Activation of mesencephalic dopamine neurons by chemical stimulation of the nucleus tegmenti pedunculopontinus pars compacta

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.

Subcellular distribution of M2 muscarinic receptors in relation to dopaminergic neurons of the rat ventral tegmental area

Acetylcholine can affect cognitive functions and reward, in part, through activation of muscarinic receptors in the ventral tegmental area (VTA) to evoke changes in mesocorticolimbic dopaminergic transmission. Among the known muscarinic receptor subtypes present in the VTA, the M2 receptor (M2R) is most implicated in autoregulation and also may play a heteroreceptor role in regulation of the output of the dopaminergic neurons. We sought to determine the functionally relevant sites for M2R activation in relation to VTA dopaminergic neurons by examining the electron microscopic immunolabeling of M2R and the dopamine transporter (DAT) in the VTA of rat brain. The M2R was localized to endomembranes in DAT-containing somatodendritic profiles but showed a more prominent, size-dependent plasmalemmal location in nondopaminergic dendrites. M2R also was located on the plasma membrane of morphologically heterogenous axon terminals contacting unlabeled as well as M2R- or DAT-labeled dendrites. Some of these terminals formed asymmetric synapses resembling those of cholinergic terminals in the VTA. The majority, however, formed symmetric, inhibitory-type synapses or were apposed without recognized junctions. Our results provide the first ultrastructural evidence that the M2R is expressed, but largely not available for local activation, on the plasma membrane of VTA dopaminergic neurons. Instead, the M2R in this region has a distribution suggesting more indirect regulation of mesocorticolimbic transmission through autoregulation of acetylcholine release and changes in the physiological activity or release of other, largely inhibitory transmitters. These findings could have implications for understanding the muscarinic control of cognitive and goal-directed behaviors within the VTA.

Different function of pedunculopontine GABA and glutamate receptors in nucleus accumbens dopamine, pedunculopontine glutamate and operant discriminative behavior

The nucleus accumbens, as the main input structure of the ventral basal ganglia loop, is described as a limbic-motor interface. Dopamine input to nucleus accumbens modulates processing of concurrent glutamate input from limbic structures carrying motor and motivational information. There is evidence that these dopamine/glutamate interactions are fundamentally involved in response selection processes. However, the pedunculopontine tegmental nucleus (PPTg) in the brainstem is connected with limbic structures as well as dopaminergic midbrain areas, which also project to the nucleus accumbens. Furthermore, behavioral studies implicate the PPTg in complex, motivated behavior. Thus, the PPTg might be involved in motivated behavior by influencing response selection processes in the nucleus accumbens. In this study we used in vivo microdialysis in freely moving rats in order to inhibit (100, 200, 300 and 400 microm baclofen) or stimulate [5, 12.5, 25 or 50 micromalpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)] the PPTg in animals that are performing an operant discrimination task for food reward. The behavioral consequences were correlated with dopamine and glutamate levels in nucleus accumbens and PPTg, respectively. PPTg inhibition by local GABAB receptors impaired the response rate and accuracy of performance in the operant discrimination task. PPTg stimulation by local AMPA receptors exclusively impaired the response rate. Both treatments blocked the performance-driven dopamine signal in nucleus accumbens, whereas glutamate in PPTg was enhanced after AMPA administration only. The data indicate that the PPTg functionally participates in a network of subcortical and cortical structures, which is responsible for the execution of motivated behavior and response selection processes.

Glutamate and GABA modulate dopamine in the pedunculopontine tegmental nucleus

The pedunculopontine tegmental nucleus (PPTg) has an important anatomical position connecting basal ganglia and limbic systems with motor execution structures in the pons and spinal cord. It receives glutamatergic and GABAergic input and has additional reciprocal connections with mesencephalic dopaminergic neurons, suggesting that the PPTg plays a key role in frontostriatal information processing. In vivo microdialysis in freely moving rats, in combination with behavioral analysis, was used in this study to investigate whether the dopaminergic input can be modulated at the level of the PPTg via N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) or GABA(B) receptors. Stimulation of the GABA(B) receptor decreased dopamine release in the PPTg while that of the AMPA and NMDA receptors increased it. A time-related comparison of the effects of NMDA (0.75 and 1 mM) and AMPA (50 and 25 microM) revealed a more long-lasting effect after AMPA stimulation than after NMDA. However, only the infusion of the GABA(B) receptor agonist baclofen (100 and 200 microM) stimulated stereotyped behavior (e.g. sniffing, digging or head movements) and contralateral circling. This study clearly demonstrates that GABAergic as well as glutamatergic terminals in the PPTg are critically involved in the modulation of the dopamine system. Moreover, a decrease in PPTg dopamine via GABA(B) receptor stimulation seems to be behaviorally relevant.

Organization of somatic motor inputs from the frontal lobe to the pedunculopontine tegmental nucleus in the macaque monkey

