Effects of MPTP on the fine structure of neurons in substantia nigra of dogs

This study evaluates fine structural changes in neurons from pars compacta of substantia nigra in dogs 1 and 4 days after administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The toxin induced a disruption and high amplitude swelling of mitochondria, and dispersion of rough endoplasmic reticulum at 4 days. Mitochondria in dendrites were less damaged than those in the soma. Swelling of myelinated axons in the nigrostriatal pathway was evident at 1 and 4 days after injection. Similar morphologic changes are produced by axotomy and inhibitors of mitochondrial function.

MPTP-induced duodenal ulcers in rat. Prevention by reuptake blockers for serotonin and norepinephrine, but not dopamine

Abnormal activity of dopamine, serotonin, and norepinephrine may contribute to the pathophysiology of duodenal ulcers. We therefore studied the effects of neuropharmacological manipulations on 1-methly-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP)-induced duodenal ulcers. Duodenal ulcers were produced in rats by 12 subcutaneous injections of a neurotoxin, MPTP, over 4 days. At an MPTP dose of 20 mg.kg. injection, duodenal ulcers developed in 91% (43 of 47) of animals with low mortality. When neuropharmacological agents were preadministered before MPTP, the following effects on duodenal ulcers incidence were obtained. MAO-B inhibitors (pargyline [55%], deprenyl [43%]) but not MAO-A inhibitors (clorgyline [91%]) significantly decreased the frequency of duodenal ulcers suggesting that, like MPTP-induced parkinsonism, formation of a toxic metabolite, probably 1-methyl-4-phenyl-pyridinium is involved. Reuptake blockers for serotonin (fluoxetine [18%], indalpine [25%]) also decreased the frequency of duodenal ulcers. Reuptake blockers for norepinephrine (desmethylimipramine [17%], tomoxepine [31%], but not amfonelic acid [82%]) decreased the frequency of duodenal ulcers. Reuptake blockers for dopamine (benztropine [73%], amfonelic acid [82%], GBR-12909 [80%]) did not protect against duodenal ulcers. However, GBR-12909 significantly decreased the severity of those duodenal ulcers that were produced. These data suggest that abnormally low levels of synaptic transmission in serotonergic and possibly noradrenergic neurons play an important role in the pathogenesis of duodenal ulcer while the role of dopamine may be limited to modulation of ulcer severity.

Studies on 4-benzyl-1-methyl-1,2,3,6-tetrahydropyridine, a nonneurotoxic analogue of the parkinsonian inducing agent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Previous reports indicate that 4-benzyl-1-methyl-1,2,3,6-tetrahydropyridine (BMTP), the benzyl analogue of the Parkinsonian inducing neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), is not neurotoxic in the C-57 black mouse even when administered at a dose 10 times greater than the dose of MPTP required to cause an 85% depletion of neostriatal dopamine. Intrastriatal microdialysis in the rat with the corresponding 4-benzyl-1-methylpyridinium ion BMP+ for 60 min, however, causes nerve terminal destruction similar to that observed following a 15-min perfusion with the 1-methyl-4-phenylpyridinium ion MPP+, the monoamine oxidase B (MAO-B) generated metabolite derived from MPTP. With the aid of purified beef liver MAO-B and synthetic standards, we observed the efficient and quantitative conversion of BMTP to the corresponding 2,3-dihydropyridinium intermediate BMDP+, which underwent further, but incomplete, oxidation to BMP+. These MPTP-type properties point to in vivo effects, such as pharmacokinetic parameters and/or alternative metabolic pathways, to account for BMTP's lack of neurotoxicity.

Bilateral recovery of striatal dopamine after unilateral adrenal grafting into the striatum of the 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'CH3-MPTP)-treated mouse

A rodent model of Parkinson's disease, the 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine treated mouse, was used to determine whether striatal dopamine levels recover following grafting adrenal medulla into the striatum. Four types of grafts were performed: 1) adult mouse adrenal medulla, 2) adult adrenal medulla that had been freeze-thawed to kill viable cells, 3) postnatal day 7 adrenal medulla, and 4) sham grafts lacking tissue. At 1 month after grafting, only postnatal day 7 grafts contained surviving cells. However, all three types of tissue grafts promoted a unilateral recovery of host dopaminergic fibers on the side of the graft. In striking contrast to the unilateral recovery of the dopaminergic fibers, striatal dopamine levels were increased bilaterally in all tissue grafted mice. These observations suggest that adrenal tissue grafted into the striatum, whether it remains viable or not, has more widespread biochemical effects on the host dopaminergic system than previously recognized. Moreover, these observations bear on mechanisms that may underlie the general recovery of motor disturbances reported in human Parkinson's disease patients who have received a striatal graft of adrenal tissue.

