In Vivo Binding of Antipsychotics to D3 and D2 Receptors: A PET Study in Baboons with [11C]-(+)-PHNO

Measuring the in vivo occupancy of antipsychotic drugs at dopamine D₂ and D₃ receptors separately has been difficult because of the lack of selective radiotracers. The recently developed [¹¹C]-(+)-PHNO is D₃-preferring, allowing estimates of the relative D₂ and D₃ binding of antipsychotic drugs. We used positron emission tomography (PET) imaging in baboons with [¹¹C]-(+)-PHNO to examine the binding of clozapine and haloperidol to D₂ and D₃ receptors. Four animals were scanned with dynamically acquired PET and arterial plasma input functions. Test and retest scans were acquired in single scanning sessions for three subjects to assess the reproducibility of [¹¹C]-(+)-PHNO scans. Four additional scans were acquired in each of three subjects following single doses of antipsychotic drugs (clozapine 0.5534 mg/kg, haloperidol 0.0109 mg/kg, two administrations per drug per subject) and compared with baseline scans. The percent change in binding (ΔBP_ND) following challenges with antipsychotic drugs was measured. A regression model, based on published values of regional D₂ and D₃ fractions of [¹¹C]-(+)-PHNO BP_ND in six brain regions, was used to infer occupancy at D₂ and D₃ receptors. BP_ND following antipsychotic challenge decreased in all regions. Estimated D₂ : D₃ selectivity was 2.38 for haloperidol and 5.25 for clozapine, similar to published in vitro values for haloperidol (3.03), but slightly higher for clozapine (2.82). These data suggest that acute doses of clozapine and haloperidol bind to D₃ receptors in vivo, and that the lack of D₃ occupancy by antipsychotics observed in some recent imaging studies may be because of other phenomena.     

Elevation of Dopamine Induced by Cigarette Smoking: Novel Insights from a [11C]-(+)-PHNO PET Study in Humans

Positron emission tomography (PET) has convincingly provided in vivo evidence that psychoactive drugs increase dopamine (DA) levels in human brain, a feature thought critical to their reinforcing properties. Some controversy still exists concerning the role of DA in reinforcing smoking behavior and no study has explored whether smoking increases DA concentrations at the D3 receptor, speculated to have a role in nicotine''s addictive potential. Here, we used PET and [¹¹C]-(+)-PHNO ([¹¹C]-(+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol) to test the hypothesis that smoking increases DA release (decreases [¹¹C]-(+)-PHNO binding) in D2-rich striatum and D3-rich extra-striatal regions and is related to craving, withdrawal and smoking behavior. Ten participants underwent [¹¹C]-(+)-PHNO scans after overnight abstinence and after smoking a cigarette. Motivation to smoke (smoking topography), mood, and craving were recorded. Smoking significantly decreased self-reported craving, withdrawal, and [¹¹C]-(+)-PHNO binding in D2 and D3-rich areas (−12.0 and −15.3%, respectively). We found that motivation to smoke (puff rate) predicted magnitude of DA release in limbic striatum, and the latter was correlated with decreased craving and withdrawal symptoms. This is the first report suggesting that, in humans, DA release is increased in D3-rich areas in response to smoking. Results also support the preferential involvement of the limbic striatum in motivation to smoke, anticipation of pleasure from cigarettes and relief of withdrawal symptoms. We propose that due to the robust effect of smoking on [¹¹C]-(+)-PHNO binding, this radiotracer represents an ideal translational tool to investigate novel therapeutic strategies targeting DA transmission.     

