Effects of activation of NMDA and AMPA glutamate receptors on the extracellular concentrations of dopamine, acetylcholine, and GABA in striatum of the awake rat: a microdialysis study

Evaluation of the potential role of glutamatergic, cholinergic, and nitrergic systems in the dopamine release induced by the pesticide glyphosate in rat striatum

Glyphosate (GLY) is a pesticide that severely alters nigrostriatal dopaminergic neurotransmission, inducing great increases in dopamine release from rat dorsal striatum. This GLY-induced striatal dopamine overflow occurs through mechanisms not yet fully understood, hence the interest in evaluating the role of other neurotransmitter systems in such effects. So, the main objective of this mechanistic study was to evaluate the possible mediation of the glutamatergic, cholinergic, and nitrergic systems in the GLY-induced in vivo dopamine release from rat dorsal striatum. The extracellular dopamine levels were measured by cerebral microdialysis and HPLC with electrochemical detection. Intrastriatal administration of GLY (5 mmol/L) significantly increased the dopamine release (1102%). Pretreatment with MK-801 (50 or 400 μmol/L), a non-competitive antagonist of NMDA receptors, significantly decreased the effect of GLY (by 70% and 74%, respectively), whereas AP-5 (400 μmol/L), a competitive antagonist of NMDA receptors, or CNQX (500 μmol/L), an AMPA/kainate receptor antagonist, had no significant effect. Administration of the nitric oxide synthase inhibitors, L-nitroarginine (L-NAME, 100 μmol/L) or 7-nitroindazole (7-NI, 100 μmol/L), also did not alter the effect of GLY on dopamine release. Finally, pretreatment of the animals with mecamylamine, an antagonist of nicotinic receptors, decreased the effect of GLY on dopamine release by 49%, whereas atropine, a muscarinic antagonist, had no significant effect. These results indicate that GLY-induced dopamine release largely depends on the activation of NMDA and nicotinic receptors in rat dorsal striatum. Future research is needed to determine the effects of this pesticide at environmentally relevant concentrations.

Neonatal Prefrontal Inactivation Results in Reversed Dopaminergic Responses in the Shell Subregion of the Nucleus Accumbens to NMDA Antagonists

Striatal dopaminergic dysregulation in schizophrenia could result from a prefronto-striatal dysconnectivity, of neurodevelopmental origin, involving N-methyl-d-aspartate (NMDA) receptors. The dorsomedian shell part of the nucleus accumbens is a striatal subregion of particular interest inasmuch as it has been described as the common target region for antipsychotics. Moreover, NMDA receptors located on the dopaminergic endings have been reported in the shell. The present study examines in adult rats the effects of early functional inactivation of the left prefrontal cortex on behavioral and dopaminergic responses in the dorsomedian shell part of the nucleus accumbens following administration of two noncompetitive NMDA receptor antagonists, ketamine, and dizocilpine (MK-801). The results showed that postnatal blockade of the prefrontal cortex led to increased locomotor activity as well as increased extracellular dopamine levels in the dorsomedian shell following administration of both noncompetitive NMDA receptor antagonists, and, more markedly, after treatment with the more specific one, MK-801, whereas decreased dopaminergic levels were observed in respective controls. These data suggest a link between NMDA receptor dysfunctioning and dopamine dysregulation at the level of the dorsomedian shell part of the nucleus accumbens. They may help to understand the pathophysiology of schizophrenia in a neurodevelopmental perspective.

Catecholamines and cognition after traumatic brain injury

Cognitive problems are one of the main causes of ongoing disability after traumatic brain injury. The heterogeneity of the injuries sustained and the variability of the resulting cognitive deficits makes treating these problems difficult. Identifying the underlying pathology allows a targeted treatment approach aimed at cognitive enhancement. For example, damage to neuromodulatory neurotransmitter systems is common after traumatic brain injury and is an important cause of cognitive impairment. Here, we discuss the evidence implicating disruption of the catecholamines (dopamine and noradrenaline) and review the efficacy of catecholaminergic drugs in treating post-traumatic brain injury cognitive impairments. The response to these therapies is often variable, a likely consequence of the heterogeneous patterns of injury as well as a non-linear relationship between catecholamine levels and cognitive functions. This individual variability means that measuring the structure and function of a person’s catecholaminergic systems is likely to allow more refined therapy. Advanced structural and molecular imaging techniques offer the potential to identify disruption to the catecholaminergic systems and to provide a direct measure of catecholamine levels. In addition, measures of structural and functional connectivity can be used to identify common patterns of injury and to measure the functioning of brain ‘networks’ that are important for normal cognitive functioning. As the catecholamine systems modulate these cognitive networks, these measures could potentially be used to stratify treatment selection and monitor response to treatment in a more sophisticated manner.

Ammonia mediates methamphetamine-induced increases in glutamate and excitotoxicity

Ammonia has been identified to have a significant role in the long-term damage to dopamine and serotonin terminals produced by methamphetamine (METH), but how ammonia contributes to this damage is unknown. Experiments were conducted to identify whether increases in brain ammonia affect METH-induced increases in glutamate and subsequent excitotoxicity. Increases in striatal glutamate were measured using in vivo microdialysis. To examine the role of ammonia in mediating changes in extracellular glutamate after METH exposure, lactulose was used to decrease plasma and brain ammonia. Lactulose is a non-absorbable disaccharide, which alters the intestinal lumen through multiple mechanisms that lead to the increased peripheral excretion of ammonia. METH caused a significant increase in extracellular glutamate that was prevented by lactulose. Lactulose had no effect on METH-induced hyperthermia. To determine if ammonia contributed to excitotoxicity, the effect of METH and lactulose treatment on calpain-mediated spectrin proteolysis was measured. METH significantly increased calpain-specific spectrin breakdown products, and this increase was prevented with lactulose treatment. To examine if ammonia-induced increases in extracellular glutamate were mediated by excitatory amino-acid transporters, the reverse dialysis of ammonia, the glutamate transporter inhibitor, DL-threo-β-benzyloxyaspartic acid (TBOA), or the combination of the two directly into the striatum of awake, freely moving rats was conducted. TBOA blocked the increases in extracellular glutamate produced by the reverse dialysis of ammonia. These findings demonstrate that ammonia mediates METH-induced increases in extracellular glutamate through an excitatory amino-acid transporter to cause excitotoxicity.

