Paul S. Fitzmaurice

Dopamine modulates the plasticity of mechanosensory responses in Caenorhabditis elegans

Dopamine‐modulated behaviors, including information processing and reward, are subject to behavioral plasticity. Disruption of these behaviors is thought to support drug addictions and psychoses. The plasticity of dopamine‐mediated behaviors, for example, habituation and sensitization, are not well understood at the molecular level. We show that in the nematode Caenorhabditis elegans, a D1‐like dopamine receptor gene (dop‐1) modulates the plasticity of mechanosensory behaviors in which dopamine had not been implicated previously. A mutant of dop‐1 displayed faster habituation to nonlocalized mechanical stimulation. This phenotype was rescued by the introduction of a wild‐type copy of the gene. The dop‐1 gene is expressed in mechanosensory neurons, particularly the ALM and PLM neurons. Selective expression of the dop‐1 gene in mechanosensory neurons using the mec‐7 promoter rescues the mechanosensory deficit in dop‐1 mutant animals. The tyrosine hydroxylase‐deficient C. elegans mutant (cat‐2) also displays these specific behavioral deficits. These observations provide genetic evidence that dopamine signaling modulates behavioral plasticity in C. elegans.

Brain serotonin transporter in human methamphetamine users

RationaleResearch on methamphetamine (MA) toxicity primarily focuses on the possibility that some of the behavioural problems in human MA users might be caused by damage to brain dopamine neurones. However, animal data also indicate that MA can damage brain serotonin neurones, and it has been suggested that cognitive problems and aggression in MA users might be explained by serotonergic damage. As information on the brain serotonin system in human MA users is fragmentary, our objective was to determine whether protein levels of serotonin transporter (SERT), a key marker of serotonin neurones, are decreased in brain of chronic MA users.MethodsSERT immunoreactivity was measured using an immunoblotting procedure in autopsied brain of 16 chronic MA users testing positive for the drug in blood and brain and matched controls.ResultsSERT levels were non-significantly decreased (−14% to −33%) in caudate, putamen and thalamus (normal in hippocampus), and, unlike the robust striatal dopamine reduction, there was marked overlap between control and MA user ranges. Concentrations of SERT were significantly decreased (−23% to −39%) in orbitofrontal and occipital cortices (normal in frontopolar and temporal cortices).ConclusionsOur data suggest that MA might modestly damage brain serotonin neurones and/or inhibit SERT protein expression, with cerebral cortex being more affected than sub-cortical regions. The SERT reduction in orbitofrontal cortex complements other data suggesting involvement of this area in MA-related behaviour. Decreased brain SERT could also be related to the clinical finding that treatment with a selective serotonin re-uptake inhibitor might increase relapse to MA.

Levels of 4-Hydroxynonenal and Malondialdehyde Are Increased in Brain of Human Chronic Users of Methamphetamine

Animal studies suggest that the widely used psychostimulant drug methamphetamine (MA) can harm brain dopamine neurones, possibly by causing oxidative damage. However, evidence of oxidative damage in brain of human MA users is lacking. We tested the hypothesis that levels of two “gold standard” products generated from lipid peroxidation, 4-hydroxynonenal (one of the most reactive lipid peroxidation aldehyde products) and malondialdehyde, would be elevated in post mortem brain of 16 dopamine-deficient chronic MA users compared with those in 21 matched control subjects. Derivatized aldehyde concentrations were determined by gas chromatography-mass spectrometry. In the MA group, we found significantly increased levels of 4-hydroxynonenal and malondialdehyde in the dopamine-rich caudate nucleus (by 67 and 75%, respectively) and to a lesser extent in frontal cortex (48 and 36%, respectively) but not in the cerebellar cortex. Approximately half of the MA users had levels of 4-hydroxynonenal falling above the upper limit of the control range in caudate and frontal cortex. A subgroup of MA users with high brain drug levels had higher concentrations of the aldehydes. Our data suggest that MA exposure in human causes, as in experimental animals, above-normal formation of potentially toxic lipid peroxidation products in brain. This provides evidence for involvement of oxygen-based free radicals in the action of MA in both dopamine-rich (caudate) and -poor (cerebral cortex) areas of human brain.

