ABSTRACT
We examined if the strength of contextual fear learning determines whether remote memories become independent of the hippocampus. Rats received 3 or 10 shocks in a single contextual fear conditioning session and then received sham or complete neurotoxic lesions of the hippocampus 7, 50, or 100 days later. Following recovery from surgery, the rats were returned to the conditioning context for a 5-min retention test. During this test, freezing, complete immobility except for breathing, was used as an index of memory. Regardless of the learning-to-surgery interval, the rats with hippocampal damage from the 3-shock condition showed little and significantly less freezing than their respective control group, suggesting profound flat graded retrograde amnesia. Similarly, each group of hippocampal-damaged rats from the 10-shock condition froze significantly less than their respective control group. However, the rats that received hippocampal damage 50 days after learning froze significantly more than the rats that received the damage 7 days after learning. The latter gradient to the retrograde amnesia did not increase with more time as the freezing was not as high in the most remote memory group (100 days). Combined, these findings suggest that a contextual fear memory acquired in a single session under stronger learning parameters remains dependent on the hippocampus.
