Role of the subthalamic nucleus in the regulation of nigral dopamine neuron activity

Impaired glutamate homeostasis and programmed cell death in a chronic MPTP mouse model of Parkinson's disease

The pathogenesis of Parkinson's disease is not fully understood, but there is evidence that excitotoxic mechanisms contribute to the pathology. However, data supporting a role for excitotoxicity in the pathophysiology of the disease are controversial and sparse. The goal of this study was to determine whether changes in glutamate signaling and uptake contribute to the demise of dopaminergic neurons in the substantia nigra. Mice were treated chronically with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and probenecid or vehicle (probenecid or saline alone). Extracellular levels of glutamate in the substantia nigra were substantially increased, and there was an increase in the affinity, but no change in the velocity, of glutamate transport after MPTP/probenecid treatment compared to vehicle controls. In addition, the substantia nigra showed two types of programmed death, apoptosis (type I) and autophagic (type II) cell death. These data suggest that increased glutamate signaling could be an important mechanism for the death of dopaminergic neurons and trigger the induction of programmed cell death in the chronic MPTP/probenecid model.

Evolution of Deep Brain Stimulation: Human Electrometer and Smart Devices Supporting the Next Generation of Therapy

Deep Brain Stimulation (DBS) provides therapeutic benefit for several neuropathologies including Parkinson's disease (PD), epilepsy, chronic pain, and depression. Despite well established clinical efficacy, the mechanism(s) of DBS remains poorly understood. In this review we begin by summarizing the current understanding of the DBS mechanism. Using this knowledge as a framework, we then explore a specific hypothesis regarding DBS of the subthalamic nucleus (STN) for the treatment of PD. This hypothesis states that therapeutic benefit is provided, at least in part, by activation of surviving nigrostriatal dopaminergic neurons, subsequent striatal dopamine release, and resumption of striatal target cell control by dopamine. While highly controversial, we present preliminary data that are consistent with specific predications testing this hypothesis. We additionally propose that developing new technologies, e.g., human electrometer and closed-loop smart devices, for monitoring dopaminergic neurotransmission during STN DBS will further advance this treatment approach.

Physiology of the normal and dopamine-depleted basal ganglia: insights into levodopa pharmacotherapy

Dopamine (DA) neurons exist in two activity states; either spontaneously firing or quiescent and nonfiring. When faced with a behavioral demand, the quiescent DA neurons can be activated to facilitate normal motor output. Levodopa appears to increase DA output by activating these nonfiring neurons; as a consequence, DA release is increased, but behavioral demand can now overwhelm the system, potentially leading to the inactivation and on/off phenomena. Levodopa administered in a pulsatile manner may also lead to the induction of synaptic plasticity within the DA systems. In the ventral mesolimbic system, this could lead to the loss of behavioral flexibility, impulsive behavior, and cognitive impairment, whereas in the dorsal nigrostriatal system, this may underlie Levodopa-induced dyskinesia. Continuous administration of Levodopa may circumvent this sensitization process, enabling a therapeutic response without limbic and motor side effects.

Phenotype, compartmental organization and differential vulnerability of nigral dopaminergic neurons

The degeneration of nigral dopaminergic (DA-) neurons is the histopathologic hallmark of Parkinson's disease (PD), but not all nigral DA-cells show the same susceptibility to degeneration. This starts in DA-cells in the ventrolateral and caudal regions of the susbtantia nigra (SN) and progresses to DA-cells in the dorsomedial and rostral regions of the SN and the ventral tegmental area, where many neurons remain intact until the final stages of the disease. This fact indicates a relationship between the topographic distribution of midbrain DA-cells and their differential vulnerability, and the possibility that this differential vulnerability is associated with phenotypic differences between different subpopulations of nigral DA-cells. Studies carried out during the last two decades have contributed to establishing the existence of different compartments of nigral DA-cells according to their neurochemical profile, and a possible relationship between the expression of some factors and the relative vulnerability or resistance of DA-cell subpopulations to degeneration. These aspects are reviewed and discussed here.

High frequency stimulation and pharmacological inactivation of the subthalamic nucleus reduces 'compulsive' lever-pressing in rats

In recent years there have been several attempts to establish high frequency stimulation (HFS) as an additional treatment strategy for obsessive-compulsive disorder (OCD). Two studies reported that bilateral HFS of the subthalamic nucleus (STN) dramatically alleviated compulsions and improved obsessions in three patients with co-morbid Parkinson's disease and OCD. A recent study reported that HFS as well as pharmacological inactivation of the STN alleviate compulsive checking in the quinpirole rat model of OCD. As the quinpirole model is based on a dopaminergic manipulation, the aim of the present study was to test whether HFS and pharmacological inactivation of the STN exert an anti-compulsive effect also in the drug-naive brain, using the signal attenuation rat model of OCD. The main finding of the present study is that both HFS and pharmacological inactivation of the STN exerted an anti-compulsive effect, although the two manipulations differed in their effects on other behavioral measures. These findings support the possibility that HFS of the STN may provide an additional therapeutic strategy for OCD.

High frequency stimulation and temporary inactivation of the subthalamic nucleus reduce quinpirole-induced compulsive checking behavior in rats

Obsessive-compulsive disorder (OCD) represents a highly prevalent and impairing psychiatric disorder. Functional and structural imaging studies implicate the involvement of basal ganglia-thalamo-cortical circuits in the pathophysiology of this disorder. In patients remaining resistant to pharmaco- and behavioral therapy, modulation of these circuits may consequently reverse clinical symptoms. High frequency stimulation (HFS) of the subthalamic nucleus (STN), an important station of the basal ganglia-thalamo-cortical circuits, has been reported to reduce obsessive-compulsive symptoms in a few Parkinson's disease patients with comorbid OCD. The present study tested the effects of bilateral HFS of the STN and of bilateral pharmacological inactivation of the STN (via intracranial administration of the GABA agonist muscimol) on checking behavior in the quinpirole rat model of OCD. We demonstrate that both HFS and pharmacological inactivation of the STN reduce quinpirole-induced compulsive checking behavior. We conclude that functional inhibition of the STN can alleviate compulsive checking, and suggest the STN as a potential target structure for HFS in the treatment of OCD.

NMDA-mediated release of glutamate and GABA in the subthalamic nucleus is mediated by dopamine: an in vivo microdialysis study in rats

The present study investigated the effects of N-methyl-D-aspartic acid.H2O (NMDA) on the dopamine, glutamate and GABA release in the subthalamic nucleus (STN) by using in vivo microdialysis in rats. NMDA (100 micromol/L) perfused through the microdialysis probe evoked an increase in extracellular dopamine in the STN of the intact rat of about 170%. This coincided with significant increases in both extracellular glutamate (350%) and GABA (250%). The effect of NMDA perfusion on neurotransmitter release at the level of the STN was completely abolished by co-perfusion of the selective NMDA-receptor antagonist MK-801 (10 micromol/L), whereas subthalamic perfusion of MK-801 alone had no effect on extracellular neurotransmitter concentrations. Furthermore, NMDA induced increases in glutamate were abolished by both SCH23390 (8 micromol/L), a selective D1 antagonist, and remoxipride (4 micromol/L), a selective D2 antagonist. The NMDA induced increase in GABA was abolished by remoxipride but not by SCH23390. Perfusion of the STN with SCH23390 or remoxipride alone had no effect on extracellular neurotransmitter concentrations. The observed effects in intact animals depend on the nigral dopaminergic innervation, as dopamine denervation, by means of 6-hydroxydopamine lesioning of the substantia nigra, clearly abolished the effects of NMDA on neurotransmitter release at the level of the STN. Our work points to a complex interaction between dopamine, glutamate and GABA with a crucial role for dopamine at the level of the STN.

