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Dopamine enables purposive behavior and adjusts vigor as a function of the relative value of actions. In Parkinson’s disease, dopamine neurons die and give rise to a series of motor and cognitive changes that interfere with the expression of volitional actions. Here we report a novel inhibitory input to dopamine neurons originated in the mesencephalic locomotor region that selectively blocks purposive behavior. GABAergic neurons of the pedunculopontine nucleus (PPN) synapse onto dopamine neurons of the substantia nigra and decrease dopamine release in the dorsal striatum. Activation of PPN neurons abolished exploratory locomotion and goal-directed actions while preserved other motor behaviors; furthermore, PPN caused a decrease in movement vigor and interrupted motor sequences presumably by modulating the immediate value of the learned action. Our results reveal an inhibitory mechanism in the midbrain that rapidly and reversibly adjusts the intrinsic value of ongoing actions.

Cognitive flexibility enables effective switching between mental processes to generate appropriate responses. Cholinergic neurons (CNs) within the pedunculopontine nucleus (PPN) are associated with many functions, but their contribution to cognitive flexibility remains poorly understood. Here we measure PPN cholinergic activities using calcium indicators during the attentional set-shifting task. We find that PPN CNs exhibit increasing activities correlated with rewards during each stage and error trials in reversal stages, indicating sensitivity to rule switching. Inhibition of PPN cholinergic activity selectively impairs reversal learning, which improves with PPN CN activation. Activation of PPN CNs projecting to the substantia nigra pars compacta, mediodorsal thalamus, and parafascicular nucleus in a time-locked manner with reward improves reversal learning. Therefore, PPN CNs may encode not only reward signals but also the information of changing reward contingency that contributes to guiding reversal learning through output projections to multiple nuclei that participate in flexibility.

The main excitatory inputs to the striatum arising from the cortex and the thalamus innervate both striatal spiny projection neurons and interneurons. These glutamatergic inputs to striatal GABAergic interneurons have been suggested to regulate the spike timing of striatal projection neurons via feedforward inhibition. Understanding how different excitatory inputs are integrated within the striatal circuitry and how they regulate striatal output is crucial for understanding basal ganglia function and related behaviors. Here, using VGLUT2 mice from both sexes, we report the existence of a glutamatergic projection from the mesencephalic locomotor region to the striatum that avoids the spiny neurons and selectively innervates interneurons. Specifically, optogenetic activation of glutamatergic axons from the pedunculopontine nucleus induced monosynaptic excitation in most recorded striatal cholinergic interneurons and GABAergic fast-spiking interneurons. Optogenetic stimulation in awake head-fixed mice consistently induced an increase in the firing rate of putative cholinergic interneurons and fast-spiking interneurons. In contrast, this stimulation did not induce excitatory responses in spiny neurons but rather disynaptic inhibitory responses ex vivo and a decrease in their firing rate in vivo, suggesting a feedforward mechanism mediating the inhibition of spiny projection neurons through the selective activation of striatal interneurons. Furthermore, unilateral stimulation of pedunculopontine nucleus glutamatergic axons in the striatum induced ipsilateral head rotations consistent with the inhibition of striatal output neurons. Our results demonstrate the existence of a unique interneuron-specific midbrain glutamatergic input to the striatum that exclusively recruits feedforward inhibition mechanisms. SIGNIFICANCE STATEMENT Glutamatergic inputs to the striatum have been shown to target both striatal projection neurons and interneurons and have been proposed to regulate spike timing of the projection neurons in part through feedforward inhibition. Here, we reveal the existence of a midbrain source of glutamatergic innervation to the striatum, originating in the pedunculopontine nucleus. Remarkably, this novel input selectively targets striatal interneurons, avoiding the projection neurons. Furthermore, we show that this selective innervation of interneurons can regulate the firing of the spiny projection neurons and inhibit the striatal output via feedforward inhibition. Together, our results describe a unique source of excitatory innervation to the striatum which selectively recruits feedforward inhibition of spiny neurons without any accompanying excitation.

