Octopamine release from centrifugal fibers of the Limulus peripheral visual system

Octopamine, a biogenic amine, is synthesized and stored within centrifugal (efferent) fibers that project from the brain to the lateral and ventral eyes of the horseshoe crab, Limulus polyphemus. The experiments described here show that depolarization of Limulus lateral and ventral eyes, produced by elevating the concentration of extracellular K+, causes the selective release of newly synthesized octopamine from centrifugal fibers in a manner that requires the influx of extracellular Ca2+. Conjugates of octopamine and tyramine that are also stored within centrifugal fibers are not released in response to K+-induced depolarization. These findings add further support to the hypothesis that octopamine is a neurotransmitter synthesized by and released from centrifugal fibers in Limulus eyes. This amine may be responsible for many of the alterations in lateral eye structure and function that are mediated by centrifugal innervation.