Single-unit activity in the primate nucleus tegmenti pedunculopontinus related to voluntary arm movement

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.

Multimodal convergence in the pedunculopontine tegmental nucleus: Motor, sensory and theta-frequency inputs influence activity of single neurons

The pedunculopontine tegmental nucleus of the brainstem (PPTg) has extensive interconnections and neuronal-behavioural correlates. It is implicated in movement control and sensorimotor integration. We investigated whether single neuron activity in freely moving rats is correlated with components of skilled forelimb movement, and whether individual neurons respond to both motor and sensory events. We found that individual PPTg neurons showed changes in firing rate at different times during the reach. This type of temporally specific modulation is like activity seen elsewhere in voluntary movement control circuits, such as the motor cortex, and suggests that PPTg neural activity is related to different specific events occurring during the reach. In particular, many neuronal modulations were time-locked to the end of the extension phase of the reach, when fine distal movements related to food grasping occur, indicating strong engagement of PPTg in this phase of skilled individual forelimb movements. In addition, some neurons showed brief periods of apparent oscillatory firing in the theta range at specific phases of the reach-to-grasp movement. When movement-related neurons were tested with tone stimuli, many also responded to this auditory input, allowing for sensorimotor integration at the cellular level. Together, these data extend the concept of the PPTg as an integrative structure in generation of complex movements, by showing that this function extends to the highly coordinated control of the forelimb during skilled reach to grasp movement, and that sensory and motor-related information converges on single neurons, allowing for direct integration at the cellular level.

Dystonia and the pedunculopontine nucleus: Current evidences and potential mechanisms

Being a major component of the midbrain locomotion region, the pedunculopontine nucleus (PPN) is known to have various connections with the basal ganglia, the cerebral cortex, thalamus, and motor regions of the brainstem and spinal cord. Functionally, the PPN is associated with muscle tone control and locomotion modulation, including motor initiation, rhythm and speed. In addition to its motor functions, the PPN also contribute to level of arousal, attention, memory and learning. Recent studies have revealed neuropathologic deficits in the PPN in both patients and animal models of dystonia, and deep brain stimulation of the PPN also showed alleviation of axial dystonia in patients of Parkinson's disease. These findings indicate that the PPN might play an important role in the development of dystonia. Moreover, with increasing preclinical evidences showed presence of dystonia-like behaviors, muscle tone changes, impaired cognitive functions and sleep following lesion or neuromodulation of the PPN, it is assumed that the pathological changes of the PPN might contribute to both motor and non-motor manifestations of dystonia. In this review, we aim to summarize the involvement of the PPN in dystonia based on the current preclinical and clinical evidences. Moreover, potential mechanisms for its contributions to the manifestation of dystonia is also discussed base on the dystonia-related basal ganglia-cerebello-thalamo-cortical circuit, providing fundamental insight into the targeting of the PPN for the treatment of dystonia in the future.

Neuronal spiking in the pedunculopontine nucleus in progressive supranuclear palsy and in idiopathic Parkinson's disease

The pedunculopontine nucleus (PPN) is engaged in posture and gait control, and neuronal degeneration in the PPN has been associated with Parkinsonian disorders. Clinical outcomes of deep brain stimulation of the PPN in idiopathic Parkinson's disease (IPD) and progressive supranuclear palsy (PSP) differ, and we investigated whether the PPN is differentially affected in these conditions. We had the rare opportunity to record continuous electrophysiological data intraoperatively in 30 s blocks from single microelectrode contacts implanted in the PPN in six PSP patients and three IPD patients during rest, passive movement, and active movement. Neuronal spikes were sorted according to shape using a wavelet-based clustering approach to enable comparisons between individual neuronal firing rates in the two disease states. The action potential widths showed a bimodal distribution consistent with previous findings, suggesting spikes from noncholinergic (likely glutamatergic) and cholinergic neurons. A higher PPN spiking rate of narrow action potentials was observed in the PSP than in the IPD patients when pooled across all three conditions (Wilcoxon rank sum test: p = 0.0141). No correlation was found between firing rate and disease severity or duration. The firing rates were higher during passive movement than rest and active movement in both groups, but the differences between conditions were not significant. PSP and IPD are believed to represent distinct disease processes, and our findings that the neuronal firing rates differ according to disease state support the proposal that pathological processes directly involving the PPN may be more pronounced in PSP than IPD.

Spike discharge characteristic of the caudal mesencephalic reticular formation and pedunculopontine nucleus in MPTP-induced primate model of Parkinson disease

The pedunculopontine nucleus (PPN) included in the caudal mesencephalic reticular formation (cMRF) plays a key role in the control of locomotion and wake state. Regarding its involvement in the neurodegenerative process observed in Parkinson disease (PD), deep brain stimulation of the PPN was proposed to treat levodopa-resistant gait disorders. However, the precise role of the cMRF in the pathophysiology of PD, particularly in freezing of gait and other non-motor symptoms is still not clear. Here, using micro electrode recording (MER) in 2 primates, we show that dopamine depletion did not alter the mean firing rate of the overall cMRF neurons, particularly the putative non-cholinergic ones, but only a decreased activity of the regular neurons sub-group (though to be the cholinergic PPN neurons). Interestingly, a significant increase in the relative proportion of cMRF neurons with a burst pattern discharge was observed after MPTP intoxication. The present results question the hypothesis of an over-inhibition of the CMRF by the basal ganglia output structures in PD. The decreased activity observed in the regular neurons could explain some non-motor symptoms in PD regarding the strong involvement of the cholinergic neurons on the modulation of the thalamo-cortical system. The increased burst activity under dopamine depletion confirms that this specific spike discharge pattern activity also observed in other basal ganglia nuclei and in different pathologies could play a mojor role in the pathophysiology of the disease and could explain several symptoms of PD including the freezing of gait. The present data will have to be replicated in a larger number of animals and will have to investigate more in details how the modification of the spike discharge of the cMRF neurons in the parkinsonian state could alter functions such as locomotion and attentional state. This will ultimely allow a better comprehension of the pathophysiology of freezing of gait.

What is the therapeutic mechanism of pedunculopontine nucleus stimulation in Parkinson's disease?

Pedunculopontine nucleus (PPN) deep brain stimulation (DBS) is an experimental treatment for Parkinson's disease (PD) which offers a fairly circumscribed benefit for gait freezing and perhaps balance impairment. The benefit on gait freezing is variable and typically incomplete, which may reflect that the clinical application is yet to be optimised or reflect a fundamental limitation of the therapeutic mechanism. Thus, a better understanding of the therapeutic mechanism of PPN DBS may guide the further development of this therapy. The available evidence supports that the PPN is underactive in PD due to a combination of both degeneration and excessive inhibition. Low frequency PPN DBS could enhance PPN network activity, perhaps via disinhibition. A clinical implication is that in some PD patients, the PPN may be too degenerate for PPN DBS to work. Reaction time studies report that PPN DBS mediates a very specific benefit on pre-programmed movement. This seems relevant to the pathophysiology of gait freezing, which can be argued to reflect impaired release of pre-programmed adjustments to locomotion. Thus, the benefit of PPN DBS on gait freezing could be akin to that mediated by external cues. Alpha band activity is a prominent finding in local field potential recordings from PPN electrodes in PD patients. Alpha band activity is implicated in the suppression of task irrelevant processes and thus the effective allocation of attention (processing resources). Attentional deficits are prominent in patients with PD and gait freezing and PPN alpha activity has been observed to drop out prior to gait freezing episodes and to increase with levodopa. This raises the hypothesis that PPN DBS could support or emulate PPN alpha activity and consequently enhance the allocation of attention. Although PPN DBS has not been convincingly shown to increase general alertness or attention, it remains possible that PPN DBS may enhance the allocation of processing resources within the motor system, or "motor attention". For example, this could facilitate the 'switching' of motor state between continuation of pattern generated locomotion towards the intervention of pre-programmed adjustments. However, if the downstream consequence of PPN DBS on movement is limited to a circumscribed unblocking of pre-programmed movement, then this may have a similarly circumscribed degree of benefit for gait. If this is the case, then it may be possible to identify patients who may benefit most from PPN DBS. For example, those in whom pre-programmed deficits are the major contributors to gait freezing.

