Decreased striatal dopaminergic responsiveness in detoxified cocaine-dependent subjects

Cocaine blocks the reuptake of dopamine, a neurotransmitter involved in the control of movement, cognition, motivation and reward. This leads to an increase in extracellular dopamine; the reinforcing effect of cocaine is associated with elevated dopamine levels in the nucleus accumbens. But addiction to cocaine involves other effects, such as craving, loss of control and compulsive drug intake; the role of the dopamine system in these effects is less well-understood. We therefore used positron emission tomography (PET) to compare the responses of cocaine addicts and normal controls to intravenous methylphenidate, a drug that, like cocaine, causes an increase in synaptic dopamine. Addicts showed reduced dopamine release in the striatum, the brain region where the nucleus accumbens is located, and also had a reduced 'high' relative to controls. In contrast, addicts showed an increased response to methylphenidate in the thalamus (a region that conveys sensory input to the cortex). This thalamic response was associated with cocaine craving and was not seen in control subjects. Thus, our findings challenge the notion that addiction involves an enhanced striatal dopamine response to cocaine and/or an enhanced induction of euphoria. Moreover, they suggest a participation of thalamic dopamine pathways in cocaine addiction, a possibility that merits further investigation.

Serotonergic modulation of dopamine measured with [11C]raclopride and PET in normal human subjects

OBJECTIVE

This study was undertaken to measure serotonergic modulation of dopamine in vivo by using positron emission tomography (PET), a radiotracer for the striatal dopamine D2 receptor ([11C]raclopride), and a pharmacologic challenge of the serotonin system (d,l-fenfluramine).

METHOD

Two PET studies using [11C]raclopride were performed in 11 normal male subjects before administration of the serotonin-releasing agent and reuptake inhibitor fenfluramine (60 mg p.o.) and 3 hours afterward. A graphical analysis method was used with the [11C]raclopride data to derive the distribution volume of D2 receptors. Plasma levels of fenfluramine, norfenfluramine, homovanillic acid (HVA), cortisol, and prolactin were determined.

RESULTS

Levels of fenfluramine and prolactin were elevated 2 hours after fenfluramine administration and remained significantly elevated during the second scan, while levels of HVA and cortisol were not altered significantly during the time of scanning. A significant decrease in the specific binding (striatum) and the nonspecific binding subtracted from the specific binding (striatum minus cerebellum) of [11C]raclopride was observed. The rate of metabolism of [11C]raclopride and the nonspecific binding (cerebellum) were not significantly altered by the fenfluramine intervention.

CONCLUSIONS

The observed decrease in [11C]raclopride binding is consistent with an increase in dopamine concentrations and with the ability of serotonin to stimulate dopamine activity. The ability to measure serotonergic modulation of dopamine in vivo may have implications for the study of etiologic and therapeutic mechanisms in schizophrenia, major depressive disorder, obsessive-compulsive disorder, and substance abuse.

Qualitative analysis of patients' work during mechanical ventilation and weaning

OBJECTIVE

To describe patients' recollections of their experiences during mechanical ventilation and weaning and to extend an evolving nursing theory of weaning.

DESIGN

A qualitative approach that used grounded theory techniques.

PATIENTS AND METHODS

We surveyed 20 hospitalized patients who had recently undergone mechanical ventilation and weaning from the ventilator in a critical care unit. Included in the analysis were any thoughts, feelings, or actions recalled by the patients that were related to the weaning experience.

RESULTS

During ventilation and weaning, patients were engaged to various degrees in what they called "work," which consisted of their efforts to assist in their adjustment and recovery. This work had four themes: sense making, enduring, preserving self, and controlling responses. These themes represented activities by which patients dealt with their personal concerns and cooperated with the therapeutic plan.

CONCLUSIONS

Despite good care during the process, mechanical ventilation and weaning are stressful experiences that require active participation by patients. Patients' work needs to be understood and supported by clinicians to facilitate recovery from mechanical ventilation and weaning.

Dopamine D2 receptor availability in opiate-dependent subjects before and after naloxone-precipitated withdrawal

Dopamine may play a role in opiate withdrawal and dependence. We measured dopamine D2 receptor availability in 11 opiate-dependent subjects using PFT and [11C]raclopride at baseline and during naloxone-precipitated withdrawal. Because [11C]raclopride is sensitive to endogenous dopamine, this strategy enabled us to test whether we could document in humans the DA reductions reported in animal models of opiate withdrawal. Results were compared with values from 11 controls, two of which also received naloxone. The ratio of the distribution volume in striatum to that in cerebellum (Bmax/Kd + 1) was used as model parameter for D2 receptor availability. Baseline measures for Bmax/Kd were lower in opiate-dependent subjects (2.44 +/- 0.4) than in controls (2.97 +/- 0.45 P < or = .009). Naloxone precipitated an intense withdrawal in the abusers but did not change the Bmax/Kd ratio. This study documents decreases in D2 receptors in opiate-dependent subjects but does not document significant changes in striatal DA concentration during acute withdrawal.

Model for estimating dopamine transporter occupancy and subsequent increases in synaptic dopamine using positron emission tomography and carbon-11-labeled cocaine

Although increases in dopamine secondary to the inhibition of the dopamine transporter appear to underlie the reinforcing properties of cocaine, there is presently no model that relates the elevation of synaptic dopamine to the transporter occupancy by cocaine. We propose such a model based on positron emission tomographic (PET) measurements of the brain concentration of cocaine and the assumption of rapid equilibrium between free cocaine and cocaine bound to the dopamine transporter. A euphorigenic dose of cocaine (about 40 mg) is predicted to occupy 80-90% of the transporters, while a perceptible dose (about 5 mg) occupies about 40% of the transporters. If reuptake of dopamine is reduced in proportion to the fraction of transporters occupied by cocaine, our model indicates that synaptic dopamine rises supra-linearly with occupancy, so that 5 and 40 mg doses of cocaine give about 2- and 10-fold increases, respectively. A consequence is that a given dose of cocaine produces a similar degree of elevation of dopamine regardless of the prior level of occupation of the transporters by cocaine. This prediction is supported by recent PET/neuropsychological studies in our laboratory where dopamine transporter occupancy was measured after giving methylphenidate intravenously to volunteers; similarly intense "highs" were reported whether the initial occupancy was zero or 75-85%. It could also explain why attempts to block the psychostimulant-induced "high" by pretreating subjects with drugs that block the dopamine transporter have been unsuccessful, and why the use of methylphenidate to treat cocaine addicts led to increased cocaine consumption.

Effects of methylphenidate on regional brain glucose metabolism in humans: relationship to dopamine D2 receptors

OBJECTIVE

The authors' goals were to determine whether baseline dopamine activity contributes to response to methylphenidate and to assess the pattern of metabolic responses associated with enhanced dopamine activity.

METHOD

They used positron emission tomography with 2-deoxy-2[18F]fluoro-D-glucose to evaluate the effects of two sequential doses of methylphenidate on brain metabolism in 15 healthy subjects. Dopamine D2 receptor availability was measured with [11C]raclopride to evaluate its relation to methylphenidate-induced metabolic changes.