To reveal the somatotopy of the pedunculopontine tegmental nucleus that functions as a brainstem motor center, we examined the distribution patterns of corticotegmental inputs from the somatic motor areas of the frontal lobe in the macaque monkey. Based on the somatotopical map prepared by intracortical microstimulation, injections of the anterograde tracers, biotinylated dextran amine and wheat germ agglutinin-conjugated horseradish peroxidase, were made into the following motor-related areas: the primary motor cortex, the supplementary and presupplementary motor areas, the dorsal and ventral divisions of the premotor cortex, and the frontal eye field. Data obtained from the present experiments were as follows: (i) Corticotegmental inputs from orofacial, forelimb, and hindlimb representations of the primary motor cortex tended to be arranged orderly from medial to lateral in the pedunculopontine tegmental nucleus. However, the distribution areas of these inputs considerably overlapped; (ii) The major input zones from distal representations of the forelimb and hindlimb regions of the primary motor cortex were located medial to those from their proximal representations, although there was a substantial overlap between the distribution areas of distal versus proximal limb inputs; (iii) The main terminal zones from the forelimb regions of the primary motor cortex, the supplementary and presupplementary motor areas, and the dorsal and ventral divisions of the premotor cortex appeared to overlap largely in the mediolaterally middle aspect of the pedunculopontine tegmental nucleus; and (iv) Corticotegmental input from the frontal eye field was scattered over the pedunculopontine tegmental nucleus.Thus, the present results indicate that the pedunculopontine tegmental nucleus is likely to receive partly separate but essentially convergent cortical inputs not only from multiple motor-related areas representing the same body part, but also from multiple regions representing diverse body parts. This suggests that somatotopical representations are intermingled rather than segregated in the pedunculopontine tegmental nucleus.

Protection against dopaminergic nigrostriatal cell death by excitatory input ablation

The importance of enhanced glutamatergic neurotransmission in the basal ganglia and related structures has recently been highlighted in the development of Parkinson's disease. The pedunculopontine tegmental nucleus (PPN) is the major origin of excitatory, glutamatergic input to dopaminergic nigrostriatal neurons of which degeneration is well known to cause Parkinson's disease. Based on the concept that an excitatory mechanism mediated by glutamatergic neurotransmission underlies the pathogenesis of neurodegenerative disorders, we made an attempt to test the hypothesis that removal of the glutamatergic input to the nigrostriatal neurons by PPN lesions might prevent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism in the macaque monkey. The PPN was lesioned unilaterally with microinjection of kainic acid, and, then, MPTP was administered systemically. In these monkeys, the degree of parkinsonian motor signs was behaviourally evaluated, and the histological changes in the dopaminergic nigrostriatal system were analysed by means of tyrosine hydroxylase immunohistochemistry. The present results revealed that nigrostriatal cell loss and parkinsonian motor deficits were largely attenuated in the MPTP-treated monkey group whose PPN had been lesioned, compared with the control, MPTP-treated monkey group with the PPN intact. This clearly indicates that the onset of MPTP neurotoxicity is suppressed or delayed by experimental ablation of the glutamatergic input to the nigrostriatal neurons. Such a protective action of excitatory input ablation against nigrostriatal cell death defines evidence that nigral excitation driven by the PPN may be implicated in the pathophysiology of Parkinson's disease.

Subthalamic stimulation-induced synaptic responses in substantia nigra pars compacta dopaminergic neurons in vitro

The subthalamic nucleus (STN) is one of the principal sources of excitatory glutamatergic input to dopaminergic neurons of the substantia nigra, yet stimulation of the STN produces both excitatory and inhibitory effects on nigral dopaminergic neurons recorded extracellularly in vivo. The present experiments were designed to determine the sources of the excitatory and inhibitory effects. Synaptic potentials were recorded intracellularly from substantia nigra pars compacta dopaminergic neurons in parasagittal slices in response to stimulation of the STN. Synaptic potentials were analyzed for onset latency, amplitude, duration, and reversal potential in the presence and absence of GABA and glutamate receptor antagonists. STN-evoked depolarizing synaptic responses in dopaminergic neurons reversed at approximately -31 mV, intermediate between the expected reversal potential for an excitatory and an inhibitory postsynaptic potential (EPSP and IPSP). Blockade of GABA(A) receptors with bicuculline caused a positive shift in the reversal potential to near 0 mV, suggesting that STN stimulation evoked a near simultaneous EPSP and IPSP. Both synaptic responses were blocked by application of the glutamate receptor antagonist, 6-cyano-7-nitroquinoxalene-2,3-dione. The confounding influence of inhibitory fibers of passage from globus pallidus and/or striatum by STN stimulation was eliminated by unilaterally transecting striatonigral and pallidonigral fibers 3 days before recording. The reversal potential of STN-evoked synaptic responses in dopaminergic neurons in slices from transected animals was approximately -30 mV. Bath application of bicuculline shifted the reversal potential to approximately 5 mV as it did in intact animals, suggesting that the source of the IPSP was within substantia nigra. These data indicate that electrical stimulation of the STN elicits a mixed EPSP-IPSP in nigral dopaminergic neurons due to the coactivation of an excitatory monosynaptic and an inhibitory polysynaptic connection between the STN and the dopaminergic neurons of substantia nigra pars compacta. The EPSP arises from a direct monosynaptic excitatory glutamatergic input from the STN. The IPSP arises polysynaptically, most likely through STN-evoked excitation of GABAergic neurons in substantia nigra pars reticulata, which produces feed-forward GABA(A)-mediated inhibition of dopaminergic neurons through inhibitory intranigral axon collaterals.

Nitric oxide facilitates N-methyl-D-aspartate-induced burst firing in dopamine neurons from rat midbrain slices

Dopamine (DA) neurons in the ventral tegmental area and substantia nigra pars compacta were induced to fire in bursts with application of N-methyl-D-aspartate (NMDA, 20 microM) and apamin (100 nM) while recording intracellularly in the rat brain slice. L-Arginine (300 microM), a substrate for nitric oxide (NO) production, increased both the number of spikes per burst and the magnitude of interburst hyperpolarizations. Nitric oxide synthase inhibitors N-nitro-L-arginine methyl ester (L-NAME, 100 microM), N-nitro-L-arginine, and 7-nitroindazole inhibited NMDA-induced burst firing by reducing the number of spikes per burst. Moreover, L-arginine (100 microM) reversed the inhibition of burst firing produced by L-NAME. These findings suggest that NO facilitates NMDA-induced burst firing in DA neurons.

