Melanized dopaminergic neurons are differentially susceptible to degeneration in Parkinson's disease

Human brain phenolsulfotransferase. Regional distribution in Parkinson's disease

Brain phenolsulfotransferase (PST) in 105.000 x g supernatant fractions prepared from post mortem human brain catalyzes the sulfate conjugation of dopamine (DA). Using 50 microM DA, the PST activity was linear up to one hour. The KM value for DA was 3.1 microM. Higher concentrations of DA from 25 b microM up caused inhibition of PST activity. Assessment of regional distribution in normal brain using 20 microM DA concentration revealed the highest PST activities in temporal and frontal cortex. About ten times lower activities were measured in parietal and occipital lobe, amygdala, hypothalamus, and hippocampus, whereas the nucleus accumbens, nucleus basalis of Meynert, caudate nucleus, and substantia nigra showed the lowest activities (about 1% of those in frontal and parietal cortex). In the brains of subjects with Parkinson's disease (PD) treated with levodopa, a significant reduction of PST activities was observed in hypothalamus, frontal and temporal cortex, amygdaloid nucleus, occipital and parietal cortex (between 20 and 38.8% of controls). Depletion of PST activity was less severe in hippocampus (46% of controls), nucleus accumbens, putamen, and substantia nigra (67 and 72% of controls, respectively). No changes were observed in the nucleus basalis of Meynert, while PST activity was increased in the caudate nucleus (174 to 203% of controls). The presented data indicate that on PD brain the PST activity is reduced in areas of the cerebral isocortex and limbic system, while in the basal ganglia it is either mildly reduced (putamen) or increased (caudate nucleus). Selective changes of PST activity in PD brain may indicate an important function of this enzyme in the metabolism and/or storage of DA under pathological conditions.

The oxidant stress hypothesis in Parkinson's disease: evidence supporting it

Oxidant stress, due to the formation of hydrogen peroxide and oxygen-derived free radicals, can cause cell damage due to chain reactions of membrane lipid peroxidation. Because the substantia nigra is rich in dopamine, which can undergo both enzymatic oxidation via monoamine oxidase and nonenzymatic autoxidation, hydrogen peroxide and oxyradicals (superoxide anion radical and hydroxyl radical) are generated in this midbrain nucleus. Although proof that oxidant stress actually causes the loss of monoaminergic neurons in patients with Parkinson's disease is lacking, there is a considerable body of evidence from studies in both animals and humans that support the concept. (1) Neurotoxins that selectively destroy the dopaminergic neurons in the nigra, such as 6-hydroxydopamine and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), appear to act via oxidant stress. (2) The substantia nigra of patients with Parkinson's disease reveals evidence of oxidant stress by the findings of increased lipid peroxidation and decreased reduced glutathione. (3) Total iron is increased and ferritin is reduced in the substantia nigra pars compacta in patients with Parkinson's disease. This combination suggests that this transition metal is in a low molecular weight form, capable of catalyzing nonenzymatic oxidative reactions, especially the conversion of hydrogen peroxide to hydroxyl radical, which is the most reactive of the oxygen radicals. (4) Neuromelanin, a product of dopamine autoxidation, can serve as a reservoir for iron, promoting the generation of oxyradicals. (5) Antioxidant defense mechanisms appear to be reduced in the parkinsonian substantia nigra with the findings of decreased activities of glutathione peroxidase and catalase.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitotoxic lesions of the pedunculopontine tegmental nucleus of the rat. I. Comparison of the effects of various excitotoxins, with particular reference to the loss of immunohistochemically identified cholinergic neurons

The pedunculopontine tegmental nucleus (PPTg) has been shown to have cholinergic connections with the thalamus and basal ganglia. The ability of various doses of the excitotoxins (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) (AMPA), folate, ibotenate, kainate, N-methyl-D-aspartate (NMDA), quinolinate and quisqualate to make lesions in the PPTg was examined, with particular reference to their ability to destroy cholinergic neurons identified using choline acetyltransferase (ChAT) immunohistochemistry. All of the toxins induced convulsive activity on recovery from surgical anesthesia and all except folate made lesions in the PPTg and surrounding structures. The size of the lesions was computed following examination of Cresyl violet stained sections. The largest lesions were made by kainate = AMPA greater than NMDA = ibotenate greater than quisqualate = quinolinate. All of the toxins destroyed cholinergic neurons, higher doses producing greater loss than lower. The ratio of cholinergic cell loss to general neuronal loss (assessed by Cresyl violet staining) was also computed, revealing marked differences between the toxins. Statistical analysis showed that there were significant differences between excitotoxins in terms of this ratio, but these were accounted for by the low dose of quinolinate (24 nmol) producing a significantly greater ratio of damage (12.18:1) than every other toxin. (Next highest ratio: quisqualate 60 nmol, 6.22:1.) Between the other toxins (kainate, AMPA, ibotenate, quisqualate, NMDA and the high dose of quinolinate) there were no statistically significant differences. Intense calcium deposits (stained by Alizarin red) were found frequently and often defined the borders of the lesion. Tyrosine hydroxylase immunohistochemistry revealed axons running below and into the area of lesioned tissue suggesting strongly that fibers were undamaged by the lesions. We conclude that in the PPTg, different excitotoxins make discriminably different lesions, both quantitatively and qualitatively. Unlike excitotoxic lesions in the basal forebrain quinolinate, not quisqualate, made the most selective lesions of cholinergic neurons and, unlike excitotoxic lesions in the septal nuclei, non-myelinated fibers were spared by ibotenate. The implications of these data for research into brainstem mechanisms of Parkinson's disease are discussed.

Dopamine transporter mRNA content in human substantia nigra decreases precipitously with age

The dopamine transporter is the primary means of inactivating synaptic dopamine as well as a major site of action for psychostimulants (such as cocaine and amphetamine) and for neurotoxins that induce parkinsonism. In the present study, a human dopamine transporter partial cDNA clone obtained by polymerase chain reaction exhibited 87% and 89% identity at the nucleic acid and amino acid levels, respectively, with transmembrane domains 3-5 of the rat homolog. This clone was used to quantitate human dopamine transporter mRNA by nuclease protection assay. The postmortem content of dopamine transporter mRNA in the substantia nigrae of 18- to 57-yr-old subjects was relatively constant, while in subjects greater than 57 yr old, a precipitous (greater than 95%) decline in substantia nigra dopamine transporter mRNA was evident. In contrast, tyrosine hydroxylase mRNA in the same samples declined in a linear manner with increasing age. In situ hybridization experiments confirmed the profound loss of dopamine transporter gene expression in melanin-positive (presumptive dopamine) nigral neurons. These data may begin to shed light on compensatory changes occurring in human dopamine neurons during normal aging.

