Putative neuroendocrine cells in the Aplysia cerebral ganglion.
We report on a special population of cells in the Aplysia cerebral ganglion that are characterized by several features compatible with neuroendocrine function. These cells can be recognized in living ganglia by their small size, white color and their typical distribution as a compact cluster in the central medial region of the dorsal ganglion surface. Upon intracellular recording, these cells generate action potentials of relatively long duration (about 25 ms), as compared with the faster action potentials of larger white cells or of non-white cells (about 4 ms). Intracellular injection of the small white cells with Lucifer yellow after recording revealed a dual projection area: single cells have one process which branches extensively into many varicose terminals as it courses through the neuropil, and then sends varicose terminals to the vascular sheath at the periphery of the ganglion. In cryostat sections, these cells were specifically characterized by their content of large granules, the staining characteristics of which distinguish them from lipochondria or lysosomes. Their ability to bind fluorochromes nonspecifically is of particular importance for the interpretation of histochemical localization studies based on immunofluorescence techniques.