Dopamine Synthesis Rate Research Articles

Long-Term Treatment with Atypical Antipsychotic Iloperidone Modulates Cytochrome P450 2D (CYP2D) Expression and Activity in the Liver and Brain via Different Mechanisms.

CYP2D enzymes engage in the synthesis of endogenous neuroactive substances (dopamine, serotonin) and in the metabolism of neurosteroids. The present work investigates the effect of iloperidone on CYP2D enzyme expression and activity in rat brains and livers. Iloperidone exerted a weak direct inhibitory effect on CYP2D activity in vitro in the liver and brain microsomes (Ki = 11.5 μM and Ki = 462 μM, respectively). However, a two-week treatment with iloperidone (1 mg/kg ip.) produced a significant decrease in the activity of liver CYP2D, which correlated positively with the reduced CYP2D1, CYP2D2 and CYP2D4 protein and mRNA levels. Like in the liver, iloperidone reduced CYP2D activity and protein levels in the frontal cortex and cerebellum but enhanced these levels in the nucleus accumbens, striatum and substantia nigra. Chronic iloperidone did not change the brain CYP2D4 mRNA levels, except in the striatum, where they were significantly increased. In conclusion, by affecting CYP2D activity in the brain, iloperidone may modify its pharmacological effect, via influencing the rate of dopamine and serotonin synthesis or the metabolism of neurosteroids. By elevating the CYP2D expression/activity in the substantia nigra and striatum (i.e., in the dopaminergic nigrostriatal pathway), iloperidone may attenuate extrapyramidal symptoms, while by decreasing the CYP2D activity and metabolism of neurosteroiods in the frontal cortex and cerebellum, iloperidone can have beneficial effects in the treatment of schizophrenia. In the liver, pharmacokinetic interactions involving chronic iloperidone and CYP2D substrates are likely to occur.

Enhanced tyrosine hydroxylase activity induces oxidative stress, causes accumulation of autotoxic catecholamine metabolites, and augments amphetamine effects in vivo.

In Parkinson's disease, dopamine‐containing nigrostriatal neurons undergo profound degeneration. Tyrosine hydroxylase (TH) is the rate‐limiting enzyme in dopamine biosynthesis. TH increases in vitro formation of reactive oxygen species, and previous animal studies have reported links between cytosolic dopamine build‐up and oxidative stress. To examine effects of increased TH activity in catecholaminergic neurons in vivo, we generated TH‐over‐expressing mice (TH‐HI) using a BAC‐transgenic approach that results in over‐expression of TH with endogenous patterns of expression. The transgenic mice were characterized by western blot, qPCR, and immunohistochemistry. Tissue contents of dopamine, its metabolites, and markers of oxidative stress were evaluated. TH‐HI mice had a 3‐fold increase in total and phosphorylated TH levels and an increased rate of dopamine synthesis. Coincident with elevated dopamine turnover, TH‐HI mice showed increased striatal production of H2O2 and reduced glutathione levels. In addition, TH‐HI mice had elevated striatal levels of the neurotoxic dopamine metabolites 3,4‐dihydroxyphenylacetaldehyde and 5‐S‐cysteinyl‐dopamine and were more susceptible than wild‐type mice to the effects of amphetamine and methamphetamine. These results demonstrate that increased TH alone is sufficient to produce oxidative stress in vivo, build up autotoxic dopamine metabolites, and augment toxicity.

Dopamina jako mediator układu pozapiramidowego współuczestniczący w regulacji ruchu

Liczne badania pokazują, że dopamina jest ważnym regulatorem kontroli motorycznej. Prawidłowe funkcjonowanie tej regulacji zależy od odpowiedniego stężenia dopaminy w prążkowiu. W artykule omówiono interakcję pomiędzy korą mózgową (obszar przedmotorowy i ruchowy) a układem pozapiramidowym, z możliwą rolą w tym zjawisku zarówno neuronów GABAergicznych, jak i glutaminergicznych. Ważną rolę dopaminy w tym mechanizmie podkreślono wykorzystując wiedzę kliniczną pochodzącą z badań pacjentów z chorobą Parkinsona, u których drastyczne obniżenie poziomu dopaminy w układzie pozapiramidowym powoduje istotne upośledzenie ich czynności motorycznych. Zmiany te można przypisać obniżonemu tempu syntezy dopaminy, co z kolei prowadzi do wyraźnego obniżenia poziomu dopaminy w prążkowiu. Z drugiej strony, zwiększona aktywność fizyczna, w szczególności długotrwale powtarzane ćwiczenia, wpływając na poziom testosteronu i BDNF, może aktywować hydroksylazę tyrozynową, a tym samym podnieść poziom dopaminy w układzie nigrostriatalnym. W konsekwencji zmiany te mogą prowadzić do poprawy sprawności ruchowej zarówno osób zdrowych, jak i pacjentów z chorobą Parkinsona.

