Abstract
Sensory responses are modulated throughout the nervous system 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 coeruleus (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 includes modulation in the glomerular layer, the site of input to the MOB. Noradrenergic terminals are found in the glomerular layer, but noradrenaline receptor activation does not seem to acutely modulate olfactory sensory neuron 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 locus coeruleus in anesthetized mice. Surprisingly, the odor-evoked intrinsic optical signals at the glomerulus were persistently weakened after LC activation. Calcium imaging selectively from olfactory sensory neurons confirmed that this effect was due to a uniform gain suppression of presynaptic input, and did not require exposure to a stimulus during the LC activation. Finally, noradrenaline antagonists prevented glomerular suppression. 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.
