Chapter 4.4 Changes in acetylcholine extracellular levels during cognitive processesHandbook of Microdialysis - Methods, Applications and Perspectives

The microdialysis studies in which acetylcholine (ACh) extracellular levels were measured during the performance of spontaneous and acquired behaviors involving cognitive processes have been reviewed. If diffusion and inactivation are kept stable, the extracellular ACh levels are an indication of the release from the neurons and depend on neuronal firing. The words “cognitive processes” comprise many brain functions including attention, information acquisition, memory formation which can be investigated in animals implanted with microdialysis probes. Novelty induces arousal and attention and a diffuse increase in ACh release from the cerebral cortex and hippocampus. Conversely, sensory stimulation induces diffuse activation of the cortical cholinergic system and a more intense ACh release from the specific receiving cortical areas. Habituation attenuates the cholinergic activation. The relationship between levels of attention and ACh release was also examined. Spatial memory formation is associated with an activation of the hippocampal cholinergic system. The acquisition of a conditioned stimulus is accompanied by an increase in ACh release occurring not only in the cortex and hippocampus but also in the nucleus accumbens, for some stimuli. Working memory and reference memory involve the cortical and hippocampal cholinergic systems, respectively, and the shifting from hippocampal to striatal cholinergic activation accompanies the changing of rat strategy in mastering a maze for food. From the papers reviewed, microdialysis appears a powerful tool for investigating the role of brain cholinergic system in specific areas and different cognitive processes. Using this tool it the extensive involvement of the brain cholinergic pathways in the ongoing cognitive activity has been confirmed