Lateral habenula stimulation inhibits rat midbrain dopamine neurons through a GABA(A) receptor-mediated mechanism

Transient changes in the activity of midbrain dopamine neurons encode an error signal that contributes to associative learning. Although considerable attention has been devoted to the mechanisms contributing to phasic increases in dopamine activity, less is known about the origin of the transient cessation in firing accompanying the unexpected loss of a predicted reward. Recent studies suggesting that the lateral habenula (LHb) may contribute to this type of signaling in humans prompted us to evaluate the effects of LHb stimulation on the activity of dopamine and non-dopamine neurons of the anesthetized rat. Single-pulse stimulation of the LHb (0.5 mA, 100 micros) transiently suppressed the activity of 97% of the dopamine neurons recorded in the substantia nigra and ventral tegmental area. The duration of the cessation averaged approximately 85 ms and did not differ between the two regions. Identical stimuli transiently excited 52% of the non-dopamine neurons in the ventral midbrain. Electrolytic lesions of the fasciculus retroflexus blocked the effects of LHb stimulation on dopamine neurons. Local application of bicuculline but not the SK-channel blocker apamin attenuated the effects of LHb stimulation on dopamine cells, indicating that the response is mediated by GABA(A) receptors. These data suggest that LHb-induced suppression of dopamine cell activity is mediated indirectly by orthodromic activation of putative GABAergic neurons in the ventral midbrain. The habenulomesencephalic pathway, which is capable of transiently suppressing the activity of dopamine neurons at a population level, may represent an important component of the circuitry involved in encoding reward expectancy.

High-frequency stimulation of the subthalamic nucleus increases glutamate in the subthalamic nucleus of rats as demonstrated by in vivo enzyme-linked glutamate sensor

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an effective therapy for Parkinson's disease; however, the mechanism whereby DBS ameliorates the symptoms of Parkinson's disease remains an area of intense research. In the present study, we investigated the hypothesis that the neurotransmitter glutamate is released within the STN during high-frequency stimulation (HFS) of the STN. Direct measurements of extracellular glutamate concentration in the STN were made using a dual enzyme-based electrochemical sensor. The studies were carried out in ketamine/xylazine anesthetized rats placed in a Kopf stereotaxic head frame. Various electrical stimulations (100-micros cathodic pulses; 100-3000 microA; 10- to 1000-Hz frequency; 5-s to 60-min stimulus durations) using bipolar stimulating electrodes were delivered to the STN. Stimulation of the STN elevated the concentration of glutamate in the STN. The concentration of glutamate rose quickly during HFS, remained elevated for the duration of stimulation, and descended slowly towards baseline upon cessation of stimulation. Elevation of the extracellular concentration of glutamate in the STN may be an important mechanism whereby DBS in the STN improves the symptoms of Parkinson's disease. Furthermore, our data argue against the hypothesis that DBS works primarily by electrotonic inhibition of the stimulated structure.

The Yin and Yang of dopamine release: a new perspective

Dopamine has undergone extensive investigation due to its known involvement in a number of neurological and psychiatric disorders. In particular, studies into pathological conditions have focused on the roles of high amplitude, phasically evoked dopamine release in regions such as the prefrontal cortex and striatum. However, research has shown that dopamine release can be more complex than just phasic release; thus, there is also a tonic, background dopamine release, with alterations in tonic dopamine release likely having unique and important functional roles. Unfortunately, however, tonic dopamine release has received relatively little attention. In this review, we summarize our recent studies and discuss how modulation of the dopamine system, both in terms of phasic activation and attenuation of tonic dopamine are important for the functions of brain regions receiving this dopamine innervation, and that imbalances in these dopamine release mechanisms may play a significant role in psychiatric disorders such as schizophrenia.

The influence of the subthalamic nucleus upon the damage to the dopamine system following lesions of globus pallidus in rats

Lesioning or stimulating the subthalamic nucleus (STN) in patients with Parkinson's disease, or in animal models of parkinsonism, alleviates many of the symptoms and so it is tempting to think of the STN as a part of the cause of Parkinson's disease. The globus pallidus (GP) is thought to have a tonic inhibitory action on the STN. An ibotenic acid injection into the GP in rats removes the cells of the GP and, over the following 6 weeks, a progressive loss of dopamine cells (counted stereologically in sections stained for tyrosine hydroxylase) develops in substantia nigra (SN). In this investigation we show that, when animals have the STN cells destroyed by very small ibotenic acid injections, their dopamine neurons are not damaged. Furthermore, if a lesion to the GP follows a lesion of STN then the dopamine cells also survive this double insult, at least for the first 3 weeks following the lesion. The experiments provide good reason to suspect that, at least in the short term, increased activity in the STN is a contributory cause of the loss of dopamine cells which follows the lesion of the GP in rats. Whether or not this is part of the mechanism of cell loss in Parkinson's disease, the rats with GP lesions at least provide an opportunity to test strategies that might protect dopamine cells from slowly developing damage. Removing the STN seems to be neuroprotective in this new model of dopamine degeneration.

Regulation of firing of dopaminergic neurons and control of goal-directed behaviors

There are several brain regions that have been implicated in the control of motivated behavior and whose disruption leads to the pathophysiology observed in major psychiatric disorders. These systems include the ventral hippocampus, which is involved in context and focus on tasks, the amygdala, which mediates emotional behavior, and the prefrontal cortex, which modulates activity throughout the limbic system to enable behavioral flexibility. Each of these systems has overlapping projections to the nucleus accumbens, where these inputs are integrated under the modulatory influence of dopamine. Here, we provide a systems-oriented approach to interpreting the function of the dopamine system, its modulation of limbic-cortical interactions and how disruptions within this system might underlie the pathophysiology of schizophrenia and drug abuse.

The involvement of dopamine in the modulation of sleep and waking

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

Synaptic activation of dendritic AMPA and NMDA receptors generates transient high-frequency firing in substantia nigra dopamine neurons in vitro

Transient high-frequency activity of substantia nigra dopamine neurons is critical for striatal synaptic plasticity and associative learning. However, the mechanisms underlying this mode of activity are poorly understood because, in contrast to other rapidly firing neurons, high-frequency activity is not evoked by somatic current injection. Previous studies have suggested that activation of dendritic N-methyl-d-aspartate (NMDA) receptors and/or G-protein-coupled receptor (GPCR)-mediated reduction of action potential afterhyperpolarization and/or activation of cation channels underlie high-frequency activity. To address their relative contribution, transient high-frequency activity was evoked using local electrical stimulation (1 s, 10-100 Hz) in brain slices prepared from p15-p25 rats in the presence of GABA and D2 dopamine receptor antagonists. The frequency, pattern, and morphology of action potentials evoked under these conditions were similar to those observed in vivo. Evoked activity and reductions in action potential afterhyperpolarization were diminished greatly by application of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or NMDA receptor selective antagonists and abolished completely by co-application of AMPA and NMDA antagonists. In contrast, application of glutamatergic and cholinergic GPCR antagonists moderately enhanced evoked activity. Dendritic pressure-pulse application of glutamate evoked high-frequency activity that was similarly sensitive to antagonism of AMPA or NMDA receptors. Taken together, these data suggest that dendritic AMPA and NMDA receptor-mediated synaptic conductances are sufficient to generate transient high-frequency activity in substantia nigra dopamine neurons by rapidly but transiently overwhelming the conductances underlying action potential afterhyperpolarization and/or engaging postsynaptic voltage-dependent ion channels in a manner that overcomes the limiting effects of afterhyperpolarization.

The Dopamine System and the Pathophysiology of Schizophrenia: A Basic Science Perspective

The dopamine system has been a subject of intense investigation due to its role in a number of normal functions and its disruption in pathological conditions. Thus, the dopamine system has been shown to play a major role in cognitive, affective, and motor functions, and its disruption has been proposed to underlie the pathophysiology of several major psychiatric and neurological disorders, including schizophrenia, Parkinson's disease, drug abuse, and attention deficit/hyperactivity disorder. Although these studies have continued to define the basic functional principles of the dopamine system in the mammalian brain, we are still at the initial stages in unraveling the complex role of this transmitter system in regulating behavioral processes. Accumulating evidence suggests that dopamine modulates excitatory and inhibitory neurotransmission, and moreover affects synaptic plasticity induced within the circuits of its target brain regions. It is this role in synaptic plasticity that has associated the dopamine system with aspects of cognitive function involving learning and memory. In this chapter, we summarize recent findings relevant to the role of the dopamine system in psychiatric disorders at cellular, anatomical, and functional levels. In particular, we will focus on the regulation of dopamine neuron activity states and how this impacts dopamine release in cortical and subcortical systems, and the physiological and behavioral impact of dopamine receptor stimulation in the postsynaptic targets of these neurons. A brief summary of recent findings regarding the development and maturation of DA system and how this relates to the pathophysiology of psychiatric disorders are given, and finally models of dopamine system disruption in schizophrenia and how therapeutic approaches impact on dopamine system dynamics is presented.

