Is the serotonin binding protein (SBP) a soluble storage form for serotonin?

The binding of serotonin to a soluble, high affinity binding protein (SBP) present in synaptosomes and assoicated with serotonergic tracts, has now been studied for the effect of drugs and, in particular, drugs interacting with contractile proteins. Vincristine, vinblastine, and cytochalasin B were found to cause 50% inhibition of serotonin binding to SBP at 1.5 X 10(-6)M, 7.5 X 10(-6)M and 50 X 10(-6)M, respectively. Colchicine did not affect the binding at 1 X 10(-3)M. When vinblastine was injected intraventricularly, the binding capacity of SBP isolated from brain 20 and 26 hours after injection was decreased 42% and 60% respectively. It is concluded that SBP is an actin-like contractile protein, distinct from microtubulin, and not associated with membranes. The storage of serotonin through binding to a protein with such properties suggests that this compartment is in some way involved in the translocation or transfer of serotonin from one part of the cell to another.