Iron-melanin complex in substantia nigra of parkinsonian brains: an x-ray microanalysis

Using energy-dispersive x-ray analysis on an electron microscope working in the scanning transmission electron microscopy mode equipped with a microanalysis system, we studied the subcellular distribution of trace elements in neuromelanin-containing neurons of the substantia nigra zona compacta (SNZC) of three cases of idiopathic Parkinson's disease (PD) [one with Alzheimer's disease (AD)] and of three controls, in Lewy bodies of SNZC, and in synthetic dopamine-melanin chemically charged or uncharged with Fe. Weak but significant Fe peaks similar to those of a synthetic melanin-Fe3+ complex were seen only in intraneuronal highly electron-dense neuromelanin granules of SNZC cells of PD brains, with the highest levels in a case of PD plus AD, whereas a synthetic melanin-Fe2+ complex showed much lower iron peaks, indicating that neuromelanin has higher affinity for Fe3+ than for Fe2+. No detectable Fe was seen in nonmelanized cytoplasm of SNZC neurons and in the adjacent neuropil in both PD and controls, in Lewy bodies in SNZC neurons in PD, and in synthetic dopamine-melanin uncharged with iron. These findings, demonstrating for the first time a neuromelanin-iron complex in dopaminergic SNZC neurons in PD, support the assumption that an iron-melanin interaction contributes significantly to dopaminergic neurodegeneration in PD and PD plus AD.

Cloning of human neurotensin/neuromedin N genomic sequences and expression in the ventral mesencephalon of schizophrenics and age/sex matched controls

A human genomic clone encompassing exons 1-3 of the neurotensin/neuromedin N gene was identified using a canine neurotensin complementary DNA probe. Sequence comparisons revealed that the 120-amino acid portion of the precursor sequence encoded by exons 1-3 is 89% identical to previously determined cow and dog sequences and that the proximal 250 bp of 5' flanking sequences are strikingly conserved between rat and human. The 5' flanking sequence contains cis-regulatory sites required for the induction of neurotensin/neuromedin N gene expression in PC12 cells, including AP1 sites and two cyclic adenosine-5'-monophosphate response elements. Oligonucleotide probes based on the human sequence were used to examine the distribution of neurotensin/neuromedin N messenger RNA in the ventral mesencephalon of schizophrenics and age- and sex-matched controls. Neurotensin/neuromedin N messenger RNA was observed in ventral mesencephalic cells some of which also contained melanin pigment or tyrosine hydroxylase messenger RNA. Neurons expressing neurotensin/neuromedin N messenger RNA were observed in the ventral mesencephalon of both schizophrenic and non-schizophrenic humans.

Is the vulnerability of neurons in the substantia nigra of patients with Parkinson's disease related to their neuromelanin content?

The contribution of neuromelanin (NM) to the pathogenesis of Parkinson's disease (PD) has long been suspected. In particular, a correlation has been reported between the estimated cell loss in the mesencephalic dopaminergic cell groups and the percentage of NM-pigmented neurons in these cell groups. To test whether the amount of pigment per cell is a critical factor or whether the presence of NM within a neuron is sufficient to account for the degeneration of dopaminergic neurons, the NM content was measured in each neuron from representative sections throughout the ventral mesencephalon of four controls subjects and four patients with PD. Intraneuronal NM was quantified by a densitometric method, using known amounts of synthetic melanin as standards. In control brains, the distribution of melanized neurons in the nigral complex showed a high proportion of lightly melanized neurons in the ventral tegmental area and the pars alpha and gamma of the substantia nigra (SN), whereas heavily melanized neurons were mostly located in the pars beta and lateralis of the SN. An inverse relationship was observed between the percentage of surviving neurons in PD compared with controls and the amount of NM they contain, suggesting that the vulnerability of the dopaminergic neurons is related to their NM content. Factors other than NM may be involved in the differential vulnerability of catecholaminergic neurons in PD. In particular, the constant topography of the cell loss suggests that cell position within the nigral complex is a key factor.

Compartmentalization of excitatory amino acid receptors in human striatum

Division of the mammalian neostriatum into two intermingled compartments called striosomes and matrix has been established by analysis of enzyme activity, neuropeptide distribution, nucleic acid hybridization, and neurotransmitter receptor binding. Striosomes and matrix are distinct with respect to afferent and efferent connections, and these regions provide the potential for modulation and integration of information flow within basal ganglia circuitry. The primary neurotransmitters of corticostriatal afferents are excitatory amino acids, but to date no correlation of excitatory amino acid receptors and striatal compartments has been described. We examined binding to the three pharmacologically distinct ionotropic excitatory amino acid receptors, N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and kainate, in human striatum using in vitro receptor autoradiography and compared the binding to striosomes and matrix histochemically defined by acetylcholinesterase activity. Our findings reveal increased binding to N-methyl-D-aspartate receptors and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors in matrix relative to striosomes and increased kainate receptor binding in striosomes relative to matrix. These results suggest that afferent input to the two striatal compartments may be mediated by pharmacologically distinct excitatory amino acid receptor subtypes.

Toxic effects of iron for cultured mesencephalic dopaminergic neurons derived from rat embryonic brains

Iron, a transition metal possibly involved in the pathogenesis of Parkinson's disease, was tested for its toxic effects toward cultures of dissociated rat mesencephalic cells. When cultures were switched for 24 h to serum-free conditions, the effective concentrations of ferrous iron (Fe2+) producing a loss of 50% of dopaminergic neurons, as quantified by tyrosine hydroxylase (TH) immunocytochemistry, TH mRNA in situ hybridization, and measurement of TH activity, were on the order of 200 microM. High-affinity dopamine (DA) uptake, which reflects integrity and function of dopaminergic nerve terminals, was impaired at significantly lower concentrations (EC50 = 67 microM). Toxic effects were not restricted to dopaminergic neurons inasmuch as trypan blue dye exclusion index and gamma-aminobutyric acid uptake, two parameters used to assess survival of other types of cells present in these cultures, were also affected. Protection against iron cytotoxicity was afforded by desferrioxamine and apotransferrin, two ferric iron-chelating agents. Normal supplementation of the culture medium by serum proteins during treatment was also effective, presumably via nonspecific sequestration. Potential interactions with DA were also investigated. Fe2+ at subtoxic concentrations and desferrioxamine in the absence of exogenous iron added to the cultures failed to potentiate or reduce DA cytotoxicity for mesencephalic cells, respectively. Transferrin, the glycoprotein responsible for intracellular delivery of iron, was ineffective in initiating selective cytotoxic effects toward dopaminergic neurons preloaded with DA. Altogether, these results suggest (a) that ferrous iron is a potent neurotoxin for dopaminergic neurons as well as for other cell types in dissociated mesencephalic cultures, acting likely via autoxidation into its ferric form, and (b) that the presence of intra- and extracellular DA is not required for the observed toxic effects.

