Tonic and stimulus-evoked nitric oxide production in the mouse olfactory bulb

Nitric oxide (NO) has been long assumed to play a key role in mammalian olfaction. This was based largely on circumstantial evidence, i.e. prominent staining for nitric oxide synthase (NOS) and cyclic guanosine 3′,5′-cyclic monophosphate (cGMP) or soluble guanylyl cyclase, an effector enzyme activated by NO, in local interneurons of the olfactory bulb. Here we employ innovative custom-fabricated NO micro-sensors to obtain the first direct, time-resolved measurements of NO signaling in the olfactory bulb. In 400 μm thick mouse olfactory bulb slices, we detected a steady average basal level of 87 nM NO in the extracellular space of mitral or granule cell layers. This NO 'tone' was sensitive to NOS substrate manipulation (200 μM l-arginine, 2 mM N^G-nitro-l-arginine methyl ester) and Mg²⁺ modulation of N-methyl-D-aspartate (NMDA) receptor conductance. Electrical stimulation of olfactory nerve fibers evoked transient (peak at 10 s) increments in NO levels 90-100 nM above baseline. In the anesthetized mouse, NO micro-sensors inserted into the granule cell layer detected NO transients averaging 55 nM in amplitude and peaking at 3.4 s after onset of a 5 s odorant stimulation. These findings suggest dual roles for NO signaling in the olfactory bulb: tonic inhibitory control of principal neurons, and regulation of circuit dynamics during odor information processing.