Biomedical subjects

P Cumming

Publications and source records attributed to P Cumming.

At least 55 records · Page 3Linked to original sources

PubMed1995

Regulation of DOPA decarboxylase activity in brain of living rat.

To test the hypothesis that L-DOPA decarboxylase (DDC) is a regulated enzyme in the synthesis of dopamine (DA), we developed a model of the cerebral uptake and metabolism of [3H]DOPA. The unidirectional blood-brain clearance of [3H]DOPA (K1D) was 0.049 ml g-1 min-1. The relative DDC activity (k3D) was 0.26 min-1 in striatum, 0.04 min-1 in hypothalamus, and 0.02 min-1 in hippocampus. In striatum, 3,4-[3H]dihydroxyphenylacetic acid ([3H]DOPAC) was formed from [3H]DA with a rate constant of 0.013 min-1, [3H]homovanillic acid ([3H]HVA) was formed from [3H]DOPAC at a rate constant of 0.020 min-1, and [3H]HVA was eliminated from brain at a rate constant of 0.037 min-1. Together, these rate constants predicted the ratios of endogenous DOPAC and HVA to DA in rat striatum. Pargyline, an inhibitor of DA catabolism, substantially reduced the contrast between striatum and cortex, in comparison with the contrast seen in autoradiograms of control rats. At 30 min and at 4 h after pargyline, k3D was reduced by 50% in striatum and olfactory tubercle but was unaffected in hypothalamus, indicating that DDC activity is reduced in specific brain regions after monoamine oxidase inhibition. Thus, DDC activity may be a regulated step in the synthesis of DA.

3,4-Dihydroxyphenylacetic Acid↗

PubMed1995

Regional striatal DOPA transport and decarboxylase activity in Parkinson's disease.

METHODS: We measured blood-brain barrier transport and decarboxylation of 6-[18F]fluoro-L-DOPA (FDOPA) using PET in patients with Parkinson's disease (n = 7, 57 +/- 7 yr) and age-matched control subjects (n = 7, 60 +/- 6 yr). To visually present regional changes of FDOPA uptake in Parkinson's disease, we introduced maps of FDOPA uptake relative to occipital cortex, averaged across control subjects and Parkinson's disease patients in an MRI-based stereotaxic coordinate space. RESULTS: There was no significant changes in the blood-to-brain transport of FDOPA (KD1) in Parkinson's disease. The KD1 values of the head of caudate were lower than those of putamen in both normal subjects and Parkinson's disease patients. In Parkinson's disease, the activity of L-DOPA decarboxylase (DDC) was differentially reduced in subdivisions of striatum. The residual DDC activity was 63% of the control value in the head of caudate nucleus, 54% in the anterior putamen and 39% in the posterior putamen. The DDC activity in frontal and occipital cortices remained unchanged by the disease. Subtraction of averaged FDOPA uptake maps (control minus Parkinson's disease) visualized a spatial pattern of pathological changes in FDOPA uptake common to Parkinson's disease patients. CONCLUSION: The striatal blood-to-brain transport of FDOPA remained unchanged while the DDC activity was differentially reduced within the striatum in Parkinson's disease. We found the FDOPA uptake maps useful in identifying altered patterns of FDOPA metabolism common in Parkinson's disease.

Biological Transport↗

PubMed1994

Elevated dopa decarboxylase activity in living brain of patients with psychosis.

The hypofrontality theory of the pathogenesis of schizophrenia predicts that cortical lesions cause psychosis. During a search for abnormalities of catecholaminergic neurotransmission in patients with complex partial seizures of the mesial temporal lobe, we discovered an increase of the rate of metabolism of an exogenous dopa tracer (6-[18F]fluoro-L-dopa) in the neostriatum of a subgroup of patients with a history of psychosis. When specifically assayed for this abnormality, patients with schizophrenia revealed the same significant increase of the rate of metabolism in the striatum. The finding is consistent with the theory that a state of psychosis arises when episodic dopamine excess is superimposed on a trait of basic dopamine deficiency in the striatum. The finding is explained by the hypothesis that cortical insufficiency, a proposed pathogenetic mechanism of both disorders, causes an up-regulation of the enzymes responsible for dopa turnover in the neostriatum as well as the receptors mediating dopaminergic neurotransmission.

Adult↗

PubMed1994

Distribution of histamine H3 binding in forebrain of mouse and guinea pig.

