Serotonin-binding glycoprotein of rat platelets

Platelets, because of their ability to take up serotonin, are often considered as models of serotonergic neurons. We have purified a glycoprotein from rat platelets that specifically binds serotonin (KD = 10 nM) but not [3H]imipramine (up to 0.2 microM). This protein is distinct from fibronectin. Antiserum raised against this protein was shown to be monospecific by double diffusion, rocket immunoelectrophoresis and immunoreactivity of protein transferred from slab gels to nitrocellulose sheets. We have called this protein serotonectin. Serotonectin-like material was localized by light- and electron-microscopic immunocytochemistry in rat platelets, using the unlabelled peroxidase-antiperoxidase complex bridge technique. All serotonectin immunoreactivity was found to be associated with the plasma membrane but no immunoreactivity was found in the surface connecting system. Washing platelets with Krebs solution removed 75% of the extractable serotonectin, indicating that it is a peripheral protein. Serotonectin was found to be circulating in the plasma, as the ease of its removal from platelet membranes suggests it must do; however, none was found in brain synaptosomes. Serotonectin was found to be synthesized in blood-forming organs (spleen, bone marrow) but not in the liver or the mucosa of the gut. Antibodies to serotonectin (4.5 mg/ml) inhibited the uptake of [14H]serotonin (0.2 microM) by rat platelets but not by synaptosomes. Serotonectin did not bind [3H]imipramine and washing the protein off platelet membranes did not impede the binding of [3H]imipramine to the membranes. Thus an action on serotonectin does not seem to be responsible for the antagonism by tricyclic antidepressants of serotonin uptake. These data suggest that serotonectin is present on the external surface of platelet plasma membranes, that it is excluded from the plasma membrane of the surface connecting system, and that it may be in equilibrium with free material in plasma.