Melanin, tyrosine hydroxylase, calbindin and substance P in the human midbrain and substantia nigra in relation to nigrostriatal projections and differential neuronal susceptibility in Parkinson's disease

The anatomy of melanin-containing neurons and other midbrain structures was examined by tyrosine hydroxylase (TH), calbindin D28k, and substance P immunostaining. Greater than 95% of cells in the substantia nigra pars compacta contained melanin, but densely packed cells in a ventral tier had a low content of melanin and loosely packed cells in a dorsal tier had a high content of melanin. Approximately 60% in the gamma group and 40% in the retrorubral nucleus had a low content of melanin. TH immunostaining was moderate in both the ventral and dorsal tiers, but more intense in the gamma group and retrorubral nucleus. Calbindin D28k was absent from the ventral and dorsal tiers, but present in the gamma group and retrorubral nucleus. In the light of primate tracing studies these findings suggest that the ventral tier of the pars compacta projects to striosomes of the striatum and the dorsal tier, gamma group and retrorubral nucleus to the matrix compartment. The ventral tier is more vulnerable than the dorsal tier in Parkinson's disease, but the cells contain less melanin. Neither tier contains calbindin D28k. This differential vulnerability between the ventral and dorsal tiers cannot be explained by melanin or calbindin D28k.

Selective loss of nigral neurons in Alzheimer's disease: a morphometric study

Loss of neurons from the substantia nigra (SN), which is sometimes observed in Alzheimer's disease (AD), was quantitatively analyzed in 10 cases of presenile AD and 19 age-matched controls. On sections from the upper and lower portions of the SN, the pigmented zone (zona compacta) and the non-pigmented zone (zona reticulata) were delineated, and these zones were partitioned into quarters: medial, mid-medial, mid-lateral and lateral. This approach clarified topographical preference of neuronal depletion in the SN of AD; namely (1) pigmented neurons were more severely affected than non-pigmented neurons, (2) neuronal depletion was more marked in the lower SN (-38%, P less than 0.001), where the pigmented neurons in the medial quarter were most severely affected (-51%, P less than 0.001), (3) in the upper SN (neuronal loss: -21%, P less than 0.01), the pigmented neurons in the mid-medial quarter were most severely affected (-43%, P less than 0.01). These findings suggest that some groups of nigral neurons are primarily involved in presenile AD. Gallyas staining after bleaching of melanin pigments uncovered a large number of neurofibrillary tangles (NFTs) mainly in the pigmented zone, especially in the medial quarter. A large number of NFTs, scarse senile plaques, and substantial depletion of neurons form an unique combination of Alzheimer pathology in the SN not well recognized so far.

Slow changes of tyrosine hydroxylase gene expression in dopaminergic brain neurons after neurotoxin lesioning: a model for neuron aging

Slow neuron regression develops during the adult phase of life in select brain systems of mammals. We describe a model in adult rats that resolves several phases in a slow atrophic process that differentially influences levels of mRNA and protein for tyrosine hydroxylase (TH). Responses of striatal dopaminergic markers to 6-hydroxydopamine (6-OHDA) lesions in rats indicated that the striatal terminals maintained TH protein, despite greater than 3-fold loss of TH mRNA in the substantia nigra pars compacta (SNC) cell bodies whose axons project to the striatum. The loss of TH mRNA/cell was progressive up to 9 months, whereas SNC cell body shrinkage stabilized by 3 months post-lesioning. Consideration of possible mechanisms in protein turnover motivated a search for PEST motifs in the TH of rats and other vertebrates that could be a point of regulation by altering the rate of TH protein turnover.

In vivo trapping of hydroxyl free radicals in the striatum utilizing intracranial microdialysis perfusion of salicylate: effects of MPTP, MPDP+, and MPP+

Increased formation of hydroxyl free radicals (.OH) reflected by .OH adduct of salicylate in brain dialysate was demonstrated during the sustained (more than 2 hours) dopamine overflow elicited by 75 nmol of 1-methyl-4-phenyldihydropyridine (MPDP+) and 1-methyl-4-phenylpyridinium (MPP+) in the rat striatum. Owing to its weak dopamine releasing action, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) did not significantly increase the .OH formation. This data suggests that sustained elevation of dopamine in the extracellular fluid elicited by MPTP analogues can be auto-oxidized, which in turn leads (possibly by indirect mechanisms) to the formation of cytotoxic .OH free radicals near the nigrostriatal terminals.

Role of iron and iron chelation in dopaminergic-induced neurodegeneration: implication for Parkinson's disease

Recent studies in Parkinson's disease suggest that the degeneration of the nigrostriatal melanin-containing dopaminergic neurons results from toxic effects of free radicals, which are generated during dopamine metabolism in the substantia nigra (SN). This has been linked to the selective accumulation of iron, a known catalyst of radical formation, in the zona compacta of the SN. We have shown that interaction of iron with melanin may result in a high affinity binding of iron to melanin (KD = 13.0 +/- 0.15 nM). Indeed, x-ray analysis of melanized dopamine neurons of parkinsonian SN has shown an interaction of iron with melanin that is absent in control brains. In the presence of excess Fe3+, melanin potentiates iron-induced lipid peroxidation. Since iron chelators prevent lipid peroxidation, we have ascertained the ability of the iron chelator deferoxamine to prevent the lesion of the nigrostriatal dopamine neuron induced by 6-hydroxy dopamine (6-OHDA). Our results demonstrated that intraventricular injection of 130 ng deferoxamine to rats prior to 250 micrograms of 6-OHDA partially prevented the decrease in striatal dopamine content caused by 6-OHDA (56% reduction vs 90%, respectively). This protection was sufficient to produce normal dopamine-related behavioral responses. These results suggest that iron and iron chelators play a crucial role in the process of dopaminergic neurodegeneration and neuroprotection. The latter is further supported by our recent findings that intranigral injection of iron (50 micrograms) resulted in a substantial selective decrease of striatal dopamine (95%) and impaired dopamine-related responses.

