SPECT imaging of dopamine receptors with [123I]epidepride: characterization of uptake in the human brain

Subacute administration of both methcathinone and manganese causes basal ganglia damage in mice resembling that in methcathinone abusers

An irreversible extrapyramidal syndrome occurs in man after intravenous abuse of “homemade” methcathinone (ephedrone, Mcat) that is contaminated with manganese (Mn) and is accompanied by altered basal ganglia function. Both Mcat and Mn can cause alterations in nigrostriatal function but it remains unknown whether the effects of the ‘homemade’ drug seen in man are due to Mcat or to Mn or to a combination of both. To determine how toxicity occurs, we have investigated the effects of 4-week intraperitoneal administration of Mn (30 mg/kg t.i.d) and Mcat (100 mg/kg t.i.d.) given alone, on the nigrostriatal function in male C57BL6 mice. The effects were compared to those of the ‘homemade’ mixture which contained about 7 mg/kg of Mn and 100 mg/kg of Mcat. Motor function, nigral dopaminergic cell number and markers of pre- and postsynaptic dopaminergic neuronal integrity including SPECT analysis were assessed. All three treatments had similar effects on motor behavior and neuronal markers. All decreased motor activity and induced tyrosine hydroxylase positive cell loss in the substantia nigra. All reduced ¹²³I-epidepride binding to D2 receptors in the striatum. Vesicular monoamine transporter 2 (VMAT2) binding was not altered by any drug treatment. However, Mcat treatment alone decreased levels of the dopamine transporter (DAT) and Mn alone reduced GAD immunoreactivity in the striatum. These data suggest that both Mcat and Mn alone could contribute to the neuronal damage caused by the ‘homemade’ mixture but that both produce additional changes that contribute to the extrapyramidal syndrome seen in man.

Dystonia-deafness syndrome caused by ACTB p.Arg183Trp heterozygosity shows striatal dopaminergic dysfunction and response to pallidal stimulation

BACKGROUND

Dystonia-deafness syndrome is a well-known clinical entity, with sensorineural deafness typically manifesting earlier than dystonia. ACTB p.Arg183Trp heterozygosity has been reported in six patients to cause combined infant-onset deafness and dystonia manifesting in adolescence or young adulthood. Three of these have received beneficial pallidal stimulation. Brain imaging to assess striatal function has not been reported previously, however. Nor has a comprehensive hypothesis been presented for how the pleiotropic manifestations of this specific beta-actin gene mutation originate developmentally.

CASE PRESENTATION

A 19-year-old girl with congenital mild dysmorphic facial features, cochlear implants for infant-onset deafness, and mild cognitive and emotional disability, presented with an adolescent-onset, severe generalized dystonia. Brain MRI and multiple single gene sequencing were inconclusive. Due to life-threatening dystonia, we implanted a neurostimulation device, targeting the postero-ventral internal pallidum bilaterally. The Burke-Fahn-Marsden Dystonia Rating Scale motor/disability scores improved from 87/25 to 21/13 at 2.5 months postoperatively, 26/14 at 3 years, and 30/14 at 4 years. Subsequent whole exome sequencing identified heterozygosity for the ACTB p.Arg183Trp variant. Brain imaging included ¹²³I-ioflupane single photon emission computed tomography (Dopamine Transporter-SPECT), SPECT with ¹²³I-epidepride (binds to dopamine type 2-receptors) and ¹⁸ Fluoro-Deoxy-Glucose (FDG)-PET. Both Epidepride-SPECT and FDG-PET showed reduced tracer uptake in the striatum bilaterally, particularly in the putamen. DaT-SPECT was slightly abnormal.

CONCLUSIONS

In this patient with dystonia-deafness syndrome caused by ACTB p.Arg183Trp heterozygosity, unprecedented brain imaging findings strongly indicate striatal neuronal/dopaminergic dysfunction as the underlying cause of the dystonia. Pallidal stimulation provided a substantial improvement of the severe generalized dystonia, which is largely sustained at 4-year follow-up, and we advise this treatment to be considered in such patients. We hypothesize that the pleiotropic manifestations of the dystonia-deafness syndrome caused by this mutation derive from diverse developmental functions of beta-actin in neural crest migration and proliferation (facial dysmorphogenesis), hair cell stereocilia function (infant-onset deafness), and altered synaptic activity patterns associated with pubertal changes in striatal function (adolescent-onset dystonia). The temporal differences in developmental onset are likely due to varying degrees of susceptibility and of compensatory upregulation of other actin variants in the affected structures.

