NMDA-receptor activity visualized with (S)-[N-methyl-11C]ketamine and positron emission tomography in patients with medial temporal lobe epilepsy

Pharmacological Characterization of [18F]-FNM and Evaluation of NMDA Receptors Activation in a Rat Brain Injury Model

PURPOSE

NMDA receptors (NMDARs) dysfunction plays a central role in the physiopathology of psychiatric and neurodegenerative disorders whose mechanisms are still poorly understood. The development of a PET (positron emission tomography) tracer able to selectively bind to the NMDARs intra-channel PCP site may make it possible to visualize NMDARs in an open and active state. We describe the in vitro pharmacological characterization of [¹⁸F]-fluoroethylnormemantine ([¹⁸F]-FNM) and evaluate its ability to localize activated NMDA receptors in a rat preclinical model of excitotoxicity.

PROCEDURES

The affinity of the non-radioactive analog for the intra-channel PCP site was determined in a radioligand competition assay using [³H]TCP ([³H]N-(1-[thienyl]cyclohexyl)piperidine) on rat brain homogenates. Selectivity was also investigated by the displacement of specific radioligands targeting various cerebral receptors. In vivo brain lesions were performed using stereotaxic quinolinic acid (QA) injections in the left motor area (M1) of seven Sprague Dawley rats. Each rat was imaged with a microPET/CT camera, 40 min after receiving a dose of 30 MBq + / - 20 of [¹⁸F]-FNM, 24 and 72 h after injury. Nine non-injured rats were also imaged using the same protocol.

RESULTS

FNM displayed IC₅₀ value of 13.0 ± 8.9 µM in rat forebrain homogenates but also showed significant bindings on opioid receptors. In the frontal and left somatosensory areas, [¹⁸F]FNM PET detected a mean of 37% and 41% increase in [¹⁸F]FNM uptake (p < 0,0001) 24 and 72 h after QA stereotaxic injection, respectively, compared to the control group.

CONCLUSIONS

In spite of FNM's poor affinity for NMDAR PCP site, this study supports the ability of this tracer to track massive activation of NMDARs in neurological diseases.

Tailoring Materials for Epilepsy Imaging: From Biomarkers to Imaging Probes

Excising epileptic foci (EF) is the most efficient approach for treating drug-resistant epilepsy (DRE). However, owing to the vast heterogeneity of epilepsies, EF in one-third of patients cannot be accurately located, even after exhausting all current diagnostic strategies. Therefore, identifying biomarkers that truly represent the status of epilepsy and fabricating probes with high targeting specificity are prerequisites for identifying the "concealed" EF. However, no systematic summary of this topic has been published. Herein, the potential biomarkers of EF are first summarized and classified into three categories: functional, molecular, and structural aberrances during epileptogenesis, a procedure of nonepileptic brain biasing toward epileptic tissue. The materials used to fabricate these imaging probes and their performance in defining the EF in preclinical and clinical studies are highlighted. Finally, perspectives for developing the next generation of probes and their challenges in clinical translation are discussed. In general, this review can be helpful in guiding the development of imaging probes defining EF with improved accuracy and holds promise for increasing the number of DRE patients who are eligible for surgical intervention.

Pharmacological and behavioral divergence of ketamine enantiomers: implications for abuse liability

Ketamine, a racemic mixture of (S)-ketamine and (R)-ketamine enantiomers, has been used as an anesthetic, analgesic and more recently, as an antidepressant. However, ketamine has known abuse liability (the tendency of a drug to be used in non-medical situations due to its psychoactive effects), which raises concerns for its therapeutic use. (S)-ketamine was recently approved by the United States' FDA for treatment-resistant depression. Recent studies showed that (R)-ketamine has greater efficacy than (S)-ketamine in preclinical models of depression, but its clinical antidepressant efficacy has not been established. The behavioral effects of racemic ketamine have been studied extensively in preclinical models predictive of abuse liability in humans (self-administration and conditioned place preference [CPP]). In contrast, the behavioral effects of each enantiomer in these models are unknown. We show here that in the intravenous drug self-administration model, the gold standard procedure to assess potential abuse liability of drugs in humans, rats self-administered (S)-ketamine but not (R)-ketamine. Subanesthetic, antidepressant-like doses of (S)-ketamine, but not of (R)-ketamine, induced locomotor activity (in an opioid receptor-dependent manner), induced psychomotor sensitization, induced CPP in mice, and selectively increased metabolic activity and dopamine tone in medial prefrontal cortex (mPFC) of rats. Pharmacological screening across thousands of human proteins and at biological targets known to interact with ketamine yielded divergent binding and functional enantiomer profiles, including selective mu and kappa opioid receptor activation by (S)-ketamine in mPFC. Our results demonstrate divergence in the pharmacological, functional, and behavioral effects of ketamine enantiomers, and suggest that racemic ketamine's abuse liability in humans is primarily due to the pharmacological effects of its (S)-enantiomer.

Activated N-methyl-D-aspartate receptor ion channels detected in focal epilepsy with [18 F]GE-179 positron emission tomography

OBJECTIVE

Imaging activated glutamate N-methyl-D-aspartate receptor ion channels (NMDAR-ICs) using positron emission tomography (PET) has proved challenging due to low brain uptake, poor affinity and selectivity, and high metabolism and dissociation rates of candidate radioligands. The radioligand [¹⁸ F]GE-179 is a known use-dependent marker of NMDAR-ICs. We studied whether interictal [¹⁸ F]GE-179 PET would detect foci of abnormal NMDAR-IC activation in patients with refractory focal epilepsy.

METHODS

Ten patients with refractory focal epilepsy and 18 healthy controls had structural magnetic resonance imaging (MRI) followed by a 90-min dynamic [¹⁸ F]GE-179 PET scan with simultaneous electroencephalography (EEG). PET and EEG findings were compared with MRI and previous EEGs. Standard uptake value (SUV) images of [¹⁸ F]GE-179 were generated and global gray matter uptake was measured for each individual. To localize focal increases in uptake of [¹⁸ F]GE-179, the individual SUV images were interrogated with statistical parametric mapping in comparison to a normal database. Additionally, individual healthy control SUV images were compared with the rest of the control database to determine their prevalence of increased focal [¹⁸ F]GE-179 uptake.

RESULTS

Interictal [¹⁸ F]GE-179 PET detected clusters of significantly increased binding in eight of 10 patients with focal epilepsy but none of the controls. The number of clusters of raised [¹⁸ F]GE-179 uptake in the patients with epilepsy exceeded the focal abnormalities revealed by the simultaneously recorded EEG. Patients with extensive clusters of raised [¹⁸ F]GE-179 uptake showed the most abnormal EEGs.