Limited recovery of striatal dopaminergic fibers by adrenal medullary grafts in MPTP-treated aging mice

Systemic injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) damages the dopaminergic (DA) nigrostriatal system in C57BL/6 mice. We have investigated the effect of MPTP neurotoxicity and subsequent adrenal medullary grafts into the striatum of young (2-3 months) and aging (12 months) mice. MPTP treatment (4 X 20 mg/kg ip given 3 or 12 h apart in young mice and 12 h apart in aging mice) resulted in 80-90% depletion of striatal DA and virtual disappearance of tyrosine hydroxylase (TH)-immunoreactive (IR) fibers in both young and aging mice 1 week following treatment. Only partial recovery of TH-IR fibers was seen 5 weeks after MPTP treatment in young mice, while virtually no recovery was seen in aging mice. Adrenal medullary minced pieces were grafted into the striatum of young and aging mice 1 week after MPTP treatment. In young mice, dense TH-IR fibers were observed in the striatum on the grafted side 4 weeks later, far denser than those in sham-operated striatum. Although this staining was most prominent around the grafts, many TH-IR fibers also were found in the ventral striatum close to the nucleus accumbens. No such increase in TH-IR fibers was found on the nongrafted side. DA concentration on the grafted side recovered to 45% of the control level. In aging mice receiving similar grafts, TH-IR fibers also were observed in the grafted striatum, but were less dense and more restricted around the site of the graft compared with young mice. DA concentration on the grafted side was 29% of the control level. We conclude that the MPTP-depleted nigrostriatal DA system in aging mouse brain can recover partially following adrenal medullary grafts, but the degree of recovery is more limited compared with that in young brain.

Attenuation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurotoxicity by tobacco smoke

Several epidemiological studies have indicated that there may be an inverse relationship between smoking and Parkinson's disease. The purpose of this study was to determine whether chronic exposure to cigarette smoke alters the parkinsonian-like neurochemical changes caused by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice. Following 4 weeks of brief, intermittent exposure to smoke, mice were treated with MPTP, 10 mg/kg. Smoke exposure was found to reduce the decrease in striatal dopamine and metabolite levels caused by MPTP. Although smoke exposure inhibited cerebral MAO-B activity, tissues from smoke-treated mice were able to metabolize MPTP in a normal fashion. This suggests that inhibition of cerebral MAO may not be a major mechanism for the apparent protective effect of cigarette smoke.

Ultrastructural changes of the substantia nigra, ventral tegmental area and striatum in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice

The substantia nigra pars compacta (SNc), ventral tegmental area (VTA) and striatum in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated young adult mice were examined for the occurrence of marked ultrastructural changes. In the median SNc dopaminergic neurons, the Golgi apparatus and rough endoplasmic reticulum were dilated and there was a decrease in the number of ribosomes in the rough endoplasmic reticulum (Nissl substance). The mitochondrial cristae were often disarranged. Characteristic electron dense deposits were often observed in the neuronal fibers, many of which were dilated. However, no clear evidence of nuclear degeneration was seen. On the other hand, ultrastructural abnormalities were not seen in the dopaminergic neurons in the VTA. In both the latero-dorsal and latero-ventral striatum, many dilated neuronal fibers, which contained only a few synaptic vesicles and sparse subcellular structures were observed, but no significant ultrastructural changes could be seen in the medial part of the striatum in MPTP-treated mice. Thus, the effect of MPTP treatment is inhomogenous within the mouse striatum, as would be expected from the appearance of ultrastructural changes in only some of the dopaminergic neurons in the SNc.