Predictive reward signal of dopamine neurons

The effects of lesions, receptor blocking, electrical self-stimulation, and drugs of abuse suggest that midbrain dopamine systems are involved in processing reward information and learning approach behavior. Most dopamine neurons show phasic activations after primary liquid and food rewards and conditioned, reward-predicting visual and auditory stimuli. They show biphasic, activation-depression responses after stimuli that resemble reward-predicting stimuli or are novel or particularly salient. However, only few phasic activations follow aversive stimuli. Thus dopamine neurons label environmental stimuli with appetitive value, predict and detect rewards and signal alerting and motivating events. By failing to discriminate between different rewards, dopamine neurons appear to emit an alerting message about the surprising presence or absence of rewards. All responses to rewards and reward-predicting stimuli depend on event predictability. Dopamine neurons are activated by rewarding events that are better than predicted, remain uninfluenced by events that are as good as predicted, and are depressed by events that are worse than predicted. By signaling rewards according to a prediction error, dopamine responses have the formal characteristics of a teaching signal postulated by reinforcement learning theories. Dopamine responses transfer during learning from primary rewards to reward-predicting stimuli. This may contribute to neuronal mechanisms underlying the retrograde action of rewards, one of the main puzzles in reinforcement learning. The impulse response releases a short pulse of dopamine onto many dendrites, thus broadcasting a rather global reinforcement signal to postsynaptic neurons. This signal may improve approach behavior by providing advance reward information before the behavior occurs, and may contribute to learning by modifying synaptic transmission. The dopamine reward signal is supplemented by activity in neurons in striatum, frontal cortex, and amygdala, which process specific reward information but do not emit a global reward prediction error signal. A cooperation between the different reward signals may assure the use of specific rewards for selectively reinforcing behaviors. Among the other projection systems, noradrenaline neurons predominantly serve attentional mechanisms and nucleus basalis neurons code rewards heterogeneously. Cerebellar climbing fibers signal errors in motor performance or errors in the prediction of aversive events to cerebellar Purkinje cells. Most deficits following dopamine-depleting lesions are not easily explained by a defective reward signal but may reflect the absence of a general enabling function of tonic levels of extracellular dopamine. Thus dopamine systems may have two functions, the phasic transmission of reward information and the tonic enabling of postsynaptic neurons.

Compensatory mechanisms in experimental and human parkinsonism: towards a dynamic approach

This paper provides an overview of the compensatory mechanisms which come into action during experimental and human parkinsonism. The intrinsic properties of the dopaminergic neurones of the substantia nigra pars compacta (SNc) which degenerate during Parkinson's disease are described in detail. It is generally considered that the nigrostriatal pathway is principally responsible for the compensatory preservation of dopaminergic function. It is also becoming clear that the morphological characteristics of dopaminergic neurones and the dual character, synaptic and asynaptic, of striatal dopaminergic innervation engender two modes of transmission, wiring and volume, and that both these modes play a role in the preservation of dopaminergic function. The plasticity of the dopamine neurones, extrinsic or intrinsic to the striatum, can thus be regarded as another compensatory mechanism. Recent anatomical and electrophysiological studies have shown that the SNc receives both glutamatergic and cholinergic inputs. The dynamic role this innervation plays in compensatory mechanisms in the course of the disease is explained and discussed. Recent developments in the field of compensatory mechanisms speak for the urgence to develop a valid chronic model of Parkinson's disease, integrating all the clinical features, even resting tremor, and illustrating the gradual evolution of nigral degeneration observed in human Parkinson's disease. Only a dynamic approach to the physiopathological study of compensatory mechanisms in the basal ganglia will be capable of elucidating these complex questions.

Locomotion and stereotypy induced by scopolamine: contributions of muscarinic receptors near the pedunculopontine tegmental nucleus

In this study, we test whether blockade of muscarinic receptors near mesopontine cholinergic cell groups may contribute to locomotor activation induced by scopolamine. Unilateral or bilateral injections of scopolamine (10-150 micrograms) into the pedunculopontine tegmental nucleus (PPT) increased horizontal locomotion by 2-15 times in a dose-related way. Unilateral or bilateral injections of scopolamine into the PPT increased stereotypic behaviors (such as sniffing in one location or over large areas), self-biting and grooming. Carbachol (4 micrograms) injected into PPT reduced locomotion for 20 min, followed by 70 min of increased locomotion. When carbachol (4 micrograms) was injected into the PPT before scopolamine (3 mg/kg, i.p.), the activating effect of scopolamine was attenuated, but not when carbachol was injected after scopolamine. Therefore, carbachol appears to compete with scopolamine for muscarinic receptors near the PPT that mediate locomotor activating effects of systemic scopolamine. Haloperidol (0.1 mg/kg, i.p.) also attenuated the stereotypy and locomotion induced by scopolamine in the PPT. We hypothesize that scopolamine acts by blocking muscarinic receptors on mesopontine cholinergic neurons, thereby disinhibiting cholinergic neurons that can activate dopamine neurons.