Heightened D3 Dopamine Receptor Levels in Cocaine Dependence and Contributions to the Addiction Behavioral Phenotype: A Positron Emission Tomography Study with [11C]-(+)-PHNO

The dopamine system is a primary treatment target for cocaine dependence (CD), but research on dopaminergic abnormalities (eg, D₂ receptor system deficiencies) has so far failed to translate into effective treatment strategies. The D₃ receptor system has recently attracted considerable clinical interest, and D₃ antagonism is now under investigation as a novel avenue for addiction treatment. The objective here was to evaluate the status and behavioral relevance of the D₃ receptor system in CD, using the positron emission tomography (PET) radiotracer [¹¹C]-(+)-PHNO. Fifteen CD subjects (many actively using, but all abstinent 7–240 days on scan day) and fifteen matched healthy control (HC) subjects completed two PET scans: one with [¹¹C]-(+)-PHNO to assess D₃ receptor binding (BP_ND; calculated regionally using the simplified reference tissue model), and for comparison, a second scan with [¹¹C]raclopride to assess D_2/3 binding. CD subjects also completed a behavioral battery to characterize the addiction behavioral phenotype. CD subjects showed higher [¹¹C]-(+)-PHNO BP_ND than HC in the substantia nigra, which correlated with behavioral impulsiveness and risky decision making. In contrast, [¹¹C]raclopride BP_ND was lower across the striatum in CD, consistent with previous literature in ⩾2 week abstinence. The data suggest that in contrast to a D₂ deficiency, CD individuals may have heightened D₃ receptor levels, which could contribute to addiction-relevant traits. D₃ upregulation is emerging as a biomarker in preclinical models of addiction, and human PET studies of this receptor system can help guide novel pharmacological strategies for treatment.     

Dopamine Response to Psychosocial Stress in Chronic Cannabis Users: A PET Study With [11C]-(+)-PHNO

A number of addictions have been linked with decreased striatal dopamine (DA) receptor availability and DA release. Stress has a key role in cannabis craving, as well as in modulation of dopaminergic signaling. The present study aimed to assess DA release in response to a laboratory stress task with [¹¹C]-(+)-PHNO positron emission tomography in cannabis users (CU). Thirteen healthy CU and 12 healthy volunteers (HV) were scanned during a sensorimotor control task (SMCT) and under a stress condition using the validated Montreal imaging stress task (MIST). The simplified reference tissue model (SRTM) was used to obtain binding potential (BP_ND) in striatal subdivisions: limbic striatum (LST), associative striatum (AST), and sensorimotor striatum (SMST). Stress-induced DA release (indexed as a percentage of reduction in [¹¹C]-(+)-PHNO BP _ND) between CU and HV was tested with analysis of variance. SMCT BP_ND was significantly higher in CU compared with HV in the AST (F=10.38, p=0.003), LST (F=4.95, p=0.036), SMST (F=4.33, p=0.048), and whole striatum (F=9.02, p=0.006). Percentage of displacement (change in BP_ND between SMCT and MIST PET scans) was not significantly different across groups in any brain region, except in the GP (−5.03±14.6 in CU, compared with 6.15±12.1 in HV; F=4.39, p=0.049). Duration of cannabis use was significantly associated with stress-induced [¹¹C]-(+)-PHNO displacement by endogenous DA in the LST (r=0.566, p=0.044), with no effect in any other brain region. In conclusion, despite an increase in striatal BP_ND observed during the control task, chronic cannabis use is not associated with alterations in stress-induced DA release.     

Exploring regulation and function of dopamine D3 receptors in alcohol use disorder. A PET [11C]-(+)-PHNO study

