Electrophysiology and ultrastructure of axons of neurosecretory cells associated with the cephalic aorta of Rhodnius prolixus

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Ultrastructural analysis of the cephalic aorta in Rhodnius prolixus revealed a large nerve attached to the ventral side of the aorta, and several smaller neuronal processes located in the wall or towards the lumen of the aorta. In cross sections at various distances posterior to the brain, this nerve decreased in size due primarily to the loss of axon profiles. Most axon profiles in the nerve contained electron-dense granules characteristic of neurosecretory cells. In the light of previously reported cobalt backfills of the nerve on the cephalic aorta, many of these axons belonged to neurosecretory cells the cell bodies of which were located in the brain, suboesophageal ganglion and retrocerebral complex. Electrical activity was recorded in situ by an extracellular suction electrode into which a portion of the cephalic aorta was looped. Electrical activity occurred in a continuous firing pattern often displayed by insect neurosecretory cells. Periods of silence were observed in only a few preparations and when present they were shorter than 1–2 s in duration. Differences in the discharge firing patterns of various electrically active units allowed us to classify this activity into three major groups of action potential. Simultaneous recordings using two suction electrodes located at various regions, demonstrated that most of the axons ended near or in the cephalic aorta while some of the axons extended along the aorta to the posterior regions of the thorax. Differences in electrical activity recorded simultaneously from the aorta and corpus cardiacum verified that axons along the aorta arose from groups of neurosecretory cells different from those associated with the distal regions of the corpus cardiacum. Changes in electrical activity as a result of transecting the brain and associated nervous structures at various locations signified that neurosecretory cells associated with the suboesophageal ganglion were the major contributors to the ongoing electrical activity. These results demonstrate that the cephalic aorta of Rhodnius prolixus is associated with several neurosecretory cells. The ability to correlate some action potential types with the structural organization of neurosecretory cell types will permit the use of the cephalic aorta as a model system in which to study the coordinated release of neurohormones from identifiable neurosecretory cell types regulating various neuroendocrine processes.