Noradrenergic modulation of behavioral odor detection and discrimination thresholds in the olfactory bulb.

The mammalian main olfactory bulb (MOB) receives a significant noradrenergic input from the locus coeruleus. Norepinephrine (NE) is involved in the acquisition of conditioned odor preferences in neonatal animals and in some species-specific odor-dependent behaviors. Thus far, the role of NE in odor processing in adult rats remains less studied. We investigated the role of noradrenergic modulation in the MOB on odor detection and discrimination thresholds using behavioral and computational modeling approaches. Adult rats received bilateral MOB injections of vehicle, NE (0.1-1000 microM), noradrenergic receptor antagonists and NE + receptor antagonists combined. NE infusion improved odor detection and discrimination as a function of NE and odor concentration. The effect of NE on detection and discrimination magnitude at any given odor concentration varied in a non-linear function with respect to NE concentration. Receptor antagonist infusion demonstrated that alpha1 receptor activation is necessary for the modulatory effect of NE. Computational modeling showed that increases in the strength of alpha1 receptor activation leads to improved odor signal-to-noise ratio and spike synchronization in mitral cells that may underlie the behaviorally observed decrease of detection and discrimination thresholds. Our results are the first to show that direct infusion of NE or noradrenergic receptor antagonists into a primary sensory network modulates sensory detection and discrimination thresholds at very low stimulus concentrations.