Neural processing in the primary auditory cortex following cholinergic lesions of the basal forebrain in ferrets

Cortical acetylcholine (ACh) release has been linked to various cognitive functions, including perceptual learning. We have previously shown that cortical cholinergic innervation is necessary for accurate sound localization in ferrets, as well as for their ability to adapt with training to altered spatial cues. To explore whether these behavioral deficits are associated with changes in the response properties of cortical neurons, we recorded neural activity in the primary auditory cortex (A1) of anesthetized ferrets in which cholinergic inputs had been reduced by making bilateral injections of the immunotoxin ME20.4-SAP in the nucleus basalis (NB) prior to training the animals. The pattern of spontaneous activity of A1 units recorded in the ferrets with cholinergic lesions (NB ACh-) was similar to that in controls, although the proportion of burst-type units was significantly lower. Depletion of ACh also resulted in more synchronous activity in A1. No changes in thresholds, frequency tuning or in the distribution of characteristic frequencies were found in these animals. When tested with normal acoustic inputs, the spatial sensitivity of A1 neurons in the NB ACh- ferrets and the distribution of their preferred interaural level differences also closely resembled those found in control animals, indicating that these properties had not been altered by sound localization training with one ear occluded. Simulating the animals' previous experience with a virtual earplug in one ear reduced the contralateral preference of A1 units in both groups, but caused azimuth sensitivity to change in slightly different ways, which may reflect the modest adaptation observed in the NB ACh- group. These results show that while ACh is required for behavioral adaptation to altered spatial cues, it is not required for maintenance of the spectral and spatial response properties of A1 neurons.

Revisiting a Telencephalic Extent of the Ascending Reticular Activating System

Is the cerebrum involved in its own activation to states of attention or arousal? "Telencephalon" is a term borrowed from embryology to identify not only the cerebral hemispheres of the forebrain, but also the basal forebrain. We review a generally undercited literature that describes nucleus basalis of Meynert, located within the substantia innominata of the ventrobasal forebrain, as a telencephalic extension of the ascending reticular activating formation. Although that formation's precise anatomical definition and localization have proven elusive over more than 70 years, a careful reading of sources reveals that there are histological features common to certain brainstem neurons and those of the nucleus basalis, and that a largely common dendritic architecture may be a morphological aspect that helps to define non-telencephalic structures of the ascending reticular activating formation (e.g., in brainstem) as well as those parts of the formation that are telencephalic and themselves responsible for cortical activation. We draw attention to a pattern of dendritic arborization described as "isodendritic," a uniform (isos-) branching in which distal dendrite branches are significantly longer than proximal ones. Isodendritic neurons also differ from other morphological types based on their heterogeneous, rather than specific afferentation. References reviewed here are consistent in their descriptions of histology, particularly in studies of locales rich in cholinergic neurons. We discuss the therapeutic implications of a basal forebrain site that may activate cortex. Interventions that specifically target nucleus basalis and, especially, the survival of its constituent neurons may benefit afflictions in which higher cortical function is compromised due to disturbed arousal or attentiveness, including not only coma and related syndromes, but also conditions colloquially described as states of cognitive "fog" or of "long-haul" mental compromise.

Characterization of basal forebrain glutamate neurons suggests a role in control of arousal and avoidance behavior