RESULTS

Methylphenidate increased brain metabolism in six subjects, decreased it in two, and did not change it in seven; however, it consistently increased cerebellar metabolism. Methylphenidate significantly increased "relative" (region relative to the whole brain) metabolism in the cerebellum and decreased it in the basal ganglia. Regional metabolic changes in the cerebellum and the frontal and temporal cortices were significantly correlated with D2 availability. Frontal and temporal metabolism were increased in subjects with high D2 receptors and decreased in subjects with low D2 receptors.

CONCLUSIONS

Methylphenidate induced variable changes in brain metabolism, but it consistently increased cerebellar metabolism. It also induced a significant reduction in relative metabolism in the basal ganglia. The significant association between metabolic changes in the frontal and temporal cortices and in the cerebellum and D2 receptors suggests that methylphenidate's metabolic effects in these brain regions are due in part to dopamine changes and that differences in D2 receptors may be one of the mechanisms accounting for the variability in response to methylphenidate.

Impact of the introduction of new medical methods on therapeutic abortions at the Royal Infirmary of Edinburgh

OBJECTIVE

To assess the impact of the introduction of new medical methods on the provision of therapeutic abortions at the Royal Infirmary Edinburgh.

DESIGN

A review of the total number of abortions performed by medical and surgical means between 1989 and 1995 (inclusive); a prospective survey of the terminations of pregnancy (< or = 9 weeks of gestation) performed over the six-month period of January to June 1994; and a questionnaire of the reasons why women chosen a particular method.

SETTING

Large teaching hospital in Scotland.

SUBJECTS

One thousand and seven women seeking early pregnancy termination between January and June 1994.

MAIN OUTCOME MEASURES

Proportion of pregnancies terminated by medical means; comparison of complete abortion rate, incidence of complications and morbidity following both medical and surgical methods (< or = 9 weeks of gestation); reasons for preference of the method of abortion.

RESULTS

Since 1991 there has been a progressive increase in the number of medical abortions performed at the Royal Infirmary of Edinburgh, and by 1994 the majority of women (57%) seeking abortion at < or = 9 weeks chose a medical method. Women who chose medical abortion had more years at full-time education and were less likely to smoke (P < 0.04). Both medical and surgical methods were highly effective (> 96% complete abortion) with a low incidence of complications and morbidity. However, women who had chosen the medical method were less likely to receive antibiotics for suspected endometritis than their surgical counterparts (chi 2, P = 0.0001).

CONCLUSIONS

If this trend towards medical methods in Edinburgh is repeated elsewhere, it will inevitably have an impact on gynaecological services by releasing staff and operating time for other purposes.

Decreases in dopamine receptors but not in dopamine transporters in alcoholics

It has been hypothesized that ethanol's actions on the dopamine (DA) system may participate in addiction. The purpose of this study was to evaluate the DA system in the brain of alcoholics. We evaluated 10 alcoholics and 17 nonalcoholics using positron emission tomography and [11C]raclopride to measure DA D2 receptors. In addition, in 5 of the alcoholics and 16 of the nonalcoholics, we also measured DA transporters with [11C]d-threo methylphenidate. The ratio of the distribution volumes in striatum to that in cerebellum, which corresponds to Bmax/Kd + 1, was used as model parameter of DA D2 receptor and transporter availability. Dopamine D2 receptor availability (Bmax/Kd) was significantly lower in alcoholics (2.1 +/- 0.5) than in nonalcoholics (2.7 +/- 0.6) (p < 0.05) and was not correlated with days since last alcohol use. Alcoholics showed DA transporter values similar to those in nonalcoholics. The ratio of DA D2 receptor to transporter availability was significantly higher in nonalcoholics (1.4 +/- 0.1) than in alcoholics (1.1 +/- 0.1) (p < 0.005). Alcoholics showed significant reductions in D2 receptors (postsynaptic marker) but not in DA transporter availability (presynaptic marker) when compared with nonalcoholics. Because D2 receptors in striatum are mainly localized in gamma-aminobutyric acid (GABA) cells these results provide evidence of GABAergic involvement in the dopaminergic abnormalities seen in alcoholics.

Caring and comfort metaphors used by patients in critical care

OBJECTIVE

To examine the meaning of metaphors used by critical care patients about their ventilator weaning experience.

DESIGN

Grounded theory.

POPULATION, SAMPLE, SETTING

In 1992-1993, from a population of mechanically ventilated patients, a convenience sample of 20 adult patients was recruited from one 14-bed multispecialty ICU in a 740-bed teaching hospital in eastern Canada.

METHODS

Transcripts of interviews were reviewed from the 20 original study transcripts, of which 18 included one or more metaphors for a total of 70. All were coded and classified with sub-themes analyzed for implicit meanings.

FINDINGS

Four categories of metaphors were Physical Discomfort, Nurse Caring, Altered Self, and Patient Work. Data provide the elements for a mid-range theory of caring.

CONCLUSIONS

Metaphors provide vivid images of significant patient concerns and are a way for people to express meaning and feeling.

CLINICAL IMPLICATIONS

People communicate about their inner world through language. Examining patients' metaphors is a valuable approach to understanding the experiential world of patients in critical care so that nursing actions can be directed toward personal needs which may not be expressed openly. Providing interventions aimed at these personal needs will help patients find suitable levels of physical and emotional comfort.

Brain monoamine oxidase A inhibition in cigarette smokers

Several studies have documented a strong association between smoking and depression. Because cigarette smoke has been reported to inhibit monoamine oxidase (MAO) A in vitro and in animals and because MAO A inhibitors are effective antidepressants, we tested the hypothesis that MAO A would be reduced in the brain of cigarette smokers. We compared brain MAO A in 15 nonsmokers and 16 current smokers with [11C]clorgyline and positron emission tomography (PET). Four of the nonsmokers were also treated with the antidepressant MAO inhibitor drug, tranylcypromine (10 mg/day for 3 days) after the baseline PET scan and then rescanned to assess the sensitivity of [11C]clorgyline binding to MAO inhibition. MAO A levels were quantified by using the model term lambda k3 which is a function of brain MAO A concentration. Smokers had significantly lower brain MAO A than nonsmokers in all brain regions examined (average reduction, 28%). The mean lambda k3 values for the whole brain were 0.18 +/- 0.04 and 0.13 +/- 0.03 ccbrain (mlplasma)-1 min-1 for nonsmokers and smokers, respectively; P < 0.0003). Tranyl-cypromine treatment reduced lambda k3 by an average of 58% for the different brain regions. Our results show that tobacco smoke exposure is associated with a marked reduction in brain MAO A, and this reduction is about half of that produced by a brief treatment with tranylcypromine. This suggests that MAO A inhibition needs to be considered as a potential contributing variable in the high rate of smoking in depression and in the development of more effective strategies for smoking cessation.

