Figure 2.

Re neurons were generally recruited in the encoding phase of trace and delay fear conditioning. A, Animals underwent a 2-d behavioral experiment, in which half of the animals in each group were killed after conditioning and the rest after retrieval test. B, c-Fos+ expression in Re neurons (red square). c-Fos signals were visualized with blue/black reaction products within the nucleus of activated neurons. C, The freezing behavior of TRACE, DELAY, UNPAIRED, and NoCOND groups. Freezing levels of the conditioning (n = 12 per group) and test trials (n = 6 per group) are presented in left panel, respectively. The middle panel shows the averaged freezing levels of the UNPAIRED group (gray line with SEM) and the divergent high (solid lines) or low (dashed lines) freezing levels of each animal during retrieval test. The right panel shows the average c-Fos+ count of the UNPAIRED group (gray bar with SEM) and the counts of high freezing (open circles; behaviors shown in solid lines) and low freezing (filled circles; behaviors shown in dashed lines) animals based on their behavior on trial 3 (n = 3 each). D, Group differences in c-Fos+ counts of the Re after conditioning (upper panel) and test (lower panel); *p < 0.05. NS, not significant.