Pedunculopontine tegmental nucleus neurons provide reward, sensorimotor, and alerting signals to midbrain dopamine neurons

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.

Pedunculopontine tegmental nucleus lesions impair probabilistic reversal learning by reducing sensitivity to positive reward feedback

Recent findings indicate that pedunculopontine tegmental nucleus (PPTg) neurons encode reward-related information that is context-dependent. This information is critical for behavioral flexibility when reward outcomes change signaling a shift in response patterns should occur. The present experiment investigated whether NMDA lesions of the PPTg affects the acquisition and/or reversal learning of a spatial discrimination using probabilistic reinforcement. Male Long-Evans rats received a bilateral infusion of NMDA (30nmoles/side) or saline into the PPTg. Subsequently, rats were tested in a spatial discrimination test using a probabilistic learning procedure. One spatial location was rewarded with an 80% probability and the other spatial location rewarded with a 20% probability. After reaching acquisition criterion of 10 consecutive correct trials, the spatial location - reward contingencies were reversed in the following test session. Bilateral and unilateral PPTg-lesioned rats acquired the spatial discrimination test comparable to that as sham controls. In contrast, bilateral PPTg lesions, but not unilateral PPTg lesions, impaired reversal learning. The reversal learning deficit occurred because of increased regressions to the previously 'correct' spatial location after initially selecting the new, 'correct' choice. PPTg lesions also reduced the frequency of win-stay behavior early in the reversal learning session, but did not modify the frequency of lose-shift behavior during reversal learning. The present results suggest that the PPTg contributes to behavioral flexibility under conditions in which outcomes are uncertain, e.g. probabilistic reinforcement, by facilitating sensitivity to positive reward outcomes that allows the reliable execution of a new choice pattern.

Dopamine Prediction Errors in Reward Learning and Addiction: From Theory to Neural Circuitry

Midbrain dopamine (DA) neurons are proposed to signal reward prediction error (RPE), a fundamental parameter in associative learning models. This RPE hypothesis provides a compelling theoretical framework for understanding DA function in reward learning and addiction. New studies support a causal role for DA-mediated RPE activity in promoting learning about natural reward; however, this question has not been explicitly tested in the context of drug addiction. In this review, we integrate theoretical models with experimental findings on the activity of DA systems, and on the causal role of specific neuronal projections and cell types, to provide a circuit-based framework for probing DA-RPE function in addiction. By examining error-encoding DA neurons in the neural network in which they are embedded, hypotheses regarding circuit-level adaptations that possibly contribute to pathological error signaling and addiction can be formulated and tested.

Reward functions of the basal ganglia

Besides their fundamental movement function evidenced by Parkinsonian deficits, the basal ganglia are involved in processing closely linked non-motor, cognitive and reward information. This review describes the reward functions of three brain structures that are major components of the basal ganglia or are closely associated with the basal ganglia, namely midbrain dopamine neurons, pedunculopontine nucleus, and striatum (caudate nucleus, putamen, nucleus accumbens). Rewards are involved in learning (positive reinforcement), approach behavior, economic choices and positive emotions. The response of dopamine neurons to rewards consists of an early detection component and a subsequent reward component that reflects a prediction error in economic utility, but is unrelated to movement. Dopamine activations to non-rewarded or aversive stimuli reflect physical impact, but not punishment. Neurons in pedunculopontine nucleus project their axons to dopamine neurons and process sensory stimuli, movements and rewards and reward-predicting stimuli without coding outright reward prediction errors. Neurons in striatum, besides their pronounced movement relationships, process rewards irrespective of sensory and motor aspects, integrate reward information into movement activity, code the reward value of individual actions, change their reward-related activity during learning, and code own reward in social situations depending on whose action produces the reward. These data demonstrate a variety of well-characterized reward processes in specific basal ganglia nuclei consistent with an important function in non-motor aspects of motivated behavior.

