Effect of circadian phase on context and cued fear conditioning in C57BL/6J mice

We examined context-dependent and tone-cued fear conditioning during the ac tivity and rest periods of C57BL/6J mice. Wheel-running activity was measur ed continuously as a marker of circadian phase. To control the effects of l ight on the response, the animals were kept in a skeleton photoperiod (two 15-min light pulses per day, indicating the beginning and the end of the da y). Half the animals were trained and tested for context-dependent fear con ditioning 2 h after the morning light pulse; the other half were trained an d tested 2 h after the evening light pulse. Animals were tested every 24 h for 5 days to analyze the conditioned response and the rate of extinction. They were then trained for tone-cued fear conditioning at the same time and tested for 5 consecutive days. A significant difference between the mornin g and the evening groups was observed in the conditioning level and extinct ion rate of context-dependent fear conditioning, but not in tone-cued fear conditioning. These results suggest a modulating effect of the biological c lock on the context fear-conditioning pathway.