Preclinical studies support an important role of dopamine D3 receptors (DRD3s) in alcohol use disorder (AUD). In animals, voluntary alcohol consumption increases DRD3 expression, and pharmacological blockade of DRD3s attenuates alcohol self-administration and reinstatement of alcohol seeking. However, these findings have yet to be translated in humans. This study used positron emission tomography (PET) and [¹¹C]-(+)-PHNO to compare receptor levels in several dopamine D2 receptor (DRD2) and DRD3 regions of interest between AUD subjects in early abstinence (n = 17; 6.59 ± 4.14 days of abstinence) and healthy controls (n = 18). We recruited non-treatment seeking subjects meeting DSM-5 criteria for AUD. We examined the relationship between DRD2/3 levels and both alcohol craving and alcohol motivation/wanting, using a cue reactivity procedure and an intravenous alcohol self-administration (IVASA) paradigm, respectively. [¹¹C]-(+)-PHNO binding levels in AUD subjects were significantly lower than binding in HCs when looking at all DRD2/3 ROIs jointly (Wilk’s Λ = .58, F(6,28) =3.33, p = 0.013, η²_p = 0.42), however there were no region-specific differences. Binding values demonstrate −12.3% and −16.1% lower [¹¹C]-(+)-PHNO binding in the SMST and SN respectively, though these differences did not withstand Bonferroni corrections. There was a positive association between [¹¹C]-(+)-PHNO binding in the SN (almost exclusively reflective of DRD3) and alpha (lower values reflect higher alcohol demand) in the APT after Bonferroni corrections (r = 0.66, p = 0.0080). This demonstrates that AUD subjects with lower DRD3 levels in the SN exhibit increased demand for alcohol. These results replicate previous findings demonstrating reduced DRD2/3 levels while also supporting a lack of DRD3 upregulation and potential downregulation in early abstinent AUD. Furthermore, the finding that binding in the SN is associated with alcohol demand warrants further examination.Subject terms: Neurotransmitters, Addiction, Translational research     

Reduced Insulin Sensitivity Is Related to Less Endogenous Dopamine at D2/3 Receptors in the Ventral Striatum of Healthy Nonobese Humans

Background:

Food addiction is a debated topic in neuroscience. Evidence suggests diabetes is related to reduced basal dopamine levels in the nucleus accumbens, similar to persons with drug addiction. It is unknown whether insulin sensitivity is related to endogenous dopamine levels in the ventral striatum of humans. We examined this using the agonist dopamine D_2/3 receptor radiotracer [¹¹C]-(+)-PHNO and an acute dopamine depletion challenge. In a separate sample of healthy persons, we examined whether dopamine depletion could alter insulin sensitivity.

Methods:

Insulin sensitivity was estimated for each subject from fasting plasma glucose and insulin using the Homeostasis Model Assessment II. Eleven healthy nonobese and nondiabetic persons (3 female) provided a baseline [¹¹C]-(+)-PHNO scan, 9 of which provided a scan under dopamine depletion, allowing estimates of endogenous dopamine at dopamine D_2/3 receptor. Dopamine depletion was achieved via alpha-methyl-para-tyrosine (64mg/kg, P.O.). In 25 healthy persons (9 female), fasting plasma and glucose was acquired before and after dopamine depletion.

Results:

Endogenous dopamine at ventral striatum dopamine D_2/3 receptor was positively correlated with insulin sensitivity (r(7)=.84, P=.005) and negatively correlated with insulin levels (r(7)=-.85, P=.004). Glucose levels were not correlated with endogenous dopamine at ventral striatum dopamine D_2/3 receptor (r(7)=-.49, P=.18). Consistently, acute dopamine depletion in healthy persons significantly decreased insulin sensitivity (t(24)=2.82, P=.01), increased insulin levels (t(24)=-2.62, P=.01), and did not change glucose levels (t(24)=-0.93, P=.36).

Conclusion:

In healthy individuals, diminished insulin sensitivity is related to less endogenous dopamine at dopamine D_2/3 receptor in the ventral striatum. Moreover, acute dopamine depletion reduces insulin sensitivity. These findings may have important implications for neuropsychiatric populations with metabolic abnormalities.     

First human evidence of d-amphetamine induced displacement of a D2/3 agonist radioligand: A [11C]-(+)-PHNO positron emission tomography study.