Visual Abstract, Striatal dopamine (DA) is a major player in action selection and reinforcement. DA release is under strong local control by striatal ACh acting at axonal nicotinic ACh receptors (nAChRs) on DA axons. Striatal nAChRs have been shown to control how DA is released in response to ascending activity from DA neurons, and they also directly drive DA release following synchronized activity in a small local cholinergic network. The source of striatal ACh has been thought to arise solely from intrinsic cholinergic interneurons (ChIs), but recent findings have identified a source of cholinergic inputs to striatum from brainstem nuclei, the pedunculopontine nucleus (PPN) and laterodorsal tegmentum (LDT). Here, we used targeted optogenetic activation alongside DA detection with fast-scan cyclic voltammetry to test whether ChIs alone and/or brainstem afferents to the striatum can account for how ACh drives and modulates DA release in rat striatum. We demonstrate that targeted transient light activation of rat striatal ChIs drives striatal DA release, corroborating and extending previous observations in mouse to rat. However, the same light stimulation targeted to cholinergic brainstem afferents did not drive DA release, and nor did it modulate DA release activated subsequently by electrical stimulation, whereas targeted activation of ChIs did so. We were unable to obtain any evidence for DA modulation by PPN/LDT stimulation. By contrast, we could readily identify that striatal ChIs alone are sufficient to provide a source of ACh that powerfully regulates DA via nAChRs.

SummaryAssimilation of novel strategies into a consolidated action repertoire is a crucial function for behavioral adaptation and cognitive flexibility. Acetylcholine in the striatum plays a pivotal role in such adaptation and its release has been causally associated with the activity of cholinergic interneurons. Here we show that the midbrain, a previously unknown source of acetylcholine in the striatum, is a major contributor to cholinergic transmission in the striatal complex. Neurons of the pedunculopontine and laterodorsal tegmental nuclei synapse with striatal cholinergic interneurons and give rise to excitatory responses that, in turn, mediate inhibition of spiny projection neurons. Inhibition of acetylcholine release from midbrain terminals in the striatum impairs action shifting and mimics the effects observed following inhibition of acetylcholine release from striatal cholinergic interneurons. These results suggest the existence of two hierarchically-organized modes of cholinergic transmission in the striatum where cholinergic interneurons are modulated by cholinergic neurons of the midbrain.

Cholinergic neurons of the brainstem have traditionally been associated with a role in wakefulness as part of the reticular activating system, but their function cannot be explained solely on the basis of their modulation of the brain state. Recent findings about their connectivity and functional heterogeneity suggest a wider role in behavior, where basal ganglia is at the center of their influence. This review focuses on recent findings that suggest an intrinsic functional organization of the cholinergic brainstem that is closely correlated with its connectivity with midbrain and forebrain circuits. Furthermore, recent evidence on the temporal structure of the activation of brainstem cholinergic neurons reveals fundamental aspects about the nature of cholinergic signaling. Consideration of the cholinergic brainstem complex in the context of wider brain circuits is critical to understand its contribution to normal behavior.

Cholinergic transmission is essential for adaptive behavior and has been suggested to play a central role in the modulation of brain states by means of the modulation of thalamic neurons. Midbrain cholinergic neurons from the pedunculopontine nucleus (PPN) and the laterodorsal tegmental nucleus (LDT) provide dense innervation of the thalamus, but a detailed connectivity mapping is missing., Cholinergic transmission is essential for adaptive behavior and has been suggested to play a central role in the modulation of brain states by means of the modulation of thalamic neurons. Midbrain cholinergic neurons from the pedunculopontine nucleus (PPN) and the laterodorsal tegmental nucleus (LDT) provide dense innervation of the thalamus, but a detailed connectivity mapping is missing. Using conditional tracing of midbrain cholinergic axons in the rat, together with a detailed segmentation of thalamic structures, we show that projections arising in PPN and LDT are topographically organized along the entire extent of the thalamus. PPN cholinergic neurons preferentially innervate thalamic relay structures, whereas LDT cholinergic neurons preferentially target thalamic limbic nuclei. Moreover, both PPN and LDT provide a dense innervation of the intralaminar thalamic nuclei. Notably, we observe a differential synaptic density that functionally dissociates between PPN and LDT innervation. Our results show that midbrain cholinergic neurons innervate virtually all thalamic structures and this innervation is functionally segregated.

