Due to its widespread expression, acetylcholine has been termed the "universal cytotransmitter". The cholinergic system regulates the synthesis, actions and degradation of acetylcholine. In this compilation, the authors begin with a summary of the current literature on this topic. Following this, a review of the results of studies examining the cholinergic hypothesis of central processes, either in stress reaction of humans or in depression induced by chronic social stresses, is provided. These studies include investigations performed by clinicians assessing the pathogenesis of depression and experiments with rodents. In the closing study, an acetylcholine (ACh) biosensor based on acetylcholine oxidase (AChOx) on EDC activated thioglycolic acid self-assembled monolayer (TGA-SAM) using a tiny micro-chip is developed.