SIGNIFICANCE

Detection of multiple foci of abnormal NMDAR-IC activation in 80% of our patients with refractory focal epilepsy using interictal [¹⁸ F]GE-179 PET could reflect enhanced neuronal excitability due to chronic seizure activity. This indicates that chronic epileptic activity is associated with abnormal NMDAR ion channel activation beyond the initial irritative zones. [¹⁸ F]GE-179 PET could be a candidate marker for identifying pathological brain areas in patients with treatment-resistant focal epilepsy.

NMDA receptor ion channel activation detected in vivo with [18F]GE-179 PET after electrical stimulation of rat hippocampus

The positron emission tomography (PET) tracer [¹⁸F]GE-179 binds to the phencyclidine (PCP) site in the open N-methyl-D-aspartate receptor ion channel (NMDAR-IC). To demonstrate that PET can visualise increased [¹⁸F]GE-179 uptake by active NMDAR-ICs and that this can be blocked by the PCP antagonist S-ketamine, 15 rats had an electrode unilaterally implanted in their ventral hippocampus. Seven rats had no stimulation, five received pulsed 400 µA supra-threshold 60 Hz stimulation alone, and three received intravenous S-ketamine injection prior to stimulation. Six other rats were not implanted. Each rat had a 90 min [¹⁸F]GE-179 PET scan. Stimulated rats had simultaneous depth-EEG recordings of induced seizure activity. [¹⁸F]GE-179 uptake (volume of distribution, V_T) was compared between hemispheres and between groups. Electrical stimulation induced a significant increase in [¹⁸F]GE-179 uptake at the electrode site compared to the contralateral hippocampus (mean 22% increase in V_T, p = 0.0014) and to non-stimulated comparator groups. Rats injected with S-ketamine prior to stimulation maintained non-stimulated levels of [¹⁸F]GE-179 uptake during stimulation. In conclusion, PET visualisation of focal [¹⁸F]GE-179 uptake during electrically activated NMDAR-ICs and the demonstration of specificity for PCP sites by blockade with S-ketamine support the in vivo utility of [¹⁸F]GE-179 PET as a use-dependent marker of NMDAR-IC activation.

Positron Emission Tomography (PET) Ligand Development for Ionotropic Glutamate Receptors: Challenges and Opportunities for Radiotracer Targeting N-Methyl-d-aspartate (NMDA), α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA), and Kainate Receptors

Ionotropic glutamate receptors (iGluRs) mediate excitatory neurotransmission within the mammalian central nervous system. iGluRs exist as three main groups: N-methyl-d-aspartate receptors (NMDARs), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), and kainate receptors. The past decades have witnessed a remarkable development of PET tracers targeting different iGluRs including NMDARs and AMPARs, and several of the tracers have advanced to clinical imaging studies. Here, we assess the recent development of iGluR PET probes, focusing on tracer design, brain kinetics, and performance in PET imaging studies. Furthermore, this review will not only present challenges in the tracer development but also provide novel approaches in conjunction with most recent drug discovery efforts on these iGluRs, including subtype-selective NMDAR and transmembrane AMPAR regulatory protein modulators and positive allosteric modulators (PAMs) of AMPARs. These approaches, if successful as PET tracers, may provide fundamental knowledge to understand the roles of iGluR receptors under physiological and pathological conditions.

First in human evaluation of [18F]PK-209, a PET ligand for the ion channel binding site of NMDA receptors

Background

Efforts to develop suitable positron emission tomography (PET) tracers for the ion channel site of human N-methyl-d-aspartate (NMDA) receptors have had limited success. [¹⁸F]PK-209 is a GMOM derivative that binds to the intrachannel phencyclidine site with high affinity and selectivity. Primate PET studies have shown that the volume of distribution in the brain was reduced by administration of the NMDA receptor antagonist MK-801, consistent with substantial specific binding. The purpose of the present study was to evaluate [¹⁸F]PK-209 in 10 healthy humans by assessing test–retest reproducibility and binding specificity following intravenous S-ketamine administration (0.5 mg ∙ kg⁻¹). Five healthy subjects underwent a test–retest protocol, and five others a baseline-ketamine protocol. In all cases dynamic, 120-min PET scans were acquired together with metabolite-corrected arterial plasma input functions. Additional input functions were tested based on within-subject and population-average parent fractions.

Results

Best fits of the brain time-activity curves were obtained using an irreversible two-tissue compartment model with additional blood volume parameter. Mean test–retest variability of the net rate of influx K_i varied between 7 and 24% depending on the input function. There were no consistent changes in [¹⁸F]PK-209 PET parameters following ketamine administration, which may be a consequence of the complex endogenous ligand processes that affect channel gating.

Conclusions

The molecular interaction between [¹⁸F]PK-209 and the binding site within the NMDA receptor ion channel is insufficiently reproducible and specific to be a reliable imaging agent for its quantification.

Trial registration

EudraCT 2014-001735-36. Registered 28 April 2014

Synthesis of carbon-14 labeled ketamine and norketamine

Ketamine is a well-known general anesthetic that inhibits cerebral NMDA receptors. Norketamine is a major circulating metabolite of this drug. A nasal spray formulation of esketamine, the S enantiomer of ketamine, is under development for the management of treatment-resistant depression. To assess the pharmacokinetic properties, C-14 labeled ketamine and norketamine were prepared separately from commercially available [¹⁴ C]CuCN through a five-step sequence with the C-14 label at the quaternary carbon of the cyclohexyl ring. Chiral resolution of [¹⁴ C]ketamine and chiral column separation of [¹⁴ C]norketamine resolved/separated the (S)-enantiomers from (R)-enantiomers.

The Role of Radionuclide Imaging in Epilepsy, Part 1: Sporadic Temporal and Extratemporal Lobe Epilepsy

Epilepsy is one of the most common yet diverse neurologic disorders, affecting almost 1%-2% of the population. Presently, radionuclide imaging such as PET and SPECT is not used in the primary diagnosis or evaluation of recent-onset epilepsy. However, it can play a unique and important role in certain specific situations, such as in noninvasive presurgical localization of epileptogenic brain regions in intractable-seizure patients being considered for epilepsy surgery. Radionuclide imaging can be particularly useful if MR imaging is either negative for lesions or shows several lesions of which only 1 or 2 are suspected to be epileptogenic and if electroencephalogram changes are equivocal or discordant with the structural imaging. Similarly, PET and SPECT can also be useful for evaluating the functional integrity of the rest of the brain and may provide useful information on the possible pathogenesis of the neurocognitive and behavioral abnormalities frequently observed in these patients.

