Neuropeptide modulation of photosensitivity. I. Presence, distribution, and characterization of a substance P-like peptide in the lateral eye of Limulus.

The lateral eye of Limulus is innervated by an efferent system of fibers containing a substance P-like peptide. They were detected and their distribution was studied using indirect immunocytochemical techniques and monoclonal and serum antibodies. These thin, efferent fibers travel up the optic nerve, cross the lateral plexus, and branch out profusely when reaching the ommatidial layer. Innervation is extended to more than one component of the ommatidia, including the pigment, retinular, and eccentric cells. Immunoreactive staining could be abolished by absorbing the antisera with as little as 1 microM synthetic substance P. The efferent character of the fibers was established by means of ligation experiments, a technique also used to determine their origin in the circumesophageal connectives. Radioimmunoassay with two different C-terminal serum antibodies confirmed the presence of substance P-like material in the eye in the amounts of 61.44 pg/micrograms of protein, or up to 18 ng/eye. Gel filtration chromatography of crude extracts of the lateral eye, followed by radioimmunoassay, revealed an elution pattern extremely similar but not identical to that of synthetic substance P. These results show that a system of efferent fibers containing a substance P-like peptide originates in cells in the circumesophageal ring and innervates the ommatidia of the lateral eye. Their distribution and origin suggest an involvement in the modulation of photosensitivity, as part of a larger, generalized, level-setting regulator that is driven by a circadian clock but can also be activated by other systems.