Sources and targets of nitric oxide signalling in insect nervous systems.

Nitric oxide (NO) is a membrane permeant signalling molecule which activates soluble guanylyl cyclase and leads to the formation of cyclic GMP (cGMP) in target cells. In the nervous system, NO/cGMP signalling is thought to play essential roles in synaptic plasticity during development and also in the mature animal. This review summarizes neurochemical, cell biological, and physiological investigations of NO/cGMP signalling in the nervous system of insects. The anatomical localization of donor and target cells suggests functions in olfaction, vision, and mechanosensation. Behavioural assays have uncovered contributions of NO signalling in oxygen sensing, habituation to chemosensory stimuli, and associative memory formation. During development, NO regulates cell proliferation, axonal outgrowth, and synaptic maturation. The cellular distribution of NO-responsive cells suggests that NO can serve as a retrograde synaptic messenger, as an intracellular messenger, and as a lateral diffusible messenger irrespective of conventional synaptic connectivity.