Mechanistic Insights into Sympathetic Neuronal Regeneration: Multitracer Molecular Imaging of Catecholamine Handling After Cardiac Transplantation.

BACKGROUND: Post-transplant reinnervation is a unique model to study sympathetic neuronal regeneration in vivo. The differential role of subcellular mechanisms of catecholamine handling in nerve terminals has not been investigated. METHODS AND RESULTS: Three different carbon-11-labeled catecholamines were used for positron emission tomography of transport (C-11 m-hydroxyephedrine, HED), vesicular storage (C-11 epinephrine, EPI), and metabolic degradation (C-11 phenylephrine). A 2-day protocol was used, including quantification of myocardial blood flow by N-13 ammonia. Resting myocardial blood flow and EPI, HED and phenylephrine retention were homogeneous in healthy volunteers (n=7). Washout was only observed for phenylephrine (T(1/2) 49±6 min). In nonrejecting, otherwise healthy heart transplant recipients (>1 year after surgery, n=10), resting myocardial blood flow was also homogenous. Regional catecholamine uptake of varying degrees was observed in the anterior left ventricular wall and septum. Overall, 24±19% of left ventricle showed HED uptake levels comparable with healthy volunteers, whereas it was only 8±7% for EPI (P=0.004 versus HED). Phenylephrine washout was not different from healthy volunteers in the area with restored EPI and HED retention (T(1/2) 41±7 min; P>0.05), but was significantly enhanced in the EPI/HED mismatch area (T(1/2) 36±8 min; P=0.008), consistent with inefficient vesicular storage and enhanced metabolic degradation. CONCLUSIONS: Regeneration of subcellular components of sympathetic nerve terminal function does not occur simultaneously. In the reinnervating transplanted heart, a region with normal catecholamine transport and vesicular storage is surrounded by a borderzone, where transport is already restored but vesicular storage remains inefficient, suggesting that vesicular storage is a more delicate mechanism. This observation may have implications for other pathologies involving cardiac autonomic innervation.

Prefrontal and Striatal Glutamate Differently Relate to Striatal Dopamine: Potential Regulatory Mechanisms of Striatal Presynaptic Dopamine Function?

Theoretical and animal work has proposed that prefrontal cortex (PFC) glutamate inhibits dopaminergic inputs to the ventral striatum (VS) indirectly, whereas direct VS glutamatergic afferents have been suggested to enhance dopaminergic inputs to the VS. In the present study, we aimed to investigate relationships of glutamate and dopamine measures in prefrontostriatal circuitries of healthy humans. We hypothesized that PFC and VS glutamate, as well as their balance, are differently associated with VS dopamine. Glutamate concentrations in the left lateral PFC and left striatum were assessed using 3-Tesla proton magnetic resonance spectroscopy. Striatal presynaptic dopamine synthesis capacity was measured by fluorine-18-l-dihydroxyphenylalanine (F-18-FDOPA) positron emission tomography. First, a negative relationship was observed between glutamate concentrations in lateral PFC and VS dopamine synthesis capacity (n = 28). Second, a positive relationship was revealed between striatal glutamate and VS dopamine synthesis capacity (n = 26). Additionally, the intraindividual difference between PFC and striatal glutamate concentrations correlated negatively with VS dopamine synthesis capacity (n = 24). The present results indicate an involvement of a balance in PFC and striatal glutamate in the regulation of VS dopamine synthesis capacity. This notion points toward a potential mechanism how VS presynaptic dopamine levels are kept in a fine-tuned range. A disruption of this mechanism may account for alterations in striatal dopamine turnover as observed in mental diseases (e.g., in schizophrenia). SIGNIFICANCE STATEMENT: The present work demonstrates complementary relationships between prefrontal and striatal glutamate and ventral striatal presynaptic dopamine using human imaging measures: a negative correlation between prefrontal glutamate and presynaptic dopamine and a positive relationship between striatal glutamate and presynaptic dopamine are revealed. The results may reflect a regulatory role of prefrontal and striatal glutamate for ventral striatal presynaptic dopamine levels. Such glutamate-dopamine relationships improve our understanding of neurochemical interactions in prefrontostriatal circuits and have implications for the neurobiology of mental disease.

Predicting the development of levodopa-induced dyskinesias: a presynaptic mechanism?

