The neurotoxicity of 5,7-dihydroxytryptamine in the mouse atrium: protection by 1-phenyl-3-(2-thiazolyl)-2-thiourea and by ethanol

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 Parkinson's or Alzheimer's 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.

Studies on the mechanism of action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

Explants of embryonic rat substantia nigra in organotypic culture are sensitive to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) at concentrations approximating the doses given in vivo to monkeys. Fluorescence microscopy and 3H-dopamine uptake measurements reveal that the toxicity is selective for dopamine neurons, whereas other neurons and cells in the culture appear normal by phase contrast microscopy. Reduced MPTP (piperidine analog) is inactive in the tissue culture model, while fully oxidized MPTP (pyridinium analog) destroys dopamine neurons. Pargyline and deprenyl, two monoamine oxidase inhibitors, inhibit the neurotoxic action of MPTP. Pargyline and deprenyl also protect monkeys in vivo. The results implicate monoamine oxidase in the mechanism of action of MPTP. Two possible mechanisms for protection by monoamine oxidase are discussed.