Louis E. DeLanney

Toxic effects of MDMA on central serotonergic neurons in the primate: importance of route and frequency of drug administration.

This study compared the toxic effects of oral versus subcutaneous and single versus multiple doses of 3,4-methylenedioxymethamphetamine (MDMA) on central serotonergic neurons in non-human primates. Orally administered MDMA was approximately one-half as effective as subcutaneously administered drug. Multiple doses were more effective than single doses, but a single 5 mg/kg dose of MDMA given orally still produced a long-lasting depletion of serotonin in the monkey brain. These results indicate that when MDMA is given to monkeys in a manner similar to that employed by humans, it exerts toxic effects on central serotonergic neurons. This suggests that humans using MDMA may be at risk for incurring central serotonergic neuronal damage.

Rapid ATP Loss Caused by 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine in Mouse Brain

Abstract: The effects of 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP) on ATP levels in different areas of mouse brain were studied after rapid fixation of cerebral tissue in situ by microwave irradiation. ATP levels in the striatum, ventral mesencephalon, and cerebellum of untreated C57BL/6 mice killed by microwave irradiation were 2‐3 times greater than values measured in the brains of animals killed by cervical dislocation. In microwaved mice, administration of MPTP (40 mg/kg s.c.) caused a 10‐20% decrease in ATP concentrations as compared to control animals injected with saline. This decrease was relatively rapid and selective because it occurred in both the striatum and ventral mesencephalon, but not in the cerebellar and frontal cortex, at 30, 60, 120, and 240 min after MPTP exposure. Furthermore, ATP loss in the striatum was prevented by mazindol, a catecholamine uptake blocker, indicating a rather selective effect of MPTP on the ATP content of dopaminergic terminals. Results of this study are consistent with mitochondrial damage in the MPTP model of parkinsonism and provide the first direct experimental evidence in vivo that a decrease in ATP may play a role in MPTP‐induced neurotoxicity.

Aging and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced degeneration of dopaminergic neurons in the substantia nigra.

This study assessed the influence of aging on substantia nigra degeneration induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Extensive neuronal degeneration was found in the substantia nigra of older (8-12 months of age) but not younger (6-8 weeks of age) mice given MPTP. Older mice did not have higher brain concentrations of either MPTP or 1-methyl-4-phenylpyridinium (MPP+), the putative toxic metabolite of MPTP, to account for the greater toxicity. In fact, older mice metabolized MPTP more rapidly than younger mice, probably because of the increase in monoamine oxidase activity that occurs with aging. Striatal synaptosomes from older mice did not accumulate more [3H]MPP+ than synaptosomes from younger mice. Thus, it is concluded that the greater neurodegenerative effect of MPTP in older animals is not due to greater levels or uptake of MPP+, but rather is related to a true increase in sensitivity of older dopaminergic cells to MPTP. For comparative purposes, the toxic effect of another dopaminergic neurotoxin, methamphetamine, was tested. Older animals were not more sensitive than young mature animals to the toxic effect of methamphetamine. This finding indicates that the increased sensitivity of older dopaminergic neurons to MPTP is selective. The link established here between aging and the neurodegenerative effect of MPTP, a toxin which produces parkinsonism in humans, provides a mechanism by which an age-related neurodegenerative disorder such as Parkinsons disease could be caused by an MPTP-like toxin in the environment.

Chapter 36: Astrocytes and Parkinson's disease

Publisher Summary This chapter discusses astrocytic reaction in human Parkinsons disease (PD) and in 1 methyl-4-phenyl-l, 2, 3, 6-tetrahydropyridin (MPTP)-induced Parkinsonism in relation to the chronic and acute stages of the disease process. Although noradrenergic, cholinergic, and other neurotransmitter systems besides dopamine (DA) are involved in PD, abnormalities in these pathways are generally thought to be of less importance than the degeneration of the dopaminergic nerve cells in substantia nigra (SN) and the nigrostriatal pathway. The chapter focuses on the events that occur in astrocytes in the SN and the striatum and on the acute effect of MPP+ on astrocytes in the MPTP model in the squirrel monkey. In contrast to MPTP-induced Parkinsonism, PD is a chronic progressive process. Since active nerve cell degeneration continues to take place throughout the course of the illness, glial scars may not be the only sign of the disease process at the time of postmortem observation. There is some evidence that astrocytes may have a function in DA metabolism in the normal as well as in the parkinsonian neostriatum. Astrocytls in the human striatum, when examined by electron microscopy, often contain neuromelanin, a marker for catecholamine activity.

