Immunohistochemical and biochemical evidence for the presence of serotonin-containing neurons and nerve fibers in the octopus arm

The octopus arm contains a tridimensional array of muscles with a massive sensory-motor system. We herein provide the first evidence for the existence of serotonin (5-HT) in the octopus arm nervous system and investigated its distribution using immunohistochemistry. 5-HT-like immunoreactive (5-HT-lir) nerve cell bodies were exclusively localized in the cellular layer of the axial nerve cord. Those cell bodies emitted 5-HT-lir nerve fibers in the direction of the sucker, the intramuscular nerves cords, the ganglion of the sucker, and the intrinsic musculature. Others 5-HT-lir nerve fibers were observed in various tissues, including the cerebrobrachial tract, the skin, and the blood vessels. 5-HT was detected by high-performance liquid chromatography in various regions of the octopus arm at levels matching the density of 5-HT-lir staining. The absence of 5-HT-lir interconnections between the cerebrobrachial tract and the other components of the axial nerve cord suggests that two types of 5-HT-lir innervation exist in the arm. One type, which originates from the brain, may innervate the periphery through the cerebrobrachial tract. Another type, which originates in the cellular layer of the axial nerve cord, may form an intrinsic network in the arm. In addition, 5-HT-lir fibers likely emitted from the neuropil of the axial nerve cord were found to project into cells showing staining for peripheral choline acetyltransferase, a marker of sensory cells of the sucker. Taken together, these observations suggest that intrinsic 5-HT-lir innervation may participate in the sensory transmission in the octopus arm.