PET tracers for imaging brain α7 nicotinic receptors: an update

Positron emission tomography (PET) molecular imaging of brain targets is a powerful tool to diagnose, follow up, and develop treatments and personalized medicine for a number of acute and chronic brain disorders. The availability of β+ emitter tracers labelled with [(11)C] or [(18)F] having optimal characteristics of affinity and selectivity for alpha-7 nicotinic receptors (α7R) has received considerable attention, due to the major implication of these receptors in brain functions. The aim of this review is to identify the interest and need for the in vivo exploration of α7R by PET molecular imaging, which tools are currently available for this and how to progress.

Neuroinflammation and β amyloid deposition in Alzheimer's disease: in vivo quantification with molecular imaging

BACKGROUND/AIMS

Neuroinflammation plays a crucial role in the pathogenesis of Alzheimer's disease (AD). Its relationship with underlying β amyloid deposition remains unclear. In vivo visualization of microglial activation has become possible with the development of molecular imaging ligands when used with positron emission tomography (PET). The translocator protein (TSPO) is upregulated during neuroinflammation. Consequently, targeting TSPO with radiolabeled ligands for PET is an attractive biomarker for neuroinflammation.

METHODS

A review of the research literature on PET imaging which studied in vivo neuroinflammation in AD subjects and its relationship with amyloid load was performed, including papers published between 2001 and 2012.

RESULTS

Six studies were included using either [(11)C]PK-11195 or another non-TSPO radioligand that binds to the monoaminooxidase B. All the studies evaluated amyloid load with [(11)C]PIB. Microglial activation and astrocytosis are potentially early phenomena in AD. However, the individual levels of amyloid deposition and microglial activation were not correlated.

CONCLUSION

Noninvasive in vivo molecular imaging to visualize neuroinflammation in AD may contribute to our understanding of the kinetics of neuroinflammation and its relationship to the hallmarks of the disease. Both are important for the development of future therapeutic modalities and for quantifying the efficacy of future disease-modifying treatments.

The specific PKR inhibitor C16 prevents apoptosis and IL-1β production in an acute excitotoxic rat model with a neuroinflammatory component

The double-stranded RNA-dependent protein kinase (PKR), an apoptotic inducer, regulates much pro-inflammatory cytokine production. The purpose of this study was to evaluate in vivo the effects of the specific PKR inhibitor C16 in the striatum in an acute excitotoxic rat model with an important neuroinflammatory component. Inflammation was induced by unilateral striatal injection of quinolinic acid (QA) in 10-week-old normotensive rats. Animals were separated into groups receiving either vehicle or C16 for both sham and QA rats. The effects were assessed in ipsi- and contralateral striata by immunoblotting for PKR activation, by Luminex assay for cytokine levels and by immunofluorescent staining for cleaved caspase-3 to detect neuronal apoptosis. The highest dose of C16 (600μg/kg; C16-2) in QA rats reduced expression of the active catalytic domain of the PKR vs. that in vehicle-injected QA rats. A robust increase of IL-1β levels on the contralateral side of QA rats was prevented by C16-2 (97% inhibition). Macroscopic and microscopic observation of cerebral tissue (Hematoxylin & Eosin staining) revealed that tissue integrity was more preserved with C16-2 treatment than its vehicle in QA rats. Furthermore, C16-2 treatment decreased by 47% the neuronal loss and by 37% the number of positive cleaved caspase-3 neurons induced by QA injection. In conclusion, C16 prevented not only the PKR-induced neuronal loss but also the inflammatory response in this acute excitotoxic in vivo model, highlighting its promising neuroprotective properties to rescue acute brain lesions.

Multiple-Dose Plasma Pharmacokinetic and Safety Study of LY450108 and LY451395 (AMPA Receptor Potentiators) and Their Concentration in Cerebrospinal Fluid in Healthy Human Subjects

The objective of this study was to measure the steady-state cerebrospinal fluid (CSF) concentration of LY450108 and LY451395 (positive modulators of AMPA receptors) in healthy subjects after the administration of 1 mg and 5 mg. Secondary objectives included the evaluation of safety, pharmacokinetics, and steady-state ratio of plasma:CSF concentrations of LY450108 and LY451395 after multiple dosing. This study was an open-label, multiple oral dose study evaluating 1 mg and 5 mg LY450108 and 1 mg and 5 mg LY451395 in 12 (3 subjects per dosing group) healthy subjects, aged 18 to 49 years. Twelve healthy male subjects completed the study. LY450108 and LY451395 were quantifiable in CSF after 1-mg and 5-mg multiple-dose administrations with plasma:CSF ratio of 82:1 and 44:1, respectively. LY450108 and LY451395 1 mg and 5 mg were measured in the CSF. Single and multiple oral doses of LY450108 and LY451395 were determined to be safe and well tolerated in healthy subjects.

