We assessed the use of manganese (Mn)
for tracing cortical and subcortical connections
in living monkeys. Eleven Mn injections were made
into cortical and subcortical structures with
known anterograde connectivity, including visual
cortex (V1/V2), posterior parietal cortex (PPC), superior temporal gyrus (STG), dorsal temporal pole (TG), orbitofrontal cortex (area 12o), and ventral striatum
and pallidum. Mn-labeled
projections were demonstrated by comparing 0.75 mm3 T1-weighted
magnetic resonance images (MRI), obtained 24 to 96 hours after the
injection, with a prior baseline scan. For three injections (two V1/V2 and
TG), we co-injected histological tracers (fluororuby
[FY] and lucifer yellow [LY]) to compare the
distribution of these tracers with that of Mn.
Finally, we examined single-unit responses following injections at two PPC
Mn injections labeled expected cortical and subcortical sites. For example, the V1/V2 injections
(n=3) labeled lateral pulvinar, lateral geniculate nucleus (LGN), and several cortical areas
including V4. The TG injection labeled medial pulvinar,
amygdala, the upper bank
of the superior temporal sulcus, STG, and orbitofrontal cortex. At each of these sites there was
a close correspondence between the distribution of Mn
labeling observed with MRI and the distribution of LY- and FY-labeled axon
terminals observed in histological sections.
As early as 24 hours after an injection, single-unit responses at the
injection site were comparable to those observed at the same site just
prior to the injection.
We conclude that Mn tracing is an effective method
for identifying anterograde neuronal connections
in non-human primates. Since this technique can be performed in vivo
and does not interfere with subsequent neuronal activity, it should provide
an unprecedented opportunity to study structure-function relationships in
the same animals.
Support Contributed By: NIH, McDonnell Ctr for
Higher Brain Function