Excitatory amino acid-induced excitation of dopamine-containing neurons in the rat substantia nigra: modulation by kynurenic acid

D-Serine and D-Cycloserine Reduce Compulsive Alcohol Intake in Rats

There is considerable interest in NMDAR modulators to enhance memory and treat neuropsychiatric disorders such as addiction, depression, and schizophrenia. D-serine and D-cycloserine, the NMDAR activators at the glycine site, are of particular interest because they have been used in humans without serious adverse effects. Interestingly, D-serine also inhibits some NMDARs active at hyperpolarized potentials (HA-NMDARs), and we previously found that HA-NMDARs within the nucleus accumbens core (NAcore) are critical for promoting compulsion-like alcohol drinking, where rats consume alcohol despite pairing with an aversive stimulus such as quinine, a paradigm considered to model compulsive aspects of human alcohol use disorders (AUDs). Here, we examined the impact of D-serine and D-cycloserine on this aversion-resistant alcohol intake (that persists despite adulteration with quinine) and consumption of quinine-free alcohol. Systemic D-serine reduced aversion-resistant alcohol drinking, without altering consumption of quinine-free alcohol or saccharin with or without quinine. Importantly, D-serine within the NAcore but not the dorsolateral striatum also selectively reduced aversion-resistant alcohol drinking. In addition, D-serine inhibited EPSCs evoked at -70 mV in vitro by optogenetic stimulation of mPFC-NAcore terminals in alcohol-drinking rats, similar to reported effects of the NMDAR blocker AP5. Further, D-serine preexposure occluded AP5 inhibition of mPFC-evoked EPSCs, suggesting that D-serine reduced EPSCs by inhibiting HA-NMDARs. Systemic D-cycloserine also selectively reduced intake of quinine-adulterated alcohol, and D-cycloserine inhibited NAcore HA-NMDARs in vitro. Our results indicate that HA-NMDAR modulators can reduce aversion-resistant alcohol drinking, and support testing of D-serine and D-cycloserine as immediately accessible, FDA-approved drugs to treat AUDs.

Psychopharmacological effects of acute exposure to kynurenic acid (KYNA) in zebrafish

A metabolite of the kynurenine pathway, kynurenic acid (KYNA) is an important endogenous neuromodulator and neuroprotector, that also exerts neurotropic effects following exogenous administration. In humans and animals, KYNA regulates affective and cognitive responses, acting mainly as an antagonist of glutamatergic receptors. However, the complete psychopharmacological profile of KYNA (which includes the activity of several neurotransmitter receptors) is poorly understood, and merit further studies. Aquatic models are rapidly emerging as useful tools in translational psychopharmacology research. Here, we exposed adult zebrafish (Danio rerio) to exogenous KYNA for 20 min, and assessed their behavior in the novel tank test. Exposure to KYNA (20 mg/L) in this paradigm evoked overt effects in fish, including decreased latency to enter the top half of the tank, increased number of top entries and longer top duration. In contrast, locomotor activity indices (swimming distance and velocity) were not affected by KYNA in this study. Overall, our results show KYNA has an anxiolytic-like pharmacological effect in zebrafish, and therefore strongly support the utility of zebrafish models in neurotropic drug screening, including drugs acting at central glutamatergic system. Robust phenotypic differences evoked by KYNA, revealed here using three-dimensional (3D) reconstructions of zebrafish locomotion in X, Y and time (Z) coordinates, confirm this notion, also demonstrating the value of 3D-based phenotyping approaches for high-throughput drug screening using zebrafish models.

The possible role of the kynurenine pathway in anhedonia in adolescents

To address the heterogeneous nature of adolescent major depression (MDD), we investigated anhedonia, a core symptom of MDD. We recently reported activation of the kynurenine pathway (KP), a central neuroimmunological pathway which metabolizes tryptophan (TRP) into kynurenine (KYN) en route to several neurotoxins, in a group of highly anhedonic MDD adolescents. In this study, we aimed to extend our prior work and examine the relationship between KP activity and anhedonia, measured quantitatively, in a group of MDD adolescents and in a combined group of MDD and healthy control adolescents. Thirty-six adolescents with MDD (22 medication-free) and 20 controls were included in the analysis. Anhedonia scores were generated based on clinician- and subject-rated assessments and a semi-structured clinician interview. Blood KP metabolites, collected in the AM after an overnight fast, were measured using high-performance liquid chromatography. The rate-limiting enzyme of the KP, indoleamine 2,3-dioxygenase (IDO), was estimated by the ratio of KYN/TRP. Pearson correlation tests were used to assess correlations between anhedonia scores and KP measures while controlling for MDD severity. IDO activity and anhedonia scores were positively correlated in the group psychotropic medication-free adolescents with MDD (r = 0.42, P = 0.05) and in a combined group of MDD subjects and healthy controls (including medicated patients: r = 0.30, P = 0.02; excluding medicated patients: r = 0.44, P = 0.004). In conclusions, our findings provide further support for the role for the KP, particularly IDO, in anhedonia in adolescent MDD. These results emphasize the importance of dimensional approaches in the investigation of psychiatric disorders.

