The pedunculopontine tegmental nucleus-A functional hypothesis from the comparative literature

Chronic MPTP administration regimen in monkeys: a model of dopaminergic and non-dopaminergic cell loss in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder clinically characterized by cardinal motor deficits including bradykinesia, tremor, rigidity and postural instability. Over the past decades, it has become clear that PD symptoms extend far beyond motor signs to include cognitive, autonomic and psychiatric impairments, most likely resulting from cortical and subcortical lesions of non-dopaminergic systems. In addition to nigrostriatal dopaminergic degeneration, pathological examination of PD brains, indeed, reveals widespread distribution of intracytoplasmic inclusions (Lewy bodies) and death of non-dopaminergic neurons in the brainstem and thalamus. For that past three decades, the MPTP-treated monkey has been recognized as the gold standard PD model because it displays some of the key behavioral and pathophysiological changes seen in PD patients. However, a common criticism raised by some authors about this model, and other neurotoxin-based models of PD, is the lack of neuronal loss beyond the nigrostriatal dopaminergic system. In this review, we argue that this assumption is largely incorrect and solely based on data from monkeys intoxicated with acute administration of MPTP. Work achieved in our laboratory and others strongly suggest that long-term chronic administration of MPTP leads to brain pathology beyond the dopaminergic system that displays close similarities to that seen in PD patients. This review critically examines these data and suggests that the chronically MPTP-treated nonhuman primate model may be suitable to study the pathophysiology and therapeutics of some non-motor features of PD.

White matter tract alterations in Parkinson's disease patients with punding

OBJECTIVE

To assess brain white matter tract alterations in patients with Parkinson's disease and punding (PD-punding) compared with controls and PD cases without any impulsive-compulsive behaviour.

METHODS

Forty-nine PD patients (21 PD-punding and 28 PD with no impulsive-compulsive behaviours) and 28 controls were consecutively recruited. Clinical, cognitive and psychopathological evaluations were performed. Diffusion tensor MRI metrics of the main white matter tracts were assessed using a tractography approach.

RESULTS

Compared with controls, both PD groups showed white matter microstructural alterations of the left pedunculopontine tract and splenium of the corpus callosum. PD-punding patients showed a further damage to the right pedunculopontine tract and uncinate fasciculus, genu of the corpus callosum, and left parahippocampal tract relative to controls. When adjusting for depression and/or apathy severity, a greater damage of the genu of the corpus callosum and the left pedunculopontine tract was found in PD-punding compared with patients with no impulsive-compulsive behaviours.

CONCLUSIONS

PD-punding is associated with a disconnection between midbrain, limbic and white matter tracts projecting to the frontal cortices. These alterations are at least partially independent of their psychopathological changes. Diffusion tensor MRI is a powerful tool for understanding the neural substrates underlying punding in PD.

Activation of Pedunculopontine Glutamate Neurons Is Reinforcing

Dopamine transmission from midbrain ventral tegmental area (VTA) neurons underlies behavioral processes related to motivation and drug addiction. The pedunculopontine tegmental nucleus (PPTg) is a brainstem nucleus containing glutamate-, acetylcholine-, and GABA-releasing neurons with connections to basal ganglia and limbic brain regions. Here we investigated the role of PPTg glutamate neurons in reinforcement, with an emphasis on their projections to VTA dopamine neurons. We used cell-type-specific anterograde tracing and optogenetic methods to selectively label and manipulate glutamate projections from PPTg neurons in mice. We used anatomical, electrophysiological, and behavioral assays to determine their patterns of connectivity and ascribe functional roles in reinforcement. We found that photoactivation of PPTg glutamate cell bodies could serve as a direct positive reinforcer on intracranial self-photostimulation assays. Further, PPTg glutamate neurons directly innervate VTA; photostimulation of this pathway preferentially excites VTA dopamine neurons and is sufficient to induce behavioral reinforcement. These results demonstrate that ascending PPTg glutamate projections can drive motivated behavior, and PPTg to VTA synapses may represent an important target relevant to drug addiction and other mental health disorders.

SIGNIFICANCE STATEMENT

Uncovering brain circuits underlying reward-seeking is an important step toward understanding the circuit bases of drug addiction and other psychiatric disorders. The dopaminergic system emanating from the ventral tegmental area (VTA) plays a key role in regulating reward-seeking behaviors. We used optogenetics to demonstrate that the pedunculopontine tegmental nucleus sends glutamatergic projections to VTA dopamine neurons, and that stimulation of this circuit promotes behavioral reinforcement. The findings support a critical role for pedunculopontine tegmental nucleus glutamate neurotransmission in modulating VTA dopamine neuron activity and behavioral reinforcement.

