In vivo release of endogenous amino acids from the rat striatum: further evidence for a role of glutamate and aspartate in corticostriatal neurotransmission

Social isolation of adolescent male rats increases anxiety and K+ -induced dopamine release in the nucleus accumbens: Role of CRF-R1

Early life adversity can disrupt development leading to emotional and cognitive disorders. This study investigated the effects of social isolation after weaning on anxiety, body weight and locomotion, and on extracellular dopamine (DA) and glutamate (GLU) in the nucleus accumbens (NAc) and their modulation by corticotropin releasing factor receptor 1. On the day of weaning, male rats were housed singly or in groups for 10 consecutive days. Anxiety-like behaviors were assessed by an elevated plus maze (EPM) and an open field test (OF). Neurotransmitter levels were measured by in vivo microdialysis. Single-housed rats spent less time, and entered more, into the closed arms of an EPM than group-housed rats. They also spent less time in the center of an OF, weighed more and showed greater locomotion. In the NAc, no differences in CRF, or in basal extracellular DA or GLU between groups, were observed. A depolarizing stimulus increased DA release in both groups but to higher levels in isolated rats, whereas GLU increased only in single-housed rats. Blocking CRF-R1 receptors with CP-154,526 decreased DA release in single-housed but not in group-housed rats. The corticotropin releasing factor receptor type 1 receptor antagonist also decreased GLU in group-housed animals. These results show that isolating adolescent rats increases anxiety, body weight and ambulation, as well as the sensitivity of dopaminergic neurons to a depolarizing stimulus. This study provides further evidence of the detrimental effects of social isolation during early development and indicates that dysregulation of the CRF system in the NAc may contribute to the pathologies observed.

A Review of Neurotransmitters Sensing Methods for Neuro-Engineering Research

Neurotransmitters as electrochemical signaling molecules are essential for proper brain function and their dysfunction is involved in several mental disorders. Therefore, the accurate detection and monitoring of these substances are crucial in brain studies. Neurotransmitters are present in the nervous system at very low concentrations, and they mixed with many other biochemical molecules and minerals, thus making their selective detection and measurement difficult. Although numerous techniques to do so have been proposed in the literature, neurotransmitter monitoring in the brain is still a challenge and the subject of ongoing research. This article reviews the current advances and trends in neurotransmitters detection techniques, including in vivo sampling and imaging techniques, electrochemical and nano-object sensing techniques for in vitro and in vivo detection, as well as spectrometric, analytical and derivatization-based methods mainly used for in vitro research. The document analyzes the strengths and weaknesses of each method, with the aim to offer selection guidelines for neuro-engineering research.

In Vivo Chemical Monitoring at High Spatiotemporal Resolution Using Microfabricated Sampling Probes and Droplet-Based Microfluidics Coupled to Mass Spectrometry

An essential approach for in vivo chemical monitoring is to use sampling probes coupled with analytical methods; however, this method traditionally has limited spatial and temporal resolution. To address this problem, we developed an analytical system that combines microfabricated push-pull sampling probes with droplet-based microfluidics. The microfabricated probe provides spatial resolution approximately 1000-fold better than that of common microdialysis probes. Microfabrication also facilitated integration of an extra channel into the probe for microinjection. We created microfluidic devices and interfaces that allowed manipulation of nanoliter droplet samples collected from the microfabricated probe at intervals of a few seconds. Use of droplet-based microfluidics prevented broadening of collected zones, yielding 6 s temporal resolution at 100 nL/min perfusion rates. Resulting droplets were analyzed by direct infusion nanoelectrospray ionization (nESI) mass spectrometry for simultaneous determination of glutamine, glutamate, γ-aminobutyric acid, and acetylcholine. Use of low infusion rates that enabled nESI (50 nL/min) was critical to allowing detection in the complex samples. Addition of ¹³C-labeled internal standards to the droplet samples was used for improved quantification. Utility of the overall system was demonstrated by monitoring dynamic chemical changes evoked by microinjection of high potassium concentrations into the brain of live rats. The results showed stimulated neurochemical release with rise times of 15 s. This work demonstrates the potential of coupling microfabricated sampling probes to droplet-based mass spectrometric assays for studying chemical dynamics in a complex microenvironment at high spatiotemporal resolution.

Neuronal activity mediated regulation of glutamate transporter GLT-1 surface diffusion in rat astrocytes in dissociated and slice cultures

The astrocytic GLT‐1 (or EAAT2) is the major glutamate transporter for clearing synaptic glutamate. While the diffusion dynamics of neurotransmitter receptors at the neuronal surface are well understood, far less is known regarding the surface trafficking of transporters in subcellular domains of the astrocyte membrane. Here, we have used live‐cell imaging to study the mechanisms regulating GLT‐1 surface diffusion in astrocytes in dissociated and brain slice cultures. Using GFP‐time lapse imaging, we show that GLT‐1 forms stable clusters that are dispersed rapidly and reversibly upon glutamate treatment in a transporter activity‐dependent manner. Fluorescence recovery after photobleaching and single particle tracking using quantum dots revealed that clustered GLT‐1 is more stable than diffuse GLT‐1 and that glutamate increases GLT‐1 surface diffusion in the astrocyte membrane. Interestingly, the two main GLT‐1 isoforms expressed in the brain, GLT‐1a and GLT‐1b, are both found to be stabilized opposed to synapses under basal conditions, with GLT‐1b more so. GLT‐1 surface mobility is increased in proximity to activated synapses and alterations of neuronal activity can bidirectionally modulate the dynamics of both GLT‐1 isoforms. Altogether, these data reveal that astrocytic GLT‐1 surface mobility, via its transport activity, is modulated during neuronal firing, which may be a key process for shaping glutamate clearance and glutamatergic synaptic transmission. GLIA 2016;64:1252–1264

Vesicular uptake and exocytosis of L-aspartate is independent of sialin

The mechanism of release and the role of l-aspartate as a central neurotransmitter are controversial. A vesicular release mechanism for l-aspartate has been difficult to prove, as no vesicular l-aspartate transporter was identified until it was found that sialin could transport l-aspartate and l-glutamate when reconstituted into liposomes. We sought to clarify the release mechanism of l-aspartate and the role of sialin in this process by combining l-aspartate uptake studies in isolated synaptic vesicles with immunocyotchemical investigations of hippocampal slices. We found that radiolabeled l-aspartate was taken up into synaptic vesicles. The vesicular l-aspartate uptake, relative to the l-glutamate uptake, was twice as high in the hippocampus as in the whole brain, the striatum, and the entorhinal and frontal cortices and was not inhibited by l-glutamate. We further show that sialin is not essential for exocytosis of l-aspartate, as there was no difference in ATP-dependent l-aspartate uptake in synaptic vesicles from sialin-knockout and wild-type mice. In addition, expression of sialin in PC12 cells did not result in significant vesicle uptake of l-aspartate, and depolarization-induced depletion of l-aspartate from hippocampal nerve terminals was similar in hippocampal slices from sialin-knockout and wild-type mice. Further, there was no evidence for nonvesicular release of l-aspartate via volume-regulated anion channels or plasma membrane excitatory amino acid transporters. This suggests that l-aspartate is exocytotically released from nerve terminals after vesicular accumulation by a transporter other than sialin.

Amygdalofugal axon terminals immunoreactive for L-aspartate or L-glutamate in the nucleus accumbens of rats and domestic chickens: a comparative electron microscopic immunocytochemical study combined with anterograde pathway tracing

Several studies have shown that L-aspartate (Asp) is present in synaptic vesicles and released exocytotically from presynaptic terminals, possibly by Ca(2+)-dependent corelease of Asp and L-glutamate (Glu). It has been demonstrated that both excitatory amino acids (EAAs) are released from the rat striatum as part of corticostriatal neurotransmission. The single or colocalized occurrence of Asp and Glu in specific synaptic boutons of the chicken medial striatum/nucl. accumbens has been demonstrated by our group using ultrastructural immunocytochemistry. However, evidence for the presence of EAAs in any specific striatal pathway was only circumstantial. Here, we report on the distribution of Asp and Glu in specific synaptic terminals of the amygdalostriatal pathway, both in rat and chicken brains, combining anterograde tracing with postembedding immunogold labeling of Asp or Glu. Immunoreactivity for Asp and Glu was observed in amygdalofugal terminals with asymmetrical synaptic junctions (morphologically representing excitatory synapses) in both species. The postsynaptic targets were either dendritic spines or small dendrites, whereas axosomatic or axo-axonic connections were not observed. Ultrastructurally, the synaptic terminals immunoreactive for Asp were indistinguishable from those immunoreactive for Glu. The findigs are consistent with an Asp-Glu corelease mechanism, with a distinct synaptic contingent, evolutionarily conserved in the amygdalostriatal pathway.

