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 [³H]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 [¹¹C]cocaine or [¹¹C]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. Received: 17 February 1999 / Final version: 17 April 1999     

Ketamine does not decrease striatal dopamine D2 receptor binding in man

Abstract Rationale. A glutamate–dopamine interaction has been implicated in the psychosis-like effects of glutamate N-methyl-D-aspartate (NMDA) receptor antagonists, such as phencyclidine and ketamine. However, recent imaging studies addressing striatal glutamate–dopamine interaction directly in vivo in man have been controversial. Objectives. To examine whether the NMDA receptor antagonist ketamine in high subanesthetic concentrations decreases striatal [¹¹C]raclopride binding potential in man. To further evaluate whether changes in striatal [¹¹C]raclopride binding are associated with ketamine-induced behavioral effects. Methods. The effect of computer-driven subanesthetic ketamine infusion on striatal dopamine release was studied in healthy male subjects using a controlled study design. Dopamine release was studied using positron emission tomography and the [¹¹C]raclopride displacement paradigm. A conventional region of interest-based analysis and voxel-based analysis were applied to the positron emission tomography data. Results. The average plasma ketamine concentration was 293±29 ng/ml. Ketamine did not alter striatal [¹¹C]raclopride binding. Ketamine induced typical behavioral effects, such as hallucinations but there was no correlation between these effects and displacement of [¹¹C]raclopride binding. Conclusions. This controlled study indicates that ketamine does not decrease striatal [¹¹C]raclopride binding. Striatal dopamine release is of minor importance in the psychosis-like effects of ketamine. Electronic Publication     

Manganese induced brain lesions inMacaca fascicularis as revealed by positron emission tomography and magnetic resonance imaging

A series of positron emission tomography scans was made on two monkeys during a 16-month period when they received manganese(IV)oxide by subcutaneous injection. The distribution of [¹¹C]-nomifensine uptake, indicating dopamine terminals, was followed in both monkey brains. The brain distributions of [¹¹C]-raclopride, demonstrating D₂ dopamine receptors, and [¹¹C]-l-dopa, as a marker of dopamine turnover, were followed in one monkey each. The monkeys developed signs of poisoning namely unsteady gait and hypoactivity. The [¹¹C]-nomifensine uptake in the striatum was reduced with time and reached a 60% reduction after 16 months exposure. This supports the suggestion that dopaminergic nerve endings degenerate during manganese intoxication. The [¹¹C]-l-dopa decarboxylation was not significantly altered indicating a sparing of [¹¹C]-l-dopa decarboxylation during manganese poisoning. A transient decrease of [¹¹C]-raclopride binding occurred but at the end of the study D₂-receptor binding had returned to starting values. The magnetic resonance imaging (MRI) revealed that the manganese accumulated in the globus pallidus, putamen and caudate nucleus. There were also suggestions of gliosis/edema in the posterior limb of the internal capsule. MRI might be useful to follow manganese intoxication in humans as long as the scan is made within a few months of exposure to manganese, i. e. before a reversal of the manganese accumulation.     

Long-Term Exposure to Oral Methylphenidate or dl-Amphetamine Mixture in Peri-Adolescent Rhesus Monkeys: Effects on Physiology,Behavior, and Dopamine System Development

The stimulants methylphenidate and amphetamine are used to treat children with attention deficit/hyperactivity disorder over important developmental periods, prompting concerns regarding possible long-term health impact. This study assessed the effects of such a regimen in male, peri-adolescent rhesus monkeys on a variety of cognitive/behavioral, physiological, and in vivo neurochemical imaging parameters. Twice daily (0900 and 1200 hours), for a total of 18 months, juvenile male monkeys (8 per group) consumed either an unadulterated orange-flavored solution, a methylphenidate solution, or a dl-amphetamine mixture. Doses were titrated to reach blood/plasma levels comparable to therapeutic levels in children. [¹¹C]MPH and [¹¹C]raclopride dynamic PET scans were performed to image dopamine transporter and D₂-like receptors, respectively. Binding potential (BP_ND), an index of tracer-specific binding, and amphetamine-induced changes in BP_ND of [¹¹C]raclopride were estimated by kinetic modeling. There were no consistent differences among groups on the vast majority of measures, including cognitive (psychomotor speed, timing, inhibitory control, cognitive flexibility), general activity, physiological (body weight, head circumference, crown-to-rump length), and neurochemical (ie, developmental changes in dopamine transporter, dopamine D₂ receptor density, and amphetamine-stimulated dopamine release were as expected). Cytogenetic studies indicated that neither drug was a clastogen in rhesus monkeys. Thus, methylphenidate and amphetamine at therapeutic blood/plasma levels during peri-adolescence in non-human primates have little effect on physiological or behavioral/cognitive development.     

