S. Goulet et al., ASPIRATION LESIONS OF THE AMYGDALA DISRUPT THE RHINAL CORTICOTHALAMICPROJECTION SYSTEM IN RHESUS-MONKEYS, Experimental Brain Research, 119(2), 1998, pp. 131-140
In macaque monkeys, aspiration but not excitotoxic lesions of the medi
al temporal lobe limbic structures, the amygdala and hippocampus, prod
uce a severe impairment in visual recognition memory. Furthermore, cer
tain ventromedial cortical regions, namely the rhinal (i.e., entorhina
l and perirhinal) cortex, are now known to be critical for visual reco
gnition memory. Because the route taken by temporal cortical efferent
fibers, especially perirhinar efferents, passes nearby the amygdala, i
t is possible that inadvertent damage to these fibers is produced by t
he aspirative but not the excitotoxic process, thereby accounting at l
east in part for the different behavioral outcomes of the two types of
lesion. To test this idea, we assessed the integrity of the rhinal co
rticothalamic projection system after aspiration lesions of the amygda
la. Three rhesus monkeys with unilateral amygdala removals received bi
laterally symmetrical injections of a retrograde fluorescent tracer in
to the medial portion of the mediodorsal nucleus of the thalamus. Retr
ogradely labeled cells were identified using conventional fluorescence
microscopy techniques. In all three cases, the rhinal cortex of the i
ntact hemispheres contained moderate numbers of retrogradely labeled c
ells. By contrast, the rhinal cortex of the amygdalectomized hemispher
es consistently contained few retrogradely labeled cells, and a direct
comparison of the two hemispheres showed this difference to be statis
tically significant. A similar asymmetric pattern was observed for are
a TE but not for the cortex lining the dorsal bank of the superior tem
poral sulcus, nor for the rostral cingulate motor area, which was exam
ined as a control. The results indicate that aspiration lesions of the
amygdala not only remove the cell bodies of the amygdala, as intended
, but also inadvertently disrupt projection fibers arising from cells
in the rhinal cortex and area TE that pass nearby or through the amygd
ala en route to the thalamus. Behavioral studies examining the effects
of aspiration lesions of the amygdala in nonhuman primates need to ta
ke these findings into consideration.