Higher cortical and lower subcortical metabolism in detoxified methamphetamine abusers

OBJECTIVE

Methamphetamine has raised concerns because it may be neurotoxic to the human brain. Although prior work has focused primarily on the effects of methamphetamine on dopamine cells, there is evidence that other neuronal types are affected. The authors measured regional brain glucose metabolism, which serves as a marker of brain function, to assess if there is evidence of functional changes in methamphetamine abusers in regions other than those innervated by dopamine cells.

METHOD

Fifteen detoxified methamphetamine abusers and 21 comparison subjects underwent positron emission tomography following administration of [(18)F]fluorodeoxyglucose.

RESULTS

Whole brain metabolism in the methamphetamine abusers was 14% higher than that of comparison subjects; the differences were most accentuated in the parietal cortex (20%). After normalization for whole brain metabolism, methamphetamine abusers exhibited significantly lower metabolism in the thalamus (17% difference) and striatum (where the differences were larger for the caudate [12%] than for the putamen [6%]). Statistical parametric mapping analyses corroborated these findings, revealing higher metabolism in the parietal cortex and lower metabolism in the thalamus and striatum of methamphetamine abusers.

CONCLUSIONS

The fact that the parietal cortex is a region devoid of any significant dopaminergic innervation suggests that the higher metabolism seen in this region in the methamphetamine abusers is the result of methamphetamine effects in circuits other than those modulated by dopamine. In addition, the lower metabolism in the striatum and thalamus (major outputs of dopamine signals into the cortex) is likely to reflect the functional consequence of methamphetamine in dopaminergic circuits. These results provide evidence that, in humans, methamphetamine abuse results in changes in function of dopamine- and nondopamine-innervated brain regions.

Imaging brain cholinergic activity with positron emission tomography: its role in the evaluation of cholinergic treatments in Alzheimer's dementia

One of the strategies in the treatment of Alzheimer's disease is the use of drugs that enhance cholinergic brain function, since it is believed that cholinergic dysfunction is one of the factors that contributes to cognitive deterioration. Positron emission tomography is a medical imaging method that can be used to measure the concentration, kinetics, and distribution of cholinergic-enhancing drugs directly in the human brain and assess the effects of the drugs at markers of cholinergic cell viability (vesicular transporters, acetylcholinesterase), at muscarininc and nicotinic receptors, at extracellular acetylcholine, at markers of brain function (glucose metabolism and blood flow), and on amyloid plaque burden in vivo in the brains of patients with Alzheimer's disease. In addition, these measures can be applied to assess the drugs' pharmacokinetic and pharmacodynamic properties in the human brain. Since the studies are done in living human subjects, positron emission tomography can evaluate the relationship between the drugs' biological, behavioral, and cognitive effects; monitor changes in brain function in response to chronic treatment; and determine if pharmacologic interventions are neuroprotective. Moreover, because positron emission tomography has the potential to identify Alzheimer's disease during early disease, it can be used to establish whether early interventions can prevent or delay further development.

Effects of route of administration on cocaine induced dopamine transporter blockade in the human brain

The route of administration influences the reinforcing effects of cocaine. Here we assessed whether there were differences in the efficacy of cocaine to block the dopamine transporters (major target for cocaine's reinforcing effects), as a function of route of administration. Positron emission tomography and [11C]cocaine, a dopamine transporter radioligand, were used to compare the levels of dopamine transporter blockade induced by intravenous, smoked and intranasal cocaine in 32 current cocaine abusers. In parallel, the temporal course for the self-reports of "high" were obtained. Cocaine significantly blocked dopamine transporters. The levels of blockade were comparable across all routes of administration and a dose effect was observed for intravenous and intranasal cocaine but not for smoked cocaine. For equivalent levels of cocaine in plasma and DAT blockade, smoked cocaine induced significantly greater self reports of "high" than intranasal cocaine and showed a trend for a greater effect than intravenous cocaine. The time to reach peak subjective was significantly faster for smoked (1.4+/-0.5 min) than for intravenous cocaine (3.1+/-0.9 min), which was faster than intranasal cocaine (14.6+/-8 min). Differences in the reinforcing effects of cocaine as a function of the route of administration are not due to differences in the efficacy of cocaine to block the dopamine transporters. The faster time course for the subjective effects for smoked than intravenous and for intravenous than for intranasal cocaine highlights the importance of the speed of cocaine's delivery into the brain on its reinforcing effects.