A Review of the Pedunculopontine Nucleus in Parkinson's Disease

The pedunculopontine nucleus (PPN) is situated in the upper pons in the dorsolateral portion of the ponto-mesencephalic tegmentum. Its main mass is positioned at the trochlear nucleus level, and is part of the mesenphalic locomotor region (MLR) in the upper brainstem. The human PPN is divided into two subnuclei, the pars compacta (PPNc) and pars dissipatus (PPNd), and constitutes both cholinergic and non-cholinergic neurons with afferent and efferent projections to the cerebral cortex, thalamus, basal ganglia (BG), cerebellum, and spinal cord. The BG controls locomotion and posture via GABAergic output of the substantia nigra pars reticulate (SNr). In PD patients, GABAergic BG output levels are abnormally increased, and gait disturbances are produced via abnormal increases in SNr-induced inhibition of the MLR. Since the PPN is vastly connected with the BG and the brainstem, dysfunction within these systems lead to advanced symptomatic progression in Parkinson's disease (PD), including sleep and cognitive issues. To date, the best treatment is to perform deep brain stimulation (DBS) on PD patients as outcomes have shown positive effects in ameliorating the debilitating symptoms of this disease by treating pathological circuitries within the parkinsonian brain. It is therefore important to address the challenges and develop this procedure to improve the quality of life of PD patients.

On the Role of the Pedunculopontine Nucleus and Mesencephalic Reticular Formation in Locomotion in Nonhuman Primates

The mesencephalic reticular formation (MRF) is formed by the pedunculopontine and cuneiform nuclei, two neuronal structures thought to be key elements in the supraspinal control of locomotion, muscle tone, waking, and REM sleep. The role of MRF has also been advocated in modulation of state of arousal leading to transition from wakefulness to sleep and it is further considered to be a main player in the pathophysiology of gait disorders seen in Parkinson's disease. However, the existence of a mesencephalic locomotor region and of an arousal center has not yet been demonstrated in primates. Here, we provide the first extensive electrophysiological mapping of the MRF using extracellular recordings at rest and during locomotion in a nonhuman primate (NHP) (Macaca fascicularis) model of bipedal locomotion. We found different neuronal populations that discharged according to a phasic or a tonic mode in response to locomotion, supporting the existence of a locomotor neuronal circuit within these MRF in behaving primates. Altogether, these data constitute the first electrophysiological characterization of a locomotor neuronal system present within the MRF in behaving NHPs under normal conditions, in accordance with several studies done in different experimental animal models.

SIGNIFICANCE STATEMENT

We provide the first extensive electrophysiological mapping of the two major components of the mesencephalic reticular formation (MRF), namely the pedunculopontine and cuneiform nuclei. We exploited a nonhuman primate (NHP) model of bipedal locomotion with extracellular recordings in behaving NHPs at rest and during locomotion. Different MRF neuronal groups were found to respond to locomotion, with phasic or tonic patterns of response. These data constitute the first electrophysiological evidences of a locomotor neuronal system within the MRF in behaving NHPs.

Rethinking the Pedunculopontine Nucleus: From Cellular Organization to Function

The pedunculopontine nucleus (PPN) has long been considered an interface between the basal ganglia and motor systems, and its ability to regulate arousal states puts the PPN in a key position to modulate behavior. Despite the large amount of data obtained over recent decades, a unified theory of its function is still incomplete. By putting together classical concepts and new evidence that dissects the influence of its different neuronal subtypes on their various targets, we propose that the PPN and, in particular, cholinergic neurons have a central role in updating the behavioral state as a result of changes in environmental contingencies. Such a function is accomplished by a combined mechanism that simultaneously restrains ongoing obsolete actions while it facilitates new contextual associations.

Differential Control of Dopaminergic Excitability and Locomotion by Cholinergic Inputs in Mouse Substantia Nigra

Understanding how dopaminergic (DA) neurons of the substantia nigra pars compacta (SNc) govern movements requires a detailed knowledge of how different neurotransmitter systems modulate DA neuronal excitability. We report a heterogeneity of electrophysiological properties between medial and lateral SNc neurons modulated by cholinergic neurotransmission. Lateral DA neurons received mainly excitatory (nicotinic or glutamatergic) mediated cholinergic neurotransmission. Medial DA neurons received predominantly GABAergic currents mediated by presynaptic nicotinic receptors or biphasic GABAergic and nicotinic neurotransmission conveyed by GABA and ACh corelease, which inhibited DA neurons. To examine whether cholinergic signaling in the SNc controls mouse behavior, we used optogenetics in awake behaving mice and found that activation of cholinergic terminals in the medial SNc decreased locomotion, whereas activation in the lateral SNc increased locomotion. Our findings provide novel insights on how cholinergic inputs to subregions of the SNc regulate the excitability of DA neurons differentially, resulting in different patterns of motor behavior.

The Cellular Diversity of the Pedunculopontine Nucleus: Relevance to Behavior in Health and Aspects of Parkinson's Disease

The pedunculopontine nucleus (PPN) is a rostral brainstem structure that has extensive connections with basal ganglia nuclei and the thalamus. Through these the PPN contributes to neural circuits that effect cortical and hippocampal activity. The PPN also has descending connections to nuclei of the pontine and medullary reticular formations, deep cerebellar nuclei, and the spinal cord. Interest in the PPN has increased dramatically since it was first suggested to be a novel target for treating patients with Parkinson's disease who are refractory to medication. However, application of frequency-specific electrical stimulation of the PPN has produced inconsistent results. A central reason for this is that the PPN is not a heterogeneous structure. In this article, we review current knowledge of the neurochemical identity and topographical distribution of neurons within the PPN of both humans and experimental animals, focusing on studies that used neuronally selective targeting strategies to ascertain how the neurochemical heterogeneity of the PPN relates to its diverse functions in relation to movement and cognitive processes. If the therapeutic potential of the PPN is to be realized, it is critical to understand the complex structure-function relationships that exist here.

The Pedunculopontine Tegmental Nucleus as a Motor and Cognitive Interface between the Cerebellum and Basal Ganglia

As an important component of ascending activating systems, brainstem cholinergic neurons in the pedunculopontine tegmental nucleus (PPTg) are involved in the regulation of motor control (locomotion, posture and gaze) and cognitive processes (attention, learning and memory). The PPTg is highly interconnected with several regions of the basal ganglia, and one of its key functions is to regulate and relay activity from the basal ganglia. Together, they have been implicated in the motor control system (such as voluntary movement initiation or inhibition), and modulate aspects of executive function (such as motivation). In addition to its intimate connection with the basal ganglia, projections from the PPTg to the cerebellum have been recently reported to synaptically activate the deep cerebellar nuclei. Classically, the cerebellum and basal ganglia were regarded as forming separated anatomical loops that play a distinct functional role in motor and cognitive behavioral control. Here, we suggest that the PPTg may also act as an interface device between the basal ganglia and cerebellum. As such, part of the therapeutic effect of PPTg deep brain stimulation (DBS) to relieve gait freezing and postural instability in advanced Parkinson’s disease (PD) patients might also involve modulation of the cerebellum. We review the anatomical position and role of the PPTg in the pathway of basal ganglia and cerebellum in relation to motor control, cognitive function and PD.