Differential associations of dopamine synthesis capacity with the dopamine transporter and D2 receptor availability as assessed by PET in the living human brain

BackgroundThe dopamine (DA) neurotransmission has been implicated in fundamental brain functions, exemplified by movement controls, reward-seeking, motivation, and cognition. Although dysregulation of DA neurotransmission in the striatum is known to be involved in diverse neuropsychiatric disorders, it is yet to be clarified whether components of the DA transmission, such as synthesis, receptors, and reuptake are coupled with each other to homeostatically maintain the DA neurotransmission. The purpose of this study was to investigate associations of the DA synthesis capacity with the availabilities of DA transporters and D2 receptors in the striatum of healthy subjects. MethodsFirst, we examined correlations between the DA synthesis capacity and DA transporter availability in the caudate and putamen using PET data with L-[β-11C]DOPA and [18F]FE-PE2I, respectively, acquired from our past dual-tracer studies. Next, we investigated relationships between the DA synthesis capacity and D2 receptor availability employing PET data with L-[β-11C]DOPA and [11C]raclopride, respectively, obtained from other previous dual-tracer assays. ResultsWe found a significant positive correlation between the DA synthesis capacity and DA transporter availability in the putamen, while no significant correlations between the DA synthesis capacity and D2 receptor availability in the striatum. ConclusionThe intimate association of the DA synthesis rate with the presynaptic reuptake of DA indicates homeostatic maintenance of the baseline synaptic DA concentration. In contrast, the total abundance of D2 receptors, which consist of presynaptic autoreceptors and postsynaptic modulatory receptors, may not have an immediate relationship to this regulatory mechanism.

Postsynaptic Mechanisms Render Syn I/II/III Mice Highly Responsive to Psychostimulants.

BackgroundSynapsins are encoded by SYN I, SYN II, and SYN III, and they regulate neurotransmitter release by maintaining a reserve pool of synaptic vesicles.MethodsPresynaptic dopamine responses to cocaine were examined by microdialysis, and postsynaptic responses were evaluated to various dopamine receptor agonists in the open field with SynI/SynII/SynIII triple knockout mice.ResultsTriple knockout mice showed enhanced spontaneous locomotion in a novel environment and were hyper-responsive to indirect and direct D1 and D2 dopamine agonists. Triple knockout animals appeared sensitized to cocaine upon first open field exposure; sensitization developed across days in wild-type controls. When mutants were preexposed to a novel environment before injection, cocaine-stimulated locomotion was reduced and behavioral sensitization retarded. Baseline dopamine turnover was enhanced in mutants and novel open field exposure increased their striatal dopamine synthesis rates. As KCl-depolarization stimulated comparable dopamine release in both genotypes, their readily releasable pools appeared indistinguishable. Similarly, cocaine-induced hyperlocomotion was indifferent to blockade of newly synthesized dopamine and depletion of releasable dopamine pools. Extracellular dopamine release was similar in wild-type and triple knockout mice preexposed to the open field and given cocaine or placed immediately into the arena following injection. Since motor effects to novelty and psychostimulants depend upon frontocortical-striatal inputs, we inhibited triple knockout medial frontal cortex with GABA agonists. Locomotion was transiently increased in cocaine-injected mutants, while their supersensitive cocaine response to novelty was lost.ConclusionsThese results reveal presynaptic dopamine release is not indicative of agonist-induced triple knockout hyperlocomotion. Instead, their novelty response occurs primarily through postsynaptic mechanisms and network effects.

Evaluation of 6-11C-Methyl-m-Tyrosine as a PET Probe for Presynaptic Dopaminergic Activity: A Comparison PET Study with β-11C-l-DOPA and 18F-FDOPA in Parkinson Disease Monkeys.

