Pedunculopontine nucleus in the squirrel monkey: distribution of cholinergic and monoaminergic neurons in the mesopontine tegmentum with evidence for the presence of glutamate in cholinergic neurons

Normal and pathological gait: what we learn from Parkinson's disease

Gait and balance disorders represent a major therapeutic challenge in Parkinson's disease (PD). These symptoms respond poorly to dopaminergic treatments, except in the early phase of the disease. Currently, no other treatment is particularly efficient and rehabilitation appears to be the most effective approach. Since these gait and balance deficits are resistant to dopaminergic drugs, their occurrence could be related to the development of extradopaminergic lesions in PD patients. We provide a comprehensive description of the clinical features of gait and balance disorders in PD. We also highlight the brain networks involved in gait and balance control in animals and humans with a particular focus on the relevant structures in the context of PD, such as the mesencephalic locomotor region. We also review other neuronal systems that may be involved in the physiopathology of gait and balance disorders in PD (noradrenergic and serotoninergic systems, cerebellum and cortex). In addition, we review recent evidence regarding functional neurosurgery for gait disorders in PD and propose new directions for future therapeutic research.

Biology of Parkinson's disease: pathogenesis and pathophysiology of a multisystem neurodegenerative disorder

Parkinson's disease (PD) is the second most common movement disorder. The characteristic motor impairments - bradykinesia, rigidity, and resting tremor - result from degenerative loss of midbrain dopamine (DA) neurons in the substantia nigra, and are responsive to symptomatic treatment with dopaminergic medications and functional neurosurgery. PD is also the second most common neurodegenerative disorder. Viewed from this perspective, PD is a disorder of multiple functional systems, not simply the motor system, and of multiple neurotransmitter systems, not merely that of DA. The characteristic pathology - intraneuronal Lewy body inclusions and reduced numbers of surviving neurons - is similar in each of the targeted neuron groups, suggesting a common neurodegenerative process. Pathological and experimental studies indicate that oxidative stress, proteolytic stress, and inflammation figure prominently in the pathogenesis of PD. Yet, whether any of these mechanisms plays a causal role in human PD is unknown, because to date we have no proven neuroprotective therapies that slow or reverse disease progression in patients with PD. We are beginning to understand the pathophysiology of motor dysfunction in PD, but its etiopathogenesis as a neurodegenerative disorder remains poorly understood.

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.

Dynamic regulation of midbrain dopamine neuron activity: intrinsic, synaptic, and plasticity mechanisms

Although the roles of dopaminergic signaling in learning and behavior are well established, it is not fully understood how the activity of dopaminergic neurons is dynamically regulated under different conditions in a constantly changing environment. Dopamine neurons must integrate sensory, motor, and cognitive information online to inform the organism to pursue outcomes with the highest reward probability. In this article, we provide an overview of recent advances on the intrinsic, extrinsic (i.e., synaptic), and plasticity mechanisms controlling dopamine neuron activity, mostly focusing on mechanistic studies conducted using ex vivo brain slice preparations. We also hope to highlight some unresolved questions regarding information processing that takes place at dopamine neurons, thereby stimulating further investigations at different levels of analysis.

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.

Multiple origins of cholinergic innervation of the cochlear nucleus

Acetylcholine (Ach) affects a variety of cell types in the cochlear nucleus (CN) and is likely to play a role in numerous functions. Previous work in rats suggested that the acetylcholine arises from cells in the superior olivary complex, including cells that have axonal branches that innervate both the CN and the cochlea (i.e. olivocochlear cells) as well as cells that innervate only the CN. We combined retrograde tracing with immunohistochemistry for choline acetyltransferase to identify the source of ACh in the CN of guinea pigs. The results confirm a projection from cholinergic cells in the superior olivary complex to the CN. In addition, we identified a substantial number of cholinergic cells in the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT) that project to the CN. On average, the PPT and LDT together contained about 26% of the cholinergic cells that project to CN, whereas the superior olivary complex contained about 74%. A small number of additional cholinergic cells were located in other areas, including the parabrachial nuclei.The results highlight a substantial cholinergic projection from the pontomesencephalic tegmentum (PPT and LDT) in addition to a larger projection from the superior olivary complex. These different sources of cholinergic projections to the CN are likely to serve different functions. Projections from the superior olivary complex are likely to serve a feedback role, and may be closely tied to olivocochlear functions. Projections from the pontomesencephalic tegmentum may play a role in such things as arousal and sensory gating. Projections from each of these areas, and perhaps even the smaller sources of cholinergic inputs, may be important in conditions such as tinnitus as well as in normal acoustic processing.

Cholinergic cells in the tegmentum send branching projections to the inferior colliculus and the medial geniculate body

The pontomesencephalic tegmentum (PMT) provides cholinergic input to the inferior colliculus (IC) and the medial geniculate body (MG). PMT cells are often characterized as projecting to more than one target. The purpose of this study was to determine whether individual PMT cholinergic cells, (1) innervate the auditory pathways bilaterally via collateral projections to left and right auditory thalamus; or, (2) innervate multiple levels of the auditory pathways via collateral projections to the auditory thalamus and inferior colliculus. We used multiple retrograde tracers to identify individual PMT cells that project to more than one target. We combined the retrograde tracer studies with immunohistochemistry for choline acetyltransferase to determine whether the projecting cells were cholinergic. We found that individual PMT cells send branching axonal projections to two or more auditory targets in the midbrain and thalamus. The collateral projection pattern that we observed most frequently was to the ipsilateral IC and ipsilateral MG. Cells projecting to both MGs were somewhat less common, followed by cells projecting to the contralateral IC and ipsilateral MG. Both cholinergic and non-cholinergic cells contribute to each of these projection patterns. Less often, we found cells that project to one IC and both MGs; there was no evidence for non-cholinergic cells in this projection pattern. It is likely that collateral projections from PMT cells could have coordinated effects bilaterally and at multiple levels of the ascending auditory pathways.

