Lee Ang

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.

Striatal dopaminergic and serotonergic markers in human heroin users.

To establish whether chronic opiate exposure might impair brain dopaminergic or serotonergic function in humans, we assessed biochemical indices of monoaminergic neurotransmitter activity and integrity in post mortem striatum of nine chronic heroin users and 14 control subjects. Striatal levels of the vesicular monoamine transporter were normal, suggesting that the density of dopamine nerve terminals is not reduced in heroin users. In nucleus accumbens, levels of tyrosine hydroxylase protein (-25%) and those of the dopamine metabolite homovanillic acid (-33%) were reduced significantly together with a trend for decreased dopamine (-32%) concentration. These changes could reflect either a compensatory downregulation of dopamine biosynthesis in response to prolonged dopaminergic stimulation caused by heroin, or reduced axoplasmic transport of tyrosine hydroxylase. Striatal levels of serotonin were either normal or elevated whereas concentrations of the serotonin metabolite 5-hydroxyindoleacetic acid were decreased by 27–38%. Our data suggest that chronic heroin exposure might produce a modest reduction in dopaminergic and serotonergic activity that could affect motivational state and impulse control, respectively.

Brain proteasomal function in sporadic Parkinson's disease and related disorders

Because genetic defects relating to the ubiquitin–proteasome system were reported in familial parkinsonism, we evaluated proteasomal function in autopsied brains with sporadic Parkinsons disease. We found that proteasome peptidase activities in a fraction specific to the proteasome were preserved in five brain areas (including the striatum) of Parkinsons disease where neuronal loss is not observed. Striatal protein levels of two proteasome subunits were normal in Parkinsons disease but reduced mildly in disease controls (multiple system atrophy). Our brain data suggest that a systemic, global disturbance in the catalytic activity and degradation ability of the proteasome itself is unlikely to explain the cause of Parkinsons disease.

Brain antioxidant systems in human methamphetamine users

Animal data suggest that the widely abused psychostimulant methamphetamine can damage brain dopamine neurones by causing dopamine‐dependent oxidative stress; however, the relevance to human methamphetamine users is unclear. We measured levels of key antioxidant defences [reduced (GSH) and oxidized (GSSG) glutathione, six major GSH system enzymes, copper–zinc superoxide dismutase (CuZnSOD), uric acid] that are often altered after exposure to oxidative stress, in autopsied brain of human methamphetamine users and matched controls. Changes in the total (n = 20) methamphetamine group were limited to the dopamine‐rich caudate (the striatal subdivision with the most severe dopamine loss) in which only activity of CuZnSOD (+ 14%) and GSSG levels (+ 58%) were changed. In the six methamphetamine users with severe (− 72 to − 97%) caudate dopamine loss, caudate CuZnSOD activity (+ 20%) and uric acid levels (+ 63%) were increased with a trend for decreased (− 35%) GSH concentration. Our data suggest that brain levels of many antioxidant systems are preserved in methamphetamine users and that GSH depletion, commonly observed during severe oxidative stress, might occur only with severe dopamine loss. Increased CuZnSOD and uric acid might reflect compensatory responses to oxidative stress. Future studies are necessary to establish whether these changes are associated with oxidative brain damage in human methamphetamine users.

Nigral glutathione deficiency is not specific for idiopathic Parkinson's disease.

The consistent findings of decreased levels of the major antioxidant glutathione in substantia nigra of patients with idiopathic Parkinsons disease (PD) has provided most of the basis for the oxidative stress hypothesis of the etiology of PD. To establish whether a nigral glutathione deficiency is unique to PD, as is generally assumed, or is present in other Parkinsonian conditions associated with nigral damage, we compared levels of reduced glutathione (GSH) in postmortem brain of patients with PD to those with progressive supranuclear palsy (PSP) and multiple system atrophy (MSA). As compared with the controls, nigral GSH levels were decreased in the PD and PSP patient groups (P < 0.05 for PD [−30%], PSP [−21%]), whereas a similar decrease in the MSA patient group did not reach statistical significance (P = 0.078, MSA [−20%]). GSH levels were normal in all examined normal and degenerating extra‐nigral brain areas in PSP and MSA. A trend for decreased levels of uric acid (antioxidant and product of purine catabolism) also was observed in nigra of all patient groups (−19 to −30%). These data suggest that glutathione depletion, possibly consequent to overutilisation in oxidative stress reactions, could play a causal role in nigral degeneration in all nigrostriatal dopamine deficiency disorders, and that antioxidant therapeutic approaches should not be restricted to PD.

