An investigation into the role of the pedunculopontine tegmental nucleus in the mediation of locomotion and orofacial stereotypy induced by d-amphetamine and apomorphine in the rat

Brainstem Circuits for Locomotion

Locomotion is a universal motor behavior that is expressed as the output of many integrated brain functions. Locomotion is organized at several levels of the nervous system, with brainstem circuits acting as the gate between brain areas regulating innate, emotional, or motivational locomotion and executive spinal circuits. Here we review recent advances on brainstem circuits involved in controlling locomotion. We describe how delineated command circuits govern the start, speed, stop, and steering of locomotion. We also discuss how these pathways interface between executive circuits in the spinal cord and diverse brain areas important for context-specific selection of locomotion. A recurrent theme is the need to establish a functional connectome to and from brainstem command circuits. Finally, we point to unresolved issues concerning the integrated function of locomotor control.

Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Effects of pedunculopontine nucleus cholinergic lesion on gait and dyskinesia in hemiparkinsonian rats

Pedunculopontine nucleus (PPN) cholinergic neurons are implicated in freezing of gait in Parkinson's disease (PD) and motor stereotypy in normal animals, but the causal role of these neurons on specific gait parameters and treatment-induced dyskinesia remains speculative. Therefore, we examined whether selective cholinergic lesion of the rostral PPN affects PD motor and gait deficits, L-DOPA-induced dyskinesia and motor improvement, and DA-agonist-induced dyskinesia. Sprague-Dawley rats were assigned to one unilaterally lesioned group: Sham lesion, PPN cholinergic lesion with diphtheria urotensin II fusion toxin, medial forebrain bundle dopamine lesion with 6-hydroxydopamine, or dual acetylcholine and dopamine lesion. We used gait analysis and forepaw adjusting steps to examine PD gait and motor deficits. Forepaw adjusting steps were also used to assess motor improvement with L-DOPA treatment. The abnormal involuntary movements scale measured L-DOPA and dopamine D1- and D2-receptor agonist-induced dyskinesia. Lesions, verified via tyrosine hydroxylase and choline acetyltransferase immunohistochemistry reduced an average of 95% of nigral dopamine neurons and 80% of PPN cholinergic neurons, respectively. Rats receiving acetylcholine and dual lesion demonstrated enhanced freezing, and acetylcholine lesioned rats exhibited increased print area and stand index. Dopamine and dual lesion produced similar forepaw adjusting steps task on and off L-DOPA. Relative to DA lesioned rats, dual lesioned rats displayed reduced L-DOPA and DA agonist-induced dyskinesia at specific time points. Our results indicate that PPN cholinergic neurons affect gait parameters related to postural stability. Therefore, therapeutically targeting PPN cholinergic neurons could reduce intractable postural instability in PD without affecting motor benefits or side effects of L-DOPA treatment.

Pedunculopontine Nucleus Degeneration Contributes to Both Motor and Non-Motor Symptoms of Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by hypokinetic motor features; however, patients also display non-motor symptoms like sleep disorders. The standard treatment for PD is dopamine replacement with L-DOPA; however, symptoms including gait deficits and sleep disorders are unresponsive to L-DOPA. Notably, these symptoms have been linked to aberrant activity in the pedunculopontine nucleus (PPN). Of late, clinical trials involving PPN deep brain stimulation (DBS) have been employed to alleviate gait deficits. Although preclinical evidence implicating PPN cholinergic neurons in gait dysfunction was initially promising, DBS trials fell short of expected outcomes. One reason for the failure of DBS may be that the PPN is a heterogenous nucleus that consists of GABAergic, cholinergic, and glutamatergic neurons that project to a diverse array of brain structures. Second, DBS trials may have been unsuccessful because PPN neurons are susceptible to mitochondrial dysfunction, Lewy body pathology, and degeneration in PD. Therefore, pharmaceutical or gene-therapy strategies targeting specific PPN neuronal populations or projections could better alleviate intractable PD symptoms. Unfortunately, how PPN neuronal populations and their respective projections influence PD motor and non-motor symptoms remains enigmatic. Herein, we discuss normal cellular and neuroanatomical features of the PPN, the differential susceptibility of PPN neurons to PD-related insults, and we give an overview of literature suggesting a role for PPN neurons in motor and sleep deficits in PD. Finally, we identify future approaches directed towards the PPN for the treatment of PD motor and sleep symptoms.

Dichotomy between motor and cognitive functions of midbrain cholinergic neurons

Cholinergic neurons of the pedunculopontine nucleus (PPN) are interconnected with all the basal ganglia structures, as well as with motor centers in the brainstem and medulla. Recent theories put into question whether PPN cholinergic neurons form part of a locomotor region that directly regulates the motor output, and rather suggest a modulatory role in adaptive behavior involving both motor and cognitive functions. In support of this, experimental studies in animals suggest that cholinergic neurons reinforce actions by signaling reward prediction and shape adaptations in behavior during changes of environmental contingencies. This is further supported by clinical studies proposing that decreased cholinergic transmission originated in the PPN is associated with impaired sensorimotor integration and perseverant behavior, giving rise to some of the symptoms observed in Parkinson's disease and progressive supranuclear palsy. Altogether, the evidence suggests that cholinergic neurons of the PPN, mainly through their interactions with the basal ganglia, have a leading role in action control.

The Effects of Amphetamine and Methamphetamine on the Release of Norepinephrine, Dopamine and Acetylcholine From the Brainstem Reticular Formation

Amphetamine (AMPH) and methamphetamine (METH) are widely abused psychostimulants, which produce a variety of psychomotor, autonomic and neurotoxic effects. The behavioral and neurotoxic effects of both compounds (from now on defined as AMPHs) stem from a fair molecular and anatomical specificity for catecholamine-containing neurons, which are placed in the brainstem reticular formation (RF). In fact, the structural cross-affinity joined with the presence of shared molecular targets between AMPHs and catecholamine provides the basis for a quite selective recruitment of brainstem catecholamine neurons following AMPHs administration. A great amount of investigations, commentary manuscripts and books reported a pivotal role of mesencephalic dopamine (DA)-containing neurons in producing behavioral and neurotoxic effects of AMPHs. Instead, the present review article focuses on catecholamine reticular neurons of the low brainstem. In fact, these nuclei add on DA mesencephalic cells to mediate the effects of AMPHs. Among these, we also include two pontine cholinergic nuclei. Finally, we discuss the conundrum of a mixed neuronal population, which extends from the pons to the periaqueductal gray (PAG). In this way, a number of reticular nuclei beyond classic DA mesencephalic cells are considered to extend the scenario underlying the neurobiology of AMPHs abuse. The mechanistic approach followed here to describe the action of AMPHs within the RF is rooted on the fine anatomy of this region of the brainstem. This is exemplified by a few medullary catecholamine neurons, which play a pivotal role compared with the bulk of peripheral sympathetic neurons in sustaining most of the cardiovascular effects induced by AMPHs.