Concomitant blockade of 5-HT(1A) receptor and 5-HT transporter: use of the Hunter Serotonin toxicity criteria in a clinical pharmacology study

There is a potential risk that 5-HT(1A) receptor blockade combined with blockade of the 5-HT transporter by an SSRI may cause a toxic increase in 5-HT within the synapse, sparking concern for 'serotonin syndrome', a rare but potentially life threatening condition. We evaluated the safety and pharmacodynamics of the combination of the 5-HT(1A) antagonist lecozotan and the SSRI citalopram in a well-controlled Clinical Pharmacology Unit setting using the Hunter Serotonin Toxicity Criteria (HSTC), a set of validated decision rules featuring neurological and body temperature measurements, to detect any clinically relevant serotonin toxicity. Forty-three young healthy male subjects were randomized, to 2 parallel double-blind treatment groups following a 10-day citalopram 40 mg run-in period: citalopram 40 mg/lecozotan 10mg or citalopram 40 mg/placebo for 9 days. Overall, the combined administration of active drugs was well tolerated, however, one subject experienced moderate hyperreflexia, tremor of the hands, and sweating of hands and feet after 3 days of combined treatment. The event prompted treatment withdrawal and was regarded as mild serotonin toxicity, as per the HSTC. The onset of the event was around the time of peak plasma concentrations (t(max)) of both lecozotan and citalopram, and its time course corresponds to the well-defined PK profile of lecozotan. No evidence of a PK interaction was detected trough lecozotan and citalopram plasma concentrations analysis. The utility of the HSTC in detecting the non-discrete group of symptoms commonly referred to as "serotonin toxicity" was demonstrated in this clinical pharmacology study combining two 5-HT agents in a clinically controlled setting.

Spatio-temporal diffusion of dynamic PET images

Positron emission tomography (PET) images are corrupted by noise. This is especially true in dynamic PET imaging where short frames are required to capture the peak of activity concentration after the radiotracer injection. High noise results in a possible bias in quantification, as the compartmental models used to estimate the kinetic parameters are sensitive to noise. This paper describes a new post-reconstruction filter to increase the signal-to-noise ratio in dynamic PET imaging. It consists in a spatio-temporal robust diffusion of the 4D image based on the time activity curve (TAC) in each voxel. It reduces the noise in homogeneous areas while preserving the distinct kinetics in regions of interest corresponding to different underlying physiological processes. Neither anatomical priors nor the kinetic model are required. We propose an automatic selection of the scale parameter involved in the diffusion process based on a robust statistical analysis of the distances between TACs. The method is evaluated using Monte Carlo simulations of brain activity distributions. We demonstrate the usefulness of the method and its superior performance over two other post-reconstruction spatial and temporal filters. Our simulations suggest that the proposed method can be used to significantly increase the signal-to-noise ratio in dynamic PET imaging.

Basal ganglia neuroprotection with anticonvulsants after energy stress: a comparative study

The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model provides a valuable paradigm of the energy deficiency disorders found in childhood. In such disorders, anticonvulsants may provide neuroprotection by modulating cellular energy consumption and by exerting favorable pleiotropic effects on neuronal survival. To verify such hypothesis, we tested the effects of levetiracetam, vigabatrin, gabapentine, pregabaline, tiagabine, clonazepam and lamotrigine on neuroprotection in the MPTP mouse model. The membrane dopamine transporter (DAT) density, which provides a reliable index of dopaminergic neurons survival in the basal ganglia, was assessed by semi-quantitative autoradiography of the striatum. Unlike all other anticonvulsants tested, lamotrigine provided a significant and dose-dependent neuroprotection in these experimental conditions. Lamotrigine, a widely used and well-tolerated molecule in children, could provide neuroprotection in various energy deficiency disorders.

MPTP intoxication in mice: a useful model of Leigh syndrome to study mitochondrial diseases in childhood

The basal ganglia, which are interconnected in the striato-nigral dopaminergic network, are affected in several childhood diseases including Leigh syndrome (LS). LS is the most common mitochondrial disorder affecting children and usually arise from inhibition of the respiratory chain. This vulnerability is attributed to a particular susceptibility to energetic stress, with mitochondrial inhibition as a common pathogenic pathway. In this study we developed a LS model for neuroprotection trials in mice by using the complex I inhibitor MPTP. We first verified that MPTP significantly inhibits the mitochondrial complex I in the brain (p = 0.018). This model also reproduced the biochemical and pathological features of LS: MPTP increased plasmatic lactate levels (p = 0.023) and triggered basal ganglia degeneration, as evaluated through dopamine transporter (DAT) autoradiography, tyrosine hydroxylase (TH) immunohistochemistry, and dopamine dosage. Striatal DAT levels were markedly decreased after MPTP treatment (p = 0.003). TH immunoreactivity was reduced in the striatum and substantia nigra (p = 0.005), and striatal dopamine was significantly reduced (p < 0.01). Taken together, these results confirm that acute MPTP intoxication in young mice provides a reproducible pharmacological paradigm of LS, thus opening new avenues for neuroprotection research.

Chronic dietary alpha-linolenic acid deficiency alters dopaminergic and serotoninergic neurotransmission in rats

This study examined the effects of dietary alpha-linolenic acid [18:3(n-3)] deficiency on dopaminergic serotoninergic neurotransmission systems in 60-d-old male rats. Rats were fed semipurified diets containing either peanut oil [the (n-3)-deficient group] or peanut plus rapeseed oil (control group). We measured the densities of the serotonin-2 (5-HT2) receptors and the dopamine-2 (D2) receptors by autoradiography and membrane-binding assays in relation to the fatty acid composition and levels of endogenous monoamines in three cerebral regions: the frontal cortex, the striatum and the cerebellum. Long-term feeding of the (n-3)-deficient diet induced a significantly higher 5-HT2 receptor density in the frontal cortex compared with the control rats without any difference in the endogenous serotonin concentrations. The results also showed some modification of dopaminergic neurotransmission specifically in the frontal cortex in the rats deficient in alpha-linolenic acid, with a significantly lower density of D2 receptors and a significantly lower concentration of endogenous dopamine than in control animals. Moreover, there were lower levels of (n-3) fatty acids in all the regions studied in the deficient rats, balanced by greater levels of (n-6) fatty acids. These results suggest that chronic consumption of an alpha-linolenic acid-deficient diet could induce modifications of the neurotransmission pathways; this might induce the behavioral disturbances previously described in this fatty acid-deficient animal model.