Focal not widespread grafts induce novel dyskinetic behavior in parkinsonian rats

Dyskinesias are a common consequence of dopaminergic therapy in patients with Parkinson's disease. Little is known about the influence of cellular replacement strategies upon drug-induced dyskinesias. In the current study, we employed parkinsonian rats to test whether the distribution of dopamine neuron grafts could differentially alter striatal circuitry and levodopa-induced dyskinesias. Specifically, we compared behavioral and neurochemical consequences of dopamine reinnervation restricted to a focal region of the striatum to innervation encompassing the majority of the striatum by distributing the same number of cells into single locus or multiple locations. Both the single-site and widespread grafts reduced pregraft dyskinesias and normalized FosB/DeltaFosB in the dorsal two-thirds of the lateral striatum. However, single-site DA graft recipients developed a robust, novel forelimb-facial stereotypy and upregulated FosB/DeltaFosB expression in the ventrolateral striatum, an area associated with movements of tongue and forelimbs. The onset of forelimb-facial stereotypy correlated with measures of increased graft function.

Morphine-induced c-fos mRNA expression in striatofugal circuits: modulation by dose, environmental context, and drug history

Opiates and psychostimulants produce many shared behavioral and neurobiological adaptations, such as behavioral sensitization and the induction of immediate early genes in the caudate-putamen (CPu). Previous studies indicate that factors such as dose, the environmental context surrounding drug administration and drug history can influence both morphine- and psychostimulant-induced behavioral sensitization. In addition, these factors can modulate the ability of psychostimulants to engage striatofugal circuits in the CPu. The present study, therefore, sought to examine whether these factors have similar influences over the ability of morphine to engage cortico-striatofugal circuits. We report that, when given in the home cage, morphine produced a small, but significant increase in the number of c-fos+ striatonigral cells and c-fos+ cells in cingulate cortex, but had no effect on the number of c-fos+ striatopallidal cells. When given in a novel test environment, however, morphine dramatically increased the number of c-fos+ striatonigral cells in a dose-dependent fashion, and this effect was maintained following repeated treatment. Unexpectedly, morphine treatment in a novel environment produced a dose-dependent reduction in the number of c-fos+ striatopallidal cells and c-fos+ cells in cingulate cortex, relative to exposure to novelty alone-effects that were reversed by repeated morphine treatment. We suggest that alterations in c-fos expression patterns in striatofugal circuits following morphine administration may be involved in drug-experience-dependent plasticity.

Haloperidol impairs auditory filial imprinting and modulates monoaminergic neurotransmission in an imprinting-relevant forebrain area of the domestic chick

In vivo microdialysis and behavioural studies in the domestic chick have shown that glutamatergic as well as monoaminergic neurotransmission in the medio-rostral neostriatum/hyperstriatum ventrale (MNH) is altered after auditory filial imprinting. In the present study, using pharmaco-behavioural and in vivo microdialysis approaches, the role of dopaminergic neurotransmission in this juvenile learning event was further evaluated. The results revealed that: (i) the systemic application of the potent dopamine receptor antagonist haloperidol (7.5 mg/kg) strongly impairs auditory filial imprinting; (ii) systemic haloperidol induces a tetrodotoxin-sensitive increase of extracellular levels of the dopamine metabolite, homovanillic acid, in the MNH, whereas the levels of glutamate, taurine and the serotonin metabolite, 5-hydroxyindole-3-acetic acid, remain unchanged; (iii) haloperidol (0.01, 0.1, 1 mm) infused locally into the MNH increases glutamate, taurine and 5- hydroxyindole-3-acetic acid levels in a dose-dependent manner, whereas homovanillic acid levels remain unchanged; (iv) systemic haloperidol infusion reinforces the N-methyl-d-aspartate receptor-mediated inhibitory modulation of the dopaminergic neurotransmission within the MNH. These results indicate that the modulation of dopaminergic function and its interaction with other neurotransmitter systems in a higher associative forebrain region of the juvenile avian brain displays similar neurochemical characteristics as the adult mammalian prefrontal cortex. Furthermore, we were able to show that the pharmacological manipulation of monoaminergic regulatory mechanisms interferes with learning and memory formation, events which in a similar fashion might occur in young or adult mammals.

Dopamine gating of glutamatergic sensorimotor and incentive motivational input signals to the striatum

Dopamine (DA) neurons of the substantia nigra (SN) and ventral tegmental area (VTA) respond to a wide category of salient stimuli. Activation of SN and VTA DA neurons, and consequent release of nigrostriatal and mesolimbic DA, modulates the processing of concurrent glutamate inputs to dorsal and ventral striatal target regions. According to the view described here, this occurs under conditions of unexpected environmental change regardless of whether that change is rewarding or aversive. Nigrostriatal and mesolimbic DA activity gates the input of sensory, motor, and incentive motivational (e.g. reward) signals to the striatum. In light of recent single-unit and brain imaging data, it is suggested that the striatal reward signals originate in the orbitofrontal cortex and basolateral amygdala (BLA), regions that project strongly to the striatum. A DA signal of salient unexpected event occurrence, from this framework, gates the throughput of the orbitofrontal glutamate reward input to the striatum just as it gates the throughput of corticostriatal sensory and motor signals needed for normal response execution. Processing of these incoming signals is enhanced when synaptic DA levels are high, because DA enhances the synaptic efficacy of strong concurrent glutamate inputs while reducing the efficacy of weak glutamate inputs. The impairments in motor performance and incentive motivational processes that follow from nigrostriatal and mesolimbic DA loss can be understood in terms of a single mechanism: abnormal processing of sensorimotor and incentive motivation-related glutamate input signals to the striatum.

Opposed Behavioural Outputs of Increased Dopamine Transmission in Prefrontocortical and Subcortical Areas: A Role for the Cortical D-1 Dopamine Receptor

The possibility that the dopaminergic neurons innervating the medial prefrontal cortex (mPFC) can inhibit locomotor behaviour has been suggested in several studies. The evidence remains indirect, however, because the manipulations tested aimed exclusively at permanently depleting mPFC dopamine. Here we demonstrate in rats that acute increases in dopamine transmission in this site by local injections of amphetamine inhibit the known locomotor-activating effects of amphetamine in the nucleus accumbens (N.Acc.). Further, intra-mPFC injections of the D-1 dopamine receptor antagonist SCH-23390, but not other dopamine antagonists with greater affinities for noradrenergic, serotonergic and D-2 dopamine receptors, enhanced the locomotion induced by intra-N.Acc. amphetamine. These findings provide direct evidence for the inhibition of locomotor activity by mPFC dopamine and suggest that it is acting at D-1 dopamine receptors in this site.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Altered striatal amino acid neurotransmitter release monitored using microdialysis in R6/1 Huntington transgenic mice

Huntington's disease is an autosomal dominant disease which presents with striatal and cortical degeneration causing involuntary movements, dementia and emotional changes. We employed 16-week-old transgenic Huntington mice (R6/1 line developed by Bates and coworkers) that express exon 1 of the mutant human Huntington gene with 115 CAG triplet repeats. At this age, R6/1 mice do not exhibit an overt neurological phenotype nor any striatal neuronal loss. Using microdialysis, we monitored basal and intrastriatal N-methyl D-aspartate (NMDA, 100 microM, 15 min)- and KCl (100 mM, 15 min)-induced increases in local aspartate, glutamate and GABA release in halothane-anaesthetized transgenic mice and wild-type controls. Basal striatal dialysate glutamate levels were reduced by 42% in R6/1 mice whilst aspartate and GABA levels did not differ from those observed in control mice. Intrastriatal NMDA was associated with significantly greater aspartate (at 15 min) and GABA (at 30 min) levels in the R6/1 mice compared to controls, whilst glutamate release rapidly increased to the same extent in both groups. Intrastriatal KCl was associated with enhanced increases (30 min) in local aspartate and glutamate release in the R6/1 mice above those observed in controls whilst the rapid increase (15 min) in GABA release was similar in both groups. The results provide compelling evidence for specific alterations in both basal, as well as NMDA- and KCl-induced, release of striatal amino acid neurotransmitters in this transgenic model of Huntington's disease, even in the absence of manifest neurodegeneration.

