Cholinergic brainstem neurons modulate cortical gamma activity during slow oscillations

Integrating brainstem and cortical functional architectures

The brainstem is a fundamental component of the central nervous system, yet it is typically excluded from in vivo human brain mapping efforts, precluding a complete understanding of how the brainstem influences cortical function. In this study, we used high-resolution 7-Tesla functional magnetic resonance imaging to derive a functional connectome encompassing cortex and 58 brainstem nuclei spanning the midbrain, pons and medulla. We identified a compact set of integrative hubs in the brainstem with widespread connectivity with cerebral cortex. Patterns of connectivity between brainstem and cerebral cortex manifest as neurophysiological oscillatory rhythms, patterns of cognitive functional specialization and the unimodal-transmodal functional hierarchy. This persistent alignment between cortical functional topographies and brainstem nuclei is shaped by the spatial arrangement of multiple neurotransmitter receptors and transporters. We replicated all findings using 3-Tesla data from the same participants. Collectively, this work demonstrates that multiple organizational features of cortical activity can be traced back to the brainstem.

The mesopontine tegmentum in reward and aversion: From cellular heterogeneity to behaviour

The mesopontine tegmentum, comprising the pedunculopontine tegmentum (PPN) and the laterodorsal tegmentum (LDT), is intricately connected to various regions of the basal ganglia, motor systems, and limbic systems. The PPN and LDT can regulate the activity of different brain regions of these target systems, and in this way are in a privileged position to modulate motivated behaviours. Despite recent findings, the PPN and LDT have been largely overlooked in discussions about the neural circuits associated with reward and aversion. This review aims to provide a timely and comprehensive resource on past and current research, highlighting the PPN and LDT's connectivity and influence on basal ganglia and limbic, and motor systems. Seminal studies, including lesion, pharmacological, and optogenetic/chemogenetic approaches, demonstrate their critical roles in modulating reward/aversive behaviours. The review emphasizes the need for further investigation into the associated cellular mechanisms, in order to clarify their role in behaviour and contribution for different neuropsychiatric disorders.

The Mesencephalic Locomotor Region: Multiple Cell Types, Multiple Behavioral Roles, and Multiple Implications for Disease

The mesencephalic locomotor region (MLR) controls locomotion in vertebrates. In humans with Parkinson disease, locomotor deficits are increasingly associated with decreased activity in the MLR. This brainstem region, commonly considered to include the cuneiform and pedunculopontine nuclei, has been explored as a target for deep brain stimulation to improve locomotor function, but the results are variable, from modest to promising. However, the MLR is a heterogeneous structure, and identification of the best cell type to target is only beginning. Here, I review the studies that uncovered the role of genetically defined MLR cell types, and I highlight the cells whose activation improves locomotor function in animal models of Parkinson disease. The promising cell types to activate comprise some glutamatergic neurons in the cuneiform and caudal pedunculopontine nuclei, as well as some cholinergic neurons of the pedunculopontine nucleus. Activation of MLR GABAergic neurons should be avoided, since they stop locomotion or evoke bouts flanked with numerous stops. MLR is also considered a potential target in spinal cord injury, supranuclear palsy, primary progressive freezing of gait, or stroke. Better targeting of the MLR cell types should be achieved through optimized deep brain stimulation protocols, pharmacotherapy, or the development of optogenetics for human use.

Gut-Initiated Alpha Synuclein Fibrils Drive Parkinson's Disease Phenotypes: Temporal Mapping of non-Motor Symptoms and REM Sleep Behavior Disorder

Parkinson's disease (PD) is characterized by progressive motor as well as less recognized non-motor symptoms that arise often years before motor manifestation, including sleep and gastrointestinal disturbances. Despite the heavy burden on the patient's quality of life, these non-motor manifestations are poorly understood. To elucidate the temporal dynamics of the disease, we employed a mouse model involving injection of alpha-synuclein (αSyn) pre-formed fibrils (PFF) in the duodenum and antrum as a gut-brain model of Parkinsonism. Using anatomical mapping of αSyn-PFF propagation and behavioral and physiological characterizations, we unveil a correlation between post-injection time the temporal dynamics of αSyn propagation and non-motor/motor manifestations of the disease. We highlight the concurrent presence of αSyn aggregates in key brain regions, expressing acetylcholine or dopamine, involved in sleep duration, wakefulness, and particularly REM-associated atonia corresponding to REM behavioral disorder-like symptoms. This study presents a novel and in-depth exploration into the multifaceted nature of PD, unraveling the complex connections between α-synucleinopathies, gut-brain connectivity, and the emergence of non-motor phenotypes.

The Basal Ganglia and Mesencephalic Locomotor Region Connectivity Matrix

Although classically considered a relay station for basal ganglia (BG) output, the anatomy, connectivity, and function of the mesencephalic locomotor region (MLR) were redefined during the last two decades. In striking opposition to what was initially thought, MLR and BG are actually reciprocally and intimately interconnected. New viral-based, optogenetic, and mapping technologies revealed that cholinergic, glutamatergic, and GABAergic neurons coexist in this structure, which, in addition to extending descending projections, send long-range ascending fibers to the BG. These MLR projections to the BG convey motor and non-motor information to specific synaptic targets throughout different nuclei. Moreover, MLR efferent fibers originate from precise neuronal subpopulations located in particular MLR subregions, defining independent anatomo-functional subcircuits involved in particular aspects of animal behavior such as fast locomotion, explorative locomotion, posture, forelimb- related movements, speed, reinforcement, among others. In this review, we revised the literature produced during the last decade linking MLR and BG. We conclude that the classic framework considering the MLR as a homogeneous output structure passively receiving input from the BG needs to be revisited. We propose instead that the multiple subcircuits embedded in this region should be taken as independent entities that convey relevant and specific ascending information to the BG and, thus, actively participate in the execution and tuning of behavior.

The regulation of the pedunculopontine tegmental nucleus in sleep-wake states

The pedunculopontine tegmental nucleus (PPTg) plays a vital role in sleep/wake states. There are three main kinds of heterogeneous neurons involved: cholinergic, glutamatergic, and gamma-aminobutyric acidergic (GABAergic) neurons. However, the precise roles of cholinergic, glutamatergic and GABAergic PPTg cell groups in regulating sleep-wake are unknown. Recent work suggests that the cholinergic, glutamatergic, and GABAergic neurons of the PPTg may activate the main arousal-promoting nucleus, thus exerting their wakefulness effects. We review the related projection pathways and functions of various neurons of the PPTg, especially the mechanisms of the PPTg in sleep-wake, thus providing new perspectives for research of sleep-wake mechanisms.

Integrating brainstem and cortical functional architectures

The brainstem is a fundamental component of the central nervous system yet it is typically excluded from in vivo human brain mapping efforts, precluding a complete understanding of how the brainstem influences cortical function. Here we use high-resolution 7 Tesla fMRI to derive a functional connectome encompassing cortex as well as 58 brainstem nuclei spanning the midbrain, pons and medulla. We identify a compact set of integrative hubs in the brainstem with widespread connectivity with cerebral cortex. Patterns of connectivity between brainstem and cerebral cortex manifest as multiple emergent phenomena including neurophysiological oscillatory rhythms, patterns of cognitive functional specialization, and the unimodal-transmodal functional hierarchy. This persistent alignment between cortical functional topographies and brainstem nuclei is shaped by the spatial arrangement of multiple neurotransmitter receptors and transporters. We replicate all findings using 3 Tesla data from the same participants. Collectively, we find that multiple organizational features of cortical activity can be traced back to the brainstem.