PET and SPECT exploration of central monoaminergic transporters for the development of new drugs and treatments in brain disorders

Membrane and vesicular monoaminergic transporters, responsible for the homeostasis of neurotransmitter pools at nerve endings, are very involved in the physiology and diseases of central nervous system. Recent progresses of cerebral molecular imaging using SPECT and PET methods allow the extend of in vivo exploration of these transporters. For this aim, an increasing number of radiopharmaceuticals labelled with [123I], [99mTc], [11C] or [18F] have been developed such as cocaine derivatives for the DAT, compounds from the diphenyl sulfide family for the SERT, and dihydrotetrabenazine derivatives for the VMAT2. These functional imaging methods can be very useful in several neurological and psychiatric disorders which involve the monoaminergic neurotransmission systems such as Parkinson's disease, ADHD, depression and autism. For example, the DAT is a specific index of the density of dopaminergic endings which progressively degenerate in Parkinson's disease. In vivo exploration of this transporter can therefore be a relevant way (i) to realize an early detection of the loss of dopaminergic neurons, (ii) to assess the progression of the disease, (iii) to validate and improve the efficacy of new therapeutic strategies such as neuroprotection and neuroreparation. In all, the extend of in vivo exploration of monoamine transporters will allow great progress for (1) knowledge of physiopathological mechanisms of brain disorders, (2) early diagnosis of cerebral dysfunctions, allowing early use of new therapies, (3) selection of homogenous classes of subjects for therapeutic assays, (4) objectiveness of drug-molecular target interaction, (5) follow-up of disease evolution and treatment.

Cholinergic nicotinic receptor involvement in movement disorders associated with Lewy body diseases. An autoradiography study using [(125)I]alpha-conotoxinMII in the striatum and thalamus

The presence of alpha6 subunit containing nicotinic acetylcholine receptors on nigrostriatal dopaminergic neurons has been demonstrated in rodents and monkeys. [(125)I]alpha-conotoxinMII is a radioligand that binds to alpha6, and also alpha3 subunits of nicotinic acetylcholine receptors (nAChRs). In the present study, we have compared the distribution of [(125)I]alpha-conotoxinMII binding in post mortem human tissue from four groups of patients: individuals with dementia with Lewy bodies displaying extra-pyramidal features (DLB + EPF), DLB without extra-pyramidal features (DLB - EPF) Parkinson's disease without dementia (PD) and age-matched controls. Reduced binding was observed in the putamen and caudate in PD and both DLB groups. In DLB patients, the decline was greater in DLB + EPF compared to DLB - EPF group. The declines in nicotinic receptor binding in the striatum were in part paralleled by reductions in the striatal dopamine transporter. In the thalamus, [(125)I]alpha-conotoxinMII binding was significantly reduced in the centromedian nucleus in both DLB groups, and also in the parafascicular nucleus in the DLB - EPF group. In DLB + EPF and PD patients, there was decreased binding in the ventral lateral nucleus. This study demonstrates alterations of alpha6 and/or alpha3 nAChRs binding in DLB and PD, which are likely to relate to extra-pyramidal symptoms.

Motor behaviour deficits and their histopathological and functional correlates in the nigrostriatal system of dopamine transporter knockout mice

Chronic dysregulation of dopamine homeostasis has been shown to induce behavioural impairment in dopamine transporter knockout mutant mice arising from the dysfunction of the mesolimbic and hypothalamo-infundibular system. Here, we assessed whether there are also any motor consequences of a chronic and constitutive hyperdopaminergia in the nigrostriatal system in dopamine transporter knockout mutant mice. For this, we analysed motor performances using tests assessing balance, coordinated motor skills (rotarod, pole test), stride lengths and locomotor activity. Dopamine transporter knockout mutant mice were markedly hyperactive in the open field with central compartment avoidance, as previously shown. However, sensorimotor integration was also found to be altered in dopamine transporter knockout mutant mice which displayed a reduced fore- and hind-limb mean stride length, impaired motor coordination on the pole test and reduced rearings in the open field. Moreover, dopamine transporter knockout mutant mice showed a slower task acquisition on the rotarod. Six-week-old dopamine transporter knockout wild type mice having the same femur size as adult dopamine transporter knockout mutant mice ruled out a possible size-effect bias. Whilst there was no significant difference in the striatal volume, we found a slight but significant reduction in neuronal density in the striatum but not in the nucleus accumbens of dopamine transporter knockout mutant mice. There was a reduced binding in the striatum and nucleus accumbens of dopamine(1) receptors ([(3)H]SCH 23390) and dopamine(2) receptors ([(3)H]YM-09151-2). There was no significant difference in the number of dopaminergic neurons in the substantia nigra between dopamine transporter knockout mutant mice and dopamine transporter knockout wild type mice. These results suggest an impaired functioning of the nigrostriatal system in dopamine transporter knockout mutant hyperdopaminergic mice, as illustrated by motor and sensorimotor integration deficits, despite their apparent hyperactivity. These dysfunctions may arise from combined striatal cell loss and/or functional changes of dopaminergic neurotransmission.