Predominant surface distribution of neurokinin-3 receptors in non-dopaminergic dendrites in the rat substantia nigra and ventral tegmental area

Neurokinin-3 (NK(3)) receptors are prevalent within the substantia nigra (SN) and ventral tegmental area (VTA), where their activation can affect motor and motivational behaviors as well as cardiovascular function and stress responses. These actions are mediated, in part, by dopaminergic neurons in each region. To determine the relevant sites for activation of these receptors, we examined the electron microscopic localization of NK(3) receptors and tyrosine hydroxylase (TH), the catecholamine synthesizing enzyme in dopaminergic neurons in the SN and VTA of rat brain. In each region, immunogold-silver labeling for NK(3) receptors was detected in many somatodendritic profiles, some of which contained TH-immunoreactivity. NK(3)-immunogold particles were largely associated with endomembranes resembling smooth endoplasmic reticulum, and only occasionally located on the plasma membrane in TH-labeled dendrites. In comparison with these dendrites, non-TH immunoreactive dendrites contained significantly more total (VTA) and more plasmalemmal (VTA and SN) NK(3)-immunogold particles. In each region, NK(3) gold particles also were seen in axonal as well as glial profiles, some of which contacted TH-immunoreactive dendrites. The NK(3)-labeled axon terminals formed either symmetric or asymmetric, excitatory-type synapses, the latter of which were significantly more prevalent in the VTA, compared with SN. These results provide the first ultrastructural evidence indicating that NK(3) receptors are available in cytoplasmic reserve in dopaminergic neurons, but more immediately accessible at the plasmalemmal surface of non-dopaminergic dendrites in both the SN and VTA. The activation of these receptors, together with the NK(3) receptors in either the presynaptic axon terminals or glia may contribute to the diverse physiological effects of tachykinins in each region, and most prominently involving excitatory inputs to the VTA.

Subthalamic nucleus lesions increase impulsive action and decrease impulsive choice - mediation by enhanced incentive motivation?

The subthalamic nucleus (STN) is traditionally thought of as part of a system involved in motor control but recent evidence suggests that it may also play a role in other psychological processes. Here we examined the effects of STN lesions on two measures of impulsivity and found that STN lesions increased 'impulsive action' (produced behavioral disinhibition), as measured by performance on a differential reinforcement of low rates of responding task, but decreased 'impulsive choice' (impulsive decision making), as measured by a delay discounting task. In addition, amphetamine and food restriction increased 'impulsive action' and decreased 'impulsive choice' to a greater extent in STN-lesioned animals than in sham controls. We speculate that these apparently discrepant effects may be because STN lesions enhance the incentive salience assigned to rewards. These findings suggest that the STN may serve as a novel target for the treatment of psychological disorders characterized by deficits in behavioral control, such as drug addiction and attention deficit hyperactivity disorder.

Control of the Subthalamic Innervation of the Rat Globus Pallidus by D2/3 and D4 Dopamine Receptors

The effects of activating dopaminergic D2/3 and D4 receptors during activation of the subthalamic projection to the globus pallidus (GP) were explored in rat brain slices using the whole cell patch-clamp technique. Byocitin labeling and both orthodromic and antidromic activation demonstrated the integrity of some subthalamopallidal connections in in vitro parasagittal brain slices. Excitatory postsynaptic currents (EPSCs) that could be blocked by CNQX and AP5 were evoked onto pallidal neurons by local field stimulation of the subthalamopallidal pathway in the presence of bicuculline. Bath application of dopamine and quinpirole, a dopaminergic D2-class receptor agonist, reduced evoked EPSCs by about 35%. This effect was only partially blocked by sulpiride, a D2/3 receptor antagonist. The sulpiride-sensitive reduction of the subthalamopallidal EPSC was associated with an increase in the paired-pulse ratio (PPR) and a reduction in the frequency but not the mean amplitude of spontaneous EPSCs (sEPSCs), indicative of a presynaptic site of action, which was confirmed by variance–mean analysis. The sulpiride-resistant EPSC reduction was mimicked by PD 168,077 and blocked by L-745,870, selective D4 receptor agonist and antagonist, respectively, suggesting the involvement of D4 receptors. The reduction of EPSCs produced by PD 168,077 was not accompanied by changes in PPR or the frequency of sEPSCs; however, it was accompanied by a reduction in mean sEPSC amplitude, indicative of a postsynaptic site of action. These results show that dopamine modulates subthalamopallidal excitation by presynaptic D2/3 and postsynaptic D4 receptors. The importance of this modulation is discussed.

An increase in AMPA and a decrease in SK conductance increase burst firing by different mechanisms in a model of a dopamine neuron in vivo

A stylized, symmetric, compartmental model of a dopamine neuron in vivo shows how rate and pattern can be modulated either concurrently or differentially. If two or more parameters in the model are varied concurrently, the baseline firing rate and the extent of bursting become de-correlated, which provides an explanation for the lack of a tight correlation in vivo and is consistent with some independence of the mechanisms that generate baseline firing rates versus bursts. We hypothesize that most bursts are triggered by a barrage of synaptic input and that particularly meaningful stimuli recruit larger numbers of synapses in a more synchronous way. An example of concurrent modulation is that increasing the short-lived AMPA current evokes additional spikes without regard to pattern, producing comparable increases in spike frequency and fraction fired in bursts. On the other hand, blocking the SK current evokes additional bursts by allowing a depolarization that previously produced only a single spike to elicit two or more and elongates existing bursts by the same principle, resulting in a greater effect on pattern than rate. A probabilistic algorithm for the random insertion of spikes into the firing pattern produces a good approximation to the pattern changes induced by increasing the AMPA conductance, but not by blocking the SK current, consistent with a differential modulation in the latter case. Furthermore, blocking SK produced a longer burst with a greater intra-burst frequency in response to a simulated meaningful input, suggesting that reduction of this current may augment reward-related responses.

The hippocampus modulates dopamine neuron responsivity by regulating the intensity of phasic neuron activation

Aberrant dopamine (DA) signaling has been advanced as a contributing factor to the pathophysiology of a number of psychiatric conditions including schizophrenia; however, the many factors involved in regulating DA system responsivity have not been completely delineated to date. We have shown previously that DA neuron activity states are independently regulated by distinct afferent pathways. We now provide evidence that these pathways interact to control the population of neurons that are phasically activated. As shown previously, infusions of NMDA into the ventral subiculum (vSub) increases the number of spontaneously active DA neurons (population activity), while having no effect on firing rate or average bursting activity. In contrast, NMDA activation of the pedunculopontine tegmental nucleus (PPTg) results in a significant increase in DA neuron burst firing without significantly affecting population activity. However, simultaneous excitation of the vSub and PPTg induces a significant increase in both DA neuron population activity and burst firing resulting in a approximately 4-fold increase in the number of high-bursting neurons observed per electrode track. These data suggest that DA neuron population activity is not simply associated with the tonic release of DA in forebrain regions, but rather represents a recruitable pool of DA neurons that can be further modulated by excitatory inputs to induce a graded phasic response. Taken as a whole, we propose that the synchronous activity of distinct afferent inputs to the VTA phasically activates selective populations of DA neurons, and hence may be a site of pathological regulation underlying aberrant DA signaling.

