Three-dimensional distribution of NO sources in a primary mechanosensory integration center in the locust and its implications for volume signaling.

Nitric oxide (NO) is an evolutionarily conserved mediator of neural plasticity. Because NO is highly diffusible, signals from multiple sources might combine in space and time to affect the same target. Whether such cooperative effects occur will depend on the effective signaling range and on the distances of NO sources to one another and to their targets. These anatomical parameters have been quantified in only few systems. We analyzed the 3D architecture of NO synthase (NOS) expression in a sensory neuropil, the ventral association center (VAC) of the locust. High-resolution confocal microscopy revealed NOS immunoreactive fiber boutons in submicrometer proximity to both the axon terminals of sensory neurons and their postsynaptic target, interneuron A4I1. Pharmacological manipulation of NO signaling affected the response of A4I1 to individual wind-puff stimuli and the response decrement during repetitive stimulation. Mapping NOS immunoreactivity in defined volumes around dendrites of A4I1 revealed NOS-positive fiber boutons within 5 mum of nearly every surface point. The mean distances between neighboring NOS-boutons and between any point within the VAC and its nearest NOS-bouton were likewise about 5 mum. For an NO signal to convey the identity of its source, the effective signaling range would therefore have to be less than 5 mum, and shorter still when multiple boutons release NO simultaneously. The architecture is therefore well suited to support the cooperative generation of volume signals by interaction between the signals from multiple active boutons.