Rapid ATP Loss Caused by Methamphetamine in the Mouse Striatum: Relationship Between Energy Impairment and Dopaminergic Neurotoxicity

Abstract: To study the relationship between energy impairment and the effects of α‐methamphetamine (METH) on dopaminergic neurons, ATP and dopamine levels were measured in the brain of C57BL/6 mice treated with either a single or four injections of METH (10 mg/kg, i.p.) at 2‐h intervals. Neither striatal ATP nor dopamine concentrations changed after a single injection of METH, but both were significantly decreased 1.5 h after the multiple‐dose regimen. The effects of METH on ATP levels appear to be selective for the striatum, as ATP concentrations were not affected in the cerebellar cortex and hippocampus after either a single or multiple injections of METH. In a second set of experiments, an intraperitoneal injection of 2‐deoxyglucose (2‐DG; 1 g/kg), an inhibitor of glucose uptake and utilization, was given 30 min before the third and fourth injections of METH. 2‐DG significantly potentiated METH‐induced striatal ATP loss at 1.5 h and dopamine depletions at 1.5 h and 1 week. These results indicate that a toxic regimen of METH selectively causes striatal energy impairment and raise the possibility that perturbations of energy metabolism play a role in METH‐induced dopaminergic neurotoxicity.

Protection against DSP-4-induced neurotoxicity by deprenyl is not related to its inhibition of MAO B

Clinical studies suggest that deprenyl may retard the progression of Parkinsons disease, an effect that may be related to its monoamine oxidase (MAO) inhibiting properties. Deprenyl also protects against the neurodegenerative effects of the noradrenergic toxin DSP-4. In this study we investigated the role of MAO B inhibition in this protection. C57BL/6 mice were given DSP-4 (50 mg/kg i.p.) 1 h. 24 h or 4 days after the administration of deprenyl (10 mg/kg i.p.) or the selective MAO B inhibitor MDL 72974 (1.25 mg/kg), and then killed 1 week later for assay of hippocampal norepinephrine. The MAO B inhibiting effects of deprenyl or MDL 72974 were also determined after these same intervals of time. Deprenyl and MDL 72974 produced comparable degrees of enzyme inhibition 1 h (greater than 95%), 24 h (greater than 90%) or 4 days (greater than 70%) after their administration. Given 1 h before, deprenyl totally blocked the norepinephrine-depleting effects of DSP-4, but this protection declined sharply when 24 h or 4 days was allowed to elapse between deprenyl and DSP-4 administration. MDL 72974 failed to protect at any time point. In vitro, we detected no activity using DSP-4 as a substrate for MAO. These findings suggest that the ability of deprenyl to protect against DSP-4-induced neuronal degeneration may not depend on its MAO B inhibiting properties.

Dopamine uptake blockers protect against the dopamine depleting effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in the mouse striatum

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a recently described neurotoxin, produces a marked dopamine (DA) depletion in the mouse striatum. In this study, a series of DA uptake blockers was tested for their ability to prevent this effect of MPTP. The agents tested (amfonelic acid, benztropine, bupropion and mazindol) completely protected against DA depletion in the mouse striatum when given before DA-depleting doses of MPTP were administered, whereas atropine and trihexyphenidyl (which were employed for comparative purposes) did not. DA uptake blocking agents appear to represent a second general class of compounds, monoamine oxidase inhibitors being the first, which protect against the biologic effects of MPTP in the mouse.

Aging of the nigrostriatal system in the squirrel monkey.

