Neurosecretion. XVI. Protrusions of bounding membranes of neurosecretory granules

Protrusions of bounding membranes of neurosecretory granules, comparable to those demonstrated by Castel (1977) in the mammalian neurohypophysis, were observed under various experimental conditions in the corpora cardiaca of the insects Leucophaea maderae and Periplaneta americana. Electrical stimulation in vitro of the nervus corporis cardiaci I, which elicited a marked rise in the amount of neurohormone discharged, as determined by bioassay, also yielded a significantly larger number of membrane protrusions than were observed in unstimulated controls. However, no comparable response was obtained in glands subjected to stimulation of hormone release by exposure to serotonin or high potassium concentrations. On the other hand, membrane protrusions were numerous under certain conditions not expected to stimulate neurohormone release, i.e., in tissue exposed to a zinc iodide mixture without prior fixation. The present results support the conclusion drawn by Castel that these configurations appear to be related to the process by which neurosecretory material is discharged. Too transient to be much in evidence under physiological condition, they become more prominent not only after appropriate acceleration of the rate of release, but also when such membrane arrangements are "frozen" by procedures that interfere with the regular milieu.