Modulation of midbrain dopamine neurotransmission by serotonin, a versatile interaction between neurotransmitters and significance for antipsychotic drug action

Schizophrenia has been associated with a dysfunction of brain dopamine (DA). This, so called, DA hypothesis has been refined as new insights into the pathophysiology of schizophrenia have emerged. Currently, dysfunction of prefrontocortical glutamatergic and GABAergic projections and dysfunction of serotonin (5-HT) systems are also thought to play a role in the pathophysiology of schizophrenia. Refinements of the DA hypothesis have lead to the emergence of new pharmacological targets for antipsychotic drug development. It was shown that effective antipsychotic drugs with a low liability for inducing extra-pyramidal side-effects have affinities for a range of neurotransmitter receptors in addition to DA receptors, suggesting that a combination of neurotransmitter receptor affinities may be favorable for treatment outcome.This review focuses on the interaction between DA and 5-HT, as most antipsychotics display affinity for 5-HT receptors. We will discuss DA/5-HT interactions at the level of receptors and G protein-coupled potassium channels and consequences for induction of depolarization blockade with specific attention to DA neurons in the ventral tegmental area (VTA) and the substantia nigra zona compacta (SN), neurons implicated in treatment efficacy and the side-effects of schizophrenia, respectively. Moreover, it has been reported that electrophysiological interactions between DA and 5-HT show subtle, but important, differences between the SN and the VTA which could explain (in part) the effectiveness and lower propensity to induce side-effects of the newer atypical antipsychotic drugs. In that respect the functional implications of DA/5-HT interactions for schizophrenia will be discussed.

Basal ganglia control of substantia nigra dopaminergic neurons

Although substantia nigra dopaminergic neurons are spontaneously active both in vivo and in vitro, this activity does not depend on afferent input as these neurons express an endogenous calcium-dependent oscillatory mechanism sufficient to drive action potential generation. However, afferents to these neurons, a large proportion of them GABAergic and arising from other nuclei in the basal ganglia, play a crucial role in modulating the activity of dopaminergic neurons. In the absence of afferent activity or when in brain slices, dopaminergic neurons fire in a very regular, pacemaker-like mode. Phasic activity in GABAergic, glutamatergic, and cholinergic inputs modulates the pacemaker activity into two other modes. The most common is a random firing pattern in which interspike intervals assume a Poisson-like distribution, and a less common pattern, often in response to a conditioned stimulus or a reward in which the neurons fire bursts of 2-8 spikes time-locked to the stimulus. Typically in vivo, all three firing patterns are observed, intermixed, in single nigrostriatal neurons varying over time. Although the precise mechanism(s) underlying the burst are currently the focus of intensive study, it is obvious that bursting must be triggered by afferent inputs. Most of the afferents to substantia nigra pars compacta dopaminergic neurons comprise monosynaptic inputs from GABAergic projection neurons in the ipsilateral neostriatum, the globus pallidus, and the substantia nigra pars reticulata. A smaller fraction of the basal ganglia inputs, something less than 30%, are glutamatergic and arise principally from the ipsilateral subthalamic nucleus and pedunculopontine nucleus. The pedunculopontine nucleus also sends a cholinergic input to nigral dopaminergic neurons. The GABAergic pars reticulata projection neurons also receive inputs from all of these sources, in some cases relaying them disynaptically to the dopaminergic neurons, thereby playing a particularly significant role in setting and/or modulating the firing pattern of the nigrostriatal neurons.

Neuronal activity in the subthalamic nucleus modulates the release of dopamine in the monkey striatum

The primate subthalamic nucleus (STN) is commonly seen as a relay nucleus between the external and internal pallidal segments, and as an input station for cortical and thalamic information into the basal ganglia. In rodents, STN activity is also known to influence neuronal activity in the dopaminergic substantia nigra pars compacta (SNc) through inhibitory and excitatory mono- and polysynaptic pathways. Although the anatomical connections between STN and SNc are not entirely the same in primates as in rodents, the electrophysiologic and microdialysis experiments presented here show directly that this functional interaction can also be demonstrated in primates. In three Rhesus monkeys, extracellular recordings from SNc during microinjections into the STN revealed that transient pharmacologic activation of the STN by the acetylcholine receptor agonist carbachol substantially increased burst firing of single nigral neurons. Transient inactivation of the STN with microinjections of the GABA-A receptor agonist muscimol had the opposite effect. While the firing rates of individual SNc neurons changed in response to the activation or inactivation of the STN, these changes were not consistent across the entire population of SNc cells. Permanent lesions of the STN, produced in two animals with the fiber-sparing neurotoxin ibotenic acid, reduced burst firing and firing rates of SNc neurons, and substantially decreased dopamine levels in the primary recipient area of SNc projections, the striatum, as measured with microdialysis. These results suggest that activity in the primate SNc is prominently influenced by neuronal discharge in the STN, which may thus alter dopamine release in the striatum.

