Oleh Hornykiewicz

Biochemical aspects of Parkinson's disease

The importance of the striatal dopamine (DA) deficiency and the DA substituting property of levodopa for the pathophysiology and therapy of Parkinsons disease (PD) is reiterated. In addition, it is shown that in PD, significantly reduced DA levels are also found in the nucleus accumbens, external and internal segments of the globus pallidus, the substantia nigra reticulata, and the subthalamic nucleus. It is proposed that, in addition to the critical role played by the striatal DA loss, the DA changes in the extrastriatal nuclei of the basal ganglia are importantly involved in the pathophysiologic mechanisms resulting in the parkinsonian movement disorder, and that the therapeutic and/or side effects of DA substitution therapy may, in part, be mediated through these brain regions which, like the striatum, suffer DAergic deafferentation in PD. From observations in brain of patients with secondary parkinsonism, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine parkinsonism in the rhesus monkey, as well as the regional DA transporter distribution in the primate substantia nigra, it is concluded that PD may be caused by any exogenous and/or endogenous toxin using the transporter system for DA and to some degree the other brain monoamines (noradrenaline, serotonin), to enter, and damage, the respective monoamine neurons. Based on converging evidence, the view is advanced that endogenous, genetically based(excessive) formation, or accumulation, of toxic DA transporter substrates, such as isoquinoline or β-carboline derivatives, may in fact represent the primary cause of substantia nigra cell degeneration in patients with PD.

Biochemical Evidence of Dysfunction of Brain Neurotransmitters in the Lesch-Nyhan Syndrome

Different brain regions were removed post mortem from three patients with the Lesch-Nyhan syndrome and were examined for alterations in hypoxanthine-guanine phosphoribosyl transferase (HGPRT), adenine phosphoribosyl transferase, and biochemical indexes of norepinephrine, dopamine, serotonin, gamma-aminobutyric acid (GABA), and acetylcholine neuron function, as compared with age-matched controls. The level of HGPRT activity in the material from patients with the Lesch-Nyhan syndrome was less than 1 per cent of control levels, whereas adenyl phosphoribosyl transferase was not significantly altered. All biochemical aspects of the function of dopamine-neuron terminals in the striatum (except dihydroxyphenylacetic acid levels) were decreased to 10 to 30 per cent of the control values. Serotonin and 5-hydroxyindoleacetic acid levels were increased, striatal choline acetyltransferase levels were low, and striatal glutamic acid decarboxylase and guanylate cyclase activities were unaltered. The disruption of the balance between the functions of GABA, dopamine, and acetylcholine neurons in the extrapyramidal system probably accounts for some of the symptoms observed in the Lesch-Nyhan syndrome (e.g., choreoathetosis).

Glutathione peroxidase activity in Parkinson's disease brain

Glutathione peroxidase is an enzyme of major importance in the detoxification of peroxides in brain. Using the spectrophotometric procedure of Paglia and Valentine [8] and Beutler [2] we measured the activity of this enzyme in autopsied brain from 12 patients dying with idiopathic Parkinsons disease and 11 neurologically normal adults matched with respect to age and postmortem interval. In the Parkinsons disease patients glutathione peroxidase activity was slightly but significantly reduced in several brain areas including substantia nigra. Although the magnitude of the glutathione peroxidase deficiency in Parkinsons disease substantia nigra was small (19% reduction), coupled with the reported marked deficiency of reduced glutathione [9] it may represent one of the contributing factors leading to nigral dopamine neurone loss.

Aging Produces a Specific Pattern of Striatal Dopamine Loss: Implications for the Etiology of Idiopathic Parkinson's Disease

Abstract: To examine the possible causal contribution of normal or accelerated aging to the neurodegenerative process of Parkinsons disease, we measured the influence of aging on subregional striatal dopamine and homovanillic acid levels in postmortem brain of 23 neurologically and psychiatrically normal human subjects 14–92 years old. We observed a significant decline in striatal dopamine levels and increase in the homovanillic acid/dopamine molar ratios with increasing age. The dopamine loss, on average, was of the same magnitude in the caudate nucleus and the putamen (‐60% in the 84‐year‐old group as compared with the 22‐year‐old group), with the caudal component of both nuclei being more affected than the rostral subdivisions. The level of subregional dopamine metabolism, as measured by the homovanillic acid/dopamine ratio, in our young individuals (mean age, 22 years) was found to be inversely correlated to the degree of subregional dopamine loss suffered by the individuals in the older age groups. We conclude the following: (a) Striatal subdivisions with physiologically higher dopamine metabolism are not at a greater risk of suffering dopamine neuronal damage with advancing age, as would seem to be implied by the oxidative stress hypothesis; thus, formation of dopamine‐derived oxy radicals in the human striatum appears unlikely to be a primary factor responsible for the age‐related striatal dopamine loss. (b) The regional and subregional pattern of striatal dopamine loss in normal aging differs substantially from the pattern typically observed in idiopathic Parkinsons disease; therefore, the cause of idiopathic Parkinsons disease cannot be primarily an age‐dependent neurodegenerative process.

Effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on the regional distribution of brain monoamines in the rhesus monkey

In an attempt to define neurochemically the part played by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) as a potential Parkinsons disease-inducing neurotoxin, we measured the tissue concentrations of the monoamines dopamine, noradrenaline and serotonin in 45 brain regions in nine rhesus monkeys (Macaca mulatta) receiving repeated intramuscular injections of a total amount of 2.1-7.5 mg/kg MPTP-HCl. Four monkeys treated with MPTP during a period of one to five weeks developed permanent Parkinsonism, and five animals receiving the neurotoxin during a period of two to seven months remained asymptomatic. We found that, compared with the distribution pattern established in the brain of seven normal (drug-free) rhesus monkeys, in the MPTP-treated monkeys none of the three major brain monoamine neuron systems was completely resistant to the neurotoxin. In addition, each brain monoamine had a characteristic regional pattern of MPTP-induced changes. As expected, the most significant alterations were found within the nigrostriatal dopamine system, i.e. profound dopamine loss in caudate nucleus, putamen and substantia nigra. However, many extrastriatal regions of the subcortex and brainstem also suffered significant loss of dopamine, with the noradrenaline loss in the regionally subdivided brainstem being less widespread, and the serotonin levels least affected. Thus, in subcortex/brainstem the ranking order of sensitivity to MPTP was: dopamine greater than noradrenaline much greater than serotonin. In the cerebral (neo- and limbic) cortex, all three monoamine neuron systems suffered widespread statistically significant losses. The ranking order of MPTP sensitivity of the cortical monoamines was: noradrenaline greater than serotonin greater than dopamine. In the cerebellar cortex, dopamine and noradrenaline concentrations were significantly reduced, whereas the serotonin level remained unchanged. A remarkable observation was that many of the subcortical and cortical changes found in the symptomatic monkeys were also found in the asymptomatic animals. Our data are compatible with several possible mechanisms by which MPTP may have produced the observed patterns of monoamine loss in the brain of the rhesus monkey. Our study demonstrates that in the rhesus monkey MPTP mimicked, in addition to the profound striatal dopamine loss, some of the extrastriatal dopamine, noradrenaline and serotonin changes often seen in the brain of patients with idiopathic Parkinsons disease. However, using our treatment regimen, we have not been able to reproduce in the rhesus monkey the inter-regional pattern of striatal dopamine loss typical of idiopathic Parkinsons disease, i.e. a significantly greater loss of dopamine in the putamen compared with the caudate nucleus.

Marked disparity between age-related changes in dopamine and other presynaptic dopaminergic markers in human striatum.

Because age‐related changes in brain dopaminergic innervation are assumed to influence human disorders involving dopamine (DA), we measured the levels of several presynpatic DAergic markers [DA, homovanillic acid, tyrosine hydroxylase (TH), aromatic l‐amino acid decarboxylase (AADC), vesicular monoamine transporter 2 (VMAT2), and dopamine transporter (DAT)] in post‐mortem human striatum (caudate and putamen) from 56 neurologically normal subjects aged 1 day to 103 years. Striatal DA levels exhibited pronounced (2‐ to 3‐fold) post‐natal increases through adolescence and then decreases during aging. Similarly, TH and AADC increased almost 100% during the first 2 post‐natal years; however, the levels of TH and, to a lesser extent, AADC then declined to adult levels by approximately 30 years of age. Although VMAT2 and DAT levels closely paralleled those of TH, resulting in relatively constant TH to transporter ratios during development and aging, a modest but significant decline (13%) in DAT levels was observed in only caudate during aging. This biphasic post‐natal pattern of the presynaptic markers suggests that striatal DAergic innervation/neuropil appears to continue to develop well past birth but appears to become overelaborated and undergo regressive remodeling during adolescence. However, during adulthood, a striking discrepancy was observed between the loss of DA and the relative preservation of proteins involved in its biosynthesis and compartmentation. This suggests that declines in DA‐related function during adulthood and senescence may be explained by losses in DA per se as opposed to DAergic neuropil.

The role of brain edema in epileptic brain damage induced by systemic kainic acid injection.

