPs; n = 2 males; Macaca mulatta) encode learning a new association through spike rate modulation. However, the power of low-frequency local field potential (LFP) oscillations decreases in response to novel, not-yet-learned stimuli but then increase as learning progresses. Both NBM and the dorsolateral prefrontal cortex encode confidence in novel associations by increasing low- and high-frequency LFP power in anticipation of expected rewards. Finally, NBM high-frequency power dynamics are anticorrelated with spike rate modulations. Therefore, novelty, learning, and reward anticipation are separately encoded through differentiable NBM signals. By signaling both the need to learn and confidence in newly acquired associations, NBM may play a key role in coordinating cortical activity throughout the learning process.SIGNIFICANCE STATEMENT Degradation of cells in a key brain region, the nucleus basalis of Meynert (NBM), correlates with Alzheimer's disease and Parkinson's disease progression. To better understand the role of this brain structure in learning and memory, we examined neural activity in the NBM in behaving nonhuman primates while they performed a learning and memory task. We found that single neurons in NBM encoded both salience and an early learning, or cognitive state, whereas populations of neurons in the NBM and prefrontal cortex encode learned state and reward anticipation. The NBM may thus encode multiple stages of learning. These multimodal signals might be leveraged in future studies to develop neural stimulation to facilitate different stages of learning and memory.

Bilateral Pathways from the Basal Forebrain to Sensory Cortices May Contribute to Synchronous Sensory Processing

Sensory processing in the cortex should integrate inputs arriving from receptive fields located on both sides of the body. This role could be played by the corpus callosum through precise projections between both hemispheres. However, different studies suggest that cholinergic projections from the basal forebrain (BF) could also contribute to the synchronization and integration of cortical activities. Using tracer injections and optogenetic techniques in transgenic mice, we investigated whether the BF cells project bilaterally to sensory cortical areas, and have provided anatomical evidence to support a modulatory role for the cholinergic projections in sensory integration. Application of the retrograde tracer Fluor-Gold or Fast Blue in both hemispheres of the primary somatosensory (S1), auditory or visual cortical areas showed labeled neurons in the ipsi- and contralateral areas of the diagonal band of Broca and substantia innominata. The nucleus basalis magnocellularis only showed ipsilateral projections to the cortex. Optogenetic stimulation of the horizontal limb of the diagonal band of Broca facilitated whisker responses in the S1 cortex of both hemispheres through activation of muscarinic cholinergic receptors and this effect was diminished by atropine injection. In conclusion, our findings have revealed that specific areas of the BF project bilaterally to sensory cortices and may contribute to the coordination of neuronal activity on both hemispheres.

Multiple Mechanisms for Processing Reward Uncertainty in the Primate Basal Forebrain

The ability to use information about the uncertainty of future outcomes is critical for adaptive behavior in an uncertain world. We show that the basal forebrain (BF) contains at least two distinct neural-coding strategies to support this capacity. The dorsal-lateral BF, including the ventral pallidum (VP), contains reward-sensitive neurons, some of which are selectively suppressed by uncertain-reward predictions (U(-)). In contrast, the medial BF (mBF) contains reward-sensitive neurons, some of which are selectively enhanced (U(+)) by uncertain-reward predictions. In a two-alternative choice-task, U(-) neurons were selectively suppressed while monkeys chose uncertain options over certain options. During the same choice-epoch, U(+) neurons signaled the subjective reward value of the choice options. Additionally, after the choice was reported, U(+) neurons signaled reward uncertainty until the choice outcome. We suggest that uncertainty-related suppression of VP may participate in the mediation of uncertainty-seeking actions, whereas uncertainty-related enhancement of the mBF may direct cognitive resources to monitor and learn from uncertain-outcomes.