Feed-forward metabotropic signaling by Cav1 Ca2+ channels supports pacemaking in pedunculopontine cholinergic neurons

Like a handful of other neuronal types in the brain, cholinergic neurons (CNs) in the pedunculopontine nucleus (PPN) are lost during Parkinson's disease (PD). Why this is the case is unknown. One neuronal trait implicated in PD selective neuronal vulnerability is the engagement of feed-forward stimulation of mitochondrial oxidative phosphorylation (OXPHOS) to meet high bioenergetic demand, leading to sustained oxidant stress and ultimately degeneration. The extent to which this trait is shared by PPN CNs is unresolved. To address this question, a combination of molecular and physiological approaches were used. These studies revealed that PPN CNs are autonomous pacemakers with modest spike-associated cytosolic Ca2+ transients. These Ca2+ transients were partly attributable to the opening of high-threshold Cav1.2 Ca2+ channels, but not Cav1.3 channels. Cav1.2 channel signaling through endoplasmic reticulum ryanodine receptors stimulated mitochondrial OXPHOS to help maintain cytosolic adenosine triphosphate (ATP) levels necessary for pacemaking. Inhibition of Cav1.2 channels led to the recruitment of ATP-sensitive K+ channels and the slowing of pacemaking. A 'side-effect' of Cav1.2 channel-mediated stimulation of mitochondria was increased oxidant stress. Thus, PPN CNs have a distinctive physiological phenotype that shares some, but not all, of the features of other neurons that are selectively vulnerable in PD.

Integrating brainstem and cortical functional architectures

The brainstem is a fundamental component of the central nervous system yet it is typically excluded from in vivo human brain mapping efforts, precluding a complete understanding of how the brainstem influences cortical function. Here we use high-resolution 7 Tesla fMRI to derive a functional connectome encompassing cortex as well as 58 brainstem nuclei spanning the midbrain, pons and medulla. We identify a compact set of integrative hubs in the brainstem with widespread connectivity with cerebral cortex. Patterns of connectivity between brainstem and cerebral cortex manifest as multiple emergent phenomena including neurophysiological oscillatory rhythms, patterns of cognitive functional specialization, and the unimodal-transmodal functional hierarchy. This persistent alignment between cortical functional topographies and brainstem nuclei is shaped by the spatial arrangement of multiple neurotransmitter receptors and transporters. We replicate all findings using 3 Tesla data from the same participants. Collectively, we find that multiple organizational features of cortical activity can be traced back to the brainstem.

Dopaminergic Dependency of Cholinergic Pallidal Neurons

We employed the whole-cell patch-clamp method and ChAT-Cre mice to study the electrophysiological attributes of cholinergic neurons in the external globus pallidus. Most neurons were inactive, although approximately 20% displayed spontaneous firing, including burst firing. The resting membrane potential, the whole neuron input resistance, the membrane time constant and the total neuron membrane capacitance were also characterized. The current-voltage relationship showed time-independent inward rectification without a "sag". Firing induced by current injections had a brief initial fast adaptation followed by tonic firing with minimal accommodation. Intensity-frequency plots exhibited maximal average firing rates of about 10 Hz. These traits are similar to those of some cholinergic neurons in the basal forebrain. Also, we examined their dopamine sensitivity by acutely blocking dopamine receptors. This action demonstrated that the membrane potential, excitability, and firing pattern of pallidal cholinergic neurons rely on the constitutive activity of dopamine receptors, primarily D2-class receptors. The blockade of these receptors induced a resting membrane potential hyperpolarization, a decrease in firing for the same stimulus, the disappearance of fast adaptation, and the emergence of a depolarization block. This shift in physiological characteristics was evident even when the hyperpolarization was corrected with D.C. current. Neither the currents that generate the action potentials nor those from synaptic inputs were responsible. Instead, our findings suggest, that subthreshold slow ion currents, that require further investigation, are the target of this novel dopaminergic signaling.

Frequency-specific modulation of oscillatory activity in the rat auditory cortex by vagus nerve stimulation

BACKGROUND

We previously found that vagus nerve stimulation (VNS) strengthened stimulus-evoked activity in the superficial layer of the sensory cortex but not in the deep layer, suggesting that VNS altered the balance between the feedforward (FF) and feedback (FB) pathways. Band-specific oscillatory activities in the cortex could serve as an index of the FF-FB balance, but whether VNS affects cortical oscillations along sensory pathways through neuromodulators remains unclear.

HYPOTHESIS

VNS modulates the FF-FB balance through the cholinergic and noradrenergic systems, which modulate stimulus gain in the cortex.

METHODS

We investigated the effects of VNS using electrocorticography in the auditory cortex of 34 Wistar rats under general anesthesia while presenting click stimuli. In the time-frequency analyses, the putative modulation of the FF and FB pathways was estimated using high- and low-frequency power. We assessed, using analysis of variance, how VNS modulates auditory-evoked activities and how the modulation changes with cholinergic and noradrenergic antagonists.

RESULTS

VNS increased auditory cortical evoked potentials, consistent with results of our previous work. Furthermore, VNS increased auditory-evoked gamma and beta powers and decreased theta power. Local administration of cholinergic antagonists in the auditory cortex selectively disrupted the VNS-induced increase in gamma and beta power, while noradrenergic antagonists disrupted the decrease in theta power.

CONCLUSIONS

VNS might strengthen the FF pathway through the cholinergic system and attenuate the FB pathway through the noradrenergic system in the auditory cortex. Cortical gain modulation through the VNS-induced neuromodulatory system provides new mechanistic insights into the effect of VNS on auditory processing.

Feed-forward metabotropic signaling by Cav1 Ca 2+ channels supports pacemaking in pedunculopontine cholinergic neurons

Like a handful of other neuronal types in the brain, cholinergic neurons (CNs) in the pedunculopontine nucleus (PPN) are lost in the course of Parkinson's disease (PD). Why this is the case is unknown. One neuronal trait implicated in PD selective neuronal vulnerability is the engagement of feed-forward stimulation of mitochondrial oxidative phosphorylation (OXPHOS) to meet high bioenergetic demand, leading to sustained oxidant stress and ultimately degeneration. The extent to which this trait is shared by PPN CNs is unresolved. To address this question, a combination of molecular and physiological approaches were used. These studies revealed that PPN CNs are autonomous pacemakers with modest spike-associated cytosolic Ca 2+ transients. These Ca 2+ transients were attributable in part to the opening of high-threshold Cav1.2 Ca 2+ channels, but not Cav1.3 channels. Nevertheless, Cav1.2 channel signaling through endoplasmic reticulum ryanodine receptors stimulated mitochondrial OXPHOS to help maintain cytosolic adenosine triphosphate (ATP) levels necessary for pacemaking. Inhibition of Cav1.2 channels led to recruitment of ATP-sensitive K + channels and slowing of pacemaking. Cav1.2 channel-mediated stimulation of mitochondria increased oxidant stress. Thus, PPN CNs have a distinctive physiological phenotype that shares some, but not all, of the features of other neurons that are selectively vulnerable in PD.