Quantitative analysis of striatal dopamine D2 receptors with 123 I-iodolisuride SPECT in degenerative extrapyramidal diseases

123I-Iodolisuride has high specific affinity for binding on dopamine D2 receptors in the striatum and has been used in a few single photon emission computed tomography (SPECT) studies of extrapyramidal disorders. The diagnosis of Parkinson's disease (PD) is very difficult in the first 5 years of evolution, with 15-25% false positive diagnoses. The aim of this study was therefore to determine the value of iodolisuride SPECT in discriminating Parkinson's from the most frequent Parkinson-plus syndromes (PPS). Seventeen patients with an extrapyramidal syndrome had a SPECT examination 1 h after injection of 180-185 MBq of 123I-iodolisuride. They were followed under dopaminergic treatment for at least 2 years. After 2 years, they were separated in two groups according to specific clinical criteria and sensitivity to dopaminergic treatment: nine patients had PD (age = 59.8+/-8.8 years; Hoehn and Yahr = 1.8+/-0.7; evolution = 4.3+/-3 years) and eight had PPS (age = 71.6+/-7.3 years; Hoehn and Yahr = 2.9+/-2.0; evolution = 4.1+/-1.5 years). The binding potential of iodolisuride in the striatum was assessed by considering the striatum (S)/occipital lobe (O) ratio at the pseudo-equilibrium 1 h after injection. The S/O ratio was statistically different between PD and PPS (1.97+/-0.3 vs. 1.65+/-0.2 (P<0.02)). Iodolisuride SPECT could differentiate both groups with a sensitivity of 88.8% and a specificity of 75%. Iodolisuride is a good specific D2 receptor ligand for SPECT and complements specific clinical criteria for the diagnosis of Parkinson's disease and differentiation between different extrapyramidal disorders.

Relationship between the appearance of symptoms and the level of nigrostriatal degeneration in a progressive 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned macaque model of Parkinson's disease

The concept of a threshold of dopamine (DA) depletion for onset of Parkinson's disease symptoms, although widely accepted, has, to date, not been determined experimentally in nonhuman primates in which a more rigorous definition of the mechanisms responsible for the threshold effect might be obtained. The present study was thus designed to determine (1) the relationship between Parkinsonian symptom appearance and level of degeneration of the nigrostriatal pathway and (2) the concomitant presynaptic and postsynaptic striatal response to the denervation, in monkeys treated chronically with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine according to a regimen that produces a progressive Parkinsonian state. The kinetics of the nigrostriatal degeneration described allow the determination of the critical thresholds associated to symptom appearance, these were a loss of 43.2% of tyrosine hydroxylase-immunopositive neurons at the nigral level and losses of 80.3 and 81.6% DA transporter binding and DA content, respectively, at the striatal level. Our data argue against the concept that an increase in DA metabolism could act as an efficient adaptive mechanism early in the disease progress. Surprisingly, the D(2)-like DA receptor binding showed a biphasic regulation in relation to the level of striatal dopaminergic denervation, i.e., an initial decrease in the presymptomatic period was followed by an upregulation of postsynaptic receptors commencing when striatal dopaminergic homeostasis is broken. Further in vivo follow-up of the kinetics of striatal denervation in this, and similar, experimental models is now needed with a view to developing early diagnosis tools and symptomatic therapies that might enhance endogenous compensatory mechanisms.

Polyunsaturated fatty acids and cerebral function: focus on monoaminergic neurotransmission

More and more reports in recent years have shown that the intake of polyunsaturated fatty acids (PUFA) constitutes an environmental factor able to act on the central nervous system (CNS) function. We recently demonstrated that the effects of PUFA on behavior can be mediated through effects on the monoaminergic neurotransmission processes. Supporting this proposal, we showed that chronic dietary deficiency in alpha-linolenic acid in rats induces abnormalities in several parameters of the mesocortical and mesolimbic dopaminergic systems. In both systems, the pool of dopamine stored in presynaptic vesicles is strongly decreased. This may be due to a decrease in the number of vesicles. In addition, several other factors of dopaminergic neurotransmission are modified according to the system affected. The mesocortical system seems to be hypofunctional overall [e.g., decreased basal release of dopamine (DA) and reduced levels of dopamine D2 (DAD2) receptors]. In contrast, the mesolimbic system seems to be hyperfunctional overall (e.g., increased basal release of DA and increased levels of DAD2 receptors). These neurochemical changes are in agreement with modifications of behavior already described with this deficiency. The precise mechanisms explaining the effects of PUFA on neurotransmission remain to be clarified. For example, modifications of physical properties of the neuronal membrane, effects on proteins (receptors, transporters) enclosed in the membrane, and effects on gene expression and/or transcription might occur. Whatever the mechanism, it is therefore assumed that interactions exist among PUFA, neurotransmission, and behavior. This might be related to clinical findings. Indeed, deficits in the peripheral amounts of PUFA have been described in subjects suffering from neurological and psychiatric disorders. Involvement of the monoaminergic neurotransmission function has been demonstrated or hypothesized in several of these diseases. It can therefore be proposed that functional links exist among PUFA status, neurotransmission processes, and behavioral disorders in humans. Animal models are tools of choice for the understanding of such links. Improved prevention and complementary treatment of neurological and psychiatric diseases can be expected from these studies.

