STORAGE OF SEROTONIN AND SEROTONIN BINDING-PROTEIN IN SYNAPTIC VESICLES

We have used the newly introduced method of DeLorenzo & Freedman (1978) for isolating synaptic vesicles to determine if such vesicles contain both serotonin (5‐HT) and serotonin binding protein (SBP). Two fractions were obtained. A 55, 000 g fraction was morphologically heterogeneous and contained coated vesicles. A 135, 0000 vesicle (dia. 51.3 nm) fraction was homogeneous in ultra‐structure and contained no coated vesicles. The specific activity of SBP in this fraction was much higher than that in the supernatant. Unlike SBP, very little lactic dehydrogenase activity appeared in the 135, 000 g fraction. Qualitative and quantitative differences were observed between the polypeptide profiles of soluble proteins extracted from the vesicles and supernatant proteins on SDS gels. Therefore, entrapment of cytosol in the vesicles of the 135, 000 g fraction was minimal. The 5‐HT concentration of the 135, 000 g vesicles was 5.5 ng/mg protein and in the supernatant, 11.3 ng/mg protein. The ATP concentration in the 135, 000 g vesicle fraction was only 0.8 ng/mg Pr. Rabbit spinal cords were transected in order to determine if SBP is moved proximo‐distally in axons by rapid axonal transport as would be predicted for a constituent of synaptic vesicles. SBP accumulated above the cut at a rate consistent with fast transport (78 mm/day). SBP activity fell caudal to the point of transection and there was no evidence, such as an accumulation below the lesion, that might indicate retrograde transport of SBP. These experiments indicate that SBP is probably synthesized in the cell bodies of serotonergic neurons and some is rapidly transported down axons to be stored in terminals in vesicles. Copyright © 1979, Wiley Blackwell. All rights reserved