SIGNIFICANCE STATEMENT

To survive in an uncertain world, we must approach uncertainty and learn from it. Here we provide evidence for two mostly distinct mechanisms for processing uncertainty about rewards within different subregions of the primate basal forebrain (BF). We found that uncertainty suppressed the representation of certain (or safe) reward values by some neurons in the dorsal-lateral BF, in regions occupied by the ventral pallidum. This uncertainty-related suppression was evident as monkeys made risky choices. We also found that uncertainty-enhanced the activity of many medial BF neurons, most prominently after the monkeys' choices were completed (as they awaited uncertain outcomes). Based on these findings, we propose that different subregions of the BF could support action and learning under uncertainty in distinct but complimentary manners.

Descending projections from the basal forebrain to the orexin neurons in mice

The orexin (hypocretin) neurons play an essential role in promoting arousal, and loss of the orexin neurons results in narcolepsy, a condition characterized by chronic sleepiness and cataplexy. The orexin neurons excite wake-promoting neurons in the basal forebrain (BF), and a reciprocal projection from the BF back to the orexin neurons may help promote arousal and motivation. The BF contains at least three different cell types (cholinergic, glutamatergic, and γ-aminobutyric acid (GABA)ergic neurons) across its different regions (medial septum, diagonal band, magnocellular preoptic area, and substantia innominata). Given the neurochemical and anatomical heterogeneity of the BF, we mapped the pattern of BF projections to the orexin neurons across multiple BF regions and neuronal types. We performed conditional anterograde tracing using mice that express Cre recombinase only in neurons producing acetylcholine, glutamate, or GABA. We found that the orexin neurons are heavily apposed by axon terminals of glutamatergic and GABAergic neurons of the substantia innominata (SI) and magnocellular preoptic area, but there was no innervation by the cholinergic neurons. Channelrhodopsin-assisted circuit mapping (CRACM) demonstrated that glutamatergic SI neurons frequently form functional synapses with the orexin neurons, but, surprisingly, functional synapses from SI GABAergic neurons were rare. Considering their strong reciprocal connections, BF and orexin neurons likely work in concert to promote arousal, motivation, and other behaviors. J. Comp. Neurol. 525:1668-1684, 2017. © 2016 Wiley Periodicals, Inc.

Mechanisms and use of neural transplants for brain repair

Under appropriate conditions, neural tissues transplanted into the adult mammalian brain can survive, integrate, and function so as to influence the behavior of the host, opening the prospect of repairing neuronal damage, and alleviating symptoms associated with neuronal injury or neurodegenerative disease. Alternative mechanisms of action have been postulated: nonspecific effects of surgery; neurotrophic and neuroprotective influences on disease progression and host plasticity; diffuse or locally regulated pharmacological delivery of deficient neurochemicals, neurotransmitters, or neurohormones; restitution of the neuronal and glial environment necessary for proper host neuronal support and processing; promoting local and long-distance host and graft axon growth; formation of reciprocal connections and reconstruction of local circuits within the host brain; and up to full integration and reconstruction of fully functional host neuronal networks. Analysis of neural transplants in a broad range of anatomical systems and disease models, on simple and complex classes of behavioral function and information processing, have indicated that all of these alternative mechanisms are likely to contribute in different circumstances. Thus, there is not a single or typical mode of graft function; rather grafts can and do function in multiple ways, specific to each particular context. Consequently, to develop an effective cell-based therapy, multiple dimensions must be considered: the target disease pathogenesis; the neurodegenerative basis of each type of physiological dysfunction or behavioral symptom; the nature of the repair required to alleviate or remediate the functional impairments of particular clinical relevance; and identification of a suitable cell source or delivery system, along with the site and method of implantation, that can achieve the sought for repair and recovery.

Relationship between cortical state and spiking activity in the lateral geniculate nucleus of marmosets

KEY POINTS

How parallel are the primate visual pathways? In the present study, we demonstrate that parallel visual pathways in the dorsal lateral geniculate nucleus (LGN) show distinct patterns of interaction with rhythmic activity in the primary visual cortex (V1). In the V1 of anaesthetized marmosets, the EEG frequency spectrum undergoes transient changes that are characterized by fluctuations in delta-band EEG power. We show that, on multisecond timescales, spiking activity in an evolutionary primitive (koniocellular) LGN pathway is specifically linked to these slow EEG spectrum changes. By contrast, on subsecond (delta frequency) timescales, cortical oscillations can entrain spiking activity throughout the entire LGN. Our results are consistent with the hypothesis that, in waking animals, the koniocellular pathway selectively participates in brain circuits controlling vigilance and attention.