After chronic levodopa use, many patients with Parkinson disease (PD) develop involuntary movements. Whether disabling or minor, levodopa-induced dyskinesia (LID) constitutes an undesirable outcome calling for better treatment strategies. Options for managing LID include delaying its onset by combining a dopaminergic agonist with levodopa from the start(1) or symptomatic control using amantadine. However, fundamental questions about LID remain: by what mechanisms does it develop, and why don't all patients go on to experience LID after sustained levodopa exposure?

Does human presynaptic striatal dopamine function predict social conformity?

Socially desirable responding (SDR) is a personality trait which reflects either a tendency to present oneself in an overly positive manner to others, consistent with social conformity (impression management (IM)), or the tendency to view one's own behaviour in an overly positive light (self-deceptive enhancement (SDE)). Neurochemical imaging studies report an inverse relationship between SDR and dorsal striatal dopamine D2/3 receptor availability. This may reflect an association between SDR and D2/3 receptor expression, synaptic dopamine levels or a combination of the two. In this study, we used a [¹8F]-DOPA positron emission tomography (PET) image database to investigate whether SDR is associated with presynaptic dopamine function. Striatal [¹8F]-DOPA uptake, (k(i)(cer), min⁻¹), was determined in two independent healthy participant cohorts (n=27 and 19), by Patlak analysis using a cerebellar reference region. SDR was assessed using the revised Eysenck Personality Questionnaire (EPQ-R) Lie scale, and IM and SDE were measured using the Paulhus Deception Scales. No significant associations were detected between Lie, SDE or IM scores and striatal [¹8F]-DOPA k(i)(cer). These results indicate that presynaptic striatal dopamine function is not associated with social conformity and suggests that social conformity may be associated with striatal D2/3 receptor expression rather than with synaptic dopamine levels.

[Cardiac sympathetic innervation imaging with myocardial MIBG scintigraphy]. / Studio dell'innervazione cardiaca mediante scintigrafia miocardica con MIBG.

Metaiodobenzylguanidine (MIBG) was developed initially as a tracer for oncological imaging; when labeled with 123 I or 131 I, it may detect APUDomas, such as pheochromocytomas and paragangliomas. In the last years, MIBG has found an important role also in neurology and cardiology, as cardiac innervation tracer. Actually, MIBG cardiac imaging is a universally accepted method to estimate cardiac sympathetic innervations. This review covers the role of MIBG cardiac imaging in Parkinson disease and parkinsonisms, from the pathophysiological premises for cardiac denervation to new emerging data.

Presynaptic striatal dopamine dysfunction in people at ultra-high risk for psychosis: findings in a second cohort.

BACKGROUND: Using positron emission tomography (PET), we previously observed increases in 3,4-dihydroxy-6-[(18)F]fluoro-L-phenylalanine ((18)F-DOPA) uptake in the striatum of subjects at ultra-high risk (UHR) for psychosis, indicating elevated presynaptic dopamine synthesis capacity. The purpose of this study was to test if this finding would be replicated in a second UHR cohort. METHODS: (18)F-DOPA PET was used to estimate dopamine synthesis capacity in the striatum of an entirely new cohort of 26 individuals at UHR for psychosis (14 males, mean±SD age = 22.7±4.7 years) and 20 healthy volunteers matched for age and gender (11 males, mean±SD age = 24.5±4.5 years). RESULTS: Dopamine synthesis capacity was elevated in the whole [t(44) = 2.6; p = .01, effect size = .81] and associative striatum [t(44) = 2.6; p = .01, effect size = .73] of UHR compared with control subjects. When the two samples were combined to give a final sample of 32 control and 50 UHR subjects, the higher levels of dopamine synthesis capacity in the UHR group reached significance across the whole [F(1,81) = 11.0; p = .001], associative [F(1,81) = 12.7; p = .001], and sensorimotor [F(1,81) = 4.7; p = .03], but not the limbic [F(1,81) = 2.1; p = .2], striatum. CONCLUSIONS: The findings indicate that elevated dopamine synthesis capacity in the dorsal striatum is a robust feature of individuals at UHR for psychosis and provide further evidence that dopaminergic abnormalities precede the onset of psychosis.

Progressive supranuclear palsy: in vivo SPECT imaging of presynaptic vesicular acetylcholine transporter with [123I]-iodobenzovesamicol.