Change in expression of vesicular protein synapsin II by chronic treatment with D2 allosteric modulator PAOPA

The hallmark symptoms of schizophrenia include profound disturbances in thought, perception, cognition etc., which negatively impacts an individual's quality of life. Current antipsychotic drugs are not effective in treating all symptoms of this disorder, and often cause severe movement and metabolic side effects. Consequently, there remains a strong impetus to develop safer and more efficacious therapeutics for patients, as well as elucidating the etiology of schizophrenia. Previous work in our lab has introduced a novel candidate for the treatment of this disease: the dopamine D2 receptor (D2R) allosteric modulator, 3(R)-[(2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide (PAOPA). We have previously shown that PAOPA, by selectively modulating D2R, can ameliorate schizophrenia-like symptoms in animal models, although the precise mechanism is presently not understood. Synapsin II is a presynaptic vesicular protein which has been strongly implicated in schizophrenia, as it is reduced in the prefrontal cortex of patients, and knockdown of this protein elicits schizophrenia-like phenotypes in animal models. Given the therapeutic effects of PAOPA and the role of synapsin II in schizophrenia, the objective of this study was to investigate the effect of chronic administration of PAOPA (45 days) on neuronal synapsin II protein expression in rodents. Immunoblot results revealed that the synapsin IIa, but not the IIb isoform, was increased in the dopaminergic regions of the striatum, nucleus accumbens, and medial prefrontal cortex. The results of this study implicate a role for modulation of synapsin II as a possible therapeutic mechanism of action for potential antipsychotic drug PAOPA.

Effects of the dopamine D2 allosteric modulator, PAOPA, on the expression of GRK2, arrestin-3, ERK1/2, and on receptor internalization

The activity of G protein-coupled receptors (GPCRs) is intricately regulated by a range of intracellular proteins, including G protein-coupled kinases (GRKs) and arrestins. Understanding the effects of ligands on these signaling pathways could provide insights into disease pathophysiologies and treatment. The dopamine D2 receptor is a GPCR strongly implicated in the pathophysiology of a range of neurological and neuropsychiatric disorders, particularly schizophrenia. Previous studies from our lab have shown the preclinical efficacy of a novel allosteric drug, 3(R)- [(2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide (PAOPA), in attenuating schizophrenia-like behavioural abnormalities in rodent models of the disease. As an allosteric modulator, PAOPA binds to a site on the D2 receptor, which is distinct from the endogenous ligand-binding site, in order to modulate the binding of the D2 receptor ligand, dopamine. The exact signaling pathways affected by this allosteric modulator are currently unknown. The objectives of this study were to decipher the in vivo effects, in rats, of chronic PAOPA administration on D2 receptor regulatory and downstream molecules, including GRK2, arrestin-3 and extracellular receptor kinase (ERK) 1/2. Additionally, an in vitro cellular model was also used to study PAOPA’s effects on D2 receptor internalization. Results from western immunoblots showed that chronic PAOPA treatment increased the striatal expression of GRK2 by 41%, arrestin-3 by 34%, phospho-ERK1 by 51% and phospho-ERK2 by 36%. Results also showed that the addition of PAOPA to agonist treatment in cells increased D2 receptor internalization by 33%. This study provides the foundational evidence of putative signaling pathways, and changes in receptor localization, affected by treatment with PAOPA. It improves our understanding on the diverse mechanisms of action of allosteric modulators, while advancing PAOPA’s development into a novel drug for the improved treatment of schizophrenia.