Kynurenines, Parkinson's disease and other neurodegenerative disorders: preclinical and clinical studies

The kynurenine pathway is the main pathway of tryptophan metabolism. L-kynurenine is a central compound of this pathway since it can change to the neuroprotective agent kynurenic acid or to the neurotoxic agent quinolinic acid. The break-up of these endogenous compounds' balance can be observable in many disorders. It can be occur in neurodegenerative disorders, such as Parkinson's disease, Huntington's and Alzheimer's disease, in stroke, in epilepsy, in multiple sclerosis, in amyotrophic lateral sclerosis, and in mental failures, such as schizophrenia and depression. The increase of QUIN concentration or decrease of KYNA concentration could enhance the symptoms of several diseases. According to numerous studies, lowered KYNA level was found in patients with Parkinson's disease. It can be also noticeable that KYNA-treatment prevents against the QUIN-induced lesion of rat striatum in animal experiments. Administrating of KYNA can be appear a promising therapeutic approach, but its use is limited because of its poorly transport across the blood-brain barrier. The solution may be the development of KYNA analogues (e.g. glucoseamine-kynurenic acid) which can pass across this barrier and disengaging in the brain, then KYNA can exert its neuroprotective effects binding at the excitatory glutamate receptors, in particular the NMDA receptors. Furthermore, it seems hopeful to use kynurenine derivatives (e.g. 4-chloro-kynurenine) or enzyme inhibitors (e.g. Ro-61-8048) to ensure an increased kynurenic acid concentration in the central nervous system.

Excitatory and inhibitory responses of dopamine neurons in the ventral tegmental area to nicotine

In the present electrophysiological study the mechanisms by which nicotine activates dopamine neurons in the ventral tegmental area in anesthetized Sprague-Dawley rats were analyzed. Intravenous administration of nicotine caused a dose-dependent increase in firing rate and percentage of spikes fired in bursts of ventral tegmental area dopamine neurons. However, this activation was preceded by an instantaneous but short-lasting inhibition of the firing rate. The excitation of dopamine neurons by nicotine (1.5-400 microg/kg i.v.) was antagonized and even reversed into an inhibitory response by elevated levels (four-fold) of the endogenous glutamate receptor antagonist kynurenic acid, as induced by a potent inhibitor of kynurenine 3-hydroxylase (PNU 156561A, 40 mg/kg, i.v., 5-9 h). The antagonistic action induced by PNU 156561A pretreatment was prevented by administration of D-cycloserine (128 mg/kg, i.v., 5 min). Administration of the GABA(B)-receptor antagonist CGP 35348 (200 mg/kg, i.v., 3 min) facilitated the nicotine-induced increase in burst firing activity of dopamine neurons and antagonized the short-lasting decrease in firing rate by nicotine. The results of the present study show that nicotine produces both inhibition and excitation of ventral tegmental area dopamine neurons, actions that appear to be related to the release of GABA and glutamate, respectively. Whereas the excitatory action of nicotine may be associated with motivational processes underlying learning and cognitive behavior, the inhibitory action of the drug may play a more prominent role in the situation of a profound dysregulation of the mesocorticolimbic dopamine system and may help to explain the high prevalence of tobacco-smoking in schizophrenics.

Differential expression of AMPA receptor subunits in dopamine neurons of the rat brain: a double immunocytochemical study

We have examined the distribution of dopamine neurons expressing alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunits (glutamate receptors 1, 2/3 and 4) in the A8-A15 regions of the rat brain using double immunofluorescence. The distribution of glutamate receptor 1- or 2/3-like immunoreactive neurons completely overlapped that of tyrosine hydroxylase-like immunoreactive neurons in dopamine cell groups in the retrorubral field (A8), the substantia nigra (A9), the ventral tegmental area and the nucleus raphe linealis (A10), and the rostral hypothalamic periventricular nucleus (A14, A15). In the caudal hypothalamic periventricular nucleus (A11), arcuate nucleus (A12) and zona incerta (A13), the distribution was partially overlapping. Neurons double-labeled for tyrosine hydroxylase and glutamate receptor 1 or 2/3 immunoreactivities were, however, exclusively found in certain dopamine cell regions: in areas A14-A15, 85-88% of tyrosine hydroxylase-containing neurons expressed glutamate receptor 1 and 22-25% expressed glutamate receptor 2/3, while in areas A8-A10, 20-43% expressed glutamate receptor 1 and 63-84% expressed glutamate receptor 2/3. In contrast, the double-labeled neurons were hardly detected in the A11-A13 regions. No tyrosine hydroxylase-positive neurons displayed glutamate receptor 4 immunoreactivity, though a partially overlapping distribution of tyrosine hydroxylase- and glutamate receptor 4-immunopositive neurons was also seen in regions A8-10, A11 and A13. The present study has demonstrated the morphological evidence for direct modulation of dopamine neurons via AMPA receptors in rat mesencephalon and hypothalamus. This distribution may provide the basis for a selective dopamine neuron loss in neurodegenerative disorders, such as Parkinson's disease.