Mesencephalic representations of recent experience influence decision making

Decisions are influenced by recent experience, but the neural basis for this phenomenon is not well understood. Here, we address this question in the context of action selection. We focused on activity in the pedunculopontine tegmental nucleus (PPTg), a mesencephalic region that provides input to several nuclei in the action selection network, in well-trained mice selecting actions based on sensory cues and recent trial history. We found that, at the time of action selection, the activity of many PPTg neurons reflected the action on the previous trial and its outcome, and the strength of this activity predicted the upcoming choice. Further, inactivating the PPTg predictably decreased the influence of recent experience on action selection. These findings suggest that PPTg input to downstream motor regions, where it can be integrated with other relevant information, provides a simple mechanism for incorporating recent experience into the computations underlying action selection.

DOI:http://dx.doi.org/10.7554/eLife.16572.001

The decisions we make are influenced by recent experience, yet it is not known how this experience is represented in the brain. For decisions about when, where and how to move, researchers have hypothesized that recent experience might influence activity in a region of the brainstem – the central trunk of the brain – that is known to be involved in movement.

When deciding when, where and how to move, several areas of the brain are involved in selecting the optimal action. Recent studies suggest that groups of neurons known as locomotor brainstem nuclei may also contribute to making decisions about movements.

Thompson et al. investigated whether a brainstem locomotor area called the pedunculopontine tegmental (PPTg) nucleus in mice might contribute to decision making rather than just conveying the selected response. The mice were trained to recognize particular odors and move to either the left or right to collect a food reward. While the mice were selecting an action, the activity of neurons in the PPTg nucleus reflected the action they had chosen on a previous experience and the outcome of that choice (i.e. whether they received a reward). These representations of past experiences influenced the upcoming decision the mice were about to take.

The findings of Thompson et al. suggest that the PPTg nucleus might play a critical role in the process of selecting the optimal action. Future work will examine what kinds of information about the environment or recent experience have the biggest effect on the activity of this region.

DOI:http://dx.doi.org/10.7554/eLife.16572.002

The primate pedunculopontine nucleus region: towards a dual role in locomotion and waking state

The mesencephalic reticular formation (MRF) mainly composed by the pedunculopontine and the cuneiform nuclei is involved in the control of several fundamental brain functions such as locomotion, rapid eye movement sleep and waking state. On the one hand, the role of MRF neurons in locomotion has been investigated for decades in different animal models, including in behaving nonhuman primate (NHP) using extracellular recordings. On the other hand, MRF neurons involved in the control of waking state have been consistently shown to constitute the cholinergic component of the reticular ascending system. However, a dual control of the locomotion and waking state by the same groups of neurons in NHP has never been demonstrated in NHP. Here, using microelectrode recordings in behaving NHP, we recorded 38 neurons in the MRF that were followed during transition between wakefulness (TWS) and sleep, i.e., until the emergence of sleep episodes characterized by typical cortical slow wave activity (SWA). We found that the MRF neurons, mainly located in the pedunculopontine nucleus region, modulated their activity during TWS with a decrease in firing rate during SWA. Of interest, we could follow some MRF neurons from locomotion to SWA and found that they also modulated their firing rate during locomotion and TWS. These new findings confirm the role of MRF neurons in both functions. They suggest that the MRF is an integration center that potentially allows to fine tune waking state and locomotor signals in order to establish an efficient locomotion.

Effect of Subthalamic Nucleus Stimulation on Pedunculopontine Nucleus Neural Activity

BACKGROUND

The pedunculopontine nucleus has recently been proposed as an alternative target for deep brain stimulation for the treatment of medically intractable Parkinson's disease. The suggested indication for pedunculopontine nucleus deep brain stimulation is severe and medically intractable axial symptoms such as gait and postural impairment.

OBJECTIVE

Our goal in this study was to describe the effects of subthalamic nucleus stimulation on pedunculopontine nucleus electrophysiological activity.

METHODS

Fourteen male Wistar rats were divided into a sham stimulation group and an experimental group. In both groups, electrodes were implanted bilaterally into the subthalamic nucleus and into the right pedunculopontine nucleus. Microelectrode recordings were carried out in both groups prior to and during subthalamic nucleus stimulation.

RESULTS

Subthalamic nucleus stimulation produced no clear inhibition of neuronal firing in the pedunculopontine nucleus. However, we found that stimulation of the subthalamic nucleus at 60 Hz produces some entrainment of pedunculopontine nucleus neuronal firing and a shift of subthalamic nucleus firing patterns to more tonic and random patterns. These results are consistent with the effects of deep brain stimulation on neuronal activity in the subthalamic nucleus and globus pallidus internus.

CONCLUSION

The result of this study provides additional evidence to improve our understanding of the mechanism of subthalamic nucleus-deep brain stimulation, and its physiological consequences.

An integrative role for the superior colliculus in selecting targets for movements

A fundamental goal of systems neuroscience is to understand the neural mechanisms underlying decision making. The midbrain superior colliculus (SC) is known to be central to the selection of one among many potential spatial targets for movements, which represents an important form of decision making that is tractable to rigorous experimental investigation. In this review, we first discuss data from mammalian models-including primates, cats, and rodents-that inform our understanding of how neural activity in the SC underlies the selection of targets for movements. We then examine the anatomy and physiology of inputs to the SC from three key regions that are themselves implicated in motor decisions-the basal ganglia, parabrachial region, and neocortex-and discuss how they may influence SC activity related to target selection. Finally, we discuss the potential for methodological advances to further our understanding of the neural bases of target selection. Our overarching goal is to synthesize what is known about how the SC and its inputs act together to mediate the selection of targets for movements, to highlight open questions about this process, and to spur future studies addressing these questions.

The integrative role of the pedunculopontine nucleus in human gait

The brainstem pedunculopontine nucleus has a likely, although unclear, role in gait control, and is a potential deep brain stimulation target for treating resistant gait disorders. These disorders are a major therapeutic challenge for the ageing population, especially in Parkinson's disease where gait and balance disorders can become resistant to both dopaminergic medication and subthalamic nucleus stimulation. Here, we present electrophysiological evidence that the pedunculopontine and subthalamic nuclei are involved in distinct aspects of gait using a locomotor imagery task in 14 patients with Parkinson's disease undergoing surgery for the implantation of pedunculopontine or subthalamic nuclei deep brain stimulation electrodes. We performed electrophysiological recordings in two phases, once during surgery, and again several days after surgery in a subset of patients. The majority of pedunculopontine nucleus neurons (57%) recorded intrasurgically exhibited changes in activity related to different task components, with 29% modulated during visual stimulation, 41% modulated during voluntary hand movement, and 49% modulated during imaginary gait. Pedunculopontine nucleus local field potentials recorded post-surgically were modulated in the beta and gamma bands during visual and motor events, and we observed alpha and beta band synchronization that was sustained for the duration of imaginary gait and spatially localized within the pedunculopontine nucleus. In contrast, significantly fewer subthalamic nucleus neurons (27%) recorded intrasurgically were modulated during the locomotor imagery, with most increasing or decreasing activity phasically during the hand movement that initiated or terminated imaginary gait. Our data support the hypothesis that the pedunculopontine nucleus influences gait control in manners extending beyond simply driving pattern generation. In contrast, the subthalamic nucleus seems to control movement execution that is not likely to be gait-specific. These data highlight the crucial role of these two nuclei in motor control and shed light on the complex functions of the lateral mesencephalus in humans.