Presence of NMDA-type glutamate receptors in cingulate corticostriatal terminals and their postsynaptic targets

The glutamatergic projection from the anterior cingulate cortex to the medial caudate-putamen nucleus (CPN) has been implicated in motor and cognitive functions, many of which are potently modulated by activation of N-methyl-D-aspartate subtype of glutamate receptors (NMDARs). To determine the functional sites for NMDAR activation within this circuitry, we combined anterograde transport of biotinylated dextran amine (BDA) from deep layers of the rat anterior cingulate cortex with immunogold labeling of NMDAR subunit, NMDAR1, in the dorsomedial CPN. BDA-containing axons were seen in patch-like clusters in a neuropil that showed more uniform immunogold-silver labeling for NMDAR1. Electron microscopy of these regions showed that BDA-labeling was present exclusively in axons and terminals, 23% (98 of 421) of which also contained NMDAR1-immunoreactivity (IR). BDA-labeled terminals often apposed NMDAR1-immunoreactive neuronal and glial profiles. These terminals also formed asymmetric excitatory-type synapses with dendritic spines. Of 155 anterogradely labeled axon terminals forming asymmetric synapses, 34% were with NMDAR1-labeled, and 66% with unlabeled dendritic spines. These results provide ultrastructural evidence for the involvement of NMDARs in presynaptic regulation of glutamate transmission, and in postsynaptic modulation of the excitability of spiny neurons in patch-like compartments of the dorsomedial CPN. These dual NMDAR-mediated actions are likely to play a major role in the acquisition of new behaviors and reward-related processes that have been associated with cortical input to the striatal patch compartments.

Role of pertussis toxin-sensitive G-proteins in synaptic transmission and plasticity at corticostriatal synapses

The role of pertussis toxin (PTX)-sensitive G-proteins in corticostriatal synaptic transmission and long-term synaptic depression (LTD) was examined using extracellular field potential and whole cell voltage-clamp recordings in striatal slices. High-frequency stimulation (HFS) produced LTD, defined as long-lasting decreases both in synaptically driven population spikes (PSs) measured with field potential recording and in excitatory postsynaptic currents (EPSCs) measured with whole cell recording. Striatal LTD could not be induced in slices obtained from rats that had received a unilateral intrastriatal injection of PTX. However, LTD could be induced in slices obtained from paired control slices. Furthermore, striatal LTD was prevented by pretreatment with N-ethylmaleimide (NEM), another compound that disrupts the function of PTX-sensitive G-proteins. NEM, itself, also potentiated PS and EPSC amplitudes. In addition, NEM increased the frequency and amplitude of both spontaneous and miniature EPSCs and decreased the paired-pulse facilitation ratio, suggesting that it may act on both pre- and postsynaptic sites. The findings suggest that PTX-sensitive G-proteins have multiple roles at corticostriatal synapses, including regulation of synaptic transmission at both pre- and postsynaptic sites, and a key role in striatal LTD.

Taurine release is enhanced in cell-damaging conditions in cultured cerebral cortical astrocytes

The release of preloaded [3H]taurine from cultured cerebral cortical astrocytes was studied under various cell-damaging conditions, including hypoxia, ischemia, aglycemia and oxidative stress, and in the presence of free radicals. Astrocytic taurine release was enhanced by K+ (50 mM), veratridine (0.1 mM) and the ionotropic glutamate receptor agonist kainate (1.0 mM). Metabotropic glutamate receptor agonists had only weak effects on taurine release. Similarly to the swelling-induced taurine release the efflux in normoxia seems to be mediated mainly by DIDS-(diisothiocyanostilbene-2,2'-disulphonate) and SITS-(4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate) sensitive CI- channels, since these blockers were able to reduce both basal and K+ -stimulated release. The basal release of taurine was moderately enhanced in hypoxia and ischemia, whereas the potentiation in the presence of free radicals was marked. The small basal release from astrocytes signifies that taurine release from brain tissue in ischemia may originate from neurons rather than glial cells. On the other hand, the release evoked by K+ in hypoxia and ischemia was greater than in normoxia, with a very slow time-course. The enhanced release of the inhibitory amino acid taurine from astrocytes in ischemia may be beneficial to surrounding neurons, outlasting the initial stimulus and counteracting overexcitation.

Differential regulation of glutamate, aspartate and gamma-amino-butyrate release by N-methyl-D-aspartate receptors in rat striatum after partial and extensive lesions to the nigro-striatal dopamine pathway

The in vivo microdialysis methodology was used to assess the effect of N-methyl-D-aspartate (NMDA) receptor ligands on glutamate (GLU), aspartate (ASP) and gamma-aminobutyrate (GABA) extracellular levels in the striatum of anaesthetized rats, after damage to the dopamine (DA) nigrostriatal pathway by injections of different doses of 6-hydroxydopamine (6-OH-DA) seven days earlier. The 6-OH-DA treated rats were divided into two groups, corresponding to animals with 20-80% (partial) and 85-99% (extensive) striatal DA tissue depletion, respectively. In rats with partial DA depletion, the striatal extracellular ASP levels significantly increased after intrastriatal dialysis perfusion with MK-801 (100 microM), an antagonist of NMDA receptors. In addition, a change in the pattern of local NMDA (500 microM)- induced efflux of ASP was observed in the striatum of these rats. However, in these partially DA-depleted striata no changes were found in basal extracellular levels of GLU, ASP and GABA or in NMDA- and MK-801-mediated effluxes of GLU and GABA relative to striata from sham rats. In contrast, rats with extensive striatal DA depletion exhibited a significant increase in ASP and GABA extracellular striatal levels, after intrastriatal dialysis perfusion with NMDA. In addition, the MK-801-mediated stimulation of extracellular ASP levels was accentuated along with the appearance of a MK-801 mediated increase in extracellular striatal GLU. Finally, basal extracellular levels of ASP, but not of GLU and GABA, were found to increase in extensive DA-depleted striata when compared to sham and partially DA-depleted striata. Thus, a differential regulation of basal and NMDA receptor-mediated release of transmitter amino acids occur seven days after partial and extensive DA-depleted striatum by 6-OH-DA-induced lesions of the nigrostriatal DA pathway. These findings may have implications as regards the participation of NMDA receptors in the compensatory mechanisms associated with the progress of Parkinson's disease, as well as in the treatment of this neurological disorder.

Simultaneous determination of basal and evoked output levels of aspartate, glutamate, taurine and 4-aminobutyric acid during microdialysis and from superfused brain slices

A HPLC method, involving pre-column derivatisation with o-phthalaldehyde and fluorescence detection, is described. It allows the resolution of aspartate, glutamate, taurine and GABA, in a single run with detection limits of 3.2, 1.7, 1.4 and 2 fmol/microl of perfusate, respectively. It is sufficiently sensitive and rapid (15 min) for the determination "on line" of the four amino acids in perfusates obtained during in vivo microdialysis experiments. The procedure has been used to determine basal, K+ - or veratridine-stimulated release of these amino acids in different brain areas during microdialysis and from perfused tissue slices.

Effect of excitatory amino acid neurotransmitters on acid secretion in the rat stomach

Excitatory amino acids (EAAs), in particular, L-aspartate (L-Asp) neurons and their processes, were localized in the rat stomach using a immunohistochemical method with specific antibodies against either L-Asp or its synthesizing enzyme, aspartate aminotransferase (AAT). Myenteric ganglia and nerve bundles in the circular muscle and in the longitudinal muscle were found to be AAT- or L-Asp-positive. In addition, AAT- or L-Asp-positive cells were also found in the muscle layer and the deep mucosal layer. The distribution of AAT- or L-Asp-positive cells in both the mucosal and muscle layers was heterogeneous in the stomach. In addition, L-Asp at 10(-6) M negligibly influenced acid secretion in an everted preparation of isolated rat stomach. However, according to our results, L-Asp markedly inhibited the histamine-stimulated acid secretion, but not the oxotremorine- or the pentagastrin-stimulated acid secretion. Furthermore, L-Asp also inhibited histamine-induced elevation of cAMP. L- Asp itself did not affect the cAMP level although it elevated the cGMP level in the stomach. Moreover, either (+)2-amino-5-phosphonovaleric acid or (+/-)3-(2-carboxypiperazin-4-yl)prophyl-1-phosphonic acid, i.e. two specific antagonists for N-methyl-D-aspartic acid (NMDA) receptors, blocked the inhibitory effect of L-Asp on histamine-stimulated acid secretion or histamine-induced elevation of cAMP. Since cAMP has been strongly implicated as the second messenger involved in histamine-induced acid secretion, we believe that L-Asp regulates acid secretion in the stomach by inhibiting histamine release through the NMDA receptors, subsequently lowering the level of cAMP and ultimately reducing acid secretion.

