Imaging D2 receptor occupancy by endogenous dopamine in humans

The impact of endogenous dopamine on in vivo measurement of D2 receptors in humans was evaluated with single photon emission computerized tomography (SPECT) by comparing the binding potential (BP) of the selective D2 radiotracer [123I]IBZM before and after acute dopamine depletion. Dopamine depletion was achieved by administration of the tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine (AMPT), given orally at a dose of 1 g every six hours for two days. AMPT increased [123I]IBZM BP by 28 +/- 16% (+/- SD, n = 9). Experiments in rodents suggested that this effect was due to removal of endogenous dopamine rather than D2 receptor upregulation. Synaptic dopamine concentration was estimated as 45 +/- 25 nM, in agreement with values reported in rodents. The amplitude and the variability of the AMPT effect suggested that competition by endogenous dopamine introduces a significant error in measurement of D2 receptors in vivo with positron emission tomography (PET) or SPECT. However, these results also imply that D2 receptor imaging coupled with acute dopamine depletion might provide estimates of synaptic dopamine concentration in the living human brain.     

In vivo imaging of brain nicotinic acetylcholine receptors with 5-[123I]iodo-A-85380 using single photon emission computed tomography

The distribution and kinetics of 5-[123I]iodo-A-85380, a novel ligand for brain nicotinic acetylcholine receptors (nAChRs), were evaluated in the Rhesus monkey using single photon emission computed tomography (SPECT). Peak levels of radioactivity were measured in brain at 90 min after injection of the tracer. Accumulation of radioactivity was highest in the thalamus, intermediate in the frontal cortex and basal ganglia, and lowest in the cerebellum. The ratio of specific to nonspecific binding (V3") in the thalamus, estimated from the (thalamic-cerebellar)/cerebellar radioactivity ratio, reached a value of 6 at 4 h post-injection. Specific binding was reduced by subcutaneous injection of 1 mg/kg cytisine at 2.25 h after injection of radiotracer. At 2.5 h after cytisine administration, radioactivity in the thalamus was reduced by 84%, in the frontal cortex, by 76%, and in the basal ganglia, by 57% of the level measured at the time of cytisine administration, demonstrating that the binding was reversible. On the basis of these findings, together with other data indicating high affinity, receptor subtype selectivity, low nonspecific binding and lack of toxicity in animals, 5-[123I]iodo-A-85380 appears to be a promising ligand for SPECT imaging of nAChRs in the human brain.     

Iodine-123-iodobenzamide binding in parkinsonism: reduction by dopamine agonists but not L-Dopa

The aim of the present study was to investigate the effect of treatment with L-Dopa or a dopamine agonist, or both, on specific striatal 123I-iodobenzamide (IBZM) binding using an intraindividual longitudinal design. METHOD: We prospectively studied the effect of dopaminomimetic treatment on specific [123I]IBZM binding measured by SPECT in 29 patients with a clinical diagnosis of Parkinson's disease, none of whom had previously received dopaminomimetic drugs. The patients had been selected on the basis of normal subsequent specific [123I]IBZM binding, semiquantitatively calculated as the basal ganglia/frontal cortex ratio, and a positive response to the dopamine agonist apomorphine before initiation of dopaminomimetic therapy. A second 123I-IBZM SPECT investigation was performed after 3-6 mo of treatment with L-Dopa or a dopamine agonist, or both. RESULTS: Specific [123I]IBZM binding was unchanged in 10 patients treated with L-Dopa alone. However, after treatment with a dopamine agonist there was a significant decline in specific [123I]IBZM binding (p < 0.05). After treatment with a combination of L-Dopa and a dopamine agonist, specific [123I]IBZM binding was reduced without reaching a level of significance (p = 0.08). CONCLUSION: Short-term treatment with a dopamine agonist but not with L-Dopa reduces specific [123I]IBZM binding. Therefore, before performing an [123I]IBZM SPECT scan in patients previously treated with dopaminomimetic drugs, dopamine agonists should be discontinued.     