Electrophysiological characterization of substantia nigra dopaminergic neurons in partially lesioned rats: Effects of subthalamotomy and levodopa treatment

Progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta is the main histopathological characteristic of Parkinson's disease. We studied the electrophysiological characteristics of the spontaneous activity of substantia nigra pars compacta dopaminergic neurons in rats with a partial, unilateral, 6-hydroxydopamine lesion of the nigrostriatal pathway. In addition, the effects of subthalamotomy and prolonged levodopa treatment on the activity of dopaminergic neurons were investigated. As a result of the lesion ( approximately 50% neuronal loss), the number of spontaneously active neurons was significantly reduced. Basal firing rate, burst firing and responsiveness to intravenously administered apomorphine remained unchanged. In contrast, the variation coefficient, a measure of interspike interval regularity, was significantly increased. Ibotenic acid (10 microg) lesion of the ipsilateral subthalamic nucleus in lesioned rats did not modify the electrophysiological parameters. However, prolonged levodopa treatment (100 mg/kg/day + benserazide 25 mg/kg/day, 14 days) reversed the irregularity observed in cells from lesioned rats, while it induced an irregular firing pattern in cells from intact rats. Our results using an experimental model of moderate Parkinson's disease indicate that surviving substantia nigra pars compacta dopaminergic neurons fire irregularly. In this model, subthalamotomy does not modify the firing pattern while levodopa treatment efficiently restores normal firing of SNpc neurons and does not appear to be toxic to them.

Glycine release in the substantia nigra: Interaction with glutamate and GABA

Previous studies have reported a high number of glycine (GLY) receptors in the substantia nigra (SN) but a low number of GLY-neurons, suggesting that taurine, a partial agonist of GLY-receptors, is the natural substrate for SN GLY-receptors. By using microdialysis to quantify amino acids in the extracellular space of the SN, we observed an extracellular pool of GLY in the rat that increased after depolarizing with high-K+ in a Ca2+-dependent manner and that diffuses through the extracellular space. GLY markedly increased after blocking either the tricarboxylic cycle with fluorocitrate or the glutamine synthetase activity with MSO. Because these products act selectively on glial cells, their effects show glia as a key cell in maintaining the extracellular pool of GLY in the SN. Extracellular GLY was modified by glutamate and glutamate receptor agonists. The local administration of GLY modified the extracellular concentration of GABA. Taken together, the complex regulation of the extracellular level of GLY, its possible glial origin and interaction with glutamate and GABA suggest a volume transmitter role for GLY in the SN, a possibility which also agrees with the recent finding of GLY-transporters in this centre.

Subthalamic nucleus lesions enhance the psychomotor-activating, incentive motivational, and neurobiological effects of cocaine

The subthalamic nucleus (STN) is traditionally thought to be involved in motor control, and dysfunction of the STN is thought to contribute to movement disorders. Here, we show that the STN also plays an important role in motivational processes and the response to drugs of abuse. Specifically, bilateral STN lesions produced a dose-dependent increase in the psychomotor-activating effects of cocaine, the rate at which animals acquired cocaine self-administration, and the motivation for cocaine assessed using a progressive ratio schedule. Furthermore, bilateral STN lesions enhanced the ability of cocaine to induce gene expression in the nucleus accumbens and caudate-putamen, two structures known to be involved in mediating the psychomotor-activating and incentive motivational effects of drugs of abuse. These findings suggest that engagement of the STN serves to dampen the psychomotor-activating and incentive motivational effects of drugs of abuse. Thus, the STN may serve as a novel target for therapeutic interventions aimed at treating drug dependence.

Dopaminergic and GABAergic modulation of glutamate release from rat subthalamic nucleus efferents to the substantia nigra

The regulation of the glutamatergic projection from the subthalamic nucleus (STN) to the substantia nigra (SN) was investigated using dual-probe microdialysis in the awake behaving rat. Reverse dialysis of the cholinergic receptor agonist carbachol (1 mM) into the STN caused an increase in the extracellular concentrations of glutamate and dopamine in the SN. The increase in glutamate was transient and returned toward basal values despite the continued perfusions of the STN with carbachol. Carbachol-stimulated glutamate release was prolonged by perfusion of the selective D2 dopamine receptor antagonist raclopride (100 microM) into the SN and was attenuated by the perfusion of the selective D2-like receptor agonist quinpirole (10 microM). In contrast, perfusion of the D1 dopamine receptor antagonist SCH-23390 (100 microM) did not alter the carbachol-stimulated glutamate release even though it increased basal glutamate concentrations. Perfusion of the GABAA receptor antagonist bicuculline (10 microM) into the SN prolonged the carbachol-stimulated glutamate release in similar fashion as raclopride. The present findings suggest that somatodendritically released dopamine in the SN regulates glutamate release from subthalamic axon terminals by differentially activating dopamine D2 and D1 receptors. Activation of D2 heteroreceptors, located on STN axon terminals, provides a negative feedback control on stimulated subthalamic glutamate release, while D1 receptor activation preferentially regulates basal glutamate concentrations. The findings of the present study also indicate that GABA exerts an inhibitory control on glutamate release in the SN through GABAA receptors.

Dopamine efflux in the rat striatum evoked by electrical stimulation of the subthalamic nucleus: potential mechanism of action in Parkinson's disease

The precise mechanism whereby continuous high-frequency electrical stimulation of the subthalamic nucleus ameliorates motor symptoms of Parkinson's disease is unknown. We examined the effects of high-frequency stimulation of regions dorsal to and within the subthalamic nucleus on dopamine efflux in the striatum of urethane-anaesthetized rats using constant potential amperometry. Complementary extracellular electrophysiological studies determined the activity of subthalamic nucleus neurons in response to similar electrical stimulation of the subthalamic nucleus. High-frequency stimulation of the subthalamic nucleus increased action potential firing in the subthalamic nucleus only during the initial stimulation period and was followed by a cessation of firing over the remainder of stimulation. Electrical stimulation of the subthalamic nucleus with 15 pulses elicited stimulus-time-locked increases in striatal dopamine efflux with maximal peak effects occurring at 50 Hz frequency and 300 microA intensity. Extended subthalamic nucleus stimulation (1000 pulses at 50 Hz; 300 microA) elicited a similar peak increase in striatal dopamine efflux that was followed by a relatively lower steady-state elevation in extracellular dopamine over the course of stimulation. In contrast, extended stimulation immediately adjacent and dorsal to the subthalamic nucleus resulted in an 11-fold greater increase in dopamine efflux that remained elevated over the course of the stimulation. Immunohistochemical staining for tyrosine hydroxylase revealed catecholaminergic fibers running immediately dorsal to and through the subthalamic nucleus. Taken together, these results suggest that enhanced dopamine release within the basal ganglia may be an important mechanism whereby high-frequency stimulation of the subthalamic nucleus improves motor symptoms of Parkinson's disease.

Control of the subthalamic innervation of substantia nigra pars reticulata by D1 and D2 dopamine receptors

The effects of activating dopaminergic D1 and D2 class receptors of the subthalamic projections that innervate the pars reticulata of the subtantia nigra (SNr) were explored in slices of the rat brain using the whole cell patch-clamp technique. Excitatory postsynaptic currents (EPSCs) that could be blocked by 6-cyano-7-nitroquinoxalene-2,3-dione and D-(-)-2-amino-5-phosphonopentanoic acid were evoked onto reticulata GABAergic projection neurons by local field stimulation inside the subthalamic nucleus in the presence of bicuculline. Bath application of (RS)-2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine hydrochloride (SKF-38393), a dopaminergic D1-class receptor agonist, increased evoked EPSCs by approximately 30% whereas the D2-class receptor agonist, trans-(-)-4aR-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-1H-pyrazolo(3,4-g)quinoline (quinpirole), reduced EPSCs by approximately 25%. These apparently opposing actions were blocked by the specific D1- and D2-class receptor antagonists: R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetra-hydro-1H-3-benzazepinehydrochloride (SCH 23390) and S-(-)-5-amino-sulfonyl-N-[(1-ethyl-2-pyrrolidinyl)-methyl]-2-methoxybenzamide (sulpiride), respectively. Both effects were accompanied by changes in the paired-pulse ratio, indicative of a presynaptic site of action. The presynaptic location of dopamine receptors at the subthalamonigral projections was confirmed by mean-variance analysis. The effects of both SKF-38393 and quinpirole could be observed on terminals contacting the same postsynaptic neuron. Sulpiride and SCH 23390 enhanced and reduced the evoked EPSC, respectively, suggesting a constitutive receptor activation probably arising from endogenous dopamine. These data suggest that dopamine presynaptically modulates the subthalamic projection that targets GABAergic neurons of the SNr. Implications of this modulation for basal ganglia function are discussed.