Dopamine receptor-mediated regulation of striatal cholinergic activity: positron emission tomography studies with norchloro[18F]fluoroepibatidine

Large numbers of in vitro studies and microdialysis studies suggest that dopaminergic regulation of striatal acetylcholine (ACh) output is via inhibitory dopamine D2 receptors and stimulatory dopamine D1 receptors. Questions remain as to the relative predominance of dopamine D2 versus D1 receptor modulation of striatal ACh output under physiological conditions. Using positron emission tomography, we first demonstrate that norchloro[18F]fluoroepibatidine ([18F]NFEP), a selective nicotinic ACh receptor (nAChR) ligand, was sensitive to changes of striatal ACh concentration. We then examined the effect of quinpirole (D2 agonist), raclopride (D2 antagonist), SKF38393 (D1 agonist), and SCH23390 (D1 antagonist) on striatal binding of [18F]NFEP in the baboon. Pretreatment with quinpirole increased the striatum (ST) to cerebellum (CB) ratio by 26+/-6%, whereas pretreatment with raclopride decreased the ST/CB ratio by 22+/-2%. The ratio of the distribution volume of [18F]NFEP in striatum to that in cerebellum, which corresponds to (Bmax/K(D)) + 1 (index for nAChR availability), also showed a significant increase (29 and 20%; n = 2) and decrease (20+/-3%; n = 3) after pretreatment with quinpirole and raclopride, respectively. However, both the D1 agonist and antagonist had no significant effect. This suggests that under physiological conditions the predominant influence of endogenous dopamine on striatal ACh output is dopamine D2, not D1, receptor-mediated.

Cocaine abusers show a blunted response to alcohol intoxication in limbic brain regions

Cocaine and alcohol are frequently used simultaneously and this combination is associated with enhanced toxicity. We recently showed that active cocaine abusers have a markedly enhanced sensitivity to benzodiazepines. Because both benzodiazepines and alcohol facilitate GABAergic neurotransmission we questioned whether cocaine abusers would also have an enhanced sensitivity to alcohol that could contribute to the toxicity. In this study we compared the effects of alcohol (0.75 g/kg) on regional brain glucose metabolism between cocaine abusers (n = 9) and controls (n = 10) using PET and FDG. Alcohol significantly decreased whole brain metabolism and this effect was greater in controls (26+/-6%) than in abusers (17+/-10%) even though they had equivalent levels of alcohol in plasma. Analysis of the regional measures showed that cocaine abusers had a blunted response to alcohol in limbic regions, cingulate gyrus, medial frontal and orbitofrontal cortices.

CONCLUSIONS

The blunted response to alcohol in cocaine abusers contrasts with their enhanced sensitivity to benzodiazepines suggesting that targets other than GABA-benzodiazepine receptors are involved in the blunted sensitivity to alcohol and that the toxicity from combined cocaine-alcohol use is not due to an enhanced sensitivity to alcohol in cocaine abusers. The blunted response to alcohol in limbic regions and in cortical regions connected to limbic areas could result from a decreased sensitivity of reward circuits in cocaine abusers.

Evidence that gingko biloba extract does not inhibit MAO A and B in living human brain

Extracts of Ginkgo biloba have been reported to reversibly inhibit both monoamine oxidase (MAO) A and B in rat brain in vitro leading to speculation that MAO inhibition may contribute to some of its central nervous system effects. Here we have used positron emission tomography (PET) to measure the effects of Ginkgo biloba on human brain MAO A and B in 10 subjects treated for 1 month with 120 mg/day of the Ginkgo biloba extract EGb 761, using [11C]clorgyline and [11C]L-deprenyl-D2 to measure MAO A and B respectively. A three-compartment model was used to calculate the plasma to brain transfer constant K1 which is related to blood flow, and lambdak3, a model term which is a function of the concentration of catalytically active MAO molecules. Ginkgo biloba administration did not produce significant changes in brain MAO A or MAO B suggesting that mechanisms other than MAO inhibition need to be considered as mediating some of its CNS effects.