Cholinergic and non-cholinergic projections from the pedunculopontine and laterodorsal tegmental nuclei to the medial geniculate body in Guinea pigs

The midbrain tegmentum is the source of cholinergic innervation of the thalamus and has been associated with arousal and control of the sleep/wake cycle. In general, the innervation arises bilaterally from the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT). While this pattern has been observed for many thalamic nuclei, a projection from the LDT to the medial geniculate body (MG) has been questioned in some species. We combined retrograde tracing with immunohistochemistry for choline acetyltransferase (ChAT) to identify cholinergic projections from the brainstem to the MG in guinea pigs. Double-labeled cells (retrograde and immunoreactive for ChAT) were found in both the PPT (74%) and the LDT (26%). In both nuclei, double-labeled cells were more numerous on the ipsilateral side. About half of the retrogradely labeled cells were immunonegative, suggesting they are non-cholinergic. The distribution of these immunonegative cells was similar to that of the immunopositive ones: more were in the PPT than the LDT and more were on the ipsilateral than the contralateral side. The results indicate that both the PPT and the LDT project to the MG, and suggest that both cholinergic and non-cholinergic cells contribute substantially to these projections.

Functional topography of respiratory, cardiovascular and pontine-wave responses to glutamate microstimulation of the pedunculopontine tegmentum of the rat

Functionally distinct areas were mapped within the pedunculopontine tegmentum (PPT) of 42 ketamine/xylazine anesthetized rats using local stimulation by glutamate microinjection (10 mM, 5-12 nl). Functional responses were classified as: (1) apnea; (2) tachypnea; (3) hypertension (HTN); (4) sinus tachycardia; (5) genioglossus electromyogram activation or (6) pontine-waves (p-waves) activation.We found that short latency apneas were predominantly elicited by stimulation in the lateral portion of the PPT, in close proximity to cholinergic neurons. Tachypneic responses were elicited from ventral regions of the PPT and HTN predominated in the ventral portion of the antero-medial PPT. We observed sinus tachycardia after stimulation of the most ventral part of the medial PPT at the boundary with nucleus reticularis pontis oralis, whereas p-waves were registered predominantly following stimulation in the dorso-caudal portion of the PPT. Genioglossus EMG activation was evoked from the medial PPT. Our results support the existence of the functionally distinct areas within the PPT affecting respiration, cardiovascular function, EEG and genioglossus EMG.

Ethanol action on dopaminergic neurons in the ventral tegmental area: interaction with intrinsic ion channels and neurotransmitter inputs

The dopaminergic system originating in the midbrain ventral tegmental area (VTA) has been extensively studied over the past decades as a critical neural substrate involved in the development of alcoholism and addiction to other drugs of abuse. Accumulating evidence indicates that ethanol modulates the functional output of this system by directly affecting the firing activity of VTA dopamine neurons, whereas withdrawal from chronic ethanol exposure leads to a reduction in the functional output of these neurons. This chapter will provide an update on the mechanistic investigations of the acute ethanol action on dopamine neuron activity and the neuroadaptations/plasticities in the VTA produced by previous ethanol experience.

Projections from auditory cortex to midbrain cholinergic neurons that project to the inferior colliculus

We have shown that auditory cortex projects to cholinergic cells in the pedunculopontine tegmental nucleus (PPT) and laterodorsal tegmental nucleus (LDT). PPT and LDT are the sources of cholinergic projections to the inferior colliculus, but it is not known if the cortical inputs contact the cholinergic cells that project to the inferior colliculus. We injected FluoroRuby into auditory cortex in pigmented guinea pigs to label cortical projections to PPT and LDT. In the same animals, we injected Fast Blue into the left or right inferior colliculus to label PPT and LDT cells that project to the inferior colliculus. We processed the brain to identify cholinergic cells with an antibody to choline acetyltransferase, which was visualized with a green fluorescent marker distinguishable from both FluoroRuby and Fast Blue. We then examined the PPT and LDT to determine whether boutons of FluoroRuby-labeled cortical axons were in close contact with cells that were double-labeled with the retrograde tracer and the immunolabel. Apparent contacts were observed ipsilateral and, less often, contralateral to the injected cortex. On both sides, the contacts were more numerous in PPT than in LDT. The results indicate that auditory cortex projects directly to brainstem cholinergic cells that innervate the ipsilateral or contralateral inferior colliculus. This suggests that cortical projections could elicit cholinergic effects on both sides of the auditory midbrain.

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.