Relationship between psychostimulant-induced "high" and dopamine transporter occupancy

The ability of cocaine to inhibit the dopamine transporter (DAT) appears to be crucial for its reinforcing properties. The potential use of drugs that produce long-lasting inhibition of the DAT as a mean of preventing the "high" and reducing drug-seeking behavior has become a major strategy in medication development. However, neither the relation between the high and DAT inhibition nor the ability to block the high by prior DAT blockade have ever been demonstrated. To evaluate if DAT could prevent the high induced by methylphenidate (MP), a drug which like cocaine inhibits the DAT, we compared the responses in eight non-drug-abusing subjects between the first and the second of two MP doses (0.375 mg/kg, i.v.) given 60 min apart. At 60 min the high from MP has returned to baseline, but 75-80% of the drug remains in brain. Positron-emission tomography and [11C]d-threo-MP were used to estimate DAT occupancies at different times after MP. DAT inhibition by MP did not block or attenuate the high from a second dose of MP given 60 min later, despite a 80% residual transporter occupancy from the first dose. Furthermore some subjects did not perceive a high after single or repeated administration despite significant DAT blockade. These results indicate that DAT occupancy is not sufficient to account for the high, and that for DAT inhibitors to be therapeutically effective, occupancies > 80% may be required.

Distribution volume ratios without blood sampling from graphical analysis of PET data

The distribution volume ratio (DVR), which is a linear function of receptor availability, is widely used as a model parameter in imaging studies. The DVR corresponds to the ratio of the DV of a receptor-containing region to a nonreceptor region and generally requires the measurement of an arterial input function. Here we propose a graphical method for determining the DVR that does not require blood sampling. This method uses data from a nonreceptor region with an average tissue-to-plasma efflux constant k2 to approximate the plasma integral. Data from positron emission tomography studies with [11C]raclopride (n = 20) and [11C]d-threo-methylphenidate ([11C]dMP) (n = 8) in which plasma data were taken and used to compare results from two graphical methods, one that uses plasma data and one that does not. k2 was 0.163 and 0.051 min-1 for [11C]raclopride and [11C]dMP, respectively. Results from both methods were very similar, and the average percentage difference between the methods was -0.11% for [11C]raclopride and 0.46% for [11C]dMP for DVR of basal ganglia (BG) to cerebellum (CB). Good agreement between the two methods was also achieved for DVR images created by both methods. This technique provides an alternative method of analysis not requiring blood sampling that gives equivalent results for the two ligands studied. It requires initial studies with blood sampling to determine the average kinetic constant and to test applicability. In some cases, it may be possible to neglect the k2 term if the BG/CB ratio becomes reasonably constant for a sufficiently long period of time over the course of the experiment.

PET evaluation of the dopamine system of the human brain

Dopamine plays a pivotal role in the regulation and control of movement, motivation and cognition. It also is closely linked to reward, reinforcement and addiction. Abnormalities in brain dopamine are associated with many neurological and psychiatric disorders including Parkinson's disease, schizophrenia and substance abuse. This close association between dopamine and neurological and psychiatric diseases and with substance abuse make it an important topic in research in the neurosciences and an important molecular target in drug development. PET enables the direct measurement of components of the dopamine system in the living human brain. It relies on radiotracers which label dopamine receptors, dopamine transporters, precursors of dopamine or compounds which have specificity for the enzymes which degrade dopamine. Additionally, by using tracers that provide information on regional brain metabolism or blood flow as well as neurochemically specific pharmacological interventions, PET can be used to assess the functional consequences of changes in brain dopamine activity. PET dopamine measurements have been used to investigate the normal human brain and its involvement in psychiatric and neurological diseases. It has also been used in psychopharmacological research to investigate dopamine drugs used in the treatment of Parkinson's disease and of schizophrenia as well as to investigate the effects of drugs of abuse on the dopamine system. Since various functional and neurological parameters can be studied in the same subject, PET enables investigation of the functional integrity of the dopamine system in the human brain and investigation of the interactions of dopamine with other neurotransmitters. Through the parallel development of new radiotracers, kinetic models and better instruments, PET technology is enabling investigation of increasingly more complex aspects of the human brain dopamine system. This paper summarizes the different tracers and experimental strategies developed to evaluate the various elements of the dopamine system in the human brain with PET and their applications to clinical research.

Measuring age-related changes in dopamine D2 receptors with 11C-raclopride and 18F-N-methylspiroperidol

This study investigates the rate of age-related dopamine D2 receptor loss as determined by positron emission tomography (PET) and 11C-raclopride and compares it with D2 loss previously estimated with 18F-N-methylspiroperidol (NMS). Dopamine D2 receptors were measured with 11C-raclopride in 24 healthy volunteers (24-73 years of age) using the ratio of the distribution volume in striatum to that in cerebellum (Bmax/Kd + 1). The results were compared with those obtained in 20 healthy male volunteers (20-49 years of age) in whom D2 receptors were measured with NMS using the ratio index (slope of the striatum-to-cerebellum ratio as a function of time). Findings of correlational analysis between age and dopamine D2 receptor availability were significant for both ligands. Estimates of dopamine D2 receptor loss per decade corresponded to 7.9% for the 11C-raclopride study and 7.8% for the NMS study. Both ligands documented significant age-related decreases in dopamine D2 receptors that occurred relatively early in life (40 years of age).

Age associated decrements in dopamine D2 receptors in thalamus and in temporal insula of human subjects

This study evaluates the effects of age on DA D2 receptors in extrastriatal regions. DA D2 receptor availability was evaluated in 42 healthy male subjects (mean age 41 +/- 16, range 21 - 86 year old) with positron emission tomography (PET) and [11C]raclopride. Estimates of Bmax/Kd were obtained using the ratio of the distribution volume in the region of interest (caudate, putamen, thalamus, frontal, occipital cortices, temporal insula, cingulate and orbitofrontal gyri) to that in cerebellum. Correlations between age and D2 receptors were significant in putamen (r = -0.58, p < or = 0.0001), caudate (r = -0.54, p < or = 0.0002), thalamus (r = -0.33, p < or = 0.03) and temporal insula (r = -0.39, p < or = 0.01) but not in any of the frontal regions. The decrease in DA D2 receptor availability was 6.6% per decade in caudate, 8.2% in putamen, 7.6% in thalamus and 13% in temporal insula. This study indicates that D2 losses with age are not limited to striatum and involve also thalamic as well as temporal cortical regions.

MR-PET image coregistration for quantitation of striatal dopamine D2 receptors

PURPOSE

Our goal was to assess the utility of MR-PET image coregistration to quantify dopamine D2 receptors in striatum.

METHOD

Twenty-nine normal subjects were investigated with PET and [11C]raclopride and with MRI. D2 receptors were quantified using the ratio of the distribution volume in striatum to that in cerebellum. Measures obtained using regions selected directly from the PET images were compared with those obtained from MR images and then projected to coregistered PET images.

RESULTS

There were no differences between measures selected from the PET images (3.9 +/- 0.5) and those from the MR images (3.9 +/- 0.65). The values for these two measures were significantly correlated and corresponded to r = 0.9, p < 0.0001.

CONCLUSION

Regions of interest selected directly from PET images, where there is a large contrast between the region of interest and background, as for the case of dopamine D2 ligands, are almost identical to those obtained from coregistered MR images.

Who gives advice about postpartum contraception?

Women were interviewed to determine what advice they received about postpartum contraception and what they thought of it. Only 4% of women discussed postpartum contraception antenatally. Up to 84% discussed the issue with a midwife on the postnatal ward but discussion was often felt to be brief, limited and frequently held as the mother was leaving the hospital. Obstetricians appeared to have little interest in the subject and only 50% of mothers left the hospital with supplies of a contraceptive. Almost all women discussed contraception with their general practitioner at the postnatal check but a significant number felt that the choice of method was limited to condoms or pills. The postnatal check is traditionally held at six weeks--two to three weeks after the recommended time for starting contraceptive precautions. Women with short inter-pregnancy intervals were younger, less likely to be married and more likely to default from postnatal follow-up. Pregnant women should be offered the opportunity during the antenatal period to discuss postpartum contraception with someone who has a special interest in the subject. The postnatal ward is not an appropriate setting for discussion about future contraception.