Regional Distribution of Cocaine in Postmortem Brain of Chronic Human Cocaine Users

We measured concentrations of cocaine and its major metabolites (benzoylecgonine, ecgonine methylester, norcocaine, and cocaethylene) in 15 autopsied brain regions of 14 human chronic cocaine users. Only slight differences were observed in concentrations of cocaine and its metabolites amongst the examined brain areas. Although it is likely that some postmortem redistribution of the drug must have occurred, our data are consistent with the possibility that behaviorally relevant doses of cocaine are widely distributed throughout the brain of humans who use the drug on a chronic basis. Consideration should therefore be given to the possible pharmacological and toxicological actions of cocaine in both striatal and extra-striatal brain areas in human users of the drug.

The human nucleus accumbens is highly susceptible to G protein down-regulation by methamphetamine and heroin.

Abstract: Although the nucleus accumbens is assumed to be a critical brain “pleasure center,” its function in humans is unknown. As animal data suggest that a unique feature of this small brain area is its high sensitivity to down‐regulation of an inhibitory G protein by drugs of abuse, we compared G protein levels in postmortem nucleus accumbens with those in seven other brain regions of chronic users of cocaine, methamphetamine, and heroin, and of matched controls. Biochemical changes were restricted to the nucleus accumbens in which concentrations of Gαi1 and/or Gαi2 were reduced by 32‐49% in the methamphetamine and heroin users. This selective responsiveness to these abused drugs implies a special role for the human nucleus accumbens in mechanisms of drug reinforcement and suggests that some features of the drug‐dependent state (e.g., tolerance) might be related to inhibition of Gαi‐linked receptor activity.

Brain choline acetyltransferase activity in chronic, human users of cocaine, methamphetamine, and heroin

Cognitive impairment has been reported in some chronic users of psychostimulants, raising the possibility that long-term drug exposure might damage brain neuronal systems, including the cholinergic system, which are responsible for normal cognition. We measured the activity of choline acetyltransferase (ChAT), the marker enzyme for cholinergic neurones, in autopsied brain of chronic users of cocaine, methamphetamine, and, for comparison, heroin. As compared with the controls, mean ChAT levels were normal in all cortical and subcortical brain areas examined. However, the two of 12 methamphetamine users, who had the highest brain/blood drug levels at autopsy, had a severe (up to 94%) depletion of ChAT activity in cerebral cortex, striatum, and thalamus. Based on the subjects examined in the present study, our neurochemical data suggest that brain cholinergic neurone damage is unlikely to be a typical feature of chronic use of cocaine, methamphetamine, or heroin, but that exposure to very high doses of methamphetamine could impair, at least acutely, cognitive function requiring a normal nucleus basalis cholinergic neuronal system. Reduced brain ChAT might be explained in part by a hyperthermia-related mechanism as low ChAT levels have also been observed in brain of some patients with neuroleptic drug-associated hyperthermia. Studies of cognitive and brain cholinergic status in high dose users of MA are warranted.

Abnormal activity of membrane phospholipid synthetic enzymes in the brain of patients with Friedreich's ataxia and spinocerebellar atrophy type-1

Much evidence, derived from biochemical studies of both blood and autopsied brain, has suggested that phospholipid metabolism is abnormal in patients with Friedreichs ataxia (FA), a disorder characterized by severe neuronal loss in the spinal cord and lower brain stem with no, or only modest, damage in other brain regions. To establish the cause of our recent finding of reduced brain levels of phospholipids in FA, we assayed activities of 10 phospholipid‐metabolizing enzymes in the autopsied cerebellar cortex of patients with the disorder and, for comparison, in a group of patients with spinocerebellar ataxia type 1 (SCA‐1), a disease characterized, unlike FA, by marked neuronal loss in the cerebellar cortex. Enzyme activities were also measured in four brain areas which are relatively unaffected morphologically in both FA and SCA‐1. We found that ethanolamine kinase activity was increased in multiple brain regions of patients with FA (increased 31%–137%) and, more modestly, in SCA‐1 (increased 39%–60%), suggesting a nonspecific enhancement of phosphoethanolamine production in both disorders. In contrast, the activity of phosphoethanolamine cytidylyltransferase (PECT), the rate‐limiting enzyme of phosphatidylethanolamine synthesis, was significantly and markedly decreased by 35%–78% in the cerebellar, frontal, and occipital cortices of patients with FA but was normal in SCA‐1. Reduced PECT activity in FA may explain the lower brain levels of phosphatidylethanolamine in the disorder. Moreover, because decreased PECT activity in FA occurs in brain regions having no, or only modest, morphologic damage, this may represent a systemic change consequent to the frataxin gene defect. Our data also suggest that therapeutic intervention in FA designed to increase synthesis of membrane phospholipids may warrant further investigation.