Neurophysiological and Morphological Plasticity in Rat Hippocampus and Medial Prefrontal Cortex Following Trace Fear Conditioning

The current study utilized trace fear conditioning as a model system to examine how associative learning affects both synaptic and intrinsic plasticity of hippocampal CA1 neurons in the same behavioral characterized rats. Rats received a one-day 10-trial trace fear conditioning, followed by a brief memory test in a novel context the next day. Brain slices were prepared immediately after the test for electrophysiological recordings. Synaptic plasticity was studied by stimulating Schaffer collateral pathway and recorded in stratum radiatum in CA1 area. Intrinsic excitability of CA1 neurons was studied by using sharp electrode intracellular recording. Analysis with the data we found that the behavioral performance (percent freezing during memory test) was significantly correlated with the amount of LTP (r = 0.64, p < 0.05) and the size of post-burst AHP (r = -0.55, p < 0.05). The significant correlation was only observed in conditioned rats but not pseudoconditioned rats, suggesting it is learning specific. In addition, the rats that displayed higher level of freezing during memory test (good learners) had significantly greater LTP in hippocampus than the rats that displayed lower level of freezing (poor learners). Furthermore, neurons from good learners displayed enhanced intrinsic excitability than the neurons from poor learners and other control rats, as evidenced by smaller post-burst AHPs [F(4,52) = 4.77, p < 0.01] and reduced spike-frequency adaptation [F(3,52) = 5.62, p < 0.01]. Finally, the percent LTP was significantly correlated with the size of post-burst AHP (r = -0.38, p < 0.05). Thus these data suggest that within