Association between age-related decline in brain dopamine activity and impairment in frontal and cingulate metabolism

OBJECTIVE

Despite the well-documented loss of brain dopamine activity with age, little is known about its functional consequences in healthy individuals. This study investigates the relationship between measures of brain dopamine D(2) receptors (molecules that transmit dopamine signals) and regional brain glucose metabolism (a marker of brain function) in healthy individuals.

METHOD

Thirty-seven healthy volunteers aged 24-86 years underwent positron emission tomography scans after injection of [(11)C]raclopride to assess dopamine D(2) receptors and [(18)]fluorodeoxyglucose to assess regional brain glucose metabolism. Two methods used to assess the correlations between metabolism and dopamine D(2) receptors-pixel-by-pixel correlations and correlations in preselected regions of interest-were then compared.

RESULTS

D(2) receptors as well as frontal and cingulate metabolism declined with age. Regardless of the method used, significant correlations between metabolism and D(2) receptors were found in the frontal cortex (Brodmann's areas 6, 7, 8, 9, 10, 11, 44, 45, 47), anterior cingulate gyrus (areas 24, 32), temporal cortex (area 21), and caudate. These correlations remained significant after removing age effects (partial correlation).

CONCLUSIONS

These results provide the first link between age-related declines in brain dopamine activity and frontal and cingulate metabolism, which supports the need to investigate the therapeutic utility of interventions that enhance dopamine function in the elderly. The fact that correlations remained significant after removing age effects suggests that dopamine may influence frontal, cingulate, and temporal metabolism regardless of age.

Imaging the functioning human brain

One of the most exciting methodological advances for brain research field arises in functional brain imaging, which enables us to localize and characterize neural activity and biochemical events in the living human brain. Recently developed event-related functional MRI makes it possible to visualize the brain activity associated with cognitive processes with the temporal resolution of the hemodynamic response. In addition, the high sensitivity and selectivity of positron-emission tomography allow us to probe the neurochemical processes at the molecular level. Positron-emission tomography also has been applied to investigate the effects of therapeutic drugs as well as the effects of drugs of abuse.

Dopamine-transporter occupancy after intravenous doses of cocaine and methylphenidate in mice and humans

OBJECTIVES

Recent studies using positron emission tomography (PET) have established the relationship between an intravenous dose of cocaine and the percentage occupancy of the dopamine transporter in humans, and have documented the requirement of more than 50% occupancy for perception of the "high". The present experiments were conducted to examine dose-occupancy and dose-effect relationships in mice for cocaine and also for methylphenidate, a dopamine uptake blocker used in pediatric psychiatry.

METHODS

Percentage occupancies of the dopamine transporter by cocaine and methylphenidate were estimated after intravenous injection in mice from the displacement of in vivo binding of [(3)H]cocaine from the striatum. Locomotor activity was measured in a photocell apparatus.

RESULTS

The relationship between drug doses (milligrams of hydrochloride salt per kilogram body weight) and percentage occupancy of the dopamine transporter was indistinguishable for cocaine and methylphenidate, and corresponded to about 50% occupancy at 0.25 mg/kg and about 80% at 1 mg/kg. This was similar to the relationship between drug dose and transporter occupancy, previously measured in human and baboons using [(11)C]cocaine or [(11)C]d-threo-methylphenidate and PET. Methylphenidate increased locomotor activity in the mice substantially more than cocaine at the same dose and the same degree of dopamine-transporter receptor occupancy.

CONCLUSIONS

The range of dopamine-transporter occupancy required for behavioral activation in the mice was thus similar to that previously reported for experience of a cocaine- or methylphenidate-induced "high" in human subjects. Our results are consistent with other studies in which both cocaine and methylphenidate were evaluated in animal behavioral assays and were found to have very similar psychopharmacological properties.

Prediction of reinforcing responses to psychostimulants in humans by brain dopamine D2 receptor levels

OBJECTIVE

This study assessed whether brain dopamine D2 receptor levels, which show significant intersubject variability, predict reinforcing responses to psychostimulants in humans.

METHOD

[11C]Raclopride and positron emission tomography were used to measure D2 receptor levels in 23 healthy men (mean age = 34 years, SD = 7) who had no drug abuse histories in order to assess if there were differences between the subjects who liked and those who disliked the effects of intravenous methylphenidate (0.5 mg/kg).