Transient high-frequency firing in a coupled-oscillator model of the mesencephalic dopaminergic neuron

Dopaminergic neurons of the midbrain fire spontaneously at rates <10/s and ordinarily will not exceed this range even when driven with somatic current injection. When driven at higher rates, these cells undergo spike failure through depolarization block. During spontaneous bursting of dopaminergic neurons in vivo, bursts related to reward expectation in behaving animals, and bursts generated by dendritic application of N-methyl-d-aspartate (NMDA) agonists, transient firing attains rates well above this range. We suggest a way such high-frequency firing may occur in response to dendritic NMDA receptor activation. We have extended the coupled oscillator model of the dopaminergic neuron, which represents the soma and dendrites as electrically coupled compartments with different natural spiking frequencies, by addition of dendritic AMPA (voltage-independent) or NMDA (voltage-dependent) synaptic conductance. Both soma and dendrites contain a simplified version of the calcium-potassium mechanism known to be the mechanism for slow spontaneous oscillation and background firing in dopaminergic cells. The compartments differ only in diameter, and this difference is responsible for the difference in natural frequencies. We show that because of its voltage dependence, NMDA receptor activation acts to amplify the effect on the soma of the high-frequency oscillation of the dendrites, which is normally too weak to exert a large influence on the overall oscillation frequency of the neuron. During the high-frequency oscillations that result, sodium inactivation in the soma is removed rapidly after each action potential by the hyperpolarizing influence of the dendritic calcium-dependent potassium current, preventing depolarization block of the spike mechanism, and allowing high-frequency spiking.

Subcellular distribution and plasticity of neurokinin-1 receptors in the rat substantia nigra and ventral tegmental area

Neurokinin-1 receptors show activity-dependent changes in their surface distributions that are critical in spinal pain mechanisms, and also may play an important role in the motor and affective behaviors influenced by dopaminergic projections from the substantia nigra and ventral tegmental area. To determine the relevant sites for neurokinin-1 receptor activation in these midbrain regions, we examined the electron microscopic immunolabeling of neurokinin-1 receptors and the dopamine-synthesizing enzyme, tyrosine hydroxylase in normal rats. We also examined whether neurokinin-1 receptor distributions in one or both regions are affected by (1) startle-evoking intense auditory stimulation or (2) acute administration of apomorphine, a dopamine D1/D2 agonist that enhances startle while paradoxically reducing the prepulse inhibition produced by low intensity conditioning stimuli in rat models of schizophrenia. In each region, neurokinin-1 immunogold was located on the plasma membrane and endomembranes of somatodendritic profiles with or without tyrosine hydroxylase. As compared with controls, animals receiving intense auditory stimulation either alone or together with smaller low intensity prepulses showed a significant increase in neurokinin-1-plasmalemmal labeling in non-dopaminergic dendrites of both regions, and a reduction in this labeling in dopaminergic dendrites of the ventral tegmental area. Both effects were diminished following apomorphine administration. In absence of the intense auditory stimulation, however, apomorphine increased neurokinin-1-immunogold particles on the plasma membrane of the non-dopaminergic dendrites exclusively in the substantia nigra. Our results are the first to show that neurokinin-1 receptors have plasmalemmal distributions in dopaminergic and non-dopaminergic neurons that can be differentially modified by startle-evoking auditory stimulation. They suggest that while apomorphine can independently affect neurokinin-1 receptor trafficking in substantia nigra motor circuits, its effects on neurokinin-1 receptor distributions in the ventral tegmental area are exclusively dependent on sensory activation.

Independent presynaptic and postsynaptic mechanisms regulate endocannabinoid signaling at multiple synapses in the ventral tegmental area

Dopamine (DA) neurons in the ventral tegmental area have been implicated in psychiatric disorders and drug abuse. Understanding the mechanisms through which their activity is regulated via the modulation of afferent input is imperative to understanding their roles in these conditions. Here we demonstrate that endocannabinoids liberated from DA neurons activate cannabinoid CB1 receptors located on glutamatergic axons and on GABAergic terminals targeting GABA(B) receptors located on these cells. Endocannabinoid release was initiated by inhibiting either presynaptic type-III metabotropic glutamate receptors or postsynaptic calcium-activated potassium channels, two conditions that also promote enhanced DA neuron excitability and bursting. Thus, activity-dependent release of endocannabinoids may act as a regulatory feedback mechanism to inhibit synaptic inputs in response to DA neuron bursting, thereby regulating firing patterns that may fine-tune DA release from afferent terminals.

High-frequency stimulation in Parkinson's disease: more or less?

Deep-brain stimulation at high frequency is now considered the most effective neurosurgical therapy for movement disorders. An electrode is chronically implanted in a particular area of the brain and, when continuously stimulated, it significantly alleviates motor symptoms. In Parkinson's disease, common target nuclei of high-frequency stimulation (HFS) are ventral thalamic nuclei and basal ganglia nuclei, such as the internal segment of the pallidum and the subthalamic nucleus (STN), with a preference for the STN in recent years. Two fundamental mechanisms have been proposed to underlie the beneficial effects of HFS: silencing or excitation of STN neurons. Relying on recent experimental data, we suggest that both are instrumental: HFS switches off a pathological disrupted activity in the STN (a 'less' mechanism) and imposes a new type of discharge in the upper gamma-band frequency that is endowed with beneficial effects (a 'more' mechanism). The intrinsic capacity of basal ganglia and particular STN neurons to generate oscillations and shift rapidly from a physiological to a pathogenic pattern is pivotal in the operation of these circuits in health and disease.

Group III metabotropic glutamate-receptor-mediated modulation of excitatory transmission in rodent substantia nigra pars compacta dopamine neurons

Glutamate plays an important role in the regulation of dopamine neuron activity. In particular, the glutamatergic input from the subthalamic nucleus is thought to provide control over dopamine neuron firing patterns. The degeneration of dopamine neurons in the substantia nigra pars compacta (SNc) observed in Parkinson's disease (PD) is believed to be due to a complex interplay of factors, including oxidative stress and mitochondrial dysfunction. Although glutamate is not the primary cause of cell death in PD, there is evidence suggesting excessive glutamate release onto dopamine neurons may play a role in continued degeneration. Although many studies have focused on the role of glutamate in the SNc, little work has been directed at exploring the modulatory control of glutamate release in this region. Previous studies have found a high-potency inhibitory effect of nonselective group III mGluR agonist on glutamatergic transmission in the SNc. Using whole-cell patch-clamp methods and novel pharmacological tools, we have determined that mGluR4 mediates the group III mGluR modulation of excitatory transmission in the rat SNc. The group III mGluR-selective agonist l-(+)-2-amino-4-phosphonobutyric acid inhibits excitatory transmission in the SNc at low micromolar concentrations with a maximal inhibition occurring at 3 muM. This effect was potentiated by the mGluR4-selective allosteric modulator N-phenyl-7-(hydroxymino)cyclopropa[b]chromen-1a-carboxamide and was not mimicked by the mGluR8-selective agonist (S)-3,4-dicarboxyphenylglycine. Interestingly, in an attempt to employ knockout mice to confirm the role of mGluR4, we discovered an apparent species difference suggesting that in mice, both mGluR4 and mGluR8 modulate excitatory transmission in the SNc.

Subthalamic nucleus lesions alter basal and dopamine agonist stimulated electrophysiological output from the rat basal ganglia

The subthalamic nucleus (STN) is an important link in the "indirect" striatal efferent pathway. To assess its role on basal ganglia output via the substantia nigra pars reticulata (SNr), we monitored the single unit activities of SNr neurons in chloral hydrate-anesthetized rats 5-8 days after bilateral kainic acid lesions (0.75 microg/0.3 microl/side) of the STN. Consistent with loss of an excitatory input, the average basal firing rate of SNr neurons was significantly reduced in STN-lesioned animals. Moreover, the lesions modified the responses of SNr neurons to individual and concurrent stimulation of striatal D1 and D2 receptors. Bilateral striatal infusions of the D1/D2 agonist apomorphine (10 microg/microl/side) into the ventral-lateral striatum (VLS) were previously shown to cause significant increases in SNr cell firing (to 133% of baseline) in normal rats. However, in STN-lesioned rats, identical infusions caused no overall change in SNr activity (mean, 103% of basal rates). Conversely, selective stimulation of striatal D2 receptors by bilateral co-infusion of the D2 agonist quinpirole and the D1 antagonist SCH 23390 that previously caused little change in SNr firing in normal rats significantly inhibited their firing in STN-lesioned rats. Finally, the modest excitatory responses of SNr neurons to selective stimulation of striatal D1 receptors by co-infusions of SKF 82958 with the D2 antagonist YM09151-2 were not altered by lesions of the STN. These results implicate the STN as a mediator of excitatory response of SNr neurons to D2, and mixed D1/D2, dopamine receptor agonists in normal rats, and challenge conventional views on the role of the STN and the "indirect" pathway in regulating dopamine-stimulated output from the SNr.