Synthesis and in vivo evaluation of [O‐methyl‐11C] 2‐(4‐methoxyphenyl)‐N‐(4‐methylbenzyl)‐N‐(1‐methyl‐ piperidin‐4‐yl)acetamide as an imaging probe for 5‐HT2A receptors

2‐(4‐Methoxyphenyl)‐N‐(4‐methylbenzyl)‐N‐(1‐methylpiperidin‐4‐yl)acetamide (AC90179, 4 ), a highly potent and selective competitive 5‐HT_2A antagonist, was labeled by [¹¹C]‐methylation of the corresponding desmethyl analogue 5 with [¹¹C]methyl triflate. The precursor molecule 5 for radiolabeling was synthesized from p‐tolylmethylamine in three steps with 46% overall yield. [¹¹C]AC90179 was synthesized in 30 min (30 ± 5% yield, EOS) with a specific activity of 4500 ± 500 Ci/mmol and >99% chemical and radiochemical purities. Positron emission tomography studies in anesthetized baboon revealed that [¹¹C] 4 Penetrates the blood–brain barrier (BBB) with a rapid influx and efflux of the tracer in all brain regions. Due to lack of tracer retention or specific binding, [¹¹C] 4 cannot be used as PET ligand for imaging 5‐HT_2A receptors. Copyright © 2006 John Wiley & Sons, Ltd.     

Methylphenidate and its ethanol transesterification metabolite ethylphenidate: brain disposition, monoamine transporters and motor activity

Ethylphenidate is formed by metabolic transesterification of methylphenidate and ethanol. Study objectives were to (a) establish that ethylphenidate is formed in C57BL/6 (B6) mice; (b) compare the stimulatory effects of ethylphenidate and methylphenidate enantiomers; (c) determine methylphenidate and ethylphenidate plasma and brain distribution and (d) establish in-vitro effects of methylphenidate and ethylphenidate on monoamine transporter systems. Experimental results were that: (a) coadministration of ethanol with the separate methylphenidate isomers enantioselectively produced l-ethylphenidate; (b) d and dl-forms of methylphenidate and ethylphenidate produced dose-responsive increases in motor activity with stimulation being less for ethylphenidate; (c) plasma and whole-brain concentrations were greater for ethylphenidate than methylphenidate and (d) d and DL-methylphenidate and ethylphenidate exhibited comparably potent low inhibition of the dopamine transporter, whereas ethylphenidate was a less potent norepinephrine transporter inhibitor. These experiments establish the feasibility of the B6 mouse model for examining the interactive effects of ethanol and methylphenidate. As reported for humans, concurrent exposure of B6 mice to methylphenidate and ethanol more readily formed l-ethylphenidate than d-ethylphenidate, and the l-isomers of both methylphenidate and ethylphenidate were biologically inactive. The observed reduced stimulatory effect of d-ethylphenidate relative to d-methylphenidate appears not to be the result of brain dispositional factors, but rather may be related to its reduced inhibition of the norepinephrine transporter, perhaps altering the interaction of dopaminergic and noradrenergic neural systems.     

“In-loop” carbonylation—A simplified method for carbon-11 labelling of drugs and radioligands