How conditioned stimuli acquire the ability to activate VTA dopamine cells: a proposed neurobiological component of reward-related learning

The ability to learn about conditioned stimuli (CS) associated with rewards is a crucial adaptive mechanism. Activity in the mesocorticolimbic dopamine (DA) system, as well as in the ventral tegmental area (VTA), is correlated with responding to and learning about CSs. The mechanism by which VTA neurons become activated by signals associated with conditioned stimuli is not fully understood. Our model suggests that NMDA receptor stimulation in the VTA allows originally weak glutamate signals carrying information about environmental stimuli, coincident with strong excitation correlated with primary rewards, to be strengthened and thereby acquire the ability to activate VTA neurons in themselves, producing approach. Furthermore, once synaptic strengthening occurs, the model suggests that NMDA receptor stimulation in VTA is not necessary for the expression of reward-related learning. In this review we survey evidence that VTA cells respond to cues associated with primary rewards, that this responding is acquired, and that the VTA possesses the attributes to function as a site of integration of signals of primary and conditioned stimuli.

Projections from auditory cortex to cholinergic cells in the midbrain tegmentum of guinea pigs

Anterograde and retrograde tracing techniques were used to characterize projections from the auditory cortex to the pedunculopontine and laterodorsal tegmental nuclei (PPT and LDT, respectively) in the midbrain tegmentum in guinea pigs. For anterograde tracing, tetramethylrhodamine dextran (FluoroRuby) was injected at several sites within auditory cortex. After sufficient time for transport, the brain was processed for immunohistochemistry with anti-choline acetyltransferase to reveal presumptive cholinergic cells. Anterogradely labeled axons were observed ipsilaterally and, in smaller numbers, contralaterally, in both the pedunculopontine and laterodorsal tegmental nuclei. In all four nuclei, tracer-labeled boutons appeared to contact immunolabeled (i.e., cholinergic) cells. The contacts occurred on cell bodies and dendrites. The results were similar following injections that spread across multiple auditory cortical areas or injections that were within primary auditory cortex. In order to confirm the anterograde results, in a second series of experiments, retrograde tracers were deposited in the pedunculopontine tegmental nucleus. These injections labeled layer V pyramidal cells in the auditory cortex. The results suggest an excitatory projection from primary auditory cortex bilaterally to cholinergic cells in the midbrain tegmentum. Such a pathway could allow auditory cortex to activate brainstem cholinergic circuits, possibly including the cholinergic pathways associated with arousal and gating of acoustic stimuli.

Osmosensitive response of glutamate in the substantia nigra

Previous studies have suggested the increase of extracellular glutamate (GLU) in the substantia nigra (SN) as a cause of dopamine-cell degeneration (excitotoxicity) in Parkinson's disease (PD). However, the mechanisms involved in this increase remain unknown. The present work studied osmoregulation as a cause of GLU release in the SN. Microdialysis was used to change extracellular osmolarity, to administer drugs and to quantify the extracellular non-synaptic GLU (EnS-GLU). Two osmolarity modifications were performed, a moderate decrease (5%) resembling physiological modifications and a substantial decrease (>or=20% decrease) similar to that observed under pathological conditions. Hypo-osmolarity induced a dose-response (285-80 mOsm) increase of EnS-GLU which was detected after small osmolarity modifications (15 mOsm) and which was very marked (>1000%) after more intense osmolarity changes. This response disappeared after pre-treating rats with a P2 purinergic-receptor antagonist (pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid; 1 mM) suggesting ATP involvement in the osmosensitive EnS-GLU response. The EnS-GLU increase observed after administration of ATP (0.1-100 microM) and 2-methylthioadenosine triphosphate tetrasodium (P2-receptor agonist; 100 microM) and the lack of effects of adenosine administration (1 mM) suggest that the ATP action on P2 receptors is an amplificatory mechanism in the osmosensitive EnS-GLU response. The marked action of osmolarity on extracellular Glu suggests osmolarity regulation as a possible source for excitotoxicity in the SN.

Hypocretin mechanisms in nicotine addiction: evidence and speculation

BACKGROUND

The hypocretin/orexin system has been implicated in arousal mechanisms, sleep, and sleep disorders, including narcolepsy, and more recently in drug addiction. Theoretically, hypocretin (hcrt) mechanisms appear to be potential substrates for nicotine addiction: arousal and attentional mechanisms influence use and withdrawal symptoms, and hcrt systems overlap anatomically with a number of brain regions associated with nicotine addiction.

OBJECTIVE

This review summarizes the studies that have examined hcrt mechanisms in the effects of nicotine and describes hcrt innervation of, and effects in, several brain regions implicated in nicotine addiction. The review speculates on the possible mechanisms by which hcrt may contribute to nicotine addiction in these regions, with the objective of encouraging research in this area.

RESULTS

In a small literature, both experimenter-administered and self-administered nicotine have been shown to elicit or depend on hcrt signaling. However, although untested in experimental designs, there is compelling evidence that hcrt mechanisms in the ventral tegmental area, the pontine region, thalamocortical circuits, the prefrontal cortex, and the amygdala could have a broad influence on nicotine addiction.

CONCLUSIONS

Evidence reviewed leads to the conclusion that hcrt mechanisms could mediate several dimensions of nicotine addiction, including a multi-faceted regulation of mesocorticolimbic dopaminergic function, but beyond dopaminergic mechanisms, hcrt could influence nicotine use and relapse during abstinence through broadly based arousal/attentional effects. These speculative ideas need to be examined experimentally; the potential gains are a more thorough understanding of the pathophysiology of nicotine addiction, and the discovery of novel targets for the development of pharmacotherapeutics.