Imaging the competition between D(2/3) radioligands and endogenous dopamine is so far the only way to measure dopamine release in the living human brain. The dopamine D(2) receptor exists in a high (D(2)(high)) and a low-affinity state for dopamine. Under physiological conditions, dopamine is expected to bind to D(2)(high) only. [(11)C]-(+)-4-propyl-9-hydroxynaphthoxazine ((+)-PHNO) is the first D(2/3) agonist radioligand for positron emission tomography (PET) imaging in humans. Since [(11)C]-(+)-PHNO is expected to bind preferentially to D(2)(high), it should be particularly vulnerable to competition with endogenous dopamine. Nine healthy subjects participated in two PET scans, one after administration of d-amphetamine and one after placebo. [(11)C]-(+)-PHNO PET test re-test variability was determined in 11 healthy subjects. Binding potentials (BPs) were calculated for caudate, putamen, ventral striatum, and globus pallidus. d-Amphetamine led to a significant decrease of [(11)C]-(+)-PHNO BPs in caudate (-13.2%), putamen (-20.8%), and ventral striatum (-24.9%), but not in globus pallidus (-6.5%). d-Amphetamine-induced displacement correlated with serum d-amphetamine levels in all regions but caudate. This is the first report on competition between endogenous dopamine and a D(2/3) agonist radioligand in humans. [(11)C]-(+)-PHNO PET might be a superior measure for release of endogenous dopamine than PET employing conventional D(2/3) antagonist radioligands.     

A PET Study on the Acute Effect of Ethanol on Striatal D2 Dopamine Receptors with [11C]Raclopride in Healthy Males

The effect of acute oral ethanol intake (1·0 g/kg) on cerebral D2-receptors ([¹¹C]raclopride binding) was studied in seven healthy volunteers, using water and alcohol in two separate 59-min PET sessions. In the alcohol experiments, the blood ethanol concentration at the beginning of the imaging was 26·4±3·8 mmol/l and remained stable during the PET session. Ethanol was not found to influence binding of [¹¹C]raclopride to the dopamine D2 receptors in the human striatum, as indicated by the unchanged ratio B_max/K_d of the whole (left and right) striatum. The T_max values of the control and ethanol experiments did not differ in the whole striatum, but the right to left difference of striatal T_max was turned from negative to positive by ethanol (P=0·02). However, the difference between hemispheres in B_max/K_d was not significantly altered by ethanol intake. There was considerable interindividual variation in the response of all the above parameters to acute ethanol exposure. According to the present results, the acute effects of peroral ethanol exposure on striatal D2 receptor binding potential are of relatively small magnitude in man. However, the changes in T_max suggest that ethanol may influence the right–left difference of [¹¹C]raclopride binding. © 1997 by John Wiley & Sons, Ltd.     

Dopamine D1 receptor imaging in the rodent and primate brain using the isoquinoline +-[11C]A-69024 and positron emission tomography

In vivo pharmacokinetic and brain binding characteristics of (+)-[(11)C]A-69024, a high-affinity-D1-selective dopamine receptor antagonist, were assessed with micro-PET and beta-microprobes in the rat and PET in the baboon. The biodistribution of (+)-[(11)C]A-69024 in rats and baboons showed a rapid brain uptake (reaching a maximal value at 5 and 15 min postinjection in rats and baboons, respectively), followed by a slow wash out. The region/cerebellum concentration ratio was characterized by a fourfold higher uptake in striatum and a twofold higher uptake in cortical regions, consistent with in vivo specific binding of the radiotracer in these cerebral regions. Furthermore, this specific (+)-[(11)C]A-69024 binding significantly correlated with the reported in vitro distribution of dopamine D1-receptors. Finally, the specific uptake of the tracer in the striatum and cortical regions was completely prevented by either a pretreatment with large doses of nonradioactive (+/-)A-69024 or of the D1-selective antagonist SCH23390, resulting in a similar uptake in the reference region (cerebellum) and in other brain regions. Thus, (+)-[(11)C]A-69024 appears to be a specific and enantioselective radioligand to visualize and quantify brain dopamine D1 receptors in vivo using positron emission tomography.     