Transition-metal mediated carbonylation with ¹¹C-labelled carbon monoxide ([¹¹C]CO) is a versatile method for introducing ¹¹C (t_1/2 = 20.3 min) into drugs and radioligands for subsequent use in positron emission tomography (PET). The aim of the current study was to perform the ¹¹C-carbonylation reaction on the interior surface of a stainless-steel loop used for high performance liquid chromatography (HPLC). In the experimental setup, cyclotron produced ¹¹C-labelled carbon dioxide ([¹¹C]CO₂) was converted to [¹¹C]CO by reduction over heated Molybdenum and swept into an HPLC loop pre-charged with the appropriate reaction mixture. Following a 5 min reaction, the radiochemical purity (RCP) and the trapping efficiency (TE) of the reaction mixture was determined. After optimization, [¹¹C]N-Benzylbenzamide was obtained in quantitative radiochemical yield (RCY) following a 5 min reaction at room temperature. The methodology was further applied to label [¹¹C]benzoic acid (RCP≥99%, TE>91%), [¹¹C]methyl benzoate (RCP≥99%, TE>93%) and [¹¹C]phthalide (RCP≥99%, TE>88%). A set of pharmaceuticals was finally radiolabelled using non-optimized conditions. Excellent yields were obtained for the histamine-3 receptor radioligand [¹¹C]AZ13198083, the oncology drug [¹¹C]olaparib and the dopamine D2 receptor radioligand [¹¹C]raclopride, whereas a moderate yield was observed for the high-affinity dopamine D2 receptor radioligand [¹¹C]FLB457. The presented “in-loop” process proved efficient for diverse ¹¹C-carbonylations, providing [¹¹C]amides, [¹¹C]esters and [¹¹C]carboxylic acids in moderate to excellent RCYs. Based on the advantages associated with performing the radiolabelling step as an integrated part of the purification system, this methodology may become a valuable addition to the toolbox of methodologies used for ¹¹C-carbonylation of drugs and radioligands for PET.     

Cortical glutamate–dopamine interaction and ketamine-induced psychotic symptoms in man

Rationale The noncompetitive glutamate N-methyl-d-aspartate receptor antagonist ketamine induces transient psychotic symptoms in man. Involvement of dopaminergic mechanisms in these effects has been suggested. Objectives The purpose of this article is to study the effects of ketamine on extrastriatal dopamine receptor availability in healthy subjects and extracellular dopamine levels in rat cortex. Materials and methods The effect of computer-driven subanesthetic ketamine infusion on cortical dopamine release was studied in healthy male subjects using a controlled study design. Dopamine D2/D3 receptor availability was quantified using positron emission tomography (PET) and [¹¹C]FLB 457. A conventional region of interest-based analysis and voxel-based analysis was applied to the PET data. The ketamine-induced cortical dopamine release in rats was studied using in vivo microdialysis. Results Ketamine infusion reduced significantly the [¹¹C]FLB 457 binding potential (BP) in the posterior cingulate/retrosplenial cortices, suggestive of increased dopamine release. This brain imaging finding was further supported by a microdialysis experiment in rats showing that ketamine increased the extracellular dopamine concentration by up to 200% in the retrosplenial cortex. Ketamine-induced psychotic symptoms were associated with changes in the [¹¹C]FLB 457 BP in the dorsolateral prefrontal and anterior cingulate cortices. Conclusions Our results suggest that cortical dopaminergic mechanisms have a role in the emergence of ketamine-induced psychosis-like symptoms in man. The glutamate–dopamine interaction in the posterior cingulate during ketamine infusion is well in line with the recent functional and structural imaging studies suggesting involvement of this cortical area in the development of schizophrenic psychosis.     

6‐[123I]Iodo‐2‐[[4‐(2‐methoxyphenyl) piperazin‐1‐yl]methyl]imidazo[1,2‐a]pyridine as potential SPECT agent for imaging dopamine D4 receptor: synthesis and in vivo evaluation in a nonhuman primate

The dopamine D₄ receptor subtype has drawn considerable attention due to its potential implication in schizophrenia and erectile dysfunction. 6‐[¹²³I]Iodo‐2‐[[4‐(2‐methoxyphenyl)piperazin‐1‐yl]methyl]imidazo[1,2‐a]pyridine [¹²³I]‐1, a potent and selective new dopamine D₄ agonist, was synthesized by classical iododestannylation using sodium [¹²³I]iodide and chloramine‐T. Radiolabeling yield varied from 34 to 38% with a radiochemical purity exceeding 99%. Despite a good in vitro profile, in vivo evaluation of this radioligand in baboon showed no brain uptake after i.v. injection. Copyright © 2008 John Wiley & Sons, Ltd.     

Estimating Endogenous Dopamine Levels at D2 and D3 Receptors in Humans using the Agonist Radiotracer [11C]-(+)-PHNO