The basal forebrain (BF) is involved in arousal, attention, and reward processing but the role of individual BF neuronal subtypes is still being uncovered. Glutamatergic neurons are the least well-understood of the three main BF neurotransmitter phenotypes. Here we analyzed the distribution, size, calcium-binding protein content and projections of the major group of BF glutamatergic neurons expressing the vesicular glutamate transporter subtype 2 (vGluT2) and tested the functional effect of activating them. Mice expressing Cre recombinase under the control of the vGluT2 promoter were crossed with a reporter strain expressing the red fluorescent protein, tdTomato, to generate vGluT2-cre-tdTomato mice. Immunohistochemical staining for choline acetyltransferase and a cross with mice expressing green fluorescent protein selectively in GABAergic neurons confirmed that cholinergic, GABAergic and vGluT2+ neurons represent distinct BF subpopulations. Subsets of BF vGluT2+ neurons expressed the calcium-binding proteins calbindin or calretinin, suggesting that multiple subtypes of BF vGluT2+ neurons exist. Anterograde tracing using adeno-associated viral vectors expressing channelrhodopsin2-enhanced yellow fluorescent fusion proteins revealed major projections of BF vGluT2+ neurons to neighboring BF cholinergic and parvalbumin neurons, as well as to extra-BF areas involved in the control of arousal or aversive/rewarding behavior such as the lateral habenula and ventral tegmental area. Optogenetic activation of BF vGluT2+ neurons elicited a striking avoidance of the area where stimulation was given, whereas stimulation of BF parvalbumin or cholinergic neurons did not. Together with previous optogenetic findings suggesting an arousal-promoting role, our findings suggest that BF vGluT2 neurons play a dual role in promoting wakefulness and avoidance behavior.

Intravenous and Non-invasive Drug Delivery to the Mouse Basal ForebrainUsing MRI-guided Focused Ultrasound

Basal forebrain cholinergic neurons (BFCNs) regulate circuit dynamics underlying cognitive processing, including attention, memory, and cognitive flexibility. In Alzheimer's disease and related neurodegenerative conditions, the degeneration of BFCNs has long been considered a key player in cognitive decline. The cholinergic system thus represents a key therapeutic target. A long-standing obstacle for the development of effective cholinergic-based therapies is not only the production of biologically active compounds but also a platform for safe and efficient drug delivery to the basal forebrain. The blood-brain barrier (BBB) presents a significant challenge for drug delivery to the brain, excluding approximately 98% of small-molecule biologics and nearly 100% of large-molecule therapeutic agents from entry into the brain parenchyma. Current modalities to achieve effective drug delivery to deep brain structures, such as the basal forebrain, are particularly limited. Direct intracranial injection via a needle or catheter carries risks associated with invasive neurosurgery. Intra-arterial injection of hyperosmotic solutions or therapeutics modified to penetrate the BBB using endogenous transport systems lack regional specificity, which may not always be desirable. Intranasal, intrathecal, and intraventricular administration have limited drug distribution beyond the brain surface. Here, we present a protocol for non-invasively, locally, and transiently increasing BBB permeability using MRI-guided focused ultrasound (MRIgFUS) in the murine basal forebrain for delivery of therapeutic agents targeting the cholinergic system. Ongoing work in preclinical models and clinical trials supports the safety and feasibility of MRIgFUS-mediated BBB modulation as a promising drug delivery modality for the treatment of debilitating neurological diseases.

Chronic traumatic encephalopathy and the nucleus basalis of Meynert

Due to the growing number of chronic traumatic encephalopathy (CTE) cases in the military and contact sports, defining the cellular and molecular substrate of this disorder is crucial. Most classic neuropathological investigations describe cortical tau and, to a lesser extent, amyloid lesions, which may underlie the clinical sequela associated with CTE. The application of modern molecular biologic technology to postmortem human brain tissue has made it possible to evaluate the genetic signature of specific neuronal phenotypes at different stages of CTE pathology. Most recently, molecular pathobiology has been used in the field of CTE, with an emphasis on the cholinergic neurons located within the nucleus basalis of Meynert, which develop tau pathology and are associated with cognitive dysfunction similar to that found in Alzheimer's disease (AD). Quantitative findings derived from single-cell transcript investigations provide clues to our understanding of the selective vulnerability of neurons containing AD-like tau pathology at different stages of CTE. Since human tissue-based studies provide a gold standard for the field of CTE, continued molecular pathological studies are needed to reveal novel drug targets for the treatment of this disorder.

