Dopamine receptor mapping with PET imaging in Parkinson's disease

Parkinson's disease (PD) is a chronic neurodegenerative disorder characterised pathologically by the loss of dopaminergic neurons in the substantia nigra pars compacta. These neurons project to the striatum, and their loss leads to alterations in the activity of the neural circuits that regulate movement. The striatal output of the circuit related to the control of movement is mediated by two pathways: the direct striatal pathway, which is mediated through facilitation of D1 receptors, and the indirect striatal pathway, mediated through D2 receptors. Positron emission tomography (PET) molecular imaging is a powerful in vivo technique in which using selective dopaminergic radioligands has been employed to investigate the dopaminergic system in humans. In this article we aim to review the role of PET imaging in understanding the postsynaptic dopaminergic mechanisms in PD. PET studies have allowed us to gain important insights into the functions of the dopaminergic system, the mechanisms of drug-induced motor and non-motor complications, and the placebo effect in PD.

Serotonergic mechanisms responsible for levodopa-induced dyskinesias in Parkinson's disease patients

Levodopa-induced dyskinesias (LIDs) are the most common and disabling adverse motor effect of therapy in Parkinson's disease (PD) patients. In this study, we investigated serotonergic mechanisms in LIDs development in PD patients using 11C-DASB PET to evaluate serotonin terminal function and 11C-raclopride PET to evaluate dopamine release. PD patients with LIDs showed relative preservation of serotonergic terminals throughout their disease. Identical levodopa doses induced markedly higher striatal synaptic dopamine concentrations in PD patients with LIDs compared with PD patients with stable responses to levodopa. Oral administration of the serotonin receptor type 1A agonist buspirone prior to levodopa reduced levodopa-evoked striatal synaptic dopamine increases and attenuated LIDs. PD patients with LIDs that exhibited greater decreases in synaptic dopamine after buspirone pretreatment had higher levels of serotonergic terminal functional integrity. Buspirone-associated modulation of dopamine levels was greater in PD patients with mild LIDs compared with those with more severe LIDs. These findings indicate that striatal serotonergic terminals contribute to LIDs pathophysiology via aberrant processing of exogenous levodopa and release of dopamine as false neurotransmitter in the denervated striatum of PD patients with LIDs. Our results also support the development of selective serotonin receptor type 1A agonists for use as antidyskinetic agents in PD.

Smoking-induced change in intrasynaptic dopamine concentration: effect of treatment for Tobacco Dependence

The aim of this study was to determine whether standard treatments for Tobacco Dependence affect smoking-induced changes in intrasynaptic dopamine (DA) concentration. Forty-three otherwise healthy adult cigarette smokers (10 to 40 cigarettes per day) were treated with either practical group counseling (PGC) psychotherapy (n=14), bupropion HCl (n=14), or matching pill placebo (n=15) (random assignment) for 8 weeks. Before and after treatment, each subject underwent a bolus-plus-continuous-infusion (11)C-raclopride positron emission tomography (PET) scanning session, during which he or she smoked a regular cigarette. The PET scanning outcome measure of interest was percent change in smoking-induced (11)C-raclopride binding potential (BP(ND)) in the ventral caudate/nucleus accumbens (VCD/NAc), as an indirect measure of DA release. Although the entire study sample had a smaller mean smoking-induced reduction in VCD/NAc BP(ND) after treatment (compared to before treatment), this change was highly correlated with smaller total cigarette puff volumes (and not other treatment variables). These data indicate that smoking-induced DA release is dose-dependent, and is not significantly affected by reductions in daily smoking levels or treatment type.