Plasma fatty acid levels in autistic children

Phospholipid fatty acids are major structural components of neuronal cell membranes, which modulate membrane fluidity and hence function. Evidence from clinical and biochemical sources have indicated changes in the metabolism of fatty acids in several psychiatric disorders. We examined the phospholipid fatty acids in the plasma of a population of autistic subjects compared to mentally retarded controls. Our results showed a marked reduction in the levels of 22: 6n-3 (23%) in the autistic subjects, resulting in significantly lower levels of total (n-3) polyunsaturated fatty acids (PUFA) (20%), without significant reduction in the (n-6) PUFA series, and consequently a significant increase in the (n-6)/(n-3) ratio (25%). These variations are discussed in terms of potential differences in PUFA dietary intake, metabolism, or incorporation into cellular membranes between the two groups of subjects. These results open up interesting perspectives for the investigation of new biological indices in autism. Moreover, this might have new therapeutic implications in terms of child nutrition.

mRNA D(2) dopaminergic receptor expression after hypoxia-ischemia in rat immature brain

Previous studies have shown a reduction of dopaminergic D(2) receptors (D(2)R) in the striatum after hypoxia-ischemia in newborn rats. We show here an early and transient reduction of mRNA D(2)R in nonatrophic brains following hypoxia-ischemia. The left carotid artery of P7 rats was ligated followed by hypoxia for 2 h. The rats were sacrificed after 24 h, 48 h and 14 days. D(2)R mRNA was studied by in situ hybridization, the cell number by conventional histology, and neuronal and astrocyte differentiation by immunohistochemistry. A 20% reduction of striatal mRNA D(2)R occurred 24 h after hypoxia-ischemia, whereas no reduction was observed after 48 h and 14 days. There were no differences in total cell number and in the expression of neuronal (MAP-1, MAP-2) and astrocyte (GFAP) markers between both brain hemispheres nor between control and hypoxia-ischemia animals. The early decrease in mRNA D(2)R could explain the delayed reduced D(2)R after neonatal hypoxia-ischemia.

Synthesis of tropane and nortropane analogues with phenyl substitutions as serotonin transporter ligands

The effects of structural modifications of 2 beta-carbomethoxy-3 beta-phenyl tropane analogues were evaluated on in vitro affinity to the dopamine (DAT) and serotonin (5-HTT) transporters in rat brain tissue. The introduction of a large alkyl group at the 4'-position of the phenyl ring, affording 2 beta-carbomethoxy-3 beta-(4'-alkylphenyl) tropane, diminished the affinity for the DAT whereas moderate 5-HTT affinity was obtained. The introduction of an iodine at the 3'-position of the 4'-alkylphenyl, affording 2 beta-carbomethoxy-3 beta-(3'-iodo-4'-alkylphenyl) tropane, and N-demethylation, affording 2 beta-carbomethoxy-3 beta-(3'-iodo-4'-alkylphenyl) nortropane, improved affinity and specificity for the 5-HTT. It could be assumed from these results that the combination of these three modifications of tropane structure yielded highly selective compounds for the 5-HTT. Of the new compounds synthesized, the most selective cocaine derivative, 2 beta-carbomethoxy-3 beta-(3'-iodo-4'-isopropylphenyl) nortropane (8d) labeled with iodine-123 or carbon-11, could be a potential ligand for exploration of the 5-HT transporter by SPET or PET.

Pallidal border cells: an anatomical and electrophysiological study in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkey

A dopamine transporter-radioligand binding study demonstrated a dopaminergic innervation around the pallidal complex in the normal monkey (n=5), i.e. where a subpopulation of pallidal neurons known as "border cells" is classically identified. Surprisingly, this peripallidal binding persists in monkeys rendered parkinsonian (n=5) with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment. The border cell electrophysiological activity was then analysed in normal and parkinsonian monkeys (n=2), either in the untreated state or following administration of levodopa. Pallidal border cell firing frequency was significantly decreased after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment (8.9+/-0.7 vs 31.4+/-1.6Hz, P<0.05). This decrease was partly corrected by levodopa administration (19.2+/-1.0Hz, P<0.05 vs both normal and parkinsonian situations). The peripallidal dopaminergic innervation suggests that pallidal border cells are under a direct dopaminergic control, arising from the ventral tegmental area and/or the basal forebrain magnocellular complex, the role of which remains unknown. Moreover, the relative sparing of these dopaminergic fibers in parkinsonian monkeys suggests that they would exhibit specific adaptive properties totally different from those described in the nigrostriatal pathway.

L-arginine and nitric oxide-related compounds in plasma: comparison of normal and arginine-free diets in a 24-h crossover study

The amino acid L-arginine is the precursor of nitric oxide (NO), a powerful vasodilator with antiplatelet properties. The availability of L-arginine has been suggested to be a rate-limiting factor in the production of NO in conditions such as hypercholesterolemia. It was speculated that fluctuations in plasma concentrations of L-arginine during the day may be dependent upon dietary intake of the amino acid, or other variables, and might modify the elaboration of endogenous NO. Over a 24-h period, the plasma concentrations of L-arginine and NO-related compounds (NOx) were measured during an L-arginine and nitrate/nitrite-free diet (diet A) or a nitrate/nitrite-free diet with a fixed amount of L-arginine intake (3.8 g/d) (diet B) in eight healthy volunteers during a 2-day crossover study. Subjects were randomly selected to begin with diet A or diet B and consumed the other diet on the second day. During diet A, plasma L-arginine decreased significantly from 09.00 to 16.00 (21.4+/-2.0 to 11.9+/-1.1 microg/ml), rose slightly in the evening (to 16.6+/-1.7 microg/ml) and gradually increased during the night. During diet B, plasma L-arginine showed a peak after each meal (approximately 23 microg/ml). Plasma NOx concentrations measured by chemiluminescence did not show any circadian variation on either diet. Plasma L-arginine concentrations change during the day and are influenced by dietary intake. Importantly, plasma NOx do not seem to vary with this pattern in healthy individuals.