We recently developed a novel PET probe, 6-(11)C-methyl-m-tyrosine ((11)C-6MemTyr), for quantitative imaging of presynaptic dopamine synthesis in the living brain. In the present study, (11)C-6MemTyr was compared with β-(11)C-l-DOPA and 6-(18)F-fluoro-l-dopa ((18)F-FDOPA) in the brains of normal and Parkinson disease (PD) model monkeys (Macaca fascicularis). PD model monkeys were prepared by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration, and (11)C-β-CFT was applied to assess neuronal damage as dopamine transporter (DAT) availability. (11)C-6MemTyr, β-(11)C-l-DOPA, or (18)F-FDOPA was injected with and without carbidopa, a specific inhibitor of peripheral aromatic L-amino acid decarboxylase. In normal and PD monkeys, the dopamine synthesis rates calculated using PET probes were analyzed by the correlation plot with DAT availability in the striatum. In normal monkeys, whole-brain uptake of β-(11)C-l-DOPA and (18)F-FDOPA were significantly increased by carbidopa at the clinical dose of 5 mg/kg by mouth. In contrast, (11)C-6MemTyr was not affected by carbidopa at this dose, and the metabolic constant value of (11)C-6MemTyr in the striatum was significantly higher than those of the other 2 PET probes. Significant reduction of the presynaptic DAT availability in the striatum was detected in MPTP monkeys, and correlation analyses demonstrated that (11)C-6MemTyr could detect dopaminergic damage in the striatum with much more sensitivity than the other PET probes. (11)C-6MemTyr is a potential PET probe for quantitative imaging of presynaptic dopamine activity in the living brain with PET.

MAOA-VNTR polymorphism modulates context-dependent dopamine release and aggressive behavior in males

A recent [18F]FDOPA-PET study reports negative correlations between dopamine synthesis rates and aggressive behavior. Since dopamine is among the substrates for monoamine oxidase A (MAOA), this investigation examines whether functional allelic variants of the MAOA tandem repeat (VNTR) promotor polymorphism, which is known to modulate aggressive behavior, influences dopamine release and aggression in response to violent visual stimuli.We selected from a genetic prescreening sample, strictly case-matched groups of 2×12 healthy male subjects with VNTRs predictive of high (MAOA-High) and low (MAOA-Low) MAOA expression. Subjects underwent pairs of PET sessions (dopamine D2/3 ligand [18F]DMFP) while viewing a movie of neutral content, versus violent content. Directly afterwards, aggressive behavior was assessed by the Point Subtraction Aggression Paradigm (PSAP). Finally, PET data of 23 participants and behavioral data of 22 participants were analyzed due to post hoc exclusion criteria.In the genetic prescreening sample MAOA-Low carriers had significantly increased scores on the Buss–Perry Aggression Questionnaire. In the PET-study-group, aggressive behavior under the emotional neutral condition was significantly higher in the MAOA-Low group. Interestingly, the two MAOA-groups showed inverse dopaminergic and behavioral reactions to the violent movie: The MAOA-High group showed higher dopamine release and increased aggression after the violent movie; MAOA-Low subjects showed decreases in aggressive behavior and no consistent dopamine release.These results indicate a possible impact of the MAOA-promotor polymorphism on the neurobiological modulation of aggressive behavior. However, the data do not support approaches stating that MAOA-Low fosters aggression by a simple pro-dopaminergic mechanism.

The effect of central noradrenergic system lesion on dopamine (DA) and serotonin (5-HT) synthesis rate following administration of 5-HT3 receptor ligands in chosen parts of the rat brain

