Markus Beu

Cortical GABA, striatal dopamine and midbrain serotonin as the key players in compulsive and anxiety disorders--results from in vivo imaging studies.

Various factors are discussed in the pathophysiology of anxiety disorders, including dysfunctions of the (DA)ergic, serotonin (5-HT)ergic and GABAergic system. We assessed the contribution of the individual synaptic constituents by subjecting all available in vivo imaging studies on patients with anxiety disorders to a retrospective analysis. On a total of 504 patients with obsessive-compulsive disorder (OCD), generalized anxiety disorder (GAD), panic disorder (PD), phobia, or posttraumatic stress-disorder (PTSD) and 593 controls, investigations of VMAT2, DAT, SERT, D1, D2, 5-HTIA, 5-HT2A, GABA(A), and NK1 receptor binding in neostriatum, ventral striatum, thalamus, neocortex, limbic system, cingulate, midbrain/ pons or cerebellum were performed using either PET or SPECT. Separate analyses of the individual disorders showed significant decreases of striatal D2 receptors in OCD (-18%), mesencephalic SERT in OCD (-13%), frontocortical GABAA receptors in PD (-13%) and temporocortical GABAA receptors in GAD (-16%). Pooling of all disorders yielded a significant reduction of mesencephalic SERT (-13%), mesencephalic (-27%) as well as cingulate 5-HT1A receptors (-18%), striatal D2 receptors (-21%) and frontal (-14%), temporal (-14%), occipital (-13%) and cingulate GABAA receptors (-15%). The results show that DA, 5-HT, and GABA play a major role in all subtypes of anxiety disorders. In particular, the findings imply that the regulation state of DA as modulated by GABA and 5-HT may be crucial for the development of anxiety- and compulsion-related disorders. As GABA and 5-HT inhibit DAergic neurotransmission, the reductions of GABAA, 5-HT1A and SERT can be assumed to result in an enhanced activity of the mesolimbic DAergic system. This notion is also reflected by the decrease of striatal D2 receptor binding, which is indicative of an increased availability of synaptic DA.

Modulation of the neuronal circuitry subserving working memory in healthy human subjects by repetitive transcranial magnetic stimulation

We studied the effect of repetitive transcranial magnetic stimulation (rTMS) on changes in regional cerebral blood flow (rCBF) as revealed by positron emission tomography (PET) while subjects performed a 2-back verbal working memory (WM) task. rTMS to the right or left dorsolateral prefrontal cortex (DLPFC), but not to the midline frontal cortex, significantly worsened performance in the WM task while inducing significant reductions in rCBF at the stimulation site and in distant brain regions. These results for the first time demonstrate the ability of rTMS to produce temporary functional lesions in elements of a neuronal network thus changing its distributed activations and resulting in behavioral consequences.

Diagnostic Accuracy of Combined FP-CIT, IBZM, and MIBG Scintigraphy in the Differential Diagnosis of Degenerative Parkinsonism: A Multidimensional Statistical Approach

In vivo molecular imaging of pre- and postsynaptic nigrostriatal neuronal degeneration and sympathetic cardiac innervation with SPECT is used to distinguish idiopathic Parkinson disease (PD) from atypical parkinsonian disorder (APD). However, the diagnostic accuracy of these imaging approaches as stand-alone procedures is often unsatisfying. The aim of this study was therefore to evaluate to which extent diagnostic accuracy can be increased by their combined use together with a multidimensional statistical algorithm. Methods: The SPECT radiotracers 123I-(S)-2-hydroxy-3-iodo-6-methoxy-N-[1-ethyl-2-pyrrodinyl)-methyl]benzamide (IBZM), 123I-N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)nortropan (FP-CIT), and meta-123I-iodobenzylguanidine (MIBG) were used to assess striatal postsynaptic D2 receptor binding, striatal presynaptic dopamine transporter binding, and myocardial adrenergic innervation, respectively. Thirty-one PD and 17 APD patients were prospectively investigated. PD and APD diagnoses were established using consensus criteria and reevaluated after 37.4 ± 12.4 and 26 ± 11.6 mo in PD and APD, respectively. Test accuracy (TA) for PD–APD differentiation was computed for all logical (Boolean) combinations of imaging modalities by receiver-operating-characteristic analysis—that is, after multidimensional optimization of cutoff values. Results: Analysis showed moderate TA for PD–APD differentiation using each molecular approach alone (IBZM, 79%; MIBG, 73%; and FP-CIT, 73%). For combined use, the highest TA resulted under the assumption that at least 2 of the 3 biologic markers had to be positive for APD using the following cutoff values: 1.46 or less for IBZM, less than 2.10 for FP-CIT, and greater than 1.43 for MIBG. This algorithm distinguished APD from PD with a sensitivity of 94%, specificity of 94% (TA, 94%), positive predictive value of 89%, and negative predictive value of 97%. Conclusion: Results suggest that the multidimensional combination of FP-CIT, IBZM, and MIBG scintigraphy is likely to significantly increase TA in differentiating PD from APD. The differential diagnosis of degenerative parkinsonism may thus be facilitated.

