Intracellular proteins that bind serotonin in neurons, paraneurons and platelets

1. Properties of serotonin binding proteins (SBP) extracted from neuroectodermal derivatives (brain, myenteric plexus, thyroid parafollicular cells) were found to differ from serotonin binding proteins extracted from cells of mesodermal (platelets) or entodermal (intestinal mucosa) origin. 2. Serotonin binding proteins extracted from brain, myenteric plexus, and thyroid parafollicular cells all had similar properties. 3. Binding of tritiated serotonin to serotonin binding protein is inhibited by a drug that interferes with intracellular storage of serotonin (reserpine) or by exposing the protein to extracellular salt concentrations (Krebs solution). 4. It is proposed that serotonin binding protein is a neuron-specific storage protein for serotonin that serves to decrease intravesicular osmotic pressure, and that serotonin dissociates from a complex with the protein when the lumen of synaptic vesicles is exposed to extracellular fluid at the time of exocytosis.