The release of amino acids from rat neostriatum and substantia nigra in vivo: a dual microdialysis probe analysis

It has previously been demonstrated, in dual probe microdialysis studies, that stimulation of the neostriatum with kainic acid causes the release of GABA both locally within the neostriatum and distally in the substantia nigra, observations that are consistent with the known anatomy of the basal ganglia. The object of the present study was to further examine the characteristics of GABA release and to determine whether taurine, which has been proposed to be present in striatonigral neurons, has similar characteristics of release, and to examine the release of excitatory amino acids under the same conditions. To this end, dual probe microdialysis studies were carried out on freely-moving rats. The application of kainic acid to neostriatum enhanced the release of GABA, taurine, aspartate and glutamate locally in the neostriatum and distally in the substantia nigra. The distal release of each amino acid in the substantia nigra was sensitive to the administration of 6,7-dinitroquinoxaline-2,3-dione and tetrodotoxin to the neostriatum. Similarly the local release of GABA, aspartate and glutamate but not taurine was sensitive to the intrastriatal application of 6,7-dinitroquinoxaline-2,3-dione or tetrodotoxin. It is concluded that the release of taurine from the substantia nigra has similar characteristics to that of GABA and may be released from the terminals of striatonigral neurons following the stimulation of their cell bodies in the neostriatum. The release of taurine in the neostriatum however, is likely to be mediated mainly by different mechanisms and not related to neuronal activity. The release of excitatory amino acids is likely to involve indirect effects in the neostriatum and polysynaptic pathways in the substantia nigra.

Synaptic vesicular localization and exocytosis of L-aspartate in excitatory nerve terminals: a quantitative immunogold analysis in rat hippocampus

To elucidate the role of aspartate as a signal molecule in the brain, its localization and those of related amino acids were examined by light and electron microscopic quantitative immunocytochemistry using antibodies specifically recognizing the aldehyde-fixed amino acids. Rat hippocampal slices were incubated at physiological and depolarizing [K+] before glutaraldehyde fixation. At normal [K+], aspartate-like and glutamate-like immunoreactivities were colocalized in nerve terminals forming asymmetrical synapses on spines in stratum radiatum of CA1 and the inner molecular layer of fascia dentata (i.e., excitatory afferents from CA3 and hilus, respectively). During K+ depolarization there was a loss of aspartate and glutamate from these terminals. Simultaneously the immunoreactivities strongly increased in glial cells. These changes were Ca2+-dependent and tetanus toxin-sensitive and did not comprise taurine-like immunoreactivity. Adding glutamine at CSF concentration prevented the loss of aspartate and glutamate and revealed an enhancement of aspartate in the terminals at moderate depolarization. In hippocampi from animals perfused with glutaraldehyde during insulin-induced hypoglycemia (to combine a strong aspartate signal with good ultrastructure) aspartate was colocalized with glutamate in excitatory terminals in stratum radiatum of CA1. The synaptic vesicle-to-cytoplasmic matrix ratios of immunogold particle density were similar for aspartate and glutamate, significantly higher than those observed for glutamine or taurine. Similar results were obtained in normoglycemic animals, although the nerve terminal contents of aspartate were lower. The results indicate that aspartate can be concentrated in synaptic vesicles and subject to sustained exocytotic release from the same nerve endings that contain and release glutamate.

Glutamatergic involvement in psychomotor stimulant action

The sympathomimetic psychomotor stimulants, including cocaine, amphetamines, and the phenylethylamine amphetamine-like derivatives, exert actions in mammalian systems that implicate involvement of the excitatory neurotransmitter, glutamate and its receptors. Despite evidence that psychomotor stimulants do not directly stimulate glutamate receptors, blockade of acute lethal, convulsive, circulatory, thermoregulatory, locomotor and stereotypical responses, as well as interference with slowly developing behavioral sensitization and brain monoaminergic neurotoxicities, can be achieved by receptor antagonists at both N-methyl-D-aspartate and AMPA/kainate glutamate receptor subtypes. Alterations in glutamatergic neurobiology, including elevations in extracellular glutamate levels, changes in glutamate receptor properties and glutamatergic neuronal degeneration, have also been attributed to psychomotor stimulant administration. Blockade of glutamate receptors offers therapeutic options in management of psychomotor stimulant toxicity.

Effects of NMDA antagonism on striatal dopamine release in healthy subjects: application of a novel PET approach

Agents that antagonize the glutamatergic N-methyl-d-aspartate (NMDA) receptor, such as phenylcyclidine (PCP) and ketamine, produce a behavioral state in healthy volunteers that resembles some aspects of schizophrenia. A dysfunction in NMDA-dopaminergic interactions has been proposed as a mechanism for these behavioral effects. In this study, we examined the effects of ketamine on striatal dopamine release in healthy human subjects with a novel 11C-raclopride/PET displacement paradigm and compared these effects to administration of saline and the direct-acting dopamine agonist amphetamine. We found that the percent decreases (mean +/- SD) in specific 11C-raclopride binding from baseline for ketamine (11.2 +/- 8.9) was greater than for saline (1.9 +/- 3.7) (t = 2.4, df = 13, P = 0.003) indicating that ketamine caused increases in striatal synaptic dopamine concentrations. Ketamine-related binding changes were not significantly different than the decreases in percent change (mean +/- SD) in specific 11C-raclopride binding caused by amphetamine (15.5 +/- 6.2) (t = 1.3, df = 19, P = 0.21). Ketamine-induced changes in 11C-raclopride-specific binding were significantly correlated with induction of schizophrenia-like symptoms. The implications of this brain imaging method for studies of schizophrenia and the mechanism of action of antipsychotic drugs are discussed.

Preferential blockade of cholecystokinin-8S-induced increases in aspartate and glutamate levels by the CCK(B) receptor antagonist, L-365,260, in rat brain

In the present studies, the ability of a locally delivered cholecystokinin (CCK) receptor agonist and systemically delivered antagonists to modulate extracellular levels of aspartate and glutamate in the frontal cortex of anaesthetised rats and frontal cortex and caudate-putamen of freely moving rats was investigated using an in vivo microdialysis technique. In the anaesthetised rats, local application of sulphated CCK octapeptide (CCK-8S, 10 microM) into the frontal cortex enhanced extracellular aspartate levels to a maximum of 265+/-16% of the basal levels, whereas glutamate levels were increased to a maximum of 168+/-7% of the basal levels. Given 40 min prior to the cortical perfusion of 10 microM of CCK-8S, the CCK(B) receptor antagonist, L-365,260 (20 mg/kg, s.c.), limited the rise in cortical aspartate by over half to 170+/-10% of the basal levels. However, this same dose of L-365,260 still allowed CCK-8S to increase glutamate by 44+/-15% above the basal levels. Whereas the enhanced glutamate levels were totally unaffected by systemic administration of the CCK(A) receptor antagonist, L-364,718 (20 mg/kg, -40 min, s.c.), this treatment was able to limit the elevation in aspartate to 220+/-4% of the basal levels. In the freely moving rats, local perfusion of CCK-8S (10 microM) increased aspartate and glutamate levels to maxima of 275+/-12% and 225+/-14% of the basal levels, respectively, in the frontal cortex. In the caudate-putamen, aspartate and glutamate levels were also elevated by CCK-8S (10 microM) to 248+/-15% and 185+/-12% of the basal levels, respectively. The respective increase in aspartate and glutamate induced by CCK-8S (10 microM) were limited to 140+/-10% and 124+/-6% (frontal cortex), of the basal levels, and 162+/-15% and 143+/-8% (caudate-putamen), by 40 min pretreatment with L-365,260 (20 mg/kg, s.c.). In conclusion, CCK-8S was able to enhance both aspartate and glutamate overflow in the frontal cortex of anaesthetised rats, and frontal cortex and caudate-putamen of freely moving rats. These increases were preferentially offset by the selective CCK(B) receptor antagonist, L-365,260, since no influence could be discerned using the selective CCK(A) receptor antagonist, L-364,718.

Ultrastructural immunocytochemical localization of mu-opioid receptors in dendritic targets of dopaminergic terminals in the rat caudate-putamen nucleus

Many motor effects of opiates acting at mu-opioid receptors are thought to reflect functional interactions with dopaminergic inputs to the caudate-putamen nucleus. We examined the cellular and subcellular bases for this interaction in the rat caudate-putamen nucleus by dual immunocytochemical labelling for mu-opioid receptors and tyrosine hydroxylase, a marker mainly for dopamine in this region. mu-Opioid receptor-like immunoreactivity showed a patchy distribution by light microscopy. Within the patches, electron microscopy revealed that immunogold labelling for mu-opioid receptors was mainly distributed along extrasynaptic plasma membranes of medium spiny neurons. In contrast, immunoperoxidase labelling for tyrosine hydroxylase was exclusively located in axons and axon terminals without detectable mu-opioid receptor-like immunoreactivity. Forty-six percent of the total mu-opioid receptor-labelled neuronal profiles (n = 1441) were in contact with tyrosine hydroxylase-immunoreactive axons and terminals. These contacts were characterized by closely apposed parallel plasma membrane segments, without well-defined synaptic junctions, or with punctate symmetric specializations. From 639 noted appositions, over 90% were between mu-opioid receptor-labelled dendrites and/or dendritic spines and tyrosine hydroxylase-containing terminals. The dendritic spines containing mu-opioid receptor-like immunoreactivity often received asymmetric synapses characteristics of excitatory inputs from unlabelled terminals. Axon terminals containing mu-opioid receptor-like immunoreactivity formed asymmetric synapses with dendritic spines, or apposed tyrosine hydroxylase-labelled terminals. Our results suggest that, in striatal patch compartments, mu-agonists and dopamine dually modulate the activity of single spiny neurons mainly through changes in their postsynaptic responses to excitatory inputs. In addition, our findings implicate mu-opioid receptors and dopamine in the presynaptic regulation of excitatory neurotransmitter release within the striatal patch compartments.