Effects of Muscarinic Acetylcholine m1 and m4 Receptor Blockade on Dyskinesia in the Hemi-Parkinsonian Rat

Standard treatment for Parkinson's disease (PD) is L-DOPA, but with chronic administration the majority of patients develop L-DOPA-induced dyskinesia (LID). Emerging evidence implicates the cholinergic system in PD and LID. Muscarinic acetylcholine receptors (mAChR) are known to modulate movement and of late have been implicated as possible targets for LID. Therefore the current study investigated the role of M₁ and M₄ mAChRs in LID, on motor performance following L-DOPA treatment, and sought to identify brain sites through which these receptors were acting. We first administered M₁R-preferring antagonist trihexyphenidyl (0, 0.1, and 1.0 mg/kg, i.p.) or the M₄R-preferring antagonist tropicamide (0, 10, and 30 mg/kg, i.p.) before L-DOPA, after which LID and motor performance were evaluated. Both compounds worsened and extended the time course of LID, while M₁R blockade improved motor performance. We then evaluated the effects of tropicamide and trihexyphenidyl on dyskinesia induced by D₁R agonist SKF81297 or D₂R agonist quinpirole. Surprisingly, both M₁R and M₄R antagonists reduced D₁R agonist-induced dyskinesia but not D₂R agonist-induced dyskinesia, suggesting that mAChR blockade differentially affects MSN firing in the absence of postsynaptic DA. Finally, we evaluated effects of striatum- or PPN-targeted tropicamide microinfusion on LID and motor performance. Despite prior evidence, M₄R blockade in either site alone did not affect the severity of LID via local striatal or PPN infusions. Taken together, these data suggest M₄R as a promising therapeutic target for reducing LID using more selective compounds.

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

We present data from animal studies showing that the pedunculopontine tegmental nucleus-conserved through evolution, compartmentalized, and with a complex pattern of inputs and outputs-has functions that involve formation and updates of action-outcome associations, attention, and rapid decision making. This is in contrast to previous hypotheses about pedunculopontine function, which has served as a basis for clinical interest in the pedunculopontine in movement disorders. Current animal literature points to it being neither a specifically motor structure nor a master switch for sleep regulation. The pedunculopontine is connected to basal ganglia circuitry but also has primary sensory input across modalities and descending connections to pontomedullary, cerebellar, and spinal motor and autonomic control systems. Functional and anatomical studies in animals suggest strongly that, in addition to the pedunculopontine being an input and output station for the basal ganglia and key regulator of thalamic (and consequently cortical) activity, an additional major function is participation in the generation of actions on the basis of a first-pass analysis of incoming sensory data. Such a function-rapid decision making-has very high adaptive value for any vertebrate. We argue that in developing clinical strategies for treating basal ganglia disorders, it is necessary to take an account of the normal functions of the pedunculopontine. We believe that it is possible to use our hypothesis to explain why pedunculopontine deep brain stimulation used clinically has had variable outcomes in the treatment of parkinsonism motor symptoms and effects on cognitive processing. © 2016 International Parkinson and Movement Disorder Society.

Investigating complex basal ganglia circuitry in the regulation of motor behaviour, with particular focus on orofacial movement

Current concepts of basal ganglia function have evolved from the essentially motoric, to include a range of extramotoric functions that involve not only dopaminergic but also cholinergic, γ-aminobutyric acid (GABA)ergic and glutamatergic mechanisms. We consider these mechanisms and their efferent systems, including spiralling, feed-forward striato-nigro-striatal circuitry, involving the dorsal and ventral striatum and the nucleus accumbens (NAc) core and shell. These processes are illustrated using three behavioural models: turning-pivoting, orofacial movements in rats and orofacial movements in genetically modified mice. Turning-pivoting indicates that dopamine-dependent behaviour elicited from the NAc shell is funnelled through the NAc-nigro-striato-nigro-pedunculopontine pathway, whereas acetylcholine-dependent behaviour elicited from the NAc shell is funnelled through the NAc-ventral pallidum-mediodorsal thalamus pathway. Cooperative/synergistic interactions between striatal D1-like and D2-like dopamine receptors regulate individual topographies of orofacial movements that are funnelled through striatal projection pathways and involve interactions with GABAergic and glutamatergic receptor subtypes. This application of concerted behavioural, neurochemical and neurophysiological techniques implicates a network that is yet broader and interacts with other neurotransmitters and neuropeptides within subcortical, cortical and brainstem regions to 'sculpt' aspects of behaviour into its topographical collective.

Striatal patch compartment lesions reduce stereotypy following repeated cocaine administration

Stereotypy can be characterized as inflexible, repetitive behaviors that occur following repeated exposure to psychostimulants, such as cocaine (COC). Stereotypy may be related to preferential activation of the patch (striosome) compartment of striatum, as enhanced relative activation of the patch compartment has been shown to positively correlate with the emergence of stereotypy following repeated psychostimulant treatment. However, the specific contribution of the patch compartment to COC-induced stereotypy following repeated exposure is unknown. To elucidate the involvement of the patch compartment to the development of stereotypy following repeated COC exposure, we determined if destruction of this sub-region altered COC-induced behaviors. The neurons of the patch compartment were ablated by bilateral infusion of the neurotoxin dermorphin-saporin (DERM-SAP; 17 ng/μl) into the striatum. Animals were allowed to recover for eight days following the infusion, and then were given daily injections of COC (25mg/kg) or saline for one week, followed by a weeklong drug-free period. Animals were then given a challenge dose of saline or COC, observed for 2h in activity chambers and sacrificed. The number of mu-labeled patches in the striatum were reduced by DERM-SAP pretreatment. In COC-treated animals DERM-SAP pretreatment significantly reduced the immobilization and intensity of stereotypy but increased locomotor activity. DERM-SAP pretreatment attenuated COC-induced c-Fos expression in the patch compartment, while enhancing COC-induced c-Fos expression in the matrix compartment. These data indicate that the patch compartment contributes to repetitive behavior and suggests that alterations in activity in the patch vs matrix compartments may underlie to this phenomenon.

Deep brain stimulation of different pedunculopontine targets in a novel rodent model of parkinsonism

The pedunculopontine tegmental nucleus (PPTg) has been proposed as a target for deep brain stimulation (DBS) in parkinsonian patients, particularly for symptoms such as gait and postural difficulties refractory to dopaminergic treatments. Several patients have had electrodes implanted aimed at the PPTg, but outcomes have been disappointing, with little evidence that gait and posture are improved. The PPTg is a heterogeneous structure. Consequently, exact target sites in PPTg, possible DBS mechanisms, and potential benefits still need systematic investigation in good animal models. We have investigated the role of PPTg in gait, developed a refined model of parkinsonism including partial loss of the PPTg with bilateral destruction of nigrostriatal dopamine neurons that mimics human pathophysiology, and investigated the effect of DBS at different PPTg locations on gait and posture using a wireless device that lets rats move freely while receiving stimulation. Neither partial nor complete lesions of PPTg caused gait deficits, underlining questions raised previously about the status of PPTg as a motor control structure. The effect of DBS in the refined and standard model of parkinsonism were very different despite minimal behavioral differences in nonstimulation control conditions. Anterior PPTg DBS caused severe episodes of freezing and worsened gait, whereas specific gait parameters were mildly improved by stimulation of posterior PPTg. These results emphasize the critical importance of intra-PPTg DBS location and highlight the need to take PPTg degeneration into consideration when modeling parkinsonian symptoms. They also further implicate a role for PPTg in the pathophysiology of parkinsonism.