To map the unknown distribution of histamine H3 binding sites, we compared autoradiographs of the binding of the H3 agonist N alpha-[3H]methylhistamine ([3H]NAMH) in the forebrain of mouse and guinea pig. Saturation binding revealed the presence of a single site with apparent affinity of 0.75 nM in the mouse and 1.2 nM in the guinea pig. Binding constants were nearly identical in male and female mice. The rank order of [3H]NAMH binding in mouse brain was striatum > insular cortex > neocortex, as previously observed in the rat. In the guinea pig, the rank order was insular cortex > striatum approximately neocortex. The Bmax of [3H]NAMH in insular cortex of both species was close to 60 fmol/mg. Thus, the guinea pig differs from the rodent in that the highest density of H3 binding was present in cortical rather than subcortical structures.

Animals↗

PubMed1994

Histamine H3 binding sites in rat brain: localization in the nucleus of the solitary tract.

The distribution of high-affinity binding sites for the histamine H3 agonist [3H]-N alpha-methylhistamine ([3H]NAMH, 4 nM) in the rat brain was examined by autoradiography. Binding in all brain regions was displaceable with the H3 antagonist thioperamide (2 microM). The pattern of H3 binding was heterogeneous: highest levels were in the striatum and substantia nigra, which receives sparse histaminergic innervations. Intermediate H3 binding was present in neocortex and diencephalon, while low binding was evident in the medial septum and dorsal hippocampus. Hitherto unreported H3 binding sites were noted in the dorsolateral tegmentum and in the nucleus of the solitary tract (NTS). Because the NTS receives a dense histaminergic innervation, [3H]NAMH binding in the NTS was further studied by Scatchard analysis. The presence of saturable binding (Bmax = 10 +/- 4 fmol/mg, Kd = 0.6 +/- 0.3 nM, n = 3) suggested that histamine may act via H3 receptors in the NTS.

Animals↗

PubMed1994

Subclasses of histamine H3 antagonist binding sites in rat brain.

Histamine H3 antagonists have been reported to discriminate subclasses of histamine H3 agonist binding in rat cortical membranes. This phenomenon was investigated by autoradiography of cryostat sections of rat forebrain labelled with [3H]N alpha-methylhistamine ([3H]NAMH, 4 nM). Displacement curves with thioperamide detected a single site in cortex and striatum (pIC50 = 8.18 +/- 0.03). However, Hill coefficients (nH = 0.51 +/- 0.12) suggested the possible presence of multiple binding sites. Displacement with burimamide was consistent with two site models in all brain regions examined (pIC50(A) = 7.9 +/- 1.5; pIC50(B) = 5.6 +/- 0.7), except for the medial septum where a single site was detected. Elsewhere, the relative abundance of the two sites displaced by burimamide (H3A:H3B) appeared to be 1:2. Thioperamide may have failed to discriminate two sites because the IC50s were too similar to be distinguished in the present autoradiographic study.

Animals↗

PubMed1994

Brain uptake of alpha-[14C]methyl-para-tyrosine in the rat.

The blood-brain permeabilities of L-[3H]tyrosine and the tyrosine hydroxylase (TH) inhibitor alpha-[14C]methyl-para-tyrosine ([14C]AMPT) were determined in rat striatum, a brain region rich in TH activity, and in other brain regions containing relatively little TH activity. In striatum, the unidirectional clearance rate (K1) for L-[3H]tyrosine (6.2 ml hg-1 min-1) was significantly greater than the rates for L-[14C]AMPT (2.8 ml hg-1 min-1) and D-[14C]AMPT (0.8 ml hg-1 min-1). The apparent volume of distribution (Vf) for L-[14C]AMPT in striatum (72.5 +/- 4.0 ml hg-1) did not differ from the Vf in other brain regions. The homogeneous distribution of L-[14C]AMPT in rat brain indicates that labeled AMPT is unsuitable for the study of TH in vivo by quantitative autoradiography.

Animals↗

PubMed1994

A kinetic analysis of 6-[18F]fluoro-L-dihydroxyphenylalanine metabolism in the rat.

A previous study of the metabolism of 6-[18F]-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) in rats pretreated with carbidopa contained information amenable to kinetic analysis. Using these data, tracer transfer coefficients and metabolic rate constants were estimated. After intravenous injection, FDOPA in circulation was O-methylated (k0D = 0.055 min-1), and the metabolite (O-methyl-FDOPA) escaped from plasma with a rate constant (k-1M) of 0.01 min-1. The initial clearance of FDOPA to striatum (K1D) was 0.07 ml g-1 min-1, and the equilibrium distribution volume (VeD) was 0.67 ml g-1. The initial clearance of O-methyl-FDOPA to striatum (K1M) was 0.08 ml g-1 min-1, and the equilibrium distribution volume (VeM) was 0.75 ml g-1. The rate constant of FDOPA decarboxylation (k3D) was 0.17 min-1 in striatum. The elimination of 6-[18F]fluorodopamine (FDA) from striatum suggested an apparent rate constant for monoamine oxidase activity (k7') of 0.055 min-1. 6-[18F]Fluorohomovanillic acid (FHVA) was formed from 6-[18F]fluoro-L-3,4-dihydroxyphenylacetic acid with a rate constant (k11) of 0.083 min-1, and FHVA was eliminated from striatum (k9) with a rate constant of 0.12 min-1. The steady-state concentration ratios of FDA and its metabolites were shown to be functions of these rate constants.