Analysis of cerebrospinal fluid γ-aminobutyric acid by capillary electrophoresis with laser-induced fluorescence detection

The measurement of γ-aminobutyric acid (GABA) is suitable for investigating various neurological disorders. In this study, a sensitive and selective method for free GABA quantification in cerebrospinal fluid (CSF) has been standardised. This method is based on CE with LIF detection using 4-fluoro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-F) as a derivatisating agent. The reaction conditions (NBD-F concentration, pH, temperature and reaction time) and the electrophoretic parameters (run buffer composition and pH and separation voltage) were optimised to obtain the maximum derivatisation efficiency and electrophoretic resolution. The best resolution was obtained using 200 mM sodium borate, 10 mM SDS, 8.5 mM β-CD, pH 10 and 20 kV voltage. The method was linear in the concentration range of 2.5-1000 nM with good inter- and intra-assay precision values. The effects of CSF handling on free GABA concentrations were also evaluated. Our results show that the time delay between CSF collection and freezing strongly increases the CSF GABA values. Age-related reference values were established in 55 paediatric controls. The influence of antiepileptic therapy on free CSF GABA was studied in 38 neuropaediatric patients. Significantly, higher GABA values were obtained in patients taking valproic acid or vigabatrin therapy, which are antiepileptic drugs that modulate GABA metabolism.

Ketamine increases striatal dopamine release and hyperlocomotion in adult rats after postnatal functional blockade of the prefrontal cortex

Schizophrenia is a complex psychiatric disorder that may result from defective connectivity, of neurodevelopmental origin, between several integrative brain regions. Different anomalies consistent with brain development failures have been observed in patients' left prefrontal cortex (PFC). A striatal dopaminergic functional disturbance is also commonly acknowledged in schizophrenia and could be related to a dysfunctioning of dopamine-glutamate interactions. Non-competitive NMDA antagonists, such as ketamine, can induce psychotic symptoms in healthy individuals and worsen these symptoms in patients with schizophrenia. Our study set out to investigate the consequences of neonatal functional blockade of the PFC for dopaminergic and behavioral reactivity to ketamine in adult rats. Following tetrodotoxin (TTX) inactivation of the left PFC at postnatal day 8, dopaminergic responses induced by ketamine (5 mg/kg, 10 mg/kg, 20 mg/kg sc) were monitored using in vivo voltammetry in the left part of the dorsal striatum in freely moving adult rats. Dopaminergic responses and locomotor activity were followed in parallel. Compared to PBS animals, in rats microinjected with TTX, ketamine challenge induced a greater release of dopamine in the dorsal striatum for the highest dose (20 mg/kg sc) and the intermediate dose (10mg/kg sc). A higher increase in locomotor activity in TTX animals was observed only for the highest dose of ketamine (20 mg/kg sc). These data suggest transient inactivation of the PFC during early development results in greater behavioral and striatal dopaminergic reactivity to ketamine in adulthood. Our study provides an anatomo-functional framework that may contribute toward a better understanding of the involvement of NMDA glutamatergic receptors in schizophrenia.

Ethanol-induced alterations of amino acids measured by in vivo microdialysis in rats: a meta-analysis

PURPOSE

In recent years in vivo microdialysis has become an important method in research studies investigating the alterations of neurotransmitters in the extracellular fluid of the brain. Based on the major involvement of glutamate and γ-aminobutyric acid (GABA) in mediating a variety of alcohol effects in the mammalian brain, numerous microdialysis studies have focused on the dynamical behavior of these systems in response to alcohol.

METHODS

Here we performed multiple meta-analyses on published datasets from the rat brain: (i) we studied basal extracellular concentrations of glutamate and GABA in brain regions that belong to a neurocircuitry involved in neuropsychiatric diseases, especially in alcoholism (Noori et al., Addict Biol 17:827-864, 2012); (ii) we examined the effect of acute ethanol administration on glutamate and GABA levels within this network and (iii) we studied alcohol withdrawal-induced alterations in glutamate and GABA levels within this neurocircuitry.

RESULTS

For extraction of basal concentrations of these neurotransmitters, datasets of 6932 rats were analyzed and the absolute basal glutamate and GABA levels were estimated for 18 different brain sites. In response to different doses of acute ethanol administration, datasets of 529 rats were analyzed and a non-linear dose response (glutamate and GABA release) relationship was observed in several brain sites. Specifically, glutamate in the nucleus accumbens shows a decreasing logarithmic dose response curve. Finally, regression analysis of 11 published reports employing brain microdialysis experiments in 104 alcohol-dependent rats reveals very consistent augmented extracellular glutamate and GABA levels in various brain sites that correlate with the intensity of the withdrawal response were identified.

CONCLUSIONS

In summary, our results provide standardized basal values for future experimental and in silico studies on neurotransmitter release in the rat brain and may be helpful to understand the effect of ethanol on neurotransmitter release. Furthermore, this study illustrates the benefit of meta-analyses using the generalization of a wide range of preclinical data.

α7 neuronal nicotinic receptor agonist (TC-7020) reverses increased striatal dopamine release during acoustic PPI testing in a transgenic mouse model of schizophrenia

Genetic and post mortem evidence has implicated the α7 neuronal nicotinic receptor (NNR) in the etiology of schizophrenia and related disorders. In schizophrenia, enhanced subcortical dopamine (DA) correlates with positive and cognitive of the disease, including impairments in sensorimotor gating. We measured the levels of extracellular DA and DA metabolites during an acoustic test session of prepulse inhibition (PPI) of the startle response, a measure of sensorimotor gating, by microdialysis and HPLC-EC in a transgenic mouse model of schizophrenia. In th-fgfr1(tk-) mice, blockade of fibroblast growth factor receptor 1 (FGFR1) signaling during development in catecholaminergic neurons results in reduced size and density of midbrain DA neurons of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA). These mice displayed reduced PPI and enhanced startle response relative to control mice as well as a potentiation of DA release in the dorsal striatum during a 30 minute PPI test session. Acute administration of a partial α7 NNR agonist TC-7020 (1.0 mg/kg) normalized PPI and startle deficits and attenuated increases of DA release during acoustic PPI testing. These results provide direct evidence of elevated striatal dopaminergic transmission with impaired sensorimotor gating that may underlie cognitive and positive symptoms and motor deficits in schizophrenia and related disorders. Also, systemic targeting of alpha7 NNRs may ameliorate these deficits by functionally suppressing striatal DA activity.

Regulation of glutamate release by α7 nicotinic receptors: differential role in methamphetamine-induced damage to dopaminergic and serotonergic terminals

Regulation of glutamate release is an important underlying mechanism in mediating excitotoxic events such as damage to dopamine (DA) and serotonin (5-HT) neurons observed after exposure to methamphetamine (Meth). One way to regulate glutamate release may be through the modulation of α7 nicotinic acetylcholine (nACh) receptors. Meth administration is known to increase acetylcholine release; however, it is unknown whether Meth increases glutamate release and causes long-term damage to both DA and 5-HT terminals through the activation of α7 nACh receptors. To test this hypothesis, the α7 nACh receptor antagonist, methyllycaconitine (MLA), was administered before the administration of repeated doses of Meth while simultaneously monitoring extracellular striatal glutamate with in vivo microdialysis. In addition, the subsequent long-term decreases in markers of dopaminergic and serotonergic terminals, including DA reuptake transporter (DAT), serotonin reuptake transporter (SERT), vesicular monoamine transporter-2, vesicular DA, and vesicular 5-HT content in the rat striatum, were measured. The results show that MLA pretreatment prevented Meth-induced increases in striatal glutamate and protected against the subsequent long-term decreases in striatal DAT and vesicular DA content without affecting the hyperthermia produced by Meth. In contrast, the Meth-induced decreases in striatal SERT immunoreactivity and vesicular 5-HT content were not affected by MLA. This suggests that the α7 nACh receptor differentially mediates glutamate-dependent damage to DA but not 5-HT terminals in a manner that is independent of hyperthermia. Furthermore, antagonism of α7 nACh receptors may be a possible therapeutic strategy for decreasing extracellular glutamate and preventing the excitotoxic damage observed in other DA-related neurodegenerative disorders.

