HISTOCHEMICAL-LOCALIZATION OF ACETYLCHOLINESTERASE IN THE LATERAL EYEAND BRAIN OF LIMULUS-POLYPHEMUS - MIGHT ACETYLCHOLINE BE A NEUROTRANSMITTER FOR LATERAL INHIBITION IN THE LATERAL EYE

The distribution of acetylcholinesterase (AChE) in the lateral eye and brain of the horseshoe crab was investigated with histochemical means using standard controls to eliminate butyrylcholinesterase and nonspe cific staining. Intense staining was observed in the neural plexus of the lateral compound eye, in the lateral optic nerve, and in various n europils of the brain. Nerve fibers with moderate to weak staining wer e widespread in the brain. No somata were stained in either the latera l eye or the brain. The distribution of acetylcholinesterase in the su praesophageal ganglia and nerves of the giant barnacle was also invest igated for comparison. Although both the median optic nerve of the bar nacle and the lateral optic nerve of the horseshoe crab appear to cont ain the fibers of histaminergic neurons, only the lateral optic nerve of the horseshoe crab shows AChE staining. Other parts of the barnacle nervous system, however, showed intense AChE staining. These results along with the histochemical controls eliminate the possibility that s ome molecule found in histaminergic neurons accounted for the AChE sta ining but support the possibility that acetylcholine might be involved as a neurotransmitter in lateral inhibition in the horseshoe crab ret ina. Two reasonable neurotransmitter candidates for lateral inhibition , histamine and acetylcholine, must now be investigated.