Relationship of estrogen synthesis capacity in the brain with obesity and self-control in men and women.

Gonadal hormones are linked to mechanisms that govern appetitive behavior and its suppression. Estrogens are synthesized from androgens by the enzyme aromatase, highly expressed in the ovaries of reproductive-aged women and in the brains of men and women of all ages. We measured aromatase availability in the amygdala using positron emission tomography (PET) with the aromatase inhibitor [11C]vorozole in a sample of 43 adult, normal-weight, overweight, or obese men and women. A subsample of 27 also completed personality measures to examine the relationship between aromatase and personality traits related to self-regulation and inhibitory control. Results indicated that aromatase availability in the amygdala was negatively associated with body mass index (BMI) (in kilograms per square meter) and positively correlated with scores of the personality trait constraint independent of sex or age. Individual variations in the brain's capacity to synthesize estrogen may influence the risk of obesity and self-control in men and women.

It's Not What You Take Up, It's What You Keep: How Discoveries from Diverse Disciplines Directed the Development of the FDG PET/CT Scan.

Although imaging glucose metabolism with positron emission tomography combined with X-ray CT (FDG-PET/CT) has become a standard diagnostic modality for the discovery and surveillance of malignant tumors and inflammatory processes, its origins extend back to more than a century of notable discoveries in the fields of inorganic and organic chemistry, nuclear physics, mathematics, biochemistry, solute transport physiology, metabolism, and imaging, accomplished by pioneering and driven investigators, of whom at least ten were recipients of the Nobel Prize. These tangled and diverse roots eventually coalesced into the FDG-PET/CT method, that through its many favorable characteristics inherent in the isotope used (18F), the accurate imaging derived from coincidence detection of positron annihilation radiation combined with computed tomography, and the metabolic trapping of 2-deoxy-2-[18F]fluoro-D-glucose (FDG) in tissues, provides safety, sensitivity, and specificity for tumor and inflammation detection. The authors hope that this article will increase the appreciation among its readers of the insight, creativity, persistence, and drive of the many investigators who made this technique possible. This article is followed by a review of the many applications of FDG-PET/CT to the gastrointestinal tract and hepatobiliary system (Mandelkern in Dig Dis Sci 2022).

Inhibition of food craving is a metabolically active process in the brain in obese men.

OBJECTIVE: Obesity is associated with impaired inhibitory control over food intake. We hypothesized that the neural circuitry underlying inhibition of food craving would be impaired in obesity. Here we assessed whether obese men show altered brain responses during attempted cognitive inhibition of craving when exposed to food cues. METHODS: Sixteen obese men (32 ± 8.7 years old, BMI = 38.6 ± 7.2) were compared with 11 age-matched non-obese men (BMI 24.2 ± 2.5) using PET and FDG. Brain glucose metabolism was evaluated in a food deprived state: no food stimulation, food stimulation with no inhibition (NI), and food stimulation with attempted inhibition (AI), each on a separate day. Individualized favorite food items were presented prior to and after FDG injection for 40 min. For AI, participants were asked to attempt to inhibit their desire for the food presented. Self-reports for hunger and food desire were recorded. RESULTS: Food stimulation compared with no stimulation increased glucose metabolism in inferior and superior frontal gyrus, default mode network and cerebellum, in both groups. For both groups, AI compared with NI-suppressed metabolism in right subgenual anterior cingulate, orbitofrontal areas, bilateral insula, and temporal gyri. There was a stimulation-by-group interaction effect in obese (but not in non-obese) men showing increased metabolism in pregenual anterior cingulate cortex (pgACC) and caudate during AI relative to NI. Changes in the food desire from NI to AI correlated negatively with changes in metabolism in pgACC/caudate in obese but not in non-obese men. CONCLUSIONS: Obese men showed higher activation in pgACC/caudate, which are regions involved with self-regulation and emotion/reward during AI. Behavioral associations suggest that successful AI is an active process requiring more energy in obese but not in non-obese men. The additional required effort to increase cognitive control in response to food stimulation in obese compared with non-obese men may contribute to their uncontrolled eating behavior.