A Comparison of Dopaminergic and Cholinergic Populations Reveals Unique Contributions of VTA Dopamine Neurons to Short-Term Memory

We systematically compare the contributions of two dopaminergic and two cholinergic ascending populations to a spatial short-term memory task in rats. In ventral tegmental area dopamine (VTA-DA) and nucleus basalis cholinergic (NB-ChAT) populations, trial-by-trial fluctuations in activity during the delay period relate to performance with an inverted-U, despite the fact that both populations have low activity during that time. Transient manipulations reveal that only VTA-DA neurons, and not the other three populations we examine, contribute causally and selectively to short-term memory. This contribution is most significant during the delay period, when both increases and decreases in VTA-DA activity impair short-term memory. Our results reveal a surprising dissociation between when VTA-DA neurons are most active and when they have the biggest causal contribution to short-term memory, and they also provide support for classic ideas about an inverted-U relationship between neuromodulation and cognition.

Basal forebrain activation enhances between-trial reliability of low-frequency local field potentials (LFP) and spiking activity in tree shrew primary visual cortex (V1)

Brain state has profound effects on neural processing and stimulus encoding in sensory cortices. While the synchronized state is dominated by low-frequency local field potential (LFP) activity, low-frequency LFP power is suppressed in the desynchronized state, where a concurrent enhancement in gamma power is observed. Recently, it has been shown that cortical desynchronization co-occurs with enhanced between-trial reliability of spiking activity in sensory neurons, but it is currently unclear whether this effect is also evident in LFP signals. Here, we address this question by recording both spike trains and LFP in primary visual cortex during natural movie stimulation, and using isoflurane anesthesia and basal forebrain (BF) electrical activation as proxies for synchronized and desynchronized brain states. We show that indeed, low-frequency LFP modulations ("LFP events") also occur more reliably following BF activation. Interestingly, while being more reliable, these LFP events are smaller in amplitude compared to those generated in the synchronized brain state. We further demonstrate that differences in reliability of spiking activity between cortical states can be linked to amplitude and probability of LFP events. The correlated temporal dynamics between low-frequency LFP and spiking response reliability in visual cortex suggests that these effects may both be the result of the same neural circuit activation triggered by BF stimulation, which facilitates switching between processing of incoming sensory information in the desynchronized and reverberation of internal signals in the synchronized state.

Developmental specification of forebrain cholinergic neurons

Striatal cholinergic interneurons and basal forebrain cholinergic projection neurons, which together comprise the forebrain cholinergic system, regulate attention, memory, reward pathways, and motor activity through the neuromodulation of multiple brain circuits. The importance of these neurons in the etiology of neurocognitive disorders has been well documented, but our understanding of their specification during embryogenesis is still incomplete. All forebrain cholinergic projection neurons and interneurons appear to share a common developmental origin in the embryonic ventral telencephalon, a region that also gives rise to GABAergic projection neurons and interneurons. Significant progress has been made in identifying the key intrinsic and extrinsic factors that promote a cholinergic fate in this precursor population. However, how cholinergic interneurons and projection neurons differentiate from one another during development, as well as how distinct developmental programs contribute to heterogeneity within those two classes, is not yet well understood. In this review we summarize the transcription factors and signaling molecules known to play a role in the specification and early development of striatal and basal forebrain cholinergic neurons. We also discuss the heterogeneity of these populations and its possible developmental origins.