ABSTRACT

The major afferent cortical pathway in the visual system passes through the dorsal lateral geniculate nucleus (LGN), where nerve signals originating in the eye can first interact with brain circuits regulating visual processing, vigilance and attention. In the present study, we investigated how ongoing and visually driven activity in magnocellular (M), parvocellular (P) and koniocellular (K) layers of the LGN are related to cortical state. We recorded extracellular spiking activity in the LGN simultaneously with local field potentials (LFP) in primary visual cortex, in sufentanil-anaesthetized marmoset monkeys. We found that asynchronous cortical states (marked by low power in delta-band LFPs) are linked to high spike rates in K cells (but not P cells or M cells), on multisecond timescales. Cortical asynchrony precedes the increases in K cell spike rates by 1-3 s, implying causality. At subsecond timescales, the spiking activity in many cells of all (M, P and K) classes is phase-locked to delta waves in the cortical LFP, and more cells are phase-locked during synchronous cortical states than during asynchronous cortical states. The switch from low-to-high spike rates in K cells does not degrade their visual signalling capacity. By contrast, during asynchronous cortical states, the fidelity of visual signals transmitted by K cells is improved, probably because K cell responses become less rectified. Overall, the data show that slow fluctuations in cortical state are selectively linked to K pathway spiking activity, whereas delta-frequency cortical oscillations entrain spiking activity throughout the entire LGN, in anaesthetized marmosets.

Afferent and Efferent Connections of the Cortex-Amygdala Transition Zone in Mice

The transitional zone between the ventral part of the piriform cortex and the anterior cortical nucleus of the amygdala, named the cortex-amygdala transition zone (CxA), shows two differential features that allow its identification as a particular structure. First, it receives dense cholinergic and dopaminergic innervations as compared to the adjacent piriform cortex and amygdala, and second, it receives projections from the main and accessory olfactory bulbs. In this work we have studied the pattern of afferent and efferent projections of the CxA, which are mainly unknown, by using the retrograde tracer Fluorogold and the anterograde tracer biotinylated dextranamine. The results show that the CxA receives a relatively restricted set of intratelencephalic connections, originated mainly by the olfactory system and basal forebrain, with minor afferents from the amygdala. The only relevant extratelencephalic afference originates in the ventral tegmental area (VTA). The efferent projections of the CxA reciprocate the inputs from the piriform cortex and olfactory amygdala. In addition, the CxA projects densely to the basolateral amygdaloid nucleus and the olfactory tubercle. The extratelencephalic projections of the CxA are very scarce, and target mainly hypothalamic structures. The pattern of connections of the CxA suggests that it is indeed a transitional area between the piriform cortex and the cortical amygdala. Double labeling with choline acetyltransferase indicates that the afferent projection from the basal forebrain is the origin of its distinctive cholinergic innervation, and double labeling with dopamine transporter shows that the projection from the VTA is the source of dopaminergic innervation. These connectivity and neurochemical features, together with the fact that it receives vomeronasal in addition to olfactory information, suggest that the CxA may be involved in processing olfactory information endowed with relevant biological meaning, such as odors related to reproductive or defensive behaviors.

