Brain monoamine oxidase B and A in human parkinsonian dopamine deficiency disorders.

See Jellinger (doi:10.1093/awx190) for a scientific commentary on this article. The enzyme monoamine oxidases (B and A subtypes, encoded by MAOB and MAOA, respectively) are drug targets in the treatment of Parkinson's disease. Inhibitors of MAOB are used clinically in Parkinson's disease for symptomatic purposes whereas the potential disease-modifying effect of monoamine oxidase inhibitors is debated. As astroglial cells express high levels of MAOB, the enzyme has been proposed as a brain imaging marker of astrogliosis, a cellular process possibly involved in Parkinson's disease pathogenesis as elevation of MAOB in astrocytes might be harmful. Since brain monoamine oxidase status in Parkinson's disease is uncertain, our objective was to measure, by quantitative immunoblotting in autopsied brain homogenates, protein levels of both monoamine oxidases in three different degenerative parkinsonian disorders: Parkinson's disease (n = 11), multiple system atrophy (n = 11), and progressive supranuclear palsy (n = 16) and in matched controls (n = 16). We hypothesized that if MAOB is 'substantially' localized to astroglial cells, MAOB levels should be generally associated with standard astroglial protein measures (e.g. glial fibrillary acidic protein). MAOB levels were increased in degenerating putamen (+83%) and substantia nigra (+10%, non-significant) in multiple system atrophy; in caudate (+26%), putamen (+27%), frontal cortex (+31%) and substantia nigra (+23%) of progressive supranuclear palsy; and in frontal cortex (+33%), but not in substantia nigra of Parkinson's disease, a region we previously reported no increase in astrocyte protein markers. Although the magnitude of MAOB increase was less than those of standard astrocytic markers, significant positive correlations were observed amongst the astrocyte proteins and MAOB. Despite suggestions that MAOA (versus MAOB) is primarily responsible for metabolism of dopamine in dopamine neurons, there was no loss of the enzyme in the parkinsonian substantia nigra; instead, increased nigral levels of a MAOA fragment and 'turnover' of the enzyme were observed in the conditions. Our findings provide support that MAOB might serve as a biochemical imaging marker, albeit not entirely specific, for astrocyte activation in human brain. The observation that MAOB protein concentration is generally increased in degenerating brain areas in multiple system atrophy (especially putamen) and in progressive supranuclear palsy, but not in the nigra in Parkinson's disease, also distinguishes astrocyte behaviour in Parkinson's disease from that in the two 'Parkinson-plus' conditions. The question remains whether suppression of either MAOB in astrocytes or MAOA in dopamine neurons might influence progression of the parkinsonian disorders.

Dopamine and noradrenaline, but not serotonin, in the human claustrum are greatly reduced in patients with Parkinson's disease: possible functional implications.

In the human brain, the claustrum is a small subcortical telencephalic nucleus, situated between the insular cortex and the putamen. A plethora of neuroanatomical studies have shown the existence of dense, widespread, bidirectional and bilateral monosynaptic interconnections between the claustrum and most cortical areas. A rapidly growing body of experimental evidence points to the integrative role of claustrum in complex brain functions, from motor to cognitive. Here, we examined for the first time, the behaviour of the classical monoamine neurotransmitters dopamine, noradrenaline and serotonin in the claustrum of the normal autopsied human brain and of patients who died with idiopathic Parkinson's disease (PD). We found in the normal claustrum substantial amounts of all three monoamine neurotransmitters, substantiating the existence of the respective brain stem afferents to the claustrum. In PD, the levels of dopamine and noradrenaline were greatly reduced by 93 and 81%, respectively. Serotonin levels remained unchanged. We propose that by virtue of their projections to the claustrum, the brain stem dopamine, noradrenaline and serotonin systems interact directly with the cortico-claustro-cortical information processing mechanisms, by-passing their (parallel) routes via the basal ganglia-thalamo-cortical circuits. We suggest that loss of dopamine and noradrenaline in the PD claustrum is critical in the aetiology of both the motor and the non-motor symptoms of PD.

Low levels of astroglial markers in Parkinson's disease: relationship to α-synuclein accumulation.