Dopamine D2 receptor binding in adrenal medulla: characterization using [3H]spiperone

The possibility that dopamine may function as a neuromodulator or neurotransmitter in the adrenal gland, and not merely serve as a precursor to the catecholamines, has been suggested. If this hypothesis is correct, receptors for dopamine should be identifiable in the adrenal. The present work demonstrates the existence of a high-affinity receptor in adrenal medulla using [3H]spiperone as the radioligand to label the receptors. [3H]Spiperone bound rapidly, reversibly, and with high affinity to bovine adrenal medullary membranes. Scatchard analysis yielded a Kd of 0.09 nM and a Bmax of 51 fmol/mg protein. In competition binding experiments, dopaminergic antagonists were at least 100 times more potent in displacing [3H]spiperone from its binding sites than adrenergic or serotonergic receptor antagonists. Similarly, agonists at the dopamine receptor more readily competed for [3H]spiperone binding than other receptor agonist drugs tested. Furthermore, D2 selective antagonists and agonists were much more potent than D1 receptor ligands. These results suggest that [3H]spiperone may bind to a high-affinity D2 dopamine receptor in adrenal medulla.     

Reduction of [11C]CO2 to [11C]CO using solid supported zinc

A new method for the reduction of no‐carrier‐added [¹¹C]carbon dioxide into [¹¹C]carbon monoxide ([¹¹C]CO) is described, in which the reductant (zinc) is supported on fused silica particles. Using this setup, which allows for a reduction temperature (485°C) well above the melting point for zinc (420°C), radiochemical yields of up to 96% (decay‐corrected) were obtained. A slight decrease in radiochemical yield was observed upon repeated [¹¹C]CO productions (93 ± 3%, n  = 20). The methodology is convenient and efficient and provides a straightforward path to no‐carrier‐added production of [¹¹C]CO.     

Imaging Nicotine- and Amphetamine-Induced Dopamine Release in Rhesus Monkeys with [11C]PHNO vs [11C]raclopride PET

The radiotracer [¹¹C]PHNO may have advantages over other dopamine (DA) D₂/D₃ receptor ligands because, as an agonist, it measures high-affinity, functionally active D₂/D₃ receptors, whereas the traditionally used radiotracer [¹¹C]raclopride measures both high- and low-affinity receptors. Our aim was to take advantage of the strength of [¹¹C]PHNO for measuring the small DA signal induced by nicotine, which has been difficult to measure in preclinical and clinical neuroimaging studies. Nicotine- and amphetamine-induced DA release in non-human primates was measured with [¹¹C]PHNO and [¹¹C]raclopride positron emission tomography (PET) imaging. Seven adult rhesus monkeys were imaged on a FOCUS 220 PET scanner after injection of a bolus of [¹¹C]PHNO or [¹¹C]raclopride in three conditions: baseline; preinjection of nicotine (0.1 mg/kg bolus+0.08 mg/kg infusion over 30 min); preinjection of amphetamine (0.4 mg/kg, 5 min before radiotracer injection). DA release was measured as change in binding potential (BP_ND). Nicotine significantly decreased BP_ND in the caudate (7±8%), the nucleus accumbens (10±7%), and in the globus pallidus (13±15%) measured with [¹¹C]PHNO, but did not significantly decrease BP_ND in the putamen or the substantia nigra or in any region when measured with [¹¹C]raclopride. Amphetamine significantly reduced BP_ND in all regions with both radiotracers. In the striatum, larger amphetamine-induced changes were detected with [¹¹C]PHNO compared with [¹¹C]raclopride (52–64% vs 33–35%, respectively). We confirmed that [¹¹C]PHNO is more sensitive than [¹¹C]raclopride to nicotine- and amphetamine-induced DA release. [¹¹C]PHNO PET may be more sensitive to measuring tobacco smoking-induced DA release in human tobacco smokers.     

