Major goals of research into the neurobiology of learning and memory a
re to identify (1) brain areas/circuitries that subserve different mne
monic functions and (2) chemistries that encode the memory trace. The
discovery that activity modulates neuronal gene expression provided te
chniques attendant to the first goal and candidates for cellular chang
es pertinent to the second. Studies in our laboratories have exploited
activity-regulated changes in c-fos gene expression to map regions en
gaged in two-odor discrimination learning, with particular interest in
neuronal groups in hippocampus and amygdala. The results of these stu
dies demonstrate that the subdivisions of hippocampus and amygdala do
not act in concert across behaviors but are differentially activated d
epending on task demands. In hippocampus, preferential activation of f
ield CA3 was uniquely associated with initial learning of an odor pair
, whereas predominant activation of CA1 occurred with exploration of a
novel field and with overtrained responding to odors. The reappearanc
e of precisely the same balance of subfield activation within disparat
e behavioral contexts was taken to suggest that the hippocampus has ba
sic modes of function that recur in different circumstances and make r
ather generalized contributions to behavior. Within the amygdala, the
basolateral division was most prominently active during task acquisiti
on but not during performance of the well-learned discrimination. Inde
ed, the amygdala appeared to play the dominant role relative to hippoc
ampus in the early stages of associating positive and negative valence
s with discriminative cues. These results demonstrate that the balance
of neuronal activity both within and between limbic structures change
s across sequential stages of odor learning in a fashion that is likel
y to define behavioral output. (C) 1998 Academic Press.