Multiple neuromodulators regulate olfactory system operation, as they do many other functional computations within the brain.  In this review, the authors review key principles of neuromodulation within the early olfactory system and argue that studying individual neuromodulators separately may be limiting our ability to understand the full capacities of neuromodulation in natural contexts.

Linster C, Cleland TA (2016)  Neuromodulation of olfactory transformations.  Current Opinion in Neurobiology 40:170-177.

Abstract:  The olfactory bulb and piriform cortex are the best studied structures of the mammalian olfactory system and are heavily innervated by extrinsic neuromodulatory inputs. The state-dependent release of acetylcholine, norepinephrine, serotonin, and other neuromodulators into these olfactory structures alters a constellation of physiological parameters in neurons and synapses that together modify the computations performed on sensory signals. These modifications affect the specificity, detectability, discriminability, and other properties of odor representations and thereby govern perceptual performance. Whereas different neuromodulators have distinct cellular effects, and tend to be associated with nominally different functions, it also is clear that these purported functions overlap substantially, and that ad hoc hypotheses regarding the roles of particular neuromodulators may have reached the limits of their usefulness.