GABAB receptor-dependent modulation of network activity in the rat prefrontal cortex in vitro

GABA (gamma-aminobutyric acid) can mediate inhibition via pre- and post/extrasynaptic GABA receptors. In this paper we demonstrate potentially post/extrasynaptic GABA(B) receptor-dependent tonic inhibition in L2/3 pyramidal cells of rat medial prefrontal cortex (mPFC) in vitro. First, we show via voltage-clamp experiments the presence of a tonic GABA(B) receptor-dependent outward current in these neurons. This GABA(B)ergic current could be induced by ambient GABA when present at sufficient concentrations. To increase ambient GABA levels in the usually silent slice preparation, we amplified network activity and hence synaptic GABA release with a modified artificial cerebrospinal fluid. The amplitude of tonic GABA(B) current was similar at different temperatures. In addition to the tonic GABA(B) current, we found presynaptic GABA(B) effects, GABA(B)-mediated inhibitory postsynaptic currents and tonic GABA(A) currents. Second, we performed current-clamp experiments to evaluate the functional impact of GABA(B) receptor-mediated inhibition in the mPFC. Activating or inactivating GABA(B) receptors led to rightward (reduction of excitability) or leftward (increase of excitability) shifts, respectively, of the input-output function of mPFC L2/3 pyramidal cells without effects on the slope. Finally, we showed in electrophysiological recordings and epifluorescence Ca(2+)-imaging that GABA(B) receptor-mediated tonic inhibition is capable of regulating network activity. Blocking GABA(B) receptors increased the frequency of excitatory postsynaptic currents impinging on a neuron and prolonged network upstates. These results show that ambient GABA via GABA(B) receptors is powerful enough to modulate neuronal excitability and the activity of neural networks.

Cadmium effects on 24h changes in glutamate, aspartate, glutamine, GABA and taurine content of rat striatum

This work evaluates the possible changes in 24 h variations of striatal aspartate, glutamate, glutamine, gamma-aminobutyric acid (GABA) and taurine content after oral cadmium treatment. Male rats were submitted to cadmium exposure at two doses (25 and 50 mg/L of cadmium chloride (CdCl(2))) in the drinking water for 30 days. Control rats received cadmium-free water. After the treatment, rats were killed at six different time intervals throughout a 24 h cycle. Differential effects of cadmium on 24 h amino acid fluctuations were observed. Metal exposure modified the daily pattern of the amino acids concentration found in control animals, except for GABA and taurine with the lowest dose used. Exposure to 25 mg/L of CdCl(2) decreased mean content of aspartate, as well as GABA concentration. These results suggest that cadmium exposure affects 24 h changes of the studied amino acids concentration in the striatum, and those changes may be related to alterations in striatal function.

Involvement of NMDA receptors in the ventrolateral striatum of rats in apomorphine-induced jaw movements

The role of NMDA receptors in the ventrolateral striatum to modulate dopamine receptor-mediated jaw movements was investigated in freely moving rats, using a magnetic sensor system combined with intracerebral microinjection of drugs. Apomorphine (1mg/kg i.v.) induced repetitive jaw movements that were reduced, in a dose-dependent manner, by bilateral microinjections of the NMDA receptor agonist NMDA (0.1 and 1mug/0.2mul bilaterally) into the ventrolateral striatum. Apomorphine-induced repetitive jaw movements were also reduced, in a dose-dependent manner, by bilateral microinjections of the NMDA receptor antagonists d-APV (0.01 and 0.1mug) or MK-801 (0.5 and 5mug). The inhibitory effect of NMDA (1mug) was reduced by co-administration of MK-801 (0.5mug). Microinjections of drugs into the ventrolateral striatum in the absence of apomorphine did not affect jaw movements. These results suggest that NMDA receptors in the ventrolateral striatum play an important modulatory role in the expression of dopamine receptor-mediated jaw movements. However, similar effects of NMDA and NMDA antagonists echo previous paradoxical findings and indicate that interactions between dopamine and NMDA receptors are complex and multifaceted. Cellular mechanism(s) may involve differential effects of NMDA agonism and antagonism on dopamine D1-like vs D2-like receptors and, possibly, on related GABAergic processes.

Novel environment and GABA agonists alter event-related potentials in N-methyl-D-aspartate NR1 hypomorphic and wild-type mice

Clinical and experimental data suggest dysregulation of N-methyl-d-aspartate receptor (NMDAR)-mediated glutamatergic pathways in schizophrenia. The interaction between NMDAR-mediated abnormalities and the response to novel environment has not been studied. Mice expressing 5 to 10% of normal N-methyl-d-aspartate receptor subunit 1 (NR1) subunits [NR1(neo)(-/-)] were compared with wild-type littermates for positive deflection at 20 ms (P20) and negative deflection at 40 ms (N40) auditory event-related potentials (ERPs). Groups were tested for habituation within and across five testing sessions, with novel environment tested during a sixth session. Subsequently, we examined the effects of a GABA(A) positive allosteric modulator (chlordiazepoxide) and a GABA(B) receptor agonist (baclofen) as potential interventions to normalize aberrant responses. There was a reduction in P20, but not N40 amplitude within each habituation day. Although there was no amplitude or gating change across habituation days, there was a reduction in P20 and N40 amplitude and gating in the novel environment. There was no difference between genotypes for N40. Only NR1(neo)(-/-) mice had reduced P20 in the novel environment. Chlordiazepoxide increased N40 amplitude in wild-type mice, whereas baclofen increased P20 amplitude in NR1(neo)(-/-) mice. As noted in previous publications, the pattern of ERPs in NR1(neo)(-/-) mice does not recapitulate abnormalities in schizophrenia. In addition, reduced NR1 expression does not influence N40 habituation but does affect P20 in a novel environment. Thus, the pattern of P50 (positive deflection at 50 ms) but not N100 (negative deflection at 100 ms) in human studies may relate to subjects' reactions to unfamiliar environments. In addition, NR1 reduction decreased GABA(A) receptor-mediated effects on ERPs while causing increased GABA(B) receptor-mediated effects. Future studies will examine changes in GABA receptor subunits after reductions in NR1 expression.