The role of basal forebrain cholinergic neurons in fear and extinction memory

Cholinergic input to the neocortex, dorsal hippocampus (dHipp), and basolateral amygdala (BLA) is critical for neural function and synaptic plasticity in these brain regions. Synaptic plasticity in the neocortex, dHipp, ventral Hipp (vHipp), and BLA has also been implicated in fear and extinction memory. This finding raises the possibility that basal forebrain (BF) cholinergic neurons, the predominant source of acetylcholine in these brain regions, have an important role in mediating fear and extinction memory. While empirical studies support this hypothesis, there are interesting inconsistencies among these studies that raise questions about how best to define the role of BF cholinergic neurons in fear and extinction memory. Nucleus basalis magnocellularis (NBM) cholinergic neurons that project to the BLA are critical for fear memory and contextual fear extinction memory. NBM cholinergic neurons that project to the neocortex are critical for cued and contextual fear conditioned suppression, but are not critical for fear memory in other behavioral paradigms and in the inhibitory avoidance paradigm may even inhibit contextual fear memory formation. Medial septum and diagonal band of Broca cholinergic neurons are critical for contextual fear memory and acquisition of cued fear extinction. Thus, even though the results of previous studies suggest BF cholinergic neurons modulate fear and extinction memory, inconsistent findings among these studies necessitates more research to better define the neural circuits and molecular processes through which BF cholinergic neurons modulate fear and extinction memory. Furthermore, studies determining if BF cholinergic neurons can be manipulated in such a manner so as to treat excessive fear in anxiety disorders are needed.

Reorganization of Motor Cortex by Vagus Nerve Stimulation Requires Cholinergic Innervation

BACKGROUND

Vagus nerve stimulation (VNS) paired with forelimb training drives robust, specific reorganization of movement representations in the motor cortex. The mechanisms that underlie VNS-dependent enhancement of map plasticity are largely unknown. The cholinergic nucleus basalis (NB) is a critical substrate in cortical plasticity, and several studies suggest that VNS activates cholinergic circuitry.

OBJECTIVE

We examined whether the NB is required for VNS-dependent enhancement of map plasticity in the motor cortex.

METHODS

Rats were trained to perform a lever pressing task and then received injections of the immunotoxin 192-IgG-saporin to selectively lesion cholinergic neurons of the NB. After lesion, rats underwent five days of motor training during which VNS was paired with successful trials. At the conclusion of behavioral training, intracortical microstimulation was used to document movement representations in motor cortex.

RESULTS

VNS paired with forelimb training resulted in a substantial increase in the representation of proximal forelimb in rats with an intact NB compared to untrained controls. NB lesions prevent this VNS-dependent increase in proximal forelimb area and result in representations similar to untrained controls. Motor performance was similar between groups, suggesting that differences in forelimb function cannot account for the difference in proximal forelimb representation.

CONCLUSIONS

Together, these findings indicate that the NB is required for VNS-dependent enhancement of plasticity in the motor cortex and may provide insight into the mechanisms that underlie the benefits of VNS therapy.

New insights into the human brain proteome: Protein expression profiling of deep brain stimulation target areas

Deep brain stimulation (DBS) is a neurosurgical procedure that provides therapeutic benefits for movement and affective disorders. The nucleus basalis of Meynert (NBM) and substantia nigra (SN) are considered target areas to apply DBS. Even though the degeneration of NBM and SN underlies the cognitive decline observed in neurological diseases, the protein knowledge derived from both areas is scarce. We have characterized the proteome present in both subcortical brain areas using the Triple TOF 5600 mass spectrometer, identifying 2775 and 3469 proteoforms in NBM and SN respectively. Data mining of MS-generated proteomic data have revealed that: i) 675 proteins tend to localize to synaptic ending, ii) 61% of the global dataset is also present in human CSF and/or plasma, and iii) 894 proteins have not been previously identified in healthy brain by MS. The correlation of NBM and SN proteomic expression profiles with human brain transcriptome data available at Allen Brain Atlas has revealed protein evidence for 250 genes considered with brain-wide expression and 112 genes with region-specific expression in human brain. In addition, protein datasets have been classified according to their chromosomal origin, increasing the current proteome coverage in healthy human brain.

BIOLOGICAL SIGNIFICANCE

The nucleus basalis of Meynert and substantia nigra are brain areas of clinical interest to apply the deep brain stimulation (DBS) technology in neurosurgery. Our proteomic characterization has revealed 675 proteins involved in the regulation of synaptic transmission, electrical machinery, and neurotransmitter release in both DBS target areas. Moreover, 2599 identified proteins present capacity to be secreted to the CSF and plasma. Our data contribute to a further step towards the characterization of the anatomical atlas of the human brain proteome, detecting 652 proteins that are common between different basal ganglia structures. This article is part of a Special Issue entitled: HUPO 2014.