SPECT imaging of dopamine transporters in human brain with iodine-123-fluoroalkyl analogs of beta-CIT

Iodine-123-2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane (beta-CIT) is a useful SPECT tracer for imaging the dopamine transporter. Its slow kinetics, however, necessitate imaging on the day after the injection. Two N-omega-fluoroalkyl analogs of beta-CIT, the fluoropropyl and fluoroethyl compounds (beta-CIT-FP and beta-CIT-FE, respectively), characterized by faster kinetics in baboons, were tested in humans as potential tracers for the dopamine transporter. Four healthy volunteers were injected with [123I]-beta-CIT-FP and another four were injected with [123I]beta-CIT-FE. SPECT data were acquired for 1149 +/- 590 min and 240 +/- 30 min, respectively. Both tracers demonstrated high brain uptake (6.37% +/- 0.37% and 7.8% +/- 1.5% of the injected dose, respectively). Activity concentrated with time in the striatal area, reaching a peak within 30 min, with little or no washout for [123I]beta-CIT-FP and a faster washout for [123I]beta-CIT-FE (14.7% +/- 6.9%). Occipital and midbrain activity showed similar patterns, displaying a peak within 15 min and rapid washout, followed by stable levels at approximately 100 min for both tracers. The ratio of peak specific striatal-to-peak specific midbrain activity was 9.1 +/- 1.8 for [123I]beta-CIT-FP and 7.7 +/- 0.7 for [123I]beta-CIT-FE, showing high in vivo selectivity for the dopamine transporter. These preliminary results suggest that both compounds could be used as SPECT (labeled with 123I) or PET (labeled with 18F) radiotracers to image the dopamine transporters in the living human brain.     

In vitro affinity of piribedil for dopamine D3 receptor subtypes, an autoradiographic study

Receptor binding autoradiography, using the selective ligand [3H]7-OH-(R)DPAT (R(+)-2-dipropylamino-7-hydroxy 1,2,3,4-tetrahydronaphthalene), showed that piribedil is a potent inhibitor at dopamine D3 receptors in limbic regions (island of Calleja), with affinity (IC50) between 30 and 60 nM. The in vitro IC50 of piribedil for inhibition of [3H]spiperone binding to receptors of the dopamine D2-like family (D2, D3 and D4), ranged between 10(-7) and 10(-6) M in different brain regions (medial and lateral caudate putamen, olfactory tubercles, and nucleus accumbens). At the highest concentration tested (10(-5 M) piribedil inhibited dopamine D1 receptor binding by < 50%. It is concluded that piribedil has 20 times higher affinity for dopamine D3 than for dopamine D2-like receptors, and very low affinity for the dopamine D1 receptor subtype in rat brain. How this pattern of receptor affinity is related to the pharmacological profile of piribedil deserves further investigation.     

Up-regulation of estrogen receptor mRNA and estrogen receptor activity by estradiol in liver of rainbow trout and other teleostean fish

[125I]- and [123I]NNC 13-8241 were prepared from the trimethyltin precursor and radioactive iodide using the chloramine-T method. The total radiochemical yields of [125I]- and [123I]NNC 13-8241 were 60-70% and 40-50% respectively, with radiochemical purity higher than 98%. In binding studies with [125I]NNC 13-8241 in rats in vitro and in vivo a high uptake of radioactivity was demonstrated in brain regions known to have a high density of benzodiazepine (BZ) receptors such as the occipital and frontal cortex. SPECT examination with [123I]NNC 13-8241 in a Cynomolgus monkey demonstrated a high uptake of radioactivity in the occipital and frontal cortex. After displacement with flumazenil radioactivity in these brain regions was reduced to the level of a central region including the pons. Four hours after injection about 80% of the radioactivity in monkey plasma represented unchanged radioligand. This low degree of metabolism indicates that NNC 13-8241 is metabolically more stable than the radioligands hitherto developed for imaging of BZ-receptors in the primate brain.     

