Evaluation of the antifungal effect of plant extracts on oral Candidaspp. - a critical methodological analysis of the last decade

INTRODUCTION

In 2022, the World Health Organization published a report encouraging researchers to focus on Candida spp. to strengthen the global response to fungal oral infections and antifungal resistance. In the context of innovative research, it seems pertinent to investigate the antifungal potential of natural extracts of plants and the methodology involved in the recent reports. The aim of this systematic review is to identify the current state of in vitro research on the evaluation of the ability of plant extracts to inhibit Candida spp.

MATERIAL AND METHODS

A bibliographic search has been developed to on a 10-year period to identify which plant extracts have an antifungal effect on the Candida spp. found in the oral cavity.

RESULTS

A total of 20 papers were reviewed and fulfilled all the selection criteria and were included in the full data analysis.

DISCUSSION

Plants have been tested in a wide range of states - whole extracts, extraction of particular components such as flavonoids or polyphenols, or even using the plant to synthesize nanoparticles. Of forty-five plants tested, five of them did not show any effect against Candida spp., which weren't part of the same family. There is a wide range of plant that exhibit antifungal proprieties.

CONCLUSION

Many plants have been tested in a wide range of states - whole extracts, extraction of components such as flavonoids or polyphenols, or even using the plant to synthetize nanoparticles. The combination of plants, the addition of plants to a traditional antifungal and the interference with adhesion provided by some plants seem to be promising strategies. Nonetheless, on contrary to drugs, there is a critical lack of standardization on methodologies and protocols, which makes it difficult to compare data and, consequently, to conclude, beyond doubts, about the most promising plants to fight Candida spp. oral infections.

Effects of dopamine on the in vivo binding of dopamine D2 receptor radioligands in rat striatum

The effects of moderate changes in extracellular dopamine concentrations on the in vivo binding of specific dopaminergic D2 radioligands with different affinities and kinetics were investigated in rats. Either [125I]NCQ298 (Kd = 19 pM), or [25I]iodolisuride (Kd = 0.27 nM) or [3H]raclopride (Kd = 1.5 nM) were administered intravenously (IV) to animals 1 h after the intraperitoneal (IP) injection of either alpha-methyl-p-tyrosine (AMPT) (250 mg/kg) or nomifensine (15 mg/kg), or saline. The kinetics of radioactivity concentration in the striatum, cerebellum, and plasma were measured for up to 4 h after [125I]NCQ298 or [125I]iodolisuride injection and up to 1.5 h after [3H]raclopride injection. For each tracer, the striatum-to-cerebellum radioactivity concentration ratios (S/C) and the binding potential (BP), calculated as the association to dissociation binding rate constant ratios (k3/k4), were assessed and related to the changes in extracellular dopamine concentration induced by drug treatments. Results show that S/C and BP of [3H]raclopride were significantly diminished by pretreatment with nomifensine, a drug that increases extracellular dopamine concentration. Nomifensine pretreatment induced no changes in the in vivo binding indexes of the high affinity [125I]NCQ298 and a slight but not significant decrease of the binding indexes of 125I]iodolisuride. Treatment with AMPT, which induced a 40% reduction in dopamine concentration, did not change [125I]NCQ298 binding indexes but slightly increased those of [3H]raclopride and [125I]iodolisuride. In conclusion, the change of dopamine concentration induces modification of radiotracer kinetics. Thus, the combined use of tracers with high and low affinities could allow us to obtain information both on receptor density and neurotransmitter release in vivo. However, as indicated by the [3H]raclopride study with AMPT, small changes in the concentration of intrasynaptic dopamine cannot be easily detected.

Cholinergic neurotransmission studied in vivo using positron emission tomography or single photon emission computerized tomography

During the past decade, considerable efforts have been made in the development of radiopharmaceuticals for the in vivo study of the cholinergic neurotransmission using positron emission tomography or single photon emission computerized tomography. The main cholinergic radioligands, labelled with positron- or gamma-photon-emitting radionuclides, are reviewed with respect to use as in vivo markers of either acetylcholinesterase, vesicular acetylcholine transporter, brain and heart muscarinic receptors, or cholinergic nicotinic receptors. The main results obtained in the in vivo study of the physiology, pharmacology or pathology of the different steps of the cholinergic neurotransmission using single photon emission computerized tomography and positron emission tomography are discussed.