Using positron emission tomography (PET) and an acute dopamine depletion challenge it is possible to estimate endogenous dopamine levels occupying dopamine D_2/3 receptors (D_2/3R) in humans in vivo. Our group has developed [¹¹C]-(+)-PHNO, the first agonist radiotracer with preferential in vivo affinity for D₃R. Thus, the use of [¹¹C]-(+)-PHNO offers the novel possibility of (i) estimating in vivo endogenous dopamine levels at D_2/3R using an agonist radiotracer, and (ii) estimating endogenous dopamine levels at D₃R in extrastriatal regions such as the substantia nigra, hypothalamus, and ventral pallidum. Ten healthy participants underwent a [¹¹C]-(+)-PHNO PET scan under baseline conditions and another under acute endogenous dopamine depletion achieved via oral administration of alpha-methyl-para-tyrosine (64 mg/kg). [¹¹C]-(+)-PHNO binding was sensitive to acute dopamine depletion, allowing in vivo estimates of endogenous dopamine in D₂R-rich regions (caudate and putamen), mixed D_2/3R-rich regions (ventral striatum and globus pallidus), and extrastriatal D₃R-rich regions (hypothalamus and ventral pallidum). Dopamine depletion decreased self-reported vigor, which was correlated with the reduction in dopamine levels in the globus pallidus. [¹¹C]-(+)-PHNO is a suitable radiotracer for use in estimating endogenous dopamine levels at D₂R and D₃R in neuropsychiatric populations.     

A rapid one‐step radiosynthesis of [11C]‐d‐threo‐methylphenidate

Carbon‐11 labelled d‐threo‐methylphenidate ([¹¹C]‐(1)), a radiopharmaceutical commonly used to image the dopamine transporter, has been labelled in one step using [¹¹C]‐methyl triflate. No protection of the nitrogen atom on the piperidine ring of d‐threo‐ritalinic acid·HCl was required, as O‐methylation was favored over N‐methylation by performing the methylation in buffered solutions which effectively protonate and protect the amine functionality. The reaction was effectively carried out at room temperature in solution by captive solvent ‘LOOP’ methods or using conventional glass vials. Copyright © 2010 John Wiley & Sons, Ltd.     

Cardiovascular effects of methylphenidate in humans are associated with increases of dopamine in brain and of epinephrine in plasma

Rationale. The cardiovascular effects of psychostimulant drugs (methylphenidate, amphetamine, cocaine) have been mostly associated with their noradrenergic effects. However, there is some evidence that dopaminergic effects are involved in the cardiovascular actions of these drugs. Here, we evaluated this association in humans. Methods. Positron emission tomography (PET) and [¹¹C]raclopride, a dopamine (DA) D2 receptor radioligand that competes with endogenous DA for occupancy of the D2 receptors, were used to measure changes in brain DA after different doses of intravenous methylphenidate in 14 healthy subjects. Cardiovascular (heart rate and blood pressure) and catecholamine (plasma epinephrine and norepineprhine) responses were determined in parallel to assess their relationships to methylphenidate-induced changes in brain DA. Results. Methylphenidate administration significantly increased heart rate, systolic and diastolic blood pressures and epinephrine concentration in plasma. The increases in blood pressure were significantly correlated with methylphenidate-induced increases of DA in striatum (r>0.78, P<0.001) and of plasma epinephrine levels (r>0.82, P<0.0005). In turn methylphenidate-induced DA increases in striatum were correlated with increases of epinephrine in plasma (r=0.85, P<0.0001). Subjects in whom methylphenidate did not increase DA had no change in blood pressure or in plasma epinephrine concentration. Discussion. These results are consistent with the hypothesis that methylphenidate-induced increases in blood pressure are in part due to its central dopaminergic effects. They also suggest that methylphenidate's pressor effects may be in part mediated by DA-induced increases in peripheral epinephrine. Electronic Publication     

Effects of phencyclidine on [3H]catecholamine and [3H]serotonin uptake in synaptosomal preparations from rat brain

Phencyclidine inhibited uptake in vitro of [³H]norepinephrine (ic₅₀ 0.52 μM), [³H]dopamine (ic₅₀ 0.73 μM) and [³H]serotonin (ic₅₀ 0.80 μM) in crude synaptosomal preparations from rat brain through a competitive mechanism. Phencyclidine was fairly similar in potency to d-amphetamine and methylphenidate in inhibiting catecholamine uptake but was 8 times more potent than d-amphetamine and 34 times more potent than methylphenidate in inhibiting [³H]serotonin uptake.     

