Research reportNoradrenergic effects on olfactory perception and learning
Introduction
Norepinephrine (NE) plays important roles in the detection, learning and memorization of olfactory sensory stimuli (Linster et al., 2011). As in other systems, NE has been implicated in signal to noise ratio (S/N) modulation of olfactory stimuli, plasticity associated with olfactory learning and the ability to discriminate odorants. We here review its role in adult rodent olfactory processing with emphasis on olfactory behavioral studies. NE’s role in neonatal olfactory learning has been described in detail elsewhere and seems unrelated to adult processes due to substantial changes in the olfactory and noradrenergic system after a critical period (Moriceau et al., 2009a, Moriceau et al., 2009b). The modulation of olfactory guided behaviors in adult rodents is complex and variable, ranging from effects on detection thresholds to effects on memory duration, mediated by common underlying neural and synaptic mechanisms.
Section snippets
Olfactory guided behaviors
Olfaction is an important sense for most animals, implicated and necessary for multiple processes such as acquiring food, recognizing mates and conspecifics and identifying potential dangers. These processes involve detection of an odorant, often at low concentrations and masked by background odorants, identifying this odorant, and possibly memorizing it (Cleland, 2011). Some of these processes are non-associative, e.g. recognition of a previously encountered odor, some are associative, e.g.
Odor detection
Odor detection is the first step in odor processing. The olfactory system is built to facilitate odor detection with a very high ratio of sensory to principal neuron projections facilitating the detection of even very low concentration odors (Cleland and Linster, 2005). Odor detection abilities could potentially be modulated by NE via increased excitability of principal cells (Linster et al., 2011), increased overall inhibitory tone leading to a decrease in baseline spontaneous activity (Nai et
Experimental considerations
As reviewed in Fig. 1, labs employ a variety of behavioral paradigms to assess the role of NE modulation for olfactory processing. Because task parameters such as odor concentration, choice of odorants, intertrial intervals, to just name a few have a significant effect on talk outcomes (Cleland et al., 2009, Freedman et al., 2013, Hackett et al., 2015) it can be difficult to directly compare results from different studies. Similarly, there are numerous ways to influence NE modulation during a
Conclusion
The behavioral studies reviewed in this paper show that NE modulation can effect all aspects of olfactory processing: detection, encoding, retrieval, specificity and duration of an encoded odor memory. Typically, non-reward associative tasks are more strongly affected by manipulations of the NE system then reward motivated tasks, a phenomenon we are currently investigating in more detail. To better understand these data, one has to correlate these results with cellular, synaptic and network
Funding
Original research reviewed in this article was funded by NIH/NIDCD Grant DC008702.
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