Advances of Molecular Imaging in Epilepsy

Positron emission tomography (PET) is a neuroimaging method that offers insights into the molecular functioning of a human brain. It has been widely used to study metabolic and neurotransmitter abnormalities in people with epilepsy. This article reviews the development of several PET radioligands and their application in studying the molecular mechanisms of epilepsy. Over the last decade, tracers binding to serotonin and γ-aminobutyric acid (GABA) receptors have been used to delineate the location of the epileptic focus. PET studies have examined the role of opioids, cannabinoids, acetylcholine, and dopamine in modulating neuronal hyperexcitability and seizure termination. In vivo analyses of drug transporters, e.g., P-glycoprotein, have increased our understanding of pharmacoresistance that could inform new therapeutic strategies. Finally, PET experiments targeting neuroinflammation and glutamate receptors might guide the development of novel biomarkers of epileptogenesis.

Comparisons of the Effects of Anesthesia and Stress on Release of Tumor Necrosis Factor-α, Leptin, and Nitric Oxide in Adult Male Rats

Bacterial lipopolysaccharide (LPS) stimulates massive release of tumor necrosis factor-alpha (TNF-α) together with nitric oxide (NO) and a lessor release of leptin. We hypothesized that other types of stress such as that of surgery might also release these cytokines and NO. Adult male rats were anesthetized with ketamine/acepromazine/xylazine anesthesia (90 + 2 + 6 mg/ml, respectively) and an external jugular catheter was inserted for removal of blood samples (0.6 ml) at various times postoperatively. Plasma TNF-α was almost undetectable in decapitated rats and was near zero immediately following the placement of the jugular catheter (time zero [to]). As the rats awakened from anesthesia, there was a rise in TNF-α at 30 min that peaked at 2 hr with a 400-fold increase and then precipitously declined 40-fold to a level still greater than zero at 3 hr. At 6 hr on the following morning, TNF-α values were near zero, but following connection of tubing and withdrawal of the initial blood sample, there was a 100-fold increase 1 hr later, followed by a decline over the next 3 hr. In contrast, plasma [NO3/NO2] from decapitated rats was 117 μM. Values at t0 were decreased and plummeted 4-fold within 30 min, then rose slightly in the ensuing 3 hr. At 6 hr on the next day [NO3/NO2] values were lower than at t0 and declined gradually during the next 4 hr. Leptin gradually declined from pre-operative concentrations, reaching a minimum at 3 hr and its concentration was unaffected by the bleeding stress of the second day. We conclude that release of TNF-α, [NO3/NO2], and leptin are neurally controlled since plasma levels of all three declined as a result of anesthesia. TNF-α secretion was remarkably stress responsive, whereas NO release appeared to be suppressed by the combined operative and bleeding stress, and leptin was stress unresponsive.

Neuroimaging of epilepsy

Imaging is pivotal in the evaluation and management of patients with seizure disorders. Elegant structural neuroimaging with magnetic resonance imaging (MRI) may assist in determining the etiology of focal epilepsy and demonstrating the anatomical changes associated with seizure activity. The high diagnostic yield of MRI to identify the common pathological findings in individuals with focal seizures including mesial temporal sclerosis, vascular anomalies, low-grade glial neoplasms and malformations of cortical development has been demonstrated. Positron emission tomography (PET) is the most commonly performed interictal functional neuroimaging technique that may reveal a focal hypometabolic region concordant with seizure onset. Single photon emission computed tomography (SPECT) studies may assist performance of ictal neuroimaging in patients with pharmacoresistant focal epilepsy being considered for neurosurgical treatment. This chapter highlights neuroimaging developments and innovations, and provides a comprehensive overview of the imaging strategies used to improve the care and management of people with epilepsy.

NMDA receptor binding in focal epilepsies

OBJECTIVE

To demonstrate altered N-methyl-d-aspartate (NMDA) receptor availability in patients with focal epilepsies using positron emission tomography (PET) and [(18)F]GE-179, a ligand that selectively binds to the open NMDA receptor ion channel, which is thought to be overactive in epilepsy.

METHODS

Eleven patients (median age 33 years, 6 males) with known frequent interictal epileptiform discharges had an [(18)F]GE-179 PET scan, in a cross-sectional study. MRI showed a focal lesion but discordant EEG changes in two, was non-localising with multifocal EEG abnormalities in two, and was normal in the remaining seven patients who all had multifocal EEG changes. Individual patient [(18)F]GE-179 volume-of-distribution (VT) images were compared between individual patients and a group of 10 healthy controls (47 years, 7 males) using Statistical Parametric Mapping.

RESULTS

Individual analyses revealed a single cluster of focal VT increase in four patients; one with a single and one with multifocal MRI lesions, and two with normal MRIs. Post hoc analysis revealed that, relative to controls, patients not taking antidepressants had globally increased [(18)F]GE-179 VT (+28%; p<0.002), and the three patients taking an antidepressant drug had globally reduced [(18)F]GE-179 VT (-29%; p<0.002). There were no focal abnormalities common to the epilepsy group.

CONCLUSIONS

In patients with focal epilepsies, we detected primarily global increases of [(18)F]GE-179 VT consistent with increased NMDA channel activation, but reduced availability in those taking antidepressant drugs, consistent with a possible mode of action of this class of drugs. [(18)F]GE-179 PET showed focal accentuations of NMDA binding in 4 out of 11 patients, with difficult to localise and treat focal epilepsy.

Human Radiation Dosimetry for the N-Methyl-D-Aspartate Receptor Radioligand 11C-CNS5161

(11)C-CNS5161 (N-(2-chloro-5-methylthiophenyl)-N'-(3-methylthiophenyl)-N'-(11)C-methylguanidine) has been successfully used in PET imaging of N-methyl-d-aspartate (NMDA) receptors. However, no human dosimetry data have been published. We are planning to use this radiotracer for investigating NMDA receptor function in systemic lupus erythematosus, traumatic brain injury, and Parkinson disease. We have therefore undertaken (11)C-CNS5161 PET imaging to measure the whole-body distribution of this radionuclide and to estimate radiation dose to various organs.

METHODS

Dynamic PET studies of the whole body were performed on 5 healthy adults. Regions of interest were drawn over the visualized structures. Resultant time-activity curves were generated and used to determine residence times for dosimetry calculations. S factors were implemented within the OLINDA/EXM software for each structure or organ.

RESULTS

For (11)C-CNS5161, organ doses ranged from 0.0002 to 0.0393 mGy/MBq (0.0006-0.1455 rad/mCi). The critical organ for radiation burden was the lungs, with a dose of 0.0393 mGy/MBq (0.1455 rad/mCi). Radiation doses to the reproductive and blood-forming organs were 0.0023, 0.0002, and 0.0020 mGy/MBq (0.0086, 0.0006, and 0.0074 rad/mCi) for the ovaries, testes, and red marrow, respectively. The effective dose equivalent was 0.0106 mSv/MBq (0.0392 rem/mCi).