New Repeat Polymorphism in the AKT1 Gene Predicts Striatal Dopamine D2/D3 Receptor Availability and Stimulant-Induced Dopamine Release in the Healthy Human Brain.

The role of the protein kinase Akt1 in dopamine neurotransmission is well recognized and has been implicated in schizophrenia and psychosis. However, the extent to which variants in the AKT1 gene influence dopamine neurotransmission is not well understood. Here we investigated the effect of a newly characterized variant number tandem repeat (VNTR) polymorphism in AKT1 [major alleles: L- (eight repeats) and H- (nine repeats)] on striatal dopamine D2/D3 receptor (DRD2) availability and on dopamine release in healthy volunteers. We used PET and [11C]raclopride to assess baseline DRD2 availability in 91 participants. In 54 of these participants, we also measured intravenous methylphenidate-induced dopamine release to measure dopamine release. Dopamine release was quantified as the difference in specific binding of [11C]raclopride (nondisplaceable binding potential) between baseline values and values following methylphenidate injection. There was an effect of AKT1 genotype on DRD2 availability at baseline for the caudate (F(2,90) = 8.2, p = 0.001) and putamen (F(2,90) = 6.6, p = 0.002), but not the ventral striatum (p = 0.3). For the caudate and putamen, LL showed higher DRD2 availability than HH; HL were in between. There was also a significant effect of AKT1 genotype on dopamine increases in the ventral striatum (F(2,53) = 5.3, p = 0.009), with increases being stronger in HH > HL > LL. However, no dopamine increases were observed in the caudate (p = 0.1) or putamen (p = 0.8) following methylphenidate injection. Our results provide evidence that the AKT1 gene modulates both striatal DRD2 availability and dopamine release in the human brain, which could account for its association with schizophrenia and psychosis. The clinical relevance of the newly characterized AKT1 VNTR merits investigation.SIGNIFICANCE STATEMENT The AKT1 gene has been implicated in schizophrenia and psychosis. This association is likely to reflect modulation of dopamine signaling by Akt1 kinase since striatal dopamine hyperstimulation is associated with psychosis and schizophrenia. Here, using PET with [11C]raclopride, we identified in the AKT1 gene a new variable number tandem repeat (VNTR) marker associated with baseline striatal dopamine D2/D3 receptor availability and with methylphenidate-induced striatal dopamine increases in healthy volunteers. Our results confirm the involvement of the AKT1 gene in modulating striatal dopamine signaling in the human brain. Future studies are needed to assess the association of this new VNTR AKT1 variant in schizophrenia and drug-induced psychoses.

Synthesis of l-[4-11 C]Asparagine by Ring-Opening Nucleophilic 11 C-Cyanation Reaction of a Chiral Cyclic Sulfamidate Precursor.

The development of a convenient and rapid method to synthesize radiolabeled, enantiomerically pure amino acids (AAs) as potential positron emission tomography (PET) imaging agents for mapping various biochemical transformations in living organisms remains a challenge. This is especially true for the synthesis of carbon-11-labeled AAs given the short half-life of carbon-11 (11 C, t1/2 =20.4 min). A facile synthetic pathway to prepare enantiomerically pure 11 C-labeled l-asparagine was developed using a partially protected serine as a starting material with a four-step transformation providing a chiral five-membered cyclic sulfamidate as the radiolabeling precursor. Its structure and absolute configuration were confirmed by X-ray crystallography. Utilizing a [11 C]cyanide nucleophilic ring opening reaction followed by selective acidic hydrolysis and deprotection, enantiomerically pure l-[4-11 C]asparagine was synthesized. Further optimization of reaction parameters, including base, metal ion source, solvent, acid component, reaction temperature and reaction time, a reliable two-step method for synthesizing l-[4-11 C]asparagine was presented: within a 45±3 min (n=5, from end-of-bombardment), the desired enantiomerically pure product was synthesized with the initial nucleophilic cyanation yield of 69±4 % (n=5) and overall two-step radiochemical yield of 53±2 % (n=5) based on starting [11 C]HCN, and with radiochemical purity of 96±2 % (n=5).