Astroglial ablation prevents MPTP-induced nigrostriatal neuronal death

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a potent neurotoxin which destroys nigrostriatal dopamine neurons, resulting in irreversible idiopathic parkinsonism. MPTP displays dopaminergic neurotoxicity to humans, monkeys, cats and rodents. The oxidative conversion of MPTP to 1-methyl-4-phenylpyridine (MPP+) is responsible for the generation of its neurotoxicity. This metabolism is mediated by the action of monoamine oxidase B, which in the substantia nigra pars compacta (SNc) is localized specifically in astroglia. Employing various combinations of intra-SNc injections of MPTP and the astroglia-specific toxin, L-alpha-aminoadipic acid (L-alpha-AA), we examined the effects of selective astroglial ablation on MPTP-induced nigrostriatal neuronal death in the rat. Varying nigrostriatal cell loss was assessed primarily by the aid of fluorescent retrograde axonal tracing. Treatment with MPTP alone caused tremendous nigrostriatal cell loss, while intra-SNc co-injections of MPTP and L-alpha-AA produced protection against MPTP neurotoxicity in a dose-dependent fashion. Similar effects of L-alpha-AA occurred in the SNc pretreated with the gliotoxin just prior to or 1 day before MPTP administration. However, this preventive action by L-alpha-AA was considerably reduced 3 days after its intra-SNc injection. Interestingly, 7 days following L-alpha-AA pretreatment, nigrostriatal cell loss was even enhanced rather than attenuated by MPTP administered into the SNc. Thus, our data provide clear morphological evidence for the critical importance of the presence of astroglia in the onset of MPTP neurotoxicity.

[Study of an MPTP-induced parkinsonian animal model in the rhesus monkey and the mechanism of the action of MPTP]

In this paper are presented the data of the use of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to create a parkinsonian animal model in rhesus monkey. We studied the mechanism of the action of MPTP through testing monoamine oxidase B (MAO-B) inhibitor, deprenyl and L-prolyl-L-leucyl-glycinamide (PLG) against the neurotoxicity of MPTP. The results indicated that: (1) the use of MPTP can establish a useful parkinsonian animal model in rhesus monkey; (2) pretreatment with deprenyl can effectively prevent the neurotoxicity of MPTP; and (3) whether PLG can prevent or alleviate the neurotoxicity of MPTP requires further study.

MPTP toxicity in the mouse brain and vitamin E

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) caused transient alterations in vitamin E levels in every brain region examined. However, vitamin E returned to normal levels within a few hours in all brain regions but the substantia nigra, where at 2 days vitamin E levels first rose above normal levels. Vitamin E deficient mice were much more susceptible to MPTP toxicity than controls, in terms of lethality and DOPAC depletion in the substantia nigra. However, in the same vitamin E deficient mice, the striatum was partially protected from neurotransmitter and metabolite depletion by MPTP. The mechanism of toxicity of MPTP may differ in the striatum and the midbrain.

Behavioural effects and supersensitivity in the rat following intranigral MPTP and MPP+ administration

Unilateral intranigral injections of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and MPP+ (1-methyl-4-phenylpyridine) were given to young rats and unilateral intranigral injections of MPTP were given to old rats. MPTP in old rats and MPP+ in young rats induced ipsiversive circling for at least one week after injection and contraversive circling after the systemic administration of apomorphine; the number of D-2 receptors (Bmax) in the striatum of the injected hemisphere increased compared with that of control rats. MPTP in young rats induced only short-lasting ipsiversive circling and no contraversive circling after apomorphine; the number of striatal D-2 receptors did not increase. These results suggest that the neurotoxicity of MPTP is age-dependent in the rat, and that MPTP has neurotoxic effects on the nigrostriatal dopaminergic system in old rats and induces dopamine receptor supersensitivity in the denervated striatum.

Induction of chorea and dystonia in parkinsonian primates

Administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in primates induced a parkinsonian syndrome that could be reversed by levodopa treatment. Animals quickly developed an apparent restlessness ("akathisia") of the lower limbs after as little as five doses. After 4-10 weeks of regular levodopa therapy, animals developed "peak dose" choreiform movements in the lower limbs that spread, with time, to involve the upper limbs and orofacial musculature. With further treatment (5-21 months), animals developed "peak dose" dystonia that variably involved the limbs and orofacial musculature. These conditions represent novel models of levodopa-induced chorea and dystonia in humans. They depend on the same underlying neuropathology and treatment regimen as their human counterparts. It is to be anticipated that these models of dyskinesia will be useful in determining the mechanisms underlying chorea and dystonia in humans and are ideally suited for experimental evaluation of new treatment strategies.

