ACTIVATION OF LOCUS-COERULEUS ENHANCES THE RESPONSES OF OLFACTORY-BULB MITRAL CELLS TO WEAK OLFACTORY NERVE INPUT

The main olfactory bulb (MOB) receives a dense projection from the pon tine nucleus locus coeruleus (LC), the largest collection of norepinep hrine (NE)-containing cells in the brain. LC is the sole source of NE innervation of MOB. Previous studies of the actions of exogenously app lied NE on mitral cells, the principal output neurons of MOB, are cont radictory. The effect of synaptically released NE on mitral cell activ ity is not known, nor is the influence of NE on responses of mitral ce lls to olfactory nerve inputs. The goal of the present study was to as sess the influence of LC activation on spontaneous and olfactory nerve -evoked activity of mitral cells. In methoxyflurane-anesthetized rats, intracoerulear microinfusions of acetylcholine (ACh) (200 mM; 90-120 nl) evoked a four- to fivefold increase in LC neuronal discharge, and a transient EEG desynchronization and decrease in mitral cell discharg e. LC activation increased excitatory responses of mitral cells evoked by weak (i.e., perithreshold) nasal epithelium shocks (1.0 Hz) in 17/ 18 cells (mean increase=67%). The discharge rate of mitral cells at th e time that epithelium-evoked responses were increased did not differ significantly from pre-LC activation baseline values. Thus, changes in mitral baseline activity do not account for the increased response to epithelium stimulation. These findings suggest that increased activit y in LC-NE projections to MOB may enhance detection of relatively weak odors.