Veratridine-stimulated release of amine conjugates from centrifugal fibers in the Limulus peripheral visual system

Centrifugal fibers that originate in the brain and project to the Limulus peripheral visual system synthesize and store octopamine and conjugates of octopamine and tyramine. In a previous study we showed that depolarization, induced by elevating extracellular K+, stimulated a preferential release of octopamine from these fibers. Here we show that veratridine-induced depolarization stimulates a rapid, transient release of octopamine and a delayed, sustained release of amine conjugates. Veratridine-stimulated release of both octopamine and amine conjugates depends on the influx of extracellular Ca2+ and is blocked by tetrodotoxin or the absence of extracellular Na+. The depolarization-stimulated release of amine conjugates raises the possibility that these molecules serve as intercellular messengers in the Limulus peripheral visual system.