NADPH diaphorase activity in the antennae of the hawkmoth Manduca sexta

Biochemical and physiological studies suggested that increases in the levels of cyclic GMP in insect antennal receptor cells play a role in olfactory adaptation. As inositol-trisphosphate-dependent Ca2+ influx appears to precede the increase in intracellular cyclic GMP levels, it was hypothesized that a Ca2+-dependent mechanism might stimulate the guanylyl cyclase. The present study used histochemical staining for NADPH diaphorase to examine whether antennal receptor neurones of male Manduca sexta could contain nitric oxide synthase. This Ca2+/calmodulin-dependent enzyme is a prerequisite for nitric-oxide-dependent stimulation of guanylyl cyclase and possesses NADPH diaphorase activity. It was found that a subpopulation of olfactory receptor neurones as well as mechano-, thermo- and hygroreceptors on the moth antenna are NADPH-diaphorase-positive. Staining was also seen in non-neuronal cells. In the developing antenna, the NADPH-diaphorase-dependent staining was first observed at pupal stage 13-14, at approximately the same time as the antennal receptor neurones became physiologically active. The number and location of stained receptor cells was highly variable, and significantly more pheromone-sensitive sensilla were NADPH-diaphorase-positive in pheromone-stimulated antennae. This suggests that the enzyme is transiently activated by pheromone rather than being continuously active.