RESULTS

Subjects who liked the effects of methylphenidate had significantly lower D2 receptor levels (mean = 2.72 Bmax/Kd, SD = 0.3) than subjects who disliked its effects (mean = 3.16, SD = 0.3). Moreover, the higher the D2 levels found, the more intense were methylphenidate's unpleasant effects.

CONCLUSIONS

These results provide preliminary evidence that D2 receptor levels predict response to psychostimulants in humans and that low D2 receptors may contribute to psychostimulant abuse by favoring pleasant response.

Positron emission tomography studies of dopamine-enhancing drugs

Although PET is technologically complex because the restricted time scale requires that radioisotope production, radiotracer synthesis, and PET imaging be carried out in the same place, the payoff is that compounds labeled with these isotopes can be used to track the distribution and movement of drugs in the brain and also measure drug effects on specific molecular targets in the human brain. Provided that appropriate radiotracers are available, one can determine the amount of a drug that gets into the brain, the minimum effective dose, the duration of action, or the binding site occupancy required to elicit a particular therapeutic or behavioral effect with a relatively small number of PET studies. Because studies are carried out directly in humans, the relationship of these parameters to behavior and to therapeutic efficacy can be evaluated. The possibilities are enormous and are largely driven by advances in PET technology (including radiotracer chemistry and instrumentation) that synergize with advances in neuropharmacology.

Methylphenidate and cocaine have a similar in vivo potency to block dopamine transporters in the human brain

The reinforcing effects of cocaine and methylphenidate have been linked to their ability to block dopamine transporters (DAT). Though cocaine and methylphenidate have similar in vitro affinities for DAT the abuse of methylphenidate in humans is substantially lower than of cocaine. To test if differences in in vivo potency at the DAT between these two drugs could account for the differences in their abuse liability we compared the levels of DAT occupancies that we had previously reported separately for intravenous methylphenidate in controls and for intravenous cocaine in cocaine abusers. DAT occupancies were measured with Positron Emission Tomography using [11C]cocaine, as a DAT ligand, in 8 normal controls for the methylphenidate study and in 17 active cocaine abusers for the cocaine study. The ratio of the distribution volume of [11C]cocaine in striatum to that in cerebellum, which corresponds to Bmax/Kd +1, was used as measure of DAT availability. Parallel measures were obtained to assess the cardiovascular effects of these two drugs. Methylphenidate and cocaine produced comparable dose-dependent blockade of DAT with an estimated ED50 (dose required to block 50% of the DAT) for methylphenidate of 0.07 mg/kg and for cocaine of 0.13 mg/kg. Both drugs induced similar increases in heart rate and blood pressure but the duration of the effects were significantly longer for methylphenidate than for cocaine. The similar in vivo potencies at the DAT for methylphenidate than for cocaine are in agreement with their reported relative in vitro affinities (Ki 390 nM and 640 nM respectively), which is likely to reflect the similar degree of uptake (8-10% of the injected dose) and regional distribution of these two drugs in the human brain. Thus, differences in the in vivo potency of these two drugs at the DAT cannot be responsible for the differences in their rate of abuse in humans. Other variables i.e. longer duration of methylphenidate's side effects may counterbalance its reinforcing effects.

PET and drug research and development

The use of PET to examine the behavioral, therapeutic and toxic properties of drugs and substances of abuse is emerging as a powerful new scientific tool. PET provides a new perspective on drug research by virtue of its ability to directly assess both pharmacokinetic and pharmacodynamic events in humans and in animals. These parameters can be assessed directly in the human body both in healthy volunteers and in patients. Moreover, the new generation of high-resolution, small-animal cameras hold the promise of introducing imaging in the early stages of drug development and make it possible to carry out longitudinal studies in animals and to study genetically altered animals. This places PET in a unique position to contribute significantly to the process of drug development through understanding the molecular mechanisms underlying drug action while addressing some very practical questions such as determining effective drug doses for clinical trials for new drugs, determining the duration of drug action and examining potential drug interactions.