Upper brainstem cholinergic neurons project to ascending and descending circuits

BACKGROUND

Based on their anatomical location, rostral projections of nuclei are classified as ascending circuits, while caudal projections are classified as descending circuits. Upper brainstem neurons participate in complex information processing and specific sub-populations preferentially project to participating ascending or descending circuits. Cholinergic neurons in the upper brainstem have extensive collateralizations in both ascending and descending circuits; however, their single-cell projection patterns remain unclear because of the lack of comprehensive characterization of individual neurons.

RESULTS

By combining fluorescent micro-optical sectional tomography with sparse labeling, we acquired a high-resolution whole-brain dataset of pontine-tegmental cholinergic neurons (PTCNs) and reconstructed their detailed morphology using semi-automatic reconstruction methods. As the main source of acetylcholine in some subcortical areas, individual PTCNs had abundant axons with lengths up to 60 cm and 5000 terminals and innervated multiple brain regions from the spinal cord to the cortex in both hemispheres. Based on various collaterals in the ascending and descending circuits, individual PTCNs were grouped into four subtypes. The morphology of cholinergic neurons in the pedunculopontine nucleus was more divergent, whereas the laterodorsal tegmental nucleus neurons contained richer axonal branches and dendrites. In the ascending circuits, individual PTCNs innervated the thalamus in three different patterns and projected to the cortex via two separate pathways. Moreover, PTCNs targeting the ventral tegmental area and substantia nigra had abundant collaterals in the pontine reticular nuclei, and these two circuits contributed oppositely to locomotion.

CONCLUSIONS

Our results suggest that individual PTCNs have abundant axons, and most project to various collaterals in the ascending and descending circuits simultaneously. They target regions with multiple patterns, such as the thalamus and cortex. These results provide a detailed organizational characterization of cholinergic neurons to understand the connexional logic of the upper brainstem.

Evidence for upregulation of excitatory synaptic transmission in the substantia nigra in Schizophrenia: a postmortem ultrastructural study

The dopamine hypothesis of schizophrenia suggests that psychotic symptoms originate from dysregulation of dopaminergic activity, which may be controlled by upstream innervation. We hypothesized that we would find anatomical evidence for the hyperexcitability seen in the SN. We examined and quantified synaptic morphology, which correlates with function, in the postmortem substantia nigra (SN) from 15 schizophrenia and 12 normal subjects. Synapses were counted using stereological techniques and classified based on the morphology of the post-synaptic density (PSD) and the presence or absence of a presynaptic density. The density and proportion of excitatory synapses was higher in the schizophrenia group than in controls, while the proportion (but not density) of inhibitory synapses was lower. We also detected in the schizophrenia group an increase in density of synapses with a PSD of intermediate thickness, which may represent excitatory synapses. The density of synapses with presynaptic densities was similar in both groups. The density of synapses with mixed morphologies was higher in the schizophrenia group than in controls. The human SN contains atypical synaptic morphology. We found an excess amount and proportion of excitatory synapses in the SN in schizophrenia that could result in hyperactivity and drive the psychotic symptoms of schizophrenia. The sources of afferent excitatory inputs to the SN arise from the subthalamic nucleus, the pedunculopontine nucleus, and the ventral tegmental area (VTA), areas that could be the source of excess excitation. Synapses with mixed morphologies may represent inputs from the VTA, which release multiple transmitters.

A whole new world: embracing the systems-level to understand the indirect impact of pathology in neurodegenerative disorders

The direct link between neuropathology and the symptoms that emerge from damage to the brain is often difficult to discern. In this perspective, we argue that a satisfying account of neurodegenerative symptoms most naturally emerges from the consideration of the brain from the systems-level. Specifically, we will highlight the role of the neuromodulatory arousal system, which is uniquely positioned to coordinate the brain's ability to flexibly integrate the otherwise segregated structures required to support higher cognitive functions. Importantly, the neuromodulatory arousal system is highly heterogeneous, encompassing structures that are common sites of neurodegeneration across Alzheimer's and Parkinson's disease. We will review studies that implicate the dysfunctional interactions amongst distributed brain regions as a side-effect of pathological involvement of the neuromodulatory arousal system in these neurodegenerative disorders. From this perspective, we will argue that future work in clinical neuroscience should attempt to consider the inherent complexity in the brain and employ analytic techniques that do not solely focus on regional functional impairments, but rather captures the brain as an inherently dynamic, distributed, multi-scale system. Through this lens, we hope that we will devise new and improved diagnostic markers and interventional approaches to aid in the treatment of neurodegenerative disorders.

The subcortical and neurochemical organization of the ventral and dorsal attention networks

Attention is a core cognitive function that filters and selects behaviourally relevant information in the environment. The cortical mapping of attentional systems identified two segregated networks that mediate stimulus-driven and goal-driven processes, the Ventral and the Dorsal Attention Networks (VAN, DAN). Deep brain electrophysiological recordings, behavioral data from phylogenetic distant species, and observations from human brain pathologies challenge purely corticocentric models. Here, we used advanced methods of functional alignment applied to resting-state functional connectivity analyses to map the subcortical architecture of the Ventral and Dorsal Attention Networks. Our investigations revealed the involvement of the pulvinar, the superior colliculi, the head of caudate nuclei, and a cluster of brainstem nuclei relevant to both networks. These nuclei are densely connected structural network hubs, as revealed by diffusion-weighted imaging tractography. Their projections establish interrelations with the acetylcholine nicotinic receptor as well as dopamine and serotonin transporters, as demonstrated in a spatial correlation analysis with a normative atlas of neurotransmitter systems. This convergence of functional, structural, and neurochemical evidence provides a comprehensive framework to understand the neural basis of attention across different species and brain diseases.

Alterations of sleep oscillations in Alzheimer's disease: A potential role for GABAergic neurons in the cortex, hippocampus, and thalamus

Sleep abnormalities are widely reported in patients with Alzheimer's disease (AD) and are linked to cognitive impairments. Sleep abnormalities could be potential biomarkers to detect AD since they are often observed at the preclinical stage. Moreover, sleep could be a target for early intervention to prevent or slow AD progression. Thus, here we review changes in brain oscillations observed during sleep, their connection to AD pathophysiology and the role of specific brain circuits. Slow oscillations (0.1-1 Hz), sleep spindles (8-15 Hz) and their coupling during non-REM sleep are consistently reduced in studies of patients and in AD mouse models although the timing and magnitude of these alterations depends on the pathophysiological changes and the animal model studied. Changes in delta (1-4 Hz) activity are more variable. Animal studies suggest that hippocampal sharp-wave ripples (100-250 Hz) are also affected. Reductions in REM sleep amount and slower oscillations during REM are seen in patients but less consistently in animal models. Thus, changes in a variety of sleep oscillations could impact sleep-dependent memory consolidation or restorative functions of sleep. Recent mechanistic studies suggest that alterations in the activity of GABAergic neurons in the cortex, hippocampus and thalamic reticular nucleus mediate sleep oscillatory changes in AD and represent a potential target for intervention. Longitudinal studies of the timing of AD-related sleep abnormalities with respect to pathology and dysfunction of specific neural networks are needed to identify translationally relevant biomarkers and guide early intervention strategies to prevent or delay AD progression.

