Striatal and frontal cortex binding of 11-C-labelled clozapine visualized by positron emission tomography (PET) in drug-free schizophrenics and healthy volunteers

The binding of 11C-labelled clozapine in the brain was studied in three drug-free schizophrenic patients and in three healthy volunteers. High radioactivities were found in the striatum and in the frontal cortex. The rate constantk ₃, which is proportional to receptor association rate and the number of receptors, was lower in the frontal cortex compared to the striatum. No obvious difference between the two brain areas was seen for the dissociation rate constant from the receptors (k ₄). Two schizophrenic patients were reexamined after pretreatment with haloperidol, one after 6 weeks of treatment with a low oral dose, the other one after an IV injection 1 h before 11C-clozapine was given. After haloperidol pretreatment, the binding of 11C-clozapine in striatum and frontal cortex was reduced, more pronounced in the striatum, indicating competition for D-2 dopamine binding sites. Our finding indicates that clozapine has an affinity for a receptor population in the frontal cortex that is predominantly not of the dopamine-D2 type. This feature might be of importance for the unique clinical profile of the drug.     

Regional distribution of the opioid receptor agonist N-(methyl-11C)pethidine in the brain of the rhesus monkey studied with positron emission tomography

The regional distribution and kinetics in the brain of Rhesus monkeys of N-(methyl-11C)-pethidine have been studied by positron emission tomography, PET. 11C-Pethidine reached the brain with peak radioactivities appearing within 6-10 min. after administration. Highest radioactivities were measured in areas corresponding to the thalamus, the striatal area and also the lowest transection of the temporal lobes, with an uptake of 2.7-3.1 times the homogenous dilution of the radioactive dose. Low radioactivities were seen in the cerebellum and the occipital lobes. This distribution corresponds to the regional density of opioid receptors using in vitro binding techniques. The 11C-pethidine derived radioactivity left the brain with an initial half-life of 40-60 min., followed by an elimination which paralleled the plasma elimination of unlabelled pethidine. After pretreatment of the monkey with a small dose of naloxone, the radioactivities decreased about 40% in areas corresponding to the thalamus, striatum and lowest section of the temporal lobes, indicating competition for the same binding sites. By the use of a three-compartment model, it was possible to get an estimate of 11C-pethidine receptor binding characteristics in the brain. The ratio of Kon/Koff, equal to Bmax./Kd, was 0.06-0.1. This indicates that pethidine is bound with low affinity to the opioid receptors and is a poor ligand for studies of opioid receptor function with PET. Brain kinetics of 11C-pethidine is mainly determined by its blood kinetics.     

11C-SCH 39166, a selective ligand for visualization of dopamine-D1 receptor binding in the monkey brain using PET

The new selective D₁-dopamine receptor antagonist SCH 39166 was labelled with the positron emitting isotope¹¹C and used as ligand for visualization of dopamine-D₁ receptor binding in Cynomolgus monkeys by PET. After intravenous administration of the ligand a marked uptake of radioactivity was recorded in the D₁-dopamine receptor-rich striatum and neocortex but not in the dopamine receptor-poor cerebellum. The uptake of radioactivity in striatum and neocortex was markedly displaced after the intravenous injection of a high dose of the D₁-dopamine receptor antagonist SCH 23390 but not after the 5-HT₂ receptor antagonist ketanserine.¹¹C-SCH 39166 should be a useful tool to explore D₁-dopamine receptor characteristics in the living human brain by PET.     

Swim stress alters in vivo binding of [3H]N-methylspiperone.