Chronic MPTP treatment reproduces in baboons the differential vulnerability of mesencephalic dopaminergic neurons observed in Parkinson's disease

Chronic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to baboons was shown previously to result in a motor syndrome and a pattern of striatal dopaminergic fibre loss similar to those observed in idiopathic Parkinson's disease. In the present study, tyrosine hydroxylase-immunoreactive neurons were quantified in the mesencephalon of control (n = 4) and chronically MPTP-treated (n = 3) baboons. MPTP induced a significant reduction in neuronal cell density in the substantia nigra (63.8% reduction) and the ventral tegmental area (53.1%). Within the substantia nigra, obvious mediolateral and dorsoventral gradients of neuronal cell loss were observed. First, the pars lateralis was more affected than the lateral divisions of the pars compacta (89.6% vs 73.8% cell loss), which in turn were more depleted than the medial divisions (60.1% reduction). Second, the ventral regions of the pars compacta were more degenerated than the dorsal parts (82.4 vs 51.5% decrease). This regional pattern is strikingly similar to that observed in Parkinson's disease and indicates that two subpopulations of dopaminergic neurons are distinguishable on the basis of their differential vulnerability to MPTP. Finally, the present study confirms that chronic mitochondrial complex I inhibition using MPTP in primates is sufficient to reproduce the typical dopaminergic cell loss and striatal fibre depletion observed in Parkinson's disease.

A primate model of Huntington's disease: functional neural transplantation and CT-guided stereotactic procedures

In this article, we show that 1) computed tomographic (CT)-guided stereotactic infusion of an excitotoxin into the striatum of a nonhuman primate provides a useful neuropathologic and behavioral model for Huntington's disease. 2) High-resolution positron emission tomography (PET) can be used to image the decreased glucose utilization and the preservation of dopaminergic terminals in the lesioned striatum by using 2-fluoro-deoxy-D-glucose (2FDG) and N-(C-11)-methyl-2-beta-carbomethoxy-3-beta-phenyl tropane (CPT) as tracers. 3) Transplantation of cross-species striatal fetal tissue into the lesioned caudate-putamen reduces many of the abnormal motor movements and behavioral changes seen in the Huntington's disease primate model. 4) Graft rejection results in the return of the abnormal signs of the pregrafted state. These results indicate that treatment of the neuronal deficit in Huntington's disease can involve intervention at the local neuronal circuit level. CT-guided stereotactic implantation of cells that might protect or replace this defective circuitry may eventually provide an effective treatment for Huntington's disease.

Modeling analysis of [11C]flumazenil kinetics studied by PET: application to a critical study of the equilibrium approaches

The multi-injection modeling approach was used for the in vivo quantitation of benzodiazepine receptors in baboon brain using positron emission tomography (PET) and [11C]flumazenil (RO 15-1788) as a specific ligand. The model included three compartments (plasma, free, and bound ligand) and five parameters (including the benzodiazepine receptor concentration). The plasma concentration after correction for the metabolites was used as the input function. The experimental protocol consisted of four injections of labeled and/or unlabeled ligand. This protocol allows the evaluation, from a single experiment, of the five model parameters in various regions of interest. For example, in the temporal cortex, the concentration of receptor sites available for binding (B'max) and the equilibrium dissociation constant (Kd) were estimated to be 70 +/- 15 pmol/ml and 15.8 +/- 2.2 nM, respectively. The validity of the equilibrium approach, which is the most often used quantitation method, has been studied from simulated data calculated using these model parameters. The equilibrium approaches consist of reproducing in PET studies the experimental conditions that permit the use of the usual in vitro methods such as Scatchard analysis. These approaches are often open to criticism because of the difficulty of defining the notion of equilibrium in in vivo studies. However, it appears that the basic relation of Scatchard analysis is valid over a broader range of conditions than those normally used, such as the requirement of a constant bound/free ratio. Simulations showed that the values of the receptor concentration (B'max) and the equilibrium dissociation constant (Kd) found using Scatchard analysis are always underestimated. These simulations also suggest an explanation concerning the dependency of B'max and Kd on the time point employed for the Scatchard analysis, a phenomenon found by several authors. To conclude, we propose new protocols that allow the estimation of the B'max and Kd parameters using a Scatchard analysis but based on a protocol including only one or two injections. These protocols being entirely noninvasive, it thus becomes possible to investigate possible changes in receptor density and/or affinity in patients.