UP-DOWN states and ripples differentially modulate membrane potential dynamics across DG, CA3, and CA1 in awake mice

Hippocampal ripples are transient population bursts that structure cortico-hippocampal communication and play a central role in memory processing. However, the mechanisms controlling ripple initiation in behaving animals remain poorly understood. Here we combine multisite extracellular and whole-cell recordings in awake mice to contrast the brain state and ripple modulation of subthreshold dynamics across hippocampal subfields. We find that entorhinal input to the dentate gyrus (DG) exhibits UP and DOWN dynamics with ripples occurring exclusively in UP states. While elevated cortical input in UP states generates depolarization in DG and CA1, it produces persistent hyperpolarization in CA3 neurons. Furthermore, growing inhibition is evident in CA3 throughout the course of the ripple buildup, while DG and CA1 neurons exhibit depolarization transients 100 ms before and during ripples. These observations highlight the importance of CA3 inhibition for ripple generation, while pre-ripple responses indicate a long and orchestrated ripple initiation process in the awake state.

Structural connections between the noradrenergic and cholinergic system shape the dynamics of functional brain networks

Complex cognitive abilities are thought to arise from the ability of the brain to adaptively reconfigure its internal network structure as a function of task demands. Recent work has suggested that this inherent flexibility may in part be conferred by the widespread projections of the ascending arousal systems. While the different components of the ascending arousal system are often studied in isolation, there are anatomical connections between neuromodulatory hubs that we hypothesise are crucial for mediating key features of adaptive network dynamics, such as the balance between integration and segregation. To test this hypothesis, we estimated the strength of structural connectivity between key hubs of the noradrenergic and cholinergic arousal systems (the locus coeruleus [LC] and nucleus basalis of Meynert [nbM], respectively). We then asked whether the strength of structural LC and nbM inter-connectivity was related to individual differences in the emergent, dynamical signatures of functional integration measured from resting state fMRI data, such as network and attractor topography. We observed a significant positive relationship between the strength of white-matter connections between the LC and nbM and the extent of network-level integration following BOLD signal peaks in LC relative to nbM activity. In addition, individuals with denser white-matter streamlines interconnecting neuromodulatory hubs also demonstrated a heightened ability to shift to novel brain states. These results suggest that individuals with stronger structural connectivity between the noradrenergic and cholinergic systems have a greater capacity to mediate the flexible network dynamics required to support complex, adaptive behaviour. Furthermore, our results highlight the underlying static features of the neuromodulatory hubs can impose some constraints on the dynamic features of the brain.

Effect of MK-801, an antagonist of NMDA receptor in the pedunculopontine tegmental nucleus, on cardiovascular parameters in normotensive and hydralazine hypotensive rats

Objectives

In the present study, the cardiovascular effects of glutamate NMDA receptor of the pedunculopontine tegmental nucleus (PPT) in normotensive and hydralazine (HLZ) hypotensive rats were evaluated.

Materials and Methods

In the normotensive condition, MK-801(1 nmol; an NMDA receptor antagonist) and L-glutamate (L-Glu, 50 nmol an agonist) alone and together were microinjected into the nucleus using a stereotaxic device. In hypotensive condition, 2 min after induction of hypotension by HLZ (10 mg/kg, intravenous), drugs, same as in normotensive condition, were microinjected into the PPT. Recorded mean arterial pressure (MAP), systolic blood pressure (SBP), and heart rate (HR) were recorded throughout the experiment by a Power lab apparatus that was connected to a catheter inserted into the femoral arty. The cardiovascular changes (Δ) induced by microinjection drugs were computed and statistically analyzed.

Results

In the normotensive condition, L-Glu significantly increased ΔMAP and ΔSBP (P<0.001) and decreased ΔHR (P<0.01) compared with the control. MK-801 alone significantly increased HR (P<0.05) while co-injected with L-Glu + MK-801 it significantly attenuated the L-Glu effect on ΔMAP and ΔSBP but augmented ΔHR (P<0.01). In the hydralazine hypotension condition, L-Glu significantly improved hypotension (P<0.01) and deteriorated bradycardia induced by HLZ (P<0.05). MK-801 alone did not significantly affect ΔMAP, ΔSBP, and ΔHR but when co-injected with L-Glu (L-Glu + MK-801) it could significantly attenuate the cardiovascular effect of L-Glu in the PPT.

Conclusion

We found that activation of NMDA receptors of the glutamatergic system in the PPT evoked blood pressure and inhibited HR in both normotensive and hypotensive conditions in rats.

Targeted activation of midbrain neurons restores locomotor function in mouse models of parkinsonism

The pedunculopontine nucleus (PPN) is a locomotor command area containing glutamatergic neurons that control locomotor initiation and maintenance. These motor actions are deficient in Parkinson’s disease (PD), where dopaminergic neurodegeneration alters basal ganglia activity. Being downstream of the basal ganglia, the PPN may be a suitable target for ameliorating parkinsonian motor symptoms. Here, we use in vivo cell-type specific PPN activation to restore motor function in two mouse models of parkinsonism made by acute pharmacological blockage of dopamine transmission. With a combination of chemo- and opto-genetics, we show that excitation of caudal glutamatergic PPN neurons can normalize the otherwise severe locomotor deficit in PD, whereas targeting the local GABAergic population only leads to recovery of slow locomotion. The motor rescue driven by glutamatergic PPN activation is independent of activity in nearby locomotor promoting glutamatergic Cuneiform neurons. Our observations point to caudal glutamatergic PPN neurons as a potential target for neuromodulatory restoration of locomotor function in PD.

Here, the authors use cell-type specific stimulation of brainstem neurons within the caudal pedunculopontine nucleus to show that activation of excitatory neurons can normalize severe locomotor deficit in mouse models of parkinsonism. The study defines a potential target for neuromodulatory restoration of locomotor function in Parkinson’s disease.

Local sleep: A new concept in brain plasticity

Traditionally, sleep and wakefulness have been considered as two global, mutually exclusive states. However, this view has been challenged by the discovery that sleep and wakefulness are actually locally regulated and that islands of these two states may often coexist in the same individual. Importantly, such a local regulation seems to be the key for many essential functions of sleep, including the maintenance of cognitive efficiency and the consolidation of new skills and memories. Indeed, local changes in sleep-related oscillations occur in brain areas that are used and involved in learning during wakefulness. In turn, these changes directly modulate experience-dependent brain adaptations and the consolidation of newly acquired memories. In line with these observations, alterations in the regional balance between wake- and sleep-like activity have been shown to accompany many pathologic conditions, including psychiatric and neurologic disorders. In the last decade, experimental research has started to shed light on the mechanisms involved in the local regulation of sleep and wakefulness. The results of this research have opened new avenues of investigation regarding the function of sleep and have revealed novel potential targets for the treatment of several pathologic conditions.