Pedunculopontine and laterodorsal tegmental nuclei contain distinct populations of cholinergic, glutamatergic and GABAergic neurons in the rat

The pedunculopontine tegmental nucleus (PPTg) and laterodorsal tegmental nucleus (LDTg) provide cholinergic afferents to several brain areas. This cholinergic complex has been suggested to play a role in sleep, waking, motor function, learning and reward. To have a better understanding of the neurochemical organization of the PPTg/LDTg we characterized the phenotype of PPTg/LDTg neurons by determining in these cells the expression of transcripts encoding choline acetyltransferase (ChAT), glutamic acid decarboxylase (GAD) or the vesicular glutamate transporters (vGluT1, vGluT2 and vGluT3). Within the PPTg/LDTg complex we found neurons expressing ChAT, vGluT2 or GAD transcripts, these neuronal phenotypes were intermingled, but not homogeneously distributed within the PPTg or LDTg. Previous studies suggested the presence of either glutamate or gamma-aminobutyric acid (GABA) immunolabeling in a large number of PPTg/LDTg cholinergic neurons, leading to the widespread notion that PPTg/LDTg cholinergic neurons co-release acetylcholine together with either glutamate or GABA. To assess the glutamatergic or GABAergic nature of the PPTg/LDTg cholinergic neurons, we combined in situ hybridization (to detect vGluT2 or GAD transcripts) and immunohistochemistry (to detect ChAT), and found that over 95% of all PPTg/LDTg cholinergic neurons lack transcripts encoding either vGluT2 mRNA or GAD mRNA. As the vast majority of PPTg/LDTg cholinergic neurons lack transcripts encoding essential proteins for the vesicular transport of glutamate or for the synthesis of GABA, co-release of acetylcholine with either glutamate or GABA is unlikely to be a major factor in the interactions between acetylcholine, glutamate and GABA at the postsynaptic site.

The pedunculopontine nucleus in Parkinson's disease: primate studies

Gait freezing and poor balance are two of the most disabling symptoms of advanced Parkinson's disease (PD), and also of other untreatable progressive neurological disorders, such as multi-system atrophy (MSA) and progressive supranuclear palsy (PSP). In PD, these symptoms are currently inadequately managed by drugs and also the present surgical treatment of deep brain stimulation (DBS) of the sub-thalamic nucleus (STN) and the globus pallidus internus (GPi). The pedunculopontine nucleus (PPN) has been implicated in these symptoms. The PPN is in the upper brain stem. The major inhibitory input is from the GPi and substantia nigra reticulata (SNr), and bilateral output is to the substantia nigra compacta (SNc), thalamus and spinal cord. Stimulation of the PPN in the decerebrate rat, cat and dog induced gait-like movements. In autopsy studies in PD, MSA, PSP and the DYT-1 dystonic brain, the PPN is degenerate. Autoradiography of the MPTP-Parkinsonian primate shows excessive inhibition in the PPN. Lesions of the PPN in the normal primate induced PD-type bradykinesia, which was persistent with bilateral lesions. In the MPTP-primate model, microinjections of the gamma aminobutyric acid A (GABA) antagonist bicuculine into the PPN reversed Parkinsonian akinesia implying that stimulation of this region might have a therapeutic role in drug resistant PD. Low frequency (5-10Hz) stimulation of the PPN in the same model reversed akinesia independently of L-dopa; moreover, l-dopa and stimulation effects were additive, implying the involvement of non-dopaminergic pathways.

Four excitatory postsynaptic ionotropic receptors coactivated at the motoneuron-Renshaw cell synapse

Renshaw cells (RCs) are spinal interneurons excited by collaterals of the axons of motoneurons (MNs). They respond to a single motoneuronal volley by a surprisingly long (tens of milliseconds) train of action potentials. We have analyzed this synaptic response in spinal cord slices of neonatal mice in light of recent observations suggesting that the MN axons release both acetylcholine and glutamate. We found that the RC synaptic current involves four components of similar amplitudes mediated by two nicotinic receptors (nAChRs, tentatively identified as alpha(7) homomers and alpha(4)beta(2) heteromers) and two glutamate receptors (AMPARs and NMDARs). The decay time constants of the four components cover a wide range: from 3.6 +/- 2.2 ms (alpha(7) nAChRs) to 54.6 +/- 19.5 ms (NMDARs, at -45 mV). The RC discharge can be separated into an initial doublet of high-frequency action potentials followed by later spikes with a variable latency and longer interspike intervals. The initial doublet involves the four ionotropic receptors as well as endogenous voltage-dependent conductances. The late discharge depends on NMDARs, but these receptors must be primed by the initial depolarization. The activation of the NMDARs is prolonged by the fact that their slow deactivation is further slowed by depolarization. The formation of the initial doublet is favored by hyperpolarization, whereas the late discharge is favored by depolarization. This suggests that in physiological conditions the pattern of discharge of the RC in response to a MN input may alternate between a phasic and a tonic response.

Sources of cholinergic input to the inferior colliculus

We combined retrograde tracing with immunohistochemistry for choline acetyltransferase to identify the source of cholinergic input to the inferior colliculus (IC) in guinea pigs. Injection of a retrograde tracer into one IC labeled cells in many brainstem nuclei. Retrogradely-labeled cells that were also immunoreactive for choline acetyltransferase were identified in two nuclei in the midbrain tegmentum: the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT). More PPT and LDT cells project ipsilaterally than contralaterally to the IC and, on both sides, there are more projecting cells in the PPT than in the LDT. Double-labeled cells were not found in any other brainstem nucleus. A common feature of cholinergic cells in PPT and LDT is collateral projections to multiple targets. We placed different retrograde tracers into each IC to identify cells in PPT and LDT that project to both ICs. In both PPT and LDT, a substantial proportion (up to 57%) of the immunoreactive cells that contained tracer from the contralateral IC also contained tracer from the ipsilateral IC. We conclude that acetylcholine in the IC originates from the midbrain tegmental cholinergic nuclei: PPT and LDT. These nuclei are known to participate in arousal, the sleep/wake cycle and prepulse inhibition of acoustic startle. It is likely that the cholinergic input to the IC is directly associated with these functions.