Neurotransmitter release from high-frequency stimulation of the subthalamic nucleus

Object. High-frequency stimulation (HFS) delivered through implanted electrodes in the subthalamic nucleus (STN) has become an established treatment for Parkinson disease (PD). The precise mechanism of action of deep brain stimulation (DBS) in the STN is unknown, however. In the present study, the authors tested the hypothesis that HFS within the STN changes neuronal action potential firing rates during the stimulation period by modifying neurotransmitter release.

Methods. Intracellular electrophysiological recordings were obtained using sharp electrodes in rat STN neurons in an in vitro slice preparation. A concentric bipolar stimulating electrode was placed in the STN slice, and electrical stimulation (pulse width 50–100 µsec, duration 100–2000 µsec, amplitude 10–500 µA, and frequency 10–200 Hz) was delivered while simultaneously obtaining intracellular recordings from an STN neuron.

High-frequency stimulation of the STN either generated excitatory postsynaptic potentials (EPSPs) and increased the action potential frequency or it generated inhibitory postsynaptic potentials and decreased the action potential frequency of neurons within the STN. These effects were blocked after antagonists to glutamate and γ-aminobutyric acid were applied to the tissue slice, indicating that HFS resulted in the release of neurotransmitters. Intracellular recordings from substantia nigra pars compacta (SNc) dopaminergic neurons during HFS of the STN revealed increased generation of EPSPs and increased frequency of action potentials in SNc neurons.

Conclusions. During HFS of STN neurons the mechanism of DBS may involve the release of neurotransmitters rather than the primary electrogenic inhibition of neurons.

Innervation of the substantia nigra

This review describes inputs to neurons in the substantia nigra and contrasts them with the action of agonists for the putative receptors through which they act. Special emphasis is placed on gamma-aminobutyric acid (GABA) afferents. Dopamine released from the somato-dendritic compartment of dopamine neurons and endocannabinoids released from dopamine and GABA neurons serve as retrograde signals to modulate GABA release. The release may be fostered by Ca(2+) release from intracellular Ca(2+) stores, which in turn may be influenced by the inputs.

Extracellular taurine in the substantia nigra: Taurine-glutamate interaction

Taurine has been proposed as an inhibitory transmitter in the substantia nigra (SN), but the mechanisms involved in its release and uptake remain practically unexplored. We studied the extracellular pool of taurine in the rat's SN by using microdialysis methods, paying particular attention to the taurine-glutamate (GLU) interaction. Extracellular taurine increased after cell depolarization with high-K(+) in a Ca(2+)-dependent manner, being modified by the local perfusion of GLU, GLU receptor agonists, and zinc. Nigral administration of taurine increased the extracellular concentration of gamma-aminobutyric acid (GABA) and GLU, the transmitters of the two main inputs of the SN. The modification of the glial metabolism with fluocitrate and L-methionine sulfoximine also changed the extracellular concentration of taurine. The complex regulation of the extracellular pool of taurine, its interaction with GABA and GLU, and the involvement of glial cells in its regulation suggest a volume transmission role for taurine in the SN.

Glutamate receptors and Parkinson's disease: opportunities for intervention

Parkinson's disease is a debilitating neurodegenerative movement disorder that is the result of a degeneration of dopaminergic neurons in the substantia nigra pars compacta. The resulting loss of striatal dopaminergic tone is believed to underlie a series of changes in the circuitry of the basal ganglia that ultimately lead to severe motor disturbances due to excessive basal ganglia outflow. Glutamate plays a central role in the disruption of normal basal ganglia function, and it has been hypothesised that agents acting to restore normal glutamatergic function may provide therapeutic interventions that bypass the severe motor side effects associated with current dopamine replacement strategies. Analysis of the effects of glutamate receptor ligands in the basal ganglia circuit suggests that both ionotropic and metabotropic glutamate receptors could have antiparkinsonian actions. In particular, NMDA receptor antagonists that selectively target the NR2B subunit and antagonists of the metabotropic glutamate receptor mGluR5 appear to hold promise and deserve future attention.

Ablation of the subthalamic nucleus protects dopaminergic phenotype but not cell survival in a rat model of Parkinson's disease

Inhibition or ablation of the hyperactive subthalamic nucleus (STN) in Parkinson's disease (PD) does not only reverse motor deficits, silencing the glutamatergic output of the subthalamic nucleus, but has also been implicated to have neuroprotective effects on nigral neurons in animal models of Parkinson's disease. Ablation of the subthalamic nucleus has been shown to increase the number of tyrosinhydroxylase-immunopositive cells and partially restores behavioral deficits in animal models of Parkinson's disease. However, it is unclear whether subthalamic nucleus ablation indeed prevents cell death or whether the effect is due to the rescue of the dopaminergic (DA) phenotype of impaired cells by upregulating tyrosine hydroxylase (TH). We therefore investigated the potential neuroprotective effects of a preceding subthalamic nucleus lesion on 6-hydroxydopamine (6-OHDA)-induced nigral cell death and compared the retrograde tracer fluorogold (FG) as a marker of cell survival with tyrosinhydroxylase immunoreactivity as a marker of the dopaminergic phenotype. In the present study, we show that ablation of the subthalamic nucleus does not affect the number of fluorogold-labeled cells but increases the number of tyrosinhydroxylase-positive neurons in subthalamic nucleus-lesioned hemiparkinsonian animals and leads to partial behavioral recovery of the rats. We conclude that subthalamic nucleus ablation exerts neuroprotective properties on the dopaminergic nigrostriatal pathway against 6-hydroxydopamine toxicity in terms of rescuing the neurotransmitter phenotype in the remaining neurons rather than enhancing the total number of nigral cells.

Electrophysiological Interactions between Striatal Glutamatergic and Dopaminergic Systems

Glutamatergic and dopaminergic systems play a primary role in frontal-subcortical circuits involved in motor and cognitive functions. Considerable evidence has emerged indicating that the complex interaction between these neurotransmitter systems within the dorsal striatum and nucleus accumbens is critically involved in the gating of information flow in these highly integrative brain regions. As a result, disruptions of the interaction between glutamate and dopamine has been proposed as a pathological basis for a number of disorders, including the pathophysiology of schizophrenia. In this chapter, we discuss recent studies that have significantly advanced our understanding of the reciprocal interactions between glutamatergic and dopaminergic systems within the striatal complex in the normal brain and in pathological states.