Both nicotinic and muscarinic receptors in ventral tegmental area contribute to brain-stimulation reward

Cholinergic neurons of the pedunculopontine tegmental nucleus (Ch5) and laterodorsal tegmental nucleus (Ch6) monosynaptically activate dopamine neurons of the substantia nigra and ventral tegmental area (VTA) via nicotinic and muscarinic receptors. The nicotinic receptors near the VTA have been proposed to be important for nicotine self-administration in rats and for tobacco smoking in humans. Nicotinic and muscarinic blockers were microinjected into the VTA of rats trained to lever-press for lateral hypothalamic stimulation via an ipsilateral electrode. The competitive nicotinic blocker dihydro-beta-erythroidine (DH beta E; 5-60 micrograms) shifted rate-frequency curves to the right by a mean of 6-27% in a dose-related manner; the noncompetitive nicotinic blocker mecamylamine (10-300 micrograms) produced similar shifts of 7-21%. Atropine (30 micrograms) shifted the curves to the right by a mean of 82% in three of the sites tested with DH beta E. All blockers decreased maximum bar-pressing rates significantly in some sites when the shifts were large. Therefore, nicotinic receptors in the VTA make small contributions to the maintained rewarding effect of brain-stimulation reward in rats, but muscarinic receptors in the VTA appear to be more important.

Single-unit activity in the primate nucleus tegmenti pedunculopontinus related to voluntary arm movement

In the pedunculopontine tegmental nucleus (PPN), single-unit activity was recorded in two monkeys trained to manipulate an on-off lever with a hand. Among 280 neurons recorded, a change in the firing rate related to the lever-off movement was observed in 125 neurons for the contralateral limb movement (53%) and in 96 neurons for the ipsilateral limb movement (48%). The changes were an increase in the firing rate in 122 neurons and a decrease in 99 neurons. These changes in the firing rate related to the task often occurred for both the contralateral and ipsilateral limb movements. The change of activity preceded the movement onset for both contralateral and ipsilateral arm movements. These findings suggest that in primates the PPN contributes to coordination of upper limb movements on both sides.

Excitotoxic lesions of the pedunculopontine tegmental nucleus produce contralateral hemiparkinsonism in the monkey

Dopaminergic nigrostriatal neurons, degeneration of which causes Parkinson's disease, are known to receive excitatory input almost exclusively from the pedunculopontine tegmental nucleus (PPN). We report here that excitotoxic lesions of the PPN produce abnormal motor signs relevant to hemiparkinsonism in the macaque monkey. Under the guidance of extracellular unit recordings, the electrophysiologically identified PPN was injected unilaterally with kainic acid. These PPN-lesioned monkeys exhibited mild to moderate levels of flexed posture and hypokinesia in the upper and lower limbs contralateral to the lesion. In most of the monkeys, such pathophysiological events were gradually improved and became stationary in 1-2 weeks. The hemiparkinsonian symptoms observed after PPN destruction might be ascribed to a decrease in nigrostriatal neuron activity due to excitatory input ablation.

Increased striatal dopamine efflux follows scopolamine administered systemically or to the tegmental pedunculopontine nucleus

The cholinergic cells of the tegmental pedunculopontine nucleus monosynaptically excite dopaminergic neurons of the substantia nigra. In vivo electrochemical methods were used to monitor dorsal striatal dopamine efflux in awake rats following intraperitoneal scopolamine injections and following the direct application of scopolamine to the tegmental pedunculopontine nucleus. Systemic injections of scopolamine (1.0, 3.0 or 10.0 mg/kg) resulted in dose-related increases in peak striatal dopamine oxidation currents of between 1.1 and 2.0 nA. Increases began within 10-20 min after injection and peaked after 40-90 min. Unilateral microinjections of scopolamine into the tegmental pedunculopontine nucleus (10, 50 or 100 micrograms/0.5 microliter) resulted in dose-related increases in dopamine oxidation currents that peaked 60-90 min postinjection (2.9-5.0 nA). Carbachol (4.0 micrograms/0.5 microliter) injected unilaterally into the tegmental pedunculopontine nucleus 20 min before 100 micrograms tegmental pedunculopontine nucleus scopolamine, or injected bilaterally 20 min before 3.0 mg/kg systemic scopolamine, attenuated the increases produced by scopolamine alone. The carbachol preinjection tests suggest that the effects of both systemic and tegmental pedunculopontine nucleus scopolamine treatments are mediated largely by muscarinic receptors near the tegmental pedunculopontine nucleus. These findings are consistent with the proposal that enhanced activation of substantia nigra dopamine cells results from scopolamine-induced disinhibition of the tegemental pedunculopontine nucleus cholinergic cell group via blockade of their inhibitory autoreceptors.

Subpallidal outputs to the nucleus accumbens and the ventral tegmental area: anatomical and electrophysiological studies

The goal of this study was to investigate the functional organization of the subpallidal-->accumbens direct and indirect feedback loops by both anatomical and electrophysiological methods. The results of the dextran-conjugated rhodamine injections into the subpallidal area has shown three distinct projections: (1) a substantial pathway from the subpallidal area to the ventral tegmental area, (2) a more diffuse rostral projection from the subpallidal area to the core area of the nucleus accumbens, and (3) a sparse pathway projecting rostrodorsally from the subpallidal area toward the thalamic regions. Electrical or chemical stimulation of the subpallidal region, which was studied by the axonal tracer, evoked inhibitory responses in the majority (60 and 80%, respectively) of the accumbens and ventral tegmental area neurons in a standard extracellular recording study. Less than 1/3 of the accumbens or ventral tegmental area cells showed an increase in the mean firing rate. The majority (77.5%) of all responded neurons had a latency of less than 10 ms. Furthermore, injection of glutamate into the subpallidal area not only altered the firing pattern of the accumbens neurons, but also attenuated their excitatory responses elicited by the electrical stimulation of the ventral subiculum. Our results indicate that the subpallidal area plays a predominantly inhibitory role in the ventral tegmental area-accumbens-subpallidal circuitry, presumably by its GABAergic projections, and may also modulate subicular input into the nucleus accumbens.