Microdialysis as a tool for in vivo study of dopamine transporter function in rat brains

The role of dopamine as a major modulator of CNS function is well-known, and the homeostasis of dopamine is considered to be of major importance in the pathogenesis of several psychiatric and neurological diseases. Few methods are currently available for in vivo study of dopamine transporter function, which regulates extracellular levels of dopamine. Adapting the 'indicator diffusion' method applied to the microdialysis technique, we present here a suitable method for this functional investigation. We measured the cellular extraction of [3H]-MPP+, which is known to accumulate in the dopaminergic neurones through the DAT in the rat striatum, using [14C]-mannitol as reference substance characterized by absence of cellular accumulation. The cellular extraction was 0.41 and was almost abolished in the presence of the dopamine-uptake inhibitor cocaine, reaching 0.07. This suggested that extraction of [3H]-MPP+ was due to cellular uptake by dopamine transporters. Tissue analysis confirmed that [3H]-MPP+ was internalized in cells and that such transport was stopped by cocaine. Moreover, [3H]-MPP+ extraction was dramatically decreased after lesioning the nigro-striatal pathway with 6-hydroxydopamine, whereas [14C]-mannitol extraction was unchanged. It is concluded that the presented method can be used to study the functioning of the dopamine transporter in live animals.

Neuroprotection of nerve growth factor-loaded microspheres on the D2 dopaminergic receptor positive-striatal neurones in quinolinic acid-lesioned rats: a quantitative autoradiographic assessment with iodobenzamide

Huntington's disease (HD) results from the degeneration of striatal neurones, mainly gamma-aminobutyric acid (GABA)ergic projection neurones and lately cholinergic interneurones. The use of trophic factors as agents able to prevent such neural degeneration is a promising strategy. The aim of this study was to validate nerve growth factor-loaded (NGF-loaded) poly-D,L-lactide-co-glycolide (PLGA) microspheres for treatment of HD in a rat model with quinolinic acid lesion using autoradiographic study of D2 dopaminergic receptors (D2R). This target is expressed by about half of striatal neurones and its scintigraphic exploration has already been performed for the follow-up of this degenerative process. Ex vivo autoradiography of D2R performed with iodobenzamide, the widely used ligand for single photo emission computerized tomography, revealed slight neuroprotection. Moreover, tolerance of microspheres was demonstrated by in vitro autoradiography with the marker of gliosis, [(3)H]-PK 11195.

Chronic n-3 polyunsaturated fatty acid deficiency alters dopamine vesicle density in the rat frontal cortex

We studied the effects of a chronic deficiency in n-3 polyunsaturated fatty acids (n-3 PUFA) on the vesicle dopaminergic compartment in the frontal cortex of rats. Electronic micrographic analysis showed that the synaptic density and the clear vesicle density were similar in deficient and control rats. However, dopaminergic immunolabeling revealed a significantly decreased number of gold-labeled vesicles in the dopaminergic presynaptic terminals of the deficient rats. These findings demonstrate that dopamine cortical vesicles are specifically decreased in n-3 PUFA deficiency. The mechanism leading to this modification could involve several abnormalities (vesicle turn-over, membrane fluidity, vesicular monoamine transporter). This reduction in the dopaminergic vesicle pool constitutes the first structural support for the previously described modifications of dopamine metabolism in the frontal cortex. Such changes in dopamine neurotransmission could be involved in behavioral abnormalities occurring in n-3 PUFA deficient rats.

Nicotine impairs endothelium-dependent dilatation in human veins in vivo

BACKGROUND

Cigarette smoking is associated with impaired endothelium-dependent dilatation in human veins and arteries. An in vivo study in animals suggests that nicotine may contribute to this abnormality. We tested the hypothesis that local administration of nicotine at a dose reproducing the plasma concentration observed during smoking would impair endothelium-dependent vasodilatation in human veins in vivo.

METHODS

We studied 11 healthy nonsmokers with the dorsal hand vein compliance technique. After 70% to 80% preconstriction with phenylephrine, endothelium-dependent venous relaxation was assessed by infusion of bradykinin (1 to 278 ng/min), a potent vasodilator acting primarily in this model through endothelial release of nitric oxide and prostanoids. Sodium nitroprusside (0.0001 to 3166 ng/min) was used to test endothelium-independent relaxation. Dose-response curves were constructed before and during nicotine coinfusion at a rate of 40 ng/min, reproducing a plasma concentration of 15 ng/mL.

RESULTS

After a 10-minute preinfusion, nicotine administration was associated with a loss in sensitivity to bradykinin (P < .001). After 30 and 60 minutes of preinfusion with nicotine, the venorelaxant effect of bradykinin was further reduced (P < .001). A similar inhibition of the response to bradykinin by nicotine persisted in the presence of indomethacin (INN, indomethacin). Coinfusion of nicotine did not attenuate sodium nitroprusside-induced venodiiation.