Stable parkinsonian syndrome and uneven loss of striatal dopamine fibres following chronic MPTP administration in baboons

The progressive degeneration of dopamine neurons observed in idiopathic Parkinson's disease was mimicked by injecting low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to baboons, on a chronic basis. Five Papio papio baboons were treated on two different regimens (chronic intravenous administration at weekly intervals for 20-21 months or, daily MPTP treatment for five days followed five to six months later by chronic weekly injections for 5-21.5 months). All animals were assessed for motor symptoms during and after neurotoxic treatment. Both regimens invariably resulted in the appearance of a progressive and irreversible syndrome characterized by action and resting tremor, cogwheel rigidity, postural impairments, hypokinesia and bradykinesia. In some animals, symptoms of resting tremor and rigidity initially restricted to one side of the body became bilateral within a few months of treatment. Subtle abnormalities that may be found in idiopathic Parkinson's disease such as alterations of the blink reflex response were also noted. Neuropathological examination of caudate nucleus, putamen, substantia nigra and ventral tegmental area in brain sections stained for tyrosine hydroxylase showed a typical uneven striatal dopamine fibre loss and a neuronal depletion in the dopaminergic mesencephalic cell groups that reproduce those observed in idiopathic Parkinson's disease. Immunocytochemical observations and behavioural data show that chronic rather than acute MPTP injection regimens can replicate most of the neuropathological and the clinical features typical of idiopathic Parkinson's disease, possibly by increasing the ability of this neurotoxin to target specific subpopulations of mesencephalic dopaminergic neurons.

Magnetic resonance imaging to monitor pathology of caudate-putamen after excitotoxin-induced neuronal loss in the nonhuman primate brain

We used MR imaging to locate and monitor in vivo the pathological events taking place 2 to 4 weeks after unilateral striatal injections of ibotenic acid (IA) in the Papio papio baboon. As early as 2 weeks after IA injections, excitotoxic lesions in the caudate and the putamen were directly visualized on T1-weighted images as small areas of low signal intensity. On T2-weighted images, the lesion sites were visualized as areas of high-intensity signal, spreading over larger areas than the corresponding regions in T1-weighted images. These alterations of T2-values in the lesioned striatum persisted 4 weeks after surgery. However, as the striatal degeneration progressed from 2 to 4 weeks after lesion, the size of the areas of high signal intensity on T2-weighted images decreased, whereas the same regions appeared essentially unmodified on T1-weighted images. A marked enlargement of the ipsilateral lateral ventricle (a characteristic of excitotoxic striatal lesions) could be detected 4 weeks after surgery, on both axial T1- and T2-weighted images. Comparisons of MR images with postmortem anatomical data indicated that areas of increased T1 values corresponded to regions of severe neuronal depletion (a direct result of the excitotoxic lesion), whereas areas of increased T2 values corresponded to regions of increased content in astrocytes and ferritin and probably in the early period following lesion (2 weeks) to a superimposed edema.

Intrastriatal transplantation of cross-species fetal striatal cells reduces abnormal movements in a primate model of Huntington disease

Huntington disease is a neurological movement disorder involving massive neuronal death in the caudate-putamen region of the brain. Neither preventive nor curative therapy exists for this disease. The implantation of cross-species striatal neural precursor cells into the lesioned striatum of nonhuman primates (baboons) reduced the abnormal movements seen in the disease model. These abnormal movements reappeared after immunological rejection of the implanted striatal cells and were not modified by transplantation with nonstriatal cells. These findings encourage further experimentation toward the use of cell sources other than human fetal cells in a potential clinical application to Huntington disease.

Quantitative evaluation of benzodiazepine receptors in live Papio papio baboons using positron emission tomography

