DIFFERENTIAL PATTERNS OF C-FOS MESSENGER-RNA EXPRESSION IN AMYGDALA DURING SUCCESSIVE STAGES OF ODOR DISCRIMINATION-LEARNING

Expression of the activity-dependent gene c-fos was used to assess rel ative levels of neuronal activation in the amygdala and related struct ures of rats at different stages of odor discrimination learning, In s itu hybridization was used to evaluate c-fos mRNA content within the a mygdalar subdivisions, the bed nucleus of the stria terminalis, and th e hippocampus, After initial exploration of the test apparatus, c-fos mRNA levels were increased in the medial and, to lesser extent, basola teral subdivisions and remained low in the central division, The balan ce of amygdala to hippocampal labeling favored hippocampus, Rats engag ed in familiar nose-poke responses had comparably elevated labeling in the medial and basolateral divisions and low labeling densities in th e central division, The ratio of hippocampal to amygdala labeling was at control levels, Rats required to switch from ad libitum responding to cued responding to odors had high basolateral to medial labeling ra tios, This was in marked contrast to the medial dominance found in con trol and exploration rats, Hybridization was substantially more dense in basolateral amygdala than in hippocampal CA1; this imbalance was un ique to the group required to form first associations between odors an d rewards. Rats performing an overtrained odor discrimination had the least differentiation between amygdalar subdivisions of any behavioral group, The hippocampus-to-amygdala labeling ratio favored hippocampus and was nearly identical to the ratio in exploration rats, These resu lts demonstrate that the balance of activity within and between Limbic structures shifts according to behavioral demands, It is suggested th at the balances reflect the availability of pertinent afferent cues, i nteractions between hippocampus and the extended amygdala, and relativ e levels of activity in the diffuse projections to the limbic system.