CONCLUSION

The radiation dosimetry for (11)C-CNS5161 for a standard single injection of 555 MBq (15 mCi) will result in an effective dose equivalent of 5.9 mSv (0.59 rem) and a lung dose of 21.8 mGy (2.18 rad) in young, healthy subjects.

Application of cross-species PET imaging to assess neurotransmitter release in brain

RATIONALE

This review attempts to summarize the current status in relation to the use of positron emission tomography (PET) imaging in the assessment of synaptic concentrations of endogenous mediators in the living brain.

OBJECTIVES

Although PET radioligands are now available for more than 40 CNS targets, at the initiation of the Innovative Medicines Initiative (IMI) "Novel Methods leading to New Medications in Depression and Schizophrenia" (NEWMEDS) in 2009, PET radioligands sensitive to an endogenous neurotransmitter were only validated for dopamine. NEWMEDS work-package 5, "Cross-species and neurochemical imaging (PET) methods for drug discovery", commenced with a focus on developing methods enabling assessment of changes in extracellular concentrations of serotonin and noradrenaline in the brain.

RESULTS

Sharing the workload across institutions, we utilized in vitro techniques with cells and tissues, in vivo receptor binding and microdialysis techniques in rodents, and in vivo PET imaging in non-human primates and humans. Here, we discuss these efforts and review other recently published reports on the use of radioligands to assess changes in endogenous levels of dopamine, serotonin, noradrenaline, γ-aminobutyric acid, glutamate, acetylcholine, and opioid peptides. The emphasis is on assessment of the availability of appropriate translational tools (PET radioligands, pharmacological challenge agents) and on studies in non-human primates and human subjects, as well as current challenges and future directions.

CONCLUSIONS

PET imaging directed at investigating changes in endogenous neurochemicals, including the work done in NEWMEDS, have highlighted an opportunity to further extend the capability and application of this technology in drug development.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

Initial evaluation of 18F-GE-179, a putative PET Tracer for activated N-methyl D-aspartate receptors

N-methyl D-aspartate (NMDA) ion channels play a key role in a wide range of physiologic (e.g., memory and learning tasks) and pathologic processes (e.g., excitotoxicity). To date, suitable PET markers of NMDA ion channel activity have not been available. (18)F-GE-179 is a novel radioligand that selectively binds to the open/active state of the NMDA receptor ion channel, displacing the binding of (3)H-tenocyclidine from the intrachannel binding site with an affinity of 2.4 nM. No significant binding was observed with 10 nM GE-179 at 60 other neuroreceptors, channels, or transporters. We describe the kinetic behavior of the radioligand in vivo in humans.

METHODS

Nine healthy participants (6 men, 3 women; median age, 37 y) each underwent a 90-min PET scan after an intravenous injection of (18)F-GE-179. Continuous arterial blood sampling over the first 15 min was followed by discrete blood sampling over the duration of the scan. Brain radioactivity (KBq/mL) was measured in summation images created from the attenuation- and motion-corrected dynamic images. Metabolite-corrected parent plasma input functions were generated. We assessed the abilities of 1-, 2-, and 3-compartment models to kinetically describe cerebral time-activity curves using 6 bilateral regions of interest. Parametric volume-of-distribution (V(T)) images were generated by voxelwise rank-shaping regularization of exponential spectral analysis (RS-ESA).

RESULTS

A 2-brain-compartment, 4-rate-constant model best described the radioligand's kinetics in normal gray matter of subjects at rest. At 30 min after injection, 37% of plasma radioactivity represented unmetabolized (18)F-GE-179. The highest mean levels of gray matter radioactivity were seen in the putamina and peaked at 7.5 min. A significant positive correlation was observed between K1 and V(T) (Spearman ρ = 0.398; P = 0.003). Between-subject coefficients of variation of V(T) ranged between 12% and 16%. Voxelwise RS-ESA yielded similar V(T)s and coefficients of variation.

CONCLUSION

(18)F-GE-179 exhibits high and rapid brain extraction, with a relatively homogeneous distribution in gray matter and acceptable between-subject variability. Despite its rapid peripheral metabolism, quantification of (18)F-GE-179 VT is feasible both within regions of interest and at the voxel level. The specificity of (18)F-GE-179 binding, however, requires further characterization with in vivo studies using activation and disease models.

Subanesthetic doses of ketamine transiently decrease serotonin transporter activity: a PET study in conscious monkeys

Subanesthetic doses of ketamine, an N-methyl-D-aspartic acid (NMDA) antagonist, have a rapid antidepressant effect which lasts for up to 2 weeks. However, the neurobiological mechanism regarding this effect remains unclear. In the present study, the effects of subanesthetic doses of ketamine on serotonergic systems in conscious monkey brain were investigated. Five young monkeys underwent four positron emission tomography measurements with [(11)C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)benzonitrile ([(11)C]DASB) for the serotonin transporter (SERT), during and after intravenous infusion of vehicle or ketamine hydrochloride in a dose of 0.5 or 1.5 mg/kg for 40 min, and 24 h post infusion. Global reduction of [(11)C]DASB binding to SERT was observed during ketamine infusion in a dose-dependent manner, but not 24 h later. The effect of ketamine on the serotonin 1A receptor (5-HT1A-R) and dopamine transporter (DAT) was also investigated in the same subjects studied with [(11)C]DASB. No significant changes were observed in either 5-HT1A-R or DAT binding after ketamine infusion. Microdialysis analysis indicated that ketamine infusion transiently increased serotonin levels in the extracellular fluid of the prefrontal cortex. The present study demonstrates that subanesthetic ketamine selectively enhanced serotonergic transmission by inhibition of SERT activity. This action coexists with the rapid antidepressant effect of subanesthetic doses of ketamine. Further studies are needed to investigate whether the transient combination of SERT and NMDA reception inhibition enhances each other's antidepressant actions.

Diminution of the NMDA receptor NR2B subunit in cortical and subcortical areas of WAG/Rij rats

Modulation of glutamatergic NMDA receptors affects the synchronization of spike discharges in in WAG/Rij rats, a valid genetic animal model of absence epilepsy. In this study, we describe the alteration of NR2B subunit of NMDA receptors expression in WAG/Rij rats in different somatosensory cortical layers and in hippocampal CA1 area. Experimental groups were divided into four groups of six rats of both WAG/Rij and Wistar strains with 2 and 6 months of age. The distribution of NR2B receptors was assessed by immunohistochemical staining in WAG/Rij and compared with age-matched Wistar rats. The expression of NR2B subunit was significantly decreased in different somatosensory cortical layers in 2- and 6-month-old WAG/Rij rats. In addition, the distribution of NR2B in hippocampal CA1 area was lower in 6-month-old WAG/Rij compared with age-matched Wistar rats. The reduction of NR2B receptors in different brain areas points to disturbance of glutamate receptors expression in cortical and subcortical areas in WAG/Rij rats. An altered subunit assembly of NMDA receptors may underlie cortical hyperexcitability in absence epilepsy.