Although gliosis is a normal response to brain injury, reports on the extent of astrogliosis in the degenerating substantia nigra in Parkinson's disease (PD) are conflicting. It has also been recently suggested that accumulation of nigral α-synuclein in this disorder might suppress astrocyte activation which in turn could exacerbate the degenerative process. This study examined brain protein levels (intact protein, fragments, and aggregates, if any) of astroglial markers and their relationship to α-synuclein in PD and in the positive control parkinson-plus conditions multiple system atrophy (MSA) and progressive supranuclear palsy (PSP). Autopsied brain homogenates of patients with PD (n=10), MSA (n=11), PSP (n=11) and matched controls (n=10) were examined for the astroglial markers glial fibrillary acidic protein (GFAP), vimentin, and heat shock protein-27 (Hsp27) by quantitative immunoblotting. As expected, both MSA (putamen>substantia nigra>caudate>frontal cortex) and PSP (substantia nigra>caudate>putamen, frontal cortex) showed widespread but regionally specific pattern of increased immunoreactivity of the markers, in particular for the partially proteolyzed fragments (all three) and aggregates (GFAP). In contrast, immunoreactivity of the three markers was largely normal in PD in brain regions examined with the exception of trends for variably increased levels of cleaved vimentin in substantia nigra and frontal cortex. In patients with PD, GFAP levels in the substantia nigra correlated inversely with α-synuclein accumulation whereas the opposite was true for MSA. Our biochemical findings of generally normal protein levels of astroglial markers in substantia nigra of PD, and negative correlation with α-synuclein concentration, are consistent with some recent neuropathology reports of mild astroglial response and with the speculation that astrogliosis might be suppressed in this disorder by excessive α-synuclein accumulation. Should astrogliosis protect, to some extent, the degenerating substantia nigra from damage, therapeutics aimed at normalization of astrocyte reaction in PD could be helpful.

The profile of mephedrone on human monoamine transporters differs from 3,4-methylenedioxymethamphetamine primarily by lower potency at the vesicular monoamine transporter.

Mephedrone (4-methylmethcathinone, MMC) and 3,4-methylenedioxymethamphetamine (MDMA) are constituents of popular party drugs with psychoactive effects. Structurally they are amphetamine-like substances with monoamine neurotransmitter enhancing actions. We therefore compared their effects on the human monoamine transporters using human cell lines stably expressing the human noradrenaline, dopamine and serotonin transporter (NET, DAT and SERT); preparations of synaptic vesicles from human striatum in uptake experiments; and a superfusion system where releasing effects can be reliably measured. MMC and MDMA were equally potent in inhibiting noradrenaline uptake at NET, with IC50 values of 1.9 and 2.1 µM, respectively. Compared to their NET inhibition potency, both drugs were weaker uptake inhibitors at DAT and SERT, with MMC being more potent than MDMA at DAT (IC50: 5.9 vs 12.6 µM) and less potent than MDMA at SERT (IC50: 19.3 vs 7.6 µM). MMC and MDMA both induced concentration-dependently [(3)H]1-methyl-4-phenylpyridinium-release from NET-, DAT or SERT-expressing cells which was clearly transporter-mediated release as demonstrated by the selective inhibitory effects of nmolar to low µmolar concentrations of desipramine, GBR 12909 and fluoxetine, respectively. MMC and MDMA differed most in their inhibition of [(3)H]dopamine uptake by synaptic vesicles from human striatum with MDMA being 10-fold more potent than MMC (IC50: 20 vs 223 µM) and their ability to release [(3)H]dopamine from human vesicular monoamine transporter expressing SH-SY5Y neuroblastoma cells in which MDMA seems to have a stronger effect. Our findings give a molecular explanation to the lower long-term neurotoxicity of MMC compared to MDMA.

Is Parkinson's disease a vesicular dopamine storage disorder? Evidence from a study in isolated synaptic vesicles of human and nonhuman primate striatum.