Orexin axons in the rat ventral tegmental area synapse infrequently onto dopamine and gamma-aminobutyric acid neurons

Cells in the ventral tegmental area (VTA) facilitate motivated behaviors, and the activity of VTA neurons is regulated by dense projections from the lateral hypothalamic area (LHA). Orexin (Orx) neurons in the lateral and perifornical hypothalamus play important roles in arousal, feeding, and energy metabolism. Orx cells contribute substantially to the LHA projection to the rat midbrain. However, the morphological features of Orx fibers in the VTA and whether they synapse onto dopamine (DA) or gamma-aminobutyric acid (GABA) neurons have not yet been investigated. We utilized immunoperoxidase and immunogold-silver staining to examine the morphological features and synaptic incidence of Orx-labeled axons in the VTA. We then combined immunoperoxidase labeling for Orx with immunogold-silver labeling for GABA or for tyrosine hydroxylase (TH) in DA neurons. Electron microscopic analysis revealed that most Orx-labeled axons in the VTA were passing fibers. The less common Orx varicosities were occasionally apposed to TH- or GABA-labeled dendrites without synapsing. Only a small proportion of Orx-positive axons synapsed onto dendrites or soma. The synapses included both asymmetric and symmetric types and targeted TH- and GABA-labeled profiles with equal frequency. These findings suggest that most Orx fibers in the VTA are axons passing to caudal brainstem structures. However, Orx does mediate some direct synaptic influence on VTA DA and GABA neurons. Additional nonsynaptic effects are suggested by the presence of numerous dense-cored vesicles. These studies have important implications for understanding the mechanisms whereby Orx can alter behavior through regulating VTA DA and GABA cell activity.

Pre- and post-synaptic sympathetic function in human hibernating myocardium

PURPOSE

Impaired pre-synaptic noradrenaline uptake-1 mechanism has been reported in a swine model of hibernating myocardium (HM). To ascertain whether adrenergic neuroeffector abnormalities are present in human HM, we combined functional measurements in vivo using cardiovascular magnetic resonance (CMR) and positron emission tomography (PET) to assess pre- and post-synaptic sympathetic function.

METHODS

Twelve patients with coronary artery disease and chronic left ventricular (LV) dysfunction underwent CMR at baseline and 6 months after bypass for assessment of regional and global LV function and identification of segments with reversible dysfunction. Before surgery, myocardial noradrenaline uptake-1 ([(11)C]meta-hydroxy-ephedrine; HED) and beta-adrenoceptor (beta-AR) density ([(11)C]CGP-12177) were measured with PET. Patient PET data were compared with those in 18 healthy controls.

RESULTS

The volume of distribution (V(d)) of HED in HM (47.95+/-28.05 ml/g) and infarcted myocardium (42.69+/-25.76 ml/g) was significantly reduced compared with controls (66.09+/-14.48 ml/g). The V(d) of HED in normal myocardium (49.93+/-20.48 ml/g) of patients was also lower than that in controls and the difference was close to statistical significance (p=0.06). Myocardial beta-AR density was significantly lower in HM (5.49+/-2.35 pmol/g), infarcted (4.82+/-2.61 pmol/g) and normal (5.86+/-1.81 pmol/g) segments of patients compared with healthy controls (8.61+/-1.32 pmol/g).

CONCLUSION

Noradrenaline uptake-1 mechanism and beta-AR density are reduced in the myocardium of patients with chronic LV dysfunction and evidence of HM. The increased sympathetic activity to the heart in these patients is a generalised rather than regional phenomenon which is likely to contribute to the remodelling process of the whole LV rather than playing a causative role in HM.

Hardwiring the brain: endocannabinoids shape neuronal connectivity

The roles of endocannabinoid signaling during central nervous system development are unknown. We report that CB(1) cannabinoid receptors (CB(1)Rs) are enriched in the axonal growth cones of gamma-aminobutyric acid-containing (GABAergic) interneurons in the rodent cortex during late gestation. Endocannabinoids trigger CB(1)R internalization and elimination from filopodia and induce chemorepulsion and collapse of axonal growth cones of these GABAergic interneurons by activating RhoA. Similarly, endocannabinoids diminish the galvanotropism of Xenopus laevis spinal neurons. These findings, together with the impaired target selection of cortical GABAergic interneurons lacking CB(1)Rs, identify endocannabinoids as axon guidance cues and demonstrate that endocannabinoid signaling regulates synaptogenesis and target selection in vivo.