Activity in mouse pedunculopontine tegmental nucleus reflects action and outcome in a decision-making task

Recent studies across several mammalian species have revealed a distributed network of cortical and subcortical brain regions responsible for sensorimotor decision making. Many of these regions have been shown to be interconnected with the pedunculopontine tegmental nucleus (PPTg), a brain stem structure characterized by neuronal heterogeneity and thought to be involved in several cognitive and behavioral functions. However, whether this structure plays a general functional role in sensorimotor decision making is unclear. We hypothesized that, in the context of a sensorimotor task, activity in the PPTg would reflect task-related variables in a similar manner as do the cortical and subcortical regions with which it is anatomically associated. To examine this hypothesis, we recorded PPTg activity in mice performing an odor-cued spatial choice task requiring a stereotyped leftward or rightward orienting movement to obtain a reward. We studied single-neuron activity during epochs of the task related to movement preparation, execution, and outcome (i.e., whether or not the movement was rewarded). We found that a substantial proportion of neurons in the PPTg exhibited direction-selective activity during one or more of these epochs. In addition, an overlapping population of neurons reflected movement direction and reward outcome. These results suggest that the PPTg should be considered within the network of brain areas responsible for sensorimotor decision making and lay the foundation for future experiments to examine how the PPTg interacts with other regions to control sensory-guided motor output.

Asymmetric pedunculopontine network connectivity in parkinsonian patients with freezing of gait

Freezing of gait is one of the most debilitating symptoms in Parkinson's disease as it causes falls and reduces mobility and quality of life. The pedunculopontine nucleus is one of the major nuclei of the mesencephalic locomotor region and has neurons related to anticipatory postural adjustments preceding step initiation as well as to the step itself, thus it may be critical for coupling posture and gait to avoid freezing. Because freezing of gait and postural impairments have been related to frontal lesions and frontal dysfunction such as executive function, we hypothesized that freezing is associated with disrupted connectivity between midbrain locomotor regions and medial frontal cortex. We used diffusion tensor imaging to quantify structural connectivity of the pedunculopontine nucleus in patients with Parkinson's disease with freezing of gait, without freezing, and healthy age-matched controls. We also included behavioural tasks to gauge severity of freezing of gait, quantify gait metrics, and assess executive cognitive functions to determine whether between-group differences in executive dysfunction were related to pedunculopontine nucleus structural network connectivity. Using seed regions from the pedunculopontine nucleus, we were able to delineate white matter connections between the spinal cord, cerebellum, pedunculopontine nucleus, subcortical and frontal/prefrontal cortical regions. The current study is the first to demonstrate differences in structural connectivity of the identified locomotor pathway in patients with freezing of gait. We report reduced connectivity of the pedunculopontine nucleus with the cerebellum, thalamus and multiple regions of the frontal cortex. Moreover, these structural differences were observed solely in the right hemisphere of patients with freezing of gait. Finally, we show that the more left hemisphere-lateralized the pedunculopontine nucleus tract volume, the poorer the performance on cognitive tasks requiring the initiation of appropriate actions and/or the inhibition of inappropriate actions, specifically within patients with freezing. These results support the notion that freezing of gait is strongly related to structural deficits in the right hemisphere's locomotor network involving prefrontal cortical areas involved in executive inhibition function.

Reward prediction-related increases and decreases in tonic neuronal activity of the pedunculopontine tegmental nucleus

The neuromodulators serotonin, acetylcholine, and dopamine have been proposed to play important roles in the execution of movement, control of several forms of attentional behavior, and reinforcement learning. While the response pattern of midbrain dopaminergic neurons and its specific role in reinforcement learning have been revealed, the roles of the other neuromodulators remain elusive. Reportedly, neurons in the dorsal raphe nucleus, one major source of serotonin, continually track the state of expectation of future rewards by showing a correlated response to the start of a behavioral task, reward cue presentation, and reward delivery. Here, we show that neurons in the pedunculopontine tegmental nucleus (PPTN), one major source of acetylcholine, showed similar encoding of the expectation of future rewards by a systematic increase or decrease in tonic activity. We recorded and analyzed PPTN neuronal activity in monkeys during a reward conditioned visually guided saccade task. The firing patterns of many PPTN neurons were tonically increased or decreased throughout the task period. The tonic activity pattern of neurons was correlated with their encoding of the predicted reward value; neurons exhibiting an increase or decrease in tonic activity showed higher or lower activity in the large reward-predicted trials, respectively. Tonic activity and reward-related modulation ended around the time of reward delivery. Additionally, some tonic changes in activity started prior to the appearance of the initial stimulus, and were related to the anticipatory fixational behavior. A partially overlapping population of neurons showed both the initial anticipatory response and subsequent predicted reward value-dependent activity modulation by their systematic increase or decrease of tonic activity. These bi-directional reward- and anticipatory behavior-related modulation patterns are suitable for the presumed role of the PPTN in reward processing and motivational control.

Reduction of [11C](+)3-MPB binding in brain of chronic fatigue syndrome with serum autoantibody against muscarinic cholinergic receptor

Background

Numerous associations between brain-reactive antibodies and neurological or psychiatric symptoms have been proposed. Serum autoantibody against the muscarinic cholinergic receptor (mAChR) was increased in some patients with chronic fatigue syndrome (CFS) or psychiatric disease. We examined whether serum autoantibody against mAChR affected the central cholinergic system by measuring brain mAChR binding and acetylcholinesterase activity using positron emission tomography (PET) in CFS patients with positive [CFS(+)] and negative [CFS(−)] autoantibodies.

Methodology

Five CFS(+) and six CFS(−) patients, as well as 11 normal control subjects underwent a series of PET measurements with N-[¹¹C]methyl-3-piperidyl benzilate [¹¹C](+)3-MPB for the mAChR binding and N-[¹¹C]methyl-4-piperidyl acetate [¹¹C]MP4A for acetylcholinesterase activity. Cognitive function of all subjects was assessed by neuropsychological tests. Although the brain [¹¹C](+)3-MPB binding in CFS(−) patients did not differ from normal controls, CFS(+) patients showed significantly lower [¹¹C](+)3-MPB binding than CFS(−) patients and normal controls. In contrast, the [¹¹C]MP4A index showed no significant differences among these three groups. Neuropsychological measures were similar among groups.

Conclusion

The present results demonstrate that serum autoantibody against the mAChR can affect the brain mAChR without altering acetylcholinesterase activity and cognitive functions in CFS patients.