Gamma band directional interactions between basal forebrain and visual cortex during wake and sleep states

The basal forebrain (BF) is an important regulator of cortical excitability and responsivity to sensory stimuli, and plays a major role in wake-sleep regulation. While the impact of BF on cortical EEG or LFP signals has been extensively documented, surprisingly little is known about LFP activity within BF. Based on bilateral recordings from rats in their home cage, we describe endogenous LFP oscillations in the BF during quiet wakefulness, rapid eye movement (REM) and slow wave sleep (SWS) states. Using coherence and Granger causality methods, we characterize directional influences between BF and visual cortex (VC) during each of these states. We observed pronounced BF gamma activity particularly during wakefulness, as well as to a lesser extent during SWS and REM. During wakefulness, this BF gamma activity exerted a directional influence on VC that was associated with cortical excitation. During SWS but not REM, there was also a robust directional gamma band influence of BF on VC. In all three states, directional influence in the gamma band was only present in BF to VC direction and tended to be regulated specifically within each brain hemisphere. Locality of gamma band LFPs to the BF was confirmed by demonstration of phase locking of local spiking activity to the gamma cycle. We report novel aspects of endogenous BF LFP oscillations and their relationship to cortical LFP signals during sleep and wakefulness. We link our findings to known aspects of GABAergic BF networks that likely underlie gamma band LFP activations, and show that the Granger causality analyses can faithfully recapitulate many known attributes of these networks.

Cell type-specific long-range connections of basal forebrain circuit

The basal forebrain (BF) plays key roles in multiple brain functions, including sleep-wake regulation, attention, and learning/memory, but the long-range connections mediating these functions remain poorly characterized. Here we performed whole-brain mapping of both inputs and outputs of four BF cell types - cholinergic, glutamatergic, and parvalbumin-positive (PV+) and somatostatin-positive (SOM+) GABAergic neurons - in the mouse brain. Using rabies virus -mediated monosynaptic retrograde tracing to label the inputs and adeno-associated virus to trace axonal projections, we identified numerous brain areas connected to the BF. The inputs to different cell types were qualitatively similar, but the output projections showed marked differences. The connections to glutamatergic and SOM+ neurons were strongly reciprocal, while those to cholinergic and PV+ neurons were more unidirectional. These results reveal the long-range wiring diagram of the BF circuit with highly convergent inputs and divergent outputs and point to both functional commonality and specialization of different BF cell types.

Cholinergic profiles in the Goettingen miniature pig (Sus scrofa domesticus) brain

Central cholinergic structures within the brain of the even-toed hoofed Goettingen miniature domestic pig (Sus scrofa domesticus) were evaluated by immunohistochemical visualization of choline acetyltransferase (ChAT) and the low-affinity neurotrophin receptor, p75^NTR . ChAT-immunoreactive (-ir) perikarya were seen in the olfactory tubercle, striatum, medial septal nucleus, vertical and horizontal limbs of the diagonal band of Broca, and the nucleus basalis of Meynert, medial habenular nucleus, zona incerta, neurosecretory arcuate nucleus, cranial motor nuclei III and IV, Edinger-Westphal nucleus, parabigeminal nucleus, pedunculopontine nucleus, and laterodorsal tegmental nucleus. Cholinergic ChAT-ir neurons were also found within transitional cortical areas (insular, cingulate, and piriform cortices) and hippocampus proper. ChAT-ir fibers were seen throughout the dentate gyrus and hippocampus, in the mediodorsal, laterodorsal, anteroventral, and parateanial thalamic nuclei, the fasciculus retroflexus of Meynert, basolateral and basomedial amygdaloid nuclei, anterior pretectal and interpeduncular nuclei, as well as select laminae of the superior colliculus. Double immunofluorescence demonstrated that virtually all ChAT-ir basal forebrain neurons were also p75^NTR -positive. The present findings indicate that the central cholinergic system in the miniature pig is similar to other mammalian species. Therefore, the miniature pig may be an appropriate animal model for preclinical studies of neurodegenerative diseases where the cholinergic system is compromised. J. Comp. Neurol. 525:553-573, 2017. © 2016 Wiley Periodicals, Inc.