Stereological estimations and neurochemical characterization of neurons expressing GABAA and GABAB receptors in the rat pedunculopontine and laterodorsal tegmental nuclei

To better understand GABAergic transmission at two targets of basal ganglia downstream projections, the pedunculopontine (PPN) and laterodorsal (LDT) tegmental nuclei, the anatomical localization of GABAA and GABAB receptors was investigated in both nuclei. Specifically, the total number of neurons expressing the GABAA receptor γ2 subunit (GABAAR γ2) and the GABAB receptor R2 subunit (GABAB R2) in PPN and LDT was estimated using stereological methods, and the neurochemical phenotype of cells expressing each subunit was also determined. The mean number of non-cholinergic cells expressing GABAAR γ2 was 9850 ± 1856 in the PPN and 8285 ± 962 in the LDT, whereas those expressing GABAB R2 were 7310 ± 1970 and 9170 ± 1900 in the PPN and LDT, respectively. In addition, all cholinergic neurons in both nuclei co-expressed GABAAR γ2 and 95-98% of them co-expressed GABAB R2. Triple labeling using in situ hybridization revealed that 77% of GAD67 mRNA-positive cells in the PPT and 49% in the LDT expressed GABAAR γ2, while 90% (PPN) and 65% (LDT) of Vglut2 mRNA-positive cells also expressed GABAAR γ2. In contrast, a similar proportion (~2/3) of glutamatergic and GABAergic cells co-expressed GABAB R2 in both nuclei. The heterogeneous distribution of GABAAR and GABABR among non-cholinergic cells in PPN and LDT may give rise to physiological differences within each neurochemical subpopulation. In addition, the dissimilar proportion of GABAAR γ2-expressing glutamatergic and GABAergic neurons in the PPN and LDT may contribute to some of the functional differences found between the two nuclei.

Functional diversity for body actions in the mesencephalic locomotor region

The mesencephalic locomotor region (MLR) is a key midbrain center with roles in locomotion. Despite extensive studies and clinical trials aimed at therapy-resistant Parkinson's disease (PD), debate on its function remains. Here, we reveal the existence of functionally diverse neuronal populations with distinct roles in control of body movements. We identify two spatially intermingled glutamatergic populations separable by axonal projections, mouse genetics, neuronal activity profiles, and motor functions. Most spinally projecting MLR neurons encoded the full-body behavior rearing. Loss- and gain-of-function optogenetic perturbation experiments establish a function for these neurons in controlling body extension. In contrast, Rbp4-transgene-positive MLR neurons project in an ascending direction to basal ganglia, preferentially encode the forelimb behaviors handling and grooming, and exhibit a role in modulating movement. Thus, the MLR contains glutamatergic neuronal subpopulations stratified by projection target exhibiting roles in action control not restricted to locomotion.

Alteration of Mesopontine Cholinergic Function by the Lack of KCNQ4 Subunit

The pedunculopontine nucleus (PPN), a structure known as a cholinergic member of the reticular activating system (RAS), is source and target of cholinergic neuromodulation and contributes to the regulation of the sleep–wakefulness cycle. The M-current is a voltage-gated potassium current modulated mainly by cholinergic signaling. KCNQ subunits ensemble into ion channels responsible for the M-current. In the central nervous system, KCNQ4 expression is restricted to certain brainstem structures such as the RAS nuclei. Here, we investigated the presence and functional significance of KCNQ4 in the PPN by behavioral studies and the gene and protein expressions and slice electrophysiology using a mouse model lacking KCNQ4 expression. We found that this mouse has alterations in the adaptation to changes in light–darkness cycles, representing the potential role of KCNQ4 in the regulation of the sleep–wakefulness cycle. As cholinergic neurons from the PPN participate in the regulation of this cycle, we investigated whether the cholinergic PPN might also possess functional KCNQ4 subunits. Although the M-current is an electrophysiological hallmark of cholinergic neurons, only a subpopulation of them had KCNQ4-dependent M-current. Interestingly, the absence of the KCNQ4 subunit altered the expression patterns of the other KCNQ subunits in the PPN. We also determined that, in wild-type animals, the cholinergic inputs of the PPN modulated the M-current, and these in turn can modulate the level of synchronization between neighboring PPN neurons. Taken together, the KCNQ4 subunit is present in a subpopulation of PPN cholinergic neurons, and it may contribute to the regulation of the sleep–wakefulness cycle.

Mutual Interactions between Brain States and Alzheimer's Disease Pathology: A Focus on Gamma and Slow Oscillations

Brain state varies from moment to moment. While brain state can be defined by ongoing neuronal population activity, such as neuronal oscillations, this is tightly coupled with certain behavioural or vigilant states. In recent decades, abnormalities in brain state have been recognised as biomarkers of various brain diseases and disorders. Intriguingly, accumulating evidence also demonstrates mutual interactions between brain states and disease pathologies: while abnormalities in brain state arise during disease progression, manipulations of brain state can modify disease pathology, suggesting a therapeutic potential. In this review, by focusing on Alzheimer's disease (AD), the most common form of dementia, we provide an overview of how brain states change in AD patients and mouse models, and how controlling brain states can modify AD pathology. Specifically, we summarise the relationship between AD and changes in gamma and slow oscillations. As pathological changes in these oscillations correlate with AD pathology, manipulations of either gamma or slow oscillations can modify AD pathology in mouse models. We argue that neuromodulation approaches to target brain states are a promising non-pharmacological intervention for neurodegenerative diseases.

Modulation of Prefrontal Cortex Slow Oscillations by Phasic Activation of the Locus Coeruleus

Cortical slow rhythmic activity, a hallmark of deep sleep, is observed under urethane anesthesia. Synchronized fluctuations of the membrane excitability of a large neuronal population are reflected in the extracellular Local Field Potential (LFP), as high-amplitude slow (∼1 Hz) oscillations (SO). The SO-phase indicates the presence (Up) or absence (Down) of neuronal spiking. The cortical state is controlled by the input from thalamic and neuromodulatory centers, including the brainstem noradrenergic nucleus Locus Coeruleus (LC). The bidirectional modulation of neuronal excitability by noradrenaline (NA) is well known. We have previously shown that LC phasic activation caused transient excitability increase in the medial prefrontal cortex (mPFC). In the present study, we characterized the effect of LC phasic activation on the prefrontal population dynamics at a temporal scale of a single SO cycle. We applied short (0.2 s) trains of electric pulses (0.02-0.05 mA at 20-50 Hz) to the LC cell bodies and monitored a broadband (0.1 Hz-8 kHz) mPFC LFP in urethane-anesthetized rats. The direct electrical stimulation of LC (LC-DES), applied during the Up-phase, enhanced the firing probability in the mPFC by ∼20% and substantially prolonged Up-states in 56% of trials. The LC-DES applied during Down-phase caused a rapid Down-to-Up transition in 81.5% of trials. The LC-DES was more effective at a higher frequency, but not at a higher current. Our results suggest that transient NA release, coupled to SO, may promote synaptic plasticity and memory consolidation by sustaining a depolarized state in the mPFC neurons.

Cortical and subcortical hemodynamic changes during sleep slow waves in human light sleep

EEG slow waves, the hallmarks of NREM sleep are thought to be crucial for the regulation of several important processes, including learning, sensory disconnection and the removal of brain metabolic wastes. Animal research indicates that slow waves may involve complex interactions within and between cortical and subcortical structures. Conventional EEG in humans, however, has a low spatial resolution and is unable to accurately describe changes in the activity of subcortical and deep cortical structures. To overcome these limitations, here we took advantage of simultaneous EEG-fMRI recordings to map cortical and subcortical hemodynamic (BOLD) fluctuations time-locked to slow waves of light sleep. Recordings were performed in twenty healthy adults during an afternoon nap. Slow waves were associated with BOLD-signal increases in the posterior brainstem and in portions of thalamus and cerebellum characterized by preferential functional connectivity with limbic and somatomotor areas, respectively. At the cortical level, significant BOLD-signal decreases were instead found in several areas, including insula and somatomotor cortex. Specifically, a slow signal increase preceded slow-wave onset and was followed by a delayed, stronger signal decrease. Similar hemodynamic changes were found to occur at different delays across most cortical brain areas, mirroring the propagation of electrophysiological slow waves, from centro-frontal to inferior temporo-occipital cortices. Finally, we found that the amplitude of electrophysiological slow waves was positively related to the magnitude and inversely related to the delay of cortical and subcortical BOLD-signal changes. These regional patterns of brain activity are consistent with theoretical accounts of the functions of sleep slow waves.