PURPOSE: To evaluate the integrity of brain cholinergic pathways in vivo in patients with progressive supranuclear palsy (PSP) by measuring the vesicular acetylcholine transporter expression at single photon emission computed tomography (SPECT) with [123I]-iodobenzovesamicol. MATERIALS AND METHODS: All participants provided informed written consent according to institutional human ethics committee guidelines. Ten patients with PSP and 12 healthy volunteers underwent dynamic [123I]-iodobenzovesamicol SPECT and magnetic resonance (MR) imaging. CT and MR images were used to register the dynamic SPECT image to the Montreal Neurologic Institute brain template, which includes the regions of interest of the striatum and the septo-hippocampal, innominato-cortical, and ponto-thalamic cholinergic pathways. For each region of interest, pharmacokinetic modeling of regional time activity curves was used to calculate [123I]-iodobenzovesamicol to vesicular acetylcholine transporter binding potential value, proportional to vesicular acetylcholine transporter expression. RESULTS: When compared with control participants, patients with PSP had binding potential values that were unchanged in the striatum and septohippocampal pathway, significantly lower in the anterior cingulate cortex (P=.017) in the innominatocortical pathway, and significantly decreased in the thalamus (P=.014) in the pontothalamic cholinergic pathway. In addition, binding potential values in the thalamus were positively correlated with those in the pedunculopontine nucleus (ρ=0.81, P<.004) and binding potential values in both the thalamus (ρ=-0.88, P<.001) and pedunculopontine nucleus (ρ=-0.80, P<.010) were inversely correlated with disease duration. CONCLUSION: Cholinergic pathways were differentially affected in the PSP group, with a significant alteration of pontothalamic pathways that increased with disease progression at both cell body and terminal levels, while the innominatocortical pathway was only mildly affected, and the septohippocampal pathway and the striatum were both preserved.

[The clinical significance of MIBG myocardial scintigraphy in Parkinson disease].

Meta-iodobenzylguanidine (MIBG) myocardial scintigraphy can assess postganglionic presynaptic cardiac sympathetic nerve endings. Reduced cardiac MIBG uptake on MIBG myocardial scintigraphy has been reported in patients with Parkinson disease (PD), dementia with Lewy bodies (DLB), pure autonomic failure (PAF), and familial PD linked to SNCA duplication. This imaging procedure is a sensitive diagnostic tool that might differentiate PD and DLB from other movement disorders from Alzheimer disease (AD). We recently reported cardiac sympathetic denervation in PD, DLB, PAF, and familial PD linked to SNCA duplication which accounts for the reduced cardiac MIBG uptake in these disorders. The patients with PD, DLB, PAF and familial PD linked to SNCA duplication have Lewy bodies in the nervous system, whereas patients with multiple system atrophy (MSA), progressive supranuclear palsy (PSP), corticobasal degeneration, AD, and parkin-associated PD do not. However, in patients with MSA or PSP, cardiac sympathetic denervation was associated with the presence of Lewy bodies in the nervous system. Therefore, cardiac sympathetic denervation is closely related to the presence of Lewy bodies in the wide range of neurodegenerative processes. Thus, we conclude that reduced cardiac MIBG uptake is a potential biomarker for the presence of Lewy bodies in the nervous system. We infer that MIBG myocardial scintigraphy is a noninvasive tool for detecting Lewy bodies during life.

Presynaptic 5-HT1A is related to 5-HTT receptor density in the human brain.

5-Hydroxytryptamine (5-HT or serotonin) is an important neurotransmitter for a number of brain functions and widely distributed throughout the brain. Physiological and pharmacological relationship between 5-HT1A receptors and serotonin transporter (5-HTT) in the regulation of 5-HT neurotransmission has now been documented. A relationship between 5-HT1A receptors and 5-HTT is also suggested by the pathophysiology of depression and the mechanism of action of antidepressants. We have scanned 42 healthy adults with both [11C] WAY-100635 and [11C] DASB to investigate the anatomical co-distribution of multiple serotonergic markers. We hypothesized that lower 5-HTT densities in the dorsal raphe nucleus (DRN) and limbic regions will be accompanied by lower 5-HT1A receptor density in the same regions, contributing to the 5-HT1A receptor desensitization. In addition, variations in DRN 5-HT1A receptor density can theoretically influence the density and/or function of other serotonin receptor subtypes and the 5-HTT consequent to changes in serotonergic tone. In a comparatively large sample of volunteers, we have shown that the relationship between 5-HT1A and 5-HTT PET indices was complex. We were unable to demonstrate robust, intra-regional relationships between 5-HT1A and 5-HTT densities. Inter-regionally, DRN 5-HT1A receptors were related to cortical (temporal and frontal regions) and paralimbic (insula), but not limbic 5-HTT. This latter finding may reflect differences in 5-HT tone between individuals, and highlights probable substrates sensitive to variations in DRN 5-HT function.