Antipsychotic occupancy of dopamine receptors in schizophrenia

Antipsychotic drugs were introduced in the early 50s on the basis of clinical observations in patients with schizophrenia. Experimental studies later revealed that antagonism at the D(2) dopamine receptor is a common characteristic of all antipsychotic drugs. In the 80s, the advent of brain imaging technologies such as positron emission tomography (PET) allowed for direct noninvasive studies of drug binding in treated patients. The concept receptor occupancy is defined as the fraction (%) of a receptor population that is occupied during treatment with an unlabelled drug. With regard to antipsychotic drugs, the radioligand [(11) C]-raclopride has been the most widely used for binding to the D(2) /D(3) -dopamine receptors. The present review discusses the contribution from molecular imaging to the current understanding of mechanism of action (MoA) of antipsychotic drugs. Consistent initial PET-findings of high D2-receptor occupancy in the striatum of patients responding to different antipsychotic drug treatments provided clinical support for the dopamine hypothesis of antipsychotic drug action. It has subsequently been demonstrated that patients with extrapyramidal syndromes (EPS) have higher occupancy (above 80%) than patients with good response but no EPS (65-80%). The PET-defined interval for optimal antipsychotic drug treatment has been implemented in the evolvement of dose recommendations for classical as well as more recently developed drugs. Another consistent finding is lower D(2) -occupancy during treatment with the prototype atypical antipsychotic clozapine. The MoA of clozapine remains to be fully understood and may include nondopaminergic mechanisms. A general limitation is that currently available PET-radioligands are not selective for any of the five dopamine receptor subtypes. Current attempts at developing such ligands may provide the tools required to refine further the MoA of antipsychotic drugs.

In vivo tomographic imaging studies of neurodegeneration and neuroprotection: a review

Noninvasive tomographic imaging methods including positron emission tomography (PET) and single photon emission computed tomography (SPECT) are extremely sensitive and are capable of measuring biochemical processes that occur at concentrations in the nanomolar range. Inherent to neurodegenerative processes is neuronal loss. Thus, PET or SPECT monitoring of biochemical processes altered by neuronal loss (changes in neurotransmitter turnover, alterations in receptor, transporter or enzyme concentrations) can provide unique information not attainable by other methods. Such imaging techniques can also be used to longtitudinally monitor the effects of neuroprotective treatments. This review highlights current imaging probes used to evaluate patients with specific neurodegenerative disorders (e.g., Alzheimer's Disease, Parkinson's Disease, Huntington's Chorea), including those that image receptors of the dopaminergic, cholinergic and glutamatergic systems. Areas of future research focus are also defined. It is clear that monitoring the progression of neurodegenerative disorders and the impact of neuroprotective treatments are two different but related goals for which noninvasive imaging via PET and SPECT methods plays a powerful and unique role.

[SPECT with 123I-IBZM: utility in differential diagnosis of degenerative Parkinsonisms and establishment of quantification method]

OBJECTIVES

To assess utility of SPECT with 123I-Iolopride ( 123I-IBZM) in the differential diagnosis of patients with Parkinsonian symptoms and try to establish an adequate quantification method.

MATERIAL AND METHOD

We analyzed a total of 34 patients who underwent a study with 123I-IBZM SPECT. Studies were analyzed qualitatively (visually) and quantitatively, using different quantification methods. We used different sums of slices (2, 3, 4, and 7 slices) with different cortical regions as a reference (frontal and occipital regions). Results were analyzed statistically. The final diagnosis of patients was established by two neurologists, specialized in movement disorders.

RESULTS

Studies were visually assessed as normal in 24 cases and as pathologic in the other 10 cases. Scintigraphic studies had an adequate diagnostic correlation in 33 of the 34 patients. Four of the 8 methods used in the quantification were statistically significant in the differentation between normal and pathological. The use of different cortical brain regions as reference did not improve differentation between normal and pathologic studies. Global quantitative assessment of the studies showed that normal studies had higher values than pathological ones, with important overlapping between both categories.

CONCLUSIONS

123I-IBZM SPECT is an effective diagnostic tool in the establishment of the differential diagnosis in patients with Parkinson's disease and Parkinson-Plus. Quantification of these studies had limited utility since the overlapping of index values between normal and pathological restricts their use in individual cases.

In vivo affinity of [18F]fallypride for striatal and extrastriatal dopamine D2 receptors in nonhuman primates

RATIONALE

[18F]Fallypride is a new and promising radiotracer, suitable for imaging D2 receptors with Positron Emission Tomography (PET) in both striatal and extrastriatal regions. The high signal to noise ratio of [18F]fallypride has been attributed to its high affinity for D2 receptors (K(D) of 0.03 nM, measured in vitro at room temperature).