N-ethylmaleimide irreversibly inhibits the binding of [3H]threo-(+-)-methylphenidate to the stimulant recognition site

N-Ethylmaleimide, a nonspecific protein modifier which reacts selectively with the sulfhydryl group of cysteinyl residues under controlled conditions, irreversibly inhibited the binding of [3H]threo-(+/-)-methylphenidate to a subset of stimulant binding sites in striatal tissue membranes from the rat in vitro. The inhibition was marked by a decrease in the Bmax of binding of the radiolabelled stimulant drug, while the KD remained unchanged. Pretreatment with excess unlabelled methylphenidate afforded complete protection from inactivation of the binding site by N-ethylmaleimide. Uptake of [3H]dopamine into striatal synaptosomes was likewise reduced after treatment with N-ethylmaleimide; pretreatment with large concentrations of methylphenidate provided partial protection from inactivation of transport. These findings suggest that the stimulant recognition site on the dopamine transport complex contains one or more cysteinyl residues.     

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 of¹¹C 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.     

Gas phase production of [11C]methyl iodide‐d3. Synthesis and biological evaluation of S‐[N‐methyl‐11C]citalopram and deuterated analogues

Three ¹¹C‐labelled tracers for the serotonin reuptake site, S‐[N‐methyl‐¹¹C]citalopram ( [¹¹C]‐4 ), S‐[N‐methyl‐d₃‐¹¹C]citalopram ( [¹¹C]‐12 ), and S‐[N‐methyl‐¹¹C]citalopram‐α,α‐d₂ ( [¹¹C]‐13 ) were synthesized and the distribution of radioactivity after injection of radioligand was examined ex vivo in rats. The deuterated analogue of (S)‐desmethylcitalopram, (S)‐1‐(4‐fluorophenyl)‐1‐(3‐methylamino‐[3‐d₂]‐propyl)‐1,3‐di‐hydro‐isobenzofuran‐5‐carbonitrile ( 11 ), was synthesized in a multi‐step synthesis from escitalopram ( 4 ) and used as precursor in the synthesis of [¹¹C]‐13 . In analogy with the reported gas phase synthesis of [ ¹¹ C]methyl iodide the first gas phase synthesis of [¹¹C]Methyl iodide‐d₃ is reported. The ¹H/²H kinetic isotope effect related to the synthesized compounds were investigated in ex vivo rat studies, where the brain regions of interest to cerebellum ratios of the tracers [¹¹C]‐4 , [¹¹C]‐12 and [¹¹C]‐13 were compared. The ex vivo data indicated no significant differences in binding in any of the investigated brain regions after injection of the three tracers. Copyright © 2004 John Wiley & Sons, Ltd.     

Synthesis and evaluation of [O‐methyl‐11C]4‐[3‐[4‐(2‐methoxyphenyl)‐ piperazin‐1‐yl]propoxy]‐4‐aza‐tricyclo[5.2.1.02,6]dec‐8‐ene‐3,5‐dione as a 5‐HT1A receptor agonist PET ligand

4‐[3‐[4‐(2‐Methoxyphenyl)piperazin‐1‐yl]propoxy]‐4‐aza‐tricyclo[5.2.1.02,6]dec‐8‐ene‐3,5‐dione (4), a potent and selective 5‐HT_1A agonist, was labeled by ¹¹C‐methylation of the corresponding desmethyl analogue 3 with ¹¹C‐methyl triflate. The precursor molecule 3 was synthesized from commercially available endo‐N‐hydroxy‐5‐norbornene‐2,3‐dicarboximide in two steps with an overall yield of 40%. Radiosynthesis of ¹¹C‐4 was achieved in 35 min in 20±5% yield (n=6) at the end of synthesis with a specific activity of 2600±250 Ci/mmol. In vivo positron emission tomography (PET) studies in baboon revealed rapid uptake of the tracer into the brain. However, lack of specific binding indicates that ¹¹C‐4 is not useful as a 5‐HT_1A agonist PET ligand for clinical studies. Copyright © 2008 John Wiley & Sons, Ltd.     

Improved synthesis and application of [11C]benzyl iodide in positron emission tomography radiotracer production