GABAB/NMDA receptor interaction in the regulation of extracellular dopamine levels in rodent prefrontal cortex and striatum

Deficits in N-methyl-D-aspartate receptor (NMDAR)-mediated neurotransmission may underlie dopaminergic hyperactivity in schizophrenia. Dysregulation of the GABAergic system has also been implicated. In this study we investigated a role for GABA(B) receptors as an intermediate step in the pathway leading from NMDAR stimulation to DA regulation. Since glycine (GLY) has been found to ameliorate treatment resistant negative symptoms in schizophrenia, we treated a group of rats with 16% GLY food for 2 weeks. DA levels in prefrontal cortex (PFC) and striatum (STR) were assessed by dual-probe microdialysis and HPLC-EC in freely moving rats. Infusion of the GABA(B) receptor agonists SKF97541 and baclofen into PFC and STR significantly reduced basal DA, an effect that was reversed by the antagonist, CGP52432. In PFC, GABA(B) agonists also reduced AMPH-induced DA release following treatment with either 1 or 5 mg/kg AMPH. Similar effects were seen following subchronic glycine treatment in the absence, but not presence of CGP52432 during 5 mg/kg AMPH treatment. In STR SKF97541 decreased only the 1 mg/kg AMPH-induced DA release. Subchronic GLY treatment in STR leads to a significant reduction in basal DA levels, but did not affect AMPH (5 mg/kg)-induced release. Our findings support a model in which NMDA/glycine-site agonists modulate DA release in part through presynaptic GABA(B) receptors on DA terminals, with both GABA(B) ligands and GLY significantly modulating AMPH-induced DA release. Both sites, therefore, may represent appropriate targets for drug development in schizophrenia and substance abuse disorders.

Effects of stimulation of the centromedian nucleus of the thalamus on the activity of striatal cells in awake rhesus monkeys

Although the existence of a massive projection from the caudal intralaminar nuclei of the thalamus [i.e. the centromedian (CM) and parafascicular nuclei] to the striatum is well documented, the effects of CM activation upon striatal cells remain poorly understood. Therefore, we studied the effects of electrical stimulation of CM on the electrophysiological activity of striatal neurons, and on striatal levels of gamma-aminobutyric acid (GABA) and acetylcholine in rhesus monkeys. Striatal cells did not respond to single-pulse stimulation (bipolar biphasic stimulation, 175-500 muA), but the large majority of recorded neurons responded to burst stimulation (100 Hz, 1 s, 150-175 muA) of CM, often with a delay of tens of milliseconds. Striatal phasically active neurons, which likely correspond to projection neurons, responded mainly with increases in firing (13/28 cells), while tonically active neurons (likely cholinergic interneurons) often showed combinations of increases and decreases in firing (24/46 cells). In microdialysis studies, CM stimulation led to a reduction of striatal acetylcholine levels. This effect was prevented by addition of the GABA-A receptor antagonist gabazine to the microdialysis fluid. We conclude that CM stimulation frequently results in striatal response patterns with excitatory and inhibitory components. Under the conditions chosen here, the specific patterns of striatal responses to CM stimulation are likely the result of striatal processing of thalamic inputs. Through these indirect effects, local CM stimulation may engage large portions of the striatum. These effects may be relevant in the interpretation of the therapeutic effects of CM stimulation for the treatment of neurological disorders.

MK801- and scopolamine-induced amnesias are reversed by an Amazonian herbal locally used as a "brain tonic"

RATIONALE

Traditional remedies prepared from Ptychopetalum olacoides (PO) are used throughout the Amazon to alleviate age-related conditions. These formulas are mainly used by elders, and alleged effects may be related to the anticholinesterase properties identified in a standardized ethanol extract of this species [P. olacoides standardized ethanol extract (POEE)].

OBJECTIVES

To further characterize the potential of this extract for developing drugs useful to treat cognitive deficits, the effects of POEE on scopolamine (scop)- and MK801-induced amnesias (acquisition, consolidation, and retrieval) in mice were investigated.

RESULTS

Scop (3.0 mg/kg, ip) significantly impaired memory (all three phases) in the step-down inhibitory avoidance protocol. As expected, MK801 (0.1 mg/kg, ip) was amnesic regarding acquisition and consolidation, but not retrieval. POEE (100 mg/kg, ip) reversed the scop-induced impairment in all three phases of long-term and short memories, whereas only the memory consolidation deficit was reversed with MK801-induced amnesia.

CONCLUSIONS

This study complements previously reported promnesic properties of this plant extract and suggests that POEE may be further developed for treating conditions associated with cognitive deficits, especially those linked with cholinergic malfunction.

Circadian control of the daily plasma glucose rhythm: an interplay of GABA and glutamate

The mammalian biological clock, located in the hypothalamic suprachiasmatic nuclei (SCN), imposes its temporal structure on the organism via neural and endocrine outputs. To further investigate SCN control of the autonomic nervous system we focused in the present study on the daily rhythm in plasma glucose concentrations. The hypothalamic paraventricular nucleus (PVN) is an important target area of biological clock output and harbors the pre-autonomic neurons that control peripheral sympathetic and parasympathetic activity. Using local administration of GABA and glutamate receptor (ant)agonists in the PVN at different times of the light/dark-cycle we investigated whether daily changes in the activity of autonomic nervous system contribute to the control of plasma glucose and plasma insulin concentrations. Activation of neuronal activity in the PVN of non-feeding animals, either by administering a glutamatergic agonist or a GABAergic antagonist, induced hyperglycemia. The effect of the GABA-antagonist was time dependent, causing increased plasma glucose concentrations only when administered during the light period. The absence of a hyperglycemic effect of the GABA-antagonist in SCN-ablated animals provided further evidence for a daily change in GABAergic input from the SCN to the PVN. On the other hand, feeding-induced plasma glucose and insulin responses were suppressed by inhibition of PVN neuronal activity only during the dark period. These results indicate that the pre-autonomic neurons in the PVN are controlled by an interplay of inhibitory and excitatory inputs. Liver-dedicated sympathetic pre-autonomic neurons (responsible for hepatic glucose production) and pancreas-dedicated pre-autonomic parasympathetic neurons (responsible for insulin release) are controlled by inhibitory GABAergic contacts that are mainly active during the light period. Both sympathetic and parasympathetic pre-autonomic PVN neurons also receive excitatory inputs, either from the biological clock (sympathetic pre-autonomic neurons) or from non-clock areas (para-sympathetic pre-autonomic neurons), but the timing information is mainly provided by the GABAergic outputs of the biological clock.

Inhibition of stimulated dopamine release and hemodynamic response in the brain through electrical stimulation of rat forepaw

The subcortical response to peripheral somatosensory stimulation is not well studied. Prior literature suggests that somatosensory stimulation can affect dopaminergic tone. We studied the effects of electrical stimulation near the median nerve on the response to an amphetamine-induced increase in synaptic dopamine. We applied the electrical stimulation close to the median nerve 20 min after administration of 3mg/kg amphetamine. We used fMRI and microdialysis to measure markers of dopamine (DA) release, together with the release of associated neurotransmitters of striatal glutamate (Glu) and gamma-aminobutyric acid (GABA). Changes in cerebral blood volume (CBV), a marker used in fMRI, indicate that electrical stimulation significantly attenuated increased DA release (due to AMPH) in the striatum, thalamus, medial prefrontal and cingulate cortices. Microdialysis showed that electrical stimulation increased Glu and GABA release and attenuated the AMPH-enhanced DA release. The striatal DA dynamics correlated with the CBV response. These results demonstrate that electrical stimulation near the median nerve activates Glu/GABA release, which subsequently attenuate excess striatal DA release. These data provide evidence for physiologic modulation caused by electroacupuncture at points near the median nerve.