Ultra-fast biosensors and multi-photon microscopy in the future of brain studies

The direct, highly selective and sensitive real-time imaging of neuro- and biochemical mediators is the only way to clarify precisely the chemistry of the brain and to discover the key molecular targets involved in regulation of brain homeostasis. To realize that, we need: high-speed deep-tissue imaging techniques with high spatial and temporal resolution; and ultra-fast and highly selective molecular sensors, giving a possibility to monitor target molecules directly in their physiological environment; in addition, these molecular sensors have to be comparatively small and permeable for blood-brain barrier, to be applicable in brain studies. The present view accents on the perspectives for development of direct approach for investigation of function/flow coupling phenomenon in the brain, based on the current progress in development of ultra-fast molecular sensors for direct visualization of biochemical mediators (e.g., nitric oxide, Ca ions), and high-speed two-photon/multi-photon deep-tissue imaging.

NGL family PSD-95-interacting adhesion molecules regulate excitatory synapse formation

Synaptic cell adhesion molecules (CAMs) regulate synapse formation through their trans-synaptic and heterophilic adhesion. Here we show that postsynaptic netrin-G ligand (NGL) CAMs associate with netrin-G CAMs in an isoform-specific manner and, through their cytosolic tail, with the abundant postsynaptic scaffold postsynaptic density-95 (PSD-95). Overexpression of NGL-2 in cultured rat neurons increased the number of PSD-95-positive dendritic protrusions. NGL-2 located on heterologous cells or beads induced functional presynaptic differentiation in contacting neurites. Direct aggregation of NGL-2 on the surface membrane of dendrites induced the clustering of excitatory postsynaptic proteins. Competitive inhibition by soluble NGL-2 reduced the number of excitatory synapses. NGL-2 knockdown reduced excitatory, but not inhibitory, synapse numbers and currents. These results suggest that NGL regulates the formation of excitatory synapses.

PET measures of pre- and post-synaptic cardiac beta adrenergic function

Positron Emission Tomography was used to measure global and regional cardiac beta-adrenergic function in 19 normal subjects and 9 congestive heart failure patients. [(11)C]-meta-hydroxyephedrine was used to image norepinephrine transporter function as an indicator of pre-synaptic function and [(11)C]-CGP12177 was used to measure cell surface beta-receptor density as an indicator of post-synaptic function. Pre-synaptic, but not post-synaptic, function was significantly different between normals and CHF patients. Pre-synaptic function was well matched to post-synaptic function in the normal hearts but significantly different and poorly matched in the CHF patients studied. This imaging technique can help us understand regional sympathetic function in cardiac disease.

Functional motor neurons differentiating from mouse multipotent spinal cord precursor cells in culture and after transplantation into transected sciatic nerve

OBJECT

One of the current challenges in neurobiology is to ensure that neural precursor cells differentiate into specific neuron types, so that they can be used for transplantation purposes in patients with neuron loss. The goal of this study was to determine if spinal cord precursor cells could differentiate into motor neurons both in culture and following transplantation into a transected sciatic nerve.

METHODS

In cultures with trophic factors, neurons differentiate from embryonic precursor cells and express motor neuronal markers such as choline acetyltransferase (ChAT), Islet-1, and REG2. Reverse transcription-polymerase chain reaction analysis has also demonstrated the expression of Islet-1 in differentiated cultures. A coculture preparation of neurospheres and skeletal myocytes was used to show the formation of neuromuscular connections between precursor cell-derived neurons and myocytes both immunohistochemically and electrophysiologically. Following various survival intervals, precursor cells transplanted distal to a transection of the sciatic nerve differentiated into neurons expressing the motor neuron markers ChAT and the alpha1 1.2 (class C, L-type) voltage-sensitive Ca++ channel subunit. These cells extended axons into the muscle, where they formed cholinergic terminals.

CONCLUSIONS

These results demonstrate that motor neurons can differentiate from spinal cord neural precursor cells grown in culture as well as following transplantation into a transected peripheral nerve.