Modifications of precentral cortex discharge and EMG activity in monkeys with MPTP-induced lesions of DA nigral neurons

1. Individual neurons were recorded extracellularly in the precentral forelimb area of two monkeys trained to perform rapid, large amplitude flexion and extension movements of the contralateral forearm in response to auditory signals. Electromyographic (EMG) activity in the biceps/triceps muscles was recorded separately under the same conditions. The dopaminergic (DA) neurons of the substantia nigra (SN) were destroyed selectively by repeated series of intravenous injections of MPTP. The lesion was verified on serial slices using both tyrosine hydroxylase immunocytochemistry and classical staining methods. 2. In normal monkeys, the frequency of firing of precentral neurons shows rapid changes shortly before the onset of displacement. In our sample (n = 102), most of the neurons (49%) tested during movement in both directions (flexion, extension) showed a reciprocal pattern of activity for the two directions of movement, a small percentage (19%) exhibited a change for only one direction (unidirectional neurons), and the remaining 32% displayed a similar change for both directions of movement (bidirectional neurons). 3. In MPTP-treated monkeys, movement-related modification of neuronal activity was more gradual, beginning earlier and lasting longer relative to the onset of movement. The cellular reaction time (the time between the auditory cue and a significant change in neuronal activity) was not significantly altered. Spontaneous firing of precentral neurons (n = 124) did not increase significantly, and the dynamic discharge rate was unchanged after the nigral lesion. However, only 18% of cortical neurons still presented a reciprocal pattern of discharge for the two directions of movement, while the percentage of unidirectional neurons increased (50%), and the percentage of bidirectional neurons remained the same (32%). 4. After MPTP treatment, alterations in movement parameters and EMG activity were observed. Mean reaction time and movement duration increased by 20-25% and 25-30% respectively. The movements were slower and were associated with a generalized depression in the shape and the amplitude of EMG activity in the agonist muscle. 5. The neuronal basis for the observed central and peripheral disturbance in the MPTP-treated monkeys is discussed. We conclude that SN lesion leads to two main disturbances of cortical activity: i) the loss of the reciprocal pattern of response of movement-related cortical cells, and ii) an inability of the motor cortex to modify its activity in response to peripheral input.

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurotoxicity in the rat: characterization and age-dependent effects

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a potent dopaminergic toxin that has been found to produce Parkinson's disease-like symptoms in humans and monkeys. The neurotoxic effects of MPTP appear to be reduced in rodents where multiple dosing procedures are required to demonstrate long-lasting neuronal deficits. In the present study, the neurotoxic effects of MPTP were further characterized in the rat. Following the repeated administration of MPTP, pronounced (60-80%) and dose-dependent depletions of striatal dopamine and serotonin concentrations were found in the rat brain. Time-course studies revealed that while striatal dopamine concentrations remained consistently reduced for at least 8 weeks following MPTP treatment, striatal serotonin depletions as well as MPTP-induced monoamine depletions in other brain regions were transient in nature. Pretreatment with the MAO-B inhibitor pargyline afforded a selective and complete protection of striatal dopamine levels without significantly affecting MPTP-induced striatal serotonin depletions. Similarly, treatment with ascorbic acid was found to selectively attenuate MPTP-induced dopamine depletions in rats. The neurotoxic effects of MPTP were also found to increase in the developing rat. No significant brain monoamine depletions were observed in neonatal rats following the repeated administration of MPTP. However, MPTP-induced neurotoxicity progressively increased in older rats. The present results indicate that when appropriate treatment procedures are used, a pronounced, selective, age-dependent, and long-lasting MPTP-induced reduction in striatal dopamine concentrations can be observed in the rat brain. The present results are discussed in reference to the putative mechanisms and species differences of MPTP-induced neurotoxicity.

