Light Microscopy of Living Neurosecretory Cells of theCorpus cardiacumofSchistocerca gregaria

Postembryonic proliferation of neuroendocrine cells expressing adipokinetic hormone peptides in the corpora cardiaca of the locust

Neuroendocrine glands that synthesize and secrete peptide hormones regulate the levels of these peptide messengers during development. In this article we describe a mechanism for regulating neuropeptide levels in the corpora cardiaca of the locust Schistocerca gregaria, a neuroendocrine gland structurally analogous to the vertebrate adenohypophysis. A set of five colocalized peptide hormones of the adipokinetic hormone family is synthesized in intrinsic neurosecretory cells in the corpora cardiaca. During postembryonic development there are progressive changes in the absolute and relative levels of these five peptide hormones. We show that the ability of the gland to increase peptide synthesis is due to a 100-fold increase in the number of cells which make up the gland. The gland grows by the addition of new cells derived from symmetrical division of undifferentiated precursor cells within the corpora cardiaca. We show, using double-label immunocytochemistry, that cells born in the glandular lobe mature into cells that express adipokinetic hormone peptides. The pattern of cell birth and peptide expression can account for the dramatic increase in postembryonic peptide levels.

Octopamine and cyclic AMP mediate release of adipokinetic hormone I and II from isolated locust neuroendocrine tissue

Octopamine serves as a neurotransmitter in the glandular lobe of the locust corpus cardiacum where it regulates adipokinetic hormone (AKH) secretion from intrinsic neurosecretory cells. Two AKHs (AKH I and II) from the corpus cardiacum of Locusta have been sequenced and synthesized. We have now demonstrated that octopamine mediates release of both AKH I and II from Locusta corpora cardiaca in vitro. Octopamine, IBMX, and forskolin have previously been shown to elevate levels of cAMP in the glandular lobe. In this paper we demonstrate that IBMX and forskolin mediate secretion of AKH I and AKH II thus mimicking the effects of octopamine in this tissue. The cAMP analogs dibutyryl cAMP and 8-bromo cAMP also elicit release of AKH and a subthreshold concentration of IBMX potentiates the effects of octopamine. These observations demonstrate that cAMP participates in regulating AKH release and support the hypothesis that octopamine mediates hormone release at least in part via changes in intracellular levels of cAMP. The release of AKH I and AKH II is apparently regulated by similar mechanisms. The hyperlipemic activity of AKH II released into the perfusates following stimulation by octopamine and agents which elevate cAMP levels is significantly lower than that of AKH I. This differential response is probably due both to the reduced lipid-mobilizing effect of AKH II relative to AKH I, as well as to the release of greater amounts of AKH I.

Electrical coupling and gap junctions between neurosecretory cells in an insect

Intracellular injections of Lucifer yellow into single neurosecretory cells in the posterior (glandular) lobe of the corpus cardiacum of locusts resulted in dye transfer between these cells. Passage of hyperpolarizing or depolarizing current into one cell resulted in changes in the membrane potential of other cells. Freeze-fracture replicas of these cells revealed gap junctions which probably mediate the electrical and dye coupling. It seems apparent from these data that coupling may synchronize the neurosecretory cells' activity.

Metamorphosis of the cerebral neuroendocrine system in the moth Manduca sexta

We have examined the morphology and neuronal elements of the cerebral neuroendocrine system in the larval, pupal, and adult stages of the moth Manduca sexta with a variety of neuroanatomical techniques. The larval brain contains several discrete groups of neurosecretory and non-neurosecretory cells that project to the associated neurohemal organs (the corpora cardiaca-allata complex, or CC-CA) and to a variety of more peripheral structures. A previously undescribed set of cells in the subesophageal ganglion have also been found that project out the neurosecretory nerves. During metamorphosis, the cerebral neuroendocrine system undergoes a dramatic structural reorganization, including the reduction or loss of many larval nerves and a repositioning of the cell groups and their dendritic fields. Despite these changes, most of its central elements are retained. In addition, by the completion of adult development a new cluster of cells can be found on either side of the dorsal midline of the brain. We have also determined the relative contributions of the different cell groups to the moth neuroendocrine system by intracellular iontophoresis of dye into individual cells. Within the dorsal protocerebrum, five separate morphological types of cells can be recognized, each with a distinctive pattern of dendritic arborization in the brain and terminal neurohemal processes that project to the CC, the CA, the aorta, or to a combination of these regions. The large intrinsic cells of the CC have also been filled, revealing an unusual set of morphological features in these peripheral neurosecretory cells.