Abnormal brain tryptophan metabolism and clinical correlates in Tourette syndrome

Symptoms in Tourette syndrome (TS) are likely related to abnormalities involving multiple neurotransmitter systems in striatal-thalamo-cortical circuitry. Although prior studies have found abnormal levels of tryptophan, serotonin, and their metabolites in blood, cerebrospinal fluid and brain tissue of TS patients, understanding of focal brain disturbances and their relationship to clinical phenotype remains poor. We used alpha-[(11)C]methyl-L-tryptophan (AMT) positron emission tomography (PET) to assess global and focal brain abnormalities of tryptophan metabolism and their relationship to behavioral phenotype in 26 children with TS and nine controls. Group comparisons on regional cortical and subcortical AMT uptake revealed decreased AMT uptake in bilateral dorsolateral prefrontal cortical and bilaterally increased uptake in the thalamus (P = 0.001) in TS children. The ratio of AMT uptake in subcortical structures to dorsolateral prefrontal cortex was significantly increased bilaterally (P < 0.01) in TS patients also. Behaviorally defined subgroups within the TS sample revealed differences in the pattern of AMT uptake in the fronto-striatal-thalamic circuit. This study demonstrates cortical and subcortical abnormalities of tryptophan metabolism in TS and provides neuroimaging evidence for a role of serotonergic mechanisms in the pathophysiology of TS.

Glutamate-dopamine-GABA interactions in the aging basal ganglia

The study of neurotransmitter interactions gives a better understanding of the physiology of specific circuits in the brain. In this review we focus mostly on our own results on the interaction of the neurotransmitters glutamate, dopamine and GABA in the basal ganglia during the normal process of aging. We review first the studies on the action of endogenous glutamate on the extracellular concentrations of dopamine and GABA in the neostriatum and nucleus accumbens during aging. It was found that there exists an age-related change in the interaction of glutamate, dopamine and GABA and that these effects of aging exhibit a dorsal-to-ventral pattern of effects with no changes in the dorsal parts (dorsal striatum) and changes in the most ventral parts (nucleus accumbens). Second we reviewed the data on the effects of different ionotropic and metabotropic glutamate receptor agonists on the extracellular concentrations of dopamine and GABA in the nucleus accumbens. The results obtained clearly show the different contribution of each glutamate receptor subtype in the age-related changes produced on the interaction of glutamate, dopamine and GABA in this area of the brain. Third the effects of an enriched environment on the action of AMPA and NMDA-receptor agonists in the nucleus accumbens of rats during aging are also evaluated. Finally, and since the nucleus accumbens has been suggested to play a role in emotion and motivation and also motor behaviour, we speculated on the possibility of a specific contribution for the different glutamatergic pathways terminating in the nucleus accumbens and their interaction with a decreased dopamine playing a relevant role in motor behaviour during aging.

Chronic treatment with a dopamine uptake blocker changes dopamine and acetylcholine but not glutamate and GABA concentrations in prefrontal cortex, striatum and nucleus accumbens of the awake rat

The present study was aimed to investigate the effects of a chronic treatment with the dopamine uptake blocker nomifensine on the in vivo extracellular concentrations of dopamine, acetylcholine, glutamate and GABA in the prefrontal cortex, striatum and nucleus accumbens. Male Wistar rats received intraperitoneal (i.p.) daily injections of nomifensine (10 mg/kg) or saline for 22 days. Microdialysis experiments were performed on days 1, 8, 15 and 22 of treatment to evaluate the effects of the injection of nomifensine or saline. Motor activity of the animals was monitored during microdialysis experiments. Injections of nomifensine increased extracellular concentration of dopamine in striatum and nucleus accumbens, but not in prefrontal cortex. Acetylcholine concentrations in striatum but not in nucleus accumbens were increased by nomifensine on days 15 and 22 of treatment. In prefrontal cortex, nomifensine increased acetylcholine levels without differences among days. No changes were found on glutamate and GABA concentrations in the three areas studied. Injections of nomifensine also increased spontaneous motor activity and stereotyped behaviour without differences among days. These results show that systemic chronic treatment with a dopamine uptake blocker produces differential effects on extracellular concentrations of dopamine and acetylcholine, but not glutamate and GABA, in different areas of the brain.

Aging, plasticity and environmental enrichment: structural changes and neurotransmitter dynamics in several areas of the brain

Cajal was probably the first neurobiologist to suggest that plasticity of nerve cells almost completely disappeared during aging. However, we know today that neural plasticity is still present in the brain during aging. In this review we suggest that aging is a physiological process that occurs asynchronously in different areas of the brain and that the rate of that process is modulated by environmental factors and related to the neuronal-synaptic-molecular substrates of each area. We review here some of the most recent results on aging of the brain in relation to the plastic changes that occur in young and aged animals as a result of living in an enriched environment. We highlight the results from our own laboratory on the dynamics of neurotransmitters in different areas of the brain. Specifically we review first the effects of aging on neurons, dendrites, synapses, and also on molecular and functional plasticity. Second, the effects of environmental enrichment on the brain of young and aged animals. And third the effects of an enriched environment on the age-related changes in neurogenesis and in the extracellular concentrations of glutamate and GABA in hippocampus, and on dopamine, acetylcholine, glutamate and GABA under a situation of acute mild stress in the prefrontal cortex.

Capillary hydrophilic interaction chromatography/mass spectrometry for simultaneous determination of multiple neurotransmitters in primate cerebral cortex

A diverse array of neurotransmitters and neuromodulators control and affect brain function. A profound understanding of the signaling pathways and the neural circuits underlying behavior is therefore likely to require the tracking of concentration changes of active neurochemicals. In the present study, we demonstrate the feasibility of a method allowing the simultaneous determination of the concentrations of six neurotransmitters: acetylcholine, serotonin, dopamine, gamma-aminobutyric acid (GABA), glutamate and aspartate, in the extracellular brain fluid (EBF). We used hydrophilic interaction chromatography (HILIC) coupled to tandem mass spectrometry (MS/MS) to analyze the EBF from the monkey brain. A push-pull sampling method was used to collect EBF from the prefrontal cortex (PFC) of conscious monkeys at flow rates in the range of low nL/min. The detection limits of acetylcholine, serotonin, dopamine, GABA, glutamate and aspartate were 0.015, 0.15, 0.3, 1.2, 6 and 15 femtomoles, respectively, allowing us to quantitatively determine the concentrations of these six neurotransmitters simultaneously from 500 nL in vivo samples. We conclude that HILIC/MS/MS combined with the push-pull sampling method represents a sensitive technique for simultaneous monitoring of neurotransmitters from EBF samples.

