This image is the electrostatic potential of the neurotransmitter 5HT (5-hydroxytryptamine or serotonin).

The cationic charge on the nitrogen of the amine sidechain (blue color) is considered to be important for the molecular attraction between the molecule and the negative charge (red color) on the aspartate residue in the third transmembrane domain of the receptor. (See this link to receptor binding graphic)