Dose-dependent destruction of the coeruleus-cortical and nigral-striatal projections by MPTP

In order to determine whether 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces neuronal death or the loss of tyrosine hydroxylase (TH) immunoreactivity, 4 catecholaminergic nuclei in the mouse: substantia nigra compacta (SNc), locus coeruleus (LC), ventral tegmental area (VTA) and the A13 nucleus in the hypothalamus were quantitatively examined. Serial sections were taken through the restrocaudal extent of each nucleus: alternate sections were incubated with TH antiserum and reacted with an immunoperoxidase technique while the alternate set was Nissl stained. Counts and 3 dimensional reconstructions of TH reactive somata were made for each nucleus for saline-treated controls and mice treated with different doses of MPTP (37.5, 75, 150 and 300 mg/kg). TH-positive neurons were counted along with their counterparts on the Nissl-stained alternative sections to both identify the catecholaminergic neurons and to measure their destruction. Concentrations of striatal dopamine and cortical norepinephrine were measured for all dosages of MPTP in order to determine the relationship between dosage, target tissue neurotransmitter concentration and neuronal destruction. By 20 days after MPTP injection there was a dose-dependent random loss of TH-immunoreactive neurons that was almost identical in all 4 nuclei examined. Analysis of the Nissl versus TH cell counts revealed that MPTP resulted in neuronal destruction in the SNc and the LC rather than just a loss of TH immunoreactivity. There was no difference in sensitivity to MPTP between the SNc and the LC. Decreases in cortical norepinephrine concentrations were about one third of the decreases of LC neuronal counts for all MPTP doses; while decreases in striatal dopamine and SNc cell loss was similar to the LC for the two lower doses of MPTP but for the higher doses, the relationship approached or exceeded a one to one ratio. Hence estimates of neuronal death based upon target tissue transmitter concentrations could not be made using the same relationship for SNc and the LC catecholaminergic neurons and use of the same relationship for higher MPTP dosages results in an underestimate of LC neuronal destruction relative to that in the SNc.