Animals↗

PubMed1993

Altered histamine H3 binding in rat forebrain after reserpine treatment.

Groups of 6 rats were treated for 5 days with either reserpine hydrochloride (5 mg/kg i.p., per diem), or saline. Regional binding of the histamine H3 agonist N alpha-[3H]methyl-histamine ([3H]NAMH) was determined in forebrain sections by quantitative autoradiography and Scatchard analysis. Highest maximal binding was in nucleus accumbens (107 +/- 18 fmol/mg) and corpus striatum (58 +/- 9 fmol/mg), where the apparent affinity was close to 4 nM. Maximal binding of [3H]NAMH in the insular cortex (39 +/- 6 fmol/mg) was higher than in other cortical areas examined. Reserpine treatment produced a 50% decrease in both the Bmax and the apparent Kd in the corpus striatum and nucleus accumbens, but binding parameters in the cortex and septum were unaltered. Therefore, the response of H3 receptors in rat forebrain to reserpine treatment for 5 days was regionally heterogenous such that maximal [3H]NAMH binding was typically higher in insular cortex (36 +/- 6 fmol/mg) than in corpus striatum (24 +/- 3 fmol/mg) of reserpine-treated rats.

Animals↗

PubMed1993

Kinetics of the uptake of [3H]paroxetine in the rat brain.

Paroxetine, an antidepressant with a high affinity for serotonin (5-HT) re-uptake sites, is a potential tracer of these sites. We determined the kinetic properties of [3H]paroxetine in rat brain in vivo. Relative to [14C]iodo-antipyrine, the brain uptake index (BUI) of [3H]paroxetine was 60-70%. The unidirectional blood clearance of [3H]paroxetine were 0.05-0.12 ml g-1 min-1, lower than expected from the BUI values. The steady state volume of distribution was 3.5 ml hg-1 in the diencephalon and 1.8 ml g-1 in the cerebellum, suggesting a binding potential of unity. Autoradiographs at four hours after [3H]paroxetine injection (300 microCi, i.p.) revealed heterogeneous binding consistent with the calculated binding potentials. Binding was nearly absent from cerebellum and was highest in the dorsal raphe, superior colliculus, dorsal hypothalamus, and entorhinal cortex, but did not reach equilibrium in four hours of tracer circulation. The specific binding relative to vermis was displaced by pretreatment with fluoxetine (10 mg/kg, i.p.).

8-Hydroxy-2-(di-n-propylamino)tetralin↗

PubMed1993

Human striatal L-dopa decarboxylase activity estimated in vivo using 6-[18F]fluoro-dopa and positron emission tomography: error analysis and application to normal subjects.

DOPA decarboxylase is the enzyme directly responsible for the synthesis of the neurotransmitters dopamine and serotonin, and indirectly of noradrenaline, in brain. We used the decarboxylation coefficient (k3D) of 6-[18F]fluoro-DOPA (FDOPA) to denote the relative activity of L-DOPA decarboxylase in vivo in the human brain. To determine the relative enzyme activity with positron emission tomography (PET), we evaluated the model that separates the metabolism into compartments of nondiffusible and diffusible (i.e., transient) tracer metabolites. Error analysis indicated that the least-squares optimization alone was not sufficient to yield accurate estimates of k3D in the presence of the inherent error of PET. To improve the accuracy of the k3D estimates by optimizing the number of parameters, we introduced biological constraints which included a tracer partition volume (Ve) common to frontal cortex and striatum, and a fixed ratio (q) between the blood-brain barrier transport coefficients of O-methyl-[18F]fluoro-DOPA and FDOPA, the two sources of radioactivity in plasma. We found that a two-step analysis yielded sufficiently accurate estimates of k3D. The two steps include the initial estimation of the partition volume in frontal cortex and the subsequent use of this value to determine k3D in striatum and other structures. We studied twelve healthy controls (age 45 +/- 15 years). The average k3D value was 0.081 +/- 0.024 min-1 (coefficient of variation (COV) 30%) for caudate nucleus, 0.074 +/- 0.013 min-1 (COV 18%) for putamen, and 0.010 +/- 0.005 min-1 (COV 50%) for cerebral cortex.