Dynamic Precision Phenotyping Reveals Mechanism of Crop Tolerance to Root Herbivory.

The western corn rootworm (WCR; Diabrotica virgifera virgifera LeConte) is a major pest of maize (Zea mays) that is well adapted to most crop management strategies. Breeding for tolerance is a promising alternative to combat WCR but is currently constrained by a lack of physiological understanding and phenotyping tools. We developed dynamic precision phenotyping approaches using 11C with positron emission tomography, root autoradiography, and radiometabolite flux analysis to understand maize tolerance to WCR Our results reveal that WCR attack induces specific patterns of lateral root growth that are associated with a shift in auxin biosynthesis from indole-3-pyruvic acid to indole-3-acetonitrile. WCR attack also increases transport of newly synthesized amino acids to the roots, including the accumulation of Gln. Finally, the regrowth zones of WCR-attacked roots show an increase in Gln turnover, which strongly correlates with the induction of indole-3-acetonitrile-dependent auxin biosynthesis. In summary, our findings identify local changes in the auxin biosynthesis flux network as a promising marker for induced WCR tolerance.

Decreased dopamine brain reactivity in marijuana abusers is associated with negative emotionality and addiction severity.

Moves to legalize marijuana highlight the urgency to investigate effects of chronic marijuana in the human brain. Here, we challenged 48 participants (24 controls and 24 marijuana abusers) with methylphenidate (MP), a drug that elevates extracellular dopamine (DA) as a surrogate for probing the reactivity of the brain to DA stimulation. We compared the subjective, cardiovascular, and brain DA responses (measured with PET and [(11)C]raclopride) to MP between controls and marijuana abusers. Although baseline (placebo) measures of striatal DA D2 receptor availability did not differ between groups, the marijuana abusers showed markedly blunted responses when challenged with MP. Specifically, compared with controls, marijuana abusers had significantly attenuated behavioral ("self-reports" for high, drug effects, anxiety, and restlessness), cardiovascular (pulse rate and diastolic blood pressure), and brain DA [reduced decreases in distribution volumes (DVs) of [(11)C]raclopride, although normal reductions in striatal nondisplaceable binding potential (BPND)] responses to MP. In ventral striatum (key brain reward region), MP-induced reductions in DVs and BPND (reflecting DA increases) were inversely correlated with scores of negative emotionality, which were significantly higher for marijuana abusers than controls. In marijuana abusers, DA responses in ventral striatum were also inversely correlated with addiction severity and craving. The attenuated responses to MP, including reduced decreases in striatal DVs, are consistent with decreased brain reactivity to the DA stimulation in marijuana abusers that might contribute to their negative emotionality (increased stress reactivity and irritability) and addictive behaviors.

Alcohol decreases baseline brain glucose metabolism more in heavy drinkers than controls but has no effect on stimulation-induced metabolic increases.

During alcohol intoxication, the human brain increases metabolism of acetate and decreases metabolism of glucose as energy substrate. Here we hypothesized that chronic heavy drinking facilitates this energy substrate shift both for baseline and stimulation conditions. To test this hypothesis, we compared the effects of alcohol intoxication (0.75 g/kg alcohol vs placebo) on brain glucose metabolism during video stimulation (VS) versus when given with no stimulation (NS), in 25 heavy drinkers (HDs) and 23 healthy controls, each of whom underwent four PET-(18)FDG scans. We showed that resting whole-brain glucose metabolism (placebo-NS) was lower in HD than controls (13%, p = 0.04); that alcohol (compared with placebo) decreased metabolism more in HD (20 ± 13%) than controls (9 ± 11%, p = 0.005) and in proportion to daily alcohol consumption (r = 0.36, p = 0.01) but found that alcohol did not reduce the metabolic increases in visual cortex from VS in either group. Instead, VS reduced alcohol-induced decreases in whole-brain glucose metabolism (10 ± 12%) compared with NS in both groups (15 ± 13%, p = 0.04), consistent with stimulation-related glucose metabolism enhancement. These findings corroborate our hypothesis that heavy alcohol consumption facilitates use of alternative energy substrates (i.e., acetate) for resting activity during intoxication, which might persist through early sobriety, but indicate that glucose is still favored as energy substrate during brain stimulation. Our findings are consistent with reduced reliance on glucose as the main energy substrate for resting brain metabolism during intoxication (presumably shifting to acetate or other ketones) and a priming of this shift in HDs, which might make them vulnerable to energy deficits during withdrawal.

