Abstract

We provide evidence herein that blockade of NA signaling in the olfactory bulb through α and β receptors impairs, but does not impede an adult mouse's ability to discriminate between odor cues. We also characterized the modulation of activity in mitral cells that occurs in awake-behaving mice engaged in an olfactory discrimination task. We then blockade NA signaling within the bulb while recording from mitral cells in behaving animals and determine what normal modulations are impaired. We found that both SMC odor responsiveness and divergent responses to the two odors being discriminated increased during the discrimination task. The data also showed that the increases in responsiveness and divergence were transient, diminishing by the end of the discrimination task and gone the following day. A set of reversal experiments showed that many mitral cells reverse the valence of their divergent response, indicating that the olfactory bulb plays some role in coding the meaning of an odor in addition to odor quality. The divergence in individual cell responses was not dependent upon noradrenaline signaling but did persist for a shorter duration. Noradrenergic antagonism caused a drastic reduction in the normal transient increase of single cell responses to the rewarded and unrewarded odors. We provide evidence that mitral cell synchrony exists in the mouse olfactory bulb. It does not appear that a dynamic modulation mitral cell synchrony exists on the time scale of an olfactory learning session. The results presented below redefine the function of the olfactory bulb as a transiently modifiable (active) filter used by higher cortical structures to shape odor representations at the output of the olfactory bulb in behaviorally meaningful ways.