Movement- and behavioral state-dependent activity of pontine reticulospinal neurons

Forty-five years ago Shik and colleagues were the first to demonstrate that electrical stimulation of the dorsal pontine reticular formation induced fictive locomotion in decerebrate cats. This supraspinal motor site was subsequently termed the "mesencephalic locomotor region (MLR)". Cholinergic neurons of the pedunculopontine tegmental nucleus (PPT) have been suggested to form, or at least comprise in part, the neuroanatomical basis for the MLR, but direct evidence is lacking. In an effort to clarify the location and activity profiles of pontine reticulospinal neurons supporting locomotor behaviors, we employed in the present study a retrograde tracing method in combination with single-unit recordings and antidromic spinal cord stimulation as well as characterized the locomotor- and behavioral state-dependent activities of both reticulospinal and non-reticulospinal neurons. The retrograde labeling and antidromic stimulation responses suggested a candidate group of reticulospinal neurons that were non-cholinergic and located just medial to the PPT cholinergic neurons and ventral to the cuneiform nucleus (CnF). Unit recordings from these reticulospinal neurons in freely behaving animals revealed that the preponderance of neurons fired in relation to motor behaviors and that some of these neurons were also active during rapid eye movement sleep. By contrast, non-reticulospinal neurons, which likely included cholinergic neurons, did not exhibit firing activity in relation to motor behaviors. In summary, the present study provides neuroanatomical and electrophysiological evidence that non-cholinergic, pontine reticulospinal neurons may constitute the major component of the long-sought neuroanatomic MLR in mammals.

A neural correlate of predicted and actual reward-value information in monkey pedunculopontine tegmental and dorsal raphe nucleus during saccade tasks

Dopamine, acetylcholine, and serotonin, the main modulators of the central nervous system, have been proposed to play important roles in the execution of movement, control of several forms of attentional behavior, and reinforcement learning. While the response pattern of midbrain dopaminergic neurons and its specific role in reinforcement learning have been revealed, the role of the other neuromodulators remains rather elusive. Here, we review our recent studies using extracellular recording from neurons in the pedunculopontine tegmental nucleus, where many cholinergic neurons exist, and the dorsal raphe nucleus, where many serotonergic neurons exist, while monkeys performed eye movement tasks to obtain different reward values. The firing patterns of these neurons are often tonic throughout the task period, while dopaminergic neurons exhibited a phasic activity pattern to the task event. The different modulation patterns, together with the activity of dopaminergic neurons, reveal dynamic information processing between these different neuromodulator systems.

Muscarinic receptor occupancy and cognitive impairment: a PET study with [11C](+)3-MPB and scopolamine in conscious monkeys

The muscarinic cholinergic receptor (mAChR) antagonist scopolamine was used to induce transient cognitive impairment in monkeys trained in a delayed matching to sample task. The temporal relationship between the occupancy level of central mAChRs and cognitive impairment was determined. Three conscious monkeys (Macaca mulatta) were subjected to positron emission tomography (PET) scans with the mAChR radioligand N-[(11)C]methyl-3-piperidyl benzilate ([(11)C](+)3-MPB). The scan sequence was pre-, 2, 6, 24, and 48 h post-intramuscular administration of scopolamine in doses of 0.01 and 0.03 mg/kg. Occupancy levels of mAChR were maximal 2 h post-scopolamine in cortical regions innervated primarily by the basal forebrain, thalamus, and brainstem, showing that mAChR occupancy levels were 43-59 and 65-89% in doses of 0.01 and 0.03 mg/kg, respectively. In addition, dose-dependent impairment of working memory performance was measured 2 h after scopolamine. A positive correlation between the mAChR occupancy and cognitive impairment 2 and 6 h post-scopolamine was the greatest in the brainstem (P<0.00001). Although cognitive impairment was not observed 24 h post-scopolamine, sustained mAChR occupancy (11-24%) was found with both doses in the basal forebrain and thalamus, but not in the brainstem. These results indicate that a significant degree of mAChRs occupancy is needed to produce cognitive impairment by scopolamine. Furthermore, the importance of the brainstem cholinergic system in working memory in monkey is described.

Putative cholinergic interneurons in the ventral and dorsal regions of the striatum have distinct roles in a two choice alternative association task

The striatum consists of GABAergic projection neurons and various types of interneurons. Despite their relative scarcity, these interneurons play a key role in information processing in the striatum. One such class of interneurons is the cholinergic tonically active neurons (TANs). In the dorsal striatum, TANs are traditionally considered to be responsive to events of motivational significance. However, in recent years, studies have suggested that TANs are not exclusively related to reward and reward-predicting stimuli, but may contribute to other processes, including responses to aversive stimuli, detecting the spatial location of stimuli and generating movement. Currently there is little data concerning TAN activity in the ventral striatum (VS) of behaving animals. Here, we simultaneously recorded neurons in the ventral and the dorsolateral (DLS) regions of the striatum while animals performed a two choice alternative association task. Our data show that a large percentage of the putative TANs in both regions responded around movement initiation and execution. The majority of these neurons exhibited directional selectivity which was stronger in DLS relative to VS. In addition, the preferred directions in VS were mostly contralateral to the recording site whereas the observed preferred directions in DLS were equally distributed contralaterally and ipsilaterally to the recording site. The most interesting difference between DLS and VS was that DLS TANs maintained activity alterations throughout the movement whereas TANs in VS exhibited short-lasting phasic activity alterations that were maintained throughout the movement by different neurons. Our findings suggest that coding of movement by TANs in both regions overlaps to some degree, yet the differences in response patterns support the notion that the TANs in DLS participate in the motor loop whereas TANs in VS convey event-related information such as movement initiation, movement direction, and end of movement.

Internal pallidum and substantia nigra control different parts of the mesopontine reticular formation in primate

The locomotor area has recently emerged as a target for deep brain stimulation to lessen gait disturbances in advanced parkinsonian patients. An important step in choosing this target is to define anatomical limits of its 2 components, the pedunculopontine nucleus and the cuneiform nucleus, their connections with the basal ganglia, and their output descending pathway. Based on the hypothesis that pedunculopontine nucleus controls locomotion whereas cuneiform nucleus controls axial posture, we analyzed whether both nuclei receive inputs from the internal pallidum and substantia nigra using anterograde and retrograde tract tracing in monkeys. We also examined whether these nuclei convey descending projections to the reticulospinal pathway. Pallidal terminals were densely distributed and restricted to the pedunculopontine nucleus, whereas nigral terminals were diffusely observed in the whole extent of both the pedunculopontine nucleus and the cuneiform nucleus. Moreover, nigral terminals formed symmetric synapses with pedunculopontine nucleus and cuneiform nucleus dendrites. Retrograde tracing experiments confirmed these results because labeled cell bodies were observed in both the internal pallidum and substantia nigra after pedunculopontine nucleus injection, but only in the substantia nigra after cuneiform nucleus injection. Furthermore, anterograde tracing experiments revealed that the pedunculopontine nucleus and cuneiform nucleus project to large portions of the pontomedullary reticular formation. This is the first anatomical evidence that the internal pallidum and the substantia nigra control different parts of the brain stem and can modulate the descending reticulospinal pathway in primates. These findings support the functional hypothesis that the nigro-cuneiform nucleus pathway could control axial posture whereas the pallido-pedunculopontine nucleus pathway could modulate locomotion.