IntroductionSince little has been known about the effect of the central noradrenergic system on the reactivity of serotonin 5-HT3 receptors, the aim of the current study was to find out whether this reactivity could be altered by chemical damage to the system in adult rats in early developmental stage. Materials and methodsAdult male Wistar rats with central noradrenergic lesion induced by DSP-4 on day 1 and 3 of life were injected with analgesic model substance – morphine, serotoninergic 5-HT3 receptor agonist (1-phenylbiguanide, PBG), 5-HT3 receptor antagonist (ondansetron) or both compounds jointly followed by decarboxylase inhibitor of aromatic amino acids (NSD-1050). After 30min following NSD-1050 injection, the animals were decapitated using a guillotine. Chosen cerebral structures were dissected, and the contents of 5-hydroxytryptofan (5-HTP) and l-dihydroxyphenylalanine (l-DOPA) were determined using high-pressure liquid chromatography with electrochemical detection (HPLC/ED). ResultsNeither PBG nor morphine affected l-DOPA contents in the hippocampus in control rats; however, DSP-4 lesion caused a significant decrease in the synthesis rate of DA in this structure. Hippocampal contents of 5-HTP increased after morphine or PBG administration, and central noradrenergic lesion attenuated this effect. Morphine or PBG decreased cerebellar DA synthesis rate in control rats and DSP-4 lesion did not modify it. Cerebellar levels of 5-HTP increased after morphine or PBG challenge in control rats. DSP-4 lesion intensified the effect of morphine and attenuated that of PBG. Ondansetron abolished the effects mediated by PBG. We did not observe any impact of PBG or ondansetron on DA and 5-HT synthesis in the striatum. ConclusionDamage to the central noradrenergic system in rat newborns, through altered reactivity of central 5-HT3 receptors, results in permanent disorders in serotoninergic transmission in hippocampus and cerebellum as well as dopaminergic transmission in hippocampus, which may attenuate the activity of the descending pathways that derive from these structures.

Influence of O-Methylated Metabolite Penetrating the Blood–Brain Barrier to Estimation of Dopamine Synthesis Capacity in Human L-[β-11C]DOPA PET

O-methyl metabolite (L-[β-(11)C]OMD) of (11)C-labeled L-3,4-dihydroxyphenylalanine (L-[β-(11)C]DOPA) can penetrate into brain tissue through the blood-brain barrier, and can complicate the estimation of dopamine synthesis capacity by positron emission tomography (PET) study with L-[β-(11)C]DOPA. We evaluated the impact of L-[β-(11)C]OMD on the estimation of the dopamine synthesis capacity in a human L-[β-(11)C]DOPA PET study. The metabolite correction with mathematical modeling of L-[β-(11)C]OMD kinetics in a reference region without decarboxylation and further metabolism, proposed by a previous [(18)F]FDOPA PET study, were implemented to estimate radioactivity of tissue L-[β-(11)C]OMD in 10 normal volunteers. The component of L-[β-(11)C]OMD in tissue time-activity curves (TACs) in 10 regions were subtracted by the estimated radioactivity of L-[β-(11)C]OMD. To evaluate the influence of omitting blood sampling and metabolite correction, relative dopamine synthesis rate (kref) was estimated by Gjedde-Patlak analysis with reference tissue input function, as well as the net dopamine synthesis rate (Ki) by Gjedde-Patlak analysis with the arterial input function and TAC without and with metabolite correction. Overestimation of Ki was observed without metabolite correction. However, the kref and Ki with metabolite correction were significantly correlated. These data suggest that the influence of L-[β-(11)C]OMD is minimal for the estimation of kref as dopamine synthesis capacity.

Assessment of dopamine (DA) synthesis rate in selected parts of the rat brain with central noradrenergic lesion after administration of 5-HT3 receptor ligands

The study objective was to determine the effect of central noradrenergic system lesions performed in the early extrafetal life period on dopamine synthesis in the rat brain. The content of L-dihydroxyphenylalanine (L-DOPA) was assessed in the frontal lobe, thalamus, hypothalamus and brain stem of rats by high-pressure chromatography with electrochemical detection (HPLC/ED) after administration of 5-HT3 receptor ligands. Adult male Wistar rats which underwent central noradrenergic lesions by DSP-4 administration (50 mg/kg m.c. i.p.) on day 1 and 3 of life received i.p. injections of the aromatic amino acid decarboxylase inhibitor (NSD-1050) in a dose of 100 mg/kg b.w. Next, 30 min after NSD-1050 injection, the animals were decapitated by guillotine. Selected brain structures were dissected and L-DOPA content was determined by HPLC/ED. A statistically significant reduction was found in DA synthesis in the group of animals with DSP-4 lesions induced by PBG (1-phenylbiguanide, 7.5 mg/kg b.w. i.p.) and ondansetron (1.0 mg/kg b.w. i.p.). Morphine and PBG had no major effect on DA synthesis in the cerebral cortex of both control animals and in rats with noradrenergic lesions. The assessment of the effect of DSP-4 lesions on L-DOPA content in the brain stem after administration of morphine (7.5 mg/kg b.w. s.c.), PBG (7.5 mg/kg b.w. i.p.) or ondansetron (1.0 mg/kg b.w. i.p.) separately or jointly showed a statistically significant increase in the synthesis of DA in animals with DSP-4 lesions, as compared to the control group exposed to 0.9% NaCl and morphine. The analysis of the effect of DSP-4 lesions on L-DOPA content in the thalamus and hypothalamus revealed no statistically significant differences between the control groups of rats and those with DSP-4 lesions. As shown by this model, permanent noradrenergic lesions in animals in the early extra-fetal period result in increased reactivity of the central dopamine system.