Neural mechanisms of dystonia: evidence from a 2-deoxyglucose uptake study in a primate model of dopamine agonist-induced dystonia

The neural mechanisms that mediate dystonia were investigated in a novel experimental primate model of dopamine agonist-induced dystonia. This condition was produced by long-term (15 months) dopamine agonist therapy of a macaque monkey that had been rendered hemiparkinsonian by unilateral infusion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine into the right common carotid artery. The 2-deoxyglucose (2-DG) metabolic mapping technique was applied to the animal during the expression of active unilateral dystonia, and regional brain uptake of 2-DG was assessed autoradiographically. The results demonstrate that dystonia is associated with marked increases in 2-DG uptake in the constituent nuclei of the basal ganglia (caudate nucleus, putamen, medial and lateral segments of the globus pallidus) and in the subthalamic nucleus, but decreased uptake in the structures that receive output of the basal ganglia (ventral anterior/ventral lateral thalamic complex and lateral habenula). Based on these findings it is suggested that dystonia is characterized by increased activity in the putaminopallidal and pallidosubthalamic pathways, and decreased activity in the subthalamopallidal and pallidothalamic pathways.

Improvements in MPTP-induced object retrieval deficits and behavioral deficits after fetal nigral grafting in monkeys

Improvements in MPTP-induced deficits were only found in subjects that received fetal substantia nigra transplants into the caudate nucleus. The MPTP-induced deficits were assessed using an object retrieval task that examined cognitive and subtle motor performance and by behavioral observation to determine the overall status of the subjects. Subjects that were also moderately or severely impaired by MPTP administration but that received inappropriate donor cells or implant sites (cerebellum to CN or SN to cortex) did not show any evidence of behavioral recovery. These subjects could not respond on the task in the months after grafting and were sacrificed, showing no improvements in parkinsonian signs or healthy behavior signs, up to 5-6 months after surgery. Grafting of SN cells into the striatum of non-MPTP lesioned subjects failed to modify normal behavior or induce abnormal behavior determined by our 2 behavioral assessment methods. In those monkeys that received the appropriate transplants, TH immunohistochemistry revealed that cells of the fetal substantia nigra grafted into the caudate nucleus survived and extended neurites into the host striatum. Indeed, grafted dopamine neurons were often associated with appreciable innervation of the caudate nucleus and appeared to be well incorporated into the host brain. In contrast, examination of the striatum of subjects in the inappropriate-graft group (e.g., cerebellar cells grafted into the caudate) showed no evidence of TH staining within the graft or host caudate nucleus. This indicated that there was no evidence of dopamine neurons present in the grafted tissue and that the mere presence of a fetal graft did not appear to induce sprouting in these MPTP-treated subjects. Although behavioral recovery occurred in only those monkeys that received appropriate transplants (fetal SN to host CN) and not in those that received inappropriate grafts (fetal cerebellum to CN or fetal SN to cortex), the CSF HVA levels did not distinguish those monkeys with improved parkinsonism from those that remained severely parkinsonian. The finding that in some SN-CN grafted subjects reported here, there was evidence of increased dopamine and lowered HVA/dopamine ratio in the vicinity of the SN grafts (cf. Elsworth et al., 1990b) is consistent with the hypothesis that graft-derived or graft-induced dopamine production is responsible for behavioral recovery. In addition, the finding that CSF HVA levels in non-MPTP lesioned subjects were unchanged by fetal SN grafts further indicates that CSF HVA levels may not be sufficiently sensitive to changes in central dopamine production to reflect release of dopamine from relatively small grafts that may, in lesioned subjects, modify behavior.(ABSTRACT TRUNCATED AT 400 WORDS)

MPTP is proconvulsant acutely but has no long-term effect in rodent models of seizure and epilepsy

1-Methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) was used to lesion the substantia nigra of rodents to look for changes in various animal models of epilepsy and seizures. MPTP, acutely administered to C57BL/6J mice, could cause seizures at high doses and enhanced maximal electroshock seizures at lower doses. Older mice were more sensitive to MPTP toxicity. MPTP given over 1 week to produce a 75% drop of striatal dopamine had no effect on seizure thresholds to pentylenetetrazol or picrotoxin and did not change the maximal electroshock seizure. Epileptic gerbils given maximally tolerated doses of MPTP had only a slight striatal dopamine reduction (32%) while seizures remained unaltered. The data are consistent with the hypothesis that chronic dysfunction of dopamine containing substantia nigra neurons have no significant influence on seizures in these animal models.