The pedunculopontine nucleus--auditory input, arousal and pathophysiology

This review describes the role of the pedunculopontine nucleus (PPN) in various functions, including sleep-wake mechanisms, arousal, locomotion and in several pathological conditions. Special emphasis is placed on the auditory input to the PPN and the possible role of this nucleus in the manifestation of the P1 middle latency auditory evoked response. The importance of these considerations is evident because the PPN is part of the cholinergic arm of the reticular activating system. As such, the auditory input to this region may modulate the level of arousal of the CNS and, consequently, abnormalities in the processing of this input can be expected to have serious consequences on the level of excitability of the CNS. The involvement of the PPN in such disorders as schizophrenia, anxiety disorder and narcolepsy is discussed.

Investigations of fetal development models for prenatal drug exposure and schizophrenia. Prenatal d-amphetamine effects upon early and late juvenile behavior in the rat

Recent evidence suggests that mid-pregnancy is a critical period for production of fetal abnormalities that cause behavioral and neuropathological changes in adult offspring. The present experiments provide an animal model of these effects by treating pregnant Sprague-Dawley rats during gestational days 11-14 with d-amphetamine (AM). Offspring were tested for neurological signs, foraging activity, reversal learning, and sensitivity to amphetamine challenge. In the Early Juvenile period, postnatal days (PND) 20-30, female AM offspring initially showed reductions in rearing, holepoking, and midfield activity. On later trials, and as young adults, AM females showed signs of locomotor hyperactivity despite continued poor foraging efficiency, and were also more sensitive to a 1.0 mg/kg d-amphetamine challenge. AM males showed initially slower and more perseverative responding than controls, but then developed excessive response switching. These changes continued during tests for Retention, Reversal, and Extinction in the Late Juvenile/Early Adult stage (PND 50-90), when both AM-exposed sexes showed increased eating time, significantly more perseverative lateral turning preference (right or left), and slower reversal learning than controls. Behavioral data were consistent with aberrations in thalamo-frontal and mesolimbic/nigrostriatal projection systems that have been reported in AM animals and which are also affected by maternal drug abuse and schizophrenia.

The pedunculopontine tegmental nucleus: a role in cognitive processes?

The cholinergic pedunculopontine tegmental nucleus, located in the brainstem and part of the reticular formation, has been traditionally linked to motor function, arousal and sleep. Its anatomical connections, however, raise the possibility that the pedunculopontine tegmental nucleus is also involved in other aspects of behaviour such as motivation, attention and mnemonic processes. This is of obvious importance, since the pedunculopontine tegmental nucleus undergoes degeneration in human neurodegenerative disorders also characterized by attentional and/or mnemonic deficits. Moreover, recent behavioural animal work suggests that cognitive processes may be represented in the pedunculopontine tegmental nucleus. The difficulty that faces research in this area, however is the possible influence of cognition by other processes, such as arousal state, motivation and motor function. Nevertheless, by reviewing the literature, the pedunculopontine tegmental nucleus seems to be involved in attentional and possibly also in learning processes. These processes could be mediated by influencing cortical function via the thalamus, basal forebrain and basal ganglia. The involvement of the pedunculopontine tegmental nucleus in mechanisms of memory, however, seems to be rather unlikely.

Pedunculopontine nucleus in the squirrel monkey: cholinergic and glutamatergic projections to the substantia nigra

The distribution and chemospecificity of the pedunculonigral neurons have been studied in squirrel monkeys (Saimiri sciureus) with cholera toxin subunit B (CTb) and fluorogold (FG) as retrograde tracers combined with immunohistochemistry for choline acetyltransferase (ChAT), glutamate, and the calcium binding protein calbindin D-28k. The injection of either CTb or FG into the substantia nigra produces prominent retrograde cell labeling in the mesopontine tegmentum. Labeled neurons are particularly numerous at the level of the decussation of the superior cerebellar peduncle, where they abound principally in the pars dissipata of the pedunculopontine nucleus (PPN). A significant proportion of retrogradely labeled neurons in the PPN display ChAT immunoreactivity. Within the entire PPN, approximately 25% of the retrogradely labeled neurons express ChAT immunoreactivity, but proportions of doubly labeled neurons are about 35%, 25%, and 15% in the rostral, middle, and caudal thirds of the PPN, respectively. These doubly labeled neurons are scattered among numerous retrogradely labeled neurons that are ChAT-negative and whose number increases along the rostrocaudal extent of the PPN. Several retrogradely labeled neurons in the PPN display glutamate immunoreactivity, but very few express calbindin. This study provides the first direct evidence for the involvement of cholinergic and glutamatergic neurons in the pedunculonigral projection in primates. Furthermore, the fact that some neurons of the PPN display both ChAT and glutamate immunoreactivity indicates that single neurons in the mesopontine tegmentum may exert a two-fold effect upon dopaminergic neurons of the substantia nigra. This dual cholinergic and glutamatergic pedunculonigral projection may play a crucial role in the functional organization of primate basal ganglia.