Functional imaging: PET

Among various neuroimaging techniques used for the evaluation of children with intractable epilepsy, positron emission tomography (PET) employing various PET tracers plays a very important role, especially in localizing areas of focal cortical dysplasia. This is particularly important in infants, where incomplete myelination may limit the structural information provided by MRI. In children with tuberous sclerosis, PET can differentiate between epileptogenic and nonepileptogenic tubers, previously not thought to be possible with neuroimaging. PET may reveal cortical or subcortical abnormalities in various epilepsy syndromes, such as infantile spasms and Landau-Kleffner syndrome. Various other applications of PET have included the investigation of epileptic networks, secondary epileptic foci, dual pathology, and neuroinflammation. Finally, PET can also be used to evaluate various cognitive processes and their underlying neurological substrates and can help in addressing the issue of brain plasticity and reorganization, related to epilepsy.

PET and SPECT tracers for glutamate receptors

Radioligands for PET imaging of glutamate receptors will have the potential for studying neurological and neuropsychiatric disorders and their diagnosis and therapeutic intervention. Glutamate is the major excitatory neurotransmitter in the brain and is implicated in the pathophysiology of many neurodegenerative and neuropsychiatric disorders. Glutamate and its receptors are potential targets in the treatment of these disorders. Glutamate signaling is mediated through ionotropic and metabotropic receptors. The abundant concentration of these receptors can facilitate their in vivo quantification using positron emission tomography (PET). Glutamate receptors are a potentially important set of targets for monitoring disease progression, for evaluating the effect of therapy and for new treatment development based on the quantification of receptor occupancy. Here, we review the PET and single-photon emission computed tomography (SPECT) radioligands that have been developed for imaging glutamate receptors in living brain.

Increased neurokinin-1 receptor availability in temporal lobe epilepsy: a positron emission tomography study using [(11)C]GR205171

PURPOSE

Activation of the neurokinin-1 (NK1) receptor by neuropeptide substance P (SP) induces and maintains epileptic activity in various experimental models of epilepsy. The primary objective of this study was to investigate whether neurobiological changes linked to NK1-SP receptor system are associated with hyperexcitability in patients with temporal lobe epilepsy (TLE). A secondary objective was to investigate the relationship between seizure frequency and NK1 receptor availability.

METHODS

A positron emission tomography study was conducted with the selective NK1 receptor antagonist [(11)C]GR205171 in nine patients with TLE and 18 healthy control participants. Parametric PET images were generated using the Patlak graphical method, with cerebellum as reference region. Data analyses including group comparisons were performed using statistical parametric mapping.

RESULTS

Patients with TLE showed increased NK1 receptor availability in both hemispheres with the most pronounced increase in anterior cingulate gyrus ipsilateral to seizure onset. A positive correlation between NK1 receptor availability and seizure frequency was observed in the medial temporal lobe and in the lentiform nucleus ipsilateral to the seizure onset.

CONCLUSION

Our results suggest that there is an intrinsic network using the NK1-SP receptor system for synaptic transmission and epileptiform activity in TLE.

Acute S-ketamine application does not alter cerebral [18F]altanserin binding: a pilot PET study in humans

Modeling short-term psychotic states with subanaesthetic doses of ketamine provides substantial experimental evidence in support of the glutamate hypothesis of schizophrenia. Ketamine exerts its pharmacological effects both directly via interactions with glutamate receptors and indirectly by stimulating presynaptic release of endogenous serotonin (5-HT). The aim of this feasibility study was to examine whether acute ketamine-induced 5-HT release interferes with the binding of the 5-HT(2A) receptor (5-HT(2A)R) radioligand [(18)F]altanserin and positron emission tomography (PET). Two subjects treated with ketamine and one subject treated with placebo underwent [(18)F]altanserin PET at distribution equilibrium conditions. Robust physiological, psychopathological and cognitive effects were present at ketamine plasma concentrations exceeding 100 microg/l during >70 min. Notwithstanding, we observed stable radioligand binding (changes +/-95% CI of -1.0 +/- 1.6% and +4.1 +/- 1.8% versus -1.2 +/- 2.6%) in large cortical regions presenting high basal uptake of both, [(18)F]altanserin and ketamine. Marginal decreases of 4% of radioligand binding were observed in the frontal lobe, and 8% in a posteriorily specified frontomesial subregion. This finding is not compatible with a specific radioligand displacement from 5-HT(2A)R which should occur proportionally throughout the whole brain. Instead, the spatial pattern of these minor reductions was congruent with ketamine-induced increases in cerebral blood flow observed in a previous study using [(15)O]butanol PET. This may caused by accelerated clearance of unspecifically bound [(18)F]altanserin from cerebral tissue with increased perfusion. In conclusion, this study suggests that [(18)F]altanserin PET is not sensitive to acute neurotransmitter fluctuations under ketamine. Advantageously, the stability of [(18)F]altanserin PET towards acute influences is a prerequisite for its future use to detect sub-acute and chronic effects of ketamine.

PET tracer technology for monitoring focal epilepsies

Studies using positron emission tomography (PET) have advanced our pathophysiological and biochemical understanding of focal and generalized epilepsies. H(2) (15)O PET allows quantification of cerebral blood flow and (18)F-fluorodeoxyglucose-PET quantification of cerebral glucose metabolism. Neurotransmitters are directly responsible for modulating synaptic activity and newer PET tracers can provide information about synaptic activity and specific ligand-receptor relationships, which are important for epileptogenesis and the spread of epileptic activity.

In vitro and in vivo characterization of [3H]CNS-5161—A use-dependent ligand for theN-methyl-d-aspartate receptor in rat brain

Glutamate is the major excitatory neurotransmitter in the brain. Glutamate activation of the N-methyl-D-aspartate (NMDA) receptor subtype is thought to mediate important physiological and pathological processes, including memory formation and excitotoxicity. The goal of the present work was to characterize and validate a candidate agent for noninvasive positron emission tomography (PET) imaging of this receptor. [(3)H]-labeled N-[3-(3)H]-methyl-3-(thiomethylphenyl)cyanamide (CNS-5161) was incubated with rat brain homogenates at increasing concentrations, temperatures, and times to establish the binding kinetics and affinity of the ligand in vitro. Nonspecific binding was measured with 100 microM MK-801. The compound was also injected i.v. in rats pretreated with saline, NMDA, MK801, or a combination, and organ and brain regional uptake was assessed at various times after injection by autoradiography or dissection. Blood and brain samples were assayed for metabolites by high-performance liquid chromatography. CNS-5161 binds brain membranes with high affinity (K(d) < 4 nM) and fast association and dissociation kinetics. Specific binding increased in the presence of glutamate and glycine. Intravenous administration in control rats resulted in a heterogeneous brain distribution with hippocampus and cortex > thalamus > striatum > cerebellum, and a cortex/cerebellum ratio of 1.4. Pretreatment with NMDA increased the hippocampus-to-cerebellum ratio to 1.6-1.9 while MK801 abolished this increase, resulting in ratios close to 1. Thus, CNS-5161 binds preferentially to the activated state of the NMDA receptor channel in vitro and in vivo. The high affinity and fast kinetics make it compatible with PET imaging of a carbon-11 labeled CNS-5161.