Differential coupling of rat D2 dopamine receptor isoforms expressed in Spodoptera frugiperda insect cells

By using a baculovirus expression system, the two isoforms of the rat D2 dopamine receptor were expressed at densities ranging up to 15 pmol/mg of protein. D2L and D2S dopamine receptors expressed in aline of Spodoptera frugiperda (Sf9) insect cells Sf9cells, displayed high affinity for the antagonists spiroperidol and (+)-butaclamol and the agonist N-propylnorapomorphine. Antisera raised against the D2 receptor immunoprecipitated binding sites for a radiolabeled D2 antagonist from solubilized extracts of infected Sf9cells. In immunoblots of Sf9cells infected with recombinant D2 baculovirus, these antisera recognized a major species of protein of approximately 46 kDa. Photoaffinity-labeling of infected Sf9cells using N-(p-azido-m-[125I]iodophenethyl)spiperone also identified a protein of this size, suggesting that D2 receptors expressed in Sf9cells are largely unglycosylated. In cells expressing receptors at a density greater than 1 pmol/mg, GTP-sensitive, high-affinity binding of agonists was not detected in studies of the inhibition of the binding of a radiolabeled D2 antagonist. When expression levels were under 1 pmol/mg, the binding of agonists was sensitive to the addition of guanine nucleotides, indicating that D2 receptors were coupled to endogenous G proteins. Endogenous G proteins enable both isoforms of D2 receptors to couple to the inhibition of adenylyl cyclase activity. The high-affinity state of the D2 receptor was directly measured using a radiolabeled agonist. Although the density of receptors increased with longer times after infection, the density of high-affinity sites reached a maximum of approximately 40 fmol/mg 30 to 36 hr after infection. Coexpression of D2 receptors and G protein subunits in Sf9cells dramatically increased the density of high-affinity sites, whereas the total density of receptors was unchanged, confirming that D2 receptors in Sf9 cells can exist in the high-affinity-coupled state, but that appropriate G proteins are expressed at relatively low levels. The density of D2S receptors converted to a coupled, agonist-preferring state when coexpressed with G proteins subunits (alpha i1, beta 1 and gamma 2) was 5 times greater than that of D2L receptors expressed under the same conditions, consistent with the hypothesis that D2 dopamine receptor isoforms differentially couple to alpha i1.     

An improved method for rapid and efficient radioiodination of iodine-123-IQNB

The SPECT radioligand, 3-quinuclidinyl-4-[123I]iodobenzilate ([123I]IQNB), binds to muscarinic receptors and has generated interest as a potential agent for clinical SPECT. Unfortunately, cumbersome and inefficient radioiodination procedures have limited the practicality of [123I]IQNB SPECT imaging. METHODS: We report a rapid (5 min) and simple radioiodination procedure for preparing [123I]IQNB from a tri-n-butylstannyl precursor in a no-carrier-added reaction that yields high specific activity with radiochemical yield exceeding 60%. The radiochemical purity of the final product exceeds 95%. RESULTS: We have used this procedure to radioiodinate the four stereoisomers of [123I]IQNB. The procedure is highly reliable and reproducible. SPECT studies on a healthy human volunteer at 1, 2, 6 and 24 hr after injection of each of the four stereoisomers reveal expected differences in the kinetic and binding characteristics of the four stereoisomers. (R,S)-[123I]IQNB appears to be the SPECT agent of choice. CONCLUSION: Radioiodination of [123I]IQNB from our tri-n-butylstannyl precursor is simpler, more efficient and less expensive than previous techniques. The potential exists for a "kit" which would be practical in a typical clinical setting.     