Effects of kainic acid, quisqualic acid, and their antagonist, pCB-PzDA, on rat electrocorticograms and monoamine metabolite levels in rat striatum

The action of kainic acid (KA), quisqualic acid (QA), and 1-(4-chlorobenzoyl)-piperazine-2,3-dicarboxylic acid (pCB-PzDA) was investigated in the central nervous system of male Sprague Dawley rats. Intracerebroventricularly injected KA and QA (100 nmol) induced spike discharges, and pCB-PzDA (100 nmol) suppressed electrocorticograms for one hour. pCB-PzDA enhanced the KA-induced spike discharges and inhibited those induced by QA. 2,3-Di-hydroxyphenylacetic acid(DOPAC) and homovanillic acid (HVA) levels were increased transiently by 10 nmol and continuously by 100 nmol of KA. KA dose-dependently increased 5-hydroxyindoleacetic acid (5-HIAA) levels 2 hours after administration. While 10 nmol of QA slightly increased the HVA level, 100 nmol of QA significantly increased DOPAC, HVA, and 5-HIAA levels. DOPAC and HVA levels were increased by 100 nmol of pCB-PzDA, although this agent inhibited KA-induced increases in DOPAC, HVA, and 5-HIAA levels. On the other hand, while pCB-PzDA first inhibited QA-induced increases in DOPAC, HVA and 5-HIAA levels for one hour, DOPAC and HVA levels thereafter increased additively. These findings suggest that pCB-PzDA may act not only as a NMDA antagonist, but that it may also act directly on dopaminergic neurons.

Collateral axons of cholinergic pontine neurones projecting to midline, mediodorsal and parafascicular thalamic nuclei in the rat

The organization of collateral axons projecting from neurones in the pontine laterodorsal tegmental nucleus (LDTg) has been examined using combinations of retrograde neuronal tracers with immunocytochemical markers for the acetylcholine-synthesizing enzyme choline acetyltransferase (CHAT), focussing on projections to the midline, mediodorsal and parafascicular thalamic nuclei and the ventral tegmental area. 25-59% of LDTg neurones projecting to the mediodorsal nucleus provided collaterals to the midline nuclei. Virtually all (87-96%) of these double retrogradely labelled neurones appeared cholinergic. 9-18% of LDTg neurones projecting to the parafascicular nuclei also provided a collateral to the midline nuclei and 50-78% of these double retrogradely labelled neurones stained for CHAT. 26-29% of the single LDTg neurones which projected collaterals to both the mediodorsal and midline nuclei, were found to project a third collateral to the ventral tegmental area. These anatomical findings, taken together with functional evidence, suggest that cholinergic terminals arising from LDTg are involved in coordinating thalamic mechanisms of brain state control; and in regulating dopaminergic pathways, both directly and via the thalamus.

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.

Ascending afferent regulation of rat midbrain dopamine neurons

Standard, extracellular single-unit recording techniques were used to examine the electrophysiological and pharmacological responsiveness of midbrain dopamine (DA) neurons to selected, ascending afferent inputs. Sciatic nerve stimulation-induced inhibition of nigrostriatal DA (NSDA) neurons was blocked by both PCPA (5-HT synthesis inhibitor) and 5,7-DHT (5-HT neurotoxin), suggesting mediation by a serotonergic (5-HT) system. Direct stimulation of the dorsal raphe (which utilizes 5-HT as a neurotransmitter and inhibits slowly firing NSDA neurons) inhibited all mesoaccumbens DA (MADA) neurons tested. Paradoxically, DPAT, a 5-HT1A agonist which inhibits 5-HT cell firing, enhanced sciatic nerve stimulation-induced inhibition of NSDA neurons. MADA neurons were not inhibited by sciatic nerve stimulation and, therefore, could not be tested in this paradigm. In contrast to the dorsal raphe, electrical stimulation of the pedunculopontine tegmental nucleus preferentially excited slowly firing NSDA and MADA neurons. Thus, both excitatory and inhibitory ascending afferents influence the activity of midbrain DA neurons, and intact 5-HT systems are necessary for sciatic nerve stimulation to alter DA cell activity. However, the role that 5-HT plays in mediating peripheral sensory input remains unclear.

A cholinergic input to the substantia nigra pars compacta increases striatal dopamine metabolism measured by in vivo voltammetry

3,4-Dihydroxyphenylacetic acid (DOPAC) and ascorbic acid (AA) were measured by differential pulse voltammetry in the neostriatum of anesthetized rats. Physostigmine (2.3 nmol) applied into the substantia nigra pars compacta (SNc), increased DOPAC concentration in the ipsilateral neostriatum, but did not modify AA levels. The largest increase of striatal DOPAC (37 +/- 8% above basal) was observed when physostigmine was applied at less than 0.5 mm from SNc, and decreased with increasing distance of the injection site from the pars compacta region. Chemical stimulation of the pedunculopontine tegmental nucleus (PPN) with kainic acid (2.3 nmol) increased both DOPAC and AA concentration in the ipsilateral neostriatum. Pretreatment with the muscarinic antagonist scopolamine (5 mg/kg, i.p.) inhibited the increase of striatal DOPAC from 20 to 70 min after kainic acid injection into the PPN, whereas the increase of AA was reduced from 90 to 160 min. By contrast, the nicotinic antagonist mecamylamine (4 mg/kg, i.p.) did not inhibit neither DOPAC nor AA increase elicited by the chemical stimulation of PPN. These results support the existence of cholinergic neurotransmission within the SNc that increases the firing rate of nigrostriatal dopaminergic neurons, enhancing dopamine turnover in neostriatum without changes in AA release. They also suggest that the PPN could be the origin of cholinergic afferents to the SNc that modulate the activity of dopaminergic neurons, through activation of muscarinic cholinergic receptors. Finally, the activation of a multisynaptic loop involving a cholinergic pathway which modulates the activity of the glutamatergic corticostriatal neurons is postulated to explain the increase of AA in neostriatum observed after PPN stimulation.