Efficient measurement of endogenous neurotransmitters in small localized regions of central nervous systems in vitro with HPLC

High performance liquid chromatography (HPLC) is widely used to determine neurotransmitter concentrations in the central nervous system (CNS). Finding the optimal methods to sample from CNS tissue poses a challenge for neuroscientists. Here, we describe a method that allows assay of neurotransmitters (or other chemicals) in small regions (down to 180mum in diameter) in in vitro preparations concurrently with electrophysiological recordings. The efficiency for measuring small amounts of chemicals is enhanced by a sample collecting pipette with filter paper at the tip that makes close contact with the target region in CNS tissue. With a wire plunger in the calibrated pipette controlled by a microsyringe pump, there is virtually no dead volume. Samples in a volume of 10muL (taken, e.g., at 2muL/min over 5min) can be injected into a HPLC machine with microbore columns. We demonstrate the effectiveness of this method by measuring acetylcholine (ACh) in the ventral horn and its surrounding areas of the spinal cord in en bloc brainstem-spinal cord preparations. In control conditions, endogenous ACh levels in these regions were detectable. Application of neostigmine (an inhibitor of acetylcholinesterases (AChEs)) increased ACh concentrations, and at the same time, induced tonic/seizure-like activity in efferent motor output recorded from cervical ventral nerve roots. Higher ACh concentrations in the ventral horn were differentiated from nearby regions: the lateral and midline aspects of the ventral spinal cord. In addition, ACh in the preBötzinger Complex (preBötC) and the hypoglossal nucleus in medullary slice preparations can also be measured. Our results indicate that the method proposed in this study can be used to measure neurotransmitters in small and localized CNS regions. Correlation between changes in neurotransmitters in target regions and the neuronal activities can be revealed in vitro. Our data also suggest that there is endogenous ACh release in spinal ventral motor columns at fourth cervical (C4) level that regulates the respiratory-related motor activity.

Mediation of glutamatergic receptors and nitric oxide on striatal dopamine release evoked by anatoxin-a. An in vivo microdialysis study

In this work, the involvement of ionotropic glutamatergic receptors and nitric oxide on striatal dopamine release induced by anatoxin-a was investigated in conscious and freely-moving rats. To study the participation of glutamatergic receptors, the effects of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine (MK-801) and d(-)-2-amino-5-phosphonopentanoic acid (APV), were examined. The perfusion of 3.5 mM anatoxin-a increased the extracellular dopamine levels to 701% relative to the basal. When CNQX was administered with 3.5 mM anatoxin-a, the increase of dopamine levels was 29% smaller than that observed with anatoxin-a alone. When MK-801 and APV were administered, the effect of anatoxin-a was attenuated 26% and 25% respectively in terms of that observed with anatoxin-a alone. And with CNQX plus MK-801, the effect of anatoxin-a was 53% inhibited in terms of the effect of anatoxin-a alone. These results suggest that the striatal dopamine release induced by anatoxin-a is partly mediated by activation of both ionotropic glutamatergic receptors. Since the neuronal form of nitric oxide synthase (nNOS) produces nitric oxide (NO) primarily in response to activation of NMDA receptors, it was tested if NO could play any role in the effect of anatoxin-a. Treatment with NOS inhibitors, L-nitro-arginine methyl ester (L-NAME) and d(-)-2-amino-5-phosphonopentanoic acid (7-NI), induced decreased anatoxin-a effects of 22% and 26% respectively. In conclusion, the present in vivo results demonstrate that anatoxin-a induced an indirect activation of ionotropic glutamatergic receptors (NMDA and AMPA/kainite receptors), which stimulate striatal dopamine release. On the other hand, activation of NMDA receptors may elicit NO increased levels enhancing dopamine release.

Environmental enrichment reduces the function of D1 dopamine receptors in the prefrontal cortex of the rat

Environmental enrichment produces changes in spontaneous and psychostimulant-induced motor activity. Dopamine in the prefrontal cortex (PFC), through the activation of D1 receptors, has been suggested to play a role in modulating motor activity. The present study investigated the effects of environmental enrichment on spontaneous motor activity, prefrontal acetylcholine release following local D1 receptor stimulation and D1 receptor expression in the PFC. Male wistar rats (3 months of age) were housed in enriched or isolated conditions during 90 days. Animals were then implanted with guide cannulae to perform microdialysis experiments in the PFC. Spontaneous motor activity and acetylcholine extracellular concentrations were monitored simultaneously. Also spontaneous motor activity was measured in an open field. On completion of the experiments, the density of D1 receptors in the PFC was studied by immunocytochemistry. Rats housed in an enriched environment showed significantly lower spontaneous motor activity in the open field compared to isolated animals. Perfusion of the D1 agonist SKF38393 (50 microM; 40 min) in the PFC produced long lasting increases of spontaneous motor activity and of local dialysate concentrations of acetylcholine in both groups of rats. However, increases of both motor activity and acetylcholine concentrations were significantly lower in enriched compared to isolated animals. Moreover, the density of D1 receptors in the PFC was significantly reduced in animals housed in an enriched environment. These results are the first evidence suggesting that environmental enrichment during adult life changes the function of D1 dopamine receptors in the PFC.

Stimulation of D2 receptors in the prefrontal cortex reduces PCP-induced hyperactivity, acetylcholine release and dopamine metabolism in the nucleus accumbens

The aim of the present study was to investigate the effects of stimulation of D2 receptors in the prefrontal cortex (PFC) on spontaneous motor activity and the hyperactivity induced by the psychomimetic phencyclidine (PCP). In addition, the effects of prefrontal D2 stimulation under PCP treatment on dialysate concentrations of acetylcholine, choline, dopamine, DOPAC and HVA in the nucleus accumbens were also investigated. Sprague-Dawley male rats were implanted with guide cannulae to perform bilateral injections into the medial PFC of the D2 agonist quinpirole (1.5 and 5 microg/side). Horizontal and vertical spontaneous motor activity and the motor activity induced by systemic injections of the PCP (5 mg/kg i.p.) were monitored in the open field. PFC injections of quinpirole (1.5 and 5 microg/side) significantly decreased horizontal and vertical spontaneous motor activity in a dose-related manner. These effects were blocked by the D2 antagonist raclopride (5 microg/side). Microinjections of quinpirole (1.5 and 5 microg/side) into the PFC also significantly attenuated the hyperactivity produced by PCP (5 mg/kg i.p.). PCP also increased dialysate concentrations of acetylcholine, and dopamine metabolites in the nucleus accumbens. These increases were also reduced by injections of quinpirole (5 microg/side) into the PFC. These results suggest that the stimulation of prefrontal D2 receptors plays an inhibitory role in regulating spontaneous and PCP-induced motor activity and also in the neurochemical changes produced by PCP in the nucleus accumbens.