Haloperidol effects on striatal dopamine and DOPAC levels and subcellular distribution

Signaling involving dopaminergic neurons in the brain is important for determining the saliency of sensory input, the urgency of sensory import, the actual value of an experience as compared to its expected value, and the process of converting motivation to action. Drugs that are antagonists for the dopamine D2 receptor subtype are clinically effective in treating schizophrenia and a variety of other mental conditions. These drugs have a substantial effect on the levels and sub-cellular distribution of dopamine and its major metabolite DOPAC. Currently, there is no quantitative analysis of how these various changes can be accomplished. The goal of this work is to explain how the changes in dopamine and DOPAC levels and subcellular location elicited by the prototypical antipsychotic drug haloperidol occur. A computational model of a dopaminergic striatal varicosity was used. In the model, dopamine can be located in cytosol, vesicles, extracellular compartment, and a DOPAC-synthesis-secretory compartment. DOPAC can be located in cytosol, extracellular, and DOPAC-synthesis-secretory compartment. The model has two distinct sites for dopamine and DOPAC synthesis. The cytosolic site synthesizes dopamine for use in the signaling pool and metabolizes a portion of dopamine that is transiently located in cytosol. The DOPAC-synthesis-secretory complex site is hypothesized to be located on the plasma membrane and to be dedicated to the synthesis of DOPAC to be secreted into the extracellular compartment. The model also has enzymes involved in dopamine and DOPAC synthesis, transporters that move dopamine between compartments, and exocytosis. Haloperidol induces a small increase in extracellular dopamine, a somewhat larger increase in extracellular DOPAC, a larger increase in tissue DOPAC, a large increase in rate of dopamine synthesis, and no change in level of dopamine. The increase in extracellular dopamine was simulated by increasing rate of secretion of dopamine into extracellular space by exocytosis. An increase in synthesis of signaling dopamine was required to maintain the elevated levels of extracellular dopamine. The increase in extracellular DOPAC was simulated by increasing rate of dopamine synthesis in the DOPAC-synthesis-secretory compartment. With only these manipulations, the model failed to match experimental data in terms of increase in total tissue DOPAC and increase in the rate of dopamine synthesis. These discrepancies were resolved by increasing the passive diffusion of dopamine from vesicles to cytosol and further increasing the rate of dopamine synthesis for the signaling pool. Literature data indicate that haloperidol does accumulate in and partially alkalinize amine neurotransmitter storage vesicles, which would increase passive diffusion of dopamine from the vesicles. I conclude that the increases in extracellular dopamine and DOPAC are likely mediated by antagonism of dopamine D2 receptors but that the large increase in tissue DOPAC and much of the increase in dopamine synthesis result from physico-chemical properties of haloperidol rather than block of dopamine D2 receptors. Furthermore, the results suggest dopaminergic varicosities possess a mechanism for monitoring amount of stored dopamine and modulating dopamine synthesis to maintain constant stores.

Relation between Presynaptic and Postsynaptic Dopaminergic Functions Measured by Positron Emission Tomography: Implication of Dopaminergic Tone

Both presynaptic and postsynaptic dopaminergic functions can be estimated by positron emission tomography (PET). While both presynaptic and postsynaptic dopaminergic functions would be regulated by corresponding genes and related to personality traits, the relation between presynaptic and postsynaptic functions in terms of interindividual variation has hardly been investigated. In the present study, both striatal dopamine D(2) receptor binding and endogenous dopamine synthesis rate were measured in the same healthy subjects using PET with [(11)C]raclopride and l-[β-(11)C]DOPA, respectively, and these two parameters were compared. Two PET studies with [(11)C]raclopride and l-[β-(11)C]DOPA were performed sequentially at rest condition on 14 healthy men. For [(11)C]raclopride PET, the binding potential was calculated by the reference tissue model method. For l-[β-(11)C]DOPA PET, the endogenous dopamine synthesis rate was estimated by graphical analysis. A significant negative correlation was observed between the binding potential of dopamine D(2) receptors and endogenous dopamine synthesis rate (r = -0.66, p < 0.05). Although the interindividual variation of binding potential of [(11)C]raclopride would be due to both the interindividual difference in the receptor density and that in the concentration of endogenous dopamine in the synaptic cleft, the negative correlation between parameters for both presynaptic and postsynaptic functions might indicate a compensative relation between the two functions.

