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.