c-fos protein-like immunoreactivity: distribution in the human brain and over-expression in the hippocampus of patients with Alzheimer's disease

c-fos protein-like immunoreactivity was investigated in the human brain post mortem, using a polyclonal antiserum raised against the N-terminal conserved peptide of c-fos protein. Immunostaining was found in the cerebral cortex, hippocampus, striatum, thalamus and cerebellum but not in the upper brainstem and the adrenal gland. c-fos-like immunoreactivity predominated in neuronal elements, but was also observed in neuropil and glial cells. In addition to a nuclear localization, the staining could be seen in neuronal dendrites (i.e. in the pyramidal cells of hippocampus or in some cortical areas). In order to analyse the effect of brain injury on c-fos expression, the characteristics of the immunostaining were analysed in the hippocampus of patients deceased with Alzheimer's disease known to be associated with a preferential vulnerability of the pyramidal neurons. No staining was observed in the senile plaques or in neurofibrillary tangles, the histopathological stigmata of the disease. Densitometric measurement of the intensity of c-fos-like staining revealed a significant increase in the hilus, the fimbria and the CA1 field of the pyramidal layer in brains of patients with Alzheimer's disease compared to controls. These modifications may result from a suffering stage of hippocampal cells or from a compensatory mechanism in the still surviving neurons not yet affected by the pathological process.

Co-expression of tyrosine hydroxylase messenger RNA 1 and 2 in human ventral mesencephalon revealed by digoxigenin- and biotin-labelled oligodeoxyribonucleotides

In situ hybridization experiments, using oligodeoxyribonucleotides specific for the two major expressed human tyrosine hydroxylase mRNAs, were performed on human brain sections at the level of the mesencephalon. The specificity of the probes was ascertained by Northern blot experiments carried out with independently in vitro synthesized human tyrosine hydroxylase mRNAs. For in situ hybridization experiments, oligodeoxyribonucleotides were labelled with nucleotides tagged with digoxigenin or biotin molecules. The hybridized oligonucleotides were detected by antibodies coupled with peroxidase and alkaline phosphatase enzymes, which yield, with appropriate substrates, brown and purple products, respectively. The simultaneous detection of the two mRNAs with digoxigeninated and biotinylated probes revealed that these two mRNAs are co-expressed in single cells. The purple product obtained with alkaline phosphatase exhibits a discrete distribution within the dopaminergic cells suggesting these mRNAs are associated with sub-cellular structures. Finally, a heterogeneity in the intensity of the labelling of reactive cells with both probes was visualized as well as the expression of the two mRNA species in neurites.

Why are nigral catecholaminergic neurons more vulnerable than other cells in Parkinson's disease?

Although the cause of neuronal death in Parkinson's disease remains unknown, a hyperoxidation phenomenon has been implicated as a potential cytotoxic mechanism. Catecholaminergic neurons containing neuromelanin, an autoxidation byproduct of catecholamines, are more vulnerable in Parkinson's disease than nonmelanized catecholaminergic neurons. High levels of CuZn superoxide dismutase mRNA have been observed in the substantia nigra, suggesting that high levels of oxygen free radicals are indeed produced in the structure. Catecholaminergic neurons surrounded by a low density of glutathione peroxidase cells are more susceptible to degeneration in Parkinson's disease than those well protected against oxidative stress. The nigral content in iron, a compound that exacerbates the production of free radicals in catecholaminergic neurons, is increased in Parkinson's disease. Altogether these data suggest that hyperoxidation may participate in the selective vulnerability of catecholaminergic neurons in Parkinson's disease.

Iron-melanin interaction and lipid peroxidation: implications for Parkinson's disease

The vulnerability of substantia nigral (SN) melaninized dopamine neurons to neurodegeneration in Parkinson's disease and the selective increases of iron and basal lipid peroxidation in SN indicate that iron-melanin interaction could be crucial to the pathogenesis of this disease. The present study describes, for the first time, the identification and characterization of a high-affinity (KD = 13 nM) and a lower affinity (KD = 200 nM) binding site for iron on dopamine melanin. The binding of iron to melanin is dependent on pH and the concentration of melanin. Iron chelators, U74500A, desferrioxamine, and to less extent 1,10-phenanthroline and chlorpromazine, but not the Parkinson-inducing neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, can inhibit the binding of iron to melanin and iron-induced lipid peroxidation. Although melanin alone diminishes basal lipid peroxidation in rat cortical homogenates, it can also potentiate that initiated by iron, a reaction inhibited by desferrioxamine. In the absence of an identifiable exogenous or endogenous neurotoxin in idiopathic Parkinson's disease, iron-melanin interaction in pars compacta of SN may be a strong candidate for the cytotoxic component of oxygen radical-induced neurodegeneration of melaninized dopamine neurons.

Cysteinyldopamine is not incorporated into neuromelanin

The pigment of human substantia nigra, neuromelanin, has been thought to be derived from dopamine. To examine the genesis of neuromelanin, we advanced a new hypothesis that neuromelanin is formed by oxidation of dopamine and cysteinyldopamine. On the basis of this hypothesis, synthetic neuromelanins were obtained by tyrosinase oxidation of dopamine in the presence of various ratios of cysteine and were hydrolyzed with hydriodic acid to obtain 4-amino-3-hydroxyphenylethylamine (AHPEA). The AHPEA content in these synthetic melanins was shown to be proportional to the sulfur content. Eleven samples of human substantia nigra were treated as well and contents of AHPEA were found to be only trace amounts. These results suggest that cysteinyldopamine may not be incorporated into neuromelanin.

Alterations of dopaminergic and noradrenergic innervations in motor cortex in Parkinson's disease

The motor areas of the cerebral cortex contain dense dopaminergic and noradrenergic innervation in humans. We looked for changes of these innervations in cases with Parkinson's disease (PD). The density of fibers immunolabeled with tyrosine hydroxylase or dopamine-beta-hydroxylase was evaluated in the primary motor, premotor, and prefrontal cortical regions in 6 cases with PD and 7 control cases. Reductions of both noradrenergic and dopaminergic cortical innervations were observed, with similar magnitudes of reduction found in the motor and prefrontal regions of the cortex. Depletion of noradrenergic innervation was diffuse, involving all cortical laminae. Depletion of dopaminergic innervation was laminar specific, with the most significant reductions in layers I and II; reductions in layers V and VI were either less marked (prefrontal cortex) or not detectable (primary motor). The results suggest the existence of two separate mesocortical dopaminergic systems in humans, with the one distributing to upper cortical layers being preferentially involved in PD.