OBJECTIVES

We sought to further characterize this tracer in terms of its in vivo affinity, possible affinity differences between brain regions and dependence of in vitro affinity on temperature.

METHODS

PET scans were performed in baboons over a wide range of concentrations to measure the in vivo K(D) of [18F]fallypride in striatal and extrastriatal regions. Several analytical approaches were used, including nonlinear kinetic modeling and equilibrium methods. Also, in vitro assays were performed at 22 and 37 degrees C.

RESULTS

No significant differences in the in vivo K(D) were detected between regions. In vivo K(D) of [18F]fallypride was 0.22+/-0.05 nM in striatum, 0.17+/-0.05 nM in thalamus, and 0.21+/-0.07 nM in hippocampus. These values were intermediate between in vitro K(D) measured at 22 (0.04+/-0.03 nM) and 37 degrees (2.03+/-1.07 nM).

CONCLUSION

The in vivo affinity of [18F]fallypride was not as high as previously estimated from in vitro values. This property might contribute to the favorable kinetic properties of the tracer. The in vivo affinity was similar between striatal and extrastriatal regions. This result indicates that the measured regional in vivo affinities of this tracer are not affected by putative regional differences in endogenous dopamine, and that [18F]fallypride is an appropriate tool to provide unbiased estimates of the occupancy of D2 receptors by antipsychotic drugs in striatal and extrastriatal regions.

Distribution of dopamine D2-like receptors in the human thalamus: autoradiographic and PET studies

The distribution of dopamine (DA) D(2)-like receptors in the human thalamus was studied using in vitro autoradiographic techniques and in vivo positron emission tomography in normal control subjects. [(125)I]Epidepride, which binds with high affinity to DA D(2) and D(3) receptors, was used in autoradiographic studies to determine the distribution and density of D(2)-like receptors, and the epidepride analogue [(18)F]fallypride positron was used for positron emission tomography studies to delineate D(2)-like receptors in vivo. Both approaches revealed a heterogeneous distribution of thalamic D(2/3) receptors, with relatively high densities in the intralaminar and midline thalamic nuclei, including the paraventricular, parataenial, paracentral, centrolateral, and centromedian/parafascicular nuclei. Moderate densities of D(2/3) sites were seen in the mediodorsal and anterior nuclei, while other thalamic nuclei expressed lower levels of D(2)-like receptors. Most thalamic nuclei that express high densities of D(2)-like receptors project to forebrain DA terminal fields, suggesting that both the thalamic neurons expressing D(2)-like receptors and the projection targets of these neurons are regulated by DA. Because the midline/intralaminar nuclei receive prominent projections from both the ascending reticular activating core and the hypothalamus, these thalamic nuclei may integrate activity conveying both interoceptive and exteroceptive information to telencephalic DA systems involved in reward and cognition.

Prefrontal DA transmission at D1 receptors and the pathology of schizophrenia

The current view on the dopamine (DA) hypothesis of schizophrenia postulates a cortical/subcortical imbalance: subcortical mesolimbic DA projections might be hyperactive, resulting in hyperstimulation of D2 receptors and positive symptoms, whereas mesocortical DA projections to the prefrontal cortex might be hypoactive, resulting in hypostimulation of D1 receptors, negative symptoms, and cognitive impairment. Although the subcortical abnormalities are relatively well established now, the evidence for cortical hypodopaminergia is just emerging. This article will review current evidence for prefrontal hypodopaminergia in schizophrenia, with special emphasis on positron emission tomography (PET) studies measuring cortical D1 receptors in schizophrenia. The presentation of the clinical data will be introduced by a brief overview of the function of prefrontal DA systems, both at the cellular and cognitive level. The impact of antipsychotic drugs on prefrontal DA function will also be reviewed. We will conclude with the formulation of several models of altered prefrontal DA transmission at D1 receptors in schizophrenia.

Striatal and extra-striatal D(2)/D(3) dopamine receptor occupancy by quetiapine in vivo. [(123)I]-epidepride single photon emission tomography(SPET) study

BACKGROUND

Selective action at limbic cortical dopamine D(2)-like receptors could mediate atypical antipsychotic efficacy with few extrapyramidal side-effects.