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.

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 Parkinsons 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.

Low striatal serotonin transporter protein in a human polydrug MDMA (ecstasy) user: a case study

Evidence that the widely used methamphetamine analog MDMA (3,4-methylenedioxymethamphetamine, ecstasy) might damage brain serotonin neurones in humans is derived from imaging investigations showing variably decreased binding of radioligands to the serotonin transporter (SERT), a marker of serotonin neurones. However, in humans, it is not known whether low SERT binding reflects actual loss of SERT protein itself. As this question can only be answered in post-mortem brain, we measured protein levels of SERT and that of the rate-limiting serotonin-synthesizing enzyme tryptophan hydroxylase (TPH) in autopsied brain of a high-dose MDMA user. As compared with control values, SERT protein levels were markedly (−48% to −58%) reduced in striatum (caudate, putamen) and occipital cortex and less affected (−25%) in frontal and temporal cortices, whereas TPH protein was severely decreased in caudate and putamen (−68% and −95%, respectively). The magnitude of the striatal SERT protein reduction was greater than the SERT binding decrease typically reported in imaging studies. Although acknowledging limitations of a case study, these findings extend imaging data based on SERT binding and suggest that high-dose MDMA exposure could cause loss of two key protein markers of brain serotonin neurones, a finding compatible with either physical damage to serotonin neurones or downregulation of components therein.

Brain dopamine-stimulated adenylyl cyclase activity in Parkinson's disease, multiple system atrophy, and progressive supranuclear palsy

The dopamine D1 receptor is considered to participate in levodopas antiparkinsonian action and levodopa‐induced dyskinesias. We examined the functional status of the D1 receptor in brain of patients with Parkinsons disease (PD), multiple system atrophy (MSA), and progressive supranuclear palsy (PSP). Dopamine‐stimulated adenylyl cyclase activity was significantly increased in putamen (+43%) and frontal cortex (+52%) in PD, normal in PSP, but decreased by 47% in putamen in MSA. The supersensitive dopamine D1 receptors in both striatum and cerebral cortex in PD might compensate for dopamine deficiency, but could also contribute to long‐term complications of levodopa therapy.

Normal glutathione levels in autopsied brain of chronic users of heroin and of cocaine

BACKGROUND Animal studies suggest that exposure to either of the two widely used drugs of abuse, heroin or cocaine, causes depletion of the antioxidant, reduced glutathione, a hallmark of oxidative stress, in the brain. However, the relevance of the animal findings to the human is uncertain and clinical trials with the antioxidant GSH precursor n-acetylcysteine have produced mixed results in cocaine dependence. METHODS Our major objective was to compare glutathione levels, determined by an HPLC-coulometric procedure, in autopsied brain of chronic heroin (n = 11) and cocaine users (n = 9), who were positive for the drugs in the brain, to those of matched controls (n = 16). Six brain regions were examined, including caudate, hippocampus, thalamus and frontal, temporal and insular cortices. RESULTS In contrast to experimental animal findings, we found no statistically significant difference between mean levels of reduced or oxidized glutathione in the drug user vs. control groups. Moreover, no correlation was found between levels of drugs in the brain and those of glutathione. CONCLUSIONS Acknowledging the many generic limitations of an autopsied human brain study and the preliminary nature of the findings, our data nevertheless suggest that any oxidative stress caused by heroin or cocaine in chronic users of the drugs might not be sufficient to cause substantial loss of stores of glutathione in the human brain, at least during early withdrawal. These findings, requiring replication, might also have some relevance to future clinical trials employing glutathione supplement therapy as an anti-oxidative strategy in chronic users of the two abused drugs.