Selective lesions of the cholinergic neurons within the posterior pedunculopontine do not alter operant learning or nicotine sensitization

Cholinergic neurons within the pedunculopontine tegmental nucleus have been implicated in a range of functions, including behavioral state control, attention, and modulation of midbrain and basal ganglia systems. Previous experiments with excitotoxic lesions have found persistent learning impairment and altered response to nicotine following lesion of the posterior component of the PPTg (pPPTg). These effects have been attributed to disrupted input to midbrain dopamine systems, particularly the ventral tegmental area. The pPPTg contains a dense collection of cholinergic neurons and also large numbers of glutamatergic and GABAergic neurons. Because these interdigitated populations of neurons are all susceptible to excitotoxins, the effects of such lesions cannot be attributed to one neuronal population. We wished to assess whether the learning impairments and altered responses to nicotine in excitotoxic PPTg-lesioned rats were due to loss of cholinergic neurons within the pPPTg. Selective depletion of cholinergic pPPTg neurons is achievable with the fusion toxin Dtx-UII, which targets UII receptors expressed only by cholinergic neurons in this region. Rats bearing bilateral lesions of cholinergic pPPTg neurons (>90% ChAT+ neuronal loss) displayed no deficits in the learning or performance of fixed and variable ratio schedules of reinforcement for pellet reward. Separate rats with the same lesions had a normal locomotor response to nicotine and furthermore sensitized to repeated administration of nicotine at the same rate as sham controls. Previously seen changes in these behaviors following excitotoxic pPPTg lesions cannot be attributed solely to loss of cholinergic neurons. These findings indicate that non-cholinergic neurons within the pPPTg are responsible for the learning deficits and altered responses to nicotine seen after excitotoxic lesions. The functions of cholinergic neurons may be related to behavioral state control and attention rather than learning.

Enhanced consumption of salient solutions following pedunculopontine tegmental lesions

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

The disrupted basal ganglia and behavioural control: an integrative cross-domain perspective of spontaneous stereotypy

Spontaneous stereotypic behaviour (SB) is common in many captive animal species, as well as in humans with some severe psychiatric disorders, and is often cited as being related to general basal ganglia dysfunction. Despite this assertion, there is little in the literature examining SB specifically in terms of the basal ganglia mechanics. In this review, we attempt to fill this gap by offering an integrative, cross-domain perspective of SB by linking what we currently understand about the SB phenotype with the ever-growing literature on the anatomy and functionality of the basal ganglia. After outlining current models of SB from different theoretical perspectives, we offer a broad but detailed overview of normally functioning basal ganglia mechanics, and attempt to link this with current neurophysiological evidence related to spontaneous SB. Based on this we present an empirically derived theoretical framework, which proposes that SB is the result of a dysfunctional action selection system that may reflect dysregulation of excitatory (direct) and inhibitory (indirect and hyperdirect) pathways as well as alterations in mechanisms of behavioural switching. This approach also suggests behaviours that specifically become stereotypic may reflect inbuilt low selection threshold behavioural sequences associated with early development and the species-specific ethogram or, low threshold behavioural sequences that are the result of stress-induced dopamine exposure at the time of performance.

Striatal patch compartment lesions alter methamphetamine-induced behavior and immediate early gene expression in the striatum, substantia nigra and frontal cortex

Methamphetamine (METH) induces stereotypy, which is characterized as inflexible, repetitive behavior. Enhanced activation of the patch compartment of the striatum has been correlated with stereotypy, suggesting that stereotypy may be related to preferential activation of this region. However, the specific contribution of the patch compartment to METH-induced stereotypy is not clear. To elucidate the involvement of the patch compartment to the development of METH-induced stereotypy, we determined if destruction of this sub-region altered METH-induced behaviors. Animals were bilaterally infused in the striatum with the neurotoxin dermorphin-saporin (DERM-SAP; 17 ng/μl) to specifically ablate the neurons of the patch compartment. Eight days later, animals were treated with METH (7.5 mg/kg), placed in activity chambers, observed for 2 h and killed. DERM-SAP pretreatment significantly reduced the number and total area of mu-labeled patches in the striatum. DERM-SAP pretreatment significantly reduced the intensity of METH-induced stereotypy and the spatial immobility typically observed with METH-induced stereotypy. In support of this observation, DERM-SAP pretreatment also significantly increased locomotor activity in METH-treated animals. In the striatum, DERM-SAP pretreatment attenuated METH-induced c-Fos expression in the patch compartment, while enhancing METH-induced c-Fos expression in the matrix compartment. DERM-SAP pretreatment followed by METH administration augmented c-Fos expression in the SNpc and reduced METH-induced c-Fos expression in the SNpr. In the medial prefrontal, but not sensorimotor cortex, c-Fos and zif/268 expression was increased following METH treatment in animals pre-treated with DERM-SAP. These data indicate that the patch compartment is necessary for the expression of repetitive behaviors and suggests that alterations in activity in the basal ganglia may contribute to this phenomenon.

The Pedunculopontine Tegmental Nucleus: A Second Cholinergic Source for Frequency-Specific Auditory Plasticity

Cholinergic modulation is essential for many brain functions and is an indispensable component of the prevalent models attempting to understand the neural mechanism responsible for learning-induced auditory plasticity. Unlike the cholinergic basal forebrain, the cholinergic pedunculopontine tegmental nucleus (PPTg) has received little attention. This study was designed to confirm whether the PPTg enables frequency-specific plasticity in the ventral division of the medial geniculate body of the thalamus (MGBv). Using the mouse model, we paired electrical stimulation of the PPTg with tone stimulation to help define the role of the PPTg. The receptive fields of MGBv neurons were examined before and after the paired stimulation; they were quantified in this study by best frequency (BF), response threshold, dynamic range, and spike number. We found that the electrical stimulation of the PPTg together with a tone presentation shifted the BFs of MGBv neurons upward when the frequency of the paired tone was higher than that of the control BF. Similarly, the BFs shifted downward when the frequency of the paired tone was lower than that of the control BF. The BFs of MGBv neurons, however, remained unchanged when the frequency of the paired tone was the same as that of the control BF. There was a linear relationship between the BF shift of MGBv neurons and the difference between the frequency of the paired tone and the control BF of MGBv neurons. Highly frequency specific changes were also observed in the response threshold, dynamic range, and spike number. This frequency-specific plasticity was largely eliminated by the microinjection of the muscarinic receptor antagonist atropine into the MGBv before the paired stimulation. Our findings suggest that the PPTg, like the cholinergic basal forebrain, is an important cholinergic source that enables frequency-specific plasticity in the central auditory system.

Expression of c-fos mRNA in the basal ganglia associated with contingent tolerance to amphetamine-induced hypophagia

Tolerance to the hypophagic effect of psychostimulants is contingent on having access to food while intoxicated. Rats given chronic injections of such drugs with access to food learn to suppress stereotyped movements, which interfere with feeding. In contrast, controls given the drug after food access do not learn to suppress stereotypy and, therefore, do not become tolerant. To determine the role of the basal ganglia in this phenomenon, we used in situ hybridization to measure the expression of c-fos mRNA, a marker for neural activation, in the brains of tolerant and nontolerant rats. Rats given chronic amphetamine injections prior to food access learned to suppress stereotyped movements, whereas yoked controls given the drug after feeding did not. Following an acute injection of amphetamine, both of these groups had higher levels of c-fos mRNA than saline-treated controls throughout the striatum, in the nucleus accumbens core, the ventral pallidum and layers V-VI of the motor cortex. In contrast, tolerant rats, which had learned to suppress stereotypy, had higher levels of c-fos mRNA than both amphetamine- and saline-treated controls in the entopeduncular nucleus, globus pallidus, subthalamic nucleus, pedunculopontine nucleus, nucleus accumbens shell, olfactory tubercle, somatosensory cortex, and layers II-IV of motor cortex. These data suggest that the learned suppression of amphetamine-induced stereotypy involves the activation of dorsal striatal pathways previously implicated in response selection as well as the ventral striatum, long implicated in appetitive motivation and reinforcement.