Properties of distinct ventral tegmental area synapses activated via pedunculopontine or ventral tegmental area stimulation in vitro

Anatomical studies indicate that synaptic inputs from many cortical and subcortical structures converge on neurons of the ventral tegmental area (VTA). Although in vitro electrophysiological studies have examined synaptic inputs to dopamine (DA) and non-DA neurons in the VTA, they have largely relied upon local electrical stimulation to activate these synapses. This provides little information regarding the distinct properties of synapses originating from different brain areas. Using whole-cell recordings in parasagittal rat brain slices that preserved subcortical axons from the pedunculopontine nucleus (PPN) to the VTA, we compared these synapses with those activated by intra-VTA stimulation. PPN-evoked currents demonstrated longer latencies than intra-VTA-evoked currents, and both VTA and PPN responses were mediated by GABA(A) and AMPA receptors. However, unlike VTA-evoked currents, PPN currents were exclusively mediated by glutamate in 25-40% of the VTA neurons. Consistent with a cholinergic projection from the PPN to the VTA, nicotinic acetylcholine receptors (nAChR) were activated by endogenous acetylcholine released during PPN, but not VTA, stimulation. This was seen as a reduction of PPN-evoked, and not VTA-evoked, synaptic currents by the alpha7-nAChR antagonist methyllycaconitine (MLA) and the agonist nicotine. The beta2-nAChR subunit antagonist dihydro-beta-erythroidine had no effect on VTA- or PPN-evoked synaptic currents. The effects of MLA on PPN-evoked currents were unchanged by the GABA(A) receptor blocker picrotoxin, indicating that alpha7-nAChRs presynaptically modulated glutamate and not GABA release. These differences in physiological and pharmacological properties demonstrate that ascending PPN and presumed descending inputs to VTA utilize distinct mechanisms to differentially modulate neuronal activity and encode cortical and subcortical information.

Altered neuronal activity relationships between the pedunculopontine nucleus and motor cortex in a rodent model of Parkinson's disease

The pedunculopontine nucleus (PPN) is a new deep brain stimulation (DBS) target for Parkinson's disease (PD), but little is known about PPN firing pattern alterations in PD. The anesthetized rat is a useful model for investigating the effects of dopamine loss on the transmission of oscillatory cortical activity through basal ganglia structures. After dopamine loss, synchronous oscillatory activity emerges in the subthalamic nucleus and substantia nigra pars reticulata in phase with cortical slow oscillations. To investigate the impact of dopamine cell lesion-induced changes in basal ganglia output on activity in the PPN, this study examines PPN spike timing with reference to motor cortex (MCx) local field potential (LFP) activity in urethane- or ketamine-anesthetized rats. Seven to ten days after unilateral 6-hydroxydopamine lesion of the medial forebrain bundle, spectral power in PPN spike trains and coherence between PPN spiking and PPN LFP activity increased in the approximately 1 Hz range in urethane-anesthetized rats. PPN spike timing also changed from firing predominantly in phase with MCx slow oscillations in the intact urethane-anesthetized rat to firing predominantly antiphase to MCx oscillations in the hemi-parkinsonian rat. These changes were not observed in the ketamine-anesthetized preparation. These observations suggest that dopamine loss alters PPN spike timing by increasing inhibitory oscillatory input to the PPN from basal ganglia output nuclei, a phenomenon that may be relevant to motor dysfunction and PPN DBS efficacy in PD patients.

Regulation of cholinergic phenotype in developing neurons

Specification of neurotransmitter phenotype is critical for neural circuit development and is influenced by intrinsic and extrinsic factors. Recent findings in rat hypothalamus in vitro suggest the role of neurotransmitter glutamate in the regulation of cholinergic phenotype. Here we extended our previous studies on the mechanisms of glutamate-dependent regulation of cholinergic phenotypic properties in hypothalamic neurons. Using immunocytochemistry, electrophysiology, and calcium imaging, we demonstrate that hypothalamic expression of choline acetyltransferase (the cholinergic marker) and responsiveness of neurons to acetylcholine (ACh) receptor agonists increase during chronic administration of an N-methyl-D-aspartate receptor (NMDAR) blocker, MK-801, in developing rats in vivo and genetic and pharmacological inactivation of NMDARs in mouse and rat developing neuronal cultures. In hypothalamic cultures, an inactivation of NMDA receptors also induces ACh-dependent synaptic activity, as do inactivations of PKA, ERK/MAPK, CREB, and NF-kappaB, which are known to be regulated by NMDA receptors. Interestingly, the increase in cholinergic properties in developing neurons that is induced by NMDAR blockade is prevented by the blockade of ACh receptors, suggesting that function of ACh receptor is required for the cholinergic up-regulation. Using dual recording of monosynaptic excitatory postsynaptic currents, we further demonstrate that chronic inactivation of ionotropic glutamate receptors induces the cholinergic phenotype in a subset of glutamatergic neurons. The phenotypic switch is partial as ACh and glutamate are coreleased. The results suggest that developing neurons may not only coexpress multiple transmitter phenotypes, but can also change the phenotypes following changes in signaling in neuronal circuits.