Apamin-induced irregular firing in vitro and irregular single-spike firing observed in vivo in dopamine neurons is chaotic

Dopamine neurons of the substantia nigra often fire action potentials irregularly in vivo, but in vitro fire in a regular, pacemaker-like firing pattern. Bath application of apamin, a blocker of calcium-activated potassium channels, can shift a dopamine neuron from pacemaker-like to irregular firing. To determine whether the irregular firing was caused by intrinsic cellular mechanisms rather than random synaptic input or some other form of noise, spike density functions of interspike interval records were analyzed using non-linear forecasting methods to quantify any non-linear (non-periodic) structure. Intrinsic cellular mechanisms are capable of producing chaotic firing, which is deterministic, non-linear, and loses predictability exponentially with increasing forecast time.To determine whether forecasting spike density functions could reliably measure predictability, forecasting was first applied to spike density functions produced by computer simulations of pacemaker-like, chaotic, and random firing, as well as pacemaker-like and chaotic firing that were randomly synaptically driven. Exponential loss of predictability was successfully detected in both chaotic and randomly driven chaotic firing. Predictability scaled faster than exponentially for random spiking, and linearly (slower than exponentially) for randomly driven pacemaker firing. The method was then applied to experimental records of apamin-induced irregular firing of rat dopaminergic neurons of the substantia nigra in vitro and in vivo. Exponential loss of predictability was detected in both cases, consistent with chaotic firing. Experimental records of pacemaker-like firing in vitro showed linear scaling, consistent with a randomly driven pacemaker. Several schemes for neural encoding of synaptic inputs have been suggested, such as rate codes or temporal codes. However, our results suggest that under some conditions, the irregular firing of dopamine neurons does not reflect the random temporal dynamics of its inputs, but rather the intrinsic, deterministic dynamics of dopamine cells themselves, under the tonic neuromodulatory influence of apamin in vitro and possibly that of an unidentified endogenous modulatory substance in vivo.

Afferent modulation of dopamine neuron firing patterns

In recent studies examining the modulation of dopamine (DA) cell firing patterns, particular emphasis has been placed on excitatory afferents from the prefrontal cortex and the subthalamic nucleus. A number of inconsistencies in recently published reports, however, do not support the contention that tonic activation of NMDA receptors is the sole determinate of DA neuronal firing patterns. The results of work on the basic mechanism of DA firing and the action of apamin suggest that excitatory projections to DA neurons from cholinergic and glutamatergic neurons in the tegmental pedunculopontine nucleus, and/or inhibitory GABAergic projections, are also involved in modulating DA neuron firing behavior.

Blockade of SK-type Ca2+-activated K+ channels uncovers a Ca2+-dependent slow afterdepolarization in nigral dopamine neurons

Sharp electrode current-clamp recording techniques were used to characterize the response of nigral dopamine (DA)-containing neurons in rat brain slices to injected current pulses applied in the presence of TTX (2 microM) and under conditions in which apamin-sensitive Ca2+-activated K+ channels were blocked. Addition of apamin (100-300 nM) to perfusion solutions containing TTX blocked the pacemaker oscillation in membrane voltage evoked by depolarizing current pulses and revealed an afterdepolarization (ADP) that appeared as a shoulder on the falling phase of the voltage response. ADP were preceded by a ramp-shaped slow depolarization and followed by an apamin-insensitive hyperpolarizing afterpotential (HAP). Although ADPs were observed in all apamin-treated cells, the duration of the response varied considerably between individual neurons and was strongly potentiated by the addition of TEA (2-3 mM). In the presence of TTX, TEA, and apamin, optimal stimulus parameters (0.1 nA, 200-ms duration at -55 to -68 mV) evoked ADP ranging from 80 to 1,020 ms in duration (355.3 +/- 56.5 ms, n = 16). Both the ramp-shaped slow depolarization and the ensuing ADP were markedly voltage dependent but appeared to be mediated by separate conductance mechanisms. Thus, although bath application of nifedipine (10-30 microM) or low Ca2+, high Mg2+ Ringer blocked the ADP without affecting the ramp potential, equimolar substitution of Co2+ for Ca2+ blocked both components of the voltage response. Nominal Ca2+ Ringer containing Co2+ also blocked the HAP evoked between -55 and -68 mV. We conclude that the ADP elicited in DA neurons after blockade of apamin-sensitive Ca2+-activated K+ channels is mediated by a voltage-dependent, L-type Ca2+ channel and represents a transient form of the regenerative plateau oscillation in membrane potential previously shown to underlie apamin-induced bursting activity. These data provide further support for the notion that modulation of apamin-sensitive Ca2+-activated K+ channels in DA neurons exerts a permissive effect on the conductances that are involved in the expression of phasic activity.

