Changes in perforant path: dentate gyrus evoked potentials during classical fear conditioning in the anesthetized rat

Murphy, Philip A. (2005) Changes in perforant path: dentate gyrus evoked potentials during classical fear conditioning in the anesthetized rat. Masters thesis, Memorial University of Newfoundland.

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Abstract

Neural changes accompanying fear conditioning in the hippocampus, specifically the dentate gyrus, have been described in awake rats. Doyere et al. (1995) examined the time course of synaptic modifications in perforant path-dentate gyrus connections during learning. They found an increase in slope (m V /ms) of the field excitatatory synaptic potential (EPSP), a reflection of system drive, for the conditioned group while a decrease was noted for the pseudoconditioned group. As for the reactivity of the granule cells in the dentate gyrus, population spike decreases were found in both groups. The primary goal of this study was to apply the conditioning methods of Doyere et al. (1995) and measure the perforant path-dentate gyrus responses during an anesthetized state using urethane. -- Following a 30 min initial baseline period, animals in the conditioned group received 32 tone-footshock pairings over a 90 min conditioning period, followed by a 60 min rest period, and then a 90 min extinction period during which 32 tones were presented. Mean EPSP slope and population spike responses were plotted over time and compared to the responses recorded in a pseudoconditioned group where animals received 32 deliberate CS-US unpairings during the 90 min conditioning period. -- The EPSP slope responses in both groups did not vary significantly from baseline level over the 300 min period and were not affected by the manner in which tone and footshock presentations were received. As for the population spike response, no group differences were noted during the conditioning period but a significant group by block interaction was found for the 60 min post-conditioning period. In contrast to the slope responses, significant group effects were found in the extinction period and the final 30 min period of recording. Consistent results were noted following analysis of the calculated ratios of the population spike/slope responses. Correlation analyses of the slope and population spike suggested that conditioning led to an increase in cell excitability such that a smaller slope was associated with a larger population spike. This effect occurred after conditioning and disappeared with extinction. -- The overall pattern suggested that conditioning changes perforant path-dentate gyrus connections in the urethane anesthetized rat and that the changes are unlike those that occur during the awake state. While EPSP slope changes were non significant in both pseudoconditioned and conditioned rats, the difference in spike amplitude profile with conditioning relative to pseudoconditioning implies that pairing-related modifications can occur in the anesthetized state.

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