The binding of the 11C-labeled benzodiazepine antagonist Ro 15-1788 (flumazenil) was measured in the neocortex of live Papio papio baboons by positron emission tomography. This allowed us to calculate in vivo (i.e., at physiological temperature, neurotransmitters concentrations, and ionic environment) the apparent density of available benzodiazepine receptors (B'max) and the dissociation constant of Ro 15-1788 (Kd). By coadministering increasing doses of unlabeled Ro 15-1788 with [11C]Ro 15-1788 and assuming that nonsaturable radioactivity indicated the free ligand concentration, we were able to obtain saturation isotherms. We showed that a state of quasiequilibrium was reached 50 min after the administration of the radioligand. Linear Scatchard plots allowed us to calculate B'max at 78 and 50 pmol/ml of cerebral tissue in the occipital and frontal cortices, respectively. In both these areas, Kd is on the order of 6 nM, with a Hill number very close to unity. This indicates that Ro 15-1788 binds in vivo with high affinity to an homogeneous population of saturable sites. A similar measurement was carried out on a naturally photosensitive P. papio baboon. Absolute values of B'max, Kd, and Hill number were similar to those of the control baboons. Although results concerning this baboon can only be considered as a case report, this similarity may suggest that its epileptic syndrome is not related to a large change in B'max or Kd, at least in occipital and frontal cortices. Our results showed that quantitative estimation by positron emission tomography of some characteristics of benzodiazepine receptors is possible in live baboons and may represent a supplementary tool for investigating further the molecular mechanisms of benzodiazepine receptor function in physiological and physiopathological conditions. We suggest that a similar method of quantification of classic in vivo [3H]Ro 15-1788 binding could be usefully adapted when studying rodent models of epilepsy, stress, and other neuropsychological disorders. On the other hand, the similarity between the B'max and Kd values we obtained in baboons and those recently reported in humans using similar methods emphasizes that most of the in vivo characteristics of the benzodiazepine receptors of baboons are very close to those of human benzodiazepine receptors. This confirms that P. papio baboons are a suitable animal model for studying the pharmacology of benzodiazepine receptor ligands before clinical applications in humans.

A primate model of Huntington's disease: behavioral and anatomical studies of unilateral excitotoxic lesions of the caudate-putamen in the baboon

Unilateral caudate-putamen (CP) lesions induced by the glutamate receptor agonist ibotenic acid in baboons produced a neuropathological and behavioral model of Huntington's disease (HD) in the nonhuman primate. Neuropathological evaluation of the lesioned caudate-putamen revealed a neurodegenerative pattern resembling HD. The ibotenic acid-infused CP areas showed a neuronal loss in Nissl-stained sections and a marked astrocytic gliosis by immunohistochemical staining for glial-fibrillary-acidic protein. Acetylcholinesterase fiber staining was severely reduced in the lesioned CP, while afferent dopaminergic fibers, as shown by tyrosine hydroxylase staining, were relatively spared. There was a moderate reduction of met-enkephalin staining in the globus pallidus-pars lateralis ipsilateral to the ibotenic acid lesion, indicating a partial denervation of this structure following the lesion. In the behavioral studies a dyskinetic syndrome with features in common with HD was provoked in the lesioned animals following dopamine receptor agonist administration (1-2 mg/kg apomorphine). The symptoms included hyperkinesia, chorea, dystonia, postural asymmetries, head, and orofacial dyskinesia. The apomorphine test was highly reproducible and individual animals responded with a similar set and incidence of dyskinesia in successive tests. Since the behavioral observations following excitotoxic caudate-putamen damage parallel symptoms in HD patients given dopamine stimulatory drugs, a hypothesis is presented for the observed abnormal movements suggesting that the CP lesions reduce movement thresholds while the activation of dopaminoceptive regions induces dyskinesias.

A primate model of Huntington's disease: cross-species implantation of striatal precursor cells to the excitotoxically lesioned baboon caudate-putamen

Ibotenic acid was injected unilaterally into the baboon caudate-putamen (CP) to achieve a neural degeneration model in the primate, with a neuropathology similar to Huntington's disease. Four to six weeks later injections of cell suspensions of striatal precursor cells, obtained by dissection of the fetal rat striatal region (13-15 days gestational age), were made into the excitotoxically lesioned CP of 3 baboons immunosuppressed by Cyclosporin A. Morphological analysis indicated that in one of the baboons, which had the largest lesion of the CP and the shortest survival time (6 weeks after implantation), there was a surviving striatal implant. The implanted neurons grew in high densities in cellular aggregates within the host gliotic CP. These neurons had a neuronal size phenotypical for rat striatum, i.e. on average about a 25% smaller neuronal cell diameter than a similar population in the baboon caudate-putamen. Glial-fibrillary-acid-protein immunoreactivity was present on large astrocytes within the striatal implant, with a distinct border towards the lesion-induced astrogliosis of the host. Neuronal markers for acetylcholinesterase and Leu-enkephalin were distributed in a typical patchy manner in the striatal implants along with fiber staining for tyrosine-hydroxylase-like immunoreactivity (TH) possibly derived from afferent host dopaminergic axons. Some of these fibers in the implants came from intrinsic TH-positive neuronal somata, probably of neocortical fetal origin and transiently expressing the enzyme. In conclusion, the results indicate that neuronal replacement can be achieved by cross-species implantation of fetal striatal precursor cells to the previously neuron depleted primate CP under immunosuppression but that the survival and growth of such implants may be variable and subject to unfavourable trophic conditions.