Opioid receptor imaging and displacement studies with [6-O-[11C] methyl]buprenorphine in baboon brain

Buprenorphine (BPN) is a mixed opiate agonist-antagonist used as an analgesic and in the treatment of opiate addiction. We have used [6-O-[11C]methyl]buprenorphine ([11C]BPN) to measure the regional distribution in baboon brain, the test-retest stability of repeated studies in the same animal, the displacement of the labeled drug by naloxone in vivo, and the tissue distribution in mice. The regional distribution of radioactivity in baboon brain determined with PET was striatum > thalamus > cingulate gyrus > frontal cortex > parietal cortex > occipital cortex > cerebellum. This distribution corresponded to opiate receptor density and to previously published data (37). The tracer uptake in adult female baboons showed no significant variation in serial scans in the same baboon with no intervention in the same scanning session. HPLC analysis of baboon plasma showed the presence of labeled metabolites with 92% +/- 2.2% and 43% +/- 14.4% of the intact tracer remaining at 5 and 30 min, respectively. Naloxone, an opiate receptor antagonist, administered 30-40 min after tracer injection at a dose of 1.0 mg/kg i.v., reduced [11C]BPN binding in thalamus, striatum, cingulate gyrus, and frontal cortex to values 0.25 to 0.60 of that with no intervention. There were minimal (< 15%) effects on cerebellum. Naloxone treatment significantly reduced the slope of the Patlak plot in receptor-containing regions. These results demonstrate that [11C]BPN can be displaced by naloxone in vivo, and they affirm the feasibility of using this tracer and displacement methodology for short-term kinetics studies with PET. Mouse tissue distribution data were used to estimate the radiation dosimetry to humans. The critical organ was the small intestine, with a radiation dose estimate to humans of 117 nrad/mCi.

Dopamine transporters decrease with age

Postmortem studies have documented degeneration of dopamine cells with age, but the changes that occur in healthy aging individuals is less clear. The purpose of this study was to evaluate the extent to which age-induced changes in dopamine transporters occur in subjects with no evidence of motor impairment.

METHODS

We evaluated 23 right-handed healthy volunteers (age range 20-74 yr) using PET and [11C]d-threo-methylphenidate. The ratio of the distribution volume for [11C]d-threo-methylphenidate in striatum to that in cerebellum was used as model parameter for dopamine transporter availability (Bmax/Kd + 1).

RESULTS

Dopamine transporter availability was significantly lower in subjects > 40 yr of age than in those < 40 yr. Estimates of dopamine transporter availability showed a significant negative correlation with age both for the putamen (r = -0.72, p < 0.0001) and the caudate (r = -0.74, p < 0.0001). Dopamine transporter availability was higher in the left than in the right putamen but did not differ between the left and right caudate.

CONCLUSION

This study documents a 6.6% decrease per decade of life in striatal dopamine transporters of healthy volunteers.

Cocaine uptake is decreased in the brain of detoxified cocaine abusers

Binding of [11C]cocaine in brain was measured with positron emission tomography in 12 detoxified cocaine abusers and in 20 controls to evaluate if there were changes in cocaine binding and in dopamine (DA) transporter availability associated with chronic cocaine use. Nine controls and 10 cocaine abusers had an additional scan with [18F]N-methylspiroperidol to measure dopamine D2 receptors. Cocaine abusers had significantly lower uptake of [11C]cocaine in brain (6.2 +/- 1% dose/cc tissues) than controls (7.7 +/- 2%). The distribution volumes (DV) for [11C]cocaine were reduced in basal ganglia (BG), cortex, thalamus, and cerebellum (CB) of cocaine abusers. However there were no differences in the ratio of the DV in BG to that in CB, which is an estimate of DA transporter availability. Values for DA D2 receptor availability were decreased in cocaine abusers and did not correlate with estimates of dopamine transporter availability. In summary, detoxified cocaine abusers showed decreased uptake of cocaine in brain but did not show changes in DA transporter availability.

Inhibition of monoamine oxidase B in the brains of smokers

The massive health problem associated with cigarette smoking is exacerbated by the addictive properties of tobacco smoke and the limited success of current approaches to cessation of smoking. Yet little is known about the neuropharmacological actions of cigarette smoke that contribute to smoking behaviour, or why smoking is so prevalent in psychiatric disorders and is associated with a decreased risk of Parkinson's disease. Here we report that brains of living smokers show a 40% decrease in the level of monoamine oxidase B (MAO B; EC 1.4.3.4) relative to non-smokers or former smokers. MAO B is involved in the breakdown of dopamine, a neurotransmitter implicated in reinforcing and motivating behaviours as well as movement. MAO B inhibition is therefore associated with enhanced activity of dopamine, as well as with decreased production of hydrogen peroxide, a source of reactive oxygen species. We propose that reduction of MAO B activity may synergize with nicotine to produce the diverse behavioural and epidemiological effects of smoking.

Development of drug targeting based on recombinant expression of the chicken avidin gene

The chemistry required for covalent biotinylation of drugs, radiopharmaceuticals and other ligands is highly developed, and a large number of biotinylated reagents can be readily synthesized. In order to investigate whether expression of avidin cDNA in mammalian cells might be useful as part of a drug targeting strategy, we transiently expressed the avidin gene in two human tumor cell lines (the cervical carcinoma cell line, HeLa, and the liver derived line, Hep G2). Avidin protein as detected by either immunohistochemistry or binding of streptavidin-biotin complexes was present and functional following transient expression. This result indicated that the mechanisms underlying avidin oligomerization which are necessary for proper protein folding are present within mammalian carcinoma cell lines. Next, we generated a producer cell line (derived from psi2) capable of releasing a recombinant retrovirus encoding chicken avidin, and a tumorigenic murine breast cancer cell line (16/C) with stable avidin expression. We show that these cell lines are suitable for conferring functional expression of avidin in vitro. These experiments establish a means by which avidin gene expression can be explored as a mechanism for targeted gene delivery of biotin-derivitized drugs in vitro, and have important implications for utilization of this strategy in vivo.

Temporal relationships between the pharmacokinetics of methylphenidate in the human brain and its behavioral and cardiovascular effects

Positron emission tomography was used to compare the pharmacokinetics of [11C]methylphenidate in the human brain with the temporal course of the subjective and cardiovascular effects observed after intravenous methylphenidate (0.5 mg/kg). Four subjects were tested twice with [11C]methylphenidate, at baseline and after methylphenidate. All subjects showed almost identical uptake of 11C labeled drug in brain, as well as a very similar decrease in binding of [11C]methylphenidate in basal ganglia, after pretreatment with methylphenidate. In contrast, the magnitude of the behavioral and cardiovascular changes induced by methylphenidate varied among the subjects. The temporal course for methylphenidate effects paralleled closely the pharmacokinetics of [11C]methylphenidate in brain for the perception of "restlessness" and for changes in systolic blood pressure and heart rate. In contrast, methylphenidate induced "high", "anxiety" and changes in diastolic blood pressure decreased rapidly despite long lasting binding of the drug in brain. These results indicate that binding of methylphenidate in brain does not appear to predict individual responses to the drug and that more than one neurotransmitter and/or adaptation process are likely to be involved in the behavioral and cardiovascular effects of methylphenidate.