Cholinergic Neurons in the Basal Forebrain Promote Wakefulness by Actions on Neighboring Non-Cholinergic Neurons: An Opto-Dialysis Study

Understanding the control of sleep-wake states by the basal forebrain (BF) poses a challenge due to the intermingled presence of cholinergic, GABAergic, and glutamatergic neurons. All three BF neuronal subtypes project to the cortex and are implicated in cortical arousal and sleep-wake control. Thus, nonspecific stimulation or inhibition studies do not reveal the roles of these different neuronal types. Recent studies using optogenetics have shown that "selective" stimulation of BF cholinergic neurons increases transitions between NREM sleep and wakefulness, implicating cholinergic projections to cortex in wake promotion. However, the interpretation of these optogenetic experiments is complicated by interactions that may occur within the BF. For instance, a recent in vitro study from our group found that cholinergic neurons strongly excite neighboring GABAergic neurons, including the subset of cortically projecting neurons, which contain the calcium-binding protein, parvalbumin (PV) (Yang et al., 2014). Thus, the wake-promoting effect of "selective" optogenetic stimulation of BF cholinergic neurons could be mediated by local excitation of GABA/PV or other non-cholinergic BF neurons. In this study, using a newly designed opto-dialysis probe to couple selective optical stimulation with simultaneous in vivo microdialysis, we demonstrated that optical stimulation of cholinergic neurons locally increased acetylcholine levels and increased wakefulness in mice. Surprisingly, the enhanced wakefulness caused by cholinergic stimulation was abolished by simultaneous reverse microdialysis of cholinergic receptor antagonists into BF. Thus, our data suggest that the wake-promoting effect of cholinergic stimulation requires local release of acetylcholine in the basal forebrain and activation of cortically projecting, non-cholinergic neurons, including the GABAergic/PV neurons.

SIGNIFICANCE STATEMENT

Optogenetics is a revolutionary tool to assess the roles of particular groups of neurons in behavioral functions, such as control of sleep and wakefulness. However, the interpretation of optogenetic experiments requires knowledge of the effects of stimulation on local neurotransmitter levels and effects on neighboring neurons. Here, using a novel "opto-dialysis" probe to couple optogenetics and in vivo microdialysis, we report that optical stimulation of basal forebrain (BF) cholinergic neurons in mice increases local acetylcholine levels and wakefulness. Reverse microdialysis of cholinergic antagonists within BF prevents the wake-promoting effect. This important result challenges the prevailing dictum that BF cholinergic projections to cortex directly control wakefulness and illustrates the utility of "opto-dialysis" for dissecting the complex brain circuitry underlying behavior.

Basal Forebrain Cholinergic Neurons Primarily Contribute to Inhibition of Electroencephalogram Delta Activity, Rather Than Inducing Behavioral Wakefulness in Mice

The basal forebrain (BF) cholinergic neurons have long been thought to be involved in behavioral wakefulness and cortical activation. However, owing to the heterogeneity of BF neurons and poor selectivity of traditional methods, the precise role of BF cholinergic neurons in regulating the sleep-wake cycle remains unclear. We investigated the effects of cell-selective manipulation of BF cholinergic neurons on the sleep-wake behavior and electroencephalogram (EEG) power spectrum using the pharmacogenetic technique, the 'designer receptors exclusively activated by designer drugs (DREADD)' approach, and ChAT-IRES-Cre mice. Our results showed that activation of BF cholinergic neurons expressing hM3Dq receptors significantly and lastingly decreased the EEG delta power spectrum, produced low-delta non-rapid eye movement sleep, and slightly increased wakefulness in both light and dark phases, whereas inhibition of BF cholinergic neurons expressing hM4Di receptors significantly increased EEG delta power spectrum and slightly decreased wakefulness. Next, the projections of BF cholinergic neurons were traced by humanized Renilla green fluorescent protein (hrGFP). Abundant and highly dense hrGFP-positive fibers were observed in the secondary motor cortex and cingulate cortex, and sparse hrGFP-positive fibers were observed in the ventrolateral preoptic nucleus, a known sleep-related structure. Finally, we found that activation of BF cholinergic neurons significantly increased c-Fos expression in the secondary motor cortex and cingulate cortex, but decreased c-Fos expression in the ventrolateral preoptic nucleus. Taken together, these findings reveal that the primary function of BF cholinergic neurons is to inhibit EEG delta activity through the activation of cerebral cortex, rather than to induce behavioral wakefulness.