Aminobutyric acid (GABA)ergic neurons are widely distributed in brainstem structures involved in the regulation of the sleep-wake cycle, locomotion, and attention. These brainstem structures include the pedunculopontine nucleus (PPN), which is traditionally characterized by its population of cholinergic neurons that have local and wide-ranging connections. The functional heterogeneity of the PPN is partially explained by the topographic distribution of cholinergic neurons, but such heterogeneity might also arise from the organization of other neuronal populations within the PPN. To understand whether a topographical organization is also maintained by GABAergic neurons, we labeled these neurons by in situ hybridization for glutamic acid decarboxylase mRNA combined with immunohistochemistry for choline acetyltransferase to reveal cholinergic neurons. We analyzed their distribution within the PPN by using a method to quantify regional differences based on stereological cell counts. We show that GABAergic neurons of the rat PPN have a rostrocaudal gradient that is opposite to that of cholinergic neurons. Indeed, GABAergic neurons are predominantly concentrated in the rostral PPN; in addition, they form, along with cholinergic neurons, a small, high-density cluster in the most caudal portion of the nucleus. Thus, we provide evidence of heterogeneity in the distribution of different neuronal populations in the PPN and show that GABAergic and cholinergic neurons define neurochemically distinct areas. Our data suggest that the PPN is neurochemically segregated, and such differences define functional territories. J. Comp. Neurol. 515:397– 408, 2009.

Neurons in the pedunculopontine nucleus (PPN) are highly heterogeneous in their discharge properties, their neurochemical markers, their pattern of connectivity and the behavioural processes in which they participate. Three main transmitter phenotypes have been described, cholinergic, GABAergic and glutamatergic, and yet electrophysiological evidence suggests heterogeneity within these subtypes. To gain further insight into the molecular composition of these three populations in the rat, we investigated the pattern of expression of calcium binding proteins (CBPs) across distinct regions of the PPN and in relation to the presence of other neurochemical markers. Calbindin- and calretinin-positive neurons are as abundant as cholinergic neurons, and their expression follows a rostro-caudal gradient, whereas parvalbumin is expressed by a low number of neurons. We observed a high degree of expression of CBPs by GABAergic and glutamatergic neurons, with a large majority of calbindin- and calretinin-positive neurons expressing GAD or VGluT2 mRNA. Notably, CBP-positive neurons expressing GAD mRNA were more concentrated in the rostral PPN, whereas the caudal PPN was characterized by a higher density of CBP-positive neurons expressing VGluT2 mRNA. In contrast to these two large populations, in cholinergic neurons expression of calretinin is observed only in low numbers and expression of calbindin is virtually non-existent. These findings thus identify novel subtypes of cholinergic, GABAergic and glutamatergic neurons based on their expression of CBPs, and further contribute to the notion of the PPN as a highly heterogeneous structure, an attribute that is likely to underlie its functional complexity.

The pathophysiology of Parkinson's disease is reviewed in light of recent advances in the understanding of the functional organization of the basal ganglia (BG). Current emphasis is placed on the parallel interactions between corticostriatal and corticosubthalamic afferents on the one hand, and internal feedback circuits modulating BG output through the globus pallidus pars interna and substantia nigra pars reticulata on the other. In the normal BG network, the globus pallidus pars externa emerges as a main regulatory station of output activity. In the parkinsonian state, dopamine depletion shifts the BG toward inhibiting cortically generated movements by increasing the gain in the globus pallidus pars externa-subthalamic nucleus-globus pallidus pars interna network and reducing activity in "direct" cortico-putaminal-globus pallidus pars interna projections. Standard pharmacological treatments do not mimic the normal physiology of the dopaminergic system and, therefore, fail to restore a functional balance between corticostriatal afferents in the so-called direct and indirect pathways, leading to the development of motor complications. This review emphasizes the concept that the BG can no longer be understood as a "go-through" station in the control of movement, behavior, and emotions. The growing understanding of the complexity of the normal BG and the changes induced by DA depletion should guide the development of more efficacious therapies for Parkinson's disease.