What do physicians tell patients with end-stage COPD about intubation and mechanical ventilation?

BACKGROUND

At some point in time, many patients with end-stage COPD require intubation and mechanical ventilation (MV) to sustain life. MV decisions are most effective when the patient and physician have discussed the options in advance. The purpose of this study was to examine how the physician perceives the decision-making process.

METHODS

Fifteen respirologists were interviewed to elicit information regarding intubation and MV, and the exchange of information between patients and physicians. Emergent themes were coded using a qualitative approach and were verified by a blinded researcher.

RESULTS

Respondents included ten academic and five community-based respirologists from seven hospitals. Most physicians were men with between 4 and 37 years experience. Narratives were very similar in content and seemed well rehearsed. Approach and delivery, however, were unique to each physician. Fourteen respirologists emphasized the importance of knowing patients as individuals prior to initiating this discussion. This period of familiarization often dictated when the physician believed the ventilation discussion is appropriate. Individual physician comfort also appeared to affect the timing of the discussion. Physicians discussed the many elements that make the MV discussion difficult for physicians and patients. Intubation details included a tube being placed down the throat, the discomfort associated with the tube, the inability to speak, and the availability of pain reducing medication. All physicians discussed the possibility of death with their patients, although many preferred euphemisms in initial discussions. All physicians indicated that intubation is presented as the patient's choice. However, all but one physician commonly framed their discussions in order to influence patient choice. The positive or negative framing seemed contingent on the physician's expectations for that patient.

CONCLUSIONS

Our interviews demonstrated considerable agreement between physicians about the content and timing of the intubation MV discussion. Physicians all agreed that knowing the patient and his or her situation was important in determining the timing of the intubation and MV discussion. Practice style and individual physician comfort with end-of-life decisions may influence the timing of the discussion and possibly the number of patients who are finally approached. All physicians advocated a shared decision-making approach, but they strongly influence the deliberation process. Thus, the decision-making model seemed to be physician driven in this study.

[18F]haloperidol binding in baboon brain in vivo

The binding of [18F]haloperidol to dopamine D2 and to sigma recognition sites in baboon brain was examined using positron emission tomography (PET). Studies were performed at baseline and after treatment with either haloperidol (to evaluate saturability), (+)-butaclamol (which has specificity for dopamine D2 receptors) or (-)-butaclamol (which has specificity for sigma sites). Binding was widespread. Treatment with (-)-butaclamol had no effect, whereas (+)-butaclamol selectively reduced the uptake in striatum. Haloperidol increased the clearance rate from all brain regions. These results indicate that the binding profile of [18F]haloperidol does not permit the selective examination of either dopamine D2 or sigma sites using PET.

Carbon-11-d-threo-methylphenidate binding to dopamine transporter in baboon brain

The more active d-enantiomer of methylphenidate (dl-threo-methyl-2-phenyl-2-(2-piperidyl)acetate, Ritalin) was labeled with 11C(t1/2:20.4 min) to characterize its binding, examine its specificity for the dopamine transporter and evaluate it as a radio-tracer for the presynaptic dopaminergic neuron.

METHODS

PET studies were carried out in the baboon. The pharmacokinetics of [11C]dl-threo-methylphenidate, [11C]l-threo-MP and with its racemate ([11C]dl-threo-methylphenidate, [11C]MP). Nonradioactive methylphenidate was used to assess the reversibility and saturability of the binding. GBR 12909, 3 beta-(4-iodophenyl)tropane-2-carboxylic acid methyl ester (beta-CIT), tomoxetine and citalopram were used to assess the binding specificity.

RESULTS

The ratio between radioactivity in the striatum and that in the cerebellum (ST/CB) after injection of [11C]d-threo-MP was higher than that for [11C]MP and [11C]l-threo-MP (3.3 for d-, 2.2 for racemic and 1.1 for l- in the same baboon). Most of the striatal binding of [11C]d-threo-MP was displaceable by injection of nonradioactive MP. Pretreatment with nonradioactive MP (0.5 mg/kg), GBR12909 (1.5 mg/kg) and RTI-55 (0.3 mg/kg) markedly reduced striatal but not cerebellar uptake of [11C]d-threo-MP. In all cases, the ST/CB after pretreatment was reduced by about 60% compared to 43% for [11C]MP. The ratios of distribution volumes at steady-state for the ST/CB for the three separate studies in the same baboon were reduced by about 50%, as compared with 37% for [11C]MP. In contrast, pretreatment with tomoxetine (3.0 mg/kg) or citalopram (2.0 mg/kg) did not change [11C]d-threo-MP kinetics; the ST/CB after pretreatment was similar to that for the control.

CONCLUSION

These results demonstrate the saturable, reversible and specific binding of [11C]d-threo-MP to the dopamine transporter on the baboon brain, suggesting that [11C]d-threo-MP will be a useful PET tracer for the presynaptic dopaminergic neuron in living human brain.

A new PET ligand for the dopamine transporter: studies in the human brain

Carbon-11-d-threo-methylphenidate, the active enantiomer of methylphenidate (ritalin), has been shown to bind uniquely to the dopamine transporter in the baboon brain. This study characterizes its binding in the human brain and measures its test-retest reproducibility.

METHODS

Studies were done in seven normal controls, each of whom was scanned with [11C]d-threomethylphenidate on two different occasions. Six subjects were scanned twice 3-5 wk apart without intervention to assess reproducibility. One subject was scanned sequentially before and after treatment with methylphenidate to assess binding saturability. Graphical analysis was used to obtain tissue distribution volumes (DV). The ratio of the DV in the basal ganglia (BG) to that in cerebellum (CB) (DVBG/DVCB), which corresponds to (Bmax/Kd) + 1 was used to estimate dopamine transporter availability.

RESULTS

Highest tracer uptake occurred in the basal ganglia, where activity peaked 7-11 min postinjection. The half-clearance time for the tracer in brain regions other than the basal ganglia was 74 min. In the basal ganglia, only 10%-15% of the activity had cleared at 74 min. Time-activity curves for [11C]d-threo-methylphenidate in the basal ganglia and cerebellum were highly reproducible. The average percent change for the absolute value for DVBG/DVCB was 6.5% +/- 4% (range 0-12%). Methylphenidate pretreatment decreased basal ganglia uptake but not cortical or cerebellar binding and reduced DVBG/DVCB by 62% and Bmax/Kd by 91%.

CONCLUSION

These studies demonstrate that [11C]d-threo-methylphenidate binding in the human brain is reversible, highly reproducible and saturable. Thus, it is an appropriate PET ligand to measure dopamine transporter availability.