PET studies of peripheral catechol-O-methyltransferase in non-human primates using [18F]Ro41-0960

We previously reported the results of PET (positron emission tomography) studies of [18F]Ro41-0960, a potent COMT inhibitor, in baboon brain. Here we report an evaluation of the pharmacokinetics and specificity of binding of [18F]Ro41-0960 in the peripheral organs of baboon. We observed a rapid clearance of the tracer from the heart and no significant uptake in the lung. In contrast, there was a high uptake and slow clearance in both kidney and liver, consistent with a high level of COMT in these peripheral organs. We also observed a dose-dependent inhibition of [18F]Ro41-0960 uptake by unlabeled Ro41-0960 (ED50 was 0.5 mg/kg in liver, and <0.01 mg/kg in kidney), with a halftime for recovery of COMT of about 25 h at the dose of 2 mg/kg of unlabeled Ro41-0960. This indicates a reversible tight binding interaction between COMT and Ro41-0960 in both liver and kidney and suggests that [18F]Ro41-0960 may be a useful radiotracer for future examination of the functional activity of COMT in the human body.

Comparable changes in synaptic dopamine induced by methylphenidate and by cocaine in the baboon brain

Though the blockade of dopamine transporters (DAT) is associated with cocaine's and methylphenidate's reinforcing effects, it is the stimulation of dopamine (DA) receptors, achieved by increases in synaptic DA, that enables these effects to occur. Positron emission tomography (PET) and [11C]raclopride were used to assess the levels of occupancy of DA D2 receptors by dopamine achieved by doses of cocaine or methylphenidate previously documented to block over 70% of DAT. Studies were performed in five baboons using a paired scan protocol designed to measure DA D2 receptor availability (Bmax/Kd) at baseline conditions and after intravenous administration of either cocaine or methylphenidate. Cocaine (1-2 mg/kg) or methylphenidate (0.5 mg/kg) administered 5 min prior to [11C]raclopride decreased Bmax/Kd by 29+/-3% and 32 + 4%, respectively. Smaller reductions in Bmax/Kd (13% for cocaine given 30 min before [11C]raclopride and 25+/-10% for methylphenidate given 40 min before [11C]raclopride) were seen with longer periods between drug and radioligand. These observations are consistent with the slower striatal clearance kinetics of [11C]methylphenidate than [1C]cocaine observed in previous PET experiments and with the approximately twofold higher potency of methylphenidate than cocaine in in vitro experiments. Though the elevation of synaptic DA induced by >70% occupancy of DAT by these drugs lead to a modest increase in occupancy of D2 receptors (25-30%), further studies are required to assess if this is an underestimation because of differences in D2 receptor binding kinetics between raclopride and DA.

Blockade of striatal dopamine transporters by intravenous methylphenidate is not sufficient to induce self-reports of "high"

The reinforcing effects of cocaine and methylphenidate have been linked to their ability to block dopamine transporters (DAT). Using positron emission tomography (PET), we previously showed that intravenous cocaine induced a significant level of DAT blockade, which was associated with the intensity for self-reports of "high" in cocaine abusers. In this study, we measured DAT occupancies after intravenous methylphenidate and assessed whether they also were associated with the "high". Occupation of DAT by intravenous MP was measured with PET using [11C]cocaine, as a DAT ligand, in eight normal control subjects tested with different methylphenidate doses. The ratio of the distribution volume of [11C]cocaine in striatum to that in cerebellum, which corresponds to Bmax/Kd + 1, was used as measure of DAT availability. In parallel, self-reports of "high" were measured. Methylphenidate produced a dose-dependent blockade of DAT with an estimated ED50 of 0.075 mg/kg. DAT occupancies were significantly correlated with the "high" (p <.03). However, four of the eight subjects, despite having significant levels of DAT blockade, did not perceive the "high". Methylphenidate is as effective as cocaine in blocking DAT in the human brain (cocaine ED50 = 0.13 mg/kg), and DAT blockade, as for cocaine, was also associated with the "high". However, the fact that there were subjects who despite significant DAT blockade did not experience the "high" suggests that DAT blockade, although necessary, is not sufficient to produce the "high".

