Autoradiographic localization of newly synthesized octopamine to retinal efferents in the Limulus visual system

The biogenic amine octopamine is synthesized from both tyrosine and tyramine in the lateral, median, and ventral eyes of Limulus. The autoradiographic studies presented here were designed to locate the sites of octopamine synthesis in the ventral and lateral eyes. We found that efferent fibers, which project to ventral and lateral eyes from the central nervous system, became intensely and selectively labeled during in vitro incubations with 3H-tyramine. In the ventral eye, more than 95% of the efferent fibers were labeled. Results of biochemical analyses suggested that most of the radioactive substance within these efferent fibers was newly synthesized octopamine. The selective labeling of efferent fibers during incubation with 3H-tyramine was used as an anatomical tool to study the number and distribution of efferent fibers within the ventral eye. Light microscopic (LM) reconstructions of the distribution of label in serial longitudinal sections through ventral optic nerves together with electron microscopic (EM) autoradiographic analyses revealed between 70 and 200 efferent axons. The results of these studies and of reconstructions of efferent innervation to photoreceptor somata suggest that each ventral photoreceptor cell or each small cluster of cells is innervated by a separate efferent fiber. Both LM reconstructions and EM analyses showed that efferent fibers ramify extensively and specifically in and near the internal rhabdom of ventral photoreceptor cells. In EM autoradiographs of lateral eyes incubated with 3H- tyramine, the silver grains that were located over ommatidia were concentrated exclusively over efferent fibers. All of these efferent fibers, which lay near rhabdoms and in partitions between retinular cells, were labeled. The results of our present studies support our hypothesis that octopamine is a neurotransmitter in Limulus retinal efferent fibers. This amine may modulate the biochemistry and physiology of ventral photoreceptor cells and may mediate many of the known effects of circadian efferent innervation to the lateral eye.