Dopamine and glutamate control each other's release in the basal ganglia: a microdialysis study of the entopeduncular nucleus and substantia nigra

This study utilized microdialysis in conscious rats to investigate dopaminergic control of excitatory amino acid release in the entopeduncular nucleus (EPN), and glutamatergic control of dopamine release in the substantia nigra pars reticulata (SNr). EPN dialysates contained both glutamate and aspartate, which were elevated by dopamine depletion with reserpine and 6-hydroxydopamine (6-OHDA), reduced by the D2/3 agonist LY 171555 and unaffected by the D1 agonist SKF 38393, in line with current theory. The D2/3 agonist RU 24213 was behaviourally active but paradoxically increased glutamate and aspartate release in EPN, possibly via kappa opioid receptor blockade. 6-OHDA-hemilesioned rats also showed a significant increase in glutamate and aspartate contralaterally, suggesting that nigrostriatal dopamine affects EPN neurotransmission bilaterally. In reserpine-treated rats, basal levels of dopamine in the SNr were greatly reduced, and were further lowered by focal application of NMDA antagonists, suggestive of the removal of a high glutamatergic tone. A threshold amount of L-DOPA applied to the SNr elevated dopamine output about two-fold and 5-HT output about 13-fold, indicating L-DOPA effects the release of monoamines other than dopamine. Concomitant addition of the NMDA antagonists potentiated these releases synergistically, suggesting that this could be how they facilitate the antiparkinsonian action of L-DOPA.

On the release of glutamate and aspartate in the basal ganglia of the rat: interactions with monoamines and neuropeptides

Using highly sensitive analytical procedures, glutamate (Glu), aspartate (Asp) and several putative neurotransmitters and metabolites can be monitored simultaneously in the extracellular space of neostriatum, substantia nigra and cerebral cortex of the rat by in vivo microdialysis. Glu and Asp are found at sub-micromolar concentrations in all investigated brain regions. In order to ascertain their neuronal origin, we have extensively studied the sensitivity of extracellular Glu and Asp levels to: (i) K(+)-depolarization, (ii) Na(+)-channel blockade, (iii) removal of extracellular Ca2+, (iv) depletion of presynaptic vesicles, and (v) integrity of neuronal pathways. The relevance of these criteria for several neurotransmitters monitored simultaneously or in parallel experiments has also been examined. The functional interactions among different neuronal pathways in the basal ganglia are studied by using selective pharmacological treatments, administered systemically, or locally via intracerebral injections or the microdialysis perfusion medium. Immunohistochemical evidence for the existence of Glu and/or Asp neuronal pathways in the basal ganglia of the rat is presented, discussing especially new findings indicating the existence of a Glu-independent Asp system, intrinsic to the neostriatum of the rat. The clinical relevance of these interactions is discussed, focusing on the implications for the treatment of neurodegenerative disorders affecting the basal ganglia.

Effect of perinatal asphyxia on systemic and intracerebral pH and glycolysis metabolism in the rat

The effects of perinatal asphyxia on systemic and brain pH and glycolysis metabolism were studied in the rat. Perinatal asphyxia was induced by immersing pup-containing uterus horns, obtained by cesarean section from rats within the last day of gestation, in a water bath at 37 degrees C for various periods of time (0-23 min). Subcutaneous levels of pyruvate (Pyr), lactate (Lact), glutamate (Glu), and aspartate (Asp) were monitored with microdialysis 40-80 min after delivery. In parallel experiments, the pups were sacrificed 40 min after delivery and the heart and brain were removed for measuring pH. Brain (striatum) Pyr, Lact, Glu, and Asp levels were also analyzed. A decrease in the rate of survival was first observed following asphyctic periods longer than 16 min, and no survival could be observed after 22 min of asphyxia. In control (cesarean-delivered) pups, heart and brain pH were 7.36 +/- 0.01 (N = 8) and 7.30 +/- 0.01 (N = 8), respectively. Significant decreases in pH were first observed following 5-6 and 10-11 min of asphyxia, in heart and brain, respectively. In both regions pH decreased along with the length of asphyxia, but a decrease below 7 was only observed in the brain, following asphyctic periods longer than 16 min. A significant increase in subcutaneous Lact levels was first observed following 2-3 min of asphyxia, with a maximum after 20-21 min of asphyxia. In the brain, the increase in Lact levels was delayed compared to that observed in subcutaneous tissue. Pyr and Asp levels increased in subcutaneous tissue following perinatal asphyxia and decreased in brain tissue following > 15 min of asphyxia. Glu levels were increased subcutaneously by moderate (5-16 min) asphyctic periods, but, in the brain, were only transiently increased by 10-11 min of asphyxia. Thus, changes in systemic pH, glycolysis, and excitatory amino acid metabolism are observed following shorter asphyctic periods than are changes in the brain. In particular, increases in subcutaneous Lact levels precede: (i) a decrease in brain pH, (ii) an increase in brain Lact levels, (iii) a decrease in the rate of survival, and, probably, (iv) brain damage. It is suggested that monitoring Lact levels by subcutaneous microdialysis is a useful method for predicting the outcome produced by hypoxic-ischemic insults.

Extracellular levels of glutamate and aspartate in the entopeduncular nucleus of the rat determined by microdialysis: regulation by striatal dopamine D2 receptors via the indirect striatal output pathway?

This study used intracerebral microdialysis to monitor the outputs of excitatory amino acids in the entopeduncular nucleus (EPN) of conscious or halothane-anaesthetized rats, in an attempt to obtain direct biochemical evidence for the theory that neuronal inputs to the EPN by the indirect striatal output pathway are glutamatergic and regulated primarily by dopamine D2 receptors in the striatum. In dopamine-intact animals, both glutamate and asparate were readily detectable in EPN dialysates. Recoveries of both amino acids were increased bilaterally by local perfusion with veratridine (100 microM, given under halothane anaesthesia), pretreatment with reserpine (4 mg/kg, i.p., 24 h beforehand), unilateral pretreatment of the medial forebrain bundle with 6-OHDA (8 microg/4 microl), and by the systemic (1 mg/kg, i.p.) or bilateral intrastriatal (7 microg/0.5 microl under halothane anaesthesia) administration of the dopamine D2 receptor antagonist haloperidol, but not raclopride (2 mg/kg, i.p.). The dopamine D1 receptor antagonist SCH 23390 was ineffective both systemically (0.25 mg/kg, i.p.) and intrastriatally (0.125 microg/0.5 microl/side), as also were control intrastriatal injections of saline (0.5 microl/side). By contrast, the dopamine D2/3 receptor agonist quinpirole (4 mg/kg, i.p.) lowered the outputs of glutamate and aspartate in the EPN of reserpine-treated and normal individuals, whilst the dopamine D1 receptor agonist SKF 38393 (30 mg/kg, i.p.) was inactive; however, both drugs caused behavioural arousal. The dopamine D2/3 receptor agonist RU 24213 reversed reserpine-induced akinesia, yet paradoxically increased glutamate (not aspartate) output in the EPN still further. The combination of benserazide (30 mg/kg, i.p.) and L-DOPA (50 mg/kg, i.p.) evoked intense contraversive circling in unilaterally 6-OHDA-lesioned rats, together with a drop in EPN glutamate (but not aspartate) output in the intact but not lesioned hemisphere. These results offer biochemical support for the hypothesis that excitatory neurones innervating the EPN via the indirect striatal output pathway, may utilise glutamate and/or aspartate as their neurotransmitter. They further endorse the view that the EPN receives information from striatal D2 and not D1 receptors via excitatory synapses, which become hyperactive following dopamine depletion or inactivation, and which are subject to control by the contralateral as well as by the ipsilateral hemisphere. The results obtained with RU 24213 and L-DOPA, however, indicate that dopaminergic behaviours can also occur independently of glutamate or aspartate release in the EPN.