Calcium Imaging of Basal Forebrain Activity during Innate and Learned Behaviors

The basal forebrain (BF) plays crucial roles in arousal, attention, and memory, and its impairment is associated with a variety of cognitive deficits. The BF consists of cholinergic, GABAergic, and glutamatergic neurons. Electrical or optogenetic stimulation of BF cholinergic neurons enhances cortical processing and behavioral performance, but the natural activity of these cells during behavior is only beginning to be characterized. Even less is known about GABAergic and glutamatergic neurons. Here, we performed microendoscopic calcium imaging of BF neurons as mice engaged in spontaneous behaviors in their home cages (innate) or performed a go/no-go auditory discrimination task (learned). Cholinergic neurons were consistently excited during movement, including running and licking, but GABAergic and glutamatergic neurons exhibited diverse responses. All cell types were activated by overt punishment, either inside or outside of the discrimination task. These findings reveal functional similarities and distinctions between BF cell types during both spontaneous and task-related behaviors.

Modulation of Specific Sensory Cortical Areas by Segregated Basal Forebrain Cholinergic Neurons Demonstrated by Neuronal Tracing and Optogenetic Stimulation in Mice

Neocortical cholinergic activity plays a fundamental role in sensory processing and cognitive functions. Previous results have suggested a refined anatomical and functional topographical organization of basal forebrain (BF) projections that may control cortical sensory processing in a specific manner. We have used retrograde anatomical procedures to demonstrate the existence of specific neuronal groups in the BF involved in the control of specific sensory cortices. Fluoro-Gold (FlGo) and Fast Blue (FB) fluorescent retrograde tracers were deposited into the primary somatosensory (S1) and primary auditory (A1) cortices in mice. Our results revealed that the BF is a heterogeneous area in which neurons projecting to different cortical areas are segregated into different neuronal groups. Most of the neurons located in the horizontal limb of the diagonal band of Broca (HDB) projected to the S1 cortex, indicating that this area is specialized in the sensory processing of tactile stimuli. However, the nucleus basalis magnocellularis (B) nucleus shows a similar number of cells projecting to the S1 as to the A1 cortices. In addition, we analyzed the cholinergic effects on the S1 and A1 cortical sensory responses by optogenetic stimulation of the BF neurons in urethane-anesthetized transgenic mice. We used transgenic mice expressing the light-activated cation channel, channelrhodopsin-2, tagged with a fluorescent protein (ChR2-YFP) under the control of the choline-acetyl transferase promoter (ChAT). Cortical evoked potentials were induced by whisker deflections or by auditory clicks. According to the anatomical results, optogenetic HDB stimulation induced more extensive facilitation of tactile evoked potentials in S1 than auditory evoked potentials in A1, while optogenetic stimulation of the B nucleus facilitated either tactile or auditory evoked potentials equally. Consequently, our results suggest that cholinergic projections to the cortex are organized into segregated pools of neurons that may modulate specific cortical areas.

A Role for the Transcription Factor Nk2 Homeobox 1 in Schizophrenia: Convergent Evidence from Animal and Human Studies