Distribution and morphology of cholinergic, putative catecholaminergic and serotonergic neurons in the brain of the Egyptian rousette flying fox, Rousettus aegyptiacus

Over the past decade much controversy has surrounded the hypothesis that the megachiroptera, or megabats, share unique neural characteristics with the primates. These observations, which include similarities in visual pathways, have suggested that the megabats are more closely related to the primates than to the other group of the Chiropteran order, the microbats, and suggests a diphyletic origin of the Chiroptera. To contribute data relevant to this debate, we used immunohistochemical techniques to reveal the architecture of the neuromodulatory systems of the Egyptian rousette (Rousettus aegypticus), an echolocating megabat. Our findings revealed many similarities in the nuclear parcellation of the cholinergic, putative catecholaminergic and serotonergic systems with that seen in other mammals including the microbat. However, there were 11 discrete nuclei forming part of these systems in the brain of the megabat studied that were not evident in an earlier study of a microbat. The occurrence of these nuclei align the megabat studied more closely with primates than any other mammalian group and clearly distinguishes them from the microbat, which aligns with the insectivores. The neural systems investigated are not related to such Chiropteran specializations as echolocation, flight, vision or olfaction. If neural characteristics are considered strong indicators of phylogenetic relationships, then the data of the current study strongly supports the diphyletic origin of Chiroptera and aligns the megabat most closely with primates in agreement with studies of other neural characters.

GABA distribution in lamprey is phylogenetically conserved

The localization of gamma-aminobutyric acid (GABA) has been well described in most classes of vertebrates but not in adult lampreys. The question if the GABA distribution is similar throughout the vertebrate subphylum is therefore still to be addressed. We here investigate two lamprey species, the sea lamprey, Petromyzon marinus, and the river lamprey, Lampetra fluviatilis, and compare the GABA pattern with that of other vertebrates. The present immunohistochemical study provides an anatomical basis for the general distribution and precise localization of GABAergic neurons in the adult lamprey forebrain and brainstem. GABA-immunoreactive cells were organized in a virtually identical manner in the two species. They were found throughout the brain, with the following regions being of particular interest: the granular cell layer of the olfactory bulb, the nucleus of the anterior commissure, the septum, the lateral and medial pallia, the striatum, the nucleus of the postoptic commissure, the thalamus, the hypothalamus, and pretectal areas, the optic tectum, the torus semicircularis, the mesencephalic tegmentum, restricted regions of the rhombencephalic tegmentum, the octavolateral area, and the dorsal column nucleus. The GABA distribution found in cyclostomes is very similar to that of other classes of vertebrates, including mammals. Since the lamprey diverged from the main vertebrate line around 450 million years ago, this implies that already at that time the basic vertebrate plan for the GABA innervation in different parts of the brain had been developed.

Topography of cortical and subcortical connections of the human pedunculopontine and subthalamic nuclei

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is the most common surgical therapy for Parkinson' s disease (PD). DBS of the pedunculopontine nucleus (PPN) is emerging as a promising surgical therapy for PD as well. In order to better characterize these nuclei in humans, we determined the anatomical connections of the PPN and STN and the topography of these connections using probabilistic diffusion tractography. Diffusion tractography was carried out in eight healthy adult subjects using diffusion data acquired at 1.5 T MRI (60 directions, b=1000 s/mm(2), 2 x 2 x 2 mm(3) voxels). The major connections that we identified from single seed voxels within STN or PPN were present in at least half the subjects and the topography of these connections within a 36-voxel region surrounding the initial seed voxel was then examined. Both the PPN and STN showed connections with the cortex, basal ganglia, cerebellum, and down the spinal cord, largely matching connections demonstrated in primates. The topography of motor and associative brain areas in the human STN was strikingly similar to that shown in animals. PPN Topography has not been extensively demonstrated in animals, but we showed significant topography of cortical and subcortical connections in the human PPN. In addition to demonstrating the usefulness of PDT in determining the connections and topography of small grey matter structures in vivo, these results allow for inference of optimal DBS target locations and add to our understanding of the role of these nuclei in PD.

Glutamate synaptic inputs to ventral tegmental area neurons in the rat derive primarily from subcortical sources

Dopamine and GABA neurons in the ventral tegmental area project to the nucleus accumbens and prefrontal cortex and modulate locomotor and reward behaviors as well as cognitive and affective processes. Both midbrain cell types receive synapses from glutamate afferents that provide an essential control of behaviorally-linked activity patterns, although the sources of glutamate inputs have not yet been completely characterized. We used antibodies against the vesicular glutamate transporter subtypes 1 and 2 (VGlut1 and VGlut2) to investigate the morphology and synaptic organization of axons containing these proteins as putative markers of glutamate afferents from cortical versus subcortical sites, respectively, in rats. We also characterized the ventral tegmental area cell populations receiving VGlut1+ or VGlut2+ synapses according to their transmitter phenotype (dopamine or GABA) and major projection target (nucleus accumbens or prefrontal cortex). By light and electron microscopic examination, VGlut2+ as opposed to VGlut1+ axon terminals were more numerous, had a larger average size, synapsed more proximally, and were more likely to form convergent synapses onto the same target. Both axon types formed predominantly asymmetric synapses, although VGlut2+ terminals more often formed synapses with symmetric morphology. No absolute selectivity was observed for VGlut1+ or VGlut2+ axons to target any particular cell population. However, the synapses onto mesoaccumbens neurons more often involved VGlut2+ terminals, whereas mesoprefrontal neurons received relatively equal synaptic inputs from VGlut1+ and VGlut2+ profiles. The distinct morphological features of VGlut1 and VGlut2 positive axons suggest that glutamate inputs from presumed cortical and subcortical sources, respectively, differ in the nature and intensity of their physiological actions on midbrain neurons. More specifically, our findings imply that subcortical glutamate inputs to the ventral tegmental area expressing VGlut2 predominate over cortical sources of excitation expressing VGlut1 and are more likely to drive the behaviorally-linked bursts in dopamine cells that signal future expectancy or attentional shifting.