The cause of degeneration of nigrostriatal dopamine (DA) neurons in idiopathic Parkinson's disease (PD) is still unknown. Intraneuronally, DA is largely confined to synaptic vesicles where it is protected from metabolic breakdown. In the cytoplasm, however, free DA can give rise to formation of cytotoxic free radicals. Normally, the concentration of cytoplasmic DA is kept at a minimum by continuous pumping activity of the vesicular monoamine transporter (VMAT)2. Defects in handling of cytosolic DA by VMAT2 increase levels of DA-generated oxy radicals ultimately resulting in degeneration of DAergic neurons. Here, we isolated for the first time, DA storage vesicles from the striatum of six autopsied brains of PD patients and four controls and measured several indices of vesicular DA storage mechanisms. We found that (1) vesicular uptake of DA and binding of the VMAT2-selective label [(3)H]dihydrotetrabenazine were profoundly reduced in PD by 87-90% and 71-80%, respectively; (2) after correcting for DA nerve terminal loss, DA uptake per VMAT2 transport site was significantly reduced in PD caudate and putamen by 53 and 55%, respectively; (3) the VMAT2 transport defect appeared specific for PD as it was not present in Macaca fascicularis (7 MPTP and 8 controls) with similar degree of MPTP-induced nigrostriatal neurodegeneration; and (4) DA efflux studies and measurements of acidification in the vesicular preparations suggest that the DA storage impairment was localized at the VMAT2 protein itself. We propose that this VMAT2 defect may be an early abnormality promoting mechanisms leading to nigrostriatal DA neuron death in PD.

Reduced noradrenaline, but not dopamine and serotonin in motor thalamus of the MPTP primate: relation to severity of parkinsonism.

We recently found severe noradrenaline deficits throughout the thalamus of patients with Parkinson's disease [C. Pifl, S. J. Kish and O. Hornykiewicz Mov Disord. 27, 2012, 1618.]. As this noradrenaline loss was especially severe in nuclei of the motor thalamus normally transmitting basal ganglia motor output to the cortex, we hypothesized that this noradrenaline loss aggravates the motor disorder of Parkinson's disease. Here, we analysed noradrenaline, dopamine and serotonin in motor (ventrolateral and ventroanterior) and non-motor (mediodorsal, centromedian, ventroposterior lateral and reticular) thalamic nuclei in MPTP-treated monkeys who were always asymptomatic; who recovered from mild parkinsonism; and monkeys with stable, either moderate or severe parkinsonism. We found that only the symptomatic parkinsonian animals had significant noradrenaline losses specifically in the motor thalamus, with the ventroanterior motor nucleus being affected only in the severe parkinsonian animals. In contrast, the striatal dopamine loss was identical in both the mild and severe symptom groups. MPTP-treatment had no significant effect on noradrenaline in non-motor thalamic nuclei or dopamine and serotonin in any thalamic subregion. We conclude that in the MPTP primate model, loss of noradrenaline in the motor thalamus may also contribute to the clinical expression of the parkinsonian motor disorder, corroborating experimentally our hypothesis on the role of thalamic noradrenaline deficit in Parkinson's disease.

Thalamic noradrenaline in Parkinson's disease: deficits suggest role in motor and non-motor symptoms.

The thalamus occupies a pivotal position within the corticobasal ganglia-cortical circuits. In Parkinson's disease (PD), the thalamus exhibits pathological neuronal discharge patterns, foremost increased bursting and oscillatory activity, which are thought to perturb the faithful transfer of basal ganglia impulse flow to the cortex. Analogous abnormal thalamic discharge patterns develop in animals with experimentally reduced thalamic noradrenaline; conversely, added to thalamic neuronal preparations, noradrenaline exhibits marked antioscillatory and antibursting activity. Our study is based on this experimentally established link between noradrenaline and the quality of thalamic neuronal discharges. We analyzed 14 thalamic nuclei from all functionally relevant territories of 9 patients with PD and 8 controls, and measured noradrenaline with high-performance liquid chromatography with electrochemical detection. In PD, noradrenaline was profoundly reduced in all nuclei of the motor (pallidonigral and cerebellar) thalamus (ventroanterior: -86%, P = .0011; ventrolateral oral: -87%, P = .0010; ventrolateral caudal: -89%, P = .0014): Also, marked noradrenaline losses, ranging from 68% to 91% of controls, were found in other thalamic territories, including associative, limbic and intralaminar regions; the primary sensory regions were only mildly affected. The marked noradrenergic deafferentiation of the thalamus discloses a strategically located noradrenergic component in the overall pathophysiology of PD, suggesting a role in the complex mechanisms involved with the genesis of the motor and non-motor symptoms. Our study thus significantly contributes to the knowledge of the extrastriatal nondopaminergic mechanisms of PD with direct relevance to treatment of this disorder.

