Nadph-diaphorase activity in corpus allatum cells of the cockroach, Diploptera punctata

Homology of insect corpora allata and vertebrate adenohypophysis?

Animal species of various phyla possess neuroendocrine glands whose hormonal products regulate developmental and physiological mechanisms and directly impact behavior. Two examples, the corpora allata of insects and the vertebrate adenohypophysis have previously been regarded as analogous tissues that evolved independently from diffuse epidermal nerve nets of early metazoans. More recent developmental and functional studies accumulated evidence suggesting that the bilaterian nervous systems including its modern parts (e.g. pallium or cortex and mushroom bodies) and its neuroendocrine appendages (that are considered to be more ancient structures) possess a single evolutionary origin. The corpora allata of insects and the vertebrate adenohypophysis share a number of characteristics in respect of morphology, control of hormone release by RFamides, metabolites produced by closely related cytochrome P450 enzymes and gene expression during embryonic development. This review incorporates latest findings into an extensive description of similarities between insect corpora allata and vertebrate adenohypophysis that should encourage further studies about the onto- and phylogenetic origin of these neuroendocrine glands.

Nitric oxide/cGMP signaling in the corpora allata of female grasshoppers

The corpora allata (CA) of various insects express enzymes with fixation resistant NADPHdiaphorase activity. In female grasshoppers, juvenile hormone (JH) released from the CA is necessary to establish reproductive readiness, including sound production. Previous studies demonstrated that female sound production is also promoted by systemic inhibition of nitric oxide (NO) formation. In addition, allatotropin and allatostatin expressing central brain neurons were located in close vicinity of NO generating cells. It was therefore speculated that NO signaling may contribute to the control of juvenile hormone release from the CA. This study demonstrates the presence of NO/cGMP signaling in the CA of female Chorthippus biguttulus. CA parenchymal cells exhibit NADPHdiaphorase activity, express anti NOS immunoreactivity and accumulate citrulline, which is generated as a byproduct of NO generation. Varicose terminals from brain neurons in the dorsal pars intercerebralis and pars lateralis that accumulate cGMP upon stimulation with NO donors serve as intrinsic targets of NO in the CA. Both accumulation of citrulline and cyclic GMP were inhibited by the NOS inhibitor aminoguanidine, suggesting that NO in CA is produced by NOS. These results suggest that NO is a retrograde transmitter that provides feedback to projection neurons controlling JH production. Combined immunostainings and backfill experiments detected CA cells with processes extending into the CC and the protocerebrum that expressed immunoreactivity against the pan-neural marker anti-HRP. Allatostatin and allatotropin immunopositive brain neurons do not express NOS but subpopulations accumulate cGMP upon NO-formation. Direct innervation of CA by these peptidergic neurons was not observed.

Interactions between epithelial nitric oxide signaling and phosphodiesterase activity in Drosophila

Signaling by nitric oxide (NO) and guanosine 3',5'-cyclic monophosphate (cGMP) modulates fluid transport in Drosophila melanogaster. Expression of an inducible transgene encoding Drosophila NO synthase (dNOS) increases both NOS activity in Malpighian (renal) tubules and DNOS protein in both type I (principal) and type II (stellate) cells. However, cGMP content is increased only in principal cells. DNOS overexpression results in elevated basal rates of fluid transport in the presence of the phosphodiesterase (PDE) inhibitor, Zaprinast. Direct assay of tubule cGMP-hydrolyzing phosphodiesterase (cG-PDE) activity in wild-type and dNOS transgenic lines shows that cG-PDE activity is Zaprinast sensitive and is elevated upon dNOS induction. Zaprinast treatment increases cGMP content in tubules, particularly at the apical regions of principal cells, suggesting localization of Zaprinast-sensitive cG-PDE to these areas. Potential cross talk between activated NO/cGMP and calcium signaling was assessed in vivo with a targeted aequorin transgene. Activated DNOS signaling alone does not modify either neuropeptide (CAP2b)- or cGMP-induced increases in cytosolic calcium levels. However, in the presence of Zaprinast, both CAP2b-and cGMP-stimulated calcium levels are potentiated upon DNOS overexpression. Use of the calcium channel blocker, verapamil, abolishes the Zaprinast-induced transport phenotype in dNOS-overexpressing tubules. Molecular genetic intervention in the NO/cGMP signaling pathway has uncovered a pivotal role for cell-specific cG-PDE in regulating the poise of the fluid transporting Malpighian tubule via direct effects on intracellular cGMP concentration and localization and via interactions with calcium signaling mechanisms.

New techniques for whole-mount NADPH-diaphorase histochemistry demonstrated in insect ganglia

Fixation-resistant NADPH-diaphorase (NADPHd) activity is used widely as a marker for nitric oxide synthase (NOS). In frozen sections, NADPHd histochemistry yields high anatomic definition. In whole-mounts, however, poor penetration of the reagents, background staining, and tissue opacity severely limit its application. Here we report a combination of new methods that significantly improves whole-mount NADPHd staining. We demonstrate these methods in the thoracic ganglia of a large insect, the locust Schistocerca gregaria, in which NADPHd has been analyzed previously using both whole-mounts and serial section reconstructions. The penetration of the staining reagents was markedly improved after fixation in methanol/formalin compared to phosphate-buffered formaldehyde. Methanol/formalin also reduced nonspecific NADPHd and enhanced the selective staining. Penetration was further enhanced by incubation regimens that exploit the temperature- or pH-dependence of NADPHd. In combination with methanol/formalin fixation, this permitted staining to develop evenly throughout these comparatively large invertebrate ganglia. These improvements were complemented by a new clearing technique that preserves the NADPHd staining, gives excellent transparency, and avoids distortion of specimen morphology. The new methods revealed the three-dimensional architecture of NADPHd expression in locust ganglia in unprecedented detail and may similarly improve whole-mount detection of NADPHd in other invertebrate and vertebrate preparations.