Catecholaminergic neurons in the parabrachial nucleus of normal individuals and patients with idiopathic Parkinson's disease

The parabrachial nucleus is believed to play a role in autonomic regulation. We have used the Fontana-Masson ammoniacal silver nitrate method and a tyrosine hydroxylase-immunostaining technique to demonstrate the presence of neuromelanin-containing catecholaminergic neurons in the parabrachial nucleus of normal individuals. In addition, we also show that there is a significant reduction of these catecholaminergic neurons and presence of Lewy bodies in the parabrachial nucleus of patients with idiopathic Parkinson's disease. These findings may be related to the several autonomic disturbances that may occur in idiopathic Parkinson's disease.

Smoker's melanosis may explain the lower hearing loss and lower frequency of Parkinson's disease found among tobacco smokers--a new hypothesis

A new hypothesis is presented explaining the preventive effect of tobacco smoking found on noise induced hearing loss and on the frequency of Parkinson's disease. The hypothesis is based on the finding of a melanocyte stimulation of tobacco smoking in the human oral mucosa, resulting in a higher melanin content in the epithelial cells, and a higher frequency of visible oral melanin pigmentation--smoker's melanosis. The preventive influence of smoking found in the cochlea and substantia nigra may also be due to a higher melanin content and to the ability of melanin to strongly bind specific chemical agents for a long time. Melanin may in this way act as a scavenger against cell toxic factors in these organs.

Medullary catecholaminergic neurons in the normal human brain and in Parkinson's disease

Parkinson's disease is thought to cause degeneration of melanin-pigmented catecholaminergic neurons throughout the brainstem, but little quantitative information is available on the fate of catecholaminergic neurons associated with the dorsal vagal complex or medullary reticular formation. We therefore examined these neurons in the normal human medulla and in the brains of patients with Parkinson's disease, using both a melanin stain and immunohistochemical methods with an antiserum against tyrosine hydroxylase. The greatest numbers of catecholaminergic neurons in the ventrolateral reticular formation (A1/C1 group) were located in the far rostral medulla, whereas the largest populations of catecholaminergic cells in the dorsal vagal complex (A2/C2 group) were found at the level of the area postrema. No loss of cells was observed in the A1/C1 group in the parkinsonian brains. In contrast, the A2/C2 group showed moderate loss of neurons, most marked at the level of the area postrema. This difference was entirely due to the loss of neurons in the medial component of the A2 group, a population that normally is only lightly pigmented, while the heavily pigmented neurons in the ventral and intermediate components of the A2 complex were unaffected. Parkinson's disease causes degeneration only of selected populations of medullary catecholaminergic neurons, without apparent relationship to the extent of melanin pigmentation.

Pathology of Parkinson's disease. Changes other than the nigrostriatal pathway

In Parkinson's disease (PD), in addition to degeneration of the nigrostriatal dopaminergic pathway, a variety of neuronal systems are involved, causing multiple neuromediator dysfunctions that account for the complex patterns of functional deficits. Degeneration affects the dopaminergic mesocorticolimbic system, the noradrenergic locus ceruleus (oral parts) and motor vagal nucleus, the serotonergic raphe nuclei, the cholinergic nucleus basalis of Meynert, pedunculopontine nucleus pars compacta, Westphal-Edinger nucleus, and many peptidergic brainstem nuclei. Cell losses in subcortical projection nuclei range from 30 to 90% of controls; they are more severe in depressed and demented PD patients. Most of the lesions are region-specific, affecting not all neurons containing a specific transmitter or harboring Lewy bodies. In contrast to Alzheimer's disease (AD), subcortical system lesions in Parkinson's disease appear not to be related to cortical pathology, suggesting independent or concomitant degeneration. The pathogenesis of multiple-system changes contributing to chemical pathology and clinical course of Parkinson's disease are unknown.

Anatomy, pigmentation, ventral and dorsal subpopulations of the substantia nigra, and differential cell death in Parkinson's disease

In six control subjects pars compacta nerve cells in the ventrolateral substantia nigra had a lower melanin content than nerve cells in the dorsomedial region. This coincides with a natural anatomical division into ventral and dorsal tiers, which represent functionally distinct populations. In six cases of Parkinson's disease (PD) the ventral tier showed very few surviving nerve cells compared with preservation of cells in the dorsal tier. In 13 subjects without PD, but with nigral Lewy bodies and cell loss, the degenerative process started in the ventral tier, and spread to the dorsal tier. This pattern of selective degeneration of nigrostriatal neurons is not seen in ageing or after acute administration of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine).

The iron chelator desferrioxamine (Desferal) retards 6-hydroxydopamine-induced degeneration of nigrostriatal dopamine neurons

A selective increase in content of iron in the pars compacta of the substantia nigra has been implicated in the biochemical pathology of Parkinson's disease. Iron is thought to induce oxidative stress by liberation of oxygen free radicals from H2O2. Because 6-hydroxydopamine (6-OHDA) is thought to induce nigrostriatal dopaminergic neuronal lesions via metal-catalyzed free radical formation, the effect of the iron chelator desferrioxamine was investigated on 6-OHDA-induced dopaminergic neuron degeneration in the rat. Intracerebroventricular injection of 6-OHDA (250 micrograms) caused a 88, 79, and 70% reduction in striatal tissue content of dopamine (DA), 3,4-dihydroxyphenylacetic acid, and homovanillic acid (HVA), respectively, and a 2.5-fold increase in DA release as indicated by the HVA/DA ratio. Prior injection of desferrioxamine (130 ng i.c.v.) resulted in a significant protection (approximately 60%) against the 6-OHDA-induced reduction in striatal DA content and a normalization of DA release. Dopaminergic-related behavioral responses, such as spontaneous movements in a novel environment and rearing, were significantly impaired in the 6-OHDA-treated group. By contrast, the desferrioxamine-pretreated rats exhibited almost normal behavioral responses. The ability of iron chelators to retard dopaminergic neurodegeneration in the substantia nigra may indicate a new therapeutic strategy in the treatment of Parkinson's disease.