Organization and physiology of the substantia nigra

A comprehensive review of the literature on the anatomy, electrophysiology and pharmacology of the substantia nigra is presented. A diagram is developed taking into account the interneuronal interactions of neurotransmitters and receptors that control firing rates and neurotransmitter releases. The central features of the diagram are a positive dopaminergic feedforward process and a positive feedback mechanism mediated by extrasynaptic substance P diffusing from striatal terminals to dopaminergic dendrites of the zona compacta neurons. This loop can enhance the transmission of information from the striatum through the pars reticulata output neurons. The loop is controlled at the level of zona compacta neurons by a negative feedback supported by the dendritic release of dopamine and boosted by pedunculopontine activation mediated by muscarinic receptors. The output of the loop is controlled by two negative feedforward processes, both involving GABAergic striatonigral afferents. Application of the model to pharmacological studies of diverse behaviors including seizures, turning, and conditioned behaviors reveals unforseen relationships and may offer insights into, and directions for, further analysis of the mechanisms and functions involved.

Role of the mesopontine area in the circling behavior induced by apomorphine in rats bearing unilateral lesion of the entopeduncular nucleus

The role of the mesopontine area and superior colliculus in turning behavior induced by systemic administration of apomorphine was studied in rats bearing a unilateral entopeduncular lesion. Bilateral electrolytic damage of the superior colliculus resulted in an enhancement of the ipsilateral circling response to apomorphine, perhaps as a consequence of an increased locomotor drive in such animals. Bilateral electrolytic lesions of the mesopontine area decreased apomorphine-induced turning in entopeduncular rats, while a bilateral kainic acid lesion of the same region was ineffective. It was concluded that the pedunculopontine nucleus and adjacent reticular formation are not an essential link for the striopallidal complex output mediating circling in this model. Fibers running through this region could be implicated in the expression of the behavior under study. Since a unilateral electrolytic lesion of the mesopontine area contralateral to the damaged entopeduncular nucleus reduced drug-induced turning, we propose that an uncrossed pathway from the intact striopallidal complex mediates circling in our rats.

Dopaminergic involvement in locomotion elicited from the ventral pallidum/substantia innominata

Microinjection of the indirect GABAA antagonist, picrotoxin, or the mu opioid agonist, Tyr-D-Ala-Gly-NMe-Phe-Gly-ol (DAGO), into the ventral pallidum and substantia innominata (VP/SI) increases locomotor activity in rats. The VP/SI has direct and indirect projections to the region of the ventral mesencephalon containing dopamine perikarya, and to certain dopamine terminal fields, including the nucleus accumbens. Thus, it is possible that modulation of the mesocorticolimbic dopamine system by pharmacological stimulation in the VP/SI may play a role in the locomotor stimulant response. It was shown that pretreatment with dopamine receptor antagonists, either peripherally or microinjected into the nucleus accumbens significantly attenuated the motor stimulant effect of DAGO or picrotoxin injection into the VP/SI. Injection of either picrotoxin or DAGO into the VP/SI increased the levels of dopamine metabolites in the nucleus accumbens and prefrontal cortex. Thus, the motor stimulant response following pharmacological stimulation of the VP/SI appears to be mediated by increased dopamine neurotransmission via feedback mechanisms to the mesocorticolimbic dopamine system.

Enhanced survival of cultured dopamine neurons by treatment with soluble extracts from chemically deafferentiated striatum of adult rat brain

A soluble fraction was extracted from a chemically deafferentiated striatum of adult Wistar rats after unilateral lesioning of the nigrostriatal pathway by 6-hydroxydopamine (6-OHDA) injection. The soluble extract from the lesioned side enhanced the survival of cultured mesencephalic dopamine (DA) neurons of 14-day-old rat embryos as evidenced by quantitative counting of tyrosine hydroxylase-like immunoreactive cells. The neurotrophic activity of this striatal extract for DA neurons was highest 14 days after 6-OHDA injection and became negligible in 28 days. The extract showed no promoting effects on cultured gamma-aminobutyric acid (GABA)-containing mesencephalic neurons. These observations indicate that the striatum of adult rats may initiate de novo synthesis of trophic substance(s) for DA neurons but not for GABA neurons when subjected to nigral dopaminergic deafferentiation.

Decrease and recovery of choline acetyltransferase activity in medial thalamus and ventral tegmental area after destruction of pedunculopontine nucleus areas in the rat

Choline acetyltransferase activities were measured in various forebrain regions 3, 7, 14, 21 and 28 days after bilateral lesions of the pedunculopontine nucleus area with ibotenic acid. The decrease was predominant in the ventral tegmental area and medial thalamus (especially at the caudal level). The decrease recovered as rapidly as 21 days after the lesions. It is concluded that the ascending cholinergic pedunculopontine projection is topographically organized within the thalamus and it appears that the effect of the experimental destruction of the projection recovered within a month.