Cortical astrocytes independently regulate sleep depth and duration via separate GPCR pathways

Non-rapid eye movement (NREM) sleep, characterized by slow-wave electrophysiological activity, underlies several critical functions, including learning and memory. However, NREM sleep is heterogeneous, varying in duration, depth, and spatially across the cortex. While these NREM sleep features are thought to be largely independently regulated, there is also evidence that they are mechanistically coupled. To investigate how cortical NREM sleep features are controlled, we examined the astrocytic network, comprising a cortex-wide syncytium that influences population-level neuronal activity. We quantified endogenous astrocyte activity in mice over natural sleep and wake, then manipulated specific astrocytic G-protein-coupled receptor (GPCR) signaling pathways in vivo. We find that astrocytic Gi- and Gq-coupled GPCR signaling separately control NREM sleep depth and duration, respectively, and that astrocytic signaling causes differential changes in local and remote cortex. These data support a model in which the cortical astrocyte network serves as a hub for regulating distinct NREM sleep features.

Sleep has many roles, from strengthening new memories to regulating mood and appetite. While we might instinctively think of sleep as a uniform state of reduced brain activity, the reality is more complex. First, over the course of the night, we cycle between a number of different sleep stages, which reflect different levels of sleep depth. Second, the amount of sleep depth is not necessarily even across the brain but can vary between regions.

These sleep stages consist of either rapid eye movement (REM) sleep or non-REM (NREM) sleep. REM sleep is when most dreaming occurs, whereas NREM sleep is particularly important for learning and memory and can vary in duration and depth. During NREM sleep, large groups of neurons synchronize their firing to create rhythmic waves of activity known as slow waves. The more synchronous the activity, the deeper the sleep.

Vaidyanathan et al. now show that brain cells called astrocytes help regulate NREM sleep. Astrocytes are not neurons but belong to a group of specialized cells called glia. They are the largest glia cell type in the brain and display an array of proteins on their surfaces called G-protein-coupled receptors (GPCRs). These enable them to sense sleep-wake signals from other parts of the brain and to generate their own signals. In fact, each astrocyte can communicate with thousands of neurons at once. They are therefore well-poised to coordinate brain activity during NREM sleep.

Using innovative tools, Vaidyanathan et al. visualized astrocyte activity in mice as the animals woke up or fell asleep. The results showed that astrocytes change their activity just before each sleep–wake transition. They also revealed that astrocytes control both the depth and duration of NREM sleep via two different types of GPCR signals. Increasing one of these signals (Gi-GPCR) made the mice sleep more deeply but did not change sleep duration. Decreasing the other (Gq-GPCR) made the mice sleep for longer but did not affect sleep depth.

Sleep problems affect many people at some point in their lives, and often co-exist with other conditions such as mental health disorders. Understanding how the brain regulates different features of sleep could help us develop better – and perhaps more specific – treatments for sleep disorders. The current study suggests that manipulating GPCRs on astrocytes might increase sleep depth, for example. But before work to test this idea can begin, we must first determine whether findings from sleeping mice also apply to people.

Cholinergic modulation of Up-Down states in the mouse medial entorhinal cortex in vitro

Cholinergic tone is high during wake and rapid eye movement sleep and lower during slow wave sleep (SWS). Nevertheless, the low tone of acetylcholine during SWS modulates sharp wave ripple incidence in the hippocampus and slow wave activity in the neocortex. Linking the hippocampus and neocortex, the medial entorhinal cortex (mEC) regulates the coupling between these structures during SWS, alternating between silent Down states and active Up states, which outlast neocortical ones. Here, we investigated how low physiological concentrations of acetylcholine (ACh; 100-500 nM) modulate Up and Down states in a mEC slice preparation. We find that ACh has a dual effect on mEC activity: it prolongs apparent Up state duration as recorded in individual cells and decreases the total synaptic charge transfer, without affecting the duration of detectable synaptic activity. The overall outcome of ACh application is excitatory and we show that ACh increases Up state incidence via muscarinic receptor activation. The mean firing rate of principal neurons increased in around half of the cells while the other half showed a decrease in firing rate. Using two-photon calcium imaging of population activity, we found that population-wide network events are more frequent and rhythmic during ACh and confirmed that ACh modulates cell participation in these network events, consistent with a role for cholinergic modulation in regulating information flow between the hippocampus and neocortex during SWS.

Determinants of seeding and spreading of α-synuclein pathology in the brain

In Parkinson's disease (PD), fibrillar forms of α-synuclein are hypothesized to propagate through synaptically coupled networks, causing Lewy pathology (LP) and neurodegeneration. To more rigorously characterize the determinants of spreading, preformed α-synuclein fibrils were injected into the mouse pedunculopontine nucleus (PPN), a brain region that manifests LP in PD patients and the distribution of developing α-synuclein pathology compared to that ascertained by anterograde and retrograde connectomic mapping. Within the PPN, α-synuclein pathology was cell-specific, being robust in PD-vulnerable cholinergic neurons but not in neighboring noncholinergic neurons. While nearly all neurons projecting to PPN cholinergics manifested α-synuclein pathology, the kinetics, magnitude, and persistence of the propagated pathology were unrelated to the strength of those connections. Thus, neuronal phenotype governs the somatodendritic uptake of pathological α-synuclein, and while the afferent connectome restricts the subsequent spreading of pathology, its magnitude and persistence is not a strict function of the strength of coupling.

Unexpected awakenings in severe dementia from case reports to laboratory

OBJECTIVE

Case report notions of unexpected memory retrieval in patients with severe dementia near to death are starting to alter the central "irreversible" paradigm of dementia and locate dementia as a problem of memory retrieval, not consolidation. We suggest that the most likely central tenet of this paradoxical memory retrieval is the fluctuation of neuromodulators projecting from the brain stem to the medial prefrontal cortex and the hippocampus. The neuromodulation-centric explanation of this phenomenon aims to open the "irreversible" paradigm of dementia up for discussion and suggest a plausible treatment strategy by questioning how the devastating process of death fluctuates memory performance in severe dementia.

BACKGROUND

Supporting demented patients, who are mostly unresponsive, without making demands or asking a question and regarding them as valuable human beings unexpectedly improve their memory performance around the time of death.

NEW LUCIDITY HYPOTHESIS

Around the time of death, neurological signs (hyper-arousal and -attention) of demented people point out that neurotransmitter discharges are dramatically changed. Relatively resistant neuromodulator circuits to neurodegeneration can maintain optimal levels of arousal and attention for memory processing. In this way, unexpected episodes of lucidity can be triggered. Also, corticotropin-releasing peptides might increase mental clarity by increasing the excitability of the neuromodulator circuits. The science of memory retrieval is more complicated and nuanced than retrieval observations in case reports, but the rapid development of new techniques holds promise for future understanding of lucidity in severe dementia.