Functional neuroimaging in the preoperative evaluation of children with drug-resistant epilepsy

FUNCTIONAL NEUROIMAGING: Although the primary imaging modality in the management of epilepsy is magnetic resonance imaging MRI, functional neuroimaging with positron-emission tomography (PET) and single photon emission computed tomography (SPECT) often provides complementary information and, in a number of situations, provides unique information that cannot be obtained with MRI. The most commonly used PET tracers used for epilepsy evaluation are 2-deoxy-2-[(18)F]fluoro-D: -glucose (FDG) and [(11)C]flumazenil (FMZ). Recently, interictal PET with alpha-[(11)C]methyl-L: -tryptophan was found to be highly specific for the epileptic focus and can differentiate between epileptogenic and nonepileptogenic lesions in the same patient (e.g., in patients with tuberous sclerosis).

DISCUSSION

In this review, we discuss clinical applications of these three PET tracers in drug-resistant temporal and extratemporal lobe epilepsy, selected epilepsy syndromes of childhood, lesional and nonlesional epilepsy, and the challenges of imaging secondary epileptic foci. A brief discussion of SPECT applications in epilepsy is also included. With further development of new tracers highly sensitive and specific for epileptogenic brain regions, the presurgical evaluation of refractory epilepsy will be greatly facilitated. Approximately 0.5 to 1.0% of the population suffer from epilepsy, of which 15-20% are intractable. Infants and children, whose seizures have a focal onset are refractory to anticonvulsants and are prolonged, tend to have the worst cognitive outcome [Meador KJ, Neurology 58 (Suppl 5):S21-S26, 2002]. Seizures themselves affect the developing brain and contribute to an adverse neurologic outcome (Holmes, Pediatric Neurology 33:1-110, 2005).

CONCLUSION

Therefore, in treating children with intractable epilepsy, it is important to consider seizure control and to give allowance for normal cognitive development.

Epilepsy surgery: perioperative investigations of intractable epilepsy

Recent advances in our understanding of the basic mechanisms of epilepsy have derived, to a large extent, from increasing ability to carry out detailed studies on patients surgically treated for intractable epilepsy. Clinical and experimental perioperative studies divide into three different phases: before the surgical intervention (preoperative studies), on the intervention itself (intraoperative studies), and on the period when the part of the brain that has to be removed is available for further investigations (postoperative studies). Before surgery, both structural and functional neuroimaging techniques, in addition to their diagnostic roles, could be used to investigate the pathophysiological mechanisms of seizure attacks in epileptic patients. During epilepsy surgery, it is possible to insert microdialysis catheters and electroencephalogram electrodes into the brain tissues in order to measure constituents of extracellular fluid and record the bioelectrical activity. Subsequent surgical resection provides tissue that can be used for electrophysiological, morphological, and molecular biological investigations. To take full advantage of these opportunities, carefully designed experimental protocols are necessary to compare the data from different phases and characterize abnormalities in the human epileptic brain.

Evaluation of (R)-[11C]verapamil as PET tracer of P-glycoprotein function in the blood-brain barrier: kinetics and metabolism in the rat

There is evidence that P-glycoprotein (P-gp) in the blood-brain barrier (BBB) may be involved in the aetiology of neurological disorders. For quantification of P-gp function in vivo, (R)-[11C]verapamil can be used as a positron emission tomography (PET) tracer, provided that a mathematical model describing kinetics of uptake and clearance of verapamil is available. To develop and validate such a model, the kinetic profile and metabolism of (R)-[11C]verapamil have to be known. The aim of this study was to investigate the presence of labeled metabolites of [11C]verapamil in the plasma and (brain) tissue of Wistar rats. For this purpose, extraction and high-performance liquid chromatography (HPLC) methods were developed. The radioactive metabolites of (R)-[11C]verapamil in the liver were N-dealkylated compounds, O-demethylated compounds and a polar fraction formed from N-demethylation products of (R)-[11C]verapamil. Apart from this [11C] polar fraction, other radioactive metabolites of [11C]verapamil were not detected in the brain tissue. Thirty minutes after injection, unmetabolized (R)-[11C]verapamil accounted for 47% of radioactivity in the plasma and 69% in the brain. Sixty minutes after injection, unmetabolized (R)-[11C] verapamil was 27% and 48% in the plasma and the brain, respectively.

Imaging structure and function in refractory focal epilepsy

Over the past decade there have been many advances in data acquisition and analysis for structural and functional neuroimaging of people with epilepsy. New imaging sequences and analysis techniques have increased the resolution of images such that underlying structural pathology can be seen in many patients with "cryptogenic" epilepsy. When an epileptogenic lesion is present, antiepileptic drugs alone rarely prevent seizures. However, the success of surgical treatment is improved when a structural lesion has been identified. Lesions might not overlap with the area of the cortex generating seizures and may continue into areas sustaining normal functions. To prevent postsurgical morbidity, the spatial relation between functionally important areas and the epileptogenic lesion must be assessed before surgery. In this review we describe the potential of different neuroimaging techniques to show lesions, assess neuronal function, and assist with the prognosis of postsurgical outcome in patients with refractory focal epilepsy.

PET in pediatric diseases

FDG-PET is being increasingly applied to pediatric conditions, particularly in oncology. PET and PET/CT scanning in children are not currently supported by Centers for Medicare and Medicaid Services unless the disease condition coincides with a reimbursed adult condition. The recent merger of the Children's Cancer Group and the Pediatric Oncology Group to form the Children's Oncology Group creates an opportunity to examine the use of FDG-PET in the management of childhood tumors in multi-institutional, cooperative efforts. The interest in incorporating PET imaging technology in pediatric medicine has been evidenced by several recent review articles summarizing the ongoing progress in this area. Future data will show that FDG-PET provides useful diagnostic information and can play a pivotal role in the clinical management and care of children with disease.