Cholinergic neurons in the rostral brainstem, including the pedunculopontine nucleus (PPN), are critical for switching behavioural state from sleep to wakefulness, and their presumed inactivity during sleep is thought to promote slow cortical rhythms that are characteristic of this state. However, it is possible that the diminished activity of cholinergic brainstem neurons during slow-wave sleep continues to have a functional impact upon ongoing cortical activity. Here we show that identified cholinergic projection neurons in the PPN fire rhythmically during cortical slow oscillations, and predominantly discharge in time with the phase of the slow oscillations supporting nested gamma oscillations (30-60 Hz). In contrast, PPN non-cholinergic neurons that are linked to cortical activity fire in the opposite phase and independent of nested gamma oscillations. Furthermore, cholinergic PPN neurons emit extensive local axon collaterals (as well as long-range projections), and increasing cholinergic tone within the PPN enhances the nested gamma oscillations without producing sustained cortical activation. Thus, in addition to driving global state transitions in the cortex, cholinergic PPN neurons also play an active role in organizing cortical activity during slow-wave sleep. Our results suggest that the role of the PPN in sleep homeostasis is more diverse than previously conceived. The functions supported by nested gamma oscillations during sleep (i.e. consolidation, plasticity) are critically dependent on the gating of the underlying cortical ensembles, and our data show that cholinergic PPN neurons have an hitherto unappreciated influence on this gating process.

Dopamine neurons in the ventral tegmental area (VTA) receive cholinergic innervation from brainstem structures that are associated with either movement or reward. Whereas cholinergic neurons of the pedunculopontine nucleus (PPN) carry an associative/motor signal, those of the laterodorsal tegmental nucleus (LDT) convey limbic information. We used optogenetics and in vivo juxtacellular recording and labeling to examine the influence of brainstem cholinergic innervation of distinct neuronal subpopulations in the VTA. We found that LDT cholinergic axons selectively enhanced the bursting activity of mesolimbic dopamine neurons that were excited by aversive stimulation. In contrast, PPN cholinergic axons activated and changed the discharge properties of VTA neurons that were integrated in distinct functional circuits and were inhibited by aversive stimulation. Although both structures conveyed a reinforcing signal, they had opposite roles in locomotion. Our results demonstrate that two modes of cholinergic transmission operate in the VTA and segregate the neurons involved in different reward circuits.

Acetylcholine in the striatal complex plays an important role in normal behavior and is affected in a number of neurological disorders. Although early studies suggested that acetylcholine in the striatum (STR) is derived almost exclusively from cholinergic interneurons (CIN), recent axonal mapping studies using conditional anterograde tracing have revealed the existence of a prominent direct cholinergic pathway from the pedunculopontine and laterodorsal tegmental nuclei to the dorsal striatum and nucleus accumbens. The identification of the importance of this pathway is essential for creating a complete model of cholinergic modulation in the striatum, and it opens the question as to whether other populations of cholinergic neurons may also contribute to such modulation. Here, using novel viral tracing technologies based on phenotype-specific fluorescent reporter expression in combination with retrograde tracing, we aimed to define other sources of cholinergic innervation of the striatum. Systematic mapping of the projections of all cholinergic structures in the brain (Ch1 to Ch8) by means of conditional tracing of cholinergic axons, revealed that the only extrinsic source of cholinergic innervation arises in the brainstem pedunculopontine and laterodorsal tegmental nuclei. Our results thus place the pedunculopontine and laterodorsal nuclei in a key and exclusive position to provide extrinsic cholinergic modulation of the activity of the striatal systems.

The basal ganglia are a group of subcortical nuclei classically thought to be involved in the control of movement, and they have reciprocal connections with the cortex, thalamus and structures in the brainstem. Recent findings suggest that the basal ganglia interact with structures involved in the control of the sleep-waking cycle. The pedunculopontine tegmental nucleus (PPN) maintains a close relationship with the basal ganglia and is intimately involved in the regulation of wakefulness and REM sleep. This study evaluated changes in activity of PPN neurons following striatal kainic acid-induced lesions. Rats were injected in the anterodorsal striatum with either kainic acid or vehicle and allowed to recover for 7 or 30 days. The results showed an increase in the number of c-Fos+ cells in the PPN 30 days but not 7 days after the striatal lesion, when motor hyperactivity was no longer detected. In addition, we found a significant correlation between the ventricular brain ratio, as an indicator of lesion size, and the number of c-Fos+ cells in the PPN. Furthermore, the spatial distribution of cell types suggested that most c-Fos+ cells in the PPN were not cholinergic. These results provide new insights into the functional relationship between the basal ganglia and the PPN and suggest that the striatum, through its indirect influence on the PPN, may contribute to the regulation of wakefulness and cortical activation.

