Micro-COPS developed multiple complex new approaches to study MG biology - e.g. an electrophysiological secretion assay based on light-mediated Ca2+ uncaging and cell capacitance measurements, a cell lineage tracing assay to assess the origin and fate of MG cells and the effect of neuronal activity, novel microscopic and imaging methods to study MG biology, and a theoretical framework that links the function of synapses between nerve cells to the number of transmitter receptors. Further, Micro-COPS generated multiple new genetically modified mouse lines that allow to follow gene expression and transcriptional control in MG or to modulate levels of intracellular second messengers. Micro-COPS identified a novel, activity-dependent, neuromodulatory feedback process, where MG sense neuronal activity via neuronal transmitters and in turn suppress neuronal activity. This mechanism protects the brain from excessive activation, may play a major role in maintaining physiological sleep behaviour, and may regulate neuronal responses to sensory inputs. Micro-COPS also identified a novel MG population in a brain region called the striatum, which controls movements and behaviour in response to the release of the neurotransmitter dopamine. This MG subpopulation expresses the dopamine D1 receptor and plays a critical role in dopamine-induced neuronal function and dopamine-dependent behaviours, such as addiction. Furthermore, Micro-COPS discovered that MG release the messenger TNFα and thereby regulate the function and plasticity of synapses between nerve cells. This process involves massive changes in the modification of cellular proteins via phosphorylation and plays a key role in the control of sleep. Finally, we developed a mouse model that expresses Gq-DREADD under the control of an inducible MG-specific promoter. This model allows to selectively activate MG without affecting other brain cell types. Using these mice, Micro-COPS discovered that specific activation of MG profoundly affects the synapses between nerve cells, specifically their stability, and thereby controls learning and memory.