Serotonylation is a receptor independent signaling mechanism by which serotonin activates intracellular processes by creating long lasting covalent bonds upon proteins. It occurs through the modification of proteins by the attachment of serotonin  on their glutamine residues. This happens through the enzyme transglutaminase and the creation of glutamyl-amide bonds.  This  process occurs following serotonin transportation  into the cell rather on plasma membranes as with the brief interactions that serotonin has when it activates 5-HT receptors.