A functional dissociation of the anterior and posterior pedunculopontine tegmental nucleus: excitotoxic lesions have differential effects on locomotion and the response to nicotine

Excitotoxic lesions of posterior, but not anterior pedunculopontine tegmental nucleus (PPTg) change nicotine self-administration, consistent with the belief that the anterior PPTg (aPPTg) projects to substantia nigra pars compacta (SNC) and posterior PPTg (pPPTg) to the ventral tegmental area (VTA). The VTA is a likely site both of nicotine’s reinforcing effect as well as its actions on locomotion. We hypothesized that pPPTg, but not aPPTg lesions, would alter locomotion in response to repeated nicotine administration by virtue of the fact that pPPTg appears to be more closely related to the VTA than is the aPPTg. Following excitotoxic lesions of aPPTg or pPPTg, rats were habituated to experimental procedures. Repeated (seven of each) nicotine (0.4 mg/kg) and saline injections were given following an on-off procedure. Measurement of spontaneous locomotion during habituation showed that aPPTg but not pPPTg lesioned rats were hypoactive relative to controls. Following nicotine, control rats showed locomotor depression for the first 2 days of treatment followed by enhanced locomotion relative to activity following saline treatment. Rats with aPPTg lesions showed a similar pattern, but the pPPTg lesioned rats showed no locomotor depression following nicotine treatment. These data confirm the role of the pPPTg in nicotine’s behavioural effects—including the development of sensitization—and demonstrate for the first time that excitotoxic lesions of the aPPTg but not pPPTg generate a deficit in baseline activity. The finding that anterior but not posterior PPTg affects motor activity has significance for developing therapeutic strategies for Parkinsonism using deep brain stimulation aimed here.

Modulation of respiratory pattern and upper airway muscle activity by the pedunculopontine tegmentum: role of NMDA receptors

The pedunculopontine tegmental nucleus (PPT) is postulated to have important functions relevant to the regulation of rapid eye movement (REM) sleep and arousal, and various motor control systems including respiration. We have recently shown that pharmacologic activation of a neuronal subpopulation within the PPT, induced by micropipette injection of glutamate in nanoliter volumes, can produce respiratory rhythm disturbances and changes in genioglossus muscle activity in anesthetized rats. The aim of this study was to determine whether the respiratory pattern disturbance and increased genioglossus muscle tone induced by glutamate injection within the PPT are mediated by activation of N-methyl-D-aspartate (NMDA) receptors within the PPT. Experiments were performed in eight adult male spontaneously breathing Sprague-Dawley rats anesthetized using nembutal. Respiratory movements were monitored by piezoelectric strain gauge. Three-barrel glass pipettes were used to pressure inject glutamate (as a probe for respiratory modulating sites), ketamine (an NMDA channel blocker), and oil-red dye (to aid in histological verification of the injection sites) within the PPT. Electroencephalograms were recorded from the sensorimotor cortex, the hippocampus, and the pons, contralateral to the injection site. Electromyograms (EMGs) were recorded from the genioglossus muscle. The typical response to glutamate injection within the PPT respiratory-modulating region was immediate apnea followed by tachypnea and increased genioglossal tonic activity. The noncompetitive NMDA receptor channel-antagonist ketamine, injected at the same site and in the same volume as glutamate (5 nl), blocked respiratory dysrhythmia and genioglossal EMG responses to subsequent glutamate injections. For the first time, the present results suggest that respiratory rhythm and upper airway muscle tone are controlled by the activation of pedunculopontine tegmental nucleus NMDA receptors.

The laterodorsal tegmentum contributes to behavioral sensitization to amphetamine

A critical event in the development of behavioral sensitization is a transient increase in excitatory drive to dopamine neurons of the ventral tegmental area (VTA). This is likely to be due, in part, to the ability of drugs of abuse to produce long-term potentiation, expressed as increased AMPA receptor transmission, at excitatory synapses onto VTA dopamine neurons. We investigated the role of the laterodorsal tegmentum (LDT) in behavioral sensitization because LDT neurons provide an important source of excitatory drive to VTA dopamine neurons, through mixed glutamate and cholinergic inputs. To test the role of the LDT in amphetamine sensitization, ibotenic acid or sham lesions of the LDT were performed 1 week before the first of six daily amphetamine injections. When challenged with amphetamine 13 days after the last injection, sham rats expressed sensitization of stereotypy and post-stereotypy locomotor hyperactivity, whereas the latter was attenuated by ibotenic acid lesions of the LDT. To determine whether plasticity occurs in the LDT during amphetamine sensitization, we used a previously developed microdialysis assay in which increased ability of AMPA to activate a pathway serves as a marker for long-term potentiation. Two days after discontinuing repeated saline or amphetamine injections, the responsiveness of LDT-VTA neurons to AMPA was determined by microinjecting AMPA (0.4 nmol) into the LDT and measuring glutamate efflux in the ipsilateral VTA. Glutamate efflux was transiently increased in both groups but a delayed group difference was apparent with relatively higher glutamate efflux in amphetamine rats 30-60 min after AMPA injection. In parallel experiments, dopamine efflux in the nucleus accumbens (NAc) following intra-LDT AMPA declined in saline rats but remained relatively stable in amphetamine rats. Both results suggest relatively greater excitability of the LDT-VTA-NAc pathway after repeated amphetamine treatment. Our results provide the first evidence that neuronal plasticity in the LDT contributes to behavioral sensitization.

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.

The pedunculopontine tegmental nucleus and responding for sucrose reward

Pedunculopontine tegmental nucleus (PPTg) lesions in rodents lead to increased sucrose consumption, but the psychological deficit behind this remains uncertain. To understand better the relationship between consumption of, and motivation for, sucrose, the authors trained rats to traverse a runway for 20% or 4% sucrose solution; after 7 days, concentrations were reversed. Control rats consumed more 20% than 4% sucrose solution and promptly altered run times in response to concentration change. PPTg-lesioned rats consumed normal quantities of 4% but more 20% sucrose solution than controls and took longer to alter their runway time following the concentration change. These data suggest that lesions of the PPTg do not alter motivation per se and might be better understood as inducing a response selection deficit.

Unilateral lesion of the pedunculopontine nucleus induces hyperactivity in the subthalamic nucleus and substantia nigra in the rat

Recent data suggest a role for the pedunculopontine nucleus (PPN) in the pathophysiology of Parkinson's disease. Although there is anatomical evidence that the PPN and the basal ganglia are reciprocally connected, the functional importance of these connections is poorly understood. Lesioning of the PPN was shown to induce akinesia in primates, whereas in the 6-hydroxydopamine rat model the PPN was found to be hyperactive. As both nigrostriatal dopamine depletion and lesioning of the PPN were shown to induce akinesia and parkinsonism, the present study was performed in order to investigate the changes in neuronal activity of the subthalamic nucleus (STN) and the substantia nigra pars reticulata (SNr) after unilateral ibotenic acid lesioning of the PPN and after unilateral 6-hydroxydopamine lesioning of the substantia nigra pars compacta (SNc). The firing rate of STN neurones significantly increased from 10.2 +/- 6.2 (mean +/- SD) to 14.6 +/- 11.7 spikes/s after lesion of the PPN and to 18.6 +/- 14.5 spikes/s after lesion of the SNc. The activity of the SNr significantly increased from 19.6 +/- 10.5 to 28.7 +/- 13.4 spikes/s after PPN lesioning and to 23.5 +/- 10.8 spikes/s after SNc lesioning. Furthermore, PPN lesion decreased the number of spontaneously firing dopaminergic SNc cells, while having no effect on their firing rate. The results of our study show that lesion of the PPN leads to hyperactivity of the STN and SNr, similar to the changes induced by lesion of the SNc. Moreover, the decreased activity of SNc cells observed after PPN lesion might be at the origin of activity changes in the STN and SNr.