Increasing incidence of Parkinsons disease with advancing age suggests that age‐related processes predispose the nigrostriatal dopaminergic system to neurodegeneration. Several hypotheses concerning the effects of aging on nigrostriatal neurons were assessed in this study using a non‐human primate model. First, we examined the possibility that the total number of dopaminergic neurons decline in the substantia nigra as a function of age. Stereological counting based on both tyrosine hydroxylase immunoreactivity (TH‐ir) and neuromelanin (NM) content revealed no difference in cell number between young, middle‐aged and old squirrel monkeys. We then determined whether advancing age changed the relative proportion of neurons characterized by 1) TH‐ir in the absence of NM, 2) the presence of both TH‐ir and NM, or 3) NM without TH‐ir. Indeed, a progressive age‐related depletion of TH only cells was paralleled by an increase in NM only neurons. The possibility that these changes could underlie a functional impairment of the nigrostriatal system was supported by striatal dopamine measurements showing a decrease in older monkeys. Finally, we tested the hypotheses that aging may enhance cell vulnerability to injury and that different dopaminergic subpopulations display varying degrees of susceptibility. When monkeys were exposed to the neurotoxicant 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine, cell loss was markedly more pronounced in older animals, and the ranking of vulnerability was TH only < TH/NM < NM only cells. The data indicate that, even in the absence of an overall neuronal loss, changes in the characteristics of dopaminergic cells reflect functional deficits and increased vulnerability to injury with age. NM content appears to be an important marker of these age‐related effects. J. Comp. Neurol. 471:387–395, 2004.

The relationships between aging, monoamine oxidase, striatal dopamine and the effects of MPTP in C57BL/6 mice: a critical reassessment

Although the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice have been reported to increase with age, they have not been characterized in the full spectrum of ages. Thus, in spite of a considerable body of scientific literature on the subject, previous reports leave unanswered the question of whether or not the increased susceptibility of fully mature mice is part of the aging process or simply a consequence of maturation. In the present study, the age-related effects of MPTP on striatal dopamine were studied in groups of C57BL/6 mice from young maturity to old age. The major increase in the effects of MPTP occurred between 2 and 10 months of age (equivalent to adolescence and young adulthood in humans). A slight additional increase was observed between 10 and 16 months (young adulthood and middle age) and the dopamine-depleting effects of MPTP significantly declined in truly aged animals (24 months). Of note also is the fact that normal concentrations of striatal dopamine did not decline in the later ages. Additional studies indicated that while neuronal sensitivity to the effects of 1-methyl-4-phenylpyridinium (MPP+; the putative toxic metabolite of MPTP) appears to remain constant, age-related changes in the activity of striatal monoamine oxidase type B (MAO B) paralleled the dopamine-depleting effects of MPTP in the 4 age groups. Indeed, MAO B activity increased between 2 and 16 months and declined slightly, but significantly, between 16 and 24 months. This pattern of age-related changes in MAO B, striatal dopamine and the sensitivity of the nigrostriatal system to toxic insult may provide insights into factors which have been implicated in age-related neurodegeneration and idiopathic Parkinsons disease.

Effects of 1-Methyl-4-Phenyl- 1,2,3,6-Tetrahydropyridine and 1 -Methyl-4-Phenylpyridinium Ion on ATP Levels of Mouse Brain Synaptosomes

Mouse brain synaptosomes, essentially devoid of mitochondrial contamination, were used as a model to study the effects of 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP) and its toxic metabolite 1‐methyl‐4‐phenylpyridinium ion (MPP+) on the levels of ATP of neuronal terminals. Similar to known inhibitors of ATP synthesis, both MPTP and MPP+ caused a dramatic depletion of synaptosomal ATP. This depletion was dose dependent and occurred as a relatively early biochemical event in the absence of any apparent damage to synaptosomal membranes. MPP+ was more effective than its parent compound in decreasing ATP; it induced a significant loss at concentrations (10–100 γM) similar to those it reaches in the brain in vivo. MPTP‐induced ATP depletion was completely prevented by the monoamine oxidase B inhibitor deprenyl, which, on the contrary, was ineffective against MPP+. As expected in view of the heterogeneous population of nerve terminals present in our synaptosomal preparations, the catecholamine uptake blocker mazindol did not significantly affect the ATP loss caused by both compounds. Data indicate that (1) administration of MPTP may cause a depletion of ATP within neuronal terminals resulting from the generation of MPP+, and (2) exposure to the levels of MPP+ reached in vivo may cause biochemical changes that are nonselective for dopaminergic terminals.