Afferent and efferent connections of the laterodorsal tegmental nucleus in the rat

The connections of the laterodorsal tegmental nucleus (LDTg) have been investigated using anterograde and retrograde lectin tracers with immunocytochemical detection. Inputs to LDTg were found from frontal cortex, diagonal band, preoptic areas, lateral hypothalamus, lateral mamillary nucleus, lateral habenula; the interpeduncular nucleus, ventral tegmental area, substantia nigra and retrorubral fields; the medial terminal nucleus, interstitial nucleus, supraoculomotor central grey, medial pretectum, nucleus of the posterior commissure, paramedian pontine reticular formation, paraabducens and paratrochlear region; the parabrachial nuclei and nucleus of the tractus solitarius. Terminal labelling from PHA-L injections of LDTg was found in infralimbic, cingulate and hippocampal cortex, lateral septum, septofimbrial and triangular nuclei, horizontal limb of diagonal band and preoptic areas; in the anterior, mediodorsal, reuniens, centrolateral, parafascicular, paraventricular and laterodorsal thalamic nuclei, rostral reticular thalamic nucleus, and zona incerta; the lateral habenula and the lateral hypothalamus. A number of brainstem structures apparently associated with visual functions were also innervated, mainly the superior colliculus, medial pretectum, medial terminal nucleus, paramedian pontine reticular formation, inferior olive, supraoculomotor, paraabducens and supragenual regions, prepositus hypoglossi and nucleus of the posterior commissure. Also innervated were substantia nigra compacta, ventral tegmental area, interfascicular nucleus, interpeduncular nucleus, dorsal and medial raphe, pedunculopontine tegmental region, parabrachial nuclei, and nucleus of the tractus solitarius. These findings suggest the LDTg to be a highly differentiated part of the ascending "reticular activating" system, concerned not only with specific cortical and thalamic regions, especially those associated with the limbic system, but also with the basal ganglia, and visual (particularly oculomotor) mechanisms. Additional links with the habenula-interpeduncular system are discussed in this context.

Interleukin 3 as a trophic factor for central cholinergic neurons in vitro and in vivo

We have found that interleukin 3 (IL-3), a growth factor for hematopoietic cells, is a novel trophic factor for mouse and rat central cholinergic neurons. It enhanced neurite outgrowth and elevated choline acetyltransferase activity. The effect seems to be specific for cholinergic neurons, since somatostatin release and glutamic acid decarboxylase and 2',3'-cyclic nucleotide 3'-phosphodiesterase activities were not significantly influenced by IL-3. In vivo, IL-3 was infused into the lateral ventricles of rats after unilateral axotomy of the septohippocampal pathways. Two weeks later, the IL-3-treated animals showed significant numbers of acetylcholinesterase-positive neurons remaining in the septal region.

Lateral hypothalamus-zona incerta region as an output station for the catalepsy induced by the blockade of striatal D1 and D2 dopamine receptors

Our previous study reported that the blockade of GABAA receptors of the lateral hypothalamus-zona incerta region (LH-ZI) by local injections of bicuculline methiodide inhibited the haloperidol-induced catalepsy. The aim of the present study was to determine (1) whether the blockade of GABAA receptors of the LH-ZI may counteract the catalepsy evoked by SCH 23390 and by sulpiride, and (2) whether the GABAA receptors of the LH-ZI affect the function of the striatal dopaminergic system. Bicuculline methiodide (2.5 and 5 ng/side) injected bilaterally into the LH-ZI inhibited in a dose-dependent manner the catalepsy induced by SCH 23390 administered peripherally (0.2 mg/kg s.c.). SCH 23390 (2 micrograms/side) and sulpiride (1 microgram/side) injected bilaterally into the rostroventral part of the striatum induced potent catalepsy. The catalepsy induced by injection of SCH 23390 (2 micrograms) and sulpiride (1 microgram) into the striatum was inhibited by bicuculline methiodide (2.5 ng and 5 ng) injected into the LH-ZI. Neither bicuculline (5 ng/side) nor muscimol (50 ng/side) injected bilaterally into the LH-ZI changed the levels of dopamine and its intraneuronal metabolite, 3,4-dihydroxyphenyl-acetic acid, or the concentration of noradrenaline and 5-hydroxyindole-acetic acid measured in the striatum and nucleus accumbens by HPLC with an electrochemical detection. It is concluded that GABAA receptors of the LH-ZI are an output station for the catalepsy induced by the blockade of the striatal D2 and D1 dopamine receptors.

Pedunculopontine tegmental nucleus-induced inhibition of muscle activity in the rat

The connections of the pedunculopontine tegmental nucleus (PPN) have led us to propose that this structure mediates striatally induced inhibition of muscle activity by directing basal ganglia output to an inhibitory reticulospinal system (nucleus reticularis gigantocellularis and ventralis, nrGi-V). We conducted experiments in order to examine the effects of electrical stimulation of the PPN on the activity of selected neck and shoulder muscles. PPN stimulation at low rates (0.1 Hz) elicited bilateral muscle excitation. As the rate of stimulation was increased (e.g. to 10 Hz), less excitation was observed. Anodal DC current inactivation of the nrGi-V during concurrent 10 Hz PPN stimulation resulted in an augmentation of muscle activity above the levels observed during 10 Hz PPN stimulation alone. PPN stimulation (10 Hz) also profoundly inhibited cortically-induced muscle activity. Further support for our proposal stems from increased baseline activity (0.1 Hz PPN-induced excitation) in animals with ibotenic acid lesions of the PPN as compared to normal animals. Apparently, destruction of the PPN releases the musculature from tonic and/or phasic inhibition. A model is discussed which attempts to account for both the rate-dependent changes in excitation and the inhibition of cortically induced muscle activity.

Impairment of active avoidance produced after destruction of pedunculopontine nucleus areas in the rat

Pedunculopontine nucleus areas were lesioned bilaterally in the rat by local injection of kainic acid. When rats so treated were tested for active avoidance behavior in shuttle boxes we found that their avoidance acquisition was completely abolished, whereas their intertrial locomotor activity and escape behavior were not affected. In the lesioned rats, choline acetyltransferase activity in the medial thalamus and substantia nigra decreased, but not in the lateral thalamus, hippocampus, or cerebral cortex. These findings suggest that the cholinergic pedunculopontine-medial thalamus projection plays an important role in memory acquisition in the rat.