Olfactory bulb excitability selectively modified in behaving rats after local 6-hydroxydopamine treatment

Previous studies have shown that the olfactory bulb (OB) responsiveness was selectively enhanced towards food odor in free-moving hungry rats. this modulation of the olfactory input depended on the action of centrifugal fibers. The present study investigated the possible involvement of the noradrenergic (NA) projections from the locus coeruleus to the OB in this adaptative control of the OB excitability. After plugging of the olfactory lumen, 17 rats received discrete injections of 6-hydroxydopamine solution (10 micrograms of the salt; 40 nmol free base) into one OB (6-OHDA bulb) and the vehicle into the contralateral side (control bulb); 10 sham operated rats received intrabulbar isotonic saline solution injected bilaterally (sham bulbs). In the 12 6-OHDA bulbs with the noradrenaline content significantly reduced, the mean dopamine and noradrenaline levels were, respectively, 90% and 34% of the control values. One week after treatment, the average irritability score was significantly higher in the 6-OHDA treated animals than in the sham-operated ones. The main electrophysiological effect of the treatment was to amplify the selective enhancement of OB responsiveness to food odor normally occurring in hungry animals; this effect was restricted to the 6-OHDA bulbs. The results discussed considering the possible involvement of NA fibers in selective attention to odors at the OB level.