Relative involvement of globus pallidus and subthalamic nucleus in the regulation of somatodendritic dopamine release in substantia nigra is dopamine-dependent

Previously, we have shown that GABA(A) receptors and glutamate receptors in substantia nigra play distinct roles in the regulation of somatodendritic dopamine release. GABAergic input to substantia nigra was found to be the primary determinant of the level of spontaneous somatodendritic dopamine release. In contrast, acute blockade of dopamine receptors by systemic haloperidol administration produced an increase in somatodendritic dopamine release in substantia nigra that was found to be dependent exclusively upon activation of nigral glutamate receptors. The focus of the present study was to identify anatomical structures that may participate in the differential regulation of somatodendritic dopamine release by GABA and glutamate under these two conditions. To this end, we pharmacologically inhibited the activity of either globus pallidus or subthalamic nucleus using microinfusion of the GABA(A) receptor agonist muscimol. The effects of these manipulations on spontaneous efflux of somatodendritic dopamine and on increases in this measure produced by systemic haloperidol administration were determined in ipsilateral substantia nigra using in vivo microdialysis. As observed previously, administration of haloperidol (0.5 mg/kg, i.p.) significantly increased extracellular dopamine in substantia nigra. Microinfusion of muscimol (400 ng/200 nl) into globus pallidus also produced a significant increase in somatodendritic dopamine efflux. When haloperidol was administered systemically in conjunction with microinfusion of muscimol into globus pallidus, an increase in nigral dopamine efflux was observed that was significantly greater than that which was produced singly by muscimol microinfusion into globus pallidus or by systemic haloperidol administration. The additive nature of the increases in somatodendritic dopamine release produced by these two manipulations indicates that independent neural circuitries may be involved. Inactivation of subthalamic nucleus by microinfusion of muscimol (200 ng/100 nl) had no effect on spontaneous somatodendritic dopamine efflux. Muscimol application into subthalamic nucleus, however, completely abolished the stimulatory effect of systemic haloperidol on dendritic dopamine efflux in substantia nigra. The present data extend our previous findings by demonstrating: 1) an important involvement of globus pallidus efferents in the GABAergic regulation of somatodendritic dopamine efflux in substantia nigra under normal conditions and, 2) an emergent predominant role of subthalamic nucleus efferents in the glutamate-dependent increase in somatodendritic dopamine efflux observed after systemic haloperidol administration. Thus, the relative influence of globus pallidus and subthalamic nucleus in the determination of the level of somatodendritic dopamine release in substantia nigra qualitatively varies as a function of dopamine receptor blockade. These findings are relevant to current models of basal ganglia function under both normal and pathological conditions, e.g. Parkinson's disease.

Fulfenamic acid sensitive, Ca(2+)-dependent inward current induced by nicotinic acetylcholine receptors in dopamine neurons

Nicotinic acetylcholine receptors (nAChRs) exhibit high Ca(2+) permeabilities and the Ca(2+)-influx through the nAChRs may be involved in regulation of a variety of signal processing in the postsynaptic neurons. The mesencephalic dopamine (DA) neurons receive cholinergic inputs from the brainstem and express abundant nAChRs. Here we report that the Ca(2+)-influx induced by a transient pressure application of ACh activates an inward current mediated by nAChRs and subsequently an inward current component that is sensitive to fulfenamic acid (FFA) and phenytoin, presumably a Ca(2+)-activated nonselective cation current in the DA neurons in the midbrain slices of the rat. The FFA- and phenytoin-sensitive current exhibits a negative slope conductance below -40 mV, suggesting its role in significant enhancement of depolarizing responses. In the current clamp recordings with perforated patch clamp configuration, bath application of carbachol markedly enhanced the glutamate-induced depolarization, which led to a long-lasting depolarizing hump. Activation of nAChRs is involved in this process, in cooperation with muscarinic receptors that suppress afterhyperpolarization caused by Ca(2+)-activated K(+)-channels. The long-lasting depolarizing hump was suppressed by FFA. All these results suggested a potential role of the FFA-sensitive current triggered by nAChR activation in marked enhancement of the excitatory synaptic response in DA neurons.

Cocaine-induced psychomotor activity is associated with its ability to induce c-fos mRNA expression in the subthalamic nucleus: effects of dose and repeated treatment

Factors that modulate the psychomotor activating effects of amphetamine and cocaine, such as environmental novelty and dose, also regulate the ability of these drugs to induce c-fos mRNA expression in the subthalamic nucleus (STN). We hypothesized therefore that engagement of the STN may be important for stimulant-induced psychomotor activation. To further test this hypothesis we examined whether repeated treatment with cocaine, which enhances its psychomotor activating effects (i.e. produces behavioural sensitization), also enhances its ability to induce c-fos expression in the STN. In addition, given that STN activity is thought to be influenced by preproenkephalin mRNA-containing (ENK+) neurons in the caudate-putamen, we also examined whether repeated cocaine treatment alters c-fos expression in ENK+ cells. We report that: (i) cocaine pretreatment enhances the ability of a cocaine challenge to induce c-fos mRNA expression in the STN, and this effect is most robust at challenge doses where behavioural sensitization is observed; (ii) the ability of cocaine to induce c-fos in the STN is independent of the ability of cocaine to engage ENK+ cells. These results support the idea that the STN is involved in stimulant-induced psychomotor activation and sensitization, but suggest that stimulant-induced engagement of the STN is not dependent on ENK+ cells in the caudate-putamen. These findings may have implications concerning the neurobiological mechanisms underlying the behavioural effects of psychostimulant drugs.

Amphetamine-induced c-fos mRNA expression in the caudate-putamen and subthalamic nucleus: interactions between dose, environment, and neuronal phenotype

When administered in a novel environment relatively low doses of amphetamine induce c-fos mRNA in the subthalamic nucleus (STN) and in preproenkephalin mRNA-containing (ENK+) neurons in the caudate-putamen (CPu). When administered at home, however, low doses of amphetamine do not produce these effects. Environmental novelty also facilitates the behavioral effects of acute and repeated amphetamine, but this is dose-dependent. The purpose of the present experiment therefore was to determine if the effect of context on amphetamine-induced c-fos expression is also dose-dependent. It was found that: (i) No dose of amphetamine tested (1-10 mg/kg) induced c-fos in many ENK+ cells when given at home. (ii) When given in a novel environment low to moderate doses of amphetamine (1-5 mg/kg) induced c-fos in substantial numbers of ENK+ cells, but the highest dose examined (10 mg/kg) did not. (iii) Environmental novelty enhanced the ability of low to moderate doses of amphetamine to induce c-fos in the STN, but the highest dose of amphetamine induced robust c-fos mRNA expression in the STN regardless of context. The results do not support the idea that engaging ENK+ cells, at least as indicated by c-fos mRNA expression, is critical to produce robust behavioral sensitization, but do suggest a possible role for the STN. Furthermore, the results highlight the importance of drug-environment interactions on the neurobiological effects of drugs, and have implications for thinking about the circuits by which context modulates the acute and long-lasting consequences of amphetamine treatment.

Phasic activation of substantia nigra and the ventral tegmental area by chemical stimulation of the superior colliculus: an electrophysiological investigation in the rat

The source of short-latency visual input to midbrain dopaminergic (DA) neurons is not currently known; however, the superior colliculus (SC) is a subcortical visual structure which has response latencies consistently shorter than those recorded for DA neurons in substantia nigra and the ventral tegmental area. To test whether the SC represents a plausible route by which visual information may gain short latency access to the ventral midbrain, the present study examined whether experimental stimulation of the SC can influence the activity of midbrain DA neurons. In urethane-anaesthetized rats, 63 pairs of extracellular recordings were obtained from neurons in the SC and ipsilateral ventral midbrain, before and after local disinhibitory injections of the GABA antagonist bicuculline (20-40 ng/200-400 nL saline) into the SC. Neurons recorded from substantia nigra and the ventral tegmental area were classified as putative DA (25/63, 39.7%) or putative non-DA (38/63, 60.3%). In nearly half the cases (27/63, 42.8%), chemical stimulation of the SC evoked a corresponding increase in neural activity in the ventral midbrain. This excitatory effect did not distinguish between DA and non-DA neurons. In 6/63 cases (9.5%), SC activation elicited a reliable suppression of activity, while the remaining 30/63 cases (47.6%) were unaffected. In almost a third of cases (16/57, 28.1%) intense phasic activation of the SC was associated with correlated phasic activation of neurons in substantia nigra and the ventral tegmental area. These data suggest that the SC is in a position to play an important role in discriminating the appropriate stimulus qualities required to activate DA cells at short latency.