Injection of glutamate into the pedunculopontine tegmental nuclei of anesthetized rat causes respiratory dysrhythmia and alters EEG and EMG power

The pedunculopontine tegmental nucleus (PPT) has been shown to have important functions relevant to the regulation of behavioral states and various motor control systems, including breathing control. Our previous work has shown that the activation of neurons within the PPT, a structure that is typically active during rapid eye movement (REM) sleep, can produce respiratory disturbances in freely moving and anesthetized rats. The aim of this study was to test the hypothesis that respiratory modulation by the PPT in anesthetized rats can be evoked in the absence of other signs of an REM-sleep-like state. We characterized electroencephalogram (EEG) and electromyogram (EMG) changes during respiratory disturbances induced by glutamatergic stimulation of the PPT in spontaneously breathing, adult male Sprague-Dawley rats anesthetized with a ketamine/xylazine combination or with nembutal. Respiratory movements were monitored by a piezoelectric strain gauge. Two-barrel glass pipettes were used to pressure inject glutamate, to probe for respiratory effective sites within the PPT, and to inject oil red dye at the end of the experiments for histological verification of the injection sites. The EEGs were recorded from the sensorimotor cortex, hippocampus, and from the pons contralateral from the injection site. The EMGs were recorded from the genioglossus muscle. The initial response to glutamate injection into the respiratory modulating region of the PPT was always a respiratory pattern disturbance. Subsequent activation of EMG and EEG often occurred in ketamine/xylazine-anesthetized rats, but REM-sleep-like patterns were not observed. Respiratory pattern and EMG power changes in nembutal-anesthetized rats were similar, but EEG activation was never observed. Thus, we conclude that respiratory suppression produced by the local activation of PPT neurons may not necessarily be accompanied by an REM-sleep-like cortical state in this anesthetized model.

Excitotoxic lesions of the pedunculopontine tegmental nucleus in rats impair performance on a test of sustained attention

Recent research has suggested that the pontomesencephalic tegmentum might be an important part of a network underlying sustained attention. The largest structure of the pontomesencephalic tegmentum is the pedunculopontine tegmental nucleus, which has ascending connections to thalamus and with corticostriatal systems. In this experiment we examined the performance of rats following bilateral excitotoxic lesions of the pedunculopontine tegmental nucleus on a test of sustained attention previously used to examine frontal cortical function. After an initial period of darkness, the rats had to attend continuously to a dim stimulus light that would, at unpredictable intervals, become transiently brighter. During this period of increased stimulus brightness the rats could press a lever to obtain a food reward. Rats were trained to a criterion level of performance before lesions were made. After surgery, sham lesioned rats (n=7) resumed accurate responding, with an average successful detection rate of approximately 70%. Pedunculopontine lesioned rats (n=7), however, only achieved a successful detection rate of approximately 40%. When the duration of the bright target stimulus was increased from 1.5 to 4 s, the performance of the pedunculopontine lesioned rats significantly improved. The observation that an increase in brightness duration caused a marked improvement in lesioned rats' performance suggests strongly that the impairment was in attention rather than motor ability or simple sensory processing. These data are taken to be consistent with the hypothesis that the pedunculopontine tegmental nucleus is an important part of a network maintaining attention.

An examination of d-amphetamine self-administration in pedunculopontine tegmental nucleus-lesioned rats

The pedunculopontine tegmental nucleus (PPTg) has long been suggested to have a role in reward-related behaviour, and there is particular interest in its possible role in drug reward systems. Previous work found increased i.v. self-administration (IVSA) of d-amphetamine following PPTg lesions when training had included both operant pre-training and priming injections. The present study examined the effect of excitotoxin lesions of the PPTg on d-amphetamine IVSA under three training conditions. Naive: no previous experience of d-amphetamine or operant responding. Pre-trained: given operant training with food before lesion surgery took place. Primed: given single non-contingent d-amphetamine infusion (0.1 mg/0.l ml) at the start of each session. Rats in all conditions were given either ibotenate or phosphate buffer control lesions of the PPTg before d-amphetamine (0.1 mg/0.1 ml infusion) IVSA training took place. Rats received eight sessions of training under a fixed ratio (FR2) schedule of d-amphetamine IVSA, followed by four sessions under a progressive ratio (PR5) schedule. In the naive condition, PPTg-lesioned rats were attenuated in their responding under FR2, and took significantly fewer infusions under PR5 than the control group. Under FR2 in the pre-trained condition, there was no difference between PPTg excitotoxin and control lesioned rats; however, PPTg-lesioned rats took significantly fewer infusions under the PR5 schedule. In the primed condition, there were no differences between PPTg-lesioned and control rats under either FR2 or PR5 schedules. These data demonstrate that operant training prior to PPTg lesion surgery corrects some, but not all, of the deficits seen in the naive condition. PPTg-lesioned rats in both naive and pre-trained conditions showed reduced responding for d-amphetamine under a PR5 schedule. These deficits are overcome by priming with d-amphetamine. We suggest that alterations in striatal dopamine activity following PPTg lesions underlie these effects.

Gabaa receptors in the pedunculopontine tegmental nucleus play a crucial role in rat shell-specific dopamine-mediated, but not shell-specific acetylcholine-mediated, turning behaviour

The role of GABA(A) receptors in the pedunculopontine tegmental nucleus in turning behaviour of rats was studied. Unilateral injection of the GABA(A) receptor agonist, muscimol (25-100 ng), into the pedunculopontine tegmental nucleus dose-dependently produced contraversive pivoting, namely tight head-to-tail turning marked by abnormal hindlimb backward stepping. This effect was GABA(A) receptor specific, since it was prevented by the GABA(A) receptor antagonist, bicuculline (50 ng), which alone did not elicit turning behaviour. Unilateral injection of a mixture of dopamine D(1) ((+/-)-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol [SKF 38393], 5 microg) and D(2) (quinpirole, 10 microg) receptor agonists into the nucleus accumbens shell has been found to elicit contraversive pivoting, whilst unilateral injection of the acetylcholine receptor agonist (carbachol, 5 microg) into the same site is known to elicit contraversive circling, namely turning marked by normal stepping. The pivoting induced by a mixture of SKF 38393 (5 microg) and quinpirole (10 microg) injected into the nucleus accumbens shell was significantly inhibited by bicuculline (50 ng) injected into the pedunculopontine tegmental nucleus, whereas muscimol (25 ng) had no effect. Neither muscimol (25 ng) nor bicuculline (50 ng) modulated the contraversive circling induced by carbachol (5 microg) injected into the nucleus accumbens shell. It is therefore concluded that unilateral stimulation of GABA(A) receptors in the pedunculopontine tegmental nucleus can elicit contraversive pivoting and that the pedunculopontine tegmental nucleus is one of the output stations of the accumbens region that mediates shell-specific, dopaminergic pivoting, but not of the accumbens region that mediates shell-specific, cholinergic circling.