Contributions of Mossy Fiber and CA1 Pyramidal Cell Sprouting to Dentate Granule Cell Hyperexcitability in Kainic Acid–Treated Hippocampal Slice Cultures

Axonal sprouting like that of the mossy fibers is commonly associated with temporal lobe epilepsy, but its significance remains uncertain. To investigate the functional consequences of sprouting of mossy fibers and alternative pathways, kainic acid (KA) was used to induce robust mossy fiber sprouting in hippocampal slice cultures. Physiological comparisons documented many similarities in granule cell responses between KA- and vehicle-treated cultures, including: seizures, epileptiform bursts, and spontaneous excitatoty postsynaptic currents (sEPSCs) >600pA. GABAergic control and contribution of glutamatergic synaptic transmission were similar. Analyses of neurobiotin-filled CA1 pyramidal cells revealed robust axonal sprouting in both vehicle- and KA-treated cultures, which was significantly greater in KA-treated cultures. Hilar stimulation evoked an antidromic population spike followed by variable numbers of postsynaptic potentials (PSPs) and population spikes in both vehicle- and KA-treated cultures. Despite robust mossy fiber sprouting, knife cuts separating CA1 from dentate gyrus virtually abolished EPSPs evoked by hilar stimulation in KA-treated but not vehicle-treated cultures, suggesting a pivotal role of functional afferents from CA1 to dentate gyrus in KA-treated cultures. Together, these findings demonstrate striking hyperexcitability of dentate granule cells in long-term hippocampal slice cultures after treatment with either vehicle or KA. The contribution to hilar-evoked hyperexcitability of granule cells by the unexpected axonal projection from CA1 to dentate in KA-treated cultures reinforces the idea that axonal sprouting may contribute to pathologic hyperexcitability of granule cells.

In vivo evaluation of [11C]N-(2-chloro-5-thiomethylphenyl)-N′- (3-methoxy-phenyl)-N′-methylguanidine ([11C]GMOM) as a potential PET radiotracer for the PCP/NMDA receptor

The development of imaging methods to measure changes in NMDA ion channel activation would provide a powerful means to probe the mechanisms of drugs and device based treatments (e.g., ECT) thought to alter glutamate neurotransmission. To provide a potential NMDA/PCP receptor PET tracer, we synthesized the radioligand [11C]GMOM (ki = 5.2 +/-0.3 nM; log P = 2.34) and evaluated this ligand in vivo in awake male rats and isoflurane anesthetized baboons. In rats, the regional brain uptake of [11C]GMOM ranged from 0.75+/-0.13% ID/g in the medulla and pons to 1.15+/-0.17% ID/g in the occipital cortex. MK801 (1 mg/kg i.v.) significantly reduced (24-28%) [11C]GMOM uptake in all regions. D-serine (10 mg/kg i.v.) increased [11C]GMOM %ID/g values in all regions (10-24%) reaching significance in the frontal cortex and cerebellum only. The NR2B ligand RO 25-6981 (10 mg/kg i.v.) reduced [11C]GMOM uptake significantly (24-38%) in all regions except for the cerebellum and striatum. Blood activity was 0.11+/-0.03 %ID/g in the controls group and did not vary significantly across groups. PET imaging in isoflurane-anesthetized baboons with high specific activity [11C]GMOM provided fairly uniform regional brain distribution volume (VT) values (12.8-17.1 ml g(-1)). MK801 (0.5 mg/kg, i.v., n = 1, and 1.0 mg/kg, i.v., n = 1) did not significantly alter regional VT values, indicating a lack of saturable binding. However, the potential confounding effects associated with ketamine induction of anesthesia along with isoflurane maintenance must be considered because both agents are known to reduce NMDA ion channel activation. Future and carefully designed studies, presumably utilizing an optimized NMDA/PCP site tracer, will be carried out to further explore these hypotheses. We conclude that, even though [11C]GMOM is not an optimized PCP site radiotracer, its binding is altered in vivo in awake rats as expected by modulation of NMDA ion channel activity by MK801, D-serine or RO 25-6981. The development of higher affinity NMDA/PCP site radioligands is in progress.

Schizophrenia, dissociative anaesthesia and near-death experience; three events meeting at the NMDA receptor

The three events, viz. schizophrenia, dissociative anaesthesia and Near-Death Experience, despite their seemingly unrelated manifestation to each other, have nevertheless similar functional basis. All three events are linked to the glutamate sensitive N-methyl-D-aspartate (NMDA) receptor complex, which serves as their common functional denominator. Arguments and speculations are presented in favor of the view that, the three events might be considered as functional models of each other. Antagonism to the recognition NMDA-site of the receptor induces dissociative anaesthesia and precipitates Near-Death Experience. Agonist reinforcement at the modulatory glycine-site of the receptor counteracts negative symptoms of schizophrenia. Both types of challenges towards the receptor are compatible with a glutamate deficiency concept which underlies the meeting of the three events at the NMDA receptor.

Qualitative and quantitative imaging of the hippocampus in mesial temporal lobe epilepsy with hippocampal sclerosis

MR imaging allows the in vivo detection of hippocampal sclerosis (HS) and has been instrumental in the delineation of the syndrome of mesial temporal lobe epilepsy with HS (mTLE-HS). MR features of HS include hippocampal atrophy with an increased T2 signal. Quantitative MR imaging accurately reflects the degree of hippocampal damage.Ictal single photon emission computed tomography (SPECT) in mTLE-HS shows typical perfusion patterns of ipsilateral temporal lobe hyperperfusion, and ipsilateral frontoparietal and contralateral cerebellar hypoperfusion. Interictal 18fluoro-2-deoxyglucose positron emission tomography (PET) shows multiregional hypometabolism, involving predominantly the ipsilateral temporal lobe. 11C-flumazenil PET shows hippocampal decreases in central benzodiazepine receptor density. Future strategies to study the etiology and pathogenesis of HS should include longitudinal MR imaging studies,MR studies in families with epilepsy and febrile seizures, stratification for genetic background, coregistration with SPECT and PET, partial volume correction and statistical parametric mapping analysis of SPECT and PET images.

Flumazenil positron emission tomography and other ligands for functional imaging

Most PET receptor studies in idiopathic generalized epilepsy conducted to date include only small numbers of patients and should be interpreted with caution. Differences between earlier and later studies can largely be explained by different inclusion criteria and improving methodology. The finding of some increase of GABAA receptor binding in IGE has a potential pathologic basis in microdysgenesis. Future studies aiming to elucidate the pathophysiology of IGEs may benefit from the use of subtype-specific opioid ligands, available now, and GABAB ligands, if and when they become available.

Neuroreceptor imaging of stress and mood disorders

Techniques such as positron emission tomography and single photon emission computed tomography allow for the imaging of neurotransmitter receptors and transporters in the brain. These tools have been used to investigate serotonergic, dopaminergic, and opioidergic function in healthy subjects as well as in patients with major depressive disorder, bipolar disorder, and other mood disorders. Pharmacologic challenges, such as amphetamine challenge, and physiologic stressors, such as pain challenge, have been used to further examine the function of these neurotransmitter systems. Neuroimaging of patient populations before and after medication treatment may be useful to understand changes in neurotransmission that accompany disease remission. As new radiotracers with higher selectivity for the various receptors and transporters are developed, imaging techniques may provide new insights into the pathophysiology of mood disorders, leading to improved diagnosis and treatment.