Orexin receptor activity in the basal forebrain alters performance on an olfactory discrimination task

Cholinergic innervation of the prefrontal cortex is critical for various forms of cognition, although the efferent modulators contributing to acetylcholine (ACh) release are not well understood. The main source of cortical ACh, the basal forebrain, receives projections from lateral and perifornical hypothalamic neurons releasing the peptides orexin (orexin A; OxA, and orexin B; OxB), of which OxA is hypothesized to play a role in various cognitive functions. We sought to assess one such function known to be susceptible to basal forebrain cholinergic manipulation, olfactory discrimination acquisition, and reversal learning, in rats following intra-basal forebrain infusion of OxA or the orexin 1 receptor (OxR1) antagonist SB-334867. OxA administration facilitated, while OxR1 antagonism impaired performance on both the acquisition and reversal portions of the task. These data suggest that orexin acting in the basal forebrain may be important for cortical-dependant executive functions, possibly through the stimulation of cortical ACh release.

Maternal choline supplementation improves spatial mapping and increases basal forebrain cholinergic neuron number and size in aged Ts65Dn mice

Down syndrome (DS) is marked by intellectual disability (ID) and early-onset of Alzheimer's disease (AD) neuropathology, including basal forebrain cholinergic neuron (BFCN) degeneration. The present study tested the hypothesis that maternal choline supplementation (MCS) improves spatial mapping and protects against BFCN degeneration in the Ts65Dn mouse model of DS and AD. During pregnancy and lactation, dams were assigned to either a choline sufficient (1.1g/kg choline chloride) or choline supplemented (5.0g/kg choline chloride) diet. Between 13 and 17months of age, offspring were tested in the radial arm water maze (RAWM) to examine spatial mapping followed by unbiased quantitative morphometry of BFCNs. Spatial mapping was significantly impaired in unsupplemented Ts65Dn mice relative to normal disomic (2N) littermates. Additionally, a significantly lower number and density of medial septum (MS) hippocampal projection BFCNs was also found in unsupplemented Ts65Dn mice. Notably, MCS significantly improved spatial mapping and increased number, density, and size of MS BFCNs in Ts65Dn offspring. Moreover, the density and number of MS BFCNs correlated significantly with spatial memory proficiency, providing support for a functional relationship between these behavioral and morphometric effects of MCS for trisomic offspring. Thus, increasing maternal choline intake during pregnancy may represent a safe and effective treatment approach for expectant mothers carrying a DS fetus, as well as a possible means of BFCN neuroprotection during aging for the population at large.

Gamma band plasticity in sensory cortex is a signature of the strongest memory rather than memory of the training stimulus

Gamma oscillations (∼30-120Hz) are considered to be a reflection of coordinated neuronal activity, linked to processes underlying synaptic integration and plasticity. Increases in gamma power within the cerebral cortex have been found during many cognitive processes such as attention, learning, memory and problem solving in both humans and animals. However, the specificity of gamma to the detailed contents of memory remains largely unknown. We investigated the relationship between learning-induced increased gamma power in the primary auditory cortex (A1) and the strength of memory for acoustic frequency. Adult male rats (n=16) received three days (200 trials each) of pairing a tone (3.66 kHz) with stimulation of the nucleus basalis, which implanted a memory for acoustic frequency as assessed by associatively-induced disruption of ongoing behavior, viz., respiration. Post-training frequency generalization gradients (FGGs) revealed peaks at non-CS frequencies in 11/16 cases, likely reflecting normal variation in pre-training acoustic experiences. A stronger relationship was found between increased gamma power and the frequency with the strongest memory (peak of the difference between individual post- and pre-training FGGs) vs. behavioral responses to the CS training frequency. No such relationship was found for the theta/alpha band (4-15 Hz). These findings indicate that the strength of specific increased neuronal synchronization within primary sensory cortical fields can determine the specific contents of memory.