Behavioural sensitisation to repeated d-amphetamine: effects of excitotoxic lesions of the pedunculopontine tegmental nucleus

The pedunculopontine tegmental nucleus (PPTg) interacts with anatomical systems thought to be involved in mediating sensitisation of the locomotor response to repeated d-amphetamine. The PPTg has direct and indirect connections with the nucleus accumbens and prefrontal cortex, and also influences midbrain dopamine activity through direct projections to substantia nigra and ventral tegmental area. In this experiment, the development of behavioural sensitisation to the locomotor stimulant effects of repeated d-amphetamine was examined in rats bearing excitotoxic lesions of the PPTg, and sham-lesioned controls. Rats were given repeated d-amphetamine (1.5 mg/kg i.p.) treatment in an on-off procedure, with saline and d-amphetamine given on alternate days, such that rats received a total of seven d-amphetamine and seven saline treatments. Locomotor responses were measured in photocell cages. On the first day of d-amphetamine treatment, there was no difference between excitotoxin and sham-lesioned rats. Development of sensitisation to the locomotor stimulant effects of d-amphetamine was delayed in PPTg-lesioned rats, relative to the sham-lesioned control rats. However, there was no difference between lesion and control groups in the locomotion seen on saline-treatment days. These data suggest that the PPTg is involved in the development of behavioural sensitisation to the locomotor stimulant effects of repeated d-amphetamine, and indicate that traditional striatal circuitry models of the mechanisms underlying sensitisation should be extended to include the PPTg.

Examination of the role of the pedunculopontine tegmental nucleus in radial maze tasks with or without a delay

Two radial maze tasks, random foraging and delayed spatial win-shift, have been used to investigate, in rats, the functions and inter-relationships of structures connected through the corticostriatal loops, such as the prelimbic cortex, nucleus accumbens, ventral pallidum and mediodorsal thalamus. The random foraging task is designed to investigate animals' ability to use spatial information to guide foraging on-line. The delayed spatial win-shift task requires, in addition, that animals hold spatially relevant information in working memory across a delay period. The pedunculopontine tegmental nucleus receives direct output from ventral striatal systems and might therefore be expected to share functional properties with them. In the present experiments we have examined the performance of rats bearing bilateral excitotoxic lesions of the pedunculopontine tegmental nucleus on both of these tasks. In acquisition tests rats were given bilateral lesions before any training took place, while in retention tests appropriate training to predetermined criterion levels of performance took place before lesions were made. In both tasks, and in both acquisition (no prelesion training) and retention (prelesion training) tests, rats with pedunculopontine lesions made significantly more errors in selecting arms to enter than did control rats. There was no motor impairment present in pedunculopontine tegmental nucleus-lesioned rats - on the contrary, on measures of speed (latency to make first arm choice and the mean time for subsequent choices) pedunculopontine-lesioned rats were slightly faster than control rats. We suggest that the pedunculopontine tegmental nucleus shares functional properties with frontostriatal systems and that it forms part of a brainstem-directed stream of striatal outflow different to the cortical re-entrant system via the thalamus.

The effect of excitotoxic lesions of the pedunculopontine tegmental nucleus on performance of a progressive ratio schedule of reinforcement

The pedunculopontine tegmental nucleus has connections with sites in both dorsal and ventral striatum, and a number of studies have suggested that it has a role in reward-related behaviour. The present experiment aimed to investigate the perception of reward in pedunculopontine tegmental nucleus-lesioned rats responding for food under a progressive ratio schedule, which measures willingness to work for a given reward. Rats were trained on a progressive ratio-5 schedule for food reward, then given ibotenic acid or sham lesions of the pedunculopontine tegmental nucleus. Their performance under this schedule was examined again following recovery from surgery. Compared with sham-lesioned rats, those with lesions of the pedunculopontine tegmental nucleus showed significantly reduced breaking points and significantly longer post-reinforcement pauses. However, there was no difference between the groups in their latency to collect food pellets once earned, suggesting that pedunculopontine tegmental nucleus excitotoxin and sham-lesioned rats were equally motivated by the presence of food. Excitotoxin-lesioned rats made significantly more responses on the control lever and more entries to the food hopper as progressive ratio increment increased, but did not differ from controls when the schedule requirement was low. These results are interpreted as indicating no global loss of motivation, since lesioned rats performed normally at low schedule requirements, and were as fast as controls to collect pellets. But as the schedule requirement increased, excitotoxin-lesioned rats showed reductions in responding on the active lever (that is, a reduction in breaking point) and an increase in inappropriate responses towards the food hopper and the control lever.We consider these data to indicate that the behavioural deficits in pedunculopontine-lesioned rats arise not from a sensory or hedonic change, but from alteration in the control of motor output.

An examination of the effects of bilateral excitotoxic lesions of the pedunculopontine tegmental nucleus on responding to sucrose reward

The effects of bilateral excitotoxic lesions of the pedunculopontine tegmental nucleus (PPTg) on sucrose intake were examined in three experiments. First, in tests of conditioned place preference using 20% sucrose as the reinforcer, it was shown that lesioned rats, regardless of whether they were food deprived or non-deprived, formed normal place preferences and showed normal amounts of locomotion. However, consumption of 20% sucrose in the pairing trials was increased in the deprived PPTg lesioned rats compared to their matched controls. A second experiment showed that sucrose consumption in the home cage was increased in both deprived and non-deprived PPTg lesioned rats, but only when the concentration of sucrose was greater than 12%: below this there were no differences in intake between the lesioned and control rats. In a third home cage experiment, it was again shown that non-deprived PPTg lesioned rats increased their consumption of 20% sucrose compared to controls. PPTg lesioned rats concomitantly reduced their intake of lab chow such that overall energy intake remained the same as that of control rats. These data are taken to suggest (i) that bilateral excitotoxic lesions of the PPTg increase consumption of sucrose selectively in conditions of high motivational excitement; (ii) that the perception of the rewarding value of 20% sucrose, as judged by place preference, is not affected by these lesions; and (iii) that PPTg lesioned rats are able to adjust their energy intake to accommodate increased sucrose loads. These data are consistent with the hypothesis that bilateral excitotoxic lesions of the PPTg do not affect energy balance regulation or judgment of the hedonic value of sucrose, but that they do affect the control of responding in the face of high levels of motivational excitement.

Excitotoxic lesions of the pedunculopontine differentially mediate morphine- and d-amphetamine-evoked striatal dopamine efflux and behaviors

Cholinergic and glutamatergic cells in the pedunculopontine tegmental nucleus are a principal source of excitatory input to midbrain dopamine neurons projecting to the striatum. Disruption of these brainstem inputs has been shown to respectively enhance and reduce psychostimulant and opiate self-administration in rats. In the present study, d-amphetamine- and morphine-induced behaviors and dorsal striatal dopamine efflux, measured using in vivo chronoamperometry, were investigated 21 days after bilateral excitotoxic (ibotenate) lesions of the pedunculopontine in rats. Compared to sham-operated controls, pedunculopontine lesions enhanced stereotyped behaviors induced by a challenge injection of d-amphetamine (1.5 mg/kg, i.p.) to an extent that markedly interfered with the expression of locomotor behavior. A significant augmentation in striatal dopamine efflux was also observed in these lesioned animals under urethane anesthesia in response to a similar challenge injection of d-amphetamine (1.5 mg/kg, i.v.) 2 days following these behavioral observations. In direct contrast, pedunculopontine lesions in a separate group of rats significantly attenuated morphine-induced (2 mg/kg, i.p.) stereotyped activity, although no significant differences were observed in locomotion compared to sham-operated animals. Under urethane anesthesia, these lesions attenuated striatal dopamine efflux evoked by a similar challenge injection of morphine (2 mg/kg, i.v.). These findings indicate that the pedunculopontine differentially mediates the pharmacological actions of two diverse drugs of abuse on striatal dopamine neurotransmission and resultant behaviors. These results also imply that the pedunculopontine tegmental nucleus may serve as a major striatal-motor interface in the processing of salient environmental stimuli, and their incentive rewarding impact on dopamine-mediated behavioral responses.