The effect of swim stress on the in vivo binding of [3H]N-methylspiperone in the striatum of the mouse was investigated. Mice were forced to swim for 5 min at 18 degrees C and the time course of radioactivity in the striatum and cerebellum, following intravenous injection of [3H]N-methylspiperone was measured. The ratio of radioactivity in the striatum to that in the cerebellum was plotted as a function of time for the estimation of in vivo binding to dopamine D2 receptors. Immediately after the swim stress, a significant decrease in binding to D2 receptors in vivo was observed. Neither the KD nor Bmax determined by in vitro binding were altered by swim stress. The time course of the changes in binding, within a 24 hr period, following the swim stress was also studied and a rapid reversal of binding, within 1 hr after the swim stress was observed. In vivo binding of [3H]N-methylspiperone in the cerebral cortex, which appeared to involve serotonin receptors, as well as D2 receptors, was not significantly altered by the swim stress. A saturation study of in vivo binding indicated that the decreases in binding to D2 receptors, due to swim stress, were primarily caused by changes in the apparent affinity rather than in the number of binding sites available in vivo. These results support the hypothesis that micro-environmental factors, including the diffusion barrier to the synapse, might be altered by swim stress.     

In vivo blockade of dopaminergic receptors from different rat brain regions by classical and atypical neuroleptics

The effect of haloperidol, chlorpromazine, clozapine, benzamides (sulpiride and isosulpride), 6-chloropyrimidines (mezilamine, UK 177) on the in vivo binding of [³H]spiroperidol to striatum, tuberculum olfactorium, frontal cortex and cerebellum in the rat brain was investigated. Since these neuroleptics of various chemical classes were able to prevent the selective retention of [³H]spiroperidol in the striatum and tuberculum olfactorium without modifying the level in the cerebellum, it has been assumed that [³H]spiroperidol is a suitable ligand to label dopaminergic receptors in the living animal. All the neuroleptics (except the benzamides) were able to displace [³H]spiroperidol from its receptors in the frontal cortex, suggesting a serotoninergic component in neuroleptic binding sites. Classical neuroleptics (haloperidol, chlorpromazine, UK 177) or atypical neuroleptics (clozapine, sulpiride, isosulpride, mezilamine) did not induce a selective blockade of dopaminergic receptors in the striatum or in the limbic system, respectively. These results indicate that there is no correlation between the selective regional stimulation of dopamine turnover after neuroleptics and the in vivo blockade of postsynpatic dopaminergic receptors.     

[11C]Ro 15-4513, a ligand for visualization of benzodiazepine receptor binding

Ro 15-4513, a partial inverse agonist at the benzodiazepine (BZ) receptor site was labelled with¹¹C and used for in vitro autoradiography on human post mortem brain sections and for positron emission tomography (PET) on Cynomolgus monkeys. The total radiochemical yield of [¹¹C]Ro 15-4513 was 30–40% with an overall synthesis time of 40 min. The specific radioactivity was about 1000 Ci/mmol at end of synthesis. In vitro autoradiography showed that [¹¹C]Ro 15-4513 bound specifically predominately in the neocortex of the human brain. Specific binding was also demonstrated in the basal ganglia and the cerebellar cortex. Flumazenil (Ro 15-1788) and clonazepam inhibited the binding in cerebral regions, but a significant proportion in the cerebellum was not inhibited by these agents. This proportion may represent ₆-containing BZ receptors. PET examination of [¹¹C]Ro 15-4513 binding in Cynomolgus monkeys demonstrated high uptake of radioactivity in neocortex. The uptake of radioactivity was markedly displaced by high doses of Ro 15-4513 or clonazepam. [¹¹C]Ro 15-4513 should be a useful ligand to examine BZ receptor characteristics in the living human brain by PET.     