Cholinergic excitation from the pedunculopontine tegmental nucleus to the dentate nucleus in the rat

In spite of the existence of pedunculopontine tegmental nucleus (PPTg) projections to cerebellar nuclei, their nature and functional role is unknown. These fibers may play a crucial role in postural control and may be involved in the beneficial effects induced by deep-brain stimulation (DBS) of brainstem structures in motor disorders. We investigated the effects of PPTg microstimulation on single-unit activity of dentate, fastigial and interpositus nuclei. The effects of PPTg stimulation were also studied in rats whose PPTg neurons were destroyed by ibotenic acid and subsequently subjected to iontophoretically applied cholinergic antagonists. The main response recorded in cerebellar nuclei was a short-latency (1.5-2 ms) and brief (13-15 ms) orthodromic activation. The dentate nucleus was the most responsive to PPTg stimulation. The destruction of PPTg cells reduced the occurrence of PPTg-evoked activation of dentate neurons, suggesting that the effect was due to stimulation of cell bodies and not due to fibers passing through or close to the PPTg. Application of cholinergic antagonists reduced or eliminated the PPTg-evoked response recorded in the dentate nucleus. The results show that excitation is exerted by the PPTg on the cerebellar nuclei, in particular on the dentate nucleus. Taken together with the reduction of nicotinamide adenine dinucleotide phosphate-diaphorase-positive neurons in lesioned animals, the iontophoretic experiments suggest that the activation of dentate neurons is due to cholinergic fibers. These data help to explain the effects of DBS of the PPTg on axial motor disabilities in neurodegenerative disorders.

Decoding brain state transitions in the pedunculopontine nucleus: cooperative phasic and tonic mechanisms

Cholinergic neurons of the pedunculopontine nucleus (PPN) are most active during the waking state. Their activation is deemed to cause a switch in the global brain activity from sleep to wakefulness, while their sustained discharge may contribute to upholding the waking state and enhancing arousal. Similarly, non-cholinergic PPN neurons are responsive to brain state transitions and their activation may influence some of the same targets of cholinergic neurons, suggesting that they operate in coordination. Yet, it is not clear how the discharge of distinct classes of PPN neurons organize during brain states. Here, we monitored the in vivo network activity of PPN neurons in the anesthetized rat across two distinct levels of cortical dynamics and their transitions. We identified a highly structured configuration in PPN network activity during slow-wave activity that was replaced by decorrelated activity during the activated state (AS). During the transition, neurons were predominantly excited (phasically or tonically), but some were inhibited. Identified cholinergic neurons displayed phasic and short latency responses to sensory stimulation, whereas the majority of non-cholinergic showed tonic responses and remained at high discharge rates beyond the state transition. In vitro recordings demonstrate that cholinergic neurons exhibit fast adaptation that prevents them from discharging at high rates over prolonged time periods. Our data shows that PPN neurons have distinct but complementary roles during brain state transitions, where cholinergic neurons provide a fast and transient response to sensory events that drive state transitions, whereas non-cholinergic neurons maintain an elevated firing rate during global activation.

Mesocorticolimbic dopamine functioning in primary psychopathy: A source of within-group heterogeneity

Despite similar emotional deficiencies, primary psychopathic individuals can be situated on a continuum that spans from controlled to disinhibited. The constructs on which primary psychopaths are found to diverge, such as self-control, cognitive flexibility, and executive functioning, are crucially regulated by dopamine (DA). As such, the goal of this review is to examine which specific alterations in the meso-cortico-limbic DA system and corresponding genes (e.g., TH, DAT, COMT, DRD2, DRD4) might bias development towards a more controlled or disinhibited expression of primary psychopathy. Based on empirical data, it is argued that primary psychopathy is generally related to a higher tonic and population activity of striatal DA neurons and lower levels of D2-type DA receptors in meso-cortico-limbic projections, which may boost motivational drive towards incentive-laden goals, dampen punishment sensitivity, and increase future reward-expectancy. However, increasingly higher levels of DA activity in the striatum (moderate versus pathological elevations), lower levels of DA functionality in the prefrontal cortex, and higher D1-to-D2-type receptor ratios in meso-cortico-limbic projections may lead to increasingly disinhibited and impetuous phenotypes of primary psychopathy. Finally, in order to provide a more coherent view on etiological mechanisms, we discuss interactions between DA and serotonin that are relevant for primary psychopathy.