Influence of neonatal treatment with the pyrethroid insecticide cypermethrin on the development of dopamine receptors in the rat kidney

The influence of neonatal treatment with the pyrethroid insecticide cypermethrin ((R,S)alpha-cyano-3-phenoxybenzyl (1R,S)-cis-trans-3-(2,2-dichloro-vinyl)-2,2-dimethylcyclopropane carboxylate) on postnatal development of renal dopamine receptors was investigated by radioligand binding assay techniques. Treatment with cypermethrin was made on rats from the 10th to the 16th day after birth. Dopamine D1- and D2-like receptors were assayed in frozen sections of kidney of 21-, 30-, 60- and 90-day-old rats using as ligands of dopamine D1- and D2-like receptors [3H]([R](+)-(chloro-2,3,4,5,-tetrahydro-5-phenyl-1,4,-benzazepinal hemimaleate) (SCH 23390) and [3H]spiperone, respectively. Treatment with cypermethrin was without effect on the affinity (Kd value) or the density (Bmax value) of dopamine D1- and D2-like receptors of rats of 21 days of age. In older groups, treatment with the compound reduced the affinity and increased the density of dopamine D1-like receptors, whereas it was without effect on the affinity of dopamine D2-like receptors and decreased their density. These findings indicate that neonatal treatment with the pyrethroid insecticide cypermethrin induces long-lasting impairment of renal dopamine D1- and D2-like receptors and that kidney is a target of the toxic action of the compound. Renal dopamine receptor changes caused by cypermethrin are consistent with possible alterations of renal tubular function and of sympathetic neuroeffector modulation. The above data suggest also that, different from the adult, neonatal exposure to pyrethroid insecticides may induce toxic effects.     

The interdisciplinary team''s approach to lupus nephritis

BACKGROUND: Animal and postmortem studies indicate that neuroleptic therapy may induce D2 dopamine receptor up-regulation in the basal ganglia. METHODS: To address this phenomenon in a clinical study, we investigated the D2 dopamine receptor binding in 15 DSM-III-R schizophrenics in the drug-naive state and 3 days after completion of a standardized neuroleptic therapy (benperidol 12-16 mg/day, for 25 days) using single photon emission computed tomography (SPECT). SPECT scans were obtained 2 hours after intravenous injection of 185 MBq 123I-iodobenzamide. For analysis, basal ganglia to frontal cortex (BG/FC) ratios were calculated and the patient sample was subgrouped into patients with a favorable versus a poor treatment response. RESULTS: Neuroleptic treatment led to decreased BG/FC ratios in patients with a favorable response, but increased ratios in the poor responders (df = 1, F = 4.1, p = .06). Changes of BG/FC ratios were significantly correlated with extrapyramidal side effects but not with neurological soft signs. CONCLUSIONS: Our findings suggest that neuroleptic therapy may induce D2 dopamine receptor up-regulation in a subgroup of patients characterized by poor treatment response and pronounced extrapyramidal side effects.     

Regional distribution of dopamine D4 receptors in rat forebrain

Binding of the D2-like (D2/D3/D4) radioligand [3H]nemonapride under selective conditions (with 300 nM S[-]-raclopride and other masking agents to occlude D2/D3 receptors and non-specific binding sites) revealed a subset of raclopride-insensitive binding sites considered D4-like receptors. These sites were stereoselective to R(-)-N-n-propylnorapomorphine (NPA) over its S(+)-NPA in a similar fashion to cloned D4 receptors expressed in cell lines. In addition, the highly D4-selective agent L-745,870 displaced 74-83% of these sites in rat brain regions, suggesting that most were D4 receptors. These apparent D4 receptors represented a relatively high proportion of D2-like receptors in hippocampus, dorsolateral frontal, medial prefrontal and entorhinal cortex, but fewer in caudate-putamen and nucleus accumbens.     