Diffuse cortical reduction of neuronal activity in unipolar major depression: a retrospective analysis of 337 patients and 321 controls

Reduction of neuronal activity in frontocortical and limbic circuits is considered a characteristic of depression. We aimed to test this hypothesis by pooling all available data from experimental literature. All investigations were included comparing regional cerebral blood flow (rCBF) or glucose metabolism (rCMRGlc) between acutely depressed unipolar major depressive patients and healthy controls. For cortical and subcortical regions we computed the percentage difference between depressives (n=337) and controls (n=321). In patients with unipolar major depression rCBF and rCMRGlc were lowered in left (−4.4%, P=0.022) and right frontal (−3.2%, P=0.053), left (−1.7%, P= 0.061) and right temporal (−3.0%, P=0.003), left (−6.5%, P= 0.002), and right parietal (−8.8%, P=0.001), and left (−6.6%, P= 0.083) and right occipital cortex (−4.2%, P=0.02). Moreover, there were reductions in left (−6.3%, P=0.029) and right basal ganglia (−4.8%, P=0.002), left (−3.4%, P=0.114) and right thalamus (−3.1%, P=0.036), and left limbic system (−2.2%, P=0.127). Parameters were increased by 1.0% (P=0.714) only in the right limbic system. There were no hemispheric asymmetries (P>0.05). Moreover, there was no indication for an anterior-posterior gradient (P>0.05), and thus no ‘hypofrontality’. In contrast to the current view, the data indicate a diffuse cortical rather than regionalized reduction of neuronal activity in unipolar major depression.

[123I]Iodobenzamide binding to the rat dopamine D2 receptor in competition with haloperidol and endogenous dopamine--an in vivo imaging study with a dedicated small animal SPECT.

PurposeThis study assessed [123I]iodobenzamide binding to the rat dopamine D2 receptor in competition with haloperidol and endogenous dopamine using a high-resolution small animal SPECT.MethodsSubsequent to baseline quantifications of D2 receptor binding, imaging studies were performed on the same animals after pre-treatment with haloperidol and methylphenidate, which block D2 receptors and dopamine transporters, respectively.ResultsStriatal baseline equilibrium ratios (V3″) of [123I]iodobenzamide binding were 1.42±0.31 (mean±SD). After pre-treatment with haloperidol and methylphenidate, V3″ values decreased to 0.54±0.46 (p<0.0001) and 0.98±0.48 (p=0.009), respectively.ConclusionThe decrease in [123I]iodobenzamide binding induced by pre-treatment with haloperidol reflects D2 receptor blockade, whereas the decrease in receptor binding induced by pre-treatment with methylphenidate can be interpreted in terms of competition between [123I]IBZM and endogenous dopamine. Findings show that multiple in vivo measurements of [123I]iodobenzamide binding to D2 receptors in competition with exogenous and endogenous ligands are feasible in the same animal. This may be of future relevance for the in vivo evaluation of novel radioligands as well as for studying the interrelations between pre- and/or postsynaptic radioligand binding and different levels of endogenous dopamine.

