While the role of platelets in the immune system is well known, how they act in modulating neurological interactions is something that has yet to be investigated. Do platelets influence behavior to any extent? That is the question behind the study, coordinated by Cristina Limatola of the Department of Physiology and Pharmacology at Sapienza University and recently published in the journal “Cell Reports”. The research shows that platelets, which store serotonin, a neurotransmitter that regulates mood, sleep, appetite and memory, play a crucial role in modulating neurological and immune responses. The experiment showed that reducing or altering the number of platelets in mice reduces brain serotonin, affecting fear-related behaviors. Indeed, the brain modulates responses based on past experiences: events associated with danger activate defensive reactions. Serotonin in the brain regulates these processes, and its deficiency can block the activity of inhibitory neurons in the hippocampus, which controls memory. This leads to impaired memory formation and fear responses even in the presence of harmless stimuli. The identified mechanism involves Natural Killer cells, which stimulate serotonin production in the gastrointestinal tract, affecting the amount of serotonin in platelets and the brain. Reduction of these cells or platelets leads to less serotonin in the brain, altering neurotransmission and plasticity in the hippocampus, affecting fear-related behaviors.