Chronic repetitive transcranial magnetic stimulation failed to change dopamine synthesis rate: Preliminary L‐[β‐11C]DOPA positron emission tomography study in patients with depression

We have examined the effects of repetitive transcranial magnetic stimulation (rTMS) on central dopaminergic function in patients with depression using positron emission tomography with L-[β-11C]DOPA, a ligand to assess the rate of endogenous dopamine synthesis. Eight patients were treated with 10-daily sessions of rTMS over the left dorsolateral prefrontal cortex. Positron emission tomography scanning was performed in each patient twice, before the first session and 1 day after the last session. Although four out of eight patients responded to rTMS, there were no changes in the striatal dopamine synthesis rate (k) following rTMS. These results suggest that chronic rTMS had a limited effect on the dopaminergic system.

Current State of Agonist Radioligands for Imaging of Brain Dopamine D2/D3 Receptors In Vivo with Positron Emission Tomography

Dopamine (DA) is known to play an important role in numerous brain functions and has been suggested to be involved in several neuropsychiatric and neurological disorders. From early on, positron emission tomography (PET) studies of the DA system have yielded high interest. Currently, several aspects of the functionality of DA neurons can be imaged, including the DA synthesis rate and the expression of DA-related proteins (receptors and transporters). A more recent application of radioligands targeting DA receptors is to study the endogenous neurotransmitter levels in vivo in brain. In vitro binding studies have suggested that in general DA receptors, as many other G-protein coupled receptors, exist in two affinity states for agonist binding. The high affinity state is thought to represent the functional state of the receptor, and the proportion between high and low affinity states may change with the development of disease. PET imaging with agonist radioligands may provide information on the existence of the high affinity state in vivo. In addition, DA receptor agonist radioligands may be superior tools for measuring changes in endogenous DA levels as antagonist radioligands inherently bind to both the low and the high affinity state. This review primarily summarizes the present status of the D(2)/D(3) receptor agonist PET radioligands. In addition several PET studies evaluating the utility of the agonist concept are discussed.

In vivo detection of neuropathologic changes in presymptomatic MAPT mutation carriers: A PET and MRI study

Background Microglial activation and disrupted neurotransmissions may herald symptomatic manifestations in neurodegenerative tauopathies. Methods We investigated microglial activation with [ 11C]DAA1106 positron emission tomography (PET), striatal dopaminergic function with l-[β- 11C]dopa PET, acetylcholinesterase (AChE) activity with [ 11C] N-methylpiperidin-4-yl acetate PET, and morphologic brain changes with MRI in three persons (aged 38–41 years) with frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), who were presymptomatic gene carriers (PGCs) from an American kindred with pallidopontonigral degeneration. The results from these 3 PGCs were compared with [ 11C]DAA1106 PET results from age-matched 9 healthy volunteers (HV), and with l-[β- 11C]dopa and [ 11C]MP4A PET results from 10 HV. Values considered significant were more than 2 SDs greater or less than the normal control mean, as the number of subjects was small for group comparisons. Results Glial activities were increased in the frontal cortex of one PGC, the occipital cortex of two PGCs, and the posterior cingulate cortex of one PGC, although none of the PGCs showed overt glial activation in the brain. Only one of the PGCs showed reduced AChE activity in the temporo-parietal cortex. Three PGCs showed low dopamine synthesis rates in the putamen. Hippocampal atrophy was observed in two PGCs. Conclusions Hippocampal atrophy and striatal dopaminergic dysfunction may be early disease processes in the pathogenesis of FTDP-17. Neuroinflammation may also be an in vivo signature of tau pathology at a prodromal stage, although current PET techniques may not constantly reveal it as the earliest neuroimaging abnormality.