LFP Oscillations in the Mesencephalic Locomotor Region during Voluntary Locomotion

Oscillatory rhythms in local field potentials (LFPs) are thought to coherently bind cooperating neuronal ensembles to produce behaviors, including locomotion. LFPs recorded from sites that trigger locomotion have been used as a basis for identification of appropriate targets for deep brain stimulation (DBS) to enhance locomotor recovery in patients with gait disorders. Theta band activity (6–12 Hz) is associated with locomotor activity in locomotion-inducing sites in the hypothalamus and in the hippocampus, but the LFPs that occur in the functionally defined mesencephalic locomotor region (MLR) during locomotion have not been determined. Here we record the oscillatory activity during treadmill locomotion in MLR sites effective for inducing locomotion with electrical stimulation in rats. The results show the presence of oscillatory theta rhythms in the LFPs recorded from the most effective MLR stimulus sites (at threshold ≤60 μA). Theta activity increased at the onset of locomotion, and its power was correlated with the speed of locomotion. In animals with higher thresholds (>60 μA), the correlation between locomotor speed and theta LFP oscillations was less robust. Changes in the gamma band (previously recorded in vitro in the pedunculopontine nucleus (PPN), thought to be a part of the MLR) were relatively small. Controlled locomotion was best achieved at 10–20 Hz frequencies of MLR stimulation. Our results indicate that theta and not delta or gamma band oscillation is a suitable biomarker for identifying the functional MLR sites.

A Network Model of the Emotional Brain

Emotion is often understood in terms of a circumscribed set of cortical and subcortical brain regions. I propose, instead, that emotion should be understood in terms of large-scale network interactions spanning the entire neuroaxis. I describe multiple anatomical and functional principles of brain organization that lead to the concept of 'functionally integrated systems', cortical-subcortical systems that anchor the organization of emotion in the brain. The proposal is illustrated by describing the cortex-amygdala integrated system and how it intersects with systems involving the ventral striatum/accumbens, septum, hippocampus, hypothalamus, and brainstem. The important role of the thalamus is also highlighted. Overall, the model clarifies why the impact of emotion is wide-ranging, and how emotion is interlocked with perception, cognition, motivation, and action.

Disynaptic Subthalamic Input to the Posterior Cerebellum in Rat

In the last decade, the interplay between basal ganglia and cerebellar functions has been increasingly advocated to explain their joint operation in both normal and pathological conditions. Yet, insight into the neuroanatomical basis of this interplay between both subcortical structures remains sparse and is mainly derived from work in primates. Here, in rodents, we have studied the existence of a potential disynaptic connection between the subthalamic nucleus (STN) and the cerebellar cortex as has been demonstrated earlier for the primate. A mixture of unmodified rabies virus (RABV: CVS 11) and cholera toxin B-subunit (CTb) was injected at places in the posterior cerebellar cortex of nine rats. The survival time was chosen to allow for disynaptic retrograde transneuronal infection of RABV. We examined the STN for neurons infected with RABV in all nine cases and related the results with the location of the RABV/CTb injection site, which ranged from the vermis of lobule VII, to the paravermis and hemispheres of the paramedian lobule and crus 2a. We found that cases with injection sites in the vermis of lobule VII showed prominent RABV labeling in the STN. In contrast, almost no subthalamic labeling was noted in cases with paravermal or hemispheral injection sites. We show circumstantial evidence that not only the pontine nuclei but also the pedunculotegmental nucleus may act as the intermediary in the connection from STN to cerebellar cortex. This finding implies that in the rat the STN links disynaptically to the vermal part of lobule VII of the cerebellar cortex, without any major involvement of the cerebellar areas that are linked to sensorimotor functions. As vermal lobule VII recently has been shown to process disynaptic input from the retrosplenial and orbitofrontal cortices, we hypothesize that in the rat the subthalamic input to cerebellar function might be used to influence more prominently non-motor functions of the cerebellum than motor functions. This latter aspect seems to contradict the primate results and could point to a more elaborate interaction between basal ganglia and cerebellum in more demanding motor tasks.

Antisaccades in Parkinson disease

Objective:

To describe the relation between gaze and posture/gait control in Parkinson disease (PD) and to determine the role of the mesencephalic locomotor region (MLR) and cortex-MLR connection in saccadic behavior because this structure is a major area involved in both gait/postural control and gaze control networks.

Methods:

We recruited 30 patients with PD with or without altered postural control and 25 age-matched healthy controls (HCs). We assessed gait, balance, and neuropsychological status and separately recorded gait initiation and eye movements (visually guided saccades and volitional antisaccades). We identified correlations between the clinical and physiologic parameters that best characterized patients with postural instability. We measured resting-state functional connectivity in 2 pathways involving the frontal oculomotor cortices and the MLR and sought correlations with saccadic behavior.

Results:

Patients with PD with postural instability showed altered antisaccade latencies that correlated with the stand-walk-sit time (r = 0.78, p < 0.001) and the duration of anticipatory postural adjustments before gait initiation (r = 0.61, p = 0.001). Functional connectivity between the pedunculopontine nucleus (PPN) and the frontal eye field correlated with antisaccade latency in the HCs (r = −0.54, p = 0.02) but not in patients with PD.

Conclusions:

In PD, impairment of antisaccade latencies, a simple and robust parameter, may be an indirect marker correlated with impaired release of anticipatory postural program. PPN alterations may account for both antisaccade and postural impairments.