Excitatory amino acids and monoamines in parahippocampal gyrus and frontal cortical pole of adults with Down syndrome

Aspartate (ASP), glutamate (GLU), noradrenaline (NA), dopamine (DA) and its acidic metabolites DOPAC and HVA, serotonin (5-HT) and its metabolite 5-HIAA were simultaneously investigated in post-mortem tissue samples from right parahippocampal gyrus (temporal cortex) and frontal cortical pole (frontal cortex) of adults with Down syndrome (DS), and of neurologically healthy controls by use of high performance liquid chromatography (HPLC). In parahippocampal gyrus, ASP, GLU, NA, DOPAC and 5-HT levels were significantly decreased in patients with DS, compared to levels found in control subjects (approximately 50%). No significant changes were observed in frontal pole. ASP and GLU levels were significantly lower in parahippocampal gyrus than in frontal pole of DS, a regional distribution that could not be observed in control subjects. In conclusion, the results of this study suggest that the temporal cortex would be more affected than the frontal cortex in adult patients with DS, a finding in line with reports showing a marked hypometabolism and extensive cell loss in temporal cortex of DS, and with those showing that parahippocampal gyrus abnormality may correlate with the extent of mental retardation affecting this type of patients.

Effects of secretogranin II-derived peptides on the release of neurotransmitters monitored in the basal ganglia of the rat with in vivo microdialysis

In vivo microdialysis was used to study the effect of secretogranin II-derived peptides on dynorphin B (Dyn B), dopamine, gamma-aminobutyric acid (GABA), glutamate and aspartate release in the substantia nigra and neostriatum of halothane-anaesthesized rats. In the substantia nigra, local infusion of secretoneurin (secretogranin II 154-186) (1-50 microM) increased, in a concentration-dependent manner, extracellular aspartate, glutamate, Dyn B, dopamine and GABA levels. The effect was particularly prominent on aspartate and glutamate levels which, following 50 microM of secretoneurin, were increased by > 20 and > 10 fold, respectively. However, the effect of secretoneurin on Dyn B release appeared to be more specific, since a significant increase (> 20 fold) was already observed following 1 microM of secretoneurin. In the neostriatum, Dyn B, glutamate, aspartate and GABA levels were also increased by local secretoneurin infusion, but the effect was less prominent than in the substantia nigra. In the substantia nigra, only Dyn B levels were significantly increased following infusion of 10 microM of the secretoneurin-C terminal (secretoneurin-15C), whereas Dyn B and GABA levels were increased by the same concentration of the secretogranin II C terminus (YM). Only glutamate and aspartate levels were increased by local infusion of 10 microM of secretogranin II 133-151 (LF), a peptide adjacent to secretoneurin in the primary amino acid sequence. In the neostriatum, Dyn B and GABA levels were increased by 10 microM of secretoneurin-15C. Dyn B levels were also increased by 10 microM of YM, and glutamate and aspartate levels were increased by 10 microM of both YM and LF. Thus secretogranin II-derived peptides affect extracellular levels of several putative neurotransmitter systems monitored in the basal ganglia of the rat with in vivo microdialysis. The effect of Dyn B appears to be specific and related to a physiological role of secretoneurin, since (i) it occurs in an area where secretoneurin-immunocytochemistry has been observed, (ii) is exerted at comparatively low concentrations, and (iii) is mimicked by secretoneurin-15C. The increases in excitatory amino acid levels produced by high concentrations of secretoneurin and other secretogranin II-derived peptides reflect, perhaps, a potential neurotoxicity produced by abnormal accumulation of these peptides.

Modulation of neurotransmitter release by cholecystokinin in the neostriatum and substantia nigra of the rat: regional and receptor specificity

The effect of cholecystokinin peptides on the release of dynorphin B, aspartate, glutamate, dopamine and GABA in the neostriatum and substantia nigra of the rat was investigated using in vivo microdialysis. Sulphated cholecystokinin-8S in the dialysis perfusate (1-100 microM) induced a concentration-dependent increase in extracellular dynorphin B and aspartate levels, both in the neostriatum and substantia nigra. Striatal dopamine levels were only increased by 100 microM of cholecystokinin-8S, while in the substantia nigra they were increased by 10-100 microM of cholecystokinin-8S. Extracellular GABA and glutamate levels were increased following 100 microM of cholecystokinin-8S only. Striatal cholecystokinin-8S administration also produced a significant increase in nigral dynorphin B levels. Local cholecystokinin-4 (100 microM) produced a moderate, but significant, increase of extracellular dynorphin B and aspartate levels in the neostriatum and substantia nigra. No effect was observed on the other neurotransmitters investigated. A 6-hydroxydopamine lesion of the nigrostriatal dopamine pathway did not affect the increases in dynorphin B and aspartate levels produced by local administration of cholecystokinin-8S. Basal extracellular GABA levels were increased significantly in both the neostriatum and substantia nigra ipsilateral to the lesion. Nigral glutamate and aspartate levels were also increased in the lesioned substantia nigra, but in the lesioned neostriatum aspartate levels were decreased. The cholecystokinin-B antagonist L-365,260 (20 mg/kg, s.c.), but not the cholecystokinin-A antagonist L-364,718 (devazepide; 20 mg/kg, s.c.), significantly inhibited the effect of cholecystokinin-8S on striatal dynorphin B and aspartate levels. In the substantia nigra, however, the effect of cholecystokinin-8S on dynorphin B and aspartate levels was inhibited to a similar extent by both L-365,260 and L-364,718. Pretreatment with L-364,718, but not with L-365.260, prevented the increase in nigral dopamine levels produced by nigral cholecystokinin-8S administration. Taken together, these results suggest that cholecystokinin-8S modulates dynorphin B and aspartate release in the neostriatum and substantia nigra of the rat via different receptor mechanisms. In the neostriatum, the effect of cholecystokinin-8S on dynorphin B and aspartate release is mediated via the cholecystokinin-B receptor subtype, while in the substantia nigra, cholecystokinin-8S modulates dynorphin B and aspartate release via both cholecystokinin-A and cholecystokinin-B receptor subtypes. Cholecystokinin-8S modulates dopamine release mainly in the substantia nigra, via the cholecystokinin-A receptor subtype.

Evidence for aspartate-immunoreactive neurons in the neostriatum of the rat: modulation by the mesencephalic dopamine pathway via D1-subtype of receptor

Aspartate-like immunoreactivity was visualized in the neostriatum of rats using indirect immunofluorescence techniques and antibodies raised against aspartate conjugated to keyhole limpet hemocyanine. In normal rats only a few aspartate-positive cell bodies with limited processes were observed. A moderate increase was seen after treatment with (+)methamphetamine and haloperidol. A dramatic increase in the number and fluorescence intensity was observed in the unilaterally 6-hydroxy-dopamine lesioned rats after multiple injections of the D1-dopamine receptor agonist SKF 38393. In these rats strongly fluorescent processes as well as extensive terminal varicose fibre networks were observed. This increase could partly be blocked by the D1-dopamine receptor antagonist SCH 23390. Using a modified technique the aspartate-positive cell bodies and processes were observed even when the antiserum was diluted 1:80,000. Positive cell bodies and fibres were also seen on the ipsilateral side outside the neostriatum, for example in the islet of Calleja and in the piriform cortex. The aspartate-positive cells were negative for dopamine- and cyclic AMP-regulated phosphoprotein-32, a marker for neurons bearing dopamine D1-receptor subtype. A proportion of the aspartate-positive neurons (20%) contained neuropeptide tyrosine-like immunoreactivity. On adjacent sections there was a marked up-regulation of preprodynorphin-like immunoreactivity. The up-regulation of dynorphin and aspartate was only observed when there was an almost complete denervation of the neostriatum as visualized with antiserum to tyrosine hydroxylase, a marker for dopamine fibres. The present results raise the possibility that aspartate may act as a neurotransmitter released from interneurons in the neostriatum.

Adenosine transport inhibitors enhance high K(+)-evoked taurine release from rat hippocampus

We examined the effects of Ca(2+)-free medium containing 20 mM Mg2+, a non-selective adenosine receptor antagonist, theophylline, and adenosine transport inhibitors, dipyridamole and nitrobenzylthioinosine, on high K(+)-evoked spreading depression, glutamate, and taurine release from the rat hippocampus using brain microdialysis. High K+ alone perfusion evoked spreading depression and increased glutamate release followed by taurine efflux. Perfusion of Ca(2+)-free medium with high K+ never evoked spreading depression and decreased the high K(+)-evoked taurine release. Perfusion of theophylline (1 mM) increased the occurrence of high K(+)-evoked spreading depression and glutamate release, but did not modify taurine release. In contrast, simultaneous perfusion of dipyridamole (100 microM) and nitrobenzylthioinosine (50 microM) reduced the occurrence of spreading depression and the high K(+)-evoked glutamate release, but enhanced significantly the taurine efflux. These findings suggest that endogenous taurine with adenosine may have neuroprotective actions against high K(+)-evoked glutamate release and spreading depression in the rat hippocampus, in addition to its osmoregulatory action.