Selective increase of iron in substantia nigra zona compacta of parkinsonian brains

Histochemical and biochemical determinations of total iron, iron (II), and iron (III) contents in brain regions from Parkinson's and Alzheimer's diseases have demonstrated a selective increase of total iron content in parkinsonian substantia nigra zona compacta but not in the zona reticulata. The increase of iron content is mainly in iron (III). The ratio of iron (II):iron (III) in zona compacta changes from almost 2:1 to 1:2. This change is thought to be relevant and may contribute to the selective elevation of basal lipid peroxidation in substantia nigra reported previously. Iron may be available in a free state and thus can participate in autooxidation of dopamine with the resultant generation of H2O2 and oxygen free radicals.

Environment, genetics and idiopathic Parkinson's disease

Since Idiopathic Parkinson's disease (IPD) was first described more than 170 years ago, there have been major advances in the understanding of the etiology of the disease as well as in its treatment. This article will review current knowledge concerning the role of the environment, genetic hypotheses and the aging factor in the etiology of IPD and proposes a complex interaction involving all these factors. Hypotheses regarding mitochondrial inhibition and free radical generation in IPD are discussed in relation to the mechanism of action of neurotoxins known to produce parkinsonian syndromes.

Iron and aluminum increase in the substantia nigra of patients with Parkinson's disease: an X-ray microanalysis

The levels of different elements were studied by x-ray microanalysis in the substantia nigra and the central gray substance of patients with Parkinson's disease, progressive supranuclear palsy, and matched controls. In control brains, only iron, potassium, silicum, sodium, sulfur, and zinc were within the limit of detection of the technique. The abundance of each element was different, but their respective concentrations in the two brain regions were similar, except for sulfur levels which were higher on neuromelanin aggregates in the substantia nigra than in nigral regions lacking neuromelanin, and in the central gray substance. In Parkinson's disease, but not in progressive supranuclear palsy, nigral iron levels increased in regions devoid of neuromelanin and decreased on neuromelanin aggregates, but were unchanged in the central gray substance, when compared to control values. Concentrations of the other elements in the central gray substance and substantia nigra were not different from controls in brains from patients with Parkinson's disease and progressive supranuclear palsy. Analysis of Lewy bodies in the parkinsonian substantia nigra revealed high levels of iron and the presence of aluminum. Metal abundance was not affected in progressive supranuclear palsy, in spite of the nigral cell death. This suggests that the increased iron levels and the detection of aluminum observed in Parkinson's disease are not solely the consequence of the neuronal degeneration.

The effect of L-dopa and carbidopa treatment on the survival of rat fetal dopamine grafts assessed by tyrosine hydroxylase immunohistochemistry and [3H]mazindol autoradiography

The effect of treatment with L-3,4-dihydroxyphenylalanine (L-DOPA) and carbidopa for five weeks on the survival of rat fetal dopaminergic ventral mesencephalon cells implanted into the denervated striatum of rats with unilateral 6-hydroxydopamine nigrostriatal lesions was assessed. Rats receiving unilateral nigral 6-hydroxydopamine lesions followed by sham striatal grafts (Groups A and B) showed no recovery of (+)-amphetamine- or apomorphine-induced motor asymmetry. Rats in Group B (receiving treatment with L-DOPA and carbidopa) showed an increase in apomorphine-induced contralateral rotation and stereotypy. Animals receiving unilateral nigral 6-hydroxydopamine lesions followed by fetal striatal dopamine grafts (Groups C and D) showed complete recovery of (+)-amphetamine-induced rotation and a decrease of apomorphine-induced contralateral rotation. Treatment of animals in Groups B and D with L-DOPA (200 mg/kg per 24 h) and carbidopa (25 mg/kg per 24 h) by mouth for five weeks had no effect on the behavioural response to (+)-amphetamine. In the 6-hydroxydopamine-lesioned animals there was loss of tyrosine hydroxylase-immunoreactive cells in substantia nigra and ventral tegmental area of greater than 97% and to 66%, respectively, compared to the intact side. The number and morphology of tyrosine hydroxylase-immunoreactive cells in the intact substantia nigra and ventral tegmental area was not altered by treatment with L-DOPA and carbidopa. In the 6-hydroxydopamine-lesioned striatum of rats receiving a sham graft (Group A) or a sham graft and treatment with L-DOPA and carbidopa (Group B) there were no tyrosine hydroxylase-positive cells or fibres visible.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopaminergic innervation of the cerebral cortex: unexpected differences between rodents and primates

Until recently, views on the organization and role of the mesotelencephalic dopaminergic (DA) systems were mostly based on studies of rodents, and it was assumed that homology existed across mammalian species. However, recent studies of both human and non-human primates indicate that this might not be so. The mesocortical DA system in primates, which is directly involved in the pathophysiology of severe illnesses such as Parkinson's disease and psychoses, shows substantial differences from that of rodents. These differences include much larger, re-organized terminal fields, a different phenotype for the co-localization of neuropeptides and a very early prenatal development.

Selective loss of nigral neurons in Pick's disease: a morphometric study

Loss of neurons from the substantia nigra (SN), which is often encountered in Pick's disease, was quantitatively analyzed in 13 cases of Pick's disease and 19 age-matched controls. On sections from the upper and lower portions of the SN, the pigmented zone (zona compacta) and the non-pigmented zone (zona reticulata) were delineated, and these zones were partitioned into quarters: medial, mid-medial, mid-lateral and lateral. Neuronal loss was fairly severe and more evident in the upper section of the SN (-40%), especially in the mid-medial and lateral quarters. In the lower section (neuronal loss: -28%), the medial quarter was most severely affected. Non-pigmented neurons were preserved. Fibrillary gliosis was denser in the zona reticulata, where neuronal loss was minimal. These findings revealed a selective vulnerability of nigral neurons according to their topography and pigmentation and suggests the primary involvement of some neuronal groups (especially the pigmented neurons) of the SN in Pick's disease.