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

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

Muscle tone facilitation and inhibition after orexin-a (hypocretin-1) microinjections into the medial medulla

Orexins/hypocretins are synthesized in neurons of the perifornical, dorsomedial, lateral, and posterior hypothalamus. A loss of hypocretin neurons has been found in human narcolepsy, which is characterized by sudden loss of muscle tone, called cataplexy, and sleepiness. The normal functional role of these neurons, however, is unclear. The medioventral medullary region, including gigantocellular reticular nucleus, alpha (GiA) and ventral (GiV) parts, participates in the induction of locomotion and muscle tone facilitation in decerebrate animals and receives moderate orexinergic innervation. In the present study, we have examined the role of orexin-A (OX-A) in muscle tone control using microinjections (50 microM, 0.3 microl) into the GiA and GiV sites in decerebrate rats. OX-A microinjections into GiA sites, previously identified by electrical stimulation as facilitating hindlimb muscle tone bilaterally, produced a bilateral increase of muscle tone in the same muscles. Bilateral lidocaine microinjections (4%, 0.3 microl) into the dorsolateral mesopontine reticular formation decreased muscle rigidity and blocked muscle tone facilitation produced by OX-A microinjections into the GiA sites. The activity of cells related to muscle rigidity, located in the pedunculopontine tegmental nucleus and adjacent reticular formation, was correlated positively with the extent of hindlimb muscle tone facilitation after medullary OX-A microinjections. OX-A microinjections into GiV sites were less effective in muscle tone facilitation, although these sites produced a muscle tone increase during electrical stimulation. In contrast, OX-A microinjections into the gigantocellular nucleus (Gi) sites and dorsal paragigantocellular nucleus (DPGi) sites, previously identified by electrical stimulation as inhibitory points, produced bilateral hindlimb muscle atonia. We propose that the medioventral medullary region is one of the brain stem target for OX-A modulation of muscle tone. Facilitation of muscle tone after OX-A microinjections into this region is linked to activation of intrinsic reticular cells, causing excitation of midbrain and pontine neurons participating in muscle tone facilitation through an ascending pathway. Moreover, our results suggest that OX-A may also regulate the activity of medullary neurons participating in muscle tone suppression. Loss of OX function may, therefore, disturb both muscle tone facilitatory and inhibitory processes at the medullary level.

Determination of acetylcholine and dopamine content in thalamus and striatum after excitotoxic lesions of the pedunculopontine tegmental nucleus in rats

The pedunculopontine tegmental nucleus (PPTg) contains cholinergic neurons whose principal ascending connections are with thalamic nuclei and structures associated with the striatum. It has been hypothesized that PPTg neurons are more closely associated with the substantia nigra (and therefore striatal motor systems) than with the ventral tegmental area (and therefore limbic striatal functions). In the present experiments we have examined the hypothesis that the PPTg is similarly associated with motor nuclei in the thalamus. Rats received unilateral ibotenate lesions of PPTg and were sacrificed 1, 2, 4 or 7 days later. Discrete thalamic nuclei, and samples of caudate-putamen and nucleus accumbens, were punched out and thalamic acetylcholine (ACh) and striatal ACh and dopamine (DA) content examined. Anteroventral nucleus had decreased ACh content after PPTg lesion, but a time dependent increase was found in mediodorsal nucleus; ACh concentration was unchanged in thalamic reticular nucleus or medial geniculate. No long-term lesion-dependent changes in striatal ACh or DA content were found. The effects of PPTg lesion on thalamic ACh content are consistent with the hypothesis that it has effects on motor nuclei, but also indicate that PPTg lesions have complex and dynamic effects on thalamic ACh content.

The urotensin II receptor is expressed in the cholinergic mesopontine tegmentum of the rat

Urotensin II (UII) is a peptide known to be a potent vasoconstrictor. The urotensin II receptor (UII-R) is expressed not only in peripheral tissues but also in the brain of rodents. As a basis for studies of UII central nervous system actions, UII-R localization in the rat brain was analyzed by in situ hybridization and by in situ binding. UII-R mRNA was found in the mesopontine tegmental area colocalizing with choline acetyltransferase. Binding sites were detected throughout the brain with the highest levels found in the pedunculopontine tegmental area, the lateral dorsal tegmental area, and the lateral septal, medial habenular, and interpeduncular nuclei. The majority of these brain nuclei are sites of axonal termination originating from the mesopontine areas, suggesting that UII-R is a presynaptic receptor. This distribution of UII-R in the cholinergic mesopontine area indicates that the UII system may be involved in sensory-motor integration and perhaps in central nervous system blood flow.

The effects of excitotoxic lesions of the pedunculopontine tegmental nucleus on conditioned place preference to 4%, 12% and 20% sucrose solutions

A number of studies have suggested that the pedunculopontine tegmental nucleus (PPTg) may play a role in reward-related behaviour. The present study was intended to investigate this further using conditioned place preference. In conditioned place preference paradigms the amount of time spent in a preferred environment is proportional to the value of the reinforcement present, until a maximum is reached. In the present experiments we aimed to determine whether this relationship was affected by lesions of the PPTg by examining the formation of a conditioned place preference to either 4%, 12% or 20% sucrose solutions in food-deprived PPTg lesioned rats. The conditioned place preference apparatus had two compartments different in colour, smell and floor texture. During conditioning, rats were restricted to one compartment or the other, one of which was paired with sucrose. This was carried out during 30 min sessions, alternating conditioned or nonconditioned trials for 14 days. On the test day, rats were given access to both compartments through a connecting chamber, and were scored for side preference over 15 min. Both PPTg and sham lesioned rats showed a conditioned place preference to 12% and 20% sucrose, but no place preference was formed by either group to 4% sucrose. There was no significant difference between the groups in the place preference shown. Consumption of 4% sucrose was not affected by excitotoxic lesions of the PPTg, but PPTg lesioned rats consumed significantly more 12% and 20% sucrose than sham controls. This suggests that perception of reward value, as judged by CPP formation, is unchanged by excitotoxic lesions of the PPTg. The increased consumption of 12% and 20% sucrose shown by rats bearing such lesions is therefore not likely to be a product of altered reward perception.

Selective deficits in attentional performance on the 5-choice serial reaction time task following pedunculopontine tegmental nucleus lesions

Sustained attention requires the integrity of basal forebrain cholinergic systems. The pedunculopontine tegmental nucleus (PPTg) has direct and indirect connections (via the thalamus) with the basal forebrain, suggesting that the PPTg may also play an important role in attentional processes. We examined this hypothesis by testing the effects of PPTg lesions in rats on performance in the 5-choice serial reaction time test. Bilateral lesions reduced accuracy, increased errors of omission, and increased the latency to correct responses. The deficits were more severe when neuronal damage was bilateral and concentrated in the posterior PPTg. Attentional demands of the task were increased by decreasing the stimulus duration, the stimulus brightness, or the inter-trial interval, and by introducing random bursts of white noise. These challenges impaired performance of all animals, but the magnitude of deficit was increased in the lesioned group. Conversely, lesion-induced deficits were partially alleviated when the attentional demands of the task were reduced. This pattern of results suggests that PPTg lesions produce a global deficit in attention, rather than a specific impairment in one process. The PPTg may control attentional processes through its direct projections to the forebrain cholinergic system or, indirectly, through activation of thalamocortical projections.