Iodine-123-beta-CIT and iodine-123-FPCIT SPECT measurement of dopamine transporters in healthy subjects and Parkinson's patients

Iodine-123-beta-carbomethoxy-3 beta-(4-iodophenyltropane) (CIT) has been used as a probe of dopamine transporters in Parkinson's disease patients using SPECT. This tracer has a protracted period of striatal uptake enabling imaging 14-24 hr postinjection for stable quantitative measures of dopamine transporters, and it binds with nanomolar affinity to the serotonin transporter. Iodine-123 fluoropropyl (FP)CIT is an analog of [123I]-beta-CIT and has been shown to achieve peak tracer uptake in the brain within hours postinjection and to provide greater selectivity for the dopamine transporter. The purpose of the present study was to compare [123I]-beta-CIT with [123I]-FPCIT in a within-subject design. METHODS: Six Parkinson's disease patients and five healthy control subjects participated in one [123I]-beta-CIT and one [123I]-FPCIT SPECT scan separated by 7-21 days. Controls were imaged at 24 hr postinjection 222 MBq (6 mCi) [123I]-beta-CIT and serially from 1-6 hr postinjection 333 MBq (9 mCi) [123I]-FPCIT. Two imaging outcome measures were evaluated: (a) the ratio of specific striatal activity to nondisplaceable uptake, also designated V"3, at each imaging time point; and (b) the rate of striatal washout of radiotracer expressed as a percent reduction per hr for [123I]-FPCIT. In addition, venous plasma was obtained from the five control subjects after the [123I]-FPCIT injection for analysis of radiometabolites. RESULTS: Both [123I]-FPCIT and [123I]-beta-CIT demonstrated decreased striatal uptake in Parkinson's disease patients compared with the controls with a mean of V"3=3.5 and 6.7 for [123I]-beta-CIT (Parkinson's disease and controls, respectively) and a mean of V"3=1.34 and 3.70 for [123I]-FPCIT (Parkinson's disease and controls, respectively). For [123I]-beta-CIT, the mean Parkinson's disease values represented 52% of the control uptake, while the mean [123I]-FPCIT value for Parkinson's disease patients was 37% of the control values. Analysis of [123I]-FPCIT time-activity curves for specific striatal counts showed washout rates of 8.2%/hr for Parkinson's disease and 4.9%/hr for controls. CONCLUSION: These data suggest that SPECT imaging with [123I]-FPCIT visually demonstrates reductions in striatal uptake similar to [123I]-beta-CIT. iodine-123-FPCIT washed out from striatal tissue 15-20 times faster than [123I]-beta-CIT, and estimates of dopamine transporter loss in Parkinson's disease patients were higher for [123I]-FPCIT than for [123I]-beta-CIT. This was most likely due to the faster rate of striatal washout and establishment of transient equilibrium binding conditions at the dopamine transporter, which the modeling theory suggests produces an overestimation of dopamine transporter density with relatively greater overestimates in healthy control subjects by [123I]-FPCIT.     

Associative and limbic regions of monkey striatum express high levels of dopamine D3 receptors: effects of MPTP and dopamine agonist replacement therapies

The role of the dopamine D3 receptor subtype in the central nervous system is still not well understood. It has a distinct and restricted distribution, mostly associated with limbic territories of the striatum (olfactory tubercle and the shell of nucleus accumbens) in rat brain. Dopaminergic denervation induced by a 6-hydroxydopamine lesion of the nigrostriatal system in rat down-regulates the expression of the D3 receptor. In the present study, we investigated the functional neuroanatomy of the dopamine D3 receptor subtype in the monkey (Macaca fascicularis) basal ganglia. We also studied the effect of administration of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and chronic D1-like (SKF 82958) or D2-like (cabergoline) agonist treatments on dopamine D3 receptor levels using receptor autoradiography. Our results clearly show that the distribution of D3 receptors in the monkey is more closely related to associative and limbic components of the striatum (caudate-putamen), as compared with its sensorimotor counterpart. Hence, D3 receptors may be more specifically involved in cognitive and motivational aspects of striatal functions, which are elaborated in prefrontal, temporal, parietal, cingulate and limbic cortices. Moreover, MPTP administration significantly decreased levels of D3 receptors and this effect was reversed or compensated by a chronic treatment with a D1-like, but not a D2-like, receptor agonist. The D3 receptor may represent an important target for adjunct or direct therapy designed to improve cognitive deficits observed in patients with Parkinson's disease, schizophrenia and other illnesses with frontal lobe cognitive disturbances.     