Dose-dependent destruction of the coeruleus-cortical and nigral-striatal projections by MPTP

In order to determine whether 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces neuronal death or the loss of tyrosine hydroxylase (TH) immunoreactivity, 4 catecholaminergic nuclei in the mouse: substantia nigra compacta (SNc), locus coeruleus (LC), ventral tegmental area (VTA) and the A13 nucleus in the hypothalamus were quantitatively examined. Serial sections were taken through the rostrocaudal extent of each nucleus: alternate sections were incubated with TH antiserum and reacted with an immunoperoxidase technique while the alternate set was Nissl stained. Counts and 3 dimensional reconstructions of TH reactive somata were made for each nucleus for saline-treated controls and mice treated with different doses of MPTP (37.5, 75, 150 and 300 mg/kg). TH-positive neurons were counted along with their counterparts on the Nissl-stained alternative sections to both identify the catecholaminergic neurons and to measure their destruction. Concentrations of striatal dopamine and cortical norepinephrine were measured for all dosages of MPTP in order to determine the relationship between dosage, target tissue neurotransmitter concentration and neuronal destruction. By 20 days after MPTP injection there was a dose-dependent random loss of TH-immunoreactive neurons that was almost identical in all 4 nuclei examined. Analysis of the Nissl versus TH cell counts revealed that MPTP resulted in neuronal destruction in the SNc and the LC rather than just a loss of TH immunoreactivity. There was no difference in sensitivity to MPTP between the SNc and the LC. Decreases in cortical norepinephrine concentrations were about one third of the decreases of LC neuronal counts for all MPTP doses; while decreases in striatal dopamine and SNc cell loss was similar to the LC for the two lower doses of MPTP but for the higher doses, the relationship approached or exceeded a one to one ratio. Hence estimates of neuronal death based upon target tissue transmitter concentrations could not be made using the same relationship for SNc and the LC catecholaminergic neurons and use of the same relationship for higher MPTP dosages results in an underestimate of LC neuronal destruction relative to that in the SNc.

Toxicity of 6-hydroxydopamine and dopamine for dopaminergic neurons in culture

Toxicity of 6-hydroxydopamine (6-OHDA) and dopamine were studied in cultures of dissociated fetal rat mesencephalic cells. To assess survival and function of dopaminergic cells we quantified the number of tyrosine hydroxylase-positive cells and measured dopamine uptake. Non-dopaminergic cells were monitored by counting the number of cells visible with phase-contrast microscopy and measuring GABA uptake. 6-OHDA, in contrast to MPP+, which selectively destroyed dopaminergic neurons, was found to be a non-selective neurotoxin in this culture system. Between 10 and 100 microM, dopaminergic and non-dopaminergic cells were destroyed. At concentrations higher than 100 microM, i.e., concentrations frequently used to lesion catecholaminergic neurons in vivo, 6-OHDA resulted in structural fixation and loss of viability of dopaminergic and non-dopaminergic cells. Dopamine produced the same actions at slightly higher concentrations. One hundred to 300 microM was toxic for all cell types, and concentrations above 300 microM resulted in fixation. The findings suggest that 6-OHDA cannot be considered a selective toxin for catecholaminergic neurons in vitro. The demonstrated toxicity of dopamine tends to support speculations that processes related to dopamine metabolism may play a role in the pathogenesis of Parkinson's disease.

Neurotransmitters and neuromodulators in the basal ganglia

The basal ganglia have become a focus for work on neurotransmitter interactions in the brain. These structures contain a remarkable diversity of neuroactive substances, organized into functional subsystems that have unique developmental histories and vulnerabilities in neurodegenerative diseases. A new view of the basal ganglia is emerging on the basis of this neurochemical heterogeneity, suggesting that dynamic regulation of transmitter expression may be a key to extrapyramidal function.

A two-compartment description and kinetic procedure for measuring regional cerebral [11C]nomifensine uptake using positron emission tomography

S-[11C]Nomifensine (S-[11C]NMF) is a positron-emitting tracer suitable for positron emission tomography, which binds to both dopaminergic and noradrenergic reuptake sites in the striatum and the thalamus. Modelling of the cerebral distribution of this drug has been hampered by the rapid appearance of glucuronide metabolites in the plasma, which do not cross the blood--brain barrier. To date, [11C]NMF uptake has simply been expressed as regional versus nonspecific cerebellar activity ratios. We have calculated a "free" NMF input curve from red cell activity curves, using the fact that the free drug rapidly equilibrates between red cells and plasma, while glucuronides do not enter red cells. With this free [11C]NMF input function, all regional cerebral uptake curves could be fitted to a conventional two-compartment model, defining tracer distribution in terms of [11C]NMF regional volume of distribution. Assuming that the cerebellar volume of distribution of [11C]NMF represents the nonspecific volume of distribution of the tracer in striatum and thalamus, we have calculated an equilibrium partition coefficient for [11C]NMF between freely exchanging specific and nonspecific compartments in these regions, representing its "binding potential" to dopaminergic or noradrenergic uptake sites (or complexes). This partition coefficient was lower in the striatum when the racemate rather than the active S-enantiomer of [11C]NMF was administered. In the striatum of patients suffering from Parkinson's disease and multiple-system atrophy, the specific compartmentation of S-[11C]NMF was significantly decreased compared with that of age-matched volunteers.

MPTP-induced parkinsonism: relative changes in dopamine concentration in subregions of substantia nigra, ventral tegmental area and retrorubral field of symptomatic and asymptomatic vervet monkeys

Dopamine (DA) and homovanillic acid (HVA) concentrations were measured in subregions of substantia nigra, ventral tegmental area and retrorubral field in vervet monkeys 1 to 2 months after treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Identical MPTP treatment regimens produced animals with different degrees of parkinsonism. In asymptomatic monkeys, changes in DA and HVA concentrations in the midbrain DA regions were relatively small and involved central substantia nigra and dorsomedial ventral tegmental area. In contrast, changes in symptomatic monkeys were more severe and widespread, significantly affecting all examined subregions of substantia nigra (greater than 75% DA depletion), both dorsomedial and ventromedial ventral tegmental area and lateral, but not medial, retrorubral field. The data indicate that DA neurons in subregions of substantia nigra, ventral tegmental area and retrorubral field are not equally susceptible to MPTP toxicity. The pattern of MPTP-induced DA and HVA losses in the vervet monkey mesostriatal dopaminergic system may resemble postencephalitic Parkinson's disease more closely than idiopathic Parkinson's disease.

Tyrosine hydroxylase gene expression in human ventral mesencephalon: detection of tyrosine hydroxylase messenger RNA in neurites

The distribution of both tyrosine hydroxylase (TH) antigen and messenger RNA was investigated in the ventral mesencephalon from human brain using a 35S-labeled human TH cDNA probe and a polyclonal TH antibody. A hybridization signal was detected not only in the perikaryon but also in the neurites, most probably dendrites, of immunoreactive neurons. In cell bodies, the level of TH mRNA varies, revealing a regional heterogeneity. In the substantia nigra of patients with Parkinson's disease the number of labeled cells was markedly reduced as compared to controls.