Sodium dynamics underlying burst firing and putative mechanisms for the regulation of the firing pattern in midbrain dopamine neurons: a computational approach

A physiologically based multicompartmental computational model of a midbrain dopamine (DA) neuron, calibrated using data from the literature, was developed and used to test the hypothesis that sodium dynamics drive the generation of a slow oscillation postulated to underlie NMDA-evoked bursting activity in a slice preparation. The full compartmental model was reduced to three compartments and ultimately to two variables, while retaining the biophysical interpretation of all parameters. A phase-plane analysis then suggested two mechanisms for the regulation of the firing pattern: (1) bursting activity is favored by manipulations that enhance the region of negative slope in the whole-cell IV curve and inhibited by those manipulations, such as increasing linear currents, that tend to dampen this region and (2) assuming a region of negative slope is present in the IV curve, the bias of the system can be altered, either enabling or disabling bursting. The model provides a coherent framework for interpreting the effects of glutamate, aspartate, NMDA, and GABA agonists and antagonists under current-clamp conditions, as well as the effects of NMDA and barium under voltage-clamp conditions.

Electrical stimulation of the prefrontal cortex increases cholecystokinin, glutamate, and dopamine release in the nucleus accumbens: an in vivo microdialysis study in freely moving rats

In vivo microdialysis, radioimmunoassay, and HPLC with electrochemical or fluorometric detection were used to investigate the release of cholecystokinin (CCK), glutamate (Glu), and dopamine (DA) in nucleus accumbens septi (NAS) as a function of ipsilateral electrical stimulation of medial prefrontal cortex (mPFC). CCK was progressively elevated by mPFC stimulation at 50-200 Hz. Stimulation-induced CCK release was intensity-dependent at 250-700 microA. NAS Glu and DA levels were each elevated by stimulation at 25-400 Hz; the dopamine metabolites DOPAC and homovanillic acid were increased by stimulation at 100-400 Hz. When rats were trained to lever press for mPFC stimulation, the stimulation induced similar elevations of each of the three transmitters to those seen with experimenter-administered stimulation. Perfusion of 1 mM kynurenic acid (Kyn) into either the ventral tegmental area (VTA) or NAS blocked lever pressing for mPFC stimulation. VTA, but not NAS, perfusion of Kyn significantly attenuated the increases in NAS DA levels induced by mPFC stimulation. Kyn did not affect NAS CCK or Glu levels when perfused into either the VTA or NAS. The present results are consistent with histochemical evidence and provide the first in vivo evidence for the existence of a releasable pool of CCK in the NAS originating from the mPFC. Although dopamine is the transmitter most closely linked to reward function, it was CCK that showed frequency-dependent differences in release corresponding most closely to rewarding efficacy of the stimulation. Although not essential for the reward signal itself, coreleased CCK may modulate the impact of the glutamatergic action in this behavior.

Association between the low threshold calcium spike and activation of NMDA receptors in guinea-pig substantia nigra pars compacta neurons

The aim of this study was to examine the interaction between N-methyl-D-aspartate (NMDA) receptor activation and the low threshold calcium spike (LTS) of phasically firing neurons in the rostral part of the substantia nigra pars compacta (SNpc) in mid-brain slices. Bath perfusion of 10 microM NMDA gradually increased the LTS area and the effect reached a maximum after 6 min of perfusion. This enhancement of the LTS by NMDA was blocked both by a competitive and non-competitive NMDA receptor antagonist, 50 microM D-AP5 and 10 microM MK801, respectively, demonstrating that this effect of NMDA was mediated through NMDA receptors. Prolonged exposure to increasing concentrations of NMDA (0.1-100 microM) progressively decreased the LTS area. The higher doses led to an irreversible marked depolarization and decrease of the membrane resistance. These results suggest that the LTS of SNpc neurons can trigger a NMDA receptor-dependent response which may have physiological and pathological roles.

Modulation of dopamine neuronal activity by glutamate receptor subtypes

In vitro and in vivo electrophysiological studies have been used to assess the effects of glutamate, as well as specific agonists and antagonists for ionotropic, N-methyl-D-aspartate (NMDA), (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate, and metabotropic subtypes of the glutamate receptor, on the neuronal firing activity of midbrain, substantia nigra zona compacta (A9) and ventral tegmental area (A10), dopamine neurons. In in vitro experiments, agonists for all glutamate receptor subtypes depolarize the membrane and increase firing rate. In in vivo experiments, iontophoretic application of these agonists increases the firing rate and induces burst-firing. Studies with subtype selective antagonists suggest that a tonic glutamate tone, acting via NMDA receptors, may modulate the firing activity of some dopamine neurons. Glutamatergic afferents from the subthalamus, pedunculopontine nucleus and frontal cortex can modulate the firing activity of dopamine neurons. The role(s) of the different glutamate receptor subtypes and pathways in mediating the physiological and pathological effects on dopamine systems is an area for further investigation.