[Dual time point 18F-FDOPA PET as a tool for characterizing brain tumors]. / PET de doble fase con (18)F-FDOPA como instrumento para la caracterización de tumores cerebrales.

(18)F-FDOPA is an amino acid analogue used to evaluate presynaptic dopaminergic activity, which has aroused great interest in neuro-oncology. We have evaluated five (18)F-FDOPA PET studies of patients referred for study of parkinsonian syndrome. Two subjects had previously treated high-grade brain tumors, one nonspecific brain injury, and 2 subjects presented unexpected tumoral lesions. For all lesions SUVmax, time to SUVmax and tumor-to-normal grey matter SUVmax rate (T/N) were calculated, and 90 minutes (18)F-FDOPA kinetics were analyzed. Tumor lesions corresponded to three malignant neurocytomas, one meningioma, one pineocytoma and one intrasinusal hemangioma. Both malignant and benign tumors exhibited high uptake of (18)F-FDOPA well above the normal cortex. However, the analysis of the curve uptake displayed characteristic patterns that facilitate the characterization of tumor lesions. A dual phase maximum uptake was observed, with an early 10 minutes uptake in malignant lesions, and a late 60 to 90 minutes uptake in benign or low grade lesions.

Impact of 123I-FP-CIT (DaTSCAN) SPECT on the diagnosis and management of patients with dementia with Lewy bodies: a retrospective study.

OBJECTIVE: To assess the impact of imaging the presynaptic dopaminergic terminal using DaTSCAN single-photon emission computed tomography (SPECT) on the clinical diagnosis and subsequent management of patients with possible dementia with Lewy bodies (DLB), who were referred for imaging. METHODS: A retrospective case note study was undertaken, involving a series of consecutive patients who had undergone DaTSCAN SPECT 12-24 months earlier. RESULTS: Case notes of 80 patients were reviewed. DaTSCAN imaging results were abnormal (indicating DLB) in 20 (25%) and normal (suggesting an alternative diagnosis or absence of disease) in 60 (75%) patients. Of the 20 patients with an abnormal scan, 18 had a postscan working clinical diagnosis of DLB (90%), one had diagnosis of vascular dementia (5%) and another had no recorded outcome (5%). Fifty-eight out of the 60 patients with a normal DaTSCAN image result had an alternative clinical diagnosis of DLB (95%). Consequently, DaTSCAN findings were concordant with the outcome clinical diagnosis in 76 out of the 80 (95%) cases. Subsequent to DaTSCAN SPECT, scan findings and diagnoses were discussed with patients and/or their carers in 94% of cases and subsequent treatment options discussed in 93% of cases. CONCLUSION: It would seem that DaTSCAN imaging has a marked influence on the working clinical diagnosis and subsequent management of patients with suspected DLB.

Differences in nigro-striatal impairment in clinical variants of early Parkinson's disease: evidence from a FP-CIT SPECT study.

INTRODUCTION: In idiopathic Parkinson's disease (PD), two different clinical phenotypes are usually distinguished: a tremor dominant variant (TD) and an akinetic-rigid type (ART). TD patients are characterized by a slower disease progression and a minor cognitive impairment. Striatal density of DAT, as quantified by FP-CIT SPECT, has been reported to correlate with rigidity and akinesia but not with tremor. OBJECTIVE: To evaluate FP-CIT uptake in TD and ART phenotypes. METHODS: We retrospectively evaluated from our database the pre-synaptic nigro-striatal function of 24 patients with TD-PD and 38 patients with ART-PD who underwent a FP-CIT SPECT within 1 year from disease onset. RESULTS: Disease duration, age at the time of SPECT scan and disease severity as measured with Unified Parkinson's Disease Rating scale part III (UPDRS III) were not statistically different between the two groups. Putamen contralateral to the most clinically affected side showed a lower FP-CIT uptake in ART patients compared to TD patients. No statistically significant differences emerged when considering bilateral caudate and ipsilateral putaminal uptake, as well as asymmetry indices and caudate/putamen ratios. FP-CIT contralateral putaminal uptake correlated with the severity of rigidity and hypokinesia but not with tremor. CONCLUSIONS: These data suggest that other neurotransmitter systems apart from the nigro-striatal dopaminergic system are involved in the generation of Parkinsonian tremor, and they are consistent with previous evidence of a lack of correlation between tremor severity and FP-CIT uptake. Putaminal relative sparing in TD patients could partially explain the slower disease progression reported in this PD phenotype.