Comparison of two pet radioligands for imaging extrastriatal dopamine transporters in human brain

We compared the sensitivity of two dopamine transporter (DAT) ligands ([C-11]cocaine and [C-11]d-threo-methylphenidate) for measurement of extrastriatal DAT availability using positron emission tomography (PET) on separated groups of 10 age matched male volunteers (age range, 21-49 years). DAT availability was obtained using the ratio of the distribution volume in the region of interest to that in the cerebellum (Bmax'/Kd'+ 1). DAT availability measured with [C-11]d-threo-methylphenidate was highest in basal ganglia, followed by thalamus > temporal insula, cingulate > orbitofrontal, frontal and occipital cortices. A similar ranking order for DAT availability was obtained with [C-11]cocaine. Specific binding (Bmax'/Kd') of [C-11]cocaine in thalamus was 25-33% that of basal ganglia and [C-11]d-threo-methylphenidate in thalamus was 11-13% that of basal ganglia. The regional measures with [C-11]cocaine were significantly correlated with those of [C-11]d-threo-methylphenidate (p < or = 0.0001). These results document extrastriatal binding in human brain with two different DAT ligands.

Mechanistic positron emission tomography studies of 6-[18F]fluorodopamine in living baboon heart: selective imaging and control of radiotracer metabolism using the deuterium isotope effect

Mechanistic positron emission tomography (PET) studies using the deuterium isotope effect and specific pharmacological intervention were undertaken to examine the behavior of 6-[18F]fluorodopamine (6-[18F]-FDA; 1) and (-)-6-[18F]fluoronorepinephrine [(-)-6-[18F]FNE; 2] in the baboon heart. Two regiospecifically deuterated derivatives of 6-[18F]FDA [alpha,alpha-D2 (3) and beta,beta-D2 (4)] were used to assess the contributions of monoamine oxidase (MAO) and dopamine beta-hydroxylase, respectively, to the clearance kinetics of 6-[18F]FDA. Compound 3 showed a reduced rate of clearance, consistent with MAO-catalyzed cleavage of the alpha C-D bond, whereas compound 4 showed no change, indicating that cleavage of the beta C-D bond is not a rate-limiting step. Pretreatment with pargyline, an MAO inhibitor, also decreased the rate of clearance. Desipramine and tomoxetine [norepinephrine (NE) uptake inhibitors], but not GBR-12909 (a dopamine uptake inhibitor), blocked the uptake of both (-)-6-[18F]FNE and 6-[18F]FDA, with (-)-6-[18F]FNE showing a higher degree of blockade. Chiral HPLC demonstrated that 6-[18F]FDA is stereoselectively converted to (-)-6-[18F]FNE in vivo in the rat heart. These studies demonstrate that (a) the more rapid clearance of 6-[18F]FDA relative to (-)-6-[18F]FNE can be largely accounted for by metabolism by MAO; (b) selective deuterium substitution can be used to protect a radiotracer from metabolism in vivo and to favor a particular pathway; (c) 6-[18F]FDA and (-)-6-[18F]FNE share the NE transporter; (d) 6-[18F]FDA is stereoselectively converted to (-)-6-[18F]FNE in vivo; and (e) the profile of radioactivity in the heart for 6-[18F]FDA is complex, probably including labeled metabolites as well as neuronal and nonneuronal uptake.

Selective reduction of radiotracer trapping by deuterium substitution: comparison of carbon-11-L-deprenyl and carbon-11-deprenyl-D2 for MAO B mapping

Recent human PET studies with the monoamine oxidase B (MAO B) tracer [11C]L-deprenyl show that the rapid rate of radiotracer trapping relative to transport reduces the sensitivity of the tracer in regions of high MAO B concentration. This study investigates the use of deuterium substituted L-deprenyl ([11C]L-deprenyl-D2) to reduce the rate of trapping in tissue and to improve sensitivity.

METHODS

Five normal subjects (43-64 yr) were studied with [11C]L-deprenyl and [11C]L-deprenyl-D2 on the same day. Time-activity data from different brain regions and the arterial plasma were analyzed using a three-compartment model as well as graphical analysis for irreversible systems.

RESULTS

For both tracers, maximum radioactivity accumulation occurred at about 5 min. For [11C]L-deprenyl, 11C concentration peaked at 5 min and remained constant throughout the study. With [11C]L-deprenyl-D2, peak 11C concentration also occurred at about 5 min but was followed by an initial washout. Carbon-11 concentration generally plateaued from 30 to 60 min. The plateau for [11C]L-deprenyl was higher than the plateau for [11C]L-deprenyl-D2. Data analysis by a three-compartment model and by graphical analysis showed that deuterium substitution: (a) does not affect plasma to tissue transport (K1); (b) reduces the rate of trapping of 11C in all brain regions; (c) facilitates the separation of model terms related to radiotracer delivery from radiotracer trapping in tissue; and (d) improves tracer sensitivity.

CONCLUSION

This study demonstrates that deuterium substitution causes a significant reduction in the rate of trapping of labeled deprenyl, providing a direct link between radiotracer uptake and MAO B in the human brain and enhancing tracer sensitivity to changes in MAO B concentration.

Carbon-11-cocaine binding compared at subpharmacological and pharmacological doses: a PET study

We have characterized cocaine binding in the brain to a high-affinity site on the dopamine transporter using PET and tracer doses of [11C]cocaine in the baboon in vivo. The binding pattern, however, of cocaine at tracer (subpharmacological) doses may differ from that observed when the drug is taken in behaviorally active doses particularly since in vitro studies have shown that cocaine also binds to low affinity binding sites.

METHODS

PET was used to compare and characterize [11C]cocaine binding in the baboon brain at low subpharmacological (18 micrograms average dose) and at pharmacological (8000 micrograms) doses. Serial studies on the same day in the same baboon were used to assess the reproducibility of repeated measures and to assess the effects of drugs which inhibit the dopamine, norepinephrine and serotonin transporters. Time-activity curves from brain and the arterial plasma input function were used to calculate the steady-state distribution volume (DV).

RESULTS

At subpharmacological doses, [11C]cocaine had a higher binding and slower clearance in striatum than in other brain regions. At pharmacological doses, [11C]cocaine had a more homogeneous distribution. Bmax/Kd for sub-pharmacological [11C]cocaine corresponded to 0.5-0.6 and for pharmacological [11C]cocaine it corresponded to 0.1-0.2. Two-point Scatchard analysis gave Bmax = 2300 pmole/g and Kd' = 3600 nM. Bmax/Kd for sub-pharmacological doses of [11C]cocaine was decreased by cocaine and drugs that inhibit the dopamine transporter, to 0.1-0.2, but not by drugs that inhibit the serotonin or the norepinephrine transporter. None of these drugs changed Bmax/Kd for a pharmacological dose of [11C]cocaine.

CONCLUSION

At subpharmacological doses, [11C]cocaine binds predominantly to a high-affinity site on the dopamine transporter.

Effects of sodium bicarbonate, disodium carbonate, and a sodium bicarbonate/carbonate mixture on the PCO2 of blood in a closed system

The effects of adding Carbicarb--a 50:50 mixture of sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3)--or sodium bicarbonate or disodium carbonate individually to whole blood were examined in vitro. Adding HCl to blood in an open system rapidly decreased the pH and [HCO3-] without increasing partial pressure of carbon dioxide (PCO2), whereas adding HCl to blood in a closed system markedly decreased pH and increased PCO2. Adding sodium bicarbonate to blood caused a rapid and predictable increase in PCO2 that was linearly related to the pH of the blood at the time of addition. Adding disodium carbonate to blood caused a rapid and predictable fall in PCO2 that was linearly related to the initial PCO2 of the blood at the time of addition. Adding Carbicarb to blood caused relatively little change in PCO2 over a wide range of initial pH and PCO2 values.