Measurement of neurotransmitters from extracellular fluid in brain by in vivo microdialysis and chromatography–mass spectrometry

During the last three decades, a great deal of information has been discovered about chemical neurotransmission. However, the most important processes, namely the complex nature of neuronal circuitry, the "cross talk" between multiple neurotransmitter systems, and the varying effects neurochemicals have at different receptors, are still being explored. Techniques such as microdialysis are routinely employed to measure neurotransmitter levels in living tissue systems. Moreover, microdialysis studies have proven to be valuable in the investigation of neurodegenerative and psychiatric disease pathology, as well as in identifying novel drugs to treat such disorders. One particular challenge in performing these experiments is the requirement to couple microdialysis to sophisticated analytical equipment. Recently, considerable attention has been focused on the development of chromatographic-mass spectrometric techniques to provide more sensitive and accurate measurements of neurochemicals collected from in vivo microdialysis experiments. This review will provide a brief overview of the microdialysis technique, as well as how microdialysis and chromatography-mass spectrometry are being used to measure extracellular levels of neurotransmitters. The primary emphasis of this review will be on how these applications are used to measure levels of acetylcholine (ACh), dopamine, norepinephrine and gamma-aminobutyric acid (GABA).

Changes in dopamine and acetylcholine in striatum of the awake rat after chronic treatment with a dopamine uptake blocker

The effects of chronic treatment with a dopamine uptake blocker on dopamine and acetylcholine extracellular concentrations in striatum of the awake rat was studied. Male Wistar rats received daily injections (i.p.) of the dopamine uptake blocker nomifensine (10 mg/kg) during 22 days. Control group was injected with vehicle (saline). Microdialysis experiments were performed on days 1, 8, 15 and 22 of treatment. Nomifensine injections increased extracellular concentration of dopamine in striatum in all days of treatment without differences among days. In contrast, acetylcholine levels showed no changes in days 1 and 8 but increased in days 15 and 22 of treatment. These results shows that chronic treatment with a dopamine uptake inhibitor, nomifensine, has no effects on dopamine release but it increases acetylcholine release in striatum of the awake rat. These results would help to further understand the effects of chronic dopamine uptake inhibition.

Methamphetamine-induced spectrin proteolysis in the rat striatum

Methamphetamine (METH) is a widely abused psychostimulant. Multiple high doses of METH cause long-term toxicity to dopamine (DA) and serotonin (5-HT) nerve terminals in the brain, as evidenced by decreases in DA and 5-HT content, decreases in tyrosine and tryptophan hydroxylase activities, decreases in DA and 5-HT re-uptake sites, and nerve terminal degeneration. Multiple high doses of METH are known to elicit a rapid increase in DA release and hyperthermia. Although METH also produces a delayed and sustained rise in glutamate, no studies have shown whether METH produces structural evidence of excitotoxicity in striatum, or identified the receptors that mediate this toxicity directly, independent of alterations in METH-induced hyperthermia. These experiments investigated whether METH can cause excitotoxicity as evidenced by cytoskeletal protein breakdown in a glutamate receptor-dependent manner. METH increased calpain-mediated spectrin proteolysis in the rat striatum 5 and 7 days after METH administration without affecting caspase 3-dependent spectrin breakdown. This effect was completely blocked with the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, GYKI 52466, but not the NMDA receptor antagonist, MK-801. However, AMPA or NMDA receptor antagonism did not attenuate the METH-induced depletions of the dopamine transporter (DAT). Independent mechanisms involved in mediating spectrin proteolysis and DAT protein loss are discussed.

Dopamine transporters are involved in the onset of hypoxia-induced dopamine efflux in striatum as revealed by in vivo microdialysis

Although many studies have revealed alterations in neurotransmission during ischaemia, few works have been devoted to the neurochemical effects of mild hypoxia, a situation encountered during life in altitude or in several pathologies. In that context, the present work was undertaken to determine the in vivo mechanisms underlying the striatal dopamine efflux induced by mild hypoxaemic hypoxia. For that purpose, the extracellular concentrations of dopamine and its metabolite 3,4-dihydroxyphenyl acetic acid were simultaneously measured using brain microdialysis during acute hypoxic exposure (10% O(2), 1h) in awake rats. Hypoxia induced a +80% increase in dopamine. Application of the dopamine transporters inhibitor, nomifensine (10 microM), just before the hypoxia prevented the rise in dopamine during the early part of hypoxia; in contrast the application of nomifensine after the beginning of hypoxia, failed to alter the increase in dopamine. Application of the voltage-dependent Na(+) channel blocker tetrodotoxin abolished the increase in dopamine, whether administered just before or after the beginning of hypoxia. These data show that the neurochemical mechanisms of the dopamine efflux may change over the course of the hypoxic exposure, dopamine transporters being involved only at the beginning of hypoxia.

Role of NMDA and AMPA glutamate receptors in the induction and the expression of dopamine-mediated sensitization in 6-hydroxydopamine-lesioned rats

Rats with unilateral 6-hydroxydopamine (6-OHDA) lesions exhibit behavioral sensitization following repeated treatment with dopamine agonists, a phenomenon called "priming." Priming has two distinct phases: induction and expression. Priming induction using three injections with D1/D2 agonist apomorphine (0.5 mg/kg) or D1 agonist SKF38393 (10 mg/kg) allows priming expression, robust contralateral rotational behavior and striatal Fos expression, following a challenge with the D2 agonist quinpirole (0.25 mg/kg). We examined the roles of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) glutamate receptors on dopamine agonist priming. Administration of the NMDA antagonist (+)5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate (MK801) (0.5 mg/kg) blocked apomorphine-priming of quinpirole-mediated responses, while MK801 dose-dependently attenuated SKF38393-priming of quinpirole-mediated striatal Fos expression and had no effect on SKF38393-priming of quinpirole-mediated rotational behavior. In contrast, administration of the AMPA antagonist 2,3-dihydroxy-6-nitro-7sulfamoyl-benzo[f]quinoxaline (NBQX) (5 or 10 mg/kg) potentiated apomorphine- and SKF38393-priming of quinpirole-mediated striatal Fos expression, but had no effect on their priming of quinpirole-mediated rotational behavior. In SKF38393-primed 6-OHDA rats, administration of MK801 (0.5 mg/kg) blocked the expression of quinpirole-mediated responses, while administration of NBQX (10 mg/kg) or the noncompetitive AMPA antagonist 4-(8-methyl-9H-1,3-dioxolo[4,5-h][2,3]benzodiazepin-5-yl)-benzenamine dihydrochloride (GYKI52466) (5 or 15 mg/kg) had no effect. These results suggest that NMDA and AMPA glutamate receptors have differing roles in dopamine agonist priming-with NMDA receptors required for D1/D2 priming induction and D2-mediated priming expression, and AMPA receptors inhibiting priming induction of D2-mediated immediate early gene expression in the striatum, but not affecting priming induction of D2-mediated rotational behavior or the expression of D2-mediated responses.