PET studies of the uptake of (S)- and (R)-[11C]nicotine in the human brain: difficulties in visualizing specific receptor binding in vivo

(S)- and (R)-[¹¹C]nicotine were synthesized by methylation of (S)- and (R)-nornicotine using [¹¹C]methyl iodide. Following their intravenous injection in tracer doses to smoking and nonsmoking healthy males the radioactivity in arterial blood showed a sharp peak at about 1 min followed by a plateau level for the remaining 50 min of recording. Uptake in the brain, as measured by positron emission tomography (PET), was rapid with a peak at 5 min followed by a steady decline towards the end of the measurement. The regional distribution of radioactivity followed essentially the distribution of gray matter with high uptake in the cortex, the thalamus and the basal ganglia and low uptake in the pons, cerebellum and white matter. Levels of the labelled natural enantiomer, (S)-[¹¹C]nicotine, were higher than those of the synthetic enantiomer, (R)-[¹¹C]nicotine, particularly in the smokers. The time-activity curves of (S)-[¹¹C]nicotine uptake were not changed by co-administration of 1.0 mg of unlabelled nicotine with the labelled nicotine. Similarly administration of unlabelled nicotine at the peak of radioactivity, 6 min following (S)-[¹¹C]nicotine, had no effect on the time-activity curves. Thus essential criteria for visualizing receptor binding with the PET technique could not be fulfilled. Calculation of kinetic constants using a two-compartment model gave values indicating that the brain uptake of [¹¹C]nicotine is mainly determined by the cerebral blood flow, extraction of the tracer over the blood-brain barrier and unspecific binding. Thus¹¹C-labelled nicotine does not seem to be a suitable tracer for PET studies of nicotinic cholinergic receptors in the human brain.     

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.     

PET analysis of alcohol interaction with the brain disposition of [11C]flumazenil

Acute alcohol administration to rats has in preliminary studies been reported to drastically increase the binding of the benzodiazepine (BZ) receptor antagonist [³H]flumazenil (Ro 15-1788) to central BZ receptors. In the present study the effect of acute alcohol ingestion on the disposition of [¹¹C]flumazenil in the human brain and plasma was examined by positron emission tomography (PET) in four healthy volunteers. Neocortex, cerebellum and pons (reference region) were delineated using X-ray computerized tomography (CT). Alcohol did not increase either total radioactivity uptake or specific [¹¹C]flumazenil binding in neocortex or cerebellum. However, alcohol had a small but significant effect on [¹¹C]flumazenil in arterial blood. After alcohol the plasma radioactivity peak was higher, more narrow and occurred earlier than in the control experiments. The present experiments contradict the view that alcohol directly affects central BZ receptor binding in man. Thus the dramatic increase of flumazenil binding in rat brain reported previously could not be observed in the human brain.     

Reduction of in vivo binding of [3H]paroxetine in mouse brain by 3,4-methylenedioxymethamphetamine.

The effects of 3,4-methylenedioxymethamphetamine (MDMA) on the in vivo binding of [3H]paroxetine, a potent and selective 5-hydroxytryptamine (5-HT; serotonin) uptake inhibitor, in the brain of the mouse were studied. The distribution of radioactivity in the brain of the mouse, after intravenous administration of [3H]paroxetine, was significantly altered by pretreatment with MDMA (15 mg/kg, i.p., 3 hr before). The hypothalamus/cerebellum and cerebral cortex/cerebellum ratios, as a function of time, were significantly decreased after the pretreatment with MDMA, indicating that the in vivo binding of [3H]paroxetine to uptake sites for 5-HT in the brain of the mouse was significantly decreased by MDMA. These ratios could reflect those of the total binding, to the non-specific binding and free ligand, since the cerebellum has very low levels of binding for [3H]paroxetine. Furthermore, these ratios decreased after pretreatment with MDMA, in a dose-dependent manner. However, the binding of [3H]paroxetine to membranes from the brain of the mouse in vitro was not altered by treatment with MDMA. The discrepancy between the in vivo binding and in vitro binding of [3H]paroxetine in the brain of the mouse is discussed.     