Recovery of dopamine transporters with methamphetamine detoxification is not linked to changes in dopamine release.

Methamphetamine's widepread abuse and concerns that it might increase Parkinson's disease led us to assess if the reported loss of dopamine transporters (DAT) in methamphetamine abusers (MA) reflected damage to dopamine neurons. Using PET with [(11)C]cocaine to measure DAT, and with [(11)C]raclopride to measure dopamine release (assessed as changes in specific binding of [(11)C]raclopride between placebo and methylphenidate), which was used as a marker of dopamine neuronal function, we show that MA (n=16), tested during early detoxification, had lower DAT (20-30%) but overall normal DA release in striatum (except for a small decrease in left putamen), when compared to controls (n=15). In controls, DAT were positively correlated with DA release (higher DAT associated with larger DA increases), consistent with DAT serving as markers of DA terminals. In contrast, MA showed a trend for a negative correlation (p=0.07) (higher DAT associated with lower DA increases), consistent with reduced DA re-uptake following DAT downregulation. MA who remained abstinent nine-months later (n=9) showed significant increases in DAT (20%) but methylphenidate-induced dopamine increases did not change. In contrast, in controls, DAT did not change when retested 9 months later but methylphenidate-induced dopamine increases in ventral striatum were reduced (p=0.05). Baseline D2/D3 receptors in caudate were lower in MA than in controls and did not change with detoxification, nor did they change in the controls upon retest. The loss of DAT in the MA, which was not associated with a concomitant reduction in dopamine release as would have been expected if DAT loss reflected DA terminal degneration; as well as the recovery of DAT after protracted detoxification, which was not associated with increased dopamine release as would have been expected if DAT increases reflected terminal regeneration, indicate that the loss of DAT in these MA does not reflect degeneration of dopamine terminals.

Addiction circuitry in the human brain.

A major challenge in understanding substance-use disorders lies in uncovering why some individuals become addicted when exposed to drugs, whereas others do not. Although genetic, developmental, and environmental factors are recognized as major contributors to a person's risk of becoming addicted, the neurobiological processes that underlie this vulnerability are still poorly understood. Imaging studies suggest that individual variations in key dopamine-modulated brain circuits, including circuits involved in reward, memory, executive function, and motivation, contribute to some of the differences in addiction vulnerability. A better understanding of the main circuits affected by chronic drug use and the influence of social stressors, developmental trajectories, and genetic background on these circuits is bound to lead to a better understanding of addiction and to more effective strategies for the prevention and treatment of substance-use disorders.

Investigation of SN2 [11C]cyanation for base-sensitive substrates: an improved radiosynthesis of L-[5-11C]-glutamine.

Carbon-11 (ß(+) emitter, t1/2 = 20.4 min) radiolabeled L-glutamine is a potentially useful molecular imaging agent that can be utilized with positron emission tomography for both human oncological diagnosis and plant imaging research. Based upon a previously reported [(11)C]cyanide end-capping labeling method, a systematic investigation of nucleophilic cyanation reactions and acidic hydrolysis reaction parameters, including base, metal ion source, phase transfer catalyst, solvent, reaction temperature and reaction time, was conducted. The result was a milder, more reliable, two-step method which provides L-[5-(11)C]-glutamine with a radiochemical yield of 63.8 ± 8.7% (range from 51 to 74%, n = 10) with >90% radiochemical purity and >90 % enantiomeric purity. The total synthesis time was 40-50 min from the end of bombardment. In addition, an Fmoc derivatization method was developed to measure the specific activity of this radiotracer.