Neuronal Reward and Decision Signals: From Theories to Data

Rewards are crucial objects that induce learning, approach behavior, choices, and emotions. Whereas emotions are difficult to investigate in animals, the learning function is mediated by neuronal reward prediction error signals which implement basic constructs of reinforcement learning theory. These signals are found in dopamine neurons, which emit a global reward signal to striatum and frontal cortex, and in specific neurons in striatum, amygdala, and frontal cortex projecting to select neuronal populations. The approach and choice functions involve subjective value, which is objectively assessed by behavioral choices eliciting internal, subjective reward preferences. Utility is the formal mathematical characterization of subjective value and a prime decision variable in economic choice theory. It is coded as utility prediction error by phasic dopamine responses. Utility can incorporate various influences, including risk, delay, effort, and social interaction. Appropriate for formal decision mechanisms, rewards are coded as object value, action value, difference value, and chosen value by specific neurons. Although all reward, reinforcement, and decision variables are theoretical constructs, their neuronal signals constitute measurable physical implementations and as such confirm the validity of these concepts. The neuronal reward signals provide guidance for behavior while constraining the free will to act.

Parallel basal ganglia circuits for voluntary and automatic behaviour to reach rewards

The basal ganglia control body movements, value processing and decision-making. Many studies have shown that the inputs and outputs of each basal ganglia structure are topographically organized, which suggests that the basal ganglia consist of separate circuits that serve distinct functions. A notable example is the circuits that originate from the rostral (head) and caudal (tail) regions of the caudate nucleus, both of which target the superior colliculus. These two caudate regions encode the reward values of visual objects differently: flexible (short-term) values by the caudate head and stable (long-term) values by the caudate tail. These value signals in the caudate guide the orienting of gaze differently: voluntary saccades by the caudate head circuit and automatic saccades by the caudate tail circuit. Moreover, separate groups of dopamine neurons innervate the caudate head and tail and may selectively guide the flexible and stable learning/memory in the caudate regions. Studies focusing on manual handling of objects also suggest that rostrocaudally separated circuits in the basal ganglia control the action differently. These results suggest that the basal ganglia contain parallel circuits for two steps of goal-directed behaviour: finding valuable objects and manipulating the valuable objects. These parallel circuits may underlie voluntary behaviour and automatic skills, enabling animals (including humans) to adapt to both volatile and stable environments. This understanding of the functions and mechanisms of the basal ganglia parallel circuits may inform the differential diagnosis and treatment of basal ganglia disorders.

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.

Reward processing by the lateral habenula in normal and depressive behaviors

The brain reward circuit has a central role in reinforcing behaviors that are rewarding and preventing behaviors that lead to punishment. Recent work has shown that the lateral habenula is an important part of the reward circuit by providing 'negative value' signals to the dopaminergic and serotonergic systems. Studies have also suggested that dysfunction of the lateral habenula is associated with psychiatric disorders, including major depression. Here, we discuss insights gained from neuronal recordings in monkeys regarding how the lateral habenula processes reward-related information. We then highlight recent optogenetic experiments in rodents addressing normal and abnormal functions of the habenula. Finally, we discuss how deregulation of the lateral habenula may be involved in depressive behaviors.