Investigating the Dopaminergic Synapse In Vivo. II. Molecular Imaging Studies in Small Laboratory Animals

Dopaminergic synaptic function may be assessed either at the presynaptic terminal or at the postsynaptic binding sites using molecular in vivo imaging methods. Apart from the density of binding sites, parameters such as alterations in dopamine synthesis, dopamine storage or dopamine release can be quantified either by application of specific radiotracers or by assessing the competition between the exogenous radioligand and endogenous dopamine. The performance of animal studies allows the induction of specific short-term or long-term synaptic conditions via pharmacological challenges or infliction of neurotoxic lesions. Therefore, small laboratory animals such as rats and mice have become invaluable models for a variety of human disorders. This article gives an overview of those small animal studies which have been performed so far on dopaminergic neurotransmission using in vivo imaging methods, with a special focus on the relevance of findings within the functional entity of the dopaminergic synapse. Taken together, in vivo investigations on animal models of Parkinsons disease showed decreases of dopamine storage, dopamine release and dopamine transporter binding, no alterations of dopamine synthesis and DA release, and either increases or no alterations of D2 receptor binding, while in vivo investigations of animal models of Huntingtons disease. showed decreases of DAT and D1 receptor binding. For D2 receptor binding, both decreases and increases have been reported, dependent on the radioligand employed. Substances of abuse, such as alcohol, amphetamine and methylphenidate, led to an increase of dopamine release in striatal regions. This held for the acute application of substances to both healthy animals and animal models of drug abuse. Findings also showed that chronic application of cocaine induced long-term reductions of both D1 and D2 receptor binding, which disappeared after several weeks of withdrawal. Finally, preliminary results yielded the first evidence that acute pplication of haloperidol might induce a reduction of dopamine transporter binding, indicating an enhancement of dopamine release into the synaptic cleft. It is remarkable to what degree the findings obtained with small animal imaging devices correspond to the results of clinical and experimental studies on humans. This agreement underlines the validity of small animal imaging methods and demonstrates the feasibility of further investigations on animal models of human diseases.

Pretreatment with Haloperidol Reduces 123I-FP-CIT Binding to the Dopamine Transporter in the Rat Striatum: An In Vivo Imaging Study with a Dedicated Small-Animal SPECT Camera

Synaptic dopamine is mainly regulated by presynaptic dopamine transporter (DAT) activity. We hypothesized that variations in synaptic dopamine are reflected by variations of DAT radioligand binding. The effect of haloperidol, which increases synaptic dopamine concentrations, was therefore assessed in the rat striatum using 123I-N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)-nortropane (123I-FP-CIT) as a DAT radioligand. Methods: Striatal 123I-FP-CIT binding was measured in 24 rats under baseline conditions (no pretreatment) and at 1 h after injection of haloperidol or a vehicle (1 mg/kg) using a small-animal SPECT camera. Results: Baseline equilibrium ratios (V3″) were 1.32 ± 0.24 (mean ± SD). After the haloperidol injection, V3″ decreased to 0.99 ± 0.38 (P2-tailed < 0.0001), corresponding to a mean reduction of DAT binding by 25%. Conclusion: Our results are indicative of competition between the DAT ligand 123I-FP-CIT and synaptic dopamine elevated by haloperidol, suggesting that the assessment of 123I-FP-CIT binding may be suitable to study variations in synaptic dopamine in vivo.

Effects of l-DOPA on striatal iodine-123-FP-CIT binding and behavioral parameters in the rat

PurposeThe effect of clinical L-3,4-dihydroxyphenylalanine (L-DOPA) doses on the binding of [123I]N-&OHgr;-fluoropropyl-2&bgr;-carbomethoxy-3&bgr;-(4-iodophenyl)nortropane ([123I]FP-CIT) to the rat dopamine transporter (DAT) was investigated using small animal single-photon emission computed tomography. Materials and methodsDAT binding was measured at baseline, after challenge with the aromatic L-amino acid decarboxylase inhibitor benserazide, and after challenge with either 5 or 10 mg/kg L-DOPA plus benserazide. For baseline and challenges, striatal equilibrium ratios (V3′′) were computed as an estimation of the binding potential. Moreover, striatal V3′′ values were correlated with parameters of motor and exploratory behavior. ResultsV3′′ differed significantly between baseline and either dose of L-DOPA/benserazide. Moreover, V3′′ differed significantly between L-DOPA treatment groups. After 5 mg/kg L-DOPA/benserazide, DAT binding was inversely correlated with sitting duration (1–5 min) and sitting frequency (10–15 min). After 10 mg/kg L-DOPA/benserazide, an inverse correlation was found between DAT binding and sitting duration (1–30 min), whereas DAT binding and duration of ambulatory activity (1–30 min) as well as head and shoulder motility (10–15 min) exhibited a positive correlation. ConclusionChallenge with 5 and 10 mg/kg L-DOPA/benserazide led to mean reductions in DAT binding by 34 and 20%, respectively. Results indicate a biphasic response with a higher effect on DAT after the lower dose of L-DOPA. The reduction in DAT binding may be interpreted in terms of competition between [123I]FP-CIT and endogenous dopamine. Moreover, there is preliminary evidence of an association between striatal DAT and motor and exploratory parameters.