Metabotropic glutamate receptor-mediated inhibition and excitation of substantia nigra dopamine neurons

Microiontophoretic drug application and extracellular recording techniques were used to evaluate the effects of the selective metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate(1S,3R-ACPD) on dopamine (DA) neurons in the substantia nigra zona compacta (SNZC) of chloral hydrate-anesthetized rats. 1S,3R-ACPD had a biphasic effect on the firing rate of DA cells, initially decreasing, then increasing the firing rate. 1S,3R-ACPD also increased the burst-firing activity of DA neurons. Application of the ionotropic receptor (iGluR) agonists (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) or N-methyl-D-aspartate (NMDA) increased the firing rates of neurons which had responded to 1S,3R-ACPD, indicating that mGluRs and iGluRs reside on the same neurons. The initial inhibitory period was not antagonized by systemic haloperidol or iontophoretic bicuculline, indicating a lack of DA or gamma-amino-n-butyric acid (GABA) involvement in this effect. Combined application of the AMPA antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX), and the NMDA antagonist, (I)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphoric acid (CPP), at currents which antagonized AMPA and NMDA, did not antagonize either the inhibitory or excitatory effects of 1S,3R-ACPD. Application of the metabotropic antagonist (S)-4-carboxy-phenylglycine antagonized both the inhibitory and excitatory effects of 1S,3R-ACPD. These results indicate that mGluRs may play a role in the modulation of dopaminergic activity in the SNZC.

Spontaneous bursting activity of dopaminergic neurons in midbrain slices from immature rats: role of N-methyl-D-aspartate receptors

Dopamine neurons in midbrain coronal slices from adult rats (40-70 days old) discharged only in pacemaker-like mode. Irregular or bursting mode was never observed. In contrast, dopamine neurons in slices from immature rats (15-21 days old) exhibited not only pacemaker-like firing (53.4% of neurons), but also irregular and bursting patterns (28.3 and 18.3%, respectively). Glutamate and kainate increased the firing rate but failed to induce bursts in dopamine neurons from either adult or immature rats. N-Methyl-D-aspartate augmented the firing rate in all neurons from adult rats and produced a modest increase of bursts in only three out of 18 cells. In slices from immature rats, N-methyl-D-aspartate activated the discharge rate in all neurons and also induced bursts in 37 and 53% of pacemaker and irregular neurons, respectively, and increased the occurrence of spikes in bursts in 76% of spontaneously bursting neurons. The selective N-methyl-D-aspartate receptor antagonist (+/-)2-amino,5-phosphonopentanoic acid prevented N-methyl-D-aspartate-induced changes and also reduced spontaneous bursts, suggesting that bursting discharge is mediated by N-methyl-D-aspartate receptor activation. While pacemaker neurons from immature and from adult rats exhibited the same sensitivity to N-methyl-D-aspartate-induced stimulation of firing rate, spontaneously bursting neurons were more sensitive than pacemaker neurons from either immature or adult rats. The present study indicates that spontaneous bursting, dependent on N-methyl-D-aspartate receptor activation, is present, and may be induced, in dopamine neurons in slices from immature rats. Its absence from cells in slices from adult rats may reflect a reduced sensitivity of N-methyl-D-aspartate receptors on dopamine or the loss of the N-methyl-D-aspartate-activated burst generator.

Phencyclidine increases forebrain monoamine metabolism in rats and monkeys: modulation by the isomers of HA966

The noncompetitive NMDA receptor antagonist phencyclidine (PCP) has psychotomimetic properties in humans and activates the frontal cortical dopamine innervation in rats, findings that have contributed to a hyperdopaminergic hypothesis of schizophrenia. In the present studies, the effects of the enantiomers of 3-amino-1-hydroxypyrrolid-2-one (HA966) on PCP-induced changes in monoamine metabolism in the forebrain of rats and monkeys were examined, because HA966 has been shown previously to attenuate stress- or drug-induced activation of dopamine systems. In rats, PCP (10 mg/kg, i.p.) potently activated dopamine (DA) turnover in the medial prefrontal cortex (PFC) and nucleus accumbens. Serotonin utilization was also increased in PFC. Pretreatment with either R-(+)HA966 (15 mg/kg, i.p.) or S-(-)HA966 (3 mg/kg, i.p.) partially blocked PCP-induced increases in PFC DA turnover, whereas neither enantiomer altered the effect of PCP on DA turnover in the nucleus accumbens or the PCP-induced increases in serotonin turnover in PFC. PCP (0.3 mg/kg, i.m.) exerted regionally selective effects on the dopaminergic and serotonergic innervation of the monkey frontal cortex, effects blocked by pretreatment with S-(-)HA966 (3 mg/kg, i. m.). Importantly, these data demonstrate that in the primate, PCP has potent effects on dopamine transmission in the frontal cortex, a brain region thought to be dysfunctional in schizophrenia. In addition, a role for S-(-)HA966 as a modulator of cortical monoamine transmission in primates is posited.

