Noradrenergic plasticity of olfactory sensory neuron inputs to the main olfactory bulb.

Sensory responses are modulated by internal factors including attention, experience, and brain state. This is partly due to fluctuations in neuromodulatory input from regions such as the noradrenergic locus ceruleus (LC) in the brainstem. LC activity changes with arousal and modulates sensory processing, cognition, and memory. The main olfactory bulb (MOB) is richly targeted by LC fibers and noradrenaline profoundly influences MOB circuitry and odor-guided behavior. Noradrenaline-dependent plasticity affects the output of the MOB; however. it is unclear whether noradrenergic plasticity also affects the input to the MOB from olfactory sensory neurons (OSNs) in the glomerular layer. Noradrenergic terminals are found in the glomerular layer, but noradrenaline receptors do not seem to acutely modulate OSN terminals in vitro. We investigated whether noradrenaline induces plasticity at the glomerulus. We used wide-field optical imaging to measure changes in odor responses following electrical stimulation of LC in anesthetized mice. Surprisingly, odor-evoked intrinsic optical signals at the glomerulus were persistently weakened after LC activation. Calcium imaging selectively from OSNs confirmed that this effect was due to suppression of presynaptic input and was prevented by noradrenergic antagonists. Finally, suppression of responses to an odor did not require precise coincidence of the odor with LC activation. However, suppression was intensified by LC activation in the absence of odors. We conclude that noradrenaline release from LC has persistent effects on odor processing already at the first synapse of the main olfactory system. This mechanism could contribute to arousal-dependent memories.