Clozapine binds preferentially to cortical D1-like dopamine receptors in the primate brain: a PET study

Rationale The D₁-like dopamine receptors have been suggested to play a role in the pathophysiology and treatment of schizophrenia. Previous positron emission tomography studies have demonstrated that the atypical antipsychotic clozapine occupies D₁-like dopamine receptors in the striatum in clozapine-treated patients. Objectives The aim of the present study was to compare striatal and cortical D₁-like dopamine receptor occupancy by clozapine in the primate brain. Methods Three monkeys were each examined three times at the same day with the radioligand (+)−[¹¹C]NNC 112. The first measurement was at baseline conditions, the second after 1.5 mg/kg and the third after 6 mg/kg clozapine IV. To compare regional levels of nonspecific binding in brain regions, an additional monkey was examined using the inactive enantiomer (−)−[¹¹C]NNC 112. Receptor occupancy was calculated using both the equilibrium–ratio analysis and the simplified reference tissue model. Results After 1.5 mg/kg the D₁-like dopamine receptor occupancy ranged from 30 to 38% in the striatum, whereas the range was 51 to 57% in the frontal cortex. After 6.0 mg/kg the occupancy was 53 to 64% in the striatum and 63 to 83% in the frontal cortex. The differences between striatal and cortical D₁-like receptors occupancy were between 12 and 25%. The study with (−)−[¹¹C]NNC 112 did not show regional differences in nonspecific binding that might explain the regional differences in occupancy. Conclusions The higher D₁-like dopamine receptor occupancy in the frontal cortex may reflect a different distribution of the D₁ and D₅ dopamine receptor subtypes among brain regions and different affinity of clozapine for the two subtypes. The finding supports the suggestion that binding to D₁-like dopamine receptors may explain clozapine’s atypical drug actions.     

Spiperone: a ligand of choice for neuroleptic receptors. 3. Subcellular distribution of neuroleptic drugs and their receptors in various rat brain areas.

Tissue fractionation was used as an analytical tool to study in various rat brain regions the subcellular distribution of the neuroleptic receptor and labeled neuroleptics previously injected into the animals. In order to assess the composition of the different subcellular fractions, the distribution pattern of various marker enzymes was also determined.After differential centrifugation of rat striatum homogenate, [³H]spiperone binding receptors together with 5′ nucleotidase were found to be mainly enriched in the microsomal fraction. Similarly, after injection of [³H]spiperone or [³H]pimozide into rats, the radioactivity was specifically recovered in the microsomal fraction in the striatum, the olfactory tubercle and nucleus accumbens and the frontal cortex but not in the cerebellum.After equilibration through a sucrose density gradient, the distribution pattern of [³H]spiperone revealed a main peak in a gradient region of low density very close to that of 5′ nucleotidase. The present results indicate that the receptor sites of neuroleptic drugs in the brain dopaminergic areas is associated with membrane-like structures but not with mitochondria or nerve terminals containing dopamine.     

Regional in vivo binding of [3H]N-propylnorapomorphine in the mouse brain. Evidence for labelling of central dopamine receptors

Tain vein injections of [³H]N-propylnorapomorphine ([³H]NPA) in male mice resulted in a dose-related accumulation of radioactivity in the following brain regions: striatum (max), olfactory tubercle and cerebellum (min). The specific binding was saturable with increasing concentrations of the drug and stereospecifically displaced by (+)butaclamol. Dopamine agonists (apomorphine, NPA and bromocriptine) and antagonists (spiperone, haloperidol, (+)butaclamol and l-sulpiride) all caused dose-dependent prevention of [³H]NPA binding. Mianserin, phenoxybenzamine and propranolol did not prevent the in vivo [³H]NPA binding suggesting that [³H]NPA binds specifically to dopamine receptors in the striatum and the olfactory tubercle of the mouse.     