Pentylenetetrazol-induced seizure is not mediated by benzodiazepine receptors in vivo

The selective benzodiazepine antagonist RO 15-1788, labelled with carbon 11 [11C] RO 15-1788, as a specific marker, together with positron emission tomography, allows the in vivo study of benzodiazepine receptors in primates. In addition, when coupled with recordings of electroencephalographic activity, this method offers the feasibility of studying the correlation between occupancy of benzodiazepine receptors and the convulsant action of drugs acting at the benzodiazepine-GABA receptor complex in vivo. The present study showed that convulsant doses of pentylenetetrazol (PTZ) could affect the binding of [11C] RO 15-1788 in vivo in two ways, depending on the doses tested: at concentrations of 20 and 30 mg/kg, pentylenetetrazol increased the binding of [11C] RO 15-1788 whereas larger concentrations displaced the binding of [11C] RO 15-1788. The direct correlation between the occupancy of respective benzodiazepine receptors, afforded by increasing convulsant doses of pentylenetetrazol, revealed that competitive interaction with benzodiazepine receptors was not necessary for pentylenetetrazol to induce the appearance of seizures in vivo.

[76Br]bromolisuride: a new tool for quantitative in vivo imaging of D-2 dopamine receptors

Bromolisuride, an ergoline derivative, was labeled with the positron emitter radionuclide, bromine 76. In vitro and in vivo binding and competition studies in rats demonstrated a high affinity (KD = 0.3 nM) and a high specificity of this new radioligand for D-2 dopamine receptors. PET kinetic studies in baboons showed an accumulation of [76Br]bromolisuride in the striatum which reached a maximum 30 min post-injection and which could be displaced by haloperidol. All these results indicated that this new ligand is certainly suitable for the non-invasive in vivo quantitative imaging of D-2 dopamine receptor sites in human brain.

Localization of 11C-radiopharmaceuticals in the Greene melanoma of hamsters

Seventy Syrian golden hamsters bearing SC transplants of Greene melanoma were used to evaluate the degree of tumour uptake of several 11C-radiopharmaceuticals selected for their potential specificity for melanoma. Tissue distribution studies were performed at 30 and 60 min after IV injection of 11C-compounds and compared with the 24-h uptake of 67Ga-citrate. Gamma camera images were also compared. The highest tumour uptake at 1 h was observed with 11C-methionine (2.42% +/- 0.72%) and although activity in liver, spleen and kidney exceeded that in melanoma the tumour was demonstrated on gamma camera imaging. Melanoma localisation of 11C-chlorpromazine, 11C-flunitrazepam and 11C-ketanserine was comparable at 1% of the dose injected per gram of tumour. High activity in other organs, particularly liver, exceeded uptake in melanoma and attempts at tumour imaging were unsuccessful. Tumour accumulation of 11C-methiodide quinuclidinyl benzylate (MQNB), an 11C-imidazobenzodiazepine (Ro-15-1788) and 14C-pimozide was low and imaging studies were not attempted. None of the 11C-radiopharmaceuticals evaluated for melanoma affinity matched that of 67Ga-citrate. The 24-h tumour uptake of 67Ga-citrate was 4.07% +/- 1.37% dose injected per gram which allowed delineation of the melanoma by gamma camera imaging.

Synthesis of ethyl 8-fluoro-5,6-dihydro-5-[11C]methyl-6-oxo-4H-imidazo [1,5-a] [1,4]benzodiazepine-3-carboxylate (RO 15.1788-11C): a specific radioligand for the in vivo study of central benzodiazepine receptors by positron emission tomography

A method of labelling ethyl 8-fluoro-5,6-dihydro-5-[11C]methyl-6-oxo-4H-imidazo[1,5-a][1,4] benzodiazepine-3-carboxylate (RO 15.1788 11C), a benzodiazepine antagonist with carbon-11 has been developed. RO 15.1788-11C was prepared by methylation of the nor derivative by I11CH3. About 100 mCi (maximum 153 mCi, 5.66 GBq) of the chemically and radiochemically pure labelled product were obtained within 25 min with a specific activity on average of 1100 mCi/mumol (maximum 1740 mCi/mu mol--64.4 GBq/mu mol). Preliminary results obtained after i.v. administration in the baboon have shown RO 15.1788-11C to be of interest as a benzodiazepine radioligand for the in vivo study of benzodiazepine receptors by positron emission tomography.