The pedunculopontine nucleus as an additional target for deep brain stimulation

The pedunculopontine nucleus has been suggested as a target for DBS. In this paper we propose a single compartment computational model for a PPN Type I cell and compare its dynamic behavior with experimental data. The model shows bursts after a period of hyperpolarization and spontaneous firing at 8 Hz. Bifurcation analysis of the single PPN cell shows bistability of fast and slow spiking solutions for a range of applied currents. A network model for STN, GPe and GPi produces basal ganglia output that is used as input for the PPN cell. The conductances for projections from the STN and the GPi to the PPN are determined from experimental data. The resulting behavior of the PPN cell is studied under normal and Parkinsonian conditions of the basal ganglia network. The effect of high frequency stimulation of the STN is considered as well as the effect of combined high frequency stimulation of the STN and the PPN at various frequencies. The relay properties of the PPN cell demonstrate that the combined high frequency stimulation of STN and low frequency (10 Hz, 25 Hz, 40 Hz) stimulation of PPN hardly improves the effect of exclusive STN stimulation. Moreover, PPN-DBS at low stimulation amplitude has a better effect than at higher stimulation amplitude. The effect of PPN output on the basal ganglia is investigated, in particular the effect of STN-DBS and/or PPN-DBS on the pathological firing pattern of STN and GPe cells. PPN-DBS eliminates the pathological firing pattern of STN and GPe cells, whereas STN-DBS and combined STN-DBS and PPN-DBS eliminate the pathological firing pattern only from STN cells.

Involvement of the human pedunculopontine nucleus region in voluntary movements

OBJECTIVE

The pedunculopontine nucleus region (PPNR) is being investigated as a target for deep brain stimulation (DBS) in Parkinson disease (PD), particularly for gait and postural impairment. A greater understanding of how PPNR activities and oscillations are modulated with voluntary movements is crucial to the development of neuromodulation strategies.

METHODS

We studied 7 patients with PD who underwent DBS electrode implantations in the PPNR. PPNR local field potential and EEG were recorded while patients performed self-paced wrist and ankle movements.

RESULTS

Back-averaging of the PPNR recording showed movement-related potentials before electromyography onset. Frequency analysis showed 2 discrete movement-related frequency bands in the theta (6- to 10-Hz) and beta (14- to 30-Hz) ranges. The PPNR theta band showed greater event-related desynchronization with movements in the ON than in the OFF medication state and was coupled with the sensorimotor cortices in the ON state only. Beta event-related desynchronization was observed in the PPNR during the premovement and movement execution phases in the OFF state. In contrast, premovement PPNR beta event-related synchronization occurred in the ON state. Moreover, beta band coherence between the PPNR and the midline prefrontal region was observed during movement preparation in the ON but not the OFF state.

CONCLUSIONS

Activities of PPNR change during movement preparation and execution in patients with PD. Dopaminergic medications modulate PPNR activities and promote the interactions between the cortex and PPNR. Beta oscillations may have different functions in the basal ganglia and PPNR, and may be prokinetic rather than antikinetic in the PPNR.

Physiological identification of the human pedunculopontine nucleus

BACKGROUND

The pedunculopontine nucleus (PPN) is a brainstem structure with widespread connections to the basal ganglia. Despite the recent introduction of PPN deep brain stimulation (DBS) for the treatment of gait disorders, little is known about its physiology in humans.

METHODS

Single unit discharge characteristics of neurons in the PPN region were analysed in four patients and PPN local field potentials (LFP) in one patient, recorded during the course of DBS implantation. Two patients had Parkinson disease, and two had non-sinemet responsive parkinsonism. Cell locations were plotted in the coordinate system of a human brainstem atlas.

RESULTS

Fifty-six units in the PPN region were studied, of which 32 mapped to within PPN boundaries. The mean (SD) discharge rate of neurons in the PPN was 23.2 (15.6) Hz. Spontaneous neuronal firing rate and burst discharge rate were significantly different between neurons in the region dorsal to PPN and those in the PPN. Responses to passive movement of contralateral and ipsilateral limbs were found. Theta and beta band oscillations were present in the PPN LFP.

CONCLUSION

PPN discharge characteristics may prove useful in the electrophysiological identification of PPN during DBS implantation surgery.

Characterization of oculomotor and visual activities in the primate pedunculopontine tegmental nucleus during visually guided saccade tasks

The pedunculopontine tegmental nucleus (PPTN) has anatomical connections with numerous visuomotor areas including the basal ganglia, thalamus, superior colliculus and frontal eye field. Although many anatomical and physiological studies suggest a role for the PPTN in the control of conditioned behavior and associative learning, the detailed characteristics of saccade- and visual-related activities of PPTN neurons remain unclear. We recorded the activity of PPTN neurons in monkeys (Macaca fuscata ) during visually guided saccade tasks, and examined the response properties of saccade- and visual-related activities such as time course, direction selectivity and contextual modulation. Saccade-related activity occurred either during saccade execution or after saccade end. The preferred directions of the neuronal activity were biased toward the contralateral and upward sides. Half of the saccade-related neurons showed activity modulation only for task saccades and not for spontaneous saccades outside the task. Visually-responsive neurons responded with short latencies. Some responded to the appearance of the visual stimulus in a directionally selective manner, and others responded to both the appearance and disappearance of the visual stimulus in a directionally non-selective manner. Many of these neurons exhibited distinct visual responses to the appearance of two different stimuli presented under different stages of the task, whereas a population of the neurons responded equally to the disappearance of the two stimuli. Thus, many PPTN neurons exhibited context-dependent activity related to the visuomotor events, consistent with a role in controlling conditioned behavior.

Different pedunculopontine tegmental neurons signal predicted and actual task rewards

The dopamine system has been implicated in guiding behavior based on rewards. The pedunculopontine tegmental nucleus (PPTN) of the brainstem receives afferent inputs from reward-related structures, including the cerebral cortices and the basal ganglia, and in turn provides strong excitatory projections to dopamine neurons. This anatomical evidence predicts that PPTN neurons may carry reward information. To elucidate the functional role of the PPTN in reward-seeking behavior, we recorded single PPTN neurons while monkeys performed a visually guided saccade task in which the predicted reward value was informed by the shape of the fixation target. Two distinct groups of neurons, fixation target (FT) and reward delivery (RD) neurons, carried reward information. The activity of FT neurons persisted between FT onset and reward delivery, with the level of activity associated with the magnitude of the expected reward. RD neurons discharged phasically after reward delivery, with the levels of activity associated with the actual reward. These results suggest that separate populations of PPTN neurons signal predicted and actual reward values, both of which are necessary for the computation of reward prediction error as represented by dopamine neurons.

Connectivity of the human pedunculopontine nucleus region and diffusion tensor imaging in surgical targeting

OBJECT

The pedunculopontine nucleus (PPN) region of the brainstem has become a new stimulation target for the treatment of gait freezing, akinesia, and postural instability in advanced Parkinson disease (PD). Because PD locomotor symptoms are probably caused by excessive gamma-aminobutyric acidergic inhibition of the PPN, low-frequency stimulation of the PPN may overcome this inhibition and improve the symptoms. However, the anatomical connections of this region in humans are not known in any detail.

METHODS

Diffusion weighted magnetic resonance (MR) images were acquired at 1.5 teslas, and probabilistic tractography was used to trace the connections of the PPN region in eight healthy volunteers. A single seed voxel (2 x 2 x 2 mm) was chosen in the PPN just lateral to the decussation of the superior cerebellar peduncle, and the Diffusion Toolbox of the Oxford Centre for Functional Magnetic Resonance Imaging of the Brain was used to process the acquired MR images. The connections of each volunteer's PPN region were analyzed using a human brain MR imaging atlas.

RESULTS

The PPN region was connected with the cerebellum and spinal cord below and to the thalamus, pallidum, subthalamic nucleus, and motor cortex above. The regions of the primary motor cortex that control the trunk and upper and lower extremities had the highest connectivity compared with other parts of motor cortex.