Schizophrenia is a highly heritable disorder with diverse mental and somatic symptoms. The molecular mechanisms leading from genes to disease pathology in schizophrenia remain largely unknown. Genome-wide association studies (GWASs) have shown that common single-nucleotide polymorphisms associated with specific diseases are enriched in the recognition sequences of transcription factors that regulate physiological processes relevant to the disease. We have used a “bottom-up” approach and tracked a developmental trajectory from embryology to physiological processes and behavior and recognized that the transcription factor NK2 homeobox 1 (NKX2-1) possesses properties of particular interest for schizophrenia. NKX2-1 is selectively expressed from prenatal development to adulthood in the brain, thyroid gland, parathyroid gland, lungs, skin, and enteric ganglia, and has key functions at the interface of the brain, the endocrine-, and the immune system. In the developing brain, NKX2-1-expressing progenitor cells differentiate into distinct subclasses of forebrain GABAergic and cholinergic neurons, astrocytes, and oligodendrocytes. The transcription factor is highly expressed in mature limbic circuits related to context-dependent goal-directed patterns of behavior, social interaction and reproduction, fear responses, responses to light, and other homeostatic processes. It is essential for development and mature function of the thyroid gland and the respiratory system, and is involved in calcium metabolism and immune responses. NKX2-1 interacts with a number of genes identified as susceptibility genes for schizophrenia. We suggest that NKX2-1 may lie at the core of several dose dependent pathways that are dysregulated in schizophrenia. We correlate the symptoms seen in schizophrenia with the temporal and spatial activities of NKX2-1 in order to highlight promising future research areas.

Optogenetic Dissection of the Basal Forebrain Neuromodulatory Control of Cortical Activation, Plasticity, and Cognition

The basal forebrain (BF) houses major ascending projections to the entire neocortex that have long been implicated in arousal, learning, and attention. The disruption of the BF has been linked with major neurological disorders, such as coma and Alzheimer's disease, as well as in normal cognitive aging. Although it is best known for its cholinergic neurons, the BF is in fact an anatomically and neurochemically complex structure. Recent studies using transgenic mouse lines to target specific BF cell types have led to a renaissance in the study of the BF and are beginning to yield new insights about cell-type-specific circuit mechanisms during behavior. These approaches enable us to determine the behavioral conditions under which cholinergic and noncholinergic BF neurons are activated and how they control cortical processing to influence behavior. Here we discuss recent advances that have expanded our knowledge about this poorly understood brain region and laid the foundation for future cell-type-specific manipulations to modulate arousal, attention, and cortical plasticity in neurological disorders.

SIGNIFICANCE STATEMENT

Although the basal forebrain is best known for, and often equated with, acetylcholine-containing neurons that provide most of the cholinergic innervation of the neocortex, it is in fact an anatomically and neurochemically complex structure. Recent studies using transgenic mouse lines to target specific cell types in the basal forebrain have led to a renaissance in this field and are beginning to dissect circuit mechanisms in the basal forebrain during behavior. This review discusses recent advances in the roles of basal forebrain cholinergic and noncholinergic neurons in cognition via their dynamic modulation of cortical activity.

Neurons in the Primate Medial Basal Forebrain Signal Combined Information about Reward Uncertainty, Value, and Punishment Anticipation

It has been suggested that the basal forebrain (BF) exerts strong influences on the formation of memory and behavior. However, what information is used for the memory-behavior formation is unclear. We found that a population of neurons in the medial BF (medial septum and diagonal band of Broca) of macaque monkeys encodes a unique combination of information: reward uncertainty, expected reward value, anticipation of punishment, and unexpected reward and punishment. The results were obtained while the monkeys were expecting (often with uncertainty) a rewarding or punishing outcome during a Pavlovian procedure, or unexpectedly received an outcome outside the procedure. In vivo anterograde tracing using manganese-enhanced MRI suggested that the major recipient of these signals is the intermediate hippocampal formation. Based on these findings, we hypothesize that the medial BF identifies various contexts and outcomes that are critical for memory processing in the hippocampal formation.

Distinct neuronal populations in the basal forebrain encode motivational salience and movement