The basal ganglia are more highly interconnected with the pedunculopontine tegmental nucleus (PPN) than with any other brain region. Regulation and relay of basal ganglia activity are two key functions of the PPN. The PPN provides an interface for the basal ganglia to influence sleep and waking, and the two structures are similarly implicated in learning, reward and other cognitive functions. Perturbations of basal ganglia activity have consequences for the PPN and vice versa, exemplified by their interdependencies in motor function and Parkinson's disease. Thus, close anatomical and physiological links between the PPN and basal ganglia make it increasingly difficult to consider the two as separate functional entities.

Immunocytochemical labeling of the transcription factor c-Fos was used to determine whether wheel running (WR) activates nucleus accumbens (NAcc) neurons in naive male Balb/c mice. The results indicate compared to novel environment exploration and forced locomotor activity, WR increases c-Fos labeling in the middle level of the core but not the shell of the NAcc. These data show that WR activates middle-core NAcc neurons more effectively than other locomotor activities. The middle core of the NAcc may be a critical region for the self-reinforcing properties of WR.

The major afferent innervation of the basal ganglia is derived from the cortex and the thalamus. These excitatory inputs mainly target the striatum where they innervate the principal type of striatal neuron, the medium-sized spiny neurons (MSNs), and are critical in the expression of basal ganglia function. The aim of this work was to test directly whether corticostriatal and thalamostriatal terminals make convergent synaptic contact with individual direct and indirect pathway MSNs. Individual MSNs were recorded in vivo and labelled by the juxtacellular method in the striatum of BAC transgenic mice in which green fluorescent protein reports the expression of dopamine D1 or D2 receptors. After recovery of the neurons, the tissue was immunolabelled for vesicular glutamate transporters type 1 and 2, as markers of cortical and thalamic terminals, respectively. Three of each class of MSNs were reconstructed in 3D and second-order dendrites selected for electron microscopic analysis. Our findings show that direct and indirect pathway MSNs, located in the matrix compartment of the striatum, receive convergent input from cortex and thalamus preferentially on their spines. There were no differences in the pattern of innervation of direct and indirect pathway MSNs, but the cortical input is more prominent in both and synaptic density is greater for direct pathway neurons. The 3D reconstructions revealed no morphological differences between direct and indirect MSNs. Overall, our findings demonstrate that direct and indirect pathway MSNs located in the matrix receive convergent cortical and thalamic input and suggest that both cortical and thalamic inputs are involved in the activation of MSNs.

Cholinergic transmission in the striatal complex is critical for the modulation of the activity of local microcircuits and dopamine release. Release of acetylcholine has been considered to originate exclusively from a subtype of striatal interneuron that provides widespread innervation of the striatum. Cholinergic neurons of the pedunculopontine (PPN) and laterodorsal tegmental (LDT) nuclei indirectly influence the activity of the dorsal striatum and nucleus accumbens through their innervation of dopamine and thalamic neurons, which in turn converge at the same striatal levels. Here we show that cholinergic neurons in the brainstem also provide a direct innervation of the striatal complex. By the expression of fluorescent proteins in choline acetyltransferase (ChAT)::Cre+transgenic rats, we selectively labeled cholinergic neurons in the rostral PPN, caudal PPN, and LDT. We show that cholinergic neurons topographically innervate wide areas of the striatal complex: rostral PPN preferentially innervates the dorsolateral striatum, and LDT preferentially innervates the medial striatum and nucleus accumbens core in which they principally form asymmetric synapses. Retrograde labeling combined with immunohistochemistry in wild-type rats confirmed the topography and cholinergic nature of the projection. Furthermore, transynaptic gene activation and conventional double retrograde labeling suggest that LDT neurons that innervate the nucleus accumbens also send collaterals to the thalamus and the dopaminergic midbrain, thus providing both direct and indirect projections, to the striatal complex. The differential activity of cholinergic interneurons and cholinergic neurons of the brainstem during reward-related paradigms suggest that the two systems play different but complementary roles in the processing of information in the striatum.

