Effect of local 6-OHDA and 5,6-DHT injections into the rat olfactory bulb on neophobia and learned aversion to a novel food.

This study investigated the role of the noradrenergic (NA) and the serotoninergic (5-HT) centrifugal fibers on the olfactory bulb (OB). For this purpose, behavioral responses largely depending on olfactory information were measured in rats after discrete bilateral injections into the OB with either 6-hydroxydopamine (6-OHDA) or 5,6-dihydroxytryptamine (5,6-DHT) solutions. The evaluation of the level of endogenous 5-HT in the OB was measured by high performance liquid chromatography. The NA fibers appeared lesioned after the 6-OHDA treatment only, whereas the 5-HT level was significantly reduced in 5,6-DHT treated bulbs only (34% of the control value). The animals were fed on usual stock-diet (S) and had the choice between S and a novel food (V) on neophobia test, and again on the following day after aversive conditioning (0.9 mEq/kg LiCl i.p.). The behavioral results showed that the drug administrations were followed by a significant effect in one case only: the intake of V was significantly enhanced in 6-OHDA treated animals during neophobia test. Thus, neophobia was reduced by 6-OHDA. The data are interpreted as follows: (1) the lesion of the bulbar NA or 5-HT fibers does not induce total anosmia; and (2) the NA innervation of the OB could play a role in the modulation of behavioral responses to biological odorants.