Neuropathology of immunohistochemically identified brainstem neurons in Parkinson's disease

Regional loss of immunohistochemically identified neurons in serial sections through the brainstem of 4 patients with idiopathic Parkinson's disease was compared with equivalent sections from 4 age-matched control subjects. In the Parkinson brains, the catecholamine cell groups of the midbrain, pons, and medulla showed variable neuropathological changes. All dopaminergic nuclei were variably affected, but were most severely affected in the caudal, central substantia nigra. The pontine noradrenergic locus ceruleus showed variable degrees of degeneration. There was also a substantial loss of substance P-containing neurons in the pedunculopontine tegmental nucleus. However, the most severely affected cell group in the pons was the serotonin-synthesizing neurons in the median raphe. In the medulla, substantial neuronal loss was found in several diverse cell groups including the adrenaline-synthesizing and neuropeptide Y-containing neurons in the rostral ventrolateral medulla, the serotonin-synthesizing neurons in the raphe obscurus nucleus, the substance P-containing neurons in the lateral reticular formation, as well as the substance P-containing neurons in the dorsal motor vagal nucleus. Lewy bodies were present in immunohistochemically identified neurons in many of these regions, indicating that they were affected directly by the disease process. These widespread but region- and transmitter-specific changes help account for the diversity of motor, cognitive, and autonomic manifestations of Parkinson's disease.

Neuromelanin synthesis in rat and human substantia nigra

A relation between neuromelanin synthesis and vulnerability of dopaminergic neurons is suggested by the fact that heavily pigmented cells are preferentially lost in aging and Parkinson's disease and that the dopaminergic neurotoxin MPP+ (1-methyl-4-phenyl-pyridine) binds to neuromelanin. To elucidate the mechanism of neuromelanin synthesis, we studied the formation of melanin in homogenates of human and rat substantia nigra tissue "in vitro". It was found that enzymatic processes accounted for 70% and 90% of the melanin formation in homogenates of human and rat tissue, respectively. The enzymatic synthesis was due to the activity of monoamine oxidase (MAO), since it was prevented by selective inhibitors of this enzyme. Both MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and MPP+ inhibited melanin formation, probably due to their ability to inhibit MAO. No evidence was found for involvement of cytochrome P-450 monooxigenases, which have been postulated to exist in central catecholaminergic neurons. Proadifen reduced melanin formation, not necessarily because it is an inhibitor of P-450 monooxigenases, but rather as it is also a potent inhibitor of MAO. Some antioxidants like ascorbic acid, but not agents destroying hydrogen peroxide, inhibited melanin formation. The findings suggest that the formation of neuromelanin in the substantia nigra involves MAO and non-enzymatic oxidative processes.

Loss of brainstem serotonin- and substance P-containing neurons in Parkinson's disease

Using postmortem immunohistochemical analysis, we have identified degeneration of several different neuronal cell groups in the brainstem of patients dying with idiopathic Parkinson's disease. We report the first chemically identified loss of presumed serotonin neurons in the median raphe nucleus of the pons and of substance P-containing preganglionic neurons in the dorsal motor vagal nucleus. This evidence is concordant with other evidence that the primary neuropathological process is not confined either to a single pathway or to neurons containing a particular transmitter. Rather it appears that Parkinson's disease affects several classes of neurons in localized areas of the brainstem.

Neurotensin receptors in Parkinson's disease and progressive supranuclear palsy: an autoradiographic study in basal ganglia

The technique of receptor autoradiography was used to study the distribution of neurotensin receptors in post mortem brain tissues from patients affected by Parkinson's disease, progressive supranuclear palsy and from age-matched controls. [125I]Neurotensin was used as ligand. Significant receptor decreases were found in the substantia nigra, both pars compacta and reticulata, and in the putamen in Parkinson's disease and progressive supranuclear palsy. In addition, significant decreases of neurotensin receptors were found in the ventral tegmental area, nucleus accumbens and dorsal part of caudate head in patients with Parkinson's disease but not in patients with progressive supranuclear palsy, indicating differential involvement of neurotensin receptors in these two neurological disorders. In addition, both in Parkinson's disease and progressive supranuclear palsy the decrement of striatal neurotensin binding sites was less than expected from the reported decrease of dopamine content in this nucleus, suggesting only partial localization of neurotensin receptors on mesostriatal dopaminergic projections.

Microtopography of D1 dopaminergic binding sites in the human substantia nigra: an autoradiographic study

The autoradiographic distribution of D1 dopaminergic binding sites was studied in the human ventral mesencephalon using the D1 antagonist [3H]SCH 23390. [3H]SCH 23390 binding was characterized by a single class of sites with a Kd of 2.5 nM and a Bmax of 31 fmol/mg of tissue. The density of [3H]SCH 23390 binding sites was high in the substantia nigra, moderate in the ventral tegmental area and low in the peri- and retrorubral field (catecholaminergic region A8). Binding densities were similar in pars compacta and pars reticulata of the substantia nigra, except for a peak value of high [3H]SCH 23390 in the pars reticulata, at a level just ventral to a zone of hyperdensity of melanized dopaminergic neurons in the pars compacta. The anatomical organization of the human ventral mesencephalon was analysed on adjacent sections stained for acetylcholinesterase histochemistry and tyrosine hydroxylase, substance P, dynorphin B, somatostatin and methionine-enkephalin immunohistochemistry, respectively. The similarity in distribution of [3H]SCH 23390 binding sites and substance P or dynorphin B immunoreactivity suggests that D1 binding sites are mainly located on the striatonigral projections. In accordance with these results: (1) the density of [3H]SCH 23390 binding sites was reduced in the substantia nigra of a patient with Huntington's chorea, a disease associated with a degeneration of striatonigral neurons; (2) the density of [3H]SCH 23390 binding sites was unaffected in the substantia nigra of a patient with Parkinson's disease, a disorder characterized by a marked loss in nigral tyrosine hydroxylase-positive neurons. [3H]SCH 23390 binding sites showed a characteristic, heterogeneous distribution within the human ventral mesencephalon, confirming data obtained in other species. The preferential localization of D1 dopamine receptors on striatonigral projections in human brain suggests that pharmacological manipulation of these receptors modulates the activity of striatonigral pathways, thereby affecting the various outputs of the nigral complex.