Comparative studies of epibatidine derivatives [18F]NFEP and [18F]N-methyl-NFEP: kinetics, nicotine effect, and toxicity

We have previously shown that [18F]norchlorofluoroepibatidine ([18F]NFEP) would be an ideal radiotracer for positron emission tomography (PET) imaging of nicotinic acetylcholine receptors (nAChR); however, its high toxicity is a limiting factor for human studies. We, therefore, synthesized its N-methyl derivative ([18F]N-Me-NFEP) and carried out comparative studies. The distribution volumes for different brain regions were higher for [18F]N-Me-NFEP than those for [18F]NFEP (average: 52.5+/-0.9 vs. 36.4+/-0.7 for thalamus), though the distribution volume (DV) ratios were similar (3.93+/-0.27 vs. 3.65+/-0.19 for thalamus to cerebellum). Treatment with nicotine reduced the binding of both radiotracers. Toxicology studies in awake rats showed that N-methyl-NFEP has a lower mortality (0 vs. 30%) and smaller effect on plasma catecholamines than NFEP at a dose of 1.5 microg/kg. However, marked alterations in cardiorespiratory parameters were observed after injection of N-methyl-NFEP (0.5 microg/kg, IV) to an awake dog. Our results suggest that although the binding characteristics of [18F]NFEP and [18F]N-Me-NFEP appear to be ideally suited for PET imaging studies of the human brain, their relatively small safety margin will limit their use in humans.

Association of methylphenidate-induced craving with changes in right striato-orbitofrontal metabolism in cocaine abusers: implications in addiction

OBJECTIVE

The authors have shown that decreases in dopamine D2 receptors in cocaine abusers were associated with decreased metabolism in the cingulate and prefrontal and orbitofrontal cortices. To assess whether increasing dopamine would reverse these metabolic decrements, they measured the effects of methylphenidate, a drug that increases dopamine, on brain glucose metabolism in 20 cocaine abusers.

METHOD

The subjects underwent two [18F]fluorodeoxyglucose positron emission tomography scans, one after two sequential placebo injections and one after two intravenous doses of methylphenidate. D2 receptors were measured with [11C]raclopride to evaluate their relation to methylphenidate-induced metabolic changes.

RESULTS

Methylphenidate induced variable changes in brain metabolism: subjects with the higher D2 measures tended to increase metabolism, whereas those with the lower D2 measures tended to decrease metabolism. Methylphenidate's effects were significant for increases in metabolism in the superior cingulate, right thalamus, and cerebellum. Methylphenidate-induced changes in the right orbitofrontal cortex and right striatum were associated with craving, and those in the prefrontal cortex were associated with mood.

CONCLUSIONS

Although methylphenidate increased metabolism in the superior cingulate, it only increased metabolism in orbitofrontal or prefrontal cortices in the subjects in whom it enhanced craving and mood, respectively. This indicates that dopamine enhancement is not sufficient per se to increase metabolism in these frontal regions. Activation of the right orbitofrontal cortex and right striatum (brain regions found to be abnormal in compulsive disorders) in the subjects reporting craving may be one of the mechanisms underlying compulsive drug administration in addicted persons. The predominant correlation of craving with right but not left brain regions suggests laterality of reinforcing and/or conditioned responses.

Substrates and inhibitors of human T-cell leukemia virus type I protease

HTLV-I is an oncogenic retrovirus that is associated with adult T-cell leukemia. HTLV-I protease and HTLV-I protease fused to a deca-histidine containing leader peptide (His-protease) have been cloned, expressed, and purified. The refolded proteases were active and exhibited nearly identical enzymatic activities. To begin to characterize the specificity of HTLV-I, we measured protease cleavage of peptide substrates and inhibition by protease inhibitors. HTLV-I protease cleavage of a peptide representing the HTLV-I retroviral processing site P19/24 (APQVLPVMHPHG) yielded Km and kcat values of 470 microM and 0.184 s-1 while cleavage of a peptide representing the processing site P24/15 (KTKVLVVQPK) yielded Km and kcat values of 310 microM and 0.0060 s-1. When the P1' proline of P19/24 was replaced with p-nitro-phenylalanine (Nph), the ability of HTLV-I protease to cleave the substrate (APQVLNphVMHPL) was improved. Inhibition of HTLV-I protease and His-protease by a series of protease inhibitors was also tested. It was found that the Ki values for inhibition of HTLV-I protease and His-protease by a series of pepsin inhibitors ranged from 7 nM to 10 microM, while the Ki values of a series of HIV-1 protease inhibitors ranged from 6 nM to 127 microM. In comparison, the Ki values for inhibition of pepsin by the pepsin inhibitors ranged from 0.72 to 19.2 nM, and the Ki values for inhibition of HIV-1 protease by the HIV protease inhibitors ranged from 0.24 nM to 1.0 microM. The data suggested that the substrate binding site of HTLV-I protease is different from the substrate binding sites of pepsin and HIV-1 protease, and that currently employed HIV-1 protease inhibitors would not be effective for the treatment of HTLV-I infections.

