Effect of intrastriatal and intranigral administration of synthetic neuromelanin on the dopaminergic neurotoxicity of MPTP in rodents

Previous studies showed that the neurotoxin MPTP and its toxic metabolites bind with high affinity to neuromelanin (NM). Therefore, the presence of NM in human and primate but not in rodent substantia nigra, theoretically may be responsible for the species-selective dopaminergic (DA) toxicity of MPTP. We measured DA levels in rodent striatum 7 days after an acute single challenge with MPTP (40 mg/kg, s.c.) given alone or 24 h following unilateral intrastriatal injections of synthetic DA-NM in mice and intrastriatal or intranigral pigment administration in rats. Ipsilateral striatal DA levels were unaffected in control rodents treated with unilateral intrastriatal or intranigral DA-NM. In mice, systemic MPTP produced marked striatal DA depletions which were mildly increased in the striata given prior DA-NM injections. In rats, a species resistant to MPTP, administration of toxin did not affect striatal DA levels. However, after pretreatment with unilateral intrastriatal DA-NM, MPTP induced mild DA falls in ipsilateral striata. By contrast, intranigral administration of DA-NM followed by MPTP, did not alter ipsilateral striatal DA in rats. The findings suggest that intrastriatal DA-NM in mice and rats may augment or initiate, respectively, MPTP-induced damage to sensitive DA-nerve-terminals perhaps by its action as a depot for binding and protracted release and action of the toxin. Lack of effect of intranigral DA-NM which is retained extraneuronally suggests that role of NM in the toxicity of MPTP may depend on its location within DA cell bodies in the nigra.