Lesions of the pedunculopontine tegmental nucleus reduce paradoxical sleep (PS) propensity: evidence from a short-term PS deprivation study in rats

Cholinergic neurons in the mesopontine tegmentum are thought to play a critical role in the generation of paradoxical sleep (PS). However, no study has yet examined whether lesions of these neurons cause deficits of PS in the rat. We describe here the effects of lesions of the pedunculopontine tegmental nucleus (PPT) on spontaneous PS and on PS propensity, expressed during and after a short period of PS deprivation. Lesions were induced by bilateral injections of ibotenate. PS deprivation was performed manually by gently waking rats each time they showed polygraphic signs of PS. Two weeks after lesions, an 8-h baseline recording was performed; the following day, rats were PS deprived for 6 h and polygraphic recordings were then continued for 2 h, to examine recovery sleep. The same protocol was repeated 1 week later. Compared with controls and with rats with limited PPT lesions, rats bearing > 60% NADPH-diaphorase-positive cell loss within the PPT showed unaffected PS under baseline conditions. However, they made fewer attempts to enter PS during deprivation and they exhibited an attenuated rebound increase in PS time after deprivation. The number of PS attempts and the magnitude of PS rebound were negatively correlated with the percent loss of diaphorase-positive neurons within the PPT. Thus, PS propensity that accumulated as a result of PS deprivation was reduced after extensive PPT lesions. In summary, although spontaneous PS was found to be unaltered, the PS deprivation procedure used in this study demonstrated the dysfunctioning of PS caused by PPT lesions.

Brainstem regions with neuronal activity patterns correlated with priming of locomotor stepping in the anesthetized rat

Locomotor stimulation in the perifornical hypothalamus produces a transient facilitation of subsequent locomotion, a priming effect, such that stepping to a second train of stimulation occurs with a shorter latency of onset and increased amplitude. Neurons responsible for the initiation of this facilitated stepping presumably respond to locomotor stimulation with a similar priming effect, i.e. either a shorter latency or a larger change in activity rate. This study used anesthetized rats (urethane, 800mg/kg) to compare brainstem regions in terms of the relative rates of occurrence of single neurons that showed both specific responses to locomotor stimulation and also priming effects. Specific responses were characterized by a progressive increase in activity prior to the first step (a Type I pattern). In that they co-varied in time with the increased probability of stepping onset, Type I responses were more specific than Type II responses, which peaked early in the stimulation train several seconds before the onset of stepping. Regions with high proportions of neurons showing Type I responses and priming effects included the anterior dorsal tegmentum lateral to the central gray, the oral pontine reticular nucleus and the medial gigantocellular nucleus. Few Type I neurons showed a modulation of activity related to the step cycle. Type I primed neurons were uncommon in the cuneiform and the pedunculopontine regions, but neurons showing other patterns (decreases and antidromic responses) were relatively prevalent there. The ventral tegmental area was generally unresponsive. The results indicate that stepping elicited by perifornical stimulation in the anesthetized rat is mediated by circuits that differ at midbrain levels from the circuits implicated in other types of locomotion. Two regions, the anterior dorsal tegmentum and the oral pontine reticular nucleus, warrant further attention to determine their possible roles in the initiation of locomotion.

The pedunculopontine tegmental nucleus and the role of cholinergic neurons in nicotine self-administration in the rat: a correlative neuroanatomical and behavioral study

The objective of this study was to determine whether the pedunculopontine tegmental nucleus plays a role in the maintenance of nicotine self-administration, and whether the ascending cholinergic projection from this nucleus to midbrain dopamine neurons in the ventral tegmental area might be involved. Studies were done with rats trained to self-administer nicotine intravenously. Self-administration was examined before and after the pedunculopontine tegmental nucleus was lesioned with the ethylcholine mustard aziridinium ion, a selective cholinergic toxin. Lesions were assessed qualitatively and quantitatively in histological sections stained for either nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry to identify cholinergic neurons, or for Nissl. Self-administration was also tested after an acute manipulation in which microinfusions of the nicotinic cholinergic antagonist dihydro-beta-erythroidine were made into the pedunculopontine tegmentum. Infusions of neurotoxin into the pedunculopontine tegmentum reduced nicotine self-administration behaviour when tested weeks later. Toxin treatment reduced the number of cholinergic neurons in the tegmentum, while largely sparing the non-cholinergic population in this area. Lesions were limited to the pedunculopontine area and did not extend to the neighboring laterodorsal tegmental nucleus or to the substantia nigra. Acute manipulation of the pedunculopontine tegmental nucleus with microinfusions of dihydro-beta-erythroidine also produced an attenuation of nicotine self-administration. Collectively these data show that the pedunculopontine tegmental nucleus is part of the neuronal circuitry mediating nicotine self-administration, and that the population of cholinergic neurons is likely a critical element.

Role of the laterodorsal tegmental nucleus in scopolamine- and amphetamine-induced locomotion and stereotypy

Scopolamine (1.5 mg/kg; i.p.) or amphetamine (3 mg/kg; i.p.) increases locomotion and stereotyped behavior patterns in rats. Previous studies suggest that scopolamine acts via muscarinic receptors near the midbrain-pons border. In this study, unilateral microinjections in N-methyl-scopolamine (2.5-10 microg) into the laterodorsal tegmental nucleus (LDT) increased locomotion. Bilateral ibotenate lesions of the LDT attenuated scopolamine-induced locomotion by 68% 7 days postlesion, and by 35% 28 days postlesion. LDT lesions reduced scopolamine-induced stereotypy less than locomotion. The sensitization to amphetamine observed on repeated tests was attenuated by LDT lesions for stereotypy, but not for locomotion. These findings suggest that scopolamine induces locomotion largely, but not exclusively, by blocking muscarinic receptors in LDT.

Frontal syndrome as a consequence of lesions in the pedunculopontine tegmental nucleus: a short theoretical review

In this review, it is argued that the consequence of bilateral damage to the pedunculopontine tegmental nucleus (PPTg) in experimental animals is the production of a form of frontal syndrome. Frontal syndrome is a term used to describe the behavioural consequences of damage to the frontal lobes in human patients. These behavioural changes can be classified as disinhibition of behaviour (a release of behavioural control), the production of inappropriate behaviour (which in patients can be either inappropriate actions or verbal behaviour), and the production of perseverative behaviour (the maintenance of an action beyond the point at which it should have been terminated). The psychological changes which underlie these behavioural changes are thought to involve executive functions, which include such things as the prospective planning of sequences of actions, attentional shifting and working memory. In this review, I attempt to demonstrate two things: first, that there are significant anatomical connections from frontostriatal systems to the PPTg. The motor cortex projects directly to the PPTg while the prefrontal cortex contacts it via striatal circuitry, forming clear routes by which the frontal lobes can communicate with the PPTg. Second, having established the existence of connections between frontostriatal systems and the PPTg, behavioural data are described. Experimental animals bearing bilateral lesions of the PPTg have been examined in a wide variety of tasks. Animals bearing such lesions are not impaired in basic processes of feeding, drinking, locomotion, or grooming and simple observation of lesioned rats' normal behaviour reveals no obvious gross impairment in function. However, the results of more subtle tests reveal a wide variety of deficits in various tasks. The outcome of these experiments are in many ways contradictory, but in the vast majority of cases, the changes can be described as involving disinhibition of behaviour, the release of inappropriate behaviour, and the production of perseverative behaviour. Anatomical and behavioural data support the conclusion that there are functional connections between frontal systems and the PPTg. This review also discusses what psychological processes might be served by such connections.