MAJOR CHALLENGE FOR THE MODEL

There is no an animal or human model to test this hypothesis; however, the similarities between neurological signs (instantaneous cognitive fluctuations) of delirium and paradoxical lucidity could provide a unique window to understand neural events of terminal lucidity on a modified animal model of delirium. Likewise, similarities between unexpected consciousness signs of terminal lucidity and lucid dreaming suggest that lucid dreaming episodes might be considered a human model for terminal lucidity research.

Calcium, Bioenergetics, and Parkinson's Disease

Degeneration of substantia nigra (SN) dopaminergic (DAergic) neurons is responsible for the core motor deficits of Parkinson’s disease (PD). These neurons are autonomous pacemakers that have large cytosolic Ca²⁺ oscillations that have been linked to basal mitochondrial oxidant stress and turnover. This review explores the origin of Ca²⁺ oscillations and their role in the control of mitochondrial respiration, bioenergetics, and mitochondrial oxidant stress.

Up and Down States and Memory Consolidation Across Somatosensory, Entorhinal, and Hippocampal Cortices

In the course of a day, brain states fluctuate, from conscious awake information-acquiring states to sleep states, during which previously acquired information is further processed and stored as memories. One hypothesis is that memories are consolidated and stored during "offline" states such as sleep, a process thought to involve transfer of information from the hippocampus to other cortical areas. Up and Down states (UDS), patterns of activity that occur under anesthesia and sleep states, are likely to play a role in this process, although the nature of this role remains unclear. Here we review what is currently known about these mechanisms in three anatomically distinct but interconnected cortical areas: somatosensory cortex, entorhinal cortex, and the hippocampus. In doing so, we consider the role of this activity in the coordination of "replay" during sleep states, particularly during hippocampal sharp-wave ripples. We conclude that understanding the generation and propagation of UDS may provide key insights into the cortico-hippocampal dialogue linking archi- and neocortical areas during memory formation.

Multiparametric assessment of the impact of opsin expression and anesthesia on striatal cholinergic neurons and auditory brainstem activity

Recent developments in genetic engineering have established murine models that permit the selective control of cholinergic neurons via optical stimulation. Despite copious benefits granted by these experimental advances, the sensory physiognomy of these organisms has remained poorly understood. Therefore, the present study evaluates sensory and neuronal response properties of animal models developed for the study of optically induced acetylcholine release regulation. Auditory brainstem responses, fluorescence imaging, and patch clamp recording techniques were used to assess the impact of viral infection, sex, age, and anesthetic agents across the ascending auditory pathway of ChAT-Cre and ChAT-ChR2(Ai32) mice. Data analyses revealed that neither genetic configuration nor adeno-associated viral infection alters the early stages of auditory processing or the cellular response properties of cholinergic neurons. However, anesthetic agent and dosage amount profoundly modulate the response properties of brainstem neurons. Last, analyses of age-related hearing loss in virally infected ChAT-Cre mice did not differ from those reported in wild type animals. This investigation demonstrates that ChAT-Cre and ChAT-ChR2(Ai32) mice are viable models for the study of cholinergic modulation in auditory processing, and it emphasizes the need for prudence in the selection of anesthetic procedures.

Selective neuronal vulnerability in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease, disabling millions worldwide. Despite the imperative PD poses, at present, there is no cure or means of slowing progression. This gap is attributable to our incomplete understanding of the factors driving pathogenesis. Research over the past several decades suggests that both cell-autonomous and non-cell autonomous processes contribute to the neuronal dysfunction underlying PD symptoms. The thesis of this review is that an intersection of these processes governs the pattern of pathology in PD. Studies of substantia nigra pars compacta (SNc) dopaminergic neurons, whose loss is responsible for the core motor symptoms of PD, suggest that they have a combination of traits-a long, highly branched axon, autonomous activity, and elevated mitochondrial oxidant stress-that predispose them to non-cell autonomous drivers of pathogenesis, like misfolded forms of alpha-synuclein (α-SYN) and inflammation. The literature surrounding these issues will be briefly summarized, and the translational implications of an intersectional hypothesis of PD pathogenesis discussed.

Modeling uncertainty-seeking behavior mediated by cholinergic influence on dopamine

Recent findings suggest that acetylcholine mediates uncertainty-seeking behaviors through its projection to dopamine neurons - another neuromodulatory system known for its major role in reinforcement learning and decision-making. In this paper, we propose a leaky-integrate-and-fire model of this mechanism. It implements a softmax-like selection with an uncertainty bonus by a cholinergic drive to dopaminergic neurons, which in turn influence synaptic currents of downstream neurons. The model is able to reproduce experimental data in two decision-making tasks. It also predicts that: (i) in the absence of cholinergic input, dopaminergic activity would not correlate with uncertainty, and that (ii) the adaptive advantage brought by the implemented uncertainty-seeking mechanism is most useful when sources of reward are not highly uncertain. Moreover, this modeling work allows us to propose novel experiments which might shed new light on the role of acetylcholine in both random and directed exploration. Overall, this study contributes to a more comprehensive understanding of the role of the cholinergic system and, in particular, its involvement in decision-making.

Pedunculopontine Nucleus Degeneration Contributes to Both Motor and Non-Motor Symptoms of Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by hypokinetic motor features; however, patients also display non-motor symptoms like sleep disorders. The standard treatment for PD is dopamine replacement with L-DOPA; however, symptoms including gait deficits and sleep disorders are unresponsive to L-DOPA. Notably, these symptoms have been linked to aberrant activity in the pedunculopontine nucleus (PPN). Of late, clinical trials involving PPN deep brain stimulation (DBS) have been employed to alleviate gait deficits. Although preclinical evidence implicating PPN cholinergic neurons in gait dysfunction was initially promising, DBS trials fell short of expected outcomes. One reason for the failure of DBS may be that the PPN is a heterogenous nucleus that consists of GABAergic, cholinergic, and glutamatergic neurons that project to a diverse array of brain structures. Second, DBS trials may have been unsuccessful because PPN neurons are susceptible to mitochondrial dysfunction, Lewy body pathology, and degeneration in PD. Therefore, pharmaceutical or gene-therapy strategies targeting specific PPN neuronal populations or projections could better alleviate intractable PD symptoms. Unfortunately, how PPN neuronal populations and their respective projections influence PD motor and non-motor symptoms remains enigmatic. Herein, we discuss normal cellular and neuroanatomical features of the PPN, the differential susceptibility of PPN neurons to PD-related insults, and we give an overview of literature suggesting a role for PPN neurons in motor and sleep deficits in PD. Finally, we identify future approaches directed towards the PPN for the treatment of PD motor and sleep symptoms.

Dichotomy between motor and cognitive functions of midbrain cholinergic neurons

Cholinergic neurons of the pedunculopontine nucleus (PPN) are interconnected with all the basal ganglia structures, as well as with motor centers in the brainstem and medulla. Recent theories put into question whether PPN cholinergic neurons form part of a locomotor region that directly regulates the motor output, and rather suggest a modulatory role in adaptive behavior involving both motor and cognitive functions. In support of this, experimental studies in animals suggest that cholinergic neurons reinforce actions by signaling reward prediction and shape adaptations in behavior during changes of environmental contingencies. This is further supported by clinical studies proposing that decreased cholinergic transmission originated in the PPN is associated with impaired sensorimotor integration and perseverant behavior, giving rise to some of the symptoms observed in Parkinson's disease and progressive supranuclear palsy. Altogether, the evidence suggests that cholinergic neurons of the PPN, mainly through their interactions with the basal ganglia, have a leading role in action control.