Imaging and quantitation of dopamine transporters with iodine-123-IPT in normal and Parkinson's disease subjects

Iodine-123-N-(3-iodopropene-2-yl)-2beta-carbomethoxy-3beta-( 4-chlorophenyl) tropane (123I-IPT) is a new dopamine transporter ligand that selectively binds the dopamine reuptake sites. Transporter concentrations have been known to decrease in Parkinson's disease patients. The purpose of this study was to evaluate the usefulness of IPT as an imaging agent for measuring changes in transporter concentrations in Parkinson's disease. METHODS: IPT labeled with 6.78 +/- 0.67 mCi 123I was injected intravenously as a bolus into eight normal controls (mean age 41 +/- 12 yr) and 17 Parkinson's disease patients (mean age 55 +/- 9 yr). Dynamic SPECT scans of the brain were then performed for 5 min each over 120 min on a triple-headed gamma camera equipped with medium-energy collimators. Regions of interest were drawn on the middle set of the image at the level of the basal ganglia (BG) for each subject. Time-activity curves were generated for the left BG, right BG and occipital cortex (OCC). The empirical ratios between BG-OCC and OCC, which represent specific-to-nonspecific binding ratios, were computed at various time points. The statistical parameter k3/k4 was estimated by two methods: a variation of the graphic method that derives the ratio of ligand distribution volumes (R[V]) and the area ratio method (R[A]), in which the ratio is calculated from the areas under the specific and nonspecific binding activity curves. RESULTS: The mean (BG-OCC)/OCC ratio for normal controls (3.07 +/- 0.73) was significantly higher than that for Parkinson's disease patients at 115 min (1.10 +/- 0.56) (p = 2.76 x 10[-5]). The mean R(V) and R(A) for normal controls were 2.06 +/- 0.27 and 1.50 +/- 0.15, respectively. The mean R(V) and R(A) for Parkinson's disease patients were 0.78 +/- 0.31 and 0.65 +/- 0.24, respectively. Both R(V) and R(A) for normal controls were significantly higher than those for Parkinson's disease patients (p values for R(V) and R(A) were 1.91 x 10(-8) and 3.46 x 10(-10), respectively). The R(V) has linear relationships with both R(A) and (BG-OCC)/OCC ratio at 115 min. The R(V) has a higher correlation (r = 0.99) with R(A) than it does with (BG-OCC)/OCC (r = 0.93). CONCLUSION: The R(V), R(A) and (BG-OCC)/OCC for Parkinson's disease patients were clearly separated from those of normal controls, and they may be useful outcome measures for clinical diagnosis. The simplest (BG-OCC)/OCC ratio, requiring a single late time point, could be useful in clinical situations, whereas R(V) or R(A) is preferred when the dynamic data are available. The findings suggest that 123I-IPT is a useful tracer for diagnosing Parkinson's disease and studying dopamine reuptake sites.     