Positron emission tomography has increased the demand for new carbon‐11 radiolabeled tracers and building blocks. A promising radiolabeling synthon is [¹¹C]benzyl iodide ([¹¹C]BnI), because the benzyl group is a widely present functionality in biologically active compounds. Unfortunately, synthesis of [¹¹C]BnI has received little attention, resulting in limited application. Therefore, we investigated the synthesis in order to significantly improve, automate, and apply it for labeling of the dopamine D₂ antagonist [¹¹C]clebopride as a proof of concept. [¹¹C]BnI was synthesized from [¹¹C]CO₂ via a Grignard reaction and purified prior the reaction with desbenzyl clebopride. According to a one‐pot procedure, [¹¹C]BnI was synthesized in 11 min from [¹¹C]CO₂ with high yield, purity, and specific activity, 52 ± 3% (end of the cyclotron bombardment), 95 ± 3%, and 123 ± 17 GBq/µmol (end of the synthesis), respectively. Changes in the [¹¹C]BnI synthesis are reduced amounts of reagents, a lower temperature in the Grignard reaction, and the introduction of a solid‐phase intermediate purification. [¹¹C]Clebopride was synthesized within 28 min from [¹¹C]CO₂ in an isolated decay‐corrected yield of 11 ± 3% (end of the cyclotron bombardment) with a purity of >98% and specific activity (SA) of 54 ± 4 GBq/µmol (n = 3) at the end of the synthesis. Conversion of [¹¹C]BnI to product was 82 ± 11%. The reliable synthesis of [¹¹C]BnI allows the broad application of this synthon in positron emission tomography radiopharmaceutical development. Copyright © 2015 John Wiley & Sons, Ltd.     

Positron emission tomographic measure of brain dopamine dependence to nicotine as a model of drugs of abuse

Rationale  Nicotine/tobacco are prototypic substances used throughout the world. Nicotine abstinence produces some depressive-like effects which are treated by the dopamine (DA) and norepinephrine reuptake inhibitor bupropion. A quantitative measure of the regional brain utilization of these catecholamines (CA) during nicotine dependence and withdrawal is important. Objective  The aim of this study was to prove that regional brain DA utilization by nicotine can be quantified by positron emission tomography (PET) using l-[β-¹¹C]DOPA. Materials and methods  Eight young Macaca mulatta monkeys were given 0.9% NaCl or nicotine in doses of 32 or 100 μg/kg i.m. bid for 9 days to produce minimal dependence. On the tenth day, PET measurements were repeated before and after i.v. nicotine administration. PET studies were done in habituated, trained, and fully conscious animals. Results  Compared to a 0.9% NaCl control, acute i.v. nicotine as a bolus plus infusion for 30 min in similar doses to maintain a steady-state level for 30 min did not affect the utilization rate constant (k ₃) in dorsal or ventral striatum as measured by l-[β-¹¹C]DOPA. When monkeys were given nicotine bid repeatedly after overnight nicotine abstinence, CA utilization was reduced. A subsequent nicotine dose normalized utilization to slightly above control levels. Changes in ventral striatum were similar to those in dorsal striatum. The reduced rate of utilization demonstrated with l-[β-¹¹C]DOPA after overnight nicotine abstinence and its reversal by nicotine the next day provides an important PET measure of brain nicotine dependence and withdrawal. This method can be applied to other substances of abuse that release DA. Research was done at the Central Research Laboratory, Hamamatsu Photonics K.K.     

Involvement of Recognition and Interaction of Carnitine Transporter in the Decrease of l-Carnitine Concentration Induced by Pivalic Acid and Valproic Acid

Purpose Prodrugs with pivalic acid and valproic acid decrease l-carnitine concentration in plasma and tissues by urinary excretion of acylcarnitine as pivaloylcarnitine (PC) and valproylcarnitine (VC), respectively. We investigated the role of the Na⁺/l-carnitine cotransporter in the porcine kidney epithelial cell line, LLC-PK₁ for the decrease of l-carnitine concentration.Methods The uptake of l-[³H]carnitine, acetyl-l-[³H]carnitine (AC), l-[³H]PC and l-[³H]VC were investigated in LLC-PK₁ cells seeded in a 6-well culture plate.Results l-Carnitine and AC uptake in LLC-PK₁ cells exhibited Na⁺ dependency. The K _m values for l-carnitine and AC uptake were 11.0 and 8.18 μM, respectively. These results indicated expression of Na⁺/l-carnitine cotransporter in LLC-PK₁ cells. PC and VC inhibited Na⁺/l-carnitine cotransporter in the competitive (K _i = 90.4 μM) and noncompetitive (K _i = 41.6 μM) manners, respectively. PC and VC uptake by Na⁺/l-carnitine cotransporter were not observed in LLC-PK₁ cells.Conclusions These data suggested that PC and VC formed in the body could not be reabsorbed in the kidney, resulting in the decrease of l-carnitine concentration. In addition, inhibition of l-carnitine reabsorption by VC with lower K _i value could induce the decrease of l-carnitine concentration. Collectively, the recognition and interaction of Na⁺/l-carnitine cotransporter are important factors for carnitine homeostasis.