The test-retest reliability of 18F-DOPA PET in assessing striatal and extrastriatal presynaptic dopaminergic function.

Brain presynaptic dopaminergic function can be assessed using 18F-DOPA positron emission tomography (PET). Regional 18F-DOPA utilization may be used to index dopaminergic abnormalities over time or dopaminergic response to treatment in clinical populations. Such studies require prior knowledge of the stability of the 18F-DOPA signal in the brain regions of interest. Test-retest reliability was examined in eight healthy volunteers who each received two 18F-DOPA PET scans, approximately 2 years apart. 18F-DOPA utilization (k(i)(cer)) was determined using graphical analysis relative to a reference tissue input (Patlak and Blasberg, 1985). Reproducibility (measured as the within-subjects variation) and reliability (measured as intraclass correlation coefficients, ICCs) of 18F-DOPA k(i)(cer) were assessed in the structural and functional subdivisions of the striatum and select extrastriatal brain regions. Voxel-based median ICC maps were used to visualize the distribution of 18F-DOPA k(i)(cer) reliability across the brain. The caudate and putamen, and associative and sensorimotor, striatal subdivisions showed good reliability across the two scan sessions with bilateral ICCs ranging from 0.681 to 0.944. Reliability was generally lower in extrastriatal regions, with bilateral ICCs ranging from 0.235 in the amygdala to 0.894 in the thalamus. These data confirm the utility of 18F-DOPA PET in assessing dopaminergic function in the striatum and select extrastriatal areas but highlight the limitations in using this approach to measure dopaminergic function in low uptake extrastriatal brain areas. This information can be used to optimize the experimental design of future studies investigating changes in brain dopaminergic function with 18F-DOPA.

Dopaminergic function and progression of Parkinson's disease: PET findings.

Recent PET studies in Parkinson's disease (PD) have shown that the progression of neurodegeneration follows an exponential decay pattern. Most of the damage to the nigrostriatal dopamine system occurs during the presymptomatic phase of the disease and the first few years following symptom onset. The progressive loss of dopaminergic neurons is accompanied by several functional adaptive changes in surviving nerve terminals, which lead to increased dopamine turnover and raise the risk of treatment-related motor complications. Younger PD patients seem to have more efficient compensatory mechanisms and a slower rate of progression of neurodegeneration.

Pre- and post-synaptic dopamine imaging and its relation with frontostriatal cognitive function in Parkinson disease: PET studies with [11C]NNC 112 and [18F]FDOPA.

Frontostriatal cognitive dysfunction is common in Parkinson disease (PD), but the explanation for its heterogeneous expressions remains unclear. This study examined the dopamine system within the frontostriatal circuitry with positron emission tomography (PET) to investigate pre- and post-synaptic dopamine function in relation to the executive processes in PD. Fifteen non-demented PD patients and 14 healthy controls underwent [(18)F]FDOPA (for dopamine synthesis) and [(11)C]NNC 112 (for D(1) receptors) PET scans and cognitive testing. Parametric images of [(18)F]FDOPA uptake (K(i)) and [(11)C]NNC 112 binding potential (BP(ND)) were calculated using reference tissue models. Group differences in K(i) and BP(ND) were assessed with both volume of interest and statistical parametric mapping, and were correlated with cognitive tests. Measurement of [(18)F]FDOPA uptake in cerebral cortex was questionable because of higher K(i) values in white than adjacent gray matter. These paradoxical results were likely to be caused by violations of the reference tissue model assumption rendering interpretation of cortical [(18)F]FDOPA uptake in PD difficult. We found no regional differences in D(1) receptor density between controls and PD, and no overall differences in frontostriatal performance. Although D(1) receptor density did not relate to frontostriatal cognition, K(i) decreases in the putamen predicted performance on the Wisconsin Card Sorting Test in PD only. These results suggest that striatal dopamine denervation may contribute to some frontostriatal cognitive impairment in moderate stage PD.

GAP-43 is critical for normal targeting of thalamocortical and corticothalamic, but not trigeminothalamic axons in the whisker barrel system.