Medial prefrontal cortical D1 receptor modulation of the meso-accumbens dopamine response to stress: an electrochemical study in freely-behaving rats

Voltammetry was used to study the role of prefrontal cortex (PFC) dopamine (DA) in modulating the nucleus accumbens (NAcc) DA response to stress. Signal increases elicited in NAcc by 15 min of restraint were monitored in freely-behaving rats following intra-PFC microinjections of D1 and D2 receptor-selective drugs. The exact site of injection was first determined by assessing the electrochemical response to stress at two dorsal-ventral levels of PFC. Consistent with previous reports, a pronounced stress response was observed ventrally at sites within the infralimbic PFC but not dorsally within the superficial layers of PFC. When microinjected into the infralimbic PFC, the D1 receptor antagonist SCH 23390 significantly enhanced the NAcc stress response. While the D1 receptor agonist SKF 38393 tended to decrease the NAcc stress response, it failed to do so reliably. Neither sulpiride (D2 receptor antagonist) nor quinpirole (D2 receptor agonist) had a significant effect. Finally, systemic administration of the selective DA uptake inhibitor GBR 12909 dose-dependently potentiated stress-induced signal increases in NAcc and in PFC, indicating that the electrochemical responses to stress in both regions were due primarily to increases in extracellular DA levels. Together, these data add to other evidence indicating that the PFC exerts an inhibitory influence on subcortical DA transmission. Specifically, the present results suggest that the NAcc DA response to stress is dampened by the concurrent activation of meso-PFC DA neurons and that this action is mediated, at least in part, by D1 receptors in PFC.

Electrochemical evidence of increased dopamine transmission in prefrontal cortex and nucleus accumbens elicited by ventral tegmental mu-opioid receptor activation in freely behaving rats

Chronoamperometry was used in combination with monoamine-selective electrodes to monitor, in nucleus accumbens (NAcc) and prefrontal cortex (PFC) of freely behaving rats, changes in dopamine (DA)-like electrochemical signals elicited by unilateral ventral tegmental microinjections of the selective mu-opioid receptor agonist D-Ala, N-Me-Phe-Gly-Ol-Enkephalin (DAMGO; 0.01, 0.1, and 1.0 nmol). The results show that DAMGO dose-dependently increased electrochemical signals both in Nacc and PFC within a few minutes of injection. While DAMGO elicited signal increases of comparable amplitudes in both regions, the increases recorded in PFC were significantly longer lasting than those in NAcc; at the highest dose tested (1.0 nmol), DAMGO produced signal increases that lasted (mean +/- sem) 129 +/- 7.3 min in PFC and 96 +/- 12.5 min in NAcc. Pretreatment with the opioid receptor antagonist, naloxone (2 mg/kg, sc), significantly attenuated the peak amplitude and reduced the duration of DAMGO-induced (0.1 nmol) signal increases both in PFC and NAcc. In contrast, pretreatment with apomorphine (50 micrograms/kg, sc), a D1/D2 DA receptor agonist, significantly reduced the duration and the rate of rise of the signal increases in both regions but had little effect on the peak increases in signal. Unilateral ventral tegmental DAMGO administration (0.01, 0.1, and 1.0 nmol) also caused dose-dependent increases in contraversive circling the duration of which approximated that of the signal increases recorded in NAcc. However, differences in the time courses of DAMGO-induced contraversive circling and signal increases in NAcc suggest that the behavioral stimulant effect of ventral tegmental mu-opioid receptor activation may not be mediated exclusively by meso-NAcc DA neurons. The results of this study suggest that enkephalins modulate the activity of meso-PFC DA neurons and that behaviorally relevant activation of mu-opioid receptors in the ventral tegmental area increases DA transmission in PFC to a same, if not to a greater extent as in NAcc. These findings are discussed in relation to evidence indicating that the response of meso-NAcc DA neurons to a variety of stimuli, including drugs of abuse, is indirectly regulated by a DA-sensitive neurons in PFC.

Effect of local cholecystokinin-8 administration on extracellular levels of amino acids and glycolytic products monitored by in vivo microdialysis in the fronto-parietal cortex of the rat

The effects of local cholecystokinin-8 (CCK-8) administration on cortical extracellular levels of amino acids, catecholamines and metabolism products were studied in the halothane anaesthetized rat by in vivo microdialysis. CCK-8 (10 microM), administrated via a microdialysis probe, produced a significant increase in the levels of aspartate, glutamate and gamma-aminobutyric acid (GABA), but not of 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), lactate and pyruvate, which were simultaneously monitored. The increase in aspartate and glutamate levels produced by CCK-8 was about 3-4-fold. The effect of CCK-8 on aspartate levels was significantly inhibited by the CCKB antagonist, L-365,260 (20 mg kg-1, s.c.), but not by the CCKA antagonist, L-364,718 (20 mg kg-1, s.c.). In contrast, the increase in glutamate levels was inhibited by both L-365,260 and L-364,718. GABA was slightly, but significantly increased (approximately 30%), by local CCK-8 and was inhibited by both CCK antagonists. The present results show that CCK-8 exerts a strong modulatory action on both aspartate and glutamate release in rat cortex. While the effect of CCK-8 on aspartate is selectively mediated via CCKB receptor subtype, the effect of CCK-8 on glutamate appears to be mediated via both CCKA and CCKB receptor subtypes.

Characterisation of extracellular amino acids in striatum of freely moving rats by in vivo microdialysis

To investigate the characteristics of extracellular amino acids released from the striatum, we performed in vivo microdialysis in non-anaesthetised, freely moving rats. Amino acids were determined after precolumn derivatisation with o-phthalaldehyde by high-performance liquid chromatography and fluorescence detection. The omission of Ca2+ in the perfusion medium partially decreased the basal concentration of aspartate and glutamate. This shows that a small fraction of basal concentration of aspartate and glutamate is of neuronal origin. The effect of high K+ and veratrine stimulation was evaluated in the presence or absence of Ca2+ or tetrodotoxin (1 microM). High K+ and veratrine caused a remarkable increase in the aspartate and glutamate efflux. The omission of Ca2+ only partially decreased K(+)-stimulated aspartate and glutamate efflux. Tetrodotoxin completely antagonised veratrine-stimulated aspartate and glutamate efflux. Although glycine and taurine releases were stimulated by high K+ and veratrine, their release was not always antagonised with Ca2+ omission or tetrodotoxin inclusion. Thus, the neuronal origin of stimulated release of glycine and taurine is unclear. Although tetrodotoxin sensitivity and Ca2(+)-dependency are regarded as a basic criterion for classical neurotransmitters in microdialysis experiments, they should not be adapted to the physiological characteristics of the release of amino acids.

Chronic treatment with a classical neuroleptic alters excitatory amino acid and GABAergic neurotransmission in specific regions of the rat brain

The purpose of the following experiments was to describe some of the neurochemical changes that occur in the basal ganglia of rats exposed chronically to a classical neuroleptic, fluphenazine, and to relate these changes to extrapyramidal motor dysfunction. For these studies a combination of behavioural, receptor autoradiographic and in situ hybridization methods were employed. Preliminary pharmacological studies on GABA receptors showed that incubation in Tris-acetate rather than Tris-citrate buffer increased the number of binding sites labelled by [3H]muscimol by over 120% without affecting binding affinity or selectivity. The results of experiments with fluphenazine showed that treatment for six months increased the frequency of vacuous chewing movements compared to controls. In the striatum, changes in GABA transmission were observed in fluphenazine-treated rats with increases in glutamate decarboxylase mRNA levels in the caudate nucleus, dorsal shell and core of the accumbens and decreases in [3H]muscimol binding in the caudate and dorsal shell regions. These data suggest that fluphenazine treatment increased GABA transmission in specific subregions of the caudate and accumbens nuclei. In addition, glutamate decarboxylase mRNA levels were elevated in the entopeduncular nucleus of fluphenazine-treated animals. Autoradiographic analysis of excitatory amino acid binding showed that fluphenazine exposure decreased [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid binding in entopeduncular nucleus and in the ventrolateral thalamic nucleus and decreased [3H]dizocilpine maleate binding in the medial geniculate nucleus. These experiments show that in addition to altering GABA transmission, chronic neuroleptic exposure alters excitatory amino acid transmission in specific regions of the basal ganglia-thalamocortical motor system. The neuroleptic dependent increases in glutamate decarboxylase mRNA levels in the entopeduncular nucleus may reflect changes in neurotransmission in the indirect pathway connecting the major input and output nuclei of the basal ganglia. Changes in some of these brain regions may be related to the occurrence of extrapyramidal motor disturbances.