Selective D1- and D2-dopamine receptor blockade both induces akathisia in humans — a PET study with [11C]SCH 23390 and [11C]raclopride

Pharmacological effects were recorded and time course for receptor binding in brain was followed by positron emission tomography after IV injection of the selective D₁-dopamine receptor antagonist SCH 23390 in four healthy subjects in doses of 310–810 µg. Akathisia, the syndrome of motor restlessness, appeared after the three highest doses. The akathisia was transient and occurred only when [¹¹C]SCH 23390 binding in the basal ganglia was at a high level with a central D₁-dopamine receptor occupancy of 45–59%. The D₂-dopamine receptor antagonist [¹¹C]raclopride was injected IV into 20 healthy subjects and 13 schizophrenic patients. Akathisia appeared in 14 healthy subjects and 7 patients and coincided with maximal [¹¹C]raclopride binding in the basal ganglia. The findings for [¹¹C]raclopride and [¹¹C]SCH 23390 are the first demonstration of a relationship between time courses for radioligand binding in the human brain and simultaneously induced pharmacological effects.     

Dopamine D2 and D3 receptor occupancy in normal humans treated with the antipsychotic drug aripiprazole (OPC 14597): a study using positron emission tomography and [11C]raclopride.

Aripiprazole (OPC 14597) is an antipsychotic drug that has high affinity for dopamine D2 and D3 receptors and the dopamine autoreceptor. It is being developed for treatment of patients with schizophrenia. The purpose of this study was to determine whether a dose response following graduated doses of aripiprazole could be quantified and correlated with its occupancy of the D2 and D3 dopamine receptors in the brain of living humans. Dopamine D2 and D3 receptor occupancy in fifteen normal male human brains was measured using positron emission tomography (PET) with [11C]raclopride. PET studies were performed before and after two weeks of administration of aripiprazole. The dopamine D2 receptor occupancy was quantified with two kinetic modeling methods without using a blood input function. Administration of aripiprazole for 14 days resulted in a dose-dependent receptor occupancy between 40 - 95% after the administration of 0.5mg, 1 mg, 2 mg, 10 mg, and 30 mg per day. These results suggest that an adequate occupancy can be obtained, and this may be useful to predict an appropriate therapeutic dose for an individual patient. Interestingly, even at striatal D2 receptor occupancy values above 90%, which occurred with the higher doses, extrapyramidal side effects (EPS) were not observed. This underlines aripiprazole's unique mechanism of action as a partial dopamine receptor agonist, which might become a novel principle in the treatment of schizophrenia.     

Dopamine D2 receptors selectively labeled by a benzamide neuroleptic: [3H]-YM-09151-2

Summary In order to label dopamine D₂ receptors selectively we tritiated the potent benzamide neuroleptic, YM-09151-2 (26.7 Ci/mmol). The binding of [³H]-YM-09151-2 to canine striatal membranes was saturable and specific with a K _D of 57 pmol/l and B _max of 36 pmol/g tissue as determined by Scatchard analysis. The K _D, but not the B _max, of [³H]-YM-09151-2 increased 6-fold in the absence of sodium chloride. [³H]-YM-09151-2 labeled 40% more sites than [³H]-spiperone in the same tissue homogenate. [³H]-YM-09151-2 binding was inhibited by dopaminergic drugs in a concentration and stereoselective manner with the appropriate dopamine D₂ receptor profile. Thus, dopamine agonists inhibited [³H]-YM-09151-2 binding to canine striatal membranes with the following rank order of potency: (–)-N-n-propylnorapomorphine > apomorphine > (±)-6,7-dihydroxy-2-aminotetralin > (+)-N-n-propylnorapomorphine > dopamine > (–)-noradrenaline > serotonin > (–)-isoprenaline. Dopaminergic antagonists competed for [³H]-YM-09151-2 binding with the following order of potency: spiperone > (+)-butaclamol > haloperidol > clebopride > (–)-sulpiride > SCH-23390 > (–)-butaclamol. Furthermore, dopamine agonists recognized 2 states of the receptor labeled by [³H]-YM-09151-2, D ₂ ^high and D ₂ ^low . The D ₂ ^high state of the receptor could be converted to D ₂ ^low by guanine nucleotides and sodium ions as is the case for [³H]-spiperone binding to D₂ receptors. [³H]-YM-09151-2 appears to be a more selective ligand for dopamine D₂ receptors than [³H]-spiperone, since YM-09151-2 displays approximately 9-fold lower affinity than spiperone for cortical serotonergic (S₂) receptors. [³H]-YM-09151-2 may become a useful tool for the selective characterization of dopamine D₂ receptors.Abbreviations used (±)ADTN (±)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene - NPA N-n-propylnorapomorphine - Gpp(NH)p 5-guanylylimidodiphosphate     