Heterogeneous intrastriatal pattern of proteins regulating axon growth in normal adult human brain.

There is much controversy regarding the extent of axon regeneration/sprouting ability in adult human brain. However, intrinsic differences in axon/neurite growth capability amongst striatal (caudate, putamen, nucleus accumbens) subdivisions could conceivably underlie, in part, their differential vulnerability in degenerative human brain disorders. To establish whether the distribution of axon growth markers in mature human striatum might be uniform or heterogeneous, we measured the intra-striatal pattern, in autopsied brain of normal subjects (n=40, age 18-99), of proteins involved in regulating axon growth. These proteins included polysialylated neural cell adhesion molecule (PSA-NCAM), microtubule-associated proteins TUC-4 (TOAD/Ulip/CRAMP-4) and doublecortin (DCX), and Bcl-2. The distribution of the marker proteins within the striatum was heterogeneous and inversely related to the pattern of dopamine loss previously characterized in Parkinson's disease (PD), with levels in nucleus accumbens>caudate>putamen, ventral>dorsal, and rostral putamen>caudal. In contrast, distribution of glial markers including glial fibrillary acidic protein (GFAP) and human leukocyte antigens (HLA-DRα and HLA-DR/DQ/DPß), other Bcl-2 family proteins, and control proteins neuron-specific enolase and α-tubulin in the striatum was either homogeneous or had a pattern unmatched to dopamine loss in PD. The putamen also showed more marked age-dependent decreases in concentrations of PSA-NCAM, TUC-4, and DCX and increases in GFAP levels than caudate. We conclude that the intrastriatal pattern of several key axon growth proteins is heterogeneous in adult human brain. Further investigation will be required to establish whether this pattern, which was inversely correlated with the pattern of dopamine loss in PD, is involved to any extent in the pathophysiology of this degenerative disorder.

Metabotropic glutamate receptor type 5 in levodopa-induced motor complications.

Metabotropic glutamate receptors type 5 (mGluR5) are implicated in regulation of synaptic plasticity and learning, and were the focus of our investigation in human Parkinson's disease (PD) patients with dyskinesias and wearing-off, and in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys with dyskinesias. Using the selective mGluR5 ligand [(3)H]ABP688 autoradiography, we measured mGluR5 in brain slices from 11 normal and 14 PD patients and from MPTP monkeys, in relation to motor complications (dyskinesias and wearing-off) associated with treatment with l-dopa. In 16 monkeys with a bilateral MPTP lesion and four controls, [(3)H]ABP688 specific binding was elevated in the striatum of dyskinetic l-dopa-treated MPTP monkeys but not in MPTP monkeys without dyskinesias compared to controls. PD patients with motor complications (either dyskinesias or wearing-off) had higher [(3)H]ABP688 specific binding compared to those without motor complications and controls in putamen, external and internal globus pallidus. Elevated glutamatergic transmission as measured with increased mGluR5 specific binding was associated with motor complications and its antagonism could be targeted for their treatment.

A brief history of levodopa.