Postsynaptic integration of glutamatergic and dopaminergic signals in the striatum

The aim of this study was to achieve a better understanding of the integration in striatal medium-sized spiny neurons (MSNs) of converging signals from glutamatergic and dopaminergic afferents. The review of the literature in the first section shows that these two types of afferents not only contact the same striatal cell type, but that individual MSNs receive both a corticostriatal and a dopaminergic terminal. The most common sites of convergence are dendritic shafts and spines of MSNs with a distance between the terminals of less than 1-2 microns. The second section focuses on synaptic transmission and second messenger activation. Glutamate, the candidate transmitter of corticostriatal terminals, via different types of glutamate receptors can evoke an increase in intracellular free calcium concentrations. The net effect of dopamine in the striatum is a stimulation of adenylate cyclase activity leading to an increase in cAMP. The subsequent sections present information on calcium- and cAMP-sensitive biochemical pathways and review the regional and subcellular distribution of the components in the striatum. The specific biochemical reaction steps were formalized as simplified equilibrium equations. Parameter values of the model were chosen from published experimental data. Major results of this analysis are: at intracellular free calcium concentrations below 1 microM the stimulation of adenylate cyclase by calcium and dopamine is at least additive in the steady state. Free calcium concentrations exceeding 1 microM inhibit adenylate cyclase, which is not overcome by dopaminergic stimulation. The kinases and phosphatases studied can be divided in those that are almost exclusively calcium-sensitive (PP2B and CaMPK), and others that are modulated by both calcium and dopamine (PKA and PP1). Maximal threonine-phosphorylation of the phosphoprotein DARPP requires optimal concentrations of calcium (about 0.3 microM) and dopamine (above 5 microM). It seems favourable if the glutamate signal precedes phasic dopamine release by approximately 100 msec. The phosphorylation of MAP2 is under essentially calcium-dependent control of at least five kinases and phosphatases, which differentially affect its heterogeneous phosphorylation sites. Therefore, MAP2 could respond specifically to the spatio-temporal characteristics of different intracellular calcium fluxes. The quantitative description of the calcium- and dopamine-dependent regulation of DARPP and MAP2 provides insights into the crosstalk between glutamatergic and dopaminergic signals in striatal MSNs. Such insights constitute an important step towards a better understanding of the links between biochemical pathways, physiological processes, and behavioural consequences connected with striatal function. The relevance to long-term potentiation, reinforcement learning, and Parkinson's disease is discussed.

Differential behavioral effects following microinjection of an NMDA antagonist into nucleus accumbens subregions

Recent studies have demonstrated the existence of two distinct regions within the nucleus accumbens (N.Acc) known as "core" and "shell". In order to investigate whether the behavioral functions of excitatory amino acid receptors differed between these two subregions, rats were administered microinjections of 2-amino-5-phosphonovaleric acid (AP-5), a competitive NMDA antagonist (0, 0.05, 0.2, 0.5, 1.0 microgram/0.5 microliter) into selected central and medial regions of the accumbens. The central and medial sites were assumed to correspond approximately to core and shell subregions, respectively. The animals were tested in two exploratory tasks: the open field and a novel object test. In the open field test, AP-5 significantly decreased peripheral locomotion and center rearing frequency in the central but not the medial group. Locomotion and rearing were not affected by AP5 infusion into a control site, the anterior dorsal striatum (ADS). In the novel object test, animals were tested in the same open field, with prior habituation, and with several novel objects placed within it. In this test, infusions of AP-5 (0, 1.0 microgram/0.5 microliter) decreased the number and duration of contacts with the novel objects in the central but not the medial group. In addition, peripheral and center locomotion were decreased by AP-5 infusions into the central site, whether objects were present or not. In contrast, AP-5 infusions into the medial site elicited an increase in peripheral locomotion in both stimulus conditions. These findings provide behavioral-pharmacological evidence that the central and medial subregions of the nucleus accumbens can be differentiated.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory amino acid receptors mediate the orofacial stereotypy elicited by dopaminergic stimulation of the ventrolateral striatum

The present experiments examined the role of excitatory amino acid receptors in the orofacial stereotypy induced by direct amphetamine microinjection into the ventrolateral striatum. In these experiments, the influence of prior intra-ventrolateral striatum treatment with various excitatory amino acid antagonists on the expression of amphetamine-stimulated oral stereotypy was observed. In all experiments, behavioral observations were conducted in the home cage using a time-sampling procedure. In the first experiment, different groups of rats received bilateral microinfusions of either kynurenic acid, 2-amino-5-phosphonopentanoic acid, 6,7-dinitroquinoxaline or dizocilpine maleate. The excitatory amino acid antagonists were administered immediately prior to bilateral microinfusions of d-amphetamine. Both N-methyl-D-aspartate and non-N-methyl-D-aspartate antagonists dose-dependently attenuated or blocked the expression of dopamine-mediated stereotypy. 2-Amino-5-phosphonopentanoic acid was the most potent of these compounds, totally suppressing stereotypy at a dose of 0.3 micrograms (equivalent to 1.5 nmol). In the second experiment, the same compounds were tested for their ability to suppress physostigmine-induced mouth movements. Cholinergic stimulation of the ventrolateral striatum has previously been shown to elicit non-directed mouth movements, quite distinguishable from stimulus-directed, amphetamine-induced biting. Excitatory amino acid antagonists were administered in the same doses prior to bilateral infusion of physostigmine (2.5 micrograms/0.5 microliters). The expression of physostigmine-induced mouth movements was for the most part not affected by excitatory amino acid antagonists, although dizocilpine maleate slightly reduced this oral behavior. In a third experiment, behavior was observed following infusion of the antagonists alone, using the same doses as in the previous experiments. No behavioral alterations were observed with the exception of a small increase in nonspecific mouth movements induced by kynurenic acid and 2-amino-5-phosphonopentanoic acid. These findings indicate that the expression of dopamine-mediated oral stereotypy, induced by amphetamine stimulation of the ventrolateral striatal region, is highly dependent on activation of striatal excitatory amino acid receptors. In contrast, oral behavior induced by cholinergic stimulation of the ventrolateral region is not mediated by glutamate input. These results are discussed in relation to the synaptic organization of neuronal elements within the striatum. Moreover, the relevance to further understanding of orofacial dyskinesias is noted.

Changes in the extracellular concentrations of amino acids in the rat striatum during transient focal cerebral ischemia

Although considerable evidence supports a role for amino acids in transient global cerebral ischemia and permanent focal cerebral ischemia, effects of transient focal cerebral ischemia on the extracellular concentrations of amino acids have not been reported. Accordingly, our study was undertaken to examine the patterns of changes of extracellular glutamate, aspartate, GABA, taurine, glutamine, alanine, and phosphoethanolamine in the striatum of transient focal cerebral ischemia, as evidence to support their pathogenic roles. Focal ischemia was induced using the middle cerebral artery occlusion model, with no need for craniotomy. Microdialysis was used to sample the brain's extracellular space before, during, and after the ischemic period. One hour of middle cerebral artery occlusion followed by recirculation caused neuronal damage that was common in the frontoparietal cortex and the lateral segment of the caudate nucleus. During 1 h of ischemia, the largest increase occurred for GABA and moderate increases were observed for taurine, glutamate, and aspartate. Alanine, which is a nonneuroactive amino acid, increased little. After recirculation, the levels of glutamate and aspartate reverted to normal baseline values right after reperfusion. Despite these rapid normalizations, neuronal damage occurred. Therefore, uptake of excitatory amino acids can still be restored after 1 h of middle cerebral artery occlusion, and tissue damage occurs even though high extracellular levels of glutamate are not maintained.

Glutamatergic mechanisms mediating stimulant and antipsychotic drug effects

This paper presents an hypothesis regarding the functions of striatal dopaminergic and glutamatergic neurotransmission. It is suggested that the principal functional role of dopaminergic neurotransmission is to regulate the efficacy of cortico-striatal and cortico-accumbens neurotransmission. Increased activity at dopamine-mediated synapses is suggested to interact with neurotransmission at adjacent cortically derived glutamate-mediated synapses, facilitating communication from the cerebral cortex, and thereby causing behavioral stimulation. Decreased activity at dopaminergic synapses, as produced by neuroleptic drugs, causes changes in the activation of cortically derived synapses in the corpus striatum and nucleus accumbens which result in behavioral sedation and decreased activity. This hypothesis suggests that activity at dopaminergic synapses produces behavioral effects only insofar as these changes modulate cortico-striatal (or cortico-accumbens) activity, and further, that the manifestations of activity in cortico-striatal systems are modulated by activity at dopaminergic synapses. It is further suggested that when neuroleptics are administered chronically, adjustments in the efficacy of cortico-striatal neurotransmission are responsible for the antipsychotic effect of neuroleptic drugs.