Autoradiographic localisation of D3-dopamine receptors in the human brain using the selective D3-dopamine receptor agonist (+)-[3H]PD 128907

The selective D₃-dopamine receptor agonist 4aR,10bR-(+)-trans-3,4,4a,10b-tetrahydro-4-[N-propyl-2,3-³H]-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol ([³H]PD 128907) was used to visualise D₃-dopamine receptors in whole hemisphere cryosections from post-mortem human brain. [³H]PD 128907 has an 18- to 40-fold selectivity for D₃- over D₂-dopamine receptors as compared to a 7- to 24-fold selectivity of the more commonly used ligand [³H]7-OH-DPAT. [³H]PD 128907 accumulated markedly in the nucleus accumbens and in the ventral parts of caudate nucleus and putamen, with a slightly heterogeneous (patch-matrix like) distribution. The binding in the lateral parts of caudate nucleus and putamen was much less dense. No binding was obtained in any other regions. A very high proportion of [³H]PD 128907 was specifically bound, as judged from the low binding remaining in the presence of the D₂/D₃-dopamine receptor antagonist raclopride. This gives the ligand a potential for the detection of low density D₃-dopamine receptors in the human brain. The binding obtained with [³H]PD 128907 was qualitatively similar to that using [³H]7-OH-DPAT in the presence of GTP. However, [³H]7-OH-DPAT labelled, in contrast to [³H]PD 128907, also D₃-dopamine receptors in neocortex. The new compound [³H]PD 128907 appears to be a suitable radioligand for autoradiographic examination of the D₃-dopamine receptor localisation in the human brain, and should also be useful for pharmacological studies of this receptor subtype. Received: 20 November 1995/Final version: 2 May 1996     

Radiosynthesis of 2-[6-chloro-2-(4-iodophenyl)imidazo[1,2-a]pyridin-3-yl]-N-ethyl-N-[11C]methyl-acetamide, [11C]CLINME,a novel radioligand for imaging the peripheral benzodiazepine receptors with PET

Recently, a new 2-(iodophenyl)imidazo[1,2-a]pyridineacetamide series has been developed as iodine-123-labelled radioligands for imaging the peripheral benzodiazepine receptors using single photon emission tomography. Within this series, 2-[6-chloro-2-(4-iodophenyl)-imidazo[1,2-a]pyridin-3-yl]-N-ethyl-N-methyl-acetamide (CLINME) was considered as an appropriate candidate for positron emission tomography imaging and was isotopically labelled with carbon-11 (T_1/2: 20.38 min) at the methylacetamide side chain from the corresponding nor-analogue using [¹¹C]methyl iodide and the following experimental conditions: (1) trapping at −10°C of [¹¹C]methyl iodide in a 1/2 (v:v) mixture of DMSO/DMF (300 µl) containing 0.7–1.0 mg of the precursor for labelling and 3–5 mg of powdered potassium hydroxide (excess); (2) heating the reaction mixture at 110°C for 3 min under a nitrogen stream; (3) diluting the residue with 0.6 ml of the HPLC mobile phase; and (4) purification using semi-preparative HPLC (Zorbax^® SB18, Hewlett Packard, 250 × 9.4 mm). Typically, starting from a 1.5 Ci (55.5 GBq) [¹¹C]CO₂ production batch, 120−150 mCi (4.44–5.55 GBq) of [¹¹C]CLINME were obtained (16–23% decay-corrected radiochemical yield, n=12) within a total synthesis time of 24–27 min (Sep-pak^®Plus-based formulation included). Specific radioactivities ranged from 0.9 to 2.7 Ci/µmol (33.3–99.9 GBq/µmol) at the end of radiosynthesis. Copyright © 2007 John Wiley & Sons, Ltd.     