Lesions causing freezing of gait localize to a cerebellar functional network

OBJECTIVE

Freezing of gait is a disabling symptom in Parkinson disease and related disorders, but the brain regions involved in symptom generation remain unclear. Here we analyze brain lesions causing acute onset freezing of gait to identify regions causally involved in symptom generation.

METHODS

Fourteen cases of lesion-induced freezing of gait were identified from the literature, and lesions were mapped to a common brain atlas. Because lesion-induced symptoms can come from sites connected to the lesion location, not just the lesion location itself, we also identified brain regions functionally connected to each lesion location. This technique, termed lesion network mapping, has been recently shown to identify regions involved in symptom generation across a variety of lesion-induced disorders.

RESULTS

Lesion location was heterogeneous, and no single region could be considered necessary for symptom generation. However, > 90% (13 of 14) of lesions were functionally connected to a focal area in the dorsal medial cerebellum. This cerebellar area overlapped previously recognized regions that are activated by locomotor tasks, termed the cerebellar locomotor region. Connectivity to this region was specific to lesions causing freezing of gait compared to lesions causing other movement disorders (hemichorea or asterixis).

INTERPRETATION

Lesions causing freezing of gait are located within a common functional network characterized by connectivity to the cerebellar locomotor region. These results based on causal brain lesions complement prior neuroimaging studies in Parkinson disease patients, advancing our understanding of the brain regions involved in freezing of gait. ANN NEUROL 2017;81:129-141.

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.

Circuits Regulating Pleasure and Happiness: The Evolution of the Amygdalar-Hippocampal-Habenular Connectivity in Vertebrates

Appetitive-searching (reward-seeking) and distress-avoiding (misery-fleeing) behavior are essential for all free moving animals to stay alive and to have offspring. Therefore, even the oldest ocean-dwelling animal creatures, living about 560 million years ago and human ancestors, must have been capable of generating these behaviors. The current article describes the evolution of the forebrain with special reference to the development of the misery-fleeing system. Although, the earliest vertebrate ancestor already possessed a dorsal pallium, which corresponds to the human neocortex, the structure and function of the neocortex was acquired quite recently within the mammalian evolutionary line. Up to, and including, amphibians, the dorsal pallium can be considered to be an extension of the medial pallium, which later develops into the hippocampus. The ventral and lateral pallium largely go up into the corticoid part of the amygdala. The striatopallidum of these early vertebrates becomes extended amygdala, consisting of centromedial amygdala (striatum) connected with the bed nucleus of the stria terminalis (pallidum). This amygdaloid system gives output to hypothalamus and brainstem, but also a connection with the cerebral cortex exists, which in part was created after the development of the more recent cerebral neocortex. Apart from bidirectional connectivity with the hippocampal complex, this route can also be considered to be an output channel as the fornix connects the hippocampus with the medial septum, which is the most important input structure of the medial habenula. The medial habenula regulates the activity of midbrain structures adjusting the intensity of the misery-fleeing response. Within the bed nucleus of the stria terminalis the human homolog of the ancient lateral habenula-projecting globus pallidus may exist; this structure is important for the evaluation of efficacy of the reward-seeking response. The described organization offers a framework for the regulation of the stress response, including the medial habenula and the subgenual cingulate cortex, in which dysfunction may explain the major symptoms of mood and anxiety disorders.

Reward and Behavioral Factors Contributing to the Tonic Activity of Monkey Pedunculopontine Tegmental Nucleus Neurons during Saccade Tasks

The pedunculopontine tegmental nucleus (PPTg) in the brainstem plays a role in controlling reinforcement learning and executing conditioned behavior. We previously examined the activity of PPTg neurons in monkeys during a reward-conditioned, visually guided saccade task, and reported that a population of these neurons exhibited tonic responses throughout the task period. These tonic responses might depend on prediction of the upcoming reward, successful execution of the task, or both. Here, we sought to further distinguish these factors and to investigate how each contributes to the tonic neuronal activity of the PPTg. In our normal visually guided saccade task, the monkey initially fixated on the central fixation target (FT), then made saccades to the peripheral saccade target and received a juice reward after the saccade target disappeared. Most of the tonic activity terminated shortly after the reward delivery, when the monkey broke fixation. To distinguish between reward and behavioral epochs, we then changed the task sequence for a block of trials, such that the saccade target remained visible after the reward delivery. Under these visible conditions, the monkeys tended to continue fixating on the saccade target even after the reward delivery. Therefore, the prediction of the upcoming reward and the end of an individual trial were separated in time. Regardless of the task conditions, half of the tonically active PPTg neurons terminated their activity around the time of the reward delivery, consistent with the view that PPTg neurons might send reward prediction signals until the time of reward delivery, which is essential for computing reward prediction error in reinforcement learning. On the other hand, the other half of the tonically active PPTg neurons changed their activity dependent on the task condition. In the normal condition, the tonic responses terminated around the time of the reward delivery, while in the visible condition, the activity continued until the disappearance of the saccade target (ST) after reward delivery. Thus, for these neurons, the tonic activity might be related to maintaining attention to complete fixation behavior. These results suggest that, in addition to the reward value information, some PPTg neurons might contribute to the execution of conditioned task behavior.