Basal forebrain (BF) is one of the largest cortically-projecting neuromodulatory systems in the mammalian brain, and plays a key role in attention, arousal, learning and memory. The cortically projecting BF neurons, comprised of mainly magnocellular cholinergic and GABAergic neurons, are widely distributed across several brain regions that spatially overlap with the ventral striatopallidal system at the ventral pallidum (VP). As a first step toward untangling the respective functions of spatially overlapping BF and VP systems, the goal of this study was to comprehensively characterize the behavioral correlates and physiological properties of heterogeneous neuronal populations in the BF region. We found that, while rats performed a reward-biased simple reaction time task, distinct neuronal populations encode either motivational salience or movement information. The motivational salience of attended stimuli is encoded by phasic bursting activity of a large population of slow-firing neurons that have large, broad, and complex action potential waveforms. In contrast, two other separate groups of neurons encode movement-related information, and respectively increase and decrease firing rates while rats maintained fixation. These two groups of neurons mostly have higher firing rates and small, narrow action potential waveforms. These results support the conclusion that multiple neurophysiologically distinct neuronal populations in the BF region operate independently of each other as parallel functional circuits. These observations also caution against interpreting neuronal activity in this region as a homogeneous population reflecting the function of either BF or VP alone. We suggest that salience- and movement-related neuronal populations likely correspond to BF corticopetal neurons and VP neurons, respectively.

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.

The ascending median raphe projections are mainly glutamatergic in the mouse forebrain

The median raphe region (MRR) is thought to be serotonergic and plays an important role in the regulation of many cognitive functions. In the hippocampus (HIPP), the MRR exerts a fast excitatory control, partially through glutamatergic transmission, on a subpopulation of GABAergic interneurons that are key regulators of local network activity. However, not all receptors of this connection in the HIPP and in synapses established by MRR in other brain areas are known. Using combined anterograde tracing and immunogold methods, we show that the GluN2A subunit of the NMDA receptor is present in the synapses established by MRR not only in the HIPP, but also in the medial septum (MS) and in the medial prefrontal cortex (mPFC) of the mouse. We estimated similar amounts of NMDA receptors in these synapses established by the MRR and in local adjacent excitatory synapses. Using retrograde tracing and confocal laser scanning microscopy, we found that the majority of the projecting cells of the mouse MRR contain the vesicular glutamate transporter type 3 (vGluT3). Furthermore, using double retrograde tracing, we found that single cells of the MRR can innervate the HIPP and mPFC or the MS and mPFC simultaneously, and these double-projecting cells are also predominantly vGluT3-positive. Our results indicate that the majority of the output of the MRR is glutamatergic and acts through NMDA receptor-containing synapses. This suggests that key forebrain areas receive precisely targeted excitatory input from the MRR, which is able to synchronously modify activity in those regions via individual MRR cells with dual projections.

Brain areas that influence general anesthesia

This document reviews the literature on local brain manipulation of general anesthesia in animals, focusing on behavioral and electrographic effects related to hypnosis or loss of consciousness. Local inactivation or lesion of wake-active areas, such as locus coeruleus, dorsal raphe, pedunculopontine tegmental nucleus, perifornical area, tuberomammillary nucleus, ventral tegmental area and basal forebrain, enhanced general anesthesia. Anesthesia enhancement was shown as a delayed emergence (recovery of righting reflex) from anesthesia or a decrease in the minimal alveolar concentration that induced loss of righting. Local activation of various wake-active areas, including pontis oralis and centromedial thalamus, promoted behavioral or electrographic arousal during maintained anesthesia and facilitated emergence. Lesion of the sleep-active ventrolateral preoptic area resulted in increased wakefulness and decreased isoflurane sensitivity, but only for 6 days after lesion. Inactivation of any structure within limbic circuits involving the medial septum, hippocampus, nucleus accumbens, ventral pallidum, and ventral tegmental area, amygdala, entorhinal and piriform cortex delayed emergence from anesthesia, and often reduced anesthetic-induced behavioral excitation. In summary, the concept that anesthesia works on the sleep-wake system has received strong support from studies that inactivated/lesioned or activated wake-active areas, and weak support from studies that lesioned sleep-active areas. In addition to the conventional wake-sleep areas, limbic structures such as the medial septum, hippocampus and prefrontal cortex are also involved in the behavioral response to general anesthesia. We suggest that hypnosis during general anesthesia may result from disrupting the wake-active neuronal activities in multiple areas and suppressing an atropine-resistant cortical activation associated with movements.