Central type benzodiazepine binding sites: a positron emission tomography study in the baboon's brain

An in vivo characterization of specific central type benzodiazepine (BZD) binding sites, labelled with [11C]Ro 15-1788 was performed, using positron emission tomography. After i.v. injection of 10 mCi [11C]Ro 15-1788 (corresponding to 1 nmol/kg), sequential quantitative tomographic slices of the brain were obtained during 80 min. In some experiments various doses of different cold drugs (BZD agonist or antagonist) were injected i.v. subsequently in order to explore the specificity of the binding of the radioligand in brain structures. The main criteria usually utilized in vitro to demonstrate a specific binding to receptors, such as regional distribution, stereospecificity and saturability of the binding and pharmacological effect linked to the receptor's occupancy, were demonstrated in the brain of a living baboon.

The kinetics and displacement of [11C]flunitrazepam in the brain of the living baboon

The distribution and kinetics of [11C]flunitrazepam in the brain were studied by positron emission tomography in the living baboon. Flunitrazepam was labelled on the methyl group with the 20 min positron emitter carbon 11. Fifteen to 25 mCi corresponding to 15-30 nmol were injected i.v. and sequential tomographic pictures of the brain were obtained. In some experiments, therapeutic doses of various benzodiazepines were injected i.v. subsequently in order to study the displacement of the radioactive ligand from brain structures. Lorazepam was shown to displace [11C]flunitrazepam from brain tissue, although other benzodiazepines (chlordiazepoxide, Ro 116896 and Ro 116893) led to a redistribution of the radioactive ligand in the body accompanied by an increase of brain radioactivity.

Brain uptake of 11C-methionine in phenylketonuria

The brain uptake of 11C-methionine was studied in 26 children with classical phenylketonuria; one adult was used as a control. Labelled methionine uptake in brain was first measured during a low phenylalanine diet and again one week later after a load of phenylalanine. Ten children aged 1 to 30 months were studied twice at intervals of several months. In children having a phenylalaninemia less than or equal to 0.3 mumoles . ml-1, a decrease in methionine brain uptake was observed with increasing age, with the largest change occurring during the first year of life. After the phenylalanine load, a mean increase in phenylalaninemia by a factor of ten was accompanied by a mean decrease in brain methionine uptake by a factor of two while blood methionine remained unchanged. Brain activity curves increased with time for children younger than one year and having phenylalaninemia less than 0.6 mumoles . ml-1. After the age of 2 most patients had a decreasing curve regardless of the blood phenylalanine level. This study indicates that 11C-methionine brain uptake may be taken as an index of blood barrier permeability to essential amino acids, and of brain maturation. The results obtained suggest that an increase in phenylalaninemia to levels greater than 0.6 mumole . ml-1 induces a modification in brain uptake of amino acids, primarily during the first two years of life.

Brain distribution and kinetics of 11C-chlorpromazine in schizophrenics: positron emission tomography studies

The positron emitter 11C (20 minutes half-life) permits the labeling of chlorpromazine (CPZ) and the study of its distribution in humans by external counting. Trace amounts of 11C-CPZ were injected intravenously into 22 schizophrenic patients all untreated for several months with neuroleptics. The brain uptake was 6.04 +/- 1.6% of the injected dose 15 minutes after the injection, and it remained constant for 45 minutes. By positron emission tomography, the drug distribution was shown to be in the gray matter, and such structures as the cortex, caudate nucleus, thalamus and putamen could be identified. This new methodology will be helpful in studying specific receptors in humans in a noninvasive way.

Labelling and metabolism of methionine-methyl-11 C

Carbon 11 which is a 20.4 minutes half life isotope emitting positrons was used for methionine labelling on the methyl group by action of 11C-methyl-iodide on DL or L homocysteine. With the method described, 20 to 30 mCi of 11C-methyl-methionine (Specific activity 50 mCi/muM) may be obtained within 25 minutes. The 20 Mev proton beam used for 11C production was 10 muA. The metabolism of 11C methionine studied on mice shows a high uptake in pancreas and to a smaller extent in brain. On humans sequential images were obtained at the head level which allowed measurement of the uptake of the radio-labelled compound in the brain so as to study the radioactivity curve in different parts of the organ.