Mice lacking the growth-associated protein GAP-43 (KO) show disrupted cortical topography and no barrels. Whisker-related patterns of cells are normal in the KO brainstem trigeminal complex (BSTC), while the pattern in KO ventrobasal thalamus (VB) is somewhat compromised. To better understand the basis for VB and cortical abnormalities, we used small placements of DiI to trace axonal projections between BSTC, VB, and barrel cortex in wildtype (WT) and GAP-43 KO mice. The trigeminothalamic (TT) pathway consists of axons from cells in the Nucleus Prinicipalis that project to the contralateral VB thalamus. DiI-labeled KO TT axons crossed the midline from BSTC and projected to contralateral VB normally, consistent with normal BSTC cytoarchitecture. By contrast, the KO thalamocortical axons (TCA) projection was highly abnormal. KO TCAs showed delays of 1-2 days in initial ingrowth to cortex. Postnatally, KO TCAs showed multiple pathfinding errors near intermediate targets, and were abnormally fasciculated within the internal capsule (IC). Interestingly, most individually labeled KO TCAs terminated in deep layers instead of in layer IV as in WT. This misprojection is consistent with birthdating analysis in KO mice, which revealed that neurons normally destined for layer IV remain in deep cortical layers. Early outgrowth of KO corticofugal (CF) axons was similar for both genotypes. However, at P7 KO CF fibers remained bundled as they entered the IC, and exhibited few terminal branches in VB. Thus, the establishment of axonal projections between thalamus and cortex are disrupted in GAP-43 KO mice.

[Quick and simple synthesis of (11)C-(+)-alpha-dihydrotetrabenazine to be used as a PET radioligand of vesicular monoamine transporters]. / Síntesis rápida y simplificada de C-(+)-alpha-dihidrotetrabenazina para su utilización como radioligando PET de los transportadores vesiculares de monoaminas.

UNLABELLED: Dihydrotetrabenazine (2-hydroxy-3-isobutyl-9,10-dimethoxy-1,3,4,6,7-hexahydro-11bH-benzo[a]-quinolizine, DTBZ) has become the ideal radioligand for the presynaptic vesicular monoamine transporter VMAT2 based on its high binding affinity and optimal lipophilicity. OBJECTIVE: To develop an automatic procedure for labelling DTBZ with carbon-11, which has been shown to be a highly effective marker for in vivo studies of neuronal losses in animal models with Parkinson's disease using positron emission tomography (PET). MATERIALS AND METHODS: We have developed a new fully automated synthesis procedure to obtain 11C-(+)DTBZ quickly and simply through labelling the precursor -(+)desmethyldihy-drotetrabenazine- at room temperature in the presence of dimethyl sulfoxide (DMSO) and potassium hydroxide (KOH), using 11CH3I as primary precursor. The final purification was carried out by solid phase extraction using commercially available cartridges and the residual solvents (DMSO and ethyl ether) were eliminated by evaporation. RESULTS: The whole procedure was automated, and after 54 syntheses, an average production of 1.94 GBq of sterile, pyrogen-free 11C-(+)DTBZ with a radiochemical purity > 99 % was obtained with 5 minutes irradiation and 6 minutes of synthesis after 11CH3I production. 11C-(+)DTBZ binding to presynaptic dopamine nerve terminals has been demonstrated by MicroPET studies in Wistar rats and M. Fascicularis monkeys. CONCLUSIONS: This new synthesis procedure is quick and simple, due to optimised techniques, which have allowed elimination of residual solvents based on their polarity for the final purification. It is also applicable to other automatic syntheses for obtaining compounds labelled by methylation reactions.

Modulation of excitation by metabotropic glutamate receptors.

Metabotropic glutamate receptors, in contrast to ionotropic glutamate receptors, do not form ion channels but instead affect intracellular chemical messenger systems. They couple via GTP-binding proteins ("G-proteins") to a variety of effectors such as ion channels and thus give glutamate, the major excitatory transmitter in the CNS, the ability to modulate processes involved in excitatory synaptic transmission. Therefore, excitatory synaptic transmission is regulated not only by the conventional GABAergic but also by the glutamatergic mechanisms themselves. Many metabotropic glutamate receptors are localized outside the immediate vicinity of transmitter release sites, thereby setting specific requirements for their activation, such as cooperation between synapses, burst activity, and glial involvement in the regulation of ambient glutamate levels.