Is methylphenidate like cocaine? Studies on their pharmacokinetics and distribution in the human brain

BACKGROUND

The purposes of this study were to investigate the pharmacokinetics of methylphenidate hydrochloride (Ritalin) in the human brain, to compare them with those of cocaine, and to evaluate whether cocaine and methylphenidate compete for the same binding sites.

METHODS

We used positron emission tomography to measure the temporal and spatial distribution of carbon 11 (11C)-labeled methylphenidate. These results were compared with those obtained previously for [11C]cocaine. Eight healthy male subjects, 20 to 51 years of age, were scanned with [11C]methylphenidate. Three were tested twice to assess test-retest variability, four were tested at baseline and after administration of methylphenidate, and one was tested with [11C]methylphenidate and [11C]cocaine. Two baboons were scanned to evaluate whether there was competition between cocaine and methylphenidate for the same binding sites in the brain.

RESULTS

The uptake of [11C]methylphenidate in the brain was high (mean +/- SD, 7.5% +/- 1.5%), and the maximal concentration occurred in striatum. Pretreatment with methylphenidate decreased binding only in striatum (40%). Although the regional distribution of [11C]methylphenidate, was identical to that of [11C]cocaine and they competed with each other for the same binding sites, these two drugs differed markedly in their pharmacokinetics. Clearance of [11C]methylphenidate from striatum (90 minutes) was significantly slower than that of [11C]cocaine (20 minutes). For both drugs, their fast uptake in striatum paralleled the experience of the "high." For methylphenidate, the high decreased very rapidly despite significant binding of the drug in the brain. In contrast, for cocaine, the decline in the high paralleled its fast rate of clearance from the brain.

CONCLUSION

We speculate that because the experience of the high is associated with the fast uptake of cocaine and methylphenidate in the brain, the slow clearance of methylphenidate from the brain may serve as a limiting factor in promoting its frequent self-administration.

Sensitivity of striatal [11C]cocaine binding to decreases in synaptic dopamine

We have previously shown that tracer concentrations of [11C]cocaine binding to the dopamine transporter (DAT) in human and baboon striatum can be visualized using positron emission tomography (PET). To determine whether the concentration of dopamine normally present in the synaptic cleft can compete with [11C]cocaine for transporter binding sites, we conducted baboon PET studies with drugs (sodium 4-hydroxybutyrate, four studies, 200 mg/kg; gamma-vinylGABA, three studies, 300 mg/kg; and citalopram, three studies, 2 mg/kg) expected to decrease synaptic dopamine. Each study involved two [11C]cocaine injections and PET scans separated by 2-4 h, with drug administration after the first injection, and without movement of the subject between scans. Time-activity data from striatum and from cerebellum were used with the arterial plasma input function to determine graphically by Logan plotting [11C]cocaine distribution volumes for the brain regions. Specific binding of [11C]cocaine to DAT in striatum was calculated as the distribution volume ratio (DVR) for striatum and cerebellum. In nine of the ten studies drug treatment produced a small increase in DVR (range, 1-11%), and in seven of these studies the increase was > or = 7%. The mean increase was 6.2 +/- 4.1%. The reproducibility of the DVR measure was assessed by comparing [11C]cocaine studies conducted without pharmacological treatments using individual baboons on separate days, and thus involving possible repositioning errors, as well as long-term changes in the state of the striatal dopamine system.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin 5-HT2 receptor availability in chronic cocaine abusers

Serotonin 5-HT2 receptor availability was evaluated in chronic cocaine abusers (n = 19) using positron emission tomography and F-18 N-methylspiperone and was compared to control subjects (n =19). 5-HT2 Receptor availability was measured in frontal, occipital, cingulate and orbitofrontal cortices using the ratio of the distribution volume in the region of interest to that in the cerebellum which is a function of Bmax/Kd. 5-HT2 Receptor availability was significantly higher in cingulate and orbitofrontal cortices than in other frontal regions or occipital cortex. The values were not different in normal subjects and cocaine abusers. These results did not show any changes in 5-HT2 receptor availability in cocaine abusers as compared to the control subjects.

Intravenous morphine pharmacokinetics in pediatric patients with sickle cell disease

To examine the pharmacokinetics of parenteral opioids, such as morphine, in patients with sickle cell disease, we determined the plasma morphine clearances in 18 patients (aged 6 to 19 years) who were receiving continuous intravenous infusions, and the pharmacokinetics of morphine in an additional six patients after single intravenous doses. Plasma morphine clearances ranged from 6.2 to 59.1 ml min-1 kg-1 (35.5 +/- 12.4, mean +/- SD) during steady-state infusions. There was a negative correlation between clearance values and age over the age range studied (p = 0.013). A significant difference (p = 0.042) was also observed in clearance values between patients who had serious adverse symptoms (23.4 +/- 10.7 ml min-1 kg-1) and those who had less serious symptoms (36.3 +/- 6.4 ml min-1 kg-1) when morphine was given at high dosage rates (> or = 0.15 mg kg-1 hr-1). Pharmacokinetic modeling of plasma morphine concentrations adequately fit a two-compartment model with a short initial distribution phase (mean half-life = 4.5 minutes) and a rapid terminal elimination half-life (77.6 +/- 19.2 minutes). These findings suggest that considerable individualization of morphine dosing may be necessary to achieve optimal analgesia and minimal adverse effects in these patients.

Long-lasting inhibition of in vivo cocaine binding to dopamine transporters by 3 beta-(4-iodophenyl)tropane-2-carboxylic acid methyl ester: RTI-55 or beta CIT

Cocaine analogs such as 3 beta-(4-iodophenyl)tropane-2 beta-carboxylic acid methyl ester (RTI-55 or beta CIT) with a higher affinity for the dopamine transporter (DAT) may be potentially useful in interfering with cocaine's actions in brain. This study evaluates the time course of the effects of RTI-55 on cocaine binding in baboon brain using PET and [11C]cocaine. [11C]Cocaine binding was measured prior to, and 90 minutes, 24 hours, 4-5 days and 11-13 days after RTI-55 (0.3 mg/kg i.v.). Parallel studies with [3H]cocaine and RTI-55 (0.5 mg/kg i.v. or 2 mg/kg i.p.) were performed in the mouse. RTI-55 significantly inhibited [11C]cocaine binding at 90 minutes and 24 hours after administration. The half-life for the clearance of RTI-55 from the DAT was estimated to be 2 to 3 days in the baboon brain. In the mouse brain, RTI-55 significantly inhibited [3H]cocaine binding at 60 and 180 minutes after administration and recovery was observed at 12 hours. These results document long-lasting inhibition of cocaine binding by RTI-55 and corroborate that binding kinetics of RTI-55 in striatum observed in imaging studies with [123I]RTI-55 represents binding to DATs.

