Contextual fear conditioning is associated with an increase of acetylcholine release in the hippocampus of rat

The effects of contextual fear conditioning on the release of acetylcholine (ACh) in the hippocampus of freely moving rats was assessed using microdia lysis. Measures were carried out during both acquisition and retention test ing (re-exposure to the conditioning chamber) and compared between animals that either received foot-shocks as unconditioned stimulus (conditioned gro up) or no foot-shocks (control group) during acquisition. Results showed th at during acquisition, hippocampal ACh extracellular level was increased wi th respect to baseline but that this increase was of similar magnitude in b oth groups. By contrast, re-exposure to the conditioning chamber the day af ter (retention testing) produced a significantly greater increase in ACh ex tracellular level in the conditioned (that, otherwise, displayed conditione d freezing behavior to contextual cues), than in the control group (which d isplayed virtually no freezing). This enhanced hippocampal ACh release seem s to result from the greater hippocampal processing of contextual stimuli i n conditioned animals with respect to controls. (C) 2000 Elsevier Science B .V. All rights reserved.