Radiolabelling and positron emission tomography of PT70, a time-dependent inhibitor of InhA, the Mycobacterium tuberculosis enoyl-ACP reductase.

PT70 is a diaryl ether inhibitor of InhA, the enoyl-ACP reductase in the Mycobacterium tuberculosis fatty acid biosynthesis pathway. It has a residence time of 24 min on the target, and also shows antibacterial activity in a mouse model of tuberculosis infection. Due to the interest in studying target tissue pharmacokinetics of PT70, we developed a method to radiolabel PT70 with carbon-11 and have studied its pharmacokinetics in mice and baboons using positron emission tomography.

An efficient and practical synthesis of [2-(11)C]indole via superfast nucleophilic [(11)C]cyanation and RANEY® Nickel catalyzed reductive cyclization.

A rapid method for the synthesis of carbon-11 radiolabeled indole was developed using a sub-nanomolar quantity of no-carrier-added [(11)C]cyanide as radio-precursor. Based upon a reported synthesis of 2-(2-nitrophenyl)acetonitrile (), a highly reactive substrate 2-nitrobenzyl bromide () was evaluated for nucleophilic [(11)C]cyanation. Additionally, related reaction conditions were explored with the goal of obtaining of highly reactive 2-(2-nitrophenyl)-[1-(11)C]acetonitrile () while inhibiting its rapid conversion to 2,3-bis(2-nitrophenyl)-[1-(11)C]propanenitrile (). Next, a RANEY® Nickel catalyzed reductive cyclization method was utilized for synthesizing the desired [2-(11)C]indole with hydrazinium monoformate as the active reducing agent. Extensive and iterative screening of basicity, temperature and stoichiometry was required to overcome the large stoichiometry bias that favored 2-nitrobenzylbromide () over [(11)C]cyanide, which both caused further alkylation of the desired nitrile and poisoned the RANEY® Nickel catalyst. The result is an efficient two-step, streamlined method to reliably synthesize [2-(11)C]indole with an entire radiochemical yield of 21 ± 2.2% (n = 5, ranging from 18-24%). The radiochemical purity of the final product was >98% and specific activity was 176 ± 24.8 GBq µmol(-1) (n = 5, ranging from 141-204 GBq µmol(-1)). The total radiosynthesis time including product purification by semi-preparative HPLC was 50-55 min from end of cyclotron bombardment.

Monoamine oxidase: radiotracer chemistry and human studies.

Monoamine oxidase (MAO) oxidizes amines from both endogenous and exogenous sources thereby regulating the concentration of neurotransmitter amines such as serotonin, norepinephrine, and dopamine as well as many xenobiotics. MAO inhibitor drugs are used in the treatment of Parkinson's disease and in depression stimulating the development of radiotracer tools to probe the role of MAO in normal human biology and in disease. Over the past 30 years since the first radiotracers were developed and the first positron emission tomography (PET) images of MAO in humans were carried out, PET studies of brain MAO in healthy volunteers and in patients have identified different variables that have contributed to different MAO levels in brain and in peripheral organs. MAO radiotracers and PET have also been used to study the current and developing MAO inhibitor drugs including the selection of doses for clinical trials. In this article, we describe the following: (1) the development of MAO radiotracers; (2) human studies including the relationship of brain MAO levels to genotype, personality, neurological, and psychiatric disorders; and (3) examples of the use of MAO radiotracers in drug research and development. We will conclude with outstanding needs to improve the radiotracers that are currently used and possible new applications.

Predominance of D2 receptors in mediating dopamine's effects in brain metabolism: effects of alcoholism.