AIMS

To test the hypothesis that quetiapine has 'limbic selective' D(2)/D(3) receptor occupancy in vivo.

METHOD

The high-affinity D(2)/D(3) ligand [(123)I]-epidepride and single photon emission tomography were used to estimate D(2)/D(3) specific binding and an index of relative percentage D(2)/D(3) occupancy in striatal and temporal cortical regions for quetiapine-treated patients (n=6). Quetiapine-, and previously studied typical-antipsychotic- and clozapine-treated patients were compared.

RESULTS

Mean (s.d.) relative percentage D(2)/D(3) receptor occupancy by quetiapine was 32.0% (14.6) in striatum and 60.1% (17.2) in temporal cortex (mean daily dose 450 mg: range 300-700 mg/day). Quetiapine treatment resulted in limbic selective D(2)/D(3) blockade similar to clozapine and significantly higher than typical antipsychotics.

CONCLUSIONS

Preliminary data suggest that limbic selective D(2)/D(3) receptor blockade is important for atypical drug action.

Specific binding and laterality of human extrastriatal dopamine D2/D3 receptors in late onset type 1 alcoholic patients

Late onset type 1 alcoholism has been suggested to be associated with decreased dopaminergic transmission. Our hypothesis was that late onset type 1 alcoholics have also abnormal extrastriatal dopamine D(2)/D(3) receptor distribution. We performed binding, heterogeneity and laterality analysis of extrastriatal and striatal dopamine D(2)/D(3) receptors in nine late onset male alcoholics and in 12 age-matched healthy males. A radioligand, [(123)I]epidepride was used in high resolution single-photon emission tomography (SPET). Specific binding of epidepride in the left temporal pole was significantly (P<0.05) lower in type 1 alcoholics (0.74+/-0.14 ml/ml) than in controls (0.89+/-0.14 ml/ml). In alcoholics, there was no normal left-to-right asymmetry of the temporal cortical heterogeneity of epidepride distribution observed in control males (0.89+/-0.19 vs. 1.10+/-0.19; P<0.05). The results suggest that the specific binding of dopamine D(2)/D(3) receptors in late type 1 alcoholics is decreased and its laterality in the temporal brain is altered from normal.

Kinetic and equilibrium analyses of [(123)I]epidepride binding to striatal and extrastriatal dopamine D(2) receptors

Quantitative SPECT measures of dopamine D(2) like receptors with [(123)I]epidepride is complicated by its high affinity and lipophilic metabolites. The purpose of this study was to use both parent (P) and lipophilic metabolites (M) as input functions in a kinetic paradigm and in comparison to the results of equilibrium studies. Kinetic studies on eleven healthy human subjects, ages 32+/- 10 were performed following i.v. injection of approximately 370 MBq of [(123)I]epidepride. Images were acquired for 13.5+/-1.0 hours. Equilibrium studies were done on seven of eleven subjects with a bolus injection of approximately 140 MBq, bolus/infusion ratio of 10 hours, and infusion for 30-32 hours. High (striatum) and low (temporal cortex) density regions were studied. Two (P and M) and one (P) input function models were applied in the kinetic studies. In receptor-rich regions, the distribution volumes in nondisplaceable compartments were fixed to those in cerebellum. In addition, in the two input function model, K(1)(P)/K(1)(M) was fixed to the values in the cerebellum. The one input function model provided V'(3) values (=f(1)*B'(max)/K(D)) which were consistent with those obtained in equilibrium studies in both receptor-rich regions, while the two input function model provided consistent values only in striatum. Poor identifiability of the rate constants of metabolites seemed to be the source of errors in the two input function model. These results suggest that correct V'(3) values can be obtained with the one input function model both in high- and low-density regions.

In vivo occupancy of striatal and temporal cortical D2/D3 dopamine receptors by typical antipsychotic drugs. [123I]epidepride single photon emission tomography (SPET) study

BACKGROUND

The dopamine hypothesis proposes that antipsychotic drugs act primarily through limbic cortical D2/D2-like dopamine receptor blockade.

AIM

To evaluate this hypothesis with the D2/D3-selective SPET probe [123I]-epidepride.

