Journal of Neuroscience, Vol 7, 2811-2820, Copyright © 1987 by Society for Neuroscience
Octopamine- and cyclic AMP-stimulated phosphorylation of a protein in Limulus ventral and lateral eyes
SC Edwards and BA Battelle
The biogenic amine octopamine (OCT) fulfills most of the criteria as a
neurotransmitter of efferent fibers that project to lateral and ventral
eyes of the horseshoe crab, Limulus polyphemus. OCT is synthesized by and
released from the efferent fibers, and OCT mimics many of the effects of
endogenous efferent activity. OCT stimulates an increase in intracellular
adenosine 3',5'-monophosphate (cAMP) in both ventral and lateral eyes, and
many of the physiological effects of OCT in these eyes appear to be
mediated via cAMP-dependent mechanisms. Here we show that OCT, acting
apparently through an OCT-specific receptor, stimulates the increased
phosphorylation of a protein with an apparent molecular weight of 122 kDa
in both ventral and lateral eyes. This protein is also phosphorylated in
response to 8-bromo cAMP and forskolin, suggesting that its phosphorylation
involves activation of a cAMP-dependent protein kinase. We present evidence
that the 122 kDa protein may be widely distributed in the Limulus visual
system but that its phosphorylation in intact tissue in response to OCT, or
agents acting through cAMP, may be restricted to portions containing
photoreceptor cell bodies. The 122 kDa protein is quantitatively a major
cellular protein in the photoreceptor cell body enriched portions of the
ventral eye, its isoelectric point is between pH 6.2 and 6.4, and it is
associated with both cell membranes and the cytoplasm. The function of this
protein is not yet known. It may be important in mediating one or more of
the effects of octopamine on Limulus vision.