In Vitro and in Vivo Evidence for a Lack of Interaction with Dopamine D2 Receptors by the Metabotropic Glutamate 2/3 Receptor Agonists 1S,2S,5R,6S-2-Aminobicyclo[3.1.0]hexane-2,6-bicaroxylate Monohydrate (LY354740) and (−)-2-Oxa-4-aminobicyclo[3.1.0] Hexane-4,6-dicarboxylic Acid (LY379268)

Some recently published in vitro studies with two metabotropic glutamate 2/3 receptor (mGluR(2/3)) agonists [(-)-2-oxa-4-aminobicyclo[3.1.0] hexane-4,6-dicarboxylic acid (LY379268) and 1S,2S,5R,6S-2-aminobicyclo[3.1.0]hexane-2,6-bicaroxylate monohydrate (LY354740)] suggest that these compounds may also directly interact with dopamine (DA) D(2) receptors. The current in vitro and in vivo studies were undertaken to further explore this potential interaction with D(2) receptors. LY379268 and LY354740 failed to inhibit D(2) binding in both native striatal tissue homogenates and cloned receptors at concentrations up to 10 microM. LY379268 and LY354740 (up to 10 microM) also failed to stimulate [(35)S]GTPgammaS binding in D(2L)- and D(2S)-expressing clones in the presence of NaCl or N-methyl-d-glucamine. In an in vivo striatal D(2) receptor occupancy assay, LY379268 (3-30 mg/kg) or LY354740 (1-10 mg/kg) failed to displace raclopride (3 microg/kg i.v.), whereas aripiprazole (10-60 mg/kg) showed up to 90% striatal D(2) receptor occupancy. LY379268 (10 mg/kg) and raclopride (3 mg/kg) blocked d-amphetamine and phencyclidine (PCP)-induced hyperactivity in wild-type mice. However, the effects of LY379268 were lost in mGlu(2/3) receptor knockout mice. In DA D(2) receptor-deficient mice, LY379268 but not raclopride blocked both PCP and d-amphetamine-evoked hyperactivity. In the striatum and nucleus accumbens, LY379268 (3 and 10 mg/kg) was without effect on the DA synthesis rate in reserpinized rats and also failed to prevent S-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine-induced reductions in DA synthesis rate. Taken together, the current data fail to show evidence of direct DA D(2) receptor interactions of LY379268 and LY354740 in vitro or in vivo. Instead, these results provide further evidence for a novel antipsychotic mechanism of action for mGluR(2/3) agonists.

Dose Optimization for Long-term rAAV-mediated RNA Interference in the Nigrostriatal Projection Neurons

Short-hairpin RNA (shRNA)-mediated gene knockdown is a powerful tool for targeted gene silencing and an emerging novel therapeutic strategy. Recent publications, however, reported unexpected toxicity after utilizing viral-mediated shRNA knockdown in vivo. Thus, it is currently unclear whether shRNA-mediated knockdown strategy can be used as a safe and efficient tool for gene silencing. In this study, we have generated rAAV vectors expressing shRNAs targeting the rat tyrosine hydroxylase (TH) mRNA (shTH) for testing the efficacy of in vivo TH knockdown in the nigral dopaminergic neurons. At high titers, not only the shTH vectors but also the scrambled and green fluorescence protein (GFP)-only controls caused cell death. In a dose-response study, we identified a dose window leading to >60% decrease in TH(+) neurons without any change in vesicular monoamine transporter-2 (VMAT2) expression. Moreover, using the safe and efficient dose, we showed that dopamine (DA) synthesis rate was significantly reduced and this lead to emergence of motor deficits in the shTH-expressing rats. Interestingly, these animals showed very robust and long-lasting recovery after a single systemic L-3,4-dihydroxyphenylalanine (L-DOPA) administration beyond what can be achieved in 6-hydroxydopamine (6-OHDA)-lesioned rats. Our results have implications for both mechanistic and therapeutic studies utilizing long-term shRNA-mediated gene silencing in the nigrostriatal projection system.

Regional dopamine synthesis in patients with schizophrenia using L-[β-11C]DOPA PET

The dopamine hypothesis has been the most widely known theory concerning schizophrenia. However, the exact mechanism including presynaptic dopaminergic activity and its relationship with symptom severity still remains to be revealed. We measured presynaptic dopamine synthesis using positron emission tomography (PET) with L-[β-11C]DOPA in 18 patients with schizophrenia (14 drug-naïve and 4 drug-free patients) and 20 control participants. Dopamine synthesis rates, expressed as ki values, were obtained using a graphical method, and the occipital cortex was used as reference region. Regions of interest were placed on the prefrontal cortex, temporal cortex, anterior cingulate, parahippocampus, thalamus, caudate nucleus, and putamen. Psychopathology was assessed with the Positive and Negative Symptom Scale (PANSS). We found significantly higher ki values in patients than in controls in the left caudate nucleus, but not in the other regions. The ki values in the thalamus exhibited a significant positive correlation with the PANSS total scores. Furthermore, a significant positive correlation was observed between the PANSS positive subscale scores and ki values in the right temporal cortex. Patients with schizophrenia showed higher dopamine synthesis in the left caudate nucleus, and dopaminergic transmission in the thalamus and right temporal cortex might be implicated in the expression of symptoms in schizophrenia.