This article highlights some landmarks in the history of levodopa, beginning with its isolation in 1910-13 from seedlings of Vicia faba to the demonstration, in 1961, of its "miraculous" effect in patients with Parkinson's disease (PD). Midway between these two time points, in 1938, L: -dopa decarboxylase was discovered, the enzyme that produces dopamine (DA) from levodopa. In 1957, DA was shown to occur in the brain, and in 1959 it was found to be enriched in the basal ganglia. At that time the striatal localization of DA, together with studies done in 1957-58 in naive and reserpine-treated animals regarding DA in the brain and the central effects of levodopa, suggested its possible involvement in "extrapyramidal control" and "reserpine parkinsonism". Following these discoveries, a study of (postmortem) brains of patients with basal ganglia disorders, including PD, was started, demonstrating, in 1960, a severe striatal DA deficit specifically in PD, thus furnishing a rational basis for the concept of "DA replacement therapy" with levodopa. Accordingly, in 1961, the first highly successful clinical trial with i.v. levodopa was carried out. In 1963, the DA deficit in the PD substantia nigra was found, indicative of a nigrostriatal DA pathway in the human brain, subsequently established in animal studies in 1964-65. In 1967, the chronic, high dose oral levodopa regimen was introduced in treatment of PD. Besides the above highlights in the history of levodopa, the article also cites critical opinions of world authorities in brain research of the time, harmful to the cause of DA, levodopa and PD. Today, the concept of DA replacement with levodopa is uncontested, with levodopa being the "gold standard" of modern drug treatment of PD.

Brain alpha-synuclein accumulation in multiple system atrophy, Parkinson's disease and progressive supranuclear palsy: a comparative investigation.

Alpha-synuclein is a major component of Lewy bodies and glial cytoplasmic inclusions, pathological hallmarks of idiopathic Parkinson's disease and multiple system atrophy, and it is assumed to be aetiologically involved in these conditions. However, the quantitative status of brain alpha-synuclein in different Parkinsonian disorders is still unresolved and it is uncertain whether alpha-synuclein accumulation is restricted to regions of pathology. We compared membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein, both the full-length 17 kDa and high molecular weight species, by western blotting in autopsied brain of patients with Parkinson's disease (brainstem-predominant Lewy body disease: n = 9), multiple system atrophy (n = 11), progressive supranuclear palsy (n = 16), and of normal controls (n = 13). Brain of a patient with familial Parkinsonism-dementia due to alpha-synuclein locus triplication (as positive control) showed increased membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein levels with abundant high molecular weight immunoreactivity. In multiple system atrophy, a massive increase in 17 kDa membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein was observed in highly pathologically affected regions, including putamen (+1760%, range +625-2900%), substantia nigra [+1000% (+356-1850%)], and white matter of internal capsule [+2210% (+430-6830%)] together with numerous high molecular weight species. Levels of 17 kDa membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein were only modestly increased in less affected areas (cerebellar cortex, +95%; caudate, +30%; with both also showing numerous high molecular weight species) and were generally normal in cerebral cortices. In both Parkinson's disease and progressive supranuclear palsy, membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein levels were normal in putamen and frontal cortex whereas a trend was observed for variably increased 17 kDa membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein concentrations [+184% (-60% to +618%)] with additional high molecular weight species in Parkinson's disease substantia nigra. No obvious correlation was observed between nigral membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein accumulation and Lewy body density in Parkinson's disease. Two progressive supranuclear palsy cases had membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein accumulation in substantia nigra similar to multiple system atrophy. Several Parkinson's disease patients had very modest high molecular weight membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein accumulation in putamen. Levels of 17-kDa membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein were generally positively correlated with those of high molecular weight membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein and there was a trend for a positive correlation between striatal dopamine loss and 17-kDa membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein concentrations in multiple system atrophy. Brain membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein accumulations in Parkinson's disease and multiple system atrophy are regionally specific, suggesting that these sporadic alpha-synucleinopathies, unlike familial Parkinsonism-dementia, are not associated with a simple global over-expression of the protein. Despite a similar extent of dopamine depletion, the magnitude of brain membrane-associated, sodium dodecyl sulfate-soluble alpha-synuclein changes is disease specific, with multiple system atrophy clearly having the most severe accumulation. Literature discrepancies on alpha-synuclein status in 'Parkinson's disease' might be explained by inclusion of cases not having classic brainstem-predominant Lewy body disease and by variable alpha-synuclein accumulation within this diagnostic classification.

Metabotropic glutamate receptor II in the brains of Parkinsonian patients.