Spatial distribution of kainate receptor subunit mRNA in the mouse basal ganglia and ventral mesencephalon

In an attempt to gain knowledge of the possible functions of kainate receptors, we have used in situ hybridization to examine the regional and cellular expression patterns of glutamate receptor subunits GluR5-7, KA1 and KA2 in the adult mouse basal ganglia, known to play a pivotal role in the translation of motivation into actions. Kainate receptor subunits were found to be differentially expressed in the circuitry forming the basal ganglia. They differ from each other in expression levels and their spatial localization. GluR6 appeared as the key subunit for the descending gamma-aminobutyric acid (GABA)ergic-glutamatergic pathways, with highest message levels in the caudate putamen, globus pallidus and subthalamic nucleus as well as in the nucleus accumbens and olfactory tubercle. GluR7 exhibited highest expression in the ascending nigrostriatal and mesolimbic dopaminergic neurons. GluR5 had a restricted distribution pattern, with high expression in the ventral pallidum, the islands of Calleja and pars compacta of the substantia nigra. KA2 was usually coexpressed with GluR6, although with a generally lower level of expression. Finally, KA1 mRNA was barely detectable in these neuronal circuits. These data suggest that kainate receptors in general may be involved in the functions associated with the basal ganglia, with a key role in the control of the central dopaminergic transmission. Thus, they might be implicated in the neurodegenerative and psychic disorders associated with an impairment of the basal ganglia.

AMPA and NMDA glutamate receptor subunits in midbrain dopaminergic neurons in the squirrel monkey: an immunohistochemical and in situ hybridization study

The objective of the present study was to analyze the cellular and subcellular localization of ionotropic glutamate receptor subunits in midbrain dopaminergic neurons in the squirrel monkey. This was achieved by means of immunohistochemistry at light and electron microscopic levels and in situ hybridization histochemistry. Colocalization studies show that nearly all dopaminergic neurons in both the ventral and dorsal tiers of the substantia nigra compacta (SNc-v, SNc-d) and the ventral tegmental area (VTA) are immunoreactive for AMPA (GluR1, GluR2/3, and GluR4) and NMDAR1 receptor subunits, but not for NMDAR2A/B subunits. The immunoreactivity of the receptor subunits is associated mainly with perikarya and dendritic shafts. Apart from the intensity of immunolabeling for the GluR4 subunit, which is quite similar for the different groups of midbrain dopaminergic neurons, the overall intensity of immunostaining for the other subunits is higher in the SNc-v and SNc-d than in the VTA. In line with these observations, in situ hybridization shows that the average level of labeling for the GluR2 and NMDAR1 subunit mRNAs is significantly higher in the SNc-v than in the VTA, and for the NMDAR1 subunit, higher in the SNc-v than in the SNc-d. In contrast, no significant difference was found for the level of GluR1 mRNA labeling among the three groups of midbrain dopaminergic neurons. At the subcellular level in the SNc-v, AMPA (GluR1 and GluR2/3) and NMDAR1 receptor subunit immunoreactivity is preferentially associated with the postsynaptic densities of asymmetric synapses, but occasionally some immunoreactivity is found along nonsynaptic portions of plasma membranes of dendrites. A small number of preterminal axons, axon terminals, and glial cell processes are also immunoreactive. Our observations indicate that the different groups of midbrain dopaminergic neurons in primates exhibit a certain degree of heterogeneity with regard to the level of expression of some ionotropic glutamate receptor subunits. The widespread neuronal and glial localization of glutamate receptor subunits suggests that excitatory amino acids may act at different levels to control the basal activity and, possibly, to participate in the degeneration of midbrain dopaminergic neurons in Parkinson's disease.

Synaptic innervation of midbrain dopaminergic neurons by glutamate-enriched terminals in the squirrel monkey