PET examination of [11C]NNC 687 and [11C]NNC 756 as new radioligands for the D1-dopamine receptor

The benzazepines NNC 687 and NNC 756 have in animal studies been described as selective D₁-dopamine receptor antagonists. Both compounds have been labeled with¹¹C for examination by positron emission tomography (PET). In the present study central receptor binding was studied in monkeys and healthy men. After IV injection of both radioligands in Cynomolgus monkeys radioactivity accumulated markedly in the striatum, a region with a high density of D₁-dopamine receptors. This striatal uptake was displaced by high doses of the selective D₁-antagonist SCH 23390 (2 mg/kg) but not by the 5HT₂-antagonist ketanserin (1.5 mg/kg) or the selective D₂-antagonist raclopride (3 mg/kg). The cortical uptake after injection of [¹¹C]NNC 687 was not reduced in displacement experiments with ketanserin. The cortical uptake of [¹¹C]NNC 756 was reduced in displacement and protection experiments with ketanserin by 24–28% (1.5 mg/kg), whereas no reduction could be demonstrated on striatal uptake. In healthy males both compounds accumulated markedly in the striatum. For [¹¹C]NNC 687 the ratio of radioactivity in the putamen to cerebellum was about 1.5. For [¹¹C]NNC 756 the ratio was about 5. This ratio of 5 for [¹¹C]NNC 756 is the highest obtained so far for PET radioligands for the D₁-dopamine receptor.     

Synthesis of the serotonin transporter ligand (±)‐10‐methyl 3‐[6‐nitro‐(2‐quinolinyl)]‐3,10‐diazabicyclo‐[4.3.1]‐decane ([11C‐methyl]NS 2495) and first in vivo results

The serotonin transporter ligand (±)‐10‐[¹¹C]‐methyl 3‐[6‐nitro‐(2‐quinolinyl)]‐3,10‐diazabicyclo‐[4.3.1]‐decane ([¹¹C‐methyl]NS 2495) was synthesized via a methylation reaction with [¹¹C]methyl iodide. The radiochemical purity exceeded 99% and the specific radioactivity was found to be 1.8 GBq/μmol at 40 min after the end of bombardment. The uptake of the tracer in the brain of a living pig was recorded by positron emission tomography (PET), first in a baseline condition, and again after treatment with citalopram (1 mg/kg, i.v.) to displace the specific binding. The distribution volume relative to the metabolism‐corrected arterial input was high in pig brain, ranging from 75–150 ml g^?1; treatment with citalopram uniformly reduced the distribution volume to 75 ml g^?1. Binding potential (pB) maps generated using the cerebellum as a reference tissue showed highest binding in the mesencephalon and cingulate cortex, where the magnitude of pB was close to 0.6. Thus, the pattern of binding in vivo agrees with the known pattern of serotonin innervations in pig brain. However, the specific binding was incompletely displaced by pre‐treatment with citalopram. Thus, [¹¹C‐methyl]NS 2495 can label serotonin transporters in a PET study of the brain of a living pig, but full displacement by cold citalopram was not obtained in vivo, possibly reflecting binding sites which are inaccessible to citalopram. Copyright © 2003 John Wiley & Sons, Ltd.     

Regional in vivo binding of the substituted benzamide [3H]eticlopride in the rat brain: evidence for selective labelling of dopamine receptors