International audience; The activity of the basal ganglia is altered in Parkinson's disease (PD) as a consequence of the degeneration of dopamine neurons in the substantia nigra pars compacta. This results in aberrant discharge patterns and expression of exaggerated oscillatory activity across the basal ganglia circuit. Altered activity has also been reported in some of the targets of the basal ganglia, including the pedunculopontine nucleus (PPN), possibly due to its close interconnectivity with most regions of the basal ganglia. However, the nature of the involvement of the PPN in the pathophysiology of PD has not been fully elucidated. Here, we recorded local field potentials in the motor cortex and the PPN in the 6-hydroxydopamine (6-OHDA)-lesioned rat model of PD under urethane anesthesia. By means of linear and nonlinear statistics, we analyzed the synchrony between the motor cortex and the PPN and the delay in the interaction between these two structures. We observed the presence of coherent activity between the cortex and the PPN in low (5-15 Hz)- and high (25-35 Hz)-frequency bands during episodes of cortical activation. In each case, the cortex led the PPN. Dopamine depletion strengthened the interaction of the low-frequency activities by increasing the coherence specifically in the theta and alpha ranges and reduced the delay of the interaction in the gamma band. Our data show that cortical inputs play a determinant role in leading the coherent activity with the PPN and support the involvement of the PPN in the pathophysiology of PD.

The pedunculopontine nucleus (PPN) is composed of neurons with different connectivity patterns that express different neurochemical markers, display distinct firing characteristics and are topographically organized in functional domains across its rostro-caudal axis. Previous reports have shown that the caudal region of the PPN is interconnected with motor regions of both the basal ganglia and brainstem/medulla. The co-distribution of ascending and descending motor outputs raises the question as to whether the PPN provides a coordinated or differential modulation of its targets in the basal ganglia and the medulla. To address this, we retrogradely labeled neurons in the two main PPN pathways involved in motor control and determined whether they project to one or both structures, their neurochemical phenotype, and their activity in normal and dopamine depleted rats, as indicated by Egr-1 expression. We show that ascending and descending motor pathways from the PPN arise largely from separate neurons that intermingle in the same region of the PPN, but have a distinct neurochemical composition and are differentially regulated in the Parkinsonian state. Thus, neurons projecting to the subthalamic nucleus consist of cholinergic, calbindin- and calretinin-expressing neurons, and Egr-1 is upregulated following a 6-hydroxydopamine lesion. In contrast, a larger proportion of neurons projecting to the gigantocellular nucleus are cholinergic, none express calbindin and the expression of Egr-1 is not changed by the dopamine lesion. Our results suggest that ascending and descending motor connections of the PPN are largely mediated by different sets of neurons and there are cell type-specific changes in Parkinsonian rats.

Neurons in the pedunculopontine nucleus (PPN) are highly heterogeneous in their discharge properties, their neurochemical markers, their pattern of connectivity and the behavioural processes in which they participate. Three main transmitter phenotypes have been described, cholinergic, GABAergic and glutamatergic, and yet electrophysiological evidence suggests heterogeneity within these subtypes. To gain further insight into the molecular composition of these three populations in the rat, we investigated the pattern of expression of calcium binding proteins (CBPs) across distinct regions of the PPN and in relation to the presence of other neurochemical markers. Calbindin- and calretinin-positive neurons are as abundant as cholinergic neurons, and their expression follows a rostro-caudal gradient, whereas parvalbumin is expressed by a low number of neurons. We observed a high degree of expression of CBPs by GABAergic and glutamatergic neurons, with a large majority of calbindin- and calretinin-positive neurons expressing GAD or VGluT2 mRNA. Notably, CBP-positive neurons expressing GAD mRNA were more concentrated in the rostral PPN, whereas the caudal PPN was characterized by a higher density of CBP-positive neurons expressing VGluT2 mRNA. In contrast to these two large populations, in cholinergic neurons expression of calretinin is observed only in low numbers and expression of calbindin is virtually non-existent. These findings thus identify novel subtypes of cholinergic, GABAergic and glutamatergic neurons based on their expression of CBPs, and further contribute to the notion of the PPN as a highly heterogeneous structure, an attribute that is likely to underlie its functional complexity.