Characterization of pre- and post-synaptic metabotropic glutamate receptor-mediated inhibitory responses in substantia nigra dopamine neurons

Two inhibitory responses mediated by both pre- and post-synaptic metabotropic glutamate receptors (mGluRs) were investigated in dopamine neurons of the substantia nigra using whole-cell patch recordings. (2R,4R)-APDC, a group II mGluR agonist, and L-2-amino-4-phosphonobutyrate (L-AP4), a group III mGluR agonist, reversibly suppressed the amplitude of excitatory postsynaptic currents (EPSCs). However, (S)-3,5-DHPG, a group I mGluR agonist, exhibited less inhibitory action on the EPSCs. LY341495, a highly potent group II mGluR antagonist, antagonized the broad spectrum mGluR agonist, 1S,3R-ACPD-induced suppression of EPSCs. In acutely dissociated dopamine neurons, glutamate (Glu) in the presence of CNQX and AP-5 evoked an outward current accompanied by an increase in K(+) conductance. (S)-3,5-DHPG, but not (2R,4R)-APDC or L-AP4, also induced an outward current. Glu-induced outward current (I(Glu-out)) was partially inhibited by LY367385, a selective mGluR1 antagonist, but not by MPEP, a selective mGluR5 antagonist. Ryanodine and cyclopiazonic acid blocked the I(Glu-out). In the presence of caffeine, Glu failed to induce a current. Charybdotoxin, but not apamin or iberiotoxin, inhibited the I(Glu-out). Taken together, both group II and III mGluRs are mainly involved in the presynaptic inhibition of Glu release to dopamine neurons, while group I mGluRs, including at least mGluR1, participate in the hyperpolarization of dopamine neurons mediated by the opening of charybdotoxin-sensitive Ca(2+)-activated K(+) channels.

Firing modes of midbrain dopamine cells in the freely moving rat

There is a large body of data on the firing properties of dopamine cells in anaesthetised rats or rat brain slices. However, the extent to which these data relate to more natural conditions is uncertain, as there is little quantitative information available on the firing properties of these cells in freely moving rats. We examined this by recording from the midbrain dopamine cell fields using chronically implanted microwire electrodes. (1) In most cases, slowly firing cells with broad action potentials were profoundly inhibited by the dopamine agonist apomorphine, consistent with previously accepted criteria. However, a small group of cells was found that were difficult to classify because of ambiguous combinations of properties. (2) Presumed dopamine cells could be divided into low and high bursting (>40% of their spikes in bursts) groups, with the majority having low bursting rates. The distribution of burst incidence was similar to that previously reported with chloral hydrate anaesthesia, but the average intraburst frequency was higher in the conscious animal at rest and was higher again in bursts triggered by salient stimuli. (3) There was no evidence for spike frequency adaptation within bursts on average, consistent with the hypothesis that afterhyperpolarisation currents may be disabled during behaviourally induced bursting. (4) Presumed dopamine cells responded to reward-related stimuli with increased bursting rates and significantly higher intraburst frequencies compared to bursts emitted outside task context, indicating that modulation of afferent activity might not only trigger bursting, but may also regulate burst intensity. (5) In addition to the irregular single spike and bursting modes we found that extremely regular (clock-like) firing, previously only described for dopamine cells in reduced preparations, can also be expressed in the freely moving animal. (6) Cross-correlation analysis of activity recorded from simultaneously recorded neurones revealed coordinated activity in a quarter of dopamine cell pairs consistent with at least "functional" connectivity. On the other hand, most dopamine cell pairs showed no correlation, leaving open the possibility of functional sub-groupings within the dopamine cell fields. Taken together, the data suggest that the basic firing modes described for dopamine cells in reduced or anaesthetised preparations do reflect natural patterns of activity for these neurones, but also that the details of this activity are dependent upon modulation of afferent inputs by behavioural stimuli.

A dual probe characterization of dialysate amino acid levels in the medial prefrontal cortex and ventral tegmental area of the awake freely moving rat

Dual probe microdialysis was employed to characterize the origins of dialysate glutamate, aspartate and gamma-aminobutyric acid (GABA) in the medial prefrontal cortex (mPfc) and to investigate functional interactions between the mPfc and ventral tegmental area (VTA) in awake, freely moving rats. Perfusion with elevated potassium (K(+); KCl, 100 mM, 20 min), low Ca(2+) (0.1 mM, 60 min) or tetrodotoxin (TTX, 10 microM, 100 min) was performed in the mPfc and dialysate levels of glutamate, aspartate and GABA were measured locally and in the VTA. Elevated K(+) in the mPfc rapidly increased dialysate glutamate and aspartate locally (+90+/-10 and +41+/-9% from basal, respectively) and in the VTA (+71+/-14 and +42+/-14%, respectively). MPfc GABA was also rapidly increased (+241+/-62%) while VTA GABA was not affected. Perfusion with low Ca(2+) in the mPfc decreased local glutamate, aspartate and GABA (-26+/-8; -35+/-7 and -45+/-8%, respectively) and decreased only GABA (-40+/-5%) in the VTA. Intra-mPfc TTX increased glutamate and aspartate locally (+82+/-23 and +54+/-27%, respectively) and in the VTA (+84+/-18 and +38+/-17%, respectively). In contrast, intra-mPfc TTX decreased local GABA (-33+6%) while VTA GABA levels were not affected. Taken together, these data confirm the influence of the mPfc upon the ipsilateral VTA and provide evidence for two neuronal pools which contribute to basal extracellular mPfc and VTA glutamate, aspartate and GABA levels, the first pool derived from Na(+)- and Ca(2+)-dependent release and the second derived from voltage-dependent reuptake.

Synaptic regulation of somatodendritic dopamine release by glutamate and GABA differs between substantia nigra and ventral tegmental area

Midbrain dopamine (DA) cells of the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) exhibit somatodendritic release of DA. To address how somatodendritic release is regulated by synaptic glutamatergic and GABAergic input, we examined the effect of ionotropic-receptor antagonists on locally evoked extracellular DA concentration ([DA]o) in guinea pig midbrain slices. Evoked [DA]o was monitored with carbon-fiber microelectrodes and fast-scan cyclic voltammetry. In SNc, evoked [DA]o was 160% of control in the presence of the AMPA-receptor antagonist, GYKI-52466, or the NMDA-receptor antagonist, AP5. Similar increases were seen with the GABAA-receptor antagonist, picrotoxin, or the GABA(B)-receptor antagonist, saclofen. The increase seen with GYKI-52466 was prevented when both picrotoxin and saclofen were present, consistent with normal, AMPA-receptor mediated activation of GABAergic inhibition. The increase with AP5 persisted, however, implicating NMDA-receptor mediated activation of another inhibitory circuit in SNc. In the VTA, by contrast, evoked [DA]o was unaffected by GYKI-52466 and fell slightly with AP5. Neither picrotoxin nor saclofen alone or in combination had a significant effect on evoked [DA]o. When GABA receptors were blocked in the VTA, evoked [DA]o was decreased by 20% with either GYKI-52466 or AP5. These data suggest that in SNc, glutamatergic input acts predominantly on GABAergic or other inhibitory circuits to inhibit somatodendritic DA release, whereas in VTA, the timing or strength of synaptic input will govern whether the net effect on DA release is excitatory or inhibitory.

Distinct roles for nigral GABA and glutamate receptors in the regulation of dendritic dopamine release under normal conditions and in response to systemic haloperidol

The regulation of dendritic dopamine release in the substantia nigra (SN) likely involves multiple mechanisms. GABA and glutamate inputs to nigrostriatal dopamine neurons exert powerful influences on dopamine neuron physiology; therefore, it is probable that GABA and glutamate likewise influence dendritic dopamine release, at least under some conditions. The present studies used in vivo microdialysis to determine the potential roles of nigral GABA and glutamate receptors in the regulation of dendritic dopamine release under normal conditions and when dopamine signaling in the basal ganglia is compromised after systemic haloperidol administration. Nigral application of the GABA(A) receptor antagonist bicuculline by reverse dialysis significantly increased spontaneous dopamine efflux in the SN. However, spontaneous dopamine efflux in the SN was not significantly affected by local application of the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione or (+/-)-3-[2-carboxypiperazine-4-yl]-propyl-1-phosphonic acid. Systemic haloperidol administration significantly increased the extracellular dopamine measured in the SN. Blockade of nigral GABA(A) receptors by local bicuculline application did not alter this effect of systemic haloperidol, despite the bicuculline-induced increase in spontaneous dendritic dopamine efflux. In contrast, nigral application of either glutamate receptor antagonist significantly attenuated the increases in dendritic dopamine efflux elicited by systemic haloperidol. These data suggest that under normal conditions, activity of GABA afferents to SN dopamine neurons is an important determinant of the spontaneous level of dendritic dopamine release. Circuit-level changes in the basal ganglia involving an increased glutamatergic drive to the SN appear to underlie the increase in dendritic dopamine release that occurs in response to systemic haloperidol administration.