Edema formation and blood-brain barrier permeability was studied in animals with epileptic seizures induced by subcutaneous injection of kainic acid. Brain edema was most pronounced between 3 and 24 h after kainic acid injection. It was reflected by massive swelling of perineuronal and perivascular astroglia. Three hours after kainic acid perivascular astroglia swelling resulted in disturbance of local microcirculation in the affected brain areas. In addition, compression of drainage veins by the edematous brain induced focal perivenous hemorrhages similar to herniation damage in human brain edema. Tracer studies with sodium fluorescein, Evans blue, albumin and horseradish peroxidase revealed only a mild increase in the permeability of cerebral vessels, topographically unrelated to areas of brain edema. This finding indicates the presence of cytotoxic brain edema in kainic acid-induced epileptic brain damage. Treatment of brain edema with dexamethasone did not influence the incidence and severity of kainic acid-induced epileptic brain damage. However, in 54% of animals injected with kainic acid, lesions were completely prevented by treatment of brain edema with mannitol. The present results indicate that brain edema plays an important role in the pathogenesis of epileptic brain damage following systemic kainic acid intoxication. It is suggested that in this model of limbic epilepsy the brain edema is due to the massive ionic imbalance elicited in the affected brain regions by the kainic acid-induced persistent neuronal excitation.

Clinical-pathological study of levodopa complications.

We sought to determine the continued benefit and the pattern of motor complications of long‐term levodopa treatment in Parkinsons disease. Patients were evaluated between 1968 and 1996. Only those who had an adequate levodopa trial and in whom autopsy revealed Lewy body Parkinsons disease were included. Total levodopa and mean daily dose were calculated in each case. Dyskinesia, wearing‐off and on‐off were collectively classified as motor adverse effects and reported as cumulative incidence. Forty‐two patients (male, 30; female, 12) with mean 15.9 years of illness and 9.1 years follow‐up received on average 500‐mg levodopa daily over 9.8 years. Seventeen of 21 patients assessed during the last 18 months of life reported some motor benefit. Adverse effects were seen in 71.4% of patients. The most common was dyskinesia, in 61.9%; wearing‐off in 35.7%; and on‐off in 16.7% of patients. The earliest adverse effect was dyskinesia and the last to emerge was on‐off. Isolated dyskinesia was seen in 35.7% and wearing‐off in 7.1% of patients; 15.5% of patients developed dyskinesia after 2.6 years and 31% after 6.4 years on levodopa. We concluded that levodopa benefit declined and adverse effects increased with time. Dyskinesia was the earliest and the most common isolated adverse effect.

Proposal for a noradrenaline hypothesis of schizophrenia

In this article, we have reevaluated the role of noradrenergic dysfunction in the pathogenesis of schizophrenia in the light of todays neuroscience and clinical data. Neurophysiological, psychophysiological, psychopharmacological, and biochemical findings that have accumulated in last decades indicate that certain noradrenergic dysfunctions play important roles in the pathogenesis of the disorder. Moreover, these findings provide us with consistent evidence for the existence of two syndromes generated by either overactivity or underactivity of the central noradrenaline (NA) system. The former appears to correspond to the type I syndrome (positive symptoms) and the latter to the type II syndrome (negative symptoms). We conclude that the involvement of brain NA in cerebral metabolism and blood flow as well as the amines role in brain development and neuronal differentiation may provide the mechanisms underlying the disease process in schizophrenia. Development of chemical agents acting specifically on the brain noradrenergic mechanisms may be a promising approach to novel treatments of the disorder.

Brain Neurotransmitters in Dystonia Musculorum Deformans

We examined histologically and biochemically the brains of two patients with generalized childhood-onset dystonia musculorum deformans. We found no important histologic changes in the basal ganglia, cerebral cortex, higher brain-stem nuclei, locus ceruleus, or raphe nuclei. Similarly, the activity of choline acetyltransferase and the levels of gamma-aminobutyric acid and glutamic acid in the cerebral cortex and basal ganglia were within the control range. In contrast, the norepinephrine concentrations were markedly and consistently decreased in the lateral and posterior hypothalamus, mamillary body, subthalamic nucleus, and locus ceruleus. The serotonin level was subnormal in the dorsal raphe nucleus, as was the dopamine level in the nucleus accumbens and, in one of the two cases, in the striatum. Elevated concentrations of norepinephrine were found in the septum, thalamus, colliculi, red nucleus, and dorsal raphe nucleus; of serotonin, in the globus pallidus, subthalamic nucleus, and locus ceruleus; and of 5-hydroxyindoleacetic acid, in the globus pallidus, subthalamic nucleus, and nuclei raphe centralis inferior and obscurus. The level of homovanillic acid showed little consistent change in the regions examined. We conclude that some of these monoamine changes, especially the pronounced apparent disturbance of noradrenergic brain mechanisms, may represent a basic neurochemical abnormality in dystonia musculorum deformans and may thus be relevant to the pathoneurophysiology and treatment of this disorder.