CONCLUSIONS

These findings suggest that connections of the PPN region with the primary motor cortex, basal ganglia, thalamus, cerebellum, and spinal cord may play important roles in the regulation of movement by the PPN region. Diffusion tensor imaging tractography of the PPN region may be used preoperatively to optimize placement of stimulation electrodes and postoperatively it may also be useful to reassess electrode positions.

High-frequency stimulation of the subthalamic nucleus modulates the activity of pedunculopontine neurons through direct activation of excitatory fibres as well as through indirect activation of inhibitory pallidal fibres in the rat

Recent data suggest a potential role of pedunculopontine nucleus (PPN) electrical stimulation in improving gait and posture in Parkinson's disease. Because the PPN receives fibres from the subthalamic nucleus (STN), we investigated the effects of STN-high-frequency stimulation (HFS) on PPN neuronal activity in intact rats and in rats bearing either an ibotenate lesion of the entopeduncular nucleus (EP) or a lesion of the substantia nigra (SN). The main response of PPN neurons to STN single-shock stimulations in the three experimental groups was a short latency (4.5 +/- 2.1 ms) and brief (15.3 +/- 6.5 ms) excitation. This response was maintained during 1-5 s of STN-HFS (130 Hz, 60 micros, 100-1000 microA). In EP-lesioned rats the percentage (75.0%) of PPN neurons showing a modulation of activity following STN-HFS was significantly higher compared with that observed in intact (39.7%) and in SN-lesioned rats (35.4%). Furthermore, in EP-lesioned rats the most frequent response of PPN neurons following STN-HFS was a 5-20 s excitation, which was present in 76.6% of responsive neurons in comparison to 15.4% and 9.1% of neurons responsive in intact and in 6-hydroxydopamine-lesioned rats, respectively. Neurons responsive to STN-HFS in the three experimental groups showed either a sharp positively skewed distribution of interspike intervals or multisecond oscillations in autocorrelograms. The results support that STN-HFS modulates the PPN through a balance of excitatory and inhibitory influences, which may be independent from the dopaminergic nigral neurons. In the absence of inhibitory EP fibres, the direct excitatory influence exerted by the STN on the PPN appears to predominate.

How best to consider the structure and function of the pedunculopontine tegmental nucleus: Evidence from animal studies

This review presents the hypothesis that the best way to consider the pedunculopontine tegmental nucleus is by analogy with the substantia nigra. The substantia nigra contains two main compartments: the pars compacta and the pars reticulata. The former contains dopamine neurons that project widely within the basal ganglia while the latter is in receipt of corticostriatal output. Similarly, the PPTg contains the Ch5 acetylcholine containing neurons that project to the thalamus and corticostriatal systems (notably the pars compacta of substantia nigra and the subthalamic nucleus) while the non-cholinergic neurons of the pedunculopontine are in receipt of corticostriatal output. Assessment of the location, composition and connections of the pedunculopontine tegmental nucleus is made to support the hypothesis that it has structural similarities with substantia nigra. Assessment of the motor, sensory and cognitive functions of the pedunculopontine is also made, suggesting functional similarities exist also. Having a clear model of pedunculopontine structure and function is a matter of some importance. It is clearly involved in Parkinson's disease and could potentially be a target for therapeutic intervention. If this is to be realized it will be best to have as clear an understanding as possible of pedunculopontine structure and function in order to maximize positive benefits.

Implantation of human pedunculopontine nucleus: a safe and clinically relevant target in Parkinson's disease

The peduncolopontine nucleus modulates locomotor activity and dysfunction in this nucleus may be responsible for the gait and postural impairments seen in Parkinson's disease and other movement disorders. We report the first surgical exploration and implantation of deep brain stimulating electrodes of the peduncolopontine nucleus area in two Parkinson's disease patients to examine the safety and the potential benefit of chronic electrical stimulation at this site. Under local anesthesia, the peduncolopontine nucleus was approached from a coronal burr hole using a trajectory that was 78-80 degrees and 62-64 degrees on the coronal and sagittal planes. Microrecordings helped to identify neurons in peduncolopontine nucleus and the adjacent substantia nigra pars reticulata. Chronic deep brain stimulating electrodes were implanted within the peduncolopontine nucleus in a manner similar to that practiced with deep brain stimulating surgery at other targets. Peduncolopontine nucleus neurons were characterized by small and broad multiunits (230 muV, 2.5 ms, 14.6 Hz). Caudal to this area, neurons firing at higher frequency, approximately 70 Hz, characteristic of nigral neuronal discharges, were encountered, followed by 2 mm of cells similar to those recorded in the dorsal peduncolopontine nucleus area. After deep brain stimulating electrodes implantation, acute intraoperative stimulation (up to 3 V) was performed with two stimulation frequencies in each session. Stimulation at 80 Hz has little discernable effect. On the other hand, stimulation at 10 Hz fostered a subjective feeling of 'well-being' and a time-locked amelioration of the clinical scores. These findings demonstrate that the stereotactic approach of peduncolopontine nucleus is safe. The target may reliably be identified by microrecordings. Low-frequency stimulation may produce acute improvements in motor function.

The pedunculopontine tegmental nucleus and experimental parkinsonism

In this article, I review a series of my experiments on the role of the pedunculopontine tegmental nucleus (Ppn) in voluntary movements. I sought to elucidate the functional role of the Ppn in the control of voluntary movements using macaque monkeys. I propose a functional model of basal ganglia circuitry in which I emphasize the role of the Ppn and discuss the underlying mechanisms of parkinsonism.

Changes in the excitability of hindlimb motoneurons during muscular atonia induced by stimulating the pedunculopontine tegmental nucleus in cats

We have previously reported that electrical stimulation delivered to the ventral part of the pedunculopontine tegmental nucleus (PPN) produced postural atonia in acutely decerebrated cats [Neuroscience 119 (2003) 293]. The present study was designed to elucidate synaptic mechanisms acting on motoneurons during postural atonia induced by PPN stimulation. Intracellular recording was performed from 72 hindlimb motoneurons innervating extensor and flexor muscles, and the changes in excitability of the motoneurons following the PPN stimulation were examined. Repetitive electrical stimulation (20-50 microA, 50 Hz, 5-10 s) of the PPN hyperpolarized the membrane potentials of both the extensor and flexor motoneurons by 2.0-12 mV (6.0 +/- 2.3 mV, n = 72). The membrane hyperpolarization persisted for 10-20 s even after termination of the stimulation. During the PPN stimulation, the membrane hyperpolarization was associated with decreases in the firing capability (n = 28) and input resistance (28.5 +/- 6.7%, n = 14) of the motoneurons. Moreover the amplitude of Ia excitatory postsynaptic potentials was also reduced (44.1 +/- 13.4%, n = 14). After the PPN stimulation, these parameters immediately returned despite that the membrane hyperpolarization persisted. Iontophoretic injections of chloride ions into the motoneurons reversed the polarity of the membrane hyperpolarization during the PPN stimulation. The polarity of the outlasting hyperpolarization however was not reversed. These findings suggest that a postsynaptic inhibitory mechanism, which was mediated by chloride ions, was acting on hindlimb motoneurons during PPN-induced postural atonia. However the outlasting motoneuron hyperpolarization was not due to the postsynaptic inhibition but it could be due to a decrease in the activity of descending excitatory systems. The functional role of the PPN in the regulation of postural muscle tone is discussed with respect to the control of behavioral states of animals.