METHOD

[123I]-epidepride SPET scans were performed on 12 patients with schizophrenia treated with antipsychotics and II age-matched healthy controls. [123I]-epidepride 'specific binding' to D2/D3 dopamine receptors was estimated, and relative percentage D2/D3 receptor occupancy by typical antipsychotic drugs determined.

RESULTS

Mean (s.d.) daily dose was 669.12 (516.8) mg chlorpromazine equivalents. Mean percentage D2/D3 receptor occupancy was 81.6 (8.1) and 73.2 (13.9) in the temporal cortex and striatum respectively.

CONCLUSIONS

Typical antipsychotic drug treatment is associated with substantial temporal cortical D2/D3 receptor occupancy. The relationship between this and efficacy is poor in patients with treatment-resistant schizophrenia.

Neurochemical brain imaging investigations of schizophrenia

Neurochemical brain imaging methods developed over the past 20 years offer significant promise for elucidating the biochemical underpinnings of schizophrenia. The two general methodologies used for these studies have been: 1) radiotracer imaging: PET (positron emission tomography) and SPECT (single photon emission computed tomography); and 2) NMR (nuclear magnetic resonance) imaging: fMRI (functional magnetic resonance imaging) and MRS (magnetic resonance spectroscopy). Despite conflicting findings, striatal D2 receptor density may be elevated in some, but not all patients. Elevated synthesis, and increased release of dopamine after amphetamine challenge have also been reported. Imaging of cortical 5-HT2A receptors suggests that this system is unaffected, in conflict with findings of postmortem studies. Although prior postmortem studies suggested an increase in cortical GABAA receptors, three SPECT studies have found no significant changes. MRS studies have shown decreased levels of NAA (N-acetyl-aspartate) moieties in hippocampus and frontal cortex of schizophrenic patients, which is consistent with the reported loss of neurons and neuropil in postmortem brains. In conclusion, developments in radiotracer and NMR imaging have provided promising leads to the biochemical abnormalities associated with schizophrenia. Future significant understanding is likely to occur with the development of new probes and enhanced instrument technology, when applied with an appreciation of the heterogeneity of the disorder and the need for careful clinical assessment of patients.

Imaging the pre- and postsynaptic side of striatal dopaminergic synapses in idiopathic cervical dystonia: a SPECT study using [123I] epidepride and [123I] beta-CIT

There is increasing evidence that a dysfunction of the dopaminergic system may be involved in the pathogenesis of idiopathic dystonia. To visualize possible alterations of the pre- and postsynaptic side of striatal dopaminergic synapses, SPECT studies using the radiotracers [123I] epidepride and [123I] beta-CIT were performed in 10 patients with idiopathic cervical dystonia. Eleven age- and sex-matched subjects served as controls. [123I] Epidepride is a new highly affine marker of D2 receptors, and [123I] beta-CIT binds to dopamine transporters on dopaminergic nerve endings. [123I] Epidepride binding was significantly reduced in both striata of dystonia patients compared with controls (p < 0.05). In contrast, striatal [123I beta-CIT uptake did not differ from controls. We conclude that dopaminergic dysfunction in idiopathic focal dystonia mainly involves postsynaptic mechanisms and suggest a disturbance of the indirect pathway of the motor circuit resulting in a disinhibited thalamocortical stimulation.

Striatal and extrastriatal imaging of dopamine D2 receptors in the living human brain with [123I]epidepride single-photon emission tomography

The iodine-123 labelled ligand benzamide epidepride was evaluated as a probe for in vivo imaging of striatal and extrastriatal dopamine D2 receptor sites in the human brain. Four healthy males were imaged with a high-resolution single-photon emission tomography scanner. Striatal radioactivity peaked at 3 h after injection. The specific binding in the striatum was 0.91+/-0.03 at 3 h and this ratio steadily increased with time. Extrastriatal radioactivity was highest in the thalamus, in the midbrain and in the temporal cortex, and peaked at 45-60 min after injection of tracer. A smaller amount of radioactivity was found in the parietal, frontal and occipital cortices. Two radioactive metabolites were observed, of which one was more lipophilic than the parent compound. The radiation burden to the patient was 0.035 mSv/MBq (effective dose equivalent). The preliminary results showed that [123I]epidepride can be used for imaging striatal and extrastriatal dopamine D2 receptor sites in the living human brain.