Isoforms
There are at least six H3 receptor isoforms in the human, and more than 20 discovered so far. In rats there have been six H3receptor subtypes identified so far. Mice also have three reported isoforms. These subtypes all have subtle difference in their pharmacology (and presumably distribution, based on studies in rats) but the exact physiological role of these isoforms is still unclear.
Read more about this topic: Histamine H3 Receptor