Freezing and hypokinesia of gait induced by stimulation of the subthalamic region

We report a case of a Parkinson's disease patient treated by bilateral deep brain stimulation of the subthalamic nucleus, who developed freezing and hypokinesia of gait induced by stimulation through a left-side misplaced electrode which was more antero-medial than the planned trajectory. Subsequently, correct repositioning of the left electrode afforded complete relief of gait disturbances. Freezing and hypokinesia of gait may be side effects of deep brain stimulation of the subthalamic region due to current spreading antero-medially to the subthalamic nucleus. These side effects are not subject to habituation and restrict any increase in stimulation parameters. We hypothesize that pallidal projections to the pedunculopontine nucleus could be responsible for these gait disturbances in our patient.

The involvement of dopamine in the modulation of sleep and waking

Dopamine (DA)-containing neurons involved in the regulation of sleep and waking (W) arise in the ventral tegmental area (VTA) and the substantia nigra pars compacta (SNc). The VTA and SNc cells have efferent and afferent connections with the dorsal raphe nucleus (DRN), the pedunculopontine and laterodorsal tegmental nuclei (PPT/LDT), the locus coeruleus (LC), the lateral and posterior hypothalamus (LH), the basal forebrain (BFB), and the thalamus. Molecular cloning techniques have enabled the characterization of two distinct groups of DA receptors, D(1)-like and D(2)-like receptors. The D(1) subfamily includes the D(1) and D(5) receptors, whereas the D(2) subfamily comprises the D(2), D(3), and D(4) receptors. Systemic administration of a selective D(1) receptor agonist induces behavioral arousal, together with an increase of W and a reduction of slow wave sleep (SWS) and REM sleep (REMS). Systemic injection of a DA D(2) receptor agonist induces biphasic effects, such that low doses reduce W and increase SWS and REMS (predominant activation of the D(2) autoreceptor), whereas large doses induce the opposite effect (predominant facilitation of the D(2) postsynaptic receptor). Compounds with DA D(1) or D(2) receptor blocking properties augment non-REMS and reduce W. Preliminary findings tend to indicate that the administration of a DA D(3)-preferring agonist induces somnolence and sleep in laboratory animals and man. DA neurons in the VTA and the SNc do not change their mean firing rate across the sleep-wake cycle. It has been proposed that DA cells in the midbrain show a change in temporal pattern rather than firing rate during the sleep-wake cycle. The available evidence tends to indicate that during W there occurs an increase of burst firing activity of DA neurons, and an enhanced release of DA in the VTA, the nucleus accumbens (NAc), and a number of forebrain structures. A series of structures relevant for the regulation of the behavioral state, including the DRN, LDT/PPT, LC, and LH, could be partly responsible for the changes in the temporal pattern of activity of DA neurons.

Connections of the basal ganglia with the limbic system: implications for neuromodulation therapies of anxiety and affective disorders

The basal ganglia are best known for their role in motor planning and execution. However, it is currently widely accepted that they are also involved in cognitive and emotional behaviors. Parts of the basal ganglia play a key role in reward and reinforcement, addictive behaviors and habit formation. Pathophysiological processes underlying psychiatric disorders such as depression, obsessive compulsive disorder and even schizophrenia involve the basal ganglia and their connections to many other structures and particularly to the prefrontal cortex and the limbic system. In this article, we aim, on the basis of current research, to describe in a succinct manner the most important connections of the basal ganglia with the limbic system which are relevant to normal behaviors but also to psychiatric disorders. Currently, we possess sufficiently powerful tools that enable us to modulate brain networks such as cortex stimulation (CS) or deep brain stimulation (DBS). Notably, neuromodulation of basal ganglia function for the treatment of movement disorders has become a standard practice, which provides insights into the psychiatric problems that occur in patients with movement disorders. It is clear that a sound understanding of the currently available knowledge on the circuits connecting the basal ganglia with the limbic system will provide the theoretical platform that will allow precise, selective and beneficial neuromodulatory interventions for refractory psychiatric disorders.

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.