Modulation of basal ganglia group II metabotropic glutamate receptors (mGluR2/3) is a potential therapeutic alternative to levodopa in Parkinson disease (PD). We used receptor-binding autoradiography of the mGluR2/3-selective radioligand [H]LY341495 in postmortem brain specimens from PD patients (n = 14) and controls (n=11) to investigate possible contributions of changes in ligand binding of this receptor to levodopa-associated motor complications experienced premortem in PD patients. The PD patients included those with and without histories of dyskinesias and those with and without "wearing off," which is defined as a reduced period of benefit from levodopa. Specific binding of [H]LY341495 to mGluR2/3 in the basal ganglia was higher in the caudate nucleus than the putamen and lower by approximately half in the external and internal globus pallidus (GPi) in controls. [H]LY341495-specific binding was reduced in the caudate and GPi in patients without wearing-off (-22% caudate, -30% GPi), compared with controls and with patients who had experienced wearing-off; there were no differences among PD patients with or without dyskinesias. These data suggest that an adaptive downregulation of mGluR2/3 in PD patients without wearing-off may compensate for increased glutamate. They indicate a key role for mGluR2/3 in control of movement and the potential for mGluR2/3-targeted drugs in the management of wearing-off fluctuations in PD.

Decreased binding of the D3 dopamine receptor-preferring ligand [11C]-(+)-PHNO in drug-naive Parkinson's disease.

The D(3) dopamine (DA) receptor is a member of the D(2)-like DA receptor family. While the D(2) receptor is abundant especially in motor-regions of the striatum, the D(3) receptor shows a relative abundance in limbic regions and globus pallidus. This receptor is of current interest in neurology because of its potential involvement in psychiatric and motor complications in Parkinson's disease and the possibility that dopamine D(3)-preferring agonist therapy might delay progression of the disorder. Preclinical data indicate that striatal levels of the D(3) (but not the D(2)) DA receptor are decreased following lesion of nigrostriatal DA neurons; at present, there are no in vivo data on this receptor subtype in Parkinson's disease. The objective of this positron emission tomography study was to compare [(11)C]-(+)-PHNO (D(3) versus D(2) preferring) and [(11)C]raclopride (D(3) = D(2)) binding in brain of non-depressed, non-demented, dopaminergic drug-naïve patients with early-stage Parkinson's disease (n = 10), relative to matched-controls (n = 9). Parkinson's disease was associated with a trend for bilaterally decreased [(11)C]-(+)-PHNO (but not [(11)C]raclopride) binding in the D(3)-rich ventral striatum (-11%, P = 0.07) and significantly decreased binding in globus pallidus (-42%, P = 0.02). In contrast, in the primarily D(2)-populated putamen, both [(11)C]-(+)-PHNO (25%, P = 0.02) and [(11)C]raclopride (25%, P < 0.01) binding were similarly increased, especially on the side contra-lateral to the symptoms. In the midbrain, presumably containing D(3) receptors localized to the substantia nigra, [(11)C]-(+)-PHNO binding was normal. Decreased [(11)C]-(+)-PHNO to [(11)C]raclopride ratio correlated with motor deficits and lowered-mood (P < 0.02). Our imaging data suggest that brain DA neuron loss in the human causes region-specific differential changes in DA D(2) and D(3) receptors with D(3) receptor 'downregulation' possibly related to some motor and mood problems in Parkinson disease. D(3) receptor levels might be a determinant vulnerability factor underlying side-effects associated with treatment; hence, these initial findings provide valuable baseline information to understand the role of D(3) receptors in response to Parkinson's disease medication.

Basic research on dopamine in Parkinson's disease and the discovery of the nigrostriatal dopamine pathway: the view of an eyewitness.

The article recapitulates some of the historical facts that led up to the recognition of dopamine (DA) as a biologically active substance in the brain and its crucial role in Parkinson's disease (PD). Three events to which the writer has been an eyewitness are specially highlighted and placed in their proper historical perspective: (1) the discovery of the striatal DA deficit in the PD brain; (2) the development of the DA replacement treatment with L-dopa, and (3) the 'birth' of the nigrostriatal DA pathway. The opposition to the new observations and their unexpected and far-reaching consequences will be illustrated by briefly discussing the strongly negative opinions expressed by some famous brain scientists of the day about the relationship between the substantia nigra, PD, and the DA-containing nigrostriatal fiber connection.