Test/retest reproducibility of iodine-123-betaCIT SPECT brain measurement of dopamine transporters in Parkinson's patients

Iodine-123-beta-CIT has been used as a probe of dopamine transporters in Parkinson's disease patients using SPECT. We studied the test/retest reproducibility of SPECT measures in Parkinson's disease patients and healthy controls obtained after injection of [123I])beta-CIT in part to assess the utility of this tracer for longitudinal evaluation of striatal dopamine transporters as a marker of disease progression. METHODS: Seven Parkinson's disease patients and seven healthy control subjects participated in two [123I]beta-CIT SPECT scans separated by 7-21 days. Subjects were imaged at 24 hr post injection of 360 MBq (9.7 mCi) of [123I]beta-CIT. Two outcome measures were evaluated; 1) the ratio of specific striatal (activity associated with DA transporter binding) to nondisplaceable uptake, also designated V3," and 2) the total specific striatal uptake (%SSU) expressed as a percentage of injected radiotracer dose. For both measures, test/retest variability was calculated as the absolute difference of test minus retest divided by the mean of test/retest and expressed as a percent. In addition, the reproducibility of left and right striatal asymmetry and putamen:caudate ratios were determined. RESULTS: The two outcome measures demonstrated excellent test/retest reproducibility for both the Parkinson's disease and healthy subject groups with variability of striatal V3" = 16.8 +/- 13.3% and percent striatal uptake = 6.8 +/- 3.4% for Parkinson's disease patients and V3" = 12.8 +/- 8.9% and %SSU = 7.0 +/- 3.9% for control subjects. There were no statistically significant differences in test/retest variability between control subjects and Parkinson's disease patients for either outcome measure. The reproducibility of left/right asymmetry indices and putamen-to-caudate ratios showed no patient versus control subject differences. The asymmetry index had greater test/retest variability than the other outcome measures. CONCLUSION: These data suggest that SPECT imaging performed at 24 hr postinjection of [123I]beta-CIT permits calculation of reliable and reproducible measures of dopamine transporters in both Parkinson's disease patients and control subjects and supports the feasibility of using [123I]beta-CIT in the evaluation of disease progression in Parkinson's disease.     

Functional role and pharmacological regulation of the dopaminergic system of the brain

The dopaminergic systems of the brain are thought to play a major role in the regulation of motor, cognitive, neuroendocrine functions and in the pathogenesis of several pathological conditions, including neurodegenerative diseases, affective disorders, schizophrenia, drug addiction, etc. Functional, biochemical, and pharmacological heterogeneity of dopamine receptors, which were divided into D1-like (D1 and D5 subtypes) and D2-like (D2, D3, and D4) families of receptors, has been postulated. The paper concerns the recent advances in the study of the structure and function of two main dopaminergic brain systems, i.e. nigrostriatal and mesolimbic. The problem of autoreceptor regulation of dopaminergic neurotransmission, particularly the processes of dopamine synthesis, release, and metabolism is discussed. The involvement of D2 and D3 dopamine autoreceptors in the control of these processes and differences in the mode of action of typical neuroleptics are analyzed. It is hypothesized that dopamine D3 autoreceptor is preferentially involved in the regulation of dopamine release while D2 one is responsible for the control of dopamine synthesis and metabolism in rat basal ganglia in vivo.     

Euphorigenic doses of cocaine reduce [123I]beta-CIT SPECT measures of dopamine transporter availability in human cocaine addicts

The in vivo potency of euphorigenic doses of intravenous cocaine for displacing [123I]beta-CIT ([123I]2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane) binding to striatal dopamine transporters (DAT) was assessed in human cocaine addicts using single photon emission computed tomography (SPECT). Cocaine-dependent subjects (n = 6) were injected with [123I]beta-CIT and imaged 24 h later under equilibrium conditions. Sequential cocaine infusions (0.28 +/- 0.03 and 0.56 +/- 0.07 mg/kg) produced significant (P < 0.0005) reductions in the specific to non-specific equilibrium partition coefficient, V3" (6 +/- 6 and 17 +/- 3%), a measure proportional to DAT binding potential. Regression analysis of the logit transformed data enabled reliable determination of the Hill coefficient (0.51) and 50% displacement (ED50) dose of cocaine (2.8 mg/kg). These preliminary data suggest that cocaine produces behavioral effects in humans at measurable levels of DAT occupancy.     