Activity of neurons in the pedunculopontine nucleus during a food-related operant conditioned reflex

The activity of 91 neurons in the compact and diffuse zones of the pedunculopontine nucleus of freely mobile rabbits was studied during performance of a food-related operant conditioned reflex. A total of 37.4% of the neurons recorded showed reactivity to the conditioned stimulus, which is evidence that the pedunculopontine nucleus is involved in operant learning. A significant predominance of excitatory responses to the conditioned stimulus and to food reinforcement was demonstrated. The main patterns of cell responses were identified, these reflecting the nature of the effect of the conditioned stimulus on neuron activity, the structure of the behavioral act, and the properties of the reinforcement, and demonstrating a relationship between the pedunculopontine nucleus and the processes of attention, motor learning, and reinforcement. Differences were seen in the associative reactive properties of the compact and diffuse zones of the pedunculopontine nucleus to the conditioned stimulus and reinforcement, which is evidence for the functional diversity of this formation and suggests a leading role for the cholinergic compact zone in food-related operant learning and reinforcement, the diffuse zone having a leading role in food-related classical conditioned-reflex learning.

Pedunculo-pontine control of visually guided saccades

The cholinergic pedunculopontine tegmental nucleus (PPTN) is one of the major ascending arousal systems in the brainstem, and it is linked to motor, limbic and sensory centers. Despite an abundance of anatomical and physiological data, however, the functional role of PPTN neurons in behavioral control is still unresolved. In this chapter, we hypothesize that the PPTN is implicated in the integrative control of movement, particularly the reinforcement of tasks performed during conscious behavior. We present a new model of the PPTN's involvement in the control of arousal, attention and reinforcement aspects of motor behavior, with a focus on the control of saccadic eye movements.

Neuron activity in the pedunculopontine nucleus during an operant conditioned defensive reflex

The activity of 109 neurons in the compact and diffuse zones of the pedunculopontine nucleus was studied in freely mobile rabbits during the acquisition and performance of a defensive operant conditioned reflex. A total of 47% of the neurons recorded showed responsive properties to the conditioned stimulus, which is evidence for the involvement of the pedunculopontine nucleus in operant learning. A significant predominance of excitatory conditioned reflex responses to the conditioned stimulus was demonstrated, showing that the nature of the influence of the pedunculopontine nucleus on projection structures during learning is mostly excitatory. The main patterns of cell responses to the conditioned stimulus were identified, these reflecting the nature of the influence of the conditioned stimulus on neuron activity, the structure of the behavioral act, and the properties of the reinforcement, suggesting a relationship between the pedunculopontine nucleus and the processes of attention, motor learning, and reinforcement. A significant decrease in the reactivity of neurons in the pedunculopontine nucleus to the conditioned stimulus as a result of specialization due to learning was demonstrated. Differences in the associative reactive properties of the compact and diffuse zones of the pedunculopontine nucleus to the conditioned stimulus were identified, which is evidence for the functional heterogeneity of this formation and suggests a leading role for the cholinergic compact zone in operant defensive behavior.

The role of the pedunculopontine tegmental nucleus in experimental parkinsonism in primates

To clarify the role of the pedunculopontine tegmental nucleus (PPN) in motor behavior, we have conducted a series of experiments in primates. In the first part, PPN was damaged locally with kainic acid, which resulted in mild hemiparkinsonism in the contralateral limbs. In the second part, muscimol (a GABA agonist) was locally injected into the PPN area in monkeys who had been trained to perform a lever-pull movement with an arm, resulting in a slowness of movement and a delay of the movement onset. In the third part, a dopaminergic neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was systemically injected in monkeys with prior PPN lesioning. These monkeys developed no, or if any, very mild parkinsonism. PPN lesioning was supposed to have protected the nigral neurons from the MPTP- toxicity. The PPN facilitates the motor system through its nigral projection. The decreased activity of the PPN may underlie the pathophysiology of parkinsonism.

Contribution of pedunculopontine tegmental nucleus neurons to performance of visually guided saccade tasks in monkeys

The cholinergic pedunculopontine tegmental nucleus (PPTN) is one of the major ascending arousal systems in the brain stem and is linked to motor, limbic, and sensory systems. Based on previous studies, we hypothesized that PPTN would be related to the integrative control of movement, reinforcement, and performance of tasks in behaving animals. To investigate how PPTN contributes to the behavioral control, we analyzed the activity of PPTN neurons during visually guided saccade tasks in three monkeys in relation to saccade preparation, execution, reward, and performance of the task. During visually guided saccades, we observed saccade-related burst (26/70) and pause neurons (19/70), indicating that a subset of PPTN neurons are related to both saccade execution and fixation. Burst neurons exhibited greater selectivity for saccade direction than pause neurons. The preferred directions for both burst and pause neurons were not aligned with either horizontal or vertical axes, nor biased strongly in either the ipsilateral or the contralateral direction. The spatial representation of the saccade-related activity of PPTN neurons is different from other brain stem saccade systems and may therefore relay saccade-related activity from different areas. Increasing discharges were observed around reward onset in a subset of neurons (22/70). These neurons responded to the freely delivered rewards within ~140 ms. However, during the saccade task, the latencies of the responses around reward onset ranged between 100 ms before and 200 ms after the reward onset. These results suggest that the activity observed after appropriate saccade during the task may include response associated with reward. We found that the reaction time to the appearance of the fixation point (FP) was longer when the animal tended to fail in the ensuring task. This reaction time to FP appearance (RTFP) served as an index of motivation. The RTFP could be predicted by the neuronal activity of a subset of PPTN neurons (13/70) that varied their activity levels with task performance, discharging at a higher rate in successful versus error trials. A combination of responses related to saccade execution, reward delivery, and task performance was observed in PPTN neurons. We conclude from the multimodality of responses in PPTN neurons that PPTN may serve as an integrative interface between the various signals required for performing purposive behaviors.

Sensory-motor gating and cognitive control by the brainstem cholinergic system

As an essential component of ascending activating systems, cholinergic neurons with diffuse projections are supposed to be involved in the regulation of cognitive processes such as attention, consciousness, learning, and memory. As for the role of cholinergic projections from the basal forebrain nuclei to cerebral cortical regions including hippocampus, a couple of models have been proposed that acetylcholine facilitates extrinsic inputs to the cortex and inhibits intracortical processing. In this review, to explore the possibility that there exists a generalized principle on the role of cholinergic systems in the brain, we summarized the knowledge so far obtained on the action of a brainstem cholinergic nucleus, the pedunculopontine tegmental nucleus (PPTN) at their target regions. By in vitro experiments we clarified that cholinergic inputs to the intermediate layer of the superior colliculus, presumably originating from the PPTN, facilitate generation of its motor outputs for the initiation of saccades. Furthermore, cholinergic inputs may enhance excitatory responses of mesopontine dopaminergic cells, for instance to reward-related signals. In addition, we observed that PPTN neurons showed multi-modal activities in behaving monkeys; their activities were related to execution and preparation of saccades, the level of task performance, and reward. The multi-modal activities encoded in the PPTN may suggest that PPTN associates movement-related activities with those related to task performance and reward. Together with the already reported facilitatory action on the sensory processing at the visual thalamus, these observations suggest that the brainstem cholinergic system facilitates the central processes for motor command generation and extrinsic sensory processing. For our final goal of exploring the general working principle of the cholinergic systems, further studies are needed to clarify the effects of the brainstem cholinergic system on the intrinsic processing in the brain.