Effects of the metabotropic glutamate receptor agonist, ACPD, on the extracellular concentrations of GABA and acetylcholine in the prefrontal cortex of the rat during the normal process of aging

The aim of the present study was to investigate the effects of activation of metabotropic glutamate receptors (mGluR) on the extracellular concentrations of GABA and acetylcholine in the prefrontal cortex of freely moving rats of different groups of age. Perfusion, through the microdialysis probe, of the agonist of mGluR, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD; 100, 500 and 1000 microM), in the prefrontal cortex of young rats produced a dose-related increase of the dialysate concentrations of GABA. The effects of perfusion of ACPD on the concentrations of GABA were attenuated in middle-aged rats. In the prefrontal cortex of aged rats, perfusion of ACPD produced no changes in dialysate concentrations of GABA at any of the doses used. Conversely, perfusion of ACPD (100, 500 and 1000 microM) in the prefrontal cortex of young, middle-aged and aged rats did not modify the dialysate concentrations of acetylcholine. Basal concentrations of acetylcholine in the prefrontal cortex of middle-aged and aged rats were significantly lower than those in young rats. In contrast, basal dialysate concentrations of GABA were not significantly different in young, middle-aged and aged rats. These results suggest that the interaction GABA-glutamate in the prefrontal cortex, mediated by mGluRs, changes with age.

Modulation of striatal dopamine release by glycine transport inhibitors

Traditional models of schizophrenia have focused primarily upon dopaminergic (DA) dysregulation. In contrast, more recent models focus on dysfunction of glutamatergic systems, acting particularly through N-methyl-D-aspartate (NMDA) receptors. NMDA receptors in brain are regulated by glycine, acting via a strychnine-insensitive regulatory site, and by glycine (GlyT1) transporters that maintain low glycine levels in the immediate vicinity of the NMDA receptor complex. The present study investigates the role of NMDA receptors in the modulation of striatal dopamine release in vitro, and of glycine transport inhibitors (GTIs) as potential psychotherapeutic agents in schizophrenia. In striatum, NMDA receptors exert dual excitatory/inhibitory effects, with inhibition reflecting activity of local GABAergic feedback regulation. We have previously demonstrated effectiveness of glycine in regulating [3H]DA release both in vivo and in vitro, consistent with its beneficial clinical effects. In the present study, similar effects were observed for the high-affinity GTI (+)N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy-)propyl]sarcosine (NFPS), and for a range of high-affinity GTIs with appropriate rank order of potency. In addition, (+)NFPS significantly stimulated NMDA-induced [3H]GABA release. Effects, of GTIs, were blocked by the glycine-site antagonists L689,560 and HA-966, and the GABA(B) antagonists phaclofen and CGP 52432, confirming the roles of both the NMDA-associated glycine-site and presynaptic GABA(B) receptors in NMDA receptor-mediated regulation of striatal DA release in vitro. Endogenous DA hyperactivity is associated with prominent positive symptoms in schizophrenia. The present results are consistent with recent clinical studies showing significant effectiveness of glycine-site agonists and GTIs in reduction of persistent positive, as well as negative, symptoms in schizophrenia.

Monoaminergic dysregulation in glutathione-deficient mice: Possible relevance to schizophrenia?

Several lines of research have implicated glutathione (GSH) in schizophrenia. For instance, GSH deficiency has been reported in the prefrontal cortex of schizophrenics in vivo. Further, in rats postnatal GSH-deficiency combined with hyperdopaminergia led to cognitive impairments in the adult. In the present report we studied the effects of 2-day GSH-deficiency with L-buthionine-(S,R)-sulfoximine on monoaminergic function in mice. The effect of GSH-deficiency per se and when combined with the amphetamine and phencyclidine (PCP) models of schizophrenia was investigated. GSH-deficiency significantly altered tissue levels of dopamine (DA), 5-hydroxytryptamine (5-HT) and their respective metabolites homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in a region-specific fashion. The effects of GSH-deficiency on tissue monoamines were distinct from and, generally, did not interact with the effects of amphetamine (5 mg/kg; i.p.) on tissue monoamines. Microdialysis studies showed that extracellular DA-release after amphetamine (5 mg/kg, i.p.) was two-fold increased in the nucleus accumbens of GSH-deficient mice as compared with control mice. Basal DA was unaltered. Further, extracellular levels of HVA in the frontal cortex and hippocampus and 5-HIAA in the nucleus accumbens were elevated by GSH-deficiency per se. Spontaneous locomotor activity in the open field was unchanged in GSH-deficient mice. In contrast, GSH-deficiency modulated the locomotor responses to mid-range doses of amphetamine (1.5 and 5 mg/kg, i.p.). Further, GSH-deficient mice displayed an increased locomotor response to low (2 and 3 mg/kg, i.p.) doses of phencyclidine (PCP). In conclusion, the data presented here show that even short-term GSH-deficiency has consequences for DA and 5-HT function. This was confirmed on both neurochemical and behavioral levels. How GSH and the monoamines interact needs further scrutiny. Moreover, the open field findings suggest reduced or altered N-methyl-d-aspartate (NMDA) receptor function in GSH-deficient mice. Thus, GSH-deficiency can lead to disturbances in DA, 5-HT and NMDA function, a finding that may have relevance for schizophrenia.

Analysis of acetylcholine and choline in microdialysis samples by liquid chromatography/tandem mass spectrometry

A sensitive liquid chromatography/electrospray ionisation tandem mass spectrometric (LC/ESI-MS/MS) method was developed for the analysis of acetylcholine and choline in microdialysis samples. A Ringer's solution that contains high (150 mM) concentrations of inorganic salts was used to extract acetylcholine and choline from a rat or mouse brain. The separation of acetylcholine, choline, an internal standard acetyl-beta-methylcholine, endogenous compounds and inorganic cations was achieved with hydrophilic interaction chromatography using a diol column. The eluent consisted of 20 mM ammonium formate (pH 3.3) and acetonitrile (20:80) which is favourable for the ESI process. Limits of detection (signal-to-noise (S/N) ratio = 3) of 0.02 nM (0.2 fmol) for acetylcholine and 1 nM (10 fmol) for choline were observed using standards diluted in Ringer's solution. A good linearity was obtained from the limit of quantitation: 0.1 nM (S/N ratio = 10) to 50 nM (r = 0.999) for acetylcholine and within the concentration range of 100-3500 nM (r = 0.998) for choline. The between-day repeatability of the method was good; RSD was 3.1% at 1 nM level of acetylcholine and 3.5% at 1000 nM level of choline. The recoveries for addition of 1 or 2.5 nM acetylcholine and 0.2 or 1 microM choline in microdialysis balancing samples were between 93 and 101% indicating that no suppressing endogenous compounds were co-eluting with acetylcholine or choline. The developed method was applied to the analysis of microdialysis balancing samples collected from rat and mouse brains.