The ventral tegmentum and dopamine: A new wave of diversity

Projection systems arising from the ventral tegmental area (VTA) and the substantia nigra (SN) have a critical role in a broad range of functions, as well as in the etiology, symptoms and treatment of neurological and psychiatric diseases. Mostly studied for its dopamine neurons, the ventral tegmentum is in fact heterogeneous at cellular and functional levels. This special issue of Neuroscience gathered some experts in the field to review the connectivity of the ventral mesencephalic dopaminergic complex, its cellular heterogeneity with attention given to glutamate neurons, the D2 autoreceptor and the cholinergic controls of dopamine activity, the influence of neurotrophins, the controls of bursting activity and the heterogeneity of neuronal activity across traits and states, the pedunculopontine tegmental and the sensory controls of dopamine activity, the sex-dependent diversity, the links between circadian and dopamine systems, the functional antero-posterior heterogeneity of the VTA and the role of its GABA tail (tVTA/rostromedial tegmental nucleus (RMTg)), the functional heterogeneity of the VTA outputs, the place of dopamine in cortico-basal ganglia circuitry, the different roles of the D1 and D2 striatal pathways and the role of dopamine in associative learning and memory. Recent progress also highlights the need for molecular markers of functional subpopulations within the ventral tegmentum, for deeper developmental knowledge of this region, and for a single cell level of connectomic. It also raises the question of inter-individual, sex, strain and species heterogeneity, and conversely the question of data generalization in a context of human pathology models, which warrant comparative studies and translational effort.

Enhanced consumption of salient solutions following pedunculopontine tegmental lesions

Rats with lesions of the pedunculopontine tegmental nucleus (PPTg) reliably overconsume high concentration sucrose solution. This effect is thought to be indicative of response-perseveration or loss of behavioral control in conditions of high excitement. While these theories have anatomical and behavioral support, they have never been explicitly tested. Here, we used a contact lickometer to examine the microstructure of drinking behavior to gain insight into the behavioral changes during overconsumption. Rats received either excitotoxic (ibotenic acid) damage to all PPTg neuronal subpopulations or selective depletion of the cholinergic neuronal sub-population (diphtheria toxin-urotensin II (Dtx-UII) lesions). We offered rats a variety of pleasant, neutral and aversive tastants to assess the generalizability and specificity of the overconsumption effect. Ibotenic-lesioned rats consumed significantly more 20% sucrose than sham controls, and did so through licking significantly more times. However, the behavioral microstructure during overconsumption was unaffected by the lesion and showed no indications of response-perseveration. Furthermore, the overconsumption effect did not generalize to highly consumed saccharin. In contrast, while only consuming small amounts of quinine solution, ibotenic-lesioned rats had significantly more licks and bursts for this tastant. Selective depletion of cholinergic PPTg neurons had no effect on consumption of any tastant. We then assessed whether it is the salience of the solution which determines overconsumption by ibotenic-lesioned rats. While maintained on free-food, ibotenic-lesioned rats had normal consumption of sucrose and hypertonic saline. After mild food deprivation ibotenic PPTg-lesioned rats overconsumed 20% sucrose. Subsequently, after dietary-induced sodium deficiency, lesioned rats consumed significantly more saline than controls. These results establish that it is the salience of the solution which is the determining factor leading to overconsumption following excitotoxic PPTg lesion. They also find no support for response-perseveration contributing to this effect. Results are discussed in terms of altered dopamine (DA) and salience signaling.

Dopamine system: manager of neural pathways

There are a growing number of roles that midbrain dopamine (DA) neurons assume, such as, reward, aversion, alerting and vigor. Here I propose a theory that may be able to explain why the suggested functions of DA came about. It has been suggested that largely parallel cortico-basal ganglia-thalamo-cortico loops exist to control different aspects of behavior. I propose that (1) the midbrain DA system is organized in a similar manner, with different groups of DA neurons corresponding to these parallel neural pathways (NPs). The DA system can be viewed as the "manager" of these parallel NPs in that it recruits and activates only the task-relevant NPs when they are needed. It is likely that the functions of those NPs that have been consistently activated by the corresponding DA groups are facilitated. I also propose that (2) there are two levels of DA roles: the How and What roles. The How role is encoded in tonic and phasic DA neuron firing patterns and gives a directive to its target NP: how vigorously its function needs to be carried out. The tonic DA firing is to provide the needed level of DA in the target NPs to support their expected behavioral and mental functions; it is only when a sudden unexpected boost or suppression of activity is required by the relevant target NP that DA neurons in the corresponding NP act in a phasic manner. The What role is the implementational aspect of the role of DA in the target NP, such as binding to D1 receptors to boost working memory. This What aspect of DA explains why DA seems to assume different functions depending on the region of the brain in which it is involved. In terms of the role of the lateral habenula (LHb), the LHb is expected to suppress maladaptive behaviors and mental processes by controlling the DA system. The demand-based smart management by the DA system may have given animals an edge in evolution with adaptive behaviors and a better survival rate in resource-scarce situations.