Fos immunoreactivity in some locomotor neural centres of 6OHDA-lesioned rats

In this study, we explore Fos expression (a measure of cell activity) in three nuclei associated with locomotion, namely the zona incerta, pedunculopontine tegmental nucleus and cuneiform nucleus (the latter two form the mesencephalic locomotor region) in hemiparkinsonian rats. Sprague-Dawley rats had small volumes of either saline (control) or 6 hydroxydopamine (6OHDA) injected into the medial forebrain bundle, the major tract carrying dopaminergic nigrostriatal axons. After various post-lesion survival periods, ranging from 2 h to 28 days, rats were perfused with formaldehyde and their brains processed for routine tyrosine hydroxylase and Fos immunocytochemistry. Our results showed a significant increase (P < 0.05) in the number of strongly labelled Fos+ cells in the cuneiform nucleus in the 6OHDA-lesioned cases compared to the controls after 7 and 28 days survival periods. By contrast, there were no significant differences (P > 0.05) in the number of strong-labelled Fos+ cells in the zona incerta and pedunculopontine nucleus of 6OHDA-lesioned rats compared to controls at any survival period. Many of the Fos+ cells within the pedunculopontine and cuneiform nuclei were glutamatergic (35-60%), while none or very few were nitric oxide synthase+. In conclusion, we reveal an increase in the number of strongly labelled Fos+ cells within the cuneiform nucleus of the so-called defensive locomotive system in 6OHDA-lesioned rats. In relation to Parkinson disease, we suggest that this increase is associated with the akinesia or lack of movement seen in patients.

Pedunculopontine neurons are involved in network changes in the kindling model of temporal lobe epilepsy

It is well known that epileptogenesis is associated with widespread neuronal network changes in brain regions adjacent to the seizure focus but also in remote structures including basal ganglia. Besides the superior colliculus, the pedunculopontine tegmental nucleus (PPN) is one of three main target regions of basal ganglia output activity and is reciprocally connected with the substantia nigra pars reticulata (SNr), which is critically involved in seizure propagation and manipulation. We here tested the hypothesis if, apart from the traditional view that the superior colliculus mediates seizure-gating mechanisms of the SNr, the PPN is involved in kindling-induced network changes. Rats were electrically kindled via the ipsilateral basolateral amygdala. In vivo extracellular single unit recordings of right PPN neurons were performed in kindled rats 1 day after a generalized seizure in order to examine kindling-associated rather than seizure-associated activity changes. The main findings of the study were (1) a seizure-outlasting drastically reduced firing rate of PPN neurons and (2) an increase in burst and irregular firing pattern in kindled rats compared with sham-kindled and naïve controls. These changes are likely to be caused by an altered inhibitory input from the SNr. Furthermore, kindling caused (3) the oscillation frequency of PPN neurons to shift towards lower frequencies. The kindling-induced activity changes were found to be anatomically restricted to the PPN, indicating that network changes follow distinct anatomical routes. We demonstrated that the PPN is strongly affected by the functional reorganization of neurocircuitry associated with kindling. The underlying mechanisms are discussed. The findings are relevant for a better understanding of kindling-associated network changes and might provide new targets for therapeutic manipulations in epilepsies.

Deep brain stimulation

Deep brain stimulation (DBS) has provided remarkable benefits for people with a variety of neurologic conditions. Stimulation of the ventral intermediate nucleus of the thalamus can dramatically relieve tremor associated with essential tremor or Parkinson disease (PD). Similarly, stimulation of the subthalamic nucleus or the internal segment of the globus pallidus can substantially reduce bradykinesia, rigidity, tremor, and gait difficulties in people with PD. Multiple groups are attempting to extend this mode of treatment to other conditions. Yet, the precise mechanism of action of DBS remains uncertain. Such studies have importance that extends beyond clinical therapeutics. Investigations of the mechanisms of action of DBS have the potential to clarify fundamental issues such as the functional anatomy of selected brain circuits and the relationship between activity in those circuits and behavior. Although we review relevant clinical issues, we emphasize the importance of current and future investigations on these topics.

Cell survival patterns in the pedunculopontine tegmental nucleus of methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys and 6OHDA-lesioned rats: evidence for differences to idiopathic Parkinson disease patients?

We explore the patterns of cell loss in the pedunculopontine tegmental nucleus (PpT), a major locomotor and muscle tone suppression centre of the brainstem, in two animal models of Parkinson disease, namely MPTP (methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-treated monkeys and 6-hydroxydopamine(6OHDA)-lesioned rats. Although there have been many studies documenting the loss of dopaminergic cells from the substantia nigra in these animal models, there has been little, if any, documentation of a loss of cells in the PpT. Results were obtained from macaque monkeys (Macaca fascicularis) and Sprague-Dawley rats. For the monkey series, animals were injected intramuscularly with MPTP (0.2 mg/kg) for 8 days consecutively and then allowed to survive for 21 days thereafter. Each monkey underwent behavioural assessment for parkinsonian symptoms. For the rat series, 6OHDA was injected into the midbrain using stereotactic coordinates. Rats were then allowed to survive for either 7, 14, 28, or 84 days thereafter. Monkey and rat brains were aldehyde-fixed and processed for routine tyrosine hydroxylase (TH; to label nigral dopaminergic cells) and nitric oxide synthase (NOs; to label PpT cholinergic cells) immunocytochemistry. In monkeys, the morphology, distribution and number of NOs(+) cells in the controls and MPTP-treated cases were very similar. In fact, in terms of number, there was only a 1% difference in the mean cell number between the controls and MPTP-treated cases. A comparable pattern was evident in 6OHDA-lesioned rats; there was no substantial difference in morphology, distribution and number of NOs(+) cells on the 6OHDA-lesioned cases when compared to the controls at each of the survival periods post-surgery. In summary, we show no loss of the large cholinergic/NOs(+) cells in the PpT in two animal models of Parkinson disease. This is in contrast to the heavy loss of these cells reported by previous findings in idiopathic Parkinson disease in patients.

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.