The novel substituted benzamide eticlopride, (S)-(-)-5-chloro-3-ethyl-N-[(1-ethyl-2-pyrrolidinyl)methyl]-6-methoxy salicylamide hydrochloride (A38503; FLB 131), was radiolabelled to high specific activity and used for in vivo receptor binding studies in the rat brain. Intravenous injections of [3H]eticlopride resulted in a rapid accumulation of radioactivity in several brain regions: striatum greater than olfactory tubercle greater than septum greater than substantia nigra greater than frontal cortex greater than cerebellum. Approximately 95% of the radioactivity recovered from the striatum was in the form of authentic eticlopride, as determined by thin-layer chromatography. Two hours after injection, the ratio between the amount of radioactivity present in the striatum and in the cerebellum was approximately 10:1. The in vivo binding of [3H]eticlopride was saturable in all dopamine-rich areas, with a very low proportion of non-specific binding. The specific in vivo binding of [3H]eticlopride was blocked by several dopamine antagonists, including haloperidol, (+)-butaclamol, spiperone, d,l-sulpiride and remoxipride. The dopamine agonist N-n-propylnorapomorphine, but not apomorphine, was found to be a potent blocker of in vivo [3H]eticlopride binding. Serotonin and noradrenaline receptor antagonists did not prevent the in vivo binding of [3H]eticlopride. Autoradiographic analysis of the in vivo [3H]eticlopride binding showed a high density of binding sites in the striatum, nucleus accumbens and the olfactory tubercle. Moderate binding was found in the hippocampal formation and in the entorhinal area, but little or no binding was detected in other cortical regions. [3H]Eticlopride binding in all these areas was blocked by pretreatment with (+)-butaclamol. Taken together, these findings show that the substituted benzamide compound [3H]eticlopride passes readily into the brain and binds with high specificity to dopamine or neuroleptic receptors in dopamine-rich brain areas. Thus, eticlopride may be a useful tool in studies of dopamine D-2 receptors in vivo.     

Alterations of the dopaminergic and glutamatergic neurotransmission in adult rats with postnatal ibotenic acid hippocampal lesion

In 6-week and 8-week-old rats (pre- and postpubertally) with neonatal excitotoxic lesions of the ventral hippocampus with ibotenic acid (IBO), we have studied apomorphine-induced motor activity and glutamate and dopamine D₁ and D₂ binding sites in the hippocampus, striatum, nc. accumbens and frontal cortex as well as K⁺-stimulated (³H)-D-aspartate release from hippocampal and frontal cortical slices. Specific glutamate binding was enhanced in the frontal cortex of 8-week-old IBO-treated animals, whereas that in other brain regions remained unchanged. Both D₁ and D₂ binding sites were downregulated in the striatum without changes in other brain structures. In 6-week-old rats, neither the glutamate nor the dopamine binding sites were altered. The amino acid release from hippocampal and frontal cortical slices of adult IBO treated rats was significantly decreased in comparison to controls, whereas in 6-week-old rats, no significant alterations were detectable. The additionally monitored motor activity was enhanced only in adult IBO-lesioned rats after apomorphine pretreatment. The present data are in agreement with the hypothesis of hyperactive dopamine and hypoactive glutamate systems in schizophrenia and are discussed in the light of schizophrenia-like behavioral changes in rats after postnatal hippocampal IBO lesion. Received: 10 August 1998/Final version: 11 February 1999     

NCQ 298, a new selective iodinated salicylamide ligand for the labelling of dopamine D2 receptors

NCQ 298 ((S)-3-iodo-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5,6-dimethoxysalicylamide) has an iodine substituent. We have labelled NCQ 298 with¹²³I and¹²⁵I, and used the radioligands as tracers in receptor studies in vitro, in vivo in autoradiography and in SPECT studies on Cynomolgus monkeys.[¹²⁵I]NCQ 298 bound in vitro to a single binding site with a K^D=19 pM. NCQ 298 has thus a 10-fold higher affinity for the dopamine D₂ receptors than the corresponding des-5-methoxy compound FLA 961 (IBZM), previously used in SPECT studies. The binding of [¹²⁵I]NCQ 298 was entirely reversible (T1/2=17.5 min at 37° C). Autoradiographical studies in vitro on rat and monkey brain tissue sections showed a distinct binding in caudate-putamen, nucleus accumbens, substantia nigra, and in layer 5 of the cerebral cortex. In vivo binding studies in mice showed a ratio of 10 between [¹²⁵I]NCQ 298 binding in striatum and cerebellum. Binding was displaced by the selective dopamine D₂ receptor antagonist raclopride. In SPECT studies with [¹²³I]NCQ 298 in two Cynomolgus monkeys, radioactivity accumulated in the basal ganglia. The measured striatum to cerebellum ratio was about 15 after 3 h. A monkey brain phantom was constructed for the determination of conversion factors from pixel events to actual radioactivity. The resulting, corrected striatum to cerebellum ratio obtained was 30. After administration of 12 mg raclopride to one of the monkeys there was a substantial decrease in striatal radioactivity. [¹²⁵I]NCQ 298 is a suitable ligand for the labelling of dopamine D₂ receptors in vitro and in vivo. The specific properties of [¹²³I]NCQ 298 suggest that this compound is a useful ligand for quantitative SPECT studies of dopamine D₂ receptors in man.     