CONCLUSION

The results show that acute local exposure to nicotine in vivo is associated with an impaired response to endothelium-derived nitric oxide in human veins. This finding may provide further insight into the pathophysiology of smoking-induced endothelial dysfunction.

A selective radiobrominated cocaine analogue for imaging of dopamine uptake sites: pharmacological evaluation and PET experiments

(E)-N-(3-bromoprop-2-enyl)-2beta-carbomethoxy-3beta-4'-tolyl -nortropane or PE2Br, an analogue of cocaine was labelled with the positron emitter 76Br (T1/2=16 h) for pharmacological evaluation in the rat and PET investigation in the monkey. [76Br]PE2Br was obtained by electrophilic substitution from the tributylstannyl precursor with radiochemical yield of 80%. In vivo biodistribution studies of [76Br]PE2Br (20 MBq/nmol) in rats showed a high uptake in the striatum (2.2% ID/g tissue at 15 min p.i.). The striatum to cerebellum radioactivity ratio was 6 at 1 hour p.i. Striatal uptake of [76Br]PE2Br was almost completely prevented by pretreatment with GBR 12909, but citalopram and maprotiline had no effect, confirming the selectivity of the radioligand for the dopamine transporter. PET imaging of the biodistribution of [76Br]PE2Br in the baboon demonstrated rapid and high uptake in the brain (5% ID at 3 min p.i.). The striatal radioactivity concentration reached a plateau at 20 min p.i. (7% ID/100 mL). The uptake in the cortex and cerebellum was very low. A significantly higher uptake in the thalamus was observed. At 1h p.i., the striatum to cerebellum ratio and thalamus to cerebellum ratio were 8 and 1.9 respectively. In competition experiments the radioactivity in the striatum and the thalamus was displaced by 5 mg/kgof cocaine and 5 mg/kg of GBR 12909, but citalopram and maprotiline had no effect. These results showed that [76Br]PE2Br is in vivo a potent and selective radioligand suitable for PET imagingof the dopamine transporter.

Modification of dopamine neurotransmission in the nucleus accumbens of rats deficient in n-3 polyunsaturated fatty acids

We studied the effects of a diet chronically deficient in alpha-linolenic acid, the precursor of long-chain n-3 polyunsaturated fatty acids, on dopaminergic neurotransmission in the shell region of the nucleus accumbens of rats. In vivo microdialysis experiments showed increased basal levels of dopamine and decreased basal levels of metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in awake rats from the deficient group compared to controls. The release of dopamine under KCl stimulation was similar in both dietary groups. By contrast, the release of dopamine from the vesicular storage pool under tyramine stimulation was 90% lower in the deficient than in the control rats. Autoradiographic studies in the same cerebral region revealed a 60% reduction in the vesicular monoamine transporter sites in the deficient group. Dopamine D(2) receptors were 35% increased in these rats compared to controls, whereas no change occurred for D(1) receptors and membrane dopamine transporters. These results demonstrated that chronic n-3 polyunsaturated fatty acid deficiency modifies several factors of dopaminergic neurotransmission in the nucleus accumbens. These findings are in agreement with the changes in dopaminergic neurotransmission already observed in the frontal cortex, and with the behavioral disturbances described in these deficient rats.

Pharmacological characterization of (E)-N-(3-iodoprop-2-enyl)-2beta-carbomethoxy-3beta-(4'-methylphenyl)n ortropane as a selective and potent inhibitor of the neuronal dopamine transporter

The pharmacological properties of the iodinated derivative of cocaine (E)-N-(3-iodoprop-2-enyl)-2beta-carbomethoxy-3beta-(4'-me thylphenyl)nortropane (PE2I) were evaluated in vitro in the rat. Binding experiments on rat striatal membranes showed that PE2I selectively recognized the dopamine transporter (DAT) according to a single binding site model with high affinity (K(d) = 4 nM, B(max) = 12 pmol/mg protein). In the cortical membranes, the binding of PE2I was also selectively associated with the DAT (IC(50) for GBR 12909 = 6 nM versus more than 1000 nM for paroxetine), with similar affinity to that of the striatum. Autoradiographic experiments on rat brain sections with [(125)I]PE2I were in agreement with the localization of the DAT. In addition, PE2I was shown to be a potent inhibitor of dopamine uptake, with IC(50) values similar to those for GBR 12909 and 2beta-carbomethoxy-3beta-(4'-iodophenyl)-tropane (beta-CIT) (2-6 nM). All of these findings, combined with previously published data, support the use of PE2I as a selective and potent tool to study the DAT both in vivo and in vitro.

Heparin-induced vasodilation in human hand veins

OBJECTIVE

To investigate whether heparin produces vasodilation in human veins and to explore the underlying mechanisms.

METHODS

Eleven healthy volunteers were studied with the dorsal hand vein compliance technique. Dose-response curves to heparin and enoxaparin were generated. Dose-response curves to heparin were also constructed before and after heparin was infused with the nitric oxide synthase inhibitor N(G)-monomethyl-l-arginine (L-NMMA) or combined histamine H1- and H2-receptor blockade.

RESULTS

Heparin but not enoxaparin caused significant dose-dependent relaxation with an average apparent maximal response (at an infusion rate of 20 IU/min) of 47% +/- 23%. L-NMMA attenuated heparin-induced relaxation (P < .001). The combination of H1-and H2-receptor antagonists attenuated heparin-induced relaxation to a lesser extent (P < .05). Heparin-induced relaxation decreased by 52%, 73%, and 35% in the presence of L-NMMA, indomethacin (INN, indometacin) plus L-NMMA, and combined H1- and H2-receptor blockade, respectively.