Brain states are dynamically shaped by distinct neuronal systems across the brain as a result of an interplay between their intrinsic activity and the environmental demand. Subsets of brainstem and forebrain structures influence the manifestation of specific brain states (e.g., sleep or wakefulness) and contribute to their cyclic alternation. Recent evidence, however, shows that such functional partition is not observed in the brainstem, where neuronal subpopulations engage in particular patterns of activity that contribute to the emergence of phasic components during the cortical slow oscillations. Cholinergic neurons of the pedunculopontine nucleus are functionally associated with the induction of the waking state but discharge during the phase of the slow oscillations that support neuronal activity. Here, we discuss the impact of the phasic signals arising from subcortical structures on the modulation of cortical slow oscillations and their functional significance.

Neurons in the pedunculopontine nucleus (PPN) exhibit a wide heterogeneity in terms of their neurochemical nature, their discharge properties, and their connectivity. Such characteristics are reflected in their functional properties and the behaviors in which they are involved, ranging from motor to cognitive functions, and the regulation of brain states. A clue to understand this functional versatility arises from the internal organization of the PPN. Thus, two main areas of the PPN have been described, the rostral and the caudal, which display remarkable differences in terms of the distribution of neurons with similar phenotype and the projections that originate from them. Here we review these differences with the premise that in order to understand the function of the PPN it is necessary to understand its intricate connectivity. We support the case that the PPN should not be considered as a homogeneous structure and conclude that the differences between rostral and caudal PPN, along with their intrinsic connectivity, may underlie the basis of its complexity.

Brain states are dynamically shaped by distinct neuronal systems across the brain as a result of an interplay between their intrinsic activity and the environmental demand. Subsets of brainstem and forebrain structures influence the manifestation of specific brain states (e.g., sleep or wakefulness) and contribute to their cyclic alternation. Recent evidence, however, shows that such functional partition is not observed in the brainstem, where neuronal subpopulations engage in particular patterns of activity that contribute to the emergence of phasic components during the cortical slow oscillations. Cholinergic neurons of the pedunculopontine nucleus are functionally associated with the induction of the waking state but discharge during the phase of the slow oscillations that support neuronal activity. Here, we discuss the impact of the phasic signals arising from subcortical structures on the modulation of cortical slow oscillations and their functional significance.

The pedunculopontine nucleus (PPN) is critically involved in brain-state transitions that promote neocortical activation. In addition, the PPN is involved in the control of several behavioral processes including locomotion, motivation and reward, but the neuronal substrates that underlie such an array of functions remain elusive. Here we analyzed the physiological properties of non-cholinergic PPN neurons in vivo across distinct brain states, and correlated these with their morphological properties after juxtacellular labeling. We show that non-cholinergic neurons in the PPN whose firing is not strongly correlated to neocortical activity are highly heterogeneous and are composed of at least three different subtypes: (1) “quiescent” neurons, which are nearly silent during slow-wave activity (SWA) but respond robustly to neocortical activation; (2) “tonic firing” neurons, which have a stationary firing rate that is independent of neocortical activity across different brain states; and (3) “irregular firing” neurons, which exhibit a variable level of correlation with neocortical activity. The majority of non-cholinergic neurons have an ascending axonal trajectory, with the exception of some irregular firing neurons that have descending axons. Furthermore, we observed asymmetric synaptic contacts within the PPN arising from the axon collaterals of labeled neurons, suggesting that excitatory, non-cholinergic neurons can shape the activity of neighboring cells. Our results provide the first evidence of distinct firing properties associated with non-cholinergic neuronal subtypes in the PPN, suggesting a functional heterogeneity, and support the notion of a local network assembled by projection neurons, the properties of which are likely to determine the output of the PPN in diverse behavioral contexts.

The notion of the pedunculopontine nucleus (PPN) is being updated in light of increasing evidence that identifies it as a key structure in the processing of exteroceptive information and its transmission to the forebrain, influencing the activity of PPN’s multiple targets (from basal ganglia to the cerebral cortex). Here we review the anatomical evidence supporting the existence of a local network that forms the basis for such processing functions and propose a more complex relationship with its efferent systems. We also identify some of the critical issues that remain to be answered in order to have a comprehensive understanding of the roles of the PPN. A better insight into this connectivity will also help to understand the contribution of the PPN to the diverse systems it influences.