In vivo visualization of central muscarinic receptors using [11C]quinuclidinyl benzilate and positron emission tomography in baboons.

The muscarinic antagonist, quinuclidinyl benzilate (QNB), labeled with carbon 11 was used as a radioligand to visualize in vivo by positron emission tomography (PET) the central muscarinic acetylcholine receptors (mAChR) in baboons (Papio papio). The binding characteristics of [11C]QNB showed its specific binding to central mAChR. [11C]QNB brain uptake was high in cerebral cortex and striatum, areas that are rich in mAChR, whereas it decreased rapidly in cerebellum, evidencing non-specific binding in this structure that is almost devoid of mAChR. These results are consistent with the known cerebral distribution of mAChR in primates. [11C]QNB specific cerebral binding was enhanced by pretreatment with methyl-QNB, a peripherally acting muscarinic antagonist. Specifically labeled binding sites alone were blocked by prior administration of dexetimide, a muscarinic antagonist. Specific radioactivity was driven out from mAChR-rich regions by atropine and dexetimide, drugs with high affinity for mAChR. This competition was stereospecific since only dexetimide, the pharmacologically active isomer of benzetimide, was able to compete with the radioligand on its binding sites. A relationship between the occupancy of [11C]QNB-labeled receptors by atropine or dexetimide and the concomitant induction of a pharmacological effect was also detected by simultaneous PET scanning and electroencephalographic recording. Since mAChR form an important part of choline receptors in the central nervous system, [11C]QNB appears to be a suitable radiotracer to monitor cerebral physiological or pathological phenomena linked to the cholinergic system in living subjects.     

The Involvement of Sigma and Phencyclidine Receptors in the Action of Antipsychotic Drugs

Abstract: An atypical antipsychotic drug clozapine and a selective sigma antagonist BMY 14802 were significantly less effective in the behavioural experiments (against apomorphine, d-amphetamine and MK-801), as well in the radioligand binding studies against ³H-spiperone (dopamine₂-receptors) and ³H-haloperidol (sigma receptors) in the rat brain, as compared to a typical antipsychotic compound haloperidol. Contrary to haloperidol and BMY 14802, clozapine was a relatively selective antagonist of MK-801-induced motor excitation in the mouse. A nearly 3-fold lower dose of clozapine was needed to block the effect of MK-801 (6.4 μmol/kg) as compared to the action of amphetamine (17 μmol/kg). Haloperidol and clozapine, but not BMY 14802, antagonized apomorphine-induced aggressiveness in the rat. After long-term treatment (for 15 days) with BMY 14802 (10 mg/kg daily), haloperidol (0.5 mg/kg daily) and clozapine (10 mg/kg daily) the motor depressant effect of apomorphine (0.15 mg/kg) was reversed. Chronic haloperidol treatment, but not administration of BMY 14802 and clozapine, increased the number of dopamine₂-receptors in the rat brain. BMY 14802 caused upregulation of sigma receptors in frontal cortex, whereas haloperidol induced the opposite change in cerebellum. Repeated treatment with clozapine significantly augmented the motor stimulating effect of MK-801 in rats. Simultaneously with a behavioural change the density of ³H-TCP binding sites in the rat forebrain was elevated after long-term treatment with clozapine, probably indicating the involvement of PCP binding sites at NMDA channel in the action of clozapine.