Hippocampal acetylcholine efflux increases during negative patterning and elemental discrimination in rats

Abstract

The purpose of this paper is to examine whether hippocampal acetylcholine (ACh) efflux increases during negative patterning (NP) discrimination tasks. For these tasks, a rat's response was rewarded when either a single stimulus A (tone) or stimulus B (light) was presented, but was not rewarded when the compound stimulus AB (tone + light) was presented to the NP group of rats. An elemental discrimination (E) task was given to another group (E group). In the E group, the rat's response was rewarded when one of two stimuli (e.g., tone) was presented, but not rewarded when the other stimulus (e.g., light) was presented. After reaching a learning criterion, a guide cannula was implanted into dorsal hippocampus under anesthesia. In test sessions, rats were given the same task as before the guide cannula implantation, and ACh efflux was measured. Hippocampal ACh efflux increased during both NP and E tasks. In addition, the magnitude of increase was higher in the NP group than in the E group. Thus, over all our results demonstrate that task difficulty is a critical factor that relates to the difference in ACh efflux in the hippocampus.