Agonist and Antagonist Effects of Aripiprazole on D2-Like Receptors Controlling Rat Brain Dopamine Synthesis Depend on the Dopaminergic Tone

Background:

The atypical antipsychotic drug aripiprazole binds with high affinity to a number of G protein coupled receptors, including dopamine D₂ receptors, where its degree of efficacy as a partial agonist remains controversial.

Methods:

We examined the properties of aripiprazole at D₂-like autoreceptors by monitoring the changes of dopamine synthesis in adult rat brain striatal minces incubated ex vivo. The effects of the dopaminergic tone on the properties of aripiprazole were assayed by comparing a basal condition (2mM K⁺, low dopaminergic tone) and a stimulated condition (15mM K⁺, where dopamine release mimics a relatively higher dopaminergic tone). We also used 2 reference compounds: quinpirole showed a clear agonistic activity and preclamol (S-(-)-PPP) showed partial agonism under both basal and stimulated conditions.

Results:

Aripiprazole under the basal condition acted as an agonist at D₂-like autoreceptors and fully activated them at about 10nM, inhibiting dopamine synthesis similarly to quinpirole. Higher concentrations of aripiprazole had effects not restricted to D₂-like autoreceptor activation. Under the stimulated (15mM K⁺) condition, nanomolar concentrations of aripiprazole failed to decrease dopamine synthesis but could totally block the effect of quinpirole.

Conclusions:

Under high dopaminergic tone, aripiprazole acts as a D₂-like autoreceptor antagonist rather than as an agonist. These data show that, ex vivo, alteration of dopaminergic tone by depolarization affects the actions of aripiprazole on D₂-like autoreceptors. Such unusual effects were not seen with the typical partial agonist preclamol and are consistent with the hypothesis that aripiprazole is a functionally selective D₂R ligand.     

Autoradiography of dopamine D2 receptors using [3H]YM-09151-2.

Rat brain dopamine D2 receptors were labeled autoradiographically using the potent and selective benzamide, [3H]YM-09151-2. Saturation binding data, quantified from autoradiograms, showed specific binding of [3H]YM-09151-2 to striatal D2 receptors with a KD of 82 pM and Bmax of 0.696 pmol/mg protein. Binding equilibrium occurred within 3 h, and a dissociation constant of 15 pM was obtained in kinetic studies. Antagonist competition curves were monophasic and displayed an order of potency expected for D2 receptors: (+)-butaclamol greater than (-)-sulpiride greater than SCH 23390 greater than mianserin. Competition by dopamine resulted in a shallow, biphasic displacement curve. The distribution of [3H]YM-09151-2 binding sites matched the known pattern for D2 receptors, with dense labeling in striatum, olfactory tubercles and nucleus accumbens, and moderate levels in substantia nigra pars compacta and mamillary nuclei. Much lower levels of non-specific binding were observed than are typically obtained when using [3H]spiperone as the ligand. The coincident high affinity and selectivity of [3H]YM-09151-2 for D2 sites should make this compound a preferred choice for tritium-based D2 autoradiography.