CONCLUSION

Heparin is an endothelium-dependent venodilator in humans. The mechanism of heparin-induced relaxation involves an increased availability of nitric oxide, possibly partially related to local release of histamine.

PET examination of three potent cocaine derivatives as specific radioligands for the serotonin transporter

Several positron emission tomography (PET) radioligands based on the aryl tropane structure have been used for studies on monoamine reuptake sites. RTI-364, RTI-330, and RTI-357 (3-beta-(4'-n-propyl-,4'-iso-propyl-, and 4'-iso-propenyl-phenyl)nortropane-2-beta-carboxylic acid methyl ester) are three recently synthesized cocaine analogues with higher affinity for the serotonin (5-HTT) than the dopamine transporter (DAT). Unlabelled RTI-364 and RTI-330 were prepared in a two-step synthesis. The key step was the addition of the appropriate propyl Grignard reagent to anhydroecgonine methyl ester. RTI-357 was prepared in a three-step synthesis with a palladium-catalyzed coupling reaction of beta-CIT and isopropenylzinc bromide as key step. Hydrolysis of the ester functions gave the carboxylic acid analogues of RTI-364, RTI-330, and RTI-357, which were labelled with 11C using [11C]methyl iodide in dimethyl formamide (DMF) and tetrabutylammonium hydroxide (TBAH) as base. All three compounds entered the monkey brain in a high degree (approximately 5-10%). There was a low uptake of [11C]RTI-364 in serotonin-rich brain areas, whereas [11C]RTI-330 and [11C]RTI-357 showed a marked uptake of radioactivity in the thalamus and the brainstem, regions known to contain serotonin transporters. Transient equilibrium was reached at 15 and 40 min for [11C]RTI-330 and [11C]RTI-357, respectively. After pretreatment with citalopram, the ratio of radioactivity in the thalamus and the brainstem to the cerebellum were markedly reduced for [11C]RTI-357 but not for [11C]RTI-330. The results indicate that [11C]RTI-357 is a potential PET radioligand for quantitation of the serotonin reuptake site.

Pharmacokinetics of intravenous and oral L-arginine in normal volunteers

AIMS

Recent studies in patients with cardiovascular diseases suggest potential for the use of orally administered L-arginine, the precursor of nitric oxide, as a therapeutic agent. This crossover study was designed to examine the pharmacokinetics of single i.v. and oral doses of L-arginine in healthy volunteers (n = 10).

METHODS

A preliminary control study (n = 12) was performed to assess the variation in plasma L-arginine concentrations when ingesting a normal diet. The observed variation was taken into account when interpreting the pharmacokinetic data obtained after exogenous administration.

RESULTS

The mean baseline plasma concentration of L-arginine in the control study was 15.1+/-2.6 microg ml(-1). After intravenous administration (30 g over 30 min), the plasma concentration reached 1390+/-596 microg ml(-1). The disappearance of l-arginine appeared biphasic, with an initial rapid disappearance due to concentration-dependent renal clearance followed by a slower fall in plasma concentrations due to nonrenal elimination. The peak concentration after oral administration (10 g) was 50.0+/-13.4 microg ml(-1), occurring 1 h after administration. Renal elimination was not observed after oral administration of this dose. The absolute bioavailability of a single oral 10 g dose of L-arginine is approximately 20%.

CONCLUSIONS

This study provides basic knowledge of L-arginine pharmacokinetics in healthy humans. Intravenous and oral administrations show at minimum a biphasic pattern. Further studies will assess whether a similar profile is observed when the drug is administered to patients.

Propranolol in hypoglycaemia unawareness

The effect of propranolol on the occurrence of hypoglycaemic symptoms was assessed in insulin-dependent diabetic patients with hypoglycaemia unawareness. A double-blind, randomised parallel group study (2:1 fashion) was conducted over 4-week period. The propranolol group (n = 9) received 20 mg (week 1 and 2) and 30 mg (week 3 and 4) twice daily, and the other group (n = 5) a matched placebo for 4 weeks. Patients included had experienced at least two severe hypoglycaemic episodes (coma or seizures) during the previous year, which were characterised by a lack of adrenergic symptoms and required the assistance of another person. The mean number of hypoglycaemias during the study period was similar in both groups (placebo: 13 +/- 2 propranolol: 11 +/- 1), whereas the number of totally asymptomatic hypoglycaemias (< 0.6 g/l) was lower on propranolol than on placebo (3 +/- 1 vs 8 +/- 3, NS) and the number of symptomatic hypoglycaemias was higher (7.2 +/- 2 vs 4.6 +/- 1, NS). Subjective evaluation of treatment by the investigators showed 0/5 successes in the placebo group and 5/9 in the propranolol group (chi2 = 4.32, p = 0.038). The main advantage of propranolol over placebo was an increased incidence of sweating. The ratio [number of hypoglycaemias with sweating/total number of hypoglycaemias] being higher with propranolol (0.28 +/- 0.08 vs 0.06 +/- 0.02, p = 0.06). This pilot study suggests that beta-blockers may be useful in restoring adrenergic symptoms during hypoglycaemia in insulin-dependent diabetic patients without warning symptoms of hypoglycaemia. This beneficial effect seems to be predominantly related to an increase in hypoglycaemia-induced sweating. A larger study is needed to confirm or invalidate these preliminary results.