Calbindin (CB) is a calcium binding protein reported to protect dopaminergic neurons from
degeneration. Although a direct link between CB content and differential vulnerability of
dopaminergic neurons has long been accepted, factors other than CB have also been
suggested, particularly those related to the dopamine transporter. Indeed, several studies
have reported that CB levels are not causally related to the differential vulnerability
of dopaminergic neurons against neurotoxins. Here we have used dual stains for
tyrosine hydroxylase (TH) and CB in 3 control and 3 MPTP-treated monkeys to visualize
dopaminergic neurons in the ventral tegmental area (VTA) and in the dorsal and ventral
tiers of the substantia nigra pars compacta (SNcd and SNcv) co-expressing TH and CB.
In control animals, the highest percentages of co-localization were found in VTA (58.2%),
followed by neurons located in the SNcd (34.7%). As expected, SNcv neurons lacked CB
expression. In MPTP-treated animals, the percentage of CB-ir/TH-ir neurons in the VTA
was similar to control monkeys (62.1%), whereas most of the few surviving neurons in
the SNcd were CB-ir/TH-ir (88.6%). Next, we have elucidated the presence of CB within
identified nigrostriatal and nigroextrastriatal midbrain dopaminergic projection neurons. For
this purpose, two control monkeys received one injection of Fluoro-Gold into the caudate
nucleus and one injection of cholera toxin (CTB) into the postcommissural putamen,
whereas two more monkeys were injected with CTB into the internal division of the globus
pallidus (GPi). As expected, all the nigrocaudate- and nigroputamen-projecting neurons
were TH-ir, although surprisingly, all of these nigrostriatal-projecting neurons were negative
for CB. Furthermore, all the nigropallidal-projecting neurons co-expressed both TH and
CB. In summary, although CB-ir dopaminergic neurons seem to be less prone to MPTPinduced
degeneration, our data clearly demonstrated that these neurons are not giving rise
to nigrostriatal projections and indeed CB-ir/TH-ir neurons only originate nigroextrastriatal
projections