The excitatory amino acid, glutamate, has long been thought to be a transmitter that plays a major role in the control of the firing pattern of midbrain dopaminergic neurons. The present study was aimed at elucidating the anatomical substrate that underlies the functional interaction between glutamatergic afferents and midbrain dopaminergic neurons in the squirrel monkey. To do this, we combined preembedding immunocytochemistry for tyrosine hydroxylase and calbindin D-28k with postembedding immunostaining for glutamate. On the basis of their ultrastructural features, three types (so-called types I, II, and III) of glutamate-enriched terminals were found to form asymmetric synapses with dendrites and perikarya of midbrain dopaminergic neurons. The type I terminals accounted for more than 70% of the total population of glutamate-enriched boutons in contact with dopaminergic cells in the dorsal and ventral tiers of the substantia nigra pars compacta as well as in the ventral tegmental area, whereas 5-20% of the glutamatergic synapses with dopaminergic neurons involved the two other types of terminals. The major finding of our study is that the glutamate-enriched boutons were involved in 70% of the axodendritic synapses in the ventral tegmental area. In contrast, less than 40% of the boutons in contact with dopaminergic dendrites were immunoreactive for glutamate in the dorsal and ventral tiers of the substantia nigra pars compacta. Approximately 50% of the terminals in contact with the perikarya of the different populations of midbrain dopaminergic neurons displayed glutamate immunoreactivity. In conclusion, our findings provide the first evidence that glutamate-enriched terminals form synapses with midbrain dopaminergic neurons in primates. The fact that the proportion of glutamatergic boutons in contact with dopaminergic cells is higher in the ventral tegmental area than in the substantia nigra pars compacta suggests that the different groups of midbrain dopaminergic neurons are modulated differently by extrinsic glutamatergic afferents in primates.

Evidence for N-methyl-D-aspartate and AMPA subtypes of the glutamate receptor on substantia nigra dopamine neurons: possible preferential role for N-methyl-D-aspartate receptors

The present studies utilized extracellular single-unit recordings in chloral hydrate-anesthetized rats to evaluate the contribution of N-methyl-D-aspartate (NMDA) and (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) subtypes of glutamate receptors to the excitatory effects of glutamate on substantia nigra dopamine neurons. Iontophoretic administration of NMDA, AMPA and glutamate increased the firing rate and amount of burst-firing of dopamine neurons. Iontophoretic application of the NMDA antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-l-phosphonic acid (CPP) inhibited the excitatory effect of NMDA and glutamate, but not that of AMPA. Iontophoretic application of the AMPA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)-quinoxaline (NBQX), inhibited the excitatory effect of AMPA and glutamate, but not that of NMDA. CPP produced a greater antagonism of the glutamate excitation than did NBQX. In addition, CPP, but not NBQX, reduced the firing rate and burst-firing of a subpopulation of DA neurons. These data indicate that both NMDA and AMPA receptors are present on substantia nigra dopamine neurons and suggest that NMDA receptors may be more sensitive than AMPA receptors to endogenous glutamate and that a tonic glutamate tone, acting via NMDA receptor stimulation, may modulate the firing rate and burst-firing activity of some dopamine neurons.

N-methyl-D-aspartic acid biphasically regulates the biochemical and electrophysiological response of A10 dopamine neurons in the ventral tegmental area: in vivo microdialysis and in vitro electrophysiological studies

The effects of local perfusion of the ventral tegmental area (VTA) with N-methyl-D-aspartic acid (NMDA) on extracellular dopamine concentrations in the nucleus accumbens were investigated by using in vivo microdialysis in halothane anaesthetized rats. The electrophysiological response of VTA dopamine neurons to NMDA were also assessed in an in vitro rat brain slice preparation. In both preparations NMDA elicited a biphasic response. Exposure of the VTA to low doses of NMDA (< 100 microM) elicited increases in dialysate dopamine levels in the nucleus accumbens and increases in the firing rate of VTA dopamine neurons. Larger doses (> 100 microM) resulted in profound reductions in both dopamine release in the accumbens and firing in the VTA. A strong correlation between the ability of NMDA to influence dopamine release in the accumbens and the firing rate in the VTA was observed. Perfusion with the non-competitive NMDA receptor antagonist PCP eliminated the NMDA-induced increases in extracellular dopamine in the accumbens. These data suggest that dopamine release in the accumbens and the firing rate of dopamine neurons can be both increased or decreased depending upon the magnitude of glutamatergic stimulation within the VTA.

Immunocytochemical localization of the quinolinic acid synthesizing enzyme, 3-hydroxyanthranilic acid oxygenase, in the rat substantia nigra

Quinolinic acid, an endogenous excitatory amino acid receptor agonist, may play a role in several brain diseases. In the present study, the immunocytochemical localization of 3-hydroxyanthranilic acid oxygenase (3HAO), the enzyme responsible for the synthesis of quinolinic acid, was examined in the adult rat substantia nigra at the light and electron microscopic levels. 3HAO-immunoreactivity was detected exclusively in astrocytes. Labeling was present in cell bodies and in fine glial processes, which frequently encircled capillaries and partially enveloped neuronal somata. Notably, 3HAO-labeled processes were in close contact with several types of synaptic profiles. Often, they partially engulfed asymmetric synapses, characteristic of excitatory neurotransmission. In addition, they were found in apposition to putative dopaminergic cell bodies. These data provide an anatomical basis for the idea that functional interactions may occur between glial processes which synthesize quinolinic acid, and synaptic profiles, many of which presumably utilize excitatory neurotransmitters.