Kevin T. Finnegan

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.

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.

Blockade of behavioral sensitization to cocaine and amphetamine by inhibitors of protein synthesis

Anisomycin and cycloheximide were used to investigate the role of protein synthesis in the mechanism of behavioral sensitization to the stereotypic effects of cocaine and amphetamine in mice. The drugs completely antagonize induction and partially block expression of the sensitization. Because these drugs were found to be neither antidopaminergic nor antiglutamatergic, it seems that they disrupt sensitization at a novel locus. The antagonism of expression is limited to that quantitative fraction of the response derived from the sensitization reaction; the acute response is unaffected by the inhibitors of protein synthesis. The results differ from those obtained with haloperidol which can completely block either the acute or sensitized response to the stimulants. These results suggest that the sensitized response is functionally different from that of the acute response. The blockage of sensitization induction by the protein synthesis inhibitors may be related to other reports that the stimulants induce the transcription of immediate early genes; however, the relationship between the activation of immediate early genes and behavioral sensitization remains to be determined.

The evolution of nigrostriatal neurochemical changes in the MPTP-treated squirrel monkey

The MPTP-treated monkey has become an important model for the study of Parkinsons disease. However, studies on the acute evolution of the neurotoxic effects of MPTP in primates are lacking. In the present study, 17 squirrel monkeys were given a single subcutaneous injection of MPTP (2.5 mg/kg). The behavioral effects and the concentrations of dopamine (DA), dihydroxyphenylacetic acid and homovanillic acid were determined in caudate, putamen and substantia nigra 1, 3, 5 (n = 3/time point) and 10 days (n = 6) after drug administration. Two animals were studied neuropathologically 8 and 9 days after MPTP. Profound parkinsonism was evident in all animals after 1 day and neuropathological examination revealed severe nerve cell destruction in the substantia nigra. Surprisingly, although 50-75% reductions in nigral DA were observed 1 and 3 days after MPTP, caudate DA was not reduced and putaminal DA was increased at these time points. The temporal sequence of these events differs markedly from that which occurs in the MPTP-treated mouse and suggests that, in the monkey, nigral cell bodies may represent an important initial site of MPTP-induced damage. Five and 10 days after MPTP, nigral DA depletions remained greater than 60% of control and striatal DA was reduced 50-85%. At these time points, the putamen was always more affected than the caudate. This interregional pattern of striatal DA deficits is similar to that seen in idiopathic Parkinsons disease.

Orally administered MDMA causes a long-term depletion of serotonin in rat brain

Recent studies suggest that 3,4-methylenedioxymethylamphetamine (MDMA), when administered subcutaneously, is toxic to central serotonergic neurons in rats. Because humans typically self-administer this drug orally, we compared this route to the s.c. route of administration. Orally administered MDMA produced a dose-related depletion of serotonin comparable to that produced by the s.c. route. These findings suggest that MDMA, when given orally, retains it neurotoxic activity and that humans using MDMA may be at risk for developing a persistent depletion of brain serotonin.

The N-methyl-d-aspartate (NMDA) receptor antagonist, dextrorphan, prevents the neurotoxic effects of 3,4-methylenedioxymethamphetamine (MDMA) in rats

Using the systemically active, non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist dextrorphan, we explored the role of the NMDA receptor-calcium channel complex in the toxic mechanism of action of 3,4-methylenedioxymethamphetamine (MDMA). Rats were treated with MDMA, dextrorphan, or the combination of MDMA and increasing doses of dextrorphan, and then killed 10 days later for the assay of serotonin and dopamine in the striatum, hippocampus, and cortex. Dextrorphan totally prevented the serotonin-depleting effects of MDMA in the straitum, with a lessened but still significant blockade noted in the hippocampus and cortex. These findings may provide a clue to the molecular events underlying MDMA-induced neurotoxicity.

Dextromethorphan protects against the neurotoxic effects ofp-chloroamphetamine in rats

Dextromethorphan, an agent that blocks the neuronal-damaging effects of hypoxemia in vitro, was tested for its ability to prevent the neurotoxic effects of p-chloroamphetamine (PCA). Rats were treated with either saline, PCA, dextromethorphan, or the combination of PCA and increasing doses of dextromethorphan. Dextromethorphan provided a dose-related protection against the serotonin (5-HT)-depleting effects of PCA. These observations may offer a clue as to the mechanism responsible for PCA-induced neurotoxicity.

The amine-depleting effects of 5,7-dihydroxytryptamine (5,7-DHT) in C57BL/6 mice do not increase with age.

A recent approach to identifying the factors that predispose neurons to an early death in Parkinsons or Alzheimers disease has been to study how the effect of specific neurotoxins are altered by increasing maturity. We determined the dose-related serotonin and norepinephrine-depleting effects of the selective neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), in C57BL/6 mice of 2 different ages. Norepinephrine and serotonin in the hippocampus were assayed 1 week after the intracerebroventricular (i.c.v.) administration of 5,7-DHT. 5,7-DHT produced an equivalent, dose-related depletion of hippocampal norepinephrine in both age groups. Since the effects of 5,7-DHT on noradrenergic neurons may, at least in part, depend on the monoamine oxidase (MAO)-generated formation of hydrogen peroxide and associated oxy-radicals, this result suggests that noradrenergic neurons do not become more vulnerable to oxidative stress with aging. We also found that the noradrenergic-depleting effects of 5,7-DHT were blocked by the non-selective MAO inhibitor pargyline (50 mg/kg, i.p.), while the selective MAO B inhibitor deprenyl (10 mg/kg, i.p.) failed to prevent this depletion. These latter results suggest that it is the A form of MAO that plays an important role in the mechanism of 5,7-DHT-induced noradrenergic toxicity. Somewhat unexpectedly, older mice were found to be less susceptible to the serotonin-depleting effects of 5,7-DHT. Although the mechanism by which this compound damages serotonergic neurons is uncertain, our results show that the increased susceptibility of serotonergic neurons to 5,7-DHT in young animals extends well beyond the neonatal period.

The dopamine-depleting effect of 6-hydroxydopamine does not increase with aging.

The dopamine-depleting effects of intracerebroventricularly administered 6-hydroxydopamine (6-OHDA) were studied in young mature (6-8 weeks), older (8-12 months) and aged (18-24 months) mice. No differences were noted between age groups. To rule out the possibility that higher levels of monoamine oxidase (which degrades 6-OHDA) in older animals might be masking an increased sensitivity of older neurons to 6-OHDA, experiments were repeated after treatment with pargyline (50 mg/kg). Again, no differences between age groups were noted. We conclude that aging does not increase the sensitivity of dopaminergic neurons to 6-OHDA.

Effects of L-type calcium channel antagonists on the serotonin-depleting actions of MDMA in rats

The calcium channel antagonists verapamil nifedipine and flunarizine all increased the threshold for convulsions induced by N-methyl-D-aspartate in rats. By contrast, only flunarizine blocked the long-term serotonin-depleting effects of 3,4-methylenedioxymethamphetamine. Flunarizine was also the only drug that antagonized methamphetamine-induced stereotypy. These findings suggest that calcium influx through L-type channels does not participate in the neurotoxic mechanism of MDMA, and that the neuroprotective actions of flunarizine are probably related to its anti-dopaminergic activity.