Dopamine signals through D1-like and D2-like receptors, which can stimulate or inhibit, respectively, neuronal activity. Here we assessed the balance between D1 or D2 receptor signaling in the human brain and how it is affected in alcoholism. Using PET, we measured the relationship between changes in dopamine and brain glucose metabolism induced by methylphenidate in controls and alcoholics. We show that methylphenidate induced significant DA increases in striatum, amygdala, and medial orbitofrontal cortex, whereas it decreased metabolism in these brain regions. Methylphenidate-induced dopamine increases were greater in controls than in alcoholics, whereas methylphenidate-induced metabolic decreases were greater in alcoholics. For both groups, methylphenidate-induced dopamine increases were associated with decreases in regional brain metabolism, and the correlations were strongest in subthalamic nuclei, anterior cingulate, and medial orbitofrontal cortex. These correlations were more extensive and robust and the slopes steeper in alcoholics than in controls despite their attenuated dopamine responses to methylphenidate, which suggests an impaired modulation of dopamine signals in the brain of alcoholic subjects. These findings are consistent with a predominant inhibitory effect of dopamine in the human brain that is likely mediated by the prominence of dopamine D2/D3 receptors.

Dynamic functional imaging of brain glucose utilization using fPET-FDG.

Glucose is the principal source of energy for the brain and yet the dynamic response of glucose utilization to changes in brain activity is still not fully understood. Positron emission tomography (PET) allows quantitative measurement of glucose metabolism using 2-[(18)F]-fluorodeoxyglucose (FDG). However, FDG PET in its current form provides an integral (or average) of glucose consumption over tens of minutes and lacks the temporal information to capture physiological alterations associated with changes in brain activity induced by tasks or drug challenges. Traditionally, changes in glucose utilization are inferred by comparing two separate scans, which significantly limits the utility of the method. We report a novel method to track changes in FDG metabolism dynamically, with higher temporal resolution than exists to date and within a single session. Using a constant infusion of FDG, we demonstrate that our technique (termed fPET-FDG) can be used in an analysis pipeline similar to fMRI to define within-session differential metabolic responses. We use visual stimulation to demonstrate the feasibility of this method. This new method has a great potential to be used in research protocols and clinical settings since fPET-FDG imaging can be performed with most PET scanners and data acquisition and analysis are straightforward. fPET-FDG is a highly complementary technique to MRI and provides a rich new way to observe functional changes in brain metabolism.

Kinetic analysis of [11C]vorozole binding in the human brain with positron emission tomography.

Using positron emission tomography, we investigated the kinetics of [11C]vorozole ([11C]VOR), a radiotracer for the enzyme aromatase that catalyzes the last step in estrogen biosynthesis. Six subjects were scanned under baseline conditions followed by retest 2 weeks later. The retest was followed by a blocking study with 2.5 mg of the aromatase inhibitor letrozole. The binding potential (BP(A)ND) was estimated from a Lassen plot using the total tissue distribution volume (VT) for baseline and blocked. for the thalamus was found to be 15 times higher than that for the cerebellum. From the letrozole studies, we found that [11C]VOR exhibits a slow binding compartment (small k4) that has a nonspecific and a blockable component. Because of the sensitivity of VT to variations in k4, a common value was used for the four highest binding regions. We also considered the tissue uptake to plasma ratio for 60 to 90 minutes as an outcome measure. Using the ratio method, the difference between the highest and lowest was 2.4 compared to 3.5 for the VT. The ratio method underestimates the high regions but is less variable and may be more suitable for patient studies. Because of its kinetics and distribution, this tracer is not a candidate for a bolus infusion or reference tissue methods.

Therapeutic doses of buspirone block D3 receptors in the living primate brain.