Synaptic mitochondrial changes in the motor cortex following unilateral cortical lesions and motor skills training in adult male rats

Unilateral lesions of the forelimb sensorimotor cortex and motor skills training interact to enhance synaptic plasticity in layer V of the motor cortex contralateral to the lesion in male rats. In this study, we assessed the metabolic correlates of these synaptic changes by quantifying the number and size of mitochondria in synaptic axonal terminals with perforated or non-perforated post-synaptic densities (PSDs) and single or multiple post-synaptic contacts. The number of synaptic mitochondria per neuron was significantly greater in rats with lesions. Neither manipulation affected mitochondrial size or number of mitochondria per synapse. Independent of experimental condition, terminals with perforated PSDs had more mitochondria than those with non-perforated PSDs and, among those with non-perforated PSDs, terminals with multiple synaptic contacts had more mitochondria than those forming single synapses.

Elevated intrasynaptic dopamine release in Tourette's syndrome measured by PET

OBJECTIVE

Dopaminergic abnormalities in frontal-subcortical circuits have been hypothesized as the underlying pathophysiologic mechanism in Tourette's syndrome. The objective of this study was to test the hypothesis that presynaptic dopamine release from the striatum is abnormal in adults with Tourette's syndrome.

METHOD

Seven adults with Tourette's syndrome and five age-matched comparison subjects each received two positron emission tomography (PET) scans with high specific activity [11C]raclopride. The first scan followed an intravenous injection of saline; the second followed an intravenous injection of amphetamine. The relative dopamine release was estimated as the percentage difference in binding potential between the postsaline and postamphetamine scans.

RESULTS

Binding potential determined after the initial [11C]raclopride scan did not significantly differ between Tourette's syndrome and comparison subjects. After amphetamine challenge, the mean value of intrasynaptic dopamine in the putamen (as determined by true equilibrium bolus estimation) increased by 21% in the subjects with Tourette's syndrome and did not change in the comparison subjects; the mean values increased by 16.9% and decreased by 1.8%, respectively, when measured by the constrained method. Dopamine release in the caudate region was not significantly different in the Tourette's syndrome and comparison subjects.

CONCLUSIONS

Greater putamen dopamine release was seen in adults with Tourette's syndrome than in comparison subjects after a pharmacologic challenge with amphetamine. These results suggest that the underlying pathobiology in Tourette's syndrome is a phasic dysfunction of dopamine transmission.

Schizophrenia is associated with elevated amphetamine-induced synaptic dopamine concentrations: evidence from a novel positron emission tomography method

A major line of evidence that supports the hypothesis of dopamine overactivity in schizophrenia is the psychomimetic potential of agents such as amphetamine that stimulate dopamine outflow. A novel brain imaging method provides an indirect measure of in vivo synaptic dopamine concentration by quantifying the change in dopamine receptor radiotracer binding produced by agents that alter dopamine release but do not themselves bind to dopamine receptors. The purpose of this investigation is (i) to determine the sensitivity (i.e., amount of dopamine reflected in radiotracer binding changes) of this method by examining the relationship between amphetamine-induced changes in simultaneously derived striatal extracellular dopamine levels with in vivo microdialysis and striatal binding levels with the dopamine D2/D3 positron-emission tomography radioligand [11C]raclopride in nonhuman primates, and (ii) to test the hypothesis of elevated amphetamine-induced synaptic dopamine levels in schizophrenia. In the nonhuman primate study (n = 4), doubling the amphetamine dose produced a doubling in [11C]raclopride specific binding reductions. In addition, the ratio of percent mean dopamine increase to percent mean striatal binding reduction for amphetamine (0.2 mg/kg) was 44:1, demonstrating that relatively small binding changes reflect large changes in dopamine outflow. In the clinical study, patients with schizophrenia (n = 11) compared with healthy volunteers (n = 12) had significantly greater amphetamine-related reductions in [11C]raclopride specific binding (mean +/- SEM): -22.3% (+/-2.7) vs. -15.5% (+/-1.8),P = 0.04, respectively. Inferences from the preclinical study suggest that the patients' elevation in synaptic dopamine concentrations was substantially greater than controls. These data provide direct evidence for the hypothesis of elevated amphetamine-induced synaptic dopamine concentrations in schizophrenia.