The pedunclopontine nucleus and Parkinson's disease

In the last decade, scientific and clinical interest in the pedunculopontine nucleus (PPN) has grown dramatically. This growth is largely a consequence of experimental work demonstrating its connection to the control of gait and of clinical work implicating PPN pathology in levodopa-insensitive gait symptoms of Parkinson's disease (PD). In addition, the development of optogenetic and chemogenetic approaches has made experimental analysis of PPN circuitry and function more tractable. In this brief review, recent findings in the field linking PPN to the basal ganglia and PD are summarized; in addition, an attempt is made to identify key gaps in our understanding and challenges this field faces in moving forward.

Neuromodulatory Influences on Integration and Segregation in the Brain

Cognitive function relies on the dynamic cooperation of specialized regions of the brain; however, the elements of the system responsible for coordinating this interaction remain poorly understood. In this Opinion article I argue that this capacity is mediated in part by competitive and cooperative dynamic interactions between two prominent metabotropic neuromodulatory systems - the cholinergic basal forebrain and the noradrenergic locus coeruleus (LC). I assert that activity in these projection nuclei regulates the amount of segregation and integration within the whole brain network by modulating the activity of a diverse set of specialized regions of the brain on a timescale relevant for cognition and attention.

Local and Relayed Effects of Deep Brain Stimulation of the Pedunculopontine Nucleus

Our discovery of low-threshold stimulation-induced locomotion in the pedunculopontine nucleus (PPN) led to the clinical use of deep brain stimulation (DBS) for the treatment of disorders such as Parkinson's disease (PD) that manifest gait and postural disorders. Three additional major discoveries on the properties of PPN neurons have opened new areas of research for the treatment of motor and arousal disorders. The description of (a) electrical coupling, (b) intrinsic gamma oscillations, and (c) gene regulation in the PPN has identified a number of novel therapeutic targets and methods for the treatment of a number of neurological and psychiatric disorders. We first delve into the circuit, cellular, intracellular, and molecular organization of the PPN, and then consider the clinical results to date on PPN DBS. This comprehensive review will provide valuable information to explain the network effects of PPN DBS, point to new directions for treatment, and highlight a number of issues related to PPN DBS.

Cholinergic neurons in the pedunculopontine tegmental nucleus modulate breathing in rats by direct projections to the retrotrapezoid nucleus

KEY POINTS

Cholinergic projections from the pedunculopontine tegmental nucleus (PPTg) to the retrotrapezoid nucleus (RTN) are considered to be important for sleep-wake state-dependent control of breathing. The RTN also receives cholinergic input from the postinspiratory complex. Stimulation of the PPTg increases respiratory output under control conditions but not when muscarinic receptors in the RTN are blocked. The data obtained in the present study support the possibility that arousal-dependent modulation of breathing involves recruitment of cholinergic projections from the PPTg to the RTN.

ABSTRACT

The pedunculopontine tegmental nucleus (PPTg) in the mesopontine region has important physiological functions, including breathing control. The PPTg contains a variety of cell types, including cholinergic neurons that project to the rostral aspect of the ventrolateral medulla. In addition, cholinergic signalling in the retrotrapezoid nucleus (RTN), a region that contains neurons that regulate breathing in response to changes in CO₂ /H⁺ , has been shown to activate chemosensitive neurons and increase inspiratory activity. The present study aimed to identify the source of cholinergic input to the RTN and determine whether cholinergic signalling in this region influences baseline breathing or the ventilatory response to CO₂ in conscious male Wistar rats. Retrograde tracer Fluoro-Gold injected into the RTN labelled a subset of cholinergic PPTg neurons that presumably project directly to the chemosensitive region of the RTN. In unrestrained awake rats, unilateral injection of the glutamate (10 mm/100 nL) in the PPTg decreased tidal volume (V_T ) but otherwise increased respiratory rate (f_R ) and net respiratory output as indicated by an increase in ventilation (V_E ). All respiratory responses elicited by PPTg stimulation were blunted by prior injection of methyl-atropine (5 mm/50-75 nL) into the RTN. These results show that stimulation of the PPTg can increase respiratory activity in part by cholinergic activation of chemosensitive elements of the RTN. Based on previous evidence that cholinergic PPTg projections may simultaneously activate expiratory output from the pFRG, we speculate that cholinergic signalling at the level of RTN region could also be involved in breathing regulation.

Mechanisms of decreased cholinergic arousal in focal seizures: In vivo whole-cell recordings from the pedunculopontine tegmental nucleus

Focal limbic seizures often impair consciousness/awareness with major negative impact on quality of life. Recent work has shown that limbic seizures depress brainstem arousal systems, including reduced action potential firing in a key node: cholinergic neurons of the pedunculopontine tegmental nucleus (PPT). In vivo whole-cell recordings have not previously been achieved in PPT, but are used here with the goal of elucidating the mechanisms of reduced PPT cholinergic neuronal activity. An established model of focal limbic seizures was used in rats following brief hippocampal stimulation under light anesthesia. Whole-cell in vivo recordings were obtained from PPT neurons using custom-fabricated 9-10 mm tapered patch pipettes, and cholinergic neurons were identified histologically. Average membrane potential, input resistance, membrane potential fluctuations and variance were analyzed during seizures. A subset of PPT neurons exhibited reduced firing and hyperpolarization during seizures and stained positive for choline acetyltransferase. These PPT neurons showed a mean membrane potential hyperpolarization of -3.82 mV (±0.81 SEM, P < .05) during seizures, and also showed significantly increased input resistance, fewer excitatory post-synaptic potential (EPSP)-like events (P < .05), and reduced membrane potential variance (P < .01). The combination of increased input resistance, decreased EPSP-like events and decreased variance weigh against active ictal inhibition and support withdrawal of excitatory input as the dominant mechanism of decreased activity of cholinergic neurons in the PPT. Further identifying synaptic mechanisms of depressed arousal during seizures may lead to new treatments to improve ictal and postictal cognition.

EEG dissociation induced by muscarinic receptor antagonists: Coherent 40 Hz oscillations in a background of slow waves and spindles

Mesopontine and basal forebrain cholinergic neurons are involved in the control of behavioral states and cognitive functions. Animals treated with cholinergic muscarinic receptor antagonists display a dissociated state characterized by behavioral wakefulness (W) associated with high amplitude slow oscillations and spindles in the electroencephalogram (EEG), similar to those that occur during non-REM (NREM) sleep. Oscillations in the gamma frequency band (≈ 40 Hz) of the EEG also play a critical role during W and cognition. Hence, the present study was conducted to determine the effect of muscarinic antagonists on the EEG gamma band power and coherence. Five cats were implanted with electrodes in different cortices to monitor the EEG. The effects of atropine and scopolamine on power and coherence within the low gamma frequency band (30-45 Hz) from pairs of EEG recordings were analyzed and compared to gamma activity during sleep and W. Muscarinic antagonists induced a NREM sleep-like EEG profile that was accompanied by a large increase in gamma power and coherence. The values of gamma coherence were similar to that occurring during alert W (AW), and greater than in quiet W, NREM and REM sleep. We conclude that under atropine or scopolamine, functional interactions between cortical areas in the gamma frequency band remain high, as they are during AW. This significant functional connectivity at high frequency may explain why the animals remain awake in spite of the presence of slow waves and spindles.