Dopamine D3 receptor (D3R) antagonists may be effective medications for multiple substance use disorders (SUDs). However, no selective D3R antagonists are currently available for clinical testing. Buspirone, originally characterized as a 5-HT1A partial agonist and used as an anxiolytic, also binds to D3R and D4R with high affinity, with lower affinity to D2R, and interferes with cocaine reward. Here we used PET with [11C]PHNO (D3R-preferring radioligand), [11C]raclopride (D2R/D3R radioligand) and [11C]NNC-112 (D1R radioligand) to measure occupancy of oral and parenteral buspirone in the primate brain. Intramuscular buspirone (0.19 and 0.5 mg/kg) blocked both [11C]PHNO and [11C]raclopride binding to striatum, exhibiting high occupancy (50-85%) at 15 min and rapid wash-out over 2-6 h. In contrast, oral buspirone (3 mg/kg) significantly blocked [11C]PHNO binding in D3-rich regions (globus pallidum and midbrain) at 3 h, but had minimal effects on [11C]raclopride binding (28-37% at 1 h and 10% at 3 h). Buspirone did not block [11C]NNC-112. Our findings provide evidence that i.m. buspirone blocks D3R and D2R, whereas oral buspirone is more selective towards D3R blockade in vivo, consistent with extensive first pass metabolism and supporting the hypothesis that its metabolites (5- and 6'-hydroxybuspirone) merit evaluation for treating SUDs. They also indicate that for oral buspirone to achieve greater than 80% sustained D3R occupancy, as might be needed to treat addiction, higher doses (at least three-fold) than those used to treat anxiety (maximal 60 mg) will be required. Nonetheless, based on previous clinical studies, these doses would be safe and well tolerated.

Addiction: beyond dopamine reward circuitry.

Dopamine (DA) is considered crucial for the rewarding effects of drugs of abuse, but its role in addiction is much less clear. This review focuses on studies that used PET to characterize the brain DA system in addicted subjects. These studies have corroborated in humans the relevance of drug-induced fast DA increases in striatum [including nucleus accumbens (NAc)] in their rewarding effects but have unexpectedly shown that in addicted subjects, drug-induced DA increases (as well as their subjective reinforcing effects) are markedly blunted compared with controls. In contrast, addicted subjects show significant DA increases in striatum in response to drug-conditioned cues that are associated with self-reports of drug craving and appear to be of a greater magnitude than the DA responses to the drug. We postulate that the discrepancy between the expectation for the drug effects (conditioned responses) and the blunted pharmacological effects maintains drug taking in an attempt to achieve the expected reward. Also, whether tested during early or protracted withdrawal, addicted subjects show lower levels of D2 receptors in striatum (including NAc), which are associated with decreases in baseline activity in frontal brain regions implicated in salience attribution (orbitofrontal cortex) and inhibitory control (anterior cingulate gyrus), whose disruption results in compulsivity and impulsivity. These results point to an imbalance between dopaminergic circuits that underlie reward and conditioning and those that underlie executive function (emotional control and decision making), which we postulate contributes to the compulsive drug use and loss of control in addiction.

Evidence that sleep deprivation downregulates dopamine D2R in ventral striatum in the human brain.

Dopamine D2 receptors are involved with wakefulness, but their role in the decreased alertness associated with sleep deprivation is unclear. We had shown that sleep deprivation reduced dopamine D2/D3 receptor availability (measured with PET and [(11)C]raclopride in controls) in striatum, but could not determine whether this reflected dopamine increases ([(11)C]raclopride competes with dopamine for D2/D3 receptor binding) or receptor downregulation. To clarify this, we compared the dopamine increases induced by methylphenidate (a drug that increases dopamine by blocking dopamine transporters) during sleep deprivation versus rested sleep, with the assumption that methylphenidate's effects would be greater if, indeed, dopamine release was increased during sleep deprivation. We scanned 20 controls with [(11)C]raclopride after rested sleep and after 1 night of sleep deprivation; both after placebo and after methylphenidate. We corroborated a decrease in D2/D3 receptor availability in the ventral striatum with sleep deprivation (compared with rested sleep) that was associated with reduced alertness and increased sleepiness. However, the dopamine increases induced by methylphenidate (measured as decreases in D2/D3 receptor availability compared with placebo) did not differ between rested sleep and sleep deprivation, and were associated with the increased alertness and reduced sleepiness when methylphenidate was administered after sleep deprivation. Similar findings were obtained by microdialysis in rodents subjected to 1 night of paradoxical sleep deprivation. These findings are consistent with a downregulation of D2/D3 receptors in ventral striatum with sleep deprivation that may contribute to the associated decreased wakefulness and also corroborate an enhancement of D2 receptor signaling in the arousing effects of methylphenidate in humans.