Almost all G-protein coupled receptors (GPCRs) undergo rapid phosphorylation upon activation by agonists. These phosphorylations events occur at multiple sites on the receptor (mostly on serine and threonine) and can result in receptor G-protein uncoupling, receptor internalization and activation of G-protein independent pathways. The G-protein coupled receptor kinases (GRKs) have classically been involved in this process, but other families of kinases have recently been implicated in this phenomenon. In the case of the M3-muscarinic receptor, the kinases known to be involved in its agonist-dependent phosphorylation include GRK2 and GRK6 but also CK1 and CK2. In order to investigate the role of phosphorylation in the M3-muscarinic receptor signalling, a knock-in Mouse model was designed, in which the M3 receptor was mutated on 15 serines within its third intracellular loop. These M3 receptor phosphorylation-deficient animals presented a deficiency in fear conditioning memory. When the Mice were confronted to a stressful event announced by a warning tone and then subjected again to the same warning tone 24h later, the M3 knock-in animals did not freeze in anticipation after hearing the second tone as much as the wild type animals did. This finding led us to focus our study on the Hippocampus, a region of the brain known to be involved in formation of new memories and learning, and particularly to further assess the potential differences in M3 receptor signalling between the knock-in and the wild type animals in terms of M3 receptor distribution, phosphorylation pattern, coupling to downstream effectors and potential involvement in the modulation of synaptic transmission in the Hippocampus.