Surprisingly rich projection from locus coeruleus to the olfactory bulb in the rat.

The brainstem nucleus, locus coeruleus (LC), is the major, if not the sole, source of noradrenergic (NE) innervation of the telencephalon. It is generally held that LC neurons project diffusely to the entire neuroaxis and this had been the basis for theories that postulate 'general' functions (sleep, attention, learning, etc.) for LC. We report that at least 40% of all LC neurons project to the olfactory bulb; the projection is almost 10 times greater than to any other part of the cerebral cortex. This unsuspectedly rich LC-olfactory bulb connection is consistent with current theories which implicate LC neurons in heightened sensory vigilance and trophic regulation of connectional development and plasticity.