Chronic nicotine modifies the effects of morphine on extracellular striatal dopamine and ventral tegmental GABA

Previously, we have shown that 7-week oral nicotine treatment enhances morphine-induced behaviors and dopaminergic activity in the mouse brain. In this study, we further characterized the nicotine-morphine interaction in the mesolimbic and nigrostriatal dopaminergic systems, as well as in the GABAergic control of these systems. In nicotine-pretreated mice, morphine-induced dopamine release in the caudate putamen and nucleus accumbens was significantly augmented, as measured by microdialysis. Chronic nicotine treatment did not change basal extracellular concentrations of dopamine and its metabolites in the caudate putamen and nucleus accumbens, nor did it affect the rate of dopamine synthesis, as assessed by 3-hydroxybenzylhydrazine dihydrochloride-induced DOPA accumulation. GABAergic control of dopaminergic activity was studied by measuring extracellular GABA in the presence of nipecotic acid, an inhibitor of GABA uptake. Acute (0.3 mg/kg or 0.5 mg/kg i.p.) and chronic nicotine, as well as morphine (15 mg/kg s.c.) in control mice decreased nipecotic acid-induced increase in extracellular GABA in the ventral tegmental area/substantia nigra (VTA/SN). In contrast, in nicotine-treated mice, morphine increased GABA levels in the presence of nipecotic acid. We did not find any alterations in GABA(B)-receptor function after chronic nicotine treatment. Thus, our data show that chronic nicotine treatment sensitizes dopaminergic systems to morphine and affects GABAergic systems in the VTA/SN.

Subnanomolar dopamine D3 receptor antagonism coupled to moderate D2 affinity results in favourable antipsychotic-like activity in rodent models: I. neurochemical characterisation of RG-15

RG-15 (trans-N-[4-[2-[4-(3-cyano-5-trifluoromethyl-phenyl)-piperazine-1-yl]-ethyl]-cyclohexyl]-3-pyridinesulfonic amide dihydrochloride) displayed subnanomolar affinity to human and rat dopamine D3 receptors (pKi 10.49 and 9.42, respectively) and nanomolar affinity to human and rat D2 receptors (pKi 8.23 and 7.62, respectively). No apparent interactions were found with the other 44 receptors and four channel sites tested in this study. RG-15 inhibited dopamine-stimulated [35S]GTPgammaS binding in membranes from rat striatum, in murine A9 cells expressing human D2L receptors and in CHO cells expressing human D3 receptors (IC50 values were 21.2, 36.7 and 7.2 nM, respectively). In these tests RG-15 showed the highest affinity toward D3 receptors when compared to amisulpride, haloperidol and SB-277011. RG-15, similar to haloperidol and amisulpride, dose-dependently inhibited in vivo [3H]raclopride binding in mouse striatum, enhanced dopamine turnover and synthesis rate in mouse and rat striatum and olfactory tubercle. SB-277011 did not change [3H]raclopride binding in mouse striatum nor biosynthesis or turnover rates in either region in mice or rats. RG-15 and haloperidol, but not SB-277011, antagonised dopamine synthesis inhibition induced by the D3/D2 full agonist 7-OH-DPAT in GBL-treated mice. RG-15, but not SB-277011, elevated plasma prolactin levels. In vitro receptor binding and functional experiments demonstrated that RG-15 had an antagonist profile on both D3 and D2 receptors. with high selectivity for dopamine D3 receptors over D2 receptors. However, in vivo, its neurochemical actions were similar to those of D2 receptor antagonists. Neurochemical comparison of RG-15 with antagonists having a different affinity and selectivity toward D3 and D2 receptors indicate that D3 receptors have little, if any, role in the control of presynaptic dopamine biosynthesis/release in dopaminergic terminal regions.