Spontaneous and evoked release of prothoracicotropin from multiple neurohemal organs of the tobacco hornworm

Release of neurohormone from putative cephalic neurohemal organs was directly demonstrated in an insect. The prothoracicotropic hormone (PTTH) of the tobacco hornworm, Manduca sexta, was measured indirectly by its ability to stimulate the secretion of alpha-ecdysone by inactive prothoracic glands; the ecdysone was measured by radioimmunoassay. The PTTH released spontaneously from intact brain-retrocerebral complexes was localized to the retrocerebral complex by placing a waxy barrier across the nerves connecting the corpora cardiaca to the brain. Isolated corpora allata spontaneously released much more PTTH than did either isolated corpora cardiaca or isolated brains. Media containing 100 mM potassium stimulated PTTH release from both isolated corpora allata and isolated corpora cardiaca. In calcium-free media, spontaneous PTTH release was diminished and release could not be stimulated by high potassium. These results indicate that depolarization of the neurosecretory cells is correlated with calcium-dependent neurohormone release and that there are multiple neurohemal organs for PTTH. The biological activities of stored and circulating PTTH are compared.