Preferential loss of serotonin markers in caudate versus putamen in Parkinson's disease.

Interest in serotonergic involvement in Parkinson's disease (PD) has focussed recently on the possibility that the remaining serotonin neurons innervating striatum (caudate and putamen) might release dopamine as a 'false transmitter'--an action that could have both beneficial and harmful (e.g. promotion of levodopa-induced dyskinesias) consequences. Evidence for a brain serotonergic disturbance in PD is derived in large part from findings of decreased binding of different radioligands to the serotonin transporter (SERT), one 'marker' of serotonin neurons. However, it is not known whether the reported changes in SERT binding reflect actual changes in levels of SERT protein or whether concentrations of all serotonin markers are similarly and markedly decreased in the two striatal subdivisions. We measured levels of SERT immunoreactivity, and for comparison, protein levels of tryptophan hydroxylase (TPH; the marker synthetic enzyme) using a Western blot procedure, as well as concentrations of serotonin, its metabolite 5-hydroxyindoleacetic acid (5-HIAA), and dopamine by HPLC in post-mortem striatum of patients with PD and normal controls. Whereas concentrations of dopamine were severely decreased (caudate, -80%; putamen, -98%) and showed little (caudate) or no (putamen) overlap between individual control and patient values, levels of all four serotonin markers were less markedly reduced (-30% to -66%) with some patients having distinctly normal levels. Unlike the preferential loss of dopamine in putamen, the caudate was affected more than putamen by loss of all serotonin markers: serotonin (-66% versus -51%), 5-HIAA (-42% versus -31%), SERT (-56% versus -30%) and TPH (-59% versus -32%). Striatal serotonin concentration was similar in the subset of patients reported to have had dyskinesias versus those not reported to have had this drug complication. Previous findings of decreased SERT binding are likely explained by loss of SERT protein. Reduced striatal levels of all of the key serotonergic markers (neurotransmitter and metabolite, transporter protein, synthesizing enzyme protein) provide strong evidence for a serotonergic disturbance in PD, but with some patients affected much more than others. The more marked caudate reduction suggests that raphe neurons innervating this area are more susceptible to 'damage' than those innervating putamen and that any functional impairment caused by striatal serotonin loss might primarily involve the caudate. Questions related to the, as yet undetermined, clinical consequences in PD of a striatal serotonin deficiency (caudate: cognitive impairment?) and preservation (putamen: levodopa-induced dyskinesias?) should be addressed in prospective brain imaging and pharmacological studies.

Marked dissociation between high noradrenaline versus low noradrenaline transporter levels in human nucleus accumbens.

We recently identified a noradrenaline-rich caudomedial subdivision of the human nucleus accumbens (NACS), implying a special function for noradrenaline in this basal forebrain area involved in motivation and reward. To establish whether the NACS, as would be expected, contains similarly high levels of other noradrenergic markers, we measured dopamine-beta-hydroxylase (DBH) and noradrenaline transporter in the accumbens and, for comparison, in 23 other brain regions in autopsied human brains by immunoblotting. Although the caudomedial NACS had high DBH levels similar to those in other noradrenaline-rich areas, the noradrenaline transporter concentration was low (only 11% of that in hypothalamus). Within the accumbens, transporter concentration in the caudal portion was only slightly (by 30%) higher than that in the rostral subdivisions despite sharply increasing rostrocaudal gradients of noradrenaline (15-fold) and DBH. In contrast, the rostrocaudal gradient in the accumbens for the serotonin transporter and serotonin were similar (2-fold increase). The caudomedial NACS thus appears to represent the only instance in human brain having a striking mismatch in high levels of a monoamine neurotransmitter versus low levels of its uptake transporter. This suggests that noradrenaline signalling is much less spatially and temporally restricted in the caudomedial accumbens than in other noradrenaline-rich brain areas.