Release of [3H]serotonin and its binding protein from enteric neurons

The release of [3H]5-HT and its binding protein, SBP, from the guinea pig enteric nervous system was analyzed. Release of both [3H]5-HT and [3H]NE from strips of longitudinal muscle with adherent myenteric plexus preloaded with the respective radioactive amine was evoked by high K+ and the ionophore X537A. However, Ca2+-dependence could not be shown for [3H]5-HT release by either agent or for [3H]NE release by X537A. However, Ca2+-dependence (as well as inhibition of release by high Mg2+ and tetrodotoxin) could be demonstrated for the release of radioactivity evoked by electrical field stimulation of everted segments of ileum preloaded by perfusion through the serosal lumen with [3H]5-HT. Light and electron microscopic radioautography revealed that the sources of released radioactivity were axons, especially axonal varicosities containing a mixture of small clear and large dense-cored vesicles. SPB, but not the cytosol marker protein, lactic dehydrogenase, was spontaneously released from the perfused everted ileum. A marked increase in SBP (but not LDH) release was provoked by electrical field stimulation at 10 Hz, and this increased release (but not the spontaneous release) was Ca2+-dependent. It is concluded that SBP and 5-HT are probably stored together, at least in part in vesicles, and that both can be released by exocytosis from depolarized axon terminals.