Levodopa-induced changes in synaptic dopamine in patients with Parkinson's disease as measured by [11C]raclopride displacement and PET

Changes in striatal binding of [11C]raclopride, a dopamine D2 receptor antagonist, induced by acute levodopa administration, were evaluated with PET in 10 patients with idiopathic Parkinson's disease (PD). The patients were scanned on two occasions: drug-free and 15 minutes after a 5-minute intravenous infusion of 3 mg/kg levodopa. Levodopa administration produced reductions in striatal [11C]raclopride uptake index with a rostrocaudal gradient. The most pronounced reduction was found in the posterior putamen (to 82% of baseline), followed by the anterior putamen (to 88% of baseline) and the caudate nucleus (to 94% of baseline). The magnitude of [11C]raclopride uptake index reduction correlated with drug-free disability. Moreover, in four hemiparkinsonian patients, a reduction in [11C]raclopride uptake index was measured in the putamen contralateral to the parkinsonian symptoms. The present results demonstrate a positive correlation between striatal dopaminergic nerve-terminal deficiency and the capacity for levodopa to increase synaptic dopamine and displace [11C]raclopride binding, which corresponds to an accelerated amine turnover in dopamine-depleted striatal tissue. We therefore suggest that dopaminergic degeneration in PD is paralleled by a progressive acceleration of amine turnover. This mechanistic consequence of nigrostriatal degeneration, the selective restoration of synaptic dopaminergic neurotransmission in denervated striatal subregions, may explain the effectiveness of levodopa in producing symptomatic benefits in early PD. However, we also suggest that in the vastly denervated striatum, as in advanced PD, an excessive acceleration of amine turnover results in swings in levodopa-induced synaptic dopamine levels that are far beyond normal. This phenomenon most likely plays a key role in the pathogenesis underlying the development of motor-response complications in PD.

Morphologic changes in superior vestibulo-ocular neurons and vestibular nerve following labyrinthectomy in the cat

Morphologic changes in ipsilateral superior vestibulo-ocular neurons (SVON) and the vestibular nerve were measured in 4 cats 8 weeks after labyrinthectomy and 4 cats 1 year after labyrinthectomy. There is a 20% decrease in SVON size and 30% decrease in rough endoplasmic reticulum and ribosomes with no change in the volume fractions of Golgi apparatus or mitochondria. In the central nervous system degeneration of the vestibular nerve terminals ipsilateral to the labyrinthectomy was represented by a 25% loss of synaptic profiles (SP) on SVON at 8 weeks and 57% loss of SP at 1 year after labyrinthectomy. There was no significant loss of fiber number in the vestibular nerve at 8 weeks post lesion but a 35% loss of fibers primarily of the large size at 1 year post lesion.

Positron emission tomographic scanning demonstrates a presynaptic dopaminergic lesion in Lytico-Bodig. The amyotrophic lateral sclerosis-parkinsonism-dementia complex of Guam

We performed positron emission tomography using 18F-6-fluorodopa on four Guamanians with an amyotrophic lateral sclerosis syndrome, eight Guamanians with parkinsonism, and seven clinically normal Guamanians; the results were compared with those of nine Vancouver control subjects. The Guamanian subjects had all been exposed to similar Chamorro lifestyles. The scans were analyzed using a graphic method that calculates a constant for whole striatal 18F-6-fluorodopa uptake. The parkinsonian subjects all had significantly reduced striatal 18F-6-fluorodopa uptake. The group with amyotrophic lateral sclerosis had significantly reduced uptake that was intermediate between that of the control group and the parkinsonian group. Two Guamanian normal subjects had reduced striatal 18F-6-fluorodopa uptake. The nigrostriatal dopaminergic lesion in Guamanian parkinsonism is similar to that found in idiopathic parkinsonism. The nigrostriatal lesions in the subjects with amyotrophic lateral sclerosis and the Guamanian normal subjects are examples of subclinical neuronal damage demonstrable in living subjects with positron emission tomography.