Transplacental effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on brain dopaminergic neurons in the mouse. An immunohistochemical study

Immunohistochemical studies of monoamine neurons were performed to evaluate toxic effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on young adult mice and compare them with those of their offspring. Mice, 9-11 weeks old (C57BL/6J), injected subcutaneously with a large dose of MPTP (17 mg/kg per day) during pregnancy on Day 9 and 12 of gestation (G9 and G12) miscarried and were examined at 13 weeks of age. Conversely, mice treated during pregnancy with sequential low dose of MPTP (2.8 mg/kg per day at G9-G17 for 8 days) successfully delivered their babies and were examined at the age of 15 weeks. Baby mice were examined at 1 and 6 weeks of age. The tyrosine hydroxylase-, aromatic L-amino acid decarboxylase- and dopamine (DA)-immunoreactive density of caudoputamen was reduced in 13-week-old mice treated with high dose of MPTP but not in the 15-week-old mothers exposed to a low dose of MPTP as compared to their respective controls. The DA-immunoreactive density of the caudoputamen was the only staining that was reduced in both 1- and 6-week-old baby mice. In conclusion, these results demonstrate that MPTP injected to pregnant mice causes a DA depletion in the striatum of their offspring indicating a transplacental effect of MPTP. The findings also indicate that fetal brain is more susceptible to MPTP toxicity than the brain of young pregnant mice.

Transplantation of human sympathetic neurons and adrenal chromaffin cells into parkinsonian monkeys: no reversal of clinical symptoms

Cultured human fetal sympathetic ganglion explants or adrenal chromaffin cell aggregates were implanted into the left striatum of monkeys whose left nigrostriatal pathway had been lesioned with the neurotoxin MPTP. There was no clinical reversal of parkinsonian symptoms and PET scans did not show increased striatal fluorodopa uptake from pre-implant levels. At sacrifice, left striatal contents of dopamine were not statistically different from MPTP-treated but non-implanted controls. Histological examinations revealed pockets of extrinsic cells which were found at the end of needle tracks. There was no evidence of immune rejection. The extrinsic cells did not stain for tyrosine hydroxylase or neurofilament, suggesting that they were not dopaminergic neurons. The failure to reverse clinical parkinsonian symptoms highlights the stage of infancy of neural implantation in Parkinson's disease.

MDL 72,974: a potent and selective enzyme-activated irreversible inhibitor of monoamine oxidase type B with potential for use in Parkinson's disease

MDL 72,974, (E)-2-(4-fluorophenethyl)-3-fluoroallylamine, was designed to be a selective inhibitor of monoamine oxidase type B (MAO-B). In vitro, the compound inhibits rat brain mitochondrial MAO in a concentration and time-dependent fashion and shows marked selectivity for the B form (IC50 = 680 and 3.6 nM for MAO-A and MAO-B, respectively). After oral administration to rats, the compound shows preferential inhibition of brain MAO-B with ED50 values of 8 and 0.18 mg/kg p.o. for the A and B forms, respectively. Selectivity is retained on repeat dosing. MDL 72,974 did not significantly potentiate the cardiovascular effects of intraduodenually-administered tyramine in anaesthetized rats and had only minor indirect sympathomimatic effects in the pithed rat. At MAO-B selective doses the neurotoxic effect of MPTP in mice was blocked.

Evidence for an epileptogenic action of 1-methyl-4-phenyl-1,2,3,6,-tetrahydropyridine

Based on behavioral observation alone, poorly characterized paroxysmal motor abnormalities have been reported in mice after acute MPTP administration. This study investigated electroencephalographic (EEG) and behavioral effects of acute MPTP in young and older mice. Single MPTP injections (30 mg/kg i.p.) produced limbic and/or generalized seizures in older (6 mo) and frank epileptiform interictal hippocampal spikes in younger (6-8 wks) mice. These latter showed behavioral seizures only after the 3rd drug administration. Studies of the acute effects of MPTP must take in consideration that seizures "per se" modify brain biochemical and metabolic activity and alter the permeability of the blood brain barrier, adding several confounding variables.