Adult↗

PubMed1993

6-[18F]fluoro-L-dopa metabolism in living human brain: a comparison of six analytical methods.

In 11 normal volunteers and six patients with Parkinson's disease, we compared six different analyses of dopaminergic function with L-3,4-dihydroxy-6-[18F]fluorophenylalanine (FDOPA) and positron emission tomography (PET). The caudate nucleus, putamen, and several reference regions were identified in PET images, using magnetic resonance imaging (MRI). The six analyses included two direct determinations of DOPA decarboxylase activity (k3D, k3*), the slope-intercept plot based on plasma concentration (K), two slope-intercept plots based on tissue content (k3r, k3s), and the striato-occipital ratio [R(T)]. For all analyses, the difference between two groups of subjects (normal volunteers and patients with Parkinson's disease) was larger in the putamen than in the caudate. For the caudate nucleus, the DOPA decarboxylase activity (k3D, k3*), tissue slope-intercept plots (kr3, ks3); and striato-occipital ratio [R(T)] analyses significantly discriminated between the normal volunteers and the patients with Parkinson's disease (p < 0.005) [with least significance for k3* (p < 0.05)], while the plasma slope-intercept plot (K) failed to do so. For the putamen, the values for k3D, k3*, K, k3r, k3s, and R(T) of normal volunteers were significantly higher than those of patients (p < 0.005) [with least significance for K (p < 0.025)]. Linear correlations were significant between k3D and k3s; k3D and k3r; k3D and R(T); and k3D and k3*, in this order of significance. We found no correlation between k3D and K values in the caudate nucleus.

Adult↗

PubMed1993

Pharmacokinetics of plasma 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine ([18F]Fdopa) in humans.

Like native DOPA, [18F]-6-fluoro-L-3,4-dihydroxyphenylalanine ([18F]FDOPA) is subject to methylation and decarboxylation. To determine the rates of formation and elimination of [18F]FDOPA metabolites, plasma from human subjects undergoing positron emission tomographic (PET) studies was analyzed by high-performance liquid chromatography (HPLC). In addition to the principal metabolite O-methyl-[18F]FDOPA (OMe[18F]FDOPA), two decarboxylated metabolites were detected in plasma from carbidopa pretreated subjects. The concentrations of each metabolite during 90 min following tracer injection could be described as a function of the concentration of [18F]FDOPA, and two rate constants; k0, the rate of formation, and k-1, the rate of clearance. Plasma metabolite time series generated from total plasma activity curves and measured rate constants were in close agreement with the actual concentrations determined by HPLC fractionation. Population means for k0 (0.011 +/- 0.002 min-1) and k-1 (0.010 +/- 0.003 min-1) were used to generate "simulated" plasma curves. The measured and generated plasma curves were used as inputs for estimation of partition and decarboxylation coefficients of [18F]FDOPA in brain. The use of generated input functions from normal population means of transfer coefficients did not introduce a systematic error into the estimate of the enzyme activity. However, the high variability of these estimates in patients precludes the use of this technique as an alterative to individual HPLC measurements.

Adult↗

PubMed1993

Striatal L-dopa decarboxylase activity in Parkinson's disease in vivo: implications for the regulation of dopamine synthesis.

L-DOPA is a large neutral amino acid subject to transport out of, as well as into, brain tissue. Competition between dopamine synthesis and L-DOPA egress from striatum must favor L-DOPA egress if decarboxylation declines relatively more than transport in Parkinson's disease. To test this hypothesis, we injected patients with Parkinson's disease with a radiolabeled analogue of L-DOPA and recorded regional brain radioactivity as a function of time by means of positron emission tomography. We simultaneously estimated the activity of the decarboxylating enzyme and the amino acid transport. In the striatum of patients, we found the L-DOPA decarboxylase activity to be reduced in the head of the caudate nucleus and the putamen. However, the rate of egress of the DOPA analogue was unaffected by the disease and thus inhibited dopamine synthesis more than predicted in the absence of L-DOPA egress.

Corpus Striatum↗

PubMed1992

Inhibition of histamine-N-methyltransferase (HNMT) by fragments of 9-amino-1,2,3,4-tetrahydroacridine (tacrine) and by beta-carbolines.