Pharmacological challenge and synaptic response - assessing dopaminergic function in the rat striatum with small animal single-photon emission computed tomography (SPECT) and positron emission tomography (PET).

Abstract Disturbances of dopaminergic neurotransmission may be caused by changes in concentrations of synaptic dopamine (DA) and/or availabilities of pre- and post-synaptic transporter and receptor binding sites. We present a series of experiments which focus on the regulatory mechanisms of the dopamin(DA)ergic synapse in the rat striatum. In these studies, DA transporter (DAT) and/or D2 receptor binding were assessed with either small animal single-photon emission computed tomography (SPECT) or positron emission tomography (PET) after pharmacological challenge with haloperidol, L-DOPA and methylphenidate, and after nigrostriatal 6-hydroxydopamine lesion. Investigations of DAT binding were performed with [123I]N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)nortropane ([123I]FP-CIT). D2 receptor bindingd was assessed with either [123I](S)-2-hydroxy-3-iodo-6-methoxy-N-[(1-ethyl-2-pyrrolidinyl)methyl]benzamide ([123I]IBZM) or [18F]1[3-(4′fluorobenzoyl)propyl]-4-(2-keto-3-methyl-1-benzimidazolinyl)piperidine ([18F]FMB). Findings demonstrate that in vivo investigations of transporter and/or receptor binding are feasible with small animal SPECT and PET. Therefore, tracers that are radiolabeled with isotopes of comparatively long half-lives such as 123I may be employed. Our approach to quantify DAT and/or D2 receptor binding at baseline and after pharmacological interventions inducing DAT blockade, D2 receptor blockade, and increases or decreases of endogenous DA concentrations holds promise for the in vivo assessment of synaptic function. This pertains to animal models of diseases associated with pre- or postsynaptic DAergic deficiencies such as Parkinson’s disease, Huntington’s disease, attention-deficit/hyperactivity disorder, schizophrenia or drug abuse.

Intranasal Dopamine Reduces In Vivo [123I]FP-CIT Binding to Striatal Dopamine Transporter: Correlation with Behavioral Changes and Evidence for Pavlovian Conditioned Dopamine Response

Purpose: Dopamine (DA), which does not cross the blood-brain barrier, has central and behavioral effects when administered via the nasal route. Neither the mechanisms of central action of intranasal dopamine (IN-DA), nor its mechanisms of diffusion and transport into the brain are well understood. We here examined whether IN-DA application influences dopamine transporter (DAT) binding in the dorsal striatum and assessed the extent of binding in relation to motor and exploratory behaviors. We hypothesized that, based on the finding of increased extracellular DA in the striatum induced by application of IN-DA, binding of [123I]FP-CIT to the DAT should be decreased due to competition at the receptor. Methods: Rats were administered 3 mg/kg IN-DA and vehicle (VEH), with IN-DA injection either preceding or following VEH. Then motor and exploratory behaviors (traveled distance, velocity, center time, sitting, rearing, head-shoulder motility, grooming) were assessed for 30 min in an open field prior to administration of [123I]FP-CIT. DAT binding after IN-DA and VEH was measured with small animal SPECT 2 h following administration of the radioligand. Results: (1) After IN-DA application, striatal DAT binding was significantly lower as compared to VEH, indicating that the nasally delivered DA had central action and increased DA levels comparable to that found previously with L-DOPA administration; and (2) DAT binding in response to intranasal VEH was lower when IN-DA application preceded VEH treatment. This finding is suggestive of Pavlovian conditioning of DA at the level of the DAT, since the DA treatment modified (decreased) the binding in response to the subsequent VEH treatment. VEH treatment also reduced motor and exploratory behaviors more when applied before, as compared to when it followed IN-DA application, also indicative of behavioral Pavlovian conditioning akin to that found upon application of various psychostimulant drugs. Conclusions: The results: (a) demonstrate a direct central action of intranasally applied DA on the DAT in the dorsal striatum, indicating enhanced DA availability; and (b) provide first evidence of a Pavlovian conditioned DA response at the DAT. The latter results have relevance to understanding neurochemical mechanisms that underlie placebo action in the treatment of Parkinsonian patients.