Evaluation of age-related changes in serotonin 5-HT2 and dopamine D2 receptor availability in healthy human subjects

We assessed the relation between serotonin 5-HT2 receptor availability and aging and compared it with that for dopamine D2 receptors on 19 healthy male volunteers (age range, 21-49 years) using positron emission tomography (PET) and F-18 N-methylspiperone (NMS). 5-HT2 Receptor availability was obtained using the ratio of the distribution volume in the region of interest to that in the cerebellum (Bmax'/Kd' + 1). 5-HT2 Receptor measures were obtained in frontal and occipital cortices. D2 receptor availability in striatum was measured using the "ratio index". 5-HT2 Receptor availability decreased significantly with age. This effect was significantly more accentuated for 5-HT2 receptor availability in the frontal (r = 0.92, p < or = 0.0001) than in the occipital (r = 0.67, p < or = 0.0016) cortex (df = 1, p < 0.025). Dopamine D2 receptors were also found to decrease significantly with age (r = 0.63, p < or = 0.007). In a given subject, striatal D2 receptor availability significantly correlated with 5-HT2 receptor availability in the frontal (r = 0.51, p < or = 0.035) but not in the occipital cortex. These results document a decline in 5-HT2 and D2 receptor availability with age and showed an association between frontal 5-HT2 and striatal D2 receptors.

Serotonergic modulation of striatal dopamine measured with positron emission tomography (PET) and in vivo microdialysis

Positron emission tomography and in vivo microdialysis were used to study serotonin's role in modulating striatal dopamine. Serial PET studies were performed in adult female baboons at baseline and following drug treatment, using the dopamine (D2) selective radiotracer, 11C-raclopride. The serotonergic system was manipulated by administration of the selective 5-HT reuptake inhibitor, citalopram, or by serotonergic (5-HT2) receptor blockade (using altanserin, a 5-HT2 antagonist). 11C-Raclopride time-activity data from striatum and cerebellum were combined with plasma arterial input functions and analyzed by calculating a distribution volume as described previously (Logan et al., 1990). Additionally, in vivo microdialysis studies were performed in awake freely moving rats using similar pharmacologic challenges plus SR 46349B, a new highly selective 5-HT2 receptor antagonist. Altanserin and SR 46349B increased extracellular striatal dopamine concentrations (35% and 910%, respectively) while altanserin decreased striatal 11C-raclopride binding (37%). Citalopram, however, decreased extracellular striatal dopamine concentrations (50%) and increased 11C-raclopride binding (33%). These data demonstrate that 5-HT-selective drugs produce changes in striatal dopamine that can be measured noninvasively with PET. Furthermore, the PET data obtained from anesthetized baboons is consistent with in vivo microdialysis data obtained from awake and freely moving rats. Finally, these studies have implications for understanding the therapeutic efficacy of atypical neuroleptics and their utility for treating schizophrenia and affective disorders.

Effects of blood flow on [11C]raclopride binding in the brain: model simulations and kinetic analysis of PET data

To assess the stability of different measures of receptor occupancy from [11C]raclopride (a D2 antagonist) studies with positron emission tomography, we analyze data from five test/retest studies in normal volunteers in terms of individual model parameters from a three-compartment model, the distribution volume (DV) and the ratio of DVs from a receptor-containing region of interest to a non-receptor-containing region. Large variations were found in the individual model parameters, limiting their usefulness as an indicator of change in receptor systems. The DV ratio showed the smallest variation. Individual differences were reflected in the greater intersubject variation in DV than intrasubject variation. The potential effects of blood flow on these measurements were addressed both experimentally and by simulation studies using three models that explicitly incorporate blood flow into a compartmental model that also includes receptor-ligand binding. None of the models showed any variation in the DV with changes in blood flow as long as flow was held constant during the simulation. Experimentally, blood flow was significantly reduced by hyperventilation in a human subject. The DV was found to be reduced relative to baseline in the hyperventilation study, but the DV ratio remained unchanged. The effect of elevated and reduced flow was also tested in two baboon experiments in which PCO2 was varied. Some variability in the DV ratio was observed but was not correlated with changes in blood flow. This raises the possibility that other factors indirectly related to changes in blood flow (or PCO2) may cause changes in DV, and these effects need to be considered when evaluating experimental results.

Pharmacokinetics and in vivo specificity of [11C]dl-threo-methylphenidate for the presynaptic dopaminergic neuron

dl-threo-Methylphenidate (Ritalin) was labeled with carbon-11 (t1/2:20.4 minutes) in order to measure its pharmacokinetics, to evaluate it as a radiotracer for the presynaptic dopaminergic neuron, and to examine its sensitivity to the loss of dopaminergic neurons. Positron emission tomographic (PET) studies were carried out in the baboon to determine specificity for the presynaptic dopaminergic neuron and in humans to assess sensitivity to neuronal loss. Studies with [11C]dl-threo-methylphenidate ([11C]MP) in baboon demonstrated high regional uptake in the striatum. Peak uptake (0.04%/cc) occurred at 5-15 minutes post-injection. The half-time for clearance from peak uptake for [11C]MP was 60 minutes and the ratio between the radioactivity in the striatum and that in the cerebellum (ST/CB) ranged from 2.2 to 2.6 at 40 minutes. Repeated measures in the same baboon showed < or = 8% variability in the ST/CB ratio. Pretreatment with unlabeled methylphenidate (0.5 mg/kg) or GBR12909 (1.5 mg/kg) 30 minutes prior to [11C]MP injection markedly reduced the striatal but not the cerebellar uptake of [11C]MP, demonstrating the saturable and specific binding of [11C]MP to a site on the dopamine transporter in the brain. In both cases, the ratio of striatum to cerebellum (ST/CB) after pretreatment was reduced by about 43%. The ratios of distribution volumes at the steady-state for the striatum to cerebellum (ST/CB) for these two separate studies in the same baboon were reduced by 37 and 38%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Slow recovery of human brain MAO B after L-deprenyl (Selegeline) withdrawal

L-Deprenyl (Selegeline) is an enzyme-activated irreversible inhibitor of monoamine oxidase B (MAO B; EC 1.4.3.4). It is used to treat Parkinson's disease at a dose of 5 mg twice a day. Since enzyme inhibition is irreversible, the recovery of functional enzyme activity after withdrawal from L-deprenyl requires the synthesis of new enzyme. We have measured a 40 day half-time for brain MAO B synthesis in Parkinson's disease and in normal subjects after withdrawal from L-deprenyl. This is the first measurement of the synthesis rate of a specific protein in the living human brain. L-Deprenyl is currently used by 50,000 patients with Parkinson's disease in the United States and its use is expected to increase with reports that it may be beneficial in Alzheimer's disease. The slow turnover of brain MAO B suggests that the current clinical dose of L-deprenyl may be excessive and that the clinical efficacy of reduced dosing should be evaluated. Such an evaluation may have mechanistic importance as well as an impact on reducing the side effects and the costs arising from excessive drug use.

Decreased dopamine transporters with age in health human subjects

The effects of aging on brain dopamine transporters was evaluated in 26 healthy male volunteers (age range, 21-63 years) using positron emission tomography and [11C]cocaine. The ratio of the distribution volume for [11C]cocaine in basal ganglia to that in cerebellum was used as a model parameter for dopamine transporter availability and showed a significant negative correlation with age (r = 0.65, p < 0.0005). This results document an age-related decline in dopamine transporters in healthy individuals.