Efferent Innervation to Limulus Eyes In Vivo Phosphorylates a 122 kD Protein

Efferent fibers innervate all of the eyes of the horseshoe crab, Limulus polyphemus. Driven by a circadian clock located in the central nervous system, the activity of the fibers at night is responsible for anatomical, biochemical, and physiological changes in the eyes, which increase their ability to detect and respond to light. We showed previously that octopamine, a putative efferent neurotransmitter, stimulates the phosphorylation of a 122 kD protein in in vitro preparations of both ventral and lateral eyes by means of a cAMP-dependent mechanism. We now report that phosphorylation of the 122 kD protein in the lateral eye is enhanced in vivo: (1) at night, in correlation with efferent nerve input activated by the circadian clock; and (2) during the day, in response to electrical stimulation of efferent axons. We show further that the 122 kD protein is enriched in, and may be restricted to, tissues that contain photoreceptors. We postulate that this protein is involved in the efferent-stimulated increase in retinal sensitivity.