Imaging the PCP site of the NMDA ion channel

The N-methyl-D-aspartate (NMDA) ion channel plays a role in neuroprotection, neurodegeneration, long-term potentiation, memory, and cognition. It is implicated in the pathophysiology of several neurological and neuropsychiatric disorders including Parkinson's Disease, Huntington's Chorea, schizophrenia, alcoholism and stroke. The development of effective radiotracers for the study of NMDA receptors is critical for our understanding of their function, and their modulation by endogenous substances or therapeutic drugs. Since the NMDA/PCP receptor lies within the channel, it is a unique target and is theoretically accessible only when the channel is in the active and "open" state, but not when it is in the inactive or "closed" state. The physical location of the NMDA/PCP receptor not only makes it an important imaging target but also complicates the development of suitable PET and SPECT radiotracers for this site. An intimate understanding of the biochemical, pharmacological, physiological and behavioral processes associated with the NMDA ion channel is essential to develop improved imaging agents. This review outlines progress made towards the development of radiolabeled agents for PCP sites of the NMDA ion channel. In addition, the animal and pharmacological models used for in vitro and in vivo assessment of NMDA receptor targeted agents are discussed.

Imaging the epileptic brain with positron emission tomography

Positron emission tomography (PET) has an established role in the noninvasive localization of epileptic foci during presurgical evaluation. [18F]fluorodeoxyglucose (FDG) PET is able to lateralize and regionalize potentially epileptogenic regions in patients who have normal MR imaging and is also useful in the evaluation of various childhood epilepsy syndromes, including cryptogenic infantile spasms and early Rasmussen's syndrome. Novel PET tracers that were developed to image neurotransmission related to gamma-aminobutyric acid (GABA) [with [11C]flumazenil] and serotonin-mediated [with alpha-[11C]methyl-L-tryptophan (AMT)] function provide increased specificity for epileptogenic cortex and are particularly useful when FDG PET shows large abnormalities of glucose metabolism. Detailed comparisons of PET abnormalities with intracranial electroencephalographic findings also improve our understanding of the pathophysiology of human epilepsy.

Prognostic value of concordant seizure focus localizing data in the selection of temporal lobectomy candidates

This study was performed to test the hypotheses that (a) resection of the temporal lobe epileptic focus, amenable to noninvasive as opposed to invasive localization, is associated with superior seizure outcome and (b) that quadruple (versus lesser degrees of) concordance of seizure focus localizing data predicts superior seizure-free outcome. Eighty-three patients underwent invasive (subdural-EEG) and/or noninvasive (video/scalp-EEG, SPECT, PET, MRI, neuropsychological testing) evaluation. All patients underwent anterior temporal lobectomy and amygdalohippocampectomy (ATL/AH) and seizure outcome was assessed at minimum one-year follow-up. At 34.8 +/- 2.5 months following ATL/AH, outcome was superior for patients in whom the seizure focus was amenable to noninvasive compared to invasive localization (80% versus 40% seizure-free, X2 = 14.03, p < 0.05). Seizure outcome was superior for patients with quadruple, compared to all lesser degrees of, concordance of seizure focus localizing data (85% versus 51% seizure-free, X2 = 7.34, p < 0.05). Post-ATL/AH, seizure outcome is superior in patients (1) harboring an epileptic focus amenable to noninvasive localization and (2) with quadruple concordance of seizure focus localizing data. These findings support the development of temporal lobectomy selection criteria including up to four invasive and/or noninvasive concordant seizure focus localizing techniques.

Synthesis and in vitro evaluation of N,N'-diphenyl and N-naphthyl-N'-phenylguanidines as N-methyl-D-aspartate receptor ion-channel ligands

A series of N,N'-diphenyl and N-naphthyl-N'-phenyl guanidine derivatives was synthesized as potential N-methyl-D-aspartate (NMDA) receptor positron emission tomography (PET) ligands. The affinity of the different compounds was determined using in vitro receptor binding assays, and their log P values were estimated using HPLC analysis. The effect of N'-3 and N'-3,5 substitution on affinity and lipophilicity was examined. The K(i) values ranged from 1.87 to 839nM, while log P values between 1.22 and 2.88 were observed.

Low concentration of DL-2-amino-5-phosphonovalerate induces epileptiform activity in guinea pig hippocampal slices

PURPOSE

The specific mechanisms by which low concentrations of cyclosporine induce seizures and low concentrations of phencyclidine provoke behavioral excitation remain to be elucidated. Both compounds block N-methyl-d-aspartate (NMDA) receptors. The aim of this study was to determine if low concentrations of the NMDA-receptor blockers increase the seizure susceptibility.

METHODS

Guinea pig hippocampal slices were exposed to artificial cerebrospinal fluid containing the NMDA blocker dl-2-amino-5-phosphono-valerate (APV; 0.1-10 microM). Extracellular field potentials were recorded from CA1 and CA3 regions.

RESULTS

Low concentrations of APV induced epileptiform burst discharges (0.1-0.25 microM), whereas higher doses failed to decrease the seizure threshold (1-10 microM).

CONCLUSIONS

The results indicate that the excitatory effect of low concentrations NMDA blockers may play a role in the neurotoxicity of aforementioned substances.

PET in psychopharmacology

Emission tomography techniques and, in particular, positron emission tomography (PET) enable the in vivo study of several physiological and neurochemical variables in human subjects using methods originally developed for quantitative autoradiography. In particular, PET allows one to evaluate in human subjects: (a) the effect of specific neurochemical challenges on regional brain function at rest or under activation; (b) the activity of neurotransmitters and the regional expression of specific molecular targets during pathology including their modulation by drug treatment; (c) the kinetics of drug disposition and activity directly in the target organ. This is of primary interest in the field of biological psychiatry and in psychoactive drugs development, where it is particularly difficult to reproduce human diseases using animal models in view of the peculiarity of this field and the large heterogeneity of each psychiatric illness also inside the same clinical definition. The aim of this paper is to review the principal strategies and the main results of the use of PET in psychopharmacology.

CPD - education and self-assessment: functional imaging in epilepsy

Functional imaging plays a growing role in the clinical assessment and research investigation of patients with epilepsy. This article reviews the literature on functional MRI (fMRI) investigation of EEG activity, fMRI evaluation of cognitive and motor functions, magnetic resonance spectroscopy (MRS), single photon emission computed tomography (SPECT) and positron emission tomography (PET) in epilepsy. The place of these techniques in clinical evaluation and their contribution to a better neurobiological understanding of epilepsy are discussed.