Dopamine transporter occupancies in the human brain induced by therapeutic doses of oral methylphenidate

OBJECTIVE

The therapeutic effects of methylphenidate in the treatment of attention deficit disorder have been attributed to its ability to increase the synaptic concentration of dopamine by blocking the dopamine transporters. However, the levels of dopamine transporter blockade achieved by therapeutic doses of methylphenidate are not known. This study measured, for the first time, dopamine transporter occupancy by orally administered methylphenidate in the human brain and its rate of uptake in the brain.

METHOD

Positron emission tomography (PET) and [11C]cocaine were used to estimate dopamine transporter occupancies after different doses of oral methylphenidate in seven normal subjects (mean age=24 years, SD=7). In addition, the pharmacokinetics of oral methylphenidate were measured in the baboon brain through use of PET and [11C]methylphenidate administered through an orogastric tube.

RESULTS

At 120 minutes after administration, oral methylphenidate produced a dose-dependent blockade of dopamine transporter; means=12% (SD= 4%) for 5 mg, 40% (SD=12%) for 10 mg, 54% (SD=5%) for 20 mg, 72% (SD=3%) for 40 mg, and 74% (SD=2%) for 60 mg. The estimated dose of oral methylphenidate required to block 50% of the dopamine transporter corresponded to 0.25 mg/kg. Oral methylphenidate did not reach peak concentration in brain until 60 minutes after its administration.

CONCLUSIONS

Oral methylphenidate is very effective in blocking dopamine transporters, and at the weight-adjusted doses used therapeutically (0.3 to 0.6 mg/kg), it is likely to occupy more than 50% of the dopamine transporters. The time to reach peak brain uptake for oral methylphenidate in brain corresponds well with the reported time course to reach peak behavioral effects.

Differences in regional brain metabolic responses between single and repeated doses of methylphenidate

Studies investigating the acute effects of drugs of abuse on human brain metabolism have measured single doses whereas these drugs are mostly taken repeatedly. Here we compared the brain metabolic response to intravenous methylphenidate when given after a single dose to that when given after two sequential doses. Methylphenidate-induced changes in metabolism differed; whereas single doses tended to decrease metabolism, repeated doses tended to increase it, and these differences were significant in frontal, parietal and occipital cortices and hippocampus. This indicates that methylphenidate's metabolic effects vary with acute previous exposure and highlights the importance of studying drugs after single and repeated administration.

Parallel loss of presynaptic and postsynaptic dopamine markers in normal aging

Aging of the human brain is associated with a decline in dopamine (DA) function, generally interpreted as reflecting DA cell loss. Positron emission tomography studies revealed that in healthy individuals, the age-related losses in DA transporters (presynaptic marker) were associated with losses in D2 receptors (postsynaptic marker) rather than with increases as is known to occur with DA cell loss. This association was specific for DA synaptic markers, because they were not correlated with striatal metabolism. Furthermore, the association was independent of age, suggesting that a common mechanism regulates the expression of receptors and transporters irrespective of age.

In vitro and ex vivo autoradiographic studies of nicotinic acetylcholine receptors using [18F]fluoronochloroepibatidine in rodent and human brain

A fluorine-18-labeled analog of the potent nicotinic agonist epibatidine is a candidate radioligand for positron emission tomographic (PET) studies of nicotinic acetylcholine receptors (nAcChR). Following intravenous administration of [18F]exo-2-(2'-fluoro-5'-pyridinyl)-7-azabicyclo[2.2.1]heptane (NFEP), high uptake in thalamus was visualized in sections of mouse and rat brain by autoradiography using a phosphor imaging device. Binding of [18F]NFEP to rat thalamic homogenate was consistent with a single class of binding site with a Kd value of 71 pM. In vitro autoradiography of thaw-mounted sections of human thalamus revealed a heterogeneous pattern of binding; Bmax values for ventrolateral nucleus, insular cortex and dorsomedial nucleus, and internal capsule were 20, 8, and 3 pmol/cc of tissue, respectively. However, similar Kd values close to 50 pM were calculated for all regions. These studies support the suitability of [18F]NFEP as a radioligand for PET studies of nAcChR in the living human brain.