PET measurement of dopamine D2 receptor-mediated changes in striatopallidal function

This study was designed to validate an in vivo measurement of the functional sensitivity of basal ganglia neuronal circuits containing dopamine D2 receptors. We hypothesized that a D2 agonist would decrease striatopallidal neuronal activity, and hence regional cerebral blood flow (rCBF) over the axon terminals in the globus pallidus. Quantitative pallidal blood flow was measured using positron emission tomography (PET) with bolus injections of H215O and arterial sampling in six baboons before and after intravenous administration of the selective D2 agonist U91356a. We also tested whether the response to U91356a was modified by previous acute administration of various antagonists. Another baboon had serial measurements of blood flow under identical conditions, but received no dopaminergic drugs. In all animals that received U91356a, pallidal flow decreased in a dose-related manner. Global CBF had a similar response, but the decline in pallidal flow was greater in magnitude and remained significant after accounting for the global effect. A D2 antagonist, but not antagonists of D1, serotonin-2, or peripheral D2 receptors, prevented this decrease. This work demonstrates and validates an in vivo measure of the sensitivity of D2-mediated basal ganglia pathways. It also supports the hypothesis that activation of the indirect striatopallidal pathway, previously demonstrated using nonselective D2-like agonists, can be mediated specifically by D2 receptors. We speculate that the U91356a-PET technique may prove useful in detecting functional abnormalities of D2-mediated dopaminergic function in diseases such as parkinsonism, dystonia, Tourette syndrome, or schizophrenia.     

2-[123I]-iodolisuride SPET visualizes dopaminergic loss in de-novo parkinsonian patients: is it a marker of striatal pre-synaptic degeneration?

The aim of this study was to assess the correlation between the functional integrity and density of striatal dopaminergic receptors and clinical data in 15 de-novo patients with idiopathic Parkinson's disease by single photon emission tomography (SPET) using 2-[123I]-iodolisuride (ILIS), a tracer based on the D2-dopamine receptor agonist lisuride. Deficient striatal uptake of ILIS correlated with the severity of the disorder, scored by the Unified Parkinson's Disease Rating Scale (UPDRS) (n = 15; ratio of ILIS uptake: basal ganglia/cerebellum [B/C] & UPDRS I-III, Spearman R = -0.562, P = 0.013), Beck's Depression Inventory (BDI) (n = 12; B/C & BDI, Spearman R = -0.825, P = 0.0009) and the ZUNG Depression Scale (ZDS) (n = 11; B/C & ZDS, Spearman R = -0.7425, P = 0.008). Experimental data indicate that lisuride shows a higher affinity for pre-synaptic dopaminergic autoreceptors than for post-synaptic D2-dopamine receptors under conditions of low applied ILIS concentrations as in this study. From the results of this study and these experimental data, we speculate that ILIS-SPET can visualize pre-synaptic striatal dopaminergic degeneration in Parkinson's disease.     

3H]YM-09151-2 (nemonapride), a potent radioligand for both sigma 1 and sigma 2 receptor subtypes

Using K+ phosphate buffer with 25 nM spiperone, [3H]YM-09151-2 binding showed a high affinity for sigma receptors but no affinity for D2 dopamine or 5-HT1A receptors in rat brain. The order of pKi values of various sigma compounds at [3H]YM-09151-2 binding sites and stereoisomer selectivity were consistent with previous studies using other sigma ligands such as (+)-[3H]SKF-10047, [3H]DTG and (+)-[3H]3-PPP. Although Scatchard analysis fitted a one-site model, competition between [3H]YM-09151-2 and (+)-pentazocine revealed two sites, sigma 1 and sigma 2 receptors, at which the Ki values of YM-09151-2 were 8.4 nM and 9.6nM, respectively. Autoradiography using [3H]YM-09151-2 also showed a characteristic distribution of sigma receptors in rat brain. [3H]YM-09151-2 is, therefore, a potent and useful radioligand for sigma 1/sigma 2 receptor subtypes.