Distribution of pontomesencephalic cholinergic neurons projecting to substantia nigra differs significantly from those projecting to ventral tegmental area

Locations of pontomesencephalic cholinergic projection neurons from the laterodorsal tegmental (LDTg) and pedunculopontine tegmental (PPTg) nuclei to midbrain dopaminergic nuclei were mapped. Stereotaxic microinjections of Fluoro-Gold- or rhodamine-labeled microspheres were made either to substantia nigra (SN) or ventral tegmental area (VTA) in rat. Choline acetyltransferase was visualized immunohistochemically. Labeled cells were digitally mapped at multiple levels of the nuclei using an interactive computer/microscope system. SN-projecting neurons were distributed predominantly ipsilaterally in distinct regions of the PPTg: either at its rostral pole or caudally in an area ventromedial to the superior cerebellar peduncle. Few SN-projecting neurons were found in LDTg. VTA-projecting neurons were distributed bilaterally throughout the cholinergic group, primarily in the densest regions of the LDTg and caudal PPTg. Neurons were not strictly segregated into these patterns. Scattered cells belonging to either projection could be found throughout the cholinergic group on either side. Hierarchical log-linear analysis showed these differences in topographic distribution to be statistically significant. Subtraction of cell density images demonstrated well delineated regions of the cholinergic group where the projections were predominately either to SN or VTA. These data indicate a high degree of internal organization within the pontomesencephalic cholinergic group based on the location of efferent projections to SN or VTA. These findings support the concept that this cholinergic group is functionally organized in a manner which selectively innervates motor (SN) and limbic (VTA) dopaminergic nuclei.

Distribution of pontomesencephalic cholinergic neurons projecting to substantia nigra differs significantly from those projecting to ventral tegmental area

Locations of pontomesencephalic cholinergic projection neurons from the laterodorsal tegmental (LDTg) and pedunculopontine tegmental (PPTg) nuclei to midbrain dopaminergic nuclei were mapped. Stereotaxic microinjections of Fluoro-Gold- or rhodamine-labeled microspheres were made either to substantia nigra (SN) or ventral tegmental area (VTA) in rat. Choline acetyltransferase was visualized immunohistochemically. Labeled cells were digitally mapped at multiple levels of the nuclei using an interactive computer/microscope system. SN-projecting neurons were distributed predominantly ipsilaterally in distinct regions of the PPTg: either at its rostral pole or caudally in an area ventromedial to the superior cerebellar peduncle. Few SN-projecting neurons were found in LDTg. VTA-projecting neurons were distributed bilaterally throughout the cholinergic group, primarily in the densest regions of the LDTg and caudal PPTg. Neurons were not strictly segregated into these patterns. Scattered cells belonging to either projection could be found throughout the cholinergic group on either side. Hierarchical log-linear analysis showed these differences in topographic distribution to be statistically significant. Subtraction of cell density images demonstrated well delineated regions of the cholinergic group where the projections were predominately either to SN or VTA. These data indicate a high degree of internal organization within the pontomesencephalic cholinergic group based on the location of efferent projections to SN or VTA. These findings support the concept that this cholinergic group is functionally organized in a manner which selectively innervates motor (SN) and limbic (VTA) dopaminergic nuclei.