Dopamine neurons in the midbrain respond to behavioral events and environmental stimuli. Their different patterns of activation in turn modulate the activity of forebrain regions and modulate the expression of selective behavioral responses. However, their activity is closely dependent on the cholinergic systems in the brainstem. Ascending cholinergic projections from the pedunculopontine and laterodorsal tegmental nuclei target dopaminergic neurons in the substantia nigra compacta and ventral tegmental area following a topographical gradient. These projections, by means of the activation of acetylcholine receptors, influence the firing of dopamine neurons and therefore their responsiveness, ultimately affecting the release of dopamine in their forebrain targets. Brainstem cholinergic neurons are thus in a position to critically influence the activity of dopaminergic neurons in the midbrain, and thereby have a critical role in the expression of behavior.

The dense interconnections of the PPN and basal ganglia enable the two structures to maintain a close functional relationship that has significant bearing on a wide range of behaviours, including sleep/arousal, locomotion, and posture. Our synthesis of the literature suggests that it is no longer sufficient to view PPN neurons as playing roles in only one of two distinct effector systems, namely the RAS and MLR, but that individual PPN neurons are ideally suited to play central roles in both systems. The impact of this hypothesis on our understanding of basal ganglia function is difficult to predict at this time. What is certain, however, is that an appreciation of the functional integration of the PPN and basal ganglia is critical for a better understanding of the roles played by each circuit in behaviour. The cellular heterogeneity present in the PPN indicates that the development of a realistic scheme for functional integration is a major challenge for the future. In meeting this challenge, it is imperative that correlative analyses of the physiology, neurochemistry and morphology of the non-cholinergic neurons, as well as the cholinergic neurons, of the PPN are undertaken. Furthermore, it is important that we consider the different functional aspects of the PPN at the level of single identified neurons. Using this and other approaches, it will be possible to dissect the influences of the basal ganglia and PPN as they work together as a single ‘system’, rather than two separate entities.

The basal ganglia are a group of subcortical nuclei classically thought to be involved in the control of movement, and they have reciprocal connections with the cortex, thalamus and structures in the brainstem. Recent findings suggest that the basal ganglia interact with structures involved in the control of the sleep-waking cycle. The pedunculopontine tegmental nucleus (PPN) maintains a close relationship with the basal ganglia and is intimately involved in the regulation of wakefulness and REM sleep. This study evaluated changes in activity of PPN neurons following striatal kainic acid-induced lesions. Rats were injected in the anterodorsal striatum with either kainic acid or vehicle and allowed to recover for 7 or 30 days. The results showed an increase in the number of c-Fos+ cells in the PPN 30 days but not 7 days after the striatal lesion, when motor hyperactivity was no longer detected. In addition, we found a significant correlation between the ventricular brain ratio, as an indicator of lesion size, and the number of c-Fos+ cells in the PPN. Furthermore, the spatial distribution of cell types suggested that most c-Fos+ cells in the PPN were not cholinergic. These results provide new insights into the functional relationship between the basal ganglia and the PPN and suggest that the striatum, through its indirect influence on the PPN, may contribute to the regulation of wakefulness and cortical activation.

Huntington's disease (HD) patients show severe diurnal choreic movements, while during slow-wave sleep (SWS) abnormal movements subside. Sleep disturbances in HD, including irregular delta activity and decreases in SWS, have also been reported. Striatal excitotoxic lesions have been shown to induce increased nocturnal spontaneous locomotor activity in rodents. In order to characterize the changes in circadian activity and sleep patterns and their correlation with motor activity after striatal excitotoxic lesions, Sprague–Dawley rats were implanted and lesioned; their locomotor and EEG activities were recorded for either 4 or 24 h during baseline or 7 and 30 days post-lesion. Locomotor activity increased significantly at 7 days post-lesion during the dark phase of the light–dark cycle. In contrast, total time spent in wakefulness (W) increased at 30 days post-lesion during the light phase of the cycle. This increase was at the expense of SWS duration. No disruption of the circadian curves was observed. Increases in the number of W-bouts and decreases in the duration of SWS-bouts were also observed. These results suggest the possible participation of the striatum in the regulation of the sleep–waking cycle, independent of locomotor activity. The increase in W could be due to loss of inhibition of target structures involved in regulation of the sleep–waking cycle.