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.
Arrestins participate in the termination of phototransduction in both vertebrates and invertebrates. However, the visual arrestins of invertebrates and vertebrates differ significantly from one another in that the invertebrate visual arrestins become phosphorylated rapidly in response to light while those in the photoreceptors of vertebrates do not. In an effort to understand the functional relevance of arrestin phosphorylation, we examined this process in the photoreceptors of the horseshoe crab Limulus polyphemus. We report that Limulus visual arrestin can be phosphorylated at three sites near its C-terminus and show that arrestin molecules phosphorylated on one, two, and three sites are normally present in both light- and dark-adapted photoreceptors. Light adaptation increases the amount of arrestin phosphorylated at three sites.
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