The H3 histamine receptor (H3R) is a G protein-coupled receptor (GPCR) mainly expressed in the central nervous system (CNS) and a key modulatory site for several neurotransmitters. It acts as an autoreceptor on histaminergic neurons, inhibiting the release and synthesis of histamine, but it is also present as a heteroreceptor on other non-histaminergic neurons, where it modulates the release of neurotransmitters such as acetylcholine, dopamine or ...