Histamine-N-methyltransferase (HNMT), the major enzyme for the metabolism of histamine in rat brain, is potently inhibited by 9-amino-1,2,3,4-tetrahydroacridine (tacrine). Structural fragments of tacrine were less potent inhibitors of rat brain HNMT than was tacrine itself. Harmaline and a number of other beta-carbolines inhibited HNMT with IC50 values in the range of 1-10 microM. HNMT inhibition by harmaline was competitive with respect to both substrates, S-adenosylmethionine and histamine (Ki = 1.4 microM). These findings are discussed in the context of mechanisms for HNMT inhibition.

Animals↗

PubMed1992

Formation and clearance of interstitial metabolites of dopamine and serotonin in the rat striatum: an in vivo microdialysis study.

In vivo microdialysis was employed in order to characterize the steady-state kinetics of the turnover of specific dopamine and serotonin metabolites in the rat striatum 48 h after surgery. Inhibitors of monoamine oxidase (MAO; pargyline) and catechol-O-methyltransferase (COMT; Ro 40-7592) were administered, either separately or in conjunction, at doses sufficient to block these enzymes in the CNS. In some experiments, the acid metabolite carrier was blocked with probenecid. Temporal changes were then observed in the efflux of interstitial dopamine, 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA). The fractional rate constants for the accumulation or disappearance of the metabolites could be determined after pharmacological blockade of catabolic enzymes or the acid metabolite carrier. Interstitial 5-HIAA was found to be cleared with a half-life of approximately 2 h. After blockade of either MAO or COMT, HVA disappeared with a half-life of 17 min. Experiments employing probenecid suggested that some of the interstitial HVA was cleared by the acid metabolite carrier, the remainder being cleared by a probenecid-insensitive process, possibly conjugation. After MAO inhibition, DOPAC disappeared with an apparent half-life of 11.3 min. The rate of 3-MT accumulation after pargyline indicated that the majority of interstitial HVA (> 95%) is formed from DOPAC rather than 3-MT. The formation of 3-MT from interstitial dopamine, calculated from the accumulation rate of 3-MT after pargyline, appeared to follow first-order kinetics (k = 0.1 min-1).

Animals↗

PubMed1991

High affinity histamine binding site is the H3 receptor: characterization and autoradiographic localization in rat brain.

The binding of the histamine autoreceptor (H3) agonist [3H]-N alpha-methyl-histamine ([3H]-N-MeHA) was examined in 25 micron thick rat forebrain sections. The specific binding was saturable and of high affinity: Scatchard analysis indicated a Kd of 2 nM and a Bmax of 25 +/- 3 fmol/section. Under similar conditions, [3H]-histamine [( 3H]-HA) bound with a Kd of 8 nM and a Bmax of 20 +/- 2 fmol/section. Competition studies indicated that both ligands bound an identical site which had the pharmacological characteristics of the H3 binding site. The high affinity binding of [3H-N-MeHA was sensitive to the presence of 5'-guanylyl-imidodiphosphate, indicating that the binding site is likely coupled to a G-protein. Autoradiographic studies indicated the [3H]-N-MeHA binding to be greatest in the nucleus accumbens, striatum, substantia nigra pars reticulata, and certain cortical areas. Striatal quinolinic acid lesions greatly reduced binding in both the striatum and ipsilateral substantia nigra, while 6-hydroxydopamine lesions of the nigrostriatal dopamine system were without effect on binding. Therefore, most of the H3 binding sites in the basal ganglia are on striatonigral projection neurons. Cortical quinolinic acid lesions greatly reduced H3 binding in cortex, indicating that the binding in cortex, as in striatum, is largely on intrinsic neurons, rather than on afferents such as histamine nerve terminals.

Animals↗

PubMed1991

Characterization of extracellular histamine in the striatum and bed nucleus of the stria terminalis of the rat: an in vivo microdialysis study.

The intracerebral microdialysis technique, coupled with a sensitive radioenzymatic assay, was employed to study histamine release in the striatum and in the bed nucleus of the stria terminalis (BNST) in conscious, freely moving rats. In these brain regions, extracellular histamine concentrations decreased by 20% when calcium was omitted from the perfusion solution. Extracellular histamine was insensitive to the addition of tetrodotoxin to the perfusion medium. In striatum, extracellular histamine concentrations declined in an apparent biexponential manner after the administration of alpha-fluoromethylhistidine, an inhibitor of histamine synthesis. The half-lives for the disappearance of histamine were 32 min and 7.7 h, indicating the presence of at least two histamine pools. Histidine loading resulted in a nearly twofold increase in histamine outflow in striatum. In the BNST, yohimbine increased the extracellular histamine content by 50%, suggesting that histamine release is subject to alpha 2-adrenergic regulation in vivo. The extent to which histamine detected in cerebral microdialysis samples is of neurogenic origin remains to be established.

Animals↗